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Removal of Synthetic Dyes by Dried Biomass of Freshwater Moss Vesicularia Dubyana: A Batch Biosorption Study

Removal of Synthetic Dyes by Dried Biomass of Freshwater Moss Vesicularia Dubyana: A Batch... environments Article Removal of Synthetic Dyes by Dried Biomass of Freshwater Moss Vesicularia Dubyana: A Batch Biosorption Study 1 , 2 1 1 1 , 1 Martin Pipíška , Martin Valica , Denisa Partelová , Miroslav Horník *, Juraj Lesný and Stanislav Hostin Department of Ecochemistry and Radioecology, University of SS. Cyril and Methodius in Trnava, Nam. J. Herdu 2, 91701 Trnava, Slovakia; pipiskam@gmail.com (M.P.); martin.valica@outlook.sk (M.V.); d.partelova@gmail.com (D.P.); juraj.lesny@ucm.sk (J.L.); stanislav.hostin@ucm.sk (S.H.) Department of Chemistry, Trnava University in Trnava, Priemyselná 4, 91843 Trnava, Slovakia * Correspondence: miroslav.hornik@ucm.sk; Tel.: +421-335-565-392 Received: 27 November 2017; Accepted: 6 January 2018; Published: 9 January 2018 Abstract: In this work the biosorption of cationic dyes thioflavin T (TT) and methylene blue (MB) from single and binary solutions on dried biomass of freshwater moss Vesicularia dubyana as a function of contact time, pH, and biomass or sorbate concentration has been investigated. The prediction of maximum sorption capacities using adsorption isotherm models were also realized. Biosorption of TT and MB is a rapid process strongly affected by solution pH. Maximum sorption capacities Q max calculated from Langmuir isotherm were 119  11 mg/g for TT and 229  9 mg/g for MB. In binary mixture, the presence of MB caused significant decrease of TT sorption, advocating the competitive sorption between TT and MB. Results revealed that V. dubyana biomass exhibited significantly higher affinity to thiazine dye MB in comparison with benzothiazole dye TT from both single and binary solutions. Based on the obtained results, the competitive effects in binary system can substantially influence the sorption process and should be thoroughly evaluated before application of selected adsorbents for removal of basic dyes from colored effluents. Keywords: biosorption; cationic dyes; moss; Vesicularia dubyana 1. Introduction Dyes are the important class of synthetic organic compounds used in many industries such as manufacture of pulp and paper, leather tanning or textile dying and in manufacture of dyestuffs. Ghaly et al. [1] pointed out that the textile industry is one of the major industries in the world and it plays a major role in the economy of many countries. The textile industry utilizes various chemicals (particularly synthetic dyes) and large amounts of water during the production process. The global textile dyes market was estimated to reach USD 4.7 billion in 2015, and is projected to reach USD 6.4 billion by 2019 and USD 8.75 billion by 2023 [2,3]. Industrial application of synthetic dyes is associated with the release of a huge amount of more or less colored effluents into the environment [4,5]. Wastewaters from the textile industry contain a large amount of dyes and chemicals containing trace metals such as Cr, As, Cu and Zn which are capable of harming the environment and human health [1]. During textile dyeing, a wide range of various dyes in a short time period are used, therefore effluents are extremely variable in composition and require an unspecific treatment processes [5]. Conventional physico-chemical (dilution, adsorption, coagulation and flocculation, oxidation, reverse osmosis and ultrafiltration) and biological (aerobic activated sludge and anaerobic processes) treatment technologies presently employed for color removal have several disadvantages such as long operational time, low specificity, formation and disposal of sludge and high Environments 2018, 5, 10; doi:10.3390/environments5010010 www.mdpi.com/journal/environments Environments 2018, 5, 10 2 of 12 cost (see e.g., critical reviews [1,6,7]. Consequently, the development of efficient clean-up technologies is of major interest. Highly efficient removal of synthetic dyes from effluents has been attempted by bioadsorption and non-conventional biosorbents have been searched in recent years that are easy available, renewable and environmentally friendly that can successfully replace the classical adsorbents [8]. To date, diverse algae, macroalgae, plant biomasses, agricultural residues as well as other biomaterials have been explored as adsorbents for removal of acidic, basic and reactive dyes from aqueous solutions [9–17]. Authors have reported that the biosorption capacity is highly dependent on solution pH, biosorbent dosage, temperature and concentration of other solutes present in solution. Since industrial effluents may contain several synthetic dyes, it is necessary to study the simultaneous sorption of two or more dyes and to quantify the mutual effect of one dye on the other. However, contradictory findings related to biosorption of dyes in multi-dye sorption systems were recently published. Albadarin et al. [18] who studied the simultaneous sorption of methylene blue and alizarin red S by olive stone biomass revealed only the limited competition between dyes. Remenárová et al. [19] confirmed that thioflavin T (TT) significantly affected biosorption of malachite green (MG) by moss R. squarrosus in binary system TT + MG. The competitive effect of MG on TT was less pronounced. On the contrary, Giwa et al. [20] found that the presence of rhodamine B and methylene blue had a synergetic effect on the maximum monolayer capacity of the sawdust of Parkia biglobosa for Acid Blue 161 dye in binary and ternary systems. To ensure the applicability of biosorption technology for colored effluents, more works are still needed for the sorption of a mixture of dyes at various operating conditions. Considering the above mentioned aspects, this study concerns the biosorption of cationic dyes thioflavin T and methylene blue from aqueous solutions by dried biomass of freshwater moss Vesicularia dubyana. The influence of variables (contact time, pH, biomass dosage) controlling the biosorption process has been considered in both single and binary dye systems. 2. Materials and Methods 2.1. Moss Biomass The biomass of freshwater moss Vesicularia dubyana was used as a sorbent of cationic dyes thioflavin T (TT) and methylene blue (MB). Vital moss biomass was cultivated in diluted Hoagland medium [21] under artificial illumination (2000 lx) at 22  2 C. Before use in sorption experiments, the biomass was thoroughly rinsed in deionized water (3 times) and dried at 60 C for 48 h. 2.2. Reagents and Instruments Thioflavin T (C.I. 49005, M (C H ClN S) 318.86, CAS 2390-54-7) and methylene blue (C.I. 17 19 2 52015, M (C H N SCl) 319.86; CAS 61-73-4) were purchased from Fluka (USA). All chemicals r 16 18 3 were of analytical grades. Cationic dyes solutions were prepared in deionized water (conductivity 0.054 S/cm; Millipore Simplicity). An UV-VIS spectrophotometer Cary 50 (Varian, Australia) was used for determination of TT and MB concentrations in solutions and for establishing calibration curves for TT and MB at maximum absorbances TT = 412 nm and MB = 650 nm. The pH effect on MB and TT concentration determination was taken into account. 2.3. Sorption Kinetics in Single and Binary Solutions in Batch System Dried biomass of V. dubyana (0.5 g/L) was added to 20 mL of solutions containing defined concentrations of thioflavin T (TT), methylene blue (MB) or their mixtures, respectively. Solution pH was adjusted to 6.0. The flasks were incubated on a rotary shaker (250 rpm) at 25 C. At the time intervals 10, 20, 40, 60, 120, 240, 360 and 1440 min, aliquot samples were obtained and remaining Environments 2018, 5, 10 3 of 12 concentration of TT and MB determined. All experiments were performed in duplicate series. The TT and MB uptake was calculated according to Equation (1). Q = (C C ) (1) t 0 t where Q represents the amount of TT or BB sorbed by moss biomass (mg/g d.w.) from single or binary solutions at time t; C and C represent the initial concentration of dyes in solution and the 0 t concentration of dyes at time t (mg/g). V is solution volume (L) and M is the amount of moss biomass (g; d.w.). 2.4. Influence of pH and Biomass Dosage To analyze the influence of pH, dried biomass (0.5 g/L) was shaken in 20 mL of TT and MB (C = 40 mg/L or C = 80 mg/L) or TT + MB (C = 40 mg/L and C = 40 mg/L) solutions 0 TT 0 MB 0 TT 0 MB of desired pH for 2 h on a rotary shaker at 250 rpm and 25 C. In order to eliminate interference of buffer components on cationic dyes biosorption, the non-buffered solutions in deionized water were adjusted to the desired pH values by adding 0.1 M HCl or 0.1 M NaOH. To analyze the influence of biomass dosage, dried biomass of desired amount (C = 0.25 to 4.0 g/L) was shaken in 20 mL of TT and MB (C = 40 mg/L or C = 80 mg/L) or TT + MB (C = 40 mg/L 0 TT 0 MB 0 TT and C = 40 mg/L) solutions for 2 h on a rotary shaker at 250 rpm, pH 6.0 and 25 C. 0 MB At the end of the experiments, aliquot samples were obtained and remaining concentration of TT and MB determined. All experiments were performed in duplicate series. The TT and MB uptake was calculated according to Equation (1). 2.5. Sorption Equilibrium in Single and Binary Solutions in Batch System Dried biomass of V. dubyana (0.5 g/L) was added to 20 mL of solutions with initial dye concentrations C in single system ranging from 20 to 200 mg/L (TT) and 20 to 320 mg/L (MB) and in binary system TT + MB from 40 to 160 mg/L. In both single and binary solutions pH was adjusted to 6.0. Flasks were incubated on a rotary shaker (250 rpm) at 25 C. After 2 h of exposure, the remaining concentration of TT and MB was determined. All experiments were performed in duplicate series. The TT and MB uptake was calculated according to Equation (1). Equilibrium data in both single and binary systems were analyzed using adsorption isotherm models according to Langmuir and Freundlich. To calculate the corresponding parameters of isotherms non-linear regression analysis was performed by OriginPro 2016 (OriginLab Corporation, Northampton, MA, USA). 2.6. Potentiometric Titration Potentiometric titration experiment was carried out according to the modified procedure described by Zhang et al. [22]. Dried biomass of freshwater moss V. dubyana (0.30 g) were protonated with 100 mL of equimolar 0.1 mol/L HCl and NaCl for 2 h on the rotary shaker (200 rpm) at 25 C. Subsequently, moss biomass was separated by centrifugation and transferred into 100 mL of 0.1 mol/L NaCl and agitated under the same conditions. Potentiometric titration was performed in an Erlenmeyer flask with glass electrode (three-point calibrated with buffers—pH 4.0, 7.0 and 10.0) by dropwise addition of 0.1 mol/L mixture of NaOH and NaCl into the bacterial suspension. The titration was conducted in the pH range of 2.0–11.0 and the potentiometric titration curve was obtained by plotting the volume of 0.l mol/L NaOH addition as titrant solution against the pH values measured. ProtoFit ver. 2.1 was used to analyse the titration data. Environments 2018, 5, 10 4 of 12 on V. dubyana biomass are occupied. Initial phase is followed by slower gradual uptake till Environments 2018, 5, 10 4 of 12 equilibrium is reached. In case of MB, the maximum uptake in single system at initial concentration 80 mg/L was 3. Results and Discussion observed after 120 min of exposure (130 ± 5 mg/g; d.w.). Uptake of TT at initial concentration 40 mg/L increased rapidly in the first 90 min and after 120 min reached 43.5 ± 2.7 mg/g. The final equilibrium 3.1. Biosorption Kinetics in Single and Binary Systems was reached within the 2 h and after this time, there was no considerable increase in both TT and MB sorption until the end of experiments. Our findings are in agreement with other studies of cationic As expected, the uptake of cationic dyes TT and MB by dried biomass of freshwater moss dyes sorption by various types of biomass. Similar behavior of MB sorption kinetics by green alga V. dubyana from both single and binary solutions was a rapid process (Figure 1A,B). At the initial phase Enteromorpha spp. [23] and red (Gracilaria parvispora), brown (Nizamuddinia zanardinii) and green (Ulva of uptake, driving force is high (uptake increased linearly) and available high affinity binding sites on fasciata) macroalga [9] was observed. Hameed [24] found that at higher initial concentration of MB V. dubyana biomass are occupied. Initial phase is followed by slower gradual uptake till equilibrium longer contact time is needed to reach equilibrium when grass waste was used as sorbent. is reached. A 140 120 TT MB TT MB 40 40 20 20 0 0 0 200 400 600 800 1000 1200 1400 1600 0 200 400 600 800 1000 1200 1400 1600 Time [min] Time [min] Figure 1. Kinetics of thioflavin T (TT) and methylene blue (MB) biosorption by dried biomass of moss Figure 1. Kinetics of thioflavin T (TT) and methylene blue (MB) biosorption by dried biomass of moss V. dubyana (CB = 0.5 g/L) from single (A. C0 TT = 40 mg/L or C0 MB =80 mg/L) and binary (B. C0 TT = 40 V. dubyana (C = 0.5 g/L) from single (A. C = 40 mg/L or C =80 mg/L) and binary (B. C = B 0 TT 0 MB 0 TT mg/L and C0 MB = 40 mg/L) solutions at 25 °C and pH 6.0. Error bars represent standard deviation of 40 mg/L and C = 40 mg/L) solutions at 25 C and pH 6.0. Error bars represent standard deviation 0 MB the mean (±SD, n = 2). of the mean (SD, n = 2). Biosorption of TT and MB by V. dubyana biomass from binary solution is shown in Figure 1B. In case of MB, the maximum uptake in single system at initial concentration 80 mg/L was Although the same kinetic profile as in the case of dye sorption in single systems was observed, an observed after 120 min of exposure (130  5 mg/g; d.w.). Uptake of TT at initial concentration 40 mg/L evident decrease of TT and MB biosorption capacities in comparison with single systems were increased rapidly in the first 90 min and after 120 min reached 43.5  2.7 mg/g. The final equilibrium recorded as a result of competitive effects (see discussion below). The final equilibrium of both TT was reached within the 2 h and after this time, there was no considerable increase in both TT and MB and MB was reached within 120 min. At initial dye concentration 40 mg/L, maximum sorption of TT sorption until the end of experiments. Our findings are in agreement with other studies of cationic (28.3 ± 0.3 mg/g d.w.) in binary system was significantly lower than maximum sorption of MB (62.7 dyes sorption by various types of biomass. Similar behavior of MB sorption kinetics by green alga ± 0.3 mg/g, d.w.). The similar kinetic profile in binary (MB + rhodamine B) and single biosorption Enteromorpha spp. [23] and red (Gracilaria parvispora), brown (Nizamuddinia zanardinii) and green (Ulva system observed Fernandez et al. [25] using Cupressus sempervirens cone chips. fasciata) macroalga [9] was observed. Hameed [24] found that at higher initial concentration of MB It is evident that V. dubyana biomass exhibited significantly higher affinity to thiazine dye MB in longer contact time is needed to reach equilibrium when grass waste was used as sorbent. comparison with benzothiazole dye TT from both single and binary solutions. In both dye molecules Biosorption of TT and MB by V. dubyana biomass from binary solution is shown in Figure 1B. the positive charge is present on quaternary nitrogen =N =, however in TT quaternary N is located in Although the same kinetic profile as in the case of dye sorption in single systems was observed, an evident position 3, substituted by one methyl group and in MB quaternary N is located in position 3 or 7 and decrease of TT and MB biosorption capacities in comparison with single systems were recorded as a result substituted by 2 methyl groups. Therefore, we suppose that among other mechanisms, steric effects of competitive effects (see discussion below). The final equilibrium of both TT and MB was reached within in dye molecules can affect dye affinity to moss biomass as well. 120 min. At initial dye concentration 40 mg/L, maximum sorption of TT (28.3  0.3 mg/g d.w.) in binary system was significantly lower than maximum sorption of MB (62.7  0.3 mg/g, d.w.). The similar 3.2. Influence of pH kinetic profile in binary (MB + rhodamine B) and single biosorption system observed Fernandez et al. [25] The pH of dye solution is one of the crucial parameters affecting the sorption process through using Cupressus sempervirens cone chips. controlling both the ionization of dye molecules and the degree of ionization of functional groups It is evident that V. dubyana biomass exhibited significantly higher affinity to thiazine dye MB in present on biomass surface. comparison with benzothiazole dye TT from both single and binary solutions. In both dye molecules The influence of solution pH (2.0 to 7.0) on TT and MB sorption by V. dubyana biomass from the positive charge is present on quaternary nitrogen =N =, however in TT quaternary N is located in single solutions is shown in Figure 2A. The lowest sorption capacities Q for both TT (QTT = 12 mg/g position 3, substituted by one methyl group and in MB quaternary N is located in position 3 or 7 and d.w.) and MB (QMM = 4 mg/g d.w.) dyes were observed at low initial pH value (pH0 = 2.0) which could substituted by 2 methyl groups. Therefore, we suppose that among other mechanisms, steric effects in be closely related to protonation of binding sites on biomass surface. The low adsorption capacity of dye molecules can affect dye affinity to moss biomass as well. Q [mg/g; d.w.] Q [mg/g; d.w.] Environments 2018, 5, 10 5 of 12 3.2. Influence of pH The pH of dye solution is one of the crucial parameters affecting the sorption process through controlling both the ionization of dye molecules and the degree of ionization of functional groups present on biomass surface. The influence of solution pH (2.0 to 7.0) on TT and MB sorption by V. dubyana biomass from single solutions is shown in Figure 2A. The lowest sorption capacities Q for both TT (Q = 12 mg/g d.w.) TT and MB (Q = 4 mg/g d.w.) dyes were observed at low initial pH value (pH = 2.0) which could EnvironmMM ents 2018, 5, 10 0 5 of 12 be closely related to protonation of binding sites on biomass surface. The low adsorption capacity of MB can also be explained by the fact that MB is mainly found as undissociated species MB (99% at 0 MB can also be explained by the fact that MB is mainly found as undissociated species MB (99% at pH = 2.0 and 86% at pH = 3.0) [26]. Obtained results indicate that an increase in pH has a positive pH = 2.0 and 86% at pH = 3.0) [26]. Obtained results indicate that an increase in pH has a positive effect on TT and MB sorption, since the competitions between dye cations and protons for the binding effect on TT and MB sorption, since the competitions between dye cations and protons for the binding sites decreases and such curves (Figure 2A) represent a typical cationic dye sorption behavior [10,18]. sites decreases and such curves (Figure 2A) represent a typical cationic dye sorption behavior [10,18]. 140 140 TT TT 120 120 MB MB 23 456 7 23 45 67 pH pH Figure Figure 2. 2. Influence Influence of of initial initial pH pHon 0 onthioflavin thioflavin T (TT) T (TT) and m and methylene ethylene blue ( blue M(MB) B) biosorptio biosorption n by biomass by biomass 3 3 of moss V. dubyana (CB = 0.5 g/dm ) from single (C0 TT = 40 mg/L or C0 MB 80 mg/L; A) and binary (C0 TT of moss V. dubyana (C = 0.5 g/dm ) from single (C = 40 mg/L or C 80 mg/L; A) and binary B 0 TT 0 MB = 40 mg/L and C0 MB = 40 mg/L; B) solutions at 25 °C for 2 h. Error bars represent standard deviation (C = 40 mg/L and C = 40 mg/L; B) solutions at 25 C for 2 h. Error bars represent standard 0 TT 0 MB of the mean (±SD, n = 2). deviation of the mean (SD, n = 2). The highest Q values were recorded at pH > 4.0 with maximum QTT = 44 ± 1 mg/g d.w. and QMM The highest Q values were recorded at pH > 4.0 with maximum Q = 44  1 mg/g d.w. and = 123 ± 3 mg/g d.w. at pH0 = 7.0. Similarly, Albadarin et al. [23] observed TT maximum biosorption Q capac = 123ity of  3 mg/g methylene blue by olive d.w. at pH = 7.0. Similarly stone by-p , Albadarin roducts atet pal H. = 7 [23.]2 observed and maxim maximum um sorptbiosorption ion of MM 0 capacity thiofl of avin methylene T by agric blue ultur by alolive by-pro stone ductby-pr s from t oducts he hoat p (pH Humulus lup = 7.2 and ulus maxi L.) w mum as fo sorption und witof hin t thioflavin he range of initial values of pH 4.0–7.0 [27]. T by agricultural by-products from the hop (Humulus lupulus L.) was found within the range of initial The same behavior was observed in binary sorption system with equimolar initial values of pH 4.0–7.0 [27]. concentrations of TT and MB. Dye sorption capacities increased from 8.2 ± 0.1 mg/g to 22.7 ± 0.4 mg/g The same behavior was observed in binary sorption system with equimolar initial concentrations (TT) and 10.5 ± 1.4 mg/g to 54.6 ± 0.1 mg/g (MB) with increasing solution pH from 2.0 to 4.0 and of TT and MB. Dye sorption capacities increased from 8.2  0.1 mg/g to 22.7  0.4 mg/g (TT) and remained almost stable till pH 7.0. From Figure 2B it is evident that the biomass of V. dubyana 10.5  1.4 mg/g to 54.6  0.1 mg/g (MB) with increasing solution pH from 2.0 to 4.0 and remained exhibited significantly higher affinity toward MB. almost stable till pH 7.0. From Figure 2B it is evident that the biomass of V. dubyana exhibited As was mentioned earlier, the dye sorption capacities are to a great extent affected by the significantly higher affinity toward MB. dissociation of functional groups present on moss surface. From the potentiometric titration curve As was mentioned earlier, the dye sorption capacities are to a great extent affected by the dissociation (Figure 3A), it is evident that from pH ~ 4.0 a small addition of NaOH caused a dramatic changes in solution pH what indicates that relevant acidic functional groups (carboxyl, phosphoryl) present on of functional groups present on moss surface. From the potentiometric titration curve (Figure 3A), it is biomass surface are dissociated. To qualitative characterization of functional groups presented on the evident that from pH ~4.0 a small addition of NaOH caused a dramatic changes in solution pH what surface of moss biomass and to determination of binding sites concentration (CAn) the prediction indicates that relevant acidic functional groups (carboxyl, phosphoryl) present on biomass surface modelling was used. The titration curve shows a relatively unpronounced inflection point, which are dissociated. To qualitative characterization of functional groups presented on the surface of moss predicts the existence of several functional groups. According to the ProtoFit prediction and obtained biomass and to determination of binding sites concentration (C ) the prediction modelling was used. An values of residual sum of squares (RSS), it was found that the titration profile was best described by The titration curve shows a relatively unpronounced inflection point, which predicts the existence of a non-electrostatic model for characterization of the four binding sites. The predicted functional several functional groups. According to the ProtoFit prediction and obtained values of residual sum groups with relevant pKa values and concentrations of binding sites (CAn) presented on the surface of of squares (RSS), it was found that the titration profile was best described by a non-electr+ostatic m + odel moss biomass are listed in Table 1. In connection to these results, we can suppose that MB and TT for ch adsorb aractee rd iz on sur ation o face f thand b e four alb ainced ndina neg g sitea st.iv Te h cha e pr re ge diof biomass. C cted function oa nsequently, l groups wone of the possib ith relevant pKa lv e alues mechanism of MB and TT biosorption by freshwater moss V. dubyana is the electrostatic attraction. and concentrations of binding sites (C ) presented on the surface of moss biomass are listed in Table 1. An Q [mg/g; d.w.] Q [mg/g; d.w.] Environments 2018, 5, 10 6 of 12 Table 1. Predicted functional groups according to pKa values and concentration of binding sites (CAn) presented on the surface of moss biomass (V. dubyana) using modelling software ProtoFit ver. 2.1. Functional Group pKa CAn (mmol/g) COOH 2.34–6.25 0.98 PO3H2 6.36–8.72 0.69 NH2 10.7–11.5 0.61 OH 8.36–12.0 0.64 During and after cationic dyes biosorption, the pH of the dye solutions in both single and binary sorption systems changed (Figure 4B). The solution pH increased from initial pH0 values 4.0, 5.0 and 6.0 to pHeq 5.2, 6.0 and 6.1 (MB), 6.6, 6.8 and 7.0 (TT) and 5.8, 6.4, 6.6 (TT + MB). On the contrary, a Environments 2018, 5, 10 6 of 12 slight decrease of solution pH was observed from pH0 = 7.0 to pHeq = ~6.6 probably as a result of a TT–hydrogen and/or MB–hydrogen ion-exchange (Figure 3B). This equilibrium value of pHeq also corresponds to the value of pHzpc = 7.78 predicted within the potentiometric titration and ProtoFit + + In connection to these results, we can suppose that MB and TT adsorbed on surface and balanced a analysis, whereby the pHzpc represents the value of pH at which the biosorbent shows a net zero negative charge of biomass. Consequently, one of the possible mechanism of MB and TT biosorption by surface charge. freshwater moss V. dubyana is the electrostatic attraction. 10 8 TT A B MB TT+MB without biomass 123456789 10 11 12 23 4 5 67 8 pH pH Figure 3. Potentiometric titration curve for dried biomass of freshwater moss V. dubyana (3.0 g/L) Figure 3. Potentiometric titration curve for dried biomass of freshwater moss V. dubyana (3.0 g/L) obtained by gradual addition of titrant 0.1 M NaOH at background electrolyte 0.1 M NaCl and obtained by gradual addition of titrant 0.1 M NaOH at background electrolyte 0.1 M NaCl and reaction reaction temperature 25 °C (A). The change of initial pH0 of dye (single and binary) solutions after temperature 25 C (A). The change of initial pH of dye (single and binary) solutions after biosorption biosorption and in control experiments in deionized water and without biomass (B). and in control experiments in deionized water and without biomass (B). 3.3. Influence of Biomass Dosage Table 1. Predicted functional groups according to pKa values and concentration of binding sites (C ) An To investigate the biomass dosage on TT and MB removal, various amounts of dried moss presented on the surface of moss biomass (V. dubyana) using modelling software ProtoFit ver. 2.1. biomass were added to both single and binary dye solutions. From Figure 4A it is a clearly seen that the sorption capacity Q of both TT and MB decreased from 241 mg/g to 118 mg/g (MB) and from 50.6 Functional Group pKa C (mmol/g) An mg/g to 2.6 mg/g (TT) with increasing concentration of moss biomass CB in single solutions. Different COOH 2.34–6.25 0.98 relationship between the sorption capacities of dyes Q and the concentration of biomass CB was PO H 6.36–8.72 0.69 3 2 NH 10.7–11.5 0.61 observed in binary sorption system 2 TT + MB (Figure 4B). The sorption capacity of TT changed only OH 8.36–12.0 0.64 slightly with increasing concentration of moss biomass in solution. On the contrary, the sorption capacity Q of MB decreased significantly (from 107 to 52.8 mg/g) with increasing CB. We suppose that competitive effects between cation dyes and binding sites play an important role and influence the During and after cationic dyes biosorption, the pH of the dye solutions in both single and binary sorption behavior. sorption systems changed (Figure 4B). The solution pH increased from initial pH values 4.0, 5.0 and 6.0 to Tabaraki and Sadeghinejad [28] pointed out that an increase of biosorbent dose generally pH 5.2, 6.0 and 6.1 (MB), 6.6, 6.8 and 7.0 (TT) and 5.8, 6.4, 6.6 (TT + MB). On the contrary, a slight decrease eq increases the overall amount of dye biosorbed (removal efficiency), due to the increased surface area Environments 2018, 5, 10 7 of 12 of solution pH was observed from pH = 7.0 to pH = ~6.6 probably as a result of a TT–hydrogen and/or 0 eq of biosorbent which in turn increases the number of binding sites. Thus, the decrease in the amount MB–hydrogen ion-exchange (Figure 3B). This equilibrium value of pH also corresponds to the value eq of dye sorbed per gram of biosorbent with increase in the biosorbent 2 dose is mainly due to observed negative aggregation and changes in specific surface area (m /g) as well as changes in of pH = 7.78 predicted within the potentiometric titration and ProtoFit analysis, whereby the pH zpc zpc insaturation of binding sites through the sorption process [29]. However, Kumar and Porkodi [30] effective mixing of biomass in sorption systems when higher biomass concentrations were used. represents the value of pH at which the biosorbent shows a net zero surface charge. 250 120 A B TT TT MB MB 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 0,00,5 1,01,5 2,0 2,5 3,03,5 4,0 C [g/L] C [g/L] B B Figure 4. The effect of biomass concentration C on thioflavin T (TT) and methylene blue (MB) Figure 4. The effect of biomass concentration CB B on thioflavin T (TT) and methylene blue (MB) biosorption biosorption by by dried dried biomass biomass of V of . dubyana V. dubyanfr a om from single single ( ( CC0 TT = 40 = 40 mg/L or mg/L or C0C MB = 80 mg/L; = 80 mg/L; (A) a( nA d) and 0 TT 0 MB binary (C0 TT = 40 mg/L and C0 MB = 40 mg/L; (B) solutions at 25 °C, pH 6.0 for 2 h. Error bars represent binary (C = 40 mg/L and C = 40 mg/L; (B) solutions at 25 C, pH 6.0 for 2 h. Error bars 0 TT 0 MB standard deviation of the mean (±SD, n = 2). represent standard deviation of the mean (SD, n = 2). 3.4. Sorption Equilibrium in Single and Binary Systems The adsorption equilibrium data of TT and MB from both single and binary sorption systems were described by adsorption isotherms according to Langmuir and Freundlich. The isotherm parameters obtained by non-linear regression analysis are reported in Table 2. Coefficients of determination (R ) related to the Langmuir model applied to TT and MB biosorption 2 2 2 data (R = 0.965 for TT; R = 0.990 for MB) were higher than those for the Freundlich model (R = 0.955 for TT; R = 0.938 for MB). In addition, the biosorption of methylene blue and thioflavin T by agricultural by-products from the hop (Humulus lupulus L.) [27] and methylene blue by water hyacinth biomass [31], agro-waste oil tea shell [13], Anethum graveolens biomass [32] and Tremella fuciformis biomass [33] was very well fitted by Langmuir model. Table 2. Langmuir and Freundlich equilibrium parameters (±SD) obtained by non-linear regression analysis for TT and MB biosorption by V. dubyana biomass in single systems. Langmuir Freundlich Dye 2 2 Qmax (mg/g) b (L/mg) R K (L/g) 1/nR TT 119 ± 11 0.04 ± 0.01 0.965 15.5 ± 3.8 2.58 ± 0.38 0.955 MB 229 ± 9 0.07 ± 0.01 0.990 45.2 ± 11.5 3.27 ± 0.59 0.938 TT: Thioflavin T; MB: methylene blue. Maximum sorption capacity Qmax and constant b, calculated from Langmuir isotherm model enable to characterize the affinity of V. dubyana biomass towards TT and MB (Table 2) in single sorption systems. Qmax for TT (119 ± 11 mg/g, d.w.) was significantly lower in comparison with Qmax for MB (229 ± 9 mg/g, d.w.). The affinity constant b of the isotherm corresponds to the initial gradient, which indicates the V. dubyana biomass affinity at low concentrations of both TT and MB. Accordingly, a higher initial gradient corresponds to a higher affinity constant b. It is evident, that Langmuir isotherm for MB is steeper at lower equilibrium concentrations than those for TT (Figure 5A,B). The difference in the b values 0.04 ± 0.01 L/mg (TT) and 0.07 ± 0.01 L/mg (MB), confirmed the higher affinity of V. dubyana biomass to MB in comparison with TT. This was also reflected by higher Qmax for MB (Table 2). The comparison of maximal sorption capacities Qmax determined from Langmuir model with those of other authors is reported in Table 3. Although the data about TT (bio)sorption are very Q [mg/g; d.w.] Added volume of 0.1 M NaOH [mL] pH eq Q [mg/g; d.w.] Environments 2018, 5, 10 7 of 12 3.3. Influence of Biomass Dosage To investigate the biomass dosage on TT and MB removal, various amounts of dried moss biomass were added to both single and binary dye solutions. From Figure 4A it is a clearly seen that the sorption capacity Q of both TT and MB decreased from 241 mg/g to 118 mg/g (MB) and from 50.6 mg/g to 2.6 mg/g (TT) with increasing concentration of moss biomass C in single solutions. Different relationship between the sorption capacities of dyes Q and the concentration of biomass C was observed in binary sorption system TT + MB (Figure 4B). The sorption capacity of TT changed only slightly with increasing concentration of moss biomass in solution. On the contrary, the sorption capacity Q of MB decreased significantly (from 107 to 52.8 mg/g) with increasing C . We suppose that competitive effects between cation dyes and binding sites play an important role and influence the sorption behavior. Tabaraki and Sadeghinejad [28] pointed out that an increase of biosorbent dose generally increases the overall amount of dye biosorbed (removal efficiency), due to the increased surface area of biosorbent which in turn increases the number of binding sites. Thus, the decrease in the amount of dye sorbed per gram of biosorbent with increase in the biosorbent dose is mainly due to insaturation of binding sites through the sorption process [29]. However, Kumar and Porkodi [30] observed negative aggregation and changes in specific surface area (m /g) as well as changes in effective mixing of biomass in sorption systems when higher biomass concentrations were used. 3.4. Sorption Equilibrium in Single and Binary Systems The adsorption equilibrium data of TT and MB from both single and binary sorption systems were described by adsorption isotherms according to Langmuir and Freundlich. The isotherm parameters obtained by non-linear regression analysis are reported in Table 2. Coefficients of determination (R ) related to the Langmuir model applied to TT and MB biosorption 2 2 2 data (R = 0.965 for TT; R = 0.990 for MB) were higher than those for the Freundlich model (R = 0.955 for TT; R = 0.938 for MB). In addition, the biosorption of methylene blue and thioflavin T by agricultural by-products from the hop (Humulus lupulus L.) [27] and methylene blue by water hyacinth biomass [31], agro-waste oil tea shell [13], Anethum graveolens biomass [32] and Tremella fuciformis biomass [33] was very well fitted by Langmuir model. Table 2. Langmuir and Freundlich equilibrium parameters (SD) obtained by non-linear regression analysis for TT and MB biosorption by V. dubyana biomass in single systems. Langmuir Freundlich Dye 2 2 Q (mg/g) b (L/mg) K (L/g) 1/n R R max TT 119  11 0.04  0.01 0.965 15.5  3.8 2.58  0.38 0.955 MB 229  9 0.07  0.01 0.990 45.2  11.5 3.27  0.59 0.938 TT: Thioflavin T; MB: methylene blue. Maximum sorption capacity Q and constant b, calculated from Langmuir isotherm model max enable to characterize the affinity of V. dubyana biomass towards TT and MB (Table 2) in single sorption systems. Q for TT (119  11 mg/g, d.w.) was significantly lower in comparison with Q for max max MB (229  9 mg/g, d.w.). The affinity constant b of the isotherm corresponds to the initial gradient, which indicates the V. dubyana biomass affinity at low concentrations of both TT and MB. Accordingly, a higher initial gradient corresponds to a higher affinity constant b. It is evident, that Langmuir isotherm for MB is steeper at lower equilibrium concentrations than those for TT (Figure 5A,B). The difference in the b values 0.04  0.01 L/mg (TT) and 0.07  0.01 L/mg (MB), confirmed the higher affinity of V. dubyana biomass to MB in comparison with TT. This was also reflected by higher Q for max MB (Table 2). Environments 2018, 5, 10 8 of 12 limited, our results indicate that V. dubyana biomass exhibited the highest Qmax in comparison with other organic and inorganic sorbents. Similarly, in case of MB biosorption the moss biomass has a high Qmax when compared with different sorbents of plant origin. Environments 2018, 5, 10 8 of 12 120 250 A B Freundlich Freundlich Langmuir Langmuir 0 20 40 60 80 100 120 140 160 0 50 100 150 200 250 C [mg/L] C [mg/L] eq eq Figure 5. Fit of the Langmuir and Freundlich isotherms of thioflavin T (TT); (A) and methylene blue Figure 5. Fit of the Langmuir and Freundlich isotherms of thioflavin T (TT); (A) and methylene blue (MB); (B) sorption by dried biomass of freshwater moss V. dubyana (CB = 0.5 g/dm ) from single (MB); (B) sorption by dried biomass of freshwater moss V. dubyana (C = 0.5 g/dm ) from single sorption systems after 2 h interactions at 25 °C and pH 6.0. sorption systems after 2 h interactions at 25 C and pH 6.0. Table 3. Comparison of Qmax values of different sorbents determined from Langmuir isotherm for TT The comparison of maximal sorption capacities Q determined from Langmuir model with max and MB. those of other authors is reported in Table 3. Although the data about TT (bio)sorption are very limited, our results indicate Sorbent that V. dubyana Qma biomass x MB (mg/g exhibited ) Qmax TT the(mg/g) pH highest Q T in(°C) comparison Reference with other max organic and inorganic sorbents. Similarly, in case of MB biosorption the moss biomass has a high Q V. dubyana 229 119 6.0 25 present study max when compar hoped leaf b with iom dif ass fer ent sorbents18 of4plant origin. 77.6 6.0 25 [27] Rhytidiadelphus squarrosus - 98.9 4.0 25 [34] Table 3. Comparison of Q values of different sorbents determined from Langmuir isotherm for TT max montmorillonite * - 95.2 6.0 25 [35] and MB. Fomitopsis carnea - 21.9 - 30 [14] Corn husk 47.95 - 6.0 25 [8] Sorbent Q (mg/g) Q (mg/g) pH T ( C) Reference max MB max TT Oak acorn peel 120.5 - 7.0 24 [36] V. dubyana 229 119 6.0 25 present study Salvia miltiorrhiza 100 - 7.0 [12] hop leaf biomass 184 77.6 6.0 25 [27] Eucalyptus bark 204.8 - 9.9 30 [37] Rhytidiadelphus squarrosus - 98.9 4.0 25 [34] Platanus leaf 99.1 - 7.0 30 [38] montmorillonite * - 95.2 6.0 25 [35] Fomitopsis Banacarnea na stalk - 243.9 21.9 - - - 30 30 [39] [14 ] Corn husk 47.95 - 6.0 25 [8] * Cetylpyridinium modified. Oak acorn peel 120.5 - 7.0 24 [36] Salvia miltiorrhiza 100 - 7.0 [12] In comparison with single sorption systems, in binary or multicomponent mixtures cationic dyes Eucalyptus bark 204.8 - 9.9 30 [37] may interact or compete for binding sites of moss biomass. Therefore, the behavior of each species in Platanus leaf 99.1 - 7.0 30 [38] Banana stalk 243.9 - - 30 [39] a multicomponent system depends strongly on the number and properties of other species present. In addition, the solution pH, the physical and chemical properties of both the sorbent and sorbate * Cetylpyridinium modified. significantly influenced the sorption process. The simultaneous Langmuir and Freundlich sorption isotherms of MB and TT from binary system MB + TT (concentration ratio C0 TT:C0 MB = 1:1) by V. In comparison with single sorption systems, in binary or multicomponent mixtures cationic dubyana biomass are shown in Figure 6A,B. Table 4 presents isotherm parameters calculated by non- dyes may interact or compete for binding sites of moss biomass. Therefore, the behavior of each linear regression analysis. species in a multicomponent system depends strongly on the number and properties of other species present. In addition, the solution pH, the physical and chemical properties of both the sorbent and sorbate significantly influenced the sorption process. The simultaneous Langmuir and Freundlich sorption isotherms of MB and TT from binary system MB + TT (concentration ratio C :C = 1:1) 0 TT 0 MB by V. dubyana biomass are shown in Figure 6A,B. Table 4 presents isotherm parameters calculated by non-linear regression analysis. Q [mg/g; d.w.] eq Q [mg/g; d.w.] eq Environm Environments ents 2018 2018, ,5 5,, 10 10 9 of 9 of 12 12 TT TT B MB MB 80 80 20 20 0 20 406080 100 120 140 0 20 40 60 80 100 120 140 C [mg/L] C [mg/L] eq eq Figure 6. Fit of the Langmuir (A) and Freundlich (B) isotherms of thioflavin T (TT) and methylene blue Figure 6. Fit of the Langmuir (A) and Freundlich (B) isotherms of thioflavin T (TT) and methylene blue (MB) biosorption by dried biomass of freshwater moss V. dubyana (CB = 0.5 g/dm ) from binary sorption (MB) biosorption by dried biomass of freshwater moss V. dubyana (C = 0.5 g/dm ) from binary sorption system TT + MB (concentration ratio C0 TT:C0 MB = 1:1) after 2 h interactions at 25 °C and pH 6.0. system TT + MB (concentration ratio C :C = 1:1) after 2 h interactions at 25 C and pH 6.0. 0 TT 0 MB Table 4. Langmuir and Freundlich equilibrium parameters (±SD) obtained by non-linear regression Table 4. Langmuir and Freundlich equilibrium parameters (SD) obtained by non-linear regression analysis for TT and MB biosorption by V. dubyana biomass in binary system TT + MB. analysis for TT and MB biosorption by V. dubyana biomass in binary system TT + MB. Langmuir Freundlich Dye Langmuir Freundlich 2 2 Dye Qmax (mg/g) b (L/mg) R K (L/g) 1/n R 2 2 Q (mg/g) b (L/mg) K (L/g) 1/n R R max TT 49.3 ± 6.2 0.03 ± 0.01 0.905 6.92 ± 1.46 2.77 ± 0.36 0.972 TT 49.3  6.2 0.03  0.01 0.905 6.92  1.46 2.77  0.36 0.972 MB 160 ± 17 0.05 ± 0.02 0.950 25.4 ± 0.72 2.67 ± 0.05 0.999 MB 160  17 0.05  0.02 0.950 25.4  0.72 2.67  0.05 0.999 TT + MB 232 ± 29 0.013 ± 0.004 0.955 16.7 ± 1.9 2.29 ± 0.12 0.839 TT + MB 232  29 0.013  0.004 0.955 16.7  1.9 2.29  0.12 0.839 1 2 Sorption of individual dye in binary system TT + MB. Total TT and MB sorption (Qeq = Qeq(TT) + Qeq 1 2 Sorption of individual dye in binary system TT + MB. Total TT and MB sorption (Q = Q (TT) + Q (MB)) in eq eq eq (MB)) in binary system TT + MB. binary system TT + MB. It is evident that sorption of both TT and MB increased with increasing of dyes concentrations It is evident that sorption of both TT and MB increased with increasing of dyes concentrations in in binary mixture. However, the coexistence of MB and TT influenced the sorption capacity of each binary mixture. However, the coexistence of MB and TT influenced the sorption capacity of each other, other, advocating the competitive sorption between TT and MB. The sorption of MB is less affected advocating the competitive sorption between TT and MB. The sorption of MB is less affected by the by the presence of TT than the sorption of TT by the presence of MB in solution. presence of TT than the sorption of TT by the presence of MB in solution. Maximum sorption capacities Qmax of cationic dyes in binary system TT + MB calculated from Maximum sorption capacities Q of cationic dyes in binary system TT + MB calculated from max Langmuir model reached 49.3 ± 6.2 mg/g (TT) and 160 ± 17 mg/g (MB) and values of affinity Langmuir model reached 49.3  6.2 mg/g (TT) and 160  17 mg/g (MB) and values of affinity parameter b (0.03 ± 0.01 L/mg for TT; 0.05 ± 0.02 L/mg for MB) indicating markedly higher affinity of parameter b (0.03  0.01 L/mg for TT; 0.05  0.02 L/mg for MB) indicating markedly higher affinity of moss biomass to MB. However, both Qmax values in binary system are significantly lower in moss biomass to MB. However, both Q values in binary system are significantly lower in comparison max comparison with Qmax values obtained in single sorption systems (Table 1). The sorption isotherms of with Q values obtained in single sorption systems (Table 1). The sorption isotherms of total dye max total dye sorption (data calculated as Qeq = Qeq(TT) + Qeq(MB); Ceq = Ceq(TT) + Ceq(MB)) were also sorption (data calculated as Q = Q (TT) + Q (MB); C = C (TT) + C (MB)) were also constructed eq eq eq eq eq eq constructed and evaluated (not shown). From Table 4 it is clear that maximum sorption capacity Qmax and evaluated (not shown). From Table 4 it is clear that maximum sorption capacity Q for MB from max for MB from single sorption system calculated from Langmuir model (Table 2) is almost equal to total single sorption system calculated from Langmuir model (Table 2) is almost equal to total Q (TT + max Qmax(TT + MB) in binary system. This confirms the hypothesis that both TT and MB in binary mixture MB) in binary system. This confirms the hypothesis that both TT and MB in binary mixture TT + MB are TT + MB are sequestered by identical binding sites on biomass surface and observed differences in sequestered by identical binding sites on biomass surface and observed differences in individual dye individual dye sorption capacities Q are due to (i) mutual competitive effects between dyes and (ii) sorption capacities Q are due to (i) mutual competitive effects between dyes and (ii) different affinity different affinity of biomass to cationic dyes. Remenárová et al. [19] described competitive effects of biomass to cationic dyes. Remenárová et al. [19] described competitive effects during sorption in during sorption in binary system TT + MG (malachite green) by moss R. squarrosus. They revealed binary system TT + MG (malachite green) by moss R. squarrosus. They revealed that TT significantly that TT significantly affected biosorption of MG in binary system TT + MG, the competitive effect of affected biosorption of MG in binary system TT + MG, the competitive effect of MG on TT was less MG on TT was less pronounced. On the contrary, Albadarin et al. [18] observed that the total pronounced. On the contrary, Albadarin et al. [18] observed that the total adsorbed quantity of single adsorbed quantity of single dyes is only slightly larger than a mixture of two components with the dyes is only slightly larger than a mixture of two components with the same concentration in binary same concentration in binary system MB and alizarin red S (ARS). This indicating the presence of system MB and alizarin red S (ARS). This indicating the presence of limited number of active sites by limited number of active sites by which the two dyes can be sequestrated and for which they will, to which the two dyes can be sequestrated and for which they will, to some extent, compete for in binary some extent, compete for in binary system. Based on obtained data and results of other authors we system. Based on obtained data and results of other authors we suppose that variance in affinity in suppose that variance in affinity in multicomponent dye systems could be attributed to the different chemical structure of dye molecules and physicochemical properties of sorbent used. Tabaraki et al. Q [mg/g; d.w.] eq Q [mg/g; d.w.] eq Environments 2018, 5, 10 10 of 12 multicomponent dye systems could be attributed to the different chemical structure of dye molecules and physicochemical properties of sorbent used. Tabaraki et al. [28] stated that the chemical structure of dye molecules, the number of sulfonic groups, the basicity and molecular weight of molecules significantly influenced biosorption capacity and affinity. 4. Conclusions In the present study, dried biomass of freshwater moss V. dubyana has been used as biosorbent for cationic dyes methylene blue and thioflavin T removal from both single and binary systems. Results revealed that an increase in pH has a positive effect on TT and MB sorption and the electrostatic attraction is one of the possible mechanism of MB and TT removal by moss V. dubyana. The experimental equilibrium biosorption data of TT and MB from single systems were well described by Langmuir isotherm and maximum sorption capacities Q were 119  11 mg/g for TT and 229  9 mg/g for MB. max In binary mixture, the presence of MB caused significant decrease of TT sorption indicating higher affinity of biomass to MB. We conclude that in binary systems the competitive effects can substantially influence the sorption process and should be thoroughly evaluated before application of selected adsorbents for removal of basic dyes from wastewaters containing mixtures of dyes. Acknowledgments: This work was supported by the project of the Cross-border Co-operation Programme and co-financed with European Regional Development Fund (ERDF), the grant number HUSK/1101/1.2.1/0148 as well as the project of the Operational Program Research and Development and co-financed with European Regional Development Fund (ERDF), the grant number ITMS 26220220191. Author Contributions: Martin Pipíška and Miroslav Horník conceived and designed the experiments; Denisa Partelová and Martin Valica performed the experiments; Denisa Partelová and Miroslav Horník analyzed the experimental data; Juraj Lesný and Stanislav Hostin contributed materials and analysis tools; Martin Pipíška and Miroslav Horník wrote the paper. 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Sorption equilibrium and kinetics of basic dye from aqueous solution using banana stalk waste. J. Hazard. Mater. 2008, 158, 499–506. [CrossRef] [PubMed] © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Environments Multidisciplinary Digital Publishing Institute

Removal of Synthetic Dyes by Dried Biomass of Freshwater Moss Vesicularia Dubyana: A Batch Biosorption Study

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environments Article Removal of Synthetic Dyes by Dried Biomass of Freshwater Moss Vesicularia Dubyana: A Batch Biosorption Study 1 , 2 1 1 1 , 1 Martin Pipíška , Martin Valica , Denisa Partelová , Miroslav Horník *, Juraj Lesný and Stanislav Hostin Department of Ecochemistry and Radioecology, University of SS. Cyril and Methodius in Trnava, Nam. J. Herdu 2, 91701 Trnava, Slovakia; pipiskam@gmail.com (M.P.); martin.valica@outlook.sk (M.V.); d.partelova@gmail.com (D.P.); juraj.lesny@ucm.sk (J.L.); stanislav.hostin@ucm.sk (S.H.) Department of Chemistry, Trnava University in Trnava, Priemyselná 4, 91843 Trnava, Slovakia * Correspondence: miroslav.hornik@ucm.sk; Tel.: +421-335-565-392 Received: 27 November 2017; Accepted: 6 January 2018; Published: 9 January 2018 Abstract: In this work the biosorption of cationic dyes thioflavin T (TT) and methylene blue (MB) from single and binary solutions on dried biomass of freshwater moss Vesicularia dubyana as a function of contact time, pH, and biomass or sorbate concentration has been investigated. The prediction of maximum sorption capacities using adsorption isotherm models were also realized. Biosorption of TT and MB is a rapid process strongly affected by solution pH. Maximum sorption capacities Q max calculated from Langmuir isotherm were 119  11 mg/g for TT and 229  9 mg/g for MB. In binary mixture, the presence of MB caused significant decrease of TT sorption, advocating the competitive sorption between TT and MB. Results revealed that V. dubyana biomass exhibited significantly higher affinity to thiazine dye MB in comparison with benzothiazole dye TT from both single and binary solutions. Based on the obtained results, the competitive effects in binary system can substantially influence the sorption process and should be thoroughly evaluated before application of selected adsorbents for removal of basic dyes from colored effluents. Keywords: biosorption; cationic dyes; moss; Vesicularia dubyana 1. Introduction Dyes are the important class of synthetic organic compounds used in many industries such as manufacture of pulp and paper, leather tanning or textile dying and in manufacture of dyestuffs. Ghaly et al. [1] pointed out that the textile industry is one of the major industries in the world and it plays a major role in the economy of many countries. The textile industry utilizes various chemicals (particularly synthetic dyes) and large amounts of water during the production process. The global textile dyes market was estimated to reach USD 4.7 billion in 2015, and is projected to reach USD 6.4 billion by 2019 and USD 8.75 billion by 2023 [2,3]. Industrial application of synthetic dyes is associated with the release of a huge amount of more or less colored effluents into the environment [4,5]. Wastewaters from the textile industry contain a large amount of dyes and chemicals containing trace metals such as Cr, As, Cu and Zn which are capable of harming the environment and human health [1]. During textile dyeing, a wide range of various dyes in a short time period are used, therefore effluents are extremely variable in composition and require an unspecific treatment processes [5]. Conventional physico-chemical (dilution, adsorption, coagulation and flocculation, oxidation, reverse osmosis and ultrafiltration) and biological (aerobic activated sludge and anaerobic processes) treatment technologies presently employed for color removal have several disadvantages such as long operational time, low specificity, formation and disposal of sludge and high Environments 2018, 5, 10; doi:10.3390/environments5010010 www.mdpi.com/journal/environments Environments 2018, 5, 10 2 of 12 cost (see e.g., critical reviews [1,6,7]. Consequently, the development of efficient clean-up technologies is of major interest. Highly efficient removal of synthetic dyes from effluents has been attempted by bioadsorption and non-conventional biosorbents have been searched in recent years that are easy available, renewable and environmentally friendly that can successfully replace the classical adsorbents [8]. To date, diverse algae, macroalgae, plant biomasses, agricultural residues as well as other biomaterials have been explored as adsorbents for removal of acidic, basic and reactive dyes from aqueous solutions [9–17]. Authors have reported that the biosorption capacity is highly dependent on solution pH, biosorbent dosage, temperature and concentration of other solutes present in solution. Since industrial effluents may contain several synthetic dyes, it is necessary to study the simultaneous sorption of two or more dyes and to quantify the mutual effect of one dye on the other. However, contradictory findings related to biosorption of dyes in multi-dye sorption systems were recently published. Albadarin et al. [18] who studied the simultaneous sorption of methylene blue and alizarin red S by olive stone biomass revealed only the limited competition between dyes. Remenárová et al. [19] confirmed that thioflavin T (TT) significantly affected biosorption of malachite green (MG) by moss R. squarrosus in binary system TT + MG. The competitive effect of MG on TT was less pronounced. On the contrary, Giwa et al. [20] found that the presence of rhodamine B and methylene blue had a synergetic effect on the maximum monolayer capacity of the sawdust of Parkia biglobosa for Acid Blue 161 dye in binary and ternary systems. To ensure the applicability of biosorption technology for colored effluents, more works are still needed for the sorption of a mixture of dyes at various operating conditions. Considering the above mentioned aspects, this study concerns the biosorption of cationic dyes thioflavin T and methylene blue from aqueous solutions by dried biomass of freshwater moss Vesicularia dubyana. The influence of variables (contact time, pH, biomass dosage) controlling the biosorption process has been considered in both single and binary dye systems. 2. Materials and Methods 2.1. Moss Biomass The biomass of freshwater moss Vesicularia dubyana was used as a sorbent of cationic dyes thioflavin T (TT) and methylene blue (MB). Vital moss biomass was cultivated in diluted Hoagland medium [21] under artificial illumination (2000 lx) at 22  2 C. Before use in sorption experiments, the biomass was thoroughly rinsed in deionized water (3 times) and dried at 60 C for 48 h. 2.2. Reagents and Instruments Thioflavin T (C.I. 49005, M (C H ClN S) 318.86, CAS 2390-54-7) and methylene blue (C.I. 17 19 2 52015, M (C H N SCl) 319.86; CAS 61-73-4) were purchased from Fluka (USA). All chemicals r 16 18 3 were of analytical grades. Cationic dyes solutions were prepared in deionized water (conductivity 0.054 S/cm; Millipore Simplicity). An UV-VIS spectrophotometer Cary 50 (Varian, Australia) was used for determination of TT and MB concentrations in solutions and for establishing calibration curves for TT and MB at maximum absorbances TT = 412 nm and MB = 650 nm. The pH effect on MB and TT concentration determination was taken into account. 2.3. Sorption Kinetics in Single and Binary Solutions in Batch System Dried biomass of V. dubyana (0.5 g/L) was added to 20 mL of solutions containing defined concentrations of thioflavin T (TT), methylene blue (MB) or their mixtures, respectively. Solution pH was adjusted to 6.0. The flasks were incubated on a rotary shaker (250 rpm) at 25 C. At the time intervals 10, 20, 40, 60, 120, 240, 360 and 1440 min, aliquot samples were obtained and remaining Environments 2018, 5, 10 3 of 12 concentration of TT and MB determined. All experiments were performed in duplicate series. The TT and MB uptake was calculated according to Equation (1). Q = (C C ) (1) t 0 t where Q represents the amount of TT or BB sorbed by moss biomass (mg/g d.w.) from single or binary solutions at time t; C and C represent the initial concentration of dyes in solution and the 0 t concentration of dyes at time t (mg/g). V is solution volume (L) and M is the amount of moss biomass (g; d.w.). 2.4. Influence of pH and Biomass Dosage To analyze the influence of pH, dried biomass (0.5 g/L) was shaken in 20 mL of TT and MB (C = 40 mg/L or C = 80 mg/L) or TT + MB (C = 40 mg/L and C = 40 mg/L) solutions 0 TT 0 MB 0 TT 0 MB of desired pH for 2 h on a rotary shaker at 250 rpm and 25 C. In order to eliminate interference of buffer components on cationic dyes biosorption, the non-buffered solutions in deionized water were adjusted to the desired pH values by adding 0.1 M HCl or 0.1 M NaOH. To analyze the influence of biomass dosage, dried biomass of desired amount (C = 0.25 to 4.0 g/L) was shaken in 20 mL of TT and MB (C = 40 mg/L or C = 80 mg/L) or TT + MB (C = 40 mg/L 0 TT 0 MB 0 TT and C = 40 mg/L) solutions for 2 h on a rotary shaker at 250 rpm, pH 6.0 and 25 C. 0 MB At the end of the experiments, aliquot samples were obtained and remaining concentration of TT and MB determined. All experiments were performed in duplicate series. The TT and MB uptake was calculated according to Equation (1). 2.5. Sorption Equilibrium in Single and Binary Solutions in Batch System Dried biomass of V. dubyana (0.5 g/L) was added to 20 mL of solutions with initial dye concentrations C in single system ranging from 20 to 200 mg/L (TT) and 20 to 320 mg/L (MB) and in binary system TT + MB from 40 to 160 mg/L. In both single and binary solutions pH was adjusted to 6.0. Flasks were incubated on a rotary shaker (250 rpm) at 25 C. After 2 h of exposure, the remaining concentration of TT and MB was determined. All experiments were performed in duplicate series. The TT and MB uptake was calculated according to Equation (1). Equilibrium data in both single and binary systems were analyzed using adsorption isotherm models according to Langmuir and Freundlich. To calculate the corresponding parameters of isotherms non-linear regression analysis was performed by OriginPro 2016 (OriginLab Corporation, Northampton, MA, USA). 2.6. Potentiometric Titration Potentiometric titration experiment was carried out according to the modified procedure described by Zhang et al. [22]. Dried biomass of freshwater moss V. dubyana (0.30 g) were protonated with 100 mL of equimolar 0.1 mol/L HCl and NaCl for 2 h on the rotary shaker (200 rpm) at 25 C. Subsequently, moss biomass was separated by centrifugation and transferred into 100 mL of 0.1 mol/L NaCl and agitated under the same conditions. Potentiometric titration was performed in an Erlenmeyer flask with glass electrode (three-point calibrated with buffers—pH 4.0, 7.0 and 10.0) by dropwise addition of 0.1 mol/L mixture of NaOH and NaCl into the bacterial suspension. The titration was conducted in the pH range of 2.0–11.0 and the potentiometric titration curve was obtained by plotting the volume of 0.l mol/L NaOH addition as titrant solution against the pH values measured. ProtoFit ver. 2.1 was used to analyse the titration data. Environments 2018, 5, 10 4 of 12 on V. dubyana biomass are occupied. Initial phase is followed by slower gradual uptake till Environments 2018, 5, 10 4 of 12 equilibrium is reached. In case of MB, the maximum uptake in single system at initial concentration 80 mg/L was 3. Results and Discussion observed after 120 min of exposure (130 ± 5 mg/g; d.w.). Uptake of TT at initial concentration 40 mg/L increased rapidly in the first 90 min and after 120 min reached 43.5 ± 2.7 mg/g. The final equilibrium 3.1. Biosorption Kinetics in Single and Binary Systems was reached within the 2 h and after this time, there was no considerable increase in both TT and MB sorption until the end of experiments. Our findings are in agreement with other studies of cationic As expected, the uptake of cationic dyes TT and MB by dried biomass of freshwater moss dyes sorption by various types of biomass. Similar behavior of MB sorption kinetics by green alga V. dubyana from both single and binary solutions was a rapid process (Figure 1A,B). At the initial phase Enteromorpha spp. [23] and red (Gracilaria parvispora), brown (Nizamuddinia zanardinii) and green (Ulva of uptake, driving force is high (uptake increased linearly) and available high affinity binding sites on fasciata) macroalga [9] was observed. Hameed [24] found that at higher initial concentration of MB V. dubyana biomass are occupied. Initial phase is followed by slower gradual uptake till equilibrium longer contact time is needed to reach equilibrium when grass waste was used as sorbent. is reached. A 140 120 TT MB TT MB 40 40 20 20 0 0 0 200 400 600 800 1000 1200 1400 1600 0 200 400 600 800 1000 1200 1400 1600 Time [min] Time [min] Figure 1. Kinetics of thioflavin T (TT) and methylene blue (MB) biosorption by dried biomass of moss Figure 1. Kinetics of thioflavin T (TT) and methylene blue (MB) biosorption by dried biomass of moss V. dubyana (CB = 0.5 g/L) from single (A. C0 TT = 40 mg/L or C0 MB =80 mg/L) and binary (B. C0 TT = 40 V. dubyana (C = 0.5 g/L) from single (A. C = 40 mg/L or C =80 mg/L) and binary (B. C = B 0 TT 0 MB 0 TT mg/L and C0 MB = 40 mg/L) solutions at 25 °C and pH 6.0. Error bars represent standard deviation of 40 mg/L and C = 40 mg/L) solutions at 25 C and pH 6.0. Error bars represent standard deviation 0 MB the mean (±SD, n = 2). of the mean (SD, n = 2). Biosorption of TT and MB by V. dubyana biomass from binary solution is shown in Figure 1B. In case of MB, the maximum uptake in single system at initial concentration 80 mg/L was Although the same kinetic profile as in the case of dye sorption in single systems was observed, an observed after 120 min of exposure (130  5 mg/g; d.w.). Uptake of TT at initial concentration 40 mg/L evident decrease of TT and MB biosorption capacities in comparison with single systems were increased rapidly in the first 90 min and after 120 min reached 43.5  2.7 mg/g. The final equilibrium recorded as a result of competitive effects (see discussion below). The final equilibrium of both TT was reached within the 2 h and after this time, there was no considerable increase in both TT and MB and MB was reached within 120 min. At initial dye concentration 40 mg/L, maximum sorption of TT sorption until the end of experiments. Our findings are in agreement with other studies of cationic (28.3 ± 0.3 mg/g d.w.) in binary system was significantly lower than maximum sorption of MB (62.7 dyes sorption by various types of biomass. Similar behavior of MB sorption kinetics by green alga ± 0.3 mg/g, d.w.). The similar kinetic profile in binary (MB + rhodamine B) and single biosorption Enteromorpha spp. [23] and red (Gracilaria parvispora), brown (Nizamuddinia zanardinii) and green (Ulva system observed Fernandez et al. [25] using Cupressus sempervirens cone chips. fasciata) macroalga [9] was observed. Hameed [24] found that at higher initial concentration of MB It is evident that V. dubyana biomass exhibited significantly higher affinity to thiazine dye MB in longer contact time is needed to reach equilibrium when grass waste was used as sorbent. comparison with benzothiazole dye TT from both single and binary solutions. In both dye molecules Biosorption of TT and MB by V. dubyana biomass from binary solution is shown in Figure 1B. the positive charge is present on quaternary nitrogen =N =, however in TT quaternary N is located in Although the same kinetic profile as in the case of dye sorption in single systems was observed, an evident position 3, substituted by one methyl group and in MB quaternary N is located in position 3 or 7 and decrease of TT and MB biosorption capacities in comparison with single systems were recorded as a result substituted by 2 methyl groups. Therefore, we suppose that among other mechanisms, steric effects of competitive effects (see discussion below). The final equilibrium of both TT and MB was reached within in dye molecules can affect dye affinity to moss biomass as well. 120 min. At initial dye concentration 40 mg/L, maximum sorption of TT (28.3  0.3 mg/g d.w.) in binary system was significantly lower than maximum sorption of MB (62.7  0.3 mg/g, d.w.). The similar 3.2. Influence of pH kinetic profile in binary (MB + rhodamine B) and single biosorption system observed Fernandez et al. [25] The pH of dye solution is one of the crucial parameters affecting the sorption process through using Cupressus sempervirens cone chips. controlling both the ionization of dye molecules and the degree of ionization of functional groups It is evident that V. dubyana biomass exhibited significantly higher affinity to thiazine dye MB in present on biomass surface. comparison with benzothiazole dye TT from both single and binary solutions. In both dye molecules The influence of solution pH (2.0 to 7.0) on TT and MB sorption by V. dubyana biomass from the positive charge is present on quaternary nitrogen =N =, however in TT quaternary N is located in single solutions is shown in Figure 2A. The lowest sorption capacities Q for both TT (QTT = 12 mg/g position 3, substituted by one methyl group and in MB quaternary N is located in position 3 or 7 and d.w.) and MB (QMM = 4 mg/g d.w.) dyes were observed at low initial pH value (pH0 = 2.0) which could substituted by 2 methyl groups. Therefore, we suppose that among other mechanisms, steric effects in be closely related to protonation of binding sites on biomass surface. The low adsorption capacity of dye molecules can affect dye affinity to moss biomass as well. Q [mg/g; d.w.] Q [mg/g; d.w.] Environments 2018, 5, 10 5 of 12 3.2. Influence of pH The pH of dye solution is one of the crucial parameters affecting the sorption process through controlling both the ionization of dye molecules and the degree of ionization of functional groups present on biomass surface. The influence of solution pH (2.0 to 7.0) on TT and MB sorption by V. dubyana biomass from single solutions is shown in Figure 2A. The lowest sorption capacities Q for both TT (Q = 12 mg/g d.w.) TT and MB (Q = 4 mg/g d.w.) dyes were observed at low initial pH value (pH = 2.0) which could EnvironmMM ents 2018, 5, 10 0 5 of 12 be closely related to protonation of binding sites on biomass surface. The low adsorption capacity of MB can also be explained by the fact that MB is mainly found as undissociated species MB (99% at 0 MB can also be explained by the fact that MB is mainly found as undissociated species MB (99% at pH = 2.0 and 86% at pH = 3.0) [26]. Obtained results indicate that an increase in pH has a positive pH = 2.0 and 86% at pH = 3.0) [26]. Obtained results indicate that an increase in pH has a positive effect on TT and MB sorption, since the competitions between dye cations and protons for the binding effect on TT and MB sorption, since the competitions between dye cations and protons for the binding sites decreases and such curves (Figure 2A) represent a typical cationic dye sorption behavior [10,18]. sites decreases and such curves (Figure 2A) represent a typical cationic dye sorption behavior [10,18]. 140 140 TT TT 120 120 MB MB 23 456 7 23 45 67 pH pH Figure Figure 2. 2. Influence Influence of of initial initial pH pHon 0 onthioflavin thioflavin T (TT) T (TT) and m and methylene ethylene blue ( blue M(MB) B) biosorptio biosorption n by biomass by biomass 3 3 of moss V. dubyana (CB = 0.5 g/dm ) from single (C0 TT = 40 mg/L or C0 MB 80 mg/L; A) and binary (C0 TT of moss V. dubyana (C = 0.5 g/dm ) from single (C = 40 mg/L or C 80 mg/L; A) and binary B 0 TT 0 MB = 40 mg/L and C0 MB = 40 mg/L; B) solutions at 25 °C for 2 h. Error bars represent standard deviation (C = 40 mg/L and C = 40 mg/L; B) solutions at 25 C for 2 h. Error bars represent standard 0 TT 0 MB of the mean (±SD, n = 2). deviation of the mean (SD, n = 2). The highest Q values were recorded at pH > 4.0 with maximum QTT = 44 ± 1 mg/g d.w. and QMM The highest Q values were recorded at pH > 4.0 with maximum Q = 44  1 mg/g d.w. and = 123 ± 3 mg/g d.w. at pH0 = 7.0. Similarly, Albadarin et al. [23] observed TT maximum biosorption Q capac = 123ity of  3 mg/g methylene blue by olive d.w. at pH = 7.0. Similarly stone by-p , Albadarin roducts atet pal H. = 7 [23.]2 observed and maxim maximum um sorptbiosorption ion of MM 0 capacity thiofl of avin methylene T by agric blue ultur by alolive by-pro stone ductby-pr s from t oducts he hoat p (pH Humulus lup = 7.2 and ulus maxi L.) w mum as fo sorption und witof hin t thioflavin he range of initial values of pH 4.0–7.0 [27]. T by agricultural by-products from the hop (Humulus lupulus L.) was found within the range of initial The same behavior was observed in binary sorption system with equimolar initial values of pH 4.0–7.0 [27]. concentrations of TT and MB. Dye sorption capacities increased from 8.2 ± 0.1 mg/g to 22.7 ± 0.4 mg/g The same behavior was observed in binary sorption system with equimolar initial concentrations (TT) and 10.5 ± 1.4 mg/g to 54.6 ± 0.1 mg/g (MB) with increasing solution pH from 2.0 to 4.0 and of TT and MB. Dye sorption capacities increased from 8.2  0.1 mg/g to 22.7  0.4 mg/g (TT) and remained almost stable till pH 7.0. From Figure 2B it is evident that the biomass of V. dubyana 10.5  1.4 mg/g to 54.6  0.1 mg/g (MB) with increasing solution pH from 2.0 to 4.0 and remained exhibited significantly higher affinity toward MB. almost stable till pH 7.0. From Figure 2B it is evident that the biomass of V. dubyana exhibited As was mentioned earlier, the dye sorption capacities are to a great extent affected by the significantly higher affinity toward MB. dissociation of functional groups present on moss surface. From the potentiometric titration curve As was mentioned earlier, the dye sorption capacities are to a great extent affected by the dissociation (Figure 3A), it is evident that from pH ~ 4.0 a small addition of NaOH caused a dramatic changes in solution pH what indicates that relevant acidic functional groups (carboxyl, phosphoryl) present on of functional groups present on moss surface. From the potentiometric titration curve (Figure 3A), it is biomass surface are dissociated. To qualitative characterization of functional groups presented on the evident that from pH ~4.0 a small addition of NaOH caused a dramatic changes in solution pH what surface of moss biomass and to determination of binding sites concentration (CAn) the prediction indicates that relevant acidic functional groups (carboxyl, phosphoryl) present on biomass surface modelling was used. The titration curve shows a relatively unpronounced inflection point, which are dissociated. To qualitative characterization of functional groups presented on the surface of moss predicts the existence of several functional groups. According to the ProtoFit prediction and obtained biomass and to determination of binding sites concentration (C ) the prediction modelling was used. An values of residual sum of squares (RSS), it was found that the titration profile was best described by The titration curve shows a relatively unpronounced inflection point, which predicts the existence of a non-electrostatic model for characterization of the four binding sites. The predicted functional several functional groups. According to the ProtoFit prediction and obtained values of residual sum groups with relevant pKa values and concentrations of binding sites (CAn) presented on the surface of of squares (RSS), it was found that the titration profile was best described by a non-electr+ostatic m + odel moss biomass are listed in Table 1. In connection to these results, we can suppose that MB and TT for ch adsorb aractee rd iz on sur ation o face f thand b e four alb ainced ndina neg g sitea st.iv Te h cha e pr re ge diof biomass. C cted function oa nsequently, l groups wone of the possib ith relevant pKa lv e alues mechanism of MB and TT biosorption by freshwater moss V. dubyana is the electrostatic attraction. and concentrations of binding sites (C ) presented on the surface of moss biomass are listed in Table 1. An Q [mg/g; d.w.] Q [mg/g; d.w.] Environments 2018, 5, 10 6 of 12 Table 1. Predicted functional groups according to pKa values and concentration of binding sites (CAn) presented on the surface of moss biomass (V. dubyana) using modelling software ProtoFit ver. 2.1. Functional Group pKa CAn (mmol/g) COOH 2.34–6.25 0.98 PO3H2 6.36–8.72 0.69 NH2 10.7–11.5 0.61 OH 8.36–12.0 0.64 During and after cationic dyes biosorption, the pH of the dye solutions in both single and binary sorption systems changed (Figure 4B). The solution pH increased from initial pH0 values 4.0, 5.0 and 6.0 to pHeq 5.2, 6.0 and 6.1 (MB), 6.6, 6.8 and 7.0 (TT) and 5.8, 6.4, 6.6 (TT + MB). On the contrary, a Environments 2018, 5, 10 6 of 12 slight decrease of solution pH was observed from pH0 = 7.0 to pHeq = ~6.6 probably as a result of a TT–hydrogen and/or MB–hydrogen ion-exchange (Figure 3B). This equilibrium value of pHeq also corresponds to the value of pHzpc = 7.78 predicted within the potentiometric titration and ProtoFit + + In connection to these results, we can suppose that MB and TT adsorbed on surface and balanced a analysis, whereby the pHzpc represents the value of pH at which the biosorbent shows a net zero negative charge of biomass. Consequently, one of the possible mechanism of MB and TT biosorption by surface charge. freshwater moss V. dubyana is the electrostatic attraction. 10 8 TT A B MB TT+MB without biomass 123456789 10 11 12 23 4 5 67 8 pH pH Figure 3. Potentiometric titration curve for dried biomass of freshwater moss V. dubyana (3.0 g/L) Figure 3. Potentiometric titration curve for dried biomass of freshwater moss V. dubyana (3.0 g/L) obtained by gradual addition of titrant 0.1 M NaOH at background electrolyte 0.1 M NaCl and obtained by gradual addition of titrant 0.1 M NaOH at background electrolyte 0.1 M NaCl and reaction reaction temperature 25 °C (A). The change of initial pH0 of dye (single and binary) solutions after temperature 25 C (A). The change of initial pH of dye (single and binary) solutions after biosorption biosorption and in control experiments in deionized water and without biomass (B). and in control experiments in deionized water and without biomass (B). 3.3. Influence of Biomass Dosage Table 1. Predicted functional groups according to pKa values and concentration of binding sites (C ) An To investigate the biomass dosage on TT and MB removal, various amounts of dried moss presented on the surface of moss biomass (V. dubyana) using modelling software ProtoFit ver. 2.1. biomass were added to both single and binary dye solutions. From Figure 4A it is a clearly seen that the sorption capacity Q of both TT and MB decreased from 241 mg/g to 118 mg/g (MB) and from 50.6 Functional Group pKa C (mmol/g) An mg/g to 2.6 mg/g (TT) with increasing concentration of moss biomass CB in single solutions. Different COOH 2.34–6.25 0.98 relationship between the sorption capacities of dyes Q and the concentration of biomass CB was PO H 6.36–8.72 0.69 3 2 NH 10.7–11.5 0.61 observed in binary sorption system 2 TT + MB (Figure 4B). The sorption capacity of TT changed only OH 8.36–12.0 0.64 slightly with increasing concentration of moss biomass in solution. On the contrary, the sorption capacity Q of MB decreased significantly (from 107 to 52.8 mg/g) with increasing CB. We suppose that competitive effects between cation dyes and binding sites play an important role and influence the During and after cationic dyes biosorption, the pH of the dye solutions in both single and binary sorption behavior. sorption systems changed (Figure 4B). The solution pH increased from initial pH values 4.0, 5.0 and 6.0 to Tabaraki and Sadeghinejad [28] pointed out that an increase of biosorbent dose generally pH 5.2, 6.0 and 6.1 (MB), 6.6, 6.8 and 7.0 (TT) and 5.8, 6.4, 6.6 (TT + MB). On the contrary, a slight decrease eq increases the overall amount of dye biosorbed (removal efficiency), due to the increased surface area Environments 2018, 5, 10 7 of 12 of solution pH was observed from pH = 7.0 to pH = ~6.6 probably as a result of a TT–hydrogen and/or 0 eq of biosorbent which in turn increases the number of binding sites. Thus, the decrease in the amount MB–hydrogen ion-exchange (Figure 3B). This equilibrium value of pH also corresponds to the value eq of dye sorbed per gram of biosorbent with increase in the biosorbent 2 dose is mainly due to observed negative aggregation and changes in specific surface area (m /g) as well as changes in of pH = 7.78 predicted within the potentiometric titration and ProtoFit analysis, whereby the pH zpc zpc insaturation of binding sites through the sorption process [29]. However, Kumar and Porkodi [30] effective mixing of biomass in sorption systems when higher biomass concentrations were used. represents the value of pH at which the biosorbent shows a net zero surface charge. 250 120 A B TT TT MB MB 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 0,00,5 1,01,5 2,0 2,5 3,03,5 4,0 C [g/L] C [g/L] B B Figure 4. The effect of biomass concentration C on thioflavin T (TT) and methylene blue (MB) Figure 4. The effect of biomass concentration CB B on thioflavin T (TT) and methylene blue (MB) biosorption biosorption by by dried dried biomass biomass of V of . dubyana V. dubyanfr a om from single single ( ( CC0 TT = 40 = 40 mg/L or mg/L or C0C MB = 80 mg/L; = 80 mg/L; (A) a( nA d) and 0 TT 0 MB binary (C0 TT = 40 mg/L and C0 MB = 40 mg/L; (B) solutions at 25 °C, pH 6.0 for 2 h. Error bars represent binary (C = 40 mg/L and C = 40 mg/L; (B) solutions at 25 C, pH 6.0 for 2 h. Error bars 0 TT 0 MB standard deviation of the mean (±SD, n = 2). represent standard deviation of the mean (SD, n = 2). 3.4. Sorption Equilibrium in Single and Binary Systems The adsorption equilibrium data of TT and MB from both single and binary sorption systems were described by adsorption isotherms according to Langmuir and Freundlich. The isotherm parameters obtained by non-linear regression analysis are reported in Table 2. Coefficients of determination (R ) related to the Langmuir model applied to TT and MB biosorption 2 2 2 data (R = 0.965 for TT; R = 0.990 for MB) were higher than those for the Freundlich model (R = 0.955 for TT; R = 0.938 for MB). In addition, the biosorption of methylene blue and thioflavin T by agricultural by-products from the hop (Humulus lupulus L.) [27] and methylene blue by water hyacinth biomass [31], agro-waste oil tea shell [13], Anethum graveolens biomass [32] and Tremella fuciformis biomass [33] was very well fitted by Langmuir model. Table 2. Langmuir and Freundlich equilibrium parameters (±SD) obtained by non-linear regression analysis for TT and MB biosorption by V. dubyana biomass in single systems. Langmuir Freundlich Dye 2 2 Qmax (mg/g) b (L/mg) R K (L/g) 1/nR TT 119 ± 11 0.04 ± 0.01 0.965 15.5 ± 3.8 2.58 ± 0.38 0.955 MB 229 ± 9 0.07 ± 0.01 0.990 45.2 ± 11.5 3.27 ± 0.59 0.938 TT: Thioflavin T; MB: methylene blue. Maximum sorption capacity Qmax and constant b, calculated from Langmuir isotherm model enable to characterize the affinity of V. dubyana biomass towards TT and MB (Table 2) in single sorption systems. Qmax for TT (119 ± 11 mg/g, d.w.) was significantly lower in comparison with Qmax for MB (229 ± 9 mg/g, d.w.). The affinity constant b of the isotherm corresponds to the initial gradient, which indicates the V. dubyana biomass affinity at low concentrations of both TT and MB. Accordingly, a higher initial gradient corresponds to a higher affinity constant b. It is evident, that Langmuir isotherm for MB is steeper at lower equilibrium concentrations than those for TT (Figure 5A,B). The difference in the b values 0.04 ± 0.01 L/mg (TT) and 0.07 ± 0.01 L/mg (MB), confirmed the higher affinity of V. dubyana biomass to MB in comparison with TT. This was also reflected by higher Qmax for MB (Table 2). The comparison of maximal sorption capacities Qmax determined from Langmuir model with those of other authors is reported in Table 3. Although the data about TT (bio)sorption are very Q [mg/g; d.w.] Added volume of 0.1 M NaOH [mL] pH eq Q [mg/g; d.w.] Environments 2018, 5, 10 7 of 12 3.3. Influence of Biomass Dosage To investigate the biomass dosage on TT and MB removal, various amounts of dried moss biomass were added to both single and binary dye solutions. From Figure 4A it is a clearly seen that the sorption capacity Q of both TT and MB decreased from 241 mg/g to 118 mg/g (MB) and from 50.6 mg/g to 2.6 mg/g (TT) with increasing concentration of moss biomass C in single solutions. Different relationship between the sorption capacities of dyes Q and the concentration of biomass C was observed in binary sorption system TT + MB (Figure 4B). The sorption capacity of TT changed only slightly with increasing concentration of moss biomass in solution. On the contrary, the sorption capacity Q of MB decreased significantly (from 107 to 52.8 mg/g) with increasing C . We suppose that competitive effects between cation dyes and binding sites play an important role and influence the sorption behavior. Tabaraki and Sadeghinejad [28] pointed out that an increase of biosorbent dose generally increases the overall amount of dye biosorbed (removal efficiency), due to the increased surface area of biosorbent which in turn increases the number of binding sites. Thus, the decrease in the amount of dye sorbed per gram of biosorbent with increase in the biosorbent dose is mainly due to insaturation of binding sites through the sorption process [29]. However, Kumar and Porkodi [30] observed negative aggregation and changes in specific surface area (m /g) as well as changes in effective mixing of biomass in sorption systems when higher biomass concentrations were used. 3.4. Sorption Equilibrium in Single and Binary Systems The adsorption equilibrium data of TT and MB from both single and binary sorption systems were described by adsorption isotherms according to Langmuir and Freundlich. The isotherm parameters obtained by non-linear regression analysis are reported in Table 2. Coefficients of determination (R ) related to the Langmuir model applied to TT and MB biosorption 2 2 2 data (R = 0.965 for TT; R = 0.990 for MB) were higher than those for the Freundlich model (R = 0.955 for TT; R = 0.938 for MB). In addition, the biosorption of methylene blue and thioflavin T by agricultural by-products from the hop (Humulus lupulus L.) [27] and methylene blue by water hyacinth biomass [31], agro-waste oil tea shell [13], Anethum graveolens biomass [32] and Tremella fuciformis biomass [33] was very well fitted by Langmuir model. Table 2. Langmuir and Freundlich equilibrium parameters (SD) obtained by non-linear regression analysis for TT and MB biosorption by V. dubyana biomass in single systems. Langmuir Freundlich Dye 2 2 Q (mg/g) b (L/mg) K (L/g) 1/n R R max TT 119  11 0.04  0.01 0.965 15.5  3.8 2.58  0.38 0.955 MB 229  9 0.07  0.01 0.990 45.2  11.5 3.27  0.59 0.938 TT: Thioflavin T; MB: methylene blue. Maximum sorption capacity Q and constant b, calculated from Langmuir isotherm model max enable to characterize the affinity of V. dubyana biomass towards TT and MB (Table 2) in single sorption systems. Q for TT (119  11 mg/g, d.w.) was significantly lower in comparison with Q for max max MB (229  9 mg/g, d.w.). The affinity constant b of the isotherm corresponds to the initial gradient, which indicates the V. dubyana biomass affinity at low concentrations of both TT and MB. Accordingly, a higher initial gradient corresponds to a higher affinity constant b. It is evident, that Langmuir isotherm for MB is steeper at lower equilibrium concentrations than those for TT (Figure 5A,B). The difference in the b values 0.04  0.01 L/mg (TT) and 0.07  0.01 L/mg (MB), confirmed the higher affinity of V. dubyana biomass to MB in comparison with TT. This was also reflected by higher Q for max MB (Table 2). Environments 2018, 5, 10 8 of 12 limited, our results indicate that V. dubyana biomass exhibited the highest Qmax in comparison with other organic and inorganic sorbents. Similarly, in case of MB biosorption the moss biomass has a high Qmax when compared with different sorbents of plant origin. Environments 2018, 5, 10 8 of 12 120 250 A B Freundlich Freundlich Langmuir Langmuir 0 20 40 60 80 100 120 140 160 0 50 100 150 200 250 C [mg/L] C [mg/L] eq eq Figure 5. Fit of the Langmuir and Freundlich isotherms of thioflavin T (TT); (A) and methylene blue Figure 5. Fit of the Langmuir and Freundlich isotherms of thioflavin T (TT); (A) and methylene blue (MB); (B) sorption by dried biomass of freshwater moss V. dubyana (CB = 0.5 g/dm ) from single (MB); (B) sorption by dried biomass of freshwater moss V. dubyana (C = 0.5 g/dm ) from single sorption systems after 2 h interactions at 25 °C and pH 6.0. sorption systems after 2 h interactions at 25 C and pH 6.0. Table 3. Comparison of Qmax values of different sorbents determined from Langmuir isotherm for TT The comparison of maximal sorption capacities Q determined from Langmuir model with max and MB. those of other authors is reported in Table 3. Although the data about TT (bio)sorption are very limited, our results indicate Sorbent that V. dubyana Qma biomass x MB (mg/g exhibited ) Qmax TT the(mg/g) pH highest Q T in(°C) comparison Reference with other max organic and inorganic sorbents. Similarly, in case of MB biosorption the moss biomass has a high Q V. dubyana 229 119 6.0 25 present study max when compar hoped leaf b with iom dif ass fer ent sorbents18 of4plant origin. 77.6 6.0 25 [27] Rhytidiadelphus squarrosus - 98.9 4.0 25 [34] Table 3. Comparison of Q values of different sorbents determined from Langmuir isotherm for TT max montmorillonite * - 95.2 6.0 25 [35] and MB. Fomitopsis carnea - 21.9 - 30 [14] Corn husk 47.95 - 6.0 25 [8] Sorbent Q (mg/g) Q (mg/g) pH T ( C) Reference max MB max TT Oak acorn peel 120.5 - 7.0 24 [36] V. dubyana 229 119 6.0 25 present study Salvia miltiorrhiza 100 - 7.0 [12] hop leaf biomass 184 77.6 6.0 25 [27] Eucalyptus bark 204.8 - 9.9 30 [37] Rhytidiadelphus squarrosus - 98.9 4.0 25 [34] Platanus leaf 99.1 - 7.0 30 [38] montmorillonite * - 95.2 6.0 25 [35] Fomitopsis Banacarnea na stalk - 243.9 21.9 - - - 30 30 [39] [14 ] Corn husk 47.95 - 6.0 25 [8] * Cetylpyridinium modified. Oak acorn peel 120.5 - 7.0 24 [36] Salvia miltiorrhiza 100 - 7.0 [12] In comparison with single sorption systems, in binary or multicomponent mixtures cationic dyes Eucalyptus bark 204.8 - 9.9 30 [37] may interact or compete for binding sites of moss biomass. Therefore, the behavior of each species in Platanus leaf 99.1 - 7.0 30 [38] Banana stalk 243.9 - - 30 [39] a multicomponent system depends strongly on the number and properties of other species present. In addition, the solution pH, the physical and chemical properties of both the sorbent and sorbate * Cetylpyridinium modified. significantly influenced the sorption process. The simultaneous Langmuir and Freundlich sorption isotherms of MB and TT from binary system MB + TT (concentration ratio C0 TT:C0 MB = 1:1) by V. In comparison with single sorption systems, in binary or multicomponent mixtures cationic dubyana biomass are shown in Figure 6A,B. Table 4 presents isotherm parameters calculated by non- dyes may interact or compete for binding sites of moss biomass. Therefore, the behavior of each linear regression analysis. species in a multicomponent system depends strongly on the number and properties of other species present. In addition, the solution pH, the physical and chemical properties of both the sorbent and sorbate significantly influenced the sorption process. The simultaneous Langmuir and Freundlich sorption isotherms of MB and TT from binary system MB + TT (concentration ratio C :C = 1:1) 0 TT 0 MB by V. dubyana biomass are shown in Figure 6A,B. Table 4 presents isotherm parameters calculated by non-linear regression analysis. Q [mg/g; d.w.] eq Q [mg/g; d.w.] eq Environm Environments ents 2018 2018, ,5 5,, 10 10 9 of 9 of 12 12 TT TT B MB MB 80 80 20 20 0 20 406080 100 120 140 0 20 40 60 80 100 120 140 C [mg/L] C [mg/L] eq eq Figure 6. Fit of the Langmuir (A) and Freundlich (B) isotherms of thioflavin T (TT) and methylene blue Figure 6. Fit of the Langmuir (A) and Freundlich (B) isotherms of thioflavin T (TT) and methylene blue (MB) biosorption by dried biomass of freshwater moss V. dubyana (CB = 0.5 g/dm ) from binary sorption (MB) biosorption by dried biomass of freshwater moss V. dubyana (C = 0.5 g/dm ) from binary sorption system TT + MB (concentration ratio C0 TT:C0 MB = 1:1) after 2 h interactions at 25 °C and pH 6.0. system TT + MB (concentration ratio C :C = 1:1) after 2 h interactions at 25 C and pH 6.0. 0 TT 0 MB Table 4. Langmuir and Freundlich equilibrium parameters (±SD) obtained by non-linear regression Table 4. Langmuir and Freundlich equilibrium parameters (SD) obtained by non-linear regression analysis for TT and MB biosorption by V. dubyana biomass in binary system TT + MB. analysis for TT and MB biosorption by V. dubyana biomass in binary system TT + MB. Langmuir Freundlich Dye Langmuir Freundlich 2 2 Dye Qmax (mg/g) b (L/mg) R K (L/g) 1/n R 2 2 Q (mg/g) b (L/mg) K (L/g) 1/n R R max TT 49.3 ± 6.2 0.03 ± 0.01 0.905 6.92 ± 1.46 2.77 ± 0.36 0.972 TT 49.3  6.2 0.03  0.01 0.905 6.92  1.46 2.77  0.36 0.972 MB 160 ± 17 0.05 ± 0.02 0.950 25.4 ± 0.72 2.67 ± 0.05 0.999 MB 160  17 0.05  0.02 0.950 25.4  0.72 2.67  0.05 0.999 TT + MB 232 ± 29 0.013 ± 0.004 0.955 16.7 ± 1.9 2.29 ± 0.12 0.839 TT + MB 232  29 0.013  0.004 0.955 16.7  1.9 2.29  0.12 0.839 1 2 Sorption of individual dye in binary system TT + MB. Total TT and MB sorption (Qeq = Qeq(TT) + Qeq 1 2 Sorption of individual dye in binary system TT + MB. Total TT and MB sorption (Q = Q (TT) + Q (MB)) in eq eq eq (MB)) in binary system TT + MB. binary system TT + MB. It is evident that sorption of both TT and MB increased with increasing of dyes concentrations It is evident that sorption of both TT and MB increased with increasing of dyes concentrations in in binary mixture. However, the coexistence of MB and TT influenced the sorption capacity of each binary mixture. However, the coexistence of MB and TT influenced the sorption capacity of each other, other, advocating the competitive sorption between TT and MB. The sorption of MB is less affected advocating the competitive sorption between TT and MB. The sorption of MB is less affected by the by the presence of TT than the sorption of TT by the presence of MB in solution. presence of TT than the sorption of TT by the presence of MB in solution. Maximum sorption capacities Qmax of cationic dyes in binary system TT + MB calculated from Maximum sorption capacities Q of cationic dyes in binary system TT + MB calculated from max Langmuir model reached 49.3 ± 6.2 mg/g (TT) and 160 ± 17 mg/g (MB) and values of affinity Langmuir model reached 49.3  6.2 mg/g (TT) and 160  17 mg/g (MB) and values of affinity parameter b (0.03 ± 0.01 L/mg for TT; 0.05 ± 0.02 L/mg for MB) indicating markedly higher affinity of parameter b (0.03  0.01 L/mg for TT; 0.05  0.02 L/mg for MB) indicating markedly higher affinity of moss biomass to MB. However, both Qmax values in binary system are significantly lower in moss biomass to MB. However, both Q values in binary system are significantly lower in comparison max comparison with Qmax values obtained in single sorption systems (Table 1). The sorption isotherms of with Q values obtained in single sorption systems (Table 1). The sorption isotherms of total dye max total dye sorption (data calculated as Qeq = Qeq(TT) + Qeq(MB); Ceq = Ceq(TT) + Ceq(MB)) were also sorption (data calculated as Q = Q (TT) + Q (MB); C = C (TT) + C (MB)) were also constructed eq eq eq eq eq eq constructed and evaluated (not shown). From Table 4 it is clear that maximum sorption capacity Qmax and evaluated (not shown). From Table 4 it is clear that maximum sorption capacity Q for MB from max for MB from single sorption system calculated from Langmuir model (Table 2) is almost equal to total single sorption system calculated from Langmuir model (Table 2) is almost equal to total Q (TT + max Qmax(TT + MB) in binary system. This confirms the hypothesis that both TT and MB in binary mixture MB) in binary system. This confirms the hypothesis that both TT and MB in binary mixture TT + MB are TT + MB are sequestered by identical binding sites on biomass surface and observed differences in sequestered by identical binding sites on biomass surface and observed differences in individual dye individual dye sorption capacities Q are due to (i) mutual competitive effects between dyes and (ii) sorption capacities Q are due to (i) mutual competitive effects between dyes and (ii) different affinity different affinity of biomass to cationic dyes. Remenárová et al. [19] described competitive effects of biomass to cationic dyes. Remenárová et al. [19] described competitive effects during sorption in during sorption in binary system TT + MG (malachite green) by moss R. squarrosus. They revealed binary system TT + MG (malachite green) by moss R. squarrosus. They revealed that TT significantly that TT significantly affected biosorption of MG in binary system TT + MG, the competitive effect of affected biosorption of MG in binary system TT + MG, the competitive effect of MG on TT was less MG on TT was less pronounced. On the contrary, Albadarin et al. [18] observed that the total pronounced. On the contrary, Albadarin et al. [18] observed that the total adsorbed quantity of single adsorbed quantity of single dyes is only slightly larger than a mixture of two components with the dyes is only slightly larger than a mixture of two components with the same concentration in binary same concentration in binary system MB and alizarin red S (ARS). This indicating the presence of system MB and alizarin red S (ARS). This indicating the presence of limited number of active sites by limited number of active sites by which the two dyes can be sequestrated and for which they will, to which the two dyes can be sequestrated and for which they will, to some extent, compete for in binary some extent, compete for in binary system. Based on obtained data and results of other authors we system. Based on obtained data and results of other authors we suppose that variance in affinity in suppose that variance in affinity in multicomponent dye systems could be attributed to the different chemical structure of dye molecules and physicochemical properties of sorbent used. Tabaraki et al. Q [mg/g; d.w.] eq Q [mg/g; d.w.] eq Environments 2018, 5, 10 10 of 12 multicomponent dye systems could be attributed to the different chemical structure of dye molecules and physicochemical properties of sorbent used. Tabaraki et al. [28] stated that the chemical structure of dye molecules, the number of sulfonic groups, the basicity and molecular weight of molecules significantly influenced biosorption capacity and affinity. 4. Conclusions In the present study, dried biomass of freshwater moss V. dubyana has been used as biosorbent for cationic dyes methylene blue and thioflavin T removal from both single and binary systems. Results revealed that an increase in pH has a positive effect on TT and MB sorption and the electrostatic attraction is one of the possible mechanism of MB and TT removal by moss V. dubyana. The experimental equilibrium biosorption data of TT and MB from single systems were well described by Langmuir isotherm and maximum sorption capacities Q were 119  11 mg/g for TT and 229  9 mg/g for MB. max In binary mixture, the presence of MB caused significant decrease of TT sorption indicating higher affinity of biomass to MB. We conclude that in binary systems the competitive effects can substantially influence the sorption process and should be thoroughly evaluated before application of selected adsorbents for removal of basic dyes from wastewaters containing mixtures of dyes. Acknowledgments: This work was supported by the project of the Cross-border Co-operation Programme and co-financed with European Regional Development Fund (ERDF), the grant number HUSK/1101/1.2.1/0148 as well as the project of the Operational Program Research and Development and co-financed with European Regional Development Fund (ERDF), the grant number ITMS 26220220191. Author Contributions: Martin Pipíška and Miroslav Horník conceived and designed the experiments; Denisa Partelová and Martin Valica performed the experiments; Denisa Partelová and Miroslav Horník analyzed the experimental data; Juraj Lesný and Stanislav Hostin contributed materials and analysis tools; Martin Pipíška and Miroslav Horník wrote the paper. 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Sorption equilibrium and kinetics of basic dye from aqueous solution using banana stalk waste. J. Hazard. Mater. 2008, 158, 499–506. [CrossRef] [PubMed] © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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EnvironmentsMultidisciplinary Digital Publishing Institute

Published: Jan 9, 2018

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