Preparation of Mesoporous Biochar from Cornstalk for the Chromium (VI) Elimination by Using One-Step Hydrothermal Carbonation

Chao Wang; Jun Xie; Mingdong Zheng; Jinbo Zhu; Changliang Shi

doi:10.1155/2021/3418887

Preparation of Mesoporous Biochar from Cornstalk for the Chromium (VI) Elimination by Using One-Step Hydrothermal Carbonation

Wang, Chao;Xie, Jun;Zheng, Mingdong;Zhu, Jinbo;Shi, Changliang 2021-10-05 00:00:00 Hindawi Journal of Analytical Methods in Chemistry Volume 2021, Article ID 3418887, 10 pages https://doi.org/10.1155/2021/3418887 Research Article Preparation of Mesoporous Biochar from Cornstalk for the Chromium (VI) Elimination by Using One-Step Hydrothermal Carbonation 1 1 1 1 2 Chao Wang , Jun Xie , Mingdong Zheng , Jinbo Zhu , and Changliang Shi Department of Materials Science and Engineering, Anhui University of Science & Technology, Huainan, China College of Chemistry and Chemical Engineering, Henan Polytechnic University, Jiaozuo, China Correspondence should be addressed to Jun Xie; jxie@aust.edu.cn Received 18 May 2021; Revised 17 August 2021; Accepted 16 September 2021; Published 5 October 2021 Academic Editor: Alessandro Buccolieri Copyright © 2021 Chao Wang et al. +is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Hydrothermal carbon (HTC) was prepared by the one-step hydrothermal method for Cr (VI) removal from wastewater, which was considered a “green chemistry” method. +e speciﬁc surface area (S ) of HTC was 85 m /g with the pore size in range of BET 2.0–24.0 nm. FT-IR spectra analysis showed that the HTC had abundant chemical surface functional groups. +e inﬂuence of adsorption parameters such as pH, HTC dosage, Cr (VI) concentration, and contact time on the removal eﬃciency of Cr (VI) had been investigated. When the initial concentration was 50 mg/L, pH � 6, amount of adsorbent was 0.2 g/50 ml, and adsorption time was 90 min; the Cr (VI) absorbed rate of HTC reached 98%. Batch adsorption experiments indicated that Cr (VI) adsorption data of HTC ﬁtted the Freundlich isothermal and pseudo-second-order kinetic models. Overall, our ﬁndings provide a promising material in treatment of Cr (VI)-rich wastewater and give a clear picture of its application, which is worthy of further study. exchange had been devoted to eliminating Cr (VI) from 1. Introduction industry wastewaters [10–14]. However, numerous ap- Heavy metal contamination in water is becoming a con- proaches were indicated as invalid and expensive, which led cerning global environmental issue. Heavy metal pollutants, to secondary pollution to the environment as a consequence such as chromium, arsenic, cadmium, nickel, copper, and in some cases [15]. Adsorption had been regarded as the lead, are diﬃcult to be removed or degraded from the water most eﬀective method aiming to eliminate the contaminants mostly due to their high stability [1]. Among these heavy within the aqueous systems [16]. Hence, the investigation in metals, chromium was noticed as a hazardous pollutant appropriate adsorbent emerged the importance of devel- introduced by multiple industrial processes including oping a considerable adsorption technology [16, 17]. Acti- electroplating, leather tanning, pigment, and production vated carbons have better adsorption capacity and Cr (VI) paint [2]. In wastewater, chromium existed principally with absorbed rate resulted from the large surface area and volume, which have been widely employed to eliminate Cr two comparatively stable forms including hexavalent Cr (VI) and trivalent Cr (III) [3, 4], in which Cr (VI) was relatively (VI) ions from wastewater resulted from its low cost, high hazardous than Cr (III), especially in the human body [5, 6]. adsorption eﬃciency, and simple operation [18, 19]. Un- Moreover, Cr (VI) revealed a nature of absorption and fortunately, activated carbons are commonly prepared via accumulation within human bodies such as the stomach, chemical or physical activation methods. In general, biomass kidneys, and liver especially, which caused the severe so- materials were employed as precursors [20], which require matic damages [5, 7–9]. high energy consumption and cause damage to the envi- Massive eﬀorts focused on precipitation, electrochemical ronment [21–23]. +erefore, it is particularly important to recovery, solvent extraction, membrane separation, and ion ﬁnd a synthesis method with low energy consumption. 2 Journal of Analytical Methods in Chemistry Recently, hydrothermal carbonization (HTC) processes as a diphenylcarbazide were acquired from Beijing Chemical nascent technology to produce functional materials was Reagents Company. Potassium dichromate (K Cr O ) with 2 2 7 reported resulting from its low cost, simplicity in operation, a certain quantity was dissolved into deionized water to and high energy eﬃciency [24, 25]. One was also be classiﬁed prepare the stock solution and then diluted to the required as “green” because no organic solvents or surfactants were concentration for further analysis. needed in the treatment process [25]. Biochar is produced from agriculture and forest waste 2.2. Synthesis of HTC. Diﬀerent synthesis methods of HTC which contain mainly carbon. Biochar-type materials have were considered [32–34]. +e preparation of biomass porous raised increasing attention attributed to their unique mes- carbon usually requires a high temperature calcination oporous structure, high ion exchange capacity, and high process [21–23], while our hydrothermal carbon (HTC) is speciﬁc surface which allow them to be widely used for prepared by a one-step hydrothermal method, which is applications in greenhouse gas reduction, soil improvement, much simpler. A representative process is selected, as shown and remediation of contaminated soil [26–28]. Guo et al. in Figure 2. 15 g corn stalk had been dissolved into 80 mL of reported the Cr (VI) adsorption on the HTC prepared from dilute sulfuric acid (1.84 M) with stirring to form a ho- rice husk at 650–850 C and found adsorption eﬃciency is mogenous solution. +e mixture was subsequently removed related to the micoporous and mesoporous [29]. Anand- to 120 mL Teﬂon-lined stainless steel autoclave and then kumar and Mandal used AC prepared from bael fruit at placed in an electronic heating furnace (preheated at 190 C) 600 C to eliminate Cr (VI) with a considerable value of for 12 h. After cooled to room temperature, the solid product 17.27 mg/g in adsorption capacity that had been acquired was gathered by a ﬁlter ﬂask and cleaned by a deionized [30]. Also, several activated carbon adsorbents from agri- ethanol solution. HTC products had been dried in a vacuum cultural and biological wastes including almond shells, oven at 120 C for 8 h. straw, waste tea, coconut shells, cactus leaves [31, 32], and algae had been introduced in chromium (VI) elimination from water solutions. However, the production process of 2.3. Characterization Techniques. +e porous texture of the biochar is relatively complex. +e biomass was ﬁrst pre- HTC was analyzed by N sorption at −196 C, using an heated at low temperature and followed by chemical acti- automatic adsorption system (Quanta Chrome, America). vation. Last, the biomass was hydrothermally carbonized at Before measurements, the sample was degassed at 120 C for high temperatures such as 600, 800, and even 1000 C. 5 h. +e speciﬁc surface area of the sample was calculated by In this work, the mesoporous biochar was acquired using the Brunauer–Emmett–Teller (BET) method using the ad- one-step hydrothermal carbonization from cornstalk at sorption data at the relative pressure (P/P ) range of relatively mild temperature conditions (190 C) at self-gen- 0.05–0.3. +e total pore volume was calculated at P/P � 0.99, erated pressures. +e eﬀects of several important operating and the pore size distribution curve was computed using the parameters on the removal of Cr (VI) from aqueous solu- BJH model. −1 tion, such as pH, adsorption dosage, initial concentration of Infrared spectra (5000–0 cm ) were recorded using a the solution, and contact time, were studied by batch ex- Bruker VERTEX 70 FT-IR spectrometer. +e sample was periments. Furthermore, the adsorption kinetics and iso- prepared by mixing an oven-dried (at 105 C) sample with therms of Cr (VI) on HTC at diﬀerent temperatures were spectroscopy-grade KBr in an agate mortar. also analyzed and discussed. +e HTC sample was dried in a fan-forced oven under air at 80 C. About 0.1-0.2 g of each sample was weighed into 2. Experimental tin foil cups and combusted with an oxygen catalyst at 1150 C. +e ultimate analyses were conducted on a +ermo 2.1. Materials. +e raw material of this experiment is corn Scientiﬁc FLASH 2000 autoanalyzer. straw, which was collected in Huainan City, Anhui Province. Powdered HTC sample was placed onto the adhesive TGA, EDS, and XRD of corn straw are shown in carbon tape on an aluminum stub followed by sputter Figures 1(a), 1(b), and 1(c), respectively. According to the coating with gold. +e surface morphology of the sample was TGA curve, the weight loss of corn straw can be divided into observed on a UL-TRA55 scanning electron microscope three stages: A (gasiﬁcation stage), B (thermal cracking (SEM) operated at 2 kV. stage), and C (carbonization stage). +e decomposition temperature exceeds 200 C and has thermal stability. +e characteristic peaks of C and O elements in EDS showed that 2.4. Adsorption Experiments. Adsorption characteristic had the main component of corn straw was cellulose. +e XRD been achieved under the batch mode at 35 C. First, a certain diﬀraction pattern shows that the diﬀraction peak is 14.8 , amount of 1 M HCl and 1 M NaOH had been applied to ° ° 16.5 , and 22.5 , representing (101) and (002) crystal planes modify the pH value of each solution followed by mixed with of cellulose type I crystal structure, respectively, indicating the adsorbent. +e inﬂuence of pH on the adsorbability of that the crystal structure has not changed. Cornstalk was HTC had been illustrated to determine an optimal pH value. acquired from local natural resources and was cleaned and +e eﬀects of initial adsorbate concentration, dosage, and dried to constant weight at room temperature. Sulfuric acid contacting duration on the adsorption performance were (98 wt.%), phosphoric acid, sodium hydroxide, acetone, then studied at the optimum pH. +e adsorbent was potassium dichromate, and analytical grade 1,5- employed to mix with 50 mL of adsorbate solutions with a Journal of Analytical Methods in Chemistry 3 250 °C Gasiﬁcation water loss Au Pyrolysis Si Al Ca Au 0 Carbonization 50 100 150 200 250 300 350 400 450 500 02468 10 0 102030405060 Temperature (°C) Energy (keV) 2θ (degree) TGA of Corn Straw EDS of Corn Straw XRD of Corn Straw (a) (b) (c) Figure 1: +e TGA (a), EDS (b), and XRD (c) curves of corn straw. High-Temperature homogenous reaction at 190 °C for 12 h solution Dissolved in dilute sulfuric acid (1.8 M) Stirring Corn stalk 15 g dried in a vacuum oven at washed with 120 °C for 8 h deionized water and ethanol Hydrothermal carbon (HTC) Figure 2: Synthesis process of the HTC. particular initial concentration. Specimens were designed to c − c􏼁 0 t η � . (2) be collected at diﬀerent intervals of time, and the adsorbents were extracted by ﬁltration. Whereafter, the ﬁltrates had been investigated in the residuary chromium (VI) con- centration with the UV spectrophotometer (TU-1880, 3. Results and Discussion Beijing, China) under the wavelength of 540 nm. +e amount of adsorbed had been evaluated via the following 3.1. Pore Structure Characterization. +e curves of the pore equation: size distribution (PSD) and corresponding nitrogen ad- sorption-desorption isotherm for HTC samples are shown v c − c􏼁 0 t (1) q � , in Figure 3. According to the IUPAC classiﬁcation, it can be seen from Figure 1(a) that the HTC exhibits a type IV where q is the amount of Cr (VI) ions adsorbed per unit isotherm curve. +ere was no signiﬁcant increase in nitrogen gram of hydrothermal carbon (mg/g) at any time (t), c is the uptake when the relative pressure was below 0.01, the ap- ﬁnal Cr (VI) concentration after a certain period (mg/L), c parent increase in nitrogen adsorption at the range from 0.1 is the initial Cr (VI) concentration (mg/L), and v is the initial to 1.0 in relative pressure, and the capillary hysteresis loop solution volume (L); m is the HTC mass (g). +e percentage indicates the existence of developed mesoporous structure in of removed metal ions in the solution was calculated using the sample. Figure 3(b) shows its pore size distribution in the following equation: 2.0–24.0 nm, in which multiple peaks are shown, indicating Weight (%) Intensity 4 Journal of Analytical Methods in Chemistry 300 0.030 0.025 0.020 0.015 0.010 0.005 0.000 0.0 0.2 0.4 0.6 0.8 1.0 0 4 8 12 16 20 24 Relative pressure (P/P ) Pore width (nm) YMJG-190 ºC -12 h YMJG-190 ºC-12 h (a) (b) Figure 3: N adsorption-desorption isotherms (a) and pore sizes distribution curves (b) of YMJG. that the HTC has a broad size distribution. Table 1 provides were formed on the surface of the sample. +is may be the textural characteristics of the HTC which exists an S because the HTC synthesized from cornstalk was not BET 2 3 and overall pore volume (V ) of 85 m /g, 0.042 cm /g, re- completely carbonized [41]. +is phenomenon is interesting spectively. +ese values are even higher than the reports in due to fact that some porosity structures could be formed some kinds of literature [35, 36] at higher hydrothermal during the hydrothermal carbonization of biomass materials temperatures. +e large S and broad distribution of pore under common conditions [41–43]. Figure 5(a) shows that BET size is necessary for the Cr (VI) ions reaching the interior of there are some pores formed on the surface of the sample. the material, thus achieving maximum absorption. Besides, many small spheres cover the inner wall of the aperture (Figure 5(b)). +e reason is that the cornstalk underwent isomerization, fragmentation, dehydration, po- 3.2. FT-IR Spectra and Ultimate Analyses of the HTC- lymerization, and carbonization to form the spheres on the Cornstalk. Structure analysis of the HTC-cornstalk at the surface of the sample [44]. initial and end states of the adsorption had been revealed via FT-IR spectra and is shown in Figure 4. O-H (bonded) 3.4. Batch Adsorption Studies stretching vibration is correlative to the band between 3700 −1 −1 and 3030 cm . Moreover, peaks of 2929 and 2376 cm 3.4.1. Eﬀect of pH and HTC-Cornstalk Dosage. At the range implied the stretching vibration of C-H and C-N, respec- of 1.0–9.0 in pH values of the solution, Cr (VI) absorption −1 tively, while 1700 and 1600 cm were associated with C � O induced by HTC at 35 C had been observed. Figure 6(a) and C � C stretching [36]. +e absorption band between 995 −1 uncovers the impact of the pH value on Cr (VI) adsorption and 1242 cm is attributed to the C-O stretching vibration −1 under the conditions of 50 mg/L in the initial Cr (VI) of esters, aliphatic, or alcohols [37]. Band at 620 cm was concentration and 90 min in the contacting duration. +e corresponded to the aromatic ring (C-H) bending vibration adsorbability sharply decreases with the increase of the pH [38]. After adsorption, the peaks at 3450, 2929, and value from 4.0 to 9.0 which implied the strong correlation −1 2376 cm shift towards a lower wavenumber of 3429, 2918, −1 between adsorbing behaviors of Cr (VI) and the pH value of and 2355 cm , respectively. +e absolute values of bands the solution. Only 22% Cr (VI) ions had been eliminated in −1 between 1700 and 1600 cm are smaller after adsorption. the case of pH � 9 instead of over 82% while in a slightly +is suggests that there were possible interactions between acidic solution with pH � 6.0. Moreover, an even higher these groups and Cr (VI). However, the absolute values of −1 removal rate of the Cr (VI) ions of 98% had been achieved the band at 1112 and 640 cm become larger, which in- when the pH value was 1.0. +e surface functional groups in dicates that the HTC-cornstalk has higher functional groups the adsorbent and metal solution chemistry are highly re- after Cr (VI) adsorption. One is given in Table 2, in which lated to the pH, which can largely aﬀect metal adsorption the carbon content was increased during the HTC process, ability [40, 45, 46]. Under the strong acid condition, the whereas the oxygen and hydrogen contents were decreased, removal rate of Cr (VI) ion was higher; however, it would which is consistent with other reports [39, 40]. bring acid pollution to the environment. +us, the optimum pH for the adsorption experiment is chosen as 6. 3.3. SEM Analysis. +e SEM images of HTC-cornstalk at +e eﬀect of the adsorbent dosage was also investigated diﬀerent magniﬁcations are shown in Figures 5(a) and 5(b), at 35 C, as shown in Figure 6(b). Results indicated that the respectively. It can be observed that the uniform spheres removed Cr (VI) ions quantity was enlarged as the 3 -1 Volume adsorbed (cm ·g ) 3 -1 Incremental pore volume (cm ·g ) Journal of Analytical Methods in Chemistry 5 Table 1: Textural characteristics. Speciﬁc surface Total pore Micropore Mesopore Average pore size Sample 2 −1 3 −1 3 −1 area (m ·g ) volume (cm ·g ) volume (cm ·g ) rate (%) (nm) HTC 85 0.42 0.003 99.93 3.41 minor eﬀect would appear even by further increasing the contacting duration. Desorption ratio of Cr (VI) ions and adsorption quantity in 150 min was reached almost 98%, 12.29 mg/g, respectively. C-H 3.5. Adsorption Kinetic Analysis. A kinetic study on ad- C-N C=O sorption was carried out to reveal the adsorption rates and C-H the controlling adsorption mechanism. Pseudo-ﬁrst-order O-H (equation (3)) and pseudo-second-order modes (equation (4)) are commonly used to ﬁt experimental data [48]: C-O ln q − q 􏼁 � ln q − k t, (3) e e 1 4000 3600 3200 2800 2400 2000 1600 1200 800 400 -1 Wavenumber (cm ) t 1 1 � + , (4) q k q q q Before adsorption 2 e e e Aer adsorption where k is the rate constant of the pseudo-ﬁrst-order model −1 Figure 4: FT-IR spectra of the HTC-cornstalk. (min ), k is the rate constant of the pseudo-second-order model (g/mg. min), and q and q are the values of the amount adsorbed per unit mass at equilibrium and at any Table 2: Ultimate analysis of samples. time t, respectively. +e experimental data and the ﬁtting by Ultimate analysis (wt%) the two equations are shown in Figure 8, and the ﬁtted Sample C% H% O% N% kinetic parameters are given in Table 3. Cornstalk 43.17 5.60 50.55 0.65 One emerged in Figure 8 and Table 3 that both models HTC-cornstalk 63.85 4.13 29.81 0.32 were well-ﬁtted with the experimental data. Furthermore, By diﬀerence. the calculated adsorption capacities (q , cal) in the models were consistent with the experimental adsorption capacities increasing adsorbent dosage. Increasing the amount of (q , exp). However, the pseudo-second-order model HTC-cornstalk from 0.05/50 to 0.2/50 (g/mL) largely en- exhibited a higher correlation coeﬃcient (R ) which was hanced the elimination percentage of Cr (VI) ions from 36% employed to estimate the consistency of the ﬁtted models to 98%, resulted from the increase of the active sites which with experimental results than that of the pseudo-ﬁrst-order were available for the occupation of Cr (VI) ions. A further model. Although, in consideration of a particular kinetic increase in the adsorbent dosage did not have any eﬀect. model, a relatively high R was not irrefragable evidence of a better ﬁtting [49]. +e Cr (VI) adsorption onto HTC- cornstalk in this work was considered that was apropos to 3.4.2. Eﬀect of Initial Cr (VI) Concentration and Contact the pseudo-second-order kinetic model. Time. +e adsorption performance of the HTC-cornstalk inﬂuenced by the initial Cr (VI) concentrations from 30 to 90 mg/L was studied at 35 C with pH � 6. Results are shown in 3.6.AdsorptionIsothermAnalysis. To investigate the Cr (VI) Figure 7(a). +e HTC-cornstalk dosage and the contacting adsorbability of HTC, adsorption isotherms had been car- duration were kept at 0.2 g/50 mL, 90 min, respectively. One ried out with various initial concentrations of Cr (VI) (20, was introduced that the removal eﬃciency and adsorbability of 30, 40, 50, 60, 70, 80, 90, and 100 mg/L) at diﬀerent tem- ° ° ° HTC increased along with the increase of Cr (VI) concen- peratures (35 C, 45 C, and 55 C). tration. However, while the Cr (VI) concentration hit over Results had been ﬁtted with Langmuir (5) and 50 mg/L, the removal eﬃciency steadily reduced as a conse- Freundlich (6) isotherm models: quence. With a high Cr (VI) ions concentration, the adsorption c c 1 e e rate was restricted because the adsorption was saturated and � + , (5) q q k q e max 1 max the desorption rate was higher than adsorption [47]. Figure 7(b) shows the impact of contacting duration on lg q � lg + lgk, (6) the Cr (VI) adsorption within the aqueous solution with 50 mg/L in the initial Cr (VI) concentration under the same conditions. One was cleared that the adsorption procedure where q refers to the quantity of adsorbed metal ions per approached the equilibrium state within 150 min or less. +e unit mass of adsorbent (mg/g), c represents the solute Transmittance 6 Journal of Analytical Methods in Chemistry (a) (b) 2 μm 200 nm Figure 5: SEM images of HTC-cornstalk. 0.05 0.10 0.15 0.20 0.25 0.30 0.35 1 23456789 HTC-cornstalk mass (g) pH Cr (VI) removal Cr (VI) removal (a) (b) Figure 6: Eﬀect of initial solution pH on the removal eﬃciency of Cr (VI) from aqueous solutions. (a) Volume, 50 mL; agitation speed, 120 rpm; HTC dosage, 4 g/L; eﬀect of initial solution of the HTC dosage on the removal eﬃciency of Cr (VI) from aqueous solutions. (b) pH, 6; volume, 50 mL; agitation speed, 120 rpm; Cr (VI) concentration, 50 mg/L; contact time, 90 min. 18 105 14.0 13.5 13.0 12.5 12 12.0 11.5 11.0 10.5 65 6 70 10.0 30 40 50 60 70 80 90 20 40 60 80 100 120 140 160 180 200 220 + -1 Cr (mg·L ) t (min) Cr (VI) removal Cr (VI) removal q q e e (a) (b) Figure 7: Eﬀect of initial Cr (VI) concentration (a) and contacting duration (b) on the Cr (VI) adsorbability of HTC. Removal efficiency (%) Removal efficiency (%) -1 q (mg·g ) Removal efficiency (%) Removal efficiency (%) -1 q (mg·g ) e Journal of Analytical Methods in Chemistry 7 y=0.07085x+1.28243 R =0.96238 -1 y=-0.05818x+2.64437 R =0.98889 -2 -3 -4 0 20 40 60 80 100 120 0 20 40 60 80 100 120 140 160 t (min) t (min) (a) (b) Figure 8: Pseudo-ﬁrst-order kinetics (a) and pseudo-second-order kinetics (b) for Cr (VI) adsorption onto HTC at 35 C (initial Cr (VI) concentration 50 mg/L). Table 3: Adsorption kinetics parameters of Cr (VI) on HTC-cornstalk. Pseudo-ﬁrst-order kinetic model Pseudo-second-order kinetic model q . (mg/g) e exp −1 2 2 q . (mg/g) k (min ) R q . (mg/g) k (g/mg min) R e cal 1 e cal 2 12.29 14.07 0.06 0.96 14.11 0.004 0.99 1.4 0.09 1.3 R =0.9957 0.08 R =0.95738 1.2 R =0.98027 1.1 0.07 1.0 0.06 R =0.8871 0.9 R =0.7249 0.05 0.8 R =0.37933 0.7 0.04 0.6 0.0 0.5 1.0 1.5 2.0 -0.6 -0.4 -0.2 0.0 0.2 0.4 c (mg/L) lg c e e 35 °C y=0.01993x+0.0464 35 °C y=0.721x+1.153 45 °C y=0.02168x+0.03769 45 °C y=0.699x+1.193 55 °C y=0.0199x+0.04146 55 °C y=0.759x+1.223 (a) (b) Figure 9: Langmuir isotherm model (a) and Freundlich isotherm model (b) for Cr (VI) adsorption onto HTC. concentration in the bulk solution (mg/L) at equilibrium state, As revealed above, R values of higher than 0.9 were q (mg/g) and k represent the Langmuir constants related to obtained in the Freundlich model which indicated an ex- max 1 the saturated metal ions adsorbability and the adsorption free cellent consistency while ﬁtting instead of many poor ones in energy, respectively, and the constants k and n in the the Langmuir model which were ﬁtted at three diﬀerent Freundlich model represent the strength and the distribution of temperatures. +erefore, the Freundlich model was selected the adsorptive bonds. Experimental data ﬁtted with Langmuir to express the Cr (VI) adsorption of HTC-cornstalk. and Freundlich equations are shown in Figure 9. Furthermore, Freundlich as an empirical equation had been employed to the ﬁtted results are given in Table 4 in detail. illustrate the exponential distribution sites, energies, and the -1 ln (q -q) c ·q e e e t (q) lg q e 8 Journal of Analytical Methods in Chemistry Table 4: +e parameters of the isotherm adsorption model. Langmuir modes Freundlich models Temperature ( C) 2 2 q (mg/g) K (L/mg) R K (mg/g) 1/n R max l 35 50.14640 0.42987 0.87281 14.49226 0.72818 0.98489 45 46.11816 0.57533 0.68560 15.63765 0.69809 0.95072 55 50.23585 0.48011 0.27588 17.50070 0.81531 0.91757 heterogeneity of the adsorbent surface [50]. +e adsorption molecular simulation technology,” Chemistry Letters, vol. 49, no. 12, pp. 1452–1455, 2020. capacities (K) increased with increasing temperatures. +e [4] S. Rangabhashiyam, S. Sayantani, and P. Balasubramanian, values of n at diﬀerent temperatures are greater than unity, “Assessment of hexavalent chromium biosorption using indicating favorable adsorption of Cr (VI) [30, 51]. biodiesel extracted seeds of Jatropha sp., Ricinus sp. and Pongamia sp,” International Journal of Environmental Science 4. Conclusion and Technology, vol. 16, no. 10, pp. 5707–5724, 2019. [5] A. Hariharan, V. Harini, S. Sandhya, and S. Rangabhashiyam, A high-eﬃciency HTC adsorbent was successfully synthe- “Waste musa acuminata residue as a potential biosorbent for sized by the one-step hydrothermal method for the uptake of the removal of hexavalent chromium from synthetic waste- Cr (VI). +e adsorption property of HTC was studied by water,” Biomass Conversion and Bioreﬁnery, pp. 1–14, 2020. varying pH, HTC dosage, Cr (VI) concentration, and contact [6] A. H. M. G. Hyder, S. A. Begum, and N. O. Egiebor, “Ad- time. +e Cr (VI) adsorption capability of HTC was pH- sorption isotherm and kinetic studies of hexavalent chro- dependent, which was favored with a low pH range. +e Cr mium removal from aqueous solution onto bone char,” (VI) removal eﬃciency of HTC reached 98% under optimal Journal of Environmental Chemical Engineering, vol. 3, no. 2, pp. 1329–1336, 2015. experiment conditions. A pseudo-second-order kinetic [7] S. Nayak, S. Rangabhashiyam, P. Balasubramanian, and model could describe Cr (VI) adsorption behavior on HTC P. Kale, “A review of chromite mining in Sukinda valley of well, while the Freundlich adsorption isotherm was more India: impact and potential remediation measures,” Inter- suitable for Cr (VI) adsorption onto the adsorbent. +us, the national Journal of Phytoremediation, vol. 22, no. 8, HTC was a potentially eﬀective and sustainable adsorbent pp. 804–818, 2020. for application in Cr (VI) removal from wastewater [8] S. Rangabhashiyam, N. Selvaraju, M. B. Raj, A. P. K. Muhammed, K. D. Amith, and E. R. Ushakumary, Data Availability “Hydrous cerium oxide nanoparticles impregnated Enter- omorpha sp. for the removal of hexavalent chromium from +e data used to support the ﬁndings of this study are in- aqueous solutions,” Journal of Environmental Engineering, cluded within the article. vol. 142, no. 9, Article ID C4015016, 2016. [9] V. Hasija, P. Raizada, P. Singh et al., “Progress on the pho- tocatalytic reduction of hexavalent Cr (VI) using engineered Conflicts of Interest graphitic carbon nitride,” Process Safety and Environmental Protection, vol. 152, pp. 663–678, 2021. +e authors declare that they have no conﬂicts of interest. [10] M. Ciopec, C. M. Davidescu, A. 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Publisher: Hindawi Publishing Corporation
Copyright: Copyright © 2021 Chao Wang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ISSN: 2090-8865
eISSN: 2090-8873
DOI: 10.1155/2021/3418887
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Abstract

Hindawi Journal of Analytical Methods in Chemistry Volume 2021, Article ID 3418887, 10 pages https://doi.org/10.1155/2021/3418887 Research Article Preparation of Mesoporous Biochar from Cornstalk for the Chromium (VI) Elimination by Using One-Step Hydrothermal Carbonation 1 1 1 1 2 Chao Wang , Jun Xie , Mingdong Zheng , Jinbo Zhu , and Changliang Shi Department of Materials Science and Engineering, Anhui University of Science & Technology, Huainan, China College of Chemistry and Chemical Engineering, Henan Polytechnic University, Jiaozuo, China Correspondence should be addressed to Jun Xie; jxie@aust.edu.cn Received 18 May 2021; Revised 17 August 2021; Accepted 16 September 2021; Published 5 October 2021 Academic Editor: Alessandro Buccolieri Copyright © 2021 Chao Wang et al. +is is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Hydrothermal carbon (HTC) was prepared by the one-step hydrothermal method for Cr (VI) removal from wastewater, which was considered a “green chemistry” method. +e speciﬁc surface area (S ) of HTC was 85 m /g with the pore size in range of BET 2.0–24.0 nm. FT-IR spectra analysis showed that the HTC had abundant chemical surface functional groups. +e inﬂuence of adsorption parameters such as pH, HTC dosage, Cr (VI) concentration, and contact time on the removal eﬃciency of Cr (VI) had been investigated. When the initial concentration was 50 mg/L, pH � 6, amount of adsorbent was 0.2 g/50 ml, and adsorption time was 90 min; the Cr (VI) absorbed rate of HTC reached 98%. Batch adsorption experiments indicated that Cr (VI) adsorption data of HTC ﬁtted the Freundlich isothermal and pseudo-second-order kinetic models. Overall, our ﬁndings provide a promising material in treatment of Cr (VI)-rich wastewater and give a clear picture of its application, which is worthy of further study. exchange had been devoted to eliminating Cr (VI) from 1. Introduction industry wastewaters [10–14]. However, numerous ap- Heavy metal contamination in water is becoming a con- proaches were indicated as invalid and expensive, which led cerning global environmental issue. Heavy metal pollutants, to secondary pollution to the environment as a consequence such as chromium, arsenic, cadmium, nickel, copper, and in some cases [15]. Adsorption had been regarded as the lead, are diﬃcult to be removed or degraded from the water most eﬀective method aiming to eliminate the contaminants mostly due to their high stability [1]. Among these heavy within the aqueous systems [16]. Hence, the investigation in metals, chromium was noticed as a hazardous pollutant appropriate adsorbent emerged the importance of devel- introduced by multiple industrial processes including oping a considerable adsorption technology [16, 17]. Acti- electroplating, leather tanning, pigment, and production vated carbons have better adsorption capacity and Cr (VI) paint [2]. In wastewater, chromium existed principally with absorbed rate resulted from the large surface area and volume, which have been widely employed to eliminate Cr two comparatively stable forms including hexavalent Cr (VI) and trivalent Cr (III) [3, 4], in which Cr (VI) was relatively (VI) ions from wastewater resulted from its low cost, high hazardous than Cr (III), especially in the human body [5, 6]. adsorption eﬃciency, and simple operation [18, 19]. Un- Moreover, Cr (VI) revealed a nature of absorption and fortunately, activated carbons are commonly prepared via accumulation within human bodies such as the stomach, chemical or physical activation methods. In general, biomass kidneys, and liver especially, which caused the severe so- materials were employed as precursors [20], which require matic damages [5, 7–9]. high energy consumption and cause damage to the envi- Massive eﬀorts focused on precipitation, electrochemical ronment [21–23]. +erefore, it is particularly important to recovery, solvent extraction, membrane separation, and ion ﬁnd a synthesis method with low energy consumption. 2 Journal of Analytical Methods in Chemistry Recently, hydrothermal carbonization (HTC) processes as a diphenylcarbazide were acquired from Beijing Chemical nascent technology to produce functional materials was Reagents Company. Potassium dichromate (K Cr O ) with 2 2 7 reported resulting from its low cost, simplicity in operation, a certain quantity was dissolved into deionized water to and high energy eﬃciency [24, 25]. One was also be classiﬁed prepare the stock solution and then diluted to the required as “green” because no organic solvents or surfactants were concentration for further analysis. needed in the treatment process [25]. Biochar is produced from agriculture and forest waste 2.2. Synthesis of HTC. Diﬀerent synthesis methods of HTC which contain mainly carbon. Biochar-type materials have were considered [32–34]. +e preparation of biomass porous raised increasing attention attributed to their unique mes- carbon usually requires a high temperature calcination oporous structure, high ion exchange capacity, and high process [21–23], while our hydrothermal carbon (HTC) is speciﬁc surface which allow them to be widely used for prepared by a one-step hydrothermal method, which is applications in greenhouse gas reduction, soil improvement, much simpler. A representative process is selected, as shown and remediation of contaminated soil [26–28]. Guo et al. in Figure 2. 15 g corn stalk had been dissolved into 80 mL of reported the Cr (VI) adsorption on the HTC prepared from dilute sulfuric acid (1.84 M) with stirring to form a ho- rice husk at 650–850 C and found adsorption eﬃciency is mogenous solution. +e mixture was subsequently removed related to the micoporous and mesoporous [29]. Anand- to 120 mL Teﬂon-lined stainless steel autoclave and then kumar and Mandal used AC prepared from bael fruit at placed in an electronic heating furnace (preheated at 190 C) 600 C to eliminate Cr (VI) with a considerable value of for 12 h. After cooled to room temperature, the solid product 17.27 mg/g in adsorption capacity that had been acquired was gathered by a ﬁlter ﬂask and cleaned by a deionized [30]. Also, several activated carbon adsorbents from agri- ethanol solution. HTC products had been dried in a vacuum cultural and biological wastes including almond shells, oven at 120 C for 8 h. straw, waste tea, coconut shells, cactus leaves [31, 32], and algae had been introduced in chromium (VI) elimination from water solutions. However, the production process of 2.3. Characterization Techniques. +e porous texture of the biochar is relatively complex. +e biomass was ﬁrst pre- HTC was analyzed by N sorption at −196 C, using an heated at low temperature and followed by chemical acti- automatic adsorption system (Quanta Chrome, America). vation. Last, the biomass was hydrothermally carbonized at Before measurements, the sample was degassed at 120 C for high temperatures such as 600, 800, and even 1000 C. 5 h. +e speciﬁc surface area of the sample was calculated by In this work, the mesoporous biochar was acquired using the Brunauer–Emmett–Teller (BET) method using the ad- one-step hydrothermal carbonization from cornstalk at sorption data at the relative pressure (P/P ) range of relatively mild temperature conditions (190 C) at self-gen- 0.05–0.3. +e total pore volume was calculated at P/P � 0.99, erated pressures. +e eﬀects of several important operating and the pore size distribution curve was computed using the parameters on the removal of Cr (VI) from aqueous solu- BJH model. −1 tion, such as pH, adsorption dosage, initial concentration of Infrared spectra (5000–0 cm ) were recorded using a the solution, and contact time, were studied by batch ex- Bruker VERTEX 70 FT-IR spectrometer. +e sample was periments. Furthermore, the adsorption kinetics and iso- prepared by mixing an oven-dried (at 105 C) sample with therms of Cr (VI) on HTC at diﬀerent temperatures were spectroscopy-grade KBr in an agate mortar. also analyzed and discussed. +e HTC sample was dried in a fan-forced oven under air at 80 C. About 0.1-0.2 g of each sample was weighed into 2. Experimental tin foil cups and combusted with an oxygen catalyst at 1150 C. +e ultimate analyses were conducted on a +ermo 2.1. Materials. +e raw material of this experiment is corn Scientiﬁc FLASH 2000 autoanalyzer. straw, which was collected in Huainan City, Anhui Province. Powdered HTC sample was placed onto the adhesive TGA, EDS, and XRD of corn straw are shown in carbon tape on an aluminum stub followed by sputter Figures 1(a), 1(b), and 1(c), respectively. According to the coating with gold. +e surface morphology of the sample was TGA curve, the weight loss of corn straw can be divided into observed on a UL-TRA55 scanning electron microscope three stages: A (gasiﬁcation stage), B (thermal cracking (SEM) operated at 2 kV. stage), and C (carbonization stage). +e decomposition temperature exceeds 200 C and has thermal stability. +e characteristic peaks of C and O elements in EDS showed that 2.4. Adsorption Experiments. Adsorption characteristic had the main component of corn straw was cellulose. +e XRD been achieved under the batch mode at 35 C. First, a certain diﬀraction pattern shows that the diﬀraction peak is 14.8 , amount of 1 M HCl and 1 M NaOH had been applied to ° ° 16.5 , and 22.5 , representing (101) and (002) crystal planes modify the pH value of each solution followed by mixed with of cellulose type I crystal structure, respectively, indicating the adsorbent. +e inﬂuence of pH on the adsorbability of that the crystal structure has not changed. Cornstalk was HTC had been illustrated to determine an optimal pH value. acquired from local natural resources and was cleaned and +e eﬀects of initial adsorbate concentration, dosage, and dried to constant weight at room temperature. Sulfuric acid contacting duration on the adsorption performance were (98 wt.%), phosphoric acid, sodium hydroxide, acetone, then studied at the optimum pH. +e adsorbent was potassium dichromate, and analytical grade 1,5- employed to mix with 50 mL of adsorbate solutions with a Journal of Analytical Methods in Chemistry 3 250 °C Gasiﬁcation water loss Au Pyrolysis Si Al Ca Au 0 Carbonization 50 100 150 200 250 300 350 400 450 500 02468 10 0 102030405060 Temperature (°C) Energy (keV) 2θ (degree) TGA of Corn Straw EDS of Corn Straw XRD of Corn Straw (a) (b) (c) Figure 1: +e TGA (a), EDS (b), and XRD (c) curves of corn straw. High-Temperature homogenous reaction at 190 °C for 12 h solution Dissolved in dilute sulfuric acid (1.8 M) Stirring Corn stalk 15 g dried in a vacuum oven at washed with 120 °C for 8 h deionized water and ethanol Hydrothermal carbon (HTC) Figure 2: Synthesis process of the HTC. particular initial concentration. Specimens were designed to c − c􏼁 0 t η � . (2) be collected at diﬀerent intervals of time, and the adsorbents were extracted by ﬁltration. Whereafter, the ﬁltrates had been investigated in the residuary chromium (VI) con- centration with the UV spectrophotometer (TU-1880, 3. Results and Discussion Beijing, China) under the wavelength of 540 nm. +e amount of adsorbed had been evaluated via the following 3.1. Pore Structure Characterization. +e curves of the pore equation: size distribution (PSD) and corresponding nitrogen ad- sorption-desorption isotherm for HTC samples are shown v c − c􏼁 0 t (1) q � , in Figure 3. According to the IUPAC classiﬁcation, it can be seen from Figure 1(a) that the HTC exhibits a type IV where q is the amount of Cr (VI) ions adsorbed per unit isotherm curve. +ere was no signiﬁcant increase in nitrogen gram of hydrothermal carbon (mg/g) at any time (t), c is the uptake when the relative pressure was below 0.01, the ap- ﬁnal Cr (VI) concentration after a certain period (mg/L), c parent increase in nitrogen adsorption at the range from 0.1 is the initial Cr (VI) concentration (mg/L), and v is the initial to 1.0 in relative pressure, and the capillary hysteresis loop solution volume (L); m is the HTC mass (g). +e percentage indicates the existence of developed mesoporous structure in of removed metal ions in the solution was calculated using the sample. Figure 3(b) shows its pore size distribution in the following equation: 2.0–24.0 nm, in which multiple peaks are shown, indicating Weight (%) Intensity 4 Journal of Analytical Methods in Chemistry 300 0.030 0.025 0.020 0.015 0.010 0.005 0.000 0.0 0.2 0.4 0.6 0.8 1.0 0 4 8 12 16 20 24 Relative pressure (P/P ) Pore width (nm) YMJG-190 ºC -12 h YMJG-190 ºC-12 h (a) (b) Figure 3: N adsorption-desorption isotherms (a) and pore sizes distribution curves (b) of YMJG. that the HTC has a broad size distribution. Table 1 provides were formed on the surface of the sample. +is may be the textural characteristics of the HTC which exists an S because the HTC synthesized from cornstalk was not BET 2 3 and overall pore volume (V ) of 85 m /g, 0.042 cm /g, re- completely carbonized [41]. +is phenomenon is interesting spectively. +ese values are even higher than the reports in due to fact that some porosity structures could be formed some kinds of literature [35, 36] at higher hydrothermal during the hydrothermal carbonization of biomass materials temperatures. +e large S and broad distribution of pore under common conditions [41–43]. Figure 5(a) shows that BET size is necessary for the Cr (VI) ions reaching the interior of there are some pores formed on the surface of the sample. the material, thus achieving maximum absorption. Besides, many small spheres cover the inner wall of the aperture (Figure 5(b)). +e reason is that the cornstalk underwent isomerization, fragmentation, dehydration, po- 3.2. FT-IR Spectra and Ultimate Analyses of the HTC- lymerization, and carbonization to form the spheres on the Cornstalk. Structure analysis of the HTC-cornstalk at the surface of the sample [44]. initial and end states of the adsorption had been revealed via FT-IR spectra and is shown in Figure 4. O-H (bonded) 3.4. Batch Adsorption Studies stretching vibration is correlative to the band between 3700 −1 −1 and 3030 cm . Moreover, peaks of 2929 and 2376 cm 3.4.1. Eﬀect of pH and HTC-Cornstalk Dosage. At the range implied the stretching vibration of C-H and C-N, respec- of 1.0–9.0 in pH values of the solution, Cr (VI) absorption −1 tively, while 1700 and 1600 cm were associated with C � O induced by HTC at 35 C had been observed. Figure 6(a) and C � C stretching [36]. +e absorption band between 995 −1 uncovers the impact of the pH value on Cr (VI) adsorption and 1242 cm is attributed to the C-O stretching vibration −1 under the conditions of 50 mg/L in the initial Cr (VI) of esters, aliphatic, or alcohols [37]. Band at 620 cm was concentration and 90 min in the contacting duration. +e corresponded to the aromatic ring (C-H) bending vibration adsorbability sharply decreases with the increase of the pH [38]. After adsorption, the peaks at 3450, 2929, and value from 4.0 to 9.0 which implied the strong correlation −1 2376 cm shift towards a lower wavenumber of 3429, 2918, −1 between adsorbing behaviors of Cr (VI) and the pH value of and 2355 cm , respectively. +e absolute values of bands the solution. Only 22% Cr (VI) ions had been eliminated in −1 between 1700 and 1600 cm are smaller after adsorption. the case of pH � 9 instead of over 82% while in a slightly +is suggests that there were possible interactions between acidic solution with pH � 6.0. Moreover, an even higher these groups and Cr (VI). However, the absolute values of −1 removal rate of the Cr (VI) ions of 98% had been achieved the band at 1112 and 640 cm become larger, which in- when the pH value was 1.0. +e surface functional groups in dicates that the HTC-cornstalk has higher functional groups the adsorbent and metal solution chemistry are highly re- after Cr (VI) adsorption. One is given in Table 2, in which lated to the pH, which can largely aﬀect metal adsorption the carbon content was increased during the HTC process, ability [40, 45, 46]. Under the strong acid condition, the whereas the oxygen and hydrogen contents were decreased, removal rate of Cr (VI) ion was higher; however, it would which is consistent with other reports [39, 40]. bring acid pollution to the environment. +us, the optimum pH for the adsorption experiment is chosen as 6. 3.3. SEM Analysis. +e SEM images of HTC-cornstalk at +e eﬀect of the adsorbent dosage was also investigated diﬀerent magniﬁcations are shown in Figures 5(a) and 5(b), at 35 C, as shown in Figure 6(b). Results indicated that the respectively. It can be observed that the uniform spheres removed Cr (VI) ions quantity was enlarged as the 3 -1 Volume adsorbed (cm ·g ) 3 -1 Incremental pore volume (cm ·g ) Journal of Analytical Methods in Chemistry 5 Table 1: Textural characteristics. Speciﬁc surface Total pore Micropore Mesopore Average pore size Sample 2 −1 3 −1 3 −1 area (m ·g ) volume (cm ·g ) volume (cm ·g ) rate (%) (nm) HTC 85 0.42 0.003 99.93 3.41 minor eﬀect would appear even by further increasing the contacting duration. Desorption ratio of Cr (VI) ions and adsorption quantity in 150 min was reached almost 98%, 12.29 mg/g, respectively. C-H 3.5. Adsorption Kinetic Analysis. A kinetic study on ad- C-N C=O sorption was carried out to reveal the adsorption rates and C-H the controlling adsorption mechanism. Pseudo-ﬁrst-order O-H (equation (3)) and pseudo-second-order modes (equation (4)) are commonly used to ﬁt experimental data [48]: C-O ln q − q 􏼁 � ln q − k t, (3) e e 1 4000 3600 3200 2800 2400 2000 1600 1200 800 400 -1 Wavenumber (cm ) t 1 1 � + , (4) q k q q q Before adsorption 2 e e e Aer adsorption where k is the rate constant of the pseudo-ﬁrst-order model −1 Figure 4: FT-IR spectra of the HTC-cornstalk. (min ), k is the rate constant of the pseudo-second-order model (g/mg. min), and q and q are the values of the amount adsorbed per unit mass at equilibrium and at any Table 2: Ultimate analysis of samples. time t, respectively. +e experimental data and the ﬁtting by Ultimate analysis (wt%) the two equations are shown in Figure 8, and the ﬁtted Sample C% H% O% N% kinetic parameters are given in Table 3. Cornstalk 43.17 5.60 50.55 0.65 One emerged in Figure 8 and Table 3 that both models HTC-cornstalk 63.85 4.13 29.81 0.32 were well-ﬁtted with the experimental data. Furthermore, By diﬀerence. the calculated adsorption capacities (q , cal) in the models were consistent with the experimental adsorption capacities increasing adsorbent dosage. Increasing the amount of (q , exp). However, the pseudo-second-order model HTC-cornstalk from 0.05/50 to 0.2/50 (g/mL) largely en- exhibited a higher correlation coeﬃcient (R ) which was hanced the elimination percentage of Cr (VI) ions from 36% employed to estimate the consistency of the ﬁtted models to 98%, resulted from the increase of the active sites which with experimental results than that of the pseudo-ﬁrst-order were available for the occupation of Cr (VI) ions. A further model. Although, in consideration of a particular kinetic increase in the adsorbent dosage did not have any eﬀect. model, a relatively high R was not irrefragable evidence of a better ﬁtting [49]. +e Cr (VI) adsorption onto HTC- cornstalk in this work was considered that was apropos to 3.4.2. Eﬀect of Initial Cr (VI) Concentration and Contact the pseudo-second-order kinetic model. Time. +e adsorption performance of the HTC-cornstalk inﬂuenced by the initial Cr (VI) concentrations from 30 to 90 mg/L was studied at 35 C with pH � 6. Results are shown in 3.6.AdsorptionIsothermAnalysis. To investigate the Cr (VI) Figure 7(a). +e HTC-cornstalk dosage and the contacting adsorbability of HTC, adsorption isotherms had been car- duration were kept at 0.2 g/50 mL, 90 min, respectively. One ried out with various initial concentrations of Cr (VI) (20, was introduced that the removal eﬃciency and adsorbability of 30, 40, 50, 60, 70, 80, 90, and 100 mg/L) at diﬀerent tem- ° ° ° HTC increased along with the increase of Cr (VI) concen- peratures (35 C, 45 C, and 55 C). tration. However, while the Cr (VI) concentration hit over Results had been ﬁtted with Langmuir (5) and 50 mg/L, the removal eﬃciency steadily reduced as a conse- Freundlich (6) isotherm models: quence. With a high Cr (VI) ions concentration, the adsorption c c 1 e e rate was restricted because the adsorption was saturated and � + , (5) q q k q e max 1 max the desorption rate was higher than adsorption [47]. Figure 7(b) shows the impact of contacting duration on lg q � lg + lgk, (6) the Cr (VI) adsorption within the aqueous solution with 50 mg/L in the initial Cr (VI) concentration under the same conditions. One was cleared that the adsorption procedure where q refers to the quantity of adsorbed metal ions per approached the equilibrium state within 150 min or less. +e unit mass of adsorbent (mg/g), c represents the solute Transmittance 6 Journal of Analytical Methods in Chemistry (a) (b) 2 μm 200 nm Figure 5: SEM images of HTC-cornstalk. 0.05 0.10 0.15 0.20 0.25 0.30 0.35 1 23456789 HTC-cornstalk mass (g) pH Cr (VI) removal Cr (VI) removal (a) (b) Figure 6: Eﬀect of initial solution pH on the removal eﬃciency of Cr (VI) from aqueous solutions. (a) Volume, 50 mL; agitation speed, 120 rpm; HTC dosage, 4 g/L; eﬀect of initial solution of the HTC dosage on the removal eﬃciency of Cr (VI) from aqueous solutions. (b) pH, 6; volume, 50 mL; agitation speed, 120 rpm; Cr (VI) concentration, 50 mg/L; contact time, 90 min. 18 105 14.0 13.5 13.0 12.5 12 12.0 11.5 11.0 10.5 65 6 70 10.0 30 40 50 60 70 80 90 20 40 60 80 100 120 140 160 180 200 220 + -1 Cr (mg·L ) t (min) Cr (VI) removal Cr (VI) removal q q e e (a) (b) Figure 7: Eﬀect of initial Cr (VI) concentration (a) and contacting duration (b) on the Cr (VI) adsorbability of HTC. Removal efficiency (%) Removal efficiency (%) -1 q (mg·g ) Removal efficiency (%) Removal efficiency (%) -1 q (mg·g ) e Journal of Analytical Methods in Chemistry 7 y=0.07085x+1.28243 R =0.96238 -1 y=-0.05818x+2.64437 R =0.98889 -2 -3 -4 0 20 40 60 80 100 120 0 20 40 60 80 100 120 140 160 t (min) t (min) (a) (b) Figure 8: Pseudo-ﬁrst-order kinetics (a) and pseudo-second-order kinetics (b) for Cr (VI) adsorption onto HTC at 35 C (initial Cr (VI) concentration 50 mg/L). Table 3: Adsorption kinetics parameters of Cr (VI) on HTC-cornstalk. Pseudo-ﬁrst-order kinetic model Pseudo-second-order kinetic model q . (mg/g) e exp −1 2 2 q . (mg/g) k (min ) R q . (mg/g) k (g/mg min) R e cal 1 e cal 2 12.29 14.07 0.06 0.96 14.11 0.004 0.99 1.4 0.09 1.3 R =0.9957 0.08 R =0.95738 1.2 R =0.98027 1.1 0.07 1.0 0.06 R =0.8871 0.9 R =0.7249 0.05 0.8 R =0.37933 0.7 0.04 0.6 0.0 0.5 1.0 1.5 2.0 -0.6 -0.4 -0.2 0.0 0.2 0.4 c (mg/L) lg c e e 35 °C y=0.01993x+0.0464 35 °C y=0.721x+1.153 45 °C y=0.02168x+0.03769 45 °C y=0.699x+1.193 55 °C y=0.0199x+0.04146 55 °C y=0.759x+1.223 (a) (b) Figure 9: Langmuir isotherm model (a) and Freundlich isotherm model (b) for Cr (VI) adsorption onto HTC. concentration in the bulk solution (mg/L) at equilibrium state, As revealed above, R values of higher than 0.9 were q (mg/g) and k represent the Langmuir constants related to obtained in the Freundlich model which indicated an ex- max 1 the saturated metal ions adsorbability and the adsorption free cellent consistency while ﬁtting instead of many poor ones in energy, respectively, and the constants k and n in the the Langmuir model which were ﬁtted at three diﬀerent Freundlich model represent the strength and the distribution of temperatures. +erefore, the Freundlich model was selected the adsorptive bonds. Experimental data ﬁtted with Langmuir to express the Cr (VI) adsorption of HTC-cornstalk. and Freundlich equations are shown in Figure 9. Furthermore, Freundlich as an empirical equation had been employed to the ﬁtted results are given in Table 4 in detail. illustrate the exponential distribution sites, energies, and the -1 ln (q -q) c ·q e e e t (q) lg q e 8 Journal of Analytical Methods in Chemistry Table 4: +e parameters of the isotherm adsorption model. Langmuir modes Freundlich models Temperature ( C) 2 2 q (mg/g) K (L/mg) R K (mg/g) 1/n R max l 35 50.14640 0.42987 0.87281 14.49226 0.72818 0.98489 45 46.11816 0.57533 0.68560 15.63765 0.69809 0.95072 55 50.23585 0.48011 0.27588 17.50070 0.81531 0.91757 heterogeneity of the adsorbent surface [50]. +e adsorption molecular simulation technology,” Chemistry Letters, vol. 49, no. 12, pp. 1452–1455, 2020. capacities (K) increased with increasing temperatures. +e [4] S. Rangabhashiyam, S. Sayantani, and P. 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Journal

Journal of Analytical Methods in Chemistry – Hindawi Publishing Corporation

Published: Oct 5, 2021

References

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H. Xiyili, S. Çetintaş, and D. Bingöl
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Preparation of Mesoporous Biochar from Cornstalk for the Chromium (VI) Elimination by Using One-Step Hydrothermal Carbonation

Preparation of Mesoporous Biochar from Cornstalk for the Chromium (VI) Elimination by Using One-Step Hydrothermal Carbonation

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Preparation of Mesoporous Biochar from Cornstalk for the Chromium (VI) Elimination by Using One-Step Hydrothermal Carbonation

Preparation of Mesoporous Biochar from Cornstalk for the Chromium (VI) Elimination by Using One-Step Hydrothermal Carbonation

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