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A simple method to valorize silica sludges into sustainable coatings for indoor humidity buffering

A simple method to valorize silica sludges into sustainable coatings for indoor humidity buffering In this study, the production of indoor humidity-buffering coatings (IHC-s) from recycling waste silica sludges by using a room-temperature sol-gel method which is a simple and energy-efficient route is reported. The properties of these IHC-s are identified by scanning electron microscope, X-ray diffraction, X-ray fluorescence spectrometer, laser particle size analyzer, N adsorption-desorption isotherms and toxicity characteristic leaching procedure (TCLP). − 2 The moisture adsorption-desorption tests show that the IHC-s have moisture buffering values of ca. 270–316 g m and moisture contents of 23.6–26.7% in the range of 50–90% relative humidity (RH). Furthermore, the humidity buffering capacities, moisture adsorption-desorption rate and stability are significantly superior to commercially available coatings in the range of 50–75% RH. The enhancement may be due to the formation of porous structure in the coatings via the dispersed waste silica sludges and gypsum which transformed from bassanite by self- assembly process. Most importantly, the prepared IHC-s show surpassing antimicrobial efficacy (> 99.99%) and no detectable leaching heavy metals based on TCLP tests, which provides an economic and environmental-friendly route for recovering and valorizing industrial wastes. Keywords: Valorization, Mesoporous/microporous structure, Indoor humidity control, Industrial sludges, Sol-gel 1 Introduction arecommonlyusedtoregulate indoor RH [6]. To The energy consumption by buildings was estimated have a better living space, it is urgent to create an to be ca. 40% of global energy and over 50% of them energy-saving technology for the control of indoor in the buildings comes from heating and air- humidity. The humidity buffering materials (HBMs) conditioning machines [1]. In order to cut down the have attracted much attraction due to its zero-energy CO emissions from the extensive energy consump- consumption [7–14]. Generally, the diatomite [15, 16] tion in the buildings, the development of passive ma- is frequently used as starting materials to fabricate terials for the buildings that can adjust the indoor HBMs because of its superior properties such as low humidity naturally and keep the comfortable level of toxicity, lightweight, abundance and high porosity living environment is crucial [2–4]. In Taiwan, the [17–22]. For example, it was proposed that HBMs average annual relative humidity (RH) is usually were prepared by high-temperature sintering diatom- higher than 75%, which is much higher than the suit- itewithvolcanicash [23]. Escalera et al. [15]reported able RH (40–70%) for people [5]. Therefore, dehu- that the sintering of diatomite with Brazil nut shell midifiers which consume a large amount of energy ash to produce brick-type HBMs. In the recent years, many different materials were developed for HBM ap- plications. The synthesis of metal-organic frameworks * Correspondence: shliu@mail.ncku.edu.tw [24, 25] which have larger surface area (S )and 1 BET Department of Environmental Engineering, National Cheng Kung University, pore volume (V ) was reported and tested as HBMs Tainan 70101, Taiwan total Department of Biology, Universitas Airlangga, Surabaya 60115, Indonesia © The Author(s). 2022 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. Kuok et al. Sustainable Environment Research (2022) 32:8 Page 2 of 9 for moderating indoor moisture variation. A smart comparison between the prepared IHC-s with commer- wall-brick HBM prepared from sepiolite and CaCl cially available coatings was carried out. exhibits a superior adsorption-desorption content with antifouling and antifungal properties [26]. Bioinspired 2 Materials and methods ant-nest-like hierarchical porous materials were pro- 2.1 IHC-s fabrication posed for narrowing indoor humidity fluctuation [27]. Industrial waste silica sludges were obtained from the A renewable bamboo charcoal loaded with silver plants which produced precipitated silica (i.e., enhance- doped titanium dioxide was prepared and served as ment silica fume) in Taiwan. As can be seen in Fig. 1, HBMs to improve indoor environment quality [28]. the waste silica sludges were dried at 105 °C for 2 d and However, the aforementioned HBMs may suffer from grounded to the powders which were sieved with a filter the energy-intensive and complex preparation routes of 100 mesh (0.149 mm). The IHC-s was prepared by as well as high cost. To fulfil the large-scale applica- using a facile sol-gel method. In a typical run, the tions, the development of a simple, rapid, low- weighed waste silica sludges (0–60 g), bassanite (35–90 temperature and cost-effective method to prepare g) and kaolin (5 g) were mixed entirely. The sodium sili- HBMs should be highlighted. cates (5 g) and acrylic resins (65 g) were totally liquefied It was estimated that ca. 5.5 Mt of industrial inor- in deionized water and then sol-gelled together with ganic sludges were generated annually in Taiwan. aforementioned components for 30 min. The resultant Landfills are the common methods for practical dis- mixture was coated on a plastic plate and then cured at posal of these wastes. Although the sludges may be 25 °C for 24 h with 75% RH. The humidity buffering recycled as soil additives, adsorbents and construction properties were studied based on the recovery ratios of materials [29–31], the recovery ratios and amounts waste silica sludges, i.e., the prepared IHC-s (s = weight are still low. In this way, some environmental prob- percentages (%) of waste silica sludges). The commercial lems, for instance, the overload of landfill and im- coatings made of diatomite were obtained from Econ- proper disposal may happen. Form the viewpoint of phoenix Company. The main chemical compositions of circular economy, an economic feasible and environ- commercial coatings can be seen in Table S1 of Supple- mentally friendly method should be developed to re- mental Materials. cover these industrial wastes efficiently. In this work, the indoor humidity-buffering coatings 2.2 IHC-s characterizations (IHC-s) were fabricated by recovering industrial sludges Morphologies of prepared samples were investigated by (i.e., waste silica sludges) which were generated from scanning electron microscope (SEM, AURIGA). precipitated silica producing plants. The physicochemi- Chemical structures of samples were identified by a var- cal properties of waste silica sludges and prepared IHC-s iety of spectroscopies, i.e., X-ray diffraction (XRD, were investigated by employing a series of analytic tech- PANalytical X’Pert PRO), X-ray fluorescence spectrom- niques and spectroscopic instruments. Humidity buffer- eter (XRF, PANalytical Epsilon 4), laser particle size ana- ing performance of the prepared IHC-s was explored by lyser (Beckman Couter LS-230) and N adsorption- moisture adsorption-desorption tests. Additionally, tox- desorption isotherms (Micromeritics ASAP 2020). The icity characteristic leaching procedure (TCLP) tests of TCLP tests were carried out by employing the Standard these IHC-s were also performed. The performance Method (NIEA R201.14C). The heavy metals of waste Fig. 1 A schematic diagram of IHC-s preparation Kuok et al. Sustainable Environment Research (2022) 32:8 Page 3 of 9 silica sludges and coatings were identified by inductively 3 Results and discussion coupled plasma-optical emission spectrometer (ICP- 3.1 Textural properties of waste silica sludges OES, JY ULTIMA 2000). Industrial waste silica sludges, with their main chemical compositions shown in Table S1, are the key raw mate- 2.3 Humidity buffering performance rials for the IHC-s. As observed, the dominant chemical To perform moisture adsorption-desorption tests, the element of waste silica sludges is silicon (ca. 98.8%) IHC-s were covered onto the plate (100 × 100 mm) with which is originated from the manufacture of precipitated the coating thickness of 2 mm. Humidity buffering prop- silica. Additionally, a small amount of sulfur elements erties of coatings were evaluated by moisture (ca. 0.98%) observed in the waste silica sludges is attrib- adsorption-desorption tests and response to humidity uted to the usage of sulfuric acid during the process. variation. For moisture adsorption-desorption tests, Some heavy metals such as calcium, alumina, iron, mag- three different coatings were fabricated, dried and nesium, copper and arsenic are found in the waste silica weighted (m ). Before moisture adsorption, the coatings sludges. As shown in Fig. 2, the waste silica sludges have were cured at 25 °C under 50% RH for 48 h. Afterwards, a broad XRD diffraction peak at 2θ =22 , indicating the moisture adsorption and desorption of these coatings existence of amorphous silica [32]. As observed in were carried out at 90 and 50% RH, respectively. As a re- Fig. 3A, the particle size of waste silica sludges is mostly sult, the weights (m and m ) of moisture adsorption at located between 10 and 50 μm. The averaged particle a1 a2 90% RH for 24 h and the weights (m and m ) of mois- size of waste silica sludges is calculated to be ca. d1 d2 ture desorption at 50% RH for 24 h can be obtained. The 26.8 μm. The SEM image (Fig. 3B) of waste silica sludges moisture adsorption capacities of coatings (m , m , m shows the formation of aggregated silica particles with 1 2 3 and m ) were calculated based on the following Eqs. (1– amorphous structure. The porous structure of waste 4). Accordingly, moisture buffering capacities (W ,g silica sludges (i.e., specific S and V ) are deter- a BET total − 2 m ) are attained by taking the averaged values by using mined by N adsorption at 77 K and the results are sum- Eq. (5). marized in Table S2. The result shows that waste silica 2 − 1 3 sludges have an S of 59 m g and V of 0.64 cm BET total − 1 m ðÞ g ¼ m −m ð1Þ g . In addition, waste silica sludges exhibit a type-IV 1 a1 0 isotherm (Fig. 3C) with pore size distribution of 10–100 m ðÞ g ¼ m −m ð2Þ 2 a1 d1 nm (Fig. 3D). The presence of meso-macropore in the waste silica sludges is responsible for the moisture ad- m ðÞ g ¼ m −m ð3Þ 3 a2 d1 sorption and desorption performance as discussed in the following section. m ðÞ g ¼ m −m ð4Þ 4 a2 d2 3.2 Physicochemical properties of IHC-s m þ m þ m þ m 1 2 3 4 As reported earlier [33], transformation of CaSO ·0.5H O W ¼ ð5Þ a 4 2 4  A (bassanite) into CaSO ·2H O (gypsum) via self-assembly 4 2 2 process (as indicated in the Eq. (7)) and the formed where A (m ) is the surface area of prepared coatings. Furthermore, moisture adsorption content (u) can be obtained according to Eq. (6). m −m 1 0 uðÞ % ¼ ð6Þ where m is the sample weight after drying and m is the 0 1 sample weight after moisture adsorption. For response to humidity variation, the adsorption- desorption of moisture was conducted in the humidity range of 50–75% RH, i.e., moisture adsorption at 75% RH for 24 h and then desorption at 50% RH for 24 h. Hygroscopic sorption properties of coatings was investi- gated by measuring moisture contents of coatings in dif- ferent RH (40, 50, 75, 85 and 90%). Durability of coatings was also performed via four cyclic moisture Fig. 2 XRD patterns of waste silica sludges, bassanite, gypsum adsorption-desorption tests for at least 96 h in the range and IHC-s of 50–75% RH. Kuok et al. Sustainable Environment Research (2022) 32:8 Page 4 of 9 Fig. 3 A particle size distribution, (B) SEM image, (C)N adsorption-desorption isotherms and (D) pore size distributions of waste silica sludges Fig. 4 SEM images of (A) bassanite, (B) gypsum, and (C) IHC-0 and (d) IHC-48 Kuok et al. Sustainable Environment Research (2022) 32:8 Page 5 of 9 Fig. 5 Photographs of (A) IHC-58, (B) IHC-78 and (B) IHC-88 gypsum can serve as the skeleton structure which provides adhered by the resin and then consolidate between the mechanical properties such as hardness, compressive cross-linking structure of rod-like particles of gypsum, strength and porosity for the coating. as shown in Fig. 4D. The mesoporous and microporous structure can be developed by the dispersed waste silica CaSO  0:5H O þ nH O→CaSO  2H O sludges and gypsum. Therefore, the higher ratios of 4 2 2 4 2 þðÞ n‐1:5 H O: ð7Þ waste silica sludges can supply more mesopores for the IHC-s coatings. However, the excessive waste silica As shown in Fig. 4A and B, the bassanite with the sludges (> 60 wt%) can make coating surface crack, as brick-like morphology is hydrated into gypsum with displayed in Fig. 5. In addition, the surface cracking also rod-like particles during casting process. The gypsum may be due to the insufficiency of gypsum, which make with rod-like particles is also observed for IHC-0 (with- the skeleton weak and thus excessive agglomeration of out waste silica sludges), as can be seen in Fig. 4C. This waste silica sludges by resins. In this study, different ra- indicates that the crystallization of gypsum can occur tios of waste silica sludges (0–58 wt%) were used and even in the existence of the acrylic resin, which also can fabricated as IHC-s. be confirmed by XRD pattern (see Fig. 2). The charac- The porous structure of IHC-s and commercial coat- teristic peaks of gypsum are located at 11.6, 20.7, 23.3, ings is investigated by N adsorption-desorption iso- 26.6, 29, 31, 33.4, 35.9 and 40.6°, suggesting the forma- therms. All the samples exhibit Type-IV isotherms due tion of gypsum in the IHC-s. The waste silica sludges to the presence of mesoporous structure, as can be seen are amorphous SiO with micro-sized particles which in Fig. 6A. Also, the Type-H3 hysteresis loops can be ob- can suspend completely in the water. The dissolution of served for IHC-s with different amounts of waste silica SiO (0.01–0.012% by weight in water at 25 °C) produces sludges because slit-shaped pores are formed in the monomeric form, i.e., Si (OH) and the solid phase [34]. presence of gypsum and waste silica sludges particles. The dispersed waste silica sludges in the solvent can be Accordingly, the S and V of IHC-s with different BET total Fig. 6 A N adsorption-desorption isotherms and (B) pore size distributions of IHC-s and commercial coatings 2 Kuok et al. Sustainable Environment Research (2022) 32:8 Page 6 of 9 Table 1 Porous properties of IHC-s samples Samples IHC-0 IHC-38 IHC-48 IHC-58 Commercial coatings 2 −1 S (m g ) 3.9 10.0 13.1 13.5 2.0 BET 3 −1 V (cm g ) 0.049 0.147 0.193 0.206 0.505 total ratios of waste silica sludges and commercial coatings adsorption the higher porosity of samples. The pristine are summarized in Table 1. As a result, the S and IHC-0 shows lower adsorption rate that the water drop BET V of coatings are increased as the ratios of waste sil- cannot be adsorbed completely over 130 s. Upon adding total ica sludges are increased. The S values of IHC-s (s = waste silica sludges, the initial contact angles become BET 38, 48 and 58 wt%) are measured to be 10.0, 13.1 and smaller and the adsorption rates for water drop are in- 2 − 1 13.5 m g , respectively, which are larger than original creased. Therefore, the more ratios of waste silica 2 − 1 2 IHC-0 (3.9 m g ) and commercial coatings (2.0 m sludges result in the increased pore volumes that pro- − 1 g ). The V values are also increased as amounts of mote the ability for liquid water adsorption. total waste silica sludges are increased (from 0.049 to 0.206 3 − 1 cm g ). The above result suggests that the addition of 3.3 Moisture adsorption-desorption capacity tests waste silica sludges can increase the V and S .It The moisture buffering performance (in the range of total BET should be noted that the V value of commercial 50–90% RH) of the IHC-s is shown in Table 2. The total coatings is greater than those of IHC-s. As observed in moisture buffering capacities and content values of IHC- − 2 Fig. 6B, the pore volumes of commercial coatings are 0 are 217 g m and 16.3%, respectively. The moisture mostly attributed to the contribution of macropore (> buffering capacities of IHC-s gradually increase as the 50 nm). However, the mesopores (i.e., 2–50 nm) in the amounts of waste silica sludges increase, i.e., the per- IHC-s are the most effective pores for moisture adsorp- formance of samples with waste silica sludges is better tion. Therefore, the presence of waste silica sludges can than that of IHC-0. As a result, the moisture buffering increase the volumes of mesopores, as evidenced in the capacities of IHC-38, IHC-48 and IHC-58 can reach − 2 pore size distributions of IHC-s (see Fig. 6B). 270, 304 and 316 g m , respectively. Also, moisture Surface contact angles of IHC-s samples are shown in contents of IHC-38, IHC-48 and IHC-58 are 23.6, 25.6 Fig. 7. The adsorption time relates to the porous proper- and 26.7%, respectively. The moisture buffering capaci- ties of materials, i.e., the faster rate of water drop ties and contents of IHC-s are remarkably superior to Fig. 7 Surface contact angle of (A) IHC-0, (B) IHC-38, (C) IHC-48 and (D) IHC-58 Kuok et al. Sustainable Environment Research (2022) 32:8 Page 7 of 9 − 2 − 1 Table 2 Moisture buffering capacities and contents of IHC-s observed for IHC-38 (17.3 g m h ). The results show and commercial coating that the adsorption and desorption rates of IHC-s (38– Sample IHC-0 IHC-38 IHC-48 IHC-58 Commercial coatings 58%) are higher than that of IHC-0 and commercial − 2 coatings. Hygroscopic curves of the IHC-s coatings are W (g m ) 216.9 270.3 303.7 316.1 58.7 shown in Fig. 8C. The IHC-48 coatings start to adsorb u (%) 16.3 23.6 25.6 26.7 11.4 moisture at the RH of 40%. It can be seen that no sig- nificant difference between IHC-48 and commercial − 2 those of commercially available coatings (59 g m and coatings can be observed in the humidity range of < 11.4%), which are possibly due to the unique porous 50%. While relative humidity is higher than 75%, the properties. Moisture buffering ability has the positive adsorbed capacities are increased sharply upon the correlation with S and V of coatings. addition of waste silica sludges. Note that curves of ad- BET total Humidity buffering performance (in the range of 50– sorption and desorption are not overlapped due to the 75% RH) of IHC-s and their corresponding results are hysteresis effect. Durability of adsorption-desorption shown in Fig. 8A. Except to IHC-0 coating, IHC-s (s = process for IHC-48 was performed by cyclic tests, as 38, 48 and 58%) have the higher moisture buffering cap- presented in Fig. 8D. The moisture buffering capacities acities in the range of 50–75% RH, indicating the waste of IHC-48 coatings keep stable for four cyclic runs (i.e., silica sludges possess the positive effect on moisture ad- the entire run time = 96 h). In other words, the moisture sorption. In addition, the benchmarks of adsorbed cap- can be adsorbed by IHC-48 spontaneously and then des- acities for HBMs issued by Japanese Industrial Standards orbed from pores entirely. − 2 − 2 (JIS, see Table S3) are 29 g m (Level 1) and 50 g m As shown in Table S4, leaching contents of heavy (Level 2) for the adsorption time of 12 h. The IHC-38, metals (Cu, Cr, Cd, Ni, Ba, Co and Pb) for IHC-48 were IHC-48 and IHC-58 samples possess moisture buffering investigated by the TCLP tests. No detectable heavy − 2 capacities of 49, 48 and 46 g m , respectively for the ad- metals are observed in leaching solutions of IHC-48 sorption time of 12 h, which reach the requirement of coatings. Moreover, the antibacterial property of IHC-48 Level 1 and close to Level 2. Also, adsorption-desorption is evaluated via the standard method from JIS (JIS Z gradients of various coatings are shown in Fig. 8B. The 2801). As observed in Table S5, the bacteria concentra- − 1 IHC-48 coatings have the highest adsorption rate (20.1 g tion of 99 CFU mL observed for IHC-48 after the con- − 2 − 1 6 m h ) and the optimal desorption rate can be tact time of 24 h is significantly lower 6.0 × 10 CFU Fig. 8 A Moisture adsorption-desorption performance, (B) gradient, (C) hygroscopic curves f IHC-s and commercial samples and (D) a cyclic test of IHC-48 Kuok et al. Sustainable Environment Research (2022) 32:8 Page 8 of 9 − 1 mL for blank. The R factor (i.e., the decimal logarithm Funding This work was supported by Ministry of Science and Technology of Taiwan of the bacteria concentrations between the reference and (MOST 107–2221-E-006-009-MY3). the IHC-48 samples) of antimicrobial activity is ca. 4.8 which is greater than the threshold value (i.e., 2) of Japa- Availability of data and materials nese Industrial Standards (JIS Z 2801). It is worth noting All data supporting the conclusions of this article are included in this that the IHC-48 recycled from industrial wastes pos- manuscript. sesses an excellent moisture buffering ability which can reach the benchmark of JIS and is also superior to the Declaration commercial coatings. More importantly, the simple and Competing interests energy-saving sol-gel method to prepare IHC-48 coat- The authors declare they have no competing interests. ings with an excellent antimicrobial efficacy (> 99.99%) and environmental friendliness (non-detectable heavy Received: 8 September 2021 Accepted: 29 December 2021 metal leaching) recycled from inorganic wastes under room temperature may be a promising candidate for practical applications in the indoor coatings. References 1. Hou PM, Zu K, Qin MH, Cui SQ. A novel metal-organic frameworks based humidity pump for indoor moisture control. Build Environ. 2021;187:107396. 2. Kreiger BK, Srubar WV. Moisture buffering in buildings: a review of 4 Conclusions experimental and numerical methods. Energ Buildings. 2019;202:109394. The humidity buffering coatings, which are the new ap- 3. Wan H, Sun ZW, Huang GS, Xu XH, Yu JH. 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A simple method to valorize silica sludges into sustainable coatings for indoor humidity buffering

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Abstract

In this study, the production of indoor humidity-buffering coatings (IHC-s) from recycling waste silica sludges by using a room-temperature sol-gel method which is a simple and energy-efficient route is reported. The properties of these IHC-s are identified by scanning electron microscope, X-ray diffraction, X-ray fluorescence spectrometer, laser particle size analyzer, N adsorption-desorption isotherms and toxicity characteristic leaching procedure (TCLP). − 2 The moisture adsorption-desorption tests show that the IHC-s have moisture buffering values of ca. 270–316 g m and moisture contents of 23.6–26.7% in the range of 50–90% relative humidity (RH). Furthermore, the humidity buffering capacities, moisture adsorption-desorption rate and stability are significantly superior to commercially available coatings in the range of 50–75% RH. The enhancement may be due to the formation of porous structure in the coatings via the dispersed waste silica sludges and gypsum which transformed from bassanite by self- assembly process. Most importantly, the prepared IHC-s show surpassing antimicrobial efficacy (> 99.99%) and no detectable leaching heavy metals based on TCLP tests, which provides an economic and environmental-friendly route for recovering and valorizing industrial wastes. Keywords: Valorization, Mesoporous/microporous structure, Indoor humidity control, Industrial sludges, Sol-gel 1 Introduction arecommonlyusedtoregulate indoor RH [6]. To The energy consumption by buildings was estimated have a better living space, it is urgent to create an to be ca. 40% of global energy and over 50% of them energy-saving technology for the control of indoor in the buildings comes from heating and air- humidity. The humidity buffering materials (HBMs) conditioning machines [1]. In order to cut down the have attracted much attraction due to its zero-energy CO emissions from the extensive energy consump- consumption [7–14]. Generally, the diatomite [15, 16] tion in the buildings, the development of passive ma- is frequently used as starting materials to fabricate terials for the buildings that can adjust the indoor HBMs because of its superior properties such as low humidity naturally and keep the comfortable level of toxicity, lightweight, abundance and high porosity living environment is crucial [2–4]. In Taiwan, the [17–22]. For example, it was proposed that HBMs average annual relative humidity (RH) is usually were prepared by high-temperature sintering diatom- higher than 75%, which is much higher than the suit- itewithvolcanicash [23]. Escalera et al. [15]reported able RH (40–70%) for people [5]. Therefore, dehu- that the sintering of diatomite with Brazil nut shell midifiers which consume a large amount of energy ash to produce brick-type HBMs. In the recent years, many different materials were developed for HBM ap- plications. The synthesis of metal-organic frameworks * Correspondence: shliu@mail.ncku.edu.tw [24, 25] which have larger surface area (S )and 1 BET Department of Environmental Engineering, National Cheng Kung University, pore volume (V ) was reported and tested as HBMs Tainan 70101, Taiwan total Department of Biology, Universitas Airlangga, Surabaya 60115, Indonesia © The Author(s). 2022 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. Kuok et al. Sustainable Environment Research (2022) 32:8 Page 2 of 9 for moderating indoor moisture variation. A smart comparison between the prepared IHC-s with commer- wall-brick HBM prepared from sepiolite and CaCl cially available coatings was carried out. exhibits a superior adsorption-desorption content with antifouling and antifungal properties [26]. Bioinspired 2 Materials and methods ant-nest-like hierarchical porous materials were pro- 2.1 IHC-s fabrication posed for narrowing indoor humidity fluctuation [27]. Industrial waste silica sludges were obtained from the A renewable bamboo charcoal loaded with silver plants which produced precipitated silica (i.e., enhance- doped titanium dioxide was prepared and served as ment silica fume) in Taiwan. As can be seen in Fig. 1, HBMs to improve indoor environment quality [28]. the waste silica sludges were dried at 105 °C for 2 d and However, the aforementioned HBMs may suffer from grounded to the powders which were sieved with a filter the energy-intensive and complex preparation routes of 100 mesh (0.149 mm). The IHC-s was prepared by as well as high cost. To fulfil the large-scale applica- using a facile sol-gel method. In a typical run, the tions, the development of a simple, rapid, low- weighed waste silica sludges (0–60 g), bassanite (35–90 temperature and cost-effective method to prepare g) and kaolin (5 g) were mixed entirely. The sodium sili- HBMs should be highlighted. cates (5 g) and acrylic resins (65 g) were totally liquefied It was estimated that ca. 5.5 Mt of industrial inor- in deionized water and then sol-gelled together with ganic sludges were generated annually in Taiwan. aforementioned components for 30 min. The resultant Landfills are the common methods for practical dis- mixture was coated on a plastic plate and then cured at posal of these wastes. Although the sludges may be 25 °C for 24 h with 75% RH. The humidity buffering recycled as soil additives, adsorbents and construction properties were studied based on the recovery ratios of materials [29–31], the recovery ratios and amounts waste silica sludges, i.e., the prepared IHC-s (s = weight are still low. In this way, some environmental prob- percentages (%) of waste silica sludges). The commercial lems, for instance, the overload of landfill and im- coatings made of diatomite were obtained from Econ- proper disposal may happen. Form the viewpoint of phoenix Company. The main chemical compositions of circular economy, an economic feasible and environ- commercial coatings can be seen in Table S1 of Supple- mentally friendly method should be developed to re- mental Materials. cover these industrial wastes efficiently. In this work, the indoor humidity-buffering coatings 2.2 IHC-s characterizations (IHC-s) were fabricated by recovering industrial sludges Morphologies of prepared samples were investigated by (i.e., waste silica sludges) which were generated from scanning electron microscope (SEM, AURIGA). precipitated silica producing plants. The physicochemi- Chemical structures of samples were identified by a var- cal properties of waste silica sludges and prepared IHC-s iety of spectroscopies, i.e., X-ray diffraction (XRD, were investigated by employing a series of analytic tech- PANalytical X’Pert PRO), X-ray fluorescence spectrom- niques and spectroscopic instruments. Humidity buffer- eter (XRF, PANalytical Epsilon 4), laser particle size ana- ing performance of the prepared IHC-s was explored by lyser (Beckman Couter LS-230) and N adsorption- moisture adsorption-desorption tests. Additionally, tox- desorption isotherms (Micromeritics ASAP 2020). The icity characteristic leaching procedure (TCLP) tests of TCLP tests were carried out by employing the Standard these IHC-s were also performed. The performance Method (NIEA R201.14C). The heavy metals of waste Fig. 1 A schematic diagram of IHC-s preparation Kuok et al. Sustainable Environment Research (2022) 32:8 Page 3 of 9 silica sludges and coatings were identified by inductively 3 Results and discussion coupled plasma-optical emission spectrometer (ICP- 3.1 Textural properties of waste silica sludges OES, JY ULTIMA 2000). Industrial waste silica sludges, with their main chemical compositions shown in Table S1, are the key raw mate- 2.3 Humidity buffering performance rials for the IHC-s. As observed, the dominant chemical To perform moisture adsorption-desorption tests, the element of waste silica sludges is silicon (ca. 98.8%) IHC-s were covered onto the plate (100 × 100 mm) with which is originated from the manufacture of precipitated the coating thickness of 2 mm. Humidity buffering prop- silica. Additionally, a small amount of sulfur elements erties of coatings were evaluated by moisture (ca. 0.98%) observed in the waste silica sludges is attrib- adsorption-desorption tests and response to humidity uted to the usage of sulfuric acid during the process. variation. For moisture adsorption-desorption tests, Some heavy metals such as calcium, alumina, iron, mag- three different coatings were fabricated, dried and nesium, copper and arsenic are found in the waste silica weighted (m ). Before moisture adsorption, the coatings sludges. As shown in Fig. 2, the waste silica sludges have were cured at 25 °C under 50% RH for 48 h. Afterwards, a broad XRD diffraction peak at 2θ =22 , indicating the moisture adsorption and desorption of these coatings existence of amorphous silica [32]. As observed in were carried out at 90 and 50% RH, respectively. As a re- Fig. 3A, the particle size of waste silica sludges is mostly sult, the weights (m and m ) of moisture adsorption at located between 10 and 50 μm. The averaged particle a1 a2 90% RH for 24 h and the weights (m and m ) of mois- size of waste silica sludges is calculated to be ca. d1 d2 ture desorption at 50% RH for 24 h can be obtained. The 26.8 μm. The SEM image (Fig. 3B) of waste silica sludges moisture adsorption capacities of coatings (m , m , m shows the formation of aggregated silica particles with 1 2 3 and m ) were calculated based on the following Eqs. (1– amorphous structure. The porous structure of waste 4). Accordingly, moisture buffering capacities (W ,g silica sludges (i.e., specific S and V ) are deter- a BET total − 2 m ) are attained by taking the averaged values by using mined by N adsorption at 77 K and the results are sum- Eq. (5). marized in Table S2. The result shows that waste silica 2 − 1 3 sludges have an S of 59 m g and V of 0.64 cm BET total − 1 m ðÞ g ¼ m −m ð1Þ g . In addition, waste silica sludges exhibit a type-IV 1 a1 0 isotherm (Fig. 3C) with pore size distribution of 10–100 m ðÞ g ¼ m −m ð2Þ 2 a1 d1 nm (Fig. 3D). The presence of meso-macropore in the waste silica sludges is responsible for the moisture ad- m ðÞ g ¼ m −m ð3Þ 3 a2 d1 sorption and desorption performance as discussed in the following section. m ðÞ g ¼ m −m ð4Þ 4 a2 d2 3.2 Physicochemical properties of IHC-s m þ m þ m þ m 1 2 3 4 As reported earlier [33], transformation of CaSO ·0.5H O W ¼ ð5Þ a 4 2 4  A (bassanite) into CaSO ·2H O (gypsum) via self-assembly 4 2 2 process (as indicated in the Eq. (7)) and the formed where A (m ) is the surface area of prepared coatings. Furthermore, moisture adsorption content (u) can be obtained according to Eq. (6). m −m 1 0 uðÞ % ¼ ð6Þ where m is the sample weight after drying and m is the 0 1 sample weight after moisture adsorption. For response to humidity variation, the adsorption- desorption of moisture was conducted in the humidity range of 50–75% RH, i.e., moisture adsorption at 75% RH for 24 h and then desorption at 50% RH for 24 h. Hygroscopic sorption properties of coatings was investi- gated by measuring moisture contents of coatings in dif- ferent RH (40, 50, 75, 85 and 90%). Durability of coatings was also performed via four cyclic moisture Fig. 2 XRD patterns of waste silica sludges, bassanite, gypsum adsorption-desorption tests for at least 96 h in the range and IHC-s of 50–75% RH. Kuok et al. Sustainable Environment Research (2022) 32:8 Page 4 of 9 Fig. 3 A particle size distribution, (B) SEM image, (C)N adsorption-desorption isotherms and (D) pore size distributions of waste silica sludges Fig. 4 SEM images of (A) bassanite, (B) gypsum, and (C) IHC-0 and (d) IHC-48 Kuok et al. Sustainable Environment Research (2022) 32:8 Page 5 of 9 Fig. 5 Photographs of (A) IHC-58, (B) IHC-78 and (B) IHC-88 gypsum can serve as the skeleton structure which provides adhered by the resin and then consolidate between the mechanical properties such as hardness, compressive cross-linking structure of rod-like particles of gypsum, strength and porosity for the coating. as shown in Fig. 4D. The mesoporous and microporous structure can be developed by the dispersed waste silica CaSO  0:5H O þ nH O→CaSO  2H O sludges and gypsum. Therefore, the higher ratios of 4 2 2 4 2 þðÞ n‐1:5 H O: ð7Þ waste silica sludges can supply more mesopores for the IHC-s coatings. However, the excessive waste silica As shown in Fig. 4A and B, the bassanite with the sludges (> 60 wt%) can make coating surface crack, as brick-like morphology is hydrated into gypsum with displayed in Fig. 5. In addition, the surface cracking also rod-like particles during casting process. The gypsum may be due to the insufficiency of gypsum, which make with rod-like particles is also observed for IHC-0 (with- the skeleton weak and thus excessive agglomeration of out waste silica sludges), as can be seen in Fig. 4C. This waste silica sludges by resins. In this study, different ra- indicates that the crystallization of gypsum can occur tios of waste silica sludges (0–58 wt%) were used and even in the existence of the acrylic resin, which also can fabricated as IHC-s. be confirmed by XRD pattern (see Fig. 2). The charac- The porous structure of IHC-s and commercial coat- teristic peaks of gypsum are located at 11.6, 20.7, 23.3, ings is investigated by N adsorption-desorption iso- 26.6, 29, 31, 33.4, 35.9 and 40.6°, suggesting the forma- therms. All the samples exhibit Type-IV isotherms due tion of gypsum in the IHC-s. The waste silica sludges to the presence of mesoporous structure, as can be seen are amorphous SiO with micro-sized particles which in Fig. 6A. Also, the Type-H3 hysteresis loops can be ob- can suspend completely in the water. The dissolution of served for IHC-s with different amounts of waste silica SiO (0.01–0.012% by weight in water at 25 °C) produces sludges because slit-shaped pores are formed in the monomeric form, i.e., Si (OH) and the solid phase [34]. presence of gypsum and waste silica sludges particles. The dispersed waste silica sludges in the solvent can be Accordingly, the S and V of IHC-s with different BET total Fig. 6 A N adsorption-desorption isotherms and (B) pore size distributions of IHC-s and commercial coatings 2 Kuok et al. Sustainable Environment Research (2022) 32:8 Page 6 of 9 Table 1 Porous properties of IHC-s samples Samples IHC-0 IHC-38 IHC-48 IHC-58 Commercial coatings 2 −1 S (m g ) 3.9 10.0 13.1 13.5 2.0 BET 3 −1 V (cm g ) 0.049 0.147 0.193 0.206 0.505 total ratios of waste silica sludges and commercial coatings adsorption the higher porosity of samples. The pristine are summarized in Table 1. As a result, the S and IHC-0 shows lower adsorption rate that the water drop BET V of coatings are increased as the ratios of waste sil- cannot be adsorbed completely over 130 s. Upon adding total ica sludges are increased. The S values of IHC-s (s = waste silica sludges, the initial contact angles become BET 38, 48 and 58 wt%) are measured to be 10.0, 13.1 and smaller and the adsorption rates for water drop are in- 2 − 1 13.5 m g , respectively, which are larger than original creased. Therefore, the more ratios of waste silica 2 − 1 2 IHC-0 (3.9 m g ) and commercial coatings (2.0 m sludges result in the increased pore volumes that pro- − 1 g ). The V values are also increased as amounts of mote the ability for liquid water adsorption. total waste silica sludges are increased (from 0.049 to 0.206 3 − 1 cm g ). The above result suggests that the addition of 3.3 Moisture adsorption-desorption capacity tests waste silica sludges can increase the V and S .It The moisture buffering performance (in the range of total BET should be noted that the V value of commercial 50–90% RH) of the IHC-s is shown in Table 2. The total coatings is greater than those of IHC-s. As observed in moisture buffering capacities and content values of IHC- − 2 Fig. 6B, the pore volumes of commercial coatings are 0 are 217 g m and 16.3%, respectively. The moisture mostly attributed to the contribution of macropore (> buffering capacities of IHC-s gradually increase as the 50 nm). However, the mesopores (i.e., 2–50 nm) in the amounts of waste silica sludges increase, i.e., the per- IHC-s are the most effective pores for moisture adsorp- formance of samples with waste silica sludges is better tion. Therefore, the presence of waste silica sludges can than that of IHC-0. As a result, the moisture buffering increase the volumes of mesopores, as evidenced in the capacities of IHC-38, IHC-48 and IHC-58 can reach − 2 pore size distributions of IHC-s (see Fig. 6B). 270, 304 and 316 g m , respectively. Also, moisture Surface contact angles of IHC-s samples are shown in contents of IHC-38, IHC-48 and IHC-58 are 23.6, 25.6 Fig. 7. The adsorption time relates to the porous proper- and 26.7%, respectively. The moisture buffering capaci- ties of materials, i.e., the faster rate of water drop ties and contents of IHC-s are remarkably superior to Fig. 7 Surface contact angle of (A) IHC-0, (B) IHC-38, (C) IHC-48 and (D) IHC-58 Kuok et al. Sustainable Environment Research (2022) 32:8 Page 7 of 9 − 2 − 1 Table 2 Moisture buffering capacities and contents of IHC-s observed for IHC-38 (17.3 g m h ). The results show and commercial coating that the adsorption and desorption rates of IHC-s (38– Sample IHC-0 IHC-38 IHC-48 IHC-58 Commercial coatings 58%) are higher than that of IHC-0 and commercial − 2 coatings. Hygroscopic curves of the IHC-s coatings are W (g m ) 216.9 270.3 303.7 316.1 58.7 shown in Fig. 8C. The IHC-48 coatings start to adsorb u (%) 16.3 23.6 25.6 26.7 11.4 moisture at the RH of 40%. It can be seen that no sig- nificant difference between IHC-48 and commercial − 2 those of commercially available coatings (59 g m and coatings can be observed in the humidity range of < 11.4%), which are possibly due to the unique porous 50%. While relative humidity is higher than 75%, the properties. Moisture buffering ability has the positive adsorbed capacities are increased sharply upon the correlation with S and V of coatings. addition of waste silica sludges. Note that curves of ad- BET total Humidity buffering performance (in the range of 50– sorption and desorption are not overlapped due to the 75% RH) of IHC-s and their corresponding results are hysteresis effect. Durability of adsorption-desorption shown in Fig. 8A. Except to IHC-0 coating, IHC-s (s = process for IHC-48 was performed by cyclic tests, as 38, 48 and 58%) have the higher moisture buffering cap- presented in Fig. 8D. The moisture buffering capacities acities in the range of 50–75% RH, indicating the waste of IHC-48 coatings keep stable for four cyclic runs (i.e., silica sludges possess the positive effect on moisture ad- the entire run time = 96 h). In other words, the moisture sorption. In addition, the benchmarks of adsorbed cap- can be adsorbed by IHC-48 spontaneously and then des- acities for HBMs issued by Japanese Industrial Standards orbed from pores entirely. − 2 − 2 (JIS, see Table S3) are 29 g m (Level 1) and 50 g m As shown in Table S4, leaching contents of heavy (Level 2) for the adsorption time of 12 h. The IHC-38, metals (Cu, Cr, Cd, Ni, Ba, Co and Pb) for IHC-48 were IHC-48 and IHC-58 samples possess moisture buffering investigated by the TCLP tests. No detectable heavy − 2 capacities of 49, 48 and 46 g m , respectively for the ad- metals are observed in leaching solutions of IHC-48 sorption time of 12 h, which reach the requirement of coatings. Moreover, the antibacterial property of IHC-48 Level 1 and close to Level 2. Also, adsorption-desorption is evaluated via the standard method from JIS (JIS Z gradients of various coatings are shown in Fig. 8B. The 2801). As observed in Table S5, the bacteria concentra- − 1 IHC-48 coatings have the highest adsorption rate (20.1 g tion of 99 CFU mL observed for IHC-48 after the con- − 2 − 1 6 m h ) and the optimal desorption rate can be tact time of 24 h is significantly lower 6.0 × 10 CFU Fig. 8 A Moisture adsorption-desorption performance, (B) gradient, (C) hygroscopic curves f IHC-s and commercial samples and (D) a cyclic test of IHC-48 Kuok et al. Sustainable Environment Research (2022) 32:8 Page 8 of 9 − 1 mL for blank. The R factor (i.e., the decimal logarithm Funding This work was supported by Ministry of Science and Technology of Taiwan of the bacteria concentrations between the reference and (MOST 107–2221-E-006-009-MY3). the IHC-48 samples) of antimicrobial activity is ca. 4.8 which is greater than the threshold value (i.e., 2) of Japa- Availability of data and materials nese Industrial Standards (JIS Z 2801). It is worth noting All data supporting the conclusions of this article are included in this that the IHC-48 recycled from industrial wastes pos- manuscript. sesses an excellent moisture buffering ability which can reach the benchmark of JIS and is also superior to the Declaration commercial coatings. More importantly, the simple and Competing interests energy-saving sol-gel method to prepare IHC-48 coat- The authors declare they have no competing interests. ings with an excellent antimicrobial efficacy (> 99.99%) and environmental friendliness (non-detectable heavy Received: 8 September 2021 Accepted: 29 December 2021 metal leaching) recycled from inorganic wastes under room temperature may be a promising candidate for practical applications in the indoor coatings. References 1. Hou PM, Zu K, Qin MH, Cui SQ. A novel metal-organic frameworks based humidity pump for indoor moisture control. Build Environ. 2021;187:107396. 2. Kreiger BK, Srubar WV. Moisture buffering in buildings: a review of 4 Conclusions experimental and numerical methods. Energ Buildings. 2019;202:109394. The humidity buffering coatings, which are the new ap- 3. Wan H, Sun ZW, Huang GS, Xu XH, Yu JH. 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Sustainable Environment Research – Springer Journals

Published: Jan 21, 2022

Keywords: Valorization; Mesoporous/microporous structure; Indoor humidity control; Industrial sludges; Sol-gel

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