Montmorillonite-based porous adsorbent prepared by gel casting method was used to adsorb Cr(III)-organic complexes in tanning wastewater with the initial concentration of 10 mg L−1. The as-porous adsorbent exhibited an excellent performance of separation. Meanwhile, its structural and morphology were characterized by TG, BET, XRD, SEM, EDS and XPS, showing that the porous adsorbent was complete hollow ball structure. And the adsorption results showed that the adsorption amount of porous adsorbent rapidly declined from 9.58 to 5.28, and only Cr(III) was adsorbed, when the molar ratio of Cr(III) and citrate was more than 1:5. Increased of pH was beneficial to the adsorption of Cr(III)-citrate in range from 2.46 to 7.12. Furthermore, the best removal efficiency of porous adsorbent for Cr(III)-organic complex used up to 97% and remaining above 84% after five cycles. Finally, the adsorption mechanism of Cr(III) on porous adsorbent is also provided. Therefore, a kind of easy separated porous adsorbent was prepared in this study so as to treat tanning wastewater containing Cr(III)-organic complex following alkali precipitation, as well as solving the problem of difficult recycle.
Research Article
Montmorillonite-Based Porous Composites as Adsorbents for Removal of Cr(III)-Organic Complexes in Tannery Wastewater
https://doi.org/10.21203/rs.3.rs-285779/v1
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Zirconium oxide
montmorillonite
porous adsorbent
Cr(III)-organic complex
adsorption
wastewater
Tanning industry plays an important role in developing countries [1], and chromium(III) sulfate (Cr(OH)SO4∙nH2O) has always been considered to be the most effective tanning agent [2]. Metallic chromium (Cr) is mainly discharged from the tanning, retanning and washing stages of leather. There are two mainly existing forms of Cr in solution, i.e., Cr(III) and Cr(VI), and Cr(VI) is more toxic [3]. Therefore, it is urged to eliminate it from wastewater before discharging to the environment. At present, a rich array of approaches including alkali precipitation [4], photocatalytic degradation [5], membrane filtration [6] and adsorption [7, 8] have been developed to remove Cr(III) from tanning wastewater and Cr(III) can be readily precipitated in alkaline solution [9]. However, the concentration of chromium in the supernatant after alkali precipitation is usually failed to meet the value of 1.5 mg L−1 of Chinese environmental protection agency (EPA) [10]. The primary contributor is that a large amount of chemicals, such as organic acids, organic additives, macromolecular organics, collagen hydrolysis macromolecule (− COOH and –OH), are added during the tanning process [11]. Moreover, these organics can coordinate with Cr(III) to form a linear, network-like stable chromium-organic complex through adsorption, charge attraction, and complexation reactions [12]. Compared with other methods, adsorption is a more economical and efficient method to remove chromium from wastewater and thus it is widely used following alkali precipitation [13].
Nowadays, activated carbons, chitosan, natural zeolites and clay minerals are normally applied to remove heavy metals from wastewater [14–17]. Due to its higher adsorption efficiency and larger interlayer cation exchange capacities, richer extensive sources and lower cost, montmorillonite (MMT) is considered to be an important adsorbent to remove heavy metals from wastewater [18, 19]. MMT is a yellowish powder with a special 2:1 nanometer layered structure and a large number of cations, as well as selective adsorption properties for Cr(III) [20]. The ideal physical and chemical properties can be used to improve its adsorption performance, mainly contributing by its interlayer cations which are in favor of ion exchange action [21]. Zirconium-based oxides are also regarded as attractive adsorption materials in the field of water treatment, due to its stable, non-toxic and water insoluble properties [22, 23]. Previous researchers have demonstrated that zirconium oxide loaded MMT (ZrO2-MMT) is feasible and has an excellent adsorption capacity for Cr ions [24]. In most circumstances, adsorbent is applied in form of powder to treat wastewater. However, powder is easy to agglomerate in water, resulting in greatly reducing for specific surface area as well as adsorption capacity. Furthermore, it is difficult to separate powder adsorbent from wastewater, limiting its reusing and recycling. Therefore, it is very valuable to develop a new form of adsorbent to overcome these problems.
Gel casting technique has been used to prepare porous materials, which can effectively support the porous structure before sintering process [25, 26]. Previous reports reveal that the porous adsorbent shows high adsorption capacity, excellent reuse performance and free separation property when removing contaminants from water [19, 26, 27]. However, removing Cr(III)-organic complexes from tanning wastewater by porous adsorbents is rarely reported.
Herein, the objective of this work is to prepare MMT-based porous adsorbent by gel casting method, then adsorbing Cr(III)-organic complex in tanning wastewater following alkali precipitation. Formate and citrate are chose as organic ligands, exploring the effects of pH, temperature and molar ratio on the adsorption capability of MMT-based porous adsorbent. Since a large number of Na+ and Ca2+ were introduced by alkali precipitation in the tanning wastewater, the effects of interference ions (e.g., Na+ and Ca2+) are discussed in the adsorption process. Furthermore, industrial wastewater often consist of multiple coexisting anion and cation ions (e.g., Cl−, SO42−, HPO42−, Zn(II) and Cr(VI)), and they compete the active adsorption sites with Cr(III). Therefore, the effects of interference ions are also discussed in the adsorption process. In addition, the MMT-based porous adsorbent shows an excellent separation performance and reuse performance during the adsorption-desorption process.
2.1 Preparation of porous adsorbent
ZrO2-MMT composite (Zr(SO4)2 and MMT in the mass ratio of 3:5) was prepared by ion-exchange method [24]. Acrylamide monomer (12.2 g), N,N'-methylenebisacrylamide (MBAM, as cross linker, 1.25 g) and polyacrylic acid salt (as dispersing agent, 1.25 mL) were added to 45 mL of distilled water under stirring. 25 g of ZrO2-MMT was slowly added to the mixture under stirring to form the slurry. Polystyrene ball (as pore-making agent, 115 mL) and triethanolamine dodecyl sulfate (as foamer, 2.5 g) were added to the slurry and stirred for 1 h. Ammonium persulfate (APS, as initiator, 0.0287 g) and N,N,N',N',-tetramethylethylenediamine (TEMED, as catalyst, 0.88 mL) were added to the slurry and stirred for 30 min. Subsequently, the slurry was injected into the mold, drying for 4 h at 50°C. Finally, the dry body was calcinated for 8 h at temperatures of 600, 700 and 800°C.
2.2 Adsorption experiments
Adsorption experiments was carried out to evaluate the adsorption performance of the porous adsorbent. The influences of various factors, such as molar ratio (1:0, 1:1, 1:3, 1:5, 1:10, 1:15, 1:20), temperature (20, 40, 60, 80°C) and pH (2.46, 4.53, 5.45, 6.47, 7.12) on adsorption capacity were examined. A certain amount of organic ligands were added to Cr2(SO4)3 solution (10 mg L−1) to prepare Cr(III)-formate and Cr(III)-citrate complex solution. After absorption, the complex solution was filtered through filter membrane.
To evaluate the effect of interfering cations on Cr(III)-citrate (1:5) adsorption, of the porous adsorbent (1 g L− 1) was added into Cr(III)-citrate solutions (10 mg L− 1) containing 0 ~ 25 mmol L− 1 interfering cations (Na+ and Ca2+), preparing by dissolving Na2SO4 and CaCl2. In the interference experiments, interfering anion ions (10 mg L− 1), including Cl−, SO42− and HPO42− were added into Cr(III) solutions (10 mg L− 1) in form of NaCl, Na2SO4 and Na2HPO4. In addition, ZnCl2 and K2Cr2O7 were selected to carry out coexisting cations experiment, which contained 10 mg L− 1 of Zn(II), and Cr(VI), in 10 mg L− 1 Cr(III) solution. Furthermore, 1 M H2SO4 or 1 M NaOH solution was used to adjust its pH.
To evaluate the recyclability of the porous adsorbent, five times adsorption-desorption experiments were carried out. The consumed porous adsorbent was immersed in 0.1 M H2SO4 solution and shook for 24 h. Then the desorbed porous adsorbent was filtrated, washed and dried for the next cycle. Finally, the inductively coupled plasma (ICP) technique (Varian Liberty 200 ICP-OES) was used to measure the concentration of Cr(III) in solution.
3.1 Preparation process of the porous adsorbent
The preparation process of the porous adsorbent is shown in Fig. 1. Briefly, ZrO2-MMT composite, organic monomers and polystyrene balls are mixed to prepare slurry in the prescribed sequence. Then the injection mold is formed into a wet body, as shown in Fig. 1. Next, the wet body is dried at 50 °C to obtain a dry body. The temperature is raised from 20 to 380 °C (for 360 min) for glue discharge (Fig. S1). Therefore, the position of the polystyrene spheres is released and produced a porous structure after the glue discharge. Finally, the porous adsorbent is obtained after thermal treatment at 600 °C as shown in Fig. 1.
3.2 TG, FTIR and XRD spectra analysis
Fig. 2a presents the thermal analysis curves of the porous adsorbent. Firstly, the TG curve of dry body slow declines from 35 to 100 °C, mainly due to dehydration of dry body and the desorption process of adsorbed water molecules between the MMT surface and the layer [28]. Then the mass of dry body significantly decreases in the range of 100~330 °C, mainly because of the decomposition of polyacrylamide at 200 °C. When the temperature increases to 300 °C, it decomposes into NH3, and it decomposes into H2, CO and NH3 above 300 °C. Subsequently, the polystyrene sphere is severely degraded in the range of 330~450 °C. Therefore, the mass loss of dry body is presumably due to the decomposition of organic substances such as polyacrylamide and polystyrene spheres in the slurry [28].
Fig. 2b displays the FTIR spectra of MMT, ZrO2-MMT and porous adsorbent. For MMT, the peaks at 466, 1042, 1634 and 3451 cm−1 are attributed to the vibration of Al–O, stretching vibration of Si–O, bending vibration of H–OH and stretching vibration of O–H, respectively. Compared to MMT, the bands of Si–O–Si in the ZrO2-MMT show a shift to 504 cm−1, indicating ZrO2 enters into the layers structure of MMT [29]. In addition, the FTIR spectra of ZrO2-MMT and porous adsorbent show an important band at 504 cm−1 corresponding to the vibration of the Zr–O bond [30]. Furthermore, compare with the pristine MMT, the XRD spectra of the porous adsorbent show obvious diffraction peaks at 30.50º and 50.58º (Fig. S2), corresponding to the (101) and (211) diffraction planes of ZrO2, respectively [30,31]. This result also indicates that ZrO2 exists in the layer structure of the porous adsorbent. Compared with ZrO2-MMT, few changes are observed in the FTIR spectrum of porous adsorbent when the powder is prepared as porous material. This reason could be due to attributed to the carbonized adding organics.
XPS analysis is used to discuss the chemical and structural information of the porous adsorbent. The full spectra of XPS reveal the existence of elements Zr, Al, Si, C, Na and O (Fig. 2c). The peaks of binding energy (182.8 eV) can be assigned to Zr 3d (Fig. 2d), indicating the existence of ZrO2.
3.3 SEM-EDS and BET analysis
The morphological feature of porous adsorbent is shown in Fig. 3. After thermal treating at 600 °C, there is a hollow sphere structure inside the porous adsorbent, forming an integral pore structure (Fig. 3a and b). Fig. 3c and d showing the wall of the porous adsorbent, it is in clear porous structure with certain interlayer spacing as well as smooth surface. With the increase of temperature, the pore structure becomes not integral (Fig. S3). The wrinkled layered structure of the spherical wall section is clearly seen in Fig. 3e and f. The thickness of the wall is about 12~20 μm, which suggests that the layer structure of MMT in porous adsorbent remains integral.
The specific surface area, pore size and pore volume of MMT, ZrO2-MMT and porous adsorbent are listed in Table S1. The specific surface area of the porous adsorbent (38.80 m2 g−1) is almost equivalent to that of ZrO2-MMT (40.56 m2 g−1), indicating that the gel casting method is feasible for ZrO2-MMT molding. In addition, the value of pore size (13.93 nm) also proves the existence of mesoporous. The pore volumes of MMT and ZrO2-MMT are 0.0048 and 0.0028 cm3 g−1, respectively. The reduced pore volume is probably caused by the existence of ZrO2 in the MMT sheet, proving that ZrO2 has entered into the layers structure of MMT.
3.4 Adsorption experiments
As shown in Fig. 4a, the adsorption amount of the porous adsorbent is not serious changed with increasing molar ratio of formate and Cr(III), which indicates that formate plays a less important role in the process. With a changed molar ratio of Cr(III) and citrate ranging from 1:1 to 1:5, the adsorption amount of Cr(III)-citrate is no corresponding changed. This result can be attributed to strong positively charged, causing by the existing of Cr(III) in form of Cr(H2O)63+ in aqueous solution [32]. While the adsorption amount of Cr(III)-citrate rapidly declined when the mole ratio of Cr(III) and citrate is more than 1:5. Since there are three carboxyl groups in its molecular structure, citrate has the stronger coordination function for Cr(III). In the adsorption process, the movement of Cr(III) is restricted by citrate, which is increased in direct proportion to since there are three carboxyl groups in its molecular structure, citrate the concentration of since there are three carboxyl groups in its molecular structure, citrate in the system. Furthermore, the complexation reaction of Cr3+ with citric acid is as follows:
When the molar ratio of citrate and Cr(III) increases, the equilibrium shift to form of the complex, resulting a decreased concentration of Cr(III) in the solution, as well as decreasing the amount of Cr(III) being adsorbed.
To explore the adsorption performance of the adsorbent for organic ligands, then total organic carbon (TOC) is employed to measure total carbon of organic ligands in solution before and after adsorption. There is no external carbon source before and after adsorbing of Cr(III)-organic complexes on porous adsorbent. As can be seen from Fig. 4b, there is no change in the amount of TOC before and after adsorption. The result reveals that the porous adsorbent adsorbs Cr(III), whereby organic ligands are not adsorbed further illustrating that the increased organic ligands can restrict the movement of Cr(III) and impair the adsorption amount of porous adsorbent.
The adsorption curves of the porous adsorbent at various temperatures are depicted in Fig. 4c. With the increased temperature, the adsorption amount of Cr(III)-citrate is not regularly changed. It can be explained that the carboxyl group in the molecular structure of citrate plays more important role in forming Cr(III) than the force generated by the thermal motion on Cr(III). With the increasing temperature, the adsorption amount of Cr(III) alone slightly increases followed by gradually decreasing, which is mainly due to the fact that valency forces of Cr(III) in the solution becomes stronger. It is beneficial for Cr(III) to overcome the electrostatic repulsion and space hindrance during the adsorption process. As a result, the active sites of porous adsorbent are increased, which is in favor of Cr(III) combination. However, the adsorption amount of Cr(III) on the porous adsorbent decreases, it can be attributed to more intensely moved Cr(III) as well as partially desorbed Cr(III).
The concentration of Cr(III) in the Cr(III)-citrate system is basically about 10 mg L−1 without adding adsorbent (Fig. 4d). With increasing pH, the concentration of Cr(III)-citrate significantly decreases after adding adsorbent. It is explained that the increased pH impairs the complexation force between citrate and Cr(III). When pH is in range from 4.53 to 7.12, the residual concentration of Cr(III) is slightly changed. It may be due to the fact that the adsorbed Cr(III) is not able to desorb at higher pH. The as-prepared porous adsorbent obtains complete hollow ball structure and large specific surface area, which is beneficial for adsorbing Cr(III)-citrate at high pH. In addition, zeta potential of the porous adsorbent is negatively charged form with pH in the range from 2.89 to 11.22, which is in favor of adsorbing of positive charged heavy metals.
3.5 Influence of interfering ions and reusability of the porous adsorbent3
Various coexisting cations in the tanning wastewater interferes the adsorption amount of porous adsorbent for Cr(III)-citrate. Na+ and Ca2+ are chose as interfering cations to investigate their effects on Cr(III)-citrate adsorption (Fig. 5a). Na+ and Ca2+ can interfere the up taking Cr(III)-citrate, mainly due to the fact that Na+ and Ca2+ compete active adsorption sites with Cr(III). Fig. 5b shows that the adsorption amount is not significantly inhibited in the presence of these competing anions (Cl−, SO42−), except for HPO42− ion. The result is mainly due to the similar tetrahedral structure of chromium and phosphate. Fig. 5c shows Cr(III) effectively removed by the porous adsorbent, but it has low adsorption efficiency for other metal ions, such as Zn(II) and Cr(VI).
Fig. 5d depicts that the change of adsorption capacity of the porous adsorbent in five times adsorption-desorption cycles. The results show that the removal efficiency of the porous adsorbent for Cr(III)-citrate decreases from 97% to 84% as time going suggesting that porous adsorbent has good stability and reusability. Furthermore, the porous adsorbent is easier to separate from the effluent than powder, and do not produce secondary pollution (Fig. S4).
The adsorbed porous adsorbent is immersed in 1 M H2SO4 solution and shaken for 24 h. Then, 0.1 M H2SO4 or 0.1 M NaOH solution is used to adjust pH value (2~3). The hydrolysis formula of chromium is as follows [33]:
7[Cr(H2O)6]3+⇌[Cr(H2O)6OH]2++29H2O+Cr2(OH)24++Cr3(OH)45++Cr3++7H+ (2)
According to the requirement, desorption solution of chromium is prepared with different acidity and alkalinity, which is reused for leather tanning. This process realizes the function reuse of waste and has a good economic value in industrial production.
3.6 Adsorption mechanism
3.6.1 Adsorption kinetic
As shown in Fig. S5a, when the time reaches 70 min, the adsorption equilibrium of Cr(III) on porous adsorbent is established. The fitting parameters are exhibited in Table S2. The adsorption data fittings of kinetic models of Cr(III) on porous adsorbent are showed in Fig. 6a and b. The R2 (R2 = 0.9924) value of pseudo-second-order kinetic model is much higher than the pseudo-first-order kinetic model (R2 = 0.8567). Further, the calculated value of pseudo-second-order kinetic equation is 11.11 mg g−1, which is closer to the experimental value of 10.46 mg g−1. Therefore, the pseudo-second-order kinetic equation can better fit the adsorption process, which reveals that the adsorption process is mainly chemical adsorption.
3.6.2 Adsorption isotherm
Fig. S5b shows the effect of the initial concentration of Cr(III). The values of fitting parameters are exhibited in Table S3. The linear data fittings by the isotherm models are shown in Fig. 7a and b. The experimental data agrees well with both Langmuir (R2 = 0.9935) and Freundlich (R2 = 0.8174) models, the theoretical maximum adsorption capacity qm (149.25 mg g−1) is closer to the equilibrium adsorption amount qe (129.63 mg g−1). Therefore, the Langmuir isotherm model can appropriately describe the adsorption process. Accordingly, the adsorption process of porous adsorbent for Cr(III) is mainly monolayer adsorption. The value of n is the adsorption strength. The good adsorption property of porous adsorbent is proved by the value of n = 3.25 (between 1 and 10).
3.6.3 Thermodynamics
The relevant parameters are presented in Table S4. Plot of ln Ka versus 1000/T is shown in Fig. S6. The positive value of ΔH reflects the fact that adsorption of Cr(III) on porous adsorbent is an endothermic process. The negative value of ΔG implies the feasibility and spontaneity of adsorbing Cr(III) on the surface of porous adsorbent. During the adsorption process, the increased randomness of the system solid/solution interface is shown by the positive values of ΔS. This result indicates that porous adsorbent has high affinity for Cr(III).
3.6.4 XPS, Zeta and EDS analysis
As shown in Fig. 8a, the peak of Cr element appears after adsorbing Cr(III), which illustrated that Cr(III) has been adsorbed by porous adsorbent. In addition, in the high-resolution spectrum of Cr 2p, the peaks of binding energy (575.2 eV) of Cr 2p appear after adsorption (Fig. 8b), which corresponds to Cr(III), indicating that the porous adsorbent has adsorption capacity for Cr(III). Additionally, the zeta potential of MMT and porous adsorbent at different pH are depicted in Fig. 8c. The zeta potential of the porous adsorbent is negatively charged form with pH in the range of 2.89~11.22, which is beneficial to adsorb heavy metals in solutions.
Fig. 8d and e show the SEM images before and after adsorbing of Cr(III)-organic complexes on porous adsorbent. It can be seen that the layer surface of the porous adsorbent is smooth and clear in Fig. 8d. The surface of the porous adsorbent appears fragments and becomes relatively rough after adsorption (Fig. 8e), indicating that the porous adsorbent has an adsorption capacity for Cr(III). In addition, the adsorbed Cr(III) ions are also confirmed by EDS analysis (insets in Fig. 8d and e). Fig. S7b clearly shows the presence of Cr element after adsorption. It also proves that Cr(III) has been adsorbed by porous adsorbent.
3.7 Comparison of adsorption capacity of Cr(III) adsorption on various adsorbents
Table 1. Maximum adsorption capacity (qm) of Cr(III) by various adsorbents in other reports.
|
Adsorbents |
qm (mg g−1) |
References |
|
Humic acid modified Ca-montmorillonite |
15.66 |
[34] |
|
Silica/sulfonate (SFS) |
72.8 |
[35] |
|
Corncob waste activated carbon |
84.55 |
[36] |
|
MAC-pseudo-non-imprinted adsorbent |
69.61 |
[37] |
|
Gelatin-Montmorillonite Nanocomposite |
52.91 |
[38] |
|
Fe3O4–SiO2 sub-micron particles |
9.00 ± 3.00 |
[39] |
|
Porous adsorbent |
149.25 |
This work |
The comparison of the maximum adsorption capacities for Cr(III) on various adsorbents reported in the literature are listed in Table 1 [34-39]. As shown in Table 1, it can be determined that the adsorption performance of the porous adsorbent is better than many other adsorbents.
In summary, montmorillonite-based porous adsorbent is successfully prepared by gel casting method. The adsorbent is characterized via TG, BET, XRD, SEM, EDS and XPS. The results indicate that the gelatinization temperature of dry body is 380 °C, and the specific surface area of porous adsorbent is basically consistent with that of pristine MMT. In addition, adsorption test results show that the adsorption performance of the adsorbent is significantly influenced, when the molar ratio of Cr(III) and citrate is greater than 1:5. Temperature plays no evident role during the adsorption process. Increased pH is beneficial to adsorb Cr(III)-citrate as pH rises from 2.46 to 7.12. The pseudo-second-order kinetic equation and the Langmuir isotherm equation can better describe the adsorption process. In terms of adsorbing Cr(III), the porous adsorbent is superior to many other adsorbents. The removal efficiency decreases from 97% to 84% after five adsorption-desorption cycles when Cr(III)-citrate is 10 mg L−1. Our results exhibit that montmorillonite-based porous adsorbent has an excellent reuse performance and easily separating property in the adsorption process, which can deeply treat tanning wastewater following alkali precipitation. Finally, the adsorption mechanism reveals that electrostatic attraction may contribute to the adsorption of Cr(III). The results of EDS and XPS analyses reveal that Cr(III) is adsorbed on the porous adsorbent. Therefore, montmorillonite-based porous adsorbent is a promising material for removing Cr(III)-organic complex with low concentration (10 mg L−1) in tanning wastewater.
Acknowledgements
The research was supported by Liaoning Province (Jinzhou) Fur Green Manufacturing Industry Technology Innovation Strategic Alliance [201854], Research on resource recycling technology of tanned chromium-containing waste dander [2018020190-301], The "Seedling" Project named "Research on Graphene-based 3D Material Construction and Key Technologies for Industrialization Application" for Youth Science and Technology Talents of Education Department of Liaoning Province (LQ2020011), Study on the deep removal of heavy metal chromium from industrial wastewater in the upper reaches of Daling River estuary [BDHYYJY2020013] (This study were supported by the Open Fund of Institute of Ocean Research, Bohai University), National Natural Science Foundation of China [21878024], Innovation Team Project of Liaoning Province [LT2015001], and Innovation Team Project of Liaoning Higher University [2018479-14].
Conflict of Interest Statement
We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service or company. No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part.
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