Introduction

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. However, Cr(III) intake is associated with chronic diseases such as cardiovascular disease, diabetes, and neurological diseases [3,4,5,6], and Cr(VI) is even more toxic [7]. Therefore, it is urged to eliminate it from wastewater before discharging to the environment. At present, a rich array of approaches including alkali precipitation [8], photocatalytic degradation [9], membrane filtration [10] and adsorption [11, 12] have been developed to remove Cr(III) from tanning wastewater and Cr(III) can be readily precipitated in alkaline solution [13]. However, the concentration of chromium in the supernatant undergoing alkali precipitation is usually failed to meet the value of 1.5 mg L−1 proposed by Chinese environmental protection agency (EPA) [14]. 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 [15]. 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 [16]. 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 [17].

Nowadays, activated carbons, chitosan, natural zeolites and clay minerals are normally applied to remove heavy metals from wastewater [18,19,20,21]. 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 [22, 23]. 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) [24]. 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 [25]. 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 [26, 27]. Previous researchers have demonstrated that zirconium oxide loaded MMT (ZrO2-MMT) is feasible and has an excellent adsorption capacity for Cr ions [28]. In most circumstances, the adsorbent is applied in form of powder to treat wastewater. However, the powder is easy to agglomerate in water, resulting in greatly reducing in the 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 [29, 30]. Previous reports reveal that the porous adsorbent shows high adsorption capacity, excellent reuse performance and free separation property when removing contaminants from water [23, 30, 31]. Gustafsson et al. [32] studied the mechanisms and modeling of Cr(III) complexation to natural organic matter (NOM). This study suggests that Cr(III)-NOM complexes are important for chromium speciation in many environments. Chen et al. [33] studied tributyl hexadecyl phosphonium solidification strategy to construct gold nanoprobes for the detection of aqueous Cr(III)-organic complexes, and Cr(III)-citrate is selected for the forthcoming study. However, removing Cr(III)-formate and Cr(III)-citrate complexes from tanning wastewater by porous adsorbents is rarely reported.

Herein, the objective of this work is to prepare MMT-based porous adsorbent in method of gel casting, then adsorbing Cr(III)-organic complex in tanning wastewater undergoing alkali precipitation. Formate and citrate are used 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+ are 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 consists 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.

Experimental Details

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 [28]. 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 in condition of stirring. 25 g of ZrO2-MMT was slowly added to the mixture to form the slurry, together with stirring. 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 at 50 °C for 4 h. The temperature was raised from 20 to 380 °C for glue discharge (for 360 min) (Fig. S1). Finally, the dry body was calcinated at temperatures of 600, 700 and 800 °C, for 8 h. An optimum calcination temperature of 600 °C was obtained.

Adsorption Experiments

Adsorption experiments were 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 adsorbing with Cr(III)-citrate (1:5), 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 used to carry out coexisting cations experiment, which contained 10 mg L−1 of Zn(II), Cr(VI) and Cr(III), respectively. 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 shaken 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 the solution.

Results and Discussion

Preparation process of the porous adsorbent

Fig. 1
figure 1

Schematic illustration of the prepared porous adsorbent: a step 1 added Zr-MMT to acrylamide monomer mixture and stirred; step 2 added the polystyrene ball to the slurry and stirred; b the slurry was injected into the mold, dried at 50 °C for 4 h, then heat-treated at 600, 700 or 800 °C for 8 h

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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.

TG, FTIR and XPS Spectra Analysis

Fig. 2
figure 2

a TG curves of the MMT and porous adsorbent; b FTIR spectra of MMT, ZrO2-MMT and porous adsorbent; c XPS survey spectrum of porous adsorbent; d high resolution XPS spectrum of Zr 3d

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Figure 2a presents the TG curves of the MMT and porous adsorbent. The weight loss of MMT is about 7.4% between 35 and 200 °C, which is adsorbed water. And the weight loss is about 2.8% between 200 and 700 °C, which is lattice water. Because the main component of MMT is aluminosilicate, so the thermal stability is good. At the same time, the TG curve of the dry body slowly declines from 35 to 100 °C, mainly due to dehydration of the dry body and the desorption process of adsorbed water molecules between the MMT surface and the layer [34]. 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 then 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 the dry body is presumably due to the decomposition of organic substances such as polyacrylamide and polystyrene spheres in the slurry [34].

Figure 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 with MMT, the bands of Si–O–Si in the ZrO2-MMT shift to 504 cm−1, indicating ZrO2 enters into the layers structure of MMT [35]. 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 [36]. Furthermore, compared 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 [36, 37]. 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, due to adding carbonized 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.

SEM-EDS and BET analysis

Fig. 3
figure 3

SEM images of the: a and b hollow sphere structure, scale bar = 200 and 500 μm; c and d wall, scale bar = 1 μm and 500 nm; e and f spherical wall section, scale bar = 1 and 10 μm

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The morphological feature of porous adsorbent is shown in Fig. 3. Subjecting to thermal treatment at 600 °C, there is a hollow sphere structure inside the porous adsorbent, forming an integral pore structure (Fig. 3a and b). The wall of the porous adsorbent is observed in Fig. 3c and d, and it is in a clear porous structure with certain interlayer spacing as well as a 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 presented in Fig. 3e and f. The thickness of the wall is about 12–20 μm, suggesting that the layer structure of MMT in porous adsorbent remains integral.

The data on specific surface area, pore size and pore volume of MMT, ZrO2-MMT and porous adsorbent, is 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 of MMT.

Adsorption Experiments

Fig. 4
figure 4

a The effect of molar ratio of organics and Cr(III); b TOC changes of Cr(III)-organic complex adsorbed on porous adsorbent; c The adsorption curves of Cr(III)-citrate and Cr(III) on the porous adsorbent at various temperatures; d The effect of pH on the concentration of Cr(III) in Cr(III)-citrate solution before and after addition adsorbent (adsorbent dose = 1 g L−1; adsorption time = 60 min)

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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 not 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 an aqueous solution [38]. 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, and they are positive proportion relation. Since there are three carboxyl groups in its molecular structure. Furthermore, the complexation reaction of Cr3+ with citric acid is as follows:

(1)

When the molar ratio of citrate and Cr(III) increases, the equilibrium shifts to the 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 numbers of active site with 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 with pH in the range from 2.89 to 11.22, in favor of adsorbing of positive charged heavy metals.

Influence of interfering ions and reusability of the porous adsorbent

Fig. 5
figure 5

a Effect of competing cations (Na+ and Ca2+) on adsorbing Cr(III)-citrate with porous adsorbent (adsorbent dose = 1 g L−1; C0 = 10 mg L−1; adsorption time = 60 min; pH 6.5); b Effects of the coexisting anions and the c coexisting cations on adsorbing Cr(III); d Recycling of porous adsorbent (adsorbent dose = 1 g L−1; C0 = 10 mg L−1; adsorption time = 60 min; pH 6.5)

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Various coexisting cations in the tanning wastewater interferes the adsorption amount of porous adsorbent for Cr(III)-citrate. Na+ and Ca2+ are used as interfering cations to investigate their effects on adsorbing Cr(III)-citrate (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). Figure 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. Figure 5c shows Cr(III) is effectively removed with the porous adsorbent, but it performs low adsorption efficiency for other metal ions, such as Zn(II) and Cr(VI).

Figure 5d depicts that the change of adsorption capacity of the porous adsorbent in five times adsorption-desorption cycles. The removal efficiency of five times adsorption-desorption cycles is 97%, 95%, 91%, 89 and 84%, respectively. 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 [39]:

$$7[Cr({\text{H}}_2{\text{O}})6]^{3+} \rightleftharpoons [{\text{Cr}}({\text{H}}_2{\text{O}})6{\text{OH}}]^{2+}+29{\text{H}}_2{\text{O}}+{\text{Cr}}_2({\text{OH}})^{24+}+{\text{Cr}}_3({\text{OH}})^{45+}+{\text{Cr}}^{3+}+7{\text{H}}^{+}$$
(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.

Adsorption Mechanism

Adsorption Kinetic

Fig. 6
figure 6

The kinetic adsorption data plots for adsorbing Cr(III) on the porous adsorbent a pseudo-first-order equation and b pseudo-second-order equation

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Table 1 The first-order, second-order kinetic parameters on adsorbing Cr(III) with porous adsorbent
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$${log}\left({q}_{e}-{q}_{t}\right)={log}{q}_{e}-\frac{{k}_{1}}{2.303}t$$
(3)
$$\frac{t}{{q}_{t}}=\frac{1}{{q}_{e}}t+\frac{1}{{k}_{2}{q}_{e}^{2}}$$
(4)

where t is the adsorption time (min); qe represents the adsorption amount of porous adsorbent for Cr(III) at equilibrium adsorption (mg g−1); k1 (min−1) and k2 (g mg−1 min−1) represent the rate constants of the Pseudo-first-order and Pseudo-second-order kinetic models.

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 1. 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 that of 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.

Adsorption isotherm

Fig. 7
figure 7

Linear regression fitting of the adsorption data on Cr(III) with the porous adsorbent using (a) Langmuir equation and (b) Freundlich equation

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Table 2 The adsorption isotherm parameters on adsorbing Cr(III) with porous adsorbent
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In order to evaluate the mechanism on adsorbing Cr(III) with porous adsorbent, the adsorption data is fitted by the linear forms of the Langmuir and Freundlich isotherm models as follows:

$$\frac{{C}_{e}}{{q}_{e}}=\frac{1}{{q}_{m}b}+\frac{{C}_{e}}{{q}_{m}}$$
(5)
$${log}{q}_{e}=\frac{1}{n}{log}{C}_{e}+{log}k$$
(6)

where b is the Langmuir constant, (L mg−1); qm represents the maximum adsorption capacity of porous adsorbent, (mg g−1); Ce is the concentration of Cr(III) at equilibrium adsorption, (mg L−1); k and n represent the Freundlich constants.

Figure S5b shows the effect of the initial concentration of Cr(III). The values of fitting parameters are exhibited in Table 2. 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).

Thermodynamics

Table 3 The adsorption thermodynamic parameters on adsorbing Cr(III) with porous adsorbent
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XPS, Zeta and EDS Analysis

Fig. 8
figure 8

a XPS survey spectra of porous adsorbent before and after adsorbing Cr(III); b high resolution XPS spectrum of Cr 2p; c Zeta potential of MMT and porous adsorbent with different pH values; SEM images and EDS analysis (insets) of porous adsorbent: d before adsorbing Cr(III), scale bar = 200 nm and e after adsorbing Cr(III), scale bar = 200 nm

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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 [40, 41]. 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 to Cr(III) [42, 43]. Therefore, the valence state of Cr(III)-organic complexes did not change before and after adsorption. Additionally, the zeta potential of MMT and porous adsorbent at different pH is depicted in Fig. 8c. The zeta potential of the porous adsorbent is negatively charged with pH in the range of 2.89–11.22, which is beneficial to adsorb heavy metals in solutions.

Figure 8d and e show the SEM images before and after adsorbing Cr(III)-organic complexes with the 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 to Cr(III). In addition, the result of adsorbing Cr(III) ions is 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.

Comparisons of Adsorption Capacity of Various Adsorbents to Cr(III)

Table 4 Maximum adsorption capacity (qm) to Cr(III) by with various adsorbents in other reports
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The comparison of the maximum adsorption capacity to Cr(III) on various adsorbents reported in the literature are listed in Table 4 [16, 44,45,46,47,48]. As shown in Table 4, the adsorption capacity of this porous adsorbent is the largest and outperforms many other adsorbents. To sum up, montmorillonite-based porous composites not only show good adsorption performance, but also separate easily, which will not cause secondary pollution.

Conclusions

In summary, montmorillonite-based porous adsorbent is successfully prepared via gel casting method. The adsorbent is characterized by 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. If the calcination temperature is 600 °C, the pore structure in the porous material is complete, for the pore wall and section lamellar structure are clear, and the pore channel can be seen, and the adsorption performance is the best. In addition, adsorption test results show that the 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 the concentration of 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 undergoing alkali precipitation. Finally, the adsorption mechanism reveals that electrostatic attraction may contribute to the adsorption of Cr(III). The results of EDS and XPS analysis 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 from tanning wastewater with low concentration (10 mg L−1).

Supplementary Information.

The analytical methods of montmorillonite-based porous composites are available in the supplementary information. The thermal treatment flow chart of the adsorbent is shown in Fig. S1. XRD patterns of pristine MMT and porous adsorbent are shown in Fig. S2. SEM images of the hollow sphere structure at 700 and 800 °C are shown in Fig. S3. Easily separable process of adsorbed porous adsorbent is shown in Fig. S4. The effect of time (adsorbent dose = 1 g L−1; C0 = 10 mg L−1) and initial Cr(III) concentration (adsorbent dose = 1 g L−1; adsorption time = 60 min; C0 = 50–160 mg L−1) on Cr(III) adsorption are shown in Fig. S5a and b. Plot of ln Ka versus 1000/T for Cr(III) adsorption on porous adsorbent is shown in Fig. S6. EDS spectra of adsorbent before and after adsorbing Cr(III) are shown in Fig. S7. Specific surface area, pore size and pore volume of MMT, Zr-MMT and porous adsorbent are listed in Table S1.