Enhanced adsorption performance of La(III) and Y(III) on kaolinite by.pdf
- 1. Applied Clay Science 229 (2022) 106693 Available online 31 August 2022 0169-1317/© 2022 Elsevier B.V. All rights reserved. Research Paper Enhanced adsorption performance of La(III) and Y(III) on kaolinite by oxalic acid intercalation expansion method Weisha Dou a , Zhaoping Deng a,* , Jianping Fan b , Quanzhi Lin a , Yuhang Wu a , Yanlin Ma a , Zepeng Li a a College of Materials & Chemistry and Chemical Engineering, Chengdu University of Technology, Chengdu 610059, China b College of Energy, Chengdu University of Technology, Chengdu 610059, China A R T I C L E I N F O Keywords: Kaolinite H2C2O4 intercalation Interlayer expansion Adsorption REEs A B S T R A C T This paper reported a new method to enhance adsorption performance of La(III) and Y(III) on kaolinite by enlarging the interlayer space of kaolinite. The intercalation method could enlarge the interlayer space of kaolinite effectively in previous research literatures, whereas there were few studies on utilizing further reaction of intercalators to expand and exfoliate kaolinite by generating gas. The expanded kaolinite (EKH) involved in this study could enhance adsorption performance of La(III) and Y(III) significantly. The EKH in a state between extended and exfoliated was obtained undergoing two steps. Firstly, H2C2O4 was inserted into the kaolinite layers through the secondary substitution liquid phase intercalation method. Secondly, the Kaol-H2C2O4 (KH) intercalation complex was rapidly calcined with H2C2O4 and Na2CO3, the power provided by the neutralization reaction promoted the thermal decomposition of H2C2O4 between the kaolinite layers to produce a large amount of gas. XRD showed that the interlayer space of the KH intercalation complex expanded from 0.717 nm to 1.112 nm. The specific surface area and total pore volume of the EKH were 1.5 and 2.2 times higher than unmodified kaolinite respectively. The surface of EKH possessed higher zeta potential with more negative charges. The re sults of batch adsorption experiments showed that the adsorption capacity (101.5 mg/g and 78.9 mg/g) of EKH to La(III) and Y(III) was much higher than that of kaolinite (17.1 mg/g and 9.8 mg/g). Pseudo-second-order model and Langmuir model manifest the adsorption kinetics and isotherm. Therefore, the EKH prepared by the intercalation expansion method could be prospectively applied to the treatment of La(III) and Y(III) in in dustrial wastewater. 1. Introduction Clay minerals are layered silicate minerals universally existing in soils, sedimentary rocks and sediments, which are formed by the weathering, hydrothermal alteration and deposition of aluminosilicate rocks (Obaje et al., 2013). Clay minerals are usually composed of sili con‑oxygen tetrahedral sheets (SiO4) and aluminum‑oxygen octahedral sheets (AlO6) stacked through shared oxygen connections (Bhattachar yya and Gupta, 2008). According to the different way of the stacked structural unit layers and the interlayer forces, the basic structural types of layered silicates are mainly divided into 1:1 type (TO type, kaolinite, halloysite) and 2:1 type (TOT type, montmorillonite) (Zhu et al., 2016). Furthermore, clay minerals are abundant in nature and are widely used in papermaking, ceramics, construction, catalysis and other industries (Zhu et al., 2015; Zhang et al., 2016; Aboudi Mana et al., 2017). It also plays an important role in geological formation processes and the extraction of oil and natural gas (Awad et al., 2019). Most of the clay minerals are inexpensive, easy availability, environmental friendliness and possess high specific surface area and exchange capacity (Uddin, 2017). The application has been extended to the field of environmental remediation for adsorbing heavy metals, dyes, rare earth metals and other pollutants in wastewater (Kausar et al., 2018; Wang et al., 2018; Gu et al., 2019; Gil et al., 2021). With the rapid development of industry, emerging pollutants (ECs) such as liquid rare earth elements (REEs) wastes in discarded electronic equipment and high concentrations of REEs contained in wastewater discharged from mining and mineral processing seriously affect human health and damage the ecological environment (Rim, 2016; Gwenzi et al., 2018). The concentration of REEs in the wastewater was usually up to 100–500 mg/L (Fawzy et al., 2022). Therefore, the recovery of * Corresponding author. E-mail address: [email protected] (Z. Deng). Contents lists available at ScienceDirect Applied Clay Science journal homepage: www.elsevier.com/locate/clay https://doi.org/10.1016/j.clay.2022.106693 Received 23 April 2022; Received in revised form 3 August 2022; Accepted 24 August 2022
- 2. Applied Clay Science 229 (2022) 106693 2 REEs from industrial wastewater is crucial for environment protection. Compared with extraction, ion exchange, chemical precipitation, membrane filtration and other methods, adsorption is a green, simple, and efficient method (Nageeb, 2013; Anastopoulos et al., 2016; Liu and Chen, 2021; Novikau and Lujaniene, 2022). Kaolinite (chemical formula is Al2O3⋅2SiO2⋅2H2O) lacks exchangeable cations due to strong hydrogen-bonds interaction between layers and possesses a small spe cific surface area (Zhang et al., 2021), so the adsorption performance is not good. It’s necessary to improve the adsorption performance of kaolinite by modification. At present, heat treatment (Cheng et al., 2021), acid activation (Fang et al., 2021), hydroxyl metal cation pil laring (Turgut Basoglu and Balci, 2010; Kumararaja et al., 2017; Najafi et al., 2021), organic intercalation exfoliation (Yilmaz et al., 2019; Lai et al., 2020) and other methods were used to enlarge the kaolinite layer space and specific surface area, and increase the adsorption active sites. Intercalation is a method in which organic molecules insert into the interlayers of kaolinite without destroying its layered structure, which can enlarge the interlayer space (Slaný et al., 2022). Since the asym metric distribution of atoms on both sides of kaolinite interlayer makes the interlayer polar, only small molecules with strong polarity (such as DMSO, Urea, etc.) can directly insert into the kaolinite layers. The hydrogen bonds between kaolinite layers can be reduced or destroyed by external force after intercalation, so that it can be exfoliated into lamellae or single-layer nano-kaolinite to improve the adsorption per formance (Cheng et al., 2015). Zhang et al. (2017) prepared intercalation-exfoliation kaolinite with KAc, which enhanced the ability of kaolinite to fix sodium by enlarging the interlayer space and hydroxyl loss. Liu et al. (2019) exfoliated kaolinite using the gas generated by the Fenton reaction between H2O2 and Fe2+ in the Kaol-DMSO intercalation complex to enhance its adsorption capacity of Pb2+ . Maged et al. (2020) prepared the Kaol-KAc intercalation complex by secondary substitution liquid phase interca lation method and proved that it has improvement of Pb2+ in waste water. Compared with exfoliation, expansion is a state of enlarging the interlayer domains of kaolinite without totally breaking the layered structure to separate them, which can expose more surfaces between layers and form porous channels for adsorption. Ding et al. (2021) used the energy released by the violent reaction of Urea and KCIO3 to drive the thermal decomposition of Urea between the Kaol-Urea layers to generate a large amount of NH3, thereby forming a layer-expanded structure aluminosilicate material. Therefore, the physical and chemi cal properties of kaolinite could be improved by intercalation expansion method and applied to the adsorption of rare earth cations (La3+ , Y3+ ) in industrial wastewater. At present, there was no research on using environment friendly and inexpensive H2C2O4 as the expansion agent to increase the surface utilization rate of expanded minerals to increase the adsorption capacity of REEs significantly. In this paper, the KH intercalation complex was prepared by the secondary substitution liquid phase intercalation method, and the power provided by the neutralization reaction of H2C2O4 and Na2CO3 pro moted the thermal decomposition of H2C2O4 between the Kaolinite layers to generate a large amount of gas at high temperature, thereby expanding the kaolinite to enhance the adsorption capacity of La(III), Y (III). The intercalated and expanded kaolinite was characterized by X- ray diffraction (XRD), Scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), Brunauer Emmet Teller (BET), Thermo gravimetric-Differential thermal analysis (TG-DTA), Zeta po tential to analyze its structure, morphology, and adsorption mechanism. In the batch adsorption study, the effects of pH, initial concentration and contact time on the adsorption were explored, and the kinetics and isotherm model were used to fit the experimental data. 2. Materials and methods 2.1. Materials and reagents La(NO3)3⋅6H2O (AR, 99%) and Y(NO3)3⋅6H2O (AR, 99.5%), dimethyl sulfoxide (DMSO, AR), ethanol (C2H5OH, AR, 99.7%) were purchased from Shanghai McLean Biochemical Co., Ltd.; Kaolinite, anhydrous oxalic acid (H2C2O4, AR ≥ 99.5%), anhydrous sodium car bonate (Na2CO3, AR ≥ 99.8%), and hydrochloric acid (HCI, 0.1 M) were purchased from Chengdu Kelong Chemical Co., Ltd. 2.2. Preparation of oxalic acid intercalated and expanded kaolinite Preparation of precursor Kaol-DMSO (KD): 10 g of kaolinite was added to a mixture of 100 mL DMSO and 9 mL water (9%) under stirring magnetically at 80 ◦ C for 24 h, and filter the milky white viscous solu tion with ethanol. The filter cake was dried in an oven at 60 ◦ C for 12 h to obtain white powder KD. Preparation of Kaol-H2C2O4 (KH) intercalation complex: 1.5 g of precursor KD was added to 25 mL oxalic acid solution (2 mol/L) under stirring magnetically at room temperature for 24 h, and filter the milky white solution with ethanol. The filter cake was dried in an oven at 60 ◦ C for 12 h to obtain white powder KH. Preparation of expanded kaolinite (EKH): 0.5 g of KH, 0.4 g of Na2CO3 and 0.5 g of H2C2O4 were mixed and grinded in an agate mortar for 30 min, then placed in a ceramic crucible and calcined in a muffle furnace at 400 ◦ C for 30 min (heating rate was 5 ◦ C/min), cool to room temperature to obtain EKH. The experimental flow diagram of modifi cation kaolinite was shown as Fig. 1: 2.3. Characterization The prepared samples were characterized by X-ray diffraction (XRD, DX-2700, China) using Cu-Kα radiation (λ = 1.5406 Å) operating at 40 kV and 30 mA. The patterns were measured in the 2θ range of 5–80◦ at the scanning rate of 0.07◦ /s. A scanning electron microscope (SEM, ZEISS Gemini 300, Germany) was used to observe the microstructure and morphology of the samples equipped with an EDS elemental composition analyzer. Simultaneous thermal analyzer (STA409 PC, Germany) was used to analyze the physical and chemical changes of samples. The samples were heated from room temperature to 800 ◦ C at a heating rate of 10 ◦ C/min. The specific surface area and pore size dis tribution were measured by nitrogen adsorption/desorption (Micro meritics ASAP 2460, American). Before each measurement, sample was degassed in the facility over 6 h at 200 ◦ C. The specific surface area of samples was calculated by Brunauer Emmet Teller (BET) method. Fourier transform infrared spectroscopy (FTIR) was collected on a Nicolet iS5 spectrometer (Thermo Scientific, American), which were recorded between 4000 and 400 cm− 1 with 32 scans per spectrum at a resolution of 4 cm− 1 . And samples were prepared as KBr pellets (1 mg of sample was mixed with 200 mg of KBr). Zeta potential of samples was tested by Zetasizer Nano ZS90 (Malvern, England). 2.4. Batch adsorption studies The stock solution of La(III) and Y(III) with a concentration of 1000 mg/L was prepared by dissolving an appropriate amount of La (NO3)3⋅6H2O and Y(NO3)3⋅6H2O in ultrapure water. The different con centrations of La(III) and Y(III) for adsorption study were all prepared by dilution of the stock solution. 25 mg of adsorbent was added to 25 mL of solutions with known concentration, and placed in a constant temper ature water bath for static adsorption at 30 ◦ C, took the supernatant after reaching the adsorption equilibrium, and measured the concentration of La(III) and Y(III) solutions by ICP-OES (Thermo ICAP 6000, American). Adsorption isotherm studies were conducted with concentrations ranging from 20 mg/L to 500 mg/L. The adsorption kinetics study was W. Dou et al.
- 3. Applied Clay Science 229 (2022) 106693 3 performed with 300 mg/L solution in the range of 20–1440 min contact time. The initial solution pH (1–6) was adjusted with 0.1 mol/L HCl solution and the influence of pH on the adsorption performance was studied in 300 mg/L solution. The adsorption capacity was calculated by the following equation: Qe = (C0 − Ce) × V m (1) where Qe is the equilibrium adsorption capacity of La(III) and Y(III) (mg/g), C0 and Ce are the initial and adsorption equilibrium concen trations of La(III) and Y(III) respectively (mg/L), V is the volume of the rare earth solution (mL), and m is the weight of the adsorbent (g). 3. Results and discussion 3.1. XRD analysis The intercalation effect of kaolinite could be characterized by interlayer space and intercalation rate (Xue et al., 2016). Changes in interlayer space indicated whether organic molecules were intercalated between the kaolinite layers. The interlayer domain (along the C-axis direction) expanded and increased after organic molecules were inserted into the interlayer domain of kaolinite, and the d001-value of XRD could directly reflect this change. According to the Bragg formula (2dsinθ = nλ), when the d-value of the interlayer space increased, the 2-Theta (degree) decreased accordingly. As shown in Fig. 2a, the diffraction at 2θ = 12◦ was the characteristic (001) reflection of the raw kaolinite, and its basic interlayer space was 0.717 nm. The base reflection intensity of the kaolinite was significantly weakened after the intercalation of DMSO and H2C2O4, the (001) reflection shifted to a lower diffraction (2θ = 7◦ ), and new reflections with d-values of 1.078 nm and 1.112 nm appeared respectively. The diffraction peaks were symmetrical and sharp. This indicated that DMSO and H2C2O4 successfully inserted into the kaolinite interlayer and the kaolinite interlayer space increased significantly after intercalation. The interlayer space could qualitatively evaluate the intercalation effect, but the intercalation rate could quantitatively reflect the intercalation situation. The intercalation rates of KD and KH intercalation complex were 89.55% and 89.08% respectively, which were calculated by the following formula: Fig. 1. The experimental flow diagram of modification kaolinite. Fig. 2. XRD patterns of (a) kaolinite intercalated complex and (b) expanded kaolinite. W. Dou et al.
- 4. Applied Clay Science 229 (2022) 106693 4 RI = IC (IC + Ik) (2) where IC is the newly emerged d001 diffraction peak intensity of the intercalated complex, and Ik is the diffraction peak intensity of the re sidual kaolinite. After H2C2O4 inserted into the interlayer of kaolinite, it was decomposed into CO and CO2 through high temperature calcination and the power provided by the neutralization reaction of H2C2O4 and Na2CO3 to achieve the purpose of expanding kaolinite. As shown in Fig. 2b, the XRD pattern of kaolinite was almost the same as that of raw kaolinite after direct calcination. The reflection caused by H2C2O4 intercalation disappeared (d = 1.112 nm) after calcination of KH intercalation complex alone, high temperature made the H2C2O4 ther mally decompose into gas between the kaolinite layers, but the char acteristic diffraction peak at 2θ = 12◦ was still obvious, indicating that the kaolinite was not expanded. The intensity of (001) reflection of the EKH obtained by calcination mixed with H2C2O4 and Na2CO3 was obviously weakened, indicating that the kaolinite layers had been suc cessfully expanded. 3.2. SEM and EDX analysis The SEM images of the raw kaolinite, the intercalated complex and the expanded kaolinite were shown in Fig. 3. The raw kaolinite showed a thicker, closely packed layered structure and partially exhibited a hexagonal shape (Fig. 3a) (Du et al., 2010). After the intercalation of DMSO and H2C2O4 (Fig. 3b, c), the layered structure remained, but it could be observed that the close-packed layers were stretched and the interlayer space expanded, which was corresponding with the XRD re sults. After the KH intercalation complex was calcined with H2C2O4 and Na2CO3, the morphology of kaolinite changed greatly, and the pore structure appeared obviously (Fig. 3d). The gas generated by the ther mal decomposition of H2C2O4 between the layers expanded the original ordered layered structure, at the same time doesn’t exfoliate it into a single-layer sheet, thus achieving the purpose of expanding kaolinite. This special morphology of pores and gaps was beneficial to the adsorption of rare earth metal cations. The chemical compositions on the surfaces of the raw kaolinite, the intercalated complex and the expanded kaolinite were further shown in Fig. 4. The EDX results showed that O, Si, and Al were the most abundant components in kaolinite. The element components of C and S appeared after the intercalation of DMSO and H2C2O4, indicating that they were successfully inserted between the kaolinite layers. The reduction of C element composition indicated that the interlayer H2C2O4 was decom posed into gas to achieve the purpose of expansion. 3.3. TG-DTA analysis Thermal analysis could reflect the compositional changes of mate rials at different temperatures and the reaction processes involved. Fig. 5 Fig. 3. The SEM images of (a) raw kaolinite, (b) Kaol-DMSO intercalation complex, (c) Kaol-H2C2O4 intercalation complex, (d) expanded kaolinite. W. Dou et al.
- 5. Applied Clay Science 229 (2022) 106693 5 showed the TG-DTA curves of Kaolinite, KD, KH and EKH respectively. In the TG curves of Kaolinite (Fig. 5a), there was an obvious mass loss of 10.7% between 450 ◦ C and 550 ◦ C, which was due to the dehydrox ylation of kaolinite. There were four endothermic peaks on the corre sponding DTA curve, which were the removal of free water between kaolinite layers (around 100 ◦ C), the removal of adsorbed water (200 ◦ C–250 ◦ C), the dehydroxylation (520 ◦ C) and the transformation of the kaolinite phase to metakaolinite (600 ◦ C–750 ◦ C) respectively (Ptáček et al., 2014). Two mass loss stages appeared in the TG curve of KD (Fig. 5b), which were caused by DMSO decomposition (6.3%, decomposition temperature around 189 ◦ C) and kaolinite dehydrox ylation (8.6%), corresponding to the endothermic peaks around 200 ◦ C and 520 ◦ C on the DTA curve. There were four mass loss stages in the TG curve of KH (Fig. 5c), which were the removal of interlayer free water (3.7%), the removal of adsorbed water (7.6%), the decomposition of oxalic acid (8.4%, decomposition temperature over 150 ◦ C) and the dehydroxylation (6.7%). The endothermic peaks around 100 ◦ C, 200 ◦ C–300 ◦ C and around 500 ◦ C on the DTA curve correspond to each other. Therefore, the temperature should be 150 ◦ C–450 ◦ C when mixing H2C2O4, Na2CO3 and KH for calcination. Two mass loss stages appeared in the TG curve of EKH (Fig. 5d), which were the residual H2C2O4 decomposition (6.7%) and kaolinite dehydroxylation (3.5%), corre sponding to the endothermic peak at 100 ◦ C -300 ◦ C on the DTA curve. 3.4. FTIR analysis The absorption bands in the high, medium and low wavenumber regions of the Fourier transform infrared spectrum could indicate the functional group structure and composition characteristics of the materials. As shown in Fig. 6a, in the FTIR spectrum of raw kaolinite, the band at 3716 cm− 1 was attributed to the OH stretching vibration of outer hydroxyl, and the band at 3620 cm− 1 belonged to the OH stretching vibration of inner hydroxyl. The bands at 3470 cm− 1 and 1634 cm− 1 were corresponded to the OH stretching and bending vi bration of adsorbed water respectively. The wide absorption bands near 930 cm− 1 might be the Si–O or AI–O stretching vibrations. The outer hydroxyl was located in the interlayer of kaolinite, and was sensitive to the intercalated organic molecules (Zhang et al., 2015). Therefore, the absorption bands weakened to 3702 cm− 1 and 3701 cm− 1 respectively after DMSO and H2C2O4 intercalated the interlayer. The inner hydroxyl was located in the interlayer structural unit of kaolinite (between Si–O tetrahedron and Al–O octahedron), which was not easily affected by external conditions. In the FTIR spectruma of KD and KH intercalation complex, the new bands at 3541 cm− 1 , 3497 cm− 1 and 3544 cm− 1 , 3504 cm− 1 indicated that DMSO and H2C2O4 formed new hydrogen bonds with kaolinite. The bands at 3021 cm− 1 , 2937 cm− 1 and 3023 cm− 1 , 2936 cm− 1 were assigned to the CH stretching vibration of DMSO and the OH stretching vibration of H2C2O4 respectively (Slaný et al., 2019). The bands at 1429 cm− 1 and 1317 cm− 1 were assigned to the CH3 asymmetric and sym metric deformations of DMSO. The bands at 1729 cm− 1 and 1696 cm− 1 belonged to the C– –O stretching vibration of H2C2O4, and the band at 1411 cm− 1 was assigned to the OH bending vibration of H2C2O4 mole cule. The absorption bands near 912 cm− 1 were due to the S– –O bond in the DMSO molecule. These absorption bands indicated that the DMSO and H2C2O4 were successfully intercalated the interlayer of kaolinite. In the FTIR spectrum of the EKH obtained after calcination, the charac teristic bands belonged to DMSO and H2C2O4 all disappeared, indicating Fig. 4. EDX spectra of (a) raw kaolinite, (b) Kaol-DMSO intercalation complex, (c) Kaol-H2C2O4 intercalation complex, (d) expanded kaolinite. W. Dou et al.
- 6. Applied Clay Science 229 (2022) 106693 6 that the H2C2O4 in the intercalated complex generated gas to expand the kaolinite. The new band at 2496 cm− 1 might be the associative hydrogen bond of the carboxyl group in the neutralization reaction residual H2C2O4. Therefore, the sample needed to be washed with water to remove excess H2C2O4. 3.5. BET analysis The specific surface area and pore structure could directly reflect the adsorption performance of the materials. Materials with large specific surface area and high porosity tended to display better adsorption per formance. The BET specific surface area and pore structure parameters of raw kaolinite (NK), KD, KH and EKH were listed in Table 1. After intercalation and expansion modification, the specific surface area increased from 15.60 m2 /g of NK to 23.59 m2 /g of EKH, and the total pore volume of pores increased from 0.0663 cm3 /g to 0.1453 cm3 /g. This indicated that the porosity of kaolinite was improved, which could provide more active sites for adsorption. The N2 adsorption-desorption Fig. 5. TG and DTA curves of (a) Kaolinite, (b) Kaol-DMSO intercalation complex, (c) Kaol-H2C2O4 intercalation complex and (d) expanded Kaol-H2C2O4. Fig. 6. (a) FTIR spectra of raw kaolinite, kaolinite intercalation complex and expanded kaolinite; (b) Zeta potential diagrams of NK, EKH, and Washed EKH. W. Dou et al.
- 7. Applied Clay Science 229 (2022) 106693 7 isotherms and pore size distributions of NK and EKH were shown in Fig. 7a and b. The isotherms of NK and EKH belonged to IV type iso therms, that was the pore structure type of mesoporous materials (average pore diameter 2–50 nm) (Yu et al., 2007), and both had obvious H3 hysteresis loops, illustrated that there were many slits- shaped pores in the structure. Compared with the pore size distribu tion of NK at 30 nm, the EKH showed a bimodal pore size distribution at 3 nm and 30 nm, indicated that it formed abundant mesopores, which was conducive to the subsequent adsorption of rare earth metal cations. 3.6. Zeta potential Zeta potential was the potential of the shear plane of colloid surface, which could reflect the charge on the surface of materials and its adsorption properties (Zhen et al., 2017). The NK, EKH, and washed EKH all appeared obviously electronegativity when dispersed in water (pH = 7.0), and the values of zeta potential were − 13.5 mV, − 38.7 mV, and − 23.0 mV respectively (Fig. 6b). The absolute values of zeta po tential of EKH and washed EKH were much higher than that of NK, indicated that EKH and washed EKH possessed much more negative surface charges than NK. The generation of negative charges on the NK surface was mainly due to the hydrolysis of surface and edge hydroxyl groups of Kaolinite. The EKH treated with the intercalation and expansion of DMSO and H2C2O4 generated more negative charges due to the formation of new hydrogen bonds and more space between the layers exposed after expansion, which was beneficial to the adsorption of rare earth metal cations. 3.7. Explanation of the mechanism of intercalation-expansion kaolinite The essence of Kaolinite intercalation reaction was the breaking of interlayer hydrogen bonds and the formation of new hydrogen bonds with organic molecules (Fig. 8). As proton acceptors, DMSO and H2C2O4 contained proton-accepting functional groups S– –O and C– –O, which formed hydrogen bonds with the outer hydroxyl of Kaolinite (S-O-HO-AI and C-O-HO-AI). Since only small molecules with strong polarity could directly insert into the Kaolinite layers, it was necessary to use DMSO to destroy the hydrogen bonds between the Kaolinite layers, weaken its cohesion energy, and then introduced H2C2O4 for substitution. The high temperature rapid calcination and the power provided by the neutrali zation reaction of H2C2O4 and Na2CO3 promoted the H2C2O4 between the KH intercalation complex interlayer to generate a large amount of gas, thereby effectively expanded the interlayer space of Kaolinite. The reactions involved in the calcination process were as follows: H2C2O4 + Na2CO3→Na2C2O4 + CO2 + H2O (3) H2C2O4→CO + CO2 + H2O (4) 3.8. Batch adsorption studies 3.8.1. Effect of initial pH The initial pH of La(III) and Y(III) solutions had a great influence on the adsorption of rare earth metal cations on the EKH surface and the ionization degree of the adsorbate (Zhu et al., 2015). In order to avoid the hydrolysis of La(III) and Y(III) to form the precipitation of La(OH)3 and Y(OH)3, the effect of pH on the adsorption capacity in the range of 1–6 was discussed. As shown in Fig. 9a, the adsorption capacity increased with the increasing of pH, because a large amount of H+ competed with La(III) and Y(III) for binding to the negative charge on the EKH surface under acidic conditions. At pH = 5, the maximum adsorption capacity was 90.4 mg/g (La) and 66.5 mg/g (Y), so the pH = 5 was selected as the initial pH of La(III) and Y(III) solutions for sub sequent adsorption experiments. 3.8.2. Morphology of EKH after adsorption It could be seen from Fig. 9b that La(III) and Y(III) were adsorbed on the surface of EKH and in the pore structure created by expansion, probably due to the enhanced surface complexation. The appearance of Y elemental component in EDX further indicated that Y was successfully adsorbed on EKH. 3.8.3. Adsorption kinetics The adsorption kinetics study could evaluate the adsorption effi ciency of the adsorbent and the controlling factors of the adsorption process (Ryu et al., 2021a). Therefore, it was necessary to discuss the effect of contact time on the adsorption capacity. The adsorption ki netics experiments were conducted with 25 mg EKH in 25 mL La(III) and Y(III) solutions with an initial concentration of 300 mg/L, and the experimental conditions were pH = 5 and T = 303 K. Fig. 10a,b showed the effect of contact time range of 20–1440 min on La(III) and Y(III) adsorption. The adsorption capacity increased rapidly within 4 h, and Table 1 Specific surface area and pore structure parameters of the samples. Sample BET surface area (m2 /g) Pore volume (cm3 /g STP) Average pore diameter (nm) NK 15.60 ± 0.05 0.0663 32.04 Kaol- DMSO 17.55 ± 0.20 0.1132 28.16 Kaol- H2C2O4 20.64 ± 0.08 0.1406 19.20 EKH 23.59 ± 0.14 0.1453 17.01 Fig. 7. N2 adsorption-desorption isotherms and pore size distributions of (a) raw kaolinite and (b) expanded kaolinite. W. Dou et al.
- 8. Applied Clay Science 229 (2022) 106693 8 the adsorption capacity reached 83% of the saturated adsorption ca pacity at 4 h. After 4 h, the growth rate of the adsorption capacity slowed down and finally reached the equilibrium adsorption. The adsorption kinetics data of La(III) and Y(III) on EKH were fitted by pseudo-first-order model and pseudo-second-order model, and the relevant fitting parameters obtained were shown in Table 2. The pseudo- first-order and pseudo-second-order kinetics equations were as follows: Qt = Qe ( 1 − e1/K1t ) (5) t Qt = 1 K2Qe 2 + t Qe (6) where Qt is the adsorption capacity at time t (h) (mg/g), Qe is the equilibrium adsorption capacity (mg/g), K1 and K2 are the Pseudo-first- order constants (min− 1 ) and Pseudo-second-order constants (g⋅mg− 1 ⋅min− 1 ) respectively. The fitted data showed that the correlation coefficient of the pseudo- second-order model (R2 = 0.99) was higher than that of the pseudo-first- order model (R2 = 0.98), and the calculated Qe value was closer to the experimental Qe value, so the pseudo-second-order model was able to better describe the adsorption kinetics of EKH. 3.8.4. Adsorption isotherm The adsorption isotherm represented the concentration relationship of the adsorbate in the liquid-solid two-phase at a certain temperature, and could reflect the macroscopic characteristics such as adsorption capacity, adsorption strength and adsorption state (Ma et al., 2021; Li et al., 2022). Fig. 10c, d showed the adsorption capacity of EKH in different initial La(III) and Y(III) concentration range from 20 mg/L to 500 mg/L. The adsorption capacity increased with the initial concen tration, which because the larger concentration gradient difference provided more impetus for the diffusion of La(III) and Y(III) into EKH. When the concentration reached 400 mg/L, the adsorption active sites in EKH were gradually saturated, and the adsorption capacity reached 101.5 mg/g (La) and 78.9 mg/g (Y) respectively. The adsorption isotherm data of La(III) and Y(III) on EKH were fitted by the Langmuir model and the Freundlich model, and the relevant fitting parameters obtained were shown in Table 3. The Langmuir and Freundlich equations were as follows: Qe = QmKLCe 1 + KLCe (7) Qe = KFCe 1/n (8) where Qe is the equilibrium adsorption capacity (mg/g), Qm is the maximum adsorption capacity (mg/g), Ce is the equilibrium concen trations of La (III) and Y(III) (mg/L), KL and KF are the Langmuir con stants (L/mg) and Freundlich constants (L/g) respectively, n is the dimensionless constant. The fitted data showed that the correlation coefficient (R2 = 0.99) of the Langmuir model was higher than that of the Freundlich model (R2 = 0.98), so the Langmuir model could better describe the adsorption process between La(III) and Y(III) on the EKH, which was homogeneous monolayer adsorption rather than heterogeneous multilayer adsorption. 3.9. Comparison with other adsorbents The adsorption capacity of EKH were compared with other Fig. 8. The mechanism diagram of intercalation-expansion Kaolinite. Fig. 9. (a) Effect of initial pH on the adsorption capacity of La (III) and Y(III) on EKH; (b) The SEM image and EDX spectrum of EKH after adsorption. W. Dou et al.
- 9. Applied Clay Science 229 (2022) 106693 9 adsorbents reported in literature as shown in Table 4. At present, the adsorbents used to treat industrial wastewater mainly included metal organic framework materials (MOFs), nanomaterials (magnetism Fe0 ), hydrogel materials, mesoporous and porous materials (mesoporous SiO2), etc. Although these materials possessed high stability, specific surface area and adjustable pore size, there were problems such as expensive, complicated operation and difficult replacement of adsor bents. Montmorillonites, smectite, halloysite and other clay minerals was also used to adsorb REEs, however the adsorption capacity was not high enough (Bradbury and Baeyens, 2002; Coppin et al., 2002). In this study, the raw materials of EKH were inexpensive and the preparation method was simple, meanwhile the adsorption performance of EKH Fig. 10. Effect of contact time (a), (b) and initial concentration (c), (d) on the adsorption capacity and the adsorption kinetics and isotherm fitted curves of La and Y. Table 2 Pseudo-first-order, Pseudo-second-order model parameters. REE Pseudo-first-order model Pseudo-second-order model Qe(mg/g) K1(min− 1 ) R2 RMSE* Qe(mg/g) K2(g⋅mg− 1 ⋅min− 1 ) R2 RMSE* La 87.1904 0.5165 0.9882 9.4031 98.2143 6.27 × 10− 3 0.9946 4.9503 Y 63.6839 0.5174 0.9807 8.3230 71.9281 8.65 × 10− 3 0.9932 3.3781 * RMSE: Root mean square error. Table 3 Langmuir, Freundlich isotherm parameters. REE Langmuir Freundlich Qm(mg/g) KL(L/mg) R2 RMSE* n KF(L/g) R2 RMSE* La 140.5727 4.82 × 10− 3 0.9956 6.1690 1.8018 4.5804 0.9755 28.8889 Y 119.5347 3.50 × 10− 3 0.9980 1.9488 1.5731 1.5770 0.9783 17.5878 * RMSE: Root mean square error. W. Dou et al.
- 10. Applied Clay Science 229 (2022) 106693 10 enhanced significantly. Thus, EKH was expected to be applied in the field of environmental remediation. Compared with traditional methods of modified kaolinite (such as heat treatment, acid activation, hydroxyl cation pillared, etc.), the experimental equipment involved in this method was simple and the experimental conditions were easy to control. Different from the pre vious reaction that only intercalated (Kaol-KAc, Kaol-NMF) or further utilized intercalators (such as Fenton reaction, violent reaction of KCIO3 and urea) to expand and exfoliate kaolinite, this study proposed for the first time to insert H2C2O4 between kaolinite layers and skillfully use gases generated from the neutralization reaction of Na2CO3 and H2C2O4 and thermal decomposition of H2C2O4 to expand without completely exfoliating kaolinite. 4. Conclusion In this study, a new method was proposed to enlarge the interlayer space of kaolinite and increase its adsorption active sites. The H2C2O4 was inserted into the kaolinite layers and the KH intercalation complex was rapidly calcined with H2C2O4 and Na2CO3 to obtain the EKH. XRD showed the interlayer space of KD and KH increased significantly. SEM characterized the interlayer expansion of KD, KH, EKH and the forma tion of pores. TG-DTA determined the calcination temperature of KH by analyzing the decomposition of materials at each temperature stage. FTIR demonstrated the successful intercalation of DMSO and H2C2O4 by the red shift of hydrogen bonds and the characteristic functional groups. BET indicated the specific surface area and pore volume of EKH were significant increased to increase the adsorption active sites. Zeta po tential showed that KH and EKH possessed more negative charges on the surface to improve the adsorption performance of La(III) and Y(III). The formation of kaolinite intercalation complex was due to the formation of hydrogen bonds between DMSO and H2C2O4 with the kaolinite Al–O octahedral hydroxyl groups, while calcination promoted the decompo sition of H2C2O4 between kaolinite layers to generate a large amount of CO and CO2, and the EKH exposed more inner layers surface and form an intersecting surface, which was beneficial to improve the adsorption performance. Batch adsorption experiments showed that the adsorption capacity of La(III) and Y(III) on the EKH after washing were as high as 101.5 mg/g and 78.9 mg/g respectively, which were 6 times and 8 times higher than unmodified kaolinite. The pseudo-second-order kinetics and Langmuir model could describe the adsorption kinetics and isotherm well. Therefore, the H2C2O4 intercalated and expanded kaolinite re ported in this paper had great application prospects in the treatment of La(III) and Y(III) in industrial wastewater. The EKH with excellent adsorption performance was obtained by the reaction of intercalators on the basis of H2C2O4 intercalation, which overcame the problems of instability and poor adsorption performance of intercalation complex. In the future, more research could focus on the expansion and exfoliation of other clay minerals driven by gas generated by intercalators. CRediT authorship contribution statement Weisha Dou: Conceptualization, Writing – original draft. Zhaoping Deng: Writing – review & editing. Jianping Fan: Investigation. Quanzhi Lin: Data curation. Yuhang Wu: Supervision. Yanlin Ma: Visualization. Zepeng Li: Supervision. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Data availability Data will be made available on request. References Aboudi Mana, S.C., Hanafiah, M.M., Chowdhury, A.J.K., 2017. 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Adsorbent Adsorbate Adsorption capacity (mg/g) Reference MIL-101-PMIDA Lu(III), Y (III) 63.4, 25.3 Ryu et al., 2021b EDTA-β-cyclodextrin La(III) 47.78 Zhao et al., 2016 Acid-activated kaolinite Cu(II) 42.01 Chai et al., 2020 Fe3O4(CA) NPs La(III), Y (III) 35.8 Ashour et al., 2017 Magnetic silica nanocomposite (P507) La(III) 55.9 Wu et al., 2013 halloysite, kaolinite Eu(III) 4.2, 1.6 Yang et al., 2019; Zhou et al., 2022 Acid-activated montmorillonites La(III), Y (III) 23.6, 15.8 Fang et al., 2021 EKU MB 37 Ding et al., 2021 H2O2-DK Pb(II) 9 Liu et al., 2019 EKH La(III), Y (III) 101.5, 78.9 This work W. Dou et al.
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