Enhanced adsorptive performance of Cr(VI) onto layered double hydroxide-bentonite composite: Isotherm, kinetic and thermodynamic studies

Functionalization of layered double hydroxides on bentonite for cesium and iodine retention in high-level radioactive waste disposal

Bentonite is regarded as an adequate buffer material in deep geological repositories and its swelling properties serve to prevent the penetration of groundwater into the repository and to minimize the release of radionuclides. However, bentonite is rarely effective in removing anionic radionuclides due to its permanent negative surface charge. The aim of this study was to enhance the anion removal ability of bentonite by incorporating layered double hydroxides (LDH) with a high anion exchange capacity. The functionalization of CuAlBi LDH on bentonite (CuAlBi LDH@Ben) revealed an effective approach for removing both cesium and iodine from aqueous solutions. The peak shift of the Si-O stretching band to higher frequencies, the vertically oriented platelet morphology, and the increase in specific surface area provide confirmation that LDH platelets grow on the surface of montmorillonite. The CuAlBi LDH@Ben demonstrates enhanced anion retention performance in bentonite without impacting its retention behavior toward cations, as evidenced by Kd values of 1943.1 mL/g for Cs+, 442.4 mL/g for I-, and 650.7 mL/g for IO3-, respectively.

Effective nitrate removal with high N2 selectivity by active-site-rich particle electrode of bentonite-based Cu-Fe LDH composite

A novel bentonite-based Cu-Fe layered double hydroxide (LDH) composite particle electrode (CuFe-LDH/BT) was fabricated and used as the catalyst to remove nitrate-nitrogen (NO3--N) in three-dimensional electrochemical (3D/E) system. The results showed that the prepared CuFe-LDH/BT exhibited the highest catalytic activity when the molar ratio of copper to iron was 3:1, the dosage of bentonite (BT) was 1 g, liquid-phase synthesis pH was 10, and liquid-phase synthesis temperature was 40 °C. The prepared composite particle electrode was characterized by X-Ray Diffraction (XRD), Scanning electron microscopy (SEM), Brunauer-Emmett-Teller method (BET), Electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS). The characterization results indicated that LDH structure was successfully formed in CuFe-LDH/BT, and CuFe-LDH/BT had obvious layered structure, high specific surface area and excellent conductivity. Under the reaction conditions of CuFe-LDH/BT dosage of 3 g/L, current density of 8 mA/cm2 and initial pH of NO3--N solution of 7, in the range of NO3--N concentration of 50∼200 mg/L, the maximum removal efficiency of NO3--N could reach 100% at reaction time of 240 min, and the maximum N2 selectivity was 83.41%. The recycling test showed that CuFe-LDH/BT maintained high activity after 3 reuses. The possible reaction mechanism of NO3--N removal in the 3D/E system catalyzed by CuFe-LDH/BT was explored. In summary, the 3D/E system catalyzed by CuFe-LDH/BT can achieve the effective removal of NO3--N in water body.

Recent advances of application of bentonite-based composites in the environmental remediation

Bentonite-based composites have been widely utilized in the removal of various pollutants due to low cost, environmentally friendly, ease-to-operate, whereas the recent advances concerning the application of bentonite-based composites in environmental remediation were not available. Herein, the modification (i.e., acid/alkaline washing, thermal treatment and hybrids) of bentonite was firstly reviewed; Then the recent advances of adsorption of environmental concomitants (e.g., organic (dyes, microplastics, phenolic and other organics) and inorganic pollutants (heavy metals, radionuclides and other inorganic pollutants)) on various bentonite-based composites were summarized in details. Meanwhile, the effect of environmental factors and interaction mechanism between bentonite-based composites and contaminants were also investigated. Finally, the conclusions and prospective of bentonite-based composites in the environmental remediation were proposed. It is demonstrated that various bentonite-based composites exhibited the high adsorption/degradation capacity towards environmental pollutants under the specific conditions. The interaction mechanism involved the mineralization, physical/chemical adsorption, co-precipitation and complexation. This review highlights the effect of different functionalization of bentonite-based composites on their adsorption capacity and interaction mechanism, which is expected to be helpful to environmental scientists for applying bentonite-based composites into practical environmental remediation.

Cellulose nanocrystals intercalated clay biocomposite for rapid Cr(VI) removal

The application of bentonite (Bt) as an adsorbent for heavy metals has been limited due to its hydrophobicity and insufficient surface area. Herein, we present cellulose nanocrystal (CNC) modified Bt composite (CNC@Bt) with enhanced efficiency for Cr(VI) removal. CNC@Bt exhibited an increased specific surface area and a porous structure, while maintaining the original crystal structure of Bt. This was achieved through a synergistic function of ion exchange, hydrogen bonding, electrostatic interactions, and steric hindrance. The adsorption of Cr(VI) by CNC@Bt followed the pseudo-second-order kinetic and Langmuir isotherm adsorption model. Moreover, the process was endothermic and spontaneous. At an initial Cr(VI) concentration of 20 mg/L and pH = 4.0, 10 g/L CNC@Bt achieved a removal rate of 92.7%, and the adsorption capacity was 1.85 mg/g, significantly higher than bare Bt (37.9% and 0.76 mg/g). The removal efficiency remained consistently above 80% over a wide pH range, indicating the potential practical applicability of CNC@Bt. With its fast adsorption rate, pH adaptability, and stable performance, CNC@Bt presents promising prospects for the rapid treatment of Cr-contaminated wastewater.

Efficient removal of Cr (VI) from aqueous solution using memory effect property of layered double hydroxide material

Treating wastewater containing pollutants with layered double hydroxide (LDH) material attracts excellent interest. LDH materials are known by the memory effect property, which leads to the reconstruction of the LDH structure after its calcination and rehydration. In this study, LDH material was prepared, calcined, and then rehydrated in an aqueous Cr(VI) solution. XRD, FTIR, and SEM-EDS analysis confirm the successful reconstruction of LDH-loading chromium on its surface and layered space. Response surface methodology (RSM) results showed that LDH mass, contact time, and chromium concentration are the main factors controlling the removal of Cr(VI). The heterogeneous sorption of chromium was described by fitting the equilibrium data to the Freundlich model. Analytical techniques, thermodynamic data, activation, and adsorption energies confirm that the removal process of Cr(VI) is endothermic, spontaneous, and physical nature. LDH exhibits good reusability performance with only a 7% reduction of initial adsorption capacity after five cycles of the calcination-rehydration process. These results show that the memory effect of LDH is helpful for the intercalation and the removal of emergent pollutants, especially for wastewater treatment.

Structured LDH/Bentonite Composites for Chromium Removal and Recovery from Aqueous Solutions

This study focuses on chromium removal through adsorption and ion exchange using structured calcined layered double hydroxide (LDH) (MgAl)-bentonite composites. Firstly, the powders were structured into granulates to study the effect on Cr sorption kinetics to circumvent the limitations of working with powders in real-life applications. Secondly, the regeneration of the structured composites was optimized to enable multi-cycling operation, which is the key for their applicability beyond laboratory scale. Firstly, the LDH/bentonite ratio was optimized to obtain the best performance for the removal of Cr3+ and Cr6+ species. In powder form, the calcined adsorbent containing 80 wt% LDH and 20 wt% bentonite performed best with an adsorption capacity of 48 and 40 mg/g for Cr3+ and Cr6+, respectively. The desorption was optimized by studying the effect of the NaCl concentration and pH, with a 2 M NaCl solution without pH modification being optimal. The kinetic data of the adsorption and desorption steps were modelled, revealing a pseudo-second order model for both. This was also demonstrated using XRD and Raman measurements after the Cr3+ and Cr6+ adsorption tests, indicating successful uptake and revealing the adsorption mechanism. Finally, five consecutive adsorption-desorption cycles were performed, each showing nearly 100% adsorption and desorption.

Application of functionalized layered double hydroxides for heavy metal removal: A review

Layered double hydroxides (LDHs) are ionic laminar composites composed of positively charged brucite-like layers with an interlayered region containing charged compensating anions and solvation molecules. Such functional LDHs materials present a strong potential for heavy metal treatment especially for wastewater and soil, due to the large surface area and layered structure. This paper started with the background of techniques for heavy metals treatment and then discussed the potential environmental toxic effects, feasibility, stability of LDH composites. The preparation strategies of LDHs composites, and their application were summarized, followed by main mechanisms involving chelation, complexation, surface precipitation, ion exchange. This work also presented the potential environmental toxic effects, feasibility, stability of LDHs composites, reuse of waste liquid and the ratio adjustment of M²⁺ and N³⁺ for LDHs synthesis. While most efforts focused on improving the absorption capacity of LDHs by composites construction, ignoring the toxicity effects and detailed mechanism investigation. Based on a thorough review of the latest development, the challenges and perspectives would be proposed, offering promising insights on environmental purification via LDHs based materials.

Insight into ANN and RSM Models’ Predictive Performance for Mechanistic Aspects of Cr(VI) Uptake by Layered Double Hydroxide Nanocomposites from Water

Mathematical predictive models are vital tools for understanding of pollutant uptake during adsorptive water and wastewater treatment processes. In this study, applications of CoAl-LDH and its bentonite-CoAl intercalated LDH (bentonite-CoAl-LDH) for uptake of Cr(VI) from water were modeled using response surface methodology (RSM) and artificial neural network (ANN), and their performance for predicting equilibrium, thermodynamics and kinetics of the Cr(VI) uptake were assessed and compared based on coefficient of determination (R2) and root mean square error (RMSE). The uptake of Cr(VI) fits well quartic RSM polynomial models and ANN models based on Levenberg–Marquardt algorithms (ANN-LMA). Both models predicted a better fit for the Langmuir model compared to the Freundlich model for the Cr(VI) uptake. The predicted non-linear Langmuir model contestant (KL) values, for both the RSM and ANN-LMA models yielded better ΔG°, ΔH and ΔS predictions which supported the actual feasible, spontaneous and greater order of reaction as well as exothermic nature of Cr(VI) uptake onto the tested adsorbents. Employing the linear Langmuir model KL values dwindles the thermodynamic parameter predictions, especially for the RSM models. The excellent kinetic parameter predictions for the ANN-LMA models further indicate a mainly pseudo-second-order process, thus confirming the predominant chemisorption mechanism as established by the Cr(VI) speciation and surface charges for the Cr(VI) uptake by both CoAl-LDH and bentonite-CoAl-LDH. The ANN-LMA models showed consistent and insignificant decline in their predictions under different mechanistic studies carried out compared to the RSM models. This study demonstrates the high potential reliability of ANN-LMA models in capturing Cr(VI) adsorption data for LDHs nanocomposite heavy metal uptake in water and wastewater treatment.

Sewage Sludge ZnCl2-Activated Carbon Intercalated MgFe-LDH Nanocomposites: Insight of the Sorption Mechanism of Improved Removal of Phenol from Water

Keywords: sludge-activated carbon; MgFe layered double hydroxide; nanocomposite materials; phenol aqueous uptake; mechanistic studies; reusability performance.