Applications of aluminium (Al) salt or lanthanum (La) modified bentonite (LMB) have become popular methodologies for immobilizing phosphorus (P) in eutrophic lakes. The presence of humic substances, has been shown to inhibit this form of treatment due to the complexation with La/Al. However, the effects of other dissolved organic matter (DOM), especially that derived from phytoplankton (the dominant source in eutrophic lakes) are unknown. In this study, the interaction with La/Al of Suwannee River Standard Humic Acid Standard II (SRHA) and algae-derived DOM (ADOM) were investigated and compared. Differed to SRHA which was dominated by polyphenol-like component (76.8%, C1-SRHA), majority in ADOM were protein-like substance, including 41.9% tryptophan-like component (C2-ADOM) and 21.0% tyrosine-like component (C3-ADOM). Two reactions of complexation and coprecipitation were observed between SRHA/ADOM and La/Al. Complexation dominated at low metal inputs less than 10 μM and coprecipitation was the main reaction at higher metal inputs. For ADOM, the tryptophan-like component (C2-ADOM) was the important component to react with metal. The reaction rate for C2-ADOM with La were about two-third of that for C1-SRHA, indicating that the influence of C2-ADOM was significant during the P immobilization by La/Al-based treatment in eutrophic lakes. The P removal data in the presence of ADOM confirmed the significant inhibition of ADOM. In addition, based on the composition of coprecipitates and relatively biodegradable character of tryptophan-like substances (C2-ADOM), the coprecipitation of ADOM was assumed to reduce the stability of precipitated P in eutrophic lakes. The release of P from the potential biodegradation of the coprecipitates and thus the possible decline of the performance of P immobilization by La/Al-based treatments is an important work in the future.

Heavy metal pollution is a serious environmental problem globally, particularly in mines and tailings ponds. In this study, based on laboratory and field tests, the migration of heavy metal contaminants in a tailings pond and the retention behavior of a compacted bentonite engineered barrier system on the heavy metal contaminants were analyzed by a numerical simulation. The results demonstrate that the hydraulic conductivity of compacted bentonite is lower than that of the tailings from the laboratory tests. The hydraulic conductivity of the tailings sand decreased with an increase in the dry density and increased with an increase in the concentration of the chemical solution, which could be attributed to the large amounts of fine-grained soil contained in the tailings, according to the grain size distribution test. The hydraulic conductivity of the tailings from the engineering geological survey was between 2.0 × 10−6 and 9.0 × 10−5 m/s, and followed the order: tail coarse sand ＞ tail silty sand ＞ tail medium sand ＞ tail fine silt. The numerical simulation of the seepage could satisfactorily describe the actual working condition of the tailings dam. With the groundwater seepage, the migration range of the heavy metal contaminant in the researched tailings pond reached a maximum of 45 m for 5 years. The retention efficiencies of the 0.2 m engineered barrier against the heavy metal contaminant for 15 and 30 years were 45.4% and 57.2%, respectively. Moreover, the retention efficiency would exceed 87% when the engineered barrier thickness is increased to 0.5 m. The results of model validation show that the calculated results are in good agreement with the measured ones. These findings can provide effective ideas for the prevention and control of environmental pollution in mines and tailings ponds.

Increasing applications of rare earth elements (REEs) and improving technologies have led to increased demand. Because of their limited availability and depletion of most resources, the recovery of these elements from waste has become important. The use of cost-effective materials for this purpose and the high value that can potentially be recovered would be beneficial and attractive to many industries using REEs. In this study, natural zeolite and bentonite were used in batch studies to recover REEs (La, Y, Lu, Sm, Pr, Tm, Ce, Nd, Yb, Gd, Eu, Er, Ho, Dy, and Sc) from aqueous solutions. The effect of adsorbent dosage, pH, concentration, contact time, and competing ions on recovery was investigated. Desorption studies were conducted using ammonium sulphate. Adsorption onto zeolite was found to increase with pH, whereas uniform adsorption was observed for bentonite, except at pH 2 (16% less efficiency). The pH values of 6.2 and 3.2 were selected as the optimum for zeolite and bentonite, respectively. For zeolite, the average adsorption efficiencies for REEs at 0.5, 1.0, 2.0, 5.0, and 10 mg L⁻¹ were found to be 91, 96, 89, 40, and 20% respectively but, > 98% adsorption efficiencies were achieved with bentonite at all concentrations. The zeolite and bentonite adsorption data were better described by Langmuir though, for bentonite, the coefficients of determination (R² values) for the Freundlich and Dubinin-Radushkevich isotherm models were also significant. There was no significant difference (p > 0.05) on the adsorption of the elements in the presence of competing ions. Bentonite proved to perform better, most likely as a result of its higher surface area. Generally, the good adsorption performance of both adsorbents in their natural forms makes them an attractive and potential cheap option for the recovery of REEs from wastewaters.

Efficacy of activated bentonite, activated carbon, and natural phosphate under experimental conditions was tested as low-cost adsorbents for spiramycin antibiotic removal from aqueous solution. Equilibrium kinetic and isotherm adsorption process are well described by pseudo-second order and Langmuir isotherm models for activated bentonite and activated carbon, while natural phosphate follows pseudo-first order and Freundlich models, respectively. Obtained results revealed that activated bentonite has the highest adsorption capacity (260.3 mg/g) as compared to activated carbon (80.3 mg/g) and natural phosphate (1.7 mg/g). The adsorption capacity decreases for all adsorbents in the presence of NaCl. The adsorption processes are facilitated in the alkaline pH range for activated bentonite and activated carbon, whereas, for natural phosphate, the acidic pH range is favorable. They are involving ion exchange and hydrogen bond mechanisms as well as Van der Waals forces and also π interactions for activated carbon. Thermodynamic calculation shows that spiramycin adsorption is endothermic and spontaneous on all adsorbents. The activated bentonite reusability is more efficient by more than 95% in two-step desorption using NaOH and HCl eluents compared to activated carbon. Thus, activated bentonite is a promising adsorbent for spiramycin removal from aqueous solution.

Robust and simple composite films for the removal of methyl orange (MO) and Cr(VI) have been prepared by combining chitosan, saponin, and bentonite at a specific ratio. There are several composite films (chitosan-saponin-bentonite (CSB)) prepared; among them, the composite films CSB₂:₃ and CSB₁:₁ have the highest removal efficiency toward MO and Cr(VI) where the maximum removal is 70.4% (pH 4.80) and 92.3% (pH 5.30), respectively. It was found that different types of adsorbate have different thermodynamic properties of the adsorption process; the adsorption of MO onto CSB₂:₃, chitosan, and acid-activated bentonite (AAB) proceeded endothermically, while the adsorption of Cr(VI) onto CSB₁:₁, chitosan, and AAB proceeded exothermically. The parameters of the adsorption were modeled by using isotherm and kinetic equations. The models of Langmuir, Freundlich, Redlich-Peterson, Sips, and Toth were used for fitting the adsorption isotherm data at a temperature of 30, 45, and 60 °C; all of the isotherm models could represent the data well. The result indicates that CSB₂:₃ has the highest adsorption capacity toward MO with qₘ of 360.90 mg g⁻¹ at 60 °C; meanwhile, CSB₁:₁ has the highest adsorption capacity toward Cr(VI) with qₘ 641.99 mg g⁻¹ at 30 °C. The pseudo-second-order model could represent the adsorption kinetics data better than the pseudo-first-order equation. The adsorption mechanism was proposed, and the thermodynamic properties of the adsorption were also studied.

In this study, organoclay and clay-biopolymer composites were evaluated for the adsorption process of an anionic red dye, Allura Red (AR), in aqueous solution. For this purpose, the cationic exchange capacity (CEC) of a natural bentonite was calculated, and it was modified with the cationic surfactant hexadecyltrimethylammonium bromide (OB). Furthermore, a commercial montmorillonite modified with dimethyldialkyl ammonium (OM) was also employed. These organo-modified clays were used for the synthesis of two series of composites, with alginate as the polymer matrix, and were identified as OBC and OMC, respectively; composites were obtained in the wet (W) and dry (D) states. The adsorbent materials were characterized by means of infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and dispersive energy spectroscopy (EDS). Afterwards, kinetics and isotherms studies were performed in batch systems, with dye solutions of different concentrations, without pH adjustment. Some parameters were evaluated, such as the variation of the pH value of the solution and the concentration of the dye with the organoclays and composites. Chemisorption was considered as the main mechanism that follows the adsorption processes of AR. Results demonstrated that the pseudo-second order was the kinetics model that best described the adsorption process of the AR dye, with both, hydrated and dry composites. Finally, the Freundlich and the Langmuir–Freundlich isotherms were the best models that described the hydrated and dry composites behavior, respectively.

This study analyzed the performance of organophilic clays obtained from the chemical modification of sodium bentonite clay when applied to the adsorption of herbicide 2,4-dichlorophenoxyacetic acid (2,4-D). Kinetic curves and equilibrium isotherms were obtained in order to determine time and adsorption capacity of the material, as well as understand the mechanisms involved in this phenomenon. The results showed that the most predictive kinetic model for experimental data was of pseudo-second order (R² > 0.98), and that external mass transfer is the dominant factor in the time of operation. Isotherms were obtained at temperatures of 298, 308, and 318 K, under which the Dubinin-Radushkevich model was shown to have a good fit to data (R² > 0.96), according to mathematical adjustments. The maximum adsorption capacity obtained experimentally was 50.36 mg g⁻¹, found at a temperature of 298 K, being higher or compatible with other materials reported in the literature. With help of the thermodynamic studies on the process, it was observed that the adsorption of 2,4-D in organophilic clays refers to a spontaneous (ΔG°ₐdₛ < 0), exothermal (ΔH°ₐdₛ = − 9.99 kJ mol⁻¹) process of physical nature. Lastly, it was observed that the adsorbent can be easily regenerated when subjected to eluents such as mixtures containing fractions of ethanol/water (desorption = 95%).

Three different types of zirconium-modified bentonites (ZrMBs) including zirconium-modified original bentonite (ZrMOB), zirconium-modified magnesium-pretreated bentonite (ZrMMgB), and zirconium-modified calcium-pretreated bentonite (ZrMCaB) were synthesized and used as active covering materials to suppress the release of phosphorus (P) from sediments. To assess the covering efficiency of ZrMBs to inhibit P release from sediments, we examined the impact of ZrMB covering layer on P mobilization in sediments at different depths as well as the release of P through the interface between sediment and overlying water (SWI) by use of simulating P release control experiments and diffusive gradients in thin films (DGT) technology. The results showed that the amount of soluble reactive P (SRP) in the overlying water greatly decreased after covering with ZrMBs. Moreover, both pore water SRP and DGT-liable P (DGT-P) in the top sediments decreased after capping with ZrMBs. An obvious stratification of DGT-P was observed along the vertical direction after covering with ZrMBs, and static and active layers were found in the top sediment and in the lower sediment directly below the static layer, respectively. Furthermore, ZrMB covering led to the change of P species from easily released P to relatively or very stable P, making P in the top sediment more stable compared to that without ZrMB covering. Besides, an overwhelming majority of P immobilized by ZrMBs is hard to be re-released into the water column in a common environment. Overall, the above results demonstrate that sediment covering with ZrMBs could effectively prevent the transport of SRP from sediments into the overlying water through the SWI, and the control of P transport into the overlying water by ZrMB covering could be mostly due to the immobilization of pore water SRP, DGT-P, and mobile P in the top sediment by ZrMBs.

Bentonite is a porous clay material that shows good performance for adsorbing heavy metals and other pollutants for wastewater remediation. In our previous study, magnetic bentonite (M-B) was prepared to solve the separation problem and improve the operability. In this study, we investigated the influence of various parameters on the Pb(II) adsorption of M-B, and it showed effective performance. About 98.9% adsorption removal rate was achieved within 90 min at adsorbent dose of 10 g/L for initial Pb(II) concentration of 200 mg/L at 40 °C and pH 5. The adsorption kinetic fit well by the pseudo-second-order model, and also followed the intra-particle diffusion model up to 90 min. Moreover, adsorption data were successfully reproduced by the Langmuir isotherm; the maximum adsorption capacity was calculated as 80.40 mg/g. The mechanism of interaction between Pb(II) ions and M-B was ionic exchange, surface complexation, and electro-static interactions. Thermodynamics study indicated that the reaction of Pb(II) adsorption on M-B was endothermic and spontaneous; increasing the temperature promoted adsorption. This study was expected to provide a reference and theoretical basis for the treatment of Pb-containing wastewater using bentonite materials.

The removal of three over-the-counter pharmaceuticals from aqueous solution using four different adsorbents was analyzed. To study the effect of infused pharmaceutical and adsorbent on the adsorption system, both the concentration of drug and adsorbent dosage were varied, with constant temperature and pressure at different contact time. Adsorption kinetics, isotherm models, and ANOVA allegorized a generic trend for pharmaceutical removal efficiency of the adsorbents that varied as follows: activated carbon > fly ash > bentonite > sugar cane bagasse ash. The Tempkin model appears to fit the isotherm data better than Freundlich and Langmuir. Correspondingly, the kinetic studies implied a pseudo-second-order fit, to understand the mechanism by which the solute accumulates on the surface of a solid and gets adsorbed to the surface via intra-particle diffusion. Furthermore, some special cases of removal tendencies were noted based on sorbate-sorbent interaction. Effectively, it was observed that at an adsorbent loading of 2 g and initial concentration of 0.2 mmol L⁻¹, bentonite, fly ash, and activated carbon were able to strip more than 80% of all pharmaceuticals from urine. A framework for the highest significance of the experiments was obtained using response surface methodology by the combination of ciprofloxacin-bentonite followed by paracetamol-activated carbon and ibuprofen-activated carbon. Quasi-Newton and Bayesian regression methods were implemented on Langmuir isotherm by designing the neural network for the batch adsorption experiments. Based on the numerical calculations and graphical representations, the proposed model leads to the result that error is minimized and the values are optimized for different pharmaceuticals such as paracetamol, ibuprofen, ciprofloxacin that can be removed from wastewater streams by locally available adsorbents. Graphical abstract