Layered double hydroxides are antitype 2/1 clay minerals that can be synthesized rapidly under laboratory conditions. Due to their high anion exchange capacities, layered double hydroxides have been investigated as potential adsorbents for removal of anionic contaminants from aqueous systems. In this study, uncalcined and calcined layered double hydroxides were prepared and characterized by X-ray diffraction, with the products evaluated for their ability to adsorb As(III) in aqueous solutions. Results indicated that As(III) could be adsorbed on chloride layered double hydroxide and calcined layered double hydroxide, but no adsorption occurred for carbonate layered double hydroxide. The adsorption isotherms of As(III) on chloride layered double hydroxide and calcined layered double hydroxide were typical L and H-type curves, respectively. The adsorption of As(III) on calcined layered double hydroxide was a slow process and reached a quasi-equilibrium after a 20 hr reaction time. The layered double hydroxides had high pH buffering capacities and the As(III) adsorption on calcined layered double hydroxide was a function of pH. Competing anions strongly affected adsorption, with As(III) adsorption increasing in the order: HPO(2-)4 < SO(2-)4 < CO(2-)3 < F- < Cl- < Br- approximately equals I- < NO(-)3. Adsorbed As(III) on calcined layered double hydroxide could be desorbed by different anions, but there was no systematic relationship between As(III) desorption and anion affinities for the calcined layered double hydroxide. Calcination immobilized the As(III) adsorbed on calcined layered double hydroxides. Although layered double hydroxides could be recycled and used as an adsorbent, its adsorption efficiency was reduced with successive treatments.

Adsorption and desorption kinetics of As(V) on Fe and Mn oxide mixture was studied. A predictive kinetics model based on component additivity approach was developed. The kinetics model predicted the kinetic data of As(V) well for the mixed oxides. Ferrihydrite and δ‐MnO2 controlled As(V) reaction kinetics differently. Arsenate [As(V)] soil contamination is a great concern worldwide because of its high toxicity, carcinogenicity, and mobility. The adsorption of As(V) on soil minerals such as ferrihydrite and δ‐MnO2 controls the transport and bioavailability of As(V) in the soil environment. A large number of studies have investigated the equilibrium and mechanisms of As(V) adsorption onto ferrihydrite and δ‐MnO2 individually. However, both ferrihydrite and δ‐MnO2, commonly coexisting within soils has been overlooked and little information is available for the kinetics of As(V) adsorption and desorption in this mixed mineral system. Therefore, in this work, kinetics of As(V) adsorption and desorption onto ferrihydrite and δ‐MnO2 mixed minerals is studied using the stirred‐flow method. A kinetics model for the mixed mineral system was developed using the component additivity approach, which incorporated the nonlinear binding of As(V) to both ferrihydrite and δ‐MnO2 simultaneously. The kinetics model successfully predicted the kinetics of As(V) adsorption and desorption in the mixed mineral system under varying solution chemistry conditions and properly accounted for the heterogeneity of the binding sites of both ferrihydrite and δ‐MnO2. For As(V) adsorption kinetics, the ferrihydrite bidentate non‐protonated sites (Fh‐bi‐np) played a dominant role, followed by the δ‐MnO2 binding sites and the ferrihydrite bidentate protonated sites (Fh‐bi‐p). After the 4‐h desorption, As(V) was mainly retained on ferrihydrite binding sites. This study helps to quantitatively elucidate the kinetic behavior and mechanisms of As(V) in the Fe oxide and Mn oxide systems, thus enhancing our understanding of the fate and transport of As in soil environments.

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