Selective and Efficient Removal of Toxic Oxoanions of As(III), As(V), and Cr(VI) by Layered Double Hydroxide Intercalated with MoS₄^2–

Abstract

We demonstrate the highly selective and exceptionally efficient capture of toxic oxoanions of As(III)/As(V) (HAsO32–/HAsO42–) and Cr(VI) (CrO42–) by the Mg/Al layered double hydroxide (Mg/Al-LDH) intercalated with MoS42– (MoS4-LDH). The MoS4-LDH exhibits very high removal rates (>99%) of As(III), As(V), and Cr(VI) from complex aqueous solutions. In the presence of many competing nontoxic anions such as SO42–, NO3–, and Cl–, remarkable selectivity for HAsO32–, HAsO42–, and CrO42– is observed. Highly concentrated As(III) and Cr(VI) can be rapidly reduced to <10 ppb, much below the permitted level for drinking water. The maximum adsorption capacities for As(III), As(V), and Cr(VI) are 99, 56, and 130 mg/g, respectively. Sorption isotherms for As(III) and As(V) agree with the Langmuir model, suggesting a monolayer adsorption on the sorbent. The adsorptions of As(V) and Cr(VI) are exceptionally rapid, showing >93% removals within 1 min and >96% removal within 5 min. Sorption kinetics for these oxoanions follows a pseudo-second-order model consistent with a chemisorption involving possible As–S and Cr–S bonding. The Cr(VI) adsorption is accompanied by an oxidation/reduction reaction with MoS42– resulting in Cr(III) species. The order of rate constants (k2) is Cr(VI) > As(V) > As(III), which reflects their respective adsorption rates. The S–As and S–Cr interactions and the reduction of Cr(VI) to Cr(III) contribute to the effective uptakes. These results make the MoS4-LDH material a compelling candidate for application in remediation of water polluted with arsenic and hexavalent chromium.