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

We demonstrate the highly selective and exceptionally efficient capture of toxic oxoanions of As(III)/As(V) (HAsO₃^2-/HAsO₄^2-) and Cr(VI) (CrO₄^2-) by the Mg/Al layered double hydroxide (Mg/Al-LDH) intercalated with MoS₄^2- (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 SO₄^2-, NO₃^-, and Cl^-, remarkable selectivity for HAsO₃^2-, HAsO₄^2-, and CrO₄^2- 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 MoS₄^2- 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.