Evaluation of Zn-Al-SO₄ layered double hydroxide for the removal of arsenite and arsenate from a simulated soil solution: Isotherms and kinetics

The main objective of the present study was to evaluate the effectiveness of synthetic Zn-Al-SO₄ layered double hydroxide (LDH) for the removal of arsenite and arsenate from a simulated soil solution. The Zn-Al-SO₄ was synthesized using a hydrothermal method and the adsorption of arsenic was studied using Langmuir and Freundlich models. Moreover, the kinetics of adsorption was tested using pseudo-first, pseudo-second, intraparticle diffusion and Elovich models. The results of the experiments revealed that the adsorption isotherms of arsenite and arsenate on the Zn-Al-SO₄ LDHs can be described well with Langmuir isotherm and maximum adsorption capacities were calculated to be 34.24 and 47.39mgg^-1 for arsenite and arsenate, respectively. The adsorption kinetics of arsenate followed pseudo-second order while arsenite uptake showed better correlation with intra-particle diffusion model. Furthermore, the effects of two major coexisting and competing divalent anions such as SO₄^2- and CO₃^2- on the uptakes of arsenite and arsenate were studied and the results showed that CO₃^2- had greater adverse effect on the uptakes of arsenite and arsenate by Zn-Al-SO₄ than SO₄^2-. Based on X-ray diffraction results, the main underplaying adsorption mechanism of arsenite and arsenate might be the exchange on the external surfaces and edges of Zn-Al-SO₄ LDHs, but possibly some topotactic exchange may occur through the substitution of arsenic anions with sulfate in the interlayer region.