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

Abstract

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 SO42− and CO32− on the uptakes of arsenite and arsenate were studied and the results showed that CO32− had greater adverse effect on the uptakes of arsenite and arsenate by Zn–Al–SO4 than SO42−. 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.