Evaluation of Zn-Al-SO_4 layered double hydroxide for the removal of arsenite and arsenate from a simulated soil solution: Isotherms and kinetics

摘要

The main objective of the present study was to evaluate the effectiveness of synthetic Zn-Al-SO_4 layered double hydroxide (LDH) for the removal of arsenite and arsenate from a simulated soil solution. The Zn-Al-SO_4 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-SO_4 LDHs can be described well with Langmuir isotherm and maximum adsorption capacities were calculated to be 34.24 and 47.39 mg g~(-1) for arsenite and arsenate, respectively. The adsorption kinetics of arsenate followed pseudo-second order while arsenite uptake showed better correlation with intraparticle diffusion model. Furthermore, the effects of two major coexisting and competing divalent anions such as SO_4~(2-) and CO_3~(2-) on the uptakes of arsenite and arsenate were studied and the results showed that CO_3~(2-) had greater adverse effect on the uptakes of arsenite and arsenate by Zn-Al-SO_4 than SO_4~(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-SO_4 LDHs, but possibly some topotactic exchange may occur through the substitution of arsenic anions with sulfate in the interlayer region.