New data on arsenic sorption properties of Zn-Al sulphate layered double hydroxides: Influence of competition with other anions

The adequacy of synthetic Zn–Al-sulphate LDHs to remove arsenic from aqueous systems was tested through sorption experiments, using a series of aqueous solutions with dissolved HAsO4 2− together with other anions (Cl−, SO4 2−, MoO4 2−, HCO3−, CO3 2−) to assess their competition influence on the As removing process. The competitors were added into the solution both simultaneously and afterwards with respect to HAsO4 2− in order to verify the effectiveness and the possible reversibility of the As sorption process. The results showed that only carbonates species, in particular in the fully deprotonated form CO3 2−, affect significantly the otherwise high efficacy of the sorption process. In fact, up to ~90% of HAsO4 2− can be removed from the solution, decreasing to ~60% in the presence of CO3 2−, whilst up to ~30% of HAsO4 2− can be desorbed when CO3 2− is added afterwards into the solution. Considering the very restricted range of pH where HAsO4 2− and CO3 2− are simultaneously the predominant species in the solution (~10 < pH < ~11.5), Zn–Al-sulphate LDHs could be successfully used for the treatment of As contaminated waters with pH ranging from circum-neutral to moderately alkaline.