The influence of sulfate on selenate sorption on Mg-Al-CO3 layered double hydroxides prepared by fine inorganic sol-gel synthesis studied by X-ray photoelectron spectroscopy

Abstract

Layered double hydroxides (LDHs) are considered as the next generation (synthetic) inorganic anion exchangers, however, success in bringing this class of inorganic materials to society on industrial scales depends on advancement of the knowledge on their atomic level structure and the interfacial processes. Majority of sorption studies focus on the speciation in liquid adsorbate; fewer researches deal with an involvement of various adsorbent phases in adsorption mechanisms. In this work, X-ray photoelectron spectroscopy was applied to reveal a difference in selenate removal speciation-level pathways (with a focus on solid phase) altered by its major competitor, sulfate. It was defined that sulfate greatly influenced the mechanism of selenate sorption and slightly affected the removal capacity of this Mg-Al-CO3 LDHs (synthesized by fine inorganic sol-gel synthesis) to selenate, especially at pH 7. Among the three Al-containing phases (Al(OH)3, AlOOH and Al2O3) and three Mg-based species (Mg(OH)2, MgO, Mg[H2O]2+), Al(OH)3 played a major role in selenate removal at equilibrium in all batches except for one experimental condition. At pH = 8.5 in SO42−-containing adsorbate, Mg(OH)2 species played a greater role in selenate removal than Al(OH)3 phase. In sulfate-free batches, an involvement of Al(OH)3 species was stronger than in sulfate-containing suspensions. Aqueous sulfate increased participation of the interlayer carbonate in anion exchange with aqueous selenate compared with the sulfate-free solutions. Notably, at equal concentrations of H+ and OH− regardless the presence of sulfate, anion exchange processes with aqueous selenate were gentler than at domination of any, H+ or OH−. However, at pH = 7, the competition of selenate and sulfate was also the strongest.