Removal of Harmful Anions in Aqueous Solution with Various Layered Double Hydroxides

Abstract

Four types of layered double hydroxides (LDHs) were synthesized by a co-precipitation method. The ability of various LDHs to remove anionic species including As(III), As(V), B, Cr(VI) and Se(IV) was investigated. Removal tests were performed primarily by a batch operation. The Cr(VI) removal was conducted by both batch and column studies. For the column removal tests, LDH pellets were prepared using a granulator and 5% poly-vinyl alcohol solution as binder.
Among the LDHs tested, Mg-Al-NO₃⁻ LDH showed the optimal removal of anionic species, and it demonstrated excellent removal of As(V), Se(IV) and Cr(VI) particularly at the low anionic concentrations. The order of anionic removal by the LDHs is as follows; Mg-Al-NO₃⁻ LDH > Mg-Al-Cl⁻ LDH >> Mg-Al-SO₄²⁻ LDH > Mg-Al-CO₃²⁻ LDH. The selective uptake of anion had the following order; As(V) > Se(IV) > Cr(VI) > As(III) > B. This study has demonstrated the uptake of harmful anions is influenced by the two specific factors, (1) the originally intercalated anions in the LDH, and (2) valency of anionic target species in aqueous solution. Column study also demonstrated that about 350 times Cr(VI) in solution per unit volume of LDH pellet can be removed, though this resulted in minor change in pH of the solution and greater NO₃⁻ elution. This study has revealed the ability of LDH to remove harmful anionic species and its practical application in wastewater treatment.