Removal of Toxic Heavy Metal Ions in Aqueous Solution with Mg/Al Type Hydrotalcite Derived from Wastes

Abstract

The inorganic anion exchanger, hydrotalcite (HT) was synthesized by using aluminum dross and MgCl₂ waste solution discharged from an Al regeneration factory. The ability of the obtained CO₃²⁻ type HT and its calcination product to remove various toxic heavy metal ions such as As(III), As(V), Se(IV) and Cr(VI) from aqueous solution was investigated.
The amount of As(III) removed with the calcination product of CO₃²⁻ type HT at 773 K is larger than that with the CO₃²⁻ type HT. In the removal of As(V), no marked difference was found between the CO₃²⁻ type HT and calcination product. In the same way, it is possible to remove Se(IV) and Cr(VI) with the CO₃²⁻ type HT and the calcination product. In using the calcination product for the removal of As(III), As(III) is directly incorporated with HT calcination product in a rehydration process, and the reaction is predominant over the anion exchange reaction between OH⁻ and As(III). About 90% of As(III) removal can be obtained in the initial pH range from 4 to 11. These phenomena are related to the pH buffer effect of HT, by which the equilibrium pH after As(III) removal is adjusted to be about 11. The largest amount of Cr(VI) removal can be obtained with the calcination product at 673 K. The HT derived from the above wastes, which has both anion exchange ability and pH buffer action, can be used as an excellent removal agent of various toxic heavy metal ions.