Removal of arsenic(V) ion from aqueous solutions by lanthanum compounds

A new adsorption process for the removal of As(V) ion from aqueous solutions has been studied using lanthanum hydroxide (LH), lanthanum carbonate (LC) and basic lanthanum carbonate (BLC). These La compounds were effective in removing As ion to decrease the concentration down to < 0.001 mM. Dissolution of these La compounds was measured in the pH range of 2 to 12. The dissolution was appreciable at initial pH <4.3, <4.3 and <4.0 for LH, LC and BLC, respectively. Kinetic study showed that the As removal was a first-order reaction in the neutral pH range and the rate constants were in the order of LH > LC > BLC. The As removal was highly pH-dependent. The optimum pH range was 3-8, 4-7 and 2-4 for LH, LC and BLC, respectively. The following two mechanisms are proposed: (i) adsorption by exchange of CO₃ and/or OH group with As ions in the neutral to alkaline pH range where La does not dissolve and (ii) precipitation of insoluble lanthanum arsenate, LaAsO₄, in the acid pH range.