Effects of pH on Heavy Metal Sorption on Mineral Apatite

Abstract

The sorption of aqueous Pb, Cd, and Zn onto a mineral apatite from North Carolina was investigated in relation to a wide range of pH. The effects of pH on solid-phase precipitation were particularly emphasized. The heavy metals were applied as single or multiple species. Solution pH greatly affected metal sorption mechanism by apatite and metal−apatite reaction products. The sorption of aqueous Pb was primarily through a process of the dissolution of apatite followed by the precipitation of variable pyromorphite-type minerals under acidic condition or of hydrocerussite [Pb3(CO3)2(OH)2] and lead oxide fluoride (Pb2OF2) under alkaline condition. Otavite (CdCO3), cadmium hydroxide [Cd(OH)2], and zincite (ZnO) were formed in the Cd or Zn system, especially under alkaline condition; while hopeite [Zn3(PO4)2·4H2O] might precipitate only under very acidic condition. Alternative sorption mechanisms other than precipitation of the crystalline phases were important in reducing Cd and Zn concentrations by the apatit...