Effects of pH on Heavy Metal Sorption on Mineral Apatite

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 Pb 3 (CO 3 ) 2 (OH) 2 and lead oxide fluoride (Pb 2 OF 2 ) under alkaline condition. Otavite (CdCO 3 ), cadmium hydroxide Cd(OH) 2 , and zincite (ZnO) were formed in the Cd or Zn system, especially under alkaline condition; while hopeite Zn 3 (PO 4 ) 2 ·4H 2 O 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 apatite and might include ion exchange, adsorption, absorption, complexation, coprecipitation, or precipitation of amorphous phases. Removal of Cd and Zn by the apatite was pH dependent, whereas removal of Pb was not. The removals were about 0.729 mmol of Pb, 0.489−1.317 mmol of Cd, and 0.596−2.187 mmol of Zn/g of apatite, representing removal of 99.9%, 37.0−99.9%, and 27.0−99.9% of heavy metals from solution, respectively.