Influence of carbonate on the reaction of heavy metals in soils

The reaction of Cu, Zn and Cd with soils with carbonate contents ranging from 0 to 75 mg g^?1 was studied before and after removal of soil carbonates with acetate buffer at pH 5. Treatment with acetate buffer caused a strong decrease in metal retention by those soils containing carbonates, although if no carbonate was originally present, the treatment caused little effect or even an increase in the amounts sorbed. Before the treatment, adsorption of increasing amounts of Cu and Zn was accompanied by a continuous increase in Ca + Mg released, and those soils containing carbonate released Ca + Mg in excess of their exchangeable amounts, due to dissolution of carbonates and/or penetration of the heavy metal into the carbonate structure. It is suggested that Cu was preferentially retained by the treated soils through precipitation of Cu oxide, and by adsorption on the soil carbonates in the case of the original samples. Zn was removed from the solution by the original carbonate soils through formation of ZnCO₃. Treated soils were likely to retain Zn by cation exchange and/or adsorption. Adsorption was probably the main process involved in retention of Cd. In all cases pH was the master variable in controlling the extent and probably the nature of the reaction.