Bioavailability and solubility equilibria of heavy metals in soils

Metal adsorption by garden soils as a function of soil solution pH, indicated increasing metal adsorption with pH. At pH >5.0 more Pb was adsorbed than Zn or Cd. Soluble Zn, Pb and Cd concentrations in soils after 600 hours of reactions decreased almost linearly with pH. This was attributed to increasing charge density of the soils and the hydrolysis of metal cations. At pH below 6.5, the likely control on Zn2+ activities in soil solution was Franklinite (ZnFe2O4), but between pH 7 and 9, Zn2+ activities in soil solution approached the solubility isotherm of Hopeite (Zn3(PO4)2. The log Pb2+ - pH activity isotherms indicated that at pH < 6, Pb2+ activities in soil solution were close to chloropyromorphite [Pb5(PO4)3Cl] solubility, but as pH increased from 6 to 8, the activities of Pb2+ approached the solubility isotherm of Cerrusite (PbCO3). The log Cd2+-pH activity isotherms indicated that below pH 6.8, Cd2+ activities were either in equilibrium with or slightly oversaturated to Cd3(PO4)2 solubility. The dominant inorganic Zn complex was ZnSO4o followed by ZnOH+, ZnCl+ and Zn(OH)2o in that order.