Bioavailability and Solubility Equilibria of Heavy Metals in Soils : Dynamics of cadmium, lead and zinc

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