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EFFECT OF CROPPING SYSTEMS ON ADSORPTION OF METALS BY SOILS: II. EFFECT OF pH

The effect of pH on adsorption of Cd, Cu, Ni, Pb, and Zn by soils under different cropping systems was investigated. Plots of metal adsorption vs. pH (unadjusted) were generated for 24 soils, 12 from each of two long-term cropping systems. Two soils, one from each site, were selected to study the metal adsorption over a range of adjusted pH values. Results showed that differences in metal adsorption were dependent on the initial heavy-metal concentration. At low concentrations, all the added metals were adsorbed regardless of the solution pH. At high concentrations, however, metal adsorption by soils was strongly related to solution pH; metal adsorption increased with increasing solution pH. A decrease in solution pH with increasing initial metal concentration was observed for all soils and metals. In general, a decrease in solution pH for a given amount of metal added followed the sequence: Cu = Pb > Cd = Ni = Zn. Metal solubility diagrams showed that, in general, metal adsorption by soils could not be attributed to precipitation. Although some of the adsorption behavior of these metals was consistent with that based on metal hydrolysis, other results showed that this mechanism could not explain metal adsorption. Metal distribution coefficients, Kd, decreased sharply with increasing initial metal concentration and followed the sequence: Pb ≥ Cu [U00BB] Cd = Ni = Zn. Also, Kd values were related to solution pH. At high solution pH, part of the Pb and Cu adsorption could be explained by adsorption of hydroxocomplexes from hydrolysis reactions of these metals. Plots of Kd vs. initial metal concentration suggested that Cd, Ni, and Zn adsorption could be attributed to ion-exchange reactions.