SOLUBILITY CONTROL OF CU, ZN, CD AND PB IN CONTAMINATED SOILS

We developed a semiempirical equation from metal complexation theory w hich relates the metal activity of soil solutions to the soil's pH, or ganic matter content (OM) and total metal content (MT) The equation ha s the general form: pM = a + bpH - c log(MTOM-1), where pM is the nega tive logarithm (to base 10) of the metal activity, and a, b and c are constants. The equation successfully predicted free Cu2+ activity in s oils with a wide range of properties, including soils previously treat ed with sewage sludge. The significant correlation of pCu to these mea sured soil properties in long-contaminated soils suggests that copper activity is controlled by adsorption on organic matter under steady st ate conditions. An attempt was made from separate published data to co rrelate total soluble Cu, Zn, Cd and Pb in soils to soil pH, organic m atter content and total metal content. For Cu, the total Cu content of the soil was most highly correlated with total soluble Cu. Similarly, total soluble Zn and Cd were correlated with total metal content, but were more strongly related to soil pH than was soluble Cu. Smaller me tal solubility in response to higher soil pH was most marked for Zn an d Cd, metals that tend not to complex strongly with soluble organics. The organic matter content was often, but not always, a statistically significant variable in predicting metal solubility from soil properti es. The solubility of Pb was less satisfactorily predicted from measur ed soil properties than solubility of the other metals. It seems that for Cu at least, solid organic matter limits free metal activity, whil st dissolved organic matter promotes metal solubility. in soils well-a ged with respect to the metal pollutant. Although total metal content alone is not generally a good predictor of metal solubility or activit y, it assumes great importance when comparing metal solubility in soil s having similar pH and organic matter content.