CHEMICAL ASPECTS OF HEAVY-METAL SOLUBILITY WITH REFERENCE TO SEWAGE-SLUDGE AMENDED SOILS

The long term benefits of applications of sewage sludges to land as an alternative source for plant nutrients are frequently limited by pote ntially toxic contents of heavy metals. While upper limits for metal c ontents in amended soils have been defined in both North America and E urope, there has been little attention paid to the fate of the metals if soil management practices are changed and the solubility and hence the mobility of the metals increased. This study investigated the role of changes in pH and additions of chloride ions to the content of sol uble Cd, Co, Cr, Cu, Pb, Ni, V and Zn in soils to which sewage sludge had been applied. The contents of soluble metals ranged from less than 1 mu mol L(-1) for V to 500 mu mol L(-1) for Zn. For all the metals, contents were greater in the presence of CT ions and increased markedl y as the pH decreased below about pH 5. Contents of V, Cr, Cu and Pb i ncreased at pH's above 7. As all metal contents were undersaturated wi th respect to hydroxide or carbonate precipitates, the changes in thei r contents were probably related to desorption from variable charge si tes on mineral and/or humic surfaces.