Assessment of the interactions of metals and nitrilotriacetic acid in soil/sludge mixtures

The solution phase forms of Cu, Mn, Ni, and Zn in digested sewage sludge and a soil/sludge mixture were investigated. Gel filtration chromatographic analysis indicated that Cu and possibly Ni were maintained in solution by association with a soluble, high molecular weight organic fraction; Mn solubility was due to the presence of unbound inorganic species and soluble Zn was distributed equally between the two forms. Speciation of the metals in the solution phase of the soil/sludge mixture generally reflected that of the sludge. However, the total amount of soluble Mn in the soil/sludge mixture was approximately 25 times greater than in the sludge and was attributed to heavy metal induced release of indigenous soil Mn. Increases in the quantities of soluble Ni and Zn in both the sludge and the soil/sludge mixture following equilibration with 40 mg L^?1 NTA were due to conversion of solid phase forms to soluble metal-NTA complexes. The preferential complexation of NTA with Cu already present in soluble organic forms resulted in a change in speciation without a corresponding increase in solubility. The importance of changes in speciation with regard to potential metal mobility and bioavailability within sludge-amended soil is discussed.