MEASURING CADMIUM ION ACTIVITIES IN SLUDGE-AMENDED SOILS

Trace metal activities are important in relation to metal bioavailability in soils, but they are difficult to measure. We compared Cd2+ activities determined by four different methods in laboratory prepared solutions and soil saturation extracts. The methods include (i) the ion selective electrode, (ii) the Cd2+ addition method, (iii) the metal-ion chelate equilibria method, and (iv) GEOCHEM-PC, a computer chemical speciation program. The laboratory prepared solutions of known Cd concentrations ranged from 10(-8) to 10(-4) M Cd in a simple noncomplexing inorganic matrix to a complex matrix of inorganic and organic Ligands that resembled a soil solution. There was good agreement between Cd2+ activities determined by the ion selective electrode and GEOCHEM-PC in test solutions for Cd concentrations >10(-6.5) M. There was also a good agreement between Cd2+ activities determined by the addition method and GEOCHEM-PC for test solutions of 10(-8) and 10(-7) M Cd and soil saturation extracts. Cadmium ion activities determined by the chelation method did not agree well with those determined by the addition method or GEOCHEM-PC.