The bioavailability of cadmium from sewage sludge-amended soils.

Abstract

Cadmium is a potentially toxic trace element present, at low concentrations, in a variety of environmental media. The processing of waste-waters produces sewage sludge, in which cadmium may be present at high concentrations. Sewage sludge applied to soils used for the production of food crops represents a means for the elevation of human dietary exposure to cadmium. This study has examined aspects of the bioavailability of cadmium from sludge-amended soils. A number of sludge-amended and control soils were sampled and placed in tubs in the field. Duplicates of each soil were limed to a mean pH(H 20) of 6.9. Concentrations of cadmium in the edible tissues of cabbages and lettuces were lower for those plants grown on limed soils. Concentrations of cadmium in peeled potato tubers were not always reduced by the application of lime. Cadmium bioavailability was estimated by four soil test reagents, DTPA, EDTA-(Na) 2 , CaCl2 and NH4NO3 . The DTPA test proved to be the best indicator of plant cadmium concentrations. Multivariate statistics were used to develop models, based upon soil variables, for the prediction of plant cadmium concentrations and CF-values. Models were tested against an independent data set. Data quality for cadmium analyses was assessed by the routine use of certified reference materials. Methods of ETA-AAS analysis were developed and optimised for the control of interferences. Preliminary cadmium speciation studies, using SEC-ICP-MS, are described. Cadmium-binding species in the cytosol extract of potato tubers do not appear to survive in vitro gastro-intestinal enzymolysis. Cadmium- binding species in aqueous extracts of both intrinsically and extrinsically labelled samples of cooked potato tuber have molecular weights of < 2 x 10 3 daltons. An iterative model was written to examine the influence of soil and plant variables on the cadmium burden of the plough layer through time