A laboratory study of the immobilisation of cadmium in soils

Abstract

Traces of cadmium ion in solution, alone and together with different cadmium bonding agents (K 2CO 3, CaCO 3, KH 2PO 4, Ca 3(PO) 4, Fe(NO 3) 3 and Fe(OH) 3), were added to three soils of pH 5·0, 7·0 and 7·6, respectively, and incubated aerobically at 45°C for 6 days. The extent to which the different binding agents immobilised cadmium in the treated soils was determined by measuring the amounts extracted by CH 3COONH 4, CaCl 2, NH 4Cl, Ca(NO 3) 2, (CH 3COO) 2Ca or KNO 3 solutions. The extracted cadmium was determined by AAS. Recoveries by the different extractants, which are representative of those proposed for the determination of ‘available cadmium’ in soils, showed a systematic variation for each of the treated soils. The extent to which these differences arise from pH and the complexation of the cadmium is discussed. Markedly different amounts of cadmium were immobilised in the different soils, under the same conditions and treatments. CaCO 3 and Ca 3(PO 4) 2 were the most efficient in immobilising cadmium in the acid soil, K 2CO 3 and Ca 3(PO 4) 2 were best with the neutral soil and Fe(NO 3) 3, Fe(OH) 3 and Ca 3(PO 4) 2 with the calcareous soil. Thus, under the aerobic conditions likely to be found in dry farmlands, the cadmium immobilisation mechanisms appear to be associated with precipitation on carbonate and phosphate surfaces; in calcareous soils, the surface hydrolysis of cadmium on ferric hydroxide seems most important.