A laboratory study of the immobilisation of cadmium in soils

Traces of cadmium ion in solution, alone and together with different cadmium bonding agents (K₂CO₃, CaCO₃, KH₂PO₄, Ca₃(PO)₄, Fe(NO₃)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 CH₃COONH₄, CaCl₂, NH₄Cl, Ca(NO₃)₂, (CH₃COO)₂Ca or KNO₃ 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. CaCO₃ and Ca₃(PO₄)₂ were the most efficient in immobilising cadmium in the acid soil, K₂CO₃ and Ca₃(PO₄)₂ were best with the neutral soil and Fe(NO₃)₃, Fe(OH)₃ and Ca₃(PO₄)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.