KINETICS OF CADMIUM RELEASE FROM SOILS AS INFLUENCED BY ORGANIC-ACIDS- IMPLICATION IN CADMIUM AVAILABILITY

The low-molecular-weight organic acids (LMWOAs) secreted by plant root s modify the mobility of Cd through formation of soluble complexes in the soil rhizosphere; the kinetics of Cd release as influenced by orga nic acids and its impact on Cd bioaccumulation remains to be establish ed. The influence of LMWOAs (10(-3) and 10(-2) M), viz., acetic, citri c, oxalic, fumaric, and succinic acids, which are present in the soil rhizosphere, on the kinetics of Cd release from selected soils (Lusela nd, Waitville, and Jedbergh) of Saskatchewan, Canada, was investigated at 25 degrees C and at an ionic strength of 0.1 M NaNO3. It was obser ved that the Cd release from the soils increased initially up to a rea ction period of about 2 h and then slowly decreased with time. With ti me, the LMWOAs were apparently microbially degraded and the soil solut ion Cd2+, which was transformed from the Cd-LMWOA complexes by microbi al degradation, was adsorbed onto the negatively charged soil particle s. Increasing amounts of Cd were released from the soils to the soil s olution with the renewal of LMWOAs after every 2 h. The short-term (re action period of 0.25 to 1 h) kinetics of Cd release from the soils wa s described by a parabolic diffusion kinetic model. The overall diffus ion coefficient of the Cd release from the soil and the Cd released by the renewal of LMWOAs followed the same trend as the Cd availability index of the soils: Luseland much greater than Waitville > Jedbergh. T hus, the results presented in this investigation indicate the importan ce of the kinetics of the Cd release from the soils by the LMWOAs excr eted by root exudates in understanding the availability of soil Cd.