Kinetics of Cadmium Release from Soils as Influenced by Organic Acids: Implication in Cadmium Availability

Abstract

The low‐molecular‐weight organic acids (LMWOAs) secreted by plant roots modify the mobility of Cd through formation of soluble complexes in the soil rhizosphere; the kinetics of Cd release as influenced by organic acids and its impact on Cd bioaccumulation remains to be established. The influence of LMWOAs (10−3 and 10−2 M), viz., acetic, citric, oxalic, fumaric, and succinic acids, which are present in the soil rhizosphere, on the kinetics of Cd release from selected soils (Luseland, Waitville, and Jedbergh) of Saskatchewan, Canada, was investigated at 25°C and at an ionic strength of 0.1 M NaNO3. It was observed that the Cd release from the soils increased initially up to a reaction period of about 2 h and then slowly decreased with time. With time, the LMWOAs were apparently microbially degraded and the soil solution Cd2+, which was transformed from the Cd‐LMWOA complexes by microbial degradation, was adsorbed onto the negatively charged soil particles. Increasing amounts of Cd were released from the soils to the soil solution with the renewal of LMWOAs after every 2 h. The short‐term (reaction period of 0.25 to 1 h) kinetics of Cd release from the soils was described by a parabolic diffusion kinetic model. The overall diffusion 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 >> Waitville > Jedbergh. Thus, the results presented in this investigation indicate the importance of the kinetics of the Cd release from the soils by the LMWOAs excreted by root exudates in understanding the availability of soil Cd.

Effect of organic acids on adsorption and desorption of rare earth elements

“…The concentrations of LMWOAs were reported to be in the range of 10 À7 -10 À4 mol/L in the soil pore solutions, even higher in the rhizosphere soils (Stevenson, 1994;Krishnamurti et al, 1997). The concentrations of 1-10 mM of organic acids were used in the sorption and desorption of cadmium (Krishnamurti et al, 1997;Naidu and Harter, 1998).…”

“…The concentrations of 1-10 mM of organic acids were used in the sorption and desorption of cadmium (Krishnamurti et al, 1997;Naidu and Harter, 1998). Therefore, a concentration of 10 À2 mol/L of LMWOAs was chosen in this study to simulate the adsorption of REEs by and desorption from the rhizosphere soils.…”

Effects of low-molecular-weight organic acids and residence time on desorption of Cu, Cd, and Pb from soils

“…Increasing evidences have suggested that the anions of LMWOAs may play an important role in the fate of metals (Naidu and Harter, 1998;Glover et al, 2002;Collins et al, 2003;Onyatta and Huang, 2003). LMWOAs can form soluble complexes with metal cations (Robert and Berthelin, 1994;Stevenson and Fitch, 1994), thus modify the fixation of metals by soil organic matter, oxides, clays, and enhance their release (Krishnamurti et al, 1997;Chen et al, 2003). Such properties can be used in phytoremediation to increase metal bioavailability and hence the bioaccumulation of metals into plants (Wu et al, 2003), minimizing the environmental risk.…”

Effects of EDTA and low molecular weight organic acids on soil solution properties of a heavy metal polluted soil

“…Of these compounds, low molecular weight organic acids (LMWOA) are of particular importance due to their metal chelating/ complexing properties for mobilization of mineral nutrients (Marschner et al, 1989;Zhang et al, 1991;Jones et al, 1996) and heavy metals (Mench et al, 1988;Mench and Martin, 1991). Krishnamurti et al (1997) demonstrated that various LMWOA were able to influence the rate of Cd release from different soils and increase the solubility of Cd in bulk soil through the formation of soluble Cd-LMWOA complexes.…”