The effect of crop exudates and EDTA on cadmium adsorption by agricultural podsolic soil: implications on groundwater

Abstract

Sorption studies were conducted to investigate the effect of the presence of organic acids or exudates released by crops [acetic acid (AA), citric acid (CA) and oxalic acid (OA)] and ethylenediaminetetraacetic acid (EDTA) on the adsorption of carcinogenic cadmium (Cd) by an agricultural podsolic soil acting as a natural adsorbent. Powder X-ray diffraction, X-ray fluorescence, Fourier transform infrared spectroscopy and cation exchange capacity were used to characterize the soil. The effect of pH and changing concentration of the organic acids and Cd on the adsorption was investigated. PHREEQC (short for pH, REdox and EQuilibrium in C language) geochemical modeling was used to determine the speciation of Cd. The podsolic soil was effective as a natural clay adsorbent in capturing Cd, thus preventing mobility to groundwater. In the presence of crops organic acids and EDTA, the adsorption capacity decreased at all initial concentrations of Cd and the extent of reduction followed the sequence: OA < AA < CA < EDTA. The adsorption of Cd onto the soil was best described by Freundlich isotherm in the presence of all acids. The adsorption of Cd decreased as the concentration of crop organic acids increased. Moreover, EDTA totally inhibited the adsorption of Cd at all concentrations. The adsorption of Cd increased in the presence of CA and EDTA at pH greater than 5. The capturing of Cd by the soil will be decreased by the presence of organic acids, thus increasing mobility to groundwater which should be protected from such pollutants.