Effect of soil properties on sorption and mobility of cadmium in selected aridisols and andisols

Abstract

Concern over environmental quality has generated interest in the chemistry of Cd in soils. When Cd sorption and the influence of soil characteristics on the process are better understood, Cd contamination of ground water and plant availability may be assessed more accurately. Therefore, a series of experiments, including laboratory batch studies, soil thin layer chromatography studies and selective ion electrode procedures were conducted to evaluate the effect of soil properties on Cd sorption and mobility by fourteen different Aridisols and Andisols having a range of chemical properties. In addition, surface complexation models were used to model the amount of Cd adsorbed onto each of the humic acid and hydrous ferric oxide surfaces. Sorption isotherms were obtained using batch experiments by in which 25 ml of solution containing a total of 0.8, 1.6, 3.2, 6.4, and 12.8 ppm of Cd²⁺ to 0.5 g were added to soil samples. The results of the batch experiments indicate that in all the soils used the Cd sorption is best described by the Freundlich sorption isotherm. The maxima soil sorption capacities were significantly correlated with the free iron oxides. The Cd activities in the soils varied from 10⁻⁷·⁴⁴ to 10⁻⁴·⁸² M. It increased with increasing total Cd added and were inversely related to the soil pH The Cd mobility retardation factor (Rf) obtained from soil thin layer chromatography ranged from 0.25 to 0.95. It showed that Cd was slightly mobile in 64%, moderately mobile in 29% and very mobile in 7% of the soils. The Cd Rf indicates that Cd mobility would decrease with increasing amounts of iron oxide fractions; silt % and exchangeable Mg²⁺ in the soils. The non electrostatic and diffuse layer models results indicate that humic materials are an important factor in Cd sorption at pH values greater than 3 and hydrous ferric oxide surfaces are important at pH values greater than 7. The ion activity products of Cd(OH)₂ ranged from 2.3 x 10⁻²⁵ M to 5.6 x 10⁻¹⁶ M, while the solid activity coefficients of adsorbed Cd (SACCd2+) ranged from 3.9 x 10⁻⁶ to 4.6. The SAC(Cd₂₊) values were significantly correlated yet negative with silt %, CEC, Ald, Alo, O.C and iron fractions.