INFLUENCE OF ELECTROLYTE-COMPOSITION AND PH ON CADMIUM SORPTION BY ANACID SANDY SOIL

Extraction of soil with CaCl2 has been recommended as a measure of bio availability of heavy metals. Interpretation of soil extraction data i n terms of plant uptake potential may improve when the chemical behavi our of heavy metals in these extracts is ascertained. The effect of pH , Cd complexation by Cl, and competition between Cd and Ca on Cd sorpt ion was studied at an ionic strength of 0.03 M. Sorption of cadmium wa s measured in 0.01 M CaCl2, in 0.01 M Ca(NO3)2, in a mixture of 0.02 M NaCl and 0.01 M NaNO3, and in 0.03 M NaNO3, at different values of pH ranging from 3.8 to 4.9. Adsorption isotherms were all linear, with a negative intercept on the y-axis. This intercept indicated (linear) d esorption of only part of the initial soil Cd content. About 50% of th e Cd in solution was complexed in the presence of 0.02 M Cl at ionic s trength of 0.03. Due to competition between Cd and Ca, sorption of Cd was reduced by 80% in the Ca-electrolytes as compared with the Na-elec trolytes. Sorption was highly sensitive to pH as each 0.5 unit increas e in pH resulted in twice as much sorption of Cd. An empirical factor in the sorption equation that accounts for this effect of pH showed a similar response to changes in pH as a mechanistic factor. This mechan istic factor was developed by assuming that Cd and protons sorb onto t he same sites and that a two-site Langmuir sorption isotherm for proto ns was able to describe the titration curve of the soil. This similari ty may explain the successful application of the empirical factor in t his and previous studies.