Influence of electrolyte composition and pH on cadmium sorption by an acid sandy soil

Abstract

SUMMARY: Extraction of soil with CaCl 2 , has been recommended as a measure of bioavailability of heavy metals. Interpretation of soil extraction data in terms of plant uptake potential may improve when the chemical behaviour of heavy metals in these extracts is ascertained. The effect of pH, Cd complexation by Cl, and competition between Cd and Ca on Cd sorption was studied at an ionic strength of 0.03 m . Sorption of cadmium was measured in 0.01 m CaCl 2 , in 0.01 m Ca(NO 3 ) 2 , in a mixture of 0.02 m NaCl and 0.01 m NaNO 3 , and in 0.03 m NaNO 3 , 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) desorption of only part of the initial soil Cd content. About 50% of the Cd in solution was complexed in the presence of 0.02 m Cl at ionic strength 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‐electrolytes. Sorption was highly sensitive to pH as each 0.5 unit increase 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 mechanistic factor was developed by assuming that Cd and protons sorb onto the same sites and that a two‐site Langmuir sorption isotherm for protons was able to describe the titration curve of the soil. This similarity may explain the successful application of the empirical factor in this and previous studies.