Effect of pH on the Desorption of Trace Metals by Different LMWOAs from a Field Soil

Abstract

Effects of initial pH on desorption of copper (Cu), cadmium (Cd), and lead (Pb) by different low molecular weight organic acids (LMWOAs) from soils were examined. Cu, Cd, and Pb removal by LMWOAs was highly dependent on the initial pH. With an increase in pH, the amounts of the three trace metals desorbed by malonic acid and DL-hydroxybutanedioic acid sharply decreased, and desorption of Cd and Pb by malonic acid followed the same trend. These results indicate that pH was the dominant factor governing the release of trace metals by LMWOA extraction. When the pH of the solutions was not adjusted, malonic acid was found to be most effective in extracting Cd and Cu from the field soil. Desorption of both Cu and Cd from soil followed a descending order (malonic acid >citric acid > DL-hydroxybutanedioic acid) that was consistent with the acids’ acidity from strong to weak. However, Pb desorption followed a release order (citric acid > malonic acid > DL-hydroxybutanedioic acid) that was well matched by the same sequence in the stability constants of their produced Pb-ligand complexes.