Properties of and heavy metal complexation by aqueous humic extracts

Aqueous extracts of terrestrial and underwater soils were prepared by Soxhlet extractions at temperatures between 100 and 37°C (under reduced pressure). From the law of mass action, stability constants were derived for the complexation of Cd, Cu, and Pb by these extracts. The activity of the heavy metals was determined polarographically, whereas the concentration of acid functional groups was used for ligand activity. Visible spectra, electrical conductivity, organic matter contents, and complexation constants depended on extraction temperature as well as on soil properties. In most cases, complex stability increased with increasing pH of the polarographic medium, but sometimes maximum stability was found around pH 7.