Physico-chemical processes of humic acid-trace element interactions

Abstract

Experiments were carried out under various pH conditions since this factor may strongly influence the equilibrium of various physico-chemical forms in which a certain trace element may occur in the aquatic environment. Pick-up processes were investigated for iron, cobalt, zinc, ruthenium and mercury under various experimental conditions. The rate and type of bonding or association of trace metals in humic acids was investigated by means of their replacement with other ions. For this reason the pick-up of a radionuclide of a certain trace metal by humic acid was performed, and when equilibrium was reached, the humic acids were transferred to another system where a known concentration of another ion was present. Replacement reactions took place and their yields were followed by radioactivity measurements in the liquid phase. The shape of each curve indicates the specific kinetics of the interaction of a particular trace element with humic acids under experimental conditions. Speed, total yield and time needed to reach equilibrium are specific for each particular case. The purpose of our work is to investigate the nature of trace-element interactions with humic acids. Having in mind the large variations that may occur in the constitution of humic acids, our attention focussed primarily on the search for the role these chemical variations and the different processes of their formation play on the ability of humic acids to bind trace elements.

Our results demonstrated that charge-dependent processes, such as ion exchange and sorption-desorption, present a major interaction mechanism in humic acidtrace element interaction. Another important mechanism by which some trace elements are associated with humic acids seems to be chelation.