Reduction and reoxidation of humic acid: influence on speciation of cadmium and silver.
Naturally occurring variations of redox conditions are considered to affect the interactions between trace metals and humic substances in a 2-fold manner. First, additional proton binding sites of humic substances formed under reducing conditions may also act as binding sites for trace metals. Second, reduced humic substances may transfer electrons to redox-active trace metals. In this study, we investigated the influence of electrochemical reduction of a purified soil humic acid on the binding of two chalcophile metal cations of environmental concern, Cd(2+) and Ag(+), with metal titrations conducted under monitored redox conditions. The binding of cadmium to reduced humic acid was slightly enhanced compared to humic acid reoxidized by O(2) and quantitatively in excellent agreement with the increase in binding sites formed upon reduction. Competitive experiments with calcium indicated that sulfur-containing sites played a minor role in cadmium binding, although sulfur K-edge XANES revealed that 36% of humic sulfur was in a reduced oxidation state. In all experiments with silver, the formation of Ag(0) was detected with transmission electron microscopy. Free Ag(+) activities under reducing conditions were controlled by Ag(0) formation rather than by binding to humic acid.