Np(V) reduction by humic acid: contribution of reduced sulfur functionalities to the redox behavior of humic acid.
The role of sulfur-containing functional groups in humic acids for the Np(V) reduction in aqueous solution has been studied with the objective to specify individual processes contributing to the overall redox activity of humic substances. For this, humic acid model substances type M1-S containing different amounts of sulfur (1.9, 3.9, 6.9 wt.%) were applied. The sulfur functionalities in these humic acids are dominated by reduced-sulfur species, such as thiols, dialkylsulfides and/or disulfides. The Np(V) reduction behavior of these humic acids has been studied in comparison to that of the sulfur-free humic acid type M1 at pH 5.0, 7.0 and 9.0 under anaerobic conditions by means of batch experiments. For Np redox speciation in solution, solvent extraction and ultrafiltration were applied. In addition, redox potentials of the sample solutions were monitored. At pH 5.0, both rate and extent of Np(V) to Np(IV) reduction were found to increase with increasing sulfur content of the humic acids. At pH 7.0 and 9.0, sulfur functional groups had only a slight influence on the reduction behavior of humic acid toward Np(V). Thus, in addition to quinoid moieties and non-quinoid phenolic OH groups, generally acknowledged as main redox-active sites in humic substances, sulfur functional groups have been identified as further redox-active moieties of humic substances being active especially in the slightly acidic pH range as shown for Np(V). Due to the low sulfur content of up to 2 wt.% in natural humic substances, their contribution to the total reducing capacity is smaller than that of the other redox-active functional groups.