Interaction of Modified Humic Substances with Np(V): Influence of Monomer Nature on Redox Properties of Humic Substances

The complexation of radionuclides with humic substances (HS), which are ubiquitous in the environment, is recognized as an important factor for the safety assessment of geological disposal of radioactive wastes. HS possess a whole variety of functional groups with prevailing contribution of carboxyls and hydroxyls. This provides for their redox and complexation properties. As a result, HS impact substantially speciation of heavy metals and radionuclides in the contaminated environments. Interactions of HS with neptunium (Np) are of particular concern due to high radiotoxicity and long half-life of this actinide. The most stable oxidation state of neptunium is (V) presented by dioxocation NpO2. The latter has a low ion charge which results in high migration ability of these species. Interactions with mineral phases, complexation with different ligands and redox transformation can strongly affect transport of Np(V). Besides inorganic ligands such as carbonate, sulfate and phosphate, HS play an important role in speciation of Np in the environment. Most of the models of HS–metal ion interaction assume that carboxylic groups act as major complexing sites for metal ions (1, 2). At the same time, phenolic and quinonoid groups are considered to be responsible for redox properties of HS (3). In our previous publication (4) we have demonstrated that incorporation of additional hydroquinone groups into backbone of leonardite humic acids (HA) resulted to a substantial increase in reducing ability of the humic materials with respect to NpO2. The goal of this research was to find out if the kinetics of this reaction can be improved by changing structure of the quinonoid center, nominally, by using methylsubstituted hydroquinone. The idea behind using the substituted hydroquinone for improving kinetics was the known fact that the presence of methyl-substituent in the hydrioquinone ring hinders formation of quinhydrone (5). The stability of quinhydrone slows down substantially reactions with participation of non-substituted hydroquinones.