Np(V) reduction by humic acids - Impact of sulfur functionalities of humic acids

The migration behavior of actinide contaminants is influenced by humic substances (fulvic acids and humic acids (HA)) due to their complexing and redox properties and their ability to form colloids. The redox activity of humic substances is attributed to the reversible hydroquinone/quinone redox couple with semiquinone-type free radicals as significant electron donor/acceptor intermediate species and furthermore, to the oxidation of phenolic OH groups to phenoxy radicals. Also nitrogen and sulfur containing functional groups of HA are discussed as redox-active groups. While sulfur functional groups of HA have not been studied as reducing agents toward actinides like U, Np or Pu so far, organic compounds containing sulfur in the reduced form as thiol groups (R-SH) were found to reduce Np(VI).
In the present work, the influence of sulfur functional groups of HA on their capability to reduce Np(V) in aqueous solution has been studied. For this, HA model substances type M1-S with different sulfur contents (1.9, 3.9, 6.9 wt.%) were applied. Reduced sulfur species, such as thiols, dialkylsulfides and/or disulfides, were determined as the dominating sulfur functionalities in the HA. Their Np(V) reduction behavior was studied in comparison to a sulfur-free HA type M1.
At pH 5.0, the Np(V) reduction strongly increases with increasing content of reduced sulfur functional groups of the HA. That means that the sulfur functionalities contribute to the reduction capability of HA at pH 5.0. In contrast to this, at pH 7.0 and 9.0, the sulfur functionalities of HA have only a slight influence on the Np(V) reduction (not shown). This implies that the contribution of the sulfur functionalities to the reduction capability of HA toward Np(V) depends on pH value.