Neptunium(V) sorption onto granite and its mineral constituents in the absence and presence of humic acid

The sorption of neptunium(V) onto granite and its main mineral constituents quartz, orthoclase, albite, biotite, and muscovite is studied under anaerobic conditions as a function of pH in a series of batch equilibrium experiments. Furthermore, the effect of humic acid on the neptunium(V) sorption is studied applying a 14C-labeled synthetic humic acid (type M42) for the experiments. The data suggest that the neptunium sorption onto granite is affected by both the pH and the presence of organic material. In the absence of humic acid, the neptunium sorption starts between pH 7 and pH 8 and increases with increasing pH value. Due to addition of humic acid the neptunium sorption onto granite is decreased in the neutral to alkaline pH range, which is attributed to complexation of neptunium by humic acid in solution. Thus, in this pH range the neptunium mobility is increased by humic substances. The neptunium(V) sorption onto granite and onto its mineral constituent biotite is nearly equal, suggesting that the sorption of neptunium(V) is controlled by the minor amount of biotite which, however, is reactive.