Aqueous uptake of uranium(VI) by pyrite under Boom Clay conditions

The aqueous uptake of uranium(VI) on crushed natural pyrite was investigated under Boom Clay conditions, i.e., in a ~ 0.014 mol/L NaHCO3 electrolyte with special emphasis on the role of dissolved organic matter (~ 70 ppm DOC). Macroscopic batch experiments with different initial concentrations of uranium(VI) (10-8 – 10-4 mol/L) were combined with XANES-EXAFS spectroscopy on the solid phase to elucidate the speciation of uranium in these systems, and to gain insight into the major reaction mechanisms playing between uranium and pyrite. It was found that the predominant valence state of uranium in solution is likely to be uranium(VI), while on the solid phase mixtures of uranium(IV) (amorphous UO2 nanocolloids) and uranium(VI) (adsorbed onto FeIII oxide sites) were observed. These observations indicate that in these conditions, the uranium solid-liquid distribution is governed by both reduction and adsorption processes.