The influence of natural organic matter on the speciation and solubility of Eu in Boom Clay porewater

Abstract

AbstractThe influence of natural organic matter (NOM) on the speciation and solubility of europium (Eu) was studied under geochemical conditions representative for the Boom Clay. Different organic matter types were used, and analysis was performed both after 0.45 μm microfiltration and after 30000 MWCO ultrafiltration to distinguish between larger colloids (assumed to be immobile underin situconditions) and small dissolved species.Equilibrium was approached from undersaturation starting from synthesised Eu(OH)3(s), which, during the experiment, transformed into EuOHCO3(s), in agreement with thermodynamic considerations. In the absence of NOM, the Eu solution concentrations after 0.45 μm filtration exceeded the thermodynamic solubility of EuOHCO3(s) by several orders of magnitude, indicating the presence of inorganic Eu colloids. In the presence of NOM, the Eu solubility increased with increasing NOM concentration as was expected, but, surprisingly, was dependent on the operational size cut-off: at an identical NOM concentration in the filtrate, the Eu solution concentration after 0.45 μm filtration was consistently higher compared to the Eu concentration after 30000 MWCO filtration. This latter observation necessitates detailed knowledge concerning the pore size cut-off of Boom Clay underin situconditions in order to use the correct Eu-NOM complexation constant and/or maximum solubility in transport calculations. At higher NOM concentrations (TOC>30 mg/L) the Eu solubility after 0.45 μm filtration was seemingly independent of the NOM concentration.In contrast, after 30000 MWCO ultrafiltration, the Eu solution increased linearly with increasing DOC, from the expected thermodynamic solubility (∼5×10−7 mol L−1) at 0 mg L−1DOC to ∼3×10−5 mol L−1at 80 mg L−1DOC. All of the data sets were modelled using the Nagra/PSI database [8] for solubility, hydrolysis and inorganic aqueous complexation reactions, and fitted organic complexation reactions between Eu3+and NOM functional groups. Both a free ligand approach (with electrostatic correction) and the humic ion-binding model VI [23], which was for the first time successfully introduced into Phreeqc geochemical code, were tested and provided equally good fits to the data.