Investigation of U(VI) Adsorption in Quartz–Chlorite Mineral Mixtures

A batch and cryogenic laser-induced time-resolved luminescence spectroscopy investigation of U­(VI) adsorbed on quartz-chlorite mixtures with variable mass ratios have been performed under field-relevant uranium concentrations (5 × 10^–7 M and 5 × 10^–6 M) in pH 8.1 synthetic groundwater. The U­(VI) adsorption K_d values steadily increased as the mass fraction of chlorite increased, indicating preferential sorption to chlorite. For all mineral mixtures, U­(VI) adsorption K_d values were lower than that calculated from the assumption of component additivity possibly caused by surface modifications stemming from chlorite dissolution; The largest deviation occurred when the mass fractions of the two minerals were equal. U­(VI) adsorbed on quartz and chlorite displayed characteristic individual luminescence spectra that were not affected by mineral mixing. The spectra of U­(VI) adsorbed within the mixtures could be simulated by one surface U­(VI) species on quartz and two on chlorite. The luminescence intensity decreased in a nonlinear manner as the adsorbed U­(VI) concentration increased with increasing chlorite mass fraction–likely due to ill-defined luminescence quenching by both structural Fe/Cr in chlorite, and trace amounts of solubilized and reprecipitated Fe/Cr in the aqueous phase. However, the fractional spectral intensities of U­(VI) adsorbed on quartz and chlorite followed the same trend of fractional adsorbed U­(VI) concentration in each mineral phase with approximate linear correlations, offering a method to estimate of U­(VI) concentration distribution between the mineral components with luminescence spectroscopy.