Insight into sequestration and release characteristics of uranium(VI) on phlogopite in the presence of humic acid

Abstract

Knowledge of the sorption speciation and surface configuration of uranium(VI) at the mineral/water interface is essential to construct reliable retention and migration models. However, the ubiquitously existing natural organic substances at U(VI)-contaminated sites readily interact with U(VI) and interfere with the environmental fate of U(VI). In this work, the adsorption behavior and mechanism of U(VI) on phlogopite in the presence of humic acid (HA) were investigated by combining batch experiments, cryogenic time-resolved laser fluorescence spectroscopy (TRLFS), and extended X-ray absorption fine structure (EXAFS) spectroscopy. The batch sorption experiments at different HA concentrations suggested that HA had little effect at pH < 4 but suppressed U(VI) sorption on phlogopite from pH 4 to 12. Fluorescence spectral characteristics indicated the formation of multiple surfaces and aqueous U(VI)-humate species, whose abundances varied with pH. The TRLFS coupled with EXAFS spectra suggested that the HA-U(VI) hybrids preferentially bind to surface sites via U(VI) rather than HA. The humate uranium species increased uranium release and migration risk in the natural environment. These findings elucidate the species characteristics and environmental behavior of U(VI) in the presence of natural humic acid and provide guidance for remediation treatments and safety assessment of uranium-contaminated sites.