Effects of Fulvic Acid on Uranium(VI) Sorption Kinetics

Abstract

This study focuses on the effects of fulvic acid (FA) on uranium(VI) sorption kinetics to a silica sand. Using a tritium-labeled FA in batch experiments made it possible to investigate sorption rates over a wide range of environmentally relevant FA concentrations (0.37-23 mg L(-1) TOC). Equilibrium speciation calculations were coupled with an evaluation of U(VI) and FA sorption rates based on characteristic times. This allowed us to suggest plausible sorption mechanisms as a function of solution conditions (e.g., pH, U(VI)/FA/surface site ratios). Our results indicate that U(VI) sorption onto silica sand can be either slower or faster in the presence of FA compared to a ligand-free system. This suggests a shift in the underlying mechanisms of FA effects on U(VI) sorption, from competitive sorption to influences of U(VI)-FA complexes, in the same system. Changes in metal sorption rates depend on the relative concentrations of metals, organic ligands, and mineral surface sites. Hence, these results elucidate the sometimes conflicting information in the literature about the influence of organic matter on metal sorption rates. Furthermore, they provide guidance for the selection of appropriate sorption equilibration times for experiments that are designed to determine metal distribution coefficients (Kd values) under equilibrium conditions.

Influence of Carbon and Microbial Community Priming on the Attenuation of Uranium in a Contaminated Floodplain Aquifer

“…The increased labile carbon and corresponding fulvic acid within PR sediments is related to the biomass and associated cell exudates generated during the addition of acetate to stimulate U bioreduction over a 2-year experimental period (Williams et al 2011). The fulvic acid concentration within PR sediments may be sufficiently high to overcome competitive sorption effects, resulting in faster sorption of U(VI)-fulvic acid complexes onto sediments as shown by Tinnacher et al (2013). This might explain the initial difference in U concentrations for batch experiments, but not the time length for which U(VI) re-sorption or reduction occurred in these experiments, with NR sediments lagging behind PR sediments by 2 weeks after acetate stimulation.…”

“…Our batch studies assess geochemical trends in U and carbon over a longer period than that of previous studies (Gu et al 2005a;Luo and Gu 2009), who tracked changes for less than 10 d, in order to ensure dissolution was not rate-limited by concentration or surface competition (Tinnacher et al 2013). Our batch studies assess geochemical trends in U and carbon over a longer period than that of previous studies (Gu et al 2005a;Luo and Gu 2009), who tracked changes for less than 10 d, in order to ensure dissolution was not rate-limited by concentration or surface competition (Tinnacher et al 2013).…”

“…Abiotically, certain mobile humic fractions complex with U(VI) (Czerwinski et al 1994;Lenhart et al 2000) affecting its mobility in oxygenated sediments (McCarthy et al 1998;Sachs and Bernhard 2011). It is becoming clear that NOM concentration in conjunction with both U concentration and surface site competition strongly govern the rate for which U(VI) dissolution occurs (Tinnacher et al 2013). Under anaerobic conditions, increased dissolution and mobilization of U(IV) can also occur in the presence of NOM (Mibus et al 2007), with humic acids influencing dissolution rates to a greater degree than fulvics (Luo and Gu 2009).…”

“…In order to better differentiate how U is retained or released from reduced sediments that occur naturally vs. those that are primed with carbon as a result of previous bioremediation events, we conducted a series of characterization studies and laboratory experiments to investigate differences in U release and sequestration over a 70-d period in batch and over 70-pore volumes in column experiments. Our batch studies assess geochemical trends in U and carbon over a longer period than that of previous studies (Gu et al 2005a;Luo and Gu 2009), who tracked changes for less than 10 d, in order to ensure dissolution was not rate-limited by concentration or surface competition (Tinnacher et al 2013). We also include an assessment of microbial community dynamics using molecular tools to better understand the potential biotic role in U dissolution.…”

Biogeochemical aspects of uranium mineralization, mining, milling, and remediation

“…Soluble HA can complex U(VI), increasing its solubility and reducing its ability to adsorb onto Fe oxides and clays at circumneutral pH (Křepelová et al, 2006). Natural organic matter can also affect the rate and extent of U sorption onto silica sand (Tinnacher et al, 2013). The increased solubility of U(VI) in the presence of HA also may impair reduction to U(IV) by solid phase Fe(II).…”