Uranium Speciation and Bioavailability in Aquatic Systems: An Overview

Abstract

The speciation of uranium (U) in relation to its bioavailability is reviewed for surface waters (fresh-and seawater) and their sediments. A summary of available analytical and modeling techniques for determining U speciation is also presented. U(VI) is the major form of U in oxic surface waters, while U(IV) is the major form in anoxic waters. The bioavailability of U (i.e., its ability to bind to or traverse the cell surface of an organism) is dependent on its speciation, or physicochemical form. U occurs in surface waters in a variety of physicochemical forms, including the free metal ion (U 4+ or UO 2 2+ ) and complexes with inorganic ligands (e.g., uranyl carbonate or uranyl phosphate), and humic substances (HS) (e.g., uranyl fulvate) in dissolved, colloidal, and/or particulate forms. Although the relationship between U speciation and bioavailability is complex, there is reasonable evidence to indicate that UO 2 2+ and UO 2 OH + are the major forms of U(VI) available to organisms, rather than U in strong complexes (e.g., uranyl fulvate) or adsorbed to colloidal and/or particulate matter. U(VI) complexes with inorganic ligands (e.g., carbonate or phosphate) and HS apparently reduce the bioavailability of U by reducing the activity of UO 2 2+ and UO 2 OH + . The majority of studies have used the results from thermodynamic speciation modeling to support these conclusions. Time-resolved laser-induced fluorescence spectroscopy is the only analytical technique able to directly determine specific U species, but is limited in use to freshwaters of low pH and ionic strength. Nearly all of the available information relating the speciation of U to its bioavailability has been derived using simple, chemically defined experimental freshwaters, rather than natural waters. No data are available for estuarine or seawater. Furthermore, there are no available data on the relationship between U speciation and bioavailability in sediments. An understanding of this relationship has been hindered due to the lack of direct quantitative U speciation techniques for particulate phases. More robust analytical techniques for determining the speciation of U in natural surface waters are needed before the relationship between U speciation and bioavailability can be clarified.

Products of abiotic U(VI) reduction by biogenic magnetite and vivianite

“…The role of phosphate in the speciation of hexavalent uranium is well studied (Markich, 2002). However its role in the speciation of reduced uranium in environmentally relevant systems is less well known.…”

Phosphate regulates uranium(VI) toxicity to Lemna gibba L. G3

“…Generally, uranium occurs in surface waters in a variety of physicochemical forms, including the free metal ion (U 4þ or UO 2 2þ ) and complexes with inorganic ligands (e.g., uranyl carbonate or uranyl phosphate), and humic substances (e.g., uranyl fulvate) in dissolved, colloidal, or particulate forms (Markich, 2002). The major bioavailable forms of U(VI) are likely UO 2 2þ and UO 2 OH þ rather than uranium complexed or adsorbed to colloidal or particulate matter (unpublished data).…”

“…The major bioavailable forms of U(VI) are likely UO 2 2þ and UO 2 OH þ rather than uranium complexed or adsorbed to colloidal or particulate matter (unpublished data). U(VI) complexed with inorganic ligands (e.g., carbonate, sulfate, or phosphate) and humic substances apparently reduce the bioavailability of uranium by reducing the activity of UO 2 2þ and UO 2 OH þ (Kohler et al, 1996;Dodge and Francis, 1997;Markich, 2002).…”

Interaction of Uranium with Bacterial Cell Surfaces: Inferences from Phosphatase-Mediated Uranium Precipitation

“…It is important to understand how these different U species interact with bacterial cellular surfaces, especially for designing biological wastewater treatment systems. However, studies evaluating the effect of aqueous U speciation have been largely limited to biosorption, bioaccumulation, and bioreduction (9,15,16,20).Among the biological mechanisms involved in metal remediation, enzymatic bioprecipitation of heavy metals as metal phosphates is particularly attractive and is considered to be a promising approach for biological treatment of U effluents due to its high efficiency (14, 21). Bioprecipitation of metals as phosphates is mediated by phosphatases that cleave a phosphomonoester substrate (such as ␤-glycerophosphate) to release the phosphate moiety, which in turn precipitates heavy metals, such as U, Cd, Ni, Am, etc., from solutions (22, 23).…”