Uranium speciation and bioavailability in aquatic systems: an overview.

Scott J Markich

doi:10.1100/tsw.2002.130

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 or UO2) 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 UO2 and UO2OH 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 UO2 and UO2OH. 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.

Highlights

The bioavailability of uranium (U) is dependent on its speciation, or physicochemical form
Using multiple linear regression analysis, combined with speciation modeling, Markich et al.[98] provided evidence to show that, under the prescribed experimental conditions, the biological response (BR) of V. angasi to U was related to the activity of particular U species (i.e., BR ∝ 1.86 × UO22+ + UO2OH+) and not the total U concentration (Fig. 5)
The results showed that there was no significant (p > 0.05) difference in the mean duration of valve opening (DVO) of V. angasi exposed to U in both waters (Fig. 6)

Summary

INTRODUCTION

The bioavailability of uranium (U) (i.e., its ability to bind to or traverse the cell surface of an organism) is dependent on its speciation, or physicochemical form. Surface waters and sediments contaminated with anthropogenic U, principally from the nuclear fuel cycle (e.g., mining and milling of U ore and reprocessing of waste) and from the combustion of petrofuels (e.g., coal) and the manufacturing and application of phosphatic fertilizers (e.g., superphosphate), pose potential ecological risks[6,7] Such risks are usually evaluated, at least in the first instance, by determining the total U concentration in water and/or sediments, and comparing these values with established guidelines or standards (where available) for protecting aquatic ecosystems. The average concentration of U (as 238U, the most abundant isotope) in riverwater is 0.3 μg l-1[15,16] but typically ranges from 0.01 to 6.6 μg l-1 depending on contact time with the Ubearing strata, the U content of the strata, the amount of evaporation, and the availability of complexing ions[11]. Substantial U enrichment (up to 100 times) has been reported for sediments sampled from anoxic environments (e.g., swamps or deep ocean basins)[20]

SPECIATION METHODS

Analytical Methods

Method

Computational Methods

Findings

SUMMARY