Impact of environmentally based chemical hardness on uranium speciation and toxicity in six aquatic species

Abstract

Treated effluent discharge from uranium (U) mines and mills elevates the concentrations of U, calcium (Ca), magnesium (Mg), and sulfate (SO42–) above natural levels in receiving waters. Many investigations on the effect of hardness on U toxicity have been experiments on the combined effects of changes in hardness, pH, and alkalinity, which do not represent water chemistry downstream of U mines and mills. Therefore, more toxicity studies with water chemistry encountered downstream of U mines and mills are necessary to support predictive assessments of impacts of U discharge to the environment. Acute and chronic U toxicity laboratory bioassays were realized with 6 freshwater species in waters of low alkalinity, circumneutral pH, and a range of chemical hardness as found in field samples collected downstream of U mines and mills. In laboratory-tested waters, speciation calculations suggested that free uranyl ion concentrations remained constant despite increasing chemical hardness. When hardness increased while pH remained circumneutral and alkalinity low, U toxicity decreased only to Hyalella azteca and Pseudokirchneriella subcapitata. Also, Ca and Mg did not compete with U for the same uptake sites. The present study confirms that the majority of studies concluding that hardness affected U toxicity were in fact studies in which alkalinity and pH were the stronger influence. The results thus confirm that studies predicting impacts of U downstream of mines and mills should not consider chemical hardness.

Bioaccumulation and toxicity of uranium, arsenic, and nickel to juvenile and adult Hyalella azteca in spiked sediment bioassays

“…The present higher toxicity values for adults based on body burden, overlying water, sediment porewater, and solids compared with juveniles further indicate that juveniles are more sensitive to metals than adults ( Table 2). The present results are well aligned with findings reported elsewhere suggesting that early life stages are the most sensitive to metals [15,35,52,53]. Juveniles are likely more sensitive to metals because they are smaller, and their detoxification mechanisms are sometimes immature and less able to cope with sudden increases in metal uptake.…”

The role of sediment properties and solution pH in the adsorption of uranium(VI) to freshwater sediments

“…Stock solutions of U(VI) were prepared from uranyl nitrate hexahydrate (UO2(NO3)2•6H2O (Strem Chemicals, Inc., Newburyport, MA, USA) and ultra-pure water (18 M-cm). Test solutions were reconstituted from ultra-pure water with 0.01 M NaNO3-0.005 M CaSO4 to mimic water quality conditions (i.e., conductivity, total hardness, alkalinity, and major ions; Table S2) found downstream of Saskatchewan U mining and milling operations (EARMP, 2014;Goulet et al, 2015).…”

“…The pH of the suspensions were adjusted daily with 0.1 M HNO3 and/or NaOH (less than 0.05% of the total solution volume) to the desired pH values of 6, 7 and 8 until a stable pH was achieved (maximum up to 10 days). These pH values represent the range of overlying water pH typically found downstream of Saskatchewan U mines (EARMP, 2014;Goulet et al, 2015).…”

Predicting the bioavailability of sediment‐bound uranium to the freshwater midge (Chironomus dilutus) using physicochemical properties

“…Other water chemistry variables such as pH, hardness, and alkalinity can also influence the sorption and bioavailability of U [33,47,48]. The sorption of U is significantly pH-dependent, with peak sorption occurring at circumneutral pH values [15,36], which can influence the availability of U.…”

“…U-contaminated field sediments Test conditions. The physicochemical properties of the Ucontaminated field sediments are presented in Table 1, and the associated water chemistry from the 10-d laboratory bioaccumulation sediment tests and field conditions are presented in Supplemental Data, Other water chemistry variables such as pH, hardness, and alkalinity can also influence the sorption and bioavailability of U [33,47,48]. The sorption of U is significantly pH-dependent, with peak sorption occurring at circumneutral pH values [15,36], which can influence the availability of U.…”