A procedure for quantitation of total oxidized uranium for bioremediation studies

A procedure was developed for the quantitation of complexed U(VI) during studies on U(VI) bioremediation. These studies typically involve conversion of soluble or complexed U(VI) (oxidized) to U(IV) (the reduced form which is much less soluble). Since U(VI) freely exchanges between material adsorbed to the solid phase and the dissolved phase, uranium bioremediation experiments require a mass balance of U in both its soluble and adsorbed forms as well as in the reduced sediment bound phase. We set out to optimize a procedure for extraction and quantitation of sediment bound U(VI). Various extractant volumes to sediment ratios were tested and it was found that between 1:1 to 8:1 ratios (v/w) there was a steady increase in U(VI) recovered, but no change with further increases in v/w ratio. Various strengths of NaHCO3, Na-EDTA, and Na-citrate were used to evaluate complexed U(VI) recovery, while the efficiency of a single versus repeated extraction steps was compared with synthesized uranyl-phosphate and uranyl-hydroxide. Total recovery with 1 M NaHCO3 was 95.7% and 97.9% from uranyl-phosphate and uranyl-hydroxide, respectively, compared to 80.7% and 89.9% using 450 mM NaHCO3. Performing the procedure once yielded an efficiency of 81.1% and 92.3% for uranyl-phosphate and uranyl-hydroxide, respectively, as compared to three times. All other extractants yielded 7.9-82.0% in both experiments. Biologically reduced U(IV) was treated either alone or mixed with uncontaminated sediment slurries to ensure that the procedure was not interfering with subsequent U(IV) quantitation. While U(VI) was recovered, it represented 0.07% of the total uranium alone or 7.8% when mixed with sediments. Total uranium recovered did not change. The procedure was then used to monitor changes in complexed U(VI) levels during uranium-reduction in pure culture and sediments. There was no appreciable complexed U(VI) concentration in pure culture. In sediments however, once soluble U(VI) levels and reduction rates decreased, complexed U(VI) levels began to decrease while U(IV) levels continued to increase. This indicated that once soluble U(VI) was nearly exhausted, sorbed U(VI) became bioavailable and was reduced microbiologically. Typically, uranium is quantified in two steps, soluble U(VI) and U(IV). However, the present study shows that after successive washings with water to remove soluble U(VI), a significant pool of oxidized uranium remains which may be mistakenly quantified as U(IV). This procedure can be used to quantified this pool, does not interfere with U(IV) quantitation, and has an overall efficiency of 95.8%. (C) 2003 Elsevier Science B.V. All rights reserved.