Bioligand-mediated partitioning of radionuclides to the aqueous phase

Abstract

The aqueous-phase partitioning of ⁵⁹Fe, ¹⁴⁷Pm, ²³⁴Th and ²⁴¹Am by complexing compounds from subsurface bacteria has previously been studied in the presence of quartz sand. In this study the aqueous-phase partitioning of pico-to submicromolar amounts of ⁵⁹Fe, ¹⁴⁷Pm, ²³⁴Th and ²⁴¹Am was analyzed in the presence of TiO₂ and exudates from three species of subsurface bacteria: Pseudomonas fluorescens, Pseudomonas stutzeri, and Shewanella putrefaciens. All were grown under aerobic conditions and P. stutzeri and S. putrefaciens were grown under anaerobic conditions as well. The supernatants of the aerobic and anaerobic cultures were collected and radionuclide was added. TiO₂, with BET surface area of 49.9 m²·g⁻¹, was added to the supernatant radionuclide mix, and the pH was adjusted to approximately 8. After incubation, the amount of radionuclide in the liquid phase of the samples and controls was analyzed using scintillation method. Two types of values were calculated: solution% = the activity maintained in solution relative to the total activity, and Q-values = the quotient between the activity in samples and the activity in controls. Aerobic supernatants had solution% values between 89% and 100% for ⁵⁹Fe and between 18 and 43% for ²³⁴Th. The solution% values for ²⁴¹Am and ¹⁴⁷Pm were less than 2% overall, but the Q-values were between 34 and 115 times more ²⁴¹Am in bacterial supernatants than in controls. The corresponding values for ¹⁴⁷Pm ranged from 6 to 20 times more than in the control. The solution% values for all elements in the presence of anaerobic supernatants were below 2%, but the Q-values clustered around 7 for ⁵⁹Fe and ranging from 2 to 29 for ^234Th, indicated that anaerobic supernatants partitioned these elements to the aqueous phase. Both aerobic and anaerobic supernatants tested positive for complexing compounds when analyzed, using the Chrome Azurol S assay. Complexation with excreted organic ligands is most likely the reason for the higher amounts of metals in samples than in the controls. Hence, aerobically and anaerobically excreted organic ligands seem able to influence the mobility of radionuclides in aerobic and anaerobic environments contaminated with these compounds.