Comparing the Efficiency of Various Microbial and Nanoparticulate Treatments aimed at Remediating Uranium and Strontium, with an Emphasis on Endpoint Identification and Recalcitrance to Oxidative Dissolution

Abstract

Radionuclide-polluted groundwater is present at numerous nuclear facilities across the world. Aside from the environmental problems associated with their migration, the practical and financial impacts surrounding conventional radionuclide remediation methods present further issues. Laboratory experiments encompassing pure culture, sediment microcosm and flowthrough column studies have shown that indigenous sediment bacteria can remove radionuclides including uranium and strontium from contaminated groundwater. For example the biostimulation of anaerobic bacteria with various organic electron donors has been shown to reduce soluble U(VI) to sparingly-soluble U(IV) biominerals. This enzymatic-mechanism of precipitating high oxidation state radionuclide contaminants as their reduced valence forms extends to other fission products (e.g. Tc(VII) to Tc(IV), and Np(V) to Np(IV)) and has been considered for prospective in-situ biotechnological remediation strategies. Enhancing the recalcitrance of reduced uranium (U(IV)) (bio)minerals is the subject of a significant body of academic research, given their susceptibility to oxidative dissolution. Strontium is a redox inactive element, lending to its contrasting biogeochemical cycling in the environment. In the environment, strontium’s solubility is most strongly mediated by the soil constituents possessing highly-reactive surfaces. These include silicate clay minerals and iron oxides, whose surfaces are prone to sorbing dissolved ionic species depending on their pH-dependant surface charge. Sorbed Sr2+ cations remain susceptible to being released back into solution via ion exchange with other divalent cations at mineral surfaces. Precipitating strontium biominerals through microbially generated ligands, such as carbonate and phosphate, is a wellstudied method for enhancing strontium removal. In this study, the susceptibility of various (a)biotically-derived uranium and strontium (bio)remediation endpoints to oxidative dissolution was assessed using sediment microcosms. After the simultaneous removal of U and Sr using a range of biotic and abiotic treatment methods, namely organic electron donors and functionalised nanoparticles respectively, separate reoxidation batch microcosm experiments were set up to investigate how recalcitrant these phases were to oxidising conditions, induced either aerobically or through the addition of nitrate. Of the 6 experimental microcosms, 3 treatment methods resulted in the enhanced cotreatment of U and Sr, compared with a control microcosm not amended with any treatment. The compounds that enhanced uranium and strontium removal were Carbo-iron, Nanofer25S and glycerol phosphate. Glycerol phosphate additions concurrently precipitated amorphous U(IV)-phosphate and strontium-incorporated calcium phosphate biominerals. The strontiumphosphate biominerals proved highly recalcitrant to oxidative dissolution, with the (a)biotically induced oxidising conditions further enhanced strontium removal as opposed to releasing strontium back into solution. The microcosms amended with Carbo-iron produced a carbon and iron-associated U(IV) species, by far the least susceptible to remobilisation under aerobic conditions, and also one of the most recalcitrant to nitrate-induced oxidation (along with glycerol phosphate and Nanofer25S treatments). Transferring the end-product of glycerol phosphate treatment (U(IV)-phosphate) to aerobic conditions resulted in a steady increase in dissolved uranium concentration, although the endpoint was highly resistant to nitrate-induced oxidation. Lastly, the potential of urea to stimulate bacterial ureolysis in an indigenous microbial community was examined, again using Sellafield-sediment microcosms. It was hypothesised that the ureolytic activity would generate sufficient alkalinity, favouring calcium carbonate precipitation, capable of coprecipitating aqueous Sr2+ ions. However, attempts to induce strontium-incorporated calcium carbona

Date of Award	6 Jan 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Jonathan Lloyd (Supervisor)