Biostimulation by Glycerol Phosphate to Precipitate Recalcitrant Uranium(IV) Phosphate

Abstract

Stimulating the microbial reduction of aqueous uranium(VI) to insoluble U(IV) via electron donor addition has been proposed as a strategy to remediate uranium-contaminated groundwater in situ. However, concerns have been raised regarding the longevity of microbially precipitated U(IV) in the subsurface, particularly given that it may become remobilized if the conditions change to become oxidizing. An alternative mechanism is to stimulate the precipitation of poorly soluble uranium phosphates via the addition of an organophosphate and promote the development of reducing conditions. Here, we selected a sediment sample from a U.K. nuclear site and stimulated the microbial community with glycerol phosphate under anaerobic conditions to assess whether uranium phosphate precipitation was a viable bioremediation strategy. Results showed that U(VI) was rapidly removed from solution and precipitated as a reduced crystalline U(IV) phosphate mineral similar to ningyoite. This mineral was considerably more recalcitrant to oxidative remobilization than the products of microbial U(VI) reduction. Bacteria closely related to Pelosinus species may have played a key role in uranium removal in these experiments. This work has implications for the stewardship of uranium-contaminated groundwater, with the formation of U(IV) phosphates potentially offering a more effective strategy for maintaining low concentrations of uranium in groundwater over long time periods.

Rapid Hydrolysis of Organophosphates Induced by U(IV) Nanoparticles: A Kinetic and Mechanistic Study using Spectroscopic Analysis

“…Understanding the speciation, mechanism, and kinetics of U transformation under various conditions relevant to natural and engineered systems is essential to predicting and controlling the migration of U in contaminated environments. Recently, bioreduction accompanied by the phosphate-precipitated U(VI) phase has been considered as a potential pathway for U transformation to U(IV)-phosphate mineral phases that could further limit U mobility [12,14]. Previous studies have shown that such processes can compete with or limit uraninite formation during the bioreduction of aqueous U(VI) in the presence of dissolved phosphates [54].…”

“…Murray et al observed that U can be strongly adsorbed on apatite, which has a high capacity for retaining U by phosphate mineralization [7]. Biotic approaches for immobilizing subsurface U were also examined using microbial phosphatase activities that promote the in situ sequestration of U as an insoluble phosphate mineral [11,12]. In the presence of organophosphates under reducing conditions, the biomineralization of U(VI)-phosphate minerals has been shown to be more effective for removing U than bioreduction to U(IV) species; e.g., uraninite.…”

“…This is because biogenic U(IV) is considered to be prone to oxidative remobilization after exposure to oxygen or nitrate [13]. Furthermore, the ningyoite-like U(IV)-phosphate precipitates obtained by stimulating microbial U(VI) reduction in supplement with organophosphate (e.g., glycerol phosphate) exhibit high resistance to oxidative remobilization [12]. Also, mononuclear U(IV)-phosphate complexes and nanocrystalline U(IV) minerals were identified as major forms of U in laboratory bioreduction settings as well as in lake sediments possessing rich organic matter [14,15].…”

Uranium Bioreduction and Biomineralization

“…Stimulating a sediment microbial community with G2P under anaerobic conditions led to the formation of crystalline U(IV) phosphate minerals (e.g. ningyoite), which were more recalcitrant to oxidative remobilization than the products of microbial U(VI) reduction (Newsome et al, 2015b). An isolated strain (Serratia sp.)…”

Removal of Soluble Strontium via Incorporation into Biogenic Carbonate Minerals by Halophilic Bacterium Bacillus sp. Strain TK2d in a Highly Saline Solution

“…Recently, various methods that use biomineralization by microorganisms to remove radionuclides from contaminated solutions have been studied (6)(7)(8). Using biogenic minerals, dispersed radionuclides at low concentrations can be removed for a long period at a low cost.…”