(25f) Removal of Radioactive Uranium From Groundwater Using Nanoparticle Technology and Bioremediation Strategies

In recent times, water contamination from radiological materials has become a significant hazard to human health. Wastewaters containing uranium and other actinides generated in nuclear reactors in processes related to generating energy and manufacturing nuclear weapons have been discharged for over 50 years.  Many sites are contaminated with toxic uranium owing to solubilization of natural uranium, production of nuclear energy, and mining of uranium. Consequently, uranium is one of the most common radionuclides in soils, sediments, and groundwater at several U.S. Department of Energy (DOE) sites and is therefore of particular environmental concern. Acid in situ mining technology has been adopted as the main approach for exploiting uranium deposits as nuclear fuel.  After the  leaching process, the groundwater is generally characterized by very low pH and high levels of radioactive uranium.  The radionuclide occurs as hexavalent uranyl (UO₂²⁺) that is extremely mobile, contaminating the groundwater resources.  Generally, the long-lived  radionuclides of the ²³⁸U₉₂ series are extremely hazardous owing to their chemical toxicity and high radioactivity.  Ingestion of uranium from contaminated water can cause adverse health effects including several forms of cancer, severe kidney or liver damage, birth defects, and even death.  The World Health Organization has recognized  U(VI) as a human carcinogen, and has recommended that its concentration in drinking water be below 15 micrograms per liter. Some of the DOE sites have uranium contamination in both groundwater and soil.  Traditional ex situ remediation approaches based on pump-and-treat methods such as lime neutralization, anion exchange, activated aluminum and biosorption are extremely expensive and limited  due to poor extraction efficiency, inhibitory competing ions, and massive waste production.  Furthermore, bringing the radioactive contaminants to the surface level shall increase the health and safety risk factors for the cleanup workers and general public.  Therefore, efficient and economical strategies are needed for treatment of uranium contamination in groundwater and prevent its further spread into deep subsurface.

In nature, uranium is present in two valence states, namely, hexavalent uranium, U(VI), and tetravalent uranium, U(IV).  In groundwater at low pH values, the hexavalent form prevails, forming a highly soluble and mobile ionic compound, uranyl (UO₂²⁺);  however,  at higher pH levels U(VI) can form carbonate and hydroxide compounds.  Meanwhile, tetravalent uranium is very insoluble in water, as it forms uraninite, UO₂.  Reduction of uranium from U(VI) to the U(IV) form can prevent its  migration through groundwater and effectively protect our drinking water sources. The present work investigates the removal of uranium from groundwater by two methods: use of zero-valent iron nanoparticles (nZVIs or nFe⁰) and bioremediation by microbial reduction using sulfate reducing bacteria (SRB).

Nano-scale zero-valent iron (nZVI) is of increasing interest for use in a variety of environmental remediation, water and waste water treatment applications.  The  nZVI particles  are highly mobile with high surface-to-volume ratio, and various types of nZVI have been  widely studied as chemical reductants and adsorbents  for environmental applications.  This can be attributed to their low production costs and high efficiencies for the removal of a wide range of contaminants including heavy metals and chlorinated compounds.  Their use for next-generation remediation technologies is directed at improving the efficacy, versatility and economics of treatment. The application of nZVI particles shall not only reduce U (VI) to insoluble U(IV), but  will also result in the adsorption and precipitation of uranyl ions onto these particles.    The fundamental concept is that nZVIs are a source of aqueous  Fe(II), Fe(III), hydrogen, free radicals, and other species including precipitates of iron-hydroxy complexes that can significantly contribute to uranium removal.    It must be noted that the bulk of the U(VI) is in contact with the reactive sites of the nZVI (nFe⁰) particles, so that reduction of U(VI) to U(IV) occurs as the UO₂²⁺species is transformed to UO₂(s), and the nZVI (nFe⁰) is oxidized to the ferrous iron (Fe²⁺).  Another reaction occurs between the nZVI and the protons (H⁺) in solution to yield the ferrous ions (Fe²⁺), which in turn undergoes oxidation to the ferric form (Fe³⁺).  Simultaneously, the U(VI) is reduced to U(IV) in the redox coupling reactions.  The products UO₂(s) and Fe(OH)₃ (s) are synchronously coprecipitated and uranyl-ferric metal complexes are formed resulting in uranium immobilization.  

Biological remediation technologies using microbial populations have proven efficient and cost-effective in the treatment waters contaminated with uranium.  Among various subsurface microorganisms,  the sulfate reducing bacteria (SRB) are a major group responsible for heavy metal immobilization and groundwater detoxification.  It is well documented that various  types  of SRB including  several trains of  Desulfovibrio have been successful in the reduction of uranium from the U(VI) to U(IV) form by transforming uranyl (UO₂²⁺) to uranite  (UO₂)..  Additionally, when SRB are artificially stimulated in the subsurface using nutrient injection, multiple benefits might be derived besides uranium reduction. Other heavy metals and radionuclides such as  chromium Cr(VI), U(VI), and technetium Tc(VII) could  also undergo reduction to less soluble forms.  Bioremediation of uranium using SRB essentially involves microbial reduction of U(VI) to U(IV) through the release of enzymes, as well as through chemical reduction by interaction with microbial by-products.  Therefore, the  reduction of U(VI) to U(IV) using SRB involves at least these three important processes: (1) U(VI) binding to the cell surface and to extracellular biopolymers (biosorption); (2) chemical reduction of U(VI) by microbially generated hydrogen sulfide (H₂S); and (3) bioreduction of U(VI), which is enzymatic dissimilatory metal reduction with U(VI) acting as a terminal electron acceptor. Additionally, sorption onto bio-surfaces, including polymers and dead cells can contribute to the removal and immobilization of uranium.  Chemical reduction by the H₂S produced by SRB is an important aspect of this research, as these bacteria can substitute uranium (VI) as an electron acceptor to produce energy.  However, certain process variables including pH, nutrients, temperature, and uranium concentration must be investigated and carefully controlled for achieving maximum removals of uranium. One of the major challenge sin the implementation of the technology  is to make the SRB reduction of uranium  a more efficient and economical process by optimally using carbon sources and electron donors such as methanol, ethanol, etc.

The experimental methodology for this study uses two parallel columns packed with aquifer sand media – one containing nZVI particles, and the other  a microbial biofilm of  SRB.  The uranium concentrations in the aqueous phase were determined by liquid scintillation counting (LSC).  This analytical technique achieved a low uranium detection limit of 2 micrograms per liter.  The  continuous flow column experiments  determined uranium removal rates as a function of time, and evaluated the overall process dynamics and efficiencies for the two scenarios (using  nZVI particles  and employing a  a SRB biofilm).  The presentation shall involve detailed investigation of the mechanisms associated with uranium removal in the two different processes – sand column containing nZVIs, and sand column with a biofilm constituted by SRB.  It will also include the optimization of the sand column with SRB biofilm using appropriate carbon source and electron acceptor for optimizing the process efficiency and economics. Furthermore, the discussion will include the presentation of a mathematical model for forecasting the removal of uranium in abiotic (nZVI) and biotic (SRB) soil columns.