Bioremediation and bioreduction of dissolved U(VI) by microbial mat consortium supported on silica gel particles

The goal of this research was to simulate natural attenuation processes for uranium (U) by constructing a bioremediation system based on microbial mats. Ancient microbial consortia, such as microbial mats, have evolved the capacity to manage hostile environments and potentially bioremediate metal-contaminated water. To facilitate the engineering applications of microbial mats, the constituent microbial groups of the mat consortium were immobilized with required nutrients in silica gel. Resulting silica mat particles (SMP) were tested for efficiency in removing and, subsequently, reducing U(VI) to U(IV). Uranium-containing groundwaters from a Superfund Site (0.2 mg U(VI) L-1, 293 mg HCO3 L-1 at pH 8.2) and synthetic waters (similar to 0.05-2 mg U(VI) L-1, pH 8-9) were assessed with this system. Over 80% of the dissolved U(VI), present as mostly U(VI)-carbonate (Superfund) or U(VI)-hydrolysis species (synthetic), was removed by the SMP within 15 min of treatment. X-ray absorption near-edge structure spectroscopy studies showed that the U sequestered in the SMP was reduced to U(IV) within 24 h of exposure. Effective sequential batch treatments and maintenance of a low redox environment by nutrient additions demonstrated the potential for long-term durability and capacity for continuous use of this system. Drying the SMP produced a hard compact product (1% of original weight). Capacity for on-site generation of SMP, relative low-cost constituent materials, the simplicity of management, and the formation of a stable compact disposal product indicate this system has great potential for the field remediation of U-contaminated waters.