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Mixed-species microbial mats that were dominated by the cyanobacterium Oscillatoria sp. and contained heterotrophic and purple autotrophic bacteria were constructed for specific bioremediation applications. When the mats were challenged with metals, production and secretion of metal-binding extracellular polysaccharide bioflocculants were observed. The concentration of these negatively charged polysaccharides was correlated with the removal of manganese from the water column beneath a surface microbial mat. Bioflocculants from an Oscillatoria sp. that was isolated from the mat were collected and concentrated for characterization. A chromatographic analysis revealed a heterogeneous population of polysaccharides with respect to charge density and molecular size. The subpopulation of polysaccharides which exhibited the highest level of flocculating activity was polyanionic and had a molecular weight of more than 200,000. A glycosyl analysis of the bioflocculants revealed the presence of galacturonic acid (2.2%) and glucuronic acid (1.86%). The presence of these components, which were negatively charged at the pH levels generated by the mats during photosynthesis (pH > 7.5), may account for the metal-binding properties of the mats.

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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). Uraniumcontaining groundwaters from a Superfund Site (0.2 mg U(VI) L -1 , 293 mg HCO 3 L -1 at pH 8.2) and synthetic waters (∼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.