Bioremediation of Mercury
A number of naturally occurring Hg biotransformations may have applications in the bio-remediation of Hg-contaminated soil and water. Mercury-resistant bacteria have been isolated by Glombitza at the Akademie der Wissenschaften (137,138). These Hg-resistant bacteria can accumulate up to 2%-4% by weight of nonvolatile Hg from aerated solutions containing 50-100 mg LHg2+ (139). Biomass can be suspended in a feed solution within a bioreactor and provided a C source (e.g., methanol or acetate) plus additional nutrients. The Hg-laden biomass is separated from solution and thermally decomposed at 400°-500°C to recover the Hg distillate.
Another technique being investigated to remediate Hg is biosorption. One such technology, which employs immobilized algae, is marketed by Bio-Recovery Systems under the name Alga-SORB (139). The product is prepared by heating the algae (Chlorella vulgaris) to 300°-500°C and immobilizing them on a silicate-based matrix. Alga-SORB shows a strong adsorption for Hg2+ independent of pH over a range of 2-6. Mercury adsorption occurs as a result of interactions with ''soft'' ligands (forms covalent complexes with ''soft'' functional groups that contain N or S) present in the cell wall. In an E.P.A.-
sponsored SITE demonstration, Alga-SORB columns treated with 0.14-1.6 mg Hg resulted in an effluent consistently below 10 |g L_1. Unfortunately, biosorption is not effective in removing organically bound Hg compounds such as phenylmercuric acetate or MeHg (139). The biosorption systems are only effective on Hg in its cationic form (i.e., Hg2+). Therefore, Hg covalently bound to organic carbon such as MeHg must be hydrolyzed to the free Hg ion prior to its removal by biosorption.
The microbiological reduction of Hg2+ to Hg0, followed by volatilization, is another potentially useful process to remediate Hg-contaminated waste streams. To be used as a method to remove Hg, volatile Hg0 must be captured, requiring a containment system (see proposed bioreactor, Fig. 7). Unfortunately, this design would be limited in the volume of water that could be treated.
Some of the more well-characterized Hg treatment systems have been developed with Chlorella emersonii (140) and Hg-reducing bacteria, including Xanthomonas mal-tophilia, Aeromonas hydrophila, Alcaligenes eutrophus, Pseudomonas paucimobilis, and Microbacterium sp. (Gesellschaft für Biotechnologische Forschung [GBF], Institute of Biotechnological Research, Braunschweig, Germany; 139). When Chlorella emersonii was immobilized on calcium alginate beads it was capable of removing 99% of the Hg from a 1-mg solution in 12 days. The system devised by GBF worked by coating a porous support with bakers yeast and then exposing it to nonsterile Hg feed. Mercury-resistant bacteria then colonized the particles. Loading rates of 2-48 mg Hg2+ hresulted
in effluent concentrations of 50-100 |g LAnother system, created by C. Hansen (Department of Nutrition and Food Science, Utah State University), utilized a bacterial consortium isolated from municipal activated sludge (139). In this system, a feed solution containing Hg2+ is added to a fluidized bed, where the Hg2+ is reduced to Hg0 as it contacts biofilm-coated sand. Effluent concentrations of 10 |g LHg2+ were achieved when the influent concentration was 2 mg L_1.
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