565 1-5667-0608-4/01/$0.00+$1.50© 2004 by CRC Press LLC 14 The Future, Emerging Mitigation and Remediation Technologies CONTENTS 14.1 Emerging Mitigation and Remediation Technologies........................56514.1.1 Process Description ...................................................................56714.1.2 Key Design Criteria ...................................................................56814.1.3 Photoreduction of Cr(VI)..........................................................56914.1.4 Pollution Prevention Trends ....................................................570Bibliography ........................................................................................................57214.2 Conclusions ...............................................................................................573 14.1 Emerging Mitigation and Remediation Technologies Stephen M. Testa, James F. Begley, and James Jacobs Some of the emerging technologies for the mitigation and remediation ofCr(VI) include microbial strategies for in-situ and on-site bioremediationstrategies, and use in permeable reactive barriers. Discovery of microorgan-isms capable of reducing Cr(VI) to Cr(III) have significant potential indevelopment of in-situ or on-site bioremediation strategies. In 1977, the firstreported bacterial strains, Pseudomonas , were isolated from chromate-con-taminated sewage sludge (Romanenko and Korenkov, 1977). Since 1977,several other chromate reducing strains have been reported including otherstrains of Pseudomous , as well as strains of Achromobacter , Aeromonas , Bacillus , Desulfomamaculum , Enterobacter , Escherichia , and Micrococus species(Gvozdyak et al., 1986; Horitsu et al., 1987; Bopp et al., 1983; Wang et al.,1989; and Fude et al., 1994).An indigenous bacterium capable of reducing and immobilizing Cr to aninsoluble Cr(III) precipitate on its surface, thus removing Cr(VI) from solution, L1608_C14.fm Page 565 Monday, July 26, 2004 2:41 PM566 Chromium(VI) Handbook has been isolated at a wood preserving site located in Acton, Ontario (McLeanand Phipps, 1999). The operation utilized a copper-arsenate-chromate solutionto preserve the wood, which resulted in soil and groundwater contamination.A yet unidentified gram-negative strain, tolerant to high concentrations ofCr(VI) [up to about 500 milligrams per liter (mg/l)], and possibly Cu and As(up to about 40 mg/l), has been noted. McLean and Phipps (1999) state thatthe high tolerance ensures that unpredicted release of adsorbed metals willnot inhibit the reduction reaction. Laboratory studies showed the bacteriumexhibited a broad range of reduction efficiency under minimal nutrient con-ditions at temperatures between 4 and 37 ∞ C, pH 4 to pH 9, and under aerobicand anaerobic conditions. The exact mechanism by which the indigenousmicroorganism reduces aqueous Cr(VI) to Cr(III) remains uncertain since acombination of biochemical and surface mediated reactions have been impli-cated in the process.In the consideration of permeable reactive barriers in the deep subsurface,metabolic capabilities of dissimilatory metal-reducing bacteria (DMRB) haveshown merit (Gerlach et al., 1999). These capabilities have the potential tocreate zones of reduced indigenous metals (i.e., Fe(II)) in the path of agroundwater contaminant plume, thus forming redox-reactive barriers.Essentially, starved cells of Shewanella alga BrY were resuscitated with arti-ficially associated Fe(II), which in turn almost instantaneously reducedCr(VI) to Cr(III), the Cr(III) precipitating onto the sand media. In batch andcolumn studies, the microbially-generated surface-associated Fe(II), pro-duced from indigenous Fe(III), has been shown to reduce Cr(VI) to Cr(III),resulting in the precipitation of Cr(III) on existing surfaces, forming stableend products, and eliminating Cr(VI) from the water phase.In organic-carbon-poor subsurface environments, sucrose-amended, yeastextract-amended, and lactate-amended systems were observed to be effectivefor the microbial reduction of Cr(VI) (Hong and Sewell, 1999). Dependingon the supply of the electron donor (i.e., lactate), a reduction sequence ofnitrate, Cr(VI), and sulfate was observed. Acetate and benzoate amendedsystems were found not to be as effective as electron donors, allowing foronly 34.5 and 13.7% removal of Cr(VI), versus 100% removal utilizingsucrose, lactate, and yeast extract-amended systems.Use of the bacterium Pseudomonas Putida ATCC17484 was deemed success-ful for the biodegradation of naphthalene with the presence of Cr(VI)(Ghoshal et al., 2001). The presence of Cr(VI) however inhibited bacterialgrowth and reduced the biodegradation rate of the naphthalene. Completereduction of Cr(VI) was achieved at concentrations up to 6.3 mg/l. In anotherstudy, a bench-scale treatability study was performed to evaluate the feasi-bility of in-situ bioremediation of perchlorate (ClO 4 - ) and Cr(VI) (Perlmutter,2001). Two media (sand and gravel), and four electron donors (acetate,molasses, composted manure, and concentrated fruit juice) were considered.Acetate and molasses were found to be acceptable electron donors. It wasdetermined that acetate may be required to initiate the treatment system,but molasses would be used as the long-term electron donor. It was further L1608_C14.fm Page 566 Monday, July 26, 2004 2:41 PMThe Future, Emerging Mitigation and Remediation Technologies 567noted that the Cr(VI) concentrations used in this study (up to 8.0 mg/l) didnot inhibit ClO 4 - reduction (originally set at 1500 mg/l) and was routinelyreduced to below its analytical detection limit during the study.Another emerging technology is an enhanced anaerobic treatment ofCr(VI) contaminated groundwater using biostimulation of indigenous soilbacteria by adding an alkane gas. An alkane gas, such as propane is used asa growth substrate to promote conditions leading to the reduction the Cr(VI).With this approach, the added substrate serves as both the electron donorand primary growth substrate for the bacteria. Cr(VI) is potentially the finalelectron acceptor in the process.Cr(VI) is known to be reduced both aerobically and anaerobically in dif-ferent bacterial systems (Suthersan, 2002). While consuming propane, themicrobes use up the oxygen and create the geochemical conditions necessaryfor the reduction of Cr(VI).Microbial transformation of Cr(VI) varies with the oxidation state. Ingroundwater, the predominant form