Metals bioremediation

Keywords Biosorption, Heavy metal, Bioremediation, Biotransformation, Genetically modified microbes... 

Current approaches to metal bioremediation are based upon the complexation, oxidation-reduction, and methylation reactions just discussed. Until recently, interest was focused on technologies that could be applied to achieve in situ immobilization of metals. However, within the last few years, the focus has begun to shift toward actual metal removal, because it is difficult to guarantee that metals will remain immobilized indefinitely. 

Pal A, Paul AK. Microbial extraceUular polymeric substances central elements in heavy metal bioremediation. Indian J Microbiol 2008 48 49-64. 

A major concern when remediating wood-treatment sites is that pentachlorophenol was often used in combination with metal salts, and these compounds, such as chromated copper—arsenate, are potent inhibitors of at least some pentachlorophenol degrading organisms (49). Sites with significant levels of such inorganics may not be suitable candidates for bioremediation. 

A more constrained opportunity for nitrate bioremediation arose at the US-DoE Weldon Spring Site near St. Louis, Missouri. This site had been a uranium and thorium processing faciUty, and treatment of the metal had involved nitric acid. The wastestream, known as raffinate, was discharged to surface inpoundments and neutralized with lime to precipitate the metals. Two pits had nitrate levels that requited treatment before discharge, but heavy rains in 1993 threatened to cause the pits to overflow. Bioremediation by the addition of calcium acetate as a carbon source successfully treated more than 19 million liters of water at a reasonable cost (75). 

R. E. Hiachee, D. B. Andersoa, F. B. Mettiag, r., and G. D. Sayles, eds.. Applied Biotechnology for Site Kemediation, Lewis PubUshers, Ann Arbor, Mich., 1994. J. L. Means and R. E. Hiachee, eds.. Emerging Technology for Bioremediation of Metals, Lewis PubUshers, Ann Arbor, Mich., 1994. 

PermeOx is also used to improve the bioremediation of soils contaminated with creosote or kerosene (see Bioremediation (Supplement)), to deodori2e sewage sludges and wastewater (see Odormodification), and to dechloriaate wastewater and effluents. A special formulation of calcium peroxide, made by FMC and sold ia the United States under the trademark Trap2ene, is used for removing metal ions from acidic waste streams such as coal ash leachate and acid mine drainage (see Wastes, industrial). 

These results may be viewed in the wider context of interactions between potential ligands of multifunctional xenobiotics and metal cations in aquatic environments and the subtle effects of the oxidation level of cations such as Fe. The Fe status of a bacterial culture has an important influence on synthesis of the redox systems of the cell since many of the electron transport proteins contain Fe. This is not generally evaluated systematically, although the degradation of tetrachloromethane by a strain of Pseudomonas sp. under denitrifying conditions clearly illustrated the adverse effect of Fe on the biotransformation of the substrate (Lewis and Crawford 1993 Tatara et al. 1993). This possibility should therefore be taken into account in the application of such organisms to bioremediation programs. 

A further application of the manipulation of microbial activity in the rhizo-sphere is their potential to remediate contaminated land. Bioremediation involves the u.se of microorganisms that break down contaminants. Radwan et al. (255) found that the soil associated with the roots of plants grown in soil heavily contaminated with oil in Kuwait was free of oil residues, presumably as a result of the ability of the resident rhizosphere microflora to degrade hydrocarbons. The use of plants as a means to accumulate pollutants such as heavy metals (256,257) to degrade hydrocarbons and pesticides (255) is already widely implemented and has proven to be successful. In some cases, there is no doubt that it is the plant itself that is responsible for the removal of the contaminants. However, in most... 

Sandrin, T. and Hoffman, D., Bioremediation of organic and metal co-contaminated environments Effects of metal toxicity, speciation, and bioavailability on biodegradation, in Environmental Bioremediation Technologies, Singh, S.N. and Tripathi, R.D., Eds, Springer, Berlin, Germany, 2007, pp. 1-34. 

Jonioh, V., Obbard, J.P., and Stanforth, R.R., Impact of treatment additives used to reduce lead solubility and microbial toxicity in contaminated soils, in Bioremediation of Metals and Inorganic Compounds, Leeson, A. and Alleman, B.C., Eds, Battelle Press, Columbus, OH, 1999, pp. 7-12. 

Bioremediation is not restricted only to biodegradable organic contaminants. New techniques are currently under development for the bioremediation of metal-contaminated sites. Microbial activity can alter the oxidation state of some elements, reducing or increasing their mobility, and this transformation can be used for remediation purposes. 

It has long been known that certain microbes can alter the oxidation state of some toxic metals, mainly by reducing them to a lower oxidation state, and this chemical transformation can be used for the bioremediation of contaminated soils. Three main mechanisms are involved in the... 

Biological activity can be used in two ways for the bioremediation of metal-contaminated soils to immobilize the contaminants in situ or to remove them permanently from the soil matrix, depending on the properties of the reduced elements. Chromium and uranium are typical candidates for in situ immobilization processes. The bioreduction of Cr(VI) and Ur(VI) transforms highly soluble ions such as CrO and UO + to insoluble solid compounds, such as Cr(OH)3 and U02. The selenate anions SeO are also reduced to insoluble elemental selenium Se°. Bioprecipitation of heavy metals, such as Pb, Cd, and Zn, in the form of sulfides, is another in situ immobilization option that exploits the metabolic activity of sulfate-reducing bacteria without altering the valence state of metals. The removal of contaminants from the soil matrix is the most appropriate remediation strategy when bioreduction results in species that are more soluble compared to the initial oxidized element. This is the case for As(V) and Pu(IV), which are transformed to the more soluble As(III) and Pu(III) forms. This treatment option presupposes an installation for the efficient recovery and treatment of the aqueous phase containing the solubilized contaminants. 

Bioremediation also has its limitations. Some chemicals are not amenable to biodegradation, for instance, heavy metals, radionuclides, and some chlorinated compounds. In some cases, the microbial metabolism of the contaminants may produce toxic metabolites. Bioremediation is a scientifically intensive procedure that must be tailored to site-specific conditions, and usually requires treatability studies to be conducted on a small scale before the actual cleanup of a site.13 The treatability procedure is important, as it establishes the extent of degradation and evaluates the potential use of a selected microorganism for bioremediation. A precise estimate on vessel size or area involved, speed of reaction, and economics can therefore be determined at the laboratory stage. 

Three cyanide-degrading nitrilases were recently cloned and purified and their kinetic profiles were evaluated in order to better understand their applicability to cyanide bioremediation. CynD from Bacilluspumilus Cl and DyngD from Pseudomonas stutzeri exhibit fairly broad pH profiles with >50% activity retained across pH 5.2 to pH 8.0 while the CHT (NHase) from Gloeocercospora sorghi exhibited a more alkaline pH activity profile with almost all of its activity retained at pH 8.5, slightly lower thermal tolerance, and quite different metal tolerance compared with the two bacterial enzymes [46]. 

McLean JS, Lee J-U, Beveridge TL (2002) Interactions of bacteria and environmental metals, fine-grained mineral development and bioremediation strategies. In Huang PM, Bollag J-M, Senesi N (eds) Interactions between soil particles and microorganisms. Impact on the terrestrial ecosystem, vol 8, IUPAC series on analytical and physical chemistry of environmental systems. Wiley Chichester UK, pp 227-261... 

Bioremediation of sites that are contaminated with toxic metals is an important issue in environmental restoration. Bacteria have long been known for their ability to Itake up metals from their immediate environment (Borrok and Fein 2004). The efficiency of bacterial cells in concentrating metals is related to their large surface area-to-volume ratio and high surface density of charge. The cell surfaces of all bacteria are negatively... 

---