Most of the evolving technologies have been investigated only on the laboratory scale, but some have already been tested on a pilot or even technical scale. However, detailed experience and wide commercial use are not available. Until now heavy metals have been treated with physicochemical technologies. Although these elements are not 'degradable', they are not biochemically inert. Several microbiological transformations are known that mainly alter the physicochemical behavior of metals , including
• solubilization and sorption (bioleaching, biosorption, phytoremediation)
• precipitation (bioprecipitation)
• volatilization by alkylation
Solubilization was originally developed to support the mining of metals (Me) by leaching (bioleaching) . Various metabolites play an important role in bioleaching, e.g., surfactants, chelators, and organic acids. Many bacteria, e.g., Thiobacillus sp. and Leptospirillum ferrooxidans are reported to be able to leach metals. The metals, which occur mainly in solid ore deposits (MS2), are solubilized by two indirect mechanisms: via thiosulfate or via polysulfides and sulfur. Some metal sulfides are chemically attacked by Fe(III) hexahydrate ions, resulting in the formation of thiosulfate, which is oxidized to sulfuric acid. Other metal sulfides are attacked by Fe(III) and protons, which leads to the formation of polysulfides as intermediates and finally to elemental sulfur, which is biooxidized to sulfuric acid. The two mechanisms can be simplified by the following equations :
• thiosulfate mechanism (FeS2, MoS2, WS2):
S2O32- + 8 Fe3+ + 5 H2O ^ 2 SO42- + 8 Fe2+ + 10 H+ (2)
• polysulfide mechanism (e.g., ZnS, CuFeS2, PbS):
MeS + Fe3+ + H+ ^ Me2+ + 0.5 H2Sn + Fe2+ (n > 2) (3)
The bioleaching method requires pumping of the groundwater and removal of the solubilized metals in a groundwater treatment plant. For treatment, biosorption may be used. In summary, although bioleaching is a pump-and-treat process, the higher solubility of the 'contaminants', i.e., the solubilized metals, may result in enhanced removal of the contaminants compared to classical pump-and-treat procedures.
Another technology for removing dissolved contaminants from soil and ground-water is phytoremediation: contaminants are taken up through the roots of plants and trees. The choice of plants depends on the characteristics of the contaminants and the soil as well as on the three-dimensional distribution of the contaminants, be cause the efficiency of phytoremediation is restricted to the depth of root growth. Some contaminants, e.g., nitroaromatics, are taken up but are not transported within the plant. Transformation of the contaminants is usually not effective enough for final elimination and is therefore of minor importance. For example, heavy metals can be accumulated without significant transformation by some plants to a very high extent. Hence, for final removal of the contaminants, the plants have to be harvested and eliminated. Because of the high water demand of some trees, phytoremediation can also be used for a hydraulic limitation of a pollution plume.
A main problem in the mining industry is acidification and contamination of groundwater with dissolved metal ions together with sulfate. With bioprecipitation, both problems can be solved together. Addition of an organic substrate (represented by 'CH2O') leads to the formation of sulfide and an increase in the pH. The metals (Me2+) are precipitated as a nontoxic metal sulfide in an abiotic reaction:
Many metals can also be precipitated as metal carbonates or hydroxides. The reactions are disturbed by the presence of oxygen. However, O2 is readily consumed when an organic substrate is added. Most formed metal sulfides are very stable even if the redox milieu recovers after finalization of the organic substrate addition. Re-mobilization can occur when the pH drops to low values. If excess formation of H2S occurs, a second, aerobic treatment phase is required, in which surplus H2S is reox-idized to sulfate. With bioprecipitation, contamination with at least the following metals can be treated: Pb, Zn, Cu, Cd, Ni. The mobile, very toxic, hexavalent chromium ion (Cr6+) has been reported to serve as an alternative electron acceptor for dis-similatory Fe(III)-reducing bacteria, which use reduction of Cr6+ to the less mobile, nontoxic, trivalent chromium (Cr3+), which precipitates as chromium hydroxide to gain energy for growth. Chromium hydroxide is stable also in aerobic environments. Comparable reactions are reported for uranium (U6+). Fe(II) and S2- can also reduce Cr(VI) in nonenzymatic reactions.
In addition to these redox reactions, microorganisms can form and degrade orga-nometals. Alkylation and dealkylation are carried out by a wide range of microorganisms, which changes several important parameters such as toxicity, volatility, and water solubility of the contaminants . For example, arsenic can be removed from the vadose zone by this bioprocess. For microorganisms this process is linked to detoxification of the environment, because the volatile transformation products evaporate readily into the atmosphere. However, the volatile products are rather toxic. Therefore, using this bioprocess as a remediation technology requires extraction of the volatile products (e.g., by soil vapor extraction). The extracted contaminated soil vapor can be treated chemically: the compound is dealkylated and the product is removed in a gas scrubber. The following microbial pathways for reducing inorganic arsenic to volatile organic compounds can occur. Arsenate (As5+) and arsenite (As3+) are transformed to arsine (AsH3) by bacteria, preferably under anaerobic conditions.
Fungi and some bacteria transform arsenate and arsenite to di- and trimethylarsine in an aerobic environment. The process can take place only when an appropriate C source is available.
In addition, several other biotransformation reactions forming a variety of other products can occur. For example, As5+ can be consumed as an electron acceptor during the oxidation of organic matter, resulting in its reduction to the more soluble and more toxic As3+ . Until now, the necessary environmental conditions for a controlled microbial arsenic volatilization have not been investigated. Although such reduction processes (leading to gaseous As3- compounds) are well documented from laboratory experiments to occur with significant transformation rates, substantial natural arsenic losses in the field have not yet been observed . The bio-process that adds methyl groups to metal ions, forming volatile organometals that volatilize from the environmental compartment, is a general detoxification process used by bacteria. It is well documented, e.g., for mercury (Hg) .
Concerning radioactive metals, we should note that a bacterium, Deinococcus ra-diodurans, was isolated that transforms, e.g., toluene, even under high doses of y radiation. Only the organic pollutants were degraded; the radionuclide of course remained unaffected. The survival of this microorganism in a radioactive environment led to speculation that probably some as-yet unknown bacteria exist which will allow complete removal of radionuclides from the environment by bioleaching. Future progress may also be achieved by constructing genetically engineered microorganisms (GEM) that contain, e.g., a complete degradation sequence for final mineralization of contaminants, specific genes to resist unfavorable environments, or other improved biochemical features, like a deficiency in adhesion, which will be useful for bioaugmentation.
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