2 The 4 Technology Solutions
IN-SITU BIOREMEDIATION OF METALS CONTAMINATED GROUNDWATER
An On-Line Version of a Column First Published in:
The National Environmental Journal Jan./Feb. 1994 Vol. 4 No. 1 Pages 24-25
by: David B. Vance
Significant technical developments have improved our ability to remediate groundwater contaminated with hydrocarbons. There has not been similar growth in alternatives for sites contaminated with metals. This will change over the next decade as remediation of metal contaminated groundwater becomes a more pressing issue. The purpose of this column is to describe innovative biological in-situ metal remediation methods. Physical/chemical developments will be discussed in the next column.
When groundwater is contaminated with metals, remediation alternatives are limited to mobilization or immobilization. Unlike hydrocarbons, biodegradation into innocuous carbon dioxide and water is not possible. Irrespective of the available reactions, the same metal will be still ultimately be present.
The choice of mobilization or immobilization is dependent upon the hydrogeologic setting of the site, the chemical properties of the metal contaminant, the geochemistry of the site soils and groundwater, and the current and future use of the contaminated groundwater. Generally, it is easier to immobilize a metal than it is to mobilize and recover it. However, an immobilized metal may still be a source of future problems if subsurface ambient conditions change.
Metal mobilization and recovery is vulnerable to the same limitations encountered with pump and treat systems for hydrocarbons. Adsorption to components of the soil matrix can significantly increase the required duration for operation of the recovery system.
Aggressive metal mobilization (beyond that achievable with passive pump and treat) is an approach to be applied only under extreme conditions. However given proper geochemical conditions, what may be possible is the use of complexing ligands to enhance the mobility of particularly troublesome metal contaminants (such as radionucleides). In such an application it would be essential to maintain hydraulic control of the site. Injection wells would introduce organic complexing agents (such as gluconate) to mobilize the targeted metals. Groundwater, with the solubilized metals, could then be recovered. Once the metal removal process has been completed, conventional in-situ bioremediation could be used to degrade the residual concentrations of the organic complexing agent.
Another aggressive approach to metal mobilization is methylation. Mercury can be methylated by a number of microorganisms (methanobacterium for example), resulting in the conversion of Hg+2 salts to extremely volatile methylmercury compounds. Arsenic, selenium and tellurium can undergo similar reactions. Such volatilized methylated metals are very mobile in the subsurface. In addition, the methylated forms of these metals are extremely toxic. The use of this approach would be predicated on a substantial need for total removal of the metal and the ability to exercise complete hydraulic control of groundwater and soil gas at the site.
A critical component of applying metal immobilization will be the use of risk assessments to establish the safety of allowing the presence of contaminating metals in an insoluble form within the aquifer matrix.
Mechanisms for microbial mediated metal immobilization can be divided into two categories, active and passive.
Active mechanisms operate directly on the metal and include:
- intracellular accumulation; and
Passive mechanisms act indirectly by modifying the surrounding environment. They include:
- complexing with extracellular biological chelates;
- biosorption to cell surfaces; and
- destruction of complexing ligands.
Precipitation - In anaerobic environments sulfate reducing bacteria such as Desulfovibrio and Desulfotomaculum produce hydrogen sulfide by the reaction SO42- + 10H = H2S + 4H2O. The hydrogen sulfide reacts with soluble metals forming insoluble metal sulfides.
The following solubility products (in parts per billion) illustrate that point:
- Cadmium Sulfide 8.7 X 10-7;
- Copper Sulfide 8.8 X 10-14;
- Lead Sulfide 4.3 X 10-6;
- Mercury Sulfide 1.1 X 10-16.
Intracellular Accumulation - This is a two stage process. First, metal ions are bound passively to the surface of the bacterial cell wall by physical/chemical processes. Then the metal ions are transferred to the interior of the cell by microbial energy systems normally associated with magnesium and potassium transport. Pseudomonas and Thiobacillusare examples of common bacterial species that are capable of this process.
Oxidation and Reduction - There are many known metal redox reactions mediated by microbes. For example: Chromate ions (Cr+6) can be reduced to insoluble trivalent chromium (Cr+3) with soluble reductase enzymes generated by Escherichia coli (E. coli). Soluble uranium ions (U+6) precipitate as the mineral uraninite when reduced to the U+4 species. This reduction reaction can be driven by reductase enzymes produced by the bacteria Desulfovibrio vulgaris.
Extracellular Complexation - Extracellular complexation occurs from interactions of metals with extracellular polymers excreted by bacteria or from organic matter accumulated from the dead microbes.
Extracellular polymers include:
- proteins, and
- nucleic acids.
Indigenous bacteria can be stimulated to specifically produce extracellular polysaccharides.
Another class of microbial chelating agents are siderophores, which are low molecular weight ligands synthesized and excreted by bacteria for capturing and supplying iron to support metabolic activity. Other metals may also complex with these ligands. Siderophore producers include: Psuedomonus, Actinomyces, and Azotobacter.
Cell Wall Binding - There are three mechanisms for the binding of metals to bacterial cell walls:
- Ion exchange reactions with peptidoglycan and teichoic acid (important biochemical components of the bacterial cell wall);
- precipitation through nucleation reactions; and
- complexation with nitrogen and oxygen ligands.
Gram positive bacteria have high adsorptive capacity, particularly Bacillus, due to the high peptidoglycan and teichoic acid content in their cell walls. In general, gram negative bacterial cell walls are low in these components and exhibit poor metal adsorption.
Destruction of Complexing Ligands - Metals associated with the plating, mining and nuclear industries are some times released into the environment complexed with organic ligands such as EDTA or cyanide. These organic ligands can be biodegraded by conventional in-situ bioremediation methods to destroy the ligand and immobilize the metal.
The use of microbiological methods for environmental remediation is not an entirely new concept. Microbial interactions with metals have long been recognized and since the 1960's work in the field by biohydrometallurgists and biogeochemists has exploded. Given adequate hydraulic controls, this is a technology that may be easier to physically implement than in-situ bioremediation of hydrocarbons. The success of the latter has been contingent upon the ability to induce adequate mass transfer of oxygen into the impacted zone, often difficult. Stimulation of microbes for the manipulation of metals in-situ is usually dependent upon the mass transport of soluble salts, a much easier task. However, in practical application it is likely that several of the methods described above would be used. Implementation of that complex process should be by individuals knowledgeable of those methods.
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