INTRODUCTION
Rapid industrialisation and urbanisation have resulted in elevated emission of toxic heavy metals and radionuclides entering the biosphere. An inorganic toxicant may be cationic such as metallic ions of mercury, cadmium, chromium, lead, nickel and uranium, etc. Toxic inorganic may also be alkylated or aromatized forms of metal ions such as methyl-mercury and phenyl-mercury. Heavy toxic metals and radionuclides seriously influence the metabolism of living organisms and cause permanent threatening of health. The increasing amounts of toxic metals emitted into the biosphere as a result of industrial activities indicate potential hazard to ecosystem. Solid and/or liquid wastes containing toxic heavy metals may be generated in various industrial processes, e.g., in chemical manufacturing, electric power generating, coal and ore mining, melting and metal refining, metal plating, and others. Domestic waste sludge may also contain undesirable level of toxic metals.
Aqueous effluents that are produced during nuclear power and defence activities frequently contain radionuclides (Faison et al., 1990). These effluents derive from various sources, including reactor coolant water, evaporator condensate, and fuel reprocessing waste. Metals contained within these wastes include the fission product strontium and caesium. Removal of radionuclides is a critical element of the treatment of nuclear industry effluents. 90Sr poses a particular health hazard because of its tendency to be incorporated into bone. The chemical behaviour of 137Cs mimics that of sodium and potassium, which are readily absorbed by cells.
Conventional techniques to remove toxic metals and radionuclides, e.g., ion exchange and precipitation, lack specificity and are ineffective at low concentrations of metal ions.
Environmental biotechnology is the technology of applying mainly microorganisms to improve the quality of the environment. Bioremediation is the use of living organisms to reduce or eliminate environmental hazards resulting from the accumulation of toxic chemicals and other hazardous wastes. This technology is based on the utilisation of naturally occurring or genetically engineered microorganisms to transform organic and inorganic compounds. Biological methods compete with physical and chemical technologies. For an economical biological process of remediation and elimination of toxic metal contaminants, the following features are essential: inexpensive biomass, high metal binding capacity, selective metal binding, effective desorption methods, recycling of desorbents, and repeated use of biomass.
Microbial transformation of toxic metals and radionuclides may affect their solubility, mobility, and bioavailability (Francis, 1997). Several of the key microbial processes may affect mobilisation or immobilisation of toxic elements by one or more of the following mechanisms:
| – | chelation of elements by metabolites, |
| – | oxidation-reduction of metals which affect the solubility or valence state, |
| – | changes in pH which affect the ionic state, |
| – | biosorption by functional groups on the cell surface, |
| – | bioaccumulation by an energy-dependent transport system, |
| – | immobilisation due to formation of stable materials, |
| – | biomethylation, |
| – | biodegradation of organic complex of metals and radionuclides. |
Fig. 1. Metal processing mechanisms of microorganisms
Table 1. Examples of toxic heavy metals accumulating microorganisms
|
Organism |
Element |
|
Citrobacter sp. |
Lead, Cadmium |
|
Thiobacillus ferrooxidans |
Silver |
|
Bacillus cereus |
Cadmium |
|
Bacillus subtilis |
Chromium |
|
Pseudomonas aeruginosa |
Uranium |
|
Micrococcus luteus |
Strontium |
|
Rhisopus arrhizus |
Mercury |
|
Aspergillus niger |
Thorium |
|
Saccharomyces cerevisiae |
Uranium |
1. MOBILISATION
Dissolution of toxic metals and radionuclides is due to oxidation-reduction reaction and production of mineral or organic acid metabolites, as well as lowering of the pH.
1.1. Enzymatic oxidation
Inorganic compounds that can exist in more than one oxidation state and in which the higher oxidation state is less soluble, enzymatic oxidation may be a useful way for removing the inorganic species from solution. Much of the available information on the dissolution of metals deals with the ore leaching by autotrophic microorganisms such as Thiobacillus ferrooxidans, T. thiooxidans. The role of Thiobacillus ferrooxidans in the removal of uranium from ore is due to indirect and direct actions.
Indirect action:
Direct action:
Ferrous iron thus produced is a carrier for electrons for uranium oxidation and is reoxidized by Thiobacillus ferrooxidans:
1.2. Enzymatic reduction
In case of inorganic compounds that can exist in more than one oxidation state and whose reduced state is insoluble, enzymatic reduction may be useful in removing the species from solution. Enzymatic reduction by facultative and obligate anaerobic microorganisms may be a potential application of in situ bioremediation.
Examples of bacterial reduction:
1.3. Complexation
The use of complexation agents may be useful in mobilizing toxic inorganic compounds to facilitate their removal from solid waste (Birch and Bachofen, 1990).
Complexes can be represented as follows:
Metal + Ligand ⇔ Metal complex
Microbial complexing agents can be low molecular weight organic acids and alcohols, high molecular weight ligands, siderophores, and toxic metal binding compounds. Low molecular weigh compounds as various organic acids (citric acid, tricarboxylic acids) released during microbial degradation have been found to have metal complexation ability. Some amino acids formed by bacteria can also be complexing agents. The rank order of the complexing ability of organic acids:Tricarboxylic acids > dicarboxylic acids > monocarboxylic acids
The microbial degradation of organic compounds, mainly cellulose and lignin, releases large macromolecular compounds. They are differentiated into three fractions: compounds soluble in alkali as humic acids, compounds soluble in acids as fulvic acids, and residual humic fraction that cannot be extracted by either dilute alkali or acid. These macromolecules are termed humates. Their structures have not yet been fully characterized, although certain functional groups, such as carboxyl, alcohol, and phenol are common to all humic macromolecules. The complexation of heavy metals and radionuclides to humates is highly pH-dependent.1.4. Siderophores
When microorganisms are grown in an iron deficient medium, they produce specific iron chelators, so called siderophores, in the medium. They play an important role in the complexation of toxic metals and radionuclides and increase their solubility (Neilands, 1983). Siderophores are compounds that possess catecholate, phenolate or hydroxamate as their binding groups. Over the past few years many siderophore or siderophore-like compounds have been identified from various biological systems. Although siderophores are primarily specific for Fe(III), they can also complex other metals and radionuclides.
2. IMMOBILISATION
Immobilisation of toxic metals and radionuclides are brought about by precipitation, biosorption and bioaccumulation. These processes have received considerable attention because of their potential application of waste water treatment containing toxic metals and radionuclides.
2.1. Precipitation
Sulfate reduction is an example for the precipitation of metallic ions in solution. Most metal sulfides are quite insoluble in aqueous solution. The stability of these sulfides depends on the maintenance of anoxic conditions. Sulfate reducing bacteria are being used in engineered natural systems such as constructed wetland to treat metal contaminants. Microbial degradation of organo-phosphates to ortho-phosphate can lead to metal precipitation throught formation of metal-phosphates, especially above pH 7. Intracellular phosphates may also immobilise metals.
2.2. Biosorption
Biosorption of toxic metals and radionuclides is based on non-enzymatic processes such as
adsorption (Fig. 2). Adsorption is due to the non-specific binding of ionic species to cell surface-associated or extracellular polysaccharides and proteins (Mullen and et al. 1989; Volesky, 1990). Bacterial cell walls and envelopes, and the walls of fungi, yeasts, and algae are efficient metal biosorbents that bind charged groups. The cell walls of Gram-positive bacteria bind larger quantities of toxic metals and radionuclides than the envelopes of the Gram-negative bacteria. Bacterial sorption of some metals can be described by the linearised Freundlich adsorption equation,
where S is the amount of metal absorbed in µmol · g–1, C is the equilibrium solution concentration in µmol · l –1, K and n are the Freundlich constants (Mullen et al. 1989). Metallic ions can be removed by alive or dead biomass. Biosorbents may be regenerated by treatment with acid or with certain chelating agents. Besides of bacteria, waste fungal biomass deriving from several industrial fermentations may provide an economical source of biosorptive materials. Many species have high cell wall chitin contents, and this polymer of N-acetyl-glucosamine is an effective biosorbent.
Fig. 2. Sites of biosorption of toxic metals
Fig. 3. Two-stage biosorption process
2.3. Bioaccumulation
Bioaccumulation has been described for such metals as mercury, lead, silver, cadmium, nickel, 137cesium, 60cobalt, 85stroncium, plutonium, and uranium. Intracellular accumulation of toxic elements is carried out by an energy dependent transport system (Gadd, 1988).
Potential mechanisms of toxic metal flux across membranes can be ion pumps, ion channels, carrier mediated transport, endocytosis, complex permeation, and lipid permeation.
Permeabilization of cell membranes to toxic elements can result in further exposure of intracellular metal binding sites and increase passive accumulation. The most widely used living cell system is capable to remove toxic metals from effluents of waste water treatment. The metals and radionuclides are released inside the cells, incorporated into biochemical pathways, or trapped in an inactive form by complexation with another high affinity ligand.
CONCLUSION
The potential application of biotechnology for the treatment of various forms of toxic solid and liquid wastes has advantages and disadvantages. The advantages of microbial treatment are: environment friendliness, self-reproducibility, adaptability, recyclisation of bioproducts, specificity, and good cost/benefit ratio. The disadvantages include the slowness of the processes, and the difficulty to control the processes.
However, due to the increasing need for the safe removal of toxic metals and radionuclides and the continued public interest to environmental problems, the microbial processes may play an important role in the future of waste management. Commercial utilization of these tools is rare, but the development of these topics of biotechnology is desirable on both environmental and economic aspects.
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