Microbial Interactions with Inorganic Pollutants and Xenobiotics: Remediation Methods

Microbial Interactions with Inorganic Pollutants

Microbial interactions with inorganic pollutants: Industrial activity increases microbial interactions, but a harmless product may be contaminated with a toxic end product.

Effects in Mines: Acid Mine Drainage

Coal mines and metal holdings undergo autooxidation of sulfides (S = Fe + H₂O), which produces sulfuric acid. Acid rain drainage contaminates aquifers and plants.

Nitrate Transformation by Microbes

Due to excessive fertilizer use in agriculture, nitrate [NO₃^-] is transformed into toxic masses (NO₂^- and N₂O) through denitrification.

• NO₃^- to NO₂^- reduction produces toxicity in fodder (causing animal death) and vegetables (intoxicating humans).
• NO₂^- inhibits Clostridium botulinum.
• Amines and nitrites react in food, forming N-nitrosamines, which are carcinogenic.
• N₂O reduces the ozone layer.

Remediation

Use urea and ammonium fertilizers in moderation. Add nitrapyrin, which inhibits Nitrosomonas.

Biological Remediation

Use biological fertilizers (Azotobacter, Rhizobium, mycorrhizae).

Interaction of Microorganisms with Metals and Metalloids

Some metals, such as mercury, selenium and arsenic, are methylated by microbes, increasing toxicity and biomagnification through the trophic chain. Newer technologies are being used to address this.

Heavy metals in sulfide-rich sediments remain immobile under neutral or alkaline pH. However, toxic forms are mobile and bioaccumulative, moving from animal to plant to human (e.g., cadmium).

Biological Removal of Heavy Metals

1. First, the metal binds to the cell surface through cation exchange with the cellulose capsule.
2. Second, the metal passes into the cell.
3. Third, precipitation of metals occurs through products formed by microorganisms (S^2-, PO₄^3-).
4. Fourth, biotransformation takes place.

Applications: Eliminate metals from industrial effluents, sludge, mine accident water, and jewelry waste. Biopelicula is a third-based approach.

Air Pollution Contaminants

Sulfur compounds (S₁₈ and S₁₉) arise from industrial coal use and directly affect humans. Contaminants include:

• CO: Produced by incomplete combustion; involved in hydrocarbon reactions.
• NO₂: Causes acid rain and produces secondary oxidizing contaminants (O₃) that damage plants and humans.
• SO₂: Produced by fuel combustion; causes acid rain that damages plants.
• Pb: Produced by fuel combustion; damages human flora and fauna.
• Particulate matter: Causes black soot and respiratory problems.

Atmospheric Bioremediation

Volatile organic compounds and gases are treated in bioreactors. Bio-supported systems transform gases into harmless products.

• Biofilters: Air passes through a moist biological bed. Advantages include introducing oxygen and neutralizing pH.
• Bioscrubbers: Air is sprayed with bacterial suspensions in a reactor, effectively removing volatile organic compounds that are soluble in water.

Aquifer Contamination

The objective is to isolate contaminated wells and sensitive areas using physical barriers, degradation agents, and emulsifiers. Halocarbons are a complex problem; anaerobic conditions degrade some through cometabolism. Methanotrophs (methane monooxygenase) degrade dichloroethylene and vinyl chloride.

Leaching (Lixiviation)

Leaching is a process in which a pulverized ore is treated with a solvent that dissolves the desired part. Uses include mining to extract gold and copper at reduced cost. In situ uranium leaching offers environmental advantages, with less rock disturbance and simpler cleanup.

Municipal Solid Waste (MSW) Leachate

Organic matter in MSW forms black leachate with a foul smell, which is toxic and carries harmful substances into soils and water. Wet waste and rain accelerate the process. Appropriate treatment and management include waterproof liners and drainage channels.

Biolixiviation

Biolixiviation is a natural solution-based process used in industrial mining. Bacteria (mainly Thiobacillus ferrooxidans) oxidize minerals, releasing free metals present in ores.

• Advantages: compatible with the environment, no SO₂ emissions, does not require pyrometallurgical processes, reduces energy consumption, and has low reagent and operating costs.
• Drawbacks: reactions are slow, and solutions are very dilute, so the metals must be recovered and the water treated before recirculation.

Xenobiotics

Xenobiotics are synthetic chemical compounds that do not occur naturally. Microbial enzymes often lack the ability to degrade them, so they persist in the environment.

• Recalcitrant: Structurally resistant, poorly soluble in water, toxic, and resistant to biodegradation.
• Bioaccumulative: Selected xenobiotics accumulate in animal tissues.

Biological Pest Control

Viral: Viruses are highly antagonistic to pests. Examples include baculoviruses, togaviruses, parvoviruses, picornaviruses, and rhabdoviruses (e.g., Cydia pomonella). They act specifically against larvae of lepidopteran pests and forest insects.

Operation

1. Bio-insecticide is produced by inoculating occlusion bodies.
2. The bio-insecticide is sprayed and the occlusion bodies remain on leaves.
3. Alkaline pH is not established; the larva eats the occlusion bodies.
4. The bodies dissolve in the alkaline intestine, releasing viruses.
5. Viruses enter the intestinal cells, replicate, and invade the hemolymph and tissues.
6. In late stages, the viruses synthesize protein occlusion bodies, the insect dies, and the viruses are released to infect other larvae.

Another form of infection is transovarial transmission and parasitoids.

Fungal: Spores of antagonistic fungi are inoculated. The spore penetrates the insect cuticle, germinates, and forms mycelia. Blastospores are transported through the hemolymph and affect other tissues, causing death. Fungal structures emerge through the anus, mouth, and cuticle segments to release spores. Insect spectrum specificity includes Beauveria bassiana, Verticillium lecanii, Metarhizium anisopliae, Coelomomyces, and Arthrobotrys.

Halocarbons

Halocarbons are compounds containing one or more halogen atoms. They are very stable, used as plastics, refrigerants, solvents, and pesticides, and are highly recalcitrant contaminants.

Biodegradation Mechanisms

1. Cometabolism: Transformation by an enzyme acting on another compound, following the normal metabolic route.
2. Photodegradation: Photocatalysis using photoreactive substances and solar energy, which eliminates compounds before biodegradation.

Types: Haloalkanes contain carbon bonded to halogens such as chlorine, bromine, and fluorine. Examples include Freon and tetrachloroethylene, used in refrigerants and fluorocarbons.

• Anaerobic biodegradation: Reductive dehalogenation by hydrogenolysis and haloelimination (sulfate-reducing and methanogenic bacteria).
• Aerobic biodegradation: Dioxygenase and monooxygenase enzymes introduced by methanotrophic bacteria consortia.

Haloaromatics such as halobenzenes are biodegraded by anaerobic hydrogenolysis (sulfate-reducing consortia and methanogens) and by dihaloelimination (Pseudomonas and Alcaligenes). Halophenols, used as wood colorants and preservatives, are also biodegradable.

Chlorophenols such as pentachlorophenol are biodegraded through aromatic ring rupture. Anaerobically, they produce CH₄ and CO₂ (methanogenic and sulfate-reducing bacteria). Aerobically, they produce CO₂ and H₂O (Pseudomonas, Flavobacterium, and Microbacterium chlorophenolicum).

Nitroaromatics are used in solvents, pesticides, and explosives. Nitrobenzene and phenol are very biodegradable; anaerobic reduction of the nitro group is best achieved by sulfate-reducing and methanogenic bacteria and Pseudomonas.

Polychlorinated biphenyls (PCBs): Used in fluids and oils, they bioaccumulate. Anaerobic dehalogenation is possible, and aerobic degradation is complete in vitro by Phanerochaete and Arthrobacter.

Organochlorine pesticides such as DDT are persistent, soluble in fats, and not very stable, yet they are biodegradable through hydrogenolysis and basic haloelimination under anaerobic conditions.

Synthetic Polymers

Biodegradable polymers include bioplastics produced by bacteria (lactic acid, starch-based plastics, polyhydroxyalkanoates), which are easy to mold and useful for many applications. The main problem is the cumulative accumulation of polyvinyl chloride, polyethylene, and polystyrene, which harms animals. Bacteria such as Pseudomonas, Bacillus, Alcaligenes, and Azotobacter degrade some of them.

Alkylbenzene Sulfonate Detergents

Anionic surfactants emulsify fats. Biodegradable alkylbenzene sulfonates are linear, while branched forms degrade more slowly. Aerobic degradation is carried out by consortia of microorganisms.