Biodegradation and Bioremediation

Xenobiotics: Concepts, Sources, and EPA Regulations

• Etymology and Definition:
      * The term "Xenobiotic" is derived from the Greek word "XENOS", which translates to "foreign" or "strange."
      * Xenobiotics are man-made chemicals that do not occur naturally in the environment.
      * They are typically synthesized for specific agricultural or industrial applications. Examples include aromatics, pesticides, hydrocarbons, plastics, and lignin.
      * They are frequently referred to as RECALCITRANTS because they possess a high level of resistance to environmental degradation.

• Primary Sources of Xenobiotics:
      * Petrochemical Industry: Includes oil and gas industries and refineries. It produces fundamental chemicals such as vinyl chloride and benzene ($C_{6}H_{6}$).
      * Plastic Industry: This sector is closely linked to petrochemicals. It utilizes complex organic compounds, including plasticizers and cross-linking agents.
      * Pesticide Industry: This is the most common source of xenobiotics. The chemical structures are often based on benzene and its various derivatives.
      * Paint Industry: Major ingredients include solvents like xylene, toluene, methyl ethyl ketone, and methyl compounds.
      * Other Industrial Sources: These include the electronic industry, textile industry, pulp and paper industry, cosmetics, pharmaceutical industry, and wood preservation processes.

• Environmental Protection Agency (EPA) List of Organic Pollutants:
      * The EPA identifies several organic pollutants injected into the environment via human activity:
          1. Acenaphthene
          2. Benzidine
          3. Carbon tetrachloride ($CCl_{4}$)
          4. Chlorinated phenols
          5. Dichlorobenzene
          6. Hexachloroethane
          7. Naphthalene
          8. Polynucleated aromatic hydrocarbons (e.g., benzopyrine, toluene)
          9. Polychlorinated biphenyls (PCBs)
          10. Hexachlorocyclohexane ($HCH$)
          11. Pesticides (e.g., aldrin, $DDT$, endrin)

Fundamental Principles of Biodegradation

• IUPAC Definition: According to the International Union of Pure and Applied Chemistry, biodegradation is the "Breakdown of a substance catalyzed by enzymes of microorganisms like bacteria or fungi."

• Functional Definition: It is the capacity of microorganisms to transform toxic xenobiotic chemicals into simpler, non-toxic compounds through the synthesis of specific enzymes.

• Influencing Factors: The efficiency of biodegradation is affected by several variables:
      * Substrate specificity
      * Nutrition sources
      * Temperature
      * $pH$ levels

• Microbial Utilization of Contaminants:
      * Primary Substrate: The contaminant is available in sufficient quantity to serve as the sole source of energy for the microbe.
      * Secondary Substrate: The contaminant provides energy but is not present in a high enough concentration to be the primary source.
      * Co-metabolic Substrate: The utilization of a compound by a microbe that relies on a different primary substrate for its main energy needs.

Microbial Agents in Bioremediation

• Aerobic Bacteria:
      * Genera: Pseudomonas, Alcaligenes, Sphingomonas, Rhodococcus, and Mycobacterium.
      * Capabilities: These are known to degrade pesticides and hydrocarbons, including alkanes and polyaromatics. They often use the contaminant as their sole source of carbon and energy.

• Methanotrophs:
      * Definition: Aerobic bacteria that utilize methane ($CH_{4}$) as their source of carbon and energy.
      * Mechanism: They possess the enzyme methane monooxygenase, which has a broad substrate range. This allows them to act against various compounds, such as chlorinated aliphatics (e.g., trichloroethylene and $1,2\text{-dichloroethane}$).

• Anaerobic Bacteria:
      * Used less frequently than aerobic bacteria.
      * Applications: Effective for the bioremediation of polychlorinated biphenyls (PCBs) in river sediments, trichloroethylene (TCE), and chloroform.

• Fungi:
      * Capable of degrading a diverse range of toxic or persistent environmental pollutants.

Classification of Contaminants by Degradability

• Bio-degradable:
      * Petroleum products: Gas, diesel, fuel oil.
      * Crude oil compounds: Benzene, toluene, xylene, naphthalene.
      * Specific pesticides: Malathion.
      * Industrial solvents and coal compounds: Phenols, cyanide in coal tars, and coke waste.

• Partially Degradable / Persistent:
      * TCE (trichloroethane): Significant threat to groundwater.
      * PCE (perchloroethane): Common dry-cleaning solvent.
      * PCBs: Though degraded in laboratories, they often resist degradation in field work.
      * Elements: Arsenic ($As$), Chromium ($Cr$), Selenium ($Se$).

• Not Degradable / Recalcitrant:
      * Heavy metals and radioactive elements: Uranium ($U$), Mercury ($Hg$).
      * Persistent Pesticides: $DDT$.

Bioremediation Strategies: In Situ and Ex Situ

• Bioremediation Definition: A waste management technique that involves treating contaminated media (water, soil, subsurface material) by altering environmental conditions to stimulate microbial growth and degrade pollutants. It utilizes naturally occurring bacteria, fungi, or plants.

• In Situ Bioremediation: involves cleaning up contamination directly at the site where it occurred.
      * Intrinsic Bioremediation (Bioattenuation): Relies on the inherent capacity of indigenous microorganisms already present in the environment. It utilizes existing microflora, nutrients, and natural ventilation.
      * Accelerated / Engineered In Situ Bioremediation: Involves adding substrates, nutrients, or specific microbes to the environment to accelerate microbial growth and toxic breakdown.
      * Biostimulation: Supplying oxygen and nutrients (e.g., Phosphorus, Nitrogen, Glutamate, Sulphur, Acetate) via aqueous solutions to stimulate indigenous bacteria. It is most effective when paired with Bioaugmentation.
      * Biosparging: Injecting air under pressure below the water table to increase groundwater oxygen levels, enhancing the degradation of contaminants (e.g., Benzene, Toluene, VOCs, Chlorinated Ethanes, Petroleum Hydrocarbons, Vinyl Chloride).
      * Bioventing: Providing air and nutrients (including $H_{2}O_{2}$ and $O_{2}$) through wells into contaminated soil to stimulate indigenous bacteria.
      * Bioaugmentation: The addition of indigenous or exogenous microorganisms to contaminated sites. Microbes used include B. licheniformis, B. thuringiensis, P. polymyxa, Flavobacterium, Arthrobacter, Trichoderma, and Pseudomonas sp. This targets Chloroethane, chloromethane, thioacetate, phenols, tetrachloroethylene, and trichloroethylene.

• Ex Situ Bioremediation: involves removing contaminated material to be treated elsewhere. This method is expensive and can damage the local environment due to physical removal.
      * Solid Phase Treatment: Takes place above ground under controlled moisture, heat, temperature, and nutrients.
      * Slurry Phase Treatment: Occurs in a Bioreactor where contaminated soil is mixed with water, nutrients, oxygen, and other additives for optimal degradation.

Specific Ex Situ Methods: Composting, Landfarming, and Biopiling

• Composting:
      * Combines contaminated soil with non-hazardous organic bulking agents (e.g., straw, manure, agricultural waste, wood waste).
      * The organic material supports high microbial populations and elevated temperatures ($45\,^{\circ}\text{C} \text{ to } 65\,^{\circ}\text{C}$).
      * The process takes approximately $20 \text{ to } 40\text{ days}$. Temperature rise indicates microbial degradation activity.
      * Static pile composting: Piles are aerated via blowers or vacuum pumps.
      * Mechanically agitated in-vessel composting: Material is placed in a vessel for mixing and aeration.
      * Windrow composting: Long piles (windrows) are mixed using tractors. This is the most common and cost-effective method.

• Landfarming:
      * Contaminated soil is excavated and spread in a layer approximately $18\,\text{inches}$ thick.
      * Relies on indigenous microbes and is periodically tilled for aeration.
      * Focuses more on volatilization of lighter hydrocarbons.
      * Includes a collection system for leachate (the liquid containing contaminants that seep through the soil).

• Biopiling:
      * A hybrid of landfarming and composting.
      * Contaminated soil is stacked up to $20\,\text{feet}$ high over a layer of nutrients.
      * Aeration, nutrients, and microbes are provided through perforated pipes.
      * The setup is covered with a plastic sheet to induce volatilization. Gasses are captured in off-gas treatment units and condensates in collection tanks.

Advanced Bioremediation Techniques: Myco- and Phytoremediation

• Mycoremediation (Fungal Bioremediation):
      * Utilizes fungal mycelium which releases extracellular enzymes and acids.
      * Fungi are unique in their ability to degrade wood.
      * Groups: White rot fungi (containing lignin-modifying enzymes) and Brown rot fungi.
      * Contaminants targeted: PCBs, dioxins, pesticides, chlorophenols, pulp and paper mill effluents, dyestuffs, and heavy metals.
      * Species: Pleurotus ostreatus, Irpex lacteus, Trametes versicolor, Agaricus bisporus, Pleurotus pulmonarius.

• Phytoremediation (Plant-based Bioremediation):
      * Phytoextraction/accumulation: Plants like Helianthus annuus (sunflower) remove metals (Pb, Cd, Cu, Ni, Zn, Cr) from soil.
      * Rhizofiltration: Using plant roots (e.g., Sunflowers, Indian mustard) to treat groundwater and surface water.
      * Phytovolatilization: Contaminants are absorbed, volatilized, and transpired into the atmosphere. Used for Hg, Se, and As removal by Brassica juncea, Alfalfa, and Arabidopsis thaliana.
      * Phytostimulation/Rhizodegradation: Breakdown of contaminants in the rhizosphere by microbes stimulated by plant root zones.
      * Phytodegradation: Plants like Hybrid poplars, Black willow, Algae, and Stonewort degrade complex organic compounds and solvents into simpler molecules.
      * Phytostabilization: In-place inactivation of metals (As, Pb, Cd, Cr, Cu, Zn, Hg) via precipitation and complexation. Brassica juncea, grasses, and Hybrid poplars are used.

Application to Specific Contaminants

• Pesticides:
      * Microbial degradation: Genera Flavobacterium, Pseudomonas, Rhodococcus.
      * Biotransformation: Enzymes oxidation, reduction, or hydrolysis.
      * Phytoaccumulation: Crops like brinjal, spinach, radish, and rice can accumulate $DDT$ and benzene hexachloride.

• Oil Spills:
      * Remediation involves seeding with naturally occurring microbes. Over $70$ genera, including Nocardia, Bacillus, Streptomyces, and Coryneforms, can degrade petroleum.

• Plastics:
      * Endophytes: Pestalotiopsis microspore can degrade polyester polyurethane.
      * Microbes: Bacillus subtilis, Aspergillus niger, Aspergillus nidulans, Aspergillus flavus, Aspergillus glaucus, Penicillium species, and Pseudomonas sp. use plastics as carbon or energy sources.

• Genetically Engineered Microorganisms (GEMs):
      * Used to enhance degradation capabilities, often considered a form of bioaugmentation.

Limitations and Disadvantages

• Process Speed: Bioremediation is inherently slow, taking days to months.

• Efficiency Limits: Heavy metals are often not removed completely, and the process may not capture all quantities of a contaminant.

• Site Requirements: In situ methods require soil with high permeability.

• Biological Risks: Lab-grown strains (GEMs) may be consumed by soil protozoa or fail to compete with/survive indigenous microbes.

• Environmental Risks: Contaminant solubility might increase during the process, leading to leaching and further damage.

• Contact Issues: GEMs may fail to make physical contact with the specific compounds slated for degradation.