Steps and Approaches in Bioremediation, Bioleaching, Removal of Xenobiotics and Petroleum Degradation

Steps and Approaches in Bioremediation

1. Co-metabolism/ 2^°metabolism

Some xenobiotics compounds (like halogenated hydrocarbons, cyclohexanes) are degraded by microbes or microbial activity but these compounds are rarely used as the source of energy or carbon by them. The degradation of such compounds therefore depends upon the presence of another compound which includes necessary enzymerequired for the degradation of the xenobiotics. Hence the degradation of xenobiotics compounds/pollutants depends on a substrate which is called co-metabolite and such degradation is known as co-metabolism. Co-metabolism can be defined as the degradation of a compound that serve as a primary energy source. Co-metabolism is one form of secondary substrat transformation in which enzyme produced for primary substrate degradation are capable of degrading secondary substrate even though the degradation of secondary substrate does not provide energy or carbon to microorganisms.

Examples: Polynuclear aromatic hydrocarbons (PAH), pesticides are degraded by this mechanism.

Addition of methane or mathanol supports methanotrophic activity that has been found effective in degradation of chlorinated solvent like vinyl chloride by co-metabolism.

2. Aerobic and Anaerobic metabolism

Although oxygen is not a limiting factor, it is one of the most essential elements for micrpbial degradation or bioremediation. Oxygen is necessary for the initial breakdown of pollutants. Moreover, a complete degradtion of pollutants require aerobic metabolism. Most of the bioremediation method rely on enhanced supply of oxygen to the contaminated sites., e.g., petroeun degradation.

Anaerobic metabolism results in low degradtion and requires a longer period of time. Anaerobic bacteria use electron acceptors like nitrate, iron, sulphate, etc. or organic copmounds. But the energy yield from bacteria is less than aerobic metabolism. Anaerobic degradation is obligatory to degrade the pollutants at the site where oxygen transfer or supply is not possible, for example, in deeper sediments. The pollutants are remediate by one of the following pathways:

3. Genetic Engineering

Microorganisms are able to degrade many organic and inorganic pollutants or xenobiotics by their enzymatic activities directly or through co-metabolism. However, the microorganisms can be genetically modified to enhance its enzymatic capability to degrade a wide range of pollutants. Genetically engineered microorganisms have the advantage of possessing high growth rate, resistance to toxicity and higher affinity to pollutants. The gene responsible for producing the enzyme, required to catalyse the breakdown of xenobiotics can even be artificially introduced to icroorganisms that lack it. However, alot of debate exists over the safety, persistance, ecological damage and containments associated with the release of genetically modified microorganisms in the environment.

Bioleaching

Definitions

Bioleaching is the process described as dissolution or recovery of metals from their mineral sources by naturally ocurring microorganisms.

Bioleaching is an ecofriendly and conventional method for the recovery of metals from ores by application of microorganisms. This process is much safer than traditional leaching ny the us of chemicals.

Bioleaching is the use of microorganisms to produce soluble forms of metals from thair ores in the form of concentrated solution, which are recovered into their solid form by hydrometallurgical processes.

For example: Copper sulphite which is water insoluble and present in mineral sources is microbially oxidised to copper sulphate which is water soluble, This process is much safer than the use of chemicals to extract metals.

Metal leaching microorganisms

The microbial extraction of metals is usually done through leaching by acidophilic sulfur oxidizing and iron oxidizing bacteria (chemoautotrophs), The process is currently applied for the recovery of :

Copper

Molybdenum

Gold

Nickel

Antimony

Tin

Cadmium

Lead

Zinc

Arsenic, etc.

When the microorgansims are employed to leach metals (usually from low grade ores), the ores are broken up and heaped into piles. Microorganisms are added to heaps of ore and then water is pumped to the tops of heaps. The water trickles down carrying soluble metal forms. The leachate is collected and processed for the recovery of metals.

Advantages of Bioleaching

• Environmentally safe
• Economical
• Application for metal recovery from low-grade ores

Xenobiotics

Xenobiotics broadly referred to the man-made or synthetic unnatural and foreign chemicals that are present in theenvironment at unnaturally high concentration, for example, pesticides, herbicides, solvents, refrigerants, etc. The xenobiotic compounds are either not produced naturally or produced at much lower cincentration than by man.

Recalcitrant Xenobiotics

Recalcitrant xenobiotics are the compounds that resist biodegradation or that do not easily undergo biodegradation and thereby persist in the environment for a long period of time.

Types

1. Halocarbons

Solvents: chloroform

Condenser: Freons, CF₄, CCl₃F

Insecticides: DDT, BHC, Lindane

Herbicides: Dalopon, 2,4-D

1. PCBs: Polychlorinated biphenyls

• Synthetic polymers: Plastics like polyethylene, PVC, Polystyrene, Nylon, etc.

1. Alkyl benzyl sulphonate: Detergents
2. Oil mixture
3. Others like aliphatic and aromatic compounds with nitro, silphonate, methoxy, amino, carbamoyl group substitution

Hazards from Xenobiotics

1. Toxicity
2. Carcinogenicity

• Progressive build-up in environment (pollution)

1. Biomagnifications

Removal of Xenobiotics

Biodegradation of xenobiotics is an effective method of removal of xenobiotics/recalcitrant. The degradation and subdequent removal occurs through a variety of metabolic pathways.

1. Removal of Alkane and Aromatic Hydrocarbons

Addition of –OH group by oxygenase

Oxidation of –OH to -COOH

Opening of the ring structure

Degradation of linear molecules by β-oxidation

Degradation of Petroleum

Petroleum is a complex mixture of aliphatic, alicyclic and aromatic hydrocarbons and a trace amount of non-hydrocarbon compounds. Petroleum hydrocarbons are formed from the anaerobic conversion of biomass under high temperature and pressure. Their biodegradation is relatively slow. The petroleum processing activities introduce them into water and which is destructive to birds and various forms of marine lives.

Petroleum hydrocarbons are degraded with an initial attack monoxygenase or dioxygenase (have strict requirements for molecular O₂). For open chain petroleum hydrocarbons, the terminal methyl group is oxidized to carboxylic acid group.

Reference

Atlas, RM and R Bartha. Microbial Ecology:Fundamentals and Applications. The Benjamin Cummins Publication co. Inc., 1998.

Gordis, L. Epidemiology. third edition. 2004.

Maier, RM, IL Pepper and CP Gerba. Environmental Microbiology. Academic press Elsevier Publication, 2006.

park, K. Park's Text Book of social and prevention Medicine. 18th edition. 2008.