Bio-remediation: some new developments

Context

A team led by SuchetaTripathy from the Structural Biology and Bio-Informatics Division at Indian Institute of Chemical Biology (CSIR-IICB), Kolkata, isolated a fungus, Arthriniummalaysianum , and used the fungus biomass to remove Cr(VI).

Sources of Hexavalent chromium

• Hexavalent chromium is used in textile dyes, wood preservation, anti-corrosion products, chromate conversion coatings, and a variety of niche uses
• In India the particularly untreated tannery waste is an important source of the same.
• Industrial uses of hexavalent chromium compounds include chromate pigments in dyes, paints, inks, and plastics;
• chromates added as anticorrosive agents to paints, primers, and other surface coatings;
• and chromic acid electroplated onto metal parts to provide a decorative or protective coating.
• Hexavalent chromium can be formed when performing “hot work” such as welding on stainless steel or melting chromium metal.

Harmful effects of Hexavalent chromium

In it’s untreated form, hexavalent chromium is a

neurotoxic, genotoxic and a known carcinogen

New technology for removal of hexavalent chromium from industrial effluents

The team isolated a fungus, Arthriniummalaysianum , and used the fungus biomass to remove Cr(VI

Importantly, the heat-dried fungal biomass converts Cr(VI)— to a non-toxic trivalent form of chromium, thus eliminating the problems of disposing Cr(VI)-containing waste

The positively charged functional groups found on the surface of the fungus binds to Cr(VI) ions. These groups were found intact even after heat drying. The functional groups have more ability to adsorb the negatively charged Cr(VI) through strong electrostatic attraction in acidic conditions

What is bioremediation?

Bioremediation is a natural process that involves the use of biological entities to neutralize the contaminated site.

It is a “treatment that uses naturally occurring organisms to break down hazardous substances into less toxic or non toxic substances”.

Bioremediation is the technology that uses microorganism metabolism to remove pollutants. It uses relatively lowcost, low-technology techniques, which generally have a high public acceptance and can often be carried out on site.

This technology includes

• biostimulation (stimulating viable native microbial population),
• bioaugmentation (artificial introduction of viable population),
• bioaccumulation (live cells),
• biosorption (dead microbial biomass),
• phytoremediation (plants) and
• rhizoremediation (plant and microbe interaction
• Bioventing
• Bioleaching
• Land farming

Uses of microorganisms for waste treatment

Bioremediation can be used at the site of contamination (in situ) or on contamination removed from the original site (ex situ). In the case of contaminated soil, sediments, and sludges, it can involve land tilling in order to make the nutrients and oxygen more available to the microorganisms

The microorganisms that can be used are

• In the presence of oxygen. Examples of aerobic bacteria recognized for their degradative abilities are Pseudomonas, Alcaligenes, Sphingomonas, Rhodococcus, and Mycobacterium. These microbes have often been reported to degrade pesticides and hydrocarbons, both alkanes and compounds. Many of these bacteria use the contaminant as the sole source of carbon and energy.
• In the absence of oxygen. Anaerobic bacteria are not as frequently used as aerobic bacteria. There is an increasing interest in anaerobic bacteria used for bioremediation of polychlorinated biphenyls (PCBs) in river sediments, dechlorination of the solvent trichloroethylene (TCE), and chloroform.
• Ligninolytic fungi. Fungi such as the white rot fungus Phanaerochaetechrysosporium have the ability to degrade an extremely diverse range of persistent or toxic environmental pollutants. Common substrates used include straw, saw dust, or corn cobs.
• Aerobic bacteria that grow utilizing methane for carbon and energy. The initial enzyme in the pathway for aerobic degradation, methane monooxygenase, has a broad substrate range and is active against a wide range of compounds, including the chlorinated aliphatics.

Examples of bioremediation techniques

Biosparging.

Biosparging involves the injection of air under pressure below the water table to increase groundwater oxygen concentrations and enhance the rate of biological degradation of contaminants by naturally occurring bacteria. Biosparging increases the mixing in the saturated zone and thereby increases the contact between soil and groundwater. The ease and low costof installing small-diameter air injection points allows considerable flexibility in the design and construction of the system.

Bioventing

is a promising new technology that stimulates the natural in-situ biodegradation of any aerobicallydegradable compounds in NAPL within the soil by providing oxygen to existing soil microorganisms. In contrast to soil-vapor extraction (SVE), bioventing uses low air-flow rates to provide only enough oxygen to sustain microbial activity. Oxygen is most commonly supplied through direct air injection into residual contamination in soil by means of wells. Adsorbed fuel residuals are biodegraded, and volatile compounds also are biodegraded as vapors move slowly through biologically active soil.[5]

Bioaugmentation

Bioaugmentation is the introduction of a group of natural microbial strains or a genetically engineered variant to treat contaminated soil or water.  It is commonly used in municipal wastewater treatment to restart activated sludge bioreactors. Most cultures available contain a research based consortium of Microbial cultures, containing all necessary microorganisms.

At sites where soil and groundwater are contaminated with chlorinated ethenes, such as tetrachloroethylene and trichloroethylene, bioaugmentation is used to ensure that the in situ microorganisms can completely degrade these contaminants to ethylene and chloride, which are non-toxic

Biopiling

Biopile treatment is a full-scale technology in which excavated soils are mixed with soil amendments, placed on a treatment area, and bioremediated using forced aeration. The contaminants are reduced to carbon dioxide and water. The basic biopile system includes a treatment bed, an aeration system, an irrigation/nutrient system and a leach ate collection system. Moisture, heat, nutrients, oxygen, and pH are controlled to enhance biodegradation

Ex-Situ Bioremediation

Composting is a process by which organic wastes are degraded by microorganisms, typically at elevated temperatures.Typical compost temperatures are in the range of 55° to 65° C. The increased temperatures result from heat produced by microorganisms during the degradation of the organic material in the waste. Windrow composting has been demonstrated using the following basic steps. First, contaminated soils are excavated and screened to remove large rocks and debris

Bioreactors

Slurry reactors or aqueous reactors are used for ex situ treatment of contaminated soil and water pumped up from a contaminated plume. Bioremediation in reactors involves the processing of contaminated solid material (soil, sediment, sludge) or water through an engineered containment system.

Phytoextraction

This process has been tried more often for extracting heavy metals than for organics, We can find five types of phytoremediation techniques, classified based on the contaminant fate: phytoextraction, phytotransformation, phytostabilization, phytodegradation, rhizofiltration, even if a combination of these can be found in natureThe main advantage of phytoextraction is environmental friendly. The traditional methods those are used for cleaning up the heavy metal contaminated soil are responsible for disruption of soil structure and reduce soil productivity, whereas phytoextraction can clean up the soil without causing any kind of harm to the soil quality.

Way forward

Bioremediation can be used

To degrade highly toxic heavy metals,chemicals,effluents,;to remove Heavy metals from tanneries, and to treat Oil spills. Hence, for the processes harmful for the humans, microorganisms may be used to reduce the toxic compounds to less toxic ones.