Bioremediation as an Emerging Technology for the Detoxification of Obsolete Pesticides

By Ashwani Sharma
October 2008

The Author is a Senior Research Fellow at the School of Environmental Sciences, Jawaharlal Nehru University, in New Delhi, India. → See also:

Introduction

Bioremediation as defined by the American Academy of Microbiology is “the use of living organisms to reduce or eliminate environmental hazards resulting from accumulations of toxic chemicals or other hazardous wastes” (Gibson and Sayler, 1992). Bioremediation is not a new technology. This is evident in that humankind has practiced composting, sewage treatment and fermentation since the beginning of recorded history. All of these processes utilize microbial processes in degradation process. The modern use of bioremediation began with the opening of the first biological sewage treatment plant in Sussex, UK, in 1891. The use of this technology in cleaning up pollutant spills is gaining popularity. Over the past ten years an increase in the types of contaminants to which bioremediation is applied has been evident.

Indigenous and enhanced micro-organisms have been shown to degrade industrial solvents, polychlorinated biphenyls, explosives and many different agricultural chemicals, including pesticides. Pilot, demonstration, and full-scale applications of bioremediation have been carried out, but on a limited scale. Internationally micro-organisms have been identified and employed, to transform and degrade contaminants. Admittedly the application of this technology to in situ bioremediation of polluted sites has been limited. At present, bioremediation is often the preferred method for remediation of especially petroleum hydrocarbons, because it is cost effective, and it converts the petroleum hydrocarbons into the harmless by-products carbon dioxide and water. Bioremediation can be used to degrade concentrated organic contaminants near their source or as a secondary remediation strategy following specific physical or chemical methods. It can also be applied for sequestration of metals and radio-nuclides through microbially mediated transformation processes as well as for remediation of large plumes of diluted contaminants that are broadly dispersed in the environment.

However, the full benefits of bioremediation have not been realized because processes and organisms that are effective in controlled laboratory tests are not always equally effective in full-scale applications. The failure to perform optimally in the field setting stems from a lack of predictability due, in part, to inadequacies in the fundamental scientific understanding of how and why these bioremediation processes work (Forsyth et al., 1995).

Micro-organisms degrade or transform contaminants by a variety of mechanisms. Petroleum hydrocarbons for example are converted to carbon dioxide and water or are used as a primary food source by bacteria, which use the energy to generate new cells. Where the hydrocarbons are chlorinated the degradation takes place as a secondary or co-metabolic process rather than a primary metabolic process. In such a case enzymes, which are produced during aerobic utilization of carbon sources such as methane, degrade the chlorinated compounds. Under aerobic conditions, a chlorinated solvent such as trichloroethylene can be degraded through a sequence of metabolic steps, where some of the intermediary by-products may be more hazardous than the parent compound (e.g., vinyl chloride).

The impressive capabilities of micro-organisms and plants to degrade and transform contaminants should provide tremendous benefits in the clean-up of pollutants from spills and storage sites. These remediation ideas have provided the foundation for many ex situ waste treatment processes (including sewage treatment) and a host of in situ bioremediation methods that are in practice today (Hinchee et al., 1994).

Over the past ten years, progress has been made in expanding the number and type of contaminants to which bioremediation can be applied and in the number of practical methods for implementing in situ bioremediation. Techniques such as hydrofracturing have been developed for improved delivery of nutrients to micro-organisms in low-permeability geologic media. Methods have been developed for creating passive treatment systems such a bio-filters (Taylor et al., 1993). Novel concepts for using microbial produced biopolymers as in situ plugging agents have also been explored. Bioremediation is a more cost effective method of remediation as compared to incineration or physical and chemical remediation methods (Saaty and Booth, 1994: Wijesinghe et al., 1992; Atlas, 1995). This technology has the potential to be one of the most cost effective technologies for dealing with environmental remediation problems.

In situ or Ex situ

In situ bioremediation refers to below ground methods applied at the site of contamination whereas ex situ refers to above-ground bioremediation, where the sediment or water has been extracted from the subsurface. The most widespread use of ex situ bioremediation is the cleanup of storage tank leaks and oil spills in pipelines, tank farms and petroleum refineries. The technique, known as a solid-phase bioremediation is similar to composting, in which soils are physically and chemically manipulated to stimulate breakdown of target organics by resident and exogenous microbes. In soil piles it is necessary to install piping networks to distribute air and allow addition of water and supplemental additives. Application of nutrients, pH regulating chemicals, and microbial strains may be performed by spraying (in water slurry) or direct application with tillage to distribute the additives into the soil. For more heavily contaminated soils and sludge’s, slurry processes are used. These are modified versions of the activated sludge process used in various industrial wastewater treatment plants. The soil or sludge is fluidised with water and treated in an on site bioreactor. These systems are easily monitored and addition of nutrients and microbial strains can be made to closely control and enhance the bioprocess.

The third major category of on site bioremediation involves pumping groundwater to the surface for treatment in an above-ground bioreactor. The effluent from the bioreactor, containing oxygen, nutrients and acclimated micro-organisms, is then injected back into the ground to remediate the contaminated soils associated with the groundwater. This process combines ex situ groundwater treatment with in situ soil treatment.

In situ bioremediation has several advantages over ex situ techniques:

Biodegradation of Pesticides

Microbial degradation is an important step in the disappearance and, in most cases detoxification of pesticides. Herbicide biodegradation may prevent the problem of environmental pollution but it can also reduce the effectiveness of a compound in controlling targeted pests. Many soil applied pesticides are degraded more rapidly following repeated application at the same site (Racke and Coats, 1990).

In a survey of soils from commercial fields, there was evidence that enhanced biodegradation of the compound has been induced by normal field applications, in some soils by a single previous treatment (Walker et al., 1993). Several herbicides are prone to degradation, including members of the thiocarbamate group, the ureas, linuron and monolinuron, the amides, propyzamide and napropamide, and the triazinones, chloridazon and metamitron (Roberts et al., 1991).

Factors responsible for the enhanced degradation are micro-organisms present in the soil, able to degrade applied pesticide. Walker et al., 1993, gave evidence, that bacteria are responsible for the process. The rate of degradation in soil was unaffected by treatment of the soils with the antifungal antibiotic cycloheximide, but was inhibited by the antibacterial antibiotic chloramphenicol. Usually mixed cultures of bacteria able to degrade pesticides are isolated from these soils by enrichment culture.

Early research indicated that napropamide was relatively stable to degradation in the soil, but the results of recent laboratory and field experiments have indicated that the rate of degradation is increased significantly in the soil treated previously with the same compound. Mixed cultures of bacteria were isolated from the soils by enrichment culture on a liquid mineral medium with napropamide as a sole carbon and nitrogen source (Walker et al., 1993). The related herbicide diphenamid is also susceptible to enhanced degradation, suggesting that substituted amides may be particularly prone to this phenomenon.

Enhanced degradation of linuron occurred in soils, following repeated applications of the herbicide. These effects were more pronounced in laboratory incubations than in field persistence studies. Direct isolation of bacteria from soil failed to identify bacteria with the ability to degrade the herbicide. Sequential enrichment culture in a mineral base medium led to the isolation of an apparently stable mixed culture of bacteria, which would degrade the herbicide (Roberts et al., 1991).

Metamitron is a selective, pre and post-emergence herbicide, used for weed control in sugar beet, which acts by inhibiting electron transport in photosynthesis. The compound is degraded to desaminometamitron in various soils and by different soil microbes. Parekh et al., 1994, isolated metamitron-degrading bacteria Rhodococcus sp. from the treated soils by enrichment culture. The herbicide was completely degraded within 7 days at 25o C in laboratory tests. A pure culture of Arthrobacter sp. was shown to degrade metamitron, in the presence of alternative sources of carbon and nitrogen, within 2 weeks of incubation in the dark.

Bacteria capable of degrading carbofuran as sole carbon and nitrogen sources were isolated from liquid cultures of treated soils (Parekh et al., 1994). Similar types of carbofuran-degrading bacteria were isolated from different soils. All isolates were Gram negative, aerobic rods that hydrolyzed the insecticide to carbofuran phenol. Racke and Coats, 1990, imply the presence of carbofuring degrading bacteria in soils without a history of previous treatment. It is possible that many soils contain a small population of bacteria that have the ability to degrade certain compounds. The presence of such bacteria in untreated soils could be due to contamination from surrounding treated areas or they may be an inherent part of the soil microflora, which degrade naturally occurring analogous compounds. Chary et al., 1992, detected carbofuran degrading bacteria in 48% of samples from previously untreated plots.

Walker et al., 1993, isolated a mixed microbial culture able to degrade the herbicide to a single degradation product – napthoxypropionic acid. Addition of this culture to a previously untreated soil introduced rapid degrading ability. These results gave hope for use of augmentation for the treatment of sites contaminated by carbofuran and other pesticides.

Atrazine is a broad-leaf, pre-emergence herbicide. It is a leading member of a class of triazine ring-containing herbicides that includes simazine and terbuthylazine. Atrazine has been found to be less biodegradable than other pesticides. However, a number of different bacteria have been identified that are capable of metabolizing atrazine to ammonia and carbon dioxide.. Organisms that can initiate the pathway are given, but other organisms may also carry out later steps. This pathway demonstrates microbial biodegrative diversity.

With some chemicals in some soils the waste is consumed completely, or has been reduced to an acceptable concentration level. In other kinds of soils, or when the toxic agents have low water solubility, or have been adsorbed in the soil matrix to a high degree, the micro-organisms would not be able to achieve sufficient degradation.

In soils contaminated with pesticides, a unique problem exists because of the presence of herbicides; chemicals designed to prohibit the growth of vegetation. Herbicide tolerant plants can survive at these sites and are ideal candidates for testing whether vegetation can be used to enhance microbial degradation of pesticide wastes.

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