Remediation strategies of hazardous agrochemicals
3.1 Physical remediation methods
A number of physical and chemical methods are available to remove contaminants from soil. Non biological methods such as low temperature thermal desorption has often been used to remediate pesticide-contaminated sites. This technology involves the removal of organic compounds including pesticides using temperature and volatilization. It requires highly specialized facilities and demands comparatively high cost. Air sparging method is more often used to remove hydrocarbons by injecting large volumes of pressurized air into contaminated soil, removing volatile organic compounds that might otherwise be removed by carbon filtering systems. Another popular method is incineration which is applied to the contaminated soil using heat and oxygen as the oxidizing compounds. Each technology has its advantage as well as limitation for the treatment of specific contaminant (Frazar 2000, Yao et al. 2012).
3.2 Biological methods
Compensation of high cost technologies can be done by replacing the physical and chemical remediation techniques with “bioremediation” techniques. Utilization of biota to remediate the contamination from the medium is a superior concept. The soil organisms not only indicate the toxicity but also eliminate those contaminants from trophic spheres.
Discrimination between natural biodegradation and bioremediation is a difficult task. The process of bioremediation accelerates the rate of the natural microbial degradation of pollutants by providing these microorganisms with nutrients, carbon sources or electron donors. The process demands the addition of indigenous microorganisms having characteristics to degrade the desir ed contaminant at a faster rate. Production of H:0 and C02 without producing the toxic intermediates is the unique feature of bioremediation (Frazar 2000).
Bioremediation and its types
Bioremediatiou technologies can be broadly classified as in situ or ex situ. In situ bioremediation involves treating the contaminated material at the site, while ex situ deals with the removal of the contaminated material to be treated outside the site. Ex situ bioremediation technologies include bioreactors, biofilters, and land fanning, whereas in situ methods include bioventing, biosparging, biostimulation and liquid delivery systems. In situ technology is more popular due to its less equipment requirement, lower cost and eco-friendly nature. However, this treatment has limited application in fields. Bioremediation processes may be either aerobic or anaerobic based on the contaminated site and types of contamination (Master et al. 2002, Chowdhury et al. 2012).
Principles of bioremediation
Micro organisms are mostly used as bioremediatiou tool because of their ability to degr ade different substrates than the natural carbon sources. Capability of adaptation through mutation is an important characteristic of micro-organisms, which in turn helps to develop the ability of degr ading toxic or complex compounds, probably because of the evolution of more adequate transport systems.
Biodegradation of xenobiotic compound involves the breakdown of toxic compounds to less complex compounds and ultimately to water and C02 by the action of microbes. The complete breakdown of pesticides into inorganic compounds is known as biomineralization. The degradation process may be complete or partial, which conventionally leads to formation of less toxic organic compounds, referred to as partial biodegradation. Metabolic activities of bacteria, fungi, actinobacteria, and plants play significant role in the degradation process. Although most of the pesticides are biodegradable, certain pesticides cannot degrade easily and are called recalcitrant. The degradation process of xenobiotic compounds depends upon the physical, chemical, and microbiological characteristics of the soil and the chemical properties of the pollutants. Pesticide degradability decreases as molecular weight and degree of branching increase. Pesticide metabolism completes in three steps. It includes phase I, the transformation of the original toxic compound through oxidation, reduction, or hydrolysis reactions. The second phase which includes the conjugation of pesticide or its metabolites to sugars and amiuoacids, results in more water solubility and lesser toxicity in compounds. Phase III involves the conversion of metabolites into less toxic secondary conjugates and stimulation of intra- or extracellular enzymes such as hydrolases, peroxidases, and oxygenases by soil fauna (Chino-Flores et al. 2011, Abdel-Razek et al. 2013).