Environmental remediation of pesticides
The techniques used for remediation of other organic pollutants can also be extended for remediation of pesticide-contaminated soil (Castelo-Grande et al. 2010). Nevertheless, some particular method has been developed due to the specific condition that arises as a result of the contamination of soil by pesticides. As pesticides contaminated the soil through diffuse pollution, so the remediation measures have to be different as compared to point-source contamination. On the contrary, since pesticides are used in agricultural soil, maintenance of soil properties is important and aggressive technologies used for remediation of industrial polluted soils cannot be replicated in agricultural soils. Furthermore, choosing of appropriate remediation method which can protect the human and environmental health is also important as the level of pesticides contamination may exceed the standard safe limit. Remediation techniques involving physical, chemical, and biological methods including adsoiption, oxidation, catalytic degradation, membrane filtration, and biological treatment have been evolving continuously for remediation of pesticide-contaminated environment.
Based on the level of pesticide contamination and then risk, money and time involved, remediation of contaminated soil may be carried out in situ, or the contaminated matrix may be transported to special reactors or vessels (er situ) where treatment will be carried out.
In general, the following is the broad categorization of pesticide remediation technology which is also given in Figure 2. These are:
- • Chemical and physical methods or a combination of both
- • Biological methods: bioremediation approach
- • Thermal destruction methods
Figure 2. Flow chart of different remediation measures for pesticide contamination.
Chemical and physical remedial techniques
The objective of remediation involving chemical and/or physical methods is to transform the chemical environment of the pesticides so that it can not enter into the different elements of soil system including plants, groundwater, or soil organisms. Such preventive steps include decreasing mobility or utilizing any change of chemical constiments of the pesticides. Here we are discussing the different physico-chemical methods involved in the remediation of pesticide-contaminated soil and the biological method is given hi detail in the subsequent sections.
3.1.1 Chemical reduction
In redox reactions, one reactant loses electrons (is oxidized), and the other gains electrons (is reduced). Degradation of pesticides by reduction is enhanced by aerobic environments. Nano?scale zero-valent iron (nZVI) is the most commonly used chemical reductant for halogenated, more particularly chlorinated pesticides in soil. Several reports are available where reductive delialogenation of persistent pesticides on the surface of zero-valent iron (ZVT) becomes a promising technique for treating the contaminated soil (Sayles et al. 1997, El-Temsah et al. 2016). The addition of Fe or Al salts to zero-valent iron (nZVI) enhances the metolachlor degradation by creating pH and redox conditions that favor the formation of green rusts (Satapanajaru et al. 2003). However, caution should be taken to avoid the negative environmental effects of nZVI when it is used in an open enviromnent.
3.1.2 Chemical oxidation
The main purpose of this method is to mineralize the pesticides in contaminated soil to C02, water, and inorganics, or converting them to non or less toxic metabolites. Ozone, hydrogen peroxide, hypochlorites, chlorine, and chlorine dioxide are the most commonly used oxidizing agents (Pavel and Gavrilescu 2008). However, sometime these conventional oxidizing agent may not be able to completely degrade the pesticides and consequently, they are combined with iron salts, semiconductors (such as TiO,) and/or ultraviolet-visible light irradiation for better remediation and the process is known as “advanced oxidation process” (AOPs) (Gimeuez et al. 2015). The use of different approaches in the AOP field for pesticide degradation has been studied by several workers (Parker et al. 2017. Korntckou et al. 2017, Saylor et al. 2019). However, the success of this method in the real field will provide a reference for its applications.
3.1.3 Soil washing
The main principle of this technique is to separate the contaminants like pesticides from soils and sediments by using physical, chemical techniques or a combination of both the techniques. It involves the extraction of pesticides from contaminated soil with the help of a solution that efficiently transfers the pesticides from solid soil phase to liquid solvent phase. It can be considered as an off-site technology. Here the dug soil sample is processed in special extractor units which can be considered as a soil-liquid extraction operation unit, where the pesticides are transferred from the solid soil to the liquid washing fluid. For pesticide remediation by soil washing techniques, surfactant solutions are the most commonly used liquid phases. The efficiency of the combination of soil washing and electrolysis with diamond electrodes to remove atrazine using sodium dodecyl sulfate (dos Santos et al. 2015a, b) as a surfactant can be cited. Jinzliong et al. (2017) treated the organochlorine pesticides (OCPs) contaminated soils by washing soil with triton X-100 (TX-100) surfactant coupled with adsorption treatment of the solution with activated carbon. Cotillas et al. (2018) reported that the addition of electrolytic processes during the soil washing process efficiently enhances the removal of pesticides from soil.
3.1.4 Chemical extraction-solvent extraction
Supercritical fluid extraction (SFE) can be applied for the extraction of pesticides from contaminated soil. This method is modem and has a high solvency and recovery capacity, being composed of a principal solvent agent, for instance, the methanol, often used as an aid to carbon dioxide (CO,) (Castelo-Grande et al. 2005). Pesticides get solubilized in CO, when it passes through contaminated soil and the collected solvent is disposed of safely. Solvent extraction can be used to recover a wide variety of substances. Due to the high solvency power of CO„ the method can be applied for remediation of a wide variety of pesticides without disturbing soil characteristics, such as stmctures and nutrients. However, remediation potential depends on soil properties (pH, moisture, organic matter content) and type of extraction method used. The selective extraction of persistent organic pollutants including DDT reported (Bielska et al. 2013) the reduction in remediation when total organic matter and other soil properties in the analyzed soils were higher.
3.1.5 Soil flushing
This is an in situ chemical method of remediation of pesticide-contaminated soil. Here, pesticides are extracted from the contaminated soil by injection of fluid. When fluid is injected into the contaminated soil, the adsorbed pesticides get desorbed from the soil and retrieved in the fluid. The process is continuous as the contaminated fluid is purified and re-injected or circulated to extract the pesticides and, re-extracted, and re-purified. Chemistry of pesticides particularly water solubility and volatilization influences significantly the success of this remediation method (dos Santos et al. 2016). However, this method performs poorly or fails in low permeable soils and with pesticides adsorbed strongly on the solid soil surface.