Since Second World War, pesticides have been used to help increasing agricultural production and preserve food quality for longer periods of time. In fact, the agricultural production during that period has doubled, and pesticides were responsible for 30% of that increase. Although pesticides have been benefiting agriculture by increasing production and fighting many plant and human diseases (such as malaria), their indiscriminate, and many times irresponsible use has made them an environmental problem. This is mainly due to their properties, such as high retention time in soil, low vapour pressure, moderate absorption by organic matter and clay, and high drainage potential, which leads to groundwater contamination. Thus, pesticides are two-edged swords, since they help to diminish hunger and fight some diseases, saving many lives, but their accumulation in ecosystems leads to their incorporation in the food chain, with all associated health problems. Some pesticides, such as atrazine, can exhibit phenomena called bio magnification, which consists on the ability of a pollutant to concentrate in animal tissues as it moves up in the food chain.
The soil, besides being an important support for all ecosystems, is a non-renewable natural resource, because the time necessary to form 1 cm of forest soil is estimated to be 200–400 years. Nevertheless, its contamination by organic compounds such as polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs) and pesticides, among others, is a growing problem, in spite of the increasing awareness of this fact by many countries.
Pesticides are classified, according to their purpose in sight as: bactericides, insecticides, fungicides, herbicides and others.
Atrazine (2-chloro-4-(ethylamino)-6-(isopropylamino)-S-triazine), a white, crystalline solid, slightly volatile, is a herbicide from the group of the S-triazines (i.e., symmetrical triazines), which was discovered in 1952 by scientists from J.R. Geigy Ltd. in Switzerland. Its first application was done in 1954. The S-triazines are strong inhibitors of the photosynthesis by interfering with the Hill reaction (which is a water-splitting (photolysis) light-initiated reaction that results in the production of free oxygen by the plants). Atrazine is used in corn, sugar cane, sorghum and pineapple cultures, among others, and presents slight to moderate toxicity for humans and other animals, causing abdominal pains, diarrhoea and vomits, being also considered potentially carcinogenic by the Environmental Protection Agency (EPA from USA).Atrazine has also the power to increase the toxicity of arsenic in human cells.
Due to its low vapour pressure and Henry constant, atrazine is easily drained, moving in the aqueous phase of the soil and contaminating the groundwater. Also, due to its Kow value (water/octanol partition coefficient), atrazine is a herbicide that has the capacity to be adsorbed by organic matter, argyle and fat tissues. In 1988, more than half of the American states had the groundwater contaminated with atrazine, while, for the surface water, it was estimated that the Mississippi river transported around 160 t per year of atrazine to the Mexican Golf. The main reason for the large use of atrazine, in spite of its risks, resides in its low price, when compared to other herbicides.
A study carried out in Portugal on the contamination of groundwater by pesticides in the regions of “Beira Litoral” and “Ribatejo e Oeste”, detected the presence of pesticides in all of the 79 places studied. Atrazine appeared in 70% of the samples analysed, followed by its metabolites desethyl atrazine (DEA) and deisopropyl atrazine (DIA), which appeared in 56 and 48% of the samples. Even though some others pesticides were found, they had lower occurrences. Because of its structure, atrazine is not easily degraded. Its partial degradation is possible by fungus but its total mineralization is not possible by a single microorganism. In fact, it is necessary to have two or more different kind of microorganisms capable of the degradation of the atrazine to achieve total mineralization, although some reports point out that, in some cases even the presence of various microorganisms is not enough to attain this. After the application of a herbicide, a large number of phenomena will dictate the route, which it will follow. These phenomena include retention (adsorption, absorption), transformation (decomposition, degradation), transportation (volatilization, lixiviation, superficial drainage) and the interaction among all these processes. There are also some other aspects that must be taken into consideration, including the structure and properties of the pesticide and environmental characteristics, such as the weather and the localization of the area, among others. For all the pointed above, we may conclude that, the behaviour of pesticides is very complex and is influenced by many different variables.
There are many techniques available for soil decontamination, all of them having some advantages and disadvantages. Supercritical extraction (SCE) is a technique that presents some important and unique advantages over the other decontamination processes, among which we stand out the low impact in the structure of the soil and on the environment. The first studies on supercritical extraction were carried out by Hanna and Hogart in 1880: they investigated the solubility of many different inorganic salts in solvents under supercritical conditions. In the 1970’s, due to the energy crisis, the interest in supercritical extraction has increased, a tendency that continues till nowadays, mainly due to environmental concerns. Supercritical extraction is a unit operation in which a supercritical fluid (SCF), which may be defined as any substance at a temperature and pressure above its critical point, is used as the extracting solvent. SCFs are particularly good solvents because their capacity for dissolving substances is close to that of the liquids, but their viscosity and diffusion coefficient
are close to those of the gases, thus improving the transport and mass transfer characteristics of these fluids. Furthermore, since the surface tension of SCFs is equal to zero, these fluids are particularly suitable for the extraction of substances from solid matrices, such as soil. Another advantage in the use of SCFs is the possibility of changing their dissolving power by changing the pressure and/or temperature of the fluid, thus allowing the fractional extraction and separation of solutes, and the complete recovery of the solvent by simple pressure adjustments. Of all the SCFs that have been studied, carbon dioxide (CO2) is the most commonly used because of its low critical temperature (TC = 304.2 K) and pressure (PC = 7.39MPa), non-toxicity, availability and low cost.
The supercritical extraction with CO2 has been successfully applied to the removal of a variety of contaminants from soils, even the most persistent to treat, such as PBCs and PAHs. SCE is also being used by EPA as an analytical technique to determine the contents of polycyclic aromatic hydrocarbon (PAHs) and total petroleum hydrocarbons (TPH) in soils. The high potential of SCE as an environmental technique is shown by new applications that are appearing, such as the decontamination of soils with plutonium by SCE, which has been studied by researchers at INEEL (Idaho National Engineering and Environmental Laboratory).
Supercritical extraction in solid matrices
SCE has received a lot of attention as a potential technique for soil remediation. The extraction of a substance from a solid matrix requires the following steps: desorption from the matrix, solubilisation in the SCF, transport by the extracting fluid and precipitation of the contaminant.
All these steps must be taken into consideration when trying to implement an experimental protocol for the extraction of contaminants, such as pesticides, from solid matrices by SCE. Therefore, even though the knowledge of the solubility of pesticides in the SCF is important for the design of these processes, it is not enough, because variables such as: the interaction between the pesticide and the solid matrix (soil), pH, content of organic matter, and type of soil (content of clay, argyle); and phenomena like van der Waals forces, hydrogen bonds, charge transfer and dipole-induced, must be taken into consideration. Then, the extraction of contaminants from soils is very complex, due to the large number of simultaneous phenomena occurring, and therefore, it is a common practice to begin the study of the applicability of a new technique for soil remediation by studying the extraction of the pollutants from previously contaminated sand, instead of soil.
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