Aplicación de técnicas de remediación (fotocatálisis heterogénea y solarización) para minimizar la presencia de residuos de insecticidas en agua y suelo

Abstract

ABSTRACT The application of pesticides to protect crops against pests and diseases is a common agricultural practice. Pesticides have been the cause of the increase in food production in recent decades, but their use have produced in some cases a lot of adverse effects that may affect the health of people who handle them, the environment and the consumer. Agriculture is one of the few activities where chemicals are deliberately introduced in the environment, and the consequence of their application is the generation of residues. These substances can cause serious pollution problems due to their persistence and accumulation in the soil (increasing their persistence in areas where they have been treated), their mobility in water, soil and air (favoring the contamination of remote territories from the original area of application, which include courses of surface and groundwater), their transformation products (presenting sometimes higher stability and toxicity than the starting materials), and the possibility of accumulation in organisms exposed due to their ability to biomagnification (may cause toxicological problems in organisms exposed). Of all the risks associated with the use of pesticides, which generates more concern is their presence in the final product and its impact on consumer health. The misconception that both soil and water are able to dilute and purify all our wastes has provoked an alarming deterioration in the quality and availability of these essential resources. Several studies have shown the presence of residual concentrations of pesticides in soil and surface- and groundwater all over the planet, which has generated a great social concern. As a result, in recent decades it has driven the development of new technologies whose objectives are the detoxification and reclamation of the polluted resources by anthropogenic action. In this context, we propose two remediation techniques, heterogeneous photocatalysis and solarization, for the elimination or at least reduction of the residue levels of certain insecticides in water and soil, respectively. Heterogeneous photocatalysis is an Advanced Oxidation Process (AOPs) based on catalytic and photochemical processes able to produce deep changes in the chemical structure of the IV pollutants. In this process, the semiconductor material, which act as photocatalysts, absorb UV radiation and generate hydroxyl radicals (¿¿OH) when they are in contact with water. This radical is a strong oxidant specie, capable of quickly degrading many organic substances to carbon dioxide, water and mineral salts. On the other hand, solarization is a disinfection technique in which a soil, previously moistened, is covered with clear plastic and exposed to sunlight during the months of higher radiation. During this process the soil temperature reaches lethal levels for many fungi, bacteria, nematodes and weed seeds; besides causing complex changes in the chemical, physical and biological soil properties. Although it was originally conceived as non-chemical alternative to methyl bromide, recent studies suggest that it can affects the persistence of pesticides in the soil. As a consequence, it could be used for decontamination purposes. With this aim, in this work the degradation of eight insecticides (acetamiprid, imidacloprid, thiamethoxam, spirodiclofen, spiromesifen, spirotetramat, chlorantraniliprole and flubendiamide) in water and soils, using two remediation techniques, has been studied. To degrade these compounds in water, the heterogeneous photocatalysis process was optimized by using different combinations between four catalysts (TiO2, ZnO, ZnTiO3 y Zn2TiO4) and one oxidizing agent (Na2S2O8). For soil remediation, the effect of solarization on the persistence and degradation of these pollutants was assessed. The results obtained in photocatalysis experiments show that ZnO/Na2S2O8 system is more effective on the oxidation of pollutants, being acetamiprid, imidacloprid, tiametoxam, spiromesifen and spirotetramat totally degraded under artificial light irradiation after 5, 2, 5, 60 and 5 min, respectively, and decreasing the residual levels of chlorantraniliprole, spirodiclofen and flubendiamide to values below 0.3, 0.5 and 5 %, respectively, after 120 min of illumination. Regarding the study of the effectiveness of solarization, the results obtained show that it enhances dissipation rates compared with the untreated soils, which can be attributed to an increase in soil temperature, caused by the plastic coverage, since it is directly related to processes that can contribute to pesticide biodegradation, mainly as a result of increased microbial activity, desorption and the action of catalytic substances.