Regeneración de aguas contaminadas con insecticidas, mediante TiO2/Na2S2O8/UV, para su reutilización en cultivos hortícolas

Abstract

Climate change, desertification and world population increase have caused the need of water reuse for the irrigation of crops. Agro-wastewater with pesticides could represent a risk to human health and environment due to their release in the soil and their plants uptake. Scientific literature is replete with works that study the elimination of different pollutants from water through different Advanced Oxidation Processes being, heterogeneous photocatalysis, one of them. In this process, a semiconductor oxide absorbs UV-VIS radiation direct- or indirectly, generating mainly hydroxyl radicals (HO•), a strongly oxidizing species capable of eliminating almost any organic molecule. This is an interesting technology for the removal of pollutants, such as pesticides, from agricultural wastewater. The main objective of this Dissertation was the regeneration of agro-wastewater polluted with four insecticides (chlorantraniliprole (CLO), imidacloprid (IMI), pirimicarb (PIR) and thiamethoxam (TIA)) and their main intermediate transformation products (TPs) by heterogeneous photocatalysis. In addition to the subsequent water reuse in the irrigation of horticultural crops (lettuce and pepper). Firstly, an analytical methodology was developed for the simultaneous determination of insecticide residues and their TPs in water, soil, lettuce and pepper samples using HPLC/MS-QqQ. Secondly, the behaviour (adsorption, degradation and leaching) of the four insecticides and their TPs in a characteristic soil from the Campo de Cartagena area (Murcia, SE of Spain) was assessed. Subsequently, the effect of some of the most influential factors in heterogeneous photocatalysis for the elimination of the target insecticides, at laboratory scale, was evaluated: nature and concentration of the photocatalyst (TiO2 and ZnO), concentration of the oxidant (PS, Na2S2O8), UV-A radiation source (artificial with LED technology and solar) and the water matrix complexity. Afterwards, the removal of insecticide residues in agro-wastewater was carried out using the most convenient photocatalytic system (TiO2/PS/UV-A solar) at pilot plant. Finally, a number of experimental horticultural crops with different types of water were irrigated: (i) unpolluted irrigation water, (ii) polluted irrigation water with commercial phytosanitary products and (iii) regenerated water by solar heterogeneous photocatalysis, to evaluate, in harvested agricultural products, the presence of pollutant residues and the impact on their productivity and quality. Regarding to the results of the insecticide soil mobility, with reference to the calculated leaching indices (GUS and ELI) in our experimental conditions, IMI and TIA were catalogued, as leachable and potentially groundwater pollutants, while CLO and PIR were non-leachable and with low potential for groundwater contamination. Concerning the heterogeneous photocatalytic process, once the main factors of the treatment were optimized, at laboratory scale, an efficient removal of the studied insecticides was achieved by means of TiO2/PS/UV-A solar. Following, at pilot plant, the needed amount of agro-wastewater to carry out the irrigation of different lettuce and pepper crops, on field scale, was decontaminated; practically without insecticides nor their TPs residues. When crops were irrigated with this water, neither the studied insecticides nor their TPs were detected in lettuce, pepper and soil samples; except CLO which was detected in the soil of the pepper crop. However, the presence of these pollutants in agricultural products and soil was observed when the agro-wastewater was not subjected to the photocatalysis process, causing a potential risk to human health and environment. Finally, the impact of reuse of regenerated water by solar heterogeneous photocatalysis on the productivity and quality of horticultural products (lettuce and pepper) was evaluated. It becomed positive, since hardly any significant differences were observed for all the measured parameters when compared with those horticultural crops irrigated with unpolluted irrigation water. In conclusion, solar heterogeneous photocatalysis can be applied in a sustainable way, contributing to the water utilisation in areas with water scarcity and the use of renewable energies, mainly in areas with high solar radiation. Therefore, the novelty of this Dissertation reside in the reuse of agro-wastewater for agricultural purposes without human nor environmental risk.