Effect of Sunlight and UV-C Disinfection Dose Irradiation on the Degradation of Organophosphorous Pesticide Dichlorvos

A. CRUZ-ALCALDE; CARMEN SANS; A. IRIEL; NAHUEL J BUSTOS; ALICIA FERNÁNDEZ CIRELLI

Lisboa

Congreso; 18th European Meeting on Environmental Chemistry (EMEC); 2017

Dichlorvos (2,2-Dihclorovinyl dimethyl phosphate, DDVP) is a chlorinated organophosphorus pesticide, employed as insecticide in agriculture, as well as to treat parasite infections in livestock and domestic animals [1]. The widespread employment of this insecticide has caused its detection in surface waters worldwide. DDVP is classified by the World Health Organization (WHO) as a highly hazardous pesticide [2] and it has been banned in the European Union.Despite the risks potentially posed by its presence in water compartments, little is known about the behavior and fate of DDVP under natural sunlight and UV-C irradiation, being the latter increasingly adopted as a primary disinfection method for drinking water and wastewater effluent. This study investigated the photodegradation mechanism of DDVP under sunlight and UV-C irradiation and the role that reactive oxygen species (ROS) plays in it. To simulate sunlight effect on DDVP, pesticide water solutions were irradiated by a Xe-OP lamp (Phillips 1kW), in a SOLAR BOX® (Co.fo.me.gra 220 V, 50 Hz). The photonic flow of the Xe-lamp in the 290-400 nm range was 2.98 μEinstein s−1. The UV-C photoreactor used to simulate disinfection doses irradiations was equipped with one low pressure UV-C (254 nm) lamp (4W, 30% UVC efficiency, Philips TUV G4T5), with a photon flow of 0.17 μEinstein s-1. The role of oxygen in the phototransformation of DDVP was examined with dissolved oxygen (at a concentration of 6.03 ± 0.5 mg L-1) and without oxygen, in N2 bubbled solutions (O2 concentration < 0.3 ± 0.1 mg L-1). The contribution of ROS was monitored by adding radical scavengers into reaction solutions. Concretely, tert-butanol (3mmol L-1) and sodium azide (2.5 mgL-1) were used to deplete hydroxyl radical (HO?) and singlet oxygen (1O2) reaction pathways, respectively. Results showed that photodegradation of DDVP occur under simulated sunlight, providing an alternative degradation mechanism to hydrolysis in water. In N2-saturated solution virtually no direct photolysis was produced under simulated sunlight irradiation, and therefore the most relevant contribution to pesticide depletion could be associated with the presence of dissolved oxygen.The experimental evidence points to a photoionization of DDVP and the scavenging of hydrated electrons by oxygen generating the formation of superoxide radicals (O2?‾). It is not probable that O2?‾ reacts with the pesticide but other oxidant species like hydroxyl radicals (HO?) resulting from different radical reactions. The possible contribution of singlet oxygen generation (O21) by photosensitization of excited DDVP could also account to the decomposition of organophosphorous pesticide in aqueous solutions. DDVP degradation rate is significantly increased under UV-C irradiation at disinfection doses (40-100 mJ cm-2), due to the contribution of direct photolysis. However, dissolved oxygen still plays an important role in the pesticide degradation.AcknowledgementsThe research leading to these results has received funding from the People Programme (Marie Curie Actions) of the European Union´s Programme H2020-MSCA-RISE-2015 under REA grant agreement n° 690618.References[1] T. Oncescu, M.I. Stefan, P. Oancea, Environ. Sci. Pollut. Res. 17 (2010) 1158?1166.[2] World Health Organization, The WHO Recommended Classification of Pesticides By Hazard and Guidelines To Classification, 2009.