ABSTRACT

This work is based on the study of the photodegradation of commercial pesticides (Laition®, Ultracid®, Sevnol® and Metasystox®), which have been selected because of their use in citric cultivars of Valencia, together with their corresponding active ingredient. Two different advanced oxidation methods have been employed, solar photocatalysis with TiO2 and photo-Fenton process, as a pre-treatment to detoxify and enhance the biodegradability of those persistent pollutants in order to allow coupling a conventional biological treatment.

Studies on the photodegradation  of pesticides have been carried at laboratory scale (using a solar simulator or in open reactors submitted to sunlight)  in order to determine the experimental conditions before it was scaled up to a pre-industrial pilot plant based on CPC-technology. Different analytical techniques have been employed to follow the photo-oxidative reaction: DOC, HPLC, surface tension, ionic-chromatography, GC-MS… Based on these results, reaction kinetics has been determined and main intermediates have been detected. Carbamate pesticides have been observed to be more difficult to degrade than the organophosphorated ones and the commercial formulae require longer periods of treatment than pure active ingredients. 

Removal of the active ingredients needs shorter irradiation periods than mineralization of organic matter, and it is closely related with detoxification of the solutions. For this reason, toxicity has been determined out according to different analytical methods (inhibition of the respiration of activated sludge, inhibition of BOD5 and inhibition of  the Vibrio fisheri bacteria), as weel as biodegradability (BODst, BOD5/COD ratio and the Zahn-Wellens test) during the photocatalytic process, in order to determine the minimum irradiation needed to couple a biological treatment. In general, accumulation of toxic intermediates has not been detected, although when chlorinated compounds were present in the commercial formulae, some toxicity remains after elimination of the pesticide.