Influence of humic substances on the photolysis of aqueous pesticide residues Jose ´ R. Garbin b , De ´bora M.B.P. Milori a , Marcelo L. Simo ˜es a , Wilson T.L. da Silva a , Ladislau Martin Neto a, * a Embrapa Instrumentac ¸a ˜o Agropecua ´ ria, P.O. Box. 741, 13560-970, Sa ˜o Carlos-SP, Brazil b Universidade de Sa ˜o Paulo, IFSC/IQSC/EESC, P.O. Box. 369, 13560-970, Sa ˜o Carlos-SP, Brazil Received 4 July 2005; received in revised form 7 July 2006; accepted 10 July 2006 Available online 8 September 2006 Abstract The present work investigated the direct and indirect photolysis of pesticide residues (atrazine, imazaquin, iprodione), in aqueous solutions and under UV–visible radiation (280–480 nm). Different kinds of humic substances (HS) were added to samples in order to evaluate their behaviour as possible photocatalysts and their effect on the photolysis of pesticides. The fulvic acids were purchased from the International Humic Substances Society, and they were added to samples in concentrations ranging from 1 to 150 mg l 1 . Titanium dioxide was used as the photocatalyst, in concentration ranging from 10 to 150 mg l 1 . Pesticides photolysis were measured by UV–vis- ible absorption spectroscopy and differential pulse polarography with all used pesticides, reaching total degradation after 2 h of irradi- ation, thus indicating a fast direct photolysis. Photocatalysis by TiO 2 could increase the pesticides photolysis rate up to 40%. This effect, however, was not observed for imazaquin photolysis. Again, except for imazaquin, HS presence showed a positive effect in increasing pesticide degradation, but only within specific concentration ranges (below 10 mg l 1 for iprodione and about 30 mg l 1 for atrazine). Above these ranges HS induce a decrease in the pesticides photolysis rate. Spin-trapping measurements by electronic paramagnetic res- onance spectroscopy, using the spin-trap DMPO, showed that HS are able to photogenerate hydroxyl radicals, increasing the pesticides molecule degradation. However, the HS also react with the photogenerated hydroxyl radical, influencing the pesticide photolysis, leading to a decrease in the photolysis rate and causing it to be strongly dependent on the nature and concentration of residues in the water to be treated. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Photodegradation; Pesticides; Electron paramagnetic resonance; Hydroxyl radical; Spin-trapping 1. Introduction The fast development of societies, in the last two centu- ries, has generated serious environmental contamination problems, mainly with reference to drinking water, which tends to become a limiting factor for the life quality in the planet. Currently, pesticides and chemical fertilizer residues are considered agents of concern as they are the second larg- est source of contamination in drinking water in developing countries (Lagaly, 2001). They rarely cause a prompt man- ifestation of diseases, however they present a cumulative effect in the human organism. The consequences seen on human beings exposed to small doses of these pollutants, for large periods of time, are still unknown (Biradar and Rayburn, 1995; Paoline et al., 2004). Conventional treatments, as chlorination and filtration, are ineffective to this kind of residues and generate by- products which need further treatment with additional costs (Kolpin et al., 1998). Several alternative methods have been proposed to solve the problem and special atten- tion has been given to the use of UV light to break chem- ical bonds and promote the mineralization of the residues 0045-6535/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2006.07.017 * Corresponding author. Tel.: +55 16 3374 2477; fax: +55 16 3372 5958. E-mail addresses: garbin@cnpdia.embrapa.br (J.R. Garbin), mar- tin@cnpdia.embrapa.br (L.M. Neto). www.elsevier.com/locate/chemosphere Chemosphere 66 (2007) 1692–1698