The effect of dissolved humic acid (HA) on two types of hydrolysis reactions was investigated: (I) dehydrochlorination of gamma-hexachlorocyclohexane (HCH) and 1,1,2,2-tetrachloroethane (TeCA) as a reaction involving hydroxide ions (OH(-)) and (II) hydrolysis of 1-octyl acetate (OA) which is catalyzed by H(+) at the applied pH value (pH 4.5). The rate of TeCA hydrolysis was not affected by addition of 2 g l(-1) of HA at pH 10 (k' = 0.33 h(-1)) but HCH hydrolysis was significantly inhibited (k' = 4.6 x 10(-3) h(-1) without HA and 2.8 x 10(-3)h(-1) at 2 g l(-1) HA). HCH is sorbed by 51% whereas TeCA sorption is insignificant at this HA concentration. Sorbed HCH molecules are effectively protected due to electrostatic repulsion of OH(-) by the net negative charge of the HA molecules. In contrast, OA hydrolysis at pH 4.5 (k' = 1.6 x 10(-5) h(-1)) was drastically accelerated after addition of 2 g l(-1) HA (k' = 1.1 x 10(-3) h(-1)). The ratio of the pseudo-first-order rate constants of the sorbed and the freely dissolved ester fraction is about 70. H(+) accumulation in the microenvironment of the negatively charged HA molecules was suggested to contribute to the higher reaction rate for the sorbed fraction in case of this H(+)-catalyzed reaction. Analogous effects from anionic surfactants are known as micellar catalysis.
"In addition, previous studies reported the influence of HSs on the hydrolysis (the major transformation pathway for numerous of pesticides or pharmaceuticals in the environment (Arias-Estevez et al., 2008b; Georgi et al., 2008)) xenobiotics such as herbicides, pollutants and diverse hydrophobic compounds (Georgi et al., 2008; Perdue and Wolfe, 1982; Sabadie, 1997; Salvestrini et al., 2008) showing a high association for these compounds by the HSs micellar aggregates. On the other hand, the pesticides carbofuran and carbofuranderivatives (Scheme 1) are widely used in agriculture as systemic insecticide with plenty of adverse effects upon ecosystem because their nature as endocrine disrupters (Gupta, 1994; Krol et al., 2001). "
[Show abstract][Hide abstract] ABSTRACT: " Influence of humic substances in stability of carbofuran and their derivatives. " Inhibitory effects for 3-hydroxy-carbofuran and 3-keto-carbofuran were observed. " Non-significant effect upon the carbofuran stability was found. " These behaviors were rationalized in terms of the micellar pseudophase model. a b s t r a c t The influence of humic aggregates in water solution upon the chemical stability of carbofuran (CF) and the carbofuran-derivatives, 3-hydroxy-carbofuran (HCF) and 3-keto-carbofuran (KCF), has been investi-gated in basic media. An inhibition upon the basic hydrolysis of 3-hydroxy-carbofuran and 3-keto-carbo-furan ($1.7 and $1.5-fold, respectively) was observed and it was rationalized in terms of the micellar pseudophase model. Nevertheless, non-significant effect upon the carbofuran stability was found in the presence of humic substances. These behaviors have been compared with the corresponding ones in other synthetic colloidal aggregates.
"This could be due to the negative charges associated with organic particles, which result in a decrease of the effective surface pH and/or stabilization of the transition species during reaction (Noblet et al., 1996). Organic matter effects may also be explained by a micellar catalysis model (Georgi et al., 2008). Furthermore, soil amendment with organic matter favors biodegradation and may hinder the effect of amendment on chemical hydrolysis reactions, as observed for triasulfuron by Said-Pullicino et al. (2004). "
[Show abstract][Hide abstract] ABSTRACT: Despite constraining legislation and increasing efficiency of pesticides (therefore decrease in the applied amounts), their use still cause a contamination of environment (air, soil and water). Therefore, in a global context of increasing production, a more significant decrease of pesticides use in agriculture is required to prevent additional contamination of agrosystems. To conciliate agricultural and environmental interests, a better understanding of the fate of pesticides is needed, in particular because it will determine the exposure and consequently the impact of pesticides on the target and non-target organisms. This goal requires new efforts of research at different scales (from molecular to field scale). Following application, most of the pesticides reach the soil either after direct application or foliage wash-off. As a major interface between other environmental compartments, the soil plays a preponderant buffering role in the fate of pesticides. Apart volatilization, the main processes that control the fate of pesticides in soils are retention on soil particles and degradation (biotic and abiotic). These coupled bio-physico-chemical processes can lead to a transitory or permanent accumulation of pesticides in soils or, on the contrary, to their elimination from the environment. They determine the pesticide concentration in the soil solution, and have a large influence on pesticide transfer toward ground or surface waters and on their ecotoxicological impacts on soil organisms as well. The main difficulties in studying and predicting the retention and degradation of pesticides in soils are the chemical diversity of pesticides and reactivities, the high diversity of soils and their heterogeneous composition and structure. In addition, the pedoclimatic conditions, in particular soil temperature and water content, have a strong influence on retention and degradation because of their effect on soil biological, chemical and physical properties. Therefore, the objective of this chapter is to provide an overview of the factors involved in the retention and degradation of pesticides in soils and to discuss and clarify the needs of new integrated approach. In particular, this work will discuss (i) the pertinent scales (among elementary constituents, aggregates and mesoscopic scales) for both retention and degradation studies, (ii) the integrative properties that should be considered, such as hydrophobicity of the organo-clay granulometric fraction or soil structure (iii) the primordial role of water.
Pesticides in the Modern World - Risks and Benefits, 10/2011: chapter 29: pages 535-560; Intech OpenAcess Publisher., ISBN: 978-953-307-458-0
[Show abstract][Hide abstract] ABSTRACT: The effect of sorption to dissolved humic acids (HAs) on the chlorination of PAHs in aqueous solution was studied. The addition of HA accelerated the chlorination of fluoranthene and naphthalene in hypochlorite solutions at pH 5, the stronger effect being observed for fluoranthene that is sorbed to a higher extent than naphthalene. Sorption coefficients (K(DOC)) of the analytes were determined by solid-phase microextraction (SPME). The observed rate constant for fluoranthene chlorination is, for example, larger by a factor of 5 in the presence of 10 mg L(-1) of an aquatic HA as compared to HA-free solution (k' = 0.02 h(-1) at 60 mg L(-1) active chlorine, pH 5, without HA). While Cl2 is the dominant reactive species in pure aqueous solution for both PAHs, the reaction of fluoranthene seems to involve an additional pathway of chlorination by HOCl in the presence of HA. It was found that not only did HA not protect PAHs from the electrophilic attack of the chlorinating species, but the sorption of PAHs on the hydrophobic domains of the HA favored instead the extent of the chlorination reaction.
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