Environmental Engineering Reference
In-Depth Information
residue in the water body, its movement to the atmosphere through volatilization from
the water surface, and sorption and desorption to the particles present in the water.
Abiotic transformation may include chemical (mainly hydrolysis and thermolysis) and
photochemical reactions. Both types of reactions are degradative pathways for many pesti-
cides in the aquatic environment. Information about these degradation routes is necessary
to estimate the persistence of these compounds and to identity the factors that influence
their behavior in the aquatic media.
Regarding hydrolysis reactions, pH of water is responsible for the transformation of
some of the pesticides in solution, especially in conjunction with pH extremes (García-
Repetto et al. 1994). Furthermore, organic compounds sensitive to pH give rise to their
rapid degradation, even with a slight variation of pH. For example, many organophos-
phorus pesticides (Dannenberg and Pehkonen 1998; García-Repetto et al. 1994; Santos
et al. 1998), carbamate pesticides, (Sanz-Asencio et al. 1997), and pyrethroid pesticides
(Al-Mughrabi et al. 1992) present a chemical structure that makes them susceptible to
hydrolysis in an aqueous media, and therefore, they have a low environmental persistence.
The cleavage of the ester group, present in these classes of pesticides, leads to the forma-
tion of two new degradation products.
Photochemical reactions are one of the major transformation processes affecting the fate
of the pesticides in natural water (Dimou et al. 2004a; Marcheterre et al. 1988; Neilson and
Allard 2008; Saha and Kulshrestha 2002). Two ways of photodegradation reactions occur in
sunlit natural water. In direct photolysis, organic compounds absorb light and, as a conse-
quence, undergo transformation. For this to occur in water, the Sun's emission (290-800 nm)
needs to fit the adsorption spectrum of the pesticide. In indirect photochemical reactions,
organic chemicals are transformed by energy transfer from another excited species (e.g.,
components of natural organic matter) or by reaction with very reactive, short-lived species
formed in the presence of light (e.g., hydroxyl radicals, single oxygen, ozone, and peroxy
radicals). Absorption of actinic radiation by nitrate and dissolved organic matter (DOM)
leads to the formation of most of these species. Therefore, the composition of the aquatic
media plays an important role in the phototransformation of pesticides in this compartment.
Dissolved substances present in natural waters are responsible for the different photolysis
rates of pesticides observed between natural and distilled water (Dimou et al. 2005; Durand
et al. 1991; Sevilla-Morán et al. 2010b).
The hydroxyl radical, OH˙, is the most reactive of the aforementioned reactive interme-
diates due to its nonselective and highly electrophilic nature. Its concentration in surface
water is reported to be at 10 −14 −10 −18 M (Brezonik and Fulkerson-Brekken 1998). The types
of reactions where OH˙ is involved include H-abstraction and addition to double bonds.
Different parameters affect the photolysis of pesticides in the aquatic environment.
Thus, the photodegradation behavior of many pesticides in aquatic media has been inves-
tigated under various experimental conditions such as different types of aqueous matrix
(ground, surface, and mineral water) (Konstantinou et al. 2001; Lin et al. 1999; Sakkas
et al. 2002a), light sources (natural sunlight, xenon arc lamps, etc.) (Elazzouzi et al. 1999;
Sakellarides et al. 2003; Zamy et al. 2004), and light intensities, as well as under the influ-
ence of substances dissolved in natural water (humic acid (HA), nitrate, and iron ions)
(Bachman and Patterson 1999; Dimou et al. 2004b; Wilson and Mabury 2000). Ideally,
solar radiation should be used in the studies of environmental photochemistry; however,
meteorological conditions in most countries and a slow degradation rate do not permit
reproducible experimentation. In a first approach to the study of photochemical behavior
of organic compounds in different matrices, it is common to conduct the degradation
under controlled conditions. Generally, the use of xenon arc lamp, with light above 290 nm
Search WWH ::




Custom Search