Environmental Engineering Reference
In-Depth Information
The widespread use of pesticides inevitably leads to their presence in surface waters
and groundwater. Pesticides can be transformed in the environment into a large number
of degradation products, commonly defined as transformation products (TPs), although
other terms, such as metabolites or pesticide derivatives can be used. Pesticide transfor-
mation is any process in which change in the molecular structure of the pesticide takes
place. Most of the applied pesticides are ultimately degraded into universally present com-
pounds, such as carbon dioxide, ammonia, water, mineral salts, and humic substances
(Somasundaram and Coats 1991). The rates at which transformation reactions occur in
aquatic systems vary considerably for individual chemicals and under different envi-
ronmental conditions. Some compounds undergo the transformation immediately after
or even before application, whereas complete mineralization of some pesticides, such as
dichlorodiphenyltrichloroethane (DDT) or mirex, can take decades (Larson et al. 1997).
From the same parent compound, different TPs can be created by various transformation
processes. Once a pesticide is introduced in the aquatic environment, it can be subjected to
a variety of processes: physical (accumulation, deposition, dilution and diffusion); chemi-
cal (hydrolysis and oxidation); photochemical (photolysis and photodegradation), and bio-
chemical (biodegradation, biotransformation and bioaccumulation) processes. Photolysis
and hydrolysis are main abiotic processes that take place in the aquatic environment
(Barceló and Hennion 1997). In photolysis, the light energy and intensity, as well as duration
of the sunlight, affect the rate of pesticide degradation. Many environmental parameters,
such as pH, temperature, water type, and the presence of humic substances can catalyze or
hinder the hydrolysis or photolysis of pesticides (Lartiges and Garrigues 1995; Bachman
and Patterson 1999; Iesce et al. 2006).
Many modern-day pesticides are degraded by microbial or chemical processes into
nontoxic products. In some instances, transformation products can be more toxic and
pose a greater risk to the environment than the parent compound (Belfroid et al. 1998).
Moreover, TPs can have different properties that enable them to occur in areas not reached
by the parent compounds. The process of degradation generally increases the water solu-
bility and the polarity of the compound. The increase in solubility is caused by the loss
of carbon, the incorporation of oxygen, and the addition of carboxylic acid functional
groups. For example, atrazine degrades through a combination of physical, chemical, and
biochemical processes. The three major degradation products of atrazine are deethylatra-
zine (DEA), deisopropylatrazine (DIA), and hydroxyl-atrazine (HA) (Kruger et al. 1993).
Atrazine's maximum solubility is 33 mg/L, while DEA (loss of two carbon atoms) and
DIA (loss of three carbon atoms) have solubilities of 670 mg/L and 3200 mg/L, respec-
tively (Thurman and Meyer 1996). Due to their mobility in soil and water environment,
TPs can reach groundwater more easily than parent compounds (Martínez Vidal et al.
2009). Table 9.2 shows a brief summary of TPs that have been studied in the recent years
as well as their concentrations found in studied waters.
Two of the most widely used classes of agricultural herbicides are triazines and chlo-
roacetamides ( Table 9.1 ). The difference between these two classes of herbicides is that
a substantially larger fraction of chloroacetamides undergoes transformation in soils
than in the case of triazines (Hladik et al. 2005). Thus, triazine pesticide TPs are found
in lower concentrations than parent compounds, while the opposite is true for chloro-
acetamide pesticide TPs. Chloroacetamide TPs are readily formed, but once they are
transported in an oligotrophic environment, they are likely to be persistent. The two
most commonly investigated classes of chloroacetamide TPs, ionic ethanesulfonic acid
(ESA) and oxanilic acid (OA), are not likely to present a risk to human health (Heydens
et al. 1996, 2000). However, some neutral chloroacetamide derivatives were found to
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