Surfactant Effects on Environmental Behavior of Pesticides - Reviews of Environmental Contamination and Toxicology - page 83

1988; Lee et al. 1989; Mishael et al. 2002). The exchange profiles of HDTMA +
were more clearly observed for its adsorption to several clay samples, and the
higher-charge expandable clay minerals could adsorb more HDTMA + (Jaynes and
Boyd 1991). Depending on the length of an alkyl chain, spacing of silicate layers,
and mineral surface charge, alkyltrimethylammonium cations are considered to be
incorporated as monolayer, bilayer, pseudo-trimolecular layer, and paraffin
complex.
The surfactants dissolved in pore water or surface water and their parts adsorbed
on soil and sediment would be degraded by various microbes to more hydrophilic
degradates. Existing reviews on microbial degradation of surfactants show more
facile degradation under aerobic conditions than anaerobic (Krogh et al. 2003; Ying
2006). LAS is aerobically degraded with half-lives of a few days in river water to
about a month in soil and is not degraded anaerobically; alkyl sulfates are readily
biodegradable irrespective of areobicity. Cationic surfactants are susceptible to aer-
obic biodegradation with half-lives about 1 hr but increase of non-methyl substitu-
tion at the nitrogen reduces their biodegradability. The nonionic alcohol ethoxylates
are degradable in either condition but alkylphenoxy ethoxylates having a longer
ethoxy unit become difficult to be metabolized. Each surfactant is generally metab-
olized by consortia because of the limited metabolic capacity of an individual
microorganism (Van Ginkel 1996). Typical metabolic pathways of some surfactants
under aerobic conditions are summarized in Fig. 6.
Quaternary trimethylammonium salts are usually degraded by monooxygenase
via central fission of the C alkyl — N bond to form trimethyl amine and the corre-
sponding alkanal, which is further oxidized to the acid followed by stepwise
β

-oxidation at the far end of the alkyl chain
was reported for DTMA Br. Alkyl sulfates are most rapidly biodegradable via
enzymatic cleavage of the sulfate ester bond by alkylsulfatases to form the corre-
sponding alcohol and sulfate ion. The major microbial degradation pathway of
LAS is the successive

-oxidation. As a minor pathway, ω

-oxidation at the far end of the alkyl chain to the corre-
sponding acid, followed by

ω

-oxidation. The final product is a sulfophenyl
alkanoate that is further degraded via desulfonation and ring cleavage by
dioxygenases. One of the major aerobic routes for alcohol and alkylphenoxy
ethoxylates is the oxidation of the terminal ethoxy unit to the carboxylic acid,
followed by stepwise

β

-oxidation finally to form the alkanoic acid, alkylphenol,
and alkylphenoxyacetic acid. In the case of alcohol ethoxylates, either central
fission to produce the corresponding alkanoic acid and polyethyleneglycol or

β

-
oxidation at the alkyl chain has been reported. Recently, a unique nitration reaction
at the phenoxy ring in soil/sewage sludge mixture has been reported for the possible
metabolites of alkylphenoxy ethoxylates (Telscher et al. 2005).

ω -/ β

Behavior in Water

Because the chemical class of most surfactants is quaternary ammonium, sulfate,
sulfonate, or ether, they are resistant to hydrolysis under environmental conditions.