Surfactant Effects on Environmental Behavior of Pesticides - Reviews of Environmental Contamination and Toxicology - page 138

Simple Organic Chemicals
The mechanism of the surfactant-modified biodegradation of a chemical has been
investigated extensively on PAHs and long-chain alkanes by using isolated
microbes, microbial consortia, and intact soils. Laha and Luthy (1992) reported that
several types of nonionic surfactants at concentrations above cmc inhibit the bacte-
rial mineralization of phenanthrene, and their presence at lower concentrations
shows an insignificant effect or delayed mineralization. They proposed the reduc-
tion of microbial enzymatic activity or less transport of a chemical from micelles
to the cells as possible reasons (Laha and Luthy 1991). Inhibition of bacterial
growth with an effect on enzymatic activity by nonionic surfactants was reported
for dibutyl phthalate-degrading soil bacteria (Chao and Lin 2006). The promoted
mineralization of phenanthrene and biphenyl at lower concentrations of nonionic
surfactants than cmc has been shown by Aronstein et al. (1991), and their enhanced
desorption from soil was considered to increase bioavailability to microbes.
Increased solubilization by surfactants could facilitate the mineralization of decane
by the two gram-negative bacterial strains, even in the presence of micelles with the
biodegradation rate following the Monod equation (Bury and Miller 1993). Zhang
and Miller (1994) examined the effects of rhamnolipid on the biodegradation of
octadecane by four isolated microorganisms producing the biosurfactant and
showed that promotion and inhibition of biodegradation are highly dependent on
both surfactant concentration and bacterial species.
The effect of surfactant on biodegradation has been also theoretically investigated
by many researchers. For less water-soluble chemicals such as PAHs, the dissolution
rate from their solid states becomes a rate-determining step of microbial degradation.
In the biodegradation of solid phenanthrene and naphthalene by two Pseudomonas
strains, Volkering et al. (1995) reported the enhanced dissolution of PAHs above cmc
by Triton X-100 and Brij 35 together with more biodegradation, but bioavailability of
the micellized PAHs to the microbes was considered less than those dissolved in an
aqueous phase (see Fig. 10d). Similar results were reported by Grimberg et al. (1996),
and the excess amount of surfactants forming more micelles was found to reduce
bacterial growth by reducing bioavailable phenanthrene. Furthermore, Mulder et al.
(1998) demonstrated through the enhanced biodegradation of naphthalene in the
presence of six nonionic surfactants that a mechanistic mass-transfer model well
describes the dissolution of solid naphthalene by the surfactants and that the biomass
formation rate by a Pseudomonas strain increases concomitantly with the mass-
transfer rate under naphthalene dissolution-limited conditions.
Assuming that the transfer of a chemical between phases is instantaneous relative
to biodegradation and the surfactant does not alter the specific activity of biomass,
Guha and Jaffé (1996a,b) have kinetically analyzed the micellar effect of nonionic
surfactants on the biodegradation of 14 C-phenanthrene by a mixed enrichment culture
isolated from a petroleum-contaminated soil. They separated step (e) in Fig. 10 into
the mass transfer of the filled micelle to a cell surface, the attachment of the filled
micelle to the cell surface as a hemimicelle, and the transfer of a chemical from the
hemimicelle to the cell. The second process was defined as the ratio of the micellar