| Abstract |
- Surfactants, or surface active agents, are the active ingredients in detergents used for
domestic cleaning applications, such as dish washing and laundering. They are effective cleaning
agents by concentrating at interfacial regions, lowering the surface tension of water, and forming
micelles that solubilize oils (West, 1992). Surfactants are amphiphilic, because they are composed
of both hydrophilic and hydrophobic moieties. The hydrophobic moiety interacts with non-polar
phases, such as oils, while the hydrophilic moiety interacts with polar phases, such as water. Oils
are removed from surfaces by partitioning into the hydrophobic interior of the surfactant micelle.
Upon rinsing with water, the micelles carrying the oils are washed away to waste water treatment
facilities, septic tanks, or drain fields.
Linear alkylbenzenesulfonates (LAS) are the main anionic surfactants used in detergent
formulations (Figure 1). LAS is a mixture of homologues that range between 10 and 14 carbon
atoms on the aliphatic chain (Swisher, 1987). Each LAS homologue consists of different isomers,
defined by the position of the phenyl ring on the aliphatic chain. The homologues and isomers
have different sorption and biodegradation characteristics. For example, SchOberl (1989) found
that longer-chain LAS homologues and isomers biodegrade faster than shorter-chain LAS
homologues and isomers. Hand and Williams (1987) found that longer LAS alkyl-chain
homologues and isomers have higher sediment partition coefficients than their shorter chain
1
counterparts. LAS also is generally considered to be biodegradable in aerobic environments and
persistent in anaerobic environments (Leisinger et al., 1981). The reason LAS biodegradation is
limited to aerobic conditions is because molecular oxygen is required for the first step in its
biodegradation pathway.
As LAS is a constituent of detergents used in everyday cleaning, it is found in most
municipal sewage effluents. Because LAS is an anthropogenic chemical and does not occur
naturally, it is a good indicator of groundwater contamination (LeBlanc, 1984). An excellent site
for the study of LAS behavior in groundwater is the U.S. Geological Survey's (USGS) Cape Cod
Toxic Water Research Site, located on Cape Cod, Massachusetts, where sewage effluent from a
waste water treatment facility is discharged onto a shallow groundwater aquifer (LeBlanc, 1984)
(Figure 2). The groundwater moves at a rate of 0.2 to 0.7 m/day and has a temperature range of
9.5 to 14°C (Harvey et al., 1994). The waste water treatment facility, located on the Otis Air
National Guard Base, has discharged secondary sewage effluent onto 4.86 hectares of rectangular
sand beds since 1936 (LeBlanc, 1984). The result of this long-term sewage discharge is a large
cigar-shaped that is currently 4000 m long, 762 to 914 m wide and 23 m thick. The plume
contains nitrate, ammonia, LAS, chloride, boron, sulfate, and phosphate (LeBlanc, 1984). In
addition, alkylbenzenesulfonate (ABS), a branched-chain non-biodegradable analog of LAS, was
discharged in the sewage effluent until 1964 and is found in the farthest down-gradient (2-3 km)
portion of the plume (LeBlanc, 1984). After 1964, LAS replaced ABS because LAS is more
readily biodegradable.
Although metabolites of LAS have been detected in the plume, Field et al, (1992)
estimated the rate at which LAS is biodegraded to be slow as LAS was detected after a 2.7 to 4.6
2
year residence time in the sewage contaminated groundwater. Low temperature (9.5 to 14°C)
and oxygen conditions (<1 mg/L) in the groundwater are thought to hinder the biodegradation of
LAS (Barber et al., 1995). Because LAS biodegradation has not been fully characterized in the
Cape Cod site, research was aimed at characterizing LAS biodegradation at 25°C and 10°C in lab
incubations with groundwater collected from the Cape Cod site.
|
|---|
