Review articleFate and effects of linear alkylbenzene sulphonates (LAS) in the terrestrial environment
Introduction
Linear alkylbenzene sulphonates (LAS) are a group of anionic surfactants, characterised by having both a hydrophobic and a hydrophilic group. The basic structure of the LAS molecule is a benzene ring connected to an alkyl chain of different length (the hydrophobic end) and a sodium sulphate group (the hydrophilic end). The LAS-group consists of different homologues, i.e. different lengths of the alkyl chain, and of different isomers, i.e. different position of the benzene ring on a fixed alkyl chain. LAS used in commercial products tend to be a mixture of homologues. The chemical structure of LAS is presented in Fig. 1.
Being easily degradable under aerobic conditions, LAS was introduced as a substitute for the slowly biodegradable alkyl benzene sulphonates (ABS) in the mid-sixties. Today LAS is one of the major ingredients of synthetic detergents and surfactants and is used world-wide for both domestic and industrial applications. The annual world wide consumption of LAS is approximately 2×106 t per year (de Wolfe and Feijtel, 1997). Together with other synthetic surfactants, e.g. alcohol ethoxylates, alkane sulphonates and alcohol sulphates, the total consumption of surfactants is approximately 7×106 t per year which is about the same level as the consumption of traditional soap (8.5×106 t per year).
After use and disposal, LAS may enter the environment by one of several routes, including by direct discharge to surface water or discharge to water from sewage treatment plants. However, in recent years waste water treatment plants have significantly reduced the amount of LAS discharged to surface waters. LAS may enter the soil compartment by irrigation with wastewater or by sewage sludge application. LAS may also reach the soil environment, when anionic surfactants are used as emulsifying, dispersing and spreading agents in the processing of fertilisers and distribution of pesticides in agriculture (Spurrier and Jackobs, 1955; Parr and Norman, 1965; Ernst et al., 1971; Pfahler et al., 1981and Khanislamova, 1991) and as pesticides in horticulture (Hislop et al., 1977). Furthermore, though still at an early scientific state, anionic surfactants are used in the remediation of contaminated soil by mobilising heavy metals, PAHs etc. (Burchfield et al., 1994; Scheibenbogen et al., 1994).
The fate and effects of LAS in the aquatic environment have been studied extensively (e.g. Abel, 1974; Schöberl, 1989; Waters et al., 1989; Lewis, 1991; Painter, 1992; Blok and Balk, 1993and Malcolm et al., 1995), whereas the terrestrial environment has received considerably less attention (Kloepper-Sams et al., 1996; de Wolfe and Feijtel, 1997). This review summarises the present knowledge of fate and effects of LAS in the terrestrial environment.
Section snippets
Fate in waste water treatment plants
The vast majority of surfactants and detergents used in households or industry reach waste water treatment plants (WWTP). In the WWTP a relatively large portion of LAS will be removed from the waste water either by adsorption to solids or organic material, or by precipitation with calcium or magnesium. LAS homologs with longer alkyl chains have a higher adsorptive tendency to solids as well as a greater insolubility when bound to calcium (Prats et al., 1993). A two- to threefold increase in Kd
Biological effects
The primary effect of surfactants in general, is the disruption of biomembranes and denaturation of proteins (Swisher, 1987; Schwunger and Bartnik, 1980). The toxicity of LAS is influenced by the position of the benzene ring. Terminal ring position results in higher toxicity compared to central placement of the ring (Prats et al., 1993). The toxicity also increases with increasing chain length, as long as the surfactant remains soluble (Baker et al., 1941; Schöberl et al., 1988; Painter, 1992;
Ecological risk
LAS may enter the terrestrial environment by several routes. However, the use of sewage sludge as fertiliser on agricultural land is by far the dominating input. The load of LAS in sewage sludge may be considerable with concentrations of more than 10 000 mg kg−1 dry wt. Since only Denmark presently have a cut-off value for LAS in sewage sludge used as fertiliser (currently these are 2600 mg kg−1 dry wt. and will be lowered to 1300 mg kg−1 dry wt. in the year 2000), it is not possible to
Summary
Linear alkylbenzene sulphonates (LAS) are used in large amounts in modern society, and can hence be found in relatively high concentrations in sewage sludge. The concentration in sewage sludge is very dependent upon the treatment of sewage in the waste water treatment plants. The degradation is fairly rapid under aerobic conditions, but LAS is only very slowly degraded under anaerobic conditions. In the terrestrial environment LAS generally disappears rapidly with half-lives in the range of
Acknowledgements
The financial support given by AISE, CESIO, ECOSOL and CLER is acknowledged.
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