Fate and effects of linear alkylbenzene sulphonates (LAS) in the terrestrial environment

Abstract

Linear alkylbenzene sulphonates (LAS) are a group of anionic surfactants, characterised by having both a hydrophobic and a hydrophilic group. LAS is one of the major ingredients of synthetic detergents and surfactants and is used world-wide for both domestic and industrial applications. LAS is relatively rapidly aerobically degraded, but only very slowly or not at all degraded under anaerobic conditions. Therefore, LAS can be found in very high concentrations in most sewage sludge and enter the soil compartment as a result of sludge application. LAS can be found in elevated concentrations in soil immediately after sludge amendment, but a half-life of approximately 1–3 weeks will generally prevent accumulation in soil and biota. The concentration in soils that have not received sewage sludge recently, is generally less than 1 mg kg⁻¹ and not more than 5 mg LAS kg⁻¹. This is below the lowest concentration of LAS where effects have been observed in the laboratory. The laboratory data is in accordance with field studies using aqueous solutions of the sodium salt of LAS. However, observations of the ecological impact of sewage sludge applications or application of LAS spiked into sludge indicates a lower toxicity of LAS when applied in sludge. On the basis of the information reviewed in this paper, it is concluded that LAS can be found in high concentrations in sewage sludge, but that the relatively rapid aerobic degradation and the reduced bioavailability when applied via sludge, most likely will prevent LAS from posing a threat to terrestrial ecosystems on a long term basis.

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.