Reduction of aquatic toxicity of linear alkylbenzene sulfonate (LAS) by biodegradation

Abstract

Partial biodegradation of LAS is shown to significantly reduce the specific toxicity (i.e. per unit weight) of the remaining LAS to Daphnia magna (water fleas) and Pimephales promelas (fathead minnows). This results from the fact that the longer homologs and more terminal isomers, which are the more toxic, are also the more rapidly degraded under bacterial action. The acute aquatic LC 50 of LAS may range from ∼0.5 to ∼50 mg/l depending mainly upon the chain length of the particular homolog. A high molecular weight commercial type LAS with LC 50 around 2 mg/l before biodegradation may show Daphnia LC 50's of 30–40 mg/l. for the LAS remaining after 80–85% degradation. A further contribution to this toxicity reduction may occur if the methylene blue analytical method is used to determine the amount of LAS remaining, since some of the biodegradation intermediates show methylene blue activity but no significant toxicity. For example, sulfophenylundecanoate, a model of early intermediates, shows Daphnia and fathead lc 50's ∼200 and ∼75 mg/l., respectively. Sulfophenylbutyrate, modeling somewhat later intermediates, gives lc 50 values around 5000–10,000 mg/l. Dialkyl tetralin/indane sulfonates (the major non-linear components in commercial LAS) exhibit 1/2–1/10 the toxicity of the corresponding LAS homologs. These results re-emphasize that analysis simply for methylene blue active substances (MBAS) gives no basis for predicting the aquatic toxicity of an environmental sample. And furthermore, that meaningful water quality criteria and standards cannot be established in terms of MBAS content while based on toxicity studies on intact, undegraded LAS.