Mechanisms of Bacterial Biodegradation of Alkyl Sulphate and Alkylpolyethoxy Sulphate Surfactants

Abstract

Sulphated surfactants, including long-chain alkyl sulphates and polyethoxy sulphates, are important components of modern detergent formulations. Depending on the alkyl chain these compounds are readily biodegradable by microorganisms in soil and natural waters. From a biochemical viewpoint biodegradation is initiated by different mechanisms. Alkyl sulphates are attacked initially by a range of sulphatases of differing specificities, which liberate long-chain alcohols that can be assimilated via central metabolic pathways. In contrast the biodegradation of alkylpolyethoxy sulphates is less straightforward. Our studies using sodium dodecyl triethoxy sulphate (SDTES) have shown that primary biodegradation can be initiated by sulphatase or etherase attack. Some bacteria possess an etherase system which produces a mixture of mono-, di- and triethylene glycol monosulphates; whereas in others the etherase acts almost exclusively on the alkyl-ether bond generating triethylene glycol monosulphate. In pure cultures of single isolates these sulphated glycols and their oxidised derivatives are not broken down further. However, in environmentally-relevant test systems of mixed cultures of glycol sulphates are metabolised to C02 and inorganic sulphate. Hydrophobic metabolites containing the alkyl portion of the parent surfactant are readily biodegradable in mixed culture systems. Therefore, in environmental terms, the alkylpolyethoxy sulphates can be regarded as completely biodegradable.