Fate and effect of monoalkyl quaternary ammonium surfactants in the aquatic environment

The effect or the alkyl chain of quaternary ammonium-based surfactants on t heir aquatic toxicity and aerobic biodegradability has been studied. Two fa milies of monoalkylquats surfactants were selected: alkyl trimethyl ammoniu m and alkyl benzyl dimethyl ammonium halides. Acute toxicity tests on Daphn ia magna and Photobacterium phosphoreum were carried out and EC50 values in the range of 0.1-1 mg/l were obtained for the two series of cationic surfa ctants. Although the substitution of a benzyl group for a methyl group incr eases the toxicity, an incremental difference in toxicity between homologs of different chain length were not observed. Biodegradability of the differ ent homologs was determined not only in standard conditions but also in coa stal water, both tests yielding similar results. An increase in the alkyl c hain length or the substitution of a benzyl group for a methyl group reduce s the biodegradation rate. The degradation of these compounds in coastal wa ters was associated with an increase in bacterioplankton density, suggestin g that the degradation takes place because the compound is used as a growth substrate. (C) 2000 Elsevier Science Ltd. All rights reserved.