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

Abstract

The effect of the alkyl chain of quaternary ammonium-based surfactants on their aquatic toxicity and aerobic biodegradability has been studied. Two families of monoalkylquats surfactants were selected: alkyl trimethyl ammonium and alkyl benzyl dimethyl ammonium halides. Acute toxicity tests on Daphnia 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 surfactants. Although the substitution of a benzyl group for a methyl group increases the toxicity, an incremental difference in toxicity between homologs of different chain length were not observed. Biodegradability of the different homologs was determined not only in standard conditions but also in coastal water, both tests yielding similar results. An increase in the alkyl chain length or the substitution of a benzyl group for a methyl group reduces the biodegradation rate. The degradation of these compounds in coastal waters was associated with an increase in bacterioplankton density, suggesting that the degradation takes place because the compound is used as a growth substrate.

Introduction

The environmental acceptability of a compound depends on the Predicted Environmental Concentration:Predicted No Effect Concentration ratio, i.e. it depends on both exposure and effects. Therefore, it requires an understanding of the physico-chemical properties, the environmental fate and behavior, and the bioaccumulation potential of the substance; and also requires an assessment of the toxicity data in aquatic organisms, from laboratory studies and, where available, field observations (Brown, 1996, Cartwright and Lewsi, 1996, Britton, 1998).

Quaternary ammonium-based surfactants (QACs) are molecules with at least one hydrophobic hydrocarbon chain linked to a positively charged nitrogen atom, the other alkyl groups being mostly short-chain substituents such as methyl or benzyl groups. QACs are mainly produced from natural fats and oils, resulting in mixed alkyl chain lengths in most products. Because of the positive charge, cationic surfactants have a strong affinity for negatively charged surfaces, to which their application as a fabric softener or as a disinfectant is attributed. After use, the residual product is discharged to sewage treatment plants or surface waters, and finally to coastal waters.

The most extensively studied of the QACs is the dihydrogenated tallow dimethyl ammonium chloride (DHTDMAC) due to its wide use as a fabric softener until a few years ago. Many reports on its biodegradability (Larson, 1983, Larson and Vashon, 1983, Sullivan, 1983, Vives-Regp et al., 1987) and aquatic toxicity (Lewis and Wee, 1983, Vives-Rego et al., 1986, Roghair et al., 1992, Versteeg and Shorter, 1992) have been published. Furthermore, a review of the environmental impact of this chemical was published by the European Centre for Ecotoxicological and Toxicological Safety Assessment of Chemicals (ECETOC, 1993).

The alkyl chain length not only determines the physical-chemical properties (water solubility, octanol/water partition coefficient, adsorption/partition coefficient on sediments, sludges and soils) of a surfactant (Boethling and Lynth, 1992, Garcı́a et al., 1997), but also may have a decisive role in the fate and effects of these compounds in the environment. In aerobic biodegradation tests, the resistance of quaternary alkyl ammonium salts to biological biodegradation increases strongly with the number of long alkyl chains linked to the nitrogen atom (Van Ginkel and Kolvenbach, 1991). On the other hand, in anaerobic conditions, monoalkylquats resulted recalcitrant under anaerobic conditions and inhibition of methanogenic gas production decreased with increasing the alkyl chain length (Garcı́a et al., 1999).

The purpose of this study was to investigate the effect of the hydrophobic moiety of monoalkyl quaternary ammonium surfactants on their toxicity to aquatic organisms and degradability by bacteria (in both fresh and seawater). To determine the aquatic toxicity of these cationic surfactants not only acute toxicity tests but also the monitoring of their effects on marine bacterioplankton by flow cytometry have been carried out. For this study two series of pure homologs of QACs were tested.