Flow microcalorimetry investigation of the influence of surfactants on a heterogeneous aerobic culture

Abstract

The influence of various surfactants on the biological activity of a mixed aerobic culture has been investigated by using flow microcalorimetry. The response of the culture to the addition of homologous n-alkylcarboxylates (C2 to C16) and n-alkylpyridinium bromides (Cll to C14) has been examined under endogenous and substrate saturation conditions, and inhibitory concentrations (MIC or the concentration which decreased the initial activity (heat flux) of the culture by 50%) were determined for each state. Under both conditions, the n-alkylpyridinium bromides were found to be more toxic than the n-alkylcarboxylates of identical chain length, thus confirming that the head group of the amphiphiles plays an important role in the microbial toxicity of surfactants. The relationship observed between the concentration at which 50% of the activity is lost and the chain length of the surfactant further confirms that cellular toxicity is also dependent on surfactant hydrophobicity. In relation to the biodegradability of surfactants in mixed aerobic cultures, the low concentration effects of n-alkylcarboxylates on endogenous culture were investigated in some detail. There appear to be compounded indications that these surfactants are rapidly metabolized by the microorganisms of the mixed culture, at least for homologs lower than Cl0. Approximately 3 x 101 tons of surfactants, of which 65% are anionic, 28% are nonionic, and 7% are cationic, are produced annually in the United States (34). In many of their uses, surface-active agents are likely to be released into the environment and particularly into wastewater streams and treatment systems. It has been estimated that the average concentration of anionic surfactants in sewage in the United * Corresponding author. zations from absolute ethanol. The white precipitates were then dried at 115°C for 12 h. The purity of these salts was found to be between 98 and 99% by titration with standardized HCl. (ii) n-Alkylpyridinium bromide salts. n-Alkylpyridinium bromide salts were prepared by a procedure described by Knight and Shaw (21). Mixtures of n-bromoalkanes and pyridine in a molar ratio 1:1.1 were heated for 3 h at 150°C. The waxlike solids were recrystallized once in ether-ethanol (5:1) and three times in acetone (UV grade) and finally dried under vacuum for 48 h at 105°C. The purity of these salts, as determined by bromide titrations, was found to be between 98 and 99.5%. Culture. The heterogeneous aerobic culture used in this study was grown continuously in a 6-liter fermentor (working volume, 4 liters) maintained at 25°C and under an aeration rate of ca. 2.5 liters min-1. The composition of the sterile growth medium was identical to that used by Beaubien et al. (4), consisting of the following (in grams liter-'): beef extract, 22.0; peptone, 20.0; tryptic soy broth, 20.0; urea, 7.26; NaCl, 1.70; CaC12, 0.730; and MgSO4. 7H20, 0.480. The mixture was steam sterilized at 121°C for 20 min and stored at 4°C until used. Medium and tap water were continuously pumped into the ferment...