A limitation of the microtox® test for toxicity measurements of nonionic surfactants

Abstract

Abstract-The Microtox" test was successfully used to measure EC50 values of nonionic polyethoxylate surfactants. However, toxicity measurements of real samples that contain surfactants above a particular concentration, termed the critical toxicity concentration (CTC) are not valid. These samples require dilution before the test is performed, and because the relationship between toxicity and concentration is not linear above the CTC, the EC50 cannot be extrapolated back to give the toxicity of the original concentrated sample and a true estimation of toxicity is therefore not possible. This phenomenon may be related to the minimum surface tension requirement of the bacteria or other physical properties of the surfactant such as the tendency to assemble at interfaces and surfaces and the tendency to form micelles.

Toxicity of nonionic surfactants

“…CTC values were consistently lower than the CMC values for all surfactants although they are of the same magnitude. This was also observed by Sherrard et al [2]. These authors indicated that an association between surface tension and toxicity of the surfactant existed and they concluded that the non-micellar molecules produce a concentration-related toxic response.…”

“…The concentration at this point is termed as the Critical Toxicity Concentration (CTC) and the proportionality between log Γ and concentration no longer holds beyond this point. The curve plateaus beyond this point [2].…”

“…Because of the widespread use of non-ionic surfactants, their discharge into the environment is of concern [2]. As such biodegradability and toxicity of these surfactants are continuously being assessed.…”

“…Toxicity has been measured by using small crustaceans such as Daphnia magna, Ceriodaphnia dubia, fathead minnows (Pimephales promelas), and marine luminescent bacteria [3]. Several different commercial systems have been used to determine acute toxicity, such as Microtox ® [2], Toxalert ® [3] and Spirotox [4]. Algal tests are also common [5].…”

Bioremediation of a Soil Industrially Contaminated by Wood Preservatives—Degradation of Polycyclic Aromatic Hydrocarbons and Monitoring of Coupled Arsenic Translocation

“…It is also possible that the surfactant itself has contributed to the increased toxicity, something that has been observed in an earlier study where the nonionic surfactant sodium dodecyl sulfate gave a toxic response in concentrations much lower than those utilized in the present study (Mazidji et al 1992). It is likely that the toxicity is a result of the very low surface tension, which implies damage to cell functions of the microorganisms (Sherrard et al 1996). In the present study, the concentration of surfactant was well above the CMC, and consequently, the surface tension was very low and could possibly explain the differences in Microtox® data observed between treatments receiving surfactant and those who did not.…”