Investigation of the effects of water quality factors in a test medium on algal growth inhibition by hexadecyltrimethylammonium chloride

Abstract

Cationic surfactants such as hexadecyltrimethylammonium salts (HTAC) are known to exhibit high toxicity to aquatic organisms; however, the extent to which water quality conditions influence their apparent toxicity has not been sufficiently clarified. In this study, we investigated the effects of selected water quality factors on algal growth inhibition by HTAC using the freshwater green alga Raphidocelis subcapitata. Algal growth inhibition tests were conducted using standard OECD medium and modified OECD media in which dissolved organic carbon (DOC), pH, hardness (Ca²⁺/Mg²⁺), or sodium concentration was independently adjusted. The results showed that algal growth inhibition by HTAC was significantly reduced under high-DOC conditions achieved by the addition of humic acid, compared with the standard OECD medium. In contrast, no significant effects of pH, water hardness, or sodium concentration on HTAC-induced growth inhibition were detected under the tested conditions. These findings suggest that the algal toxicity of HTAC is highly dependent on DOC concentration in the test medium, likely due to reduced bioavailability through sorption to organic matter. Consideration of DOC is therefore essential for reliable algal toxicity testing of HTAC.