Reduced toxicity of diuron to the freshwater green alga Pseudokirchneriella subcapitata in the presence of black carbon

Black carbon (BC) is known to act as supersorbent for many organic contaminants. Its presence in surface waters at a level of a few mg/L, which may occur, e.g., after storm events in urban areas, might result in a reduced bioavailability of many contaminants and thus greatly impact their potential toxicity. Photosynthesis-inhibiting phenyl urea derivatives, such as diuron, are widely used as herbicides and diuron is regularly measured in European freshwater systems. In this study, the toxicity of diuron to the freshwater green alga Pseudokirchneriella subcapitata was investigated in the presence of BC in its native and combusted form. As a toxicity endpoint, the in vivo chlorophyll fluorescence was determined and used to indicate the bioavailability of diuron. Fifty milligrams native BC/L reduced effects of 5mugdiuron/L on photosynthesis by 10+/-2%, whereas photosynthesis was completely restored in the presence of the same concentration of combusted BC, suggesting a significantly enhanced adsorption of diuron to the BC fraction compared to the organic carbon fraction. Assuming an environmentally realistic concentration of approximately 1.5mg of combusted BC/L, diuron toxicity would be reduced by approximately 20% in surface waters due to the presence of BC. Higher BC concentrations after storm events might reduce the toxicity even further. A calculation of the Freundlich sorption coefficient K(F,BC,tox) via the toxicity endpoint, resulted in a log K(F,BC,tox) of the combusted BC of 5.7, which is comparable to values obtained by classical sorption experiments. This study contributes to a refined risk assessment of micropollutants in surface waters taking into account the presence of potentially relevant sorbents and, consequently, reduced bioavailability.