Influence of natural organic matter on ecotoxicity, ingestion and depuration of MXenes (Ti3C2Tx) in D. magna and R. subcapitata

The two-dimensional inorganic advanced materials MXenes (M_n+1X_nT_x) have been gaining traction due to their unique characteristics making them chemically and mechanically stable with diverse applications for, e.g., energy storage, water purification, and photocatalysis. Although the properties and potential applications of MXenes have been widely researched, a need for more information regarding the potential ecotoxicological effects of the materials remains due to their possible widespread use. The purpose of this study was to assess the short-term toxicity of Ti₃C₂T_x-MXenes towards the freshwater algae (Raphidocelis subcapitata) and the crustacean (Daphnia magna), as well as their bioaccumulation behaviour in ingestion and depuration studies with D. magna. The ecotoxicity of MXenes was investigated in different test setups, including a 24 h aging experiment of MXenes following exposure to the water accommodated fraction with and without the presence of the chemical pentachlorophenol (PCP) and natural organic matter (NOM). Generally, it was found that MXenes (aged and not aged) did not affect the mobility of D. magna in the concentration range tested (highest test concentration: 100 mg/L) while algal growth was affected (EC₅₀-values of 81 mg/L and 41 mg/L, with and without aging, respectively). Furthermore, the results indicated that the presence of MXenes did not affect the toxicity of PCP towards algae. Contrastingly, daphnia tests of PCP solutions with MXenes present showed lower toxicity than solutions without MXenes, suggesting a decrease in bioavailability of PCP in the presence of MXenes. Addition of NOM to algal tests resulted in decreased toxicity of MXenes. Additionally, the ingestion and depuration tests with D. magna indicated that the presence of NOM lowered the organisms ingestion of MXenes. In the presence of a food source (algae) D. magna depurated the residual MXenes faster than in experiments where no food was present.