Metal sorption onto nanoscale plastic debris and trojan horse effects in Daphnia magna: Role of dissolved organic matter

There is a debate on whether the Trojan horse principle is occurring for nanoscale plastic debris (NPD < 1 µm). It is realized that NPD have a high capacity to sorb environmental contaminants such as metals from the surrounding environment compared to their microplastic counterparts, which influences the sorbed contaminants' uptake. Herein, we studied the influence of dissolved organic matter (DOM) on the time-resolved sorption of ionic silver (Ag⁺) onto polymeric nanomaterials, as models of NPD, as a function of particle size (300 and 600 nm) and chemical composition [polystyrene (PS) and polyethylene (PE)]. Subsequently, the toxicity of NPD and their co-occurring (adsorbed and absorbed) Ag⁺ on Daphnia magna was determined. Silver nitrate was mixed with 1.2 × 10⁵ NPD particles/mL for 6 days. The extent of Ag⁺ sorption onto NPD after 6 days was as follows: 600 nm PS-NPD > 300 nm PS-NPD > 300 nm PE-NPD. The presence of DOM in the system increased the sorption of Ag⁺ onto 300 nm PS-NPD and PE-NPD, whereas DOM decreased the sorption onto 600 nm PS-NPD. Exposure to 1 mg/L NPD or 1 µg/L Ag⁺ was not toxic to daphnids. However, the mixture of these concentrations of PS-NPD and Ag⁺ induced toxicity for both sizes (300 and 600 nm). The addition of DOM (1, 10 and 50 mg/L) to the system inhibited the combined toxicity of Ag⁺ and NPD regardless of the size and chemical composition. Taken together, in natural conditions where the concentration of DOM is high e.g. in freshwater ecosystems, the sorption of metals onto NPD depends on the size and chemical composition of the NPD. Nevertheless, under realistic field conditions where the concentration of DOM is high, the uptake of contaminants in D. magna that is influenced by the Trojan horse principles could be negligible.