Adsorption kinetics of different mercury species on three kinds of micro-/nano-plastics in micro-polluted aquatic environments and their combined toxicity.

The contamination of micro-plastics (MPs)/nano-plastics (NPs) in environment poses a global concern, necessitating a scientific evaluation of their potential risks to ecosystems and organisms. We herein investigated the adsorption kinetics of different mercury species including inorganic mercury (Hg2+), methylmercury (MeHg) and ethylmercury (EtHg) on three different MPs, such as polyethylene MPs (PE MPs), polypropylene MPs (PP MPs) and polystyrene MPs (PS MPs) in micro-polluted water in detail, and further evaluated the individual and combined cytotoxicity of polystyrene NPs (PS NPs)/polypropylene NPs (PP NPs) and different mercury species. The results indicated that EtHg undergoes demethylation partly to form Hg2+ during the adsorption process on PP MPs, and the adsorption kinetics of Hg2+, MeHg and EtHg on three MPs fitted with pseudo-first-order (PFO) model in initial stage and simultaneously fitted with pseudo-second-order (PSO) model during whole adsorption process. This suggested that adsorption of mercury species on three MPs we driven mainly by physical process in initial stage and by chemical process during whole absorption process. In natural micro-polluted water, the equilibrium adsorption capacities (Qe) of Hg2+ on three MPs (156.3-270.3 ng/g) are much higher than that of MeHg (5.562-78.13 ng/g) and EtHg (7.831-70.42 ng/g). Cytotoxicity experiments revealed that PP NPs and PS NPs themselves have little cytotoxicity, but the presence of them can enhance the cytotoxicity of mercury species, showing NPs size-depended and mercury species-depended synergistic toxic effect. The findings of this study provided valuable insights for scientifically evaluating the potential risk of MPs/NPs to ecosystems and organisms.