Environmental processes and toxicity of metallic nanoparticles in aquatic systems as affected by natural organic matter

The environmental processes of metallic nanoparticles (MNPs) have been widely investigated in the literature where the importance of natural organic matter (NOM) was often highlighted. This review thoroughly summarized the emerging efforts in understanding interactions between MNPs and NOM. The main environmental processes influenced by NOM are adsorption, stabilization/aggregation, dissolution and surface transformation. Adsorption, competition and desorption irreversibility of NOM on the MNP surface were discussed with regard to NOM properties. For soluble MNPs, their dissolution was generally promoted after interacting with NOM although dissolution suppression was also reported. Besides dissolution, two main surface transformation processes including oxidation and sulfidation can occur on the MNP surface in the presence of NOM. The relationship between NOM-induced dispersion and other processes such as adsorption, dissolution and surface transformation was discussed. Due to the alteration of the above environmental processes, NOM can have an impact on the toxicity of MNPs on aquatic organisms. Therefore, the toxicity of MNPs in the presence of different types of NOM was addressed. It is concluded that NOM can alter MNP toxicity through changing suspension stabilization of MNPs, bioavailability of metal ions dissolved from MNPs, electrostatic interactions and steric repulsion between MNPs and organisms, and MNP-induced reactive oxygen species generation. However, the impact of MNP transformation induced by NOM on toxicity needs further investigations. Moreover, mesocosm studies and mathematical models are suggested to examine the environmental processes of MNPs in the presence of NOM in large-scale aquatic systems. Several key research challenges are provided at the end of this review.