Effects And Mechanism Of Dissolved Organic Matter And Photoaging On The Suspension And Bioavailability Of Nanoplastics

Since the concept of“Microplastics（MPs）”was proposed in Science in 2004,microplastics have received extensive attention as a new type of pollutant.A large number of studies have shown that microplastics will be further decomposed into nanoplastics（Nanoplastics,NPs）with smaller particle size and more active properties through biological or abiotic effects.The size effect of NPs may give them unique environmental chemical behavior and bioavailability.NPs entering the environment,their physicochemical properties and environmental behavior are susceptible to environmental components（e.g.,minerals,organic matter）and biotic or abiotic aging.However,there are few reports on the regulation mechanism of different types of organic matter and photochemical aging on the suspension properties and bioavailability of NPs.Based on this,this experiment used polyethylene terephthalate（PET）as raw material to prepare NPs,and took PET-NPs as the research object to develop different types of dissolved organic matter（DOM）and its different molecular weight fractionated（M_f）on the physicochemical properties,suspension properties,and bioavailability of NPs under photoaging conditions.Through the classical theory of colloid chemistry（Derjaguin-Landau-Verwey-Overbeek,DLVO）model analysis,the regulatory effects of DOM and photoaging on the suspension and migration of NPs in the environment were revealed.Toxicity,influencing factors and related mechanisms of aquatic organisms including invertebrates.Finally,using rice seedlings as the test plants,the exposure experiments of NPs were carried out,and the inhibitory effects and internal mechanisms of photoaging and DOM on the bioavailability of NPs were explored.The major results in this study are shown as follows:（1）The aggregation of PET-NPs in the environment was strongly affected by the electrolyte,and the Critical Coagulation Concentration（CCC）of NaCl and CaCl₂ solutions were 98 mM and 0.65 mM,respectively.After the addition of Suwannee River Humic acid（SRHA）and Biochar HA（BCHA）,the CCC of PET-NPs in NaCl and CaCl₂ solutions became smaller and followed the classical DLVO theory,and the destabilization effects of different electrolytes on PET-NPs conformed to the Schulze-Hardy rule.The aggregation of PET-NPs promoted by M_f-SRHA and M_f-BCHA may be caused by van der Waals interaction;the electric double layer of PET-NPs was compressed,and electrostatic charge shielding reduced the effective repulsion between PET-NPs,resulting in its stability decrease.At the same time,the bridged effect of Ca²⁺with M_f-SRHA and M_f-BCHA was also an important reason for the instability of PET-NPs,which further promoted the aggregation of PET-NPs.（2）Due to the increase of surface oxidation functional groups of PET-NPs after photoaging,the electrostatic repulsion between particles increased,resulting in enhanced stability.The CCC of PET-NPs in NaCl and CaCl₂ solutions after aging were 145 mM and 5.4 mM,respectively.After the addition of M_f-SRHA and M_f-BCHA,the CCC of aged PET-NPs in NaCl and CaCl₂solutions after aging became larger,and the Zeta potential did not change significantly.The results of adsorption experiments showed that the adsorption capacity of M_f-SRHA and M_f-BCHA by aged PET-NPs increased significantly.This indicated that the larger steric hindrance caused by the thickening of the adsorption layer was the main mechanism to inhibit the aggregation of aged PET-NPs;Fourier transform ion cyclotron resonance-Mass Spectrometry analysis（FT-ICR MS）showed that the macromolecular BCHA contained more condensed aromatic hydrocarbons and unsaturated hydrocarbons,and had strong hydrogen bonding and Lewis acid-base interactions with the aged PET-NPs,which enhanced the stability of aged PET-NPs.The aggregates of aged PET-NPs with M_f-SRHA or M_f-BCHA remained highly stable in solutions reaching the NaCl solubility limit,while the CCC increased more in CaCl₂ solutions,which further inhibited the aggregation of PET-NPs.（3）Meta-analysis results showed that NPs significantly reduced the survival,behavior and reproduction of fish and aquatic invertebrates by 56.1%,24.2%and 36.0%,respectively.NPs can disrupt the storage and distribution of energy in fish and aquatic invertebrates,and oxidative stress and oxidative damage in vivo were increased by 72.0%and 9.6%,respectively;while the antioxidant prevention system and neurotransmission were significantly reduced by 24.4%and15.9%,respectively.Through heterogeneity analysis,it was found that the toxicity of NPs to fish and aquatic invertebrates strongly depended on NPs size,concentration and functional group,and different biological indicators show different sensitivities.Compared with rice seedlings exposed to NPs alone,rice seedlings exposed to M_f-BCHA or M_f-SRHA in combination with NPs had better plant growth,longer root length,larger root surface area,and greater root freshness.Confocal laser microscopy showed that NPs could be absorbed by rice at seedling stage and adhered tightly to the root surface,while M_f-SRHA and M_f-BCHA both inhibited the absorption of NPs by rice.In particular,the large aggregates formed by aged NPs with M_f-BCHA with larger adsorption affinity further weakened the uptake of NPs by rice.This study will provide data support or theoretical reference for the physicochemical properties and suspension characteristics of NPs under the influence of DOM and photoaging conditions.At the same time,this research work also helps people to more accurately understand the potential toxicity mechanism of NPs to aquatic organisms,and provides a theoretical basis for the environmental risk assessment and management of NPs.