Exposure Order to Photoaging and Humic Acids Significantly Modifies the Aggregation and Transformation of Nanoplastics in Aqueous Solutions

Abstract

The colloidal stability of nanoplastics in aqueous solutions is greatly regulated by photoaging and dissolved organic matter (DOM). However, how the exposure order to sunlight and DOM modifies the environmental behavior of nanoplastics is seldomly determined. Here, with two different exposure orders, we investigated the impact of molecular-weight (MW)-fractionated humic acids (HAs) derived from biochar and the Suwannee River, respectively, on the aggregation of poly(ethylene terephthalate) nanoplastics (PET-NPs) in mono-and divalent electrolyte solutions. For exposure pattern (i) (photoaging followed by HA coating), photoaged PET-NPs had more oxidized surfaces and exhibited 22−320% higher binding affinity to HAs (especially the higher MW fractions) than the pristine counterparts, which greatly improved the dispersion of PET-NPs. For exposure pattern (ii) (HA coating followed by photoaging), HA-PET assemblies were formed, the dispersion of which increased with increasing irradiation time and was significantly higher than that of the samples in the exposure pattern (i) at the end of the experiment. This high dispersion of photoaged HA-PET assemblies was ascribed to the extra oxidation of PET by reactive oxygen species generated in the PET-HA interfaces during photoaging. These findings highlight the "active nature" of HA-PET assemblies, which provide new insight into the reaction of HA with nanoplastics beyond adsorption in the natural environment.

Multifaceted Aquatic Environmental Differences between Nanoplastics and Microplastics: Behavior and Fate

“…The addition of Suwannee River humic acid (SRHA) under simulated seawater showed that the critical coagulation concentration (CCC) value of PET-NP was 98 mmol/L for Na + at pH = 5.8, and the CCC value of PET-NP decreased after the addition of different molecular weights of SRHA in NaCl solution, implying that the stability of NPs decreased and became vulnerable to aggregation. The smaller molecular weight HA resulted in the greatest decrease in the CCC of PET-NP, from 98 mmol/L to 53 mmol/L, while the larger molecular weight HA introduced a higher steric hindrance along with a decrease in the ζ potential of PET-NP, which partially counteracted the aggregation effect …”

“…The smaller molecular weight HA resulted in the greatest decrease in the CCC of PET-NP, from 98 mmol/L to 53 mmol/L, while the larger molecular weight HA introduced a higher steric hindrance along with a decrease in the ζ potential of PET-NP, which partially counteracted the aggregation effect. 70 The formation of eco-corona also leads to changes in the ecotoxicity of NPs and their capacity to carry pollutants, which can alter the "Trojan horse effect" of NPs. 66 The "Trojan horse effect" refers to NPs acting as carriers for other pollutants and transporting them.…”