Humic Acid and Fulvic Acid Hinder Long-Term Weathering of Microplastics in Lake Water

We investigated the photoaging of polypropylene (PP) microplastics (MPs) in lake water. The results showed that photoaging of PP MPs was significantly inhibited in lake water compared with ultrapure water after 12 d of ultraviolet (UV) irradiation, and humic acid and fulvic acid, rather than carbonate (CO32-), nitrate (NO3-), or chloride (Cl-) ions, were identified as the primary contributors to the observed inhibition. Mechanisms for the roles of humic acid (Suwannee River humic acid) and fulvic acid (Pony Lake fulvic acid) in reducing the rates of photodegradation showed that humic acid and fulvic acid acted as both reactive oxygen species (ROS) scavengers (e.g., of (OH)-O-center dot) (dominant contribution) and optical light filters. As ROS scavengers, humic acid and fulvic acid significantly decreased the capacity for the formation of (OH)-O-center dot and O-2(center dot-) by PP MPs under irradiation. In addition, the chromophores in humic acid and fulvic acid competed for photons with MPs through the light-shielding effect, thereby causing less fragmentation of PP particles and changes in other properties (melting temperature, contact angle, and surface zeta potential). The proposed mechanisms for inhibition by humic acid and fulvic acid will aid our efforts to assess the duration of aging and alterations of MP properties during long-term weathering in natural waters.

Automated summary

The photoaging of polypropylene microplastics in lake water was found to be significantly inhibited by humic acid and fulvic acid, acting as scavengers of reactive oxygen species and optical light filters. This inhibition mechanism reduces the formation of harmful oxidants and competes with the microplastics for light, resulting in less fragmentation and changes in properties.