Heavy metals trigger distinct molecular transformations in microplastic-versus natural-derived dissolved organic matter
Xianbao Zhong, Kaiying Zhao, Mengyuan Wu, Yaohui Zhang, Chunbo Ma, Hexiang Liu, Bokun Chang, Xiaohui Lian, Yujing Li, Zixuan Huang, Lang Zhu, Ming Zhang, Chi Zhang, Yajun Yang, Jialong Lv
Abstract
Dissolved organic matter (DOM) is a key determinant of heavy metal fate in aquatic environments, influencing their mobility, toxicity, and bioavailability. Derived from natural sources such as soil and vegetation decomposition, natural DOM (N-DOM) typically features humic-like substances with abundant oxygen-containing functional groups that stabilize heavy metals through complexation. However, microplastic-derived DOM (MP-DOM), increasingly prevalent due to plastic degradation, may interact differently with heavy metals, potentially exacerbating environmental risks amid rising plastic pollution. Yet, how heavy metals drive molecular transformations in MP-DOM versus N-DOM remains unclear, hindering accurate pollution assessments. Here, we compare interactions between N-DOM and MP-DOM with cadmium, chromium (Cr), copper, and lead from both fluorescence and molecular perspectives. Our results show that N-DOM, dominated by humic-like substances (46.0-57.3 %), lignin-like (55.0-64.9 %), and tannin-like (10.1-17.6 %) compounds, forms more stable heavy metal complexes via carboxyl, phenolic hydroxyl, and ether groups than MP-DOM. By contrast, MP-DOM-enriched in protein/phenolic-like substances (13.8-24.0 %), condensed aromatic (12.1-28.5 %), and protein/aliphatic-like (8.6-12.4 %) compounds-yields less stable complexes and is highly susceptible to Cr-induced oxidation. Mass-difference network analysis and density functional theory calculations further reveal that both DOM types undergo heavy-metal-triggered decarboxylation and dealkylation, but N-DOM retains complex structures, whereas MP-DOM degrades into smaller, hazardous molecules such as phenol and benzene. This study underscores the potential for heavy metals to exacerbate the ecological risks associated with the transformation of MP-DOM, providing crucial insights to inform global risk assessment and management strategies in contaminated waters where plastic and metal pollution co-occur.