Heavy metals trigger distinct molecular transformations in microplastic-versus natural-derived dissolved organic matter

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.

Keywords: Complexation; Dissolved organic matter; Heavy metal; Microplastic; Molecular transformation.

Graphical abstract

Fig. 1

a, Scanning electron microscopy (SEM) image and elemental mapping of carbon (C) and oxygen (O) elements. The red dashed circle indicates the local area of the microplastic selected for magnified SEM imaging. b, Fourier transform infrared (FTIR) spectra. The vertical dashed lines represent different infrared wavelengths. c, Variation in dissolved organic carbon (DOC) of microplastic-dissolved organic matter (MP-DOM) during the ultraviolet aging of polypropylene (PP), polyethylene (PE), and polyvinyl chloride (PVC) microplastics. Here, aged PP-DOM (from polypropylene), aged PE-DOM (from polyethylene), and aged PVC-DOM (from polyvinyl chloride) represent the dissolved organic matter extracted from ultraviolet-aged microplastics.

Fig. 2

Spectral and molecular characterization of dissolved organic matter (DOM). a, Fourier transform infrared (FTIR) spectra; b, Ultraviolet–visible absorption spectra, with the inset showing a magnified view at 254 nm for the six types of DOMs; c, Synchronous fluorescence spectra; d, Excitation–emission matrix spectrograms, where the letters A–F denote the characteristic fluorescence peaks of the six types of DOMs; e, Van Krevelen diagrams of the compounds and the percentages of different molecular classes in the DOMs. The six types of DOMs include SY-DOM (from river water), CJ-DOM (from sediment), TR-DOM (from soil), PP-DOM (from polypropylene), PE-DOM (from polyethylene), and PVC-DOM (from polyvinyl chloride).

Fig. 3

Fluorescence quenching curves of dissolved organic matter (DOM) components titrated with cadmium (Cd), chromium (Cr), copper (Cu), and lead (Pb). Components 1–4 correspond to the four fluorescent constituents of natural dissolved organic matter, namely SY-DOM, CJ-DOM, and TR-DOM. Components 5–7 represent the three fluorescent constituents of microplastic-derived dissolved organic matter, including PP-DOM, PE-DOM, and PVC-DOM.

Fig. 4

Two-dimensional synchronous fluorescence correlation spectroscopy: a, synchronous spectra; b, asynchronous spectra.

Fig. 5

Two-dimensional Fourier-transform infrared correlation spectroscopy: a, synchronous spectra; b, asynchronous spectra.

Fig. 6

Removal (red), retention (blue), and production (grey) of compounds in van Krevelen diagrams for the six types of DOMs before and after quenching by Cd, Cr, Cu, and Pb. The six types of DOMs include SY-DOM, CJ-DOM, TR-DOM, PP-DOM, PE-DOM, and PVC-DOM.

Fig. 7

Kendrick mass defect of COO (KMD(COO)) versus carbon number and their scaled extended segments in the formula plots for the SY-DOM (a) and PP-DOM (b) compounds before and after quenching by Cd, Cr, Cu, and Pb.

Fig. 8

a–f, Radar plots of possible types of reactions between precursor-products reacted with Cd, Cr, Cu, and Pb for SY-DOM (a), CJ-DOM (b), TR-DOM (c), PP-DOM (d), PE-DOM (e), and PVC-DOM (f). g–j, The reactions of PE-DOM with Cd (g), Cr (h), Cu (i), and Pb (j), computational analysis of possible transformation pathway networks.

Fig. 9

The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), electrostatic potential distribution, and possible reaction pathways of typical compounds in SY-DOM and CJ-DOM (a), TR-DOM (b), PVC-DOM (c), as well as PP-DOM and PE-DOM (d) during the reaction with heavy metals.