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Methylmercury formation in water triggered by light-independent plastic weathering

Nature Geoscience volume 18, pages 862–868 (2025) Cite this article

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Abstract

Plastic debris has recently been identified as a hotspot for abiotic metal transformations, triggered primarily by photo-weathering under sunlight. Here we perform a set of experiments with freshwater in the field and laboratory to explore metal transformations under dark conditions. We demonstrate that light-independent weathering of plastics leads to methylation of inorganic mercury (Hg(II)) in waterbodies. We propose that methylation occurs via an abiotic pathway involving three chain reaction steps, namely the release of plastic-derived dissolved organic matter (P-DOM), complexation of P-DOM with Hg(II) and intramolecular transfer of methyl groups. P-DOM is released during the light-independent oxidation of plastics via reactive oxygen species. Density functional theory simulations confirm the thermodynamic feasibility of the intramolecular transfer of methyl groups to Hg(II), upon its complexation with oxygen-containing groups in P-DOM. Model estimates demonstrate that polypropylene in freshwater produces methylmercury via this abiotic pathway with Hg(II) methylation potentials from 2.8 × 10−5% per day to 5.5 × 10−2% per day in China and 4.0 × 10−6% per day to 7.5 × 10−3% per day in other regions of the world. Plastic debris is therefore a hidden driver of abiotic methylmercury formation in dark waters. Our study uncovers a pathway through which the ongoing plastic pollution alters mercury cycling, posing a burgeoning threat to planetary health.

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Fig. 1: PP loading to freshwaters increases MeHg concentrations after 42 days.
The alternative text for this image may have been generated using AI.
Fig. 2: Methylation potentials of Hg(II) during plastic weathering.
The alternative text for this image may have been generated using AI.
Fig. 3: The key role of ROS in the light-independent PP weathering.
The alternative text for this image may have been generated using AI.
Fig. 4: P-DOM triggers MeHg formation.
The alternative text for this image may have been generated using AI.
Fig. 5: Chemical features of P-DOM.
The alternative text for this image may have been generated using AI.
Fig. 6: The pathway of plastic-induced abiotic Hg(II) methylation.
The alternative text for this image may have been generated using AI.

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Data availability

All data are available in the Article and its Supplementary Information. Source data are provided with this paper. These data are available via figshare at https://doi.org/10.6084/m9.figshare.29242877 (ref. 50).

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Acknowledgements

This work was supported by the National Natural Science Foundation of China (grant nos 42222702, 42225701 and 42207457), the Natural Science Foundation of Jiangsu Province (grant no. BK20220092) and Jiangsu Funding Program for Excellent Postdoctoral Talent (grant no. 2022ZB464).

Author information

Authors and Affiliations

  1. State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing, China

    Yingnan Huang, Cun Liu, Fei Dang & Yujun Wang

  2. University of Chinese Academy of Sciences, Beijing, China

    Yingnan Huang, Cun Liu, Fei Dang & Yujun Wang

  3. Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, China

    Zhineng Hao

  4. School of Environment, Nanjing University, State Key Laboratory of Pollution Control and Resource Reuse, Nanjing, China

    Xiao Zhang & Huan Zhong

  5. State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, China

    Bo Meng

  6. Stockbridge School of Agriculture, University of Massachusetts, Amherst, MA, USA

    Baoshan Xing

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  1. Yingnan Huang
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Contributions

Y.H., F.D., H.Z. and B.X. conceptualized and designed the study. Experiments were conducted by Y.H. and X.Z. FT-ICR-MS analyses were performed by Z.H. DFT calculations were conducted by C.L. The paper was prepared and revised by Y.H., F.D. and H.Z. with contributions from Y.W., B.M. and B.X.

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Correspondence to Fei Dang or Huan Zhong.

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Nature Geoscience thanks Dana Kühnel and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editors: Xujia Jiang and Carolina Ortiz Guerrero, in collaboration with the Nature Geoscience team.

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Extended data

Extended Data Fig. 1 Minimal impact of microbial methylation or stainless-steel containers on MeHg formation.

(a) Net MeHg production in freshwaters from 15 distinct sites with and without PP after 42 days. The bottom and top of the boxes show the first and third quartiles, respectively, the bar in the middle shows the median, the black solid dot shows the average value, and the whiskers show the minimum and maximum values (n=15 sites). Individual site values are overlaid as circles. Each circle represents geometric mean MeHg production of triplicates for individual freshwater sites. (b) Hg(II) methylation potentials using PP weathered outdoors under darkness for 42 days. (c) Hg(II) methylation potentials with PP subjected to accelerated alkaline weathering in the laboratory. In panels (b) and (c), samples were either unfiltered or filtered through 0.22-μm glass fiber filters before reaction with Hg(II) at 1.7 μM (b) for 6 h at 25 °C and (c) for 4 h at 70 °C. No significant difference between treatments (P = 0.262 and P = 0.288) based on the two-tailed independent-sample t-test. (d) Minimal impact of stainless-steel containers on net MeHg production. Freshwater samples from Shenzhen (SZ) and Hefei (HF) were spiked with PP at 1,200 items L−1. Samples were incubated in darkness for 42 days in three container types: stainless-steel containers, stainless-steel containers with silcosteel coating, and quartz glass containers. No significant difference across different container types with HF (P = 0.749, two-tailed independent-sample t-test) and SZ freshwater (P = 0.404, one-way ANOVA with a Tukey’s post-hoc test). Note that stainless-steel containers with silcosteel coating were not available in sufficient quantities for HF freshwater samples. In panels (b) to (d), the ambient MeHg concentrations in the matrices were subtracted from all the data. The bars show the mean ± s.d. of the incubation replicates (n = 3, also shown as circles). Concentrations below the method detection limit (MDL, 1.0×10−1 pM) were substituted with the MDL value for calculation.

Source data

Extended Data Fig. 2 The Gibbs free energies and activation energies for the reactions between Hg(II) and P-DOM with different functional groups.

(a) The probability of a reaction between sulfur- or nitrogen-containing P-DOM and Hg(II) is minimal, as evidenced by the positive Gibbs free energies. (b) The reaction between oxygen-containing P-DOM and Hg(II) is thermodynamically favorable under both neutral and alkaline conditions.

Source data

Extended Data Fig. 3 The effect of oxygen on MeHg formation.

Methylation potential of Hg(II) is 2-fold greater under an air atmosphere in the presence of dissolved O2 than under a N2 atmosphere. Ambient MeHg concentrations in the matrices were subtracted from all data. The bars show the mean ± s.d. of the incubation replicates (n = 3, also shown as circles). Bars labeled with different letters indicate statistically significant differences (P < 0.001) between treatments according to two-tailed independent-sample t-test.

Source data

Extended Data Fig. 4 The proposed solutions for inhibiting unintentional Hg(II) methylation in plastic vials during sample digestion.

PP 1, PTFE 1-2, or PFA underwent alkaline weathering in the presence of Hg(II) at 1.7 μM and Ag(I) at 200 μM for 4 h at 70 °C. Most unintentional Hg(II) methylation (> 97%) was inhibited. Ambient MeHg concentrations in the matrices were subtracted from all data. The bars show the mean ± s.d. of the incubation replicates (n = 3, also shown as circles). Bars labeled with different letters indicate statistically significant differences between treatments (that is, without Ag(I) vs. with Ag(I)) for each plastic type: P < 0.001 (PP 1), P < 0.001 (PTEFE 1), P < 0.001 (PTFE 2), and P < 0.001 (PFA) (two-tailed independent-sample t-test). PTFE: polytetrafluoroethylene (Teflon); PP: polypropylene; PFA: perfluoroalkoxy, which is a copolymer of tetrafluoroethylene and perfluorinated vinyl ether. The PTFE 1 and PTFE 2 were produced by different manufacturers (Supplementary Table 3).

Source data

Extended Data Table 1 Characteristics of freshwater samples collected across China
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Supplementary information

Supplementary Information (download PDF )

Supplementary Methods 1–4, Texts 1–6, Tables 1–6 and Figs. 1–11.

Source data

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Huang, Y., Liu, C., Hao, Z. et al. Methylmercury formation in water triggered by light-independent plastic weathering. Nat. Geosci. 18, 862–868 (2025). https://doi.org/10.1038/s41561-025-01766-5

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