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Photolytic degradation of methylmercury enhanced by binding to natural organic ligands

Abstract

Methylmercury is a neurotoxin that accumulates in food webs and poses a significant risk to human health1. In natural water bodies, methylmercury concentrations remain low due to the degradation of methylmercury into inorganic mercury by sunlight, a process known as photodecomposition. Rates of photodecomposition are relatively rapid in freshwater lakes2,3,4, and slow in marine waters5, but the cause of this difference is not clear. Here, we carry out incubation experiments with artificial freshwater and seawater samples to examine the mechanisms regulating methylmercury photodecomposition. We show that singlet oxygen—a highly reactive form of dissolved oxygen generated by sunlight falling on dissolved organic matter—drives photodecomposition. However, in our experiments the rate of methylmercury degradation depends on the type of methylmercury-binding ligand present in the water. Relatively fast degradation rates (similar to observations in freshwater lakes) were detected when methylmercury species were bound to sulphur-containing ligands such as glutathione and mercaptoacetate. In contrast, methylmercury–chloride complexes, which are the dominant form of methylmercury in marine systems, did not degrade as easily. Our results could help to explain why methylmercury photodecomposition rates are relatively rapid in freshwater lakes and slow in marine waters.

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Figure 1: Concentration dependence of MeHg photodegradation rates.
Figure 2: MeHg degradation through generation of 1O2 from photosensitized humic acid.
Figure 3: Effect of ligand complexation on degradation of MeHg.

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References

  1. Mergler, D. et al. Methylmercury exposure and health effects in humans: A worldwide concern. Ambio 36, 3–11 (2007).

    Article  Google Scholar 

  2. Lehnherr, I. & St Louis, V. L. Importance of ultraviolet radiation in the photodemethylation of methylmercury in freshwater ecosystems. Environ. Sci. Technol. 43, 5692–5998 (2009).

    Article  Google Scholar 

  3. Sellers, P., Kelly, C. A., Rudd, J. W. M. & MacHutchon, A. R. Photodegradation of methylmercury in lakes. Nature 380, 694–697 (1996).

    Article  Google Scholar 

  4. Hammerschmidt, C. R. & Fitzgerald, W. F. Photodecomposition of methylmercury in an Arctic Alaskan lake. Environ. Sci. Technol. 40, 1212–1216 (2006).

    Article  Google Scholar 

  5. Whalin, L., Kim, E-H. & Mason, R. Factors influencing the oxidation, reduction, methylation and demethylation of mercury species in coastal waters. Mar. Chem. 107, 278–294 (2007).

    Article  Google Scholar 

  6. Marvin-Dipasquale, M. C. & Oremland, R. S. Bacterial methylmercury degradation in Florida Everglades peat sediment. Environ. Sci. Technol. 32, 2556–2563 (1998).

    Article  Google Scholar 

  7. Oremland, R. S., Culbertson, C. W. & Winfrey, M. R. Methylmercury decomposition in sediments and bacterial cultures: Involvement of methanogens and sulfate reducers in oxidative demethylation. Appl. Environ. Microbiol. 57, 130–137 (1991).

    Google Scholar 

  8. Sellers, P., Kelly, C. A. & Rudd, J. W. M. Fluxes of methylmercury to the water column of a drainage lake: The relative importance of internal and external sources. Limnol. Oceanogr. 46, 623–631 (2001).

    Article  Google Scholar 

  9. Hoigne, J. & Bader, H. in Organometals and Organo-Metalloids; Occurrence and Fate in the Environment (eds Brinckman, F. E. & Bellama, J. M.) 292–313 (ACS Symposium Series, Vol. 82, American Chemical Society, 1978).

    Google Scholar 

  10. Chen, J., Pehkonen, S. O. & Lin, C-J. Degradation of monomethylmercury chloride by hydroxyl radicals in simulated natural waters. Water Res. 37, 2496–2504 (2003).

    Article  Google Scholar 

  11. Hoigne, J. Comment on ‘Degradation of monomethylmercury chloride by hydroxyl radicals in simulated natural waters’. Water Res. 38, 3470–3471 (2004).

    Article  Google Scholar 

  12. Zepp, R. G., Hoigne, J. & Bader, H. Nitrate-induced photooxidation of trace organic-chemicals in water. Environ. Sci. Technol. 21, 443–450 (1987).

    Article  Google Scholar 

  13. Southworth, B. A. & Voelker, B. M. Hydroxyl radical production via the photo-Fenton in the presence of fulvic acid. Environ. Sci. Technol. 37, 1130–1136 (2003).

    Article  Google Scholar 

  14. Vermilyea, A. W. & Voelker, B. M. Photo-Fenton reaction at near neutral pH. Environ. Sci. Technol. 43, 6927–6933 (2009).

    Article  Google Scholar 

  15. Suda, I., Suda, M. & Hirayama, K. Degradation of methyl and ethyl mercury by singlet oxygen generated from sea water exposed to sunlight or ultraviolet light. Arch. Toxicol. 67, 365–368 (1993).

    Article  Google Scholar 

  16. Grandbois, M., Latch, D. E. & McNeill, K. Microheterogeneous concentrations of singlet oxygen in natural organic matter isolate solutions. Environ. Sci. Technol. 42, 9184–9190 (2008).

    Article  Google Scholar 

  17. Latch, D. E. & McNeill, K. Microheterogeneity of singlet oxygen distributions in irradiated humic acid solutions. Science 311, 1743–1747 (2006).

    Article  Google Scholar 

  18. Waples, J. S., Nagy, K. L., Aiken, G. R. & Ryan, J. N. Dissolution of cinnabar (HgS) in the presence of natural organic matter. Geochim. Cosmochim. Acta 69, 1575–1588 (2005).

    Article  Google Scholar 

  19. Brezonik, P. L. Chemical Kinetics and Process Dynamics in Aquatic Systems (CRC Press, 1993).

    Google Scholar 

  20. Schmidt, R. & Afshari, E. Collisional deactivation of O-2(1-Delta-G) by solvent molecules—comparative experiments with O-16(2) and O-18(2). Ber. Bunsenges. Phys. Chem. Chem. Phys. 96, 788–794 (1992).

    Article  Google Scholar 

  21. Boreen, A. L., Edhlund, B. L., Cotner, J. B. & McNeill, K. Indirect photodegradation of dissolved free amino acids: The contribution of singlet oxygen and the differential reactivity of DOM from various sources. Environ. Sci. Technol. 42, 5492–5498 (2008).

    Article  Google Scholar 

  22. Zepp, R. G., Schlotzhauer, P. F. & Sink, R. M. Photosensitized transformations involving electronic-energy transfer in natural waters: Role of humic substances. Environ. Sci. Technol. 19, 74–81 (1985).

    Article  Google Scholar 

  23. Hintelmann, H., Keppel-Jones, K. & Evans, R. D. Constants of mercury methylation and demethylation rates in sediments and comparison of tracer and ambient mercury availability. Environ. Toxicol. Chem. 19, 2204–2211 (2000).

    Article  Google Scholar 

  24. Amirbahman, A., Reid, A. L., Haines, T. A., Kahl, J. S. & Arnold, C. Association of methylmercury with dissolved humic acids. Environ. Sci. Technol. 36, 690–695 (2002).

    Article  Google Scholar 

  25. Hsu-Kim, H. Stability of metal-glutathione complexes during oxidation by hydrogen peroxide and Cu(II)-catalysis. Environ. Sci. Technol. 41, 2338–2342 (2007).

    Article  Google Scholar 

  26. Abedinzadeh, Z., Gardes-Albert, M. & Ferradini, C. Kinetic study of the oxidation mechanism of glutathione by hydrogen peroxide in neutral aqueous medium. Can. J. Chem. 67, 1247–1255 (1989).

    Article  Google Scholar 

  27. Ni, B., Kramer, J. R., Bell, R. A. & Werstiuk, N. H. Protonolysis of the Hg–C bond of chloromethylmercury and dimethylmercury. A DFT and QTAIM study. J. Phys. Chem. A 110, 9451–9458 (2006).

    Article  Google Scholar 

  28. Tossell, J. A. Theoretical study of photodecomposition of methyl Hg complexes. J. Phys. Chem. A 102, 3587–3591 (1998).

    Article  Google Scholar 

  29. Hintelmann, H., Welbourn, P. M. & Evans, R. D. Measurement of complexation of methylmercury(II) compounds by freshwater humic substances using equilibrium dialysis. Environ. Sci. Technol. 31, 489–495 (1997).

    Article  Google Scholar 

  30. Karlsson, T. & Skyllberg, U. Bonding of p.p.b. levels of methyl mercury to reduced sulfur groups in soil organic matter. Environ. Sci. Technol. 37, 4912–4918 (2003).

    Article  Google Scholar 

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Acknowledgements

We thank K. Linden for assistance with ultraviolet photolysis experiments and K. McNeill and B. M. Voelker for helpful discussions regarding this study. This work was supported by Duke’s Pratt School of Engineering and Duke’s Center for Comparative Biology of Vulnerable Populations funded by the National Institute of Environmental Health Science.

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T.Z. carried out all experiments and data analysis. H.H-K. conceived the study, supervised the research and carried out speciation calculations. Both authors drafted the manuscript.

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Correspondence to Heileen Hsu-Kim.

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The authors declare no competing financial interests.

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Zhang, T., Hsu-Kim, H. Photolytic degradation of methylmercury enhanced by binding to natural organic ligands. Nature Geosci 3, 473–476 (2010). https://doi.org/10.1038/ngeo892

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