What do we know about abiotic processes involving organohalogens in seawater?

In seawater, methyl halides (CH₃X, X = Cl, Br, I) undergo S_N2 reactions with the main seawater inorganic nucleophiles, Cl^- and H₂O:

The rate constants for these reactions are temperature-dependent, and lifetimes for CH₃I and CH₃Br have been estimated to vary between a few days in tropical waters and months in polar waters^4,5, whilst reaction half-life for CH₃Cl hydrolysis has been calculated as 2.5 years⁶.

Only a few laboratory studies have been carried out to test the photochemical pathway for VOHs. Moore and Zafiriou³ suggested that CH₃I is produced photochemically in filtered seawater in presence of dissolved organic matter (DOM) according to the following mechanism:

They also suggested that this pathway could be significant in open oceans, an hypothesis supported by measurements carried out by Happel and Wallace⁷ in oceanic areas with low biological productivity.

Measurements of polyhalogenated halocarbons carried out in oceanic waters have often suggested a photochemical source. For example, high surface maxima of CH₂ICl have been observed in correspondence of CH₂I₂ subsurface maxima, suggesting production of CH₂ICl by photochemical reaction of CH₂I₂ with Cl^{- 8,9}. Moore and Tokarczyk⁸ also reported high concentrations of CHBr₂Cl and CHBrCl₂ in deep oceanic waters, possibly due to successive reaction of CHBr₃ with Cl^-.

Future work

As part of the EU-funded THALOZ (Tropospheric Halogens - Effect on Ozone) project, we will investigate in the laboratory the main chemical and photochemical reactions of organohalogens with seawater inorganic nucleophiles and with marine DOM. The rates of organohalogen release from these experiments will be compared with release rates from macroalgae and phytoplankton determined in this laboratory. Production and degradation rates will be incorporated into an existing mathematical model, that will be tested against measured seawater organohalogen concentrations from shipboard cruises.

References

[1] Nightingale, P.D., Malin, G. and Liss, P.S., 1995. Production of chloroform and other low-molecular-weight halocarbons by some species of macroalgae. Limnology and Oceanography, 40: 680-689.
[2] Scarratt, M.G. and Moore, R.M., 1998. Production of methyl bromide and methyl chloride in laboratory cultures of marine phytoplankton II. Marine Chemistry, 59: 311-320.
[3] Moore, R.M. and Zafiriou, O.C., 1994. Photochemical Production of Methyl-Iodide in Seawater. Journal of Geophysical Research-Atmospheres, 99: 16415-16420.
[4] Elliott, S. and Rowland, F.S., 1995. Methyl Halide Hydrolysis Rates in Natural-Waters. Journal of Atmospheric Chemistry, 20: 229-236.
[5] Jeffers, P.M. and Wolfe, N.L., 1996. On the degradation of methyl bromide in sea water. Geophysical Research Letters, 23: 1773-1776.
[6] Zafiriou, O.C., 1975. Reaction of methyl halides with seawater and marine aerosols. Journal of Marine Research, 53: 75-81.
[7] Happell J.D. and Wallace, D.W.R., 1996. Methyl iodide in the Greenland/Norwegian Seas and the tropical Atlantic Ocean: Evidence for photochemical production. Geophysical Research Letters, 23: 2105-2108.
[8] Moore, R.M. and Tokarczyk, R., 1993. Volatile Biogenic Halocarbons in the Northwest Atlantic. Global Biogeochemical Cycles, 7: 195-210.
[9] Yamamoto, H., Yokouchi, Y., Otsuki, A. and Itoh, H., 2001. Depth profiles of volatile halogenated hydrocarbons in seawater in the Bay of Bengal. Chemosphere, 45: 371-377.