Alessandra Leri: XANES Spectroscopy to Illuminate Biogeochemical Cycling of Halogens in Environmental Systems

The halogens chlorine and bromine have high electron affinities and exist in seawater mainly as chloride and bromide anions, which have generally been considered unreactive in the environment. Using Cl and Br K-edge XANES spectroscopy, we have measured high concentrations of organo-chlorine and -bromine in naturally degraded particulate organic matter (POM) from oceanic sediment traps. While organobromine speciation in marine POM is exclusively aromatic, organochlorine is fractionated into aliphatic and aromatic particles that appear in a heterogeneous distribution. The major precursor of sediment trap material is phytoplankton biomass, the detritus of which under-goes oxidative breakdown as part of the marine carbon cycle. We hypothesized that unsaturated lipid and protein moieties in phytoplankton detritus would be susceptible to halogenation through oxidative degradation mechanisms. Using model experiments, we showed that algal particulates are readily chlorinated and brominated through various abiotic pathways, including photochemical and Fenton-like reactions. These processes produce organohalogens in particulate algal detritus at levels exceeding 0.1% by mass. In contrast with the exclusively aromatic organobromine observed in natural marine POM, the lab-based experiments generate aliphatic organobromine in algal particulates; however, the aliphatic organobromine produced appears to be labile and susceptible to debromination on relatively short (3-week) timescales under highly oxidizing conditions. These findings have implications for the transformation and stabilization of marine organic carbon prior to sedimentary burial.
We have also measured high concentrations of non-volatile organochlorine and -bromine in several varieties of edible kelps. Such compounds are likely to contribute to organohalogen body burden in humans and other organisms.

References:

1. A. Leri, M. Dunigan, R. Wenrich, and B. Ravel (2019). Particulate organohalogens in edible brown seaweeds. Food Chemistry 272, 126. https://doi.org/10.1016/j.foodchem.2018.08.050
2. A. Leri, L. Mayer, K. Thornton, P. Northrup, M. Dunigan, K. Ness, and A. Gellis (2015). A marine sink for chlorine in natural organic matter. Nature Geoscience 8, 620. https://www.nature.com/articles/ngeo2481
3. A. Leri, L. Mayer, K. Thornton, and B. Ravel (2014). Bromination of marine particulate organic matter through oxidative mechanisms. Geochimica et Cosmochimica Acta 142, 53. https://doi.org/10.1016/j.gca.2014.08.012
4. A. Leri and B. Ravel (2014). Sample thickness and quantitative concentration measurements in Br K-edge XANES spectroscopy of organic materials. Journal of Synchrotron Radiation 21, 623. https://doi.org/10.1107/S1600577514001283