Halogen dynamics in environmental systems: An x-ray spectroscopic study

Abstract

The paradigmatic view of halogens as unreactive species in the environment has been overturned by recent studies revealing natural production of organochlorine (Cl_org) and organobromine (Br_org) compounds, which were previously believed to appear in the environment due almost exclusively to anthropogenic pollution. Characterization of natural organohalogen molecules and elucidation of natural halogenation mechanisms have proved challenging, largely due to the chemical heterogeneity of natural organic matter. This dissertation reports robust analyses of Cl_org and Br_org fluxes in soil and sediment systems, shedding light on the natural halogenation processes occurring as part of the biogeochemical halogen cycles.

High Cl_org levels detected in the soil organic horizon have given rise to the hypothesis that C-Cl bonds form naturally during oxidative decay of plant material. To probe Cl dynamics in natural systems, quantitative analytical methods were developed based on synchrotron X-ray absorption spectroscopy (XAS). Total Cl_org and inorganic chloride (Cl_inorg) concentrations were quantified in plant litter as a function of decay time and degree of exposure to the soil microbial community. These in situ measurements reveal that (Cl_inorg) and soluble aromatic Cl_org leach from plant material during initial weathering stages, leaving refractory aliphatic Cl_org in the solid phase that persists through further degradation. Contact with soil microorganisms produces additional, stable aromatic Cl _org in leaf tissue. Spectromicroscopic analyses of leaf litter complement the bulk quantitative results. These findings shed light on relative contributions of biotic and abiotic processes to natural chlorination of phyto-organic molecules, providing new insight into Cl transformations in soil systems and implicating Cl_org as a constituent in a complex biogeochemical Cl cycle.

The geochemistry of Br in different sedimentary and geological environments was also examined through XAS-based techniques. Br was found to exist in a highly heterogeneous distribution, with Br_org appearing ubiquitously in samples from diverse geographical locations. In coastal and marine sediments, Br_org is strongly associated with organic matter as well as with metal cations such as Fe, Ca, and Zn. The chemical state of Br varies with sediment column depth, supporting a general hypothesis of biogeochemical Br cycling with possible implications for C storage.