Organobromine chemistry

Distribution and Geochemistry of Organobromine in Terrestrial Soils and Marine Sediments

Organobromine (OBr) compounds, widely recognized as persistent, toxic anthropogenic pollutants, are also produced in the environment by myriad organisms with bromoperoxidative capabilities [1-4]. Although the production of many different classes of OBr compounds has been well documented [5], particularly for marine ecosystems, the fate of the C-Br bond following the decomposition of biological material remains largely unexplored. Using synchrotron-based X-ray absorption spectroscopy (XAS), we are evaluating the scope of OBr distribution in coastal and marine sediments and in terrestrial soils from diverse locations around the globe.

By inducing element-specific core level transitions, XAS provides a means to determine the oxidation state and coordination environment of Br in heterogeneous natural samples without employing extraction procedures that may result in incomplete recoveries or chemical alterations. The Br K-absorption edge and associated spectral features correspond to electronic transitions from the Br-1s core orbitals to vacant atomic and molecular orbitals with Br-4p character (Figure 1).

Figure 1a

Figure 1b

Figure 1: Br K-edge X-ray absorption near-edge fine structure (XANES) spectra of model compounds. a) Sodium bromate. b) 4-bromophenol. c) 1-bromoeicosane. d) KBr (aqueous).



The energy of the Br transition shifts to higher energy with an increase in Br oxidation state. For instance, sodium bromate, with Br in the +V state, exhibits an absorption maximum at 13,478.1, whereas Br(–I) in KBr (aq) absorbs at 13,477.8 eV (Figure 1; a,d). Compared with inorganic Br - compounds, a C-Br bond results in the appearance of lower-energy features around 13,474 eV, corresponding to 1s ® p* or s* molecular orbital transitions (Figure 1; b,c). In addition, Br atoms connected to aliphatic carbon display absorption maxima ~0.6 eV lower in energy than Br connected to aromatic C (Figure 1 inset; b vs. c). Features in the post-edge region of the spectra point to additional aspects of electronic structure. Using this information, the distinction between inorganic Br - and Br atoms bound to aliphatic or aromatic C in natural samples becomes manifest and can be quantified relative to total Br via least-squares fitting with model compounds.

In this manner, we have detected OBr in deep-sea and coastal sediments, terrestrial soils, and weathering plant material. While the concentrations of Br in terrestrial soils are generally low, with an average worldwide value of 0.85 mg/kg, the predominant form of Br in soil organic matter appears to be OBr, as evidenced by the X-ray spectra of mulch collected from the forest floor (Figure 2) and humic substances isolated from soil (Figure 3; c,d).


Figure 2


Figure 2: Br K-edge X-ray absorption near-edge fine structure (XANES) spectra of soil mulch from the forest floor in the Pine Barrens of NJ. Each "layer" roughly represents one year of leaf litter deposition. a) Middle Mulch Layer. b) Bottom Mulch Layer. c) Soil-Mulch Interface. (The top layer, representing the most recent year's deposition, yielded a Br signal insufficient for spectral acquisition.)

Figure 3


Figure 3: Br K-edge X-ray absorption near-edge fine structure (XANES) spectra of isolated humic substances. (HA = humic acid, FA = fulvic acid.) a) Suwannee River FA. b) Suwannee River HA. c) Soil FA. d) Soil HA. e) Lake Fryxell FA. f) Peat HA. g) Peat FA. h) Leonardite HA.


In coastal and marine sediments, with higher background Br concentrations, OBr was detected in the ppm range, and Br speciation was shown to change with depth. In estuarine sediments collected on Cape Cod, MA, for example, OBr accounts for 60% of total Br at the sediment-water interface, 30-40% at 4-20 cm, and 5-20% at 20-25 cm, with inorganic Br -constituting the remainder (Figure 4).


Figure 4


Figure 4: Br K-edge X-ray absorption near-edge fine structure (XANES) spectra of well-mixed estuarine sediment sections from a 25-cm core collected on Cape Cod, MA. a) 0-3.5 cm. b) 4-9 cm. c) 9-16 cm. d) 16-20 cm. e) 20-25 cm.


To shed further light on the geochemistry of Br, we documented its distribution by generating X-ray fluorescence image “maps” of undisturbed sediment sections, examples of which are shown for the Cape Cod sediments (Figure 5A).


Figure 5A-a

Figure 5A-c

Figure 5A-b


Figure 5A: Br Ka fluorescence images of undisturbed estuarine sediment sections from a 25-cm core collected on Cape Cod, MA. Lighter color denotes greater Br intensity. a) 0-1 cm depth (10 x 7 mm2 map). b) 10-13 cm (5 x 13 mm2). c) 23-25 cm (8 x 8 mm2)


Figure 5B


Figure 5B: Br K-edge micro-X-ray absorption near-edge fine structure (m-XANES) spectra of spots identified by X-ray fluorescence imaging.


The distribution of Br becomes increasingly uniform with sediment column depth. At the sediment-water interface (0-1 cm), where dark organic matter predominates among the sand in the sample, Br fluorescence appears most intense in large clusters (Figure 5A; a). X-ray spectra acquired with a microfocused beam at both the clusters and diffuse areas reveal aromatic OBr contributions varying from 55 to 100% (eg. Figure 5; red and blue circles). Fewer high-intensity Br clusters appear 10-13 cm down the sediment column, where the presence of organic material among the sand grains is more sparse than at the interface (Figure 5A; b). At this depth, OBr again dominates the micro-X-ray speciation results (Figure 5; pink and red boxes). At 23-25 cm, the distribution of Br appears more uniform and diffuse (Figure 5A; c), with Br intensity greatest in the spaces between mineral grains, where the speciation of Br is chiefly inorganic (eg. Figure 5; yellow oval). More reducing conditions further down the sediment column could encourage reductive debromination of C-Br moieties by microorganisms, with consequent release of Br - ion into interstitial waters [6,7]. OBr does appear at 23-25 cm at a few localized hot spots (eg. Figure 5; gray oval).

Sediments such as these appear to act as a key locus for the transformation of Br between inorganic and organic forms, functioning as both sources and sinks of OBr. The C-Br bond is omnipresent in the numerous sedimentary and geological environments we have examined, implicating OBr as an important player in natural Br cycling.


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