Bromination of Coastal Organic Matter in the Presence of Iron

Abstract

As sea levels rise in coastal wetland areas, seawater reacts with natural organic matter (NOM) in previously freshwater wetlands to form organobromine compounds, which are toxic and environmentally damaging. A possible catalyst of bromination reactions is Fenton’s Reagent, formed from Fe ions and hydrogen peroxide (H2O2). To study these reaction mechanisms, NOM was reacted with various salt solutions, H2O2 and different forms of Fe, including aqueous Fe ions and Fe clay minerals. The extent of bromide conversion to organobromine was analyzed using a combination of x-ray spectroscopy techniques: XRF, XANES, and micro x-ray spectromicroscopy mapping. As confirmed by all three methods, the total amount of organobromine in oxic samples decreased when Fe2+or Fe3+ions were present. This suggests that aqueous Fe2+/Fe3+ions are not catalysts of natural bromination, but rather hinder the catalytic effects of H2O2 and any other catalysts already present in the NOM. However, when mineral forms of iron were tested, magnetite (Fe3O4) showed an ability to increase bromination of NOM. Ferrihydrite showed no such ability. These results suggest that while aqueous Fe(II) ions might decrease bromine incorporation, Fe(II) in a mineral form can catalyze the incorporation of Br salts into decomposing plant matter in natural systems.