Environmental Factors Influencing the Formation of Organobromines in Coastal Soils

Abstract

Global warming leads to sea level rise, facilitating saltwater intrusion into freshwater wetlands and bringing salt ions, in particular halides such as bromide, into contact with soil organic matter (SOM). Reactions between bromide and SOM form organobromines such as 1,2-dibromoethane (BrCH2CH2Br). Organobromines may then degrade into volatile halocarbons like bromoform or methyl bromide that not only destroy stratospheric ozone but also contribute to further global warming. Thus, I conducted laboratory simulations of natural bromination reactions to investigate the impacts of environmental factors on organobromine formation in coastal soils. Four main environmental factors were tested: 1) increasing bromide (Br-) concentration in solution; 2) changing concentrations of other ions that may compete with Br- in sorption or in reactions, such as sulfate and bicarbonate; 3) changing pH; and 4) varying oxygen availability. Factors were first tested in isolation (salt solutions), then in combination (seawater samples). I hypothesized that organobromination would be enhanced with increasing Br- concentration and decreasing abundances of competing ions. X-ray fluorescence spectroscopy (XRF), measuring bromine (Br) concentrations incorporated into soil, and X-ray absorption near-edge structure spectroscopy (XANES), assessing Br speciation, were used to test my hypothesis. Results from reactions with salt solutions were mostly consistent with my hypothesis. However, samples reacted with seawater generally showed decreases in organobromine percent composition as salinity increased. My results suggest that soil depth and choice of salt solution or seawater samples are additional important variables affecting organobromination. I propose further experiments working towards more generalizable projections of effects of sea level rise on coastal soils.