Abiotic Dehalogenation of 1,4-Dichlorobenenze with Reactive Iron Mineral Coatings in a Contaminated Sediment Core

Subsurface systems contaminated by dichlorobenzene (DCB) is a significant issue due to its past use as a solvent, a chemical intermediate, and a degreasing agent. As a result of its widespread release into the environment, understanding natural attenuation processes is important. However, with a stable aromatic ring, (a)biotic degradation is a challenge. This study focuses on 1,4-DCB as it is one of the constituents of concerns at a site with historical contamination. An 18.3 m anoxic core was collected at the site with the sediment redox condition preserved. After subsampling the core into a total of 225 5.08 cm thick samples, five redox transition zones (RTZs) were identified through screening analyses, which included sediment Fe and S concentrations, sediment pH, sediment oxidation-reduction potential, total volatile organic compounds in the sediment headspace, and abundant bacteria. Reactive Fe mineral coatings in the sediments from five RTZs have been characterized using field emission scanning electron microscopy with energy dispersive X-ray analysis, X-ray diffraction, and sequential extraction. The RTZs abundant with reduced iron mineral coatings including Fe sulfide minerals were targeted for this investigation: mackinawite (FeS), pyrite (FeS₂), and siderite (FeCO₃). Batch studies conducted in the glovebox revealed pseudo-first-order rate expressions where 90% degradation of 1,4-DCB was achieved between 52 d and 202 d for the three RTZs studied. The zone most rich in the poorly crystalline Fe sulfides revealed the greatest rates of dehalogenation.