Reductive transformation of chlorinated dioxins in sediments: An assessment of the contribution of chemical and microbial catalysis.

The estuarine sediments are the ultimate sinks of chlorinated dioxins whose fate is of utmost concern due to toxicity and bioaccumulation issues. Dioxin sources and sinks tend to be dominated by 2,3,7,8-TCDD and the higher chlorinated congeners. Recent evidence suggests that outgassing processes may be responsible for the accumulation of diCDD at the air-water interface. Under reducing conditions prevailing in sediments, dioxins may undergo transformation reactions, including dechlorination. Specific emphasis was placed on assessing the extent and rates of biotic and abiotic dechlorination. Moreover, biotic and abiotic dechlorination signatures were developed to aid in dioxin pattern interpretation. Estuarine Passaic River (NJ) sediment-derived microbial communities were used to evaluate the effect of pore-water chemistry and a brominated priming compound on dechlorination of both heptaCDD isomers. Biotic processes effected dechlorination. yields of up to 31%, resulting in the endpoint of either monoCDD or diCDD. Lateral (2,3,7,8 positions) dechlorination was the predominant pathway, resulting in the minor formation of 2,3,7,8-TCDD. The effects of pore-water chemistry on dechlorination patterns were minimal, despite differences in community structure. However, the priming compound shifted both the structure and function (exhibited dechlorination activity) of the microbial community. The impact of polymeric humic materials or dissolved organic matter (DOM) on reductive dechlorination of chlorinated dioxins was investigated using Aldrich humic acid (AHA) and polymaleic acid (PMA) as model compounds. The dechlorination yields were dependent on the humic structure and bulk reductants. DiCDD constituted the endpoint of dechlorination under all conditions studied, and an overall preference for peri(1,4,6,9)-dechlorination was observed, resulting in higher 2,3,7,8/non-2,3,7,8-TCDD/PCDD ratios than in biotic systems. The associated rates of dechlorination were on the order of pM-nM/d and followed thermodynamic considerations as rates decreased with a decrease in level of chlorination. These results corroborate experimental evidence for potential 2,3,7,8-TCDD fluxes in sediments, which appear to reflect abiotic dechlorination signatures, and lend credence to the potential for dioxin cycling in waterways sediments. Whether sediment-mediated reactivity constitutes a significant natural attenuation process in the overall fate of dioxins has to be considered in light of temporal and spatial analysis of dioxin patterns.PhDApplied SciencesBiological SciencesEnvironmental engineeringMicrobiologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/132571/2/9977158.pd