Project Summary (2022-2027)

Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs, also abbreviated as PCDD/F) are among the most challenging environmental contaminants. Over the past 20 years, anaerobic microorganisms such as Dehalococcoides mccartyi and enrichments have been shown to use a number of PCDD/Fs as electron acceptors. Under laboratory conditions, the respiration rates of these organohalide-respiring bacteria (OHRB) yield PCDD/F half-lives of the order of months to a few years. Hence, OHRB, if stimulated, can play a larger role in attenuating PCDD/Fs in situations where more intrusive solutions such as dredging and capping are not practicable because of cost or other factors.

The ability of D. mccartyi to respire PCDD/Fs is closely linked to the high numbers of cobalt-containing proteins (mostly reductive dehalogenases - RdhAs), the genes and proteins associated with the salvaging pathway for cobalamins, and hydrogenases to utilize the electron donor (H2). All three are critical subunits of the respiration complex. Cobalamins are corrinoids (compounds containing four reduced pyrrole rings with cobalt at the center of the ring) with 5,6- dimethylbenzimidazol (5,6-DMBI) as the lower ligand. A Cbl with cyanide as the upper ligand of cobalt is called CN-Cbl or vitamin B12 (B12). All PCDD/F respirers known so far are auxotroph for Cbl. They cannot synthesize Cbl and must salvage it using the Cbl salvage pathway. Yet, Chloroflexi – the phylum all D. mccartyi belong to has the highest number of cobalt-containing genes (mostly rdhs) among all known bacteria and archaea.

The potential for the presence of OHRB at the two Superfund sites - Tittabawassee and Saginaw Rivers, MI (TR/SR), and Passaic River, NJ (PR) is high. The project’s work has already identified a dominant phylotype, PRdAO, at the PR site. Overall, these OHRB combined with amendments including activated carbon (AC), hydrogen release compounds (HRC), and B12 have the potential to reduce the overall toxicity equivalency quotient (TEQ) and health risks associated with PCDD/Fs. Such AC-based approaches are already being implemented for other organohalides such as polychlorinated biphenyls. Integrative mathematical models play a key role in developing optimal solutions for PCDD/F remediation.

Accordingly, this project has the following four specific aims.

• Aim 1: Quantify the parameters for integrative remediation of PCDD/Fs in the presence of exogenous B12, HRC, D. mccartyi, and AC.
• Aim 2: Identify native OHRB and Rdhs-mediated pathways for dechlorination and detoxification of PCDD/Fs at the TR/SR/PR Superfund sites.
• Aim 3: Determine the effect of AC, bioaugmentation, and salinity on native PCDD/F-dechlorinating OHRB, dechlorination pathways, and rates in microcosms.
• Aim 4: Develop and validate site-specific integrative remediation models for PCDD/F involving biotic and abiotic interventions.

This new comprehensive model integrates the collective knowledge into a mathematical tool for predicting reduction in TEQ and health risk in response to interventions. Overall, this project provides novel and powerful approaches, design principles, and site-specific models for economical remediation of PCDD/Fs.