Debromination and sorption of polybrominated diphenyl ethers by nanoscale zero-valent iron and its composites [electronic resource]

Polybrominated diphenyl ethers (PBDEs) are widely used as flame-retardants and are receiving increasing attention as persistent organic pollutants (POPs) because of their ubiquitous presence and persistence in the environment, bioaccumulation and toxicity properties. Nanoscale zerovalent iron (nZVI) is a strong reducing agent for an array of organic contaminants. This research focused on PBDE reaction kinetics, pathways and mechanisms with several nZVI-relevant remediation materials that may safely mitigate PBDEs. One objective of this research was to synthesize or develop nZVI remediation materials, including nZVI, palladized bimetallic nanoparticles (nZVI/Pd) and nZVI/Pd impregnated activated carbon (nZVI/Pd-AC). A second objective of this research was aimed at providing a more in-depth understanding and evaluation of reaction kinetics, pathways and mechanisms of PBDEs by those materials. To realize these goals, this research characterized the materials synthesized, analyzed the effectiveness of each material on PBDE debromination, and evaluated the effects of catalyst and particle properties, as well as the reaction preferences and mechanisms involved. nZVI was synthesized and characterized to comprehensively assess the degradation rates, preference and mechanisms for reaction with PBDEs. nZVI debrominated the selected PBDEs into lower brominated compounds and diphenyl ether, a completely debrominated form of PBDEs. The effectiveness of nZVI towards debromination increased with increasing bromine substitutes in PBDEs. To assess degradation pathway, the reaction of 2,3,4-tribromo diphenyl ether (BDE 21) was investigated more thoroughly, and a susceptibility of the meta-bromine by nZVI was observed. The stepwise debromination from n-bromo- to (n-1)-bromodiphenyl ether was observed as the dominant reaction process, although simultaneous multistep debromination was likely for di-BDEs that have two bromines adjacent to each other on the same phenyl ring. The heat of formation (Hf) and the energy of the lowest unoccupied molecular orbital (ELUMO) are useful descriptors of relative reaction rates among PBDE homologue groups. A good correlation between PBDE activity and respective ELUMO indicated that the main debromination mechanism by ZVI is direct electron transfer. The effect of particle properties and the catalyst were studied with two commercially available nZVI slurries, N25 and N25S with an organic stabilizer, and palladium (Pd). A main factor contributing to the decrease of laboratory-synthesized nZVI activity was likely the result of the drying and stabilization processes. The organic stabilizer polyacrylic acid on the commercial nZVI slowed the PBDE reduction, probably due to its hindrance effect for sorption and surface reaction. Palladization of nZVI promoted reaction kinetics with an optimum Pd loading at 0.3 Pd/Fe wt%, and changed the reaction preference to para-bromines, resulting in PBDEs with less estrogenic potencies. A wide range of PBDEs that are environmental-abundant were debrominated to DE in one week by N25 and nZVI/Pd, where nZVI/Pd reacts more completely and effectively. Step-wise major PBDE debromination pathways by unamended and palladized Fe0 were compared. In addition to galvanic couple formation between Pd and iron, it was found that a greater role of H atom transfer is induced by Pd. Moreover, steric hindrance and rapid sequential debromination of adjacent bromines play an important role in the pathways for palladized nZVI, indicating the importance of surface precursor complex formation. nZVI/Pd-AC particles were synthesized and characterized to evaluate their effectiveness in PBDE debromination. Difficulty with in-situ synthesis of a significant fraction of zero-valent iron within the micro-porous material was demonstrated. X-ray fluorescence mapping of nZVI/Pd-AC showed that Pd mainly deposits on the outer part of particles, and Fe was present throughout the activated carbon particles. While BDE 21 was sorbed onto activated carbon composites quickly, debromination was slower compared to reaction with freely dispersed nZVI/Pd. According to the distribution of reaction intermediates, BDE 21 reacted on both iron and palladium surfaces. Results demonstrated that activated carbon introduces a retarding effect on the reaction, which is caused by the heterogenous distribution of nZVI and Pd on AC and/or immobilizing of hydrophobic organic contaminants at the sorption sites. Overall, the results of this research suggest that nZVI and nZVI/Pd-assisted debromination is a feasible PBDE remediation material that can fully debrominate PBDEs. Increasing the surface activity of nZVI is essential to debrominate PBDEs effectively. Palladium promotes debromination kinetics and reduces the toxicity of by-product during debromination. Activated carbon (AC) can effectively reduce PBDE concentrations in the liquid phase by strong sorption, though it retards reaction by reducing the availability of PBDEs to nZVI particles impregnated in AC.

Advisor: Luthy Richard G. primary advisor. Thesis advisor
Fendorf Scott advisor. Thesis advisor
Reinhard Martin