Palladium/magnesium bimetallic systems for dechlorination of polychlorinated biphenyls

Polychlorinated biphenyls (PCBs), a family of 209 congeners, are recalcitrant environment pollutants and suspected carcinogens prevalent in nation's surface waters, sediments and superfund sites. The threat of PCBs, current state and critical aspects of in-situ reclamation technologies that warrant research focus has been discussed. Toward this end, an effective system has been devised for PCB dechlorination by modifying magnesium (Mg) through the deposition of very small amounts of palladium (Pd). Every Pd/Mg bimetallic particle functioned as numerous nanoscale batteries generating electrons that eventually led to PCB dechlorination. Through our study, we demonstrate Mg to be a promising substrate in Pd-based bimetallic systems for PCB dechlorination. It is the first instance of relating the unique corrosion properties of Mg to distinctive advantages it offers in synthesis, storage and application of bimetallic particles in environmental systems, aspects which have been onerous in Fe-based systems. Also, Mg at a high oxidation potential of 2.37V provides a greater thermodynamic force for dechlorination. These advantages of Mg, coupled with its natural abundance, low density and cost and environmentally friendly nature further strengthen its candidature. The Pd/Mg systems were enhanced using novel techniques of nano-synthesis to obtain tailor-made bimetallic particles with small Pd islands and maximized reduction potential at drastically reduced Pd requirements.;A better understanding of the unexplored Pd/Mg systems was sought by studying their behavior in presence of naturally abundant anions such as sulfate, chloride, nitrate, hydroxide and carbonate and organic solvents that may accompany PCB contamination. While there have been studies on some of these parameters influencing corrosion of pure Mg, literature on Mg galvanically coupled with Pd remains limited. Insights on effect of initial PCB concentration and system pH on dechlorination rates were also provided.;For PCBs undergoing catalytic hydrodechlorination (HDCl), conflicting accounts occur on the reactivity of substituted chlorines and the ensuing dechlorination pathways. In order to understand these relationships, intermediates and dechlorination pathways of carefully selected 17 congeners were investigated. The preferential site of electrophilic attack and its mechanistic aspects were interpreted in terms of steric, inductive and resonance stabilization. The trends for electrophilic substitution were consistently p- > m- > o-positions indicating that more toxic 'coplanar' PCBs were preferentially reduced. Dechlorination rates and pathways were influenced both by inductive effect of Cl that governs the stability of the intermediates and by steric effects primarily effecting the adsorption step. In the absence of steric effects, dechlorination occurred preferentially from the less substituted phenyl ring.;Finally, we evaluated the ability of Pd/Mg to dechlorinate (i) PCBs spiked in clean clays and sediments and (ii) historically contaminated aged sediments (Waukegan Harbor, WHS). While (i) Pd/Mg completely dechlorinated multi-component commercial PCB mixtures, and (ii) active components in extraction media did not inhibit dechlorination, sulfide in WHS poisoned Pd, compromising Pd/Mg reactivity. We discuss major factors limiting Pd/Mg performance in sediments, proposing possible solutions. Recommendations to improve Mg based systems were made focusing on their nanotechnological aspects including synthesis techniques, challenges in nanoscaling Mg and potential applications in broader environmental spheres.