Electrochemically mediated reduction of epichlorohydrin pollutant by Ni(I)(hexamethylcyclam) in aqueous media

Abstract

Epichlorohydrin (ECH) is a volatile organic compound and toxic to human health. Here we investigated clean mediated electrocatalytic dechlorination of ECH in aqueous solutions using electrogenerated [Nickel(I)(hexamethylcyclam)]+ ([Ni(I)(hmc)]+) at glassy carbon cathode. Ni(II)/Ni(I) redox behavior of [Ni(II)(hmc)]2+, examined first by cyclic voltammetry in ECH free-aqueous solutions of various anions (Cl−, Br−, ClO4−, SO42−, phosphate, NO3−), showed only Cl− could provide most stable and reactive low-valent nickel species with highest versatility toward activation of ECH dechlorination. Cyclic voltammetry experiments over a range of scan rates and ECH concentrations elucidated rate limiting mechanism for Ni(I) catalyzed ECH dechlorination, and estimated catalytic reaction rate constant (kcat). ECH concentration-dependent kcat and icat results revealed ECH dechlorination by Ni(I) followed Michaelis-Menten type inner-sphere electron transfer kinetics at low ECH concentrations (< 20mM), but outer-sphere mechanism at higher concentrations. Interestingly, GC/MS showed complete destruction of ECH and production of three non-chloro products within 3-h bulk electrolysis. A possible mechanism was proposed for the overall Ni(I)(hmc)-catalyzed ECH dechlorination. Fast mediation kinetics, total removal of ECH, non-chloro organics as the only dechlorination products, and highly stable [Ni(I)(hmc)]+ mediator indicate electrocatalytic mediation using [Ni(I)(hmc)]+ in aqueous NaCl solution may be feasible for dechlorination of other organic chloro compounds.