Elucidating the dechlorination mechanism of hexachloroethane by Pd-doped zerovalent iron microparticles in dissolved lactic acid polymers using chromatography and indirect monitoring of iron corrosion.

The degradation mechanism of the pollutant hexachloroethane (HCA) by a suspension of Pd-doped zerovalent iron microparticles (Pd-mZVI) in dissolved lactic acid polymers and oligomers (referred to as PLA) was investigated using gas chromatography and the indirect monitoring of iron corrosion by continuous measurements of pH, oxidation-reduction potential (ORP), and conductivity. The first experiments took place in the absence of HCA, to understand the evolution of the Pd-mZVI/PLA/HO system. This showed that the evolution of pH, ORP, and conductivity is related to changes in solution chemistry due to iron corrosion and that the system is initially cathodically controlled by H mass transport to Pd surfaces because of the presence of an extensive PLA layer. We then investigated the effects of Pd-mZVI particles, temperature, initial HCA concentration, and PLA content on the Pd-mZVI/PLA/HCA/HO system, to obtain a better understanding of the degradation mechanism. In all cases, HCA dechlorination first requires the production of atomic hydrogen H-involving the accumulation of tetrachloroethylene (PCE) as an intermediate-before its subsequent reduction to non-chlorinated C and C compounds. The ratio between Pd-mZVI dosage, initial HCA concentration, and PLA content affects the rate of H generation as well as the rate-determining step of the process. A pseudo-first-order equation can be applied when Pd-mZVI dosage is much higher than the theoretical stoichiometry (600 mg for [HCA] = 5-20 mg L). Our results indicate that the HCA degradation mechanism includes mass transfer, sorption, surface reaction with H, and desorption of the product.