Pentachlorophenol Dechlorination Using Zero Valent Iron and Nickel-Iron Bimetallics

Abstract

Zero Valent Iron (ZVI) systems are used to treat chlorinated phenols (CPs) contaminated waters. In these systems CPs removal occurs by different pathways with a complex dependence on many variables. This research studied pentachlorophenol (PCP) removal by ZVI and Ni/Fe bimetals to decipher some of the complex dependencies and resolve contradictions reported in the literature. The influence of key variables was studied; including the mineralogy of the iron oxides on the ZVI surface, methods of preparing Ni/Fe bimetallics, dissolved oxygen (DO), and the presence of nitrate. PCP removal was also compared with that of lower CPs including tetra-, tri- and di-chlorinated phenols. Batch and column experiments were performed. Gas chromatography was used to analyse the solution phase CPs and Raman spectroscopy was used to identify the oxides on the ZVI surface. At least two different theories have been promoted about the fundamental CP removal processes in ZVI based systems; CP dechlorination and CP incorporation with the oxides which includes sorption, co-precipitation and/or physical entrapment. This thesis provides insights that both incorporation and dechlorination can be dominant depending on the system characteristics. For example, CPs with higher degree of chlorination tended to exhibit greater dechlorination while CPs with lower degree of chlorination showed greater incorporation with the oxides. Nitrate competed with PCP for electrons, led to the formation of lepidocrocite, hematite, goethite within the first 24 hours of reaction and significantly decreased the PCP dechlorination using ZVI and bimetallic system of ZVI mixed with Ni (Nis/Fe). When nitrate was present, PCP removal occurred mainly due to incorporation with the oxides and only partial PCP dechlorination transpired. Further, the DO competed with PCP for electrons and led to the formation of maghemite, hematite, lepidocrocite, which resulted in significantly lower PCP dechlorination by ZVI or Ni coated ZVI (Nic/Fe) bimetallic system. In preparing the Nic/Fe, immersing ZVI that had coatings of wustite/magnetite or only magnetite in a NiSO4/H2SO4 solution resulted in Ni0 depositing, as discrete Ni particles or a thin film, respectively. The deposited Ni0 particles led to enhanced PCP dechlorination, while, the thin film led to decreased PCP dechlorination compared to the unmodified ZVI. The Nis/Fe bimetal, prepared with ZVI consisted of only magnetite, resulted in a significant increase in PCP dechlorination compared to the Nic/Fe and ZVI systems. The Nis/Fe bimetal achieved complete dechlorination of PCP and 2,3,4,6-tetrachlorophenol as well as their lower CPs daughter products with phenol and/or cyclohexanol being the final product(s).