Chlorophenols Dechlorination Water Treatment Using Ni-Iron Bimetallic Systems: Implications of the Degree of Chlorination, Nickel Coating, and Iron Oxide Phases

Abstract

Chlorophenols (CPs) have been detected as contaminants causing groundwater and potable water pollution. Toxicity of CPs increases with the increase in the degree of chlorination. This study aimed at investigating the efficiency of Nickel coated iron (Ni/Fe) bimetal for CP dechlorination water treatment. The emphasis was on the effect of degree of chlorination, Nickel coating, and iron oxide films on possible removal mechanisms of CPs such as dechlorination and sorption, co-precipitation, physical entrapment with iron oxides (i.e. incorporation). Batch experiments were conducted with Ni/Fe and four CPs [pentachlorophenol (PCP), 2,3,4,6-tetrachlorophenol (2,3,4,6-TeCP), 2,4,6-trichlorophenol (2,4,6-TCP), 2,4-dichlorophenol (2,4-DCP)], single CP at a time, under identical, and anoxic conditions. Reactivity of CPs with Ni/Fe was PCP > 2,3,4,6-TeCP > 2,4,6-TCP ≈ 2,4-DCP with 55, 46, 34, and 30% of the respective compounds initially introduced to each system removed from the solution after 25 days. Dechlorination of PCP and 2,3,4,6-TeCP resulted in accumulation of lower CPs and trace amounts of phenol with 93–96% mass balance. The 2,4,6-TCP and 2,4-DCP concentrations decreased over time, but a noticeable increase in the corresponding dechlorination products was not observed. Furthermore, the reaction of 2,4,6-TCP and 2,4-DCP resulted in the formation of iron oxides (akaganeite, hematite, lepidocrocite, goethite, and wustite) causing Ni/Fe surface passivation. Incorporation with such iron oxides was a significant removal process of 2,4,6-TCP and 2,4-DCP by Ni/Fe. The higher pKa of 2,4,6-TCP (5.97-7.42) and 2,4-DCP (7.68) and passive oxides could lead to 2,4,6-TCP and 2,4-DCP and their degradation products towards a greater affinity for incorporation with the iron oxides. In conclusion, dechlorination potential of Ni/Fe was greater for CPs with a high number of chlorine (PCP, TeCP). The CPs with a low number of chlorine (TCP, DCP), when reacted with Ni/Fe, demonstrated a low preference for dechlorination but a greater affinity for removal by incorporation with the iron oxides.