Effect of Degree of Chlorination on Chlorophenols Dechlorination Water Treatment with Zero Valent Iron

Abstract

Chlorophenols (CPs) are known for causing water pollution. Zero-Valent Iron (ZVI) can be used to treat water via CP dechlorination. Incorporation (sorption, co-precipitation and/or physical entrapment) of CPs with iron oxides is a concurrent process occurring with dechlorination, which can be influenced by the physical-chemical properties of CPs and leads to concerns on CPs dechlorination with ZVI. This study aimed at investigating the effect of degree of chlorination on CPs incorporation and dechlorination when using ZVI. Pentachlorophenol (PCP), 2,3,4,6-Tetrachlorophenol (2,3,4,6-TeCP), 2,4,6-Trichlorophenol (2,4,6-TCP) and 2,4-Dichlorophenol (2,4-DCP) were chosen to study dechlorination and incorporation of CPs using ZVI. Experiments were carried out under identical settings in batch reactors under anoxic conditions. CP dechlorination was assessed using Gas chromatography-Mass spectrometry to quantify the respective CPs and all dechlorination products formed. Raman spectroscopy was used to identify the iron oxides formed on the ZVI. CPs dechlorination increased with increasing degree of chlorination except for with 2,4-DCP. The sequence of CPs dechlorination was PCP>2,3,4,6-TeCP>2,4-DCP>2,4,6-TCP with accumulation of lower CPs over 25 days reaction. CPs with lower degree of chlorination showed greater tendency for incorporation with the oxides. 2,4,6-TCP and 2,4-DCP concentrations in the solutions decreased over time but there was no increase in dechlorination products resulting in poor mass balance. Higher pKa values of 2,4,6-TCP (5.97-7.42) and 2,4-DCP (7.68) could lead to greater affinity of these CPs and/or their degradation products for incorporation with the oxides hindering dechlorination. The formation of oxides (akaganeite, hematite, lepidocrocite, goethite, wustite) observed with 2,4,6-TCP and 2,4-DCP could also limit the dechlorination and lead to greater incorporation of higher amount of these CPs with the oxides. This study demonstrated that CP dechlorination using ZVI increases with the increase in degree of chlorination of CPs. Further, the CPs tested showed increasing affinity for incorporation with the oxides and decreased dechlorination as the degree of chlorination decreases.