Sulfate Radical-based Oxidation Of Chlorophenols Using Zero-valent Iron/sodium Peroxydisulfate System

Advanced oxidation processes based on the generation of sulfate radicals (SO4-) have been developed in recent years and have been gaining increased attention for wastewater treatment. 4-Chlorophenol (4-CP) and 2,4,6-trichlorophenol (2,4,6-TCP) which possess chemical toxicity are typical chlorophenols and recalcitrant organic pollutants. The innovative approach described in this study provides a very promising and environmentally friendly technology for water treatment. The iron/sodium peroxydisulfate (PDS) systems are involved in Fe(II) and zero-valent iron (ZVI). These processes are based on the generation of sulfate radicals, which are powerful oxidizing species found in nature for the oxidation of 4-CP and 2,4,6-TCP. These studies are focused on the oxidation degradation of 4-CP and 2,4,6-TCP in terms of Fe(II)/PDS, PDS/ZVI amd UV/PDS/ZVI as the homogeneous and heterogeneous system. The main studies are as followed:(1) The Fe(Ⅱ)/PDS for the oxidation of 4-CP was investigated in order to examine the potental for the ferrous ion-activated PDS. The effects of temperature, pH, the initial concentrations of Fe(II), PDS and citric acid on the degradation efficiencies of 4-CP were studied. The results showed that the degradation efficiency of 4-CP was significantly enhanced as temperature increased in the range of 30-70℃. According to the Arrhenius Equation, the activation energy of 4-CP in the heat-activated PDS system was 110.2 kJ/mol. The effect of pH on the degradation efficiency of 4-CP followed the order:acidity> neutrality > basicity. In the PDS/Fe(II) system, ferrous ion played an important role in generating sulfate radicals at ambient temperature. The degradation efficiency of 4-CP increased first, then decreased with increasing concentration of Fe(II). Moreover, the degradation efficiency of 4-CP increased then kept constant with increasing concentration of PDS. The optimum experimental condition is established and the addition of probe compounds proved the formation of sulfate radicals. Furthermore, the iron availability in the aqueous solution was manipulated with the optimum amount of citric acid, as a chelating agent. The degradation efficiency of 4-CP was 50.9%in the PDS/Fe(II)/citric acid system, which was superior to 43.5%at 50℃at the same initial concentration of PDS. This study provided some fundamentals for in-depth investigation in the Iron/PDS system.(2) In order to improve the degradation efficiency of 4-CP and avoid the pollution of additional ions, ZVI-mediated decomposition of PDS that resulted in the generation of sulfate radicals was reported for the oxidation of 4-CP at ambient temperature and near neutral pH. The effects of ZVI loading, pH, the initial concentrations of target compound on the degradation efficiencies of 4-CP and reaction mechanism were investigated. The results showed that ZVI significantly improved the degradation efficiencies of 4-CP at ambient temperature. The degradation efficiency of 4-CP increased first, then decreased with increasing ZVI loading. The optimum loading of ZVI was around 0.20 g/L and 88%removal of 4-CP was observed in 1 h of reaction time. The concentration of Fe(II) in aqueous solution increased when the loading of ZVI increased. The effect of pH on the degradation of 4-CP in the PDS/ZVI system was slight and the solutiong kept at initial pH 6.0. The addition of methanol and tert-butyl alcohol as hydroxyl radical and sulfate radical scavengers proved the presence of sulfate radicals in the PDS/ZVI system. The degradation of 4-CP was accompanied by the formation of hydroquinone,1,4-benzoquinone, and small molecule compounds such as oxalic acid and succinic acid. Chloride ion release and formation of oxidation intermediates were evidence of 4-CP degradation involving sulfate radicals. Therefore, hydroquinone pathway was regarded as the main step in the oxidation of 4-CP.(3.) The photodegradation of two selected chlorophenols,4-CP and 2,4,6-TCP, in aqueous solutions of PDS/ZVI under UV irradiation at near neutral pH was investigated in order to improve the degradation efficiency of contaminants. The performance of UV/PDS/ZVI process was compared with other processes such as UV alone, UV/ZVI, UV/PDS, and PDS/ZVI in terms of 4-CP and 2,4,6-TCP degradation. In the case of 4-CP, the degradation efficiencies followed the order:UV/PDS/ZVI> UV/PDS> UV/ZVI> UV. It was evident that the addition of UV was highly effective in enhancing the degradation of 4-CP. Quantitative data of intermediates such as 1,4-benzoquinone and hydroquinone showed that the formation of intermediates was relatively small in the UV/PDS/ZVI, suggesting that the UV/PDS/ZVI process strongly accelerated the degradation of intermediates. In the case of 2,4,6-TCP, it was found that the photoassisted PDS/ZVI process significantly accelerated the degradation of 2,4,6-TCP in comparison with the dark reaction. The synergy effect of UV and PDS/ZVI process in the degradation of chlorophenols was observed. The effects of PDS concentration on the degradation of 2,4,6-TCP were also examined. Degradation mechanism of organic pollutant in the UV/PDS/ZVI system was proposed. The present study can provide a novel route for wastewater treatment using UV/PDS/ZVI.