Synergistic Degradation Of P-Chlorophenol In The Heterogeneous Iron Oxide/Persulfate/Bisulfite System

In recent years,advanced oxidation technology based on sulfate radical(SO₄^·-)has attracted much attention due to its high oxidation efficiency and a wide working p H range,and how to achieve efficient activation of oxidants such as persulfate（PDS）has become a research hotspot.Transition metal ions or elementary substances are commonly used as activation materials,but the catalytic efficiency of metal oxides tends to be low.Therefore,in this paper,a heterogeneous system of iron oxides/PDS/bisulfite（HSO₃^-）was established,in which the efficient reduction-catalysis-oxidation process between iron oxides,PDS and HSO₃^-could realize the high-efficency activation of PDS and the synergistic degradation of p-chlorophenol（4-CP）,a typical refractory organic pollutant.The dissolved-oxygen-dependency heterogeneous activation mechanism of PDS under the effect of the crystal facet of iron oxides was systematicly investigated.The main results are as follows:（1）The results of the comparative experiments showed that theα-FeOOH/PDS/HSO₃^-system had an obvious synergistic effect on the degradation of4-CP.It was found that SO₄^·-in this system is mainly generated through two pathways:the conversion of SO₃^·-and activation of PDS by Fe²⁺,respectively.XPS,ATR-FTIR analyses and DFT calculation showed that HSO₃^-was adsorbed on the surface of goethite to form the interface≡Fe（III）-S（IV）complexes,which promoted the dissolution of iron ions by complexation or reduction mechanisms,and further initiated the heterogeneous process of PDS.Futhermore,dissolved oxygen played a key role in this activation process of PDS.On the one hand,it accelerated the formation and decomposition of≡Fe（III）-S（IV）complexes and induced the subsequently activation reactions of PDS,on the other hand,it also promoted the transformation of SO₃^·-to SO₄^·-.（2）The catalytic performance of different crystalline iron oxides in the system（Fe_xO_y/PDS/HSO₃^-）and the effects of Fe_xO_y dosage,PDS and HSO₃^-concentration,initial p H and reaction temperature were investigated.The results showed that the catalytic abilities of four iron oxides were in the order of:Fe₃O₄>α-FeOOH>γ-Fe₂O₃>α-Fe₂O₃.The characterization analyses and DFT calculations showed that the ability of the crystal surface to adsorb HSO₃^-was the key factor that determined the catalytic activation of PDS by iron oxide.The catalytic ability of magnetite hexahedron{110}crystal face was obviously stronger than that of octahedron（311）and（511）crystal face,while the catalytic activity of hematite nanoplate{001}crystal face was weaker than that of nanocube（110）and（012）plane.Moreover,the activation process of PDS by{001}-exposed hematite only occurred on the water-solid interface,while the magnetite exposed with the（311）and（511）planes had very weak ability to adsorb and complex HSO₃^-at the interface and could mainly activate PDS homogeneously by the leached ferrous ions.（3）The effect of common inorganic anions on the degradation of 4-CP in the Fe₃O₄/PDS/HSO₃^-system was investigated,and it was found that F^-,SO₄^2-and Cl^-could obviously inhibit the degradation of 4-CP,while NO₃^-and HCO₃^-showed slight negligible effects.The continuous flow column reactor based on Fe₃O₄/PDS/HSO₃^-system was established to treat simulated 4-CP wastewater under two different water backgrounds.The results showed that PDS and HSO₃^-in the influent could undergo catalytic oxidation reactions at the Fe₃O₄ interface and in the solution to achieve the rapid generation of SO₄^·-and the efficient degradation of 4-CP.However,the degradation rate of 4-CP gradually decayed after 24 hours of operation.Furthermore,we investigated the long-term treatment 4-CP in the continuous flow column reactor under the desulfurization wastewater background,and it was found that the removal ratio of 4-CP was significantly inhibited（～40%）,but the degradation ratio of 4-CP could still remain stable at about 20%after 213.5 hours.The results indicated that the Fe₃O₄/PDS/HSO₃^-system had a certain extent of feasibility in high-salt wastewater.