Degradation Of Typical Organic Contaminants In Water By Iron/Carbon-based Heterogeneous Catalysts Activated Sodium Persulfate

Persulfate?PS?oxidation method based on sulfate free radical?SO₄^-·?possess several advantages such as high efficiency of treatment,short processing cycle,and a wide range of application has become the preferred method for treatment of refractory organic pollutants.The employment of metallic iron and non-metallic carbon-based heterogeneous PS activation systems can reduce the dissolution of metal ions via the immobilization of active catalysis constituents,and can avoid the secondary pollution as well as the competitive quenching effect against free radicals without the introduction of anions and the organic complexing agent,hence are reliable alternative methods to traditional homogeneous systems such as ferrous ion,chelated ferrous and ferric ion activated PS.Based on this,this article focused on the research and development of high-efficiency Fe/C based heterogeneous catalysts?Fe₃O₄,Fe/SGAC and Fe-MIL53?and its application on PS activation and recalcitrant contaminant degradation.The impact mechanism of synthesis parameters on structural characteristics and catalytic capacities of these three catalysts,the activation and degradation performance,as well as the mechanism of these catalysts toward PS were all investigated.The specific research content,as well as the main conclusion,are listed as follows:?1?In order to solve the problem of the quenching reaction and the secondary pollution that exists in the homogeneous system and increase the removal efficiency of the organic contaminant,nano iron oxide:Fe₃O₄ nanoparticles were synthesized using chemical coprecipitation method and were used to construct the heterogeneous activation system.The effect of the different molar ratio of the iron salt in precursor solutions,the synthesis temperature,and time on the catalytic capacities of the products was studied.The results have shown that the increase of Fe^II/Fe^III molar ratio and synthesis temperature may inhibit the catalytic capacity while prolonging synthesis time in a proper range can enhance the catalytic capacity of Fe₃O₄.The catalytic capacity of Fe₃O₄ performs the best when the product was synthesized using Fe^II/Fe^III=1/2 at the temperature of 60°C for 2 h.DEP degradation rate achieves 90%after reaction for 60 min under optimal conditions.?2?To confirm the feasibility of using Fe₃O₄ as a heterogeneous PS catalyst,the activation performance of Fe₃O₄ towards PS was assessed systematically.The activation performance of Fe₃O₄ towards PS was evaluated systematically,the major active free radical species were identified,and the activated mechanism was explored.The results show the activation system can be most effective under the pH range of 3⁹.And can still be able to be effective when using high initial PS concentration at p H 11.But the surface of nano Fe₃O₄ is easy to be oxidized and can generate corrosion products like?-Fe₂O₃ and?-FeOOH during the activation process,therefore causes the irreversible inactivation of the catalyst.Both SO₄^·-and hydroxyl radical?·OH?are the main active free radical that responsible for the degradation of DEP,but other reactive oxygen species?ROS?such as superoxide radical?O₂^-·?and hydroperoxy radical?HO₂·?also participate in the activation process.?3?Focus on the problem that nano Fe₃O₄ cannot be stably stored in the air,and is easy to be oxidized when activating PS,this paper presents the synthesis of an iron oxide supported granular activated carbon?Fe/SGAC?using impregnation-deposition-reduction-oxidation method.The elementary constituents and oxidation state of Fe/SGAC was analyzed,the influence of different iron or sulfur content on the structure,density of surface functional groups,as well as catalytic capacity of the product were investigated.The results show that the surface of Fe/SGAC contains constituents like Fe₂O₃/FeOOH,Fe₂?SO₄?₃,FeO/Fe₃O₄,and Na₂S₄O₆,which act as new active sites for persulfate and help to improve the catalytic capacity of Fe/SGAC compare with raw GAC and undoped FeGAC.But iron/sulfur loaded or doped content should not be too high.The optimal content of iron and sulfur are 0.5%and 5%,respectively.This activation system can oxidize OG effectively,and the removal of OG was mainly due to the oxidation by free radicals or non-free-radicals reactive species,rather than the adsorption by the catalysts.?4?To clarify the feasibility of using Fe/SGAC as a novel heterogeneous PS catalyst,the activation performance of Fe/SGAC towards PS was estimated systematically.The main active free radical species was identified,and the activation mechanism was also proposed.The results illustrate that this system function well in a wide pH range of 3 to 11.In an optimal condition,OG can be almost totally removed within 180 min.The iron-contained active constituents and basic functional groups on the surface of Fe/SGAC consumed abundantly during the activation process.Meanwhile,the BET and porosity of Fe/SGAC declined,leading to the gradually reduce the catalytic capacity of Fe/SGAC during the reuse experiments.The activation mechanism of Fe/SGAC toward PS include two different pathways:non-radical activation and radical activation.The degradation of OG was not only attributed to the oxidation by SO₄^-·and·OH but also ascribed to the oxidation by non-radical reactive species.?5?Due to the problem that nano Fe₃O₄ is not stable and the worse reusability of Fe/SGAC,an iron-based metal-organic framework:Fe-MIL53 was synthesized using the facile hydrothermal method and used as a novel PS heterogeneous catalyst.The catalytic capacities of different Fe-MIL53 synthesized under different crystallization and vacuum activation conditions were investigated.The reason that causes the discrepancy of catalytic capacity was analyzed.Results indicate that the crystallization and vacuum activation condition determines the BET and relative amount of different oxidation state of Fe coordinatively unsaturated site?CUS?,thus influences the catalytic capacity of Fe-MIL53.Samples that synthesized under150°C for 5 h,with a post-vacuum heating at 170°C for 12 h possess the best structure and reactivity.Fe-MIL53 possess a mix-valent state of Fe CUS.The higher the amount ratio of Fe^II/Fe^III CUS,the stronger the catalytic capacity of Fe-MIL53.?6?To understand the feasibility of using Fe-MIL53 as a novel heterogeneous PS catalyst,the activation performance of Fe-MIL53 towards PS was assessed systematically.The dominant active free radical species were identified,the transmutation mechanism of the Fe CUS valence state,as well as the transformation of free radical species,were elaborated.The results manifested that this system was most applicable in an acidic environment.In optimum condition,OG decolorization rate achieves 98%after reaction for 90 min,and the COD removal rate of the wastewater reaches 74%after reaction for 120 min.This system also has applicability for OG wastewater of higher concentration.The Fe CUS active site,especially Fe^III CUS was largely consumed,the morphology of Fe-MIL53 tended to restructure,and the crystallization structure changed a lot during the activation process.But since Fe^II and Fe^III CUS could transfer to each other continuously within a long reaction period,Fe-MIL53 possess a good cyclic catalytic reactivity.Heterogeneous activation is the major reaction mechanism that involved in the activation process.Homogeneous activation only played an auxiliary role in the activation process.Compared with nano Fe₃O₄ and Fe/SGAC,Fe-MIL53 possess the characteristics of high catalytic efficiency,high stability and can be able to use at least five cycles,thus it is the best persulfate catalyst in this study.