The Mechanism Of Heterogeneous Transition Metal Catalytic Activation Of Persulfate And Its Preliminary Exploration On The Treatment Of Refractory Organic Pollutants

The contamination of organic matters in water bodies,especially these refractory organic matters,has been widely concerned for a long time.Persulfate-based advanced oxidation processes(AOPs)are widely employed for the control of refractory organic pollution in recent years,among which the heterogeneous catalytic activation of persulfate has been most extensively studied with the advantages of low energy consumption,easy operation,simple equipment requirements.In the system of heterogeneous catalytic activation of persulfate,improving the catalytic efficiency of heterogeneous catalysts has always been the research subject.The main purpose of this work is to improve the catalytic efficiency of transition metal catalysts.A series of highly effective catalysts were prepared by modification of heterogeneous transition metal catalysts by means of transition metal oxide composite,non-redox metal oxides introduction,and foreign ions doping.The intrinsic catalytic mechanism in each reaction system was revealed.Moreover,the optimum reaction systems were selected and the oxidation capacity and characteristics of these reaction systems in actual wastewater treatment were compared.The main conclusions are summarized as follows:(1)As for Cu Co@Zn O,the addition of Cu O significantly promoted the activation efficiency of Co O on PMS.Under conditions of 0.2 g L^-1 catalysts,2 m M PMS,and 40ppm 4-CP,Cu Co@Zn O/PMS system demonstrated excellent performance that 96.0%4-CP could be removed.The main reactive oxygen species(ROS)in Cu Co@Zn O/PMS system included HO^·,SO₄^·-,and ¹O₂.The synergism between Cu/Co mainly comes from two aspects:1)The Co/Cu bimetallic oxides compound introduced more Lewis acid sites on the catalyst surface,thus forming more abundant surface hydroxyl groups,which were conducive to the adsorption and activation of PMS;2)The electron transfer between Cu(I)and Co(III)avoided the inefficient consumption of PMS and increased the yield of SO₄^·-,and thus improved the stoichiometric efficiency of PMS.(2)The introduction of non-redox metal oxides M_xO_y(M=Mg,Zn,Ca,Ba,Al)significantly affected the activation efficiency and pathway of transition metal catalyst Cu@Fe₃O₄ for PMS.Compared with Cu@Fe₃O₄,the introduction of Mg O,Ca O,and Zn O significantly improved the catalytic efficiency,the degradation rate of ACE increased from0.004 to 0.029?0.242 min^-1 in various reaction systems.In Cu@Fe₃O₄/PMS system,a classic sulfate radical based oxidation process was observed.After the addition of non-redox metal oxides,the oxidation mechanism in Cu M@Fe₃O₄/PMS system was transformed into a ¹O₂-dominated non-radical oxidation mechanism.The mechanism study showed that the introduction of M_xO_y promoted the generation of oxygen vacancies(O_v)on the surface of the catalyst,which was conducive to the adsorption and activation of PMS.Moreover,the introduction of M_xO_y promoted the generation of Cu³⁺on the catalyst surface and changed the direction of electron transfer between the catalyst and PMS.the differing reducibility of Cu³⁺led to differences in the catalytic properties of Cu M@Fe₃O₄.(3)A Step-by-step oxidation was found in OMS-2/PMS system.In the first stage,OMS-2-mediated electron transfer dominated the oxidation process.In the second stage,¹O₂-dominated reaction process that mainly occurred in solution.Further study found that the physicochemical properties of the catalyst and the concentration of PMS changed significantly during the reaction,causing the adsorption and desorption of O₂^·-on the catalyst surface,which further promoted the generation of the step-by-step oxidation.(4)A series of M-OMS-2 catalysts were synthesized by doping metal ions M(M=Mg²⁺,Ca²⁺,Zn²⁺,Cu²⁺,Fe³⁺)into OMS-2.A step-by-step oxidation phenomenon was observed in all M-OMS-2/PMS system.In the first stage,M-OMS-2-mediated electron transfer dominated the oxidation process.In the second stage,¹O₂-dominated reaction process that mainly occurred in solution.During the reaction,the physicochemical properties of the catalyst and the concentration of PMS changed significantly,resulting in the adsorption and desorption of O₂^·-on the catalyst surface,which further promoted the generation of the step-by-step oxidation mechanism.The beneficial effect of redox-inactive metals doping is related to the enhancement of the conductivity and reducibility of catalysts.In contrast,the adverse effect of redox metals doping is due to the significant decline of the dispersion of Mn.(5)A series of Co₉S₈ catalysts were prepared by adjusting the S doping ratio in Co₃O₄.The experimental results showed that S doping exhibited significant effect on the catalytic efficiency.The degradation efficiencies of ACE(within 40 min)in Co₃O₄/PMS,1C-Co₉S₈/PMS,2C-Co₉S₈/PMS,and 3C-Co₉S₈/PMS systems were 77.3%,42.5%,88.1%,and 92.0%,respectively.¹O₂,OH^·,and SO₄^·-were identified as main ROS.Interestingly,the contribution of ¹O₂ decreased with the increasing S doping ratio,while the contribution of SO₄^·-increased with the increasing S doping ratio.On the one hand,S replaced lattice oxygen,and thus reduced the production of ¹O₂.On the other hand,introduced S^2-can replace PMS to reduce Co³⁺to Co²⁺/Co⁰,and generated Co²⁺could continue to react with PMS to generate SO₄^·-,which enhanced the conversion efficiency between PMS and SO₄^·-.(6)In the treatment of actual wastewater,the reaction system with nonradical oxidation mechanism(¹O₂ oxidation,direct electron transfer)is less affected by complex water quality conditions and exhibited higher mineralization for refractory organic pollutants.In the reaction system with free radical oxidation mechanism,the conversion rate of the organic pollutants to the small molecule intermediates is faster.Based on the above results,this study employed several methods to improve the catalytic efficiency of heterogeneous transition metal catalysts for PMS activation and deeply analyzed the intrinsic mechanism of activation efficiency improvement,which provided a new idea for the development of high efficiency heterogeneous transition metal catalysts.More importantly,our study expanded the prospect of application of heterogeneous transition metal catalyst activated persulfate system in the treatment of refractory organic pollution.