Effective Activation Of Peroxymonosulfate By Cobalt-carbon Based Catalysts For Degradation Of Chlorophenols Organic Pollutants

Chlorophenolic organic pollutants(CPs)have anti-chemical/biodegradability,environmental mobility,long-distance migration,and long-lasting properties,which may cause cancer,teratogenicity,mutagenicity and other potential hazards to the human body.The development of effective methods of governance,enabling them to be converted into less harmful intermediates or even fully mineralized into CO2,H22O has become the main research goal.At present,the treatment methods for CPs mainly include adsorption,biodegradation and chemical oxidation method.Among them,the advanced oxidation technology based on sulfate radical(SO4·-)(SR-AOPs)in the chemical oxidation method has the advantages of strong oxidizing ability,high selectivity and wide application pH range,and become an emerging new effective treatment of CPs technology.So4·-can be generated by the activation of peroxymonosulfate(PMS)or persulfate(PS).Compared with the activation methods such as heat activation,alkali activation and UV activation,the transition metal ions Mn+(Fe2+,Co2+,Mn2+,Cu+ etc.)act as a homogeneous catalyst to activate PMS/PS has been widely used because it does not require external conditions,high efficiency,and mild reaction conditions.Among them,Co2+/PMS is the best match which has high catalytic efficiency.However,it is necessary to develop high-efficiency heterogeneous catalysts because of the disadvantages in the practical application of difficult recovery,difficult separation,and potential health hazards caused by metal ion residuesRecently,carbonaceous materials such as activated carbon,carbon nanotubes,graphene oxide,porous carbon,etc.have a wide range of applications in SR-AOPs due to the metal-free nature,large surface area,adjustable electronic and physicochemical characteristics and excellent chemical stability.They can activate PMS/PS through catalytic sites such as zigzag edge defects,surface oxygen-containing functional groups such as ketone groups and sp2 hybrid carbon.The introduction of heteroatom doping into the carbon matrix to produce defects to adjust the physicochemical properties of the material is a method of modifying the carbon material.Nitrogen modification destroys the electronic layout of sp2-hybrid carbon,which breaks the chemical inertness of graphene,induces positive charge of adjacent sp2 carbon atoms,enhances electron transfer between carbon and PMS,and improves the catalytic efficiency of carbon nanomaterials.However,in view of its still limited catalytic efficiency,combinating with transition metal cobalt and forming cobalt-carbon composites as a heterogeneous catalyst for PMS/PS activation has great research prospects.Cobalt-carbon based catalysts can combine the advantages of cobalt and carbon materials in SR-AOPs.The carbon material has a large surface area,high adsorption capacity and a large number of active groups on the surface,ensuring stable fixation and excellent dispersion of the cobalt active material,and also significantly promoting the accumulation of contaminants on the catalyst surface and approaching the active oxidant.Cobalt active species dispersed on carbon materials can avoidaggregation during the catalytic process and keep high activity.Strong covalent bond interactions between cobalt and carbon support such as Co-N-C or Co-O-C can also reduce the loss of cobalt ions.In addition,the special electron transport effect between the cobalt and carbon plays an important role in PMS/PS activation.The synergistic coupling between cobalt and carbon support can significantly improve the ability of cobalt-carbon based catalysts to activate PMS/PS,which makes cobalt-carbon based catalysts the focus of research in SR-AOPs.Based on the above research background and current situation,we prepared three kinds of cobalt-carbon composites with different structures as heterogeneous catalysts to remove chlorophenol organic pollutants(monochlorophenol and 2,4-dichlorophenol)by activating PMS.Free radicals or non-radical active species were in the degradation process.The interaction between the catalyst and chlorophenols was analyzed experimentally and theoretically,and the effect of this interaction on the degradation rate of the pollutants was studied.The active sites of the prepared cobalt-carbon-based catalysts were analyzed by comparing the changes of catalysts before and after catalysis such as element valence states,and their catalytic mechanisms were obtained.The main contents of the paper are summarized as follows:1.Cobalt doped g-C3N4 activation of peroxymonosulfate for monochlorophenols degrdationThe intrinsic electron-rich holes of graphite-phase carbon nitride(g-C3N4)provide a suitable attachment site for the positive-valent transition metal ions,which provides a highly dispersed metal-regulated active site for heterogeneous catalytic applications This study investigated cobalt-doped g-C3N4(CCN)as a high-efficiency catalyst to activate PMS to degrade three kinds of monosubstituted chlorophenolic contaminants(MCP),including ortho-chlorophenols,meta-chlorophenols and para-chlorinated.The effects of Co doping amount,PMS concentration,catalyst loading and initial pH of the solution on the activity of the catalyst were investigated in detail.The experimental results show that the CCN/PMS catalytic system can effectively degrade MCP.The degradation reaction follows the pseudo-first order reaction in kinetic.It was determined by free radical quenching experiments and ESR spectroscopy that SO4·-is the main active free radical in the degradation process.The degradation rate of MCPs by CCN/PMS was 2-chlorophenol>3-chlorophenol>4-chlorophenol.We analyzed the interaction between catalyst and MCPs from the combination of experiment and theory,and discussed the effect of this interaction on the degradation process of MCPs It was found that the order of degradation rate of monochlorophenol was not only affected by the structural characteristics of MCPs,but also related to the adsorption behavior of CCN based on intermolecular interaction.Analysis of MCP degradation products by GC-MS revealed that 1,4-benzoquinone or 1,4-benzoquinone chloride was the maj or intermediate product2.Highly dispersed CoO nanodots embedded in N doped porous carbon as catalyst for the removel of chlorophenol by adsorption and degradationPorous carbon has better adsorption capacity and more active sites than g-C3N4 in SR-AOPs.We chose N-doped porous carbon as the carrier of the cobalt active species.Highly dispersed CoO nanodots(?6.5 nm)embedded nitrogen-doped porous carbon catalyst(CoO-N-C)was successfully prepared from a biomass Schiff-base polymer by a simple hydrothermal-calcination method.It was used to remove 4-CP by adsorption and degradation.The results show that CoO-N-C exhibited excellent adsorption efficiency for 4-CP by large surface area and abundant adsorption sites generated by CoO nanodot embedding and heteroatom N doping,and its saturated adsorption amount for 4-CP reached 129.39 mg/g.In addition,CoO-N-C also had higher catalytic performance on PMS activation than pure C and N-doped C.The affecting factors for the catalytic performance of CoO-N-C/PMS such as PMS concentration,catalyst loading,initial pH and inorganic anions were systematically studied.Through the combination of adsorption and degradation treatment,the TOC removal rate of 4-CP by CoO-N-C/PMS was close to 99.7%.The non-radical mechanism(single-line oxygen,1O2)was found to be the major oxidation process for 4-CP degradation.The introduction of CoO nanodots not only adds the radical catalytic process to CoO-N-C/PMS,but also enhances its non-radical catalytic process.The embedded CoO nanodots and doped nitrogen,especially pyridine N and graphite N,resulting in an increase of the active site and catalytic activity,play the synergistic roles in the catalytic process3.Co/N co-doped porous carbon activated peroxymonosulfate to degrade 2,4-dichlorophenolThe kind of the cobalt species also affects the catalytic performance of the catalyst We adjusted the doping form of Co in this work.The Co/N co-doped porous carbonaceous catalyst(CoNC)was prepared by using glucose as the C source,branched-chain polyethyleneimine(PEI)as the N source and the complexing agent for Co2+,followed by 180? hydrothermal-800 ? calcination and washed with acid to remove the unstable cobalt particles.It was used to degrade 2,4-DCP.In CoNC,Co mainly exists in the form of Co-Nx.The effects of the doping amount of cobalt on the physicochemical properties and catalytic properties of the catalyst were specifically studied.The results show that the doping amount of cobalt directly affects the morphology,specific surface area,pore structure,degree of graphitization and the number of active sites of the catalyst,thus affecting its catalytic performance.When the doping amount of cobalt was 0.33 at%,the obtained CoNC had the largest specific surface area and pore volume,the highest degree of graphitization(45.44%),and the best catalytic performance.In addition,the free radical catalytic pathway and the non-radical catalytic pathway had been shown to exist simultaneously in the CoNC/PMS catalytic system,while non-free radicals dominated.The synergistic catalysis of Co-Nx and N doped in CoNC gives it excellent catalytic performance.