Study On Properties And Mechanism Of Activated Peroxymonosulfate Of Iron-based/Diatomite Composite Material

In recent years,the emission of a large amount of Persistent Organic Pollutants（POPs）causes significant hazards to human life health and puts forward new challenges to environmental treatment technologies.Sulfate Radical-Based advanced Activation Processes（SR-AOPs）have attracted more and more attention.Among persulfate activators,iron-based activators have the most application potential.However,traditional iron-based activators have disadvantages,such as harsh application conditions,high metal dissolution rate,low activation efficiency and being prone to secondary pollution,which limit the practical application of SR-AOPs technology.Diatomite is a natural siliceous porous mineral.A series of persulfate activation materials with high activation efficiency,low metal dissolution rate and strong environmental adaptability is expected to obtain by diatomite carrier with it nanopore structure.In this thesis,three peroxymonosulfate activators,nano zero valent iron/diatomite composite,nano Co Fe₂O₄/diatomite composite and F-N-C/diatomite composite,were prepared by liquid-phase reduction precipitation method,citric acid calcination method and solution diffusion method respectively with diatomite carrier.The influence law of process conditions on the structure of composites and the structure-activity relationship were obtained.The crystallographic features,microtopography,elemental distribution,surface chemical states and pore structures of the composites were analyzed and characterized.The reactive group species as well as bisphenol A degradation process intermediates were speculated.The effects of environmental factors such as p H value,composite dosages,bisphenol A concentration and anionic species on the degradation of bisphenol A by composite/peroxymonosulfate system were investigated,the application basis of composite/peroxymonosulfate system was obtained.The activation performance enhancement mechanism of the composites/peroxymonosulfate system was also explored.The main achievements of this thesis are as follows:（1）The optimized preparation conditions of nano zero valent iron/diatomite composite are as follows:loading amount（mass）40%,amount ratio of BH₄^-to Fe²⁺is 4:1,the volume ratio of deoxygenated water to ethanol is 1:1.The quasi first-order kinetic constant of bisphenol A degradation by nano zero valent iron/diatomite composite/PMS system was 3 times that of zero valent iron/PMS system.The enhancement mechanism of the activation performance of nano zero valent iron/diatomite composites is as follows:The introduction of diatomite carrier increased the adsorption capacity of composites and increased the active sites.Hydroxyl radicals play a major role with sulfate radicals during degradation,and singlet oxygen participates in the reaction.The process of bisphenol A degradation consists of hydroxylation reaction,branch chain scission reaction and ring-opening reaction.The effect of application conditions on the bisphenol A degradation process by composite/peroxymonosulfate system was as following rules:the degradation ability was the best at neutral p H,the acidic and alkaline environment had an inhibitory effect on the degradation process.The degradation rate of bisphenol A decreased with the increase of contaminant concentration,increased with the increase of peroxymonosulfate concentration,first increased and then decreased with the increase of composites dosage,and the highest when the composite dosage was 0.25 g/L.（2）The optimized preparation conditions of nano Co Fe₂O₄/diatomite composites are as follows:loading amount（mass）40%,amount ratio of citric acid to metal ion is 4:1,calcination temperature 400℃and calcination time 2 h.The quasi first-order kinetic constant of bisphenol A degradation by Co Fe₂O₄/diatomite composite/peroxymonosulfate system was 5.3 times higher than that of Co Fe₂O₄/peroxymonosulfate system.The enhancement mechanism of the activation performance of nano Co Fe₂O₄/diatomite composites is as follows:The introduction of diatomite carrier increased the adsorption capacity and active sites of composites.The degradation process of bisphenol A was composed of the non-radical path（singlet oxygen）and the radical path（sulfate radical and hydroxyl radical）,and the non-radical path was dominant.The degradation process consisted of benzene ring hydroxylation,multi-molecular coupling,ring-opening reaction and carbon skeleton oxidation.The dissolution rate of iron and cobalt ions was much lower than that of a pure Co Fe₂O₄ activator.The effects of application conditions on the degradation of bisphenol A are as follows:the degradation ability was the best under weak alkaline conditions.Acidic and strong alkaline environments hindered the activation process.The degradation rate of bisphenol A decreased with the increase of pollutant concentration,increased with the increase of persulfate concentration and dosage of composites.Anions had little effect on the degradation process.（3）The optimized preparation conditions of F-N-C/diatomite composites are as follows:the mass ratio of diatomite to iron nitrate and zinc nitrate is 10.0:1.0:29.7,the reaction time is 6 h,and the calcination temperature is 900℃.The quasi first-order kinetic constants of bisphenol A degradation by composite/peroxymonosulfate system were 11.8times and 9.6 times higher than those of F-N-C/peroxymonosulfate system and physical mixed sample/peroxymonosulfate system.The enhancement mechanism of the activation performance of F-N-C/diatomite composites is as follows:the construction of single atom and composite pore structure improves the adsorption performance and active sites of the composite.The activation process consists of the non-radical path（singlet oxygen,tetravalent iron）and the radical path（superoxide radical）,and the non-radical path was dominant.The degradation process of bisphenol A consisted of multi molecule coupling,benzene ring hydroxylation,branch chain breaking,ring-opening reaction and carboxylation reaction.The effects of application conditions on the degradation of bisphenol A are as follows:the degradation ability was the best under acidic conditions,and the activation process was hindered under alkaline conditions.The degradation rate of bisphenol A decreased with the increase of pollutant concentration,increased with the increase of peroxymonosulfate concentration and composite materials.The degradation rate of bisphenol A was the highest when the concentration of peroxymonosulfate was greater than 2 mmol/L.Anions have little effect on the degradation process.