Synthesis And Property Of High-performance Heterogeneous Fenton Catalysts Based On Ferrihydrite

Ferrihydrite?Fh?is a typical nano-mineral widely distributed in the surface environment.Fh is an important geological adsorber and natural catalyst with nano-size particles?2?6 nm?,large specific surface areas?³00 m²/g?,low crystallinity,and high reactivity.As s highly efficient heterogeneous Fenton catalyst,Fh can activate H2O2 to produce hydroxyl radicals?·OH?with high oxidation capacity,affecting the migration and transformation of organic and inorganic contaminants in the environment,and has been used for environmental pollution control.However,like many heterogeneous Fenton systems,Fh-based system also has two key problems that restrict its heterogeneous Fenton reactivity:slow reduction rate of Fe?III?and low effective utilization rate of H2O2.To solve the above problems,two approaches are proposed in this study.One is to inject electrons in the reaction process to accelerate the reduction of Fe?III?and reduce the inefficient decomposition of H2O2;The other is to improve the electron transfer rate in the reaction process to promote the decomposition of H2O2.Therefore,by introducing a variety of nano-materials with photocatalytic activity in visible light,such as Ag/AgCl and Ag/AgBr,as well as carbon materials with excellent electron conduction ability,such as CNTs and HTC,we prepared four new and efficient heterogeneous Fenton/photo-Fenton composite catalysts,i.e.,Ag/AgCl/Fh,Ag/AgBr/Fh,CNTs/Fh and HTC/Fh.The structure,morphology and electron transfer characteristics of these catalysts were studied by various characterization methods.The Fenton reactivity of various composite catalysts was evaluated by Fenton catalysis experiments.By measuring the decomposition rate of H2O2,regeneration concentration of Fe?II?and species and concentration of reactive oxygen species?ROS?in these reaction processes,the catalytic mechanism of different heterogeneous Fenton reaction was clarified.This study provides a theoretical basis for the development of environmental functional materials based on nano-minerals and a new perspective for understanding the supergene environment behavior of nano-minerals.Some valuable research results are obtained as follows:1.For the first time,Ag/AgCl composites were introduced into the Fenton catalytic system of Fh,effectively improving the Fe?II?regeneration rate and Fenton reactivity.In this study,Ag/AgCl/Fh composite catalyst was successfully prepared by loading AgCl on the surface of Fh and then photo-deposition of Ag nanoparticles.Ag nanoparticles in Ag/AgCl/Fh can generate photo-generated electrons through the surface plasmon resonance?SPR?effect,and then these electrons are injected into Fh to accelerate the reduction of Fe?III?.AgCl can inhibit the photo-generated electron-hole combination generated by Ag nanoparticles,so as to achieve the efficient output of photo-generated electrons.Under the synergistic effect of Ag nanoparticles and AgCl,the reduction rate of Fe?III?in Fh was effectively improved,and the decomposition of H2O2 was promoted to produce ROS,significantly enhancing the Fenton catalytic activity.The Kapp of 6%Ag/AgCl/Fh was about 0.0506 min^-1,approximately 5.1 times of that of Fh(0.0093 min^-1).In addition,the prepared Ag/AgCl/Fh composite catalyst is little affected by the pH value of the solution and can exhibit good catalytic activity in a wide pH range.2.Ag/AgBr/Fh Fenton catalytic system was constructed with high efficient Fe?III?reduction and H2O2 decomposition achieved by the photo-generated electrons of plasmonic material and semiconductor.Under visible light irradiation,the SPR effect of Ag nanoparticles and semiconductor properties of AgBr can generate photo-generated electrons.In addition,the Schottky barrier formed at the Ag-AgBr interface promotes the separation of photo-generated electron-hole pairs of AgBr.The electrons generated by Ag nanoparticles and AgBr are simultaneously transferred to Fh,which on the one hand accelerate the redox cycle of Fe?III?/Fe?II?in Fh,and on the other hand reduce the consumption of H2O2 used for reducing Fe?III?and then improves the effective utilization rate of H2O2.The decomposition rate of H2O2,the formation concentration of·OH,the concentration of Fe?II?in the catalyst and the Fenton catalytic activity of these catalysts all conform to the trends:Ag/AgBr/Fh>AgBr/Fh>Fh.3.It was found that the excellent electron conduction ability of CNTs can promote the electron transfer of H2O2 to Fh,thus accelerating Fe?III?reduction and improving the Fenton catalytic activity of Fh.CNTs/Fh composite catalyst was prepared by introducing oxygen-containing functional groups on the surface of CNTs and then simply stirring with Fh.The Kapp of 3%CNTs/Fh was up to 0.0811 min^-1,approximately 7.1 times higher than that of Fh(0.0114 min^-1),and the degradation rate of BPA was still up to 99.6%after used for 4 cycles,indicating that CNTs/Fh has excellent Fenton catalytic activity and good stability.Combining the results of catalytic experiments and DFT calculations,we propose that CNTs can be used as a“bridge”to facilitate electron transfer of H2O2 to Fh and accelerate the reduction of Fe?III?to Fe?II?.In addition,the Fe–O–C bond formed between CNTs and Fh may also promote the decomposition of H2O2 by reducing the redox potential of Fe?III?/Fe?II?,thus significantly enhancing the catalytic activity of the composite catalyst.4.A new method for improving Fh heterogeneous Fenton catalytic activity was first proposed by using carboxy-rich HTC microspheres.HTC microspheres prepared by hydrothermal regulation of functional groups on the surface of HTC microspheres is rich in carboxyl groups,showing a disordered graphite structure.In the HTC/Fh catalytic reaction system,the decomposition of H2O2,the surface Fe?II?of the catalyst and the·OH generated on the catalyst were all much higher than that in the Fh system,indicating that HTC could promote the decomposition of H2O2,thus accelerating the reduction of Fe?III?,and the generated Fe?II?continued to react with H2O2 to generate·OH,thus significantly enhancing the heterogeneous Fenton reactivity of Fh.The K_app of 10%HTC/Fh was 0.0152 min^-1,about 2.3 times of Fh(0.0067 min^-1).Compared with CNTs/Fh,the catalytic activity of HTC/Fh was lower,which may be related to the particle size,morphology,and specific surface area of these two carbon materials.Tubular CNTs have a larger specific surface area,so they can better contact with Fh,which is more beneficial for the electrons transfer from H2O2 to Fh.In addition,this study can enlighten the study of the geochemical behavior of iron?oxyhydr?oxides-carbon particles-H2O2 in their coexistence system.