| The treatment of toxic and harmful organic pollutants in water has always been one of the research hot topics in the field of environmental protection.As a green and efficient technology for organic pollutant removal.photo-Fenton and photocatalytic technology have received more and more attention,and have important significance in the removal of refractory pollutants.Graphite phase carbon nitride(g-C3N4)and bismuth vanadate(BiVO4)both have narrow band gaps,and the conduction bands potential of g-C3N4(-1.1 V vs NHE)and BiVO4(-0.32 V vs NHE)are both more negative than the reduction potential of O2/H2O2(0.69 V),so the conduction band electrons of these semiconductors have enough energy to reduce O2 into H2O2.This paper focuses on the efficient and green removal of environmental organic pollutants,using g-C3N4,and BiVO4 as light-responsive materials,respectively,and comparatively studied the kinetics and mechanism of photo-Fenton and photocatalytic degradation of pollutants.It was found that under different catalyst systems and different organic pollutants,the photocatalytic process always showed zero-order kinetic characteristics and the photo-Fenton process was first-order kinetic characteristics.The main conclusions of the paper are as follows:(1)A series of photocatalyst materials doped with different concentrations of iron were prepared by doping trace iron in exfoliated layered g-C3N4,and the photo-Fenton and photocatalytic degradation reactions on this material were compare timely studied.When the Fe loading reached 0.054 wt%with 10 mmol/L H2O2 was added,the best removal efficiency of Rhodamine 6G(Rh 6G)reached 60%within 0.5 h and?100%within 1 h of light illumination.However,using g-C3N4 without Fe dopping as a control experiment,the removal efficiency of Rh 6G was only 25%within 0.5 h and 55%within 1 h of light illumination.After 5 consecutive cycles,the photo-Fenton removal efficiency of Rh 6G still exceeded 88%within 1 h.The kinetic model verifies that the photo-Fenton reaction conforms to first-order kinetics,and the photocatalytic reaction conforms to zero-order kinetics.According to the experimental results of the scavenger,the sequence of the main active species in the degradation process of Rh 6G is:·O2->·OH>>h+,and the detailed mechanical steps of the photo-Fenton degradation reaction were proposed.(2)Layered TE-g-C3N4 was prepared by inserting H2SO4 into a B-g-C3N4 and thermal exfoliation.The exfoliated TE-g-C3N4 can generate H2O2 in situ under light illumination.Then the in situ generated H2O2 can be used as the "start button" for photo-Fenton degradation of Rhodamine B(RhB)reaction.Thus a novel H2O2-free photo-Fenton reaction was proposed for the first time.The removal efficiency reached 90%within 1 h and?100%within 2 h of light illumination using TE-g-C3N4 with Fe3+ as catalyst.With strong contrast,the removal efficiency of RhB using TE-g-C3N4 without Fe3+ is just about 55%after 1 h and 95%after 2 h of light illumination,respectively.After 5 cycles,the removal efficiency of photo-Fenton degradation still exceeded 90%.Moreover,from the results of scavenger experiments and electron spin resonance spectroscopy(ESR)test,it can be seen that the order of main active species in the photo-Fenton degradation of RhB is:OH>h+>>·O2-,while the sequence in photocatalytic degradation is h+>>·OH>·O2-.On this basis,detailed steps of the possible mechanism of photo-Fenton and photocatalytic degradation reaction are proposed.The reason why photo-Fenton reaction is faster than photocatalysis can be well explained by the machnism steps proposed.Once more,it is obtained by data fitting that the photo-Fenton reaction conforms to first-order kinetics,but the photocatalytic reaction conforms to zero-order kinetics.In order to verify the universality of this regularity,we use the same dye with different catalysts and different dyes with different catalysts for confirmation.Using B-g-C3N4 and the g-C3N4 catalyst prepared in Chapter 3 as catalysts and dyes crystal violet(CV),malachite green(MG),and methylene blue(MB)as pollutants,we concluded that the photo-Fenton processes always conform to first-order kinetics but the photocatalytic processes conform to zero-order kinetics,indicating that the rule has certain universality.(3)The monoclinic BiVO4 catalyst with an octahedral structure similar to the "ice sugar"structure was successfully synthesized by hydrothermal method,and it was found that this material system can also realize the photo-Fenton degradation reaction without addition of H2O2.Using the obtained BiVO4 as a light absorbing materials,photo-Fenton and photocatalytic degradation of RhB,Rh 6G and norfloxacin(NOR)were compared.After 20 minutes of light illumination,the photo-Fenton removal efficiencies of RhB,Rh 6G and NOR were 80%,30%and 85%,respectively,and the removal efficiencies within 1 h reached 100%,70%and 98%.After five consecutive cycles,the removal efficiency of RhB still exceeds 90%.Comparing the photo-Fenton degradation curves of RhB and Rh 6G,we found that the removal efficiency of the RhB reached 80%after the light irradiation for 20 min,which was 2.7 times that of the Rh 6G,and the obtained kinetic rate constants were 0.0437 min-1 and 0.0136 min-1,respectively,which may be caused by the coordination difference between RhB or Rh 6G and Fe3+.Therefore,we speculate that the carboxyl group in the RhB molecular structure is easier to coordinate with Fe3+ than the amino group in the Rh 6G structure,which causes the degradation reaction rate of RhB to be much higher than that of Rh 6G.Again the rule that photo-Fenton conforms to first-order kinetics and photocatalysis does zero-order kinetics worked using BiVO4 as photocatalyst and RhB,Rh 6G or NOR as pollutant,rspectively.The analysis of the experimental data based on ESR and a series of scavengers shows that the order of the active species in photo-Fenton degradation of Rh 6G is·OH>h+>·O2-,and the corresponding reaction mechanism was proposed. |
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