| Photocatalytic oxidation that utilizes sunlight and dissolved oxygen in water to generate reactive oxygen species(ROS)is one of the best solutions for wastewater treatment.However,due to the rapid recombination of photoinduced charge carriers and low production of hydroxyl radical(·OH)being the most effective ROS,photocatalytic decontamination is far from practical application.In order to boost the·OH yield and consequently increase the overall oxidation capacity,herein we couple a low concentration of ozone with visible light and use graphitic carbon nitride(g-C3N4)catalysts to build a photocatalytic ozonation(Vis/O3/g-C3N4)system,which has proven to be capable of rapidly mineralizing a wide variety of organic pollutants in water.The main contents and conclusions are as follows.(1)Bulk g-C3N4 was used in visible-light photocatalytic ozonation and triggered a super synergy between photocatalysis and ozonation toward much faster degradation of oxalic acid.The apparent pollutant removal rate constant in Vis/03/bulk g-C3N4 can reach up to 95.8 times as high as the sum of those in photocatalytic oxidation(Vis/O2/bulk g-C3N4)and ozonation,and photocatalytic ozonation under visible light(420-800 nm)is more robust than that under UV with the same light intensity.As a result of high conduction band minimum position and relatively narrow bandgap,bulk g-C3N4 has proven to be more active than W03 and Ti02,the state-of-the-art catalysts in the field of visible-light photocatalytic ozonation.(2)Nanoporous g-C3N4 was synthesized via a non-template method,and the catalytic oxidation of p-hydroxybenzoic acid(PHBA)was investigated in various processes.It is found that photocatalytic ozonation(Vis/O3/nanoporous g-C3N4)leads to faster and much more complete mineralization of PHBA.The final total organic carbon removal by Vis/O3/nanoporous g-C3N4 was about 45%higher than the sum of those in Vis/02/nanoporous g-C3N4 and ozonation.The significantly enhanced mineralization of PHBA in photocatalytic ozonation is caused by(i)the direct oxidation of aromatic compounds into carboxylic acids by ozone,and more importantly by(ii)further oxidation of these ozone-refractory carboxylic acids into CO2 and H2O by abundant ·OH generated in this process.(3)We developed an in-situ electron paramagnetic resonance(EPR)technique,by which the photoexcitation of electrons,their further trapping by dissolved O2 and O3,and the evolution of ROS upon aqueous bulk and nanosheet g-C3N4 suspensions were semiquantitatively visualized.The presence of only 2.1 mol%O3 in the inlet O2 gas stream can trap one to two times more photoexcited electrons than pure O2,and shifts the ·OH generation pathway from the inefficient three-electron reduction of oxygen(O2?·O2-?HO2·?H2O2?·OH)to the robust one-electron reduction of ozone(O3?·O3-?HO3·?·OH),consequently greatly improving the yield of ·OH.Furthermore,a series of g-C3N4 materials with well-tuned band structures were synthesized,and the correlation of activity with band structure was revealed.The upshift of conduction band edge benefits to improve the catalytic activity,accompanied by which narrowing of band gap could further improve the reactivity of g-C3N4 by improving the overall number of photoexcited electrons.(4)The structural and catalytic stabilities of g-C3N4 in photocatalytic ozonation were comprehensively studied.We have,for the first time,shown the chemical instability of g-C3N4 toward ·OH attack,and revealed its decomposition pathway.On a positive note,'OH usually preferentially attack micromolecular organics in wastewater rather than g-C3N4,which obviously inhibits the decomposition of g-C3N4;therefore,the catalytic activity of g-C3N4 and stable operation of photocatalytic ozonation can be preserved. |
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