| Advanced oxidation technology based on hydroxyl radical HO·,superoxide anion radical O2~-·,singlet oxygen ~1O2 has the advantages of fast oxidation rate and convenient operation in the treatment of refractory organic wastewater.However,the traditional homogeneous Fenton technology based on iron has some disadvantages,such as narrow pH range,strongly acid effluent,large amount of iron sludge after treatment,and difficulty in separating and reusing catalysts.Graphite-phase carbon nitride g-C3N4contains metal coordination cavities and a narrow band gap(2.7 eV),which makes it a good heterogeneous Fenton photocatalytic material which can utilize visible light.The incorporation of metal into g-C3N4 not only overcomes the shortcomings of homogeneous Fenton,but also reduces the recombination of photogenerated carriers,thus improves photocatalytic activity of g-C3N4.On the basis of iron-doped g-C3N4,copper was innovatively introduced to synthesize iron-copper bimetallic doped modified g-C3N4,in order to give full play to the advantages of copper,which has a wide pH range and can reduce trivalent iron as divalent iron to break the restrictive step of Fenton reaction.In this paper,the effects of different working conditions and environmental conditions on the performance of this heterogeneous catalyst in aniline degradation were studied.After identifying the main free radicals and determining the concentration changes of various substances in the reaction process,the mechanism of degradation reaction was put forward.First,the formation of g-C3N4 was confirmed by X-ray diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscopy(TEM),Fourier transform infrared spectroscopy(FTIR)and X-ray photoelectron spectroscopy(XPS).The fact that the doped iron existed in the form of Fe(III)-N and Fe2O3,while the doped copper existed in the form of Cu(I)-N bond was also proven.The diffuse reflectance spectroscopy(DRS)proved that metal doping was beneficial to broaden the absorption range of g-C3N4.The photoluminescence spectra(PL)as well as photocurrent response curves(I-t curve)also showed that the introduction of metals contributed to the separation of photogenerated electrons and holes.After defining the physical properties of the material,the optimum material4Fe1Cu-CN was selected according to the removal rate of aniline and the amount of metal dissolution.It was found that under the conditions of 12 W visible light irradiation,pH=7,T=25°C,the removal rate of aniline degraded by 50 mmol/L hydrogen peroxide catalyzed by 0.5 mg/L 4Fe1Cu-CN for 6 hours was 79.9%,the concentration of dissolved iron was 0.251 mg/L,and the concentration of dissolved copper was 0.079 mg/L.Compared with 4Fe-CN,the removal rate was increased by 7.2%and the dissolution rate of iron was reduced by 0.154 mg/L.By comparing the influence of pH on 4Fe-CN and 4Fe1Cu-CN,it is proved that copper can improve the pH tolerance of materials.The suitable pH range of 4Fe-CN is 5.0-9.0,while that of4Fe-1Cu-CN is 3.0-11.0.And the catalytic performance of the material is not affected by common inorganic anions.Through mass spectrometry analysis,it was determined that the degradation intermediate products included phenol,nitrobenzene and aminophenol,but there was a phenomenon that carbon on g-C3N4 entered the aqueous phase,which led to the increase of TOC in the solution.The material retained 65.7%activity after 4 times of reuse,and the removal rate of aniline decreased by about 14.2%.The reduced catalytic activity was due to the loss of Fe2O3 and the oxidation of some monovalent copper to divalent copper.In view of the special phenomenon that bicarbonate increases the catalytic performance of 4Fe1Cu-CN and decreases that of 4Fe-CN,this paper studies the variation of the concentration of HO·,~1O2,hydrogen peroxide,dissolved metal and nitrate with the presence or absence of bicarbonate,and proposes respective catalytic mechanisms for two different systems.When there is no bicarbonate in the system,the degradation of aniline by hydrogen peroxide catalyzed by iron-copper doped g-C3N4 is based on heterogeneous catalysis.During the reaction,the valence change of metal on the coordination g-C3N4 catalyzes hydrogen peroxide to produce HO·and ~1O2.Among them,the degradation of aniline by HO·accounts for 25.83%and that by ~1O2 for20.57%.Both free radicals reached their maximum concentration at 30 min,and tended to be stable and maintain dynamic equilibrium after 60 min.With the increase of reaction time,hydrogen peroxide decomposes continuously,and the concentration of dissolved metal and nitrate increases.When there is bicarbonate in the system,the degradation of aniline by hydrogen peroxide catalyzed by iron-copper doped g-C3N4 is a homogeneous catalytic process dominated by copper.After the reaction,the dissolution of Cu is 14.93 times as much as that without bicarbonate.The concentration changes of various substances under different concentrations of bicarbonate were compared.It was found that when the bicarbonate concentration was 10 mM,the total production of HO·was the highest,and the degradation rate of aniline,metal dissolution and nitrate production were higher.This indicated that HO·was an important factor affecting the degradation of aniline and the stability of materials.When the bicarbonate concentration increases to 50 mM and 100 mM,excessive bicarbonate will consume HO·,resulting in the decrease of total HO·production,aniline degradation rate,metal dissolution and nitrate production. |
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