Studies On Catalysis Mechanisms In Pulsed Discharge Plasma Combined With TiO₂ For Degradation Organic Pollutants In Wastewater

The combination of pulsed discharge plasma with TiO2 catalysis has great potential to enhance the removal efficiency of organic pollutants in wastewater and the utilization efficiency of the discharge energy,which is a promising wastewater treatment technology.However,much effort has been devoted to optimizing the discharge conditions and improving the synergetic effects of plasma-induced TiO2 photocatalytic,the catalysis mechanism in underwater pulsed discharge plasma combination with TiO2 is not yet fully understood.The oxidants and reductants in pulsed discharge plasma system are different from those in traditional photocatalytic system.There are some other chemical and physical effects other than UV light,such as high-energy electrons,O3,excited nitrogen atom,and nitrate,may be have effect on TiO2 catalytic mechanism including charge carriers and radical generation.In this work,a synergistic system of pulsed discharge combined with TiO2 photocatalysis was established,phenol was used as the chemical probe to characterize the activity of TiO2 in pulsed discharge system.The paper explored the effect of active species generated by discharge and Fe3+ ions on TiO2 catalytic mechanism in the pulsed discharge system.The excitation effect of high energy electron,the modifcation effect of excited nitrogen atom,and the synergetic between Fe3+ and TiO2 were analyzed.The main results are summarized as follows:1.The pulsed discharge plasma combination with TiO2 has been developed to optimize the activity of TiO2 by varying discharge conditions of pulsed voltage,discharge modes,air flow rate and solution conductivity.The experimental results showed that the activity of TiO2 has been found to be most effective and the enhanced removal efficiency was 10%at applied voltage of 20 kV,TiO2 dosage of 0.2 g/L,air flow rate of 0.05 m3/h,and solution conductivity of 10 ?S/cm under air atmosphere discharge.2.The excited role of high-energy electrons from plasma channel in TiO2 charge carriers and radical generation has investigated through electron scavenger addition,EPR and XPS analysis.The results showed that TiO2 could be activated by high-energy electrons to initiate pairs of electron-hole,and the electrons on the conduction band(CB)could be trapped on the oxygen vacancy and the Ti4+OH groups located at the TiO2 surface.In the case of air,the CB electrons mainly transferred to dissolved oxygen and O3,resulting in more ·OH.Meanwhile,the CB electrons directly transferred to p-benzoquinone adsorbated on TiO2 as electron acceptor which ineffectively consume CB electrons,thus suppressing the formation rate of ·OH.3.The effect of nitrogen-based reactive species on TiO2 characterization and the reduction reaction between conduction band electron and nitrite in pulsed discharge plasma system has investigated through XRD,XPS,UV-vis analysis and by-products measurement.The experimental results showed that excited nitrogen from plasma discharge were doped on TiO2 particles,which narrowed the band gap of TiO2 from 3.0 eV to 2.8 eV and enlarged the absorption edge at around 440 nm,therefore the photocatalytic activity in the visible light was enhanced.Meanwhile,the nitrate could be changed to nitrite using CO2-· which was a product of formic oxidation on the hole of TiO2,then nitrite react with H2O2 for the enhancement of the ·OH formation rate in the plasma/TiO2 system.4.The synergetic effect of TiO2 and Fe3+ in pulsed discharge plasma has systematically investigated by XPS,UV-vis spectra,XRD,photocourrent analysis and phenolic by-products,Fe2+ ion measurement.The experimental results showed that a significantly synergistic effect occoured between Fe3+ and TiO2.Fe3+ ions and excited nitrogen from plasma discharge were doped on TiO2 particles,which narrowed the band gap of TiO2 from 3.0 eV to 2.0 eV and enlarged the absorption edge at around 620 nm,and therefore enhanced the photocatalytic activity in the visible light.The co-doping of Fe3+ and nitrogen also significantly increased the separation rate of photo-generated electrons and holes and prolonged their lifetime.Meanwhile,the Fe3+ ions could be changed to Fe2+ ions using photoelectron on TiO2 surface and hydroquinone,inducing photo-Fenton reaction for the enhancement of the ·OH formation rate in the plasma/TiO2/Fe3+ system.