Study On Sonocatalytic Degradation Of Organic Pollutants Induced By Several Semiconductor Composites

Nowadays,the organic pollutant wastewater discharged from industrial production leads to the increasingly serious water pollution.Removing these pollutants from industrial effluents is a major concern in the current wastewater treatment area.In recent years,the photocatalytic degradation of organic contaminants has been extensively studied.However,the actual wastewater has some characteristics,such as low-transparency and high density.The light cannot effectively irradiate the photocatalyst surface.Therefore,the effect of photocatalytic degradation is not obvious in practical application.Considering above reasons,the sonocatalytic degradation,based on the combination of semiconductor photocatalyst and ultrasonic irradiation,is deemed to be a suitable and efficient approach to treat wastewater containing low-transparency and high-density of organic contaminants due to the vigoroso penetrating power of ultrasound.The mechanism of sonocatalytic degradation reaction derives from ultrasonic cavitation effect which can cause sonoluminescence.Sonoluminescence can generate wide wavelength scope of lights.These lights can motivate matched semiconductor sonocatalyst to implement the photocatalytic reaction.Since the occurrence of sonocatalytic reaction is also associated with light,therefore,it is very important to find a resultful and stable semiconductor sonocatalyst.Generally,the wide band-gap semiconductors as sonocatalysts have powerful oxidation-reduction ability.However,they can only be motivated by high energy ultraviolet lights.The amount of ultraviolet light in sonoluminescence is very small,which greatly limit its applications.And that,the narrow-band semiconductors can utilize a largr number of low energy lights in sonoluminescence,but they have not powerful oxidation-reduction ability.In addition,narrow-band semiconductors own electrons（e^-）and holes（h⁺）easily recombine,suppressing the sonocatalytic performances of semiconductor sonocatalysts.Thus,the sonocatalytic degradation efficiency of organic pollutant treatment in wastewater is affected.In order to improve the efficiency of organic pollutant treatment,it is necessary to solve these problems that the utilization of light is low and the electrons（e^-）and holes（h⁺）are easily recombined in the sonocatalytic degradation reaction.In this work,the following four ways were used to solve the above problems:1)two different frequencies of ultrasound wave（40 KHz and 25 KHz）as illumination source are used.When two different frequent ultrasound waves are used in the same medium,the superposition phenomenon can occur in the focal acoustic field.Such superposition produces the large amplitude vibration waves in ultrasound wave spreading process,which promotes the increase of cavitation bubble number and kind.2)The band gap of broadband semiconductors is changed using the way of ion doping,leading red shifting of the catalyst absorption edge.In particular,the use of rare earth metal ion cyclic redox couple not only can enlarge light response scope,but also restrain own electrons（e^-）and holes（h⁺）recombination.The high-valent oxidizing ions(Ce⁴⁺)can get an electron（e^-）and there are reduced to low-valent reducing ions(Ce³⁺).The generated low-valent reducing ions(Ce³⁺)as electron donors easily give electron（e^-）and are oxidized by oxygen（O₂）,forming high-valent state oxidizing ion(Ce⁴⁺).The electrons（e^-）were continuously consumpted by the circular reaction.Therefore,the reorganisation of electrons（e^-）and holes（h⁺）are inhibited.3)The coated Z-scheme sonocatalytic system was constructed.The electrons（e^-）of the first semiconductor valence band（VB）are transferred to the conduction band（CB）of the second semiconductor to recombine with the holes（h⁺）on conduction band（CB）,which suppresses the recombination of electrons（e^-）and holes（h⁺）.At the same time,electrons（e^-）on more negative conduction band（CB）and holes（h⁺）on more positive valence band（VB）are retained.In addition,in coated Z-scheme sonocatalytic system,Pd nanorods as co-catalyst and conductive passageway are joined to transfer electrons（e^-）.4)Er³⁺:Y₃Al₅O₁₂ as up-conversion luminescence agent was used to convert the indirectly utilized visible lights from ultrasonic cavitation effect into directly utilized ultraviolet lights.The performance of the sonocatalyst was improved by these four methods.In the second chapter,Ce⁴⁺-doped BaZrO₃ composite was prepared via hydrothermal method.The sonocatalytic activity of prepared Ce⁴⁺-doped BaZrO₃composite was evaluated through dual-frequent sonocatalytic degradation of norfloxacin（NOR）.Under dual-frequent ultrasonic irradiate,the effect of Ce/Zr molar proportions for sonocatalytic activity of Ce⁴⁺-doped BaZrO₃ was investigated.The effects of ultrasonic irradiation time,used times and single/dual-frequent ultrasonic frequent for sonocatalytic degradation of norfloxacin（NOR）were also studied.In addition,the generated reactive oxygen species（ROS）during the dual-frequent sonocatalytic degradation process of norfloxacin（NOR）were confirmed by using different trapping agents.Finally,the possible mechanism for the dual-frequent sonocatalytic degradation of norfloxacin（NOR）caused by Ce⁴⁺-doped BaZrO₃ was proposed.The experimental results show that the Ce⁴⁺-doped BaZrO₃ displays a good sonocatalytic activity under dual-frequent ultrasonic irradiation.Under optimal conditions,the sonocatalytic degradation ratio of norfloxacin（NOR）can reach 73.90%.In the third chapter,the coated Z-scheme Pd-BaZrO₃@WO₃ composite was prepared via hydrothermal and sol-gel methods.The sonocatalytic activity of prepared coated Z-scheme Pd-BaZrO₃@WO₃ composite was evaluated through sonocatalytic degradation of diazinon.The effect of Pd-BaZrO₃ and WO₃ mass proportions for sonocatalytic activity of coated Z-scheme Pd-BaZrO₃@WO₃ was investigated.The effects of ultrasonic irradiation time,used times and catalyst dosages for sonocatalytic degradation of diazinon were also researched.At the same time,the produced intermediates were detected in the degradation process of diazinon by using gas chromatography-mass spectrometer（GC-MS）.The generated reactive oxygen species（ROS）during the sonocatalytic degradation process of diazinon were confirmed by using different trapping agents.Finally,the possible mechanism and process for the sonocatalytic degradation of diazinon caused by coated Z-scheme Pd-BaZrO₃@WO₃ was proposed.The experimental results show that the coated Z-scheme Pd-BaZrO₃@WO₃ displays a good sonocatalytic activity under ultrasonic irradiation.Under optimal conditions,the sonocatalytic degradation ratio of diazinon can reach 73.90%.In the fourth chapter,the coated Z-scheme Er³⁺:Y₃Al₅O₁₂/Pd-CdS@BaTiO₃composite was prepared via hydrothermal and sol-gel methods.The sonocatalytic activity of prepared coated Z-scheme Er³⁺:Y₃Al₅O₁₂/Pd-CdS@BaTiO₃ composite was evaluated through sonocatalytic degradation of chlortetracycline（CTC）.The effects of different sonocatalyst,ultrasonic irradiation time,degradation concentration,used times and catalyst dosages for sonocatalytic degradation of chlortetracycline（CTC）were also researched.The generated reactive oxygen species（ROS）during the sonocatalytic degradation process of chlortetracycline（CTC）were confirmed.Finally,the possible mechanism for the sonocatalytic degradation of chlortetracycline（CTC）caused by coated Z-scheme Er³⁺:Y₃Al₅O₁₂/Pd-CdS@BaTiO₃ was proposed.The experimental results show that the coated Z-scheme Er³⁺:Y₃Al₅O₁₂/Pd-CdS@BaTiO₃ displays a good sonocatalytic activity under ultrasonic irradiation.Under optimal conditions,the sonocatalytic degradation ratio of diazinon can reach 84.02%.