Theoreticaland Experimental Research On Wastewater Treatment By Laser-Induced Cavitation

With the development of society and economy,organic dye is becoming an environmental burden in this era.Explore a new wastewater treatment method for the effective reuse of water resources is necessary.Organic dye wastewater has serious impact on environment,such as toxicity,carcinogenicity,high chemical load,etc.Traditional physical,chemical and biological degradation methods have some limitations,such as demanding a large area and leading in secondary pollution.In recent years,scholars have proposed hydraulic and ultrasonic cavitation degradation methods to solve these problems.However,cavitation erosion may affect the service life of the equipment.This work proposes a novel degradation method to deal with organic wastewater by using laser-induced cavitation.The hydroxyl radical produced by laser plasma and collapse bubbles are used to decompose organic molecules.Advantages exist including non-contact degrading,high fading velocity,short degradation time,low energy consumption,etc.Main work in this study is as follows:(1)Builds laser cavitation degradation experimental platform: Technical route and experimental procedure was drew up,and key problems were proposed and solve accordingly.Materials,experimental equipment and characterization equipment have been listed,and a experimental platform has been built.Specific experimental and simulation schemes have been drew up,including experimental and simulational schemes for laser-induced cavitation pulsation,experimental schemes for laser cavitation degradation and a laser-photocatalysis combined degradation method.(2)Characteristics of laser-induced bubbles in aqueous solution: The effects of laser energy on maximum radius,maximum volume and pulsation period of laser-induced bubbles have been studied by experiments and simulations.The pulsation mechanism of laser-induced bubbles near a solid boundary and in the infinite domain in aqueous solution was revealed.Results indicate that: A single bubble generated near a solid boundary has a larger size than the bubbles in a bubble string in the infinite domain.The maximum radius,volume,and pulsation period of the bubbles increase with the laser energy.However,the increasing trend slows down at high laser energy due to the plasma absorption effect.The pulsation process of laser-induced bubbles obtained by simulations is basically consistent with the experimental results captured by the high-speed camera.A linear relationship exists between the maximum bubble radius and the pulsation period.(3)Pulsation mechanism of laser-induced bubbles in suspension: Micron and nanoscale solid particles were added to aqueous solution,and the effects of laser energy and particle concentration on the maximum size and pulsation period of laser-induced bubbles have been studied.In addition,the mechanism of the particles on laser-induced bubble pulsation has been analyzed.Results indicate that: The maximum radius and volume of bubbles decrease with the particle concentration in the suspension.Rebound occurred in the suspension of micron particles,however,no rebound was found in the suspension of nanoscale particles.(4)Experiments and mechanisms of laser cavitation degradation: To study the characteristics of laser cavitation degradation,the influence of laser energy,degradation time,initial concentration,temperature,p H and particle concentration have been investigated in this work.Results indicate that: a larger laser energy may increase the degradation efficiency.However,the growth rate may slow down at high energy level.A higher initial solution concentration or p H value has a negative effect on the degradation rate,while a higher temperature may increases the degradation rate.The adjunction of solid particles may reduce the degradation efficiency.The degradation extent in the suspension of micron-scale particles rebounds at a certain concentration.However,the rebound was not observed in the suspension of nanometer-scale particles.(5)A combined degradation processing including laser-induced cavitation and photocatalysis: A combined degradation method including laser-induced cavitation and photocatalysis has been proposed.The effect of laser energy and particle concentration on the efficiency of the combined degradation method was investigated.In addition,total organic carbon(TOC)and liquid chromatograph mass spectrometer(LC-MS)were employed to analyze the intermediates,the degrading process and the degrading pathway.Finally,the energy utilization ratio and the synergetic coefficient of the combined degradation method is calculated.Results indicate that: the catalyst particles have an optimal concentration value(0.2%),and the degradation extent may decrease after the particle concentration exceeds the optimal value.The combined degradation method is capable of degrading 98.60 % of the solute within 30 min.However,it is difficult to completely decompose the intermediates in a short time.The combined degradation method in this work has higher energy utilization ratio and synergistic effect.