The Study On The Mechanism Of Breaking Methyl Orange And Benzo[a]pyrene By ClO₂ Enhanced With Hydrodynamic Cavitation

At present,a series of problems existed in the process of wastewater treatment,such as high cost,high energy consumption,and industrialization difficulties.Hydrodynamic cavitation?HC?,as a new type of energy-saving and cost-effective water treatment process,has got more and more attention of scientific and technical workers.The design parameters of cavitation device were optimized with numerical analysis of the cavitation mechanism,and the degradation technology of HC combined with green oxidant Cl O₂ was put forward.The feasibility of this new technology was confirmed by applying it to the degradation of characteristic degradant——methyl orange firstly,then focusing on the degradation of another represented pollutant——benzo[a]pyrene.The cavitation effect,degradation rate of target pollutants,degradation products and possible degradation pathways were studied in depth.Main content were as follows:?1?It was verified that the designed cavitation device could generate cavitation effects by the combined method of FLUENT simulation and methylene blue spectrophotometry from computer simulation and experimental angles,and consequently the best cavitation device with best cavitation effect was selected.?1?Different sizes of venturi tubes and orifice plates were designed.A two-dimensional turbulent model for venturi tubes and a three-dimensional turbulent model for orifice plates were set up by thek-estandard model in FLUENT 15.0 fluid dynamics simulation software.According to the medium parameters of 25°C water,appropriate boundary conditions were defined respectively.At inlet pressure of 0.4 MPa and outlet pressure of 0.15MPa,the pressure distribution,velocity distribution,turbulent kinetic energy,and vapor holdup of each cavitation devicer were simulated.The experimental results were showed as followed:Firstly,the structure parameters of venturi tube are the important influential factors of cavitation effect.Under optimal conditions that throat diameter ratio is 0.56,throat length is 15 mm,diffusion section length is 100 mm,inlet pressure is 0.4 MPa and solution temperature is 30-40°C,the best cavitation effect is obtained.Secondly,orifice opening rate and distribution manner of the orifice plates are the important influential factors of cavitation effect.The best cavitation effect is obtained under condition that the opening rate is 0.0461,inlet pressure is 0.4 MPa and solution temperature is 30-40°C.The effect of the holes arrangement on cavitation effect is:annular distribution>radiation distribution>uniform distribution.?2?The production of·OH in different cavitations devices were measured by methylene blue spectrophotometry method.It was found that all cavitations device could generate·OH by cavitation effects.The captured quantity of·OH is related to cavitation structure parameters and operating conditions such as solution temperature,inlet pressure,cavitation time,etc.In a comprehensive comparison,the orifice plate,the parameters of which are annular distribution and the hole opening rate is 0.0461,has the best cavitation effect,and under optimal operating conditions,the captured quantity of·OH is as high as 18.5?g/L.?2?The methyl orange with a concentration of 10 mg/L was degraded by hydrodynamic cavitation alone,ClO₂ oxidation alone,and two technology combined,respectively.Using the best orifice plate as the cavitation device.The HPLC and UV-visible spectrophotometry were used to test and compare the degradation effect of three different methods.BBD response surface method was used to optimize the degradation of methyl orange with combined method.?1?The degradation results of methyl orange by hydrodynamic cavitation alone indicated that the solution temperature and inlet pressure are the main factors affecting the degradation effect.The degradation rate of methyl orange was 18.5%under condition that the solution optimal temperature was 35?,the optimal inlet pressure was 0.4 MPa and the reaction time was 90 min with the method of hydrodynamic cavitation alone,and the degradation products were 4-?Dimethylamino?phenol and sodium 4-hydroxybenzene sulfonate.The degradation results of methyl orange by Cl O₂ alone oxidation indicated that the degradation rate of it increased with an increase in the concentration of ClO₂.The corresponding degradation rate of methyl orange was 45.8%under condition that the optimum concentration of ClO₂ was 8mg/L and the reaction time was 90 min.As the continuous increasing concentration of ClO₂,the rising speed of methyl orange degradation rate increased slowly obviously.The change of pH had little effect on the degradation effect.Methyl orange could be oxidized to benzenedio,phenol and chain hydrocarbon.Under the same conditions,the degradation rate of methyl orange degraded through combined experiment was as high as 90.8%.The enhancement factor was as high as 3.625 compared with the two single technology.And methyl orange could be degraded to benzenediol,phenol and benzoquinone by this combined technology.?2?BBD response surface method was used to analyze the hydrodynamic cavitation combined with ClO₂ oxidation.This method laid special stress on the effect of solution temperature,inlet pressure and reaction time on degradation rate of methyl orange.The polynomial model could predict the degradation rate of methyl orange accurately.The prediction results indicated that the 82.97%when the temperature was 38.13?,the pressure was 0.4 MPa and the reaction time was 60 min.The real experimental results showed that optimal degradation rate of methyl orange was 82.8%when the temperature was 35?,the pressure was 0.4 MPa and the reaction time was 60 min,so the theoretical analysis is in agreement with the practical experimental results.?3?The benzo[a]pyrene with a concentration of 5?g/L was degraded by hydrodynamic cavitation alone,Cl O₂ oxidation alone,and two technology combined,respectively.Comparing the degradation effects of three different technology under the optimal conditions by the test of HPLC and GC-MS and calculating the possible path of benzo[a]pyrene oxidated by·OH through the quantum chemical density functional theory to confirm the experimental result.?1?The degradation results of benzo[a]pyrene by hydrodynamic cavitation alone indicated that the solution temperature and inlet pressure were the main factors affecting the degradation effect.The degradation rate of benzo[a]pyrene was 41.85%when the solution optimal temperature was 35?,the optimal inlet pressure was 0.4 MPa and and the reaction time was 90 min.The degradation product with hydrodynamic cavitation alone was benzo[a]pyrene-4,5-dihydrodiol.The degradation results of benzo[a]pyrene by ClO₂ oxidation alone indicated that the degradation rate of benzo[a]pyrene increased with an increase in the concentration of ClO₂.The corresponding degradation rate of benzo[a]pyrene was 96.15%under condition that the optimum concentration of Cl O₂ was 5 mg/L and the reaction time was 40 min.The degradation rate of benzo[a]pyrene increased slowly with the continuous increaseofClO₂concentration.Benzo[a]pyrenecouldbeoxidizedto benzo[a]pyrene-4,5-dione by ClO₂ oxidation alone.Under the same conditions,the degradation rate of benzo[a]pyrene degraded through ClO₂ enhanced by hydrodynamic cavitation was as high as 99.2%.The enhancement factor was 1.23 compared with the two single technology,and benzo[a]pyrene could be degraded to benzo[a]pyrene-4,5-dione and and 4,5-dialdehyde chrysene with combined experiment which had a lower toxicity than the degradation products of the single technology.?2?The possible transition state structure of the reaction process of benzo[a]pyrene and·OH free radical calculated by using the quantum chemical density functional theory indicated that the addition reaction of the·OH was a process of no energy barrier.This reaction was easier than the hydrogen abstraction reaction.The calculation predicted the possible addition sites and the addition compound was benzo[a]pyrene-1,2-dihydrodiol or benzo[a]pyrene-2,8-dihydrodiol,and the addition reaction of the ortho-position was more likely to occur.