Kinetics Of The Reaction Of H₂O₂ With HOCl And Oxidation And Disinfection Efficiency And Mechanism For Additional Generated ¹O₂

The gap between water supply and water demand sometimes necessitates the use of the nearest water source to treat the water.Electrochemical water treatment technology is driven by electricity to generate water treatment chemicals,which can realize various water treatment processes such as coagulation,oxidation and disinfection.At the same time,the electrochemical water treatment technology is flexible and can be used alone or in combination with other technologies,so it is especially suitable for small-scale decentralized water treatment.When the water contains micro-concentration refractory organics,electrochemical oxidation technology is a common choice.Electrochemical oxidation can generate oxidants such as H₂O₂,·OH,and O₃ in situ,thereby degrading organic pollutants in water.However,the generation of by-products is an important problem to be solved in electrochemical oxidation technology.The formation of HOCl at anode is the main reason for the formation of chlorinated disinfection by-product in electrochemical systems.Using H₂O₂ to reduce HOCl is a feasible method to control the generation of chlorinated disinfection by-product from the source.In this paper,focusing on the reaction of H₂O₂ and HOCl,the kinetics and principle of the reaction were studied in detail,and the mechanism and yield of singlet oxygen（¹O₂）produced by the reaction were explored.Taking this reaction as a quantitative ¹O₂production system,the efficiency of ¹O₂ in oxidizing organic matter was studied,and the reaction was applied to an electrochemical system,then a ¹O₂involved electrochemical disinfection technology was proposed.In this paper,the kinetics of the reaction between HOCl and H₂O₂ was studied,and the second-order reaction rate of the reaction increased first and then decreased from p H6.5 to 12.In the reaction of HOCl and H₂O₂,the second-order reaction rate constants between the components are,195.5±3.3 M^-1s^-1 for HOCl and H₂O₂（k₁）,and 4.0×10⁷M^-1s^-1for HOCl and HO₂^-（k₂）,OCl^-and H₂O₂（k₃）are 3.5×10³ M^-1s^-1,respectively.k₂and k₃ are kinetically indistinguishable and therefore interdependent.The apparent second-order reaction rate of H₂O₂ with HOCl increases with increasing ionic strength.The apparent activation energy for the reaction of H₂O₂ and HOCl is 30.88±4.37 k J/mol.Three reaction pathways between H₂O₂ and HOCl and the corresponding intermediates and transition states are proposed.Density functional theory calculations suggest that the reaction of HOCl with HO^-is the most thermodynamically favorable route because it does not require overcoming a high free energy barrier.Due to the high energy barrier of the reaction of OCl^-with H₂O₂ pathway,the contribution of this reaction to the reaction of H₂O₂ with HOCl is negligible.Using FFA as a probe,the reaction rate of ¹O₂ between H₂O₂ and HOCl at p H 7-12 was studied,and the average yield was 92.3%.The yield of¹O₂ produced by the reaction of H₂O₂ with HOBr was 75%.Owing to the heavy atom effect,the ¹O₂ yield for the reaction of H₂O₂ with HOX（X=Cl,Br,I）decreases in the order of Cl>Br>I.¹O₂ is considered the dominant reactive oxygen species responsible for micro-pollutant degradation in many non-photochemical systems.Based on this,the characteristics of the ¹O₂-dominated H₂O₂/HOCl system for degradation of organic pollutants were investigated and compared with several non-photocatalytic systems.The self-quenching rate of ¹O₂ in water is very fast,and the first-order reaction rate is 2.76×10⁵ s^-1.The reaction of pollutants with ¹O₂ in water competes with the self-quenching reaction of ¹O₂,and the capture rate of ¹O₂ by pollutants is related to the product of the concentration of pollutants and the second-order reaction rate constant of the reaction of pollutants with ¹O₂.Increasing the concentration of pollutants is beneficial to increasing the capture ratio of ¹O₂,thereby increasing the degradation of pollutants.Due to the unreasonable quenching method and the false positive results of the EPR method,the role of ¹O₂ in non-photocatalytic systems is generally overestimated,and the relaxation luminescence of ¹O₂ is the most direct evidence for the generation of ¹O₂.The second-order reaction rate constant of the reaction between phenates and ¹O₂ can be determined by using the system of H₂O₂/HOCl to quantitatively generate ¹O₂.The measured values are generally lower than the previous ones in the photosensitization system,the possible reason is that the excited state photosensitizer also participates in the degradation of the phenate in the photosensitization system,which makes the value of the reaction rate constant high.The QSAR of the reaction of phenates with ¹O₂ was studied,and the quantum chemical descriptors E_HOMO and E_{Vertical IP} performed better than the empirical parameter Hammett constant.Due to the high energy of forming endoperoxide in the reaction between ¹O₂ and phenate,this route is unlikely to be the main route of the reaction,and the substitution reaction of phenate by ¹O₂ at ortho-para position is a more probable route.¹O₂ is an excellent disinfectant and plays an important role in photosensitizer-based sunlight disinfection.However,the use of ¹O₂ for disinfection in electrochemical systems has not been reported.The efficiency of H₂O₂ and HOCl directly mixed to produce ¹O₂disinfection was studied.The result found that the system has a good disinfection efficiency for E.coli.The mixed system of H₂O₂ and HOCl has better disinfection efficiency than a single HOCl with the same CT value,and the reason is that ¹O₂participates in the disinfection.The reaction of H₂O₂ and HOCl was applied to the electrochemical system,and the paried electrode reaction was constructed to realize and study the generation and reaction of H₂O₂ and HOCl in the electrochemical system.¹O₂can be generated in this constructed electrochemical system,and the generation of ¹O₂ is directly confirmed by detecting the near-infrared luminescence in the system.In the H₂O₂+Cl^-anodic oxidation electrochemical system under the experimental conditions,the concentration of ¹O₂ can reach 1.64 p M.¹O₂ played an important role in the inactivation of E.coli,while HOCl only played a minor role in the presence of 1 m M H₂O₂ due to its low concentration.Due to the rapid reduction of HOCl by H₂O₂,the formation of chlorinated organic by-products and Cl O₃^-in this system is greatly reduced.In this paper,the study of the reaction kinetics and reaction mechanism of H₂O₂ and HOCl is helpful for the in-depth understanding of the control of the generation of chlorinated by-products in the electrochemical system,the research on the degradation of organic pollutants by ¹O₂-dominated H₂O₂/HOCl system is conducive to the current understanding of ¹O₂ oxidation characteristics and the re-understanding of some non-photocatalytic systems,and promotes the development of non-radical oxidation systems,and the proposed electrochemical disinfection system based on the reaction of HOCl and H₂O₂ with ¹O₂ involvement is beneficial to improve the H₂O₂-based electrochemical water treatment technology and promote the development of electrochemical disinfection technology.