| The ClO2generator, using the single-stage-R5-technology based on the reaction of sodium chlorate and hydrochloric acid, has become one of the primary substitutes for chlorine disinfection equipments of drinking water, as being easy to control and with simple structure, high security, and high performance-to-price ratio.In view of the low efficiency actualities of domestic ClO2generators based on single-stage-R5-technology and some problems existing in tapwater-disinfection process for ClO2, the following aspects were researched in this paper:optimization of ClO2preparation technics based on single-stage-R5-technology; reaction kinetics of ClO2preparation; composition and toxicity of ClO2solution produced; influences of various factors on dosage, disinfection effect and water quality in ClO2dosing process; self decay law of ClO2in water; and chlorite removal by powdered activated carbon (PAC) and ascorbic acid (AsA) from water. These studies can provide reference methods or theoretical guidance for the works such as:the process modification of ClO2generator based on the single-stage-R5-technology, the perfection of the ClO2dosing and disinfecting technics, and the effective removal of ClO2-from water, and so on.The researches of the dissertation are mainly focused on:(1) The optimization of ClO2preparation technics based on single-stage-R5-technologyAccording to a design of systemic optimization of the ClO2preparation process using single-stage-R5-technology, and based on the mechanism and the interaction conditions of various substances in ClO3-/Cl-reaction system, the batch preparation experiments of ClO2were conduced to explore the effects of such parameters as mole ratio of NaC103to HCl, temperature, reaction time, and catalysts on the NaClO3conversion rate, ClO2yield, and ClO2purity. The optimum conditions obtained are that:the mole ratio of NaClO3to HCl is0.25, the temperature is50℃, the reaction time is50min, and catalyst amount of MnCl2is0.01mol·L-1. Then the continuous preparation experiments of ClO2were performed under the optimum reaction conditions. Based on the analysis of experimental results and flow conditions of the continuous reactor, it was discovered that the low efficiency of continuous reactor mainly resulted from the uneven-mixing of raw materials, the hysteresis of heating up and the back-mixing of raw materials. Finally, by means of the preheating-and-premixing treatment of raw materials and using the larger-axis-diameter-ratio reactor designed according to principle of plug flow reactor, the mixing degree of raw materials and the temperature uniformity in reactor were improved, and the back-mixing of materials was reduced as well. The results show that NaClO3conversion and ClO2yield increase significantly, and NaClO3conversion rate, C102yield and ClO2purity can be up to95%,82%and61%and above, respectively. The values increase14,11and10percentage points respectively than the best results of present literatures.(2) Reaction kinetics of the ClO2preparation based on single-stage-R5-technologyBased on the general reaction mechanism of ClO3-/Cl-system and the characteristics of high acidity reaction system of NaClO3/HCl, the theoretical reaction rate equation derived by Hong-Lenzi was simplified utilizing equivalent substitution of ClO3-for Cl2, then the rate constants were determined by means of kinetic experiments, and the empirical rate equation of this reaction was finally deduced as the formula with mixed-order (combination of the first order and the second order with respect to ClO3-). This formula is convenient for engineering use as the parameters are fewer and easy to measure.(3) The composition, toxicity and security for tapwater disinfection of ClO2solution producedThe composition, toxicity, and concentration decay trends of the produced ClO2solution were determined. The security of the residual amounts of ClO3-and ClO2-contained in this ClO2solution used for drinking water disinfection was evaluated subsequently. The results show that the quantitiies of ClO3-and ClO2-introduced in drinking water accompanying with the disinfection process of produced ClO2solution are safe within the conventional ClO2dosages range of0.5to2mg·L-1, but the pH of treated water would decrease. The produced ClO2solution has no inducing effect on the mice bone marrow polychromatic erythrocytes micronucleus, its acute oral toxicity to mice is low-toxic class, but it is non-toxic class to mice when diluted more than one times. These results have certain instruction significance for the designs of the safe ClO2dosage, safe dilution ratio of ClO2solution produced, and the secure protection class of dosing process.(4) Influences of various factors on dosage, disinfection effect and water quality in ClO2dosing processBy means of the disinfection experiments of the ClO2generator using single-stage-R5-techology in conventional water plant, the effects of such operating parameters of generator as feed flow, reaction temperature and reaction time on sterilizing efficacy and finished water quality were investigated. The corresponding safty-operating parameters with certain range for drinking water disinfection were also obtained. Meanwhile, the influence of pipeline water pressure on ClO2dosage, its distribution characteristics in pipeline water, and disinfection effect of it were investigated as well through an experiment in imitating pipeline, and then these regulations were further verified in practical pipeline water. The referential tephnics-parameters were obtained finally available for ClO2generator dosing with pressure in pipeline water (5) Kinetic study of ClO2self-decay in waterThe decay rate equation of ClO2in pure water was established by measuring decay products and analyzing decay mechanism. The rate equation parameters were determined by testing the effects of temperature and pH on the decay rate. The results show that the products of ClO2decay in pure water are mainly as: Cl2(HC10), ClO3-, and few ClO2-, somewhat different from that of ClO2decay in alkaline water. The reaction order of ClO2decay in pure water is1with respect to ClO2, and-0.155with respect to H+. Obviously, ClO2is more stable in acidic or lower temperature water than in neutral, alkalescent, or higher temperature water.(6) Chlorite removal performance by powdered activated carbon (PAC) and ascorbic acid (AsA) from waterFirstly, the isotherms and mechanism of the sorption of ClO2-on PAC from water were studied by using the beaker-stirring experiments under the pH and temperature of conventional water treatment. The sorption Characteristics and thermodynamics were investigated by using Langmuir and Freundlich models to fit and discuss the isotherms. The sorption kinetics data were analyzed by using the models of pseudo first order, pseudo second order, intraparticle diffusion, and film diffusion to recognize the mechanism and rate control steps of the sorption. The results show that the sorption process of chlorite on PAC from water is spontaneous and endothermic with the mechanism of chemisorption. The sorption of chlorite on PAC is most favorable at pH=6. The sorption isotherms are well described by Freundlich model, and the sorption kinetics fits well the pseudo-second-order model. Therefore, the experimental results prove that the chemisorption reaction is the main rate-control mechanism of chlorite sorption by PAC.Secondly, the chlorite removal performance by AsA from water and its influencing factors were studied under the conditions of conventional water treatment. The effects of pH, temperature, AsA dosage, and action time on chlorite removal were investigated. The optimal conditions of treatment were obtained as that:pH=5.0, temperature was30℃, reaction time was25min, and mass ratio of AsA to chlorite was5.5. Under the optimal conditions and within the chlorite concentrations of2-10mg·L-1in water, the chlorite removal rates were more than98%. Therefore, the contents of residual ClO2-in treated water can be limited below0.02mg·L-1, in line with the requirement of GB5749-2006. |
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