Research On The Formation And Control Of Inorganic Disinfection By-products Of Chlorine Dioxide

As an alternative class A disinfectant of chlorine in drinking water disinfection,the advantages of chlorine dioxide?ClO₂?is with high efficiency,wide spectrum and almost not producing organic halogenation disinfection by-products.However,the formation of inorganic disinfection by-products?ClO₂^–and ClO₃^–?in the disinfection process,especially the ClO₂^–which may exceed the standard,has become an important problem to limit its safe application in the field of drinking water preparation.A plenty of researches have shown that the three main ways to generate ClO₂^–are ClO₂^–preparation process,self-decay process and reaction process with reducing substances in water.Therefore,this study started with these three main ways,carried out a systematic study on the formation law and mechanism of inorganic disinfection by-products,and explored the control method of ClO₂ inorganic disinfection by-products by taking actual water samples as the object.The researches of the dissertation are mainly focused on:?1?In this paper,the effect on the ClO₂ preparation efficiency are studied,which including the reaction time,reaction temperature,concentration of H₂SO₄,concentration of Na ClO₃and the mole ratio of H₂O₂ to Na ClO₃.The optimum conditions obtained are that:the reaction time is 60min,the temperature is 75?,the H₂SO₄ concentration is 6mol/L,the Na ClO₃concentration is 450g/L.In the process of preparing ClO₂ by R11 method,ClO₂^–was almost absent from the generation fluid.ClO₃^–is the main inorganic DBPs,as long as it comes from the incomplete reactant sodium chlorate,the content of which is related to the conversion rate of sodium chlorate.?2?The reaction order of ClO₂ decay is calculated to 1 in pure water by chemical reaction kinetics experiment.The kinetic equation model of self-decay reaction related to concentration and temperature is established as v=1.30×10⁶e^-5484.72/Tc^1.14,and the activation energy indicates that ClO₂ can decay at room temperature.Only a small amount of ClO₂^–is generated by self-decay of ClO₂in pure water,and the law of which is not obvious.With the increase of decay time,ClO₃^–concentration gradually increases,promoted by the ClO₂ initial concentration,temperature and illumination.?3?The inorganic reducing substances such as Fe???,Mn???and S^2-,as well as natural organic matter?NOM?play an important role of the formation of ClO₂^–,but the reaction mechanism is different.Fe???and S^2-will continue react with ClO₂^–after reducing ClO₂.Mn???will result in the formation of Mn O₂with adsorption and catalytic oxidation so as to affect the proceeding of the reaction.NOM hardly reacts with ClO₂^–.The ClO₂^–yield rate of inorganic reducing substances?70%?is higher than that of NOM?60%?.ClO₂ oxidation can convert large molecular weight substances of NOM to small molecular weight substances and hydrophobic substances to hydrophilic substances.The medium and large molecular weights?1K-100K Da?substances of NOM has the highest ClO₂^–yield rate.?4?ClO₂ disinfection experiments of three kinds of actual water samples showed that the concentration of inorganic reducing substance and natural organic matter in water jointly determined the amount of ClO₂^–.In the conventional water treatment process for the Yangtze river,coagulation and precipitation can control the production of ClO₂^–by removing humus substances in the water,which is 14.60%lower than the direct disinfection.ClO₂ pre-oxidation and coagulation process can significantly control the formation of ClO₂^–in the disinfection stage,but it needs to add a ClO₂^–removal process.After adding Fe²⁺removal process,ClO₂^–can reduce 26.53%.When using KMn O₄ as the pre-oxidant,ClO₂ can reduce 34.69%,but too much KMn O₄can even cause the opposite result.