Study On The Rules Of Hydroxyl Radicals Generation In Electrochemical Catalytic Oxidation System And Its Applications In Wastewater Treatment

The intensification of industrial activities is making the water environment deteriorate increasingly. And some long-standing biorefractory organics do exist widely in the rivers and lakes, even in our drinking water, which bring us a lot of inestimable harms and to our production and lives. Ordinary technologies of wastewater treatment always can not deal with these kinds of wastewater effectively, their troublesomely performance even make second-pollutant. Advanced Oxidation Processes with the strong oxide hydroxyl radicals which can decompose organics completely are paid much attention by the researchers. In these years, it has come into notice that the study on application and preparation of Ti substrate coating metal-oxide electrodes with highly catalytic activity in the field of Advanced Oxidation Process and electrochemical catalytic oxidation more and more, as well as the study of radicals chemistry. The studies on the source of hydroxyl radicals, reaction mechanism and the intermediates are always the foreland in Electrochemical Oxidation Technology.In this thesis, the character of active out layer and the ability of producing hydroxyl radicals of electrode with out-layer of PbO2 doped Bi or La were studied on the base of pure Ti metal net which was used as the substrate with the intermediate layer of SnO2 doped Sb. The results show that when the PbO2 layer was doped with the oxide of La or Bi, the surface of electrode became more compact and active. The quantity of hydroxyl radicals and the capability of degradation of aromatic organic compounds were evidently superior than the electrode with nothing adulterated. La is an element in the third subgroup, there are only three electrons in the valence band. And the energy level of conducting band of it is lower than that of Pb(Ⅳ). So, the electrons are appreciated to transit from the valence band to the conducting band. When a positive ion lost an electron, it is called the"hole". The oxide of La increase the number of holes of PbO2 out layer, and these holes transfer to the surface of PbO2, then, a series of reactions take place in the system of PbO2-H2O, and then OH? and O2-? with strong oxidation were generated(chapter 3)In order to determinate the instantaneous concentration and the change rules of OH?, in this paper, salicylic acid was used as the"probe"reagent to trap OH? and then form the stable and easily determined intermediate 2,5-DHBA. Through the determination of 2,5-DHBA by HPLC with fluorescence detector, the OH? in the electrochemical catalytic oxidation system were quantified indirectly. This method allowed the direct investigation of OH? in aqueous and comparison of different OH? generated systems, and consequently provided a short cut for estimating the source of OH?, speculating on the mechanism of reactions and for the further study of electrochemical degradation of organic compounds. Virtually, the method provided an effective and reliable basis for the utilities of electrochemical catalytic oxidation in actual wastewater treatment. The results suggested that when direct current power was used as power supply and the out-put voltage was 2.20V, 150 min later the concentration of hydroxyl radicals came to 2.044μmol/L, whereas, when the effective out-put voltage of AC power with out-put frequency 523Hz was 1.17V, on the time of 60 min, the concentration of hydroxyl radicals reached 4.218μmol/L which was as ten times as that in DC power system. High frequency AC power produced a large number of OH?, these OH? were probably generated from the highly frequent exchange happened on the surface of two working electrodes, which accelerated the speed of physical adsorption and desorption of hydroxyl radicals and led to the concentration of OH? in aqueous rise sharply. Furthermore, the increase of frequency reduced the side reaction of H2O2 and the consumption of electric quantity. In additional, a couple of working electrodes enhanced the effective surface areas in a large degree, which made the efficiency of OH? generation increase evidently.In order to continue the further study of generation law of OH? in high frequency AC system, the factors of pH values, out-put frequencies and the concentrations of Na2CO3 (an scavenger of radicals) were studied in this thesis respectively. It was founded that high pH value of electrolyte and high frequency of the AC power are favorable for the generation of OH? in wastewater treatment, however, CO32- is opposite to them. Because the metal-oxide electrodes are extensively hydrophilic, when pH=11.0, a great deal of OH- groups were adsorbed in the stern layer of electrical double layer,the probability of OH? generation enormously rises then, much more OH? were trapped. The higher of out-put frequency was used, the more hydroxyl radicals would be obtained. The OH? under 523Hz out-put frequency condition were 3 times higher than that under 144Hz out-put frequency. Hoigne believed that the substance which can consume OH? and do not produce O2-? are the scavenger of radicals. The ordinary scavengers are CO32-, HCO3-, humus and so on. So, different masses of Na2CO3 were added into the electrolyte to investigate the circumstance of OH? generation. As a result, when the concentration of Na2CO3 reached 5g/L, the quantity of OH? detected descend by 89.0% than the solution without Na2CO3 in it. (chapter 4)In order to confirm the dominant role of OH? in combusting the organics by indirect oxidation of electrochemical oxidation technology, the effect factors on phenol degradation were studied in DC power system and high frequency AC power system respectively. The results show that the tides of phenol degradation were consistent with the rules of OH? generation entirely. The oxidation of aromatic compounds such as phenol was appropriate to take place, because the electrophilic reaction for aromatic ring made the displacement reaction and oxidation reaction become easy to happen. In the experiment about the effect factors of phenol degradation, the order of factors was gained as follow: initial concentration> materials of cathode>pH value> concentration of Na2SO4> current density. Through the calculation of electrochemical oxidation index, we draw a conclusion that the EOIs of phenol degradation under AC power system were at lease 2 times higher than that under DC power.(chapter 5)For fulfilling the fruits of research in actual wastewater treatment, some kinds of wastewater respectively containing phenol, nitrobenzene, emulsified oil and furfural were electrolyzed to checkout the real efficiency of organics degradation by high frequency AC power. Through the economic analysis, it was founded that there was a strong relationship between the initial COD concentration and the energy consumption per kilogram of COD. In this electrochemical system, the higher initial COD concentration of wastewater, the lower energy would be consumpted per kilogram of COD, so did the cost. So, this method is adapted to be used in the treatment of high concentration of organics. This is mainly because that increasing the concentration of organics which react with hydroxyl radicals make the probability of reaction between hydroxyl radicals and organics happen more often than before, and this reduced the lost of radicals. Secondly, because of the high concentration of organics in aquous, which made the distance between them shrinked, and the reactions between radicals and impurities (scavenger) were reduced. So, the probabilities for hydroxyl radicals to start and continue the chain reaction were increased. They all made the electrochemical efficiency increase.(chapter 6)...