A New And Secure Technology For Drinking Water Purification By Electrochemical Pre-Oxidation

The normal water treatment technology cannot meet the drinking water requirement at present because of more and more serious water source pollution. Several tough problems associated with the chlorination process are that the over standard disinfectant by-products are reported to be carcinogen and teratogenicity, and ozone disinfection process leads to generation of bromate. Development of a new drinking water treatment technology has become the most important research project because of the inevitable safety problems for the ordinary technology. There are two ways to solve the problem of disinfection byproducts. The first way is employing pre-oxidation process in advance before applying the conventional process to remove parts of pollutants. The second is employing advanced oxidation technologies to remove disinfection byproducts. The first method is considered to be better. In this dissertation, electrochemical pre-treatment of drinking water was systematically studied. The main work and results are as follows: (1) Raw water pre-oxidation by electro-synthesis of hydrogen peroxide:There are two types of electrochemical reactor for water treatment by electro-synthesis of hydrogen peroxide from water and O2. One is that the electrodes are vertically placed and water flows along the electrodes (parallel flow mode) and the other one is that water flows across the electrode mesh (cross-flow mode). The parallel flow mode with vertical electrode has low efficiency of O2utilization, thus increasing the cost of operation. The cross-flow mode can maximize the use of O2but its flow resistance is still high, thus decreasing the throughput of the treatment process. It will be better not to use diaphragm in water treatment industry due to the cost of investment. Furthermore, an intractable problem occurs associated with the disassembling and cleaning of the diaphragm when it’s contaminated. Therefore, a new membrane free electrolyzer was designed in this paper in which the dip angle of the electrodes was adjustable between0°and90°. The designed reactor improved the passage of oxygen and reduced gas resistance so as to make full use of oxygen. The electrolysis conditions of electro-synthesizing hydrogen peroxide without adding any chemical substance were systematically studied. Electro-synthesis of low concentration hydrogen peroxide for indirect oxidation in addition to direct oxidation by anode can reduce the energy consumption. The concentration of hydrogen peroxide generated on-site was5.56mg-L-1and current efficiency was above37%. The electrolysis conditions were as follows: current density:2mA·cm-2, electrode dip angle:between30and65°, water flux:250L·h-1and air flux:100L·h-1. The activated oxygen-containing species, such as O2·-、HO2-,were generated simultaneously during electro-synthesis of hydrogen peroxide. The hydroquinone was decomposed by activated oxygen-containing species through ring-opening reaction. The results indicate that, the activated oxygen-containing species like O2·-、HO2-and H2O2were generated during cathodic oxygen reduction process and these species had good indirect oxidation effect on cathode.(2) Modification of cathode material and efficient electro-synthesis of hydrogen peroxide:Graphite felt was electrochemically modified in diluted H2SO4using cyclic voltammetry for30minutes. Electro-catalytic activity of oxygen reduction and wettability were improved significantly after modification. Raman spectra result suggests an increasing disorder of the defects during the modification. The result of X-ray photoelectron spectroscopy (XPS) characterization indicates that oxygen-containing groups were introduced onto the graphite felt surface and carboxyl group exhibited the highest oxygen reduction activity. X-ray diffraction (XRD) results demonstrate that the crystallite size of graphite felt was reduced due to the etching by cyclic voltammetry. Rotating disk electrode (RDE) characterization shows that the oxygen reduction on the modified graphite felt electrode took place through approximate two-electron reaction at low potentials. Chronoamperometry (CA) results indicate that the modified graphite felt exhibited good stability.(3) Study on the electro-catalytic oxidation degradation of dye wastewater with three-dimensional electrodes:In contrast to traditional plate electrode, three-dimensional electrode has a larger area-volume ratio and higher mass transfer rate. We prepared supported activated carbon as three-dimensional particles, and methyl orange was selected as a target dyeing pollutant. The various parameters affecting the performance of the decoloration of methyl orange and removal of COD were systematically studied. The spectrophotometry affirmed more hydroxyl radicals and superoxide were generated in the three-dimensional electrode system than that for the two-dimensional system due to the formation of microelectrodes at high potentials. The removal of methyl orange in the three-dimensional electrode system was mainly dependent on the oxidation by these substances, and the degradation of the dye waste followed a pseudo-first-order reaction.(4) Studies on the removal of carbon tetrachloride by bimetal alloy: As a kind of widely employed organic chloride, carbon tetrachloride has biological toxicity, which is hard to decompose and seriously pollutes groundwater. The reductive degradation of carbon tetrachloride with bimetal alloy was investigated in this dissertation. The degradation influence factors were examined through batch test and column filtering experiments. (5) Pilot test of electrochemical pre-oxidation:The designed electrolyzer equipped with Al-Fe alloy filter column was conducted for treatment of Yellow River water. Yellow River water was first pre-oxidized by the activated oxygen-containing species through electrolysis, followed by deep oxidation with strong oxidizing hydroxyl radical, which was generated from hydrogen peroxide by means of catalysis on the surface of Al-Fe alloy. In addition, comparison between electrochemical pre-oxidation and chemical pre-oxidation was made and the cost of electrochemical pre-oxidation was also discussed.