Electrochemical Oxidation Technology For The Treatment Of Refractory Industrial Organic Wastewater

With the rapid development of the human economic society, the refractory organic wastewater from industry has become the pressing problem in the world, due to its complexity characteristic and the presence of toxic and refractory organic compounds. Owing to its high efficiency, environmental compatibility, and amenability to automation, the electrochemical method has attracted a great deal of attention and become the most promising advanced oxidation technology for the treatment of such kinds of wastewater. However, at present, low current efficiency and high energy consumption are still the main drawbacks of this technology, which hinder the wide application of this technology in the industry. Therefore, as for the treatment of refractory industrial organic wastewater, this thesis thoroughly investigated the hot spots of electrochemical oxidation technology. This study has important practical significance and academic research value on the electrochemical oxidation technology development, which can improve its industrial practicablity and promote its engineering application. This study prepared and modified the existing catalytic anode materials (PbO2 and SnO2 ceramica materials) and did research on the mechanism. Several innovative achievements were obtained on the study of a three-dimensional electrode reactor to the treatment of the actual coking wastewater. The main contents and results were summarized below:(1) As for the PbO2 electrode, the intermediate layer between Ti substrate and PbO2 film with a high oxygen evolution potential (OEP) can enhance the electrochemical catalytic activity of the whole electrode by shifting the OEP positively. The SnO2 doped with Sb was chosen as the intermediate layer in this investigation. According to Wsb/(Wsb+Wsn) (atom quality ratio) of the intermediate layer used, the PbO2 electrodes of 0.5 wt.% or 8 wt.% had the highest OEP and exhibited the highest electrocatalytic activity that 4-hour TOC removal rate of phenol in the electrolysis can attain 68% around.(2) During the electrodeposition process of PbO2 film, a low current density can help enough Pb2+ ion permeate into the porous cauliflower structure of Sb-SnO2 intermediate layer through the mass transfer and diffusion effect, and encourage the formation of the PbO2-SnO2 composited electrode with a porous surface and an interconnected architecture. The results of the electrolysis experiments and accelerated service life tests indicated that this PbO2-SnO2 composited electrode can evaluate the electrocatalytic activity of PbO2 electrode by the exhibition of the high electrocatatlytic activity of the intermediate layer and improve the service life of the electrode, which can attain 120 h in the accelerated service life tests.(3) The electrocatalytic activity of PbO2 electrode can be optimized by the control of the acidity and the concentration of F" in the electrodeposition bath. Through the micro-stucture analysis, an appropriate improvement of acidity or the content of F" in the bath could elevate the degree of crystallization of PbO2 film and small the grain size of particles. This changes in physical property of the PbO2 film can effectively increase the real surface area of the electrode surface, enhance the ability of adsorbing oxygen species and improve the electrocatalytic activity of PbO2 electrode for the oxidation of organic pollutants.(4) The SnO2 ceramic material was firstly explored to be the anode in the electrochemical oxidation process for the treatment of organic pollutants. The properties of SnO2 ceramic were characterized by component and phase analysis. The electrochemical property of SnO2 ceramic was also investigated by electrochemical tests. The results showed that SnO2 is the main components of the ceramic material, including a small amount of Fe, Cu and Sb dopping elements. The ceramic material has a similar rutile type crystal structure with SnO2 coated electrode, and the crystallinity is higher than the latter. The OEP of the SnO2 ceramic electrode was also similar with that of the SnO2 coated electrode(>2.0 V vs. SCE) and exhibited a good electrocatalytic activity during the treatment of simulated wastewater containing 200 mg/L p-nitrophenol (PNP). With a current density of 20 mA cm-2, the removal rate of PNP can attain 100%, the TOC removal rate can reach 88% round after 4-hour electrolysis, which was higher than that of PbO2 and BDD electrodes. The degradation kinetics of PNP follows a pseudo-first-order kinetics model and the electrochemical oxidation comprehensive degradation mechanism was established correspondingly. Additionally, in the electrode stability tests, the SnO2 ceramic electrode exhibited good electrochemical stability in the solution containing a certain concentration (1 M) of electrolyte, such as Na2SO4, NaOH and H2SO4.(5) As a preliminary treatment for the subsequent desalinization process, a combination of heterogeneous electrolysis and homogeneous photolysis was developed to mineralize bio-refractory organic pollutants in the wastewater. The RSM was used to assess the individual and interactive effects of several operating parameters (initial substrate concentration, pH, current density and Na2SO4 concentration) on the mineralization efficiency of p-nitrophenol. The results indicated that both pH and current density were found to be the most significant positive factors that affected this combined process. The most suitable conditions for PNP mineralization in this combined process were:a pH value around 9,0.175～0.25 M Na2SO4, a current density larger than 25 mA cm-2. The LC-MS analysis results depicted that the denitration and substitution by hydroxyl radicals on aromatic rings might be the first step of attack. But the detection of nitrated aliphatic compound indicated that there might be another aromatic ring opening pathway occurred before the nitro group released from the aromatic ring.(6) With respect to the individual electrolysis and photolysis process, a synergetic effect on the mineralization rate of PNP was found in the combined process of electrolysis and photolysis. In Na2SO4 medium, the synergetic effect was primarily attributed to the dissolved oxygen mainly provided by the side reaction of electrolysis process continuously, which can act as the electron acceptor for the degradation of organic pollutants in the photolysis reactions. Therefore, the reutilization of oxygen elevated the photolysis efficiency and improved the synergetic effect of this combined process.(7) A three dimensional electrode reactor was designed to the treatment of coking wastewater. Pyridine, quinoline and indole, were chosen as the representatives of the nitrogen heterocyclic compounds in the coking wastewater and were treated by this three-dimensional electrolysis. The mineralization results verified that these nitrogen heterocyclic compounds can be effectively mineralized in this three-dimensional electrode reactor. As for the wastewater simulated by these three organics, the 3-hour TOC removal rate can attain 70%. The order of mineralization rate of these three compounds is:indole> quinoline> pyridine. Besides that, experimental runs using actual coking wastewater were conducted to evaluate the effect of the operating parameters of this three dimensional electrode reactor, such as the anode material, the packed particle type and the current density. When the current density was 40 mA cm-2, in the three-dimensional-electrode system with RuO2 anode and GAC as the packed particle electrodes, COD removal rate can attain 73% in 4 hours and the energy consumption was 43.01 kWh (kg COD)-1 around. This three dimensional electrode technology has a good prospect for the industrial application as the pretreatment or the advanced treatment for the biochemical processes.