INTRODUCTION Vietnam Academy of Science and Technology Institute of Tropical Technology *** PHAM THI MINH Research characteristic of azo organic compounds mineralization in dye textile wastewater by e[.]
Vietnam Academy of Science and Technology Institute of Tropical Technology …… ….***………… PHAM THI MINH Research characteristic of azo organic compounds mineralization in dye textile wastewater by electro-Fenton method Major: Theoretical chemistry and Physical chemistry Code: 62 44 01 19 SUMMARY OF CHEMICAL DOCTORAL THESIS Hanoi - 2014 The thesis was completed at: Institute of Tropical Technology Vietnam Academy of Science and Technology Supervisors: Assoc Prof Nguyen Thi Le Hien Assoc Prof Dinh Thi Mai Thanh Referee 1: Assoc Prof Mai Thanh Tung Referee 2: Assoc Prof Vu Thi Thu Ha Referee 3: Assoc Prof Nguyen Thi Cam Ha The thesis will be defended in front of doctoral thesis judgement, held at Institute of Tropical Technology - Vietnam Academy of Science and Technology at A.M, May 20th , 2014 The thesis can be found at: - Vietnam Academy of Science and Technology library - Vietnam National library INTRODUCTION The urgency of the thesis Recently, the more strongly the process of industrialization andurbanization develop, the worse the pollution problem is getting One of the most industries which make highly pollution is dyeing textile industry With thousands of small manufactories from traditional villages, post-produced wastewater is hardly handled before flowing directly into lakes, rivers, which caused environmental pollution The dye textile wastewater contains a lot of organic compounds, especially azo pigment This is a kind of the stable, persistent and toxic compound Researching and treating wastewater containing azo compound is an important problem in order to remove all these substances before discharging it into the environment, protect human and ecological environment There are many treatment methods of toxic organic compounds such as the method of physics, biology or chemistry However, these direct methods are still limited In recent times, the advanced oxidation methods have been researching by the many scientists The nature of this method is the formation process of HO • with strong oxidizing ability, it can completely oxidize toxic organic compounds to form CO2 and H2O Many different advanced oxidation methods have been effectively applied such as the methods of ozone, Fenton, electro-Fenton Among them, electro-Fenton method not only is confirmed to have many advantages but also can easily combine with other advanced methods to absolutely treat the toxic organic compounds, minimize the environmental pollution Electro-Fenton method is the oxygen reduction process to create H2O2, then H2O2 will oxidates transition metal ions such as Fe2+, Cu2+ to create HO• or HO2• radicals with high oxidability The oxygen reduction creating H2O2 depends on the nature of the cathode material The recent surveys showed that the composite electrode which was made from spinel-structured complex oxides of transition metal mixing in with conductive polymers such as polyanilin, polythiophen has good catalytic ability for the oxygen reduction to create H2O2 However, the research to find out the suitable material to apply in pollute organic compounds treatment is a new research approach, which has not been much interested by domestic and foreign scientists For the reasons mentioned above, the thesis "Research characteristic of azo organic compounds mineralization in dye textile wastewater by electro-Fenton method” has been carried out The objectives of the thesis - Synthesize the spinel-structured complex oxide Cu1,5Mn1,5O4 - Synthesize Ppy and Ppy(oxit)/Ppy coatings on carbon substrate - Determine the optimal mode for azo compounds mineralization - Treat the dye textile wastewater in reality by electro-Fenton effect 3 The main contents of the thesis - Synthesize and research the composition, structure, surface morphology of spinel-structured complex oxide Cu1,5Mn1,5O4 - Synthesize and research the characteristics of Ppy and Ppy(oxide)/Ppy coatings - Electrochemical characteristics of platinum anode and carbon solution containing azo compounds cathode in - Mineralize some azo compounds by electro-Fenton methods - Treat dye textile wastewater by electro-Fenton methods Chapter OVERVIEW 1.1 The dye textile wastewater 1.1.1 Sources of dye textile wastewater The wastewater source arises in dye textile industry from the stages of starching, removing, cooking, easing, dyeing and finishing The main pollution problem in the dye textile industry is water pollution Considering two factors: the volume and the composition of wastewater, dye textile industry causes the biggest pollution 1.1.2 Characteristics of dye textile wastewater The characteriristic of dye textile wastewater in general and the dye textile wastewater in Van Phuc and Duong Noi villages in particular is all containing coloring organic compounds, thus indicators such as pH, DO, BOD, COD… are very high, exceeding allowed standards to discharge into ecological environment 1.1.3 Main pollutants in dye textile wastewater Main pollutants in textile wastewater are dyes, surface-active substances, organic halogen compounds Among them, dyes are the most difficult compositions to be treated, especially azo dyes widely-used nowadays, those account for 60-70% of the market share 1.1.4 The dyes commonly used in Vietnam Today, Vietnamese often use dyes such as direct dyes, acid dyes, active dyes, bases dyes, reverted dyes, sulfur dyes, dispersed dyes, azo dyes and pigment dyes Most of them contain azo link (- N=N-) in the molecular 1.1.5 The general concept of azo coloring compounds Azo compounds, the synthetic coloring compounds, contain the -N=N- link and popularly applied in many industries (foodstuff, printing and dyeing ) This is the kind of stable, persistent and toxic compound to human health and ecological environment 1.2 The methods of dye textile wastewater treatment 1.2.1 The actuality of dye textile wastewater pollution in Vietnam In recent years, although the environmental field has been particularly concerned by the state, a number of companies, factories and most of the handicraft dye textile villages still directly discharge post-produced untreated wastewater into rivers, lakes and canals which causes serious environmental pollution 1.2.2 The actuality of azo-inflected wastewater treatment technologies 1.2.2.1 The traditional methods The traditional methods of dye textile wastewater treatment have been applied, including: absorbing methods, physical methods, oxidative methods and microbiological methods 1.2.2.2 The advanced oxidative methods Advanced oxidative method is the process of decomposition compounds based on • OH active free radicals which are created immediately in liquid medium in the treatment process The •OH radical which is very active, can oxidate without selecting organic compounds to form CO2, H2O and mineral salts 1.2.2.3 Some common advanced oxidative processes In recent time, many advanced oxidative methods have been applied quite effectively, such as ozonation, Fenton, electro-Fenton, photo-Fenton 1.2.3 Electrochemical method Electrochemical process allows eliminating or minimizing the pollutants by directly oxidizing way of pollutants on the electrode by currents or creating oxygen agents in the environment which is capable of oxidizing toxic organic compounds There are two methods to create hydroxyl radical: directly oxidizing at the anode or indirectly oxidizing on the cathode by electro-Fenton effect 1.2.3.1 Directly electrochemical oxidation at the anode to create hydroxyl radical This method allows oxidating water to create hydroxyl radical absorbed on the anode surface with high overpotential: H2O → •OH ads + H+ + e (1) 1.2.3.2 Electro-Fenton method Electro-Fenton method is a combination of the Fenton effect and the current flowing through the anode and cathode electrodes In this method, H 2O2 and Fe2+ may be added in a solution with pH acid and imposed static current to decompose organic compounds In addition, H2O2 which can be generated by the reduction of O2 on the cathode (reaction 2) will immediately combine with Fe 2+ presented in the solution to create strong oxidizing agents, HO• ( reaction 3) O2 + 2H+ + 2e → H2O2 Fe2+ + H2O2 → Fe(OH)2+ + •OH (2) (3) Although Fe2+ is rapidly decomposed by HO• to form Fe2+ and HO- (reaction 4), the reaction still continues due to the regeneration of Fe 2+ according to the reaction 5,6,7 Fe2+ + •OH → Fe3+ + HO- (4) Fe3+ + 1e (5) → Fe2+ Fe3+ + R• → Fe2+ + R+ (6) Fe3+ + H2O2 → Fe2+ + HO2• + H+ (7) Ion Fe2+ regenerates mainly due to the reduction of Fe 3+ by H2O2 according to reaction The organic substance will be destroyed by HO• agent which was born from the reaction and HO2• agent which was born from reaction according to the diagram: Organic compounds + HO•, HO2• → products (8) Organic compounds treated by Electro-Fenton method was assessed quite effectively because the HO• radical was not only created in the solution from the reaction but also formed on the anode electrode from the H 2O oxidation (reaction 9) H2O → HO• + H+ + e (9) Dissolved oxygen reduction process that occurs according to the mechanism of receiving 2e to create H2O2 (reaction 2) depends on the nature of the cathode material Therefore, cathode material which are capable of electrochemically catalyzing for oxygen reduction in order to create H 2O2 is one of the most important factors that decide the performance of wastewater treatment by electroFenton method 1.2.4 Application of electro-Fenton effect for mineralizing azo compounds in dye textile wastewater In recent years, the researches on applications of electro-Fenton method to treat azo dyes in dye textile wastewater have been interested by many domestic and foreign scientists However, the way of researching which uses composite cathode material with the catalytic ability to oxygen reduction reaction to create H 2O2 is still very limited Hence, the thesis focused on researching to produce and using the composite cathode materials based on the basis of Cu 1,5Mn1,5O4 complex oxide and conductive Ppy 1.2 Polypyrol (Ppy) and composite Ppy(Cu1,5Mn1,5O4)/Ppy Recent researches show that Ppy coating, especially Ppy(oxide)/Ppy has good catalytic ability for the reduction of the oxygen to produce H 2O2 on the cathode, setting the basis for research of the application this material in polluted organic compounds treatment Chapter CONDITION AND EXPERIMENTAL METHOD 2.1 Chemicals and equipments * Chemicals: The chemicals which were used in this study have high-purity, including: CuSO4.5H2O, MnSO4.H2O, KOH, Pyrol, KCl, H2SO4, NaOH, Na2SO4, red methyl, orange methyl, red Congo, FeSO4.6H2O, diphenyl amine, K 2Cr2O7, Ag2SO4, HgSO4, NaOH, carbon graphite, wastewater was removed from two dye textile villages: Van Phuc and Duong Noi * Equipment: - Electrochemical Netherlands) measurement equipment AUTOLAB 30 (Eco., Co., - STEREOSCAN 440 LEICA equipment paired with LEO software - UV-Vis equipment (UV-spectrometer, CINTRA 40 – The USA) - pH measurement equipment named PHM 210 Standard pH Meter (France) - Annealed machine named DRB 200 HACH- Japan - The USA - Magnetic stirrer heating machine - Analytical Electronic scale, Toledo manufacturer (Switzerland) 2.2 Condition and experimental method 2.2.1 Synthesize complex oxides of Cu and Mn: Complex oxides of Cu and Mn were synthesized by co- precipitation method from CuSO4, MnSO4 and KOH solution 2.3.2 Electrochemical method - Static-current method: Synthesize Ppy coating and Ppy(Cu1,5Mn1,5O4)/Ppy composite at mA/cm2 in 1000 seconds - Static-potential: Identify catalytic efficiency and stability of Ppy Ppy(Cu1,5Mn1,5O4)/Ppy coatings at E = -0.5 V/SCE, in 2000 seconds, which corresponds to the oxygen reduction voltage creating H2O2 - Steady-potential method: Determine the value of feedback current at steady-state, which characterizes the electrochemical reaction speed at the researched potential 2.3.3 The analysis method: - The methods of SEM, TEM, EDX, X - Ray: Analyze the surface morphology, composition and structure of oxide and Ppy(oxit)/Ppy coating - Ultraviolet visible spectra (UV-Vis) method: This method allows evaluating the performance of azo coloring compounds degradation through absorbed intensity by the formula: (10) With: is the degraded efficiency (%), A0 is absorbed intensity at first, At is absorbed intensity at t time - Chemical oxygen demand (COD) determination method: + COD degraded performance in mineralize process was assessed by the formula: % COD = COD*100%/CODo + Current efficiency in mineralize process was assessed by the formula: H (%) = (11) - Color-grade measurement method: Allows evaluating color removal of wastewater solution before and after treatment - The methods of survey and investigation: Determine some mostly-used dyes at some traditional dye textile villages Chapter RESULTS AND DISCUSSION 3.1 Characteristic of Cu1,5Mn1,5O4 complex oxide Surface morphology, structure and molecular composition of Cu and Mn complex oxide were analyzed by scanning electron microscopy methods (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectra (EDX) and X-ray diffraction spectra (X-Ray) Results are shown in figure 3.1, 3.2, 3.3 and 3.4 The results which were measured by SEM, TEM, EDX and X-Ray methods showed that Cu and Mn complex oxide after being synthesized is small, fine particles, spinel-structured with molecular formula Cu1,5Mn1,5O4 TEM Figure 3.1 SEM image Figure 3.3 EDX spectra of Cu and Mn complex oxide Electron diffraction Figure 3.2 TEM image Figure 3.4 X-Ray spectra of Cu and Mn complex oxide 3.2 Synthesize and electrochemical catalytic ability of Ppy(Cu1,5Mn1,5O4)/Ppy coatings Ppy and 3.2.1 Synthesize Ppy and Ppy(Cu1,5Mn1,5O4)/Ppy coatings on carbon substrate electrode The voltage change in the Ppy and Ppy(Cu 1,5Mn1,5O4)/Ppy coatings in the synthesis process is shown in Figure 3.5 At the first phase of the synthesis process, voltage sharply increases to a stable value, equivalent to the polymerization process of pyrol into polypyrrole which is accumulated on the anode electrode surface With the presence of oxide synthesis voltage significantly reduced by the activation area of the electrode increases, which makes synthesis current density reduces and leads to reduced potential Figure 3.5 Synthetic Curves of Ppy and Ppy(Cu1,5Mn1,5O4)/Ppy coatings Figure 3.6 EDX spectra of Ppy(Cu1,5Mn1,5O4)/Ppy coating 3.2.2 Characteristic of Ppy Ppy(Cu1,5Mn1,5O4)/Ppy coatings 3.2.2.1 The component of Ppy(Cu1,5Mn1,5O4)/Ppy coating: The component of Ppy(Cu1,5Mn1,5O4)/Ppy coating was analyzed by EDX spectra (figure 3.6) The results showed that Cu and Mn complex oxide was present in the component of Ppy(Cu1,5Mn1,5O4)/Ppy coating after synthesizing 3.2.2.2 The electrochemical catalytic ability to oxygen reduction reaction creating hydrogen peroxide Ppy and Ppy(Cu1,5Mn1,5O4)/Ppy coatings Electrochemical catalytic ability of the C/Ppy and C/Ppy(Cu 1,5Mn1,5O4)/Ppy cathode electrodes at different pH values was represented by the curves: current – static potential, allowing the determination speed of oxidants reduction reaction on the studied electrode (figure 3.7) When there is the presence of oxygen, with high oxygen content, it led to cathode currents sharply increased starting with the potential +0.2V/SCE, corresponding to the oxygen reduction reaction to create H2O2, confirming the presence of H2O2 by KI + starch Besides, when there was oxygen aeration, the cathode current density of Ppy coating containing oxide was always greater than the cathode current density of Ppy coating without oxide This is proved that Ppy(Cu1,5Mn1,5O4)/Ppy coating had the catalytic ability for the dissolved oxygen reduction better than Ppy coating By observing the graph describing feedback cathode current density of the C/Ppy(Cu1,5Mn1,5O4)/Ppy electrode at different pH values in potential -0.5V/SCE (figure 3.8), it showed that feedback cathode current density reached the highest value at pH3 This proved that the C/Ppy(Cu1,5Mn1,5O4)/Ppy cathode electrode had the best capable of electrochemical catalyst for oxygen reduction reaction to create H2O2 at pH3 10 The results showed that: When [Fe 2+] was lower than mM, this content was not enough to completely react with H2O2 content that was generated, so the low HO• radical concentration lead to the low mineralization efficiency When Fe 2+ concentration increased to mM, %COD and H reached the highest value at any time of mineralization This proves that mM Fe 2+ concentration was enough to completely react with H2O2 that was created on the cathode electrode to create HO • radical When [Fe2+] was higher than mM, this content did not react completely with H2O2 content, the surplus Fe2+ ion content would be oxidized to create Fe3+ The surplus redox couples Fe3+/Fe2+ would create a continuously redox cycle, so currents efficiency increased Figure 3.13 Influence of Fe2+ ion concentration for the variation of %COD and H with time 3.4.3.2 Influence of cathode electrode material Influence of cathode electrodes C, C/Ppy and C/Ppy(Cu 1,5Mn1,5O4)/Ppy on performance of red methyl mineralization was surveyed in 0.05 M Na 2SO4 solution, pH3, mM Fe2+, with l/min oxygen aeration rate at mA/cm imposed current density The variation of %COD, H and decomposing efficiency () in red methyl mineralization with time was assessed by COD identify method and UV-Vis spectra analysis method The results was shown in 3.14, 3.15 and 3.16 Figures, it proves that all of them had electrochemical catalytic ability for oxygen reduction process to produce H2O2 on cathode electrode It showed that %COD in red methyl mineralization process for cases also continuously increased with mineralization time However, in case C/Ppy(Cu 1,5Mn1,5O4)/Ppy cathode electrode was used, at any mineralization time, %COD and H also reached the highest values The results proves that C/Ppy(Cu1,5Mn1,5O4)/Ppy cathode electrode material had good 14 electrochemical catalytic ability for oxygen reduction process to produce H 2O2, which was applied in red methyl mineralization by Electro-Fenton method Figure 3.14 The variation of %COD and H with time Figure 3.15 UV-Vis spectra of red metyl with time Figure 3.16 The variation of h red metyl with time 3.4.3.3 Influence of the imposed current density Influence of the imposed current density on 0,35 mM red metyl mineralization efficiency by Electro-Fenton method was conducted in 0.05 M Na 2SO4 solution, pH3, mM Fe2+, C/Ppy(Cu1,5Mn1,5O4)/Ppy cathode electrode, liter/min oxygen aeration rate The variation of %COD, H and h in red metyl mineralize process with time was assessed by COD identify method and UV-Vis spectra analysis method The results were shown in 3.17, 3.18 and 3.19 figures 15 Figure 3.17 The variation of %COD and H with Q charge Figure 3.19 The variation of h red metyl with Q charge Figure 3.18 UV-Vis spectra of red metyl with time The results of identification COD (figure 3.17) showed that, at same electrical charge values, in case mA/cm2 current density was imposed, %COD and H red metyl mineralization efficiency all reached higher values than in case imposed other current density The results were explained as following: when the imposed current density was high enough, it will be suitable for the oxygen reduction process to create hydrogen peroxide on cathode electrode, so red metyl mineralization efficiency by Electro-Fenton method increased However, when the current density was higher, H2O oxidation process would occur to create O2 on anode electrode which led to electrical consuming of mineralization, at the same time on cathode electrode occured H+ ion reduction reaction to create H leading to the decrease of the reaction speed to create H 2O2 Therefore, the speed of formation HO• radical (reaction 2) decreased which led to the decrease of %COD and H in mineralization Results of the UV-Vis spectra analysis showed that, when imposing mA/cm current density, maximum absorption intensity of methyl red at 523 nm 16 wavelength decreased rapidly with mineralization time (Figure 3.18), respectively reached the highest value (88%) compaired with imposing other current densities (figure 3.19) 3.4.3.4 Influence of oxygen aeration rate Figure 3.20 illustrated the influence of oxygen aeration rate on the variation of %COD and H with 0,35 mM red metyl mineralize time in 0.05 M Na 2SO4 solution, pH3, mM Fe2+, C/Ppy(Cu1,5Mn1,5O4)/Ppy cathode electrode, liter/min oxygen aeration rate, at mA/cm2 imposed current density With liter/min oxygen aeration rate, %COD and H at anytime of mineralization all achieved higher values than compairing with 0.5 l/min oxygen aeration speed This confirms that liter/min oxygen aeration speed which creates saturated oxygen content in solution is the optimal speed Figure 3.20 The variation of %COD and H with mineralization time By eye observing (figure 3.21) showed that, the color removal of red methyl solution clearly changed from dark pink to light pink with mineralize time, after hours of mineralization at mA/cm2 current density, the color of solution was almost completely removed The color of the solution changed very fast in a short period of mineralize process, this is explained that: at the first time of mineralize process, red methyl concentration rapidly decreased to create intermediate compounds before being completely mineralized to create CO2, H2O and minerals 17 Before treatment After 1h treatment After 2h treatment After 5h treatment Figure 3.21 The color change of methyl red solution with mineralization time 3.5 Red congo mineralization 3.5.1 Influence of cathode electrode material Influence of cathode electrode material on 0,25 mM red congo mineralization was made in conditions: 0.05M Na2SO4 solution, pH3, mM Fe2+, C/Ppy(Cu1,5Mn1,5O4)/Ppy cathode electrode, liter/min oxygen aeration rate, at mA/cm2 imposed current density Results of COD identification and UV-Vis spectra analysis were shown in figure 3.22, 3.23 and 3.24 Observing figure 3.22 showed that for all of the cases using C; C/Ppy and C/Ppy(Cu1,5Mn1,5O4)/Ppy cathode electrodes, %COD gradually increased with mineralization time However, when using C/Ppy(Cu1,5Mn1,5O4)/Ppy cathode electrode, at anytime %COD all reached higher values than the cases using C and C/Ppy cathodes Figure 3.22 The variation of % COD and H with red congo mineralize time For all types of electrodes, the red congo absorption intensity at 560nm wavelength all rapidly decreased (figure 2.23), corresponding to the degradation N = N - structure led to the discoloration of the red congo solution Besides, in the spectra, another spectrum in the ultraviolet at 320 nm wavelength with absorption intensity gradually increased This is predicted that there is the presence of 18 intermediate products containing benzene aromatic ring The results allow confirming that, in the electrolysis process, - N = N - azo link has been broken to create hydrocarbon derivatives With time, produced hydroxyl radical will destroy the benzene ring, lead to red congo was completely mineralized to create CO 2, H2O and other minerals Results of the UV-Vis spectra analysis (Figure 3.24) showed that 0.25 mM red Congo destroyed efficiency reached the highest value when using the C/Ppy(Cu1,5Mn1,5O4)/Ppy cathode electrode This reconfirmed that, C/Ppy(Cu1,5Mn1,5O4)/Ppy cathode electrode had electrochemical catalytic ability for oxygen reduction process to produce H2O2, so HO• radical produced reaction speed increased (reaction 1.6), thus red Congo destroyed efficiency increased Figure 3.23 UV-Vis spectra of red congo with time Figure 3.24 The variation of h red congo with time 3.5.2 Influence of the imposed current density Influence of current density on efficiency of red congo mineralization by ElectroFenton method was surveyed in 0.05 M Na 2SO4 solution, pH3, mM Fe2+, C/Ppy(Cu1,5Mn1,5O4)/Ppy cathode electrode Results of COD identification and UV-Vis spectra analysis were shown in figures 3.25 and 3.26 When imposing 1mA/cm2 current density, at different times, the same Q electricity charge value went through, % COD, H and h always reach higher values than those of 0.5; and 10 mA/cm2 imposed cases This result is entirely consistent with the results of red methyl mineralization This reconfirmed that mA/cm current density is the optimal value in the survey scope of the thesis 19 Figure 3.25 The variation of %COD and H with Q electricity charge Figure 3.26 The variation of h with Q electricity charge 3.6 Orange methyl mineralization Orange methyl mineralization was conducted in conditions 0.05M Na 2SO4 solution, pH3, 1,0 mM orange methyl, mM Fe 2+, C/Ppy(Cu1,5Mn1,5O4)/Ppy cathode electrode at other imposed current densities Results of COD identification and UV-Vis spectra analysis in mineralization were shown in the figures 3.27, 3.28 and 3.29 The results showed that at the same Q electricity charge values, with 1mA/cm2 imposed current density, % COD, H and orange methyl all reach much higher value than those of 0.5; and 10 mA/cm2 imposed case The results demonstrated orange methyl can be completely mineralized by Electro-Fenton method to product CO2, H2O and minerals Figure 3.27 The variation of %COD and H with Q electricity charge 20