Catalytic Function Of Fiber Degradation Of Dyes And Other Organic Pollutants

At present, water pollution has become a major issue in many countries, particularly in developing countries. Dye pollutant from dyestuffs and textile industry is a dangerous source of environment contamination. The conventional treatment technologies widely used in industry for eliminating dyes include adsorption and biological treatment. However, adsorption treatments simply transfer dyes from solution to adsorbents, and cannot completely eliminate them. Biological treatments are limited to some extent due to the long treatment period, the larger occupied area of equipments and the biological toxicity of various dyes. Recently, advanced oxidation processes (AOPs), which have high degradation rates, have received increasing attention as a promising technology for the efficient destruction of dyes and other organic pollutants, while some oxidation processes need energylight sources, like ultraviolet light, and may bring secondary pollution. Therefore, there is a clear need for more efficient and eco-friendly approaches to removing dyes and other organic pollutants from wastewater.Metallophthalocyanine derivatives (MPcs), having analogous structure to the active center of cytochrome P-450, are highly conjugated macrocyclic compounds and have been extensively studied as catalysts for a variety of applications in organic synthesis, green chemistry and environmental treatment. Recently, MPcs have received increasing attention in some catalytic oxidation systems using H₂O₂ as oxidant. We note that dyes in the textile industry is mainly used for fiber dyeing, and the mechanism of dyeing is the enrichment and fixation processes of dyes from solution to fiber in a very short time. In light of the above considerations, we demonstrated that a new method,"phase transfer in situ catalytic oxidation,"which is based on fiber supported MPcs (catalytic functional fiber), can be applied to eliminate various dyes. Because fibers have a high natural affinity to dyes by physical and chemical interaction, dyes can be enriched or adsorbed onto the fiber, and be oxidized rapidly and effectively in situ at the surface and interior of fiber. This method combines common adsorption and AOPs together. The advantage of this method is that dyes can be transferred quickly from aqueous phase to adsorbent material (fiber) and be decomposed in situ immediately in the presence of oxidant and on-site catalyst. This method resolves the problem of adsorption saturation and improves the environmental adaptability of AOPs. The oxidative process of dyes in the presence of catalytic functional fiber is a heterogeneous catalytic reaction. Catalytic functional fiber is recuperated easily from dyes solution for the reuse, and causes no secondary pollution. The phase transfer in situ catalytic oxidation has proven itself to be a feasible approach, which may be potentially applied to the elimination of widely existing pollutants. This dissertation has gained some valuable results both in academic disciplines and practical applications.1. The phthalic anhydride-urea route was employed to synthesize cobalt tetranitrophthalocyanine, which was then reduced into cobalt tetraaminophthalocyanine (CoTAPc). A novel aqueous soluble and highly reactive metallophthalocyanine derivative, cobalt tetra(2,4-dichloro-1,3,5-triazine)aminophthalocyanine (Co-TDTAPc), was prepared by modification of CoTAPc with cyanuric chloride and characterized by UV/Vis, FTIR, TGA, and so on. Because of the high reactive groups in the side chains, Co-TDTAPc was immobilized on the cellulosic fiber by covalent bond to obtain a novel cellulosic fiber supported MPcs catalysts (Co-TDTAPc-F).2. Based on"phase transfer in situ catalytic oxidation", we choose the inexpensive and eco-friendly H₂O₂ as the sole oxidant in our reaction system. The Co-TDTAPc-F/H₂O₂ system can efficiently decompose various dyes, including C. I. Acid Red 1 (AR1), C. I. Reactive Red 2, C. I. Reactive Red 24, C. I. Reactive Red 195, C. I. Reactive Yellow 145, C. I. Reactive Blue 221 and C. I. Direct Red 31. The catalytic activity of Co-TDTAPc-F in the heterogeneous system is evidently higher than that of Co-TDTAPc in the homogeneous system. This catalytic reaction can proceed at a wide pH range from acidic to alkaline in the Co-TDTAPc-F/H₂O₂ catalytic system, although lower pH resulted in higher oxidation rate. This result is distinct from the traditional Fenton system where the oxidation can only take place at the pH lower than 3. Co-TDTAPc-F is stable, causes no secondary pollution and remains efficient in repetitive test cycles with no obvious degradation of catalytic activity. This system has no negative effect caused by NaCl as encountered in other systems; instead, NaCl is an accelerant for the current catalytic oxidation. Therefore, it is an additional advantage for our system to be used practically. The intermediates and by-products formed in the catalytic oxidation of dyes were examined by HPLC, TOC, FTIR and GC/MS. The results show that the intermediates include phenol and naphthol analogy complexes. Then the opening of the phenyl-ring and naphthyl-ring occur to form small molecular biodegradable aliphatic carboxylic compounds such as oxalic acid, malonic acid and maleic acid etc. Some of the intermediates can be mineralized into CO2 and H2O. The fiber phase plays an important role in the whole catalytic oxidation process, improves the decomposition rate of dyes by concentrating dyes and catalytic active sites in the reaction system, prevents the formation of dimeric phthalocyanine and provides a microenvironment that is prosperous to the catalytic reaction. Importantly, the Co-TDTAPc-F/H₂O₂ system can overcome the effect of negative factors, such as ethanol and isopropanol. By contraries, these additives can accelerate the degradation of dyes. In addition, Co-TDTAPc-F has a high catalytic ability to degrade chlorophenols (CPs) such as 2-chlorophenol, 4-chlorophenol, 2,4-dichlorophenol and 2,4,6-trichlorophenol in the presence of H₂O₂. The process of decomposition of CPs in the Co-TDTAPc-F/H₂O₂ system includes the dechlorination of CPs and the opening of benzene rings. The main intermediates are oxalic acid, maleic acid and succinic acid, which can also be further degraded in this system. Therefore, the catalytic oxidation in the Co-TDTAPc-F/H₂O₂ system leads to a deeper oxidation.3. Carbon fiber (CF), carbon nanofiber (CNF) and multiwalled carbon nanotubes (MWNTs) used to support MPcs catalysts (CoTAPc-CF, CoTAPc-CNF and CoTAPc-MWNTs) were prepared using covalent immobilization of CoTAPc on them, and characterized by X-ray photoelectron spectroscopy, attenuated total reflection Fourier transform infrared spectra and thermogravimetric analysis. The oxidative removal of rhodamine 6G (Rh6G) in the presence of CoTAPc-CF or CoTAPc-CNF was investigated by examination of UV/Vis absorption spectra. The results showed that Rh6G was oxidized efficiently in the CoTAPc-CF/H₂O₂ or CoTAPc-CF/H₂O₂ system at neutral pH and room temperature. The introduction of CF and CNF resulted in a marked enhanced catalytic activity that CoTAPc does not have. In order to further investigate the effect of carbon fiber materials on the catalytic performance of MPcs, the more structured MWNTs with less surface defects were chosen as the support. Electron paramagnetic resonance spin-trap experiments indicated that CoTAPc-MWNTs have a novel non-radical pathway, which is different from common CoTAPc catalytic systems. On the basis of the online electrochemical measurements in the CoTAPc-MWNTs/H₂O₂ system, we inferred that MWNTs participated directly in the electron transfer during the catalytic oxidation process and generated abundant holes, which provided local high electric potential at active sites for the oxidation of the adsorbed Rh6G. In this catalytic system, MWNTs provide strong adsorption to conjugated Rh6G due to their special sp2 hybridized surface, and are able to rapidly oxidize the conjugated dye by a special electron transfer pathway. In addition, CoTAPc-MWNTs/regenerated cellulose composite fiber (CoTAPc-MWNTs-F) was prepared by solution spinning and was used to catalyze the oxidation of Rh6G. The results indicated that CoTAPc-MWNTs-F/H₂O₂ system could efficiently oxidize Rh6G in acidic, neutral and alkaline conditions, thereby improving the reaction environment of cobalt phthalocyanine and greatly broadening the scope of application of cobalt phthalocyanine in catalytic oxidation fields. The enhanced catalytic performance of cobalt phthalocyanine by inducing theÏ€-conjugated carbon fiber materials may provide a new strategy for the design of highly efficient oxidation catalysts.4. In order to further develop"phase transfer in situ catalytic oxidation"for removing other organic pollutants, activated carbon fiber (ACF) was chosen as the support for the MPcs due to its extremely high adsorption capacity and unusual chemical stability. We immobilized Co-TDTAPc covalently on ACF to obtain a novel heterogeneous catalyst, Co-TDTAPc-ACF. The Co-TDTAPc-ACF/H₂O₂ system can efficiently remove phenols such as 4-nitrophenol (4-NP) across a wide pH and temperature range. Importantly, compared with homogeneous Co-TDTAPc used alone, the introduction of ACF contributed specifically to the activity enhancement of Co-TDTAPc. This might be attributed to the fact that ACF provides a predominant enrichment of catalytic active sites and improves catalytic oxidation efficiency by concentrating substrates from solution dozens, and even hundreds, of times. Gas chromatography/mass spectrometry analysis demonstrated that most of the phenols was oxidized into less-toxic and more-biodegradable compounds, such as maleic acid, succinic acid, malic acid and adipic acid, etc. Controlled experiments showed that the presence of 2-propanol, as hydroxyl radicals scavenger, has little influence on 4-NP oxidation. The result of electron paramagnetic resonance spin-trap experiments indicated that ACF result in a different reaction pathway from free radicals pathway to non-radical mechanism, and open up a new channel to specifically enhance the catalytic activity of cobalt phthalocyanine. Repetitive tests showed that Co-TDTAPc-ACF can maintain high catalytic activity over several cycles, and it has a better regeneration capability under mild conditions. Furthermore, as an improved phase transfer catalytic oxidation system, Co-TDTAPc-ACF/H₂O₂ may be identified as a potential conventional approach to treat wastewater by combining enrichment and catalytic oxidation together. Depend on the self-reliance regeneration capability of Co-TDTAPc-ACF in the presence of H₂O₂, it is expected to provide a technology capable of being operated in a continuous and efficient mode to treat organic pollutants.