Organic Azo Dye Reducing Decolorization Research

If the waste water effluent from textile industrial contaminated with high concentration dyestuff discharged into water system without been properly treated, will not only case a aesthetically uncomfortable, but also can raise the BOD, COD value of the water which is detrimental to aquatic life, even worse most of the azo dye and it’s metabolites are carcinogenic, teratogenesis and mutagenic causing severe environmental problem. Therefore, treatment of textile industrial effluents containing azo dyes prior their discharge into the nature water system is necessary. Flocculation, adsorption, membrane separation, ozone oxidation and advanced oxidation are conventional methods used in textile effluent treatment. However, physical/chemical methods have the inherent draw backs of being unable to completely remove the recalcitrant azo dyes, generating a significant amount of sludge that may cause secondary pollution, and involving complicated procedures. These draw backs make physical/chemical methods economical inefficient. Usually the molecular structure of Azo dyes are big and complicated which is recalcitrant to bio-treatment, so bio-treatment is not a proper way for azo dye waste water treatment. In this article we compared the decolorization capability of using hydrazine hydrate, formic acid, methanol, ethanol, oxalic acid as H donor, the result indicate that using hydrazine hydrate as H donor has a very good decolorization result. Hydrazine hydrate decolorization reaction can take place under ambient pressure and temperature,even though the reaction can treat light concentration azo dye water solution, fill the blank of light concentration azo dye treatment. Hydrazine hydrate can achieve a very high decolorization ratio in p-Aminoazobenzene, Methyl-Orange, Congo-Red decolorization reactions. The reaction rate is proportional to the reaction temperature and the pH value of the solution. The dyeing assistant reagents have negative effects for the hydrazine hydrate decolorization reaction. Kinetic studies show that the azo dye decolorization reaction fallowed the first-order kinetics in the presence of hydrazine hydrate. The Methyl-Orange decolorization reaction activation energy (Ea=59.236kJ· mol-1) were calculated from the decolorization experiment data. Depend on the decolorization products we indentified, we made a simple derivation of the decolorization reaction mechanism:First, the lone pair electron on hydrazine hydrate combined with azo dyes in the azo bond; second through the transference of the hydrogen atoms from hydrazine hydrate to the azo bond, the azo bond reducing to amino reagents, then the azo dye decomposed to corresponding amines.In order to find a convenient and efficient azo dye reducing decolorization method, we investigated the effect of introducing the N-heterocyclic carbene Pd catalyst into the reducing reaction. N-heterocyclic carbene Pd catalysts are widely used in Hack and Suzuki coupling reactions and have excellent catalytic capability in these reactions. Introducing Immobilized N-heterocyclic carbene Pd catalysts in water treatment have several good qualities:1.Reuseable;2. Environmental friendly;3. Small amount of Pd catalyst are needed. Most researchers focused their work on N-heterocyclic carbene Pd catalysts’ applications in dehalogenation reactions of aromatic halide. Catalytic decolorization of azo dyes is a new research field. This paper developed an imidazole carbene palladium complex immobilized on SBA-15catalyst; the palladium content of the catalyst is1.09%. The characterizations of the catalyst show that the palladium was immobilized on SBA-15through chemical bonds. We tested the catalytic capability of the N-heterocyclic carbene Pd catalyst in azo dye reducing reactions. The H donor selection experiment result show that using formic acid as H donor the catalyst can decolorize Methyl Orange, Orange II and Ponceau S in a very short time. The reduction rate is correlated with the reaction temperature; the higher the temperature is the faster the reaction is. Furthermore some decolorization reactions under detrimental conditions (low reaction temperature, high azo dye concentrations et.) may lead to catalyst poisoning and then the decolorization rate no longer increases. The catalyst reuse experiment result indicates that the catalyst still has catalytic activity after the third run of the reaction.