Anaerobic Biodegradation Of Azo Dye In Textile Dyeing Wastewater Enhanced By Quinone-modified Chitosan

With the increasing demand for textiles,the more dye products are accumulated in textile dyeing wastewater,which is considered one of the major contamination problems in the world.Azo dyes are the most widely used dye species because of their wide spectrum and low cost.Its stability makes it difficult to be degraded by ecosystems through their self-regulation.According to the researches,there are many methods used to azo dyes degradation.Anaerobic biodegradation has become the mainstream method of azo dyes degradation,since it is a relatively simple,economical,secondary pollution-free and non-specific reduction process.Anaerobic microorganisms are more sensitive to toxic pollutants,azo dyes,than the aerobic microorganisms,which makes the metabolic rate is slower.At the same time,the electronic transmission is easily blocked under the anaerobic condition.Therefore,the anaerobic biodegradation of the azo dyes should be enhanced.Redox mediator has been proved that can enhance the anaerobic biodegradation of azo dyes.However,they might be lost with the effluent during the treatment process in a continuous flow anaerobic reactor,resulting in high treatment costs and the risk of secondary contamination.In order to solve these problems,the study of immobilized redox mediator has become one of the hot topics in recent years.Hence,in this thesis,chitosan was taken as the substrate material.The novel immobilized redox mediator,quinone-modified chitosan,had been synthesized,which was synthesized from anthraquinone-2-sulfonate sodium(AQS)by chloride acetylation and then chemically branched onto the chitosan.Through the Fourier Transform Infrared Spectroscopy(FTIR),Nuclear Magnetic Resonance Hydrogen Spectrum(H~1 NMR),Scanning Electron Microscope(SEM)and other means,the ball-forming method of the quinone-modified chitosan was determined and optimized.Then,the reaction dynamics were studied by model fitting,including adsorption and anaerobic biodegradation.Finally,the process operating parameters were optimized and the carbon balance was analyzed under optimized conditions to further research the process performance.The main conclusions we obtained were given as follows.(1)Quinone-modified chitosan was synthesized from AQS by chloride acetylation and then chemically branched onto the C6-OH of chitosan,and the chemical structure was determined.According to the analysis of FTIR and H~1 NMR,anthraquinone-2-sulfonyl chloride(ASC)and quinone-modified chitosan had obvious absorption peaks at the absorption bands of asymmetric telescopic vibration and symmetrical telescopic vibration caused by-SO₂-X,and both had distinct radon characteristic peaks of anthraquinone.The appearance of benzene ring adjacent hydrogen peak,?=6.91,7.00,7.09 ppm,in H~1 NMR proved that the ASC was successfully grafted to the C6-OH of chitosan.At the same time,compared with the quinone-modified chitosan deprotonized balls formed by sodium hydroxide,the ion crosslinking balls crosslinked by sodium tripophosphate were better mechanical and more stable due to the presence of a large number of chains.According to the analysis of FTIR,the peaks near 1384.36 cm^-1and 1071.62 cm^-1 of ion crosslinking balls had been weakened at the same time,which indicated the P=O bond was successfully introduced,(2)Quinone-modified chitosan balls were been prepared and the method had been optimized.The p H of sodium tripophosphate affected the ion crosslinking progress of the quinone-modified chitosan.With the decreasing of the p H,the degree of crosslinking was higher,the mass of a ball and the mechanical strength also increased.Moreover,both the progress of adsorption and the anaerobic biodegradation of quinone-modified chitosan balls conformed to the secondary dynamics model.The adsorption property of the low cross-linking quinone-modified chitosan balls was relatively strong,which from high to low were TPP₁₀-AQS@CS>TPP₈-AQS@CS>TPP₆-AQS@CS and the corresponding equilibrium adsorption capacity were 1.86 mg/g,1.69 mg/g,1.68mg/g for the respective.The electronic transport capabilities from high to low were TPP₆-AQS@CS>TPP₈-AQS@CS>TPP₁₀-AQS@CS(the corresponding values were 0.26±0.12 m Eq/g,0.20±0.46m Eq/g,0.18±0.14 m Eq/g for the respective).However,the enhancement of the anaerobic biodegradation from high to low were TPP₈-AQS@CS>TPP₆-AQS@CS>TPP₁₀-AQS@CS(the corresponding decolorization efficiencies were 98.34%,98.25%,96.33%for the respective).That indicated that there was synergy between adsorption and redox in the anaerobic biodegradation system of methyl orange.In this enhanced system,the enhancement of the TPP₈-AQS@CS had a better decolorization effect which made the efficiency increase from 81.6%to 98.34%.Hence,in this study,a quinone-modified chitosan sol and sodium triphosphate solution(0.027 M,p H=8)were selected to prepare the quinone-modified chitosan balls.(3)The technological parameters of anaerobic biodegradation of azo dye enhanced by quinone-modified chitosan balls were optimized,and the enhancement mechanism of quinone-modified chitosan balls was proved by carbon balance analysis under the optimal conditions.There was synergy between COD_{carbon source}:COD_{azo dye} and m _TPP-AQS@CS:m_sludge to the decolorization of azo dye,methyl orange(MO).The decolorization efficiency without adding carbon source and quinone-modified chitosan balls(COD_{carbon source}:COD_{azo dye}=0,m _TPP-AQS@CS:m_sludge=0)was only 49.82%.The decolorization efficiency adding carbon source(COD_{carbon source}:COD_{azo dye}=2.5,m _TPP-AQS@CS:m_sludge=0)was increased to 86.34%.However,the decolorization efficiency adding carbon source and quinone-modified chitosan balls(COD_{carbon source}:COD_{azo dye}=2.5,m _TPP-AQS@CS:m_sludge=2.5)was increased to 97.52%.A regression fitting model of decolorization efficiency had been established between COD_{carbon source}:COD_{azo dye} and m _TPP-AQS@CS:m_sludge.According to the mathematical model,the optimized technological parameters could be got,which COD_{carbon source}:COD_{azo dye} is 3.3839,m _TPP-AQS@CS:m_sludge is 0.1353.Under this condition,the results of carbon balance analysis indicated that quinone-modified chitosan balls could enhance the anaerobic biodegradation of MO(the decolorization efficiency increased from 87.54%to 97.92%,)while slow down the inhibition of methane-producing microbiome by the addition of MO(the accumulation methane production increased from 4.9 ml to 10.05 ml).After anaerobic biodegradation,the carbon distribution of experiment group adding quinone-modified chitosan balls was 8.7:0.7:0.6(VFA:CH₄:MO),while that without adding quinone-modified chitosan balls was 9.3:0.3:0.4.