Preparation Of Cellulose-modified Flocculant And Its Application To Dye Flocculation In Water

With the rapid development of printing and textile industry,the environmental pollution is becoming more and more serious,which has been drawn much attention.In addition to increase of the chemical oxygen demand(COD)of the water body,the dye effluents are highly visible and interfere with light penetration into water thereby retard photosynthesis and inhibit the growth of aquatic organisms seriously.Among various technologies for treatment of the dye-containing wastewater,flocculation is one of the mostly widely applied methods in terms of low-cost,easy-operation and high-efficiency.Nowdays,inorganic metal-based flocculants and synthetic polymeric ones are two of the most extensively applied flocculants.However,the increased concentration of metal ions of inorganic flocculants or residual noxious monomers in synthetic ones when these chemicals are used may have potential disadvantages to both ecosystem and human health.Therefore,development of safer,cheaper and more effective flocculants is still one of the hottest topics in flocculation.Recently,more attentions have been paid to natural polymer based flocculants in water treatment,since they are believed to be wide-resource,nontoxic,environment-friendly and high-efficient materials.Furthermore,they have been even acclaimed as "Green Flocculants of 21st Century".However,disadvantages such as poor reactivity,low molecular weight and poor solubility have limited their practical applications.Chemical modifications are employed to improve their performancesIn this work,cellulose has been modified by etherification,grafting and hydrolyzing to prepare an anionic flocculant(hydrolyzed polyacrylamide grafted carboxymethyl cellulose,CMC-g-HPAM)and a cationic one(carboxymethyl cellulose-graft-poly[(2-methacryloyloxyethyl)trimethyl ammonium chloride],CMC-g-PDMC).These two cellulose-based flocculants are applied to remove a cationic dye(methylene blue,MB)and an anionic one(acid green25,AG25)from aqueous solutions,respectively.The effects of both internal(grafting ratio and hydrolysis degree)and external factors(pH,flocculant dosage and salt additives)on flocculation of dyes have been studied systematically.Furthermore,zeta potential measurement and image analysis in combination with fractal theory has been employed to investigate the flocculation mechanism in detail.Several results have been drawn:(1)The modified anionic cellulosed-based flocculant,CMC-g-HPAM,exhibits much better flocculation performance for removal of MB from aqueous solutions and the colour removal efficiency can be more than 90%under optimal condition.At the same time,the flocs produced by this flocculant are considerably larger and denser.Investigation of pH effect indicates that higher dye removal efficiency would take place at alkaline conditions(pH=9).Furthermore,the flocculation performance of CMC-g-HPAM has been improved significantly with increasing hydrolysis time and hydrolysis degree.Based on experimental results,hydrolysis time is better controlled in 3h in order to save time and energy and maintain high flocculation efficiency.The final improved flocculation performance of CMC-g-HPAM is ascribed to the cooperative effects of the charge neutralization and bridging flocculation mechanisms.(2)The modified cationic cellulosed-based flocculant,CMC-g-PDMC,can efficiently flocculate AG25 from aqueous solutions at different pH levels and the colour removal efficiency is nearly 100%.It is due to the enhanced charge neutralization and bridging effects for improvement of both positive charge and molecular weight of the polymeric flocculants after PDMC branches grafted.Furthermore,the grafting ratio plays an important role in both flocculation performance and flocs properties.To obtain higher colour removal efficiency as well as larger and more compactness flocs,the grafting ratio of CMC-g-PDMC should be controlled around 180%.The flocculation performance of this cellulose-based flocculants with various grafting ratio shows quite different pH dependence.The dye removal efficiency of CMC-g-PDMC with high grafting ratio is pH independent for stronger positive charge and higher molecular weight,while that of CMC-g-PDMC with relatively low grafting ratio would be reduced in alkaline conditions.Moreover,the cationic cellulose-based flocculant shows good salt resistance to most salts investigated in this work.Above all,the flocculating experiments indicate that these two cellulose-based flocculants bear quite good flocculating performance for removal of dyes from aqueous solutions and show wide prospects of practical applications in terms of low-cost and environment-friendly.