Evaluation Of Starch-based Flocculants For Flocculation Of Dissolved Organic Matters From Textile Dyeing Secondary Wastewater

As one of major textile manufacturers in the world,China have generated and discharged large quantities of dyeing wastewaters every year.With the deteriorating situation of water resources and more concerns from the public,the environmental issue caused by textile dyeing effluents has become one of focuses of social attention.Therefore,the advanced treatment and reuse of textile dyeing secondary effluents can meet requirements of both alleviating water resources shortage and preventing water pollution.Among various technologies in treatment of secondary dyeing effluents,flocculation is one of the most widely applied methods in terms of low-cost,easy-operation and high-efficiency.Moreover,it can evidently alleviate the load of contaminants removal by the subsequent processes and reduce the operation cost.However,the flocculant always plays a key role in the flocculation process.Nowadays,inorganic metal-based flocculants and synthetic polymeric ones are two of the most extensively applied flocculants.However,the increased metal ions concentrations of inorganic flocculants or residual noxious monomers of synthetic polymers in water,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,the applications of natural-polymer-based flocculants in water treatment have attracted more attentions,due to their advantages of wide-resource,low cost,non-toxicity,and high efficiency.Furthermore,they have been even acclaimed as"Green Flocculants of 21st Century".However,several disadvantages of natural polymers such as poor reactivity,low molecular-weight and bad water solubility have limited their practical applications.Chemical modifications are employed to improve their application performances.In this current work,three different starch-based flocculants with various chain architectures and charge properties have been prepared through etherification,graft copolymerization,or their combinations,i.e.two cationic ones,(starch-graft-poly[(2-methacryloyloxyethyl)trimethyl ammonium chloride]and starch-3-chloro-2-hydroxypropyl trimethyl ammonium chloride,denoted as STC-g-PDMC and STC-CTA respectively),and an amphoteric flocculant(carboxymethyl starch-graft-poly[(2-methacryloyloxyethyl)trimethyl ammonium chloride],denoted as CMS-g-PDMC).Then,flocculation of dissolved organic matters(DOM)in dyeing secondary effluents by those three starch-based flocculants was investigated and compared with polyaluminium chloride(PAC)in detail.For further exploration of the flocculation mechanisms,humic acid(HA)and bovine serum albumin(BSA)were selected as main representatives of DOM in dyeing secondary effluents,i.e.protein-like extracellular and humic/fulvic-like matters respectively according to the characteristics of studied wastewater based on its three-dimensional excitation-emission matrix spectrum.In the experiments of removing two simulated pollutants from the synthetic wastewater,those three flocculants have shown far different flocculation efficiency and flocs properties due to their distinct structural features.The effects of pH,flocculant dose,and initial containment concentration including structural factor on flocculation of HA and BSA have been studied systematically.Several interesting results have been drawn as shown below,(1)On the basis of the changes of the dissolved organic concentration(DOC)and ultraviolet absorbance at 254 nm(UV254)before and after flocculation of the textile dyeing secondary effluents,STC-g-PDMC owns much better flocculation performance than other flocculants,showing the highest removal efficiency and a lowest dose.The optimal dose of STC-g-PDMC for flocculation of real dyeing secondary effluent is only one-third of that of conventional PAC.The maximal DOC and UV254 removal percentage of STC-g-PDMC are both more than 60%while those of PAC are only 20-30%.It might be attributed to both its strong cationic feature and branch chain architecture.(2)Then,HA was used to simulate the humic/fulvic-like matters in dyeing secondary effluents.On the basis of the experimental results,STC-g-PDMC and CMS-g-PDMC with strongly cationic branch chains have much better flocculation performance than PAC and STC-CTA,the latter of which features linear chain architecture and strongly cationic pieces lying on its chain backbone.It indicates that the architecture of cationic branch chains plays an important role in HA flocculation due to their significantly enhanced bridging effects.Moreover,STC-g-PDMC has higher HA removal efficiency and better floc properties than CMS-g-PDMC,suggesting that charge neutralization effects make notable contributions to HA removal and that the additional anionic pieces on CMS-g-PDMC can weaken its flocculation performance.In addition,STC-g-PDMC used as coagulant aid for PAC has also been tried,which observably reduces the optimal dose of the inorganic coagulant.(3)Finally,BSA was employed to simulate the protein-like extracellular matters in dyeing secondary effluents.According to the experimental results,STC-g-PDMC with cationic branches has remarkable advantage over other flocculants due to its more efficient charge neutralization and bridging flocculation effects.Therefore,the optimal dose of STC-g-PDMC is much lower.In addition,another interesting finding in this work was the reaggregation phenomenon after restabilization at overdoes during the flocculation of BSA effluents by STC-g-PDMC at very narrow pH range near neutral condition,which might be ascribed to some special interactions between STC-g-PDMC and amino acid fragments of BSA.Above all,the flocculating experiments indicate that STC-g-PDMC exerts quite excellent flocculating performance for flocculation of dissolved organic matters from aqueous solutions due to its distinct branch chain architecture and shows significant potential in terms of practical applications.Moreover,this obtained result is of great importance in guiding the design and selection of a suitable polymeric flocculant in treating a target wastewater.