Study On The Application Of Starch-based Flocculants In The Treatment Of Hairwork Wastewater

China is the world's largest producer of wigs,and the manufacturing generates large quantities of hairwork effluents.Hairwork wastewater is a kind of wastewater with high turbidity,high chemical oxygen demand(COD),high ammonia nitrogen,high color and complex composition.Coagulation/flocculation is the most widely applied wastewater treatment process because of high efficiency,convenience and low cost.It is usually the first step for industrial wastewater treatment to remove the solute,colloid and larger suspended particles.Also,coagulation/flocculation is an important step in the hairwork wastewater treatment process.The final coagulation/flocculation performance is usually largely dependent on the coagulants/flocculants selected.However,traditionally inorganic-metal-based and synthetic-organic-polymeric flocculants are known contributors to secondary pollution and health risks due to the residual metal ions,as well as the release of toxic organic monomers.Therefore,the research of green and efficient natural polymer-based flocculant has become a hot topic.Among natural polymer materials,starch with abundant sources and hydroxyl groups,which is easily modified,has attracted more and more attention.In fact,many starch-based flocculants with good flocculation performances have been obtained.In this study,two versions of cationic starch-based flocculants,starch-3-chloro-2-hydroxypropyl triethyl ammonium chloride(St-CTA-DQ)and starch-graft-poly[(2-methacryloyloxyethyl)trimethyl ammonium chloride](St-g-PDMC-DQ),have been successfully prepared.Fourier transform infrared spectroscopy(FTIR),1H nuclear magnetic resonance(1H NMR),and zeta-potential measurements were used to characterize the structure and solution properties of the starch-based flocculants.In addition to two environmental parameters,pH and flocculant dose,the effects of cationic group contents and distributions,on the flocculation of the hairwork effluents were investigated systematically.Besides,St-CTA-DQ and St-g-PDMC-DQ were used in combination with poly aluminum chloride(PAC),respectively.And the effects of the dose and the dosing sequence of starch-based flocculant and PAC on the combined flocculation performance were also investigated.The experimental results are as follows:(1)For the treatment of the hairwork wastewater,the coagulation properties of the two versions of starch-based flocculants and a traditional inorganic coagulant,PAC,were compared in detail.It was found that both St-CTA-DQ and St-g-PDMC-DQ showed much better effect than PAC.The optimal dose of PAC was 190 mg/L and the residual turbidity was 23.2 NTU;the optimal dose of St-CTA-DQ was 20-40 mg/L and the residual turbidity could be reduced to approximate 15 NTU;the optimal dose of St-g-PDMC-DQ was 30-40 mg/L,and the residual turbidity could be reduced to about 10 even 5 NTU.Compared to PAC,the two versions of starch-based flocculants demonstrated bigger and more compacted flocs,which was favorable for settlement and separation.On the one hand,a higher cationic content(degree of substitution for St-CTA-DQ and grafting rate for St-g-PDMC-DQ)led to better flocculation performance,i.e.,lower optimal dose as well as higher removal rates of both turbidity and UV254,demonstrating that charge neutralization was the main coagulation mechanism.On the other hand,flocculation performance worsened as the pH level increased.These phenomena were primarily due to improved charge neutralization with higher contents of cationic groups or lower pH.(2)In order to further improve the flocculation performance,the two versions of starch-based flocculants were used in combination with PAC.The experimental results show that the dose of PAC could be significantly reduced and the floc properties were also improved in the combined flocculation process.When the dose of St-CTA-DQ4 was 10 mg/L,the dose of PAC decreased by more than half.Similarly,when St-g-PDMC-DQ2 was added,the optimal dose of PAC could be reduced to 90 mg/L.In addition,the dosing sequence of PAC and the starch-based flocculants also had a significant effect on the flocculation performance.The process of adding PAC first and then polymer was better than the other two processes,i.e.PAC fed after the polymeric flocculants and adding them simultaneously.When adding PAC first,the pollutants in hairwork wastewater could be combined with Al3+ and form primary flocs by charge neutralization.Then the cationic starch-based flocculants could not only occupy the remaining negatively charged binding sites of pollutants through charge neutralization but also show effective bridging effects because of dual effects of cationic polymeric flocculants,i.e.coagulation and flocculation.In summary,these two versions of starch-based flocculants,St-CTA-DQ and St-g-PDMC-DQ,either alone or combined with PAC for flocculation of hairwork wastewater,showed good performance.Therefore,they have a good prospect in industrial wastewater treatment including hairwork wastewater.