Preparation And Properties Of Cationic Starch-based Flocculant

With the development of industrialization and the increasng demand for water resources,the amount of sewage discharged is also increasing.Under the condition of water shortage in China,the sewage treatment and recycling are particularly important.The commonly used flocculation method is a economical and convenient sewage treatment method.At present,the flocculants in common use have many disadvantages,such as large dosage,high cost,residual monomer and residual metal ions affecting water quality and difficulty in biodegradation.A kind of cationic starch flocculant was synthesized by graft copolymerization methacryloyloxyethyl trimethylammonium chloride(DMC)as monomer,ceric ammonium nitrate as initiator.The optimum synthesis conditions were studied by the Orthogonal and single factor test,and the grafting rate of the product synthesized under this conditions was107.4%.TheetherifyingagentGTAwaspreparedwith3-chloro-2-hydroxypropyltrimethylammonium chloride(CTA)in an ice bath.A cationic starch flocculant St-GTA with a degree of substitution of 0.7173 was utilized by the nucleophilic substitution reaction of starch and GTA with NaOH as a catalyst.By the reaction of these two monomers and starch,the ternary cationic starches St-GTA-g-PDMC and St-g-PDMC-GTA with nitrogen content of 4.09%and 4.17%were prepared by the single factor testes.The structures and properties of the cationic starch flocculants were characterized by IR,~1H NMR,SEM,XRD and Zeta potential2%kaolin suspension was used as the simulated water sample.When the dosage of St-g-PDMC was 8 mg/L,the turbidity could be reduced to 60.85 NTU.At this time,the zeta potential deviated from the zero point potential,and the flocculation mechanism was the charge fragmentation mechanism.When the dosage of St-GTA is 10 mg/L,the turbidity could be reduced to 46.04 NTU.At this time,the Zeta potential was on the brink of the zero point potential.The adsorption neutralization played an important role in the flocculation process..When the dosage of St-GTA-g-PDMC was 4.75 mg/L,the lowest turbidity was40.88 NTU.At this time,the Zeta potential was positive and far from the zero point potential,indicating that the flocculation mechanism was dominated by the charge fragmentation mechanism.When the dosage of St-GTA-g-PDMC was 4.5 mg/L,the minimum turbidity was 50.06 NTU,and the Zeta potential was close to zero.The neutralization effect of adsorption was the main flocculating mechanism.Adsorption bridging and net trapping existed in all the four flocculation processes.The effects of settling time,temperature,pH value and initial turbidity of the kaolin suspensions on the flocculation performance of St-GTA-g-PDMC were investigated.The results showed that when the settling time was less than 1 h,the urbidity of the upper layer of kaolin decreased obviously.When more than 1 h,the turbidity is basically stable and decreases slowly.The flocculant is little influenced by the temperature change.When the temperature was 45°C,the flocculation effect was best.When the temperature was 45°C and the dosage was only 2.5 mg/L,the turbidity of kaolin supernatant could reach 52.63NTU.Acidic or alkaline conditions could affect the performances of the flocculant,the flocculation effect was best under the neutral condition.With the initial turbidity of the kaolin suspension increasing,,the optimum amount of flocculant St-g-PDMC-GTA increased linearly,and the minimum turbidity of the supernatant liquid decreased linearly.Tested with the domestic sewage from the primary sedimentation tank of the sewage treatment plant of Qiaoxi District of Shijiazhuang City,the performance of two ternary cationic starch flocculants St-GTA-g-PDMC and St-g-PDMC-GTA were tested and compared with that of PAC and CPAM..The results showed that the flocculant effects of St-GTA-g-PDMC and St-g-PDMC-GTA were superior to that of PAC and CPAM when the flocculant was used alone.When the modified starch flocculants compounded with PAC,the dosage of PAC decreased significantly,,and the turbidity of the domestic sewage was reduced below 10 NTU.The remaining CODcr value was less than 50 mg/L and the removal rate was over 93%.The water quality achieved GB18918-2002 Class A emission standards.