Study On The Coagulation And Flocculation Properties Of Water-soluble Chitosan Derivatives

Chitosan, the deacetylated derivative of chitin, is one of the abundant renewable,nontoxic and biodegradable polymers. Chemical modification is one of the importantways to improve the application performance of chitosan. In this study, two kinds ofamphoteric chitosan-based flocculants, i.e., carboxymethyl chitosan (CMCS) andquaternized carboxymethyl chitosan (QCMC), have been prepared. CMCS wasprepared from chitosan with chloroactic acid as the carboxymethyl reagent first, andthen N-quaternary ammonium group was introduced by the reaction of CMCS with2,3-epoxypropyl trimethyl-ammonium to get QCMC. The weight-average molecularweight and deacetylation degree of chitosan, which used for the preparation of CMCSand QCMC, was23.2×10~4and92.7%, respectively. The chemical structure ofchitosan derivatives was characterized. And their coagulation properties have beeninvestigated using kaolinite suspension and four kinds of reactive dye wastewater assimulated water by jar test. The effect of experimental conditions on the coagulationefficiency was investigated in detail.The main contents and conclusions are summarized as below:(1) Carboxymethyl chitosan (CMCS) and quaternized carboxymethyl chitosan(QCMC) have been prepared and characterized by FT-IR,1H-NMR and XRD,respectively. The degree of substitution (DS) of carboxymethyl group was64.4%andthe DS of the quaternization group was98.4%. It proved that the prepared chitosanderivatives were O-carboxymethyl chitosan and O-carboxymethyl-N-(2-hydroxyl)propyl-3-trimethyl ammonium chitosan chloride.(2) The coagulation properties of CMCS and QCMC have been investigatedusing kaolinite suspension by jar test. The effects of the settling time, hydraulicconditions, pH value, dosage of the coagulant, and initial turbidities on thecoagulation properties have been investigated in detail. The results indicated that bothof CMCS and QCMC performed a coagulation process. When the concentration of coagulant was low(≦5mg/L), the turbidity removal rate of kaolinite suspensions(initial turbidity50~300NTU) reach up to70%~97%. Settling time or hydraulicconditions showed a limited effect on the coagulation performance. The optimal pHvalue range of QCMC was wider than CMCS, the turbidity can be removed byQCMC in the pH range of3~9. To reach the optimal coagulation performancerequired more CMCS and QCMC at a high pH value. The optimal CMCS dosageincreased with decreasing initial turbidity of kaolinite suspensions, the turbidityremoval rate decreased with decreasing initial turbidity when using optimal CMCSdosage. The optimal QCMC dosage increased with increasing initial turbidity ofkaolinite suspensions. It is presumed that the coagulation process induced by CMCSor QCMC is dominated by charge neutralization and bridging coagulation mechanism.And the coagulation process of kaolinite suspensions exhibits a dynamic changing,which is simultaneously responsible for the two kinds of driving forces.(3) CMCS and QCMC also have been used for the treatment of dye ReactiveBlack5, Reactive Red136, Reactive Yellow145and Reactive Blue222solutions bycoagulation. The results indicated that mixing speed play a greatest role on affectingthe coagulation in four kinds of hydraulic conditions, followed by mixing time,coagulation speed, and coagulation time in that order. The dye removal rate wasgreatly influenced by pH. The optimal pH range of four kinds of dye was3~4. Dyeremoval rate increased first and then decreased with the concentration of coagulantincreased. The optimal dosage of flocculant increased with increasing of initialconcentration of dye. The optimal range of coagulant concentration for tap water waswider and the dye removal rate also higher compared with in distilled water. CMCSand QCMC are amphoteric coagulant, their coagulation properties depend on themolecular chain and chemical structure. It is presumed that the coagulation inducedby CMCS and QCMC were dominated by charge neutralization and bridgingcoagulation mechanism. Quaternary ammonium group of QCMC has increased itscation intensity lead to the low optimal QCMC concentration.