Optimizing coagulation and flocculation process for kaolinite suspension with chitosan

Chitosans with different degrees of deacetylation (DD) and different molecular weights (Mw) have been evaluated as flocculants of kaolinite suspensions prepared with demineralized water (DW) and tap water (TW), respectively. The effects of chitosan molecular parameters (i.e. DD and Mw) and environmental conditions (i.e. pH, initial turbidity, flocculant dosage and aqueous media) on the coagulation–flocculation efficiency were investigated in detail. To optimize the selection of the chitosan molecular parameters and environmental conditions, the coagulation–flocculation efficiency was investigated using orthogonal tests. The results indicated that the effect of pH on the coagulation–flocculation efficiency of chitosan is insignificant. The optimal flocculant dosage increases with the increasing of initial turbidity. The chitosan dosage (Mw 232 kDa) required for the maximum coagulation–flocculation efficiency of kaolinite suspensions is 0.10 mg/L and the residual turbidity reaches the target value (i.e. 10 NTU) in the pH range 3–9 in both TW and DW, regardless of DD from 54.6% to 95.2%. Low molecular weight chitosan and chitooligomers (Mw below 21.1 kDa, DD 95.2%) failed to reach the target value of residual turbidity in TW under all investigated conditions, whereas good results were obtained using 0.10-0.20 mg/L in DW. The coagulation–flocculation performance in TW was very different from that in DW due to the presence of salt. Overall, the results were consistent with the destabilization of kaolinite by the combined mechanisms of charge neutralization and bridging.