Characteristics Of Floc Morphological Evolution During Enhanced Coagulation And Its Effect On Sb(V) Removal

To explore the morphological features and development of PFS flocs formedunder variable coagulation conditions，a series of enhanced coagulation jar-test andpilot-test studies have been conducted with the help of modern photographictechnology and image processing technology. Meanwhile，the effects of variations infloc morphology has been also investgated on the performance of coagulation andcorresponding antimony removal.The results showed that the fractal dimension of floc mainly depends on its Zetapotential. The floc with more compact structure has a larger fractal dimension and asmaller absolute value for Zeta potential. The floc formed under higher coagulationpH is much larger，making it easier to precipitate. It was also found that flocs withsmaller size settle more quickly as their fractal dimension increased. At pH=8.0，higher mixing intensity results in the forming of less dense floc with smaller fractaldimension；at pH=6.0，a higher mixing speed at early stage and a appropriate mixingspeed at later stage increase the fractal dimension of floc，leading to a more compactstructure during coagulation process. The growth of floc is weakened by lowtempareture，causing the formation of flocs with lower fractal dimension and smallersize than those formed under higher temperature. The size of floc formed under poorcoagulation performance would be effectively enlarged by adding coagulantpolyacrylamide (PAM)，improving precipitation performance. The morphologicalcharacteristics of floc could affect the turbidity and antimony (V) concentration insolution after coagulation，but the concentration of antimony (V) in filtrated solutionis not influenced，which implies the removal of antimony (V) may be not realized bythe sweep flocculation.The performance of antimony (V) removal by PFS was also investigated. Theresults showed that the efficiency of antimony (V) removal increases with decreasingpH value and increasing PFS dosage，and the maximum removal rate may be up to97%. However with the presence of phosphate (PO3-4)，the removal efficiency wouldbe significantly reduced，even by70%. The existence of NaHCO3and humic acidmight also reduce the antimony (V) removal efficiency by20%and10%at the most,respectively.The coagulation jar-test results indicated that the adsorption between PFS hydrosis products and antimony (V) is mainly responsible for antimony removal，andthis adsorption behavior obey the pseudo second order model and the Freundlichisotherm equation with R2≥0.9. The affinity of antimony (V) to PFS under acidiccoagulation pH is higher than that under neutral and alkaline coagulation pHs.Moreover the reaction activation energy Ea was calculated as17.09kJ/mol. It isconcluded that the adsorption of antimony (V) on PFS hydrosis products iscomplicated and belong to chemical adsorption.The conducted pilot-scale tests have also indicated that the turbidity，color，andconcentration of iron and antimony for effluent (after filtration) could be ensured tomeet the relevant requirements continuously and steadily in Standards for DrinkingWater Quality (GB5749-2006) at PFS dosages of90mg/L，70mg/L，50mg/L and30mg/L，and corresponding initial pH values (raw water pH，7.0，6.5，6.0). By overallconsideration of antimony (V) removal efficiency， and chemical dosages andoperation cost， the optimal running condition have been selected with a initial pHvalue of7.0and a PFS dosage of70mg/L，and the total cost of chemical dosages isabout0.115yuan/m3.