Removal Of Typical PFCs From Reclaimed Water By Enhanced Coagulation

AS a new group of persistent organic pollutants,perfluorinated compounds(PFCs)were produced and consumed for more than fifty years.Because of their high surface activity and stability,PFCs are widely used in production and consumption such as textile,detergent,fire retardants,petrochemical industry and so on.PFCs in water have increasingly attracted global concerns in recent years due to their global distribution,persistence,strong bioaccumulation and potential toxicity.Among all the PFCs,perfluorooctanesulfonate(PFOS)and perfluorooctanoate(PFOA)are two most typical ones in water.Municipal wastewater treatment plants(WWPT)are one of the main sources of PFCs,after reclamation the effluent of WWPT becomes reclaimed water,reclaimed water is usually used for greening and cleaning,and the uses of reclaimed water is closely related to the life and production of human beings.The existence of PFCs in reclaimed water will not only contaminate the environment again but also endanger the health of human beings directly.Therefore,it is urgent to prevent and control the pollution of PFCs in reclaimed water.In this paper,in order to adsorb PFOS and PFOA in water effectively,bentonites were modified by cetyltrimethyl ammonium bromide(CTMAB).Organic bentonites were used as coagulant-aid when we removed PFOS and PFOA from water by enhanced coagulation.A series of organic bentonites were synthesized under various conditions,after the contrast test of adsorbing PFOS/PFOA in water organic bentonite which can adsorb PFOS/PFOA most effectively was selected for adsorbent and coagulant-aid,than the chosen bentonite was analyzed by X-Ray diffraction.The adsorption characteristics and rule of organic bentonites to PFOS and PFOA were investigated with the adsorption tests.The capability and rule for removal of PFOS and PFOA from reclaimed water by enhanced coagulation with the single factor method,during the enhanced coagulation organic bentonites were used as coagulant-aid.After the enhanced coagulation with the single factor method,the Response Surface Methodology was adopted to the optimization experiment.In order to obtain the best factors combinations and the optimal response value for removal of PFOS and PFOA from reclaimed water by enhanced coagulation,the three factors and three levels orthogonal experiment was designed by Box-Behnken Design(BBD).The experimental results are as follows:(1)AS bentonites were modified by CTMAB,its adsorptive capacity for PFOS and PFOA was effectively enhanced.The result of contrast test indicated that CTMAB-Bent6 which was intercalated by both CTMAB and PFS could adsorb PFOS and PFOA most effectively.The result of X-Ray diffraction displayed that CTMAB was intercalated into the interlayer of bentonites availably,the interlayer distance of bentonites increased,and the organic modification of bentonites was successful.(2)Organic bentonites CTMAB-Bent6 could adsorb PFOS and PFOA in water rapidly,the adsorption equilibrium time was 4 h.The adsorption could be well described by the Pseudo-second-order kinetics equation.The adsorption isotherms of PFOS and PFOA were found to fit better with the Freundlich equation than the Langmuir equation.In the same condition,the degree of PFOS adsorption onto CTMAB-Bent6 was larger than that of PFOA adsorption.The pH value of solution had significant effect on the adsorption of PFOS and PFOA on CTMABBent6,an increase in pH leaded to a decrease in the adsorption of PFOS and PFOA on CTMABBent6.(3)Enhanced coagulation could remove PFOS and PFOA from reclaimed water effectively when organic bentonites CTMAB-Bent6 was used as coagulant-aid.With the increasing of PAC dose,the removal rate of PFOS/PFOA increased first and decreased later,and the optimal dosage of PAC was 10 mg/L.There was a correlation between the removal of PFOS/PFOA and the removal of turbidity.With the increasing of pH,the removal rate of PFOS and PFOA raised first and reduced later,the rest turbidity decreased,and the PFOS/PFOA removal effect was best when pH value was 7.The removal rate of PFOS/PFOA increased with the increasing of turbidity.When the initial concentration of PFOS/PFOA increased,the removal rate of PFOS / PFOA decreased.The removal rate of PFOS and PFOA attained 96.05% and 84.05%,respectively,when the initial concentration of PFOS/PFOA was 3.6 mg/L.In the same condition,enhanced coagulation could remove PFOS more effectively than PFOA.When PFOS/PFOA and phenanthrene coexisted in solution,the removal rate of PFOS / PFOA decreased,but there was little influence on the removal of turbidity.The removal effect of PFOS/PFOA by PFS was very similar to PAC.Compared with powdered activated carbon,enhanced coagulation could remove PFOS and PFOA more effectively when CTMAB-Bent6 was used as coagulant-aid.(4)The experimental scheme was designed by BBD,than a quadratic polynomial model was built.The model matched well,and the lack of fit was not significant,thus the model could be used for the prediction and analysis of the removal of PFOS/PFOA by enhanced coagulation.As show in response surface diagram and contour map,PAC dosage,pH value and turbidity all had a significant effect on the removal of PFOS/PFOA by enhanced coagulation,and pH value was the most significant factor.There was interaction between any two factors,and there was significant interaction between PAC dosage and pH value,pH value and turbidity.The optimal response value with the best factors combinations was obtained by analyzing the model,and the measured date conformed to the optimal response value.In a word,the prediction results were accurate and reliable.Due to the effective adsorption of PFOS/PFOA on the organic bentonites,enhanced coagulation can remove PFOS and PFOA from reclaimed water more effectively.In addition,bentonite is inexpensive and available,and the synthesize technique of organic bentonites is simple.In a word,the organic bentonites has considerable application value when we treat the pollution of PFOS/PFOA in reclaimed water.