Adsorption Of PFOA In Groundwater By Quaternary Ammonium Epoxide Compounds Modified Activated Carbon And Its Mechanism Analysis

As a kind of emerging persistent organic pollutant,perfluorooctanoic acid(PFOA)has received increasing attention in recent years.PFOA is widely distributed in the slow-circulating groundwater system with the features of difficulty in decomposition,and strong accumulation.PFOA is found in groundwater as the anion perfluorooctanoate,and its electronegative fluorocarbon tail and negative carboxyl will favor sorption to the surfaces with positive charge.Therefore,activated carbon tailored with positively-charged quaternary ammonium/epoxide forming compounds(QAE-AC)one of the more effective methods for PFOA removal.Wood-based activated carbon and coconut-based activated carbon were selected as the parent of the tailored carbon,and two QAE reagents(trade name QUAB)with molecular weights of 188 and 360 Da were employed to anchor onto carbon in this study for PFOA removal.The most favorable QAE-AC protocol was achieved when the 1.5 g wood-based activated carbon,pre-treated with 4 mol/L HNO3 to adjust the carbon’s slurry pH to 4.77,and tailored with the 15.8 g QUAB188,the reaction temperature was adjusted to 50℃,and the pH of the reaction solution was 12.5 for 48 h.The QAE-AC offered a bed life of 123000 BV at 50%breakthrough for PFOA removal(initial concentration was 200 ng/L)in the Rapid Small Scale Column Test(RSSCT).There was a peak PFOA bed-life at 0.8 g QAE which was followed by a minimum at 2.3 g QAE,then a steady rise in bed life for yet subsequently higher QUAB dosing applications.This study employs a simply and high repeatable pKdistribution formula to analysis the oxygen functionality of carbon.The carbon that had been pretreated with NaOH and H2O2,more than half of the functionalities were phenolic-like,after HNO3 and HCl pretreatment,the proportion of phenolic-like groups significantly decreased.As the number of phenolic carbonyl functional groups increased,the adsorption effect of PFOA decreased.After HCl pretreatment,the proportion of lactone-like was substantially increased.The results of EDs and surface charge distribution showed that,as the QAE increased,the concentration of QAE and the amount of surface positive charge on the surface of carbon also grew.When the dosage of QAE was 0.8 g or 15.8 g,the carbon variants hosted about equal mesoporosity as did the pristine wood carbon,with cumulative micropores(<20 A)of about 0.45 cm3/g and the cumulative mesoporous(20-500 A)of about 0.62 cm3/g.In contrast,the dosage of QAE was 2.3 g had lost a considerable portion of its micropores and mesoporous.The HNO3 and H2O2 pretreatment created more micropores smaller than 20 A width,but some of the mesopore were destroyed by this pretreatment.In comparison,the NaOH pretreatment created far more porosity in the 20-50 A width range.To reveal the adsorption mechanism,molecule model was used herein to provide new insight into the mechanism of PFOA removal by QAE-AC.Based on experimental results and simulation,the QAE’s epoxide functional group cross-linked with phenolics that were present along the activated carbon’s graphene edge sites.QAE is preferentially reacted with the phenolic in the micropores and mesopores of carbon,and some QAE molecules form new "pore-like structures" outside the pores with the graphene planes or other QAE molecules.This pore-like gelatinous structure hosted considerable adsorption capacity for PFOA by the quaternary ammonium.The main factors affecting the adsorption of PFOA are the pore size and the amount of positive charge on the surface of the modified carbon.PFOA was adsorbed via both hydrophobic interaction and electrostatic interaction.Mainly PFOA removal was adsorbed via hydrophobic interaction when the amount of pre-dosed QUAB188 was lower.In comparison,the process of PFOA removal included significant electrostatic interactions when the amount of pre-dosed QUAB188 was higher.Through the analysis of the groundwater,as an oxyanion with a high concentration and harmful to the human,nitrate is found that the nitrate is could be adsorbed by QAE-AC through the anion exchange.In order to evaluate the effect of nitrate on the adsorption of PFOA by QAE-AC in practical applications,nitrate was selected as the representative of oxyanion,and to investigate the adsorption efficiency of QAE-AC for PFOA and nitrate co-adsorption.The results showed that 50 mg/L nitrate diminished PFOA half-breakthrough by only 9%,a rather small change.This indicated that the nitrate oxyanion incurred only a slight effect on the PFOA removal.According to the molecule model,the favorable nitrate adsorption sites were in smaller micropores;and this meant that the nitrate sorption has little impact on the PFOA sorption which was occurring in the mesopores.Therefore,it can be considered that QAE-AC can simultaneously adsorb harmful PFOA and nitrate in water.Herein,based on the pollution statu and charge characteristics of PFOA in groundwater,a research system for quaternary ammonium salt modified activated carbon was assessed,which simultaneously removed the two pollutants of PFOA and nitrate in water.The influenced factors and molecules model were revealed the mechanism of synchronous removal.This research has developed a new technology to control the pollution of PFOA and inorganic oxyanions such as nitrate in water,and provide new ideas for water repairing and treatment.This paper also provided a pK distribution model based on Gaussian function,which provides a new method for the characterization of surface functional groups of activated carbon with convenient operation,simple calculation and strong reproducibility.