Preparation of Heterocyclic Quaternary Nitrogen-grafted Activated Carbons and Their Adsorption Properties for Perfluorocarboxylate

Author:Sun Hao

Supervisor:wang yong tian


Degree Year:2019





In recent years,Perfluorocarboxylates(PFCAs)have been detected in the groundwater from various industrial and mining areas around the world.They have been considered as the most concerned trace pollutants in the world due to their high biological toxicity,stable physicochemical properties and nondegradability in the natural environment.Granular activated carbon adsorption is considered as the most efficient treatment technology.However,due to the strong solubility of perfluorocarboxylate(especially the low carbon chain PFCAs)in water,it is mainly in the form of anions,which requires the adsorbent to have both appropriate pore structure and a large number of positive charges.Traditional quaternary ammonium loading protocol can significantly increase the positive charges number on the activated carbon surface.However,the long carbon chain greatly increases the steric hindrance in the carbon pores,which is not conducive to PFCAs adsorption.Therefore,it is necessary to develop a carbon-based adsorbent that with appropriate pore structure,abundant surface charge and minimized steric hindry to improve the PFCAs adsorption performance.The main objective of this study is to develop a novel method to remove PFCAs from the groundwater.Trifluoroacetate(TFA)and perfluorooctanoate(PFOA)were chosen as the target pollutants.In this approach,pyrrole and ammonia were seclected as the nitrogen source.Oxidation polymerization and menschutkin reaction were utilized to load positively-charged quaternary nitrogen groups on the carbon surface.This protocol avoids the adverse effect of long alkyl chain(in the quaternary ammonium molecule)on pollutants adsorption.Firstly,a polypyrrole(Ppy)grafted coconut activated carbon(AC)was prepared by the chemical oxidation(Fe3+)polymerization protocol.The surface properties of activated carbon were regulated by changing the preparation conditions.The Ppy-grafted ACP1.5 carbon obtained an adsorption capacity of 37.2 mg/g for trifluoroacetate(TFA),which is more than three times than that of the pristine coconut activated carbon(11.3 mg/g).In addition,BET,SEM and EIS were used to characterize the physical structure and electrochemical properties of the grafted Ppy polymer on ACP surface.Element composition and surface charge distribution were characterized by X-ray photoelectron spectroscopy(XPS)and surface charge titration.The diffusion behavior and adsorption mechanism of TFA on ACP surface were investigated through adsorption kinetics and thermodynamics study.Furthermore,a stable rapid small scale column tests(RSSCTs)system was established to analyze the mass transfer process of TFA in the fixed-bed operation.In order to minimize the pore volume loss in the quaternary nitrogen grafting process and further improve the TFA purification efficiency,the coal activated carbon(AW)was modified by nitric acid oxidation,thermal ammonia activation and menschutkin reaction.The influence of modification conditions on TFA adsorption performance was investigated.The AWNQ4 carbon was obtained under the optimal preparation conditions and achieved the highest monomolecular adsorption capacity of 32.9 mg/g.The physiochemical properties of the AWNQ4 carbon variants were monitored by SEM,BET,XPS,Boehm titration,surface charge titration and electrochemical cyclic voltammetry characterization.The diffusion and adsorption mechanism of TFA on the pyridinic quaternary nitrogen-grafted activated carbon were clarified through kinetics and thermodynamics studies,and the dynamic adsorption performance was also investigated by RSSCTs.Moreover,the effects of various components in groundwater on TFA adsorption were investigated;SO42-and NO3-were identified as the main groups that inhibiting TFA adsorption.The PFOA adsorption performance of the modified activated carbons was also investigated.Compared with TFA,PFOA has a higher molecular weight and larger molecular size.The loaded Ppy on activated carbon surface blocked the pore structure,which increased the steric hindrance of activated carbon adsorption and this was not conducive to the adsorption of macromolecular organics such as PFOA.However,the load of pyridinic quaternary nitrogen increased the positive charges on the carbon surface and minimized the pore volume loss.This was conducive to the adsorption of negative PFOA molecules.By optimizing the preparation conditions,the relationship between the structural characteristics of activated carbon and PFOA was determined.The AWNQ2 carbon that prepared under the best preparation conditions obtained the highest adsorption capacity for PFOA.In RSSCTs,AWNQ2 achieved the longest bed life.When processing groundwater that had been spiked with 200 ng/L PFOA,the AWNQ2 exhibited 150000 BV to 70 ng/L breakthrough,this is much better than the pristine AW did(101000 BV).The PFOA adsorption mechanism was further investigated by adsorption thermodynamics and the molecular diffusion model was established.Based on the above work,the pyridinic quaternary nitrogen-graftted activated carbon achieved high TFA and PFOA removal efficiency.In order to further improve its economic benefits,the reusability is explored through thermal and solvent regeneration,respectively.For TFA saturated activated carbon,a novel regeneration method was proposed.The mixed ethanol and NaCl solution were used as the elution agent to remove TFA and reactivate the quaternary nitrogen group at the same time.The highest regeneration efficiency of 82.9%was achieved.For PFOA saturated activated carbon,the PFOA desorption behaviors in the solvent and thermal regeneration process were explored.It was noted that thermal regeneration efficiency was higher(85%)than the solvent regeneration did.The cycle RSSCTs was conducted to verify the reusability of pyridinic quaternary nitrogen-grafted activated carbon.In this paper,activated carbon-based modified adsorbents were prepared by surface modification to achieve efficient removal of perfluorocarboxylate pollutants that with different molecular weights in groundwater.It is an economical and efficient perfluorocarboxylate pollution control technology and the adsorbent exhibited excellent regeneration efficiency.This study not only broadens the application range of granular activated carbon,but also proposes a new method to against the perfluorinated compound pollution.This dissertation includes 75 figures,41 tables and 172 references.