Wenming Hao,Basma I.Waisi ,Timothy M.Vadas ,Jeffrey R.McCutcheon

1 College of Chemical Engineering and Technology, Taiyuan University of Technology, Taiyuan 030024, China

2 Department of Chemical &Biomolecular Engineering, University of Connecticut, Storrs, CT 06269, United States

3 Department of Chemical Engineering, University of Baghdad, Baghdad 10071, Iraq

4 Department of Civil and Environmental Engineering, University of Connecticut, Storrs, CT 06269, United States

Keywords:Activated carbon nanofibers (ACNFs)Chemical activation Bisphenol-A (BPA)Fixed bed Adsorption

ABSTRACT Activated carbon nanofibers (ACNFs) with small diameter can significantly increase the accessibility of intra pores and accelerate adsorption of molecules from water.In this study,ACNFs were made by blending K2CO3 or ZnCl2 as the activating agent into the polyacrylonitrile(PAN)in dimethylformamide solution for electrospinning prior to pyrolysis.Bisphenol-A (BPA),an endocrine disruption pollutant,is widely applied in the production of polycarbonate plastics and epoxy resins.Accordingly,BPA is often used as a model contaminant commonly removed via adsorption.Batch adsorption studies were used to evaluate the kinetics and adsorption capacity of the ACNFs.Redlich-Peterson (R-P) and Langmuir models were found to fit the isotherm of BPA adsorption better than Freundlich model,showing the homogeneous nature of the PAN originated ACNFs.The adsorption kinetics was better described by the pseudo second-order model than that by the pseudo first-order model.The fitting by intraparticle diffusion model indicates the adsorption of BPA onto ACNFs is mainly controlled by pore diffusion.High pH value and ionic strength reduced BPA adsorption from aqueous solution.The breakthrough curves studied in two different fixed bed systems (cross flow bed system and packed flow bed system) confirmed the scalability of BPA removal by ACNFs in dynamic adsorption processes.The modified dose-response model predicted well the fixed-bed outlet concentration profiles.

1.Introduction

Activated carbon nanofibers (ACNFs) are an emergent carbon material which has attracted attention due to its unique properties[1].Having unique morphology with size from submicron to nanometer,high porosity,concentrated pore size distribution,ACNFs have been applied in different fields such as electrical energy conversion and storage [1-6],air pollution removal [7],capacitive desalination[8,9],gas sensor[10]and hydrogen adsorption [11].Among the different approaches of making nanofibers,electrospinning is the most preferred,since it is a simple,efficient,and scalable technology with low energy consumption [4].In a typical process,an electrical potential is applied between a droplet of polymer solution or melted polymer at the end of a capillary and a grounded collector,and a stream of nanofiber is spun from the droplet onto the collector by the static electrical force.Since the performance of carbon nanofiber highly depends on its precursor,an ideal precursor should have high carbon content,high molecular weight,and high degree of molecular orientations [12].Polyacrylonitrile (PAN) has been proven to satisfy all the requirements.PAN also has other merits such as being environmentally benign and good commercial viability [13].It has been reported that 90% of world’s total carbon fiber productions are PAN based [12].

In order to prepare ACNFs,the electrospun polymer nanofibers need to be carbonized and activated after being stabilized in an oxygen contained atmosphere.During an activation,the specific surface area and porosity of carbons are created by removing the most reactive carbon atoms from the structure [2].There are two approaches for activation.In physical activation,the carbon precursor has to be carbonized in an inert atmosphere followed with activation by agents such as CO2or H2O vapor[7,9,14].In chemical activation,the carbon precursor needs to be mixed with chemicals as activating agents such as ZnCl2[8],KOH [11,15],K2CO3[16] or H3PO4[3],and the carbonization and activation then occur during the same step under an inert atmosphere.The most important advantage of chemical activation over physical activation is that the activation temperature is lower,the time is shorter,and the yield is higher [16].The porosity development of ACNFs during activation is affected by different factors,such as activation temperature,activation time,heat rate and gas flow rate.ACNFs with nano-scaled fiber size can significantly accelerate the diffusing rate of adsorbate molecules into the intra-pores [7].Along with large surface area and pore volume,concentrated pore size,and abundant functional groups,ACNFs have a great potential of being used as adsorbents to remove pollutant molecules from water.

Bisphenol-A (BPA) is an endocrine disrupting compound,but widely applied in the production of polycarbonate plastics and epoxy resins.BPA has been found in industrial wastewater,groundwater,surface water,and even in drinking water,which is commonly studied as a model contaminant for adsorptive water purification [17-26].In the present work,we demonstrated the fabrication of ACNFs by chemical activation using ZnCl2and K2CO3as the activating agents.To our best knowledge,it is the first time to prepare ACNFs by blending ZnCl2or K2CO3into the precursor of PAN solution before the electrospinning.ACNFs with nanoscaled fiber size are expected to show better diffusion of adsorbate molecules into the intra-pores.We used BPA as a commonly studied model contaminant for adsorptive water purification to assess the capability of the fabricated ACNFs to remove compounds from aqueous solution.Batch equilibrium adsorption,kinetics,effect of pH and ionic strength were studied in detail.Both the adsorption isotherms and kinetics were fitted by different models.Moreover,the dynamic adsorption of BPA on the ACNFs was studied in two fixed-bed systems.The experimental breakthrough curves were also fitted by the modified dose-response (MDR) model.

2.Materials and Methods

2.1.Materials and solution preparation

Polyacrylonitrile(PAN,MWavg.150000 g·mol-1,Scientific Polymer Products Inc.,USA),dimethylformamide(DMF,Acros Organics,Belgium),potassium carbonate(K2CO3,≥99%,Acros Organics),zinc chloride(ZnCl2,anhydrous,≥98%,Alfa Aesar,UK)and bisphenol-A(BPA,≥97%,Alfa Aesar),were used without further purification.Salt (ZnCl2or K2CO3) and PAN (14% by mass) were mixed in DMF solution(the mass ratio of salt to PAN is 0.5:1).To facilitate mixing,salt was added in DMF first and stirred until dissolved (ZnCl2) or homogeneously dispersed(K2CO3).PAN was mixed into the above mentioned DMF solution (mixture) and dissolved under constant stirring at 60 °C for 5 h and then overnight at room temperature.The viscosities of the solutions/mixture were measured on a DV-I Prime viscometer (Brookfield,USA).

2.2.Fabrication of precursor nanofiber by electrospinning

The solution of salt and PAN in DMF was electrospun using a technique described elsewhere [5].During the spinning,the solution was charged by applying a voltage of 23.0-29.0 kV,and dispensed by a high pressure syringe pump (KD Scientific,Germany)at a constant rate of 1 ml·h-1through a 20 gauge blunt end needle.The fiber subsequently spun onto a grounded drum collector rotating at 70 r·min-1.The tip to collector distance was held constant at 18 cm.The precursor mats were all spun at room temperature under a relative humidity of 20%-40%.

2.3.Activation of carbon nanofibers

wherek2(g·mg-1·min-1) is the rate constant of pseudo secondorder adsorption.

2.4.Characterization of activated carbon nanofibers

Batch equilibrium adsorption of BPA was conducted to compare the adsorption capacity of the ACNFs.Table 1 shows the uptake of BPA from the aqueous solution with an initial concentration of 300 mg·L-1on the ACNFs after the adsorption of 24 h.From the result,it can be seen that mesopore volume (Vmeso) is more correlative with the adsorption capacity than the other pore structure parameters.As BPA has relatively large molecular size of 0.325 nm × 1.12 nm,mesopores can facilitate the diffusion of BPA molecules into the pore structure of ACNFs [33].ACNF-K-600-3 has the highest external surface area of 143 m2·g-1,which resulted in the highest BPA uptake of 180 mg·g-1among the K2CO3activated ACNFs.ACNF-Zn-700-1 has the highest adsorption capacity among the ZnCl2activated ACNFs,due to its highest external surface area from large pores.It is worth noticing that the mesopore size also plays a crucial role for the adsorption of BPA.Lage pore size facilitates the diffusion of BPA into the pores of ACNFs.Although K2CO3-800-1 had higher external surface area than K2CO3-600-2,the BPA adsorption of the former is smaller than the latter,since the pore size of K2CO3-800-1 is smaller than K2CO3-600-2.As can be seen from the pore size distribution in Figs.S2 and S3,K2CO3-800-1 have pore size mainly in 2.2 nm.While,the pore size of K2CO3-600-2 is more broadly distributed,from 2.1 to 3.3 nm.

Adsorption kinetics of BPA on ACNFs was analyzed from the adsorption of BPA with the initial concentration of 300 mg·L-1,pH=7,at different adsorption time intervals.The effect of pH on the adsorption was evaluated by tuning pH values of the solutions from 3 to 12.The influence of ionic strength was studied by adding different amounts of NaCl,MgCl2or CaCl2into the BPA solutions.

Scanning electron microscopy (SEM) images of the nanofibers before and after activation were recorded with field emission scanning electron microscopy (FESEM,JEOL 6335F,Japan).Samples were sputter coated with platinum before the imaging.Fiber size distributions and the average fiber diameter were obtained by measuring thirty individual fiber sizes using Image J software.

2.4.2.Nitrogen adsorption

Nitrogen adsorption/desorption isotherms were measured at the boiling point of liquid nitrogen (-196 °C) with an ASAP 2020 device (Micromeritics,USA).Samples were degassed under the conditions of a dynamic vacuum at a temperature of 300 °C for 5 h before the measurement.Surface areas (SBET) were calculated using the Brunauer-Emmet-Teller(BET)model.The total pore volume (Vt) was determined from the uptake of N2at a relative pressure (p/p0) of 0.99.The external surface area (Sext) and micropore volume (Vmic) were calculated using thet-plot method (according to the definition from IUPAC,micropores are smaller than 2 nm,mesopores are in the range of 2-50 nm,and macropores are larger than 50 nm).The specific microporous surface area (Smic) was approximated as the difference betweenSBETandSext.The larger pore volume (Vmeso) was calculated as the difference betweenVtandVmic.Pore size distributions were estimated from the adsorption curve of the isotherms by using density function theory(DFT) method and routines of the Micromeritics instrument.

2.4.3.Mechanical properties

In the moonlight, when all on board were asleep, excepting the man at the helm, who was steering103, she sat on the deck, gazing down through the clear water

The tensile strength and flexibility of the ACNFs were measured on a dynamic mechanical analyzer (DMA) from DMA Q800 (TA Instruments,USA) to quantify the mechanical strength of the ACNFs.The measurement of tensile strength and Young’s modulus were performed at 25°C and ambient humidity on the ACNFs samples with the sizes of 3 cm×0.6 cm.All results presented were the average of three individual measurements from different samples.

2.5.Batch adsorption of bisphenol-A

The batch equilibrium BPA adsorption test was performed in a set of 100 ml beakers containing 20 mg (corresponding to 1.0 g·L-1) of ACNFs adsorbents and 20 ml of BPA solution with a concentration of 300 mg·L-1.The solutions with pH value of 7 were stirred at 300 r·min-1for 24 h to reach adsorption equilibrium conditions.Then the solutions were separated from the adsorbents by filtration.Concentrations of equilibrium(Ce,mg·L-1)BPA solutions were determined by reading absorbance values by means of a GENESYS 10S UV/Vis spectrophotometer (Thermo Scientific,USA)at λmax=272 nm.The adsorption isotherm of BPA was measured from the adsorption of BPA with different initial concentrations(C0,mg·L-1),pH=7,using the same batch equilibrium adsorption conditions.

57.He was asleep: Sleeping potions are common in fairy tales, such as in The Twelve Dancing Princesses in which suitors are drugged to keep them from learning a secret.Return to place in story.

Many years have passed and many children have benefited from various therapeutic2 riding programs. But none touched me as much as this one boy. He required a steady horse, one with patience with his rider s inability to balance and an understanding of the boy s need to occasionally lay his face on the mane and just breathe in horse smells. We had several wonderful horses that filled the bill.

Fig.S4 shows the N2adsorption and desorption isotherms at -196 °C and the corresponding pore size distributions of the ACNFs activated by ZnCl2at different temperatures.In Fig.S4(a),the isotherm of ACNF-Zn-600-1 does not have high adsorption,since the pores were not well developed at 600°C within 1 h.When the temperature increased to 700 °C,ACNF-Zn-700-1 showed a dramatic increase of adsorption,which indicates more pores were generated.The isotherm is close to type I,which is formed by micropores.However,when the temperature increased to 800 and 900 °C,the adsorption of the isotherms decreased compared with ACNF-Zn-700-1,which could be ascribed to the burn-off of the materials at high temperatures.The small hysteresis loops in ACNF-Zn-800-1 and ACNF-Zn-900-1 are from mesopores.The pore size distributions in Fig.S4(b)shows the ACNFs activated by ZnCl2at different temperatures have mainly micropores but small amount of mesopores in the range of 2.0-3.0 nm.Fig.S5 shows the N2adsorption and desorption isotherms at-196°C and the corresponding pore size distributions of the ACNFs activated by ZnCl2at 600°C for different times.In Fig.S5(a),the adsorption of the isotherms increased when the activation time prolonged from 1 to 3 h,which means more pores were formed.The isotherms are typical type I.The pore size distributions in Fig.S5(b) showed the ACNFs activated by ZnCl2have mainly micropores but also mesopores between 2.0 and 3.5 nm.Table 1 shows the porosity parameters calculated from the isotherms.The BET surface area and pore volume increased when the activation temperature increased from 600 to 700 °C,but decreased when the temperature increased to 800 and 900 °C.ACNF-Zn-700-1 has the highest surface area of 876 m2·g-1and pore volume of 0.48 m3·g-1.Compared with the activation by K2CO3,the optimal activation temperature by ZnCl2is 700 °C,while the optimal activation temperature by K2CO3is 800 °C (ACNF-K-800-1 has the highest surface area of 846 m2·g-1).The same trend was observed for external surface areaSext.When it was activated at 600°C,the external surface area of the ACNFs had very small increase with the prolongation of activation time.When the activation temperature increased to 700°C,theSextincreased to 129 m2·g-1,and decreased when the activation time reached 800 and 900°C.Liuet al.[8]also prepared ACNFs by ZnCl2activation,but they introduced the activating agent by immersing the carbonized nanofiber precursor into ZnCl2aqueous solution,and the prepared ACNFs have much less porosity compared with that in this study.

2.6.Modeling of adsorption kinetics and equilibrium with BPA

The equilibrium adsorption amount of BPA,qe(mg·g-1),was calculated by the following equation:

whereC0andCe(mg·L-1)are the initial and equilibrium concentrations of BPA,respectively,V(L)is the volume of BPA solution,andm(g) is the amount of the adsorbent used.

The adsorption isotherm was fitted by Langmuir model,Freundlich model and Redlich-Peterson (R-P) model.Langmuir isotherm assumes that adsorption takes place at specific homogeneous sites on the surface of the adsorbent,and a monomolecular adsorbed layer was formed,which can be expressed as follows [17]:

whereqm(mg·g-1) is the maximum adsorption capacity of the adsorbent,andKL(L·mg-1) is the Langmuir adsorption equilibrium constant.

Freundlich isotherm provides an empirical equation,which assumes non-ideal adsorption takes place on a heterogeneous surface with different adsorption energy,and can be expressed as:

whereKR(L·g-1)and αR(L·mg-1)βare the adsorption R-P constants,and β is the exponent,ranges between 0 and 1.When β=0,the R-P equation become Henry’s equation which is a linear isotherm.When β=1,it is Langmuir isotherm equation.For high adsorbate concentration,the R-P equation become Freundlich isotherm equation.

The R-P isotherm combines the features of both Langmuir and Freundlich equations,which is expressed as follows [20,22]:

whereKF((mg·g-1)(L·g-1)n)is the Freundlich constants(adsorption capacity) related to binding energy,1/nreflects the adsorption intensity or surface heterogeneity [17].

They said nothing more, but went out towards their houses amongthe sand-hills. All at once, in front of one of the houses where thesea grass did not keep the sand down with its twining roots, whatseemed to be a column of smoke rose up. A gust of wind rushedbetween the hills, hurling the particles of sand high into the air;another gust, and the strings of fish hung up to dry flapped andbeat violently against the walls of the cottage; then everything wasquiet once more, and the sun shone with renewed heat.

Day by day it grew bigger and fatter, and at last one morning it said to Cola-Mattheo, the peasant, whom it always regarded as its father, Dear papa, I am now of a suitable age and wish to marry

When she had lived there some time, it happened that the King of the country was hunting in the forest, and his hunters came to the tree on which the maiden sat

The adsorption kinetic curve was fitted by the pseudo-first order model,the pseudo-second order model,and the intraparticle diffusion model.The pseudo-first order model can be expressed as[24]:

wherek1(min-1) is the rate constant of pseudo first-order adsorption;qe(mg·g-1)is the amount of BPA adsorbed at equilibrium,andqt(mg·g-1) is the amount of BPA adsorbed at timet.

The pseudo-second order model can be expressed as [24]:

The electrospun nanofiber mats were stabilized in air at 280°C for 1 h in a muffle furnace (Carbolite,Germnay) using a ramp rate of 1 °C·min-1.The stabilized nanofibers were then activated in a tube furnace (Lindberg Blue M,Thermo Scientific,USA) for 1 h at different temperatures(600,700,800 and 900°C)under the atmosphere of nitrogen gas with a ramp rate of 5°C·min-1.To assess the effect of activation time,the activation at 600°C was conducted for four lengths of time (1,2,3 and 4 h).After activation,the resulted ACNFs were washed 5 times with 2 L distilled water and dried at 100 °C overnight.

He was struck with astonishment69 at the sight of the chariot, and was gazing at it, when the Enchanter strode up to him, exclaiming: Shake hands, Cloverleaf, old fellow! Don t you know me? No, I can t say I do, replied the King, somewhat embarrassed

The intraparticle diffusion model can be applied to explore the adsorption progress.It can be represented by the following equation [27]:

wherekidis the intraparticle diffusion rate constant,andCis a constant related to the bounding layer thickness.

2.7.Dynamic adsorption of bisphenol-A

The dynamic adsorption of BPA in a solution of 10 mg·L-1was conducted using home-made fixed-bed systems.As shown in Fig.S1 (in Supplementary Material),both cross flow bed and packed bed systems were used to evaluate the performance of the BPA adsorption on the ACNF mats.In the cross flow bed module,the ACNF mats with a width of 2.8 cm and different height of 2.0,4.0 and 6.0 cm were fixed.Flow rate was set to 80 ml·h-1to evaluate the effect of bed height.The adsorption on the ACNFs mats of 2.8 cm × 2.0 cm were measured at the flow rate of 40,60 and 80 ml·h-1which was controlled by a high pressure syringe pump (KD Scientific).In the packed bed system,the ACNF mats were packed in a round shape module with a diameter of 1.5 cm.The adsorption of BPA in the packed bed was studied by changing the number of the ACNF mats layers from 1 to 3.The concentration of BPA effluent was monitored by the Thermo Scientific GENESYS 10S UV/Vis spectrophotometer at λmax=272 nm.VISIONlite software was used to collect data at intervals of 30 s.

The adsorption amount in the flow bed system was calculated by the equation [28]:whereq(t) is the adsorbed amount at timet,Ciis the inlet concentration,C(t) is the bed outlet concentration at timet,Qis the volumetric flow rate,andmis the mass of the adsorbent in the bed.Loading amount at the bed breakthrough point(qb)were estimated by assuming thatt=tboccurs whenC(t)/Ciis 0.05 [28].

The experimental breakthrough curves were fitted by modified dose-response (MDR) model.The MDR model,also known as the Yan model,was originally developed for pharmacology studies and recently used to describe the adsorption of contaminants of emerging concern [29].Moreover,it can make correct prediction of effluent concentrations at time zero (i.e.,C(0)=0).The basic form of the model is given as the following equation:

whereaMDRandbMDRare the parameters of the model.bMDRis associated to the maximum adsorption capacity (qm) by the equation:

3.Results and Discussion

3.1.Electrospun precursor fibers

DMF is commonly used solvent to make PAN solution for electrospinning.The viscosity of 14% PAN in DMF solution is 5035 mPa·s,which is influenced by the addition of salts.K2CO3is not soluble in DMF,but could reduce the viscosity of the PAN solution to 4271 mPa·s when it was homogeneously mixed in.Interestingly,ZnCl2could be dissolved in DMF,but significantly increased the viscosity of the solution to 7450 mPa·s.The viscosity of the PAN solution significantly affects the electrospinning process and the resultant fibers.Fig.1 shows the SEM images of the electrospun nanofibers of PAN with salt being added and the corresponding fiber size distributions.In Fig.1(a),micrometer-sized K2CO3particles were noticed among the nanofibers,and the fibers were randomly arranged.The diameter of the fibers was between 0.6-1.6 μm with the average diameter of 1.16 μm.However,the fibers from ZnCl2added PAN solution are much bigger than that from K2CO3added PAN solution.As shown in Fig.1(b),the large fibers were somehow aligned in an order compared with the fibers from K2CO3added PAN solution.The smaller fibers surrounded the ordered large fibers.The sizes of the fibers were also wildly dispersed with the average diameter of 4.96 μm.The difference in the morphology and size between the K2CO3added and the ZnCl2added nanofibers is largely related to the changing of the viscosity by the two salts.The increased viscosity of PAN solution by ZnCl2resulted in larger fiber size,while the decreased viscosity of PAN solution by K2CO3led to smaller fiber size.Such result is well in line with the findings in other studies [30,31].

3.2.Morphology and porosity of the ACNFs

3.2.1.ACNFs activated by K2CO3

The K2CO3mixed PAN nanofiber mats were stabilized and activated into activated carbon nanofiber mats.Fig.2 presents the SEM images and corresponding fiber size distributions of the PAN based ACNFs.The K2CO3particles disappeared after the activation and subsequent washing by distilled water.Compared with the precursor in Fig.1(a),the sizes of the fibers were decreased after activation.What’s interesting is the fibers activated in the same time period but at different temperatures have similar size,which is around 600-750 nm (Fig.2(a),(e) and (f) for ACNF-K-600-1,ACNF-K-700-1 and ACNF-K-800-1).However,keeping the activation time at 600 °C for longer activation time resulted in smaller fiber size.As shown in Fig.2(a)-(d),the fiber size decreased from 700 nm for ACNF-K-600-1 to 150 nm for ACNF-K-600-4.

Fig.2.SEM images and fiber size distributions of K2CO3 activated ACNFs from PAN with different activation time and temperature:(a)ACNF-K-600-1;(b)ACNF-K-600-2;(c)ACNF-K-600-3;(d)ACNF-K-600-4;(e)ACNF-K-700-1;(f)ACNF-K-800-1.The ACNFs were referred as ACNF-K-A-B,‘‘K”refers to the activation agent K2CO3,‘‘A”stands for the activation temperature (°C) and ‘‘B” is activation time (h).

The porosity of the K2CO3activated ACNFs was characterized by N2adsorption at-196°C.Fig.S2 shows the N2adsorption and desorption isotherms of the ACNFs activated for different times.The N2adsorption increased when the activation time increased from 1 to 3 h;however,it decreased substantially when the activation time was prolonged to 4 h.The pore size distributions in Fig.S2(b) shows that all the ACNFs activated at 600 °C have mesopores from 2.0 to 3.5 nm.Table 1 shows the data of porosity derived from the N2adsorption isotherms.The BET surface area increased from 257 m2·g-1at 1 h to 551 m2·g-1at 3 h.The pore volume increased from 0.20 to 0.32 m3·g-1.However,the BET surface area and pore volume decreased to 381 m2·g-1and 0.26 m3·g-1,respectively,when the activation time was 4 h.It is likely that the pore generation by K2CO3reached a maximum near 3 h,but subsequent reactions up to 4 h led to larger mesopores in the range of 2-4 nm,decreasing net surface area and pore volume.The external surface areaSextof the ACNFs activated at 600 °C for 1 and 2 h are quite similar,which are 83 and 95 m2·g-1,respectively.While the activation time increased to 3 and 4 h,theSextof the ACNFs increased to 143 and 125 m2·g-1,respectively,which could be ascribed to the pore expansion with long activation time.

Table 1 Porosity,mechanical properties and BPA adsorption capacity of chemically activated ACNFs from PAN

The isotherms of the ACNFs activated at different temperatures in Fig.S3(a) are close to type I which is mainly from micropores.The adsorption is higher when the activation temperature is raised from 600 to 800°C,which means activation at 800°C created more pores in the ACNFs than that at 600 °C.Pore size distributions derived from the adsorption branch of the isotherms using the DFT model are presented in Fig.S3(b).Although part of the micropore range is missing,it could still be noticed that ACNFs had mainly micropores,and small amount of mesopores between 2.0 and 3.5 nm.Porosity parameters calculated from the isotherms are presented in Table 1.As expected,the BET surface area and pore volume increased when the activation temperature increased.ACNF-K-800-1 has the highest surface area of 846 m2·g-1and pore volume of 0.46 m3·g-1.TheSextof the ACNFs activated increased gradually from 95 to 124 m2·g-1with the increase of activation temperature from 600 to 800 °C.

It had taken 40 years, but the good deed had been repaid. Nana was right. We reap exactly what we sow. “Every good deed you do will someday come back to you.”

3.2.2.ACNFs activated by ZnCl2

Since the ZnCl2impregnated precursor PAN fibers had a larger size,the resultant ACNFs also had relatively large fiber size even though they shrunk after activation.Fig.3 shows the SEM images and corresponding fiber size distributions.All the ACNFs had similar morphology.The large fibers in the precursors were still aligned in an order after activation.Keeping the activation at 600 °C,the size of the ACNFs decreased when the activation time was prolonged.ACNF-Zn-600-1 had the average fiber size of4.52 μm,while ACNF-Zn-600-3 had the average fiber size of 3.81 μm.However,there was no obvious difference in morphology among the ACNFs activated at different temperatures.

Fig.3.SEM images and fiber size distributions of ZnCl2 activated ACNFs from PAN with different activation time and temperature:(a)ACNF-Zn-600-1;(b)ACNF-Zn-600-2;(c)ACNF-Zn-600-3;(d)ACNF-Zn-700-1;(e)ACNF-Zn-800-1;(f)ACNF-Zn-900-1.The ACNFs were referred as ACNF-Zn-A-B,‘‘Zn”refers to the activation agent ZnCl2,‘‘A”stands for the activation temperature (°C) and ‘‘B” is activation time (h).

The used adsorbent was regenerated by calcination at 400°C in N2for 2 h.The regeneration test was conducted by evaluating the BPA adsorption capacity of the regenerated adsorbent.

3.3.Mechanical strength

Table 1 also shows the tensile strength and Young’s modulus of the ACNFs,both of which are averaged from three individual samples.The K2CO3activated ACNFs in this study showed higher tensile strength and Young’s modulus than the steam activated ACNFs prepared by Manickamet al.[5].The ZnCl2activated ACNFs are weaker than the K2CO3activated counterpart in general.The possible reason is that the K2CO3nanoparticles in the PAN solution reinforced the nanofiber,while the dissolved ZnCl2did not.According to Huet al.[32],the addition of nanoparticles in the precursor solution could reinforce the mechanical strength of carbon nanofibers.In the process of making nanofibers,K2CO3powder and ZnCl2powder was added into the PAN solution before electrospinning.However,ZnCl2could be dissolved in the PAN in DMF solution,while K2CO3cannot be dissolved.ACNF-Zn-600-1 is the strongest with both high tensile strength and Young’s modulus among the ZnCl2activated ACNFs.Both of the increase of activation temperature and the prolongation of activation time caused a decrease of mechanical strength and Young’s modulus,which could be attributed to the introduction of pores and the reduction of the fiber size when activation temperature was increased and activation time was prolonged.When the individual fiber became more porous,it would be weaker.

3.4.Batch study of bisphenol-A adsorption

2.4.1.Fiber morphology and size

3.4.1.Adsorption isotherms

The adsorption of BPA was studied in detail on ACNF-K-600-3,the ACNFs with the highest adsorption capacity.The isotherm was determined by subjecting a set of samples to aqueous solutions of BPA with different concentrations at room temperature and using an equilibration time of 24 h.As shown in Fig.4,like in most equilibrium adsorption process,the adsorption capacity of ACNF-K-600-3 increased with increasing BPA concentration until the adsorbent reached saturation.Such concentration dependent loading is likely due to the concentration gradient which can accelerate the diffusion of BPA into the pore structures[34].

Hansel stretched up his hand and broke off a little bit of the roof to see what it was like, and Gretel went to the casement23 and began to nibble24 at it

For a moment he was stunned12 into silence by this new learning. Finally he said quietly, I never realized that,Amy. You re in a wheelchair all the time — I never thought you d mind sitting in the boat. It s the same thing.

Fig.4.Adsorption isotherms of BPA onto ACNF-K-600-3,the K2CO3 activated ACNFs from PAN,activation conditions: 600 °C,3 h,pH=7 in the starting solution.

The adsorption capacity (183.6 mg·g-1) was comparable with the reported capacities of other adsorbents [20,24].Fig.4 shows the experimental adsorption uptake of BPA on ACNF-K-600-3 with regression analyses of three models: Langmuir isotherm,Freundlich isotherm and R-P isotherm.Table 2 shows the isotherm parameters and the values of the correlation coefficient (R2).The values ofR2are higher for R-P isotherm and Langmuir isotherm than Freundlich isotherm,indicating that R-P isotherm model and Langmuir model offer better fitting in the adsorption of BPA on the ACNFs.The maximum adsorption capacityqmcalculated by the Langmuir isotherm model is 186.8 mg·g-1,which is quite close to the experimental adsorption capacity of 183.6 mg·g-1at the equilibrium point.Interestingly,the β value of 0.987 from R-P isotherm model is very close to 1,which indicates the R-P isotherm is close to Langmuir isotherm.Langmuir isotherm based on the assumption that adsorption happens on a homogeneous surface with equivalent sites.The relatively good fitting of the experimental adsorption isotherm using Langmuir and R-P isotherm model can be explained by the homogeneous surface nature of the ACNFs made from a pure precursor of PAN based nanofibers.

Table 2 Isotherm parameters for BPA adsorption on ACNF-K-600-3

3.4.2.Adsorption kinetics

Fig.5 shows the kinetics of BPA adsorption.From the timedependent uptake curve in Fig.5(a),the equilibrium time of the adsorption appeared to be 2 h,and 90% was adsorbed within 1 h.In order to deeply understand the result of the kinetics of BPA adsorption on the ACNFs,the adsorption kinetic data were fitted using pseudo first-order,pseudo second-order,and intraparticle diffusion models.Kinetic model fitting plots and parameters are shown in Fig.5(b) and (c) and Table 3.The correlation coefficient(R2) for pseudo second-order model is 0.9996,much higher than that for pseudo first-order model,which means the pseudo second-order model is suitable to represent the adsorption kinetics.Moreover,the adsorption capacity calculated from the pseudo second-order model is 182.15 mg·g-1,which is also very close to the experimental adsorption capacity.

Table 3 Kinetic parameters for BPA adsorption on ACNF-K-600-3

Fig.5.(a)Adsorption kinetics of BPA onto ACNF-K-600-3,the K2CO3 activated ACNFs from PAN.(b)Pseudo-first order model fitting.(c)Pseudo-second order model fitting.(d)Intraparticle diffusion model fitting;activation conditions: 600 °C,3 h.Starting concentration of BPA in the batch equilibrium adsorption test is 300 mg·L-1,pH=7.

The process of adsorption happens in several steps:external diffusion,intraparticle diffusion,and the actual adsorption on the surface.The intraparticle diffusion model is used to fit the experimental data to reveal the rate-controlling step in the BPA adsorption process.As shown in Fig.5(d),the adsorption of BPA on process is divided into two steps,and the first step is the controlling step.The value of the intercept ‘‘C” is very close to zero,which means the adsorption is mainly controlled by pore diffusion.This less complex adsorption process is largely related to the narrow fiber size distribution of the ACNFs,which led to the intrafiber diffusion dominating the adsorption process.The values of rate parameterskid,1,CandR2are presented in Table 3.It can be seen that the intraparticle diffusion model fits the experimental data better than the pseudo first-order model.But pseudo secondorder model fits best the experimental data,indicating that the adsorption of BPA on the ACNFs is also related to the initial concentration of the adsorbate.

3.4.3.Effect of pH and ionic strength

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The pH of the aqueous solution affected the adsorption of BPA on the ACNFs.The effect of the initial pH of the solution on the adsorption of BPA was investigated at the pH range from 3 to 12.It can be seen in Fig.6(a),the adsorption capacity was slightly decreased when pH was decreased from 7 to 3.When pH was higher than 9,the adsorption capacity is getting lower gradually.When pH is 12,there is a dramatic decrease in the adsorption capacity.Both the surface charge density of the ACNFs and the charge of BPA are affected by the pH value of the solution.As shown in Fig.6(b),the pHpzcof ACNF-K-600-3 is approximately 1.97.When the pH of the solution is higher than 1.97,the surface charge density of the ACNFs is negative.On the other hand,the value of pKafor BPA is 9.6-10.2 [25].When the pH is above 9.6,BPA is deprotonated and in an anionic state,which result in the repulsive electrostatic interaction between the adsorbent and the adsorbate [19].The electrostatic interaction between BPA molecules and the surface of ACNFs could be the mechanism of the adsorption and explain the decreased adsorption of BPA at high pH conditions.Another important factor for the adsorption could be the hydrophobic interaction between BPA molecules and the surface of ACNFs,as the FT-IR results in Fig.S6 indicated that the chemical bonds on the ACNFs had hydrophobic characteristics.

Fig.6.(a)Influence of the pH on the adsorption of BPA onto ACNF-K-600-3(activation conditions:600°C,3 h,starting concentration of BPA is 300 mg·L-1,adsorption time is 24 h).(b) Zeta potential of ACNF-K-600-3 suspensions with different pH values.

Fig.7 shows the effect of ionic strength from Na+,Mg2+,and Ca2+on the adsorption of BPA onto ACNFs.The adsorption of BPA showed a trend of decrease while the ionic strength increased,even though there were fluctuations for the adsorption when the concentration of Na+and Mg2+were increased.The same phenomenon was also observed by others [18,19].The decrease of the BPA adsorption with the increased ionic strength could be attributed to the screening effect between the surface of ACNFs and the BPA molecules [35].The fluctuation of the adsorption of BPA was related to the small quantity of ionic strength could occupy the adsorption sites of ACNFs and decrease the adsorption capacity [19].

Fig.7.Influence of ionic strength of(a)NaCl,(b)MgCl2,(c)CaCl2 on the adsorption of BPA onto ACNF-K-600-3,activation conditions: 600 °C,3 h.Starting concentration of BPA is 300 mg·L-1.Adsorption time is 24 h.

3.4.4.Regeneration

The used adsorbent was regenerated by calcination at 400°C in N2for 2 h.After regeneration,the BPA adsorption capacity of the regenerated adsorbent was tested.The regeneration test was repeated five times following the same procedure.As shown in Fig.8,the regenerated adsorbent still kept similar capacity compared with the original adsorbent.It reached 83.7% of the original capacity after 5 times of regeneration.During the regeneration process at 400 °C,the BPA molecules adsorbed in the pores were removed,the adsorption sites were recovered.The low level of the decrease of the adsorption capacity indicates a good recycling performance.

Fig.8.Regeneration test of ACNF-K-600-3 regenerated by calcination at 400 °C in N2 for 2 h.

3.5..Dynamic adsorption of bisphenol-A

To assess the BPA adsorption performance on the ACNFs under dynamic conditions,fixed-bed adsorption measurements were performed with a bed concentration of 10 mg·L-1in two different flow bed systems:cross flow bed system and packed flow bed system,as shown in Fig.S1.Fixed-bed adsorption is commonly used for separation and purification in Industry.Employing fixed-beds adsorption could help to better understand the scalability of an adsorption process.Fig.9 shows the resulting fixed-bed outlet BPA concentration adsorption profiles.The black control of the breakthrough curves was conducted in the fixed beds flowed with BPA solution without adsorbent,as shown in Fig.S7.The residence time,velocity of the solution while using different height,and the mass of the ACNFs mats in cross flow bed system,along with the residence time,velocity of the solution while using different numbers of layers,and the mass of the ACNFs mats in the packed bed system were shown in Table 4.In Fig.9(a),the slope of the mass transfer zone displayed by the bed exit concentration profiles for BPA was decreasing when the bed height of the adsorbent increased,which could be due to the larger adsorbent height leading to higher adsorption.Fig.9(b) shows that the increased flow rate caused higher slope of the mass transfer zone,which could be attributed to the fact that the diffusion of BPA molecules into the pore structures of the ACNFs takes time,and the increased flow rate results in a shorter contact time of the solution with the adsorbent.In Fig.9(c),the slope of the mass transfer zone decreased while increasing the number of the adsorbent layers in the packed bed system,because more layers of the adsorbent gave larger mass of the adsorbent and longer contact time for the BPA solution and the adsorbent as shown in Table 4.

Table 4 Adsorption conditions and adsorption loading in the cross flow bed system and packed bed system

Fig.9.Experimental (colored dots) and modified dose-response (MDR) model(black solid line)breakthrough curves for BPA adsorption in fixed-beds with ACNFs(ACNF-K-600-3): (a) effects of bed height in cross flow bed (flow rate: 80 ml·h-1,bed height: 2,4,6 cm);(b) effects of flow rate in cross flow bed (flow rate: 40,60,80 ml·h-1,bed height:2 cm);(c)effects of layer numbers in packed flow bed(flow rate: 40 ml·h-1,adsorbent layers: 1,2,3).

The results of the loading amount at the breakthrough point(qb) in the cross flow bed and packed bed were shown in Table 4.In both systems,the flow bed with less amount of adsorbent and slow flow rate gave higher loading amount at the breakthrough point.The adsorbent with one layer got a loading amount of 39.8 mg·g-1at theqbwhich is the highest among the tests.The possible reason for the high loading amount atqbon the one layer adsorbent bed is that the low adsorbent amount can have high concentration gradient of BPA between that in the adsorbent and that in the solution.

The modified dose-response(MDR)model was typically used to predict the fixed-bed outlet concentration profiles,suggesting its suitability to be used for the design and scale-up purpose.However,MDR model is an empirically derived model,there are no physical meanings for the parameters.The MDR model breakthrough curves with the experimental breakthrough curves for BPA adsorption in the fixed-beds with ACNF-K-600-3 are shown in Fig.9.The fitting parameters of MDR model are presented in Table S1.In general,MDR model fits the experimental breakthrough curves quite well,as the regression coefficients (R2) are all above 0.98.The experimental curve for the adsorbent in packed flow bed with 3 layers of adsorbent and a flow rate of 40 ml·h-1has aR2value of 0.9987,which is the best fitting among the tests.Such a good fitting of the experimental breakthrough curves indicates that MDR model could make the prediction of the fixed-bed outlet concentration profiles in the fixed-bed system for BPA adsorption.In the cross flow bed study,the values of parameter‘‘b” increased as bed height increased.This trend was also previously reported by others[36].Through the parameters found with this model,it is possible to obtain an expression that reproduces the behavior of the bed in other experimental conditions,without the need to carry out more experiments [37].

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Inspired by the results of the influence of pH condition on the adsorption of BPA,the saturated ACNFs bed was flowed with a NaOH solution with a pH value of 12 for 6 h to regenerate the used ACNFs.Fig.S8 showed that the regenerated ACNFs did not show a full recovery after two cycles of regeneration,which could be attributed to that the adsorbed BPA molecules had strong interaction with the surface of ACNFs and could not be removed by a simple washing with basic solutions.

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4.Conclusions

Introducing K2CO3or ZnCl2as activating agent into the PAN in DMF solution for electrospinning was shown to be an effective method of making ACNFs,even though K2CO3is not soluble in DMF.The ACNFs formed after stabilization and activation had high porosity and mechanical strength.The morphology of the K2CO3activated ACNFs were different form the ZnCl2activated ones,due to the different viscosities of the starting solutions.The K2CO3activation can result in stronger mechanical strength for ACNFs than ZnCl2activation.Activation time and temperature significantly affect the porosity of ACNFs and the performance of BPA adsorption from aqueous solution.External surface area from large pores was proven to determine the adsorption capacity of BPA,which could be ascribed to that BPA molecule with large size tend to be adsorbed in large pores.The isotherm from batch adsorption was better described by Langmiur and R-P models than Freundlich model,which is ascribed to the homogeneous surface nature of the ACNFs from PAN.The kinetics study confirmed that the adsorption of BPA into ACNFs was controlled by the intra-pores and the small fiber size could reduce the diffusion mass transfer.High pH value and ionic strength in the solution was shown to hinder the adsorption of BPA into ACNFs.The dynamic adsorption results demonstrated that the adsorption of BPA into ACNFs is scalable in the fixed-bed systems.The MDR model can make a good prediction of the fixed-bed outlet concentration profiles.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Dr.Jian Ren is thanked for the help with the mechanical strength measurements.This work was financially supported by the National Science Foundation (1438518).

Supplementary Material

Supplementary material to this article can be found online at https://doi.org/10.1016/j.cjche.2023.03.017.