Dengfeng Zhang*,Peili Huo,Wei Liu

Faculty of Chemical Engineering,Kunming University of Science and Technology,Kunming 650500,China

1.Introduction

The phenolic compounds are highly carcinogenic and toxic even at low concentration[1].Thus,the removal of phenol from wastewater is extremely necessary.Hitherto,various treatment technologies including adsorption[2],biodegradation[3],oxidation[4],precipitation[5],ion exchange[6]and solvent extraction[7]have been developed for the removal of phenol from wastewater.It is widely accepted that activated carbon(AC)is the most frequently-used adsorbent since it has well-developed porous structure and specialsurface chemistry property[8].Therefore,AC has been widely used in environmental monitoring[9],environmentalprotection[10],catalystsupportand electrode materialpreparation[11,12].For phenolic compound adsorption,usage ofAC is also considered as a prevalent method[13-15].

Research has shown that the properties of the adsorbent,adsorbate and the operational parameters will in fluence the liquid-phase adsorption process[16].Among them,the porous structure and surface chemistry property of the adsorbent act dominant roles in the adsorption process[17].Thus,for the adsorption system formed by phenol and AC,most investigations were carried out to modify the porous structure and surface chemistry of AC in order to enhance phenol adsorption[2,18-20].Hitherto,it has been found that usage of chemical agents,such as potassium hydroxide(KOH)[2],zinc chloride(ZnCl2)[18],potassium carbonate(K2CO3)[19],and phosphoric acid(H3PO4)[20],can produce AC with well-developed pore morphology and enhanced phenol adsorption capacity.In addition,the introduction of some basic nitrogen-containing functional groups,such as amine,pyridinic,pyrrolic and quaternary nitrogen groups,will increase the higher π-electron density in basal plane of AC and thus lead to higher phenol adsorption[21].The introduction of some basic nitrogen-containing functional groups can be realized by the exposure of AC to gaseous ammonia(NH3)[22],urea(H2NCONH2)and melamine(C3H6N6)[23].Itis needed to notice that the abovementioned activation methods are categorized into chemical activation and they always incorporate complex procedures compared with physical activation.In order to make a supplement to the physical activation research,a thermal modified method for AC was put forward in this work.The mechanism of thermal modification was elucidated based on porous structure and surface chemistry analyses.The kinetics,equilibrium and thermodynamics behaviors of phenol adsorption on the modified AC sample were also investigated.

2.Materials and Methods

2.1.Precursors and thermal modification of AC samples

The raw AC sample was obtained from coconut shell,which underwent carbonization and steam activation in turn.The thermal modification of raw AC sample was conducted in a muffle furnace.Raw AC sample was loaded in a closed crucible.The loaded crucible was heated isothermally for 1 h.In order to investigate the heating temperature dependence of phenol adsorption,three temperature points(700,900 and 1100°C)were studied.The raw and modified AC samples were crushed and sieved into diameters between 0.3 and 1.4 mm.

All the AC samples were preserved in sealed plastic containers,and helium(He)was injected to prevent undesired oxidation.Before each adsorption test,samples were dried at 105°C for 24 h under vacuum condition.

2.2.Adsorption of phenol

Batch adsorption of phenol from the aqueous solutions were conducted at 25-55°C by agitating 12 g of each AC sample with 1200 ml of phenol solution of desired concentration in a flat bottom flask.The flask was placed in a thermostatic shaker bath at 200 r·min-1.In order to determine the phenol concentration,liquid sample was extracted and filtered.The phenol concentration of the pre-processed liquid sample was determined using a PUXI T6 UV-Vis ultraviolet spectrophotometerata wavelength of270 nm.Each experimentwas carried out in duplicates and the arithmetic mean value was adopted as the final phenol concentration.The adsorption amount of phenol on AC sample at adsorption time(t),qt,was calculated according to

where qt(mg·g-1)is the adsorption amount of phenolon AC sample at t;V(L)is the volume of the solution;C0and Ct(mg·L-1)are the initial phenol concentration and instantaneous phenol concentration corresponding to t,respectively;m(g)is the mass of AC sample.

The kinetics curves of phenol adsorption on various AC samples were generated by plotting qtversus t.

When the adsorption reach equilibrium state,the equilibrium adsorption amount of phenol(qt,mg·g-1)was given by

where Ce(mg·L-1)is the equilibrium concentration of phenol.

The adsorption isotherms of phenol adsorption on various AC samples were obtained by plotting qeversus Ce.

2.3.Pore morphology of AC samples

The pore morphologies of AC samples were analyzed using the ASAP 2020,provided by Micromeritics instruments.Prior to pore morphology analysis,all the samples were degassed under vacuum for 12 h to effectively drive out the residual gas and moisture.The nitrogen adsorption and desorption isotherms at 77 K were collected at relative pressures(p/p0)ranging from 0.005 to 0.995.In this work,the specific surface area,pore volume and pore size of each sample were analyzed based on the nitrogen adsorption and desorption data.Further calculative details can be found in[24].

2.4.Surface chemistry of AC samples

It has been pointed out that oxygen-containing functional groups have an importantimpacton phenoladsorption on carbon-based adsorbents[25-27].And oxygen-containing functional groups usually occur in the form of carboxyl,phenolic hydroxyl and lactone base[28].Thus,the amounts of the above groups on AC samples were determined by the frequently-used titration method proposed by Boehm[29].

In addition,Fourier Transform Infrared(FTIR)spectroscopy analysis using FTIRspectrometry(EQUINOX55,Bruker Corp.,Germany)was also applied to determine the surface chemistry of the AC samples.All the samples and the dried KBr were ground at a mass ratio of 1:100.The FTIR spectra were generated with 32 scans at a resolution of 4 cm-1.

2.5.X-ray diffraction analysis

X-ray diffraction(XRD)analysis using nickel- filtered Cu Kαradiation(λ=0.154056 nm)was performed on Rigaku D/Max-2550PC at 40 kV and 200 mA.The scanned angle(2θ)was limited between 5°and 75°with step size 0.01°.

3.Results and Discussion

3.1.Thermal modification temperature

The modified AC samples obtained from 700 °C,900 °C and 1100 °C together with the raw state were designated as AC1,AC2,AC3and AC0,respectively.In order to primarily explore the phenol adsorption performance of all AC samples,adsorption experiment was carried out at temperature of 25°C and initial phenol concentration of 1000 mg·L-1.As shown in Fig.1,it can be observed that the phenol adsorption of all the three AC samples after thermal modification is superior to the raw sample and the order of phenol adsorption is as follows:AC2＞AC1＞AC3＞AC0.In consideration of phenol adsorption on various modified AC samples,the optimum modification temperature is 900°C,followed by 700 °C and 1100 °C.

Fig.1.Phenol adsorption curves of various AC samples.

3.2.Characteristics of AC samples

The nitrogen adsorption-desorption isotherms of four AC samples were shown in Fig.2.In accordance with the classification approach of Brunauer,Deming,Deming,Teller(BDDT)[30],all the adsorption isotherms(adsorption branch)are categorized into type I isotherm which is described formicroporous adsorbents.The hysteresis loop constituted by adsorption branch and desorption branch is also detected for each AC sample.The shape of the hysteresis loop can help to assess to the pore shape of the adsorbent[16].Because slight difference is observed between the hysteresis loop of isotherms of the raw AC sample and the otherthree modified AC samples,itcan be concluded thatthermal modification method has a minimal effect on the pore shape of AC sample.

Based on the low temperature nitrogen adsorption and desorption data presented in Fig.2,the pore morphology parameters of AC samples including specific surface area,pore volume and pore size were given in Table 1.It can be seen that the raw sample(AC0)is a highly-developed micro-mesoporous carbon-based material with larger BET specific surface area,pore volume and smaller pore diameter in comparison with all the thermal modified AC samples.With the modification temperature increasing from 700 to 1100°C,a degradation trend in pore morphology is observed.It is reported that the heat treatment will cause the expansion of microcrystalline structure of AC[31].Therefore,it can be inferred that the pore structure of thermal modified AC samples will collapse and degrade.

Fig.2.Nitrogen adsorption-desorption isotherms of various AC samples at 77 K.

Table 1 Pore morphology parameters of AC samples

As can be seen in Fig.3,XRD spectra present two broad asymmetric peaks corresponding to 25°and 45°,respectively,which can be assigned to disordered graphitic 002 plane and 100 plane,respectively[32].Yoshizawa et al.concluded that the 002 peak confirmed the presence of micropore wall structure[33].In comparison with AC0,the intensity of AC3corresponding to 002 peak is greatly weakened,which means that thermal modification at extremely high temperature of 1100°C leads to the degradation of pore morphology.The above conclusion is also consistent with the results of pore morphology analysis.

It is acknowledged that degradation in pore morphology of all the thermal modified AC samples will lead to the decrease of phenol adsorption.However,as can be seen in Fig.1,the AC samples after thermal modification exhibitsuperior phenoladsorption capacity.Thus,itcan be deduced thatthe surface chemistry property ofthe AC samples will play a dominantand positive role in phenoladsorption process.According to the results of Boehm's titration method,the concentrations of three main oxygen-containing functional groups(phenolic hydroxyl,lactone base and carboxyl)of all the AC samples were shown in Fig.4.As shown in Fig.4,it is found that the thermal modification will cause the decrease of the total amountof carboxyl,phenolic hydroxyl and lactone base.

Fig.3.X-ray diffraction patterns of various AC samples.

Fig.4.Oxygen-containing functional groups of various AC samples.

In addition,FTIR analysis was also used to determine the surface chemistry change between the raw AC sample and thermal modified AC samples.As can be seen in Fig.5,the band intensity atapproximately 1700 cm-1attributed to the stretching vibration of CO bond in carboxylic,lactone and ester[34]of AC0is stronger than that of AC1,AC2and AC3.The bands at approximately 3306 and 3271 cm-1are assigned to phenolic hydroxyl[35].It can be seen that the intensity of phenolic hydroxyl in all the three thermal modified AC samples are lower than that of the raw sample.Thus,FTIR analysis also confirms that thermal modification leads to the decrease in main oxygen-containing functional groups.

Fig.5.FTIR spectra of various AC samples.

Ithasbeen confirmed thattwo mechanismscontribute to phenoladsorption,i.e.,π-π dispersion force interaction and the formation of electron-donor-acceptor complex[36-38].The π-π dispersion force interaction mechanism mainly exists between the π electrons in the aromatic rings of phenolmolecule and the delocalized πelectrons in basal planes of AC[39].The formation of electron-donor-acceptor complex mainly appears with electrolytes,essentially when phenol is ionized underthe experimentalconditions used[37].In the case oftemperature operated at between 25 and 55°C,the degree of ionization phenol adsorption on both raw and modified AC samples is lower and phenol mainly exists in the form of molecule[40].Thus,the π-π dispersive force interaction is dominant.It is known that oxygen atoms bound to the carbon can decrease the π electron density and weaken the dispersion forces between phenol π electron ring and carbon π electrons[41].Based on the above analyses,the lower oxygen-containing functional groups determined in this work(as shown in Fig.6)will strengthen the π-π dispersive force interaction between phenol and thermal modified AC samples(AC1,AC2and AC3)and lead to the higher phenol adsorption.Additionally,it can be seen from Fig.4 that the total amount of oxygen-containing groups of AC sample treated at 900°C(AC2)is lower than that of 700 °C(AC1)and 1100 °C(AC3),which determines the superior phenol adsorption performance of AC2as indicated in Fig.1.Thus,the adsorption behaviors including kinetics,equilibrium and thermodynamics of AC2in combination with AC0as a reference were elaborated in details.

3.3.Adsorption kinetics

The kinetics of phenol adsorption was conducted at temperature of 25 °C and initial phenol concentration range of 200-1400 mg·L-1.The kinetics curves of AC0and AC2,i.e.the amount of phenol adsorbed(qt)versus time(t),were shown in Fig.7(a)and(b),respectively.

Hitherto,Lagergren model and pseudo-second order model were always adopted to describe the adsorption kinetics of an adsorbate from aqueous solution[42,43].The equations of Lagergren model and pseudo-second order modelwere given by Eqs.(3)and(4),respectively[43-46].

where t(min)and qt(mg·g-1)are respectively the time and the amount of phenol adsorbed on AC samples at any time t;qe,expand qe,cal(mg·g-1)are the amount of phenol adsorbed at experimentalequilibrium and calculated-equilibrium,respectively;k1(min-1)and k2(g·mg-1·min-1)are Lagergren and second order rate constants.In Eqs.(3)and(4),the parameters qe,cal,k1and k2can be obtained by linear regression based on the data shown in Fig.7.The fitting results of AC0and AC2were shown in Tables 2 and 3,respectively.

As can seen in both Fig.7(a)and(b),the fitting accuracy of pseudosecond ordermodel is superiorto Lagergren model.In addition,itis also found that the correlation coefficients(R2)for the pseudo-second order model of both AC0and AC2exceed 0.99,which are higher than that of Lagergren model(ranging from 0.6994 to 0.9700)for both AC0and AC2.In addition,the calculated values of qe,calare extremely close to the experimental data,i.e.qe,exp[47],whereas the gap between qe,caland qe,expfor Lagergren model is more obvious.Thus,the pseudosecond order model is suitable to represent the adsorption kinetics of phenol on the raw and thermal modified AC samples.This result is consistent with the kinetics behavior of phenol adsorption on other AC samples[48,49].It is worth noting that the pseudo-second order model is established based on the hypothesis that the rate-limiting step may be controlled by valency forces through sharing orexchanging of electrons between adsorbent and adsorbate[50].As a result,the pseudo-second-order kinetics model well fitting to phenol adsorption on the AC samples indicates that phenol adsorption on AC samples is probably due to the interaction between the π electrons in phenolic ring of phenol and the basal plane of AC sample[41].This conclusion also supports our viewpoint in Section 3.2.

Itis also found thatfora certain initialphenolconcentration,the rate constant k2of AC2is greater than that of AC0,which means that the thermal modification can also enhance the adsorption rate of phenol on AC sample.

Fig.6.Structure of three main oxygen-containing functional groups.

Fig.7.Adsorption kinetics curves of phenol adsorption on(a)AC0 and(b)AC2.

Table 2 Fitting results of the Lagergren and pseudo-second-order kinetics models of phenol adsorption on AC0

3.4.Adsorption equilibrium

The knowledge of adsorption equilibrium is helpful to understand the mechanism of an adsorption process and the isotherm model is always treated as an effective tool.In this work,two classical isotherm models,i.e.the Freundlich and Langmuir models as respectively shown in Eqs.(5)and(6),were adopted to describe the interaction between phenol and AC[51,52].

where kF(mg·g-1· (mg·L-1)-1/n)is the Freundlich constant;1/n is the heterogeneity factor and the dimensionless parameter n for most practical systems is greater than unity[14,16],qmax(mg·g-1)is the monolayer adsorption capacity and kL(L·mg-1)is the Langmuir adsorption constant.

Table 3 Fitting results of the Lagergren and pseudo-second-order kinetics models of phenol adsorption on AC2

The parameters incorporated in Freundlich and Langmuir models can be generated through linear fitting method based on the adsorption data(shown in Fig.8)of phenol adsorption on AC samples.

Fig.8.Adsorption isotherms of phenol adsorption on AC samples.

As can be seen in Table 4,comparison of the fitting parameter R2shows that Langmuir model yields a better fitting result(0.9798-0.9993)than Freundlich model(0.9002-0.9554).Thus,it indicates that the Langmuir model rather than Freundlich model can well describe the adsorption behavior of phenol on raw and thermal modified AC samples in the range of phenol concentration and temperature explored in this work.Langmuir model assumes a localized monolayer adsorption with the allowance that one adsorbate molecule can occupy more than one adsorption site[51].Therefore,it can infer that the mechanism of phenol adsorption on AC samples is intended to obey monolayer adsorption mechanism.

Table 4 Fitting results of Freundlich and Langmuir models of phenol adsorption on AC samples

In addition,it is found that the monolayer phenol adsorption capacity(qmax)for both AC0and AC2is almost a constant which is independentwith temperature.This result is also consistentwith the definition of qmaxin Langmuir model.The average qmaxof thermal modified AC sample(AC2)is 144.93 mg·g-1,increased by 21.25%than the raw sample(119.53 mg·g-1).

3.5.Adsorption thermodynamics

Based on Langmuir model,the heat of adsorption of phenol on AC sample(ΔHL)can be calculated by Eq.(7)[25].in which:T(K)is the isothermaltemperature;R(8.314 J·mol-1·K-1)is the universal gas constant.

According to Langmuir parameter KLshown in Table 4,ΔHLof AC2was generated by linear plotting ln KLversus 1/T.The plotting result shows that ΔHLof AC2is-7.12 kJ·mol-1.The negative value of ΔHLindicates that the phenol adsorption on the modified AC sample is exothermic process and temperature increasing is not favorable to phenol adsorption.Additionally,the absolute value of ΔHadsis in the range of≤40 kJ·mol-1,which confirms that adsorption of phenol on the modified AC sample is mainly consistent with a mechanism involving physical adsorption[25].

4.Conclusions

In this work,the phenol adsorption behavior on thermal modified AC samples has been studied and the major conclusions from the test results can be summarized as follows.

In comparison with the raw sample,the thermal modification at temperature range of 700-1100°C can improve the phenol adsorption capacity.The pore morphology and surface chemistry property analyses confirm that the enhancement of phenol adsorption is attributed to the decrease in oxygen-containing functional groups on AC sample.The π-π dispersion force interaction can explain mechanism of thermal modification.For the test temperature range between 700 and 1100°C,the optimum modification temperature is 900°C and the maximum phenol adsorption capacity of the obtained AC sample can reach 144.93 mg·g-1which is higher than that of the raw sample,i.e.119.53 mg·g-1.For the raw and the optimum modified AC samples,the pseudo-second order kinetics and Langmuir models are found to fit the experimentalkinetics and equilibriumdata very well,respectively.Thermodynamics study shows that the phenol adsorption on the optimum modified AC sample is exothermic and mainly via physical adsorption.

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