Xinlong Yan *,Yanfang Li ,Xiaoyan Hu ,Rui Feng ,Min Zhou ,Dezhi Han

1 Key Laboratory of Coal Processing and Efficient Utilization(Ministry of Education),School of Chemical Engineering&Technology,China University of Mining and Technology,Xuzhou 221116,China

2 State Key Laboratory Base of Eco-chemical Engineering,College of Chemical Engineering,Qingdao University of Science and Technology,Qingdao 266042,China

Keywords:Zeolitic imidazolate framework-8 Activated carbon Carbonization Phenol Adsorption

ABSTRACT Trace zeolitic imidazolate framework-8(ZIF-8)-decorated activated carbon(AC)pellets were synthesized by a facile wet impregnation technique.After pyrolysis of the above composite material,the obtained carbon had a large surface area and pore volume,with traces of Zn on its surface.Subsequently,the capacity of the ZIF8/AC samples to adsorb and remove phenol from aqueous media was evaluated in both batch and column experimental setups.The equilibrium adsorption capacity reached 155.24 mg·g−1,which was 2.3 times greater than that of the pure AC(46.24 mg·g−1).In addition,adsorption kinetics were examined by pseudofirst and pseudosecond order models,and adsorption isotherms were fitted into Langmuir and Freundlich equations.The adsorbent could be easily filtered from the solution and washed with methanol and water,while maintaining an efficiency＞90%after 4 cycles.The above results make it a potentially reusable candidate for water purification.

1.Introduction

In recent years,water pollution represents a principal worldwide environmental threat.Phenol and its derivatives are extensively used in industry and often lead to the formation of stable chemical species[1,2],making them the major organic pollutants in wastewater.To avoid discharge into the environment,several techniques for eliminating phenols from wastewater have been applied,such as chemical oxidation,solvent extraction,photocatalytic decomposition,membrane filtration and adsorption[3–7].Among them,adsorption appears to be the most effective approach owing to its simple experimental setup,high efficiency and low cost[8,9].The chosen adsorbent often plays a critical role in efficient adsorption.Therefore,diverse materials have been utilized for phenol removal,including AC,clays,zeolites,and mesoporous metal oxides[10–13].

The large surface area(SA)and pore volume(PV),low cost and easy availability make activated carbons(AC)the most extensively studied adsorbents for wastewater treatment.However,the low adsorption capacity and poor selectivity of AC prepared by conventional methods limit its application [14–17].Therefore,various methods have been developed to modify the properties of AC,with the main focus being a porous structure and surface chemical properties.Chen et al.[18]discovered that the phenol adsorption capacity of ammonia-treated AC was superior to that of pristine AC.Gokce et al.[19] used HNO3-treated AC for phenol adsorption in aqueous solution.Despite a significant reduction in the total SA and PV of HNO3-treated AC,the adsorption capacity(expressed as mg phenol·m−2)increased.Atieh et al.[20]reported that Fe2O3,Al2O3and TiO2-modified AC exhibited greater adsorption capacity compared with that of pure AC.It was obvious that N and metal oxide modification could improve the AC removal efficiency for phenol from water.However,during the impregnation process,most of the AC pores were occluded,which was unfavorable to adsorption owing to the decreased available sites.

Recently,metal and N codoped porous carbons synthesized by pyrolysis of metal–organic frameworks(MOFs)were extensively studied because of their three-dimensional structures,diverse properties,and excellent performance in wastewater treatment [21–23].However,the difficulty of separating nanosized particles from water,as well as their high cost,hampered their widespread and large-scale applications.

In this work,trace amounts of ZIF-8 were decorated onto AC pellets with a simple impregnation method and then calcined at 925°C for 6 h under Ar gas.The obtained carbons were characterized and utilized as an adsorbent for phenol elimination from aqueous medium.The adsorption kinetics,thermodynamics,and recyclability of the obtained carbon were examined in detail.The results indicated a superior adsorption capacity of ZIF-8 decorated AC toward phenol(155.24 mg·g−1)compared to AC pellets(46.2 mg·g−1).In addition,the good stability,reusability and easy separation from a solution make ZIF-8-decorated AC a promising material for the adsorption of phenol from wastewater.

2.Experimental

2.1.Materials

The zinc nitrate hexahydrate(＞99%),methanol(≥99.5%)and phenol(99%) were purchased from Sinoreagent.The 2-methylimidazole(HmIm) (98%) was purchased from J&K Scientific Company.All compounds were p.a.grade and used without additional purification.Coconut shell-AC was supplied by Shaowu Xinsen Carbon Company.

2.2.Synthesis of ZIF-decorated AC

The selected ACs(10–20 mush)were rinsed twice with distilled H2O and dried at 80°C before further use.Different amounts of zinc nitrate(0.5,0.25,0.125 mol)and 1 mol of 2-methylimidazole were dissolved in 90 ml MeOH.A total of 1.2 g of dry AC was combined with each of the above solutions.Afterward,the mixtures were ultrasonicated for 20 min and then left overnight.After filtration and washing with methanol,the obtained particles were dried at 75°C and denoted as 12-ZIF8/AC,14-ZIF8/AC,and 18-ZIF8/AC,respectively.The above particles were heated in a tubular furnace under inert atmosphere(Ar gas flow,15 ml·min−1)at 925°C for 6 h,with a heating rate of 5°C·min−1.After returning to room temperature,the resulting solids were collected and named cal12-ZIF8/AC,cal14-ZIF8/AC and cal18-ZIF8/AC,respectively.

2.3.Characterization

Powder XRD spectra of the materials were analyzed with a Bruker D8 X-ray diffractometer (CuKαradiation).The morphology of the materials was characterized with an FEI Quanta 400 FEG or Zeiss Gemini 300 SEM.The FT-IR spectra were acquired on a Nicolet IS5 in the range of 4000–400 cm−1.Thermogravimetric analyses(TGA)were performed under a nitrogen atmosphere using a TG Netzsch STA 449 F5 analyzer with a 10°C·min−1heating rate.The nitrogen adsorption isotherms were recorded using a Quantachrome IQ2 porosimeter.The SA and pore size distribution (PSD) were evaluated by BET and NLDFT techniques,respectively.The total pore volume was estimated as the volume of liquid N2adsorbed.The micropore volume (Vmicro) was acquired by t-plot while mesopore volume was given via subtraction of Vtotalwith Vmicro.The Zinc content in the samples were quantified by an Agilent 7900 inductively coupled plasma-mass spectrometry(ICP-MS).

2.4.Adsorption studies

In a typical experiment for recording the adsorption isotherm,0.01 g of the adsorbent was shaken at a constant pace for 24 h with 50 ml of phenol solutions with varying concentration(10–240 mg·L−1)at 30°C.After separating the solid with a PTFE syringe filter(0.22 μm),the concentration of phenol was determined spectrophotometrically at the characteristic wavelength of 270nm (Persee TU-1810 UV–Vis spectrophotometer,China).The adsorption capacity(qe,mg·g−1)of phenol was obtained as:

where C0is the starting and Ceis the equilibrium amount of phenol(in mg·L−1),V is the volume of the phenol solution in liters,and m is the quantity of the adsorbent in g.

For the kinetic experiments,0.02 g of the adsorbent was shaken with 100 ml of phenol solution(60 mg·L−1),and the adsorption capacity(qt)was analyzed at various time intervals according to the method mentioned above.

2.5.The adsorption studies using a fixed-bed column

The fixed-bed column experiments were carried out in a 40 cm×1 cm glass column (height × diameter).The column was successively filled with irregular glass beads with a size of 3–4 mm,quartz wool and AC(～2 cm high).The 40 mg·L−1phenol solution was continuously supplied to the column(upward flow),with a 10 ml·min−1flow rate.Ctrefers to the outlet,while C0is the inlet adsorbate concentration.At certain time intervals,aliquots of effluent were taken,and the amount of phenol was quantified spectrophotometrically.

3.Results and Discussion

3.1.Characterization

3.1.1.XRD

The XRD patterns of all materials are represented in Fig.1.Two broad peaks at～23° and～43° could be observed in all samples and represented the characteristic diffraction peaks for amorphous AC[24].After soaking with the precursor of ZIF-8,small peaks merged at 2θ of 7°,10°,12°and 17°,which were the typical peaks of the ZIF-8 crystal[25]and suggested that ZIF-8 was successfully impregnated onto AC.In addition,the highest amount of ZIF-8 was incorporated in 14-ZIF8/AC,as it exhibited the strongest peaks relevant to those of ZIF-8.Calcination probably decomposed the ZIF-8 crystal structure,as indicated by the disappearance of characteristic XRD signals for ZIF-8,while the signals of the AC do not change.In addition,no peaks related to Zn or Zncontaining compounds could be observed,possibly because the Zn in ZIF-8 was evaporated at high temperature since the boiling point of zinc(～907°C)was lower than the calcination temperature.

3.1.2.SEM

The SEM images of the synthesized samples are given in Fig.2.Randomly distributed pores with different sizes could be seen on the AC sample.With the impregnation of ZIF-8,small rhombicdodecahedral particles emerged on the outer and inner surface of the AC,suggesting the efficient incorporation of ZIF-8 into the AC.Besides,the corresponding elemental mapping(Fig.S1)analysis disclosed the majority of zinc and nitrogen from the ZIF-8 central metal and ligand on the AC surface.Calcination caused the disappearance of these particles,and a rough surface decorated with irregular particles can be observed in cal14-ZIF8/AC since the ZIF-8 crystal decomposed at high temperatures.The distribution of the elements on cal14-ZIF8/AC was further investigated by EDX spectroscopy(Fig.2(d)-(f)).The results show that N and Zn atoms are evenly distributed on the surface of the AC.The Zinc content in 14-ZIF8/AC and cal14-ZIF8/AC determined by ICP-MS were 0.49 wt%and 0.005 wt%,as most of the Zn evaporated during calcination.Besides,the N content in the carbon surface was found to be 46.73 wt%(based on EDX results),which was contributed by the pyrolysis of ligands in the ZIFs.

Fig.1.XRD Spectra of the AC,ZIF8/AC and cal-ZIF8/AC.

Fig.2.SEM images of the(a)AC,(b)14-ZIF8/AC,and(c)cal14-ZIF8/AC along with(d-f)the elemental mapping of cal14-ZIF8/AC(1537×magnified).

3.1.3.FT-IR

The surface chemistry of the AC,14-ZIF8/AC and cal14-ZIF8/AC was further examined by FT-IR (Fig.3).All samples exerted strong and broad peaks at～3460 cm−1(see the inset of Fig.3)originating from O--H stretching vibration.The vibrations observed at 2970 and 2920 cm−1were attributed to asymmetric stretching C--H vibrations.The peak at～1620 cm−1originated from C=N vibrations [26].With the magnification of the curves in the range of 400–1500 cm−1,it was clearly shown that several characteristic peaks of ZIF-8 emerged in the spectrum of 14-ZIF8/AC(Fig.3).Signals at～1306 and～1145 cm−1originated from in-plane C--N bending vibrations,and a signal at 759 cm−1could be associated with out-of-plane C=N bending[27,28].This result further confirmed the incorporation of ZIF-8 on the surface of the AC.As expected,these peaks disappeared after calcination.

Fig.3.FT-IR signals of the AC,14-ZIF8/AC and cal14-ZIF8/AC.

Fig.4.BET isotherms and pore size distributions of the(a)AC,14-ZIF8/AC and cal14-ZIF8/AC and the(b)cal-ZIF8/AC with different Zn/HmIm ratios.

3.1.4.N2 adsorption/desorption

Fig.4(a)displays the BET isotherms of the AC,14-ZIF8/AC and cal14-ZIF8/AC.All isotherms represent an interplay of characteristic type I and type IV isotherms with a hysteresis loop,revealing the microporous and mesoporous surface of all samples.The dramatic increase in the volume of adsorbed N2within the relative pressure of 0–0.01 reveals a large microporosity in the samples.All samples exhibited comparable pore size distribution patterns.With the change of the Zn/HmIm ratio during synthesis,there was little change in the shape of the isotherms;however,the uptake of N2varied,and the cal14-ZIF8/AC showed the highest N2adsorption capacity,possibly owing to it having the highest ZIFloading content.The summary of BET SAs and PVs obtained from the isotherms is given in Table 1.With the impregnation of ZIFs on the AC,the SA of the AC decreased from 716 to 556 m2·g−1because part of the pores in the AC was blocked by the ZIF-8.After carbonization at 925 °C,the SA of the cal14-ZIF8/AC recovered to 837 m2·g−1,which was much higher than those of the AC and 14-ZIF8/AC,due to the creation of pores from the vaporized Zn in the sample.

Table 1Summary of SAs,PVs and pore diameters(PDs)of the AC,14-ZIF8/AC and cal-ZIF8/AC samples with different Zn/HmIm ratios

3.1.5.TG

The TGA curves of the AC,14-ZIF8/AC and cal14-ZIF8/AC samples are given in Fig.5.The initial weight loss stage at a temperature range from room temperature to 100°C could be attributed to the desorption of gas and adsorbed H2O.A tendency for further weight loss was observed for the AC and 14-ZIF8/AC samples,with values of 8%and 15%,respectively,for the temperature range of 200–900°C,corresponding to the decomposition of the AC surface groups and the loaded ZIF-8[29].However,the curve of cal14-ZIF8/AC remained nearly constant,as it had already been calcined at 925°C for 6 h.

3.2.Removal of phenol by adsorption

3.2.1.The kinetics of adsorption

Fig.6 illustrates the influence of equilibration time on the quantity of adsorbed phenol.A gradual increase of adsorption toward equilibrium over time was observed for all the samples.The AC showed an equilibrium adsorption capacity of 46.24 mg·g−1at approximately 300 min.With ZIF-8 decoration,the adsorption capacity of 14-ZIF8/AC decreased,possibly because of pore blockage.After calcination,the adsorption capacity of cal14-ZIF8/AC increased significantly to 85.3 mg·g−1,which was two times that of AC.Among all the calcined samples,cal14-ZIF8/AC showed the highest adsorption capacity,which should be attributed to its large SA and PV,as well as the surface functional groups.

The kinetic data of phenol adsorption were fitted into pseudofirst and pseudosecond order models (see supporting information).The straight-line plots of ln(qe−qt)vs t and t/qtvs t for different samples are displayed in Fig.S2.The rate constants(k1,k2),adsorption quantity(qe)and correlation coefficients(R2)of the two models were calculated(Table 2).For the AC and 14-ZIF8/AC samples,the pseudofirst order rate law fitted the experimental data nicely (R2＞0.9).However,for the calcined 14-ZIF8/AC samples,the phenol adsorption data fit better into the pseudosecond order rate law equation.The pseudofirst and pseudosecond order models assume physisorption and chemisorption processes[30,31],respectively.Therefore,it could be concluded that the AC and 14-ZIF8/AC adsorbed phenol via weak interaction forces,while chemical adsorption might dominate during adsorption on the calcined ZIF-8/AC samples.Besides,the kinetic results were analyzed by the intraparticle diffusion model(Fig.S4).A linear relationship between qtand t0.5with a correlation coefficient R2=0.985 can be found.However,the plot did not pass through the origin point,this is indicative of some degree of boundary layer control and then,the intraparticle diffusion is not the only rate-limiting step.

Fig.5.TGA profiles of the AC,14-ZIF8/AC and cal14-ZIF8/AC.

Fig.6.Time-dependent binding of phenol onto the(a)AC,14-ZIF8/AC and cal14-ZIF8/AC and the(b)cal-ZIF8/AC with different Zn/HmIm ratios.

Table 2The kinetic parameters for the binding of phenol onto different adsorbents

Fig.7.Adsorption isotherms of phenol on the cal-ZIF8/AC with different Zn/HmIm ratios at 30°C.(The trend lines were fitted according to ExpDec1 model(Origin)).

Table 3The phenol adsorption capacity of the cal14-ZIF8/AC and other reported carbon adsorbents

3.2.2.Adsorption isotherm

Fig.7 displays the adsorption isotherms of phenol on the different cal-ZIF8/AC samples.The adsorption capacity of the adsorbents (AC,cal12-ZIF8/AC,cal14-ZIF8/AC and cal18-ZIF8/AC) initially increased with increasing equilibrium concentration and then approached a stable value.The adsorption capacities of the calcined ZIF8/AC were apparently larger than that of the AC,and the cal14-ZIF8/AC had the best adsorption capacity (155.24 mg·g−1).This value was greater than the capacity of most of the reported carbon adsorbents(Table 3),making it a potential adsorbent for practical application.

To additionally assess the adsorption characteristics of phenol onto novel adsorbents,the data were fitted by Langmuir and Freundlich models(see supporting information).The results of the linear fit for both isotherms are given in Fig.S3,and the corresponding parameters are represented in Table 4.The higher correlation coefficient (R2＞0.97)for the Langmuir isotherm model suggested that the phenol adsorption onto the AC obeyed this law,with all the adsorption sites of AC having equal affinity to phenol [37,38].However,in the case of phenol adsorbed onto the calcined ZIF8/AC samples,the Freundlich equation was a better fit for the data,implying that the Freundlich model could better define the adsorption isotherms.In addition,the n parameter of the Freundlich model was larger than 1,suggesting that adsorption occurred on the heterogeneous surfaces of the calcined ZIF8/AC with high affinity[39,40].

Table 4Parameters of the Langmuir and Freundlich isotherm analyses

Based on the analysis of the adsorption kinetics and isotherms,phenol adsorption onto the calcined ZIF8/AC samples might be considered as a combination of physisorption and chemisorption.In addition to the suitable pore structure of the cal14-ZIF8/AC for hosting the adsorbed phenol molecules,the N-containing functional groups resulting from the pyrolysis of ZIF-8 on the surface of cal14-ZIF8/AC could interact with phenol molecules via π–π dispersion forces and the “donor-acceptor effect” [41].In addition,the residual Zn on the surface of the carbons was another potential type of active site for phenol adsorption[42,43].

3.2.3.Effect of adsorbent dose and recyclability

The removal efficiency of phenol by the cal14-ZIF8/AC was evaluated at different adsorbent doses(Fig.8).Increasing the amount of adsorbent from 0.01 to 5 g·L−1,the removal efficiency improved from 4.73 to 100%.This improvement could be associated with the increased number of adsorption sites on the surface of the material.

The recyclability of the cal14-ZIF8/AC was also examined to test its potential in practical applications.After each adsorption,the cal14-ZIF8/AC was fully washed with methanol and distilled H2O and then dried at 70°C until its next use.As shown in Fig.9,the cal14-ZIF8/AC still exhibited a comparable adsorption capacity after 4 cycles,suggesting the good reusability of cal14-ZIF8/AC in adsorptive removal of phenol from wastewater.

Fig.8.The effect of the amount of cal14-ZIF8/AC on phenol removal.

3.2.4.Fixed-bed column adsorption

Fig.10 presents the breakthrough(BT)curve of phenol adsorption on the AC and cal14-ZIF8/AC.It was observed that the breakthrough curve exhibited an“S”shape,typical for the adsorption of molecules with high affinity for the adsorbent [44,45].The outlet adsorbate concentration(Ct)of the cal14-ZIF8/AC was 12.5 mg·L−1,which was much lower than that of the AC (28.1 mg·L−1) in the first 10 min.With increasing time on stream,BT curves showed an upward trend until an equilibrium value was achieved at approximately 10 h.Consistent with the earlier results,the cal14-ZIF8/AC had a higher breakthrough capacity than that of the AC.

4.Conclusions

In this study,a facile route for the synthesis of trace ZIF-8-decorated AC pellets was developed.After carbonization,the resulting composite material showed a high SA of 837 m2·g−1and PV of 0.41 cm3·g−1,and it was further investigated as an adsorbent for phenol removal from aqueous mediums.The maximum adsorption capacity was 155.24 mg·g−1,which was approximately 2.3 times that of the AC(46.24 mg·g−1).The equilibrium adsorption could be described with the Freundlich isotherm equation,and the adsorption kinetics obeyed the pseudosecond order rate law.In addition,the adsorbent could be recycled via a simple and straightforward methanol/water-washing process.The recycled material maintained more than 90%of its initial adsorption capacity during 4 cycles,making it a promising,effective adsorbent for practical removal of phenol from the environment.

Fig.9.Recyclability of the cal14-ZIF8/AC for phenol adsorption at 30°C.

Fig.10.Breakthrough profiles for phenol adsorption on the AC and cal14-ZIF8/AC at 30°C.

Acknowledgements

This work was supported by National Natural Science Foundation of China(21606252),the Key Research and Development Program of Xuzhou(KC19214)and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2020.06.027.