Liu He,Yiyang Qiu,Chu Yao,Guojun Lan,Na Li,Huacong Zhou,Quansheng Liu,Xiucheng Sun,Zaizhe Cheng,Ying Li,

1 Institute of Industrial Catalysis, Zhejiang University of Technology, Hangzhou 310058, China

2 Inner Mongolia Key Laboratory of High-Value Functional Utilization of Low Rank Carbon Resources, Huhhot 010051, China

Keywords:Mercury-ion removal Adsorption Carbon adsorbent Defect sites

ABSTRACT Carbon is a normally used adsorbent for removal of heavy metal ion in aqueous solutions,but the efficient adsorbent needs intensive modification by heteroatom doped or supported noble metals that cause severe pollution and easy leaching of active components during use.In this paper,the role of intrinsic defects on Hg2+ adsorption for carbon adsorbent was investigated.The maximum adsorbing capacity of defectrich carbon has been improved up to 433 mg·g-1 which is comparable to most of the modified carbon adsorbents via supported metal chloride or noble metal components.The basicity is increased with the content of defective sites and the strong chemical bonding can be formed via electron transformation between the defect sites with adsorbed Hg2+.The present study gives a direction to explore cheap and easily scale-up high-performance mercury adsorbents by simply tuning the intrinsic defective structure of carbon without the necessity to support metal or other organic compounds.

1.Introduction

Hg2+is one of the most toxic heavy metal ions in wastewater and can accumulate with volatilization and biological chain,which seriously threatens the ecological environment and human health [1-3].Much research effort has been devoted to mercury control,including mining,trading,using and mercury capturing[4,5].Generally,Hg2+can be trapped by chemical precipitation[6],ion exchange[7]and adsorption[8,9].Nevertheless,considering the cost and scale of industrial use,adsorption is currently the most efficient approach to remove Hg2+from the aqueous solution[10-12].

Carbon materials have been widely used in the field of adsorption due to their excellent properties including high specific surface area,well-developed pores,acid and basic resistance,tunable surface physicochemical structure and low cost [7,13-18].Extensive studies have shown that the Hg2+adsorption performance of pristine carbon materials is poor and needs to be improved by loading noble metal,metal oxides and halogens [19-22],but the cost and easy corrosion of halogens limit the wide use of the abovemodified carbon materials in the removal of Hg2+viaadsorption method.Recently,researchers have found that constructing certain basic sites on the carbon surface can help improve the adsorption performance of non-metal carbon adsorbents for the acidic Hg2+[23-25].For example,the nitrogen atom has an extra electron compared with the carbon,which makes the surface of the nitrogendoped carbon appear basic and can effectively improve its adsorption capacity for Hg2+[26,27].Huet al.[3,28]synthesized the magnetic nitrogen-doped porous carbon by a one-step method,and the resulting adsorbent exhibited excellent Hg2+adsorption properties(429 mg·g-1)in conjunction with magnetic properties,which facilitated the separation of the adsorbent.Meanwhile,utilizing the strong interaction effect between the S and Hg atoms,Bahramifaret al.[29] prepared thiol-functionalized multi-walled carbon nanotubes (MWCNTs-sh) with a 9-fold increase in removal efficiency compared to the pristine multi-walled carbon nanotubes(MWCNTs) for Hg2+adsorption.Recently,Liet al.discovered that the alkynyl cross-linked mechanochemical graphite oxide(AMGO)with a saturation Hg2+adsorption capacities of about 923 mg·g-1can be obtainedviaa mechanochemical reaction between CaC2and graphene oxide,this is attributed to the strong interactions between Hg2+and sp hybridized carbon atoms and oxygencontaining functional groups in AMGO[30].

Defect engineering is a novel method to modify the electronic structure of carbon materials.The introduction of a certain amount of defective structures (e.g.edge sites,vacancies,topological defects) can alter the electron distribution on the carbon surface[31].The delocalized π electrons of carbon can be generated at these defective sites,making their neighbored carbon atoms have more additional electrons.The electronic state of edge defects on graphene has been studied experimentally and theoretically by several research groups.Its electron cloud is localized on edge sites,exhibiting stronger functionalization activities [32].Activated carbon,as an amorphous carbon material with composite hybridization of sp2and sp3,has rich intrinsic defective sites[33].Considering that the annual production capacity of activated carbon is over ten million tons,switching the low-cost activated carbons into high-performance Hg2+adsorbents can help achieve comprehensive treatment of mercury-containing wastewater.Even though there are a lot of experiments on this subject,the application of activated carbon to control mercury contamination is limited due to the lack of understanding of the role of defective structure on mercury-adsorption performances.

In this paper,the effect of the defective structure of activated carbons on the adsorption properties of HgCl2was investigated.The defective structure of carbon was modulated through a simple heat treatment (deoxidation) process of the activated carbon with abundance surface functional groups.The relationship between the equilibrium adsorption capacity of activated carbon and its defective content has been obtained,and the adsorption mechanism fully discussed.

2.Experimental

2.1.Materials

The activated carbons (AC) were purchased from Hangzhou Catan New Material Technology Co.,Ltd.China.HgCl2(AR,99.5%)was provided by Guizhou Zhongli Chemical Co.,Ltd.Other reagents were obtained from Shanghai Chemical Reagent Inc.of the Chinese Medicine Group.All reagents were analytical grade and used without any further purification.

2.2.Preparation of defect-rich AC materials (AC-Tx)

The commercial AC were washed with 1 mol·L-1HCl and 5%(mass) HF solutions to remove the ash and then dried in an oven at 110°C for 8 h before further processing,which was named pristine AC.Then thermal treated at different temperatures for 3 h under argon and then cooled down to ambient temperature under argon.The obtained defect-rich AC materials are denoted as AC-Tx,Tx indicates thermally treated temperatures (800,1150 and 1300°C).To illustrate the effect of thermally treated time,a sample treated at 600°C for 5 minutes was prepared and which is denoted as AC-T600-5.

2.3.Characterizations

Nitrogen adsorption isotherms were determined at -196 °C on the Quantachrome Autosorb-IQ3 apparatus.The adsorbents were outgassed at 300 °C for 8 h before adsorption measurement.The specific surface area was obtained using the Brunauer-Emmett-Teller (BET) model from adsorption data in a relative pressure ranging from 0.05 to 0.30.The total pore volume was determined from the aggregation of N2vapor adsorbed at a relative pressure of 0.99,and the pore size distribution was calculated by the BJH method [34].The morphology of adsorbent was characterized by scanning electron microscopy (SEM,FEI Nova NanoSEM 450) and high-resolution transmission electron microscope and selected area electron diffraction (HRTEM,SAED,FEI Tecnai G2 F20 microscopes operated at 200 kV).X-ray photoelectron (XPS) measurement was conducted on a Thermo Scientific K-Alpha instrument using 300w Al Kα.The binding energies were calibrated by the contaminant carbon (C 1s 284.6 eV).

X-ray powder diffraction(XRD)measurements were performed using a PANalytical Empyrean powder diffractometer at a scan rate of 5(°)·min-1with a Cu Kα radiation (λ=0.1541 nm).Raman spectroscopy was performed on powder samples using a HORIBA LabRam HRRaman spectrometer with a 532 nm excitation wavelength.

The argon temperature-programmed desorption (Ar-TPD) and carbon dioxide temperature-programmed desorption (CO2-TPD)of the samples were carried out by self-made TPD equipment.The steps of CO2-TPD are following: the sample (100 mg) was purged at 150 °C for 30 min in a U-shaped quartz tubular reactor under argon,and CO2was introduced at 50 °C for 3 h.Subsequently,the physically adsorbed CO2was removed by purging with argon at 50 °C until the baseline was flat.While the samples were cooled down to room temperature,the samples were heated to 500°C with a heating rate of 10 °C·min-1and the signal of carbon dioxide was collected by online mass spectra (QIC 20).

The content of acidic oxygen groups on the adsorbent surface was determined by Boehm titration by NaHCO3,Na2CO3,NaOH and C2H5ONa,and the surface basicity was determined by acidbase titration with HCl and NaOH [35].

2.4.Acid-base titration

An acid-base titration is adopted to determine the basicity on the surface of AC-Tx [36].The sample (100 mg) was suspended in the 5 mmol·L-1HCl solution(50 ml)and stirred for 24 h to reach equilibrium.After filtrating,2-3 drops of phenolphthalein indicator were added into the solution and titrated against the 5 mmol·L-1NaOH solution until the end-point was reached.The end point was detected by visual inspection through a sharp color change from colorless to pink.

2.5.Adsorption experiments

The adsorption of HgCl2from aqueous solutions was carried out in a conical flask containing 50 ml of HgCl2solutions(1000 mg·L-1)in contact with 100 mg adsorbent for 24 h,and the adsorption experiments were performed at room temperature (ca.25 °C).The adsorption capacity of the samples was calculated by titration with diethyldithiocarbamate(DDTC).The titration process is as follows: 10 ml test solution,15 ml concentrated HNO3,5 ml concentrated HCl and 1 g sodium tartrate was added together in a flask.Then,NH3·H2O (1:1) was added to the above flask until the pH of the solution was over 8.0,and 3 ml CCl4was added as the extractant.0.1 ml 0.1% (mass) CuCl2indicator was added and titrated against the standard DDTC solution until the endpoint was reached.The end point was detected by visual inspection through a sharp color change from colorless to yellow in the CCl4phase.

The equilibrium adsorption capacity (Qe,mg·g-1) of the adsorbents and the removal efficiency (Re,%) were calculated as:

C0(mg·L-1) is the initial concentration of HgCl2in the solution,Ce(mg·L-1) is the equilibrium concentration of HgCl2in the solution,andm(g) is the mass of the adsorbent.

2.5.1.Kinetic studies

The kinetic studies for HgCl2adsorption on AC-Tx were analyzed in terms of pseudo-first-order and pseudo-second-order kinetic equations.Their equation can be expressed as:

WhereK1(min-1) andK2(g·mg-1·min-1) are the rate constants of the pseudo-first-order and pseudo-second-order kinetic respectively.Qt(mg·g-1) denotes the adsorption capacity at timet(min)[37].

2.5.2.Adsorption isotherm models

Langmuir and Freundlich isotherm models are widely used to predict the adsorption capacity of a particular substance.The Langmuir isotherm assumes monolayer coverage of adsorbent over a homogeneous adsorbent surface,while the Freundlich isotherm supposes a heterogeneous surface with a non-uniform distribution of heat of adsorption over the surface and multilayer adsorption.Their equation can be expressed as:

Where,Ce(mg·L-1) is the equilibrium concentration,Qe(mg·g-1) is the adsorption capacity,Qm(mg·g-1)represents the maximum theoretical monolayer adsorption capacity andKLis the Langmuir equilibrium constant related to the affinity of adsorption sites.KFandnare the Freundlich adsorption constants that are related to the adsorption capacity and intensity,respectively [38].

3.Results and Discussion

3.1.Textural properties of the various carbon adsorbents

In this paper,the selected pristine AC is an activated carbon with abundant oxygen-containing functional groups with wood as raw materials.To exclude the effect of metal impurities in ACs,the AC was pretreated with dilute HCl and HF solution to remove inorganic impurities.The content of defective sites was adjustedviathe thermal treated temperature and time as described in the experimental parts.The removal of surface functional groupsviathermal treatment under inert gas can generate the surface defective sites in carbon,this has been reported in our previous work [39].The textural properties and surface functional groups as well as the defective structure has been fully characterized and discussed below to study the role of defective structure of carbon adsorbent on its adsorption properties of Hg2+in aqueous solution.

The nitrogen adsorption/desorption experiment was employed to investigate the pore structure of obtained AC-Tx,and the results are presented in Fig.S1(a) and (b) (Supplementary Material).All-adsorption isotherms of AC-Tx exhibit a significant inflection point underP/P0of 0.1 because of the capillary filling of micropores,and the isotherms can be identified as type-IV with an H4 hysteresis loop extending from middle to high pressure region(P/P0of 0.4-1.0),which indicates the existence of both mesopores and micropores on these AC-Tx samples.The pore structure for all the samples is similar which indicates that the thermal treatment doesn’t destroy the pore structure of pristine AC.The surface area and pore volume of various AC-Tx are given in Table 1.The specific surface area and pore volume decreases slightly with increasing of the thermal treated temperature from 2183 m2·g-11.82 cm3·g-1to 1529 m2·g-1and 1.31 cm3·g-1respectively.However,the surface area of all materials used in present work is above 1500 m2·g-1,therefore,the effect of porous structure can be neglected.

The morphology of AC and AC-T1300 were investigated by scanning electron microscopy(SEM)and high-resolution transmission electron microscope (HRTEM) and the corresponding images are presented in Fig.1.In the SEM pictures,the surface of the pristine AC and AC-T1300 is rough and with a large number of nanoparticles.The nanoparticle size of AC is 20-22 nm,and that of AC-T1300 is 18-20 nm,which is no difference in size between two adsorbents.From the HRTEM pictures,it can be seen that the lattice fringes of both pristine AC and AC-T1300 are disordered,indicating that the activated carbon is amorphous.The selected area electron diffraction (SAED) pattern of various carbon adsorbents is shown in inset (c,d) of Fig.1.The SAED patterns of AC and AC-T1300 have no diffraction spots,indicating that both are amorphous structures.

Fig.1.SEM images of (a) pristine AC,(b) AC-T1300,and HRTEM images of (c) pristine AC,(d) AC-T1300 with the inset showing SAED pattern.

The O element analysis,Ar-TPD and Boehm titration were used to analyze the changes of oxygen-containing functional groups in activated carbon treated under different temperatures.The O content of pristine AC is 11.5%(mass) and it decreases with the increase of thermal treated temperature as given in Table 1.When the temperature arrives at 1300 °C,the O content is down to 0.5%(mass).

The Ar-TPD profiles of AC-Tx are given in Fig.2.There is a CO desorption peak of around 700 °C which represents the decomposition of carbonyl groups,and two distinct CO2desorbing peaks around 300 °C and 690 °C of pristine AC,which represent the decompositions of carboxyl group and lactone group respectively.When pristine AC was calcined at 600 °C for 5 min,the intensity of CO and CO2desorption peak at 100-600°C of AC-T600-5 significantly decreases,demonstrating that the oxygen-containing functional groups are decomposed.With the increase of heating temperature,the desorption peaks intensity of CO and CO2for AC-T800,AC-T1150 and AC-T1300 are further reduced compared with AC-T600-5,there are only small peaks of CO signals corresponding to carbanyl groups on the carbon surface around over 800 °C.

Fig.2.(a) CO2,(b) CO Ar-TPD profiles,(c) XRD patterns,(d) Raman spectra for various AC-Tx.

The results of Boehm titration are shown in Table 2,as expected,the pristine AC has a variety of acidic functional groups on the surface containing carboxylic,lactone,phenolic and carbonyl groups,which are 0.39,0.43,0.02,0.06 mmol·g-1,respectively.After the thermal treatment,the content of acidic functional groups decreases for AC-Tx with the increase of thermal treated temperature.When the thermal treated temperature is over 1150 °C,there are no acidic functional groups detected on the adsorbents by Boehm titration.These results are consistent with Ar-TPD.

Table 2 The concentration of surface functional groups determined via Boehm titration of AC-Tx

The XRD patterns of various AC-Tx are given in Fig.2(c).There are two peaks at 26° and 44° for all adsorbents,the first peak at 26°corresponds to the(0 0 2)reflection of carbon materials,which is attributed to the stacking of the graphene,and the second peak centered on 44° is assigned to the (1 0 1) reflection originating from the in-plane structure of hexagonal symmetry.It is noteworthy that the intensity of(1 0 1)in the adsorbents is increased with thermal treatment,demonstrating that some defective sites are formed after annealing treatment in AC [39].The Raman spectra for AC-Tx are shown in Fig.2(d).Two strong bands occur at 1350 cm-1(D-band) and 1580 cm-1(G-band) for all samples,which represent disordered structure and the ideal hexagonal symmetry structure,respectively [39].The relative intensity ratio between the D band and the G band (ID/IG) can be used as a descriptor of the defect content in the carbon material.TheID/IGvalues for various AC-Tx are given in Table 1.It can be seen that theID/IGvalues of the above adsorbents are in the order of AC-T1300 (1.18) ＞AC-T1150 (1.12) ＞AC-T800 (0.89) ＞AC-T600-5 (0.81) ＞ pristine AC (0.72),which indicates that the defect amounts of AC are increased with the increase of thermal treated temperature.Fig.3 gives the relationship betweenID/IGvalues and the oxygen content of the adsorbent.TheID/IGvalues positively correlated with the decreasing of oxygen content,which confirms the role of deoxidation in the formation of defective sites foractivated carbons.This has been reported in our previous reports and other references [39,40].

Fig.3.The relationship between the value of ID/IG and the content of O for various AC-Tx.

Activated carbon,as a kind of amorphous carbon,is a solid containing a mixture of predominantly sp2hybridized carbon with sp3hybridized carbon.According to the latest conclusion obtained by our group,the content of sp3hybridized carbon by XPS C 1s results can also reveal the content of defect sites.The high-resolution XPS C 1s spectra of pristine AC,AC-T600-5,AC-T800,AC-T1150 and ACT1300 can be deconvoluted into six peaks,as shown in Fig.4(a),which are assigned to sp2(284.6 eV),sp3(285.3 eV),C—O(286.5 eV),C=O (287.8 eV),COOH (289.1 eV) and π-π*peak(290-294 eV) species,respectively.In Table 3,the values of sp3/(sp2+sp3) for pristine AC,AC-T600-5,AC-T800,AC-T1150 and AC-T1300 is 0.13,0.17,0.19,0.20 and 0.25 respectively[41],which has the same rule asID/IG.Therefore,from the above analysis,the carbon adsorbent with different defect content was prepared by calcinating under different temperatures.As the calcinating temperatures increase from 600 °C to 1300 °C,the defect content shows an upward trend.

Fig.4.(a) High-resolution spectra of C 1s for various AC-Tx;(b) CO2-TPD profiles of pristine AC and AC-T1300.

Table 3 The fitting results of various AC-Tx via XPS

In order to study the effect of defect sites on the surface chemical properties of carbon materials,the basicity of various defect-rich activated carbons was also characterized by CO2-temperature programmed desorption (CO2-TPD) technique,which is given in Fig.4(b).The CO2-TPD of AC-T1300 has an obvious desorption peak at 80°C compared to pristine AC,which can be attributed to CO2desorbed at weak base sites on the adsorbent surface[42].While for pristine AC,the CO2desorption peak at 80°C is absent,there is only one CO2desorption peak around 300°C for AC which is assigned to the decomposition of the carboxyl group on the carbon material as proved by Ar-TPD results given in Fig.2(a).Therefore,this result indicates that the defective carbon materials have basic sites compared with pristine AC.

Taking into account that CO2-TPD is only a semi-quantitative analytical method,the acid-base titration was used for accurate analysis of basic sites of carbon [36],and the result of the acidbase titration are given in Table 1.It can be found that the total surface basicity of the pristine AC is 0.01 μmol·m-2.The content of basic sites of AC-T600-5 increases to 0.03 μmol·m-2after being thermally treated at 600 °C for 5 min.The content of basic sites of AC-T1300 increases significantly and reaches 0.36 μmol·m-2.This indicates the total basicity can be improved with the thermally treatment.

3.2.Adsorption of HgCl2 by AC-Tx

The adsorption capacity of AC-Tx is shown in Table 1.The adsorption capacity of AC-Tx follows the order AC-T1300 ＞AC-T1 150 ＞AC-800 ＞AC-T600-5 ＞pristine AC.As can be seen,all adsorbents after thermal treatment have a higher adsorption capacity compared to pristine AC.

The relationship between the adsorption capacity of HgCl2andID/IGis shown in Fig.5(a).It can be seen that the adsorption capacity is positively correlated with theID/IGvalue.When theID/IGvalue is between 0.7-0.9,theQeincreases rapidly with the increase ofID/IGvalue,and when theID/IGvalue is greater than 0.9,theQeincreases slowly with the increase ofID/IGvalue.To verify the adsorption properties of carbon with Hg2+,various carbon materials were also been treated through the same way and the adsorption properties have been tested and listed in Fig.S3.It can be seen that the adsorption behavior present a similar trend with that of AC-Tx despite they have different texture properties.The texture properties of other ACs series have been provided and discussed in Table S1.We also tried to correlate the adsorption properties with the different surface oxygen groups and surface area of these carbons.The curves are provided in Fig.S4,there is not direct relationship between them.The above results further proved the role of surface defective structure with the adsorption properties of Hg2+.

Fig.5.(a) The relation of adsorption capacity of HgCl2 with the ID/IG value of various AC-Tx (Conditions: C0 (HgCl2)=1000 mg·L-1;pH=6.0;Adsorbent dosage=100 mg/50 ml;Time=1440 min);(b) The relation of surface basicity with value of ID/IG on various AC-Tx.

As known that the Hg2+is a Lewis acid,the basicity may play an important role in the adsorption properties of carbon based adsorbent.Therefore,we correlated the relationship between surface basicity and the defect content (ID/IG),which is given in Fig.5(b),it can be seen that theID/IGvalue for adsorbents and surface basicity shows a positive correlation,which indicates that the surface basicity of the carbon material increases as defect site content increases.According to the conclusions of the literature,the carbon atoms near the defect sites have higher charge density and can provide electrons as active sites in the catalytic and adsorption processes,which may explain why carbon materials with highID/IGvalue have better adsorption properties for Hg2+.

The adsorption performance of AC-T1300 and other carbonaceous sorbents for Hg2+is compared in Table 4 in terms of theQmby Langmuir model.In view of theQm,the performance of AC-T1300 already far exceeds that of commercial AC [43] and MWCNTs [18],and even outperforms some N-doped [17,28] and S-doped porous carbon[10,29].More importantly,the preparation process of AC-T1300 is simple and convenient,and can be easily scaled up for commercialization.

Table 4 Comparison of adsorption capacities of Hg2+ obtained by different adsorbents

3.3.Adsorption behavior of pristine AC and defective carbon (ACT1300)

The adsorption capacity of HgCl2on AC-T1300 is as high as 392 mg·g-1which is used for the subsequent adsorption investigation.A kinetics study of the adsorption process can be used to profile the adsorption rate and examine the adsorption mechanism.The effect of adsorption time for pristine AC and AC-T1300 was carried out with various periods ranging from 0 to 48 h with an adsorbent dosage of 100 mg.The effect of adsorption time on the adsorption rate of HgCl2on pristine AC and AC-T1300 are shown in Fig.6.At the beginning of adsorption,the adsorption capacity of AC-T1300 is much higher than that of pristine AC.The ACT1300 has high adsorption capacity is due to it’s abundant basic sites on the surface which can adsorb HgCl2chemically.With the increase of adsorption time,both the AC and AC-T1300 show an increasing trend of HgCl2adsorption performance due to the physical adsorption of HgCl2by the pore structure of adsorbents.

Fig.6.Effects of contact time on HgCl2 adsorption at AC and AC-T1300 (Conditions:C0 (HgCl2)=1000 mg·L-1;pH=6.0;Adsorbent dosage=100 mg/50 mL;Time=5-2880 min).

To evaluate the adsorption mechanism of pristine AC and ACT1300,kinetic studies are necessary.For this reason,pseudofirst-order and pseudo-second-order kinetic equations were studied for HgCl2adsorption.The pseudo-first order equation is based on adsorption capacity and is applicable when adsorption occurs in a single layer boundary by diffusion mechanism.However,pseudo-second-order kinetic equation shows the dominant chemical adsorption mechanism which controls the adsorption process.The calculated values ofQe,K1,K2andR2are listed in Table 5.Results revealed that for pristine AC,both pseudo-first-order kinetics and pseudo-second-order kinetics models could well describe the sorption mechanism,although the pseudo-second-order kinetics model gave a slightly higherR2value,which implies that there may be a combination of diffusion and chemisorption for the adsorption process of pristine AC.While for AC-T1300,the value ofR2in the pseudo-second-order model is much higher than in the pseudo-first-order model.This indicates the reaction rate is proportional to the number of active sites on the AC-T1300 surface and the rate-limiting step is chemical sorption.

Table 5 Kinetics models parameters of HgCl2 adsorption by pristine AC and AC-T1300

The experimental data relating to the HgCl2adsorption for pristine AC,and AC-T1300 were fitted by Langmuir and Freundlich adsorption isotherms models and the parameters are shown in Table 6.Results reveal that the Freundlich model (R2is 0.9942)has a better consistency with the adsorption trend than the Langmuir model(R2is 0.8297)for pristine AC,which shows the adsorption of HgCl2on pristine AC is multi-molecular layer adsorption.The maximum adsorption capacity of HgCl2on pristine AC can be calculated by the Langmuir model,which is 105 mg·g-1.For ACT1300,the correlations for Langmuir and Freundlich adsorption models are 0.9126 and 0.9201,respectively,indicating that both Langmuir and Freundlich models can describe the adsorption state of AC-T1300 well,and the maximum adsorption capacity calculated by the Langmuir model is 433 mg·g-1,showing excellent adsorption performance.

Table 6 Adsorption isotherm models parameters of HgCl2 adsorption by pristine AC and AC-T1300

Experiments were carried out to examine the effect of adsorbent dosage on HgCl2adsorption,which are presented in Fig.7(d).It can be seen that the adsorption capacity of AC-T1300 decreases as adsorbent dosage does,while the removal efficiency of HgCl2increases.

Fig.7.Kinetics of HgCl2 on AC (a) and AC-T1300 (b);(c) adsorption isotherm of HgCl2 on AC and AC-T1300;(d) effects of adsorbent dosage on HgCl2 adsorption at AC-T1300(conditions: C0 (HgCl2)=5-1000 mg·L-1;pH=6.0;adsorbent dosage=25-300 mg/50 ml;time=5-1440 min).

3.4.Adsorption mechanism

As evidenced by the above results,the chemisorption of HgCl2has occurred on the defect-rich carbon.To get a better insight into the interaction mechanism of HgCl2and carbon adsorbents,XPS spectra and TG analysis were conducted for the fresh pristine AC,AC-T1300,and used pristine AC,AC-T1300 and the results are given in Fig.8 and Fig.9.The high-resolution XPS scans of C 1s,Cl 2p and Hg 4f after adsorption (denoted as HgCl2@pristine AC and HgCl2@AC-T1300) were conducted and the results are shown in Fig.8(a)-(c).It can be seen from Fig.8(a) that the sp3peak for pristine AC,HgCl2@pristine AC,AC-T1300,and HgCl2@AC-T1300 are located at 285.2,285.4,285.26 and 285.7 eV,respectively.After HgCl2adsorption,the sp3peak for pristine AC and AC-T1300 shift to high binding energy,which may be caused by the electrons transfer from the defect sites of carbon to the HgCl2molecule.From Fig.8(b),it can be seen that the peak of C—Cl appeared in the Cl 2p spectra for pristine AC and HgCl2@pristine AC.While for HgCl2@AC-T1300,there is only the peak of Hg—Cl.The chlorine was introduced into AC when treated with hydrochloric acid,and it can removal from AC during the thermal treatment.Furthermore,from the Fig.8(c),6the peak of Hg2+at 101.65 eV and 101.02 eV can be observed from the Hg 4f spectra for HgCl2@pristine AC and HgCl2@AC-T1300,the position of Hg2+in the Hg 4f spectra for HgCl2@AC-T1300 is shift to lower binding energy than that in HgCl2@pristine AC,which is consistent with the conclusion drawn by C1s spectra.Based on the above analysis,it can be seen that electrons are provided to HgCl2from the defect sites on AC-Tx inthe process of adsorption.According to our previous results [44],the binding energy (Eb) of HgCl2at different oxygen groups,such as carboxyl,lactone,hydroxy and quinone,on the oxygen-rich carbon(AC)is below 35 kJ·mol-1,however,theEbof HgCl2at defective site (zigzag or mono-vacancy) reach 50 kJ·mol-1,which theoretically explains why the HgCl2adsorption performance of ACT1300 is much higher than that of pristine AC.

Fig.8.The high-resolution spectra of (a) C 1s,(b) Cl 2p and (c) Hg 4f for various AC-Tx.

Fig.9.TG and DTG profiles for HgCl2@pristine AC and HgCl2@AC-T1300.

TG techniques were conducted under N2flow to evaluate the thermal stability of various AC-Tx before and after adsorbed HgCl2.TG and DTG profiles of the HgCl2@pristine AC and HgCl2@ACT1300 are given in Fig.9.Three weight loss peaks in the TG diagrams of HgCl2@pristine AC,of which the peak at about 100 °C is caused by the evaporation of physically adsorbed water on the adsorbent,and the peak at 280 °C is attributed to the desorption of adsorbed HgCl2.The third peak at 700°C represents the decomposition of oxygen-containing functional groups on carbon materials.For HgCl2@AC-T1300,the mass loss peak over 700 °C disappears due to the high-temperature treatment,while a small mass loss peak appears at 205°C,which may be due to the desorption of the small amount of multilayer adsorbed HgCl2on ACT1300.The maximum mass loss peak of HgCl2@AC-T1300 is shift to 323°C,compared with HgCl2@pristine AC,which indicates that the adsorption of HgCl2on AC-T1300 is more stable and suggests a stronger interaction between AC-T1300 and HgCl2compared to pristine AC.

4.Conclusions

In this paper,the effect of defect structure on the chemical properties of activated carbon surface,and its application in HgCl2adsorption was studied.The content of surface defects in activated carbon increased with the increase of the temperature of the deoxygenation process,which was verifiedviaRaman,XRD and other characterizations.The increase of defect content can significantly change the surface basicity of activated carbon as determinedviaacid-base chemical titration.The results of HgCl2adsorption revealed that the adsorption performance of HgCl2of the defect-rich carbon material increased with the increasing the number of defect sites.The maximum adsorbing capacity of defect-rich carbon was 433 mg·g-1based on Langmuir model.And the adsorption process for AC-T1300 is kinetically faster than pristine AC.In addition,HgCl2adsorption on AC-T1300 is more stable than pristine AC by XPS and TG characterizations.The present study gives a direction to explore a cheap and easily scaleup high-performance mercury adsorbents by simply tuning the intrinsic defective structure of carbon without being necessary to support metal or chemical bonding of other organic compounds.

Data Availability

Data will be made available on request.

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

This work was funded by the 2019‘‘Rare Earth and Coal Chemical Industry”Key Science and Technology Project of Inner Mongolia Autonomous Region of China(2019ZD017),the National Natural Science Foundation of China (21908197,22108248,22208305).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2023.03.021.

Nomenclature

Cesolution concentration at adsorption equilibrium,mg·L-1

C0initial solution concentration,mg·L-1

KFthe Freundlich adsorption constant,mg1-1/n·L1/n·g-1

KLthe Langmuir equilibrium constant,L·mg-1

K1the rate constant of the pseudo-first-order kinetic,min-1

K2the rate constant of pseudo-second-order kinetic,g·mg-1-·min-1

P.V.total volume,cm3·g-1

Qethe equilibrium adsorption capacity,mg·g-1

Qmthe maximum theoretical monolayer adsorption capacity,mg·g-1

Reremoval efficiency,%

S.A.specific surface area,m2·g-1