GUO Si-jia, GUO Gui-ping

Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences, Shijiazhuang 050061, China.

Abstract: This article discussed the benzoic acid activated carbons which have changed the types and content of acid oxygen-function groups on the surface of activated carbons and their effect on the adsorption for Hg0 in simulated flue gas at 140 ℃. These surface acid oxygen function groups were identified by Boehm titration, Fourier transformation infrared spectrum,temperature programmed desorption and X-ray photoelectron spectroscopy. It indicates that the carboxyl, lactone and phenolic were formed when the benzoic acid is loaded on the surface of activated carbons. Among the surface acid oxygen function groups, the carboxyl groups enhance the adsorption capacities of Hg0 for activated carbons to a greater extent.

Keywords: Benzoic acid; Modified activated carbon; Surface acid oxygen function groups;Characterization; Adsorption for Hg0

1 Preface

As a global pollutant, Mercury has attracted attention from around the world. The element Hg lingers in the atmosphere, which travels long distance with monsoon and pollutes large areas.Hg in the atmosphere enters water through wet and dry subsidence, and converts into methylmercury under the effect of microorganisms, which enters human body through bioaccumulation and poses great harm to humans. Coal is particularly important in energy structure of China, reaching a consumption of over 1.4 billion tons per year.Study shows that coal power plants are the major source of Hg pollution. However, Hg pollution control of coal power plants in our country is still at an early stage, causing challenges to Hg pollution control. In October 9th2013, the international Minamata Convention comes into effect officially, which means the international community has reached consensus in controlling Mercury emission. The convention has a strict restriction on Hg pollution emission of coal power plants.

Hg0, Hg2+(HgCl2, HgSO4, HgO) and Hgpare the main existing forms of Hg in the flue pipes of coal-fired power plant. Among them, Hg2+and Hgpcan be effectively removed by wet desulfurization and dust collecting units (Brown T D et al. 1999).Compared with the removal efficiency of Hg2+and Hgp, the removal efficiency of Hg0by wet desulfurization and dust collectors is limited.

Activated carbon injection is an effective method for controlling Hg0in the flue pipes of coal-fired power plants (Yan R et al. 2004; Ho T et al. 2005). However, the temperature of the power plant flue pipe is between 70-180 ℃, and the adsorption capacity of activated carbon is weak at this time. Therefore, a large amount of activated carbon needs to be injected to effectively control Hg0emission, thus increasing the cost of the activated carbon injection and affecting the reuse value of fly ash. In order to reduce the amount of activated carbon injection, preparation of modified activated carbon with high adsorption capacity for Hg0under high temperature has become the focus of international research (Feng W et al. 2006;Vidic R D and Siler D P, 2001; SUN Wei et al.2006; Wang J et al. 2010; Hutson N D et al. 2007).

As a common absorbent, the activated carbon not only has a large specific surface area, but also contains a variety of acidic oxygen-containing functional groups on the surface. The study has pointed out that the acidic oxygen-containing functional groups on the surface of activated carbon have a positive effect on the adsorption of Hg0(Buczek B et al. 1999; Menendez J A et al.1996). For example, the content of acidic oxygen-containing functional groups on the surface of activated carbon treated with HNO3increased significantly, which promoted the transfer of electrons among acidic oxygen-containing functional groups and increased the adsorption and oxidation capacity for Hg0by activated carbon (Manchester S et al. 2008;Manchester S et al. 2008).

LUO Jin-ying et al. studied the absorption capacity for Hg0by modified activated carbon prepared by impregnated benzoic acid, phenol and other chemicals, and the experimental results showed that the benzoic acid promoted the adsorption capacity for Hg0while phenol inhibited the adsorption capacity for activated carbon (LUO Jin-ying and LUO Jin-jing, 2009; LUO Jin-Ying et al. 2009).

Based on this research, we have further studied the types and contents of acid functional groups on the surface of benzoic acid-modified activated carbons and their effects on the adsorption of HgO in simulated flue gas so as to provide new methods for preparing activated carbons with high adsorption capacities.

2 Experiment

2.1 Experimental materials and preparation methods

In this study, we use granular activated carbon(Calgon Carbon Co. Ltd. BPL) to prepare modified activated carbon, as shown in the parameters of Table 1. The modification method is as follows:Taking granular activated carbon, soaking them in deionized water at 100 °C for 1 h; then after heating and drying, the particles are ground into size of 100-200 mesh by a sealed sample preparation pulverizer to obtain powdered activated carbon, which is recorded as BPL and then placed in a tube furnace for calcination in an inert gas atmosphere N2; the heating process is as follows: Firstly heated to 1 000 °C at 10 °C/min,then calcined at 1 000 °C for 2 h, and finally cooled to room temperature at 10 °C/min, and recorded the sample as BPL1000. Then mix 1.00 g of BPL1000 with different concentrations of benzoic acid solution, which is vibrated for 24 h at 25oC at 125 r/min, filtered to obtain activated carbon filter cake, and dried at 105 °C, that is,benzoic acid modified activated carbon, referred to as BPLCXs; and then use a spectrophotometer(Thermo Electron Corporation, Genesys 10UV) to detect the concentration of residual benzoic acid in the filtrate at λ = 270 nm. The calculated reduction of benzoic acid in the solution is the loading amount of benzoic acid on the surface of activated carbon. The loading amount is shown in Table 2:

Table 1 Parameters of BPL

Table 2 Amount of benzoic acid on the surface of BPLCXs

2.2 Characterization method

Scanning electron microscopy SEM (LEO,LEO1530) has been applied to obtain activated carbon surface images magnified by 1 000 and 20 000 times. The BET specific surface area and pore size distribution of activated carbon were determined by specific surface area analyzer(ASAP2020C, Micromeritics). N2 was the test gas and the test temperature was 77 K. The specific surface area was calculated using the BET equation. The pore size distribution was calculated using the BJH method; the micropore specific surface area and volume were calculated using the t-plot method. The acidic oxygen-containing functional groups on the surface of activated carbon were measured by Boehm’s titration method: 0.200 g of BPLCXs was placed in a conical flask, 60.00 mL of 0.05 N NaHCO3was added and shaken at 120 r/min for 24 h at 25 °C.By dint of 0.1 N HCl back titration 20.00 mL filtrate, the carboxyl content of the activated carbon surface is obtained; if Na2CO3solution is added to the activated carbon, the content of carboxyl and lactone on the surface of the activated carbon is obtained by the same method;NaOH is added to the activated carbon to obtain the surface of the activated carbon Carboxyl,lactone and phenolic hydroxyl content. The carboxyl group, the lactone group and the hydroxyl group content can be obtained respectively by subtracting the content of the acidic oxygen-containing functional groups. The surface infrared spectrum of activated carbon was detected by Fourier transform infrared spectroscopy(NEXUS470, Thermo Nicolet) with a scanning range of 4 000 cm-1to 400 cm-1. The pretreatment method is as follows: The activated carbon is mixed with KBr powder (AR, Shanghai Sinopharm Group), diluted 300 times, and dried at 105 °C for 10 hours to remove the moisture in the sample. The CO2and CO signals desorbed at high temperature on activated carbon samples were detected using a fully automated temperature programmed chemical desorber (TPD) (AutoChem II 2920, Micromeritics). The experimental method is as follows: Take 0.200 g of activated carbon sample, helium carrier gas and purge at 50 mL/min for 30 min, and then raise the temperature to 900°C at a heating rate of 10 °C/min. During the experiment, the chamber pressure was set to 1×10-6torr and kept under a helium atmosphere with a helium flow constant at 50 mL/min. The tail is connected to a Hiden qic-20 mass spectrometer to detect the signals of CO2and CO which are the effluent gases. The binding energy of C1sand O1sof activated carbon samples was measured by X-ray photoelectron spectroscopy (PHI Quantum-2000, PHI). Instrument parameters were set as: Monochromatic AlKα1,2 X-ray source voltage of 15 kV; power 35 w; beam spot diameter of 200 μm. In order to prevent the active carbon from polluting the chamber during the vacuuming process, the activated carbon was pretreated by uniformly mixing the activated carbon sample with the KBr at 1: 200 dried at 105 °C for 10 h after the sheet was pressed at 600 Kg/m2. The experimental results need to calibrate the C1s electron binding energy of the graphite layer with 284.6 ev.

2.3 Hg0 adsorption experiments

As shown in Fig. 1, the adsorption reaction was carried out in a fixed bed adsorption unit. 20 mg of activated carbon and 2.00 g of quartz sand were uniformly mixed and filled into a diameter of 6 mm quartz tube; both ends of the quartz tube were plugged and placed into the tube furnace with the adsorption temperature of 140oC. The Hg0permeable tube was put into the U-shaped tube tail,which was filled with glass beads in the front of the U-shaped tube to control the N2 in all cross-section through the permeable tube by constant water bath temperature and stable Hg0inflow concentration. The total intake flow is set to 1.0 L/min. Mercury analyzer (JEROME431-X)was connected to the fixed bed adsorption device outlet, and the outlet automatically monitored the Hg0concentration. Each test was taken at an interval of 5 min. Before each experiment, the outlet end of Hg0concentration constant was at 10±1 μg/m31 h, and the adsorption reaction could be carried out. After the experiment, the saturated adsorption capacity of activated carbon was calculated by the adsorption curve, as shown in equation (1), the abscissa of adsorption curve is time (t/min), and the ordinate is Hg0concentration(Ciμg/m3). The total mass is 10 μg/m3× t min × 1 L/min/1 000 μg. By subtracting the area S μg between the adsorption curve and the time axis(representing the mass of Hg0not adsorbed by the activated carbon), the saturated adsorption capacity τ (μgHg0/gAC) can be obtained.

Fig. 1 Schematic of the fixed bed reactor for adsorption of Hg0

3 Results and discussion

3.1 Characterization of activated carbon sample

3.1.1 Specific surface area of activated carbon sample

Fig. 2 is a scanning electron micrograph of BPL, BPL1000 and BPLCX2, respectively. After the BPL is heated at a high temperature of 1 000 °C in an N2 atmosphere, the oxygencontaining functional groups on the surface of the BPL are pyrolyzed and desorbed, and therefore the surface of the BPL1000 is clearer than the surface of the BPL1000. Electron micrographs, before and after being loaded with benzoic acid do not change the surface morphology of activated carbon,probably because benzoic acid is adsorbed in the pores of activated carbon, and cannot be observed by scanning electron microscopy.

The BPL, BPL1000 and BPLCXs micropores,the total pore specific surface area and volume are shown in Table 3, respectively. The specific surface area of BPL is 901 m2/g. After heating at 1 000 ℃ for 2 hrs, the specific surface area of BPL1000 decreases to 838 m2/g and the specific surface area of micropore decreases from 582 m2/g to 460 m2/g. As an important pore structure of activated carbon, micropores transform into mesopores or macropores after pyrolysis, which not only leads to the decrease of specific surface area and micropore volume, but also reduces the specific surface area and total volume of activated carbon (Menendez J A et al. 1996). In addition,comparing the specific surface area and volume of BPL1000 and BPLCXs, the specific surface area and volume of BPLCXs are further decreased due to the loading of benzoic acid, from 838 m2/g in BPL1000 to 823 m2/g in BPLCX1, 782 m2/g in BPLCX2 and 598 m2in BPLCX3 /g, and the pore volume also decreases from 0.65 cm3/g to 0.58 cm3/g for BPLCX1, 0.54 cm3/g and 0.49 cm3/g for BPLCX2. By comparing the specific surface area and volume of BPLCXs, it’s found that the total specific surface area, total volume, micropore surface area and micropore volume all decrease with the increase of benzoic acid loading. For example, the micropore specific surface area of BPLCX2 and BPLCX3 are 419 m2/g and 292 m2/g,respectively, which is 41 m2/g and 167 m2/g less than that of BPL1000, respectively. The micropore volume of BPLCX3 was 0.13 cm3/g, which is 0.08 cm3/g less than the 0.21 cm3/g of BPL1000. The decrease of specific surface area and volume of modified activated carbon indicates that the loading of benzoic acid has blocked the pores of activated carbon, decreased the specific surface area and pore volume of BPLCXs, and further decreased the specific surface area and volume with the increase of benzoic acid loading.

3.1.2 Boehm titration method to measure the content of each acidic oxygen-containing functional groups

Table 4 shows the Boehm titration method for measuring the content of acidic oxygen-containing functional groups on the surface of each activated carbon. Because of the carboxyl at 100 ℃ - 250℃ pyrolysis and desorption of CO2, the lactone is under high temperature and phenolic hydroxyl will be pyrolyzed and CO2will be desorbed (Tessmer C H et al. 1997). The contents of carboxyl, lactone and phenolic hydroxyl groups on BPL surface are 0.030 mmol/g, 0.115 mmol/g and 0.105 mmol/g,respectively. After calcination at 1 000 ℃, the content of carboxyl group decreases to 0.000 mmol/g. Hydroxyl content has also dropped.BPL1000 modified by benzoic acid has led to the changes of the number of surface acidic oxygen-containing functional groups in which the content of carboxyl, lactone groups and phenolic hydroxyl increases with the increase of benzoic acid loading. It is inferred that the addition of benzoic acid increases the content of carboxyl groups as well as the number of acidic oxygen-containing functional groups on the surface of activated carbon. The activated carbon may be led by an acidic group of the surface oxygen-containing functional after pyrolysis,generating highly active reaction sites (Tessmer C H et al. 1997). When in contact with the benzoic acid of the reactive sites, complex chemical reactions occur to produce a variety of acidic oxygen-containing functional groups. However,the carboxyl group content is lower than the acid content of activated carbon adsorption on the BPLCXs surface, and the content of various acidic oxygen-containing functional groups is also lower than the loading amount of benzoic acid probably because of the active site of acid carbonylation reaction, and the Boehm titration does not measure the carbonyl content.

In order to verify the state of benzoic acid in the pores of activated carbon, BPLCX2 was calcined at 350 ℃ for N2, and white crystals were found on the tail of quartz tube loaded with BPLCX2. The white crystals were confirmed to be benzoic acid by FTIR analysis, indicating that benzene Formic acid was physically adsorbed on the surface of activated carbon. The content of various acidic oxygen-containing functional groups of BPLCX2 at this time was measured by Boehm titration. It was found that the content of carboxyl group decreased to 0.057 mmol/g, the content of lactone group and phenolic hydroxyl group decreased to 0.095 mmol/g and 0.0123 mmol/g, respectively, indicating that benzoic acid is adsorbed on the surface of activated carbon,which not only chemically reacts with active sites but also increases the content of various acidic oxygen-containing functional groups. Some benzoic acids are also physically adsorbed on the surface of the activated carbon.

Fig. 3 shows the FTIR spectra of BPL,BPL1000, and BPLCXs. The figure shows a clear peak of BPL at the vibration frequencies of 797 cm-1, 1 085 cm-1and 1 560 cm-1, representing the CH vibration peak of benzene ring (Sablinskas V et al. 2005), the CO plane vibration peak in ether or alcohol, Benzene ring skeleton (WU Jin-guang.1994) vibration peak, indicating the BPL surface distribution of aromatic, ether and alcohol-based oxygen-containing functional groups. According to the Boehm titration method, the carboxyl and lactone groups are distributed on the surface of the modified activated carbon. The vibration peak of the C = O shrinkage of the two oxygen-containing functional groups is between 1 640 cm-1and 1 780 cm-1(WU Jin-guang, 1994). However, no peak of C = O contraction vibration is observed in BPLCXs. It is speculated that the C = O contraction vibration peak is hard to detect because the loading concentration is lower than the FTIR minimum detection concentration. In order to verify this point of view, the concentration of benzoic acid was increased to 2.466 mmol/g. The FTIR spectrum showed obvious peak at 1 690 cm-1,that is, the peak of C = O contraction of carboxyl group (WU Jin-guang, 1994). The loading does increase the carboxyl content of the activated carbon surface.

Activated carbon surface acid oxygen functional groups by high temperature pyrolysis and CO2and CO desorption, and with the surface acid oxygen functional groups of different types,the pyrolysis temperature and decomposition products are not the same. Based on this principle,we used TPD to characterize BPL and BPLCXs.Fig. 4(a) shows the CO2spectrum of the oxygencontaining functional groups of BPL and BPLCXs pyrolyzed and desorbed via TPD. The amount of CO2desorbed per unit mass of BPLCXs is higher than that of BPL, and the mass of desorbed CO2increases with the increase of benzoic acid loading amount. This phenomenon proves that the surface acidity of oxygen- containing functional groups of BPLCXs increases with the increase of benzoic acid loading. Fig. 4(b), 4(c), 4(d), 4(e) and 4(f)represent the peak fitting results of CO2for pyrolysis and desorption of BPL and BPLCXs,respectively. BPL pyrolysis at 240oC, 378oC, 639oC and 786oC CO2desorption, between the carboxylic acid pyrolysis temperature 100oC-250oC, lactone-based pyrolysis temperature 190oC-650oC and anhydride pyrolysis temperature 350oC-600oC (Figueiredo J et al. 1999). So BPL acidic surface oxygen functional groups include carboxyl, lactone, anhydride and so on. From Fig.4 (c) -4 (f), it can be seen that the CO2of BPLCXs is pyrolyzed and desorbed by two different degrees of temperature with the pyrolysis temperature of 360oC-375oC and 380oC -410oC, respectively,which are different from those of acidic functional groups, indicating that the load of benzoic acid has changed the types and content of acid functional groups on the surface of BPL. In addition, from the analysis of desorption temperature, the temperature of pyrolysis CO2of BPLCXs is higher than the pyrolysis temperature of carboxylic acid(Figueiredo J et al. 1999). However, the pyrolysis temperature of lactone or anhydride indicates that the loading of benzoic acid increases the content of lactones and anhydrides on the surface of activated carbon.

Fig. 5 shows the TPD pyrolysis and desorption CO spectra of BPL and BPLCXs. Fig. 5 (a) shows the CO spectra of pyrolysis and desorption of BPL and BPLCXs. The quality of CO desorbed by BPLCXs was higher than that of BPL, and the mass of CO desorbed by pyrolysis also increased with the increase of benzoic acid loading. The pyrolysis temperatures of the oxygen functional groups on the surface of BPL were 388oC, 585oC,682oC and 867oC. The pyrolysis temperature of BPLCXs was between 400oC-410oC, 680oC-690oC and 920oC-950oC. The CO of BPLCXs was from carboxylic anhydride, Phenolic hydroxyl or carbonyl pyrolysis (Figueiredo J et al. 1999).According to the analysis of TPD spectrum, the surface of benzoic acid-modified activated carbon not only distributes carboxyl groups, but also distributes oxygen-containing functional groups such as lactone groups and acid anhydrides.

Fig. 6 shows the C1s binding energy spectra of BPL and BPLCXs. The binding energies of C1s in BPL were 284.5 ev, 286.22 ev and 287.8 ev,respectively, which were C1s in graphite layer, C1s of CO bond in ester or ether, and C1s of carbonyl or quinone group C = O; C1s in BPLCXs. It is noteworthy that a new C1s appears at the binding energy of 288.6 eV, which is the peak of C1s with carboxyl C = O (Figueiredo J et al. 1999), which proves that the load of benzoic acid causes the surface of activated carbon to appear carboxyl. Fig.7 shows the O1s binding energy spectra of BPL and BPLCXs. The binding energy of O1s is 530 ev,531.8 ev and 532.4 ev, respectively O1s of quinone group, O1s of C-O of ether bond and O1s of C = O of anhydride, lactone group and carbonyl group.The O1s binding energy of BPLCXs on the surface can be 530.9 ev, 532.7 ev and 533.4 ev, which is higher than that of O1s of BPL, indicating that the chemical state of C1s and O1s of benzoic acid-modified activated carbon is quite different from that of BPL. However, the O1s of BPLCXs were lower than the O1s binding energy of carboxylate 534.2 ev. XPS analysis results are consistent with FTIR and TPD, that is, the load of benzoic acid changes the type and content of acidic oxygen-containing functional groups on the surface of BPL, which not only increases the carboxyl content, but also increases the content of lactone, phenolic hydroxyl and anhydride on the surface of activated carbon Oxygen functional groups.

Fig. 2 SEM images of BPL, BPL1000 and BPLCXs zoomed in 1 000 and 20 000

Table 3 Specific surface areas and pore volumes of the ACs’ pores

Table 4 Content of the surface acid oxygen function groups of ACs

Fig. 3 FTIR spectrums of the BPL and BPLCXs

Fig. 4 The spectrum of CO2 desorbed from the BPL and BPLCXs

Fig. 5 Spectrums of CO desorbed from the BPL and BPLCXs

Fig. 6 XPS spectrums of C1S in BPL and BPLCXs

Fig. 7 XPS spectrums of O1s in BPL and BPLCXs

3.2 Modified activated carbon surface acidic oxygen functional group Hg0 saturated adsorption capacity under high temperature environment

Fig. 8 shows the adsorption curves of Hg0at 140oC, Hg0inflow concentration of 10 μg/m3,BPL, BPL1000 and BPLCXs to Hg0, and the saturated adsorption capacity of Hg0for each activated carbon sample is calculated by formula(1) Low order, with the ranking of BPLCX1 (35 μg Hg0/gAC)> BPLCX3 (25 μg Hg0/gAC)> BPLCX4(16 μg Hg0/gAC)> BPL (10 μg Hg0/gAC)>BPL1000 (6 μg Hg0/gAC). Among them, BPLCX2 had the highest saturated adsorption capacity,which was 5 times of the saturated adsorption capacity of BPL, indicating that the adsorption capacity of modified activated carbon on Hg0was significantly increased due to the load of benzoic acid, and the adsorption of Hg0by activated carbon was chemisorption at high temperature. Therefore,the loading of benzoic acid promotes the adsorption of HgO to the activated carbon surface.Fig. 9 shows the relationship between the content of acidic functional groups on the surface of BPL,BPL1000, BPLCX1 and BPLCX2 and the saturated adsorption capacity of Hg0. The carboxyl content shows the best linear relationship with the saturated adsorption capacity (R2 = 0.9563). There is no obvious correlation between the content of hydroxyl group and the saturated adsorption capacity. It is proved that the carboxyl group is the main acidic oxygen-containing functional group for Hg0adsorption. However, the saturated adsorption capacity of BPLCXs to Hg0does not increase further with the increasing of benzoic acid.It may be because the loading of benzoic acid not only increases the surface carboxyl content, but also adds other oxygen-containing functional groups such as lactone group, phenolic hydroxyl group. The content of other oxygen-containing functional groups on Hg0adsorption to form a competitive relationship, but will inhibit the Hg0adsorption.

Fig. 8 Adsorption of Hg0 by BPL, BPL1000, and BPLCXs

Table 5 Adsorption capacities of Hg0 for BPL, BPL1000 and BPLCXsat 140 oC

4 Conclusions

When BPL is heated at high temperature, the specific surface area and void volume will decrease. When BPL1000 is loaded with benzoic acid, the specific surface area and void volume will decrease even further, and will decrease with the increase of benzoic acid loading amount.

By Boehm titration, FTIR, TPD and XPS, it’s proved that the impregnation of benzoic acid has caused the chemical reaction on the active site of BPL1000, and the acid oxygen-containing functional groups including carboxyl group,lactone group, phenolic hydroxyl group and anhydride have been increased on the surface of benzoic acid-modified activated carbon; but some benzoic acid is still physically adsorbed on the surface of activated carbon.

Hg0adsorption experiments at 140 ℃ have proved that the increase of carboxyl content in benzoic acid-modified activated carbon has greatly promoted the saturated adsorption capacity of Hg0,indicating that carboxyl is the active site for promoting Hg0chemisorption. However, the lactone and phenolic hydroxyl don’t significantly promote Hg0adsorption.