Adsorption of Metal Ions on Nitrogen Surface Functional Groups in

An active carbon with very high nitrogen content (∼9.4 wt % daf) was also ... The nitrogen functional groups in the carbons act as surface coordinat...
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Langmuir 2002, 18, 470-478

Adsorption of Metal Ions on Nitrogen Surface Functional Groups in Activated Carbons Y. F. Jia, B. Xiao, and K. M. Thomas* Northern Carbon Research Laboratories, Department of Chemistry, Bedson Building, University of Newcastle upon Tyne, Newcastle upon Tyne, NE1 7RU, UK Received July 25, 2001. In Final Form: September 25, 2001 A commercially available coconut-shell-derived active carbon was oxidized with nitric acid, and both the original and oxidized active carbons were treated with ammonia at 1073 K to incorporate nitrogen functional groups into the carbon. An active carbon with very high nitrogen content (∼9.4 wt % daf) was also prepared from a nitrogen-rich precursor, polyacrylonitrile (PAN). These nitrogen-rich carbons had points of zero charge (pHpzc) similar to H-type active carbons. X-ray absorption near-edge structure (XANES) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and temperature-programmed desorption (TPD) were used to characterize the nitrogen functional groups in the carbons. The nitrogen functional groups present on the carbon surface were pyridinic, pyrrolic (or indolic), and pyridonic structures. The adsorption of transition metal cations Cd2+, Ni2+, and Cu2+ from aqueous solution on the suite of active carbons showed that adsorption was markedly higher for carbons with nitrogen functional groups present on the surface than for carbons with similar pHpzc values. In contrast, the adsorption characteristics of Ca2+ from aqueous solution were similar for all the carbons studied. Flow microcalorimetry (FMC) studies showed that the enthalpies of adsorption of Cd2+(aq) on the active carbons with high nitrogen contents were much higher than for nitric acid oxidized carbons studied previously, which also had enhanced adsorption characteristics for metal ion species. The enthalpies of adsorption of Cu2+ were similar to those obtained for Cd2+ for specific active carbons. The nitrogen functional groups in the carbons act as surface coordination sites for the adsorption of transition metal ions from aqueous solution. The adsorption characteristics of these carbons are compared with those of oxidized carbons.

1. Introduction Activated carbons have been successfully employed as adsorbents and catalyst supports due to their welldeveloped porous structures and large internal surface comprised of hydrophobic graphene layers and hydrophilic surface functional groups. These porous materials can be used for the adsorption of a wide range of species from both gas or liquid phases. However, adsorption from the liquid phase is more complex and the fundamental principles are much less well understood compared with gas-phase adsorption. An important aspect in the treatment of aqueous systems using active carbons is that it can be used to remove both inorganic and organic species, and this is very important in the purification of water. Applications of the adsorption of metal species from aqueous solution are for the recovery of noble metals,1-3 treatment of potable and wastewater containing trace amounts of toxic metals,4-10 and preparation of metal catalysts supported on carbon.11-16 The adsorption of

inorganic or ionic species from solution are mainly controlled by electrostatic forces, and this needs to be discussed in terms of the speciation of the adsorbate and the amphoteric nature of the carbon adsorbent, which is related to the various types of functional groups present on the surface.17 There is also the role of specific interactions between the adsorbate and adsorbent to consider. Therefore, an understanding of the mechanism of the interaction of metal species with surface functional groups is essential for tailoring the surface chemistry to suit specific applications. In comparison with the extensive research on surface oxygen functional groups,18-22 nitrogen functional groups on an active carbon surface have not been studied extensively.23-25 Reports on the effects of nitrogen functional groups present in active carbon on the adsorption of metal species are very limited.26 X-ray photoelectron spectroscopy (XPS) was used to identify the nitrogen functional groups present on carbon surfaces.23-25 However, XPS studies of carbon suffer from the lack of spectral resolution. The spectra usually have broad over-

* To whom correspondence should be addressed. (1) Jia, Y. F.; Steele, C. J.; Hayward, I. P.; Thomas, K. M. Carbon 1998, 36, 1299. (2) Laxen, P. A. Hydrometallurgy 1984, 13, 169. (3) McDougall, G. J.; Adams, M. D.; Hancock, R. D. Hydrometallurgy 1987, 18, 125. (4) Reed, B. E.; Arunachalam, S.; Thomas, B. Environ. Prog. 1994, 13, 60. (5) Taylor, R. M.; Kuennen, R. W. Environ. Prog. 1994, 13, 65. (6) Huang, C. P.; Wu, M. H. Water Res. 1977, 11, 673. (7) Netzer, A.; Hughes, D. E. Water Res. 1984, 18, 927. (8) Corapcioglu, M. O.; Huang, C. P. Water Res. 1987, 21, 1031. (9) Tan, T. C.; Teo, W. K. Water Res. 1987, 21, 1183. (10) Jayson, G. G.; Sangster, J. A.; Thompson, G.; Wilkinson, M. C. Carbon 1993, 31, 487. (11) Austin, J. M.; Groenewald, T.; Spiro, M. J. Chem. Soc., Dalton Trans. 1980, 854. (12) Albers, P.; Deller, K.; Despeyroux, B. M.; Prescher, G.; Scha¨fer, A.; Seibold, K. J. Catal. 1994, 150, 368. (13) Rondon, S.; Wilkinson, W. R.; Proctor, A.; Houalla, M.; Hercules, D. M. J. Phys. Chem. 1995, 99, 16709. (14) Sosa, R. C.; Masy, D.; Rouxhet, P. G. Carbon 1994, 32, 1369.

(15) Rodriguez-Reinoso, F. Carbon 1998, 36, 159. (16) Radovic, L. R.; Rodriguez-Reinoso, F. In Chem. Phys. Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1997; Vol. 25, p 243. (17) Radovic, L. R.; Moreno-Castilla, C.; Rivera-Utrilla, J. In Chem. Phys. Carbon; Radovic, L. R., Ed.; Marcel Dekker: New York, 2000; Vol. 27, p 227. (18) Puri, B. R. In Chemistry and Physics of Carbon; Walker, P. L., Jr., Ed.; Marcel Dekker: New York, 1970; Vol. 6, p 191. (19) Moreno-Castilla, C.; Ferro-Garcia, M. A.; Joly, J. P.; BautistaToledo, I.; Carrasco-Marin, F.; Rivera-Utrilla, J. Langmuir 1995, 11, 4386. (20) de la Puente, G.; Pis, J. J.; Menendez, J. A.; Grange, P. J. Anal. Appl. Pyrol. 1997, 43, 125. (21) Zawadzki, J. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1989; Vol. 21, p 147. (22) Jia, Y. F.; Thomas, K. M. Langmuir 2000, 16, 1114. (23) Jansen, R. J. J.; van Bekkum, H. Carbon 1995, 33, 1021. (24) Jansen, R. J. J.; van Bekkum, H. Carbon 1994, 32, 1507. (25) Vinke, P.; van der Eijk, M.; Verbree, M.; Voskamp, A. F.; van Bekkum, H. Carbon 1994, 32, 675. (26) Abotsi, G. M. K.; Scaroni, A. W. Carbon 1990, 28, 79.

10.1021/la011161z CCC: $22.00 © 2002 American Chemical Society Published on Web 12/18/2001

Adsorption of Metal Ions on N Functional Groups

lapping bands, which lead to difficulties in differentiating between the various forms of nitrogen. X-ray absorption near-edge structure (XANES) spectroscopy has advantages over XPS in determining nitrogen functionality present in coals27,28 and in chars29,30 in terms of better spectral resolution. The fundamental challenge is to maximize the capacity of the adsorbent. A number of factors influence the adsorption of metal ion species from aqueous solution, these include the surface charge on the adsorbent, the nature of the chemical interaction between the metal species and the functional groups, and the speciation of the metal in the solution as a function of pH.17 In this study, the role of nitrogen functional groups has been investigated. TPD, XANES, and FTIR were used to characterize nitrogen surface functional groups present in active carbons. These functional groups were either incorporated into the active carbon by ammonia treatment at high temperature or by carbonization of a nitrogenrich precursor (polyacrylonitrile). The influences of nitrogen functional groups on the adsorption of the transition metal cations Cd2+, Ni2+, and Cu2+ from aqueous solution were investigated. In real applications, for example, the removal of trace ionic species from effluent streams, complex mixtures of species need to be treated, and competitive adsorption takes place. Therefore, since Ca2+ is one of the common species likely to be present in aqueous effluent, the adsorption characteristics were also studied for comparison, to establish if significant competitive adsorption effects exist for surface sites in nitrogen-functionalized carbons. 2. Experimental Section 2.1. Materials Used. Chemical Modification. Commercially available coconut-shell-derived carbon C, prepared by physical activation using steam at ∼900 °C, was treated with ammonia using the following procedure: Carbon C was heated to 800 °C at a heating rate of 3 °C min-1 in ammonia (flow rate ∼200 cm3 min-1) and held at maximum temperature for 3 h. The treated carbon was cooled in ammonia to room temperature. The carbon was then Soxhlet extracted with water to remove residual ammonia and any soluble material. The sample was vacuumdried at 75 °C and designated sample code CA. Carbon C was also oxidized by refluxing in 7.5 M HNO3 for 48 h to give carbon CN2. The oxidized carbon CN2 was treated with ammonia using the same procedure as above to give a nitrogen-rich carbon designated code CN2A. In previous studies, CN2 was heat-treated to provide a suite of carbons where the oxygen functional groups of various thermal stabilities were varied progressively. The resultant carbons were designated using the code of original carbon followed by a number to indicate the heat treatment temperature in degrees Celsius; e.g., CN2-300 represents CN2 heat-treated to 300 °C and held at the heat treatment temperature (HTT) for 1 h. The adsorption characteristics of these oxidized carbons are compared with the nitrogen-functionalized carbons. Polyacrylonitrile (PAN)-Derived Carbon. Polyacrylonitrile powder was pretreated in air at 200 °C for 1 h. The pretreated PAN was heated at a heating rate of 3 °C min-1 to 900 °C in argon. The argon flow was then changed to carbon dioxide and the char was gasified at 900 °C for 4 h. This gave a carbon, designated code PANC, with a burnoff ∼60 wt %. 2.2. Characterization of the Carbon Surface. Temperature-Programmed Desorption (TPD). Temperature-programmed desorption (TPD) studies were carried out using a Thermal (27) Mitra-Kirtley, S.; Mullin, O. C.; Branthaver, J. F.; Cramer, S. P. Energy Fuels 1993, 7, 1128. (28) Mullin, O. C.; Mitra-Kirtley, S.; Elp, J. V.; Cramer, S. P. Appl. Spectrosc. 1993, 47, 1268. (29) Zhu, Q.; Money, S. L.; Russell, A. E.; Thomas, K. M. Langmiur 1997, 13, 2149. (30) Jones, J. M.; Zhu, Q.; Thomas, K. M. Carbon 1999, 37, 1123.

Langmuir, Vol. 18, No. 2, 2002 471 Science STA 1500 thermogravimetric analyzer (TGA) connected to a VG Quadrupole 300 amu mass spectrometer by a heated stainless steel capillary lined with deactivated fused silica. Approximately 7 mg of sample was placed in a sample bucket and heated from ambient temperature to 1200 °C at a heating rate of 15 °C min-1 under flowing argon (50 cm3 min-1). The evolved gas was sampled and analyzed by the mass spectrometer throughout the course of the desorption process. The mass/charge (m/z) ratios 28, 30, and 44 were monitored throughout the desorption process. Point of Zero Charge Measurements. The pH at the potential of zero charge (pHpzc) of various carbons was measured using the pH drift method.31 The pH of a solution of 0.01 M NaCl was adjusted between 2 and 12 by adding either HCl or NaOH. Nitrogen was bubbled through the solution at 25 °C to remove dissolved carbon dioxide until the initial pH stabilized. Active carbon (0.15 g) was added to 50 mL of the solution. After the pH had stabilized (typically after 24 h), the final pH was recorded. The graphs of final versus initial pH were used to determine the points at which initial pH and final pH values were equal. This was taken as the pHpzc of the carbon. X-ray Absorption Near-Edge Structure Spectroscopy (XANES). The nitrogen X-ray absorption near-edge structure (K edge) spectroscopy measurements were carried out using beam line Station 1.1 of the Synchrotron Radiation Source (SRS) at Daresbury Laboratory. The SRS storage ring was operated at 2 GeV with the ring currents 100-300 mA. The monochromator was a high-energy spherical grating monochromator with a Au grating and a slit width of 0.1 mm. Detection was by total electron yield. The carbon samples were ground finely and dispersed in carbon tetrachloride. A few drops of each slurry were applied to a high purity aluminum plate and the solvent was allowed to evaporate. The measurements were carried out at ambient temperature under high vacuum, 10-6-10-8 bar. The spectra were calibrated using acridine as a reference, which has a strong absorption at 399.7 eV. Flow Microcalorimetry (FMC) Studies. A Microscal Model 3 Vi immersion flow microcalorimeter was used to measure the enthalpy change for the adsorption of species from aqueous solution. Approximately 90 mg of carbon (particle size of 0.2120.425 mm) was used with a flow rate of 3.3 cm3 h-1. The enthalpies of adsorption were measured by the following procedure: (1) The carbon was equilibrated by flowing water through the cell until a steady baseline was achieved. (2) Cadmium or copper nitrate solution was passed over the sample and the enthalpy of adsorption was measured by calibration of the thermogram profile against a known current/voltage/time calibration peak. (3) After the adsorption and calibration were completed, the water was passed over the sample to obtain the heat of desorption. Fourier Transform Infrared Spectroscopy (FTIR). FTIR spectra were recorded on a Nicolet 20-PCIR Fourier transform infrared spectrometer with a CsI optics DTGS detector with a resolution of 4 cm-1 using KBr disks prepared by mixing 0.5% of finely ground carbon sample in KBr. Titration Studies. The selective neutralization method used to evaluate the amphoteric character of the carbon surface was the scheme suggested by Boehm.32 The amounts of various acidic functional groups were measured by selective neutralization using NaHCO3, Na2CO3, and NaOH solutions, respectively. The basicity was determined by the reaction of carbon with HCl. About 0.2 g of carbon was placed in 25 cm3 of 0.1 N of each solution, and the mixtures were allowed to stand for 72 h at room temperature. The mixtures were separated by filtering. The amount of each base neutralized by the carbon was determined by back-titration using 0.1 N HCl solution, whereas the amounts of HCl consumed by the basic groups in carbon was determined by back-titration using 0.1 N NaOH. Energy-Dispersive X-ray Analysis (EDA). EDA studies of carbons with adsorbed metal ion species were carried out using a JEOL JSM 35 scanning electronic microscope (SEM) with a LINK QX2000 EDA analyzer. 2.3. Adsorption Studies of Metal Cations. Adsorption Isotherms. Cadmium, nickel, copper, and calcium nitrate solutions (31) Lopez-Ramon, M. V.; Stoeckli, F.; Moreno-Castilla, C.; CarrascoMarin, F. Carbon 1999, 37, 1215. (32) Boehm, H. P. Adv. Catal. 1966, 16, 179.

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Table 1. Characterization Data for the Carbons Used in This Study ultimate analysis (wt % daf) sample code

C

H

N

O (by diff)

C CA CN2A PANC

96.1 95.5 90.3 85.1

0.4 0.1 0.5 0.5

0.3 2.3 5.3 9.4

3.2 2.1 3.9 5.0

pore volume (cm3 g-1) microporea total poreb (CO2, 273 K) (N2, 77 K) 0.32 0.26 0.20 0.26

D-R surface areac (m2 g-1) (CO2, 273 K)

0.32 0.30 0.30 0.30

683 546 493 545

a Micropore volume obtained from extrapolation of DR graph for carbon dioxide adsorption at 273 K. b Total pore volume obtained from extrapolation of the nitrogen adsorption data at 77 K to p/p0 ) 1. c Calculated from the DR micropore volume.

were used for studies of the adsorption of metal species from aqueous solution on the activated carbons. The mixtures of carbon and the aqueous metal nitrate solutions were adjusted to pH 4.1 using an acetic acid/acetate buffer, while the pH of the mixture was approximately 7.0 without any buffer present. The acetate buffer was prepared by mixing 90 mL of 0.2 M CH3COOH with 10 mL of 0.2 M CH3COONa solutions. The mixtures of carbon and solution were allowed to stand for 48 h in a water bath at 25 °C. The carbons were separated from the solution by filtering. The amounts of cadmium adsorbed on the carbons were determined by measuring solution concentration using a Unicam 701 inductively coupled plasma (ICP) atomic emission spectrometer (University of Newcastle upon Tyne, Chemical Analytical Service Unit).

3. Results 3.1. Characterization of Carbon. Analytical and Porous Structure Measurements. The ultimate analysis and porous structure characterization data of the carbons used in the study are given in Table 1. It is evident that treatment with ammonia at 800 °C introduced an appreciable amount of nitrogen into the coconut-shell-derived carbon C. The nitrogen content of the carbon increased from 0.3 to 2.3 wt % daf after ammonia treatment. More nitrogen was incorporated into the carbon when carbon C was oxidized using nitric acid prior to ammonia treatment. The nitrogen content of CN2A was ∼5.3 wt % daf, i.e., twice that of CA. The high amount of nitrogen (9.4 wt % daf) in the PAN-derived carbon allowed the assessment of the role of nitrogen functional groups on the adsorption of metal cations. The suite of carbons used have micropore volumes obtained from carbon dioxide adsorption at 273 K in the range 0.20-0.32 cm3 g-1 and total pore volumes obtained from nitrogen adsorption at 77 K in the range 0.30-0.32 cm3 g-1. It is apparent that both ammonia-treatment procedures did not greatly modify the porous structure of the activated carbon. Activated carbon with similar well-developed porosity was also prepared from polyacrylonitrile using carbon dioxide as activating agent. Point of Zero Charge Measurements. The graphs of final pH versus initial pH obtained using the pH drift method for active carbon, oxidized active carbon,22 heat-treated oxidized active carbons,22 nitrogen-functionalized active carbons, and PAN carbon are shown in Figure 1. The results show that the characteristic graphs for carbons PANC, CN2A, CA, C, and CN2-800 are similar, with pHpzc values varying between 7.85 and 8.66 (see Table 2). The characteristic curves are markedly different from those of acid-oxidized carbon CN2 and heat-treated oxidized carbons CN2-300, -400, -500, and -600, which have pHpzc values CN2A,

CA > C, consistent with the order of nitrogen contents of the carbons (see Table 1). The effect of incorporation of nitrogen functionality in active carbons on the adsorption of Ca2+(aq) was also investigated (see Figure 10). In contrast to the adsorption of transition metal ions, nitrogen-rich carbons CA and CN2A did not have increased adsorption of Ca2+(aq) compared with carbon C. Therefore, the nitrogen-functionalized carbons show adsorption selectivity to transition metal ions. After adsorption of transition metal ions, the nitrogenrich carbons were repeatedly washed with distilled water at room temperature. The EDA analysis of the resultant carbons shows that all the adsorbed metals were desorbed in the washing process, indicating that the adsorption was reversible. The oxidized carbon CN2 adsorbs larger amounts of Cd2+ ion than nitrogen-rich carbons, and the cadmium species adsorbed on CN2 could not be recovered by Soxhlet extraction with water.22 Flow Microcalorimetry (FMC) Studies. Information relating to the mechanisms of adsorption of transition metal ions from aqueous solution on carbons with nitrogen functional groups can be obtained using FMC to investigate the variation of enthalpies of adsorption in relation to surface coverage. The variation of the enthalpies of adsorption of Cd2+(aq) and Cu2+(aq) ions with solution equilibrium concentration for the suite of active carbons studied are shown in Figures 11 and 12, respectively. In both cases, the enthalpies of adsorption of metal ions on different carbons increase with equilibrium solution

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Figure 12. Enthalpies of adsorption of Cu2+(aq) on carbons C, PANC, CN2A, and CN2..

concentrations (or with amount of adsorption). This trend is similar to the results reported previously22 for adsorption of Cd2+(aq) on nitric acid-oxidized carbon CN2, but the enthalpies of adsorption are much higher for CN2A and PANC. At a solution equilibrium concentration of 1.0 mM Cd2+(aq), the enthalpies of adsorption for carbon C, CN2A, PANC, and CN2 were ∼38, 13, 10, and 0.15 kJ‚mol-1, respectively, while for adsorption of Cu2+(aq) ions at the same solution concentration, the corresponding values for the enthalpies of adsorption were 40, 15, 26, and 0.05 kJ‚mol-1, respectively. The enthalpies of adsorption for Cd2+(aq) and Cu2+(aq) on carbon C were the highest in both of the above series. 4. Discussion 4.1. Nitrogen Functional Group Characterization. The characterization of the nitrogen functional groups on the carbon surface is crucial to the understanding of the adsorption mechanism of metal cations on the active carbons with very high nitrogen contents. The forms of nitrogen present on the carbon surface either incorporated by ammonia treatment or inherited from the nitrogen rich precursor were shown by XANES spectroscopy to be mainly pyridinic, pyridonic, and pyrrolic (or indolic) structures. These nitrogen functional groups are usually located at the edges of the graphene layers. It has also been proposed on the basis of XPS studies that amine groups are present in the ammonia-treated carbons.37,38 However, XPS studies23 of nitrogen functionality in the active carbon obtained by HNO3 oxidation followed by ammonia treatment suggested the presence of pyridinic and pyridonic nitrogen functional groups. Recent XPS studies39,40 of carbons produced from polynuclear aromatic compounds with well-defined nitrogen functionality have shown that carbonization resulted in the conversion of pyrrolic to pyridinic and amine to pyrrolic and pyridinic functional groups. Also, increasing concentrations of quaternary and N-X (possibly N-oxide of pyridine or other oxidized nitrogen functionalities due to surface oxidation) were produced with increasing carbon heat-treatment temperature.39,40 It has been proposed that the quaternary (37) Kurth, R.; Tereczki, B. and Boehm, H. P. Extended Abstracts of the 15th Biennial Conference On Carbon; American Carbon Society: Philadelphia, PA, 1981; p 244. (38) Sto¨hr, B.; Boehm, H. P.; Schlo¨gl, R. Carbon 1991, 29, 707.

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nitrogen species observed in X-ray photoelectron spectroscopy (XPS) is nitrogen incorporated into the graphene layers.39,41 However, in previous XANES studies,29 which gave clear resolution of pyridinic, pyrrolic, and pyridone groups, the lack of appropriate reference compounds did not allow identification of the peak energy for the quaternary species. The XANES spectra show that the nitrogen functional groups in carbons PANC and CN2A are broadly similar, despite the different preparation methods [high-temperature ammonia treatment and carbonization of a nitrogenrich precursor (PAN)]. The nitric acid oxidation step, which enhanced nitrogen incorporation in the carbon by hightemperature ammonia treatment, also led to incorporation of a relatively small amount of nitrogen. However, this nitrogen functionality is different, and the XANES spectra (see Figure 4) indicate that significant amounts do not exist after ammonia treatment at high temperature. The interpretation of IR spectra is more difficult than that of XANES, due to the overlap with other bands. The pyridinic, pyrrolic (or indolic), and pyridonic structures give CdC, CdN, and CdO absorption bands in the IR region near 1500-1600 cm-1. The graphene layer sites and quinone CdO also show bands in the same region. Therefore, it is difficult to differentiate the functional groups on the basis of IR bands. The absence of the bands in the 2200 cm-1 region indicated that the CtN groups were absent in all the carbons used in this study. Distinct differences were observed in the temperatureprogrammed desorption studies of the suite of carbons used in this study, which show differences in the thermal stability of surface species. Both CA and CN2A have a heat treatment temperature (HTT) of 800 °C, whereas PAN has a HTT of 900 °C, and this difference is reflected in evolution of CO, CO2, and NO at slightly lower temperature in the TPD profiles of the former. These studies provided clear evidence for the absence of significant carbon dioxide desorption below 600 °C for CN2A, CA, and PANC, which is consistent with the absence of carboxylic acid surface groups similar to those incorporated into carbon C by nitric acid oxidation. However, the desorption of species that starts just below the HTT and is mainly above the HTT indicates that there is some decomposition and structural change above the HTT. The selective neutralization studies also support the conclusion that carboxylic acid groups are not present on the surface of PANC, CA, and CN2A. The desorption of nitrogen surface species NO, N2, and, possibly, other species such as N2O involves surface reactions. Furthermore, N2 cannot be distinguished from CO, and N2O cannot be distinguished from CO2 using quadrupole mass spectrometry.41 Therefore, only the NO TPD profiles were studied and these show that the NO profiles are shifted to slightly higher temperature similar to the observation of NO release in temperature-programmed combustion of nitrogen-containing carbons.41 However, it is not possible to make definitive conclusions regarding the nature of nitrogen surface species from TPD because of the complex nature of reactions with oxygen surface species. 4.2. Adsorption Isotherms. The adsorption capacities of CN2A, CA, and PANC for transition metal cation species do not correlate with surface area, micropore volumes, and total pore volumes of the active carbons. Nitrogen functional groups present on the carbon surface substan(39) Pels, J. R.; Kapteijn, F, Moulijn, J. A.; Zhu, Q.; Thomas, K. M. Carbon 1995, 33, 1641. (40) Stanczyk, K.; Dziembaj, R.; Ptwowarska, Z.; Witkowski, S. Carbon 1995, 33, 1383. (41) Grant, K. A.; Zhu, Q.; Thomas, K. M. Carbon 1994, 32, 883.

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tially enhanced the adsorption of Cd2+(aq), Ni2+(aq), and Cu2+(aq) under neutral or slightly basic conditions, with the carbon of highest nitrogen content (PANC) having greater adsorption capacity [∼20 times the capacity of carbon C, which had a similar pHpzc and ∼25% of the oxidized carbon CN2 (pHpzc ) 2.53)]. Temperatureprogrammed desorption and selective neutralization studies of CN2A, CA, and PANC showed that the carboxylic acidic oxygen functional groups, which had a marked effect on cadmium ion adsorption on carbon CN2, through ion exchange reactions, were absent. The pH of the solution in the range 4.0-7.0 does not greatly affect the adsorption isotherms of Cd2+(aq) on carbon C. In contrast, the adsorption isotherms of Cd2+(aq) on the nitrogen-rich carbons (CN2A, CA, and PANC) are markedly different at pH 4.1 and 7.0. At pH 4.1, the amounts adsorbed are lower and are only slightly greater than the untreated active carbon, whereas at pH 7.0, the adsorption isotherms increase sharply at low concentration before reaching a plateau. At low pH there is competition for the basic nitrogen sites between protonation and adsorption of metal ions. At pH 7.0, most of the basic nitrogen sites are available for adsorption of Cd2+(aq) ions, which can occupy a large proportion of active sites at low concentration. After reaching the plateau, most of the nitrogen sites are occupied by cadmium species with only limited sites available. Therefore, it is reasonable to conclude that decreased adsorption capacities of cadmium ions on nitrogen-rich carbons with decreasing pH are mainly due to protonation of basic nitrogen functional groups, which decreases the availability of adsorption sites. Therefore, the enhancement of the adsorption of transition metal cations can be unequivocally ascribed to the nitrogen functional groups present on the carbon surface. 4.3. Enthalpies of Adsorption. Previous studies of the heat of adsorption of cadmium nitrate on graphitized carbon black and activated carbon from 1.0 mM aqueous solution gave the enthalpies of adsorption 159 and 89 kJ‚mol-1, respectively.42 It is evident from Figures 11 and 12 that incorporation of aromatic nitrogen functional groups resulted in lower enthalpies of adsorption of Cd2+(aq) and Cu2+(aq) ions on carbon CN2A and PANC compared with carbon C. The enthalpies of adsorption of these metal ions on CN2 were 2 orders of magnitude or more lower than those of the other carbons. The enthalpies of adsorption of Cd2+(aq) and Cu2+(aq) on PANC were similar to those for CN2A. The differences in the enthalpies of adsorption of metal ions from aqueous solution on C, CN2A, PANC, and CN2 reflect the different adsorption sites available in the carbons. The carboxylic groups in CN2 undergo ionexchange reactions with M2+ ions in aqueous solution and the enthalpies of adsorption are very low. In the case of carbon C, the adsorption sites are the graphene basal planes and basic oxygen sites. 4.4. Adsorption Mechanism. The equilibrium uptakes of ionic species on active carbons are largely governed by electrostatic attraction or repulsion. Moreover, in some systems, specific interactions, e.g., complex formation, or nonspecific interactions, e.g., van der Waals interactions, may participate in the adsorption process. The following aspects need to be considered in the adsorption of metal ion species: (a) the speciation diagram of the adsorbate as a function of pH and (b) the unique amphoteric nature of the carbon adsorbent that has

separate and noncoincident acidic and basic sites. Therefore, both solution pH and metal concentration influence the extent of adsorption, with the former determining the charge density of the carbon surface.17 Both acidic and basic functional groups are present in the nitrogen-functionalized active carbons, and the adsorption properties will depend on the relative concentrations and properties of these functionalities. The relative contributions of the functional groups and the graphene layers to the surface charge are important. The contributions to the positive charge are (1) basic oxygen-containing functional groups (pyrones and chromenes), (2) protonated nitrogen functional groups, and (3) and graphene layers acting as Lewis bases. The surface charge on the adsorbent and speciation in solution is affected by the pH of the solution, and this influences the nature of the chemical interaction between the species and the functional groups on the carbon surface. Knowledge of the speciation diagram of the adsorbate and the surface chemistry of the adsorbent is necessary for an understanding of the adsorption mechanism.17 The hydrolysis product of Ni2+(aq) with known stability is Ni(OH)+. The stabilities of Ni(OH)2(aq) and Ni(OH)3- are less well established. Ni4(OH)42- is formed at high concentrations of Ni2+(0.1 M) for pH > 7.43 The hydrolysis of Cd2+(aq) starts at pH >7, and the formation of Cd2(OH)3+, Cd(OH)+, Cd(OH)2, and Cd(OH)42- has been established. The hydrolysis of Cu2+(aq) leads to the formation of Cu2(OH)22+. Mononuclear species Cu(OH)+, Cu(OH)2, and Cu(OH)3- are formed in dilute solution with increasing pH in the range 8-12, with Cu(OH)42- formed in the more alkaline solutions.43 Therefore, it is reasonable to conclude that M(OH)+ species are not important in relation to equilibrium with the surface at the metal ion concentrations and pH range (4-7) used in this study. The enthalpies of adsorption of Cd2+(aq) and Cu2+(aq) ions on CN2A and PANC are 2 orders of magnitude higher than the corresponding enthalpies of adsorption on oxidized carbon CN2 and lower than the enthalpies of adsorption on the original untreated carbon and other H-type active carbons. It is evident that the thermodynamics are consistent with a different adsorption mechanism with surface nitrogen functional groups in CN2A, CA, and PANC coordinating with these transition metal ions whereas, with oxidized carbon CN2, ion exchange takes place with carboxylic functional groups. A comparison of the Cd2+(aq) adsorption characteristics of the nitrogen-functionalized and oxidized carbons in relation to pHpzc is shown in Figure 13. It is evident that there is a general decrease in Cd2+(aq) adsorption with increasing pHpzc. However, the nitrogen functionalized carbons adsorb 10-20 times the amount of Cd2+(aq) compared with original carbon C and CN-800, which have similar pHpzc values and are carbons with similar heattreatment temperatures that have very low nitrogen contents. Therefore, this enhanced adsorption of Cd2+(aq) cannot be due to electrostatic interactions alone. Active carbons C, CA, CN2A, and PANC can be compared directly because they have similar pHpzc values (range 8.14-8.66). Figure 14 shows the variation of Cd2+ adsorbed at Cd2+(aq) equilibrium solution concentrations of 0.05 and 0.15 mM with nitrogen content of the active carbon adsorbents. It is apparent that the amount of Cd2+(aq) adsorbed increases approximately linearly with nitrogen content. There are possibly some small effects due to different relative concentrations of surface nitrogen

(42) Groszek, A. J. Proceedings of Carbon 92, Essen, Germany, Deutsche Keramische Geselleshaft, 1992, pp 278-280.

(43) Baes, C. F., Jr.; Mesmer, R. E. The Hydrolysis of Cations; Wiley: New York, 1976.

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Figure 13. The variation of Cd2+(aq) adsorbed on the suites of oxidized and nitrogen-functionalized carbons at 0.05 mM (O) and 0.15 mM (4) Cd2+(aq) solution equilibrium concentrations with pHpzc.

Jia et al.

pH on the addition of nitrogen-functionalized carbon to the solution of metal ions indicates that coordination via pyridone groups is not significant. The quaternary nitrogen has been proposed to have a positive charge,41 and therefore, electrostatic interactions are not favorable. This leaves the pyridine, amine, and pyrrole functional groups as possible surface coordination sites. There is evidence from carbonization of pure polynuclear aromatic compounds that amine groups have lower stability than pyridinic and pyrrolic groups at high heat-treatment temperatures.40 Pyridinic groups are much more basic than pyrrolic groups, and therefore, it is proposed that these act as ligands through the formation of surface species similar to coordination compounds. Cu2+(aq) was adsorbed to a greater extent than Cd2+(aq) and Ni2+(aq) on nitrogen-rich active carbons. The selectivity is due to the difference in the stability constant of the surface species formed by the transition metal ions and the nitrogen functional groups in the adsorbent. The hard/ soft acid /base principles can be used to rationalize the different extents of adsorption of Cd2+(aq), Cu2+(aq), Ni2+(aq), and Ca2+(aq) ions on CN2A, CA, and PANC. Alkali earth metal ions normally only form complexes with chelate agents such as crown ethers. Thus, the presence of nitrogen functional groups in the carbon did not increase the adsorption of Ca2+(aq) as observed above. Therefore, the competitive adsorption effects of Ca2+(aq) would be small, which is not the case for oxidized carbons, where ion exchange is the adsorption mechanism.22 5. Conclusions

Figure 14. The variation of Cd2+(aq) adsorbed on the suite of nitrogen-functionalized carbons at 0.05 mM (4) and 0.15 mM (O) Cd2+(aq) equilibrium solution concentrations.

Figure 15. A schematic diagram of possible surface nitrogen functional groups in activated carbon for adsorption of transition metal ions.

functional groups in the carbons. It is proposed that coordination to nitrogen functional groups is responsible for the enhanced metal ion adsorption. A schematic representation of the possible nitrogen functional groups present on the active carbon surface is given in Figure 15. The pyridinic, pyrrolic, and pyridonic groups were identified by XANES, and quaternary and N-X nitrogen have been shown previously to be present in PAN carbon by XPS studies.39 The small increase in

Active carbons with high nitrogen functional groups concentrations were prepared by ammonia treatment of an active carbon or by carbonization of PAN, a nitrogenrich precursor. Oxidation pretreatment of the active carbon using nitric acid enhanced the incorporation of nitrogen into the carbon by ammonia treatment. More nitrogen was incorporated into the active carbon by ammonia treatment throughout the heating process, rather than admitting ammonia at the maximum heat-treatment temperature. XANES and FTIR studies showed that pyridinic, pyrrolic, and pyridonic functional groups were present on the carbon surface. TPD and selective neutralization studies showed that carboxylic acid groups were not present in significant quantities in the carbons, which had high nitrogen contents. The presence of surface nitrogen functional groups in active carbons incorporated by ammonia treatment at high temperature markedly increased the adsorption of capacities for transition metal ion species such as Cd2+(aq), Ni2+(aq), and Cu2+(aq) under neutral or basic conditions without changing the porous structure characteristics greatly. The amounts of transition metal ions adsorbed correlated with the nitrogen contents of the nitrogen-rich carbons. In contrast, the presence of nitrogen functional groups did not enhance the adsorption of Ca2+(aq) on the carbon. The enthalpies of adsorption of Cd2+(aq) and Cu2+(aq) on the nitrogenrich carbons were intermediate between enthalpies for the untreated active carbon and the nitric acid-oxidized carbon. Measurement of the points of zero charge for the carbon adsorbents showed that the enhanced adsorption of metal ions on nitrogen-functionalized carbons cannot be explained by electrostatic interactions. The results suggest that pyridinic surface groups in the active carbons act as adsorption sites for transition metal ion species in aqueous solution by a coordination mechanism. LA011161Z