Competitive Adsorption of Aqueous Metal Ions on an Oxidized

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Competitive Adsorption of Aqueous Metal Ions on an Oxidized Nanoporous Activated Carbon B. Xiao and K. M. Thomas* Northern Carbon Research Laboratories, Department of Chemistry, School of Natural Sciences, Bedson Building, University of Newcastle upon Tyne, Newcastle upon Tyne, NE1 7RU United Kingdom Received February 2, 2004. In Final Form: March 4, 2004 Competitive adsorption is the usual situation in real applications, and it is of critical importance in determining the overall performance of an adsorbent. In this study, the competitive adsorption characteristics of all the combinations of binary mixtures of aqueous metal ion species Ca2+(aq), Cd2+(aq), Pb2+(aq), and Hg2+(aq) on a functionalized activated carbon were investigated. The porous structure of the functionalized active carbon was characterized using N2 (77 K) and CO2 (273 K) adsorption. The surface group characteristics were examined by temperature-programmed desorption, Fourier transform infrared spectroscopy, Raman spectroscopy, acid/base titrations, and measurement of the point of zero charge (pHPZC). The adsorption of aqueous metal ion species, M2+(aq), on acidic oxygen functional group sites mainly involves an ion exchange mechanism. The ratios of protons displaced to the amount of M2+(aq) metal species adsorbed have a linear relationship for both single-ion and binary mixtures of these species. Hydrolysis of metal species in solution may affect the adsorption, and this is the case for adsorption of Hg2+(aq) and Pb2+(aq). Competitive adsorption decreases the amounts of individual metal ions adsorbed, but the maximum amounts adsorbed still follow the order Hg2+(aq) > Pb2+(aq) > Cd2+(aq) > Ca2+(aq) obtained for single metal ion adsorption. The adsorption isotherms for single metal ion species were used to develop a model for competitive adsorption in binary mixtures, involving exchange of ions in solution with surface proton sites and adsorbed metal ions, with the species having different accessibilities to the porous structure. The model was validated against the experimental data.

1. Introduction The toxic heavy metals present in wastewater and effluents are a major environmental concern. Many methods have been proposed for heavy metal removal. Chemical precipitation or electrochemical methods can be used to remove aqueous heavy metal pollutants present in high concentrations from water. However, there is a requirement to remove the final traces of metal species after these treatment procedures. At very low concentrations, such pollutants can be removed by adsorption on activated carbon. Activated carbons have well-developed porous structures with specific surface chemical properties and are widely used in industry for the removal of many organic compounds from both liquid and gas phases. Activated carbons contain hydrophobic graphene layers and various hydrophilic functional groups. Organic compounds are adsorbed on the former, whereas polar species are adsorbed on the latter. Surface chemistry plays an important role in the adsorption of aqueous heavy metals on activated carbons.1-3 Chemical oxidation, which incorporates both oxygen and nitrogen functional groups on the surface of activated carbon, enhances the adsorption of aqueous metal cation species and modifies the selectivity of the activated carbon for these species.4,5 Oxygen functional groups are involved in the formation of surface complexes with aqueous metal species and ion exchange

with the displacement of protons.6 Nitrogen functional groups coordinate with aqueous metal species, but the metal ions are displaced at pH 4.1.5 Aqueous metal ions have different affinities for various functional groups such as carboxylic groups and phenolic groups on the carbon surface. Metal anionic species are adsorbed by a different mechanism; for example, the aurocyanide ion is mainly adsorbed on the graphene layers.7 Redox reactions involving the aqueous metal species have also been proposed.8-10 Several factors affect the adsorption of aqueous metal species on activated carbon, and these include the surface charge of activated carbon and the speciation of metal ions in solution. This leads to a dependence of the amount adsorbed on the point of zero charge (pHPZC), isoelectric point, and experimental conditions, such as ionic strength, pH, and concentrations of species in solution. The adsorption of aqueous metal ions is also strongly influenced by competition of different aqueous metal ions to occupy the limited number of surface sites, which decreases the removal efficiency of activated carbon for the metals of interest.11 Quantitative modeling of the adsorption of aqueous metal species has been investigated to interpret adsorption in relation to electrostatic effects, ion exchange, and coordination with functional groups on the amphoteric carbon surface.12 These models suggest that the adsorption of aqueous species onto a hydrated solid surface must overcome an extra energy barrier to complete the exchange

* Corresponding author. E-mail address: [email protected]. (1) Bautista-Toledo, I.; Rivera-Utrilla, J.; Ferro-Garcia, M. A.; Moreno-Castilla, C. Carbon 1994, 32, 93. (2) Rivera-Utrilla, J.; Ferro-Garcia, M. A. Adsorpt. Sci. Technol. 1986, 3, 293. (3) Leon y Leon, C. A.; Radovic, L. R. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1994; Vol. 24, p 213. (4) Jia, Y. F.; Thomas, K. M. Langmuir 2000, 16, 1114. (5) Jia, Y. F.; Xiao, B.; Thomas, K. M. Langmuir 2002, 18, 470.

(6) Carapcioglu, M. O.; Huang, C. P. Water Res. 1987, 21, 1031. (7) Jia, Y. F.; Steele, C. J.; Hayward, I. P.; Thomas, K. M. Carbon 1998, 36 1229. (8) Fu, R.; Zeng, H.; Lu, Y. Carbon 1993, 31, 1089. (9) Huang, C. P.; Blankenship, D. W. Water Res. 1984, 18, 37. (10) Lo´pez-Gonza´lez, J. D.; Moreno-Castilla, C.; Guerrero-Ruiz, A.; Rodriguez-Reinoso, F. J. Chem. Technol. Biotechnol. 1982, 32, 575. (11) Gabaldon, C.; Marzal, P.; Ferrer, J.; Seco, A. Water Res. 1996, 30, 3050.

10.1021/la049712j CCC: $27.50 © 2004 American Chemical Society Published on Web 04/22/2004

Competitive Adsorption of Aqueous Metal Ions

of their hydration spheres.13 The stability of surface complexes formed between metal ions and activated carbon depends on the pH of the solution and the concentration of aqueous metal ions, as well as the surface characteristics and porous structure of the activated carbon.3-5,12 In this paper, the adsorption characteristics of all the binary mixtures of Ca2+(aq), Cd2+(aq), Pb2+(aq), and Hg2+(aq) species on a functionalized activated carbon were studied. The pH of the solution was not controlled by the addition of buffer solution because the ionic components of the buffer would interfere with the metal ion competitive adsorption process. The pH was measured as a function of metal ion adsorption to quantify the displacement of protons by the metal ions. An oxidized carbon was chosen for the investigation because of the enhanced (×30) metal ion adsorption of the carbon compared with the untreated carbon.5 The physical and chemical properties of the functionalized carbon were characterized. The objective was to understand the mechanism of competitive adsorption of aqueous metal ions on activated carbons in relation to the surface charge, functional groups, porous structure of the adsorbent, and speciation in solution. 2. Experimental Section Materials Used. A commercial steam-activated coconut shell derived activated carbon supplied by Pica, Vierzon, France, was used as the starting material. The oxygen functional groups were incorporated on the carbon surface by nitric acid oxidation (7.5 M HNO3, 368 K, 72 h.). The oxidized activated carbon was Soxhlet extracted for 72 h using distilled water to remove residual acidic solution in the pores and soluble products from the activated carbon oxidation. The sample was then dried at 388 K for 24 h. The functionalized activated carbon was designated the code GN, while the original activated carbon was code G. Gas Adsorption Measurements. The micropore and total pore volumes of the activated carbons G and GN were determined using a McBain spring apparatus. The micropore volumes were obtained from the data for adsorption of CO2, at 273 K, by extrapolation of the Dubinin-Radushkevich equation to obtain the micropore volumes using FCO2 ) 1.023 g cm-3. The total pore volumes were obtained from adsorption of N2, at 77 K with the total pore volumes being calculated from the uptake of N2 at p/p0 ) 1 using FΝ2 ) 0.8081 g cm-3. Temperature-Programmed Desorption (TPD) TPD studies were carried out using a Thermal Sciences STA 1500 thermogravimetric analyzer connected to a VG Quadrupole 300 amu mass spectrometer by a heated stainless steel capillary lined with deactivated fused silica. The TPD studies were carried out at a heating rate of 15 K min-1 in argon with a flow rate of 50 mL min-1. The evolved gases were sampled continuously and analyzed by the mass spectrometer. The intensities of the m/z 28, 30, and 44 peaks were monitored throughout the desorption process. Fourier Transform Infrared Spectroscopy (FTIR). The FTIR spectra of the activated carbon were recorded using a Nicolet 20-PC FTIR spectrophotometer with CsI optics and a DTGS detector. KBr disks containing 0.5 wt % finely grounded carbon were used. The spectral resolution was 4 cm-1 in the range 5004000 cm-1. Raman Spectroscopy. The Raman spectra were recorded using a Jobin Yvon HR800 confocal Raman microscope with excitation using the 514.5-nm argon ion laser line. The spectral resolution was 4 cm-1 in the range 200-1800 cm-1. Acid/Base Titrations. The amphoteric character of the functionalized carbon surface was assessed by the selective acid/ base neutralization method developed by Boehm.14 Aliquots of 0.2 g of activated carbon were reacted with 25 mL of 0.1 N NaOH, 0.1 N Na2CO3, 0.1 N NaHCO3, and 0.1 N HCl, respectively, for (12) Huang, C. P. In Carbon Adsorption Handbook; Cheremisinoff, P. N., Ellerbusch, F., Eds.; Ann Arbor Science Publishers: Ann Arbor, 1978; p 281. (13) James, R. O.; Healy, T. W. J. Colloid Interface Sci. 1972, 40, 65. (14) Boehm, H. P. Carbon 1994, 32, 759.

Langmuir, Vol. 20, No. 11, 2004 4567 72 h. Back-titration was carried out using HCl (0.1 N) or NaOH (0.1 N) to neutralize excess acid and base for determining acid/ base consumption by the activated carbon. The consumptions of various bases and acid are related to the properties and amount of functional groups on the carbon surface. pH at the Point of Zero Charge (pHPZC). The pH at the point of zero charge (pHPZC) of the carbon samples was measured using the pH drift method.15 The pH of a solution of 0.01 M NaCl was adjusted between pH 2 and pH 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 value of the solution stabilized. A total of 0.15 g of activated carbon was added to 25 mL of the solution. After the pH had stabilized (typically after 24 h), the final pH was recorded. The graphs of final pH versus initial pH were used to determine the points at which the initial pH was equal to the final pH. This point was taken as the pHPZC of the carbon. Adsorption of Metal Ions. The Ca2+(aq), Cd2+(aq), Pb2+(aq), and Hg2+(aq) solutions were prepared from Ca(NO3)2‚4H2O, Cd(NO3)2‚4H2O, Pb(NO3)2, and Hg(NO3)2‚H2O, respectively. Both the adsorption studies of binary metal ion mixtures and those of single metal ions were carried out at 298 ( 0.1 K. A total of 0.1 g of activated carbon was added to 50 mL of the aqueous solution, and the solution was allowed to equilibrate for 48 h. This was sufficient time for all the systems to equilibrate. The initial molar concentration ratios of metal ions in the binary mixture solutions for competitive adsorption studies were 0.5, 1.0, and 2.0. The amounts of metal ions adsorbed on the carbon surface were determined by the difference of the metal ion concentrations before and after adsorption. A Unicam 701 Inductively Coupled Plasma (ICP) atomic emission spectrometer was used to measure the concentrations of aqueous metal ions in solution. The pH values of solutions before and after adsorption were also measured to investigate the displacement of protons from the functional groups on the carbon surface by the adsorption of metal ion species.

3. Results and Discussion 3.1. Characterization of Functionalized Activated Carbon. Analytical Data. The analytical data for the activated carbons used in this study are given in Table 1. The results show that the oxidation process changes the oxygen content of activated carbon G from 2.95 to 22.36 wt % (dry ash free, daf) for carbon GN, which indicates that a large amount of oxygen functional groups were incorporated on the carbon surface. The volatile matter increases on oxidation from 4.46 wt % (daf) for activated carbon G to 33.77 wt % (daf) for carbon GN. A small amount of nitrogen was incorporated during the oxidation process. Previous X-ray near-edge structure spectroscopy has shown that the nitrogen is present as pyridine N-oxide groups.4.5,16 Porous Structure Characteristics. The porous structure characteristics of the original and functionalized activated carbons obtained from N2 (77 K) and CO2 (273 K) adsorption are compared in Table 1. The total pore and micropore volumes of original carbon G were 0.484 and 0.334 cm3 g-1, respectively. After oxidation to form carbon GN, the total pore and micropore volumes were 0.366 and 0.262 cm3 g-1, respectively. It is evident that the oxidation process decreases both the total pore and micropore volumes by ∼25%. Table 1 shows that the characteristic energies and mean pore radii17 of carbons G and GN are very similar. The pore structures for both carbons may be described as nanoporous. The relatively small change in porous structure due to oxidation may be reversed to a large extent by heat treatment to 600 °C. This increases (15) Lopez-Ramon, M. V.; Stoeckli, F.; Moreno-Castilla, C.; CarrascoMarin, F. Carbon 1999, 37, 1215. (16) Zhu, Q.; Money, S. L.; Russell, A. E.; Thomas, K. M. Langmiur 1997, 13, 2149. (17) Dubinin, M. M.; Stoeckli, H. F. J. Colloid Interface Sci. 1980, 75, 34.

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Table 1. Analytical Data of the Activated Carbons Used in This Study carbons G Proximate Analysis, wt % volatile matter (daf) 4.46 ash (db) 2.73 C H N O

Ultimate Analysis, wt % daf 96.22 0.42 0.28 2.95

GN 33.77 0.49 76.34 0.89 1.05 22.36

Porous Structure Characterization; Pore Volumes,a cm3 g-1 VCO2c 0.334 0.262 VN2b 0.484 0.366 X,d nm 0.42 0.47 E0,e kJ mol-1 27.14 24.96 HCl NaOH Na2CO3 NaHCO3 pHPZC

Titrations, mequiv‚g-1 0.61 0.08 0.00 0.00 8.14

0.02 5.44 3.58 2.45 2.53

a Density of adsorbed phase: nitrogen, 0.8081 g cm-3; carbon dioxide, 1.023 g cm-3. b Obtained from Langmuir model at p/p0 ) 1. c Obtained from an intercept of the DR plot. d Mean radius of microporosity from Dubinin-Stoeckli analysis17 of CO2 adsorption data at 273 K. e Characteristic energy from Dubinin-Radushkevich analysis of CO2 adsorption data at 273 K (β )0.35).

the total pore volume of GN from 0.366 to 0.455 cm3 g-1, which is similar the total pore volume of G (see Table 1). Therefore, the oxidation treatment does not alter the pore structure greatly but mainly results in blockage of some of the pores. Previous studies have shown that nitric acid oxidized activated carbon has enhanced (×30) adsorption of Cd2+(aq) species compared with that of original carbon.4 It is apparent that this enhancement cannot be ascribed to changes in the pore structure during oxidation. Heat treatment over the temperature range 573-1073 K gradually removed the oxygen functional groups from the functionalized carbon surfaces and increased the pHpzc, while the amount of Cd2+(aq) adsorbed decreased progressively.5 TPD. Oxygen functional groups dominate the chemical and physical properties of carbon surfaces. These surface groups have different thermal stabilities and decompose to form CO and CO2 over specific temperature ranges. However, the local structure around the functional groups in heterogeneous carbon materials influences their thermal stabilities and, hence, decomposition temperature ranges. Multiple peaks may also be due to decomposition of a specific functional group in different sites/environments. Hence, the assessment of functional groups using TPD is complex and more qualitative rather than quantitative. Figure 1 shows that the TPD profile for GN contains three CO2 peaks. A Gaussian multipeak function was used in the analyses of the CO and CO2 TPD profiles. The peaks have overlapping temperature ranges of ∼400700, 470-900, and 700-1250 K centered at ∼535, 695, and 950 K, respectively. The CO profiles have two overlapping peaks with ranges of 640-850 and 650-1150 K centered at 745 and 920 K. CO2 and CO peaks were not observed in the TPD of the original carbon G below ∼1050 K. The initial CO2 TPD peak for GN at 535 K was evolved at a much lower temperature than the first CO peak and is associated with carboxylic groups, which are the least stable surface species. The second peak may be due mainly to the decomposition of anhydride and lactone groups. The CO desorbed is mainly due to the decomposition of

Figure 1. TPD profiles for CO and CO2 for carbon GN: heating rate 15 K min-1.

anhydride, carbonyl, and phenol groups.18,19 The CO shoulder peak at ∼745 K is associated with anhydride groups. Phenol, carbonyl, ether, and quinone groups may contribute to the high-temperature CO and CO2 peaks.20,21 Integration of the areas under the peaks showed that 54.3% of the oxygen was evolved as carbon dioxide. The low, intermediate, and high-temperature carbon dioxide peaks represented 13.7, 16.8, and 23.8% of the total oxygen, respectively. The CO shoulder and main peak represent 7.6 and 38.1% of the total oxygen, respectively. The relative concentrations should be regarded as semiquantitative rather than quantitative because of the complex nature of the desorption processes and the assumptions involved in the analysis of the profiles. However, the total amounts of CO and CO2 desorbed are quantitative. The amount of Cd2+(aq) adsorbed changes quite markedly with heat treatment to progressively remove oxygen surface functional groups. Heat treatment to 673 K reduces the adsorption of Cd2+(aq) to ∼20-25% of the amount adsorbed by GN while reducing the oxygen content by ∼20%. This demonstrates the marked difference in the adsorption characteristics of the various types of oxygen groups, which yield CO2 in TPD because there is little or no CO desorbed below 673 K. FTIR. A comparison of the FTIR spectra of the raw and functionalized carbon and the functionalized carbon with adsorbed metal species is shown in Figure 2. A new band at 1717 cm-1 was observed for the oxidized carbon GN compared with the untreated carbon G. This band is ascribed to the CdO stretching vibrational band of carboxylic acid groups. The band gradually decreases with increasing heat treatment temperature and disappears at ∼1073 K.4,5 Comparison of the spectrum of GN with that of the original carbon G shows that carbon GN has a stronger band at 1576 cm-1. The bands observed at 3430, 1576, and 1200 cm-1 are assigned to phenolic OsH stretching vibrations conjugated with CdO, CdO stretching vibrations, and OsH bending modes, respectively.22-24 The metal ion species (Hg, Pb, and Ca) adsorbed on the (18) Figueiredo, J. L.; Pereira, M. F. R.; Freitas, M. M. A.; Orfao, J. J. M.; Carbon 1999, 37, 1379. (19) Biniak, S.; SÄ wia¸ tkowski, A.; Pakuła, M. In Chemistry and Physics of Carbon, Radovic, L. R., Ed.; Marcel Dekker: New York, 2001; Vol. 27, p 125. (20) Otake, Y.; Jenkins, R. G. Carbon 1993, 31, 109. (21) Turner, J. A.; Thomas, K. M. Langmuir 1999, 15, 6416. (22) Zawadzki, J. In Chemistry and Physics of Carbon; Thrower, P., Ed.; Marcel Dekker: New York, 1988; p 147. (23) Starsinic, M.; Taylor, R. L.; Walker, P. L., Jr.; Painter, P. C. Carbon 1983, 21, 69. (24) Dyke, S. F.; Floyd, A. J.; Sainsbury, M.; Theobald, R. S. Organic Spectroscopy: An Introduction; Penguin: London, 1971.


Figure 2. FTIR spectra of carbons (a) Hg2+(aq) adsorbed on GN, (b) Pb2+(aq) adsorbed on GN, (c) Ca2+(aq) adsorbed on GN, (d) GN before Soxhlet extraction, (e) oxidized carbon GN, (f) GN heat treated at 1073 K, and (g) original carbon G.

surface of oxidized carbon GN primarily influence the relative intensities of the infrared peaks at 1717 and 1576 cm-1, which are characteristic of acidic carboxylic groups. The peak at 1717 cm-1 had a higher relative intensity than the peak at 1576 cm-1 for carbon GN with the metal species adsorbed whereas in the case of carbon GN, these peaks were very similar in intensity. There was no evidence for nitrate species for GN with adsorbed metal ion species. An analogous effect was observed for adsorption of Fe3+ species on oxidized carbon surfaces.25 These observations are consistent with the acidic oxygen functional groups acting as ion-exchange sites with displacement of protons by metal ions. Raman Spectroscopy. The Raman spectrum of the oxidized carbon GN has two bands at 1360 and 1600 cm-1, and it was similar to those observed for a wide range of activated carbons and cokes.26,27 The bands are designated the D and G bands, respectively. The spectrum was characteristic of the carbon structure and the broad bands obscured any differences due to the presence of large amounts of oxygen functional groups. The relative band intensity ratios ID/IG do not change markedly with metal species adsorption or surface oxidation. The adsorption of metal ions had no observable effect on the relative intensities but decreased the overall intensities of the D and G Raman bands. There was no evidence for nitrate species associated with adsorbed metal ion species. The Raman spectra are consistent with the infrared spectra, which showed only small changes in relative intensities associated with bands due to carboxylic groups. (25) Pakuła, M.; Biniak, S.; SÄ wia¸ tkowski, A. Langmuir 1998, 14, 3082. (26) Johnson, C. A.; Patrick, J. W.; Thomas, K M. Fuel 1986, 65, 1284. (27) Lespade, P.; Marchand, A.; Couzi, M.; Cruege, F. Carbon 1984, 22, 375.

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Acid and Base Titrations. The numbers of surface oxygen functional groups can be estimated by acid and base titration. Carbon G is a basic H-type carbon, while the oxidized carbon GN is an acidic L-type carbon due to the incorporation of acidic surface oxygen functional groups. Activated carbons have amphoteric properties and, therefore, may be titrated against both acids and bases. NaHCO3 neutralizes the carboxylic groups, Na2CO3 neutralizes carboxylic groups and carboxyl groups in lactone, and the strong alkali (e.g., NaOH) neutralizes all acidic groups including the phenolic hydroxyl groups on a carbon surface in the same way as for pure phenols.15 Oxidized carbon GN contains carboxylic groups, 2.5 mequiv‚g-1, lactone groups (and/or lactol), 1.1 mequiv‚g-1, and phenol groups, 1.9 mequiv‚g-1, respectively, whereas the unoxidized carbon G is a basic carbon (pHPZC ∼ 8.1). Heat treatment of carbon GN at 1073 K removes the carboxylic and lactone (or lactol) groups on the surface of functionalized carbon GN to give a basic carbon.5 However, it must be recognized that the determination of the quantities of carboxylic groups by titration methods is only semiquantitative because of the amphoteric nature of carbon surfaces. pHPZC Measurements. The original carbon G was a basic H-type carbon (pHPZC ) 8.1). The basic character of G is ascribed to the presence of chromene- and pyrone-type structures and delocalized π electrons on the basal planes of the surface of activated carbon. After oxidation to form carbon GN, the pHPZC decreased markedly to 2.5. The lower pHPZC implies that the surface of carbon GN has a net negative charge in aqueous solution, which is beneficial for the adsorption of aqueous metal cation species. Heat treatment of carbon GN to 1073 K converts carbon GN to a basic H-type carbon with a higher pHPZC value (7.9), similar to that of carbon G (8.1). The effect of oxidation of activated carbon on the pHPZC and the isoelectric point (pHIEP) has been compared previously.28-30 It was suggested that the pHPZC characterizes the acidity of all the surfaces, whereas the pHIEP is related to the acidity of the external surface of activated carbon because the latter decreased to a small extent during oxidation of activated carbon. 3.2. Speciation of Aqueous Metal Ions. The speciation of metal ions in aqueous solutions affects their interaction with surface oxygen functional groups. In the case of the metal ions present in dilute solutions, the hydrolysis products of metal ions are assumed to be primarily mononuclear.30,31 The low solution concentrations used in this study do not allow precipitation to occur, and, therefore, the speciation diagrams can be constructed from the hydrolysis equilibria of mononuclear hydroxides using global hydrolysis constants. The analyses of hydrolysis products of aqueous metal ions indicate that the hydrolysis of cadmium ions starts at pH > 7 when Cd(OH)(aq), Cd(OH)2(aq), Cd(OH)3-(aq), and Cd(OH)42-(aq) species are established in dilute solutions. At pH < 7, the aqueous cadmium ions are in the form of Cd2+(aq) ions.32-35 The lead species start to undergo (28) Moreno-Castilla, C.; Ferro-Garcia, M. A.; Joly, J. P.; BautistaToledo, I.; Carrasco-Marin, F.; Rivera-Utrilla, J. Langmuir 1995, 11, 4386. (29) Leon y Leon, C.A.; Solar, J. M.; Calemma, V.; Radovic, L. R. Carbon 1992, 30, 797. (30) Carapcioglu, M. O.; Hung, C. P. Carbon 1987, 25, 569. (31) Radovic, L. R.; Moreno-Castilla, C.; Rivera-Utrilla, J. In Chemistry and Physics of Carbon; Radovic, L. R., Ed.; Marcel Dekker: New York, 2001; Vol. 27, p 227. (32) Fergusson, J. E. The Heavy Elements: Chemistry, Environmental Impact and Health Effects; Pergamon Press: Oxford, 1990; p 48. (33) Hahne, H. C. H.; Kroontje, W. J. Environ. Qual. 1973, 2, 444.

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Table 2. Initial and Equilibrium Solution Concentration and pH Ranges for the Single Metal Ion and Binary Mixture Adsorption single metal ion MA

[MA]init, mM

[MA]e, mM

pH

Ca2+(aq) Cd2+(aq) Pb2+(aq) Hg2+(aq)

0.097-1.259 0.097 2.200 0.100-1.900 0.596-4.173

0.012-0.773 0.011-1.510 0.005-0.728 0.017-1.066

3.04-3.91 2.87-4.08 2.75-3.47 1.97-2.85

binary mixtures MA/MB

[MA]init, mM

[MB]init, mM

[MA]eq, mM

[MB]eq, mM

pH

Cd2+(aq)/Ca2+(aq) Pb2+(aq)/Ca2+(aq) Pb2+(aq)/Cd2+(aq) Hg2+(aq)/Ca2+(aq) Hg2+(aq)/Cd2+(aq) Hg2+(aq)/Pb2+(aq)

0.096-1.373 0.082-1.203 0.098-3.900 0.097-2.892 0.095-2.947 0.048-3.806

0.049-1.927 0.042-2.058 0.097-3.884 0.111-6.590 0.053-6.316 0.049-5.748

0.012-0.954 0.006-0.454 0.008-0.602 0.025-0.969 0.031-0.840 0.049-1.242

0.065-0.991 0.020-2.063 0.006-1.169 0.035-6.473 0.013-6.081 0.011-5.478

2.97-3.85 2.76-3.74 2.79-3.81 2.45-3.50 2.47-3.90 2.42-3.87

hydrolysis initially to Pb(OH)+(aq) at pH > 4-5.33,35 In the pH range 6-13, the mononuclear species Pb(OH)+(aq), Pb(OH)2(aq), and Pb(OH)3-(aq) are known to exist. A polynuclear species Pb3(OH)42+(aq) appears in the range of pH 7.6-11.35 In contrast to the Cd2+(aq) and Pb2+(aq) species, the aqueous mercury ion hydrolyzes very readily to produce primarily the neutral mononuclear species Hg(OH)2(aq) in dilute solutions. Other species produced in small amounts are Hg(OH)+(aq), Hg(OH)3-(aq), Hg2(OH)3+(aq), and probably Hg3(OH)33+(aq), but the Hg(OH)3-(aq) species only occur at a high pH (>11). The Hg2(OH)3+(aq) and Hg3(OH)33+(aq) species only occur at high mercury concentrations and at low hydroxyl concentrations before precipitation commences.35 The speciation of aqueous mercury ions indicates that the aqueous mercury species start to hydrolyze at a very low pH. The initial pH values of the binary mixture solutions decrease to values typically in the range 2.42-3.9 after competitive adsorption, as a result of the release of protons, with the amount depending on the amounts of M2+(aq) adsorbed (see Table 2). Cd2+(aq) ions are the major Cd(II) species in the corresponding binary mixture solutions at pH < 7. In the binary mixture solutions containing Pb2+(aq)/Ca2+(aq) and Pb2+(aq)/Cd2+(aq), the initial pH is ∼5.6 and a small amount of Pb(OH)+(aq) may be present in the initial solutions at this pH. The initial pH value of the binary mixture solution Hg2+(aq)/Pb2+(aq) was ∼4, and this reduces to 2.42-3.87 after mercury and lead species were adsorbed on the carbon surface. Therefore, Pb2+(aq) is the dominant Pb species in binary Hg2+(aq)/Pb2+(aq) solutions. The Hg2+(aq), Hg(OH)+(aq), and Hg(OH)2(aq) species exist in the solutions, and their distributions in aqueous solutions of pH ) 3 are approximately 40, 20, and 40%, respectively. These factors together suggest a more complicated adsorption mechanism of aqueous mercury species on functionalized carbon GN. The effect of hydrolysis of Pb2+(aq) on adsorption is much smaller than for aqueous mercury species. Many factors such as pH, ionic strength, metal ion concentration, the presence of anions, and so forth influence the forms of metal species in solution, and the surface groups of the functionalized carbon influence the adsorption of metal ions in the carbon surface so that the adsorption of the metal ions from an aqueous solution is complex. 3.3. Adsorption of Single Metal Ions. Adsorption Kinetics. The kinetics of metal ion adsorption was studied to establish an equilibrium time for the isotherm studies. Figure 3 shows kinetic profiles for Pb2+(aq) adsorption on carbon GN covering a wide range of concentrations. It is (34) Weber, W. J.; Posselt, H. S. Aqueous Environmental Chemistry of Metals; Ann Arbor Science: Ann Arbor, 1976; p 291. (35) Baes, C. F., Jr.; Mesmer, R. E. The Hydrolysis of Cations; John Wiley & Sons: New York, 1976.

Figure 3. Kinetic profiles for the adsorption of Pb2+(aq) on carbon GN at 298 K.

apparent that equilibrium was established within 24 h. Similar studies were carried out for the other metal ion species, and it was shown that a time of 48 h was suitable for all the systems. Adsorption Isotherms. The adsorption isotherms for the Ca2+(aq), Cd2+(aq), Pb2+(aq), and Hg2+(aq) solution species on carbon GN are shown in Figure 4. These isotherms reach a well-defined plateau and are Type I according to the IUPAC classification.36 The ranges for the initial and final metal ion solution concentrations and the pHs of the solutions are given in Table 2. It is evident that the pHs of the solutions vary in the range 1.97-4.08. The adsorption of aqueous species on activated carbons may be described by various isotherms including twoparameter (Langmuir37 and Freundlich)38 and threeparameter (Sips39 and Redlich-Peterson)40 isotherm equations. The shapes of the adsorption isotherms reach a plateau, and, therefore, the Freundlich equation, which predicts increasing adsorption with increasing solution concentration, can be eliminated. The isotherms for aqueous metal ion adsorption can be described by the Langmuir equation,

ni )

nm,iKiCi 1 + KiCi

(1)

where ni is the amount adsorbed and nm,i is the maximum (36) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57 (4), 603. (37) Langmuir, I. J. Am. Chem. Soc. 1916, 38, 2221. (38) Freundlich, H. M. F. Z. Phys. Chem. 1907, 57, 385. (39) Sips, R. J. Chem. Phys. 1948, 16, 490. (40) Redlich, O.; Peterson, D. L. J. Phys. Chem. 1959, 63, 1024.


Langmuir, Vol. 20, No. 11, 2004 4571

Figure 4. Adsorption of single metal ion aqueous species on carbon GN [solid line, Langmuir model; 4, Ca2+(aq); O, Cd2+(aq); b, Pb2+(aq); 0, Hg2+(aq)]. Table 3. Langmuir Parameters Obtained from Isotherms of Individual M2+(aq) Metal Ions metal ion Ca2+(aq)

Cd2+(aq)

Pb2+(aq)

Hg2+(aq)

Metal Ion Adsorption nm,i, 0.27 ( 0.01 0.34 ( 0.01 0.57 ( 0.02 1.37 ( 0.08 -1 mmol g Ki, 19.68 ( 1.99 15.75 ( 1.18 36.05 ( 4.89 7.87 ( 1.60 L mmol-1 a R 0.992 0.997 0.993 0.991 Proton Displacement Hm,i, 0.49 ( 0.02 0.64 ( 0.01 0.81 ( 0.03 0.95 ( 0.07 -1 mmol g KH,i, 16.65 ( 1.95 13.03 ( 1.15 30.94 ( 4.76 8.66 ( 2.64 L mmol-1 Ra 0.993 0.997 0.985 0.981 a

R ) regression coefficient.

amount adsorbed in mmol g-1; Ki is the adsorption constant in L mmol-1, Ci is the equilibrium concentration in mM, and i stands for the metal species; i ) Ca, Cd, Pb, and Hg. Table 3 gives the parameters derived from eq 1. It is apparent that the Langmuir model fits the isotherms for all the M2+(aq) species investigated with regression coefficients R g 0.991 and acceptable errors for both nm,i and Ki. Clearly, there is no need to use isotherm equations with more than two variable parameters to describe the isotherm. The Langmuir adsorption constant is not strongly dependent on pH or the distribution of functional group properties in the activated carbon under the conditions used in this study. The Langmuir adsorption constants have the following order for adsorption of M2+(aq) species: KPb > KCa > KCd > KHg. This is clearly different

from the order based on maximum amounts adsorbed (nm). This order for the Langmuir adsorption constant Ki is not directly related to speciation in solution because both Hg2+(aq) and Pb2+(aq) have multiple speciation in an aqueous solution under the conditions used and are at opposite ends of the order. Adsorption of aqueous metal ions on activated carbons is controlled by both electrostatic (Coulombic) and nonelectrostatic interactions. The maximum amounts adsorbed shown by the plateau differ by a factor of 5 and are in the order Hg2+(aq) > Pb2+(aq) > Cd2+(aq) > Ca2+(aq). Divalent cations, with the exception of Be2+, are all thought to have an inner hydration shell with octahedral coordination of six water molecules. Therefore, the hydrated M2+(aq) ions all have similar sizes. However, there is a weak trend in the size of hydrated metal ions: Ca2+(aq) > Cd2+(aq) ) Hg2+(aq) > Pb2+(aq).41 The outer hydration shell of ∼10 waters are only very loosely bound. The M2+(aq) cations have similar characteristics except for Ca2+, where the water molecules in the inner hydration shell exchange more rapidly with the bulk water than for the other divalent species. The largest maximum amounts adsorbed are observed for Hg2+(aq) and Pb2+(aq), where hydrolysis needs to be considered. Pb(OH)+(aq) will have a smaller hydrated ion size than Pb2+(aq) because of the lower charge. Hg(OH)+ and Hg(OH)2 are present in Hg2+(aq). The effective size of hydrate species are in the order M2+ > M(OH)+ > M(OH)2. The differences in the maximum adsorption capacity are related to the size of the effective hydrated radii of aqueous metal species present in solution in relation to the porous structure and adsorbate/adsorbent interactions. Species with smaller effective hydrated radii are able to diffuse into parts of the porous structure not available to larger M2+(aq) species, thereby resulting in increased adsorption of metal species and release of protons. Water can also diffuse into this porosity, and a local equilibrium will occur between adsorbed phase and the solution species with the formation of ionic species, which are adsorbed. Hence, Hg2+(aq) and Pb2+(aq) have the highest maximum amounts adsorbed. The stabilities of metal complexes with compounds containing carboxylic groups may be compared with amounts adsorbed under specified conditions and isotherm parameters.42,43 The order of the stability constants for Pb2+(aq), Cd2+(aq), and Ca2+(aq) with six carboxylate anions are in the same order as for the maximum amounts of metal ions adsorbed given above.44,45 It has been proposed that the reduction of Hg2+ to Hg2+ may be involved in the adsorption of mercury species on carbon surfaces, as a result of the presence of surface charge, and the π electrons of the aromatic basal planes of the carbon may be available for reduction.31,46 There is no evidence for reduced species under the conditions used in this study. Therefore, the higher amount of Hg2+(aq) adsorbed is due to electrostatic and nonelectrostatic interactions, which are related to speciation in solution, the accessibility of the species to the porous structure, and the stability of surface complexes formed. Displacement of [H+] by [M2+](aq) Species. The adsorption of metal ions on oxidized carbon surfaces results in (41) Harris, D. C. Quantitative Chemical Analysis, 5th ed.; W. H. Freeman and Co.: New York, 1999; p 180. (42) Evangelou, V. P.; Marsi, M.; Chappell, M. A. Spectrochim. Acta, Part A 2002, 58, 2159. (43) Seki, H.; Suzuki, A. J. Colloid Interface Sci. 1995, 171, 490. (44) Sillen, L.G.; Martell, A. E. Stability Constants of Metal Ion Complexes; The Chemical Society Special Publication No. 25; The Chemical Society: London, 1971. (45) Bunting, J. W.; Thong, K. M. Can. J. Chem. 1970, 48, 1654. (46) Humenick, M. J., Jr.; Schnoor, J. L. J. Environ. Eng. 1974, 100, 1249.

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Figure 5. Variation of protons displaced with amount of single metal ions adsorbed (solid line, eq 2). Table 4. Parameters for the Displacement of Protons by Metal Ion Species Obtained from Equation 2 ci, mmol g-1 Ca2+(aq) Cd2+(aq) Pb2+(aq) Hg2+(aq) Cd2+/Ca2+(aq) Pb2+/Ca2+(aq) Pb2+/Cd2+(aq) Hg2+/Ca2+(aq) Hg2+/Cd2+(aq) Hg2+/Pb2+(aq) a

gi

Ra

Single Metal Ion -0.03 ( 0.01 1.87 ( 0.07 -0.04 ( 0.02 1.97 ( 0.08 -0.04 ( 0.03 1.47 ( 0.08 -0.01 ( 0.03 0.71 ( 0.03

0.995 0.993 0.986 0.994

Binary Metal Ion Systems -0.01 ( 0.02 1.99 ( 0.08 0.01 ( 0.02 1.78 ( 0.05 -0.06 ( 0.01 1.56 ( 0.03 -0.03 ( 0.02 0.94 ( 0.04 -0.05 ( 0.02 0.87 ( 0.03 -0.05 ( 0.03 1.09 ( 0.05

0.985 0.987 0.996 0.985 0.987 0.977

R ) regression coefficient.

the displacement of protons from the surface acidic oxygen functional groups by an ion exchange mechanism. The isotherms for metal ion adsorption expressed as the amounts of protons displaced also follow the Langmuir isotherm as shown in Figure 4b. The corresponding isotherm parameters are given in Table 3. It is apparent that the values of the Langmuir adsorption constants obtained by this method are similar to the corresponding parameters determined from metal ion concentration measurements. Figure 5 shows linear graphs of the amount of H+ displaced versus metal ions adsorbed. These data cover the range of concentrations up to complete saturation of the carbon surface with M2+(aq) species. The number of protons displaced from the surface sites by metal ion species can be expressed as

Hi(n) ) gini + ci

(2)

where Hi(n) is the amount of protons displaced (mmol g-1) from the carbon surface by adsorption of amount ni (mmol g-1) of metal ions; the subscript i represents the metal ion (i ) Ca, Cd, Pb, and Hg); gi is the gradient, and ci is the intercept of the graph of eq 2. Table 4 gives the values of gi and ci for adsorption of individual metal ions. Equation 2 neglects the effect of the activity of [H+], and this is reasonable considering the dilute solutions used. The lines for Ca2+(aq), Cd2+(aq), Pb2+(aq), and Hg2+(aq) pass through the origin within experimental error. The observation of isotherms for H+ displaced versus metal ion solution concentration, which follow a Langmuir isotherm equation, is predicted from the linear relationship between M2+(aq) adsorbed and H+ desorbed, which

passes through the origin. This linearity indicates that the adsorption mechanism of metal ions associated with oxygen functional groups does not change markedly with metal ion concentrations in the dilute solutions used in this study. The results given in Table 4 show that the ratios of [H+] displaced to metal ions adsorbed (gi) obtained from the gradients of straight lines in Figure 5 were 1.87, 1.97, 1.47, and 0.71 for the Ca2+(aq), Cd2+(aq), Pb2+(aq), and Hg2+(aq) species, respectively. The ratios are in the order of Cd2+(aq) ∼ Ca2+(aq) > Pb2+(aq) . Hg2+(aq). The order based on the amount of metal ions adsorbed on the isotherm plateau, corresponding to adsorption on all available surface sites (see Table 3), are in the order Hg2+(aq) > Pb2+(aq) > Cd2+(aq) > Ca2+(aq). The maximum amounts of protons displaced vary by a factor of ∼2 compared with a factor of 5 for metal species adsorbed and have the same order. The pHpzc (2.53) for carbon GN corresponds to an H+ concentration of 2.95 mM. The carboxylic functional group concentration (2.5 mequiv‚g-1) was determined from the Boehm titration experiments. The values gCa and gCd were 1.87 and 1.97, respectively. This implies that adsorption of Ca2+(aq) and Cd2+(aq) species on carbon GN occur with one metal ion displacing approximately two protons from the carbon. The adsorption of Pb2+(aq) species displaces fewer ∼X 1.5 H+ ions, which may be partly due to the presence of a small amount of Pb(OH)+ species. The adsorption of Hg2+(aq) species from the dilute aqueous solution is complicated because of the presence of Hg2+(aq), Hg(OH)+(aq), and Hg(OH)2(aq) species. The ratio gHg ∼ 0.71 is low for ion exchange. This is possibly due to adsorption of the Hg(OH)2(aq). The observation that the amounts of protons displaced show a smaller range of values than the amounts of metal ions adsorbed indicates the importance of electrostatic effects and speciation in solution in the adsorption mechanism. 3.4. Competitive Adsorption of Aqueous Metal Ion Species and Displaced Protons. The extended Langmuir model can be used to describe competitive adsorption for ternary systems.47,48 The model can be described by the following equation:

qi )

QiKiCi n

1+

(3)

∑ KkCk

k)1

where qi is amount of component i adsorbed from the multicomponent system, Qi is the maximum amount adsorbed, Ci and Ck are the equilibrium solution concentrations of components i and k, and Ki and Kk are the adsorption constants of components i and k. The assumptions used for the derivation of eq 3 are the same as those for adsorption of single metal ions but with the additional requirement for competitive adsorption that all the species compete for the same sites. Koopal et al.49 extended previous studies of congruent affinity distributions to heterogeneous surfaces where the individual affinity distributions of the components differ and derived analytical isotherm equations for multicomponent adsorption on heterogeneous surfaces. If the adsorption characteristics of the two components, A and B, are described by the Langmuir-Freundlich isotherm, (47) Butler, J. A. V.; Ockrent, C. J. Phys. Chem. 1930, 34, 2841. (48) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed., WileyInterscience: New York, 1990. (49) Koopal, L. K.; van Riemsduk, W. H.; de Wit, J. C. M.; Benedetti, M. F. J. Colloid Interface Sci. 1994, 166, 51.


Langmuir, Vol. 20, No. 11, 2004 4573

Table 5. Parameters Obtained from Koopal Model for Competitive Adsorptiona single metal ion Ca2+(aq) Cd2+(aq) Pb2+(aq) Hg2+(aq) a

nm,i

K ˜M

K ˜H

0.27 ( 0.01 19.35 ( 1.72 0.00 ( 0.40 0.35 ( 0.01 17.13 ( 2.03 0.23 ( 0.26 0.58 ( 0.03 41.61 ( 15.04 0.32 ( 0.74 1.37 ( 0.08 8.93 ( 3.76 0.43 ( 0.22

Rb 0.996 0.998 0.982 0.988

The refinement gave R ) β ) p )1. b R) regression coefficient.

then component A, for a two-component system, can be expressed as follows:

θA )

(K ˜ ACA)R

[(K ˜ ACA)R + (K ˜ BCB)β]p

[(K ˜ ACA)R + (K ˜ BCB)β] {1 + [(K ˜ ACA)R + (K ˜ BCB)β]p} (4)

˜ A and where θA is the surface coverage of adsorbate A, K K ˜ B are the affinity coefficients, CA and CB are the concentrations of A and B in solution, R and β are related to the overall exponents, and a and b, for the single component case by the relationships a ) Rp and b ) βp, where p is a measure of the width of the intrinsic surface heterogeneity. Metal ion adsorption on activated carbon is often treated as single ion adsorption. However, it should be considered as competitive adsorption between the metal ions and the protons. Protons are displaced by metal ions and, therefore, there are no problems with different accessibilities of the ions to the porous structure. The results obtained from fitting the experimental data for metal ion and H+ concentrations to the above equation assuming that K ˜M and K ˜ H are constant for the experimental conditions shown in Table 2 are given in Table 5. It is apparent that (1) R ) β ) p ) 1, that is, all four M2+/H+(aq) systems have ideal behavior and the isotherm adsorption derived for the heterogeneous surface reduces to the extended Langmuir equation; (2) K ˜ H is small and identical within experimental error for all the systems studied; and (3) K ˜M can be regarded as constant over the pH range used and identical within experimental error to the corresponding Langmuir parameters (see Table 3). The results are quite different from previous studies of Cd2+(aq) on a humic acid where surface heterogeneity is marked with R, β, and p in the range 0.56-0.66. The adsorbent surface charges in response to changes in the solution pH and ionic strength and the resulting surface electrostatic potential influence the adsorption affinity of the ions. If the adsorption is mainly controlled by electrostatic interactions, a similar amount of metal ions should be adsorbed. The reasons for adsorption of different amounts adsorbed are (i) speciation in solution, (ii) surface heterogeneity, (iii) accessibility of species to the porous structure, and (iv) adsorbate/adsorbent interactions. The observation that all the M2+/H+(aq) systems have ideal behavior indicates that surface heterogeneity is not a major factor. The pH is below ∼4.5, which is the pKa of the COOH functional groups on carbon surfaces for all the measurements (see Table 2). Therefore, there is not significant negative charge on the carbon surface. Metal ion adsorption can occur with the COO- groups or with the other oxygen functional group. The binding constant for the +2 cations with a -1 surface site are stronger than for the +1 proton. This is shown in the competitive adsorption parameters from the Koopal competitive adsorption model. The order for Pb2+(aq), Cd2+(aq), and Ca2+(aq) is the same as that observed previously for the formation of 1:1 complexes with six carboxylate anions.45 However, the highest amounts adsorbed and protons displaced occur

Figure 6. Adsorption isotherms of (a) total ions adsorbed and (b) H+ displaced for the binary mixture system of Pb2+(aq)/ Ca2+(aq) on carbon GN with different initial concentration ratios.

where hydrolysis in solution results in M(OH)+ and M(OH)2, which have smaller effective hydrated sizes than M2+(aq) because of their lower charges, and these species have greater accessibility to the porous structure, thereby allowing a greater amount of adsorption. Hence, M2+(aq) species, which undergo hydrolysis to M(OH)+ and M(OH)2, have the highest maximum amounts of metal species adsorbed and protons displaced by the metal ion species. 3.5. Competitive Adsorption of Aqueous Metal Ion Species. The competitive adsorption of binary aqueous metal cation species in mixtures Cd2+(aq)/Ca2+(aq), Pb2+(aq)/Ca2+(aq), Pb2+(aq)/Cd2+(aq), Hg2+(aq)/Ca2+(aq), Hg2+(aq)/Cd2+(aq), and Hg2+(aq)/Pb2+(aq) on carbon GN were investigated under different initial concentration ratios (0.5, 1.0, and 2.0 mole fraction ratios). Figure 6 shows a comparison of the isotherms for total ions adsorbed versus total ions in solution and the corresponding isotherms for the displacement of protons for the Pb2+(aq)/Ca2+(aq) binary mixture system. It is apparent that the isotherms for total metal ion adsorption and H+ displaced have shapes similar to the isotherms for individual ion species. The isotherms for the metal ion components in the Pb2+(aq)/Ca2+(aq) binary mixture system are shown in Figure 7. The adsorption of Ca2+(aq) reaches a maximum and then decreases with increasing equilibrium solution concentration, while the adsorption of Pb2+(aq) increases with increasing Pb2+(aq) solution equilibrium concentration. Figure 8 shows the variation of Ca2+(aq) adsorbed with Pb2+(aq) adsorbed for the three initial concentration ratios. This shows the displacement of Ca2+(aq) adsorbed

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Xiao and Thomas Table 6. Langmuir Parameters for the Adsorption of the More Strongly adsorbed species in the binary mixtures binary solution (A/B)

mf,Aa

nm,A, mmol g-1

Cd2+/Ca2+(aq)

KA, L mmol-1

R2

0.5 1.0 2.0 Cd2+(aq)

0.28 ( 0.01 0.36 ( 0.02 0.47 ( 0.07 1

0.13 ( 0.01 0.19 ( 0.01 0.23 ( 0.01 0.34 ( 0.01

17.91 ( 3.35 22.79 ( 2.90 20.89 ( 1.90 15.75 ( 1.18

0.967 0.989 0.993 0.997

0.5 1.0 2.0 Pb2+(aq)

Pb2+/Ca2+(aq) 0.09 ( 0.01 0.47 ( 0.02 0.17 ( 0.02 0.51 ( 0.02 0.30 ( 0.03 0.56 ( 0.02 1 0.57 ( 0.02

13.37 ( 1.62 15.49 ( 1.83 18.68 ( 2.00 36.05 ( 4.89

0.991 0.990 0.990 0.993

0.5 1.0 2.0

Pb2+/Cd2+(aq) 0.10 ( 0.01 0.32 ( 0.03 0.25 ( 0.03 0.51 ( 0.03 0.32 ( 0.03 0.47 ( 0.02

21.06 ( 3.86 8.82 ( 1.82 25.77 ( 3.08

0.979 0.951 0.981

0.5 1.0 2.0 Hg2+(aq)

Hg2+/Ca2+(aq) 0.10 ( 0.01 1.18 ( 0.08 0.17 ( 0.01 1.20 ( 0.07 0.27 ( 0.02 1.27 ( 0.05 1 1.37 ( 0.08

4.64 ( 0.89 5.13 ( 0.81 4.83 ( 0.56 7.87 ( 1.60

0.986 0.987 0.991 0.991

0.5 1.0 2.0

Hg2+/Cd2+(aq) 0.12 ( 0.03 1.34 ( 0.16 0.26 ( 0.07 1.34 ( 0.19 0.42 ( 0.10 1.20 ( 0.08

2.35 ( 0.68 2.24 ( 0.70 3.43 ( 0.56

0.978 0.965 0.966

0.5 1.0 2.0

Hg2+/Pb2+(aq) 0.20 ( 0.08 1.06 ( 0.17 0.34 ( 0.10 1.06 ( 0.11 0.53 ( 0.06 1.07 ( 0.04

1.82 ( 0.66 3.44 ( 0.97 3.22 ( 0.28

0.956 0.967 0.998

a Mole fraction of species A, m f,A ) CA/(CA + CB), where CB and CA are the concentrations of the binary mixture species in solution.

Figure 7. Adsorption of (a) Ca2+(aq) and (b) Pb2+(aq) on carbon GN for the binary system of Pb2+(aq)/Ca2+(aq) with different initial concentration ratios.

(aq), Hg2+(aq)/Cd2+(aq), and Hg2+(aq)/Pb2+(aq), where Hg2+(aq) is the more strongly adsorbed species. Details of the isotherms are given in Supporting Information. At a low concentration, the amounts adsorbed for both metal ions increase with increasing concentration of metal ions in solution. As the concentration of both metal ion species in solution increases further, the adsorption sites that are also available to both adsorbed species are occupied to an increasing extent by the more strongly adsorbed species, resulting in a decrease in the amount of the more weakly adsorbed species. In the binary system Cd2+(aq)/ Ca2+(aq), the adsorption of Ca2+(aq) species does not show a clear maximum amount adsorbed because of the similar interaction to Cd2+(aq) species for the single metal ions. The investigation of all the binary mixture systems showed that the competitive abilities of aqueous metal ions follows the order

Hg2+(aq) > Pb2+(aq) > Cd2+(aq) > Ca2+(aq)

Figure 8. Variation of Ca2+(aq) adsorbed with Pb2+(aq) adsorbed on carbon GN for the binary mixture system of Pb2+(aq)/Ca2+(aq) with different initial concentration ratios.

with increasing Pb2+(aq) adsorbed with the Ca2+(aq) adsorbed passing through a maximum. This characterizes the competitive adsorption between Pb2+(aq) and Ca2+(aq), where Ca2+(aq) is the more weakly adsorbed species, which is displaced by Pb2+(aq) at high concentrations. These characteristics are also found in the other binary systems, for example, Pb2+/Cd2+(aq), where Pb2+(aq) is the more strongly adsorbed species, and Hg2+(aq)/Ca2+-

Relationship between Species Concentrations and Amounts Adsorbed during Competitive Adsorption. The use of three initial solution mole concentration ratios (0.5, 1.0, and 2.0) for determining isotherms led to the equilibrium solution concentration ratios being approximately constant over the full isotherm. All the adsorption isotherms of the more strongly adsorbed species followed a Langmuir isotherm. The Langmuir parameters and the average mole fractions at equilibrium in solution are shown in Table 6. These data allowed the influence of the competitive adsorption of the more weakly adsorbed M2+(aq) component on the Langmuir parameters to be established. In the case of the Cd2+(aq)/Ca2+(aq) binary mixture, Cd2+(aq) is the most strongly adsorbed species. The proton displacement studies show that both ions displace ∼2H+. It is evident that the competitive adsorp-


Figure 9. Protons displaced from carbon GN by competitive adsorption of Pb2+(aq) and Ca2+(aq) ions from the binary solution system Pb2+(aq)/Ca2+(aq).

tion effect of Ca2+(aq) results in a small decrease in nm,Cd, but the values of KCd are not significantly different. In both Pb2+(aq)/Ca2+(aq) and Pb2+(aq)/Cd2+(aq) binary mixtures, Pb2+(aq) is the most strongly adsorbed species. In contrast to the Cd2+(aq)/Ca2+(aq) binary mixture, where the ∼2H+ are displaced per metal ion adsorbed, Pb2+(aq) displaces ∼1.47H+ per metal ion adsorbed, over the whole concentration range. It is apparent that there are differences in both electrostatic effects and speciation of Pb2+(aq) in solution to consider. In these two cases, both the apparent nm,Pb and KPb values for the adsorption of Pb2+(aq) decrease in the presence of Ca2+(aq) and Cd2+(aq). In all three binary mixtures with Hg2+(aq) present, the Hg2+(aq) was the most strongly adsorbed species. The effects of speciation in solution and differences due to electrostatic effects due to displacement of protons are likely to be the most significant. The value of KHg changes quite markedly, whereas the values of nm,Hg do not change markedly. The proton displacement studies show that Hg2+(aq) displaces ∼0.71H+ while Pb2+(aq) displaces ∼1.47 H+. It is apparent that the different displacements of protons by the metal ions in the binary mixtures makes nm less susceptible to the competitive adsorption effect. Both nm,Hg and KHg either decrease approximately linearly with decreasing mole fraction of Hg or are constant within experimental error. Hence, the isotherms of the more strongly adsorbed species in a binary mixture follow a Langmuir adsorption isotherm with the values of nm and K depending on the mole fraction of the other species at equilibrium in the binary mixture solution. The adsorption of the more strongly adsorbed component may be suppressed by the presence of other metal ion species while the weaker adsorbed species may undergo displacement. Displacement of [H+] by [M2+](aq) species. Figure 9 shows a graph of H+ displaced versus total metal ion [Pb2+(aq) + Ca2+(aq)] adsorbed for adsorption of the Pb2+(aq)/Ca2+(aq) binary mixture on carbon GN. A linear relationship is observed between the [H+] displaced and the amount of metal ion adsorbed. This graph is similar to the graphs for proton displacement by adsorption of single metal ions shown in Figure 5. The ratio gPb/Ca of [H+] displaced to total metal ions adsorbed [Pb2+(aq) + Ca2+(aq)], obtained from the gradient of the straight line, is 1.78, which is intermediate between 1.47 and 1.87 for the adsorption of Pb2+(aq) and Ca2+(aq), respectively. Analogous results observed for other binary systems are shown in Table 4, and the linear relationships have regression coeffi-

Langmuir, Vol. 20, No. 11, 2004 4575

Figure 10. Comparison of experimental results and calculated H+ displaced for all the binary M2+(aq) systems: calculations are based on the amounts of metal ions adsorbed from the binary mixture and the proton displacement data for single metal ions in Table 3 and assume that the contributions of adsorbed components are additive.

cients R > 0.977. The corresponding graphs are given in Supporting Information. However, the amounts of metal ions displaced by the individual components of the binary mixture are different but there are clearly no major changes over the isotherm range studied. The gi values for proton displacement for single metal ions can be used to calculate the proton displacement for binary systems using the amounts of metal ions adsorbed. The proton displacements were calculated from the amounts of metal ions adsorbed and the data on displacement of protons by single metal ions given in Table 4, assuming additivity of proton displacements. These calculated values are compared with the experimental results for all six binary mixtures in Figure 10. It is apparent that the displacement of [H+] for the competitive adsorption in binary mixtures is approximately additive on the basis of the adsorption characteristics of single metal ion interactions with surface functional groups. The interaction of individual metal ions with acidic oxygen functional groups is not markedly affected by competitive adsorption. As a result, the competitive adsorption of binary metal ion mixtures can be expressed in terms of the proton displacement parameters for adsorption of single metal ions. Modeling Competitive Adsorption of Metal Ions. The ternary system involving metals A and B and H+ involves competitive adsorption. The adsorption studies of single metal ions are indicative of different accessibilities of hydrolyzed metal species to the porous structure compare with M2+(aq). The results in Table 3 indicate that the maximum amounts adsorbed (nm) for metal ions used in this study vary by a factor x ∼ 5, while the H+ displaced by the adsorption of the metal ions varies by a factor of x ∼ 2. Therefore, the sites occupied by the species are not the same and the competitive adsorption is more complicated than competition for the same sites. It is evident that eq 3 cannot be applied directly to competitive adsorption of the M2+(aq) species used in this study. Analysis of competitive adsorption in the M2+/H+(aq) single metal ion systems shows that the systems (i) follow the Langmuir isotherm and (ii) behave as though the surface is homogeneous, (iii) K ˜ M can be regarded as constant over the pH range used, (iv) K ˜ H is very much smaller than K ˜ M, (v) the Langmuir parameters obtained from the conventional Langmuir and competitive adsorption models are identical within experimental error (see

4576


Xiao and Thomas

Tables 3 and 5), and (vi) the isotherms for metal ion adsorption are directly related to the corresponding isotherms for proton displacement even where complex speciation occurs in solution. This allows the competitive adsorption of the ternary A, B, and H+ system to be described by a simpler binary metal ion model based on ideal Langmuir behavior, neglecting K ˜ H because it is very small and assuming different accessibilities to the porous structure. The displacement of the protons describes the electrostatic mechanism of adsorption and allows differences in metal species accessibility to the porous structure to be quantified. However, some metal ions may be adsorbed by ion-dipole interactions and, in the case mercury species, neutral species are adsorbed. The amounts of metal species adsorbed are linearly related to the protons displaced irrespective of the speciation in solution. Studies of the adsorption of single Hg2+(aq), which has the most complex speciation in solution, show that the isotherm for total adsorption of mercury and the electrostatic component shown by the proton displacement isotherms are identical within experimental error. Therefore, speciation in solution does not influence the shape of the isotherms in this case and the differences in accessibility to the porous structure can be related using the proton displacement data. A model for competitive adsorption was developed assuming that the adsorption of aqueous metal species occurred mainly via an ion exchange mechanism. The more strongly adsorbed metal species (A), that is, adsorbed to the larger maximum extent, can displace protons from all the sites including those available to the more weakly adsorbed species (B), whereas some of the surface sites will either not ion exchange or are not available to the more weakly adsorbed species (B). Ion exchange may also occur between the adsorbed metal ion species and the metal ion species in the binary mixture solution as well as the proton surface sites. Hence, the surface functional group sites during adsorption may be divided into two types, those available to the more strongly adsorbed metal (A) and those available to the more weakly adsorbed metal (B) with the sites available to B being a subset of the sites available to A. In the binary mixture systems, competitive adsorption takes place on the surface proton sites that can be occupied by both species, that is, sites available to B, while noncompetitive adsorption occurs on the sites only available to A. The dynamic adsorption ion exchange also occurs between sites occupied by metal ions and metal ion species in solution. Therefore, the equations for competitive adsorption of aqueous metal species in a binary mixture solution can be established according to the dynamic adsorption equilibrium similar to that used in the derivation of the Langmuir equation. The noncompetitive adsorption of A on sites not available to B can be described as follows

AT - A B )

(Am - Bm)KAHCA 1 + KAHCA

(5)

where AT is the total amount of A adsorbed, AB is the amount of A adsorbed on sites also available to B, Bm is the maximum amount of B adsorbed, CA is the concentration of A in solution, and KAH is the adsorption constant for noncompetitive exchange of A with surface proton sites. KAH is related to the ratio of the adsorption and desorption coefficients for A. In the case of competitive adsorption of A on surface sites available to B, the exchange of A in solution with

surface proton sites and adsorbed B have to be considered, and this can be described by the following equation:

AB )

(Bm - BT)KAHBCA + BTKABCA 1 + KAHBCA

(6)

where KAHB is the adsorption constant for competitive exchange of A with surface proton sites in the presence of species B, KAB is the adsorption constant for exchange of A with adsorbed B, BT is the total amount of B adsorbed, and the other parameters are as defined in eq 5. KAHB is related to the ratio of the adsorption and desorption coefficients for A on sites also available to B. The competitive adsorption of B in the presence of A involves the exchange of B in solution with surface proton sites and adsorbed A, and this can be described by the following equation:

BT )

(Bm - AB)KBHACB + ABKBACB 1 + KBHACB

(7)

where KBHA is the adsorption constant for competitive exchange of B with surface proton sites in the presence of species A, KBA is the adsorption constant for exchange of B with adsorbed A, CB is the concentration of B in solution, and the other parameters are as defined in eqs 5 and 6. KBHA is related to the ratio of the adsorption and desorption coefficients for B. Equations 5-7 can be combined to eliminate AB to give the following equations for AT and BT.

AT )

KAH(Am - Bm)CA + 1 + KAHCA φKBHABmCB 1 + KBHACB + φ(KBHA - KBA)CB BT )

KBHABmCB 1 + KBHACB + φ(KBHA - KBA)CB

(8) (9)

where

φ)

KAHBCA + KABCAKBHACB KBHACB + KAHBCAKBACB

(10)

The experimental studies have established that speciation of M2+(aq) ions in solution may give species that have different accessibilities to the porous structure and displace different amounts of protons per metal ion adsorbed from the surface. The differences must be taken into consideration using eq 2. Table 4 gives experimental data for the relationship between the protons displaced and metal ions adsorbed for the metal ion species used in this study. Thus, eq 5 becomes

H(AT) - H(AB) )

[H(Am) - H(Bm)]KAHCA 1 + KAHCA

(11)

Similarly, eqs 6-10 can be expressed in terms of protons displaced. Validation of the Model for Competitive Adsorption in Binary M2+(aq) Metal Ion Systems. The surface sites in activated carbons have a range or distribution of properties. However, the adsorption of single metal ions on carbon GN has shown that the adsorption isotherm can be accurately represented by the Langmuir isotherm. Adsorption constants for the exchange of metal ions with the


Langmuir, Vol. 20, No. 11, 2004 4577

Figure 11. Comparison of the experimental results and the model prediction for (a) Pb2+(aq)/Ca2+(aq), (b) Hg2+(aq)/Pb2+(aq), (c) Cd2+(aq)/Ca2+(aq), (d) Pb2+(aq)/Cd2+(aq), (e) Hg2+(aq)/Ca2+(aq), and (f) Hg2+(aq)/Cd2+(aq) binary metal ion systems.

noncompetitive and competitive adsorption surface proton sites for the binary mixtures are expected to be similar and also similar to the values obtained for adsorption of the individual ions. This provides additional criteria for validation of the model. The experimental data were fitted to the above equations using the KA and KB values and maximum amounts adsorbed Am and Bm fixed at the values derived from the adsorption of individual metal ions with the additional constraint that KA for adsorption of the single ion was equal to KAH and KAHB for the binary mixture adsorption. In the special cases of CA ) 0 or CB ) 0, eqs 8 and 9 reduce to the Langmuir equation for pure species A and B, respectively. When the ion exchange constants KAB )

KBA ) 0, both eqs 8 and 9 reduce to the extended Langmuir equation. Validation of the model involved fitting results and comparing the regression coefficient R2, χ2/DoF (DoF ) degrees of freedom), and average relative deviation (ARD). The ARD is defined as follows:

ARD, % )

1

∑

N

|

|

nexpt - npred nexpt

× 100

(12)

where N is the number of data points and nexpt and npred are the experimental result and the predicted result, respectively. The quality of the fit of the above model for

4578


Xiao and Thomas

Table 7. Parameters Obtained from Fitting the Experimental Data to Equations 8-10 for Competitive Adsorption Combined with Equation 2 for Proton Displacementa binary system (A/B)

KAB, L mmol-1

χ2/DoF

Cd2+(aq)/Ca2+(aq) Pb2+(aq)/Ca2+(aq) Pb2+(aq)/Cd2+(aq) Hg2+(aq)/Ca2+(aq) Hg2+(aq)/Cd2+(aq) Hg2+(aq)/Pb2+(aq)

1.41 ( 0.29 8.52 ( 1.31 5.12 ( 1.05 2.30 ( 0.33 1.54 ( 0.20 1.42 ( 0.19

0.002 0.002 0.003 0.006 0.010 0.018

a K -10 L mmol-1 in all cases. Values for K , K , n BA was Pb2+(aq) > Cd2+(aq) > Ca2+(aq) as expected from comparing the values of nm obtained from the single ion adsorption. The metal ion selectivity is related to electrostatic effects, multiple speciation of the ions in solution, the accessibility of these species to the porous structure, and the stability constants for interaction with the surface species. The application of the general ion exchange model above has shown that a simplified model can be used for the adsorption of M2+(aq) species on functionalized activated carbons. Because KBA ) 0, eqs 8-10 can be simplified and combined to eliminate AB, giving the following equations:

AT )

(Am - Bm)KAHCA + 1 + KAHCA Bm(KABKBHACB + KAHB)CA (13) 1 + KAHBCA + KBHACB + KBHAKABCACB

BT )

BmKBHACB 1 + KAHBCA + KBHACB + KBHAKABCACB

(14)

The experimental data can also be fitted to eqs 13 and 14,

giving the trends in the KAB values obtained for the more general model. It is apparent that all the binary mixtures studied can be described with good accuracy using the model described above, using the Langmuir parameters obtained from the corresponding single ion adsorption and a single parameter, KAB, for exchange of the more strongly adsorbed species in solution and the more weakly adsorbed species on the carbon surface. 4. Conclusions The adsorption of aqueous metal ions on a functionalized activated carbon prepared by oxidation with nitric acid occurs mainly by an ion exchange mechanism with the acidic oxygen functional groups incorporated on the carbon surfaces. The M2+(aq) species have different abilities to displace H+ from the adsorbent surface. The maximum amounts of metal ions adsorbed on the functionalized carbon differed by a factor of 5, while the corresponding protons displaced differ by a factor of ∼2, indicating the importance of electrostatic effects and species accessibility to the porous structure. The speciation of aqueous metal species in solution influences the adsorption of aqueous metals and results in differences in the proton exchange ratios for the metal ions. The lower ratios in the adsorption of Pb2+(aq) and Hg2+(aq) species are related to the hydrolysis of metal ion species in aqueous solution. The competitive adsorption of binary mixtures of metal ions decreases the adsorption capacity of activated carbon for individual metal ions and the extent of the decrease depends on the other metal ion present. Although the adsorption of aqueous metal ions is influenced by many factors, the ratios of protons displaced to metal ions adsorbed for individual aqueous metal ions and binary metal ion mixtures are quite consistent over a wide concentration range. This allows the hydrogen ion concentration of the binary systems with competitive adsorption to be calculated through the linear relationship between the protons displaced and the amount adsorbed in the adsorption of single metal ions. The competitive adsorption of all the binary metal ion mixtures were modeled using dynamic equilibrium methods combined with the displacement of protons to allow for different accessibilities to the porous structure. The Langmuir parameters obtained from single metal ion adsorption and parameters for ion exchange between components of the binary metal ion mixture were used in the model. The model only requires a single additional parameter for the ion exchange of the more strongly adsorbed species in solution with the more weakly adsorbed species because the other ion exchange parameter, for exchange of the more weakly adsorbed component in solution and the more strongly adsorbed species, was very small. The model fits the experimental results very well for all the binary mixtures and demonstrates the displacement of the weakly adsorbed metal ion species by the more strongly adsorbed species. The order of the metal ions based on maximum amounts adsorbed for the single ions is identical to the order based on the ion exchange constants for the binary mixtures, and this is interpreted as the competitiveness of various metal ions in the binary mixture systems. The metal ion selectivity is related to electrostatic effects, multiple speciation of the ions in solution, accessibility of the porous structure to the species present in solution, and the stability constants for interaction with the surface species. Supporting Information Available: Isotherm details and other figures, as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org. LA049712J

Competitive Adsorption of Aqueous Metal Ions on an Oxidized

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