Langmuir 1999, 15, 4803-4811
4803
Comparative Study on the Adsorption of Cyanide Gold Complexes onto Different Carbonaceous Samples: Measurement of the Reversibility of the Process and Assessment of the Active Surface Inferred by Flow Microcalorimetry S. Lagerge,*,† J. Zajac,† S. Partyka,† and A. J. Groszek‡ Laboratoire des Agre´ gats Mole´ culaires et Mate´ riaux Inorganiques, C.N.R.S., E.S.A 5072, Universite´ de Montpellier II, Case 015, Place E. Bataillon 34095, Montpellier Cedex 05, France, and Microscal Ltd., 79, Southern Row, London W10 5AL, U.K. Received February 27, 1998. In Final Form: April 7, 1999 In this paper, adsorption of potassium gold cyanide from water onto three activated carbon samples, four graphites, and two carbon blacks (Graphon and Vulcan) is compared. The present work collects the results of flow microcalorimetric studies for each carbonaceous adsorbent so as to obtain estimates of their polar and accessible apolar graphitic basal plane surface areas. The relative polar and graphitic nature of the carbons is evaluated and related where possible to their capacity of adsorption for gold complexes. Finally, the thermodynamic reversibility of gold adsorption on the carbonaceous adsorbents from an aqueous solution of potassium aurocyanide at room temperature has also been studied. A flow adsorption microcalorimetry method was used to measure the amounts of aurocyanide adsorbed and desorbed in one adsorption-desorption cycle, as well as the corresponding enthalpic changes upon adsorption and desorption. On the basis of the adsorption and enthalpy values obtained in adsorption and desorption cycles, estimates could be made of the reversible and irreversible contributions to the total adsorption capacity and the integral molar enthalpy of adsorption. The degree of adsorption irreversibility ranges from 25% for Graphon to 54% for G212. The irreversible and reversible enthalpy components have respective values of about -50 and -25 kJ mol-1. A large part of the microporous structure in activated carbons is not accessible to the adsorption of gold complexes. One part of the aurocyanide is believed to irreversibly adsorb as an unpaired anion Au(CN)2- through electrostatic interactions on the very active surface sites having polar character. The less active sites are occupied by the ion-paired neutral molecular species (KAu(CN)2) through the action of van der Waals forces and account for the reversible adsorption. In reversible and irreversible processes, the active sites are thus expected to be associated with the graphitic-like structure containing nonideal aromatic rings and polar structures located at the edge defects in the graphite structure.
Introduction The adsorption of complex gold cyanides is an important industrial application of active carbon adsorbents and has been extensively investigated all over the world.1-5 There is still no agreement on the exact mechanism of the selective adsorption of gold complexes onto activated carbons from aqueous solution. A number of adsorption mechanisms of gold from potassium aurocyanide solutions onto activated carbons have been proposed over the years.4-30 The most recent theories12,13,15,17-19,21,24,26,27 favor * To whom correspondence should be addressed. Fax: 67-1433-04. E-mail:
[email protected]. † Universite ´ de Montpellier II. ‡ Microscal Ltd. (1) Fleming, C. A. Hydrometallurgy 1992, 30, 127. (2) Fleming, C. A.; Nicol, M. J. J. S. Afr. Inst. Min. Metall. 1984, 84, 95. (3) Muir, D. M. Recovery of Gold from Cyanide Solution using Activated Carbon: A review. Paper of the Mineral Chemistry Research Unit; Murdoch University: Australia, 1984. (4) Tsuchida, N.; Muir, D. M. Metall. Trans. B 1986, 17B, 523. (5) Tsuchida, N.; Muir, D. M. Metall. Trans. B 1986, 17B, 529. (6) Abotsi, G. M. K.; Osseo-Asare, K. Int. J. Min. Process. 1986, 18, 217. (7) Cashion, J. D.; Cookson, D. J.; Brown, L. J.; Howard, D. G. In Industrial Applications of the Mo¨ ssbauer Effect; Long, G. J., Stevens, J. G., Eds.; Plenum Press: New York, 1987; p 595. (8) Fuerstenau, M. C.; Nebo, C. O.; Kelso, J. R.; Zaragoza, M. R. Metall. Process. 1987, 4, 177. (9) McDougall, G. J.; Adams, M. D.; Hancock, R. D. Hydrometallurgy 1987, 18, 125.
the adsorption of gold as Au(CN)2- without chemical change. There is much controversy about the nature of the adsorption sites in carbon adsorbents. Some of the (10) Adams, M. D.; McDougall, G. J.; Hancock, R. D. Hydrometallurgy 1987, 18, 139. (11) Adams, M. D.; McDougall, G. J.; Hancock, R. D. Hydrometallurgy 1987, 19, 95. (12) Cashion, J. D.; Mcgrath, A. C.; Volz, P.; Hall, J. S. Trans. Inst. Min. Metall. 1988, 97, C129. (13) Klauber, C. Surf. Sci. 1988, 203, 118. (14) van der Merwe, P. F.; van Deventer, J. S. J. Chem. Eng. Commun. 1988, 65, 121. (15) Cook, R.; Crathorne, E. A.; Monhemius, A. J.; Perry, D. L. Hydrometallurgy 1989, 22, 171. (16) Adams, M. D.; Fleming, C. A. Metall. Trans. B 1989, 20B, 315. (17) McGrath, A. C.; Hall, J. S.; Cashion, J. D. Hyperfine Interact. 1989, 46, 673. (18) Jones, W. G.; Klauber, C.; Linge, H. G. Nineteenth Biennial Conference on Carbon; Penn State: University Park, PA, June 1989; p 38. (19) Jones, W. G.; Linge, H. G. Hydrometallurgy 1989, 22, 231. (20) Zarrouki, M.; Thomas, G. Analusis 1990, 18, (4), 261. (21) Kongolo, K.; Bahr, A.; Friedl, J.; Wagner, F. E. Metall. Trans. B 1990, 21B, 239. (22) Groszek, A. J.; Partyka, S.; Cot, D. Carbon 1991, 29, (7), 821. (23) le Roux, J. D.; Bryson, A. W.; Young, B. D. J. S. Afr. Inst. Min. Metall. 1991, 91, (3), 95. (24) Klauber, C. Langmuir 1991, 7, 2153. (25) Ibrado, A. S.; Fuerstenau, D. W. Hydrometallurgy 1992, 30, 243. (26) Sibrell, P. L.; Miller, J. D. Miner. Metall. Process. 1992, 9, 189. (27) Ibrado, A. S.; Fuerstenau, D. W. Miner. Eng. 1995, 8, (4/5), 441. (28) Papirer, E. Carbon 1995, 33, 1331. (29) Lagerge, S.; Zajac, J.; Partyka, S.; Groszek, A. J.; Chesneau, M. Langmuir 1997, 13, 4683. (30) Lagerge, S. Ph.D. Thesis, Montpellier, 1995.
10.1021/la980243t CCC: $18.00 © 1999 American Chemical Society Published on Web 06/09/1999
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theories assume that carbon oxygen surface species or polar surface groups on the carbon play a direct role in the process4,5,16 while others maintain that the most relevant characteristic of an activated carbon for gold loading is a high concentration of accessible graphene layers.18,19,24,25,26 It is also suggested that the adsorption is more likely to take place at the edges of the graphitic crystallites.22,26 Recently, the intervention of polar surface groups which were supposed to be probably pyrone type with basic properties was broadly confirmed.18,28 However, the identity of these polar sites in carbon adsorbents which are invariably produced during the formation of active carbons by oxidation and activation of various carbonaceous materials remains a mystery. It is often emphasized that the most desirable polar sites should have basic character.4,5 Since the surface oxides taking part in aurocyanide adsorption are not known with certainty, another view is that most of the favored sites are localized either on the graphitic basal planes19,24,27 or at the edge defects in the graphite structure.26 In previous work,29 two different adsorption regimes for potassium aurocyanide complexes from deionized water at T ) 298 K and pH ) 6 onto activated carbon have been shown. For very low equilibrium concentrations, a fully irreversible gold adsorption process is occurring and has been ascribed to an anionic exchange mechanism between Au(CN)2- ions and some anions initially present on the activated carbon surface. For relatively concentrated solutions, above a given surface coverage ratio, the adsorption of gold complexes becomes partly reversible, which indicates that, in this second stage, additional gold complexes are physically adsorbed. In this second stage, both irreversible and reversible mechanisms are occurring. We prove that the reversibility is due to the physisorption of neutral molecular species (KAu(CN)2) onto less active parts of the activated carbon surface. Moreover it was postulated that the adsorption of aurocyanide anions on polar sites is the driving force,30 in agreement with other researchers.22,26,28 The first aim of this study is mainly to verify the role of polar (hydrophilic) and graphitic basal plane (hydrophobic) surface structures of the carbon adsorbents in the adsorption process of KAu(CN)2 and to estimate the amount of surface accessible to the adsorbate species. The main preoccupation in this attempt was, above all, to settle on a strategy of investigations, bearing in mind the difficulties in the analysis of the adsorption onto activated carbons due to their great chemical and structural heterogeneity.31 An original approach using model solids, presented in the following section, has been successfully used in this work. Flow microcalorimetry equipment allows determination of the specific areas of surface sites having certain common chemical characteristics in a variety of carbonaceous and mineral adsorbents.32,34,35,37 It has been used in this study so as to obtain estimates of the polar and accessible graphitic basal plane surface areas in the carbon adsorbents. Nevertheless, the exact chemical nature of the carbon surface sites responsible for aurocyanide adsorption has not been addressed. The present work collects the results of similar microcalorimetric studies for a number of carbons that can be (31) Bansal, R. C.; Donnet, J. B.; Stoeckli, F. Active Carbon; Marcel Dekker: New York and Basel, 1988. (32) Groszek, A. J. Proc. R. Soc. London A, 1970, 314, 473-498. (33) Gregg, S. J.; Sing, K. W. Adsorption, Surface Area and Porosity; Academic Press: London, 1982. (34) Groszek, A. J. Carbon 1987, 25, 717. (35) Groszek, A. J. Carbon 1989, 27, 33. (36) Groszek, A. J. Extended Abstract 20th Biennial Conf. on Carbon, June 1991. (37) Groszek, A. J.; Partyka, S. Langmuir 1993, 9, 2271.
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classified as (a) graphites, (b) carbon blacks, and (c) activated carbons. The relative polar and graphitic nature of the carbons is evaluated and related where possible to their capacity of adsorption for gold complexes. The adsorption isotherms of Au(CN)2- from pure KAu(CN)2, in deionized water solution, at pH ) 6.0 and 25 °C were also determined for each carbon sample. It is obvious that the determination of the reversible and irreversible adsorption capacities of carbon adsorbents may be of great importance to the estimation of their practical performance. The present paper also describes application of flow adsorption microcalorimetry to study the reversibility of gold adsorption on graphites, carbon blacks, carbonized coconut shells, and some active carbons in aqueous solution at room temperature. This method has already appeared to be a reliable and fast procedure to measure simultaneously the enthalpy changes and amounts of adsorption in flow adsorption systems and has been successfully used to quantify the reversible and irreversible adsorption components.47 The starting point of this fundamental adsorption study is a simple system which yields experimental results that are easy to interprete without ambiguity. Such a system must be composed of a pure solvent (deionized water), a pure solute (potassium aurocyanide), and a well-defined carbon sample, because the influence of two important factors, the presence of electrolytes in the adsorption medium and the variation of its acidity, on the gold adsorption has been confirmed by many authors. The experimental results of gold complex adsorption onto industrial activated carbons were compared with the results provided from different model solids (graphites and carbon blacks), of which chemical and structural properties could be controlled to a great extent. Materials and Experimental Procedure Materials. G209, G210, and G212 activated carbons were supplied by PICA (Vierzon, France) and are widely used in industrial gold extraction. They were derived from coconut shells and prepared by high-temperature (1173 K) steam activation. The different extents of activation were in the order G212 > G210 > G209. Well-ordered natural graphite flakes (G0) consisting entirely of ideal graphene layers were supplied by Aldrich. We also used a natural graphite ground in air (G3) and two samples of oleophilic graphite flakes (one produced from natural graphite (G1) and the other from synthetic graphite (G2)). Graphon was a graphitized carbon black, almost entirely hydrophobic, and non-microporous. All of them (G1-G3 and Graphon) were supplied by Dr. A. J. Groszek (Microscal Ltd., London, U.K.). Samples G1-G3 were washed with hydrochloric acid and subsequently dried in a current of hydrogen at 500 °C for 2 h. Samples G1 and G3 were prepared from the same natural graphite by different methods of milling so as to obtain either a predominantly nonpolar and non-microporous graphite (G1 and G2) in which most of the surface consists of basal planes or a relatively polar and partly microporous graphite (G3) with a (38) Zettlemoyer, A. C. J. Colloids Interface Sci. 1968, 28, 343-369. (39) Bolis, V.; Fubini, B.; Marchese, L.; Martra, G.; Costra, D. J. Chem. Soc., Faraday Trans. 1991, 87, 497-505. (40) Nakamoto, H.; Takahashi, H. Zeolithes 1982, 2, 67-68. (41) Lagerge, S.; Rousset, P.; Zoungrana, T.; Douillard, J. M.; Partyka, S. Colloids Surf., A 1993, 80, 261-272. (42) Kinaly, Z.; Dekany, I. J. Chem. Soc., Faraday Trans. 1 1989, 85, 3373. (43) Everett, D. H. Pure Appl. Chem. 1985, 31, 579. (44) Groszek, A. J. Flow Adsorption Microcalorimetry; private communication, to be published. (45) Chan, B. K. C. Ph.D. Thesis, University of Newcastle Upon Tyne, England, 1991, pp 111-123. (46) Steele, C. J.; Thomas, K. M. Eureka Progress Report; Northern Carbon Research Laboratories, Department of Chemistry, University of Newcastle Upon Tyne: England, 1995. (47) Zajac, J.; Groszek, A. J. Carbon 1997, 35, 1053.
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Table 1. Proximate and Elemental Analysis of the Coconut-Based Activated Carbons (G209, G210, and G212) ex-coconutactivated carbons
oxygen
carbon
hydrogen
nitrogen
moisture
volatile matter
ash
fixed carbon
G209 G210 G212
3.07 2.04 1.85
91.10 93.57 94.07
0.42 0.27 0.23
0.25 0.00 0.00
3.13 2.39 1.96
3.25 3.25 2.05
2.98 1.74 3.15
90.65 92.63 91.84
elem anal./wt %
proximate anal./wt %
Table 2. Surface Parameters of the Coconut-Based Activated Carbons: BET Specific Surface Areas (As), Polar (Spol) and Graphitic Basal Plane (Sbasal) Areas, and Porosity Characterization 77 K, N2 adsorption
flow microcalorimetry
BET surface area, As/(m2 g-1)
microporous volume, Vµ/(cm3 g-1)
mercury porosimetry mesoporous volume, V(macro+meso)/(cm3 g-1)
273 K, CO2 adsorption microporous surface, Aµ/(m2 g-1)
polar area, Spol/(m2 g-1)
basal plane area, Sbasal/(m2 g-1)
1118 1135 1688
0.27 0.27 0.39
0.39 (33 m2 g-1) 0.34 (32 m2 g-1) 0.56 (45 m2 g-1)
470 461 445
24 22 10
2640 2580 2945
G209 G210 G212
Table 3. Surface Parameters of the Graphites (G0-G3), Graphon, and Vulcan: BET Specific Surface Areas (As), Polar (Spol) and Graphitic Basal Plane (Sbasal) Surface Areas, and Porosity Characterization N2 adsorption at 77 K BET surface area, As/(m2 g-1) G0 G1 G2 G3 Graphon Vulcan
1-2 91 151 396 82 107
cumulative area pores, 1.6-30 nm (m2 g-1)
flow microcalorimetry
pore volume (N2 at NTP) (cm3 g-1)
295 91
high proportion of “edge” surface sites. Finally Vulcan was furnished by Cabot (Berre, France). It was produced by the ASTM Cabot method and was thermally treated at the temperature 1300 °C for 24 h, at atmospheric pressure. It is amorphous and hence ungraphitized. From a textural and chemical point of view, the graphites and carbon blacks exhibit less heterogeneous surfaces than the activated carbons and thus can be used as models for the adsorption studies onto active carbons. These different carbons were used without any modification. Potassium dicyanoaurate was supplied by Me´taux-Pre´cieux-Industrie (Bagneux, France) and was used in solution in deionized water without any other reagents.29 Characterization. The measurements of elemental and proximate analysis were run at the Northern Carbon Research Laboratories, University of Newcastle upon Tyne, U.K.45,46 The specific surface area (As) and microporous volume (Vµ) were determined by volumetric adsorption measurements at 77 K of nitrogen (Analsorb 9011, France) and by applying the BET and R-s methods, respectively, with molecular cross-section surface areas being 16.2 Å2 for nitrogen.33 The microporous surface area (Aµ) was also quantified by gravimetric adsorption measurements at 273 K of CO2 (Gravimetric McBain spring method) and using the D-R plot.29,31,33,48,49 The flow microcalorimeter Microscal Mark IV Flow Adsorption Microcalorimeter has been used in this work to separately determine the hydrophobic/apolar and hydrophilic/polar surface sites or contributions in a solid.34-40 The method of determining the surface area of hydrophilic and hydrophobic sites in carbon products is based on the determination of the heat of adsorption of 1-butanol from dilute solutions in n-heptane and in water solution, respectively, capable of forming a close-packed monolayer on the hydrophilic and hydrophobic surface sites.34-37 A 5% ratio of active carbon (5 mg) with particle size ranging between 200 and 400 µm dispersed in a PTFE powder (95 mg) was placed in the calorimetric cell and outgassed in situ via a vacuum system for 1 h. The samples were then equilibrated (wetted) by passing a carrier liquid (i.e., the solvent) supplied at a constant flow rate of 3.3 mL h-1 from a syringe pump for 45 min. The carbon sample was dispersed with PTFE to ensure that the heat of displacement could be measured more rapidly. The graphites and carbon blacks were used without PTFE powder. After the completion of the wetting process, the (48) Stoeckli, H. F. Carbon, 1990, 28, 1-6. (49) Dubinin, M. M. In Chemistry of Physics and Carbon; Walker, P., Ed.; Marcel Dekker: New York, 1966; Vol. 2, pp 59-74.
0.36 0.22
av pore diameter (nm)
3.7 10.5
polar area, Spol/(m2 g-1)
basal plane area, Sbasal/(m2 g-1)
3.9 5.2 54 0.14 7.2
2 93 146 174 85 105.5
flow carrier fluid was switched to 1-butanol in solution in n-heptane (2 g L-1) or water (10 g L-1). The integral heats of adsorption of 1-butanol from water and n-heptane were converted to surface areas of hydrophobic and polar sites, respectively, assuming that the heats produced by a unit area of a hydrophilic or hydrophobic surface, obtained from the adsorption onto Graphon, are equal to 149 and 18 mJ m-2, respectively.34-37 The results are reported in Tables 1-3. They have been discussed in detail in previous work.30,46 In this work, they are used in order to compare the adsorption of cyanide gold complexes onto different carbonaceous samples, and therefore it is not the purpose of this work to analyze the characterization data for each carbon adsorbent. The hydrophobic surface areas corresponding to the areas of the basal planes available were very similar for each sample and represent most of the total surface area with few polar sites (low polar surface area). The specific surface areas determined by flow microcalorimetry are much higher than those obtained by N2 adsorption. For active carbons, the surface areas obtained by flow microcalorimetry have to be interpreted with caution because of the enthalpy of adsorption of the probe (1butanol), which is significantly different in the microporous carbon structure and on the open surface. Until adsorption is claimed to occur mainly on the external carbon surface, the enthalpy of adsorption is proportional to and may be correlated with surface area. When adsorption of 1-butanol occurs in the microporous structure, the resulting enthalpies are much higher than those obtained on a nonporous surface.33-40 Consequently, the polar, basal, and thereby total surface areas obtained by flow microcalorimetry are undervalued in comparison to those obtained from N2 adsorption. Therefore, these surface areas can be used only for comparative study between similar microporous samples, as in the paper. For nonmicroporous samples, the values obtained by flow microcalorimetry and reported in Table 3 agree with those obtained from N2 adsorption isotherms at 77 K. The graphites were found to be polar to a small extent, and Graphon was almost fully hydrophobic. For Graphon, the polar area has been perfectly confirmed by chemisorption of O2. Graphon consisted of small polyhedral particles unlike G1 and G2 which were flakes with a high degree of graphiticity (ca. 98%) as measured by electron diffraction. The graphon and polar graphite (G3) graphiticities were zero and 83%, respectively. The pore size distributions in the graphite samples G1 and G2 were essentially as shown for Graphon in Figure 1 and therefore are not reported in this paper; there was no microporosity. Polar
4806 Langmuir, Vol. 15, No. 14, 1999
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Figure 1. Distribution of surface area with pore diameter of Graphon and polar graphite G3. graphite had relatively high proportions of microporosity compared to G1, G2, and Graphon. Adsorption Measurements. Adsorption isotherms were obtained using the solution depletion method, which consists of comparing the solute concentrations before and after the attainment of adsorption equilibrium.41 The experimental procedure has been widely used by many authors and is the same as that used in previous papers.29 In each case this method allowed us to construct the individual adsorption isotherms of the reduced surface excess,42,43 Γ in µmol g-1, equal to the number of moles of Au(CN)2- anions adsorbed at the interface per unit mass of the solid adsorbent from a diluted aqueous solution of KAu(CN)2, against the concentrations, Ce Au(CN)2- in µmol L-1, remaining in the equilibrium bulk solution after the attainment of the equilibrium of adsorption: Γ ) f(Ce). The adsorption experiments were carried out in stoppered Pyrex tubes to which about 0.2 g of dry carbon adsorbent was added to potassium gold cyanide solution (28 cm3) with its initial gold concentration varying between 100 and 1000 ppm for the activated carbons and between 1 and 300 ppm for the graphites and carbon blacks. The enthalpy changes accompanying interfacial phenomena were determined using a Microscal Mark IV flow adsorption microcalorimeter. Microscal’s flow adsorption microcalorimetry equipment was used to study the adsorption-desorption of KAu(CN)2 from its aqueous solution on the different carbon adsorbents at 298 K and to determine the heats of displacement of KAu(CN)2 using the procedure described previously.30,44 A 5% ratio of active carbon (5 mg) with particle size ranging between 200 and 400 µm dispersed in a PTFE powder (95 mg) was placed in the calorimetric cell and outgassed in situ via a vacuum system for 1 h. The samples were then equilibrated (wetted) by passing deionized water supplied at a constant flow rate of 3.3 mL h-1 from a syringe pump for 45 min. The graphites and Graphon were used without PTFE powder. After the completion of the wetting process, the flow carrier fluid was switched to KAu(CN)2 solution. The integral enthalpies of adsorption of KAu(CN)2 (displacement of the water by these adsorbing molecules) were determined from a 5 mmol L-1 KAu(CN)2 stock solution. The amounts of aurocyanide adsorption and desorption were determined by analyzing the effluent from the adsorbent bed placed in the flow cell using an on-line UV detector (λ ) 248 nm). The adsorbent and the KAu(CN)2 solution were allowed to remain in contact until no further change in the effluent composition was detected (after 90 min, i.e., attainment of the equilibrium of adsorption). After the attainment of the equilibrium of adsorption, the aurocyanide species were then desorbed from the carbon (displacement of the KAu(CN)2 molecules by the adsorbing solvent) by switching back to water, and the heat of desorption was measured. Solvent and solution could be interchanged sequentially, allowing adsorption-desorption cycles to be determined.
Figure 2. Adsorption isotherms of aurocyanide species per unit mass of carbonaceous adsorbent onto graphites (G1, G2, G3), Vulcan, and Graphon from deionized water at pH ) 6.0 and T ) 298 K, (a) on a linear-linear scale and (b) on a linearlog scale. All the adsorption-desorption experiments were conducted at T ) 298 K with the use of deionized water as solvent. Water was not buffered to any constant pH value; its initial pH value was 6.0.
Results and Discussion In a previous paper,29 we proposed an ion-exchange mechanism for the adsorption of gold complexes. Ionized polar groups or structural deficiencies located at the edges of some graphitic ring systems could provide positive surface charges effective for the irreversible adsorption of aurocyanide anions through electrostatic interactions. The aim of the following study is to support this view of the aurocyanide adsorption mechanism. The experimental investigation essentially consists (1) of studying the adsorption of potassium aurocyanide complexes at T ) 298 K onto different carbonaceous samples and its reversibility and (2) of determining the role of graphene layers and polar structures in the adsorption process. Adsorption Isotherms of Gold Complexes and Accessible BET Surface Area. In Figures 2 and 3 we have plotted the adsorption isotherms on a linear-linear (a) and a linear-log (b) scale. The adsorption isotherms of Au(CN)2- anions determined for the graphites, carbon blacks (Figure 2), and active carbons (Figure 3) exhibit the same shape, suggesting the same mechanism of aurocyanide adsorption. The adsorption curves consist of two distinct parts. An initial vertical section, corresponding to very low equilibrium concentrations, certainly suggests
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Figure 3. Adsorption isotherms of aurocyanide species per unit mass of carbonaceous adsorbent onto coconut-based activated carbons (G209, G210, and G212) from deionized water at pH ) 6.0 and T ) 298 K, (a) on a linear-linear scale and (b) on a linear-log scale. Table 4. Average Cross-Sectional Area (σ) per One Adsorbed Aurocyanide Species Calculated from the BET Specific Surface Areas and Maximal Amounts of Adsorption of Graphites, Carbon Blacks, and Active Carbon Samples maximal amounts of adsorption G0 G1 G2 G3 Vulcan Graphon G209 G210 G212
Γ (µmol g-1)
Γ (µmol m-2)
av cross-sectional areas, σ (from BET As) (Å2)
0 35 47 96 42 19 434 470 460
0 0.39 0.31 0.24 0.39 0.24 0.39 0.41 0.27
0 432 519 692 437 755 428 401 609
a very strong adsorption of gold species for some of the surface sites. The linear-log plots emphasize the presence of very-high-energy adsorption sites. The second part of the isotherms reaches a limiting value, Γm, characteristic of the system and corresponding to the saturation of the surface. From this approximate plateau value and on the basis of BET specific surface areas of the solid samples, an experimental cross-sectional area (σ) per adsorbed molecule has been calculated and is reported in Table 4 for all adsorbents. The average values of σ range between 400 and 755 Å2 for all adsorbents and are far from the 21 Å2 estimated for a linear and nonhydrated Au(CN)2- ion
adsorbed in a horizontal orientation13 or from the 40 Å2 obtained for the ion pair K[Au(CN)2] lying flat on the surface.26 Such high values are in agreement with the suggestion that only a small proportion of the total surface area of carbonaceous adsorbents, as determined by the BET N2 method, adsorbs aurocyanide complexes from aqueous solution. The presence of aromatic condensed ring systems in the carbon surface is a common feature of the substrates studied. It is thus interesting to note that, even in the case of nonporous graphites (G1, G2, Graphon, Vulcan), only a very small fraction of the surface is concerned with the adsorption of gold. This indicates a completely localized specific adsorption onto some sites distributed uniformly on the solid surface and that most of the graphitic planes constituting most of the surface of these graphites are not available for the adsorption of gold species. Moreover, the comparison of the maximal amount of adsorption per unit of BET surface, in Table 4, shows that Graphon, which is a nonporous and highly graphitized sample, with a surface consisting almost entirely of ideal graphitic basal planes, gives the lowest density of aurocyanide adsorption. Its surface should contain extended contiguous areas of graphitic basal planes. Perhaps such uniform structures cannot provide many active sites for adsorption. Vulcan (ungraphitized carbon black), which contains more hydrophilic sites and less contiguous areas of graphitic basal planes in comparison to Graphon, adsorbs much more aurocyanide. Quite the same gold capacities are observed for G2 and G1 although their BET surface areas differ substantially. In the turbostratic structure of active carbons, the heterogeneity of aromatic rings is much higher and, consequently, the number of surface sites responsible for aurocyanide adsorption certainly increases. The amount adsorbed is very small and almost the same (about 450 µmol g-1) for all active carbons although the BET surface areas are very large and differ substantially. This means that the activated carbons exhibit quite the same effective surface for the adsorption of gold whatever their “real” specific surface area and porous structure. This fact, in turn, suggests that the surface available for the adsorption is essentially the external surface and that residing in the larger micropores. If we consider aurocyanide anions adsorbed in a horizontal position (21 Å2),13 only a very small fraction of the total adsorption capacity of carbon for gold is utilized or most of the BET surface area is not available for the adsorption of gold complexes. Two microporous solid samples, G212 and G3, containing probably much smaller micropores than the other carbons, give relatively low adsorptive values per unit BET surface area. The G2 sample yields a gold adsorption per unit surface area similar to those on activated carbons G209 and G210 and much greater than those for G212 and G3. The nonporous samples are at least as efficient as those exhibiting a high specific surface area due to a very highly developed microporous structure. Considering the amount of adsorption at the saturation, for each activated carbon, roughly equal to 450 µmol g-1, only 4.3% of the total BET surface for G209 and G210 samples and about 3% for the G212 sample is effective. This very small effective surface can be partly explained in terms of a nonavailable part of the microporous structure. The aurocyanide adsorption seems to occur mostly in the relatively large meso- and macropores. Penetration of microporous structure by the adsorbate is greatly limited, on one hand, by the adsorbate size and, on the other hand, by a low kinetic process. It is probable that much longer times of contact between the adsorbent and the solution, allowing diffusion in mi-
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Table 5. Average Cross-Sectional Areas (σ) per One Adsorbed Aurocyanide Species Calculated from the Polar Areas (Spol) of Graphites, Carbon Blacks, and Active Carbon Sample σ (from Spol) [Å2]
G1
G2
G3
Vulcan
Graphon
G209
G210
G212
18.5
18.0
93.5
29.2
1.3
9.2
7.8
3.6
cropores to occur, would significantly increase the total surface area available for aurocyanide adsorption and give rise to a much higher amount of adsorption. However, such long contact times are of little relevance to the practical performance of active carbons in the gold extraction. However, it is probable that a specific adsorption of gold complexes onto some particular active sites can also explain the ineffectiveness of a large part of the accessible surface ofthe carbon adsorbents. Extent of the Hydrophilic Sites in Carbonaceous Surfaces on the Adsorption of Gold Complexes. It is well-known that the structural imperfections in graphites and active carbons result in many possibilities for reaction at the edge carbon atoms, so that the surface is composed primarily of oxygen-containing organic functional groups that are located mostly at the edges of broken graphitic ring systems coming from the milling.31 Vulcan is amorphous, and hence ungraphitized, but contains undoubtedly some hydrophilic areas (chemical impurities...). Even graphitized carbon blacks heated to 3000 °C still retain a small amount of polar sites on their surface.32 Surface sites of this type may interact with polar molecules by their Lewis acidic or basic functionality. For the purpose of this work, the polar surface areas Spol have been estimated from flow microcalorimetric study of the adsorption of 1-butanol from n-heptane. The results are reported in Tables 2 and 3. The adsorption of aurocyanide onto graphites and carbon blacks, reported in Figure 2, correlates directly with this parameter except for G3. Samples G1, G2, and Vulcan exhibit very similar Spol values and give similar densities of adsorption. Vulcan and Graphon are both carbon blacks, but the greater capacity of Vulcan for aurocyanide adsorption in comparison to Graphon is probably related to its higher polar surface area. Considering the hydrophilic surface sites, obtained from flow calorimetry experiments, as the effective surface for the adsorption of aurocyanide species, the calculations of the cross-sectional areas per adsorbed molecule give rise to more realistic values, as can be seen in Table 5. The values obtained from the two nonporous graphites (G1 and G2) are virtually identical (18 Å2) and very close to the geometrical cross-sectional area of a linear and nonhydrated aurocyanide anion adsorbed in a horizontal position (21 Å2).13 The value of the average surface area per one adsorbed complex is a little higher for Vulcan but remains quite close to the theoretical value. The much higher value obtained for the partly microporous graphite (G3) certainly accounts for the fact that a part of the microporous structure is not available for the adsorption of gold complexes. These results firmly indicate that the aurocyanide species is adsorbed in a horizontal orientation onto the polar surface areas of the carbon surface. The active site or effective surface for the adsorption of gold species is thus expected to be associated with the graphitelike structures containing nonideal aromatic rings. Such nonideality may be due to the presence of some structural imperfections, coming from the milling for example, or impurities in the graphite crystal structure. This is in line with a previous view proposed in the literature,26 suggesting that the adsorption of aurocyanide complex on graphitic carbons is much greater at the edge defects of the graphite planes than on the planes themselves. As expected, graphitized carbon black gives the lowest density of adsorption because it is a nonporous and highly
graphitized sample. Its surface contains extended contiguous areas of graphitic basal planes, and its basal surface area is virtually identical with the BET specific surface area. However, a very small σ value was found (1.5 Å2). Perhaps such uniform structures can provide few active sites for adsorption. Indeed, besides the polar active sites which do not exist in Graphon (polar area ) 0.14 m2 g-1), some particular distorted carbon rings may also well constitute active structures available for the strong adsorption of gold complexes. The presence of such structures, probably present in all the carbons, has been demonstrated by many authors. As in the case of fullerenes, in graphites and graphitized carbon black, the occurrence of a few rings consisting of five or seven carbons which would be more reactive than the ideal hexagonal carbon and effective for the adsorption of gold complexes has been shown. For active carbons G209-G212, the values of Spol have to be interpreted with caution because the accessibility of the microporous carbon structure to the aurocyanide adsorbate differs from its accessibility to the titrant molecule (1-butanol). Until adsorption is claimed to occur mainly on the external carbon surface, the quantity of adsorption may be correlated with Spol. The related values of the average surface area per one adsorbed complex reported in Table 5 are undervalued in comparison to the geometrical value but remain quite realistic. The difference probably resides in the fact that the aurocyanide complexes can penetrate a part of the microporous structure which is not accessible to the 1-butanol probe molecule. Flow Adsorption-Desorption Microcalorimetry Data. Flow adsorption microcalorimetry has also been used to assess the reversibility of the phenomenon and quantify reversible and irreversible components. For the purpose of the present study, the results of aurocyanide adsorption and desorption from deionized water onto three active carbons were compared with those provided from Graphon and two graphites (G0 and G1). In each case, desorption isotherms for aurocyanide anions could be constructed using the depletion technique and allowed to ensure the reliability of the microcalorimetric method.29,30 The adsorption of aurocyanide complexes was measured at the KAu(CN)2 concentration 5 mmol L-1. Determining the gold capacity at the same solute concentration in the stock solution ensures that uniform adsorption conditions prevail for all carbonaceous adsorbents. At first sight, the use of such concentrated solutions seems unreasonable because the leach slurry in a real process for large-scale recovery of gold by active carbon never contains so much gold. Nevertheless, the aurocyanide concentration in the solution percolating through the flow calorimetric cell corresponds to the concentration of the equilibrium bulk phase in the batch adsorption system. In the batch adsorption systems, it was observed that the amount of aurocyanide adsorption still slightly increases for equilibrium concentrations higher than 2500 µm L-1. This slight increase can be assigned to adsorption on lower energy adsorption sites, and the postulated maximum adsorption amounts are higher than the amounts measured at the pseudoplateaus in Figure 3. However, the fact that a pseudoplateau is observed for Ce around 2000 µmol L-129,30 enables us to perform a thermodynamic treatment to determine the enthalpy of adsorption for the lowest adsorption sites. On this basis, it is evident that
Adsorption of Cyanide Gold Complexes
Langmuir, Vol. 15, No. 14, 1999 4809
Table 6. Results of Adsorption-Desorption Cycles for Two Graphites (G0 and G1), Graphon, and Three Coconut-Based Active Carbons (G209, G210, and G212) at T ) 298 K adsorption cycle model samples ex-coconut-activated carbon
G0 G1 Graphon G209 G210 G212
desorption cycle
Γ1 (µmol g-1)
∆1,2h (kJ mol-1)
Γ2 (µmol g-1)
∆1,2h (kJ mol-1)
0 34 29.3 425.2 651.5 501
0 -26.3 -29.5 -34.1 -35.6 -35.0
0 -20 -22 -293.7 -378.8 -229.5
0 21.9 24.2 27.8 24.6 26.0
Table 7. Reversible and Irreversible Components of the Amount Adsorbed and the Integral Molar Enthalpy of Aurocyanide Adsorption Calculated from the Results Reported in Table 6 for G0, G1, Graphon, and Coconut-Based Active Carbons (G209, G210, and G212) irreversible adsorption model samples ex-coconut-activated carbons
G0 G1 Graphon G209 G210 G212
reversible adsorption
Γ1 (µmol g-1)
∆1,2h (kJ mol-1)
Γ2 (µmol g-1)
∆1,2h (kJ mol-1)
0 14 7.3 131.5 272.7 271.5
0 -32.5 -45.5 -48.2 -50.8 -42.6
0 -20 -22 -293.7 -378.8 -229.5
0 21.9 24.2 27.8 24.6 26.0
flow adsorption calorimetry experiments performed at relatively high concentrations (5 mM) prove to be relevant and give reliable adsorption results even if the maximal solubility of KAu(CN)2 at room temperature is not reached and thereby if surface saturation is probably not achieved with 5 mM solutions. Both reversible and irreversible adsorption progress with increasing concentration until the plateau adsorption region. It is thus clear that reversible and irreversible adsorption components determined at sufficiently high solute concentrations in adsorption-desorption cycles with the use of a Microscal flow microcalorimeter will be equal to the maximum gold adsorption capacities of the carbonaceous adsorbents. The comparison of aurocyanide adsorption onto active carbons with that of Graphon and graphites gives some idea of the nature of active sites which can adsorb aurocyanide complexes. The amounts of aurocyanide adsorbed at the solid-solution interface Γ and the integral molar enthalpies of displacement ∆dplH for both adsorption and desorption cycles are collected in Table 6. Neither adsorption nor enthalpy effects have been detected for ideal graphite (G0). This adsorbent is nonporous graphite composed of well-ordered natural graphite flakes which have been purified by the manufacturer to a minimum content of impurities. It may be that extensive, ideal graphite-like structures are completely inactive with respect to aurocyanide adsorption. This is in agreement with the small density of aurocyanide adsorption observed with Graphon (Figure 2). Nevertheless, it seems more probable that the experimental method applied simply fails to detect aurocyanide adsorption because of the adsorbent’s small specific surface area. For other carbon adsorbents, it is evident that the maximum quantity of aurocyanide complex adsorption per unit of mass of carbon increases with increasing BET specific surface area of the solids and reaches a maximum for activated carbons which have the highest surface area. The integral molar enthalpies of adsorption and desorption obtained with different samples are very similar, which means that the interactions between the aurocyanide adsorbate and active sites in the carbon adsorbents are of the same type. This is a clear indication that the adsorption of aurocyanide involves similar surface structures. From the above results, showing similar behavior irrespective of the nature of the carbon support, we can assume the same mechanism of adsorption of gold onto graphites, carbon blacks, and activated carbons. The flow
microcalorimetric results clearly show that the adsorption of aurocyanide species from deionized water on the different carbonaceous adsorbents at room temperature is partly reversible, because the amount of aurocyanide removed by the solvent from the carbon surface in the desorption cycle is markedly less than the amount adsorbed in the adsorption cycle. The total amount of aurocyanide adsorbed (Γtot) on the carbon surface during the first cycle (adsorption cycle) is noted Γ1 ) Γtot. During the second cycle (desorption cycle), a part of this amount of aurocyanide initially adsorbed could be removed by the solvent (deionized water) from the carbon surface and is noted Γ2 and accounts for the reversible amount of adsorption (Γrev ) -Γ2). In such a case, the amount remaining adsorbed on the carbon surface after the first desorption cycle is markedly decreased and the difference between the two values (Γ1 and Γ2) can be considered as a direct measure of the amount of aurocyanide species irreversibly adsorbed on the carbon surface (Γirr ) Γ1 + Γ2 ) Γtot - Γrev). By analogy, the enthalpy of desorption taken with the opposite sign is identified with the average enthalpy of reversible adsorption (∆1,2hrev ) -∆1,2h2) and the average enthalpy of irreversible adsorption ∆1,2hirr can be calculated as follows:
∆1,2hirr ) Γ1∆1,2h1 - Γ2∆1,2h2 Γtot∆1,2htot - Γrev∆1,2hrev ) Γ1 + Γ2 Γtot - Γrev where ∆1,2htot or ∆1,2h1 is the average enthalpy of adsorption obtained upon the first cycle (adsorption cycle). In Table 7, we have reported the reversible and irreversible components of the amount adsorbed and the integral molar enthalpy of aurocyanide adsorption calculated from the experimental results reported in Table 6. Cycle experiments carried out on the three active carbons, Graphon, and graphites demonstrate strong adsorption of the KAu(CN)2 on all the carbons with varying degrees of irreversibility except for G0. As can be seen, the adsorption at room temperature can be clearly divided into irreversible and reversible processes. The heats of adsorption follow similar trends to those for the amounts of adsorption, confirming their reversible and irreversible proportions. There are considerable variations between the various carbon substrates in their total gold adsorption capacities and in their degree of adsorption reversibility and, more
4810 Langmuir, Vol. 15, No. 14, 1999
importantly, in their ability to irreversibly adsorb gold aurocyanide from dilute solutions. It can be noticed, from the adsorption isotherms plotted on a linear-linear scale in Figure 3, that the adsorbed amounts at very low equilibrium, Ln (Ce) around -5.5, are close to those for the irreversibly adsorbed Au(CN)2- for G1, Graphon, and G209. It is interesting that the magnitude of the enthalpies related to the adsorption of gold cyanide does not depend on the presence or absence of micropores. The graphitized carbons gave similar enthalpic values to those for some of the microporous carbons. There is also no direct correlation with the accessible graphite basal planes in the adsorbents given in Tables 2 and 3 and the proportions of polar surface sites. The comparison of graphite flakes and Graphon shows that the polar sites present in the flakes radically increase the irreversible adsorption. A quarter and about 40% of the total amount of aurocyanide is irreversibly adsorbed on Graphon and G1, respectively. Among the three active carbons, the degree of irreversibility is 31% (G209), 42% (G210), and 54% (G212). Some explanation for these differences can be found by having recourse to both the polar and the graphitic nature of the carbons. Active carbons G209 and G210 having the greatest polar surface area and the smallest graphitic basal plane areas yield the highest reversible adsorption. The G210 sample, which has the same polar surface area and a greater graphitic basal plane area in comparison with G209, adsorbs much more aurocyanide irreversibly. The G210 and G212 samples adsorb similar amounts of aurocyanide irreversibly, but the latter contains a greater graphitic basal plane area and a smaller polar surface. From these flow microcalorimetric experiments, it is evident that both polar and some graphene layers are involved in the adsorption process, particularly in the ability of the carbon samples to irreversibly adsorb gold complexes and that ideal graphitic basal planes alone are fully ineffective for the adsorption of aurocyanide species. It is unlikely that hydrophilic sites alone are involved in the adsorption of gold complexes, and there is not enough evidence to neglect the role of graphitic basal planes in the phenomenon, especially when we consider that the polar groups are attached to the graphite structure and could be the source of edge defects. Consequently, it is proposed that aurocyanide adsorption occurs mainly on the heterogeneous regions of the adsorbents. One part of the aurocyanide is believed to irreversibly adsorb as an unpaired anion Au(CN)2- by an anion-exchange mechanism on the very active surface sites having polar character. The less active sites are occupied by the ionpaired neutral molecular species (KAu(CN)2) through the action of van der Waals forces and account for the reversible adsorption.29 In reversible and irreversible processes, the active sites or effective surface is thus expected to be associated with the graphitic-like structure containing nonideal aromatic rings and polar structures located at the edge defects in the graphite structure. This is in line with the view represented by Sibrell and Miller26 that the adsorption of aurocyanide complex on graphitic carbons is much greater at the edge defects of the graphite planes than on the planes themselves. The analysis of the enthalpy effects collected in Table 7 provides additional arguments. The values of ∆1,2hirr and ∆1,2hrev are still believed to reflect similar mechanisms of the phenomena both onto nonporous graphitized carbon samples and on microporous activated carbons. It should be noted that the value of ∆1,2hirr obtained under dynamic conditions with active carbon G210 is only a little greater than -55 kJ mol-1 measured in the batch microcalorimetry experiment.29 Although the irreversible aurocyanide adsorption
Lagerge et al.
is more exothermic than the reversible one, the absolute values of ∆1,2hirr are much smaller than those expected for a typical chemisorption. The discrepancy may be explained by a competitive character of the phenomenon, the property which argues in favor of the anion-exchange mechanism. Large and weakly hydrated aurocyanide anions, such as that in the form of the irreversibly adsorbed species,29,30 displace interfacial water molecules and are capable of exchanging with some simple anions previously retained at the carbon-water interface. The process is irreversible because desorption is carried out in deionized water. The enthalpy change gained on adsorption of Au(CN)2- will be diminished by the enthalpy necessary to release water molecules and exchangeable anions from the carbon surface. The more polar the active sites involved in the irreversible adsorption, the greater will be both desorption components and, hence, the smaller the total enthalpy of the process. The moderate values of ∆1,2hirr estimated for the carbonaceous adsorbents seem to show that the active sites are the graphite structures linked to a certain kind of polar group. The reversible adsorption of aurocyanide in the KAu(CN)2 form is also competitive; the adsorbate has to displace solvent molecules from the interface. The loss of enthalpy related to the desorption of interfacial water should not be great because the absolute values of ∆1,2hrev are not small at all. Therefore, the surface sites responsible for reversible adsorption are less active and more hydrophobic in nature. Following the idea of Sibrell and Miller,26 the unsymmetrical charge distribution at defects in the graphite structure and the nonparallel orientation of the potassium aurocyanide ion pair could well account for the van der Waals type adsorption of the aurocyanide adsorbate on the defect sites of the graphitic planes. Conclusion The flow microcalorimetry, applied to gold adsorption on graphites, carbon blacks, and active carbons from aqueous solutions of KAu(CN)2 at room temperature, appears particularly useful for the study of the thermodynamic reversibility of the phenomenon. The experimental results show that the adsorption of gold complexes occurs onto a very small fraction of the total carbon surface. The ineffectiveness of a great part of the BET surface area is due not only to a very small accessibility of most of the microporous structure but also to a specific adsorption onto localized active sites on the carbon surface. It appeared that the ideal graphitic basal planes are fully ineffective for aurocyanide adsorption. Flow calorimetric results show the importance of both polar and some graphitic structures. Moreover, flow microcalorimetry results clearly show that the maximum gold capacity of the carbonaceous adsorbents is composed of two parts corresponding to the amounts of aurocyanide irreversibly and reversibly adsorbed on the carbon surface. For a given composition of the adsorption medium, the proportion between these two adsorption components is a complex function of both the polar and the graphitic nature of the carbon surface. The irreversible and reversible contributions to the integral molar enthalpy of adsorption from deionized water have been estimated for -50 and -25 kJ mol-1, respectively. Since the density of adsorption on Graphon is very low, it is proposed that aurocyanide adsorption occurs mainly at some edge defects in the graphite crystal structure. One part of the aurocyanide is believed to irreversibly adsorb as an unpaired anion Au(CN)2- through electrostatic interactions on the very active surface sites having polar character. The less active
Adsorption of Cyanide Gold Complexes
sites are occupied by the ion-paired neutral molecular species (KAu(CN)2) through the action of van der Waals forces and account for the reversible adsorption. In reversible and irreversible processes, the active sites or effective surface is thus expected to be associated with the graphitic-like structures containing nonideal aromatic rings. Such nonideality may be due to the presence of some structural imperfections, polar structures, or impurities located at the edge defects in the graphite crystal structure. It appears unlikely that a particular structure present on the carbon surfaces alone accounts for the adsorption of gold. It seem more realistic to assume that many structural imperfections or polar groups different in nature are responsible for the adsorption of gold. These
Langmuir, Vol. 15, No. 14, 1999 4811
functional groups are either able to modify the graphitic ring systems, making these latter effective for gold adsorption (reversible adsorption or stabilization), or able to directly adsorb gold complexes (irreversible adsorption through electrostatic interactions). This could explain the dependence of adsorption on the physicochemical conditions of the adsorption medium and the difficulty in proposing an unequivocal model for the mechanism of gold adsorption. Acknowledgment. This work has been supported by the European research foundation. EUREKA/TEPREM. LA980243T
