Langmuir 1997, 13, 4683-4692
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A Two-Stage Adsorption of Cyanide Gold Complexes onto Activated Carbon Inferred from Various Experimental Studies S. Lagerge,† J. Zajac,† S. Partyka,*,† A. J. Groszek,‡ and M. Chesneau§ Laboratoire des Agre´ gats Mole´ culaires et Mate´ riaux Inorganiques, CNRS, URA 079, Universite´ de Montpellier II, Case 015, 2 Place E. Bataillon, 34095 Montpellier Cedex 05, France, Microscal Ltd., 79, Southern Row, London W10 5AL., U.K., and Socie´ te´ PICA, 15 route de Foe¨ cy, F-18100 Vierzon, France Received October 7, 1996. In Final Form: May 28, 1997X In this paper a fundamental study has been made of the adsorption of potassium gold cyanide from water onto industrial activated carbon widely used for the recovery of gold. We examined the thermodynamic reversibility of the adsorption of gold complexes by determining the adsorption/desorption isotherms of Au, Au(CN)2-, and potassium counterion from pure KAu(CN)2 in deionized water solution at pH ) 6.0 and T ) 298 K. This study was supplemented by the determination of pH and conductivity variations of the bulk phase, surface charge modifications of the loaded solid particles, and enthalpic changes upon gold complex adsorption. The occurrence of two distinct and successive or overlapping mechanisms of adsorption in the studied range of concentrations at room temperature has been experimentally proved. For very low equilibrium concentrations, gold is adsorbed as Au(CN)2- anions and the adsorption has been found to be fully irreversible. The experimental results are consistent with an anionic exchange mechanism between Au(CN)2- ions and some anions initially present on the activated carbon surface. The measurements of the enthalpies associated with the adsorption process confirm the occurrence of the irreversible mechanisms of adsorption. 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, gold complexes are physically adsorbed. Moreover, in this stage, the adsorption of potassium counterion is occurring and is associated with a constant value of the pH, conductivity of the bulk phase, and electrophoretic mobility of the loaded solid particles. The experimental results prove that the reversibility is due to the physisorption of neutral molecular species (KAu(CN)2) on the less active parts of the activated carbon surface.
Introduction Activated carbons have been widely employed for the adsorption of gold from potassium aurocyanide in water solutions. The problem of gold recovery, and eventually other noble metals, from their ores by adsorption onto carbonized supports is complex and requires extensive fundamental studies of the mechanism by which activated carbon adsorbs gold. The starting point of our consideration is the carbonin-pulp (CIP) process for the extraction of gold from cyanide leach liquors,1,2 which is the preferred method for the recovery of native gold from its ores in all new gold plants.3 The most important procedure now in use for the dissolution of native gold, as KAu(CN)2, from its ore is cyanidation.4-6 Basically the process consists of leaching the ore with a very dilute alkaline solution of potassium or sodium cyanide in the presence of oxygen at pH 9-10. The ability of carbon to adsorb selectively gold cyanide is very often significantly affected by most of the common impurities in the leach liquor.12 Therefore, from an * To whom correspondence should be addressed: fax, 67.14.33.04; e-mail,
[email protected]. † Laboratoire des Agregats Mole ´ culaires et Materiaux Inorganiques. ‡ Microscal Ltd. § Socie ´ te´ PICA. X Abstract published in Advance ACS Abstracts, July 15, 1997. (1) Zadra, J. B.; Angel, A. K.; Heinen, H. J. Bur. Mines Rep. Invest. 1952, No. 4843. (2) Zadra, J. B. Bur. Mines Rep. Invest. 1950, No. 11672. (3) Hall, K. B. World Min. 1974, 27, 44. (4) Dorr, J. V. N.; Bosqui, F. L. Cyanidation and Concentration of Gold and Silver Ores, 2nd ed.; McGraw-Hill Book Co., Inc.: New York, 1950; pp 177-427. (5) Thompson, P. F. Trans. Electrochem. Soc. 1947, 91, 41-71. (6) Wyman, W. I. J. Pat. Off. Soc. 1931, 13, 547-564.
S0743-7463(96)00977-8 CCC: $14.00
industrial point of view, the selective adsorption of gold is an important technological challenge. In spite of the numerous studies made on the subject, the principles governing the selective adsorption of gold complexes onto activated carbons are still not well understood and remain of vital importance for the production of improved carbon adsorbents. A number of adsorption mechanisms of gold from potassium aurocyanide solutions onto activated carbons have been proposed over the years.7-36 The two most widely accepted theories (7) Cho, E.; Pitt, C. H. Metall. Trans. B 1979, 10B, 159-189. (8) Cho, E.; Dixon, S.; Pitt, C. H. Metall. Trans. B 1979, 10B, 185189, 1979. (9) McDougall, G. J.; Hancock, R. D.; Nicol, M. J.; Wellington, O. L.; Copperthwaite, R. G. J. S. Afr. Inst. Min. Metall. 1980, 80 (9), 344-356. (10) McDougall, G. J.; Hancock, R. D. Gold Bull. 1981, 14 (4), 138153. (11) Fleming, C. A.; Nicol, M. J. J. S. Afr. Inst. Min. Metall. 1984, 84, 95. (12) 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. (13) Tsuchida, N.; Muir, D. M. Metall. Trans. B 1986, 17B, 523. (14) Tsuchida, N.; Muir, D. M. Metall. Trans. B 1986, 17B, 529. (15) Abotsi, G. M. K. Osseo-Asare, K. Int. J. Min. Process. 1986, 18, 217. (16) 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. (17) Fuerstenau, M. C.; Nebo, C. O.; Kelso, J. R.; Zaragoza, M. R. Metall. Process. 1987, 4, 177. (18) McDougall, G. J.; Adams, M. D.; Hancock, R. D. Hydrometallurgy 1987, 18, 125. (19) Adams, M. D.; McDougall, G. J.; Hancock, R. D. Hydrometallurgy 1987, 18, 139. (20) Adams, M. D.; McDougall, G. J.; Hancock, R. D. Hydrometallurgy 1987, 19, 95. (21) Cashion, J. D.; McGrath, A. C.; Volz, P.; Hall, J. S. Trans. Inst. Min. Metall. 1988, 97, C129. (22) Klauber, C. Surf. Sci. 1988, 203, 118. (23) van der Merwe, P. F.; van Deventer, J. S. J. Chem. Eng. Commun. 1988, 65, 121.
© 1997 American Chemical Society
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Table 1. BET Specific Surface Areas (As) and Porosity Characterization of Activated Carbon G210 As (BET) (m2 g-1)
77 K, N2 adsorption microporous volume (Vµ) (cm3 g-1)
1150
273 K, CO2 adsorption microporous surface (D-R equation):(Aµ) (m2 g-1)
0.27
474
mercury porosimetry mesoporous (macro + meso) volume (cm3 g-1) porous area (m2 g-1) 0.34
32
Table 2. Proximate and Elemental Analysis of Activated Carbon G210 elemental analysis/wt %
proximate analysis/wt %
oxygen
carbon
hydrogen
nitrogen
moisture
volatile matter
ash
fixed carbon
2.09
95.86
0.27
0.00
2.39
2.86
2.84
91.91
reduce to (a) adsorption involving ion pairs, Mn+[Au(CN)2-]n,9,18-20 and (b) adsorption of unpaired ions onto activated carbons. The adsorption in the form of an ion pair, Mn+[Au(CN)2-]n, with subsequent reduction of gold (1) either to gold metal or to a partially reduced state between 1 and 0 has also been proposed.10-15 The most recent theories, on X-ray photoelectron spectroscopy (XPS)9,22,24,27 and Mo¨ssbauer spectroscopy9,21,26,30,33,35,36 investigations, favor the adsorption of gold as Au(CN)2without chemical change. Some of these theories assume that carbon oxygen surface species on the carbon play a direct role in the process.7,8,13,14,25 In such a case an ion exchange mechanism occurs and Au(CN)2- is adsorbed on the activated carbon substrate via the cyanide ligand’s nitrogen atoms through oxygen functionalities on the carbon. Other authors maintain that the most relevant characteristic of an activated carbon for gold loading is a high concentration of accessible graphene layers.27,34,35 The delocalized π-electrons of the condensed aromatic ring systems can be transferred toward the vacant orbitals in the adsorbed gold complex where they take part in a donor bond with the central gold atom of the adsorbed species33 or they can be the source of the reducing action of the carbon surface.9,20,22,27,34 However Sibrell and Miller,35 considering the geometry of CN- groups, believe that the adsorption is more likely to take place at the edges carbon atoms of the graphitic ring systems. The first aim of this fundamental study is to propose a reliable general mechanism of adsorption of cyanide gold complexes and to elucidate the nature of the interactions between gold complexes and activated carbon. In this attempt, changes in the interfacial properties are investigated of an industrial activated carbon sample at the solid/water interface upon the adsorption of KAu(CN)2 complexes at free pH values and in the absence of extra salt. For this purpose, we examined in detail the thermodynamic reversibility of the adsorption of gold complexes. We separately and simultaneously studied the adsorption and desorption of complexed cyanide, gold, and potassium counterion forming a part of the KAu(CN)2 molecule. This paper reports adsorption/desorption isotherms and electrokinetic data supplemented by calori(24) Cook, R.; Crathorne, E. A.; Monhemius, A. J.; Perry, D. L. Hydrometallurgy 1989, 22, 171. (25) Adams, M. D.; Fleming, C. A. Metall. Trans. B 1989, 20B, 315. (26) McGrath, A. C.; Hall, J. S.; Cashion, J. D. Hyperfine Interact. 1989, 46, 673. (27) Jones, W. G.; Klauber, C.; Linge, H. G. Nineteenth Biennial Conference on Carbon, The Pennsylvania State University, University Park, PA, June 1989. (28) Jones, W. G.; Linge, H. G. Hydrometallurgy 1989, 22, 231. (29) Zarrouki, M.; Thomas, G. Analusis 1990, 18, (4), 261. (30) Kongolo, K.; Bahr, A.; Friedl, J.; Wagner, F. E. Metall. Trans. B 1990, 21B, 239. (31) Groszek, A. J.; Partyka, S.; Cot, D. Carbon 1991, 29, (7), 821. (32) Le Roux, J. D.; Bryson, A. W.; Young, B. D. J. S. Afr. Inst. Min. Metall. 1991, 91, (3), 95. (33) Klauber, C. Langmuir 1991, 7, 2153. (34) Ibrado, A. S.; Fuerstenau, D. W. Hydrometallurgy 1992, 30, 243. (35) Sibrell, P. L.; Miller, J. D. Miner. Metall. Process. 1992, 9, 189. (36) Ibrado, A. S.; Fuerstenau, D. W. Min. Eng. 1995, 8, (4/5), 441.
metric measurements of the enthalpy of displacement. The electrical nature of the solid/water interface is strongly influenced by the pH of the aqueous phase. Since this parameter was not adjusted in the experiments (free pH), both the surface potential and the surface charge of activated carbon particles could vary along the isotherm. To illustrate this effect, the pH values of the supernatant liquid were monitored. Comparison of data from all these measurements yields a greater insight into the possible mechanism of the adsorption process and its evolution with the surface coverage. It was not our intent either to identify the nature of the adsorption sites responsible of the adsorption or to determine the surface available for the adsorption of gold complexes. For that reason, the nature of adsorption sites (characterization of the surface groups) present on G210 has not been included in this paper. The starting point of this fundamental adsorption study is a simple system which yields the experimental results, easy to be interpreted without ambiguity. Such a system must be composed of a pure solvent, deionized water, a pure solute, KAu(CN)2, and a well-defined carbon sample, because of the influence of two important factors, the presence of electrolytes in the adsorption medium, and the variation in its acidity, on the gold adsorption, as has been confirmed by many authors.37 Material and Experimental Procedures Materials. The industrial activated carbon, G210, used in the experimental work was obtained from PICA (Vierzon, France). It is derived from coconut shells and prepared by high temperature (1173 K) steam activation. We used a powdered sample obtained by crushing and sieving to obtain average particle diameter of 10 µm as determined using light scattering analysis (Mastersizer E, Malvern Instruments). Prior to use, the carbon was washed with deionized water at 298 K and subsequently dried at 423 K for 12 h. The water-washing was stopped when the conductivity and the pH values of the suspension remained unchanged and very close to those of pure deionized water. This operation significantly reduced the amount of superficial mineral impurities. Potassium dicyanoaurate was supplied by Me´taux-Pre´cieuxIndustrie (Bagneux, France) and was used in solution in deionized water without any other reagents. In industry, sodium aurocyanide is very often preferred to potassium aurocyanide. It might be that the adsorption of potassium gold complexes would be different compared to that of sodium gold complexes. But it is rather probable that this difference will concern essentially the extent of adsorption and that the general mechanism of adsorption from pure deionized water will be the same in each case. Therefore in this paper, for practical reasons, the potassium has been used as counterion. This fundamental study should contribute to better understanding of the general mechanism of adsorption of aurocyanide complexes. Characterization. The characterization data are presented in Tables 1-3. They are given as an indication but are not discussed in detail. (37) Bansal, R. C.; Donnet, J. B.; Stoeckli, F. Active Carbon; Marcel Dekker, Inc.: New York and Basel, 1988.
Adsorption of Cyanide Gold Complexes
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Table 3. Experimental Integral Enthalpies of Immersion of Sample G210 in Different Solvents, Expressed per Unit Mass and per Unit BET Surface Areaa ∆immH (J g-1) ∆immH (mJ m-2) a
water
heptane
cyclohexane
benzene
R-pinene
-31 -27
-134 -118
-111 -98
-137 -120
-116 -102
Measurements made at 25 °C.
The specific surface area (As) and microporous volume (Vµ) were determined by volumetric adsorption measurements at 77 K of nitrogen (Analsorb 9011, France) and applying the BET and R-s methods, respectively, with molecular cross-section surface areas being 16.2 Å2 for nitrogen.38 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.37 In both cases, the sample of 0.1 g was initially outgassed at 423 K under vacuum (10-3 Torr). Finally, a mercury penetration method (Carlo Erba Model 200 Pressure Porosimeter) was used to measure the macro- and mesoporosity of the activated carbon.37 The results are presented in Table 1. The N2, 77 K, adsorption/desorption isotherm was characteristic of microporous adsorbent, i.e., of type I in the BDDT classification with a very slight hysteresis loop of type B in the De Boer classification.38 Meso- and macropore size distribution were quite broad but showed some sharp peaks in the pore size range between 4 and 10 nm with a maximum (5.5%) at 4 nm. Results of chemical analysis are shown in Table 2. The measurements of elemental and proximate analysis have been run at the Northern Carbon Research Laboratories, University of Newcastle upon Tyne, U.K. The elemental analysis reflected the empirical formula of the carbon sample under investigation. It is highly microporous with some hydrogen and oxygen surface functionality but with no nitrogen content. Immersional wetting calorimetric measurements were used to quantify the strength of the interactions between the activated carbon and pure liquids. The method adapted for this study was the microcalorimetry of immersion which allowed us to determine the enthalpies of immersion (∆immH) of the adsorbent into water and some apolar solvents. The used conduction calorimeter and its operational procedure have been characterized previously.39-43 The samples was initially heated at 433 K under vacuum (10-3 Torr) for 5 h. The immersion enthalpies, ∆immH, of the activated carbon are presented in Table 3 (in J g-1 and mJ m-2). We can observe that the ∆immH values in water are lower than those in other nonpolar liquids. It is evident that the carbon surface is mostly apolar with a high concentration of (accessible) graphene layers. The small differences in enthalpy of immersion for different nonpolar liquids are due to different cross-sectional areas of molecules (cyclohexane, heptane), different accessibility of porous structure to different immersion liquids (R-pinene), and some additional adsorption of some of them on the polar part of the surface. Since the specific surface areas have been determined by nitrogen gas adsorption, there arises an important problem of the accessibility of the solid surface to the immersion liquid. In the case of nonporous and meso- and macroporous solids, the enthalpy of immersion is simply proportional to the surface area available to the immersion liquid. However, it is not always true for microporous solids.38 Adsorption and Electrophoretic Measurements. Adsorption isotherms were obtained using the solution depletion method,39 which consists of comparing the solute concentrations before and after the attainment of adsorption equilibrium. Electrokinetic or zeta potentials are determined by micro(38) Gregg, S. J.; Sing, K. W. Adsorption, Surface Area and Porosity; Academic Press: London, 1982. (39) Lagerge, S.; Rousset, P.; Zoungrana, T.; Douillard, J. M.; Partyka, S. Colloids Surf. A 1993, 80, 261-272. (40) LaGerge, S. Ph.D. Thesis, Montpellier, 1995. (41) Calvet, E.; Prat, H Re´ cents progre` s en microcalorime´ trie; Dunod: Paris, 1958. (42) Fowkes, F. M.; Burgess, T. E. Clean Surfaces; Goldfinger, G., Ed.; Marcel Dekker: New York, 1970. (43) Partyka, S.; Rouquerol, F.; Rouquerol, J. J. Colloid Interfaces Sci. 1979, 68, 30; Proc. R. Soc. London, Ser. A 1970, 314, 473.
electrophoresis.44-46 This technique is employed to measure the mobility of small particles of chemically pure adsorbents. A correlation between the adsorption and electrophoretic results is usually examined with the aim of sheding light on the mechanism by means of which the solutes are adsorbed at the solution-solid interface. This implies the necessity of maintaining the same experimental conditions in both experiments. For this purpose, the same initial operational procedure is applied. When a solid is in contact with a solution, the preferential adsorption of one of the solution components is usually observed. The amount adsorbed per unit mass of the solid is given by the expression
Γ)
10-3M(Ci - Cae ) mS
(1)
where Γ is the reduced surface excess47,48 equal to the number of moles of solute adsorbed at the interface from a diluted aqueous solution. In this eq Ci and Cae are respectively the initial (before adsorption) and equilibrium (after the attainment of the equilibrium of adsorption) solute concentrations (mol L-1 of solution), mS is the mass of adsorbent (g), and M is the initial mass of solution (g). The adsorption experiments were carried out in stoppered Pyrex tubes of capacity 30 cm3. Each tube containing the same mass of dry activated carbon (about 0.2 g) and the same volume of aqueous solution (28 cm3) of a given molarity was equilibrated by gentle rotation for 24 h at a constant temperature in a thermostat at 298 ( 0.5 K. Subsequently the solid samples were separated from the supernatants by filtration (Nalgene filter SFCA, pore size 0.45 µm). The clear supernatants collected from each tube were analyzed for the solute content and pH, giving the gold, complexed cyanide (in the (CN)2 group), and potassium concentrations and the pH values, respectively, after the attainment of the adsorption equilibrium. Simultaneously, samples of solid suspension were collected for electrophoresis experiments. The surface excess or the amount of adsorption per unit mass of the adsorbent surface was then calculated by means of eq 1. The complexed cyanide concentration in the supernatant was measured by UV spectroscopy (Varian Cary 3E) between 210 and 300 nm (complexed CN adsorption range, λmax ) 239.6 nm) while the gold (Au) and potassium (K) concentrations were measured by flame spectroscopy (Fsp) respectively at 242.8 nm (absorption) and 766.5 nm (emission) with an air-acetylene flame. For potassium, the solutions were adjusted to 1000 ppm Cs by adding CsCl. The reproducibility between two concentration determinations was within 2% for UV spectroscopy and 1% for flame spectroscopy. Thus we are able to separately determine the adsorption of complexed CN, K+, and Au+ forming a part of the adsorbed KAu(CN)2 molecule. In order to obtain desorption data, a given amount of the supernatant liquid (28 cm3) was removed from each tube after the adsorption equilibrium had been reached and replaced by a known amount of solvent (28 cm3 of deionized water) under the thermostatic conditions. The tubes were shaken for 24 h and the solid was separated from the supernatant liquid by filtration; the concentrations of the individual elements or groups (Au+, complexed CN and K+) in the supernatant liquid were then detected as previously. (44) Somasundaran, P.; Fuerstenau, D. W. J. Phys. Chem. 1966, 70, 90. (45) Bijsterbosch, B. H. J. Colloı¨d Interface Sci. 1974, 47, 186. (46) Ducker, W. A.; Pashley, R. M.; Ninham, B. W. J. Colloı¨d Interface Sci. 1989, 128, 66. (47) Kinaly, Z.; Dekany, I. J. Chem. Soc., Faraday Trans. 1, 1989, 85, 3373. (48) Everett, D. H. Pure Appl. Chem. 1985, 31, 579.
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The amount adsorbed after desorption (Γa2) was calculated as follows
Γa2 ) Γa1 -
[Cde (M - MR + MA) - Cae (M - MR)] 10-3 (2) mS
where MR and MA are respectively the mass of the solution removed from the tube and the mass of solvent added to the tube and Cde is the final concentration of solution (after the attainment of the new adsorption equilibrium). Γa1 is the amount of adsorption previously defined as Γ in eq 1. All the adsorption and desorption data were obtained at pH 6.0 and T ) 298 K. A Rank Brothers microelectrophoresis apparatus MKII with a rectangular cell was applied to measure the average velocity U at which charged colloidal particles move under the action of a steady and weak electric field between platinum black electrodes.49-51 A sample of activated carbon suspension from the supernatant, collected after the attainment of adsorption equilibrium and filtration, was transferred to the thermostated microelectrophoresis cell. The velocity U for at least 10 particles was measured at both stationary levels and the average value taken; the polarity of electrodes was reversed after each velocity measurement. Chemical Analysis of the Bulk Phase. The water used throughout all experiments was deionized and purified with a Millipore “Super Q” system. It had a pH value of about 6.0 and conductivity which varied between 0.05 and 0.1 µS cm-1. No extra salt was added in the solution to buffer the system. Values of pH were determined with Tacussel electrode (France). The accuracy of the measurement was to 0.05 pH unit. Following the adsorption of gold complexes, the supernatant was also analyzed using conductometry and anionic chromatography techniques and by colorimetry at 612 nm with cyaniver reagent52 in order to detect the possible presence of free cyanide. Microcalorimetry of Adsorption. The enthalpy changes accompanying the adsorption of gold complexes onto activated carbon were measured by a microcalorimetric batch technique that allowed the adsorbing species to be introduced from the outside of the calorimetric cell to a homogeneous suspension of solid in the solvent.53 It is thus possible to follow the process step by step and detect enthalpy changes associated with subsequent steps. Since the whole adsorption range should be covered, i.e., from the beginning of the isotherm to its plateau saturation, a concentrated KAu(CN)2 stock solution at a molality around 7.6 × 10-3 mol L-1 Au should be used. In such a case, a correction term arising from the dilution of the gold complexes species injected into the calorimetric cell should be substracting from the total enthalpic effect. The enthalpy change upon adsorption will be called henceforth the enthalpy of displacement by reason of the competitive character of the phenomenon. A stock solution of molality 7.6 ×10-3 mol L-1 Au was injected into the calorimetric cell containing 28 g of solvent (dilution experiment) or 28 g of solvent and 0.2 g of activated carbon (adsorption experiment). The apparent differential molar enthalpy of displacement, ∆1,2h˙ , corresponding to a given adsorption step was evaluated by means of the following approximation
∆1,2h˙ )
∆expH - ni2∆dilh ∆na2
(3)
where ∆expH is the experimentally measured enthalpy change, ni2 is the number of moles of solute injected into the calorimetric cell, ∆na2 is the change in the number of moles of solute adsorbed on the surface and is determined graphically with the aid of the adsorption isotherm, and ∆dilh is the molar integral enthalpy of (49) Hunter, R. J. Foundations of Colloı¨d Science; Clarendon Press: Oxford, 1991; Vol. I. (50) Saleeb, F. Z.; de Bruyn, P. L. J. Electroanal. Chem. 1972, 37, 99. (51) O’Brien, R. W.; White, L. R. J. Chem. Soc., Faraday Trans. 2, 1978, 74, 1607. (52) Norme franc¸ aise NF T 90108. (53) Partyka, S.; Lindheimer, M.; Zaini, S.; Keh, E.; Brun, B. Langmuir 1986, 2, 101.
Figure 1. Adsorption isotherms of gold (Au), potassium counterion (K), and complexed cyanides (CN) adsorbed onto activated carbon G210 from deionized water at pH ) 6.0 and T ) 25 °C, on a linear-linear scale.
Figure 2. Adsorption isotherms of gold (Au), potassium counterion (K), and complexed cyanides (CN) adsorbed onto activated carbon G210 from deionized water at pH ) 6.0 and T ) 25 °C, on a linear-log scale. dilution for the equilibrium concentration of KAu(CN)2 in the calorimetric cell.
Results Adsorption-Desorption Data. Bearing in mind the interactions responsible for the adsorption of gold cyanide complexes, we carrefully investigated the problem of thermodynamic reversibility of the gold complexes adsorption. The experimental investigation consisted of determining the adsorption and desorption isotherms for KAu(CN)2 from aqueous solution at T ) 298 K onto the activated carbon G210. Water was not buffered to any pH value; its initial pH was 6.0. Figures 1 and 2 show the adsorption isotherms for gold (Au), potassium (K), and complexed cyanide (CN in the (CN)2 group) on a linear-linear and linear-log scale, respectively. The amounts of each compound adsorbed per unit mass of carbon (ΓAu, ΓCN, and ΓK) expressed in µmol g-1 were plotted against the gold concentrations, CeAu in µmol L-1 (Figure 1) or the corresponding log (CeAu) (Figure 2) in the equilibrium bulk solution after the attainment of the equilibrium of adsorption. For Au and CN, the adsorption isotherms consist of two distinct parts. An initial vertical section, corresponding to very low equilibrium concentrations, certainly suggests a very strong adsorption of gold complexes for some of the surface sites. The second part of the isotherms reaches a limiting value, Γm, characteristic of the system and corresponding to the saturation of the surface. The linear-log scale clearly shows that the quantities of adsorption obtained for complexed cyanide are approxi-
Adsorption of Cyanide Gold Complexes
Figure 3. Calculated log(ΓAu(CN)2-/ΓK+) values against the logarithm of equilibrium concentration of aurocyanide complexes for the adsorption of KAu(CN)2 onto activated carbon G210 from deionized water at pH ) 6.0 and T ) 25 °C.
mately twice those obtained for gold in the whole range of equilibrium concentration. This confirms that the adsorption amounts of gold and cyanide groups are equal to the stoichiometric values required for the adsorption of an aurocyanide species without any chemical change, as postulated by other authors.9,21,22,24,26,27,30,33,35,36 Therefore, we can now plot an adsorption isotherm in terms of the amounts of Au(CN)2- anions adsorbed per unit mass of the solid adsorbent, ΓAu(CN)2- expressed in µmol g-1, as a function of the corresponding concentrations, CeAu(CN)2in µmol L-1, remaining in the equilibrium bulk solution after the attainment of the adsorption equilibrium at a given temperature; ΓAu(CN)2- ) f(CeAu(CN)2-). However, we must recall that, in the present state of the knowledge, the adsorption of gold as aurocyanide species cannot be explained unequivocally. Indeed, it is also possible that, in a second step, a chemical reaction could occur between the aurocyanide complex and the surface (functional groups for example) involving a chemical change of the gold complex initially adsorbed as Au(CN)2-. But, if this were the case, the products of this surface reaction would have remained adsorbed on the carbon surface because no free cyanide (CN- ions) was detected in the bulk solution. The linear-log scale representation is particularly interesting at low coverage ratios and is in fact more appropriate to analyze the adsorption results for potassium counterion. Indeed, Figure 2 allows one to note that the potassium counterion does not adsorb in the low coverage ratios (initial part of the isotherm). Thereon, from a given value of the equilibrium concentration (Cec) or the corresponding Γc, the adsorption of potassium, very low for the reduced aurocyanide equilibrium concentrations, increases more and more until the formation of a plateau (Figure 1). In Figure 3 we have plotted the ratio of the amount of adsorption of aurocyanide species (ΓAu(CN)2-) to that of potassium (ΓK) against the equilibrium concentration of aurocyanide complexes on a log-log scale. This curve shows that following the adsorption of gold complexes, the ratio of ΓAu(CN)2- to ΓK gradually decreases reaching a limiting value roughly equal to 2, i.e. log (ΓAu(CN)2-/ΓK) ≈ 0.3; the amounts of potassium adsorption (when it occurs) give the gold to potassium ratios which vary and are not representative of any particular adsorbed gold species. The results of investigations concerning the thermodynamic reversibility are presented in Figure 4 showing the adsorption isotherms of potassium counterion and aurocyanide species, on a linear-log scale as previously determined in Figure 2, together with the desorption curves obtained following dilution with deionized water.
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Figure 4. Adsorption and desorption isotherms of potassium counterion and aurocyanide anions performed from deionized water at pH ) 6.0 and T ) 25 °C on activated carbon G210, on a linear-log scale.
The adsorption/desorption isotherms, obtained for Au(CN)2-, show three different regions. The first one is in the range of very dilute equilibrium concentrations (Ce < Cec) or low coverage ratios (Γ < Γc), where the desorption curve is shifted toward the low equilibrium concentrations compared to the adsorption isotherm while the amount of adsorption of Au(CN)2- anion remains unchanged after the dilution process indicating a fully irreversible adsorption. A second region, for the higher equilibrium concentrations in the range of Cec < Ce < 160 µmol L-1 or the corresponding Γc < Γ < 50 µmol g-1, occurs where a part of the adsorbed species is removed from the surface following the dilution in deionized water. The adsorption and desorption isotherms tend to be more and more similar when the equilibrium concentration or the coverage ratio is increasing, accounting obviously for a partly reversible adsorption of aurocyanide anions. At last, a third region occurs for Ce > 160 µmol L-1 where both isotherms become fully similar, indicating that the adsorption is fully reversible. The ratio between the desorbed complexed cyanide and desorbed gold was also found to be 2 and no free cyanide was detected in the bulk solution, which means that gold species removed from the surface is also in the form of aurocyanide groups. Moreover, Figure 4 shows that the adsorption and desorption isotherms, obtained for the potassium counterion, match very well one another in the whole range of concentration. This means that the adsorption of potassium counterion, when it occurs, is always fully reversible. pH and Conductivity Analysis. The particular behavior concerning the adsorption of potassium led us to complete the adsorption study by the chemical analysis of the bulk phase upon aurocyanide adsorption. We measured the variations of the pH and conductivity values in the bulk solution upon aurocyanide adsorption. The initial pH value of the deionized water used in this study was 6.0. When put in contact with water-washed G210, after 24 h, the pH value of the suspension (activated carbon in pure deionized water) was increased to about 6.5 (pHiep). The variation of the pH value together with that of the conductivity in the bulk phase was then measured upon adsorption of gold species. Both curves are presented in Figure 5. In this figure, we have only plotted the net variation of conductivity in the bulk phase due to the adsorption of gold complexes against the amount of adsorption of Au(CN)2- species in the whole range of adsorption. This curve is obtained by substracting the contribution due to the presence of aurocyanide species which are not adsorbed and hence remaining in the bulk
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Figure 5. Variations of the pH (open circles) and conductivity (open triangles) values in the bulk solution upon the adsorption of Au(CN)2- anions onto activated carbon G210 from deionized water at pH ) 6.0 and T ) 25 °C.
Figure 6. Chromatographic analysis of the bulk solution upon the adsorption of KAu(CN)2 onto activated carbon G210 from deionized water at pH ) 6.0 and T ) 25 °C.
solution (from Ce > 0 µmol L-1) to the total contribution (experimental value). The former contribution is obtained from a blank experiment without activated carbon (calibration curve covering the whole range of equilibrium concentration). Figure 5 shows that the adsorption of gold complexes leads to an increase of the pH to a given value of the gold equilibrium concentration (Cec) or Γc after which it remains constant. A similar trend is observed with the conductivity of the supernatant. Chromatographic Data. The chromatographic analysis is reported in Figure 6 for six different supernatants (a to f) in the irreversible part of the isotherm. The blank experiment (water-washed activated carbon in pure deionized water) does not exhibit any chromatographic peak indicating that none of the anionic species is removed from the surface following the immersion of the activated carbon in pure deionized water. This was expected because the solid sample has been previously washed with water. However, following the adsorption of gold complexes (chromatograms b-f in Figure 6), some peaks can
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Figure 7. Evolution of the electrophoretic mobility of activated carbon G210 particles upon the adsorption of Au(CN)2- anions onto activated carbon G210 from deionized water at pH ) 6.0 and T ) 25 °C.
be detected and they cannot be ascribed to the presence of aurocyanide species remaining in the bulk phase because they are also present for Ce ) 0 (chromatograms b-e in Figure 6). This also clearly indicates that certain anionic species, initially present on the carbon surface, are removed from the surface to the bulk solution following the adsorption of gold. In comparison with the chromatographic spectrum obtained with KCl solution (Figure 6a′,b′), it appears that the first peak obtained after the attainment of the equilibrium of adsorption (t ≈ 0.8 s) can be attributed to desorption of chloride ions. The peak areas were however too small for quantitative determination but were of similar magnitude. It seems therefore that the amount of chloride ions removed from the surface were roughly constant in the whole range of irreversible adsorption with the increasing amounts of adsorption of Au(CN)2-. Another large peak detected for t > 0.8 s, indicates that at least one additional anionic species is desorbed together with chloride ions during the fully irreversible adsorption. But once again, the corresponding exchanged ion could not be identified and its concentration also appeared to be roughly constant for all the supernatants. Electrophoretic Data. One of the major factors influencing the adsorption of aurocyanide complexes onto activated carbons may be that of electrical interactions within the environment of an electrical double layer at the interface carbon/water. An understanding of charge generation and the structure of the double layer, together with the relevance of measured electrokinetic potentials, is therefore pertinent to our interpretation of the adsorption process. For this purpose, we measured the electrophoretic mobility of powdered carbon particles loaded with the Au(CN)2- at different coverage ratios. Figure 7 shows the resultant electrophoretic mobility as a function of the amount of adsorption of aurocyanide species for G210. It is generally representative of surface charge variations on the external solid surface upon adsorption of gold complexes and cannot be used to explain all the phenomena measured on adsorption especially the filling of internal surface (porous surface). Although the electroporetic measurement (µ-values) cannot be seen as representative of the total surface charge variation due to the total adsorption of aurocyanide species, they remain pertinent for a better understanding of the mechanism of adsorption and allow some important conclusions to be drawn concerning the mechanism of adsorption. Indeed, if the adsorption of gold complexes occurs within the microporous structure, it is probable that the mechanism of adsorption will be similar to that occurring on the
Adsorption of Cyanide Gold Complexes
Figure 8. Differential molar enthalpies of displacement against the equilibrium amount of Au(CN)2- anions adsorbed onto activated carbon G210 from deionized water at pH ) 6.0 and T ) 25 °C.
external surface. Moreover, previous works have shown that the adsorption of gold complexes essentially occurs on the external, macro- and mesoporous, surface and that most of the microporous structures in activated carbons are not available to the adsorption of aurocyanide complexes.40 In such a case the measured electrophoretic mobility may be thought representative of as being the actual surface charge variation. We can note that the µ-value is constant along the adsorption isotherm indicating that the net external surface charge remains unchanged in the whole range of adsorption. In this figure the Γc value is indicated by the arrow as in Figure 5. Calorimetric Data. Finally, in Figure 8, we have reported the variation of the apparent differential molar enthalpy of displacement, ∆1,2h˙ , as a function of the amount of adsorption at 298 K. The microcalorimetric experiments were performed with a high sensitivity (1 mJ) at which the molar enthalpy of dilution of KAu(CN)2 in the calorimetric cell was found to be close to zero in the whole range of concentrations. Therefore, no correction term arising from the dilution of solute injected into the calorimetric cell should be substracting from the total enthalpic effects. We were essentially interested with the enthalpic effects related with the initial irreversible adsorption of aurocyanide species (0 < Γ < 240 µmol g-1 ≈ Γc). In this region of the adsorption isotherm, the experimental enthalpic curve was found to be roughly linear and highly exothermic with a value of ∆1,2h˙ around -55 kJ mol-1. This constant value of the molar enthalpy of displacement indicates a similar mechanism of adsorption in the whole range of the irreversible adsorption. Discussion Irreversible Adsorption. As stated previously, in the initial part of the isotherm (Γ < Γc ≈ 240 µmol g-1), i.e., for dilute aurocyanide equilibrium concentrations, the adsorption of potassium counterion does not occur at all (ΓK ) 0) and is associated with the fully irreversible adsorption of aurocyanide species (ΓCN ) 2ΓAu) without the detection of free cyanide in the bulk solution. These experimental results are fully consistent with an adsorption mechanism by which gold is irreversibly adsorbed as Au(CN)2- anions without the occurrence of chemical changes through an anionic exchange involving electrostatic interactions at oppositely charged surface sites and confirm some literature results.9,21,22,24,26,27,30,33,35,36 Following the anionic exchange, the potassium counterion would be released to the bulk solution with an initially preadsorbed anion, accounting for the increase of conductivity in the bulk phase (0.5 < κ < 200) as well as the
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absence of the adsorption of potassium. Indeed, the conductivity curve reported in Figure 5 follows very well the corresponding adsorption isotherm trend and clearly shows that the adsorption of Au(CN)2- anions leads to an increase of the conductivity in the bulk solution. This increase is very marked at low equilibrium concentrations and indicates that an ionic species is removed from the surface to the solution upon aurocyanide adsorption. Considering that the estimated cross sectional area per adsorbed molecule is about 20.8 Å2, as determined by Klauber,22 an estimate surface coverage could be calculated using the BET or microporous specific surface area of the solid (i.e., 1150 and 474 m2 g-1, respectively) and the irreversible amount of adsorption (240 µmol g-1). This estimate surface coverage was found to be very small and about 2.6% of the total BET surface and 6.3% of the microporous surface. The chromatographic analysis reported in Figure 6 confirms the hypothesis of the anionic exchange mechanism between the eluted anions and Au(CN)2- anions in the irreversible part of the isotherm. Moreover, this study reveals that some of the ion exchangeable sites are occupied by chlorides anions as well as by one or more different anionic species which could not be identified. In this initial region of irreversible adsorption, the invariance of the electrophoretic mobility irrespective of the amount of adsorption can be attributed to and confirm the adsorption of aurocyanide anions involving an ionexchange mechanism between Au(CN)2- anions and anions initially present in the interfacial region as preadsorbed counterions. Such electrostatic interactions can result in an irreversible adsorption. This type of mechanism does not involve any surface charge variation. The average µ-value in this region is around -4.1 × 10-4 cm2 V-1 s-1 and is very close to that of unloaded carbon particles in the pure solvent (pHe ) 6.5). The total variation of the enthalpy of displacement, reported in Figure 8, includes not only the adsorption energy of the adsorbate molecule but also some other energetic contributions arising from the desorption (displacement) of the water molecules from the surface following the adsorption of aurocyanide anions, the desorption of the exchanged-ion initially present in the interface, the dehydration of the adsorbed anions (Au(CN)2-) and the rehydration of those ions which are desorbed (exchanged). The corresponding enthalpic value is constant and about -55 kJ mol-1 in the whole range of irreversible adsorption. It is close to the values currently found in the case of electrostatic interactions and therefore is also in agreement with the anionic-exchange mechanism in the whole range of irreversible adsorption. The constant enthalpic curve indicates that the irreversible adsorption of gold complexes does not detect the assumed surface heterogeneity of the activated carbons. The sensitivity of the interactions to the surface heterogeneity is in the order van der Waals (physisorption) . electrostatic interactions > chemisorption. Thus, if the anionic-exchange occurs, it is not surprising to find a constant value. In conclusion, our experimental results indicate that gold is strongly adsorbed on active carbon through an anionic-exchange mechanism between Au(CN)2- anions and various anions initially present on the carbon surface as postulated in the literature. Further clarification is required however for the results of chromatographic investigations showing a partial release of anionic species (Cl- ...) which do not indicate clearly a 1-1 anionic exchange mechanism. Moreover the simultaneous increase of the pH value in the bulk phase following adsorption through an anionic exchange remains to be explained. According to some of the literature interpre-
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tations37 the variations of the pH values are probably due to the occurrence of an anionic exchange between Au(CN)2and interfacial OH- anions but may also be due to the adsorption of gold in the acidic form, HAu(CN)2. Because of the low ∆pH values compared to the amount of adsorption of aurocyanide species, these eventualities remain very debatable. By using the variation of pH during the adsorption of gold complexes, we could calculate an approximate amount of adsorption of hydrogen ions or the amount of desorption of hydroxide ions, and these values, very small compared to those obtained for aurocyanide adsorption, could explain neither the adsorption of gold as HAu(CN)2 nor an anionic exchange mechanism involving hydroxide ions as the exchanged anions. The starting point of our consideration is based on the site-dissociation-site-binding model for the potentialdetermining mechanism of oxides in water.54,55 These authors showed that the charge development on oxide surfaces in water could occur by direct proton transfer to the surface hydroxyl groups. If the electron density of the oxygen atoms in the M-OH groups is low, then the strength of the hydrogen bond formed with the polarized hydrogen atom will be reduced and ionization with a water molecule via the following dissociation process may occur
M - OH + H2O S M - O- + H3O+
(6)
If the electron density of the oxygen is high, then protons may become bound to the hydroxyl groups causing dissociation
M - OH + H2O S M - OH+ 2 + OH-
(7)
This model for oxides may be widened to all polar groups having atoms with high or low electron density. Therefore the values of the pHiep of the polar groups as traditionally determined in water may serve as a general guide to the ability of the surface to attract protons. In the case of active carbons, electrostatic charges or ionized polar groups located on the edges of graphene layers could provide positive active sites. Moreover, if some anions (or electrolytes) are present, they will be mainly adsorbed on the carbon surface sites and could be displaced following the adsorption process (anionic exchange mechanism). Because of the numerous mineral impurities present on the carbon surface, the water-carbon interface exhibits some exchangeable sites and the adsorption of aurocyanide anions may occur according the following mechanism
Cn+ ‚‚‚ X- + Au(CN)2-K+ S Cn+ ‚‚‚ Au(CN)2- + KX (6) where Cn+ is a positive active site on the carbon surface and X- is an anion initially adsorbed. Following the irreversible adsorption of Au(CN)2- on the carbon surface, some preadsorbed monovalent anions are exchanged in the Stern layer54,55 by similarly charged Au(CN)2- anions. Such an anionic-exchange mechanism does not strictly modify either the number of ions in the adsorbed layer or the global surface charge (σ0 + σi). It is therefore characterized by a constant electrophoretic mobility of the loaded solid particles in the suspension. Both the potassium counterions, which are not adsorbed, and the exchanged anions removed from the surface (54) Lyklema, J. Fundamentals of Interface and Colloı¨d Science; Academic Press: London, 1995; Vol. II. Davis, J. A.; James, R. O.; Leckie, J. O. J. Colloı¨d Interface Sci. 1978, 63, 480. (55) Hunter, R. J. Foundations of Colloı¨d Science; Vol.I, Clarendon Press: Oxford, 1991; Vol. I.
following the adsorption of aurocyanide anions allow the electroneutrality of the bulk phase to be satisfied. However, this 1-1 anionic-exchange mechanism cannot explain the increase of the pH unless OH- anions are partly exchanged as postulated in the literature.37 This would suppose either an exchange between Au(CN)2- anions and OH- anions initially adsorbed in the IHP54,55 or an exchange between Au(CN)2- anions and surface potential determining OH- anions. According to the triple-layer model, these eventualities remain very debatable and it appears more likely to us to explain the increase of the pH value in the bulk phase in terms of “surface charge regulation”.56,57 The adsorbed aurocyanide anions as well as the anions initially present on the carbon surface are retained essentially through electrostatic interactions. However, some differences will exist in the adsorption energy and in the state of hydration, whether on the surface or in the bulk phase, between all the ions taking part in the exchange.58,59 Thus, the occurrence of the anionic exchange implies that the aurocyanide anions have a higher capacity to neutralize the surface charge compared to the anions initially adsorbed on the carbon surface. Indeed, because of its larger size, it is probable that the aurocyanide complex is less hydrated and therefore will be adsorbed closer to the surface compared to the preadsorbed anions. Consequently, following the adsorption of Au(CN)2- the position of the σi plane will be closer to the surface plane. Moreover, if we take into account the adsorption energy, ∆Gads, which is given by the following eq60,61
∆Gads ) ∆Gelec + ∆Gspec
(7)
where ∆Gelec is the contribution of Coulombic interactions and ∆Gspec is the term related to the specific adsorption arising from all the other contribution to the free energy of adsorption, it is also probable that the later contribution (∆Gspec) will be stonger in the case when the Au(CN)2anion is adsorbed. All these factors lead to a stonger interaction between the aurocyanide anions and the charged surface sites, which, as a result, will be more strongly neutralized. In such a case, the surface charge, σ0, still remains unchanged following the anionic exchange but the term σi becomes more and more negative on account of the adsorption of Au(CN)2- anions nearer to the surface. As a result the term σ0 + σi becomes more and more negative following the adsorption. The surface potential, ψi, in turn decreases. The electroneutrality condition at the interface (σ0 + σi + σd ) 0) should generate a release of chloride and other ions, from the interface to the bulk solution, more important compared to the adsorption of Au(CN)2- at the surface. In that case it is obviously impossible for the bulk phase electroneutrality to be conserved. The system will therefore react by an adsorption of protons from the solution to the surface plane. The protons, being surface potential determining, will lead to an increase of σ0 which would thus become less negative. Thus, the excess of negative charge at the σi-plane, due to the adsorption of Au(CN)2- anions, is counteracted on (56) Zajac, J.; Trompette, J. L.; Partyka, S. Langmuir 1996, 12, 1357. (57) Goloub, P. T.; Koopal, L. K.; Bijsterbosch, B. H. Langmuir 1996, 12, 3188. (58) Kihira, H.; Matijevic, E. Langmuir 1992, 8, 2855. (59) Halliwel, R.; Nyburg, J. Trans. Farad. Soc. 1963, 59, 1126. (60) de Keizer, A.; Lyklema, J. J. Colloı¨d Interface Sci. 1980, 75 (1), 171. (61) Rendall, H. M.; Hough, D. B. Adsorption from solution at solid/ liquid interface; Parfitt, G. D., Rochester, C. H. Eds.; Academic Press: New York, 1983.
Adsorption of Cyanide Gold Complexes
one hand by a release of chloride and other ions and, on the other hand, by the adsorption of protons on the surfaceplane (σ0) in order to decrease the negative charge at the σd plane and to finally obtain σ0 + σi ) σd. Consequently, in the bulk phase, the excess of positive charge, due to the potassium counterion which does not adsorb, will be neutralized by both OH- ions generated following the adsorption of protons and the exchanged chloride ions released from the surface to the solution. This indirect anionic exchange mechanism, which involves a surface charge regulation, can explain all the experimental results. The balance sheet of such a mechanism is a partial exchange between Au(CN)2- anions and anions initially adsorbed on the carbon surface with an increase of the pH value of the suspension. Indeed, the chromatographic analysis showed that a small and constant quantity of ions, some of which were chloride ions (t ˜ 0.8 s), were exchanged but do not indicate a 1-1 anionic-exchange mechanism with these ions. Moreover, this small amount of exchanged ions shows that the main mechanism is the surface charge regulation, which explains the continuous increase of the pH in the whole range of irreversible adsorption of gold complexes. Both the OH- ions coming from the adsorption of protons and the ions removed from the surface following the exchange account for the increase of the conductivity in the bulk phase. At last, this model of adsorption mechanism implies that σ0 + σi remains constant in the whole range of adsorption accounting for a constant value of electrophoretic mobility. Partly Reversible Adsorption. For Ce > Cec, or the corresponding Γ > Γc, additional reversible adsorption takes place. The reversible contribution to the adsorption of the aurocyanide species is associated not only with the adsorption of potassium counterion but also with constant values of pH and conductivity of the bulk phase. For higher equilibrium concentrations, the adsorption of potassium gradually increases together with the reversible contribution and the electrophoretic mobility still remains constant. This is consistent with the physisorption of gold certainly as neutral molecular species, KAu(CN)2 occurring in the high concentration range. In this second part of the isotherm, neither the additionnal KAu(CN)2 molecules physisorbed onto the neutral part of the carbon surface (uncharged) do modify the total surface charge. Such a physical adsorption is always completely reversible if it is not restrained by the porous structure of the solid substrate. However, according to Figure 3 we can note that the ratio of gold to potassium (ΓAu(CN)2-/ΓK) gradually decreases reaching a limiting value of approximately 2, which is not representative of a particular adsorbed gold species. Nevertheless, a more detailed representation of the experimental adsorption isotherms allowed this point to be elucidated. The basic idea of this new and meaningful graphical representation40 was to consider the total experimental adsorption isotherm, exhibiting a fully irreversible initial part and a partly reversible final one from a given equilibrium concentration, as the sum of two distinct adsorption isotherms, each of them being representative of a particular contribution: one fully irreversible, characteristic of the specific adsorption, and the other fully reversible related to physisorption. Therefore, it appears reasonable to consider that the amounts of adsorption of aurocyanide anions remaining on the solid surface after the dilution process accounts for the irreversible part of the isotherm while the loaded gold complexes which can be removed by dilution relates to the reversible part. Therefore the reversible adsorption is obtained by substracting the aurocyanide adsorbed amount remaining on the carbon surface consecutively to the dilution process (irreversible adsorption) from the total
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Figure 9. Overall (open squares), irreversible (open circles), and reversible (open triangle) adsorption isotherms of Au(CN)2anions adsorbed onto activated carbon G210 from deionized water at pH ) 6.0 and T ) 25 °C together with the reversible adsorption isotherm of potassium counterion (crosses) performed in the same conditions.
amount of adsorption (overall adsorption isotherm). As to the amount of adsorption accounting for the irreversible isotherm, it is that obtained after desorption. In Figure 9, we have plotted both reversible and irreversible amounts of adsorption of aurocyanide anions together with the desorbed amount of potassium (reversible adsorption) against the equilibrium concentration of Au(CN)2-. Thus, we obtained two adsorption isotherms, the sum of which agrees with the experimental adsorption curve. With such a representation, the relative amounts of adsorption of gold and potassium are of great interest. Indeed, for the diluted solutions or low coverage ratio, we note that only the fully irreversible adsorption of Au(CN)2-is occurring and that potassium is not adsorbed at all. Then, from a particular value of the equilibrium concentration, Cec, the fully irreversible adsorption of Au(CN)2- anions tends to reach a plateau value while the additional reversible aurocyanide adsorption begins to take place and thereafter still increases. The adsorption of potassium counterion proceeds similarly. Moreover, the amounts of adsorption observed for potassium are as high as those obtained for gold in the same reversible adsorption isotherm resulting in two roughly identical adsorption isotherms. This clearly indicates that the loadings of gold and potassium, observed in the reversible isotherm, are very close to the stoichiometric values required for the physisorption of the neutral molecular species, KAu(CN)2, without chemical change and without variation of pH and conductivity. As for aurocyanide anions, the adsorption of potassium was found to be fully reversible in this range of concentrated solutions. In this second step, it is probable that gold is physically adsorbed as neutral molecular species involving weak interactions such as dispersion forces (London-van der Waals forces) and that this physisorption occurs most probably on less energetic or reactive parts of the carbon surface. Such physisorption is also fully consistent with the invariance of the electrophoretic mobility as is observed for the coverage ratios above Γc. The occurrence of these two successive or overlapping mechanisms of adsorption has also been experimentally proved using a flow microcalorimetry method which consists in determining not only the temperature changes on an adsorbent bed due to adsorption/desorption phenomena but also the corresponding adsorption and desorption quantities.40 Adsorption/desorption cycles have been repeated several times, which tests the reversibility of the system. The most important step in this respect is the first adsorption/desorption cycle. Repetition of the cycle revealed a partly reversibility in the interactions
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and gave the proportion of irreversible adsorption in the total adsorption measured in the first adsorption step. We postpone the discussion of these adsorption/desorption cycles to a later publication. Conclusion The experimental data show two different adsorption regimes for potassium aurocyanide complexes from deionized water onto activated carbon. They support an anionic exchange mechanism at low equilibrium concentration characterized by a fully irreversible adsorption of Au(CN)2anions and the physisorption of neutral molecular species, KAu(CN)2, at higher values. The first step has been already postulated many times. However, a new aspect of the irreversible process which has not been deeply explained before is suggested. The anion exchange between Au(CN)2- and some various anions initially adsorbed on the activated carbon surface also involve a surface charge regulation; some protons
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also adsorb together with the aurocyanide anions in order to regulate the surface charge following the anion exchange. This first mechanism is characterized by a continuous increase of the pH and a constant electrophoretic mobility of the carbon particles in the whole range of irreversible adsorption of gold complexes without the potassium counterion. For the concentrated solutions, from a given aurocyanide coverage ratio, the adsorption of gold complexes onto active carbon becomes partly reversible. By separating the experimental adsorption isotherm into two different isotherms, representative for the irreversible and reversible contributions, 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. Acknowledgment. This work has been supported by the European research foundation EUREKA/TEPREM. LA960977A
