Carbon Vol. 35, No. 9, pp. 1329-1337, 1997 Copyright 0 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0008-6223/97 $17.00 + 0.00
Pergamon PII: 80008-6223(97)00094-8
IRREVERSIBLE AND REVERSIBLE ADSORPTION OF SOME HEAVY TRANSITION METALS ON GRAPHITIC CARBONS FROM DILUTE AQUEOUS SOLUTIONS A. J. Microscal
Ltd., 79 Southern
WI0 5AL, U.K.
(Received 5 February 1997; accepted in revised form 24 April 1997) Abstract-The adsorption of various heavy transition metal compounds was studied using two different flow adsorption methodologies: flow injection adsorption and flow equilibrium adsorption analyses. In both cases the determinations were made of the heats of adsorption and the amounts of adsorption from dilute aqueous solutions. The carbons used for the work included two different types of graphitised carbon black and active carbons prepared from coconut shells and coal. The work shows that in every case a significant proportion of the solutes was adsorbed irreversibly. At the same time reversible adsorption of the solute also took place. The two graphitised carbons differed in respect of the extent of the irreversible adsorption of Hg(NO,), and KPtCl,, which appeared to be related to small differences in the amounts of polar sites on the surface of these carbons. There were also indications of the decomposition of the adsorbates
soon after saturation of the surfaces took place. These effects were much more pronounced for the adsorptions
on active carbons which contained from 2-5% of their total surfaces in the form of hydrophilic sites. Generally the heats of irreversible adsorptions were very high, even on graphitised carbon blacks, approaching -200 kJ mol 1 at surface coverages below 1% of their BET N, surface. 0 1997 Elsevier Science Ltd
Key Words-A. adsorption.
Carbon black, A. activated carbon, C. adsorption,
1. INTRODUCTION It has been known that active carbons can strongly adsorb transition metal compounds from aqueous solutions [ 1,2]. The adsorption has been strikingly illustrated by the determination of the heats of adsorption of various transition metals on graphitised carbon blacks, active carbons and graphites. Typical results of these studies are illustrated by the heats of adsorption of several metal nitrates and chlorides on Graphon shown in Fig. 1. The heaviest metals produced the highest heats of adsorptions, the interactions being especially strong for gold and mercury compounds. However, the strong adsorption was confined to a small part of the total surface in graphitized carbons suggesting that only a small part of the basal plane sites in such carbons is capable of forming a strong bond with the metal compounds. Furthermore, the adsorption of the metals was drastically reduced in the presence of n-butanol in the aqueous solutions which indicated that the blocking of the basal plane sites by a monolayer of n-butanol prevents the adsorption of the metals, i.e. the adsorption must be associated with the basal planes and not exclusively a small proportion of the polar sites present in graphitised carbon blacks such as Graphon. The present paper describes some of the more extensive work carried out recently by the author on the adsorption of potassium gold aurocyanide, silver and mercury nitrates, and potassium 1329
D. functional groups, D. heat of
chloroplatinate on graphites and active carbons to obtain more detailed information on the mechanism of adsorption of these heavy transition metals on the graphitic adsorbents. The work was carried out by using two methodologies: flow injection adsorption thermodynamics (FIAT) and flow equilibrium adsorption thermodynamics (FEAT). The first method involved injections of small quantities of the metal solutions flowing through a bed of an adsor-
Fig. 1. Injection of 0.2 [lrnol quantities of metal salts into water percolating through 70 mg of Graphon.
bent, followed by determinations of the differential heats of adsorption and the associated amounts of adsorption on the adsorbent. In the FEAT method adsorbents were saturated with a solution of the metal compounds and determinations made of the resulting integral heats and amounts of adsorption. The saturation is carried out in two or more adsorption/desorption cycles of sufficiently long duration to achieve adsorption equilibrium. At that point the heats and amounts of adsorption and desorption are equal. All the work was carried out with the Microscal Mark 4 Flow Adsorption Microcalorimeter (FAMC) with the use of downstream detectors for monitoring concentration changes in the solutions passing through the adsorbent beds to the microcalorimeter.
2.1 Apparatus The Microscal Mark 4 FAMC used in the work was recently described . In its latest version the flow adsorption system involves the use of Harvard 22 syringe pumps capable of pumping liquids through the adsorbent beds against back-pressures up to 5 barg. High back-pressures can be produced during percolation of water and aqueous solutions through carbon adsorbent beds and the pumps must be capable of producing uniform flows of liquids under these conditions. In addition, operation of down stream detectors used in the FAMC system is impeded by the presence of air/vapour bubbles that may be formed in the adsorbent bed during the adsorption processes (displacement of the strongly adsorbed gases from the micropores). In such a case backpressures are increased by pressure regulators until the detector produces a smooth base line. Generally a back-pressure of 1 barg is sufficient for this purpose, providing the adsorbent was evacuated before its wetting by the carrier liquid and the flow system is sufficiently tightly sealed to prevent any ingress of air into the adsorbent bed or the detector. Switch over from the carrier liquid supplied by pump 1 to a solution supplied by pump 2 must be carried out under conditions of equal backpressure against which the liquids are supplied by both pumps. To ensure that the pressures are sufficiently close, they have to be independently monitored (Microscal’s FAMC system uses two Druck pressure gauges) and back-pressure regulators are used to reduce the pressure differences to less than 0.05 barg. A diagram representing the system is shown in Fig. 2. The exchange of solvents can be carried out automatically by specially designed automatic sequence controllers and the adsorption and heat data stored, processed and analysed with the aid of Microscal software.
solvents and reagents
The properties of the adsorbents used in this work are given in Table I, The solvents and reagents used were all Aldrich materials with purities exceeding 99%
3. RESULTS AND DISCUSSION
3.1 Adsorption of silver and mercury nitrates on active carbon BPL A series of results obtained using the flow injection method for the adsorption of silver nitrate on active carbon BPL are shown in Fig. 3. The experiment consisted of sequential injections of 20 ~1 aliquots of a 0.01M solution of AgNO, into water percolating through a 0.17 cmm3 bed of the carbon adsorbent at the rate of 3 ml h-‘. The bed was composed of only 5.9 mg of the carbon made up to the volume of the cell (0.17 cm ^“) with PTFE powder. This was done to reduce the time of the adsorptions and to obtain conditions under which a high degree of surface coverage could be achieved by a small number of the injections. For the five injections shown in Fig. 3. the total amount of AgNO, retained irreversibly by carbon was 0.62 pmol or 105 pmol per gram. This constituted 7 1% of the total amount of the irreversible adsorption determined separately by the flow equilibrium method which was 148 jlrnol gg’ (see Table 3). It is evident that as the injection number increases, less and less silver is irreversibly adsorbed, and, at the same time, increasing amounts are reversibly adsorbed. as shown by the negative heat effects following the positive heats of adsorption recorded for each adsorption. Silver nitrate contacting the surface during the first injection is almost completely adsorbed, indicating that it is adsorbed on the most active sites present in the carbon. The time of contact between the solution and the carbon during the percolation is only 3 minutes and it is remarkable that so much of the silver compound can be adsorbed in this short time. The FIAT method, therefore, discriminates between the “strong” active sites and the weaker ones. the highest rates of adsorption being obtained on the most active sites. A plot of the amount of adsorption against the number of injections for silver and mercury nitrates is shown in Fig. 4. The amounts of mercury nitrate adsorbed in the successive injections are similar to those obtained for silver nitrate, but the heats of adsorption are much higher. Combining the heat and adsorption data the molar heat of adsorption for a total adsorption of 105 btmol g-’ is - 85.7 kJ mol -I, whereas the heat of adsorption of mercuric nitrate (a in 5 injections) is total of 97 Llrnol gg’ adsorbed - 138 kJ mol -I. The difference is illustrated by the plots in Fig. 4, confirming that the irreversible adsorption of Hg(NO,), is much more energetic than that of AgNO,.
Adsorption of some heavy transition metals on carbons
Flow Microcalorimeter System
Autommtic S9quone. Contr0ll.r
Block Heater & T*mpar*tur* Monitor
Fig. 2. Microscal flow microcalorimeter.
The adsorption of silver nitrate was also investigated by the FEAT method which involved 3 percolation cycles of its 5 millimolar solution through 7.3 mg of carbon BPL lasting 2 hours each. The heat evolution produced by the displacement of water by the silver salt stopped between 60-90 minutes of percolation and a similar indication was obtained from the record of concentration of nitrate ions emerging from the calorimetric cell as determined by an on-line UV spectrometer. The amounts of adsorption, desorption and the corresponding heat changes are given in Table 2. Nearly complete reversibility was reached in
the third cycle with the heats and amounts of adsorption becoming closely similar. It is clear that, as already indicated by the injection results, some of the silver nitrate is adsorbed irreversibly, but a major part of the total adsorption of 405 pm01 g-r was adsorbed reversibly. The irreversible adsorption is estimated from the difference between the first and second adsorption cycles and amounts to 176 pmol g -l, i.e. constitutes some 44% of the total adsorption. Clearly the adsorption of silver nitrate on the active carbon occurs on two different parts of the carbon surface, one of which
1332 Table 1, Properties
BET N, SA (m2 g_‘)
Polar surface” (mZg_‘)
Accessibleb basal plane surface (m2 g- ‘)
85 70 1000
0.25 0.14 54
85 70 83
Graphon V3G Coal-active Carbon (BPL) Coconut-active Carbon
a Determined from the heat of adsorption from heptane . b Determined from the heat of adsorption contane from n-butanol heptane .
No of Injections interacts
To investigate sites
of the more
of silver and mercury carbon BPL at 2O’C.
Table 2. Adsorption
of silver nitrate on active carbon (FEAT methodology)
O.lM nitric acid, O.lM NH,OH and in the presence of O.OlM nitric acid. The results of these experiments are given in Table 3. As can be seen, the treatment with nitric acid substantially reduced the amount of the irreversible adsorption and for the adsorption in the presence of O.OlM HNO,, the irreversible adsorption completely disappears and reversible adsorption is reduced to 40% of the original adsorption. On the other hand, the treatment of the carbon with NH,OH had little effect on the irreversible adsorption and the total adsorption increased only slightly after carried
Fig. 4. Adsorption surface
Adsorption,/ desorption (pmot g r)
Percolation sequence Silver solution Water Silver solution Water Silver solution Water
1 2 3
403 -227 255 -216 227 206
Heats of adsorption/ desorption
(Jg-‘J 14.6 -6.5 6.8 -6.5 6.3 -5.2
IO mJ tIeat of .\dcorption.
.\muunt 0. I7
1 E:xp No Molar Fig. 3. Adsorption
lleats of Adsorption
of silver nitrate
; 143 -+ 101 kJ/mol
from O.OlM solution
in water on active carbon
Adsorption Table 3. Adsorption
of some heavy transition
of silver nitrate from water carbon chemivron BPL Adsorption
AgNO, AgNO, + HNO”, AgNO,
Nil HNO, HNO, NHO‘,OH, a AgNO,
(pmol g- I)
the treatment. The results taken together strongly suggest that the active sites responsible for the strong adsorption of silver nitrate are basic in nature and capable of interacting with silver ions by an ion exchange mechanism. The equilibrium adsorption method was also used for the determination of the adsorption of mercuric nitrate on graphitised carbons and active carbon BPL. The adsorptions were determined from 1 millimolar solutions. The amounts of adsorption and the heats of adsorption are given in Table 4. The total amount of adsorption of Hg(NO,), on carbon BPL is smaller than that for AgNO, which was adsorbed from a 5 mmol solution and 76% of this amount is adsorbed irreversibly. The irreversible adsorptions are, therefore, very similar for both metals, but the reversible adsorption is much higher One of the graphitised carbons, for AgNO,. Graphon, adsorbs approximately the same amount of Hg(NO& per unit BET surface area as active carbon BPL, but another graphitised carbon V3G adsorbs much less. The molar heat of adsorption for V3G is, however, very high compared with that for Graphon (-207 and - 124 kJ mol -i, respectively). The adsorption difference in the heats of adsorption between these adsorbents seems to be in line with the areas of their polar sites which are three times as high for Graphon as they are for V3G, but it is out of step with the molar heat for carbon BPL which contains more than 100 times the area of the polar sites present in the graphitised carbons, but gives a molar heat of adsorption of - 153 kJ mol -I. It appears therefore that the active surface sites in V3G have special features responsible for their high affinity for Hg(NO,),. It is also evident that the oxidation of V3G does not increase the heat of adsorption of
Adsorbent Graphon V3G V3G (oxidised) AC BPL
of Hg(N03)2 Adsorption (pmol g-‘) 20.3 8.2 25 176
in O.OlM of HNO,.
Table 4. Adsorption
metals on carbons
A typical series of differential heats of adsorption determined by repeated injections of 20 ~1 aliquots of O.OlM K,PtCl, solutions into water percolating through a bed of 25.8 mg Graphon mixed with inert PTFE power, is shown in Fig. 5. The differential heats of adsorption were determined in conjunction with the amounts of the solute which was irreversibly adsorbed. A similar series of experiments with the active carbon are shown in Fig. 6. As can be seen in Fig. 5, graphitized carbon black gives much higher heats of differential adsorption at low surface coverages than the active carbon. Since the graphitized carbon black is virtually free from polar functional groups, it is concluded that the exceptionally strong adsorption of K,PtCl, on this carbon must take place at least partly on the graphitic basal plane sites. Some of these sites clearly show much higher affinity for the PtCl,’ ion than the majority of the basal plane surface, but the high affinity cannot be only connected with the presence of the polar oxygen containing sites. Assuming that each adsorbed molecule of K,PtClg occupies 40-50 A2 on the basal plane surface, the proportion of that surface exhibiting strong affinity for the PtCl, ion in Graphon is about 5%. This is much more than the proportion of the polar surface sites in Graphon which is less than 1%. The high heats of adsorption on the graphitized carbon suggest that the strongly adsorbed platinum ion will lead to formation of a stable catalyst, which confirms the work of A. Linares-So!sno ri ui. reporting that the catalytic activity of Pt is not only a function of the amount of O-containing groups in a carbon support , but some other factors which have not been clearly identified. 3.3 Adsorption ofpotassium aurocyanide The adsorption of the complex gold cyanides is an important industrial application of active carbon adsorbents and has been extensively investigated in the U.S.A., South Africa and Australia. There is still no agreement on the exact mechanism of the gold cyanide adsorption from aqueous solutions . One school of thought proposes the adsorption of potassium aurocyanide on the graphitic planes as the driving mechanism . Another view represented by
I mmol solutions
Heat of adsorption
(J 8’) 2.5 1.7 1.8 26.9
heat of (kJ mol
123 207 72 153
1334 Flow rate of
water - 3 ml/b ; Temperature - 20’ C
aeat of Adsorpuon
Fig. 5. Adsorption
of O.OlM solution
of K,PtCI, following injection of 20 ~II aliquots into water percolating of Graphon mixed with 80 mg of PTFE powder.
Flow r8te of w8ter - 3 ml/b ; Temperrture - 20° C
He8t of Adsorption
Solute rem8ining in solution after hljecnon of 200 Md of K$tCl, Fig. 6. Adsorption
of O.OlM solution of K,PtC16 of active carbon
] 70 mnol
following injection of 20 /cl aliquots into water percolatmg BPL mixed with 80 mg of PTFE powder.
M. Adams and C. Fleming  is that the aurocyanide ion adsorbs predominantly on polar sites in carbon adsorbents which are invariably produced during the formation of active carbons by oxidation and activation of various carbonaceous materials. An illustration of the FIAT technique applied to the adsorption of gold compounds is provided by the results shown in Fig. 7 giving the heats of adsorption of KAu(CN), on a coconut active carbon, together with the corresponding amounts of adsorption. As in the case of experiments with AgN03 only about 10 mg of the carbon was placed in the adsorbent bed and was mixed with 85 mg of PTFE powder to fill the calorimetric cell. With the flow rate of water through the adsorbent bed of 3 ml h-l the contact between the gold solution and the adsorbent
was limited to about 3 minutes so that the adsorption had to be quite fast to extract all the gold aurocyanide from solution during that short time. As can be seen, the coconut carbon was quite effective in irreversibly adsorbing a high proportion of the injected gold compound with the production of gradually decreasing heat effects. A similar series of experiments was produced with a coal derived active carbon. The amounts and heats of adsorption for both carbons are shown in Fig. 8. The coconut carbon proved to be slightly more effective in the removal of the gold solute from solution, but gave much lower heats of adsorption. The latter results indicate that the affinity of gold cyanide for the coal derived carbon is higher than that for the coconut carbon. but not the kinetics of adsorption. That indicates somewhat
Adsorption of some heavy transition metals on carbons
Heat effect produced
KAu(CN), in effluent
Fig. 7. Adsorption
of KAu(CN), coconut
from 10 mmol aliquots (20 ul) injected into water percolating through carbon mixed with 85 mg of PTFE, at a flow rate of 50 ~1 min-‘.
a bed of 10.2 mg of
Fig. 9. Adsorption/desorption cycles of KAu(CN nut carbon.
)z on coco-
No of Iajoctions Fig. 8. Irreversible
adsorption of KAu(CN), bons from water at 20°C.
on active car-
higher accessibility of the coconut carbon to the gold solute. Cyclic FEAT experiments carried out on the two carbons demonstrate strong adsorption of the solutes on both carbons with a substantial degree of irreversibility. The results for two adsorption/desorption cycles on the coconut carbon are shown in Fig. 9. As can by seen, the adsorption at room temperature falls into two categories: 1. irreversible adsorption defined as the difference in the adsorption between the first cycle and on subsequent cycles in which the adsorption becomes fully reversible, and 2. reversible adsorption which remains constant over a large number of cycles after full adsorption equilibrium is attained.
The total adsorption of KAu(CN), in the first cycle is 616 pmol g-’ out of which 178 prnol g-’ or, 29% of the total is adsorbed irreversibly. The first two cycles were not sufficiently long to achieve complete saturation of the adsorbent, as is indicated by the adsorption peaks with long tails. Desorption appears, however, complete after this period of time. Clearly the gold solute penetrates very slowly into some of the more inaccessible pores and the process is not completed in the two 90 minute contacts with the 5 mmol solution of gold aurocyanide used in the adsorption cycles. The heats of adsorption follow similar trends to the adsorptions confirming their reversible and irreversible proportions. The shapes of the peaks represent the kinetics of the adsorption process and differential quantities can be obtained from them. Most of the adsorption occurs however in the first 30 minutes of the contact and it appears, therefore, that a relatively short contact time should be sufficient to achieve a substantial degree of extrac-
Fig. 10. Adsorption/desorption
Coconut carbon Coal carbon Graphon
- 3 ml/b, Temperature
- 25 ’ C
cycles of potassium aurocyanide from its 5 millimolar solution in water on 62 mg of graphitized carbon black.
Table 5. Irreversible adsorption of KAuCN, evaluated from 2 adsorption/desorption cycles on active carbons from 5 mm01 aqueous solutions at 20°C Carbon
Adsorption (pmol g ‘) 178 396 5
tion of potassium aurocyanide from aqueous solutions. Three adsorption/desorption cycles on a graphitised carbon black (Graphon) are shown in Fig. 10. In this case the adsorption is mainly reversible after the first cycle. The FEAT adsorption experiments have been carried out on a number of different adsorbents and the irreversible and reversible parts of adsorption obtained from two adsorption/desorption cycles. The results for some of these adsorbents are listed in Table 5. There are considerable variations between the various carbons in their total gold adsorption capacities, but more importantly, in their ability to irreversibly adsorb gold aurocyanide from solution. It is interesting that the proportion of the strongly adsorbed gold cyanide does not depend on the presence or absence of micropores. The graphitised carbons produced a similar proportion of the strong adsorption as the microporous carbons. There is also no correlation with the accessible graphite basal planes in the adsorbents given in Table 1 and the proportions of polar sites in the surface of the carbons. However, graphitised carbon (Graphon) gives the lowest total adsorption, which suggests that graphitic basal planes by themselves are not sufficient to produce strong adsorption of gold cyanide. For all the graphitic
carbons investigated in this work the total amounts of adsorbed KAu(CN), covered less than 5% of the total available surface, irrespective of whether the adsorbents were microporous or not. It appears, therefore, that only a small proportion of basal plane sites are active in the adsorption process and that this activity may only be achieved by graphitic ring systems combined with activating elements such as oxygen, nitrogen or other electron donating groups.
4. CONCLUSIONS The adsorption of several heavy transition metals from water on graphitic carbons is invariably partly irreversible at room temperature. The adsorption of the transition compounds occurs only on a small percentage of the total surface available in carbon adsorbents irrespective of whether the carbons are microporous or not. Most of the graphitic basal plane surface in carbon adsorbents is inactive for the adsorption of potassium aurocyanide. This applies to irreversible and reversible adsorption. The adsorption methodologies based on flow injection analysis (FIAT) and cyclic flow equilibrium methods (FEAT) can provide useful characterisation of adsorbents in respect of their total metal adsorption capacity, capacity for strong irreversible and the adsorption and desorption adsorption, kinetics.
REFERENCES 1. Groszek, A. J., Procredings OS Curhm 92 Chnfercwcc, Essen, June 22-24, 1992, pp. 278-280 2. Groszek, A. J., Procrrdings of Twenrierh Bienniul CO~ILVetm on Crrrhon, Univ. California, Santa Barbara, 1991, pp. 56657.
of some heavy transition
3. Groszek, A. J. and Partyka, S., Langmuir, 1993, 9, 2721-2725. 4. Roman-Martinez, M. C., Cazorla Amoros, D., LinaresSolano, A., Salinas Martinez de Lecca, C., Yomashita, H. and Anpo, M., Carbon, 1995, 13, 3-13.
metals on carbons
5. Fleming, C., Hydrometallurgy, 1992, 30, 127-162. 6. Jones, W. and Linge, H., Hyrometallurgy, 1989, 22, 231-238. 7. Adams, M. and Fleming, C., Metall. Trans. B, 1989, 20B, 315-325.