Carbon. 1975, Vol. 13. pp 437-W.
Printed in Great Bntain
IODINE ADSORPTION AND STRUCTURE OF ACTIVATED CARBONS A. J. JUHOLA MSA Research Corporation, Evans City, PA 16033,U.S.A. (Received 31 March 1975) Abstract-Studies were conducted on the adsorption of iodine from saturated aqueous solutions and from saturated vapor by eight activated carbons of greatly diverse pore structures. Water adsorption data were used to determine the pore size distribution curves which provided both the distributions of the pore constriction (desorption) and cavity (adsorption) diameters. Adsorption from aqueous phase formed a unimolecular layer on the carbon surface while adsorption from saturated vapor produced pore-filling of micropores (pores less than 30 A diameter) and surface coverage of the macropores. A great deal of steric interfedence was present because of the small difference in the diameter of iodine molecule (4.94A) relative to the 10-30A diameter pores. Good correlation was attained between adsorption and pore structure when corrections were made for the steric effect and the mean diameter distributions of the constrictions and cavities were used. The model for the iodine-on-carbon adsorption resembles packing of spheres into cylinders. 1. INTRODUCTION
that both distribution curves were required to attain a good correlation with iodine adsorption. Prior to all structure and density measurements, the carbons were treated to remove water soluble ash and hydrophilic ash which would otherwise introduce errors in the pore diameter calculations based on the water
There are two important applications of iodine-on-carbon adsorption process that warrant study of the exact role played by the pore size distribution on these processes. First quality control tests to evaluate carbons for their effectiveness in aqueous phase applications, and second
nuclear power plant air cleaning applications. In the latter, interest is in two areas, (1) KI-I2 impregnants in carbon for radioactive methyl iodide exchange reactions and (2) vapor phase adsorption of elemental iodine. Very little prior work appears to have been done on correlation of pore size distribution with iodine adsorption. Studies were done at Pittsburgh Activated Carbon Company, Grant[l], which showed that the surface area of pores larger than 10 A diameter correlated with the iodine number as determined by the aqueous phase carbon evaluation tests. It is now realized that the iodine adsorption processes are considerably more complex. In this study, the relationship between the pore size distribution and aqueous phase iodine adsorption was reinvestigated. To complete the study the vapor phase iodine adsorption was investigated to considerable depth.
2.1 Activated carbon pore structure The procedures used for determining the pore size distributions were those developed by Juhola and Wiig which are in part based on calculating the pore diameters from water adsorption data using the Kelvin equation. In these structure determinations, data of both the desorption and adsorption branches of the water adsorption isotherm were used. A value of cos 0 = 0.47, as substituted into the Kelvin equation, was used in calculations involving both branches of the hysteresis loop. This is a departure from previous usage of the method where only data of the desorption branch has been used. Diameters calculated from the adsorption branch data are those of pore cavities and those from the desorption branch data are those of pore constrictions. In this study it was found
adsorption isotherms. The treatment involved leaching the carbon with 2% HCl, water, 2% Na2C0, and water, in the sequence given. In the carbons studied, the amount of ash removed would not have a significant effect on the pore volume, the largest increase would be 1.5%. The pore size distribution curves of eight carbons of greatly diverse pore structure are presented in Figs. 1, 2, and 3. The important part of each curve is that below 1000A pore diameter since essentially all of the adsorption occurs in these pores. This portion of the curve is also that determined from water adsorption data. Carbons 1 and 3 are bituminous coal base manufactured by Pittsburgh Activated Carbon Co.; 2 is anthracite coal base manufactured by Thomas Ness, Ltd.; 4 is manufactured by Pittsburgh Activated Carbon Co.; 5 by North American Carbon Co.; 6 is a special superactivated coconut carbon prepared in the MSA Research Laboratory; and 7, a lignite base, manufactured by Darco Division of Atlas Powder Co. Carbon 8 is manufactured by Sutcliffe Speakman Co. 2.2 Vapor phase adsorption Exactly 1,000g of each of the eight carbons was weighed into a weighing bottle and placed in a desiccator. A watch glass with iodine granules on it was placed in the center of the desiccator and the carbon bottles placed at equal distance around the watch glass. Periodically the weighing bottles were weighed to determine when the iodine adsorptive capacity had been reached. Weight gain was approximately 1 mg/hr for each carbon over most of the adsorption time, but when saturation was approached, the adsorption rate decreased abruptly. When it was 0.04mglhr the carbon was assumed to have reached its saturation capacity.
Table 1. Adsorptive capacity of carbons for iodine at near 22°Cin saturated iodine vapor at near 0.27 mm Hgvapor pressure
Pore diameter, A Fig. 1. Pore size distribution curves for coal base activated carbons: ---, cavity diameters calculated from adsorption side of water isotherm; -, constriction diameters calculated from desorption side of water isotherm.
Fig. 2. Pore size distribution curves carbons: water
A coconut from
Fig. 3. Pore size distribution curves for activated carbons: ---, cavity diameters calculated from adsorption side of water isotherm; -, constriction diameters calculated from desorption side of water isotherm.
Table 1 gives the saturation capacity in weights and volumes. For the latter the normal solid density of 4.93 g/cm’ was used for the conversion. 2.3 Aqueous phase adsorption Several variations of a basic procedure were used to measure iodine adsorption from aqueous solutions.
Common to all procedures, weighed quantities of each carbon, ground to powder, were shaken with measured quantities of KI-I2 solutions in stoppered flasks. After filtering off the carbon, the filtrate was titrated with standard Na&O, solution. Change in total IZ content divided by the carbon weight gave the amount adsorbed at each filtrate concentration. In the commonly used iodine number test, standardized procedures are followed consisting of the following steps: The carbon is ground to pass 95% through a 325-mesh U.S. standard sieve and the powdered carbon then dried 3 hr at 150°C in an air convection oven. A weighed quantity of the carbon, usually near 1.0 g, is placed into a glass stoppered Erlenmeyer-type flask and 10cm3 of 5% HCl acid is added to the carbon. The mixture is brought to a boil and allowed to boil 30 set and then cooled to room temperature. One hundred ml of 0.1 N IZ solution, made up of 12.69g I2 and 19.1g KI per liter, is pipetted to the acidified carbon and the mixture shaken for 30 sec. The iodine-carbon mixture is filtered through a fluted filter paper. The first 15ml of filtrate are discarded and of the remaining filtrate 50 ml are titrated with 0.1 N NazSzOl solution. The iodine number is defined as the amount of iodine adsorbed in mg/g when the equilibrium concentration is 0.02 N total iodine, hence the carbon weight must be selected to bring the final concentration near 0.02 N. In this study the interest was in determining the adsorption isotherm over the equilibrium concentration range from below 0.02 N to as close to 0.1 N as possible. The above procedure gave consistent results up to about 0.07 N equilibrium concentration, but at concentrations approaching 0.1 N large deviations in the amount adsorbed appeared which were obviously in error. This error was minimized by performing a blank test; i.e. without carbon, and equating the difference in Na2S20J titer between the blank and carbon test to the amount adsorbed. Apparently a small amount of iodine reacts with the filter paper which becomes a major source of error at concentrations near 0.1 N. In a study of iodine adsorption on carbon, Hill and Marsh found evidence that the KL formed by the reaction of KI and IZionizes by a small amount and that only the free IZ is adsorbed. The reaction and ionization constant are as follows: Klz=KItIz
Iodine adsorption and structure of activated carbons For any equilibrium condition the Concentrations components are then:
[KU UKIli - [KI~lel[U~l,~ - [KM,] = *”
where [KU, - [KM, = [KU,, U~ltr - [KLI, = [Izlc [KI], = initial concentration as weighed into solution [I& = total iodine at equilibrium as determined by titration. Whether KI,, Iz, or KI, or all three are adsorbed is important to the interpretation of the results of this study. To check the findings of Hill and Marsh, tests were conducted by the above procedure with iodine solutions of 0.1,0.2,0.3, and 0.4 N. When the amount adsorbed was plotted against total iodine [Izt KI,] in solution at equilibrium, a series of curves was obtained as shown in Fig. 4. By means of eqn (3), the results of Fig. 4 were replotted against the free iodine in solution at equilibrium. The adsorption points now fell on a common curve as shown in Fig. 5. This verifies the finding of Hill and Marsh and shows that neither KI, nor KI are adsorbed. The solubility of iodine in pure water is 0.030 g/l00 g at 25°C or 1.28 mmol/l. In a 0.1 N iodine solution containing 19.1 g KI/I the free iodine concentration is 0.84 mmol/l. In order to determine the effect of increased initial free iodine concentration. some tests were made with 0.1 N
solution containing 17.5g KI per liter, which increased the free iodine concentration to l-00 mmol/l. In these tests the solution volume was also increased to 500 ml and HCl acid treatment was eliminated since the carbons had already been acid treated. Figures 6, 7, and 8 present the adsorption isotherms determined by the various procedures described. Where there was overlapping of experimental points by the various procedures used, the agreement was good. It should be pointed out that for some of the carbons, especially carbon 8, there still were deviations in adsorption on the high side as the free iodine concentration approached that of the saturated solution, although every precaution had been taken to avoid errors that could produce them. These appear to be real and indicate an unstable state wherein the mode of adsorption changes. On the basis of adsorption theories discussed
u”, ” IO
Pzle,mmOl/l Fig. 6. Aqueous phase adsorption of iodine on coal base carbons.
Fig.7. Aqueous phase adsorption of iodine on coconut base carbons. 25
Fig. 4. Adsorption of iodine on carbon 6 from solutions of 0.1,0.2, 0.3, and 0.4 N total initial iodine concentration, [I&,, and plotted against total iodine concentration at equilibrium.
Fig. 5. Adsorption of iodine on carbon 6 from solutions of 0.1.0.2, 0.3, and 0.4 total initial iodine concentration, &],,, and plotted against free iodine concentration at equilibrium.
Fig. 8. Aqueous phase adsorption of iodine on activated carbons.
later in this paper, this change appears to be due to a change from strictly a surface coverage to pore filling. Table 2 presents the apparent saturation capacity at [I& = 1.0 mmol/l as reported in weight units and volume units, the latter based on the normal density of iodine, 4.93 g/cm3. Table2. Adsorptive capacity of carbons from aqueous KI-I, solutions when equilibrium concentration is I.0 mmolll of free iodine
OF ADSORPTION DATA WITHPORESTRUCTURE
On comparing the amounts of iodine vapor adsorbed at saturated vapor pressure (Table l), with iodine adsorbed from aqueous solution at near saturated concentration (Table 2), it was observed that considerably more iodine was adsorbed from the vapor phase than from the aqueous phase, This suggested that iodine vapor may be adsorbing by a pore filling process and iodine from the aqueous solution by surface coverage. These concepts are not departures from long known facts of adsorption. In 1945, work done on the Division 10 NDRC Project showed that chloropicrin gas fife correlated with the micropore volume, and the more recent work done at Pittsburgh Activated Carbon Co. showed that the iodine number correlated with the surface area of pores larger than 10h in diameter. Figure 9 presents a first examination of relationships that may exist between the volume of solid iodine
adsorbed and total pore volume or micropore volume. Micropore volume is defined here as pores having diameters of less than 3OA as measured at the pore constrictions. When total pore volume was plotted against iodine volume no correlation was found. For the plot of micropore volumes, seven of the points fell on a straight line, with carbon 7 showing a small deviation. This deviation in carbon 7 was due to a large surface area of macropores combined with a small micropore volume while the other carbons had relatively large micropore volumes and small macropore surface areas. Except for carbon 7, the micropore volumes were considerably larger than the iodine volumes, suggesting steric hindrance due to packing of molecules in relatively small pores. Iodine is quite incompressible, hence its adsorbed phase could be likened to rigid spheres packed in rigid cylinders, Based on this concept, a much better correlation was worked out between weight of iodine adsorbed and micropore structure. Figure 10 shows the variation in filled cylinder volume when the ratio of sphere diameter to cylinder diameter is varied over the range zero to one. At zero ratio the sphere
Fig. 10. Fraction f2 of volume of spherespackedinto cylindersof varied diameters.
volume fraction is 0.74. In these calculations, it was assumed that the iodine molecules in solid state also have this fraction. The true density of the iodine molecules is then the normal density of 4.93g/cm3 divided by 0~74 giving 6.67g/cm3. Using eqn (4) below, the weight of iodine that could occupy increments of pore volume, A V, of different fZ factors were summed up from AV!, the smallest pores, to AV,, the largest pores in which pore filling could occur. Weight iodine adsorbed = 5 667f;A V, “I
Fig.9. Correlation of total pore volume and micropore volume with solid-volume iodine as adsorbed from gas phase.
In these calculations an additional assumption was made in that, after the pore filling process stopped, adsorption continued on the macropore surface forming a unimolecular layer. The method for carrying out the latter calculations is discussed in a later section on aqueous phase adsorption. The calcula$ed or theoretical weights of adsorbed iodine are given in Table 3 in columns 3 and 4, and when
Iodine adsorption and structure of activated carbons Table 3. Adsorption of iodine vapor on carbons, experimental compared to theoretical as calculated frompore structure
compared to the experimental, show a very good correlation. Pore filling was assumed to occur in pores up to 30 A constriction diameters (micropores). In calculating the amount of adsorbed iodine in pores of cavity diameter, the micropore volume was again used. The good agreement between the experimental and theoretical results supports the contention that when incompressible substances are adsorbed in small pores, void spaces exist between the molecules which reduce the quantity that can be adsorbed.
covered by an iodine molecule when molecules assume a hexagonal arrangement in the unimolecular layer. A, is equal to 21.1 X lo-l6 cm’. A comparison of the average surface area of column 4 with the iodine unimolecular layer areas of columns 5 and 6 shows that the possible surface coverage of the iodine adsorbed from aqueous phase is consistently less than the carbon surface area, indicating that considerable steric hindrance occurs when iodine molecules are adsorbed on sharply curved surfaces. The results on the vapor phase adsorption are included here to show that all of the adsorbed iodine vapor cannot be accommodated on the surface; hence, pore filling is the only way iodine vapor adsorption can occur as already shown in a previous section. Figure 11 shows the effect the curvature of a cylinder wall has on the maximum number of spheres that can be
3.2 Aqueous phase adsorption Correlations were sought between iodine adsorptive capacity and available surface area. Table 4 presents surface areas for the eight carbons as calculated from the pore size distribution curves. The equation used was vn 4AV Surface area = C 7 vi
Fig. 1I. Effect of sphere/cylinder diameter on accommodation of sphere layer on cylinder surface.
where d is the mean pore diameter in each increment of pore volume AV, AV, is the volume increment having the smallest d and AV,,,the largest d Cylindrical pores were assumed. Column 4 gives the average surface area as calculated from pore distribution curves for the constrictions and cavities. Columns 5 and 6 give the calculated surface coverage of a unimolecular layer of iodine molecules as adsorbed from aqueous and vapor phases using the equation
in contact with the wall. By applying the steric hindrance corrections from Fig. 11, the weight of iodine that could be adsorbed on each carbon surface was calculated. The equation used for making this calculation is as follows: “” 4Mj,AV Theo. wt. adsorbed I2 = ; A I
where W is the adsorptive capacity, N the Avogadro’s number, M the iodine molecular weight and A,, the area
where j, is as defined in Fig. 11, all other terms are as previously defined. Table 5 presents the theoretical weights of iodine adsorbed as calculated with eqn (7). The average weights as given in column 5 are plotted against the experimentally determined weights in Fig. 12. The
Table 4. Carbon surface areas and calculated coverage of area by iodine as adsorbed from aqueous and vapor phase
Table5. Amount of iodine adsorbed from aqueous phase, experimental and theoretical
Unimolecular layer area = WNA,/M
A. J. JUHOLA 4. CONCLUSION
This study has contributed considerable new information on iodine adsorption and confirmed prior concepts of the nature of pore structure as originally proposed in the studies reported by Juhola and Wiig. These conclusions are summarized as follows: 1. Iodine adsorption capacity is dependent on the micropore volume and macropore surface area when carbon is equilibrated with vapor at saturation concentration. 2. Iodine adsorption capacity is dependent on the total surface area when the carbon is equilibrated at near 2.0 3.0 saturation concentration in aqueous phase. Theoretical weighton surface, g/g 3. When an incompressible substance, such as iodine is adsorbed, steric hindrance occurs; i.e. the molecules have Fig. 12. Correlation of aqueous phase iodine adsorption with theoreticallydeterminedamount. more than normal void volume between them, such as occurs when rigid spheres are packed into relatively small correlation is good, confirming that adsorption from cylinders. aqueous phase is primarily on the carbon surface but that 4. Pores consist of cavities separated by constrictions. steric hindrance reduces the number of molecules that can It is necessary to measure both the constriction and cavity be accommodated on the surface. diameter to arrive at a measured surface area that can approach the true surface area. 3.3 Iodine number 5. In aqueous phase adsorption, the KI apparently In the prior studies[l], iodine number was found to hinders pore filling by the iodine, hence surface adsorpcorrelate with carbon surface area of pores larger than tion occurs primarily. Some evidence was found for pore 10A in diameter according to the equation. filling at concentrations near saturation for some of the Iodine number, mg/g = 17t I.07 x surface area. (8) In carbons. Apparently the region at concentrations near Fig. 13, the straight line was drawn according to the above saturation is unstable where adsorption can proceed equation and the experimentally determined points of this either way for some carbons. 6. The pore cavity diameters, relative to constriction study then plotted on the graph. The correlation is good. In view of the results of this study, it appears that the diameters, were found to be the largest for the highly amount adsorbed at 0.02 N (0.11 mmol/l free iodine) is a activated carbons. Pore development on continued fractional part of the saturation capacity, and also, th! activation appears to progress to a large extent by cavity limitation on surface area to pores greater than 10 A enlargement. diameter is a compensating factor which accounts for the 7. The self consistency of the iodine adsorption, steric hindrance corrections, and pore size distribution add steric effects. further reliance to the method based on water adsorption for the determination of carbon pore size distributions. The carbon must be pretreated to remove hydrophilic ash for this method of pore size distribution measurements to be valid. At present the method based on water adsorption data is the only way that both the diameter distributions of the constrictions and cavities can be determined. I
Acknowledgement-The author wishes to give recognition to E. W. Feehan for the iodine adsorption data.
Fig. 13. Iodine number as function of carbon surface area of pores larger than 10A diameter.
REFERENCES 1. Grant R. J., Basic Concept ofAdsorption on Activated Carbon. Pittsburgh Activated Carbon Company (1961). 2. Juhola A. J. and Wiig E. O., J. Am. Chem. Sot. 71,561,2069, and 2078 (1949). 3. Hill A. and Marsh H., Carbon 6, 31(1968). 4. Juhola A. J. and Blacet F. E., OSRD Report No. 5500(1945).