T H E SORPTION OF CHLORINE BY ACTIVATED CHARCOAL' L. H. REYERSON AND A. W. WISHART School of Chemistry, University of Minnesota, Minneapolis, Minnesota Received March 16, 1958
I n a previous comniunication (5) the authors described a method for determining the sorption of chlorine by porous sorbents, which was a modification of the method given earlier by Cameron and Reyerson (l), and presented the results of a series of measurements on the sorption of chlorine by silica gel. The results of a similar series of measurements on the sorption of chlorine by activated charcoal are given a t this time. The charcoal used in this study was a part of the material prepared by Cameron for use in the work reported by Reyerson and Cameron (3, 4). It was steamactivated a t 850°C. and allowed to cool to room temperature before air was admitted. The steam-activated charcoal was then placed in a silica bulb and heated to 700°C. under high vacuum for 24 hours. Upon cooling to room temperature, oxygen was admitted. After standing several hours in oxygen the charcoal was heated once more, pumped out for 48 hours, and finally cooled in an atmosphere of nitrogen. Weighed amounts of this charcoal were placed in the glass bucket of the McBain-Bakr balance. The quantities of chlorine sorbed by this charcoal were determined a t 35.5", 51°, 73.5",and 91.5"C. over a pressure ranging from zero to about atmospheric pressure. The chlorine was purified, introduced into the system, and maintained at the various vapor pressures as previously described (5). The results are given in table 1 and presented graphically in figure 1. As shown in figure 1 a slight hysteresis exists in the desorption points near 100 mm. pressure. Slightly more chlorine was retained during desorption than was taken up during adsorption. Since the establishment of equilibrium was slow, as shown in table 1, it may well be that the observed differences were due to the fact that equilibrium was not quite attained. However, similar results were observed in the sorption of bromine by the same activated charcoal (3). It was found impossible to remove all of the sorbed chlorine under prolonged evacuation a t elevated temperatures. The final desorption points reached by two methods of drastic treatment are shown in the figure, The material here presented formed a part of a thesis submitted to the Graduate Faculty of the University of Minnesota by Arthur W. Wishart in partial fulfillment of the requirements for the degree of Doctor of Philosophy, August, 1937. 679
680
L. H. REYERSON AND A. W. WISHART
Curves of the type here obtained are typical of a Langmuir type of sorption. This was further borne out by plotting values against the X / M P / ( X / M ) . A straight line is demanded by the Langmuir theory, and this was found when the values in table 2 were plotted as shown in figure 2. It TABLE 1 Sorption of chlorine bu activated charcoal
mm
hours
millimoles
00* 81 6 227 3 318 7 52f 672 40" 103 24
I/
Isothermal a t 35.5"C.
___
C
I 1 6 0 6 9 0 001
0 0 0 0 0 0 0 0 0 0 0
0 3559 3799 3869 3962 3997 3938.t 36881 3433$ 30921: 23271:
millimoles
o.o*
0.0 5.0192 5.3579 5.4552 5.5873 5.6360 5.55261: 5.20001: 4,84051 4.3600: 3.28151
Isothermal at 73.5%. mm.
1
24 18 15 12 12 15 15 18 24 48
25.2 260.2 586.5 720.3 610.5 303.0 127.1 24.2 18.2
,
0.0* 15.3 136.7 324.3 474.8 682.0 399.9 67.1 6.8
0.0 0.2903 0.3503 0.3703 0.3775 0.3892 0.3'7351 0.34041 0.28531
0.0 4.0935 4.9399 5.2220 5.3229 5.4888 5.26731: 4.80061 4,02381
I1 24 20 16 16 12 16 20 24
0.0
0.2750 0.3438 0.3634 0.3738 0.36451: 0.35041: 0.33051: 0.2869$ 0.27301:
3.8781 4.8481 5.1245 5.2714 5.1406$ 4.94111 4.66121: 4.04541 3.84951:
hours
20 20 18 12 12 14 18 20 20
Isothermal a t 91.5"C.
_ _ _ I -
Isothermal at 51.0"C.
0.0
0.0* 25.7 142.5 248.0 566.3 721.8 591.3 330.4 138.9 31.9 10.5 0.0Ot
I
0.0 0.2663 0.3101 0.3294 0.3543 0.3587 0.3554t 0.34261 0.3211t 0.2813$ 0.24321 0.17991:
I
0.0 3.7552 4.3731 4 6455 4,9966 5.0584 5.01131 4.83091 4.52821 3.96721 3,42941 2.53691 I
20 18 15 15 12 16 18 18 20 20 36
* Readings in rows indicated in this manner were taken with the system evacuated and before admission of the halogen. t Chlorine frozen down by liquid oxygen. $ Desorption readings. appeared from these results that we were dealing with a monomolecular type of sorption. Since equilibria were established slowly, it seemed likely that the sorptions were of an activated type. Calculations of differential heats of sorption tended t o confirm this idea. Ari average of nine of these values gave 8860 calories per mole as the differential heat of sorp-
SORPTION OF CHLORINE BY ACTIVATED CHARCOAL
681
tion. This is significantly larger than the heat of vaporization at the boiling point, 4778 calories, and indicates strong forces of attraction between the chlorine atoms and sorption centers on the carbon surface. The type of sorption curve for chlorine on charcoal is fundamentally different from that of chlorine on silica gel (l),but it is almost exactly like that of bromine on activated charcoal (3). Furthermore, the quantity in millimoles of chlorine sorbed a t a given temperature is about the same as the m o u n t of bromine taken up a t a like temperature above its boiling point. In figure 3 the isotherms of the sorption of chlorine, bromine, and iodine by charcoal ar;: given. A similar comparison for the sorption of
FIG.1. Sorption of chlorine by steam-activated charcoal a t different temperatures and pressures
these halogens by si!ica gel is shown in figure 4. These two figures show the differences in the type of sorption exhibited by charcoal and silica gel for the halogens. Figure 3 shows that the sorptive capacity of charcoal for bromine and chlorine is about the same, while it is more than tenfold greater than for iodine. Since the sorptions of all of these halogens fit the Langniuir expression for a monomolecular layer of sorbed molecules, it may well be that the iodine molecules are enough larger than the bromine molecules so that steric hindrance prevents the iodine molecules from being sorbed on most of the active centers of the carbon surface. Weaker forces existing between the carbon and the iodine may account for some of the difference, but it does not seem probable that a shorter average life of
682
L. H. REYERSON AND A. W. WISHART
TABLE 2 Sorption values used in Langmuir equation _ l l _ _ l l l
Isothermal a t 35.5"C.
Isothermal a t 73.5"C. 1.582 4.958 16.257 19.923 42.423 63.920 73.677 94.221 119,286
. .... .. . ...
mm.
millimoles
18.2 24.2 25.2 127.1 260.3 303.0 586.5 610.5 720.3
3,8495 4.0454 3.8781 4.6612 4.8481 4.9111 5.1245 5.1406 5.2714
_ _ _ _ _ l _ l _
iect,hermal a t 51.0"C.
474 8 682.0
4.728 5.982 6.498 27.268 53.691 61.322 114.450 118.760 136.643
5.3229 5.4885
Isothermal at 91.5"C. 1.690 3.738 13.977 27.672 62.103 75.921 89.199 124.253
10.5 25.7 31.9 138.9 142.5 248.0 330.4 566.3 591.3 721.8
3.4294 3.7552 3.9672 4.5282 4.3731 4.6455 4.8309 4.9966 5.0113 5.0584
3.062 6.844 8.041 30.674 32.585 53.385 68.393 113.337 117.993 142.693
FIG.2. Sorption isotherms obtained by use of the Langmuir equation
SORPTION OF CHLORINE BY ACTIVATED CHARCOAL
683
iodine on the carbon surface could be responsible for the tenfold decrease in gorption. If steric hindrance prevents many of the active centers from taking up iodine but not chlorine or bromine, then it should be possible to obtain some idea concerning the spacing of active centers on activated charcoal. From viscosity measurements the radii of tke halogen molecule: are given as follows: chlorine, 1.85 A,;bromine, 2.02 A , ;and iodirx, 2.23 A. Recent x-ray studies on solid bromine (6) and solid iodine (2) give the following distanree between the halogen atoms ofJhe molecules in the crystal lattice: Br-Br = 2.27 and 1-1 = 2.70 A. However, the absence of a like value for chlorine led us to use the values from viscosity data for compari-
H. I
I
I
I
L.UU.C
Ob
FhOlVlklkllAr)l*er* ZW 400
I
600
FIG.3. Sorption of chlorine, bromine, and iodine by charcoal
son. If, for convenience, we assume the cross sections of the molecules t o be circles, then the area covered will be 1.07 X 10-l6 cm.2 by a chlorine molecule, 1.28 x 10P6cm.2 by a bromine molecule, and 1.56 x 10-16 cm.2 by an iodine molecule. If the effective area is the same as this cross section, then the critical area from the steric hindrance point of view must lie cm.2 The real areas occupied by these between 1.28 and 1.56 X molecules are probably somewhat greater than this, in view of the results of the x-ray studies and the knowledge of the probable shape of the molecules. Even so, the minimum spacing ofjhe active centers on the charcoal surface must be somewhere between 2.5 A and 3.2 A. If the chlorine and bromine molecules occupy all of the active centers in a checkerboard arrangement, then the minimum area of charcoal covered by chlorine
684
L. H. REYERSOK .4ND A . W , WISHART
molecules conies out to be 4.31 X lo6em.*per gram of charcoal. A similar calculation for bromine gives a minimum area of 5.54 x IO6cm.2per gram. It is more t,han likely that' the surface area of the charcoal used in these investigations is somewhat larger than these values. I t is also possihle that each atom of the halogen molecule is held by the carbon surface. If so, the spacing of t'he active centers may be such that they are able to hold the chlorine and homine atoms without too much st,rain but not the iodine atoms. These studies prove the very great difference that exists between the character of sorption on silica gel and charcoal. The surfaces of these two sorbents are of about the same magnitude, yet the whole character of the sorption is different. It should also be noted that the three halogens differ aniong themselves in the manner in which t,hey are sorbed b y silica
FIG.4. Sorption of chlorine, bromine, and iodine by silica gel
gel. Further work will be needed to explain these differences. It is also suggested that the tcniperature of the sorbent surface plays a distinct rblc in the sorptioii proress. The energy of the surface may be such that the amount of sorption is greatly reduced in spite of the fact that the sorbate at the teinperature studied is below the critical temperature or even near the boiling point. Further study is in progress in the hope that additional light may he thrown on the problem. SUMMARY
1. Sorption isotherms for chlorine on activated charcoal were ohtained at 35.5", 51.0", 73.5', and 91.5"C. 2 Comparison of thesc resuIts with those of hromine and iodine is made.
SORPTIOK OB CHLORIKE BY ACTIVATED CHARCOAL
685
3. Calculations show the minimum surface area of the activated charcoal t o be 5.54 X lo6 cm3. REFERESCES (1) CAMERON A K D REYERSOS: J. Phys. Chem. 39, 169 (1935). (2) HARRIS, NACK, A K D BLAKE: J. Am. Chem. SOC. 60, 1583 (1928). A S D C A M E R O N : J. Phys. Chem. 39, 181 (1935). (3) REYERSOS (4) REYERSON A S D C A M E R O N : J. Phys. Chem. 40, 233 (1936). (5) REYERSON AKD i v I S H . 4 R T : J. Phys. Chem. 41, 943 (1937). (6) VOXKEGUT AND WARREN: J. Am. Chem. SOC. 68, 2159 (1936).
