Langmuir 1991, 7, 1296-1298
1296
pNitropheno1 Adsorption from Aqueous Solution on Carbon Black. Temperature Effect M. L. Gonzhlez Martfn Departamento de F'uica, Universidad de Extremadura, 06071 Badajoz, Spain C. Valenzuela Calahorro and V. G6mez Serrano' Departamento de Qdmica Znorgbnica, Universidad de Extremadura, 06071 Badajoz, Spain Received April 6, 1990. Zn Final Form: August 3, 1990
Introduction With adsorption a t the liquid-solid interface, the temperature may influence both the adsorption process and the adsorbed state. The temperature effect promotes the diffusion of adsorptive and alters its surface ordering. The variation of temperature causes adsorbate rearrangements and also changes in the adsorption-desorption equilibrium, which depends on the thermal stability of the adsorbed state. Additionally, the thermal agitation of the solution has an incidence on the mass transport and, hence, on the adsorption by varying the kinetic of the process. The aspects of the adsorption connected with the effect of temperature are of great importance in fundamental and applied research involving activated diffusion, phase transitions, stability of the adsorbed state, etc. However, only a few studies were carried out on the subject. They dealt with properties of the adsorbed phase as melting,' ordering2s3and structuring4* or with the solubility of the adsorptive.' The focus of the present work was to study the adsorption, and ita temperature dependence, of p-nitrophenol from an aqueous solution on a series of carbon black samples, which are adsorbate-adsorbent systems used widely in adsorption studies. The adsorption isotherms were determined a t 20,30, and 40 "C. Previously Pur? carried out investigations based upon the adsorption of several solutes, including p-nitrophenol, on activated carbons and carbon black samples, but utilizing different materials and operating only at 35 "C. Experimental Section A series of carbon black samples including Sterling V (SV), Vulcan 3 (V3), Vulcan 6 (V6), Black Pearls 880 (BPSSO), Black Pearls 1300 (BP1300), and Black Pearls 2000 (BP2000), which were furnished by Cabot in Spain, are the adsorbents utilized in this work. The carbons were first sized, and the particle size lower than 0.32 mm was then chosen for further studies. The was recrystallized twice from distilled water p-nitrophenol (PNP) before being used as adsorptive. The carbons and p-nitrophenol are referred to hereafter as shown in parentheses. The adsorption of Ns at -196 "C was carried out with comparative purposes only. A conventional gravimetric apparatus for gas adsorption was the experimental device utilized. Samples were first oven-dried at 110 "C for 24 h and then out(1) Chiou, C. C. T.; Manes, M. J. J. Phys. Chem. 1974, 78, 622. (2) Mills, A. K.; Hockey, J. A. J.Chem. Soc., Faraday Trans. 1 1975, 71, 2384. (3) Mills, A. K.; Hockey, J. A. J. Chem. Soc., Faraday Trans. 1 1975, 71, 2392. (4) Everett, D. H. Prog. Colloid Polym. Sci. 1978, 65, 103. (5) Kern, H. E.; Piechocki, A.; Brauer, U.; Findenegg, G. H. Prog. Colloid Polym. Sci. 1978, 65, 118. (6) Liphard, M.; Glanz, P.; Pilarski, G.; Findenegg, G. H. Prog. Colloid Polym. Sci. 1980, 67, 131. (7) Bartell, F. E.; Thomas, L. T.; Fu, Y. J. Phys. Chem. 1951,55,1456.
( 8 ) Puri, B. R. In Activated Carbon Adsorption of Organics from Aqueous Phase; Suffet, I. H., McCurie, M. J., Eds.; Ann. Arbor Science: Ann Arbor, MI, 1980; Chapter 17.
0743-7463/91/2407-1296$02.50/0
gassed in the adsorption system at 150 "C for 12 h, at a pressure of 133 X 104 Pa. The pseudoequilibration time given to each adsorption measurement was 1 h. Adsorption isotherms for PNP were determined by first placing different amounts of carbon into a set of 250-cm3 glass flasks provided with Bakelite screw-on caps to prevent solvent losses by evaporation. The quantities of used carbon ranged between the limit values of 0.004 g for BP2000 and 3.440 g for SV. Then, to each container was added 100 cm3 of a PNP solution of concentration varying between 1.01 X lo-' mol L-I for SV and 1.05 X 10-3mol L-1 for BP2000. According to earlier results,9 the solvent was in fact a buffer solution at pH = 4 made up of acetic acid and sodium acetate. Once the phases were brought into a contact, the flasks were held in a shakerbath (Gallenkamp) with water at 20, 30, or 40 "C. The thermostatic system remained working under continuous agitation (120 oscillations/min) until the adsorption equilibrium was reached. This took between 24 h for SV and V3 and 60 h for BP2000. By use of a previously established analysis method? the absorbance measurements of the supernatant liquid were effected (after appropriated dilution where necessary) at 315 nm with the aid of a SP8-250 UV/VIS Pye-Unicam spectrophotometer.
Results and Discussion According to the main objective in this work, first we estimated the apparent specificsurface area (&ET) in order to be able to compare the adsorption capacity of the materials under study. It was accomplished by application of the modified BET equation up to C/Co N 1.0 (Cand COrepresent the equilibrium and initial concentrations of the PNP solutions), and assuming a flat surface orientation for the adsorbate molecules. Thus CX(S-C) was plotted against C / S , were X is the amount of PNP taken up and S is the solubility of PNP at each working temperature. The resulting plots at the temperature of 30 "C are shown as an example in Figures 1and 2, and they prove that the BET equation fits quite well the experimental isotherms. Once the monolayer capacity was obtained by the method was then calculated by taking the of Giles et al.,"J SBET surface area covered by each adsorbate molecule to be values are summarized in equal to 52.5 A2.'0 The SBET Table I, together with those derived from the N2 adsorption at -196 "C. (In this case the BET equation was applied up to PIP0 N 0.2, and A , was equal to 16.2 A2.) Data in Table I indicate that the carbon blacks may differ largely from the extent in which their surface area is accessible to PNP at 20,30, and 40 "C and to N2 at -196 "C. As the best illustrative example of this, the SBET value for SV is only 1.35% (PNP, 20 "C) and 2.59% (N2, -196 "C) of the corresponding value for BP2000. Differencesin the surface area depending on the adsorptive are consistent with the smaller molecular size of N2 in relation to PNP. Adsorption data are usually presented as the isotherm. This simple graphic method, displaying the variation of the adsorbed amount with relative concentration, allows us to infer easily valuable information on the adsorption process and the adsorbed state. The isotherms of SV and V3 in Figure 3 show that the adsorbed amount increases with C/Co, the augment being more marked from C/Co 0.9. The former behavior is compatible with the formation of a monomolecular layer of solute on adsorbent surface. The latter behavior was attributed by Giles et al." to the adsorption on (a) the first layer, (b) a new region of the substrate, or ( c ) a part of the original surface. The shape of isotherms reveals that the adsorption at this final stage (9) Ldpez Gonzaez, J. de D.; Valenzuela Calahorro, C.; Navarrete Guijosa, A.; Gdmez Serrano, V. An. Quim. 1988,848,47. (IO)Giles, C. H.; D'Silva, A. P.; Trivedi, A. S. J. Appl. Chem. 1970,
20, 37.
(11) Giles, C. H.; MacEwan, T. H.; Nakhwa, S. N.; Smith, D. J.Chem.
SOC.1960, 3973.
0 1991 American Chemical Society
Notes
Langmuir, Vol. 7,No. 6, 1991 1297
12.0
V6
.2 9.0 -
v3
E"
Y
? 6.0
X 1
I
I
5
0
I
1
10
1'5
+/lo*
3.0
sv
Figure 1. BET plots for the adsorption of PNP on SV (o),V3 (0)) and V6 (A). 0.0 0.0
0.2
0.4
0.6
0.1
1.0
C/C, Figure 3. Adsorption isotherms for p-nitrophenol on several carbon blacks at 20 (a),30 (A),and 40 O C (0).
Figure 2. BET plots for the adsorption of P N P on BP880 (a), BP1300 ( O ) , and BP2000 (A).
Table I. Adsorption of N, at -196 and 40
O C
and of PNP at 20,30, PNP adsorption
sample SV v3 V6 BP880 BP1300 BP2000
NP adsorption -196 39 73 101 232 548 1505
O C
O C
30 " C
14 31 53 142 352 1034
12 31 47 137 342 1037
20
40
O C
10
31 40 136 336 1037
Apparent specific surface area (m2g l )estimated by the BET method.
is not especially influenced by temperature effect (for instance, through an activated diffusion process), and therefore there is little probability that the process takes place on fresh surface. Also, the adsorbed amount at C/Co values lower than around 0.9 does not appear to be sufficiently important, particularly for SV,so that the mere rearrangement of the adsorbed state causes such a significant increase in the adsorption. In brief, the results indicate that the adsorption follows the monolayer-multilayer mechanism. Furthermore, the small extent of the
adsorption recommends that the process occurs mainly on external surface and in pores of large size. (These textural properties might stem in these materials from a fine participle s i ~ e . ~ In ~fact ~ ~an~adsorption ) isotherm similarly shaped to that of V3 waa obtained by Smith et aL14 for the system made up of benzoic acid in aqueous solution and a nonporous Graphon carbon. Moreover, the almost parallel position occupied by the isotherms from C/CoN 0.9 suggests that the thermal stability of the adsorbed state suffers no appreciable changes, although a more appropriate plot of the adsorption data would be needed to support this supposition. The isotherms for BP880 and BP1300 (see Figure 4) present a first nearly vertical branch, which ends in an inflection point leading to a well-defined plateau; at higher C / COvalues, the behavior is temperature dependent. The great slope of the initial region of the isotherms is connected with the adsorption in narrow pores since the overlapping of the adsorption potential of facing walls enhances the adsorption process. This is corroborated with the fact that isotherms with a similar shape were reported for the adsorption of PNP on activated carbons, which are essentially microporous materials,8*gJ4J6although whether or not it happens so will depend likely on the way in which the PNP molecules interact with the adsorbent, either by taking part a electrons of the aromatic ring or through any specific functional group. In the first case, simultaneous interaction of the adsorbate with active positions located in opposite walls would be possible in view of the symmetrical distribution of the electronic charge at both sides of the plane of carbon atoms, with the mentioned resulting effect on the adsorption. The plateau of the isotherms for BP880 and BP1300 extends at least up to a C/Co value around 0.8. According (12) Dacey J. R. In The Solid-Gas Inter/ace; Flood, A., Ed.;Marcel Dekker, Inc.: New York, 1967; p 995; Vol. 2. (13).Kochling,K.-H.; McEneey,B.; Muller, S.;Fitzer,E.International Committee /or Characterization and Terminology of Carbon 1986,23, 601. (14) Smith, R. N.; Geiger, C. F.; Pierce, C. J. Phys. Chem. 1963,57, 382. (15) Kipling, J. J. Adsorption from Solutions of Non-Electrolytes; Academic Press, Inc.: London, 1965; Chapter 1.
Notes
1298 Langmuir, Vol. 7, No. 6, 1991 4.0 BP2000
3 .O r
IJ) Y
-0 E
2.0
2
c/c, Figure 4. Adsorption isotherms to p-nitrophenol on several carbon blacks at 20 (a),30 (A), and 40 O C (0).
to Giles et al.," the length of the plateau correspondswith the energy barrier that must be overcome in order that the adsorption proceeds. The results show then that after the first adsorption stage, the process is highly prevented in both carbons. It occurs either because the materials have saturated their adsorption capacity at lower C/Co or because the adsorbed film consisting of solute and solvent is not sufficiently thin as to allow the formation of a second adsorbate layer or the diffusion of solution through the inner core toward uncovered surface. The adsorption in multilayer appears to have little compatibility with the size of the pores and the magnitude of the energy barrier. Therefore, the final rise of the adsorption advocates for activated diffusion. This supposition is supported by the fact that the process is strongly temperature dependent, as demonstrated by the inversion produced in the adsorption isotherms with respect to the normal order of appearance in accordance with the exothermic character of the adsorption process. According to the slope of the initial upward branch, the
isotherms of V6 and BP2000 fall between those of SV or V3 and BP880 or BP1300. Particularly the isotherm of V6 resembles more closely to that of V3, whereas the same happens for BP2000 compared with BP880 and BP1300. Differences in the isotherm shape are likely associated with the porosity of the adsorbents and, more concretely, with the size of the pores in which the adsorption occurs. In line with above results, V6 and BP2000 will contain then smaller pores than SV and V3 and larger ones than BP880 and BP1300. In fact, a similarly shaped isotherm to that of V6 was reported by Smith et al." for the adsorption of benzoic acid from aqueous solution onto an activated Saran carbon with wide pores. Furthermore, the results are consistent with the porosity distribution that possess these carbons in the regions of macropores and mesopores (up to a pore radius of 37 A), as determined by mercury porosimetry.ls Thus the plots of AV/A log r against r, where V is the cumulative pore volume and r is the pore radius, display a main maximum that shifts toward lower r values in the series SV, V3, V6, BP880, and BP13W, BP2000 presents a porosity distribution formed by the broadest range of pore sizes, being also in agreement with the previous statement. At high C/Co values, the adsorption pursues with the formation of the monolayer for V6 and probably of a further layer for BP2000. A diffusion process driving adsorptive molecules to a fresh surface appears to have little probability for the later carbon since, at least apparently, the adsorption is not prevented by pore size effect,as suggested by the absence of a plateau from the isotherm. The augmentation of the adsorption might also be connected with a rearrangement of the adsorbed state. It would be possible for this material in view of its great adsorption capacity. If so, either the onset or the extent of the process should be affected by increasing temperature. The normal tendency in that direction in relation to the adsorption would consist of the commencement of its augmentation at a lower C/Co and of the drop of its decrease. However, the opposite behavior is observed in Figure 4 for the isotherm determined at 40 "C. Finally, the increase with rising temperatures of the separation between isotherms from a certain C/Co value (it is difficult to guess for V6, while for BP2000 is around 0.6) seems to indicate a decrease in the thermal stability of the adsorbed state in SV and BP2000. (16) G o d e z Martin,M.L. DoctoralTheeis, ExtremaduraUniversity, Bndajoz, 1989.
