Environ. Sci. Technol. lQ08,22, 703-708
Chemisorption of Catechol on Gibbsite, Boehmite, and Noncrystalline Alumina Surfaces Murray B. McBrlde" and Lambert G. Wessellnkt
Department of Agronomy, Cornel1 University, Ithaca, New York The mechanism of bonding of catechol and related phenolic compounds on aluminum oxides was elucidated from sorption behavior in the presence of competing adsorbates and the nature of the infrared spectra of the surface-bound molecules. The surfaces demonstrated a high degree of selectivity toward catechol, adsorbing the molecule in the presence of a large excess of chloride. Phosphate competed effectively with catechol for sorption sites while acetate did not. Dispersive and Fourier transform infrared spectroscopy verified that catechol bound on the aluminum oxide surfaces was chemically perturbed in much the same manner as catechol chelated by A13+,suggesting that the dominant sorption process involved the formation of a 1:l bidentate complex with surface Al. The mechanism of bonding was similar for all the aluminum oxides, but the dominant crystal surfaces of the crystalline oxides were unreactive toward catechol, and adsorption was attributed to -AlOH groups situated on "edge" faces. As a result, the noncrystalline oxide was more reactive per unit of surface area than the crystalline minerals boehmite and gibbsite.
Introduction Interest in the processes of bonding between organics and mineral components of soils stems in part from the problem of leaching of organic pollutants through soils into aquifers. In aqueous media, layer silicates have little affinity for certain groups of organic compounds such as the phenols, although there is evidence that organic matter in soils and sediments plays a significant role in sorption (1). In addition, phenols are known to adsorb (with varying affinities) on oxides, depending upon the number and position of hydroxy substitutions on the benzene ring. For example, ortho-diphenolic compounds are adsorbed more strongly than other diphenols on aluminum oxide (2). Monophenolic compounds adsorb weakly, if at all, on oxides (2, 3). Circumstantial evidence, then, points to the involvement of a bidentate bond with the surface in cases where molecular geometry is favorable. However, Kummert and Stumm ( 4 ) have interpreted sorption and acidbase titration data for catechol to indicate the formation of a 1:l complex between a surface -AlOH group on hydrous 7-A1203and a single phenolate group. Oxalate appears to bond to two adjacent Fe atoms on iron oxides to form a binuclear complex (5) and may bond either by this binuclear mechanism to two A1 atoms or by bidentate coordination to a single A1 atom on gibbsite edge surfaces (6). Both mechanisms appear to be stereochemically feasible on the basis of the distances between adjacent surface A1 atoms. Thus, catechol adsorption on oxides might involve similar bidentate or binuclear bonding mechanisms in which the two phenolic ligands coordinate to two or one surface AI atom(s), respectively. Fe3+and AI3+ are expected to coordinate catechol by similar mechanisms at the oxide surfaces because the chemistry 'Present address: Department of Soil Science and Plant Nutrition, Agricultural University, De Dreijen, 6073 BC Wageningen, The Netherlands. 0013-936X/88/0922-0703$01.50/0
of catechol complexation by these two metals in solution is very similar. This study evaluates the potential significance of oxide-phenol interaction in the retention of phenols by soils, investigating the nature of the phenol-aluminum hydroxide interaction utilizing several phenols and aluminum hydroxide materials. Aluminum hydroxides were chosen in order to minimize oxidation of the phenols upon interaction with the surface, a process observed on iron and manganese oxides. A detailed study of catechol adsorption was conducted. Adsorption isotherms were obtained in the presence and absence of competing anions in order to assess the relative affinity of catechol for the surface. Analysis of the surface complexes by infrared spectroscopy enabled more direct interpretation of surface bonding to be made.
Materials and Methods Noncrystalline alumina [Al(OH),] was prepared by titrating 1.0 L of 0.5 N AlC13to pH 7 with 0.5 N NaOH. The suspension was then aged at room temperature for 48 h, dialyzed, and freeze-dried. The surface area, measured by Brunauer-Emmett-Teller (BET) analysis of Nzadsorption data obtained at 77 K for three partial pressures of N2 (P/Po= 0.1,0.2,0.3)was 207 m2/g. Infrared spectroscopy confirmed the material to be a noncrystalline hydroxide, with no well-defined OH stretch or bend vibrations that would indicate a crystalline hydroxide. Boehmite (A100H) was prepared by a modification (W. Bleam, personal communication) of the method described by Bugosh (7). Aluminum powder (4.25 g) was added to 200 mL of lob6M HgC12 After 10 min, the suspension was centrifuged and the supernatant decanted. This A1 amalgam was washed 3 times with distilled water and suspended in 250 mL of distilled water along with 12.5 mL of glacial acetic acid. This suspension was then boiled for three 8-)1periods while maintaining a constant volume with added water. Boiling allowed much of the acetic acid to escape as vapor, but H202(30%)was added at the end of the heating process to oxidize the remaining acetic acid. The faintly white suspension was heated at 165-170 "C in a pressure bomb for 8 h, after which time the translucent suspension was freeze-dried. Infrared spectroscopy confirmed the existence of OH bend and stretch vibrations that are diagnostic for boehmite. The BET surface area was measured at 216 m2/g. Boehmite is a layer structure consisting of linked chains of A10, octahedra, which run parallel to the c axis, forming an infinite two-dimensional sheet. It crystallizes in the form of thin plates with well-developed (010) faces parallel to the sheet structure. Microcrystalline gibbsite, Al(OH)3,obtained from Dr. J. Russell at the Macaulay Institute, Scotland, has been estimated by electron microphotographs t o have a surface area near 100 m2/g (8). This is much greater than the 32.5 m2/g measured by N2 adsorption, but the discrepancy probably arises from face-to-face contact of the well-formed pseudo-hexagonal platelets in the dry powder. Gibbsite is a layer structure consisting of edge-shared A106 octahedra, forming an infinite two-dimensional sheet per-
0 1988 American Chemical Society
Environ. Sci. Technol., Vol. 22, No. 6, 1988
703
pendicular to the c axis. The crystalline platelets have well-developed (001) faces parallel to the sheet structure. Isotherms were obtained for catechol adsorption on all three aluminum hydroxides. Equilibrations were conducted in duplicate or triplicate with appropriate concentrations of catechol in a total solution volume of 25 mL with 52 mg of gibbsite, 50 mg of amorphous Al(OH),, or 25 mg of boehmite. A constant electrolyte concentration of 0.1 M KCl was maintained, and the pH was controlled at a constant value of 7 by adjusting repeatedly with NaOH if necessary during the adsorption. A 6-h reaction time was chosen because it was sufficiently long to permit essentially complete adsorption ( 4 ) but was too brief to allow significant autoxidation of catechol. Samples were continuously shaken in the experiments with amorphous Al(OH), to maintain a homogeneous suspension, but only intermittent shaking was necessary for the gibbsite and boehmite experiments. Following the 6-h reaction time, the suspensions were centrifuged to separate the solids, and the supernatants were analyzed for catechol at pH 7 by UV absorbance at 275 nm. Analysis of these supernatants for total soluble Al by atomic absorption revealed no detectable dissolution of the solids by catechol, and speciation calculations confirmed that catechol should have had little effect on the solubility of A1 at pH 7. In addition, no evidence of a bathochromic shift was seen in the 275-nm absorbance of catechol, indicating that A1 had not complexed with catechol at a measurable level. The solids were frozen and freeze-dried in preparation for infrared spectroscopy. After adsorption isotherms of catechol had been measured from the above experiments, K2HP04or potassium acetate salts were added to the catechol solutions in quantities corresponding approximately to the highest molar concentration of catechol used in the isotherm, and the equilibrations were repeated under the same conditions with these competing anions present. Additional sorption experiments, using boehmite as the adsorbent, were conducted in the same manner with hydroquinone and phenol in order to evaluate the importance of the ortho substitution on phenol affinity for oxides. Infrared (IR) spectra of the freeze-dried oxide powders in KBr pellets were obtained on a Perkin-Elmer 281 spectrometer. At lower adsorption levels, conventional IR spectrometry was insufficiently sensitive to observe the peaks of adsorbed organics, and diffuse reflectance spectra of the dry powders (neat) were obtained on an FTIR spectrometer (IBM Model IR-98). Self-supporting oriented films of boehmite were made by depositing about 0.4 mL of an aqueous boehmite suspension (25 mg/mL) on a polyethylene sheet and allowing it to dry at room temperature. These films were exposed to catechol vapor in a closed chamber and immediately placed in the beam of the dispersive IR spectrometer for analysis. X-ray diffraction analysis of boehmite as the oriented film and the randomly oriented powder was done to obtain qualitative information regarding the orientation of crystallites in boehmite films. Results and Discussion Adsorption on Gibbsite. Little adsorption of catechol was measured on gibbsite as demonstrated by the isotherm in Figure 1, with the highest measured adsorption reaching 15 mmol/ kg. The maximum adsorption, based upon fitting the data to the Langmuir equation, was estimated at 23 mmol/kg. The low adsorption level, despite the relatively high surface area of this microcrystalline mineral, suggests that chemisorption occurred only at crystal edges 704
Environ. Sci. Technol., Vol. 22, No. 6, 1988
5
'W 1 -
1
2
3
4
4 equilibrium conc (X 10.M)
Figure 1. Isotherms for adsorption of catechol on gibbsite at pH 7 from an aqueous solutlon of 0.1 M KCI (O), 0.1 M KCI 4 x M potassium acetate (O), 0.1 M KCI 4 X M K,HP04 (m).
+
+
and steps and that the dominant planar (001) surface is inert. More direct confirmation of edge adsorption by catechol will be produced later in this paper. A similar conclusion regarding the nonreactive nature of the (001) surface has been made for phosphate (9) and Cu2+adsorption (10) on gibbsite, with adsorption levels of about 25 and 5 mmol/kg, respectively. Since aluminum hydroxides have zero points of charge in the 7-9 range (for example, boehmite synthesized by the method described in this paper has a zero point of charge of 8.0 by electrophoretic mobility), few positively charged surface sites (-Al-OH2+groups) exist at pH 7. This, combined with the fact that catechol adsorption was conducted at pH 7 with excess Cl- present, precludes the possibility of nonselective electrostatic adsorption of catecholate anions. Thus, adsorption of catechol on aluminum oxides is necessarily by ligand exchange of Al-coordinated OH- or HzO. In the case of gibbsite, the (001) surface comprises structural OHgroups coordinated to two A13+ions each; ligand exchange of these OH- groups by catechol is evidently not favored because of the stability of the bonding. Only edge OHgroups coordinated to single A13+ions have been considered accessible to ligand exchange (9). The fact that catechol adsorption on aluminum oxides has been shown by other workers to reach a maximum at a pH in excess of 8 ( 4 ) is an indication that adsorption occurs as the anion form of catechol. While the existence of acetate in solution (4X M) failed to diminish catechol adsorption, phosphate at the same concentration had a strong inhibitory effect on adsorption (Figure 1). Acetate and other monocarboxylic acids are known to chemisorb on oxides by coordination of the carboxylate group to surface Fe or A1 (5,6,11,12), but the rather weak adsorption of these acids, indicated by low adsorption maxima and ease of replacement by nonspecifically adsorbing anions (5,13),suggests that they should be poor competitors with ligands that form bidentate or binuclear surface complexes. In contrast, phosphate competes effectively with catechol, presumably by forming a binuclear complex with adjacent surface A1 atoms, blocking these sites for catechol adsorption. Indirectly, these sorption data suggest that both functional groups of catechol are involved in the surface bond. Adsorption on Amorphous Alumina. Much larger quantities of catechol adsorbed on the noncrystalline alumina compared to the gibbsite. The isotherms in Figure 2 represent only the lower concentration range of adsorption, Much higher levels of apparent adsorption were achieved, but this may have been due at least in part to air oxidation of catechol and formation of polymeric oxidation products observed at the higher concentrations used. Evidence for such products was seen as the devel-
m
z
400
fE
Y
300
?
8
'0
2
200
P
5 100
1
2 3 4 equilibrium COW. (X 10 M)
4
2
4
6
8
1
0
1
2
4
Flgure 2. Isotherms for adsorption of catechol on amorphous AI(OH), at pH 7 from an aqueous solution of 0.1 M KCI (0),0.1 M KCI 1.5 X lo-, M potassium acetate (O),0.1 M KCI 1.5 X lo-, M K,HP04
+
+
(W.
opment of green coloration on the alumina. Such reactions remove catechol from solution, causing an overestimation of adsorption. In any event, true chemisorption appears to be in excess of 400 mmol/kg. Phosphate, and to a lesser M), has a suppressing effect extent acetate (at 1.5 X on apparent adsorption at high concentrations, but the adsorption at lower concentrations shows no significant effect of acetate (Figure 2). Strong adsorption of catechol may occur at a limited number of sites that bond the molecule through both functional groups; this type of interaction should be relatively insensitive to the presence of acetate. While phosphate had a relatively less dramatic effect on catechol adsorption in the case of amorphous alumina than in the case of gibbsite, the much greater number of adsorption sites in the former case, in spite of the higher concentration of phosphate used, may have been able to chemisorb much of the phosphate, lowering the degree of competition by phosphate for sites. Noncrystalline alumina was a more efficient adsorbent than either boehmite or gibbsite per unit of surface area. Calculations revealed boehmite and gibbsite to have almost identical maximum adsorption densities of 0.28 molecule of catechol/nm2 while noncrystalline alumina had a much higher density of 1.2 molecules/nm2. A high surface density of edge A1-OH groups, which readily participate in ligand exchange with organic acids, is presumably responsible for the behavior of noncrystalline alumina, a material that was synthesized under conditions which minimized the formation of well-ordered surfaces. Adsorption on Boehmite. The boehmite, with a higher surface area than gibbsite and a higher crystallinity than amorphous alumina, might be expected to have an intermediate adsorption capacity for catechol. This is shown to be the case in Figure 3, with an adsorption maximum of about 100 mmol/kg estimated from the isotherm. Again, concentrations higher than those plotted in Figure 3 produced apparent additional adsorption, but autoxidation/polymerization was believed to be responsible for this further diminished concentration of catechol in solution. A green oxidation product was again noticed at the higher catechol concentrations. Phosphate (2 X M) diminished adsorption as it did for the other aluminum oxides. In Figure 4, the adsorption isotherms of catechol, hydroquinone, and phenol on boehmite are compared. Clearly, the ortho arrangement of OH groups on the aromatic ring are necessary for strong bonding, but the para arrangement does not preclude adsorption. Since phenol fails to adsorb in the presence of excess C1-, the intrinsic
equilibrium conc. (X 10 MI
Flgure 3. Isotherms for adsorption of catechol on boehmlte at pH 7 from an aqueous solution of 0.1 M KCI (O),0.1 M KCI 2 X M KZHPO, (M).
+
2o
it 1 phenol
0
2
4
6
8
10
4 equilibrium conc (XlO MI
Flgure 4. Isotherms of catechol, hydroquinone, and phenol adsorbed on boehmite at pH 7 from an aqueous 0.1 M KCI solution.
affinity of a single phenolic OH for coordination positions on surface A1 atoms must be low. Hydroquinone is only slightly more acidic than phenol. Therefore, the greater tendency of hydroquinone to adsorb relative to phenol is not likely explained by a greater ease of replacement of the proton at the phenolic group by Al, but may indicate that a multinuclear surface bond involving both phenolic groups is possible. Chemisorption of phenols requires proton removal from the phenolic group(s), and while the very low acidity of these groups suggests that this removal would require a very high pH, the formation of chelates with Als+ or Fe3+can force the removal of protons at quite low pH (14). Thus, adsorption of catechol occurs on aluminum oxides, presumably via the catecholate anion, even at low pH (4). Phenol and hydroquinone adsorption is not assisted by this mechanism, and proton removal from the phenolic groups requires a high pH. IR Spectra of Adsorbed Molecules. Assignments of IR absorption peaks for catechol are listed in Table I (15-17). In addition, the observed IR bands for the complex of soluble A P with catechol and for catechol chemisorbed on amorphous alumina (spectra depicted in Figure 5) are assigned in Table I by comparison to the spectrum of free catechol. These assignments are in reasonable agreement with those reported by Weinberg et al. (18)from inelastic electron tunneling spectroscopy for catechol chemisorbed on Al,03, with differences probably arising Envlron. Sci. Technol., Vol. 22, No. 6, 1988 705
Table I. Assignment of IR Absorption Peaks of Free, AI-Complexed, and Adsorbed Catechol
wavenumber, cm-' A1 comfree plexed adsorbed
vibration C-C ring stretch C-C ring stretch C-C ring stretch coupled to OH bend, OH bend + C-0 stretch OH bend coupled to C-0 stretch
C-H bend (in plane) C-H bend (out of plane) C-H bend (out of plane) ring bend (in plane) C-H bend (out of plane) ring bend (in plane)
1625 1605 1535 1520 1475 1370 1283 1260 1245 1190 1100 1040 940 920 850 770 740 630
1590 1495
1612 1583 1495
1460 1340
1460 1340
1260
1257
1100
1100
1025(?) 870(?)
870(?)
800(?)
800(?)
740 640
2000
1800
1600
1400
1200
WAVENUMBER (CM-l) Flgure 6. FTIR spectrum of catechol adsorbed on amorphous alumina at the 100-200 mmol/kg level.
I
L lea0
1BW
1400
1200
1000
8W
BW
WAVENUMBER (CM-')
Flgure 5. Dlspersive Infrared spectra (KBr pellets) of amorphous alumina (Ilne l),amorphous alumina equilibrated with 0.5 X lo4 M (line 2),2 X lo4 M (line 3), and 4 X lo4 M (Ilne 4)catechol. The spectrum of the AI3+-catechol complex (line 5) Is shown for comparison purposes.
from the low-pressure, low-temperature conditions of this technique. Complexation of A13+ to catechol shifts the aromatic ring C-C stretch vibrations to lower wavenumbers while reducing the multiple OH bend/C-0 stretch vibrations in the 1190-1283 cm-' region to a single absorption at 1260 cm-l. Since chelation displaces the protons from the phenolic groups, bands associated with OH bend vibrations are lost. Weinberg et al. (18) concluded that phenol, resorcinol, and hydroquinone, as well as catechol, adsorb as the phenoxide anions, but these results may not be obtained in aqueous oxide suspensions at ambient temperature. The IR spectra observed for catechol adsorbed on alumina are similar to that for the Al-catechol complex, as 706
Environ. Sci. Technol., Vol. 22, No. 6, 1988
a comparison of peak positions in Figure 5 and Table I reveals. Within the range of catechol adsorption levels investigated by IR, there was no apparent change in the spectrum as a function of surface concentration other than a gain in intensity of the absorption bands attributed to adsorbed catechol and a relative weakening of the 1640 cm-l band due to adsorbed water. Fourier transform IR spectroscopy allowed a clearer spectrum of the catechol to be obtained at the lower adsorption levels. The 1200-2000 cm-l range of this spectrum is shown in Figure 6, revealing absorption bands at the same positions seen for catechol adsorbed at the higher levels. Clearly, the bonding environment of catechol adsorbed on alumina is very similar to that of catechol complexed with Al, evidence that the catechol bonds as a 1:l complex with surface Al. The lower energy of the ring C-C bonds resulting from this coordination (see Table I) suggests that A13+ perturbs the molecule, shifting the distribution of electron density in the aromatic ring. Although the FTIR spectra were obtained under evacuated conditions, the similarity of these spectra to those determined on a conventional instrument without evacuation suggests that the inner-sphere complexes existed on air-dry oxides prior to further dehydration induced by evacuation. Their formation in aqueous suspension has not been verified in this study, but the strength of adsorption of catechol from solution implies an inner-sphere bond on hydrated oxides. The IR spectrum of boehmite (Figure 7A) revealed peaks at 1560, 1470, and 1420 cm-l that were confirmed to arise from chemisorbed acetate, an artifact of the procedure for boehmite synthesis. Repeated (3x) washing with 0.5 M NaCl followed by dialysis to remove the salt produced the spectrum in Figure 7B, evidence that most of the acetate had been desorbed. Adsorption of acetate reproduced the
A
Flgure 7. Dlspersive It7 spectrum of boehmtte (KBr pellet) before (A) and after (B) washing with NaCI.
same spectrum seen in Figure 7A. Heating the boehmite at 100 “C for several hours greatly reduced the intensity of the 1640 cm-l band, a result of the loss of adsorbed water. Weak infrared peaks at 1495 and 1255 cm-l (spectra not shown) were detected for boehmite equilibrated with catechol, evidence that the boehmite surface bonded catechol by the same mechanism as amorphous alumina. The weak catechol peaks are explained by the low level of catechol sorbed by boehmite. FTIR spectroscopy was unable to detect an adsorbed species in the case of hydroquinone adsorption on boehmite. Catechol Vapor Adsorption. Oriented self-supporting films of boehmite had IR spectra similar to that of boehmite in KBr, with the exception that a sharp peak at 3675 cm-l was evident in the films. This peak has been attributed to the stretching vibration of “free” surface OH groups coordinated to two Al ions on the (020) faces of the crystallites (19). Evidently, KBr interacts with the boehmite surfaces to alter this free OH vibration. X-ray diffraction revealed that the oriented film had an enhanced intensity of the reflection from the (020) crystal plane relative to the intensity in the powder, confirming that the (020) surface predominates in the synthetic boehmite used in this study. While other crystal faces also generate IR absorptions due to surface hydroxyls (19),these were too weak to be observed in this study. Catechol vapor adsorbed on the boehmite film, generating an IR spectrum very similar to that of catechol adsorbed from solution, with major peaks at 1495 and 1255 cm-l. As vapor adsorbed, these peaks strengthened and reached maximal intensities, while the surface OH vibration at 3675 cm-l was not noticeably affected. This result suggests that few, if any, of the (020) surface OH groups were involved in ligand-exchange reactions with chemisorbed catechol and is consistent with the conclusion by Lewis and Farmer (19)that the (020) surface is not reactive toward phosphate. However, prolonged exposure to catechol vapor generated peaks at 1280 and 1370 cm-l and eliminated the 3675 cm-l peak. The former peaks are assigned to physically adsorbed catechol since they are consistent with the spectrum of free catechol (see Table I). The loss of the free OH vibration may result from hydrogen bonding of physisorbed catechol on the (020) surface, which would broaden or shift the 3675 cm-l peak. The results of the vapor-phase adsorption studies suggest that the dominant (020) crystal face of boehmite is not reactive toward catechol other than via weak physical adsorption mechanisms. This is consistent with the fact that all OH groups on an ideal (020) surface are coordinated to two A1 atoms. It is generally believed that this type of OH, similar to that found on the (001) surface of gibbsite, does not take part in ligand-exchange reactions (9). Chemisorption of catechol, then, is suggested to occur at the less prevelant crystallite edge faces, where OH
Figure 8. Diagram of catechol formlng a (A) bidentate complex at the edge of a gibbslte plate, (B) blnuclear complex at the edge of a boehmtte crystal. Open circles depict oxygen or hydroxyl, while shaded circles symbolize AI3+.
groups coordinated to single Al atoms reside. These groups are believed to readily protonate to form reactive A10H2+ groups, which act as Lewis acid sites for catechol adsorption. Possible bonding sites at gibbsite and boehmite crystal edges are depicted diagrammatically in Figure 8, showing the feasibility of both bidentate and binuclear bonding of catechol. Calculations from accepted bond distances in catechol (20) and aluminum oxides reveal that the 0-0 distances in catechol and in adjacent surface OH groups are comparable, 2.74 A in catechol and 2.71 A on the oxide, an indication that the structures shown in Figure 8 are stereochemically reasonable. The edge sites diagrammed in Figure 8 are also believed to be responsible for oxyanion adsorption. Crystal growth theory predits that the chemically more stable (020) surfaces should predominate over these edge surfaces, with the result that chemisorption on boehmite occurs on a small fraction of the total surface.
Acknowledgments Appreciation is expressed to Tim Wachs, Department of Chemistry, Cornel1 University, for his assistance in obtaining the FTIR spectra. Registry No. AlOOH, 1318-23-6; A120H3, 1344-28-1; 2HOCeH40H, 120-80-9;C6H,0H, 108-95-2; hydroquinone, 123-31-9; Al(OH)s, 14762-49-3.
Literature Cited Isaacson, P. J.; Frink, C. R. Enuiron. Sci. Technol. 1984, 18,43-48. Bjorling, C. 0. Farm. Reuy 1949,48, 588-599. Yost, E. C.; Anderson, M. A. Enuiron. Sci. Technol. 1984, 18, 101-106. Kummert, R.; Stumm, W. J . Colloid Interface Sci. 1980, 75, 373-385. Parfitt, R. L.; Farmer, V. C.; Russell, J. D. J. Soil. Sci. 1977, 28, 29-39. Parfitt, R. L.; Fraser, A. R.; Russell, J. D.; Farmer, V. C. J. Soil Sci. 1977, 28, 40-47. Bugosh, J. J. Phys. Chem. 1961,65, 1789-1793. Russell, J. D.; Parfitt, R. L.; Fraser, A. R.; Farmer, V. C. Nature (London) 1974,248, 220-221. Parfitt, R. L. Adu. Agron. 1978, 30,1-50. McBride, M. B.; Fraser, A. R.; McHardy, W. J. Clays Clay Miner. 1984, 32, 12-18. Rochester, C. H.; Topham, S. A. J . Chem. Soc., Faraday Trans. 1 1979, 75, 872-881. Cornell, R. M.; Schindler, P. W. Colloid Polym. Sci. 1980, 258, 1171-1175. McBride, M. B. Clays Clay Miner. 1982, 30, 438-444. Envlron. Sci. Technol., Vol. 22, No. 6, 1988
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(14) Motekaitis, R. J.; Martell, A. E. Inorg. Chem. 1984, 23, (15) (16) (17) (18)
18-23. Szymanski, H. A. Interpret. Infrared Spectra 1967,3, 23. Hidalgo, A.; Otero, C. Spectrochim. Acta 1960,16,52&539. Varsanyi, G. Vibrational Spectra of Benzene Derivatives; Academic: New York, 1969. Weinberg, W. H.; Bowser, W. M.; Lewis, B. F. Proc. I n t . Conf. Solid Surf., 2nd 1974, 863-866.
(19) Lewis, D. G.; Farmer, V. C. Clay Miner. 1986,21,93-100. (20) Sofen, S. R.; Ware, D. C.; Cooper, S. R.; Raymond, K. N. Inorg. Chem. 1979,18,234-239.
Received for review October 27, 1986. Revised manuscript received November 12, 1987. Accepted January 13, 1988. This research was supported by N S F Grant EAR-8512226.
Removal of Mercury Vapor from Air with Sulfur-Impregnated Adsorbents Yoshlo Otanl, Hltoshl Eml, Chlkao Kanaoka, Ichlro Uchljlma, and Hlroshl Nlshinot
Department of Chemistry and Chemical Engineering, Kanazawa University, 2-40-20 Kodatsuno, Kanazawa 920, Japan The adsorption process of mercury vapor on sulfur-impregnated activated carbon, active alumina, and zeolite was experimentally studied by using packed beds. It was found that the sulfur-impregnated active alumina and zeolite have unusual concave breakthrough curves; i.e., the outlet concentrations are not initially equal to zero but gradually decrease to the minimum and then increase to the inlet concentration. Despite the unusual shape of the breakthrough curves, these adsorbents had an equilibrium adsorbed mass equal to the stoichiometrical value obtained from Hg S HgS. The deficiency of these adsorbents for the removal of mercury vapor (nonzero initial outlet concentration) could be covered by combining them with sulfur-impregnated activated carbon.
+
-
Table I. Physical Properties of Supporting Materials of Sulfur active alumina composition, %
A1203,90 SiOz, 10
particle size, wm bulk density, g/cm3 particle density, g/cm3 BET surface, area, m2/g pore volume, cm3/g average pore diameter, nm
74-175 0.56 3.2 320 0.80 10
zeolite 3A
activated carbon
A1203, 46 SiOz, 36 Na20, 18 0.80 2.4 100
0.45 0.4
104-147 0.43 1.7 1250 0.56
