Bonding of chlorophenols on iron and aluminum oxides

Sorption of 2,4,6-Trichlorophenol by Bacillus subtilis. Christopher J. Daughney and Jeremy B. Fein. Environmental Science & Technology 1998 32 (6), 74...
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Environ. Sci. Technol. 1991, 25,702-709

(50) Acute Toxicity to Daphnia. In EEC Commission Directive 841449, Test No. (2-2, Official Journal of the European Communities 1984, No. L251, p p 155-159. (51) Tosato, M. L.; Marchini, S.; Paolangeli, G.; Passerini, L.; Pino, A.; Skagerberg, B. In Proceedings of the 8th European Symposium on QSARs, Sorrento, Italy, September 9-13, 1990; Silipo, C., Vittoria, A., Eds., Elsevier: Amsterdam,

T h e Netherlands, in press. Received for review September 23, 1988. Revised manuscript received July 2, 1990. Accepted October 20, 1990. Financial grants from the European Economic Communities (Contract B6641-7-32-87) and the Italian Minister of Education are gratefully acknowledged.

Bonding of Chlorophenols on Iron and Aluminum Oxides King-Hsl S. Kung" and Murray 6. McBrlde

Department of Soil, Crop and Atmospheric Sciences, Cornell University, Ithaca, New York 14853

The adsorption of 10 chlorophenols on synthetic, naturally occurring iron and aluminum oxides was studied to elucidate the mechanism of binding and relative bond strength of the chlorine-substituted phenols on oxide surfaces. Surface-enhanced deprotonation of chlorophenols was identified by spectroscopic methods. Chlorophenolates were found to be chemisorbed on oxide surfaces via an inner-sphere coordination. Chlorophenols also bonded on oxides by weak physical forces (H bonding and condensation), but these types of weak bonding were identified only when adsorption occurred from the vapor phase onto dry surfaces. Physisorbed chlorophenols, unlike chemisorbed molecules, were readily removed from oxide surfaces by washing with water. Poorly crystallinzed iron and aluminum oxides showed similar mechanisms of chlorophenol binding, although the bond for chlorophenolate chemisorbed on iron oxide was stronger than that on aluminum oxide. Only physically adsorbed chlorophenols were detected on crystalline gibbsite, suggesting that the dominant (001) crystal face, with surface hydroxyl groups doubly coordinated to Al, was not specifically reactive with the chlorophenols. Chemisorption, however, was identified on the crystalline iron oxide, goethite. From the extent of perturbation of aromatic ring electrons, the surface bond strength for chlorophenolates on aluminum oxide was found to correlate with the Lewis basicity of the phenolate anions (the higher the pK, of the chlorophenols, the stronger the surface bond). Nevertheless, the amount of chlorophenol adsorbed on noncrystalline iron oxide a t controlled pH of 5.4 was limited by the extent of deprotonation (the lower the pK,, the more adsorption).

Introduction Chlorophenols are widely distributed in soils and aquatic environments, arising mostly from their use as biocides and preservatives. The U.S. EPA has listed all of these xenobiotic chemicals as toxic pollutants, some of them having been designated as priority pollutants (1). In the past decade, much effort has been directed toward the study of their sorption and degradation in the environment (2-4). In a recent study, Urich and Stone (5) proposed a mechanism of degradation involving specific adsorption of chlorophenols onto manganese oxide followed by surface oxidation. However, bonding of chlorophenols on metal oxides has not been directly confirmed experimentally. Therefore, a study involving spectroscopic methods was considered necessary to explore the interaction between chlorophenols and metal oxides. In particular, the use of self-supporting oxide films was considered essential to avoid matrix effects on the organic-oxide interaction and

* Current address: Environmental and Water Resources Engineering, The University of Michigan, Ann Arbor, MI 48109-2125. 702

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to allow direct observation of the chemical environment of the adsorbed organics and of water competition for adsorption sites. The purpose of this work was to elucidate the mechanism of surface bonding of chlorophenols on metal oxides and determine the relative strength of the oxide-organic bond. Iron and aluminum oxides were chosen as adsorbents since they are the most common metal oxides in soil and aquifer materials. Adsorption of 10 chlorinated phenols including mono-, di-, and polychlorophenols was studied from the vapor and/or solution phase. Ultraviolet (UV) spectrometry was utilized to determine the degree to which adsorption perturbed the aromatic ring electrons of chlorophenols. Fourier transform infrared spectroscopy (FTIR) of the phenol-oxide complexes allowed the type of binding mechanism to be investigated. Adsorption of chlorophenols from dilute solution was quantified by isotherms to estimate the surface reactivity.

Experimental S e c t i o n Unless otherwise noted, chlorophenols and all the other chemicals used in this work were analytical grade, commercially available reagents (from Aldrich or Eastman) and were used without further purification. 2,3,4,6-Tetrachlorophenol was technical grade (Eastman) and was purified twice by sublimation. Water was distilled, deionized, and filtered through 0.2-km Millipore filters. All experiments were conducted a t 22 f 1 "C. Oxide Preparation and Characterization. a. Aluminum Oxides. Pseudoboehmite was prepared by fast hydrolysis of aqueous AlC1, with NaOH followed by mixing and aging a t raised temperature for several days. The gellike product was then dialyzed against fresh water to remove excess salts. The final suspension, with a concentration of -40 mg/mL and pH 5.5, was used to prepare transparent self-supporting films. The X-ray diffraction (XRD) pattern of freeze-dried material confirmed this aluminum oxide to be pseudoboehmite, with peaks indicating d spacings near 6.2 (very broad peak), 3.2, 2.3, 1.8, and 1.5 A (weak peaks). This poorly crystallized material was an incompletely dehydrated boehmite (Le., pseudoboehmite), containing more sorbed water than crystalline boehmite (6). The surface area of the freeze-dried material calculated from N2 adsorption by the three-point BET method was 324 m*/g. Crystalline gibbsite used in this work was obtained from the Macaulay Institute and has been characterized before (7). This gibbsite is composed of sheets of edge-sharing A1 octahedra forming hexagonal platelets with well-developed (001) faces. The BET surface area of this gibbsite was 32.5 m2/g (8). Poorly crystallized gibbsite was prepared by titrating aqueous A1C13 solution with dilute NaOH. The milky

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white suspension was aged at room temperature for several weeks and then dialyzed and stored as suspension. XRD and TR spectroscopy indicated this material to be disordered gibbsite. The BET surface area of this material was 56 m2/g. b. Iron Oxides. Well-crystallized goethite used for self-supporting film preparation was described and characterized in a previous paper (9). Noncrystalline iron oxide was prepared by a method described previously (IO),but using Fe(NO,), reagent instead of Fe2(SO4I3. The BET surface area of this noncrystalline iron oxide was 91.5 m2/g. c. Film Preparation. Details of the method for preparing self-supporting films to be used in FTIR studies are given in Kung and McBride (9). The transparent pseudoboehmite self-supporting films were further useful for UV transmittance studies because of their lack of color and minimal light scattering. Spectroscopic Measurements. Infrared spectra measurements were made on a Fourier transform Perkin-Elmer 1720-xspectrometer with 2-cm-' resolution. UV spectra were recorded on Perkin-Elmer Lambda 4C spectrometer, and ESR spectra were obtained with a Varian E-104 (x-band) spectrometer. The IR spectrum of each pure chlorophenol was obtained by dissolving the chlorophenol in methanol, then depositing the solution onto a NaCl window, and allowing the solvent to evaporate. The UV spectra of pure chlorophenols a t fixed pH were obtained by dissolving the compounds in water and adjusting the solution pH with diluted NaOH. Spectra of the organic-oxide complexes for vapor-phase adsorption were obtained after the oxide film had been exposed to chlorophenol vapor in a closed container with the pure chlorophenol present in excess. The exposure time required to obtain a detectable spectrum varied from seconds to hours depending on the vapor pressure of the chlorophenol. Relative quantity of chlorophenol adsorbed could be controlled by increasing the time of exposure to organic vapor. Spectra of organic adsorbed from aqueous solution were obtained after the oxide film had been soaked in M chlorophenol solution for -15 min. Spectra of adsorbed chlorophenols were obtained by subtracting the spectrum of the untreated oxide from that of the organic-oxide complex. Since the final spectra were refined by subtracting the untreated oxide spectrum, this eliminated the IR bands arising from the oxide itself. Isotherm Determinations. Adsorption isotherms were obtained by adding 15 mL of 2, 4, 6, 8, and 10 x M chlorophenol solutions to preweighed 300-mg quantities of noncrystalline iron oxide in polycarbonate centrifuge tubes. After the tubes were capped and shaken for 18 h, the oxide was separated from suspension by centrifugation a t 15000 rpm for 45 min. The supernatant was analyzed for unadsorbed chlorophenol by UV spectrometry a t the appropriate detection wavelength. The amount of adsorption was calculated from the difference between initial and final concentrations. The pH was adjusted by dilute NaOH immediately after the chlorophenol solution had been added to the oxide such that the final pH of the suspension after 18 h of shaking was 5.4. The wavelengths used to measure 3-chlorophenol, 3,4dichlorophenol, 2,4-dichlorophenol, and 2,4,6-trichlorophenol concentrations were 273.5, 283.0, 283.7, and 293.0 nm, respectively. These chlorophenols were selected for isotherm determinations because of their relatively high water solubility and because they represent mono-, di-, and trisubstituted chlorophenols. Standard solutions with no oxide added were subjected to the same treatment and

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Figure 1. IR spectra of adsorbed 2-chlorophenol on noncrystallineiron oxide at increasing amount of adsorption.

were used to generate standard curves. NaClO, was used as background electrolyte to maintain an essentially constant ionic strength (0.05 M) in all isotherm determinations. From preliminary experiments, 18 h had been found to be sufficient time for the adsorption reaction. Some of the isotherm experiments were duplicated and those data are reported as average values.

Results and Discussion Monosubstituted Chlorophenols. The FTIR spectra of 2-chlorophenol (2MCP) adsorbed on noncrystalline iron oxide generated by increasing exposure time to pure 2MCP vapor are shown in Figure 1. Bands a t 1582, 1474, and 1441 cm-l are assigned to the aromatic ring C=C stretching vibrations, while bands a t 1245,1155,1126,and 1033 cm-l are assigned to ring C-H in plane bending vibrations. The band a t 1053 cm-' is assigned to a C1sensitive vibration, and a relatively broad band a t 1294 cm-' is assigned to the C-0 stretching vibration. Three bands, 1378,1338, and 1198 cm-l, appearing a t high loading (Figure lC,D) are attributed to 0-H bending and deformation vibrations. A detailed study of these IR spectra revealed at least three types of surface-organic interaction involved in adsorption from vapor phase since increasing exposure time showed three different chemical environments of the bonded organics. Lack of 0-H bending and deformation bands a t low adsorption level (Figure 1A) indicated that 2MCP was adsorbed on the surface as the 2-chlorophenolate anion. The formation of phenolate in solution requires fairly alkaline conditions (the pKa of 2MCP is 8.55). Despite the expectation that the air-dried oxide films would have acidic surfaces, the deprotonation of 2MCP on the oxide was apparently induced by direct bonding between surface Fe atoms and the phenolate anion. The formation of inner-sphere surface-organic bonding at the initial stage of adsorption was further substantiated by the shift of 1R band positions of the adsorbed organic. Environ. Sci. Technol., Vol. 25, No. 4, 1991 703

For example, shifts in the ring C=C vibrational frequencies of 2MCP from 1594,1585,1495,1481, and 1452 cm-' to 1582, 1474 and 1441 cm-' (Figure 1A) are attributed to a change of electron distribution and symmetry of the ring as 2MCP is complexed on the surface. This type of bonding between surface Fe atoms and the 2-chlorophenolate through inner-sphere coordination will be referred to hereafter as chemisorption. At medium level of adsorption, (Figure lB,C), 0-H bending and deformation bands appeared at 1378 and 1200 cm-', respectively. The appearance of phenolic 0-H vibrations indicated that BMCP was retained on the oxide surface in the protonated form. When the 0-H bending band of adsorbed 2MCP a t 1378 cm-' was compared with that of the pure organic a t 1338 cm-l, it appears that a 40-cm-l shift resulted from adsorption. Shifting to a higher wavenumber indicates the OH group is restrained and more energy is required to bend the 0-H group. Thus, the 2MCP adsorbed a t this level was interpreted to be complexed with the oxide surface through an H bond. H bonding through the hydroxyl group of phenol is consistent with the upward shift of the OH bend vibration. At this level of adsorption, the OH group of 2MCP was probably H bonded to oxide surface hydroxyl groups or BMCP molecules previously adsorbed. Organics adsorbed via H bonding are considered to be physically adsorbed. At very high loadings, the 1R spectrum was found to be similar to that of the unadsorbed 2MCP spectrum. As shown in Figwe lD, 0-H bending and deformation bands appeared at 1338 and 1198 cm-l, and ring C=C stretching vibration appeared a t 1480, 1453, 1585, and 1594 cm-l. This result suggested that the organic on the surface was essentially undisturbed and 2MCP was condensed on the oxide surface by weak physical forces such as van der Waals or dipole-dipole interactions. Since the molecules are not perturbed by the surface, it can be debated whether this type of sorption should be considered an adsorption process. Nevertheless, it probably corresponds to multilayer adsorption described in the BET model of physical adsorption. Thus, organics sorbed via condensation were also considered to be physically sorbed. Desorption of BMCP from iron oxide was studied in order to compare the relative binding strength of 2MCP adsorbed a t different levels. Three stages of treatment, (1)exposure to the atmosphere, (2) washing with distilled water, and (3) washing with methanol, were used in the desorption study. Upon exposure to the atmosphere, loss of IR bands attributed to condensed 2MCP indicated that these weakly adsorbed organics readily evaporated as the organic vapor pressure was lowered. In contrast, the 2MCP that was sorbed by H bonding did not readily desorb by evaporation in air. However, this type of BMCP was easily washed off the surface by water. Apparently, H-bonded 2MCP was not able to compete with excess water for sorption sites. Chemisorbed BMCP, as revealed by the persistence of the IR spectrum, was not washed out by water. Repeatedly washing the oxide film with water showed no evidence of removing this type of bonded organic. Even repeated washing with methanol left most of the chemisorbed BMCP on the surface. Resistance to desorption may signify a strong phenolate-Fe bond (relative to the water-Fe bond) or a substantial activation energy that must be overcome to achieve desorption. Different modes of 2MCP bonding were also identified on the poorly crystallized aluminum oxide, pseudoboehmite. Figure 2 shows the IR spectra of BMCP adsorbed on pseudoboehmite after increasing exposure time to the 704

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Figure 2. I R spectra of adsorbed 2-chlorophenol on pseudoboehmite at increasing amount of adsorption.

Figure 3. Same as Figure 2 with scale expanded to show the range around 1300 cm-' (A-C). UV spectra of the adsorbed 2-chlorophenol on pseudoboehmite, (D-F).

organic vapor. The OH bend vibration was too weak and broad to be useful in identifying different types of adsorption a t high loadings (Figure 2C). Nevertheless, more than one type of surface bonding was suggested by the band around 1300 cm-l, which was assigned to the C-0 stretching vibration. This band shifted from 1306 to 1296 cm-' as the amount of adsorption increased. In a parallel study, adsorption of 2MCP onto pseudoboehmite was monitored by UV spectrometry as well. The UV spectra obtained for the adsorption levels corresponding to those in Figure 2 are shown in Figure 3D-F. For clarity, the C-0 band region of the IR spectra was replotted on an expanded scale in Figure 3A-C. The broad UV adsorption band around 280 nm (Figure 3D-F) arises from the B band of the aromatic ring T-T* transition (12). For acidic aqueous solutions of BMCP, the maximum absorption of this B band is a t 273 nm with a shoulder around 278 nm. When the solution pH is raised, the maximum of the B band shifts to 293.5 nm. The bathochromic shift is caused by deprotonation of the phenol, making nonbonding electrons in the phenolate anion available for interaction with the *-electron system to the ring (12). On the pseudoboehmite surface, phenolate was bonded to Al, which made the nonbonding electrons less available to perturb the ring. Thus, a less pronounced shift (to 281 rather than 293.5 nm) relative to phenolate anion was observed. As evident in Figure 3, a t high absorption levels, both the IR and UV spectra suggested a second type of ad-

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Wavenumber (cm-1) Figure 4. I R spectra of increasing adsorption of 3-chlorophenol on well-crystallizedgibbsite (A-C) and on poorly crystallized gibbsite (D-E).

sorption, indicated by the new band a t 1296 cm-l (Figure 3C) and a shoulder a t 276 nm in the UV spectrum (Figure 3F). The initially adsorbed organic (responsible for the IR band at 1306 cm-' and UV band at 281 nm) was bonded by inner-sphere coordination to surface A1 atoms since it was significantly perturbed from unadsorbed 2MCP (which has IR and UV bands a t 1292 cm-l and 273 nm, respectively). The second type of adsorbed organic, responsible for the IR band at 1295 cm-' and UV band at 276 nm, may have been H bonded since the bands were only slightly perturbed relative to unadsorbed 2MCP. Chemisorption was evidently largely completed before physical adsorption became significant. For crystalline gibbsite, no detectable chemisorption was found by IR w e n a t low adsorption levels. However, poorly crystallized gibbsite showed both chemical and physical (H-bonding) adsorption. The IR spectra of adsorbed 3-chlorophenol, (3MCP), on both gibbsites is displayed in Figure 4. Increasing exposure time of crystalline gibbsite to 3MCP vapor showed only physical (H-bonding) adsorption (Figure 4A-C). In contrast, chemisorption on poorly crystallized gibbsite was identified from the disappearance of 0-H bending and deformation bands as well as the shift of the C-0 stretching vibration from 1258 to 1271 cm-' a t low adsorption levels (Figure 4D). At high loadings, however, the IR spectrum of 3MCP adsorbed on poorly crystallized gibbsite (Figure 4E) became similar to that of crystalline gibbsite (Figure 4C), a result of the saturation of chemisorption sites as adsorption levels were increased. Lack of Chemisorption on crystalline gibbsite suggests that few specific adsorption sites were available on this oxide. Since the (001) crystal face is dominant on crystalline gibbsite and all the OH groups on this face are coordinated to two A1 atoms (14), it is concluded that doubly coordinated OH groups are not labile to ligand exchange by chlorophenol. Poorly crystallized gibbsite did chemisorb chlorophenol a t low loading, presumably due to singly coordinated OH groups located on edges ( 1 2 , 24) or surface defects expected to be plentiful on this disordered mineral. This result is consistent with chemisorption studies of other organic and inorganic anions on aluminum oxides, such as phosphate (13) and catechol (8), which indicated that only singly coordinated surface hydroxyl groups were reactive. Although the (020) faces that are dominant on boehmite are similar to the (001) faces of gibbsite and are not expected to chemisorb chlorophenols, the pseudoboehmite

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1700 1600 1500 1400 1300 1200 1100 Wavenumber (cm-l) Figure 5. IR spectra of adsorbed 3-chlorophenol on noncrystalline iron oxide (A) and on goethite (B). (C) Pure 3MCP on NaCl window.

used in this work was less crystalline than boehmite. Consequently, pseudoboehmite also chemisorbed chlorophenols a t low adsorption levels, the IR spectra indicating physical adsorption ( H bonding and condensation) only a t high surface loadings. In the case of goethite, chemisorption was evident despite the high degree of crystallinity of this oxide. This is probably due to singly coordinated OH groups, which reside on all dominant crystal faces of goethite (11). As shown in Figure 5, the IR spectra of 3MCP adsorbed on both crystalline goethite and noncrystalline iron oxide indicated chemisorption. For comparison, the IR spectrum of unadsorbed 3MCP is shown in Figure 5C. Notable changes in the IR spectrum upon adsorption on the oxide surface included loss or shift of the ring stretch vibrations at 1605,1590,1487,1474, and 1445 cm-l with new bands appearing at 1584 and 1473 cm-' (Figure 5A,B). The C-0 stretching vibration band was shifted from 1249 to 1271 and 1274 cm-' upon adsorption on noncrystalline iron oxide and goethite, respectively. Other bands appearing a t 1240 and 1158 cm-l in the spectra of Figure 5 were assigned to C-H in plane vibrations. The loss of bands near 1322 and 1220 cm-l assigned to the 0-H bending and deformation vibrations, respectively, suggested that adsorption involved direct coordination of phenolate to surface Fe atoms. Disubstituted chlorophenols. The IR spectrum of 2,4-dichlorophenol (24DCP) adsorbed a t low levels on noncrystalline iron oxide from the vapor phase is shown in Figure 6A. Also shown (Figure 6C), is the IR spectrum of unadsorbed 24DCP. The OH bending and deformation bands at 1406,1329, and 1187 cm-' were absent from the spectrum of adsorbed organic, indicating inner-sphere bonding (chemisorption). Furthermore, after adsorption, the broad band at 1479 cm-l (with a shoulder around 1490 cm-l) assigned to a ring C=C stretching vibration, was replaced by a sharp band a t 1473 cm-l. Bands that arose from C-H in plane bending (1249 and 1052 cm-l) and a C1-sensitive vibration (1096 cm-I), however, remained relatively unperturbed by adsorption. The IR spectrum of 24DCP adsorbed from aqueous solution on noncrystalline iron oxide (Figure 6B) was qualitatively similar to that for 24DCP adsorbed from the vapor phase (Figure 6A), although the former spectrum Environ. Sci. Technol., Vol. 25, No. 4, 1991

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Flgure 6. I R spectra of adsorbed 2,4-dichlorophenol on noncrystalline iron oxide from vapor at low loading (A) and from aqueous solution (B). (C) Pure 24DCP on NaCl window.

was much less intense due to water competition for sorption sites. The similarity between these suggested that the process of chemisorption from aqueous solution was not fundamentally different from the process of chemisorption from vapor. However, prolonged exposure of the iron oxide to M or higher concentrations of aqueous 24DCP showed no spectral evidence of the physical adsorption that was detected upon adsorption from vapor. An attempt was also made to adsorb 24DCP from both solution and vapor phases onto crystalline gibbsite. Vapor adsorption produced 0-H bend vibrations at 1420,1409, 1359, and 1332 cm-l even a t very low adsorption levels, suggesting that physical adsorption dominated, and little or no chemisorption occurred. Solution-phase adsorption produced no spectral evidence of molecules retained on the surface, as water adsorption apparently competed effectively with the weak physical bonding of 24DCP. Poorly crystallized gibbsite and pseudoboehmite revealed binding mechanisms similar to those of noncrystalline iron oxide upon adsorption from the vapor phase. For example, the IR spectra of 24DCP adsorbed on pseudoboehmite a t different loading levels are shown in Figure 7A-C. Chemisorption was identified a t low adsorbate levels (Figure 7A), evidenced by the absence of OH vibrations. Physical adsorption (H bonding) was identified a t high loadings (Figure 7C) by the presence of a broad and weak OH bending vibration a t 1420 cm-l as well as a new band a t 1286 cm-l, which was assigned to the C-0 stretch vibration perturbed by H bonding. After repeated washing of the oxide film with water, the spectrum of Figure 7C was changed to that of Figure 7D. Disappearance of IR bands a t 1420 and 1286 cm-' upon washing of the pseudoboehmite film with water indicated that almost all of the H-bonded 24DCP was removed from the surface but most of the chemisorbed 24DCP remained (Figure 7D). This result is substantially the same as that for the desorption studies on noncrystalline iron oxide. It should be noted that although IR spectra identified chemisorption of 24DCP onto poorly crystallized aluminum oxides from vapor phase, no adsorption from aqueous M 24DCP solution at pH 5.8 was detected by the same method. This result is attributed to the low adsorption level of 24DCP on aluminum oxides and the attenuation of IR beam intensity by excess adsorbed water. Lower 24DCP adsorption on aluminum oxide compared to adsorption on iron oxide from solution under the same experimental conditions suggests that surface A1 is less able than Fe to form a chemical bond in the presence of water, which implies that the Al-phenolate bond is weaker than 706

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Figure 7. I R spectra of adsorbed 2,4-dichlorophenol on pseudoboehmite at increasing amount of adsorption (A-C). (D) After washing repeatedly with water.

the Fe-phenolate bond. This suggestion was further supported by the relative shift of the phenolic C-0 bond vibrational frequency. The C-0 bond strength is expected to reflect the phenolate-metal coordination bond strength, since forming a direct bond between phenolate and the oxide may shift electron density from the phenolate anion toward the surface metal atom. The shifted electron density is expected to weaken the C-O stretching vibration. Therefore, a more energetic C-0 stretching vibration should be expected for weak phenolate-metal bonding. Since phenolate was identified as the chemisorbed species on poorly crystallized aluminum and iron oxides, the wavenumber of the C-0 stretching vibration for a given chlorophenol could be used to compare surface bonding strength on aluminum and iron oxides. In the case of 24DCP, the C-0 stretching vibration was found a t 1306 and 1293 cm-I on aluminum and iron oxides, respectively, indicating that phenolate was coordinated more energetically on oxide surfaces of Fe than Al. The weaker bonding of phenolate on aluminum oxide may explain why little adsorption from aqueous solution was detected by IR, as the excess of water molecules may have been effective competing ligands for chemisorption sites. For 2,6-dichlorophenol, 26DCP, several mechanisms were detected for vapor-phase adsorption on poorly crystalline aluminum and iron oxides. For example, on pseudoboehmite at least two forms of adsorbed 26DCP were identified by IR after the oxide film had prolonged exposure to the organic vapor. If the film was then exposed to the atmosphere for several hours, the IR spectrum showed that most of the physically adsorbed 26DCP evaporated from the surface. Loss of physisorbed molecules from the surface was identified by a dramatic decrease in intensity of 0-H vibration bands a t 1325, 1269, and 1179 cm-' and the C-0 band a t 1240 cm-l. Bands arising from X-sensitive and C-H deformation vibrations of 26DCP were found a t 1259, 1195, and 1071 cm-l. A 1292-cm-' band, which was assigned to the C-0 stretching vibration of chemisorbed 26DCP, remained intact. The IR spectrum of 26DCP adsorbed on noncrystalline iron oxide at low adsorption levels revealed a C-0 band shifted to 1284 cm-l. This band shift suggested that 2,6-dichlorophenolate bonded more strongly on iron than aluminum oxide surfaces. Polysubstituted Chlorophenols. Adsorption mechanisms for tri- and tetrachlorophenols were found to be similar to those for mono- and dichlorophenols. For ex-

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Figure 9. UV spectra of 2,4,5-trichlorophenoI aqueous solution at pH -6 (dashed line) and pH -9 (dotted line). Solid line is the 245TCP adsorbed on pseudoboehmite surface. I

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Figure 8. I R spectra of adsorbed 2,4,6-trichlorophenoI on goethite at increasing amount of adsorption (A-C) and on noncrystalline iron oxide (D).

ample, the IR spectrum of 2,4,6-trichlorophenol (246TCP) adsorbed on crystalline goethite with increasing exposure time to the organic vapor is shown in Figure 8A-C. Chemisorption was identified from the following evidence: ring C=C stretching vibration shifted from 1473 to 1453 cm-l; lack of OH vibrations a t 1320 and 1220 cm-l; an IR spectrum similar to that for 246TCP adsorbed on noncrystalline iron oxide (Figure 8D), a mineral with a high surface density of chemisorption sites. Since chemisorption of both 26DCP and 246TCP was detected on goethite, it was concluded that C1 group substitution adjacent to the phenolic oxygen caused little steric hindrance to adsorption to chlorophenols on goethite. On the other hand, no 2,6-dimethylphenol was chemisorbed on goethite (unpublished data), possibly because the more bulky methyl groups hindered phenolate bonding a t reactive oxide sites. When the adsorption of 246TCP onto noncrystalline iron M aqueous oxide, gibbsite, and pseudoboehmite from solution a t pH 5.9 was conducted, the IR spectra of the adsorbed 246'rCP on noncrystalline iron oxide and pseudoboehmite revealed chemisorption only. Because the solution pH controls the degree of chlorophenol deprotonation, chemisorption is expected to be a function of pH. As the pK, of chlorophenols decrease with increasing chlorine substitution on the ring, more highly substituted chlorophenols will be more fully deprotonated in aqueous solution at a given pH. This may explain the fact that for the same solution concentration of 24DCP and 246TCP, chemisorption of 246TCP onto pseudoboehmite was detected by IR, but chemisorption of 24DCP was not. Again, no physical adsorption from solution phase was detected for these poorly crystallized aluminum and iron oxides. No adsorption of 246TCP onto gibbsite from solution was detected by IR, confirming that physisorption of chlorophenols could not occur in the presence of aqueous solvent. Further insight into the mechanism of chlorophenol chemisorption was provided by the shift of UV spectra of 2,4,5-trichlorophenol (245TCP) adsorbed from the vapor phase onto pseudoboehmite (Figure 9). Since the pK, of 245TCP is 7.4, the UV spectra of aqueous 245TCP solution a t pH 5.5 and 9.0 represent the spectra of the neutral molecule and the phenolate anion, respectively. Adsorption produces a shift of the UV absorption B band from 290 to 302 nm accompanied by a shift of the E, band from -225 nm (broad shoulder) to -240 nm. Compared to the position of thie B and E2 bands of fully deprotonated 245TCP (at 2145 and 310.5 nm), it was found that adsorption of aluminum oxide generated -50-6070 of the

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Figure 10. I R spectra of adsorbed 2,3,4,6-tetrachlorophenol on gibbsite (A), poorly crystallizedgibbsite (B), and noncrystalline iron oxide (C).

band shift expected if complete deprotonation of 245TCP had occurred. The significance of this UV absorption band shift will be further discussed below. The IR spectra of 2,3,4,64etrachlorophenol (2346TetCP) adsorbed from the vapor phase on pseudoboehmite and noncrystalline iron oxide are shown in Figure 10B,C. Since gibbsite adsorbs chlorophenols by physisorption only, the IR spectrum of 2346TetCP adsorbed on crystalline gibbsite is shown in Figure 10A for comparison. The IR bands for the physisorbed 2346TetCP a t 1204 and 1316 cm-' were assigned to 0-H bending and deformation, while the band at 1289 cm-l was assigned to the C-0 stretching vibration. Bands a t 1550, 1451, and 1372 cm-I were assigned to ring C=C vibration. For chemisorbed 2346TetCP the ring C = C stretching vibration was changed to 1429 and 1443 cm-l for noncrystalline iron oxide, and pseudoboehmite, respectively. The IR spectra of pentachlorophenol (PCP) adsorbed from the vapor phase on iron and aluminum oxides showed broad bands around 1570 and 1430 cm-l; the characteristic sharp IR bands of PCP could not be detected. However, from ESR spectroscopy, a free radical was identified during the adsorption of PCP on the surface of pseudoboehmite. The appearance of an organic radical suggested that PCP was oxidized on the oxide surface. Recently, oxidation of PCP by manganese oxides has been demonstrated to be feasible (5) and the dimerization and dechlorination of PCP radical cation has been reported upon oxidation by Cu(I1) adsorbed on smectite ( 4 ) . No unidentified broad IR absorption bands were found when other chlorophenols were used as adsorbates in this work, suggesting that most chlorophenols were not oxidized (at least not a t levels detectable by IR). Environ. Sci. Technol., Vol. 25, No. 4, 1991 707

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Table I. pK, and B-Band Positions of UV Adsorption (nm) for Chlorophenols in the Protonated (OH) and Deprotonated ( O - ) Forms and Adsorbed on Pseudoboehmite (AI)

0.4

0.2

2MCP 3MCP 4MCP 24DCP 26DCP 34DCP 245TCP 246TCP

0 0

20 40 60 80 100 120 Equilibrium Conc. (pmol/L)

Flgure 11. Adsorption isotherms for selected mono-, di-, and trichlorophenols on noncrystalline iron oxide at pH 5.4 in 0.05 M NaCIO, ionic media.

Isotherms. All the chlorophenols (with the possible exception of PCP) used in this work appeared to chemisorb on noncrystalline iron oxide from the vapor and solution phases, based upon the FTIR spectra. In order to quantify the adsorption of chlorophenols from aqueous solution, adsorption isotherms of selected chlorophenols at pH 5.4 in the presence of 0.05 M NaC104 were obtained, and the results are graphed in Figure 11. The amounts of chlorophenols adsorbed on noncrystalline iron oxides decreased in the order 246TCP (6.15) > 24DCP (7.85) > 34DCP (8.63) > 3MCP (9.10),where the pK, values are indicated in parentheses. Clearly, adsorption from aqueous solution was dependent on the pK, of the chlorophenols, with the more readily dissociating phenols adsorbing most readily. Since the quantitites adsorbed on the oxide were low, it is likely that only a small fraction of the available adsorption site on noncrystalline iron oxide were occupied by chlorophenolate anions in aqueous solution at pH 5.4. The effect of pH on adsorption of 24DCP on noncrystalline iron oxide was studied by adjusting the solutionphase pH between 4.6 and 6.2. At pH 4.6, the amount of organic adsorbed over the concentration range studied was too low to be detected with confidence. At pH 5.4, adsorption was detectable, as shown in Figure 11. At pH 6.2, the amount of adsorption was higher than that a t pH 5.4. Apparently, in these acidic solutions that are below the pK, of 24DCP (7.85), the extent of dissociation of the chlorophenols determined the amount of adsorption. In acidic solution, the higher the solution pH, the greater the extent of 24DCP deprotonation and the more phenolate is adsorbed on the oxide. Although the extent of adsorption may also be influenced by the hydrophobicity of the neutral phenol species, the concentrations that were used to obtain isotherms in this study were far below the water solubility of the organics. Therefore, the hydrophobic effect was not considered to be a major driving force in removing chlorophenols from solution. Chlorophenol adsorption isotherms on aluminum oxide were not measured because adsorption was expected to be lower than that on iron oxide, based on the spectral evidence of a weaker surface bond. However, the relative bond strength for different chlorophenols adsorbed on aluminum oxide was estimated from the relative perturbation of aromatic ring electron density. Surface Binding Strength. Table I summarizes the UV maximum absorption B-band positions for chlorophenol solutions and chlorophenols adsorbed on pseudoboehmite from the vapor phase. For those B bands with identifiable shoulders (or two absorption maxima), the higher wavelength peak is in parentheses. The pK, value of the chlorophenols are also listed in this table. The bathochromic shift arising from deprotonation is due to the interaction of P electrons of the aromatic ring with the nonbonding electrons. Since the UV absorption maxima for chlorophenols complexed on aluminum oxide surfaces 708

Environ. Sci. Technol., Vol. 25, No. 4, 1991

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Flgure 12. (A) The pK, of chlorophenols as a function of their shift of B bands (nm) upon deprotonation (filled circles) and adsorption (open circles). (B) Chlorophenol UV-band shift upon adsorption expressed as percent of band shift induced by deprotonation plotted against pK,.

were positioned between the absorption peaks of chlorophenols and their correspondent anions (Table I), it appears that bonding of the chlorophenolate ligands with A1 reduces the interaction of the nonbonding oxygen electrons with the aromatic ring. The magnitude of the UV band shift upon deprotonation or adsorption on aluminum oxide is plotted as a function of pK, value in Figure 12A. The B band shift upon adsorption increased as the pK, of the chlorophenol decreased. This correlation was probably inherited partially from the greater shift caused by deprotonation (Figure 12A), since this shift was also a function of pK,. However, these two functions were not parallel to each other, suggesting that interaction with A1 did have influence on the shift induced by adsorption. In Figure 12B, the band shift resulting from adsorption, expressed as a percentage of the band shift caused by phenol dissociation, is plotted against the pK, of the chlorophenols. It was found that, the higher the pK,, the lower the relative shift. A high shift percentage suggests a chemical similarity of the adsorbed molecule to the free chlorophenolate anion, that is, weak bonding between the phenolate and surface A1 atom. In contrast, a strong surface bond between the organic and oxide would give the organic less character of the free anion, i.e., weaker ring electron perturbation. Therefore, Figure 12B demonstrates that chlorophenols with high pK, values bond more strongly than those with low pK, values, presumably because a high pK, value signifies strong Lewis basicity of the phenolate ligand. It should be noted that although a stronger surface bond was indicated from spectral data for chlorophenols with high pK,, less adsorption of these organics from solution was measured. The apparent discrepancy between the amount of adsorption from solution and the surface bond strength from spectral data was attributed to the limiting conditions imposed on adsorption from solution. Adsorption of chlorophenols on the oxide involves the dissociation of the organic to its anion form and formation

of the surface metal-anion bond by the displacement of a water ligand from the metal (15). Although the formation of the surface metal-anion bond evidently facilitates the deprotonation of the chlorophenols, dissociation of the organic in solution is pH dependent. Thus, the relative ease of phenolate anion formation becomes the limiting factor for the adsorption of phenols with high pK, from acidic solution. In contrast, if deprotonation of the organic is not a limiting factor (Le., the solution pH is higher than the pK, of the organic), the amount of organic adsorbed under controlled conditions becomes solely a function of the strength of surface bond on a particular adsorbent. For example, the quantity of para-substituted benzoates adsorbed on iron oxide at pH 5.3 was demonstrated to increase in proportion to the surface Fe-carboxylate bond strength (9). An adsorption model assuming that the amount of dissociation of the benzoates would limit adsorption would have predicted a reversed order of preference. Therefore, for adsorption of chlorophenols a t a solution pH range below their pK, values, it was the extent of dissociation of the chlorophenols rather than the intrinsic strength of the surface bond that determined the final amount of adsorption. Significance. Different mechanisms of surface bonding were identified by this work, and since the bonding mechanism determined the affinity for the surface, the mobility of chlorophenols in the natural environment is likely to be controlled by the operative mechanism. For example, the formation of inner-sphere complexes a t the surface may effect the subsequent rate of desorption (16) and redox reaction ( 5 ) and may possibly attenuate the biodegradation or facilitate the transport of these pollutants by colloidal oxide particles through aquifer materials. Chlorophenols will physically adsorb on dry oxide surfaces but are readily desorbed by wetting. Chlorophenols can also be chemisorbed from both the vapor and solution phases and will not desorb by wetting or washing with methanol. Conclusions

From the spectroscopic and adsorption data, the following conclusions can be drawn concerning the bonding of chlorophenols onto iron and aluminum oxide surfaces: (1) Chemisorption (inner-sphere complexation to Fe or Al) and two type of physical adsorption (H bonding and condensation) of chlorophenols on dry oxide surfaces from the vapor phase were identified from FTIR spectroscopy. Physically adsorbed molecules were readily washed off the surface by water and were not present when adsorption from aqueous solution occurred. (2) Only physical adsorption of chlorophenols was detected spectroscopically on crystalline gibbsite, whereas chlorophenols chemisorbed on poorly crystallized aluminum and iron oxides and goethite. Thus, the reactive chemisorption sites were suggested to be surface OH

groups coordinated to a single structural A1 or Fe atom. (3) Strength of the surface bond between chlorophenolates and aluminum oxide surfaces was correlated with the pK, of the phenols. The higher the pK,, the stronger the surface bond, suggesting that bond strength was controlled by the Lewis basicity of the chlorophenolate anions. (4) Adsorption from solution under moderately acidic conditions was controlled in part by the extent of dissociation of the chlorophenol, since the more acidic chlorophenols adsorbed in the greatest quantity, and higher pH induced greater adsorption. Registry No. BMCP, 95-57-8; BMCP, 108-43-0; 4MCP, 10648-9; 24DCP, 120-83-2;26DCP, 87-65-0; 34DCP, 95-77-2;245TCP, 95-95-4; 246TCP, 88-06-2; Fe20,, 1309-37-1; pseudoboehmite, 1318-23-6;gibbsite, 14762-49-3;2,3,4,6-tetrachlorophenol, 58-90-2; goethite, 1310-14-1.

L i t e r a t u r e Cited Keith, L. H.; Telliard, W. A. Environ. Sei. Technol. 1979, 13, 416-423. Schellenberg, K.; Leuenberger, C.; Schwarzenbach, R. P. Enuiron. Sci. Technol. 1984, 18, 652-657. Zielke, R. C.; Pinnavaia, T. J. Clays Clay Miner. 1988,36, 403-408. Boyd, S. A.; Mortland, M. M. Enuiron. Sci. Technol. 1986, 20, 1056-1058. Urich, H.-J.; Stone, A. T. Enuiron. Sci. Technol. 1989,23, 421-428. Hsu, P. H. In Minerals in Soil Environments, 2nd ed.; Dixon, J. B., Weed, S. B., Eds.; Soil Science Society of America: Madison, WI, 1989; Chapter 7. Russell, J. D.; Parfitt, R. L.; Fraser, A. R.; Farmer, V. C. Nature 1974, 248, 220-221. McBride, M. B.; Wesselink, L. G. Enuiron. Sei. Technol. 1988,22, 703-708. Kung, K.-H.; McBride, M. B. Soil Sci. SOC.Am. J . 1989, 53, 1673-1678. Kung, K.-H.; McBride, M. B. Clays Clay Miner. 1989,37, 333-340. Parfitt, R. L. Adu. Agron. 1978, 30, 1-50. Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric Identification of Organic Compounds,4th ed.; Wiley: New York, 1981, Chapters 2 and 6. Lewis, D. G.; Farmer, V. C. Clay Miner. 1986,21,93-100. Davis, J. A.; Hem, J. D. In The Environmental Chemistry of Aluminum; Sposito, G., Ed.; CRC Press: Boca Raton, FL, 1989; Chapter 7. Stone, A. T.; Morgan, J. J. In Aquatic Surface Chemistry; Stumm, W., Ed.; Wiley: New York, 1987; Chapter 9. Isaacson, P. J.; Frink, C. R. Environ. Sci. Technol. 1984, 18, 43-48. Dean, J. A. Handbook of Organic Chemistry; McGraw-Hik New York, 1987; Section 8. Received for review June 11, 1990. Revised manuscript received November 8,1990. Accepted November 19,1990. This work was supported by National Science Foundation under Grant EAR8512226.

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