Langmuir 1992,8, 525-533
525
Characterization of Benzoic and Phenolic Complexes at the Goethite/Aqueous Solution Interface Using Cylindrical Internal Reflection Fourier Transform Infrared Spectroscopy. 2. Bonding Structures M. Isabel Tejedor-Tejedor,’ Eric C. Yost, and Marc A. Anderson Water Chemistry Program, 660 North Park Street, University of Wisconsin, Madison, Wisconsin 53706 Received March 15,1991.I n Final Form: September 3,1991 Bonding structures of benzoic compounds at the goethite (a-FeOOHIlwaterinterface were investigated “in situ” using cylindrical internal reflection (CIR)Fourier transform infrared (FTIR) spectroscopy. CIRFTIR spectra of phthalate (PHTH), p-hydroxybenzoate (PHB), and 2,4-dihydroxybenzoate(2,4DHB) added to goethite suspensions allowed us to identify surface complexes using methodology described in part 1 of this study. The PHTH and PHBlsurface iron complexes are mainly formed via a bidentate, binuclear structure involving both oxygens of one carboxylate each bound to one surface iron atom. The 2,4DHB/surface iron complex is formed via a chelation structure involving one carboxylicoxygen and the phenolic oxygen (bidentate, mononuclear). Results obtained from CIR-FTIR studies were consistentwith adsorption isotherm data. The type of binding is related to the quantity of species adsorbed due to the numbers of surface sites having a particular coordination possibility.
Introduction The adsorption/desorption behavior of water-soluble organics on iron and aluminum oxides has been studied extensively over the years by soil and environmental chemists whose interest has been predicated by the belief that adsorption on metal oxides is the main mechanism for controlling the transport of organics in soil and In addition, it is generally thought groundwater that many of these inorganic particles are so completely bathed with these organic species that the surfaces of these particles no longer express the chemistries of their inorganic host. Furthermore, the subject of organic adsorption/desorption onto oxides is also of great importance technologically to a variety of additional disciplines. These include the use of surfactants in the flotation of minerals, the application of drying control agents and slip casting additives in the processing of ceramics, the employment of protective antioxidative coatings, dye sensitization of semiconducting oxides for solar cells, the photodegradation of toxic organic microcontaminants, etc.3-7 Information concerning the type of interaction between the organic solute and the surface of the hydrous oxide has often been obtained using data from adsorption isotherms and to some degree from calorimetric titrations.21* Still, the basic understanding of the adsorption behavior of organic matter at the oxidelwater interface is incomplete.9
* To whom correspondence should be addressed.
(1) Kummert, R.; Stu”, W. J. Colloid Interface Sci. 1980,75,373385. (2) Stu”, W.;Kummert, R.; Sigg, L. Croat. Chem. Acta 1980,53, 291-312. (3) Furstenau, D. W.;Healy, T. V. In Adsorptiue Bubble Separation Techniques; Lemlich, R., Ed.;Academic Press: New York, 1972; pp 92131. (4) Onoda,G. Y.;Hench, L. L. CeramicProcessing BejoreFiring; Wiley: New York, 1978. (5) Liska, P. N.; Vlachopoulos, N.; Nazeeruddin, M. K.; Comte, P.; Graetzel, M. J. Am. Chem. SOC.1988, 110, 3683. (6) Matthews, R. W.J . Catal. 1988, 111, 264. (7) Sabate, J.; Anderson, M. A.; Kikkawa, H.; Edwards, M.; Hill, C. G., Jr. J. Catal. 1991,127, 167-177. (8) Machesky, M. L.; Anderson, M. A. Langmuir 1986,2, 582-587.
The type of information gained from these model-dependent techniques fails to provide an answer asto whether the organic ligand is coordinated to the metal atom of the surface (inner sphere surface complexes) or whether these ligands retain their hydration sphere and only form an ion pair with the metal atoms of the surface (outer sphere surface complexes). To answer these questions, we need to measure “in situ” some spectral property of either the ligands or the surface metal atoms that depends on the constitution and symmetry of their surroundings. Surface spectroscopy techniques such as photoelectron spectroscopy (ESCA),secondary ion spectroscopy (SIMS),’O etc. or bulk techniques such as Raman and IR spectroscopy with a methodology adequate to detect and isolate the interfacial species1’ have been used successfully to study the gas/solid interface. Unfortunately, most of these techniques require that the sample be examined under a vacuum or at least in dry conditions. To investigate the interfacial chemistry of aqueous colloidalsystems, one can utilize two of the most promising “in situ” spectroscopic techniques: extended X-ray absorption fine structure and cylindrical internal reflection Fourier transform infrared spectroscopy (CIRFTIR).I3 In part 1of this series,14we have discussed how CIR-FTIR “in situ” spectroscopy can be used routinely to obtain information on the structure of complexes formed between organic oxo anions and the surface of metal oxides suspended in aqueous suspensions. In particular, we presented a detailed account of this methodology used in the study of complex formation between phenolic and benzoic species and a-FeOOH. (9) Davis, J. A.; Gloor, R. Enuiron. Sci. Technol. 1981,15,1223-1229. (10)Hercules, D. M. Anal. Chem. 1978,50, 734-744. (11)Bell, A. T., Hair, M. L., Eds. Vibrational Spectroscopies for Adsorbed Species;ACS Symposium Series 137;American Chemical Society: Washington, DC, 1980. (12)Hayes, K. F.; Roe,A. L.; Brown, G . E., Jr.; Hodgson, K. 0.; Leckie, Parks, G . A. Science 1987,238, 783. J. 0.; (13) Tejedor-Tejedor, M. I.; Anderson, M. A. Langmuir 1986,2,203210. (14)Tejedor-Tejedor, M. I.; Yost, E. C.; Anderson, M. A. Langmuir 1990,6, 980-987.
0143-7463/9212408-0525$03.00/0 0 1992 American Chemical Society
Tejedor-Tejedor et al.
526 Langmuir, Vol. 8, No. 2,1992 In this paper we use the CIR-FTIR technique to record the aqueous solution spectra of the acid and anion forms of uncomplexed phthalic and 2,4-dihydroxybenzoic acids. We compare and contrast these spectra with the ones of the interfacial complexes formed between the a-FeOOH surface with phthalate, p-hydroxybenzoate, and 2,4-dihydroxybenzoate. In addition we examine the adsorption isotherms for some of these organics adsorbates in order to relate the type of complex with the nature and number of sites on the goethite surface.
h
0
0 -
3.
w
(15) Yost, E.C.;Tejedor-Tejedor,M. I.;Anderson, M. A. Enuiron. Sei. Technol. 1990,24,822-828. (16) Yost, E.C.Ph.D. Thesis, Universityof Wisconsin-Madison, 1986. (17) Luh, J. D.; Baker, R. A. E M . Ext. Ser. (F'urdue Uniu.) 1970,137, 534-542. (18) Scott, H. D.; Wolf, D. C.; Levy,T. L. J . Enoiron. Qual. 1982,11, 107-112.
80
C
.-0 4-
CT L
40
0 v)
-0
a Experimental Section A. Materials. Data on the morphology and preparation of the ahorbent material, a-FeOOH,as well ason the characteristics of someof the phenolicand benzoic compoundsused as adsorbates are described in part 1 of this study.'* Additional adsorbates used in this paper are sodium benzoate (Kodak Chemicals),p hydroxybenzoate(97% minimum, Kodak Chemicals),potassium hydrogen phthalate (99.95-100.05% Aldrich Chemical Co.), and 2,4-dihydroxybenzoate (97+% Aldrich Chemical Co.). The chemicals required no additional purification. B. Analytical Techniques. 1. CIR-FTIR Method. As in part 1, the infrared spectra of both organic aqueous solutions and goethite suspensionswere recorded interferometrically with a Nicolet 60SX Fourier transform infrared (M'IR) spectrometer and a Hg-Cd-Te (MCT) detector. The sampling method was a cylindricalinternal reflection (CIR)cell (SpectraTechCIRCLE System)and a ZnSe crystal rod as the internal reflection element (IRE). We employed an angle of incidenceof 37O in these studies. Spectral resolution was 4 cm-l. Single-beamIR spectra were the result of 2000 or more co-added interferograms. Experimental methods and preparation conditions used to obtain and isolate the spectrafor organic compounds in aqueoussolution,or organic compounds at the goethite-aqueous interface, are described in part 1of this study." 2. Adsorption Isotherms. Batch equilibrium adsorption experiments of benzoate, phthalate, 2,4-dihydroxybenzoate (2,4DHB) and p-hydroxybenzoate on goethite were conducted at 20 O C at pH 5.5. Preceding the addition of the adsorbate, aliquota of goethite suspensions (from 0.7 to 37.0 g/L) were brought to a fixed ionic strength of 0.01 M in KC1 and to a pH = 5.5. These suspensions were left to equilibrate for several hours in a temperature-controlled shaker (New Brunswick). Following this equilibration procedure, microliter quantities of M) were added to vigorously stirred adsorbatesolution (lPLlP3 aliquota of the preequilibrated goethite suspensions. After a few minutes,pH wasmeasured,readjusted,and allowedto reequilibrate for 3 h. The samples were next centrifuged and the supernatants filtered through a 0.05-pm Nucleopore filter. The adsorption of organicswas calculated from the difference between organic added and that remaining in the supernatant. The concentration of the organic adsorbate in the supernatant was determined using radiolabeled [W] organic procedures as in earlierworks and/or by measuring absorbanceof the compound in the ultraviolet (UV). Ultraviolet analysis utilized 5-cm or 1cm quartz cells in a double beam Varian DMS 80 UV-vis spectrometer set at 224.0 nm for benzoate, 246.6 nm in the case of 2,4DHB, and 250.0 nm for PHB. As in the radionuclide technique, UV analyses used as a blank the filtrate of a goethite suspension. The analytical wavelengths chosen were major secondarypeakslsinorder to eliminateproblems caused by slight differences in the concentration of inert electrolytes which affected absorbance. For salicylateand benzoateboth [14C]and absorption in the UV were used to evaluateadsorption;agreement between these two techniqueswas excellent.lB The equilibration time of 3 h for all organic adsorbates was selected to minimize biodegradati~n."-'~ This time frame is also sufficient for the
120
E
0
00
Equilibrium Concentration (pM)
Figure 1. Adsorption isotherms of benzoate (A), phthalate (B), and p-hydroxybenzoate on a-FeOOH (l' vs Cq): at 20 "C, pH 5.5,O.Ol M KCl, and a goethite concentrationof 37 g/L (A), 0.73 g/L (B), and 2.0 g/L (C).
-
30
0
1
I 10
I 20
I
I
I
30
40
50
Equilibrium Concentration (pM)
Figure 2. Adsorption isotherms of salicylate (A) and 2,4-dihydroxybenzoate(B)on a-FeOOH (I'vs C-): at 20 OC, pH 5.5,O.Ol M KCl, and a goethite concentration of 0.73 g/L. adsorption process to reach a steady state.I6 In the case of 2,4DHB adsorption the experimentawere performed in the dark.
Results and Discussion A. Equilibrium Adsorption. Adsorption isotherms (pH 5-5.6) are shown in Figure 1for phthalate (PHTH), p-hydroxybenzoate (PHB), and benzoate (BNZ). Figure 2 showsthe isotherms for 2,4DHB adsorption and, as well, for purpose of comparison, the adsorption isotherm of salicylate (SAL)published elsewhere.16 Except for the case of benzoate, for which we do not have enough information, the isotherms in these two figures show Langmuir behavior. The chemical affinity of the adsorbent for the adsorbate is indicated by Langmuir constant (KL). The value of this parameter for the different organics/a-FeOOH systems can be extracted by fitting the adsorption isotherm data of Figures 1and 2 to the linearized form of the Langmuir equation
r = - I / K ~r/c + rmax
(1)
where m = - ~ / K and L b = I'mu. Results from these calculations show that the chemical affiity increasesas follows: 2,4DHB (KL= 0.92) >> PHTH (KL 0.60) > SAL (KL= 0.38) > PHB (KL 0.002) >> BNZ (KL= ?). However, the adsorption value at the plateau of the PHB isotherm (Figure IC)is almost 6 times that of both 2,4PHB and SAL (Figure 2). Therefore, the chemical affinity of these organics for the goethite surface is not the major fador in determining the maximum adsorption The same conclusion is reached when density (rmax). (19) Boyd, S. A.; King, R. Soil Sci. 1984,137, 115-119.
Benzoic Compounds at the GoethitelWater Interface
Langmuir, Vol. 8, No. 2, 1992 527
for PHTH and PHB (curves comparing the values of rmax
B and C of Figure 1).
In contrast to the values of rmax that we observed for these organics on goethite, Kummert and S t u " ' found lower adsorption of PHTH (pH 4.3) than salicylate on 7-Alz03. Appelt et aLZoreported PHTH adsorption >> salicylate and p-hydroxybenzoate adsorption at pH 4.2 and 5.4 on soil. Salicylate adsorption was about 20 % less than 2,4DHB adsorption with Fe or A1 oxides found in the literature. The adsorption isotherm unfortunally does not provide insight into the cause of this lack of correlation between chemical affinity and maximum adsorption density. It may be due to the differences in the number of surface Fe atoms bound to one adsorbate ion or molecule. It may also be caused by differences in the type of surface Fe atoms used by the different adsorbates (as is the case of surface goethite complexeswith benzoate14versus the ones with salicylate15). Alternatively, the lack of correlation may be due to stereochemical effects which control the packing density of the adsorbate on goethite. This information will be obtained from the interpretation of the mid-infrared spectra, measured "in situ" using CIRFTIR, of these organic adsorbates at the goethite/ aqueous solution interface. B. CIR-FTIRSpectra. CIR-FTIR spectra of benzoic acid/benzoate, p-hydroxybenzoate, and salicylate in solution have been presented in earlier papers.14J5 The assignment of bands to the vibrational modes of these benzoic compounds have also been explained in these respective papers using information not only from the literature but as well from the effect of DzO solvent as compared to HzO and also to the effect of pH (pD) on the position and/or relative intensities of the spectral peaks.14 In addition, the CIR-FTIR spectra of phenol in HzO and DzO at different pH values (and one pD) have assisted in supporting band assignments in other related probe compounds.14 Finally, the use of 13C-labeledcarboxylic carbon in salicylate provided confirmatory data regarding the assignments of IR bands from the carboxylic and phenolic substituents.15 In this section of the present paper, we report and examine the CIR-FTIR spectra of PTHT and 2,4DHB in aqueous solutions as well as the spectra of interfacial complexes with goethite and complexes with Fe(II1) in solution of PHB, PTHT, and 2,4DHB. 1. Phthalate. Uncomplexed PHTH in Aqueous Solution. Aqueous phthalate and phthalic acid yield CIRFTIR spectra seen in Figure 3. At pH 7.3 (Figure 3c), phthalate is essentially a doubly charged anion. On the other hand, a t pH 3.7, phthalate exists as a single charged anion (Figure 3b). At pH 1.7 (Figure 3a) only phthalic acid is present in the solution (pK1 = 2.75, pK2 = 4.93 at Z = 0.1 On the basis of (a) differences in frequency values and relative intensities of the absorption bands among the spectraof different pH solutions,(b) similarities with the spectra of benzoate/benzoic acid in aqueous solution^,'^ and (c) literature data for this organic molecule in different matrices,l6 bands at 1705 and 1293 cm-1 of Figure 3a should be assigned respectively to v(C=O) and to the strongly coupled v(C-OH) and 6(C-0-H) vibrations of the two carboxylic groups. Absorption bands a t 1552 and 1382 cm-', in part c of Figure 3, pertain to the asymmetric (vmY) and symmetric (vsy) stretching of the (20) Appelt, H.; Coleman, N. T.;Pratt, P. F. Soil Sci. Am. Roc. 1975, 39.623-627. (21) Martell, A. E.;Smith, R. M. Critical Stability Constants;Plenum Press: New York, 1977; Vol. 3.
P
w
0 2
U
m U
0 v)
m
a
*
1
1900 1700 1500 1300 1100
900
W A V E N U M B E R S (cm-l) Figure 3. CIR-FTIR spectra of potassium phthalate solutions in HzO, at different pH values and I = 1 M KCI: (A) pH 1.7,O.Ol M; (B)pH 3.7, 0.1 M; (C) pH 7.3, 0.1 M. Table 1. Results from Mathematical Analysis of Absorption Bands from the Spectra of Ionic PHTH in H20/DnO Solutions (Spectra Shown in Figure 3) frequency at maximum,
intensity at maximum,
1401 1380 1361
0.045 0.025 0.004
p H = 7.3 1435-1330
1404 1385 1359
0.055 0.061 0.005
13 32 40
96 78 60
p D = 5.8 1650-1497
1604 1586 1561
0.034 0.037 0.184
11 16 30
51 1 72
1465-1330
1444 1431 1405 1384
0.014 0.002 0.114 0.071
8 15 15 17
54 51 52 38
solution frequency pH/pD range, cm-l p H = 3.7 1435-1330
cm-l
AU
% width gauss 17 43 28 70 17 58
two carboxylate groups. The band at 1403cm-l had been previously assigned to 6(C-O-H)22 and ~ , y 2of~ the carboxylic and carboxylate groups. The fact that this band is still present in the spectra of PHTH at pH 7.3 (Figure 3c) and PHTH a t pD 5.8 (Table II), where there are no 4 0 2 H groups, eliminates the possibility of the former assignment. Furthermore, the mathematical analysis using FOCAS (a Nicolet Instruments Fourier self deconvolution and curve analysis program) of the 1435-1330 cm-' region of the spectra of Figure 3b (PHTH, pH 3.7) and Figure 3c (PHTH, pH 7.31, shows that the intensity of the 1404-1401 cm-l component changes very little with a change in pH (Table I). This contrasts with the intensity of the 1385-1380 cm-l peak which multiplies by more than 2 over the same pH change. At pH 3.7 only one carboxylic group is ionized whereas at pH 7.3 both groups are ionized. Therefore, this band (1404-1401 cm-l) cannot be assigned to the V,~(COO-)mode but should be assigned to the vibration 14 of the benzene ring and the band at 13861380 cm-l to the v,(COO-). In Table I1 we provide additional information on the band assignment relating to spectra shown in Figure 3, as well as frequencies and band assignment of phthalate in D2O at pD 5.8. Two observations were obvious by comparing phthalate in HzO (22) Arenas, J. F.;Marcos, J. F.Spectrochim. Acta, Part A 1980, %A 1075-1081. (23) Lindberg, B. J. Acta Chem. Scand. 1968,22, 571-580.
Tejedor-Tejedor et al.
528 Langmuir, Vol. 8, No. 2, 1992
and D20: (1)The bands assigned to the v(COO-) modes are positioned a t higher energies in D20 (as expected for some 0.-D bond formation as opposed to 0-H bond formation2*)and (2) C-C stretching and benzene (C-H) bending modes are not changed. Fe-PHTH Complexes. Figure 4 shows the spectra of interfacial PHTH in both HzO and DzO a t different pH/ pD values as well as the one correspondingto an Fe-PTHT complex in solution at pH 1.4. All spectra consist of mainly two bands which fall near 1545 and 1408 cm-I. Although the frequencies at maximum of these bands are practically the same for all of the pH/pD conditions studied, their shapes and relative intensities are different as the pH/pD values change. For example, in the spectra of pH 3.5/pD 4.5 (Figure 4b,d) the “lower frequency band” is more asymmetrical about the central maximum and has a larger half-height width and relative intensity than in the spectra of pH 6.5/pD 6 (Figure 4c,e). These differences are even more pronounced when this band is compared with the one for the spectrum of Fe-PHTH complex in solution (Figure 4a).
The mathematical analysis of the “lower frequency band” for the spectrum of pH 3.5/pD 4.5 shows that it is the result of four components, whose maximum position, intensity, and shape are presented in Table 111. The component at 1403 (H20)-1404 (DzO) cml-l is present as well in the spectra of uncomplexed PHTH in solution and we have assigned it to the vibration 14 of the ring (see Tables I and 11). The component at 1381 (H20)-1388 (DzO) cm-I is very close in value to the vsy mode of uncomplexed carboxylate of PHTH in solution (1380 cm-l at pH 3.7; 1385 cm-l a t pD 5.8); thus, this band we assign to one uncomplexed carboxylate group of the interfacial PHTH. Neither the peak a t 1411 (H20)-1413 (DzO) cm-l nor the one at 1355 (H20)-1361 (D20) cm-l are part of the spectrum of uncomplexed PHTH in solution (see Table I); therefore, they ought to be due to the vSy mode of carboxylate groups bound to the surface Fe atoms. The band at 1411 (H20)-1413 (DzO) cm-l should be ascribed to a bidentate complex,and the band at 1355(H20)-1361 (DzO) cm-I to a monodentate complex, since the frequencies of these bands are higher and lower respectively than that of the one of uncomplexed ~ a r b o x y l a t e In . ~ the ~ spectrum of pD 6.0, the “lower frequency band” is mainly the result of the same overlapping peaks as appeared in the corresponding band in the spectrum of pD 4.5, but the relative intensity of the component at 1361 cm-l is less than half (see Table 111) (given the low absolute intensities of the bands in the spectrum of pD 6, this component is almost undistinguishable from noise). Finally, in the spectrum of the Fe-PHTH complex in solution (Figure 4a), the “lower frequency band”, that falls at 1413 cm-’, can be fitted by two curves whose frequencies at maximum are 1422 and 1406 cm-l (Table 111). The higher frequency component we attribute to the yay of a bidentate carboxylate and the lower frequency band to the vibration 14 of the ring. The second most intense band in the spectra of Figure 4, whose frequency at maximum varies between 1541cm-’ (pH 3.5, Figure 4b) and 1569cm-l (Fe-PHTH in solution, Figure 4a), is very broad and is covering the spectral range where the vasy mode of both free and bidentate carboxylate in PHTH are expected to fall. We performed the mathematical analysis of this band in the spectra of interfacial PHTH at pD 4.5 and at pD 6.0, and in the spectrum of Fe-PHTH complex in solution. The results are given in Table 111. We attribute the components at 1612-1605 cm-l and 1588-1581 cm-l, as in the case of free PHTH in solution (Table I and 111, to v(C-C) ring vibrations. The components a t 1565 and at 1563 cm-l, of the spectra of pD 4.5 and 6.0, respectively, we assign to the vasy of uncomplexed carboxylate groups and the peaks a t ~ 1 5 5 cm-l 0 and 1530 cm-l of the same spectra to the vaSy mode of bidentate carboxylate groups. The position of the band corresponding to the v(-C=O) of a monodentate carboxylate in the spectrum of pD 4.5 (Figure 4d) is not apparent. Whereas the band of the spectrum of Fe-PHTH in solution associated with the vSy mode of carboxylate (1413 cm-l) does not have a component for uncomplexed carboxylate groups, the band of this spectrum in the frequency region of the vasY mode of carboxylate (1569 cm-’) includes the peak at 1568 cm-l as the most intense constituent (Table 111). This peak we have attributed previously to free carboxylate in the case of interfacial PHTH. Thus, we are forced to conclude that the assignment of these individual components to different types of carboxylate
(24) Pinchas, S.; Lulight, I. Infrared Spectra of Labelled Compounds; Academic Press: London, 1971.
(25) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds; Wiley: New York, 1986.
Table 11. Infrared Frequencies and Band Assignment for PHTH in H2O and DzO Solutions observed frequencies,@cm-l band assignment pH = 1.7 pH = 3.7 pH = 7.3 pD = 5.8 u(C4) 1705 1709 V.W. 6(H20)* 1622 1622 u(C-C) ring, 8a 1603 1603 v(C-C) ring, 8b 1582 (ah) 1582 (ah) 1580 (sh) u(COO-),, 1556 1552 1561 v(C-C) ring, 19a 1488 1486 1486 v(C-C) ring, 19b V.W. 1443 1444 v(C-C) ring, 14 1401 1403 1405 u(COO-),y 1382 (ah) 1382 1386 6(CH) + coupled 1293 1298 402H mods: 1278 u(C-0) and 1260 (sh) 1260 6(C-O-H) 6(CH) 1141 1141 1150 1148 (C-H) inner pl bend 1075 1078 1085 1086 a Key: sh shoulder; v.w., veryweak absorption band. Result from a poor subtraction of the sovlent bending vibration.
a I
I
I
I
I
1950 1750 1550 1350 1150
WAVEN
u M BERS
1
950
(cm-1)
Figure 4. CIR-FTIR spectra of complexed phthalate (PHTH): (a) F e L P H T H in HzO solution, p H 1.4,l M KCl, [Felbd = 0.33 M, and [ P H T H I b ~ = 0.025 M; (b-e) interfacial PHTH, I = 0.01 M KC1, and r = 100 pmol/g; (b) p H 3.5, (c) p H 6.5,(d) pD 4.5, (e) pD 6.0.
Langmuir, Vol. 8, No. 2, 1992 529
Benzoic Compounds at the GoethitelWater Interface
~
Table 111. Results from Mathematical Analysis of Absorption Bands from the Spectra of Complexed PHTH (Interfacial and in Solution PHTH) (Spectra Shown in Figure 4) suspension frequency frequency at intensity at compound RH/PD range, cm-1 maximum, cm-1 maximum, AU width % 1650-1497 interfacial PHTH pD = 4.5 1605 0.003 16 1581 0.007 19 1565 0.006 22 1550 0.012 25 1530 0.011 31 ~
99 53 85 97 99
1430-1325
1413 1404 1388 1361
0.018 0.029 0.026 0.013
15 19 26 38
34 77 70 97
1650-1497
1608 1583 1563 1547 1530
0.003 0.004 0.006 0.006 0.005
22 17 16 15 17
49 49 49 51 51
1430-1340
1414 1407 1389 1362
0.004 0.005 0.004 0.001
18 19 25 14
47 44 79 47
pH = 3.5
1430-1330
1411 1403 1381 1355
0.008 0.025 0.012 0.011
14 34 37 37
99 93 75 70
pH = 1.4O
1650-1497
1612 1587 1568 1549 1527
0.030 0.033 0.043 0.026 0.025
25 22 19 24 25
99 59 54 80 76
1435-1332
1422 1413 1406
0.064 0.002 0.079
21 15 24
99 47 66
pD = 6.0
Fe-PHT H in aq solution
gauss
~~
Solution pH.
groups is not straightforward. It may well be that the triplet 1568, 1550, 1527 cm-l mainly arises from the coupling of the uasy mode of complexed carboxylates with 4C-C) ring modes due to the increase in coplanarity of this group with the ring upon complexation. The same phenomenon was observed in the spectrum of Fe-benzoate complex in s01ution.l~ In summary: (a) In the case of interfacial PHTH, this organic ligand is bound to the surface of a-FeOOH through one carboxylate group, mostly forming a bidentate complex. However, some monodentate is clearly visible in the spectra of the lower studied pH/pD conditions. (b) Although the exact value of Au (uasy- vsy of COO-) for the complexed carboxylate cannot be determined, because the true position for the uasy is unknown, Au should be larger than 117 cm-l (at pD 4.5, 1530 - 1413 cm-l, Table 111). Although, this value of Au is closer to the Au values for bridging bidentates carboxylates than to the ones for bidentate mononucleate, this does not provide definitive proof as to the type of bidentate complexes.26 (c) Under the experimental conditions at which we prepared the FePHTH complex in solution, there are not any uncomplexed carboxylate groups. Absorption bands at 1708and 1300cm-l in the spectrum shown in Figure 4a provide evidence as to the presence in solution of PHTH with -HCOz groups. Unfortunately, we do not have enough information to distinguish whether these bands come from PHTH acid, PHTH with a carboxylate group complexed with iron, or both. However, we know that the complex in solution is at equilibrium with a solid phase whose spectrum does not show bands coming from -HCOz vibrations (1708 and 1300cm-l). The rest of the absorption bands coming from the solid phase
overlap with those of the solution spectrum. It is very likely that two PHTH species are at equilibrium in solution, a diprotonated PHTH and a PHTH with the two carboxylate groups forming complexes with Fe atoms. Bands due to carboxylic groups are observed as well in the spectra of PHB (Figure 5a, 1671and 1353cm-l) and BNZ14 with iron in solution, even the Fe/organic ligand ratio is very high. 2. pHydroxybenzoate. As we mentioned before, the CIR spectra of uncomplexed PHB in aqueous solution have been presented and discussed in part 1of this series of papers. Fe-Parahydroxybenzoate (PHB) Complexes. The spectra of interfacial PHB in DzO at pD values ranging from 3.5 to 6.0, the one of Fe-PHB in solution at pD 2.2, and, for comparative propose, the already published14 spectrum of PHB at pD 6 are seen in Figure 5. The spectra of interfacial PHB for the different pD values resemble each other and they are similar to the spectrum of uncomplexed PHB a t pD 6 (monoprotonated anion). The main discrepancy between the spectra of uncomplexed and complexedPHB is connectedwith the position, profile, and relative intensity of the band assigned to the uasy of carboxylate. In the case of the uncomplexed PHB, the frequency of this band is 1540 cm-l, its intensity is about 0.7 times that of the strongest band in the spectrum, and it is rather symmetric around the central maximum, Figure 5f. On the contrary, the peaks of the interfacial PHB spectra which best match this band at 1540 cm-' have such low frequency values as 1506 cm-' (pD 3.51, 1508 cm-1, (pD 4.0), and 1510 cm-' (pD 5.1 and6) (see parts b-e of Figure 5); furthermore, they are asymmetrical and not very intense (about 20 ?% of the intensity of the strongest
Tejedor-Tejedor et al.
530 Langmuir, Vol. 8, No. 2, 1992
a
I
- f
Table IV. Results from Mathematical Analysis of Absorption Bands from the Spectra of Ionic, Complexed in Solution, and Interfacial PHB, in D2O (Spectra Shown in Figure 5 and in Ref 14) frequency frequency at intensity at range, maximum maximum, ?6 compound pD cm-1 (cm-') AU width gauss PHB in aq 6.0 1492-1320 1407 0.012 25 37 solution 16 75 1386 0.110 ~
c L
f
c
I .
r
c
0
f
c
e
W
d
0
z
~~
Fe-PHB 2.2 1450-1365 in aq solution
1425 1412 1399
0.013 0.028 0.028
12 15 15
65 98 74
interfacial PHB
3.5 1450-1315
1423 1406 1392 1375 1354
0.013 0.017 0.022 0.011 0.007
14 16 17 22 28
100 98 89 92 55
4.0 1450-1315
1423 1403 1389 1370 1350
0.013 0.019 0.027 0.013 0.008
18 19 21 24 23
95 100 61 94 98
5.1 1450-1315
1424 1402 1388 1368 1340
0.012 0.017 0.023 0.011 0.007
29 24 25 59
100 100 72 98 94
1423 1402 1388
0.0005 0.0013 0.0017
24 22 24
50 50 50
d
IS\O
1+20
I
I
1520
1320
I
1120
i
920
WAVENUMBERS (cm-')
Figure 5. CIR-FTIR spectra of p-hydroxybenzoate (PHB) in DzO: (a)FemPHB in solution, pD 2.2,l M KC1, [FeIbd = 0.034 M, and [PHBIbd = 0.017 M; (b-e) interfacial PHB, Z = 0.01 M KCI, and r = 100 pmollg; (b) pD 3.5, (c) pD 4.0, (d) pD 5.1, and (e) pD 6.0; (f) ionic PHB at pD 6.0.14
band). The highest intensity band of the spectra of complexed PHB, Figure 5, has a frequency (1407-1391 cm-l) that is just a few wavenumbers higher than that of the band attributed to uBy (COO-) of uncomplexed PHB in solution a t pD 6 (1386 cm-l, Figure 50. It is important to notice for the case of interfacial PHB that, whereas the frequency at the maximum of this band is almost the same for all pD values investigated (parts b-e of Figure 51, its symmetry increases and its bandwidth decreases with increasing pD. Furthermore, the spectra of interfacial PHB at pD 6 (Figure 5e) and Fe-PHB complex in solution (Figure 5a) are the ones that exhibit the most similar shapes and, paradoxically, the biggest difference in frequency (16 cm-1). We saw earlier that changes due to complexation, in the shape and relative intensity of the bands associated with the v(CO0-) modes, were exhibited as well by benzoic14 and PHTH. From these previous cases we learned that the mathematical analysis of the band in the region of 1508 cm-' does not provide additional structural information due to the fact that complexation causes the u,,(COO-) band to strongly couple with ring stretching modes. On the other hand, the deconvolution of the band associated with the u,(COO-) (1404-1391 cm-l), into individual components does provide some insight on the type of complexes formed by the carboxylate group. Results from the mathematical analysis of this "highest intensity band" of the spectra of uncomplexed PHB at pD 6,14 Fe-PHB in solution at pD 2.2, and interfacial PHB a t four different pD values are presented in Table IV. Data in this table show the following. (a) In the case of interfacial PHB, at pD values of 3.5,4.0, and 5.1, the band is best described by five components: the one at 1423 cm-1 we assigned to a ring C-C stretching mode; a pair of peaks at 1407-1392 cm-' (pD 3.51, 1404-1389 cm-l (pD 4.0), and 1402-1388 cm-l (pD 5.1) which we attribute to the vsy mode of a bidentate carboxylate; the component at 1376 cm-l (pD 3.5),1371 cm-I (pD 4,0), and 1368 cm-l (pD 5.1) which should be assigned to the u(C0Fe) mode of a monodentate carboxylate, since these frequency values are lower than the vgY (COO-) frequency for uncomplexed
6.0 1440-1330
33
PHB (1386 cm-l);14 and finally, bands near 1350 cm-1, may account for the contribution of neighboring bands and/or for a poor selection of the baseline. (b) The peak near 1370cm-' is absent from the set of components related with both interfacial PHB at pD 6 (adjust by 1423 cm-l and 1401-1387 cm-' peaks) and complexedPHB in solution (fitted by 1425and 1411-1399 cm-' peaks). (c) The band located at 1386cm-l, in the spectrum of free ionized PHB at pD 6,14 can be very well described by one curve (75% Gaussian). In the above paragraph, we attributed two peaks to the usy of bidentate carboxylate, meanwhile the frequency values of these two bands are different for the complex in solution (1411-1399 cm-I) than for interfacial complexes (whose values depend on pD: 1406-1392 cm-l, 1403-1389 cm-l, and 1402-1388 cm-l); the value of Au for these peaks, 14 cm-l, remains constant for all the complexes. Since the source of this doublet is the same for both interfacial and solution complexes, it cannot be attributed to two different binding sites or types of complexes. These two bands may rise from the coupling of the carboxylate mode with a u(C-C) of the ring. The phenolic stretching frequency of complexed PHB (1272-1274 cm-', Figure 5 ) is =8 cm-l higher than that of uncomplexed PHB at pD 6 (1266 cm-l) and is identical to one of the un-ionized free PHB (1271 cm-l).14 Thus, coordination of PHB with Fe through the carboxylate group may be the only reason for this small increase in the frequency of the phenolic stretching, without having to invoke its coordination with Fe. Although the ring-0-D bending (1012 cm-l for uncomplexed ionized PHB) would be a good vibration to diagnose whether or not the phenolic group is coordinated with Fe ions, this mode produces such weak bands that cannot be differentiated from noise in the spectra of interfacial PHB. Parts b and c of Figure 6 show the spectra of interfacial PHB in H20 at pH 3.6 and 5.5, respectively. Although they are too noisy to be informative in the region 1800-1450 cm-I, the absorption bands in the region below 1450 cm-' add new insight into the type of complexes that PHB forms with the Fe atoms
Langmuir, Vol. 8, No. 2, 1992 631
Benzoic Compounds at the CoethitelWater Interface
f
0
4
2
0
m
a
W
W
a
z
m
U 0
cn
m
a
a
a 0 cn
m
a
1800
1830
1460
1$90
11'20
980
W A V E N U M B E R S (cm-') Figure 6. CIR-FTIR s ectra of complexed p-hydroxybenzoate (PHB) in H20: (a) FeELPHB in solution, pH 1.6, 1 M KC1, [FeIbd = 0.06 M, and [ P H B l b ~= 0.03M (b and c) interfacial PHB, I = 0.01 M KC1, and r = 100 fimollg;(b) pH 3.6,(c) pH 5.5.
of the a-FeOOH surface. The presence of the doublet 1275-1251 cm-' (Figure 6b) indicates that the phenolic group is still protonated in the interfacial PHB;14 hence its oxygen is not coordinated with Fe. The same is true for the complex in solution at pH 1.6, Figure 6a. In summary: (a) The biggest fraction of the interfacial PHB is bound to the Fe atoms of goethite through the carboxylate group forming a bidentate complex, but as in the case of PHTH, a smaller fraction of the interfacial PHB binds to the goethite through only one oxygen of the carboxylate forming a monodentate complex. The quantity of this type of complex decreases with increasing pD. (b) Since the two stretching carboxylate vibrations couple with ring modes, the true value of its Au is not known, although it should not be smaller than =lo0 cm-l (vwy 1 1506 and usy= 1400 for pD 3.5). This value of Au does not allow us to distinguish between the formation of chelating and bridging complexes. (c) The frequencies for both the v,,(COO-) of the bidentate and the v(C-0) of the monodentate complexes decrease slightly with increasing pD (see Table IV). (d) Under the experimental conditions used in this study, the carboxylate of PHB forms bidentate complexes with Fe(II1) in solution. (e) The phenolic groupdoes not coordinate either with Fe(II1) in solution or with the atoms of the goethite surface. 2,4-Dihydroxybenzoate. 2,4DHB in Aqueous Solutions. Expectedly, the addition of a second OH to the benzene ring produces a more complex IR spectra than other benzoic compounds employed in this study (see Figure 7). We have mentioned before the couplings that take place in phenol and p-hydroxybenzoate between the in-plane phenolic bending vibration and vibration 14 of the benzene ring.14 Furthermore, in the case of salicylate, we established by isotopic studies using 13C that the v,(C-O) mode of carboxylate is coupled with the in-plane phenolic bending,15and this coupling may happen as well for other salicylate derivatives. Given the information available in the literature concerning IR band assignments for 2,4DHB, and other IR information for related compounds, we will yet need studies with 13Cand lSOmarked molecules to make definitive assignments of bands in the spectrum of this organic compound dissolved in Hz0 (parts a and b of Figure 7). Fortunately, when 2,4DHB dissolves in D20,the H atoms of the OH groups exchange with D
Idso
is's0 ldS0 l i s 0 10'60
850
W A V E N U M B E R S (c m-l) Figure 7. CIR-FTIR spectra of 0.1 M potassium 2,4-dihydroxybenzoate solutions in HzO/D20,at different pH/pD values and I = 1 M KC1: (a) pH 5.5, (b) pH 10, (c) pD 6.0,and (d) pD 10.
atoms and, in this way, the frequency value for the inplane bending modes of the phenolic groups shifts from the region of 1350 cm-l to the vicinity of lo00 cm-l. As the frequency value of this vibration in D2O is far removed from the frequency value of both, vibration 14 of the benzene ring and the symmetric stretching of the carboxylate group (which ranges between 1400and 1300cm-'), the possibility of coupling among these vibrational modes disappears. This will result in an easier band assignment of the spectral bands when 2,4DHB is dissolved in DzO in lieu of H2O. Additionally, since the intramolecular deuterium bond formed between the one oxygen of the carboxylate group and the phenolic OD group is weaker than the corresponding hydrogen bond, the frequencies of the vas and va modes of the carboxylate group are closer to the ones of a symmetric carboxylate in the case of 2,4DHB dissolved in D20 than in the case of 2,4DHB dissolved in HzO, simplifying once again the interpretation of the spectrum. Band assignments for the spectra of this organic in DzO solution will be based on our IR studies of salicylate16and p-hydro~ybenzoatel~ dissolved in D2O as well as on the very sparse literature data on the IR of 2,4DHB in KBr.26 The CIR spectrum of 2,4DHB in D2O solution at pD 6 is seen in Figure 7c. The profiles of the IR spectra of salicylate15 and 2,4DHB in D2O at pD 6 indeed resemble each other, the presence of another OD group in C(4) introduces the following changes in the spectrum of salicylate: (a) It lowers the frequencyof the stretching modes . of the carboxylate group (for 2,4DHB a t pD 6, the v ~ , and us,. are found a t 1537 and 1368 cm-l; salicylate in DzO at pD 5.8, uwy = 1545 cm-' and uSy = 1376 cm-9. (b) It decreases the frequencies of one of the components of the pair 19 of the ring (2,4DHB, 1477 cm-l; salicylate, 1459 cm-9 (c) It changes the bands belonging to the phenolic stretching and in-plane bending of salicylate (1241 and 1017 cm-l, respectively) such that they become doublets in the case of the spectrum of 2,4DHB (1289-1257 cm-' and 1040-1015 cm-l). With a change of the pD from 6 to 10,this organic passes from being a singlely charged anion, in which the carboxylic group is deprotonated,to a doubly charged anion, in which the carboxylic group and the phenolic OD substituted on C(4) are deprotonated (pK1 (26) Musso, H.Chem. Ber. 1956,88, 1915-1921.
Tejedor- Tejedor et al.
532 Langmuir, Vol. 8,No. 2, 1992
f
W
0 2
a
m
U
0 cn m
a
V k 1860
I
I
I
I
1670
1490
1310
1130
i 950
W A V E N U M B E R S (cm-') Figure 8. CIR-FTIR spectra of complexed 2,4-dihydroxybenzoate (2,4DHB) in DzO: (a) FeIL2,4DHB in solution, pD 2.2, 1 M KCl, [FeIhd = 0.76 M, and [2,4DHBIb~= 0.02 M; (b) interfacial 2,4DHB, pD 6, I = 0.01 M KCL, and r = 20 pmol/g. = 3.1, pK2 = 8.6 a t I = 0.1 M21). Spectral changes due to deprotonation of the phenolic OD will help in the interpretation of the spectrum at pD 6. Upon deprotonation, the frequency values for the pair 8 u(C-C) of the ring decrease (pD 6, Figure 7c, 1620-1587 cm-l; pD 10, Figure 7d, 1602-1557 cm-I), as well as did the frequency of the vmy of the carboxylate which drops to 1510 cm-l at pD 10 from 1537 cm-l a t pD 6 (see parts d and c of Figure 7). The decline in frequency value for these stretching modes can be interpreted in terms of an increase in resonance between the benzene ring and the carboxylate group which stabilizes the phenoxide ion. These spectral changes are similar to the ones observed in the spectrum of p-hydroxybenzoate upon deprotonation of the phenolic OD.14 Furthermore, as in the cases of phenol and p-hydro~ybenzoate,'~ at pD 10, the phenolic stretching modes should appear at higher frequency values than at pD 6 (1289-1257 cm-l, Figure 7c), and as a consequence, we assigned to these vibrations to the bands at 1326-1277 cm-' in the spectrum of Figure 7d. Finally, the doublet 1040-1015 cm-l in the spectrum of 2,4DHB at pD 6 (Figure 7c), attributed to the in-plane phenolic modes, appears as a single band, a t 1038 cm-', in the spectrum of 2,4DHB at pD 10 (Figure 7d). This band, obviously, arises from the phenolic in-plane bending mode of the OD in the ortho position. Fe2,PDHB Complexes. The CIR-FTIR spectra of Fe-2,4DHB complexes in D2O solution at pD 2.2, and 2,4DHB on the surface of a-FeOOH particles suspended in D2O at pD 6 are given in Figure 8. Changes in the spectrum of this organic upon complexation with iron (see Figures 7c and 8a) are totally comparable to the ones that take place in the case of salicylate which is known to form a complex with this metal These are as follows: (a) There is a decrease in the frequency of bands assigned to the pair 8 of v(C-C) ring modes (uncomplexed 2,4DHB, Figure 7c, 1620-1587 cm-l; complexed 2,4DHB, Figure 8a, 1604-1570 cm-I). (b) Both the vasy and usy of the carboxylate shift to a lower frequency value, with a net result of a larger Au for the complexed form (uncomplexed 2,4DHB, Figure 7c, 1537-1368 cm-1; complexed 2,4DHB, Figure 8a, 1532-1346 cm-l). (c) The highest frequency phenolic stretching vibration appears a t lower wavenumbers in the spectrum of the complexed form (1289 cm-l for free 2,4DHB versus 1273 cm-l for the complexed form).
The lower frequency one (1257 cm-l in the spectrum of free 2,4DHB) is not visible in the spectrum of complexed 2,4DHB. This may be due to a poor subtraction of the D20 bending mode (1200 cm-l 14). Unlike the Fe-salicylate complex, the complexed 2,4DHB shows an absorption band at 1022cm-l, which should be assigned to a phenolic bending vibration. In the spectrum of free 2,4DHB, there are two bands related to phenolic bending vibrations at 1040and 1015cm-l. The lower frequency one disappears upon deprotonation of the phenol group in the para position (see Figure 7c,d). These observations seem to indicate that the 1022-cm-lband is due to the phenol group in the para position, and hence, as this phenolic group is still protonated in the complexed form, it means that it does not bind to the iron ions. The spectrum of interfacial 2,4DHB, Figure 8b, exhibits peak frequencies and relative intensities similar to the spectrum of the Fe-2,4DHB complex in DzO solution. There are some differences, but these are small and may be due to noise in the spectrum of interfacial 2,4DHB. In this spectrum, the relative intensities of the bands assigned to the pair 8 of the benzene ring are different than the ones in the spectrum of the complex in solution (Figure 8a) and their frequency values are slightly lower (complex in solution, 1604-1570 cm-l; interfacial complex, 16011564cm-l). Furthermore, the vmy(COO-)appears at lower frequencies in the spectrum of the interfacial 2,4DHB (interfacial complex, 1515cm-'; complex in solution, 1532 cm-1). Although the difference in frequency values for these two bands is important, the band of the interfacial complex is broad, not very intense, and coincides with a band of a-FeOOH. Hence, its shape and frequency value may be distorted by the subtraction of the spectrum of goethite from the one of 2,4DHB-goethite. If real, these differences could be invoked in terms of an eventual dissimilarity in the electron-donor character of the OD group on C(4) in these two systems. This would not be surprising since the complex in solution is at pD 2.2 and the interfacial complex at pD higher than 6 (pD bulk solution is 6 and the particle surface is positively charged). Furthermore, the properties of the solvent (DzO) in the interfacial region are different than in the bulk solution. Unlike the case of salicylate, the frequency value for usy(COO-)is the same both for the solution and for the interfacial c0mp1ex.l~ These IR studies revealed the formation of a complex between 2,4DHB and Fe atoms of the a-FeOOH surface. This complex, as in the case of salicylate, can be described as a bidentate mononuclear complex (chelate) with respect to the entire 2,4DHB ion. In this complex, the iron atom binds one oxygen of the carboxylate group and the oxygen of the phenolic group in ortho position, resulting in the formation of a new sixatom ring. With respect to the carboxylate group the complex is monodentate mononuclear. In the case of the complex in solution, the para-substituted phenolic group does not bind with any Fe ion. Although, as we mentioned above, the bending vibration of this group is not clearly visible in the spectrum of the interfacial complex, the fact that the phenolic stretching has the same value in the spectra of these two type of complexes (1271-1273 cm-l, Figure 8) is enough evidence to decide that the parasubstituted OD is not bound in the interfacial complex.
Conclusions (1) Equilibrium adsorption studies of these benzoic compounds on goethite showed that chemical affinity is not the most important variable in controlling the maximum adsorption density.
Benzoic Compounds at the Goethite/ Water Interface
(2) CIR-FTIR data revealed the following: (a) The two benzoic compounds studied having an -OH in the ortho position to the carboxylate group, SAL and 2,4DHB, bind to the same surface Fe of goethite through one oxygen of the carboxylate group and the phenolic oxygen in the ortho position. This results in the formation of a new sixatom ring. This complex is classified as a bidentate chelate with respect to the whole ligand and as monodentate with respect to the carboxylate group. (b) BNZ, PTHT, and PHB are bound to the surface Fe of goethite through one carboxylate group, forming mainly a bidentate complex. (3) SAL and 2,4DHB, which form bidentate chelating complexes, show high chemical affinity having values for rmax of approximately 20 pmollg. Meanwhile PHTH and PHB, which form bidentate complexes through one carboxylate groups, have values for rmax of =lo0 and 130 pmol/g, respectively, although, their chemical affinity is similar in the case of PHTH and much smaller in the case ' of PHB than the chelating complexes.
Langmuir, Vol. 8, No. 2, 1992 533 (4) In a chelate structure the organic ligand forms two bonds with the same iron atom. As we mentioned in one earlier paper,la there are relatively few goethite surface iron atoms with two exchangeable OH ligands. Thus, given the high rmax values for PHTH and PHB, the carboxylate should be forming a bridging bidentate complex and not a bidentate chelate. (5) Stereochemistry may be the reason for PHTH showing asmaller rmar than PHB. Because the complexed carboxylate is coplanar with the benzene ring, a bulky group, as the case here by the addition of another carboxylate in the ortho position, can shadow some binding sites of the a-FeOOH surface (Fe atoms with A type OH) in the same row, which are separated by 3 A.16 Registry No. K(PHTH),877-24-7;Na(PHB), 114-63-6;K(2,4 DHB), 18396-42-4; FeO(OH), 20344-49-4; goethite, 1310-14-1; sodium benzoate, 532-32-1.
