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Langmuir 2002, 18, 1104-1111 31
P Nuclear Magnetic Resonance Study of the Adsorption of Phosphate and Phenyl Phosphates on γ-Al2O3 Bruce B. Johnson,*,† Alexander V. Ivanov,‡ Oleg N. Antzutkin,§ and Willis Forsling§
Division of Inorganic Chemistry, Luleå University of Technology, S-971 87 Luleå, Sweden, Colloid and Environmental Chemistry Laboratory, La Trobe University, Bendigo, P.O. Box 199 Bendigo, Victoria 3552, Australia, and Amur Institute of Integrated Research, Far East Division, Russian Academy of Sciences, Relochnyi per. 1, Blagoveshchensk 675000, Russia Received November 6, 2000. In Final Form: November 5, 2001 Adsorption of diphenyl-, phenyl- and orthophosphate on γ-Al2O3 was studied with a combination of macroscopic and 31P solid-state NMR measurements. Results for adsorption suggest that diphenyl phosphate was bound largely as an outer-sphere complex, while phenyl phosphate was held largely as inner-sphere surface complexes with an outer-sphere complex present only at higher pH values. Both the adsorption edge and the cross polarization magic angle spinning NMR spectra were consistent with the interaction between the surface and phenyl phosphate being driven by electrostatic forces. Adsorption of orthophosphate was more complex, with evidence of outer- and inner-sphere complexes and surface precipitation. Increasing the orthophosphate concentration and equilibration time tended to increase the fraction bound as a surface precipitate.
Introduction Phosphate sorption by oxides and clay minerals is of importance since it is an important determinant of the availability of phosphorus to plants and of the amount leaching from soils into waterways where it can cause significant environment damage. Both phosphate and various organophosphates interact strongly with aluminum ions in solution and are also known to adsorb strongly to the oxides and hydrous oxides of aluminum within soil and sediment systems.1-4 While there have been many studies of phosphate sorption, there are still uncertainties about the sorption mechanism. Fresh insights into the chemical processes involved have been provided in the past few years through spectroscopic studies. FTIR, in particular, has proved to be a valuable tool in studies of the bonding between phosphate and organophosphates and mineral surfaces. For example, Laiti et al.4 used diffuse reflectance FTIR to investigate the transition between orthophosphate adsorbed to aged γ-Al2O3 and the formation of a bulk AlPO4 phase over many days. FTIR was, however, unable to determine whether the AlPO4 formed at short reaction times as the spectra were dominated by the adsorbed species. More recently Barja et al.5 used in situ attenuated total reflectance FTIR (ATR-FTIR) to study the adsorption of methylphosphonate by goethite. Their study suggested a transition from a bridging bidentate complex at low coverages to a monodentate protonated complex at high coverages. Similar studies with the ATR-FTIR technique have provided useful information on the adsorbed species * Corresponding author:
[email protected]. † La Trobe University. ‡ Russian Academy of Sciences. § Luleå University of Technology. (1) Goldberg, S.; Sposito, G. Soil Sci. Soc. Am. J. 1984, 48, 772. (2) Bolan, N. S.; Barrow, N. J. J. Soil Sci. 1984, 35, 273. (3) Violante, A.; Colombo, C.; Buondonno, A. Soil Sci. Soc. Am. J. 1991, 55, 65. (4) Laiti, E.; Persson, P.; O ¨ hman, L.-O. Langmuir 1996, 12, 2969. (5) Barja, B. C.; Tejedor-Tejedor, M. I.; Anderson, M. A. Langmuir 1999, 15, 2316.
for phenylphosphonate on aged γ-Al2O3 and boehmite6 and for phosphate on TiO2.7 In principle, 31P NMR should provide a particularly valuable means of studying the sorption of phosphates by mineral surfaces. Since 31P chemical shifts can be correlated with the electronic environment around the phosphorus nucleus, this spectroscopic method should enable differentiation between outer-sphere and innersphere complexation and the formation of an AlPO4 phase. In addition it may provide information on changes in protonation of sorbed species with pH. There have been a few studies of orthophosphate sorption on soil minerals that have used 31P NMR in recent years;8-10 these have demonstrated some of its potential. For instance, Bleam et al.8 used 31P NMR to study the adsorption of orthophosphate onto boehmite (γ-AlOOH). From the change in position of the broad main peak with pH, and the results of the dipolar suppression experiment11 they argued that the phosphate was chemisorbed to the surface as a protonated complex at pH values up to 9. The identity of a second, narrower, upfield peak was not definitely determined. The authors suggested that it may arise from the presence of a surface AlPO4 phase or as an artifact from freeze-drying of samples since there were significant phosphate concentrations in the supernatant solution. In another study Lookman et al.10 resolved the 31P NMR spectrum of orthophosphate adsorbed on amorphous aluminum hydroxide into three peaks, tentatively assigned to adsorption of deprotonated and protonated species and to formation of an amorphous aluminum phosphate phase. Spectra from phosphate samples ad(6) Persson, P.; Laiti, E.; O ¨ hman, L.-O. J. Colloid Interface Sci. 1997, 190, 341. (7) Connor, P. A.; McQuillan, A. J. Langmuir 1999, 15, 2916. (8) Bleam, W. F.; Pfeffer, P. E.; Goldberg, S.; Taylor, R. W.; Dudley, R. Langmuir 1991, 7, 1702. (9) Hinedi, Z. R.; Goldberg, S.; Chang, A. C.; Yesinowski, J. P. J. Colloid Interface Sci. 1992, 152, 141. (10) Lookman, R.; Grobet, P.; Merckx, R.; Van Riemsdijk, W. H. Geoderma 1997, 80, 369. (11) Opella, S. J.; Frey, M. H. J. Am. Chem. Soc. 1979, 101, 5894.
10.1021/la001537t CCC: $22.00 © 2002 American Chemical Society Published on Web 01/18/2002
NMR Study of Phosphate Adsorption
sorbed on gibbsite also showed the formation of amorphous aluminum phosphate. Given the uncertainties of previous studies, we have chosen to investigate the adsorption of orthophosphate, phenyl phosphate, and diphenyl phosphate onto a high surface area γ-Al2O3 sample. The effects of concentration, pH, and equilibration time have all been investigated. The experiments aimed to determine the nature of the adsorbed species and surface precipitates and the conditions under which they formed. Experimental Section Materials. Suspensions of γ-Al2O3 (20 g L-1) (Sumitomo AKPG015) were prepared by mixing dry powder with 0.10 M NaCl. The specific surface area (N2 BET) of the γ-Al2O3 was 140 m2 g-1, and the surface site density12 was equal to 1.03 sites/nm2 (1.71 µmol surface sites/m2). Stock solutions of diphenylphosphoric acid H[(C6H5O)2PO2] (Aldrich), disodium phenyl phosphate dihydrate, Na2[(C6H5O)PO3]‚2H2O (Merck, 98%), and potassium dihydrogen phosphate KH2PO4 (Riedel-de Hae¨n, >99.5%) were prepared with Milli-Q (Millipore) water. The NaCl, NaOH, and HCl, used as background electrolyte and for pH adjustment, were all Merck p.a. grade. Complexes. Aluminum nitrate nonahydrate, Al(NO3)3‚9H2O, was used, together with the reagents listed above in the preparation of aluminum phosphate and aluminum complexes 1 and 2. AlPO4‚nH2O was prepared by addition of concentrated solutions of the aluminum and phosphate salts in a 1:1 molar ratio. The resultant precipitate was filtered, washed with water several times, and then air-dried. The composition was confirmed as AlPO4‚nH2O by inductively coupled plasma mass spectrometry (ICP-MS) analysis. A portion of the AlPO4‚nH2O was oven dried at ca. 250 °C to produce anhydrous AlPO4. Water loss suggested that n ≈ 6 for the hydrated sample. Complex 1. Tris(phenylphosphato)dialuminum, [Al2{(C6H5O)PO3}3]. Aluminum salt (760 mg) and 762 mg of sodium phenyl phosphate were dissolved in the minimum volume of water. These solutions were then mixed with intensive stirring, and a white precipitate was formed immediately. The precipitate was left to age for a few hours and then filtered, washed with water several times, and dried in air: yield, 61%. Solid-state CP/MAS 13C NMR data (δ, ppm/line width, ppm): (1:2:1:2), 152.9/ 2.7 (-OC-), 129.6/2.6 (o-CH-), 123.7/2.7 (p-CH-), 121.8/2.7 (mCH-). Cf. data for the initial Na2[(C6H5O)PO3]‚2H2O: (1:1:1: 1:1:1), 155.5/0.7 (-OC-), 133.4/1.0, 130.0 /0.8 (o-CH-), 122.3 /0.6, 120.9/0.7 (m-CH-), 117.3/0.9 (p-CH-). Anal. Calcd for C18H15O12P3Al2 (Mr ) 570.19): Al, 9.46%. Found: Al, 9.91%. Complex 2. Tris(diphenylphosphato)aluminum(III), [Al{(C6H5O)2PO2}3]. Aluminum salt (380 mg) and 751 mg of diphenylphosphoric acid were dissolved in the minimum volume of water. In the latter case the solution was heated gently (5055 °C) in order to improve the solubility of H[(C6H5O)2PO2]. These solutions were mixed with intensive stirring, and a white precipitate was formed immediately. It was filtered, washed with water several times, and then dried in air: yield, 63%. Solidstate CP/MAS 13C NMR data (δ, ppm/line width, ppm): (1:2:1:2), 152.1/1.3 (-OC-), 129.4/1.4 (o-CH-), 123.6/2.7 (p-CH-), 120.0/ 2.6 (m-CH-). Cf. data for the main peaks of initial H[(C6H5O)2PO2]: 150.9/1.6 (-OC-), 130.5/1.7 (o-CH-), 122.9/1.0 (m-CH-), 120.2/1.0 (p-CH-). Anal. Calcd for C36H30O12P3Al (Mr ) 774.53): Al, 3.48%. Found: Al, 3.86%. Elemental analyses of aluminum complexes 1 and 2 were carried out by high-resolution inductively coupled plasma mass spectrometry (used in medium resolution mode, ∆m/m ≈ 4500) (HR-ICP-MS) (ELEMENT, Finnigan MAT, Bremen, Germany). NMR Instrumentation. Solid-state 31P magic-angle-spinning (MAS) NMR spectra were recorded on a Chemagnetics Infinity CMX-360 (B0 ) 8.46 T) spectrometer using cross-polarization (12) Laiti, E.; O ¨ hman, L.-O.; Nordin, J.; Sjo¨berg, S. J. Colloid Interface Sci. 1995, 175, 230.
Langmuir, Vol. 18, No. 4, 2002 1105 (CP) from the protons together with proton decoupling.13 The 31P operating frequency was 145.73 MHz. The proton π/2 pulse duration was 5.0 µs. In CP/MAS experiments CP mixing time was 1.9 ms and the nutation frequency of protons during decoupling in all experiments was ωnut/2π ) 54 kHz. From 64 (for individual solid samples) to 1024 (for surface complexes) transients spaced by a relaxation delay of 2 or 3 s were accumulated. Powder samples (ca. 350 mg) were packed in zirconium dioxide double bearing 7.5 mm rotors. The spinning frequencies for all samples ranged from 5000 to 5100 Hz, stabilized to (2 Hz with an in-built stabilization device. All 31P isotropic shift data (in the deshielding, δ-scale) are given with respect to 85.5% H3PO414 (here, 0 ppm, externally referenced), which was mounted in a 5 mm glass tube and placed in a 7.5 mm rotor to avoid errors due to differences in the magnetic susceptibility. Experiments. The variation in the amount of diphenyl phosphate, phenyl phosphate, or orthophosphate adsorbed as a function of pH was determined as follows. A 50.0 mL portion of 20 g/L γ-Al2O3 suspension in 0.10 M NaCl was stirred for at least 18 h under purified N2 in a constant temperature room (23.0 ( 0.5 °C) in order to fully hydrate the surface of the substrate. The pH of the suspension was adjusted to >10 with NaOH, an aliquot of the adsorbate was added to give the required initial solution concentration, and the pH was readjusted to about 10. After an equilibration time of 1 h, a 4 mL sample was taken and firmly capped, and the pH of the remaining suspension was adjusted to about 9 with HCl. Again an equilibration time of about 1 h was allowed before a further 4 mL sample was taken, and the pH of the remaining suspension was reduced again. Through a series of such steps, samples were taken in the pH range from 10 to 4. These samples were centrifuged at 4000 rpm for 30 min, and the supernatant solutions were analyzed to determine the amount of adsorbate remaining in solution by UV-visible spectrophotometry (Perkin-Elmer Lambda 2S). Diphenyl phosphate and phenyl phosphate were determined directly at wavelengths of 262 and 263 nm, respectively, with careful background subtraction, while phosphate was determined by use of the Molybdenum Blue method at 880 nm.15 Samples for analysis by 31P NMR were prepared by a similar method. The suspension was equilibrated under N2 for at least 18 h prior to addition of the adsorbate, and the pH was then adjusted to the value required. For the short-term equilibration studies, a further 3-4 h was allowed for adsorbate equilibration before sampling. Samples containing about 1 g of substrate were centrifuged at 4000 rpm for 30 min, the supernatant was kept for analysis, and the paste was dried by vacuum desiccation at room temperature. For the long-term equilibration studies, the adsorbate/substrate systems were equilibrated as detailed above with the pH adjusted to the required value for 1-2 h after addition of the adsorbate. Twenty five milliliter aliquots of each suspension were then placed in centrifuge tubes and carefully flushed with N2 to avoid carbonate contamination, and the tubes were placed on a drum roller and allowed to tumble for a period of 10 days. Prior to centrifuging and drying each adsorbate/substrate system, the final pH value of the suspension was determined. Those values are reported in Table 1 and Figure 1. Deconvolution of the NMR Peaks. The measured 31P NMR peaks were separated into contributions from component peaks by use of the Spinsight spectrometer operating program with the in-built deconvolution routine. A combination of 85% Gaussian and 15% Lorentzian broadening gave the best fit to the experimental results. The large contribution of the Gaussian component can be justified by a wide distribution of chemical shifts of surface adsorbed species, which gives rise to a predominantly inhomogeneous broadening. The ratio 85%:15% between Gaussian and Lorentzian broadening could be varied slightly from spectrum to spectrum, adjusting other simulation parameters and keeping the same goodness-of-fit. However, for consistency we have kept the ratio fixed in all simulations. (13) Pines, A.; Gibby, M. G.; Waugh, J. S. J. Chem. Phys. 1972, 56, 1776. (14) Karaghiosoff, K. In Encyclopedia of Nuclear Magnetic Resonance; Wiley: New York, 1996; Vol. 6, p 3612. (15) Vogel, A. I. Vogel’s Textbook of Quantitative Inorganic Analysis; Longman: London, 1987.
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Johnson et al.
Table 1. 31P Chemical Shift, Line Width, and Relative Peak Intensity for Adsorption on γ-Al2O3
compound/adsorption system
31P chemical shift (ppm)
Pure Compounds KH2PO4 4.0 Na2HPO4 6.6 Na3PO4‚12H2O 7.5 10.0 K3PO4a Na2 phenyl phosphate‚2H2O 4.3 2.6 H-diphenyl phosphate -10.4
line width (ppm)
rel integral intens
1.2 2.7 1.6 1.0 0.9 1.5
0.77 0.23
12.5 9.6 9.2 10.8 6.3 5.8 8.7 2.9 4.5
0.59 0.41 0.41 0.59 0.22 0.56 0.22 0.59 0.41
Adsorbed Systems: Orthophosphate 3 mM orthophosphate, pH 4.05 0 5.7 -4.6 7.9 -10 10.3 3 mM orthophosphate, pH 7.05 0 5.7 -4.6 7.9 -10 10.3 6 mM orthophosphate, pH 3.95 0 5.7 -4.6 7.9 -10 10.3 6 mM orthophosphate, pH 6.95 0 5.7 -4.6 7.9 -10 10.3 6 mM orthophosphate, pH 8.5 0 5.7 -4.6 7.9 -10 10.3 6 mM orthophosphate, pH 4.4 0 5.7 (10 day equilibration) -4.6 7.9 -10 10.3 12 mM orthophosphate. pH 4.8 0 5.7 (10 day equilibration) -4.6 7.9 -10 10.3
0.16 0.36 0.47 0.27 0.42 0.31 0.17 0.39 0.43 0.31 0.46 0.23 0.44 0.42 0.14 0.17 0.40 0.44 0.12 0.36 0.51
Aluminum Salts -22.2 -27.2 Al phosphate‚nH2O -8.0 -13.8 Al phenyl phosphate -9.8 -15.6 -21.9 Al diphenyl phosphate -24.1 -26.7 AlPO4 (anhydrous)
Adsorbed Systems: Phenyl Phosphate and Diphenyl Phosphate 3 mM phenyl phosphate, pH 4.0 -4.9 4.7 0.49 -8.3 9.1 0.51 3 mM phenyl phosphate, pH 7.0 -4.9 4.7 0.80 -8.3 9.1 0.20 3 mM phenyl phosphate, pH 8.5 -0.3 4.0 0.11 -4.9 4.7 0.89 6 mM phenyl phosphate, pH 4.15 -4.9 4.7 0.55 (10 day equilibration) -8.3 9.1 0.45 12 mM phenyl phosphate, pH 4.5 -4.9 4.7 0.34 (10 day equilibration) -8.3 9.1 0.66 3 mM diphenyl phosphate, pH 4.0 -9.8 3 0.71 -16.8 6 0.29 a Data from: Grimmer, A.-R.; Haubenreisser, U. Chem. Phys. Lett. 1983, 99, 487.
To provide an effective analysis of the broad relatively structureless spectral lines together with their spinning sidebands, the following protocol was developed and used. The central lines of all samples were auto-deconvoluted and optimized using either three peaks (for orthophosphate) or two (for both phenyl phosphate and diphenyl phosphate). When the results for different samples were compared, the deconvoluted lines showed that both line positions and widths were approximately conserved (variations in line widths were 10-20%). Mean values for isotropic chemical shifts and line widths were calculated for all samples with the same adsorbate, and the deconvolutions were then repeated manually with chemical shifts and line widths set at the mean values. Spinning sidebands of order (1 for orthophosphate and (1 and (2 for phenyl phosphate were also deconvoluted with line
Figure 1. Adsorption of diphenyl, phenyl, and orthophosphate onto γ-Al2O3: (b) 6 mM orthophosphate; (O) 3 mM orthophosphate; (9) 6 mM phenyl phosphate; (0) 3 mM phenyl phosphate; (2) 3 mM diphenyl phosphate; (4) 0.6 mM diphenyl phosphate. Centered open symbols represent results for adsorption from 6 mM solutions after a 10 day equilibration time. The line represents the calculated concentration of Al-OH2+ species at the surface. positions corrected for the spinning frequency (frequency of the corresponding central deconvolution plus or minus the rotational frequency multiplied by the order of the spinning sideband) and with the same line widths as the central line. The integral of the central line was added to those of the spinning sidebands for each deconvolution peak, and the relative integral peak intensities were calculated from these results.
Results Adsorption Studies. The adsorption densities of orthophosphate, phenyl phosphate, and diphenyl phosphate on γ-Al2O3 as a function of initial adsorbate concentration and pH are shown in Figure 1. Each of the adsorbates had different adsorption characteristics. There was little diphenyl phosphate adsorption at any pH studied, although the adsorption density increased significantly at pH values below 6 and increased slightly at pH values above 8. At pH 7, little, if any, diphenyl phosphate was adsorbed. The maximum adsorption density measured was 0.10 µmol/m2 at pH 4, when the initial concentration of diphenyl phosphate was 3.0 mM. This represents less than 10% adsorption and corresponds to a surface concentration that is less than 6% of the concentration of available proton-active tAl-OH sites predicted from potentiometric titration studies.12 In contrast, phenyl phosphate adsorbed strongly, especially at pH values below 7. Below pH 5.5, effectively all of the phenyl phosphate was adsorbed from the 3.0 mM solution, giving an adsorption density of about 1.05 µmol/m2. Above pH 7 the amount adsorbed decreased as the pH increased, with little remaining on the surface by pH 10. The maximum adsorption from the 6.0 mM solution was achieved at pH values below 5 with the surface concentration again decreasing to zero by pH 10. The adsorption density below pH 5 (1.75 µmol/m2) corresponds closely to the total suspension concentration of protonactive tAl-OH sites (1.71 µmol/m2),12 suggesting that each phenyl phosphate species was bound to one surface site. Adsorption of orthophosphate differed from that of phenyl phosphate in two obvious respects. First there was less pH dependence, with a substantial fraction of the adsorbate remaining on the surface at pH values above
NMR Study of Phosphate Adsorption
Figure 2. 31P CP/MAS NMR spectra of pure orthophosphate species: (a) KH2PO4, (b) Na2HPO4, and (c) Na3PO4‚12H2O at the rotor frequency |ωr/2π| ) 5000 Hz (sum of 64 transients). “s” denotes spinning sidebands.
10. Second there was no evidence of saturation of the surface with phosphate species at the adsorbate:surface ratios studied in the short-term experiments. By pH 6.5 the adsorption density from the 6.0 mM solution increased beyond the total concentration of proton-active tAl-OH sites, and by pH 5 effectively 100% of the phosphate was adsorbed resulting in an adsorption density of 2.1 µmol/ m2. There is no reason to believe that the maximum adsorption density was achieved by adsorption from the 6.0 mM orthophosphate solution, even though the adsorbed concentration was more than 20% above the concentration of proton-active sites. When the adsorption densities, N, in the long-term adsorption studies with 6.0 mM phenyl phosphate and orthophosphate at about pH 4 are compared with N at the same pH in the adsorption edge experiments (Figure 1), little additional adsorption is indicated. This is hardly surprising for orthophosphate where approximately 100% adsorption was achieved in a short equilibration time. For phenyl phosphate the amount adsorbed increased to about 1.9 µmol/m2, which is slightly greater than the proton-active surface site density; however, more than 10% of the phenyl phosphate remained in solution. 31 P NMR Spectra. 31P spectra were taken of the pure adsorbates and of aluminum complexes of each of the adsorbates in order to facilitate assignment of the resonances found for the adsorbed species. A study of the spectra for the pure orthophosphates (Figure 2) shows that the peak location shifted consistently to lower shielding from 0 ppm (H3PO4) to +10.0 ppm (K3PO4) (see Table 1 and Figure 3). This shift corresponds to an increase in ionicity of the phosphates as they are progressively deprotonated. The results for the fully deprotonated phenyl phosphate (Table 1) showed higher shielding than the fully deprotonated orthophosphate, probably due to the covalent nature of the bonding between the phenyl group and the phosphate ion. Diphenyl phosphate had the highest shielding because of the presence of two covalently bound phenyl groups. 31 P spectra of aluminum phosphates are shown in Figure 4. The 31P sites of aluminum phosphates are more shielded compared with the initial sodium and potassium salts (Figure 3), which can be explained by the decease in ionic character of Al-O bonds compared with Na+‚‚‚O- and K+‚‚‚O- bonds. (On the Pauling scale, the electronegativity difference between Al and O atoms is equal to 2.0 compared with 2.6 and 2.7 for Na and O and K and O atoms, respectively.) In addition, phosphorus resonance lines of
Langmuir, Vol. 18, No. 4, 2002 1107
Figure 3. 31P chemical shifts for some orthophosphate, phenyl phosphate, and diphenyl phosphate species.
Figure 4. 31P NMR spectra for aluminum salts: (a) AlPO4‚ nH2O; (b) anhydrous AlPO4; (c) Al2(C6H5PO4)3; (d) Al(C6H5)2PO4)3. Spectra a, c, and d are cross-polarization experiments with proton decoupling (128 transients) while spectrum b is a single-pulse experiment (pulse length 5 µs, pulse delay 20 s) with proton decoupling (16 transients). Spinning frequencies are (a) 5018, (b) 5000, (c) 5005, and (d) 5025 Hz. “s” denotes spinning sidebands.
the aluminum complexes are considerably broader than those for phosphorus salts of alkali metals. This may be due to residual 31P-27Al dipole-dipole coupling, which is not fully averaged out by MAS since 27Al nuclei have a rather large quadrupolar moment (spin I ) 5/2). An analogous broadening has been observed and extensively studied for 13C nuclei directly coupled to 14N (I ) 1).16,17 The 31P NMR spectra of the hydrated and anhydrous AlPO4 samples show a marked shift to greater shielding after dehydration. When the spectrum of the anhydrous AlPO4 sample prepared in this study was compared with that of a commercial anhydrous AlPO4 sample (Aldrich), the chemical shifts were identical. These results show the (16) Hexem, J. G.; Frey, M. H.; Opella, S. J. J. Chem. Phys. 1982, 77, 3847. (17) Olivieri, A. C. Solid State Nucl. Magn. Reson. 1992, 1, 345.
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importance of preparing standards under conditions similar to those of the adsorbed samples rather than using anhydrous commercial standards. When the peak positions (Table 1) of the three air-dried aluminum phosphates (orthophosphate, phenyl phosphate, and diphenyl phosphate) are compared, a similar trend to the alkali phosphates is observed, with aluminum orthophosphate showing the least shielding and aluminum diphenyl phosphate the greatest shielding. This results from progressive replacement of more ionic Al-O bonds by covalent C-O bonds. Spectra of Adsorbed Systems. NMR spectra for all three adsorbates were characterized by broad resonance lines at all concentrations and pH values studied. Broad resonance lines are generally characteristic of the presence of a range of different chemical environments for the phosphorus atom and are expected for adsorbed systems since the substrates are heterogeneous. Generally the lines could be resolved into two or three component peaks. A two-peak fit was chosen for the phenyl phosphate and diphenyl phosphate spectra. For these systems the addition of a third peak did not significantly improve the fit to the measured resonance line; generally when a third peak was added, the positions of two of the three fitted peaks were within (0.5 ppm. For orthophosphate a threepeak fit was chosen because it generally provided a better fit to the experimental line shape. Variations in both the fitted peak positions and line widths were not significant among samples of the same adsorbate as revealed by the auto-deconvolution best fits. Therefore, both peak positions and line widths were kept fixed at the mean values for all samples, and the intensities of the lines were adjusted freely. Corresponding spinning sidebands were similarly analyzed with the positions chosen as set out in the section on deconvolution of the NMR peaks. Hence, the integrals of the three (or two) deconvolution lines can be directly compared to quantify the relative amounts of the three (or two) types of 31P sites. Since all systems contain a significant amount of 1H spins around the 31P sites (either covalently bound as in phenyl phosphate and diphenyl phosphate, or through hydrogen-bonding to H2O molecules), we do not expect differences in cross-polarization efficiency for different phosphorus sites. In fact, the shapes of cross polarization obtained at 0.8 and 1.9 ms were identical to that in the 31P single pulse experiment. Representative examples of the peak fits for the phenyl phosphate/ and orthophosphate/γ-Al2O3 systems are shown in Figure 5. The de-convolution results found for adsorption onto γ-Al2O3 are summarized in Table 1. Values for the peak position, the line width, and the relative integral peak intensity are presented. 31P spectra for the 3 and 6 mM phenyl phosphate/ and 3 mM diphenyl phosphate/γ-Al2O3 systems are shown in Figure 6, with the spectra for the 6 mM orthophosphate/γ-Al2O3 system prepared at different pH values shown in Figure 7. For diphenyl phosphate the amount adsorbed was only sufficient to give a reasonable spectrum at low pH. For this system two peaks were identified. The first, at -9.8 ppm, corresponded closely with the peak position of the pure adsorbate (-10.4 ppm). While there was a small amount of entrained supernatant (
