Langmuir 1996, 12, 2969-2975
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Surface Complexation and Precipitation at the -Orthophosphate-Aged γ-Al2O3/Water Interface
H+
Erkki Laiti,* Per Persson, and Lars-Olof O ¨ hman Department of Inorganic Chemistry, Umea˚ University, S-901 87 Umea˚ , Sweden Received December 4, 1995. In Final Form: March 19, 1996X Surface complexation of orthophosphate ions at the water-suspended-and-aged γ-Al2O3/water interface has been studied by means of a series of batch experiments in 0.1 M Na(Cl) medium at 25.0 °C in the range 4.8 < -log [H+] < 9.6. The ratio between phosphate concentration and concentration of surface active groups (tAlOH) was varied between 0.15 and 1.50. The suspensions were equilibrated for 5 h, and experimental data consisted of measured -log [H+] and nonbound phosphate analyses. The orthophosphate ions were found to bind to the surface with high affinity at -log [H+] < 7.5. In the data evaluation, contributions from electrostatic forces were accounted for by using the constant-capacitance model. The acid/base properties of the hydroxylated alumina surface (tAlOH) have been investigated earlier and are described by two intrinsic equilibrium constants, log β110 ) 7.51 and log β-110 ) -8.87 and with a specific capacitance of 1.40 F/m2. The model describing the phosphate complexation to the alumina surface is given by the following equilibria: tAlOH + H2PO4- + H+ h tAlPO4H2 + H2O (log β111(int) ) 11.49 ( 0.08); tAlOH + H2PO4- h tAlPO4H- + H2O (log β011(int) ) 5.14 ( 0.07); tAlOH + H2PO4- h tAlPO42- + H+ + H2O (log β-111(int) ) -1.82 ( 0.04). The uncertainties reported correspond to 3σ(log β). In the presence of excess phosphate and at extended equilibration periods, a slow continuing decrease in nonbound phosphate concentration was observed. By means of diffuse reflectance FTIR measurements, this phenomenon was shown to be caused by a slow transformation into an aluminum phosphate solid phase. The surface complexation reactions evaluated in this work should therefore be regarded as a metastable state, strictly valid only in freshly prepared suspensions. However, FTIR data collected at deficit phosphate conditions indicate that this phase transformation is hardly noticeable unless an excess of ligand was introduced to the system. This implies that the presented semiequilibrium model is likely to provide a thermodynamic description of the equilibria in the system for [H2PO4-]tot/[tAlOH]tot < 1.
Introduction Phosphate ions and organophosphate and -phosphonate compounds are known to strongly interact with the aluminum ion, and surface complexation of orthophosphate onto aluminum oxides has been the subject of several investigations.1-4 It is however also known that aluminum forms a number of insoluble phosphate minerals such as berlinite (AlPO4) and variscite (AlPO4‚2H2O).5 Ferguson and King6 performed calculations of the solubility of aluminum phosphate in the presence of aluminum hydroxide. Their results indicated that a phase transformation, i.e., rearrangement into a new aluminum phosphate phase, rather than formation of surface complexes might be the phosphate removal process. There are also experimental studies, based on X-ray diffraction analysis, that support these calculations.7 Hence, there seems to exist an uncertainty about how to correctly describe processes occurring in aqueous phosphate-aluminum oxide systems. The objective of the present work is to address the question of the adsorption mechanism of phosphate on aluminum oxides by means of time-resolved adsorption measurements and infrared spectroscopy and thereby to possibly distinguish between an assumed faster surface complexation process and a slower phase transformation process. We will also try to quantitatively describe the * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, May 15, 1996. (1) Muljadi, D.; Posner, A. M.; Quirk, J. P. J. Soil Sci. 1966, 17, 212. (2) Robarge, W. P.; Corey, R. B. Soil Sci. Soc. Am. J. 1979, 43, 481. (3) Goldberg, S.; Sposito, G. Soil Sci. Soc. Am. J. 1984, 48, 772. (4) Violante, A.; Colombo, C.; Buondonno, A. Soil Sci. Soc. Am. J. 1991, 55, 65. (5) Lindsay, W. L. Chemical Equilibria in Soils; Wiley: New York, 1979; p 169. (6) Ferguson, J. F.; King, T. J.sWater Pollut. Control Fed. 1977, 49, 646. (7) Veith, J. A.; Sposito, G. Soil Sci. Soc. Am. J. 1977, 41, 870.
S0743-7463(95)01507-1 CCC: $12.00
complexation process in terms of a surface complexation model using the constant capacitance approach. Materials Chemicals and Solutions. In this work, γ-Al2O3, manufactured by Sumitomo Chemical Co., Ltd., was used. This material is of high chemical purity, 99.995%, and has a surface area of 140 m2/g according to the BET method. In the preparation of the ligand solutions, sodium dihydrogen phosphate monohydrate (Merck p.a.) was used. Sodium chloride (Merck p.a.) was dried at 180 °C and used without further purification. Dilute solutions of hydrochloric acid were prepared from HCl (Merck p.a.) and standardized against Tris-(hydroxymethyl)aminomethane, “Trisma-base” (Sigma p.a.), which was dried at 105 °C overnight before use. Dilute sodium hydroxide solutions were prepared from saturated (19 M) NaOH solution. Solid NaOH (EKA Nobel p.a.) was used for the preparation of this concentrate. Dilute NaOH solutions were standardized against standardized acids. The alumina stock suspensions were prepared to obtain a solid concentration of 20.00 g/dm3. As reported in a previous investigation,8,9 γ-Al2O3 is not thermodynamically stable in aqueous solution but undergoes a slow surface transformation, with the formation of a bayerite surface layer on the particles as a result. In order to minimize any changes in surface properties during the experiments, the suspensions were allowed to age for more than one month before use. The total concentration of proton active surface hydroxyl groups was determined, as previously described in detail,8 by analyzing the number of protons that can adsorb to, or desorb from, the surface hydroxyl groups when the surface is exposed to low and high -log [H+]. The surface site density evaluated with this method was 1.03 sites/nm2. This value corresponds well to values previously reported in the literature.10,11 The standardization of the dihydrogen phosphate (8) Laiti, E.; O ¨ hman, L.-O.; Nordin, J.; Sjo¨berg, S. J. Colloid Interface Sci. 1995, 175, 230. (9) Dyer, C.; Hendra, P.; Forsling, W.; Ranheimer, M. Spectrochim. Acta, Part A 1993, 49, 691. (10) Hohl, H.; Stumm, W. J. Colloid Interface Sci. 1976, 55, 281. (11) Kummert, R.; Stumm, W. J. Colloid Interface Sci. 1980, 75, 373.
© 1996 American Chemical Society
2970 Langmuir, Vol. 12, No. 12, 1996 solution and the determination of acidity constants for the ligand were performed from a series of potentiometric data. This implies that the acidity constants and the ligand concentration were simultaneously optimized. The optimized concentration corresponded within 0.2% to the value expected from weighing. All solutions were prepared from boiled Milli-Q plus 185 water. A constant ionic strength of 0.1 M Na(Cl) was maintained in all solutions by dissolving appropriate amounts of NaCl. Solution preparation and experimentation were performed at 25.0 ( 0.2 °C. Apparatus. The -log [H+] measurements were performed with a Radiometer Copenhagen PHM 84 Research pH meter and an Ingold U402-M6-S7/100 combination glass electrode. The function of this electrode was checked through the measurement of -log [H+] in a series of phenyl phosphonate solutions8 of known composition. To calibrate the electrode, solutions of known H+ concentration in 0.1 M Na(Cl) were used. The spectrophotometer used in the analysis of phosphate was a Shimadzu UV-2100 UVvisible spectrophotometer. Two different FTIR spectrometers were used in this work: a Perkin-Elmer 2000 and a Nicolet 520 FTIR spectrometer. Both were equipped with DTGS detectors. The sample compartments were purged with dry argon or nitrogen at ambient pressure. The samples were recorded as diffuse reflectance (DR) spectra, using a Harrick diffuse reflectance unit, from a 2% by weight mixture with finely powdered KBr (Merck, IR spectroscopic grade) at a resolution of 4 cm-1. The KBr was mixed very gently with the powdered sample, and KBr was also used as background. The final DR spectra were the average of 2000 scans, and they are reported in log(Rref/Rsample) units. To isolate the spectral features of adsorbed phosphate, the spectrum of aged γ-Al2O3 was subtracted in appropriate fractions from all spectra. The subtraction factor was always in the region 0.95-1.05.
Laiti et al.
A
B
Experiments Initial Kinetic Measurements. As pointed out in the Introduction, the objective of the present study is to distinguish between the process of surface complexation and the process of phase transformation in the heterogeneous system H+-aged γ-Al2O3-H2PO4-. To determine if it is possible to make this distinction, the present investigation was initiated with a series of experiments in which phosphate removal from the aqueous phase was followed as a function of the reaction time. A series of identical batches were prepared to contain twice the amount of ligand as compared to the amount of proton active sites on the oxide and with a slightly acidic -log [H+]. The batches were placed on an end-to-end rotating test tube holder and, at regular reaction time periods, one batch was withdrawn, centrifuged, and analyzed for its aqueous phosphate concentration. This analysis was performed according to the molybdenum blue method,12 and the intensity of color was measured spectrophotometrically at 880 nm. In Figure 1, the results of these measurements are plotted as the percentage of nonreacted phosphate as a function of the reaction time. Figure 1A shows that, within a few hours, a plateau in the point curvature is reached at approximately 50%. This figure thus implies that the amount of reacted phosphate is equal to the amount of proton active surface sites on the aluminum oxide, a behavior which can be taken as support for a surface complexation with the formation of 1:1 surface complexes. Figure 1B shows the removal of phosphate in a longer time perspective. Thus, by aging the suspension for extended periods, a continuing significant decrease in the aqueous phosphate content is clearly observed. Taken together, these two figures therefore point toward a conclusion implying that both surface complexation and phase transformation are relevant processes in H+-aged γ-Al2O3-H2PO4- suspensions. In an initial reaction, (12) Vogel, A. I. Vogel’s Textbook of Quantitative Inorganic Analysis; Longman Inc.: London, 1987.
Figure 1. Percentage of nonbound phosphate, %L(aq), as a function of reaction time. The suspensions were prepared to contain [H2L-]tot/[tAlOH]tot ) 2.0 and an initial -log [H+] ) 6.
phosphate ions complex onto the oxide surface in a 1:1 stoichiometry at a fairly rapid rate and then, at a very slow rate, the alumina phase transforms into an aluminum phosphate precipitate. On the basis of these measurements it was concluded that an equilibration time of 5 h was ideally suited to monitor the semiequilibrium surface complexation process. FTIR Measurements. To support the macroscopically derived conclusions drawn above, also at the microscopic level, a series of FTIR spectra of the adsorbed phosphate were collected. According to the kinetic adsorption measurements, the IR spectra are expected to show an initial formation of surface complexes and then, in an excess of phosphate, a time dependent change into an AlPO4 phase. Consequently, the IR spectroscopic measurements will only provide relevant information if it is possible to distinguish between phosphate ions coordinated to aluminum at the surface as complexes and those in the three-dimensional array of AlPO4(s). IR studies of adsorbed phosphate, including the present work, usually focus on the strong P-O stretching bands (ν3 and ν1 modes) in the region 900-1250 cm-1. In order to do the abovementioned distinction, the -log [H+] dependence of these bands has to be studied. If an AlPO4 phase is formed, the IR features will show no variation with -log [H+], since the symmetry of the phosphate ions in the solid phase is expected to remain the same, regardless of [H+]. On the other hand, phosphate surface complexes undergo protonation reactions which cause the PO4 unit to change symmetry. The IR spectra therefore show characteristic
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stretching bands of the adsorption samples corroborates the interpretation based on the kinetic measurements that surface complexes are initially formed. The structural implications of the IR data will be discussed later in the text. Concerning the possibility of the coexistence of an AlPO4 phase in the 1:1 samples, we cannot, because of the strong overlap of bands, from the spectra determine if or how much of such a phase is formed during the fast, initial adsorption reaction. To study the possible formation of an AlPO4 phase, a suspension containing an excess of phosphate (phosphate to tAlOH ratio of 4) was allowed to equilibrate for 1 week. The spectrum of this sample shows a strong broad ν3 band centered around 1115 cm-1 and a weaker ν1 band at 900 cm-1 (Figure 2a). These IR characteristics are in good agreement with those of AlPO4(s) and also with those of other amorphous metal phosphates.14 Thus, most likely an AlPO4 phase is formed when alumina is equilibrated for an extended period of time at relatively high phosphate to tAlOH ratios. The differences between the spectrum of this phase and the spectra of the short-term equilibrated 1:1 samples should also be noted. These further support the conclusion that at low ratios surface complexes are initially formed. Surface Complexation Measurements. In the present work, the surface complexation reactions were studied by means of a series of batch experiments. These suspensions were prepared in 50 cm3 polyethylene test tubes by mixing known volumes of aged γ-Al2O3 suspension and dihydrogen phosphate solution, and the total proton concentration was set by the addition of standardized HCl or NaOH. The suspensions were equilibrated for 5 h, after which -log [H+] was measured. Directly thereafter, the suspensions were centrifuged, and the clear solutions were analyzed for phosphate. Data Treatment
Figure 2. Diffuse reflectance FTIR spectra of phosphate adsorbed on aged γ-Al2O3 at (a) pH 4.55 (4 H2PO4-:1 tAlOH, suspension equilibrated for 1 week), (b) pH 4.15 (1 H2PO4-:1 tAlOH, suspension equilibrated for 5 h), (c) pH 6.80 (1 H2PO4-:1 tAlOH, suspension equilibrated for 5 h), (d) pH 8.90 (1 H2PO4-:1 tAlOH, suspension equilibrated for 5 h) after subtraction with pure aged γ-Al2O3. (e) is the diffuse reflectance FTIR spectrum of AlPO4(s) precipitated from solution. The ordinate scale is in log(Rref/Rsample) units, and it is arbitrary.
features as a function of -log [H+], as demonstrated in a previous work.13 In Figure 2, IR spectra are presented of phosphate adsorbed to aged γ-Al2O3 at -log [H+] 4.1, 6.8, and 8.9, respectively. All samples were prepared at a 1:1 ratio between phosphate and tAlOH, and they were equilibrated for 5 h. Included in Figure 2 is also the spectrum of aluminum phosphate precipitated from homogeneous solution by mixing Al3+, H2PO4-, and OH- in the ratio 1:1:2. The clear -log [H+] dependence of the P-O (13) Persson, P.; Nilsson, N.; Sjo¨berg, S. J. Colloid Interface Sci. 1996, 177, 263.
When dry metal oxides are reacted with water, water molecules coordinate to their surfaces and hydrogen ions migrate to the surface layer of oxygen ions of the oxide. This leads to a hydroxylated surface.15 Although in reality several different types of surface hydroxyl groups are present at the surface, a simplified description involving only one kind of hydroxyl groups is often sufficient for modeling surface complexation reactions. In the case of alumina, the hydroxylated surface groups can be schematically expressed as tAlOH. These surface hydroxyl groups can bind or release hydrogen ions and also take part in complexation reactions with metal ions and ligands. Equilibria involving H+, tAlOH, and dihydrogen phosphate (H2L-) ions can be symbolized by the general equation
pH+ + q(tAlOH) + r(H2L-) h Hp(tAlOH)q(H2L)r(p-r)
βpqr (1)
Two special cases of this general reaction are
(i) the protonation/deprotonation of the hydroxylated alumina surface: pH+ + tAlOH h Hp(tAlOH)p
βp10
(2)
(14) Arlidge, E. Z.; Farmer, V. C.; Mitchell, B. D.; Mitchell, W. A. J. Appl. Chem. 1963, 13, 17. (15) Westall, J. C. In Geochemical Processes at Mineral Surfaces; Davis, J. A., Hayes, K. F., Eds.; American Chemical Society: Washington, DC, 1986; p 54.
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(ii) the protonation/deprotonation of the dihydrogen phosphate ion: pH+ + r(H2L-) h Hp(H2L)r(p-r) βp0r
(3)
It should noted that βpqr and βpq0 are conditional and must be corrected for by the Coulombic energy of the charged surface to obtain the corresponding intrinsic constants:
βpqr(int) ) βpqre(p-r)FΨ/RT
(4)
where ψ is the potential at the surface. The law of mass action and the conditions for the total concentrations of H+ (H), tAlOH (B), and H2L- (C) then give the following three equations, in which h, b, and c are the free concentrations of each component, respectively.
∑pβpq0(int)e-(pFΨ/RT)hpbq + ∑pβp0rhpcr + ∑pβpqr(int)e-((p-r)FΨ/RT)hpbqcr
H ) h - KWh-1 +
B)b+
∑qβpq0(int)e-(pFΨ/RT)hpbq + ∑qβpqr(int)e-((p-r)FΨ/RT)hpbqcr
C)c+
(5)
(6)
r ∑rβp0rhpcr + ∑rβpqr(int)e-((p-r)FΨ/RT)hpbqc(7)
According to the constant-capacitance model,16 ψ is related to the surface charge σ by the equation
ψ)
σ κ
(8)
where κ is the specific capacitance (C V-1 m-2). The surface charge (in mol dm-3) is obtained from
Tσ )
∑pβpq0(int)e-pFΨ/RThpbq + ∑pβpqr(int)e-(p-r)FΨ/RThpbqcr
(9)
or, in electrostatic quantities (C m-2)
σ)
TσF sa
(10)
s in this equation is the specific surface area (m2 g-1), and a is the concentration of solid in g dm-3. Combining eqs 8 and 10 gives
ψ)
TσF saκ
Table 1. Proposed Model in the Three-Component System H+-tAlOH-dihydrogen phosphate (H2L-)a species
(p,q,r)
H3L HL2L3tAlOH2+ tAlOtAlLH2 tAlLHtAlL2tAlLH2
1,0,1 -1,0,1 -2,0,1 1,1,0 -1,1,0 1,1,1 0,1,1 -1,1,1 1,1,1
tAlLHtAlL2-
log βpqr(int) ( 3σ
1.90 ( 0.006 -6.71 ( 0.006 -18.45 ( 0.01 7.51 ( 0.09 -8.87 ( 0.11 11.49 ( 0.08 5.14 ( 0.07 -1.82 ( 0.04 pH ) 4-7: 10.40 pH ) 8 and 9: 11.68 0,1,1 pH ) 4-7: 4.11 -1,1,1 pH ) 8 and 9: -1.71
ref this work this work this work Laiti et al.8 Laiti et al.8 this work this work this work Goldberg and Sposito3 Goldberg and Sposito3 Goldberg and Sposito3
parameter
value
ref
specific surface area site density specific capacitance
m2/g
Laiti et al.8 Laiti et al.8 Laiti et al.8
140 1.03 sites/nm2 1.40 F/m2
a The formation constants are related according to the reaction pH+ + q(tAlOH) + r(H2L-) h Hp(tAlOH)q(H2L)rp-r. The surface complexes are evaluated on the basis of the constant-capacitance model, and the intrinsic constants are given by the equation βpqr(int) ) βpqre(p-r)Fψ/RT (eq 4).
ionic strength constants from Smith and Martell.17 These binary equilibrium constants are given in Table 1. In the evaluation of the three-component data, no attempts were made to further adjust these constants or the specific capacitance for the alumina surface. The determination of the equilibrium model for the three-component system H+-tAlOH-H2L- was based on sets of data in which corresponding -log h and total concentration of nonbound ligand, L(aq), were known. The calculated value of L(aq), L(aq)calc, can be obtained from eq 7 as the sum of [H3L], [H2L-], [HL2-], and [L3-]. The computing of the three-component system was performed using combined proton balance data (emf data) and analyzed nonbound ligand data. Taken over all experimental data, the error sum of squares, U, can be defined as ∑(Wemf(Hcalc - Hexp)2 + Wanalysis(L(aq)calc - L(aq)2). Wemf and Wanalysis are the weighting factors for the emf data and the analyzed nonbound ligand data, respectively. In the calculations, values of W were set to give comparable weights to the emf and nonbound ligand data. To find the most accurate equilibrium model, i.e., the model providing the closest fit to experimental data, a systematic variation in the integer values of p, q, and r was performed. For each choice, the optimization of βpqr constants was accomplished by a stepwise variation of one or several constants until the values resulting in the lowest error sum of squares were found. These calculations were performed using the computer program LAKE.18 Calculations and Results
(11)
The acid/base properties of the aged γ-Al2O3 surface (eq 2) have been previously characterized by the authors.8 This model includes protonation/deprotonation constants of the hydroxylated surface and a corresponding specific capacitance. The equilibrium constants describing the acid/base reactions of H2PO4- (eq 3) were determined by means of a series of potentiometric titrations. The constants evaluated were in excellent agreement with the 0.1 M (16) Morel, F. M. M.; Yeasted, J. G.; Westall, J. C. In Adsorption of Inorganics at Solid-Liquid Interfaces; Anderson, M. A., Rubin, A. J., Eds.; Ann Arbor Science: Ann Arbor, MI, 1981; p 263.
Complexation of phosphate was studied in sets of batch experiments where -log h ranged between 4.8 and 9.6. This range was chosen in order to avoid dissolution of alumina at low and high -log h, respectively. The ratio between the total concentration of H2PO4- and tAlOH, C/B, was varied within a factor of ten; i.e., C/B ) 0.15, 0.30, 0.60, 1.00, and 1.50 were studied. Within these ranges, a total of 30 experimental points with measured -log h and total concentration of nonbound ligand were collected. (17) Smith, R. M.; Martell, A. E. Critical Stability Constants; Plenum Press: New York, 1989; Vol. 6. (18) Ingri, N.; Andersson, I.; Pettersson, L.; Yagasaki, A.; Andersson, L.; Holmstro¨m, K. Acta Chem. Scand., accepted for publication.
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A
Figure 3. Potentiometric data plotted as ZB versus -log h. ZB is calculated as ZB ) (H - h + KW h-1)/B (eq 12) using H+tAlOH-H3L as the zero level. The full curves were calculated using the model presented in Table 1.
B
Figure 5. Distribution diagrams (distribution of phosphate, FL, versus -log h) calculated at [H2L-]tot/[tAlOH]tot ratios 1.00 and 0.25 (A and B, respectively) at a solid concentration of 20.0 g of γ-Al2O3/dm3. Figure 4. Experimental data (phosphate analysis) presented as the fraction of nonbound phosphate, FL(aq), against -log h. The curves denote the modeled sum of [H3L], [H2L-], [HL2-], and [L3-] using the equilibrium model presented in Table 1.
The potentiometric data are presented in Figure 3 as ZB versus -log h plots. ZB is calculated from experimental data as
ZB ) (H - h + Kwh-1)/B
(12)
using H3L as the zero level for the ligand. The nonbound ligand data have been illustrated in Figure 4, as the fraction of phosphate remaining in the aqueous phase (L(aq)/C) as function of -log h. In the evaluation of experimental data, tests of combinations of complexes with different (p,q,r) compositions have been performed. The best fit to data was given by a model that assumes the formation of three complexes according to the following reactions:
tAlOH + H2L- + H+ h tAlLH2 + H2O tAlOH + H2L- h tAlLH- + H2O tAlOH + H2L- h tAlL2- + H2O + H+
β111(int) (13) β011(int) (14) β-111(int) (15)
The numerical values of these constants and their corresponding uncertainties (3σ) are given in Table 1. In addition to experimental data, Figures 3 and 4 also present the theoretical ZB curves and the fraction of nonbound phosphate curves resulting from this model. As can be seen in the figures, the model presented shows a good fit to all experimental data. The speciation of orthophosphate in the presence of alumina has been illustrated in Figure 5 in the form of
distribution diagrams. To construct these diagrams the computer program SOLGASWATER19 was used. These diagrams show that, at low -log h, most of the orthophosphate is adsorbed onto the surface of alumina, with tAlLH2 being the dominating surface complex. When -log h is raised, two deprotonized surface complexes are formed in succession and, in slightly alkaline solutions, a desorption of the ligand takes place. The distribution diagrams also show that, at -log h e 6.0 and at low C/B ratios, one would expect the phosphate ions to be almost completely bound to the surface. Discussion As already mentioned above, the kinetic measurements indicate that in acidic to neutral aqueous orthophosphatealumina suspensions a phase transformation, with the formation of a solid aluminum phosphate phase as a result, should occur. This was also confirmed by means of FTIR measurements on samples prepared in excess phosphate and aged in suspension for extended periods. However, as shown by our introductory time-resolved measurements at the [H2L-]tot/[tAlOH]tot ratio 2, this reaction proceeds at a slow rate and the surface complexation of phosphate ions onto the alumina surface could be separately studied in freshly prepared suspensions. To characterize these surface complexes, an equilibration time of 5 h was found most suitable, since it allowed the complexation reactions to reach equilibrium while the phase transformation reaction should still be negligible. Because of the necessity to limit the equilibration time, the experiments were performed with the batch titration technique and, despite its advantages, the potentiometric titration method could not be used. Due to the restricted time of equilibration, the formation constants reported describe a meta-equilibrium after 5 h and not full thermodynamic stability. However, considering the low remaining drift in -log h (19) Eriksson, G. Anal. Chim. Acta 1979, 112, 375.
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(0.04 units/h) at this point, the resulting constants describe a model very close to the complexation equilibrium. For -log h < 7, the orthophosphate was found to bind strongly to the alumina with the formation, according to the proposed model, of the surface complexes tAlLH2, tAlLH-, and tAlL2-. When the ratio [H2L-]tot/[tAlOH]tot is increased above one, nearly all of the surface sites are occupied by ligand. With a gradual increase in -log h above neutrality, the ligand was observed to desorb gradually from the surface. This is a general observation, that desorption of anions take place upon increasing -log h. Although there are several previous investigations which involve the adsorption of orthophosphate onto alumina surfaces, not many quantitative studies interpreting the results in terms of surface complexation have been reported. Goldberg and Sposito3 have published a phosphate surface complexation model for γ-Al2O3. This model was based on recalculations of experimental data in an 0.1 M NaCl ionic medium from Huang,20 and the equilibrium constants resulting from this recalculation have been included in Table 1. Compared to our investigation, it can be noted that, although assuming the same species to form, Goldberg and Sposito were unable to assign general equilibrium constants to the three reactions but were compelled to predict local, pH-dependent, values for the three species. In view of our findings, this phenomenon is probable to originate from an improper control of the delicate balance between surface complexation and precipitation. Orthophosphate complexation onto the surface of goethite (R-FeOOH) has been reported by Nilsson et al.21 Three monodentate surface complexes of the same stoichiometries as those proposed in the present work were found to provide the best explanation of the data. To enable a comparison between the stability constants in the two systems, the formation reactions can be written as
tXOH + H3L h tXLH2 + H2O; log K(Al) ) 9.59, log K(Fe) ) 10.78 tXOH + H2L- h tXLH- + H2O; log K(Al) ) 5.14, log K(Fe) ) 7.93 tXOH + HL2- h tXL2- + H2O; log K(Al) ) 4.89, log K(Fe) ) 8.87 In these equations K(Al) and K(Fe) denote the intrinsic equilibrium constants for the reactions with alumina and goethite surfaces (tXOH), respectively. As can be seen, the surface complexes of goethite are more stable than those of alumina and, moreover, the difference increases with degree of deprotonation. The implication of this is that hydrogen phosphate ions exhibit a higher acidity when bound to the goethite surface than when bound to the surface of alumina. It is important to point out that this type of direct comparison between intrinsic constants can be used to predict the general competition behavior between two surfaces only in cases where the electrostatic effects on the two surfaces are of the same magnitude. A graphical illustration of the strength of orthophosphate complexation onto alumina and goethite is presented in Figure 6. In this figure, the fraction of bound ligand is plotted against -log h at a one to one [H2L-]tot/[tXOH]tot (20) Huang, C. P. J. Colloid Interface Sci. 1975, 53, 178. (21) Nilsson, N.; Lo¨vgren, L.; Sjo¨berg, S. Chem. Speciation Bioavailability 1992, 4 (4), 122.
Laiti et al.
Figure 6. Modeled fractions of adsorbed phosphate, FL(ads), on aged γ-Al2O3 (full line) and R-FeOOH (goethite) (dotted line) as a function of -log h. The calculations have been performed at [H2L-]tot/[tMOH]tot ) 1.0 and at a solid concentration of 20.0 g/dm3.
ratio and at a solid concentration of 20.0 g/dm3. The figure shows that, at acidic and neutral -log h, the adsorption behaviors for the two systems are fairly similar. At alkaline -log h, however, phosphate is significantly more strongly adsorbed to goethite than to alumina. This can be seen as a manifestation of the stronger acidity exhibited by the hydrogen phosphate ions bound to the goethite surface. It is also of interest to compare this proposed surface complexation model with the obtained IR spectroscopic data. As pointed out above, the IR characteristics of the adsorbed phosphate ions vary with -log h (Figure 2b-d), which supports the description of the adsorption process in terms of surface complexes. The observed variations can be related to symmetry changes of the complexes and changes of the P-O force constants caused by protonation reactions. Therefore, the thermodynamic model, with complexes differing only in the degree of protonation, is in general agreement with the IR spectroscopic data. It is more difficult, however, from the IR data to determine the number and structure of the individual complexes. In a previous study of phosphate complexation onto goethite13 it was possible to distinguish three surface complexes. The success of that study relied on the fact that the PO4 unit in each surface complex had a comparatively high and well-defined symmetry. As a consequence, the P-O bands of the various complexes were more or less completely resolved, and the symmetry change from C3v to C2v and back to C3v as a function of -log h could clearly be followed. The P-O bands of phosphate on alumina are on the other hand poorly resolved, and it is not possible to assign bands to a particular complex. The reason behind this poor resolution, we believe, is caused by a low symmetry of the surface complexes and perhaps also by small random differences in the local structural environment. This in turn might be due to a weaker interaction of phosphate with alumina than with goethite. The surface complexes on goethite could retain a high symmetry, since the proton and the iron(III) surface ion seem to have a similar effect on the PO4 unit. On alumina the P-OH and P-OAl bands are different enough to introduce an asymmetry. Thus, whenever both H+ and Al(III) are coordinated to phosphate the highest possible symmetries are Cs or C1. This might explain why the IR features show little change in the acidic to neutral range but more in the slightly alkaline region. The complexes formulated as tAlOPO3H2 and tAlOPO3H- should display similar low symmetries while tAlOPO32- most likely will have a symmetry close to C3v. Consequently, the most pronounced change in the spectra is expected in the alkaline region. The inability to isolate the IR spectrum of the
H+-Orthophosphate-Aged γ-Al2O3/Water Interface
“pure” tAlOPO32- might be due to the interference and pH-lowering effect of CO2 in the IR measurements.13 This same experimental artifact may also apply to some of the data by Bleam et al.,22 in which solid-state 31PNMR was used to monitor phosphate adsorption onto boehmite (γ-AlOOH) surfaces. On the basis of samples prepared at different pH values and subsequently filtered off and freeze-dried, these authors concluded that innersphere complexes were formed between phosphate ions and the boehmite surface and, furthermore, that the degree of protonation in these complexes varied with pH. In contrast to our equilibrium model, however, which predicts a dominance of the species tAlOPO32- at pH values above ≈9, their data indicated a continued strong change in protonation degree in the pH range 9-11. In contradiction to our model, phosphate ions have often been suggested to form bridging surface complexes on metal (hydr)oxides. The poorly resolved IR bands unfortunately prevent detailed structural interpretation. However, in the present study no direct evidence, either thermodynamic or spectroscopic, to support the formation of such species was found. Furthermore, the similarities with the goethite system, which recently has been shown to be best explained by monodentate coordination,13 strongly favor an interpretation which involves monodentate surface complexes. As pointed out earlier in the text, the kinetic experi(22) Bleam, W. F.; Pfeffer, P. E.; Goldberg, S.; Taylor, R. W.; Dudley, R. Langmuir 1991, 7, 1702.
Langmuir, Vol. 12, No. 12, 1996 2975
ments and the IR measurements have shown that when an excess of orthophosphate is equilibrated with alumina, the formation of a new phase will occur. On the basis of a series of IR measurements it was however also found that the rate of this phase transformation was highly dependent on the relative phosphate to surface site ratio. Thus, IR samples prepared at a 4:1 [H2PO4-]tot/[tAlOH]tot ratio showed signs of aluminum phosphate bands after even very short equilibrium times (30 min). On the other hand, samples containing a 1:1 phosphate to surface site ratio showed only weak signs of phase transformation after 1 week of equilibration. Finally, in samples prepared at a 0.25:1 ratio, no phase transformation could be observed at all, even after several weeks of equilibration. This implies that even though we have restricted the validity of our complexation model to describe a metaequilibrium state, it may well provide a thermodynamic description of the equilibria in the system at [H2PO4-]tot/ [tAlOH]tot < 1. Acknowledgment. We are grateful to Prof. Staffan Sjo¨berg for valuable comments on the manuscript. Financial support from the Kempe Foundation for the purchase of the FT-IR/RAMAN spectrometer is hereby acknowledged. This work forms part of a program financially supported by the Swedish Natural Science Research Council. LA9515074
