Environ. Sci. Technol. 1992, 26, 908-914
Sudicky, E. A. University of Waterloo, Waterloo, Ontario, Canada, personal communication to Michael R. Anderson of the Oregon Graduate Institute, 1988. Johnson, R. L.; Pankow, J. F. Environ. Sci. Technol., preceding paper in this issue. Schwille, F. Dense Chlorinated Solvents in Porous and Fractured Media, Translated by J. F. Pankow; Lewis Publishers: Boca Raton, FL, 1988. Sudicky, E. A. Water Resour. Res. 1986,22, 2069-2082. Anderson, M. P. CRC Crit. Rev. Environ. Control 1979,9, 97-156.
(10) McIlvride, W. A.; Rector, B. M. In Proceedings of the
Second National Outdoor Action Conference on Aquifer Restoration, Ground Water Monitoring and Geophysical Methods; 1988; pp 375-390. Received for review October 25,1991. Accepted December 9,1991. This work was supported by the University Consortium Solvents-in-Groundwater Program with corporate support from Dow Chemical Corp., Ciba-Geigy Corp., General Electric Corp., Eastman Kodak Corp., and Boeing Corp.
Identification and Quantification of the “AI,,” Tridecameric Polycation Using Ferron Davld R. Parker* Department of Soil and Environmental Sciences, University of California, Riverside, California 9252 1
Paul M. Bertsch Division of Biogeochemistry, Savannah River Ecology Laboratory, University of Georgia, Aiken, South Carolina 29802
w Historically, 27A1-NMRspectroscopy has been required for definitive identification of the A104A112(OH),(H20)1J+ polycation (“A113n),but recent studies suggest that it might be equatable to the polynuclear A1 fraction that exhibits a moderate reaction rate with the spectrophotometric ferron reagent (Alb). Our objectives were to further test this correspondence and to critically evaluate the ferronAll, reaction kinetics. Partially neutralized solutions were prepared with [All, = 10-4-10-2 mol L-l, hydrolysis ratios of 0.8-2.4, and base injection rates of 0.2-20 mL h-l. These solutions were quantitatively analyzed using ferron and 27AlNMR, and the results confirmed that the Alb fraction measured with ferron corresponds to Al13in freshly prepared solutions. The apparent pseudo-first-order rate coefficient for the ferron-Alb reaction (kb)is dependent on the total ferron concentration in the reaction mixture. Examination of fitted kb values from this work and from some previous studies revealed that they are, if properly evaluated, indeed consistent and predictable, permitting near-certain identification of All? The ferron method thus offers a simple and inexpensive alternative to 27A1-NMR analyses and allows quantification of All3 at concentrations 10-100-fold lower than presently analyzable by NMR, a concentration range pertinent to natural waters.
Introduction The hydrolysis behavior of aqueous Al(II1) has been the subject of innumerable studies in recent decades, and a myriad of reaction schemes have been proposed. Available evidence suggests that, in the absence of added base, aqueous solutions of a simple A1 salt can be accurately described by consideration of the hexaaquo ion, mononuclear hydrolysis products and, in sufficiently dilute solutions with concomitant increases in pH, solid-phase trihydroxides such as gibbsite (1,2). When A1 solutions are partially neutralized via addition of base, a much more complex system results, and an abundant body of research has focused on the prevalence of one or more polynuclear hydroxo-A1 complexes (e.g., refs 1-3). Although these species have been treated thermodynamically (Le., via development of formation constants; see, e.g., refs 4-61, their critical dependence on reaction conditions clearly points to a role as metastable intermediates in the ultimate 908
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precipitation of hydroxo or oxyhydroxo solid phases (1,3, 7, 8). The composition and structure of the predominant polynuclear species has been a source of long-standing controversy, and the disparate views can be broadly categorized into two major models. For many years, a “core-links” or “gibbsite fragment” model was invoked to explain potentiometric and other data (1-3). In this model, polymeric species consisting of one to several hexagonal rings composed of octahedrally coordinated A1 with OH in bridging positions are envisioned to form following neutralization. Upon aging, and especially at high degrees of hydrolysis, further polymerization and bimensional growth of these ring structures was invoked to explain the ultimate appearance of gibbsite or a related crystalline solid phase (1,3). The model was appealing, in part, because the dioctahedral structure of gibbsite is maintained throughout the polynuclear condensation (9), and because it might adequately account for observed declines in the reactivity of polynuclears with acids and/or complexing ligands as solutions age (7, 10-12). It is noteworthy, however, that little unequivocal (i.e., spectroscopic) evidence has ever been obtained for the existence of such polynuclear species, especially in dilute solutions (i.e., total [All < ca. 1mol L-l) at room temperatures (2). The other major model, which includes the so-called “All,” polycation as the predominant hydrolyzed species, was first proposed by Johanson (13) on the basis of crystallographic data for the structure of basic aluminum sulfates precipitated from partially neutralized solutions. The existence of this species, which has an idealized structure of A104Al12(OH)(24+n)(H20)(12-n,(7-n)+, has now been confirmed by a number of investigations employing 27Aland l70nuclear magnetic resonance spectroscopy (NMR) (2, 6, 14-20). The All, polynuclear is likely of the “Keggin” structure, consisting of a symmetrical, cagelike arrangement of 1 2 octahedrally coordinated A1 atoms surrounding a single tetrahedral core atom (16,21). Although the species has been referred to by many names, we prefer to denote it as the “All3 polycation” or the “tridecamer”. Doubt has been expressed as to the general significance of the tridecamer in natural systems (3,19,22,23),in part because its confirmed occurrence has usually coincided
0013-936X/92/0926-0908$03.00/0
0 1992 American Chemical Society
with conditions of high A1 concentration and neutralization with strong base (e.g., NaOH) under laboratory conditions. Recently, however, direct 27A1-NMRevidence has been obtained for the existence of the tridecamer in an acid forest soil from Vermont (24); to date this is the only confirmed observation of this species in nature. Nonetheless, the possible existence of Al13in the environment may be significant. Recent laboratory evidence suggests that All, is more than 10-fold more toxic to plant roots than the hexaaquo A1 ion (23), a potentially important finding given the ubiquity of A1 phytotoxicity in naturally acidic soils (25). A number of reports indicate that A1 toxicities toward fish and other aquatic biota are maximal at pH values of 5.0-6.0 (26-29) where the tridecamer, if it forms, could be expected to be relatively stable in dilute, natural waters; it has, however, received little or no consideration as a possible component of such systems. The AllB polycation also has a number of industrial applications, including use as a flocculent in waste water treatment (30-32), for the construction of “pillared” 2:l phyllosilicate clays used as catalysts (33,34),as a primary constituent of pharmaceutical and cosmetic products (35-38) and ceramics (39), and in paper sizing (40, 41). Recent studies using 27AlNMR have suggested that the polynuclear fraction that exhibits a moderate reaction rate with complexing reagents such as ferron (8-hydroxy-7iodo-5-quinolinesuLfonicacid) or 8-hydroxyquinoline might be equatable to Al13 (17, 23, 30, 31, 42, 43). The ferron method has been employed in a number of studies and can conveniently differentiate between polynuclear hydroxy-Al and mononuclear species based on differential reaction kinetics (7, 10, 12, 17, 44-48). But, the calculated rate coefficients for the ferron-polynuclear reaction have been inconsistent (22),and the variability has been interpreted as evidence for polynuclear species of varying composition (see, e.g., refs 1,7,10,and 12). We postulate, however, that much of the variability may be due to (i) differences in formulation and preparation of the ferron reagent, (ii) differences in the kinetics models employed to evaluate the data (compare, e.g., refs 12, 44, and 48), and (iii) precautions that are necessary for proper kinetic analyses and meaningful comparisons across studies, but that have not always been fully recognized. The objective of this study was to further test the correspondence between A l l 3 (as quantified by nAl NMR) and the reactive polynuclear A1 fraction kinetically determined using ferron. If the two quantities are equatable, the latter method might offer a simple and inexpensive alternative for identification and quantification of the tridecamer and could facilitate investigations of A1 speciation at concentrations too dilute for analysis using NMR. Specifically, we wished to investigate the uniqueness and constancy of the rate coefficient for the Al13-ferron reaction. The results provide a critical evaluation of the method needed for an investigation of Al13formation in dilute solutions representative of natural waters that is reported in a companion paper (49).
Experimental Section Preparation of Partially Neutralized A1 Solutions. All reagents were of analytical grade or higher purity. Double deionized (DDI) water (5.0 X diluted to 1 X mol L-I, and ferron assay initiated within 5 min aParker et al. (23, 48) employed slightly different volumes such that the ferronTIAIT molar ratio ranged from 47 to 63.
kHz with a digital resolution of 2.44 Hz point-l. The probe diameter was 10 mm, and either 16000 or 32000 acquisitions per sample were obtained. Mononuclear standards (pH -2.3, [All, = 3.70 X 10+-1.11 X lo-, mol L-') prepared from pure A1 wire dissolved in dilute HC1 were run for both spectral calibration and quantification; the resonance corresponding to the hexaaquo ion, A1(HzO):+, was assigned a chemical shift (6) of 0 ppm, as established in the literature (17,50).For the partially neutralized stock solutions, the resonance at 6 = 62.5 f 0.5 ppm downfield corresponds to the tetrahedrally coordinated A1 in the center of the All, polycation (17,18). The integrated intensity of this peak was compared with the hexaaquo ion peaks (6 = 0 ppm) of at least three acidified standards to obtain a quantitative estimate and multiplied by 13 to obtain the molar concentration of atomic A1 present as the All, species (18). Ferron Analyses for Reactive Polynuclear Al. Although the kinetically based ferron method has been described in some detail previously (12,48,51), key features are restated here due to some typographical errors in ref 48, variations both in exact chemical composition (e.g., refs 10,12,17, and 52) and in the kinetic models that have been employed (e.g., refs 10,12,44, and 48), and the pivotal role the method plays in the current study and that reported in a companion paper (49). Briefly, the three stock solutions described previously (12)were prepared except that we employed twice the o-phenanthroline concentration. These stocks were filtered and combined to yield the mixed, working ferron reagent with the following composition: 1.58 mmol L-l ferron; 0.28 mmol L-l ophenanthroline; 0.95 mol L-l sodium acetate; 0.32 mol L-' NH,OH.HCl; 0.11 mol L-l HC1; with a final pH of 5.2-5.4. The working solution was prepared frequently such that a 5-7-day-old batch was always available for use (44). Volumes of sample and mixed ferron reagent were selected such that the ferronT/AIT molar ratio was always 50 (see below) and are given in Table I. These solution volumes, and the procedure outlined below, also pertain to the ferron analyses conducted in a companion investigation (49).
The reaction temperature was maintained at 25 f 0.2 "C throughout. For [AllT I 1 X mol L-', the sample was pipeted into a 1-cm path-length quartz cuvette and placed in a Beckman DU-70 UV-vis spectrophotometer equipped with a Peltier temperature controller. The appropriate volume of ferron reagent was then injected into the cuvette using an automatic pipet, with the injection providing sufficient mixing of sample and reagent (44,48). Within 3 s of injection, the kinetics scan was initiated, and absorbance increases were monitored at 363 nm at a frequency of 10 readings min-'. At [AllT = 5 X mol L-', the required volumes of ferron and sample (Table I) pre910
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Time (min) Figure 1. Typical kinetics of ferron-AI reactions at 25 O C with ferron, = 1.20 X mol L-'. (a) Raw absorbance vs time plot for an acidified, mononuclear standard, and for a partially neutralized sample. (b) Linearized, first-order plot for selected data points for the sample. Solid line represents the data segment (2-6 min) fit to eq 3 by linear regression; the dashed extension shows the extrapolation to t = 0 for calculation of [AI,]. The horizontal dashed line represents 5 % of AI, and illustrates that deviations from linearity usually occur only when reaction is nearly complete.
cluded the use of the preceding method; instead, the sample was placed in a small beaker with a stir bar and the ferron reagent added with a repipet. After mixing for several seconds, a subsample was withdrawn and transferred to a cuvette in the spectrophotometer, and the kinetics scan was initiated. The delay from reagent addition to the start of the scan was ca. 20 s and was recorded exactly for each run. For solutions with [All, > 5 X low4 mol L-l, the reagent/sample volume ratio becomes inconveniently high; these samples were thus diluted to 1 X low4mol L-' with DDIlwater, and using the appropriate sample and reagent volumes (Table I), the assay was initiated within 1 5 min to minimize shifts in speciation upon dilution. Absorbance increases were monitored for 60 rnin such that three fractions could be operationally defined: Ala, Alb, and Al, in the parlance of Smith and Hem (7), corresponding to mononuclear Al, reactive polynuclears, and unreactive A1 [representing larger polynuclears and/or solid-phase Al(OH),], respectively. Absorbance increases after 30 min were negligible (Figure la), and the Al reacted at 30 rnin was assumed to represent [Ala + Alb]. The quantity [Al,] was computed as the difference between [All, and [Ala Alb] (throughout, brackets refer to the molar concentration of atomic A1 assignable to a given fraction). Based on the reaction kinetics of acidified (pH -2.3) A1 standards, Ala was known to completely react within 1.5-3 min (Figure la). The ferron-Alb reaction was complete within ca. 10-30 min, depending on ferronT,and
+
was analyzed on the basis of the following generalized kinetic model -(d [AIb]/dt) = k [ferron]’ [AIb]” (1) where absorbance increases due to formation of the Alferron complex are assumed to directly parallel the reaction. If the ferronT/AITmolar ratio is maintained at ca. 50 or greater (12),[ferron] is effectively constant, and an apparent rate coefficient can be defined as kb = k[ferronT]7 (2) Previous research has shown that the reaction is approximately first order with respect to [AI,] (Y = l),such that eqs 1 and 2 can be combined to yield a pseudo-first-order reaction (48). After integration and conversion to common logarithms, the integrated rate expression is thus log [Alb],, = log [AI,] - (kb/2.303)t (3) where [Alb], is the concentration of unreacted Alb at any time, t. The concentration of Alb initially present, [AI,], can then be computed by extrapolating a linear segment of an appropriate plot of the Alb-ferron reaction back to t = 0, with k b calculable from the slope of the line (Figure lb). Raw absorbance data were converted to molar concentrations based on an appropriate standard curve for acidified, mononuclear standards and the [Alb],, values at each t computed based on the 30-min reading ([Ala + Alb]). The data were then plotted in accordance with eq 3 to visually evaluate the linearity of the reaction; the first 4-10 min was used, depending on ferronT. Similarly, the first 1.5-3 min of each run (depending on ferron,), within which the overlapping ferron-Ala reaction occurs, were excluded from the analyses (Figure lb), such that a data segment 2.5-7 min in duration was actually selected. Finally, this data segment was fit to eq 3 using conventional linear regression methods, with the delay between reagent addition and the acquisition of the first reading being accurately accounted for. On the basis of the resulting estimate of [Al,], the quantity [ma]was computed by difference.
Results and Discussion The 27Al-NMRspectra obtained for all samples revealed only two significant peaks: the 6 = 0 ppm peak for octahedrally coordinated mononuclear A1 (the hexaaquo ion plus mononuclear hydroxo species) and the 62.5 ppm downfield peak corresponding to the tetrahedral core of the All, polycation. Although the 6 = 0 ppm peak was somewhat broadened (as compared to the acidified standards) due to hydrolysis (50,53),we observed no evidence of the peak or shoulder -3 ppm downfield that has usually been attributed to a small oligomer (2,16,18).This is consistent with recent reports that [All, must exceed mol L-’ to obtain observable quantities of this species (5, 16), and we attribute all NMR-detectable A1 in our samples to either mononuclear Al or the tridecamer. The tetrahedral Al13peak was clearly resolvable and integrable in all samples with [All,] I-3 X mol L-l but was increasingly difficult to quantify in the more dilute solutions. We also observed that the peak tended to broaden at higher degrees of hydrolysis, such that the m = 2.4 samples were often the most difficult to quantify. mol L-l), we Consequently, at the lowest [AI], (1 x 10-~ found that m = 1.6 yielded the most reliable estimates of the tridecamer. A t m = 0.8, the low concentration of All, (25-30 % of Al,) precluded accurate quantification, while at m = 2.4, peak broadening was the obstacle. Increased numbers of acquisitions might improve sensitivity, espe-
cially on the 11.7-T instrument, but accurate peak integration may be limited by S/N ratios that will not improve substantially within the limits posed by practical counting times. Because only 1 in 13 A1 atoms in the tridecamer is detected in the downfield peak, quantification of this species is probably limited to solutions with [Al13]= 5 X 10-~ mol L-l or greater using currently available spectrometers (2, 16, 19). The ferron-Alb kinetics data were successfully fit to the linearized first-order model (eq 3) for 17 out of the 20 solutions prepared; coefficients of determination (r2)were 20.998 for these 17 samples. With the remaining three, all of which had m = 2.4, some tailing of the linearized plot occurred relatively early (i.e., within the first 6 min) in the reaction. Our interpretation of this behavior is that these samples contained a small quantity of polynuclear Al that had coalesced or aggregated into less reactive units that we will term Alb,. Presumably, this fraction had not yet become part of the incipient and virtually nonreactive solid phase and may correspond either to the “tenuous aggregates” of All, units recently proposed (21) or to “gibbsite fragment” units (3)sufficiently small to exhibit an observable reaction with ferron. Similarly, Bachelor et al. (54) also observed a transient, more slowly reacting component in some dilute solutions neutralized to pH 6.0 ( mprobably exceeded 3.0) that likely developed from All, units initially formed during neutralization. We deconvoluted the first 13-15 min of the reaction for these samples using a nonlinear regression model (48) assuming that both types of polynuclear A1 exhibited parallel pseudofirst-order reactions. The less reactive Alb2form constituted only 3-8% of AlT, with a corresponding kb2 approximately 5-10-fold less than kb, consistent with some other observations (50,54). For these three samples, the nonlinear model yielded better fits to the experimental data, as well as values of kb in better agreement with those obtained for the other 17 solutions. Overall, the 15 solutions analyzed at [ferron], = 1.20 X lo-, mol L-l yielded an average k b value of 0.416 f 0.028 min-l, while the five solutions run using a [ferron], of 1.49 X loW3mol L-l yielded an average k b of 0.630 f 0.106 min-l. All solutions contained measurable Al, (5-5670 of AlT), although many contained no visible precipitate or turbidity, consistent with a number of earlier reports (3, 8, 45, 46, 55). A comparison of the 27A1-NMRand ferron analyses reveals that the two methods yielded virtually identical estimates of [All,] and [AIb],respectively, for all solutions mol L-l (Figure where either estimate exceeded -2 X 2). A t lower concentrations, there are some deviations from the 1:l line, but no consistent trend for either fraction to exceed the other (Figure 2). Consequently, we believe that the failure to obtain exact agreement between [All,] and [Ab]is due to the relative insensitivity of 27AlNMR, which precludes accurate peak integration at the lower concentrations, rather than the presence of any reactive polynuclears other than the tridecamer. Thus, our data suggest that, in freshly prepared, NaOH-neutralized solutions with [All, between l X lo4 and l X mol L-l, the Alb fraction measured by ferron is composed solely of the Al13 polycation, and the two fractions can be considered equatable. In order to assess the utility of the ferron method for quantification of the All, polycation in more dilute solutions, we further evaluated the constancy and uniqueness of kb values. During actual analysis, the data depicted in Figure 2 encompass only two pairs of sample and reagent volumes (Table I) and, hence, only two ferron, values. We therefore utilized data from some previous studies that Environ. Sci. Technol., Vol. 26, No. 5, 1992
911
Table 11. Ferron-Based Estimates of A1 Fractions in Some P a r t i a l l y Neutralized Solutions f r o m Previous Studies
[All,, mol L-l 2.0 x 10-5 2.0 x 10-5 2.0 x 10-5 1.0x 10-4 5.0 x 10-5 1.5 X
X
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m 0.75 1.50 2.25 2.0 2.0 2.0
[ferron],," mol L 7.02 x 10-4 7.02 x 10-4 7.02 x 10-4 1.20 x 10-3 9.68 x 10-4 5.27 x 10-4
% of A1T
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25.0 41.4 36.4 62.6 72.3 63.7
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included A1T values of
