Langmuir 1998, 14, 4731-4736
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Investigation of the Surface Properties of Solid-Phase Hydrous Aluminum Oxide Species in Simulated Wastewater Using Atomic Force Microscopy Anselm Omoike, Guoliang Chen, Gary W. Van Loon, and J. Hugh Horton* Department of Chemistry, Queen’s University, Kingston, Ontario, Canada K7L 3N6 Received February 3, 1998. In Final Form: May 27, 1998 Hydrous aluminum oxide particles precipitated from simulated wastewater were examined using atomic force microscopy in order to understand their structural and coating properties. Three types of particles were examined: aluminum oxides coprecipitated by adding alum in the presence of a solution of phosphates and tannic acid; postprecipitated particles formed by adding phosphates and tannic acid to already formed aluminum oxides; and a control case consisting of particles precipitated in the absence of either phosphate or organic component. Using tapping mode and phase imaging atomic force microscopy, it was found that the postprecipitated particles had distinctly different viscoelastic properties than either of the other two particle types and also varied markedly in particle size and morphology. These observations are consistent with a model in which the postprecipitated particles are coated with an organic coating of tannic acid. The results are discussed in the context of the relative effectiveness of these aluminum oxides in removing phosphates and other contaminants from wastewater during sewage treatment.
Introduction In addition to producing real space images of surface topography, the recent advances in atomic force microscopy (AFM) have allowed imaging of the chemical properties of surfaces on the nanometer scale.1 Among these techniques, the use of phase imaging in intermittent contact or tapping mode AFM has been shown to produce nanometer scale images of variations in surface viscoelastic properties. In the phase imaging mode, the phase shift of the oscillating cantilever is measured as a function of tip position on the surface. The observed phase shift ∆φ is related to the tip-sample force interactions, albeit in a complex fashion.2,3 However, under the correct imaging conditions, this technique can not only map out the topography across the sample surface but also allow a chemical identification of surface features. Much of the previous work on phase imaging AFM data, and particularly its interpretation, has concentrated on polymer samples.4-6 In this paper, we show how the technique can be applied to a quite different chemical system; here, we have investigated the surface-coating properties of a series of hydrous aluminum oxide particles derived from the hydrolysis of aluminum during wastewater treatment. Alum [Al2(SO4)3‚nH2O] is the most widely used coagulant in water and wastewater treatment. The use of alum requires that sufficient alkalinity be present in the wastewater to produce solid hydrous aluminum oxide species. These species remove orthophosphates present * To whom correspondence should be addressed: tel (613)-5452379; fax (613)-545-6669; e-mail
[email protected]. (1) Wiesendanger, R. In Scanning Probe Microscopy and Spectroscopy: Methods and Applications; Cambridge University Press: New York, 1994. (2) Winkler, R. G.; Spatz, J. P.; Sheiko, S.; Moller, M.; Reineker, P.; Marti, O. Phys. Rev. B 1996, 54, 8908. (3) Burnham, N. A.; Kulik, A. J.; Gremaud, G.; Gallo, P.-J.; Oulevey, F. J. Vac. Sci. Technol., B 1996, 14, 794. (4) Bar, G.; Thomann, Y.; Brandsch, R.; Cantow, H. J.; Whangbo, M. H. Langmuir 1997, 13, 3807. (5) Overney, R. M.; Meyer, E.; Frommer, J.; Guntherodt, H.-J.; Fujihira, M.; Takhano, H.; Gotoh, Y. Langmuir 1994, 10, 1281. (6) Hoper, R.; Gesang, T.; Possart, W.; Hennemann, O. D.; Boseck, S. Ultramicroscopy 1995, 60, 17.
in the wastewater by forming insoluble aluminum hydroxyphosphate or other complexes. In an activated sludge plant, there are several points7 at which alum can be added, but two common addition points are (1) before the commencement of biological treatment and (2) after the aeration chamber prior to final clarification. In the former case, it is possible that the orthophosphate ions are coprecipitated with alum in the presence of dissolved organic matter. In the latter postprecipitation case, the orthophosphate and dissolved organic concentrations in the effluent have been reduced and recycling of sludge from this system is synonymous to the use of prehydrolyzed alum in treating wastewater. Depending upon where alum is added during the wastewater treatment process, the reaction between aluminum oxides and components dissolved in wastewater produces solid-phase products that could be different in chemical and surface composition. This work presents results probing (using AFM) the changes in the solid-phase reaction products formed on the addition of alum to simulated wastewater-containing phosphates. We use tannic acid in the simulated wastewater as a surrogate for the dissolved organic components in the real case. Tannic acid is well characterized and contains the same functional groupsssalicylic, carboxylic, and phenolicsas are found in the humic material that forms the largest component of dissolved organic material in wastewater. Three different solid phases of hydrous aluminum oxide species were prepared and examined, simulating various stages in the wastewater treatment process: coprecipitated particles that correspond to the addition of alum before or at the aerator, postprecipitated particles that represent the situation where solid hydrous aluminum oxide species are recycled in the biological sludge and encounter phosphate in the aerator, and finally a control experiment in which hydrous aluminum oxide (Al(OH)3) was simply precipitated from a NaHCO3 solution in the absence of phosphate or organic components. A second control consisting of tannic acid dispersed on a mica substrate was also imaged. Using phase(7) Bowker, R. P. G.; Stensel, H. D. In Phosphorus Removal From Wastewater; Noyes Data Corporation: New Jersey, USA, 1990.
S0743-7463(98)00130-9 CCC: $15.00 © 1998 American Chemical Society Published on Web 07/30/1998
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imaging AFM, we demonstrate the presence of an organic coating on the postprecipitated particles that modifies the tip-sample interaction as well as causing considerable changes in particle morphology. These results are used to explain the varying effectiveness of these particles in the wastewater treatment process. Experimental Section Particle preparation was carried out as follows. The synthetic wastewater was made up of a solution of NaHCO3 (0.208 mol L-1), KHPO4 (3.70 mol L-1), and tannic acid, C76H52O46 (0.100 mol L-1). All the chemicals used were reagent grade obtained from BDH except the alum, which was commercial grade, obtained from the Kingston Water Purification Plant, Kingston, Ontario. Distilled deionized water was used for all sample preparations. A mixer equipped with a four-blade propeller and a variable speed control was used for mixing the components in the simulated wastewater. The coprecipitated particles were synthesized by adding alum (430 mg as Al L-1) to the synthetic wastewater, and the components were allowed to age for 5 min at a stirring speed of 380 rpm. To synthesize the postprecipitated particles, the alum (430 mg as Al L-1) was added to the synthetic wastewater in the absence of phosphates and tannic acid and the mixture was allowed to age, with stirring, for 5 min before the subsequent addition of the phosphate and tannic acid components and then aged for a further 5 min. Finally, the control Al(OH)3 particles were synthesized by adding alum (430 mg L-1 as Al) to a NaHCO3 (0.208 mol L-1) solution. Samples were prepared for AFM imaging in the following manner. Aqueous dispersions of all three hydrous aluminum oxide samples were produced by placing approximately 0.35 g L-1 slurries of the particles in an ultrasonic bath for 60 min. A 30 µL portion of the dispersion was syringed onto a freshly cleaved mica substrate 1 cm2 in area. The sample was then spun at 4000 rpm on a spin coater for 60 s to ensure an even distribution of particles over the substrate surface. The sample was allowed to air-dry at for 1 h and then imaged in the AFM. The control sample consisting of tannic acid deposited on mica was prepared from a 3.8 × 10-8 mol L-1 solution of tannic acid in methanol. This solution was dispersed and spin coated onto a mica substrate in a fashion similar to the alumina particles. All AFM data shown were acquired using a PicoSPM operated in MAC mode (Molecular Imaging, Tempe, AZ), using a Nanoscope IIE controller (Digital Instruments, Santa Barbara, CA). The MAC mode is essentially the same as tapping mode, except that the cantilever is magnetically coated and is driven by an external oscillating magnetic field.8 The cantilevers had a force constant of ∼0.5 N m-1 and a resonance frequency of ∼100 kHz. All images were acquired under ambient conditions, at the fundamental resonance frequency of the Si cantilevers. Height and phase shift data were all recorded simultaneously, as a function of both cantilever oscillation amplitude (Ao) and set point ratio rsp ) Asp/Ao. Images were recorded at scan rates of 1-2 lines/s using a 30 µm × 30 µm scanner.
Results AFM Images. Figure 1 shows the postprecipitated particles dispersed on the mica substrate, acquired at various values of both cantilever oscillation amplitude (Ao) and set point r. The left image is height mode data, while the right image is phase imaging data. The image size and z scale (i.e., height or phase shift) are the same for each image. In all cases, dark regions correspond to lower values of height and phase shift, while brighter regions correspond to higher values. Larger scale images, up to 20 µm square, demonstrated that the pattern seen in Figure 1 is typical across the surface. Several observations are immediately apparent. In all cases, the contrast between height and phase shift data is reversed. However, the range of contrast varies considerably, in both height and phase imaging data, with the most contrast visible (8) Han, W.; Lindsay, S. M.; Jing, T. Appl. Phys. Lett. 1996, 69, 4111.
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at large Ao and intermediate set points and with the least at low Ao and large set points. Images were also acquired at intermediate set point values at each Ao and show trends in contrast intermediate to those images shown here. Set points of r < 0.5 generally produced images of poor quality and often showed large tip oscillations, so are not presented here. Another interesting observation is seen in parts c and d of Figure 1. Here we see two images of the same area, taken one after another. The surface features indicated with the circles in these figures change slightly from one image to another. The changes suggest that there is a tip-sample interaction taking place. While the two images are a particularly good example of this effect, it was observed in a number of images, some taken one after another at exactly the same set point and amplitude. Imaging a small region (500 nm square) of the sample in contact mode did not show any sign of the particles on the surface. Reimaging the same area in MAC mode showed that the particles had been swept out in the contact mode image region. A similar effect was observed with the tannic acid sample (Figure 4). Changes in particle shape or other evidence of tip-sample interaction of a similar nature were not observed in the case of the coprecipitated or Al(OH)3 control particles discussed below. Figure 2 shows the coprecipitated particles deposited on the mica substrate. These images were acquired at the same set point, 0.65, but three different oscillation amplitudes of 116, 58, and 29 nm (similar to the amplitudes for Figure 1, above). Note that the Z-range in the height mode is 10 times that of Figure 1, but the Z-range of the phase imaging data is the same. Images at different set points were acquired, ranging from 0.9 to 0.25 and were similar to those seen here. Figure 3 shows the control Al(OH)3 particles on the mica substrate at an intermediate oscillation amplitude of 58 nm and set point of 0.5. Images of the control particles were also acquired at set points ranging from 0.9 to 0.15 and amplitudes ranging from 20 to 105 nm. In all cases, the height and phase images looked very similar to those shown here and, indeed, to the images of the coprecipitated particles in Figure 2, and so are not repeated. Again, images taken over larger scan areas, up to 20 µm square, show that the features seen in Figures 2 and 3 are typical of the surface. Besides the general lack of dependence of image contrast on both amplitude and set point, several other observations are of note. First, the particles are in both cases roughly circular, with diameters ranging from 40 to 500 nm. Cross sectional profiles also reveal that the particles range in height from 10 to 150 nm. The coprecipitated and control particles are similar in both size and shape and show a marked contrast in morphology to the postprecipitated particles, which are extremely flat and irregular in shape. We also emphasize the observation that, regardless of size, the coprecipitated particles show little contrast with respect to the substrate in the phase imaging mode, and control particles show virtually none, other than some slight shading or brightening at the particle boundaries. This latter effect is probably simply due to the failure of the feedback loop to adequately follow the rapid changes in sample height as the tip tracks across the particles. Figure 4 shows images of the mica sample on which tannic acid was deposited from a methanol solution. Again we present images at various set points and oscillation amplitudes comparable to those obtained in the previous experiments. The Z-range in height mode (10 nm) is similar to that for the postprecipitated particles in Figure 1, while the phase imaging Z-range is the one-half that in the other figures. It proved possible to acquire higher resolution images with this sample than with the previous
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Figure 1. Height (left) (Z range ) 20 nm) and phase imaging (right) MAC mode AFM images of the postprecipitated aluminum oxide particles dispersed on a mica substrate. All images are 2 µm square and were acquired at the following tip oscillation amplitudes and set points: (a) Ao ) 104 nm, rsp ) 0.90; (b) Ao ) 104 nm, rsp ) 0.60; (c) Ao ) 46 nm, rsp ) 0.90; (d) Ao ) 46 nm, rsp ) 0.75; (e) Ao ) 22 nm, rsp ) 0.90; (f) Ao ) 22 nm, rsp ) 0.70.
ones. All the images in Figure 4 are 500 nm across. We note that the tannic acid tends to form small agglomerates about 15 nm in diameter which can exist as isolated particles or can in turn form larger agglomerates some 300-600 nm in diameter in which the smaller agglomerates can be resolved. We also observed that by reducing the tannic acid concentration in the methanol solution, the size and frequency of the larger agglomerates decreased over the surface. There are similarities in the phase contrast behavior to postprecipitated particles in Figure 1. Again, the contrast is reversed between height and phase imaging. Here, the contrast also undergoes a reversal in the height and phase images themselves as we go from set points of 0.9-0.3 at Ao ) 106 nm. Like the postprecipitated particle images in Figure 1, it proved impossible to acquire useful images at set points lower than about 0.7 for tip oscillation amplitudes less than about 100 nm. We note that for the lower Ao images in parts c and d of Figure 4 that there is considerably less contrast, especially in the phase images. Other Techniques for Particle Characterization. We report briefly here on the results for some other techniques used for characterizing the chemical composi-
tion and activity of these aluminum oxide particles. The surface reactivities of particles were measured using a ferron complexation procedure.9 Aluminum, present as a component of a solid, reacts with ferron reagent to form a colored complex, which absorbs radiation at 370 nm. To combine with ferron, Al-OH, Al-PO4, or Al-organic matter bonds at the interface between the hydrolytic product and the adjacent solution must be broken. Aluminum that is present in weakly bonded labile forms therefore would be expected to react rapidly, whereas aluminum that is strongly bound and inert in the solid would react much more slowly. An amorphous precipitate with large surface area would also be expected to react more rapidly than a more crystalline product, due to the high surface concentration of aluminum ions able to react readily with the ferron reagent in solution. Measuring the rate of reaction between aluminum species in the solids and the reagent therefore gives an indication of the surface speciation and morphology of the aluminum-containing solid. As a simple estimate of the reaction rate, the time (9) Duffy, S. J.; Van Loon, G. W. Environ. Sci. Technol. 1994, 28, 1950.
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Figure 4. Height (left) (Z range ) 10 nm) and phase imaging (right) MAC mode AFM images of tannic acid adsorbed on a mica substrate. All images are 500 nm square and were acquired at the following tip oscillation amplitudes and set points: (a) Ao ) 106 nm, rsp ) 0.90; (b) Ao ) 106 nm, rsp ) 0.30; (c) Ao ) 52 nm, rsp ) 0.90; (d) Ao ) 26 nm, rsp ) 0.90.
Figure 2. Height (left) (Z range ) 200 nm) and phase imaging (right) MAC mode AFM images of the coprecipitated aluminum oxide particles dispersed on a mica substrate. All images are 2 µm square and were acquired at the following tip oscillation amplitudes and set points: (a) Ao ) 116 nm, rsp ) 0.65; (b) Ao ) 58 nm, rsp ) 0.65; (c) Ao ) 29 nm, rsp ) 0.65.
Figure 3. Height (left) (Z range ) 200 nm) and phase imaging (right) MAC mode AFM images of the control aluminum oxide particles dispersed on a mica substrate. The images are 2 µm square and were acquired at a tip oscillation amplitude of 58 nm and set point 0.50.
required to recover 50% of the solid-phase aluminum, designated as t50, was determined. Increasing t50 values are indicative of decreasing reactivity of aluminum in the
solid phases. The t50 values obtained for solids used in the AFM experiments were in the order control < coprecipitated , postprecipitated. The t50 value obtained for the postprecipitated particles was 6.9 times greater than the value determined for the coprecipitated particles. High-resolution solid-state magic angle spinning (MAS) 27Al NMR was also performed on each of the particles. The chemical shift for the control particles was 5.18 ppm, for the coprecipitated particles was -5.32 ppm, and for the postprecipitated particles was -1.19 ppm. The position of all these peaks is consistent with the reported range of -10 to 20 ppm assigned to octahedral aluminum10 in aluminum oxides. The downfield shift observed in the latter two solids is indicative of the presence of phosphorus in the samples with the coprecipitated particles having a higher phosphorus concentration and hence a greater shift downfield. Finally, we determined the extent of tannic acid incorporation into the particles. Tannic acid has a strong adsorption band at 278 nm. The UV-visible absorption spectrum was obtained for the solution before addition both of sodium bicarbonate and of the filtrate after precipitation of the aluminum oxides. The adsorption spectrum of the filtrate indicated that essentially all of the tannic acid had been incorporated into the precipitate. Discussion Any analysis of phase imaging data must be considered with particular care, as a number of factors can affect the image contrast. The phase shift angle ∆φ can be shown to be a function of several different factors. Here, we follow (10) Duffy, S. J.; Van Loon, G. W. Can. J. Chem. 1995, 73, 1645.
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the derivation of Whangbo.4,11 Assuming the cantilever is at its resonance frequency in the free state, the phase shift angle is given by
∆φ ) Qσ/k
(1)
where Q and k are the quality factor12 and force constant of the cantilever, respectively. Both are constants in this case. The term σ is the overall force derivative experienced by the cantilever: that is, the sum of all the force derivatives acting on the cantilever
σ)
∑i
∂Fi ∂z
(2)
where z is the tip-sample separation. When the net force acting on the tip is attractive (i.e., van der Waals forces predominate) σ and the phase shift are negative, while if the net force is repulsive (indentation forces predominate), σ and the phase shift are positive. The problem, then, is to determine under what conditions each of these two effects will dominate the tip-sample interaction. At low values of Ao, the tip does not penetrate far into the surface, and thus little tip-sample repulsion is experienced: attractive van der Waals and capillary forces predominate, mainly arising from water on the surface of the sample. Practically, it has been shown for polymer surfaces that this region is important for about Ao < 40 nm and rsp > 0.8. It would appear that such behavior is similar for this surface. This is typified by parts a, e, and f of Figure 1, where the phase imaging data show little contrast, particularly in the case of Figure 1e where both requirements are met. Alternatively, at high values of Ao and moderate values of rsp, the tip will now penetrate far into the surface, and tip-sample repulsion will dominate the interaction. This is typified in Figure 1b, which was acquired at both high values of Ao and intermediate set point. Here we see the largest contrast in the phase imaging data of the coprecipitated particles. The first problem, of course, is to determine which objects, the raised or lowered ones in the height images of Figure 1, correspond to the particles and which to the substrate. By the arguments above, it should be apparent that at high values of the set point, the height mode should most closely approximate the actual sample topography. At lower set points soft samples in particular could be deformed considerably: this would lead to the tip penetrating far into the sample and the feedback loop could thus be “fooled” into showing the softer regions on the surface as a negative excursion in the height profile. Since the raised objects in parts a, c, and e of Figure 1 (high set points) all appear raised in the remaining images, this indicates that these are indeed the real particles. Since they appear as raised features in the all the height images, it would appear that the particle height is sufficiently large that any deformation effect is canceled out at lower set points. Having identified the postprecipitated particles in the height images, we turn our attention to the phase imaging data. We first note that the data demonstrate that what we are seeing is truly a single, incomplete layer of particles on the mica substrate. If we were seeing two or more layers of the same particles on top of one another we might expect there to be little contrast in the phase imaging data, as everything present would have the same (11) Magonov, S. N.; Elings, V.; Whangbo, M. H. Surf. Sci. 1997, 375, L385. (12) Chen, G. Y.; Warmack, R. J.; Thundat, T.; Allison, D. P.; Huang, A. Rev. Sci. Instrum. 1994, 65, 2532.
viscoelastic properties. We see instead that the particles show a negative excursion in the relevant phase imaging data consistent with a particle whose surface properties are quite different from the underlying mica substrate. It should also be noted that at very low values of rsp we should observe a contrast inversion in the phase imaging data. This is because at low rsp, the tip now penetrates far into the surface, increasing the tip contact radius, 〈a〉, and hence φ4. Contrast inversions have indeed been observed on polymer surfaces, but generally at lower rsp than we were able to image with the postprecipitated particles. We found that the feedback loop became unstable at low set points on the postprecipitated samples, which is presumably due to very strong tip-sample interaction as the tip penetrates the surface layer. This effect was not repeated in the case of the other samples. Figure 4 shows that in the case of the tannic acid adsorbed on mica, a constrast reversal does indeed occur between r ) 0.9 and r ) 0.3. In the case of the coprecipitated and control Al(OH)3 particles, we observe few changes in the phase imaging or height images as a function of Ao and rsp. In particular, at large to intermediate values of Ao and moderate set points, both the particle and substrate show similar phase shifts. This demonstrates that both particle and substrate are similar in their viscoelastic properties. This is consistent with the fact that both the aluminum oxide particles, and the aluminosilicate mica substrate have similar surface structural groups present. The Young’s moduli E of mica (biotite) and amorphous Al2O3, of 40 GPa13 and 280 GPa,14 respectively, are of similar orders of magnitude. Comparing these phase shift results for the control Al(OH)3 and coprecipitated particles to those for the postprecipitated particles clearly demonstrates that the latter are characterized significantly different viscoelastic properties. In addition, the tannic acid images in Figure 4 are characterized by large changes in contrast both in the phase and height data as a function of set point and tip osciallation amplitude, which are more reminiscent of the images seen in Figure 1 of the postprecipitated particles. This is good evidence that these differences may be accounted for by tannic acid coating layer on the postprecipitated particles. The tannic acid could be bound to the particle surface via an Al-O-C bond to form a tannate ligand or could simply be strongly physisorbed on the hydrous aluminum oxide particles. In any case, since tannic acid is a large organic molecule15 (molecular weight of 1701.23 g mol-1), it might be expected to form a fairly compliant organic coating. Other than their viscoelastic properties, these particles also clearly demonstrate a wide variation in morphology. First, of course, the coprecipitated and control particles are generally spherical in shape and vary widely in size. The postprecipitated particles also vary widely in their lateral dimensions but are remarkably uniform in height. A line profile across Figure 1a shows a particle height of about 2 nm. This height measurement is taken under conditions of high set point and tip oscillation amplitude which should give the closest approximation to the actual height of surface features. Nonetheless, we should emphasize that this probably underestimates the actual height of the features in the image. In addition, we have seen evidence of tip-sample interaction in the postpre(13) Touloukian, Y. S.; Ho, C. Y. In Physical Properties of Rocks and Minerals; McGraw-Hill: New York, 1981. (14) Cottrell, A. H. In The Mechanical Properties of Matter; John Wiley & Sons: New York, 1964. (15) The Aldrich Library of FT-IR Spectra, 1st ed.; Aldrich Chemical Company, Inc.: 1984; Vol. 1, p 544c.
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cipitated particle images (Figure 1). Given that the phase imaging data show that these particles are coated with a tannic acid layer, this observation at least is not surprising. Since the hydrophilic end (carboxylic acid group) of the tannic acid is presumably bound to the particle surface, this leaves a hydrophobic overlayer surrounding the particle. This would not be expected to bind strongly to the hydrophilic mica substrate, certainly less so than the oxide or hydroxyl-terminated coprecipitated and control particles which themselves appear to be much more strongly bound to the substrate. These observations suggest that we are probably not seeing individual postprecipitated particles in these images but rather aggregated particles of relatively uniform size that are loosely bound to the substrate and to one another. The postprecipitated particles are thus probably considerably smaller in size than either of the other two particle types. These observations are in agreement with other work on these chemical systems, all of which are concerned with hydrous aluminum oxide particles that are precipitated in the presence of organic acids such as tannic acid.16 In these situations, the solid has been shown (using titration methods) to have a large surface area and (using transmission electron microscopy) to consist of many small particles, sometimes aggregated together.12 The AFM observations are also consistent with the ferron agent complexation results. All else being equal, the AFM results would indicate that the reaction time t50 for the ferron complex with the postprecipitated particles should be less than that for the coprecipitated particles since the former have been shown to have a considerably larger surface area in the AFM images. Instead, the opposite behavior is observed, with the postprecipitated particles having much longer t50 times than either the coprecipitated or control particles. The presence of a tannic acid coating (16) Ng, K. F.; Kwong, K.; Huang, P. M. Geoderma 1981, 26, 179.
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on the postprecipitated solid would prevent the ferron reagent from directly contacting the aluminum atoms at the surface and lead to longer t50 times. Our observations may also provide one reason phosphate removal is limited in the case of solids where the aluminum has been prehydrolyzed in the presence of organic matter. In this situation, the coating on the surface of the inorganic precipitate could prevent entry of the phosphate to regions where it is able to form a specific bond with aluminum atoms. Conclusions Using the AFM, we have imaged a series of hydrous aluminum oxides particles derived from the hydrolysis of alum during a model wastewater treatment process. The use of phase imaging data has demonstrated that hydrous aluminum oxide particles formed in a postprecipitation process (that is, in which the particles precipitate in the absence of organic components and are subsequently aged in the presence of tannic acid and phosphates) have markedly different viscoelastic properties than particles coprecipitated in the presence of the organic and phosphate components, or indeed control Al(OH)3 particles. In the latter case, these particles show considerably less contrast with the mica substrate in phase imaging mode. These observations are attributed to the incorporation of the tannic acid as a coating layer on the postprecipitated particles. In addition, the hydrous aluminum oxide particles show considerable variations in morphology, depending on the preparation process. These results can explain the relative effectiveness of these particles in removing the phosphate components from wastewater. Acknowledgment. We thank the Natural Sciences and Engineering Research Council of Canada for financial support. LA980130+
