Langmuir 1991, 7, 451-456
451
Quantitative Characterization of Aqueous Suspensions Using Attenuated Total Reflection Fourier Transform Infrared Spectroscopy: Influence of Internal Reflection Element-Particle Interactions on Spectral Absorbance Values Lane D. Tickanen,. M. Isabel Tejedor-Tejedor, and Marc A. Anderson Water Chemistry Program, University of Wisconsin-Madison, Madison, Wisconsin 53706
660 North Park Street,
Received March 6, 1990. I n Final Form: July 13, 1990 In this paper, we analyze in a quantitative manner the absorption band intensity of the attenuated total reflection Fourier transform infrared (ATR-FTIR) spectra of colloidal particles suspended in aqueous solutions. Using a cylindrical internal reflection (CIR) cell with zinc selenide internal reflection elements (IRE'S) having different angles of incidence, we develop a methodology for determining the quantity of suspended material (goethite, a-FeOOH) that is "seen" by the technique under a variety of suspension conditions. The band intensities, the band ratios within a given spectrum, and thus the theoretical expressions for determining the concentration are shown to be dependent upon the surface properties of the suspended particles and the IRE. When the suspended particles and the IRE bear charge of like sign, a "semiinfinite thick film" model can describe the particle arrangement in the sampling range of the IRE, and the total goethite concentration can be determined in a manner similar to that for a solute in solution. When the goethite particles and the IRE are oppositely charged, quantitative interpretation of the spectra becomes more complex. At low goethite concentrations M), positively charged suspension particles become strongly concentrated near the negatively charged ZnSe surface and particles in the bulk have virtually no contribution to the absorption intensity. Hence, a "thin film" model may be used for quantification. As the goethite concentration is increased, quantification requires the development of more complex mathematical expressions. When the suspension particles are arranged in a manner that can be approximated by a homogeneous layer of intermediate thickness, absorbance band ratios will have values that range in a predictable way between the limiting thick film and thin film values. Quantitative characterization in this case requires the development of an intermediate layer model.
Introduction The quantitative determination of optical constants and the quantitative analysis of organic liquids and aqueous solutions using attenuated total reflection (ATR) Fourier transform infrared (FTIR)spectroscopy, and more recently cylindrical internal reflection (CIR) FTIR spectroscopy, has been dem~nstrated.'-~Furthermore, CIR-FTIR spectroscopy has been used to quantitatively determine the adsorption of various solute species from aqueous solutions onto the surface of a zinc selenide CIR internal reflection element (IRE).4 Up to the present time, however, the quantitative characterization of aqueous suspensions using ATR-FTIR has not been realized. In previous ~ o r k ,we ~ ?showed ~ how CIR-FTIR spectroscopy could be used to qualitatively characterize adsorption reactions and the nature of the water in the interfacial region in goethite (a-FeOOH) suspensions. However, we noted that further work would be required if the technique were to be useful for the quantitative analysis of such suspensions. Difficulties involved ex-
plaining infrared absorption intensities, which were not proportional to formal goethite concentrations in suspensions in many cases, and interpreting absorption band ratios for the various goethite absorption bands within a given spectrum, which suggested that goethite particles were not uniformly distributed within the sampling depth of the IRE for many suspension conditions. We have since noted that the values for these band ratios seem to follow reproducible trends with changes in important suspension variables. Such variables include pH, ionic strength, and the concentration of species adsorbed to the particles. Each of these variables may influence either the sign of the charge on suspended particles and on the IRE or the degree of aggregation of the suspended particles. A t pH values lower than the pH of the isoelectric point (IEP) of a given surface, the surface will, in the absence of adsorbed anions, be positively charged. Likewise, a t pH values higher than pH(IEP), the surface will be negatively charged. When pH = pH(IEP), the surface will have zero net charge. Since the pH(1EP) for the goethite used in this study is about 9.77and pH(1EP) for the ZnSe IRE is somewhat lower than 4, one can anticipate the electrostatic interactions between the suspended particles and the IRE for various ranges for pH values. Figure 1 provides an example of the variation in ATR spectra peaks that one might see for goethite suspensions under negative (pH = l l ) , neutral (pH = 9), and positive (pH = 5) surface charge conditions. In each case the IRE would be negatively charged. If the quantitative analysis of species adsorbed on suspended particles is to be possible, it is first necessary
* To whom correspondence should be addressed.
(1)Goplen, T. G.; Cameron, D. G.; Jones, R. N. Appl. Spectrosc. 1980, 34, 657.
( 2 ) Braue, E. H.; Panella, M. G. Appl. Spectrosc. 1987, 41, 105. (3)Sperline,R. P.;Muralidharan, S.;Freiser, H. Appl. Spectrosc. 1986,
40, 1019. (4) Sperline, R. P.; Muralidharan, S.; Freiser, H. Langmuir 1987, 3, 198. ( 5 ) Tejedor-Tejedor, M. I.; Anderson, M. A. Langmuir 1986, 2, 203.
(6)Zeltner, W. A.; Yost, E. C.; Machesky, M. L.; Tejedor-Tejedor, M. I.; Andereon, M. A. Characterization of Anion Binding on Goethite Using Titration Calorimetry and Cylindrical Internal Reflection-FourierTransform Infrared Spectroscopy. In Geochemical Processes at Mineral Surfaces; ACS Symposium Series 323;Davis, J. A., Hayes, K. F., Eds.; American Chemical Society: Washington, DC, 1986.
0743-7463/91/2407-0451$02.50/0
~~~~~~~
~
(7) Zeltner, W. A.; Anderson, M. A. Langmuir 1988,4,469.
0 1991 American ~
Chemical Societv
Tickanen et al.
452 Langmuir, Vol. 7, No. 3, 1991
In the strictest sense, eqs 1 and 2 apply only when the index of refraction of the study medium (n2) is constant for all r. Other expressions may be derived to describe situations where n2 is not c o n ~ t a n t . ~ J ~ It is useful to briefly review two concepts that were developedbyHarrick.l'J2 The first is the idea of "sampling depth" (d,), or the distance from the IRE where the amplitude of the evanescent field has decayed to 5 % of EO
0
0
m
The sampling depth d, actually is part of an exponential decay constant, related to the evanescent field by
r 3900
I
I
I
I
1
1740
2820
1
660
-'
WAVEN UMBER (CM )
Figure 1. Overlay of spectra for goethite showing variation of absorbance band ratios with film thickness: (a)transmission IR spectrum: (b) ATR spectrum of thin film; (c) ATR spectrum of film of intermediate thickness; (d) ATR spectrum of thick film.
to develop a rationale for quantitatively determining the concentration of suspended particles within the sampling depth of the IRE. In this paper, we propose a methodology for determining the distribution and concentration of suspended goethite particles within the sampling depth of a zinc selenide IRE under a variety of conditions. We also suggest guidelines for conducting such investigations for other systems and discuss the implications of our research regarding the quantitative analysis of adsorbed species and the characterization of water in the solidliquid interface.
Theoretical Approach and Rationale As we discussed earlier,5the spectral characteristics of ATR spectra of suspensions are related not only to the nature and concentration of species being investigated (solute molecules, ions, or suspended particles) but also to the index of refraction and to the distribution of these species within the sampling depth of the IRE. Since the "probe" in an ATR experiment is an exponentiallydecaying evanescent wave, species nearer the IRE experience a stronger electric field than those farther away and absorb radiation as follows:8 %lac exp(-yr)]' dr a = -Jt[E, coso 0 In eq 1,a is the absorbance, n21 is the ratio of the refractive index of the study medium to that of the IRE, a is the molar absorption coefficient for the species in question, and c is the concentration. 0 is the angle of incidence of the radiation (defined from the normal to the IRE surface), Eo is the amplitude of the electric field due to the infrared radiation at the IRE surface, t is the thickness of the region of interest, and r is the radial dimension outward from the surface of the IRE, extending into the study medium (for CIR systems). y is the decay constant for the evanescent field, given by 2r(sin2e - n212)1/2 (2) Y= A1
where A1 is the wavelength of the radiation inside the IRE. (8)Wendlandt, W. W.; Hecht, H. G. Reflectance Spectroscopy; Interscience: New York, 1966.
E = E, exp(-3r/dS) (4) Although strictly a definition, d, allows one to form a picture of the dimensions of the region being studied in the experiment. (Harrick and others have frequently used another parameter, the "penetration d e p t h (dJ, which is equal to d,/3. However,at this depth the evanescent field still has a value of 0.37E0,which can give rise to interactions that can contribute significantly to the total absorbance. Therefore, we will use d, exclusively in this paper.) The second concept is that of "effective thickness" (d,), which is a way of converting the interactions of the evanescent field with the study medium into a "thickness" which can be used in the Beer-Lambert law a = Nacd, (5) In eq 5, a is the absorbance, N is the number of reflections made by radiation within the portion of the IRE that contacts the sample, a is the molar absorption coefficient, and c is the concentration of absorbing species. In a transmission experiment, deis replaced by the actual path length. In ATR, de is primarily a function of E , n, and 0, and appropriate expressions for d,11J2 have been substituted into eq 5 to yield the various absorbance expressions discussed in the paragraphs below (eqs 6-10). Two limiting situations have been discussed by Harrick,11J2where appropriately selected limits on the film thickness t give rise to results that can simplify eq 1.When the thickness of the absorbing medium is much greater than d, (that is, where the evanescent field E has decayed to a value small enough to be negligible), eq 1 can be simplified to Nn,,ac E: coso 27 Substitution for y from eq 2 reveals the wavelength dependence of a in this situation a=--
(7) This is the so-called "semiinfinite thick film" case. Because the absorbance is wavelength-dependent, the profiles of spectra of thick films (and hence the ratios of absorbances for bands in a given spectrum) are distorted relative to corresponding transmission spectra, which are independent of wavelength. In the ideal thick film limiting case, (9) Hansen, W. N. J. Opt. SOC.Am. 1968,58, 380. (10) Yeh, Pochi Optical Waues in Layered Media; John Wiley and Sons: New York, 1988. (11) Harrick, N. J. Internal Reflection Spectroscopy;John Wiley and Sons: New York, 1967. (12) Harrick, N. J. Internal Reflection Spectroscopy-Reuiew and Supplement; Harrick Scientific Company: Ossining, NY, 1985.
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Langmuir, Vol. 7,No. 3, 1991 453
the amount of distortion in band ratios can be calculated by simply multiplying the band ratios measured from a transmission spectrum by the ratio of the wavelengths at which the absorbance maxima occur. Figure I d shows a CIR spectrum for a thick film of goethite. Substitution of the appropriate expression for the field EO for thick films and unpolarized radiation (since the CIR optics scramble the polarization of the radiation) yields a form from which absorbances can be directly calculated a=
NacX,n,, cos 0[(3
+ n,,’)
sin’ 0 - 2nZl2]
+
2?r(1- nz12)[(1 n,,’) sin20 - n,,’1(sin2 0 - n212)1/2 (8)
For the situation where the thickness of the film or layer approaches zero (Le. E = Eo for all t), eq 1 simplifies to
where d is the actual thickness of the thin layer. Insertion of the appropriate field expression for thin films and unpolarized radiation yields a=
equation can then be used to determine n2 and d for nonlimiting cases to which it applies. We begin by writing an absorbance band ratio expression using the equation for absorbance given earlier (eq 1) in both the numerator and the denominator and inserting the appropriate values of a and X for each peak. If one assumes that the EO field amplitude terms a t the frequencies for the two absorbance maxima will be similar and will divide out of the expression, we obtain
+ n3,’ + n3): sin’ 0 - 2n3,2] (1 - n3,’)[(1 + nSl2)sin’ 6 - n3:]
2Nacnzld cos 0[(2
(10)
This is the ”thin film” case, where the absorbance is not dependent upon the wavelength of probe radiation, and consequently the profiles of spectra obtained from thin films greatly resemble transmission spectra. For practical purposes, a film of thickness less than d,/60 can be considered as a thin film, because the field at the film boundary is still 0.95Eo. Figure l b shows a CIR spectrum of a thin film of goethite. Note the similarity to the transmission spectrum, Figure la. It is noteworthy to mention that the term “film” in both of these cases does not necessarily mean a cohesive solid phase but may imply a layer of any phase whose index of refraction is constant over a scale of homogeneity of the order of a wavelength of the probe radiation (several micrometers in the case of IR radiation). Thus, the semiinfinite layer or the thin film could be a solid, a liquid, a gas, or, as in the present case, a suspension of appropriately small particles in liquid. However, in the case of a thin film, the suspension particles must clearly have dimensions smaller than d. When the layer has a thickness that satisfies neither the thick film nor the thin film criteria or where the suspension is not homogeneous (i.e. the index of refraction changes as a function of distance from the IRE), one might expect the spectral peak ratios to be different than those obtained for the limiting cases of thick films or thin films, as is shown in Figure IC. In such nonlimiting cases, it is difficult to derive simple expressions for the absorbance because the expressions for the field at the IRE surface (Eo) and the effective thickness (de) are less tractable. Using an approach similar to that of Belali et al.,13 one can derive an expression for intermediate absorbance band ratios resulting from a single layer of uniform thickness and index of refraction. (In our case, the ratio involves two bands in the same spectrum and unpolarized radiation, whereas Belali’s ratio was for the same peak under conditions of parallel and perpendicular polarization).This (13) Belali, R.; Vigoureux,J.M.; Camelot, M. Spectrochim. Acta 1987, 43A, 1261.
(11) which can be solved numerically with the help of a computer for d and n21 for given values of 0, A, and a. By using IRE’s with different angles of incidence (e), one can vary the sampling depth (d,) and, in many systems, force the analysis into one of the limiting cases, or, in other systems, a t least use the spectra obtained for each 0 to determine the film thickness and simplify field expressions. In this work, we show that by using IRES that provide different angles of incidence, we can in many cases characterize the arrangement of suspended particles around the IRE and then determine the amount of suspended material that is detected by the technique. Our methodology is primarily applicable to suspensions containing one suspended phase which has at least two absorption bands (with known molar absorption coefficients) assigned to the bulk of the solid that lie within the working spectral range defined by the source, detector, beam splitter, IRE, and solvent. This probides a sort of internal standard that can be used for the purposes of quantification.
Experimental Section Materials and Sample Preparation. The goethite used in this study was found in previous work to consist of needle-shaped primary crystallites averaging 80 nm in length and 35 nm in width.6 Deuterium oxide was used rather than water so that we could separate the spectral bands due to the OH groups in the bulk of the goethitesolidfrom those due to interfacial OD groups, as we did in previous work.5 Suspensions were prepared by first mixing the desired amount of the dry goethite in deuterium oxide (99.8% DzO, Aldrich Chemical) and sonicating overnight to remove adsorbed water and deuterate the surface of the goethite. Samples were then centrifuged, fresh deuterium oxide was added, the solids were resuspended by sonication,and, when desired, the pD was adjusted by adding appropriatevolumes of 0.1 M DC1 or 0.1 M NaOD (prepared from Aldrich reagents). Values of pD were determined by using a pH meter and glass electrode (Orion) and correcting for the difference in redox potential for the reduction reactions of water and deuterium oxide at the electrode. CIR-FTIR Spectroscopy. The CIR infrared spectra of aqueous goethite suspensions were recorded interferometrically with a Nicolet 60 SX spectrophotometerequipped with an MCT broad band detector. Three different zinc selenide IRE’s with average angles of incidence of 39O, 4 8 O , and 74’ were used in order to study as wide a range of sampling depths as would be feasible. In each case 0 and the number of reflections (N)for each IRE were precisely determined by using the method of Sperline et a1.,3 with the values of 0 and N being determined by an iterative process that was truncated after successive values of 0 differed by less than 0.1 %. Reagent grade benzene was used for calibrationpurposes,with the 1960-cm-labsorption maximum serving as a standard. The value for the index of refraction of benzene at this frequency was estimated from a published dispersion curve.14 Although the index of refraction of every substance undergoes (14) Bertie, J. E.; Eysel, H. H. Appl. Spectrosc. 1986, 39, 392.
Tickanen e t al.
454 Langmuir, Vol. 7, No. 3, 1991 Table I. Calibration Data for CIR-FTIR Cells Using IRES of Differing Angle of Incidence. 8, deg N wavenumber, cm-l d,, pm (d,/60), pm 38.9 f 1.0 5.42 f 0.01 47.9 f 1.5 3.97
* 0.13
73.5 f 1.0 2.87 f 0.13 0
900 3150
7.47 2.14
0.144 0.036
900 3150
4.43 1.27
0.074 0.021
900 3150
2.80 0.80
0.047 0.013
Sampling depths and thin film criteria (d,/60) are also listed.
abrupt changes in the region of an absorption maximum, the change for the 1960-cm-1transition is only about 0.01 unit, which has a negligible effect on the calculated values of 8 and N . Table I lists calibration data for angle of incidence (0) and the number of internal reflections (N) for each of the IREs used in this study. Calculated values for the sampling depth (d,) and the thickness of the limiting thin film (d,/60) are also listed. Since it is inconvenient to calibrate the IREs prior to the analysis of each sample, we established the reproducibility of each value of B and N by running several analyses of benzene using each IRE. In order to establish a worst-case range of error for each IRE, we included in the studies such trials as misaligning the IRE, reversing the IRE in the optical path following alignment, and moving the IRE off center with respect to the sample cell. The standard deviations in B and N were both less than 3 % , which are higher than those noted by Sperline, but are more than adequate for the purposes of this study and indicate that it is not necessary to calibrate each IRE before every experiment. Single beam spectra of the empty cell, suspension, or supernatant samples were the result of 1000 coadded interferograms (scans) when using the 39' and 48' IRE's, which provided an adequate signal-to-noise ratio for the purposes of this work. For the 74' IRE, this number was increased to 4000-10000 interferograms because the sampling depths and number of reflections for this IRE are relatively small, and signal-to-noise ratios were inadequate for quantitative purposes when fewer scans were averaged. Techniques a n d Methods. All background, supernatant, and suspension samples were scanned by using the same methods as were described earlier,5 in order to minimize errors caused by changing the alignment of the CIR cell. Supernatant samples were prepared for analysis by centrifuging an appropriate volume of suspension for 30 min a t 8000 rpm and the filtering the resulting liquid portion through a 0.05-pm polycarbonate membrane filter (Nucleopore no. 110603). Each supernatant and suspension pair was analyzed by using each of the three IRE's on the same day, in order to minimize errors resulting from temporal changes in the suspensions. Studies of the effect of varying goethite concentration a t fixed pD were performed as follows: First, each suspension sample was split into two portions, as described earlier.5 One portion was centrifuged and filtered to remove solids and was initially used as the supernatant. Then, a n accurately measured sample of stock suspension was added to the supernatant and mixed. After this sample was scanned, additional suspension was added to produce a more concentrated sample. This process was repeated several times to produce a series of formal suspension concentrations ranging from about 10-3 mol/L to about 0.4 mol/ L. In all cases, goethite peak ratios and the absorbances from which they were obtained were based upon simple peak height measurements. Baselines were determined by varying the subtraction ratios for the supernatant over a range of 0.5-2.0 and noting those portions of the baseline in the region of the peaks that did not change position significantly. The values of absorption coefficients used for the goethite peaks at 900 and 3150 cm-l were determined to be 10200 and 6800 m-1.M-1 in a transmission experiment using KBr pellets. The p H values of the isoelectric points (IEPs) for goethite and for ZnSe were determined by mobility studies. The electrophoretic mobility of mechanically ground ZnSe was measured at p H values between 4 and 11 with a Penkem System 3000 electrokinetics analyzer, in a constant ionic strength of 0.01 M KC1.
Table 11. Variation of Absorbance Band Ratios with Film Thickness for a Variety of Suspension Conditions. samole description 8. deg ratio d, Mm n concn = 0.7 F 39 5.4 1.4 pD = 11.5 74 5.4 >d, concn = 0.4 F pD = 11.3
39 48 74
6.1 5.4 5.6
>de
1.4
concn = 0.4 F pD = 7.5
39 48 74
2.05 2.54 3.00
0.39 f 0.02
1.23 f 0.09
concn = 0.4 F pD = 6.8
39 48 74
2.75 2.89 3.51
0.45b
concn = 0.006 F pD = 5.6
39
1.49
ds), which is the product of the band ratio value for the thin film multiplied by the ratio of the wavelengths at which the absorbance maxima occur. Certain trends in the data are readily discerned. The band ratios for suspensions with pD >> pD(1EP) for goethite are all very near the thick film limit, which suggests a rather uniform distribution of goethite particles within the entire sampling depth of the IRE. The band ratios for suspensions with pD > 9.7), the IRE and the goethite particles repel one another. Individual goethite particles repel one another as well. This leads to a rather uniform distribution of particles around the IRE. For conditions where the IRE and goethite particles are oppositely charged (4.0 C pD C 9.7), the IRE should attract particles that lie within the range of the attractive electrostatic force, resulting in the goethite particles being concentrated in the region next to the surface of the IRE.
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Langmuir, Vol. 7, No. 3, 1991 455
Table 111. Calculated Values of Goethite Concentration and Refractive Index for Systems Following the Thick Film Model.
*
sample description concn = 0.4 F pD = 11.3
calcd concn, M 0.46 f 0.05
concn = 0.7F pD = 11.5 concn = 0.7 F pD = 5.3b
nz 1.4
pD = 5.6 1.5
2.1
0.85 f 0.09
1.4
concn = 0.015 F pD = 5.6
2.0
1.4
concn = 0.018 F pD = 5.6
1.5
0.9?ib
concn = 0.033 F pD = 5.6
1.5
2.6
Formal concentrations are included for the sake of comparison. Based on two data sets. Third set did not represent thick film.
The extent of concentration enhancement should be dependent upon the relative charge density on the surfaces of the IRE and the goethite particles (which are both functions of pD), the surface area of the IRE, which can provide an upper bound on the packing of particles around the IRE, and the particle concentration. For systems where pD is closer to pD(1EP) for goethite, electrostatic forces should play a lesser role in influencing the goethite particle distribution near the IRE surface. As the data in Table I1 show, the absorbance band ratios are relatively constant for pD = 7.5 and pD = 6.8 and have intermediate values that fall between the thick and thin film cases. This might suggest that nonelectrostatic forces (such as van der Waal’s forces) are attracting and holding alayer of relatively uncharged particles a t the IRE surface. (In these cases, we observed the vast majority of the particles in the suspension undergoing flocculation and precipitation during the time period of the spectroscopic analysis.) Under these conditions, one might expect the thickness of such a layer to be equal to the average aggregate size prior to flocculation. Calculated Concentrations. Tables I11 and IV list calculated goethite concentrations for those situations that lend themselves to such calculations. Table I11 lists goethite concentrations for the suspensions in which the goethite and the IRE were both negatively charged. Under circumstances where the band ratios corresponded to thick film situations, the followingexpression, which results from a straightforward rearrangement of eq 8, was used to compute concentrations: C =
2 7 ~ 4 1 -n212)[(1+ nz12)sin2 8 - n2121(sin2e - n212)1/2
NaXlnz1cos e[ (3 + nz12)sin2 0 -
(12) Since the index of refraction of the suspension a t absorbance maxima was not known, eq 1 2 was solved simultaneously for n21 and c by using the data for the various IRE’S and choosing the value of n21 that gave the least standard deviation in c. In all cases, concentrations computed in this manner were close to the formal concentration of goethite in the suspensions. Deviations may result from accumulated errors in calculating concentrations from measured absorbance maxima and determined values of and a, as well as from changes occurring in the suspension during the analysis. Table IV lists calculated goethite concentrations for suspensions where the goethite and the IRE were oppositely charged, under circumstances where the thin film concentration expression applies, i.e., low concentrations where pD 1M), the thick filmmodel may again apply, and quantitative analysis of the concentration of suspended material may be performed. When neither the thin film model nor the thick film model applies, quantitative characterization is more difficult. Even in situations where the functional form of the particle distribution around the IRE might be well defined (e.g. exponentially decaying with distance from the IRE), the effect of the particle distribution on the average refractive index of the suspension and, hence, the evanescent electric fields in the probe radiation are not easily derived. Quantification in such cases will require the development of more complex computer programs. For systems such as the one studied here, the functional form of the particle-IRE interactions may not be clearly defined under many conditions of particle concentration, pD, and ionic strength. Additional work would be necessary to determine the effects of interparticle interactions, aggregation, and precipitation of aggregates on the functional forms of models for particle distributions. In suspensions where the particles are only slightly charged, it appears that a single layer of particles may be attached to the IRE by weakly electrostatic or nonelectrostatic forces. In suchsystems, it is possible to determine the average thickness of this layer, and quantitative characterizations of the concentration in the layer will
Tickanen et al. require determinations of the electric field distribution in the layer.'3 As we discussed in the introduction to this paper, it is necessary to know the concentration of particles that are observed by using ATR-FTIR before one can determine the surface concentrations of adsorbed species. In developing a methodology for determining concentrations of suspended particles, we have concluded that additional work will be necessary if the analysis of surface species using ATR-FTIR is to be possible. In the future, studies should be conducted to determine whether IRE/particle interactions might affect the surface concentrations of adsorbed species. For example, one might speculate that electrostatic interactions between the particles and the IRE may induce adsorption or desorption of surface species, especially in the thin film cases. Preliminary results seem to indicate that such interactions do not noticeably influence the adsorption of various phosphate species on goethite. Additional work will also be required if the quantitative determination of solids concentrations in nonlimiting cases can be performed. Acknowledgment. This work was funded by a contract from the Ecological Research Division, Office of Health and Environmental Research, U.S.Department of Energy (DE-FG02-87ER60508). We gratefully acknowledge all support received.
