Environ. Sci. Technol. 1992, 26, 1775-1779
Diffusional Transport of Solutes through Clay: Use of Clay-Modified Electrodes Perlyasamy Subramanlant and Alanah Fltch"
Department of Chemistry, Loyola Unlverslty Chicago, Chicago. Illlnols 60626 Clay-modified electrodes offer a wide range of measurements to be made in studying diffusional transport of solutes through clay films. Diffusion of the void-filling species was found to be of the same order of magnitude as diffusion in aqueous environment. Diffusion of the solid surface associated species was 6 orders of magnitude less than that of the void-filling species. Thus,one expects that diffusional breaching of clay liners will occur by the void-filling soil solution species as opposed to the solid surface adsorbed species. The larger the clay barrier, then, the longer the control of diffusion by the solid surface via adsorptive processes. Diffusional processes within clay films can play an important role in transport of pollutants across clay liners below abandoned waste sites (1-6) or clay caps above polluted sediments (7). For example, C1- moved 0.75 cm in 5 years as a result of fluid flow and 83 cm as a result of diffusion below an abandoned waste site (1). Trichlorophenol (TCP) has a solution diffusion coefficient, Dsoh,of 7.24 X lO+ cm2/s which was only diminished to a value of 2 x 10-6 cm2/s in various capping materials. Since typical clay liners are 1 m thick, simple diffusive transport at a rate approaching solution values could breach a clay liner within 5 years (1). Methods of studying the diffusive transport of solutes utilize models ranging from large core volumes to clay films 10 cm thick. In this paper we demonstrate the utility of clay-modified electrodes, in which Pt electrodes are coated with films 0.3-1 pm thick dry (approximately 10-50 pm thick wet in 0.02 M NaCl), and the diffusive flux of a solute monitored by oxidation or reduction at the underlying electrode. Breakthrough curves are obtained in 1-40 min, and the information contained in these short-time breakthrough curves is related to the partition coefficient of the solute with the solid phase. Experimental Section Montmorillonite (10 pg, SWy-1 source) oven-dried clay-modified electrodes (CME) were prepared as previously described (8). An oven-dried (10 min, 100 "C) electrode was soaked for 5 min in Nz-purged electrolyte before transfer to N2-purged electrolyte containing the complex of interest. [ F ~ ( ~ P Y ) B I(91, S O[ ~F e ( ~ h e n ) ~ l S(9, O ~[ C r ( b p ~ )(C1O4I3 ~l (101,and [Fe(CN)2(b~~)21 UI), [ F e ( ~ h e n ) ~ ( C N(111, )~l [Fe(bp~)~(C-"l NO3 (111, and [ F e ( ~ h e n ) ~ ( C N ) ~(1 lN 1)o ~ were synthesized by literature procedures. NaC104, Na2S04,K3[Fe(CN),], and [ R U ( N H ~ ) ~were ] C ~used ~ as obtained from Aldrich. The pH of the electrolyte plus complex solution ranged from 6.1 to 7.1. The pH-dependent charge on the clay is predominately edge oriented. Given the similarity of these edge sites between montmorillonites and kaolinites, we expect that in this pH range (6-7) the edge sites are neutral (12-14). Present address: Institute of Agricultural Medicine and Occupational Health, Oakdale Campus, The University of Iowa, Oakdale, IA. 0013-936X/92/0926-1775$03.00/0
All cyclic voltammograms were obtained on a PAR 273 potentiostat with a Houston 2000 XY recorder. Scan rates were 50 mV/s vs SCE. The magnitudes of the peaks were computed from extrapolated baselines. The diffusion layer for this scan rate can be estimated from the effective diffusion coefficient (see below) and the time elapsed between the formal potential and the cm2/s (see beswitching potential, 10 s. For Deffof low), the diffusion layer created during the cyclic voltammetric sweep is 0.1 pm. Clay film lengths were estimated as follows. The dry film thickness of a 35-pg spin-coated electrode measured by SEM (15)was 7.5 pm. Film length has been shown to be directly proportional to the total weight of clay added (16,17), suggesting that the corresponding film thickness of a 10-pg spin-coated electrode would be 2.14 pm. For such a well-ordered film, there would be 2229 platelets of 9.6 A in dimension for the dry film. Using a moist bulk density of 1.77 g/cm3 for SWy-1 (18), the void spacing between platelets, d, is estimated to be 11.5 A: bd = (m + pAd)/(A(d
+ d3)
(1)
where bd is the moist bulk density of the clay, m is the mass of a unit cell of clay, p is the density of water, A is the area of a unit cell, and d'is the clay crystal width of the unit cell (9.6 A). The swollen film thickness would be -4.7 pm [2229 platelets X (9.6 + 11.5 A)]. The total area covered by the oven-dried f i was measured by lifting the film from the Pt surface with Scotch tape. The total average area (glass plus Pt) covered by the oven-dried f i cm2. The area of the underlying Pt elecwas 7.92 X cm2. Spin-coated, well-oriented films trode was 7.85 X generally cover 2-6 times the area of a film formed by oven drying the equivalent mass of clay, as performed in these experiments. Thus, we estimate the moist film thickness of the oven-dried 10-pg film to be on the order of 12 pm. This dimension is significantly larger than the depth of the film sampled in the cyclic voltammetric experiment. The concentration of strongly retained material within the films was estimated from the integrated area of the reduction peak at slow-scan cyclic voltammetry (5 mV/s). Partition coefficients were determined from 10 mg of clay/20 mL of electrolyte solutions, with variable amounta of complex added, accounting for electrolyte. The clay gels were equilibrated 48 h and centrifuged, and the supernatant was monitored by UV-visible spectroscopy for the dr-pr* lines on a Varian DMS-90 UV-visible spectrophotometer. The difference in absorbance was attributed to adsorbed complex. A plot of the adsorbed complex/g of clay vs the bulk solution concentration of free complex was obtained and the partition coefficient, K = (mol/kg of clay)/M, was measured from the straight fine portion of the plot. Computations were performed in a spread sheet. Results and Discussion Diffusive flux of a solute through a clay film depends on the porosity controlled by the void distance between platelets, d. Figure 1illustrates the path a solute would
0 1992 American Chemical Society
Environ. Sci. Technol., Vol. 26, No. 9, 1992
1775
Table I. Partition Coefficients Determined from Electrochemistry and Spectroscopy
complex [Ru("3)61C13 [Fe(b~y)~(CN)dN03 [Cr(bpy),l (c104)3 [Fe(bPY)3lSO1 [Fe(phen)31SO4 [F~(~PY)~(CN)~I K3Fe(CNh
K,(UV-vis) (L/kg)
D" (cm2/s)
not mead not stable 5.3 x 1039 3 x 103 29 x 103 not meas'
nae 9.8 x 10-11 7 x 10-129 (1.2 x 10-'*)h 5 x 10-12 1 x 10-12 nae
tbb(min)
3 0/6@ 3/2@ 40-60 60-80
max > 240 3
Kp (from t b ) (L/kg)
2.4 x 103 2.4 X 103/26 X lo3 32 x 103 40 x 103 ~ 1 8 x0 103
SC (g/L)
275 2.6 78 58
95% exchange), then the intercept is a measure of either Kn3/2/Kmor Kn/Km2l3. The intercept is related to the magnitude at which the trivalent species is preferred over the divalent species. The lower the value of the intercept, the stronger the preference of the clay for the trivalent species. For pure divalent exchange the slope is expected to be 29.5 mV/decade and for pure trivalent exchange 19.7 mV/decade. Slopes and intercepts for the strongly retained aromatic ring compounds are shown in Table 11, columns 2 and 3. Data for both the slopes and intercept indicate that the weakly, electrostatically,held Ru(NH3),3+appears to favor trivalent exchange. The complexes held by strong adsorbate-adsorbate, van der Waals force, appear to favor divalent exchange. Furthermore, there is excellent agreement with AE" shifting to larger values as the partition coefficients shift to larger values. Finally, the diffusion coefficient of the strongly held species can, in some cases, be directly sampled. Equation 5 indicates that a variation in the scan rate of the cyclic voltammogram can be used to obtain the diffusion coefficient of the strongly retained species if the film concentration of the complex is known. For the strongly held complexes, after the clay film is loaded, the clay-modified electrode is transferred to fresh electrolyte and a scan rate at 5 mV/s performed. The area under the peak at this low scan rate reflects the total number of electroactive species. When this value is combined with an estimated film volcm2),the concenume ( L X area = 12 krn X 7.85 X tration can be determined. Diffusion coefficients determined in this manner are shown in Table I, column 3. As expected, the stronger the interaction of the complex with the clay film, the lower the diffusion coefficient of the adsorbed species. Diffusion coefficients, compared to bulk solution, diminish by almost 6 orders of magnitude, the effect increasing as the partition coefficient increases. Summary There is excellent agreement between all of the various experiments. Three types of solutes can be identified. There are those which have no interaction with the solid surface and whose diffusion coefficient is determined primarily by the pore volume of the film. These solutes are primarily anionic (Cl-would be an example), and the representative electroactive probe A i Fe(CN):-. A second class of solutes have a very strong short-range interaction with other adsorbates at the clay surface. They exhibit a transient diffusion coefficient related to equilibrium with the clay surface resulting in diminished concentrations of the solute within the void volumes due to adsorbate-adsorbate interactions. After completion of
adsorbate-adsorbate interactions, these solutes behave as unretained materials and their diffusion through the film is determined primarily by the effective diffusion coefficient, which is dependent upon the total void volume of the film. Compounds of this type have substantial aromatic character. The larger the ring system, the stronger the interaction observed, as deduced from a comparison in breakthrough curves for Fe(phen),2+and Fe(bpy),2+. In this class of compounds, the lower the valence of the compound, the stronger the interaction at the clay surface. Adsorption is apparently preferential for the divalent complex, as might be expected from near-neighbor packing considerations at the solid surface. The stronger the adsorption of the complex, the larger the preference for the divalent species, as observed from the increasing magnitude of the potential shift. These results are consistent with spectroscopic observations of packing effects (25)and with observations concerning the preference of clays for the divalent form of Cr(bpy),,+ (26)and Fe(bpy),,+ (27) as suggested from redox anomalies of the intercalated complexes. Solubility in the aqueous phase apparently plays a strong role in the short-range interactions with adsorbates at the clay platelet. The strongest interaction obtained is for Fe(bpy)2(CN)2,which is the least soluble compound. The partition coefficient increases, the solid surface diffusion coefficient decreases, and the time to breakthrough increases as the aqueous solubility of the compound decreases. This implicates self-aggregation processes as the solute is driven from the aqueous phase into the incipient organic phase initiated by adsorption of cationic organic complexes. The behavior outlined is consistent with earlier results obtained for the quaternary ammonium class of compounds. A third class of solutes has a weak, long-range, electrostatic interaction with the clay platelet which enhances its concentration in the void or interlayer region of the film. This class of compounds maintains its affinity for the aqueous phase. This class of solutes will exhibit enhanced transport through the film due to an increase in the effective concentration driving the diffusional process. Ru(NH3),3+is representative of this class or marker ions. Clay-modified electrodes offer a wide range of measurements to be made in studying diffusional transport of solutes through clay films. Diffusion of the void-filling species was found to be of the same order of magnitude as diffusion in an aqueous environment. Diffusion of the solid surface associated species was 6 orders of magnitude less than that of the void-filling species. Thus one expects that diffusional breaching of clay liners will occur by the void-filling soil solution species as opposed to adsorbateadsorbate localized species. The larger the clay barrier, then, the longer the control of diffusion by the solid surface via adsorptive processes.
Registry No. [Ru(NH3),]C13,72954-52-0; [Fe(bpy)z(CN)z] NO3, 15225-35-1; [ C r ( b ~ y ) ~ ( C l O23539-86-8; ~)~, [ F e ( b ~ y ) ~ ] S142630~, 81-1; [Fe(phen),]S04, 14634-91-4; [Fe(bpy),(CN),], 14841-10-2; K3Fe(CN)3,13746-66-2; P t , 7440-06-4.
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Received for review January 13, 1992. Revised manuscript received April 28,1992. Accepted May 18,1992. This work was supported by NSF Grant CHEM 901 7273.
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