[advances in chemistry] aquatic humic substances volume 219 (influence on fate and treatment of...

14 Sorption of Benzidine, Toluidine, and Azobenzene on Colloidal Organic Matter

J. C. Means

Institute for Environmental Studies, Louisiana State University, Baton Rouge, LA 70803

R. D. Wijayaratne

Chesapeake Biological Laboratory, University of Maryland, Solomons, MD 20688-0038

The sorptive behavior of natural estuarine colloids was investigated with benzidine, toluidine, and azobenzene. In general, the curvilinear equilibrium sorption isotherms were well represented by the Freundlich equation, with 1/n values of 0.72-1.00. Freundlich sorption constants for each aromatic amine, normalized to the organic carbon content of the colloids (Koc), were 3420, 2010, and 1390 at pH 7.9. Sorption was largely controlled by the aqueous-phase pH, with sorption constants increasing as pH decreased to 5. We attempted to establish to what extent benzidine, its derivatives, and azobenzene as related to their molecular structure and thermodynamic properties influence the sorptive process. Interpretation of the sorption data allowed certain mechanistic hypotheses to be formulated. An experimental value for o-toluidine water solubility of 75 ppm at pH 7.9 was determined.

/ X R O M A T I C AMINES ARE IMPORTANT IN MANY AREAS of industry and re

search. They are classically important in dye chemistry (I), and the physiological activity of these compounds makes them of interest in the biomedical field. Some of the member compounds are on the U.S . Envi-

0065-2393/89/0219-0209$06.00/0 © 1989 American Chemical Society

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In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

210 AQUATIC HUMIC SUBSTANCES

ronmental Protection Agency's (EPA) list of toxic substances; 8 of the 14 chemicals controlled by the second emergency standard issued by the Occupational Safety and Health Administration (OSHA) are aromatic amines (2). Results of limited studies on human health and environmental effects have raised concerns regarding their potential for toxicity, mutagenicity, and carcinogenicity (3).

Because organic contaminants eventually find their way into water, largely as a result of industrial discharge, some attention has been focused on the fate of industrial amines in wastewaters and in the environment as a whole. Malaney et al. (4) and Baird et al. (5) have investigated the removal and fete of several aromatic amines in activated sludge reactors. They concluded, on the basis of oxygen-uptake data, that these carcinogens were toxic and probably refractory to bacterial degradation at 500-mg/L doses. In addition, they stated that many of the monoaromatic substances could be metabolized to some extent by acclimated systems. It has also been reported (6) that a strong correlation exists between the mutagenic activity of sediment extracts from the Buffalo River (New York) and the proximity of the sampling sites to a dye-manufacturing plant.

Bond-stability calculations and model reactions of aromatic amines with monomelic constituents of humic substances suggest that covalent binding of the amine residue may occur by at least two distinct mechanisms, in a hydrolyzable (probably anil and anilinoquinone) and in a nonhydrolyzable (probably heterocyclic rings and ether bonds) manner (7). More recently, Parris (8) reported the results of binding experiments with aromatic amines and compounds that serve as models of humate functional groups (e.g., carbonyls and quinones). He postulated that primary ring-substituted anilines bind covalently to soil organic matter (e.g., humâtes) via carbonyl and quinone moitiés. The proposed mechanism of binding involves two phases. A reversible rapid equilibrium is initially established with the formation of an imine linkage with the humate carbonyls. Subsequently, a slow reaction involves 1,4-addition to a quinone ring, followed by tautomerization and oxidation to give an amino-substituted quinone.

Although the sorption behavior of aniline in soils is known to a limited extent, the cycling of the broader class of compounds in aquatic environments is very poorly understood. Because aromatic amines can be introduced into waters via industrial discharge and surface run-off from land, assessing the fate of these compounds in aquatic environments is important. We recently investigated the sorptive properties of natural colloids with polynuclear aromatic hydrocarbons (PAHs) and the herbicides linuron and atrazine. Col loidal organic material was found to be a factor of 10 times better as a sorptive substrate for these compounds than soil or sediment organic matter (9-11). These results indicate that natural colloids are potentially important substrates that can significantly influence the movement and persistence of organic contaminants in aquatic environments.

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14. MEANS & WIJAYARATNE Sorption on Colloidal Matter 211

We have investigated the sorptive behavior of estuarine colloidal organic matter with azobenzene, o-toluidine, and benzidine. The dependence of sorption on aqueous-phase p H wil l be discussed. We attempted to establish to what extent the molecular structure and thermodynamic properties of benzidine and its derivatives influence the sorptive process. Interpretation of the sorption isotherm data allows formulation of certain mechanistic hypotheses that need to be investigated further. The water solubility of o-toluidine was also determined experimentally.

Experimental Methods

Materials. Benzidine was purchased from Columbia Organic Chemicals Co. (Columbia, SC); azobenzene and o-toluidine were obtained from Eastman (Rochester, NY). All chemicals were 99 + % pure. These amines were further purified by re-crystallization from a methanolic solution. The minimum purity was 99.5%, confirmed by high-performance liquid chromatographic (HPLC) and mass spectral analyses.

Distilled-in-glass-grade acetonitrile (Burdick and Jackson, Muskegon, MI), monobasic potassium phosphate (Fisher Scientific, Fairlawn, NJ), and glass-distilled nanopure water were used in preparing buffers.

Apparatus. All separations were performed on an HPLC system equipped with an M-45 solvent delivery pump (Waters Associates), a Rheodyne model 7125 rotary valve injector with a 20-μί sample loop, a fixed-wavelength (254-nm) model 440 UV absorbance detector (Waters Associates), and a Perldn-Elmer 650-10S fluorescence spectrophotometer having a 10-^L flow cell. Reverse-phase HPLC was performed under ambient conditions on a radial compression module (Waters Associates) with an Aio cartridge (Ci8 packing, 10 cm X 5 mm i.d.). Chromatograms were recorded on a Hewlett Packard model 3390A electronic integrator.

Sample Collection and Sorption Methods. Ten 20-L natural-water samples were collected in the Chesapeake Bay estuary from the mouth of the Patuxent River in Solomons, Maryland. These samples were collected at 0.5 m below the surface with acid-washed, disalled-water-rinsed Nalgene containers. Samples were filtered through 0.45-μπι filters (Millipore) to remove suspended particulate matter. The filtrates containing the dissolved organic carbon and colloidal carbon fractions were then subjected to ultrafiltration with an Amicon HiP 5 hollow-fiber filtration system, having a nominal molecular weight cutoff of5000. Thus, the original 200 L of estuarine water was divided into two fractions in this step, an ultrafiltrate (approximately 195 L) that contained the truly dissolved organic carbon and a colloidal fraction (approximately 5 L) that was enriched by a factor of 50. The colloidal fraction is never dried or concentrated on an ultrafilter membrane in this procedure, but remains in an enriched suspension.

Aliquots (1 L) of the ultrafiltrate were spiked with known amounts of the amines (below the solubility limit of the test compound), and 50-70 mL of the enriched colloid fraction was then added back to each ultrafiltrate solution contained in 2-L glass Eyrlenmeyer flasks. Triplicate spiked samples for each amine concentration were equilibrated in a table shaker for 18 h at 20 °C in the dark. Kinetic experiments indicated that this equilibration time was sufficient to reach >95% of true equilibrium. After equilibration, each sample (1.05 L) was passed through the hollow-fiber filtra-

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Table I. Chromatographic Conditions for the Separation of Aromatic Amines Condition Benzidine o-Toluidine Azobenzene Mobile-phase C H 3 = N : K H 2 P 0 4 35:65 35:65 35:65 Flow rate (mL/min) 2 2 2 Retention time (min) 4.27 7.73 8.51 UV (λ) (nm) — — 254 Fluorescence, λ β „ X e m (nm) 340, 480 280, 480 — "All separations were performed on a radial compression module (Waters Associates) with an A 1 0 cartridge (C w packing, 10 cm x 5 mm i.d.).

tion system again to yield an ultrafiltrate fraction (~1 L) that contained dissolved organic carbon and truly dissolved amine, and a colloid fraction (—50 mL) that contained the colloidal-bound amine and dissolved amine. This step took approximately 20 min, a period of time shown in control experiments to be short enough to prevent any significant redistribution of the solutes. The hollow fibers were rinsed with 20 mL of distilled water to ensure complete recovery of die colloidal fraction from the device.

Mass balances for all compounds in all experiments were greater than 97%, which is within the range of analytical error for the amine quantification. We then determined the amine concentration contained in both fractions by HPLC on a reverse-phase C l 8 ^Bondapak) column. The amount sorbed to the colloids was determined by determining the excess amount of amine contained in the colloid fraction after correcting for the amount in solution. Each fraction was analyzed direcdy by HPLC in quadruplicate. Each sorption isotherm was determined in duplicate independent experiments.

Separations were accomplished by using an isocratic mixture of acetonitrile and water at a flow rate of 2 mL/min. The column effluent was passed through an UV detector set at a wavelength of254 nm or a fluorescence spectrophotometer equipped with a 1 0 ^ L flow cell. The aromatic amines were identified on the basis of retention time by comparison with standards. These compounds were well resolved on the column; thus identification was unambiguous. The chromatographic conditions used for the separation of aromatic amines are fisted in Table I. A mass balance for each aromatic amine was performed to verify that no significant losses or degradation of material occurred during the sorption experiments. The experiments determining the effects of pH on sorption were performed by adjusting the pH of the solution with either NaOH or HC1. The resultant sorption constant was measured after equilibration by the techniques already discussed in this section.

The organic carbon content of the bulk water, ultrafiltrate, and enriched colloidal fractions was measured on an analyzer (Oceanography International) by using the persulfate method of Menzel and Vaccaro (12). Results on blanks indicated that less than 2% of the carbon in the samples originated in the filters. The water solubility of o-toluidine was determined by using the procedures outlined by Means et al. (13).

Results and Discussion

The sorption of aromatic amines by estuarine colloids produced isotherms that conformed to the Freundlich equation:

Cs — K^pw

η (D

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where Cs is the equilibrium concentration of the compound on the sorbent, Cw is the equilibrium concentration of compound in solution, and Kd and η are constants related to the sorptive capacity of the sorbent. Because the sorption data were normalized to the organic carbon content of the colloids, the partition coefficient of the Freundlich expression corresponds to the sorption constant ( K J , which is derived from the relation:

^ =

100

(2) percent organic carbon

Each sorption isotherm was plotted with the averages of duplicate experiments determined at four or five concentration points run in triplicate. These concentrations were considerably below the aqueous solubility limit of the test compounds.

The thermodynamic equilibrium constant for a sorption reaction can be defined as

Ko = - ^ ~ = ^ (3)

where a refers to the activity, *γ to the activity coefficient, and C to the concentration of the organic compound ^ g / m L ) . The subscripts s and e denote the compound-colloid complex and equilibrium solution, respectively. Because the colloid may be considered as a solid phase in this system, the activity (ac) of this component of the system is by convention equal to unity. The concentration Cs fag sorbed/g of C) was normalized to the organic carbon content of natural colloids. In ideal solutions, 7 = aim. As the solute becomes more dilute, the activity ae approaches the modality m. Therefore, by the infinite dilution convention, as the concentration (molal units) of the solute approaches 0, 7 approaches 1. Also, under these conditions the value of 1/n approaches unity. Thus equation 3 may be rewritten as

JC = — l i m £ W ae cs->o i>e

Because concentrations of organic pollutants in environmental samples are often low, equation 4 is fundamentally significant in interpreting sorption results. The standard free energy (AG 0) of the sorption reaction at equilibrium and constant Γ is related to the thermodynamic equilibrium constant by the relationship:

(5)

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Values o f - A G were obtained by extrapolating to Cs = 0 from plotted values of -RT In CJCe vs. C s . This extrapolation also allows calculation of the thermodynamic equilibrium constant.

The structures of benzidine, o-toluidine, and azobenzene are shown in Chart I. The data obtained from the sorption experiments with aromatic amines are presented in Table II. These include amine solubilities, salinities, organic carbon contents of the enriched colloidal fraction, K ^ , and àGT° values for each compound-colloid system. The water-solubility number for o-toluidine is the first experimental value to be reported. The amount of amine sorbed on colloids is reported on an organic carbon basis because the amount of carbon is the most precise measure available for monitoring the amount of colloid present in both bulk water and enriched colloidal fraction. In addition, organic matter is known to be a major source of sorptive capacity of a variety of environmental substrates for hydrophobic compounds.

The equilibrium sorption isotherms of benzidine, o-toluidine, and azobenzene at 20 °C are shown in Figures 1, 2, and 3, respectively. The sorption isotherm of o-toluidine was linear, although benzidine and azobenzene yielded curvilinear isotherms over the concentration ranges tested. The sorption data were well represented by the Freundlich equation (r 2 > 0.96),

Azobenzene

Chart I. Structures of aromatic amines.

Table II. Sorption Parameters for Aromatic Amines on Estuarine Colloids

Chemical Solubility

(ppm) Salinity

(foe)

TOC, Colloidal Fraction (mg/L) K 0 / r 2 1/n

Standard Free Energy (kcal/mot)

Benzidine* 400 19.2 31.6 3825 0.981 0.74 -4.86 o-Toluidinec 75 19.3 34.5 2014 0.994 0.99 -4.83 Azobenzene*1 300 19.4 38.3 1394 0.962 0.72 -4.72 "Ambient pH = 7.9. bpKH = 4.5, pKfi2 = 3.3. cpKfll = 4.8, pKH = 3.7. dpKa = -2.48.

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c ο .Q m ο

60000

£ 50000 D) ι_ Φ α D)

zr Ό Φ η V. Ο

CO

c D Ο Ε <

40000

30000

0 10 15 20 25 Equilibrium Solution Concentration (Mg / ml)

Figure 1, Equilibrium sorption isotherm at 20 °C of benzidine on estuarine colloids at pH 4.9.

where the exponential constant (1/n) had values of 0.74, 0.99, and 0.72 for benzidine, o-toluidine, and azobenzene, respectively. The curvilinear isotherm patterns of benzidine and azobenzene are in agreement with the class " L " isotherms defined by Giles et al. (14). They suggested that isotherm shape provides an indication of the sorption mechanism in operation for a given solute-solvent sorbent system. The L-type isotherm, the most common type, represents a relatively high affinity between the solute and the sorbent in the initial stages of the isotherm. As sorption sites are filled, the solute molecules have a decreasing probability of colliding with vacant sites. The curvilinear response may also be attributed to multiple mechanisms of sorption, as has been demonstrated with benzidine and a soil-sediment system (15).

The sorption experiments of aromatic amines with colloids under estuarine conditions (pH = 7.9, salinity ~19%o) yielded Kœ values of 3430, 2010, and 1390 for benzidine, o-toluidine, and azobenzene, respectively. These values, somewhat higher than corresponding Kœ data for soi l -sediment systems, suggest that aromatic amines are more strongly bound to natural estuarine colloids. If the Koc values reported here are converted to a mass basis by correcting for the fraction of carbon in the colloids (50% on average) (15), then they become Kp values of 1500, 1000, and 700, respectively. Zierath reported Kp values for benzidine on soils in the range of

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50000

Equilibrium Solution Concentration (14g / ml)

Figure 2. Equilibrium sorption isotherm at 20 °C of o-toluidine on estuarine colloids at pH 7.9.

50-3940 for soil with organic carbon contents of 0.15-2.4%. More recently, Johnson and Means (16) have reported Kp values in the range of 168-1230 for estuarine sediments having carbon contents of 0.9-3.0%. Estuarine colloids have been characterized as being composed of a carbohy-drate-proteinaceous matrix in association with crystalline clay minerals and trace metals (17, 18).

In previous studies we demonstrated that estuarine colloids have a high affinity for PAHs (9) and for the herbicides atrazine and linuron (10). We also found that colloidal organic matter is on the order of 10 times better as sorptive substrate than soil or sediment organic matter (11). These apparent differences in sorption affinities exhibited by natural colloids and soils-sediments for hydrophobic organic pollutants may be related to differences in positional availability of sites for hydrophobic bonding and charge density separations on the two sorbents. Soil-sediment organic matter is itself sorbed to a highly porous and irregular inorganic matrix, often occupying or Riling micropores in the structure. If it can be assumed that the soil organic matter is not a monomolecular film, the K ( X , values calculated on a total-carbon basis may be low by 1 or 2 orders of magnitude, because all the organic carbon is not available as a sorptive surface and many charged

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sites on the organic matter may already be involved in ion pairs with the inorganic matrix. The estuarine colloids used in the present and previously reported studies (9-11) have a very low inorganic content (<5% ash) and are presumably accessible for sorption from all sides in colloidal suspension. Thus, we hypothesize that the differences observed in sorptive capacities of sediment and colloidal organic matter may be explained in part by these factors.

Wijayaratne and Means (9) demonstrated, on the basis of a study of sorption of PAHs by natural estuarine colloids, that the solubility of a hydrophobic compound was significantly correlated with the colloid sorption constant (equation 6).

log Κ,, = -0 .693 log Sfog/mL) + 4.851 (r 2 = 0.985) (6)

The calculated Koc values obtained with equation 6 for benzidine, o-toluidine, and azobenzene are 1116, 3561, and 1362, respectively. The calculated K o c value of benzidine obtained with equation 6 is significantly lower than the observed value of 3430. This discrepancy suggests that benzidine sorption is enhanced above that expected on the basis of hydrophobic

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218 AQUATO HUMIC SUBSTANCES

bonding alone, but the enhancement is nevertheless associated with the organic carbon content of the substrate. The mechanisms for enhanced sorption must involve one or more specific interactions of the amine functional group (e.g., ion pair) with components of either the substrate organic matter or associated clay minerals.

Benzidine can exist in solution as both a neutral species and an ionic (cationic) species by protonation of the amino groups. Zierath et al. (15) investigated the sorption behavior of benzidine by soils and sediments and found that sorption was highly correlated with p H because the ratio of neutral to ionized benzidine molecules is controlled by the p H of the aqueous phase. Although both species are subject to sorption, the cationic form may be sorbed to a greater extent.

Presumably, the curvilinear isotherms observed in this study are caused by multiple sorption mechanisms that are dependent on the protonation of nitrogen atoms of the diamino-biphenyl compounds in solution. The extreme pH-dependence of the colloid Koc values for benzidine and o-toluidine, particularly as p H values approach the p K e values of the compounds (Figures 4 and 5), further substantiates this hypothesis. The Kœ values of benzidine

6000

Figure 4. Effects of changes in pH on the Κ α values of benzidine. Each point represents a full isotherm determined at the pH specified. Regression coeffi

cients were all greater than 0.95.

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6 0 0 0

Figure 5. Effects of changes in pH on the Κ<* values of o-toluidine. Each point represents a full isotherm determined at the pH specified. Regression coeffi


showed increases of up to 2000 units when the ambient p H was decreased from 7.9 to 5.0.

The mechanism for enhanced sorption of aromatic amines must involve specific interactions of the amine functional group with components such as carbonyl and carboxyl groups of the colloidal organic matter or associated clay minerals in possible modes leading to adduct formation. Parris (8) has shown that amines may undergo both reversible and irreversible reactions with humâtes and compounds that serve as models of humate functional groups (e.g., carbonyls and quinones) to yield a variety of products. Benzidine and other aromatic amines are known to react with components of clay minerals (15, 19, 20), especially iron. Reactions of these types may account in part for the observed increase in sorption of benzidine with estuarine colloids, particularly at low p H levels.

In contrast to the results from benzidine sorption, the Km value of3561 for o-toluidine predicted by equation 6 is higher than the observed value of 2010. From thermodynamic considerations, because the âGT° values of ben-

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zidine and o-toluidine are similar, the two compounds would be expected to show similar sorptive behavior. Furthermore, the solubility of o-toluidine (75 ppm) is considerably lower than that of benzidine (400 ppm). This solubility difference indicates that the escaping tendency, which is measured quantitatively by the chemical potential in solution, would be greater for o-toluidine. Hence, o-toluidine would be expected to show more sorption. This apparent discrepancy in sorptive behavior may be explained if molecular configurations are taken into consideration. The methyl substituents at the ortho positions of the phenyl ring in o-toluidine cause inductive and resonance effects to operate at the reaction center. Although the sorption reaction may be facilitated by a higher electron density at the amino groups, the close proximity of the methyl groups to the nitrogen atoms may sterically hinder the molecule in forming adducts with colloidal organic matter. The influence of steric factors on bonding mechanisms is particularly noticeable with aromatic amines and humâtes (8). The linearity in the sorption isotherm (Figure 2) strongly suggests that the sorption of o-toluidine by organic colloids is essentially a partitioning process at higher p H values. We observed similar sorption behavior with PAHs, a class of neutral aromatic hydrophobic compounds (9). This observation further lends support to our hypothesis that, in the case of o-toluidine, the aromatic rings are primarily responsible for the observed sorption characteristics with estuarine colloids at p H 7-8.

The value predicted for azobenzene by equation 6 was 1362. The close agreement of the observed value (1390) with the calculated value suggests that sorption is controlled by the neutral aromatic portion of the molecule. This observation is consistent with the fact that, unlike benzidine, the two doubly-bonded nitrogen atoms contained in azobenzene are sterically hindered with sorption reactions involving attack on the nitrogen atoms. Although the values increased with decreasing p H (Figure 6), the magnitude of this increase was at least 1000 units less than that observed with benzidine for corresponding p H changes. The increase in sorption observed at lower p H values cannot be attributed to protonation of the azobenzene molecule, because the pK f l value of the azo nitrogens is -2.48. Furthermore, changes occurring within the estuarine polymeric material itself may account in part for the high values observed at low p H (10).

Acknowledgments This work was performed at the Center for Environmental and Estuarine Studies of the University of Maryland and represents portions of the doctoral dissertation of R. D . Wijayaratne, who recognizes the support by a graduate research assistantship from the Center.

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2400

PH

Figure 6. Effects of changes in pH on the values of azobenzene. Each point represents a full isotherm determined at the pH specified. Regression coeffi


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6. Nelson, C. R.; Hites, R. A. Environ. Sci. Technol. 1980, 14, 1147. 7. Hsu, T.-S.; Bartha, R. Soil Sci. 1974, 116, 444. 8. Parris, G. E. Environ. Sci. Technol. 1980, 14, 1099. 9. Wijayaratne, R. D.; Means, J. C. Mar. Environ. Res. 1984, 11, 77.

10. Means, J. C.; Wijayaratne, R. D. Science (Washington, DC) 1982, 215, 968. 11. Wijayaratne, R. D.; Means, J. C. Environ. Sci. Technol. 1984, 18, 121. 12. Menzel, D. W.; Vaccaro, R. F. Limnol. Oceanogr. 1964, 9, 138. 13. Means, J. C.; Hassett, J. J.; Wood, S. G.; Banwart, W. L. In Polynuclear

Aromatic Hydrocarbons; Jones, P. W.; Leber, P., Eds.; Ann Arbor Science: Ann Arbor, 1979; p 327.

14. Giles, C. H . ; MacEwan, T. H . ; Nakhwa, S. N . ; Smith, D. J. Chem. Soc. 1960, 3, 973.

15. Zierath, D. L . ; Hassett, J. J.; Banwart, W. L. ; Wood, S. G.; Means, J. C. Soil Sci. 1980, 129, 277.

16. Johnson, W. E. . M . S. Thesis, University of Maryland, 1986. 17. Sigleo, A. C.; Hare, P. E. ; Helz, G. R. Estuarine Coastal Shelf Sci. 1983, 17,

87. 18. Means, J. C.; Wijayaratne, R. D. Bull Mar. Sci. 1984, 35, 449. 19. Theng, Β. K. G. Clay Miner. 1971, 19, 383.

20. Soloman, C. H.; Loft, B. C.; Swift, J. C. Clay Miner. 1968, 7, 389.

RECEIVED for review July 24, 1987. ACCEPTED for publication April 7, 1988.

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