Research Solute Transport in Clay Media: Effect of Humic Acid ALANAH FITCH* AND JIA DU Department of Chemistry, Loyola University of Chicago, 6525 North Sheridan Road, Chicago, Illinois 60626
The ability of humic materials to remove cations from the gallery region of clay stacks is monitored directly using clay-modified electrodes. It is found that humic materials reduce access to the inner layer region by clogging access pores and by binding cations. Humic acid is a long-chain polyanion arising from decomposition of natural matter. Natural organic matter can exist in several size fractions (MW range of 50 000-100 000 with some fractions of colloidal dimensions 60-100 Å (1, 2) with variable acidity and water solubility (3). All size fractions, including water-soluble organic material (4), complex metal ions (5). While it has been shown that proteinaceous material can be intercalated into the interlayer region, there is little data to suggest that naturally occurring organic material commonly adsorbs in the interlayer region. Such material as is observed within the interlayer is adsorbed at pH 1. If the effect of humic acid is to solely reduce Cfree,bulk, the ratio should remain constant as a function of humic acid concentration. Figure 3B shows that for the first additions of humic acid this is true. At larger additions of humic acid, however, there is a decrease in ratio, indicating that (Deff/D)1/2κ has changed. The effective diffusion coefficient of a molecule within the interlayer domain can be affected by several factors. If the molecule is subject to a strong van der Waals interaction with the face surface of the clay, the diffusion coefficient will reflect lateral hopping along the clay surface. The magnitude of this diffusion is 5-6 orders lower than diffusion in solution. Ru(NH3)63+ was chosen as a probe molecule for this experiment precisely because it does not strongly adhere to the face surfaces but is instead retained by long-range electrostatic forces (17). For molecules retained by long-range electrostatic forces, the effective diffusion coefficient within the clay depends primarily upon the available pore space and tortuosity of the film. The argument laid out above implies that the decrease in the ratio currents is a result of a decrease in the effective diffusion coefficient, Deff, within the clay film. The smaller magnitude of Deff is related to a decrease in the accessible pores in the clay-modified electrode. To confirm that this is true and that κ, the partition coefficient, is not changing with increasing amounts of added humic acid, a similar experiment was performed with Fe(CN)63-. Both Fe(CN)63and HA are anionic, and no interaction between the two is anticipated. The absorbance peak for Fe(CN)63- remained constant in height and location as a function of added humic acid (Figure 4A). Additionally, the reduction peak currents of Fe(CN)63- at a bare electrode were unaffected by the addition of humic acid (Figure 4B), indicating no interaction of Fe(CN)63- with HA. The ratio current (Figure 4B) for Fe(CN)63- at zero added HA is consistent with values obtained in other experiments (16) and indicates that the interlayer dimension is approximately 30-60 Å wide. When humic acid is added to
14
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 30, NO. 1, 1996
FIGURE 4. (A) Magnitude of 420 nm absorbance peak for HA solution (2) and HA + 2 mM Fe(CN)63- (9). The difference [(HA + Fe(CN)63-) - HA] (b) represents the constant contribution of free Fe(CN)63- to the solution absorbance. (B) Reduction peak current heights at the bare (9) and clay-modified (2) electrodes as a function of increasing amounts of added HA. The ratio current (R ) Ip,cme/Ip,bare) (b) is also shown as a function of added HA.
FIGURE 5. (A) Absorbance peaks for Fe(bpy)32+ as a function of added HA. (B) Oxidative current peaks for Fe(bpy)32+ as a function of added HA at the bare (b) and modified (9) electrodes. Ratio currents (R ) Ip,cme/Ip,bare) (2) are also plotted as a function of added HA.
the solution, the currents at the clay-modified electrode decrease disproportionate to the currents at the bare electrode, as also evidenced by the decreasing ratio currents. We interpret these results as blockage of the pores via the humic acid polyelectrolyte chain (Figure 1B, left-hand side). There is approximately a 20% decrease in ratio that occurs with the first addition of humic acid. Recall that a 50% decrease was noted for Ru(NH3)63+ (Figure 3B). The effect of humic acid on the transport of Fe(bpy)32+ through the clay film was also studied. Fe(bpy)32+ was chosen because its low water solubility (18) should cause it to preferentially move into the more nonaqueous domains defined by humic acid. We were, however, unable to measure any binding of the probe with humic acid via either absorbance (Figure 5A) or reduction peak currents (Figure 5B). The molecular diameter of the complex is approximately 11.6 Å (21) and may contribute to the lack of interaction of Fe(bpy)32+ with humic acid. The large and bulky nature of the three bipyridine ligands may also place a steric constraint on the encapsulation process. The ratio values for Fe(bpy)32+ showed a decrease (44%) with the first additions of humic acid (Figure 5B), consistent
with what was observed for Fe(CN)63- and Ru(NH3)63+. Again, since the metal complex does not appear to interact with the humic acid, we suggest that the effect of humic acid was to block access to the entry pores to the claymodified electrode. In summary, we have given the first direct measurement of the effect of humic acid in preventing access of a probe molecule into the gallery region of clays due to competitive association and have further noted that the effect is enhanced via physical blockage of entry pores.
Nomenclature HA R Deff κ ν1/2 A n C Ip
commercial (Aldrich) humic acid ratio ) peak current at clay-modified electrode/ peak current at bare electrode effective diffusion coefficient within the clay (cm2/s) partition coefficient scan rate (V/s1/2) area of the electrode number of electrons transferred in the reduction concentration (mol/cm3) peak current (A)
Literature Cited (1) Flaig, W.; Beutelspacher, H. Z. Pflanzenernaehr. Duang. Bodenkd. 1951, 52, 1. (2) Visser, S. A. Soil Sci. 1963, 96, 353.
(3) Stevenson, F. J. Humus Chemistry; Wiley: New York, 1982; p 237. (4) Fitch, A.; Helmke, P. A. Anal. Chem. 1989, 61, 1295. (5) Stevenson, F. J.; Fitch, A.; Brar, M. S. Soil Sci. 1993, 26 (9), 1775. (6) Marley, N. A.; Gaffney, J. S.; Orlandini, K. A.; Cunningham, M. M. Environ. Sci. Technol. 1993, 27, 2456-2461. (7) Liu, H.; Amy G. Environ. Sci. Technol. 1993, 27, 1553-1562. (8) Abdul, A. S.; Gibson, T. L.; Rai, D. N. Environ. Sci. Technol. 1990, 24, 328-333. (9) McCarthy, J. F.; Williams, T. M.; Liang, L.; Jarine, P. M.; Jolley, L. W.; Taylor, D. L.; Palumbo, A. V.; Cooper, L. W. Environ. Sci. Technol. 1993, 27, 667. (10) Carter, C. W.; Suffet, I. H. Environ. Sci. Technol. 1982, 16 (11), 735. (11) Chiou, C. T.; Schmedding, D. W.; Manes, M. Environ. Sci. Technol. 1982, 16 (3) 10. (12) Chiou, C. T.; Kile, D. E.; Malcolm, R. L. Environ. Sci. Technol. 1988, 22, 3. (13) Chiou, C. T.; Kile, D. E.; Brinton, T. I.; Malcolm, R. L.; Leenheer, J. A.; MacCarthy, P. Environ. Sci. Technol. 1987, 21, 1231. (14) Chiou, C. T.; Porter, P. E.; Schmedding, D. W. Environ. Sci. Technol. 1983, 17, 227. (15) Lee, S. A.; Fitch, A. J. Phys. Chem. 1990, 94, 4998. (16) Fitch, A.; Fausto, C. L. J. Electroanal. Chem. 1988, 257, 299. (17) Fitch, A.; Du, J. J. Electroanal. Chem. 1992, 319, 409. (18) Subramanian, P.; Fitch A. Environ. Sci. Technol. 1992, 26 (9), 1775. (19) Norrish, K. Trans. Faraday Soc. 1954, 18, 120. (20) Wieglos, T.; Fitch, A. Electroanalysis 1990, 2, 449. (21) Garcia Posse, M. E.; Juri, M. A. Inorg. Chem. 1984, 23, 948.
Received for review March 3, 1994. Revised manuscript received December 13, 1994. Accepted July 28, 1995.X ES940133I X
Abstract published in Advance ACS Abstracts, October 1, 1995.
VOL. 30, NO. 1, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
15
