Environ. Sci. Technol. 1998, 32, 2856-2864
Oxalate Adsorption at a Plagioclase (An47) Surface and Models for Ligand-Promoted Dissolution LISA L. STILLINGS,* JAMES I. DREVER, AND SIMON R. POULSON Department of Geology & Geophysics, University of Wyoming, Laramie, Wyoming 82071-3006
Previous work on adsorption of oxalate at aluminosilicate surfaces suggests that maximum adsorption occurs through a bidentate attachment of the organic ligand, at nearneutral pH. Rates of ligand-promoted dissolution are expected to be greatest at this pH as well. We tested this model by measuring oxalate adsorption on the surface of andesine (An47), in solutions of pH 3-5 and total oxalate concentrations of 0-8 mM. Contrary to expectation, the greatest adsorption density of 24 µmol m-2 total oxalate was observed at pH 3 and 8 mM total oxalate. Adsorption is dependent upon the activities of both oxalate (C2O42-) and bioxalate (HC2O4-) in solution and can be modeled with either a two-term Langmuir or a two-term Freundlich isotherm. A Freundlich adsorption model provided the best fit to rate data because it was not constrained to a finite number of adsorption sites, as was the Langmuir model. The two-term ligand adsorption model was incorporated into a rate model: Rtot ) kH+[H+ ads]L + kHOx 22-[Ox [HOx] + k ] where R is the net dissolution Ox tot ads ads rate of the feldspar, [iads] is the concentration of species i adsorbed to the surface, and ki is the rate constant for release of the surface complex. The model was fit to data for oxalate-promoted dissolution of andesine, resulting in estimates for the rate constants of kHOx- ) 1.16 × 10-12, kOx2) 1.05 × 10-12, and kH+ ) 9.61 × 10-13 mol of feldspar (µmol of i)-1 s-1.
Introduction A number of studies have measured the dissolution rate of feldspar in oxalic acid solutions at 25 °C (1-6). Most have concluded that the dissolution rate in the presence of oxalic acid is increased over the rate when no oxalic acid is present, although one study (2) has argued that the effect of oxalic acid on dissolution of sodic plagioclase is very small. These studies have interpreted their oxalate-enhanced rate data with the ligand-promoted model (7)
ratetot ) RH + RL where ratetot is the net, observed rate of dissolution, RH is the proton-promoted rate of dissolution, and RL is the ligandpromoted rate of dissolution. This model suggests that adsorption of an oxalate ligand to a feldspar surface increases * Corresponding author present address: U.S. Geological Survey, Mackay School of Mines-MS 176, University of NevadasReno, Reno, NV 89557-0047. Tel: (702)784-5789; fax: (702)784-5079; e-mail:
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the dissolution rate by increasing the release rate of surface, structural cations to solution. The model also implies that the two mechanisms, proton promotion and oxalate promotion, occur independently of each other and in parallel. Furrer and Stumm (7) argue that a greater enhancement of dissolution occurs with a bidentate ligand attachment rather than an monodentate attachment. Other authors (5, 8) have predicted ligand-enhanced dissolution to be greatest at pH 5-7.5, because bidentate adsorption occurs at pH values where the ligand is fully deprotonated (i.e., pH > pK2) and where oxide surfaces are positively charged. Also, ligands are not as effective as protons in promoting dissolution rates; therefore, ligand promotion is only apparent when the rates of H+ promotion are relatively low (i.e., at near-neutral pH). We tested this conceptual model by measuring oxalate adsorption at the surface of an andesine feldspar (An47), at pH 3-5 and total oxalate of 0-8 mM and by modeling oxalatepromoted dissolution of andesine under these same conditions. Although many studies have invoked surface-adsorbed oxalate as part of a dissolution mechanism, none have measured oxalate adsorption as a test of the proposed model.
Methods Starting Material. The feldspar was an andesine plagioclase (Ab51An47Or2) from Whiteface Mountain in Essex County, New York, obtained from Wards Scientific Establishment, Inc. Its chemical composition has been reported previously (9). The sample was prepared by first breaking the mineral with a hammer and jaw crusher to obtain a 16-40 mesh size. These fragments were then hand picked, and clean pieces (not stained or weathered) were retained for further crushing. The clean pieces were ground in an agate mortar and pestle and wet sieved to isolate the RT. A plot of our andesine data as a 3 3+ function of log(aH +/aAl ) shows a distinct correlation with the logarithm of the dissolution rate, with a r2 ) 0.87 (Figure 8), although the slope of the data is 0.14 and not 0.33 as predicted by Oelkers and Schott (42). The value of 0.33 is derived from the stoichiometric ratio of Al:Si in the unreacted feldspar phase (41); an estimate of 0.14 for the reaction order suggests the surface of the dissolving feldspar phase is depleted of Al, as would be expected in a surface leached layer at low pH.
FIGURE 6. Comparison of site occupancy on the andesine surface, with measured and estimated dissolution rates (Rtot). A Freundlich adsorption model was used to estimate adsorption densities. With this model, it is implicitly assumed that adsorption density can increase with increasing total oxalate in solution. Each individual experiment is plotted along the x-axis, where the first number represents the pH of the experiment and the second number is the oxalate concentration (in mM). See Figure 5 for an explanation of the 4-2 pH-Oxtot observed rate.
FIGURE 7. Variation of log dissolution rate as a function of chemical affinity, ∆Gr. The chemical affinity of an intermediate plagioclase was calculated by assuming an ideal-mixing model for the calculation of ∆G°f of the solid phase (see text). The above discussion illustrates the limitations of experiments carried out at the macroscopic scale. An increase in dissolved oxalate concentration causes: (i) An increase in adsorbed oxalate concentration. (ii) An increase in the affinity of the dissolution reaction. (iii) A decrease in the activity of free Al3+ in solution. Each of these effects could plausibly increase the dissolution rate of andesine. In principle, we should be able to use the functional relationship between dissolution rate and oxalate concentration to distinguish among the alternatives, but in our opinion the theoretical basis for the functional relationship to be expected for each mechanism is not sufficiently
FIGURE 8. Variation of log dissolution rate as a function of log (aH3 +/aAl3+), according to the model of Oelkers and Schott (42). The slope of the regression line, 0.14, is the value of n. well established. For example, transition-state theory predicts a simple relationship between chemical affinity and dissolution rate (43, 44) in which dissolution rate becomes independent of chemical affinity at relatively small degrees of undersaturation. Experiments by Burch et al. (39), however, showed that albite dissolution did not follow the predictions of the transition-state model: dissolution rate appeared to be a strong function of chemical affinity even at high degrees of undersaturation. For adsorption of oxalate, VOL. 32, NO. 19, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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we do not have a simple, unambiguous relationship between concentration adsorbed and concentration in solution, and we do not have an independently established relationship to predict the effect of adsorption of each of the oxalate species on dissolution rate. The Al inhibition model as presented by Oelkers and Schott (42) does predict a specific slope (0.33) for the plot of log(dissolution rate) vs log3 3+ (aH +/aAl ) that is different from the slope we observe (0.14). It is possible that the model could be modified to accommodate the different slope. In summary, the surface complexation model developed by Stumm and his colleagues is fully consistent with the effect of oxalate on the dissolution rate of andesine feldspar, but the explanation is not necessarily unique. More detailed information is necessary on the behavior of adosrbed oxalate on the molecular level and on the mechanism of detachment of atoms during the dissolution of feldspar. Such information may come from surface spectroscopic methods and from ab initio molecular orbital calculations that provide insights into dissolution mechanisms at the molecular scale (e.g., ref 45).
Acknowledgments Financial support from the NSF (EPSCoR OSR-9550477) and the U.S. Air Force (F49620-94-1-0194) is gratefully acknowledged.
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Received for review March 16, 1998. Revised manuscript received June 30, 1998. Accepted June 30, 1998. ES980258D
