Environ. Sci. Technol. 1996, 30, 3161-3166
Cosolvent Effect on Goethite Surface Protonation YUAN XUE AND SAMUEL J. TRAINA* Environmental Science Graduate Program, The Ohio State University, 2021 Coffey Road, Columbus, Ohio 43210
Extensive research has been conducted on the surface acidity of hydrous metal oxides in aqueous solutions. However, little information is available on mineral surface acidities in mixed solvent solutions. The present study examined goethite surface protonation in acetone-water and methanol-water mixtures with potentiometric titrimetry. Surface acidity constants, K+int ) [XOH2+]eFΨ/RT/[XOH][H+] and K-int ) [H+][XO-]e-FΨ/RT/[XOH], were calculated using constant capacitance and diffuse-layer models. The number of apparent surface sites increased with increasing cosolvent content. Linear log K+int vs 1/(dielectric constant ) was observed. Simple electrostatic correction using a modified Born equation adequately predicted changes of K+int from water to mixed solvents. Changes in K-int were less tractable, however, showing nonlinearity with 1/.
Introduction Physical-chemical interactions at solid/aqueous interfaces have been described by surface complexation models (SCMs), e.g., the constant capacitance model (CCM), the diffuse-layer model (DLM), and the triple-layer model (TLM) (1). Although phenomenological, as argued by some authors (2), these models may indicate probable mechanisms underlying surface reactions and provide a simple and practical means to simulate sorption processes in the field. Dzombak and Morel (3) have tabulated complexation constants based on DLM in an effort to make heterogeneous speciation routine. SCMs have been applied extensively in aqueous systems. In contrast, only a handful of studies have been reported for water-water-miscible organic solvent mixtures. Some researchers (4, 5) have studied the acidity of montmorillonite and kaolinite in nonaqueous media, but no attempt was made to quantify the acidity constants. Others (6) investigated the effect of methanol (MeOH) and ethanol on ionic equilibria at the hematite/water interface using a surface complexation approach. They found that the equilibrium constants for the association of counterions with surface charge groups are significantly greater in mixed solvents than in water due to lower permitivity of the former (6). A number of investigators have attempted to interpret and predict physicochemical constants in semi-aqueous solvents from those in water. Born studied the thermo* Corresponding author telephone: (614)292-9037; fax: (614)2927432; e-mail address:
[email protected].
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1996 American Chemical Society
dynamic medium effect on ions from an electrostatic perspective and derived the concise Born equation (7), which has achieved some success in aqueous and semiaqueous solutions (8-10). From a practical standpoint, water-miscible organic cosolvents can be encountered in soils and groundwaters as a result of accidental spills or leaks of storage containers and the disposal of organic solvents in landfill sites (e.g., the codisposal of organic solvents and radioactive elements at Department of Energy sites) (11). Unfortunately, sufficient data are not available to predict the behavior of these complex waste environments. The present study examines the effect of mixed solvents on the surface acidity of goethite (R-FeOOH), the most abundant iron-oxide in soils (12). This mineral has hetrogeneous surface sites due to surface oxygen atoms that form one, two, or three bonds with Fe (12), as well as structural defects and impurities (13). Goethite is important in soil/water processes and in contaminant chemistry and has often been used as a model colloid in electrical doublelayer and ion adsorption studies ((ref 14) and the references cited therein). In addition, goethite is readily synthesized (15) and has a low solubility (16) so as to diminish the effect of dissolved species on interpretation of H+ adsorption data. Methanol (MeOH)-water mixtures and acetone-water mixtures were used in the present study as model mixed solvent systems. Methanol and acetone are not typical contaminants of soils and groundwaters. However, tabulated thermodyamic data are availalble on the dielectric constants and autoprotolysis constants of MeOH-water and acetone-water solutions, making them experimentally tractable for the present study.
Materials and Methods Materials. All chemicals used were ACS reagent grade unless otherwise indicated. Goethite was prepared in polycarbonate plastic bottles with Fe(NO3)3‚9H2O (Baker Analyzed reagent) (14, 15); more details are in (ref 17). The specific surface area determined by the N2-BET method was 64.3 ( 0.8 m2 g-1. Freeze-dried goethite was resuspended in water or mixed solvent mixtures (by sonification to ensure dispersion) to provide a stock suspension of 4 g goethite L-1 for titration. Fisher Optima acetone and methanol (Fisher, Fairlawn, NJ) were used as cosolvents. (CH3)4NCl (Aldrich, Milwaukee, WI) and KCl were used as supporting electrolytes. (CH3)4NCl was chosen because of its low ionic potential, reducing the probability of specific interactions with the geothite surfaces. Standard KOH (diluted from saturated KOH solution in a glovebox under N2 and standardized against KH2PO4 by titration) and HNO3 solutions (standardized against KOH solution by titration) were used as titrants. The dissolved CO2 in all the titrant solutions of KOH was tested to be e1%, satisfactory for accurate titration experiments (18). Titration of Goethite Suspensions in Mixed Solvents. Ross Sure-flow pH glass electrodes (Orion Research, Boston, MA), designed for suspensions, and a DL70ES automatic titrator (Mettler-Toledo, Hightstown, NJ) were used to generate proton adsorption isotherms in a water-jacketed, thermostated glass titration vessel. Mixed solvent systems
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TABLE 1
TABLE 2
Properties of Cosolvent Solutions in This Study (25 °C)
Capacitance Values of Cosolvent Solutions (25 °C)
wt % H3CC(O)CH3 ea
permitivity, density (g mL-1)a -log([H+][OH-])d
0 78.38b 0.9970c 13.777
10
30
50
73.02 0.9831 14.027
61.04 0.9537 14.645
48.22 0.9159 15.354
wt % CH3OH
20
40
60
80
permitivity, ea density (g mL-1)e -log([H+][OH-])f
69.65 0.9647 13.788
60.32 0.9304 13.780
50.98 0.8897 13.783
41.64 0.8435 13.926
a From ref 30 unless or otherwise indicated. b From ref 27. c From ref 28. d Interpolated from data in ref 29. e From ref 31. f Interpolated from data in ref 32.
H3CC(O)CH3 (wt %) capacitance (F m-2)
0 2.76
10 2.57
30 2.15
50 1.70
CH3OH (wt %) capacitance (F m-2)
20 2.45
40 2.12
60 1.80
80 1.47
protolysis concentration product of H2O expressed as [H][OH] at equilibrium; and m is the weight of the sorbent. About 60 data points were acquired for each proton isotherm. Surface Complexation Modeling. A dual-pK/monosite scheme was assumed in description of the surface acidity reactions on goethite. The surface protonation reactions are then written as (1)
XOH + H+ ) XOH2+ K+int ) [XOH2+]eFΨ/RT/[XOH][H+] (2)
were prepared by mixing specific weight fractions of MeOH or acetone with HPLC-grade H2O. The specific weight fractions chosen (10, 30, and 50% acetone and 20, 40, 60, and 80% MeOH) provided cosolvent solutions with roughly equivalent dielectric constants. Selected properties of the semi-aqueous systems investigated in this study are summarized in Table 1. All experiments were carried out under N2(g) purge and at 0.1 M ionic strength. The sorbent concentration was 3.20 g L-1. The outer cell of the electrode was filled with an aliquot of the mixed solvent solution to be titrated, prior to calibration. The electrode was calibrated in aqueous and semi-aqueous solutions in terms of proton concentration by Gran titrations with HCl and KOH; more details are provided in (refs 17 and 19). Recalibration was conducted before each titration, and the calibration was checked at the end of the titration. In all cases, the difference before and after titration was
