Soil Science Society of America Journal 63:1133-1141 (1999)
© 1999 Soil Science Society of America
DIVISION S-2-SOIL CHEMISTRY
The Effect of Oxyanions on the Oxalate-Promoted Dissolution of Goethite
Matthew J. Eicka,
John D. Peakb and
Warren D. Bradyc
a Department of Crop and Soil Environmental Sciences, Virginia Tech, Blacksburg, VA 24061-0404 USA
b Department of Plant and Soil Sciences, University of Delaware, Newark, DE 19717-1303 USA
c IT Corporation, Baton Rouge, LA 70806-7742 USA
eick{at}vt.edu
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ABSTRACT
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Organic ligand–promoted dissolution of oxide minerals can be enhanced or inhibited in the presence of specifically adsorbed oxyanions. It has been proposed that an oxyanion inhibits or enhances dissolution depending on the type of surface complex formed and the strength of the bond. Mononuclear complexes (especially if they are bidentate) accelerate dissolution, while binuclear complexes inhibit dissolution. Recent spectroscopic evidence indicates that chromate and arsenate form different surface complexes depending on surface coverages. This study examined the influence of chromate and arsenate on the oxalate promoted dissolution of goethite. Based on a previous spectroscopic study, oxyanion surface coverages were varied to generate both mononuclear and binuclear surface complexes. Chromate and arsenate inhibited the oxalate promoted dissolution of goethite at all surface coverages investigated except at pH 6
. It is proposed that chromate and arsenate inhibit goethite dissolution by decreasing oxalate adsorption. This is accomplished because arsenate and chromate are more effective competitors for goethite surface sites than oxalate and upon adsorption increase the negative charge of the goethite surface. At pH 6 the adsorption of chromate and arsenate increases the negative charge of the goethite surface which in turn increases proton adsorption. Since proton adsorption is a necessary step for oxalate-promoted dissolution of goethite, and since proton activity at pH 6 is low, an increase in the negative charge of goethite upon adsorption of the oxyanions accelerates dissolution.
Abbreviations: BET, Brunauer-Emmett-Teller • CCM, constant capacitance model • DRIFT, Diffuse Reflectance Infrared Fourier Transform • ICP-AES, inductively coupled plasma atomic emission spectrometer • PZC, point of zero charge • TEM, Transmission Electron Microscopy • XRD, x-ray diffraction
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INTRODUCTION
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SOIL BIOTA has the ability to alter the chemistry of soil environments through the synthesis of organic acids. These compounds may serve to detoxify the local environment, increase the solubility of micro and macronutrients, or may be metabolic by-products. Naturally occurring organic acids have been observed to accelerate the dissolution of oxide and aluminosilicate minerals in both the laboratory (Ludwig and Casey, 1996; Xyla et al., 1992; Furrer and Stumm, 1986; Zinder et al., 1986) and the field (Bennett et al., 1996). It is now generally accepted that organic acid–promoted dissolution of most mineral surfaces is a surface-controlled process and that metal removal from a mineral surface is similar to ligand exchange in solution (Ludwig and Casey, 1996).
In natural systems, organic acids will compete with metal ions, oxyanions, and other organic ligands for reactive surface sites. It has been proposed that these competitive sorption reactions may inhibit or enhance dissolution depending on the type of the surface complex formed and the strength of the bond (Bondietti et al., 1993; Stumm, 1992; Biber et al., 1994). For example, Bondietti et al. (1993) examined the influence of phosphate, arsenate, and selenite sorption on the EDTA-promoted dissolution of 
-FeOOH. The researchers found that the oxyanions inhibited dissolution at near-neutral pH values, while they enhanced dissolution at pH values <5. They hypothesized that mononuclear complexes (especially if they are bidentate) accelerate dissolution, while binuclear complexes inhibit dissolution (Fig. 1)
. It was proposed that the inhibition of dissolution by binuclear complexes is due to the greater energy required to remove two metal atoms from the crystal lattice. Reductive dissolution of Fe oxides was also inhibited by the sorption of oxyanions (Biber et al., 1994). However, enhanced dissolution of the Fe oxides similar to Bondietti et al. (1993) was not observed. Recent spectroscopic and kinetic evidence indicates that oxyanions such as arsenate and chromate form different surface complexes on goethite depending on surface coverages (Fendorf et al., 1997; Grossl et al., 1997). At low surface coverages, monodentate complexes were more prevalent, while at higher coverages, bidentate complexes were more common, with the bidentate–binuclear complex appearing in the greatest proportion.
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Fig. 1 Schematic illustration of (a) monodentate, (b) bidentate mononuclear, and (c) bidentate binuclear surface complexes
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Based on the above research, it is clear that oxyanions can affect the rate of both reductive and nonreductive dissolution of oxide minerals. If the proposed mechanism by Bondietti et al. (1993) is accurate, then one would expect enhanced ligand-promoted dissolution of goethite at oxyanion surface coverages where mononuclear complexes dominate and reduced dissolution at surface coverages where binuclear complexes dominate. Where near equal proportions of both types of surface complexes exist, the dissolution rate may include the effect of both types of surface complexes. Whether dissolution in the presence of oxyanions is enhanced or inhibited relative to dissolution in the absence of oxyanions will depend on which complex has a greater influence on the rate. Accordingly, the objective of this research is to test the hypothesis that bidentate–binuclear surface complexes are dissolution inhibiting, while bidentate–mononuclear and monodentate complexes are dissolution enhancing. This information will provide a better understanding of weathering reactions in natural systems and may be used for possible application to industrial processes (dissolution or corrosion of materials).
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Materials and methods
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Oxide Characterization
The goethite used in all experiments was synthesized from reagent-grade Fe(NO3)3 using the method described in Schwertmann and Cornell (1991). Excess salts were removed by electrodialysis until the conductivity of the wash solution was nearly equal to that of distilled, doubly deionized water. The clean goethite precipitate was subsequently freeze-dried and stored under desiccation. The identity of the goethite was verified by x-ray diffraction (XRD) analysis, Diffuse Reflectance Infrared Fourier Transform (DRIFT) Spectroscopy, and Transmission Electron Microscopy (TEM). X-ray diffraction analysis, DRIFT spectroscopy, and TEM micrographs were diagnostic for goethite and consistent with those present in Schwertmann and Cornell (1991). The specific surface area was 55.5 m2 g-1, as determined by a five-point N2 Brunauer-Emmett-Teller (BET) gas adsorption isotherm method.
Adsorption Edges
Adsorption of chromate and arsenate was examined as a function of pH (5–11) and ionic strength (0.01 and 0.10 M) at constant adsorbate (1 mM) and adsorbent concentrations (10 g L-1). Oxalate adsorption was examined as a function of pH (3–8) at constant ionic strength (0.10 M), adsorbate (5 mM), and adsorbent concentration (10 g L-1). Adsorption edges were measured using a batch technique in a flat-bottomed, teflon-lined, water-jacketed reaction vessel (500 mL) covered with a glass lid containing ports for a stirrer, a pH electrode, N2 gas, a burette tip, and a sample pipet. An appropriate quantity of a N2-purged goethite stock suspension (20 g L-1) and 1 M NaNO3 stock suspension were added to the batch reactor and adjusted to a pH 8 to 11 using an automatic titrator (Metrohm 718 Stat Titrino, Brinkman Instruments, Westbury, NY) and dropwise addition of 0.10 M NaOH. The suspension was sufficiently mixed using an overhead-driven mechanical stirrer (RZR-2000, Caframo Ltd., Wiarton, ON, Canada) spinning at approximately five revolutions per second and allowed to equilibrate for 24 h at a constant temperature of 298 ± 0.1 K.
After 
24 h an appropriate quantity of a 0.10 M stock solution of chromate, arsenate, or oxalate (as sodium salts) was added to the suspension (total experimental suspension volume 400 mL) and allowed to equilibrate for 6 h. For the arsenate and chromate adsorption edges, a 15-mL aliquot of the suspension was removed and placed in a 50-mL N2-purged polypropylene centrifuge tube that was placed on a rotating temperature controlled shaker for 24 h. After controlled shaking, the pH was checked and the suspension was centrifuged at 20000 g for 10 min. An initial 5-mL aliquot of the supernatant was removed from the centrifuge tubes and filtered through a 0.10-µm Gelman (Gelman Sciences, Ann Arbor, MI) metrical membrane to "presaturate" the membrane and reduce the potential for metal sorption to membranes observed by Jardine et al. (1986). The remaining 10 mL of the supernatant was removed from the centrifuge tube and filtered through the same presaturated membrane into acid-washed polypropylene test tubes. For the oxalate edge, a 10-mL sample was placed in a 50-mL N2-purged polypropylene centrifuge tube and centrifuged at 20000 g for 10 min and the supernatant removed for immediate analysis. This procedure was repeated at different pH values to obtain an adsorption edge. All samples were carefully removed from the batch reactor with an electronic pipet in order to avoid changes in the total surface area/solution ratio. Aluminum foil was thoroughly wrapped around the batch reactor to exclude light. Chromium and arsenate were analyzed using a Perkin-Elmer Optima 3000 (Perkin-Elmer, Norwalk, CT) solid state inductively coupled plasma atomic emission spectrometer (ICP-AES). Oxalate was analyzed using a Dohrmann 180 (Dohrmann, Santa Clara, CA) organic C analyzer.
Surface Complexation Modeling
The constant capacitance model (CCM) was employed to model the adsorption of chromate and arsenate on goethite. This model assumes that these oxyanions are specifically adsorbed (inner-sphere); hence, background electrolyte ions do not compete with surface complexes. This is consistent with recent spectroscopic evidence indicating that both chromate and arsenate are specifically adsorbed to goethite (Waychunas et al., 1996; Sun and Doner, 1996; Fendorf et al., 1997). The CCM assumes that the net surface charge is a linear function of surface potential:
 | (1) |
where 
is the surface charge (molc L-1), C is the capacitance density (F m-2), S is the specific surface area (m2 kg-1), a is the suspension density of the solid (kg L-1), F is the Faraday constant (9.65 x 10-4 C mol-1), and 
is the surface potential (V).
The CCM was used to model the adsorption of chromate and arsenate as both monodentate and bidentate surface complexes. A surface site density of 3.82 µmole m-2, which was determined from chromate/arsenate adsorption isotherms, was used in the CCM. This surface sight density is consistent with 3 to 5.5 µmoles m-2 of reactive sites proposed by Hiemstra et al. (1989) and Barger et al. (1997). Additionally, a fixed capacitance value of 0.80 F m-2 was used in the CCM calculations (Mesuere and Fish, 1992a, 1992b). The intrinsic surface acidity constants were obtained from Goldberg (1986) and are averages from a literature compilation of experimental data (Goldberg and Sposito, 1984). Surface acidity constant values used were
. The acid dissociation constants used in the CCM were
, and
for arsenate and
and
for chromate. The least square optimization program FITEQL Version 3.2 (Herbelin and Westall, 1996) was used (i) to fit intrinsic surface complexation constants (Table 1)
to the experimental adsorption edge data, (ii) to compute changes in surface charge, and (iii) to compute the concentrations of surface complexes. The goodness of fit is indicated by the overall variance (Vy), which is the weighted sum of squares divided by the degrees of freedom (WSOS/DF). Values of Vy less than 20 denote a reasonably good fit (Herbelin and Westall, 1996).
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Table 1 Surface complexation reactions and constant capacitance model (CCM) intrinsic surface complexation constants
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Dissolution Experiments
All dissolution studies were carried out using a pH-stat technique in the presence of 5 mM oxalate. Four grams of freeze-dried goethite were carefully weighed into a 500-mL teflon liner and 330 mL of a distilled doubly deionized water were added. Forty milliliters of a 1 M NaNO3 stock solution were added to achieve a background electrolyte concentration of 0.10 M. The suspension was dispersed using a sonic dismembrator and the teflon liner was placed in a jacketed reaction vessel and allowed to hydrate for 24 h. The suspension pH was adjusted to the appropriate value and kept constant and well-stirred with the aid of an automatic titrator and mechanical stirrer, respectively. When the pH stabilized, the suspension volume was brought to 400 mL, less the quantity of oxalate and either chromate or arsenate to be added. After 24 h, a measured quantity of a 0.10 M stock solution of arsenate or chromate (as sodium chromate or arsenate) was dispensed into the reaction vessel and allowed to equilibrate for 6 h. Following this equilibration period a measured volume of a 0.1420 M oxalate stock solution was dispensed in stepwise quantities. All experiments were conducted in the dark at 298 ± 0.1 K and 0.101 MPa pressure in an N2 environment to eliminate CO2 influences.
Oxyanion concentrations for the dissolution experiments were 0.25, 1, and 5 mM. Dissolution experiments were conducted for 50 h and dissolution rates were obtained from the linear part of the experiment (>10 h). All samples were obtained using the same sampling procedure used for the adsorption edge experiments. Samples were immediately acidified using 8 M Optima HNO3. Iron was analyzed using a Perkin Elmer Optima 3000 solid state ICP-AES.
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Results
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Adsorption Experiments
The quantity of arsenate, chromate, and oxalate adsorbed on goethite as a function of pH is displayed in Fig. 2
. Oxalate, chromate, and arsenate adsorption decrease with an increase in pH; however, both chromate and oxalate exhibit a much steeper adsorption edge than arsenate. The steepness of adsorption edges can be directly related to the degree of protonation of the anion. Arsenate is a triprotic acid, while both chromate and oxalate are diprotic. The adsorption of weak acids is maximized at a pH near the acid dissociation constant or pKa value of the protonated form of the adsorbing oxyanion (McBride, 1994; Hingston et al., 1972). Therefore, triprotic acids will exhibit a much broader slope than diprotic acids. A shift in the oxalate edge to a lower pH range compared with chromate can be explained by differences in pKa2 values. These results are consistent with other investigations examining oxyanion adsorption on goethite (Hingston et al., 1972; Goldberg, 1986; Grossl et al., 1997). No difference between the adsorption edges for chromate and arsenate was measured at different concentrations of NaNO3. This continuity in measured adsorption edges suggests that both chromate and arsenate form inner-sphere surface complexes with goethite. This observation is consistent with recent spectroscopic and kinetic investigations (Waychunas et al., 1996; Sun and Doner, 1996; Fendorf et al., 1997; Grossl et al., 1997).
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Fig. 2 Equilibrium adsorption edge data for arsenate, chromate, and oxalate adsorption on goethite vs. pH. Values in legend indicate background electrolyte concentrations
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Surface Complexation Modeling
The CCM was used to model the adsorption of chromate and arsenate as both inner-sphere bidentate and monodentate surface complexes. Figure 3
displays the experimental adsorption edges plotted with CCM simulations. Both monodentate and bidentate simulations describe the experimental data reasonably well, as indicated by the Vy (Table 2)
. However, CCM simulations show that chromate was better modeled as a monodentate surface complex, while arsenate was better modeled as a bidentate surface complex. This is consistent with spectroscopic data that indicates a mixture of both monodentate and bidentate chromate and arsenate surface complexes on goethite (Fendorf et al., 1997). However, at low surface coverages a greater proportion of chromate is associated with the monodentate complex compared with arsenate. Furthermore, kinetic and equilibrium studies suggest that arsenate is more likely to form a bidentate surface complex than chromate (Grossl et al., 1997). The intrinsic equilibrium constants calculated for the formation of monodentate and bidentate surface complexes using the CCM are listed in Table 1.
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Fig. 3 Equilibrium adsorption data on goethite vs. pH and CCM simulations for monodenate and bidentate surface complexes: (a) chromate adsorption on goethite and (b) arsenate adsorption on goethite
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Table 2 Constant capacitance model (CCM) simulation surface charge as a function of pH and overall variance (Vy) results for arsenate and chromate adsorption to goethite. Value for the surface charge in the absence of the oxyanions was taken from Felmy and Rustad (1998)
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Changes in goethite surface charge upon adsorption of chromate and arsenate were determined by comparing the electrostatic data generated by the CCM to the goethite surface charge experimental data of Felmy and Rustad (1998). Electrostatic data at all pH values indicated that goethite surface charge was reduced by at least an order of magnitude after adsorption of both chromate and arsenate (Table 2). A greater reduction in surface charge was predicted with the monodentate surface complex than with the bidentate surface complex. Similarly, surface charge was predicted to be reduced more in the presence of arsenate because of the greater negative charge of the molecule. The point of zero charge (PZC) of the goethite surface was reduced after adsorption of both chromate and arsenate. Arsenate adsorption reduced PZC values from 9.3 (Felmy and Rustad, 1998) to 4.5 (bidentate) and 3.5 (monodentate), while chromate adsorption reduced PZC values to 8.0 (bidentate) and 6.0 (monodentate). These observations suggest that reduction in surface charge upon oxyanion adsorption would increase both proton and cation adsorption compared with the goethite surface alone.
Dissolution Experiments
The oxalate-promoted dissolution of goethite in the presence of 1 mM chromate and arsenate as a function of pH are displayed in Fig. 4
. Initial dissolution of the goethite surface is rapid and dissolution rates are calculated where the rate of Fe release is approximately linear and constant (>10 h). Dissolution rates decreased linearly as pH increased and at pH 7 Fe was below instrument detection limits (Fig. 5
and Table 3)
. Calculated dissolution rates were an order of magnitude or more lower than those determined by Zinder et al. (1986). This may be due to the extensive pretreatment of the goethite prior to their dissolution experiments. The addition of 1 mM of chromate and arsenate inhibited the oxalate-promoted dissolution of goethite at all pH values except 6 (Table 3 and Fig. 4). Oxyanion surface coverages varied little in the pH range of 3 to 6 (Table 3). Arsenate had a greater effect on decreasing dissolution rates compared with chromate (Table 3). The greatest reduction in dissolution rates by the oxyanions occurred at pH 3. As the pH increased from 3 to 5, the magnitude of the reduction in dissolution rates decreased and at pH 6 chromate and arsenate enhanced the dissolution rate of goethite compared with oxalate alone (Fig. 4 and Table 3). Dissolution of goethite in the presence of oxalate as a function of oxyanion surface coverage is displayed in Fig. 6
. Dissolution rates decreased linearly as oxyanion surface coverage increased (Fig. 7
and Table 3). Similar to pH dissolution experiments, arsenate had a greater effect on decreasing dissolution rates compared with chromate.
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Fig. 4 Oxalate-promoted (5 mM) dissolution of goethite in the presence of 1 mM chromate and arsenate and a background electrolyte concentation of 0.10 M NaNO3: (a) pH 3, (b) pH 4, (c) pH 5 and (d) pH 6
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Fig. 6 Oxalate-promoted (5 mM) dissolution of goethite as a function of aqueous oxyanion concentration: (a) chromate and (b) arsenate
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Discussion
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The kinetics and mechanisms of accelerated oxide mineral dissolution in the presence of oxalate has been thoroughly studied (Zinder et al., 1986; Furrer and Stumm, 1986; Mesuere and Fish, 1992a, 1992b; Biber et al., 1994; Ludwig et al., 1996). However, less research has been conducted examining the influence of specifically adsorbed oxyanions on organic acid promoted dissolution of oxide surfaces (Mesuere and Fish, 1992a, 1992b; Bondietti et al., 1993; Biber et al., 1994). With the advance in surface spectroscopies, we are able to probe the local chemistry of adsorbates and accurately determine the type of surface complex formed. The information gained from these techniques provides a more accurate interpretation of kinetic data (Junta-Rosso et al., 1997).
Both chromate and arsenate were found to inhibit the oxalate-promoted dissolution of goethite at all pH values and surface coverages investigated except pH 6. The surface coverages used in this study encompass the range of surface coverages examined by Fendorf et al. (1997) for arsenate and are less than the surface coverages examined for chromate. Fendorf et al. (1997) observed that at low surface coverages a greater proportion of arsenate and chromate were observed to be associated with mononuclear complexes compared with a binuclear complex. Additionally, at lower surface coverages a kinetic investigation indicated that mononuclear complexes were favored over binuclear complexes for chromate (Grossl et al., 1997). On the basis of the above data and the surface coverages used in this investigation, it is expected that chromate would form mononuclear surface complexes at all but the highest surface coverage investigated
. In contrast, binuclear complexes should predominate for arsenate at all but the lowest surface coverage investigated
. Although surface coverage of both oxyanions varied little as the pH was changed, it is possible that different surface complexes may result from oxyanion protonation. However, binuclear complexes should still predominate for arsenate (Waychunas et al., 1993; Manceau, 1995; Waychunas et al., 1996; Sun and Doner, 1997), while mononuclear complexes should predominate for chromate at the surface coverages investigated in this study (Fendorf et al., 1997).
The above experimental dissolution results are inconsistent with the proposed mechanism of Stumm (1992), Bondietti et al., (1993), and Biber et al., (1994). If mononuclear complexes accelerate dissolution, then we should observe accelerated dissolution in more than one experiment where the oxyanion would be expected to form a mononuclear surface complex. Furthermore, at the surface coverage of the pH 6 experiment for arsenate, binuclear surface complexes should dominate (Fendorf et al., 1997). The above experimental results can be explained using the electrostatic data generated by the CCM simulations and the solution properties of the oxyanions.
Similar to the potential-determining ions H+ and OH-, chemisorbed cations and anions can alter surface charge. Chemisorbed cations increase surface positive charge and shift the PZC to higher pH values. In contrast, chemisorbed anions increase surface negative charge and shift the PZC to lower pH values. This increase in surface negative charge is clearly demonstrated for the adsorption of chromate and arsenate on goethite (Table 2). An increase in surface negative charge will result in a decrease in oxalate adsorption (due to electrostatic repulsion) and a decrease in dissolution rates.
Additionally, solution properties of oxyanions and organic ligands like oxalate are markedly different. Many organic ligands such as oxalate form soluble multidentate complexes in solution with metals such as Fe. These complexes are often characterized by high stability constants. In contrast, soluble complexes formed by oxyanions are weak (characterized by low stability constants). Furthermore, strong surface-metal oxyanion attraction is manifested as a tendency to form insoluble precipitates (McBride, 1994). Oxyanions such as phosphate and arsenate pair strongly with Fe+3, resulting in the precipitation of insoluble Fe(PO4, AsO4) phases. Consequently, adsorption of these oxyanions extends the cross-linking or structure of the solid goethite, making it resistant to organic ligand–promoted dissolution. Mesuere and Fish (1992a, 1992b) observed similar results for chromate and oxalate adsorption on goethite. The adsorption of these oxyanions would also explain why arsenate was more effective at inhibiting dissolution compared with chromate.
Recent evidence has also demonstrated that the mechanism of ligand-promoted dissolution of oxide surfaces is quantitatively similar to the mechanism of ligand exchange in solution (Casey and Ludwig, 1995; Ludwig and Casey, 1996). The rates of ligand-promoted dissolution and the rates of ligand exchange around dissolved metal complexes vary similarly, indicating that analogous processes may be involved. Accordingly, one would expect ligands that reduce the rate of water exchange around a dissolved metal complex to also inhibit dissolution of the metal oxide. For example, the rate of water exchange around many transition elements in outersphere M
+26. (SO-24)(aq) complexes are slower than the rate of exchange around the M(OH2)2+(aq) (Margerum et al., 1978). On the basis of this evidence, one would logically predict that the adsorption of sulfate would inhibit the dissolution of transition metal oxides. The effects of sulfate adsorption in inhibiting the dissolution of Fe oxides has been demonstrated by Biber et al. (1994) on hematite. A similar mechanism may be occurring in the presence of arsenate and chromate, and this pathway may complement the previous proposed mechanisms for goethite-inhibited dissolution in the presence of the oxyanions.
If the above explanation for the observed inhibition of goethite dissolution is valid, the increase in the rate of goethite dissolution observed for both arsenate and chromate at pH 6 must be explained. The surface complexation results indicated that the adsorption of both chromate and arsenate at pH 6 increases the net negative charge of the goethite surface (Table 2). This may be accompanied by an increase in the surface protonation due to constraints on the electric double layer. Unlike the oxalate promoted dissolution of -Al2O3, the dissolution of goethite requires protonation of an oxide or hydroxide ion adjacent to the removable Fe complex (Zinder et al., 1986). Oxalate adsorption drops precipitously as the pH is raised from 3 to 7 (Fig. 2). The reduced adsorption coupled with the decrease in the activity of protons is reflected in the decrease in dissolution rates as the pH increases (Table 3). At pH 6, the dissolution rate is two orders of magnitude less than at pH 5. We were unable to quantify a dissolution rate at pH 7. It is postulated that an increase in surface protonation accompanied by chromate or arsenate adsorption is responsible for the enhanced dissolution in the presence of the oxyanions. In the absence of the oxyanions, the rate of goethite dissolution in the presence of oxalate is low due to the relatively small amount of adsorbed oxalate, as well as the reduced proton activity. Although chromate and arsenate are better competitors for surface sites than oxalate, surface site densities of these oxyanions at pH 6 are less than the available surface sites calculated from adsorption isotherms (Mesuere and Fish, 1992a; Grossl et al., 1997). Therefore, there should be available sites for oxalate adsorption. This oxalate adsorption coupled with the increased protonation of the goethite surface enhances the dissolution relative to the dissolution in the absence of the oxyanions. At pH 6, the reduced rate of oxalate promoted dissolution of goethite, in the absence of the oxyanions, is a function of the reduced activity of protons rather than a reduction in the surface concentration of oxalate. Oxyanion adsorption increases surface protonation and results in an enhancement of the oxalate-promoted dissolution rate of goethite.
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Conclusions
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The oxalate-promoted dissolution rate of goethite was found to be inhibited in the presence of chromate and arsenate at all surface coverages and pH values investigated except pH 6. It is proposed that the type of surface complex formed by the oxyanions (mono- vs. binuclear) is not responsible for inhibition or enhancement of goethite dissolution. On the basis of the electrostatic data generated by the CCM and solution analogs, we propose that chromate and arsenate inhibit goethite dissolution by decreasing oxalate adsorption. This is accomplished because arsenate and chromate are more effective competitors for goethite surface sites than oxalate and upon adsorption increase the negative charge of the goethite surface. Furthermore, arsenate and chromate may behave similarly to other ligands and reduce the rate of water exchange around Fe in goethite-reducing dissolution rates. The oxalate-promoted dissolution of goethite at pH 6 is several orders of magnitude slower than at lower pH values due to a decrease in proton activity. It is proposed that the adsorption of chromate or arsenate increases the negative charge of the goethite surface that in turn increases proton adsorption. Since proton adsorption is necessary for the oxalate-promoted dissolution of goethite, adsorption of chromate or arsenate enhances goethite dissolution at pH 6 relative to oxalate alone.
The overall dissolution rates of mineral surfaces are influenced by the effects of organic ligands, acid and base dissolution, and anion or cation inhibitors. Understanding the mechanism of these reactions will allow for more accurate prediction of weathering rates in natural environments. In natural environments, organic ligands will compete with cations and anions as well as other organic ligands for surface sites. These competitive reactions may significantly alter (decrease or increase) organic ligand–promoted dissolution rates of minerals compared with rates measured in the laboratory. Whether a cation or anion accelerates or inhibits organic ligand promoted dissolution of mineral surfaces will depend on its effect on surface charge properties, the strength of the surface interaction, and the mechanism of ligand-promoted dissolution. Further studies using various mineral surfaces, common cations and anions found in natural systems, and different naturally occurring organic ligands are necessary to provide a more complete understanding of mineral dissolution rates in natural environments.Bargar Brown Parks 1997; Manceau Charlet 1994
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ACKNOWLEDGMENTS
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MJE would like to thank John Kelly (Sandia National Laboratory) for performing the BET surface area measurements, Joanie Hagler of the Louisiana State University Agricultural Center for the numerous ICP analyses, and would like to acknowledge the donors of The Petroleum Research Fund, administered by the American Chemical Society (PRF# 31331-G2,5), for support of this research.
Received for publication August 21, 1998.
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INTRODUCTION
