Active Sites and the Non-Steady-State Dissolution of Hematite

Only 40 h of data have been shown so that the details of the transients may be more clearly illustrated; actual sampling duration averaged 69 h with a...
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Environ. Sci. Technol. 1998, 32, 2871-2875

Active Sites and the Non-Steady-State Dissolution of Hematite SHERRY D. SAMSON* AND CARRICK M. EGGLESTON Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071-3006

Transient, non-steady-state responses of hematite dissolution rate to pH-jumps, from high to low pH, contain information about dissolution mechanisms and can be used to improve our understanding of dissolution processes operating under variable natural conditions. Our data show that, following each downward pH-jump, the hematite dissolution rate jumps up but then decays exponentially to a new steady state over a period of about 36 h. This requires that, after a pH-jump, the nature of the surface Fe sites themselves, and not only surface charge, gradually changes. Our results are consistent with the depletion of a reservoir of Fe sites active for dissolution on the hematite surface after a jump to pH 1, and show that such active sites can be reproducibly regenerated during returns to higher pH. We interpret the data with regard to long-standing crystal growth and dissolution models [e.g., Burton-Cabrera-Frank, BCF (Burton, W. K.; Cabrera, N.; Frank, F. C. Philos. Trans. R. Soc. London Ser. A 1951, 243, 299-358)] that assume the existence of “adsorbed nutrient” that is structurally distinct from metal centers in the solid surface structure. The general concept behind the model should be applicable to other minerals as well as hematite.

Introduction Mineral weathering plays a significant role in the chemistry of the hydrosphere, lithosphere, biosphere, and atmosphere; weathering and mineral dissolution have thus been the focus of intensive study by geochemists. Most mineral dissolution rate studies have been done under steady-state conditions, often far from equilibrium so that rates are independent of saturation state. This approach has allowed researchers to constrain the number of independent variables, thus simplifying investigation of mineral dissolution mechanisms. For example, clear relations between dissolution rate and the abundance of surface complexes (2) have been developed. Chemical processes in natural systems at the Earth’s surface, however, are subject to periodic (e.g., seasonal, diurnal) and transient perturbations. For example, acid mine drainage undergoes sudden pH changes as it mixes with more neutral water downstream (3); acid aerosol particles fall on more neutral water (4); and diel variations in Fe(II) and Fe(III) concentrations are observed in response to daily cycles of photosynthetically active radiation (5). If the chemical response to a change in conditions is fast, (e.g., on the order of minutes), then steady-state models may reasonably * To whom correspondence should be addressed. E-mail address: [email protected]; fax: (307)766-6769; phone: (307)766-3318. S0013-936X(98)00309-5 CCC: $15.00 Published on Web 08/27/1998

 1998 American Chemical Society

approximate variable natural systems, but if the chemical response is slow relative to the abruptness of the change (e.g., on the order of hours to days) then steady-state models may not accurately represent varying natural dissolution rates. Initial transients, a common feature of mineral dissolution experiments, show that the approach of dissolution rate to steady state takes time. Initially rapid dissolution rates that decay with time to a steady state have been reported for many oxide and silicate minerals: e.g., quartz (6), corundum and kaolinite (7), goethite and δ-Al2O3 (8), hematite (9), albite (10, 11), biotite (12), diopside (13), and enstatite (14). Nonlinear kinetics have been thought to be limited to the onset of dissolution and have been variously attributed to one or more of the following (depending on the solid, the experiment, etc.): artifacts of sample preparation (e.g., grinding) (14, 15), ultrafine particles (10), development of a leached layer (16), and nonstoichiometry of the unreacted surface (17). Pretreatments such as etching with HF or preconditioning (e.g., 3-4 days of preexperiment dissolution) have been employed specifically to avoid initial transients (18). Although ultrafine particles or grinding damage should be consumed during initial dissolution, recurrent transients in response to cycles in pH (4, 19) and increases in ligand concentration (8, 20) have been reported. This suggests that in some cases transients are neither artifactual nor random, but can be well-defined, reproducible phenomena. We are using pH-jump experiments to investigate the response of hematite (R-Fe2O3) dissolution rate to abrupt changes in pH. Our results suggest that after a pH-jump the nature of surface Fe sites, not only the surface charge, changes in a predictable way. The data are consistent with the depletion and regeneration of a reservoir of dissolution-active Fe surface sites in response to pH cycles. Such active sites can be thought of in different ways; we interpret the data with regard to long-standing crystal growth and dissolution models [e.g., Burton-Cabrera-Frank, BCF (1)].

Experimental Approach Theoretical Background. Using iron(III) (hydr)oxides as an example, in the absence of adsorbing ligands, a rate law for low-pH (proton-promoted) dissolution (2) can be written:

Ratediss ) kH[>FeOH2+]n

(1)

where Ratediss is in units of mol m-2 h-1, kH (h-1) is a rate constant for proton-promoted dissolution, [>FeOH2+] is the concentration (mol m-2) of adsorbed protons (> denotes bonds to a surface) in excess of net zero charge [pHpzc ≈ 8.5 for R-Fe2O3 (21)], and n is an integer corresponding to the number of protonation steps prior to detachment of the metal center, ideally equal to the valence of the metal ion, e.g., 3 for Fe3+ (18, 22). Equation 1 assumes steady state: i.e., constant surface area, maintenance of H+ adsorption equilibria at constant pH, and rapid reprotonation and regeneration of active sites (23). A constant ratio (mole fraction, xa) of active sites to total (active and less active) sites is assumed, and xa is often implicitly included in the rate constant kH with the understanding that dissolution is favored at a relatively small fraction of active sites with relatively low activation energies (22, 24). Although we recognize that eq 1 is not intended to apply to non-steady-state, it predicts that, in response to a sudden pH change from high to low, the dissolution rate will change as fast as protons can adsorb to the mineral surface. Proton adsorption reactions are fast, VOL. 32, NO. 19, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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on the order of 10s to 100s of milliseconds (25); hence, changes in dissolution rate should be equally fast. However, if the mole fraction of active sites also changes, perhaps slowly, after a change in pH, there will be a non-steady-state period before the distribution of surface sites reaches a new steady state, during which kH will be time dependent. BCF-type theories postulate the existence of “adsorbed nutrient”. During growth, for example, Fe3+ may adsorb from solution to a hematite surface and diffuse on the surface until it either desorbs or attaches to kink sites. Fe3+ that is “extracted” from, e.g., kink sites by depolymerization during dissolution may also remain on the surface as “adsorbed” Fe3+. By “adsorbed”, we mean surface Fe3+ that has fewer bonds to the surface than Fe3+ at kink sites and that may occupy surface sites that are distinct from the hematite structure. Fe3+ has an adsorption edge (pH at which 50% of the total adsorbate present is adsorbed) on R-SiO2 and R-Al2O3 between pH 2 and 4 (ref 26 and references therein). Thus, hypothetically, if there is a pH-dependent reservoir of “adsorbed” Fe3+ that may be filled or depleted during a transition from one steady state to another and if hematite is dissolving at pH g2 and the pH is suddenly reduced to 1, we conjecture that any adsorbed iron will desorb during approach to a new steady state. Here, we imply a conceptual connection between “active” sites and such adsorbed Fe. We use pH-jump experiments with hematite in a flowthrough reactor to test two hypotheses that follow from this conjecture: (1) Transition from one steady-state dissolution rate to another in response to a pH-jump takes much longer than required for proton adsorption, and (2) responses to these pH-jumps will be consistent with a reservoir of adsorbed or active Fe3+ sites. Reactor. The net dissolution rate in the continuous-flow stirred tank reactor (CFSTR) is

Ratediss ) (q/A)[Fe]

(2)

where Ratediss is in units of mol m-2 h-1, q is flow (L h-1), A is the hematite surface area (m2), and [Fe] is the concentration of Fe in the outlet solution (mol L-1). All of the Fe in solution is derived from hematite dissolution within the reactor cell; there is no detectable Fe in the inlet solutions. The response (Figure 1a) of a CFSTR to a step-function increase in inlet concentration is

y(t) ) a[1 - exp(-t/τ)]

(3)

where y(t) is the outlet concentration (mol L-1), a is the increase in “input” concentration (mol L-1), t is time (h), and τ is the reactor residence time (h) (27). Equation 3 assumes constant reactor volume and equal input and output flow rates. As stated, there is no detectable Fe in our inlet solutions; however, an instantaneous change in the dissolution rate within the reactor cell (Figure 1b) is mathematically equivalent to a step-function increase in inlet concentration, and y(t) should respond according to eq 3. Our pre-jump Fe input concentrations were negligible relative to post-jump concentrations in all cases, so a in eq 3 can be taken as the new steady-state output concentration at pH 1. Preparation of Hematite. Hematite was prepared according to a hydrothermal gel-sol method which yields hexagonal platelets 1-2 µm in diameter with no obvious internal porosity and virtually no goethite impurity (28). We detected no goethite by SEM or powder XRD, and a TEM study of hematite made by the gel-sol method mentioned no goethite impurity (29). The synthesized hematite was suspended in deionized water, centrifuged to select particles >1 µm in diameter, and the supernatant discarded. After three washings, the remaining hematite was dried overnight 2872

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FIGURE 1. (a) The response of a CFSTR to a step-function increase in inlet concentration is an exponential increase in outlet concentration. The x-axis is in units of τ, the residence time of the reactor. It takes approximately 5 residence times for the outlet concentration to equal the new inlet concentration. (b) In our experiments, all the Fe in solution is derived from the dissolution of r-Fe2O3 within the reactor cell. Thus, a sudden drop in pH accompanied by an equally sudden increase in dissolution rate is mathematically equivalent to a step-function increase in inlet concentration. in a 90 °C oven. The multipoint N2 BET specific surface area is 4.76 m2/g. Dissolution Experiments. The pH-jump dissolution experiments were conducted in a polycarbonate, waterjacketed, continuous-flow overhead-stirred tank reactor at 25 °C. CO2 was not excluded and the reactor was pH-statted (McIntosh Analytical Systems) using HNO3. A flow rate of approximately 1 mL min-1 was maintained, and cell volume ranged from 85 to 105 mL in separate pH-jumps and (5 mL within a single pH-jump. Effluent was filtered through a 0.2 µm cellulose acetate membrane and collected continuously in acid-washed polypropylene test tubes, 15 min/tube, with a fraction collector. In each of two experiments, a single sample of 0.75 g of hematite was cycled in pH. The hematite in the first experiment was aged for 98 days (solution changed periodically) in a 0.01 M NaClO4 solution (HPLC grade, Fisher Scientific) titrated to pH 4.5 with 1 N HCl (TraceMetal, Fisher). The hematite in the second experiment was aged for 22 days (solution changed periodically) in deionized water titrated to pH 4.5 with 1 N nitric acid (TraceMetal, Fisher). The aged powders were placed in fresh pH 4.5 nitric acid solutions prior to each experiment. Experiment 1 (10 days) included jumps from pHs 4.5 and 2 to pH 1 in the following sequence of pH: 4.5, 1, 2, 1. Experiment 2 (31 days) included jumps from pHs 6, 4.5, 3, and 2.5 to pH 1 (a pH sequence of 4.5, 1, 3, 1, 2.5, 1, 6, 1). For each pH-jump, the inlet reservoir was changed simultaneously with the pH change in the CFSTR [made with HNO3, for high to low jumps, or NaOH (reagent special grade, A. C. S., Spectrum), for low to high jumps]. Total dissolved Fe effluent concentrations were measured by flame atomic absorption, or by the Ferrozine colorimetric method (30), where greater sensitivity at low concentrations was necessary (e25 ppb). Colorimetric analysis for Fe2+ confirmed its absence in the effluent, indicating that no significant reductive dissolution had taken place.

FIGURE 2. Results of r-Fe2O3 pH-jump experiments. Time zero represents the time of the pH-jump. Each data series represents the outlet concentrations (mol L-1) normalized to surface area (m2 L-1) following a jump to pH 1 from initial pHs ranging 2-6. Pre-jump concentrations were negligible in all cases. The theoretical response to a step-function increase in dissolution rate is indicated by the solid line. Only 40 h of data have been shown so that the details of the transients may be more clearly illustrated; actual sampling duration averaged 69 h with a range 50-94 h.

Results and Discussion The outlet Fe concentrations following each downward pHjump (Figure 2) exhibited the same pattern: an initial peak within approximately 2 h followed by an exponential decay to a new steady state. This is not the simple exponential increase expected for a step-function increase in dissolution rate, implying that reactions other than proton adsorption affect the response. The initial transient is reproducible. Two separate pH 4.5 to 1 jumps show little difference. Four of the pH-jumps were done with a single sample of hematite cycled in pH, which implies that the Fe involved in the transients can be regenerated in a predictable way. These results strongly suggest that the observed transients are neither the consequence of initially damaged surfaces nor the result of preferential dissolution of ultra-fine particles. Finally, the time needed to reach a new steady state can be substantial (36 h or more), affirming our first hypothesis. It is reasonable to question whether the transients resulted simply from rerelease of Fe that was readsorbed or precipitated from solution following an upward pH-jump. Two lines of evidence suggest otherwise. First, the first pH-jump in each experiment (from pH 4.5 to 1) took place after aged hematite had been placed in fresh solutions, so the transients associated with these jumps could not have resulted from such precipitation. Second, in each of the other downward pH-jumps, the total excess Fe released in each transient (see discussion below) was one to 2 orders of magnitude greater than the amount of Fe in solution within the reactor cell at the time of the preceding upward pH-jump. To examine whether the Fe involved in the transient is consistent with adsorbed Fe, we integrate the area between the observed outlet concentrations and the outlet concentrations predicted by a step-function increase in dissolution rate (e.g., the solid line in Figure 2) to find the amount of “excess” iron released during the transient as a function of initial pH (Figure 3). Actual steady-state concentrations, which varied slightly with each jump, were used in these calculations rather than the average value used for illustrative purposes in Figure 2, and an interval of 23 reactor volumes (∼35 h) following the jump was arbitrarily selected as a common termination point for all six transients. Figure 3

FIGURE 3. The amount of “excess” iron released during the transient after each pH-jump in terms of surface coverage (nmol/m2 and percent) plotted against initial pH. A monolayer is 6940 nmol/m2 (34). shows that the amount of excess Fe released is pH dependent in a way similar to the pH dependence of Fe3+ adsorption (ref 26 and references therein), supporting our second hypothesis. Such consistency by itself does not prove the existence of adsorbed Fe as opposed to a form of labile surface Fe in, e.g., a precipitated form or an altered layer. Observations of slow proton uptake by iron oxides during acid-base titration (e.g., refs 21 and 31) have been interpreted as formation of a goethite-like surface layer (31); however, more recent work was unable to confirm such a layer, and made alternative suggestions (32). Structural investigation will be the subject of another paper. It should be noted that the total amount of Fe released in these experiments is very small. In experiment 2 (a single sample of hematite cycled through four pH-jumps), the total excess Fe released was less than 70% of one monolayer, and the total Fe released, excess and steady state, was ∼160% of one monolayer. In previous experiments in which even more total dissolution occurred, there was no increase in specific surface area as measured by BET methods. VOL. 32, NO. 19, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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We postulate the existence of two forms of dissolutionactive Fe surface sites: (1) Fe at the surface of the hematite structure that is active for dissolution at steady state, constantly regenerated during dissolution, and constant over time; and (2) adsorbed (by definition, in excess of the dissolution-active Fe at steady state) Fe that can be desorbed (consumed) after a high-to-low pH-jump. One way to visualize the postulated steady state and excess active Fe sites is as follows: Fe dissolving from the hematite surface must undergo a series of depolymerization reactions wherein the bonds to lattice oxygens are replaced by bonds to hydroxyls or water. This might correspond to a sequential progression of the metal center from the bulk mineral to the following surface sites: (1) face, (2) step, (3) kink, (4) ledge, and (5) adatom (33). Although there are distinct activation energies for each type of surface site, Monte Carlo simulations favor reaction mechanisms wherein one type of surface complex accounts for the overall rate at steady state (33). Our observations suggest that, under non-steady-state conditions, the identity of the rate-determining surface complex and the mole fraction of active sites may change with time. We suggest that in a high-to-low pH-jump the rapid dissolution of the most active sites will determine the initial rate. Easily dissolved Fe will be depleted as the regeneration of the most active sites fails to keep pace with their dissolution. The surface will eventually be residually enriched in less active sites, and overall steady state will be reached when the relative amounts of the various surface sites have reached a steadystate distribution at the lower pH. Upon the return to a higher pH, the Fe in solution will be readsorbed (the extent of this readsorption will depend on the new pH). In addition (our results suggest dominantly), the “reservoir” of excess Fe may be replenished from depolymerization reactions which regenerate active sites through extraction of Fe from the hematite surface (e.g., kink sites?) into the more labile adsorbed state (e.g., surface complexes with lower lattice coordination such as ledges and adatoms). This model can be expressed as

Ratediss ) k([>Fessi] + [>Feexcess])

(4)

where Ratediss is in units of mol m-2 h-1, k (h-1) is a first-order rate constant, [>Fessi] is the concentration (mol m-2) of Fe surface sites active for dissolution at steady state at pH i (assumed constant), and [>Feexcess] is the concentration (mol m-2) of the postulated excess Fe sites. We treat dissolution, in a first approximation, as a first-order consumption of excess Fe and a pseudo-zeroth order steady state. We also, in a first approximation, assume a single k for both steady state and excess Fe dissolution. Integration of the rate law gives the concentration of excess Fe surface sites active for dissolution as a function of time (mol m-2):

[>Feexcess] ) [>Feexcess]o exp(-kt)

(5)

The change in the amount of Fe released from hematite within the reactor, as a function of time, causes a in eq 3 to become time dependent. A time-dependent term, [a](t), in moles per liter, can be derived from eqs 2, 4, and 5:

[a](t) ) (A/q)(k)[[>Fessi] + [>Feexcess]o exp(-kt)] (6) Substituting [a](t) into eq 3 couples the rate law (eq 4) to the reactor eq 3 and predicts the Fe outlet concentration (mol L-1) as a function of time, [Fe](t):

[Fe](t) ) (A/q)(k)[[>Fessi] + [>Feexcess]o exp(-kt)][1 - exp(-t/τ)] (7) 2874

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FIGURE 4. Model fits with k ) 0.21 h-1. (a) pH 4.5 to 1, replicate 1, (b) pH 2.5 to 1.

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FIGURE 5. The rate constant, k, displayed a tendency to increase with increasing initial pH. Curved lines indicate the 95% confidence interval. This is a double exponential, i.e., exponential chemical response convolved with exponential reactor response. At infinite time, the outlet concentration equals the new steadystate outlet concentration. At steady state, [>Feexcess] ) 0, and eq 4 reduces to (steady state) Ratediss ) k[>Fess1]. Using the k (0.21 h-1) obtained by least-squares fitting of eq 7, and a site density of 4.18 Fe sites nm-2 for the {001} parting plane of hematite (6.94 × 10-6 mol m-2 ) (34), we find [>Fess1] = 2% of Fe surface sites. [>Feexcess]o varies with initial pH, from a high of approximately 16% for an initial pH of 6 to a low of approximately 3% for an initial pH of 2. Of course, site density will vary from crystal face to crystal face, so these are approximations. These surface concentrations are low enough to be consistent with, but do not prove, a form of adsorbed Fe that is also active for dissolution. The model shows a reasonably good fit to the data (Figure 4). We used a single value for the rate constant to fit each pH-jump, but in some cases (e.g., Figure 4a) a slightly larger value for k would provide a better fit to the initial peak whereas

a slightly smaller k would fit the exponential decay better. The individual rate constant for each pH-jump, obtained by fitting to each data set, appears to trend upward with increasing initial pH (Figure 5). This trend suggests that more than one form of excess Fe site may exist, each described by a different k and/or mole fraction. Clearly, other models could also be used, e.g., under nonsteady-state conditions, a surface complexation approach to proton-promoted dissolution (eq 1) may simply be expanded to include the Fe surface site itself, not only the protons adsorbed to it, in the overall rate law. The important point is that non-steady-state experiments can provide a “kinetic window” into reaction mechanisms not accessible through steady-state experiments. We have shown that transitions from one steady state to another require time, and that the transient Fe release behavior in response to pH-jumps is consistent with a form of dissolution-active surface Fe that can be removed at low pH and regenerated at high pH. The general approach should be valid for other oxides as well.

Acknowledgments We thank NSF for funding under Grant EAR-9527031 to C.M.E., S. Boese for analytical assistance, L. Stillings for helpful discussions throughout the course of this work, and two anonymous reviewers whose comments greatly improved the manuscript.

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Received for review March 27, 1998. Revised manuscript received June 1, 1998. Accepted June 15, 1998. ES9803097

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