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On the thermodynamics and kinetics of scorodite dissolution
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ScienceDirect Geochimica et Cosmochimica Acta 265 (2019) 468–477 www.elsevier.com/locate/gca
On the thermodynamics and kinetics of scorodite dissolution Xiangyu Zhu a, D. Kirk Nordstrom b, R. Blaine McCleskey b, Rucheng Wang c Xiancai Lu d, Siliang Li a, H. Henry Teng a,⇑ a
Institute of Surface-Earth System Science, Tianjin University, Tianjin 300072, China b US Geological Survey, 3215 Marine St., Suite E. 127, Boulder, CO 80303, USA c State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China d Key Laboratory of Surficial Geochemistry, Ministry of Education, School of Earth Sciences and Engineering, Nanjing University, Nanjing, Jiangsu 210023, China Received 8 May 2019; accepted in revised form 6 September 2019; available online 13 September 2019
Abstract Scorodite (FeAsO42H2O)–water interaction is critical to As distribution and storage in surface environment but is inadequately understood due to ambiguities in the mineral’s stability and weathering rate at atmospheric conditions. In the present study we attempted to experimentally determine the thermodynamic and kinetic parameters needed to compute solubility and dissolution rate of scorodite at 25 °C. Experiments were carried out in low pH (1.15) solutions using specially synthesized large scorodite crystals (with Mean Diameter of 27.9 lm). Such experimental conditions ensured the results were not subject to the influence of secondary Fe-bearing phase precipitation and grain size effect. Measured equilibrium concentrations of Fe and As, along with newly published Fe-As complexes association constants, were first used to determine the solubility products Ksp and dissolution rates rn at 50–90 °C. The obtained Ksp T and rn T dependence was then used to derive the Gibbs free energy, enthalpy, entropy, heat capacity, and activation energy for the dissolution reaction. Finally, we extrapolated the measurements to 25 °C and obtained room temperature solubility and dissolution rate, scrutinized the pH effect on dissolution, and analyzed the DG rn relation of the dissolution reaction. Our results show that literature data are likely overestimated scorodite solubility at pH > 4 4.5 due to neglecting the effect of ferric iron hydrolysis. Estimated ambient condition dissolution rate is an order magnitude lower than the earlier report, implicating the importance of size effect, but is one to two orders of magnitude higher than that of common rock-forming minerals, cautioning the proposed use of scorodite for As fixation and storage. The determined rn DG relation cannot be fully fitted by the transition state model, particularly at near equilibrium, suggesting dissolution in this study may be controlled by defect-assisted surface reactions. Ó 2019 Elsevier Ltd. All rights reserved. Keywords: Arsenic; Scorodite; Thermodynamics; Kinetics; Transition state theory
1. INTRODUCTION Arsenic is a highly toxic metalloid and a world-wide concern as a human carcinogen. Indiscriminate use of arsenical pesticides during the early to mid-1900s has led to extensive contamination of soils worldwide (Smith ⇑ Corresponding author.
E-mail address: [email protected] (H.H. Teng). https://doi.org/10.1016/j.gca.2019.09.012 0016-7037/Ó 2019 Elsevier Ltd. All rights reserved.
et al., 1998). The continuous use of groundwaters with As levels above 10 ppb (current drinking water limit sanctioned by the World Health Organization) for irrigation purpose further aggravated the pollution in many regions of the globe (Berg et al., 2001; Nordstrom, 2002; Fendorf et al., 2010; Smedley and Kinniburgh, 2013; Maghakyan et al., 2017). Geochemical cycling of arsenic at surface conditions is largely controlled by As-forming and As-containing miner-
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als through dissolution, precipitation, incorporation, sorption, and desorption (Fendorf et al., 1997; Ford, 2002; Bostick and Fendorf, 2003; Amirbahman et al., 2006; Benzerara et al., 2008; Deditius et al., 2008; Catalano et al., 2011; Li et al., 2011; Zhu et al., 2014; Karimian et al., 2017; Sowers et al., 2017; Catelani et al., 2018). Of those minerals, scorodite deserves special attention because it is one of the least soluble As phases and a pivotal secondary As-bearing mineral in acidic Fe(III)-As(V)-H2O systems, such as arsenic-contaminated soil and mine wastes (Drahota and Filippi, 2009). The low solubility is in fact the basis of the suggested arsenic fixation strategy in mining industry (Riveros et al., 2001; Langmuir et al., 2006). The dissolution behavior of scorodite has been studied extensively for the past three decades (Dove and Rimstidt, 1985; Nordstrom and Parks, 1987; Robins, 1987; Krause and Ettel, 1988; Bluteau and Demopoulos, 2007; Majzlan et al., 2012). While the results laid the groundwork for understanding scorodite stability in aqueous environment, critical discrepancies exist concerning the measured mineral solubility (Ksp, reaction (2)) that varies by five orders of magnitude (1020.24±0.58 to 1025.68±0.52) from case to case (Chukhlantsev, 1956; Nordstrom et al., 2014). A number of issues may be factored into the literature Ksp’s inconsistency for scorodite. (1) The effect of crystal size on solubility. It is well-known that, due to the Gibbs-Thompson effect, solubility of small particles increases with increasing curvature (reciprocal of particle radius) of the dissolving grains (Cabrera et al., 1954; Lasaga and Blum, 1986; Teng, 2004; Navrotsky, 2004; Fan et al., 2006; Navrotsky et al., 2008). Natural scorodite is usually small (mostly nano-sized) and, despite repeated laboratory attempts, synthesized scorodite crystals rarely exceeded 1 lm in diameter (Dutrizac and Jambor, 1988; Baghurst et al., 1995; Demopoulos et al., 1995; Bluteau and Demopoulos, 2007; Gonzalez-Contreras et al., 2010; Majzlan et al., 2012; Okamura et al., 2013; Kitamura et al., 2015; Okibe et al., 2017). The only known method to produce large (up to tens of micrometers) scorodite crystals was not developed until 2008 (Fujita et al., 2008), restricting earlier work to the use of submicron sized particles. As such, it is conceivable that the wide scatter in the reported Ksp values may have at least partially originated from the crystal size effect. (2) The formation of secondary Fe-containing minerals during dissolution. Due to the very low water solubility of ferric iron hydroxide (Ksp1039), incongruent dissolution (i.e. precipitation of Fe(OH)3) will take place when scorodite dissolves at pH > 2–3. For those studies where pH was not maintained sufficiently low (see Section 4.3 for details), significant errors may have occurred in computing the thermodynamic driving force of the overall reaction (Demopoulos, 2005). (3) Overlook of certain solution chemistry effect. This concern stems from the observation that activity coefficients and ion pairs such as Fe-As complexes and Fe-Cl or Fe-SO4 complexes were rarely considered in previous studies that acquired Ksp measurements. On a related note, while significant progress was made in the past three decades to deduce the mechanisms and surface processes of mineral dissolution through quantifying relations between dissolution kinetics and solution free
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energy change on the basis of Transition State Theory (Eyring, 1935a,b; Glasstone et al., 1941; Connor et al., 1979; Lasaga, 1981; Aagaard and Helgeson, 1982; Lasaga, 1998; Lu¨ttge, 2006), step wave model (Lasaga and Luttge, 2001), and dislocation theory (Brantley et al., 1986; Lasaga and Blum, 1986; Teng, 2004), little is known in the applicability of such advance to scorodite dissolution. To the best of our knowledge, only two studies touched upon the aspects of dissolution kinetics. The first (Harvey et al., 2006) reported an empirical rate law while the second (Bluteau and Demopoulos, 2007) examined the pH and temperature effect on apparent dissolution rate (i.e. without normalization by the sample’s surface area), and none of them considered the thermodynamic effect (i.e. saturation state) on dissolution kinetics, nor the actual physical mechanisms of scorodite dissolution. In the present study, we attempt to quantify scorodite dissolution using specially synthesized large (28 lm on average, Figs. 2 and 3) crystals and low pH (1.15) solutions to eliminate particle-size effect and secondary Fe-containing phase precipitation. The experimental results are used to extract activation energy, enthalpy, entropy, and heat capacity of scorodite dissolution reaction, and to provide mechanistic insight into the dissolution process based upon the relationship between dissolution kinetics and free energy change. 2. METHODS 2.1. Scorodite synthesis and characterization Ambient condition synthesis of scorodite in the Fe(III)As(V)-H2O system is proven difficult (Dutrizac and Jambor, 1988). We used a modified Fujita et al. (2008) method where experimental solutions of 0.3 mol/L As (V) and 0.3 mol/L Fe (II) (prepared using analytical grade Na2HAsO47H2O and FeCl24H2O and adjusted to pH = 1.5 using12 M HCl) were first heated to boiling (T = 96.1– 96.3 °C) state, followed by slow Fe (II) oxidization via air (instead of oxygen) bubbling. The setup (a 1-litre conical flask attached to a condenser pipe) was sealed completely for 7 days, and the resulting precipitates were subsequently collected (via filtration using a 0.45 lm filter) and thoroughly washed (by dilute HCl solution at pH = 1.5). The final product was air dried and stored in a sealed opaque glass bottle. The synthesized products were characterized by scanning electronic microscope (SEM) (ZEISS SUPPA 55) with an oxford INCA energy dispersive spectroscopy (EDS), xray diffractometer (Rigaku D/max III, Cu ka radiation, 40 kV and 20 mA at 0.02°/0.3 s per step from 3° to 70° 2h), and thermogravimetric analysis (PerkinElmer Pyris 1 TGA). The surface area of the crystals (Se) was experimentally determined by the BET method using N2 adsorption (Quantachrome Autosorb-iQ2-XR with outgassing T of 105 °C) and further computed using particle size distribution measured by laser diffraction (Malvern Mastersizer 3000) via P 6 Vd ii 6 Se ¼ P ¼ ð1Þ q V i qDS
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where Vi is the relative volume by particle size class di, q the material density (3.27 g/cm3 for scorodite), and Ds the Sauter diameter (mean particle size assuming spherical geometry). 2.2. Dissolution experiments The dissolution chamber consisted of two 500 ml bottles each attached onto a submersible stirrer (Supplementary material, Fig. S1). The whole set-up was immersed into a water bath where the temperature was maintained at desired settings with a variability of ±0.1 °C via a LaudaBrinkmann thermostats (Model RCS 6). Ten grams of scorodite were enclosed in a double-layered 18 lm polyester mesh bag and suspended in diluted HCl solution to prevent potential physical abrasion to the crystals from the magnetic bars spinning that at pre-set rates of 400–500 rpm. The pH of the dissolution solutions was maintained at 1.15 (±0.05) throughout the experimental duration. Two sets of data were obtained in this study, a short term one collected in the initial 4–5 h of the experiments and the other at long-term upon the establishment of a steady state. Dissolution experiments ran parallel in the two 500 ml bottles at 50, 70, and 90 °C, respectively. The scorodite bags were immersed in the solution only after the desired temperature was reached and stabilized. An aliquot of 1 ml experimental solution was sampled every 15 min to 1 h in the first 6 h and every 1 or 2 days after until a steady-state was reached. The sampled solution was filtered through a 0.45 lm membrane and diluted 100% by 1% HNO3 solution for subsequent Fe and As analyses. 2.3. Solution chemistry analyses Solution pH was measured using a Thermo Scientific 815600 Orion Ross combination electrode and an Orion STAR A325 pH meter. The electrode was calibrated using the pH 1.00 and 1.68 standards (Geotech, NIST traceable)
thermally equilibrated with the experimental solutions. The As and Fe contents in the solutions were routinely determined by Inductively-coupled plasma-optical emission spectrometry (ICP-OES, Perkin-Elmer 7300 DV) using the characteristic wavelengths of 188.979 and 193.696 nm for As (detection limit 0.03 mg L1) and 238.204 and 259.939 nm for Fe (detection limit 0.002 mg L1). Typical relative standard deviation of the measurements is 5% for As and 2% for Fe. For solutions with As in the range of 0.001 mg L1 (below the ICP-OES’s detection limit), a hydride generation atomic absorption spectrometer (HGAAS, Perkin–Elmer AAnalyst 300 equipped with a FIAS– 100 flow injection system and a quartz cell) capable of detecting As at 0.0001 mg L1 levels was employed. The relative standard deviation of the HG-AAS analyses in this study was 2%. 3. EXPERIMENTAL RESULTS 3.1. Mineral characterization The synthesized crystals show a pale-yellowish green color with a garlic odor. Sharp peaks in the XRD spectra match well with the standard pattern in the PDF database (NO. 70-0825) with no identifiable minor phases found in background, confirming the crystallization of scorodite (Fig. 1). SEM-EDS analysis further reveal the absence of any meaningful chemical impurities in the crystals. Multipoint fitting BET surface area of the crystals is approximately 0.43 m2/g, comparable to the calculated value of 0.11 m2/g using particle size distribution data (Fig. 2). SEM microphotography of the harvested crystals show mostly pyramidal or pseudo-octahedral shapes with sizes ranging from 10 to 30 lm (Fig. 3), well consistent with the direct measurement (with mean particle size of 27.9 lm) and the BET surface area-based estimation (Ds = 17.3 lm). EDS analyses further confirm that the Fe to As ratio is close to the ideal stoichiometric value of unity.
Fig. 1. An XRD spectrum of the synthesized scorodite in comparison to the standard pattern in the PDF database.
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Fig. 2. Size distribution of the synthesized scorodite crystals. Listed values indicate the sizes in 10, 50, and 90 percentile along with the mean and the Sauter diameter.
Fig. 3. SEM images showing the pyramidal or pseudo-octahedral crystal morphology of the synthesized scorodite.
TGA curves indicate that the onset of weight loss in the synthesized scorodite occurs at 168 °C (Fig. 4), nearly identical to that reported by previous studies (Bluteau and Demopoulos, 2007; Le Berre et al., 2008; GonzalezContreras et al., 2010). The total weight loss amounts to 15.5%, very close to the theoretical water content of 15.6% in scorodite. 3.2. Mineral dissolution behavior As expected, the accumulated As and Fe concentrations increased with T and over time for both short (Fig. 5a) and long term (Fig. 5b). Steady states were reached within 30 days for all the temperatures. The experiment at 70 °C failed to reach completion due to apparatus malfunction at day 15 and the steady state was extrapolated from the trend shown in the 50 and 90 °C experiments. This method may introduce an estimated 1% uncertainty in the final solubility product obtained at 70 °C. Evaporation encountered at 50 and 70 °C was insignificant but became non-negligible at 90 °C. Accordingly, the data collected at 90 °C was corrected using an evaporation model (See supplementary S-1).
4. DISCUSSION 4.1. Thermodynamic analysis The Fe to As ratio maintained close to unity during the experiments indicating that the dissolution proceeded stoichiometrically. Assuming the reaction FeAsO4 2H2 O ¼ Fe3þ þ AsO3 4 þ 2H2 O
ð2Þ
held true and reached steady states in our experiments, the solubility product of scorodite was computed using the Fe and As concentrations averaged over the last 3 measurements of the experiments (121.3, 149.6, and 345.5 lmol/L for 50, 70, and 90 °C respectively), corrected by activity coefficients estimated from the Hu¨ckel equation (also known as the B dot equation. Hu¨ckel, 1925; Helgeson, 1969. Supplementary material, S-2) via the computational code of PHREEQCI after taking into consideration the recently published association constants for Fe (III)-As(V) and Fe (III)-Cl complexes (Zhu et al., 2017). Such obtained values of Ksp fit nicely to the following equation (Fig. 6):
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Fig. 4. TGA analysis showing the thermally driven weight loss in the synthesized scorodite.
Fig. 6. Dependence of calculated scorodite solubility product (log Ksp) on temperature (1/T). The solid line is a fit to Eq. (3) and the open circle is the extrapolated value at 25 °C.
where DrG, DrCp, DrS*, DrH* are the Gibbs free energy, heat capacity, entropy (0 K), and enthalpy (0 K) of the reaction, and R the gas constant (see derivation in Supplementary material, S-3), one can readily see that the heat capacity of the dissolution reaction can be regarded constant between 50 and 90 °C. In addition, a value of Ksp = 1025.81 ± 0.1 at 25 °C can be extrapolated from the fit. This value agrees well with that (1025.83 ± 0.07) given by Langmuir et al. (2006) and the recent evaluation (1025.68 ± 0.52) by Nordstrom et al. (2014). Moreover, Eq. (4) can be used to derive Gibbs free energy, enthalpy, entropy, and heat capacity (Table 1) for reaction (2). 4.2. Kinetic analysis Scorodite dissolution rates in present study were calculated following the methods reported by Chermak and Rimstidt (1990), Lasaga (1998), and Harvey et al. (2006). Briefly, the measured concentration (C) versus time (t) are first fitted to a second order polynomial equation of the form C ¼ a þ bt þ ct2
ð5Þ
where a, b, and c are fitting constants. Dissolution rate (r) is then estimated by the differential form of Eq. (5): Fig. 5. Time dependent variations of arsenic concentration in the first 4 or 5 h of scorodite dissolution experiment (a) and long-term dataset showing both arsenic and iron concentrations up to 38 days (b). Solid line is second order polynomial fittings for eye guidance.
log Ksp ¼ A1 þ A2 =T þ A3 log T
ð3Þ
where A1, A2, and A3 are constants and T the absolute temperature. Given the resemblance of Eq. (3) to the thermodynamic relation ln Ksp ¼
Dr G Dr S Dr C p Dr H Dr C p þ ln T ¼ R RT R RT
ð4Þ
r¼
dC ¼ b þ 2ct dt
ð6Þ
Final, the normalized rate rn is obtained by the following equation rn ¼
rV sol Asp mscor
ð7Þ
where Vsol (0.00055 m3 in present study) is the volume of solution, Asp (BET surface area, 0.43 m2 g1) the specific surface area of the scorodite, and mscor (10 g) the mass of scorodite.
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Table 1 Estimated Gibbs free energy, enthalpy, entropy, heat capacity, and solubility product of scorodite dissolution shown as reaction (2) at 298.15 K and in the temperature range of 298.15 K < T < 363.15 K. DrG (kJ mol1) DrH (kJ mol1) DrS (J K1 mol1) DrCp (J K1 mol1) log Ksp
298.15 K
T (298.15–363.15 K)
147.32 ± 0.57 71.53 ± 1.94 735.3 ± 6.9 1421.8 ± 67.3 25.81 ± 0.1
–495.43 + 10.258 T–1.4218 T ln T 495.43 + 1.4218 T 8836.0 + 1421.8 ln T 1421.8 ± 67.3 535.8 + 25878/T + 171 log T
The initial (i.e. when the solutions were free of As and Fe) dissolution rates in the experimental temperature range (50–90 °C) increased from 1010.3 to 108.9 mol m2 s1 with increasing temperature. The linear relation of the initial rates (ln rn, derived from the dataset shown in Fig. 5a) to 1/T (Fig. 7) suggests an Arrhenius behavior rn ¼ AeEa =RT , where A is a pre-exponential factor in mol m2 s1 and Ea the activation energy in kJ mol1) for scorodite dissolution, consistent with previous findings (Baghurst et al., 1995; Bluteau and Demopoulos, 2007). Furthermore, the value of Ea = 77.2 kJ mol1 extracted from Fig. 7 closely matches that (77 kJ mol1) reported at a similar pH (1.1) (Baghurst et al., 1995), but significantly differs from those acquired at high pHs (61 kJ mol1 at pH = 6 and 100 kJ mol1 at pH = 7–9) (Bluteau and Demopoulos, 2007), presumably indicative of the effect of secondary Fe-containing phase formed at neutral to alkaline conditions. The room temperature (25 °C) dissolution rate extrapolated from the experimental data using the Ea value comes to be log rn = 11.3 mol m2 s1, more than one magnitude lower than that (log rn = 9.86 mol m2 s1) acquired by Harvey et al. (2006). Given the similar temperature (22 vs. 25 °C) and milder pH (2 vs. 1.15) but more than 20-fold greater (9.5 vs. 0.43 m2/g) mineral surface area in Harvey et al. (2006) experiment, we argue that the rate in our dataset is likely free of the grain-size effect widely observed during crystal dissolution (Briese et al., 2017).
Thermodynamic effect on scorodite dissolution was examined by considering the dependence of rate on the free aFe3þ aAsO3 4 energy state DG ¼ RT ln of the experimental K sp solutions. Plots of rn vs. DG revealed non-linear dependence for all experiments (Fig. 8). Far from equilibrium, for example, when DG < 15 kJ mol1 for the experiment at 50 °C, the rn vs. DG curve was dominated by a plateau (Nagy and Lasaga, 1992; Hellmann and Tisserand, 2006) where the rate varied little with respect to solution saturation state; as the solution evolved to intermediate undersaturation states (15 < DG < 5 2 kJ mol1), a sharp decrease in dissolution rate (one and two orders of magnitude for 50/70 °C and 90 °C experiments, respectively) occurred. Close to and very-near equilibrium (0 > DG > 2 5 kJ mol1), the rate decrease slowed down and exhibited a somewhat linear dependence on DG. The relation between dissolution rate and solution DG is in general described by: rn ¼ k f ðDGÞ
ð8Þ
where f ðDGÞ ¼ 0 at equilibrium and f ðDGÞ ¼ 1 at far-from equilibrium (D G ! 1, rate stabilizes at k). The forms of f (DG) vary but can be mathematically derived from three models: (1) the transition state theory (TST, Eq. (9)) (Lasaga, 1981; Aagaard and Helgeson, 1982; Helgeson et al., 1984); (2) TST modified by surface defects (Eq. (10)) (Morse, 1983; Lasaga, 1998; Teng, 2004), and (3) a sigmoidal formula (a combination of (1) and (2), Eq. (11)) (Burch et al., 1993; Hellmann and Tisserand, 2006; Xu et al., 2012): f ðDGÞ ¼ ð1 eDG=RT Þ ð9Þ DG=RT n ð10Þ f ðDGÞ ¼ 1 e m1 m 2 f ðDGÞ ¼ ðk 1 =kÞ 1 eðnDG=RT Þ þ ðk 2 =kÞ 1 eDG=RT ð11Þ
Fig. 7. Plot of scorodite dissolution rate (ln rn) vs. temperature (1/ T) showing the Arrhenius behavior.
The parameter n is an apparent reaction order, k the rate constants, and m adjustable factors, all obtained as fitting constants in practice. Applying the two mechanistic models (Eqs. (9) and (10)) to our data (Fig. 8) shows that the TSTbased rate law significantly overestimates the measurements when DG < 10 kJ mol1, whereas the surface-defect equation gives a much more accurate description of the experimental results even very-near equilibrium. The empirical sigmoidal model, as expected, fits the data with high fidelity within the entire experimental saturation range. The finding that the defect-based model gives a more satisfactory approximation to the experimental measure-
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strain energy, etch pits can only form at the high energy sites, rendering a strong negative dependence of dissolution rate on DG. Finally, when close to and very-near equilibrium and |DG| is smaller than the energy barrier for defect-assisted pit nucleation, the dissolution only has a weak dependence on DG as the surface reaction is limited at pre-existing step sites. It appears that the TST model’s overestimation in dissolution rate increases with increasing temperature. For example, at |DG| = 2 kJ mol1, the discrepancy between TST-predicted and measured rates is a factor of 3 at 50 °C (Fig. 8a) but stretches to over an order of magnitude when temperature increases to 90 °C (Fig. 8c). A clear interpretation for this observation is not readily available. One explanation may be the limitation of the TST model when applied to particle detachment/attachment to solid surfaces in solution systems (Joswiak et al., 2018). The original TST theory was developed for gas particle collision reactions where the rate constant is expressed as the product of the thermal vibrational frequency v = kBT/⁄ (⁄ is the Planck’s constant) and the partition function of the activation complex (Lasaga, 1998). For heterogeneous reactions in solution, however, it is shown that the frequency term needs to incorporate solute diffusion and the geometry of the activation complex (Hill, 1975, 1976). Hence it is possible that the actual dependence of dissolution rate on T is weaker than that depicted by the linear relation of v = kBT/⁄. 4.3. Predicted pH effect on scorodite dissolution
Fig. 8. Scorodite dissolution rate (rn) vs. the corresponding Gibbs free energy difference (DG) at 50 (a), 70 (b), and 90 (c) °C. Solid circles: measured rates; dotted lines: TST fitting; dashed lines: surface-defect fitting; solid lines: sigmoidal fitting.
ments can be interpreted in the context of dislocation theory (Burch et al., 1993; Bandstra and Brantley, 2008; Xu et al., 2012). It is well understood that mineral dissolution preferentially occurs at high energy sites such as defects and dislocations where lattice strain occurs (Holdren and Berner, 1979; Helgeson et al., 1984). Far-from equilibrium, when |DG| is greater than the melting enthalpy of crystals, dissolution is controlled by spontaneous 2D surface nucleation of etch pits (Brantley et al., 1986; Lasaga and Blum, 1986; Teng, 2004) regardless further changes in DG. At intermediate equilibrium when |DG| is comparable to the
While scorodite dissolution may proceed as described by Reaction (2), the subsequent Fe3+ hydration (e.g. Fe3+ + H2O ? Fe(OH)2+ + H+) becomes non-negligible when pH > 3. The removal of ferric iron not only changes the solution supersaturation but affects aqueous As concentration due to the adsorption of arsenate on the resultant iron hydroxide. Such pH effect can be quantitatively assessed using the Ksp value and other thermodynamic parameters obtained in this study. Plotting literature data of scorodite dissolution on our calculated solubility to pH relation shows a reasonable agreement at pH < 3, but reveals one to two order of magnitude difference at high pHs (Curve A, Fig. 9). When ferrihydrite precipitation is taken into consideration, however, the reported measurements become significantly closer to the prediction (Curve B, Fig. 9), indicating that majority of the early lab data were likely subject to the effect of ferrihydrite precipitation. If goethite instead of ferrihydrite is considered, the predicted release of As overestimates the reported values when pH > 2.5 (Curve C, Fig. 9) but closely matches our data of As in a field collected acid mining drainage (AMD) solutions (pH 3, the chemical analysis of AMD was provided in the Supplementary material, S-4). Demopoulos (2005) detected the presence of highly metastable nano-sized 2-line ferrihydrite phase associated with scorodite dissolution in laboratory. In long term, however, ferrihydrite will give rise to the more stable phase goethite, a common occurrence in natural environment (Cornell et al., 1989; Hamzaoui et al., 2002; Cudennec and Lecerf, 2006). Thus, it appears
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Fig. 9. Scorodite solubility (shown as arsenic concentration) at different pH (25 °C): comparison among congruent dissolution calculation, incongruent dissolution calculation, and experimental measurement; A: scorodite congruent dissolution curve; B: reaching equilibrium with ferrihydrite during scorodite dissolution; C: reaching equilibrium with goethite during scorodite dissolution. Open and cross symbols are literature data and solid circles show the field collected acid mine drainage data in this study. The solubility products of ferrihydrite and goethite are derived from the thermodynamic properties reported by Navrotsky et al. (2008, see supplementary material S-6).
that the scorodite + goethite system may be more fitting for field settings. Finally, we estimated As adsorption using surface complexation model (see supplementary material for detailed parameter, S-5) and found that approximately 10% and 2% of the total arsenic may be surface-bound on ferrihydrite and goethite, respectively. These values are noticeable but likely insignificant if iron hydroxide is derived solely from scorodite dissolution. 4.4. Implications for environmental consequences Scorodite dissolution rate at Earth’s surface conditions estimated from the dataset reported in this study is on the order of 1011–1012 mol/m2/s. This rate is nearly 1–2 order of magnitude higher than those reported for common silicate minerals (e.g. K-feldspar, kaolinite) in ambient environment (Brantley et al., 2007). Assuming scorodite (grain size > 5 lm) is the main storage of As in a porous geological medium (e.g. aquifer) that has an average water content of 15% (w/w) and a background As concentration of 15 ppm (Smith et al., 1998), dissolution at this rate (log rn = 11.3 mol m2 s1 at 25 °C) would render initially As-free water to one with arsenic concentration surpassing the 10 ppb threshold value within 17 h. In reality this during could be significantly shorter as scorodite dissolution in natural settings is likely much (up to an order of magnitude) faster due to the small crystal sizes. This understanding, even taking into consideration of As adsorption by existing ferric oxyhydroxide or/and other minerals in nature, suggests that cautions need to be exercised when scorodite is used for As immobilization as natural waters in contact with the arsenic storage media can quickly (within days) become polluted.
5. SUMMARY Scorodite dissolution was investigated at pH = 1.15 in the temperature range of 50–90 °C. Assuming equilibria were established at the end of experiments, we extrapolated the measurements to 25 °C and obtained thermodynamic properties of the dissolution reaction at room temperature. Based upon the temperature dependence of dissolution kinetics, we also derived the activation energy of the dissolution reaction and subsequently estimated the rate of scorodite dissolution at ambient conditions. Estimated scorodite solubility at 25 °C is in good agreement with literature data at low pH conditions but is one to two orders of magnitude smaller when pH > 4–4.5. However, when ferrihydrite is allowed to precipitate, the match between estimation and the reported measurements are significantly improved, suggesting that previous studies may be subject to the effect of incongruent dissolution. On the other hand, calculated ambient condition scorodite dissolution rate is one to two orders of magnitude higher than that of common rock-forming minerals, indicating that previously assumed low solubility may not be a solid rationale for treating scorodite as a safe storage for As in natural environments or industrial settings. Finally, an analysis of the rn DG relation in the experimental duration suggests that scorodite dissolution in this study is largely a dislocation-controlled process. ACKNOWLEDGMENTS We would like to thank Kate Campbell and Tyler Kane of the USGS, and Huan Liu of Nanjing University for their assistance on many occasions during this study. We thank three anonymous reviewers whose comments and suggestions helped to improve
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the quality of this article. Xiangyu Zhu is grateful to the China Scholarship Council. This work was supported by the National Natural Science Foundation of China (Grant Nos. 41802032, 41830859, 41861144026) and the National Research Program of the USGS.
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On the thermodynamics and kinetics of scorodite dissolution Xiangyu Zhu a, D. Kirk Nordstrom b, R. Blaine McCleskey b, Rucheng Wang c Xiancai Lu d, Siliang Li a, H. Henry Teng a,⇑ a
Institute of Surface-Earth System Science, Tianjin University, Tianjin 300072, China b US Geological Survey, 3215 Marine St., Suite E. 127, Boulder, CO 80303, USA c State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China d Key Laboratory of Surficial Geochemistry, Ministry of Education, School of Earth Sciences and Engineering, Nanjing University, Nanjing, Jiangsu 210023, China Received 8 May 2019; accepted in revised form 6 September 2019; available online 13 September 2019
Abstract Scorodite (FeAsO42H2O)–water interaction is critical to As distribution and storage in surface environment but is inadequately understood due to ambiguities in the mineral’s stability and weathering rate at atmospheric conditions. In the present study we attempted to experimentally determine the thermodynamic and kinetic parameters needed to compute solubility and dissolution rate of scorodite at 25 °C. Experiments were carried out in low pH (1.15) solutions using specially synthesized large scorodite crystals (with Mean Diameter of 27.9 lm). Such experimental conditions ensured the results were not subject to the influence of secondary Fe-bearing phase precipitation and grain size effect. Measured equilibrium concentrations of Fe and As, along with newly published Fe-As complexes association constants, were first used to determine the solubility products Ksp and dissolution rates rn at 50–90 °C. The obtained Ksp T and rn T dependence was then used to derive the Gibbs free energy, enthalpy, entropy, heat capacity, and activation energy for the dissolution reaction. Finally, we extrapolated the measurements to 25 °C and obtained room temperature solubility and dissolution rate, scrutinized the pH effect on dissolution, and analyzed the DG rn relation of the dissolution reaction. Our results show that literature data are likely overestimated scorodite solubility at pH > 4 4.5 due to neglecting the effect of ferric iron hydrolysis. Estimated ambient condition dissolution rate is an order magnitude lower than the earlier report, implicating the importance of size effect, but is one to two orders of magnitude higher than that of common rock-forming minerals, cautioning the proposed use of scorodite for As fixation and storage. The determined rn DG relation cannot be fully fitted by the transition state model, particularly at near equilibrium, suggesting dissolution in this study may be controlled by defect-assisted surface reactions. Ó 2019 Elsevier Ltd. All rights reserved. Keywords: Arsenic; Scorodite; Thermodynamics; Kinetics; Transition state theory
1. INTRODUCTION Arsenic is a highly toxic metalloid and a world-wide concern as a human carcinogen. Indiscriminate use of arsenical pesticides during the early to mid-1900s has led to extensive contamination of soils worldwide (Smith ⇑ Corresponding author.
E-mail address: [email protected] (H.H. Teng). https://doi.org/10.1016/j.gca.2019.09.012 0016-7037/Ó 2019 Elsevier Ltd. All rights reserved.
et al., 1998). The continuous use of groundwaters with As levels above 10 ppb (current drinking water limit sanctioned by the World Health Organization) for irrigation purpose further aggravated the pollution in many regions of the globe (Berg et al., 2001; Nordstrom, 2002; Fendorf et al., 2010; Smedley and Kinniburgh, 2013; Maghakyan et al., 2017). Geochemical cycling of arsenic at surface conditions is largely controlled by As-forming and As-containing miner-
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als through dissolution, precipitation, incorporation, sorption, and desorption (Fendorf et al., 1997; Ford, 2002; Bostick and Fendorf, 2003; Amirbahman et al., 2006; Benzerara et al., 2008; Deditius et al., 2008; Catalano et al., 2011; Li et al., 2011; Zhu et al., 2014; Karimian et al., 2017; Sowers et al., 2017; Catelani et al., 2018). Of those minerals, scorodite deserves special attention because it is one of the least soluble As phases and a pivotal secondary As-bearing mineral in acidic Fe(III)-As(V)-H2O systems, such as arsenic-contaminated soil and mine wastes (Drahota and Filippi, 2009). The low solubility is in fact the basis of the suggested arsenic fixation strategy in mining industry (Riveros et al., 2001; Langmuir et al., 2006). The dissolution behavior of scorodite has been studied extensively for the past three decades (Dove and Rimstidt, 1985; Nordstrom and Parks, 1987; Robins, 1987; Krause and Ettel, 1988; Bluteau and Demopoulos, 2007; Majzlan et al., 2012). While the results laid the groundwork for understanding scorodite stability in aqueous environment, critical discrepancies exist concerning the measured mineral solubility (Ksp, reaction (2)) that varies by five orders of magnitude (1020.24±0.58 to 1025.68±0.52) from case to case (Chukhlantsev, 1956; Nordstrom et al., 2014). A number of issues may be factored into the literature Ksp’s inconsistency for scorodite. (1) The effect of crystal size on solubility. It is well-known that, due to the Gibbs-Thompson effect, solubility of small particles increases with increasing curvature (reciprocal of particle radius) of the dissolving grains (Cabrera et al., 1954; Lasaga and Blum, 1986; Teng, 2004; Navrotsky, 2004; Fan et al., 2006; Navrotsky et al., 2008). Natural scorodite is usually small (mostly nano-sized) and, despite repeated laboratory attempts, synthesized scorodite crystals rarely exceeded 1 lm in diameter (Dutrizac and Jambor, 1988; Baghurst et al., 1995; Demopoulos et al., 1995; Bluteau and Demopoulos, 2007; Gonzalez-Contreras et al., 2010; Majzlan et al., 2012; Okamura et al., 2013; Kitamura et al., 2015; Okibe et al., 2017). The only known method to produce large (up to tens of micrometers) scorodite crystals was not developed until 2008 (Fujita et al., 2008), restricting earlier work to the use of submicron sized particles. As such, it is conceivable that the wide scatter in the reported Ksp values may have at least partially originated from the crystal size effect. (2) The formation of secondary Fe-containing minerals during dissolution. Due to the very low water solubility of ferric iron hydroxide (Ksp1039), incongruent dissolution (i.e. precipitation of Fe(OH)3) will take place when scorodite dissolves at pH > 2–3. For those studies where pH was not maintained sufficiently low (see Section 4.3 for details), significant errors may have occurred in computing the thermodynamic driving force of the overall reaction (Demopoulos, 2005). (3) Overlook of certain solution chemistry effect. This concern stems from the observation that activity coefficients and ion pairs such as Fe-As complexes and Fe-Cl or Fe-SO4 complexes were rarely considered in previous studies that acquired Ksp measurements. On a related note, while significant progress was made in the past three decades to deduce the mechanisms and surface processes of mineral dissolution through quantifying relations between dissolution kinetics and solution free
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energy change on the basis of Transition State Theory (Eyring, 1935a,b; Glasstone et al., 1941; Connor et al., 1979; Lasaga, 1981; Aagaard and Helgeson, 1982; Lasaga, 1998; Lu¨ttge, 2006), step wave model (Lasaga and Luttge, 2001), and dislocation theory (Brantley et al., 1986; Lasaga and Blum, 1986; Teng, 2004), little is known in the applicability of such advance to scorodite dissolution. To the best of our knowledge, only two studies touched upon the aspects of dissolution kinetics. The first (Harvey et al., 2006) reported an empirical rate law while the second (Bluteau and Demopoulos, 2007) examined the pH and temperature effect on apparent dissolution rate (i.e. without normalization by the sample’s surface area), and none of them considered the thermodynamic effect (i.e. saturation state) on dissolution kinetics, nor the actual physical mechanisms of scorodite dissolution. In the present study, we attempt to quantify scorodite dissolution using specially synthesized large (28 lm on average, Figs. 2 and 3) crystals and low pH (1.15) solutions to eliminate particle-size effect and secondary Fe-containing phase precipitation. The experimental results are used to extract activation energy, enthalpy, entropy, and heat capacity of scorodite dissolution reaction, and to provide mechanistic insight into the dissolution process based upon the relationship between dissolution kinetics and free energy change. 2. METHODS 2.1. Scorodite synthesis and characterization Ambient condition synthesis of scorodite in the Fe(III)As(V)-H2O system is proven difficult (Dutrizac and Jambor, 1988). We used a modified Fujita et al. (2008) method where experimental solutions of 0.3 mol/L As (V) and 0.3 mol/L Fe (II) (prepared using analytical grade Na2HAsO47H2O and FeCl24H2O and adjusted to pH = 1.5 using12 M HCl) were first heated to boiling (T = 96.1– 96.3 °C) state, followed by slow Fe (II) oxidization via air (instead of oxygen) bubbling. The setup (a 1-litre conical flask attached to a condenser pipe) was sealed completely for 7 days, and the resulting precipitates were subsequently collected (via filtration using a 0.45 lm filter) and thoroughly washed (by dilute HCl solution at pH = 1.5). The final product was air dried and stored in a sealed opaque glass bottle. The synthesized products were characterized by scanning electronic microscope (SEM) (ZEISS SUPPA 55) with an oxford INCA energy dispersive spectroscopy (EDS), xray diffractometer (Rigaku D/max III, Cu ka radiation, 40 kV and 20 mA at 0.02°/0.3 s per step from 3° to 70° 2h), and thermogravimetric analysis (PerkinElmer Pyris 1 TGA). The surface area of the crystals (Se) was experimentally determined by the BET method using N2 adsorption (Quantachrome Autosorb-iQ2-XR with outgassing T of 105 °C) and further computed using particle size distribution measured by laser diffraction (Malvern Mastersizer 3000) via P 6 Vd ii 6 Se ¼ P ¼ ð1Þ q V i qDS
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where Vi is the relative volume by particle size class di, q the material density (3.27 g/cm3 for scorodite), and Ds the Sauter diameter (mean particle size assuming spherical geometry). 2.2. Dissolution experiments The dissolution chamber consisted of two 500 ml bottles each attached onto a submersible stirrer (Supplementary material, Fig. S1). The whole set-up was immersed into a water bath where the temperature was maintained at desired settings with a variability of ±0.1 °C via a LaudaBrinkmann thermostats (Model RCS 6). Ten grams of scorodite were enclosed in a double-layered 18 lm polyester mesh bag and suspended in diluted HCl solution to prevent potential physical abrasion to the crystals from the magnetic bars spinning that at pre-set rates of 400–500 rpm. The pH of the dissolution solutions was maintained at 1.15 (±0.05) throughout the experimental duration. Two sets of data were obtained in this study, a short term one collected in the initial 4–5 h of the experiments and the other at long-term upon the establishment of a steady state. Dissolution experiments ran parallel in the two 500 ml bottles at 50, 70, and 90 °C, respectively. The scorodite bags were immersed in the solution only after the desired temperature was reached and stabilized. An aliquot of 1 ml experimental solution was sampled every 15 min to 1 h in the first 6 h and every 1 or 2 days after until a steady-state was reached. The sampled solution was filtered through a 0.45 lm membrane and diluted 100% by 1% HNO3 solution for subsequent Fe and As analyses. 2.3. Solution chemistry analyses Solution pH was measured using a Thermo Scientific 815600 Orion Ross combination electrode and an Orion STAR A325 pH meter. The electrode was calibrated using the pH 1.00 and 1.68 standards (Geotech, NIST traceable)
thermally equilibrated with the experimental solutions. The As and Fe contents in the solutions were routinely determined by Inductively-coupled plasma-optical emission spectrometry (ICP-OES, Perkin-Elmer 7300 DV) using the characteristic wavelengths of 188.979 and 193.696 nm for As (detection limit 0.03 mg L1) and 238.204 and 259.939 nm for Fe (detection limit 0.002 mg L1). Typical relative standard deviation of the measurements is 5% for As and 2% for Fe. For solutions with As in the range of 0.001 mg L1 (below the ICP-OES’s detection limit), a hydride generation atomic absorption spectrometer (HGAAS, Perkin–Elmer AAnalyst 300 equipped with a FIAS– 100 flow injection system and a quartz cell) capable of detecting As at 0.0001 mg L1 levels was employed. The relative standard deviation of the HG-AAS analyses in this study was 2%. 3. EXPERIMENTAL RESULTS 3.1. Mineral characterization The synthesized crystals show a pale-yellowish green color with a garlic odor. Sharp peaks in the XRD spectra match well with the standard pattern in the PDF database (NO. 70-0825) with no identifiable minor phases found in background, confirming the crystallization of scorodite (Fig. 1). SEM-EDS analysis further reveal the absence of any meaningful chemical impurities in the crystals. Multipoint fitting BET surface area of the crystals is approximately 0.43 m2/g, comparable to the calculated value of 0.11 m2/g using particle size distribution data (Fig. 2). SEM microphotography of the harvested crystals show mostly pyramidal or pseudo-octahedral shapes with sizes ranging from 10 to 30 lm (Fig. 3), well consistent with the direct measurement (with mean particle size of 27.9 lm) and the BET surface area-based estimation (Ds = 17.3 lm). EDS analyses further confirm that the Fe to As ratio is close to the ideal stoichiometric value of unity.
Fig. 1. An XRD spectrum of the synthesized scorodite in comparison to the standard pattern in the PDF database.
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Fig. 2. Size distribution of the synthesized scorodite crystals. Listed values indicate the sizes in 10, 50, and 90 percentile along with the mean and the Sauter diameter.
Fig. 3. SEM images showing the pyramidal or pseudo-octahedral crystal morphology of the synthesized scorodite.
TGA curves indicate that the onset of weight loss in the synthesized scorodite occurs at 168 °C (Fig. 4), nearly identical to that reported by previous studies (Bluteau and Demopoulos, 2007; Le Berre et al., 2008; GonzalezContreras et al., 2010). The total weight loss amounts to 15.5%, very close to the theoretical water content of 15.6% in scorodite. 3.2. Mineral dissolution behavior As expected, the accumulated As and Fe concentrations increased with T and over time for both short (Fig. 5a) and long term (Fig. 5b). Steady states were reached within 30 days for all the temperatures. The experiment at 70 °C failed to reach completion due to apparatus malfunction at day 15 and the steady state was extrapolated from the trend shown in the 50 and 90 °C experiments. This method may introduce an estimated 1% uncertainty in the final solubility product obtained at 70 °C. Evaporation encountered at 50 and 70 °C was insignificant but became non-negligible at 90 °C. Accordingly, the data collected at 90 °C was corrected using an evaporation model (See supplementary S-1).
4. DISCUSSION 4.1. Thermodynamic analysis The Fe to As ratio maintained close to unity during the experiments indicating that the dissolution proceeded stoichiometrically. Assuming the reaction FeAsO4 2H2 O ¼ Fe3þ þ AsO3 4 þ 2H2 O
ð2Þ
held true and reached steady states in our experiments, the solubility product of scorodite was computed using the Fe and As concentrations averaged over the last 3 measurements of the experiments (121.3, 149.6, and 345.5 lmol/L for 50, 70, and 90 °C respectively), corrected by activity coefficients estimated from the Hu¨ckel equation (also known as the B dot equation. Hu¨ckel, 1925; Helgeson, 1969. Supplementary material, S-2) via the computational code of PHREEQCI after taking into consideration the recently published association constants for Fe (III)-As(V) and Fe (III)-Cl complexes (Zhu et al., 2017). Such obtained values of Ksp fit nicely to the following equation (Fig. 6):
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Fig. 4. TGA analysis showing the thermally driven weight loss in the synthesized scorodite.
Fig. 6. Dependence of calculated scorodite solubility product (log Ksp) on temperature (1/T). The solid line is a fit to Eq. (3) and the open circle is the extrapolated value at 25 °C.
where DrG, DrCp, DrS*, DrH* are the Gibbs free energy, heat capacity, entropy (0 K), and enthalpy (0 K) of the reaction, and R the gas constant (see derivation in Supplementary material, S-3), one can readily see that the heat capacity of the dissolution reaction can be regarded constant between 50 and 90 °C. In addition, a value of Ksp = 1025.81 ± 0.1 at 25 °C can be extrapolated from the fit. This value agrees well with that (1025.83 ± 0.07) given by Langmuir et al. (2006) and the recent evaluation (1025.68 ± 0.52) by Nordstrom et al. (2014). Moreover, Eq. (4) can be used to derive Gibbs free energy, enthalpy, entropy, and heat capacity (Table 1) for reaction (2). 4.2. Kinetic analysis Scorodite dissolution rates in present study were calculated following the methods reported by Chermak and Rimstidt (1990), Lasaga (1998), and Harvey et al. (2006). Briefly, the measured concentration (C) versus time (t) are first fitted to a second order polynomial equation of the form C ¼ a þ bt þ ct2
ð5Þ
where a, b, and c are fitting constants. Dissolution rate (r) is then estimated by the differential form of Eq. (5): Fig. 5. Time dependent variations of arsenic concentration in the first 4 or 5 h of scorodite dissolution experiment (a) and long-term dataset showing both arsenic and iron concentrations up to 38 days (b). Solid line is second order polynomial fittings for eye guidance.
log Ksp ¼ A1 þ A2 =T þ A3 log T
ð3Þ
where A1, A2, and A3 are constants and T the absolute temperature. Given the resemblance of Eq. (3) to the thermodynamic relation ln Ksp ¼
Dr G Dr S Dr C p Dr H Dr C p þ ln T ¼ R RT R RT
ð4Þ
r¼
dC ¼ b þ 2ct dt
ð6Þ
Final, the normalized rate rn is obtained by the following equation rn ¼
rV sol Asp mscor
ð7Þ
where Vsol (0.00055 m3 in present study) is the volume of solution, Asp (BET surface area, 0.43 m2 g1) the specific surface area of the scorodite, and mscor (10 g) the mass of scorodite.
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Table 1 Estimated Gibbs free energy, enthalpy, entropy, heat capacity, and solubility product of scorodite dissolution shown as reaction (2) at 298.15 K and in the temperature range of 298.15 K < T < 363.15 K. DrG (kJ mol1) DrH (kJ mol1) DrS (J K1 mol1) DrCp (J K1 mol1) log Ksp
298.15 K
T (298.15–363.15 K)
147.32 ± 0.57 71.53 ± 1.94 735.3 ± 6.9 1421.8 ± 67.3 25.81 ± 0.1
–495.43 + 10.258 T–1.4218 T ln T 495.43 + 1.4218 T 8836.0 + 1421.8 ln T 1421.8 ± 67.3 535.8 + 25878/T + 171 log T
The initial (i.e. when the solutions were free of As and Fe) dissolution rates in the experimental temperature range (50–90 °C) increased from 1010.3 to 108.9 mol m2 s1 with increasing temperature. The linear relation of the initial rates (ln rn, derived from the dataset shown in Fig. 5a) to 1/T (Fig. 7) suggests an Arrhenius behavior rn ¼ AeEa =RT , where A is a pre-exponential factor in mol m2 s1 and Ea the activation energy in kJ mol1) for scorodite dissolution, consistent with previous findings (Baghurst et al., 1995; Bluteau and Demopoulos, 2007). Furthermore, the value of Ea = 77.2 kJ mol1 extracted from Fig. 7 closely matches that (77 kJ mol1) reported at a similar pH (1.1) (Baghurst et al., 1995), but significantly differs from those acquired at high pHs (61 kJ mol1 at pH = 6 and 100 kJ mol1 at pH = 7–9) (Bluteau and Demopoulos, 2007), presumably indicative of the effect of secondary Fe-containing phase formed at neutral to alkaline conditions. The room temperature (25 °C) dissolution rate extrapolated from the experimental data using the Ea value comes to be log rn = 11.3 mol m2 s1, more than one magnitude lower than that (log rn = 9.86 mol m2 s1) acquired by Harvey et al. (2006). Given the similar temperature (22 vs. 25 °C) and milder pH (2 vs. 1.15) but more than 20-fold greater (9.5 vs. 0.43 m2/g) mineral surface area in Harvey et al. (2006) experiment, we argue that the rate in our dataset is likely free of the grain-size effect widely observed during crystal dissolution (Briese et al., 2017).
Thermodynamic effect on scorodite dissolution was examined by considering the dependence of rate on the free aFe3þ aAsO3 4 energy state DG ¼ RT ln of the experimental K sp solutions. Plots of rn vs. DG revealed non-linear dependence for all experiments (Fig. 8). Far from equilibrium, for example, when DG < 15 kJ mol1 for the experiment at 50 °C, the rn vs. DG curve was dominated by a plateau (Nagy and Lasaga, 1992; Hellmann and Tisserand, 2006) where the rate varied little with respect to solution saturation state; as the solution evolved to intermediate undersaturation states (15 < DG < 5 2 kJ mol1), a sharp decrease in dissolution rate (one and two orders of magnitude for 50/70 °C and 90 °C experiments, respectively) occurred. Close to and very-near equilibrium (0 > DG > 2 5 kJ mol1), the rate decrease slowed down and exhibited a somewhat linear dependence on DG. The relation between dissolution rate and solution DG is in general described by: rn ¼ k f ðDGÞ
ð8Þ
where f ðDGÞ ¼ 0 at equilibrium and f ðDGÞ ¼ 1 at far-from equilibrium (D G ! 1, rate stabilizes at k). The forms of f (DG) vary but can be mathematically derived from three models: (1) the transition state theory (TST, Eq. (9)) (Lasaga, 1981; Aagaard and Helgeson, 1982; Helgeson et al., 1984); (2) TST modified by surface defects (Eq. (10)) (Morse, 1983; Lasaga, 1998; Teng, 2004), and (3) a sigmoidal formula (a combination of (1) and (2), Eq. (11)) (Burch et al., 1993; Hellmann and Tisserand, 2006; Xu et al., 2012): f ðDGÞ ¼ ð1 eDG=RT Þ ð9Þ DG=RT n ð10Þ f ðDGÞ ¼ 1 e m1 m 2 f ðDGÞ ¼ ðk 1 =kÞ 1 eðnDG=RT Þ þ ðk 2 =kÞ 1 eDG=RT ð11Þ
Fig. 7. Plot of scorodite dissolution rate (ln rn) vs. temperature (1/ T) showing the Arrhenius behavior.
The parameter n is an apparent reaction order, k the rate constants, and m adjustable factors, all obtained as fitting constants in practice. Applying the two mechanistic models (Eqs. (9) and (10)) to our data (Fig. 8) shows that the TSTbased rate law significantly overestimates the measurements when DG < 10 kJ mol1, whereas the surface-defect equation gives a much more accurate description of the experimental results even very-near equilibrium. The empirical sigmoidal model, as expected, fits the data with high fidelity within the entire experimental saturation range. The finding that the defect-based model gives a more satisfactory approximation to the experimental measure-
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strain energy, etch pits can only form at the high energy sites, rendering a strong negative dependence of dissolution rate on DG. Finally, when close to and very-near equilibrium and |DG| is smaller than the energy barrier for defect-assisted pit nucleation, the dissolution only has a weak dependence on DG as the surface reaction is limited at pre-existing step sites. It appears that the TST model’s overestimation in dissolution rate increases with increasing temperature. For example, at |DG| = 2 kJ mol1, the discrepancy between TST-predicted and measured rates is a factor of 3 at 50 °C (Fig. 8a) but stretches to over an order of magnitude when temperature increases to 90 °C (Fig. 8c). A clear interpretation for this observation is not readily available. One explanation may be the limitation of the TST model when applied to particle detachment/attachment to solid surfaces in solution systems (Joswiak et al., 2018). The original TST theory was developed for gas particle collision reactions where the rate constant is expressed as the product of the thermal vibrational frequency v = kBT/⁄ (⁄ is the Planck’s constant) and the partition function of the activation complex (Lasaga, 1998). For heterogeneous reactions in solution, however, it is shown that the frequency term needs to incorporate solute diffusion and the geometry of the activation complex (Hill, 1975, 1976). Hence it is possible that the actual dependence of dissolution rate on T is weaker than that depicted by the linear relation of v = kBT/⁄. 4.3. Predicted pH effect on scorodite dissolution
Fig. 8. Scorodite dissolution rate (rn) vs. the corresponding Gibbs free energy difference (DG) at 50 (a), 70 (b), and 90 (c) °C. Solid circles: measured rates; dotted lines: TST fitting; dashed lines: surface-defect fitting; solid lines: sigmoidal fitting.
ments can be interpreted in the context of dislocation theory (Burch et al., 1993; Bandstra and Brantley, 2008; Xu et al., 2012). It is well understood that mineral dissolution preferentially occurs at high energy sites such as defects and dislocations where lattice strain occurs (Holdren and Berner, 1979; Helgeson et al., 1984). Far-from equilibrium, when |DG| is greater than the melting enthalpy of crystals, dissolution is controlled by spontaneous 2D surface nucleation of etch pits (Brantley et al., 1986; Lasaga and Blum, 1986; Teng, 2004) regardless further changes in DG. At intermediate equilibrium when |DG| is comparable to the
While scorodite dissolution may proceed as described by Reaction (2), the subsequent Fe3+ hydration (e.g. Fe3+ + H2O ? Fe(OH)2+ + H+) becomes non-negligible when pH > 3. The removal of ferric iron not only changes the solution supersaturation but affects aqueous As concentration due to the adsorption of arsenate on the resultant iron hydroxide. Such pH effect can be quantitatively assessed using the Ksp value and other thermodynamic parameters obtained in this study. Plotting literature data of scorodite dissolution on our calculated solubility to pH relation shows a reasonable agreement at pH < 3, but reveals one to two order of magnitude difference at high pHs (Curve A, Fig. 9). When ferrihydrite precipitation is taken into consideration, however, the reported measurements become significantly closer to the prediction (Curve B, Fig. 9), indicating that majority of the early lab data were likely subject to the effect of ferrihydrite precipitation. If goethite instead of ferrihydrite is considered, the predicted release of As overestimates the reported values when pH > 2.5 (Curve C, Fig. 9) but closely matches our data of As in a field collected acid mining drainage (AMD) solutions (pH 3, the chemical analysis of AMD was provided in the Supplementary material, S-4). Demopoulos (2005) detected the presence of highly metastable nano-sized 2-line ferrihydrite phase associated with scorodite dissolution in laboratory. In long term, however, ferrihydrite will give rise to the more stable phase goethite, a common occurrence in natural environment (Cornell et al., 1989; Hamzaoui et al., 2002; Cudennec and Lecerf, 2006). Thus, it appears
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Fig. 9. Scorodite solubility (shown as arsenic concentration) at different pH (25 °C): comparison among congruent dissolution calculation, incongruent dissolution calculation, and experimental measurement; A: scorodite congruent dissolution curve; B: reaching equilibrium with ferrihydrite during scorodite dissolution; C: reaching equilibrium with goethite during scorodite dissolution. Open and cross symbols are literature data and solid circles show the field collected acid mine drainage data in this study. The solubility products of ferrihydrite and goethite are derived from the thermodynamic properties reported by Navrotsky et al. (2008, see supplementary material S-6).
that the scorodite + goethite system may be more fitting for field settings. Finally, we estimated As adsorption using surface complexation model (see supplementary material for detailed parameter, S-5) and found that approximately 10% and 2% of the total arsenic may be surface-bound on ferrihydrite and goethite, respectively. These values are noticeable but likely insignificant if iron hydroxide is derived solely from scorodite dissolution. 4.4. Implications for environmental consequences Scorodite dissolution rate at Earth’s surface conditions estimated from the dataset reported in this study is on the order of 1011–1012 mol/m2/s. This rate is nearly 1–2 order of magnitude higher than those reported for common silicate minerals (e.g. K-feldspar, kaolinite) in ambient environment (Brantley et al., 2007). Assuming scorodite (grain size > 5 lm) is the main storage of As in a porous geological medium (e.g. aquifer) that has an average water content of 15% (w/w) and a background As concentration of 15 ppm (Smith et al., 1998), dissolution at this rate (log rn = 11.3 mol m2 s1 at 25 °C) would render initially As-free water to one with arsenic concentration surpassing the 10 ppb threshold value within 17 h. In reality this during could be significantly shorter as scorodite dissolution in natural settings is likely much (up to an order of magnitude) faster due to the small crystal sizes. This understanding, even taking into consideration of As adsorption by existing ferric oxyhydroxide or/and other minerals in nature, suggests that cautions need to be exercised when scorodite is used for As immobilization as natural waters in contact with the arsenic storage media can quickly (within days) become polluted.
5. SUMMARY Scorodite dissolution was investigated at pH = 1.15 in the temperature range of 50–90 °C. Assuming equilibria were established at the end of experiments, we extrapolated the measurements to 25 °C and obtained thermodynamic properties of the dissolution reaction at room temperature. Based upon the temperature dependence of dissolution kinetics, we also derived the activation energy of the dissolution reaction and subsequently estimated the rate of scorodite dissolution at ambient conditions. Estimated scorodite solubility at 25 °C is in good agreement with literature data at low pH conditions but is one to two orders of magnitude smaller when pH > 4–4.5. However, when ferrihydrite is allowed to precipitate, the match between estimation and the reported measurements are significantly improved, suggesting that previous studies may be subject to the effect of incongruent dissolution. On the other hand, calculated ambient condition scorodite dissolution rate is one to two orders of magnitude higher than that of common rock-forming minerals, indicating that previously assumed low solubility may not be a solid rationale for treating scorodite as a safe storage for As in natural environments or industrial settings. Finally, an analysis of the rn DG relation in the experimental duration suggests that scorodite dissolution in this study is largely a dislocation-controlled process. ACKNOWLEDGMENTS We would like to thank Kate Campbell and Tyler Kane of the USGS, and Huan Liu of Nanjing University for their assistance on many occasions during this study. We thank three anonymous reviewers whose comments and suggestions helped to improve
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the quality of this article. Xiangyu Zhu is grateful to the China Scholarship Council. This work was supported by the National Natural Science Foundation of China (Grant Nos. 41802032, 41830859, 41861144026) and the National Research Program of the USGS.
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