Solubility products of amorphous ferric arsenate and crystalline scorodite (FeAsO4 · 2H2O) and their application to arsenic behavior in buried mine tailings

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Abstract

Published solubility data for amorphous ferric arsenate and scorodite have been reevaluated using the geochemical code PHREEQC with a modified thermodynamic database for the arsenic species. Solubility product calculations have emphasized measurements obtained under conditions of congruent dissolution of ferric arsenate (pH < 3), and have taken into account ion activity coefficients, and ferric hydroxide, ferric sulfate, and ferric arsenate complexes which have association constants of 104.04 (FeH2AsO42+), 109.86 (FeHAsO4+), and 1018.9 (FeAsO4). Derived solubility products of amorphous ferric arsenate and crystalline scorodite (as log Ksp) are −23.0 ± 0.3 and −25.83 ± 0.07, respectively, at 25 °C and 1 bar pressure. In an application of the solubility results, acid raffinate solutions (molar Fe/As = 3.6) from the JEB uranium mill at McClean Lake in northern Saskatchewan were neutralized with lime to pH 2–8. Poorly crystalline scorodite precipitated below pH 3, removing perhaps 98% of the As(V) from solution, with ferric oxyhydroxide (FO) phases precipitated starting between pH 2 and 3. Between pH 2.18 and 7.37, the apparent log Ksp of ferric arsenate decreased from −22.80 to −24.67, while that of FO (as Fe(OH)3) increased from −39.49 to −33.5. Adsorption of As(V) by FO can also explain the decrease in the small amounts of As(V)(aq) that remain in solution above pH 2–3. The same general As(V) behavior is observed in the pore waters of neutralized tailings buried for 5 yr at depths of up to 32 m in the JEB tailings management facility (TMF), where arsenic in the pore water decreases to 1–2 mg/L with increasing age and depth. In the TMF, average apparent log Ksp values for ferric arsenate and ferric hydroxide are −25.74 ± 0.88 and −37.03 ± 0.58, respectively. In the laboratory tests and in the TMF, the increasing crystallinity of scorodite and the amorphous character of the coexisting FO phase increases the stability field of scorodite relative to that of the FO to near-neutral pH values. The kinetic inability of amorphous FO to crystallize probably results from the presence of high concentrations of sulfate and arsenate.

Introduction

Scorodite is the least soluble arsenate phase in many mine tailings systems, and as such understanding and controlling its solubility is of special relevance in efforts to limit arsenic releases to tailings pore waters and to the environment. Published solubility products for amorphous and crystalline scorodite have generally been computed from solubility measurements without properly taking into account the chemical composition and thermodynamic properties of the solutions at equilibrium. Also, the solubility of both crystalline and amorphous scorodite has usually been measured in solutions where equilibrium was approached from undersaturation only.

Published solubility data for amorphous ferric arsenate (AFA) to crystalline scorodite (scorodite(cr)) measured in starting solutions without dissolved Fe or As, are plotted versus pH in Fig. 1. The solubility data of Chukhlantsev, 1956, Makhmetov et al., 1981, Dove and Rimstidt, 1985, Robins, 1987, are given in Table 1. The data from Tozawa et al., 1978, Krause and Ettel, 1988, Robins, 1990 are listed in Table 2. Lines drawn through the data points in Fig. 1, which show the general behavior of AFA and scorodite(cr) as a function of pH, have no theoretical or statistical significance. As discussed below, the solubility data of Robins (1990), which lies between that for AFA and scorodite(cr) is apparently for a scorodite phase of intermediate crystallinity.

In their calculation of solubility products, Chukhlantsev, 1956, Krause and Ettel, 1988 ignored ion activity coefficients. Chukhlantsev (1956) also did not consider important ferric hydroxide and ferric sulfate complexes. Except for Zhu and Merkel (2001), most researchers have also ignored ferric and other metal arsenate complexes. Dove and Rimstidt (1985) neglected ferric arsenate complexes and made their four solubility measurements from pH 5.53 to 6.36, where scorodite dissolves incongruently with precipitation of a ferric oxyhydroxide phase (cf. Nordstrom and Parks, 1987, Robins, 1987). The incongruency of their experiments is evident from the molar Fe/As ratios of their final solutions which are given in Table 1. We have chosen not to use solubility measurements obtained under incongruent conditions in our determination of the solubility product of scorodite.

A somewhat similar reevaluation of published ferric arsenate/scorodite solubility data to ours was performed by Zhu and Merkel (2001). However, these authors discounted the data of Krause and Ettel (1988) for crystalline scorodite, and did not consider the solubility studies of Robins, 1990, Nishimura and Robins, 1996.

In this study (see also Langmuir et al., 1999, Mahoney and Langmuir, 2002) we have focused on the solubilities of the most and least soluble ferric arsenate phases, amorphous ferric arsenate (AFA), and well-crystallized scorodite (scorodite(cr)), respectively. Published solubility data has been reevaluated using the geochemical code PHREEQC (Parkhurst and Appelo, 1999), and considering all of the above corrections, with a modified thermodynamic data base for aqueous arsenic species discussed in this paper, and separately for comparison using revised arsenic acid constants proposed by Nordstrom and Archer (2003). We also show the application of the solubility results to scorodite precipitation during the laboratory neutralization of arsenic-rich acid mill tailings raffinates, and to measured arsenic concentrations in the pore waters of buried uranium mill tailings at the JEB tailings management facility at McClean Lake in northern Saskatchewan, Canada.

Section snippets

Auxiliary thermodynamic data

Equilibrium constants for arsenic and arsenous acids and stability constants for metal arsenate complexes used in the PHREEQC calculations of chemical equilibria are given in Table 3. Discussion of the selection and derivation of these constants is presented in Appendix A.

Studies considered in the revaluation

Because of the difficulty of making accurate activity coefficient corrections in solutions at ionic strengths above 0.1 M, we reviewed only solubility studies performed at lower ionic strengths. In our analysis, we also

Decrease in the apparent log Ksp (log IAP) of ferric arsenates with increasing pH and increasing Fe/As ratio

In systems in which the Fe/As ratio equals 1, the apparent (computed) solubility product (IAP = Ksp) of the ferric arsenates decrease with pH up to pH 4–5 in Fig. 5. That the solubility product decreases and scorodite crystallinity increases may reflect a slowing rate of scorodite precipitation in the presence of lower concentrations of dissolved Fe(III) and As(V). Also, with increasing pH, relatively amorphous scorodite becomes undersaturated and would be expected to preferentially dissolve,

Conclusions

Published solubility data for amorphous ferric arsenate and for crystalline scorodite, was revaluated using the geochemical code PHREEQC (Parkhurst and Appelo, 1999), taking into account ionic strength and ferric and arsenate aqueous complexes, and only considering studies where some measurements were made in acid solutions where congruent dissolution of the arsenate was possible, and the measured pH and concentrations of total As and Fe were reported. Computer modeling of the solubility data

Acknowledgments

We thank George Demopoulos for suggesting revisions to an earlier draft of the paper, and to Associate Editor Liane Benning and three anonymous reviewers for helpful comments that greatly improved the final version.

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