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