Jarosite dissolution II—Reaction kinetics, stoichiometry and acid flux
Introduction
Jarosite and alunite are isostructural hydrous sulfate minerals belonging to the alunite group, with a general formula of AB3(SO4)2(OH)6 (Fig. 1) (Ralph and Ralph, 1993, Barthelemy, 2000). The B cations have octahedral coordination and are commonly Fe3+ (jarosite subgroup) or Al3+ (alunite subgroup). Cations in the A site have coordination ≥ 9 and are commonly K+, Na+, or H3O+. The ideal endmember composition for jarosite sensu strictu is KFe3(SO4)2(OH)6. Jarosite and alunite minerals commonly occur in acid sulfate soils and acid mine drainage sites where pyrite has been oxidized, pH decreases below 3.7 (c.f. White et al., 1997, Bigham and Nordstrom, 2000, Jambor et al., 2000) and water is limited. Jarosite and alunite have also been described in acidic hypersaline evaporite basins in Australia such as Lake Tyrrell (Alpers et al., 1992), and in the Antarctic (Gore et al., 1996). Recently, jarosite has been detected on the surface of Mars (Klingelhöfer et al., 2004, Madden et al., 2004).
In some coastal acid sulfate soil sites in Australia jarosite is particularly important in controlling acidity because it represents a major source of acidity (cf. White et al., 1997, Tulau and Naylor, 1999, Somerville et al., 2004, Beavis et al., 2005). Remediation strategies for acid sulfate soil sites, such as flooding to prevent further oxidation of pyrite, can have disastrous results if jarosite is abundant because reaction in water releases more acid to solution (White et al., 1997) as demonstrated in Eq. (1):KFe3(SO4)2(OH)6 + 3H2O → 3Fe(OH)3 + K+ + 3H+ + 2SO42−
Alternatively, if jarosite reacts under more acidic conditions, producing soluble ferric iron instead of ferric hydroxides, this can catalyse pyrite oxidation (Singer and Stumm, 1970):FeS2 + 14Fe3+ + 8H2O → 15Fe2+ + 2SO42− + 16H+
The ferric iron controlled oxidation of pyrite is orders of magnitude faster than the reaction with molecular oxygen (Singer and Stumm, 1970). Consequently, in acid sulfate soil environments, jarosite dissolution kinetics and the release of dissolved ferric iron can be a major factor controlling pyrite oxidation and the generation of acidity under anoxic conditions.
Experimental studies on the formation and reactivity of these hydrous sulfate mineral phases yield variable results with respect to the stability of jarosite (cf. Stoffregen, 1993, Baron and Palmer, 1996 and references therein; Bigham and Nordstrom, 2000, Drouet and Navrotsky, 2003, Drouet et al., 2004). Reported values for solubility constants range over several orders of magnitude because both natural and synthetic jarosite rarely have the pure endmember composition (Bigham and Nordstrom, 2000, Dutrizac and Jambor, 2000, Papike et al., 2006a, Papike et al., 2006b, Smith et al., 2006b). Stability and reactivity are dependent on the degree of isomorphous substitution into the A and B sites. Na+ or H3O+ substitution into the A site greatly increases jarosite reactivity and solubility. In addition to these common substitutions, other ions such as NH4+, Ag+, Tl+, Pb2+, Ca2+, Sr2+, Ba2+, Cu2+, Hg2+, and the REE can substitute into the A site, Cr3+, V3+ Ga3+ and Cu2+, can substitute into the B site, and the oxyanions CrO42−, PO43−, and AsO43− can substitute for sulfate (Dutrizac and Jambor, 2000, Becker and Gasharova, 2001, Dill, 2001, Kolitsch and Pring, 2001). This is an important characteristic because it means that jarosite can act as a temporary sink for trace metals in the environment and then release them when it dissolves (Welch et al., 2007).
The purpose of this study is to characterize a natural sample of jarosite from an acid sulfate soil site, and to experimentally determine the kinetics of acid and major ion release. Previous work on this material showed that trace elements were released to solution in dissolution experiments, however, there was a fractionation between trace metals compared to major ions (Welch et al., 2007). Ions that substitute into the A site (Rb+, Sr2+ and La3+) were preferentially released to solution in the initial stage of the dissolution reaction, whereas elements that substitute into the B site were preferentially retained in the solid phase.
Sediment cores rich in jarosite were collected from an acid sulfate soils site near Kempsey, on the mid north coast of New South Wales, Australia (30° 55′ 40″S, 152° 55′ 48″E). This area is within the lower Macleay River catchment where acid sulfate soils were first identified in Australia (Walker, 1972), and where highly vulnerable acid sulfate soils ‘hotspots’ have been targeted for management. Beavis et al. (2005) showed that acid discharge events from this site are highly responsive to rainfall. However, there can be a lag between the onset of rainfall and acidic discharge, as a function of antecedent moisture conditions—the drier the conditions, the longer the lag period. Once acidic discharge is initiated, poor water quality in terms of pH can persist for extended periods of time, from weeks to months. This relationship between rainfall and acidic discharge suggests that the kinetics of jarosite dissolution and the hydraulic conductivity of the sediments are major factors controlling acid discharge into the local waterways.
Section snippets
Methods
Jarosite dissolution experiments were conducted to estimate (a) kinetics of the reaction over a range of conditions relevant to acid sulfate soils and (b) the flux of acid and major ions to solution from jarosite.
Jarosite characterization
The jarosite powder extracted from the mottles was mustard yellow in colour. Analysis of this powder in the SEM showed that the sample appeared to be > 95% jarosite that existed as individual euhedral crystals ranging in size from ~ 0.5 to 5 µm. There were minor amounts of clay (~ illite composition), gypsum, diatom fragments and quartz grains (Fig. 2) intermixed with the sample.
The chemical composition of the jarosite crystals as determined by EDXA spot analysis was consistent with the endmember
Discussion
The experimental data show that the stoichiometry and kinetics of jarosite dissolution are complex functions of solution composition. There are important implications for managing sites that contain jarosite, since jarosite reacts with water to release acid, salts, and trace metals. The rate of release of these components depends on the environmental conditions of the site, as well as on the composition of the jarosite.
Summary and conclusions
The dissolution rate of a natural jarosite sample was determined in inorganic acids at pH 3 and 4, and in oxalic acid. Dissolution rates ranged from 0.25 to 1.1 µmol/g/day in the inorganic experiments, and rates were approximately an order of magnitude greater in the oxalate experiment. The dissolution reaction was non-stoichiometric over the course of the experiment: cations in the A site (H3O+, Na+ and K+) and SO42− are preferentially leached from the mineral surface, leaving behind a
Acknowledgements
Funding was provided by the Cooperative Research Centre for Landscape Environments and Mineral Exploration (CRC LEME). We would like to thank Ulli Troitzsch and Linda McMorrow for help with analysis with sediments and solutions respectively. SEM analysis was done at the Electron Microscopy Unit at the Research School of Biological Sciences ANU. We would like to thank Sally Stowe, Frank Brink and Cheng Huang for their assistance. We would like to thank Russel and Georgina Yerbury for their
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Cited by (0)
- 1
Research School of Earth Sciences, The Australian National University, Canberra, ACT, Australia.
- 2
School of Earth Sciences, The Ohio State University, Columbus OH, 43210, USA.
- 3
Department of Earth Sciences, Simon Fraser University, Burnaby, BC, Canada.