Stoichiometry and kinetics of jarosite dissolution in acid sulfate soils

Andrew G. Christy¹, Susan A. Welch^1,2,3, Dirk Kirste^,2,4, Fern R. Beavis^1,2,
Sara G. Beavis^1,2,5

1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
2 Cooperative Centre for Landscape Environments and Mineral Exploration (CRC LEME) , Australian National University, Canberra, ACT 0200, Australia
3 The Ohio State University, Columbus, Ohio, USA
4 Simon Fraser University, Burnaby, British Columbia, Canada
5 Fenner School of Environment and Society, Australian National University, Canberra, ACT 0200, Australia

Time evolution of ferric iron and sulfate concentrations in water associated with dissolving jarosite. Stoichiometric dissolution indicated by Fe/S = 1.5 dashed line. Earliest measurements show substantial release of sulfate, followed by slower release of Fe (steep part of curve) until ferrihydrite becomes saturated (flat part of curve). Jarosite saturation ultimately achieved at far right of activity-activity diagram.

Jarosite, ideally KFe³⁺₃(SO₄)₂(OH)₆,  is stable under acid oxidising conditions and is an important mineral in acid sulfate soils. It accommodates a wide range of trace cations in solid solution, which can be remobilised if it dissolves. We have found jarosite dissolution to be strongly incongruent: large cations such as K+ can be lost or exchanged even when the iron sulfate framework remains largely intact. Rare earth cations display extreme fractionation of LREE into jarosite and exclusion of HREE from it, with the result that dissolution of jarosite produces characteristic MREE enrichment in the associated water. Long-duration kinetic studies show that the dissolution stoichiometry in a closed system evolves in a complex fashion through time, from fast release of sulfate to slower release of Fe to constant in Fe as the system becomes buffered by saturation of Fe-oxyhydroxide phases.

Welch SA, Kirste D, Christy AG, Beavis SG, Beavis F (2007) Jarosite dissolution I - trace cation flux in acid sulfate soils.  Chem. Geol. 245: 183-197.

Welch SA, Kirste D, Christy AG, Beavis FR, Beavis SG (2008) Jarosite dissolution II - reaction kinetics, stoichiometry and acid flux.  Chem. Geol. 254: 73-84.

Welch SA, Kirste D, Christy AG, Beavis F, Beavis SG (2008) Jarosite reactivity and trace metal geochemistry in acid sulfate soils. Goldschmidt 2008 Geochemistry Conference,  Vancouver, Canada, 13 -18 July, 2008. Geochim. Cosmochim.  Acta 72: A1013 Suppl. (Refereed conference abstract).