Jarosite dissolution II-Reaction kinetics, stoichiometry and acid flux

A study was undertaken to characterize a natural jarosite sample from an acid sulfate soils site and to experimentally determine the kinetics of dissolution and major ion release from jarosite under a range of different conditions. The jarosite was sourced from a degraded acid sulfate soil site that displays spatio-temporal variability in acidity on the mid north coast of NSW, Australia. Dissolution reaction rates varied as a function of solution composition but were roughly proportional to the degree of undersaturation, and the reaction pathway towards saturation was complex. The short-term reaction rates (< 12 days) decreased with increasing acidity, sulfate or iron. Over the longer term, there was a shift from congruent to incongruent dissolution, with precipitation of Fe-(OOH) solid, and with little further change in saturation index. The dissolution rates of the natural mineral were 1-3 orders of magnitude slower than experimentally determined reaction rates of synthetic jarosite samples run under similar experimental conditions, suggesting that residual solids were inhibiting reaction. The slow reaction rates of this study are probably more typical of jarosite in natural acid sulfate environments than the faster dissolution of clean synthetic jarosite. However, the shift in mechanism over a few days from near-congruent dissolution producing little acidity but much mobile Fe³⁺, to production of acidity as that Fe³⁺ precipitates, indicates that acidity can readily spread from jarosite dissolving under moderately acid conditions.