Composition and dissolution kinetics of jarosite-rich segregations extracted from an acid sulfate soil with sulfuric material

Abstract

Jarosite (KFe3(SO4)(OH)6) is known to play a key role in perpetuating chemically aggressive properties in acid sulfate soils with sulfuric material (pH < 4) – primarily through the release of retained acidity. The purpose of this study was to describe: (i) the composition of jarosite-rich segregations handpicked from sulfuric material and (ii) the dissolution chemistry of the jarosite-rich segregations under constant flow conditions and at a range of pH values. The jarosite segregations were primarily composed of K-jarosite and were admixed with minor amounts of quartz, halite, gypsum, muscovite and organic matter. The first 12–24 h of dissolution is characterised by the rapid release of Ca, Mg, Na and S, which is attributable to the dissolution of soluble minerals such as gypsum and halite. Monitoring the differences between eluant and eluate pH showed that the release of acidity due to jarosite dissolution will resist pH increases above pH 4. The Fe/S, Fe/K and S/K molar ratios suggests that dissolution was only congruent under extremely acidic conditions (pH < 2) or in the presence of a chelating agent. Incongruent dissolution was characterised by precipitation of Fe-rich solids, which immobilised trace metal(loid)s and may have slowed jarosite dissolution through surface passivation. The rate of dissolution (in terms of log10R, R = mol/m2s) ranged from −10.5 to −12.5. Moreover, log10R and pH were nonlinearly related, so that dissolution was slowest between pH 4 and 5, and increased with both increasing and decreasing pH. These results suggest that jarosite dissolution may be able to maintain acidic soil conditions (e.g. pH < 5) even if the soil is constantly flushed. This is consistent with field observations of acid sulfate soils with sulfuric materials. Therefore, flushing acidity from soils containing jarosite-rich sulfuric material may not be a viable remediation strategy. Alternatively, promoting alkaline soil conditions may convert jarosite to more chemically benign and stable Fe (hydr)oxides.