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

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.A.M.Trueman, M.J.McLaughlin, L.M.Mosley, R.W.Fitzpatric

Extreme biogeochemical effects following simulation of recurrent drought in acid sulfate soils

Droughts are increasing in frequency and severity in many regions of the world and there are uncertainties how recurrent drought will impact acid sulfate soils, which can undergo extreme and persistent acidification (pH < 4) following oxidation. Using column experiments, we induced a 9 week drought/drying and 9 week rewetting phase in two acid sulfate soil profiles with sulfuric (pH < 4) and hypersulfidic materials from the Lower River Murray region of Australia’s largest river system. These soils had not yet recovered from the extreme period of the ‘Millennium Drought’ between 2007 and 2010. pH, redox potential, dissolved metals and greenhouse gases (CO2, CH4 and N2O) were measured at multiple depths in each column every 3 weeks during the drying and rewetting phases. The solid phase of the Sulfuric and Hypersulfidic clay soil profiles in both column experiments were analysed for complete acid-base accounting (actual and retained acidity, potential acidity in the form of pyrite, and acid neutralising capacity) and reactive metals at the beginning and end of the experiment. Residual pyrite, present in high concentrations in the Sulfuric clay soil column, oxidised during the drying period, further lowering the pH (<4) and mobilising dissolved metals (Al, Cd, Cu, Ni, Zn). More reactive Fe and Al phases were formed during the drying-rewetting cycle while reactive Mn decreased. The soil pH did not recover (i.e. increase) during the rewetting phase in both the Sulfuric and Hypersulfidic clay soils, likely due to barriers to microbial reduction reactions, although Fe-oxidising bacteria were likely still active as CO2 was released. Acid sulfate soils may not recover in inter-drought periods and more severe impacts can be expected following recurrent droughts.Luke M. Mosley, Tan Dang, Michael J. McLaughlin, Rob W. Fitzpatric