Exploring the Composition, Dissolution Kinetics and Passivation of Jarosite from an Acid Sulfate Soil

Abstract

Jarosite minerals (XFe(SO4)2(OH)6, where X is typically K or Na) are common secondary reaction products of iron sulfide oxidation. Consequently, jarosite is a diagnostic feature of sulfuric material (pH < 4) in acid sulfate soils (ASS), where it constitutes the principal source of retained acidity: an operationally defined pool of potential acidity associated with the hydrolysis of Al3+ and Fe3+ from minerals such as jarosite and schwertmannite. Soils containing considerable quantities of jarosite tend to exhibit hazardous properties (e.g. acidic and metalliferous drainage) for years to decades after the sulfuric material formed. This appears to be related to the slow mineral dissolution rate coupled with the ability of jarosite to buffer the soil solution between pH 3-4. Effectively managing such soils requires an accurate understanding of jarosite-rich sulfuric material, its long-term behaviour and effects on soil and water quality, and potential remediation strategies. However, the results described in the literature concerning jarosite solubility and jarosite kinetics are highly variable. Furthermore, there are currently no effective means of directly nullifying the environmental risks associated with jarosite in soils. Conventional ASS management strategies such as liming or dewatering and filling/capping ASS may be costly, have limited ability to effectively nullify jarosite while preventing acidity export, and may limit land-use options. Mineral surface passivation techniques – where retained acidity remains in situ, but dissolution is slowed or prevented – may be an effective alternative remediation strategy. However, the efficacy of these techniques has not been trialled for jarosite in ASS. Therefore, the work described in this thesis relates to three principal objectives: (1) to characterise jarosite-rich sulfuric material, (2) to provide a better understanding of how this material affects and is affected by the chemistry of the solution in which it dissolves; (3) to test potential jarosite passivation strategies. In order to carry out these objectives, jarosite-rich segregations were extracted from an ASS in the Gillman area within the southern Barker Inlet estuary, South Australia. A range of chemical and physical assays were conducted, and showed that the jarosite-rich segregations were primarily composed of jarosite (KFe3(SO4)2(OH)6), and were admixed with minor amounts of quartz, halite, gypsum, muscovite and organic matter. A gradient perfusion technique was utilised to describe the dissolution chemistry of this jarosite sample under constant flow conditions and at a range of pH values. The rate of dissolution (R) ranged from 3.16E-13 to 3.16E-11 mol/m2s, and was strongly correlated to pH by the function: log10R = 0.099pH2 – 0.966pH – 9.914. Monitoring differences between eluant (input) and eluate (output) pH clearly demonstrated that jarosite dissolution has a strongly acidifying effect, and should buffer soil pH around 4. Regardless of pH, the initial 12-24 hours of dissolution was characterised by the rapid release of Ca, Mg, Na and S (due to the dissolution of soluble minerals such as gypsum and halite). Consequently, flushing acidity from even highly weathered soils containing jarosite-rich sulfuric material is not advisable, as the resulting acidic, saline drainage may cause severe offsite environmental impacts. However, the dissolution experiments also suggest that the dissolution of jarosite under alkaline conditions is characterised by the formation of poorly-crystalline, ferric (oxyhydr)oxides that may possibly coat jarosite surfaces and hindered further dissolution. Therefore, promoting alkaline soil conditions may reduce the export of acidity by altering jarosite, or its surface, to more chemically benign and environmentally stable ferric (oxyhydr)oxides. This study was published in Chemical Geology (Trueman et al., 2020). In light of these conclusions, I examined the composition and dissolution kinetics of jarosite that was treated with a view to produce passivating coatings of ferric (oxyhydr)oxide (e.g. goethite, 𝛼-FeOOH) and ferric phosphate (e.g. strengite, FePO4.2H2O). I confirmed that under alkaline conditions jarosite readily decomposes to yield poorly-crystalline ferric (oxyhydr)oxides (i.e. two-line ferrihydrite). Moreover, these ferric (oxyhydr)oxides reacted with phosphoric acid to produce ferric phosphate; and reacted with monoammonium phosphate (MAP) to produce spheniscidite: an extremely rare ferric hydroxyphosphate. The direct alteration of jarosite to a ferric phosphate was constrained under ambient conditions. However, at elevated temperatures, jarosite reacted with phosphoric acid to produce phosphosiderite (a metastable dimorph of strengite) and gengenbachite (KFe3(HPO4)4(HPO4)2.6H2O); and reacted with MAP to produce an unnamed ammonium ferric phosphate (H2(NH4)Fe(PO4)2). The dissolution kinetics of the NaOH- and MAP-treated jarosite was examined using column perfusion experiments, which suggest these treatments do not offer any distinct advantages in terms of curtailing the release of potentially hazardous elements. However, the study provides key insights into jarosite alteration pathways, and sets a solid foundation upon which to further explore alternative methods of remediating jarosite-rich soils. This study was published in Chemical Geology (Trueman e al. 2021). The findings presented in this thesis can be utilised to better predict the behaviour of jarosite in the environment and its effects on soil and water quality, and to design more effective remediation strategies for ASS with jarosite-rich sulfuric material. References Trueman, A.M., Fitzpatrick, R.W., Mosley, L.M., Mclaughlin, M.J., 2021. Exploring passivation-based treatments for jarosite from an acid sulfate soil. Chemical Geology, 561: 120034. https://doi.org/10.1016/j.chemgeo.2020.120034. Trueman, A.M., Mclaughlin, M.J., Mosley, L.M., Fitzpatrick, R.W., 2020. Composition and dissolution kinetics of jarosite-rich segregations extracted from an acid sulfate soil with sulfuric material. Chemical Geology, 543: 119606. https://doi.org/10.1016/j.chemgeo.2020.119606.