Exploring passivation-based treatments for jarosite from an acid sulfate soil

Abstract

Jarosite (KFe3(SO4)2(OH)6) is a common secondary reaction product of iron sulfide oxidation and can pose a considerable risk to soil and water quality. The overarching objective of this study was to explore the possibility of employing passivation-based treatments to mitigate the risks associated with jarosite-rich materials by investigating the alteration of pedogenic jarosite to a relatively benign, sparingly-soluble mineral such as goethite or strengite. Samples of jarositic phytotubules (from an acid sulfate soil with sulfuric material) were treated with: (i) alkaline solutions with a view to produce ferric (oxyhydr)oxide; (ii) phosphate solutions with a view to produce ferric phosphate; and (iii) alkaline solutions, followed by phosphate solutions, with a view to produce ferric phosphate. These treatments were conducted at both ambient (25°C) and elevated temperatures (80°C) to explore the effects of temperature on the extent of jarosite alteration. Under alkaline conditions, jarosite readily decomposed to yield poorly-crystalline ferric (oxyhydr)oxides (e.g. two-line ferrihydrite), regardless of temperature. These ferric (oxyhydr)oxides readily reacted with phosphoric acid to yield ferric phosphate; and with monoammonium phosphate (MAP) to yield spheniscidite ((NH4,K)(Fe,Al)2(PO4)2(OH).2H2O). Jarosite reacted with phosphoric acid to produce strengite or phosphosiderite (depending on the reaction temperature) and, to a lesser extent, gengenbachite (KFe3(HPO4)4(H2PO4)2.6H2O). Similarly, jarosite reacted with MAP to produce hydrogen ammonium ferric phosphate (H2(NH4)Fe(PO4)2). The direct alteration of jarosite to ferric phosphate appears to be very limited under ambient conditions. However, this study predominantly focuses on changes to the bulk chemical composition of jarosite. Consequently, further investigation (e.g. SEM analysis) is recommended to explore subtle changes in surface chemistry and the related effects on jarosite reactivity. Column perfusion experiments were also conducted to study the release of key elements during the dissolution of NaOH- and MAP-treated jarosite. The results suggest that these treatments do not considerably lower the risks to soil and water quality posed by jarosite dissolution. The practicality of the treatments is discussed, including the potential negative side effects. Further investigation is required to determine if these, or alternative, passivation treatments could be employed in the field to effectively mitigate the environmental risk associated with jarosite.