From schwertmannite to natrojarosite: Long-term stability and kinetic approach

This work examines the transformation of iron-bearing precursors to jarosite-like minerals in the absence of bacteria or other organic compounds. The composition of the aqueous solution determines the transformation, through which crystallinity and long-term stability of jarosite increase, whereas the temperature of the environment affects the kinetics of the process. Spectroscopic techniques (FTIR and XPS) were used to characterize the chemical species present on the transformed mineral surfaces. Schwertmannite is the first phase to precipitate as a result of homogeneous nucleation and growth in the bulk of the supersaturated solution. This metastable phase transforms into a crystalline Na-rich member of the (Na,H3O)Fe3(SO4)(OH)6 solid-solution family after aging for either 3 h at 70 °C or 1 day at 20 °C. XRD analyses show that the crystallinity of natrojarosite increases progressively with reaction time, although its cell parameters and crystallite size remain nearly constant during aging, which reveals the stability of the crystal structure of this secondary phase. Interestingly, the mechanisms governing the transformation from aggregates of schwertmannite into natrojarosite crystals consist of interface-coupled dissolution–precipitation reactions that involve an internal structural reorganization within the individual nanoparticles of the secondary phase, in which Fe3+ is transferred from the solid to the solution while SO4 2−, OH–, and Na+ move in the opposite direction. The spectroscopic study confirms the mineralogical results and suggests that the crystal structure of jarosite-like minerals may offer interesting geochemical information about the aqueous solutions where they were formed. The transformation kinetics and the apparent activation energy (Ea = 52.1 kJ/mol) of the transformation were estimated using the so-called “time to a given fraction” method, and a temperature-transformation-time (TTT) diagram was established in the range 20–70 °C to define the reaction pathways during the process.