Goethite is dispersed throughout Earth's surficial environment and is a major host of Ni. Here we explore Ni cycling and its negative feedback on Fe(II)-catalyzed recrystallization of goethite by (i) suspending Ni-substituted goethite in Ni(II)-free solutions and (ii) reacting pure goethite with Ni(II)aq in the presence and absence of 57Fe-enriched Fe(II)aq at pH 7.5. Roughly 10% of substituted Ni is released from the crystal structure of goethite by Fe(II)aq and accumulates in solution and at the mineral surface, whereas only 1% of Ni release occurs in Fe(II)-free controls after 41 days of reaction. Dissolved Ni readily sorbs onto pure goethite and is mostly recovered through an acid extraction, but in the presence of Fe(II)aq, a significant fraction of Ni incorporates into the mineral and is only recoverable after complete dissolution of the solid. Sequential dissolution of goethite suggests that Ni penetrates deep into the bulk structure when there are extensive amounts of recrystallization, as evidenced by the near congruent dissolution of Ni and Fe. Changes in the tracer isotope (57Fe) composition of Fe(II)aq and (Ni-substituted) goethite, along with an increase in the goethite crystallite size, show that Ni cycling is connected to Fe(II)-catalyzed recrystallization. The extent of recrystallization decreases as Ni substitution (compared to pure goethite having the same initial crystallinity) and Ni(II)aq concentration increase. Inhibition, however, is more pronounced when Ni accumulates at the mineral surface. These findings show that Fe(II)-catalyzed recrystallization can redistribute Ni among iron oxide lattices, surfaces, and aqueous solutions, but negative feedback by sorbed Ni diminishes such reactivity. Goethite recrystallization may be enhanced in open systems where labile Ni is periodically leached.
- Iron oxides
- Isotope tracer
Peter Miller (Manager)Office of the Vice-Provost (Research and Research Infrastructure)
James Griffith (Manager)Office of the Vice-Provost (Research and Research Infrastructure)