Effect of Stoichiometry on Nanomagnetite Sulfidation

Area of Science:

• Geochemistry
• Environmental Science
• Nanotechnology

Background:

• Magnetite (Mt) is typically considered unreactive to dissolved sulfide.
• Natural magnetite's variable stoichiometry (Fe(II)/Fe(III) ratio, x_stru) may influence its reactivity.
• The impact of magnetite stoichiometry on sulfidation processes is not well understood.

Purpose of the Study:

• To investigate how magnetite stoichiometry affects sulfidation processes and products.
• To determine the role of structural Fe(II)/Fe(III) ratio (x_stru) in controlling sulfide oxidation rates and extents.
• To elucidate the mechanism of interfacial electron transfer (IET) in magnetite sulfidation.

Main Methods:

• Synthesis and characterization of magnetite nanoparticles (11 ± 2 nm) with varying stoichiometries (x_stru).
• Experimental study of sulfide oxidation by magnetite nanoparticles at controlled pH (7.0-8.0) and Fe/S molar ratios (10-50).
• Analysis of reaction products, including elemental sulfur (S⁰) and iron sulfide (FeS) clusters, using surface adsorption and electron transfer measurements.

Main Results:

• Magnetite stoichiometry (x_stru) is a key factor controlling sulfide oxidation rates and extents.
• Partially oxidized magnetite (x_stru = 0.19-0.43) oxidizes sulfide to S⁰, while nearly stoichiometric magnetite (x_stru = 0.47) shows only surface adsorption.
• Higher oxidation states of magnetite correlate with increased sulfide oxidation and S⁰ production due to enhanced electron-accepting capability.
• FeS clusters formed during sulfidation can be oxidized by highly oxidized magnetite (x_stru = 0.19).
• A linear correlation between Gibbs free energy and surface area-normalized oxidation rate suggests IET dominates sulfidation under studied conditions.

Conclusions:

• Magnetite stoichiometry significantly dictates its reactivity in sulfidation reactions.
• Interfacial electron transfer (IET) is the dominant mechanism in magnetite sulfidation at high Fe/S ratios and near-neutral pH.
• Understanding magnetite stoichiometry is crucial for predicting its environmental fate and behavior in sulfidation processes.