Iron (oxyhydr)oxides are widespread in the environment and have a disproportionate effect on the fate of metal(loid)s in groundwater. Numerous studies demonstrate the effect of iron-sulfur dynamics on contaminant sequestration, and particle size further impacts the reactivity of iron (oxyhydr)oxides, including their capacity for sorption and precipitation. This study examines the role of particle size on the sulfidation of iron (oxyhydr)oxides. Synthetic hematite nanoparticles with approximate diameters of 7.1 nm and 104 nm were coated onto quartz sand and reacted with bisulfide by advective flow.
Characterization of these nanoparticles revealed hydroxyls present at the hematite surface, with large particles having a higher hydroxide content per unit of surface area. Analysis of effluent from advective flow experiments show that the hematite nanoparticles undergo reductive dissolution of iron followed by precipitation of FeS. Iron concentrations increased with increasing bisulfide concentration, and ferrous iron in the effluent was higher for small particles than large particles when eluted with 10 mM bisulfide and lower with 0.1 and 1 mM bisulfide. Mineral transformations indicative of FeS precipitation occurred at 1 mM for small particles and at 10 mM for both particle sizes.
The log kFe calculated through kinetic model were -6.2 for large particles and -7.9 for small particles and indicates that reductive dissolution is faster for large particles. The reaction rate for surface area loss, RA, over time revealed that precipitation and associated surface passivation controlled initial surface area, influent bisulfide concentration, and initial reaction rate. Initial reaction rates for area and reductive dissolution of iron appear to be impacted by surface hydroxylation. Small particles show that reaction kinetics are not simplistic, and an optimization issue is present in surface mediated reactions where particle size is a factor.
The fundamental experimentation and subsequent model developed in this study could be important to better quantifying the role of iron (oxyhydr)oxide nanoparticles, whether engineered or naturally occurring, on the sequestration of metal(loids) posing a risk to public health.
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Lad, Uma, "Scale-Dependence of Hematite Nanoparticle Sulfidation" (2021). Earth Sciences Theses and Dissertations. 20.