Characterization and surface reactivity of natural and synthetic magnetites

- Natural magnetite samples are composed of aggregated nanoparticles.
- Aggregate size ranges considerably affect magnetite adsorption behavior.
- Increasing experimental solids concentration increases the degree of aggregation.
- Therefore, this increase in solids concentration decreases As(V) adsorption.
- 30 nm particles produce the largest aggregates but no surface area blockage.

Magnetite is an Fe (II/III) oxide mineral that occurs naturally and potentially as small particles with significant surface reactivity, and although much work is reported on synthetic material, little work exists for natural samples. The goal of the present work was to carefully characterize four natural magnetite samples from an iron ore deposit and two synthetic commercial reference samples, and to compare their surface characteristics and reactivity with the aim of evaluating their geochemical behavior towards adsorption of environmentally relevant ions, as well as their potential for use as environmental remediation sorbents. The techniques used were wet chemistry, X-ray diffraction, Raman spectroscopy, magnetic measurements (hysteresis curves), dynamic light scattering, scanning electron microscopy, low and high resolution transmission electron microscopy, BET Nitrogen adsorption, and electrophoresis. In addition, their As(V) adsorption behavior was measured at pH 6, and was analyzed as related to the surface characteristics and particle aggregation behavior determined. The analyses revealed high magnetite purity in the natural samples, and specific surface areas (SSA) ranging from 1 to 8 m2/g. Small alumino-silicate impurities were found in natural magnetites, apparently occurring at the particle surfaces and thus significantly lowering their isoelectric points as compared to the pure synthetic materials. All samples are composed of aggregates of 39-52 nm magnetite particle units, but highly aggregated with very large size dispersions. The synthetic sample with the smallest particle size (30 nm in average - 39 m2/g) showed its entire surface area available for adsorption, despite its highly aggregated state observed, suggesting an open and highly dynamic aggregate framework. The other larger samples showed more complex aggregation behavior, which produced: (1) a widely variable As(V) adsorption behavior with no clear predictable pattern among samples; and (2) a large decrease of the As(V) adsorption maxima with increasing solids concentration imposed in the experimental set-up for any one particular sample. Therefore, we recommend high caution in using the BET-SSA and solids concentration parameters when performing experimental adsorption work with microsized magnetite, especially when extrapolating laboratory results to field geochemical or engineered conditions for evaluating contaminant adsorption.