Size-dependent reactivity of hematite nanoparticles: environmental implications of dissolution mechanisms and aggregation

The interfacial reactivity of iron oxides such as hematite (Α-Fe2O3) has been extensively studied because they are naturally abundant materials that are also versatile in a range of engineering applications. A poorly understood determinate of iron oxide interfacial reactivity is their particle size, a factor that has received increasing scrutiny given the emergence of nanoscience and nanotechnology. In natural systems, size-dependent trends in dissolution may have a profound influence on the rate and extent of iron redox cycling, which will indirectly effect the numerous processes in which soluble ferric [Fe(III)] and ferrous [Fe(II)] iron subsequently participate.

This work seeks to establish (i) the size-dependent dissolution of hematite nanoparticles across a range of dissolution mechanisms (e.g. proton-promoted, ligand-promoted, thermal reductive and photoreductive dissolution) and pH values, (ii) the extent to which nanoparticle aggregation effects the reactivity of hematite nanoparticles and trends observed in size-dependent reactivity of such aggregates, and (iii) how size-dependent dissolution activity of hematite impacts the production of environmentally relevant reactive oxygen species in sunlit surface waters via the photo-Fenton chemical reaction. Results herein reveal that size-dependent reactivity for two sizes of hematite nanoparticles, 8 and 40 nm, is observed throughout all dissolution processes and mechanisms investigated. Notably, this work is among the first where size-dependent reactivity is clearly observed into circumneutral and neutral pH values representative of most natural systems. Enhanced reactivity of the smaller 8 nm hematite is likely due to available reactive surface area compared to its larger analog. Under native aggregation of nanopowders (i.e., minimal ionic strength and no sonication) and induced aggregation of nanoparticle suspensions (i.e. high ionic strengths) mass normalized rates of reductive dissolution were greater for aggregates of 8 nm hematite, and aggregate size exhibited little influence on size-dependent reactivity. Finally, size-dependent reactivity also occurs in model surface water reactions, specifically the dissolved organic matter mediated photo-Fenton reaction. Smaller nanophase hematite exhibits greater rates of Fe(II) production, which in turn yields greater steady-state hydroxyl radical concentrations. Insights from this work advance current knowledge of size-dependent reactivity of natural nanomaterials and their implications for pollutant fate and elemental cycling in environmental systems.