Size Dependent Structure and Reactivity of Nanohematite

The objective of this study was to determine the effects of particle size in the nanometer range on the physicochemical and reactive properties of hematite. This research investigated changes in the surface structure and composition as a function of size using three hematite nanoparticle samples with average diameters of 3.6, 8.6, and 40nm. The hypothesis was that as size decreased in the nanoscale, inherent structural changes would induce changes in hematite chemical reactivity with respect to the processes of adsorption and dissolution. Characterization of the hematite samples indicated increased surface hydroxylation as particle size decreased and a shift to lower pH for the point of zero net proton charge (pHpznpc) of the nanohematite surface. This resulted in distorted surface binding environments including non-ideal coordination of flanking atoms. These reduced symmetry and under-coordinated binding environments can have important implications for the chemical reactivity of hematite.

Adsorption experiments were completed using the toxic metal, Pb(II) and the common soil siderophore, desferrioxamine B (DFOB). Data illustrated changes in the adsorption of Pb(II) over the pH range 3-9 with a shift in the adsorption edge attributed to the differences in surface structure. DFOB adsorption remained minimal despite changes in particle size and pH. Kinetic dissolution experiments indicated that the presence of DFOB enhanced dissolution approximately 6 fold. In addition, it was observed that smaller particles (< 10nm) released more Fe and dissolved an order of magnitude faster than the 40nm particles in surface area normalized, DFOB-mediated dissolution experiments.

This study indicates that not only does specific surface area increase as particle size decreases, but also surface compositional changes occur that have important effects on the adsorption and dissolution behavior of hematite nanoparticles. This research and others like it, suggest the importance of accounting for structural changes of nanoparticles in models used to predict the fate and transport of metals, organic ligands, radionuclides and other contaminant compounds. Moreover, dissolution results show increased Fe release from smaller (3.6 and 8.6nm) particles, which could result in more bioavailable Fe from nanohematite versus bulk. Further experimentation is necessary to understand the complexities of changes experienced in the nano range that can impact partitioning and speciation of many environmental compounds.