Insights for size-dependent reactivity of hematite nanomineral surfaces through Cu2+ sorption

Macroscopic sorption edges for Cu2+ were measured on hematite nanoparticles with average diameters of 7 nm, 25 nm, and 88 nm in 0.1 M NaNO3. The pH edges for the 7 nm hematite were shifted approximately 0.6 pH units lower than that for the 25 nm and 88 nm samples, demonstrating an affinity sequence of 7 nm > 25 nm = 88 nm. Although, zeta potential data suggest increased proton accumulation at the 7 nm hematite surfaces, changes in surface structure are most likely responsible for the preference of Cu2+ for the smallest particles. As Cu2+ preferentially binds to sites which accommodate the Jahn-Teller distortion of its coordination to oxygen, this indicates the relative importance of distorted binding environments on the 7 nm hematite relative to the 25 nm and 88 nm particles. This work highlights the uniqueness of surface reactivity for crystalline iron oxide particles with decreasing nanoparticle diameter. (c) 2006 Elsevier Inc. All rights reserved.