Nanoparticle Size Effects on Cadmium Uptake on Hematite: Revisiting the Sorption Edge Approach

Abstract

Abstract

Mineral nanoparticles (NPs) can play important roles in metal fate and transport; yet, they often exhibit different behaviors with respect to metal sorption, especially when NP size drops below a few tens of nanometers. Studies of NP size effects on metal uptake often compare sorption “edges” for different NPs at a given metal concentration over a range of pH values. Because surface area increases dramatically as NP size decreases, failure to normalize to particle surface area (rather than mass) in “edge” experiments can lead to different metal- to mineral-surface ratios in reaction vessels. We explored the effects of hematite NP size (8, 40 nm) on cadmium (Cd) uptake in systems near Cd mineral (super)saturation, comparing mass-normalized (MN) and surface area-normalized (SAN) approaches. An ∼1 pH unit shift in sorption edge was observed as a function of NP size, regardless of approach. Effects of experimental approach on mechanism were most pronounced at pH 9.0, where adsorption and minor precipitation were observed for 8 nm NPs, a more balanced mixture of adsorption and precipitation on 40 nm SAN NPs, and substantially more precipitation on 40 nm MN NPs. Experimental results thus reconfirm that NP size can affect metal sorption, underscoring the need to design experiments that explicitly account for NP surface area differences. Results also suggest that development of conceptual and quantitative models for the effects of NPs on metal mobility will need to take into account the potential effects of NP size on reactivity.