The rare earth elements (REEs), which includes scandium, yttrium and the lanthanide series elements, have similar ionic radii and valent states. As a result, the REEs commonly substitute for each other in mineral structures and display similar geochemical distribution patterns. The ionic radii of the lanthanides decrease with increasing atomic number, a phenomenon referred to as the lanthanide contraction. We exploited this systematic variation to probe the reactivity of nano-scale confined surfaces in mesoporous silica and alumina. We used commercially available mesoporous silicas with 4 nm and 8 nm pores (SBA-15), and mesoporous alumina with 3.8 nm pores (MSU-X). We reacted these mesoporous substrates with aqueous mixtures of REEs (Sc, Y, Ce. Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) to assess batch and competitive surface adsorption trends relating to substrate pore sizes.<br/>Mesoporous alumina had a slightly lower surface-area-normalized total REEs uptake, compared to both mesoporous silicas. The uptake of individual elements on mesoporous silicas varied as a function of atomic number, or ionic radii. With decreasing ionic radius (increasing z), the uptake increased. This was most pronounced at the highest tested aqueous concentration of REEs. We attribute this observation to competition for adsorption sites between the elements present in the aqueous phase, while at lower aqueous concentrations of REEs, the number of adsorption sites is likely not limited. The detailed adsorption behavior, kinetics of adsorption, and proposed chemical mechanisms for the observed size- and mineral-surface-dependent behavior will be discussed.

Probing nano-scale confinement effects with rare earth elements

256th National Meeting (2018)

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