The mineral-solution interface plays a fundamental role in many chemical and physical processes that shape almost all geochemical and biogeochemical phenomena. Great emphasis has been placed on studying the chemical reactions – protonation, adsorption, exchange, etc. that underlie the properties of this interface and on deciphering its molecular scale complexity through the lens of various experimental and computational techniques. Past studies uncovered a complex landscape of adsorption modes (inner-sphere (IS) and outer-sphere (OS)) for adsorbed ions on minerals. Recent studies have shown that for cations, dehydration energies have a significant influence on their partitioning between IS and OS species, whereby cations with increasing hydration enthalpies (ΔHhyd ) adsorb mostly as OS species on muscovite. Using flow microcalorimetry to measure the in situ enthalpies of adsorption, this study will investigate chemical trends in cation adsorption at the quartz-solution interface by probing the effect of size, ΔHhyd , and valence on the sign and size of the relative adsorption enthalpies (ΔHr.ads). Quartz is the second most abundant mineral on earth’s surface and is a well understood substrate that lends itself to modeling efforts. The cations chosen for this study are Li+, Na+, K+, Rb+, Cs+, and then Sr2+, and Y3+ listed here in an increasing order of valence and a decreasing order of DHhyd for the monovalent cations. The ΔHshyd increase with increasing valence for the chosen cations, i.e. Rb+, Sr2+ and Y3+. The ΔHr.ads of exchange were obtained using 50 mM solutions of the chloride salts in a 10-4 M HCl. The initial data shows a correspondence between the measured ΔHr.ads and the above-mentioned trends in size and hydration energies. The calorimetric measurements will be interpreted in light of molecular dynamics simulations and surface complexation models to further assess their utility in helping to decipher the interfacial reactivity of quartz.

Chemical trends in cation adsorption at the quartz-solution interface from microcalorimetry and modeling experiments

251st National Meeting (2016)

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