No abstract

The adsorption of aqueous ions onto natural mineral surfaces controls numerous mineral−water interactions and is governed by, among other numerous factors, ion dehydration and hydrolysis. This work explored the extent to which dehydration and hydrolysis affect the adsorption of three metal cations, Al 3+ , Cr 3+ , and Mn 2+ , onto quartz (SiO 2 ) and corundum (Al 2 O 3 ) surfaces at pH 3.8 through the integration of flow microcalorimetry (FMC), quartz crystal microbalance with dissipation (QCM-D) measurements and density functional theory (DFT) calculations. At pH 3.8, negligible amounts of Mn 2+ and Al 3+ are hydrolyzed, while 78% of Cr 3+ exist in hydrolyzed species. QCM-D and FMC measurements showed that Al 3+ and Cr 3+ adsorb to both surfaces, while Mn 2+ adsorbed only to Al 2 O 3 . DFT bond energy calculations confirmed the favorable bonding between the mineral surfaces and Al 3+ and Cr 3+ , and that Mn 2+ adsorption onto SiO 2 was unfavorable. Furthermore, FMC showed that on both surfaces, the adsorption of Al 3+ was endothermic and reversible, while that of Cr 3+ was exothermic and partially irreversible. Through the integration of experimental and computational methods, this work suggested that the reversible adsorption of unhydrolyzed cations (Mn 2+ and Al 3+ ) occurred through weak electrostatic interactions. The large energy cost required to dehydrate unhydrolyzed cations resulted in an endothermic adsorption process. Meanwhile, hydrolyzed Cr 3+ species can adsorb on quartz and corundum through covalent-bond formation, and thus, their adsorption was partially irreversible. Furthermore, the hydrolysis of Cr 3+ lowered the dehydration energy during adsorption, resulting in an exothermic adsorption. By using bond energies as a guide to indicate the possibility of thermodynamically favored adsorption, there was a strong agreement between the DFT and experimental techniques. The findings presented here contribute to understanding and predicting various mineral−water interfacial processes in the natural environment.

No abstract

Flow microcalorimetry was used to investigate the energetics associated with Rb, K, Na, Cl, and NO exchange at the rutile-water interface. Heats of exchange reflected differences in bulk hydration/dehydration enthalpies (Na > K > Rb, and Cl > NO) such that exchanging Na or Cl from the surface was exothermic, reflecting their greater bulk hydration enthalpies. Exchange heats were measured at pH 2, 3.25, 5.8, and 11 and exhibited considerable differences as well as pH dependence. These trends were rationalized with the aid of a molecularly constrained surface complexation model (SCM) that incorporated the inner-sphere binding observed for the cations on the rutile (110) surface. Explicitly accounting for the inner-sphere binding configuration differences between Rb, K, and Na, as well as accompanying differences in negative surface charge development, resulted in much better agreement with measured exchange ratios than by considering bulk hydration enthalpies alone. The observation that calculated exchange ratios agreed with those measured experimentally lends additional credence to the SCM. Consequently, flow microcalorimetry and surface complexation modeling are a useful complement of techniques for probing the energetics associated with ion exchange and adsorption processes and should also serve to help validate molecular simulations of interfacial energetics.

Nested electrospray ionization alternates on-demand between microscale main-channel and nanscale sub-channels.