Anions Enhance Rare Earth Adsorption at Negatively Charged Surfaces

Anions are expected to be repelled from negatively charged surfaces. At aqueous interfaces, however, ion-specific effects can dominate over direct electrostatic interactions. Using multiple in situ surface sensitive experimental techniques, we show that surface affinity of SCN^- ions are so strong that they can adsorb at a negatively charged floating monolayer at the air/aqueous interface. This extreme example of ion-specific effects may be very important for understanding complex processes at aqueous interfaces, such as chemical separations of rare earth metals. Adsorbed SCN^- ions at the floating monolayer increase the overall negative charge density, leading to enhanced trivalent rare earth adsorption. Surface sensitive X-ray fluorescence measurements show that the surface coverage of Lu³⁺ ions can be triple of the apparent surface charge of the floating monolayer in the presence of SCN^-. Comparison to NO₃^- samples show that the effects are strongly dependent to the character of the anion, providing further evidence to ion-specific effects dominating over electrostatics.