title: Zn2+ and Sr2+ adsorption at the TiO2 (110)-electrolyte interface: influence of ionic strength, coverage, and anions.
abstract: The X-ray standing wave technique was used to probe the sensitivity of Zn2+ and Sr2+ ion adsorption to changes in both the adsorbed ion coverage and the background electrolyte species and concentrations at the rutile (alpha-TiO2) (110)-aqueous interface. Measurements were made with various background electrolytes (NaCl, NaTr, RbCl, NaBr) at concentrations as high as 1 m. The results demonstrate that Zn2+ and Sr2+ reside primarily in the condensed layer and that the ion heights above the Ti-O surface plane are insensitive to ionic strength and the choice of background electrolyte (with <0.1 A changes over the full compositional range). The lack of any specific anion coadsorption upon probing with Br-, coupled with the insensitivity of Zn2+ and Sr2+ cation heights to changes in the background electrolyte, implies that anions do not play a significant role in the adsorption of these divalent metal ions to the rutile (110) surface. Absolute ion coverage measurements for Zn2+ and Sr2+ show a maximum Stern-layer coverage of approximately 0.5 monolayer, with no significant variation in height as a function of Stern-layer coverage. These observations are discussed in the context of Gouy-Chapman-Stern models of the electrical double layer developed from macroscopic sorption and pH-titration studies of rutile powder suspensions. Direct comparison between these experimental observations and the MUltiSIte Complexation (MUSIC) model predictions of cation surface coverage as a function of ionic strength revealed good agreement between measured and predicted surface coverages with no adjustable parameters.
authors.affiliation: Environmental Research Division, Argonne National Laboratory, Argonne, IL 60439, USA. zhanzhang@anl.gov
authors.forename: Z
authors.initials: Z
authors.lastname: Zhang
authors.forename: P
authors.initials: P
authors.lastname: Fenter
authors.forename: L
authors.initials: L
authors.lastname: Cheng
authors.forename: N C
authors.initials: NC
authors.lastname: Sturchio
authors.forename: M J
authors.initials: MJ
authors.lastname: Bedzyk
authors.forename: M L
authors.initials: ML
authors.lastname: Machesky
authors.forename: L M
authors.initials: LM
authors.lastname: Anovitz
authors.forename: D J
authors.initials: DJ
authors.lastname: Wesolowski