Inner-Helmholtz potential development at the hematite (α-Fe2O3) (0 0 1) surface
2011 (English)In: Geochimica et Cosmochimica Acta, ISSN 0016-7037, E-ISSN 0016-1258, Vol. 75, no 15, 4113-4124 p.Article in journal (Refereed) Published
Electric potentials of the (0 0 1) surface of hematite were measured as a function of pH and ionic strength in solutions of sodium nitrate and oxalic acid using the single-crystal electrode approach. The surface is predominantly charge-neutral in the pH 4–14 range, and develops a positive surface potential below pH 4 due to protonation of μ-OH0 sites (pK1,1,0,int = −1.32). This site is resilient to deprotonation up to at least pH 14 (−pK−1,1,0,int ≫ 19). The associated Stern layer capacitance of 0.31–0.73 F/m2 is smaller than typical values of powders, and possibly arises from a lower degree of surface solvation. Acid-promoted dissolution under elevated concentrations of HNO3 etches the (0 0 1) surface, yielding a convoluted surface populated by sites. The resulting surface potential was therefore larger under these conditions than in the absence of dissolution. Oxalate ions also promoted (0 0 1) dissolution. Associated electric potentials were strongly negative, with values as large as −0.5 V, possibly from metal-bonded interactions with oxalate. The hematite surface can also acquire negative potentials in the pH 7–11 range due to surface complexation and/or precipitation of iron species (0.0038 Fe/nm2) produced from acidic conditions. Oxalate-bearing systems also result in negative potentials in the same pH range, and may include ferric-oxalate surface complexes and/or surface precipitates. All measurements can be modeled by a thermodynamic model that can be used to predict inner-Helmholtz potentials of hematite surfaces.
Place, publisher, year, edition, pages
2011. Vol. 75, no 15, 4113-4124 p.
IdentifiersURN: urn:nbn:se:umu:diva-47885DOI: 10.1016/j.gca.2011.05.013OAI: oai:DiVA.org:umu-47885DiVA: diva2:445199