Adsorption of organic matter at mineral/water interfaces: 7. ATR-FTIR and quantum chemical study of lactate interactions with hematite nanoparticles

The interaction of the L-lactate ion (L-CH₃CH(OH)COO ^-, Lact^-1) with hematite (α-Fe₂O ₃> nanoparticles (average diameter 11 nm) in the presence of bulk water at pH 5 and 25°C was examined using a combination of (1) macroscopic uptake measurements, (2) in situ attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, and (3) density functional theory modeling at the B3LYP/6-31+G* level. Uptake measurements indicate that increasing [Lact^_1]_(aq) results in an increase in Lact^-1 uptake and a concomitant increase in Fe(III) release as a result of the dissolution of the hematite nanoparticles. The ATR-FTIR spectra of aqueous Lact^-1 and Lact^-1 adsorbed onto hematite nanoparticles at coverages ranging from 0.52 to 5.21 μmol/m² showed significant differences in peak positions and shapes of carboxyl group stretches. On the basis of Gaussian fits of the spectra, we conclude that Lact^-1 is present as both outer-sphere and inner-sphere complexes on the hematite nanoparticles. No significant dependence of the extent of Lact^-1 adsorption on background electrolyte concentration was found, suggesting that the dominant adsorption mode for Lact^-1 is inner sphere under these conditions. On the basis of quantum chemical modeling, we suggest that inner-sphere complexes of Lact^-1 adsorbed on hematite nanoparticles occur dominantly as monodentate, mononuclear complexes with the hydroxyl functional group pointing away from the Fe(III) center.