Interaction of Cr3+ with Silica Gel at the Aqueous Interface Using Fluorescence in Sodium Dodecyl Sulfate Micelles and Confocal Fluorescence Microscopy

Chemical composition and species distribution in clay surfaces are key factors which control sorption and desorption processes of Cr3+ between sediment and water. Adsorption of Cr3+ on silica gel has been studied by following the quenching of naphthalene fluorescence in aqueous sodium dodecyl sulfate (SDS) micelles, which concentrate naphthalene and Cr3+ in the micellar surface. Adsorption for pH ≤ 5 follows a Langmuir model, and experimental data fitting is successfully described by a model based on discrete sites on the surface with variable charge as a function of pH. Probably the mechanism of Cr3+ adsorption at the silica gel surface is predominantly electrostatic, with outer-sphere complexation and ion exchange with adsorbed H+ ions. Confocal laser-scanning fluorescence microscopy measurements reveal the Cr3+ spatial distribution in three dimensions. Specifically, both images and spectra measured with confocal microscopy indicate that Cr3+ is distributed across the entire particle, not just adsorption at the surface.