Sorption of Brilliant Blue FCF in soils as affected by pH and ionic strength

Abstract

Brilliant Blue FCF has been frequently used as a dye tracer to stain flow pathways in porous media. As an ionic organic molecule, the dye interacts in a complicated manner with the solid phase. For an adequate interpretation of stained flow pathways, the sorption characteristics of the dye need to be better understood. In this study, we investigate the suitability of Brilliant Blue FCF as a dye tracer in vadose zone hydrology. The objectives were to test the effect of aqueous solution chemistry on dye absorption spectra, and to evaluate the effect of ionic strength and type of cations on dye sorption to soils. Batch sorption studies were conducted with three different soils and different ionic strengths of the background solution. Ionic strength was adjusted with either CaCl 2 or KCl. Sorption isotherms conformed to the Langmuir model, with the sorption capacity ranging over one order of magnitude. Substantial sorption was found for the soil sample with the highest clay content and the lowest pH. Increasing ionic strength led to increased sorption of Brilliant Blue FCF. The type of background cation, Ca or K, did not influence sorption. In aqueous solution, the absorption spectrum of Brilliant Blue FCF is not sensitive to pH nor ionic strength.