Sorption and intraparticle diffusion of fluorescent dyes with consolidatedaquifer media

Sorption of two fluorescent dyes (fluorescein and sulforhodamine B) was eva luated with two oppositely charged, consolidated aquifer materials (sandsto ne and limestone). Fluorescein, which has an anionic carboxylic group, expe rienced negligible sorption onto negatively charged sandstone and sorbed mu ch less than sulforhodamine B, with its two sulfonic groups, onto positivel y charged Limestone. The cationic charge on sulforhodamine B caused it to a dsorb onto negatively charged sandstone. These results validate and extend previous research with pure mineral surfaces (alumina and silica), Sorption kinetic rates decreased with increasing particle size which is consistent with diffusion-limited intraparticle sorption, Tortuosity factors were dete rmined by fitting a diffusion-limited intraparticle sorption model to kinet ic data for one particle size (all other parameters being determined indepe ndently). This tortuosity factor, which was consistent with Archie's law-ba sed estimates and other independently determined parameters, was subsequent ly used to predict kinetic adsorption results for a second particle size. C lose agreement between experimental data and model predictions corroborated intraparticle diffusion-limited sorption as an important process. Diffusio n-limited sorption can impact dye transport in unconsolidated grains having internal porosity or consolidated or fractured media having dual porosity, These results thus demonstrate the importance of understanding both the eq uilibrium and kinetics of dye sorption when designing and interpreting trac er studies.