DIFFUSIVE TRANSPORT OF CYCLODEXTRIN THROUGH IGNEOUS ROCKS

Diffusion of chemical compounds from the aqueous solution into the rock matrix is an important transport processes, especially in fractured rock aquifers. Although transport by diffusion is a relative slow process, chemicals can penetrate the rock matrix to significant depths, given enough time (months to years). The rate of propagation mainly depends on the rock diffusion coefficient of the diffusing compound. Thus, the rock diffusion coefficient is an important design parameter, for example, when remediating contaminated aquifers. For this laboratory study, we have measured the diffusion coefficient for iodide and hydroxypropyl-beta-cyclodextrin (HPCD) in three igneous rocks. Iodide is a conservative tracer and iodide diffusion in various rock types has been studied previously. HPCD is a sugar-based complexing agent currently being tested for enhanced remediation of contaminated aquifers. Although considered a conservative tracer in unconsolidated rock type of aquifers, the HPCD molecule is comparably large and there is concern about pore size exclusion when HPCD is diffusing into solid rocks. The potential impact of pore size exclusion on HPCD transport is twofold, first, HPCD may not reach small pore compartments in the rock, and second, may therefore not form a complex with the contaminant it is supposed to remove. The results of the diffusion experiments will be evaluated by comparing the iodine experiments to the HPCD results and by relating them to previous findings from other types of rocks and tracers.