Investigation of mechanisms contributing to slow desorption of hydrophobic organic compounds from mineral solids

This research investigates the mechanisms contributing to the slow desorption of hydrophobic organic compounds from water-saturated mineral solids. The mechanisms investigated were adsorption-retarded aqueous diffusion, micropore diffusion, high-energy micropore adsorption, and micropore blockage by precipitated minerals. To reduce the potential confounding effects of adsorbent heterogeneity, a set of homogeneous silica gel acid glass bead adsorbents were used in the investigation. Desorption rates for the slow-desorbing fractions of chloroform (CF), trichloroethylene (TCE), and perchloroethylene (PCE) from silica gel did not conform to the pore-diffusion model for adsorption-retarded aqueous diffusion. This indicated that diffusion through adsorbent mesopores was not responsible for slow desorption from silica gel. Micropore-diffusion modeling of TCE desorption from three silica gels and microporous glass beads indicated that pores less than 2 nm in diameter were responsible for slow desorption. Desorption rates for CF, TCE, and PCE from silica gel were also measured in methanol solutions. Under methanol extraction conditions, desorption rates for all three compounds were 1-2 orders of magnitude less than under water-saturated conditions. This indicated that high-energy adsorption was not responsible for the slow-desorbing fraction, and suggested that mineral precipitation leads to blockage of intragranular micropores. The activation energy for TCE desorption from water-saturated silica gel was measured using temperature-programmed desorption. The TCE desorption activation energy of 15 kJ/mol was close to the dissolution enthalpy for silica gel of 13 kJ/mol. This supported the hypothesis that micropore blockage by precipitated minerals may be limiting contaminant desorption rates under water-saturated conditions.