Investigation of Mechanisms Contributing to Slow Desorption of Hydrophobic Organic Compounds from Mineral Solids

Abstract

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 and glass bead adsorbents were used in the investigation. Desorption rates for the slowdesorbing fractions of chloroform (CF), trichloroethylene (TCE), and perchloroethylene (PCE) from silica gel did not conform to the pore-diffusion model for adsorptionretarded 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 watersaturated 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.

Relationships between aging of PAHs and soil properties

“…Correlation analysis revealed that k rap was negatively dependent on TOC to a certain extent while no significance level was obtained. The pore distribution also exerted an important effect on the diffusion of HOCs (Pignatello and Xing, 1996;Farrell et al, 1999;Abu and Smith, 2006). Particularly, it was proposed that the pore fraction with diameters below 6 nm (hereafter specifically called PF 6 nm for short) predominately controlled the sorption of HOCs (Farrell and Reinhard, 1994;Pignatello and Xing, 1996).…”

Effect of microbial activity, soil water content and added copper on the temporal distribution patterns of HCB and DDT among different soil organic matter fractions

“…The sequestration of DDT and HCB is of considerable interest because of its effect on bioavailability and the risk-assessment paradigms used to guide management and cleanup of contaminated sites and soil. The processes of HOCs sequestration in soil are thought to be driven by partitioning into the soil organic matter (SOM) (Gustafsson et al, 1997;Cornelissen et al, 2005) and diffusing into soil micropores (Farrell et al, 1999). In soil with greater than 0.1% organic carbon content, partitioning into SOM has been found to be the dominant process (Chiou et al, 2000;Kang and Xing, 2005).…”

Impact of mineral micropores on transport and fate of organic contaminants: A review

“…The loosely bound water molecules present in the hydrophobic pore spaces are replaced by the organic molecules, while the tightly bound water molecules associated with the surface cations and surface hydroxyl groups remain after sorption. Displacement of loosely bound water in hydrophobic micropores by hydrophobic organic compounds occurs because of the lower water density in such pore spaces, which is enthalpically unfavorable (Cheng and Reinhard, 2006b;Farrell et al, 1999aFarrell et al, , 2002.…”

“…As a result, bulky organic molecules with low diffusivities may take very long time to adsorb into micropores. Sorption in mineral micropores has been widely observed for halogenated hydrocarbons (e.g., Farmer, 1993, 1995;Aochi et al, 1992Aochi et al, , 2003Ball and Roberts, 1991;Castilla et al, 2000;Cheng and Reinhard, 2006a,b, 2007, 2008Cornelissen et al, 1998;Farrell and Reinhard, 1994;Farrell et al, 1999aFarrell et al, ,b, 2002Grathwohl and Reinhard, 1993;Li and Werth, 2004;Qafoku et al, 2009;Riley et al, 2010;Steinberg et al, 1987;Werth and Reinhard, 1997a,b;Werth et al, 1997), while there have also been limited reports for aromatic hydrocarbons (Chang et al, 2003;Hatzinger and Alexander, 1997;Hundal et al, 2001) and pesticides (Ahmad et al, 2004;Wu and Laird, 2002).…”