Hydrophobicity of Clay Surfaces: Sorption of 1,2-dibromoethane and Trichloroethene

-Sorption and desorption of two neutral, nonpolar organic compounds, 1,2-dibromoethane (EDB, a soil fumigant) and trichloroethene (TCE, an industrial solvent and common ground-water contaminant), by pyrophyllite, kaolinite, illite, and smectite were investigated. For sorption, vapors of the compounds in a stream of dry N2 gas were passed through columns of the powdered clay minerals for different periods of time. The compounds retained by the days were extracted with methanol and analyzed by gas chromatography (GC). For desorption, N 2 was passed through the treated samples, and the desorbed compounds were collected in hexane traps and analyzed by GC. Initially sorption was rapid for several hours but then proceeded at a slower rate for many hours. Surprisingly, the clays sorbed large quantities of these neutral compounds; for example, the pyrophyllite, kaolinite, illite, and smectite sorbed about 3, 5, 6, and 9% EDB, respectively, by weight. The amounts of TCE sorbed, which is more volatile than EDB, were somewhat less. Only a portion of the compounds sorbed over a period of time were desorbed in the same period of time. Desorption was rapid initially but then proceeded at a slow rate; slow desorption continued for over 100 hr from samples which had undergone sorption for only 1-2 hr. A two-compartment efflux model was used to describe the sorption behavior. A rapidly desorbing component was considered to be present on the outer surfaces of the ctay aggregates and a slowly desorbing component, in the interior pores of the aggregates. Key Words--Diffusion, Halocarbons, Illite, Kaolinite, Pyrophyllite, Smectite, Sorption-desorption. I N T R O D U C T I O N Nonpolar organic molecules contain no functional groups; hence, their retention by clays is expected to be limited to physical sorption on the outer surfaces, involving van der Waals forces. Consequently, the sorbed molecules should be only weakly held, and sorption should be readily reversible. The results reported here on the sorption of two volatile nonpolar halocarbons, 1,2-dibromoethane (EDB), a soil fumigant, and trichloroethene (TCE), a common industrial solvent, by several clay minerals show that large quantities of these chemicals are sorbed and that the sorption is not readily reversible. Concern with ground-water contamination by organic chemicals applied to soils or disposed in industrial wastes has prompted numerous investigations of their reactions and movement in soils, recently reviewed by Sawhney and Brown (1989). Most investigations have been carried out in aqueous systems, and uptake and retention of organic compounds has been generally attributed to partitioning into soil organic matter (Karickhoff et al., 1979; Weber et al., 1983; Chiou et aL, 1983). The term "parti t ioning" is analogous to extraction of an organic compound from aqueous solution into an organic solvent. When nonpolar organic compounds are partitioned into solid soil organic matter their distribution is considered homogeneous throughout the volume of the solid phase. In contrast, they are assumed to sorb only on the surface of minerals. The uptake of nonionic organic cornCopyright 9 1990, The Clay Minerals Society pounds, however, cannot always be explained solely by soil organic matter (Garbarini and Lion, 1986), and significant sorption by clay minerals may occur even in aqueous solutions (MacIntyre and Smith, 1984; Mingelgrin and Gerstl, 1983). Estes et al. (1988) recently observed that slow sorption of TCE by a suspension of montmoril lonite continued over a 28-day period. In dry soils and in soils containing small amounts of organic matter, sorption of organic pollutants likely occurs on mineral surfaces (Chiou and Shoup, 1985; Chiou, 1989). Although reactions of both polar and nonpolar aromatic hydrocarbons with clay minerals have been studied by a number of investigators and have been reviewed by Pinnavaia (1983), Mortland (1986), and Raussell-Colom and Serratosa (1987), reactions involving nonpolar aliphatic halocarbons have received little attention. Steinberg et al. (1987) observed that small concentrations of the volatile, nonpolar aliphatic halocarbon, 1,2-dibromoethane (EDB), used as a soil fumigant, have persisted in soils for as long as two decades following application. This persistence was attributed to the entrapment of EDB molecules in intraaggregate micropores in soils. Because of slow diffusion of these molecules through the " immobi le aqueous phase" in the interior pores of soil aggregates, they remained out of equilibrium with the bulk soil solution. Consequently, they were removed only slowly. Similar nonequilibrium conditions during the sorption-desorption in the aqueous phase of polynuclear aromatic hydrocarbons by soils and sediments have been observed by others