Environ. Sci. Technol. 2001, 35, 3456-3461
Sorption of Phenanthrene by Reference Smectites L A K H W I N D E R S . H U N D A L , * ,† MICHAEL L. THOMPSON,† DAVID A. LAIRD,‡ AND ANA M. CARMO† Agronomy Department, Iowa State University, Ames, Iowa 50011-1010, and USDA, ARS, National Soil Tilth Laboratory, Ames, Iowa
Fate and behavior of nonionic hydrophobic organic compounds (HOCs) in the environment is mainly controlled by their interactions with various components of soils and sediments. Due to their large surface area and abundance in many soils, smectites may greatly influence the fate and transport of HOCs in the environment. We used phenanthrene as a probe to explore the potential of reference smectites to sorb HOCs from aqueous solution. Batch experiments were used to construct phenanthrene sorption isotherms, and possible sorption mechanisms were inferred from the shape of the isotherms. Our results demonstrate that smectites can retain large amounts of phenanthrene from water. Phenanthrene sorption capacities of the reference smectites investigated in this study were comparable to those of soil clays containing a considerable amount of organic matter. Hectorite exhibited the highest sorption affinity and capacity followed by Panther Creek montmorillonite. The lack of correlation between Freundlich sorption constants (K′f) and indices of charge or hydrophobicity suggests that sorption of phenanthrene by smectites is primarily a physical phenomenon. Capillary condensation into a network of nanoor micropores created by quasicrystals is likely to be a dominant mechanism of phenanthrene retention by smectites.
Introduction The retention of nonionic hydrophobic organic compounds (HOCs) such as aromatic hydrocarbons, alkylbenzenes, chlorobenzenes, and polycyclic aromatic hydrocarbons (PAHs) in sediments and soils is often attributed to partitioning of HOC molecules into organic matter (1, 2). Hassett et al. (3) studied HOC sorption by sediments and soil materials with a wide range of organic carbon (OC) contents and reported that organic matter could mask the contribution of minerals to sorption when the mass ratio of swelling clay minerals to organic carbon (cm/oc) was < 40:1. They also found that the organic-matter partitioning model failed to account for total sorption when the cm/oc ratio was g 40. Similarly, Karickhoff (4) observed that sorption affinities for simazine and biquinoline were greater than the sorption affinities estimated by a simple organic-matter partitioning model when the cm/oc ratios of the aquifer materials were about 30:1. He concluded that the relative contribution of minerals to sorption of hydrophobic organic solutes may become significant for cm/oc ratio > 60. * Corresponding author phone: (515)294-7855; fax: (515)294-3163; e-mail:
[email protected]. † Iowa State University. ‡ USDA, ARS, National Soil Tilth Laboratory. 3456
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 35, NO. 17, 2001
In contrast, several studies have reported that the sorption of HOCs, including aromatic, chlorinated, and polycyclic aromatic hydrocarbons (PAHs), by pond and river sediments and soils were independent of mineral type and content. For example, sorption coefficients have been found to be correlated with only OC even for samples with cm/oc ratios of 95 and 158 (5, 6). The lack of consensus about the contribution of the inorganic fraction to HOC retention suggests the need for further evaluation of HOC sorption by clay minerals. The clay fraction of many soils in the upper Midwest is dominated by smectites. These minerals have a large surface area and a large capacity for physical reorganization in response to different associated cations. Association of water with uncharged oxygens on siloxane surfaces (basal planes) of smectites is generally very weak (7), making the uncharged siloxane surfaces relatively hydrophobic. The basal planes of the low-charge smectites are expected to be less hydrated than those of the high-charge smectites because fewer hydrated cations are attracted to them (8). Thus, we hypothesize that low-charge smectites would have greater sorption affinities for HOCs. The extent to which surface and structural properties of smectites govern the relative affinities and capacities for sorption of HOCs from aqueous solutions is not clear. Retention of pyrene by Laponite and hectorite has been attributed to the presence of localized nonpolar sites (9). Sorption of aqueous-phase atrazine by reference smectites varies from 0 to 100% and depends on the surface charge properties of the smectites (10), solution pH, and saturating cations (11). The negative correlation between surface charge density and sorption affinity for atrazine suggests that atrazine is sorbed as an uncharged molecule rather than as an ion. The term mixed-mode sorption has been used for retention of atrazine by smectites in which hydrophobic interactions at nonpolar nanosites on smectite surfaces occur simultaneously with polar interactions with the water molecules that solvate cations near smectite surfaces (12). The objectives of this study were (i) to investigate the potential of well-characterized reference smectites to sorb a model HOC, phenanthrene, from the aqueous phase and (ii) to explore structural and surface properties that influence the sorption of HOCs by smectites. Phenanthrene sorption by soils and sediments has been extensively studied, but the focus of these investigations has not been on the contribution of inorganic components to sorption (13-17). To our knowledge, studies investigating interactions of smectites and extremely hydrophobic compounds in aqueous suspensions are scarce. The experimental data set presented in this study allows an evaluation of various factors affecting the affinity of reference smectites to retain phenanthrene in aqueous environments.
Experimental Section Sorbent Characteristics. Reference Wyoming montmorillonite (SWy-1), hectorite (SHCa-1), White montmorillonite (STx-1), and Cheto montmorillonite (SAz-1) were purchased from the Source Clays Repository of the Clay Minerals Society. Panther Creek montmorillonite was obtained from Dr. A. D. Scott’s collection, Agronomy Department, Iowa State University, Ames, IA. The smectites were chosen to represent a range of layer charge and tetrahedral charge. All five smectites were fractionated by sedimentation to collect the
