Pyrene Sorption to WaterDispersible Colloids

of the fine fraction of the soil, the normal procedure is to disperse the soil by strong chemical dispersion, isolate Chemical sorption to mobile soil colloids is a controlling factor for the clay fraction, and conduct sorption experiments on colloid-facilitated chemical transport in the vadose zone and groundwater. We investigated sorption of pyrene to soil colloid suspensions this fraction. This procedure ignores the fact that sorporiginating from soils differing in organic matter content for different tion properties of the chemically dispersed clay fraction solution chemistries. Colloids were obtained from two soils with differmay vary from the sorption properties of the mobile ent organic matter contents but similar geological histories by three colloids released by mild chemical dispersion or medifferent methods: (i) chemical dispersion, (ii) mechanical dispersion chanical stress. Colloids with high stability are often in water, and (iii) spontaneous release in water. Batch sorption experileached preferentially from a soil (Kretzschmar et al., ments were conducted at five pyrene concentrations, in either pure 1995; Kaplan et al., 1997). These colloids may have water or at two different concentrations of K and Ca2 . Generally, K gained their stability from an increased surface charge addition enhanced pyrene sorption, whereas Ca2 addition decreased due to organic coatings (Kretzschmar et al., 1995; Kaplan sorption. The chemically dispersed colloids exhibited the highest pyet al., 1997) or by an expanded Stern layer due to specific rene sorption capacity and had the most nonlinear sorption isotherms, whereas whole soil had the most linear isotherm. Model calculations adsorption of monovalent cations (e.g., Singh and Uehara, of the potential amounts of leachable pyrene illustrated the impor1999). The preferential leaching of certain colloid types tance of including both colloidand dissolved organic matter (DOM)may lead to sorption characteristics of the mobile colfacilitated transport in risk assessment models when dealing with loids differing from those of bulk soil and the clay fracpyrene transport. The leaching potential of dissolved pyrene (with tion. This key aspect of HOC sorption to colloids and, no DOMand colloid-facilitated transport) was 5% of the leaching thus, colloid-facilitated transport, has not been investipotential when both DOMand colloid-sorbed pyrene was included. gated in the past. The amount of HOC sorbed to soil material depends mainly on the organic matter content of the soil material S of hydrophobic organic compounds (HOC) (foc) (Means et al., 1980). The quality of the organic to soil colloidal material is an important process matter is a secondary determinant for the amount of when considering colloid-facilitated transport. While HOC sorbed to the soil material (Chiou et al., 1998; colloids released from soil generally contain the same Gauthier et al., 1987; Xing and Pignatello, 1997). Bed minerals and type of organic matter as the clay fraction sediment and soil organic matter (SOM) have been of the soil, the quantitative proportions of the different shown to have different sorption capacities (Kile et al., mineral and organic phases often vary (Kaplan et al., 1995). Hydrophobic organic compounds may have a 1997; Kretzschmar et al., 1999). These differences may higher affinity for certain particle size fractions. Wilcke relate to the conditions under which the colloids are et al. (1996) and Müller et al. (2000) found that polyaroreleased from soil. Natural soil colloids are released matic hydrocarbon (PAH) had a higher affinity for the from soil either by in situ mobilization from soil aggresilt fraction of the soil. This was explained by a higher gates, raindrop impact, or surface erosive flow. In situ relative concentration of aromatic organic compounds mobilization from soil aggregates results from moderate in this fraction. chemical dispersion due to lowering of the ionic strength Soil organic matter consists of mainly of humic mateby the exchange of soil water with rainwater. Release rials that may be sorbed onto the mineral phase. Soil of colloids from the soil by raindrop impact or due to organic matter may have a more or less pronounced flexierosive flow results from a combination of mild chemical ble nature. Humic material seems to be more condensed dispersion by low ionic strength rainwater and mechanitoward the center (Hayes et al., 1989) and have a diffuse cal stress. Strong chemical dispersion, by monovalent outer boundary. The expanded and condensed parts of cations and high pH, generally does not occur in agriculSOM may be described as rubbery and glassy polymer, tural soils. When investigating the sorption properties respectively (Xing and Pignatello, 1997; Leboeuf and Weber, 1997). Sorption in the rubbery part of SOM is M. Laegdsmand and P. Moldrup, Aalborg University, Dep. of Life considered to be a phase-partitioning process by solid Sciences, Environmental Engineering Section, Sohngaardsholmsvej phase dissolution. In the glassy part of SOM the sorption 57, DK-9000 Aalborg, Denmark; L.W. de Jonge, Danish Institute of Agricultural Sciences, Dep. of Agroecology, P.O. Box 50, DK-8830 process is surface adsorption by hole filling (sorption Tjele, Denmark; K. Keiding, Aalborg University, Dep. of Life Sciin nanovoids of the rigid SOM structure). Xing and ences, Chemistry Section, Sohngaardsholmsvej 57, DK-9000 Aalborg, Pignatello (1996) found an inverse relationship between Denmark. Received 9 July 2003. Special Section: Colloids and ColloidFacilitated Transport of Contaminants in Soils. *Corresponding auAbbreviations: DOC, dissolved organic C; DOM, dissolved organic thor (mette.laegdsmand@agrsci.dk). matter; DW, deionized ultrapurified water; HOC, hydrophobic organic compound; PAH, polyaromatic hydrocarbon; SOM, soil organic Published in Vadose Zone Journal 3:451–461 (2004).  Soil Science Society of America matter; SWDC, spontaneous water-dispersible colloids; TC, total colloids; TOC, total organic C; WDC, water-dispersible colloids. 677 S. Segoe Rd., Madison, WI 53711 USA 451 Published May, 2004