Quantifying the Availability of Clay Surfaces in Soils for Adsorption of Nitrocyanobenzene and Diuron SIMONE M. CHARLES, HUI LI, BRIAN J. TEPPEN, AND STEPHEN A. BOYD* Department of Crop and Soil Sciences, Michigan State University, East Lansing, Michigan 48824-1325 Coverage of clay surfaces by soil organic matter (SOM) may limit the efficacy of the soil mineral fractions for adsorption of organic contaminants and pesticides. Two methods were scrutinized for quantitatively assessing the availability of clay surfaces in a smectitic Webster A-horizon soil for sorption of p-nitrocyanobenzene (p-NCB) and diuron. One method, described previously, involves the summation of independent contributions of SOM and swelling clays to sorption of organic solutes. For this method, several assumptions must be made and/or procedural difficulties overcome in the determination of certain terms in the equation proposed for calculating the fractional availability of mineral surfaces (f a ). To alleviate the methodological limitations, we developed an alternative approach for determining f a . Good agreement between f a values was obtained from both methods for p-NCB but not diuron. For p-NCB sorption, f a values varied between 0.55 and 0.71. For diuron sorption, our alternative equation estimated f a values varied between 0.41 and 0.61; the other approach yielded negative values. The results demonstrate that SOM does reduce the availability of clay surfaces, hence, suppressing sorption by the Webster A-horizon soil. Our newly developed method provides more reasonable estimates of the availability of soil-clay surfaces for sorption than an earlier published approach. Introduction Clay minerals and soil organic matter (SOM) are generally considered the two most active components in the sorption of aqueous phase neutral organic contaminants (NOCs) by soil (1). Significant sorption of nitroaromatic compounds (NACs) from water by clays, particularly smectites, has been well documented (2-9). The extent of interaction of NOCs or other organic molecules with soil clay minerals may, however, be limited by the availability of clay sorption sites. Soil organic matter may obscure clay surfaces and thereby reduce the efficacy of the soil clay mineral fraction for adsorption of NOCs. Using scanning electron microscopy, Laird et al. (10, 11) observed that in the clay fraction of a Webster Ap-horizon soil, both large aggregated structures (5-20 μm) and discrete particles (0.5-2 μm) appeared to be covered with SOM. In soils, SOM and clay content are known to strongly correlate. Despite their known association, the influence of SOM on the sorptive properties of soil clay mineral fraction is still not fully understood though several studies utilizing synthetic clay-humic-acid (HA) complexes suggest, qualitatively, that HA may reduce the availability of clay surfaces for pesticide adsorption (9, 12-15). Describing sorption of organic and inorganic species by soils as the additive product of the isolated SOM and mineral components is inadequate because it requires unrealistic assumptions, e.g., that the components act independently when it is known that they are intimately associated in soils (16, 17). Recognizing this, Karickhoff (18) proposed an equation that might allow experimental assessment of soil- clay surface availability for sorption of NOCs, though the approach was not evaluated experimentally. This equation incorporated potential SOM blockage of sorption sites on clays by summing NOC sorption to clay and SOM: where Qwhole soil is the total NOC mass sorbed per unit mass of whole soil, Qom and Qmin are the SOM-sorbed and mineral- sorbed NOC per unit mass of the respective sorbent phase, and fmin and fom are the fractional mineral and SOM contents of soil. The term fa represents the fractional availability of sorption sites on the clay components of whole soil, i.e., the fraction of mineral sorptive surfaces available in whole soil. The term fa is a plausible refinement of simple additivity of the individual component contributions. Fractional avail- ability ranges from 0 (unavailable) to 1 (100% available). Karickhoff (18) equated the mineral fraction to the clay fraction owing to the high surface area of clays in general and of smectites in particular. Although a few studies have shown the extent of NOC sorption to soil or HA-clay complexes does not equate to the sum of sorption of the individual mineral and SOM com- ponents (9, 12-15), there appear to be no studies that have attempted to experimentally quantify the extent to which clay surfaces are available for NOC sorption in soil. The objective of this study is to quantify the fractional availability of mineral surfaces on a smectitic soil for sorption of p-NCB and diuron using eq 1 proposed by Karickhoff (18) and a newly proposed method of data analysis presented herein. Experimental Section A-horizon Webster soil was air-dried and sieved to remove coarse fragments (>2 mm). The soil was fractionated by standard procedures (19). Carbonates and SOM were se- quentially removed from the Webster soil; carbonates were removed by stirring the soil with 0.5 M sodium acetate (acidified to pH 5) while heating at 80 °C, followed by the removal of SOM using 30% H2O2 at 80 °C (denoted SOM- removed fraction). Homoionic soils and soil fractions were prepared by washing the soils with 0.1 M KCl or MgCl2 overnight, thereafter centrifuging and discarding the super- natant. This procedure was repeated four times. The soils were then washed with Milli-Q deionized H2O until a negative Cl - ion test with AgNO3 was obtained. The soils were then freeze-dried and stored at room temperature (23 ( 1 °C) until used. The SOM contents were determined as 3.9% for the whole soil, and 0.23% for the SOM-removed soil using dry combustion (20). The gravimetric clay content of the Webster soil (31%) was determined by the hydrometer method (21). p-NCB and diuron (1,1-dimethyl-3-(3,4-dichlo- rophenyl)urea) were purchased from Aldrich Chemical Co. and used as received. The clay-sized particles (<2.0 μm) were obtained from soil using wet sedimentation after removal of carbonates, SOM, and free iron oxides (FeOx)(22), then saturated with * Corresponding author phone: 517-355-0271 x1252; fax: 517- 355-0270; e-mail: boyds@msu.edu. Q whole soil ) f a Q min f min + Q om f om (1) Environ. Sci. Technol. 2006, 40, 7751-7756 10.1021/es0611700 CCC: $33.50  2006 American Chemical Society VOL. 40, NO. 24, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 7751 Published on Web 10/27/2006