Fractional Availability of Smectite Surfaces in Soils for Adsorption of Nitroaromatic Compounds in Relation to Soil and Solute Properties

Efficacy of soil clay minerals for the sorption of nonionic organic compounds may be reduced by soil organic matter (SOM) association with clays. Previously, we quantified the fractional availability ( f a ) of soil clay surfaces for sorption of para‐nitrocyanobenzene ( p ‐NCB). Removal of SOM from a soil increased p ‐NCB sorption, demonstrating that SOM reduced the availability of clay surfaces for sorption of p ‐NCB. In this study, we extended our investigation of SOM–smectite interactions by measuring the availability of smectite surfaces in four soils to four nitroaromatic compounds (NACs) including p ‐NCB, 2,4‐dinitrotoluene (2,4‐DNT), 1,4‐dinitrobenzene (1,4‐DNB), and 1,3,5‐trinitrobenzene (1,3,5‐TNB). Increased NAC sorption was consistently observed after SOM was removed from whole soils, indicating that SOM blocks the availability of soil smectite surfaces to NACs. Estimates for the f a of smectite surfaces in the four soils ranged from 1 to 0.3. These f a values were strongly negatively correlated with the ratio of SOM/smectite contents. Also, f a values differed systematically with NAC probe molecules, with SOM blocking fewer clay mineral sites from more strongly sorbing NACs. This suggests that smectite surface availability in soils may be an operationally defined quantity whose value depends on the probe molecule. Values of f a approaching 1 for strongly sorbing NACs suggest displacement of SOM from smectite surfaces by the sorbed compound.