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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 (fa) 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 fa of smectite surfaces in the four soils ranged from 1 to 0.3. These fa values were strongly negatively correlated with the ratio of SOM/smectite contents. Also, fa 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 fa approaching 1 for strongly sorbing NACs suggest displacement of SOM from smectite surfaces by the sorbed compound. [ABSTRACT FROM AUTHOR]

Coverage of clay surfaces by soil organic matter (SOM) may Iimit 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 SUM 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 (fa). To alleviate the methodological limitations, we developed an alternative approach for determining fa. Good agreement between fa values was obtained from both methods for p-NCB but not diuron. For p-NCB sorption, fa values varied between 0.55 and 0.71. For diuron sorption, our alternative equation estimated fa values varied between 0.41 and 0.61; the other approach yielded negative values. The results demonstrate that SUM 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. [ABSTRACT FROM AUTHOR]