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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 faof smectite surfaces in the four soils ranged from 1 to 0.3. These favalues were strongly negatively correlated with the ratio of SOM/smectite contents. Also, favalues 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 faapproaching 1 for strongly sorbing NACs suggest displacement of SOM from smectite surfaces by the sorbed compound.

Nitroaromatic compounds (NACs) are commonly found as soil contaminants in military training sites and manufacturing facilities, and may adversely affect human and ecosystem health. Exchangeable cation effects on p‐nitrocyanobenzene (p‐NCB) and 1,4‐dinitrobenzene (1,4‐DNB) sorption by the Webster (fine loamy, mixed, mesic Typic Haplaquoll) soil series A (WA) and B (WB) horizons were determined from batch sorption experiments. Smectite is the most abundant mineral in the horizons of this soil. Soil organic matter (SOM) removal increased p‐NCB sorption to the WA and WB horizons by ∼1.5 times, and increased 1,4‐DNB sorption to the A and B horizons by ∼1.2 to 2.2 times, respectively. Greater sorption of NACs by whole soils after SOM removal suggests that SOM suppresses p‐NCB and 1,4‐DNB sorption by soil mineral components. Clay (<2‐μm fraction) removal decreased p‐NCB sorption to the WA and WB horizons by ∼8 and 11 times, respectively; clay removal decreased 1,4‐DNB sorption to the WA and WB horizons by ∼4.8 and 6.7 times, respectively. Nitroaromatic compound sorption to different soil fractions was measured to identify the independent effects of soil components and exchangeable cations on sorption. For this purpose, 1,2,4‐trichlorobenzene (1,2,4‐TCB) and 1,4‐DNB sorption to two organic soils and a soil devoid of smectites, and p‐NCB sorption to whole soil and the soil clay‐sized fraction were determined. Exchangeable cation type was found to minimally affect sorption of 1,2,4‐TCB, p‐NCB, and 1,4‐DNB by SOM. Sorption of p‐NCB to homoionic soil clay‐sized fractions was generally greater for clays saturated with monovalent cations than clays saturated with divalent cations. Greater p‐NCB sorption followed decreases in cation hydration energy in the order Na+< Li+< < K+< Cs+and Ca2+< Ba2+< Mg2+Similar trends were observed for whole‐soil samples exchanged with these cations. This indicates that differences in nitroaromatic sorption are due to exchangeable cation effects on the clay mineral fraction.