Quantification of the availability of clay surfaces in soils for sorption of nitroaromatics

Nitroaromatics (NACs) comprise an important class of environmental contaminants that pose a potential toxic threat to ecological and human health. To improve our understanding and modeling of the fate and transport of NACs in the environment, the effect of the interaction of soil clays with SOM must be better understood.; The influence of exchangeable cation on the adsorption of p-nitrocyanobenzene (p-NCB) and 1,4-dinitrobenzene (1,4-DNB) by Webster A and B horizon soils, which contains smectite as their most abundant clay mineral, was determined. On removal of SOM from whole soils, sorption of p-NCB and 1,4-DNB increased indicating an overall negative contribution of SOM to their sorption by whole soil. The removal of the clay-sized fraction from whole soil decreased sorption of these compounds by a factor of at least 10. This suggests that the interaction between SOM and soil clays suppresses sorption of p-NCB and 1,4-DNB by soils. We also determined, independently, the effect of exchangeable cation on sorption of 1,4-DNB by SOM and on sorption of p-NCB by whole soil, the clay-sized fraction from that soil and SOM. No effect of exchangeable cation type was found for sorption of p-NCB and 1,4-DNB by SOM. For homoionic clay-sized soil fractions, the sorption of p-NCB was generally greater by monovalent-cation-exchanged clays than divalent-cation-exchanged clays, and increased with decreasing cation hydration energy (Li+ ∼ Na+ < K+ < Cs+ and Ca2+ ∼ Mg 2+ < Ba2+). The same trends were observed for homoionic soils exchanged with monovalent and divalent cations.; Two methods were used to assess the availability of clay surfaces (f a) in soils for sorption of NACs. One method offered earlier by Karickhoff (1984) involves the summation of the sorption of solutes to SOM and swelling clays. We also developed an alternative approach for determining fa that alleviates methodological limitations of the Karickhoff approach. Using three NACs (p-nitrocyanobenzene (NCB), 2,4-dinitrotoluene (DNT) and 1,4-dinitrobenzene (DNB)) and four loam soils, sorption of each NAC by whole soil and soil from which SOM was removed was determined. For each NAC, increased sorption was observed with the removal of SOM from whole soils indicating diminished availability of mineral surfaces due to obscuration or blockage by SOM; the fa values were less than 1. For each NAC, we found a strong correlation between the fa values and the relative contents of SOM and smectite in the soils (>0.60). For each soil, the fa values decreased in the order DNT > DNB > NCB. This suggests that the nature of the NAC may also influence the mechanism driving sorption site availability. We found a strong correlation between the fa values and the strength of sorption of each NAC to smectites (≥0.90 in three of four cases). Our studies reveal that for organic solutes with polar functional groups (e.g., NO2) and in soils containing smectite clays, solute uptake from water will be strongly influenced by the type of exchangeable cation(s) present, the relative contents of SOM and sorptive clays (e.g., smectites) in the whole soil as well as the nature of the solute.