Mechanisms for the adsorption of substituted nitrobenzenes by smectite clays

To more fully understand the potential for transport of nitroaromatic compo unds in soils and subsoils,the adsorption of a series of para- and meta-sub stituted nitrobenzenes (SNBs) by K-smectite clay was measured. Adsorption i sotherms were fit to the Freundlich equation, and the resultant Freundlich adsorption coefficients (log(K-f)) were positively correlated with the Hamm ett substituent constant (r(2) = 0.80). This relationship and a positive re action constant (rho = 1.15) indicate that the adsorption reaction is favor ed by electron-withdrawing substituents. These results are consistent with an electron donor (smectite)-acceptor (substituted nitrobenzene) mechanism offered previously. However, quantum calculations did not reveal any system atic relationship between the Hammett constant and the electron density on the aromatic ring, which would explain a donor-acceptor relationship. Rathe r, electron density donated by a second substituent on nitrobenzene appears to be appropriated by the nitro group leaving ring electron density unchan ged. Fourier transform infrared spectroscopy revealed shifts in the -NO2 vi brational modes of 1,3,5-trinitrobenzene (TNB) upon adsorption to K+-smecti te that were consistent with the complexation of K+ by -NO2 groups. Such TN B vibrational shifts were not observed for SWy-1 saturated with more strong ly hydrated cations (i.e., Na+, Mg2+, Ca2+, and Ba2+). The simultaneous int eraction of multiple -NO2 groups with exchangeable K+ was indicated by mole cular dynamic simulations. Adsorption of SNBs by smectite clays appears to result from the additive interactions of -NO2 groups and secondary substitu ents with interlayer K+ ions. Adsorption occurs to a greater or lesser exte nt depending on the abilities of substituents to complex additional interla yer cations and the water solubilities of SNBs. We conclude that the adsorp tion trends of SNBs on K-SAz-1 can be explained without recourse to hypothe tical electron donor-acceptor complexes.