title: Mechanisms for the adsorption of substituted nitrobenzenes by smectite clays.
abstract: To more fully understand the potential for transport of nitroaromatic compounds in soils and subsoils,the adsorption of a series of para- and meta-substituted nitrobenzenes (SNBs) by K-smectite clay was measured. Adsorption isotherms were fit to the Freundlich equation, and the resultant Freundlich adsorption coefficients (log(Kf) were positively correlated with the Hammett substituent constant (r2 = 0.80). This relationship and a positive reaction constant (p = 1.15) indicate that the adsorption reaction is favored 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 systematic relationship between the Hammett constant and the electron density on the aromatic ring, which would explain a donor-acceptor relationship. Rather, electron density donated by a second substituent on nitrobenzene appears to be appropriated by the nitro group leaving ring electron density unchanged. Fourier transform infrared spectroscopy revealed shifts in the -NO2 vibrational modes of 1,3,5-trinitrobenzene (TNB) upon adsorption to K+-smectite that were consistent with the complexation of K+ by -NO2 groups. Such TNB vibrational shifts were not observed for SWy-1 saturated with more strongly hydrated cations (i.e., Na+, Mg2+, Ca2+, and Ba2+). The simultaneous interaction of multiple -NO2 groups with exchangeable K+ was indicated by molecular dynamic simulations. Adsorption of SNBs by smectite clays appears to result from the additive interactions of -NO2 groups and secondary substituents with interlayer K+ ions. Adsorption occurs to a greater or lesser extent depending on the abilities of substituents to complex additional interlayer cations and the water solubilities of SNBs. We conclude that the adsorption trends of SNBs on K-SAz-1 can be explained without recourse to hypothetical electron donor-acceptor complexes.
authors.affiliation: Department of Crop and Soil Sciences, Michigan State University, East Lansing 48824, USA. boyds@msu.edu
authors.forename: S A
authors.initials: SA
authors.lastname: Boyd
authors.forename: G
authors.initials: G
authors.lastname: Sheng
authors.forename: B J
authors.initials: BJ
authors.lastname: Teppen
authors.forename: C T
authors.initials: CT
authors.lastname: Johnston
mesh_terms: D000327:Adsorption; D003198:Computer Simulation; D005765:Gastrointestinal Agents / Q000737:chemistry*; D008954:Models, Biological; D009578:Nitrobenzenes / Q000032:analysis / Q000737:chemistry*; D011188:Potassium / Q000737:chemistry*; D017640:Silicates; D012987:Soil / Q000032:analysis*; D012989:Soil Pollutants / Q000032:analysis*; D012995:Solubility; D017550:Spectroscopy, Fourier Transform Infrared; D014301:Trinitrobenzenes / Q000032:analysis / Q000737:chemistry*; D014867:Water / Q000032:analysis
chemical_list: D005765:Gastrointestinal Agents; D009578:Nitrobenzenes; D017640:Silicates; D012987:Soil; D012989:Soil Pollutants; D014301:Trinitrobenzenes; D014867:Water; C029222:sym-trinitrobenzene; C033214:Smectite; D011188:Potassium