Environ. Sci. Technol. 2001, 35, 4767-4772
Spectroscopic Study of Nitroaromatic-Smectite Sorption Mechanisms C L I F F T . J O H N S T O N , * ,† MAURILIO FERNANDES DE OLIVEIRA,† BRIAN J. TEPPEN,‡ GUANGYAO SHENG,§ AND STEPHEN A. BOYD‡ Crop, Soil and Environmental Sciences, 1150 Lilly Hall, Purdue University, West Lafayette, Indiana 47907-1150, Department of Crop and Soil Sciences, Michigan State University, East Lansing, Michigan 48824-1325, and Department of Crop, Soil, and Environmental Sciences, University of Arkansas, Fayetteville, Arkansas 72701
Sorption mechanisms of 1,3- and 1,4-dinitrobenzene, 1,3,5-trinitrobenzene (TNB), dinitro-o-creasol, and 6-secbutyl-2,4-dinitrophenol (DINOSEB) on smectite were investigated using FTIR spectroscopy and HPLC methods. A quantitative method was developed that established a direct link between the HPLC and the FTIR data. Freundlich sorption values ranged from 47 (L g-1) for 1,3,5-TNB to 3.7 for DINOSEB and showed that the extent of nitroaromatic compounds (NAC) sorption was strongly dependent on the number and position of the nitro substituents as well as other substituents and steric effects. The amount of 1,3,5TNB sorbed to smectite was strongly influenced by the nature of the exchangeable cation. Furthermore, the exchangeable cation significantly influenced the positions and relative intensities of the vibrational modes of the -NO2 groups. The strongest perturbations were observed for cations with lower enthalpies of hydration (e.g., K+) and included a red shift of the νasym(NO) band, a concomitant blue shift of the νsym(NO) band. These changes were accompanied by a 2-fold increase in the relative intensity of the νasym(NO) band relative to the intensity of the νsym(NO) band. Molecular quantum mechanics calculations were used to rationalize frequency shifts in terms of nitroaromatic interactions with interlayer cations. Results indicate that the sorption of NACs to smectite surfaces is controlled largely by the hydration characteristics of the exchangeable cation, which regulates both cation-nitroaromatic complexation and swelling of the smectite.
Introduction The aqueous surface chemistry of smectites and their high affinity for certain types of substituted aromatic and triazine compounds have attracted renewed interest in recent years. Using atrazine as a probe molecule, Laird and co-workers observed widely divergent sorption behavior from aqueous suspension on 13 Ca2+-saturated smectite clays (1, 2). They * Corresponding author phone: (765)496-1716; fax: (765)496-2926; e-mail:
[email protected]. † Purdue University. ‡ Michigan State University. § University of Arkansas. 10.1021/es010909x CCC: $20.00 Published on Web 11/10/2001
2001 American Chemical Society
found sorption ranging from 0 to ∼100% of the added atrazine with corresponding Freundlich sorption coefficients ranging from
