Environ. Sci. Technol. 2002, 36, 5067-5074
Spectroscopic Study of Dinitrophenol Herbicide Sorption on Smectite C . T . J O H N S T O N , * ,† G . S H E N G , ‡ B. J. TEPPEN,§ S. A. BOYD,§ AND M. F. DE OLIVEIRA† Department of Crop, Soil and Environmental Sciences, 1150 Lilly Hall, Purdue University, West Lafayette, Indiana 47907-1150, Department of Crop, Soil and Environmental Sciences, University of Arkansas, Fayetteville, Arkansas 72701, and Department of Crop and Soil Sciences, Michigan State University, East Lansing, Michigan 48824
Sorption of two dinitrophenolic herbicides, 4,6-dinitro-ocresol (DNOC) and 4,6-dinitro-2-sec-butylphenol (DINOSEB) to smectite was studied using FTIR, HPLC, and quantum chemical methods. The high affinity of DNOC and DINOSEB for smectite surfaces was attributed to site-specific interactions with exchangeable cations and nonspecific van der Waals interactions with the siloxane surface. The positions of the νasym(NO) and νsym(NO) vibrational modes were perturbed by the exchangeable cations with similar changes occurring for both alkali and alkaline earth cations as a function of ionic potential. The cationinduced changes to the vibrational bands of the NO2 groups indicate that exchangeable metal cations are coordinated to -NO2 groups. Quantum chemical methods predicted a red-shift of the νasym(NO) band and a corresponding blue-shift of the νsym(NO) modes, as was observed experimentally. The nature of the smectite surface itself did not strongly influence the vibrational modes of sorbed DNOC or DINOSEB on the basis of a comparison of DNOC sorbed to three different smectites (K-SWy-2, K-SAz1, and K-SHCa-1). FTIR spectra of DNOC and DINOSEB sorbed to a K-SWy-2 smectite were studied quantitatively using a modified form of Beer’s law. The FTIR-derived sorption isotherm of DNOC sorbed to K-SWy-2 was in good agreement with the isotherm derived from HPLC measurements. The molar absorptivity value of DNOC sorbed to K-SWy-2 smectite was 1.43 × 107 cm2/mol in good agreement with literature values for nitroaromatics (average value of 1.72 × 107 ( 0.3 cm2/mol). On the basis of this value, the limit of detection using the FTIR method of ∼5 µgDNOC gclay was determined. These two observations (sorption isotherms and molar absorptivity) provide a direct link between the macroscopic sorption results and the FTIR spectra.
Introduction Increased concern about the fate and transport of nitroaromatic compounds (NACs) in soil and subsurface environ* Corresponding author phone: (765)496-1716; fax: (765)496-2926; e-mail:
[email protected]. † Purdue University. ‡ University of Arkansas. § Michigan State University. 10.1021/es025760j CCC: $22.00 Published on Web 11/06/2002
2002 American Chemical Society
ments has prompted several recent studies that have examined NAC interaction with soil and subsurface constituents (1-7). These compounds are commonly used as pesticides, explosives, ammunition, and solvents; their uses have resulted in contamination of soils, sediments, quarries, and aquifers. Several studies have shown recently that certain NACs have a high affinity for smectites in aqueous solution with Kf/Kd values of >104 L kg-1 (1-7). NAC properties that favor sorption include the presence of two or more, nonadjacent nitro substituents on the ring and the lack of “bulky” substituents. This is illustrated by a sorption study of 31 NACs on smectite from aqueous solution where the highest sorption was observed for 1,3,5-trinitrobenzene (TNB), 4,6-dinitroo-cresol (DNOC), and 2,4,6-trinitrotoluene (TNT) with Kd values of 60 000, 37 000, and 21 500 L kg-1, respectively (1). On the other hand, significantly less sorption was reported for unsubstituted nitrobenzene (Kd ) 7.2 L kg-1) or dinitrobenzenes with bulky substituents [e.g., 4,6-dinitro-2-secbutylphenol (DINOSEB); Kd ) 64 L kg-1] (1). The significantly lower Kd value of DINOSEB was attributed to the presence of the bulkier sec-butyl group as compared to the methyl substituent for DNOC. In a recent study of DNOC and DINOSEB sorption on smectite from aqueous solution, sorption was strongly diminished at pH values above the pKa of value (4.4 and 4.6, respectively), and it was concluded that these compounds were sorbed as the neutral species (5). Sorption was controlled, in part, by the nature of the exchangeable cations. For weakly hydrated exchangeable cations (e.g., Cs+ and Ba2+), the combined size of the cations and associated water molecules does not occupy all of the available space, and a portion of the siloxane surface is “available” for NAC sorption (7, 8). For these cations, the size of the adsorptive domains for NAC sorption (i.e., area not occupied by water molecules) is correspondingly large and greater sorption occurs. For cations with larger enthalpies of hydration (e.g., Na+ and Mg2+), in contrast, minimal sorption occurs because the space available in the interlamellar region is correspondingly small because of the large hydrated radii of the interlayer cations (4). The specific mechanisms of DNOC and DINOSEB interactions with smectites in aqueous solution are not fully understood. Some understanding of the underlying sorption mechanisms was elucidated recently in a combined FTIR and sorption study of some NACs, which included 1,3- and 1,4-dinitrobenzene (DNB) and TNB, on smectites exchanged with different cations (7). In this study, sorption was controlled largely by the hydration characteristics of the exchangeable cation and by the size of the NAC and was strongly dependent on the number and position of nitro substituents as well as other substituents. The strongest spectral perturbations occurred for those cations with lower enthalpies of hydration. It was proposed that weakly hydrated cations could form stronger complexes with sorbed NAC compounds and thereby stabilize the NAC on the clay surface. Another mechanism proposed recently to account for the high affinity of NACs for smectites has been the formation of an electron donor-acceptor (EDA) surface complex between the ring of the NAC and the siloxane surface (1, 3, 9, 10). They argued against the stabilizing role of exchangeable cations in the sorption process. The role of the siloxane surface in NAC sorption mechanisms on smectite is an important issue because of the ubiquitous occurrence of clay minerals in natural systems and because many organic moieties have similar structural properties to those of NACs. VOL. 36, NO. 23, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Structures and selected chemical and physical properties of 2,4-dinitro-o-cresol (DNOC) and 2,4-dinitro-6-sec-butylphenol. In this study, we seek to develop an improved understanding of dinitrophenol herbicide sorption mechanisms on smectite using DNOC and DINOSEB. By using a combined strategy of FTIR spectroscopy and HPLC sorption methods, we have attempted to develop a quantitative spectroscopic method to study NAC sorption on smectites to complement prior sorption studies (5). In addition, molecular modeling methods were used to assist and interpret the spectral information about dinitrophenol herbicide sorption mechanisms on smectite.
Materials and Methods Three smectite clays were studied: the Wyoming montmorillonite (SWy-2) collected from Cook County, WY; a hectorite (SHCa-1) collected from San Bernardino County, CA; and the Arizona montmorillonite (SAz-1) collected from Apache County, AZ. All clays were obtained from the Source Clays Repository of The Clay Minerals Society. Complete descriptions of the physical properties of these clay samples have been previously reported (11). Prior to size fractionation, a homoionic Na-smectite clay suspension was prepared by placing 20 g of the raw clay in 1.0 L of 0.5 M NaCl for 24 h. The Na-montmorillonite suspension was washed free of excess salts by repeated centrifugation with ultrapure water (Millipore). The
