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a Dep. of Agronomy, Iowa State Univ., Ames, IA 50011
b USDA-ARS, National Soil Tilth Lab., 2150 Pammel Dr., Ames, IA 50011
c Dep. of Agronomy, Purdue Univ., West Lafayette, IN 47907
d Dep. of Crop and Soil Sciences, Michigan State Univ., East Lansing, MI 48824
* Corresponding author (david.laird{at}ars.usda.gov).
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Abbreviations: Ca-SAz-1, calcium-saturated Arizona montmorillonite reference clay (smectite) • Ca-SWy-2, calcium-saturated Wyoming montmorillonite reference clay (smectite) • DNOC, 4,6-dinitro-o-cresol • K-SAz-1, potassium-saturated Arizona montmorillonite reference clay (smectite) • K-SWy-2, potassium-saturated Wyoming montmorillonite reference clay (smectite)
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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For neutral organic compounds, soil organic matter is commonly assumed to be the predominant sorptive phase (Chiou, 2002). In soils and subsoils, however, particularly those with low organic matter contents, clay minerals can make a substantial or even dominant contribution to sorption of certain classes of organic contaminants and pesticides including nitroaromatic compounds (NACs), triazines, and dioxins (Laird et al., 1992; Sheng et al., 2001). Among various clay minerals found in soils, smectites are very important due to their widespread occurrence (Allen and Hajek, 1989), high cation exchange capacities, large surface area, reversible interlayer expansibility, and chemically active surfaces (Laird et al., 1992). Recent studies have documented the high affinity of smectite clays for the adsorption of NACs (Haderlein and Schwarzenbach, 1993; Haderlein et al., 1996; Weissmahr et al., 1997; Sheng et al., 2001, 2002; Boyd et al., 2001). The high adsorption of NACs by clays has been attributed to the formation of electron-donor–acceptor (EDA) complexes (Haderlein and Schwarzenbach, 1993; Haderlein et al., 1996). Accordingly, the presence of electron-withdrawing –NO2 groups results in an electron-deficient aromatic ring, and negatively charged siloxane Os near sites of isomorphous substitution on the clay surface were hypothesized to contribute electrons to the electron-deficient ring system of NACs thereby forming the NAC–clay EDA complex. A quantum chemical study of the adsorption of 1, 3, 5-trinitrobenzene (TNB) on the siloxane surfaces of clay minerals, however, attributed this adsorption affinity to the existence of attractive coulombic and van der Waals forces between NACs oriented coplanar to the planar siloxane clay surface (Pelmenschikov and Leszczynski, 1999).
A study by Boyd et al. (2001) concluded that adsorption of several NACs is a consequence of several additive phenomena including complexation of the –NO2 groups and secondary substituents (e.g., –COOCH3, –CN, –NO2) that interact with interlayer K+ in K-smectites, hydrophobic interactions with the siloxane surfaces, and energy gained from the partitioning of NACs to the subaqueous environment of clay interlayers. A Fourier-transform infrared study showed shifts in N–O stretching frequencies of NACs due to formation of inner and outer sphere complexes with interlayer cations having comparatively low hydration enthalpies (e.g., K+ or Cs+ but not Ca2+ or Na+) and the –NO2 groups of NACs (Johnston et al., 2001). Sheng et al. (2002) showed that sorption of two dinitrophenol herbicides was much greater by smectites exchanged with cations having comparatively lower hydration enthalpies (e.g., K+, Cs+, and NH4+). Molecular dynamic simulations showed DNOC interacting with K+ through formation of inner sphere complexation whereby one or two Os of each –NO2 group coordinated to exchangeable K+ and the simultaneous interaction between K+ and the phenolic O and adjacent –NO2 group O. Additionally, the indirect coordination of the –NO2 groups and K+ ions through the intermediation of water led to the formation of outer sphere complexes. These studies established that the size of the hydration spheres of the interlayer cations as well as the charge density of the clay were important determinants of DNOC adsorption.
Chappell et al. (2005) reported that the hydration status of smectites had a large influence on sorption of atrazine, and that sample treatments such as air drying impact the sorption affinity of smectites for organic molecules. Their study showed that drying could impact the size and nature of smectite quasicrystals, thereby affecting the sorption affinity of smectites for organic molecules. Previous adsorption studies have been conducted using smectites that were rehydrated after treatment (air dried or freeze-dried). The results of Chappell et al. (2005) suggest the importance of considering the effects of drying of smectites on adsorption of aqueous-phase organic contaminants and pesticides.
Several previous studies have demonstrated high DNOC sorption at pHs below the pKa of DNOC, where DNOC exists as a neutral species. Sheng et al. (2002) showed decreasing sorption of DNOC with increasing pH (2.5–7) on K-smectite; however, some sorption was apparent even at pHs well above the pKa of DNOC. They attributed the ability of the K-smectite to adsorb anionic DNOC to surface acidity of the smectite. Smectite surface acidity has been shown to promote protonation and adsorption of weak bases at pHs above their pKa values (Mortland and Raman, 1968; Laird and Fleming, 1999), but this effect has never been clearly demonstrated for weak acids. Alternatively, electrical neutrality could also be maintained by co-adsorption of an inorganic cation with an anionic organic molecule. The evidence for specific interaction between K+ and DNOC (Johnston et al., 2001) suggests the possibility that K+ is co-adsorbed with DNOC.
The goal of this investigation was to test the competing hypotheses that smectite surface acidity promotes protonation and adsorption of DNOC or that K+ is co-adsorbed with DNOC at pHs above the pKa. The specific objectives of this study were to quantify (i) the extent of DNOC adsorption by smectites in aqueous suspensions at varying pH; (ii) the impact of the saturating cation (K+ vs. Ca2+) on DNOC adsorption; and (iii) the effect of sample treatment (freeze-dried and rehydrated vs. continuously hydrated) on adsorption of DNOC. A mechanistic interpretation for interactions between DNOC and smectites at pHs above and below the pKa of DNOC is proposed.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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The SAz-1 clay was free of carbonates and therefore was not treated with NaOAc buffer. Otherwise, size fractionation and ion saturation to prepare the K-SAz-1 and Ca-SAz-1 suspensions were performed by the same procedures as described above. Half of each clay suspension was lyophilized, and these samples are referred to as the freeze-dried clays. The remaining halves were stored as suspensions at 
10°C and are referred to as the never-dried clay suspensions.
The 4,6-dinitro-o-cresol (Fig. 1 ) was purchased from Aldrich Chemical Co. (Milwaukee, WI) with purity >97% and was used as received. The DNOC is a weak acid with a pKa of about 4.4 and water solubility of about 198 mg L–1 (USEPA, 1988). A DNOC stock solution was prepared by dissolving 150 mg of DNOC in 1 L of deionized water. This solution had a natural pH of about 3.2. To adjust the pH of the solution to the desired value, a few drops of 0.01 M HCl, 0.01 M KOH, or 0.005 M Ca(OH)2 were added accordingly.
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Never-Dried Suspensions
The never-dried K+– and Ca2+–saturated SWy-2 and SAz-1 suspensions were adjusted to the desired solid solution concentration of 100 g clay L–1 of electrolyte. The ionic strength for the never-dried suspensions was the same as for the freeze-dried resuspended samples. The suspensions were then shaken on a side-to-side shaker for 24 h at 25°C.
Sample Preparation and Equilibration
The smectite suspensions and DNOC solutions were adjusted to the desired pH value by adding a few drops of 0.01 M HCl, 0.01 M KOH, or 0.005 M Ca(OH)2. A pH-adjusted DNOC solution of 150 mg L–1 was then added to the pH-adjusted smectite suspensions to make up a final volume of 10 mL. The final concentration of DNOC was 120 mg L–1 with a solid solution concentration of 10 g L–1. Triplicate samples were prepared. Blank suspensions without DNOC at each pH value were also prepared. The tubes were shaken for 24 h at 25°C. The mixtures were centrifuged at 11 951 x g for 25 min to separate the solid and liquid phases.
Spectrophotometric Analysis
The pH of the supernatant from each sample was recorded, and then 9 mL of supernatant was mixed with 1 mL of 0.5 M potassium phosphate buffer to adjust the pH to 7. The pH-adjusted supernatant was passed through a 0.2-µm nylon filter and collected in a quartz cuvette. The DNOC concentrations were determined by measuring absorption at 268 nm, the wavelength of maximum absorbance for DNOC, with a Milton Roy Spectronic 601 spectrophotometer equipped with an UV-visible detector (Milton Roy Co., Rochester, NY). The amount of herbicide adsorbed by the smectite was calculated from the difference between the initial concentration of DNOC and the concentration of DNOC in the final solution.
Elemental Analysis of Supernatant
Freeze-dried K-SAz-1 (150 mg) and Ca-SWy-2 (100 mg) were weighed into Corex tubes. The DNOC solutions (150 mg L–1) were prepared by dissolving DNOC in appropriate concentrations of KCl and CaCl2 solutions. A measured volume of 0.001 M KOH and 0.0005 M Ca(OH)2 was added to bring the pH of the DNOC solution to 7. The smectite–DNOC systems were pH adjusted (pH 3–7) by adding an appropriate quantity of 0.01 M HCl such that the final volume was 10 mL with a DNOC concentration of 120 mg L–1 DNOC in 1.0 mM K+ or 0.5 mM Ca2+. Controls, that is, smectites without DNOC, were prepared using the same procedure. The mixtures were equilibrated for 48 h at 25°C, then centrifuged at 11951 x g for 25 min. Samples were prepared in triplicate. Smectite–DNOC supernatants, supernatants of the controls (no DNOC), and the background electrolyte solutions used to prepare all of the samples were filtered through 0.2-µm nylon filters and then through 0.02-µm Anotop filters (Whatman, Brentford, UK) and pH tested. Elemental analysis (Ca2+ and K+) of the filtered supernatants was performed with a Thermo Jarrell Ash Model 61 E inductively coupled plasma–atomic emission spectrometer (Thermo Fischer Scientific, Waltham, MA).
The amount of Ca2+ and K+ that was displaced from the smectite into solution in the controls (smectites without DNOC) during pH adjustment was determined. These values were calculated as the difference between the measured K+ and Ca2+ concentrations of control supernatants and the background electrolyte solutions. The amount of Ca2+ and K+ that was displaced from smectite in the smectite–DNOC system during pH adjustment was determined in the same manner. Calculating the amount of K+ and Ca2+ co-adsorbed with DNOC from the actual measured values was not possible since the individual measurements were not at exactly the same pH values in the smectite–DNOC system vs. the control. Therefore, a regression equation was calculated for the systems without DNOC to predict the K+ (y = –370.05x + 2652.9; R2 = 0.98) and Ca2+ (y = –35.643x3 + 465.37x2 – 2055x + 4162.6; R2 = 0.94) concentration at each pH of the smectite–DNOC systems being analyzed. Tabulated data represent the difference between the concentrations of K+ and Ca2+ calculated from the regression equations and the corresponding measured concentrations at each pH of the smectite–DNOC systems being analyzed.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Compared with K-SWy-2, DNOC sorption by Ca-SWy-2 was lower, and diminished much more rapidly as pH increased (Fig. 2B). Freeze-dried Ca-SWy-2 adsorbed 82% of added DNOC at pH 3; however, DNOC sorption decreased to 18% at pH 4 and remained constant to pH 7. The Ca-SAz-1 sorbed 12% of added DNOC across the entire pH range. The lower adsorption of DNOC observed for Ca2+–smectite compared with the K+–smectite (Fig. 2A and 2B) is attributed to the larger enthalpy of hydration, and hence larger hydration sphere, of Ca2+ compared with K+, which inhibits direct interactions between the exchangeable cations and –NO2 groups of DNOC (Boyd et al., 2001; Johnston et al., 2001; Sheng et al., 2002; Li et al., 2004). The larger hydration sphere around Ca2+ also obscures more of the neutral siloxane surface (hydrophobic nanosites), thereby reducing the average size of the adsorption domains. This results in comparatively fewer sites in Ca2+–smectites large enough to accommodate DNOC in its most favorable orientation. Reduction in the average size of the adsorption domains also contributes to the observed lower adsorption of DNOC by the higher-charge-density clay, Ca-SAz-1 (pH 3 and 4) compared with adsorption by Ca-SWy-2, which has lower surface charge density (Fig. 2B).
Figure 2A indicates that K-smectite suspensions adsorbed substantial amounts of DNOC at pHs well above the pKa of DNOC, where the anionic (phenolate) form of the molecule predominates. In this pH range, electrostatic repulsion between anionic DNOC and the negatively charged sites on the smectite surfaces should inhibit DNOC sorption. Our data demonstrate, however, that substantial amounts of DNOC were adsorbed. Two phenomena might help resolve this observation. First, clay surface acidity might catalyze protonation of the phenolate form of DNOC. Surface acidity has been shown to promote protonation of weak bases one or two pH units above their pKa values (Mortland and Raman, 1968; Laird and Fleming, 1999). Second, anionic DNOC might be co-adsorbed with K+, resulting in the formation of a neutral K–DNOC complex. The co-adsorption of anionic DNOC and K+ is consistent with previous studies suggesting inner sphere interactions between K+ and –NO2 groups of DNOC in the interlayers of smectites (Boyd et al., 2001; Johnston et al., 2001; Sheng et al., 2002), and with the observation that high sorption of DNOC at pHs above the pKa occurred only for the K-smectite and not for Ca-smectite.
To test the hypothesis that K+ is co-adsorbed with anionic DNOC, the release or uptake of K+ and Ca2+ accompanying pH change in the presence and absence of DNOC was evaluated for the K-SAz-1 and Ca-SWy-2 systems. The amount of solution-phase K+ or Ca2+ would be expected a priori to increase with decreasing pH, assuming ion exchange of K+ or Ca2+ by H+. Interestingly, as HCl was added to lower the solution pH (pH range 3–7), substantially more K+ was displaced from the K-smectite into solution in the control systems (smectite without DNOC) than the smectite–DNOC systems (Fig. 3A ). These results indicate co-adsorption of K+ and anionic DNOC or that more K+ is retained on the clay in the presence of adsorbed DNOC. Both explanations seem plausible, since K–DNOC complexes have been shown to form in the interlayers of K-smectite accompanied by significant negative enthalpies of interaction (Boyd et al., 2001; Johnston et al., 2001; Li et al., 2004). In contrast, we observed little or no difference in the amount of Ca2+ displaced from Ca-smectite into solution between the control and DNOC-added systems (Fig. 3B), consistent with previous results indicating the inability of Ca2+ to form complexes with DNOC in Ca-smectites (Boyd et al., 2001; Johnston et al., 2001). As pH increases, there should be a systematic decrease in the amount of displaced Ca2+ (Fig. 3B), approaching zero at pH 7; however, the measured value of displaced Ca2+ at pH 7 was about 500 µmol L–1. This excess displaced Ca2+ probably represents residual CaCl2 in the freeze-dried Ca-SWy-2 due to incomplete dialysis.
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The freeze-drying treatment (Fig. 2) did not have as large an impact on the affinity of the smectites for DNOC as in a previous study where air-drying treatments were shown to substantially influence atrazine (2-chloro-4-ethylamine-6-isopropylamino-S-triazine) sorption (Chappell et al., 2005). Air drying (Chappell et al., 2005) might have promoted the formation of larger quasicrystals (with greater effectiveness as adsorbents) than the freeze-drying procedure used in this study (Oetjen and Haseley, 2003).
In summary, we observed that, as the K-smectite suspension pH increased, excess K+ was held by the clay in the presence of DNOC, compared with the same systems devoid of sorbed DNOC. It seems plausible that anionic DNOC is co-adsorbed with K+ as a neutral K–DNOC complex by smectites in aqueous suspensions at pH values above the pKa of DNOC. This could explain the observed sorption of large amounts of DNOC by K-smectites at pHs well above the pKa of DNOC, where it would exist largely in the anionic form. At pH values below the pKa of DNOC, excess K+ on the clay (compared with the control without DNOC) is probably due to the favorable energy associated with the formation of the K–DNOC complexes in the interlayers (Li et al., 2004). In these instances, the net effect of the presence of DNOC in the interlayer of SWy-2 is to enhance the nominal selectivity of K+ vs. H+, since the formation of K–DNOC complexes in smectite interlayers is energetically favorable (Li et al., 2004). Larger amounts of DNOC are adsorbed by low-charge-density smectites than by high-charge-density smectites. The difference is related to the size of the hydrophobic nanosites on smectite surfaces between exchangeable cations, which function as planar adsorption domains. The existence of adsorption sites of sufficient size for optimal DNOC orientation and adsorption diminishes with increasing surface charge density. A limited amount of DNOC is also adsorbed by Ca-smectites independent of pH or surface charge density. Exposed aluminol groups along the lateral edges of the smectite layers are the probable sites for this limited sorption.
Since DNOC is a weak acid, pH was expected a priori to be the dominant state variable controlling sorption and movement of DNOC in contaminated soils. Our results show, however, that pH is only one of many variables to consider when developing remediation strategies. In temperate regions, most soils have a pH well above the pKa of DNOC and are dominated by Ca2+, conditions that limit sorption of DNOC by soil clays. Our results suggest that the injection of K+ into a contaminant plume may be an effective means of promoting sorption and hence preventing the movement of DNOC and possibly other nitroaromatic contaminants into groundwater.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Received for publication July 6, 2006.
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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) and absence (
) of dinitro-o-cresol (DNOC), and (B) the Ca2+ concentration in solution minus background electrolyte representing Ca2+ derived from Ca2+–saturated SWy-2 smectite in the presence (