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Efficacy of soil clay minerals for the sorption of nonionic organic compounds may be reduced by soil organic matter (SOM) association with clays. Previously, we quantified the fractional availability ([f.sub.a]) of soil clay surfaces for sorption of para-nitrocyanobenzene (p-NCB). Removal of SOM from a soil increased p-NCB sorption, demonstrating that SOM reduced the availability of clay surfaces for sorption of p-NCB. In this study, we extended our investigation of SOM--smectite interactions by measuring the availability of smectite surfaces in four soils to four nitroaromatic compounds (NACs) including p-NCB, 2,4-dinitrotoluene (2,4-DNT), 1,4-dinitrobenzene (1,4-DNB), and 1,3,5-trinitrobenzene (1,3,5TNB). Increased NAC sorption was consistently observed after SOM was removed from whole soils, indicating that SOM blocks the availability of soil smectite surfaces to NACs. Estimates for the [f.sub.a] of smectite surfaces in the four soils ranged from 1 to 0.3. These [f.sub.a] values were strongly negatively correlated with the ratio of SOM/smectite contents. Also, [f.sub.a] values differed systematically with NAC probe molecules, with SOM blocking fewer clay mineral sites from more strongly sorbing NACs. This suggests that smectite surface availability in soils may be an operationally defined quantity whose value depends on the probe molecule. Values of [f.sub.a] approaching 1 for strongly sorbing NACs suggest displacement of SOM from smectite surfaces by the sorbed compound. Abbreviations: 1,3,5-TNB, 1,3,5-trinitrobenzene; 1,4-DNB, 1,4-dinitrobenzene; 2,4-DNT, 2,4-dinitrotoluene; NAC, nitroaromatic compound; NOC, nonionic organic compound; p-NCB, paranitrocyanobenzene; SOM, soil organic matter.

Understanding sorption mechanisms and processes is critical for predicting the fate of organic pollutants in soils and for development of effective remediation strategies for contaminated soils. A novel suspension x-ray diffraction (XRD) technique was used to elucidate the influence of the size and swelling of K- and Ca-smectite quasicrystals in aqueous suspensions on adsorption of 4,6-dinitro-o-cresol (DNOC). X-ray diffraction patterns of randomly oriented smectite quasicrystals in aqueous suspensions were compared with diffraction patterns for air-dried and oven-dried oriented films of the same smectites. The XRD patterns reveal that the phenolate form of DNOC, which is predominant in solution with pH [greater than or equal to] 4.4, is primarily adsorbed on external surfaces of K-smectite quasicrystals in aqueous suspensions and may enter the interlayers as a K-DNOC complex when K-smectite quasicrystals coalesce. In Ca-smectite suspensions with pH > 4.4, the phenolate form of DNOC is adsorbed only on external surfaces; however, DNOC may become entrapped between substacks within Ca-smectite quasicrystals as the smectite suspension dries to form an oriented film. The neutral form of DNOC (pH [less than or equal to] 4.4) was adsorbed in the interlayers of a low-charge-density Ca-smectite in aqueous suspension but not in the interlayers of the high-charge-density Casmectite, apparently due to steric restrictions. Clearly, XRD analysis is a valuable tool for gaining insight into interactions between smectites and organic molecules, but caution must be used in extrapolating data based on XRD patterns of air-dried and oven-dried smectite films to processes that may occur in aqueous smectite suspensions. Abbreviations: Ca-SAz-1, calcium-saturated Arizona montmorillonite; Ca-SWy-2, calcium-saturated Wyoming montmotillonite; DNB, 1,3-dinitrobenzene; DNOC, 4,6-dinitro-o-cresol; K-SAz-1, potassium-saturated Arizona montmorillonite; K-SWy-2, potassium-saturated Wyoming montmotillonite; NACs, nitroaromatic compounds; p[K.sub.a], acid dissociation constant; RH, relative humidity; SAz-1, Arizona montmorillonite reference clay (smectite); SWy-2, Wyoming montmorillonite reference clay (smectite); XRD, x-ray diffraction.

The compound 4,6-dinitro-o-cresol (DNOC) is an important pesticide that is strongly adsorbed by smectite clays. Because DNOC is a weak acid with an acid dissociation constant ([pK.sub.a]) of about 4.4, pH was hypothesized to be a dominant state variable controlling sorption. In this study, we quantified the effect of pH, saturating cation ([K.sup.+] and [Ca.sup.2+]), and freeze-drying on adsorption of DNOC by two reference smectites with different charge densities (SWy-2 and SAz-1) in dilute aqueous suspensions. The smectite-DNOC systems were adjusted from pH 3 to 7. Nearly 100% of added DNOC was adsorbed by [K.sup.+]-saturated SWy-2 at pH 3, and sorption decreased with increasing pH to 62% at pH 7. Sorption of DNOC on [K.sup.+]-saturated SAz-1 decreased from 94% at pH 3 to 31% at pH 7. Suspended [Ca.sup.2+]-saturated SWy-2 adsorbed 82% of added DNOC at pH 3 but sorption decreased to 18% for pH 4 and above. Across the entire pH range, [Ca.sup.2+]-samrated SAz-1 sorbed about 12% of added DNOC. Slightly larger amounts of DNOC were adsorbed by the 'never dried' (not freeze-dried) smectites compared with the freeze-dried and rehydrated smectites. Analysis of supernatants front the [K.sup.+]-saturated SAz-1-DNOC systems indicated co-adsorption of [K.sup.+] with DNOC in the phenolate form at pH values above the [pK.sub.a] of DNOC. At lower pH values, DNOC adsorption and complexation with interlayer [K.sup.+] resulted in less [K.sup.+] exchange by [H.sup.+] compared with the control without DNOC. These mechanisms explain the minimal influence of pH on the adsorption of DNOC by the [K.sup.+]-saturated smectites. Abbreviations: Ca-SAz-1, calcium-saturated Arizona montmorillonite reference clay (smectite); Ca-SWy-2, calcium-saturated Wyoming montmorillonite reference clay (smectite); DNOC, 4,6-dinitroo-cresol; K-SAz-1, potassium-saturated Arizona montmorillonite reference clay (smcctitc); K-SWy-2, potassium-saturated Wyoming montmorillonite reference clay (smectite).

In aqueous suspension, the affinity of many pesticides for smectites is influenced by the clay properties such as surface area, surface charge density and location, exchangeable cations, and hydration status of exchangeable cations in clay interlayers. The amount and the type of salts present in the aqueous phase influence clay quasicrystal structures and hydration status, which we hypothesize as major determinants of pesticide sorption. To test this hypothesis, we measured sorption isotherms of alachlor, atrazine, dichlobenil, and diuron by [K.sup.+]-saturated smectite (K-SWy-2) in KCI solution and [Ca.sup.2+]-saturated smectite (Ca-SWy-2) in Ca[Cl.sub.2] solution at several ionic strengths. The results indicated that pesticide sorption by K-SWy-2 significantly increased with increasing aqueous KCI concentration. In contrast, sorption by Ca-SWy-2 at different Ca[Cl.sub.2] ionic strengths remained nearly constant. The 'salting-out' effect on the reduction of dissolution of pesticides failed to account for the significantly increased sorption by K-SWy-2 in aqueous KCI solutions. We conclude that formation of better-ordered clay quasicrystals and shrinkage of clay interlayer distance owing to increased KCI ionic strength facilitate the intercalation of pesticides leading to greater sorption by the clay, while the salting-out effect plays a minor role (if any) in the observed sorption enhancement.

Nitroaromatic compounds (NACs) are commonly found as soil contaminants in military training sites and manufacturing facilities, and may adversely affect human and ecosystem health. Exchangeable cation effects on p-nitrocyanobenzene (p-NCB) and 1,4-dinitroben-zene (1,4-DNB) sorption by the Webster (fine loamy, mixed, mesic Typic Haplaquoll) soil series A (WA) and B (WB) horizons were determined from batch sorption experiments. Smectite is the most abundant mineral in the horizons of this soil. Soil organic matter (SOM) removal increased p-NCB sorption to the WA and WB horizons by ~1.5 times, and increased 1,4-DNB sorption to the A and B horizons by ~1.2 to 2.2 times, respectively. Greater sorption of NACs by whole soils after SOM removal suggests that SOM suppresses p-NCB and 1,4-DNB sorption by soil mineral components. Clay (

Cation exchange is one of the most venerable concepts in soil science, yet it needs rethinking. This paper presents an extremely simple conceptual framework for interpreting many observed trends in cation exchange. Taking the example of Cs-K exchange, the methods of computational molecular mechanics found that Cs-montmorillonite is considerably higher in energy than K-montmorillonite at constant water content, in agreement with inferences from a new thermodynamic cycle representation of cation exchange. Since montmorillonite selects [Cs.sup.+] over [K.sup.+] in real experiments, these results mean that alkali cation-exchange selectivity is controlled by selectivity of the solution phase for the more strongly hydrated cation. Thus the clay does not 'select' for [Cs.sup.+] over [K.sup.+] in any positive sense and it may he more useful to consider cation exchange as a partitioning reaction: Given two cations of equal valence, the more weakly hydrated will tend to partition into the 'subaqueous' smectite interlayer phase. This concept seems not only parsimonious, but also more accurate than other hypotheses for cation exchange selectivity that impute more favorable interactions between smectite surfaces and the selected cations; such theories err by ignoring energy changes in the solution phase. This simple partitioning concept rationally explains the alkali and alkaline earth selectivity sequences as well as the selectivities of smectites for organic cations over inorganic, for larger organic cations over smaller, and for organometallic complexes over the uncomplexed metal.

In soils, organic matter and minerals are often associated such that it is unclear how the presence of the former component influences the sorptive properties of the latter one. In this study, sorption and desorption of the herbicides 4,6-dinitro-o-cresol ([C.sub.7][H.sub.6][N.sub.2][O.sub.5]) and dichlobenil ([C.sub.7][H.sub.3][Cl.sub.2]N) by [Ca.sup.2+]-, [K.sup.+]-smectites, and humic acid--smectite complexes, was investigated using batch-equilibratious and x-ray diffraction (XRD). Greater sorption of 4,6-dinitro-o-cresol compared with dichlobenil was observed for both smectites and humic acid--smectite complexes. For both pesticides, [K.sup.+] smectites were more effective sorbents than [Ca.sup.2+] smectites, with the lower charge-density clay (SWy-2) displaying a greater sorption capacity than the higher charge-density clay (SAz-1). The presence of humic acid did not impact pesticide sorption by [K.sup.+] days, but could enhance or suppress pesticide sorption by [Ca.sup.2+]-clays. A composite model for estimating pesticide sorption, which assumes mineral and organic matter function individually as sorbent phases, predicted sorption within a factor of 0.8 to 1.5 times the measured values. Humic acid did not contribute to pesticide desorption hysteresis in [K.sup.+]-humic acid-clay complexes, but was a source of hysteresis in the corresponding [Ca.sup.2+] complexes. The basal spacings of K-SWy-2 and humic acid-modified K-SWy-2 increased gradually from approximately 10.4 to 12.2 [Angstrom] with increasing 4,6-dinitro-o-cresol loadings. Also, XRD patterns of humic acid-modified and unmodified K-SWy-2 smectite clays were found identical. These results demonstrate the intercalation of 4,6-dinitro-o-cresoi and suggest that humic acids are restricted to the external surfaces of clay tactoids. Together, these results indicate that clay mineral fractions in soils, including those with organic coatings, may play an important role in the retention of certain pesticides.

The structure of trichloroethene (TCE) adsorbed on the mineral surfaces in the interlayer space of an idealized nontronite, a 2:1 clay mineral containing structural Fe(super 2+) is optimized using quantum mechanical geometry optimizations and molecular dynamics simulations. The dechlorination reaction of TCE on nontronite mineral surfaces are studied by density functional theory with a generalized gradient approximation, employing the periodic ab initio program CASTEP.

A study of the involvement of aluminum salts in coagulating natural organic matter found in lake water.