Saturated Smectite Research Articles

Systematic comparison of the hydration and dehydration of Na+-, K+-, and NH4+-saturated montmorillonite, nontronite, hectorite, saponite, and Fe-saponite by in situ X-ray diffraction measurements

Understanding the swelling properties of smectites is important for geoengineering, biochemical applications, and planetary sciences. The aim of this study was to construct a general model to predict the swelling behavior of smectites saturated with ubiquitous monovalent cations (Na+, K+, and NH4+). The swelling behavior of homo-ionized montmorillonite, nontronite, Al-bearing nontronite, hectorite, saponite, and Fe-saponite during hydration and dehydration were examined by means of relative-humidity-controlled X-ray diffraction. The basal spacings of Na+- and K+-saturated smectites measured during dehydration exhibited very similar curves irrespective of the smectite species, whereas those for hydration were different for different smectite species under lower relative humidity. The similarities of the dehydration curves suggest that dehydration of Na+ and K+ in the smectite interlayers are solely controlled by the intrinsic nature of the interlayer cations, i.e., the hydration energies of the cations. However, hydration of dehydrated Na+ and K+ in the smectite interlayers is controlled by both the intrinsic nature of the interlayer cations and the nature of the host phase. In contrast to Na+- and K+-saturated smectites, the hydration curves of NH4+-saturated smectites were consistent with the dehydration curves. This absence of hysteresis most likely results from the fact that dehydrated NH4+ is bound to the basal oxygen via hydrogen bonding, which was supported by the different lengths of the basal spacings of smectites after complete drying. In turn, this finding suggests that the notable hysteresis observed for Na+- and K+-saturated smectites under low relative humidity was caused by smaller layer-to-layer distances of Na+- and K+- saturated smectites, which may limit the accessibility of water molecule to the interlayer of smectites for hydration. The dehydration behaviors of NH4+-saturated smectites were almost the same as those of K+-saturated smectites except for one saponite specimen, consistent with the similar ionic radii of NH4+ and K+.

Interlayer Cation Polarizability Affects Supercritical Carbon Dioxide Adsorption by Swelling Clays.

Several strategies for mitigating the build-up of atmospheric carbon dioxide (CO2) bring wet supercritical CO2 (scCO2) in contact with phyllosilicates such as illites and smectites. While some work has examined the role of the charge-balancing cation and smectite framework features on CO2/smectite interactions, to our knowledge no one has examined how the polarizability of the charge-balancing cation influences these behaviors. In this paper, the scCO2 adsorption properties of Pb2+, Rb+, and NH4+ saturated smectite clays at variable relative humidity are studied by integrating in situ high-pressure X-ray diffraction (XRD), infrared spectroscopic titrations, and magic angle spinning nuclear magnetic resonance (MAS NMR) methods. The results are combined with previously published data for Na+, Cs+, and Ca2+ saturated versions of the same smectites to isolate the roles of the charge-balancing cations and perform two independent tests of the role of charge-balancing cation polarizability in determining the interlayer fluid properties and smectite expansion. Independent correlations developed for (i) San Bernardino hectorite (SHCa-1) and (ii) Wyoming montmorillonite (SWy-2) both show that cation polarizability is important in predicting the interlayer composition (mol% CO2 in the interlayer fluid and CO2/cation ratio in interlayer) and the expansion behavior for smectites in contact with wet and dry scCO2. In particular, this study shows that the charge-balancing cation polarizability is the most important cation-associated parameter in determining the expansion of the trioctahedral smectite, hectorite, when in contact with dry scCO2. While both independent tests show that cation polarizability is an important factor in smectite-scCO2 systems, the correlations for hectorite are different from those determined for montmorillonite. The root of these differences is likely associated with the roles of the smectite framework on adsorption, warranting follow-up studies with a larger number of unique smectite frameworks. Overall, the results show that the polarizability of the charge-balancing cation should be considered when preparing smectite clays (or industrial processes involving smectites) to capture CO2 and in predicting the behavior of caprocks over time.

Plant Root Exudates Decrease Mobility of Smectite Colloids in Porous Media in Contrast to Humic Acid

Plant root exudates are primarily composed of carbohydrates (CHs), amino acids (AAs), and organic acids (OAs). Little is known about how plant root exudates influence the stability and mobility of clay colloids in the soil profile. In this study, transport behaviors of K+–saturated smectite colloids dispersed in maize (Zea mays L.) artificial root exudate (ARE) solution, humic acid (HA) solution, and deionized water through water-saturated sand columns were investigated with solution ionic strengths of 0.1 and 10 mmol L−1 KCl and pH of 5, 7, and 9. Results showed that smectite colloids were more aggregated and transported less in ARE solution, followed by water and HA solution. This trend became less apparent with increasing pH but more pronounced with increasing ionic strength, suggesting that enhanced stability and mobility of HA-dispersed smectite colloids probably resulted from increased electrostatic and/or steric repulsions. The results for CH-, AA-, or OA-dispersed smectite colloids revealed that the AA fraction was primarily responsible for enhanced colloid retention because positively charged amine groups in AAs (especially lysine) might neutralize the negative surface charge of colloids and promote intersurface bridging. The residual colloids, after flushing with deionized water (thus eliminating secondary energy minima), decreased in the order of ARE-, water-, and HA-dispersed colloids, suggesting greater retention by primary energy minima and pore straining for the ARE-dispersed smectite colloids. Overall, in contrast to HA, plant root exudates decreased the stability and mobility of smectite colloids, thus facilitating the retention of clay colloids in root zones during water percolation events.

Smectite Clays as Adsorbents of Aflatoxin B1: Initial Steps

Smectite clay has been shown to sorb aflatoxin B1 (AfB1) in animal feed 2) and thereby reduce its toxic influence on animals and its entrance to the human food chain. In an effort to find effective adsorbents, 39 samples proposed to adsorb aflatoxin were analyzed and classified into four groups based on their properties: coefficient of linear extensibility (COLE), pH, cation exchange capacity (CEC), organic carbon, microbial content and x-ray diffraction (XRD) spacings. A subset of 20 bentonite samples from commercial sources and reference minerals from 6 US-states and 2 sites in Mexico was selected for sorption determinations. A 10-fold difference in sorption based on the Langmuir equation was observed. Yet clay properties were mostly clustered and it is not clear which properties influence this variation. The basal spacing of AfB1 saturated smectites exhibited greater resistance to collapse on heating than untreated smectites indicating that AfB1 entered the interlayer galleries of the smectites. After heating the mycotoxin-clay complex the desorbed mycotoxin was altered indicating a reaction of the molecules with the clay surface. The most effective sorbent smectite samples were from three US-states (MS, ID, TX).

Quantification of drying induced acidity at the mineral–water interface using ATR-FTIR spectroscopy

Drying induced pH changes were quantified on the surface of Na +, Ca 2+, Mg 2+ and Al 3+ saturated smectite and kaolinite clays. This was achieved using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy to measure real time changes to a pH indicator, sorbed to the clay mineral surface, during wetting and drying events. Using this technique it was possible to measure how low the pH of the surface drops during dehydration, the critical water content at which acidification of the surface begins and lastly how reversible the pH decrease is. The results show that only Al 3+–smectite shows acidification below pH 4.8 with drying. The pH starts to decrease on the Al 3+–smectite surface even when significantly hydrated (gravimetric water content ∼ 125 mg/m 2), and falls to between 1.2 and 1.4 when completely air dry. The drying induced pH decrease is completely reversible on rewetting, suggesting large pH oscillations may occur on smectite surfaces with appreciable exchangeable Al 3+. Aluminium saturated kaolinite did not show significant acidification in response to drying (pH > 3.5), however, a 0.1 M AlCl 3 solution evaporated to a final pH of 2.8. The enhanced acidification observed on an Al–smectite clay compared to a solution containing free Al 3+ ions highlights the role of highly charged surfaces in the hydrolysis reaction that occurs within the hydration shell of exchangeable Al 3+ ions.

Mechanism of Dinitrophenol Herbicide Sorption by Smectites in Aqueous Suspensions at Varying pH

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 (pKa) 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+ and Ca2+), 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+–saturated SWy‐2 at pH 3, and sorption decreased with increasing pH to 62% at pH 7. Sorption of DNOC on K+–saturated SAz‐1 decreased from 94% at pH 3 to 31% at pH 7. Suspended Ca2+–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, Ca2+–saturated 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 from the K+–saturated SAz‐1–DNOC systems indicated co‐adsorption of K+ with DNOC in the phenolate form at pH values above the pKa of DNOC. At lower pH values, DNOC adsorption and complexation with interlayer K+ resulted in less K+ exchange by H+ compared with the control without DNOC. These mechanisms explain the minimal influence of pH on the adsorption of DNOC by the K+–saturated smectites.

Ionic Strength-Induced Formation of Smectite Quasicrystals Enhances Nitroaromatic Compound Sorption

Sorption of organic contaminants by soils is a determinant controlling their transport and fate in the environment. The influence of ionic strength on nitroaromatic compound sorption by K+- and Ca2+ -saturated smectite was examined. Sorption of 1,3-dinitrobenzene by K-smectite increased as KCl ionic strength increased from 0.01 to 0.30 M. In contrast, sorption by Ca-smectite at CaCl2 ionic strengths of 0.015 and 0.15 M remained essentially the same. The "salting-out" effect on the decrease of 1,3-dinitrobenzene aqueous solubility within this ionic strength range was <1.5% relative to the solubility in pure water. This decrease of solubility is insufficient to account for the observed increase of sorption by K-smectite with increasing KCl ionic strength. X-ray diffraction patterns and light absorbance of K-clay suspensions indicated the aggregation of clay particles and the formation of quasicrystal structures as KCI ionic strength increased. Sorption enhancement is attributed to the formation of better-ordered K-clay quasicrystals with reduced interlayer distances rather than to the salting-out effect. Dehydration of 1,3-dinitrobenzene is apparently a significant driving force for sorption, and we show for the first time that sorption of small, planar, neutral organic molecules, namely, 1,3-dinitrobenzene, causes previously expanded clay interlayers to dehydrate and collapse in aqueous suspension.

Effects of Increasing Potassium Chloride and Calcium Chloride Ionic Strength on Pesticide Sorption by Potassium‐ and Calcium‐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+–saturated smectite (K‐SWy‐2) in KCl solution and Ca2+–saturated smectite (Ca‐SWy‐2) in CaCl2 solution at several ionic strengths. The results indicated that pesticide sorption by K‐SWy‐2 significantly increased with increasing aqueous KCl concentration. In contrast, sorption by Ca‐SWy‐2 at different CaCl2 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 KCl solutions. We conclude that formation of better‐ordered clay quasicrystals and shrinkage of clay interlayer distance owing to increased KCl 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.

Traditional and novel methods for estimating the layer charge of smectites

Several methods for characterizing layer charge of smectites were compared. Ten samples of various Na +- and Li +-saturated smectites (four montmorillonites, one nontronite, two saponites and three hectorites) were used. The smectites included minerals isolated from bentonites, as well as some synthetic materials. Cation exchange capacities (CECs) were determined as a reference parameter. These data express the total charge of cations that balance the negative charge sites of smectites. Four CEC methods were used, based on ion exchange reactions with solutions of barium chloride, ammonium acetate, methylene blue and a chelate complex of Cu(II). The layer charge distribution in the octahedral and tetrahedral sheets was estimated from structural formulae that were calculated from chemical analyses. Charge calculation from a structural formula is sensitive to mineral impurities and depends on how the formula is calculated. Moreover, only trace deviations from an ideal structure, such as an inhomogeneous octahedral sheet may significantly affect charge values calculated from structural formulae. Layer charge distribution and average charge densities were determined using the alkylammonium method. After intercalation of n-alkylammonium ions, the interlayer distance depends on the length of alkyl chains and the layer charge of the mineral. The charge distributions of all smectite samples were heterogeneous. The layer charge determined by the alkylammonium method reflects the permanent charge. The estimated CECs calculated from alkylammonium layer charge values were systematically lower than measured CEC values, which include contributions from both the permanent and variable charge sites and some factors related to the alkylammonium method itself. The layer charge distribution was qualitatively estimated using the visible spectra of methylene blue/smectite suspensions. Trends in the spectra indicate that dye spectral properties that reflect molecular aggregation can potentially be used to qualitatively estimate or compare the layer charges of smectites. All the methods include some advantages and disadvantages and differ in their requirements for sample type, purity and composition, time and expenses needed for the analysis. In general, method selection depends on the specific application and whether the need is to characterize layer charge, charge density, charge distribution, or for comparing the charge of similar samples or samples after a chemical treatment or modification.

Thermodynamics of Nitroaromatic Compound Adsorption from Water by Smectite Clay

Nitroaromatic compounds enter the environment through their use as explosives, pesticides, solvents, and synthetic intermediates in the manufacturing of dyes, perfumes, and drugs. Recent studies have found that many nitroaromatic compounds are strongly retained by smectites, especially K+-saturated smectites. Sorption occurs when nitroaromatic compounds replace water associated with the clay and form complexes between K+ and -NO2 groups. This study seeks to further understand nitroaromatic-clay interactions from the viewpoint of energetics. Adsorption isotherms of 1,3-dinitrobenzene, 1,4-dinitrobenzene, and 1,3,5-trinitrobenzene from aqueous solution by K+- and Ca2+-saturated smectite (SWy-2) were measured at several temperatures between 4 degrees C and 37 degrees C to determine the molar differential adsorption enthalpies. Adsorption was found to be an exothermic process on both homoionic K+- and Ca2+-smectite. The smaller adsorption enthalpy on Ca-SWy-2 was consistent with its much smaller adsorption capacity for nitroaromatics compared to K-SWy-2. Our best estimate forthe enthalpy of 1,3,5-trinitrobenzene interactions with K-SWy-2 is -124 kJ/mol, which is referenced to gas-phase 1,3,5-trinitrobenzene, corrected forthe displacement of interlayer water, and can be directly compared with quantum chemical enthalpies from the literature. Our comparable estimates for 1,3- and 1,4-dinitrobenzene interaction enthalpies are near -90 kJ/mol. We conclude that our adsorption enthalpy results are consistent with the hypothesis that nitroaromatic compounds are sorbed strongly by K-smectites because they form inner- and/or outer-sphere complexes with K+ cations in clay interlayers. Indeed, the basal spacings of rewetted clay films in the presence of nitroaromatic compounds imply that water molecules cannot effectively compete with the adsorbed nitrobenzenes for reactive sites on K-SWy-2.

Sorption of Two New Sulfonylaminocarbonyltriazolinone Herbicides and their Metabolites on Organic and Inorganic Exchanged Smectites

The sorption capacity and possible mechanisms of sorption of two new sulfonylaminocarbonyltriazolinone herbicides, methyl 2-[[[(4,5-dihydro-4-methyl-5-oxo-3-propoxy-1 H -1,2,4-triazol-1-yl)carbonyl]amino]sulfonyl]benzoate (MKH 6561) and (4,5-dihydro-3-methoxy-4-methyl-5-oxo- N -[[2-(trifluoromethoxy)phenyl]sulfonyl]-1 H -1,2,4-triazole-1-carboxamide) (MKH 6562), and their potential metabolites, 2,4-dihydro-4-methyl-5-propoxy-3 H -1,2,4-triazol-3-one (triazole) and 2-trifluoromethoxybenzenesulfonamide (phenylsulfonamide) on octadecyl (C18) and dioctadecyldimethylammonium (DOD) saturated and Fe 3+ and Na + saturated smectites has been investigated. Sorption of MKH 6561 on inorganic clays and C18 organoclays was much higher than for MKH 6562, but was similar on DOD saturated clays. For C18 saturated clays, sorption of both parent compounds increased with decreasing layer charge of the organosmectite. Sorption of triazole metabolite, which was much lower than for its parent compound MKH 6561, was higher on inorganic and C18 saturated clays than on DOD saturated clays or inorganic clays. Phenylsulfonamide metabolic sorption, which was also lower than the corresponding parent compound MKH 6562, was higher on DOD saturated clays than on C18 or inorganic saturated clays. These differences in sorption behaviour are related to the diverse relative contribution of hydrophobic and polar interactions for the various compound-clay systems.

Fenuron sorption on homoionic natural and modified smectites

The adsorption isotherms of fenuron (l, l‐dimethyl‐3‐phenylurea) on three smectites (SWy and SAz montmorillonites and SH hectorite) differing in their layer charge (SH<SWy<SAz) and saturated with several inorganic and organic cations were determined. The isotherms and sorption parameters from Freundlich equation indicate low adsorptivity on inorganic clays, but medium sorption in organoclays (OCls). Fenuron adsorption on homoionic smectites increases with decreasing layer charge and hydratation power of the inorganic exchangeable cation (except Fe3+), indicating that fenuron adsorbs as neutral molecule on uncharged siloxane surface by hydrophobic bonding, with some contribution of polar bond (fenuron C=O group and water associated to exchangeable cation). In the case of Fe3+‐saturated smectite fenuron protonation, provided by the interlayer acidic environment, promotes further sorption of fenuron as cationic form. The sorption on organoclays is enhanced via hydrophobic interaction with organocations, which is favoured for high layer charge and basal spacing and organocation saturation close to CEC. Quaternary alkylamonium is more efficient in high layer charge smectite, whereas primary alkylammonium is more efficient in medium charge smectite. The low values of the maximum sorption obtained with homoionic inorganic and organic smectites (100 and 5000 μmol/Kg) represent one fenuron molecule for each 2000–200 exchange sites and indicate that fenuron sorption is mainly associated to the outer exchange sites. This low adsorptivity of fenuron, as consequence of its high water affinity (high water solubility) would suggest high mobility of fenuron in natural soil and water systems.

Relation of Water and Neutral Organic Compounds in the Interlayers of Mixed Ca/Trimethylphenylammonium-Smectites

AbstractOrganoclays were prepared by exchanging Ca2+ in a Ca2+-saturated smectite partially or fully with trimethylphenylammonium (TMPA) cations. The mechanistic function of these organoclays as adsorbents for neutral organic compounds in aqueous solution was examined. TMPA cations were found to take a random distribution on the surfaces of mixed Ca/TMPA-smectites. The presence of TMPA, and its random distribution, resulted in water associated with the clay surfaces being held more weakly. Apparently, the interspersing of TMPA and Ca2+ ions prohibits the formation of a stable network of water molecules around Ca2+. Water molecules associated with the siloxane surface in mixed Ca/TMPA-clays are removed during the adsorption of neutral organic compounds from bulk water, leaving only ∼11 strongly held water molecules around each Ca2+, as opposed to ∼58 water molecules in homoionic Ca2+-smectite. These results demonstrate that the amount of water associated with the clay surfaces and interlayers depends on the nature of the exchange cation(s), and not on the amount of available siloxane surface area by itself. We conclude that in TMPA-smectites the TMPA cations function as nonhydrated pillars, and sorption of organic solutes occurs predominantly on the adjacent siloxane surfaces, which are hydrophobic in nature. The water molecules around Ca2+ in mixed Ca/TMPA-smectites obscures some of the siloxane surfaces. This diminishes sorption capacity, in an amount roughly equivalent to the fraction of the CEC occupied by Ca2+, because organic solutes cannot displace the waters of hydration of Ca2+.

Development of Interstratification in K-and NH4-Smectite from Jelšový Potok (Slovakia) Treated by Wetting and Drying

AbstractA K+-saturated and an NH4+ -saturated smectite from Jelgov 37 Potok (Slovakia) was analysed by X-ray diffraction (XRD) after a series of I to 150 wetting and drying (WD) cycles, and the corresponding distribution functions were calculated using the Dyakonov method. From these data the thicknesses and proportions of different layers were determined. Three kinds of layers (A- 1-00 nm, B-1.26 nm and C-1-52 nm) were detected in the original samples. After 15 WD cycles only A and B type layers were identified in the K-smectite, whereas all three varieties were present during the whole WD treatment in the NH4-smectite. After initial changes the mutual proportion of layers seems to be stable. The 1.00 nm layer is the major layer type in the K-smectite and the 1.26 nm layer is the major layer type in the NH4-smectite. The layer thicknesses evolved during the WD experiments. The systematic changes observed for the NH4-smectite were greater than those for the K-smectite.

Potassium- and Ammonium-Treated Montmorillonites. II. Calculation of Characteristic Layer Charges

AbstractThe formation of an interstratified structure in dioctahedral smectite was assumed to be influenced by (1) the overall layer charge density and its distribution in the structure, (2) the solvation energy of the cation, and (3) the nature of the solvation agent. By holding factors (2) and (3) constant it was possible to calculate the average local charge densities $\overline {{\rm{QA}}}$, $\overline {{\rm{QC}}} $, and $\overline {{\rm{QE}}} $ which are necessary for formation of 10-, 14-, and 16.8-Å mixed-layer phases in potassium-treated and ethylene glycol (EG) saturated smectites. The values of $\overline {{\rm{QA}}} $, $\overline {{\rm{QC}}} $, and $\overline {{\rm{QE}}} $ were 1.99, 1.2, and 0.56 esu/unit cell, respectively. Ammonium-treated smectites saturated with EG gave corresponding mean local charge densities of 2.7, 1.6, and 0.72 esu/unit cell. Calculations were made under the limiting condition QA &gt; QC &gt; QE &gt; 0.For K-smectites saturated with EG, Qtot = 1.99pA + 1.2pC + 0.56pE, where Qtot is the total charge (esu/unit cell), and pA, pC, and pE are probability coefficients for 10-, 14-, and 16.8-Å phases in the interstratified structure. The above equation calculated with the aid of least squares and without the limiting condition yields $${{\rm{Q}}_{{\rm{tot}}}} = {\rm{2}}{\rm{.05pA + 1}}{\rm{.29pC + 0}}{\rm{.33pE}}{\rm{.}}$$ There is a good agreement between values obtained for K-smectites and those for mica, vermiculite, and montmorillonite layer charges for which the above unit-structure distances are typical.

Charge Location as a Factor in the Dehydration of 2:1 Clay Minerals

AbstractWater was removed from Cu(II)‐saturated smectites by methanol when the isomorphic substitution was in the octahedral layer. However, Cu(II)‐saturated tetrahedrally charged smectites and vermiculites were not completely dehydrated by this treatment. Similarly, when (CH3)4N+‐smectites and vermiculites were degassed, only those clays octahedrally charged were dehydrated. Deuteration of (CH3)4N‐montmorillonite and saponite showed the OD stretching frequency for D2O bonded to the silica surface was at a higher frequency for montmorillonite than saponite, indicating a stronger deuterium bond to the surface oxygen for the latter clay. These results showed water was bonded with greater energy to tetrahedrally charged clays, possibly resulting from their more localized charge on the silicate surface.