Reference Smectite Research Articles

Interactions of chlorpyrifos with colloidal materials in aqueous systems.

An understanding of sorptive processes is key to describing the fate of chlorpyrifos [O,O-diethyl-O-(3, 5, 6-trichloro-2-pyridyl) phosphorothioate] in aquatic environments. The objectives of this study were to evaluate isotherms for adsorption and desorption of chlorpyrifos on colloidal materials and to advance understanding of interaction mechanisms between chlorpyrifos and colloidal materials. Six Ca-saturated reference smectites, one Ca-saturated humic acid (Ca-humate), and one suspended sediment sample, collected from the Upper Cedar River, Iowa, were studied. A batch equilibration technique was employed to quantify adsorption and desorption isotherms for chlorpyrifos over the 0 to 100 microg L(-1) concentration range in a 0.01 M CaCl(2) background. Large differences in sorption affinity and variation in desorption hysteresis were found among the smectites. Neither chlorpyrifos adsorption nor its desorption were correlated with cation exchange capacity, surface area, or surface charge density of the smectites. The evidence suggests that physical interaction between chlorpyrifos and smectites is the dominant mechanism for adsorption of chlorpyrifos in aqueous systems. Chlorpyrifos was very strongly sorbed on Ca-humate and was not desorbed from the Ca-humate back into the aqueous solution. Chlorpyrifos was moderately sorbed on river sediment, and a large adsorption-desorption hysteresis was also found. The study implies that the nature of both organic and inorganic materials in suspended sediment can influence the adsorption-desorption behavior of chlorpyrifos in aqueous systems.

Cation and Water Content Effects on Dipole Rotation Activation Energy of Smectites

In soil science, single frequency permittivity measurements are used to determine soil water content, and single frequency bulk electrical conductivity is used to determine soil salinity. The accuracy of these measurements may be influenced by complex interactions between frequency, temperature, and water that is tightly bound to clay surfaces. The purpose of this study is to determine the effect of temperature, saturating cation, water content, smectite properties, and frequency on electrical properties of humidified clays by analyzing three different activation energies for dipole rotations, which are calculated from the temperature dependence of electrical properties. Four reference smectites saturated with K, Na, Ca, and Mg and equilibrated at relative humidities ranging from 56 to 99% were investigated over a frequency range from 3 × 105 to 1 × 109 Hz. Two of the three activation energies were found to decrease slightly as water content increased. Higher activation energies were found for smectites saturated with Mg and K and lower values for smectites saturated with Na and Ca. Trends for type of clay were variable but appeared to be influenced by both the total water content and the distribution of water between the interlayers and the external surfaces of the smectite quasi‐crystals. Changes in quasi‐crystal orientation induced by thermal cycling were also found to influence the activation energies. The results indicate complex frequency and temperature dependent interactions impact electrical properties of the clays. We conclude that no simple equation will correct for temperature and clay content effects on single frequency measurements of permittivity or bulk electrical conductivity.

Crystalline swelling of organo-modified clays in ethanol–water solutions

The swelling of organo-modified clays in organic solvents may have important consequences on bulk properties of materials based on this technology. X-ray powder diffraction was used to study the swelling of orientated films of dioctahedral smectites exchanged with benzyloctadecyldimethylammonium (BODMA) cations, when equilibrated in ethanol–water solutions. The influence of total layer charge and the location of the layer charge on swelling of two organo-modified bentonites and three organo-modified reference smectites in ethanol–water solutions were evaluated. The measured interlayer spaces of BODMA smectites equilibrated in water increased with total layer charge and with the relative amount of tetrahedral charge. At intermediate ethanol concentrations, a distinct plateau was observed in the swelling profiles of BODMA smectites having layer charges >0.8 e − per O 20(OH) 4. When equilibrated in ethanol, all BODMA smectites expanded to similar values (34–36 Å). The magnitude of the change in crystalline swelling of organo-smectites in ethanol compared to that in water was found to be inversely proportional to total layer charge density but proportional to the ratio of octahedral to tetrahedral layer charge. Calculations using NEWMOD showed that the d-spacings measured were unlikely to be due to interstratifications of fully water-solvated with fully ethanol-solvated BODMA smectite layers. Introduction of up to 2.0 M aqueous NaCl resulted in a slight (∼0.5 Å) decrease in the interlayer spacing of BODMA montmorillonite. However, 0.02 and 0.2 M NaCl added to ethanol–water mixtures enhanced interlayer expansion in these miscible solvents. The results suggest that inorganic salt depressed the activity of water in ethanol–water mixtures and thereby increased the proportion of ethanol actually penetrating the interlayer spaces of BODMA smectites. A plausible mechanism is discussed to describe the observed interlayer expansion in ethanol–water solutions and how this might impact possible applications.

Adsorption of Dinitrophenol Herbicides From Water by Montmorillonites

AbstractThe adsorption of two dinitrophenol herbicides, 4,6-dinitro-o-cresol (DNOC) and 4,6-dinitro-o-sec-butyl phenol (dinoseb), by two reference smectite clays (SWy-2 and SAz-1) was evaluated using a combination of sorption isotherms, Fourier transformation infrared (FTIR) spectroscopy, X-ray diffraction (XRD) and molecular dynamic simulations. Clays were subject to saturation with various cations, and charge reduction. The DNOC adsorption decreased with increasing pH indicating that DNOC was primarily adsorbed as the neutral species. The FTIR spectra of DNOC-clay films showed that DNOC molecules are oriented parallel to the clay surface. Interlayer cations have a strong effect on adsorption depending largely on their hydration energies. Weakly hydrated cations, e.g. K+ and Cs+, resulted in greater sorption compared to more strongly hydrated cations such as Na+ or Ca2+. Lower hydration favors direct interactions of exchangeable cations with -NO2 groups of DNOC and manifests optimal interlayer spacings for adsorption. In the presence of sorbed DNOC, an interlayer spacing for K-SWy-2 of between 12 and 12.5 Å was maintained regardless of the presence of water. This d-spacing allowed DNOC molecules to interact simultaneously with the opposing clay layers thus minimizing contact of DNOC with water. The charge density of clays also affected sorption by controlling the size of adsorption domains. Accordingly, DNOC adsorption by low-charge clay (K-SWy-2) was much higher than by high-charge clay (K-SAz-1) and Li-charge reduction greatly enhanced dinoseb adsorption by K-SAz-1. Steric constraints were also evident from the observation that adsorption of DNOC, which contains a methyl substituent, was much greater than dinoseb, which contains a bulkier isobutyl group. Adsorption of DNOC by K-SAz-1 was not affected in the presence of dinoseb, whereas dinoseb adsorption was greatly reduced in the presence of DNOC.

Sorption of phenanthrene by reference smectites.

Fate and behavior of nonionic hydrophobic organic compounds (HOCs) in the environment is mainly controlled by their interactions with various components of soils and sediments. Due to their large surface area and abundance in many soils, smectites may greatly influence the fate and transport of HOCs in the environment. We used phenanthrene as a probe to explore the potential of reference smectites to sorb HOCs from aqueous solution. Batch experiments were used to construct phenanthrene sorption isotherms, and possible sorption mechanisms were inferred from the shape of the isotherms. Our results demonstrate that smectites can retain large amounts of phenanthrene from water. Phenanthrene sorption capacities of the reference smectites investigated in this study were comparable to those of soil clays containing a considerable amount of organic matter. Hectorite exhibited the highest sorption affinity and capacity followed by Panther Creek montmorillonite. The lack of correlation between Freundlich sorption constants (K'f) and indices of charge or hydrophobicity suggests that sorption of phenanthrene by smectites is primarily a physical phenomenon. Capillary condensation into a network of nanoor micropores created by quasicrystals is likely to be a dominant mechanism of phenanthrene retention by smectites.

Potential contributions of smectite clays and organic matter to pesticide retention in soils.

Soil organic matter (SOM) is often considered the dominant sorptive phase for organic contaminants and pesticides in soil-water systems. This is evidenced by the widespread use of organic-matter-normalized sorption coefficients (K(OM)) to predict soil-water distribution of pesticides, an approach that ignores the potential contribution of soil minerals to sorption. To gain additional perspective on the potential contributions of clays and SOM to pesticide retention in soils, we measured sorption of seven pesticides by a K-saturated reference smectite clay (SWy-2) and SOM (represented by a muck soil). In addition, we measured the adsorption of atrazine by five different K-saturated smectites and Ca-saturated SWy-2. On a unit mass basis, the K-SWy-2 clay was a more effective sorbent than SOM for 4,6-dinitro-o-cresol (DNOC), dichlobenil, and carbaryl of the seven pesticides evaluated, of which, DNOC was sorbed to the greatest extent. Atrazine was sorbed to a similar extent by K-SWy-2 and SOM. Parathion, diuron, and biphenyl were sorbed to a greater extent by SOM than by K-SWy-2. Atrazine was adsorbed by Ca-SWy-2 to a much lesser extent than by K-SWy-2. This appears to be related to the larger hydration sphere of Ca(2+) (compared to that of K(+)) which shrinks the effective size of the adsorption domains between exchangeable cations, and which expands the clay layers beyond the apparently optimal spacing of approximately 12.2 A for sorption of aromatic pesticide structures. Although a simple relation between atrazine adsorption by different K-smectites and charge properties of clay was not observed, the highest charge clay was the least effective sorbent; a higher charge density would result in a loss of adsorption domains. These results indicate that for certain pesticides, expandable soil clays have the potential to be an equal or dominant sorptive phase when compared to SOM for pesticide retention in soil.

Mechanisms for adsorption of organic bases on hydrated smectite surfaces

AbstractMechanisms by which organic bases are adsorbed on hydrated smectite surfaces were investigated. Three Ca‐saturated reference smectites (Otay, SPV, and Panther Creek) were dispersed in distilled water containing 5 μmol of pyridine or 3‐butylpyridine. The pH was adjusted to between 7.5 and 3 using 0.01 M HCl. After a 2‐h equilibration, the amount of pyridine or 3‐butylpyridine adsorbed on the clay and the amount of Ca desorbed from the clay were determined. Negligible amounts of pyridine were adsorbed by the Ca‐smectites in the neutral systems (pH > 7); however, most of the added pyridine was adsorbed on the smectites in the acidified systems (pH < 5). Equivalent amounts of Ca2+ were desorbed from the clays, indicating that pyridine was adsorbed as a protonated species by cation exchange. By contrast, 40 to 90% of added 3‐butylpyridine was adsorbed on the smectites at neutral pHs, whereas only small amounts of Ca2+ were desorbed. The results suggest that 3‐butylpyridine is initially retained by hydrophobic bonding between the alkyl side chain of the molecule and hydrophobic nanosites located between the charge sites on smectite surfaces. Surface acidity catalyzed protonation 1 to 1.5 pH units above the pKa of the bases.

Hysteresis in Crystalline Swelling of Smectites

Hysteresis in crystalline swelling of smectites was investigated with respect to the surface properties of five reference smectites. First, the samples were Na saturated and equilibrated with either 3 M NaCl or distilled water to establish initial basal spacings of 15.5 and >40 Å, respectively. Then the samples were equilibrated with NaCl solutions ranging from 0.35 to 3 M, to establish osmotic control of crystalline swelling. Basal spacings of the equilibrated clays were measured by X ray diffraction, and hysteresis was quantified as the difference in weighted-average H2O activities in the equilibration solutions for the adsorption (15.5 to 19 Å) and desorption (19 to 15.5 Å) transitions. The magnitude of hysteresis was similar for all five smectites and was not significantly correlated with surface charge density, percentage of tetrahedral charge, or surface area. The cause of hysteresis in crystalline swelling is attributed to both intrinsic and extrinsic processes. Intrinsic hysteresis is an inherent consequence of changes in the expansion and attraction energy levels during swelling and is believed to be caused by rigidity of the clay-water system. Extrinsic hysteresis is caused by factors that contribute additional rigidity to smectite quasicrystals, primarily thick quasicrystals. Sample history influences the magnitude of the extrinsic hysteresis.

Atrazine Desorption from Smectites

AbstractIn soils low in organic matter, pesticide adsorption and desorption by clay minerals may strongly influence the fate of pesticides in soil environments. Atrazine (2‐chloro‐4‐ethylamino‐6‐isopropylamino‐1,3,5‐triazine) adsorption‐desorption was determined on 11 reference smectites (<2‐µm size fraction), and a soil smectite (<0.02‐µm size fraction from the Ap horizon of a fine‐loamy, mixed, mesic Typic Haplaquoll). For each clay sample, adsorption and desorption isotherms were determined using batch equilibration. Atrazine adsorption on the clays decreased with increasing surface change density (SCD) of the smectites. The desorption isotherms indicated that adsorption was generally reversible. This suggests that atrazine is primarily adsorbed on smectite surfaces through relatively weak van der Waals or H bonds. However, a small positive hysteresis was observed with some clays, and the magnitude of the hysteresis, evaluated using the ratio of the Freundlich isotherm coefficients for adsorption and desorption, increased with SCD. This suggests that atrazine is retained by stronger binding mechanisms on smectites with high SCD, in spite of its lower adsorption capacity. On clays with high atrazine adsorption coefficients, the amount of atrazine desorbed was larger than would be predicted from the adsorption isotherms, resulting in a “negative hysteresis” whereby the desorption isotherm slope was greater than the adsorption isotherm slope. “Negative hysteresis” can be explained if atrazine is assumed to be excluded from interlayer or intraquasi‐crystal water, where only external or interquasi‐crystal water would be available during the desorption process.

Optical spectroscopic studies of the sorption of UO2+2 species on a reference smectite

The speciation of UO2+2 (uranyl) on a reference smectite (SAz-1 from Cheto, Arizona, USA) has been investigated by electronic emission and Raman vibrational spectroscopies. The spectroscopic studies have been done on uranyl-bearing clays prepared from aqueous solutions of uranyl nitrate in the pH range from ~2.5 to 7 and high initial ionic strength (~0.1–0.3 M). The uranyl loading levels in these samples ranged from ~0.1% to ~53% of the reported cation exchange capacity (~1.2 meq/g). Vibronically resolved emission spectra have been obtained for all samples. These spectra vary significantly in intensity and band-shape as a function of uranyl concentration in the clays and the equilibrium pH of the solutions from which the clays were prepared. For most clay samples the measured emission spectrum is a composite of spectra from multiple uranyl emitters. At the lowest loading levels a uranyl sorption complex with an apparent vibronic spacing of ~750 cm−1 dominates the spectra. At intermediate loading levels an additional uranyl sorption complex also having an apparent vibronic spacing of ~ 750 cm−1 is present at an approximately constant concentration ratio to the species in the most dilute samples. At the highest loading levels, a uranyl sorption complex with a vibronic spacing of ~ 850 cm−1 dominates the spectra. Raman spectra have been obtained for the more concentrated uranyl/clay samples. Two distinct bands (855 cm−1 and 883 cm−1) are seen in the spectral region of the totally symmetric uranyl stretch. The 855 cm−1 band correlates with the dominant high-coverage species, while the 883 cm−1 band arises from an additional sorption complex. Comparison of these results with aqueous solution spectral data suggests that monomeric uranyl moieties are responsible for the observed spectral responses in the clay samples, and the multiple spectral components are a result of occupancy by these moieties in several structurally and/or energetically different sites within the clay. It is proposed that the uranyl species responsible for the dominant components in the emission spectra in the low and intermediate coverage clay sample are sorbed to amphoteric edge site(s). The uranyl species responsible for the dominant component in the emission spectrum and the 855 cm−1 Raman band in the high-coverage clays is proposed to be exchanged into the fixed charge site(s). The additional complex identified by the Raman band at 883 cm−1 is also proposed as a sorption complex at fixed-charge sites.

Charge Heterogeneity and Nanostructure of 2:1 Layer Silicates by High-Resolution Transmission Electron Microscopy

AbstractSeveral soil and reference smectites and vermiculites and one reference illite were examined by high-resolution transmission electron microscopy (HRTEM) to decipher the nanostructure and layer charge heterogeneity in these minerals. HRTEM results were compared with those obtained from powder X-ray diffraction (XRD) analysis. Samples were either exchanged with Na+ ions followed by equilibration with a very dilute solution of NaCl in a pressure membrane apparatus at 316 hPa (pF = 2.5) to see the effect of hydration and applied pressure on layer organization, or exchanged with dodecylammonium ions to see the expansion behavior. Oriented samples were embedded in a low viscosity resin and cut approximately 500 Å thick perpendicular to d(001) using an ultramicrotome fitted with a diamond knife. In general, Na-saturated soil clays possessed crystallites that were thinner (c-direction) and shorter (ab-direction) as compared with reference clays. In all cases, samples treated with dodecylammonium chloride exhibited nanostructures that were more disintegrated as compared with Na-saturated samples. In a soil vermiculite, dodecylammonium ion exchange showed frayed edges indicating the initiation of mica transformation to vermiculite from edge toward core. In a reference vermiculite (Transvaal) treated with dodecylammonium ions, in addition to completely expanded crystallites, a regular interstratification between expanded vermiculite and mica (phlogopite) layers was clearly observed in some crystallites. Such nanostructural details were not detected by XRD. HRTEM of the Na-treated illite showed thick crystallites having 10 Å layer separations, whereas the dodecylammonium-exchanged illite showed three types of layers with different degrees of expansion indicating charge heterogeneity in illite: 1) unexpanded (10 Å, highest charge) crystallites; 2) expanded high-charge vermiculite-like (24 Å) crystallites; and 3) occasionally expanded high-charge vermiculite-like (24 Å) layers interspersed in the matrix of 10Å crystallites.

Stability of Synthetic Sand‐Clay Aggregates after Wetting and Drying Cycles

AbstractWe hypothesized that the lack of structure and low aggregate stability characteristic of many California Central Valley soils was due, in part, to infrequent wetting and drying cycles. We created synthetic aggregates by drying a slurry of 0.1‐ to 0.5‐mm‐diam. acid‐washed quartz sand and varying amounts of Ca‐saturated reference smectite, kaolinite, or illite at 40 °C. The resulting dry mixtures were crushed and sieved, and the 0.5‐ to 1.0‐ and 1.0‐ to 2.0‐mm‐diam. synthetic “aggregates” were subjected to up to 80 wetting and drying cycles. The wet aggregate stability (Yoder method) of 1.0‐ to 2.0‐mm‐diam. aggregates with 5% clay decreased, regardless of clay type, after 32 wetting and drying cycles. The smectite and illite 0.5‐ to 1‐mm‐diam. aggregate stability did not change significantly after 32 cycles. Following a decrease after eight cycles, wet stability for the 0.5‐ to 1‐mm 5% kaolinite aggregates increased to equal the zero‐cycle stability after 32 cycles. Increasing clay content to 25% increased the initial smectite wet aggregate stability, but decreased the kaolinite and illite wet aggregate stability for both aggregate size fractions. Scanning electron micrographs of the smectite‐sand aggregates showed that the sand grains became more tightly packed and clay became more uniformly distributed over the grains and formed grain bridges with increasing number of cycles. We attribute variation in response to wetting and drying cycles to different clay‐clay and clay‐sand interactions caused by inherent structural differences among the clays.

Adsorption of Atrazine on Smectites

AbstractSmectites may strongly influence the fate of pesticides in soils due to their large surface area and abundance in agricultural soils. This research was undertaken to determine the effects of smectite properties on the affinity of smectitic clays for atrazine (2‐chloro‐4‐ethylamino‐6‐isopropylamino‐1, 3, 5 triazine). Samples of the <2‐µm fraction were separated by sedimentation from 13 reference smectites and treated with H2O2 for removal of organic matter. A soil smectite sample was separated (<0.02‐µm fraction) from a H2O2 and dithionite‐citrate‐bicarbonate‐treated Webster (fine‐loamy, mixed, mesic Typic Haplaquoll) soil by a dispersion‐centrifugation‐decantation technique. All 14 samples were Ca saturated, dialyzed free of excess electrolyte, and freeze dried. The mineralogy of each sample was evaluated by a combination of x‐ray diffraction and chemical analysis. Values for cation‐exchange capacity (CEC), surface charge density (SCD), surface area (SA), and percent tetrahedral charge (TC) were based on structural interpretations of the chemical analyses. Freundlich adsorption isotherms for atrazine adsorption on each sample were determined using the batch‐equilibration method. Smectite was the dominant mineral in 13 of the clay samples; the 14th sample was a mixture of interstratified smectite‐illite, quartz, and kaolinite. For the smectitic clays, atrazine adsorption ranged from 0 to 100%, and the logarithm of the Freundlich adsorption constant [log(Kf)] decreased linearly with the CEC (r2 = 0.82) of the clays. Stepwise multiple‐regression analysis indicated that log (Kf) values were correlated with SCD and SA values (r2 = 0.83). Inclusion of TC and suspension pH as independent variables in the regression analysis did not significantly improve the correlations. The results indicate that the affinity of smectites for atrazine decreases with increasing SCD, which suggests that atrazine is primarily adsorbed by smectites as a neutral species.

Changes in u(VI) SPeciation Upon Sorption onto Montmorillonite from Aqueous and Organic Solutions

ABSTRACTThe speciation of UO22+ and UO22+/ TBP mixtures has been investigated in solution and intercalated with the reference smectite clay SAz-1 using x-ray absorption, Raman, andluminescence spectroscopies. Neither aquated UO22+ nor its TBP complex undergoes any detectable changes in uranium oxidation state on intercalation. Further, at the pH values employed in this work, there is no evidence for hydrolysis of the uranium species to generate dimeric or higher order uranium oligomers. However, we do find indications that the structures of the solution complexes are altered on intercalation, particularly for the UO22+TBP system and for more dilute UO22+/aqueous systems. In addition, several lines of evidence suggest that, at the loading levels used in this study, the uranyl species is interacting with two or more spectroscopically distinguishable sites on SAz-1.

Spectroscopic (ESR, EXAFS) evidence of Cu for (Al-Mg) substitution in octahedral sheets of smectites

AbstractESR has been used to obtain information on the octahedral or interlamellar position of Cu(II) in natural smectites from Burkina Faso (West Africa). On the basis of 060 XRD reflections and chemical data, these smectites were found to be Al-rich and dioctahedral. After both Mehra & Jackson, and De Endredy deferrification treatments, the Cu contents remained high (4500 and 22000 p.p.m., respectively). The Cu(II) ESR spectra of these deferrated smectites were compared to those of two reference smectites for which the structural position of Cu(II) was precisely known. The interlayer Cu(II) signal was obtained on a Cu-saturated Camp Berteau montmorillonite, while the octahedral Cu(II) signal was obtained on a synthetic Cu-rich smectite. For this latter reference sample, EXAFS spectroscopy provided evidence that Cu was in six-fold coordination in the octahedral sheet only, and was not exchangeable. In agreement with the experiments by Clementz, Pinnavaia and Mortland, a shift in the g⊥ ESR signal was observed when the air-dried Cu-saturated Camp Berteau montmorillonite (g⊥ = 2·05) was soaked in water for 48 h (g⊥ = 2·13). A small shift in the opposite sense was observed for the synthetic Cu smectite (g⊥ = 2·05 for the air-dried sample, g⊥ = 2·02 for the water-soaked sample). For the two natural smectites a small shift similar to that for the synthetic Cu-smectite was observed. These results indicate that up to 10% of the Cu atoms substitute for Al-Mg-Fe atoms in the octahedral sheets of the smectites studied.

Charge Reduction, Octahedral Charge, and Lithium Retention in Heated, Li-Saturated Smectites

AbstractReference smectites were examined to determine relationships between Li uptake, cation-exchange capacity (CEC), and octahedral layer charge after Li saturation and heating at 250°C (Hofmann-Klemen effect). Direct measurements of exchangeable Li after heating led to overestimates of charge reduction due to entrapment of Li in collapsed interlayers. Expansion of interlayers by sequential washings with 1 N MgCl2, 0.01 N MgCl2, and ethanol and subsequent determinations of exchangeable Mg provided accurate measurements of reduced charge. The CEC reductions observed in dioctahedral samples as a result of Li saturation and heating equaled octahedral charge values derived from published mineral formulae, and interlayer charge estimates obtained by alkylammonium exchange confirmed that measured CEC reductions were a consequence of uniform decreases in octahedral layer charge.Dioctahedral specimens retained 1 to 10 meq/100 g of non-exchangeable Li in excess of CEC reduction and were acidified in direct proportion to their total Fe contents, apparently as a result of the deprotonation of structural hydroxyl groups. Mild acid treatment reprotonated these hydroxyl groups, released excess Li, and resulted in total Li contents comparable to measured CEC reductions. Heating (250°C) Mg-saturated hectorite induced a loss of octahedral Li, acidification, and a reduction of CEC, indicating that Mg had partially replaced octahedral Li. These results suggest that octahedral Li is mobile at low temperatures and that cation movement into or out of the octahedral sheet is favored if the layer charge is reduced.

Effect of Exchangeable Potassium on the Hydraulic Conductivity of Smectite-Sand Mixtures

AbstractChanges in hydraulic conductivity of smectite-sand mixtures (using four reference smectites) as a function of the concentration (0.01, 0.003, 0.002, 0.001 M Cl- and distilled water) and potassium adsorption ratio (of 2, 4, and 6) of the percolating solution were measured. Swelling and dispersion of the clays were evaluated from the changes in hydraulic conductivity of the mixture and from the clay concentration in the effluent.The effect of exchangeable potassium percentage (EPP) on the hydraulic conductivity of the smectites depended on the charge density of the clays. The effect of potassium at EPPs <20 on the hydraulic conductivity of smectites having high charge density was negligible. Conversely, the hydraulic conductivity of smectites having low charge density (smectites from Wyoming and Belle Fourche, South Dakota), changed markedly when leached with dilute solutions as the EPP of the clay increased. The dispersive effect of exchangeable potassium on low-charge smectites was similar to that of exchangeable sodium. The low hydration energy of the K+ cations, coupled with the strong electrostatic attraction forces between platelets of smectites with high charge density account for the “inefficiency” of K+ in dispersing these smectites.

Charge Density and Na-K-Ca Exchange on Smectites

AbstractCa-K.-, Na-K-, and Ca-Na-exchange isotherms on four reference smectites were studied. The electric charge on the four smectites was due predominantly to cationic substitution in the octahedral layer and varied between 90 and 122 meq/g (cmole(+)/kg) clay. The affinity of the clays for K increased with a decrease in the fraction of K in the exchange phase. The selectivity coefficients of the smectites for K increased with an increase in charge density. The effect was similar in the Na-K- and Ca-K-exchange isotherms. Charge densities on the smectites had no effect on the Ca-Na-exchange isotherm. The electrostatic attraction forces between the platelets increased with increase in charge density. The low hydration energy of adsorbed K was apparently not enough to overcome these attraction forces as the charge density on the smectites increased. Equilibrium between the low hydration energy of adsorbed K and the electrostatic attraction energies between the platelets is postulated to explain the results.

Layer Charge Evaluation of Expandable Soil Clays by an Alkylammonium Method

AbstractThe interlayer charge of five reference smectites (hectorite, saponite, two montmorillonites, and a nontronite) and clay samples from strongly acidic smectitic soils of Alabama, Mississippi (Ultisols), and Texas (Alfisol) were investigated by an alkylammonium method (AAM). The total layer charge of the reference smectites based on the chemical formula ranged from 0.31 to 0.50 mol(−) per O10(OH)2. The interlayer charge density by AAM linearly increased with increasing total charge. Nontronite had the highest tetrahedral charge and exhibited the highest interlayer charge density. A qualitative distinction of soil clay samples containing low charge and high charge smectite or vermiculite was based on the d‐spacings of the complexes formed by a single alkylammonium compound. Changes in relative peak intensities indicated that the low charge smectite increases whereas the high charge smectite decreases with profile depth. The low charge smectite in these soils appears to have been mostly inherited from the underlying Tertiary and Cretaceous sediments whereas the high charge smectite appears to be a weathering product of the mica present in these sediments.

Effect of Clay Mineralogy and Aluminum and Iron Oxides on the Hydraulic Conductivity of Clay-Sand Mixtures1

Changes in hydraulic conductivity and clay dispersivity of clay-sand mixtures (four reference smectites and Fithian illite) as a function of concentration (0.01 M Cl− and distilled water) and sodium adsorption ratio (SAR ≤ 30) of the percolating solution were measured. In addition, the effect of sand percentage, sand particle size, and addition of AlCl3 and FeCl3 on the hydraulic conductivity of the mixtures were measured. Clay dispersion and migration out of the 3% clay columns was substantial. The clay dispersed only in the distilled water system; dispersion increased with an increase in the percentage of exchangeable Na and was about the same for the Wyoming montmorillonite and Fithian illite. Conversely, the clay swelled in the 0.01 M Cl− solution. The swelling of the montmorillonites increased in the order: Upton, Wyoming = Belle Fourche, South Dakota > Polkville, Mississippi > Otay, California, and was higher than that of the Fithian illite. The swelling and dispersion of the clay accounted for the changes in hydraulic conductivity. Mixtures treated with FeCl3 and AlCl3 were leached with NaCl-CaCl2 solutions until the pH of the effluent exceeded 6.5. The composition of the exchangeable phase was then determined by the SAR of the leach solutions. At pH > 6.5, the polycations hydrolyzed and were present as the hydroxy-polymer species. The hydraulic conductivity of the mixtures decreased as exchangeable Na increased, but the decrease was less than in untreated mixtures, AlCl3 was more effective in maintaining hydraulic conductivity than FeCl3. In montmorillonite clay with an ESP of 20, less than 5% of a complete Al-interlayer was enough to prevent a reduction in hydraulic conductivity. Packets in the day systems tested explain the high efficiency of the Fe and Al polycations.