Reduced-charge Montmorillonite Research Articles

A novel and efficient method for synthesizing reduced charge montmorillonite

Layer charge (LC), a paramount physiochemical characteristic of montmorillonite (Mt), has been garnered considerable attention on its regulating and impact on modified Mt products. Incorporating small cations such as Li+ into the tetrahedral/octahedral sheets of Mt, or employing acid treatment, are feasible methods to reduce its LC. However, conventional techniques for producing reduced charge Mt (RC-Mt) often suffer from prolonged processing times, complex procedures, and high consumption of water and energy. Moreover, the formation mechanisms of RC-Mt remain inadequately understood. To address these challenges, this study proposes a novel and efficient method combining dry lithiation and microwave irradiation to prepare RC-Mt, and compares it with a conventional acid treatment. The research also explores the applications of RC-Mt, focusing on organic modification and the evaluation of adsorption performance. It was found that the cation exchange capacity (CEC) of RC-Mt diminished with increasing microwave power and time, LiCl dosage, HCl concentration, acidification duration and temperature. It has been demonstrated that the dry lithiation & microwave method significantly reduces the total preparation time of RC-Mt to 12 min while eliminating the generation of wastewater. The pivotal advantages of this approach include its efficiency, time-saving, and minimal environmental impact. Analytical techniques such as X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and solid-state nuclear magnetic resonance spectroscopy (SSNMR) revealed that microwave irradiation facilitates the migration of Li+ into the hexagonal cavities and octahedral vacancies of the Mt tetrahedral sheets. Additionally, acid treatment results in the dissolution of octahedral cations from Mt. The modified products of RC-Mt with lower CEC, exhibited enhanced adsorption of perchlorate ions (ClO4−). These findings offer significant insights into the modification of Mt and its potential for advanced technological applications, presenting a promising avenue for future research.

One-pot synthesis of the reduced-charge montmorillonite via molten salts treatment

Layer charge is a critical factor determining the structural characteristics and physicochemical properties of clay minerals, and fixing small cations (e.g., Li+) into the tetrahedral/octahedral sheets of clay minerals is a feasible way for reducing their layer charges. However, conventional methods for preparing reduced-charge clay minerals are usually quite complex and time-consuming. Herein, a facile one-pot treatment method for preparing reduced-charge clay minerals was successfully developed using molten salts. In particular, montmorillonite (Mt) was mixed with mono LiNO3 or LiNO3-NaNO3 mixture, which was then heated above the melting points of salts. The obtained results showed that the cation exchange capacity (CEC) of Mt decreased evidently after being treated with molten LiNO3, and the CEC value was reversely related to the heating time. The X-ray diffraction results demonstrated that Mt layers collapsed upon treatment with molten LiNO3. On the other hand, the partial replacement of LiNO3 by NaNO3 could restrict the collapse of Mt layers, as the larger ionic radius of Na+ (than Li+) helped in maintaining the interlayer spaces of Mt during the process of charge reduction. As indicated by the Fourier transform infrared spectroscopy, the structural OH stretching vibration shifted to a higher wavenumber (from 3623 to 3632 cm−1) after the molten LiNO3 treatment, which suggested that some Li+ ions migrated into the hexagonal cavities in the tetrahedral sheets of Mt, and the new band at 3670 cm−1 indicated that some Li+ ions even further migrated into the octahedral vacancies of Mt. The locations of fixed Li+ ions were further confirmed by the X-ray photoelectron spectroscopy characterization. Due to the migration of Li+ ions into the layers of Mt, the Li1s peak shifted to lower binding energy. The above results showed that the molten salt treatment might be a cost-effective and environmentally friendly approach for synthesizing reduced-charge clay minerals, and using mixed molten salts can help in maintaining the interlayer structure.

Influence of mineralogical and morphological properties on the cation exchange behavior of dioctahedral smectites

Structure and morphology determine the smectites properties. Thereby, edge sites play a key role for the reactivity and colloidal behavior of the smectite. Even so, the edge surface area represents only 1–3% of the total surface area its contribution is up to 10% of the cation exchange capacity (CEC). Na-saturated dioctahedral smectites <0.2 μm were characterized in detail with respect to their structure (layer charge, octahedral cation distribution, iron content, and layer charge distribution within tetrahedral and octahedral sheets). Furthermore, reduced charge montmorillonite (RCM) of these materials were studied. Particle dimension was estimated from argon adsorption, calculation of specific surface area and direct observation of single layers by atomic force microscopy (AFM), which also revealed morphology and particle size distribution of single layer. CEC was measured both for Na-saturated samples and RCM using the Cu-trien method over a pH range of 4–9. The Wyoming smectite with a mean layer diameter of 277 nm and a low layer charge of 0.26 eq/f.u. differs strongly from the others smectites with a mean layer diameter of about 100 nm and layer charges between 0.30 and 0.37 eq/f.u. Furthermore, the layer stacking is much higher for the Wyoming smectite in powder and RCM. Accordingly the edge properties differ between the four smectites. Based on calculation of edge site density and CEC measurements, we showed that the RCM can be used to study the edge site reactivity of the montmorillonites. The increase of CEC above their pHPZC,edge is equal for Na-saturated montmorillonite and RCM. The present study highlights the diversity in structure and morphology and resulting edge properties of four smectites.

Restricting layer collapse enhances the adsorption capacity of reduced-charge organoclays

By restricting layer collapse, and increasing the exposure of siloxane surfaces, in the interlayer space of organoclays, their capacity for adsorbing hydrophobic organic contaminants has been enhanced. The organoclays were prepared by replacing a proportion (20–80%) of the Li+ ions, occupying interlayer sites in montmorillonite, with tetramethylammonium (TMA) ions. The TMA-exchanged samples were then heated at 200°C for 12h to induce migration of most interlayer Li+ ions into the silicate layers, and as a result of which the layer charge was reduced. Finally, the remaining Li+ cations in the reduced-charge montmorillonite were exchanged with TMA. The structural and adsorptive characteristics of the novel reduced-charge organoclays (N-TMA-Mt) are compared with those of their traditional counterparts (T-TMA-Mt), obtained by direct intercalation of TMA into reduced-charge montmorillonites. As layer charge decreased, both the specific surface area and adsorption capacity (for nitrobenzene) of T-TMA-Mt increased to a maximum, and then declined. In the case of N-TMA-Mt, however, both parameters increased as layer charge decreased. When the layer charge of N-TMA-Mt decreased to approximately 60% of the value for the original montmorillonite, the adsorption capacity of the sample was greater than that of T-TMA-Mt. XRD analysis indicates that the layer structure of N-TMA-Mt is more rigid than that of T-TMA-Mt. The above results indicate that the pre-exchanged TMA cations in the interlayer space of N-TMA-Mt act as pillars, restricting layer collapse (during thermal treatment), and increasing the exposure of siloxane surfaces. As a result, the capacity of N-TMA-Mt for adsorbing hydrophobic organic contaminants is superior to that of traditional organoclays.

Enhanced fluorescence of selected cationic dyes adsorbed on reduced-charge montmorillonite

AbstractThe aggregation of three cationic dyes (CD), crystal violet (CV), Nile blue (NB) and rhodamine B (RB) in aqueous solution was studied by visible absorption spectrophotometry and compared with methylene blue (MB). The distribution of the dye species (monomers, dimers, trimers, and tetramers) in aqueous solutions with different concentrations of dye was calculated using equilibrium stepwise aggregation constants Kn. These cationic dyes were intercalated into montmorillonite (SAz-1) and its reduced charge form (RC-SAz(210)) prepared by heating lithium montmorillonite (Li/SAz-1) at 210ºC. The fluorescence of fully saturated CD/SAz and low-CD loaded CD/RC-SAz(210) complexes was studied. Visible absorption spectra of CD aqueous solutions and visible absorption spectra and X-ray diffraction patterns (d001) of the CD/SAz and CD/RC-SAz( 210) solid complexes were obtained and evaluated. Large fluorescence intensities were found for CV/RC-SAz(210) and NB/RC-SAz(210) complexes in the same way as for the complex of methylene blue with reduced-charge montmorillonite MB/RCM(210) described previously.

Fluorescence of reduced charge montmorillonite complexes with methylene blue: Experiments and molecular modeling

The intensity of fluorescence of montmorillonites fully saturated by methylene blue (MB) is very poor due to energy dissipation in MB aggregates. A series of reduced charge montmorillonites (RCM) were prepared from Na-homoionic SWy and Ca homoionic SAz with the aim to decrease the MB aggregation. Fine tuning MB adsorption degree by charge reduction and MB concentration enabled controlled production of different dye species from aggregates via dimers to monomers. It was shown that the intensity of the fluorescence of low-loaded MB–RCM complexes is enhanced by several orders of magnitude with respect to dye-saturated original montmorillonites. XRD analyses, molecular modeling, and diffuse reflectance spectroscopy revealed that low MB-loaded RCMs are very probably adsorbed mainly on the external montmorillonite surface as isolated dye molecules. Such a state cannot be achieved in the solid state without very careful tailoring of the host–guest interaction.

Fluorescence Dynamics of Coumarin C522 on Reduced-Charge Montmorillonite in Aqueous Dispersion

Solvation is an important phenomenon, especially in association with heterogeneous phase interactions. Coumarin C522, C(14)H(12)NO(2)F(3), is used as a fluorophore probe to study the interaction between coumarin and a reduced-charge montmorillonite (RCM) surface. Such hydrophilic and hydrophobic interactions are of interest for sorption processes in confined environments. The prepared RCM series with 0.00, 0.12, 0.26, 0.43, 0.66, and 0.97 Li(+) molar fractions provide different surface charges. The aqueous dispersion of the C522/water/RCM system is studied by using steady-state and time-resolved fluorescence spectroscopies. Both the Stokes shift and the dynamics of the solvation process varied as a function of surface charge. Steady-state fluorescence spectroscopy reveals that the C522 Stokes shift varies from 5,115 cm(-1) for the 0.00 Li(+) molar fraction to 3,988 cm(-1) for the 0.97 Li(+) molar fraction. Time-resolved fluorescence spectroscopy determines that the decay time T((1)) varies from 1.0 ps for the 0.00 Li(+) molar fraction to 3.6 ps for the 0.97 Li(+) molar fraction. Within the range of a few picoseconds, the dynamics of the water solvation shell may be described with H-bond rearrangement, modified with the different RCM surface charges. Two models illustrating the interactions between C522 and RCM in water are proposed which qualitatively describe the dynamics. To the best of our knowledge, this experiment is the first measurement of solvation dynamics on a montmorillonite structure surface using ultrafast laser fluorescence spectroscopy.

Optical properties of molecular aggregates of oxazine dyes in dispersions of clay minerals

The molecular aggregation of oxazine 1 (Ox1) and oxazine 4 (Ox4) in reduced charge montmorillonite (RCM) colloids was investigated by absorption and fluorescence spectroscopies. The aggregation was significantly influenced by the structure of dye cations. Presence of four hydrophobic ethyl groups attached to the ammonium substituents in Ox1 cation prevented formation of closely packed sandwich-type assemblies (H-aggregates). Significant effect of the layer charge was observed for Ox4/RCMs dispersions. Large amounts of the Ox4 H-aggregates were formed in the systems with RCMs of the highest layer charge and reflected in quenched fluorescence. The presence of J-aggregates was proven by absorption spectra for the systems with Ox4 and low-charge RCMs. The flocculation of the lowest charge RCM colloids led to an extensive reduction of the luminescence. The trends and effects of the dye molecular structure and RCM properties are compared with the results previously published for other types of dyes.

Hydration of reduced-charge montmorillonite

AbstractA series of reduced-charge montmorillonites with cation exchange capacities of 89, 73, 49 and 29% of the starting mineral was prepared from a Li-saturated smectite (Kriva Palanka, Republic of Macedonia) by heating at 110, 130, 160 and 300°C for 24 h (samples KP110 – KP300, respectively). Hydration properties of this series were investigated gravimetrically and by in situ XRD at different relative humidities (RHs). In the gravimetric experiments, higher water contents were observed for desorption than for sorption and hysteresis was present over the whole range of RHs for all the samples. The parent montmorillonite and the samples KP110 and KP130 retained similar amounts of water under the same conditions, thus showing that the decreased negative charge on the layers had minor effect on the water uptake at all investigated RHs. Significantly decreased water content was retained by KP160 while KP300 contained only 8% water at 100% RH. The d001 values of the parent montmorillonite and the samples KP110 and KP130 increased with RH, while those of KP160 and KP300 were independent of RH and remained at ∼10.4 and ∼9.6 Å, respectively.

Electronic and structural properties of reduced-charge montmorillonites

Solid state silicon ( 29 Si ) and aluminum ( 27 Al ) nuclear magnetic resonance (NMR) spectroscopies were applied to a series of reduced-charge montmorillonites (RCMs) to discern changes in electronic and structural properties that are induced by Li fixation. Room temperature 29 Si MAS NMR spectra revealed a consistent chemical shift to more negative values and increased line width of the main Q 3(0Al) Si resonance with increasing levels of Li fixation in the RCM series. A decreased line width of the octahedral Al (Al [6]) environment was observed and may be attributed to formation of a more uniform electronic environment surrounding Al [6] as charge reduction occurs. No appreciable changes in the tetrahedral Al (Al [4]) peak were observed for the series, except for line broadening. Correlations of 29 Si NMR chemical shifts with layer charge and infrared-active structural vibrations indicated that distortions in the Si–O–T bond angles (T=tetrahedral Si or Al) occurred, with the mean Si–O–T bond angle increasing, following charge reduction. These results are interpreted as evidence of a redistribution both of layer charge and an abatement of the fit between octahedral and tetrahedral sheets following Li fixation and charge reduction. Our results are consistent with the formation of pyrophyllite-like character in reduced-charge montmorillonite.

Interaction of Methylene Blue with Reduced Charge Montmorillonite

Interaction of methylene blue (MB) with reduced charge Li−montmorillonites (RCM) in the aqueous suspensions was investigated using visible absorption spectroscopy. Dye cation agglomeration and protonation at the clay surface depended very sensitively on layer charge density. With increasing layer charge reduction, the content of agglomerates of MB cations decreased in favor of monomers and the protonated form of MB. Lower negative charge density on the clay basal surface induces a greater distance between neighboring MB cations sorbed at the clay surfaces, which suppresses dye agglomeration. Since each form of MB absorbs visible light at a different wavelength, different layer charge densities induce different colors of the resulting clay−dye suspension. Therefore, visible spectra of MB−smectite suspensions may be a simple but extremely sensitive method for the detection of layer charge density of smectites.

Effect of non-swelling layers on the dissolution of reduced-charge montmorillonite in hydrochloric acid

AbstractA series of reduced-charge montmorillonites (RCM), prepared from the same parent Li-montmorillonite (Jelšový Potok, Slovakia) by heating at various temperatures (105–210°C) for 24 h, was treated with 6 m HCl at 95°C for periods up to 30 h. Reaction solutions obtained were analysed for Al, Fe, Mg and Li and the solid reaction products were investigated by FTIR spectroscopy. Both analyses provided evidence that the extent of dissolution decreased with increased amounts of Li fixed within the montmorillonite structure, i.e. with increased heating temperature. Differences in the acid dissolution process were reflected in the structural changes which occurred within the RCM samples, due presumably to different positions of fixed Li. The ethylene glycol monoethyl ether (EGME) surface areas, and XRD and HRTEM analyses of the RCM series revealed an increased amount of non-swelling layers in the samples prepared at higher temperatures, which caused a substantially slower decomposition of M7 and M8 in HCl. The calculated XRD patterns of M6 and M7 confirmed the presence of 20% and 45% pyrophyllite-like layers, respectively, in these samples. Mixed-layer pyrophyllite-like-smectite and pyrophyllite-like crystals, containing only non-swelling layers, were found in sample M8. The results confirmed that the amount of swelling layers in RCM significantly affects their dissolution rate in HCl.

Preparation and infrared spectroscopic characterization of reduced-charge montmorillonite with various Li contents

AbstractA series of reduced-charge montmorillonites (RCMs) was prepared from Li-montmorillonite from Jelšový Potok (Slovakia) by heating at various temperatures (105–210°C for 24 h. The amount of fixed Li, 0.09–0.67 per O20(OH)4, increased with increasing temperature, confirming preparation of a set of samples of variable layer charge from the same parent Li-montmorillonite by varying only the preparation temperature. Infrared spectroscopy revealed that Li was trapped in the hexagonal cavities of the tetrahedral sheet at all temperatures. Partial deprotonation of the samples, reflected in the decrease of the intensities of the OH-bending bands, was observed after treatments above 120°C. Analysis of the OH-stretching region showed Li in the previously vacant octahedra in the samples heated above 150°C. Weak inflections near 660 and 720 cm−1 confirmed development of local trioctahedral character of octahedral cations coordinated with OH groups in the sample heated at 210°C. Gradual decrease of the layer charge due to Li fixation led to a shift of the Si-O stretching band to higher frequencies and to the appearance of new, pyrophyllite-like bands at 1120 and 419 cm−1.

Orientation of Trimethylphenylammonium (TMPA) on Wyoming Montmorillonite: Implications for Sorption of Aromatic Compounds

AbstractThe orientation of TMPA cations on montmorillonite affects the adsorbate-accessible siloxane surface area and determines whether the TMPA phenyl ring can interact with other aromatic adsorbates by π-π interactions. The purpose of this study was to determine the orientation of TMPA ions in the interlayer of normal-charge and reduced-charge Wyoming montmorillonite. The orientation of TMPA's phenyl group was investigated using infrared dichroism of selected aromatic ring vibrations. X-ray diffraction (XRD) and MacEwan Fourier transforms were used to determine interlayer spacings and to ascertain whether reduced-charge Wyoming montmorillonite is a randomly interstratified mixture of layers with two different d-spacings. For normal-charge montmorillonite, the infrared results showed that the C-N bond axis is neither perpendicular nor parallel to the surface, yet X-ray data suggested that the TMPA phenyl ring is perpendicular or nearly perpendicular to the siloxane surface. In this orientation, the average adsorbate-accessible space between adjacent TMPA ions is 24 Å2, or about 1/3 of the total surface. When the phenyl ring of TMPA is perpendicular to the clay surface, aromatic compounds should be able to interact with TMPA's aromatic ring by π-π interactions, while polar compounds such as water can interact with positively charged nitrogen atom. The reduced-charge montmorillonite used in this study is a randomly interstratified mixture of about 25% collapsed layers with no adsorbed cations and 75% expanded layers that are propped open by TMPA's methyl groups, not by the aromatic ring. The adsorbate-accessible surface area on expanded layers of reduced-charge montmorillonite is 1.5 to 2 times that on normal-charge TMPA-clay, depending on the orientation of TMPA's aromatic ring.

Dehydration and dehydroxylation of reduced charge montmorillonite

Reduced charge montmorillonites (RCM) were prepared by the thermal treatment of lithiumsaturated montmorillonite. Samples prepared by mild thermal treatment with lithium contained more water sorbed than the original montmorrilonite. When RCMs were prepared, part of the lithium cations reacted with hydroxyl groups in the octahedral sheet and released protons, which reacted with the structure. Acid treatment probably enhanced the surface area. which was reflected in the amount of water sorbed. Deprotonation of hydroxyl groups was proved by the measurements of the ignition loss. The heating of lithium saturated montmorillonite at higher temperatures brough about the collapse of the interlayers and a decrease in the amount of water sorbed.

Interaction of long chain alkylammonium cations with reduced charge montmorillonite

Reduced charge montmorillonites (RCM) were prepared using lithium thermal treatment. The sorption of octylammonium (OA), dodecylammonium (DDA) and hexadecylammonium ions (HDA) on differently charged samples were studied. The amounts of DDA and HDA sorbed on each RCM exceed the cation exchange capacity (CEC) but that of OA exceeds only the CEC of samples with the lowest CEC. The sorption is affected not only by the layer charge but also by the formation of collapsed interlayer spaces in the lowest charged montmorillonites. X-ray measurements confirmed the decrease of the layer charge after lithium thermal treatment and the layer charge heterogeneity in RCMs.

Clay modified electrodes: Part 9. Electrochemical studies of the electroactive fraction of adsorbed species in reduced-charge and preadsorbed clay films

Clay modified electrodes (CME) were prepared with natural (untreated) and reduced-charge (prepared by treatment with Li+) montmorillonite. Both the values of ct, the total concentration, and c*, the electroactive concentration of adsorbed M(bpy)32+ (M = Ru, Os, or Fe), were found to be a factor of 10 smaller in the reduced-charge clay, yielding very similar c*/ct, ratios, even though the cations were restricted to the external surfaces in the reduced-charge clay. The electrochemical inactivity of the intercalated cations by itself can therefore not account for the low values of the c*/ct, ratio found in CME. The number of defects in the films appears to be more important than the specific adsorption sites in determining what proportion of the adsorbates will be electroactive. No evidence of mediated electron transport by adsorbed species was seen in films prepared with MV2+ pre-exchanged clay. However, at low scan rates, electron transfer from lattice Fe(II) resulted in initial ipa/ipc -values larger than unity for adsorbed Fe(bpy)32+. This effect was also observed for adsorbed Ru(bpy)32+ , but not for Os(bpy)32+, in both the natural and reduced-charge montmorillonite films.

Properties of Reduced Charge Montmorillonite: Tetra-Alkylammonium Ion Exchange Forms

AbstractSurface properties of reduced charge montmorillonites (RCM) whose exchange capacities were saturated with tetra-alkylammonium cations, were studied with X-ray diffraction, nitrogen, and water adsorption. Methods were utilized for preparing montmorillonite of varying charge and of swelling reduced-charge clay with ethanol. As charge reduction proceeds, all layers retain some exchangeable cations until about half the original charge is reached at which point collapsed layers begin to appear and increase to about 60 per cent at maximum charge reduction. Charge reduction thus appears to proceed in a non-homogeneous fashion. When saturating exchange sites with tetra-alkylammonium ions in ethanol solvent, intercalation of salt in uncharged layers was noted if the samples were not completely washed. Nitrogen adsorption isotherms at liquid nitrogen temperatures showed considerable interlamellar penetration of tetramethylammonium (TMA+) clays, resulting in high calculated surface areas. Much less adsorption on the tetra-n-propylammonium (TPA+) clays was observed. Surface areas were increased upon reduction of charge up to the point where collapsed layers began to be appreciable, whereupon measured areas decreased. Water adsorption isotherms reflected the hydration properties of the exchangeable cation as well as the surface available for adsorption.