Search

Montmorillonite (Mt) is a ubiquitous swelling clay mineral and major component of soft rocks, sediments, and soils with an inherent capability to sorb metal cations. This unique feature renders Mt important for the enrichment and mobilization of environmentally important metal cations, retardation of heavy metals and radionuclide ions, the evolution of clay mineral itself, soils and sediments, and other geological processes. Understanding the interfacial interactions of Mt with metal cations at the molecular level is of fundamental importance in all these processes, but still remains elusive, due to the chemical and structural complexity of Mt surfaces and the diverse chemistries of metal cations. In this Review, we aim to provide the reader with a comprehensive overview of the adsorption modes of metal cations on basal and edge surfaces of Mt, local chemical environments of the cation binding sites, the driving forces for metal sorption, and factors influencing the dynamics of cation uptake onto Mt surfaces. Various surface complexation models [i.e., nonelectrostatic model (NEM), constant capacitance model (CCM), diffuse layer model (DLM), and triple-layer model (TLM)], advanced spectroscopic techniques (i.e., NEM, CCM, DLM, and TLM), and atomistic simulation methods (i.e., MD, DFT, and FPMD) have been used in conjunction with macroscopic adsorption experiments to gain detailed insights into the interfacial interactions of metal cations on Mt. Mt adsorbs metal cations via three independent pathways: (1) cation exchange; (2) surface complexation; and (3) nucleation and surface precipitation. The principal driving force for cation exchange is electrostatic interaction, while chemical bonding governs the two other mechanisms that depend on the basal and edge surface properties of Mt. The siloxane cavities on the tetrahedral basal plane exhibit the strongest adsorption sites for cation exchange and are greatly affected by the the degree of Al3+/Si4+ tetrahedral substitutions. At the amphoteric edge surfaces bearing hydroxyl groups, metal cations could form mono/multidentate surface complexes on Mt [010] and [110] edges. Ionic strength, pH, the presence of competing cations, temperature, and layer charge have been shown to affect the adsorption mechanisms and quantity of adsorbed cations. The updated information on the interfacial interactions of metal cations with Mt basal and edge surfaces presented in this review provides an improved understanding of the enrichment of metals, formation of metal ores, and natural biogeochemical cycles, as well as may promote technological and engineering applications of this important clay mineral in environmental remediation, geological repository, petroleum exploration and extraction, and extraterrestrial research. [ABSTRACT FROM AUTHOR]

Chemical weathering is one of the most important processes on the Earth's surface, and lithium isotopes have been used as a promising tracer of silicate weathering, as the secondary clay minerals precipitated during silicate weathering are thought to preferentially incorporate the lighter 6Li isotope. However, to utilize lithium isotopes as a geochemical tracer, it is essential to quantitatively understand the partitioning of Li in clay minerals, including structurally bound Li and exchangeable interlayer ions, as well as the associated isotope fractionation. With classical molecular dynamic simulations and thermodynamic calculations, the cation exchange between Na-/K-montmorillonite and solvated Li in environmental solution are disclosed under different aqueous activity conditions. The Li+ ion, whose hydration ability is stronger than Na+ and K+ ions, is easier to swell the montmorillonite and shows less affinity for the clay. And the calculated cationic selectivity coefficients between Na+/K+ and Li+ ions are K c (Li → Na) = 2.0, K c (Li → K) = 10.0 for Arizona-type montmorillonite and K c (Li → Na) = 1.0, K c (Li → K) = 4.0 for Wyoming-type montmorillonite, which are comparable to reported experiments. The cationic partition of Li in the clay interlayer decreases when the hydration states of montmorillonite transform from triple-layer to bilayer and then to monolayer. Based on the density functional theory, the Li isotope fractionations for Li in clay interlayer sites and structural octahedral sites are further derived by thermodynamic calculations with path integral molecular dynamic simulation, which deals with the quantum anharmonicity. The behaviors of Li isotope in the bilayer hydration state closely resembles that in the aqueous solution, and only a slight Li isotope fractionation is found between the monolayer hydrated octahedral-substituted clay and solution phase (Δ7Li monolayer-aq = −2.0‰). While Li isotope fractionation between aqueous solution and mica structural sites is significant (Δ7Li structural-aq = −10.4‰) and shows an obvious quantum anharmonicity, which indicates the importance of the anharmonic treatment for Li isotope fractionation calculations. The newly calculated Li equilibrium isotope fractionation between clay and aqueous solution deviates from the experimentally determined fractionation factor, which can be attributed to the kinetic effects. Moreover, on the basis of the surface kinetic reaction model, the Li isotope fractionation becomes more pronounced with higher clay growth rate. By integrating the cationic selectivity coefficient and Li isotope fractionation, it is possible to constrain the fluid composition during Li-clay deposit and diagenesis, and to evaluate the intensity of chemical weathering processes as well as the related carbon cycle. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ranitidine (RT) removal from water by natural Ca-montmorillonite (SAz-1) was studied. • Maximum RT adsorption value was 369.2 mg/g. • Cation exchange was the main mechanism for RT removal. • Molecular morphology of RT was crucial for its interaction with SAz-1. Adsorption behavior of ranitidine hydrochloride (RT) on a Ca-montmorillonite (SAz-1) was studied in aqueous system through batch experiments. The adsorption kinetics revealed that the equilibrium reached within 0.25 h and the data fitted well to the pseudo-second order kinetic equation (R2 = 0.98). The maximum RT adsorption capacity of SAz-1 was 369.2 mg/g and the adsorption isotherm data followed the Langmuir model (R2 = 0.99). The adsorption of RT and desorption of exchangeable cations from the clay mineral were linearly correlated, suggesting that cation exchange was the dominant mechanism of RT adsorption. The XRD examination of RT-adsorbed SAz-1 samples (unsaturated/saturated) after heating enabled the calculation of RT occupied area in the interlayer of the clay mineral. The results suggested that adsorbed-RT at low loading rate could lay on the internal surfaces in a free style to reduce the basal spacing (d 001 value) of SAz-1. When the RT loading rate was increased, a limited surface space enforced more RT molecules to lay in a tilted style and caused interlayer swelling of SAz-1 increasing the d 001 value. The trend of rising decomposition temperature of RT with increasing RT loading rates confirmed intercalation of RT molecules in SAz-1. Infrared spectral analysis revealed the participation of amide and furan groups of RT in binding between RT and SAz-1. Thus, this study indicated that SAz-1 is an efficient adsorbent to remove RT from contaminated water, and the chain-like molecular structure of RT could cause an irregular change in the basal spacing of swelling type clay minerals. [ABSTRACT FROM AUTHOR]

Currently, the deep geological repository approach for spent nuclear fuel is regarded as the most dependable and secure method for permanently disposing of this kind of waste. Among its key safety components is an engineered barrier made from compacted bentonite, which isolates the encapsulated waste from the surrounding host rock. As a result, understanding how bentonites react to varying compositions of groundwater is crucial. This is where numerical modelling becomes essential. It is generally approved by the scientific community to idealise bentonite as a material structured under a double porosity system composed of the macro and microstructure. In this context, this paper illustrates the capabilities of a double-porosity reactive transport model for bentonites fully implemented in the multiphysics COMSOL platform. For this purpose, different experimental tests were simulated based on the evaluation of diffusive ion transport, mineral dissolution and cation exchange processes in MX-80 bentonite, obtaining very satisfactory results. • A new reactive transport model for bentonites was implemented in COMSOL Multiphysics. • The model includes a geochemical system composed of 14 chemical species and gypsum. • The model considers diffusive-dispersive-advective transport in double-porosity media. • The model validation was run by 3 tests set in the Task Force on Engineered Barrier. [ABSTRACT FROM AUTHOR]

This paper demonstrates that intercalating Na+ homoionic vermiculite with Fe3+ polyhydroxy cations (1:1 molar ratio OH− to Fe3+) significantly improved the affinity of the clay mineral-based sorbent toward phosphate. Kinetic experiments revealed that adsorption is fast, approaching an equilibrium within about 200 min of contact time, and that the rate-limiting step is the intraparticle diffusion. Adsorption isotherms fitted to the Freundlich equation and a two-site Langmuir model, consistent with the heterogeneity of adsorption sites. The separation factor derived from the Langmuir constant revealed that the adsorption was favorable and even irreversible for high-affinity minor adsorption sites. The adsorption capacity was 299 ± 63 μmol g−1 (9.3 ± 2.1 mg P g−1), a value similar to several other clay-based phosphate adsorbents. Application to reservoir water spiked with 10 mg L−1 in P removed about 71% of the available phosphate. [ABSTRACT FROM AUTHOR]

Selected layered clay minerals of smectite group with different chemical composition (e.g. synthetic hectorite, synthetic smectite and natural smectite) were modified by rare earth cations such as Er3+ and a stochiometric mixture of Er3+/Yb3+. Raw and modified smectites were systematically heated up to temperatures ensuring their dehydration (400 °C), dehydroxylation (800 °C) and formation of new high temperature phases (1200 °C). The products of heating were investigated by terahertz-time domain spectroscopy to follow changes of their dielectric properties in the far-infrared region. The frequency-dependent refractive index of raw and modified clays was determined in the range of 0.2–3.8 THz (6.6–126.5 cm−1). The heating at 800 °C and above resulted in increase frequency dependent index of refraction and decrease absorption coefficient in comparison to the raw clays. The ion-exchanged smectites by rare earth cations further decreased the absorption coefficient of the samples heated at 1200 °C. Raw natural smectite sample revealed a low frequency band at 0.86 THz, already reported in the literature, which shifted to 0.67 THz after sample modification by rare earth cations. Heating of modified JP to 1200 °C resolved a new absorption band at ∼3.40 THz in the ceramic phase produced from raw natural smectite. • Cation exchange reaction was used for modification of clays by selected rare earth elements. • Effect of sample heating up to 400, 800 and 1200 °C was studied. • Terahertz time domain spectroscopy was used to study the refractive index dispersion. • The frequency dependence of absorption in THz region resolved new typical absorption bands. • The high temperature treatment can be used for synthesis of new ceramic phosphor materials. [ABSTRACT FROM AUTHOR]

[Display omitted] Crystalline hydrates of swelling clay minerals (smectites) exhibit a strong coupling between their ion exchange and hydration/dehydration reactions. The uptake or removal of water from smectite interlayers as a result of a change in the environmental conditions also leads to the partitioning of cations. Three factors, the solid ion composition, the solid basal spacing/water content, and the aqueous solution composition, are all implicated in controlling the thermodynamics of ion exchange. However, conventional approaches to measuring the exchange free energy cannot separate the influence of each of these individual factors. Here, we explore the energetics of the swelling and ion exchange reactions in montmorillonite using a potential of mean force approach and the thermodynamic integration method within molecular simulations. We investigate the influence of solution and clay composition on the spontaneity of the reactions, focusing on the 2 water-layer hydration state. The swelling simulations provide the equilibrium water content, interlayer water structure, and basal spacings, while thermodynamic integration of sodium–potassium exchange in the aqueous solution and solid phase are combined to calculate ion exchange free energies as a function of solution composition. Results confirm the tendency of the clay to collapse to lower hydration states as the concentration of the solution increases. Changes to the equilibrium water content, even at fixed hydration states, and the composition of the mixed electrolyte solution play a critical role in driving ion exchange and the selectivities of the clay to the exchanged cation, while the composition of the solid phase is shown to be insignificant. These findings underscore the extreme sensitivity of clay swelling and ion exchange thermodynamics to small (tenths of an Angstrom) deviations in layer spacing. [ABSTRACT FROM AUTHOR]

[Display omitted] • Natural Na-montmorillonite (SWy-2) adsorbed 0.88 meq/g amitriptyline (AMI) from water. • AMI adsorption was fast (<4h) and exceeded the clay mineral's CEC. • AMI removal was mainly via a cation exchange mechanism. • AMI removal was unaffected by pH in the range 2–11. • Amine groups and benzene rings in AMI controlled molecular orientation in the clay interlayer. Amitriptyline (AMI) is one of the most common tricyclic antidepressant personal care medications. Due to its environmental persistence and bioaccumulation, release of AMI into the environment via wastewater streams in elevated levels could lead to significant ecological and human health impacts. In this study, the adsorption of AMI by montmorillonite (SWy-2), a naturally abundant smectite clay with sodium ions as the main interlayer cations, was investigated. Maximum AMI adsorption (276 mg/g) occurred at pH 7–8. After adsorption, examination of the adsorbent's X-ray diffraction pattern indicated that interlayer expansion had occurred, where chemical stoichiometry confirmed cation exchange as the principal adsorption mechanism. AMI adsorption reached equilibrium within 4 h, with kinetic data best fitting the pseudo-second order kinetic model (R2 = 0.98). AMI adsorption was unaffected by solution pH in the range 2–11, where adsorption was endothermic, and molecular simulations substantiated by Fourier transform infrared spectroscopy and thermogravimetric investigations indicated that the orientation of AMI molecules in the interlayer was via an amine group and a benzene ring. Overall this research shows that SWy-2 has significant potential as a low cost, effective, and geologically derived natural material for AMI removal in wastewater systems. [ABSTRACT FROM AUTHOR]

The presence of antibiotics in soil and water raises great health concerns because they may increase the development of harmful antibiotic-resistant bacteria. In this study, composite beads prepared from 2:1 or 1:1 ratios of montmorillonite (Mt) and sodium alginate (SA) were used to adsorb tetracycline hydrochloride (TC), an antibiotic commonly found in aqueous systems. The equilibrium time for TC adsorption onto the Mt/SA was 8 h, and the kinetic data were consistent with the pseudo-second-order model. The adsorption isotherm was described with the Langmuir equation. The maximum amounts of TC adsorbed were 745, 689, and 445 mg g−1 for the 2:1- and 1:1-Mt/SA and the original Mt, respectively. The Mt/SA composite beads exhibited porous structures; however, this did not play a key role in TC removal, as previously reported. Cation exchange was the major adsorption mechanism, and electrostatic attraction and hydrogen bonding between the SA and TC also contributed to TC adsorption on the Mt/SA composite beads. In addition, the migration of a small amount of TC into the inner spaces of the beads led to the intercalation of the TC in the Mt interlayers and enhanced TC adsorption by the Mt/SA composite beads. • The porous structures of composite beads adsorbed remarkable amounts of tetracycline (TC) from water. • Cation exchange was the primary adsorption mechanism of TC. • Electrostatic attraction and hydrogen bonding between alginate and TC enhance TC adsorption. • The amine and dimethylamino groups of TC orient horizontally on the internal surface of composite beads. [ABSTRACT FROM AUTHOR]

The sorption of Eu(III) onto montmorillonite samples from different deposits such as those of Kutch (India), 10th Khutor deposit (Russia), and Cortijo de Archidona (Spain) was studied over a wide range of experimental conditions by varying the total Eu concentration, pH, and ionic strength. Various bentonite samples showed quantitative sorption at all pH values at low ionic strengths of the solution because of ion exchange interactions. At a higher ionic strength (1 M NaClO 4), the sorption is pH-dependent because of the interaction with edge sorption sites and it differed from sample to sample. The difference in the mechanism of interaction at different ionic strengths was confirmed by time-resolved laser-induced fluorescence. Thermodynamic modeling was performed with all experimental data for montmorillonite samples from different sources to create a generalized model for describing their interaction with Eu(III). Unlabelled Image • Sorption of Eu(III) onto previously not studied montmorillonite is investigated. • Samples include new Kutch bentonite from India and Khakassia bentonite from Russia. • Admixture iron oxyhydroxide in Kutch bentonite influences Eu(III) sorption. • Thermodynamic model describing Eu(III)–montmorillonite interaction is proposed. • The constant for ion-exchange Eu(III) reaction with montmorillonite is calculated. [ABSTRACT FROM AUTHOR]

Abstract Highly water-soluble material of multi-imidazolium based silsesquioxane cages (material 2) is a promising candidate for modifying clay mineral interlayer space, which is successfully demonstrated by using montmorillonite. The modified montmorillonite can be easily prepared via cation exchange reaction between material 2 and sodium ions of montmorillonite, dispersed in an aqueous solution at room temperature. The modified montmorillonite formed a precipitate within a few seconds. A single layer of material 2 found to be intercalated in the modified montmorillonite with a maximum interlayer space (d -value) of 18 Å (1.8 nm) from 13 Å (1.3 nm) of the unmodified one was confirmed by transmission electron microscopy (TEM) and X-rays diffraction (XRD). Moreover, the percentage uptake and nature of incorporation of material 2 into montmorillonite layers were investigated by thermal gravimetric analyses (TGA), 29Si NMR and FT-IR spectroscopies. Graphical abstract Unlabelled Image Highlights • POSS-based ionic materials adsorbed on montmorillonite by the cation exchange. • Hydrophobically modified montmorillonite formed precipitates in aqueous solution. • The d -value of modified montmorillonite was extended by about 5 Å. • The extended spacing confirmed the presence of POSS molecules in single interlayer. [ABSTRACT FROM AUTHOR]

Graphical abstract Under low MT loading, limited MT intercalation resulted in no change in d 001 spacing compared to raw Ca-MMT (a), but intercalation was confirmed by heated XRD analyses and by using Na-MMT. Under high MT loading, extensive intercalation of MT into the interlayer of MMT was responsible for the elevated MT uptake on MMT. Highlights • Uptake of metoprolol (MTP) by montmorillonite (MMT) was fast. • MTP removal was could be as high as 1544 mmol/kg, or 40% of the mass of MMT. • Cation exchange was the main mechanism under low MTP loading levels. • Cation exchange, Hydrogen bonding, and ion-dipole interaction may be responsible for high MTP loading levels. • Intercalated MTP was thermally more stable to prevent effective thermal degradation. Abstract This study assessed the mechanisms using montmorillonite (MMT) for the uptake and removal of a β-blocker metoprolol (MTP) from aqueous solution. Uptake of MTP was fast and equilibrium could be reached in 15 min and 4 h under initial MTP tartrate concentrations of 800 and 4000 mg/L, respectively. The MTP uptake capacity was as high as 412 mg/g (corresponding to 1544 mmol/kg), suggesting that MMT was a good candidate for β-blocker removal. Equilibrium solution pH and ionic strength had strong influence on MTP uptake on MMT and MTP uptake was reduced under high pH and high ionic strength conditions. Desorption of exchangeable cations matched well with the amount of MTP uptake at low loading levels, suggesting cation exchange being responsible for the uptake of MTP. At higher MTP uptake levels, significant expansion of d 001 spacing was observed, confirming extensive intercalation and changes in interlayer configuration of MTP. The decomposition temperature of MTP intercalated into the MMT increased in comparison to that of crystalline MTP as revealed by thermogravimetric analyses. Band shift in FTIR analyses indicated that hydrogen bonding besides cation exchange and ion-dipole interaction was responsible for the high MTP loading levels. [ABSTRACT FROM AUTHOR]

Abstract The adsorption of divalent lead on the homoionic Na forms of illite (Na-IdP) and montmorillonite (Na-SWy) was investigated by batch type adsorption experiments. Ion equilibrium experiments between Pb2+ and Na+ yielded selectivity coefficients for cation exchange on the planar sites of both clay minerals. Pb adsorption edges (pH 2–11) and isotherms (pH 3, 6 and 7 and at 10−12 M < [Pb eql ] < 10−3 M) were measured and modelled with the two site protolysis non electrostatic surface complexation and cation exchange (2SPNE SC/CE) model. The surface complexation constants derived from an iterative fitting of the edges and isotherms could describe all of the Pb adsorption data sets satisfactorily. The anomalous adsorption behaviour of trace Pb on Na-IdP in the acidic region required the consideration of an additional site with a low capacity and high affinity and an associated pH independent surface complexation reaction. Adsorption experiments with Pb/Ni, Pb/Co, Pb/Zn and Pb/Eu on Na-IdP indicated, depending on the experimental set-up, partially and/or non-competitive behaviour. The competitive adsorption experiments could be quantitatively described with a modified 2SPNE SC/CE model. In the case of non-competitive adsorption additional metal specific sets of strong sites are required. In the case of partial competitiveness, a certain fraction of the strong site capacity is assumed to be competitive whereas the remaining fraction is non-competitive. Highlights • The 2SPNE SC/CE adsorption model for illite and montmorillonite was implemented with divalent Pb. • A high affinity adsorption site was necessary to quantify trace Pb uptake on illite at low pH. • At high [Pb eql ], trace Ni, Co and Eu compete partially for strong sites on illite whereas trace Zn is non-competitive. • At high [Ni eql ] or [Eu eql ], trace Pb is non-competitive on the strong sites of illite. • The degree of competiveness between Pb, Ni, Zn, Co and Eu was quantified. [ABSTRACT FROM AUTHOR]

Abstract Montmorillonite (Mt) is well known as reinforcing agent in films. Moreover, it stabilizes anthocyanin pigments. In this study, Mt has been added as both reinforcement and color stabilizer to gelatin films containing acerola juice. Films were produced with different Mt concentrations (0–6.5% on gelatin). Increasing Mt contents resulted in enhanced tensile strength and modulus, although the elongation has been decreased. Mt also reduced the water vapor permeability in up to 45%. Mt presented two effects on film color: first, it changed its color from yellowish to red; and second, it stabilized film color throughout storage, mainly when added at 3.9%. It has been suggested that metals from Mt (such as Fe3+ or Al3+) may have formed complexes to anthocyanins, changing and stabilizing their color. Graphical abstract Unlabelled Image Highlights • Acerola juice-gelatin films were produced with 0–6.5% montmorillonite (Mt). • Mt enhanced tensile and water vapor barrier properties of films. • Mt addition resulted in film color shift from yellowish to red. • Mt also stabilized film color throughout storage, mainly at 3.9%. [ABSTRACT FROM AUTHOR]

Tyramine (TY) adsorption on a Ca-montmorillonite (SAz-2) was investigated with batch experiments and complementary analyses utilizing ultra-high performance liquid chromatography, ion chromatography, X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and thermogravimetry (TG). The adsorption reached equilibrium in 8 h, complying with the pseudo-second-order rate equation, and came to an adsorption capacity of 682 mmol kg −1 at pH 6–8.1, utilizing the Langmuir isotherm model. The adsorption of TY and desorption of exchangeable cations exhibited a linear relationship with a slope of 0.9, implying that the adsorption was largely influenced by a cation exchange mechanism. The effective adsorption was further verified by the characteristic TY bands in the FTIR spectra and the signals of mass loss due to TY decomposition in the TG measurements of the clay after adsorption experiments. Intercalation of hydrated TY into the clay interlayer was confirmed by XRD and TG analyses of the heated samples loaded with TY. The adsorption reached only 0.57 cation exchange capacity of the clay which was probably limited by the low charge density of TY as compared to the negative charge density of the clay surface and by the steric effects arising from the hydration of TY that increased its molecular size. Adsorption of TY on montmorillonite can make TY more resistant to thermal decomposition and possibly better preserved in aquatic and soil environments. [ABSTRACT FROM AUTHOR]

Abstract After construction of the repository, the bentonite will undergo some chemical effects resulting from the chemistry of site water and the solution produced by cement degradation in the high-level radioactive waste repository. In this study, the swelling pressure of compacted Gaomiaozi(GMZ) bentonite was investigated with three types of pore water simulating site groundwater (synthetic Beishan Site Water - BSW) and two cement solutions (Young Cement Water – YCW and Evolved Cement Water – ECW). X-Ray diffractometry (XRD) tests were also carried out to analyse the mineralogy of the samples before and after the swelling pressure tests. Results show that the swelling pressure curve is of “double-peak” pattern, which fades with the pH of solutions increase. The swelling pressure of GMZ bentonite decreases little with BSW, significantly with YCW and slightly with ECW. At a higher dry density, the swelling pressure of compacted GMZ bentonite reaches stability later and the final swelling pressure is higher. Cation exchange takes place in the bentonite and the montmorillonite of GMZ bentonite transforms to multi-cation dominant montmorillonite with the three types of pore waters. The swelling pressure reduction affected by the cement solutions is mainly due to the montmorillonite dissolution, but the secondary minerals are in amorphous phases. The swelling property of compacted GMZ bentonite can be reduced by cement solutions in the long-term operation of repository. Highlights • The swelling pressure of bentonite decreases with cement solutions. • The swelling pressure of more compacted bentonite stabilises later. • Cation exchanges, smectite dissolution occured. • Analcime and C-S-H gels were formed after reactions. [ABSTRACT FROM AUTHOR]

This study was conducted to evaluate color and anthocyanin stability of clarified acerola juice (CAJ) as affected by montmorillonite (Mnt) at different concentrations (0–6 wt%, dry basis). While non-complexed CAJ suffered noticeable color degradation with time and pH variations, the presence of Mnt (especially at 4–6 wt%) not only changed the initial color of CAJ but also made it more stable with time and pH changes. CAJ/Mnt mixtures were ultracentrifuged in order to separate them into supernatants and anthocyanin-complexed Mnt precipitates. The supernatants presented decreasing anthocyanin contents with increasing Mnt concentrations, indicating pigment retention by the precipitates. X-ray diffraction of precipitates showed that Mnt interlayer spacing was increased by increasing anthocyanin/Mnt ratios, corroborating anthocyanin intercalation. FTIR revealed a band at 1530 cm −1 ascribed to formation of anthocyanin-Mnt complexes. Moreover, chromatograms indicated the selective adsorption of two compounds by Mnt, which were identified by LC-MS as cyanidin-3- O -rhamnoside and pelargonidin-3- O -rhamnoside. [ABSTRACT FROM AUTHOR]

• Binder-bentonite interactions induce cation exchange to increase flowability and bleeding. • Mg2+ from MgO-GGBS involves less in cation exchange than Ca2+ from OPC and CaO-GGBS. • Binders have least effect on salt-resistant bentonite due to low exchangeable Na+ ions. • Short-term reactions with binder convert montmorillonite from Na-based to Ca-based. • Liquid limit and plasticity index of bentonites decrease with binder incorporation. Bentonite is commonly used as sealing material in seepage cut-off walls to minimize the permeability of sandy soil. To further reduce permeability and stabilize the sand-bentonite mixture, binders such as ordinary Portland cement (OPC) can be added. However, OPC can interact with bentonite and cause cation exchange, resulting in increased flowability and bleeding of soil mixture. Hence, this study proposes alternative bentonites and binders to reduce the influence of the short-term binder-bentonite interactions on the workability. The performance of three bentonite types, including sodium, calcium and salt-resistant bentonites, was evaluated. The use of ground granulated blastfurnace slag (GGBS) with activators of reactive magnesia (MgO) and quicklime (CaO) as alternative binders was also studied. When compared to OPC, MgO-GGBS had flowability of 1.7%–21.7% and bleeding of 0.6%–1.4% less, whereas CaO-GGBS had flowability of 1.9%–3.7% and bleeding of 0.1%–1.6% more. Mg2+ ions from MgO-GGBS had a lower affinity to participate in cation exchange with bentonite than Ca2+ ions from OPC and CaO-GGBS. The flowability and bleeding decreased further by 0.7%–5.4% and 6.2%–30.3%, respectively, when MgO reactivity and content increased. Besides, the binder-bentonite interactions had the least influence on salt-resistant bentonite, followed by calcium bentonite and then sodium bentonite, due to its low exchangeable sodium content. The XRD analysis also revealed that the short-term interactions of binder with bentonite tended to convert Na-based montmorillonite to Ca-based montmorillonite. In summary, the use of MgO-GGBS and salt-resistant bentonite can minimize the binder-induced cation exchange effect on the workability of sand-bentonite-binder mixture in cut-off wall construction. [ABSTRACT FROM AUTHOR]

Molecular-level understanding of the exchange processes of heavy metals into the interlayer region of clay minerals help to predict the geochemical cycle of these elements. In this study, we conducted molecular dynamics by using the newly developed CLAYFF-MOH parameters to elucidate the hydration characteristics of edge surfaces. On this basis, we explored the exchange processes of a series of heavy metal cations. Results showed that Mg for Al isomorphous substitution in the secondary outermost positions would reduce the coordination number (CN) of edge Al on both edge surfaces; water films were formed onto the interface owing to H-bonding interaction. Moreover, the cation exchange rates were Pb2+ > Cr3+ > Cd2+ > Ag+ > Ni2+ > Cu2+; Ag+ showed the largest exchange amount, followed by Ni2+, Cd2+, Pb2+, Cu2+, and Cr3+. A small part of Cl− was exchanged along with multivalent cations. The simulations of Ni2+-Cd2+-Pb2+ system indicated that coexistence of divalent cations had no significant effect on the exchange, while the exchange process in Ag+-Cd2+-Cr3+ mixing system was more complicated. Electrostatic interaction was the dominant driving force responsible for exchange. Additionally, Na+ and Ag+ were complexed on the octahedral vacancy, Si–OH, and Al–OH sites. The mobilities of cations decreased with increased ionic valence increased, and overall the diffusion in the interlayer was more constrained than in external solution. This study provides fundamental molecular-level knowledge of the fate of heavy metals in natural environment. [Display omitted] • Molecular dynamics were conducted to investigate montmorillonite-water interface. • Isomorphous substitution reduced the coordination number of edge Al. • Cation exchange rates were Pb2+ > Cr3+ > Cd2+ > Ag+ > Ni2+ > Cu2+. • Cl− was exchanged into the interlayer by binding with multivalent cations. • Na+ and Ag+ were complexed on outermost octahedral vacancy, Si–OH, and Al–OH sites. [ABSTRACT FROM AUTHOR]

Polyether amine (PA) has recently been widely applied in water-based drilling fluid to improve shale stability. To study whether PA intercalation and the resulting increase of montmorillonite (Mt) d001-value have a negative effect on shale stability, a PA adsorption test on shale, a shale composites X-ray diffraction (XRD) test and a one dimensional swelling test of shale immersed in solution were conducted. PA can absorb on shale by exchanging Na + in shale, resulting in a d001-value increase of sodium montmorillonite (Na-Mt) in shale and large vertical displacements, causing lots of micro cracks to develop in the shale with the shale matrix dispersed. For shale immersed in solution with KCl and PA mixed, PA intercalation was reduced and even prevented because K + can exchange Na + in the shale first. With the cation hydration reduced by K + adsorption and the clay layer hydration decreased by PA adsorption on clay layer, vertical displacement was greatly reduced and stability was greatly enhanced for the shale immersed in solution with KCl and PA mixed. As a result, during drilling in shale containing Na-Mt, PA can cause shale instability. Using PA and KCl together can achieve better shale stability compared to using KCl only. [ABSTRACT FROM AUTHOR]

The sorption capacity of synthetic montmorillonites of the composition Na(Al,Mg)SiO(OH) · nHO (where 0 < x < 1) in relation to the methylene blue dye has been investigated. The obtained results from the data for natural samples of montmorillonite (K10) and activated carbon have been compared. The effect of the montmorillonite composition and the medium acidity on the degree of dye adsorption has been studied. The chemical composition of montmorillonite, which is optimal for solving tasks in the field of ecology and medicine, has been determined. The character of the interaction of montmorillonites with the adsorbed substance from the point of view of the physical-chemical sorption models has also been studied. [ABSTRACT FROM AUTHOR]

The structural changes along the c axis, of the Ba-exchanged montmorillonite (Swy-2-Ba), under variable relative humidity (% RH), is investigated. In this regard, the arrangement, amount and position of both exchangeable cation and the water molecules in the interlamellar space (IS), are evaluated. This aim is achieved using the X-ray diffraction (XRD) profile modeling approach that consists of comparing experimental and theoretical patterns calculated from structural models. The contributions of the hydration states and the interlayer water amounts, as a function of the % RH, are registered by quantitative XRD investigation. The validated structural models are heterogeneous, suggesting various proportions of layer types at different RH ranges, which means the coexistence of different mixed layer structure MLS packages, exhibiting different proportions of layers with contrasting hydration states. This result is attributed to the orientation of the applied hydration sequence. Indeed, the interlayer water molecule amounts, which led to the appearance of a logic hydration hysteresis, are strongly affected by hydrous perturbation. [ABSTRACT FROM AUTHOR]

Raw montmorillonite (Mt) was cation-exchanged with Ca 2 + , Mg 2 + , and Fe 3 + , and Pb 2 + was tested for its adsorption on the exchanged Mt. The d-value and the cation exchange capacity (CEC) of Ca-exchanged montmorillonite (Ca 2 + + Mt) were larger than those of Mg-exchanged montmorillonite (Mg 2 + + Mt) and Fe-exchanged montmorillonite (Fe 3 + + Mt). The Pb 2 + adsorption on the homoionic Mt was fast and the amount of Pb 2 + adsorption followed the order of Ca 2 + + Mt > Mg 2 + + Mt > Fe 3 + + Mt. Simulation of Pb 2 + adsorption on the Mt revealed a much larger energy drop when Pb 2 + is substituting for Ca 2 + in comparison to Pb 2 + exchanging for Fe 3 + . [ABSTRACT FROM AUTHOR]

Ciprofloxacin (CIP) adsorption on montmorillonite was investigated using structurally related probing compounds phenylpiperazine (PP) and fluorochloroquinolone carboxylic acid (FCQCA). Desorption of the equivalent amounts of exchangeable cations accompanying PP and CIP adsorption on montmorillonite suggested that cation exchange was the major adsorption mechanism for the CIP adsorption under acidic to neutral conditions, and the piperazine amine group was responsible for the cation exchange interaction. In contrast, the adsorption of CIP and FCQCA on montmorillonite indicated that hydrogen bonding between the keto carbonyl group and coordinated water in the interlayer of montmorillonite was mainly responsible for the CIP adsorption in alkaline aqueous solution as evidenced by Fourier transform infrared analyses. X-ray diffraction analyses demonstrated that both cation exchange and hydrogen bonding between the keto carbonyl group and coordinated water lead to the intercalation of CIP in montmorillonite. [ABSTRACT FROM AUTHOR]

The uptake of amitriptyline (AMI) from aqueous environment by Ca-montmorillonite (SAz-2) was studied in a batch system under different physicochemical conditions. The adsorbent was characterized by X-ray diffraction and Fourier transform infrared (FTIR) analyses. The AMI adsorption on SAz-2 obeyed the Langmuir isotherm with a capacity of 330mg/g (1.05mmol/g) at pH 6-7. The adsorption kinetics was fast, almost reaching equilibrium in 2h, and followed a pseudo-second-order kinetic model. Desorption of exchangeable cations correlated with the AMI adsorption well, indicating that cation exchange was the major mechanism. X-ray diffraction patterns showing significant expansions of the d001 spacing and characteristic FTIR band shifts toward higher frequencies after AMI adsorption onto SAz-2 indicated that the adsorbed AMI molecules were intercalated into the interlayers of the mineral. Thermodynamic parameters based on partitioning coefficients suggested that the AMI adsorption was an endothermic physisorption at high adsorption levels. At low and higher AMI adsorption levels, the intercalated AMI molecules take a horizontal monolayer and bilayer conformation, respectively. The higher adsorption capacity suggested that SAz-2 could be a good candidate to remove AMI from wastewater and would be an important environmental sink for the fate and transport of AMI in soils and groundwater. [ABSTRACT FROM AUTHOR]

Highlights: [•] Different sorption sites were available for enrofloxacin (EN) uptake on different types of clays. [•] EN uptake was limited to the external sites for non-swelling clays. [•] Interlayer spaces were the major host sites for EN uptake on swelling clays. [•] Cation exchange was the dominant mechanism for cationic and neutral EN. [Copyright &y& Elsevier]

The effects of heating and cation exchange on the solid acidity of montmorillonite were investigated using n-butylamine titration in non-aqueous system and diffuse reflectance Fourier transform infrared spectroscopy. The number of total, Brønsted, and Lewis acid sites showed the same modulation tendency with increasing heating temperature, reaching a maximum at 120 °C and subsequently decreasing until it reaches a minimum at 600 °C. The Lewis acid sites result from unsaturated Al cations, and their number increased with the heating temperature due to the dehydration and dehydroxylation of montmorillonite. The generation and evolution of Brønsted acidity were mainly related to interlayer-polarized water molecules. Water adsorbed on the unsaturated Al ions also acted as a Brønsted acid. The acid strength of the Brønsted acid sites was dependent on the polarization ability of the exchangeable cation, the amount of interlayer water, and the degree of dissociation of the interlayer water coordinated to exchangeable cations. All cation-exchanged montmorillonites exhibited different numbers of acid sites and various distributions of acid strength. Brønsted acidity was predominant in Al-exchanged montmorillonite, whereas the Na- and K-exchanged montmorillonites showed predominantly Lewis acidity. Moreover, Mg- and Li-exchanged montmorillonites exhibited approximately equal numbers of Brønsted and Lewis acid sites. The Brønsted acidity of cation-exchanged montmorillonite was positively correlated with the charge-to-radius ratios of the cations, whereas the Lewis acidity was highly dependent on the electronegativity of the cations. The acid strengths of Al- and Mg-exchanged montmorillonites were remarkably higher than those of monovalent cation-exchanged montmorillonites, showing the highest acid strength ( H ≤ −3.0). Li- and Na-exchanged montmorillonites exhibited an acid strength distribution of −3.0 < H ≤ 4.8, with the acid strength ranging primarily from 1.5 to 3.3 in Li-exchanged montmorillonite, whereas only weaker-strength acid sites (1.5 < H ≤ 4.8) were present in K-exchanged montmorillonite. The results of the catalysis experiments indicated that montmorillonite promoted the thermal decomposition of the model organic. The catalytic activity showed a positive correlation with the solid acidity of montmorillonite and was affected by cation exchange, which occurs naturally in geological processes. [ABSTRACT FROM AUTHOR]

Abstract: Important structural modifications occur in swelling clays upon water adsorption. The multi-scale evolution of the swelling clay structure is usually evidenced by various experimental techniques. However, the driving force behind such phenomena is still not thoroughly understood. It appears strongly dependent on the nature of the interlayer cation. In the case of montmorillonites saturated with alkaline cations, it was inferred that the compensating cation or the layer surface could control the hydration process and thus the opening of the interlayer space, depending on the nature of the interlayer cation. In the present study, emphasis is put on the impact of divalent alkaline-earth cations compensating the layer charge in montmorillonites. Since no experimental technique offers the possibility of directly determining the hydration contributions related to interlayer cations and layer surfaces, an approach based on the combination of electrostatic calculations and immersion data is developed here, as already validated in the case of montmorillonites saturated by alkaline cations. This methodology allows to estimate the hydration energy for divalent interlayer cations and therefore to shed a new light on the driving force for hydration process occurring in montmorillonites saturated with alkaline-earth cations. Firstly, the surface energy values obtained from the electrostatic calculations based on the Electronegativity Equalization Method vary from 450mJm−2 for Mg-montmorillonite to 1100mJm−2 for Ba-montmorillonite. Secondly, considering both the hydration energy for cations and layer surfaces, the driving force for the hydration of alkaline-earth saturated montmorillonites can be attributed to the interlayer cation in the case of Mg-, Ca-, Sr-montmorillonites and to the interlayer surface in the case of Ba-montmorillonites. These results explain the differences in behaviour upon water adsorption as a function of the nature of the interlayer cation, thereby allowing the macroscopic swelling trends to be better understood. The knowledge of hydration processes occurring in homoionic montmorillonites saturated with both the alkaline and the alkaline-earth cations may be of great importance to explain the behaviour of natural clay samples where mixtures of the two types of interlayer cation are present and also provides valuable information on the cation exchange occurring in the swelling clays. [Copyright &y& Elsevier]

Montmorillonite layered silicate has been commonly used to reinforce polymer matrices. Due to its swelling in water, organic modification of the mineral surface is easily achieved which makes the surface compatible with polymers. Other minerals like mica and vermiculite though can also lead to high aspect ratio platelets in nanocomposites, but they do not swell in water owing to much stronger electrostatic forces of attraction holding their platelets together (layer charge density >0.5 eq · mol−1in comparison with 0.25–0.5 eq · mol−1for montmorillonite). In current study, milling, delamination and cation exchange processing of mica and vermiculite minerals has been reported to explore their potential as reinforcement materials. Wet grinding and subsequent sieving of the coarse minerals led to fine-sized particles suitable to perform chemical delamination in water. The delamination process resulted in Li-mica and Na-vermiculite with enhanced access to the interlayer cations, thus, higher CEC. Successful surface modification of the delaminated minerals with alkyl ammonium ions could be achieved which resulted in significant enhancements in their basal plane spacing. Peak degradation temperatures of 260°C were measured for C18 and 2C18 modified vermiculite, whereas 300°C and 275°C were observed respectively for C18 and 2C18 modified mica minerals which make them suitable for compounding with polymers at high temperature. [ABSTRACT FROM AUTHOR]

Abstract: Desorption from soil clay components may affect the transport and fate of antibiotics in the environment. In this study, ciprofloxacin (CIP) desorption from a kaolinite and a montmorillonite was investigated under different pHs, different concentrations of metal cations of various valencies (Na+, Ca2+ and Al3+) and a cationic surfactant hexadecyltrimethylammonium (HDTMA), and different desorption cycles. Desorption of CIP from kaolinite and montmorillonite was strongly pH-dependent and desorption isotherms were well fitted with the Langmuir equation. The percentage of CIP desorbed increased with increasing initial CIP loadings, desorbing cation concentrations, and desorption cycles. Comparatively, CIP was more readily desorbed from kaolinite than from montmorillonite. Moreover, the hysteresis index values were all negative, suggesting that the presence of metal cations and HDTMA in solution promoted CIP desorption from clay minerals, owing to cation exchange. The XRD analyses indicated that desorption of CIP occurred from both external and interlayer surfaces of montmorillonite. Formation of Al–CIP complex on solid surface and then detachment of Al–CIP from the solid surface may contribute to the higher CIP desorption by Al3+ in comparison to Na+ and Ca2+. [Copyright &y& Elsevier]

Abstract: The intercalation of nickel and cobalt thiabendazole complexes into montmorillonite was investigated by two strategies. The first strategy consisted of exchanging sodium montmorillonite (Na+–Mt) by nickel or cobalt ions followed by complexation of these cations by thiabendazole. The second method was the direct exchange of the sodium ions by the Ni– or Co–thiabendazole complexes. The thiabendazole–montmorillonite complexes were investigated by X-ray diffraction, FTIR and TGA analysis. [Copyright &y& Elsevier]

Abstract: The Long Term Test of Buffer Material (LOT) project at the Äspö Hard Rock Laboratory, Sweden, is a series of medium-scale field experiments focused on validating models and hypotheses concerning long term processes in the bentonite buffer of a repository for high-level radioactive waste. The test parcels emplaced in crystalline bedrock consist of blocks of compacted MX80 bentonite embedding a Cu-tube equipped with a heater to simulate the heat generation from radionuclide decay. The A2 test parcel had been subjected to elevated temperature (up to 130°C) and hydration by a Na–Ca–Cl type groundwater for almost 6years when it was retrieved to be analysed. The analyses included determinations of chemical composition, cation exchange capacity (CEC), exchangeable cations and mineralogy. Both the bulk bentonite and dialysed, homo-ionic Na-clay (<2μm and <0.2μm fractions) were analysed when relevant. Sulphate was redistributed in the heated part of the buffer under the thermal and hydration gradients that prevailed during the test period. Anhydrite accumulated in the warmer parts, whereas gypsum was dissolved in the peripheral parts of the buffer where water was supplied. Carbonate dissolution increased with temperature in the warmest parts, whereas chloride behaved conservatively in all blocks. Cu was incorporated in the bentonite matrix at the surface of the Cu-tube indicating some corrosion, which may be explained by reactions in an early stage of the test when trapped oxygen existed in the system. Along with the dissolution/precipitation reactions the porewater composition changed, which resulted in replacement of exchangeable sodium by calcium and magnesium in the warmest zone. Also Mg in the clay (<2μm and <0.2μm fractions) displays a clear gradient with peak values at the heater. Because several of the alternative sinks for Mg were eliminated in the sample preparation prior to the chemical analysis (purified clay fractions, removal of carbonates, Na-saturation) the smectite is suggested a candidate sink for Mg. Parallel with the increase in Mg, a loss in Si is indicated and CEC tends to increase in the clay that had been heated at 130°C. A loss in tetrahedral Si that is balanced by Al, and replacement of Al by Mg in the octahedral sheet would imply that the layer charge of the smectite increased, which would be consistent with the higher CEC values of these samples. The changes in CEC are, however, close to the analytical resolution of the CEC method and no effect of the changes in chemical composition can be detected in the XRD-characteristics of the clay. Therefore, supplementary high-resolution analyses are required to verify whether the structure of montmorillonite has altered in the test period. [Copyright &y& Elsevier]

Abstract: The spatial organisation of swelling clay platelets in a suspension depends on the chemical composition of the equilibration solution. Individual clay platelets can be well dispersed, with surfaces entirely in contact with the external solution, or be stacked in tactoids, where part of the surfaces forms parallel alignments embedding interlayer water and cations. External and interlayer surfaces do not exhibit similar affinities for cations having different hydration and charge properties and the clay platelet stacking arrangement influences the clay affinity for these cations. This paper aims to establish the link between exchange properties and clay tactoid size and organisation for Na–Ca exchange on montmorillonite. Different montmorillonite samples behave differently with regards to Na–Ca exchange, from ideal to non-ideal exchange behaviour. A simple model coupling the tactoid stacking size to different Na/Ca relative affinities of the external and interlayer clay surfaces enables these differences to be reproduced. [Copyright &y& Elsevier]

Abstract: Swelling clays have been extensively studied to remove contaminants from aqueous solution. This research aimed at elucidating the mechanism of adsorption of acridine orange (AO), a cationic dye, on low charge montmorillonite in order to better understand the principles behind AO removal using swelling clays. Proportional desorption of exchangeable cations from the clays accompanying AO adsorption confirmed cation exchange as the most dominant mechanism for AO removal. FTIR analyses further confirmed the presence of cationic dye AOH+ on the surface and in the interlayer of the swelling clays. XRD and TG-DTG analyses revealed interlayer adsorption, thus, intercalation of AO molecules with different conformations at low and higher AO adsorption levels. Molecular simulation showed dimer and horizontal bilayer formation of intercalated AO at the low and high adsorption levels. At the intermediate AO adsorption levels, the dimer intercalated and bilayer intercalated swelling clays formed mixtures at the single crystal scale, resulting in a significant broadening of (001) peak. The results showed that the uptake of AO could far exceed the cation exchange capacity, an advantage of using swelling clays to treat wastewater containing cationic dyes. [Copyright &y& Elsevier]

Abstract: Smectites are usually used as a clay barrier at the bottom of subsurface waste landfills due to their low permeability and their capacity to retain pollutants. The Na- and Ca-saturated SWy2 montmorillonites were interacted with initial Zn(NO3)2 or Pb(NO3)2 concentrations ranging from 10−6 to 10−2 M with a solid/liquid ratio of 10gL−1 and using acetic acid/acetate as buffer at pH 5 in order to reproduce a biodegradable leachate of a young landfill. These experiments revealed that Zn and Pb sorption onto Na-SWy2 is higher than that onto Ca-SWy2 in the whole range of concentrations. Metal retention into both montmorillonites increases with the decrease in acetic acid/acetate concentration. The two-site protolysis model with no electrostatic term (2SPNE model) was used to model these experiments. As the experimental data of Zn sorption were well fitted, this model was validated and has been improved by taking into account the metal–acetate complexation in solution. In order to validate the model for Pb sorption, new selectivity coefficients have been determined, namely log K c(Pbp sp="0.25" />=0.5 for Na-montmorillonite and log K c(Pbp sp="0.25" />=0.3 for Ca-montmorillonite. [Copyright &y& Elsevier]

Abstract: Frequent detection of pharmaceuticals in surface water and wastewater attracted renewed attention on studying interactions between pharmaceuticals and sludge or biosolids generated from wastewater treatment. Less attention was focused on studying interactions between pharmaceuticals and clay minerals, important soil and sediment components. This research targeted on investigating interactions between diphenhydramine (DPH), an important antihistamine drug, and a montmorillonite, a swelling clay, in aqueous solution. Stoichiometric desorption of exchangeable cations accompanying DPH adsorption confirmed that cation exchange was the most important mechanism of DPH uptake by the swelling clay. When the solution pH was below the pK a of DPH, its adsorption on the swelling clay was less affected by pH. Increasing solution pH above the pK a value resulted in a decrease in DPH adsorption by the clay. An increase in d 001 spacing at a high DPH loading level suggested interlayer adsorption, thus, intercalation of DPH. The results from this study showed that swelling clays are a good environmental sink for weak acidic drugs like DPH. In addition, the large cation exchange capacity and surface area make the clay a good candidate to remove cationic pharmaceuticals from the effluent of wastewater treatment facilities. [Copyright &y& Elsevier]

Abstract: An increased detection of common pharmaceuticals and personal care products in surface water and wastewater has rekindled the interests of studying their interactions with solid surfaces. In this research, adsorption of chlorpheniramine (CP), an antihistamine medicine commonly used to treat allergy, onto a swelling clay mineral montmorillonite (MMT) was investigated under kinetic and different solution conditions via batch adsorption, XRD, TG–DTG, and FTIR analyses in order to elucidate the mechanism of CP adsorption on MMT, an important environmental solid matrix, and explore potential use of MMT to remove CP from water. Stoichiometric desorption of exchangeable cations suggested cation exchange as the dominant mechanism of CP adsorption on MMT. The over-balanced desorption of exchangeable cations against the amount of CP adsorption may be attributed to protonation of the two N atoms, thus resulting in a divalent cation adsorption, or adsorption of an extra H+ generated from the second deprotonation of maleic acid. Both external and internal surfaces were responsible for CP adsorption on MMT, resulting in a large CP adsorption capacity of 0.72mmolg−1, or about 20% by weight. The fast CP adsorption, large CP adsorption capacity, and negligible pH effect make MMT a good sorbent to remove CP from wastewater on one hand and an important environmental sink on the other. [Copyright &y& Elsevier]

Abstract: An organo-montmorillonite (Cloisite® 30B) was modified with fluorescent dyes to obtain photo-functional inorganic–organic complexes for real-time monitoring of polymer-clay nanocomposite processing. The photo-functionalization with 9-anthracenemethanol (anth) was tested using adsorption from solution, as well as dry and melt compounding. Rhodamine 6G Perchlorate (RhP) and Nile Blue A Perchlorate (NBAP) were adsorbed by cation exchange. XRD, TGA and spectrofluorimetry showed that cation exchange in water/ethanol at 80°C, with an added amount of RhP corresponding to 25% of the initial surfactant content, followed by washing, recovering and drying, yielded the best results. The intercalation of the fluorescent dyes rearranged the paraffinic structure of the surfactant molecules by increasing the tilting angle from 37° to 49°. [ABSTRACT FROM AUTHOR]

Abstract: Exploring the interactions between antibiotics and soils/minerals is of great importance in resolving their fate, transport, and elimination in the environment due to their frequent detection in wastewater, river water, sewage sludge and soils. This study focused on determining the adsorption properties and mechanisms of interaction between antibiotic ciprofloxacin and montmorillonite (SAz-1), a swelling dioctahedral mineral with Ca2+ as the main interlayer cation. In acidic and neutral aqueous solutions, a stoichiometric exchange between ciprofloxacin and interlayer cations yielded an adsorption capacity as high as 330mg/g, corresponding to 1.0mmol/g. When solution pH was above its pK a2 (8.7), adsorption of ciprofloxacin was greatly reduced due to the net repulsion between the negatively charged clay surfaces and the ciprofloxacin anion. The uptake of ciprofloxacin expanded the basal spacing (d 001) of montmorillonite from 15.04 to 17.23Å near its adsorption capacity, confirming cation exchange within the interlayers in addition to surface adsorption. Fourier transform infrared results further suggested that the protonated amine group of ciprofloxacin in its cationic form was electrostatically attracted to negatively charged sites of clay surfaces, and that the carboxylic acid group was hydrogen bonded to the basal oxygen atoms of the silicate layers. The results indicate that montmorillonite is an effective sorbent to remove ciprofloxacin from water. [Copyright &y& Elsevier]

Abstract: Fluoroquinolones are a group of antibiotics commonly used in human and veterinary medicine. Highly enriched in sewage sludge as well as animal manure they can enter the environment by surface application of the sludge and manure. In this study, the interaction and adsorptive removal of ciprofloxacin (CIP) with a sodium montmorillonite (MMT) were studied in batch tests and supplemented by XRD and FTIR analyses. The adsorption of CIP on MMT was instantaneous with a large rate constant and a high initial rate. Quantitative desorption of exchangeable cations confirmed cation exchange as the most important mechanism of CIP adsorption on MMT. Higher CIP adsorption was achieved when solution pH was less than the pKa2 value of CIP, above which adsorption coefficient decreased significantly. Fitting of experimental data to adsorption of different CIP species indicated that both cationic and zwitterion forms of CIP could adsorb on MMT. An increase in basal spacing after CIP uptake indicated interlayer adsorption, i.e. intercalation of CIP. The substantial adsorption of CIP when pH was greater than the pKa2 together with the disappearance of C=O stretching vibration of the –COOH group in FTIR analyses suggested that complexation may be the dominant mechanism at high pH. [ABSTRACT FROM AUTHOR]

Abstract: This paper reports the structure of polyhedral oligomeric silsesquioxane (POSS)–octaammonium in the interlayer space of montmorillonite. The ion exchange reaction was conducted with a large excess of POSS and yielded which was investigated by EDX, XRD, FTIR and TGA coupled with GC/MS. All eight aminopropyl groups of cubic POSS were flexible enough to interact with the negative sites on montmorillonite yielding an alignment of the POSS in the interlayer space. POSS–MMT complex was calcinated and characterized by XRD. Since the basal spacing did not change, the calcinated MMT was supposed to be stabilized by polyhedral oligomeric silsesquioxane pillars. This kind of open and stable pillared structure can be useful for many applications such as new supports for catalysis. [Copyright &y& Elsevier]

Abstract: Zeolite N was produced from a variety of kaolinites and montmorillonites at low temperature (<100°C) in a constantly stirred reactor using potassic and potassic+sodic mother liquors with chloride or hydroxyl anions. Reactions were complete (>95% product) in less than 20h depending on initial batch composition and type of clay minerals. Zeolite N with 1.0

Abstract: Molecular dynamics (MD) simulations of a montmorillonite/water interface at the pore scale were carried out at 0.1molL−1 NaCl concentration in order to constrain cation, anion, and water distribution and mobility influenced by the mineral surface. MD results enabled anion exclusion and cation condensation at the surface to be quantified. MD-derived values could then be compared with macroscopic model results obtained from the Modified Gouy–Chapman (MGC) theory. While the Na concentration profile is well reproduced in the diffuse layer, anion exclusion is overestimated by the MGC theory under our experimental conditions. We also showed that MD simulations can be used to constrain Basic Stern model parameters or, in combination with zeta potential measurements, can be used to constrain triple layer model (TLM) parameters by providing suitable values for the capacitance values. Na sorption intrinsic equilibrium constant values for clay basal surfaces are given accordingly. [Copyright &y& Elsevier]

Abstract: X-ray powder diffraction (XRPD) analysis demonstrated that the main montmorillonite component of Unye bentonite was more effective in the Pb2+, Cu2+, Ni2+, Co2+, Zn2+, Al3+ and Fe3+ fixation processes in the interlayer space of the mineral than the nonclay fractions. All cation-exchanged bentonites experienced slight shifts in non-lattice bands whereas the features emerged at ∼3400 and 3200cm−1 are unique to the newly inserted cations. The clay dehydration up to 150°C and then the release of the water of the cation hydration shell in the temperature range of 150–300°C were followed by dehydroxilation between 300 and 700°C, after which no water remains in the bentonite structure. Additional features observed at elevated temperatures were assigned to the cation migration and the collapse of the clay framework. The surface areas of the Zn- and Fe-bentonites and those of the Pb-, Cu-, Ni-, Co- and Al-bentonites were found to be much higher and lower than that of raw bentonite, respectively, which were explained by the emergence of the micropores and medium mesopores in the interlayer spacings of the samples, respectively. [Copyright &y& Elsevier]

Abstract: Highly saline solutions are expected to play an important role in long term safety assessment of bentonite buffer for storage of radioactive waste. Montmorillonites being the main components of bentonites are suspected to undergo mineral alteration (particularly illitization or related mineral alteration processes) upon reaction with highly saline solutions. Literature data, however, do not provide a homogenous picture, yet. Possible alteration mechanisms, therefore, are still under discussion. In the present study 36 bentonites from different deposit were reacted in a closed system with a 6 M NaCl solution at 60 °C for 5 months, respectively. Run products were washed and dialysed and finally analyzed with respect to chemical and mineralogical changes by XRF, XRD, CEC, water uptake capacity, and amount of soluble silica. All significant chemical changes could be explained by the expected cation exchange (Na+ for Ca2+/Mg2+). After the experiment, however, the exchange sites were not completely occupied by Na+ despite a 100 fold excess of Na+ compared to the CEC. The presence of carbonates, obviously, interferes with the exchange of Na+ for Ca2+/Mg2+. The presence of gypsum proved to be even more effective and caused the opposite trend (Ca2+ was exchanged for Na+). The cation exchange buffer capacity of carbonates is believed to be particularly effective during dialysis, where the Na+ excess is significantly reduced while carbonate/gypsum dissolution proceeds. The changes of the XRD patterns, particularly the position, shape, and intensity (area) of the basal reflections, could not be attributed to mineral alteration but can be explained by cation exchange, differences of the hydration state, and/or different degrees of preferred orientation, the latter can be explained by particle arrangement. The results of the present study indicate that montmorillonites are stable in NaCl solutions of moderate pH up to 60 °C which is in agreement with most of the available studies. As expected there are no indications for illitization. In addition the study clearly shows the importance of using different chemical, mineralogical, and physical methods in order to unambiguously prove a process as illitization. Literature data indicate that temperatures of >100 °C can lead to irreversible montmorillonite alteration. The same, of course, holds true for extreme pH values, drying, or the presence of K+. These alteration processes have to be considered specifically. [Copyright &y& Elsevier]

The widely favored option for the safe storage of high-level radioactive waste is a multi-barrier disposal system with clay-based geotechnical barriers e.g. bentonite. Montmorillonite as a major constituent of bentonite (up to 90 wt.-%) is an important sink for potentially released radionuclides. The adsorption of 226Ra and Ba on Na-montmorillonite (SWy-2) was investigated as function of pH (edges) and of Ba concentration (isotherms) at different ionic strengths. Ba is often used as a chemical analogue for 226Ra due to their closely related physical and chemical properties e.g. the formation of complete Ba/Ra solid solutions. The experimental Ba/Ra adsorption data could be quantitatively described by considering cation exchange reactions on the planar sites of montmorillonite. To describe the pH dependent adsorption of Ba and Ra at high ionic strength and high pH, an additional reaction involving surface complexation was necessary. The fitted selectivity coefficients (K c) for the cation exchange of Ba and Ra with respect to Na are very similar at low ionic strength (0.02 M), whereas at higher ionic strength (0.3 M), Ra appears to be more selective. • Adsorption on montmorillonite is similar but not always identical for 226Ra and Ba. • A combined cation exchange and surface complexation model describes all data. • Surface complexation is only relevant for high pH. • Higher selectivity of 226Ra is observed at higher ionic strength. • For ionic strength <0.1 M NaCl and pH < 8, Ba is a good analogue for 226Ra. [ABSTRACT FROM AUTHOR]

Abstract: The thermal stability of synthesized and commercial organically modified montmorillonites has been investigated using thermogravimetric analysis (TGA). Four organo-montmorillonites have been prepared by replacing the inorganic cations of a commercial montmorillonite with aliphatic and aromatic ammonium bromide or chloride salts. The modification has been carried out by cation exchange to obtain a high level of intercalation in the montmorillonite. Conformations of the intercalated surfactants have been characterized using Fourier transform infrared spectroscopy (FTIR). Besides fixed organic surfactant, some surfactant in excess still remained in the montmorillonite after repeated washing with water or ethanol/water. The use of a suitable eluent for the surfactants in the washing step was the key issue as shown by the changes of the first degradation peak of Cloisite-Na+-hexadecyl benzyl dimethylammonium (CLO-BDHA), Cloisite-Na+-benzyl triethylammonium (CLO-BTA) and Cloisite-Na+-hexadecyl trimethylammonium (CLO-HTA). Hexadecyl trimethylammonium (HTA) was more sensitive to water and ethanol/water as eluents. Sonication and the use of a polar activator during the modification of montmorillonite increased the amount of surfactant adsorbed by cation exchange. Ammonium acetate method has been revealed as an adequate chemical method to measure the cation exchange capacity (CEC) of inorganic and organic montmorillonites. This method has the advantage with respect to TGA that it does not measure the unfixed surfactant. [Copyright &y& Elsevier]

Abstract: Polymerization of lauryl methacrylate “to” and “from” the surface of montmorillonite platelets was studied under a range of different reaction conditions. The polymerization was performed in order to achieve better organic coverage of the platelets, thus facilitating their exfoliation in the polymer matrices. For polymerization “to” the surface, a methacrylic functionality was first generated on the clay surface which was subsequently polymerized with the external lauryl methacrylate monomer. Substantial amounts of the polymer could be attached to the surface when lower polymerization temperatures and longer reaction times were used. Bulk polymerization was more effective in increasing the amount of polymer mass on the surface. In order to achieve polymerization “from” the surface, a bicationic initiator was first ionically bound on the surface followed by polymerization with lauryl methacrylate. Under the nonliving conditions, however, no significant amount of polymer could be grown from the surface. Nitroxide-mediated living polymerization was successful in eliminating suspected termination reactions leading to substantial gains in the organic mass bound to clay surfaces. Care was taken to avoid the presence of excess of unbound ammonium ions which can interfere in the grafting of polymer chains on the surface. X-ray diffraction and transmission electron microscopy in conjunction with thermogravimetric analysis confirmed the grafting of the polymer chains on the surface. [Copyright &y& Elsevier]

Abstract: The anionic dye Congo red (CR) was protonated by dissolving it in an aqueous solution of HCl at pH 1.2. Potentiometric titration of CR with HCl, supplemented by previous spectrophotometric studies of CR aqueous solutions at different pH values, revealed that at this pH CR is protonated to a stage that it becomes positively charged (cationic, HCR). The visible spectrum of HCR solution was recorded immediately after its preparation and after different aging periods. Two tautomeric varieties of HCR were identified, ammonium-rich variety, where most amine groups and only part of the azonium groups were protonated and azonium-rich variety, where most azo groups and only part of the amine groups were protonated. The former absorbed at 520 nm and the latter between 600 and 800 nm. The fresh HCR solution showed the predomination of the ammonium-rich variety whereas after one hour aging the azonium-rich variety predominated. The protonated dye (between 7.5 and 75.0 mmol HCR per 100 g clay) was adsorbed by Na-montmorillonite by cation exchange. The visible absorption spectra of aqueous suspensions of HCR–montmorillonite complexes were recorded within two hours after adding the dye to the clay suspensions and after eight weeks of aging. Here also the two tautomeric varieties of HCR were identified. In the fresh suspensions the ammonium-rich variety predominated whereas in the aged suspensions the azonium-rich variety predominated. The transformation of the ammonium-rich variety to the azonium-rich variety is fast in aqueous solutions but is slow in aqueous clay suspensions. After air-drying samples of the HCR–montmorillonite complexes, their visible absorption spectra were recorded. The spectra showed that as a result of drying the azonium-rich variety predominated. The thermal behavior of the solid HCR–montmorillonite complexes was investigated by thermo-visible spectroscopy. By the thermal treatment the dye was deprotonated and converted in part to CR. The deprotonation of the adsorbed dye seems to be a zero order reaction, which does not depend on concentration but on the reaction time. The ratio between protonated azonium and non-protonated CR depended on the loading of the clay. With increasing loadings the azonium variety showed the most intense absorption bands. [Copyright &y& Elsevier]

Compacted bentonites are being considered in many countries as a backfill material in high-level radioactive waste disposal concepts. A knowledge of the porewater chemistry in the clay barrier is essential since the porewater composition influences the release and transport of the radionuclides. However, quantification of the water chemistry in compacted bentonite under repository conditions is difficult. The methodology followed to obtain the porewater composition of the FEBEX bentonite is described in this paper. It is based on the characterisation of the solid phase, determination of the physico-chemical properties of the montmorillonite component and geochemical modelling. The FEBEX bentonite has a high cation exchange capacity (∼1 eq/kg), high surface area (∼725 m2/g total surface area and 62 m2/g external surface area) and accessory minerals such as carbonates, sulphates, pyrite, etc.; and organic matter. The chloride inventory in the FEBEX bentonite is ∼22 mmol/kg.The montmorillonite, together with the other mineral phases present, will determine the composition of the porewater. However, in order to calculate a unique aqueous chemistry, two further quantities are required, the chloride concentration and the pH.Water vapour adsoption/desorption isotherms, together with c-lattice spacing determinations, were used to identify the different states and location of water. Most of the water in the as received bentonite resides in the interlayer space. However, the measurements indicate that about 0.053 l/kg may be regarded as free water, implying a chloride concentration of 0.42 M. The pH of the system is fixed by equilibrium with the atmosphere (PCO2=10−3.5 bar) and saturation with the carbonate phases present. The porewater calculated to be in equilibrium with the as received FEBEX bentonite powder is a Na–Ca–Mg chloride type with a high ionic strength, 0.66 M, and a pH of ∼7.4.Likewise, in order to calculate the porewater composition of compacted re-saturated bentonite, the volume of free water is required. This value is taken to be the chloride accessible porosity obtained from Cl− through-diffusion tests (due to anion exclusion, Cl− anions can not move through the interlayer and overlapping double layer regions). The amount of free water in compacted bentonite determined in this manner was 0.03 l/kg at a dry density of ρd=1650 kg/m3. The corresponding chloride concentration is thus ∼0.73 M. Arguments are presented that the initial pH is fixed in the compacted material by the high buffering capacity afforded by the amphoteric edge sites, &z.tbnd6;SOH sites, of the montmorillonite. The pH of the porewater depends directly on the speciation of these sites, i.e., the proportions of sites present as &z.tbnd6;SOH, &z.tbnd6;SOH2+, &z.tbnd6;SO−, and this is fixed in the powdered source material through equilibration with air (compaction will not alter the state of the &z.tbnd6;SOH sites). The porewater of compacted FEBEX bentonite at ρd=1650 kg/m3 was calculated to be a Na–Ca–Mg chloride type with a high ionic strength, 0.90 M, and a pH of ∼7.4. [Copyright &y& Elsevier]