Octahedral Sheet Research Articles

Crystal chemistry of cation-exchanged forms of epistolite-group minerals, Part I. Ag- and Cu-exchanged lomonosovite and Ag-exchanged murmanite

The paper presents the first data on the crystal chemistry of the cation-exchanged forms of layered titanosilicates belonging to the epistolite group. It was found that these heterophyllosilicates have high exchange capacity and selectivity for cations of chalcophile elements (Ag, Cu and Zn) and could be considered as potentially novel raw materials or, more likely, as possible prototypes of cation-selective synthetic microporous materials. The crystal structures of Ag- and Cu-exchanged forms of lomonosovite and Ag-exchanged form of murmanite were studied by single-crystal X-ray diffraction. The topology of the main structural unit, the HOH block, remains unchanged in cation-exchanged forms as compared to the initial lomonosovite Na 4 Ti 4 (Si2O 7 ) 2 O 4 • 2Na 3 PO 4 ( P -1) and murmanite Na 4 Ti 4 (Si 2 O 7 ) 2 O 4 • 4H 2 O ( P -1). In Ag-exchanged murmanite, Ag cations occupy two crystallographically non-equivalent positions: one in the heteropolyhedral ( H ) sheet and another one in the octahedral ( O ) sheet corresponding to the positions of Na in initial murmanite. The crystal structure of the Ag-exchanged form of lomonosovite is characterized by an increased unit-cell parameter c and doubled parameter b as compared to initial lomonosovite. Ten large-cation sites statistically occupied by Ag and Na correspond to the Na sites in the initial lomonosovite: six in the interlayer space, two in the H sheet and two in the O sheet. Silver significantly replaces Na in sites in the interlayer space and in the O sheet, whereas sites in the H sheet are less affected by the ion exchange. Unit-cell parameter c of the Cu-exchanged form of lomonosovite decreases by 3.58% as compared to the initial lomonosovite whereas a and b remain almost the same. The Cu cations occupy two crystallographically independent positions. The Cu(1) site, corresponding to the Na(3) site in the initial lomonosovite, is located in the interlayer space. The low-occupancy Cu(2) site is located on the inversion centre in the O sheet; this site is vacant in the initial lomonosovite. The Cu(2) site is surrounded by six O atoms forming an octahedron distorted due to the Jahn-Teller effect.

Saponites containing divalent transition metal cations in octahedral positions — Exploration of synthesis possibilities using microwave radiation and NMR characterization

The synthesis using microwave radiation of saponites containing Co2+, Zn2+ or Cu2+ in octahedral positions has been explored (in the case of Cu2+, together with Mg2+ or Ni2+). Pure saponite phases were formed under some synthesis conditions, although in other cases some impurities were also formed. Microwave treatment at 180°C favored the formation of ordered solids, but induced the crystallization of other phases in some cases. Cu2+-containing saponites were formed only if Mg2+ or Ni2+ were used as octahedral sheet co-cations. The solids were characterized by different techniques, including MAS-NMR spectroscopy, which gave very precise information in the case of Zn-saponite, allowing us to estimate the content of tetrahedral Al, but much more undefined information for samples with paramagnetic cations. Nevertheless, the presence of tetrahedral Fe3+ and octahedral Fe2+ species was evidenced. Cu2+ slightly perturbed the NMR signals, while Co2+ strongly perturbed 27Al, 23Na and especially 29Si NMR spectral signals.

Characterization and application of heat-treated and acid-leached halloysites in the removal of malachite green: adsorption, desorption, and regeneration studies

Algerian halloysite was processed at 600°C and with 5-N HCl (H600-5N), and compared with the forms treated thermally at 600°C (H600) and unmodified (H). The obtained samples were characterized by FTIR, TEM, and nitrogen adsorption, and employed as malachite green (MG) adsorbents from aqueous solutions. The effects of pH, contact time, solution concentration, and temperature were examined. A particular attention has been focused on desorption and regeneration. The ease in desorption and regeneration constitutes a major practical advantage for treating industrial effluents. Thermo-chemical activation leads to the leaching of Al ions from the octahedral sheet. As consequence, H600-5N presents a specific area of 503 m2 g−1, i.e. eight orders of magnitude higher than that of H (63 m2 g−1). For all halloysitic solids, the capacity in MG increases with decreasing adsorption temperature. The affinity sequence is H600-5N > H600 > H. The isotherms are found to be well represented by the Redlich–Peterson equation. The thermodynamic data show that the MG adsorption onto H600-5N is spontaneous and exothermic, consequence of the electrostatic attraction between positively charged molecules and negatively charged adsorption sites. Among the seven eluents used, methanol manifests the greatest desorption capacity from H600-5N, which increases overall with temperature. After four adsorption–desorption cycles, this material was easily desorbed and regenerated. On the basis of all these considerations and, also, its adsorption capacity (192.6 mg g−1 at 25°C), H600-5N appears very effective for removing dyes from wastewaters.

Effects of Fe Substitution on Lithium Incorporation into Muscovite

주요어 : 백운모, 치환, 리튬, 밀도범함수, 이팔면체, 삼팔면체, 구조적 전하 ABSTRACT : Li-bearing muscovite is commonly found along with trioctahedral lepidolite in granitic pegmatites. Structurally, Li + ions can replace K + ions in the interlayer (Int) of muscovite or incorporate into vacancies of the dioctahedral sheet (Sub). However, detailed mechanism of the lithium incorporation into muscovite is challenging to investigate using experimental techniques alone. In the current study, density functional theory (DFT) has been applied to examine the crystal structure and energy variation when Li + resides in the interlayer or the octahedral sheet. Depending on the position of Li + (i.e., Int vs. Sub), DFT showed significant differences in the mica's structures such as lattice parameters, sheet thickness, interlayer separation, and OH angles with respect to the ab plane. DFT further showed that, in pure muscovite, Li + has a lower energy when it is located in Int than Sub. By contrast, in the case of Fe 2+ substitution into the octahedral sheet, Li + has a lower energy in Sub than in Int. These results imply that Li + incorporates into the Al octahedral sheets only when the octahedral sheets possess structural charges, suggesting cation substitution in the octahedral sheets plays an important role in the Li incorporation mechanism into muscovite. They can also explain the experimental observation about the positive relationship between Fe 2+ and Li + amounts in Li-bearing muscovite.

Raman spectra of Ni–Mg kerolite: effect of Ni–Mg substitution on O–H stretching vibrations

The Ni‐rich mineral phases constituting the Ni‐ores in New Caledonia are dominated in most cases by kerolite–pimelite series, also called talc‐like minerals. Despite their economic and geologic interests, the crystallographic structures of these minerals are not fully understood. In order to improve the knowledge of their crystallographic structure, a set of natural talc‐like minerals of varying Ni/Mg ratio was selected. A combination of SEM, EMPA, TEM, and Raman analysis shows that the studied mineral contains only one mineral phase identified as the kerolite–pimelite series. Although kerolite and pimelite are not currently recognized as distinct minerals by the IMA, this study shows that these minerals exist as distinct minerals without any mixed layering with other mineral phases such as serpentine or other sheet silicates. The solid solution between the Ni and Mg endmembers was complete without any gap, corresponding to a Ni–Mg substitution in the octahedral sheet. The Raman spectra in the OH stretching vibration region of representative samples covering the whole range of Ni–Mg substitution show a continuous and significant evolution from the Mg to the Ni endmember. The signal was decomposed into nine Gaussian–Lorentzian functions, but the large overlapping of the peaks and the complexity of the band structure prevent any interpretation of the spectra from a structural point of view. Finally, the problem is formalized as a multivariate curve resolution (MCR) which is solved using the Bayesian Positive Source Separation (BPSS) algorithm. This reveals that four possible arrangements of Ni and Mg in the octahedral sheet are encountered, and these arrangements are dependent on the Ni–Mg substitution rate. It also confirms that Raman microspectroscopy can be very efficient in quick evaluation of the Ni content of the mineral. Copyright © 2015 John Wiley & Sons, Ltd.

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.

Structural and spectroscopic changes to natural nontronite induced by experimental impacts between 10 and 40 GPa

Many phyllosilicate deposits remotely detected on Mars occur within bombarded terrains. Shock metamorphism from meteor impacts alters mineral structures, producing changed mineral spectra. Thus, impacts have likely affected the spectra of remotely sensed Martian phyllosilicates. We present spectral analysis results for a natural nontronite sample before and after laboratory-generated impacts over five peak pressures between 10 and 40 GPa. We conducted a suite of spectroscopic analyses to characterize the sample's impact-induced structural and spectral changes. Nontronite becomes increasingly disordered with increasing peak impact pressure. Every infrared spectroscopic technique used showed evidence of structural changes at shock pressures above ~25 GPa. Reflectance spectroscopy in the visible near-infrared region is primarily sensitive to the vibrations of metal–OH and interlayer H2O groups in the nontronite octahedral sheet. Midinfrared (MIR) spectroscopic techniques are sensitive to the vibrations of silicon and oxygen in the nontronite tetrahedral sheet. Because the tetrahedral and octahedral sheets of nontronite deform differently, impact-driven structural deformation may contribute to differences in phyllosilicate detection between remote sensing techniques sensitive to different parts of the nontronite structure. Observed spectroscopic changes also indicated that the sample's octahedral and tetrahedral sheets were structurally deformed but not completely dehydroxylated. This finding is an important distinction from previous studies of thermally altered phyllosilicates in which dehydroxylation follows dehydration in a stepwise progression preceding structural deformation. Impact alteration may thus complicate mineral-specific identifications based on the location of OH-group bands in remotely detected spectra. This is a key implication for Martian remote sensing arising from our results.

The Influence of Water in Montmorillonite Clay on the Performance As Electrode Material for Li-Ion Battery/Capacitor

Recently, 2D materials have attracted great attention as electrode materials for lithium ion batteries due to their special electochemical properties. Clays are nature abundant materials which have 2D layered structures. Montmorillonite (MMT) is one of them and composed of units made up of two silica tetrahedral sheets with a central alumina octahedral sheet. The cation and water content in inter layer varies with source. Unlike graphene related materials, electrochemical properties of MMT has rarely been studied. Our recent research shows that MMT could be used as a green electrode material for Li-ion battery/capacitor. Here in this study, we will exam the influence of water in MMT clay on the electrochemical performance. The performance of various status of MMT, in nature or dehydrated, will be exam. Conductive agent will also be added to improve conductivity of MMT electrode. Both traditional slurry casting and electrostatic spray deposition (ESD) will be used to fabricate the electrode. Fig. 1(a) shows the TEM image of nature MMT clay and (b) cyclic voltammetry (CV) curves of slurry casting MMT electrode. More detailed results will be shown during the meeting. Figure 1

Theoretical study for the interlamellar aminoalcohol functionalization of kaolinite

Fundamental problems related to aminoalcohols intercalating on kaolinite were investigated by using density functional theory method. This study examines the adsorption modes of diethanolamine and triethanolamine on kaolinite, the role of hydrogen bonds and van der Waals (vdW) forces between aminoalcohols and interlayer of kaolinite, and the change of molecular orbital occupancies of functionalized kaolinite. Results show that functionalized kaolinite is physically intercalated and covalently grafted by aminoalcohols. Non-covalent interaction analysis provides a visualized description that intercalated aminoalcohols form strong hydrogen bonds with Al octahedral sheet, and the interaction between aminoalcohols and Si tetrahedral sheet is mainly attributed to weak vdW force. The analysis of molecular orbital occupancies for kaolinite complex showed that the functionalized kaolinite has strong chemical reactivity as electron donors on the sites of grafted or intercalated aminoalcohols for further chemical reaction with other materials.

Montmorillonite interlayer surface chemistry: effect of magnesium ion substitution on cation adsorption

Montmorillonite is a clay mineral and the main component in bentonite clay, which is utilized in various applications including its planned use as a buffer material for long-term nuclear waste disposal. In the present paper, a quantum chemical study is presented providing an insight into montmorillonite structure and its surface chemistry, which plays a key role in understanding montmorillonite behavior at the molecular level. A model is first designed by calculating the positions of Mg-substitutions in the octahedral sheet of the layer structure. This model is then used to study (1) charge distribution in the system and (2) the energetics of Na+/Ca2+ cation adsorption on the interlayer surfaces. The results show and verify that the Mg-substitutions are positive charge deficits and the only significant charge defects in the structure. Therefore, the energetics of cation adsorption is found to correlate linearly with the inverse distances between cations and Mg-substitutions in a dry, fully periodic montmorillonite lattice.

Rate and mechanism of the photoreduction of birnessite (MnO2) nanosheets

The photoreductive dissolution of Mn(IV) oxide minerals in sunlit aquatic environments couples the Mn cycle to the oxidation of organic matter and fate of trace elements associated with Mn oxides, but the intrinsic rate and mechanism of mineral dissolution in the absence of organic electron donors is unknown. We investigated the photoreduction of δ-MnO2 nanosheets at pH 6.5 with Na or Ca as the interlayer cation under 400-nm light irradiation and quantified the yield and timescales of Mn(III) production. Our study of transient intermediate states using time-resolved optical and X-ray absorption spectroscopy showed key roles for chemically distinct Mn(III) species. The reaction pathway involves (i) formation of Jahn-Teller distorted Mn(III) sites in the octahedral sheet within 0.6 ps of photoexcitation; (ii) Mn(III) migration into the interlayer within 600 ps; and (iii) increased nanosheet stacking. We propose that irreversible Mn reduction is coupled to hole-scavenging by surface water molecules or hydroxyl groups, with associated radical formation. This work demonstrates the importance of direct MnO2 photoreduction in environmental processes and provides a framework to test new hypotheses regarding the role of organic molecules and metal species in photochemical reactions with Mn oxide phases. The timescales for the production and evolution of Mn(III) species and a catalytic role for interlayer Ca(2+) identified here from spectroscopic measurements can also guide the design of efficient Mn-based catalysts for water oxidation.

New insights into the molecular structure of kaolinite–methanol intercalation complexes

A series of kaolinite–methanol complexes with different basal spacings were synthesized using guest displacement reactions of the intercalation precursors kaolinite–N-methyformamide (Kaol–NMF), kaolinite–urea (Kaol–U), or kaolinite–dimethylsulfoxide (Kaol–DMSO), with methanol (Me). The interaction of methanol with kaolinite was examined using X-ray diffraction (XRD), infrared spectroscopy (IR), and nuclear magnetic resonance (NMR). Kaolinite (Kaol) initially intercalated with N-methyformamide (NMF), urea (U), or dimethylsulfoxide (DMSO) before subsequent reaction with Me formed final kaolinite–methanol (Kaol–Me) complexes characterized by basal spacing ranging between 8.6Å and 9.6Å, depending on the pre-intercalated reagent. Based on a comparative analysis of the three Kaol–Me displacement intercalation complexes, three types of Me intercalation products were suggested to have been present in the interlayer space of Kaol: (1) molecules grafted onto a kaolinite octahedral sheet in the form of a methoxy group (Al-O-C bond); (2) mobile Me and/or water molecules kept in the interlayer space via hydrogen bonds that could be partially removed during drying; and (3) a mixture of types 1 and 2, with the methoxy group (Al-O-C bond) grafted onto the Kaol sheet and mobile Me and/or water molecules coexisted in the system after the displacement reaction by Me. Various structural models that reflected four possible complexes of Kaol–Me were constructed for use in a complimentary computational study. Results from the calculation of the methanol kaolinite interaction indicate that the hydroxyl oxygen atom of methanol plays the dominant role in the stabilization and localization of the molecule intercalated in the interlayer space, and that water existing in the intercalated Kaol layer is inevitable.

Molecular modeling of the effects of 40Ar recoil in illite particles on their K–Ar isotope dating

The radioactive decay of 40K to 40Ar is the basis of isotope age determination of micaceous clay minerals formed during diagenesis. The difference in K–Ar ages between fine and coarse grained illite particles has been interpreted using detrital-authigenic components system, its crystallization history or post-crystallization diffusion. Yet another mechanism should also be considered: natural 40Ar recoil. Whether this recoil mechanism can result in a significant enough loss of 40Ar to provide observable decrease of K–Ar age of the finest illite crystallites at diagenetic temperatures – is the primary objective of this study which is based on molecular dynamics (MD) computer simulations.All the simulations were performed for the same kinetic energy (initial velocity) of the 40Ar atom, but for varying recoil angles that cover the entire range of their possible values. The results show that 40Ar recoil can lead to various deformations of the illite structure, often accompanied by the displacement of OH groups or breaking of the Si–O bonds. Depending on the recoil angle, there are four possible final positions of the 40Ar atom with respect to the 2:1 layer at the end of the simulation: it can remain in the interlayer space or end up in the closest tetrahedral, octahedral or the opposite tetrahedral sheet. No simulation angles were found for which the 40Ar atom after recoil passes completely through the 2:1 layer. The energy barrier for 40Ar passing through the hexagonal cavity from the tetrahedral sheet into the interlayer was calculated to be 17kcal/mol. This reaction is strongly exothermic, therefore there is almost no possibility for 40Ar to remain in the tetrahedral sheet of the 2:1 layer over geological time periods. It will either leave the crystal, if close enough to the edge, or return to the interlayer space. On the other hand, if 40Ar ends up in the octahedral sheet after recoil, a substantially higher energy barrier of 55kcal/mol prevents it from leaving the TOT layer over geological time.Based on the results of MD simulations, the estimates of the potential effect of 40Ar recoil on the K–Ar dating of illite show that some of 40Ar is lost and the loss is substantially dependent on the crystallite dimensions. The 40Ar loss can vary from 10% for the finest crystallites (two 2:1 layers thickness and <0.02μm in diameter) to close to zero for the thickest and largest (in the ab plane) ones. Because the decrease of the K–Ar estimated age is approximately proportional to the 40Ar loss, the finer crystallites show lower apparent age than the coarser ones, although the age of crystallization is assumed equal for all the crystallites. From the model it is also clear that the lack of K removal from illite fringes (potentially Ar-free) strongly increases the apparent age differences among crystallites of different size.

Atom exchange between aqueous Fe(II) and structural Fe in clay minerals.

Due to their stability toward reductive dissolution, Fe-bearing clay minerals are viewed as a renewable source of Fe redox activity in diverse environments. Recent findings of interfacial electron transfer between aqueous Fe(II) and structural Fe in clay minerals and electron conduction in octahedral sheets of nontronite, however, raise the question whether Fe interaction with clay minerals is more dynamic than previously thought. Here, we use an enriched isotope tracer approach to simultaneously trace Fe atom movement from the aqueous phase to the solid ((57)Fe) and from the solid into the aqueous phase ((56)Fe). Over 6 months, we observed a significant decrease in aqueous (57)Fe isotope fraction, with a fast initial decrease which slowed after 3 days and stabilized after about 50 days. For the aqueous (56)Fe isotope fraction, we observed a similar but opposite trend, indicating that Fe atom movement had occurred in both directions: from the aqueous phase into the solid and from the solid into aqueous phase. We calculated that 5-20% of structural Fe in clay minerals NAu-1, NAu-2, and SWa-1 exchanged with aqueous Fe(II), which significantly exceeds the Fe atom layer exposed directly to solution. Calculations based on electron-hopping rates in nontronite suggest that the bulk conduction mechanism previously demonstrated for hematite1 and suggested as an explanation for the significant Fe atom exchange observed in goethite2 may be a plausible mechanism for Fe atom exchange in Fe-bearing clay minerals. Our finding of 5-20% Fe atom exchange in clay minerals indicates that we need to rethink how Fe mobility affects the macroscopic properties of Fe-bearing phyllosilicates and its role in Fe biogeochemical cycling, as well as its use in a variety of engineered applications, such as landfill liners and nuclear repositories.

Balestraite, KLi2VSi4O10O2, the first member of the mica group with octahedral V5+

A mica-group mineral characterized by a high V content and free of Al was found in the manganesiferous beds within the metacherts of the ophiolitic sequences at the Cerchiara mine, Eastern Liguria (Italy), in association with hematite, quartz, and calcite. Chemical and structural characterization supported by Raman data defines this phase as a new mineral species, which is named balestraite after Corrado Balestra, a prominent Italian amateur mineralogist. Balestraite, ideally KLi2V5+Si4O10O2, is a 1 M trioctahedral mica crystallizing in the C 2 space group, with a = 5.2024(5), b = 8.9782(7), c = 9.997(2) A, β = 100.40(2)°, V = 459.3(1) A3, Z = 2. The reduction of symmetry from the “ideal” space group C 2/ m is related to the ordering of V at only one of the two pseudo-symmetric octahedral sites. Vanadium forms very distorted octahedra with a [2+2+2] geometry characteristic of the valence state +5. The Li,V composition of the octahedral sheet, the pure tetrasilicic character of the tetrahedral sheet, and the anhydrous character produce unusual geometrical features for this mica. The occurrence of 5+ as the dominant valence state of V and the virtually complete O2− → OH− substitution at the O4 site indicate strongly oxidizing conditions of crystallization, which are consistent with balestraite occurring at the boundary between carbonate-bearing veins and hematite bands. The new mineral and name were approved by the Commission on New Minerals, Nomenclature and Classification, IMA (2013-080).

Sequestration of heavy metal cations on montmorillonite by thermal treatment

Clay minerals are low-cost, high-efficient, and environmentally-friendly adsorbents for heavy metal cations. Proper disposal of the spent clay minerals, however, is still a big challenge holding back their wide applications in environmental remediation. In this work, the spent montmorillonites (Mt) after the adsorption of Cu2+ or Cd2+ were thermally treated, with the aim of in-situ sequestrating the adsorbed heavy metal cations. The structural characteristics of the calcined Mt samples were investigated using X-ray diffraction and X-ray photon spectroscopy, and the sequestration efficiency was tested by ammonium acetate and HCl washing. The results of this work demonstrated that Cu2+ cations could migrate into both the ditrigonal cavities on the tetrahedral sheet and vacant sites on the octahedral sheet of Mt after thermal treatment due to its small ion radius. In this case, Cu2+ cations could be efficiently sequestrated on Mt layers, e.g., less than 3.2% desorption by ammonium acetate washing and less than 5% desorption by HCl washing for the 400°C treated Cu–Mt. As for Cd2+ cations, they could hardly migrate into Mt layers, and thus much higher calcination temperature (e.g., 700°C) was needed to efficiently sequestrate them on Mt. In this case, Cd2+ cations were mainly sequestrated by the collapse of Mt layers and/or the deformation of Mt structure, which could block the desorption pathway for Cd2+ cations. Interestingly, as the thermal treatment temperature increased to a critical value (i.e., 400°C for Cu–Mt and 500°C for Cd–Mt), the structural Al3+ could be preferably dissolved by HCl than the adsorbed heavy metal cations; ammonium acetate could hardly dissolve the structural Al3+. This work demonstrated that thermal treatment could be a practical method for safe disposal of the spent Mt after the adsorption of heavy metal cations.

A comparative study on the removal of Pb(II), Zn(II), Cd(II) and As(V) by natural, acid activated and calcinated halloysite

A comparative study on the removal of Pb(II), Zn(II), Cd(II) and As(V) by natural, acid activated and calcinated halloysite

Removal of selected anions by raw halloysite and smectite clay

The structure of clay minerals is made up from tetrahedral and octahedral sheets, which can be stacked in different ways, forming 1:1, 2:1 and 2:1:1 layers. Halloysite, which belongs to the kaolin group, is composed of 1:1 layers while smectite group minerals exhibit 2:1 dioctahedral layered structure. The most important feature of these minerals is the presence of numerous active centers on their surface and/or in the interlayer space, which allows them to attract and exchange ions from aqueous solutions. These makes them perfect candidates which could be used for the purification of wastewaters from harmful ions (Bhattacharyya & Gupta 2008; Lee & Tiwari 2012). The aim of this work was to examine the sorption capacity of natural halloysite and smectite clay towards P(V), As(V) and Cr(VI). Two samples used in the research came from Polish deposits. Natural halloysite (H) was obtained from Dunino deposit, while smectite clay (SC) was obtained from Belchatow Lignine Mine where it forms an overburden cover. For the both raw samples the XRD patterns and FTIR spectra were collected. The sorption of P(V), As(V) and Cr(VI) was conducted as a function of anions concentration in the range from 0.05 to 50 mmol/L for initial pH 5 in a single-element system. The suspension of H or SC and an adequate solution (solid/solution ratio: 20 g/L) was shaken for 24 h at 25°C. Afterwards the anions concentration in the supernatant solution was measured using colorimetric methods. The P(V) and As(V) concentration was determined with molybdenum blue method, while Cr(VI) concentration was measured with diphenylcarbazide method. The XRD pattern of the H sample showed a basal peak at 7.20 A, which confirms the presence of dehydrated halloysite-(7 A). In turn, the SC exhibited a peak centered at ~12.5 A with an asymmetric profile starting from ~15.0 A. Such reflection suggests the presence of smectite which has both Na + and Ca 2+ cations in its interlayer space. The peaks at 4.26 A and 3.34 A are associated with quartz. The IR spectra of the H showed bands characteristic for kaolin group minerals related to the OH-stretching region (3700-3620 cm -1 ), vibrations of water molecules (~1630 cm -1 ) and bands assigned to stretching and bending vibrations of aluminosilicate framework (1200-400 cm -1 ). The IR spectrum of SC shows bands characteristic for smectite minerals i.e. 3623 cm -1 band attributed to OH hydroxyl located inside the 2:1 layer and a broad band centered at ~3400 cm -1 due to interlayer water surrounding cations. Also the structural vibrations of the 2:1 layer may be observed in the 1200-400 cm -1 region. The results of the experiment indicated that the sorption capacity of the H sample and the SC sample were relatively high. The amount of removed P(V) was the highest for both materials, where the sorption was respectively equal to: 201 and 256 mmol/kg. The sorption capacity of As(V) for the H was equal to 168 mmol/kg, while it was significantly lower on the SC (96 mmol/kg). In the case of H the Cr(VI) sorption reached only 36 mmol/kg and for the SC it was equal to 104 mmol/kg. In most cases the sorption isotherms were fitted to the Freundlich model. The only exception was for P(V) sorption on the H sample, which was better described by Langmuir model. The specific surface areas (S BET ) of the studied materials do not differ significantly: SC = 69.10 m 2 /g and H = 49.52 m 2 /g. The sorption centers that may attract anions in both, H and SC samples, were limited, because isomorphic substitutions in tetrahedral and/or octahedral sheets generate positively charged sites, which attract cations. It is believed that the mechanism responsible for the adsorption of anions on both materials is mainly surface complexation, which occurs at the crystals edges (Bradl 2004). The sorption capacity of the H and the SC was significantly lower than that reported for hydrotalcite-based anion-exchange materials (HTLc). For comparison, the sorption capacity towards P(V), As(V) and Cr(VI) on uncalcined HTLc was as follows: 498 mmol/kg (Kuzawa et al. 2006), 596 mmol/kg (Wu et al. 2013) and 314 mmol/kg (Alvarez-Ayuso & Nugteren 2005). Nevertheless, the examined mineral samples might be useful as sorbents for industrial wastewater treatment involving the removal of P(V) and As(V).

ChemInform Abstract: Use of Montmorillonites in Organic Reactions

AbstractReview: 16 refs.

Preparation and characterization of azobenzene-smectite photoactive mineral nanomaterials

Smectites are 2:1 layered minerals built of one octahedral sheet located between two tetrahedral sheets. The layer charge, derived from the isomorphic substitutions in the mineral structure, is compensated by the interlayer cations. The capability to exchange the interlayer cations is an important property of smectites as it enables to design and produce new nanomaterials based on natural minerals through their organic modification. Such hybrid materials are highly desirable in industry and environmental protection due to their specific properties that may be controlled in nanoscale. Preparation of photoactive materials through intercalation of layered minerals, including mainly synthetic micas, with azobenzene and other azo-compounds was proposed previously (Fujita et al., 2003; Ogawa et al., 2003 ; Heinz et al., 2008) . Azobenzene molecules show a change in their shape and dimensions upon the UV irradiation, which may affect the structure of host mineral. The photoactivate materials may find application in nanotechnology as molecular nanoswitches and nanosensors controlled by UV radiation. The objective of the study was to prepare azobenzene-smectite intercalation compounds. The obtained materials were subjected to structural and chemical characterization, which is crucial for further advancement of their photoresponsive properties. Na-montmorillonite (SWy), Ca-montmorillonite (STx), beidellite (BId) and synthetic laponite (SynL) were used for the experiments. The modification procedure involved at first the intercalation of smectites with hexadecyltrimethylammonium bromide (abbreviated C16) and after that the insertion of azobenzene into the interlayer space. The reaction with C16, in amount equal to 1.0 CEC of the smectite, was performed in an aqueous suspension (20g/L) for 2 h in 60°C. The obtained organo-smectites were prepared as thin films on glass plates and reacted with azobenzene in a teflon vessel at ~100°C for 24 h.In such conditions the azobenzene vaporizes and penetrates the interlayer space of the organo-mineral. The azobenzene/smectite weight ratio was equal to 0.2. The chemical and structural analysis of all obtained samples was carried out with use of X-ray diffraction (XRD), infrared spectroscopy (FTIR) and CHN elemental analysis. The increased amount of nitrogen and carbon in modified samples confirmed the occurrence of intercalation process of both the ammonium salt and the azobenzene. Moreover, new bands appeared in the infrared spectra of the C16-smectites at ~2924 cm -1 and ~2851 cm -1 due to the C-H stretching vibrations in the C16 molecules. The spectra of azobenzene intercalation compounds showed additionally a series of bands corresponding to the vibrations characteristic for the azobenzene molecule at: ~3061 cm -1 , ~1581 cm -1 , ~1455 cm -1 , and ~1302 cm -1 . . The basal spacing of tested minerals increased after the C16 intercalation, as confirmed by the XRD analysis. The increase was equal to 6.1 A, 3.3 A, 4.1 A and 3.5 A for SWy, STx, BId and SynL samples, respectively. This suggests a nearly horizontal arrangement of C16 molecules and formation of a monolayer in the interlayer space of smectite. Introduction of azobenzene lead to a further increase of d 001 value. The increase is visibly different for all the samples and is equal to 7.0 A, 15.0 A, 21.7 A and 23.5 A for SWy, STx, BId and SynL samples, respectively . The arrangement of organic molecules in the interlayer space is influenced by a number of factors including: the type of the mineral, the layer charge and its location in the layer, or the amount and arrangement of the cationic surfactant . A correlation between the azobenzene location in the interlayer space and the photoresponse behavior of tested materials will be the subject of further studies.