Hydrotalcite minerals are layered double hydroxides (LDH) which play an important role in immobilizing hazardous compounds to decontaminate industrial wastewaters. The stability of an LDH is mostly evaluated in terms of its low solubility in water. However, the solubility of divalent trace metals immobilized by Mg-Al-type LDHs is not well known. Hydrotalcites containing Zn in solid solution, (Mg+Zn)3-Al-LDH, were synthesized by alkaline co-precipitation. A series of eleven LDH phases with Zn mole fractions XZn = Zn/(Mg+Zn) of 0–1 were characterized by powder X-ray diffractometry (XRD), Fourier-transform infrared (FTIR) spectroscopy, thermogravimetry (TGA), scanning and transmission electron microscopy (SEM/TEM), Brunauer-Emmett-Teller (BET) surface area analysis, and inductively coupled plasma mass (ICP-MS) spectrometry. The XRD analysis provided sharp characteristic spacings for d003 and d006 which occurred for all samples, confirming a layered LDH structure. Cell parameters (a, c) obeyed Vegard’s law and confirmed the formation of a regular solid-solution series without a mixing gap. An aqueous equilibrium time was determined by kinetic dissolution experiments. Steady-state solubility occurred after 120 days, but the experiments continued up to 240 days. The XRD and SEM/TEM analyses indicated no phase changes during the long-term dissolution experiments; neither were phase impurities detected after 240 days. The solubility products of the Mg- and Zn-bearing endmember compositions were calculated from experimentally determined total cation and anion concentrations using the Visual Minteq code for considering element speciation and ion pairing. The solubility product decreased as the Zn mole fraction increased, suggesting that the Zn-bearing LDH phases were more stable than the pure Mg3-Al-LDHs. Solid-solution aqueous-solution thermodynamic equilibrium modeling using the Lippmann “total solubility product” approach and applying Lippmann diagrams with logarithmic x-axes revealed a log-linear decrease in aqueous Zn solubility. The results are promising for remediation of metal-bearing liquid wastes because the metals that co-precipitated with the LDH were more strongly retained and, therefore, less soluble than the hydroxides or carbonates of the trace metal.

Understanding the reactions taking place in the hematite-oxalic acid system is important in order to clean iron oxides from filters and to remove iron from mineral concentrates. Previous studies reported the formation of an unwanted solid phase during this process. The objective of the current work, therefore, was to visualize and rationalize the iron dissolution steps taking place in the hematite–oxalic acid reaction by combining density functional theory (DFT) calculations and experimental data. The results of DFT calculations indicated that a precipitate was formed in this reaction; XRD analysis of the solid phase after the dissolution experiment revealed the formation of humboldtine as the precipitate. The attachment of oxalate on the hematite surface and the reduction of Fe(III) to Fe(II) were key steps for humboldtine formation. Both simulations and the experimental results showed that greater oxalic acid concentrations yielded more precipitate, suggesting a simple and novel route to synthesize humboldtine, a material which is relevant to the demand for clean energy.

Smectites are a group of minerals traditionally analyzed by thermal methods due to their exceptionally large adsorbed-water contents and the presence of OH groups, which makes them unique among all common soil- and rock-forming minerals. The dehydration reaction of smectite is a low-temperature endothermic effect that ends typically below 200°C. Although the removal of bulk interlayer water requires activation energy (Ea) of just above 30 kJ/mol, the removal of the last few H2O molecules attached strongly to interlayer cations requires Ea > 100 kJ/mol. Dehydroxylation is the loss of structural OH groups that proceeds as evolution of H2O molecules out of the smectite structure and occurs in the 300–900°C range. In trioctahedral species, dehydroxylation is combined with recrystallization and proceeds usually at > 700°C. In dioctahedral species, the temperature of dehydroxylation is controlled by the type of octahedral vacancy, having trans-vacant and cis-vacant distinguished by the boundary at ~ 600°C, and by the octahedral cation–OH bond strength, following the order Mg > Al > Fe. The Ea of dehydroxylation correlates linearly with the temperature of maximum dehydroxylation; from > 170 kJ/mol for Cs+-exchanged beidellite and nontronite, through ~ 300 kJ/mol in Mg-rich montmorillonite, to > 500 kJ/mol in trioctahedral saponite. Dehydration and dehydroxylation of smectites can be accompanied by a number of other phenomena, such as dehydrogenation or defluorination. At high temperatures, smectite amorphization and recrystallization occurs. Unless amorphized and/or recrystallized, smectites can undergo rehydration and rehydroxylation, which are opposite reactions to dehydration and dehydroxylation, respectively. This review discusses the details of the above-mentioned thermal reactions of smectites, focusing on thermogravimetric methods, evolved gas analysis, and structural alterations. Factors affecting the accuracy and precision of thermal analysis of smectite are discussed along with examples of best laboratory practices. The paper also provides the most recent description and critical evaluation of smectite reaction kinetics.

Water is essential for humans, animals, and plants; pollutants, usually derived from anthropogenic activities, can have a serious effect on its quality. Heavy metals are significant pollutants and are often highly toxic to living organisms, even at very low concentrations. Among the numerous removal techniques proposed, adsorption onto suitable adsorbent materials is considered to be one of the most promising. The objective of the current study was to determine the effectiveness of halloysite nanotubes (HNT) functionalized with organic amino or thiol groups as adsorbent materials to decontaminate polluted waters, using the removal of Hg2+ ions, one of the most dangerous heavy metals, as the test case. The effects of pH, ionic strength (I), and temperature of the metal ion solution on the adsorption ability and affinity of both materials were evaluated. To this end, adsorption experiments were carried out with no ionic medium and in NaNO3 and NaCl at I = 0.1 mol L−1, in the pH range 3–5 and in the temperature range 283.15–313.15 K. Kinetic and thermodynamic aspects of adsorption were considered by measuring the metal ion concentrations in aqueous solution. Various equations were used to fit experimental data, and the results obtained were explained on the basis of both the adsorbent’s characterization and the Hg2+ speciation under the given experimental conditions. Thiol and amino groups enhanced the adsorption capability of halloysite for Hg2+ ions in the pH range 3–5. The pH, the ionic medium, and the ionic strength of aqueous solution all play an important role in the adsorption process. A physical adsorption mechanism enhanced by ion exchange is proposed for both functionalized materials.

This work presents Atomistic Topology Operations in MATLAB (atom), an open source library of modular MATLAB routines which comprise a general and flexible framework for manipulation of atomistic systems. The purpose of the atom library is simply to facilitate common operations performed for construction, manipulation, or structural analysis. Due to the data structure used, atoms and molecules can be operated upon based on different chemical names or attributes, such as atom- or molecule-ID, name, residue name, charge, positions, etc. Furthermore, the Bond Valence Method and a neighbor-distance analysis can be performed to assign many chemical properties of inorganic molecules. Apart from reading and writing common coordinate files (.pdb, .xyz, .gro, .cif) and trajectories (.dcd, .trr, .xtc; binary formats are parsed via third-party packages), the atom library can also be used to generate topology files with bonding and angle information taking the periodic boundary conditions into account, and supports basic Gromacs, NAMD, LAMMPS, and RASPA2 topology file formats. Focusing on clay-mineral systems, the library supports CLAYFF (Cygan, 2004) but can also generate topology files for the INTERFACE forcefield (Heinz, 2005, 2013) for Gromacs and NAMD.

Many kaolinites are known to exhibit limited intercalation capacity which affects their usage. Some reports have linked this lack of reactivity to particular structural features or to slow kinetics; others recommended increasing intercalation temperature as a remedy. The purpose of the current study was to investigate systematically the N-methylformamide (NMF) intercalation capacity of three kaolinites differing in layer stacking order (KGa-1b, KGa-2, and Imerys Hywite Alum) in the 5–150°C temperature range. Near-infrared spectroscopy (NIR) was employed to record the full kinetics of intercalation in closed systems with excess NMF. Increasing intercalation temperature accelerated the reaction, but the NMF uptake decreased and eventually vanished. Complementary thermogravimetric analysis (TGA) confirmed this unexpected trend. All kaolinites exhibited the same behavior, but the amount of inert material was in the order of their stacking-fault concentration at all temperatures: KGa-2 > Hywite > KGa-1b. Subjecting the samples to stepwise temperature changes showed that, once intercalated, the NMF could not deintercalate and was removed from equilibrium with the surrounding fluid. Thus, intercalation capacity was not a unique feature of the material because it depended on thermal history. As stacking order and thermal history had no detectable effect on the NMF-hosting environment, the unusual temperature dependence was attributed tentatively to the adverse effect of temperature on the adsorption of NMF on the edges of the crystallites, which is a prerequisite for intercalation.

Geopolymers have the potential to function as an environmentally friendly substitute for ordinary Portland cement, with up to 80% less CO2 emission during production. The effect is best utilized for geopolymers prepared with amorphous silica instead of waterglass (Na2xSiyO2y+x) to adjust the Si:Al ratio. The reactivity of the precursors with the alkaline activator affects the final mineralogical properties of the binder. The purpose of the present study was to investigate the amount of different phases formed during geopolymerization and to understand the quantitative evolution of carbonation during geopolymer synthesis by determining the solubility of metakaolinite and amorphous SiO2 in NaOH at various concentrations. The solubility was studied by ICP-OES measurements. X-ray diffraction was used for qualitative and quantitative phase analysis of the geopolymers. The solubility of the precursors increased with calcination temperature of metakaolinite, reaction time for amorphous SiO2, and at higher NaOH concentrations. Partial dissolution resulted in free Na+, which is a source for the formation of carbonates in the geopolymers. Thermonatrite occurred prior to trona formation in all samples.

Kaolins and clays are important raw materials for production of supplementary cementitious materials and geopolymer precursors through thermal activation by calcination beyond dehydroxylation (DHX). Both types of clay contain different polytypes and disordered structures of kaolinite but little is known about the impact of the layer stacking of dioctahedral 1:1 layer silicates on optimum thermal activation conditions and following reactivity with alkaline solutions. The objective of the present study was to improve understanding of the impact of layer stacking in dioctahedral 1:1 layer silicates on the thermal activation by investigating the atomic structure after dehydroxylation. Heating experiments by simultaneous thermal analysis (STA) followed by characterization of the dehydroxylated materials by nuclear magnetic resonance spectroscopy (NMR) and scanning electron microscopy (SEM) together with first-principles calculations were performed. Density functional theory (DFT) was utilized for correlation of geometry-optimized structures to thermodynamic stability. The resulting volumes of unit cells were compared with data from dilatometry studies. The local structure changes were correlated with experimental results of increasing DHX temperature in the following order: disordered kaolinite, kaolinite, and dickite, whereupon dickite showed two dehydroxylation steps. Intermediate structures were found that were thermodynamically stable and partially dehydroxylated to a degree of DHX of 75% for kaolinite, 25% for disordered kaolinite, and 50% for dickite. These thermodynamically stable, partially dehydroxylated intermediates contained AlV while metakaolinite and metadickite contained only AlIV with a strongly distorted coordination shell. These results indicate strongly the necessity for characterization of the structure of dioctahedral 1:1 layer silicates in kaolins and clays as a key parameter to predict optimized calcination conditions and resulting reactivity.

Layer charge is an important property of 2:1 phyllosilicates originating from isomorphic substitutions in the structure, from vacancies in the octahedral sheet and from unsatisfied bonds at the edges of the crystals. It is of particular interest in the case of smectites because it affects important properties of this mineral. Several methods have been proposed for determining the layer charge of smectites, namely the structural formula method (SFM), the alkylammonium method (AAM), the NH4-method, the K-saturation method, and the O-D method. Most of these methods have been used extensively in the past and they all have advantages and shortcomings. The SFM and the AAM are based on different principles and are considered as primary methods. They have been used for a long time but they are time consuming and they provide contrasting layer charge values, with the AAM yielding consistently lower layer charge values than the SFM, especially for low-charge smectites. The remaining methods have been developed more recently and have been calibrated over their primary counterparts. They are applied easily and are capable of producing a large amount of data within a short time. In this sense they can be used both for geological interpretations and for assessment of bentonite deposits at an industrial scale.

CaMnt is much less important than NaMnt due to its limited commercial applications. The time-dependent property of NaMnt gel has been studied extensively as it is exploited in many applications such as drilling mud and viscosity-modifier applications. In contrast, the time-dependent property of CaMnt suspension and the factors affecting it are largely unknown. The speciation of Ca2+ ions is one such factor to be evaluated. In the current study, pH and solids concentration were examined and then used to validate a recent theory on clay gel time dependency. The results supported the theory that a strong electrostatic double layer (EDL) repulsion in the 3-D network is needed to reorganize the structure and drive it toward the state of minimum free energy. The 12 wt.% CaMnt (STx-1b) gel displayed time-dependent behavior at pH 5 but not at its natural pH of 8.4. At pH 5, the interlayer Ca2+ ions became fully hydrated and desorbed from the platelet surface. This enhanced the EDL repulsion between the platelets. At pH >6.5, the positively charged hydrolysis product Ca(OH)+ began to form and was adsorbed, partially neutralizing the permanent layer charge and weakening the repulsion. However, at 19.4 wt.% solids, the gel displayed pronounced time-dependent behavior despite the high natural pH of 8. The platelets were much closer together, allowing the EDL force to operate and effect structural reorganization. The microstructure of these CaMnt gels showed high platelet concentrations interacting to form a relatively open structure. The microstructure of a kaolin (KGa-2) suspension which showed no time-dependent behavior even after 1 day of ageing revealed the importance of particle morphology and layer charge on time dependency. Its 3-D structure was formed by relatively thick, layered platelets with a low layer charge.

Among all iron oxides, hematite (α-Fe2O3), goethite (α-FeOOH), and ferrihydrite (FeOOH⋅nH2O) are the most common mineral species. While immobilization of Mo6+ by surface adsorption on ferric oxides has been studied extensively, the mechanisms of incorporation in their structure have been researched little. The objective of this study was to investigate the relation between Mo content and its structural incorporation in hematite, goethite, and six-line ferrihydrite by a combination of X-ray absorption spectroscopy (XAS), powder X-ray diffraction (pXRD), and inductively-coupled plasma optical emission spectrometry (ICP-OES). Synthesized in the presence of Mo, the hematite, goethite, and six-line ferrihydrite phases incorporated up to 8.52, 0.03, and 17.49 wt. % Mo, respectively. For hematite and goethite, pXRD analyses did not indicate the presence of separate Mo phases. Refined unit-cell parameters correlated with increasing Mo concentration in hematite and goethite. The unit-cell parameters indicated an increase in structural disorder within both phases and, therefore, supported the structural incorporation of Mo in hematite and goethite. Analysis of pXRD measurements of Mo-bearing six-line ferrihydrites revealed small amounts of coprecipitated akaganéite. X-ray absorption near edge structure (XANES) measurements at the Mo L3-edge indicated a strong distortion of the MoO6 octahedra in all three phases. Fitting of extended X-ray absorption fine structure (EXAFS) spectra of the Mo K-edge supported the presence of such distorted octahedra in a coordination environment similar to the Fe position in the investigated specimen. Incorporation of Mo6+ at the Fe3+-position for both hematite and goethite resulted in the formation of one Fe vacancy in close proximity to the newly incorporated Mo6+ and, therefore, charge balance within the hematite and goethite structures.

Kaolin waste dumps contain a huge volume of material that cannot be commercialized due to the presence of variable amounts of iron minerals, which impart a yellowish color to the kaolin. Elimination of iron from kaolin using either a chemical or a biological method was the aim of this study. The chemical leaching of iron from kaolin was carried out using response surface methodology to optimize the reaction conditions. Time was found to be the most influential variable, although oxalic acid must be present to leach 32% of the iron. Biological leaching was also assayed for 14 days using a Bacillus strain. The results of bacterial leaching of iron from kaolin showed <1% iron elimination.

Clays and clay minerals dissolve over a broad pH range, such as during sediment diagenesis and in a variety of applications, including nuclear waste storage, landfills, and geopolymer binders in the construction industry. The solubility depends on process parameters (pH, temperature, pressure, etc.) and material properties (phase content, clay mineral composition, particle size, etc.). Pretreatments such as calcination or severe grinding change the material properties and could enhance solubility, which is called activation. The aim of the current study was to determine the solubility of three different clay minerals after calcination (metakaolinite, metamontmorillonite, and metaillite) in high molar alkaline solutions (NaOH) up to 10.79 mol/L and pH = 14.73. Furthermore, the solubility of an Al(OH)3 powder in alkaline solution (NaOH) was analyzed, as it can be used to adjust the Si:Al ratio of geopolymer precursors. The residues of the clay minerals after the alkaline treatment were investigated to disclose potential alterations in their phase contents. Based on the results of the thermal and alkaline activation, conclusions about the suitability as geopolymer precursors were made. All clay minerals showed an increase in solubility proportional to the concentration of the alkaline solution. The solubility decreased in the order metakaolinite > metamontmorillonite > metaillite. Thereby, dissolution was incomplete for all three clay minerals (<90%) after 7 days and congruent for metakaolinite and metaillite but incongruent for metamontmorillonite.

A geological, mineralogical, and geochemical characterization of the Tamame de Sayago (Zamora, Spain) deposit was carried out with the aim of knowing the conditions that facilitated the genesis in the same deposit of kaolinite and smectites. The alteration processes affecting a Variscan granite were deduced throughout the study of a very wide group of representative samples by X-ray diffraction (XRD), scanning electron microscopy (SEM), chemical analyses of major, minor, and trace elements, as well as δ18O, δ34S stable isotope and K/Ar dating analyses. In addition, 2D and 2.5D graphs of the kaolinite and smectite isoconcentrations were obtained from core data. According to the color and texture, two different clayey rock types were identified and named as homogeneous alteration zones (ZAHO) and heterogeneous alteration zones (ZAHE). The ZAHO are regoliths in which the granite texture is preserved, and the feldspars are almost completely kaolinized. In the ZAHE, the original texture of the granitic rock is lost, and the main clay mineral is smectite. The mineralogical composition is similar, with kaolinite, smectite, mica, quartz, scarce feldspar, and occasionally natroalunite and APS (aluminum-phosphate-sulphate); however, the mineral concentration varies considerably because ZAHO are rich in kaolin areas whereas ZAHE are bentonitic areas. Both rock types contain numerous veins and nodules. The weathering of the Paleozoic granite alongside the absence of sedimentation during the Mesozoic led to the formation of kaolinite that is preserved in ZAHO materials. Nonetheless, during the Cretaceous–Tertiary transit, the conditions of tectonic stability varied. Late Variscan faults reactivated which allowed the percolation of Mg- and Ca-rich hydrothermal fluids through the already kaolinized granite, increasing the alteration of the primary silicates and leading to the formation of smectites in ZAHE materials. The amount of smectites is greater closer to the faults. The stable isotopes indicate the meteoric nature of the low-temperature hydrothermal fluids. The K/Ar data obtained from the natroalunite of veins indicate that those hydrothermal fluids circulated in different pulses from 66.4 ± 1.7 to 58.8 ± 1.5 Ma, as a minimum. Those ages are coincident with the first formation stages of the Duero Basin.

The structure and composition of short-range ordered aluminosilicates (SROAS) may control their affinity for organic acids with potential effects on soil organic matter stabilization. Adsorption mechanisms of model organic acids were studied to resolve the effect of Si incorporation. Adsorption of oxalic, salicylic, and octanoic acid on Al-rich (Al:Si = 3.7) and Si-rich (Al:Si = 1.4) SROAS was quantified by analyses of dissolved organic carbon using catalytic high-temperature combustion. The initial pH of 5 and 6.5 increased to 6.3–8.2 during adsorption of oxalic and salicylic acid, demonstrating hydroxyl release by ligand exchange. Minor changes in pH indicated weak interactions of octanoic acid with both SROAS. Adsorbates were characterized by Fourier-transform infrared spectroscopy. Asymmetric stretching of carboxylate groups at 1720 and 1700 cm–1, and symmetric stretching at 1430 cm–1 evinced the formation of chelate complexes for oxalic acid. An absorption band centered at 1545 cm–1 indicated partial inner-sphere binding of salicylic acid on both SROAS. Silicon-rich SROAS adsorbed 80–90% less than Al-rich SROAS, suggesting that adsorption of oxalic and salicylic acid was controlled by surface aluminol groups. Fast kinetics of oxalate adsorption on Al sites was studied by a conductivity-based stopped-flow technique. Ligand exchange proceeded at a rate constant of 3.5 s–1 (25°C), similar to solute Al complexation, with an activation energy of up to 34.1 kJ mol–1. A slow process with a rate constant of 0.13 s–1 (25°C) was attributed to diffusion of oxalate at the surface or into SROAS particles. As supported by structural characterization of Si-rich SROAS, the much lower susceptibility of Si-rich SROAS to ligand exchange relates to Al speciation. The formation of tetrahedral Al precludes its complexation by carboxyl groups.