With the over-use of tetracycline (TC) and its ultimate accumulation in aquatic systems, the demand for TC removal from contaminated water is increasing due to its severe threat to public health. Clay minerals have attracted great attention as low-cost adsorbents for controlling water pollution. The objective of the present study was to measure the adsorption behavior and mechanisms of TC on allophane, a nanosized clay mineral with a hollow spherical structure; to highlight the advantage of the allophane nanostructure, a further objective was to compare allophane with halloysite and montmorillonite, which have nanostructures that differ from allophane. Structural features and surface physicochemical properties were characterized by transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), zeta potential, N2-physisorption, and acid–base titration. The adsorption data showed that TC adsorption followed the pseudo-second order and Langmuir models. The adsorption was pH dependent, as all three clay minerals performed better under neutral to weakly alkaline conditions and maintained high adsorption performance in the presence of co-existing Na+/K+/Ca2+/Mg2+ cations. Regeneration of the adsorbent was excellent, with efficiencies exceeding 75% after five recycles. By comparison, allophane always exhibited the greatest adsorption capacity, up to 796 mg g–1 at ~pH 9. The TC adsorption on allophane and halloysite was dominated by inner-sphere complexation, together with a small amount of electrostatic adsorption, while that on montmorillonite involved mainly interlayer cation exchange. The findings provide insights into the effects of nanostructures of clay minerals on their TC adsorption performance and highlight the huge potential of allophane as an efficient and inexpensive adsorbent for TC removal.

Todorokite is a common Mn oxide (with a tunnel structure) in the Earth surface environment, and can be obtained by hydrothermal treatment or refluxing process from precursor buserite with a layered structure. Several chemical reaction conditions for the phase transformation from Na-buserite to todorokite at atmospheric pressure were investigated, including temperature, pH, crystallinity of precursor Na-buserite, the amount of the interlayer Mg2+ of the Mg-buserite and clay minerals. The results showed that the conversion rate and crystallinity of todorokite decreased with falling temperature, and Mg-buserite could not be completely transformed to todorokite at lower temperatures (40°C). The poorly crystalline Na-buserite could be converted into todorokite more easily than highly crystalline Na-buserite. Todorokite can be prepared at pH 5–9, but the rate of conversion and crystallinity of todorokite did vary with pH in the order: neutral ≈ alkali > acidic. The conversion rate of todorokite decreased with decreasing interlayer Mg2+ content of the Mg-buserite. The presence of montmorillonite or goethite slowed the formation reaction of todorokite in the refluxing process, and the reaction time was prolonged when the amounts of those minerals were increased.

Soil aggregates consist of sand, silt, and clay size particles. Many of the clay size particles in soils are clay minerals, which actively influence soil behavior. The properties of clay minerals may change significantly as soil particle size decreases to the nanoscale; however, little information is available about these properties for the Ultisols in China. In the present study, the clay mineral components and structural characteristics of four particle-size fractions (i.e., <2000, 450–2000, 100–450, and 25–100 nm) of two Ultisol samples (Ult-1 and Ult-2) were investigated using elemental analysis, X-ray diffraction, Fouriertransform infrared spectroscopy, and thermal analysis. The molar SiO2 to Al2O3 ratios were lower in the nanoscale particle-size fraction (25–100 nm) than in the 450–2000 and <2000 nm fractions. This indicates greater desilicification and allitization of the smaller Ultisol particles. Furthermore, the Fe oxide and Al oxide contents increased and reached a maximum level in the 25–100 nm fraction of the two Ultisols. Goethite was mainly found in the 100–450 nm and 25–100 nm fractions. The dominant clay minerals in the Ultisol 25–100 nm fraction were kaolinite and illite with a small amount of a hydroxy-interlayered mineral in Ult-1 and gibbsite in Ult-2. The kaolinite crystallinity decreased as particle size decreased. The low crystallinity of the kaolinite in the A horizon 25–100 nm fraction was attributed to a reduction in the thickness of coherent scattering domains, as well as to decreases in OH groups and the dimensions of octahedral AlO6 sheets. A determination of the chemical and mineralogic properties of the different size fractions of the Ultisols is important to understand the desilicification and Al and Fe oxide enrichment mechanisms during soil formation. The significance of these results can help to reveal the nanoscale transformations of clay minerals. Analysis of clay mineral compositions in nanoparticles can provide the additional data needed to understand the adsorption and mobility of nutrients and pollutants.

The loss of K-bearing clay minerals has been observed over an 80 y cultivation period in Chinese rice paddies despite the use of NKP fertilizers. Clay mineral determinations were made in flood-irrigated paddies cultivated for 3, 10, 15, 30 and 80 y in clayey (45 wt.%), red soils derived from red Quaternary sediments. Three clay minerals are initially present in these soils: illite-mica, magnesian chlorite and an interstratified mica-aluminous chlorite mineral. This last phase was identified using computer simulations. The K-bearing phases (discrete mica and illite as well as interstratified mica layers) are to a large extent lost while the Fe content decreases in the soil as a whole and increases in the chlorite. The mica component in the mixed-layer mineral decreases also. These changes in clay mineralogy and relative abundance suggest a loss of potassic minerals and an increase in the formation of less siliceous, more ferro-magnesian chlorite. These changes occur over 30 y or less, a rather rapid, irreversible transformation of soil clay minerals. Such loss of potassic minerals renders the cultivation more dependent on fertilizer amendment.

The objective of this study was to design new, functional, nanostructured materials from the abundant clay mineral kaolinite, in spite of development problems with the interlayer chemistry of kaolinite because of its non-swelling properties. A particular goal of this work was to graft, in a controlled way, alkylammonium groups onto the aluminol interlayer surfaces of kaolinite. This was successfully achieved by soft-chemical approaches, more specifically by the melting intercalation process of alkylammonium ionic liquids which were synthesized for this purpose. The resulting nanohybrid materials were characterized chemically and structurally by X-ray diffraction analysis, thermal analysis (TG/DTA), 13C cross polarization magic angle spinning nuclear magnetic resonance spectroscopy, and Fourier-transform infrared spectroscopy. The amount of grafted organic material was quantified from TGA results. Alkylammonium salts with a short alkyl chain were grafted directly using a melting reaction at 180°C under N2 involving the in situ displacement of dimethylsulfoxide (DMSO) from a DMSO-kaolinite pre-intercalate; for longer alkyl chains, the grafting was done in two steps. In the first step, the corresponding amino-alcohol was grafted into the kaolinite by displacement of DMSO from the interlayer space. The second step consisted of quaternarization of the grafted material by reaction with iodomethane or iodoethane.

To solve the problem of the leakage of phase-change materials and expand the application of clay minerals in the field of phase-change materials, illite and paraffin wax were used for the first time as a new binary hybrid core material with silica as the wall material. Phase-change microcapsules were prepared using the sol–gel method and successfully applied in silicone rubber foams. Differential scanning calorimetry showed that the microcapsules had a melting temperature of 62.99°C and a latent heat capacity of 98.24 J g–1, indicating their good heat-storage capacity. Furthermore, the effectiveness of using illite as a filler for preventing paraffin leakage was demonstrated through 200 thermal cycling tests, which demonstrated a leakage rate of only 2.6% compared to 3.2% exhibited by microcapsules without illite.

To investigate the influence of clay mineral microstructures on mechanical properties across varying hydration levels, this study employed molecular dynamics simulations to conduct uniaxial tensile strength tests in three orthogonal directions (x, y, z) using illite, montmorillonite and kaolinite. The moisture content was varied from 0% to 10% in 1% increments and from 0% to 50% in 10% increments. The observations highlight the role of water molecules in disrupting the inherent microscopic atomic structure of clay minerals, leading to diminished stability and a decline in tensile strength. As moisture content increased, there was a pronounced increase in the layer spacing of all three clay minerals, indicative of their hydration expansion behaviour. Concurrently, discernible reductions in both the tensile strength and Young's modulus of the clay minerals were observed.

Fast drying (~60 min) is useful for optimizing production processes by increasing productivity and reducing costs and environmental impacts, especially in red ceramic industries in Brazil. However, suitable clays are necessary and, currently, studies focused on the plastic behaviour of clays with compositions suitable for extrusion, especially for fast drying, are scarce. Therefore, in this study, three different clays from the same mineral deposit were studied for producing clay-based structural products via fast drying. The clays were characterized according to their chemical, mineralogical and thermal properties, particle size, cation-exchange capacity, specific surface area and open pore volume distribution. Ten formulations were developed using a simplex-centroid mixture design of experiments and their plasticity index (PI) values were determined. The response surfaces of the formulations were evaluated according to their PI, while the formation characteristics were determined according to their extrusion workability factor values. Formulations F5 (50.0 wt.% yellow clay and 50.0 wt.% green clay) and F8 (66.6 wt.% yellow clay, 16.7 wt.% grey clay and 16.7 wt.% green clay; PI = 15.5–16.6%) displayed optimal extrusion properties, followed by formulations F7 (33.3 wt.% yellow clay, 33.3 wt.% grey clay and 33.3 wt.% green clay) and F10 (16.7 wt.% yellow clay, 16.7 wt.% grey clay and 66.6 wt.% green clay; PI = 13.8–14.2%), which are within acceptable extrusion index values. Thus, the chosen formulations have significant potential for use in the manufacture of fast-drying red ceramics.

Clay minerals are essential components of soil systems and understanding their role in soil structure and function is critical for soil environmental quality management and sustainable agricultural development. An in-depth study of clay minerals and the development of related materials is essential for a complete understanding and effective management of soil systems. This review is a detailed compilation of relevant studies over the past decade in this area, focusing on an overview of clay minerals and their modified materials and their regulation of soil structure and function. We focus on the direct influence of clay minerals on the physical, chemical and biological properties of soils, such as soil structure, soil fertility, plant growth, soil microbial activity and soil carbon sequestration. Finally, we concluded by summarizing the existing issues with clay mineral materials in soil improvement and by outlining potential future development trends and strategies.

Clay-mineral type and content, bulk mineralogical composition and alteration of bentonite are very important factors for the ultra-long-term stabilization of barriers and backfills in radioactive waste disposal. This study investigates the effects of clay-mineral type and content on the swelling characteristics and permeability of bentonite–sand mixtures with clay minerals using one-dimensional swelling-pressure and constant-pressure permeability tests. The hydraulic conductivity of bentonite–sand–clay mineral mixtures increased with increasing content of non-swelling alteration products of montmorillonite. Furthermore, hydraulic conductivity was comparable to that determined with the Kozeny–Carman equation for a specific surface area, suggesting that hydraulic conductivity may be estimated based on the abundance of expected alteration products of montmorillonite. This study provides a basis for evaluation of the hydraulic conductivity of bentonite–sand mixtures with known quantities of expected alteration products of montmorillonite.

In recent years, experimental and theoretical methods have provided new insights into the size, shape, reactivity, and stability of clay minerals. Although diverse and complex, the surface chemistry of all clay minerals is defined spatially on a common scale of nanometres. This review is organized around the nanoscale architecture of clay minerals examined at several different length scales. The first, and perhaps most important, is the length scale associated with Hbonding in clay minerals. Hbonding interactions define the size and shape of 1:1 phyllosilicates and dominate the surface chemistry of many clay minerals. Structural and surface OHgroups contained within and on the surface of clay minerals provide a type of ‘molecular reporter group’ and are sensitive to subtle changes in their local environment. Examples of OH-reporter group studies in clay minerals, and the spatial scales at which they provide diagnostic information, are examined. The second length scale considered here is that associated with clay–water and clay–organic interactions. Inorganic and organic solutes can be used to explore the surface chemistry of clay minerals. Similar to the use of reporter groups, molecular probes have diagnostic properties that are sensitive to changes in their molecular environment. Clay–water interactions occur at a length scale that extends from the size of the H2O molecule (~0.3 nm) to the larger scales associated with clay-swelling (>10 nm). Similarly, clay–organic interactions are also defined, in part, on the basis of their molecular size, in addition to the type of chemical bonding interactions that take place between the organic solute and the clay surface. Examples illustrating the use of clay–water and clay–organic solute interactions as molecular probes are presented. The largest scale to be considered is that of the particles themselves, with scales that approach micrometres. Recent developments in the synthesis and characterization of ultrathin hybrid films of clay minerals provide complementary information about the nature and distribution of active sites on clay minerals, as well as providing new opportunities to exploit the surface chemistry of clay minerals in the design of functional materials.

An Fe-rich smectite from Ölberg (Germany) was characterized using X-ray diffraction, infrared spectroscopy and analytical electron microscopy. Progressive reduction of the octahedral charge was performed through the Hofmann & Klemen effect at increasing temperatures. The sample was heterogeneous, consisting of two smectite populations. One population, which comprises a minor portion of the sample, is an ‘Fe3+-montmorillonite’ with little or no tetrahedral charge and Fe3+ as the major octahedral cation. The other population, a major constituent of the sample, contains less Mg and more Al than the first one and exhibits some tetrahedral charge. This fraction may be considered as an inter-grade between nontronite (dominant tetrahedral charge) and Fe3+-montmorillonite (dominant octahedral charge). These two populations may occur as separate particles but also as interstratified layers.

Cationic dye, methylene blue (MB), was intercalated into films of expandable clay minerals with low, moderate and high layer charges (saponite, montmorillonite and synthetic expandable mica-fluorohectorite, respectively) . Tilted orientation of the dye cations on the silicate surface was observed by polarized ultraviolet-visible (UV-Vis) transmission spectroscopy. The tilting angle of the dye cations increased with the formation of high-order H-aggregates that absorb light at 525 nm. These dye species were formed only on the surface of fluorohectorite, which has the highest layer charge, and exhibited dichroism. Models for the orientation of MB cations on a clay surface were proposed based on MB orientation models previously reported, and compared with the basal spacings determined by X-ray diffraction.

Brinrobertsite, an ordered, mixed-layered, dioctahedral pyrophyllite-smectite (P/S), occurs in a metabentonite in the Ordovician Nant Ffrancon Formation near Bangor, N Wales. It comprises ~30% of the metabentonite, in association with quartz (~50%) and chlorite (clinochlore; 20%) which replaced glass shards and fine-grained glass matrix. Transmission electron microscopy (TEM) images show sequences of dominant ~24 Å 001 lattice fringes inferred to correspond to 2:1 layers with alternate pyrophyllite-like (low-charge) and smectite-like (higher-charge) interlayers (i.e. R1 ordering). The hk0 diffraction patterns are mostly hexanets with some spotty circles, implying that layers are largely coherently related, but with some turbostratic stacking. Collective data show that d100 = 5.2, b = 9.1, and d001 = 24–25 Å, assuming monoclinic or pseudomonoclinic symmetry. The composition, as determined by energy dispersive spectral analysis, is (Na0.22K0.07Ca0.06)(Al3.81Mg0.08Fe0.08)(Si7.84 Al0.16)O20(OH)4·3.54H2O, as consistent with the sum of the compositions of pyrophyllite-like and smectite-like units. Water content was determined by DTA/TGA analysis. The powder diffraction patterns have a principal peak with d001 = 24.4 Å. Patterns of air-dried and glycol-saturated brinrobertsite, including Na- and Ca-saturated and untreated samples, were modelled satisfactorily as R1-ordered P/S by the program NEWMOD-for-Windows. The unique composition of brinrobertsite relative to R1 IS, which is ubiquitous in metabentonites, was caused by leaching of alkalis and alkaline-earth elements by hydrothermal fluids associated perhaps with a nearby intrusion, as demonstrated by bulk-chemical analyses of the metabentonite. The crystal structure is modelled as having Al/Si distributions symmetrical by reflection across interlayers. This causes all 2:1 layers to be equivalent in having one tetrahedral sheet with little or no Al, and the other with significant Al substitution, giving rise to alternate high- and low-charge interlayers. Geological evidence suggests that brinrobertsite is a back-reacted product of hydrothermal alteration in the sequence: glass → pyrophyllite → brinrobertsite.

Nucleation is much more important for clay minerals than for other authigenic cements as clay crystals are very small, so that a very large number of clay crystals must be nucleated. The role of this difficult kinetic step in the diagenesis of sandstones has not been considered adequately as a ratedetermining process. The relationship between pore-fluid supersaturation and the rate of nucleation of a mineral is very different from the relationship between supersaturation and the rate of crystal enlargement; thus the two processes will act at very different rates. A diagenetic model that predicts claymineral formation but omits the nucleation stage may make unreliable predictions. This may account partially for the discrepancy between numerical simulations of CO2 injection that predict high degrees of reaction between the CO2 and the host rock, and the results of studies of natural analogues that have much lower degrees of reaction.

This work describes the development of potential multi-element slow-release fertilizers obtained by mechanochemical activation of mixtures of kaolinite and ammonium or potassium monohydrogen phosphates. Preliminary results of talc amorphization have also been included. The methodology consists of milling the materials in a high-energy ball mill, where the influence of rotation and time of milling were investigated. The samples were characterized by XRD, FTIR, TGA/DTA, SEM and MAS-NMR. The experimental results explain the slow-release behaviour of the amorphous nanostructured materials in aqueous suspensions, especially the MASNMR spectra, which showed the changes in the chemical environment of the elements analysed. The materials displayed slow-release behaviour for phosphates probably because the aluminium ions in the mineral structure interact more thoroughly with phosphate than potassium or ammonium. Nevertheless, in general, all of the nutrients were released slowly.

The present study examines the characteristics of clay minerals in shale gas reservoirs and their influence on reservoir properties based on X-ray diffraction and scanning electron microscopy. These analyses were combined with optical microscopy observations and core and well-log data to investigate the genesis, distribution characteristics, main controlling factors and pore features of clay minerals of the Lower Silurian Longmaxi Formation in the East Sichuan area, China. The clay mineral assemblage consists of illite + mixed-layer illite-smectite (I-S) + chlorite. This assemblage includes three sources of clay minerals: detrital, authigenic and diagenetic minerals. The lower section of the Longmaxi Formation in the Jiaoshiba area has sealing ability which resulted in abnormal high pressures during hydrocarbon generation which inhibited illitization. Therefore, an anomalous transformation sequence is present in which the mixed-layer I-S content increases with depth. This anomalous transformation sequence can be used to infer the existence of abnormal high pressures. The detrital components of the formation also affect the clay-minerals content indirectly, especially the abundance of K-feldspar. The transformation of mixed-layer I-S to illite is limited due to the limited availability of K+, which determines the extent of transformation. Three types of pores were observed in the shale reservoir rocks of the Longmaxi Formation: interparticle (interP) pores, intraparticle (intraP) pores and organic-matter pores. The clay-mineral content controls the development of intraP pores, which are dominated by pores within clay particles. For a given clay mineral content, smectite and mixed-layer I-S were more conducive to the development of shale-gas reservoirs than other clay minerals.

In this study, we present clay mineral records from a late Pleistocene loess-paleosol sequence in the Ili Basin, Central Asia, and assess their significance for paleoclimatic reconstruction. The results show that the clay minerals are mainly illite (average 60%) and chlorite (28%), with minor kaolinite (9%) and smectite (3%). Illite was of detrital origin with no obvious modification to its crystal structure. Increases in illite content in the loess are ascribed to wind intensity rather than pedogenesis. High proportions of illite in the clay fraction, and of muscovite in the bulk samples of the paleosol units, may lead to an overestimation of the weathering intensity. Kaolinite was likely inherited from the sedimentary rocks, while chlorite might have been inherited from both sedimentary and metamorphic rocks. The paleoclimatic signals of kaolinite and chlorite were unclear, due to reworking by both fluvial and eolian systems. Smectite was more likely formed by the transformation of biotite and illite, and its variation in the loess sequence was also controlled by wind intensity; this was largely due to aggregation and is unlikely to reflect moisture changes. Although the interpretation of paleoclimate evolution may contain some uncertainties, clay mineralogy does provide the possibility of tracing dust provenance.

Norflurazon [4-chloro-5-(methylamino)-2-(α,α,α-trifluoro-m-tolyl)-3(2H)-pyridazinone] decreased the total chlorophyll content and the chlorophyll a/b ratio of nine plant species. Grain sorghum [Sorghum bicolor (L.) Moench.], wheat (Triticum aestivum L.), and sicklepod (Cassia obtusifolia L. ♯3 CASOB) were the most susceptible plants, and cotton (Gossypium hirsutum L.) was the most tolerant. The soil properties most closely correlated with norflurazon activity were organic matter content and clay component, not clay content. A high treatment rate was necessary for effective control of plant growth in soil high in organic matter content and high in montmorillonite or vermiculite. Approximately twice as much norflurazon was required to reduce chlorophyll content 50% when applied to the soil preplant incorporated as was required when applied preemergence.

Changes in paleoenvironments over the last 17,500 yr have been documented by a high-resolution clay mineralogy and grain–size records of Core KNG5 from the northern slope of the South China Sea. Our results indicate that clay minerals are mainly from the Pearl River from 17,500 to 12,500 cal yr BP, and the South China Sea modern current system began to form since 12,500 cal yr BP, as a result, Taiwan turns to be the major contributor of clay minerals after 12,500 cal yr BP. Two grain-size populations with high variability through time were identified in the 13–28 μm and 1–2.2 μm grain-size intervals. The 1–2.2 μm grain-size population are mainly controlled by provenance supply and current transport. The 13–28 μm grain-size fraction could be controlled mainly by the sea-level change. The 1–2.2 μm grain-size population record demonstrates that East Asian Summer Monsoon intensity generally follows changes in insolation and that the response is similar for a large area of China and other northern low-latitude records, implying the globality of the monsoon evolution since Holocene. The anomalous environmental conditions in the northern South China Sea may imply intensified ENSO activity during the late Holocene.