Kaolin Layer Research Articles

High-performance intercalated composite solid electrolytes for lithium metal battery

Composite solid electrolytes (CSEs) combining the advantages of both inorganic and organic solid-state electrolytes, are expected to become the most promising solid electrolyte owning to their favorable interfaces with electrodes. However, low room-temperature ionic conductivity restricts the application of CSEs in lithium metal batteries. Herein, we design an intercalated CSE based on poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP). Kaolin (2SiO2-Al2O3-2H2O), employed as an oxidative flame retardant, imparts nonflammability to the material. The polar molecule dimethyl sulfoxide (DMSO) is inserted between the kaolin layers as the pre-intercalation treatment, serving as an organic additive within the PVDF-HFP based SPE. The intercalated structure of CSE provides rapid Li+ transport channels, resulting in a high ionic conductivity (8.58 × 10−4 S cm−1) and large Li+ transference number at room temperature. The Li||Li symmetrical cell with prepared CSE exhibits outstanding cyclic stability of over 1400 h at a current density of 0.2 mA cm−2 for the capacity of 0.2 mAh cm−2. Moreover, the assembled Li||LiFePO4 cell delivers a high initial capacity of 140.5 mAh g−1 with a capacity retention of 81.2 % after 800 cycles at 0.5 C. In this paper, we present a novel approach for constructing high-performance solid-state lithium metal batteries.

Biofouling metallic membranes in algae-laden water treatment: Performance, mechanism, and the role of the pre-coated layer

Biofouling is the primary factor limiting the performance of ultrafiltration membrane systems in drinking water treatment. This study systematically investigated the application effect and biofouling behavior of metallic membranes in the treatment of algae-laden water and evaluated the mitigation effect and mechanism of the pre-coated layer on biofouling. The findings demonstrate that the metallic membrane can almost completely remove algal cells (>99.99 %) and macromolecular organics in EOM (Extracellular Organic Matter) but has limited effects on micromolecules, such as humic acid and fulvic acid, in EOM (47.29 % and 17.72 % for TOC (Total Organic Carbon) and UV254, respectively). During direct filtration, algal cells can form a cake layer in front of the metallic membrane to reduce the membrane flux, whereas the presence of EOM makes the cake layer denser and hastens the decline in flux. In addition, irreversible membrane fouling is exasperated in acidic or Ca2+-rich solutions, whereas an increase in algal cell particles exacerbates reversible membrane fouling, causing a faster decline in flux. According to the membrane mechanism analysis, the fouling mechanism ranges from complete pore blocking to standard and intermediate blocking throughout the direct filtration process. In contrast, the application of a pre-coated kaolin layer significantly decreases membrane fouling compared with direct filtration. The pre-coated layer can prevent direct contact between the foulants and the membrane surface during the early stages of filtration, resulting in a delayed complete pore blockage stage. On the other hand, XDLVO (Derjaguin-Landau-Verwey-Overbeek) data reveals that the force between kaolinite and algae (−102.323 mJ/m2) is greater than that between kaolinite and metallic membranes (−61.424 mJ/m2), resulting in better backwashing efficacy. These findings support the use of metallic membranes in drinking water treatment.

Macroencapsulated carbonate eutectic salt phase change material with high durability for high temperature heat storage

Carbonate eutectic salt is a kind of promising phase change material (PCM) for high temperature thermal storage application, which unfortunately constrained by its significant corrosiveness and low thermal conductivity. The encapsulation of salt PCM with stable inorganic shell to make capsules of millimeter to centimeter size can solve these problems, which is unfortunately technologically difficult to be prepared. In this work, a double shell strategy was proposed to coat the salt macrospheres with inner expanded graphite (EG) layer and outer ceramic kaolin layer. All raw materials were powders, which were shaped to spheres and capsules with the help of organic binder, pressed by isostatic pressing and finally sintered at high temperature for capsule formation. In a comparative case, the single kaolin shell coated capsules leaked when they were heated over the melting point of salt since that the relatively porous kaolin shell could be infiltrated with liquid salt because of their good wettability. However, the double shells coated capsules could prevent the leakage of salt since that the inner EG layer was repellent to liquid salt. The double shells coated macrocapsules showed good stability and high durability for long-term use, as confirmed by a cycling test over 300 cycles. The eutectic salt (Na2CO3/Li2CO3 = 57/43 in weight) core PCM presented a high latent heat of 313 J/g and melting point of around 498 °C. The macrocapsules with a size of 10@1.5@3 showed a high heat storage capability of 380 J/g in a temperature range of 400–600 °C. The macroencapulated carbonate eutectic salt with good heat storage capability and good thermal cycling durability can be extensively used for high-temperature heat storage applications in solar thermal power plant and industrial waste heat recovery.

The influence of goat crossbreeding (Arbia and Saanen) on the cheese yield in Algeria, Djelfa: a preliminary study

Goat's milk is an excellent source of nutrients because its composition is the closest to that of human milk. This work was concerned with the study of the influence of crosses between local (Arbia) and imported (Saanen) goat breeds raised in the Djelfa region on the yield of fresh cheese made from the milk of two goats. The cheese was made enzymatically using two enzymes of animal origin represented by the coagulating extract of the kaolin layer of chickens as well as bovine rennet, which were applied to goat milk. The obtained results showed that the cross goats cheeses presented better results compared to those of Arbia goats, where we noted cheese yields of 19.52 and 15.41, respectively, for the cheeses of cross goats and Arbia goats coagulated by the layer extract of Kaolin; likewise, the use of rennet confirmed a superiority of the cheese yield of around 17.62 for the product of crossbred goats against 14.45 characterizing that of Arbia goats.

Penetration of plasma jet into porous dielectric layer: confirmed by surface charge dissipation of silicone rubber

The penetration of plasma in the porous structure is important for its application in plasma catalysis, plasma medicine, etc. In this paper, the penetration of plasma species in the porous kaolin layer was investigated. The silicone rubber was chosen as a probe and the inorganic porous dielectric layer was constructed with granular kaolin coated on the surface of silicone rubber. AC and pulsed plasma jets were applied to the silicone rubber, and the surface charge dissipation of bulk silicone rubber was measured to characterize the changes of surface property caused by the plasma penetration. The results showed that plasma could penetrate the porous dielectric layer on the silicone rubber and interact with the surface of silicone rubber, thus accelerating the surface charge dissipation of the bulk silicone rubber. The increase of shallow traps and surface conductivity after plasma treatment was the main reason for the acceleration of surface charge dissipation. The surface charge dissipation is enhanced with the increase of treatment time and the generating voltage of plasma. The surface charge dissipation declined for silicone rubber with a thicker kaolin layer due to the blocking of the kaolin layer on the interaction of plasma and the silicone rubber. For the same kind of plasma, the charge dissipation rate was linearly related to plasma dose which was represented by the energy density of plasma applied on the coated silicone rubber. At the same energy density, the surface charge dissipation of silicone rubber after pulsed plasma treatment was faster than that of AC plasma.

STRUCTURAL AND LITHOLOGICAL MODEL OF THE SHABERIVSKE DEPOSIT OF ELUVIAL KAOLIN

Data on the zonation and material composition of the weathering crust within the Shaberivkske deposit of the eluvial kaolin, which is located in Baranivka district of Zhytomyr region and is the part of the Dubrivsko-Khmelivsky kaolin district of Volyn kaolin subprovince. The minerals of the Shaberivske deposit are normal and alkaline kaolins, which were formed due to the weathering of granitoids. Some areas of conditioned kaolin are present in the upper part of the weathering crust of gneisses. On the basis of coordinates and the description of wells the relief of a sole and a surface of a deposit of eluvial kaolins, and also lateral change of their thickness is investigated. Layer-like kaolin deposits have been found to have complex morphology and internal structure. It was established that the greatest thicknesses of kaolins are confined to depressions in the topography of their soles. The maximum thicknesses of kaolin are inherent in the areas of the weathering crust formed by the weathering of granites and migmatites. Based on the results of well testing and chemical analysis of kaolins, the distribution of the contents of Al2O3, Fe2O3, TiO2 oxides and the index of whiteness along the lateral and vertical sections of kaolins was investigated. It was established that in the greater part of the deposit, beyond the lateral, there is a direct correlation between the average content of Al2O3 and TiO2 and the reverse – between the content of Fe2O3 and TiO2 and between Al2O3 and Fe2O3. In the vertical section of the kaolin layer, the distribution of oxides of iron, titanium, aluminum and whiteness index is uneven. As a rule, these indicators do not always correlate with each other. In general, the deposit is characterized by an uneven sole and surface with elevations and depressions, an uneven lateral distribution of the thickness of the kaolin deposit. The lateral distribution of the average contents of iron oxides, titanium and the whiteness index, as well as the vertical distribution of the contents of iron oxides, aluminum, titanium and the whiteness index, is uneven. It was found that the geological heterogeneity of the rocks of the crystalline foundation was reflected in the structure and material composition of the weathering crust and, accordingly, in the structure (character of the relief of the sole and surface, thickness) and quality parameters (whiteness, harmful impurities) of eluvial kaolins.

Oxidative Torrefaction of Sunflower Husk Pellets in the Kaolin Layer

The aim of this work was to experimentally study the oxidative torrefaction of sunflower husk pellets in a kaolin layer. The kaolin layer was used to limit oxygen access to the biomass to prevent it from self-ignition. The changes in mass yield, volatiles, ash content, high calorific value (HHV), hydrophobicity, and morphology of the biomass were investigated at torrefaction temperatures 240, 260, and 280 °C, process durations 30–60 min, and kaolin layer heights 3–5 cm. The Van Krevelen diagram and the CHO index based on atomic C, H, and O data were also used for explanation. As a result of oxidative torrefaction in the kaolin layer, the atomic O/C and H/C ratios decreased and the sunflower husk position shifted towards peat. During torrefaction at 260 °C for 1 h (kaolin layer height of 3 cm) the O/C decreased by 26.9% and the H/C by 18.8%. Reducing the height of kaolin layer, as well as increasing the temperature and time of torrefaction, led to a decrease in mass yield and an increase in ash content. Torrefaction increased the FC/VM ratio from 50 to 225%, which improved the fuel properties of the biomass. A 16.5% increase in HHV was achieved compared to untreated sunflower husk. As the torrefaction intensity increased, drier fuel was formed, the moisture absorption of which decreased from 213 to 25.8%.

Kaolin/silica/mullite fiber multilayer membranes for high-efficiency gas filtration

At present, the ultra-fine particulate matter (UPM) with diameter less than 1 μm cannot be filtered efficiently and precisely by the current filtration materials, which leads to the UPM being the main source of air pollution. Here, we prepared Kaolin/silica/mullite fiber multilayer membranes (KSFMs) by a simple vacuum filtration. The UPM filtration efficiency has been significantly improved by the nanoporous kaolin layer coated on the support body constructed by Si-sol bonded mullite fiber. By controlling the size and amount of kaolin particles in the mixed suspension, the structure and filtration efficiency and filtration resistance of the multilayer membranes are optimized. The multilayer membranes exhibit high UPM filtration efficiency with 99% for 0.3 μm and low filtration resistance (lower than 70 Pa at 0.3 m/s).

Facile preparation of chitosan modified magnetic kaolin by one-pot coprecipitation method for efficient removal of methyl orange

Chitosan modified magnetic kaolin (CS/kaolin/Fe3O4) composite was prepared by a facile one-pot coprecipitation method and used for the removal of methyl orange (MO) from aqueous medium. Under alkaline condition, Fe3O4 nanoparticles were deposited on the kaolin layer by in-situ growth method and chitosan was deposited on the kaolin layer by pH-precipitation method. With the modification of CS, adsorption sites for anionic species were introduced onto the adsorbent. The prepared CS/kaolin/Fe3O4 could remove more than 94 % of MO and showed a high saturated adsorption capacity of 349.7 mg/g. The adsorption process was controlled by film diffusion and well described by Langmuir model. The thermodynamic studies indicated that the adsorption process was exothermic in nature. Furthermore, the adsorbent exhibited satisfactory recycle ability. The results suggested that the modification with CS broadened the application scope of kaolin in anionic species removal and the CS/kaolin/Fe3O4 composite could be a promising adsorbent for wastewater treatment.

Propagation and coalescence of en-echelon cracks under a far-field tensile stress regime: An experimental study

Experiments were carried out to observe the variation in propagation and linkage of parallel en-echelon cracks with varying orientation of the crack array and different relative position of the cracks within the array in an extensional regime. Two-layered analogue model, with a basal layer of pitch overlain by a layer of kaolin paste was used in the experiments. En-echelon cracks were pre-cut within the kaolin layer maintaining specific geometrical parameters of the cracks (e.g., length, centre spacing, separation) in such a manner that there was a weak (though not negligible) local tip-induced stress favouring curvature of adjacent crack tips towards one another. The results obtained were matched with natural pattern of linkage of veins, rift basins and spreading ridges, as described in the relevant literature. The experimental results showed that the final pattern of linkage between the cracks was a result of initial deflection of crack tip from its plane due to combined effect of local and far-field stress. When the deflection of tip from the crack plane was between \(0{^{\circ }}\) to \(45{^{\circ }}\), a ‘tip to wall’ linkage took place between adjacent cracks isolating a rhombohedral area in the interaction zone. The resultant structure could be geometrically comparable to a micro-plate-like structure isolated due to linkage of ridge segments initially forming an overlapping spreading centre (OSC). When the deflection of tip from the crack plane was greater than \(45{^{\circ }}\), a ‘tip to tip’ linkage between adjacent cracks took place resulting in a structure similar to a transform fault between spreading ridges and or rift basins. When effect of the remote stress opposed the tip induced stress, no linkage took place between the adjacent cracks, and finally the tips propagated straight along a plane perpendicular to the remote extension direction.

Assessment of kaolin resources using electrical resisitivity method in part of Ekiti state southwest Nigeria

Consistent building of mineral database is very crucial to ascertaining the natural resources potential of a nation which may be valuable towards its development. Essentially, the study aimed at establishing the spatial extent and average thickness occurrence of the kaolin deposit in part of Ikere-ekiti, south western Nigeria using geophysical method. Seventy seven VES measurements and two parametric soundings were made. The data were processed and interpreted using IPIwin - a forward modelling software. The evaluated geoelectric parameers were used for the generation of maps(2D and 3D) depicting the mineral occurrence variation. Three-five subsurface layers were depicted within the study area with layer resistivity ranging from 1 to 7250 ohm-meters and layer thickness that range from 0.2 to 48 meters. Layers with a resistivity range of 1-150 ohm meters have been identified as the kaolin layers. Average thickness of the kaolin layer range from 0.8 to 46 meters. Overburden thickness were found to range from 0.3 to 44 meters.The lateral extents of the kaolin occurrence in the study site have been delineated by electrical geophysical sounding method. The obtained information may serve as database for detail exploration and exploitaion programme towards harnessing the reosurces.

New insights on mineralogy and genesis of kaolin deposits: The Burela kaolin deposit (Northwestern Spain)

The Burela deposit is the largest kaolin deposit in Spain, mined for more than 50years, the product being mainly used for porcelain. Kaolin is dominantly associated with Lower Cambrian felsites, interbedded with quartzites, micaschists and metapelites (Cándana Series), and was strongly folded during the Variscan orogeny. Kaolin layers were ductile and incompetent materials among more competent ones, producing many slides with a diastrophic appearance. Consequently, kaolin outcrops are morphologically very variable– i.e. pockets – and interlayered between metapelites and/or quartzites, resulting in complication for prospection and mining.The kaolin consists mainly of kaolinite, tubular halloysite, and spherical allophane along with quartz and minor illite. The content of kaolin minerals reaches up to 90% in the finer fractions (<2μm and <1μm).Geochemical analyses of trace and REE show a close relationship between kaolin and associated rocks. Two kaolin types can be differentiated: (i) massive, associated to felsite; and (ii) related to metapelite. A temperature range from 15 to 35°C, with an average of approximately 28°C was calculated on the basis of the isotopic signatures (δ18O, δD) for the kaolin materials. This scatter suggests that if continental weathering was involved in the kaolin formation on the lower side of the temperatures, it was not the only process, especially for kaolin associate with felsites and metapelites. The higher temperatures are indicative of a hydrothermal auto-metamorphic alteration, followed by a folding of the series that induced an apparently chaotic kaolin distribution with a combined continental weathering superimposed on the previous low-temperature hydrothermal felsite transformation.

Sediment-Hosted Kaolin Deposit from Çakmaktepe (Uşak, Turkey): its Mineralogy, Geochemistry, and Genesis

AbstractBecause of their geochemical properties, the Çakmaktepe (Uşak) kaolin deposits have been considered as primary. New sedimentological, mineralogical, and geochemical data suggest that the Çakmaktepe kaolins are secondary deposits of sedimentary processes after hydrothermal alteration of the source rocks. The kaolins in the Çakmaktepe deposit were formed from the hydrothermal alteration of calcalkaline Karaboldere volcanics (KBV). The kaolinized materials were then reworked and accumulated in a lacustrine basin. The argillic alteration zones were associated with faults, and lateral zonation of minerals was observed in the KBV. Smectite was the major phyllosilicate in the ‘outer zone’. The alteration mineralogy of the ‘inner zone’ was similar to that of the Çakmaktepe kaolins and consisted mainly of kaolinite with minor amounts of smectite and alunite. The trace-element abundances in the kaolinized volcanics and the Çakmaktepe kaolins indicated hypogene conditions. The δ18O values of the Çakmaktepe kaolins ranged from 0.2 to 5.92%, which indicated that the Çakmaktepe kaolinites were formed at temperatures between 92 and 156°C, and the δD values ranged from −91.68 to −109.45‰. The irregular edge-to-face morphology, the variation in grain-size, a few broken crystals of kaolinite, the deficiency of dissolution-replacement and crystallization mechanisms, and the the low sphericity, very angular, and poorly sorted quartz crystals in the kaolins all result from transport processes. The sedimentary structures, including trough cross-lamination, tool marks, and load casts, indicate transportation by turbulent waters and deposition of kaolin layers in a shallow lake.

Viscosity of Delaminated Kaolin through Intercalation of Potassium Acetate into its Layers

In this work, the effects of potassium acetate(KAc) intercalation on particle size and viscosity of delaminated kaolin were investigated. It was found that the KAc intercalation in the presence of a small quantity of water possessed more efficiencies. Through inserting kaolin layers with potassium acetate, the average particle size of delaminated kaolin decreased from 5.48μm to 2.21μm, moreover, the viscosity at solid content of 60% decreased distinctly, from more than 10000 mPa•s to less than 200 mPa•s. The result also indicated that the calcined kaolin was delaminated by KAc efficiently, the particle size decreased greatly without harming to the dispersion ability of kaolin suspension.

Characterization of ink pigment penetration and distribution related to surface topography of paper using confocal laser scanning microscopy

The penetration of ink into paper affects the final appearance of printing and the amount of ink usage. In this work, UV-curing fluorescent rose ink was used to investigate the penetration and distribution of the ink pigments as well as their correlation with the surface topography of paper. The ink penetration and distribution were characterized with Confocal Laser Scanning Microscopy (CLSM), whereas the microstructure of the paper surface was observed using Atomic Force Microscopy (AFM). The AFM results showed that the surface of the paper coated with kaolin layer was the smoothest among the samples. Also the pore size of the calcium carbonate coating layer was smaller than that of the kaolin coating layer. Meanwhile, the pore size distribution of the calcium carbonate coating layer appeared to be relatively narrow, compared with other samples. The results of CLSM images indicated that the depth, size, and arrangement of pores affected the penetration depth and distribution of ink pigment on coated paper. The large pores led to deeper penetration of ink pigments, and uniform ink absorption occurred when the pore distribution was uniform. The UV-curing ink pigments not only set on the surface of the uncoated paper, but also they penetrated into the paper interior and adhered to the fiber surface.

Kaolin polytypes revisited ab initio at 10 GPa

Based on systematic ab initio exploration, we reported last year that two new interlayer translations for kaolinite, − a /3 and ( a + b )/3, allowed a new family of kaolin polytypes under moderate pressure. Both translations place each silicon atom of a kaolin layer on top of a hydroxyl group from the kaolin layer below, resulting in a triangular dipyramidal fivefold coordination to all silicon atoms. The predicted − a /3 translation has since been independently observed experimentally at ~7 GPa, as kaolinite III phase, by compression of a natural kaolinite (Keokuk, Iowa). Encouraged by that verification, we extend here to the entire kaolin system both translations we had predicted for kaolinite. Based on calculated enthalpies and cell volumes for models optimized with ab initio density functional theory (DFT) calculations under pressure, we report three main results. First, we predict the existence of a kaolinite IV phase at a pressure not higher than 60 GPa and its likely crystal structure. Second, we predict three novel high-pressure crystal-structure models for nacrite, ranked by their enthalpy value, one of which is likely to be observed at about 10 GPa. Finally, three other novel and ranked high-pressure crystal-structure models are reported for dickite. Our results from interpretation of ab initio DFT calculations should guide experimental studies and facilitate their interpretation.

Carbon chemistry and groundwater dynamics at natural analogue site Ruprechtov, Czech Republic: Insights from environmental isotopes

Hydrological, geochemical and environmental isotope data from groundwater wells at Ruprechtov natural analogue site were evaluated to characterize the flow pattern and C chemistry in the system. The results show that water flow in the Tertiary sediments is restricted to 1–2 m thick zones in the so-called clay/lignite horizon with two different infiltration areas in the outcropping granites in the western and south-western part of the investigated area. Generally the flow system is separated from the underlying granite by a kaolin layer up to several tens of metres thick. Differences in stable isotope signatures in the northern part of the site indicate very local connections of both flow systems via fault zones. The observed increase of δ13C values and decrease of 14C activities in dissolved inorganic C during evolution of the groundwater from the infiltration area to the clay/lignite horizon was modelled using simple open- and closed-system models as well as an inverse geochemical model (NETPATH), which included changes in other geochemical parameters. The results from both types of models provided some insights into timescales of groundwater flow, but mainly revealed that additional sources of C are active in the system. These are very likely biodegradation of dissolved and sedimentary organic C (DOC and SOC) as well as the influx of endogenous CO2. The occurrence of microbial degradation of SOC in the clay/lignite layers is indicated by the increase of biogenic DIC, DOC, and phosphate concentrations. The impact of microbial SO4 reduction on this process is confirmed by an increase of δ34S values in dissolved SO42- from the infiltration area to the clay/lignite horizon with an enrichment factor of 11‰. The very low DOC concentrations of 1–5 mg C/L in the clay/lignite horizon with an organic matter content up to 50% C is probably caused by very low availability of organic matter to the processes of degradation and DOC release, demonstrated by extraction experiments with SOC.

The temporal evolution of pedogenic Fe–smectite to Fe–kaolin via interstratified kaolin–smectite in a moist tropical soil chronosequence

Fe-rich kaolins in soils of a terrace chronosequence in the moist tropics of Costa Rica are derived from the transformation of smectite via interstratified kaolin–smectite (K–S). The smectite is ferruginous smectite (10.1 ± 3.7% Fe 2O 3) that forms early during pedogenesis from basaltic–andesitic parent material and is the dominant mineral in < 5 ka soils. Interstratified K–S with 60–90% kaolin layers is the dominant mineral in 10 ka soils, while 125 ka soils contain a physical mixture of Fe–kaolin (6.0 ± 1.9% Fe 2O 3) and K–S with > 80% kaolin layers. K–S is indicated by X-ray diffraction (XRD) data that reveal asymmetric 001 peaks between 17.5 and 17.8 Å (ethylene glycol-solvated) and irrational 00 l peaks that exhibit Méring-like behaviour, and by transmission electron microscopy–analytical electron microscopy (TEM–AEM) data that document single crystals of K–S with compositions intermediate to those of end-member crystals of smectite and kaolin. Lattice fringe images from high resolution TEM (HRTEM) show lateral transitions from smectite layers to kaolin layers via a cell-preserved, layer-by-layer transformation mechanism, and changes in the compositions of tetrahedral and octahedral sheets, notably decreases in octahedral Fe and Mg and tetrahedral Al, indicate that the reaction is accompanied by localized dissolution of smectite 2:1 layers that proceeds laterally. Fourier transform infrared (FTIR) and XRD data indicate that kaolin layers are a mixture of kaolinite and halloysite, and differential thermal analysis–thermogravimetry (DTA–TG) combined with data from XRD and TEM–AEM indicate the development of Al-hydroxy-interlayers in K–S with time. The soil-forming factors that appear to control formation of K–S are (1) basaltic–andesitic parent that produces high concentrations of base cations and Si in young (< 5 ka) soils, and (2) moist tropical climate (3085 mm MAP, 27.3 °C MAT) with a short dry season that fosters formation of smectite in < 5 ka soils. By 125 ka, soils are dominated by Fe–kaolin and K–S, helping to constrain reaction rate. The relatively high cation exchange capacity (CEC) of the Fe-rich kaolins (18 ± 12 cmol c/kg) relative to many tropical kaolinites is attributed to small amounts of octahedral Mg and tetrahedral Al inherited from precursor ferruginous smectite layers. Accordingly, the origin (from smectite) and relatively high CEC of the K–S and Fe–kaolin has potentially important implications related to clay mineral reactions and elemental cycling in moist tropical soils.

Ab initio exploration of layer slipping transformations in kaolinite up to 60 GPa

Kaolinite transformation into other kaolin layer polytypes under pressure is of interest in many materials or industrial applications as well as in seismology. Assuming that this transformation would involve a layer slipping mechanism similar to that of the reversible dickite to HP dickite transformation observed at 2 GPa, the authors undertake here an exploratory study of corresponding transformations. Neglecting contributions from third neighbour layers and beyond to differences in total energy, it is concluded that 19 stacking models of kaolin layers are prime candidates for the lowest enthalpy under moderate pressure. Ab initio compression of those models up to 60 GPa shows that, although several of the above models come close to kaolinite in enthalpy, kaolinite probably survives compression up to ∼ 12 GPa. Beyond this pressure, a new family of kaolin polytypes with lower enthalpy than kaolinite, resulting from a different layer slipping mechanism, is spontaneously produced by ab initio compression. The silicon coordination transforms gradually from tetrahedra to triangular dipyramids with no drastic change to layer architecture. Corresponding distinguishable transformations between adjacent layers resulting from this new coordination are translations -a/3 and (a+b)/3, which are not possible translations at zero pressure. Numerous low enthalpy new polytypes based on those translations are possible. Compression to 20 GPa of two kaolin polytypes among the 19 models created above have spontaneously resulted, one into the repeated (a+b)/3 polytype with symmetry Cm, and the other one into the repeated -a/3 polytype with symmetry P1. As both models derive from kaolinite by a layer slipping mechanism and have very similar enthalpies, both considerably lower than that of kaolinite, those phases and the polytypes from the same family are prime candidates for post-kaolinite phases beyond ∼ 12 GPa.

Non-invasive monitoring of fouling in hollow fiber membrane via UTDR

Fouling is the most critical problem associated with membrane separations in liquid media. But it is difficult to control the inevitable membrane fouling because of its invisibility, especially on the inside surface of hollow fiber membranes. This study describes the extension of ultrasonic time-domain reflectometry (UTDR) for the real-time measurement of particle deposition in a single hollow fiber membrane. A transducer with a frequency of 10 MHz and polyethersulfone hollow fiber membranes with 0.8 mm inside diameter (ID) and 1.2 mm outside diameter (OD) were used in this study. The fouling experiments were carried out with 1.8 g/L kaolin suspension at flow rates 16.7 and 10.0 cm/s. The results show that UTDR technique is able to distinguish and recognize the acoustic response signals generated from the interfaces water/upper outside surface of the hollow fiber, lumen upside surface/water, water/lumen underside surface and lower outside surface/water in the single hollow fiber membrane module in pure water phase. The systemic changes of acoustic responses from the inside surfaces of the hollow fiber in the time- and amplitude-domain with operation time during the fouling experiments were detected by UTDR. It is associated with the deposition and formation of the kaolin layer on the inside surfaces. Further, the acoustic measurement indicates that the deposited fouling layer is denser on the lumen underside surface of the hollow fiber than that on the lumen upside surface as a result of weight. Moreover, it is found that the fouling layer grows faster on the inside surface of the hollow fiber at a flow rate of 10.0 cm/s than that at 16.7 cm/s due to the lower shear stress. The fouling layer formed is thicker at a flow rate of 10.0 cm/s than that at 16.7 cm/s. The flux decline data and SEM analysis corroborate the ultrasonic measurement. Overall, this study confirms that UTDR measurement will provide not only a new protocol for the observation of hollow fiber membrane fouling and cleaning, but also a quantitative approach to the optimization of the membrane bioreactor system.