Colloidal Clay Research Articles

New insight into hydrodynamic and cake erosion mechanism during rotating-disk dynamic microfiltration of concentrated bentonite suspensions at different salinity conditions

The present study investigates the dynamic filtration of concentrated slurry fluids containing colloidal clay particles under different salinity, shear flow, and flux rate conditions. The dynamic filtration study was carried out by the filtration cell equipped with a rotating disk to apply shear stress over the membrane surface. A 3D CFD simulation has been implemented to model the hydrodynamic flows inside the filtration cell to obtain the wall shear stress (WSS) on the membrane surface at different disk rotation speeds. The thickness and surface patterns of fouling were captured utilizing a surface profilometer. First, the cake-filtration model was used to predict the flux rate and fouling thickness under static filtration. Then, the corresponding filtrate flux at stabilized cake thickness under shear flow conditions was calculated. The results showed that increasing the suspension salinity causes the reduction of rheological properties and increases the cake permeability due to the compression of the electrical double layer around clay particles. The results also revealed that increasing the flux rate condenses the cake and reduces erosion. Finally, a model for predicting the rate of fouling removal during dynamic filtration was developed using nonlinear regression. The results showed that a sigmoidal function with appropriate accuracy (R2>0.96) could describe fouling thickness patterns with WSS. Moreover, the mean cake thickness decreased by 80% as the WSS increased from 0 to 2 Pa, where the rotating disk speed increased from 0 to 500 rpm.

Rheological behaviour of dilute soil-water mixtures: role of interactions from colloidal and non-colloidal particles

The rheological behaviour of dilute soil-water mixtures, such as hyperconcentrated mixtures and interstitial fluids in debris flows, can significantly affect the flow dynamics of flow-type landslides. These mixtures often contain clay, silt, and sandy particles. Despite great efforts, the effects of soil type and texture on the rheological behaviour of these mixtures are still not well understood. To tackle this issue, an experimental investigation is carried out using a narrow-gap rotational viscometer to evaluate the interactions from colloidal (clay) and non-colloidal (silt and sand) particles and possibility of flow regime change. The flow curves of dilute mixtures are determined with and without a dispersion agent. A functional curve between spindle speed and Reynolds Number or Froud Number is proposed to distinguish different flow regimes in the narrow-gap rotational viscometer. Results demonstrate the applicability of the Bingham model for describing the bulk rheological behaviour of dilute mixtures. The mixture viscosity governs the contribution of the hydrodynamic effects from all particles, while the yield stress governs the contribution of the colloidal effects from colloidal particles. Results also reveal that for dilute mixtures containing particles up to medium sand size, the main cause of the flow regime change is the flow boundary conditions.

Pattern selection in radial displacements of a confined aging viscoelastic fluid

Intricate fluid displacement patterns, arising from the unstable growth of interfacial perturbations, can be driven by fluid viscoelasticity and surface tension. A soft glassy suspension ages, $i.e.$ its mechanical moduli evolve with time, due to the spontaneous formation of suspension microstructures. The shear and time-dependent rheology of an aging suspension can be exploited to generate a wide variety of interfacial patterns during its displacement by a Newtonian fluid. Using video imaging, we report a rich array of interfacial pattern morphologies: dense viscous, dendritic, viscoelastic fracture, flower-shaped, jagged and stable, during the miscible and immiscible displacements of an aging colloidal clay suspension by Newtonian fluids injected into a radial quasi-two-dimensional geometry at different flow rates. We propose a new parameter, the areal ratio, which we define as the fully-developed pattern area normalized by the area of the smallest circle enclosing it. We show that the natural logarithms of the areal ratios uniquely identify the distinct pattern morphologies, such that each pattern can be segregated in a three-dimensional phase diagram spanned by the suspension aging time, the displacing fluid flow rate, and interfacial tension. Besides being of fundamental interest, our results are useful in predicting and controlling the growth of interfaces during fluid displacements.

Efficacy of sustainable polymers to mitigate the negative effects of anisotropic clay minerals in flotation and dewatering operations

The depletion of high-grade ore resources and ongoing demand for mineral products has led to an increase in the exploitation of low-grade and complex ores, which often contain colloid clay particles (e.g., kaolinite) that are detrimental to flotation and dewatering unit operations in mineral processing. During flotation, clay particles increase the pulp viscosity and produce a slime coating by adsorption on the surface of valuable minerals resulting in the reduction of the flotation grade and recovery. Clay particles are also suspended in mine tailings that can cause dewatering challenges in flocculation operations. The clay mitigation strategies employed during flotation (dilution and use of dispersants) and dewatering (addition of flocculants) involve reagents from non-renewable sources that can have deleterious consequences on the environment and aquatic biota. As part of an effort to find environmentally benign reagents, we evaluated the performance of six sustainable polymers (protein- and polysaccharide-based biopolymers) for their potential as dispersants and flocculants of kaolinite clay particles. Their effectiveness was assessed via chalcopyrite froth flotation, settling, and turbidity tests. Zeta potential, adsorption isotherm by total organic carbon, and X-ray scattering tests were also conducted to understand the interactions between biopolymers and kaolinite mineral surfaces. At pH 7 and 10, the anionic polysaccharide pectin showed promising dispersant efficiency in flotation and the cationic protein protamine significantly improved kaolinite flocculation in dewatering operation. Outcomes of this investigation demonstrate that commercially available sustainable polymers or “biopolymers” have a significant potential to use to mitigate the negative effects of clay particles in minerals processing to reduce environmental issues arising from inorganic and synthetic organic reagents from non-renewable sources.

Functionalizing graphene with clay nanosheets as a protein carrier

In this work, a clay colloid of sodium montmorillonite nanosheets was uniquely selected as a functionalizing reagent to functionalize graphene materials. A model protein horseradish peroxidase was spontaneously adsorbed and loaded on the clay-functionalized graphene. The enzymatic protein on the clay-functionalized graphene displayed much higher enzymatic activity in comparison with the unfunctionalized one and those functionalized ones with other agents. The graphene functionalized with inorganic nanosheets of clay as a carrier for biomolecules may have many applications including sensors, nanocomposites, electrode materials and biofuel cells.

Long-term diosmectite use does not alter the gut microbiota in adults with chronic diarrhea

BackgroundDiosmectite, a natural colloidal clay, has been used worldwide for a number of approved indications, including the treatment of chronic functional diarrhea. Here, we used high-resolution whole metagenome shotgun sequencing to assess the impact of a 5 weeks administration of diosmectite (3 g/sachet, 3 sachets/day) on the fecal microbiota of 35 adults with functional chronic diarrhea.ResultsGut microbiota was not impacted by diosmectite administration. In particular, richness remained stable and no microbial species displayed a significant evolution. Segregating patients either by diosmectite response (non responder, early responder, late responder) or by nationality (Great-Britain or Netherlands) yielded the same results.ConclusionWe concluded that no microbiota-related physiological alterations are expected upon long-term treatment with diosmectite.Trial registrationClinicaltrials.gov NCT03045926

A comparative study of biopolymer adsorption on model anisotropic clay surfaces using quartz crystal microbalance with dissipation (QCM-D)

Mitigation of colloid clay particles is critical during flotation and flocculation processes in mineral processing. Most organic and inorganic mitigation reagents have negative impacts on the environment and human health; therefore, biologically derived substances have been attracting attention as alternative reagents. Given the anisotropic nature of clay surfaces, it is imperative to understand reagent adsorption on the individual edge and basal plane surfaces of clays. Quartz crystal microbalance with dissipation (QCM-D) was used in this study to determine the adsorption characteristics of three biopolymers (the protein-based biopolymer, lysozyme, and protein and polysaccharide-based oligomers; protamine and pectin) on model surfaces of the anisotropic edge and basal planes of the clays (e.g., kaolinite and serpentine). SiO2 sensor representing the tetrahedral basal plane, Al2O3 and Mg(OH)2 representing the octahedral basal planes, AlSiO and MgSiO representing the edge faces of clays were used as model surfaces of clay minerals. For kaolinite, protamine adsorbed preferentially on the silica (SiO2) tetrahedral surface at pH 7 and on the alumina (Al2O3) surface at pH 10. Protamine adsorbed primarily on magnesium hydroxide (Mg(OH)2), representative of serpentine, at pH 7 and 10. Lysozyme adsorbed preferentially and irreversibly on the edge basal plane surfaces of both clays at pH 10, while it showed a higher affinity for octahedral surfaces (alumina and magnesium hydroxide) at pH 7. In contrast, pectin adsorbed strongly on the magnesium hydroxide, representative of the basal plane surface of serpentine. An adsorption study revealed that electrostatic attraction and/or hydrogen bonding mechanisms contributed to the adsorption of biopolymers on clay surfaces. This investigation provides a fundamental and practical understanding of biopolymer interactions with clay surfaces during selective flotation and flocculation.

Impact of heavy metal cations on deposition and release of clay colloids in saturated porous media

AbstractWhile the facilitated transport of heavy metals by colloids such as clay particles has been widely recognized, the influence of heavy metals on transport of the colloids has received much less attention. This study conducted saturated column experiments to systematically examine influence of multivalent heavy metal cations on transport of natural clay colloids and fullerene nC60 nanoparticles in glass bead porous media. Results showed that the presence of Cd2+ in the solution can cause more deposition of clay colloids in glass beads compared to Ca2+ at a given ionic strength (IS), and the subsequent release by IS reduction was less, demonstrating that the presence of Cd2+ increased the irreversibility of attachment. When the glass beads were initially adsorbed with Ca2+ and Cd2+, the release of clay colloids was significantly reduced due to strong cation bridge between the clay colloids and collector surfaces via the heavy metal cations. The release of clay colloids was significantly increased upon reduction of solution IS if the Na+ was used to exchange for the Ca2+ or Cd2+ before the IS reduction. Additional experiments showed that the nC60 nanoparticles that were deposited in the presence of Fe3+ and Cu2+ cannot be released by reduction of solution IS. However, the nanoparticle release occurred when the Ca2+ was used to exchange for the Fe3+ and Cu2+. Our work was the first to reveal the influence of heavy metal cations on the irreversibility of colloid attachment, and the findings have important implication to fabrication of functional nanomaterials for stabilization of heavy metals in soil and development of mathematic models for accurate prediction of cotransport of heavy metals and colloids in subsurface environments.

Impacts of negatively charged colloidal clay particles on photoisomerization of both anionic and cationic azobenzene molecules.

Although smectite-type clays are used as heterogeneous media for photofunctional guest molecules, the guest species are limited to cationic or polar molecules because of the intrinsic negative electric charges of clay particles. Nevertheless, in this study, aqueous clay colloids are reported to affect the photoisomerization kinetics of anionic and cationic azobenzene molecules dissolved in the colloids. Under UV-light irradiation, the clay colloids decelerate trans-to-cis isomerization, while under visible-light irradiation, the clay colloids accelerate cis-to-trans isomerization. In addition, the sol–gel transition of clay colloids affects the kinetics. The results considerably expand the applicability of clay colloids as matrixes for functional organic species.

ПОРІВНЯЛЬНИЙ АНАЛІЗ РОЗМІРНИХ ХАРАКТЕРИСТИК ЧАСТИНОК ГІБРИДНИХ ТА МІНЕРАЛЬНИХ АДСОРБЕНТІВ ДЛЯ ПРОЦЕСІВ ВОДООЧИЩЕННЯ

n the work, a comparative morphometric analysis of various types of domestic adsorbents intended for use in water purification technologies is performed. Four types of adsorbents were investigated: «Möbius» carbon adsorbent, P1T1KA clay powder of the montmorillonite type, DG-100 carbon black, C-1 colloidal graphite preparation.Morphometric analysis of adsorbent particles was carried out by the method of optical polarization microscopy («Bi-olam» brand microscope). Photomicrographs of particles pre-dispersed in the immersion liquid were obtained using a digital camera with a high-resolution matrix. Quantitative assessment of the sizes of adsorbent particles was carried out by the method of digital image analysis (ImageJ software package). The area and perimeter of individual particles were determined and their equivalent diameter and shape index were calculated. Statistical processing of experimental data was carried out using the «Statistica» software package.It was established that the nature of the distribution of particles of the investigated types of adsorbents in terms of equiv-alent diameter depends on the type of sample, and may be complicated by the processes of particle aggregation.It is shown that a similar nature of distribution is observed for all investigated carbon adsorbents according to the shape index. The pronounced and largest fraction (27-37%) corresponds to particles with a shape close to round. The most acceptable adsorbents for water purification processes can be considered a colloidal graphite preparation and clay powder of the mont-morillonite type, for which there is a higher homogeneity of the distribution of particles according to the shape indicator. This, in turn, can provide higher sorption efficiency in water treatment technologies.The application of the method of optical polarization microscopy in combination with the morphometric analysis of particles makes it possible to reasonably approach the selection of the brand of adsorbents for their use in water purification technologies. Further research in this direction can be directed to the processes of surface modification of adsorbent particles to increase their specific capacity and selective ability.

Influence of natural soil colloid’s stability on transport of copper-based nanoparticles in saturated porous media

Subsurface transport of metal-based nanoparticles (NPs) is dominantly governed by soil colloids and pore water chemistry. The current understanding of colloid-aided transport of metal oxide NPs is very limited. Therefore, the present study has explored clay colloid-aided transport of CuO-NPs in saturated porous-sand media under the influence of various environmental parameters such as ionic strength (IS), and dissolved organic matter (DOM). The role of soil pore water (SPW) chemistry and natural-colloids (NC) extracted from different types of soil on the transport of CuO-NPs has also been investigated. Results suggest stable bentonite clay colloids (BCC) significantly enhance the transport of CuO-NPs in the saturated porous media, whereas an increase in the IS has decreased the mobility of CuO-NPs due to increased aggregation of BCC along with NPs. The introduction of DOM at a concentration of 1 mg/L resulted in 70.4% retention of CuO-NPs at 10 mM NaCl in the absence of BCC. Whereas, in the presence of BCC the retention has increased to 91.7% due to aggregation of BCC. In the case of SPW, the lower mobility of CuO-NPs was observed for alluvial and black soils having higher clay content with high salinity both in the presence and absence of NC. In contrast, higher mobility of CuO-NPs was found in the presence of NC in lateritic and red soils. Whereas, absence of NC in SPW from lateritic and red soils caused almost 100% retention. The impact of flow velocity was observed only in the case of a stable suspension system. The current study suggests NC extracted from four different soils has shown significant variations in their stability that impacted the co-transport of metal oxide NPs. Also, the soil having low salinity with higher clay content possess greater risk for the release of sorbed CuO-NPs in the surface and subsurface environment.

A two-way coupled model for the co-transport of two different colloids in porous media

Models for the co-transport of two different colloids commonly assume a one-way coupling. This is because often a large colloid and small colloid are involved. Therefore, they assume that the spread of smaller colloid is affected by the transport of larger colloids, but not the other way around. However, a number of studies have shown that this assumption is not valid, even for large and small colloids. Therefore, in this study, a two-way coupled model is developed to simulate the co-transport of two different colloids in porous media and their effect on each other. We have considered the interactions of the two colloids with the grain surface, kinetics of heteroaggregation (of the two colloids), and heteroaggregate deposition onto the grain surface. We assumed a first-order kinetic model to represent heteroaggregate formation and its deposition on the grain surface. The model is evaluated by fitting the experimental data reported in four different papers from the literature on the co-transport of clay colloids and viruses, bacteria and graphene oxide nanoparticles, and clay colloids and graphene oxide nanoparticles. The model performance is compared with the commonly-used one-way coupled model. The two-way coupled model is found to satisfactorily simulate most of the experimental conditions reported in the above papers, except for the co-transport of montmorillonite–adenovirus, and Staphylococcus aureus- graphene oxide nanoparticles.

Kaolinite and illite colloid transport in saturated porous media

Understanding the transport of naturally abundant clay colloids is critical in colloid associated contaminant transport in porous media. This study was focused on the impact of the median size of sand grains and the ionic strength of solution on the transport and retention behavior of clay colloids. Two clay samples of kaolinite and illite colloids were selected to represent clay samples containing 1:1 and 2:1 clay minerals, respectively. The observed retention profiles and breakthrough curves demonstrated that the impact of ionic strength on the retention behavior of clay was consistent with other colloidal colloids such as latex colloids or graphene oxide. The quantity of the retained clay increased as the median sizes of sand decreased, and the ionic strength increased from 0 to 0.1 M. However, a similar quantity of retained illite at the ionic strength of 0.01 M and 0.1 M indicates the presence of threshold ionic strength in clay colloid retention. The exponential relationship between sand-to-clay size ratio and first-order retention coefficient at given ionic strength implies the chance in long-term prediction of clay colloid transport from the observed retention profiles. • Transport of kaolinite and illite was investigated in saturated sand column. • Quantity of retained clay does not increase for ionic strength greater than 0.01 M. • Porosity of sand can be a critical factor in clay particle transport. • Impact of aggregation and aspect ratio of clay particles should be considered.

The chemical characterization and its relationship with heavy metals contamination in surface sediment of Marchica Mediterranean Lagoon (North of Morocco).

The sediments were studied according to the granulometric characteristics, the geochemical analyses of the sediments (organic matter (OM), carbonate, and pH), the analyses of heavy metals (HM) characteristics, and their contamination status in the ecological system and the mineralogical analysis of the sediments at 8 sites spread over the Marchica lagoon (NE-Morocco). Our results showed that the opening of the new wide and deep pass affected the spatial distributions of the metals, which were closely related to fine fraction and sediment OM concentration. In the north and southeast lagoon zones presented low concentrations of the HM, fine particles (clay), OM, whereas the south and the center of the lagoon were heavily loaded with HM and OM, corresponding to the trapping zones by fine particles. The results of mineralogy analyses have revealed the predominance of non-clay minerals such as quartz and calcite, and for the clay fractions of sediments showed that they are formed of illite, kaolinite, smectite, and chlorite in highly variable proportions; illite was the most dominant clay in the north-western Marchica lagoon sediments. Therefore, the HMs are fixed by clay colloids having a high cation exchange capacity with smectite-chlorite-kaolinite assembly.

Effect of clay colloid – CuO nanoparticles interaction on retention of nanoparticles in different types of soils: role of clay fraction and environmental parameters

The extensive application of metal oxide nanoparticles (NPs) in various sectors has raised concern about their subsequent release and potentially harmful impacts on the soil system. The present study has addressed the interaction of CuO NPs with bentonite clay colloids (CC) under varying environmental parameters as a model to represent the soil pore water scenario. Based on CuO – CC interaction in model and natural soil solution extracts (SSE), the role of clay fraction and their stability on CuO retention in various types of soils have been evaluated.Results suggested that increasing ionic strength (IS) in the system caused aggregation of CuO NPs, and in the presence of CC, critical coagulation concentration decreased drastically from 27.8 and 17.3 mM to 10.7 and 0.33 mM for NaCl and CaCl2 respectively, due to heteroaggregation in the system. Interestingly, in the SSE, the dominating role of ionic valency, dissolved organic carbon (DOC), and CC was observed in colloidal stabilization over IS. No significant impact of temperature was observed on the stability of CuO NPs both in model and SSE. Further, stability studies in the SSE were correlated with NPs retention behavior in soils. Observations suggest that retention of CuO NPs in soils is a function of binding of the colloidal fraction to the soil, which in turn depends on the colloidal stability. The highest retention was observed in black and laterite soils, whereas lower binding of clay fraction in red soil caused the least retention. A decrease in Kd values after a certain application concentration provided maximum sustainable application concentration of CuO NPs, which may vary with soil properties. Results suggest that the binding of clay and organic matter with a sandy matrix of soil plays a prime role in deciding the overall fate of CuO NPs in the soils.

Freeze-thaw cycles promote vertical migration of metal oxide nanoparticles in soils

Understanding the migration of engineered nanoparticles (ENPs) in soil is of great significance for evaluating the potential risks of ENPs to ecosystem. So far, their migration under freeze-thaw cycles (FTCs) has not been investigated. This study explored the impacts of FTCs on the migration of three commonly used ENPs, copper oxide (CuO-NPs), cerium oxide (CeO2-NPs), and zinc oxide (ZnO-NPs), in three types of soil. After 32 FTC cycles, the highest migration rate of ENPs was found in black soil due to its higher clay particle content. CeO2-NPs with low surface charge exhibited the highest mobility among three ENPs, which migrated to 9–11 cm layer with the concentration of 42.1 mg/kg in the black soil column. ZnO-NPs were less influenced by FTCs as they were adsorbed onto sand grains due to electrostatic interaction, which migrated to 3–5 cm layer with the concentration of 25.2 mg/kg in the black soil. Higher moisture contents (50% and 100%) resulted in increased migration depth of the ENPs in all soils. Lower freezing temperature (−25 °C) caused fragmentation of large soil particles and produced more clay colloids. FTCs promoted the movement of moisture, which penetrated the soil and thus facilitated the movement of ENPs by increasing the contents and movement of clay colloids. This work reveals the migration behavior of ENPs in soils in freeze-thaw period and provides insights into the fate and environmental risk of nanomaterial at middle and high latitudes.

Evaluating the Durability of Lime-Stabilized Soil Mixtures using Soil Mineralogy and Computational Geochemistry

Lime stabilization is a common technique used to improve the engineering properties of clayey soils. The process of lime stabilization can be split into two parts. First, the mobilization and crowding of [Formula: see text] ions or [Formula: see text]molecules from hydrated lime at net negative surface charge sites on expansive clay colloids. Second, the formation of pozzolanic products including calcium-silicate-hydrate (C-S-H) because of reactions within lime-soil mixtures. The pozzolanic reaction is generally considered to be more durable, while the [Formula: see text] adsorption has been associated with more easily reversible consistency changes. This study offers a protocol to assess whether the stabilization process is dominated by durable C-S-H (pozzolanic) reactions or a combination of cation exchange and pozzolanic reactions. Expansive clays with plasticity indices >45% from a major highway project in Texas are the focus of lime treatment in this study. The protocol consists of subjecting lime-soil mixtures to a reasonable curing period followed by a rigorous but realistic durability test and investigating the quality and quantity of the pozzolanic reaction product. Mineralogical analyses using quantitative X-ray diffraction (XRD) and thermogravimetric analysis (TGA) indicates the formation of different forms of C-S-H. In addition, geochemical modeling is used to simulate the lime-soil reactions and evaluate the effect of pH on the stability of C-S-H. The results indicate C-S-H with Ca/Si ratio of 0.66 as most the stable form of C-S-H among other forms with Ca/Si ratio ranging from 0.66 to 2.25. The effect of reducing equilibrium pH on C-S-H is also evaluated. A reduction in pH favored dissolution of all forms of C-S-H indicating the need to maintain a pH ≥ 10.

Impact of montmorillonite clay on the homo- and heteroaggregation of titanium dioxide nanoparticles (nTiO2) in synthetic and natural waters

The homoaggregation of titanium dioxide nanoparticles (nTiO2) and their heteroaggregation with ubiquitous natural clay colloids are crucial processes affecting the environmental transport and fate of nTiO2, whereas the latter has received less attention. In this study, the effects of pH, electrolytes, natural organic matter (NOM), and montmorillonite on the homo- and heteroaggregation of nTiO2 were systematically investigated. The isoelectric point of bare nTiO2 was 6.98, whereas TiO2-montmorillonite mixtures remained negative charged due to the reduced particle surface potential by heteroaggregation. Homoaggregation of nTiO2 was mainly affected by anions, whereas heteroaggregation in TiO2-montmorillonite mixtures was mainly affected by cations. Heteroaggregation between nTiO2 and montmorillonite involved the adsorption of CC/CH. Intensive aggregation of nTiO2 was observed with 4 mg/L montmorillonite, whereas with 20 mg/L montmorillonite, the aggregation was significantly inhibited by the over-coverage of montmorillonite. NOM was attached to the surface of nTiO2 with the adsorption of functional groups involving CC/CH and OCO. The addition of NOM effectively reduced the homo- and heteroaggregation of nTiO2, and the stabilizing effect was enhanced with the increased molecular weight and aromatic/aliphatic fraction in NOM. Besides electrostatic repulsion, steric repulsion could also be one of the main stabilization mechanisms of NOM. With Ca2+ in the solutions, the stabilizing effect of NOM was significantly weakened through cation bridging. The addition of montmorillonite could facilitate the aggregation of nTiO2 in the presence of NOM. The homo- and heteroaggregation of nTiO2 were also observed in 7 different types of natural waters. Homoaggregation predominated in waters with high ionic strength and low NOM contents (seawater and groundwater), whereas heteroaggregation predominated in surface freshwater and wastewater systems. The results reflect the instability of nTiO2 in natural aquatic environments and the potential risk they pose to benthic organisms.

Microfabric characteristics of kaolinite flocculates and aggregates — Insights from large-scale molecular dynamics simulations

Clay particles are usually associated in flocculates or aggregates with different microfabric manners, which will govern the macroscale mechanical behaviors of clayey soils. In this study, the characteristics of different microfabric manners of defect-free kaolinite were explored applying large-scale full-atomistic molecular dynamics simulations. Due to the special crystal structure, kaolinite particles were more likely to flocculate in Edge-to-Face (EF) and Edge-to-Edge (EE) manners than aggregate in Face-to-Face (FF) manner when analyzed from the system energy point of view. For EF and EE manners, the 1st layer of adsorbed water of every single kaolinite particle was complete while the 2nd and the 3rd layers of adsorbed water arranged in T-, L-/inverted L- or cross-shape according to the wettability performance of surrounding kaolinite particles' surfaces. A series of new schematic diagrams of different microfabric manners with adsorbed water structures were developed according to the corresponding equilibrium topologies and physicochemical characteristics. The conceptual hypothesis of the kaolinite sedimentary process was verified and improved based on the simulation results analysis. The research results enhanced the comprehension of microfabric characteristics of clay colloids. The simulation method applied in this work would foster future investigations into clay colloids in various physicochemical environments.

Fertilization effects on microbial community composition and aggregate formation in saline‐alkaline soil

Soil was sampled (0–20 cm) from a 5-year Yellow River Delta paddy field experiment: no fertilizers (Control), mineral fertilizers (NPK), and NPK plus organic fertilizer (soybean and bean litter) applied at rates of 450 and 900 kg C ha− 1 (NPKC1 and NPKC2). The aggregate size, labile carbon fractions and root growth were investigated in combination with microbial community structure by high-throughput sequencing. NPKC1 and NPKC2 fertilization decreased Na+ content, but increased available nitrogen, phosphorus and Ca2+ contents. Macroaggregate formation was increased under NPKC1 (by 28 %) and NPKC2 (by 62 %) fertilization through increase of labile carbon fractions (particularly microbial biomass carbon), as well as better root growth. NPKC1 and NPKC2 fertilization increased microbial diversity and enriched the microbial community towards taxa responsible for aggregate stability (e.g., Actinobacteria and Ascomycota). RDA revealed that dissolved organic carbon content was the main factor affecting microbial community composition. Combined mineral and organic fertilizers decreased Na+ content, increased macroaggregates due to broader microbial diversity, increased contents of labile organic carbon and Ca2+. Consequently, combined organic and mineral fertilization were the best practice to increase microbial biomass and labile carbon fractions that strongly contribute to aggregate formation in saline-alkaline soil. Soil structure is a key factor in sustainable agriculture, especially for saline-alkaline soils, because Na+ destroys clay minearls and colloids, and thus, prevents the aggregate formation. We assessed the effects of organic amendments in combination with mineral fertilizers on chemical properties, microbial community composition and aggregate formation in a typical saline-alkaline soil.