Surface force measurements at the basal planes of ordered kaolinite particles

Application of Low-Salinity Waterflooding in Heavy Oil Sandstone Reservoir: Oil Recovery Efficiency and Mechanistic Study

Low-salinity water injection (LSWI) is a recently emerged and promising technique to enhance oil recovery. In addition, it is attractive due to its relatively low-cost, environmental friendliness, and sustainability. However, the underlying mechanisms remain unclear, and very limited research has been conducted on heavy oil. To verify the feasibility of injecting a low-salinity aquifer water (LSAW) to improve the oil recovery of our target offshore heavy oil reservoir, first, a series of experiments on the core scale, including coreflooding and spontaneous imbibition experiments, were carried out. Furthermore, atomic force microscopy (AFM), contact angle, zeta potential measurement, as well as disjoining pressure calculations based on the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory were carried out to explore the underlying governing mechanism at the microscopic scale. The secondary oil recovery factors of the coreflood tests are 67.11, 70.55, and 77.18% for seawater (SW), produced water (PW), and LSAW, respectively. The additional oil recoveries by LSAW when injected in tertiary modes are 6.38% after SW injection and 5.68% after PW injection. These results indicate that compared with SW and PW which have high brine salinity, the low-salinity brine from the subsurface aquifer (LSAW) can improve oil recovery in both secondary and tertiary modes. In addition, the oil recovery factors from the spontaneous imbibition tests (27.52% by LSAW, 17.32% by PW, and 14.00% by SW) and the insignificant variation of IFTs among the three brines lead to the anticipation that the LSAW can alter the rock to a more water-wet condition compared with SW and PW, thereby giving rise to a higher oil recovery factor in the coreflooding tests. By using AFM imaging and contact angle tests, we proved that the polar asphaltene could desorb from the rock surface and consequently reduce the water contact angle substantially when subjected to low-salinity brine. Furthermore, the zeta potential and the disjoining pressure results indicate that a more repulsive force was developed between oil and the rock under the low-salinity environment, which thereby promotes asphaltene desorption and consequent wettability alteration. Our work has paved the way to apply LSWI to the offshore heavy oil sandstone reservoir.

Facet-Dependent Charge Density of Serpentine: Nanoscopic Implications for Aggregation and Entrainment of Fine Particles

Deciphering the facet-dependent surface properties of clay minerals holds vital significance in both fundamental research and practical engineering applications. To date, the anisotropic local charge density of serpentine surfaces still remains elusive, and thus, the interaction energies and associated aggregate structures between different crystal planes of serpentine cannot be quantitatively determined. In this work, different crystal planes of serpentine (i.e., SiO basal, MgOH basal, and edge) were selectively exposed, and their surface potentials and charge densities were determined using atomic force microscopy (AFM) force measurements coupled with Derjaguin-Landau-Verwey-Overbeek (DLVO) theory fitting. The SiO and edge planes consistently exhibited a permanently negative surface charge, whereas the point of zero charge (PZC) on the MgOH plane was estimated to be pH 9.0-11.0. Based on the interaction energy calculation between different serpentine planes, the aggregation structures of serpentine were predicted. Combined with scanning electron microscopy observation of freeze-dried samples, SiO-MgOH and MgOH-edge associations were found to dominate the aggregate structures at pH ≤ 9.0, thereby resulting in a stacking or "card-houses" structures. In contrast, all of the plane associations exhibited the repulsive interaction energy at pH 11.0, which led to a completely dispersed system, ultimately causing the most severe fine particle entrainment during froth flotation. Our work provides quantitative clarification of facet-dependent surface properties and aggregate structures of serpentine under different pH conditions, which will help improve the fundamental understanding of colloidal behaviors of clay minerals.

Adsorption behavior of soil fulvic acid on crystal faces of kaolinite and goethite: Described by CD-MUSIC model

The interaction between organic matter (OM) and minerals has significant impacts on the mineralization process and rate of OM, and can protect part of organic carbon in soils. In this study, the complex species of fulvic acid (FA) on the surface of minerals (kaolinite and goethite) and the corresponding thermodynamic characteristics were investigated with the CD-MUSIC model and isothermal titration calorimetry. With increasing pH, the adsorption of kaolinite for FA increased at pH < 5.5 and decreased at pH > 5.5, which might be due to the binding of carboxyl groups of FA onto the positively charged sites of O-face and edge-face. However, that of goethite consistently decreased with increasing pH from 3.5 to 9.0 due to an increase in electrostatic repulsion. The fraction of FA was mainly adsorbed on the edge-face and O-face to form inner sphere complexes on kaolinite and outer sphere complexes on goethite. Molar adsorption enthalpies indicated that FA was adsorbed through two site types on kaolinite, while that was one site type on goethite. In addition, the molar enthalpy was more closely associated with inner sphere complexation for kaolinite, while with outer sphere complexation for goethite. The findings shed new light on the adsorption behavior and complexation mechanism of OM on the crystal faces of mineral-water interface.

Ultrafine Kaolinite Removal in Recycled Water from the Overflow of Thickener Using Electroflotation: A Novel Application of Saline Water Splitting in Mineral Processing

The presence of ultrafine clay particles that are difficult to remove by conventional filtration creates many operational problems in mining processing systems. In this work, the removal of clay suspensions has been investigated using an electroflotation (EF) process with titanium electrodes. The results show that EF is a viable and novel alternative for removing ultrafine particles of kaolinite-type clay present in sedimentation tank overflows with low salt concentrations (<0.1 mol/L) in copper mining facilities based on the saline water splitting concept. Maximum suspended solid removal values of 91.4 and 83.2% in NaCl and KCl solutions, respectively, were obtained under the experimental conditions of the constant applied potential of 20 V/SHE, salinity concentration of 0.1 mol/L, and electroflotation time of 10 and 20 min in NaCl and KCl solutions, respectively. Furthermore, the visual evidence of particle aggregation by flocculation during the experiments indicates a synergy between EF and electrocoagulation (EC) that enhances the removal of ultrafine particles of kaolinite.

Amazon raw clay as a precursor of a clay-based adsorbent: experimental study and DFT analysis for the adsorption of Basic Yellow 2 dye

A clay-based adsorbent (CBA) was purified from a sustainable precursor (raw clay, RC), which was obtained from the Amazon region in Brazil. The CBA was characterized using X-ray diffraction (XRD), Fourier Transform Infrared spectroscopy (FTIR), Brunauer-Emmet-Teller surface area (S_BET, RC = 23.386 m².g^-1, CBA = 33.020 m².g^-1), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), thermogravimetric analysis (TGA), cation exchange capacity (CEC, CBA = 44.75 cmol/kg), and point of zero charge analyses (pH_PZC, CBA = 2.20). Subsequently, CBA was used to adsorb basic yellow 2 (BY2) dye from aqueous solutions. A CBA dosage (1 g/L), initial concentration of dye (C₀ = 15 mg/L), and pH (5.6) were ideal conditions for the BY2 dye removal of ~ 98%. The BY2 kinetics was better represented by the pseudo-first-order (PFO) model while the BY2 equilibrium was well represented by the Sips model, with a maximum adsorption capacity of q_ms = 18.04 mg/g at 28 °C. The negative values of ΔG° and ΔH° showed that the studied process is spontaneous and exothermic, while the values of isosteric heat (∆H_st, -16 to -20 kJ/mol) suggest a predominance of physical interactions. The molecular chemical reactivity of BY2 was investigated using quantum chemical descriptors calculated based on Density Functional Theory (DFT) optimization of the dye molecule, and the results revealed a large energy gap value (4.3900 eV) and considerable chemical hardness (η = 2.1950 eV). Therefore, the correlation between DFT and experimental results consistently sustains that BY2 dye tends to be adsorbed on the CBA surface by electrostatic interactions, thus, this is the possible adsorption mechanism of this process.

Particles Morphology Impact on Cytotoxicity, Hemolytic Activity and Sorption Properties of Porous Aluminosilicates of Kaolinite Group

A comparative study of the properties of aluminosilicates of the kaolinite (Al₂Si₂O₅(OH)₄∙nH₂O) group with different particles morphology has been carried out. Under conditions of directed hydrothermal synthesis, kaolinite nanoparticles with spherical, sponge, and platy morphologies were obtained. Raw nanotubular halloysite was used as particles with tubular morphology. The samples were studied by X-ray diffraction, SEM, solid-state NMR, low-temperature nitrogen adsorption, and the dependence of the zeta potential of the samples on the pH of the medium was defined. The sorption capacity with respect to cationic dye methylene blue in aqueous solutions was studied. It was found that sorption capacity depends on particles morphology and decreases in the series spheres-sponges-tubes-plates. The Langmuir, Freundlich, and Temkin models describe experimental methylene blue adsorption isotherms on aluminosilicates of the kaolinite subgroup with different particles morphology. To process the kinetic data, pseudo-first order and pseudo-second order were used. For the first time, studies of the dependence of hemolytic activity and cytotoxicity of aluminosilicate nanoparticles on their morphology were carried out. It was found that aluminosilicate nanosponges and spherical particles are not toxic to human erythrocytes and do not cause their destruction at sample concentrations from 0.1 to 1 mg/g. Based on the results of the MTT test, the concentration value that causes 50% inhibition of cell population growth (IC₅₀, mg/mL) was calculated. For nanotubes, this value turned out to be the smallest—0.33 mg/mL. For samples with platy, spherical and nanosponge morphology, the IC₅₀ values were 1.55, 2.68, and 4.69 mg/mL, respectively.

Error analysis in calculation and interpretation of AFM tip-surface interaction forces

This review addresses some possible errors in calculation and interpretation of AFM tip-surface interaction forces. These usually ignored errors can affect the accuracy and correctness of the interpretation results obtained from measured interaction forces, thus hindering the application of AFM technology in related fields of colloid and interface science. Based on comprehensive analysis and assessment, three important aspects in the existing literature that may introduce significant errors in calculation and interpretation of AFM tip-surface interaction forces have been identified, and corresponding reasonable suggestions have been proposed. (1) The frequently used over-approximated electrostatic force formulas can cause great errors in the electrostatic force and the fitting of surface potential and surface charge density. Therefore, adequate electrostatic force calculation methods, like linear superposition approximation (LSA) or exact numerical solutions, should be used. (2) The over-approximated AFM tip-surface interaction models (spherical tip and flat tip-flat surface interaction models (s-f and f-f)) will lead to large errors in the electrostatic force and van der Waals force, and the subsequently fitted surface potential, surface charge density, and Hamaker constant. Therefore, the conical tip with spherical end and the conical tip with flat circular end-flat surface interaction models (cs-f and cf-f) rather than the over-approximated models (s-f and f-f) should be applied. Besides, it is recommended to use cf-f instead of cs-f to measure the interaction forces for more accuracy. (3) The inaccurately obtained (usually by SEM image) AFM tip geometry parameters (radius and half angle) have significant impacts on the fitting results of surface potential, surface charge density, and Hamaker constant. More accurate AFM tip geometry parameters and reasonable assessment of errors in calculation and interpretation are necessary.

Long-Range Ionic and Short-Range Hydration Effects Govern Strongly Anisotropic Clay Nanoparticle Interactions

The aggregation of clay particles in aqueous solution is a ubiquitous everyday process of broad environmental and technological importance. However, it is poorly understood at the all-important atomistic level since it depends on a complex and dynamic interplay of solvent-mediated electrostatic, hydrogen bonding, and dispersion interactions. With this in mind, we have performed an extensive set of classical molecular dynamics simulations (included enhanced sampling simulations) on the interactions between model kaolinite nanoparticles in pure and salty water. Our simulations reveal highly anisotropic behavior, in which the interaction between the nanoparticles varies from attractive to repulsive depending on the relative orientation of the nanoparticles. Detailed analysis reveals that at large separation (>1.5 nm), this interaction is dominated by electrostatic effects, whereas at smaller separations, the nature of the water hydration structure becomes critical. This study highlights an incredible richness in how clay nanoparticles interact, which should be accounted for in, for example, coarse-grained models of clay nanoparticle aggregation.

Surface interaction between phyllosilicate particles and sustainable polymers in flotation and flocculation

Non-renewable chemical reagents are commonly used as dispersants or flocculants of phyllosilicate clay particles in several industrial fields such as water/wastewater treatment, food production, papermaking, and mineral processing. However, environmentally benign reagents are highly desired due to the non-biodegradability and negative impacts of synthetic reagents on aquatic life. In this work, the dispersion and flocculation behavior of sustainable polymers (anionic and cationic biopolymers) sourced from proteins and polysaccharides were studied in serpentine phyllosilicate suspensions using the following bench-scale tests: zeta potential, microflotation, settling and turbidity, and isotherm adsorption using total organic carbon. The anionic polysaccharide-based biopolymer pectin acted as a switchable biopolymer for serpentine. That is, it could switch from being an efficient flocculant at pH 7 to an effective dispersant at pH 10.

Biopolymers with different backbones have the potential to disperse the phyllosilicate particles in flotation or release the water trapped within tailing particles in flocculation and could decrease environmental problems of conventional reagents.

Modulation of soft glassy dynamics in aqueous suspensions of an anisotropic charged swelling clay through pH adjustment

HYPOTHESIS

Sodium-montmorillonite (Na-Mt) particles are geometrically anisometric that carry a pH dependent anisotropic surface charge. Therefore, it should be possible to manipulate the particle-particle interaction of colloidal range Na-Mt suspensions through pH changes which in turn should alter the soft glassy dynamics of Na-Mt suspensions.

EXPERIMENTS

Rheological experiments were used to probe the impact of pH mediated colloidal particle-particle interaction on the physical aging, linear viscoelastic response, and yield stress behavior of Na-Mt suspension.

FINDINGS

The temporal evolution of the storage modulus (G') was stronger in the acid regime (pH < 9.5) than the base (pH ≥ 9.5) pH regime. Horizontal shifting of the aging curves in the acid and base regimes led to aging time-H⁺ concentration and aging time-OH^- concentration superposition. An aging time-Na-Mt concentration superposition was also observed in both pH regimes. The critical stress associated with the viscosity bifurcation behavior increased linearly with G' but with different slopes for acid and base regime. We propose that positively charged patches on the Na-Mt particle edge merge with the characteristic surface as a function of H⁺ ions in the system. This leads to a strongly associated microstructure at low pH and a relatively weak but associated microstructure at natural pH, hence confirming the hypothesis.

Characterization of Natural Consolidated Halloysite Nanotube Structures

Halloysite is a unique 1:1 clay mineral frequently appearing with nanotubular morphology, and having surfaces of different polarity with interesting and important technological applications. HNTs can be consolidated naturally in the earth by pressure and thermal flows. In this study of natural consolidated HNTs, the strength and hardness of these materials were found to be dependent on the presence of impurities (gibbsite, alunite, quartz, and other silica minerals), which accounted for the increased stability of such samples. In the absence of impurities, the strength of consolidated HNTs was significantly lower. The first 3D mapping of the pore structure of natural consolidated HNT is provided. The contributions of the porosity within the nanotubes and between the nanotubes were delineated using a combination of non-invasive ultra-small and small-angle X-ray scattering (USAXS/SAXS) analyses, BET/BJH pore size analyses, and computed tomography studies. A total porosity of 40%, as determined by X-ray attenuation and He porosimetry, was found for the natural consolidated HNTs, of which about one-third was due to the inter-HNT porosity. Nano-X-ray computed tomography (nano-XCT) analyses also indicated that 76% of the inter-HNT pores were smaller than 150 nm in diameter. The intra-HNT pore size determined by combined USAXS/SAXS and BET/BJH was about 10 nm. This pore network information is essential for the utilization of natural consolidated HNTs as a model geomaterial to investigate the effects of surface characteristics on confined fluid flow.

Surface Charge Measurements with Scanning Ion Conductance Microscopy Provide Insights into Nitrous Acid Speciation at the Kaolin Mineral-Air Interface

Unique surface properties of aluminosilicate clay minerals arise from anisotropic distribution of surface charge across their layered structures. Yet, a molecular-level understanding of clay mineral surfaces has been hampered by the lack of analytical techniques capable of measuring surface charges at the nanoscale. This is important for understanding the reactivity, colloidal stability, and ion-exchange capacity properties of clay minerals, which constitute a major fraction of global soils. In this work, scanning ion conductance microscopy (SICM) is used for the first time to visualize the surface charge and topography of dickite, a well-ordered member of the kaolin subgroup of clay minerals. Dickite displayed a pH-independent negative charge on basal surfaces whereas the positive charge on edges increased from pH 6 to 3. Surface charges responded to malonate addition, which promoted dissolution/precipitation reactions. Results from SICM were used to interpret heterogeneous reactivity studies showing that gas-phase nitrous acid (HONO) is released from the protonation of nitrite at Al-OH₂⁺ groups on dickite edges at pH well above the aqueous pK_a of HONO. This study provides nanoscale insights into mineral surface processes that affect environmental processes on the local and global scale.

Rheological implications of pH induced particle-particle association in aqueous suspension of an anisotropic charged clay

Kaolinite particles are geometrically anisometric and electrostatically anisotropic. Until recently, the charge of both basal faces of kaolinite was assumed to be independent of pH, and the isoelectric point (IEP) of the edge surface was thought to occur at pH 4-6. Therefore, kaolinite suspensions were expected to have an edge-face association at low pH. However, recent atomic force microscopy (AFM) studies have shown that the kaolinite alumina basal face and edge surface carry a pH-dependent surface charge with an IEP at pH 5-6 and ∼ 3, respectively. Here, we revisit the modes of particle association in kaolinite suspensions and apply Derjaguin-Landau-Verwey-Overbeek (DLVO) theory to study the rheological implications of surface charges of various kaolinite faces from recent AFM-based studies. Specifically, aging within the linear viscoelastic region, small amplitude oscillatory shear behavior (strain amplitude and frequency response), and critical stress behavior were studied as a function of pH. Kaolinite suspensions (40 wt%) exhibited two-step structure recovery after shear rejuvenation and two-step yielding at pH less than the IEP of the alumina basal face. In addition, the storage modulus (G') and critical stress required to stabilize the flow followed non-monotonic behavior as a function of pH. At low pH, the silica face-alumina face mode of association was expected to be dominant rather than the edge-face microstructure. A peak in the G'vs. pH curve at pH 4.5-5 was correlated with the silica face-alumina face attraction estimated from DLVO theory, which passes through a maximum at approximately the same pH. Based on these observations, we propose a qualitative state diagram for kaolinite suspensions in the pH-concentration space.

Surface interaction mechanisms in mineral flotation: Fundamentals, measurements, and perspectives

As non-renewable natural resources, minerals are essential in a broad range of biological and technological applications. The surface interactions of mineral particles with other objects (e.g., solids, bubbles, reagents) in aqueous suspensions play a critical role in mediating many interfacial phenomena involved in mineral flotation. In this work, we have reviewed the fundamentals of surface forces and quantitative surface property-force relationship of minerals, and the advances in the quantitative measurements of interaction forces of mineral-mineral, bubble-mineral and mineral-reagent using nanomechanical tools such as surface forces apparatus (SFA) and atomic force microscope (AFM). The quantitative correlation between surface properties of minerals at the solid/water interface and their surface interaction mechanisms with other objects in complex aqueous media at the nanoscale has been established. The existing challenges in mineral flotation such as characterization of anisotropic crystal plane or heterogeneous surface, low recovery of fine particle flotation, and in-situ electrochemical characterization of collectorless flotation as well as the future work to resolve the challenges based on the understanding and modulation of surface forces of minerals have also been discussed. This review provides useful insights into the fundamental understanding of the intermolecular and surface interaction mechanisms involved in mineral processing, with implications for precisely modulating related interfacial interactions towards the development of highly efficient industrial processes and chemical additives.

Study of the Influence of the Crystallographic Orientation of Cassiterite Observed with Colloidal Probe Atomic Force Microscopy and its Implications for Hydrophobization by an Anionic Flotation Collector

In this study, the physicochemical behaviors of the (110), (100), as well as (001) of SnO₂ were investigated by using high-resolution direct force spectroscopy. The measurements were conducted between a silica sphere and sample surfaces in 10 mmol/L KCl between pH 3.1 and 6.2 using colloidal probe atomic force microscopy (cp-AFM-hydrophilic). Dissimilar interactions were detected on different-oriented surfaces. The pH values where the force switched from positive to negative can be clearly distinguished and be ordered as SnO₂(100) < SnO₂(001) ≈ SnO₂(110). By fitting the force curves in the Derjaguin-Landau-Verwey-Overbeck theory framework, anisotropic surface potentials were computed between the three sample surfaces following a similar trend as force interaction. To study the implication of crystallographic orientation to surfactant adsorption, we used Aerosol 22 (sulfosuccinamate) as an anionic collector for cassiterite flotation to functionalize the different samples at pH 3. The contact angle measurements, the topography visualizations by AFM, and the force measurement using cp-AFM with hydrophobized spheres (cp-AFM-hydrophobized) have shown that Aerosol 22 was adsorbed on the sample surfaces inhomogeneously. The adsorption followed the range of SnO₂(110) > SnO₂(100) > SnO₂(001) in the concentration from 1 × 10^-6 to 1 × 10^-4 mol/L.

Role of Montmorillonite, Kaolinite, or Illite in Pyrite Flotation: Differences in Clay Behavior Based on Their Structures

It is widely acknowledged that clay minerals have detrimental effects on the process of flotation, but the mechanisms involved are still not fully understood. In this work, the effects of montmorillonite, kaolinite, and illite on pyrite flotation were investigated from the perspective of various structures of clay minerals. Flotation tests suggested that the detrimental effect of clay minerals on the flotation of pyrite increased as follows: montmorillonite > kaolinite > illite. With the help of rheology measurements, it was found that montmorillonite significantly increased pulp viscosity, which in turn substantially reduced pyrite recovery and grade. Scanning electron microscopy (SEM) images suggested that montmorillonite formed the "house-of-cards" structure by edge-to-edge and edge-to-face contact, while kaolinite and illite platelets were associated mainly in the face-to-face mode. In addition, it was clearly observed by SEM-energy dispersive spectrometry that montmorillonite and kaolinite coat on the pyrite surfaces, which would lower the surface hydrophobicity of pyrite. Kaolinite covered much larger area of pyrite surface than montmorillonite owing to the positive charge occurring at the exposed aluminum-oxygen octahedral sheet of kaolinite. Although illite has a similar 2:1 structure to montmorillonite, it showed little or no effect on pyrite flotation, which was attributed to its poor swelling nature. These findings shed light on the root cause of the adverse effect of clay minerals on pyrite flotation and are expected to provide theoretical guidance for mitigating the negative effects on flotation caused by clays.

XPS and two-dimensional FTIR correlation analysis on the binding characteristics of humic acid onto kaolinite surface

The stabilization and preservation of soil organic matter have been attributed to the strong reactive sites of mineral surfaces that cause physical isolation and chemical stabilization due to the organic-mineral interface. However, much of the micro-scale knowledge about interactions between organic ligands and minerals largely remains at the qualitative level, and neglects the heterogeneity of functional groups of organic matter. Here, we report the use of molecular-scale technologies of two-dimensional FTIR Correlation Spectroscopy (2D-FTIR-CoS) and X-ray Photoelectron Spectroscopy (XPS) to directly measure the binding processes of humic acid (JGHA) groups onto kaolinite surface. The spectroscopy results showed that the carboxylate groups, aliphatic OH and aromatic structure participate in the binding of JGHA on kaolinite surface. The carboxylic and phenolic hydroxyl interact with kaolinite surface through the interfacial COAl/Si bonds. Kaolinite prefers to adsorb C-groups at pH 4.0 and O-groups at pH 8.0. The interaction of COO^- group at 1566 cm^-1 of JGHA leads to the formation of inner-sphere complex first and then outer-sphere complex with increasing contact time. The interaction of COOH group at 1261 cm^-1 with the AlOH²⁺ of kaolinite was could be ascribed to ligand exchange and/or electrostatic attraction, whose contribution was evaluated to be 13.90%, 7.65% and 0% at pH 4.0, 6.0 and 8.0, respectively. These results of molecular binding provide quantitative mechanistic insights into organic-mineral interactions and expound the effect of functional groups of HA on binding mechanisms, and thus bring important clues for better understanding the mobility and transformation of land‑carbon including mineral-bound carbon.

The Depressing Effect of Kaolinite on Molybdenite Flotation in Seawater

Copper-molybdenum grades of important mining deposits have progressively decayed, which is associated with high levels of clay minerals which affect froth flotation. The depressing effect of clay minerals on copper sulfides was previously reported but there are no systematic studies on the effect on molybdenite flotation in seawater. The objective of this work was to study the effect of kaolinite on molybdenite flotation in seawater and to evaluate the use of sodium hexametaphosphate (SHMP) as dispersant. The results of this work show that kaolinite depresses molybdenite flotation which is more significant in seawater at pH > 9. All the experimental data validate the hypothesis that kaolinite covers molybdenite, reducing its flotation recovery. The depressing effect of kaolinite on molybdenite flotation in seawater is enhanced by the magnesium and calcium hydroxo complexes at pH > 9, which induce heterocoagulation between kaolinite and molybdenite, thus reducing recovery. The attachment of the positively charged hydroxo complexes of magnesium and calcium to the molybdenite and kaolinite surfaces is diminished by SHMP. This reagent increases the repulsive forces between molybdenite and precipitates and as a result, molybdenite becomes more hydrophobic and recovery increases.

Behavior of Ag species in presence of aquatic sediment minerals - In context of aquatic environmental safety

In recent years, there has been a growth in the number of products containing Ag nanoparticles (AgNPs) in many areas and their use suggests that the water-soil environment may be exposed to the contaminant with different Ag species. Therefore, the sorption of two Ag forms (i.e. Ag(I) ions and nanoparticles - AgNPs) on clay minerals (montmorillonite and kaolinite) and iron (oxyhydr)oxides (ferrihydrite) as a function of solution:mineral ratio (100:1, 250:1, 500:1), solution pH (3.0, 5.5 and 7.0) and initial Ag concentration (0.1-100 mg/dm³) was studied using batch method. In addition the binding strength/mobility of the bonded Ag species was researched. The results show a great sorption potential of clay minerals for both Ag forms and lower sorption capacity of ferrihydrite, in particular for Ag(I) ions. The maximum sorption capacities of montmorillonite, kaolinite and ferrihydrite estimated from three-parameter isotherm model of Sips were 94.39 mg/g, 117.8 mg/g and 26.48 mg/g for AgNPs and 17.92 mg/g, 21.14 mg/g and 3.072 mg/g for Ag(I) ions, respectively. Aggregation process plays an important role in sorption and mobility of AgNPs. The sequential extraction study indicated different binding mechanisms of the Ag forms onto the clay minerals and ferrihydrite, which depended on the active sites of minerals as well as the Ag species nature in the solution. Ag(I) was weakly bound by clay minerals but presence of iron (oxyhydr)oxides decreased the Ag(I) mobility and bioavailability. On the other hand, AgNPs bound with the active centers of minerals in a very strong way and were not able to release into water. The study of the binding of Ag forms by clay minerals and (oxyhydr)oxides allows to determine the influence of their physicochemical and structural properties, including e.g. pore size on Ag sorption. These results allow these properties to be taken into account in the study of environmental samples, including waters and soils. Moreover, the results showed that in the study of behavior of Ag forms in contact with the minerals, in addition to the sorption capacity, the susceptibility to their release is very important. Studies on sorption/desorption of AgNPs and Ag(I) ions as a form of oxidation of AgNPs is important for understanding the transport and fate of the Ag species in soil, sediments and surface water because of different their behavior in contact with the minerals.

Distinct mechanisms in the heteroaggregation of silver nanoparticles with mineral and microbial colloids

Attachment to solids is an important process for determining nanomaterial transport and their fate in environments. Here we revealed distinct behaviours in the attachment of silver nanoparticles (AgNPs) to kaolin and bacterial cells. We found preferential attachment of AgNPs to the edges of kaolin. Decreasing pH or adding metal ions promoted AgNP-kaolin attachment due to the increase of positive charge on kaolin's surfaces. Multivalent cations (Mg²⁺ and Ca²⁺) induced stronger enhancement than monovalent cations (Na⁺, K⁺ and Ag⁺), which demonstrated the positive role of electrostatic interaction in AgNP-kaolin attachment. However, the presence of metal ions inhibited AgNP binding to bacterial cells. The inhibitive effect was significantly correlated with solubility product of metal ions, which implied a chemical reaction mechanism in AgNP-cell attachment. In kaolin system, humic acid (HA) can considerably inhibit AgNP attachment and diminish the enhanced effects induced by metal ions. In contrast, in bacterial cell system, HA reduced the inhibitive effect of metal ions for AgNP adsorption, although HA itself had negligible effect on AgNP-cell attachment. Taken together, our results demonstrated the contribution of electrostatic attraction versus chemical interaction to the attachment of AgNPs to kaolin or bacterial cells, providing fundamental support to understand the attachment of nanomaterials to inorganic and organic solids in the environments.

Kaolinite-Magnesite Based Ceramics. Part I: Surface Charge and Rheological Properties Optimization of the Suspensions for the Processing of Cordierite-Mullite Tapes

The present study aimed at investigating the influence of the concentration of sodium silicate and sodium hexametaphosphate on the dispersion of an aqueous kaolinitic clay slurry regarding further use for the tape casting process. The zeta potential of the kaolinitic clay slurry matched the requirements for tape casting. The addition of magnesite in the kaolinitic slurries tended to increase the zeta potential towards the required limit values. Despite this, the further addition of surfactants allowed improving the zeta potential in agreement with the tape casting conditions. Accordingly, the rheological behavior, under continuous and oscillatory flow conditions, of various mixtures of magnesite and a kaolinitic clay was studied. Regarding the pH and the zeta potential measurements, the E–F attraction prevailed at low pH value, and F–F or E–E attraction was predominant at high pH value. All slurries exhibited a shear thinning behavior, which was well-correlated by the Herschel–Bulkley model. It appeared that the best stability for the kaolinitic clay slurries was obtained while using 0.4 mass% and 1.2 mass% of sodium hexametaphosphate and sodium silicate, respectively. An increase in the magnesite concentration above 6 mass% led to a complex behavior with low cohesion energy due to the occurrence of soluble complexes.

Insight into the Crucial Role of Secondary Mineral Phases in the Transfer of Gold Nanoparticles through a Sand Column Using Online ICP-MS/spICP-MS Monitoring

Investigating the transport of engineered nanoparticles through representative soils is an important issue in assessing their mobility and fate in the environment. In this study, successive injections of gold nanoparticles (AuNPs) were performed in a quartz sand column with an eluent composed of 10^-2 M NaCl at a pH of 7.5. After this series of injections, remobilization of the AuNPs was examined by raising the eluent pH to 10. ¹⁹⁷Au and the conservative ionic tracer ⁷⁹Br were monitored simultaneously by online inductively coupled plasma mass spectrometry (ICP-MS), and the particulate nature of gold eluting from the column was confirmed by setting the ICP-MS in the "single particle" mode. The extent of AuNP attachment was greater than predicted by DLVO theory considering quartz as the sole collector, decreased with the number of injections and with particle size. In contrast with the repulsive interaction energy between the particles and the quartz surface, kaolinite, a secondary mineral of the sand, provided favorable conditions for particle attachment. The superimposed signals of ¹⁹⁷Au and ²⁷Al in the column effluent after pH increase suggest that gold nanoparticles were essentially remobilized as heteroaggregates with the kaolinite colloids they were attached to when favorable conditions for clay detachment from the sand grains were encountered.

Probing Anisotropic Surface Properties of Illite by Atomic Force Microscopy

For the purpose of understanding the colloidal behaviors of illite in mineral processing, probing the surface charging property of illite is of great significance. This research explored the edge and basal surfaces of illite using an atomic force microscope (AFM). The interaction forces between Si/Si₃N₄ probes and illite edge/basal surfaces were measured, respectively, at different pH values in 10 mM KCl solutions. Theoretical Derjaguin-Landau-Verwey-Overbeek forces were matched up with the measured forces to derive the surface potentials of the two surfaces. On the illite basal surface, an attractive force occurred at pH 3.0, while repulsive forces dominated from pH 5.0 to 10.0. On the illite edge surface, a slight attractive force was also obtained at pH 3.0. However, the interaction changed into repulsion at pH 5.0, and this repulsive force increased gradually from pH 6.0 to 10.0. Illite basal and edge surfaces were both negatively charged, but the basal surface exhibited more negative charges than the edge surface from pH 3.0 to 10.0. Increasing solution pH from 3.0 to 10.0, there was no detection of the point of zero charge (PZC) of the illite basal surface; however, the PZC of the illite edge surface should have occurred at a pH slightly lower than 3.0. This is the first time that surface potentials of illite edge and basal surfaces were attained separately by direct force measurements. These findings provide insights into the colloidal behaviors of illite in mineral processing and oil sands extraction.

Interpretation of Hydrophobization Behavior of Dodecylamine on Muscovite and Talc Surface through Dynamic Wettability and AFM Analysis

In this study, a new approach, “dynamic wettability”, and atomic force microscopy (AFM) imaging analysis techniques were successfully used to characterize the hydrophobization mechanism of the collector dodecylamine (DDA) on muscovite and talc surfaces. The attachment of bubbles to the minerals was studied through the dynamic contact angle to gain a detailed understanding of the hydrophobization mechanism of DDA on a muscovite and talc surface. AFM imaging and interaction forces were performed to explain the DDA adsorption mechanism on both minerals. Finally, flotation tests were performed to verify the effectiveness of these techniques. After treatments with DDA, the contact angles became much larger compared to initial angles, particularly for muscovite, and the attachment of bubbles on the talc surface was much easier than muscovite due to its natural hydrophobicity. From AFM imaging, both the muscovite and talc showed a similar tendency; the higher the DDA concentration, the more the adsorbed amount. However, the adsorbed amount of DDA on talc surface was obviously more than that on muscovite. As far as interaction forces are concerned, the maximum attractions occurred at certain different concentrations respectively for muscovite and talc and agreed well with the AFM-imaging results. Moreover, results obtained from flotation tests were promising and quite in agreement with the phenomenon of these techniques.

Interaction Mechanism between Molybdenite and Kaolinite in Gypsum Solution Using Kerosene as the Flotation Collector

This paper aims to understand the fundamental interaction mechanism between molybdenite and kaolinite in gypsum solution using kerosene as collector. Micro-flotation tests were conducted to study the effect of gypsum solution on the flotation performance of mixed −74 μm molybdenite and −10 μm kaolinite mineral. The results showed that the recovery of molybdenite decreased from 86% to 74% while the gypsum solution concentration increased from 0 to 800 mg/L, indicating the detrimental effect of kaolinite on molybdenite flotation could be enhanced by gypsum solution. This is mainly caused by the slime coating of kaolinite on molybdenite through dissolved calcium ion of gypsum solution. In order to confirm the slime coating phenomenon, zeta potential distribution, scanning electron microscopy (SEM) and atomic force microscopy (AFM) measurements were used to investigate interaction characteristics and mechanisms. The zeta potential distribution results revealed that mixed samples had the value between signal molybdenite and kaolinite samples in gypsum solution, which proved the coating phenomenon of kaolinite on molybdenite. Moreover, the coating phenomenon was becoming more and more obvious with the gypsum solution concentration. The coating phenomenon of kaolinite on molybdenite surface was also directly observed from SEM results. The AFM results provided further evidence for the possibility of slime coating, as the adhesion force increased with the gypsum solution concentration, which means the aggregates of molybdenite and kaolinite were becoming more stable.

Composition in the Interface between Clay Mineral Surfaces and Divalent Cation Electrolytes

The interfacial free energy of a solid, which determines its adsorption properties, depends on interactions between the surface and the fluid. A change in surface composition can completely change the behavior of the solid. Decades of work have explored adsorption and its effects at solid-fluid interfaces from the macroscopic perspective and using molecular modeling, so the concept of the electric double layer (EDL) is well established in the community. However, direct, molecular level, experimental observations of the composition within the interface region, and its change with time and conditions, are not abundant. We used cryogenic X-ray photoelectron spectroscopy (cryoXPS) to observe the composition in the clay mineral-solution interface region as a function of bulk solution composition, on illite and chlorite in MgCl₂ and CaCl₂ electrolytes, over a range of concentrations (1-125 mM), in situ, on vitrified samples. These samples were prepared from very thin smears of centrifuged wet paste that were instantaneously chilled to liquid N₂ temperature. They preserved the adsorbed solution in its amorphous state, maintaining the location of the ions and water with respect to the solid, without the disruption that occurs during drying or the rearrangement that results as water crystallizes during freezing. With decreasing ionic strength, we could directly monitor the loss of negative charge in the interface region, producing an anion deficiency, as predicted by theory. The Cl^-/Me²⁺ ratio dropped below 1 for chlorite at 12-25 mM MeCl₂ and for illite at 75-100 mM. In addition to better understanding of clay mineral behavior in solution, this work demonstrates that only those clay minerals where surface charge density is the same or lower than that for chlorite contribute to a low salinity enhanced oil recovery response (LS EOR). This explains many of the contradictory results from studies about the role of clay minerals in LS EOR.

The application of atomic force microscopy in mineral flotation

During the past years, atomic force microscopy (AFM) has matured to an indispensable tool to characterize nanomaterials in colloid and interface science. For imaging, a sharp probe mounted near to the end of a cantilever scans over the sample surface providing a high resolution three-dimensional topographic image. In addition, the AFM tip can be used as a force sensor to detect local properties like adhesion, stiffness, charge etc. After the invention of the colloidal probe technique it has also become a major method to measure surface forces. In this review, we highlight the advances in the application of AFM in the field of mineral flotation, such as mineral morphology imaging, water at mineral surface, reagent adsorption, inter-particle force, and bubble-particle interaction. In the coming years, the complementary characterization of chemical composition such as using infrared spectroscopy and Raman spectroscopy for AFM topography imaging and the synchronous measurement of the force and distance involving deformable bubble as a force sensor will further assist the fundamental understanding of flotation mechanism.

Anisotropic surface chemistry properties and adsorption behavior of silicate mineral crystals

Anisotropic surface properties of minerals play an important role in a variety of fields. With a focus on the two most intensively investigated silicate minerals (i.e., phyllosilicate minerals and pegmatite aluminosilicate minerals), this review highlights the research on their anisotropic surface properties based on their crystal structures. Four surface features comprise the anisotropic surface chemistry of minerals: broken bonds, energy, wettability, and charge. Analysis of surface broken bond and energy anisotropy helps to explain the cleavage and growth properties of mineral crystals, and understanding surface wettability and charge anisotropy is critical to the analysis of minerals' solution behavior, such as their flotation performance and rheological properties. In a specific reaction, the anisotropic surface properties of minerals are reflected in the adsorption strengths of reagents on different mineral surfaces. Combined with the knowledge of mineral crushing and grinding, a thorough understanding of the anisotropic surface chemistry properties and the anisotropic adsorption behavior of minerals will lead to the development of effective relational models comprising their crystal structure, surface chemistry properties, and targeted reagent adsorption. Overall, such a comprehensive approach is expected to firmly establish the connection between selective cleavage of mineral crystals for desired surfaces and designing novel reagents selectively adsorbed on the mineral surfaces. As tools to characterize the anisotropic surface chemistry properties of minerals, DLVO theory, atomic force microscopy (AFM), and molecular dynamics (MD) simulations are also reviewed.

Unrefined humic substances as a potential low-cost amendment for the management of acidic groundwater contamination

The present study explores a novel application of Huma-K, a commercially available, unrefined humic substance, as a promising low-cost source of organic matter for in situ remediation of contaminated acidic groundwater plumes. This can be achieved by creating a humic-rich coating on the surface of minerals which can enhance the sorption of contaminants from groundwater. Huma-K was characterized by means of scanning electron microscopy equipped with energy dispersive spectroscopy, Fourier-transform infrared analysis, and potentiometric titrations. Batch experiments were performed to investigate the sorption-desorption behavior of Huma-K and to evaluate what conditions (pH, contact time, and initial Huma-K concentration) affect these processes upon injection into aquifer sediments. As evidenced by potentiometric titrations, Huma-K possesses functional groups that have an acidic nature, with pK values in the range of 4-6 (carboxylic) and 9-10 (phenolic). Sorption, homogeneous precipitation, and surface-induced precipitation seem to be favored in the presence of sediment at pH 4, where there is less deprotonation of acidic functional groups. As the pH is increased, functional groups become negatively charged, leading to electrostatic repulsion and dissolution of Huma-K from sediment. Kinetic experiments indicate that Huma-K sorption is a slow-rate process, most likely governed by film diffusion. The enhanced sorption of Huma-K in acidic conditions suggests that it may be used to create a subsurface treatment zone in acidic aquifers for the sequestration of contaminants such as uranium. The treatment zone will persist as long as the pH does not increase sufficiently to cause soil-bound Huma-K to be released, remobilizing aqueous contaminants.

A review of the surface features and properties, surfactant adsorption and floatability of four key minerals of diasporic bauxite resources

Diasporic bauxite represents one of the major aluminum resources. Its upgrading for further processing involves a separation of diaspore (the valuable mineral) from aluminosilicates (the gangue minerals) such as kaolinite, illite, and pyrophyllite. Flotation is one of the most effective ways to realize the upgrading. Since flotation is a physicochemical process based on the difference in the surface hydrophobicity of different components, determining the adsorption characteristics of various flotation surfactants on the mineral surfaces is critical. The surfactant adsorption properties of the minerals, in turn, are controlled by the surface chemistry of the minerals, while the latter is related to the mineral crystal structures. In this paper, we first discuss the crystal structures of the four key minerals of diaspore, kaolinite, illite, and pyrophyllite as well as the broken bonds on their exposed surfaces after grinding. Next, we summarize the surface chemistry properties such as surface wettability and surface electrical properties of the four minerals, and the differences in these properties are explained from the perspective of mineral crystal structures. Then we review the adsorption mechanism and adsorption characteristics of surfactants such as collectors (cationic, anionic, and mixed surfactants), depressants (inorganic and organic), dispersants, and flocculants on these mineral surfaces. The separation of diaspore and aluminosilicates by direct flotation and reverse flotation are reviewed, and the collecting properties of different types of collectors are compared. Furthermore, the abnormal behavior of the cationic flotation of kaolinite is also explained in this section. This review provides a strong theoretical support for the optimization of the upgrading of diaspore bauxite ore by flotation and the early industrialization of the reverse flotation process.