Colloidal Clay Research Articles

Cotransport of fullerene nanoparticles and clay colloids in porous media: The relation between aggregation and transport

In the study, cotransport of fullerene nanoparticles (nC60) and mobile clay colloids (illite (ILL), kaolinite (KL), montmorillonite (ML)) in aquifer porous media and its relation to the aggregative interaction between these two types of particles was investigated. Minimal interaction occurred between nC60 and ILL, resulting in unaffected transport. Strong heteroaggregation between ML and nC60 resulted in not only significant retention of both particles during their cotransport but also the retention of nC60 in the media pre-injected with ML. Strong homoaggregation of KL caused strong straining effect and consequently retention of both KL and nC60 during their cotransport, however, the pre-emplacement of KL did not cause retention of nC60 during the sequential injection of KL and nC60. Such aggregation behaviors were well demonstrated by the adsorption-sedimentation experiment, microscopic observation, the size and zeta-potential test, and model simulation. Based on the surface chemistry analysis, divalent-cation bridging between ML and nC60 and hydrogen bonding between KL particles were responsible for the heteroaggregation and the homoaggregation, respectively. The study demonstrated the specificity of the aggregation between the mobile clays and nC60 to the chemistry of the clays and its consequent effect on the cotransport of the particles, which is critical for assessment of the environmental risk of nC60.

Manipulating crack formation in air-dried clay suspensions with tunable elasticity

Clay, the major ingredient of natural soils, is used as a rheological modifier while formulating paints and coatings. When subjected to desiccation, colloidal clay suspensions and clayey soils crack due to the accumulation of drying-induced stresses. Even when desiccation is suppressed, aqueous clay suspensions exhibit physical aging, with their elastic and viscous moduli increasing over time as the clay particles self-assemble into gel-like networks due to time-dependent inter-particle screened electrostatic interactions. The rate of evolution of the suspension structures and therefore of the mechanical moduli can be controlled by changing clay concentration or by incorporating additives. Since physical aging and desiccation should both contribute to the consolidation of drying clay suspensions, we manipulate the desiccation process via alterations of clay and additive concentrations. For a desiccating sample with an accelerated rate of aging, we observe faster consolidation into a semi-solid state and earlier onset of cracks. We estimate the crack onset time, tc, in direct visualization experiments and the elasticity of the drying sample layer, E, using microindentation in an atomic force microscope. We demonstrate that tc∝GcE, where Gc, the fracture energy, is estimated by fitting our experimental data to a linear poroelastic model that incorporates the Griffith's criterion for crack formation. Our work demonstrates that early crack onset is associated with lower sample ductility. The correlation between crack onset in a sample and its mechanical properties as uncovered here is potentially useful in preparing crack-resistant coatings and diverse clay structures.

Batch Electrocoagulation Process for the Removal of High Colloidal Clay from Open-Cast Coal Mine Water Using Al and Fe Electrodes

Open-cast coal mining operations can produce water with high amounts of total suspended solids (TSS). We tested the use of electrocoagulation for the removal of colloidal clay from mine water. Monopolar batch electrocoagulation was performed at the laboratory scale using Al and Fe electrodes and varying the DC current (0.5, 1, and 2 A) and contact time (15, 30, and 45 min). Aluminum electrode electrocoagulation with a current of 2 A and a contact time of 15 min had the greatest TSS removal efficiency (99%), with concentrations decreasing from 5,400 to 23 mg/L. The greatest removal (98 and 99%, respectively) was obtained using an Al electrode with an electric current of 0.5 and 1 A, with 30 min of contact time. With an Fe electrode, the greatest efficiency was achieved with a current of 2 A and a contact time of 30 min. The TSS removal efficiency reached 98% while the concentration dropped to 66 mg/L, followed by 89% and 95% for the 0.5 A-45 min and 1 A-15 min variations, respectively.

Liquid-Crystalline Photonic Sandwich: Electroresponsive Colloids of Clay Nanosheets Loading Photofunctional Dyes.

Colloidal clay nanosheets obtained by the delamination of layered crystals of smectite-type clay minerals in water form liquid crystals because of their shape anisotropy. Loading of organic dyes onto the liquid crystalline clay nanosheets will enable novel photonic materials, where photofunctions of the loaded dye are controlled by the liquid crystallinity of the clay nanosheets. However, adsorption of organic dyes onto the nanosheets renders the nanosheet surfaces hydrophobic, and consequently, colloidal stability of the nanosheets is lost. In this study, this drawback is overcome by sandwiching cationic stilbazolium dyes between a pair of synthetic fluorohectorite nanosheets. This is realized by the preparation of stilbazolium-clay second-stage intercalation compounds characterized by intercalation of dye cations into every other interlayer space of the hectorite clay, where nonintercalated interlayer spaces are occupied by Na+ ions. The second-stage intercalation compounds are obtained by partial ion exchange of mother clay mineral incorporating Na+ ions in all of the interlayer spaces and delaminated from the Na+-containing interlayer spaces to form clay nanosheets sandwiching the dye molecules. Aqueous colloids of the dye-sandwiching clay nanosheets form colloidal liquid crystals, and the dye-sandwiching liquid crystalline clay nanosheets respond to an applied AC electric field to be aligned parallel to the electric field. The assembled structure of the dye-sandwiching clay nanosheets under the electric field is characterized by aligned discrete clay platelets, which is somewhat different from that of a colloidal liquid crystal of clay nanosheets without dye loading characterized by macroscopic liquid crystalline domains up to submillimeters. The electric alignment of the clay nanosheets induces alteration of light absorption of the sandwiched stilbazolium molecules, which verifies a strategy of constructing stimuli-responsive photonic materials of clay-organic hybrids.

Cotransport of fullerene nanoparticles and montmorillonite colloids in porous media: Critical role of divalent cations of montmorillonite

While the cotransport of carbon nanoparticles (CNPs) and clay colloids in porous media has been widely studied, the influence of the cation exchange capacity (CEC) of clay colloids on the transport process remains unclear. In this study, batch adsorption and column transport experiments were conducted to investigate the fate and transport of CNPs and clay colloids in quartz sand, with respect to the effect of monovalent-cation exchange capacity (mono-CEC), divalent-cation exchange capacity (di-CEC) and total CEC of clays. Fullerene nanoparticles (nC60) and six types of montmorillonite (ML) with different CEC were selected as modeled CNPs and clay colloids, respectively. Transport behavior of nC60 and ML was characterized using breakthrough curves (BTCs) and fitted with two-kinetic-sites colloid transport model. Results of the adsorption experiments showed a good linear correlation between the deposition of nC60 on the sand surface and the di-CEC of ML. Transport of ML and nC60 was inhibited by each other. The calculated mass recovery of nC60, as well as the fitted maximum deposition capacity and attachment rate coefficients of nC60 exhibited a strong linear relationship with the di-CEC of ML. These results indicate that divalent cations in ML interlayers play a significant role in aggregation between nC60 and ML and their cotransport. Through measurements of the particle size and zeta potentials of sole nC60 and mixtures of ML and nC60, FTIR and XPS analysis of nC60 under different conditions, and a release experiment of nC60 in a sand column, it demonstrated cation bridging (Ca2+-π) between nC60 and ML mediated by the divalent cations in ML interlayers. The study highlighted the potential of using di-CEC of clays as an indicator to predict the mobility of nC60 in clay-containing porous media and added insights to the transport behavior of CNPs in porous media.

Bentonite and polymeric support fluids used for stabilization in excavations

Bentonite is a natural and finite mineral resource. Dilute suspensions of sodium montmorillonite clay in water represents bentonite slurries. Suspension and orientation of colloidal clay particles define rheological properties in bentonite slurry (BS). The BS has been used about seventy years to temporarily support the excavations. More recently, polymer support fluids (PSF) gained much popularity and are widely used compared to bentonite support fluids. The PSF are categorized into natural (pure) and synthetic polymers. Physico-chemical properties of PSF are different than BS irrespective of the quite similarity in the mode of action. Synthetic polymer fluids are molecularly engineered fluids that can be a popular alternative of conventional BS deployed as excavation support fluids in different foundation applications such as diaphragm wall panels and pile bores. The synthetically engineered fluids of polymers (water-soluble) are different from conventional BS. The PSF offer additional benefits because their use is cost effective, eco-friendly, and these polymers need smaller site footprint as well as easy preparation, mixing, handling, management and ultimately the final disposal. Nevertheless, synthetic polymers have advantage over bentonite, however, foundation engineers and scientists have also certain concerns about their use because of their performance related issues. For an efficient use of polymers, specific properties and in situ behavior of polymers as well as their sorption onto the soils must be recognized because the polymer concentration in the solution is decreased with time during their use. The present manuscript reviewed the relative performance of excavation support fluids and displayed an arranged marriage of physicochemical and rhelogical properties of natural and synthetic excavation support fluids used in the foundation industry. This information will be highly useful to scientific community for their future ventures and will lay a foundation to understand the mechanisms of stabilization in open and deep excavations.

Models of Pb distribution and uptake in inundated paddy and maize cropping systems

<p>High-traffic highway crossing agricultural fields impacts the quality of food crops grown on Vertisol agricultural fields in Pasuruan. Lead (Pb) released from the exhaust of motor vehicles into the air can eventually enter the soil and be absorbed by plants. This study aimed to examine the effect of Pb from exhaust motor vehicles emission on the Pb status and its behavior in soil, water, and plants. The study was initiated by conducting a survey to determine the sampling locations in selected inundated paddy and maize cropping systems. A stratified random sampling method was used to collect soil, water, and plant samples. The soil of the study area is dominated by Vertisol, with clay content ranging from 54% to 76%. The soil attributes a high cation exchange capacity ranging from 80.53 meq 100 g<sup>-1</sup> to 93.57 meq 100 g<sup>-1</sup>. Pb emitted from 2,913,000 vehicles within four months period that entered the agricultural field was not absorbed by paddy and maize crops. Pb entered the soil in the adsorbed form, and no Pb was observed in the soluble form, so it was not absorbed by the roots. In the paddy field, the total Pb of 84.33% was influenced by pollutant distance. Likewise, in the maize field, 83.18% of total Pb was influenced by pollutant distance. The far the pollutant distance from the agricultural field, the lower its total Pb. Paddy field water is adsorbed onto the colloidal clay, which is dispersed due to inundation and sloughing of the paddy, moving with the clay and then dissolved in the water flow.</p>

Colloids facilitate cadmium and uranium transport in an undisturbed soil: A comparison of soil solution isolation methods

Accurate data of cadmium (Cd) and uranium (U) leaching are needed in the context of identifying their mass balances in agricultural soils. There is some controversy related to sampling methods and the contribution of colloid facilitated transport. Here, leaching was measured in undisturbed unsaturated soils and the impact of colloids was measured with due attention to solution sampling protocols. Soils were sampled in an arable, pH neutral silty loam soil. The columns (n = 8) were irrigated and PTFE suction plates (1 μm pores) at the bottom ensured unsaturated flow. New here is that both percolates and associated suction plates were collected, the elements in the plates were recovered with acid digestion and used as a lower estimate of colloidal forms. The fraction of elements collected in the plates were 33 % (Cd) and 80 % (U) of the total mobility (=percolates + plates), illustrating colloidal transport. Composition of pore water extracted by soil centrifugation varied largely between initial and final samples and showed that colloids increased as a result of reduced solution calcium after leaching two pore volumes with low calcium water. Flow Field-Flow Fractionation (FIFFF) of the pore water and percolates revealed co-elution of U with colloidal organic matter, oxyhydroxides and clay, illustrating colloidal transport of U by these vectors. Colloidal transport of Cd was less pronounced and was dominated by organic matter. Soil extracts with 0.01 M CaCl2 have lower colloid concentration and consequently underestimate mobile U. In contrast, Cd concentrations in 0.01 M CaCl2 extracts exceed that of percolates due to chloride complexation and higher calcium, mobilizing Cd. Soil leaching experiments better indicate potential leaching losses than a single pore water composition because the former yields the time integrated data. Suction plates and/or bottom filters need to be analysed in leaching studies to account for metal transport by colloids.

Rheological signatures of a glass-glass transition in an aging colloidal clay

The occurrence of non-equilibrium transitions between arrested states has recently emerged as an intriguing issue in the field of soft glassy materials. The existence of one such transition has been suggested for aging colloidal clays (Laponite® suspensions) at a weight concentration of 3.0%, although further experimental evidences are necessary to validate this scenario. Here, we test the occurrence of this transition for spontaneously aged (non-rejuvenated) samples by exploiting the rheological tools of dynamical mechanical analysis. On imposing consecutive compression cycles to differently aged clay suspensions, we find that quite an abrupt change of rheological parameters occurs for ages around three days. For Young’s and elastic moduli, the change with the waiting time is essentially independent from the deformation rate, whereas other “fluid-like” properties, such as the loss modulus, do clearly display some rate dependence. We also show that the crossover identified by rheology coincides with deviations of the relaxation time (obtained through x-ray photon correlation spectroscopy) from its expected monotonic increase with aging. Thus, our results robustly support the existence of a glass-glass transition in aging colloidal clays, highlighting characteristic features of their viscoelastic behavior.

Formation damage induced by clay colloids deposition in Triassic clastic geothermal fields: Insights from an experimental approach

Injectivity decline problems are often encountered during geothermal water reinjection into the subsurface despite massive surface filtration. The reinjected water carrying only nano-sized particles at relatively low concentrations causes severe permeability impairment due to pore clogging by the transported particles. To provide a better understanding to injectivity decline problems, this study investigates the impact of clay minerals transport and deposition on formation damage of sandstone reservoirs. To identify the clays potentially responsible for clogging, a mineral characterization of natural samples, collected from a Triassic reservoir located in the Paris Basin (France), was realized using X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS). The obtained results show that in this type of sedimentary environments, illite particles are the most abundant clay species that are more likely to be detached from the rock structure and migrate within the geothermal fluid. Illite particles were then used in core-flooding experiments to mimic their reinjection in sandstones under different physico-chemical conditions. The obtained flow curves were further explained by aggregation experiments carried out using the dynamic light scattering (DLS) technique. The results show a clogging-favored tendency promoted mainly by the increase of ionic strength and particle concentration and the decrease of flow rate.

Dynamics and microrheology of colloidal clay-polymer glasses and gels: Size-dependent phenomena and re-entrant behavior at early aging times.

Colloidal clay Laponite forms a variety of arrested states that display interesting aging behavior. Microrheology has been applied to Laponite-based glasses and gels, but few studies evaluate the influence of probe particle size. In this work, we report the dynamics and microrheology of Laponite-polymer dispersions during aging using passive microrheology with three different probe particle sizes. At early aging times, the neat Laponite dispersion forms an arrested state; the nature of this state (e.g., a repulsive glass or gel) has remained the subject of debate. The addition of polymer retards gelation and melts the arrested state. While this melting has been observed at the macroscale and has been attributed to a re-entrant transition of a repulsive glass to a liquid state, to our knowledge, it has not been observed at the microscale. The delay of the gelation time needed to form an arrested state was found to depend on the polymer concentration and could vary from ∼24 h for neat Laponite to seven days for some Laponite-polymer samples. Significant effects of probe particle sizes are observed from the mean-squared displacement (MSD) curves as small and intermediate probe particles show diffusive motion, while the motion of large particles is restricted. By examining the factor of ⟨Δr2 (τ)⟩a, structural heterogeneity can be confirmed through the strong size-dependence displayed. Different MSD trends of probe particles are obtained at longer aging times, but no significant changes occur after 30 days of aging. Our microrheology results also reveal significant effects of probe particle size.

Coupled effects of pH and kaolinite colloids on antibiotic transport in porous media

Antibiotics can interact with natural colloids and the surrounding media upon entry into soil and groundwater systems, which significantly alters their dynamic behavior and complicates our understanding of antibiotic fate and transport in porous media. In this study, co-transport of antibiotics and kaolinite colloids was systematically investigated using combined column experiments and numerical simulation under different pH conditions. Sulfadiazine (SDZ) transport was enhanced by kaolinite colloids under neutral and alkaline conditions, which was attributed to the higher mobility of colloids as SDZ carriers, as well as competitive sorption. However, most injected SDZ was transported in a dissolved form owing to the low sorption capacity of SDZ to kaolinite colloids and quartz sand. The colloid-facilitated transport model provided a good description of total SDZ transport, but underestimated colloidal SDZ transport using parameters from kinetic sorption experiments. Kaolinite colloids significantly promoted ciprofloxacin (CIP) transport at pH 4.0, but inhibited it at pH 7.0 and 9.0. Interestingly, enhanced CIP transport was due to the decreased number of effective sorption sites on quartz sand and the increased desorption of CIP from kaolinite colloids. Under neutral and alkaline conditions, deposited colloids provided additional sorption sites for CIP, which contributed to CIP retention. Moreover, CIP significantly inhibited the transport of kaolinite colloids owing to the increases in colloidal aggregate size and zeta potential. Overall, our results highlighted the different effects of mobile and immobile colloids on antibiotic transport, in addition to the implications of antibiotic speciation and clay colloids when predicting the transport behavior of these compounds.

Cover Image, Volume 22, Issue 1

Vadose Zone JournalVolume 22, Issue 1 COVER IMAGEFree Access Cover Image, Volume 22, Issue 1 First published: 20 January 2023 https://doi.org/10.1002/vzj2.20248AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Graphical Abstract On the cover: Scanning electron microscope image of a clay colloid. See Shi et al., “Impact of heavy metal cations on deposition and release of clay colloids in saturated porous media,” https://doi.org/10.1002/vzj2.20179. Volume22, Issue1January/February 2023 RelatedInformation

Karakteristike černozemolike karbonatne livadske crnice Oglednog polja za soju i šećernu repu u Starom Bečeju (Srbija)

In this study, Fluvisol from the area of Stari Beĉej (Serbia) was investigated with the aim of evaluating selected chemical and physical indicators of soil quality affter its long-term use in crop production. Ten bulk soil samples (soil depth: 0-210 cm) were collected to analyze soil chemical properties (humus, CaCO3, pH, P2O5, K2O), texture (six samples, soil depth: 0-130 cm), dry-stable aggregate distribution and stability of soil aggregates (four samples, soil depth: 0-80 cm). Additional undisturbed core samples (100 cm3 volume) were also collected to determine soil bulk density, total porosity, field air capacity, and field water capacity. Water infiltration capacity was also measured in the field using a double ring infiltrometer. The results indicated that the soil had a two-layer profile and a silty loam texture (with a physical clay content of 41-42% and a colloidal clay content of 27-28%). The humus horizon has quite favorable chemical properties up to a depth of 60 cm, and at a depth of 60-80 cm it is alkalized under the influence of saline groundwater. From an agronomic point of view, the structure of this soil is favorable. The results of determining the stabillity soil aggregates show that the soil structure in the Ahp and Ah horizons at depth (0-40 cm) with a content of 41% of water-stable aggregates < 0.25 mm is significantly worse (less favorable) than in the surface part of the AhBca horizon (40-60 cm) with a content of ~58% of waterstable aggregates < 0.25 mm. The main physical properties are not particularly favorable in the Ah and AhBca horizons to a depth of 80 cm. The Ah horizon is very compact (bulk density: 1.33-1.38 g/cm3 ), and the air-filled porosity is very low (less than 5%) and small (5-10%) at all depths up to 80 cm, and it is lowest in the surface 0-20 cm layer, only 3.8%. Infiltration capacity is low. The reserves of soil moisture available for plant growth in the soil layer 0-100 cm are very good (> 160 mm water depth). In summary, the results of this study show that perennial tillage can lead to a loss of soil productivity and serious soil degradation.

Multiphase Coexistence in Binary Hard Colloidal Mixtures: Predictions from a Simple Algebraic Theory.

A general theoretical framework is proposed to quantify the thermodynamic properties of multicomponent hard colloidal mixtures. This framework is used to predict the phase behavior of mixtures of rods with spheres and rods with plates taking into account (liquid) crystal phases of both components. We demonstrate a rich and complex range of phase behaviors featuring a large variety of different multiphase coexistence regions, including two five-phase coexistence regions for hard rod/sphere mixtures, and even a six-phase equilibrium for hard rod/plate dispersions. The various multiphase coexistences featured in a particular mixture are in line with a recently proposed generalized phase rule and can be tuned through subtle variations of the particle shape and size ratio. Our approach qualitatively accounts for certain multiphase equilibria observed in rod/plate mixtures of clay colloids and will be a useful guide in tuning the phase behavior of shape-disperse mixtures in general.

Effects of Salinity and pH on Clay Colloid Aggregation in Ion-Adsorption-Type Rare Earth Ore Suspensions by Light Scattering Analysis

Ion-adsorption-type rare earth ores (IREOs) are an indispensable strategic resource. Rare earths can be extracted from IREOs by means of in situ leaching, which is strongly influenced by the migration of clay components. In order to clarify the effect of the interaction between mineral particles on the rare earth leaching process, the aggregation of IREO colloids was investigated in suspension after NaCl concentration and pH value were disturbed based on a light scattering method. The results show that IREO colloids are prone to unstable aggregation, which can be affected by salinity and pH in suspension. Combined with the analysis of the surface acid–base properties and the zeta potential of the IREO colloids, the long-range electrostatic attraction between mineral heterogeneous charge surfaces plays a leading role in the interaction between mineral particles. In suspension, electrolyte concentration and pH can adjust the strength of the electrostatic force and the force field overlapping between the surface double electric layers to influence the aggregation of the IREO colloids. The above conclusion can enrich and supplement the rare earth extraction theory, which has a certain guiding significance for green exploitation of IREOs.

Time‐dependent viscosity of structured clay suspensions

AbstractTime‐dependent viscosity of structured clay suspensions under a fixed shear was revealed in the prepared slurry samples of clay lenses from the Athabasca oil sands deposit, differing from thixotropic or rheopectic suspensions. At a lower salt (Na or Ca) concentration, the viscosity oscillated with time, which was slowly damped with increasing salt concentrations. The viscosity increased without reaching a maximum plateau, even after a run of a few hours. At 1000 ppm Na concentration, a maximum viscosity was reached, followed by viscosity fluctuation within a narrow range to approach an equilibrium value with time. Both sample ageing and shear history affected the slurry viscosity. It was speculated that the Brownian motion of colloidal clay particles and the applied shear induced rearrangement and continued formation of clay microstructures with time, due to the rotation of anisotropic colloidal clay particles under simple shear flow. Its implications to oil sands research and operation were briefly discussed.

Phosphorus content in water extractable soil colloids over a 2000 years chronosequence of paddy-rice management in the Yangtze River Delta, China

Soil phosphorus (P) is usually hardly present in soluble forms as most of it is strongly bound to minerals. Therefore, transformations of soil colloidal P can play a key role in enhancing soil fertility. Here, we examined water extracts from a chronosequence of soils with 2000 years of paddy rice and 700 years of non-paddy cropping, and fractionated them into <1200 nm, <450 nm, and <3 kDa size fractions using sequential ultrafiltration. Asymmetric-flow field-flow fractionation (AF4) coupled to an organic carbon detector (OCD) and inductively coupled plasma mass spectrometry (ICP-MS) was used for organic carbon and elemental screening in the 3 kDa-450 nm colloidal size range. We found that P increased in all colloidal fractions both with long-term paddy rice and non-paddy managements. Furthermore, P in soil colloids of 3 kDa-450 nm size, as fractionated by AF4, peaked in three size ranges: nanoparticles (NP, ∼3 kDa-20 nm), fine colloids (FC, ∼20–225 nm), and medium-sized colloids (MC, ∼225–450 nm). The NP fraction originally contained mainly P and Ca for tidal wetland (TW), but turned into a complex of OC-Ca-P colloidal forms immediately after onset of arable management, with little temporal change thereafter. Fine colloidal P (FC-P) was detected only after > 50 years for the non-paddy and > 100 years under paddy management, associated with oxides, organic matter and fine clay fragments in the paddy soils and additionally Ca-bound P in the non-paddy ones, respectively. Increasing portions of medium-sized colloidal P appeared after 50 years paddy and non-paddy management, though not exceeding the concentrations of FC-P and with little changes during prolonged arable land use. Overall, after initial formation of FC-P during the first 50–100 years of management, longer term dynamics of colloidal P in these arable soils was fairly independent of paddy or non-paddy managements, but remained closely coupled to the increased presence or generation of fine colloidal clays, oxides and the associated P, thus potentially linking P fertility largely to the dynamics of fine colloids.

Interaction between Gaomiaozi bentonite colloid and uranium

In a deep geological repository system, the presence of clay colloids in the geological disposal medium is considered to be a potential factor promoting nuclide migration. In this study, Gaomiaozi bentonite colloid (GMZC) was extracted using the gravity sedimentation method, and its interaction with U(VI) at neutral pH was studied. Small angle X-ray scattering (SAXS), X-ray photoelectron spectroscopy (XPS), dynamic light scattering, and other characterizations methods were used to analyze and observe the microstructure and agglomeration morphology of the bentonite colloidal particles before and after the reaction with U(VI) at the macro-meso-microscopic level. The XPS results indicate that the U(VI) reacted with the Si–O and Al–O on the GMZC particle surfaces at room temperature, and the hydrolyzed product of the U(VI) covered the colloidal surfaces in the neutral water environment, resulting in a decrease in the absolute zeta potential (from ∼31 mV to ∼19 mV). The hydrodynamic diameter of the colloidal particles did not change significantly (∼336 nm–∼360 nm), but the small-angle scattering data revealed that the fractal dimension was larger after the reaction (∼2.8–∼3.1), indicating particle agglomeration. About 14% of the GMZC particles were precipitated after reacting with the U(VI). When the temperature was increased to 55 °C and 85 °C, the precipitation did not change significantly, but the fractal dimension of the mixed system increased (from ∼3.1 to ∼3.6), and the d001 peak was not observed in the SAXS results. When the temperature was decreased by 25 °C, the hydrodynamic diameter of the zeta potential and SAXS image both returned to the level before the temperature increase, indicating that the unstable change in the colloidal system caused by the temperature increase was reversible.

Long-distance mobilization of chromium(III) in soil associated with submicron Cr2O3

Trivalent chromium is generally assumed to form insoluble species, resulting in low mobility of Cr(III) in soils. Here, we report continuous distributions (0–19 m) of a high concentration of Cr(III) in the alkaline soils of a historically industrial site for producing Na2Cr2O7, CrO3, and Cr2O3, which challenges this abovementioned conventional wisdom. The thermodynamic equilibrium model showed the low possibility of Cr(III) originating from Cr(VI) reduction under the redox conditions of this study. The AF4-MALLS-ICP-MS and μ-XRF-XANES were used to identify the particle size distribution of Cr(III)-containing colloids and Cr(III) species in mobile colloids. In any soil layer, Cr(III) accounts for 71.1–94.3% of the total Cr in submicron soil colloids and is composed of submicron intrinsic Cr2O3 (55.2%−63.8%), Cr(OH)3 (0–33.0%), and Cr(III) adsorbed by ferrihydrite (0–19.0%) and clay montmorillonite (11.1%−21.1%) colloid. On the contrary, Cr(VI) was mainly distributed in bulk soil (> 2 µm) except for the topsoil, accounting for 62.6–90.0% of total Cr(VI). Organic matter content and soil texture are the most critical factors driving the mobilization of submicron colloids in soils by principal component analysis. Humic acid (HA) formed HA-corona on Cr2O3 surface and enhanced colloidal dispersion, thereby accelerating the long-distance mobilization of submicron Cr2O3 colloids in alkaline soil layers, whereas the heteroaggregation of clay colloid with Cr2O3 was only favorable for short-distance mobilization. Our findings help to re-recognize the potential migration risks of insoluble heavy metals in soils.