Montmorillonite Colloids Research Articles

Humic acid enhances the co-transport of colloids and phosphorus in saturated porous media

Phosphorus (P) has been widely recognized as a substance that is difficult to transport due to its tendency to become easily fixed in the soil. However, many reports demonstrate that groundwater P pollution is rising in humus-rich areas. Research is urgently needed to confirm (or reject) the hypothesis that increased P pollution is related to humus, as there is currently limited quantitative research on this topic. In this study, we conducted a series of batch equilibrium adsorption-desorption experiments and column experiments to quantify the effects of montmorillonite colloids (MCs) and humic acids (HCs, the main components of humus) on the P transport behavior. The results indicate that P's adsorption and desorption behavior on MCs can be well simulated using the Langmuir and Temkin models (R2 > 0.91). Compared to the non-HC treatments, HCs significantly increased MCs' P adsorption and desorption capacity 5.18 and 7.21 times, respectively. Moreover, HCs facilitated the transport ability of the MC-P mixture through the saturated quartz sand column. In a 0.1 M NaCl solution, the MC-P mixture is nearly completely adsorbed on the surface of quartz sand, with a penetration rate of only 0.5%. In contrast, the HC-MC-P mixture can evidently penetrate further at a rate of 26.1%. The transport parameters fitted using HYDRUS-1D further indicated that the presence of humic acids significantly decreased the deposition coefficients of colloids, thereby enhancing the co-transport of colloids and P through the quartz sand porous medium. The potential mechanism of P pollution in humus-rich areas is likely enhanced by the formation of an HC-colloid-P mixture, which greatly increases the adsorption amount of P on colloids and enhances the electrostatic and spatial repulsion between colloids as well as between colloids and quartz sand. It reduces the aggregation and adsorption of colloids, ultimately transferring P into groundwater through colloid-facilitated co-transport. The findings of this study clarified the relationship between the transport of P, colloids, and HCs, which provides a theoretical basis for explaining the P pollution mechanism in humus-rich areas.

Different co-transport forms of ciprofloxacin with montmorillonite colloids in porous media as affected by pH and cations

Different co-transport forms of ciprofloxacin with montmorillonite colloids in porous media as affected by pH and cations

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.

Molecular insights into the aggregation mechanism of montmorillonite colloid due to calcium contamination: A molecular dynamics simulation study

The montmorillonite colloid is an important part of the drilling fluid system. However, the calcium contamination in reservoir can lead to severe aggregation of montmorillonite colloids, which challenges the application of drilling fluid systems in reservoirs with high salinity. To provide insight into the calcium-induced aggregation of montmorillonite colloids, in this work, all-atom molecular dynamics (MD) simulations were adopted to study the morphology and stability of montmorillonite colloid in calcium ion environment, and the cases of ion-free, sodium, and potassium were also considered as comparison. Our simulations demonstrated that montmorillonite particles could agglomerate in three modes: edge-to-edge agglomeration (EEA), partial face-to-face agglomeration (PFFA), and face-to-face agglomeration (FFA). The EEA and PFFA modes were observed in all models, but the FFA mode was the characteristic agglomeration morphology of montmorillonite particles under the effect of Ca2+, which provides implications for the failure of the drilling fluid systems due to calcium contamination. Furthermore, molecular mechanisms underlying the calcium-induced aggregation montmorillonite colloids were investigated. Firstly, the strong electrostatic attraction of Ca2+ enhanced the binding, while the relatively large hydrated radius of Ca2+ allowed increased interlayer space and reduced repulsion. Secondly, the monolayer arrangement and pronounced hydration of Ca2+ led to the elimination of multi-layer cation repulsion and an increase in hydration-induced cohesion. These results extending our knowledge of the stability of montmorillonite colloids present a supplementary perspective to the DLVO theory and will serve as a valuable reference and inspiration for future research on montmorillonite colloid.

Sorption of the β-Blocker Metoprolol on Montmorillonite Colloids as a Function of pH and Concentration of Competing Sodium Ions

Sorption of the β-Blocker Metoprolol on Montmorillonite Colloids as a Function of pH and Concentration of Competing Sodium Ions

Experimental study of montmorillonite erosion in low ionic strength water under stagnant and flow conditions in artificial fractures

Compacted bentonite has been proposed as a buffer material in several geological repository concepts for high-level radioactive waste. However, in dilute groundwater, bentonite, especially when dominated by Na-montmorillonite, has been found to be susceptible to erosion and excessive mass loss may compromise the safety of the repository. Herein, results from twelve erosion tests with either flowing or stagnant aqueous solutions, performed with custom-designed artificial fractures made of Poly(methyl methacrylate) are reported. The fracture aperture was 0.1 mm in all tests. Na-montmorillonite or an equal mix by weight of Na- and Ca-montmorillonite were employed. Eleven of the tests were performed with the fracture being vertical, and in one test the fracture was horizontal. The transparent artificial fracture setup allowed monitoring of the expansion of the montmorillonite into the fracture. In all but one test, the diameter reached a limit value, normally within one month of the start of the test. The final diameters varied between 5.9 and 8.7 cm and no clear correlation to flow rate, type of montmorillonite or ionic strength could be discerned. The initial diameter of the emplaced clay was 3.5 cm. The erosion was monitored regularly by turbidity measurements. In general, erosion rates decreased with time and were ultimately low enough that an extrapolation or upscaling of the results to repository scale and operating time would imply a total erosion below the critical value for repository safety. This was the case for all vertical fractures, regardless of montmorillonite type and ionic strength of the aqueous solution. The observed decrease in erosion rates was not due to depletion of montmorillonite at the source because at the end of each test 90% or more of the montmorillonite remained. Further, the ionic strengths were below the critical coagulation concentration, hence gelation cannot provide an explanation to the reduction in erosion rates. Finally, the diameters of the extruded montmorillonite reached the limit while the erosion rates continued to decrease. Consequently, the observed limited expansion was not due to a steady state between erosion and swelling. Rather, the expansion behaviour suggests the presence of frictional forces between the montmorillonite and the fracture walls. The results from this study are beyond the current understanding of montmorillonite colloid behaviour and cannot be explained by existing models.

Protein–Mineral Composite Particles with Logarithmic Dependence of Anticancer Cytotoxicity on Concentration of Montmorillonite Nanoplates with Adsorbed Cytochrome c

Montmorillonite (MM) colloid nanoplates have high adsorption capacity due to their large size/thickness ratio, which allows them to be used as carriers for drug delivery. Upon adsorption of the mitochondrial protein cytochrome c (cytC) onto MM plates, the composite cytC-MM particles acquire anticancer properties because of the ability of cancer cells to phagocytize submicron particles (in contrast to the normal cells). In this way, exogenous cytC can be introduced into tumor cells, thereby triggering apoptosis-an irreversible cascade of biochemical reactions leading to cell death. In the present study, we investigated the physicochemical properties of cytC-MM particles as a function of the cytC concentration in the suspension, namely, the electrophoretic mobility, the mass increment of MM monoplates upon cytC adsorption, the ratio of the adsorbed to the free cytC in the bulk, the protein density on the MM's surface, the number of cytC globules adsorbed on an MM monoplate, the concentration of cytC-MM composite particles in the suspension, and the dependence of cytotoxicity on the cytC-MM particle concentration. For this purpose, we used microelectrophoresis, static and electric light scattering, and a colon cancer cell culture to test the cytotoxic effects of the cytC-MM suspensions. The results show that the cytotoxicity depends linearly on the logarithm of the particle concentration in the cytC-MM suspension reaching 97%.

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.

Transport of montmorillonite colloid in unsaturated packed column: The combined effects of sand grain size, flow rate and colloid concentration

The transport of colloid in unsaturated porous media affects the migration of contaminants and thus is closely related to groundwater resources protection. To figure out the combined effects of grain size, colloid concentration and injection flow rate on montmorillonite colloid transport characteristics in unsaturated quartz sand, a total of 27 sets of column experiments were conducted with three kinds of quartz sand (20, 40, 60 mesh), three flow rates (1.98, 3.96, 5.94 cm3/min) and three colloid concentrations (300, 600, 900 mg/L), using three packed columns with the inner diameter of 11 cm and the height of 40 cm. The experimental results showed that the transport of Na-montmorillonite colloid particles in the unsaturated quartz sand columnoccurred with significant retention. In the 20 mesh quartz sand column, the average peak values of the penetration curves for low and high concentration colloidal solutions of 300 and 900 mg/L increased by 44% and 27%, respectively, as the flow rate increased from 1.98 to 5.94 cm3/min. The average peak value of the colloidal solution with concentration 300 mg/L increased 17% more than that of the colloidal solution with 900 mg/L for increasing flow rate. When the injection flow rate of the colloidal solution was increased from 1.98 to 5.94 cm3/min, the effect of flow rate on colloidal transport was the most obvious, followed by the effect of media particle size, while the effect of colloidal solution concentration was the least. The calculation based on the total potential energy of Derjaguin-Landau-Verwey-Overbeek (DLVO) and collision efficiency further explained the retention of colloids in unsaturated porous media.

Polarisation‐induced covalent interactions between H+ and surface O atoms promote clay aggregation

AbstractAccumulation of H+ can cause soil acidification, affect soil particle interactions, and alter soil processes. In this study, aggregation kinetics of montmorillonite colloids induced by monovalent ions of Li+ and H+ were determined by dynamic light scattering. The experimental results indicated that the critical coagulation concentration (CCC) value of Li+ was 4.5 times that of H+, indicating that H+ can strongly promote montmorillonite colloids aggregation. Joint analysis of the adsorption force type and energy of Li+ and H+ as well as the infrared spectroscopy of Li+ and H+ montmorillonite indicated that there was only electrostatic adsorption for Li+, while there was not only electrostatic adsorption but also a new type of covalent adsorption for H+: polarisation‐induced covalent adsorption. The total adsorption energy of H+ is thus much higher than that of Li+ on montmorillonite surfaces. Moreover, the polarisation‐induced covalent adsorption energy of H+ increased linearly with increasing electric field strength, which is consistent with the quantum mechanical analysis of the asymmetric orbital hybridisation of surface O atoms on montmorillonite. On this basis, the polarisation‐induced covalent adsorption energy of H+ was employed to predict the CCC value of H+ induced montmorillonite aggregation, and it was 7.40 mmol L−1. The high agreement with experimental results (7.26 mmol L−1) proved that the polarisation‐induced covalent interactions between H+ and surface O atoms promotes montmorillonite aggregation. This polarisation induced covalent effect between H+ and surface O atoms might further affect the formation and stability of soil aggregates and the pore status of soil structure, especially as soil acidification is currently becoming a global issue. The discovery from this study will improve our understanding of the effects of soil acidification on soil properties and processes.Highlights H+ greatly promoted clay aggregation. Polarization‐induced covalent between H+ and surface O presented at clay surface. Polarization‐induced covalent bonding energy increase with increasing electric field. Electrostatic and covalent bonding forces jointly determined clay aggregation.

Cotransport of nano-hydroxyapatite and different Cd(II) forms influenced by fulvic acid and montmorillonite colloids

Soil colloids can affect the cotransport of nanoparticles and pollutants. In this study, the influencing mechanisms of organic fulvic acid (FA) and inorganic montmorillonite colloid (MONT) on the cotransport of nHAP and Cd(II) were investigated. Column experiments combined with Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, attachment efficiency calculation and two-site kinetic retention model were applied to study the mechanisms. Results showed that the co-existence of FA or MONT made the transport of nHAP improved by 58–75% and 33–59%, respectively. Both of them could improve the stability of nHAP particles and enhance electrostatic repulsion between nHAP particles and sand. Retention of nHAP in the sand was mainly caused by secondary energy minimum and physical straining. The co-existence of FA or MONT changed the amount of adsorbed species of Cd(II) and decreased the retardation effect of nHAP on Cd(II) transport. With increasing FA concentration, soluble FA·Cd and suspended nHAP·FA·Cd complexes in the system increased. Transport of soluble Cd(II) and total Cd(II) were strengthened due to the concentration effect of FA and the improved stability of nHAP particles. With increasing MONT concentration, the amount of soluble Cd(II) decreased, but that of colloidal Cd(II) (nHAP·Cd and MONT·Cd) increased. Due to the stronger effect of colloidal Cd(II) change than that of the soluble Cd(II) change, the transport of total Cd(II) was improved by 34–57%. The findings of this study can help to understand the fate of nanoparticles and Cd(II) in natural water and soil.

Colloid-mediated transport of tetracycline in saturated porous media: Comparison between ferrihydrite and montmorillonite

Given the ubiquitous mineral (e.g., clays and iron oxides) playing critical roles in impacting the fate of antibiotics in the subsurface environment, the effects of two mineral colloids (i.e., ferrihydrite and montmorillonite) on tetracycline (TC, a representative of antibiotic) transport in sand columns were investigated in this study. Interestingly, the results clearly showed that ferrihydrite colloids inhibited TC transport, while montmorillonite colloids enhanced TC mobility under neutral conditions (pH 7.0). This phenomenon resulted from the positively charged ferrihydrite colloids with weak mobility, which assisted TC deposition; besides, providing additional deposition sites for TC by the deposited ferrihydrite colloids was another important mechanism. In contrast, the transport-enhancement effect of montmorillonite on TC was attributed to the strong binding affinity of TC to clay particles as well as the competition between colloids and TC for deposition sites on sand surfaces. Moreover, the transport-inhibition effect of ferrihydrite at pH 7.0 was greater than that at pH 5.0, mainly due to more colloid-associated TC under neutral conditions. Surprisingly, ferrihydrite colloids could act as carriers of antibiotics and enhanced TC transport at pH 9.0. Because the surface charge of colloids was altered to negative and could break through the column. Meanwhile, the transport-enhancement effect of montmorillonite decreased with increasing pH from 5.0 to 9.0, resulting from the decrease of colloid-adsorbed TC. Furthermore, colloid-mediated transport of TC was influenced by ionic strength, which affected the aggregation characteristics of colloids and the binding affinities of TC to minerals. These findings provide critical information for assessing the risks of antibiotics in aquatic ecosystems where abundant natural minerals are present.

Montmorillonite colloid plates with adsorbed cytochrome c: in vitro cytotoxic effect on colon cancer cell culture

BackgroundThe apoptosis (a cascade of biochemical reactions leading to suicide of damaged biological cells) is blocked in the cancer cells because of impossibility of cytochrome c (cytC) go out from the mitochondria. However, the apoptosis can be started by introducing of exogenous cytC into cytoplasm using colloid particles as a protein carrier due to ability of the cancer cells to phagocytize extracellular particles with submicron size.ResultsThe clay mineral montmorillonite (MM) were used to prepare aqueous suspension of protein/mineral composite particles by electrostatic adsorption of the positively charged cytC globules on the negatively charged MM colloid plates, and then added to colon cancel culture. The results shows out that separately cytC and MM have no effect but the composite cytC-MM particles kill 95% of the cancer cells after 96 h treatment using equine cytC which is 97% structurally identical with the human cytC. To reach this high cytotoxicity we have formulated requirements to: (a) bare colloid particles (electric charge, form and size), (b) conditions for protein adsorption (concentrations, pH, ionic strength), and (c) suspension with the composite particles (positive total charge and optimal concentration). Due to satisfying these requirements we have reached cytotoxicity which is 1/3 higher than the reached by other authors using different artificial particles. The cytotoxicity rapidly increases with concentration of the cytC-MM particles but further it shows tendency to saturation.MethodsThe optimal pH 6.5 and the 10:3 mg/mg cytC/MM concentration ratio at adsorption were found out by employing computer (protein electrostatics) and physicochemical methods (microelectrophoresis and colloid electrooptics) to prepare cytC-MM suspension. The anticancer capability of cytC-MM nanoplates were investigated using cell culture of metastasizing colon cancer.ConclusionThe in vitro experiments with colon cancer cell culture disclose that cytC-MM composite particles have potential for application in anticancer therapy of superficial neoplasms of the skin and the alimentary system (mouth cavity, esophagus, stomach, jejunum and colon).Graphic abstract

A research of colloidal silver immobilization in bionanocomposites of natural polymers and montmorillonite

Currently, colloidal silver particles are used in the creation of electronic, optical, and sensor devices of a new generation. Silver-containing bionanocomposites (BNCs) were synthesized by immobilization of colloidal montmorillonite particles containing colloidal silver in a composition of sodium alginate and sodium salt of carboxymethylcellulose. Silver-containing montmorillonite particles Ag-Mt were obtained by replacing Na+ ions in layered silicate galleries with Ag+ ions, followed by the transformation of silver ions into silver particles. The introduction of Ag+ ions into the montmorillonite structure is justified by infrared spectroscopy. When studying the strength of bionanocomposite films, it was found that with an increase in the content of Ag-Mt particles in their composition, the strength increases and the deformation decreases.It is found that the equilibrium values of the swelling constant are set in ~30 minutes. At the same time, with an increase in the Ag-Mt content in the bionanocomposite from 3 % to 10 %, the value of the equilibrium swelling coefficient (Kswell) decreases by 2.8 times. The replacement of Na+ ions with Ag+ ions in the montmorillonite structure is accompanied by a decrease in the swelling of bionanocomposites, which is explained by the lower hydration of Ag+ ions compared to Na+ ions. As another reason for the decrease in the swelling of bimonanocomposites with an increase in the proportion of Ag-Mt in their composition, enhancing their ability to structure formation in the prersence of a clay mineral is indicated.The kinetics of the release of Ag+ ions from bionanocomposites into saline has been studied. It is shown that the release of Ag+ ions increases with increasing pH of the medium

Colloid-facilitated transport of 238Pu, 233U and 137Cs through fractured chalk: Laboratory experiments, modelling, and implications for nuclear waste disposal

The influence of montmorillonite colloids on the mobility of 238Pu, 233U and 137Cs through a chalk fracture was investigated to assess the transport potential for radioactive waste. Radioisotopes of each element, along with the conservative tracer tritium, were injected in the presence and absence of montmorillonite colloids into a naturally fractured chalk core. In parallel, batch experiments were conducted to obtain experimental sorption coefficients (Kd, mL/g) for both montmorillonite colloids and the chalk fracture material. Breakthrough curves were modelled to determine diffusivity and sorption of each radionuclide to the chalk and the colloids under advective conditions. Uranium sorbed sparingly to chalk (log Kd = 0.7 ± 0.2) in batch sorption experiments. 233U(VI) breakthrough was controlled primarily by the matrix diffusion and sorption to chalk (15 and 25% recovery with and without colloids, respectively). Cesium, in contrast, sorbed strongly to both the montmorillonite colloids and chalk (batch log Kd = 3.2 ± 0.01 and 3.9 ± 0.01, respectively). The high affinity to chalk and low colloid concentrations overwhelmed any colloidal Cs transport, resulting in very low 137Cs breakthrough (1.1–5.5% mass recovery). Batch and fracture transport results, and the associated modelling revealed that Pu migrates both as Pu (IV) sorbed to montmorillonite colloids and as dissolved Pu(V) (7% recovery). Transport experiments revealed differences in Pu(IV) and Pu(V) transport behavior that could not be quantified in simple batch experiments but are critical to effectively predict transport behavior of redox-sensitive radionuclides. Finally, a brackish groundwater solution was injected after completion of the fracture flow experiments and resulted in remobilization and recovery of 2.2% of the total sorbed radionuclides which remained in the core from previous experiments. In general, our study demonstrates consistency in sorption behavior between batch and advective fracture transport. The results suggest that colloid-facilitated radionuclide transport will enhance radionuclide migration in fractured chalk for those radionuclides with exceedingly high affinity for colloids.

Assessment of the colloidal montmorillonite dispersion as a low-cost and eco-friendly nanofluid for improving thermal performance of plate heat exchanger

Synthetic nanofluids are produced by expensive and complex procedures which hinders their large-scale application in thermal systems. In this study the montmorillonite clay as a natural nanostructure material with a high dispersion ability has been introduced for achieving low cost and eco-friendly water-based green nanofluid. For this purpose, a set of montmorillonite nanofluids (MMTNFs) with different weight fractions of 0.2, 0.4, 0.6, 0.8, and 1% was mechanically prepared. The concentration-dependent properties of the samples including stability, thermal conductivity and viscosity were experimentally evaluated. The results indicated that MMTNFs were accompanied by a primary instability, but they gradually possessed excellent long-term stability. Adding montmorillonite to the water could enhance thermal conductivity up to 8.3%, which is worth with considering the naturalness, abundance and low-cost of this clay. MMTNFs exhibited excellent thermal performance when utilized as a working fluid instead of the pure water in a plate heat exchanger (PHE). The results showed that the best performance of PHE was obtained at an optimum MMT concentration (0.6 wt%). At optimal condition, the overall heat transfer coefficient, heat transfer rate, and thermal efficiency improved 27.25%, 17.65%, and 7.82%, respectively.

Effects of clay colloids on ciprofloxacin transport in saturated quartz sand porous media under different solution chemistry conditions.

Antibiotics, a highly prevalent class of environmental organic pollutants, are becoming a matter of global concern. Clay minerals that are ubiquitous in subsurface environments may play an important role in the fate and transport of antibiotics. Taking ciprofloxacin (CIP) as a model antibiotic, this work explored the role of clay colloids (kaolinite and montmorillonite) on the adsorption and transport of CIP under different chemical solution conditions. The adsorption isotherms showed that montmorillonite colloids had a larger CIP sorption capacity than kaolinite colloids. The results of transport experiments indicated that montmorillonite colloids could promote CIP transport in saturated sand columns, but the addition of kaolinite colloids affected CIP mobility to a much smaller extent. The much stronger transport-enhancement effect of montmorillonite colloids was due to CIP adsorbed strongly to the colloids and desorption hysteresis of colloid-adsorbed CIP, likely stemming from the intercalation of this antibiotic in the interlayer of montmorillonite. Interestingly, transport of clay colloids increased with the increasing pH from 5.0 to 9.0; however, CIP transport decreased with the increasing pH in the presence of clay colloids. The observations were likely attributable to pH-dependent ciprofloxacin adsorption/desorption to clay minerals. Increasing the concentrations of NaCl and CaCl2 generally decreased the contaminant-mobilizing ability of montmorillonite colloids, mainly by increasing the aggregation of colloids and thus, decreasing the transport of colloid-adsorbed CIP. Moreover, under the test conditions (1mM NaCl and pH 7.0), the presence of CIP inhibited the transport of clay colloids due to the increase in aggregate size of clay colloids with the addition of CIP. Overall, these findings suggest that clay colloids with high adsorption abilities for antibiotics in the subsurface environment may act as a carrier for certain antibiotic compounds.

Uranium(VI) adsorption on montmorillonite colloid

Montmorillonite colloid was synthesized and characterized and the adsorption of U(VI) on colloid as a function of contact time, temperature, initial concentration of U(VI) solution, pH and foreign ions was investigated through batch experiments. The results illustrate that the adsorption reaches equilibrium quickly within 30 mins. Results showed that the absorption process is influenced by pH value, temperature and ions, among which pH value is the most significant. Adsorption percentage increases obviously at pH = 4.0–7.0, and then decreases with the increase of pH value. The competitive cations also inhibit the adsorption. pH value and ions affect the surface properties and charges of the colloid and the chemical form of nuclides. The experimental data was analyzed in detail. The maximum adsorption capacity obtained by fitting the second-order kinetic model is very close to the experimental data. The Langmuir model fits the experimental data better than Freundlich models, R2 = 0.991. The results of fitting two adsorption isotherms show that the adsorption of colloids on uranium has both physical and chemical adsorption, and is mainly monolayer adsorption. Furthermore, the adsorption process is a spontaneous endothermic reaction by calculating the thermodynamic parameters. This experiment can provide some data basis for the adsorption study of U(VI) on colloids.

Effects of fulvic acid and montmorillonite colloids at different concentrations on Cd(II) sorption onto nano-hydroxyapatite

Natural colloids can influence the binding mechanisms between nano-hydroxyapatite (nHAP) and Cd(II). In this study, the effects of organic and inorganic natural colloids on Cd(II) sorption onto nHAP were compared. Different experimental approaches combined with the additivity model and the Extended-Derjaguin-Landau-Verwey–Overbeek model were used to quantify the distribution of Cd(II) in the systems of nHAP and natural colloid, and the interaction energy between particles. The results showed that both fulvic acid (FA) and montmorillonite colloid (MONT) had the promotion and inhibition effects on Cd(II) sorption onto nHAP. Coexistence of FA or MONT could stabilize nHAP particles. FA could adsorb onto nHAP particle surface via carboxylic and phenolic groups, which increased nHAP electronegativity and formed steric resistance effect. Coexistence of MONT mainly increased nHAP electronegativity. These effects prevented the reduction of the specific surface area of nHAP particles and increased the Cd(II) sorption onto nHAP. However, the inhibition effect on Cd(II) sorption was enhanced with increasing concentration of FA or MONT because more soluble FA-Cd or suspended MONT-Cd complexes formed in the system. In nHAP-FA-Cd systems, the Cd(II) sorption onto FA was well predicted but that onto solid phase was underestimated by the additivity model. In nHAP-MONT-Cd systems, Cd(II) sorbed onto mixtures of nHAP and MONT was well described by the additive model. The findings of this study can help to understand the fate of Cd(II) in natural water and soil.

Radionuclide transport in brackish water through chalk fractures

Mobility of radionuclides originating from geological repositories in the subsurface has been shown to be facilitated by clay colloids. In brackish water, however, colloids may flocculate and act to immobilize radionuclides associated with them. Furthermore, little research has been conducted on radionuclide interactions with carbonate rocks. Here, the impact of bentonite colloid presence on the transport of a cocktail of U(VI), Cs, Ce and Re through fractured chalk was investigated. Flow-through experiments were conducted with and without bentonite colloids, present as a mixture of bentonite and Ni-altered montmorillonite colloids. Ce was used as an analogue for reactive actinides in the (III) and (VI) redox states, and Re was considered an analogue for Tc. Filtered brackish groundwater (ionic strength = 170 mM) pumped from a fractured chalk aquitard in the northern Negev Desert of Israel, was used as a solution matrix.Rhenium transport was identical to that of the conservative tracer, uranine. The sorption coefficient (Kd) of U(VI), Cs and Re, calculated from batch experiments with crushed chalk, proved to be a good predictor of mass recovery in transport experiments conducted without bentonite colloids. A meaningful Kd value for Ce could not be calculated due to its precipitation as a Ce-carbonate colloids. Transport of both U(VI) and Cs was indifferent to the presence of bentonite colloids. However, the addition of bentonite in the injection solution effectively immobilized Ce, decreasing its recovery from 17-41% to 0.8–1.4%. This indicates that radionuclides which interact with clay colloids that undergo flocculation and deposition may effectively be immobilized in brackish aquifers. The results of this study have implications for the prediction of potential mobility of radionuclides in safety assessments for future geological repositories to be located in fractured carbonate rocks in general and in brackish groundwater in particular.