Secondary Minimum Research Articles

Thermodynamic prediction and experimental investigation of short-term dynamic membrane formation in dynamic membrane bioreactors: Effects of sludge properties.

In dynamic membrane bioreactors (DMBRs), a dynamic membrane (DM) forms on a support material to act as the separation membrane for solids and liquids. In this study, batch filtration tests were carried out in a DMBR using nylon mesh (25 μm) as support material to filtrate sludge suspensions of variable properties from three different sources to evaluate the effects on the short-term DM formation process (within 240 min). Furthermore, the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory was applied to analyze the sludge adhesion and cohesion behaviors on the mesh surface to predict quantitative parameters of the short-term DM formation process (including initial formation and maturation stage). The filtration results showed that the order of the initial DM formation time (permeate turbidity <1 NTU as an indicator) was as follows: sludge with poor settleability and dewaterability < normal sludge < sludge with poor flocculability. Moreover, normal sludge (regarding settleability, dewaterability, flocculability, and extracellular polymeric substance) showed a more acceptable DM formation performance (short DM formation time, low permeate turbidity, and high permeate flux) than sludge with poor settleability, dewaterability and flocculability. The influence of sludge properties on the initial DM formation time corroborates the prediction of sludge adhesion behaviors by XDLVO theory. Additionally, the XDLVO calculation results showed that acid-based interaction, energy barrier, and secondary energy minimum were important determinants of the sludge adhesion and cohesion behaviors. Therefore, short-term DM formation process may be enhanced to achieve stable long-term DMBR operation through positive modification of the sludge properties.

Investigating microalgae cell-microsphere interactions during microalgae harvesting by ballasted dissolved air flotation through XDLVO theory

Ballasted dissolved air flotation (BDAF) method is an innovative and efficient technology for harvesting microalgae. However, this flotation process is not yet fully understood. In this study, surface properties of Scenedesmus obliquus (S. obliquus), Chlorella vulgaris (C. vulgaris) and microspheres were characterized by Zeta potential and contact angle measurements, and then the attachment behavior of the microalgae cells on microspheres was investigated by thermodynamic and extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory approaches. The calculated results revealed that the van der Waals (vdW) interaction played an important role in determining the net attraction between the microalgae cell and the microsphere, while the Lewis acid-base (AB) interaction was always repulsion. Microalgae harvesting efficiency could be significantly improved by adding ferric chloride and lowering the pH value because the electrostatic (ES) interaction changed from repulsion to neutralization, and even to attraction, while the vdW and the AB interactions remained almost unchanged. The XDLVO analysis indicated that there was an effective attachment with a secondary minimum of −0.548 × 103 kT for S. obliquus-microsphere system and −0.744 × 103 kT for C. vulgaris-microsphere system. The deeper the well depth of secondary minimum, the better the attachment of microalgae cells to microspheres, which may result in a better harvesting efficiency. Finally, a suitable XDLVO prediction model was proposed to describe the relationship between microalgae cell-microsphere interactions and harvesting efficiency of BDAF.

Influence of aggregation on nanoscale titanium dioxide (nTiO2) deposition to quartz sand

Although extensive research has been conducted to investigate nTiO2 aggregation and deposition, effects of aggregation on concurrent/subsequent deposition of nTiO2, which has important implications to the fate and transport of nTiO2 in groundwater, has received only limited attention. The objective of this study was to investigate how pH, dissolved organic matter (DOM), and valence of background solution cation influence aggregation and concurrent/subsequent deposition of nTiO2. Experiments were performed to examine nTiO2 aggregation and deposition onto quartz sand with co-present illite, kaolinite, and montmorillonite colloids under various geochemical conditions. Results showed that nTiO2 formed hetero-aggregates (i.e., nTiO2-clay aggregates) at low pH when nTiO2 and clay colloids carried opposite charges, and the hetero-aggregates may either deposit or remain suspended depending on their interactions with quartz sand and Fe/Al oxyhydroxide coatings. Deposition of nTiO2 and/or nTiO2-clay aggregates occurred as a result of electrostatic attraction, secondary minimum, and potentially Mg2+ bridging. Humic acid prevented nTiO2 aggregation and deposition under most conditions. In MgCl2 solutions, however, it facilitated deposition by adsorbing to nTiO2 and Fe/Al oxyhydroxides, thereby enabling Mg2+ bridging. This study demonstrated the important and complex roles of pH, DOM, cation valence, and clay colloids in controlling aggregation and subsequent deposition of nTiO2.

Floating and Tether-Coupled Adhesion of Bacteria to Hydrophobic and Hydrophilic Surfaces.

Models for bacterial adhesion to substratum surfaces all include uncertainty with respect to the (ir)reversibility of adhesion. In a model, based on vibrations exhibited by adhering bacteria parallel to a surface, adhesion was described as a result of reversible binding of multiple bacterial tethers that detach from and successively reattach to a surface, eventually making bacterial adhesion irreversible. Here, we use total internal reflection microscopy to determine whether adhering bacteria also exhibit variations over time in their perpendicular distance above surfaces. Streptococci with fibrillar surface tethers showed perpendicular vibrations with amplitudes of around 5 nm, regardless of surface hydrophobicity. Adhering, nonfibrillated streptococci vibrated with amplitudes around 20 nm above a hydrophobic surface. Amplitudes did not depend on ionic strength for either strain. Calculations of bacterial energies from their distances above the surfaces using the Boltzman equation showed that bacteria with fibrillar tethers vibrated as a harmonic oscillator. The energy of bacteria without fibrillar tethers varied with distance in a comparable fashion as the DLVO (Derjaguin, Landau, Verwey, and Overbeek)-interaction energy. Distance variations above the surface over time of bacteria with fibrillar tethers are suggested to be governed by the harmonic oscillations, allowed by elasticity of the tethers, piercing through the potential energy barrier. Bacteria without fibrillar tethers “float” above a surface in the secondary energy minimum, with their perpendicular displacement restricted by their thermal energy and the width of the secondary minimum. The distinction between “tether-coupled” and “floating” adhesion is new, and may have implications for bacterial detachment strategies.

Differences in transport behavior of natural soil colloids of contrasting sizes from nanometer to micron and the environmental implications

Transport behaviors of nanoparticles (<100nm) and clay fractions (clay particles, <2μm; coarse clay particles, 1–2μm and fine clay particles, 0.1–1μm) extracted from two natural soils (Inceptisol from Jilin and Oxisol from Hainan, China) were investigated in saturated sand columns at 1–30mM NaCl and pH5–9. Increasing NaCl concentrations decreased the mobility, while increasing pH increased the mobility of soil particles of various sizes. At pH5 and 30mM NaCl, nanoparticles and clay fractions exhibited the different transport behaviors, and ripening was observed for Inceptisol nanoparticles while blocking for Oxisol nanoparticles in breakthrough curves (BTCs). The effluent mass recoveries (MRs) of nanoparticles were much more than that of clay particles for both two soils (>1.9-fold) at all tested conditions, except for Inceptisol at pH5 and 30mM NaCl (with comparable MR). According to Derjaguin-Landau-Verwey-Overbeek (DLVO) calculations and particle-collector size ratios, both secondary energy minimum and physical straining led to the retention of clay fractions at pH5 and 30mM NaCl, whereas primary energy minimum and straining induced by simultaneous aggregation caused the retention of nanoparticles. The experimental attachment efficiency between soil particles of various sizes and sand collector for both two soils was in the order nanoparticles<fine clay particles<clay particles<coarse clay particles, indicating that soil nanoparticles had greater mobility than clay fractions. Consequently, nanoparticles are suggested to have greater risks to transport contaminants to surface and groundwater, once they are released from soils.

Structural changes in the hot Algol OGLE-LMC-DPV-097 and its disc related to its long cycle

Abstract Double Periodic Variables (DPVs) are hot Algols showing a long photometric cycle of uncertain origin. We report the discovery of changes in the orbital light curve of OGLE-LMC-DPV-097 which depend on the phase of its long photometric cycle. During the ascending branch of the long cycle the brightness at the first quadrature is larger than during the second quadrature, during the maximum of the long cycle the brightness is basically the same at both quadratures, during the descending branch the brightness at the second quadrature is larger than during the first quadrature, and during the minimum of the long cycle the secondary minimum disappears. We model the light curve at different phases of the long cycle and find that the data are consistent with changes in the properties of the accretion disc and two disc spots. The disc’s size and temperature change with the long-cycle period. We find a smaller and hotter disc at minimum, and larger and cooler disc at maximum. The spot temperatures, locations, and angular sizes also show variability during the long cycle.

The Role of Citric Acid in the Stabilization of Nanoparticles and Colloidal Particles in the Environment: Measurement of Surface Forces between Hafnium Oxide Surfaces in the Presence of Citric Acid.

The interactions between colloidal particles and nanoparticles determine solution stability and the structures formed when the particles are unstable to flocculation. Therefore, knowledge of the interparticle interactions is important for understanding the transport, dissolution, and fate of particles in the environment. The interactions between particles are governed by the surface properties of the particles, which are altered when species adsorb to the surface. The important interactions in the environment are almost never those between the bare particles but rather those between particles that have been modified by the adsorption of natural organic materials. Citric acid is important in this regard not only because it is present in soil but also as a model of humic and fulvic acids. Here we have studied the surface forces between the model metal oxide surface hafnia in the presence of citric acid in order to understand the stability of colloidal particles and nanoparticles. We find that citric acid stabilizes the particles over a wide range of pH at low to moderate ionic strength. At high ionic strength, colloidal particles will flocculate due to a secondary minimum, resulting in aggregates that are dense and easily redispersed. In contrast, nanoparticles stabilized by citric acid remain stable at high ionic strengths and therefore exist in solution as individual particles; this will contribute to their dispersion in the environment and the uptake of nanoparticles by mammalian cells.

Concurrent aggregation and transport of graphene oxide in saturated porous media: Roles of temperature, cation type, and electrolyte concentration.

Simultaneous aggregation and retention of nanoparticles can occur during their transport in porous media. In this work, the concurrent aggregation and transport of GO in saturated porous media were investigated under the conditions of different combinations of temperature, cation type (valence), and electrolyte concentration. Increasing temperature (6-24 °C) at a relatively high electrolyte concentration (i.e., 50 mM for Na+, 1 mM for Ca2+, 1.75 mM for Mg2+, and 0.03 and 0.05 mM for Al3+) resulted in enhanced GO retention in the porous media. For instance, when the temperature increased from 6 to 24 °C, GO recovery rate decreased from 31.08% to 6.53% for 0.03 mM Al3+ and from 27.11% to 0 for 0.05 mM Al3+. At the same temperature, increasing cation valence and electrolyte concentration also promoted GO retention. Although GO aggregation occurred in the electrolytes during the transport, the deposition mechanisms of GO retention in the media depended on cation type (valence). For 50 mM Na+, surface deposition via secondary minima was the dominant GO retention mechanism. For multivalent cation electrolytes, GO aggregation was rapid and thus other mechanisms such as physical straining and sedimentation also played important roles in controlling GO retention in the media. After passing through the columns, the GO particles in the effluents showed better stability with lower initial aggregation rates. This was probably because less stable GO particles with lower surface charge densities in the porewater were filtered by the porous media, resulting in more stable GO particle with higher surface charge densities in the effluents. An advection-dispersion-reaction model was applied to simulate GO breakthrough curves and the simulations matched all the experimental data well.

Iron Oxide Colloid Mobility as Affected by Solid Matrix Wetting Properties

Core Ideas Decreasing quartz sand wettability leads to attractive hydrophobic interactions with colloids. In flow columns, transport of Fe oxide colloids is not affected by sand matrix wettability. The short‐range hydrophobic attraction is probably counteracted by colloid surface roughness. The influence of porous media wettability on the mobility of colloids is mostly unknown. In the present work, organic‐matter‐coated goethite (OMCG) colloids were percolated through three saturated soil materials differing in wettability: untreated quartz sand and two variants of hydrophobized sand. For each type of sand, three ionic strength levels were applied. Derjaguin–Landau–Verwey–Overbeek (DLVO) and Lewis acid–base extended DLVO (XDLVO) interaction energy profiles were calculated according to contact angles and zeta potentials. Flow column results elucidated that decreasing sand wettability had no relevant effect on OMCG colloid mobility. In contrast, colloid retention increased with ionic strength in each type of sand packing. Classic DLVO interactions could predict trends in colloid retention by the respective characteristics of energy barriers and secondary minima. The extension with Lewis acid–base interactions in the XDLVO approach led to the prediction of significant short‐range (∼2 nm) attractive interaction energies between colloids and hydrophobized sand, which were not reflected by colloid breakthrough behavior. This was probably due to substantial energy barriers calculated for larger distances (∼27 to ∼75 nm, depending on ionic strength) between the solid matrix and colloids. It is concluded that the distinct surface roughness of sand grains and colloids probably weakened the strength of the short‐range attractive interactions, because larger amounts of surface area were still outside the effective distance for the short‐range interactions predicted by XDLVO. Regarding colloidal mobility, we concluded for our saturated porous systems that near‐surface attractive XDLVO interaction energies between OMCG colloids and hyrophobized sand did not significantly affect colloid mobility.

Observational Study of an Unusual Cataclysmic Binary 2MASS J16211735+4412541*

Abstract We present results from an observational campaign on the close binary system 2MASS J16211735+4412541 and a preliminary model based on the photometric data gathered during the quiescent and outburst levels. The modeling, done with the Wilson–Devinney code and its improvements, failed to reproduce the observational properties of the system. A secondary minimum obtained within the stellar model that is too shallow, as well as the evidence provided by the spectroscopic observations performed at outburst and quiescence, point toward an accretion disk surrounding one component, likely a white dwarf, as the cause of the outburst. Using a simple disk model, we modeled the observed multicolor light curves taken two (2016 August) and eight (2017 March) months after the outburst. We obtained a reasonable fit to the 2016 August light curves but those from 2017 March cannot be explained with the same parameters. We conclude that J1621 is an eclipsing cataclysmic binary, with an accretion disk still present almost a year after outburst, and not a contact-type system as previously classified. The binary is seen at an inclination of about 84° and there is evidence of changing accretion rates and disk parameters as a result of the outburst. Our results indicate that more cataclysmic variables may be hidden among contact binaries.

Different transport behaviors of Bacillus subtilis cells and spores in saturated porous media: Implications for contamination risks associated with bacterial sporulation in aquifer

In this study, we compared the transport characteristics of Bacillus subtilis cells and their spores in saturated porous media. Both the bacteria cells and spores were readily transported under lower ionic strengths (IS) (1mM), featuring high mass recovery rates (>75%). The bacteria cells recovery rate declined to 58.05% when the IS increased to 100mM; but the spores recovery rate remained at 74.54%. Spores appear to be more mobile than bacterial cells over wide range of IS. Surface properties and breakthrough curves analysis of the two bio-particles (cells and spores) indicated that attachment, straining and blocking mechanisms contribute differently to deposition processes. The extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory further indicated that the particles attachment may primarily occurred at the secondary energy minimum. More important, XDLVO calculations showed that the dominant forces changed with IS, altering the efficiency of particles attachment. The electric double layer repulsion mainly controlled particle deposition at 1mM. In contrast, the van der Waals attraction and Lewis acid-base interactions dominated particle deposition at 100mM. This study showed that spores are more mobile than bacteria cells in subsurface environment. Given that spores form under harsh environmental conditions, we must consider spore formation and transport when groundwater microbial contamination occurs.

Influence of Biochar on Deposition and Release of Clay Colloids in Saturated Porous Media.

Although the potential application of biochar in soil remediation has been recognized, the effect of biochar on the transport of clay colloids, and accordingly the fate of colloid-associated contaminants, is unclear to date. This study conducted saturated column experiments to systematically examine transport of clay colloids in biochar-amended sand porous media in different electrolytes at different ionic strengths. The obtained breakthrough curves were simulated by the convection-diffusion equation, which included a first-order deposition and release terms. The deposition mechanisms were interpreted by calculating Derjaguin-Landau-Verwey-Overbeek interaction energies. A linear relationship between the simulated deposition rate or the attachment efficiency and the fraction of biochar was observed ( ≥ 0.91), indicating more favorable deposition in biochar than in sand. The interaction energy calculations show that the greater deposition in biochar occurs because the half-tube-like cavities on the biochar surfaces favor deposition in secondary minima and the nanoscale physical and chemical heterogeneities on the biochar surfaces increase deposition in primary minima. The deposited clay colloids in NaCl can be released by reduction of ionic strength, whereas the presence of a bivalent cation (Ca) results in irreversible deposition due to the formation of cation bridging between the colloids and biochar surfaces. The deposition and release of clay colloids on or from biochar surfaces not only change their mobilizations in the soil but also influence the efficiency of the biochar for removal of pollutants. Therefore, the influence of biochar on clay colloid transport must be considered before application of the biochar in soil remediation.

Stability and aggregation of nanoscale titanium dioxide particle (nTiO2): Effect of cation valence, humic acid, and clay colloids

Fate and transport of engineered nanoscale titanium dioxide (nTiO2) have received much attention during the past decade. The aggregation and stability of nTiO2 in water with complicated components, however, have not been fully examined. The objective of this paper is to determine the individual and synergistic effect of cation valence, humic acid, and clay colloids on nTiO2 stability and aggregation, and elucidate the related mechanisms. We conducted systematic laboratory experiments to determine nTiO2 stability and aggregation in NaCl and MgCl2 solutions, both in the absence and presence of humic acid and illite colloids. Results showed that Mg2+, in comparison to Na+, could make the zeta potential of nTiO2 more positive, and shift the point of zero charge of nTiO2 (pHpzc,TiO2) towards higher pH. We also found that nTiO2 are destabilized by illite colloids at pH < pHpzc,TiO2 through formation of illite-nTiO2 hetero-aggregates, but are not interfered by illite colloids at higher pH. HA was found to make nTiO2 stable via electrostatic and steric effects, both in the absence and presence of illite colloids. Calculated interaction energy based on DLVO theory revealed that instability of the nTiO2 suspensions is mainly caused by primary minima, and that secondary minima normally do not destabilize the suspension, even though they are found to promote aggregation.

A general method for the derivation of the functional forms of the effective energy terms in coarse-grained energy functions of polymers. II. Backbone-local potentials of coarse-grained O1→4-bonded polyglucose chains.

Based on the theory of the construction of coarse-grained force fields for polymer chains described in our recent work [A. K. Sieradzan et al., J. Chem. Phys. 146, 124106 (2017)], in this work effective coarse-grained potentials, to be used in the SUGRES-1P model of polysaccharides that is being developed in our laboratory, have been determined for the O⋯O⋯O virtual-bond angles (θ) and for the dihedral angles for rotation about the O⋯O virtual bonds (γ) of 1 → 4-linked glucosyl polysaccharides, for all possible combinations of [α,β]-[d,l]-glucose. The potentials of mean force corresponding to the virtual-bond angles and the virtual-bond dihedral angles were calculated from the free-energy surfaces of [α,β]-[d,l]-glucose pairs, determined by umbrella-sampling molecular-dynamics simulations with the AMBER12 force field, or combinations of the surfaces of two pairs sharing the overlapping residue, respectively, by integrating the respective Boltzmann factor over the dihedral angles λ for the rotation of the sugar units about the O⋯O virtual bonds. Analytical expressions were subsequently fitted to the potentials of mean force. The virtual-bond-torsional potentials depend on both virtual-bond-dihedral angles and virtual-bond angles. The virtual-bond-angle potentials contain a single minimum at about θ=140° for all pairs except β-d-[α,β]-l-glucose, where the global minimum is shifted to θ=150° and a secondary minimum appears at θ=90°. The torsional potentials favor small negative γ angles for the α-d-glucose and extended negative angles γ for the β-d-glucose chains, as observed in the experimental structures of starch and cellulose, respectively. It was also demonstrated that the approximate expression derived based on Kubo's cluster-cumulant theory, whose coefficients depend on the identity of the disugar units comprising a trisugar unit that defines a torsional potential, fits simultaneously all torsional potentials very well, thus reducing the number of parameters significantly.

Reconciling DLVO and non-DLVO Forces and Their Implications for Ion Rejection by a Polyamide Membrane.

Recognizing the significance of surface interactions for ion rejection and membrane fouling in nanofiltration, we revise the theories of DLVO (named after Derjaguin, Landau, Verwey, and Overbeek) and non-DLVO forces in the context of polyamide active layers. Using an atomic force microscope, surface forces between polyamide active layers and a micrometer-large and smooth silica colloid were measured in electrolyte solutions of representative monovalent and divalent ions. While the analysis of DLVO forces, accounting for surface roughness, provides how surface charge of the active layer changes with electrolyte concentration, scrutiny of non-DLVO hydration forces gives molecular insight into the composition of the membrane-solution interface. Importantly, we report an expansion of the diffuse layer at high ionic strength, consistent with the recent development of the electrical double layer theory, but in contrast to the widely accepted phenomenon of aggregation in the secondary minimum. Further, the enhanced repulsion acting on modified membranes via polyelectrolyte adsorption can be quantitatively predicted by DLVO and non-DLVO forces. This work serves to solve past misunderstandings about the interaction forces acting on nanofiltration membranes, and it provides guidance for future work on the relation between surface properties and rejection mechanisms and fouling.

Experimental assessment of graphene oxide adsorption onto sandstone reservoir rocks through response surface methodology

Abstract This study mainly deals with the adsorption behavior of the graphene oxide (GO) onto sandstone which is an important factor for applying a material in chemical enhanced oil recovery methods. The combined effects of initial GO concentration, salinity and pH of the solution are assessed by adopting response surface methodology. The results show that the GO concentration has a stronger influence on the GO adsorption than those of the initial pH and salinity. The effect of pH on the GO adsorption becomes significant at high GO concentrations and low salinities. The Derjaguin–Landau–Verwey–Overbeek (DLVO) theory is applied to explain the observed trend of GO adsorption under various salinity and pH conditions. Reduction in the height of energy barrier and formation of a secondary minimum are responsible for increasing the GO adsorption at lower pH values and moderate salinities (1 wt.%). The contact angle measurement of the rock surface treated in GO solution at optimum conditions (GO concentration of 0.89 mg/mL, salinity of 5 wt.% and pH of 6.74) shows that the adsorbed GO can alter the wettability of sandstone from strongly oil wet (150°) to intermediate conditions (90°).

Regulation of structure, rheological and surface properties of chitin nanocrystal dispersions

The hydrosols of chitin nanocrystals (ChN) are promising as modifiers of properties, drug delivery systems, and media. The purpose of this work is to clarify the mechanisms for regulating processes of stability and structure formation occurring in the ChN hydrosols under the influence of varying ionic strength. ChN were isolated from the crab shell; ChN hydrosols are obtained with different concentrations of the dispersed phase. The structure and morphology of the particles was studied by atomic force microscopy and X-ray diffraction. Hydrosols were studied by rotational viscometry, dynamic light scattering, and photometry in the presence of KCl with concentration up to 200mM. The experimental results of finding the rapid coagulation threshold are compared with calculated ones performed with the use of the modified DLVO theory. It is established that in the range of electrolyte concentrations 30∼75mM, the optical and rheological properties of the ChN hydrosols undergo crucial changes. At an electrolyte concentration of 20∼150mM, particles coagulate at a secondary potential minimum; the parallel orientation of the particles is energetically more advantageous. In systems with a higher concentration of electrolyte, there is no potential barrier; rapid barrier-free coagulation is observed, with the formation of stronger coagulation contacts and the formation of stable gels.

Transport and retention of biochar nanoparticles in a paddy soil under environmentally-relevant solution chemistry conditions

Land application of biochar has been increasingly recommended as a powerful strategy for carbon sequestration and soil remediation. However, the biochar particles, especially those in the nanoscale range, may migrate or carry the inherent contaminants along the soil profile, posing a potential risk to the groundwater. This study investigated the transport and retention of wood chip-derived biochar nanoparticles (NPs) in water-saturated columns packed with a paddy soil. The environmentally-relevant soil solution chemistry including ionic strength (0.10–50 mM), electrolyte type (NaCl and CaCl2), and natural organic matter (0–10 mg L−1 humic acid) were tested to elucidate their effects on the biochar NPs transport. Higher mobility of biochar NPs was observed in the soil at lower ionic strengths, with CaCl2 electrolyte being more effective than NaCl in decreasing biochar NPs transport. The retained biochar NPs in NaCl was re-entrained (∼57.7%) upon lowering transient pore-water ionic strength, indicating that biochar NPs were reversibly retained in the secondary minimum. In contrast, negligible re-entrainment of biochar NPs occurred in CaCl2 due to the primary minimum and/or particle aggregation. Humic acid increased the mobility of biochar NPs, likely due to enhanced electrosteric repulsive interactions. The transport behaviors of biochar NPs can be well interpreted by a two-site kinetic retention model that assumes reversible retention for one site, and irreversible retention for the other site. Our findings indicated that the transport of wood chip biochar NPs is significant in the paddy soil, highlighting the importance of understanding the mobility of biochar NPs in natural soils for accurately assessing their environmental impacts.

DEM modelling of particle-bubble capture through extended DLVO theory

This work presents a computational study based on Discrete Element Method (DEM) to investigate the capture of particles by bubbles in the presence of electrical double layer repulsion. In the DEM model a fully mobile boundary condition was assumed for the gas-liquid (bubble) interface. The forces acting on the particle were gravitational, buoyancy, hydrodynamic, as well as elastic and damping forces if/when the particle penetrated inside the bubble. Surface forces were considered and were evaluated through the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory. A preliminary theoretical analysis of the surface forces was carried out in order to determine the possibility of particle capture. This analysis included the determination of the interaction potential energy. Five different types of interaction energy curves were found. They were characterised by (1) a monotonic increase with distance; (2) the presence of a primary minimum; (3) the presence of an energy barrier and a primary minimum, (4) the presence of an energy barrier and a primary and a secondary minimum, and (5) a monotonic decrease with distance. DEM modelling was conducted to investigate the induction time for a single particle-bubble system. It was found that the induction time decreased with increasing contact angle and decreasing height of the energy barrier. In contrast, the primary minimum had very limited impact on induction time. Additionally, modelling of a more complex system consisting of a single bubble and multiple particles was also carried out. The multiple particle-single bubble simulation results showed that a decrease in induction time considerably enhanced collection efficiency. Finally, the concept of “contactless” flotation, which occurs in the case in which only a primary minimum exists, was also demonstrated through the use of DEM modelling.

Interaction of rigid C3N- with He: Potential energy surface, bound states, and rotational spectrum.

A two-dimensional rigid rotor model of the potential energy surface is developed for the collision of C3N- with He. Ab initio calculations are performed at the coupled cluster level with single and double excitations and using a perturbative treatment of triple excitations. An augmented correlation consistent polarized valence quadruple zeta basis set complemented with a set of mid-bond functions is chosen for these calculations. The global T-shaped minimum (De = 62.114 cm-1) is found at the intermolecular distance R = 6.42 a0. A secondary minimum (De = 41.384 cm-1) is obtained for the linear configuration C3N--He and for R = 9.83 a0. Calculations of the rovibrational bound states are carried out by using a discrete variable representation method based on Sturmian functions. The first theoretical prediction of the absorption spectra for the He-C3N- complex in the microwave region is also provided.