Secondary Minimum Research Articles

Quantifying bacterial attachment and detachment using leaching solutions of various ionic strengths after bacterial pulse

In this study, we quantified the attachment and detachment of bacteria during transport in order to elucidate the contributions of reversible attachment on bacterial breakthrough curves. The first set of breakthrough experiment was performed for a laboratory sand column using leaching solutions of deionized water and mineral salt medium (MSM) of 200 mM with reference to KCl solution by employing Pseudomonas putida as a model bacterium. In the second set of experiment, the ionic strengths of leaching solutions immediately after bacterial pulse were lowered to tenfold and 100-fold diluted system (2 and 20 mM MSM) to focus on the influence of physicochemical factor. Results have shown that bacterial retention occurred in the sand column due to the physical deposition and physicochemical attachment. The physicochemical attachment was attributed to the high ionic strength (200 mM MSM) of leaching solution and the formation of primary energy minimum. Replacing the 200 mM leaching solution with the lower ionic strengths after pulse resulted in the increased tailing of breakthrough curve due to the detachment from the attached bacteria. The detachment could be well explained by DLVO theory, which showed the formation of energy barrier and disappearance of the secondary minimum as the ionic strength gradually decreased. Analysis of mass recovery revealed that 12–20% of the attachment was due to physical and physicochemical attachment, respectively, where the latter consisted of 25–75% of irreversible and reversible attachment respectively.

Synergies of media surface roughness and ionic strength on particle deposition during filtration

Although it is widely believed that media/collector roughness can enhance particle deposition on surfaces, this effect has not been consistently observed nor systematically described. Here, column tests were conducted to: 1) evaluate media roughness impacts on particle deposition in the presence of an energy barrier (i.e., at low ionic strength conditions), and 2) describe the concurrent impacts of collector surface roughness and suspension fluid ionic strength on particle deposition in packed beds. This work presents a first, systematic demonstration that media/collector surface roughness consistently influences particle deposition in a non-linear, non-monotonic manner, irrespective of the presence of an energy barrier. Notably, ionic strength-associated changes in DLVO interaction energy could not solely explain observed differences in particle deposition associated with collector surface roughness. Particle-to-roughness element and particle-to-smooth/bottom surface interactions contributed to a critical roughness size associated with a minimum DLVO interaction energy; however, that critical size is not necessarily the same as the critical size associated with minimal particle deposition rates. Surface roughness and ionic strength concurrently affected particle deposition in a manner that is not simply additive; rather, particle deposition rates were highly correlated with inverse Debye-Hückel length (i.e., ln [κ−1]) using second-order polynomial functions. Notably, the secondary energy minimum alone appears inadequate for explaining the observed particle deposition behavior. These relationships may provide insight for further development of physico-chemical filtration models for describing particle deposition on surfaces.

Double layer electrical conductivity as a stability criterion for concentrated colloidal suspensions

The slightly attractive inter–particle equilibrium potential associated with electrostatically stabilized suspensions of minimum viscosity is described by the DLVO theory and commonly gauged by static ζ-potential measurements, plagued with experimental uncertainties. In this work, the electrokinetic mobility of alumina particles was measured in suspensions prepared with selected solids content and ionic strength, as well as was the electrical conductivity of each suspension and suspending liquid. Particles electrical conductivity was then calculated and related to the colloidal stability described by the DLVO theory, enabling the identification of a processing window for the stability control of concentrated suspensions. The maximum repulsive potential and distance between particles (∼46nm) corresponds to the particles maximum conductivity. When the particles conductivity is zero, the diffuse layer is fully collapsed and they stand at the minimum reversible distance (∼7nm). At the equilibrium conductivity, a potential curve is produced with a secondary attractive minimum of ∼1.5 kT at an inter–particle distance of ∼17nm, as suggested by the DLVO theory and the Equipartition of Energy theorem. The condition for accurate measurement of static ζ-potential occurs at the isoconductivity point between particles and suspending liquid.

Varijabilnost padalina u Hvaru i Crikvenici

The study analyses the precipitation variability in Hvar and Crikvenica in the period from 1931 to 1990. These stations have a maritime type of the annual course of precipitation. The minimum value of the precipitation variability in Hvar is in autumn, in November, while the secondary minimum of the variability is in spring, in April. The primary maximum of variability is in summer, most often in July, while the secondary maximum is in March. In Crikvenica the minimum values of the precipitation variability in April and November are even, and the same is true for the maximum values of the variability in September and March. The value of the annual precipitation variability is higher in Crikvenica than in Hvar although Crikvenica has higher amount of precipitation. The location of the stations included in this research is relevant. In Crikvenica the variability is higher in autumn and winter. Monthly values of the mean relative variability coincide in the cold part of the year when the variability is only slightly higher in Crikvenica, while in the warm part of the year, with the exception of September, the variability in Hvar is significantly higher.

Solvation of Mg in helium-4: Are there meta-stable Mg dimers?

Experiments with 4He nanodroplets doped with Mg atoms were interpreted as the observation of the formation of weakly bound magnesium complexes. We present results for single Mg and Mg dimer solvation using the hypernetted chain/Euler-Lagrange (HNC-EL) method as well as path integral Monte Carlo simulations. We find that the phonon-mediated, indirect Mg-Mg interaction adds an oscillatory component to the direct Mg-Mg interaction. We undertake a step-by-step examination of the ingredients of the calculation of the phonon-induced interaction, comparing the results of semi-analytic HNC-EL calculations for bulk and single impurity results with experiments as well as Monte Carlo data. We do not find evidence for a sufficiently strong secondary minimum in the effective Mg-Mg interaction to support a metastable state.

Role of solution chemistry in the retention and release of graphene oxide nanomaterials in uncoated and iron oxide-coated sand.

Understanding the fate and transport including remobilization of graphene oxide nanomaterials (GONMs) in the subsurface would enable us to expedite their benign use and evaluate their environmental impacts and health risks. In this study, the retention and release of GONMs were investigated in water-saturated columns packed with uncoated sand (Un-S) or iron oxide-coated sand (FeS) at environmentally relevant solution chemistries (1-100mM KCl and 0.1-10mM CaCl2 at pH7 and 11). Our results showed that increasing ionic strength (IS) inhibited GONMs' transport, and the impact of K+ was less than Ca2+. The positively charged iron oxide coating on sand surfaces immobilized the negatively charged GONMs (pH7) in the primary minimum, yielding hyperexponential retention profiles particularly in Ca2+. A stepwise decrease in pore-water IS caused detachment of previously retained GONMs. The mass of GONMs released during each detachment step correlated positively with the difference in secondary minimum depth (ΔΦmin2) at each IS, indicating that the released GONMs were retained in the secondary minimum. While most retained GONMs were re-entrained upon lowering pore-water IS in Un-S, decreasing IS only released limited GONMs in FeS, which were captured in the primary minimum. Introducing 1mM NaOH (pH11) released most retained GONMs in FeS; and average hydrodynamic diameters of the detached GONMs upon injecting NaOH were significantly smaller than those of GONMs in the influent and retentate, suggesting that NaOH induced GONMs disaggregation. Our findings advance current knowledge to better predict NMs' fate and transport under various solution chemistries such as during rainfall events or in the mixing zones between sea water and fresh water where transient IS changes drastically.

Transport and aggregation of rutile titanium dioxide nanoparticles in saturated porous media in the presence of ammonium

The widely used artificial nanoparticles (NPs) and the excess of ammonium (NH4+) fertilizers are easily released into the natural environment. So, clarifying the mobility of NPs in the presence of NH4+ is therefore of great urgency and high priority. Currently, few studies focus on the transport and deposition of nanoparticle titanium dioxide (nTiO2) in single and binary systems containing NH4+, especially describing this process by a mathematical model. In this work, the comparison between the transport and retention of rutile nTiO2 in single and binary electrolyte solutions of NH4Cl and/or NaCl (0.5–50 mM) were conducted at pH 6.0 and 8.0 through running the column experiments. Experimental results show that the aggregation and retention of nTiO2 in solution containing mono-valence cations obeys the order as follows: NH4+ > Na+ > Na+ + NH4+ at the same ion strength (IS). It is attributed to the lower critical coagulation concentration (CCC) of rutile nTiO2 in NH4+ than that in Na+ solution. In particular, the simultaneous presence of NH4+ and Na+ favors the transportability of nTiO2 due to the strong competitive adsorption on the surface of NPs. The two-site kinetic retention model provides the good simulation for their transport behavior. The likely mechanism is that the secondary energy minimum of nTiO2 in NH4+ system associated with the greater K2 at surface Site 2 (from model) on sand can be explained for the more reversible deposition. Ammonium leachate associated with NPs can thus be considered a serious concern.

DLVO, hydrophobic, capillary and hydrodynamic forces acting on bacteria at solid-air-water interfaces: Their relative impact on bacteria deposition mechanisms in unsaturated porous media

Experimental and modeling studies were performed to investigate bacteria deposition behavior in unsaturated porous media. The coupled effect of different forces, acting on bacteria at solid-air-water interfaces and their relative importance on bacteria deposition mechanisms was explored by calculating Derjaguin–Landau–Verwey–Overbeek (DLVO) and non-DLVO interactions such as hydrophobic, capillary and hydrodynamic forces. Negatively charged non-motile bacteria and quartz sands were used in packed column experiments. The breakthrough curves and retention profiles of bacteria were simulated using the modified Mobile-IMmobile (MIM) model, to identify physico-chemical attachment or physical straining mechanisms involved in bacteria retention. These results indicated that both mechanisms might occur in both sand. However, the attachment was found to be a reversible process, because attachment coefficients were similar to those of detachment. DLVO calculations supported these results: the primary minimum did not exist, suggesting no permanent retention of bacteria to solid-water and air-water interfaces. Calculated hydrodynamic and resisting torques predicted that bacteria detachment in the secondary minimum might occur. The capillary potential energy was greater than DLVO, hydrophobic and hydrodynamic potential energies, suggesting that film straining by capillary forces might largely govern bacteria deposition under unsaturated conditions.

Characteristics of the global thermal tropopause derived from multiple radio occultation measurements

Thermal tropopause represents the region of the atmosphere where the environmental lapse rate changes from tropospheric positive to stratospheric negative. It also defines the altitude of the atmosphere beneath which significant weather perturbations occur except occasional overshooting thunderstorms in the tropical regions. Accordingly, how the temporal and spatial variability of thermal tropopause behaves is of great concern in atmospheric research and, hence, investigated in this study by using radio occultation (RO) observations obtained from seven space missions during the period from May 2001 to April 2013 (with a total of 6,075,359 occultations). While RO observations have been demonstrated to provide precise measurements of temperature profiles of the atmosphere, their results are inter-compared before further use in our analysis, showing expected high-precision observations with mean differences<0.06K and standard deviations<1.6K in the upper troposphere and lower stratosphere. Given a rather large data set of multi- space-mission RO measurements taken globally, a very detailed description of spatial structure and variability of the tropopause is revealed, and monthly mean zonal mean tropopause parameters in each 2° latitude band from 90°S to 90°N can be obtained. Many interesting features of seasonal cycle, spatial distribution, interannual variation, and diurnal variation of the thermal tropopause are observed. For examples, except for the primary minimum in January, the equatorial tropopause temperature exhibits a secondary minimum in April, possibly caused by the strongly combined wave forcing from two hemispheres; During the boreal winter over the tropics, the distribution of tropopause temperature extrema do not totally coincide with the altitude extrema spatially, and the former has a better agreement with the locations of strong tropical convection systems; Notable zonal asymmetries in interannual characteristics are observed in both tropical and extratropical regions. In both the tropics and Arctic, close correlation of the interannual variations is revealed between tropopause parameters and stratospheric temperatures in localized regions as well as zonal mean results while no such relationship is observed in the middle latitudes; and Diurnal variation of the equatorial tropopause shows warmer temperature in the morning and cooler value at midnight.

Retention and Release of Graphene Oxide in Structured Heterogeneous Porous Media under Saturated and Unsaturated Conditions.

In this work, saturated and unsaturated structured heterogeneous sand columns were used to examine the fate of graphene oxide (GO) nanoparticles in heterogeneous porous media under various conditions. A two-domain model considering mass exchange between zones was applied to describe GO retention and transport in structured, heterogeneous porous media, which matched the transport experimental breakthroughs well. Experimental and model results showed that GO retention and transport in all the heterogeneous columns were dominated by the preferential flow phenomena. Under saturated conditions, the coarse sand with higher hydraulic conductivity was the fast-flow domain (FFD), and the fine sand was the slow-flow domain (SFD), and both FFD and SFD affected GO particles fate in structured heterogeneous media. When the heterogeneous columns were drained, the fine sand with higher moisture content became the FFD, and the coarse sand was the SFD, however, preferential flows in the FFD dominated GO retention and transport processes. For all the columns, the mobility of GO decreased with the increasing ionic strength (IS), and the previous retained particles were released by reducing solution IS, indicating part of the retained particles were trapped in the secondary minimum energy well.

Ferroelectric Switching Pathways and Energetics in (Hf,Zr)O2

The recent discovery of ferroelectricity and antiferroelectricity in doped and pure (Hf,Zr)O2 films [1] have spurred intensive research. Compared to the more traditional ferroelectrics such as PZT, (Hf,Zr)O2 films have the advantage of a larger band gap, more uniform structure (lack of layering), and are expected to meet fewer integration problems, as evidenced by other phases of (Hf,Zr)O2 being successfully integrated into the mass-manufactured devices. Unlike the widely studied perovskite-based ferroelectrics, (Hf,Zr)O2has a distorted fluorite structure. Several recent theoretical studies considered the mechanism of ferroelectric switching in this structure and arrived at somewhat conflicting conclusions. The effect of the additional “dopants” on the ferroelectric behavior is of particular interest, however, the earlier theoretical predictions of this effect do not match the experimental data. Here, we use density functional calculations (including the highly accurate hybrid HSE) to achieve a deeper understanding of the phase stability and of switching mechanisms that could help improving the ferroelectric characteristics critical for applications. Since the saturation polarization is only defined with respect to a particular switching pathway, changes to the pathway energetics could alter the observed polarization even in the absence of substantial changes to the equilibrium fully polarized state. For example, in HfO2, the calculations predict that the intrinsic spontaneous polarization of 0.51 C/m2 could be increased to 0.68 C/m2 if one could suppress switching via the normally lowest-energy pathway. The observed remanent polarization (which is a lower value due to polycrystallinity and the domain structure) would also increase. To gain insight into this possibility, we start by developing a unified view of different fluorite-based phases. Noting that the ferroelectric Pca21 and the high-pressure antiferroelectric Pbca phases are particular orderings within the same fractionally-occupied “parent” orthorhombic Pbcm lattice, we consider other possible orderings within the same “parent” lattice. In all these structures, half of the oxygen atoms remain unpolarized, and the other half decorate the polarized sites of the Pbcm structure, thus forming two sublattices. We demonstrate that this lattice also gives rise to the monoclinic P21/cphase. Next, we systematically study the energetics of possible ferroelectric switching pathways in different environments. We find that the energetically preferred transformation leading to the change in the net polarization does not connect simply the two oppositely-polarized ferroelectric decorations of the same “parent” Pbcm lattice. Instead, in the absence of defects and dopants, the lowest-energy pathway connects two ferroelectric configurations belonging to different“parent” lattices (see figure). Contrary to a trivial energy barrier profile reported earlier [2] we find that the lowest-barrier pathway has a secondary energy minimum at the transformation midpoint (cf. figure inset). In this secondary minimum region, the unpolarized oxygen sublattice rapidly develops a strong distortion, and the structure approaches that of the tetragonal P42/nmc phase. A similar pathway was recently considered in Ref.[3]; however, contrary to findings of Ref.[3] we find that at T=0K the ferroelectric structure is clearly preferred to the tetragonal one in both HfO2 and ZrO2, and is stabilized at finite temperatures by vibrational entropy. The same low energy barrier is also found in alternative polarization switching pathways that change the roles of the two oxygen sublattices. Accounting for multiple switching pathways with identical activation energy may be important when evaluating the switching mechanisms in the presence of defects. Finally, we show that in the presence of a moderate strain (particularly with a shear component), switching may also proceed via rotating the polarization direction by ~90o, rather than reversing it. This analysis builds the framework for further studies of the effect of alloying and of defects on the switching barriers. In particular, we discuss the relative effects of the oxygen vacancies and of alloying HfO2 with ZrO2.

Aggregation kinetics of OX50 hydrosols in NaCl solutions studied by dynamic light scattering

The aggregation kinetics of OX50 sols in aqueous NaCl solutions (10–4–2 × 10–1 M) has been studied for 15 days or more by dynamic light scattering. The following set of characteristics has been considered to quantitatively estimate the coagulation intensity in the disperse systems: the particle size corresponding to the maximum in the differential particle size distribution curve, the height of the maximum, the polydispersity index, and the average diameter of the intensity distribution. It has been found that slow sol coagulation proceeds via the barrierless mechanism in secondary potential minimum, which arises from the predominance of the dispersion attractive forces over structural and electrostatic repulsive forces.

BVR photometric study of NSVS 2607629. A high mass-ratio W-type W UMa system

We present the results of our investigation of the geometrical and physical parameters of the W UMa-type binary system NSVS 2607629 based on CCD BVRc light curves and their analysis with the Wilson–Devinney code. New times of minima and light elements have been determined. We find that, as seen in eccentric systems, secondary minima do not occur at phase 0.5. According to our solution, the system is found to be a high mass-ratio W-type W UMa system. Combining our photometric solution with the emprical relation obtained for W UMa type systems by Dimitrov and Kjurkchieva (2015) we derive the masses and radii of the components of this eclipsing system as M1=0.44M⊙,M2=0.73M⊙,R1=0.57R⊙ and R2=0.71R⊙. The evolutionary state of the system is also discussed.

Coupled effect of extended DLVO and capillary interactions on the retention and transport of colloids through unsaturated porous media

Colloids are potential vectors of contaminants in the subsurface environment. The knowledge of transport and retention behaviors of colloids is of primary importance for assessment and prediction of subsurface pollution risks. In this study, sand column experiments were conducted to investigate the coupled effects of various interfacial forces on the retention and transport of a hydrophilic silica colloid and a relatively hydrophobic latex colloid. Water column experiments were performed to observe the movement of colloids with air bubbles. Extended DLVO interaction energies and capillary potential energy were calculated to analyze colloid retention at air-water interface (AWI), solid-water interface (SWI), and air-water-solid interface (AWS). Results show that colloid retention decreases due to increase in electrostatic repulsion and Born repulsion as well as decrease in Lewis acid-base attraction and hydrophobic interactions. Water content effect and hydrophobic effect on colloid retention become more predominant in the solution of higher ionic strength. Colloid retention at AWI is minimal (i.e., due to nonexistence of primary and secondary minima) at the ionic strengths <75mM. Capillary potential energy (107–108 KBT) of colloids is 4–5 orders of magnitude greater than the extended DLVO interaction energy (~103 KBT), suggesting that capillary retention at AWS is the primary mechanism controlling colloid retention in unsaturated porous media. Results from this study show that immobile solid phase (e.g., soil) could be much more important than air phase in determining colloid retention in unsaturated porous media under unfavorable conditions, especially in the solutions of high ionic strengths.

NSV 1907 - A new eclipsing, nova-like cataclysmic variable

NSV 1907, formerly listed as an irregular variable in variability catalogues, was classified as an Algol-type eclipsing binary in the Catalina Surveys Periodic Variable Star Catalogue. We have identified NSV 1907 as an ultraviolet (UV) bright source using measurements from the GALEX space telescope and detected obvious out-of-eclipse variability in archival photometric data from the Catalina Sky Survey, which instigated a closer examination of the object. A spectrum and extensive multicolour photometric observations were acquired, from which we deduce that NSV 1907 is a deeply eclipsing, nova-like cataclysmic variable. Apart from the orbital variations (deep eclipses with a period of P ≈ 6.63 hours), changes in mean brightness and irregular short-term variability (flickering) were observed. The presence of a secondary minimum at phase φ ≈ 0.5 was established, which indicates a significant contribution of the companion star to the optical flux of the system. We find possible evidence for sinusoidal variations with a period of P ≈ 4.2 d, which we interpret as the nodal precession period of the accretion disc. No outbursts or VY Scl-like drops in brightness were detected either by the CSS or during our photometric monitoring. Because of its spectral characteristics and the observed variability pattern, we propose NSV 1907 as a new moderately bright long-period SW Sextantis star. Further photometric and spectroscopic observations are encouraged.

The first photometric study of semi-detached eclipsing binary V504 Cyg

In this paper, we analyze photometry of V504 Cyg semi-detached binary system. For this purpose, after taking the photometric data, primary and secondary minimum and new epoch were calculated. The period of system is found to be 0.3516916 day. Analysis of light curve was performed by PHOEBE software which uses last version of Wilson–Devinney code. 3D model of the system is worked out using Binary Maker software. The O’Connell effect in the light curve is observed and an accurate model of this system is presented by introducing four spots on the components.

V346 Centauri: Early-type eclipsing binary with apsidal motion and abrupt change of orbital period

New physical elements of the early B-type eclipsing binary V346 Cen are derived using the HARPS spectra downloaded from the ESO archive and also numerous photometric observations from various sources. A model of the observed times of primary and secondary minima that fits them best is a combination of the apsidal motion and an abrupt decrease in the orbital period from 6322123 to 6321843 (shortening by 24 s), which occurred somewhere around JD 2 439 000. Assumption of a secularly decreasing orbital period provides a significantly worse fit. Local times of minima and the final solution of the light curve were obtained with the program PHOEBE. Radial velocities of both binary components, free of line blending, were derived via 2D cross-correlation with a program built on the principles of the program TODCOR. The oxygen lines in the secondary spectra are weaker than those in the model spectra of solar chemical composition. Using the component spectra disentangled with the program KOREL, we find that both components rotate considerably faster than would correspond to the synchronization at periastron. The apside rotation known from earlier studies is confirmed and compared to the theoretical value.

The first photometric study of W UMa eclipsing binary OQ Dra

The present study is an analysis of V-band CCD observations of new W UMa contact binary OQ Dra. To carry out the analysis, Primary and secondary minimum were obtained and new epoch was calculated. The computed period of system was 0.33967 day. Light curve analysis was performed using Binary Maker 3 and PHOEBE that uses the latest Wilson–Devinney code. We obtained photometric mass ratio of qptm = 0.55.O’Connell effect also was seen in the fitted model. Finally, the best model was achieved by introducing 2 spots on each component.

Simultaneous removal of rotavirus and adenovirus from artificial ground water using hydrochar derived from swine feces.

Hydrothermal carbonization technology can convert fecal waste into a valuable carbonaceous product referred to as hydrochar. We investigated the potential of fecal waste-derived hydrochar as an adsorbent for virus removal in water treatment. Swine feces was hydrothermally treated under two conditions: at 180 °C for 2 h and 230 °C for 7 h. The resulting solid products (hydrochar) were evaluated as virus adsorbents in water treatment. Simultaneous removal of pathogenic rotavirus (RV) and human adenovirus (HAdV) was investigated using a sand column set-up of 10 cm bed height with and without hydrochar supplement (1.5%, w/w). The removal efficiency of both viruses in a hydrochar-amended column was >3 log (complete removal). The amount of virus released in deionized water when flushed into the virus-retaining columns indicated that the secondary energy minimum played a more important role in RV retention than that of HAdV. Zeta-potential and hydrophobicity measurements on hydrochar materials indicated that the improved virus removal performance of hydrochar-amended columns was induced by the provision of extra hydrophobic surfaces. This study provides evidence that fecal waste-derived hydrochar can be used as a competent virus adsorbent.

Influence of Electrolyte Concentration on the Aggregation of Colloidal Particles near Electrodes in Oscillatory Fields.

Micron-scale particles suspended in various aqueous electrolytes have been widely observed to aggregate near electrodes in response to oscillatory electric fields, a phenomenon believed to result from electrically induced flows around the particles. Previous work has focused on elucidating the effects of the applied field strength, frequency, and electrolyte type on the aggregation rate of particles, with less attention paid to the ionic strength. Here we demonstrate that an applied field causes micron-scale particles in aqueous NaCl to rapidly aggregate over a wide range of ionic strengths, but with significant differences in aggregation morphology. Optical microscopy observations reveal that at higher ionic strengths (∼1 mM) particles arrange as hexagonally closed-packed (HCP) crystals, but at lower ionic strengths (∼0.05 mM) the particles arrange in randomly closed-packed (RCP) structures. We interpret this behavior in terms of two complementary effects: an increased particle diffusivity at lower ionic strengths due to increased particle height over the electrode and the existence of a deep secondary minimum in the particle pair interaction potential at higher ionic strength that traps particles in close proximity to one another. The results suggest that electrically induced crystallization will readily occur only over a narrow range of ionic strengths.