Secondary Minimum Research Articles

Detection of Two Totally Eclipsing B-type Binaries with Extremely Low Mass Ratios

The detection of O- and B-type stars with extremely low-mass companions is very important for understanding the formation and evolution of binary stars. However, finding them remains a challenge because the low-mass components in such systems contribute such small flux to the total. During our search for pulsations among O- and B-type stars using the Transiting Exoplanet Survey Satellite (TESS) data, we found two short-period and B-type (B9) eclipsing binaries with orbital periods of 1.61613 and 2.37857 days. Photometric solutions of the two close binaries were derived by analyzing the TESS light curves with the Wilson–Devinney method. It is discovered that both of them are detached binaries with extremely low mass ratios of 0.067(2) for TIC 260342097 and 0.140(3) for TIC 209148631. The determined mass ratio indicates that TIC 260342097 is one of the lowest mass ratios among known B-type binary systems. We showed that the two systems have total eclipses with a broad and flat secondary minimum, suggesting that the photometric parameters could be derived reliably. The absolute parameters of the two binaries are estimated and it is found that the secondary components in the two systems are overluminous and oversize when compared with the normal low-mass and cool main-sequence (MS) stars. These findings may imply that the two systems are composed of a B-type MS primary and a cool pre-MS secondary with orbital periods shorter than 2.5 days. They are valuable targets to test theories of binary star formation and evolution.

Effects of low-molecular-weight organic acids on the transport of polystyrene nanoplastics: An insight at the structure of organic acids

Plastic nanoparticles are extensively used in various products, leading to inevitable pollution in soil. Understanding their transport in soils where various organic substances exist is crucial. This study examined the impact of low-molecular-weight organic acids (LMWOAs) on the transport of polystyrene nanoplastics (PS-NPs) through saturated quartz sand. The experiments involved three dibasic acids—malonic acid (MA1), malic acid (MA2) and tartaric acid (TA) — and four monobasic acids— formic acid (FA), acetic acid (AA), propanoic acid (PA) and glycolic acid (GA) —under different pH levels (4.0, 5.5, 7.0) and in the presence of cations (Na+, Ca2+). The results demonstrated that in the presence of Na+, dibasic acids significantly enhanced PS-NPs transport, with TA being the most effective, followed by MA2 and MA1. This enhancement is attributed to the adsorption of LMWOAs onto the nanoparticles and sand, creating a more negative ζ-potential, which increases the electrostatic repulsion and decreases the PS-NPs deposition, thereby facilitating the transport. Applying the Derjaguin-Landau-Verwey-Overbeek theory, higher pH levels increased the energy barrier and secondary energy minimum, decreasing PS-NPs deposition. Moreover, dibasic acids significantly enhanced the hydrophilicity of PS-NPs. Conversely, monobasic acids, except for GA, slightly reduced the hydrophilicity of PS-NPs, as indicated by a small increase in the water contact angle, hereby minimally affecting PS-NPs transport. As for GA, although it is a monobasic acid, the additional -OH group in its molecular structure promoted PS-NPs transport, similar to dibasic acids. For example, GA also significantly enhanced the hydrophilicity of PS-NPs. In the presence of Ca2+, the enhancement of PS-NPs transport by LMWOAs was comparable to that with Na+, primarily due to the complex-forming and bridging effects of Ca2+ with the organic acids and PS-NPs. These findings provide important insights into predicting and analyzing the transport behaviors of PS-NPs.

Probing the Band Splitting near the Γ Point in the van der Waals Magnetic Semiconductor CrSBr.

This study investigates the electronic band structure of chromium sulfur bromide (CrSBr) through comprehensive photoluminescence (PL) characterization. We clearly identify low-temperature optical transitions between two closely adjacent conduction-band states and two different valence-band states. The analysis on the PL data robustly unveils energy splittings, band gaps, and excitonic transitions across different thicknesses of CrSBr, from monolayer to bulk. Temperature-dependent PL measurements elucidate the stability of the band splitting below the Néel temperature, suggesting that magnons coupled with excitons are responsible for the symmetry breaking and brightening of the transitions from the secondary conduction band minimum (CBM2) to the global valence band maximum (VBM1). Collectively, these results not only reveal splitting in both the conduction and valence bands but also highlight a significant advance in our understanding of the interplay between the optical, electronic, and magnetic properties of antiferromagnetic two-dimensional van der Waals crystals.

Evolution and disappearance of the paleo-West Pacific Anomaly: Implications to the future of South Atlantic Anomaly

Palaeomagnetic and modern geomagnetic measurements indicate that the South Atlantic Anomaly (SAA) has undergone rapid changes over the past few hundred years. Its minimum intensity decreased at an average rate of 26 not yr−1, accompanied by a continuous westward drift and spatial expansion. Recently, a secondary minimum of SAA emerged near southern Africa, leading to speculation that expansion of the SAA could indicate an impending geomagnetic reversal. Here, we focus on the evolution and disappearance of the paleo-West Pacific Anomaly (WPA), as another SAA-like structure, which may have implications for the future of SAA evolution. We regard the WPA as SAA-like due to its feature and its association with a reversal flux patch on the core-mantle boundary. Consequently, we suggest that the observed evolutionary pattern in the WPA can serve as a reference for other negative anomalies, such as the SAA. By analysing models that combine datasets of archaeomagnetic and historical records, such as gufm1 and HistKalmag, it is found that the WPA occurred between 1600 and 1820 CE. Over its duration, the WPA experienced phases of rapid expansion, drift, and division. Eventually, its primary component faded away, giving rise to a new segment that continued to expand. The initial two evolutionary phases of the WPA are similar to the evolution of the SAA over the past century. According to the WPA's evolution, it suggests that the current state of the SAA may correspond to an early stage of splitting. Forecasts based on the evolution of the WPA indicate a rapid expansion of the anomalous region in the short term, followed by a gradual reduction in its primary component and continued expansion of a new local minimum. This study provides valuable insight into the evolution of the SAA and highlights the potential utility of the WPA as an evolutionary reference for such geomagnetic phenomena.

Effect of a nonwoven geotextile on nano-TiO2 transport and retention in aggregated porous media under saturated flow conditions

In this experimental work, the transport and retention of TiO2 nanoparticles (nano-TiO2) through a geotextile-aggregated porous media filtration system at two distinct flow velocities and three ionic strengths (ISs) were investigated. Experimental results showed higher nano-TiO2 recovery in the effluent and less retention when increasing the flow velocities under unfavorable conditions. The surface roughness morphology of the porous media may have a meaningful impact on the attachment of nano-TiO2. The nano-TiO2 deposition increased with IS increasing, which are qualitatively in line with the prediction calculated by the extended Derjaguin−Landau−Verwey−Overbeek (XDLVO) theory. Specially, under unfavorable physicochemical conditions (i.e. 0.1 and 5 mM), nano-TiO2 can be retained on the porous media surface at secondary minima and primary minima. While more deposition of nano-TiO2 may occur at primary minima at the IS of 25 mM (refers to favorable physicochemical conditions). And nano-TiO2 transport pathways became less dispersive and more uniform under high flow rate in the porous media with geotextile. The geotextiles can improve flow homogeneity leading to an enhancement in nano-TiO2 particles removal. Our study showed for the first time that geotextile can be served as an efficient barrier to remove the nano-TiO2 through aggregated porous media. The fitted parameters derived from HYDRUS-1D simulations indicated that both mechanisms of physicochemical attachment and straining occur under the current conditions of this work and they have the same importance in nano-TiO2 retention at column scale.

HCl trimer: HCl-stretch excited intramolecular and intermolecular vibrational states from 12D fully coupled quantum calculations employing contracted intra- and inter-molecular bases.

We present fully coupled, full-dimensional quantum calculations of the inter- and intra-molecular vibrational states of HCl trimer, a paradigmatic hydrogen-bonded molecular trimer. They are performed utilizing the recently developed methodology for the rigorous 12D quantum treatment of the vibrations of the noncovalently bound trimers of flexible diatomic molecules [Felker and Bačić, J. Chem. Phys. 158, 234109 (2023)], which was previously applied to the HF trimer by us. In this work, the many-body 12D potential energy surface (PES) of (HCl)3 [Mancini and Bowman, J. Phys. Chem. A 118, 7367 (2014)] is employed. The calculations extend to the intramolecular HCl-stretch excited vibrational states of the trimer with one- and two-quanta, together with the low-energy intermolecular vibrational states in the two excited v = 1 intramolecular vibrational manifolds. They reveal significant coupling between the intra- and inter-molecular vibrational modes. The 12D calculations also show that the frequencies of the v = 1 HCl stretching states of the HCl trimer are significantly redshifted relative to those of the isolated HCl monomer. Detailed comparison is made between the results of the 12D calculations on the two-body PES, obtained by removing the three-body term from the original 2 + 3-body PES, and those computed on the 2 + 3-body PES. It demonstrates that the three-body interactions have a strong effect on the trimer binding energy as well as on its intra- and inter-molecular vibrational energy levels. Comparison with the available spectroscopic data shows that good agreement with the experiment is achieved only if the three-body interactions are included. Some low-energy vibrational states localized in a secondary minimum of the PES are characterized as well.

Global Parameters of Eight W UMa-type Binary Systems

Abstract Multiband photometric investigations for eight binary systems of the W Ursae Majoris-type are presented. Six systems are presented for the first time to analyze their light curves. All the analyzed systems have a temperature below 5000 K and an orbital period of less than 0.28 days. We extracted primary and secondary minima from the ground-based observations of these systems. According to a few observations reported in the literature, linear fits were considered in the O − C diagrams, and new ephemerides were presented. Light curve solutions were performed using the PHysics Of Eclipsing BinariEs code. The results of the mass ratio and fillout factor indicate that the systems are contact binary stars. Six of them showed the O’Connell effect, and a cold starspot on each companion was required for light curve solutions. Their absolute parameters were estimated and evaluated by two other methods. In this study, the empirical relationship between the orbital period and semimajor axis was updated using a sample consisting of 414 contact binary systems and the Monte Carlo Markov Chain approach. Also, using Machine Learning and the Artificial Neural Network model, the relationship between P–T 1–M 1 was updated for a better estimation of the mass of the primary star.

Effects of scattering in the accretion funnel on the pulse profiles of accreting millisecond pulsars

The hotspot emission of accreting millisecond pulsars (AMPs) undergoes scattering in the accretion flow between the disk inner radius and the neutron star surface. The scattering optical depth of the flow depends on the photon emission angle, which is a function of the pulse phase, and reaches its maximum when the hotspot is closest to the observer. At sufficiently large optical depths the observed pulse profile should develop a secondary minimum, the depth of which depends on the accretion rate and the emission geometry. Such a dip evolving with the accretion rate might explain the phase shift and pulse profile evolution observed in AMPs during outbursts. Accounting for scattering is important for accurate modeling of the AMP pulse profiles in order to improve the accuracy of determination of the neutron star parameters, such as their masses and radii. In this paper we present a simplified analytical model for the Thomson optical depth of the accretion funnel, and apply it to simulating the pulse profiles. We show that scattering in the accretion funnel has a significant effect on the pulse profiles at accretion rates of M ≳ 10−10 M⊙yr−1. Our model predicts a gradual evolution of the pulse profile with the accretion rate that appears to be consistent with the observations.

Flares confirmed for the first time to be from the primary component of an eclipsing binary

Abstract Only a handful of precise observations of flare detected from a contact binary so far. This paper presents the photometric and spectroscopic analysis of a contact binary (ASAS J082243+1927.0). Two flare events detected from this object were observed in the data of TESS. The two flare events both occurred at the secondary minimum. Considering the totally eclipsing secondary minimum, it is pretty sure that the two flares are both from the primary component. This is the first time to definitely identify a flare that comes from which component in an eclipsing binary system. This system is a low mass ratio (q ∼ 0.094) and deep contact binary ($f\sim 69.4~{{\%}}$). LAMOST spectra of this object exhibit excess emissions in the Hα line, indicating its chromospheric activity. The continuous variation of the TESS light curves is explained by using the variation of a dark spot added to the secondary component. Through analyzing the relationship between the variation of the longitude of the spot and the O-C variation of the primary minima, we discover the spot rotation is prograde as viewed in the frame rotating with the orbital motion.

Investigating the Stacking Interactions Responsible for Driving 3D Self-Association of Tricarb Macrocycles.

We have investigated the noncovalent forces that play a crucial role in the three-dimensional (3D) self-association of the tricarb macrocycle (composed of alternating triazoles and carbazoles) to understand the multilayer stacks observed through electron microscopy. To explore this idea quantitatively, we have investigated a stacked dimer model of tricarb, where we consider homochiral as well as heterochiral forms of the dimer. We have computed the rotational potential energy surface of the dimer by conducting an angle-dependent scan between the two macrocycles using different levels of theory including the RI-MP2 ab initio method. We observe that dimers oriented at an angle of 60° exhibit the highest stability, while a secondary minimum is observed at an angle of 30°. While density functional theory (DFT) describes the behavior of both minima very close to that obtained with RI-MP2, semiempirical and MM models appear to obtain only a shoulder instead of the second minimum. To further understand the underlying interactions responsible for stabilizing the self-assembly of the macrocycles, we employed energy decomposition analysis (EDA) using SAPT0. This quantitative assessment allowed us to identify the major contributing noncovalent interactions, including electrostatic, exchange-repulsion, dispersion, and induction. Finally, we expanded our study to evaluate the accuracy of the MIM (molecules-in-molecules) fragmentation methodology in capturing the crucial π-stacking interactions. Our benchmarking results using the MIM method accurately replicated the angle-dependent PES results. This shows the efficacy of MIM in predicting the noncovalent interactions responsible for the construction of 3D and other higher-order nanoarchitectures for tricarb and related compounds.

Structure and Dynamics of DNA and RNA Double Helices Formed by d(CTG), d(GTC), r(CUG), and r(GUC) Trinucleotide Repeats and Associated DNA-RNA Hybrids.

Myotonic dystrophy type 1 is the most frequent form of muscular dystrophy in adults caused by an abnormal expansion of the CTG trinucleotide. Both the expanded DNA and the expanded CUG RNA transcript can fold into hairpins. Co-transcriptional formation of stable RNA·DNA hybrids can also enhance the instability of repeat tracts. We performed molecular dynamics simulations of homoduplexes associated with the disease, d(CTG)n and r(CUG)n, and their corresponding r(CAG)n:d(CTG)n and r(CUG)n:d(CAG)n hybrids that can form under bidirectional transcription and of non-pathological d(GTC)n and d(GUC)n homoduplexes. We characterized their conformations, stability, and dynamics and found that the U·U and T·T mismatches are dynamic, favoring anti-anti conformations inside the helical core, followed by anti-syn and syn-syn conformations. For DNA, the secondary minima in the non-expanding d(GTC)n helices are deeper, wider, and longer-lived than those in d(CTG)n, which constitutes another biophysical factor further differentiating the expanding and non-expanding sequences. The hybrid helices are closer to A-RNA, with the A-T and A-U pairs forming two stable Watson-Crick hydrogen bonds. The neutralizing ion distribution around the non-canonical pairs is also described.

Apsidal motion in the α CrB binary system revealed through X-ray and optical eclipse timing

The α CrB system is among the brightest known eclipsing binaries in an eccentric orbit. In such systems, the periods between successive primary (Pprim) and secondary minima (Psec) typically differ, and the difference between the two periods provides information on apsidal motion in the system. However, with a binary period of 17.36 days and eclipse durations of more than 8 h, these eclipses are difficult to observe from the ground. We present new photometric observations of primary and secondary eclipses of α CrB obtained with the TESS satellite and we re-examine the available X-ray observations of α CrB at the time of secondary optical minimum. We also combine the optical and X-ray data to determine precise values of Pprim and Psec to explore the consequences for apsidal motion in the α CrB system. We have determined an internal structure constant that is in good agreement with theoretical expectations and we show that there is no need to invoke any substantial misalignment between the rotation and orbital axes of the primary.

Humic acid-mediated transport of a typical soil passivation remediation product (chloropyromorphite) in saturated porous media

Conversion of labile Pb species into chloropyromorphite (CPY) using phosphorus-bearing amendments was considered to be an ideal strategy in soil passivation remediation. However, the fate and transport of CPY in the soil are poorly understood. This study aims to fill the knowledge gap by evaluating the fate and transport of CPY under environmentally relevant conditions of humic acid (HA), pH, electrolyte concentration, and species through the saturated sandy medium. Results showed that bare CPY colloids are basically immobile in sandy porous media while the co-existence of HA made the transport of CPY improved by 30%–93.5%. Facilitated transport of CPY was attributed to the increased stability of CPY and the repulsive interaction between CPY particles and sands due to HA adsorption. The mobility of CPY was also increased with increasing pH from 5.0 to 9.0. When the pH was 9 with a 10 mmol/L NaCl background solution, the stronger energy barrier between CPY and sand led to enhanced transport behavior. The divalent Ca2+ had a more dramatic effect than monovalent Na+ on the aggregation and sedimentation of CPY colloids due to its effectivescreening of the surface charge of CPY and bridging interaction with CPY particles. Derjaguin-Landau-Verwey-Overbeek theory and attachment efficiency calculation indicated that high energy barriers were responsible for the high mobility of CPY colloids, while the retention of CPY in sands was mainly caused by secondary energy minimum and physically straining. The findings of this work can help to evaluate the fate of soil passivation remediation products in natural water and soil.

In-Plane Rotation of Prolate Colloids Adhered to a Planar Substrate in the Presence of Flow.

Micron-size spherical polystyrene colloidal particles are mechanically stretched to a prolate geometry with desirable aspect ratios. The particles in an aqueous medium with specific ionic concentration are then introduced into a microchannel and allowed to settle on a glass substrate. In the presence of unidirectional flow, the loosely adhered particles in the secondary minimum of surface interaction potential are easily washed off, but the remnant in the strong primary minimum preferentially aligns with the flow direction and exercises in-plane rotation. A rigorous theoretical model is constructed to account for filtration efficiency in terms of hydrodynamic drag, intersurface forces, reorientation of prolate particles, and their dependence on flowrate and ionic concentration.

Light and period variations in the supersoft X-ray source QR And (= RX J0019.8 + 2156)

ABSTRACT QR And is the optically brightest supersoft X-ray source, where a massive white dwarf in a close binary accretes material from its companion star via an accretion disc. Based on the photometric observations from the TESS space telescope and the AAVSO data base, 91 eclipse timings were determined. Variations such as flaring activities, depth and profile changes in the primary and the secondary minima, and some short-term irregular fluctuations are found in the TESS light curves, which are interpreted as the activities of the accretion disc and fluctuations of the mass transfer rate. By collecting all available eclipse timings together with those newly determined, we constructed the O-C diagram and analysed the changes in the orbital period. It is discovered that the orbital period is continuously increasing at a rate of $\dot{P}= +3.7(1)\times 10^{-7} d \cdot yr^{-1}= +0.0320(8)s \cdot yr^{-1}$. The continuous increase in the orbital period is mainly caused by the mass transfer from the low-mass companion to the white dwarf on its Kelvin–Helmholtz time-scale, while wind mass loss from the accretion disc may also contribute to the period variation. In this scenario, the mass transfer rate of QR And is estimated to be in the range of about 1.2 × 10−7 to 4.2 × 10−7M⊙ × yr−1 and the mass-accretion rate is less than 1.6 × 10−7M⊙ × yr−1. It can be roughly estimated that QR And may reach to the Chandrasekhar limit in at least 1.5 million yr, then might explode as a type Ia supernova.

Total longitudinal potential energy of interaction between parallel charged rods of finite size

We present a formula for the total potential energy of interaction between two parallel charged prismatic rods of finite size as a function of longitudinal inter-rod displacement (TLPE), obtained by calculating the repulsive electrostatic potential energy (using the nonlinear Poisson-Boltzmann equation) and the attractive van der Waals (VDW) potential energy between the rods in skewed configurations. The full profile of the TLPE curves is like an inverted bell, whose (top) width is twice the rod length, and depth is the same as that of the secondary minimum of the Derjaguin-Landau-Verwey-Overbeek (DLVO) potential for the rods.Using the TLPE function, we estimated root mean square of longitudinal displacements of fluctuating rods of 250 × 50 × 50 nm3 and a scaled surface potential of 5 for lateral inter-rod separations, d = 20 and 25 nm, e.g., in 5-rod systems, which showed non-negligible values of 24–87 nm.

Low-rate ferrate dosing damages the microbial biofilm structure through humic substances destruction and facilitates the sewer biofilm control

The microbial activities in sewer biofilms are recognized as a major reason for sewer pipe corrosion, malodor, and greenhouse gas emissions. However, conventional methods to control sewer biofilm activities were based on the inhibitory or biocidal effect of chemicals and often required long exposure time or high dosing rates due to the protection of sewer biofilm structure. Therefore, this study attempt to use ferrate (Fe(VI)), a green and high-valent iron, at low dosing rates to damage the sewer biofilm structure so as to enhance sewer biofilm control efficiency. The results showed the biofilm structure started to crush when the Fe(VI) dosage was 15 mg Fe(VI)/L and the damage enhanced with the increasing dosage. The determination of extracellular polymeric substances (EPS) showed that Fe(VI) treatment at 15-45 mgFe/L mainly decreased the content of humic substances (HS) in biofilm EPS. This is because the functional groups, such as C-O, -OH, and C=O, which held the large molecular structure of HS, were the primary target of Fe(VI) treatment as suggested by 2D-Fourier Transform Infrared spectra. As a result, the coiled chain of EPS maintained by HS was turned to extended and dispersed and consequently led to a loosed biofilm structure. The XDLVO analysis suggested that both the microbial interaction energy barrier and secondary energy minimum were increased after Fe(VI) treatment, suggesting that the treated biofilm was less likely to aggregate and easier to be removed by the shear stress caused by high wastewater flow. Moreover, combined Fe(VI) and free nitrous acid (FNA) dosing experiments showed for achieving 90% inactivation, the FNA dosing rate could be reduced by 90% with the exposure time decreasing by 75% at a low Fe(VI) dosing rate and the total cost was substantially decreased. These results suggested that applying low-rate Fe(VI) dosing for sewer biofilm structure destruction is expected to be an economical way to facilitate sewer biofilm control.

Phosphate-solubilizing microorganisms regulate the release and transformation of phosphorus in biochar-based slow-release fertilizer

Coupling phosphate-solubilizing microorganisms (PSM) can improve the availability of phosphorous (P) in biochar-based slow-release P fertilizers (BPF). However, the mechanism in release and transformation of P in BPF regulated by PSM is still unclear. Herein, the biocompatibility and the adhesion behaviors of BPF and PSM (Enterobacter hormaechei Rs-198) in soil were firstly studied, and a 90 days' laboratory-scale soil incubation experiment of BPF and Rs-198 was performed to study the transformation of P of BPF. The results show that BPF has a good biocompatibility for Rs-198 due to its low aromaticity, graphitization and free radicals' content (0.084 mg/g). Rs-198 are adhered to the surface of BPF in soil due to the high negative secondary energy minimum and low total interaction energy between Rs-198 and BPF. Available P in the incubation of BPF and Rs-198 (BR treatment) is significantly higher than that of the incubation of BPF (BF treatment) at initial 60 days. However, the content of available P in BR treatment is much lower compared with that in BF treatment on day 90, which is attributed to the entrapment of released P from BPF by Rs-198 and the formation of polyphosphate (polyP) rather than bound with soil mineral. Overall, this study presents new insights into the transformation of P in BPF regulated by PSM.

Effect of decoration route on the nanomechanical, adhesive, and force response of nanocelluloses-An in situ force spectroscopy study.

Although cellulose derivatives are widely applied in high-tech materials, the relation between their force responses and their surface chemical properties in a biological environment as a function of pH is unknown. Here, interaction forces of surface modified cellulose nanocrystals (CNCs), lignin residual cellulose nanocrystals (LCNCs), and 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO)-oxidized cellulose nanofibres (TCNFs) with OSO3-, COO- and lignin chemical groups were measured using in situ peak force quantitative nanomechanical mapping and force spectroscopy in salt solution at two pH values. We found that the forces acting between the tip and CNC or LCNC are steric dominated showing long range and slow decay as a result of their low surface charge density. High Mw lignin contributed to the increased repulsion range for LCNCs compared to CNCs. The repulsion measured for TCNFs at the very short range was electrostatic force dominating showing a steep decay attributed to its high surface charge density. In the case of TCNFs, electrostatic double layer force was also evidenced by the attraction measured at secondary minima. In all the three cases the electro steric interactions are pH dependent. Dissipation maps verified that the force behavior for each material was related to structural conformation restriction of the groups at compression. The slow decayed repulsion of CNCs or LCNCs is related to a weak restriction of conformational change due to small surface groups or high molecular weight bound polymers forming flat layers, whereas the steep repulsion of TCNFs is attributed to a strong conformation restriction of carboxylic groups occurred by forming extended structure. Our results suggest that the force responses of the materials were dominated by surface charges and structural differences. TCNFs showed superior nanomechanical and repulsion properties over CNCs or LCNCs at neutral pH.

Mechanistic insights into aggregation process of graphene oxide and bacterial cells in microbial reduction of ferrihydrite.

Microbial reduction of ferrihydrite is prevalent in natural environments and plays an important role in reductive dissolution of Fe(III) minerals. With consistent release of anthropogenic graphene oxide (GO) into water bodies, new changes in the Fe(III)-reducing microorganisms/ferrihydrite binary system demand attention. Herein, we focused on the interaction of GO and bacterial cells in view of colloidal stability and interfacial forces, and on the consequences for microbial ferrihydrite reduction. The results showed that the addition of GO decreased the bioreduction efficiency of ferrihydrite down to 1/15 of the control. Meanwhile, the GO nanosheets were found not depositing on ferrihydrite but spontaneously aggregating with Shewanella spp., the representative dissimilatory Fe(III) reduction bacterial species. Using the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory and atomic force microscopy (AFM), the aggregation process can be interpreted in three steps according to the interaction energy calculation, namely, colloidal instability, reversible aggregation and irreversible aggregation. The motility of living cells seems the reason inducing the colloidal instability between GO and bacteria. While, the aggregation remains reversible even the secondary minimum achieved at the separation distance of 8.74-9.24 nm from XDLVO. When the separation distance <5.74-6.01 nm, the adhesion work predominates and causes irreversible aggregation, validated by AFM. Additionally, the probable ecological risks raised by this aggregation behavior for the imbalance of iron biogeochemical cycle were demonstrated.