Concentrations Of Divalent Cations Research Articles

Dynamic Surface Interactions Enable the Self-Assembly of Perfect Supramolecular Crystals.

Supramolecular crystals arise from noncovalent interactions between macromonomers and allow for the engineering of dynamic functional materials. For two-dimensional (2D) crystals, the substrate surface can induce the formation of new polymorphs not available in solution, adding a layer of complexity to the supramolecular self-assembly process. Despite extensive studies on the 2D self-assembly of supramolecular crystals, unknowns remain regarding substrate-monomer interactions and the effects on network self-assembly and defect repair. Here, we used a DNA-mica model system to modulate and understand the impact of substrate-monomer interactions on the crystalline order. We controlled the surface interactions by tuning the Mg2+ concentration, varying the divalent cation type, and adjusting the relative concentration of divalent and monovalent cations. The competition between monovalent and divalent cations yielded nearly defect-free crystals with minimal polygon defects. These findings highlight the critical role of surface interactions in achieving high crystalline order, which is essential for optimizing the efficiency and performance of supramolecular functional nanomaterials.

Preventing acute neurotoxicity of CNS therapeutic oligonucleotides with the addition of Ca2+ and Mg2+ in the formulation

Oligonucleotide therapeutics (ASOs and siRNAs) have been explored for modulation of gene expression in the central nervous system (CNS), with several drugs approved and many in clinical evaluation. Administration of highly concentrated oligonucleotides to the CNS can induce acute neurotoxicity. We demonstrate that delivery of concentrated oligonucleotides to the CSF in awake mice induces acute toxicity, observable within seconds of injection. Electroencephalography (EEG) and electromyography (EMG) in awake mice demonstrated seizures. Using ion chromatography, we show that siRNAs can tightly bind Ca2+ and Mg2+ up to molar equivalents of the phosphodiester (PO)/phosphorothioate (PS) bonds independently of the structure or phosphorothioate content. Optimization of the formulation by adding high concentrations (above biological levels) of divalent cations (Ca2+ alone, Mg2+ alone, or Ca2+ and Mg2+) prevents seizures with no impact on the distribution or efficacy of the oligonucleotide. The data here establishes the importance of adding Ca2+ and Mg2+ to the formulation for the safety of CNS administration of therapeutic oligonucleotides.

Theoretical and experimental investigation of the impact of oil functional groups on the performance of smart water in clay-rich sandstones

This research investigated the effect of ion concentration on the performance of low salinity water under different conditions. First, the effect of injection water composition on interparticle forces in quartz-kaolinite, kaolinite-kaolinite, and quartz-oil complexes was tested and modeled. The study used two oil samples, one with a high total acid number (TAN) and the other with a low TAN. The results illustrated that reducing the concentration of divalent ions to 10 mM resulted in the electric double layer (EDL) around the clay and quartz particles and the high TAN oil droplets, expanding and intensifying the repulsive forces. Next, the study investigated the effect of injection water composition and formation oil type on wettability and oil/water interfacial tension (IFT). The results were consistent with the modeling of interparticle forces. Reducing the divalent cation concentration to 10 mM led to IFT reduction and wettability alteration in high TAN oil, but low TAN oil reacted less to this change, with the contact angle and IFT remaining almost constant. Sandpack flooding experiments demonstrated that reducing the concentration of divalent cations incremented the recovery factor (RF) in the presence of high TAN oil. However, the RF increment was minimal for the low TAN oil sample. Finally, different low salinity water scenarios were injected into sandpacks containing migrating fines. By comparing the results of high TAN oil and low TAN oil samples, the study observed that fine migration was more effective than wettability alteration and IFT reduction mechanisms for increasing the RF of sandstone reservoirs.

Preventing acute neurotoxicity of CNS therapeutic oligonucleotides with the addition of Ca2+ and Mg2+ in the formulation.

Oligonucleotide therapeutics (ASOs and siRNAs) have been explored for modulation of gene expression in the central nervous system (CNS), with several drugs approved and many in clinical evaluation. Administration of highly concentrated oligonucleotides to the CNS can induce acute neurotoxicity. We demonstrate that delivery of concentrated oligonucleotides to the CSF in awake mice induces acute toxicity, observable within seconds of injection. Electroencephalography (EEG) and electromyography (EMG) in awake mice demonstrated seizures. Using ion chromatography, we show that siRNAs can tightly bind Ca2+ and Mg2+ up to molar equivalents of the phosphodiester (PO)/phosphorothioate (PS) bonds independently of the structure or phosphorothioate content. Optimization of the formulation by adding high concentrations (above biological levels) of divalent cations (Ca2+ alone, Mg2+ alone, or Ca2+ and Mg2+) prevents seizures with no impact on the distribution or efficacy of the oligonucleotide. The data here establishes the importance of adding Ca2+ and Mg2+ to the formulation for the safety of CNS administration of therapeutic oligonucleotides.

The complex effect of dissolved organic carbon on desorption of per- and poly-fluoroalkyl substances from soil under alkaline conditions

Per- and poly-fluoroalkyl substances (PFASs) are contaminants of emerging concern, yet the understanding of factors that control their leaching and release from contaminated soils remains limited. This study aimed to investigate the impact of dissolved organic carbon (DOC) on the release of PFASs—specifically, perfluorohexane sulfonate (PFHxS), perfluorooctane sulfonate (PFOS), and perfluorooctanoic acid (PFOA)from soils contaminated by aqueous film forming foam (AFFF). Batch aqueous leaching experiments were conducted on AFFF-contaminated soils under alkaline solution conditions (pH 9.5, 10.5, and 12) as it enhances leaching of both PFAS and DOC. Leaching of PFOS was significantly increased under alkaline conditions. Although the leaching of PFAS generally increased with pH, PFOS appeared to be more retained under the very alkaline pH conditions used in this study. At the same solution pH, leaching of PFOS and DOC was less in Ca(OH)2 than in NaOH. The retention of PFOS under these conditions may be attributable to the shielding of the negative charge of the soil components and colloids (e.g., DOC and clay minerals) in the leachates and/or the screening of negative charges on head groups of PFOS due to the high concentration of divalent cations. Solution chemistry affected desorption of PFOS more than PFHxS and PFOA. The study highlights that the influence of DOC on PFAS leaching and transport can be very complex, and depends on leachate chemistry (e.g., pH and cation type), PFAS chemistry, the magnitude of PFAS contamination and factors that influence the solid:liquid partitioning of organic carbon in soil.

Competition between Self-Assembly and Phase Separation Governs High-Temperature Condensation of a DNA Liquid.

In many biopolymer solutions, attractive interactions that stabilize finite-sized clusters at low concentrations also promote phase separation at high concentrations. Here we study a model biopolymer system that exhibits the opposite behavior, whereby self-assembly of DNA oligonucleotides into finite-sized, stoichiometric clusters tends to inhibit phase separation. We first use microfluidics-based experiments to map a novel phase transition in which the oligonucleotides condense as the temperature increases at high concentrations of divalent cations. We then show that a theoretical model of competition between self-assembly and phase separation quantitatively predicts changes in experimental phase diagrams arising from DNA sequence perturbations. Our results point to a general mechanism by which self-assembly shapes phase boundaries in complex biopolymer solutions.

Optimizing in-situ CO2 mineralisation: geomechanics and scalability in dunite and serpentinite rocks – Examples from Australia and New Zealand

The collective drive towards achieving net-zero greenhouse gas emissions by 2050 has spurred interest in engineering solutions for carbon capture and storage worldwide. One such approach involves the permanent storage of CO2 in earth-abundant Ca-, Fe-, and Mg-bearing silicate rocks and minerals as carbonates via the process of CO2 mineralisation. This necessitates a thorough understanding of carbonate conversion under geologically relevant conditions. Nevertheless, research on CO2 injection for mineralisation via naturally fractured host rocks or induced fractures, with a research emphasis on rock mechanics and stimulated reservoir volumes (SRV) within geoengineering CO2 storage, is continuously expanding. This research addresses critical challenges related to identifying favourable geographic locations for CO2 mineralisation. It specifically focuses on the abundant availability of Mg, Ca, and Fe cations for exothermic CO2 reactions and their impact on fracture conductivity during in-situ mineralisation. A comprehensive analysis of 26 dunite and serpentinite samples from diverse locations in Australia and New Zealand, including 10 from a cored drilled hole, was conducted. Quantification of divalent cation (Mg, Ca, Fe) content and cation release capacity using XRF and XRD revealed higher cation percentages in dunite samples (approximately 30 %) compared to serpentinite samples (approximately 26 %). Additionally, the study estimated the stimulated rock mass-to-CO2 sequestered ratio, RCO2, with dunite samples averaging approximately 2.20 RCO2 values and serpentinite samples averaging approximately 2.53. Geomechanical testing enabled the prediction of fracture propagation pressures during aqueous CO2 injection for in-situ mineralisation and the estimation of fracture geometries, emphasizing the role of rock stiffness in determining fracture width (averaging 6.0 mm). Furthermore, the research estimated the rock volume exposed to CO2-laden fluid during injection, particularly focusing on the GHQ-3 sample, which theoretically amounted to approximately 600 kg of rock capable of sequestering around 300 kg of CO2 for a 10 m3 fluid volume with a CO2 concentration of 1molkg−1. The study established a relationship between injected volume and CO2 uptake, suggesting the potential for significant CO2 sequestration scalability by employing horizontal wells and fracturing additional zones, thereby creating and intersecting multiple transverse fractures along a single target zone.

The impact of geochemical and life-cycle variables on carbon dioxide removal by enhanced rock weathering: Development and application of the Stella ERW model

The carbon dioxide removal (CDR) potential of enhanced rock weathering (ERW) depends on dynamic interactions between several biogeochemical and life-cycle variables. This paper reports results from a systems model developed to account for key variable interactions and provide a computational tool for optimizing ERW applications. We discuss the model development, comparisons with laboratory and field test data, and results from a series of sensitivity analyses for several hypothetical ERW applications. The simulations were performed using a model developed in Stella Architect, an object-oriented systems dynamics modeling code. The model tracks the amount of carbon dioxide consumed by the weathering of a user-specified mass of rock powder in a soil environment. The primary model variables are: rock powder particle size distribution, soil pH, soil temperature, soil pore water saturation index, and a biological weathering factor. The rock bulk composition is also a key variable as it defines the enhanced weathering potential of the rock powder. Life cycle variables such as carbon emissions from mining, rock crushing, and transport are also quantified. The model predictions are consistent with measured data. It predicts CDR values ranging from 1 to 12 t CO2 ha−1 (depending on conditions) for single applications of 20–40 t of rock ha−1 over a 10-year interval. Applying the model to a hypothetical large-scale ERW campaign in which basalt rock powder is applied to 25% and 75% of Brazilian cropland results in CDR values of 0.15 and 0.5 Gt CO2 over ten years, respectively (for a single application). Model results show that combinations of suboptimal rock type (i.e., low concentrations of divalent cations and slow weathering rates for field conditions) and suboptimal life-cycle variables (i.e., high vehicle emission factors and long rock transport distances) can result in net positive CO2 emissions for some hypothetical ERW scenarios. It is, therefore, emphasized that the quantitative co-optimization of geochemical, biological, and life-cycle variables is essential in the planning stages of ERW applications.

Development of ionic-liquid-impregnated activated carbon for sorptive removal of PFAS in drinking water treatment

Adsorption of per- and poly-fluoroalkyl substances (PFAS) on activated carbon (AC) is considerably hindered by the surface water constituents, degrading the ability of the AC adsorption process to remove PFAS in drinking water treatment. Herein, we developed ionic-liquid-impregnated AC (IL/AC) as an alternative to AC for PFAS sorption and demonstrated its performance with real surface water for the first time. Ionic liquids (ILs) of 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (IL(C2)) and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (IL(C6)) were selected from among 272 different ILs using the conductor-like screening model for realistic solvents (COSMO-RS) simulation. Impregnation of the ILs in AC was verified using various analytical techniques. Although the synthesized IL/ACs were less effective than pristine AC in treating PFAS in deionized water, their performances were less impacted by the surface water constituents, resulting in comparable or sometimes better performances than pristine AC for treating PFAS in surface water. The removal efficiencies of 10 wt% IL(C6)/AC for six PFAS were 1.40–1.96 times higher than those of pristine AC in a surface water sample containing 2.6 mg/L dissolved organic carbon and millimolar-level divalent cation concentration. PFAS partitioning from the surface water to ILs was not hindered by dissolved organic matter and was enhanced by the divalent cations, indicating the advantages of IL/ACs for treating significant amounts of PFAS in water. The synthesized IL/ACs were effective at treating coexisting pharmaceutical and personal-care products in surface water, showcasing their versatility for treating a broad range of water micropollutants.

Nanoprecipitation-Enhanced Sensitivity in Enzymatic Nanofluidic Biosensors.

Single nanochannels show unique transport properties due to nanoconfinement. It has been demonstrated that at submillimolar concentrations of divalent cations, a nanoprecipitation reaction can occur in nanochannels. Although several reports have shown, described, and modeled the nanoprecipitation process, no further advantages have been taken from this phenomenon. Here, we show that the nanoprecipitation reaction can be incorporated into enzyme-modified nanochannels to enhance the performance of small-molecule biosensors via in situ amplification reactions. Contrary to the working principle of previous enzymatic nanofluidic biosensors, the nanofluidic biosensor described in this work operates on the basis of concerted functions: pH-shifting enzymatic activity and nanoprecipitation. We show that the simple addition of Ca2+ and Mg2+ ions in the working analyte solution containing urea can lower the detection limit from the nanometer to the subnanometer regime and modulate the dynamic linear range. This approach enables the implementation of more sensitive real-time nanofluidic detection methods without increasing the complexity of the nanofluidic platform or the sensing approach. We envision that the integration of concerted functions in nanofluidic architectures will play a key role in expanding the use of these nanoscale devices for analytical purposes.

Tuning mono-divalent cation water composition by the capacitive ion-exchange mechanism

Soil salinization poses a significant challenge to agricultural activities. To address this, the agricultural industry seeks an irrigation water solution that reduces both ionic conductivity and sodium adsorption rate (SAR), thereby diminishing the risks of soil sodification and fostering sustainable crop production. Capacitive deionization (CDI) is an attractive electrochemical technology to advance this search. Recently, a one-dimensional transient CDI model unveiled a capacitive ion-exchange mechanism presenting the potential to adjust the treated water composition by modifying monovalent and divalent cation concentrations, thereby influencing the SAR index. This behavior would be achieved by using electrodes rich in surface functional groups able to efficiently capture divalent cations during conditioning and releasing them during charging while capturing monovalent ions. Beyond the theoretical modelling, the current experimental research demonstrates, for the first time, the effectiveness of the capacitive ion-exchange mechanism in a CDI pilot plant using real water samples spiked with solutions containing specific mono and divalent ions. Electrosorption experiments and computational modeling, specifically Density-Functional Theory (DFT), were used along with the analysis of the surface functional groups present in the electrodes to describe the capacitive ion-exchange phenomenon and validate the steps involved on it, highlighting the conditioning as a critical step. Various operational and flow modes confirm the versatility of CDI technology, achieving separation factors (RMg/Na) of 5–6 in batch, raising production from 0.5 to 0.8 L m−2 h−1 (batch) to 8.0–8.1 L m−2 h−1 when using single pass although reducing RMg/Na to 2. The reliability of the CDI technology in reducing SAR was also successfully tested with different influent compositions, including magnesium and calcium. Finally, the robustness of the capacitive ion-exchange mechanism was validated by a second CDI laboratory 9-cell stack cycled over 350 cycles. Our results confirm the reported theoretical model and expands the conclusions through the experiments in a pilot plant showing direct implications for employing CDI in agricultural applications.

The Effect of 8,5'-Cyclo 2'-deoxyadenosine on the Activity of 10-23 DNAzyme: Experimental and Theoretical Study.

The in vivo effectiveness of DNAzymes 10-23 (Dz10-23) is limited due to the low concentration of divalent cations. Modifications of the catalytic loop are being sought to increase the activity of Dz10-23 in physiological conditions. We investigated the effect of 5'S or 5'R 5',8-cyclo-2'deoxyadenosine (cdA) on the activity of Dz10-23. The activity of Dz10-23 was measured in a cleavage assay using radiolabeled RNA. The Density Functional Tight Binding methodology with the self-consistent redistribution of Mulliken charge modification was used to explain different activities of DNAzymes. The substitution of 2'-deoxyadenosine with cdA in the catalytic loop decreased the activity of DNAzymes. Inhibition was dependent on the position of cdA and its absolute configuration. The order of activity of DNAzymes was as follows: wt-Dz > ScdA5-Dz ≈ RcdA15-Dz ≈ ScdA15-Dz > RcdA5-Dz. Theoretical studies revealed that the distance between phosphate groups at position 5 in RcdA5-Dz was significantly increased compared to wt-Dz, while the distance between O4 of dT4 and nonbonding oxygen of PO2 attached to 3'O of dG2 was much shorter. The strong inhibitory effect of RcdA5 may result from hampering the flexibility of the catalytic loop (increased rigidity), which is required for the proper positioning of Me2+ and optimal activity.

Reexpansion of charged nanoparticle assemblies in concentrated electrolytes

Electrostatic forces in solutions are highly relevant to a variety of fields, ranging from electrochemical energy storage to biology. However, their manifestation in concentrated electrolytes is not fully understood, as exemplified by counterintuitive observations of colloidal stability and long-ranged repulsions in molten salts. Highly charged biomolecules, such as DNA, respond sensitively to ions in dilute solutions. Here, we use non-base-pairing DNA-coated nanoparticles (DNA-NP) to analyze electrostatic interactions in concentrated salt solutions. Despite their negative charge, these conjugates form colloidal crystals in solutions of sufficient divalent cation concentration. We utilize small-angle X-ray scattering (SAXS) to study such DNA-NP assemblies across the full accessible concentration ranges of aqueous CaCl2, MgCl2, and SrCl2 solutions. SAXS shows that the crystallinity and phases of the assembled structures vary with cation type. For all tested salts, the aggregates contract with added ions at low salinities and then begin expanding above a cation-dependent threshold salt concentration. Wide-angle X-ray scattering (WAXS) reveals enhanced positional correlations between ions in the solution at high salt concentrations. Complementary molecular dynamics simulations show that these ion-ion interactions reduce the favorability of dense ion configurations within the DNA brushes below that of the bulk solution. Measurements in solutions with lowered permittivity demonstrate a simultaneous increase in ion coupling and decrease in the concentration at which aggregate expansion begins, thus confirming the connection between these phenomena. Our work demonstrates that interactions between charged objects continue to evolve considerably into the high-concentration regime, where classical theories project electrostatics to be of negligible consequence.

The Effects of Magnesium, Zinc and Calcium Ions on Endotoxin-Plasmid DNA Interaction at Various Cation Concentrations, pH Values and Incubation Times

In plasmid DNA (pDNA) production from Gram-negative bacteria, endotoxin has been known as the major contaminant. The separation becomes difficult due to its ability to form a stable complex with pDNA apart from sharing common properties like surface charge, molecular size, temperature, and pH stability. This study focused on the analysis of the zeta potential values of endotoxin, the theoretical number of cations bound per molecule of endotoxin as well as the binding tendency of cations towards endotoxin in the presence of pDNA. These analyses were conducted under various experimental conditions such as types of divalent metal cation, cation concentration, pH and incubation time. The analysis of zeta potential at different cation concentrations and pH values showed that Mg2+ had the most significant effect on endotoxin surface charge. The zeta potential of endotoxin was reduced by a magnitude of 43.55 mV, from −43.53 mV to 0.02 mV in the presence of 2.0 M Mg2+, and a magnitude of 44.12 mV, from −43.53 mV to 0.59 mV at the lowest pH level. However, in the analysis of the theoretical number of cations bound per molecule of endotoxin, Zn2+ showed the highest number (0.6) compared to Ca2+ (0.12) and Mg2+ (0.05). The tendency of Zn2+ to preferentially bind with endotoxins forming a larger aggregated structure was also evident in the DNA gel electrophoresis and transmission electron microscopic analysis. The manuscript highlights the significance of cations in the binding and aggregation of endotoxins, which ultimately improves the recovery of pDNA and affects its subsequent downstream processing.

Tracking fouling layer formation in membrane distillation of landfill leachate concentrate: Insights from periodic membrane autopsies

This study tracked the fouling layer formation in the operation of membrane distillation (MD) for the treatment of landfill leachate concentrate. At periodic intervals, the membrane fouling layer was autopsied by spectroscopic and compositional analyses, to provide insight into foulant evolution and interaction mechanisms. Results show that water flux declined rapidly at the beginning of MD operation as protein and sodium chloride (NaCl) preferentially attached and crystalized onto the membrane surface, respectively, to initiate the formation of fouling layer. Such decline became relatively smooth between the water recovery of 20–30% due to the interaction of humic acid with protein which slightly aggravated organic fouling and thus increased membrane surface hydrophilicity. Nevertheless, the enrichment of calcium (Ca2+) and magnesium (Mg2+) ions with further water recovery from leachate concentrate bridged organic foulants, particularly polysaccharide and humic acid, to further thicken the hydrophilic fouling layer to reduce water flux. Indeed, compositional quantification also evidenced a considerable increase in the content of humic acid and divalent cations (Ca2+ and Mg2+) in the fouling layer after a water recovery of 30%.

Extractable latex yield from Taraxacum kok-saghyz roots is enhanced by increasing rubber particle buoyancy

Taraxacum kok-saghyz (TK) is a leading alternative rubber-producing plant that produces natural rubber (NR) latex in laticifers within its roots. Unlike the tropical rubber tree, Hevea brasiliensis, TK can be grown in temperate regions, such as the northern United States, as the basis of a domestic rubber production industry. To support such an industry, efficient, scalable rubber extraction techniques are needed. In this study, aqueous extraction was used to recover TK natural rubber latex (TNRL) from four harvests of greenhouse-produced TK roots with a new technique that improves rubber particle buoyancy in root homogenate by chelation. The new method includes harvest, cleaning, and homogenization of freshly harvested roots, followed by centrifugation, vacuum collection of latex, and purification. Treatment of homogenate with ethylenediaminetetraacetic acid (EDTA), a divalent cation chelator, allowed more than twice as much latex to be extracted from fresh homogenate. EDTA-treated rubber particles contained lower concentrations of divalent cations than untreated ones in fresh homogenate and homogenate stored for two weeks. Long term (three months) storage of root homogenate similarly yielded more latex than fresh homogenate and had low divalent cation rubber particle concentrations, indicating that heavy cations had slowly leached from the particles into the aqueous medium during storage. Rubber particle size was larger in this treatment suggestive of rubber particle agglomeration but this was prevented and reversed by EDTA. Removal of divalent cations via chelation increased particle packing density of particles in concentrated latex and appeared to increase the gel content of dried latex samples. Molecular characterization of dried samples of purified TNRL established that the rubber was of the high molecular weight needed for product manufacturing.

Confined environment facilitates stacked conformations in Holliday Junction

Holliday Junction (HJ) is an important intermediate in homologous recombination and is involved in DNA break repair. It is a highly dynamic structure and transitions between two stacked isomers through an intermediate open conformer. However, the insight into the effect of environmental confinement on HJ conformation population is incomplete. The understanding would be an important bridge between studies generally carried out in aqueous buffers and confined environments, as found in vivo. By employing fluorescence-based techniques we investigated the effect of confinement on HJ conformation using reverse micelle encapsulation as a model for confined space. We observed that inside the confinement of reverse micelle, HJ prefers to adopt stacked conformation. Most strikingly, even at lower divalent cation concentrations, the preference for stacked conformation is prevalent over an open conformation. The conformational heterogeneity also reduces with increasing degree of confinement. Our finding suggests that such confinement-induced changes in the conformer population might influence the interaction and activity of the HJ-recognizing proteins in the cellular environment.

Effects of cation valence on swelling power, solubility, pasting, gel strength characteristics of potato starch

The objective of this study was to investigate the effects of monovalent, divalent and trivalent cations on the physicochemical properties of potato starch. Cations decreased the water absorption in potato starch granules to cause lower swelling power, especially 0.02–0.04 mmol/g Fe3+ or 0.01–0.04 mmol/g Al3+, but 0.01 mmol/g trivalent cation led more amylose dissolving into the water to cause higher solubility. High concentration of divalent or trivalent cations markedly decreased the viscosity in potato starch paste during further heating and cooling, resulting in the decreases of the trough and finial viscosity, especially 0.04 mmol/g Fe3+. Higher valence of cations decreased the gel strength in potato starch, especially 0.02–0.04 mmol/g Fe3+ or Al3+. Higher valence of cations decreased the starch gel binding with the water and caused lower water holding capacity. In general, a higher valence of cations caused more changes in the physicochemical characteristics of potato starch.

Structural, dielectric and electronic properties of Ba1-xSrxZr0.3Ti0.7O3 (x = 0.0, 0.1, 0.2, 0.24, 0.27, 0.3) ceramics: Combined experimental and ab-initio study

Ba1-xSrxZr0.3Ti0.7O3 (x = 0.0, 0.1, 0.2, 0.24, 0.27 and 0.3) materials were synthesized using conventional solid state reaction route. Structural properties were investigated using X-ray diffraction analysis. All the samples reveal single phase structure. The dielectric properties of all the samples were investigated at different temperatures of 100 K to 300 K, and at different frequencies of 1 kHz to 500 kHz. The diffusivity rises with increase in Sr divalent cation concentration. It is seemed that with increase in Sr2+ concentration the transition temperature reduces for all compositions. This may because of the introduction of Sr2+ by replacing Ba2+ since Sr+2 has smaller ionic radius than Ba+2. Moreover, the dielectric constant maxima for Ba1-xSrxTi0.7Zr0.3O3 increases up to x = 0.24 and then further increment in Sr content up to x = 0.3, the maxima of dielectric constant shows decrease. Moreover, charge conduction mechanism and optical response of Ba1-xSrxZe0.3Ti0.7O3 (x = 0.0, 0.1, 0.2, 0.3) has been calculated theoretically by using CASTEP. Herein, the calculated density of states (DOS) and three dimensional integrated charge density reveal that concentration of Sr in Ba1-xSrxZe0.3Ti0.7O3 (x = 0.1, 0.2, 0.3) causes variation in energy band gap. As regards optical response of Ba1-xSrxTi0.7Zr0.3O3, with x = 0.3 the high frequency absorption decreases to minimal value in accordance to experimental findings.

Assessing spatiotemporal variability in the concentration and composition of dissolved organic matter and its impact on iron solubility in tropical freshwater systems through a machine learning approach

Dissolved organic matter (DOM) plays important roles not only in maintaining the productivity and functioning of aquatic ecosystems but also in the global carbon cycle, although the sources and biogeochemical functions of terrestrially derived DOM have not been fully elucidated, particularly in the tropics and subtropics. This study aimed to evaluate the factors influencing spatiotemporal variability in (i) the concentration and composition of DOM, including dissolved organic carbon (DOC), ultraviolet absorption coefficient at 254-nm wavelength (a254), and components identified by fluorescence excitation–emission matrix coupled with parallel factor analysis (EEM-PARAFAC), and (ii) the concentration of dissolved iron (DFe) across freshwater systems (rivers, forested streams, and dam reservoirs) on a tropical island (Ishigaki Island, Japan) based on the results of water quality monitoring at 2-month intervals over a 2-year period. Random forests (RF) machine learning algorithm was employed, with the catchment characteristics (land use, soil type) and water temperature as the predictor variables for DOM and the composition of DOM (EEM-PARAFAC components) and hydrochemistry (water temperature, pH, and concentrations of divalent cations) as the predictor variables for DFe. The RF models for DOC, a254, and three humic-like components exhibited excellent predictive performance, indicating that these DOM properties are not only seasonally variable but also strongly influenced by the compositions of land uses and soil types in the upstream watershed. Poorly drained riparian lowland soil (Gleyic Fluvisols) was identified as the most important catchment parameter that positively influences these DOM variables. The RF model also explained a large portion of the variation in DFe, while terrestrial humic-like components were the most important parameters, emphasizing their significance as organic ligands for iron. These results improve our understanding of the impacts of terrestrial DOM and iron loadings on tropical and subtropical coastal ecosystems as well as on regional and global carbon budgets.