Acidic Solution Conditions Research Articles

Covalently cross-linked graphene oxide aerogel with stable structure for high-efficiency water purification

Porous graphene-based aerogels (GAs) hold great promise for water purification due to their fascinating properties. However, weak connections between the graphene oxide nanosheets cause instability and incompressibility in GAs during practical applications. Herein, the small organic molecule of cysteamine was selected as a covalent cross-linker to fabricate GAs through a facile one-step synthesis method. The porosity and mechanical stability of GAs were simultaneously improved due to strong covalent cross-linking between the graphene oxide nanosheets via cysteamine as the linkage. The resultant graphene/cysteamine aerogels (GCAs) were compressible with excellent structural stability under different acid, base, and organic solution conditions. The maximum oil absorption capacity of the GCAs reached as high as 251 g/g and did not obviously decrease even after regeneration by squeezing. In addition, the GCAs exhibited a high adsorption capacity for common dyes, such as methylene blue (207.8 mg/g) and methyl orange (70.2 mg/g). As expected, the as-prepared aerogels with high adsorption capacities, structural stability and excellent compressibility, have great potential as recyclable and durable materials for water purification.

(C2G4)n repeat expansion sequences from the C9orf72 gene form an unusual DNA higher-order structure in the pH range of 5-6.

Massive expansion of a DNA hexanucleotide sequence repeat (C2G4) within the human C9orf72 gene has been linked to a number of neurodegenerative diseases. In sodium or potassium salt solutions, single-stranded d(C2G4)n DNAs fold to form G-quadruplexes. We have found that in magnesium or lithium salt solutions, especially under slightly acidic conditions, d(C2G4)n oligonucleotides fold to form a distinctive higher order structure whose most striking feature is an “inverted” circular dichroism spectrum, which is distinguishable from the spectrum of the left handed DNA double-helix, Z-DNA. On the basis of CD spectroscopy, gel mobility as well as chemical protection analysis, we propose that this structure, which we call “iCD-DNA”, may be a left-handed Hoogsteen base-paired duplex, an unorthodox G-quadruplex/i-motif composite, or a non-canonical, “braided” DNA triplex. Given that iCD-DNA forms under slightly acidic solution conditions, we do not know at this point in time whether or not it forms within living cells.

Carbon-nanotube-doped Pd-Ni bimetallic three-dimensional electrode for electrocatalytic hydrodechlorination of 4-chlorophenol: Enhanced activity and stability

A novel composite bimetallic electrode, palladium-nickel/multi-walled carbon nanotubes/graphite felt (Pd-Ni/MWCNTs/GF), was synthesized for the electrocatalytic hydrodechlorination of 4-chlorophenol (4-CP). GF with a three-dimensional structure was used as the electrode substrate, and doped with MWCNTs, which can improve the GF conductivity and serve as a skeleton for metal loading. Ni and Pd were deposited on the electrode surface stepwise to obtain a well-aligned, highly active and stable Pd-Ni/MWCNTs/GF electrode. The Pd-Ni/MWCNTs/GF cathode showed a high reactivity for the electrocatalytic hydrodechlorination of 4-CP; up to 100% removal of 4-CP was achieved within 30 min, and followed pseudo-first-order kinetics with a rate constant of 0.162 min−1. Compared with other cathodes, the Pd-Ni/MWCNTs/GF electrode showed superior performance in 4-CP reduction. Excessive current will lower the reaction efficiency and current efficiency because of hydrogen evolution, and acidic solution conditions are more conducive to electrocatalytic reactions. Experiments confirmed that the Ni had a small amount of loss under acidic conditions but remained stable under neutral and alkaline conditions, whereas the loss of Pd for different pH values was constantly low. In cycle tests, the bimetallic electrode exhibits a better reactivity and stability than the single-metal Pd electrode in the long-term.

Double Current Maxima Formed under Linear Potential Sweep Conditions in Acetic Acid Solutions at pH < 2.5

The kinetics of hydrogen evolution on a copper electrode in acetic acid solutions is studied by the method of linear potential sweep. At рН < 2.5, double current peaks are observed with the height directly proportional to both √v (v is the potential sweep rate) and the total concentration of proton donors and acceptors. Upon the transition from the copper electrode to the platinum electrode, the overpotential of this process considerably decreases, but the shape of current peaks remains unchanged. The analysis of kinetics of acetic acid dissociation points to the high degree of lability of this system, which allows the relationship between the surface concentrations of its components to be expressed through the corresponding equilibrium constants. At certain potentials, significant changes in the surface pH are observed, which favor the appearance of an additional current peak in the cathodic chronovoltammogram. The Tafel plots normalized with respect to the surface concentration of hydrogen ions allow the following values of kinetic parameters to be obtained: i0 ~ 0.3 nA/cm2 and αc = 0.75. Chronovoltamograms simulated with the use of these values demonstrate double maximums which adequately agree with experimental data.

Investigation on stability and degradation kinetics of febrifugine

To investigate the stability and degradation kinetics of febrifugine. The results showed that within 24 hours, febrifugine content was decreased by only 1% in mobile phase solvent, but its content was decreased to be 90% of the initial content in the water, methanol, 50% methanol and 10% acetonitrile solution. When the pH value of the solution was between 3 and 7, the retention rate of febrifugine in 24 hours was over 98%, but its content was decreased by about 12% in alkaline solution (pH 9.0). The higher the temperature, the worse the stability of febrifugine. At 40-80 ℃, the content of febrifugine was decreased to be 60% of its initial content in 10 hours, but the content was decreased by only 5% in 10 h at 20 ℃.However, no matter 40 ℃or 60 ℃, febrifugine was mainly transformed into isofebrifugine, and the total content of febrifugine and isofebrifugine was equal to their initial total content in 10 hours, while incase of 80 ℃, the total content was decreased to be 83.33% in 10 h, which suggested that the structure of febrifugine was absolutely changed, not just isomerized to be isofebrigugine at high temperature. Light had a significant impact on the stability of febrifugine. Under bright light, the content of febrifugine was reduced by about 23% in 108 h, but it only decreased by about 10% in the natural light or darkness. In artificial gastric fluid (pH 1.4) and artificial intestinal fluid (pH 6.8), the content of febrifugine was decreased by less than 5% in 10 h. After storage at high temperature(60 ℃), high humidity [(75±1)%] and strong light (3 000 lx) conditions for 10 d, the content of solid febrifugine was decreased by 0.27%, 7.6% and 5.39%, respectively. The degradation of febrifugine basically complied with the first-order reaction kinetic process in the following conditions: in water, methanol, 50%methanol and 10% acetonitrile solvents, alkaline solution (pH>7), different light intensity and different temperatures (20,40 ℃). Therefore, no matter the isolation and purification of febrifugine or the production of the related preparations, it should be done fast in the acidic solution, low temperature and dark conditions, while the febrifugine solid should be kept in dry and dark conditions.

Effect of Aging Temperature on Corrosion Behavior of Sintered 17-4 PH Stainless Steel in Dilute Sulfuric Acid Solution

The general corrosion behavior of sintered 17-4 PH stainless steel processed under different processing conditions in dilute sulfuric acid solution at 25 °C was studied by open-circuit potential measurement and potentiodynamic polarization technique. The corrosion resistance was evaluated based on electrochemical parameters, such as polarization resistance, corrosion potential, corrosion current density as well as corrosion rate. The results showed that the precipitation-hardening treatment could significantly improve the corrosion resistance of the sintered 17-4 PH stainless steel in studied environment. As far as the influence of aging temperature on corrosion behavior of the sintered 17-4 PH stainless steel is concerned, polarization resistance and corrosion rate are reduced with increasing aging temperature from 480 up to 500 °C regardless of the temperature of solution treatment. It can be concluded that the highest corrosion resistance in 0.5 M H2SO4 solution exhibits 17-4 PH after solution treatment at 1040 °C followed by aging at 480 °C.

Degradation in the fatigue crack growth resistance of human dentin by lactic acid

The oral cavity frequently undergoes localized changes in chemistry and level of acidity, which threatens the integrity of the restorative material and supporting hard tissue. The focus of this study was to evaluate the changes in fatigue crack growth resistance of dentin and toughening mechanisms caused by lactic acid exposure. Compact tension specimens of human dentin were prepared from unrestored molars and subjected to Mode I opening mode cyclic loads. Fatigue crack growth was achieved in samples from mid- and outer-coronal dentin immersed in either a lactic acid solution or neutral conditions. An additional evaluation of the influence of sealing the lumens by dental adhesive was also conducted. A hybrid analysis combining experimental results and finite element modeling quantified the contribution of the toughening mechanisms for both environments. The fatigue crack growth responses showed that exposure to lactic acid caused a significant reduction (p≤0.05) of the stress intensity threshold for cyclic crack extension, and a significant increase (p≤0.05) in the incremental fatigue crack growth rate for both regions of coronal dentin. Sealing the lumens had negligible influence on the fatigue resistance. The hybrid analysis showed that the acidic solution was most detrimental to the extrinsic toughening mechanisms, and the magnitude of crack closure stresses operating in the crack wake. Exposing dentin to acidic environments contributes to the development of caries, but it also increases the chance of tooth fractures via fatigue-related failure and at lower mastication forces.

Biopolymer-stabilized conjugated linoleic acid (CLA) oil-in-water emulsions: Impact of electrostatic interactions on formation and stability of pectin-caseinate-coated lipid droplets

Conjugated linoleic acid (CLA) may be used as a nutraceutical, supplement or pharmaceutical due to its potential health benefits. In this study, CLA oil-in-water emulsions were fabricated using biopolymers as stabilizers: sodium caseinate as an emulsifier and pectin as a coating material. The effect of electrostatic interactions on the formation of these mixed biopolymer coatings, as well as on the stability of the resulting emulsions, was determined by systematically varying pH and ionic strength. Caseinate-coated CLA droplets were stable from pH 7 to 5 due to the strong electrostatic repulsion between them, but aggregated from pH 5 to 3 due to weakening of the electrostatic repulsion near the protein’s isoelectric point. Pectin addition greatly improved emulsion aggregation stability, particularly at levels sufficient to saturate the caseinate-coated oil droplet surfaces. Indeed, emulsions with pectin-to-caseinate ratios>1:1 were stable under acidic solution conditions (pH 5 to 3). Ionic strength had a pronounced impact on droplet aggregation, which was pH dependent. At pectin-to-caseinate ratios of 2:1, emulsions were stable against NaCl addition (0 to 200mM) at pH 3.5 and 4.0 due to strong electrostatic attraction between protein and polysaccharide molecules in the interfacial complexes, but they were unstable to NaCl addition at pH 3.0, 4.5, and 5.0 due to a weaker electrostatic attraction between the biopolymers. Mixed biopolymer-coated emulsions were stable against thermal processing (90°C, 20min). These results have important implications for the development of natural CLA delivery systems for utilization in foods and other products.

Mercury Removal from Flue Gas by Aqueous Precipitation

Mercury has drawn the public’s concern due to its bio-accumulative nature and its adverse impact on human health. At the same time, concern for greenhouse gas release has also grown, leading to a range of carbon capture strategies. Previous mercury removal studies, however, focused on either dry techniques or acidic solution conditions, and a major uncertainty is how high alkalinity (which is used in carbon mineralization capture methods) may result in a different mercury removal mechanism. To examine this question in detail, in this study, a lab-scale liquid-gas scrubbing system is manufactured and installed, based on the carbon mineralization by aqueous precipitation (CMAP) process for CO 2 sequestration. Sodium hydroxide solution is selected as a typical candidate to remove mercury. Based on the test results, the CMAP process, like most other capture methods, has relatively low elemental mercury removal efficiency, i.e. less than 20%, but it can remove oxidized mercury with more than 90% efficiency. This performance is very similar to that of the traditional flue gas desulfurization processes which are operated under acidic conditions, at pH from 4 to 6. Equilibrium calculations by Visual MINTEQ predict the speciation of mercury equilibrium in the liquid phase, and the result is in agreement with the experimental findings.

Self-Replication of Transthyretin Amyloid Aggregates from Native Tetramers in vitro

The conversion of proteins from their native conformations into amyloid aggregates with their beta -sheet enriched structures is associated with an increasing number of pathological conditions including neurodegenerative disease and systemic amyloidosis. Transthyretin (TTR) is a soluble human plasma protein that is linked to both senile systemic amyloidosis (SSA) and familial amyloid polyneuropathy (FAP) in humans. In vitro experiments have indicated that TTR amyloid aggregation requires the dissociation of the native TTR tetramer into a non-native monomer state. However, typical in-vitro experiments require the use of non-physiological, acidic solution conditions. This raises the question whether the mechanisms observed for TTR amyloid formation under harsh in-vitro conditions match those active in vivo.We have previously reported that lysozyme oligomers and curvilinear fibrils were capable of prion-like self-replication from native monomers near physiological solution conditions. We therefore investigated whether TTR amyloid oligomers and their curvilinear fibrils were capable of similar self-replication from their native tetrameric counterparts in vitro. We induced the formation of TTR oligomers and curvilinear fibrils during transient thermal denaturation in physiological saline. The morphologies and tinctorial characteristics of the resulting aggregates match those for TTR oligomers and curvilinear fibrils obtained by in vitro growth from acidic solutions. After mechanical separation of these aggregates, they were seeded into solutions of native TTR tetramers at physiological conditions (pH, temperature, ionic strength). Light scattering and thioflavin T responses indicated that TTR oligomers and their curvilinear fibrils, similar to our prior observations with lysozyme, induced conversion of the native TTR tetramers into amyloids. Our observations suggest that amyloid oligomers of different proteins have the capability for autocatalytic self-replication from their native counterparts, thereby resembling the behavior of prion proteins.

Organosulfate and nitrate formation and reactivity from epoxides derived from 2-methyl-3-buten-2-ol.

Recent work has suggested that 2-methyl-3-butene-2-ol (MBO)-derived epoxide intermediates are responsible for some of the molecular species commonly found in ambient secondary organic aerosol (SOA). Nuclear magnetic resonance techniques were used to study the reaction kinetics and products of two potential MBO-derived epoxides under acidic solution conditions in the presence of sulfate and nitrate nucleophiles. These epoxides were found to undergo reasonably fast acid-catalyzed reaction at typical SOA acidities and to produce a variety of organosulfate and nitrate species. This finding supports a previous supposition that 3-methylbutane-1,2,3-triol and at least some of the MBO-derived organosulfates previously detected on SOA are formed from the reactions of these epoxides. In general, the particular MBO-derived organosulfates and nitrates produced from MBO-derived epoxides and their respective stability toward hydrolysis were similar to those found for isoprene-derived epoxides; the nucleophilic reactions were observed to be quite regiospecific, and the tertiary addition product species were found to hydrolyze on atmospherically relevant time scales.

Dissolution of intact UO2 pellet in batch and rotary dissolver conditions

Comparative dissolution of intact un-irradiated UO2 pellet of PHWR fuel dimensions was performed in batch and dynamic rotary dissolver conditions in aqueous nitric acid solutions at elevated temperatures. The extent of dissolution was estimated by determining the uranium concentration of the resulting aqueous solution. It was observed that rate of dissolution was much faster in dynamic conditions as compared to static batch conditions.

Characterization of Quintinite Particles in Fluoride Removal from Aqueous Solutions

The aim of this study was to characterize quintinite in fluoride removal from aqueous solutions, using batch experiments. Experimental results showed that the maximum adsorption capacity of fluoride to quintinite was 7.71 mg/g. The adsorption of fluoride to quintinite was not changed at pH 5–9, but decreased considerably in highly acidic (pH 11) solution conditions. Kinetic model analysis showed that among the three models (pseudo-first-order, pseudo-second-order, and Elovich), the pseudo-second-order model was the most suitable for describing the kinetic data. From the nonlinear regression analysis, the pseudo-second-order parameter values were determined to be qe=0.18 mg/g and k2=28.80 g/mg/hr. Equilibrium isotherm model analysis demonstrated that among the three models (Langmuir, Freundlich, and Redlich-Peterson), both the Freundlich and Redlich-Peterson models were suitable for describing the equilibrium data. The model analysis superimposed the Redlich-Peterson model fit on the Freundlich fit. The Freundlich model parameter values were determined from the nonlinear regression to be KF=0.20 L/g and 1/n=0.51. This study demonstrated that quintinite could be used as an adsorbent for the removal of fluoride from aqueous solutions.

PH-dependent assembly of two polyoxometalate host-guest structural isomers based on Keggin polyoxoanion templates.

Two interesting polyoxometalate-templated host–guest compounds 1 and 2 with the same chemical formulae of [Ag6(tpt)4(SiW12O40)Cl2]·2H2O have been obtained by hydrothermal reactions of monolacunary Keggin-type K8[α-SiW11O39]·13H2O with CH3COOAg and tpt ligands under different pH conditions. Compounds 1 and 2 are structural isomers. In 1, the novel chair-like tetranuclear [Ag4(μ3-Cl)2]2+ clusters and additional Ag+ ions are linked by tpt ligands to form a 2D double-layered structure featured with two different types of voids in the layer, and the double-layers are stacked into a 3D host supramolecular framework with 1D channels filled with Keggin [SiW12O40]4− polyoxoanions and water molecules as guests. In 2, Ag+ ions are linked by tpt ligands into a 3D framework of 10(3)-ths topology. The most outstanding structural feature of 2 is that upon six-fold interpenetration among 10(3)-ths nets there are still nanosized channels along the a axis to accommodate Keggin polyoxoanions and water molecules as guests. Compound 2 represents the first example of six-fold interpenetrating cationic networks templated with polyoxoanions. In this work, the obvious differences in structure between 1 and 2 show that hydrochloric acid solution conditions, which offer not only H+ to regulate the pH value but also Cl− to coordinate with Ag+ ions, play an important role in the formation of host frameworks templated by Keggin-type polyoxoanions. The photoluminescence properties of 1 and 2 have been investigated in the solid state.

Degradation of sulfadiazine antibiotics by water falling film dielectric barrier discharge

Abstract A new water falling film dielectric barrier discharge was applied to the degradation of sulfadiazine in the aqueous solution. The various parameters that affect the degradation of sulfadiazine and the proposed evolutionary process were investigated. The results indicated that the inner concentrations of 10 mg/L sulfadiazine can be all removed within 30 min. The optimum pH value was 9.10 and both strong acidic and alkaline solution conditions were not suitable for the degradation. The degradation of sulfadiazine can be enhanced by the addition of hydrogen radical scavengers, but be inhibited by adding hydroxyl radical scavengers. The water falling film dielectric barrier discharge was rather ineffective in mineralization, because of the intermediates were recalcitrant to be degraded. The existence of Fe2+ and CCl4 in the liquid phase can promote the degradation and mineralization of sulfadiazine. It was found that the degradation of SDZ was enhanced by CCl4 was mainly because of the increase of OH due to the reaction of CCl4 with H that reduce the chances of their recombination with OH. Based on the 8 intermediate products identified by LC–MS, the proposed evolution of the degradation process was investigated.

Electrical field influence on molecular mass and electrical conductivity of polyaniline

We have described the primary studies on the conductivity and molecular weight of polyaniline in an electric field as it is used in a field effect experimental configuration. We report further studies on doped in-situ deposited polyaniline. First we have chemically synthesized polyaniline by ammonium peroxodisulfate in an acidic solution, with aqueous, organic and emulsion conditions at different times. Next, we measured mass and conductivity and obtained the best time of polymerizations. Then, we repeated these reactions under different electrical fields in constant time and measured mass and conductivity. The polyaniline is characterized by gel permeation chromatography (GPC), UV-visible spectroscopy and electrical conductivity. Polyanilines with high molecular weight are synthesized under electric field M w = (5.2–6.8) × 105, with M w/M n = 2.0–2.5. The UV-visible spectra of polyanilines oxidized by ammonium peroxodisulfate and protonated with dodecylbenzenesulfonic acid (PANi-DBSA), in N-methylpyrolidone (NMP) show a smeared polaron peak shifted into the visible. Electrical conductivity of polyaniline has been studied by four-probe method. The conductivity of the films of emeraldine protonated by DBSA cast from NMP is higher than 500 S/cm under (10 kV/cm2 of potential) electric field and shows an enhanced resistance to ageing. Next, we carried chemical polymerization at the best electric field at different times. Finally, the best time and amount of electric field were determined. Polymers synthesized under an electric field probably have better physical properties regarding the existence of less branching and high electric conductivity.

Superlubricity of Si3N4 sliding against SiO2 under linear contact conditions in phosphoric acid solutions

Superlubricity phenomena, which were previously achieved merely under point contact conditions, have been observed under the linear contact condition between the silicon nitride (Si3N4) cylinder and the silica (SiO2) ring in phosphoric acid solutions. After a total 800 s running-in period and low friction stage, the friction coefficient finally declined to about 0.003. The ultralow friction coefficient could last for more than 5 h. In these experiments, a torque sensor of high accuracy was used to make sure that the measuring result was credible. The differences between superlubricity experiments in our group and others were discussed. Unlike the situations in other superlubricity experiments about silicon nitride, where the specimens were immersed in water totally, the phosphoric acid solution with volume of only 150 μL was used in these tests. It was proved that free water in the solution had a great relationship to the necessary time of acquiring superlubricity and phosphoric acid played a unique role in the superlubricity, which could not be explained properly by the hydrodynamic lubrication model proposed by others. Finally, based on the results of the friction tests and the analysis with scanning electron microscope (SEM), a modified hydrogen-bonded network and hydrated shearing layer model was proposed to explain the mechanism of superlubricity.

Enhancement in solar hydrogen generation efficiency using a GaN-based nanorod structure

A significant enhancement in solar hydrogen generation has been achieved using a GaN-based nanorod array structure as a photoelectrode in comparison with a planar one fabricated from the same parent wafer. The nanorod array structure was formed by dry etching using a self-organised nickel nanomask. Under identical illumination conditions in hydrochloric acid solution, the photoelectrode with the nanorod array structure has demonstrated a photocurrent enhancement with a factor of 6 and an enhancement in the rate of hydrogen generation with a factor of 7. The enhancement in solar hydrogen generation is attributed to a massive improvement in light absorption area, reduced travelling distance for the migration of the photogenerated carriers to the semiconductor/electrolyte interface, and surface band bending.

Automated Circular Dichroism Spectroscopy for Medium-Throughput Analysis of Protein Conformation

Circular dichroism (CD) spectroscopy is a powerful method for monitoring conformational changes of biomolecules. For peptides and proteins, it is highly sensitive to changes in secondary structure, which may be caused by alterations in amino acid composition or solution conditions (e.g., temperature, pH, salts, detergents, denaturants, and excipients), post-translational modifications, self-association, or ligand binding. The assets of CD spectroscopy are that the signal is directly linked to structure, the analyte is measured without labels and in solution, the technique requires low sample amounts, and data analysis is straightforward. However, CD spectroscopy has remained a low-throughput method because it imposes high requirements on the optical quality of sample cells and thus cannot be performed in microplate-reader format. Here, we introduce an automated CD spectrometer equipped with a low-birefringence flow-through cell that is coupled to a three-axis robotic liquid-handling system. This enables unattended CD measurements on up to 384 samples, including sample transfer from 96-well plates into the flow-through cell, data acquisition, and cell cleaning. We show that the accuracy, precision, and reproducibility afforded by the new instrument are excellent and exemplify how the advantages offered by automated CD spectroscopy can be exploited to quantify protein stability by titration with chemical denaturants.

Automated Circular Dichroism Spectroscopy for Medium-Throughput Quantification of Protein Conformation

Circular dichroism (CD) spectroscopy is a powerful method for monitoring conformational changes of biomolecules. For proteins, it is highly sensitive to changes in secondary structure, which, in turn, are influenced by amino acid composition, posttranslational modifications, solution conditions (e.g., temperature, pH, salts, detergents, denaturants, excipients, etc.), and ligand binding. The CD signal is directly linked to protein structure, the analyte is in solution and label-free, the technique requires low sample amounts, and data analysis is straightforward. However, CD spectroscopy has remained a low-throughput method because it imposes high requirements on the optical quality of sample cells and thus cannot be performed in microplate-reader format. Here, we introduce an automated CD spectrometer that is equipped with a flow-through cell and coupled to a 3-axis robotic liquid handler. This enables completely unattended CD measurements on up to 384 samples, including sample transfer from 96-well plates into the flow-through cell, data acquisition, and cell cleaning. We demonstrate that the accuracy, precision, and data quality of the automated CD spectrometer are as good as those of a conventional, manually operated instrument and exemplify how the advantages offered by automated CD spectroscopy can be exploited in protein-unfolding experiments using chemical denaturants.