Lower Free Energy Research Articles

Atomization‐Induced High Intrinsic Activity of a Biocompatible MgAl‐LDH Supported Ru Single‐Atom Nanozyme for Efficient Radicals Scavenging

AbstractDeveloping efficient nanozymes to mimic natural enzymes for scavenging reactive radicals remains a significant challenge owing to the insufficient activity of conventional nanozymes. Herein, we report a novel Ru single‐atom nanozyme (SAE), featuring atomically dispersed Ru atoms on a biocompatible MgAl‐layered double hydroxide (Ru1/LDH). The prepared Ru1/LDH SAE shows high intrinsic peroxidase (POD)‐like catalytic activity, which outperforms the Ru nanoclusters (NCs) nanozyme by a factor of 20 and surpasses most SAEs. The density functional theory calculations reveal that the high intrinsic POD‐like activity of Ru1/LDH can be attributed to a heterolytic path of H2O2 dissociation on the single Ru sites, which requires lower free energy (0.43 eV) compared to the homolytic path dissociation on Ru NC (0.63 eV). In addition, the Ru1/LDH SAE shows excellent multiple free radicals scavenging ability, including superoxide anion radical (O2⋅−), hydroxyl radical (⋅OH), nitric oxide radical (NO⋅) and 2, 2‐diphenyl‐1‐picrylhydrazyl radical (DPPH⋅). Given the advantages of Ru1/LDH with high enzymatic activities, biosafety, and ease to scale up, it paves the way for exploring SAEs in the practical biological immunity system.

Low‐Coordinated Single‐Atom Catalysts Modulated by Metal Ionic Liquids for Efficient CO2 Electroreduction

AbstractModulating the coordination environment of single‐atom catalysts (SACs) is an attractive approach for maximizing the catalytic activity of single‐atom centers. Currently, the synthesis of low‐coordinated SACs is mainly confined to increasing the pyrolysis temperature (≥900 °C) to control C─N volatile fragments. Herein, a novel and universal strategy for the low‐coordinated SACs modulation is presented using transition metal (e.g., Ni, Co, Zn) ionic liquid precursors under relatively mild temperature of 600 °C, which regulates the L‐shell electronic structure and decreases nearly 50% electrophilic reactivity by ionization of 4‐position N atom, thereby orienting synthesis of the SACs with metal‐N3 centers. The Ni‐N3 SACs exhibit exceptional CO2 electroreduction performance of 99.7% CO Faraday efficiency with an ultra‐high CO partial current density of 467.55 mA·cm−2 as well as a CO production rate up to 10417.51 µmol·h−1·cm−2 in flow cell. The superior catalytic activity achieves over twofold increase compared with the Ni‐N4 SACs prepared by non‐metal ionic liquid precursors due to the lower free energy of the key intermediate *COOH and the stronger adsorption energy.

Understanding the desalination mechanism of a two-dimensional graphene-like membrane using data-driven design

Global freshwater resources are not inexhaustible, and the deterioration in water quality and the shortage of drinkable water are serious challenges for human beings. To date, the development of two-dimensional (2D) nanomaterials provides an essential guarantee for seawater desalination technology. However, 2D nanomaterials are still lacking in desalination efficiency. Herein, we used a data-driven design to analyze the desalination mechanism of a 2D graphene-like membrane. A 2D graphene-like desalination membrane (Desa-C) with a multiporous structure has been screened first through machine learning (ML). Then, the desalination efficiency and other comprehensive properties of mechanics, adsorption, diffusion, and electron are analyzed based on molecular dynamics simulations and first-principles density functional theory. The results indicate that the low free energy barrier of water molecules, abundant salt ions adsorption sites, and periodically distributed 0.53 nm nanopores endow Desa-C with ideal permeability and selectivity; In particular, the salt ions rejection efficiency could constantly be ensured at 100 % within 125 MPa pressure, and this value holds 97 % even if the pressure increases to 145 MPa. Moreover, the Desa-C membrane featuring metallicity could achieve a self-cleaning effect with reverse voltage conditions, which prolongs the lifetime of the desalination membrane. These findings have potential application values in desalination technology and provide a positive theoretical contribution to the innovation of desalination membranes.

Atmospheric oxidation of (E)-β-farnesene initiated by hydroxyl radical: New formation mechanism of HOMs

(E)-β-farnesene is one of the acyclic sesquiterpenes, whose atmospheric oxidation has been determined as a significant contributor to SOA formation due to the formed highly oxidized multifunctional molecules (HOMs). In this work, the atmospheric oxidation of (E)-β-farnesene initiated by OH radical was comprehensively investigated using quantum chemical calculation. For the initial reactions, OH addition reactions are dominant over H atom abstraction reactions because of lower free energy barriers. The first-generation products involving acetone, (E)− 4-methyl-8-methylenedeca-4,9-dienal, 4-methylenehex-5-enal (P3), 6-methylhept-5-en-2-one (P4), formaldehyde, (E)− 7,11-dimethyldodeca-1,6,10-trien-3-one and (E)− 6,10-dimethyl-2-methyleneundeca-5,9-dienal are produced from the subsequent reactions of the OH-(E)-β-farnesene adducts with NO and HO2. The second-generation products, such as 4-oxopentanal, 4-methylenehex-5-enal, (E)− 4-methyl-8-oxodeca-4,9-dienal, formaldehyde and (E)− 4-methyl-8-methylenenon-4-enedial, are also determined from the successive oxidations of P2 and P3 under high and low NOx conditions. For the formation of HOMs, the H atom shift is the bottleneck for the autooxidation mechanism while our newly proposed mechanism, i.e. the multiple H abstractions by OH radical and reactions with O2 and HO2, is a more thermodynamically favorable formation pathway of the HOMs. Due to the intricate oxidation mechanisms and products, it is difficult to comprise the oxidation mechanisms of sesquiterpenes (including (E)-β-farnesene) in the atmospheric chemical models to thoroughly evaluate their contributions to SOA formation. We expect that this comprehensive oxidation mechanism of (E)-β-farnesene from a theoretical perspective would be conducive to clarifying the atmospheric fate of (E)-β-farnesene and even sesquiterpenes.

A strong soy protein-based adhesive with excellent water retention

Traditional synthetic resin adhesives release harmful substances and can be replaced with more sustainable, eco-friendly, and healthier soy protein-based adhesives. However, the low water retention of these adhesives leads to an unstable bonding performance, which hampers their widespread application. Enhancing the water retention of the adhesive while maintaining its bonding and coating performance poses a significant challenge. In this work, a “bond-stitch” topology was designed and constructed by free radical copolymerization between acrylic acid and zwitterionic sulfobetaine methacrylate. This topology was combined with lithium chloride and a self-made epoxy crosslinking agent to construct a water-retaining and enhancing structure in soy protein adhesive systems, achieving simultaneous improvement in water retention, bonding strength, and coating performance. The adhesive exhibited an average shear strength of 1.77 MPa for wood and 1.82 MPa for stainless steel. Additionally, the coating performance was significantly improved, allowing uniform coating on wet poplar veneers (up to 120% moisture content) and low surface free energy substrates. Importantly, the adhesive allowing over 75% of the mass to remain after 24 h of storage in an open environment. This strong water-retaining and enhancing structure construction strategy offers a promising way to develop bio-based adhesives and hydrogels, promoting their widely application.

Enhancement by pyrazolones of colistin efficacy against mcr-1-expressing E. coli: an in silico and in vitro investigation.

Owing to the emergence of antibiotic resistance, the polymyxin colistin has been recently revived to treat acute, multidrug-resistant Gram-negative bacterial infections. Positively charged colistin binds to negatively charged lipids and damages the outer membrane of Gram-negative bacteria. However, the MCR-1 protein, encoded by the mobile colistin resistance (mcr) gene, is involved in bacterial colistin resistance by catalysing phosphoethanolamine (PEA) transfer onto lipid A, neutralising its negative charge, and thereby reducing its interaction with colistin. Our preliminary results showed that treatment with a reference pyrazolone compound significantly reduced colistin minimal inhibitory concentrations in Escherichia coli expressing mcr-1 mediated colistin resistance (Hanpaibool et al. in ACS Omega, 2023). A docking-MD combination was used in an ensemble-based docking approach to identify further pyrazolone compounds as candidate MCR-1 inhibitors. Docking simulations revealed that 13/28 of the pyrazolone compounds tested are predicted to have lower binding free energies than the reference compound. Four of these were chosen for in vitro testing, with the results demonstrating that all the compounds tested could lower colistin MICs in an E. coli strain carrying the mcr-1 gene. Docking of pyrazolones into the MCR-1 active site reveals residues that are implicated in ligand-protein interactions, particularly E246, T285, H395, H466, and H478, which are located in the MCR-1 active site and which participate in interactions with MCR-1 in ≥ 8/10 of the lowest energy complexes. This study establishes pyrazolone-induced colistin susceptibility in E. coli carrying the mcr-1 gene, providing a method for the development of novel treatments against colistin-resistant bacteria.

Self-assembly behavior of fluorinated amphiphilic copolymer in aqueous solution and its properties in the solid film state

Owing to the high electronegativity of fluorine atoms, incorporation of fluorinated moieties in a polymer can confer unique properties such as low surface energy and self-assembling ability in solution. In this study, a series of amphiphilic copolymers were prepared by radical polymerization using three monomers: 2-((2,3,3,3- tetrafluoro- 2- (1,1,2,3,3,3- hexafluoro −2- (perfluoroethoxy) propoxy) propanoyl)oxy) ethyl acrylate (FHEMA), maleic anhydride (MAH), and acrylic acid (AA). The PFHEMA block provided low surface tension, and the MAH and AA blocks (containing carboxyl) increased solubility. The properties of the polymer (PFHEMA-co-PMAH-co-PAA) can be regulated by changing the PFHEMA length. Characterization by Fourier-transform infrared spectroscopy (FTIR) and gel permeation chromatography (GPC) indicated successful polymerization (FHA01 and FHA02). When the FHEMA/MAH/AA molar ratio was 3:8:9 (FAA02), the surface tension of the hydrolyzed amphiphilic copolymers FAA02 at 300 mg·mL−1 in aqueous solution was 15.76 mN·m−1, the lowest value due to a higher fluorine content than FAA01. FAA02 self-assembled in water to form pH-sensitive micelles with diameters greater than 140 nm. For the FAA02 film, both the F/C ratio and arrangement of fluorine-containing side chains decreased with increasing film depth. Thus, this type of fluorine-containing low surface free energy material can be developed into a high performance coating material.

In silico identification of cyclosporin derivatives as potential inhibitors for RdRp of rotavirus by molecular docking and molecular dynamic studies

Rotavirus is one of the most common gastrointestinal viral diseases. Till date, there are only two vaccines available in the markets, which are specifically to be administered to young babies. In this study, VP1 RdRp is selected as potential target to carry out inhibitory activities. Cyclosporin A (Cys A) derivatives were designed via FBDD, pharmacokinetics, molecular docking, molecular dynamics (MD) simulation and molecular mechanics generalized born surface area was applied on these compounds. The results from these investigations were analyzed and it was found that the considered derivatives in this study were nontoxic and docking results revealed that the derivatives made some important bonds inside the active site of the receptors within a catalytic triad (Serine–Histidine–Aspartate). After analyzing the mean values of root mean square density (RMSD), root mean square fluctuation (RMSF), radius of gyration (RoG) and solvent accessible surface area (SASA) at 100 ns MD simulation of the selected compounds, it was found that compound 1 exhibits RMSD of 0.74 ± 0.10 Å, RMSF of 0.85 ± 0.15 Å, RoG of 16.45 ± 0.40 Å, SASA of 66.55 ± 0.35 nm2 and ΔGbind of −32.76 ± 0.02 kcal/mol. Therefore, the study revealed that amongst the designed and reported compounds, compound 1 was more stable within the active region of the RdRp and also this compound possesses lower binding free energy as compared to other selected compounds and Cys A as well. Communicated by Ramaswamy H. Sarma

Optimizing piezoelectric performance of complex perovskite through increasing diversity of B-site cations

Piezoelectric materials with unique properties are hugely appealing for high sensitivity sensors and ultrasonic transducers. Here, Pb(Ni1/3Nb2/3)O3-Pb(Yb1/2Nb1/2)O3-PbTiO3 piezoelectric system was designed and investigated in detail. The optimized piezoelectric response (d33 = 826 pC/N, d33* = 1017 pm/V) together with good temperature stability and negligible performance degradation up to 106 cycles was achieved in 0.58PNN-0.10PYN-0.32PT composition. Rayleigh analysis revealed the amplification of intrinsic and extrinsic contributions in MPB region due to more spontaneous polarization orientations and an increase in chemical disorder of system after the introduction of PYN. The origin of high piezoelectric response was investigated through the combination of scanning electron microscopy and piezoresponse force microscopy. The diversity of B-site cations was shown to enhance local structural heterogeneity, weaken long-range ferroelectric ordered domains, and lower free energy barrier via the formation of nanodomain configuration that promoted polarization rotation, thereby improving piezoelectric properties of the material. Therefore, it is suggested that increasing the diversity of B-site cations is an effective strategy for achieving outstanding piezoelectric performance.

Synthesis, Reaction Mechanism, DNA Cleavage and Antitumor Properties of Naphthoquinone‐Based Enediynes

AbstractUnder physiological conditions, natural enediynes antibiotics generate free radicals through a cycloaromatization reaction. The resulting highly reactive radicals can abstract hydrogen atoms on the phosphate backbone of DNA, leading to high cytotoxicity. On the other hand, anthraquinone‐fused compounds such as daunomycin and adriamycin can insert into DNA through planar aromatic ring structures, resulting in strong DNA damage. To this end, a series of naphthoquinone‐based enediynes were designed and synthesized by adjusting the type of terminal alkyne to modulate their water solubility and radical generation property. Theoretical calculations suggest that the naphthoquinone‐based enediyne generates radicals through a rearrangement (1,3‐proton transfer) followed by Myers‐Saito cyclization processes, with a much lower activation free energy barrier than Bergman cyclization (24.2 kcal/mol vs 39.8 kcal/mol). Differential scanning calorimetry and electron paramagnetic resonance results showed that naphthoquinone‐based enediynes undergo cycloaromatization at relatively low temperatures, resulting in the generation of highly reactive radical species. The enediynes with optimal water solubility displayed strong DNA cleavage ability at micromolar level and high cytotoxicity towards Hela cells, providing a half‐inhibitory concentration of 8.19 μM.

Improvement of the solubility and anticancer activity of 6,8-dibromochrysin by encapsulation into β-cyclodextrin and its derivatives

Background6,8-dibromochrysin (BrCN), a halogenated chrysin, possesses a profound bioactivity against dengue and Zika viruses and various types of cancer. However, the pharmaceutical applications of BrCN are limited by its poor aqueous solubility. MethodsMolecular encapsulation of BrCN with beta-cyclodextrin (βCD) and its derivatives—2,6-di-O-methyl-beta-cyclodextrin (DMβCD) and 2-hydroxypropyl-beta-cyclodextrin (HPβCD)—was performed to enhance its water solubility, and the resulting inclusion complexes were characterized theoretically and experimentally. Significant findingsMolecular docking showed that BrCN adopted two possible orientations, namely chromone ring insertion (C-form) and phenyl ring insertion (P-form) inside the lipophilic cavities of βCD, DMβCD, and HPβCD. Molecular dynamics (MD) simulations and the molecular mechanics with generalized Born and surface area solvation (MM-GBSA) method suggested that βCD derivatives show better BrCN-encapsulating potential as evidenced by their lower binding free energy values. Phase solubility study indicated a 1:1 stoichiometry between BrCN and the hosts (AL type). The resulting inclusion complexes were characterized using scanning electron microscopy and differential scanning calorimetry. We noted that encapsulation with βCD, DMβCD, and HPβCD resulted in an approximately 27-fold, 92-fold and 104-fold increase in the aqueous solubility of BrCN, respectively and enhanced its anticancer activity against A549 and H1975 cancer cells.

Exploring Regio- and Stereoselectivity in [3+2] Cycloaddition: Molecular Electron Density Theory Approach for Novel Spirooxindole-Based Benzimidazole with Pyridine Spacer

A new ethylene derivative was synthesized as a precursor for the [3+2] cycloaddition (32CA) reaction to access a novel spirooxindole embodied with benzimidazole with a pyridine spacer. The chalcone derivatives 3a–j is obtained with condensation of the acetyl derivative with aryl aldehydes. The one-pot multi-component reaction of the ethylene derivative, 5-Cl-isatin, and octahydroindole-2-carboxylic acid enables the construction of a highly functionalized quaternary center spirooxindole scaffold in a high chemical yield. A study using the Molecular Electron Density Theory (MEDT) explains the complete regio- and stereoselectivity of the reaction, resulting in the exclusive formation of the ortho/endo-cycloadduct under kinetic control. The low activation Gibbs free energy is the result of the supernucleophilic character of the in situ-generated azomethine ylide and the strong electrophilic character of the ethylene derivatives.

Synthesis and Evaluation of Some Sulfonamide-Substituted of 1,3,5-Triphenyl Pyrazoline Derivatives as Tyrosinase Enzyme Inhibitors

Pyrazoline is well-known as heterocycles compound that can exhibit many biological effects. In this work, we synthesized a series of sulfonamide-substituted 1,3,5-triphenyl pyrazoline compounds as a promising tyrosinase inhibitor agent. These compounds prepared by multicomponent reaction of corresponding aldehyde, ketone, and hydrazine using seal-vessel reactor. Pyrazolines compound were tested for their tyrosinase inhibitor activity through in vitro assay. The test result found that compound 4c, 4d, and 4e possessed better tyrosinase inhibitory activity compared to the reference drug, kojic acid. Compound 4c exhibited the strongest tyrosinase inhibitory effect with an IC50 value of 30.14 µM. The results suggested that hydroxyl and methoxy substituent at para position are preferable. Furthermore, molecular docking studies result match the pattern of in vitro assay where the compound will provide a stronger binding interaction and lower binding free energies.

Genomic Characteristics of the New HIV-1 CRF07_BC K28E32 Variant.

Accompanied with the appearance and prevalence of the new K28E32 variant among men who have sex with men, HIV-1 circulating recombinant form 07_BC (CRF07_BC) was becoming the most predominant subtype circulating in China. The K28E32 variant with five specific mutations in reverse transcriptase coding region appears to have significantly higher in vitro HIV-1 replication ability than the wild-type strain. In this study, we characterized the special mutations/substitutions in the K28E32 variant at the genomic level. Ten specific mutations that rarely appeared in other six main HIV-1 subtypes/CRFs (A-D, CRF01_AE, and CRF02_AG) were identified in the coding genes/regions of the K28E32 variant, including S77L and a novel seven-amino acid detection (32DKELYPL38) (p6Δ7) in p6, I135L in integrase, T189S in Vif, H/Y15L/F in Vpr, I264V/A and LV/LI328-329VG in gp41, and H82C and S97P in Rev. The special locations of the novel p6Δ7, and gp41 mutations I264V/A and LV/LI328-329VG in crucial protein functional domains suggest that these mutations might be functionally important to the K28E32 variant. Furthermore, eight specific substitutions were identified in Rev responsive element (RRE) of the K28E32 variant, and were revealed to increase the stability of RRE structure with a lower minimum free energy. Whether these mutations/substitutions contribute to improved transmissibility of the CRF07_BC K28E32 variant needs to be further confirmed.

Fabrication of glycerol functionalized silica nanoparticles via pickering emulsion for durable, superhydrophobic fabrics

Stable oil-in-water Pickering emulsions were successfully developed via the simplest approach: amine-terminated polydimethylsiloxane with very high viscosity and low surface free energy was encapsulated by glycerol functionalized silica NPs using a rotor-stator device. The factors affecting the Pickering emulsions performance were investigated. At a certain process (silica NPs concentration: 3 wt%, Oil/water ratio: 1:20, emulsifying speed: 15,000 rpm, emulsifying time: 5 min), it gave smaller and more stable emulsions. Then the Pickering emulsion was applied on plain weave cotton and polyester fabrics through conventional pad-dry-cure method. The coated substrates showed superhydrophobicity and maintained good water-repellency even after 30 standard laundering cycles, which are attributed to the synergistic effect of low surface free energy composite and hierarchical roughness. By contrast, fabrics coated with classical emulsifiers stabilizing Pickering emulsion adopting the same recipes were quickly wetted by liquid droplets.

A label-free colorimetric aptasensor based on an engineered chimeric aptamer and Au@FeP nanocomposites for the detection of kanamycin

The abuse of antibiotics such as kanamycin (KAN) has caused severe environmental pollution and health threat, attracting considerable attention for the monitoring of antibiotic residues. Here, a novel bivalent anti-KAN chimeric aptamer (Chi-Apt) was rationally designed through an efficient post-screening strategy that connected two homogenous anti-KAN aptamers with a poly-T linker. The engineered Chi-Apt achieved 34-fold enhanced binding affinity for KAN that was revealed by biolayer interferometry (BLI) measurement compared with the original aptamer. Meanwhile, molecular docking simulation and isothermal titration calorimetry (ITC) assay demonstrated that the engineered Chi-Apt has lower binding free energies and stronger non-covalent binding forces for KAN than the original aptamer. The engineered Chi-Apt was further evaluated by developing a label-free colorimetric aptasensor based on [email protected] nanocomposites for KAN detection. The aptasensor exhibited a lower limit of detection (LOD) than that achieved by the colorimetric aptasensor based on the original aptamer, with a LOD of 0.98 nM and a dynamic response range of 2.4–150 nM. Moreover, good recoveries of 103.3–110.1% in tap water and 107.6–112.7% in milk samples were obtained by the proposed colorimetric aptasensor. The recoveries detected by the proposed colorimetric aptasensor in real samples were consistent with that obtained by HPLC analysis, suggesting the good practicality and accuracy of the colorimetric aptasensor. This study may provide an efficient strategy for engineering high affinity aptamers to construct sensitive biosensing platforms against other targets.

Molecular Docking Study of Ferulic Acid Analog Compounds as Lung Antifibrotic at TGF-β1 Receptors

Pulmonary fibrosis is one of the conditions that occur in Post-COVID Syndrome patients. Development of specific treatment as an agent to treat pulmonary fibrosis is still ongoing. Ferulic acid is a compound that has a potential lung antifibrotic agent through inhibition of TGF-β1-mediated signaling. Molecular docking studies using the AutoDock Tools program version 1.5.7 were conducted on ferulic acid and its analogs to determine the activity of compounds as antifibrotic. Furthermore, the interaction of the compound with the receptor was visualized using the Discovery Studio 2021 program. The results showed that ferulic acid and its analogs have lower free energy than pirphenidone which is a standard lung antifibrotic drug. 4- benzoyloxy-3-methoxycinnamic acid is the compound that has the greatest activity. The group that contributes to the ligand-receptor interaction is the aromatic group with π interaction. Keywords: Molecular docking, AutoDock, antifibrotic, TGF-β1, ferulic acid

NiB monolayer: A topological metal with high NORR electrocatalytic perfomance

Electrochemical reduction reaction of NO (NORR) offers a promising strategy for effective removal of NO coupled with synthesis of NH3. In this study, the newly reported NiB monolayer with topological metal properties is proved to have excellent catalytic activity and high selectivity toward NO-to-NH3 conversion by using density functional theory (DFT) and the constant potential method (CPM). Calculations reveal that the peculiar building blocks of Ni-B triangles and tetragons can facilitate NO adsorption and activation via an electron donation-back-donation interaction mechanism. The predicted free energy profiles of the four considered NORR pathways toward synthesis of NH3 are generally going downhill under the applied potential of 0 V, and an optimal reduction pathway was screened out with a maximum kinetic barrier of 0.16 eV during the hydrogenation process. The low adsorption free energy of H atom together with the existing relatively higher energy barriers for the formation of N2O/N2 guarantee the hydrogen evolution reaction (HER) and the partial reduction channel are less competitive than NH3 production. These findings enrich the potential application of two dimensional topological materials in electrocatalysis.

Phase, chemical constitution and effective depth of oxide overgrowth and its influence on SmPrCo5 magnetic properties

Ternary Sm-Pr-Co magnets with CaCu5-type hexagonal structure exhibit superior magnetic properties and investigating their oxidation behavior is crucial for assessing their stability under the service conditions. In this regard, here thermal oxidation of sintered SmPrCo5 magnet is carried out by varying the oxidation temperature from 373 to 973 K for 5 hr under atmospheric pressure. Owing to their lower negative Gibbs free energy, rare earth oxides of Sm2O3 and Pr2O3 are found to initially form, making cobalt free from SmPrCo5 lattice. Cobalt is then observed to migrate to the oxide surface, because of the difference in chemical potential between the surface and inner layers, leading to Co3O4 formation. Moreover, here a comprehensive discussion on the occurrences of different oxide phases and their depth distributions in the oxide layer are being made. Finally, a correlation between these oxide phases and the alterations in their magnetic properties is discussed.

Geological control on carbon isotope equilibrium and kinetic fractionation of CH4-CO2-HCO3− in microbial coalbed and shale gas systems

Microbial coalbed gas (CBG) and shale gas (SG), predominately composed of methane (CH4) and carbon dioxide (CO2), are important economic resources and potent greenhouse gases. Although isotopic equilibrium of CH4 and CO2 has been observed in microbial CBG and SG basins, it is difficult to judge under what geological conditions equilibrium is achieved. Moreover, the effects of CO2 dissolution on the isotopic fractionation process need to be considered. We use data from eight microbial CBG and SG basins to discuss the geological conditions in which equilibrium and kinetic isotopic fractionation in CH4-CO2-HCO3− system is achieved. Based on isotopic equilibrium temperatures calculated using computer codes developed in MatLab software, we show that, in deep and closed reservoirs, the CH4-CO2 and CH4-HCO3− are close to carbon isotope equilibrium. In contrast, in shallow and open reservoirs, they are in disequilibrium. The CO2-HCO3− is in disequilibrium in most reservoirs. We propose that both low free energy gradients and long coexisting time of CH4 and CO2/HCO3− are necessary to attain isotopic equilibrium. However, it is difficult to accurately estimate the timescale for attaining isotope equilibrium among them. In general, a closed and deep CBG/SG reservoir is likely to be geologically and geochemically stable over long timescales, favoring isotopic equilibrium of CH4-CO2 and CH4-HCO3−. However, a shallow and open reservoir is unfavourable for their isotopic equilibrium due to shorter timescales for the coexistence of CH4-CO2-HCO3−. Using data from systems close to equilibrium, we estimated the percentage of CO2 in total CH4 and CO2 in CBG reservoirs in various basins to be from 27% to 50%, where methanogenesis is mainly by CO2 reduction. This is significantly higher than the CO2 content (1% to 15%) in gaseous CH4 and CO2 in these basins but is consistent with those (36% to 48%) from culture experiments for coal conversion by methanogenesis. Further study shows that 53–99% of the CO2 formed during CBG generation has dissolved into groundwaters to form dissolved inorganic carbon (DIC) in CBG reservoirs. We propose that CO2 dissolution likely has significantly affect the abundance and isotopic compositions of gaseous CO2 in subsurface.