Search

Objective The Corona Virus Disease-19 (COVID-19) pandemic has evolved into the largest public health event in the world. Earlier COVID-19 studies have reported that the pandemic caused widespread impacts on mental health and sleep in the general population. However, it remains largely unknown how the prevalence of mental health problems and sleep disturbance developed and interacted in adolescents at different times in the epidemic. Methods 831 teenagers (aged 14–19) underwent a longitudinal follow-up study to evaluate the prevalence of mental health problems and sleep disturbance among adolescents before, during, and after the COVID-19 breakout in China and to explore the interaction between mental health and sleep across the three measurements. The chronotype, anxiety and depression level, sleep quality, and insomnia were investigated during each measurement. Results The adolescents had delayed sleep onset and sleep offset time, longer sleep duration during the quarantine than before and after the epidemic, whereas their chronotype tended to morning type during the epidemic. Yet, the highest prevalence of anxiety, depression, poor sleeper, and insomnia symptoms were observed before but not during the COVID-19 breakout. The females and adolescents who were eveningness type showed significantly higher anxiety and depression levels, poorer sleep quality, and severe insomnia status than the males and the intermediate and morning types. Sleep disturbance was positively associated with mental problems among three measurements. Pre-measured depression level significantly predicted sleep disturbance level at follow-ups. Conclusion These findings suggested that adolescents' high prevalence of mental health and sleep problems occurred before the COVID breakout and decreased during and after the epidemic. Gender and chronotype were significant risk factors associated with affective and sleep disturbances. Depression positively predicted later sleep problems, but not vice versa.

It is important to study the mechanism of wettability of Zhaozhuang coal with surfactant compoundings in order to prevent dust disaster in the process of coal seam mining. By means of molecular simulation, the amounts of water absorbed by different systems, which are composed of monomer surfactant molecules and Zhaozhuang coal molecules or compounding surfactant molecules and Zhaozhuang coal molecules, were compared in this paper. The simulation results show that the system composed of 0.2% sodium dodecyl benzene sulfonate (SDBS) and 0.3% alcohol polyoxyethylene ether (AEO3) had the maximum water absorption amount of 479, which is significantly better than that of other compounding methods and of each monosurfactant system. Analysis results show that the interference of non-ionic surfactant can greatly reduce the electrostatic repulsion between ionic surfactants and make the adsorption sites on the Zhaozhuang coal molecule more compact. The experimental results show that the decreasing percentage of the contact angle of this type of compounding solution on Zhaozhuang coal was 83.74%, which is the best of the six compounding methods. It has a high degree of consistency with the simulation results, and the molecular simulation method applied in this paper shows that it is convenient and accurate. This research plays a guiding role in dustproof work in Zhaozhuang coal mining.

The ability of coal to adsorb methane depends on the coal microstructure; however, the research on its exploration is still underway. In this paper, a new method was adopted to investigate the evolution characteristics of the crystallite structure of eight different rank coals and its influence on the methane adsorption capacity. The crystallite lattice parameters, including d002, Lc, La, Nave, and fa, were determined by curve fitting analysis of X-ray diffraction (XRD) spectra. The methane adsorption experiments were carried out through a static capacity method, and the methane adsorption parameters (VL, PL) were measured. Correlations were established for the crystallite lattice parameters and the methane adsorption parameters. From the results obtained, there is a good negative linear relationship between VL and d002 and a good exponential relationship between PL and d002, indicating that the increasing d002 can weaken the methane adsorption capacity. VL displays an exponential increase with increasing Lc and Nave, whilePL presents a linear decrease, but reverse variations are emerged in the process of change for both, and the methane adsorption capacity is weaken temporarily. VL presents a lognormal distribution with increasing La, and the minimum value appears at La = 1.85–1.9 nm. VL and PL both obey lognormal distribution with increasing La/Lc, but their trends are completely opposite, and the methane adsorption capacity is the strongest at La/Lc = 0.85–0.9. As fa increases, VL and PL present an overall exponential increase and an overall exponential decrease, respectively, but reverse changes also emerge. The methane adsorption is related to the crystallite structure characteristics of coal. Finally, the influence mechanism of the crystallite structure evolution on the methane adsorption capacity was analyzed, which has great significance for prevention of gas disasters in underground coal mines.

Four coals samples at different ranks were analyzed by Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and solid-state13C nuclear magnetic resonance (NMR). The calculated coal molecular model was constructed according to the experimental data. The mode of evolution of four coal molecules with different metamorphic degrees was explored. The results indicate that the nanostructures of these four coal molecules mainly consist of aromatic structures, aliphatic structures and oxygen-containing functional groups. The coal metamorphic degree is the most important factor affecting the evolution of the coal molecular nanostructure. By increasing the coal rank, the aromatic carbon content and aromatic system increase, while the aliphatic carbon content and aliphatic system decrease, and the species and content of oxygen containing functional groups are also reduced. During the evolution of the molecular microcrystalline structure, the degree of vertical order of the aromatic structural unit, the flatness of the aromatic structural unit (La), the average crystallite stacking height (Lc), and the average number of crystallites in a stack (n) increase, while the interlayer distance between aromatic sheets (d002) decreases; the short-range ordering of the coal structure is mainly caused by changes in the orientational arrangement from intramolecular aromatic layers to intermolecular aromatic layers when low-rank coal molecules evolve to high rank coal molecules. The structural evolution mechanism of coal molecules of different ranks has been revealed through the analysis of the mode of evolution of the molecular structure the coal. This study enables us to better understand the nanostructure evolution mechanism of coal molecules at different ranks.

Coal and gas outbursts have occurred in the Zhaozhuang coal mine (a high gas coal mine) since its construction, and these outbursts pose a great threat to lives and property. Therefore, the combination of methane desorption experiments and molecular simulations was adopted to investigate the effects of moisture on the methane desorption characteristics of the Zhaozhuang coal, and the microscopic mechanism was analyzed in this paper. The aim of this work is to provide a method for predicting the effect of moisture on coal seam methane desorption, and quantitatively evaluating the control effect of hydraulic measures on coal and gas outbursts from a molecular perspective. The experimental data (methane desorption amount) were measured under various adsorption equilibrium pressures (0.3 MPa, 0.4 MPa, and 0.5 MPa) and moisture contents (0%, 5%, and 9.8%). Based on the Zhaozhuang coal molecular model, the molecular simulation process was conducted using Materials studio software, which can better match the experimental conditions by setting various pressure parameters (0.3 MPa, 0.4 MPa, and 0.5 MPa) and calculating various numbers of H2O molecules (0, 8, and 16) on the coal molecule. The results show that methane desorption decreases with the increasing moisture content in the experiment. The number of CH4 molecules adsorbed on the coal molecule decreases with the increasing number of H2O molecules in the simulation. This phenomenon can be explained by the competitive adsorption between CH4 and H2O molecule: the interaction between coal molecule and H2O molecules is stronger. The affinity between CH4 molecules and the coal molecule is reduced, and CH4 molecules are less aggregated around the coal molecule because of the presence of H2O molecules.

The adsorption mechanism of methane molecules and coal molecules at different ranks is of significant practical and theoretical importance in prevention gas disaster and methane storage. In this paper, four coal samples at different ranks were selected for experiments. The real coal molecules at different ranks were constructed. The relationship between the coal components and the gas adsorption capacity was analyzed. The adsorption mechanism was studied by performing combined Grand Canonical Monte Carlo(GCMC) and Density Functional Theory (DFT)simulation at temperatures of 298 K and at pressure range of 0–100 bar. The simulation results display a good agreement with the experimental results. In the process of methane adsorption simulation, methane molecules are preferentially adsorbed on the edge of the unit cell and then adsorbed inside the coal molecules. The methane molecules in the Up orientation have stronger adsorption capacity then Down orientation. The adsorption capacity order is as follows: aromatic structure > oxygen functional groups. In the aromatic structure, the adsorption capacity of adsorption sites is ranked as T > B > C. In the oxygen functional groups, the adsorption of methane by the coal molecule is caused by both the adsorption position and the adsorption direction. Under the influence of coal molecules, the structure and the Mulliken charge of methane molecules change significantly during the adsorption simulation. The adsorption forces between the coal molecules and methane molecules are mainly induced force, dispersion force, orientaion force and electrostatic force. This study enables us to better understand the adsorption mechanism of coal molecules at different ranks.