Harnessing coal and coal waste for environmental conservation: A review of photocatalytic materials.

Fossil fuels, especially coal, have played a pivotal role in driving technological and economic advancements over the past century, though accompanied by numerous environmental challenges. Rapid progress in green and sustainable energy sources, including tidal, wind, and solar energy, coupled with growing environmental concerns, the conventional coal industry is experiencing a sustained decline in both size and financial viability. This situation necessitates the urgent adoption of advanced approaches to coal utilization. Beyond serving as an energy source, coal and its by-products, known as coal waste, can serve as valuable resources for the development of advanced materials, including photocatalysts. The advancement of photocatalytic materials derived from coal and coal waste can capitalize on these natural carbon and mineral sources, providing a viable solution to numerous environmental challenges. Currently, research in this domain remains in its early stages, with existing studies primarily focusing on specific types of photocatalysts or particular aspects of the fabrication process. Therefore, available coal-based and coal waste-based photocatalytic materials were systematically examined and categorized into six types according to their composition and dimensional/structural characteristics. Each type of photocatalytic material was introduced, along with common fabrication and characterization technologies. Representative works were discussed in detail to highlight the unique features of different types of coal-based and coal waste-based photocatalytic materials. Furthermore, the promising applications of these materials in environmental protection and pollution treatment were summarized, while also addressing the challenges and prospects in this research field. This review comprehensively overviews the fundamental knowledge and recent advancements in photocatalytic materials derived from coal and coal waste, with the goal of catalyzing the development of next generation photocatalysts and contributing to the transformation of the conventional coal industry.

Effect of Fatty Acid Unsaturation on the Intermolecular Weak Interaction at the Low-Rank Coal-Water Interface: Density Functional Theory Calculations and Experimental Study.

The study on the effect of fatty acid saturation on low-rank coal (LRC) flotation is still limited. In this investigation, density functional theory (DFT) combined with Zeta potential and Fourier transform infrared spectroscopy (FTIR) was used to study the mechanism of intermolecular weak interaction at the LRC-water interface of fatty acids (decanoic acid (DA), undecylenic acid (UA), and phenyl propionic acid (PA)) with different saturations and different dodecane (D) composition hydrocarbon oil-fatty acid mixed collectors (D-DA, D-UA, D-PA). The findings demonstrated that the hydrogen bond interaction and electrostatic interaction between the UA/PA with unsaturated bonded carbon chains and the LRC molecular fragments/water molecules were stronger than DA without a saturated bond carbon chain, and UA/PA strengthened its interaction with water molecules on the whole, even PA molecules would preferentially interact with water molecules. The unsaturated bond had a minimal impact on the adsorption of the LRC hydrophobic site, and the strength of the hydrogen bond between the mixed collector and LRC is D-DA > D-UA > D-PA. In the actual flotation process, the strong hydrogen bonding and electrostatic interaction between UA/PA and water molecules weaken the collection performance of the mixed collector D-UA/D-PA for LRC, which also confirmed the research results of DFT, FTIR, and Zeta.

Preparation and characterization of high-ash coal slime-based soil amendment as well as investigations of its adsorption performance and mechanisms towards heavy metals in soil.

In this study, high-ash coal slime-based mineral soil amendment (MSA) was prepared via the hydrothermal method using high-ash coal slime as raw material, supplemented with activator calcium oxide and additive KOH solution. After hydrothermal treatment at 230 °C for 5 h, the original crystalline phase (quartz and kaolinite) of the high-ash slime was completely transformed into hydrotalcite zeolite, tobermorite, and silicate of potassium aluminosilicate, which has the largest specific surface area. The adsorption of Pb2+ and Cd2+ was adherent to the kinetic equation of secondary adsorption and Freundlich models, and the removal of Pb2+ and Cd2+ reached up to 362.58 mg g-1 and 64.67 mg g-1. The successive releases of SiO2 and CaO from MSA conformed to the Elovich equation, whereas the releases of SiO2 in Cd-containing environments and CaO in Pb- and Cd-containing environments more closely conformed to the power function; the releases of K2O all conformed to the first-order kinetic equation. The presence of Pb2+ and Cd2+ in the environment promotes the release of potassium and calcium elements with MSA's ion-exchange ability, and attenuates the release of silicon elements. Combining Pb2+ and Cd2+ with silicon resulted in the intolerant precipitation of 3PbO·2SiO2 and Cd2SiO4. The mineral precipitation mechanism is the most important mechanism of MSA in immobilizing heavy metals, accounting for 72.7%-80.5% of the total adsorption. Further contaminated soil immobilization experiments also showed that the application of MSA significantly reduced the bioavailability of soil heavy metals. When the MSA addition amount was 1.6%, the residual state increased by 63.58%. In conclusion, preparing MSA may effectively utilize coal-based solid waste with high added value.

Preparation of a Lignin-Based Cationic Flocculant and Its Application in Kaolin Suspension Treatment.

The preparation of an environmentally friendly and efficient flocculant for solid-liquid separation in industrial wastewater is highly important. In this study, a novel cationic flocculant (AL-g-PAMA) was synthesized by a thermal initiation method using alkali lignin (AL) as the main chain and acrylamide (AM) and methacrylamido propyl trimethyl ammonium chloride (MAPTAC) as the grafted side chains. The structure, thermal stability, and surface morphology of the copolymers were investigated by various characterization methods. The results indicated the successful synthesis of AL-g-PAMA. AL-g-PAMA was applied to improve solid-liquid separation in kaolin suspensions. The results showed that AL-g-PAMA had excellent flocculation-sedimentation and dewatering efficiency. When the dosage of AL-g-PAMA #5 was 600.0 g/t(s), the thickness of the compressed layer was 2.2 cm, the floc settling velocity was 24.1 cm/min, and the transmittance of the supernatant was 84.0%. The moisture content of the filter cake decreased from 55.0% to 43.4% after treatment with AL-g-PAMA #5. The results of zeta potential and focused beam reflectance measurement (FBRM) analysis indicated that bridging and electroneutralization were the main flocculation mechanisms. Therefore, this study extends the potential for using lignin as a bioflocculant and provides a feasible approach to efficiently purify high-turbidity wastewater.

Resource Recovery of Spent Lithium-Ion Battery Cathode Materials by a Supercritical Carbon Dioxide System.

The increasing global market size of high-energy storage devices due to the boom in electric vehicles and portable electronics has caused the battery industry to produce a lot of waste lithium-ion batteries. The liberation and de-agglomeration of cathode material are the necessary procedures to improve the recycling derived from spent lithium-ion batteries, as well as enabling the direct recycling pathway. In this study, the supercritical (SC) CO2 was innovatively adapted to enable the recycling of spent lithium-ion batteries (LIBs) based on facilitating the interaction with a binder and dimethyl sulfoxide (DMSO) co-solvent. The results show that the optimum experimental conditions to liberate the cathode particles are processing at a temperature of 70 °C and 80 bar pressure for a duration of 20 min. During the treatment, polyvinylidene fluoride (PVDF) was dissolved in the SC fluid system and collected in the dimethyl sulfoxide (DMSO), as detected by the Fourier Transform Infrared Spectrometer (FTIR). The liberation yield of the cathode from the current collector reaches 96.7% under optimal conditions and thus, the cathode particles are dispersed into smaller fragments. Afterwards, PVDF can be precipitated and reused. In addition, there is no hydrogen fluoride (HF) gas emission due to binder decomposition in the suggested process. The proposed SC-CO2 and co-solvent system effectively separate the PVDF from Li-ion battery electrodes. Thus, this approach is promising as an alternative pre-treatment method due to its efficiency, relatively low energy consumption, and environmental benign features.

Effect of Carboxyl Group Position on Assembly Behavior and Structure of Hydrocarbon Oil-Carboxylic Acid Compound Collector on Low-Rank Coal Surface: Sum-Frequency Vibration Spectroscopy and Coarse-Grained Molecular Dynamics Simulation Study.

In this work, the assembly behavior and structure of a compound collector with different carboxyl group positions at the low-rank coal (LRC)-water interface were investigated through coarse-grained molecular dynamics simulation (CGMD) combined with sum-frequency vibration spectroscopy (SFG). The choice of compound collector was dodecane +decanoic acid (D-DA) and dodecane +2-butyl octanoic acid (D-BA). CGMD results showed that the carboxyl group at the carbon chain's middle can better control the assembly process between carboxylic acid and D molecules. SFG research found that the carboxyl group at the carbon chain's termination had a greater impact on the displacement of the methyl/methylene symmetric stretching vibration peak, while the carboxyl group at the carbon chain's middle had a greater impact on the displacement of the methyl/methylene asymmetric stretching vibration peak. The spatial angle calculation results revealed that the methyl group's orientation angle in the D-BA molecule was smaller and the carboxyl group's orientation angle in the BA molecule was bigger, indicating that D-BA spread more flatly on the LRC surface than D-DA. This meant that the assembled structure had a larger effective adsorption area on the LRC surface. The flotation studies also verified that the assembly behavior and structure of D-BA with the carboxyl group at the carbon chain's middle at the LRC-water interface were more conducive to the improvement of flotation efficiency. The study of interface assembly behavior and structure by CGMD combined with SFG is crucial for the creation of effective compound collectors.

Study on preparation of UV-CDs/Zeolite-4A/TiO2 composite photocatalyst coupled with ultraviolet-irradiation and their application of photocatalytic degradation of dyes.

In this work, ultraviolet irradiation was employed to assist in the preparation of a novel photocatalyst composite in the form of carbon dots/zeolite-4A/TiO2, using coal tailings as the source of silicon-aluminum and carbon. The composite was designed for the degradation of methylene blue under 500 W of UV light irradiation. Zeolite-4A was used as a support for the well-dispersed carbon dots and TiO2 nanoparticles. The as-prepared composites were subjected to thorough characterization, confirming the successful formation of zeolite-4A with a cube structure, along with the loading of TiO2 and coal-based CDs in the composites. The experimental results demonstrated that the UV-CZTs nanocomposites exhibited a remarkable removal efficiency of 90.63% within 90 min for MB. The corresponding rate constant was exceptionally high at 0.0331 min-1, surpassing that of the Dark-CZTs and pure TiO2. This significant enhancement was possibly due to the synergistic effect of adsorption photocatalysis of the UV-CZTs, combined with the excellent electron-accepting capabilities of the coal-based CDs, which led to highly improved charge separation. An investigation of the spent photocatalyst's recyclability revealed that it retained a remarkable 82.94% MB removal efficiency after five consecutive cycles, signifying the stability of the composite. Trapping experiments also elucidated the primary reactive species responsible for MB degradation, which were identified as photo-generated holes and ⸱O2- species. By this process, the hydroxyl radicals generated in the system successfully promoted the transformation of coal tailings to coal-based zeolite and coal-based CDs. Coal-based zeolite served as an excellent carrier of titanium dioxide, which improved its dispersibility. The inhibition of e--h+ recombination of titanium dioxide by introducing coal-based CDs improved the photocatalytic ability of titanium dioxide. Through this study, coal tailings, as a coal processing waste, were transformed into high-value materials, and relevant photocatalytic composite materials could be prepared with broad application prospects.

Hydrothermal alkaline synthesis and release properties of silicon compound fertiliser using high-ash coal slime.

High-ash coal slime is difficult to utilise as a boiler fuel, and its accumulation results in environmental pollution. In this study, we describe a new method for the preparation of high-ash coal slime silica compound fertiliser (HASF) using CaO-KOH mixed hydrothermal method to optimize the utilization of this industrial waste and relieve the pressure on the fertiliser industry. The coal slime (D0) used in this study and its dry basis ash content by 1 mol/L and 4 mol/L sulfuric acid pre-activation (D1, D4) were greater than 85%. The effective silicon content of D0, D1, and D4 silica compound fertilisers reached 30.24%, 31.24%, and 17.35%, respectively, and the sums of effective silica-calcium-potassium oxides were 57.28%, 58.87%, and 48.16%, respectively, under the optimal reaction conditions of 230 °C, 15 h, and 1 mol/L KOH, which met the market requirements, as determined using single-factor experiments. We used XRD, FTIR, and SEM-EDS analysis techniques to demonstrate that tobermorite and leucite were the main mineral phases of the compound fertiliser, and activated coal slime D4, which contains only quartz single crystals, required more demanding reaction conditions in the synthesis reaction. Subsequently, the cumulative release pattern of HASF silica was well described by the power function equation via repeated extraction and dissolution experiments, with the dissolution rate following D4 > D1 ≈ D0. Furthermore, 4 mol/L sulfuric acid pre-activation resulted in the enrichment of HASF combined with organic matter and increased the slow-release rate of HASF silica. Thus, the synthesized HASF could have potential application prospects in soil improvement and fertilisation.

Structural Characterization and Molecular Model Construction of High-Ash Coal from Northern China.

High-ash coal, also known as low-grade coal, has becomes a viable alternative in recent years to high-quality coal because available resources have become increasingly scarce due to extensive mining activity. This work aims to provide a comprehensive understanding of the structural characteristics of high-ash coal and construct a plausible molecular structure to elucidate its chemical reactivity in future applications. Its properties were investigated using Solid-state 13C nuclear magnetic resonance (13C NMR), X-ray photoelectron spectroscopy analysis (XPS), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FT-IR). The molecular structure was constructed and validated using Material Studio, LAMMPS Software Package, and MATLAB program. The characterization results revealed that high-ash coal contains 72.15% aromatic carbon, significantly surpassing the percentage of aliphatic carbon (27.85%). The ratio of bridgehead carbon to peripheral aromatic carbon was calculated as 0.67, indicating that the pentacene is the main carbon skeleton form in the high-ash coal structure. Furthermore, oxygen-containing functional groups presented as C=O/O-C-O, C-O, and COO- within the structure along with pyridine and pyrrolic structures. Consequently, the molecular structure comprises pentacene with aliphatic carbon chains, such as methylene, that connect the benzene rings and form a three-dimensional network. The results of a simulated IR spectrum and contact angle simulation aligned with the experimental results, validating the molecular structure of high-ash coal. The chemical formula for the high-ash coal model was determined as C203H189N7O61S with a molecular weight of 3734.79.

Adsorption of Different Ionic Types of Polyacrylamide on Montmorillonite Surface: Insight from QCM-D and Molecular Dynamic Simulation.

This study investigates the interaction between montmorillonite and polyacrylamide (PAM) with different ionic types using quartz crystal microbalance with dissipation monitoring (QCM-D) and molecular dynamics (MD) simulations. The goal was to understand the effect of ionicity and ionic type on polymer deposition on montmorillonite surfaces. The results of the QCM-D analysis showed that a decrease in pH led to an increase in the adsorption of montmorillonite on the alumina surface. The ranking of adsorption mass on alumina and pre-adsorbed montmorillonite alumina surfaces was found to be cationic polyacrylamide (CPAM) > polyacrylamide (NPAM) > anionic polyacrylamide (APAM). The study also found that CPAM had the strongest bridging effect on montmorillonite nanoparticles, followed by NPAM, while APAM had a negligible bridging effect. The MD simulations showed that ionicity had a significant influence on the adsorption of polyacrylamides. The cationic functional group N(CH3)3+ had the strongest attraction interaction with the montmorillonite surface, followed by the hydrogen bonding interaction of the amide functional group CONH2, and the anionic functional group COO- had a repulsive interaction. The results suggest that at high ionicity levels, CPAM can be adsorbed on the montmorillonite surface, while at low ionicity levels, APAM may still be adsorbed with a strong coordination trend.

The Synergistic Effects of Al3+ and Chitosan on the Solid-Liquid Separation of Coal Wastewater and Their Mechanism of Action.

It is important to identify an environmentally friendly and efficient flocculant that can replace polyacrylamide for the solid-liquid separation of coal wastewater. In this study, to explore whether chitosan can be used as an environmentally friendly and efficient flocculant for the solid-liquid separation of coal wastewater, AlCl3-chitosan was used to conduct flocculation-sedimentation and dewatering tests under different chitosan dosages and shear-strength conditions for the prepared coal wastewater. Focused beam reflectance was measured to dynamically monitor the number of refractory fine particles, and the settled flocs were photographed and analyzed with microscopy to explore the effect of AlCl3-chitosan on the flocculation settlement effect and floc characteristics. The synergistic mechanisms of AlCl3 and chitosan were investigated using quartz crystal dissipative microbalance and zeta potential measurement. The results showed that the addition of chitosan can significantly improve the flocculation-sedimentation and dewatering effects of coal wastewater. A reasonable dosage under a certain shear strength is conducive to the reduction of fine slime particles, which results in a compact floc structure, increases the floc size, and improves the settling effect. The synergistic effect of AlCl3-chitosan improved the electric neutralization and adsorption bridging abilities of the chitosan, and the mixed solution of AlCl3 and chitosan had stronger adsorption on the carbon surface. This study provides a new approach to the selection of flocculants for coal wastewater treatment.

Laboratorial Investigation of Coal Fire Extinguishing and Re-burning Risk in Underground Coal Mines.

The extinguishing and re-burning of the closed fire area in an underground coal mine were investigated by laboratory-scale physical simulation. Temperatures in the center of the fire source were recorded, and the typical cooling process was observed to include the rapid cooling stage (900-400 °C) and dilatory cooling stage (400-100 °C). With the increase of coal mass from 20 to 80 kg, the rate of cooling decreases and the time required for fire extinguishing increases by 69.5%-193.2%. At temperatures ranging between 500 and 100 °C, yields of CO and H2 show strong correlation with the attenuation of the coal fire, and the trend in the yield of H2 might be used as the optimal indicator considering the different amounts of coal. A significant difference appears in the concentration of H2 released by samples of different dosages of coal in the early stage of cooling, especially when the temperature exceeds 200 °C. During the extinguishing process, micropores in coal fused into mesopores and macropores, while the content of O-containing groups fluctuated significantly. Variations of elemental C and O also indirectly reflect the combustion state in the fire cooling. Taking the experimental reactor as a physical model, the time required for the fire area from closure to safe re-opening is deduced, that is, t = Cm ln (T 1 - T ∞)/(T 2 - T ∞ ). The calculated results were compared with the changes in measured temperatures, providing a theoretical foundation for the re-opening prediction of mine fire areas.

Effects of modified fly ash doped carbon paste electrodes and metal film electrodes on the determination of trace cadmium(ii) by anodic stripping voltammetry.

Fly ash, as waste from coal combustion, has been effectively modified to improve its adsorption to remove toxic metals, and modified fly ash could be used for electrode modification to improve the sensitivity of the electrode. In this paper, a modified fly ash doped carbon paste electrode for the detection of trace cadmium was first and successfully developed. Several parameters affecting the anodic stripping voltammetric response of Cd(ii) were optimized, such as the composition of the paste, pH of the measurement solution, the concentration of Sb(iii) (or Sb(iii) and Bi(iii)), deposition potential and deposition time. Compared with Sb/MFA-CPE, the square wave anodic stripping voltammetry (SWASV) response of Cd(ii) at Sb/MMFA-CPE had a higher linear range and lower sensitivity. Relative to MFA, MMFA-CPE, due to the introduction of CTAB, provided a larger effective area for interacting with analytes, more binding sites and further facilitating electron transfer at the electrode, and amplified the electrochemical signal. Compared with Sb/MMFA-CPE, the SWASV response of Cd(ii) at Sb-Bi/MMFA-CPE had a higher linear range and similar sensitivity, since mechanisms at bismuth and antimony film electrodes were different. Besides, the electrode reactions of Cd(ii) at bismuth film electrodes involved adsorption phenomena while they were free of adsorption at antimony film electrodes.

Characterisation of floc size, effective density and sedimentation under various flocculation mechanisms.

Floc structure plays an important role in the separation of coal wastewater. In this study, a camera-based method is used to evaluate quantitatively the structural characteristics of flocs generated by different coagulants and flocculants. The correlations between particle size, settlement velocity and effective density of coal tailings flocs are analysed. The results show that the statistical settling velocity increases linearly with floc size, while the effective density decreases with increase in floc size. Different flocculation mechanisms lead to diverse growth abilities of flocs. When the flocculant is used alone, the quality of the flocs generated by the flocculants, cationic polyacrylamide (CPAM) and non-ionic polyacrylamide (NPAM), is better than that generated by anionic polyacrylamide (APAM). However, the combination of trivalent cations and APAM yields a much better effect than that obtained using CPAM and NPAM. Flocs become larger and more compact when treated with a coagulant combined with a flocculant.