On the state of the art of crystalline structure reconstruction of coal fly ash: A focus on zeolites

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.

Effects of calcium-to-silicon ratio on the properties of fly ash-based tobermorite and its removal performance of Zn2+ and Mn2

The effects of calcium-to-silicon ratio on the properties of fly ash (FA)-based tobermorite and its removal performance of Zn2+ and Mn2+ were studied. The calcium-to-silicon ratio had a significant effect on the structural properties of the tobermorite samples. The specific surface area, pore volume, and average pore size of mesoporous tobermorite samples with different calcium-to-silicon ratios (0.8TOB, 1.2TOB, and 1.6TOB) were much larger than those of FA, and those of 1.2TOB were the largest, which were 53.29 m2/g, 0.448 cm3/g, and 30.50 nm, respectively. The removal efficiencies of Zn2+ and Mn2+ by 1.2TOB were 84.19% and 47.67%, respectively, which were much higher than those of 0.8TOB (60.62% and 42.41%), 1.6TOB (46.69% and 24.31%), and FA (4.13% and 6.95%). The adsorption of Zn2+ and Mn2+ by 0.8TOB, 1.2TOB, and 1.6TOB was corresponding to the pseudo-second-order kinetic model and Langmuir isotherm model. Particularly, 1.2 TOB showed the highest maximum adsorption capacities of Zn2+ and Mn2+ calculated from the Langmuir model, which were 129.70 mg/g and 82.09 mg/g, respectively. Moreover, the adsorption mechanisms might be due to the combination with -OH and the interlayer adsorption of the samples. This research provides new insight into the fly ash-based adsorbents towards Zn2+ and Mn2+ in wastewater.

Coal fly ash and bottom ash low-cost feedstocks for CO2 reduction using the adsorption and catalysis processes

Combustion of fossil fuels, industry and agriculture sectors are considered as the largest emitters of carbon dioxide. In fact, the emission of CO2 greenhouse gas has been considerably intensified during the last two decades, resulting in global warming and inducing variety of adverse health effects on human and environment. Calling for effective and green feedstocks to remove CO2, low-cost materials such as coal ashes "wastes-to-materials", have been considered among the interesting candidates of CO2 capture technologies. On the other hand, several techniques employing coal ashes as inorganic supports (e.g., catalytic reduction, photocatalysis, gas conversion, ceramic filter, gas scrubbing, adsorption, etc.) have been widely applied to reduce CO2. These processes are among the most efficient solutions utilized by industrialists and scientists to produce clean energy from CO2 and limit its continuous emission into the atmosphere. Herein, we review the recent trends and advancements in the applications of coal ashes including coal fly ash and bottom ash as low-cost wastes to reduce CO2 concentration through adsorption and catalysis processes. The chemical routes of structural modification and characterization of coal ash-based feedstocks are discussed in details. The adsorption and catalytic performance of the coal ashes derivatives towards CO2 selective reduction to CH4 are also described. The main objective of this review is to highlight the excellent capacity of coal fly ash and bottom ash to capture and selective conversion of CO2 to methane, with the aim of minimizing coal ashes disposal and their storage costs. From a practical view of point, the needs of developing new advanced technologies and recycling strategies might be urgent in the near future to efficient make use of coal ashes as new cleaner materials for CO2 remediation purposes, which favourably affects the rate of global warming.

Co-utilization of iron ore tailings and coal fly ash for porous ceramsite preparation: Optimization, mechanism, and assessment

Maximizing the utilization of industrial by-products, such as iron ore tailings (IOTs) and coal fly ash (CFA), is crucial toward sustainable development. This study provides a meticulous insight into the optimization, mechanism, and assessment of the co-utilization of IOTs and CFA for the preparation of porous ceramsite. Micro-CT results revealed that the prepared ceramsite exhibited an exceptional porosity, peaking at 56.98%, with a wide range of pore diameters (3.55-959.10 μm) under optimal conditions (IOTs content at 76%, preheating at 550 °C for 15 min, and sintering at 1177 °C for 14 min), while maintaining good mechanical properties (water adsorption of 1.28%, comprehensive strength of 8.75 MPa, apparent density of 1.37 g/cm3, and bulk density of 0.62 g/cm3). The primary parameters affecting the porosity were identified and ranked as follows: sintering temperature > IOTs content > sintering time. The formation and growth of pores could be attributed to the equilibrium relationship between the liquid-phase surface tension and the gas expansion force, accompanied by pore wall thinning and pore merging. Notably, the prepared ceramsite is both ecologically feasible and economically rewarding, boasting a profit margin of 9.47 $/ton. The comprehensive life cycle assessment (LCA) conducted further highlights the potential of its large-scale implementation for promoting sustainable development. This study provides an innovative strategy for the co-utilization of IOTs and CFA, with advantages such as cost-effectiveness, ecological feasibility and scalability of production.

Evaluating the metal recovery potential of coal fly ash based on sequential extraction and machine learning

Given the depletion of metal resources and the potential leaching of toxic elements from solid waste, secondary recovery of metal from solid waste is essential to achieve coordinated development of resources and the environment. In this study, hybrid models combining the gradient boosting decision tree and particle swarm optimization algorithm were constructed and compared based on two different datasets. Additionally, a new, quantitative evaluation index for metal recovery potential (MRP) was proposed. The results showed that the model constructed using more elemental properties could more accurately predict metal fractions in coal fly ash (CFA) with an R² value of 0.88 achieved on the testing set. The MRP index revealed that the DAT sample had the greatest recovery potential (MRP = 43,311.70). Ca was easier to recover due to its high concentration and presence mostly in soluble fractions. Model post-analysis highlighted that the elemental properties and total concentrations generally exerted a greater influence on the metal fractions. The innovative evaluation strategy based on machine learning and sequential extraction presented in this work provides an important reference for maximizing metal recovery from CFA to achieve environmental and economic benefits with the goal of sustainable development.

A green and multi-win strategy for coal fly ash disposal by CO2 fixation and mesoporous silica synthesis

Coal combustion provides plenty of energy, along with enormous coal fly ash (CFA) and CO₂ emission. CFA could be recycled for mesoporous silica synthesis, but expensive templates are usually needed. In this work, we proposed a multi-win strategy using CO₂ as the precipitator and template. Mesoporous silica powders, with a maximum specific surface area of 355.45 m²/g, a pore volume of 0.73 cm³/g, and an average pore size of around 7.67 nm, were synthesized. The influences of silicon concentration, CO₂ flow rate, and ultrasound were investigated. In addition, the Na₂CO₃ by-product was produced with a purity of over 92 %. By averagely calculating, 1 ton CFA could generate 285 kg mesoporous silica and 1.02 t crude Na₂CO₃. Around 433 kg of CO₂ could be absorbed. Therefore, multi-goals of CFA disposal, CO₂ storage, and valuable silica materials production were realized, and the study could pave the way for large-scale industrial applications.

Green Approach for Rare Earth Element (REE) Recovery from Coal Fly Ash

Due to the growing demands of rare earth elements (REEs) and the vulnerability of REEs to potential supply disruption, there have been increasing interests in recovering REEs from waste streams such as coal fly ash (CFA). Meanwhile, CFA as a large industrial waste stream in the United States (U.S.) poses significant environmental and economic burdens. Recovery of REEs from CFA is a promising solution to the REE scarcity issue and also brings opportunities for CFA management. This study demonstrates a green system for REE recovery from Class F and C CFA that consists of three modules: REE leaching using citrate, REE separation and concentration using oxalate, and zeolite synthesis using secondary wastes from Modules I and II. In Module I, ∼10 and 60% REEs were leached from the Class F and C CFA samples, respectively, using citrate at pH 4. In Module II, the addition of oxalate selectively precipitated and concentrated REEs from the leachate via the formation of weddellite (CaC₂O₄·2H₂O), while other trace metals remained in solution. In Module III, zeolite was synthesized using wastes from Modules I and II. This study is characterized by the successful recovery of REEs and upcycling of secondary wastes, which addresses both REE recovery and CFA management challenges.

Performance and mechanism of lanthanum-modified zeolite as a highly efficient adsorbent for fluoride removal from water

Defluoridation of water is still challenging due to the fluoride pollution of both groundwater and surface water worldwide. In this study, lanthanum-modified zeolite (LMZ) was synthesized from coal fly ash and was investigated for fluoride removal from water by conducting batch and column experiments. Our results indicated that the process of fluoride adsorption was endothermic and the adsorption kinetics on LMZ followed the pseudo-second-order model. A higher temperature increased both the capacity and the rate of adsorption. The maximum fluoride adsorption capacity of LMZ reached 141.5 mg/g with a F/La molar ratio of 4.21, as estimated from the Langmuir model which best fitted the isotherm data. Fluoride adsorption greatly depended on pH, with optimal performance being achieved within ∼5.0-∼7.0. The point of zero charge of LMZ was pH 8.8, at which only bicarbonate ions greatly affected fluoride removal. However, no competing effect was observed at pH 6.3 for all tested anions including chloride, sulphate, nitrate, bicarbonate and acetate. The dominant adsorption mechanism was the ligand exchange of fluoride with hydroxyls on LMZ, as illustrated by the rise in pH due to fluoride adsorption and by the molecular scale spectroscopic FTIR, Raman and XPS studies. Fluoride adsorbed on LMZ was successfully desorbed using NaOH solution, and regenerated LMZ could be reused. The results of column studies showed that LMZ granulated with alginate performed well in treating F^--containing water. In conclusion, LMZ is a promising material for efficient defluoridation from water.

Ammonium continuous removal by zeolite P synthesized using fly ash combined with bacteria in aqueous solution

The new composite product synthesized by zeolite P and bacteria consisting of nitrobacteria and denitrobacteria can efficiently and continuously remove ammonium in solution through zeolite adsorption and bacteria degradation. In this study, we used fly ash to prepared zeolite P, and then combined bacteria to synthesize the composite product. The adsorption efficiency and mechanism of products for ammonium were further studied by batch and dynamic experiments, and adsorption model. The zeolite P with a relative crystallinity of 84.7% was synthesized using fly ash by an alkali fusion-hydrothermal method. The synthetic zeolite P could combine with bacteria to be prepared an integral adsorption composite that had hierarchical pore structure including macropores, mesopores, and micropores, and its maximum compressive strength reached 106.2 N. The zeolite P could remove ammonium from solution, and Freundlich, Temkin, and Dubinin-Radushkevich models as well as thermodynamic models all showed that the ammonium adsorption by zeolite was mainly physical adsorption. Thus, the adsorbed ammonium was easy to be desorbed and became the nitrogen source for bacteria in composites. By batch experiments, the ammonium adsorption rate of composite product was significantly improved (P < 0.05) compared with zeolite P due to zeolite adsorption and the bacteria degradation. Through dynamic experiments, the composite product could efficiently and continuously remove ammonium from solution than zeolite P and bacteria alone. Therefore, the composite product could form a stable system for the adsorption, desorption, and degradation of ammonium in solution.

The drivers of collaborative innovation of the comprehensive utilization technologies of coal fly ash in China: a network analysis

Coal consumption brings a lot of coal fly ash (CFA). It requires interdisciplinary efforts in research, policy, and practice to improve the utilization of CFA. Although there have been a lot of achievements in technological innovation, the utilization of CFA is still difficult to match its output. So, it is urgent to explore how to guide its effective innovation. This paper uses social network analysis to discuss the characteristics of the collaborative innovation network of CFA comprehensive utilization technology in China. Then, this paper uses regression analysis to explore the differences in innovation performance under different research and development (R&D) backgrounds. The results show that (1) based on the network-level indicators, the collaborative innovation scale has an obvious trend of expanding. Partnerships increased from 20 to 574. Meanwhile, the network shows obvious scale-free and "small-world" characteristics, indicating that innovation resources are concentrated in a few organizations. (2) Based on the node-level indicators, the major contributor has shifted from universities and research institutions to enterprises. Enterprises account for the highest proportion (73%) and have the highest centrality (8.3). The betweenness centrality of the universities is 265, and only 14% of the organizations are universities which means universities play an important role in connecting different organizations in the network, but their participation in the collaborative innovation is insufficient. (3) Based on the collaborative relationship-level indicators, the cooperation is lack of depth. Only a small number of organizations, especially enterprises, have stable partners, showing the characteristic of "low cooperation width and high cooperation depth," which means fewer partners but more frequently collaborative innovation. (4) Based on the innovation performance, the innovation performance under the category of cooperative R&D, especially industry-academy cooperation, is better than that of independent R&D. But, industry-academy cooperation only occupied 43% of collaborative relationships in the network. Finally, this paper puts forward suggestions for governments from five aspects: decentralization, defining roles of enterprise and university, encouraging collaboration, changing the idea of the patent application, and promoting deeper cooperation.

Solving two industrial waste issues simultaneously: Coal gasification fine slag-based hierarchical porous composite with enhanced CO2 adsorption performance

The coal gasification fine slag (FS) is industrial waste consisting of residual carbon (RC) and slag particles (SP). High-performance hierarchical porous composite (HPC) was synthesized by chemical activation followed by hydrothermal treatment from FS. It has been realized that SP could play the positive role in developing pore structure of HPC which makes CO₂ uptake have a sharp increase. Through taking the advantage of SP, HPC has an obvious increase in surface area and pore volume from 914 to 1932 m²/g and 0.617 to 1.332 cm³/g, respectively. The changes of pore structure were mainly attributed to the two factors in a synergetic way. First, hydrothermal treatment could decrease the content of SP which acted as a ballast to reduce the surface area of activated carbon. Second, the formed silicate/aluminosilicate deposits on the carbon surface and forms silicate/aluminosilicate film on internal surface of carbon particles. This makes the meso-macropores change to micro- mesopores leading to the increase of the surface area and pore volume. The porous composites also showed high CO₂ adsorption performance. The 4-FSAC-HPC sample exhibits the highest CO₂ uptake of 3.25 mol/kg and 1.41 mol/kg at 25 and 50 °C, respectively, which is an increase of around 52% and 83% comparing with that of FSAC sample. Besides, CO₂ adsorption up to saturation can be achieved in less than 3 min. CO₂ uptake of the 4-FSAC-HPC sample was well regeneration efficiencies above 98% after 10 cycles adsorption/desorption. This work provides a new approach to prepare high-performance porous compositions using the material which contains not only carbon component but mineral matters. Furthermore, it realizes solving two industrial waste issues (fine slag and CO₂) simultaneous.

A review on the applications of microbially induced calcium carbonate precipitation in solid waste treatment and soil remediation

Improper disposal and accumulation of solid waste can cause a number of environmental problems, such as the heavy metal contamination of soil. Microbially induced calcium carbonate precipitation (MICP) is considered as a promising technology to solve many environmental problems. Calcium-based solid waste can be utilized as an alternative source of calcium for the MICP process, and carbonate-based biominerals can be used for soil remediation, solid waste treatment, remediation of construction concrete, and generation of bioconcrete. This paper describes the metabolic pathways and mechanisms of microbially induced calcium carbonate precipitation and highlights the value of MICP for solid waste treatment and soil remediation applications. The factors affecting the effectiveness of MICP are discussed and analyzed through an overview of recent studies on the application of MICP in environmental engineering. The paper also summarizes the current challenges for the large-scale application of this innovative technology. In prospective study, MICP can be an effective alternative to conventional technologies in solid waste treatment, soil remediation and CO₂ sequestration, as it can reduce negative environmental impacts and provide long-term economic benefits.

Green and Moderate Activation of Coal Fly Ash and Its Application in Selective Catalytic Reduction of NO with NH3

Coal fly ash (CFA) is an ideal source for the preparation of heterogeneous catalysts due to its abundant silicon and aluminum oxides, but its activity needs to be improved. In this study, a green and moderate approach for CFA activation was proposed, and a series of CFA-based catalysts were prepared for NO selective catalytic reduction (SCR). The results indicated that CFA could be well activated via mechanochemical activation with 3 h of milling duration in 1 mol/L of acetic acid, and 90% of NO removal was achieved over the CFA-based catalyst in 250 to 375 °C. Two activating mechanisms, i.e., the enhanced CFA fragmentation and the motivated Al dissolution, were revealed during the mechanochemical activation. The former facilitated the formation of mesopores and the exposure of Fe components in CFA fragments, which enhanced the capacity of oxygen storage over the as-activated catalyst. The latter motivated the formation of Si-OH groups, which promoted the migration of electrons and the dispersion of V species, thereby increasing the capacity of NH₃ adsorption over the as-obtained catalyst. Therefore, the performance of NO reduction was improved. The proposed activating approach could be a promising integration for CFA disposal and NO removal from inside coal-fired power plants.

Removal of fluoride from water using aluminum hydroxide-loaded zeolite synthesized from coal fly ash

The removal of fluoride from wastewater is essential as the excess accumulation of fluoride in environment is harmful to the health of humans. In this study, the defluorination of water by aluminum hydroxide-coated zeolite (AHZ), which was synthesized from coal fly ash, was investigated in batches. The Langmuir maximum adsorption capacity of fluoride by AHZ reached 18.12 mg/g. Aluminum hydroxide was shown to be the major component that adsorbed fluoride. More than 92% removal of fluoride was achieved within 2 h, and the fluoride adsorption kinetics were well fitted to a pseudo-second-order model. The point of zero charge (pH_pzc) of the AHZ was determined to be 5.52. Fluoride adsorption by AHZ depended greatly on pH, and maximum performance was obtained at pH 5.5-6.5. The AHZ showed good selectivity for the adsorption of fluoride in the presence of chloride, nitrate, sulfate, bicarbonate, and acetate ions, and the fluoride was nearly exhausted at a sufficiently high dose. The release of OH^- due to fluoride adsorption was confirmed. FTIR and XPS studies further illustrated that the adsorption mechanism of fluoride adsorption on AHZ was ligand exchange with hydroxyl groups and the formation of F-Al bonds.