Photocatalytic reduction of Cr(VI) over cinder-based nanoneedle in presence of tartaric acid: Synergistic performance and mechanism

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.

Rapid photocatalytic reduction of hexavalent chromium over Z-scheme MgIn2S4/BiPO4 heterojunction: Performance, DFT calculation and mechanism insight

The accumulation of highly fluid and biotoxic hexavalent chromium (Cr(VI)) impairs water ecosystems. It is urgent to quickly reduce Cr (VI) to trivalent chromium (Cr (III)) in wastewater. Hereby, Z-scheme MgIn₂S₄/BiPO₄ heterojunction was prepared, and MB-30 (mass ratio of BiPO₄ to composite) presented a rapid Cr(VI) (10 mg L^-1) removal efficiency of 100% within 10 min, its kinetic rate constant was 9.0 and 30.1 folds that of MgIn₂S₄ and BiPO₄, respectively. After four rounds, MB-30 maintained a high removal rate of 93.18% and stabilized crystal texture. First-principles calculations revealed that the formation of Z-scheme heterojunction could ameliorate charge generation, detachment, migration capability, and light utilization. Meanwhile, the coupling of S and O in the two components produced a tight S-O bond, which acted as an atomic-level access to promote carrier migration. The findings were consistent with the structure superiority and optical and electronic properties of MB-30. The Z-scheme pattern was substantiated based on multifarious experiments, which exhibited an elevated reduction potential while emphasizing the significance of interfacial chemical bond and the internal electric field (IEF) on carrier detachment and migration.

Manganese-doped hydroxyapatite as an effective adsorbent for the removal of Pb(II) and Cd(II)

The water polluted by lead(Pb(II)) and cadmium(Cd(II)) seriously endangers ecological safety and needs to be solved urgently. Because of the relatively low adsorption rate of pure hydroxyapatite for heavy metals, a series of manganese-doped hydroxyapatites (MnHAPs) were prepared by using manganese, a common impurity in hydroxyapatite, as a doping element to improve the adsorption performance. The structural and functional groups of the materials with different Mn/(Ca +Mn) molar ratios (0%, 5%, 10%, 20%, and 30%) were investigated by scanning electron microscope (SEM), Brunauer-Emmett-Teller (BET), X-Ray diffraction (XRD), Raman spectrometer and Fourier transform infrared spectroscopy (FTIR) characterization. The presence of manganese influenced the formation and growth of hydroxyapatite crystals, resulting in lattice distortion and a large number of lattice defects in materials. Among them, manganese-doped hydroxyapatite with a Mn/(Ca +Mn) molar ratio of 10% (MnHAP-10) could most effectively remove Pb(II) and Cd(II), with the adsorption capacity of 1806.09 mg g^-1 for Pb(II) at pH = 5 and 176.88 mg g^-1 for Cd(II) at pH = 5.5. Then the adsorption behavior and mechanism were further researched systemically. It was concluded that the immobilization of Pb(II) by MnHAP-10 was mainly through dissolution precipitation and ion exchange, while Cd(II) was adsorbed by ion exchange and electrostatic interaction. In conclusion, MnHAP-10 has the potential to be applied as an effective adsorbent for the removal of Pb(II) and Cd(II) pollution.

Influence of citrate/tartrate on chromite crystallization behavior and its potential environmental implications

The ferrite process has been developed to purify wastewater containing heavy metal ions and recycle valuable metals by forming chromium ferrite. However, organic matter has an important influence on the crystallization behavior and stability of chromite synthesized from chromium-containing wastewater. We focused on the influence and effect mechanism of two typical organic acid salts (citrate (CA) and tartrate (TA)) on the process of chromium mineralization. It was found that the presence of organic matter leads to the increase of the residual content of Cr in CA system (0.50 mmol/L) and TA system (0.61 mmol/L) in the solution, and the removal of chromium was mainly due to the surface adsorption of Fe(III) hydrolysate. The decreased crystallinity of mineralized products is ascribed to the completion of organic compounds with Fe(II) and Fe(III), which hinders the formation of ferrite precursors. There was bidentate and monodentate chelation between -COO- and metal ions in the CA system and TA system respectively, which resulted in a stronger affinity between CA and iron. This study provides the underlying mechanism for Cr(III) solid oxidation by the ferrite method in an organic matter environment and is of great significance to prevent and control chromium pollution in the environment.

Mechanistic study of Cr (VI) removal by modified alginate/GO composite via synergistic adsorption and photocatalytic reduction

A novel composite material was prepared by blending graphene oxide into polyethyleneimine grafted sodium alginate. The synthesized material was investigated as adsorbent and photocatalyst for the removal of hexavalent chromium (Cr (VI)) from aqueous solutions. The composite material has shown remarkable removal efficiency for Cr (VI) in high initial concentration solutions as the removal rate reached 86.16% and 99.92% for adsorption and photoreduction, respectively. We discovered experimentally that the adsorption was dominated via electrostatic interaction while the blending of GO could contribute in stimulating electrons for the photoreduction process. Moreover, the photoreduction can alter the surface charge of chromium species, thus electrostatic repulsion could regenerating the active sites of composite spontaneously. The conduction band energy was calculated as -2.04 eV, which proved that blending GO can narrow the bandgap of the composite material, thus enhance the light response and the photoreduction ability towards Cr (VI).

Comparison of the photoactivity of several semiconductor oxides in floating aerogel and suspension systems towards the reduction of Cr(VI) under visible light

A massive amount of research has been done over the last three decades to develop photoactive materials which could be suitable for real-world use in water remediation sector. Water-floating photocatalysts could be one of the best options due to their technological characteristics in terms of efficiency and reasonability including a high oxygenation of the photocatalyst surface, a fully sunlight irradiation, easy recovery and reuse. In the present study, aerogel water-floating based materials were fabricated using poly(vinyl alcohol) and polyvinylidene fluoride as a polymer platform, and loaded with different semiconductors such as g-C₃N₄, MoO₃, Bi₂O₃, Fe₂O₃ or WO₃. The photocatalytic efficiencies of aerogel floating materials and the suspension of above-mentioned semiconductors were compared evaluating the photoreduction of Cr(VI) under visible light (λ > 420 nm). The results showed that Fe₂O₃ suspension was the most efficient but the slowest in floating system. On the contrary, g-C₃N₄ exhibited a good performance in suspension system, and on top of that it was very effective in floating system, wherein it ensures a total reduction of 10 ppm-Cr(VI) to Cr(III) within 20 min.