Synthesis of in-situ Al3+-defected iron oxide nanoflakes from coal ash: A detailed study on the structure, evolution mechanism and application to water remediation

The recovery of value-added materials from coal ash waste is of highly economic value and sustainable significance. However, researches on the synthesis of defect-engineering nanomaterials from coal ash are still blank. Herein, iron oxide (Fe_1.72Al_0.28O_3, simplified as FAO) nanoflakes were successfully synthesized from a brown coal fly ash (BCFA) waste. The obtained FAO nanoflakes possess a round-shape morphology with a diameter of around 300 nm and 50 nm in thickness. With the progress of hydrothermal treatment, the impure Al³⁺ gradually replaced part of the Fe³⁺ in the α-Fe₂O₃ crystal. Specifically, Al³⁺ was preferentially adsorbed on the (001) facet, hindering the growth of Fe³⁺ on the [001] direction and thus causing the flattening of the resultant FAO. The introduced Al³⁺ also serves as the disordered defects on the hematite surface, leading to decreased crystal parameters for hematite, the formation of a compact first shell and a reduced periodical symmetry for the central cation Fe³⁺. The defects were also found to significantly improve the adsorption capacity of the resultant FAO for Cr(VI), As(V), As(III) and Congo red in waste water, with the maximum adsorption capacity of 68.3, 80.6, 61.1 and 213.8 mg g^-1, respectively. Cyclic tests also confirmed a relatively strong stability for the as-synthesised adsorbents.

Synthesis of zeolites Na-A and Na-X from tablet compressed and calcinated coal fly ash

Zeolites Na-A and Na-X are important synthetic zeolites widely used for separation and adsorption in industry. It is of great significance to develop energy-efficient routines that can synthesize zeolites Na-A and Na-X from low-cost raw materials. Coal fly ash (CFA) is the major residue from the combustion of coal and biomass containing more than 85% SiO₂ and Al₂O₃, which can readily replace the conventionally used sodium silicate and aluminate for zeolite synthesis. We used Na₂CO₃ to replace the expensive NaOH used for the calcination of CFA and showed that tablet compression can enhance the contact with Na₂CO₃ for the activation of CFA through calcination for the synthesis of zeolites Na-A and Na-X under mild conditions. We optimized the control variables for zeolite synthesis and showed that phase-pure zeolite Na-A can be synthesized with CFA at reactant molar ratio, hydrothermal reaction temperature and reaction time of 1.3Na₂O: 0.6Al₂O₃: 1SiO₂: 38H₂O at 80°C for 6 h, respectively, while phase-pure zeolite Na-X can be synthesized at 2.2Na₂O: 0.2Al₂O₃: 1SiO₂: 88H₂O at 100°C for 8 h, respectively. The composition, morphology, specific surface area, vibration spectrum and thermogravimetry of synthesized Na-A and Na-X were further characterized.

Synthesis of zeolites Na-A and Na-X from tablet compressed and calcinated coal fly ash

Zeolites Na-A and Na-X are important synthetic zeolites widely used for separation and adsorption in industry. It is of great significance to develop energy-efficient routines that can synthesize zeolites Na-A and Na-X from low-cost raw materials. Coal fly ash (CFA) is the major residue from the combustion of coal and biomass containing more than 85% SiO₂ and Al₂O₃, which can readily replace the conventionally used sodium silicate and aluminate for zeolite synthesis. We used Na₂CO₃ to replace the expensive NaOH used for the calcination of CFA and showed that tablet compression can enhance the contact with Na₂CO₃ for the activation of CFA through calcination for the synthesis of zeolites Na-A and Na-X under mild conditions. We optimized the control variables for zeolite synthesis and showed that phase-pure zeolite Na-A can be synthesized with CFA at reactant molar ratio, hydrothermal reaction temperature and reaction time of 1.3Na₂O: 0.6Al₂O₃: 1SiO₂: 38H₂O at 80°C for 6 h, respectively, while phase-pure zeolite Na-X can be synthesized at 2.2Na₂O: 0.2Al₂O₃: 1SiO₂: 88H₂O at 100°C for 8 h, respectively. The composition, morphology, specific surface area, vibration spectrum and thermogravimetry of synthesized Na-A and Na-X were further characterized.