Combining mechanisms of black carbon and magnetic minerals in power plant fly ash

Black carbon (BC), one of the pollutants emitted from fossil fuel combustion, is closely associated with minerals and other hazardous substances. To date, little is known about the mechanisms between BC and magnetic minerals. Accordingly, further investigating the association between magnetic minerals and BC is necessary. In this work, the extraction of BC from fly ash and the magnetic fraction from BC was achieved by flotation and magnetic separation, respectively. The morphology, mineralogical composition, and magnetic properties of BC and magnetic fraction were characterized by FTIR, XRD, SEM-EDS, and vibrating sample magnetometer (VSM). The results show that BC and magnetic minerals have similar mineral compositions, rich in quartz, mullite, magnetite, and hematite. The magnetic minerals have prominent spherical characteristics and are distributed on the surface and inside the pores of BC with irregular honeycomb features. The VSM and XRD analyses show that Fe3O4 is the primary magnetic material. Moreover, large amounts of C, O, and Fe around and on the surface of magnetic spheres were detected by EDS, indicating that the spherical particles may be the structure of BC-coated Fe3O4. Pyrolysis experiments showed that the yield of the magnetic fraction in the pyrolysis product reached 60 %, far exceeding the theoretical yield of 12 % based on 5 % of doped Fe. This further proves that Fe3O4 was combined with a large number of organics during its formation, which may be due to coating and chemical adsorption. Quantum chemical calculations also confirmed this chemical adsorption between Fe3O4 with BC based on density flooding theory, in which adsorption energies ranged from -213.374 KJ/mol to -827.741 KJ/mol.