RESUMO
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.
RESUMO
This study aims to evaluate the activity concentrations of 238U, 226Ra, 232Th, 228Th and 40K along beaches close to the plutonic rocks of the Atticocycladic zone that ranged from 15 to 628, 12-2292, 16-10,143, 14-9953 and 191-1192 Bq/kg respectively. A sample from island of Mykonos contained the highest 232Th content measured in sediments of Greece. The heavy magnetic fraction and the heavy non-magnetic fraction as well as the total heavy fraction, were correlated with the concentrations of the measured radionuclides in the bulk samples. The heavy fractions seem to control the activity concentrations of 238U and 232Th of all the samples, showing some local differences in the main 238U and 232Th mineral carrier. Similar correlations have been found between 238U, 232Th content and rare earth elements concentrations. The measured radionuclides in the beach sands were normalized to the respective values measured in the granitic rocks, which at least in most cases are their most probable parental rocks, so as to provide data upon their enrichment or depletion. Since the Greek beaches are among the most popular worldwide the annual effective dose equivalent received due to sand exposure has been estimated and found to vary between 0.002 and 0.379 mSv y-1 for tourists and from 0.018 to 3.164 mSv y-1 for local people working on the beach. The values corresponding to ordinary sand samples are orders of magnitude lower than the limit of 1 mSv y-1, only in the case of heavy minerals-rich sands the dose could reach or exceed the recommended maximum limit.