Fe3O4, with a lattice parameter a = 8.357 Å and average particle size of 12.5 ± 3.6 nm, was successfullyencapsulated within a graphitic structure by a hierarchical co-assembly approach, followed by thermalannealing. The resulting material was denoted as MGNC—magnetic graphitic nanocomposite. MGNC pos-sesses average core size of 109 ± 35 nm (mainly composed by agglomerates of magnetic nanoparticles),stability up to 400◦C under oxidizing atmosphere, a micro-mesoporous structure with a fairly developedspecific surface area (SBET= 330 m2g−1) and neutral character (pHPZC= 7.1).Catalytic wet peroxide oxidation (CWPO) experiments performed with a 4-nitrophenol (4-NP)/Fe3O4mass ratio fixed at 36.6, allowed to achieve high efficiency of catalyst usage throughout the wide range of4-NP concentration considered (200 mg L−1–5 g L−1). The inclusion of Fe3O4nanoparticles in a graphiticstructure during the synthesis of MGNC was found to (i) enhance the catalytic activity in CWPO whencompared to Fe3O4, due to increased adsorptive interactions between the surface of the catalyst andthe pollutant molecules, while (ii) strongly limiting the leaching of Fe species from Fe3O4to the treatedwater, due to the confinement effect caused by the carbon shell.As a result of these effects, unprecedented pollutant mass removals were obtained − rangingfrom 5000 mg g−1h−1, when the CWPO process is performed with [4-NP]0= 200 mg L−1at pH = 3, to1250 mg g−1h−1, when [4-NP]0= 5 g L−1. High efficiency of H2O2consumption is obtained when MGNC isapplied in the CWPO of 4-NP solutions at pH = 3, with TOC removals per unit of H2O2decomposed ( H2O2)in the range 64–100%. In addition, the MGNC catalyst is also active at pH = 6; in this case a pollutant massremoval of 2090 mg g−1h−1was obtained.Although MGNC partially deactivates through successive reusability cycles, the pollutant mass removalobtained at the end of the fourth cycle is still very high when 200 mg L−14-NP solutions are considered(4808 mg g−1h−1, representing only a ca. 4% decrease when compared to the first cycle). A higher deac-tivation of the MGNC catalyst is observed when 5 g L−14-NP solutions are employed. Nevertheless, thepollutant mass removal obtained at the end of the third cycle is still high (551 mg g−1h−1).
This work was financially supported by: Project POCI-01-0145-FEDER-006984 – Associate Laboratory LSRE-LCM funded by FEDER through COMPETE2020 – Programa Operacional Competitividadee Internacionalização (POCI) – and by national funds through FCT –Fundação para a Ciência e a Tecnologia. R.S. Ribeiro acknowledgesthe FCT individual Ph.D. grant SFRH/BD/94177/2013, with financing from FCT and the European Social Fund (through POPH and QREN).A.M.T. Silva acknowledges the FCT Investigator 2013 Programme(IF/01501/2013), with financing from the European Social Fund andthe Human Potential Operational Programme.
