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[en] Hydroxyapatite (HA, Ca10(PO4)6(OH)2) nanoparticles were synthesized using calcining calcium dihydrogenphosphate (Ca(H2PO4)2 . H2O), calcium hydroxide (Ca(OH)2), and polyethylene glycol (PEG) at 900 deg. C in an oxygen atmosphere. This one-step process yields HA nanoparticles with similar particle sizes (e.g., 50-80 nm) that are well-crystallized and non-aggregated. PEG is an important factor in controlling the particle size, crystal phase, and degree of aggregation in these HA particles. This conclusion is supported by results from a field-emission scanning electron microscope (FE-SEM), X-ray diffractometry (XRD), energy dispersive X-ray analysis (EDS), a high-resolution transmission electron microscope (HR-TEM), and dynamic light scattering (DLS).
[en] Landfill, a matured and economically appealing technology, is the ultimate approach for the management of municipal solid wastes. However, the inevitable generation of leachate from landfill requires further treatment. Among the various leachate treatment technologies available, advanced oxidation processes (AOPs) are among powerful methods to deal with the refractory organic constituents, and the Fenton reagent has evolved as one promising AOPs for the treatment of leachates. Particularly, the combination of UV-radiation with Fenton's reagent has been reported to be a method that allows both the photo-regeneration of Fe2+ and photo-decarboxylation of ferric carboxylates. In this study, Fenton and photo-Fenton processes were fine tuned for the treatment of leachates from the Colmenar Viejo (Madrid, Spain) Landfill. Results showed that it is possible to define a set of conditions under which the same COD and TOC removals (approx 70%) could be achieved with both the conventional and photo-Fenton processes. But Fenton process generated an important quantity of iron sludge, which will require further disposal, when performed under optimal COD removal conditions. Furthermore conventional Fenton process was able to achieve slightly over an 80% COD removal from a 'young' leachate, while for 'old' and 'mixed' leachates was close to a 70%. The main advantage showed by the photo-assisted Fenton treatment of landfill leachate was that it consumed 32 times less iron and produced 25 times less sludge volume yielding the same COD removal results than a conventional Fenton treatment.
[en] The photoassisted reduction of metal ions and organic dye by metal-deposited Degussa P25 TiO2 nanoparticles was investigated. Copper and silver ions were selected as the target metal ions to modify the surface properties of TiO2 and to enhance the photocatalytic activity of TiO2 towards methylene blue (MB) degradation. X-ray powder diffraction (XRPD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) were used to characterize the crystallinity, chemical species and morphology of metal-deposited TiO2, respectively. Results showed that the particle size of metal-deposited TiO2 was larger than that of Degussa P25 TiO2. Based on XRPD patterns and XPS spectra, it was observed that the addition of formate promoted the photoreduction of metal ion by lowering its oxidation number, and subsequently enhancing the photodegradation efficiency and rate of MB. The pseudo-first-order rate constant (kobs) for MB photodegradation by Degussa P25 TiO2 was 3.94 x 10-2 min-1 and increased by 1.4-1.7 times in kobs with metal-deposited TiO2 for MB photodegradation compared to simple Degussa P25 TiO2. The increase in mass loading of metal ions significantly enhanced the photodegradation efficiency of MB; the kobs for MB degradation increased from 3.94 x 10-2 min-1 in the absence of metal ion to 4.64-7.28 x 10-2 min-1 for Ag/TiO2 and to 5.14-7.61 x 10-2 min-1 for Cu/TiO2. In addition, the electrons generated from TiO2 can effectively reduce metal ions and MB simultaneously under anoxic conditions. However, metal ions and organic dye would compete for electrons from the illuminated TiO2.
[en] Highlights: • PFOA removal and defluorination with AC/PS are 12 and 19 times higher than PS only • AC can activate PS to accelerate the decomposition and mineralization of PFOA. • With AC/PS, a lower reaction temperature and a shorter reaction time would suffice • A 2-cycle schematic reaction mechanism was proposed to describe PS oxidation of PFOA. -- Abstract: Treatment of persistent perfluorooctanoic acid (PFOA) in water using persulfate (PS) oxidation typically requires an elevated temperature or UV irradiation, which is energy-consuming. Under relatively low temperatures of 25–45 °C, activated carbon (AC) activated PS oxidation of PFOA was evaluated for its potential of practical applications. With presence of AC in PS oxidation, PFOA removal efficiency at 25 °C reached 682% with a high defluorination efficiency of 549% after 12 h and few intermediates of short-chain perfluorinated carboxylic acids (PFCAs) were found. The removal and defluorination rates with the combined AC/PS system were approximately 12 and 19 times higher than those of the PS-only system, respectively. Activated carbon not only removes PFOA through adsorption, but also activates PS to form sulfate radicals that accelerate the decomposition and mineralization of PFOA. The activation energy for PS oxidation of PFOA was reduced from 668 to 261 kJ/mol by the catalytic effect of AC, which implies a lower reaction temperature and a shorter reaction time would suffice. A 2-cycle schematic reaction mechanism was used to describe PS oxidation of PFOA with the generation of various intermediates and end-products