Catalytic wet peroxide oxidation: a route towards the application of hybrid magnetic carbon nanocomposites for the degradation of organic pollutants. A review

Several motivations have prompted the scientific community towards the application of hybrid magnetic carbon nanocomposites in catalytic wet peroxide oxidation (CWPO) processes. The most relevant literature on this topic is reviewed, with a special focus on the synergies that can arise from the combination of highly active and magnetically separable iron species with the easily tuned properties of carbon-based materials. These are mainly ascribed to increased adsorptive interactions, to good structural stability and low leaching levels of the metal species, and to increased regeneration and dispersion of the active sites, which are promoted by the presence of the carbon-based materials in the composites. The most significant features of carbon materials that may be further explored in the design of improved hybrid magnetic catalysts are also addressed, taking into consideration the experimental knowledge gathered by the authors in their studies and development of carbon-based catalysts for CWPO. The presence of stable metal impurities, basic active sites and sulphur-containing functionalities, as well as high specific surface area, adequate porous texture, adsorptive interactions and structural defects, are shown to increase the activity of carbon materials when applied in CWPO, while the presence of acidic oxygen-containing functionalities has the opposite effect.

Hybrid magnetic graphitic nanocomposites for catalytic wet peroxide oxidation applications

Hybrid magnetic graphitic nanocomposites (MGNC) prepared by inclusion of magnetite nanoparticles (obtained by coprecipitation) into an organic-organic self-assembly system, followed by calcination, revealed high activity for the catalytic wet peroxide oxidation (CWPO) of 4-nitrophenol solutions (5 g L-l), with pollutant removais up to 1245 mg g-' h-l being obtained when considering the mass ratio [pollutant]/[catalyst] =10. The stability of the MGNC catalyst against metal leaching was ascribed to the confinement effect of the carbon based material. These observations, together with the magnetically recoverable characteristics of MGNC, open new prospects for the wide use of this catalyst in highly efficient CWPO applications.

Core-shell nanocomposites prepared by hierarchical co-assembly: the role of the carbon shell in catalytic wet peroxide oxidation processes

The diffraction pattern of Fe3O4 (not shown) confirmed the presence of only one phase, corresponding to magnetite with a lattice parameter a = 8.357 Å and a crystallite size of 16.6 ± 0.2 nm. The diffraction pattern of MGNC (not shown) confirmed the presence of a graphitic phase, in addition to the metal phase, suggesting that Fe3O4 nanoparticles were successfully encapsulated within a graphitic structure during the synthesis of MGNC. The core-shell structure of MGNC is unequivocally demonstrated in the TEM micrograph shown in Fig. 1b. Characterization of the MGNC textural and surface chemical properties revealed: (i) stability up to 400 oC under oxidizing atmosphere; (ii) 27.3 wt.% of ashes (corresponding to the mass fraction of Fe3O4); (iii) a micro-mesoporous structure with a fairly well developed specific surface area (SBET = 330 m2 g-1); and (iv) neutral character (pHPZC = 7.1). In addition, the magnetic nature of MGNC (Fig. 2) is an additional advantage for possible implementation of in situ magnetic separation systems for catalyst recovery.

Development of magnetic carbon xerogels for catalytic wet peroxide oxidation

Hybrid magnetic carbon composites have been recently proposed as the next step in the evolution of catalysts for catalytic wet peroxide oxidation (CWPO), with several synergistic effects arising from the combination of the high catalytic activity of metal species with the proven catalytic properties of carbon-based materials in CWPO [1]. Bearing this in mind, this work sought the development of novel magnetic carbon xerogels, composed by interconnected carbon microspheres with iron (Fe) and/or cobalt (Co) microparticles embedded in their structure. As inferred from the extensive characterization performed, materials with distinctive properties were obtained upon inclusion of different metal precursors during the sol-gel polymerization of resorcinol and formaldehyde, followed by thermal annealing.

Role of nitrogen doping on the performance of carbon nanotube catalysts: a catalytic wet peroxide oxidation application

Four magnetic carbon nanotube samples (CNTs: undoped, completely N-doped and two selectively N-doped) have been synthesized by chemical vapor deposition. The materials were tested in the catalytic wet peroxide oxidation (CWPO) of highly concentrated 4 nitrophenol solutions (4-NP, 5 g L-1). Relatively mild operating conditions were considered (atmospheric pressure, T = 50 ºC, pH = 3), using a catalyst load of 2.5 g L-1 and the stoichiometric amount of H2O2 needed for the complete mineralization of 4-NP. N doping was identified to influence considerably the CWPO performance of the materials. In particular, undoped CNTs, with a moderate hydrophobicity, favor the controllable and efficient decomposition of H2O2 into highly reactive hydroxyl radicals (HO•), thus showing high catalytic activity for 4-NP degradation. On the other hand, the completely N-doped catalyst, fully hydrophilic, favors a quick decomposition of H2O2 into non-reactive O2 and H2O species. The selectively N-doped amphiphilic catalysts, i.e. hybrid structures containing undoped sections followed by N-doped ones, provided intermediate results, namely: a higher N content favored H2O2 decomposition towards non-reactive H2O and O2 species, whilst a lower N content resulted in the formation of HO•, increasing 4-NP mineralization. Catalyst stability and reusability were also investigated by consecutive CWPO runs.

Magnetic carbon xerogels for the catalytic wet peroxide oxidation of sulfamethoxazole in environmentally relevant water matrices

Novel magnetic carbon xerogels consisting of interconnected carbon microspheres with iron and/or cobalt microparticles embedded in their structure were developed by a simple route. As inferred from the characterization data, materials with distinctive properties may be directly obtained upon inclusion of iron and/or cobalt precursors during the sol-gel polymerization of resorcinol and formaldehyde, followed by thermal annealing. The unique properties of these magnetic carbon xerogels were explored in the catalytic wet peroxide oxidation (CWPO) of an antimicrobial agent typically found throughout the urban water cycle – sulfamethoxazole (SMX). A clear synergistic effect arises from the inclusion of cobalt and iron in carbon xerogels (CX/CoFe),the resulting magnetic material revealing a better performance in the CWPO of SMX at the ppb level(500 microg L−1) when compared to that of monometallic carbon xerogels containing only iron or cobalt.This effect was ascribed to the increased accessibility of highly active iron species promoted by the simultaneous incorporation of cobalt.The performance of the CWPO process in the presence of CX/CoFe was also evaluated in environmentally relevant water matrices, namely in drinking water and secondary treated wastewater, considered in addition to ultrapure water. It was found that the performance decreases when applied to more complex water and wastewater samples. Nevertheless, the ability of the CWPO technology for the elimination of SMX in secondary treated wastewater was unequivocally shown, with 96.8% of its initial content being removed after 6 h of reaction in the presence of CX/CoFe, at atmospheric pressure, room temperature(T = 25◦C), pH = 3, [H2O2]0= 500 mg L−1and catalyst load = 80 mg L−1. A similar performance (97.8% SMX removal) is obtained in 30 min when the reaction temperature is slightly increased up to 60◦C in an ultra-pure water matrix. Synthetic water containing humic acid, bicarbonate, sulphate or chloride, was also tested. The results suggest the scavenging effect of the different anions considered, as well as the negative impact of dissolved organic matter typically found in secondary treated wastewater, as simulated by the presence of humic acid.An in-situ magnetic separation procedure was applied for catalyst recovery and re-use during reusability cycles performed to mimic real-scale applications. CWPO runs performed with increased SMX concentration (10 mg L−1), under a water treatment process intensification approach, allowed to evalu-ate the mineralization levels obtained, the antimicrobial activity of the treated water, and to propose adegradation mechanism for the CWPO of SMX.

Carbon nanotubes as catalysts for wet peroxide oxidation: structure-reactivity relationships

Magnetic neat and N-doped carbon nanotubes with different properties have been synthesized by chemical vapour deposiüon and tested in the catalytic wet peroxide oxidation of 4-nitrophenol solutions (5 g L') at relatively mild operating conditions (atmospheric pressure, T = 50 °C, pH = 3)~using a catalyst load = 2.5 g L-' and [H202]o = 17.8 g L-1. The results demonstrate that the catalyst hydrophobicity/ hydrophilicity is a detenninant property in the CWPO reaction, since it affects the rate ofH202 decomposition. The controlled formation ofreactive radicais (HO* and HOO*) at hydrophobic surfaces avoids the formation of non-reactive species (02 and H20), increasing.

Metal-free catalysts derived from lignin for efficient wet peroxide oxidation

In this work, a wheat and hemp lignin (Sarkanda, Granit S.A.) has been used as raw material for the development of metal-free activated carbons. These materials were tested in the catalytic wet peroxide oxidation (CWPO) of 4-nitrophenol (4-NP; 5 g L-1) during 24 h experiments conducted at relatively mild operating conditions (p = 1 atm, t = 50 °C, pH = 3, catalyst load = 2.5 g L-1 and [H2O2]0 = 17.8 g L-1). First, the lignin was carbonized under N2 atmosphere followed by the activation of the obtained non-porous carbon (LG) under air atmosphere at different temperatures (150 to 350 ºC), leading to the generation of significant porosity.