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•Photocatalytic degradation and mineralization kinetics of DEET were studied.•Degradation products were elucidated using HR-LC–MS and GC–MS techniques.•Hydroxy-methylcyclohexadienyl ...radicals were detected as initial DPs by EPR.•The acute toxicity of DEET and its TPs was assessed along photocatalytic treatment.
Analytical and electron paramagnetic resonance (EPR) spectroscopic methods were systematically used for the kinetic and mechanistic investigation of the photocatalytic degradation of N,N-diethyl-m-toluamide (DEET), in aqueous TiO2 suspensions under simulated solar light. The degradation of DEET followed first-order kinetics while enhanced reduction (>85%) of total organic carbon (TOC) and stoichiometric transformation of nitrogen to nitrate and ammonium ions took place after 240min of irradiation. Numerous different structures of transformation products (TPs), with at least one isomer for the majority of them, were identified with high resolution accurate mass liquid chromatography (HR-LC–MS) and gas chromatography mass spectrometry (GC–MS). Low temperature EPR spectroscopy was used to study the photoinduced radicals created during the initial events of the photocatalytic oxidation. Two kinds of aromatic ring carbon-centered radicals i.e. hydroxy-methylcyclohexadienyl radicals have been resolved at 77K. The second-transient conformation of the radicals is maximized after 5min. of irradiation and then slowly decays. On the basis of identified products and radicals, a proposed pathway of photocatalytic degradation of DEET is presented, involving mono- and polyhydroxylation and/or oxidation, dealkylation and continuously the opening of the aromatic ring. Scavenging experiments indicated the contribution of OH as the main species in the DEET oxidation while O2- contributes also to the degradation in a lesser extend after the initial steps of the reaction. Finally, toxicity studies based on luminescence of Vibrio fischeri bacteria before and after the photocatalytic treatment were also performed.
The MIL-100(Fe) metal–organic framework presents a high As(III) uptake capacity of 120 mg g–1. Mechanistic insights into the role of Fe sites versus carbon sites on As(III) uptake are provided by ...a comparative study of a series of MIL-100(Fe) calcinated at 600, 800, and 900 °C. Using powder X-ray diffraction, TEM, scanning electron microscopy, and N2-porosimetry, we have mapped the morphology evolution of the materials. Fourier transform infrared spectroscopy, thermogravimetric analysis, and electron paramagnetic resonance show that noncalcined MIL-100(Fe) bears Fe3+ atoms; however, after carbonization, a porous carbon matrix is formed bearing zero-valent iron cores coated with an Fe-oxide layer and iron carbide. The relative proportion of these phases depends on the calcination temperature, that is, 600, 800, and 900 °C. A comprehensive surface complexation model is presented, allowing a quantitative description of the As(III) adsorption on Fe sites and carbon sites. More specifically, As(III) uptake can be attributed to specific FeOH sites, located inside the pores and carbon C x OH2 sites located on the surface. Confinement inside the pores is found to be responsible for the lateral interactions among the adsorbed H3AsO3 species. The As(III) uptake of MIL-100(Fe) is 3- to 10-fold higher versus pertinent adsorbent materials, such as graphite/graphite oxide, activated carbon, and pyrolytic carbon, and comparable with that of MIL-101(Cr).
•Mn-catalysts were synthesized by covalent grafting onto activated carbon.•These catalysts are effective and selective towards epoxides with H2O2 and CH3COONH4.•They are re-usable and provide ...considerably high TOFs.•The function of CH3COONH4 which is determinant for the catalysis is proposed.
A new synthetic methodology to covalently anchor MnII-Schiff-base catalysts onto activated carbon (ACox) has been applied, resulting in heterogeneous MnII-L@ACox materials. These catalysts are effective and selective towards epoxides with H2O2, in the presence of CH3COONH4, as co-catalyst, providing TONs equivalent-to/or higher than the homologous MnII-L@SiO2 catalysts for certain substrates. Moreover MnII-L@ACox catalysts are re-usable and kinetically faster than the corresponding MnII-L@SiO2 catalysts resulting in considerably higher TOFs. Combining catalytic and EPR spectroscopic data we propose a catalytic reaction mechanism which elucidates the co-catalytic function of CH3COONH4 which is of key importance for the successful performance of the studied MnII-catalysts.
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•Investigation of sulfate radical generating oxidants with TiO2 photocatalysis.•HSO5− coupled with UVA/TiO2 was the most energy efficient system.•Addition of oxidants prolonged the ...life-time of the formed radicals.•New intermediates based on consecutive hydroxyl substitutions of MC-LR were detected.•Toxicity studies on the treated samples indicated loss of MC-LR toxic properties.
This study investigated the coupling of sulfate radical generating oxidants, (persulfate, PS and peroxymonosulfate, PMS) with TiO2 photocatalysis for the degradation of microcystin-LR (MC-LR). Treatment efficiency was evaluated by estimating the electrical energy per order (EEO). Oxidant addition at 4 mg/L reduced the energy requirements of the treatment by 60% and 12% for PMS and PS, respectively compared with conventional photocatalysis. Quenching studies indicated that both sulfate and hydroxyl radicals contributed towards the degradation of MC-LR for both oxidants, while Electron Paramagnetic Resonance (EPR) studies confirmed that the oxidants prolonged that lifetime of both radicals (concentration maxima shifted from 10 to 20 min), allowing for bulk diffusion and enhancing cyanotoxin removal. Structural identification of transformation products (TPs) formed during all treatments, indicated that early stage degradation of MC-LR occurred mainly on the aromatic ring and conjugated carbon double bonds of the ADDA amino acid. In addition, simultaneous hydroxyl substitution of the aromatic ring and the conjugated double carbon bonds of ADDA (m/z = 1027.5) are reported for the first time. Oxidant addition also increased the rates of formation/degradation of TPs and affected the overall toxicity of the treated samples. The detoxification and degradation order of the treatments was UVA/TiO2/PMS > UVA/TiO2/PS>> UVA/TiO2.
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•In situ growth of small magnetite nanoparticles at natural clay nanotubes (halloysite).•Novel, facile synthetic strategy involving a modified wet-impregnation method.•Efficient ...catalytic decomposition of pentachlorophenol by magnetite@halloysite hybrids.•Catalytic efficiency of hybrids significant higher compared to free nanoparticles.•Hybrids can be reused for multiple catalytic cycles without noteworthy loss of activity.
Halloysite clay are a very attractive class of alumino-silicate based, natural nanotubes possessing high-aspect ratio, significant thermal and mechanical stability, as well as tunable surface chemistry. We report a novel, facile, synthetic approach involving a modified wet-impregnation method for the in situ synthesis of small, magnetite nanoparticles at the surface of natural halloysite nanotubes. In addition to their magnetic properties, the synthesized magnetite-halloysite hybrids are evaluated for the first time against the catalytic decomposition of pentachlorophenol from reaction solutions at room temperature. Their performance was found superior compared to free, self-supported NPs synthesized with previously reported methods. Very interestingly, after their first catalytic evaluation cycle and because of their magnetic properties the hybrids could be easily recovered from their corresponding reaction solution. The halloysite-nanoparticle hybrids are also very promising in terms of sustainability, since we demonstrate that they can be re-collected, cleaned and re-used for multiple catalytic cycles without any significant loss in their catalytic activity.
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•Mn-catalysts were synthesized by covalent grafting onto SBA-15, MCM-41 and CMK-3.•These catalysts are effective and selective towards epoxides with H2O2 and CH3COONH4.•MnII-L@CMK-3 ...provides high TONs and is kinetically very fast demonstrating extremely high TOFs.
The development of new functional catalytic materials prepared via appropriate chemical modification of mesoporous silica SBA-15, MCM-41 or carbon nanomaterials CMK-3, are presented. Their synthesis has been carried out via two synthetic approaches: (a) a two steps procedure which includes grafting of the Schiff base ligand 1,3-bis3-aza-3-(1-methyl-3-oxobut-1-enyl)-prop-3-en-1-yl-2-(4-hydroxy-phenyl)-1,3-imidazolidine (L) onto the suppors and subsequent metalation of the so-formed hybrid material, and (b) an one step procedure which allows covalent grafting of the entire MnII-Schiff base catalyst onto the carbonaceous support. The resulting single-site heterogeneous catalysts were characterized and evaluated for alkene epoxidation with H2O2 in the presence of CH3COONH4 as additive. They are efficient and selective towards formation of epoxides. The highest TONs have been achieved by L@MCM-41-MnII and MnII-L@CMK-3. Moreover, MnII-L@CMK-3 is operative for a second use and kinetically very fast, demonstrating remarkably high TOFs 65–634h−1 that is correlated to its practically zero porosity. Based on the present data, the textural features of the obtained catalysts are discussed in correlation with their catalytic performance.
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•Comparative catalytic, spectroscopic study of homologous non-heme Mn- and Fe-oxidation catalysts.•EPR and Low-Temp. UV–vis allows detection of transient intermediate species and ...determination of thermodynamic barriers.•FeIIIOOH and MnIVO, rate-limiting intermediates respectively, face Ea=91kJ/mole and 55kJ/mole respectively.
Two sets of homologous Mn- and Fe-catalysts, MnIILCl2, FeIILCl2 and MnIIL(OAc)2, FeIIL(OAc)2 have been synthesized. A detailed comparative study of their catalytic oxidative performance with H2O2, in tandem with EPR and Low-Temperature UV–vis spectroscopies has been carried out. The Metal-L(OAc)2 and Metal-LCl2 catalysts did not show any difference in their catalytic behavior i.e. there is no effect of the labile ligands on the studied catalysis. It is found that the Mn-catalysts consistently outcompeted the homologous Fe-catalysts i.e. TOFs (Mn)=162 vs. TOFs (Fe)=16. We found that the Fe-catalyst faces a significantly higher activation barrier than the Mn-catalyst i.e. Ea(FeIIL(OAC)2)=91KJ/mol≫Ea(MnIIL(OAC)2)=55kJ/mole, while the free-energy difference, ΔG(FeIIL(OAC)2)∼ΔG(MnIIL(OAC)2)∼−145kJ/mole, did not make difference. Taken altogether the present data clarify that the main thermodynamic barrier, ultimately determining the overall catalytic performance, of these homologous Mn- and Fe-catalysts is the activation energy for the transient intermediates i.e. MnII to MnIVO for the Mn-catalysts and FeII to FeIIIOOH for the Fe-catalysts. A unified/consistent catalytic thermodymanic concept is discussed, that bears relevance to the catalytic behavior of many non-heme Mn- vs. Fe-oxidation catalysts.
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•We present the manufacturing and application of a novel material, PyrC350®.•PyrC350® is developed from pyrolytic-tire char (PyrC).•PyrC350® is an efficient low-cost Arsenite (AsIII) ...adsorbent.•A theoretical Surface Complexation Model has been developed that explains the adsorption mechanism.•We provide a comparison of AsIII-uptake effectiveness in conjunction with a cost analysis.
A novel material, PyrC350®, has been developed from pyrolytic-tire char (PyrC), as an efficient low-cost Arsenite As(III) adsorbent from water. PyrC350® achieves 31mgg−1 As(III) uptake, that remains unaltered at pH=4–8.5. A theoretical Surface Complexation Model has been developed that explains the adsorption mechanism, showing that in situ formed Fe3C, ZnS particles act cooperatively with the carbon matrix for As(III) adsorption. Addressing the key-issue of cost-effectiveness, we provide a comparison of As(III)-uptake effectiveness in conjunction with a cost analysis, showing that PyrC350® stands in the top of effectiveness/cost vs. existing carbon-based, low-cost materials.
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•Pyrolytic char/Ν,F-TiO2 and activated char/TiO2 catalysts were prepared by sol–gel.•Photocatalytic activity was tested towards phenol degradation in aqueous ...suspensions.•Char/N,F-TiO2 0.2/2 catalyst showed the highest photocatalytic efficiency.•Char/N,F-TiO2 catalysts showed a superior activity than activated char/TiO2.•Increased e−–h+ separation and visible light absorption was observed for Char/N,F-TiO2 catalysts.
Pyrolytic char/Ν-TiO2 (CTN), pyrolytic char/Ν-F-TiO2 (CTNF) and activated pyrolytic char-TiO2 (ACT) composite catalysts have been successfully prepared by a sol–gel method. TiO2 anatase was the major phase formed in all doped catalysts with a small fraction of brookite phase observed for CTN and ACT. UV–vis diffuse reflectance spectroscopy showed a decrease of band gap energy for all doped CTN and CTNF catalysts. The photocatalytic activity of the studied photocatalysts was systematically investigated under visible light and simulated solar light irradiation, by monitoring the degradation of phenol in aqueous suspensions. CTN and CTNF catalysts showed a better photocatalytic efficiency compared to ACT catalysts. The CTNF composite with a ratio of pyrolytic char/Ν-F-TiO2=0.2/2 was found to be the most effective catalyst for phenol removal. According to our results, the pyrolytic char presented favorable adsorption characteristics for phenol and acts as an electron-acceptor from doped-TiO2 particles resulting in decreased e− − h+ pair recombination and subsequently in enhanced production of HO. Moreover, it was revealed that Ν-F modification promotes the e− − h+ separation in TiO2 concomitantly with an enhancement in the visible spectral absorbance.
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•A Mn-catalyst was synthesized by covalent grafting onto pyrolytic carbon surface.•The pyrolytic carbon which used as supporting matrix was derived from waste tyres.•It was active, ...for degradation of methyl orange with NaIO4, stable and recyclable.
A MnII-Schiff-base catalyst was synthesized and covalently immobilized onto Pyrolytic Carbon surface from waste tyres (PCox), resulting in the heterogeneous catalyst MnII-L@PCox which was evaluated for degradation of methyl orange (MO) using NaIO4 as oxidant. Importantly, no additive as co-catalyst was needed. For comparison, a reference heterogeneous catalyst MnII-L@ACox was also tested, using Activated Carbon (ACox) as carbon matrix. The catalytic and recyclability data of MnII-L@PCox catalyst demonstrate that it is superior, in terms of TONs and TOFs vs. MnII-L@ACox. To study the reaction path, electron paramagnetic resonance spectroscopy was used to monitor the redox evolution of the Mn-centers. Furthermore, a full mapping of the catalytic degradation of MO and product formation was carried out using LC-MS and HPLC. Combining catalytic and spectroscopic data we discuss the protective effect of the PCox matrix on Mn-centers; it allows their rapid redox evolution to higher oxidation states.
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