Microwave-induced Degradation Research Articles

Cartap wastewater treatment by a microwave-induced catalytic oxidation procedure over Ce/Co/γ-Al2O3

Microwave-assisted catalytic technology is potentially promising in the application of water treatment due to its merits of high reaction rates, simple equipment and operations. In case of the cartap wastewater rarely studied, the easy transformation of cartap into metabolites could induce higher toxicity. This study aimed to develop a novel catalyst Ce/Co/γ-Al2O3 to dispose cartap effectively. The catalyst was prepared by loading Ce and Co on the surface of the support Al2O3, followed by characterization with X-ray diffraction, scanning and transmission electron microscopies. Based on the treatment results, the microwave-induced catalytic oxidation procedure over Ce/Co/γ-Al2O3 could decompose the contaminants efficiently and fast in the case of a low oxidant dosage and a wide pH range. Under the conditions of pH of 5, microwave power of 450 W, catalyst dosage of 0.7 g and hydrogen peroxide dosage of 0.2 mL, the activity of Ce/Co/γ-Al2O3 catalyst could reach up to 86.08%. Importantly, the Ce/Co/γ-Al2O3 catalyst displayed excellent stability after using for 6 times in the degradation process. This work paved a new way for the effective catalyst design for efficient microwave-induced oxidative degradation.

Microwave-induced degradation as a novel treatment for destruction of decabromodiphenyl ether sorbed on porous minerals

• Highly hydrophobic BDE-209 sorbed efficiently on the porous minerals evaluated. • Microwave irradiation caused rapid degradation of BDE-209 sorbed on porous minerals. • Surface cations in mineral micropores greatly impacted degradation of sorbed BDE-209. • Full debromination of sorbed BDE-209 occurred under continuous microwave irradiation. • The treatment is promising for full destruction of BDE-209 and other PBDE congeners. Decabromodiphenyl ether (deca-BDE or BDE-209) is a major brominated flame retardant released from plastics in thermal processing of e-waste, and there is a significant need for developing effective technologies for its destruction. This study reports a novel treatment for complete debromination and destruction of BDE-209 by adsorbing it onto porous minerals followed by microwave irradiation. Being highly hydrophobic, BDE-209 could be sorbed efficiently by porous mineral sorbents, with the density and type of surface cations of the sorbents apparently playing little role. Microwave irradiation caused rapid degradation of the BDE-209 sorbed on the porous minerals, and the fastest degradation occurred on the microporous minerals, with the density and type of surface cations present in the mineral micropores had significant impact on the degradation rate. Evolution of the degradation intermediates during the course of microwave irradiation indicates that the sorbed BDE-209 underwent pyrolytic decomposition, involving a series of reactions, including cleavage of the ether bond, debromination, hydroxylation, ring cleavage, and oxidation. Microwave-induced degradation could cause full mineralization of the sorbed BDE-209 without producing lower brominated congeners, which is supported by the results of density functional theory (DFT) calculations and infrared spectra. Together, these findings demonstrate that microporous mineral sorption coupled with microwave irradiation could be a rapid and efficient technology for destruction of BDE-209 and other polybrominated diphenyl ethers (PBDEs).

Synthesis of a Novel Catalyst MnO/CNTs for Microwave-Induced Degradation of Tetracycline

Microwave-induced catalytic degradation (MICD) has been considered as one of the most prospective approaches to remove organic contaminants from water. High-performance catalysts, ideally offering efficient degradation ability, are essential to this process. This work reports the fabrication of manganese oxide on carbon nanotubes (MnO/CNTs) as an efficient catalyst under microwave irradiation (MI) to remove tetracycline (TC) from aqueous solution. The hybrid MnO/CNTs structure shows excellent performance in TC degradation. Combining experimental characterization and theoretical calculations, synergistic mechanisms are revealed: (i) Strong MnO/CNTs interaction stabilizes Mn(II) through interfacial bonding; (ii) high-spin states associated with low coordinated Mn(II) play a major role in MICD; and (iii) superoxide radicals (•O2−) and hydroxyl radicals (•OH) induced by microwave input are identified as the major active species.

Microwave-induced degradation of N-nitrosodimethylamine (NDMA) sorbed in zeolites: Effect of mineral surface chemistry and non-thermal effect of microwave

N-nitrosodimethylamine (NDMA), a highly toxic and carcinogenic disinfection byproduct, cannot be efficiently removed by conventional water treatment processes, while sorption by microporous minerals, followed by destruction with microwave irradiation has been developed as a promising alternative. This work examined the impact of surface cation density and type on sorption of NDMA on dealuminated ZSM-5 zeolites, and on microwave-induced degradation of the sorbed NDMA, as well as the non-thermal effect of microwave. NDMA sorption was found to increase with the framework Si/Al ratio of dealuminated ZSM-5 zeolites (i.e., decreasing density of surface cations), and was also influenced by the type of surface cations. Degradation of the sorbed NDMA proceeded faster with increases in the power level of microwave and in the micropores with higher densities of surface cations, which was attributed to the increases in the number and/or temperature of micro-scale “hot spots” formed by microwave dielectric superheating of the surface cations. The degradation rate of sorbed NDMA was also found to be inversely correlated with the hydration free energy of surface cations present in the zeolite micropores. The apparent activation energy estimated from the temperature dependence of NDMA degradation in the micropores under microwave irradiation was 18.2 kJ/mol, which is much lower than those of typical thermolysis reactions and is indicative of significant contribution from the non-thermal effect of microwave.

Degradation of N-nitrosodimethylamine (NDMA) and its precursor dimethylamine (DMA) in mineral micropores induced by microwave irradiation

Removal of N-nitrosodimethylamine (NDMA) in drinking water treatment poses a significant technical challenge due to its small molecular size, high polarity and water solubility, and poor biodegradability. Degradation of NDMA and its precursor, dimethylamine (DMA), was investigated by adsorbing them from aqueous solution using porous mineral sorbents, followed by destruction under microwave irradiation. Among the mineral sorbents evaluated, dealuminated ZSM-5 exhibited the highest sorption capacities for NDMA and DMA, which decreased with the density of surface cations present in the micropores. In contrast, the degradation rate of the sorbed NDMA increased with the density of surface cations under microwave irradiation. Evolutions of the degradation products and C/N ratio indicate that the sorbed NDMA and DMA could be eventually mineralized under continuous microwave irradiation. The degradation rate was strongly correlated with the bulk temperature of ZSM-5 and microwave power, which is consistent with the mechanism of pyrolysis caused by formation of micro-scale “hot spots” within the mineral micropores under microwave irradiation. Compared to existing treatment options for NDMA removal, microporous mineral sorption coupled with microwave-induced degradation has the unique advantages of being able to simultaneously remove NDMA and DMA and cause their full mineralization, and thus could serve as a promising alternative method.

Performance of a novel microwave-based treatment technology for atrazine removal and destruction: Sorbent reusability and chemical stability, and effect of water matrices

Transition metal-exchanged dealuminated Y zeolites were used to adsorb atrazine from aqueous solutions, followed by regeneration of the sorbents and destruction of the sorbed atrazine with microwave irradiation. Exchange of copper and iron into the zeolite's micropores significantly enhanced its sorption capacity and selectivity toward atrazine, and increased the microwave-induced degradation rate of the sorbed atrazine by 3–4-folds. Both the copper- and iron-exchanged zeolites could be regenerated and reused multiple times, while the catalytic activity of the latter was more robust due to the much greater chemical stability of Fe3+ species in the micropores. The presence of humic acid, and common cations and anions had little impact on the sorption of atrazine on the transition metal-exchanged zeolites. In the treatment of atrazine spiked in natural surface water and groundwater samples, sorptive removal of atrazine was found to be impacted by the level of dissolved organic carbon, probably through competition for the micropore spaces and pore blocking, while the water matrices exhibited no strong effect on the microwave-induced degradation of sorbed atrazine. Overall, iron-exchanged dealuminated Y zeolites show great potential for removal and destruction of atrazine from contaminated surface water and groundwater in practical implementation of the novel treatment technology.

Catalytic effect of transition metals on microwave-induced degradation of atrazine in mineral micropores

With their high catalytic activity for redox reactions, transition metal ions (Cu2+ and Fe3+) were exchanged into the micropores of dealuminated Y zeolites to prepare effective microporous mineral sorbents for sorption and microwave-induced degradation of atrazine. Due to its ability to complex with atrazine, loading of copper greatly increased the sorption of atrazine. Atrazine sorption on iron-exchanged zeolites was also significantly enhanced, which was attributed to the hydrolysis of Fe3+ polycations in mineral micropores and electrostatic interactions of protonated atrazine molecules with the negatively charged pore wall surface. Copper and iron species in the micropores also significantly accelerated degradation of the sorbed atrazine (and its degradation intermediates) under microwave irradiation. The catalytic effect was attributed to the easy reducibility and high oxidation activity of Cu2+ and Fe3+ species stabilized in the micropores of the zeolites. It was postulated that the surface species of transition metals (monomeric Cu2+, Cu2+–O–Cu2+ complexes, FeO+, and dinuclear Fe–O–Fe-like species) in the mineral micropores were thermally activated under microwave irradiation, and subsequently formed highly reactive sites catalyzing oxidative degradation of atrazine. The transition metal-exchanged zeolites, particularly the iron-exchanged ones, were relatively stable when leached under acidic conditions, which suggests that they are reusable in sorption and microwave-induced degradation. These findings offer valuable insights on designing of effective mineral sorbents that can selectively uptake atrazine from aqueous solutions and catalyze its degradation under microwave irradiation.

Impact of Surface Chemistry on Microwave-Induced Degradation of Atrazine in Mineral Micropores

Surface chemistry determines the interactions of sorbate and solvent molecules with the pore wall surfaces of microporous minerals, and affects the transmission and absorption of microwave radiation for a given solvent-sorbate-sorbent system. The sorption and microwave-induced degradation of atrazine in the micropores of nine Y zeolites with different densities (0.16-2.62 site/nm(2)) and types (Mg(2+), Ca(2+), H(+), Na(+), and NH(4)(+)) of surface cations were studied. The influence of the content of cosorbed water in the mineral micropores on atrazine degradation rate was also examined. The results indicate the presence of surface cations at around 0.23 site/nm(2) on the pore wall surface was optimal for atrazine degradation, probably due to formation of insufficient number of "hot spots" with too few cations but excessive competition for microwave energy with too many hydrated cations. Atrazine degraded faster in the presence of cations with lower hydration free energies, which could be attributed to less microwave energy consumption to desorb the bounded water molecules. Reducing the content of coadsorbed water in the micropores also increased atrazine degration rate because of less competition for microwave energy from water. Such mechanistic understanding can guide the design and selection of microporous minerals in the practical application of microwave-induced degradation.

Microwave-induced Degradation of Chlorobenzene Using Fe0 and TiO2 Coated on Cordierites as Catalyst

In this research, MW is used to irradiate and induce Fe0/cordierite (Fe0 coated cordierite particles) and TiO2/cordierite (TiO2 coated cordierite particles) for improving the catalytic decomposition rate to remove chlorobenzene (CB) dissolved in aqueous solution. Laboratory results show that when 30 W of microwave energy is applied for 300 sec the irradiation of microwave than without microwave irradiation improves the CB removal efficiency by 3.2 times (57.4% vs. 18.2%) for Fe0/cordierite and 2.7 times (43.3% vs. 15.8%) for TiO2/cordierite. Hence, applying the microwave induced cordierite coated Fe0 or TiO2 particles is a new application that has been proved effective to remove chlorine-containing organic pollutants.

Efficient degradation of crystal violet in magnetic CuFe2O4 aqueous solution coupled with microwave radiation

Nanoscale copper ferrite was prepared by co-precipitation method, while citrate acid assisted method was used as reference. Microwave-induced degradation of crystal violet was performed with synthesized copper ferrite, and the behavior of copper ferrite in this process was studied by X-ray photoelectron spectroscopy, SEM/EDS and vector network analyzer. Microwave radiation could greatly enhance the activity of copper ferrite in organic oxidation. The variant of copper and iron on the surface and in the inner core of copper ferrite was studied here. Copper ferrite presents relatively low dielectric loss. Meanwhile, microwave radiation makes a faster degradation than conventional heating process, indicating an indispensable non-thermal effect of microwave with copper ferrite in the process. Microwave induced holes could be responsible for the efficient degradation. The effect of annealing on crystallization and degradation process was considered here, and the intermediates and products were studied by GC–MS and LC–MS to provide a comprehensively evaluation of degradation.

Microwave-Induced Degradation of Atrazine Sorbed in Mineral Micropores

The herbicide atrazine is a common pollutant in reservoirs and other sources of drinking water worldwide. The adsorption of atrazine from water onto zeolites CBV-720 and 4A, mesoporous silica MCM-41, quartz sand, and diatomite, and its microwave-induced degradation when sorbed on these minerals, were studied. Dealuminated HY zeolite CBV-720 exhibited the highest atrazine sorption capacity among the mineral sorbents because of its high micropore volume, suitable pore sizes, and surface hydrophobicity. Atrazine sorbed on the minerals degraded under microwave irradiation due to interfacial selective heating by the microwave, while atrazine in aqueous solution and associated with PTFE powder was not affected. Atrazine degraded rapidly in the micropores of CBV-720 under microwave irradiation and its degradation intermediates also decomposed with further irradiation, suggesting atrazine could be fully mineralized. Two new degradation intermediates of atrazine, 3,5-diamino-1,2,4-triazole and guanidine, were first identified in this study. The evolution of degradation intermediates and changes in infrared spectra of CBV-720 after microwave irradiation consistently indicate the creation of microscale hot spots in the micropores and the degradation of atrazine following a pyrolysis mechanism. These results indicate that microporous mineral sorption coupled with microwave-induced degradation could serve as an efficient treatment technology for removing atrazine from drinking water.