Environmental remediation by an integrated microwave/UV illumination technique: VI. A simple modified domestic microwave oven integrating an electrodeless UV-Vis lamp to photodegrade environmental pollutants in aqueous media

A simple device is described based on a modified domestic microwave oven that incorporates an UV-Vis lamp encased in Teflon to photodegrade environmental pollutants in aqueous media. The performance of this device was examined using the photodegradation of the agrochemical pollutant 2,4-dichlorophenoxyacetic acid (2,4-D) as the test process driven by a coupled photocatalytic/microwave method in an aqueous TiO₂ dispersion. The aqueous dispersion was contained in a high-pressure Teflon batch (TB) reactor that also integrated a double glass cylindrical plasma lamp (DGCPL) as the source of the UV-Vis radiation. This DGCPL lamp contained mercury gas with a minute amount of neon gas and was powered solely by microwave radiation. The coupled microwave-UV-Vis irradiation of the TB-DGCPL reactor led to an enhancement of the decomposition of the 2,4-D target substrate in the modified microwave oven relative to the photocatalytic method alone. Specifically, the rates of degradation were 2×10⁻³ mM min⁻¹ (photocatalytic/microwave method (PD/MW)) and 1.1×10⁻³ mM min⁻¹ (photocatalytic method (PD)) even though the light irradiance was some six-fold greater in the latter method. That is, the coupled PD/MW method was about 10 times more efficient than the PD method alone.     

Surfactant assisted hydrothermal synthesis of SnO nanoparticles with enhanced photocatalytic activity

SnO nanoparticles have been successfully synthesized in the presence of Triton-X 100 (TX-100) surfactant via hydrothermal method for the first time, and the photocatalytic activity under UV and visible light irradiation for the degradation of Methylene Blue (MB) and Rhodamine B (RdB) organic textile dyes was investigated. The structural, morphological and chemical characterizations were investigated by using X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), selected area electron diffraction (SAED), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), Fourier Transform Infrared Spectroscopy (FTIR), UV–vis. diffuse reflectance spectroscopy (UV–vis DRS) and photoluminescence (PL) analysis. The results reveal that the addition of surfactant, TX-100, in the precursor solutions leads to reduction in crystallite size with significant changes in morphological structure of SnO nanoparticles. The synthesized SnO nanoparticles show excellent photocatalytic activity under UV or visible light irradiation. MB and RdB dyes degraded completely under UV irradiation after 90 and 150 min, respectively. Also, MB and RdB dyes degraded only 150 min later under visible light illumination with a little amount of photocatalyst (0.8 g/L). Hence, this work explores the facile route to synthesizing efficient SnO nanoparticles for degrading organic compound under both UV and visible light irradiations.     

Photocatalytic degradation of paraquat and genotoxicity of its intermediate products

The photocatalytic degradation of paraquat (1,1-dimethyl-4,4′-bipyridylium dichloride) aqueous solutions in the presence of polycrystalline TiO₂ Degussa P25 irradiated by near-UV light was investigated. The substrate and total organic carbon concentrations were monitored by UV spectroscopy and TOC measurements, respectively: the complete photocatalytic mineralization of paraquat (20 ppm) was achieved after ca. 3 h of irradiation by using 0.4 g l⁻¹ of catalyst amount at natural pH (ca 5.8). On the contrary no significant photodegradation of paraquat was observed in the absence of TiO₂ under similar experimental conditions. To evaluate the genotoxicity of paraquat and its intermediates produced during heterogeneous photocatalytic treatment, in vitro tests such as Ames test, with and without rat liver microsomal fractions (S9 mix), and micronucleus test, were used. Results obtained with Salmonella typhimurium (strain TA100) showed that paraquat and photocatalytic products were unable to induce gene mutations when photocatalysis was used in the presence of the optimum amount of TiO₂, i.e. 0.4 g l⁻¹, whereas an increase of revertants his⁺ per plate was observed after 300 min irradiation in the presence of very low amount of TiO₂ (0.04 g l⁻¹). The negative results from micronucleus test suggest that mutagenic, but non-clastogenic, late intermediates of paraquat photo-oxidation were formed when the photocatalytic runs of paraquat degradation were carried out by using 0.04 g l⁻¹ of photocatalyst.     

Synthesis,characterization and photocatalytic properties of CaNb2O6 with ellipsoid-like plate morphology

Ellipsoid-like two-dimensional (2D) plates of calcium niobate (CaNb₂O₆) were synthesized via the hydrothermal route without any surfactants or templates by controlling the reaction conditions, viz. the pH value, reaction time and temperature. The prepared powders were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), fluorescence spectroscopy, and diffuse reflectance UV–vis spectroscopy. It was found that the plates were consisted of uniaxially aligned nanorods and can absorb UV light with wavelengths of less than 340 nm. Compared with the powder of the same material prepared by the solid-state reaction method, the ellipsoid-like 2D plates exhibited a much lower room temperature luminescence intensity and higher photocatalytic activity for the degradation of Rhodamine B dye solution under UV light irradiation. The enhanced photocatalytic activity of the plates was assigned to their higher optical absorption capability, higher diffusion rate of charge carriers and higher surface area resulting from their reduced dimensionality.     

Synthesis,characterization and photocatalytic performance of W,N, S-tri-doped TiO2 under visible light irradiation

W–S–N-tri-doped TiO₂ photocatalysts (WSNTiO₂) were prepared by a simple sol–gel method. Tungstic acid, sodium sulfate and urea were used as tungsten, sulfur and nitrogen sources, respectively. The morphology and microstructure characteristics of the photocatalysts were evidenced by means of XRD, BET, TEM, SEM and UV–vis DRS techniques. The XRD results show that the main crystal phase of samples is anatase. It was also found that the tri-doping of TiO₂ increases its BET specific surface area from 95 to 121 m²·g⁻¹. Besides, it was shown that tri-doping narrows the band gap of TiO₂ effectively, which has greatly improved the photocatalytic activity in the visible light region. The photocatalytic activity of tri-doped TiO₂ powders was compared to that of bi-doped ones through the degradation of Congo Red (CR) under visible irradiation. Thus, the prepared 0.5% W–N–S–TiO₂ heat treated at 450 °C showed the best photocatalytic activity compared to the prepared pure TiO₂, Degussa P25, and co-doped samples (WNTiO₂ and WSTiO₂). In particular, a Congo Red degradation rate of approximately 99% was reached after only 35 min of visible light irradiation in the presence of 0.5% of WNSTiO₂. Total organic carbon (TOC) removal of CR was up to 72% and confirmed its significant mineralization in the presence of 0.5% of WNSTiO₂ photocatalyst.     

Atomic layer deposition of TiO2−xNx thin films for photocatalytic applications

Titanium dioxide (TiO₂) is recognized as the most efficient photocatalytic material, but due to its large band gap energy it can only be excited by UV irradiation. Doping TiO₂ with nitrogen is a promising modification method for the utilization of visible light in photocatalysis. In this work, nitrogen-doped TiO₂ films were grown by atomic layer deposition (ALD) using TiCl₄, NH₃ and water as precursors. All growth experiments were done at 500 °C. The films were characterized by XRD, XPS, SEM and UV–vis spectrometry. The influence of nitrogen doping on the photocatalytic activity of the films in the UV and visible light was evaluated by the degradation of a thin layer of stearic acid and by linear sweep voltammetry. Light-induced superhydrophilicity of the films was also studied. It was found that the films could be excited by visible light, but they also suffered from increased recombination.     

Recycling of poly(ethylene terephthalate) waste through methanolic pyrolysis in a microwave reactor

In this study, the methanolic pyrolysis (methanolysis) of poly(ethylene terephthalate) (PET) taken from waste soft-drink bottles, under microwave irradiation, is proposed as a recycling method with substantial energy saving. The reaction was carried out with methanol with and without the use of zinc acetate as catalyst in a sealed microwave reactor in which the pressure and temperature were controlled and recorded. Experiments under constant temperature or microwave power were carried out at several time intervals. The main product dimethyl-terephthalate was analyzed and identified by FTIR and DSC measurements. It was found that PET depolymerization, is favored by increasing temperature, time and microwave power. High degrees of depolymerization were measured at temperatures near 180 °C and at microwave power higher than 150 W. Most of the degradation was found to occur during the initial 5–10 min. Compared to conventional pyrolysis methods, microwave irradiation during methanolic pyrolysis of PET certainly results in shorter reaction times supporting thus the conclusion that this method is a very beneficial one for the recycling of PET wastes.     

Photocatalytic activity of nitrogen and copper doped TiO2 nanoparticles prepared by microwave-assisted sol-gel process

Cu and N-doped TiO₂ photocatalysts were synthesized from titanium (IV) isopropoxide via a microwave-assisted sol-gel method. The synthesized materials were characterized by X-ray diffraction, UV-vis diffuse reflectance, photoluminescence (PL) spectroscopy, SEM, TEM, FT-IR, Raman spectroscopy, photocurrent measurement technique, and nitrogen adsorption–desorption isotherms. Raman spectra and XRD showed an anatase phase structure. The SEM and TEM images revealed the formation of an almost spheroid mono disperse TiO₂ with particle sizes in the range of 9-17 nm. Analysis of N₂ isotherm measurements showed that all investigated TiO₂ samples have mesoporous structures with high surface areas. The optical absorption edge for the doped TiO₂ was significantly shifted to the visible light region. The photocurrent and photocatalytic activity of pure and doped TiO₂ were evaluated with the degradation of methyl orange (MO) and methylene blue (MB) solution under both UV and visible light illumination. The doped TiO₂ nanoparticles exhibit higher catalytic activity under each of visible light and UV irradiation in contrast to pure TiO₂. The photocatalytic activity and photocurrent ability of TiO₂ have been enhanced by doping of the titania in the following order: (Cu, N) - codoped TiO₂ > N-doped TiO₂ > Cu-doped TiO₂ > TiO₂. COD result for (Cu, N)-codoped TiO₂ reveals ∼92% mineralization of the MO dye on six h of visible light irradiation.     

Preparation of a new lightly cross-linked liquid crystalline polyamide by interfacial polymerization. Application to the obtainment of microcapsules with photo-triggered release

Microcapsules based on a new liquid crystalline lightly cross-linked polyamide, in which the state of order can be triggered by means of external stimuli, such as temperature and light, were prepared by interfacial polymerization. This polyamide exhibited a nematic phase up to 166 °C and it started to decompose at 340 °C; morphological variations of the film were observed by Scanning Electron Microscopy in correspondence to the clearing temperature; moreover, by continuous irradiation with UV light at room temperature, the polymer underwent E–Z photoisomerization. The prepared microcapsules contained either toluene, or concentrated solutions of naphthalene or β-carotene, as the core; in all cases, their outer surface appeared smooth and dense, while heterogeneities could be seen on the inner face. Capsule diameter lay in the range 30–120 μm, depending on the encapsulated material, with quite narrow size distributions. To the authors’ knowledge, this is the first example of microcapsules whose shell is completely constituted by a liquid crystalline lightly cross-linked polymer. Release experiments of β-carotene were performed in water and in tetrahydrofuran. β-Carotene release in water at 20 °C was strongly influenced by UV irradiation: in the absence of irradiation, it was practically negligible while, when microcapsules were submitted to continuous irradiation with UV light, β-carotene was quickly released and reached 100% release after 5 min. Preliminary experiments concerning the effect of temperature and of a swelling solvent, such as THF, on release, were also performed.     

The effect of silver doping on photocatalytic properties of titania multilayer membranes

In this investigation an Ag doped titania multilayer membrane is successfully fabricated via the sol–gel processing method. The doped membrane is characterized via X-ray Diffraction and N₂-sorption techniques and the photocatalytic properties of the membrane are investigated via methyl orange degradation. The properties included high surface area (101 m²/g), small pore size (3.1 nm), and active anatase crystal phase. The prepared titania membrane has a high photocatalytic activity and decomposes methyl orange by 50% after 9 h of UV irradiation. The prepared membrane can be applied in the development of efficient photocatalytic systems for the treatment of water. Due to the high photoactivity of the prepared titania membrane, this study reveals the possibility of combining two processes for removal of organic pollutants: the photocatalytic process and the membrane separation process. In the combining process the lifetime of the membrane increases and the quality of water is enhanced.     

Amberlite IR-120 catalyzed,microwave-assisted rapid synthesis of 2-aryl-benzimidazoles

An expeditious synthesis of 2-aryl-benzimidazoles by the condensation of o-phenylenediamine with various araldehydes is described. This greener protocol is catalyzed by Amberlite IR-120, and proceeds efficiently in the absence of any organic solvent under microwave irradiation within 3–5 min.     

Cr(VI) reduction at rutile-catalyzed cathode in microbial fuel cells

Cathodic reduction of hexavalent chromium (Cr(VI)) and simultaneous power generation were successfully achieved in a microbial fuel cell (MFC) containing a novel rutile-coated cathode. The selected rutile was previously characterized to be sensitive to visible light and capable of both non-photo- and photocatalysis. In the MFCs containing rutile-coated cathode, Cr(VI) was rapidly reduced in the cathode chamber in presence and absence of light irradiation; and the rate of Cr(VI) reduction under light irradiation was substantially higher than that in the dark. Under light irradiation, 97% of Cr(VI) (initial concentration 26 mg/L) was reduced within 26 h, which was 1.6× faster than that in the dark controls in which only background non-photocatalysis occurred. The maximal potential generated under light irradiation was 0.80 vs. 0.55 V in the dark controls. These results indicate that photocatalysis at the rutile-coated cathode in the MFCs might have lowered the cathodic overpotential, and enhanced electron transfer from the cathode to Cr(VI) for its reduction. In addition, photoexcited electrons generated during the cathode photocatalysis might also have contributed to the higher Cr(VI) reduction rates when under light irradiation. This work assessed natural rutile as a novel cathodic catalyst for MFCs in power generation; particularly it extended the practical merits of conventional MFCs to cathodic reduction of environmental contaminants such as Cr(VI).     

Synthesis of solar light driven nanorod-zinc oxide for degradation of rhodamine B,industrial effluent and contaminated river water

Surface water contamination by various dyes and pigments is a global problem caused by rapid industry, particularly textile/dyeing. Bangladesh's export-oriented textile sector has exploded in recent decades, polluting local waterways significantly. In this study, nano-ZnO were prepared using surfactant-assisted sol–gel, hydrothermal and thermal methods. SEM, XRD, reflectance spectrophotometer, EDS and adsorption tests were used to characterize the synthesized nano-ZnO. BET isotherms were used to determine the surface area, pore volume, and pore size of the as-prepared nano-ZnO. The mixed surfactant assisted-sol gel method produced nanorod-ZnO, whereas the hydrothermal and/or thermal methods yielded clusters of needles ZnO, as proven by SEM images. XRD data revealed that the synthesized nanorod-ZnO had a mainly wurtzite crystalline structure and their size was estimated using the Scherrer equation to be about 23.90 nm. EDS spectra confirmed the synthesis of pure nanorod-ZnO. Using a UV–visible reflectance spectrophotometer, the band gap energy of the as-prepared nanorod-ZnO was found to be 3.35 eV. According to BET isotherms, the BET and Langmuir surface areas were 4 and 5.4 m²/g, respectively. Prior to analyzing photodegradation, the RB was adsorbing in the presence of various doses of the nanorod-ZnO in the dark, but no adsorption was observed. The photocatalytic activities of the synthesized nano-ZnO were compared to TiO₂ (anatase) for the degradation of RB in an aqueous system under solar light, UV, fluorescence, and tungsten filament light irradiation. Nanorod-ZnO showed exceptional photocatalytic activity in degrading RB in an aqueous solution under solar light irradiation. The results suggest that 0.01 g/50 mL nanorod-ZnO with a solution pH of 7.8 is the best combination for complete degradation of 2.00 × 10^-5 M RB under solar light irradiation. When nano-ZnO was exposed to light, the inhibiting effect of ethanol and/or tert-butanol on the degradation of RB confirmed the formation of mostly hydroxyl free radicals. The synthesized nanorod-ZnO shown substantial photocatalytic activity in the removal of pollutants from industrial effluents and contaminated river water under solar light irradiation. A mechanism of excellent photocatalytic activity of the nanorod-ZnO is discussed.     

Microwave-assisted C–H bond activation using a commercial microwave oven for rapid deuterium exchange labeling (C–H → C–D) in carbohydrates

Studies have shown that facile hydrogen → deuterium exchange in two model carbohydrates via stereospecific C–H bond activation could be achieved using a pre-sonicated Raney Nickel^® catalyst and microwave irradiation. Using a simple commercial microwave oven and a silica-gel bath, monosaccharide and disaccharide samples underwent isotopic exchange using microwave irradiation for sequential 15 s intervals. The influence of chilling between irradiation intervals was examined. The results revealed increasing levels of ²H incorporation without either epimerization or concomitant decomposition seen earlier in non-optimized experiments.     

Photochemical vapor generation of carbonyl for ultrasensitive atomic fluorescence spectrometric determination of cobalt

UV photochemical vapor generation (photo-CVG) as sample introduction was first adapted for determination of ultratrace cobalt by atomic fluorescence spectrometry (AFS). Cobalt volatile species can be generated when the buffer system of formic acid and formate containing Co (II) is exposed to UV radiation. The generated gaseous products were separated from liquid phase within a gas–liquid separator and then transported to AFS for determination of cobalt. Factors affecting the efficiency of photo-CVG were investigated in detail, including type and concentration of low molecular weight (LMW) organic acid, buffer system, UV irradiation time, reaction temperature, carrier gas flow rate and hydrogen flow rate. With 4% (v/v) HCOOH and 0.4 mol L^− 1 HCOONa buffer solution, 150 s irradiation time and 15 W low pressure mercury lamp, a generation efficiency of 23–25% was achieved. A limit of detection (LOD) of 0.08 ng mL^− 1 without any pre-concentration procedure and a precision of 2.2% (RSD, n = 11) at 20 ng mL^− 1 were obtained under the optimized conditions. The proposed method was successfully applied in the analysis of several simple matrix real water samples.     

Photochemistry with microwaves: Catalysts and environmental applications

Microwave radiation has recently become an active source of thermal energy in numerous chemical reactions. As such, the microwave energy is not ordinarily and is not likely to be used to drive photochemical reactions. Accordingly, is the role of microwaves then relegated solely to be a source of heat? They do not have to be since photochemical reactions can be activated indirectly by microwaves using the UV light emitted from certain gas-fills excited by microwave radiation. This article examines the microwave radiation not only as a dielectric heat source but also a source of vacuum-UV radiation and UV light through microwave discharge electrodeless lamp devices, which in some cases (depending on design) can also serve as photoreactors.     

Microwave-assisted C-C coupling reaction using polymer-supported electron-rich oxime palladacycles in aqueous condition

Previously reported polymer-supported oxime palladacycle catalyst has shown effective catalytic activity in the Suzuki coupling reaction. In this work, the effect of microwave on reaction rate and induction period was examined by comparing them with the conventional heating. Undoubtedly, microwave heating effectively enhanced the catalytic performance in the Suzuki reaction. In-depth observation of the kinetic profiles revealed that the microwave irradiation not only reduced the overall reaction time but also significantly decreased the induction period to less than 3 min. Finally, the performance of the catalyst in water, coupling of potassium phenyltrifluoroborate and aryl bromides, and recyclability were also examined.     

Built-in piezoelectric field improved photocatalytic performance of nanoflower-like Bi2WO6 using low-power white LEDs

Photocatalysis technology has been proved to be a potential strategy for removal of organic dyes, however high-power light sources are generally necessary to initiate photocatalytic reaction. In this work, we employed an excellent photocatalyst of Bi₂WO₆ with visible light harvest and meanwhile an intrinsic ferroelectricity, which realized the efficient degradation of organic dye via the synergetic photopiezocatalysis. Through coupling the illumination by a low-power (9 W) LED and the ultrasonic vibration (120 W) by an ultrasonic cleaner, the nanoflower-like Bi₂WO₆ composed of ultrathin nanosheets showed a much more enhanced photopiezocatalysis performance for purification of organic dye than the individual photocatalysis and piezocatalysis. Furthermore, the high mineralization efficiency and the good durability of the Bi₂WO₆ catalyst were demonstrated. The possible mechanism of photopiezocatalysis was finally proposed, where the ultrasound-induced piezoelectric field in Bi₂WO₆ drove photo-generated electrons and holes to diffuse along opposite directions, consequently promoting the separation efficiency of charge carriers. This work indicates that the synergetic photopiezocatalysis by coupling irradiation and ultrasonic vibration is a promising strategy to purify organic pollutants in wastewater.     

影响TiO2薄膜光催化降解亚甲基蓝的因素

 用中频交流反应磁控溅射技术制备了具有良好光催化性能的TiO2薄膜,考察了紫外光源、反应器和溶液浓度对亚甲基蓝溶液光催化降解特性的影响. 结果表明,紫外光源对TiO2薄膜光催化降解性能有较大的影响; 在计算光催化降解速率时应充分考虑光解和光氧化的影响. 低压汞灯TUV为光催化降解的较好选择. 动态反应系统可以有效避免光解,显著提高光催化反应速度. TiO2薄膜具有很好的光催化性能,且性能稳定.     

Synthesis and characterization of highly-active nickel and lanthanum co-doped SrTiO3

A series of highly-active nickel and lanthanum co-doped SrTiO₃ photocatalysts were synthesized via sol–gel process and their photocatalytic activities were evaluated by degradation of methylene blue (MB). The obtained samples were found by XRD, XPS and UV–vis to have a perovskite structure in which Ni and La atoms were incorporated into SrTiO₃. After Ni and La doped into SrTiO₃, the absorption edge of SrTiO₃ powder was greatly shifted from 380 nm to 700 nm. Under a 100 W incandescent lamp irradiating for 14 h, a 100% of MB was degraded, which is much higher than those of pure SrTiO₃ and commercial Degussa P25. The optimal range of Ni and La dopants is 0.1–1.0 mol%. The formation of a new absorption edge and the large surface area may be the main reasons for the high activity.