Microwave Heating Time Research Articles

Microwave‐assisted degradation of chitosan with hydrogen peroxide treatment using Box‐Behnken design for enhanced antibacterial activity

SummaryLow molecular mass (MM) chitosan with high degree of deacetylation (DDA) has excellent bioactivity including antioxidant, antibacterial and encapsulation properties. In this work, to reduce the MM of chitosan, microwave‐assisted heating treatment (MAHT) conditions were investigated using three factors at three levels Box‐Behnken design (BBD). Microwave heating (MH) time, H2O2 concentration and solid‐to‐liquid ratio significantly affected the DDA and MM of chitosan. The antibacterial activities of chitosan before and after degradation were investigated based on minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC). The results showed that a second‐order polynomial equation fitted the observed values using multiple regression analysis and had a high coefficient of determination (R2 = 0.9591 and 0.9161 for the DDA and MM of chitosan, respectively). An optimisation study was performed using Derringer's desirability function methodology, and the optimal conditions were 80‐s MH time, 1.5% H2O2 concentration and 1:40 solid‐to‐liquid ratio. The MIC and MBC of chitosan before and after degradation against Escherichia coli and Salmonella typhimurium were 0.031 and 0.063 mg mL−1, and 0.25 and 0.125 mg mL−1, respectively. The optimised DDA and MM of chitosan were 90.58 ± 2.04% and 124.25 ± 14.36 kDa, respectively, which significantly reduces the use of oxidant reagent.

Migration of DEHP from plastic to food simulants under microwave heating

The migration of plasticizer DEHP from the plastic products (4 kinds of commonly used plastic food containers under microwave heating: plastic wrap, food bags, ordinary plastic boxes, microwave special plastic boxes) through food contact materials to food simulants (isooctane, 10% ethanol-water solution (v/v), 3% acetic acid-water solution (w/w) and distilled water) was studied under microwave heating (power levels of 400 W). The results shows that the DEHP mobility increases with the increase of microwave heating time, DEHP mobility in isooctane and 3% acetic acid-water solution (w/w) is significantly greater than in 10% ethanol-water solution (v/v) and distilled water; the order of DEHP mobility in isooctane is plastic wrap>food bag>common plastic box>microwave-safe plastic box, while in 3% acetic acid (w/w), the order is food bag>common plastic box>microwave-safe plastic box>plastic wrap.

NMR relaxometry as a tool to understand the effect of microwave heating on starch-water interactions and gelatinization behavior

Starch-water interactions during gelatinization by microwave heating was investigated by Low-field 1H Nuclear Magnetic Resonance (LF NMR) relaxometry experiments. Effect of microwave heating time on longitudinal (T1) and transverse relaxation times (T2), gel matrix structures, changes in proton populations and firmness values were evaluated. One to one corn starch (CS) to water ratio samples had the lowest T2 change during gelatinization indicating lower granule expansion which was attributed to the high amylose content of CS. Non-Negative-Least-Square (NNLS) analysis of the transverse relaxation curves provided detailed information on starch-water interactions during gelatinization. Rice starch (RS) samples had relatively lower changes in peak areas which was related to the better hydration of RS granules. While microscope images revealed the changes in the granule structures after distinct gelatinization intervals, firmness measurements supported the structural changes. With increasing microwave heating time, both T1 and T2 showed a decreasing trend. Two distinct proton populations detected on relaxation spectrum indicated the exchanging protons during gelatinization. A noticeable reduction in firmness at the later moments of microwave heating was detected. The results showed that starch gelatinization by microwave heating had some distinct features in terms of mechanism of the gelatinization and could be investigated by NMR relaxometry.

Effect of microwave heating time on bonding strength and corrosion resistance of Ca-P composite layers for carbon/carbon composites

Three coated Ca-P composite layers were fabricated by microwave-assisted hydrothermal (MH) and supersonic atmospheric plasma spray (SAPS) method. This multilayer coating aims to promote the bonding strength as well as the corrosion property against stimulated body fluid (SBF) on carbon/carbon (C/C) composites. The morphologies and the chemical composition of the Ca-P composite layers were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The bonding strength was evaluated by scratch test and the bioactivity was measured using SBF. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were investigated for the corrosion resistance. The results reveal that Ca-P composite layers exhibit excellent bonding strength which reaches to the loading force of 39.5 N. In SBF solution, the Ca-P composite layers can induce a new and uniform apatite layer. Additionally, potentiodynamic polarization and EIS results indicate that adding MH influences the corrosion resistance of the Ca-P composite layers. The above conclusion illustrate that MH approach could become a very useful route to enhance the long-term protection and corrosion resistance of the layers on C/C substrates for orthopedic application.

Microwave Treatment for Pest Control: Coleoptera Insects in Wooden Objects

ABSTRACTBy far the most common species of woodworms perceived to be the main cause of damage to wood, and in particular to wooden art objects, are Anobium punctatum de Geer and Hylotrupes bajulas of Coleoptera family. In search of non-chemical methods for eradicating these wood pests, microwave (MW) heating can be an effective tool to help control the insects in their different life stages (eggs, larvae, and adult). The effect of 2.45 GHz frequency MW exposure on wooden art objects is described and discussed. MW heating time is reported in connection with lethal temperature of the pest and moisture content of the treated wooden object. The treatment was carried out using Microwood 12, especially conceived for portability, easy operation, and control. Microwood 12 consists of a switch mode high-voltage power supply and an industrial continuous wave magnetron connected to a rectangular-shaped horn antenna. The measure of temperature and control of operation is done via a built-in infrared thermometer.

Novel edible oil sources: Microwave heating and chemical properties

The aim of this work was to investigate the effect of various microwave heating times (1, 3, 5, 10, and 15min) on the chemical properties of novel edible oil sources, including Mashhadi melon (Cucumis melo var. Iranians cv. Mashhadi), Iranian watermelon (Citrullus lanatus cv. Fire Fon), pumpkin (Cucurbita pepo subsp. pepo var. Styriaca), and yellow apple (Malus domestica cv. Golden Delicious) seed oils. The evaluated parameters were peroxide value (PV), conjugated diene (CD) and triene (CT) values, carbonyl value (CV), p-anisidine value (AnV), oil stability index (OSI), radical scavenging activity (RSA), total tocopherols, total phenolics, as well as chlorophyll and carotenoid contents. Results showed that extended microwave heating involves decreased quality of the seed oils, mainly due to the formation of primary and secondary oxidation products. Microwave heating time also affects the total contents of chlorophylls, carotenoids, phenolics and tocopherols, which clearly decrease by increasing the exposure time. The order of oxidative stability of the analyzed edible oils was pumpkin>Mashhadi melon>Iranian watermelon>yellow apple. The obtained results demonstrated the promising potential of these novel edible oils for different food applications.

Factors optimization of super fast removal of heavy metals from aqueous solution using microwave-enforced sorption on the surface of a novel nano-composite

Abstract The validity of microwave-enforced sorption (MES) technique as a green, efficient and fast approach was investigated in this work to enforce adsorptive binding, extraction and removal of Pb(II), Cu(II) and Hg(II) ions from aqueous solution. For this purpose, a newly designed nano-composite was synthesized using microwave-assisted heating technique via covalent bonding of nanolayers of 3-aminopropyltrimethoxysilane (SiNH2) and salicylaldehyde (SA) on the surface of nanotitanium oxide (NTiO2) for the formation of NTiO2-SiN = SA nanomaterial. The proposed nano-composite was characterized by different technique such as FT-IR, SEM, TEM, TGA and XRD analysis. The investigated metal ions were allowed to heat and surface react with NTiO2-SiN = SA nano-composite inside a microwave oven for a short period of time, 5–40 s, to enforce mass transfer of metal ions from solution onto the surface. The optimization of different important experimental parameters such as microwave heating time, mass of nano-composite, initial metal ion concentration, initial pH of metal ion solution and coexisting ions were explored and executed. The MES capacity values of Pb(II), Cu(II) and Hg(II) were characterized as 2900, 1200 and 400 μmol g−1, respectively using 5 mg of NTiO2-SiN = SA nano-composite and 20 s of microwave heating. The potential suitability of MES technique for adsorptive removal and extraction of low concentration levels (mg L−1) of Pb(II), Cu(II) and Hg(II) from tap water and industrial wastewater were also investigated in this study.

Microwave aqueous synthesis of hydroxyapatite bilayer coating on magnesium alloy for orthopedic application

Magnesium and magnesium alloys as promising biodegradable implants gain increasing interest. To improve their corrosion resistance and cytocompatibility, hydroxyapatite (HA) coatings with double-layer structure were synthesized on magnesium alloy through a rapid microwave aqueous chemical route. The coating prepared with microwave heating time of 10min consisted of cotton-like HA as the surface layer and strip-like HA as the bottom layer, and the coating thickness was ∼10.0μm. The growth kinetic, electrochemical property, degradation, mineralization and cytocompatibility of HA coated and naked magnesium alloys were investigated. Potentiodynamic polarization test demonstrated that corrosion current density of HA coated magnesium alloy prepared with microwave heating time of 10min was ∼100 times lower than that of the naked counterpart. Immersion test indicated that the strip-like HA layer with thickness of ∼7.7μm and low solubility endowed magnesium alloy with initial-stage protection; the nano-scale cotton-like HA layer exhibited high HA mineralization capacity. The mineralized layer reached ∼12.7μm in thickness after 24days’ immersion, eventually integrated with the original HA bilayer coating, providing favorable long-term protection for magnesium alloy. Moreover, HA coated magnesium alloy prepared with microwave heating time of 10min enhanced osteoblast proliferation than the naked counterpart. These attractive results indicated that this HA bilayer coating is a potential protective coating on biodegradable magnesium and magnesium alloys for orthopedic application.

Microwave heating time dependent synthesis of various dimensional graphene oxide supported hierarchical ZnO nanostructures and its photoluminescence studies

Microwave heating reaction time dependent various graphene oxide based zinc oxide (G-ZnO) heterostructures such as graphene oxide-ZnO microcubes (GZMC), graphene oxide-ZnO nanoflakes (GZNF) and graphene oxide-ZnO nanoneedles (GZNN) are synthesized by simple and cost effective microwave assisted exfoliation method. These heterostructures supported on graphene oxide nanosheets (GNSs) represent three dimensional (3D) ZnO microcubes and various confined two dimensional (2D) nanoflakes and one dimensional (1D) ZnO nanoneedles like structures. The recorded PL intensity variations show the strong evidence of the interfaces interaction between graphene oxide and ZnO heterostructures. However the differences in the PL intensities are also caused by the 3D and various confined G-ZnO heterostructures. The photoluminescence characterization of GZMC, GZNF and GZNN nanostructures exhibit a decrement in the PL intensity. The PL intensity of the GZNN is lowered by 67.50% and 39.7%to the GZMC and GZNF nanostructures respectively. The results show that ZnO heterostructures grown on GNSs with different morphologies and dimensionalities exhibit the variation in PL intensity due to preventing a direct recombination of the electrons and holes in ZnO. A tentative growth mechanism has been given for the growth of various graphene based zinc oxide heterostructures.

Extraction Optimization of Phenolic Antioxidants from microwave treated Nelumbo nucifera Seed Flour

Background: Nelumbo nucifera seeds are medicinally important due to the presence of phytochemicals with strong antioxidant potential. Extraction of these phytochemicals has been a major problem faced by the researchers. Objective: The purpose of the study was to optimize the process conditions for extraction of phenolic antioxidants from N. nucifera seed. Methods: Process variables for microwave assisted extraction of phenolic antioxidants from N. nucifera seed flour were optimized by response surface methodology using a multi-factorial central composite design based on five levels of each of four input variables including X1: particle size (in terms of mesh No.), X2: microwave treatment period (MTP), X3: solvent concentration (SC) and X4: extraction period (EP). Total extract yield was calculated as Total extractable components (TEC) and the extracts were analyzed for total phenolic content (TPC) and total antioxidant activity (TAOA). Results: Data analysis showed a significant increase (p<0.05) in TEC, TPC and TAOA in response to increase in the levels of selected factors. Optimum levels of extraction variables to achieve maximum level of response parameters were found to be: 125.96, 2.45 min, 55.21% and 3.87 h for TEC, 61.73, 1.66 min, 80.08% and 4.98 h for TPC and 60.89, 2.37 min, 44.98% and 1.123 h for TAOA. Conclusion: The extraction of phenolic antioxidants from N. nucifera seed flour is significantly increased in response to an increase in mesh No. (decrease in particle size), MTP, SC and EP but up to a certain limit. The decrease in extraction yield and antioxidant activity at higher levels of studied factors may be attributed to the possible decomposition of phenolic compounds due to prolonged duration of microwave heating and prolonged extraction time.

Microwave-assisted preparation of almond shell-based activated carbon for methylene blue adsorption

Abstract This study reports the synthesis of a novel adsorbent, hereinafter named almond shell-based activated carbon (ASAC), from waste almond shells (AS) via exposure to microwave radiation and the use of ZnCl2 as a chemical activator. Synthetic conditions were further elaborated using the response surface methodology to optimize the adsorption capacity of ASAC for methylene blue (MB) dye. The optimized ASAC preparation conditions were found to be as follows: a mass (ZnCl2/AS) ratio of 3:1 (w/w) and microwave heating time period of 15 min at 900 W. A maximum ASAC yield of 39.67% and a MB adsorption capacity of 314.20 mg/g were obtained. ASAC was also characterized by N2 adsorption-desorption measurement, scanning electron microscopy (SEM), Fourier-transform infrared spectrometry (FTIR), X-ray photoelectron spectroscopy (XPS), and point of zero charge measurement. Optimized ASAC had a Brunauer-Emmett-Teller surface area of 839.60 m2/g and a total volume of 0.406 cm3/g. FTIR and XPS analysis exhibited a decline in oxygen-containing groups of ASAC as compared with AS. The adsorption behavior of ASAC for MB was fitted well to the pseudo-second-order model and the Langmuir isotherm model. These findings support the ease of preparation and cost effectiveness that make ASAC promising for use in environmental remediation.

Spectroscopic Studies on the Effect of Some Ferrocene Derivatives in the Formation of Silver Nanoparticles.

Silver nanoparticles were prepared by microwave assisted method using silver nitrate as precursor in the presence of some ferrocene derivatives. The formation of the silver nanoparticles was monitored using UV-Vis spectroscopy. The UV-Vis spectroscopy revealed the formation of silver nanoparticles by exhibiting typical surface plasmon absorption band. The position of plasmon band (406-429 nm) was observed to depend on the nature of a particular ferrocene derivative used. TEM images indicated that the nanoparticles were spherical in shape and well-dispersed. Quantum dots (3.2 nm) were prepared by using ferrocenecarboxylic acid. The surface plasmon absorption band has shown red shift with increasing concentration of ferrocene derivative. For different duration of microwave heating time, intensity of absorption spectra in general was found to increase except in presence of ferrocene carbaldehyde where it decreased. Time-dependent spectra have indicated almost stable position of the surface plasmon band with increasing time of observation confirming that the as prepared silver nanoparticles did not aggregate with lapse of time.

Theoretical investigation of temperature distribution uniformity in wood during microwave drying in three-port feeding circular resonant cavity

ABSTRACTThe temperature distribution within wood samples during microwave drying is significantly influenced by the dimensions of the wood sample, microwave frequency, heating time, and moisture content of the wood. Here, the temperature distribution inside the wood during microwave drying has been studied by finite element analysis in a three-port feeding circular resonant cavity. With an increasing radius of the wood, the calculated temperature variation coefficient decreased from 54.3 to 23.5% and the energy efficiency increased from 18 to 95%. When the radius of the wood was 0.8 times that of the circular cavity, the drying process was optimal. The theoretical calculations indicated that the length of the wood also affects the temperature variation coefficient. When the length of wood was more than the height of waveguide, the temperature variation coefficient was less than 40.5% and the energy efficiency was more than 90%. Other factors such as microwave frequency, heating time, and moisture content also influenced the uniformity of temperature distribution. This study helps to better understand the microwave drying process to facilitate its further applications for wood drying.

Decomposition of the mixed rare earth concentrate by microwave-assisted method

A novel process was proposed to strengthen the decomposition of the mixed rare earth concentrate by utilizing the microwave radiation. Mineralogical information on the mechanisms by which microwave heating improved the leaching behavior of rare earth elements (REEs), and an interpretation of the interrelationship between mineralogy, decomposition process, and leaching process were provided in this study. The influences of the temperature, time of microwave heating and contents of NaOH (mass ratio of NaOH to mixed rare earth concentrate) on the decomposition of mixed rare earth concentrate were investigated. The results revealed that the temperature was the main factor affecting the decomposition process. The recovery of REEs by hydrochloric acid leaching reached 93.28% under the microwave heating conditions: 140 °C, 30 min and 35.35% NaOH. The BET specific surface area and SEM analysis indicated that the particles of mixed rare earth concentrate were non-hole, while the particles presented a porous structure after heating the concentrate by microwave radiation. For the microwave treated sample after water leaching, the BET specific surface area was 11.04 m2/g, which was higher than the corresponding values (6.94 m2/g) for the mixed rare earth concentrate. This result could be attributed to the phase changes of bastnaesite and monazite, and a number of cracks induced by thermal stress. The increase of BET specific surface area resulted in an increase of the recovery of REEs by promoting interaction within the system of acid leaching.

Effect of microwave irradiation on the preparation of iron oxide/arenaceous clay sorbent for hot coal gas desulfurization

For applications in the high temperature coal gas desulfurization process, iron oxide/arenaceous clay sorbents were prepared by microwave irradiation and conventional heating, respectively. The desulfurization performances of sorbents were evaluated using a fixed-bed reactor with simulated coal gas. The effects of calcination temperature and microwave heating time on the properties of sorbents were also investigated. The microwave-irradiated sorbents exhibit sulfur capacity and reactivity greater than those prepared by conventional heating. X-ray diffraction, X-ray photoelectron spectroscopy, a scanning electron microscope, a transmission electron microscope and N2 adsorption were used to characterize the physical and structural properties of the sorbents. Several benefits have been observed using microwave heating rather than conventional heating for the preparation of sorbents. The results indicate that microwave irradiation leads to better inner structure of sorbent and more uniform distribution of the active component with smaller particle size, which facilitate chemical reaction and mass transfer. Microwave irradiation also results in greater outer layer electron density and higher contents of surface metal element, both of which are beneficial to the increase of absorption efficiency and desulfurization reactivity. Then the microwave-irradiated sorbents were tested for three successive sulfurization–regeneration cycles, the 3rd regenerated sorbent still performed well with no apparent deterioration, it proved that the sorbent has good regenerability and is suitable for hot gas desulfurization.

Development of high porosity structures of activated carbon via microwave-assisted regeneration for H2S removal

In this study, commercial coconut shell activated carbon (CSAC) was utilized for H2S adsorption application. The regeneration of the CSAC adsorbent was implemented via microwave-assisted method in order to investigate the H2S regeneration efficiency. The H2S-saturated CSAC adsorbent was subjected to experience three different regeneration variables; microwave heating temperature (100–500°C), N2 flow rate (100–500ml/min) and microwave heating time (5–25min). The effect of the variables on the CSAC’s regeneration efficiency was analyzed and discussed. The results indicated that the regeneration of H2S-saturated CSAC adsorbent via microwave-assisted method could improve the porosity of the CSAC and resulted in high H2S regeneration efficiency (efficiency of H2S breakthrough capacity=1000% and efficiency of total H2S adsorption capacity=400%). The regenerated CSAC adsorbent was tested for a few regeneration cycles, which resulted in a decrease in the H2S regeneration efficiency. The regenerated CSAC adsorbent was characterized by using nitrogen adsorption/desorption analysis, elemental analysis and SEM analysis. This study emphasizes on the important key-factors in the employment of microwave-assisted method for CSAC regeneration in the H2S adsorption application.

Green and rapid synthesis of 3D Fe2(MoO4)3 by microwave irradiation to detect H2S gas

A three-dimensional flower-shaped iron molybdate microsphere (3D-FIMMS) and a zero-dimensional iron molybdate nanoparticle (0D-IMNP) were successfully synthesized by a rapid and environment friendly microwave irradiation method. The 3D-FIMMS crystal growth was investigated by changing the microwave heating time. The gas sensing performances of various morphologies of iron molybdate were studied. The results showed that 3D-FIMMS sensors exhibited excellent gas sensing properties compared with the 0D-IMNP sensors, in which response value reached 13.5 for 50ppm H2S gas. Meanwhile, the 3D-FIMMS sensors displayed high selectivity to H2S. The experimental results indicated an efficient and eco-friendly approach to realize promising sensing performance to H2S gas.

Oil extraction from sheanut (Vitellaria paradoxa Gaertn C.F.) kernels assisted by microwaves.

Shea butter, is highly solicited in cosmetics, pharmaceuticals, chocolates and biodiesel formulations. Microwave assisted extraction (MAE) of butter from sheanut kernels was carried using the Doehlert's experimental design. Factors studied were microwave heating time, temperature and solvent/solute ratio while the responses were the quantity of oil extracted and the acid number. Second order models were established to describe the influence of experimental parameters on the responses studied. Under optimum MAE conditions of heating time 23min, temperature 75°C and solvent/solute ratio 4:1 more than 88% of the oil with a free fatty acid (FFA) value less than 2, was extracted compared to the 10h and solvent/solute ratio of 10:1 required for soxhlet extraction. Scanning electron microscopy was used to elucidate the effect of microwave heating on the kernels' microstructure. Substantial reduction in extraction time and volumes of solvent used and oil of suitable quality are the main benefits derived from the MAE process.

Effects of optical and magnetic fields on the electrical characteristics of colloidal graphene quantum dots

The effects of optical, magnetic fields and microwave heating time (MHT) have a pronounced influence on the electrical performance of graphene quantum dots (GQDs).

A Technique for Localized Rapid Soot Oxidation Using Metal Aided Microwave Radiation

The regeneration of soot filter using microwave heating has several drawbacks, which include non-uniform regeneration, ineffective on very small soot thickness, and could lead to excessive heating, which damages the filter that is loaded with high layer of soot. This paper proposes a novel technique for localized rapid heating and oxidation of any thickness of solid soot, and saving of the consumed microwave energy. This technique was applied by inserting a metal in the accumulated soot, located inside a closed multi-mode microwave cavity, attached to a mono-mode waveguide, connected with a magnetron with a capacity of 2.45 GHz and 800 W. The electric field, generated heat, and temperature variation with time till the attaining of the oxidation temperature of the soot during microwave heating were simulated. The simulation methodology is based on the concept of electromagnetic-thermal energy conversion by coupling Maxwell with heat transfer equations. Finite element method was used with frequency domain for electromagnetic analysis and time domain for thermal analysis. In agreement with experimental results, the predicted microwave heating time to attaining the soot ignition/oxidation temperature was reduced from 8.5 to 0.3 seconds. Based on the presented technique, metallic electrodes can be inserted on the soot filter surfaces as local heating electrodes to enhance the microwave heating of the soot filter regeneration.