Evaluation of non-thermal effects by microwave irradiation in hydrolysis of waste-activated sludge

Study and application status of the nonthermal effects of microwaves in chemistry and materials science - a brief review

Microwaves (MWs) are widely known and used in human life and production activities based on their thermal effects. In contrast, their nonthermal effects are still under debate. Fortunately, the nonthermal effects of MWs have been investigated by an increasing number of researchers and have shown great potential in industrial production. In this review, typical studies that demonstrate the nonthermal effects of MWs in chemistry and materials science are introduced and discussed, and the applications of and the harms that are caused by these effects are summarized to facilitate the safe use of these MW effects. The mechanisms of the nonthermal effects of MWs that have been proposed by researchers with various backgrounds are presented. Because some researchers did not detect nonthermal effects of MWs, four typical relevant studies are identified and introduced. Various types of MW reactors (single-mode and multimode reactors and reactors without a MW cavity) are summarized and compared. Finally, possible directions for future research on the nonthermal effects of MWs are proposed.

This work focuses on summary and analysis of the nonthermal effect of microwaves in chemistry and materials science.

Efficient biocatalytic strategy for one-pot Biginelli reaction via enhanced specific effects of microwave in a circulating reactor

A one-pot efficient biocatalytic strategy for the synthesis of 3,4-dihydropyrimidin-2-(1H)-ones was developed in a circulating microwave reactor selecting α-chymotrypsin as the promiscuous biocatalyst. In the circulating reaction system, the combination of microwave heating and external cooling could avoid the denaturation and inactivation of enzyme, and greatly improved the radiation power of microwave, thus improving the specific effects of microwave. During the reaction process, the microwave radiation power was automatically adjusted by adjusting the speed of the reaction mixture circulation. When the microwave power was maintained at 110 W, the best results could be obtained with the highest yield of 96% at 55 °C in 50 min, and the reaction had a wide range of substrates. But no obvious product was detected in a tank microwave reactor at 55 °C for 100 min, under this condition, the microwave power was maintained at about 3 W. As a contrast, the reaction only obtained 63% yield in 55 °C oil bath for 96 h.

Energy efficient sludge solubilization by microwave irradiation under carbon nanotube (CNT)-coated condition

Microwaves (MW) have great potential for sludge solubilization, and carbon materials can act as good microwave absorbers and heat transfer media because of their high dielectric loss tangent and thermal conductivity. In this study, carbon nanotube-coated MW vessels were developed by preparing a silane-CNT mixture and spray coating. In addition, sludge solubilization by microwave irradiation was performed to evaluate the effects of the CNT-coating at different initial total suspended solid (TSS) concentrations, target temperatures, and MW irradiation times in the uncoated and CNT-coated MW vessels. The sludge solubilization efficiency increased with increasing MW irradiation time and temperature and followed a first-order reaction in both vessels. However, the energy requirement to maintain the temperature was reduced in the CNT-coated MW vessel compared to the uncoated vessel. In addition, the Arrhenius equation revealed the catalytic site in the CNT-coated MW vessel to have a temperature of around 130 °C at an average sludge temperature of 100 °C. The maximum chemical oxygen demand (COD) solubilization and soluble COD (sCOD) increase per MW energy used were 1.64 and 1.67 times higher in the CNT-coated MW vessel than in the uncoated vessel, respectively. The increase in soluble total nitrogen and phosphorus in the CNT-coated MW vessel was attributed to cell wall destruction and intracellular protoplast dissolution, because of the acceleration of the MW thermal effect and high conductivity of CNTs, as well as the MW-induced cell wall and membrane disruption by hot spots on the CNT surface. This suggests that CNTs can be applied to increase the energy efficiency in MW-based pretreatment methods.