An insight into green microwave-assisted techniques: degradation and microextraction

GOMEZ, NATALIA; AGUINAGA, MAITE; LLAMAS, NATALIA E.; GARRIDO, MARIANO; ACEBAL, CAROLINA C.; DOMINI, CLAUDIA

Advances in Microwave Chemistry

CRC PressINC (Taylor and Francis)

Año: 2018; p. 119 - 179

The microwave (MW) comprise the region of the electromagnetic spectrum x between 0.1 and 100 cm (frequency range of 300 GHz to 300 MHz, respectively). This kind of energy has been successfully applied as a potent tool in chemistry, due to its rapid and efficient selective heating properties. MW irradiation combines both thermal and non-thermal effects. The heating basically involves two processes: dipolar polarization and ionic conduction. Under MW irradiation, polar molecules orientate according to the rapid changes of the alternating electric field. Thus, they suffer rotation, friction and collisions resulting in the generation of heat. Moreover, the electric conductor materials are polarized under the alternating electric field, so the ionic conduction takes place. In this manner, the MW radiation is able to rapidly and simultaneously heats the bulk of the material and could accelerate the rates, enhance the yields and promote certain pathways in different chemical reactions. On the other hand, some authors report the so-called non-thermal effect of the MW radiation, which appears when some molecules are polarized under the electromagnetic field and are aligned leading to possible breakage of hydrogen bonds. Other authors link the non-thermal effects (also called not purely thermal or specific MW effects) with the ?hotspots? theory. Under MW irradiation, some materials known as electromagnetic wave absorbers (conductive polymer, carbon, SiC, ferrite, TiO2, magnetic metal and metal alloys) could strongly absorb MW with the consequent generation of active ?hot spots? on the surface of the materials. These hotspots, which can reach temperatures around 1400 K, would accelerate the movement of electrons and increase the oxidative capacity in degradation reactions. All these characteristics have made MW applications increase in the last decades whether in domestic, industrial or medical fields. This chapter presents a review divided into two parts: the first section discusses the applications reported on using MW irradiation, particularly the ones involving their environmental use for improving the degradation efficiency of pollutants in water and wastewaters. The second section is focused on the different types of microextraction assisted by MW. Figure 1 offers a summary of the number of publications found in the literature devoted to MW-assisted degradation and microextraction.