3. Metallic nanomaterials
Metallic nanomaterials have a metallic element as one of the
components. To date, nano titanium dioxide (nano-TiO2) [70], nano
zinc oxide (nano-ZnO) [71] and nano magnesium oxide
(nano-MgO) [72] and one pure metal nanomaterial,
nano-zero-valent iron (nZVI) [73], have been studied for dye
adsorption. These metals are not from the same group in the periodic
table, and their physical and chemical properties as well as
their adsorption mechanisms are significantly different. Some
adsorbents, such as nano-TiO2, rely on physical and structural morphology,
resulting in a predominantly physical mechanism for dye
adsorption. In contrast, adsorbents, such as nZVI, adsorb dyes
through an oxidation–reduction mechanism, resulting in predominantly
chemical adsorption. The mechanism of these nanomaterials
in adsorption will be discussed in the next few sections.