Nano zinc oxide
Nano zinc oxide (nano-ZnO) is typically used as a photocatalytic
nanomaterial in the degradation of dyes. However, nano-ZnO has
also been used as a nanocomposite together with chitosan for
the removal of acid black 26 and direct blue 78 [71]. In most conventional
chitosan adsorption studies, the time required to achieve
equilibrium is usually at least 7 h, regardless of the type of dye
[107]. However, by immobilizing nano-ZnO on the surface of chitosan,
equilibrium was achieved in only 3 min [71]. In addition
to strong electrostatic attraction between the positive charged
NH3-group from chitosan and the negatively charged anionic dye,
the physical properties of nano-ZnO also contributed to the rapid
adsorption of these dyes. These properties include high surface
charge of nano-ZnO (+40 mV at pH 2) [108] and high surface area
of nanoscale ZnO. Nano-ZnO was used to adsorb acid black 26 and
direct blue 78 at initial dye concentrations of 25 ppm, 0.5 g/L
adsorbent and pH 2. The adsorption capacities of nano-ZnO on acid
black 26 and direct blue 78 were 52.63 and 34.48 mg/g, respectively
[71]. The adsorption capacities, however, decreased signifi-
cantly at pH higher than 2, due to increasing numbers of
negative charged ions and the resulting increase in electrostatic
attraction. As such, it was concluded that the main adsorption
mechanism for nano-ZnO on these dyes is due to electrostatic
attraction.