Agriculture Reference
In-Depth Information
increasing TiO
2
concentration within the optimal value and decreased over the
optimal value.
Auxiliary oxidants such as H
2
O
2
can promote photocatalytic oxidation rate and
have two functions during the photocatalytic oxidation process: (1) they can
promote oxidation degradation as good oxidants, and (2) they can receive
photogenerated electrons from the valence band to form hydroxyl radicals with
strong oxidizing ability. Thus, auxiliary oxidants not only increase the oxidative
capacity of photocatalytic degradation but also promote the reaction process.
Similar to aeration process, increasing the oxygen content in the reaction solution
promotes production of oxidizing substances, subsequently enhancing the oxidation
effects.
12.8 Photocatalytic Reactor
Currently, in the field of photocatalysis, there has been much focus on the prepa-
ration of materials and improvements, and less attention has been given to the
engineering applications of photocatalytic technology, which has limited the tran-
sition of this technology from the laboratory to engineering applications.
Photocatalytic reactors are mainly classified into floating and fixed reactors.
According to the status of the photocatalyst, the TiO
2
photocatalytic reactors are
generally divided into floating and fixed reactors. The floating photocatalytic
reactor is generally cylindrical and filled with TiO
2
powder, and the pollutants
and TiO
2
powder are mixed by gases or stirring. The characteristics of this reactor
include large surface area, good mass transfer effects, and low degradation rate.
However, light penetration decreases with the increasing concentration of TiO
2
powder, and the separation of TiO
2
powder from the solution is difficult. As a result,
TiO
2
is generally recycled by filtration and centrifugation, which confer high
operational complexity and cost. An et al. (
2004
) designed a new type of floating
photocatalytic reactor, in which gas distribution was achieved by using porous
platen. The reactor promoted separation of photogenerated electrons and holes by
impressed voltage to improve the photocatalytic efficiency. Within 120 min of
operation, 500 mL of 0.25 mmol/L quinoline was degraded in the reactor, with a
degradation rate of 93 %. Fixed photocatalytic reactors, on the other hand, are more
commonly applied, in which TiO
2
is fixed on the surface of the substrate in a certain
manner. Mansilla et al. (
2007
) coated TiO
2
on the surface of a glass and achieved a
degradation of 18 mg/L of flumequine under UV light irradiation in 20 min, with
the degradation rate reaching 80 %.
Recently, researchers tried to combine the advantages of floating and fixed
reactors by employing magnetic TiO
2
photocatalyst and designed a batch reactor,
known as a magnetic separation photocatalytic reactor, characterized by recover-
ability of the photocatalyst. In this novel reactor, the TiO
2
powder is suspended in
the system and separated as well as recycled by a strong magnetic field after the
treatment process, thus overcoming disposal problems. The steps involved in this
