Environmental Engineering Reference
In-Depth Information
thereby improving the adsorption of 4-cp. electron-hole recombination was also minimized under a pH of 5, as suggested by
the increased degradation rate of 4-cp. However, according to liu et al. [57], high levels (i.e., above the optimum level) of dop-
ant ions such as Zn
2+
limit the diffusion of Zn
2+
on the TiO
2
lattice. Therefore, many ions are deposited on the particle surface,
thereby inhibiting photocatalytic activity. Thus, the optimum Zn
2+
ion content was set at 0.5 mol% in the photocatalytic degra-
dation of rhodamine b.
Immobilizing TiO
2
on a rigid support, such as Ac [6, 49, 62], perlite granules [60], Sic [51], SiO
2
spheres [61], and magnetic
cores, [7] enhances photocatalytic degradation of organic contaminants and the separation properties of suspended TiO
2
photo-
catalysts from the solution. li et al. [6] doped TiO
2
on Ac grains to hybridize the photocatalytic activity of TiO
2
with adsorptiv-
ity of Ac for photocatalytic degradation of methyl orange. Ac enhanced the photoefficiency of TiO
2
photocatalysts by preventing
the recombination of electron-hole pairs and extending its adsorption of methyl orange. Methyl orange was then transferred to
TiO
2
and photocatalytically degraded. He et al. [62] investigated the use of TiO
2
on Ac in removing rhodamine b under
microwave irradiation. rhodamine b is stable in aqueous solutions under microwave irradiation. However, the degradation
efficiency of rhodamine b improved significantly to 96% 20 min after Ac-supported TiO
2
was introduced into the solution, in
which rhodamine b was mineralized to cO
2
and water after the conjugated structure of rhodamine b was destroyed by photo-
catalysts. perlite and Sic are also effective supports for TiO
2
in the photocatalytic degradation of phenol [60] and 2-propanol
[51]. Hosseini et al. [60] revealed that the rate of phenol degradation is positively affected by UV light intensity. Yamashita
et al. [51] indicated that for photocatalytic degradation, the hydrophobic surface of TiO
2
-Sic photocatalysts is preferred over
that of 2-propanol diluted in water, which produces cO
2
and water.
core-shell systems, composed of nanosized photocatalysts as the shell and a core with support, are required for easily
reclaiming photocatalysts after photocatalytic degradation by either sedimentation or filtration of the aqueous solution. The
properties of core-shell photocatalysts can be varied depending on the application, including the size, composition, and thick-
ness of both the core and the shell. Using an inexpensive core in these systems reduces the preparation cost compared with
non-core-shell systems, which use solely pure photocatalysts. Fu et al. [7] reported on using nanoscale hexaferrites (baFe
12
O
19
)
as a magnetic core to enhance the separation properties of photocatalysts from the aqueous solution. They also indicated that
the TiO
2
shell is responsible for the photocatalytic degradation of organic contaminants. Wilhelm et al. [61] coated TiO
2
on SiO
2
spheres to photodegrade rhodamine b and found that rhodamine b was decomposed completely to colorless products, such as
cO
2
, mineral acids, and water, after illumination. Sun et al. [49] found that nanosized Sn(IV)/TiO
2
/Ac photocatalysts enhance
the photodegradation activity of orange G and exhibit effective separation from the aqueous solution. Tin, at a dosage of approx-
imately 2.5 % and pH of about 2, exhibited the highest photocatalytic activity. However, the degradation efficiency dropped
with SO
4
2−
and H
2
pO
4
−
in the solution because these ions are deposited on the surface of photocatalysts and reduce vacancies
for orange G adsorption, thereby reducing photodegradation activity.
17.2.2.2 Zinc Oxide Nanoparticles
ZnO is a cheap and nontoxic photocatalyst used in degrading and mineralizing environ-
mental contaminants to cO
2
, water, and mineral acids [43]. ZnO has approximately the same band-gap energy as TiO
2
(3.2 eV)
and is thus expected to exhibit photocatalytic degradation activity similar to that of TiO
2
[16, 17]. However, ZnO nanoparticles
outperform TiO
2
nanoparticles in photocatalytically degrading wastewater contaminants. Sobana et al. [43] and chen et al. [63]
found that ZnO is more reactive than TiO
2
in the photocatalytic degradation of dyes under UV light irradiation. Moreover, the
photodegradation rate is faster in alkaline conditions. The surface of ZnO in such conditions is negatively charged, favoring the
absorption of dye with a positive charge. Thus, more active OH• radicals are generated, thereby enhancing activity. Furthermore,
the efficiency of degrading estrone in water using ZnO is higher than that of degradation using TiO
2
[64]. The efficiency is
higher because ZnO exhibits higher UV absorbance than TiO
2
within the wavelength range of 320-385 nm. The excellent
activity of ZnO is also due to its intrinsic properties and unique open structure, which acts as a harvesting site for photogene-
rated electrons during optical excitation, thereby enhancing charge separation. Therefore, ZnO is a suitable alternative to TiO
2
because its photocatalytic degradation outperforms that of TiO
2
and both have a similar photocatalytic degradation mechanism
[65]. Jang et al. [66] compared the photocatalytic degradation capacity of methylene blue between ZnO and ZnO nanocrystal-
line particles. ZnO nanoparticles exhibited greater photocatalytic degradation capacity than ZnO nanocrystalline particles
because of the larger surface area provided by the former. The photodegradation efficiency of the dye increases by increasing
photocatalyst loading (i.e., expanding the total active surface area) or decreasing the initial concentration of methylene blue
solution. However, increasing photocatalyst loading decreases degradation activity because UV light penetration decreases
upon loading beyond the optimum limit.
researchers have made some modifications to increase the efficiency of ZnO in photocatalytically degrading contaminants.
Wu et al. [67] found that ZnO coated with TiO
2
degraded phenol better than pure ZnO. Moreover, the light absorption scope of
composite nanoparticles is wider than that of pure TiO
2
. Yu et al. [68] investigated the performance of pure and Ag-modified
ZnO as photocatalysts in the photocatalytic degradation of rhodamine b dye. Activity increased after incorporation of Ag into
