Environmental Engineering Reference
In-Depth Information
eficiency in the TiO
2
/ U V/O
2
system using suspension anatase TiO
2
was very close to that
obtained for the supported TiO
2
system. In the suspension system, 77% of lindane was
eliminated after 150 min of irradiation. Hiskia et al. (1997) studied the photodegradation of
lindane in aqueous solution, using near-visible and UV light (>320 nm) and in the presence
of the polyoxometalate
PW O
12
3−
, it is converted to CO
2
and HCl. Initial photodecomposi-
tion took place within a few minutes, both in the presence and absence of oxygen, and
the effective mineralization in the absence of oxygen suggests that OH
∙
radicals act as the
primary oxidant, in this case.
Guo et al. (2000) have reported that microporous polyoxometalates (H
3
PW
12
O
40
/SiO
2
and
H
4
SiW
12
O
40
/SiO
2
) were synthesized by encapsulating H
3
PW
12
O
40
and H
4
SiW
12
O
40
into a
silica matrix via hydrolysis of tetraethyl orthosilicate. The degradation study was carried
out with different concentrations of HCH (0-30 mg/L); a plot of (initial HCH degradation
rate)
r
o
vs. Co (initial HCH concentration) exhibited a straight line; and the linear correla-
tion coeficients (
R
) were 0.995 and 0.998 for H
3
PW
12
O
40
and H
4
SiW
12
O
40
, respectively. This
indicated that the disappearance of HCH followed a Langmuir-Hinshelwood irst-order
kinetic law for the initial HCH concentration chosen. Guillard et al. (1996) identiied the
intermediate products formed in a photocatalytically treated aqueous solution contain-
ing 1 g/dm
3
of lindane. Several intermediate by-products have been identiied during the
degradation of lindane such as chlorocyclohexanes, chlorobenzenes, chlorophenols, chlo-
ropropanes, chloropropanones, and the pentachlorocyclohexanone isomer. The detection
of heptachlorocyclohexane in the system showed that chlorine and hydrogen atoms were
not only abstracted from the CHCl groups constituting lindane but also added to them.
Nienow et al. (2008) have reported that the aqueous solutions of lindane were photo-
lyzed (
k
= 254 nm) under a variety of solution conditions. The initial concentrations of
hydrogen peroxide (H
2
O
2
) and lindane varied from 0 to 20 mM and 0.21 to 0.22 μM, respec-
tively. Lindane rapidly reacted, and the maximum reaction rate constant (9.7 × 10
−3
s
−1
) was
observed at pH 7 with an initial H
2
O
2
concentration of 1 mM. Thus, 90% of the lindane is
destroyed in ~4 min under these conditions. In addition, within 15 min, all chlorine atoms
were converted to chloride ions, indicating that chlorinated organic by-products did not
accumulate. Antonaraki et al. (2010) have carried out a detailed examination of the inter-
mediates formed during the photocatalytic degradation of lindane (initial concentration
2.4 × 10
−5
M) using
PW O
40
− −
×( )
catalyst. Lindane was completely mineralized into
CO
2
, Cl
−
, and H
2
O. The intermediates were identiied using gas chromatography-mass
spectrometry (GC-MS) analysis. The intermediates identiied were aromatic compounds
such as dichlorophenol, trichlorophenols, tetrachlorophenol, hexachlorobenzene, di- and
trichloro-benzenodiol, and nonaromatic cyclic compounds such as pentachlorocyclohex-
ene, tetrachlorocyclohexene, heptachlorocyclohexane, aliphatic compounds, tetrachloro-
ethane, and the condensation products polychlorinated biphenyls.
Dionysiou et al. (2000) have reported on the degradation of lindane using TiO
2
immo-
bilized on a continuous low rotating disc and have achieved 63% lindane (initial con-
centration 0.016 mM) degradation. The rotating disc photoreactor operated at a hydraulic
residence time of 0.25 days and at a disk angular velocity of 12 rpm. Cao et al. (2008) have
reported on the degradation of lindane (0.0172 mM) using 4.2 mM persulfate
S
2
2
( )
and
0.108 mM ferrous (Fe
2+
) ions in aqueous solution. Persulfate was activated by ferrous iron
and produced highly potent sulfate radicals
SO
3
4
7 0
M
12
40
( )
with standard potential at 2.6 V. The
rate of lindane oxidation was proportional to the concentration of persulfate in the aque-
ous solution. The inal products of lindane degradation were Cl
−
ion and CO
2
, signifying
complete lindane oxidation. Compared with the classical Fenton's reaction, persulfate has
a relatively longer lifetime in water.
