Environmental Engineering Reference
In-Depth Information
attack [101]. On the other hand, the disulfane radical (c) can break down
to elemental sulfur and MBT after H-transfer. In experiments performed
by Kirouani-Harani [96], the irradiation of MBT in water at pH 7 also led
to BT and OHBT. This result correlates with those obtained by Brownlee
et al. [93], and is also consistent with the previous mechanism (Fig. 11). MBTS
also might act as quencher for singlet oxygen as reported by Foote [102]
and Foote and Peters [103]. By these means, reaction of MBT with singlet
oxygen cannot be excluded since there are many C
=
S, C
=
Oexchangereac-
tions reported for thiones after treatment with singlet oxygen [96, 104, 105].
In such cases the transformation of MBT to OHBT can also occur as a dir-
ect photochemically induced reaction with singlet oxygen. Another similar
photoconversion mechanism of MBT in concentrated organic solutions of
benzene, toluene, alcohols, or acetonitrile is reported elsewhere [106]. MBT
was found to be phototransformed not only to MBTS dimer but also to
BTSO
2
- O
2
STB dimer, as well as to BT and OHBT. The direct photolysis of
MBT in water is also reported to take place through either the singlet ex-
cited state and the resulting primary intermediates of solvated electrons and
benzothiazolyl radicals, as well the triplet excited state. However, the desul-
furization steps need further clarification. Episulfide or episulfoxide might be
involved as intermediates.
Solar photodegradation of MBT was performed in Milli-Q purified water
at pH 8.0 and in a natural water sampled from a lake [106]. Desulfurization
intobenzothiazolewasclearlyfavoredinthelattercaseandthedisappearance
of MBT was reported to be fourfold faster in the natural than in the Milli-
Q water. The results of this study showed that sensitized reactions mediated
by the chromophoric components of natural waters significantly contribute to
the transformation of MBT in the aquatic environment.
Finally, the estimated quantum yield (
)forMBTusing320 nm radia-
tion was evaluated to be in the range 0.023-0.112 in pure water [96]. Malouki
et al. [106] also investigated the direct photolysis of MBT using 313 nm radi-
ation. The anionic form was found to be photoconverted into BT and OHBT
when irradiated in aerated medium with similar quantum efficiency (
Φ
= 0.02).
Solutions of BT and OHBT were further irradiated, but no degradation of
these compounds was observed in aqueous solution. All bands exhibited no
absorption changes over several hours, indicating the stability of these com-
pounds [96]. The environmentally important conclusion to be drawn from
these findings is that BT and OHBT are the anticipated stable products of
MBT and TCMTB photolysis in aquatic environments.
Φ
3.9
Triphenylboron-pyridine
Hydrolysis, thermolysis, and photolysis are competing abiotic decomposition
pathways of triphenylboron-pyridine (TPBP). TPBP is unstable in artificial
