Chemistry Reference
In-Depth Information
NHCH
2
CH
3
H
Cl
N
N
O
O
N
HO
C
C
OH
N
N
HN
N
NH
N
C
NHCH(CH
3
)
2
O
OH
Atrazine
Cyanuric acid
Structure 19.1
Atrazine.
Structure 19.2
cynauric Acid.
Due to the usually complex nature of these pollutants, a large number of more or less persistent reaction
intermediates are formed during the treatment and a significant number of mineralization end-products are
obtained. Thus, a careful analytical monitoring is essential in order to control all the transformation steps. The
formation and fate of the more abundant reaction intermediates is usually followed by HPLC, leading to the
observation of typical bell-shaped profiles in all the cases. An interesting point is that some intermediates
can possess higher stability than the starting pollutant, thus exhibiting a higher persistency in the irradiated
waste, whereas other derivatives are formed and degraded simultaneously with about the same degradation
rate of the substrate.
Figure 19.6 illustrates the behaviour of the three major intermediates found during the treatment of
wastewaters containing chloramben residues, and Scheme 19.1 depicts the early steps of its photocatalytic
transformation. All the intermediates that are formed are less persistent than the initial substrate.
Table 19.1 reports the results obtained during the photocatalytic treatment of various pesticides. The
persistence of substrate and intermediates should suggest the proper treatment time.
19.8.4
The peculiar behaviour of triazine herbicides
The photocatalytic treatment of pesticides containing the triazine ring is a typical example of the possible
limits exhibited by the
•
OH attack [42]. The mechanism of degradation of atrazine (6-chloro-N-ethyl-N'-(1-
methylethyl)-1,3,5-triazine-2,4-diamine) involves various steps including dealkylation, dechlorination and
oxidation (of aliphatic carbon and of amino groups), but the degradation stops when the very stable compound
2,4,6-trihydroxy-1,3,5-triazine (cyanuric acid) is formed. This compound is, anyway, less toxic than atrazine
(see Structures 19.1 and 19.2).
19.8.5
Treatment of aqueous wastes containing organic solvent residues
Photocatalysis can also be successfully applied to treat both gaseous and aqueous streams containing organic
solvents. If the treatment of laboratory wastewaters is considered, the experimental approach is essentially
that previously described. Table 19.2 summarizes some of the reported applications in view of the abatement
of the organic solvents residues prior to the discharge of the waste.
19.8.6
Treatment of surfactant-containing aqueous wastes
The use of aqueous surfactant solutions as alternative solvents capable of replacing the widely used but more
dangerous organic liquids is of great concern in the frame of the safer green chemistry practice. In particular,
the environmental applications of such solvents has become increasingly popular and various applications
