Chemistry Reference
In-Depth Information
19.8.1
Percolates containing soluble aromatic contaminants
The analysis of aqueous percolates in industrial areas is a usual task in order to verify the possible risk of
contamination of subsurface water sources. Aromatic sulfonates, widely used as intermediates in dyes
production, are a group of hydrophilic pollutants of great environmental concern, found in the percolates
basin within certain abandoned industrial areas. Samples of these aqueous percolates were analysed and
t
he
possible photocatalytic treatment of such wastes was reported [30]. The obtained results suggest that the
residual samples could be also readily treated in the laboratory before discharge.
A typical example of aromatic sulfonate found is the 2,6-naphthalenedisulfonate (2,6-NdS). Figures 19.4
and 19.5 show the evolution of the photocatalytic treatment of 2,6-NdS in closed and aerated cells respectively.
The trend of the TOC decrease is faster in the second case, thus confirming the crucial role played by dissolved
oxygen and by surface-adsorbed oxygen in these treatments. A favourable effect is also observed on the
desulfonation, with the corresponding increase of the sulfate formation rate.
It is noteworthy that the complete abatement of 2,6-NdS occurs after 50 min irradiation, but the
stoichiometric concentration of sulfate and the complete mineralization were achieved only after roughly 3 h,
thus supporting the hypothesis of the presence of reaction intermediates still containing sulfonate groups, for
which the degradation proceeds at a slower rate.
The substrate degradation rate increases by increasing the content of dissolved air within the irradiated cell
(Figure 19.5). Under these conditions the complete mineralization of the starting substrate, with the
corresponding stoichiometric formation of sulfate, can be achieved after around 2 h.
Complex percolate samples containing several different aromatic sulfonates (ca. 80 mg l
−1
of TOC) can
be readily treated in a closed laboratory reactor (capacity 500 ml, Hg-lamp 125 W). The observed TOC
decrease is roughly 84
after 8 h irradiation. If bubbled air flows continuously in the reactor, the
TOC decrease becomes roughly 97
%
%
after less than 5 h.
19.8.2
Photocatalytic destruction of aromatic amine residues in aqueous wastes
Aromatic amines have been recognized as high priority pollutants. The presence of such compounds, even at
very low concentration levels, is often investigated in a variety of environmental and waste samples. Moreover
these compounds can be also introduced as chemical reagents in analytical methods, usually as chelating
ligands or coupling reagents in some spectrophotometric determinations. [31].
Photocatalysis is effective for the abatement of aniline and various aniline derivatives in aqueous wastes
saturated with air [32]. The following equation describes the stoichiometric mineralization of aniline:
C
6
H
5
−NH
2
+
9O
2
→
6 CO
2
+
3 H
2
O
+
H
+
+
NO
3
−
The influence of pH on the primary degradation kinetics is relevant; at low pH values (< 2) the half-life of
aniline is about 10 times longer than at a pH around 6. This significant effect can be attributed to the repulsion
operating between the anilinium cation and the positively charged semiconductor particles On the other hand
the observed large increase of degradation rate at higher pH values (ca. 10) is also due to the increase of the
HO• concentration in basic solutions.
Different degradation steps can be clearly recognized: working at pH around 6 in Solarbox, the degradation
of aniline (ca. 20 mg l
−1
) in the presence of suspended TiO
2
(200 mg l
−1
) is observed after around 40 min
irradiation, the complete mineralization of its organic carbon occurs in less than 1 h, whereas the two main
transient intermediates, 4-hydroxyaniline and 1,3,4-trihydroxybenzene, reach their maximum concentrations
after 20 and 30 min irradiation, respectively, and are in turn completely degraded after roughly 55 min. This
information is crucial to define the irradiation time since, under the reported working conditions at least 1 h
irradiation becomes necessary in order to obtain a safe waste to be discharged.
