Chemistry Reference
In-Depth Information
solid matrices, such as soils, sediments, sludges, plants, foods, and so on, and enhancing the sensitivity of
various analytical methods. It is well known that the extraction of hydrophobic species from many complex
samples using organic solvents suffer of some drawbacks, being usually time-consuming and requiring the
handling of relatively large amounts of more or less toxic, hazardous and expensive solvents. Microwave-
assisted solvent extraction (MAE) [59], accelerated solvent extraction (ASE) [60] and supercritical fluid
extraction (SFE) [61] have been developed in recent years and proposed to overcome most of the mentioned
problems.
The combination of the MAE approach with the use of aqueous surfactant solutions as extracting phase is,
in particular, a promising new extraction approach. The term microwave-assisted micellar extraction (MAME)
was suggested some years ago to indicate this peculiar procedure [62], capable of offering recovery efficiencies
comparable to those obtained using classical extraction methods at lower costs. For example, the cost of a
MAME determination of PAHs in sediments has been estimated to be about 80-90 times lower than the
Soxhlet extraction using CH
2
Cl
2
.
Other examples of surfactant-based procedures, successfully applied to replace the organic solvents, are
the preconcentration/extraction steps exploiting the so-called cloud-point phenomenon, based on the thermal
phase separation occurring in aqueous solutions of non ionic surfactants, and the formation of ionic surfactant
coacervates under high ionic strength conditions [63].
In all these cases, the laboratory aqueous wastes contain relevant concentrations of surfactants which can
originate some problems during the photocatalytic treatment since the surfactant itself is degraded. In
particular it was found in most cases a neat reduction of the degradation rate of the pollutants, if compared
with the one observed in pure water. This surfactant inhibition effect, which can be attributed to the competition
of surfactant molecules for the active sites of the semiconductor particles, must be carefully considered when
the treatment of surfactant-containing wastes is necessary.
19.8.7
Degradation of aqueous solutions of azo-dyes
Azo-dyes represent approximately 50-70
of the dyes available on the market today, followed by the
anthraquinone group. The photocatalytic decolourization of different azo-dyes has been reported in the
literature and recently reviewed [64]. Most of these papers examine in detail the primary process under
different working conditions [65,66,67]. On the contrary, the reaction mechanisms involved in the
photocatalytic degradation of dyes and the identification of major transient intermediates have been only
more recently recognized as very important aspects of these processes [68-70].
%
19.8.8
Treatment of laboratory waste containing pharmaceuticals
The heterogeneous photocatalysis could be an efficient tool also in case of the treatment of laboratory wastes
containing pharmaceuticals [71].
Work on this topic often offers scarce knowledge on photocatalytic intermediates of pharmaceuticals;
Lambropoulou
et al
. identified 17 products of photocatalytic degradation of bezafibrate [72], Zhang
et al
.
proposed a pathway for photocatalytic degradation of acetaminophen via direct hole oxidation and
ipso
-
substitution [73]. As far as atenolol is concerned, Liu
et al
. [74] observed its slow direct photolysis, whereas
for propranolol three photodegradation products of aromatic ring oxidation and opening were identified. In
some experiments Medana
et al
. [75] identified several transformation products of atenolol, mainly mono-,
di- and tri-hydroxylated derivatives.
A further study [76] describes the oxidation of atenolol at pilot-scale using Compound Parabolic Collector
(CPC) under natural illumination. Two widely used AOP systems, TiO
2
and photo-Fenton solar photocatalysis,
were investigated. Special attention was paid to identify the main intermediate products of atenolol formed
