Chemistry Reference
In-Depth Information
The addition of K
2
S
2
O
8
has been, for example, successfully applied to increase the degradation rate and the
mineralization extent of pollutants having very different molecular structures, such as organophosphorous
compounds [17], chlorinated aromatics [18], anthraquinonic dyes [19] and others. Hydrogen peroxide can
also be applied to accelerate the photocatalytic degradation of several compounds [20-25], however the
results obtained using this cheap and common reagent are in general less satisfactory if compared with
those obtained using peroxydisulfates [26,27].
19.7
Analytical control of the photocatalytic treatment
The degradation of the target compounds has to be carefully monitored in order to adjust the mass balance
for the photocatalytic decomposition of the contaminants and to ensure that the organic compounds have
not disappeared in some other way (evaporation, adsorption in the reactor, adsorption in the catalyst, etc.)
besides photocatalysis. This monitoring is usually performed by liquid chromatography (HPLC) with UV
detection since direct injection of the aqueous sample into the analytical column is allowed, avoiding sample
pre-treatment except for the filtration necessary to remove TiO
2
. Gas chromatography can be proposed as
an alternative when pre-concentration is needed.
It is also very important to assess the extent of mineralization of the organic C, in order to establish the
moment at which wastewater can be considered completely decontaminated. The basic techniques for the
determination of the residual TOC in water are mainly based on the conversion of organic compounds to CO
2
and then measured using non-dispersive infrared absorption. Since many aqueous samples contain HCO
3
-
and/or CO
3
2−
, it is usually necessary to remove these species using a gas stripping technique before measuring
the TOC.
In view of a safe application of the technique to treat the wastes, the identification and monitoring of
transient intermediates is fundamental; they could be in principle, even more toxic and persistent than the
parent compounds. Gas or liquid chromatography coupled to mass spectrometry (GC/MS and LC/MS) are
suitable techniques for achieving very useful structural information.
In case of degradation products present at sub-mg l
−1
level, the sample can be enriched by liquid-liquid
extraction using an appropriate solvent; however, solid-phase extraction (SPE) is gaining in acceptance
mainly because SPE generates less matrix interference and a wide range of new adsorbents are commercially
available.
As additional information, in order to guarantee that the treatment is correct, toxicity tests can be performed,
particularly if incomplete mineralization is observed at the time corresponding to the total disappearance
of the substrate.
19.8
Examples of possible applications of photocatalysis
to the treatment of laboratory wastes
The photocatalytic degradation and mineralization of a great number of organic pollutants was reported since
the beginning of the environmental application studies, starting around 1980. Among them, the following
classes were investigated in detail: aliphatic and aromatic hydrocarbons, haloaliphatic compounds (in
particular chlorinated solvents), haloaromatic derivatives (halophenols, chlorobenzenes, PCBs, dioxins),
aliphatic and aromatic amines, phenols, non chlorinated solvents, surfactants, pesticides, dyes, and so on
[1-5,28,29].
Selected examples of photocatalytic treatments of aqueous laboratory wastes containing some of the
mentioned pollutants are examined in the following sections.
