Photodegradation of 4-chlorophenol sensitized by wastederived soluble organic substances using experimental design
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Abstract
Advanced Oxidation Techniques represent an alternative to more traditional water treatments since they are able to promote the degradation of organic substrates, leading to their complete mineralization. These processes are based on the production of highly reactive species (mainly radicals) and are often light assisted. Among the compounds able to photogenerate reactive species,
humic and fulvic acids (HS) have been widely studied in order to understand the water autopurification mechanisms. It is thus in principle possible to propose the use of HS in wastewater photodegradation treatments; however neither waters nor soils can be considered as exploitable source of HS. The organic fraction of urban wastes has been demonstrated to be an interesting
source of soluble organic substances (SOS), structurally similar to HS and exhibiting good photosensitizing properties.
In the present research the photodegradation of 4-chlorophenol (4-CP), a toxic and hardly biodegradable pollutant, has been studied in the presence of one type of SOS. In order to optimize experimental conditions and to evaluate possible synergistic effects with other photoactive
compounds (TiO2 and H2O2) a chemometric approach has been chosen. Two designs of experiments (D-Optimal, DoE) have been planned and the following parameters have been considered: 4-CP concentration (from 10 to 100 mg L-1), SOS concentration (from 0 to 1000 mg L-1), TiO2
concentration (from 0 to 500 mg L-1) or H2O2 concentration (from 0 to 10-2 M), cut-off filter for the
light source (340 nm, 400 nm and without filter). By mean of the Modde software, 39 experiments were selected for the TiO2-DoE and 25 experiments for the H2O2-DoE. The resulting response surfaces evidenced that, in the presence of TiO2, a competition for the active photogenerated species seems to take place between 4-CP and SOS, since also SOS can be in turn photodegraded.
On the contrary, a synergistic effect between SOS and H2O2 occurs, when the 400 nm cut-off filter is used. After three hours of irradiation 25% of 4-CP abatement was achieved in the presence of 500 mg L-1 of SOS, 35 % was achieved in the presence of 5 mM of H2O2, whereas 45% of 4-CP abatement was obtained in the presence of both SOS and H2O2 at the above mentioned concentrations.
These results encourage the use of chemometric tools and give useful information for a possible scale-up of the process in pilot plants exploiting the solar light as radiation source