11.3 Thermal-Assisted Photo-Fenton Reaction
The effect of the reaction temperature on the degradation rate of formic acid, using
the Fenton and photo-Fenton processes, was investigated [30]. First, for both
reactions, a stirred tank laboratory photoreactor irradiated from the bottom was
used to evaluate the kinetic parameters between 298 and 328 K. Afterward, the
proposed kinetic model was used to predict the conversion of the organic pollutant in
a flat-plate solar photoreactor. The previously reported radiation field and mass
balances have been used to compute the formic acid and hydrogen peroxide concentrations as a function of time in the solar reactor. Theoretical and experimental
results show that UV solar radiation improves the effectiveness of the Fenton
process. At lower temperatures, the pollutant conversion enhancement is significant,
but this effect is less important at higher temperatures. For instance, experimental
conversion enhancements after 20 min are 186.0, 74.0, and 7.4% for 298, 313, and
328 K, respectively.
11.4 The Detrimental Factors
Although HO
• is a powerful species that can nonselectively react with most organics,
the Fenton reaction could not reach deep mineralization of target pollutants (usually
less than 50% CO 2 yield). The final products formed are usually low-molecularweight organic acids (LMWOAs), such as oxalic, malonic, and acetic acids. Consistent with this is that the Fenton’s reagent is powerful at initial stage and gradually
loses its capability for organic destruction with reaction time. It is commonly
accepted that such deactivation is due to the interaction of Fe
3+ with LMWOAs
(the degradation intermediates), which is unfavorable to either Fe
3+ /Fe
2+ recycling
or to HO
• generation, although the exact mechanism is still obscure. Effective
circumvention of the blockage from these LMWOA intermediates by an appropriate
pathway may improve the iron cycle and achieve continuous production of HO
• for
target pollutant mineralization.
The influence of low-molecular-weight organic acids (LMWOAs), such as
malonic acid, ethylenediaminetetraacetic acid, and oxalic acid, on the Fenton degradation of organic pollutants was examined under visible irradiation (λ > 450 nm)
[31]. The Fenton degradation of malachite green in the dark was completely blocked
in the presence of LMWOAs. It was found that either visible light irradiation or the
addition of hydroquinone could initiate the dye degradation, but the mineralization
yield was almost zero. An important result was that the dye mineralization in the
presence of LMWOAs could be achieved when both visible irradiation and hydroquinone were introduced (Fig. 11.6). Similar results were obtained with colorless
pollutants, such as benzyltrimethylammonium chloride and 2, 4, 5-trichlorophenol.
The coupling visible irradiation and hydroquinone could be a strong and universal
11.4 The Detrimental Factors
271
The effect of the reaction temperature on the degradation rate of formic acid, using
the Fenton and photo-Fenton processes, was investigated [30]. First, for both
reactions, a stirred tank laboratory photoreactor irradiated from the bottom was
used to evaluate the kinetic parameters between 298 and 328 K. Afterward, the
proposed kinetic model was used to predict the conversion of the organic pollutant in
a flat-plate solar photoreactor. The previously reported radiation field and mass
balances have been used to compute the formic acid and hydrogen peroxide concentrations as a function of time in the solar reactor. Theoretical and experimental
results show that UV solar radiation improves the effectiveness of the Fenton
process. At lower temperatures, the pollutant conversion enhancement is significant,
but this effect is less important at higher temperatures. For instance, experimental
conversion enhancements after 20 min are 186.0, 74.0, and 7.4% for 298, 313, and
328 K, respectively.
11.4 The Detrimental Factors
Although HO
• is a powerful species that can nonselectively react with most organics,
the Fenton reaction could not reach deep mineralization of target pollutants (usually
less than 50% CO 2 yield). The final products formed are usually low-molecularweight organic acids (LMWOAs), such as oxalic, malonic, and acetic acids. Consistent with this is that the Fenton’s reagent is powerful at initial stage and gradually
loses its capability for organic destruction with reaction time. It is commonly
accepted that such deactivation is due to the interaction of Fe
3+ with LMWOAs
(the degradation intermediates), which is unfavorable to either Fe
3+ /Fe
2+ recycling
or to HO
• generation, although the exact mechanism is still obscure. Effective
circumvention of the blockage from these LMWOA intermediates by an appropriate
pathway may improve the iron cycle and achieve continuous production of HO
• for
target pollutant mineralization.
The influence of low-molecular-weight organic acids (LMWOAs), such as
malonic acid, ethylenediaminetetraacetic acid, and oxalic acid, on the Fenton degradation of organic pollutants was examined under visible irradiation (λ > 450 nm)
[31]. The Fenton degradation of malachite green in the dark was completely blocked
in the presence of LMWOAs. It was found that either visible light irradiation or the
addition of hydroquinone could initiate the dye degradation, but the mineralization
yield was almost zero. An important result was that the dye mineralization in the
presence of LMWOAs could be achieved when both visible irradiation and hydroquinone were introduced (Fig. 11.6). Similar results were obtained with colorless
pollutants, such as benzyltrimethylammonium chloride and 2, 4, 5-trichlorophenol.
The coupling visible irradiation and hydroquinone could be a strong and universal
11.4 The Detrimental Factors
271