A. Arques

Organic photocatalysts for the oxidation of pollutants and model compounds

Organic Photocatalysts for the Oxidation of Pollutants and Model Compounds M. Luisa Marin, Lucas Santos-Juanes, Antonio Arques, Ana M. Amat, and Miguel A. Miranda* Instituto Universitario Mixto de Tecnología Química-Departamento de Química (UPV-CSIC), Avda. de los Naranjos s/n, E-46022, Valencia, Spain Departamento de Ingeniería Textil y Papelera, Universidad Polit ecnica de Valencia, Campus de Alcoy, Plaza Ferr andiz y Carbonell s/n, E-03801 Alcoy, Spain

Ozonisation coupled with biological degradation for treatment of phenolic pollutants: a mechanistically based study.

Phenolic acids constitute an important group of pollutants which are reluctant to biological treatment. Solutions containing a mixture of cinnamic acid, p-coumaric acid, caffeic acid and ferulic acid were submitted to ozonisation. Then, the changes in biodegradability along the process were studied by means of respirometry. There is an optimum ozone dosage in the interval 3-5 min of treatment which allows to achieve the maximum increase in biodegradability (more than 10 times) and a high efficiency of the ozonisation process (COD decreases to a half of its initial value). Further ozonisation does not help to increase biodegradability and is clearly disadvantageous. Similar results are obtained with actual samples of olive oil wastewaters. This behaviour is explained based on the formation of highly biodegradable benzaldehydes as key ozonisation intermediates, in the early reaction stages.

Photochemical fate of a mixture of emerging pollutants in the presence of humic substances

The photodegradation of a mixture of the emerging pollutants (EPs) clofibric acid, amoxicillin, acetamiprid, acetaminophen, carbamazepine, and caffeine was studied under irradiation with a xenon lamp. The quantum efficiencies of the EPs were determined when irradiated individually. Experiments with the mixture of the EPs showed that indirect photoprocesses attributable to interaction between EPs can either enhance the photodegradation rate by photosensitization or decrease it by quenching processes. The addition of humic substances (HS) to the solutions resulted in an increase of indirect photoprocesses with higher effects on acetaminophen and carbamazepine; this was more remarkable when a filter was used to cut off radiation in the range 280-295xa0nm. Experiments carried out with chemical probes indicated that the triplet excited states of HS play a major role in the photosensitization process, although the contribution of other species cannot be completely ruled out. Additionally, V. fischeri toxicity tests showed a synergistic effect produced by the mixture of EPs before irradiation. Photodegradation resulted in an enhanced toxicity of the solution at the initial steps of the process, which was associated both with synergistic effects and with the formation of toxic photodegradation by-products of clofibric acid.

Pyrylium salt-photosensitised degradation of phenolic contaminants present in olive oil wastewaters with solar light: Part II. Benzoic acid derivatives

Abstract Solar light photodegradation, catalysed by a pyrylium salt, of seven benzoic acids present in olive oil mill, has been studied. Significant percentages of photodegradation (20–40%) have been achieved after 6xa0h of solar exposure for six of the acids, even though they were expected to be difficult to oxidise, due to the presence of an electron-withdrawing carboxylic acid group directly attached to the aromatic ring. Quenching constants for the electron-transfer process between the substrate and the excited catalyst were calculated by means of fluorescence measurements for syringic acid (66×10 9 xa0M −1 xa0s −1 ), gallic acid (51×10 9 xa0M −1 xa0s −1 ), veratric acid (51×10 9 xa0M −1 xa0s −1 ), vanillic acid (48×10 9 xa0M −1 xa0s −1 ), protocatechuic acid (37×10 9 xa0M −1 xa0s −1 ) and p -hydroxybenzoic acid (15×10 9 xa0M −1 xa0s −1 ); no quenching was found for benzoic acid. These photophysical measurements are in good correlation with the yields obtained in the pyrylium salt photocatalysed degradation of those phenolic acids.

A reliable monitoring of the biocompatibility of an effluent along an oxidative pre-treatment by sequential bioassays and chemical analyses.

A new approach to assess biocompatibility of an effluent, based on combination of different bioassays and chemical analyses, has been tested using a mixture of four commercial pesticides treated by a solar photo-Fenton as target effluent. A very fast elimination of the pesticides occurred (all of them were below detection limit at t30W=36 min), but mineralisation was a more time-consuming process, due to the formation of organic intermediates and to the presence of solvents, as shown by GC-MS analysis. Measurements based on activated sludge indicated that detoxification was coincident with the removal of the active ingredients, while more sensitive Vibrio fischeri bacterium showed significant toxicity until the end of the experiment, although the effluent might be compatible with biological processes. Biodegradability of the solutions was enhanced by the photochemical process, to reach BOD5/COD ratios above 0.8. Longer time bioassays, such as the Zahn-Wellens test, support the applicability of coupling photochemical with activated sludge-based biological processes to deal with these effluents.

Pyrylium salt-photosensitized degradation of phenolic contaminants derived from cinnamic acid with solar light: Correlation of the observed reactivities with fluorescence quenching

Abstract Solar light photodegradation of three cinnamic acid derivatives (ferulic acid, caffeic acid and p -coumaric acid) present in olive oil industry wastewater has been achieved using 2,4,6-triphenylpyrylium hydrogen sulfate as an electron transfer photosensitiser. The parent cinnamic acid undergoes no photodegradation under the employed reaction conditions. The relative reactivities are in agreement with expectations based on qualitative reasoning: the substrates with two electron-donating groups, ferulic and caffeic acid, react faster than p -coumaric acid, which has only one activating substituent; while the non-activated cinnamic acid is nearly unreactive. The data obtained from fluorescence measurements show that the excited singlet state of the pyrylium salt is quenched by the acids, with the following rate constants: 3.6×10 9 xa0M −1 xa0s −1 for cinnamic acid, 4.4×10 10 xa0M −1 xa0s −1 in the case of p -coumaric acid, 8.7×10 10 xa0M −1 xa0s −1 for caffeic acid, and 1.0×10 11 xa0M −1 xa0s −1 for ferulic acid. These values are in good agreement with the order of reactivities observed in the preparative solar reactions.

Reactivity of neonicotinoid insecticides with carbonate radicals

The reaction of three chloronicotinoid insecticides, namely Imidacloprid (IMD), Thiacloprid (THIA) and Acetamiprid (ACT), with carbonate radicals (CO·₃⁻) was investigated. The second order rate constants (4 ± 1) × 10⁶, (2.8 ± 0.5) × 10⁵, and (1.5 ± 1) × 10⁵ M⁻¹ s⁻¹ were determined for IMD, THIA and ACT, respectively. The absorption spectra of the organic intermediates formed after CO·₃⁻ attack to IMD is in line with those reported for α-aminoalkyl radicals. A reaction mechanism involving an initial charge transfer from the amidine nitrogen of the insecticides to CO·₃⁻ is proposed and further supported by the identified reaction products. The pyridine moiety of the insecticides remains unaffected until nicotinic acid is formed. CO·₃⁻ radical reactivity towards IMD, ACT, and THIA is low compared to that of HO• radicals, excited triplet states, and ¹O₂, and is therefore little effective in depleting neonicotinoid insecticides.

p-Coumaric acid photodegradation with solar light, using a 2,4,6-triphenylpyrylium salt as photosensitizer: A comparison with other oxidation methods

Abstract p-Coumaric acid has been used as a probe in order to study the effect of solar light catalysed by 2,4,6-triphenylpyrylium salts on phenolic compounds present in olive oil industry wastewaters. The results are very satisfactory, and important degradation yields are achieved. Methylene blue has also been used as a photocatalyst, but it results in slower degradation. Other advanced oxidation methods (ozone and/or UV radiation) have been tested as well; as expected, p-coumaric acid abatement is much faster (100 times), but ozone and UV are dangerous and expensive for industrial uses. In contrast with other phenolic acids, ozone and UV do not show an important synergistic effect in p-coumaric acid oxidation. This could be due to differences in the absorption spectra. Major p-coumaric acid oxidation intermediates have been identified and quantitated by HPLC; on the basis of these data, a reaction mechanism is proposed.

Pyrylium salt-photosensitized degradation of phenolic contaminants present in olive oil wastewater with solar light: Part III. Tyrosol and p-hydroxyphenylacetic acid

Abstract Photodegradation of tyrosol and p-hydroxyphenylacetic acid, present in olive oil wastewater, can be achieved using 2,4,6-triphenylpyrylium salt as solar photocatalyst. Fluorescence quenching measurements support the involvement of electron transfer as the key step in this process, with formation of substrate-derived radical cations. Important degradation yields are achieved. Other advanced oxidation methods (ozone and/or UV-radiation) have been tested as well; as expected, tyrosol and p-hydroxyphenylacetic acid abatement is faster, but ozone and UV are dangerous and expensive for industrial uses. However, ozone and UV do not show an important synergistic effect in tyrosol and p-hydroxyphenylacetic acid oxidation. p-Hydroxymandelic acid and p-hydroxybenzaldehyde have been detected and identified by HPLC as intermediates; on the basis of these data, a reaction mechanism involving oxidation at the benzylic position is proposed.

Stability and performance of silica gel-supported triphenylpyrylium cation as heterogeneous photocatalyst

Pyrylium salts are known to be good photocatalysts in aqueous solutions, able to achieve the photodegradation of several phenolic pollutants. Nevertheless, they exhibit a limited stability in these media. The rate of the hydrolytic reaction depends on the pH of the solution, the intensity of irradiation and the pyrylium concentration. Silica gel plates have been employed as inorganic support of the pyrylium cation in order to improve its stability and performance as a heterogeneous catalyst. The resulting material has been tested using ferulic acid as a model compound. Important degradation yields have been achieved (up to 80% after 6 h of solar irradiation). The catalyst is stable under these conditions and can be easily recovered for further use.