A.M. Amat

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.

Reactivity of hydroxyl radicals with neonicotinoid insecticides: mechanism and changes in toxicity

The reactivity of hydroxyl radicals (HO ) towards three neonicotonoid insecticides, namely imidacloprid, thiacloprid and acetamiprid was investigated. These radicals were generated by photolysis of H(2)O(2) solutions. Flash photolysis experiments were used to determine the rate constants of 5.5 x 10(10) M(-1)s(-1), 6 x 10(10) M(-1)s(-1), and 7.5 x 10(10) M(-1)s(-1), for the reactions of HO with acetamiprid, imidacloprid, and thiacloprid, respectively. Continuous irradiation experiments in the absence and presence of H(2)O(2) allowed the identification and toxicity evaluation of the primary photo- and oxidation products of the insecticides. In all cases, the less toxic 6-chloronicotinic acid was found to be the major product at higher degrees of oxidation. The results reported here indicate that the half life of the insecticides due to their reaction with HO radicals in natural aquatic reservoirs may vary between 5 h and 19 days, and therefore the hydroxyl radical-mediated oxidation may be a significant abiotic elimination route. However, elimination of the insecticide under such conditions might not improve the quality of the contaminated water, as the primary products of degradation still show considerable toxicity to Vibrio fischeri assays.

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.

Effect of inorganic ions on the solar detoxification of water polluted with pesticides.

The effect of eleven inorganic ions (Cl(-), NO(3)(-), SO(4)(2-); PO(4)(3-), Na(+); NH(4)(+), Ca(2+), Mg(2+), Zn(2+), Cu(2+) and Al(3+)) on the photo-Fenton elimination of pesticides has been investigated. Phosphate and chloride have been demonstrated to have an inhibitory role; on the other hand, the reaction was accelerated in the presence of Cu(2+), most probably due to a copper-driven Fenton-like process. The solar photo-Fenton treatment of a mixture of four commercial pesticides was studied at pilot plant scale in the presence of chlorides. Samples with coincident dissolved organic carbon (DOC) showed similar chemical composition, which resulted in a comparable biocompatibility, however longer irradiation periods were needed to reach the desired mineralization when Cl(-) was present. It was demonstrated that the chemical process was able to improve significantly the biocompatibility of the effluent, as shown by the inhibition of respiration of activated sludge, BOD/COD ratio and Zahn-Wellens test.

Some ozone advanced oxidation processes to improve the biological removal of selected pharmaceutical contaminants from urban wastewater

Removal of nine pharmaceutical compounds—acetaminophen (AAF), antipyrine (ANT), caffeine (CAF), carbamazepine (CRB), diclofenac (DCF), hydrochlorothiazide (HCT), ketorolac (KET), metoprolol (MET) and sulfamethoxazole (SMX)—spiked in a primary sedimentation effluent of a municipal wastewater has been studied with sequential aerobic biological and ozone advanced oxidation systems. Combinations of ozone, UVA black light and Fe(III) or Fe3O4 constituted the chemical systems. During the biological treatment (hydraulic residence time, HRT = 24 h), only AAF and CAF were completely eliminated, MET, SMX and HCT reached partial removal rates and the rest of compounds were completely refractory. With any ozone advanced oxidation process applied, the remaining pharmaceuticals disappear in less than 10 min. Fe3O4 or Fe(III) photocatalytic ozonation leads to 35% mineralization compared to 13% reached during ozonation alone after about 30-min reaction. Also, biodegradability of the treated wastewater increased 50% in the biological process plus another 150% after the ozonation processes. Both untreated and treated wastewater was non-toxic for Daphnia magna (D. magna) except when Fe(III) was used in photocatalytic ozonation. In this case, toxicity was likely due to the ferryoxalate formed in the process. Kinetic information on ozone processes reveals that pharmaceuticals at concentrations they have in urban wastewater are mainly removed through free radical oxidation.

Confirming Pseudomonas putida as a reliable bioassay for demonstrating biocompatibility enhancement by solar photo-oxidative processes of a biorecalcitrant effluent.

Experiments based on Vibrio fischeri, activated sludge and Pseudomonas putida have been employed to check variation in the biocompatibility of an aqueous solution of a commercial pesticide, along solar photo-oxidative process (TiO(2) and Fenton reagent). Activated sludge-based experiments have demonstrated a complete detoxification of the solution, although important toxicity is still detected according to the more sensitive V. fischeri assays. In parallel, the biodegradability of organic matter is strongly enhanced, with BOD(5)/COD ratio above 0.8. Bioassays run with P. putida have given similar trends, remarking the convenience of using P. putida culture as a reliable and reproducible method for assessing both toxicity and biodegradability, as a substitute to other more time consuming methods.

Involvement of triplet excited states in the electron transfer photodegradation of cinnamic acids using pyrylium and thiapyrylium salts as photocatalysts

The mechanistic pathway for degradation of cinnamic acids using 2,4,6-triphenylpyrylium as well as 2,4,6-triphenyl(thia)pyrylium salts (,) as solar photocatalysts has been unambiguously established. Results obtained in steady-state experiments have been correlated with time-resolved photophysical studies. High percentages of photodegradation (60-70%) were achieved when aqueous solutions of caffeic and ferulic acids (,) as model pollutants were submitted to irradiation in the presence of ,. Electron-transfer quenching of both the singlet and triplet excited states of , by , has been proved, and the quenching rate constants (close to diffusion control) have been determined. However, the percentages of singlet quenching by ,, even at relatively high concentrations of the model pollutants, is lower than 5%. In addition to this, growth of the signal corresponding to the pyranyl radical occurs in the microsecond timescale, incompatible with the singlet state as precursor. Thus, photodegradation of , mainly involves the triplet state of the photocatalysts.