R. Vicente

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.

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.

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.

Effect of organic species on the solar detoxification of water polluted with pesticides

The effect of organic species on a solar-driven photo-Fenton treatment of a mixture of pesticides (methyl-oxydemethon, methidathion, carbaryl and dimethoate) has been studied in this paper. Triethoxyisododecyl alcohol, acetophenone and ethylenediaminetetraacetic acid (EDTA) have been used as examples of surfactants, solvents and complexing agents, respectively. An inhibitory effect on mineralization as well as on the elimination of the pesticides was observed in the case of the aliphatic surfactants, most probably due to the competition between the pesticides and the added organic matter for reaction with the relatively unselective hydroxyl radical. A methodology combining chemical analyses and bioassays was tested in order to explore the applicability of coupling a photo-Fenton process with a biological treatment in the presence of the surfactant. Despite the complexity of the mixture under study, a reliable monitoring of the process was accomplished; the biocompatibility of the mixture was enhanced and the optimal irradiation intensity was achieved just after complete removal of the pesticides.

Humic-like substances from urban waste as auxiliaries for photo-Fenton treatment: a fluorescence EEM-PARAFAC study

In this work, analysis of excitation-emission-matrices (EEM) has been employed to gain further insight into the characterization of humic like substances (HLS) obtained from urban wastes (soluble bio-organic substances, SBOs). In particular, complexation of these substances with iron and changes along a photo-Fenton process have been studied. Recorded EEMs were decomposed by using parallel factor analysis (PARAFAC). Three fluorescent components were identified by PARAFAC modeling of the entire set of SBO solutions studied. The EEM peak locations (λex/λem) of these components were 310-330 nm/400-420 nm (C1), 340-360 nm/450-500 nm (C2), and 285 nm/335-380 nm (C3). Slight variations of the maximum position of each component with the solution pH were observed. The interaction of SBO with Fe(iii) was characterized by determining the stability constants of the components with Fe(iii) at different pH values, which were in the order of magnitude of the ones reported for humic substances and reached their highest values at pH = 5. Photochemical experiments employing SBO and Fe(iii), with and without H2O2, showed pH-dependent trends for the evolution of the modeled components, which exhibited a strong correlation with the efficiency reported for the photo-Fenton processes in the presence of SBO at different pH values.

New Route for Valorization of Oil Mill Wastes: Isolation of Humic-Like Substances to be Employed in Solar-Driven Processes for Pollutants Removal

The valorization of olive oil mill solid wastes (OMW) has been addressed by considering it as a possible source of humic-like substances (HLSs), to be used as auxiliary substances for photo-Fenton, employing caffeine as a target pollutant to test the efficiency of this approach. The OMW-HLS isolation encompassed the OMW basic hydrolysis, followed by ultrafiltration and drying. OMW-HLS structural features have been investigated by means of laser light scattering, fluorescence, size exclusion chromatography, and thermogravimetric analysis; moreover, the capability of OMW-HLS to generate reactive species under irradiation has been investigated using spin-trap electronic paramagnetic resonance. The caffeine degradation by means of photo-Fenton process driven at pH = 5 was significantly increased by the addition of 10 mg/L of OMW-HLS. Under the mechanistic point of view, it could be hypothesized that singlet oxygen is not playing a relevant role, whereas other oxidants (mainly OH• radicals) can be considered as the key species in promoting caffeine degradation.

SBO in Water Detoxification: Photo-Fenton Processes at Mild Conditions

The implementation of a photo-Fenton process at mild acidic conditions is a potential environmental application for SBOs. The Fenton reagent (sacrificial amounts of hydrogen peroxide and catalytic iron salts) has been demonstrated as an efficient method for the removal of toxic xenobiotics that is enhanced upon irradiation; sunlight can be used for this purpose. In order to avoid precipitation of iron at pH above 3, several strategies have been tested. One of them involves formation of photoactive iron complexes. Humic substances have been employed for this purpose, due to their ability to complex iron. Because of its similar chemical properties, SBO are candidates for this purpose. Experiments carried out with different pollutants have shown that SBO are not good photocatalysts because of the strong screen effect associated to their color, while they are useful to drive a photo-Fenton at milder pH, as they are good complexing agents for iron. Doehlert matrixes have been employed to determine that the pH range for efficient photo-Fenton can be extended to values of ca. 5 and that optimal SBO concentration is approximately 20 mg/L. Finally, SBO have demonstrated to be non toxic, scarcely biodegradable and relatively resistant to oxidizing conditions.