Search

The use of iron-based nanomaterials for environmental remediation processes has recently received considerable attention. Here, we employed core-shell magnetite-humic acids nanoparticles as a heterogeneous photosensitizer and iron source in photo-Fenton reaction for the degradation of the psychiatric drug carbamazepine (CBZ). CBZ showed low photodegradation rates in the presence of the magnetic nanoparticles, whereas the addition of hydrogen peroxide at pH = 3 to the system drastically increased the abatement of the contaminant. The measured Fe 2+ and Fe 3+ profiles point to the generation of Fe 3+ at the surface of the nanoparticles, indicating a heterogeneous oxidation of the contaminant mediated by hydroxyl radicals. Products with a higher transformation degree were observed in the photo-Fenton procedure and support the attack of the HO • radical on the CBZ molecule. Promising results encourage the use of the nanoparticles as efficient iron sources with enhanced magnet-sensitive properties, suitable for applications in photo-Fenton treatments for the purification of wastewater.

Objectives: Catechins are important components of human diet and have received special attention due to their antioxidant capacity. The purpose of this paper was to study the antioxidant action of (+)-catechin (CTQ) in the presence of vitamin B 2 (riboflavin) as light-absorbing agent. Furthermore, two model compounds, catechol (CTC) and resorcinol (RSC), were selected in order to elucidate the reactive target of the CTQ molecule. The influence of pH-medium was investigated., Methods: Stationary photolysis, polarographic detection of dissolved oxygen, reactive oxygen species (ROS)-scavengers, time-resolved near-IR phosphorescence detection, stationary, and time-resolved fluorescence and laser flash photolysis techniques were employed., Results: CTQ interacts with riboflavin under visible-light photoirradiation as well as with different ROS which are generated in this mechanism. Radical-scavenging activity increases with increasing of pH-medium., Discussion: pH-effect of the medium on radical-scavenging activity comes from the increased electron-donating ability of CTQ upon deprotonation. These results are very interesting due to the fact that the pH of the food products displays important variations. The [Formula: see text]-scavenging ability of CTQ, would be equal to the additive contribution of each reactive center, CTC, and RSC, present at the molecule of CTQ. However, CTQ would have a moderate ability to removal of [Formula: see text]-species at pH 7.

Some organic contaminants dissolved in natural waters tend to adsorb on suspended particles and sediments. In order to mimic the photodegradation routes in natural waters of bromoxynil (BXN) adsorbed on silica, we here prepare and characterize silica nanoparticles modified with BXN (NP-BXN). We measure the direct photolysis quantum yield of aqueous BXN at 307 nm (0.064 ± 0.001) and detect the formation of bromide ions as a reaction product. Under similar conditions the photolysis quantum yield of BXN bonded to NP-BXN is much lower (0.0021 ± 0.0004) and does not lead to formation of bromide ions. The rate constant of the reaction of NP-BXN with the excited triplet states of riboflavin, a molecule employed as a proxy of chromophore dissolved organic matter (DOM) was measured in laser flash-photolysis experiments. The rate constants for the overall (kt) and chemical interaction (kr) of singlet oxygen with NP-BXN were also measured. Kinetic computer simulations show that the relevance of the direct and indirect (through reactions with reactive species generated in photoinduced processes) photodegradation routes of BXN is very much affected by sorption on silica. Immobilization of the herbicide on the particles, on one hand, affects the photolysis mechanism and lowers its photolysis quantum yield. On the other hand, the results obtained in aqueous suspensions indicate that immobilization also lowers the rate of collisional encounter, which affects the quenching rate constants of excited triplet states and singlet oxygen with the herbicide.

The production of dichlorophen (2,2'-methylenebis(4-chlorophenol), DCP) and its use as an anthelmintic and in pesticide products result in its direct release to the environment. To the purpose of modelling the possible photodegradation routes of DCP sorbed on sediments or suspended particles, the synthesis and characterization of silica nanoparticles modified with DCP (NP-DCP) is reported. The reactivity of NP-DCP with the excited states of riboflavin, a sensitizer usually present in natural waters, and with singlet oxygen were investigated. Comparison of the kinetic results obtained here to those previously reported for irradiated aqueous solutions of DCP allowed the discussion of the effect of adsorption of the pesticide on its photodegradation. We show with the aid of computer simulations that in natural waters the relevance of the different photodegradation routes dichlorophen is very much affected by attachment to sediments., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

Within the context of environmentally friendly methods for the elimination of surface-water pollutants, the photodegradation of the phenolic pesticides bromoxynil (BXN) and dichlorophen (DCP) under simulated natural conditions has been studied. The work was done in the presence of the visible-light absorber photosensitizer riboflavin (Rf), usually present in trace quantities in natural waters. Under aerobic conditions, an efficient photooxidation of both pesticides was observed. The relatively intricate photochemical mechanism involves pesticide and oxygen consumption and, to a lesser extent, Rf degradation. The kinetic and mechanistic study supports that both H(2)O(2) and singlet molecular oxygen, O(2)((1)Δ(g)), are involved in the process. Kinetic data for the O(2)((1)Δ(g))-mediated oxidation indicate that BXN and DCP are photodegraded with this species faster than the parent compound phenol, very frequently employed as a model for aquatic contaminants, likely due to their lower pK(a) values. This observation allows the design of phenolic pesticides with different photodegradation rates under environmental conditions., (Copyright © 2010 Elsevier B.V. All rights reserved.)

The acaricide abamectin is a mixture of two colorless homologues in a molar ratio of at least 4:1 with the same structure of macrocyclic lactone. The kinetics of its degradation under direct (254 nm) and dye-sensitized (>400 nm) photoirradiation in methanol solution has been studied by UV-vis spectrophotometry, potentiometric detection of dissolved oxygen, stationary fluorescence, laser flash photolysis, and time-resolved detection of singlet molecular oxygen (O2((1)Delta(g))) phosphorescence. The results indicate that the degradation is very efficient under direct irradiation with UV light (254 nm), with a quantum yield of 0.23. On the contrary, under visible-light irradiation, using the natural pigment riboflavin or the synthetic dye rose bengal as sensitizers, the degradation is very inefficient and proceeds through a O2((1)Delta(g))-mediated mechanism, with a bimolecular rate constant for the overall O2((1)Delta(g)) quenching (the sum of physical and chemical quenching) of 5.5 x 10(5) M(-1) s(-1). This value is similar to those reported for the rate constants of the reactions of O2((1)Delta(g)) with isolated double bonds or conjugated dienes and points to similar processes in the case of abamectin.

The kinetics of rose bengal-sensitized photooxidation of tyrosine and several tyrosine-derivatives (tyr-D) named tyrosine methyl ester, tyrosine ethyl ester and tyrosine benzyl ester was studied in buffered pH 11 water, and buffered pH 11 micellar aqueous solutions of 0.01 M cetyltrimethylammonium chloride (CTAC) and 0.01 M-octylphenoxypolyethoxyethanol [triton X100 (TX100)]. Through time-resolved phosphorescence detection of singlet molecular oxygen (O(2)((1)Delta(g))) and polarographic determination of oxygen consumption, the respective bimolecular rate constants for reactive (k(r)) and overall (k(t)) quenching of the oxidative species by tyr-D were evaluated. Both rate constants behave in different fashion depending on the particular reaction medium. k(r)/k(t) values, increase in the sense CTAC<

The present work studies the visible-light-promoted photodegradation of the colorless fungicide carbendazim (methyl 2-benzimidazolecarbamate) and several 2-substituted benzimidazoles (SBZ's), in water or water-methanol solution, in the presence of air and, as a photosensitizer, the synthetic xanthene dye Rose Bengal (RB) or the natural pigment riboflavin (Rf). The results indicate that the degradation of each particular SBZ depends on its chemical structure and on the sensitizer employed. In the presence of RB, the degradation always operates via a singlet molecular oxygen (O(2)((1)Delta(g)))-mediated mechanism, through a highly efficient process, as deduced from the comparison of the rate constants for physical and chemical quenching of O(2)((1)Delta(g)). In the presence of Rf, the visible-light irradiation of any of the studied SBZ's produces a series of competitive processes that depend on the relative concentrations of Rf and SBZ. These processes include the quenching of excited singlet and triplet Rf states by the SBZ and the generation of both O(2)((1)Delta(g)) and superoxide radical anion (O(2)(-)), the latter generated by electron transfer from excited Rf species to the dissolved oxygen. The overall result is the photodegradation of the SBZ and the photoprotection of the sensitizer.

The kinetics and mechanism of the Riboflavin (Rf)-promoted photochemical degradation with visible light of the herbicide Norflurazon (NF) has been studied by time-resolved and stationary techniques. Using light of wavelength higher than 400 nm--a region where NF is totally transparent--and with concentrations of Rf and NF of ca. 0.02 and 1 mM, respectively, only the excited triplet state of Rf ((3)Rf*) is quenched by NF, in competition with dissolved ground state triplet oxygen, O(2)((3)Sigma(g)(-)). NF degradation mainly occurs by reaction with superoxide radical anion O(2)(-) formed through two electron transfer steps: from NF to (3)Rf*, yielding Rf radical anion, and from this anion to O(2)((3)Sigma(g)(-)), regenerating ground state Rf. Although singlet molecular oxygen is also produced, NF only quenches this oxidative species in a physical mode. The global result is the photoprotection of the sensitiser and the photodegradation of NF.