Gilles Mailhot

Assessment of the Fe(III)–EDDS Complex in Fenton-Like Processes: From the Radical Formation to the Degradation of Bisphenol A

The present work describes, for the first time, the use of a new and strong complexing agent, ethylenediamine-N,N-disuccinic acid (EDDS) in the homogeneous Fenton process. The effect of H(2)O(2) concentration, Fe(III)-EDDS concentration, pH value, and oxygen concentration on the homogeneous Fenton degradation of bisphenol A (BPA) used as a model pollutant, was investigated. Surprisingly, the performance of BPA oxidation in an EDDS-driven Fenton reaction was found to be much higher at near neutral or basic pH than at acidic pH. Inhibition and probe studies were conducted to ascertain the role of several radicals (e.g., (•)OH, HO(2)(•)/O(2)(•-)) on BPA degradation. This unexpected effect of pH on Fenton reaction efficiency could be due to the formation of HO(2)(•) or O(2)(•-) radicals and to the presence of different forms of the complex Fe(III)-EDDS as a function of pH. Indeed, the reduction of Fe(III)-EDDS to Fe(II)-EDDS is a crucial step that governs the formation of hydroxyl radical, mainly responsible for BPA degradation. In addition to its ability to maintain iron in soluble form, EDDS acts as a superoxide radical-promoting agent, enhancing the generation of Fe(II) (the rate limiting step) and therefore the production of (•)OH radicals. These results are very promising because they offer an important new treatment option at higher range of pH values and more particularly at pHs encountered in natural conditions.

Treatment of municipal wastewater treatment plant effluents with modified photo-Fenton as a tertiary treatment for the degradation of micro pollutants and disinfection.

The goal of this paper was to develop a modified photo-Fenton treatment able to degrade micro pollutants in municipal wastewater treatment plant (MWTP) effluents at a neutral pH with minimal iron and H(2)O(2) concentrations. Complexation of Fe by ethylenediamine-N,N-disuccinic acid (EDDS) leads to stabilization and solubilization of Fe at natural pH. Photo-Fenton experiments were performed in a pilot compound parabolic collector (CPC) solar plant. Samples were treated with solid phase extraction (SPE) and analyzed by HPLC-Qtrap-MS. The rapid degradation of contaminants within the first minutes of illumination and the low detrimental impact on degradation of bicarbonates present in the water suggested that radical species other than HO(•) are responsible for the efficiency of such photo-Fenton process. Disinfection of MWTP effluents by the same process showed promising results, although disinfection was not complete.

Photochemical production of organic matter triplet states in water samples from mountain lakes, located below or above the tree line

The production of triplet states (T(*)) of chromophoric dissolved organic matter (CDOM), reacting with the probe molecule 2,4,6-trimethylphenol (TMP) was measured upon irradiation of water samples, taken from lakes located in a mountain area (NW Italy) between 1450 and 2750 m above sea level. The lakes are located below or above the tree line and surrounded by different vegetation types (trees, alpine meadows or exposed rocks). The most photoactive samples belonged to lakes below the tree line and their fluorescence spectra and CDOM optical features suggested the presence of a relatively elevated amount of humic (allochthonous) material. The lowest (negligible) photoactivity was found for a lake surrounded by exposed rocks. Its CDOM showed an important autochthonous contribution (due to in-lake productivity) and considerably higher spectral slope compared to the other samples, suggesting low CDOM molecular weight and/or aromaticity. Among the samples, CDOM photoactivity (measured as the rate of TMP-reactive T(*) photoproduction) decreased with changing vegetation type in the order: trees, meadows, rocks. It could be connected with decreasing contribution from catchment runoff and increasing contribution from autochthonous processes and possibly precipitation.

Effect of dissolved organic compounds on the photodegradation of the herbicide MCPA in aqueous solution

This work shows that the addition of phenol and 2-propanol as model organic compounds significantly decreases the direct photolysis quantum yield of 4-chloro-2-methylphenoxyacetic acid (MCPA) upon UVB irradiation in aqueous solution. Laser flash photolysis data suggest that 2-propanol is able to decrease the formation of the MCPA excited states under irradiation. A decrease from 0.54 to 0.34 of the photolysis quantum yield of the anionic form of MCPA (which prevails over the undissociated one in surface waters) could have a considerable impact on the MCPA lifetime in ecosystems where the direct photolysis is the main phototransformation pathway. In surface water bodies where the direct photolysis has comparable kinetics as the reaction with OH, a decrease of the quantum yield would enhance the relative importance of the OH pathway, which yields considerably less toxic intermediates than the direct photolysis.

Photochemistry of 1-nitronaphthalene: a potential source of singlet oxygen and radical species in atmospheric waters.

1-Nitronaphthalene (1NN) was used as a model of nitro-PAHs to investigate photosensitized reactions in aqueous solution in the presence of oxygen and halides. Laser flash photolysis (LFP) was employed to investigate electron transfer between halide anions and the triplet state of 1NN, leading to the formation of dihalogen radical anions (X(2)(*-)) in solution. The experiments were performed in the absence or presence of oxygen, showing a bimolecular quenching rate constant for the triplet state of 1NN ((3)1NN) by oxygen of (1.95 +/- 0.05) x 10(9) M(-1) s(-1). The decay of (3)1NN was observed to strongly depend on the pH of the medium. At pH > 2, (3)1NN decayed with a pseudofirst order rate constant close to 6.0 x 10(5) s(-1). The rate constant was markedly enhanced at lower pHs, reaching 2.0 x 10(6) s(-1) at pH approximately 0.1, which suggests formation of the protonated triplet state ((3)1NNH(+)) at low pHs. Furthermore, we showed that the photoreactions of (3)1NN in the presence of oxygen are potential sources of HO(2)(*), (1)O(2), and possibly (*)OH in aqueous media. In Milli-Q water (pH approximately 6.5) and in the presence of halide anions, the quenching rate constant of (3)1NN was evaluated to be (2.9 +/- 0.4) x 10(4) M(-1) s(-1) for chloride, (7.5 +/- 0.2) x 10(8) M(-1) s(-1) for bromide, and (1.1 +/- 0.1) x 10(10) M(-1) s(-1) for iodide. Also in this case a pH-dependent reactivity was evidenced, and the quenching rate constant was (7.7 +/- 1.2) x 10(5) M(-1) s(-1) with chloride at pH 1.1.

Sulfate Radical Photogeneration Using Fe-EDDS: Influence of Critical Parameters and Naturally Occurring Scavengers.

In this study, the activation of persulfate induced by Fe(III)-ethylenediamine-N,N′-disuccinic acid (EDDS) under dark and irradiation conditions and the reactivity of the generated sulfate radical (SO4•–) under a wide range of experimental conditions were investigated by means of experimental kinetic analyses and modeling. The Fe(III)-EDDS induced activation of persulfate was found to be efficient across a wide range of pH value (3–7), whereas the second order rate constant of SO4•– with 4-tert-butylphenol (4tBP) kSO4•–,4tBP = (4.21 ± 0.22) × 109 M–1 s–1 was found to be unchanged between pH 2.5 and 8.5. Experimental and theoretical investigations showed clearly that the 4tBP degradation was enhanced in the presence of chloride (10 mM), whereas an almost complete inhibition was observed in the presence of carbonates (10 mM). For the first time, second order rate constants evaluated by laser flash photolysis experiments revealed that SO4•– has a similar reactivity with EDDS (6.21 × 109 M–1 s–1) and 4tBP (4....

Photochemical processes involving the UV absorber benzophenone-4 (2-hydroxy-4-methoxybenzophenone-5-sulphonic acid) in aqueous solution: reaction pathways and implications for surface waters.

The sunlight filter benzophenone-4 (BP-4) is present in surface waters as two prevailing forms, the singly deprotonated (HA-) and the doubly deprotonated one (A(2-)), with pKa2 = 7.30 ± 0.14 (μ ± σ, by dissociation of the phenolic group). In freshwater environments, BP-4 would mainly undergo degradation by reaction with ·OH and direct photolysis. The form HA(-) has a second-order reaction rate constant with ·OH (k(·OH)) of (1.87 ± 0.31)·10(10) M(-1) s(-1) and direct photolysis quantum yield Φ equal to (3.2 ± 0.6)·10(-5). The form A(2-) has (8.46 ± 0.24)·10(9) M(-1) s(-1) as the reaction rate constant with ·OH and (7.0 ± 1.3)·10(-5) as the photolysis quantum yield. The direct photolysis of HA(-) likely proceeds via homolytic breaking of the O-H bond of the phenolic group to give the corresponding phenoxy radical, as suggested by laser flash photolysis experiments. Photochemical modelling shows that because of more efficient direct photolysis (due to both higher sunlight absorption and higher photolysis quantum yield), the A(2-) form can be degraded up to 3 times faster than HA(-) in surface waters. An exception is represented by low-DOC (dissolved organic carbon) conditions, where the ·OH reaction dominates degradation and the transformation kinetics of HA(-) is faster compared to A(2-). The half-life time of BP-4 in mid-latitude summertime would be in the range of days to weeks, depending on the environmental conditions. BP-4 also reacts with Br2(·-), and a rate constant k(Br2(·-),BP-4) = (8.05 ± 1.33)·10(8) M(-1) s(-1) was measured at pH 7.5. Model results show that reaction with Br2(·-) could be a potentially important transformation pathway of BP-4 in bromide-rich (e.g. seawater) and DOM-rich environments.

Formation of hydroxyl radicals by irradiated 1-nitronaphthalene (1NN): oxidation of hydroxyl ions and water by the 1NN triplet state

The excited triplet state of 1-nitronaphthalene ((3)1NN*) reacts with OH(-) with a second-order reaction rate constant of (1.66 ± 0.08)×10(7) M(-1) s(-1) (μ±σ). The reaction yields the ˙OH radical and the radical anion 1NN(-)˙. In aerated solution, the radical 1NN(-)˙ would react with O(2) to finally produce H(2)O(2) upon hydroperoxide/superoxide disproportionation. The photolysis of H(2)O(2) is another potential source of ˙OH, but such a pathway would be a minor one in circumneutral (pH 6.5) or in basic solution ([OH(-)] = 0.3-0.5 M). The oxidation of H(2)O by (3)1NN*, with rate constant 3.8 ± 0.3 M(-1) s(-1), could be the main ˙OH source at pH 6.5.

Effect of ethylenediamine-N,N′-disuccinic acid on Fenton and photo-Fenton processes using goethite as an iron source: optimization of parameters for bisphenol A degradation

The main disadvantage of using iron mineral in Fenton-like reactions is that the decomposition rate of organic contaminants is slower than in classic Fenton reaction using ferrous ions at acidic pH. In order to overcome these drawbacks of the Fenton process, chelating agents have been used in the investigation of Fenton heterogeneous reaction with some Fe-bearing minerals. In this work, the effect of new iron complexing agent, ethylenediamine-N,N-disuccinic acid (EDDS), on heterogeneous Fenton and photo-Fenton system using goethite as an iron source was tested at circumneutral pH. Batch experiments including adsorption of EDDS and bisphenol A (BPA) on goethite, H2O2 decomposition, dissolved iron measurement, and BPA degradation were conducted. The effects of pH, H2O2 concentration, EDDS concentration, and goethite dose were studied, and the production of hydroxyl radical (•OH) was detected. The addition of EDDS inhibited the heterogeneous Fenton degradation of BPA but also the formation of •OH. The presence of EDDS decreases the reactivity of goethite toward H2O2 because EDDS adsorbs strongly onto the goethite surface and alters catalytic sites. However, the addition of EDDS can improve the heterogeneous photo-Fenton degradation of BPA through the propagation into homogeneous reaction and formation of photochemically efficient Fe-EDDS complex. The overall effect of EDDS is dependent on the H2O2 and EDDS concentrations and pH value. The high performance observed at pH 6.2 could be explained by the ability of O2•− to generate Fe(II) species from Fe(III) reduction. Low concentrations of H2O2 (0.1xa0mM) and EDDS (0.1xa0mM) were required as optimal conditions for complete BPA removal. These findings regarding the capability of EDDS/goethite system to promote heterogeneous photo-Fenton oxidation have important practical implications for water treatment technologies.

Photocatalytic behavior of nanosized TiO2 immobilized on layered double hydroxides by delamination/restacking process

IntroductionEfficient immobilization of TiO2 nanoparticles on the surface of Mg2Al-LDH nanosheets was performed by delamination/restacking process.Experimental partThe structural and textural properties of as-prepared nanocomposite were deeply analyzed using different solid-state characterization techniques such as: X-ray powder diffraction, Fourier transform infrared spectroscopy, and Raman spectroscopies, chemical analysis, X-ray photoelecton spectroscopy, N2 adsorption–desorption, and electronic microscopy.Results and discussionThe photocatalytic properties of immobilized TiO2 nanoparticles on Mg2Al were investigated using the photodegradation of two model pollutants: Orange II and 4-chlorophenol, and compared with pure colloidal TiO2 solution.ConclusionIt appears that Orange II photodegradation was systematically faster and more efficient than 4-chlorophenol photodegradation regardless of the medium pH. Moreover under slightly basic conditions, even if the TiO2 photocatalytic efficiency decreases, photodegradation performed in presence of easily recovered TiO2/Mg2Al1.5 nanocomposite gives rise to comparable or better results than pure TiO2.