Esther Oliveros

Industrial waste water treatment: large scale development of a light-enhanced Fenton reaction

The feasibility of a large scale application of the light-enhanced Fenton reaction has been investigated for the treatment of a highly contaminated industrial waste water containing toxic aromatic amines (dimethyl anilines or xylidines) as the main pollutants. The Fenton reagent, a combination of hydrogen peroxide and a ferrous salt, is a potent oxidizing agent of organic compounds in acidic aqueous solution, and UV/visible irradiation may significantly enhance the degradation rates. Preliminary laboratory tests on the model compounds, 2,4- and 3,4-xylidine have been performed for selecting appropriate experimental conditions. Subsequent experimentation on the industrial waste water at a large scale level (3000 mg C l−1, 500 l) has been carried out using an experimental design methodology for the simulation and the evaluation of the effects of the two critical factors, hydrogen peroxide and ferrous ion concentrations. The results indicate that the light-enhanced Fenton reaction is a most effective treatment process under acidic conditions and is a realistic alternative to adsorption of xylidines on activated carbon as used at present.

Fluorescence of pterin, 6-formylpterin, 6-carboxypterin and folic acid in aqueous solution: pH effects

Steady-state and time-resolved studies have been performed on four compounds of the pterin family (pterin, 6-carboxypterin, 6-formylpterin and folic acid) in aqueous solution, using the single photon counting technique. The fluorescence characteristics (spectra, quantum yields, lifetimes) of these compounds and their dependence on the pH have been investigated. Most pterins can exist in two acid-base forms over the pH range between 3 and 13. Emission spectra and excitation spectra were obtained for both forms of each compound studied. Fluorescence quantum yields (phi(F)) in acidic and basic media were measured. The phi(F) of folic acid (< 0.005 in both media) is very low compared to those of pterin (0.27 in basic media and 0.33 in acidic media), 6-carboxypterin (0.18 in basic media and 0.28 in acidic media) and 6-formylpterin (0.07 in basic media and 0.12 in acidic media). The variation in integrated fluorescence intensity and fluorescence lifetimes (tau(F)) was analysed as a function of pH. Dynamic quenching by OH- was observed and the corresponding bimolecular rate constants for quenching of fluorescence (k(q)) were calculated. The reported values for k(q) (M(-1) s(-1)) are 3.6 x 10(9), 1.9 x 10(9) and 1.1 x 10(10) M(-1) s(-1) for pterin, 6-carboxypterin and 6-formylpterin, respectively.

VOC photodegradation at the gas–solid interface of a TiO2 photocatalyst: Part I: 1-butanol and 1-butylamine

Abstract The gas–solid heterogeneous photocatalytic oxidation of 1-butanol and 1-butylamine in air was investigated using supported TiO2 as a catalyst. The supported catalyst was prepared using a sol–gel method and irradiated employing two different light sources, a medium pressure mercury lamp or a xenon-chloride excimer lamp. The experimental set-up was especially designed for generating a gas stream containing stable and defined concentrations of the model pollutants. The gas stream at the reactor exit was analyzed on line by gas chromatography and the structures of the intermediates were established by gas chromatography coupled with mass spectrometry. Six major intermediates (butanal, butanoic acid, 1-propanol, propanal, ethanol and ethanal) were identified in the case of the photocatalytic degradation of 1-butanol. 1-Butylamine was less efficiently adsorbed on the catalyst and its degradation was slower. Three intermediates could be identified in this case (N-butylidene-1-butylamine, N-ethylidene-1-butylamine and N–butylformamide). Based on these results, a degradation mechanism is proposed for both compounds. Mineralization could be achieved under various conditions of concentrations and flow rates and was confirmed by infrared spectroscopy.

Singlet oxygen (1Δg) production by pterin derivatives in aqueous solutions

Six compounds of the pterin family (pterin, 6-carboxypterin, 6-formylpterin, folic acid, biopterin and neopterin) have been investigated for their efficiencies of singlet oxygen (O2(1Δg)) production and quenching in aqueous solutions. The quantum yields of 1O2 production (ΦΔ) have been determined by measurements of the 1O2 luminescence in the near-infrared (1270 nm) upon continuous excitation of the sensitizer. Under our experimental conditions, all studied compounds (except folic acid) are relatively efficient 1O2 sensitizers with ΦΔ values of up to 0.47. Results show that the nature of the substituent at position 6 on the pterin moiety, as well as the pH, affect considerably the capacity of pterins to produce 1O2. All compounds investigated are efficient 1O2 quenchers: the rate constant of 1O2 total quenching (kt) by folic acid (3.0(± 0.3) × 107 M−1 s−1) is one order of magnitude larger than those for the other pterin derivatives investigated (1.4(± 0.1) × 106 M−1 s−1to 2.9(± 0.3) × 106 M−1 s−1).

Modeling the kinetics of a photochemical water treatment process by means of artificial neural networks

Abstract We have investigated the kinetics of the degradation of 2,4-dimethyl aniline (2,4-xylidine), chosen as a model pollutant, by the photochemically enhanced Fenton reaction. This process, which may be efficiently applied to the treatment of industrial waste waters, involves a series of complex reactions leading eventually to the mineralization of the organic pollutant. A model based on artificial neural networks has been developed for fitting the experimental data obtained in a laboratory batch reactor. The model can describe the evolution of the pollutant concentration during irradiation time under various conditions. It has been used for simulating the behavior of the reaction system in sensitivity studies aimed at optimizing the amounts of reactants employed in the process — an iron(II) salt and hydrogen peroxide. The results show that the process is much more sensitive to the iron(II) salt concentration than to the hydrogen peroxide concentration, a favorable condition in terms of economic feasibility.

Applications of singlet oxygen reactions: mechanistic and kinetic investigations

Analytical tools and methods for mechanistic and kinetic investigations of singlet oxygen reactions are presented and their limitations discussed in view of their application in developmental work and complex biological and biomimetic systems.

Large scale development of a light-enhanced fenton reaction by optimal experimental design

Photochemical advanced oxidation processes (AOP) for the treatment of waste waters containing biocidal or non-biodegradable organic compounds rely mainly on the production of a potent oxidizing agent, the hydroxyl radical. The Fenton9s reagent, a mixture of hydrogen peroxide and ferrous ion, represents one possible source of hydroxyl radicals and it has been shown recently that degradation rates may be significantly enhanced by UV/visible irradiation. In this context, we have investigated the feasibility of a large scale application of the light-enhanced Fenton reaction to the treatment of a highly contaminated industrial waste water containing toxic aromatic amines (dimethyl anilines or xylidines) as the main pollutants. Following preliminary laboratory tests performed on the model compounds, 2,4- and 3.4-xylidines (200 mg/L, 2 L), experimentation on the industrial waste water at a large scale level (3000 mgC/L, 500 L) has been carried out using the experimental design methodology for the simulation and the evaluation of the effects of the two critical factors, hydrogen peroxide and ferrous ion concentrations. The light-enhanced Fenton reaction has been proven to be a most effective treatment process under acidic conditions and might be an alternative to adsorption of xylidines on activated carbon as used at present.

How to evaluate photochemical methods for water treatment

Photochemical advanced oxidation processes (AOPs) generally imply generation of hydroxyl radicals which are initiating the oxidative degradation by well defined reactions (hydrogen abstraction, addition and electron transfer) with available organic substrates. This limitation of the scope of applications may be avoided in implementing combinations of different photochemical and/or thermal processes. A simple evaluation of photochemical AOPs is based on the absorption spectrum of the oxidant to be added and on the spectral distribution of the emission of commercially available light sources. Dominating light absorption, in particular in the UV-C spectral domain, by the solutes of the aqueous system to be treated may lead to exclude some of the degradation processes, as excitation of the oxidant and, consequently, production of the initiator become inefficient with increasing inner filter effects. The evaluation of photochemical AOPs in terms of volume independent rates is convenient and highly advocated, but such comparisons should only be made for processes applied to a restricted number of model substrates which are to be degraded in optimized equipment. Taking into account the volume independent rates determined in the range of realistic pollutant concentrations, the number of m3 of contaminated water of known pollutant nature and concentration to be treated per unit of time, the list of a commercially available light sources and their geometry, a final selection of the degradation process or of a combination of processes may be made, and the total electrical energy required and the number of photochemical reactors to be built may be calculated.

Photocatalytic degradation of 2,4-dihydroxybenzoic acid in water: efficiency optimization and mechanistic investigations

Abstract The experimental design and response surface methodologies have been applied to the investigation of the TiO 2 -photocatalytic degradation of 2,4-dihydroxybenzoic acid (DHBA) in aqueous solutions. It has been shown that a quadratic polynomial model may be used to assess adequately the efficiency of the photocatalytic degradation as a function of DHBA and TiO 2 concentrations. Moreover, the study of response surfaces leads to optimal conditions for investigating the catalyst efficiency. A degradation mechanism involving the cleavage of the DHBA aromatic ring at an early stage of the reaction is proposed. Such a mechanism accounts for the absence of any aromatic intermediate in the degradation process.

Nonradiative and radiative deactivation of singlet molecular oxygen (O2(a1Δg)) in micellar media and microemulsions

The effects of microheterogeneous media (micelles and microemulsions) on the lifetime and, to our knowledge for the first time, on the emission of singlet molecular oxygen (O2 (a1Ag), denoted as 1O2) were investigated. Micellar media and various types of microemulsions based on anionic (sodiumdodecyl sulfate), cationic (cetyltrimethylammonium chloride) or nonionic (Triton X-100) surfactants were formulated for this purpose. The nonradiative and radiative deactivation rate constants (k(d) and k(e), respectively) were determined in selected microheterogeneous media and in the pure solvents used for their formulation, by combining steady-state and time-resolved 1O2, luminescence detection techniques. We have shown that a simple additive model, as used in homogeneous mixtures of solvents, was inadequate for predicting values of k(d) and k(e) in organized media. In contrast, both 1O2 lifetimes (taudelta = 1/k(d)) and k(e) in the microheterogeneous systems investigated could be predicted with good precision from the composition of the media and the taudelta and k(e) values in the pure solvents, using a two-pseudophase kinetic model for the 1O2 distribution. Such a model takes into account the average times spent by 1O2 in the aqueous and lipophilic pseudo-phases of the organized media, the corresponding equilibrium constant (Keq) depending on the nature of the system.