Benito Acedo

Oxidation of atrazine in water by ultraviolet radiation combined with hydrogen peroxide

The oxidation of atrazine in water by means of direct photolysis at 254 mm and with hydrogen peroxide combined with a u.v. radiation has been studied. The influence of bicarbonate/carbonate ions and a commercial humic substance on the oxidation rate has been observed. Depending on the initial atrazine concentration conversions higher than 99% can be achieved from both types of oxidation in less than 15 min. The oxidation rate is especially fast with the combination of hydrogen peroxide and u.v. radiation. The humic substance applied slows down the rates of both types of processes because it absorbs radiation and scavenges hydroxyl radicals. There is an optimum initial concentration of hydrogen peroxide, about 0.01 M, above which the oxidation rate of atrazine decreases. The quantum yield and hydroxyl radical reaction rate constant of atrazine were found to be 0.05 mol photon −1 and 1.8 × 1020M−1s−1, respectively.

Advanced oxidation of atrazine in water—I. Ozonation

Abstract The ozonation of atrazine in water has been studied under different conditions of ozone partial pressure, pH, temperature and presence of hydroxyl radical scavengers. The process mainly develops through radical reactions, even at pH 2, probably due to the presence of traces of impurities in the starting material. The rate constant of the direct reaction between ozone and atrazine at pH 2 has been determined and expressed as a function of temperature. Thus, at 20°C the rate constant was found to be 4.5 M−1 s−1 the energy of activation being 18.8 kJ · mol−1. A model formed by mol balance equations of atrazine and ozone, the latter in the liquid and gas phases, with the ozone decomposition rate based on the Staehelin and Hoigne mechanism allowed the determination of the concentrations of these species at different experimental conditions. The aqueous solution of atrazine is characterized by a hydroxyl radical initiating rate factor which accounts for other radical reactions not included in the basic ozone decomposition mechanism. With this factor deviations between experimental and calculated concentrations of atrazine are less than ± 15% in most of the cases.

Advanced oxidation of atrazine in water—II. Ozonation combined with ultraviolet radiation

The combination of ozone with ultraviolet radiation (254 nm) to oxidise atrazine is investigated. The importance of the three routes of atrazine elimination: direct photolysis, direct ozonation and hydroxyl radical oxidation is established in percentages. At neutral pH and 20°C 87% of the oxidation rate is due to the radical way while direct ozonation only represents 1%. Mass transfer and kinetic data obtained in previous works [Beltran F. J., Ovejero G. and Acedo B. (1993) Wat. Res. 27, 1013–1021; Beltran F. J., Garcia-Araya J. F. and Acedo B. (1994) Wat. Res. 28, 2153–2164.] have been applied to mol. balance equations of atrazine, ozone (both in the gas and water) and hydrogen peroxide to obtain their corresponding concentrations in different conditions. It was necessary to include a hydroxyl radical initiating rate factor to account for radical ways not included in the basic mechanism of ozone decomposition. Comparison between different ways of oxidation and photolysis of atrazine (ozone, ozone/u.v. radiation, hydrogen peroxide/u.v. radiation) is also presented.

Chemical and photochemical degradation of acenaphthylene. Intermediate identification.

Removal of acenaphthylene from water has been carried out by means of different treatments combining UV radiation, ozone and hydrogen peroxide. Ozonation alone or in conjunction with hydrogen peroxide (10(-3) M) resulted in the highest elimination rates. Thus, conversions as high as 95-100% were obtained in less than 3 min with an ozone dose of 4.1x10(-3) mol O(3) h(-1) (flow rate 2x10(-2) m(3) h(-1)). Slightly lower efficiencies were experienced when using systems containing UV radiation. By considering the kinetics of the direct photolysis of acenaphthylene and the UV/H(2)O(2) system the photochemical reaction quantum yield φ(A) (4.0+/-0.1x10(-3) mol/photon) and the rate constant of the reaction of acenaphthylene with the hydroxyl radical k(OH,A) (8.0+/-0.5x10(9) M(-1) s(-1)) were calculated. Intermediates identified by GC/MS were in many cases similar regardless of the oxidation treatment used. Most of these by-products constituted oxygenated species of the parent compound (mainly ketones, aldehydes and carboxylic acids) that further degraded to low molecular, harmless end products.

Wet Air Oxidation Of Wastewater From Olive Oil Mills

The oxidation of wastewater from olive oil mills has been carried out in the liquid phase at high temperatures and pressures. Synthetic urban wastewater has been used to dilute the raw effluent (dilution rate 1:10). Experiments conducted using air as the oxygen source showed a positive effect of the previous neutralization of the wastewater if compared to the oxidation conducted at the original pH of the effluent (pH = 5.3). In terms of chemical oxygen demand depletion and final biodegradability characteristics of the effluent, the use of free radical promoters, for instance hydrogen peroxide, resulted in a significant enhancement of the process. Experiments completed in the presence of two commercially available catalysts (platinum supported on alumina and copper oxide supported on active carbon) showed not only an improvement in the chemical oxygen demand removal rate but also a high degree of the mineralization of the wastewater contaminant load.

Pyruvic Acid Removal from Water by the Simultaneous Action of Ozone and Activated Carbon

Activated carbon (AC) has been used to catalyze the ozonation of pyruvic acid in water. Pyruvic acid conversions were found to be 9 and 37% after 90 min of single ozonation and single adsorption with 40 gL−1 AC, respectively, while 82% was reached at the same conditions during the AC catalytic ozonation. Also, for similar conditions, mineralization reached values of 67% in the AC catalytic ozonation against hardly 5% in the non-catalytic experiment. The process likely develops through both adsorption of ozone and pyruvic acid on the AC surface and generation of hydroxyl radicals that eventually is the responsible oxidizing species. Rate constants for both non-catalytic ozonation and AC-Ozone catalytic surface reaction, at 20°C and pH 7.5, were found to be 0.025 min−1 and 87.9 Lg−1s−1, respectively. For AC concentrations higher than 2.5 gL−1 gas-liquid mass transfer of ozone constituted the limiting step. At lower concentrations, internal diffusion plus surface reaction controlled the process rate.

Kinetic modelling of aqueous atrazine ozonation processes in a continuous flow bubble contactor

The ozonation of atrazine in different waters (ultrapure and surface waters) has been studied in continuous bubble contactors with kinetic modelling purposes. Three ozonation processes have been considered: ozonation alone and combined with hydrogen peroxide or UV radiation. The kinetic models are based on a molecular and free radical mechanism of reactions, reaction rate and mass transfer data and non-ideal flow analysis models for gas and water phases through the contactors (the tanks in series model and the dispersion model). The models predict well the experimental concentrations of atrazine, dissolved ozone and hydrogen peroxide both at non-steady state and steady state regimes. From both experimental and calculated results, atrazine conversions are observed to be highly dependent on the nature of water where ozonation is carried out. As far as removal of atrazine and oxidation intermediates are concerned, ozone combined with UV radiation resulted in the most effective ozonation process among the three studied.

Contribution of free radical oxidation to eliminate volatile organochlorine compounds in water by ultraviolet radiation and hydrogen peroxide

Abstract The UV/H 2 O 2 oxidation of trichloroethylene, TCE, and 1,1,1 trichloroethane, TCA, in water was studied. Oxidation rates depend on the initial hydrogen peroxide concentration with maximum values in the presence of concentrations of 10 −2 M which represents rates of about 5 and 250 times higher than those obtained from volatilization alone for TCA and TCE, respectively. In the case of TCE, contribution of direct photolysis is negligible. In natural waters, oxidation rates of TCA and TCE slightly decreases compared to those in laboratory prepared waters. When concentrations of hydrogen peroxide applied are higher than 10 −2 M, TCA is mainly removed by volatilization since hydrogen peroxide consumes most of radicals generated. On the contrary, free radical oxidation continues to be the principal step of removal in the case of TCE since the rate constant of its reaction with hydroxyl radicals is approx. 65 times higher than that of the reaction OH-TCA. Experiments with high concentration of hydrogen peroxide (>10 −2 M) allow to determine the rate constants of TCE-OH and TCA-OH reactions that were found to be 1.8×10 9 and 2.0×10 7 M −1 s −1 , respectively.

Two-Step Wastewater Treatment: Sequential Ozonation - Aerobic Biodegradation

Abstract Ozonation of wastewater from olive related industries has been carried out after dilution with synthetic urban wastewater. The advantages of the application of different acidic and basic cycles during the ozonation process have been shown. Biodegradability of the final effluent measured as the biological oxygen demand to chemical oxygen demand ratio has significantly been increased (100–144% for table olive wastewater and 24–60% for olive oil wastewater). Aerobic biological experiments conducted by using non-acclimated microorganisms confirmed the suitability of the biodegradation after the chemical oxidation pre-treatment. A kinetic model based on a free radical mechanism has been used to simulate experimental results. Both chemical oxygen demand and dissolved ozone concentration profiles are well fitted by the model. The aerobic biodegradation process has been modeled by utilizing the Monod equation.

Kinetics of the Ozone-p-Chlorobenzoic Acid Reaction

The kinetics of p-chlorobenzoic acid ozonation was investigated. From experiments carried out at acidic pH it was suggested that the direct rate constant between the ozone molecule and p-chlorobenzoic acid is higher than the reported value of 0.15 M−1s−1. Runs completed in homogeneous contact pattern allowed for the calculation of the direct rate constant with a value of approximately 4.5 M−1s−1. Additionally, some competitive experiments were carried out in the presence of a reference compound (atrazine or simazine). From these competitive runs conducted at pH 1, values of 3.1 and 6.7 M−1s−1 were obtained by considering the presence of hydroxyl radicals. If the occurrence of these radicals is ruled out, the new values obtained are 3.6 and 2.2 M−1s−1. In any case, apparently the ozonation of p-chlorobenzoic acid seems to be faster than stated in the literature. However, the addition of tert-butyl alcohol either in homogeneous or heterogeneous experiments completely inhibited the p-chlorobenzoic acid oxidation, suggesting the generation of radical species even at strongly acidic conditions.