Degradation of atrazine using solar driven fenton-like advanced oxidation processes

Atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) was degraded using cobalt-peroximonosulfate (Co/PMS) advanced oxidation process (AOP). Three Co concentrations (0.00, 0.25 and 0.50 mM) and five peroximonosulfate (PMS) concentrations (0, 5, 8, 16 and 32 mM) were tested. Maximum degradation reached was 88% using dark Co/PMS in 126 minutes when 0.25 mM of cobalt and 32 mM of PMS were used. Complete atrazine degradation was achieved when the samples were irradiated by the sun under the same experimental conditions described. Tests for identification of intermediate products allowed identification and quantification of deethylatrazine in both dark and radiated conditions. Kinetic data for both processes was calculated fitting a pseudo-first order reaction rate approach to the experimental data. Having kinetic parameters enabled comparison between both conditions. It was found that the kinetic approach describes data behavior appropriately (R² ≥ 0.95). Pseudo-kinetic constants determined for both Co/PMS processes, show k value of 10^?4 for Co/PMS and a k value of 10^?3 for Co/PMS/ultraviolet (UV). This means, that, with the same Co/PMS concentrations, UV light increases the reaction rate by around one order of magnitude than performing the reaction under dark conditions.     

Kinetics of oxidative decolorization and mineralization of Acid Orange 7 by dark and photoassisted Co2+-catalyzed peroxymonosulfate system

A kinetic model was proposed and used to interpret the experimental data for degradation of Acid Orange 7 (AO7) in aqueous solution induced by Co(2+)/peroxymonosulfate (Co/PMS) reagent in terms of both decolorization and mineralization. The pseudo first-order decolorization rate constants are related to [Co(2+)], [PMS], reciprocal of [H(+)] and [AO7](0). Activation energy of the AO7 decolorization process was determined to be 75.7 kJ mol(-1). UV and visible light can accelerate the decolorization and mineralization process due to different mechanisms. In the combined UV/Co/PMS system, UV light can decompose PMS to generate hydroxyl ((*)OH) and sulfate radicals (SO(4)(*-)), while in the Vis/Co/PMS system, excited AO7 molecules can transfer electrons to PMS or Co(3+) and thus accelerate the decomposition of PMS and catalytic cycle of Co(3+)/Co(2+).     

An improved UV/Fe3+ process by combination with electrocatalysis for p-nitrophenol degradation

p-nitrophenol (PNP) was investigated as a model pollutant under the improved UV/Fe3+ process by combination with electrocatalysis. In the individual UV/Fe3+ process, PNP degradation rate was dependent on Fe(III) concentration and decreased during degradation due to the depletion of ferric ion and thus it was very difficult to reach the quick mineralization of organics. These drawbacks could be significantly overcome in the modified UV/Fe3+ process, and synergetic effects for PNP and COD removal were observed at two investigated Fe(III) concentrations. The enhancements on the degree of conversion for PNP and COD in presence of 0.5 mM Fe(III) were 184% and 242%, respectively, and PNP of initial concentration of 1.0 mM could be completely removed within 1 h. Thus such a process would be very attractive to the rapid mineralization of the biorefractory compounds for wastewater treatment. The possible reasons for the synergetic effects were the electrochemical regeneration of ferric ion and the role of the oxygen that formed on the anode. Based on degradation intermediates identification and synergetic effect probe, a general reaction pathway for PNP degradation in the improved process was proposed.     

CuO / 过硫酸氢钾体系催化氧化苯酚简

本论文通过直接沉淀法制备了CuO催化剂,结合过硫酸氢钾,在常温常压下催化氧化处理苯酚模拟废水。采用电子显微镜（SEM）、X射线粉末衍射（XRD）对催化剂进行了表征,并研究了反应过程中各影响因素对降解效率的影响。实验结果表明,在催化剂用量为0.2 g/L,氧化剂浓度为0.25 g/L,pH值为7,反应时间为60 min的条件下,浓度为50 mg/L的苯酚降解率可达100%,TOC去除率达84%。进一步实验表明,催化剂具有良好的重复使用能力。最后,通过自由基捕捉实验,考察了体系中的自由基种类,并根据实验结果,讨论了CuO/过硫酸氢钾体系的催化降解机理。     

Rapid dye degradation with reactive oxidants generated by chloride-induced peroxymonosulfate activation

Transition-metal is known to catalyze peroxymonosulfate (PMS) decomposition to produce sulfate radicals. Here we report reactions between PMS and chloride, without a need of transition metals, also can be used to degrade organic dye pollutant (Rhodamine B, (RhB)). Some important operating parameters, such as dosages of PMS and Cl^?, pH of solution, temperature, ionic strength, and several common cations, were systematically investigated. Almost complete decoloration of RhB was achieved within 5 min ([PMS]?=?0.5 mM, [Cl^?]?=?120 mM, and pH 3.0), and RhB bleaching rate increased with the increased dosages of both PMS and chloride ion, following the pseudo-first-order kinetic model. However, the total organic carbon (TOC) removal results demonstrated that the decoloration of RhB was due to the destruction of chromophore rather than complete degradation. RhB decoloration could be significantly accelerated due to the high ionic strength. Increasing of the reaction temperature from 273 K to 333 K was beneficial to the RhB degradation, and the activation energy was determined to be 32.996 kJ/mol. Bleaching rate of RhB with the examined cations increased with the order of NH₄ ⁺?<?Na⁺?<?K⁺?<?Al³⁺?<?Ca²⁺?<?Mg²⁺. Some major degradation products of RhB were identified by GC-MS. The present study may have active technical implications for the treatment of dyestuff wastewater in practice.     

Comparison of advanced oxidation processes and identification of monuron photodegradation products in aqueous solution

The photodegradation of monuron (3-(4-chlorophenyl)-1,1-dimethylurea) in aqueous solutions under simulated solar irradiation has been conducted by different advanced oxidation processes (UV/H(2)O(2), UV/H(2)O(2)/Fe(2+), UV/H(2)O(2)/TiO(2), UV/TiO(2), dark H(2)O(2)/Fe(3+)). The degradation rates were always higher for the homogeneous catalysis in photo-Fenton reactions (UV/H(2)O(2)/Fe(2+)) compared to the heterogeneous photocatalytic systems (TiO(2)/UV and UV/H(2)O(2)/TiO(2)). Optimal concentrations of Fe(2+) and H(2)O(2) for the abatement of the herbicide in the photo-Fenton system were found to be 1 mM Fe(II) and 10 mM H(2)O(2). Several intermediary products were identified using large volume injection micro-liquid chromatography with UV detection (mu-LC-UV), mu-LC-MS and GC-MS techniques and a degradation mechanism has been proposed.     

Synergy of ozonation and photocatalysis to mineralize low concentration 2,4-dichlorophenoxyacetic acid in aqueous solution

Concentration of 2,4-dichlorophenoxyacetic acid (2,4-D) may affect its degradation kinetics in advanced oxidation systems, and combinations of two or more systems can be more effective for its mineralization at low concentration levels. Degradations and mineralizations of 0.045mM 2,4-D using O(3), O(3)/UV, UV/TiO(2) and O(3)/UV/TiO(2) systems were compared, and influence of reaction temperature on the mineralization in O(3)/UV/TiO(2) system was investigated. 2,4-D degradations by O(3), O(3)/UV and UV/TiO(2) systems were similar to the results of earlier investigations with higher 2,4-D concentrations. The degradations and total organic carbon (TOC) removals in the four systems were well described by the first-order reaction kinetics. The degradation and removal were greatly enhanced in O(3)/UV/TiO(2) system, and further enhancements were observed with larger O(3) supplies. The enhancements were attributed to hydroxyl radical (()OH) generation from more than one reaction pathway. The degradation and removal in O(3)/UV/TiO(2) system were very efficient with reaction temperature fixed at 20 degrees C. It was suspected that reaction temperature might have influenced ()OH generation in the system, which needs further attention.     

Homogeneous degradation of 1,2,9,10-tetrachlorodecane in aqueous solutions using hydrogen peroxide, iron and UV light

The homogeneous degradation of the polychlorinated n-alkane, 1,2,9,10-tetrachlorodecane (T4C10), was studied in aqueous solutions of hydrogen peroxide, including Fenton and photo-Fenton reaction conditions. All solutions were adjusted to a pH of 2.8 and an ionic strength of 0.1 M NaClO4 prior to photolysis. T4C10 (2 x 10(-6) M) was substantially degraded by the H2O2/UV system (1.0 x 10(-2) M H2O2), with 60% disappearance in 20 min of irradiation in a photoreactor equipped with 300 nm lamps of light intensity 3.6 x 10(-5) Ein L(-1) min(-1) (established by ferrioxalate actinometry). The reaction produced stoichiometric amounts of chloride ion indicating complete dechlorination of the chlorinated n-alkane. T4C10 degraded very slowly under Fenton (Fe2+/H2O2/dark) and Fenton-like (Fe3+/H2O2/dark) conditions. However, when the same solutions were irradiated, T4C10 degraded more rapidly than in the H2O2/UV system, with 61% disappearance in 10 min of exposure. The rapid degradation is related to the enhanced degradation of hydrogen peroxide to oxidizing *OH radicals under photo-Fenton conditions. Degradation was inhibited in both the H2O2/UV and photo-Fenton systems by the addition of KI and tert-butyl alcohol due to *OH scavenging.     

Degradation of azo dye Procion Red MX-5B by photocatalytic oxidation

The photocatalytic oxidation (PCO) of a monoazo dye Procion Red MX-5B under various physico-chemical conditions was investigated. Degradation of the dye by PCO was enhanced by augmentation in UV intensity, titanium dioxide and hydrogen peroxide concentrations but was inhibited by increase in initial dye concentration. The PCO process was affected by pH in a peculiar way. In the presence of 100 mg/l of TiO2 and the absence of H2O2, the highest reaction rate was observed when the initial pH was 10. With 500 mg/l of TiO2 and 10 mM of H2O2, the reaction was the fastest at initial pH of 3-5. The optimal conditions for the degradation of the dye, at an UV intensity of 17 mW/cm2, were determined to be: TiO2 concentration, 500 mg/l; initial H2O2 concentration, 10 mM; initial pH, 5.0. Monitoring of TOC loss showed that the dye was mineralized by 90% within 80 min under these conditions. Nevertheless, the persistence of a low level of TOC indicated that mineralization was not complete and dead-end product(s) which was (were) resistant to PCO might have accumulated.     

The role of organic ligands in ferrous-induced photochemical degradation of 2,4-dichlorophenoxyacetic acid

Recent studies have shown that hydrogen peroxide is generated in a ferrioxalate-induced photoreductive reaction, but information about the effect of organic ligands on the photochemical behaviour of ferrous species is limited. The degradation of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) by a ferrous-catalyzed oxidation in the presence of various ligands such as formate, citrate, malelate, oxalate, and ethylenediaminetetra-acetic acid (EDTA) was studied. The experiments were conducted under either dark or irradiated (350n m) conditions. Forty-two percent and 34% of 2,4-D were removed by the Fe(2+)/oxalate/UV and Fe(2+)/citrate/UV processes, respectively, after 30 min of reaction and oxidative intermediates were obtained in both cases. The presence of hydroxylated intermediates suggests that 2,4-D may be attacked by hydroxyl radicals, which are the products of the photo-Fenton-like reaction. As such, hydrogen peroxide was produced by the photolysis of ferrous oxalate or ferrous citrate, referred to hereafter as photogenerated H(2)O(2). As expected, the total removal percentage of 2,4-D jumped to 97% when 1mM of hydrogen peroxide (so-called spiked H(2)O(2)) was externally added to the reaction vessel to initiate the Fe(2+)/oxalate/UV process. Therefore, the treatment of 2,4-D by the Fe(2+)/oxalate/H(2)O(2)/UV system can be operated in two steps: the photolysis of ferrous oxalate first, followed by adding the spiked H(2)O(2) sometime after the commencement of the reaction. A two-phase model has been developed to describe this tandem ferrous-catalyzed photooxidation, which would help to achieve the mineralization of 2,4-D.     

UV/ozone degradation of gaseous hexamethyldisilazane (HMDS)

As a carcinogen, hexamethyldisilazane (HMDS) is extensively adopted in life science microscopy, materials science and nanotechnology. However, no appropriate technology has been devised for treating HMDS in gas streams. This investigation evaluated the feasibility and effectiveness of the UV (185+254nm) and UV (254nm)/O(3) processes for degradation of gaseous HMDS. Tests were performed in two batch reactors with initial HMDS concentrations of 32-41mgm(-3) under various initial ozone dosages (O(3) (mg)/HMDS (mg)=1-5), atmospheres (N(2), O(2), and air), temperatures (28, 46, 65 and 80 degrees C), relative humilities (20%, 50%, 65%, 99%) and volumetric UV power inputs (0.87, 1.74, 4.07 and 8.16Wl(-1)) to assess their effects on the HMDS degradation rate. Results indicate that for all conditions, the decomposition rates for the UV (185+254nm) irradiation exceeded those for the UV (254nm)/O(3) process. UV (185+254nm) decompositions of HMDS displayed an apparent first-order kinetics. A process with irradiation of UV (185+254nm) to HMDS in air saturated with water at temperatures of 46-80 degrees C favors the HMDS degradation. With the condition as above and a P/V of around 8Wl(-1), k was approximately 0.20s(-1) and a reaction time of just 12s was required to degrade over 90% of the initial HMDS. The main mechanisms for the HMDS in wet air streams irradiated with UV (185+254nm) were found to be caused by OH free-radical oxidation produced from photolysis of water or O((1)D) produced from photolysis of oxygen. The economic evaluation factors of UV (185+254nm) and UV (254nm)/O(3) processes at different UV power inputs were also estimated.     

A bench-scale investigation of land treatment of soil contaminated with diesel fuel.

A bench-scale investigation (soil pan testing) was conducted with the objective of studying degradation rates of diesel contaminated soil (2500 and 10,000 ppm by weight of total petroleum hydrocarbons (TPH) to dry weight of soil) under different treatment conditions over a 17 week testing period. The greatest degradation of the diesel contaminated soil was obtained with the addition of nutrients (Co = 10,000 ppm of TPH; k = 0.19 week-1). 'k' for soil not amended with nutrients was 0.07 week-1. The control cell (C0 = 2500 ppm TPH), with sodium azide (to suppress degradation) was compared with an experimental cell of 2500 ppm initial concentration of TPH without nutrient amendment. The control cell exhibited a relatively low uniform degradation (k = 0.08 week-1) of TPH over the duration of the experiment with reasonable first-order kinetic regression statistics.     

Photocatalytic degradation of the herbicide erioglaucine in the presence of nanosized titanium dioxide: comparison and modeling of reaction kinetics

The present work mainly deals with photocatalytic degradation of a herbicide, erioglaucine, in water in the presence of TiO2 nanoparticles (Degussa P-25) under ultraviolet (UV) light illumination (30 W). The degradation rate of erioglaucine was not so high when the photolysis was carried out in the absence of TiO2 and it was negligible in the absence of UV light. We have studied the influence of the basic photocatalytic parameters such as pH of the solution, amount of TiO2, irradiation time and initial concentration of erioglaucine on the photodegradation efficiency of erioglaucine. A kinetic model is applied for the photocatalytic oxidation by the UV/TiO2 system. Experimental results indicated that the photocatalytic degradation process could be explained in terms of the Langmuir-Hinshelwood kinetic model. The values of the adsorption equilibrium constant, K, and the second order kinetic rate constant, k, were 0.116 ppm-1 and 0.984 ppm min-1, respectively. In this work, we also compared the reactivity between the commercial TiO2 Degussa P-25 and a rutile TiO2. The photocatalytic activities of both photocatalysts were tested using the herbicide solution. We have noticed that photodegradation efficiency was different between both of them. The higher photoactivity of Degussa P-25 compared to that of rutile TiO2 for the photodegradation of erioglaucine may be due to higher hydroxyl content, higher surface area, nano-size and crystallinity of the Degussa P-25. Our results also showed that the UV/TiO2 process with Degussa P-25 as photocatalyst was appropriate as the effective treatment method for removal of erioglaucine from a real wastewater. The electrical energy consumption per order of magnitude for photocatalytic degradation of erioglaucine was lower with Degussa P-25 than in the presence of rutile TiO2.     

Hydroxyl radical scavenging role of chloride and bicarbonate ions in the H2O2/UV process

Simultaneous effect of inorganic anions, such as chloride and bicarbonate ions, on the scavenging of hydroxyl radicals (HO*) in the H2O2/UV process is the focus of this paper. The model compound of n-chlorobutane (BuCl) was used as the probe of HO*. By changing the pH conditions (2-9) and the concentrations of NaCl (0.25-2500 mM) and NaHCO3 (25 mM), the variation of HO* concentrations and the rate of H2O2 decomposition were compared. In general, the BuCl and H2O2 follow closely the first-order reaction within the first 10 and 40 min, respectively. In the presence of chloride alone at the pH range of 2-6, the HO* concentration in the reaction mixture increases with the increase of pH, and the HO* concentration at pH = 6 is 100 times of that at pH = 2. Including bicarbonate species in the solution, the peak HO* concentration was found at a certain pH, which shifts from 4, 5, to 5-7, as the molar ratios of chloride/bicarbonate species increase from 1 to 100. In addition, without bicarbonate species HO* concentration decreases significantly with increasing chloride concentration but remained rather unchanged beyond 1250 mM. In contrast, the HO* scavenging in the presence of bicarbonate species became relatively significant only when the chloride concentration reached beyond 250 mM. Throughout all experiments of different water quality conditions, the H2O2 decomposition rate remains rather unchanged.     

Post-treatment of bio-treated acrylonitrile wastewater using UV/Fenton process: degradation kinetics of target compounds

In this study, post-treatment of bio-treated acrylonitrile wastewater was performed using the UV/Fenton process. Five target compounds (furmaronitrile, 3-pyridinecarbonitrile, 1,3-dicyanobenzene, 5-methyl-1H-benzotriazole, and 7-azaindole) were selected as target compounds and their degradation kinetics were examined. Under optimal reaction conditions (H₂O₂ dosage 3.0 mM, Fe²⁺ dosage 0.3 mM, and initial pH 3.0), more than 85% of total organic carbon (TOC) was eliminated in 30 min when a 10-W UV lamp was employed, and the electrical energy per order of magnitude for TOC removal was as low as 2.96 kWh m^?3. Furthermore, the target compounds and the toxicity were largely removed from the bio-treated effluent. Size exclusion chromatography with organic carbon detector analysis revealed that organic components with a wide range of molecular weights were greatly reduced after the UV/Fenton process. A simplified pseudo steady-state (SPSS) model was applied to predict the degradation of target compounds during the UV/Fenton process. The concentrations of generated hydroxyl radicals were estimated to be 3.06 × 10^?12 M, 6.37 × 10^?12 M, and 10.9 × 10^?12 M under 5-, 10-, and 15-W UV lamps, respectively. These results demonstrate that the proposed SPSS model fitted well with experimental data on the post-treatment of real wastewater, and consequently indicate that this model can be a useful tool in the prediction of degradation of target compounds during the UV/Fenton process.

     

Synthesis of magnetic CoFe2O4/ordered mesoporous carbon nanocomposites and application in Fenton-like oxidation of rhodamine B

CoFe₂O₄/ordered mesoporous carbon (OMC) nanocomposites were synthesized and tested as heterogeneous peroxymonosulfate (PMS) activator for the removal of rhodamine B. Characterization confirmed that CoFe₂O₄ nanoparticles were tightly bonded to OMC, and the hybrid catalyst possessed high surface area, pore volume, and superparamagnetism. Oxidation experiments demonstrated that CoFe₂O₄/OMC nanocomposites displayed favorable catalytic activity in PMS solution and rhodamine B degradation could be well described by pseudo-first-order kinetic model. Sulfate radicals (SO₄ ⁻·) were verified as the primary reactive species which was responsible for the decomposition of rhodamine B. The optimum loading ratio of CoFe₂O₄ and OMC was determined to be 5:1. Under optimum operational condition (catalyst dosage 0.05 g/L, PMS concentration 1.5 mM, pH 7.0, and 25 °C), CoFe₂O₄/OMC-activated peroxymonosulfate system could achieve almost complete decolorization of 100 mg/L rhodamine B within 60 min. The enhanced catalytic activity of CoFe₂O₄/OMC nanocomposites compared to that of CoFe₂O₄ nanoparticles could be attributable to the increased adsorption capacity and accelerated redox cycles between Co(III)/Co(II) and Fe(III)/Fe(II).

     

Oxidation of TNT by photo-Fenton process

A series of photo-Fenton reactions have been performed for the degradation of 2,4,6-trinitrotoluene (TNT) in a 4.2-l reactor. The degradation reaction rate of TNT followed a pseudo-first-order behavior; and the rate constants for 2.4mW cm(-2)UV only, 2.4mW cm(-2)UV/H(2)O(2), Fenton, photo-Fenton (2.4mW cm(-2)) and photo-Fenton (4.7mW cm(-2)) were 0.002min(-1), 0.007min(-1), 0.014min(-1), 0.025min(-1) and 0.037min(-1), respectively. Increasing the intensity of UV light, and the concentrations of ferrous ions and hydrogen peroxide promoted the oxidation rate under the experimental conditions in this study. The weighting factor (f), the Fe(II)-promoted efficiency (r) and the promoted-UV light efficiency (p) were calculated to clarify their effects on the TNT oxidation. Moreover, the inhibition effect of hydroxyl radical was also observed in both Fenton and photo-Fenton oxidation when the concentration of Fe(II) were higher than 2.88mM. Solid phase micro-extraction was first applied to the separation of the organic byproducts from TNT oxidation. GC/MS was employed to identify the byproducts during the Fenton and photo-Fenton oxidation of TNT. These compounds were clarified as 1,3,5-trinitrobenzene, 1-methyl-2,4-dinitrobenzene 2,5-dinitrobenzoic acid and 1,3-dinitrobenzene. By these byproducts, the mechanisms of the methyl group oxidation, decarboxylation, aromatic ring breakage, and hydrolysis can be recognized and demonstrated. The pathway of TNT oxidation by photo-Fenton process was also proposed in this study.     

Photo-Fenton-assisted ozonation of p-Coumaric acid in aqueous solution

The degradation of p-Coumaric acid present in olive oil mill wastewater was investigated as a pretreatment stage to obtain more easily biodegradable molecules, with lower toxicity that facilitates subsequent anaerobic digestion. Thus, photo-Fenton-assisted ozonation has been studied and compared with ozonation at alkaline pH and conventional single ultraviolet (UV) and acid ozonation treatments. In the combined process, the overall kinetic rate constant was split into various components: direct oxidation by UV light, direct oxidation by ozone and oxidation by hydroxyl radicals. Molecular and/or radical ozone reaction was studied by conducting the reaction in the presence and absence of tert-butylalcohol at pHs 2, 7 and 9. Ozone oxidation rate increases with pH or by the addition of Fenton reagent and/or UV radiation due to generation of hydroxyl radicals, *OH. Hydrogen peroxide and ferrous ion play a double role during oxidation since at low concentrations they act as initiators of hydroxyl radicals but at high concentrations they act as radical scavengers. Finally, the additional levels of degradation by formation of hydroxyl radicals have been quantified in comparison to the conventional single processes and an equation is proposed for the reaction rate as a function of studied operating variables.     

Dechlorination of trichloroethylene formed from 1,1,2,2-tetrachloroethane by dehydrochlorination in Portland cement slurry including Fe(II)

Transformation of 1,1,2,2-tetrachloroethane (1,1,2,2-TeCA) by Fe(II) in 10% cement slurries was characterized using a batch reactor system. 1,1,2,2-TeCA was completely converted to trichloroethylene (TCE) within 1h in all experiments, even in controls with cement that did not include Fe(II). Therefore, complete degradation of 1,1,2,2-TeCA depends on the behavior of TCE. The half-life of TCE was observed to be 15d when concentrations of Fe(II) and 1,1,2,2-TeCA were 98mM and 0.245mM, respectively. The kinetics of TCE removal was observed to be dependent on Fe(II) dose, pH and initial substrate concentration. Pseudo-first-order rate constants linearly increased with Fe(II) dose up to 198mM when initial target concentration was 0.245mM. Pseudo-first-order kinetics generally described the degradation reactions of TCE at a specific initial concentration, but a modified Langmuir-Hinshelwood model was necessary to describe the degradation kinetics of TCE over a wide range of initial concentrations. A surface reaction of TCE on active solids, which were formed from Fe(II) and products of cement hydration appears to control observed TCE degradation kinetics.     

Modelling the environmental degradation of water contaminants. Kinetics and mechanism of the riboflavin-sensitised-photooxidation of phenolic compounds

The aerobic visible-light-photosensitised irradiation of methanolic solutions of either of the phenolic-type contaminants model compounds (ArOH) p-phenylphenol (PP), p-nitrophenol (NP) and phenol (Ph), and for two additional phenolic derivatives, namely p-chlorophenol (ClP) and p-methoxyphenol (MeOP), used in some experiments, was carried out. Employing the natural pigment riboflavin (Rf) as a sensitiser, the degradation of both the ArOH and the very sensitiser was observed. A complex mechanism, common for all the ArOH studied, operates. It involves superoxide radical anion (O₂^√−) and singlet molecular oxygen (O₂(¹Δ_g)) reactions. Maintaining Rf in sensitising concentrations levels (≈0.02 mM), the mechanism is highly dependent on the concentration of the ArOH. Kinetic experiments of oxygen and substrate consumption, static fluorescence, laser flash photolysis and time-resolved phosophorescence detection of O₂(¹Δ_g) demonstrate that at ArOH concentrations in the order of 10 mM, no chemical transformation occurs due to the complete quenching of Rf singlet excited state. When ArOH is present in concentrations in the order of mM or lower, O₂^√− is generated from the corresponding Rf radical anion, which is produced by electron transfer reaction from the ArOH to triplet excited Rf. The determined reaction rate constants for this step show a fairly good correlation with the electron-donor capabilities for Ph, PP, NP, ClP and MeOP. In this context, the main oxidative species is O₂^√−, since O₂(¹Δ_g) is quenched in an exclusive physical fashion by the ArOH. The production of O₂^√− regenerates Rf impeding the total degradation of the sensitiser. This kinetic scheme could partially model the fate of ArOH in aquatic media containing natural photosensitisers, under environmental conditions.