UV H2O2 Research Articles

Surface disinfection of wheat kernels using gas phase hydroxyl-radical processes: Effect on germination characteristics, microbial load, and functional properties.

Wheat kernels harbor a diverse microflora that can negatively affect the suitability of the grains for further processing. To reduce surface microflora, a kernel disinfection method is required that does not affect grain functionality. Three different versions of gas phase hydroxyl-radical processes were compared with the common method for grain disinfection, that is, a bleach treatment. The gas phase hydroxyl-radicals are generated by the UV-C mediated degradation of hydrogen peroxide and/or ozone in a near water-free process. It was found that treating kernels with a bleach solution could reduce total aerobic count (TAC) and fungal count to below the level of enumeration. In comparison, the gas phase hydroxyl-radical treatment, that is, H2 O2 -UV-ozone treatment, could support a 1.3 log count reduction (LCR) in TAC and a 1.1 LCR in fungal count. The microbial load reduction for the wholemeal samples was less pronounced as endophytic microorganisms were less affected by all treatments, hinting at a limited penetration depth of the treatments. Despite reducing the microbial load on the kernel surface through the bleach and H2 O2 -UV-ozone treatments, none of these treatments resulted in a reduced microbial count on grains that underwent sprouting after the treatments. No negative effect on germination power or development of the seedling was observed for any of the treatments. The gluten aggregation behavior and xylanase activity of the wholemeal also remained unchanged after the gas phase hydroxyl-radical treatments. Our findings suggest that UV-H2 O2 -ozone treatment shows promise for dry-kernel disinfection, but further optimization of the processing parameters is required.

Removal of Colours in Textile Wastewater by Electrochemical Methods: A Review

Dye wastewater from industries poses significant health hazards to the environment, so it is important to limit its discharge into receiving waters. This overview discusses several feasible, low-cost treatment methods, which includes biological, chemical and physical approaches. Adsorption and membrane filtration are the two common and widely acceptable physical methods, whereas chemical (or widely known as oxidative methods include Fenton treatment, ozone treatment, H2O2 UV irradiation, hydrogen peroxide, NaOCl, ion-exchange, electrocoagulation, reverse process, nanofiltration, etc.). Biodegradation can occur either aerobically or anaerobically, and both of these are examples of the biological techniques. When it comes to the treatment of wastewater effluents from the textile industry, there are primarily two areas of concern: selecting the appropriate technology and developing an appropriate treatment strategy. However, The treatment of dye solutions typically depends heavily on electrochemical processes.

Application of the enhanced sono-photo‑Fenton-like process in the presence of persulfate for the simultaneous removal of chromium and phenol from the aqueous solution

This study was conducted to investigate the enhanced sono-photo‑Fenton-like process (SPFP) in the presence of persulfate, as a method of Advanced Oxidation Processes (AOPs) to generate oxidative radicals (O2−, OH, SO4−) for the simultaneous removal of chromium and phenol from aqueous solutions. The removal performance of chromium and phenol by UV alone, UV/H2O2 (UH), UV/H2O2/Fe0(UHF), UV/H2O2/Fe0/S2O8 (UHFS) and UV/H2O2/Fe0/S2O8/US (UHFSU) were estimated (12.5, 6.5 %), (35.1, 31 %), (52.5, 66.1 %), (74.9, 79.1 %) and (100, 100 %), respectively under selected conditions (phenol/chromium molar ratio, 2.0; pH, 3.0; H2O2/Fe0 molar ratio, 2.0; S2O8, 0.75 mM L−1 and reaction time, 4 min). Also, according to obtained results, robs value for UHFSU process in the removal of (chromium and phenol) was about (14.62, 30.84), (10, 4), (7.64, 2.47) and (3.2, 1.3) times than that of the UV alone, UH, UHF and UHFS, respectively. The study of the energy consumption for the various processes indicated the UHFSU process compared with other processes, has very low energy consumption (kWh m-3), which proves the process justification economically. The addition of the various scavengers (ammonium oxalate, 2-propanol and tert-butanol) was diminished the removal performance; it clearly proved oxidative radicals (O2−, OH, SO4−) plays a noticeable role in the removal of chromium and phenol.

Performance intensification of BzP photo-catalytic degradation through adding exogenous oxidant

This study was conducted to investigate the photocatalytic removal of Benzylparaben (BzP) using UVC/ZnO (UZ) process in the presence of H2O2 (UZH) from aqueous solutions. The results indicated that mean size and surface area of nano-ZnO were 30 nm and 150 m2/g, respectively. The maximum degradation and mineralization of BzP (15 mg L−1) by UZ process under optimum operational conditions (pH: 9.0, catalyst dose: 1 g L−1 within 90 min reaction), were over 67.37% and 42% of TOC, respectively. The addition of H2O2 (up to 10 mmol L-1) along with UV/ZnO resulted to 100% and 55% removal of BzP and TOC during 25 min of reaction time, respectively. The kinetic studies indicated robs value for UZH process was about 1101.6, 413.12, 367.22, 27.31 and 18.99 times than that of the ZnO, H2O2, UV, UZ and UV/H2O2 (UH), respectively. The values of energy consumption for UV, UZ, UH and UZH were calculated 1280, 95.2, 66.2 and 3.48 kW h m-3, respectively. BzP was converted to a biodegradable waste after a 90 min and 25 min irradiation time in the UV/ZnO and UV/ZnO/H2O2 processes, respectively.

Free radical generation by non-equilibrium atmospheric pressure plasma in alcohol–water mixtures: an EPR-spin trapping study

Free radical species in aqueous solution—various alcohol–water reaction mixtures—by exposure to non-equilibrium cold atmospheric pressure Ar plasma (CAP), were monitored using electron paramagnetic resonance spin-trapping techniques with 3, 5-dibromo-4-nitrosobenzene sulfonate as a water soluble nitroso spin trap. The major radical species were formed by H-abstraction from alcohol molecules due to ·OH radicals. In the ethanol–water mixture ·CH2CH2OH produced by H abstraction from CH3 group of the ethanol and ·CH3 radicals were detected. The latter was due to the decomposition of unstable CH3·CHOH to form the ·CH3 radicals and the stable formaldehyde by C–C bond fission. These intermediates are similar to those observed by reaction with ·OH radicals generation in the H2O2–UV photolysis of the reaction mixtures. The evidence of ·CH3 radical formation in the pyrolytic decomposition of the reaction mixtures by exposure to ultrasound or in methane irradiated with microwave plasma have been reported previously. However, the pyrolytic ·CH3 radicals were not found in both plasma and H2O2–UV photolysis condition. These results suggests that free radicals produced by Ar-CAP are most likely due to the reaction between abundant ·OH radicals and alcohol molecules.

Effect of operating conditions and interfering substances on photochemical degradation of a cationic surfactant

ABSTRACTThis work investigates the degradation kinetics of a recalcitrant organic pollutant, cetyltrimethylammonium bromide (CTAB), using direct UV and UV–H2O2 advanced oxidation processes. Direct photolysis at 253.7 nm showed only 55% degradation up to fluence dose of 40.65 J/cm2 for an initial CTAB concentration of 100 mg/L. The apparent fluence-based pseudo-first-order rate constant and quantum yield were 2.29(±0.325) × 10−5 cm2/mJ and 0.305(±0.043) mol/Einstein, respectively. In case of UV–H2O2, >99% degradation was observed up to a fluence dose of 0.79 J/cm2. The rate constant was ∼200 times higher compared to direct photolysis, which was due to hydroxyl radical generation in the UV–H2O2 process. The second-order hydroxyl radical rate constant for CTAB was found to be 1.59(±0.18) × 109 M−1 s−1. The effects of H2O2 dose, initial CTAB concentration and relevant water quality parameters (pH, alkalinity and nitrate concentrations) were studied; all of these influenced the rate constants. CTAB degradation was also examined in the municipal wastewater matrix. It is concluded that UV–H2O2 represents an efficient treatment process for CTAB in environmental matrices.

Modeling the sonophoto-degradation/mineralization of carbamazepine in aqueous solution

The photo-degradation process of the antiepileptic drug carbamazepine (CBZ) is studied under different conditions. Reactions were carried out under UV–H2O2, US–UV–H2O2 and US–UV–H2O2–Fe systems. An important synergistic effect between sonolysis and UV irradiation of 27.7% was quantified using the first order rate constants for carbamazepine degradation.An empirical model that includes the scavenger effect was applied and found to reproduce both degradation and mineralization of CBZ satisfactorily.Comparison experiments of scavenger-loaded conditions demonstrated that the carbamazepine photodegradation occurs mainly through a radical mechanism in two steps: during the first 10–15min, CBZ is completely degraded, whereas TOC barely changes, confirming that intermediates are not easy to mineralize. During this phase, dissolved oxygen (DO) remains in low values and the pair O2−/HO2 is an active contributor to CBZ degradation. Mineralization is low and the scavenger effect from OH radicals is relatively high (kscv=0.0014min−1mM−1). From that moment, intermediates are formed and OH radicals are responsible for increased mineralization rate with a gradual decrease in the scavenger effect (kscv=0.0004min−1mM−1).Experiments performed under the US–UV–H2O2–Fe system proved the important contribution of OH radicals. The maximum mineralization (93% in 35min) was reached for [Fe]=15ppm and [H2O2]=680ppm.A study of the flow pattern inside the reactor showed that improvement in mineralization rate with US radiation cannot be attributed to a positive effect in mixing.

Coupling UV-H2O2 to accelerate dimethyl phthalate (DMP) biodegradation and oxidation.

Dimethyl phthalate (DMP), an important industrial raw material, is an endocrine disruptor of concern for human and environmental health. DMP exhibits slow biodegradation, and its coupled treatment by means of advanced oxidation may enhance its biotransformation and mineralization. We evaluated two ways of coupling UV-H2O2 advanced oxidation to biodegradation: sequential coupling and intimate coupling in an internal circulation baffled biofilm reactor (ICBBR). During sequential coupling, UV-H2O2 pretreatment generated carboxylic acids that depressed the pH, and subsequent biodegradation generated phthalic acid; both factors inhibited DMP biodegradation. During intimately coupled UV-H2O2 with biodegradation, carboxylic acids and phthalic acid (PA) did not accumulate, and the biodegradation rate was 13 % faster than with biodegradation alone and 78 % faster than with biodegradation after UV-H2O2 pretreatment. Similarly, DMP oxidation with intimate coupling increased by 5 and 39 %, respectively, compared with biodegradation alone and sequential coupling. The enhancement effects during intimate coupling can be attributed to the rapid catabolism of carboxylic acids, which generated intracellular electron carriers that directly accelerated di-oxygenation of PA and relieved the inhibition effect of PA and low pH. Thus, intimate coupling optimized the impacts of energy input from UV irradiation used together with biodegradation.

Efficiency of sequential ozone and UV-based treatments for the treatment of secondary effluent

Sequential ozone and UV-based processes (VUV irradiation and UV/H2O2) for the treatment of secondary effluent were investigated. The effectiveness of these sequential treatments was compared with the individual treatments in terms of reduction in DOC, COD, UV254 and colour and impact on BDOC. The greatest decrease in DOC, COD and UV254 was achieved for samples treated with ozone before UV/H2O2 (UVH), where a synergistic benefit was observed. By monitoring the formation of hydroxyl radicals (HO) during UVH and ozonation it was demonstrated that UVH treatments generated more hydroxyl radicals than ozonation, which explained the greater extent of mineralisation attained during UVH treatment. Sequential O3-UVH treatment (for 5 and 10min, respectively) enhanced the formation of biodegradable compounds such that when followed by biological treatment up to 73% of the DOC could be removed from the secondary effluent. Furthermore, application of the O3-UVH treatment achieved 2.5 times greater mineralisation than ozone alone. This improvement, however, required 4.5 times greater electrical energy input (EEI) for the same treatment time of 15min.

Modeling and geometry optimization of photochemical reactors: Single- and multi-lamp reactors for UV–H2O2 AOP systems

This paper deals with the modeling and optimization of a photochemical reactor for the production of hydroxyl radicals. These radicals can be used in advanced oxidation processes for water purification. The UV-lamps used in the photochemical reactor lead to a relatively high energy and operating cost. Minimization of these costs is possible via a model-based optimization of the reactor geometry. First, a model is built taking into account the reaction kinetics, the reactor geometry and the radiation intensity distribution in the reactor. Afterwards, two different configurations of photochemical reactors are optimized. The first modeled reactor configuration is a single-lamp reactor and consists of a cylindrical UV-lamp that is present inside a quartz sleeve, outside of which the water flows between it and the external reactor wall (annular channel reactor). A rigorous optimization problem is formulated consisting of the calculation of the optimal reactor geometry (length and radius) in such a way that, for a given reactor volume and a given flow rate of the water to be treated, the mean outlet concentration of an organic pollutant is minimized. By solving this optimization problem, it has been shown that an optimal reactor length exists. If the absorption coefficient of the water or the reaction rate constant increases, the optimal reactor length increases as well. The second modeled reactor configuration is similar to the first one but now contains multiple lamps positioned symmetrically in a circular pattern (i.e. multi-lamp reactor). The optimization problem consists of the calculation of the optimal lamp position inside the reactor in such a way that the mean outlet concentration of an organic pollutant is minimized. The lamp position is given by the radial distance between the centers of the reactor and a lamp. Again a well-posed optimization problem is obtained and an optimal lamp position exists. If the number of lamps or the reaction rate constant increases or the absorption coefficient of the water decreases, the optimal distance between the centers of the reactor and the lamp increases.

Selective photodegradation and backfilling for regeneration of the inorganic–organic hybrid composite Fe3O4@C18ADB@Zn2SiO4 which captures organic pollutants from aqueous solution

The Fe3O4@C18ADB@Zn2SiO4 hybrid composite was synthesized, and it efficiently adsorbed organic pollutants. The resultant dye sludge was degraded by UV–H2O2 photocatalysis in the presence of Na2SO4, and the sorbent was regenerated by backfilling with C18ADB.

UV–H2O2 degradation of methyl orange catalysed by H3PW12O40/activated clay

A catalyst consisting of phosphotungstic acid (H3PW12O40) combined with activated clay was prepared by the impregnation method, and an experiment was carried out to evaluate the catalytic activity of the H3PW12O40/activated clay for the degradation of methyl orange (MO) in the UV–H2O2 process. The degradation ratio of MO can be affected by H2O2 concentration, reaction time, catalyst dosage, pH and temperature. The reaction temperature should be controlled at less than 70 °C, and the catalyst has a wide applicable pH range in the UV–H2O2 process. Hydroxyl radicals were generated in the UV–H2O2 system under the action of H3PW12O40/activated clay, and MO was degraded by hydroxyl radicals. Compared with traditional catalysts used in UV–H2O2 systems, H3PW12O40/activated clay has certain advantages for its practical application.

A comparative study of UV–Fenton, UV–H2O2 and Fenton reaction treatment of landfill leachate

In this study, laboratory experiments were conducted to compare the efficacy of several H2O2‐based advanced oxidation processes (AOPs): UV–Fenton, UV–H2O2, and Fenton reagent, to treat landfill leachate with the objective of removing COD and improving the leachate’s overall biodegradability (evaluated in terms of BOD5/COD ratio). The results indicated that, at optimized experimental conditions of 25 °C, pH 4.0, 4000 mg L−1 H2O2, 30 mg L−1 Fe(II) (for UV–Fenton and Fenton ) and 60 min reaction time, the UV–Fenton system showed the highest COD removal. The COD removal rate in the H2O2‐based AOPs was strongly dependent on the concentration of H2O2 and the pH value. Temperature had little effect on overall COD removal rate in the Fenton and Fenton‐like systems. All three systems would result in a significant increase in the ratio of BOD5/COD, from 0.17 to about 0.60, at the applied H2O2 dosage of 4000 mg L−1. The increase in biodegradability of the AOP leachate indicates that these AOPs would be beneficial for the subsequent biological treatment process. The UV–Fenton process was found to be the most effective approach of these H2O2‐based AOPs to enhance the biodegradability and eliminate the colour of the leachate.

Decoloration of methylene blue simulated wastewater using a UV-H2O2 combined system

Methylene blue simulated wastewater was treated with a UV-H2O2 combined system. Influences of factors such as reaction time, initial pH value and H2O2 dosage were investigated, and the reaction kinetics of the process was explored. Results showed that the degradation of methylene blue happened only in the presence of both conditions: UV irradiation and H2O2 addition. Initial pH and H2O2 dosage had a remarkable influence on the degradation efficiency. Through several groups of univariate experiments, the optimum pH and H2O2 dosage of the photolysis process were found to be 4–5 and 0.165 mL 30% H2O2 per milligram of methylene blue, respectively. The photolysis process was relatively fast at the initial stage and, 20 min later, it was approximately in accordance with the first-order kinetic equation.

Degradation of acidic Orange G dye using UV-H2O2 in batch photoreactor

Degradation of Orange G dye has been investigated using UV irradiation with hydrogen peroxide (H2O2) in a batch photoreactor. UV irradiation and H2O2 resulted in significant photodegradation of the dye although the effect individual reaction was very little. The degradation was studied to elucidate the effect of various process parameters such as pH, concentration of dye, dose of H2O2, TiO2 and light intensity/light source with reflecting or non reflecting surface of photoreactor. Results showed the efficient degradation of Orange G for typical process conditions.Keywords: Orange G, UV/H2O2, photoreactor, degradation, process parameters.

Decolourization and removal of some organic compounds from olive mill wastewater by advanced oxidation processes and lime treatment

Olive mill wastewater (OMW) generated by the olive oil extracting industry is a major pollutant, because of its high organic load and phytotoxic and antibacterial phenolic compounds which resist biological degradation. Mediterranean countries are mostly affected by this serious environmental problem since they are responsible for 95% of the worldwide olive-oil production. There are many methods used for OMW treatment, such as adsorption, electro coagulation, electro-oxidation, biological degradation, advanced oxidation processes (AOPs), chemical coagulation, flocculation, filtration, lagoons of evaporation and burning systems, etc. Currently, there is no such economical and easy solution. The aim of this study was to evaluate the feasibility of decolourization and removal of phenol, lignin, TOC and TIC in OMW by UV/H2O2 (AOPs). The operating parameters, such as hydrogen peroxide dosage, times, pH, effect of UV and natural sunlight were determined to find the suitable operating conditions for the best removal. Moreover, there is no study reported in the literature related to the use of OMW was obtained from an olive-oil producing plant (Muğla area of Turkey) which uses a modern production process. No chemical additives are used during olive oil production. This study was realised by using two different UV sources, while taking the time and energy consumption into consideration. These two sources were mercury lamps and natural sunlight. Before starting AOPs experiments, one litre of OMW was treated by adding lime until a pH of 7.00. Then, 100 ml was taken from each sample, and 1 to 10 ml of a 30% H2O2 (Riedel-deHaen) solution was added. These solutions in closed vessels were laid in the natural sunlight for a week and their compositions and colour changes were analysed daily by UV-Vis spectrophotometer. At the end of the one-week period, they were treated with lime. In this study, the effect of changes in the initial pH, times and H2O2 concentrations on removal was investigated. At the end of all experiments, changes in colour, phenol, lignin, TOC and TIC concentrations were analysed according to standard methods. In the samples exposed to natural sunlight and having an H2/OMW ratio of 3 ml/100 ml, a significant colour removal was achieved approximately 90% of the time at the end of 7 days. When the same samples were treated with lime (pH: up to 7), 99% efficiency was achieved. When phenol and lignin removals were examined in the same concentration, phenol and lignin removal were found 99.5%, 35%, respectively. However, for maximum lignin removal, more use of H2O2 (10 ml H2O2/100 ml OMW) was found to be necessary. Under these conditions, it was found that lignin can be removed by 70%, but to 90% with lime, at the end of a seven-day period. Rate constants obtained in the experiments performed with direct UV were found to be much higher than those of the samples exposed to natural sunlight (k(a)lignin=0.3883>> k(b)lignin=0.0078; k(a)phenol=0.5187>> k(b)phenol = 0.0146). Moreover, it should be remembered in this process that energy consumption may induce extra financial burden for organisations. It was found, in general, that colour, lignin, total organic carbon and phenol were removed more efficiently from OMW by using H2O2 UV and lime OMW. Moreover, in the study, lime was found to contribute, both initially and after radical reactions, to the efficiency to a great extent. Another result obtained from the study is that pre-purification carried out with hydrogen peroxide and lime may constitute an important step for further purification processes such as adsorption, membrane processes, etc.

Removal of dimethyl phthalate from water by UV–H2O2 process

This study aims to demonstrate the removal efficiency of dimethyl phthalate from water using the UV–H2O2 photooxidation process. Pure water samples, spiked with 20 ppm dimethyl phthalate (DMP) were treated by the combined effect of UV photolysis as well as hydrogen peroxide oxidation mechanisms. In the experiments, the concentration of hydrogen peroxide was varied from 34 to 136 ppm and a low-pressure mercury UV lamp of 100 mW power output was used to provide the necessary radiation. The effects of initial concentration of H2O2, UV exposure time, pH, and temperature were investigated. The results showed that about 60% of DMP were removed directly by activation caused by UV light radiation intensity after an exposure time of 1 h. However, the removal efficiency increased when the DMP-spiked water was dosed with H2O2 prior to irradiating with UV light (i.e., UV–H2O2). More than 98% of DMP was removed after 45 min when the UV-irradiated solution was dosed with 136 ppm of H2O2. The results also showed that lowering the pH and increasing the temperature enhanced the removal of DMP by UV–H2O2 process. Key words: ultraviolet light, hydrogen peroxide, hydroxyl radical, phthalates, UV–H2O2 process, advanced oxidation processes.

Degradation of nitroaromatic compounds by the UV-H2O2 process using polychromatic radiation sources.

The UV-H2O2 process, a standard advanced oxidation process (AOP) for water treatment, has been applied to the degradation of a series of nitroaromatic compounds (nitrobenzene, 1-chloro-2,4-dinitrobenzene, 2,4-dinitrophenol, 3-nitrophenol, 4-nitrophenol and 4-chloro-3,5-dinitrobenzoic acid) using polychromatic radiation sources. The optimal concentration of hydrogen peroxide ([H2O2]OPT) leading to the fastest degradation rate of a given substrate (S) was determined experimentally and estimated using a simplified kinetic model based on the main reactions involved in the first stages of the oxidation. We have shown that, under conditions of monochromatic irradiation, the ratio ROPT (= [H2O2]OPT/[S]0) is given by a simple mathematical expression containing only a few parameters, whereas, under conditions of polychromatic irradiation, ROPT is expressed by a complex mathematical equation (involving the spectral distribution of the lamp emission and the absorption spectra of H2O2 and the substrate). Two numerical analysis procedures are proposed for obtaining the bimolecular rate constants for the reaction of hydroxyl radicals with a substrate (kS) from this equation. The rate constants, kS, determined for the substrates investigated in this work are in agreement with the expected reactivity trend, taking into account the effects of substituents on the distribution of electron density in the aromatic ring. The methods proposed in this work offer a double advantage: i) a standard AOP may be used to evaluate the rate constants of reaction of substrates with hydroxyl radicals under polychromatic as well as under monochromatic irradiation, ii) optimal amounts of additive may be obtained using only a few parameters as predictive tools.

AOP/biofilm processes for DOX precursors

Ozone–hydrogen peroxide did not enhance performance, but ultraviolet–hydrogen peroxide performed comparably to ozone, and it is a much simpler process.Batch and continuous‐flow, bench‐scale experiments were performed to assess the performance of two advanced oxidation processes in combination with subsequent biodegradation for the removal of total organic carbon (TOC) and disinfection byproduct (DBP) precursors, as characterized by dissolved organic halogen formation potential (DOXFP). Ozone–hydrogen peroxide (O3–H2O2) and ultraviolet–hydrogen peroxide (UV–H2O2) were studied over a range of oxidation conditions for their ability to promote TOC and DOXFP removal in a subsequent biodegradation step. For the two waters studied, from Lake Austin and Lake Houston in Texas, the addition of H2O2 did not enhance performance after biodegradation relative to O3 alone, indicating that O3–H2O2 oxidation is not worthwhile in waters of this type, i.e., those with low specific UV absorbance. UV–H2O2–biodegradation performed comparably to O3–biodegradation, which suggests it is an alternative technology that may be worth considering for DBP precursor removal, especially for utilities that are interested in avoiding the operational complexities of ozonation.

UV–H2O2 oxidation of monoazo dyes in aqueous media: a kinetic study

During the past few years attention has been drawn on chemical techniques that could be used to discolour textile wastewaters. We have studied the photooxidation of several aqueous monoazodyes solutions in the presence of hydrogen peroxide. Irradiation was carried out with a low-pressure mercury lamp. We observed that the reaction is pseudo-first-order with respect to the dye concentration. The effects of hydrogen peroxide and dye dosages, temperature, pH, and chemical structure of the organic matter were also investigated. Results suggest that molecules, hydrazone form of which predominates, are the most sensitive to °OH oxidation. If a labile hydrogen atom is present in the chromophore, it is the primary target of the attack.