This paper reports on recent developments in homogeneous Advanced Oxidation Processes (AOPs) for the treatment of water and wastewater. It has already been established that AOPs are very efficient compared to conventional treatment methods for degradation and mineralization of recalcitrant pollutants present in water and wastewater. AOPs generate a powerful oxidizing agent, hydroxyl radical, which can react with most of the pollutants present in wastewater. Therefore, it is important to discuss recent developments in AOPs. The homogeneous AOPs such as O3, UV/O3, UV/O3/H2O2, and UV/H2O2, Fe2+/H2O2, UV/Fe2+/H2O2 on the degradation of pollutants are discussed in this paper. The influence on the process efficiency of various experimental parameters such as solution pH, temperature, oxidant concentration, and the dosage of the light source is discussed. A list of contaminants used for degradation by various AOPs and the experimental conditions used for the treatment are discussed in detail.
Wastewater is water that contains various pollutants, which means it cannot be used like pure water and should not be disposed of in a manner dangerous to humans, living organisms, and the environment. Water pollution has a serious impact on all living creatures, adversely affecting water use for drinking, household needs, recreation, fishing, transportation, and commerce. It has been estimated that the total global volume of wastewater produced in 1995 was in excess of 1,500 km3 [
In the past, economically viable chlorination has been used for water treatment. Yet the potentially adverse health effects of the by-products formed, together with raised drinking water standards, have led researchers to search for effective and economical alternatives to chlorinating drinking water [
Among the chemical methods, oxidation is efficient and applicable to large scale wastewater treatment. Generally air, oxygen, ozone, and oxidants such as NaOCl and H2O2 are used for chemical treatment. The oxidation potential of some of the oxidants is listed in Table
The oxidation potential of various reactive species.
Substance | Potential (V) |
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Hydroxyl radical (
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2.86 |
Oxygen (O) | 2.42 |
Ozone molecule (O3) | 2.07 |
Hydrogen peroxide (H2O2) | 1.78 |
Chlorine (Cl2) | 1.36 |
Chlorine dioxide (ClO2) | 1.27 |
Oxygen molecule (O2) | 1.23 |
Advanced oxidation processes (AOPs) for wastewater treatment have received a great deal of attention in recent years. AOPs generate the highly reactive hydroxyl radical (•OH) to degrade the recalcitrant chemicals present in wastewater [
AOPs such as ozonation (O3), ozone combined with hydrogen peroxide (O3/H2O2) and UV irradiation (O3/UV) or both (O3/H2O2/UV), ozone combined with catalysts (O3/catalysts), UV/H2O2, Fenton and photo-Fenton processes (Fe2+/H2O2 and Fe2+/H2O2/UV), and the ultrasonic process and photocatalysis have been successfully used for wastewater treatment [
Wastewater was treated using the Fenton process or homogeneous AOP employing iron salt with hydrogen peroxide. The combination of Fenton’s reagent with UV light is called a photo-Fenton reaction. UV light irradiation enhances the efficiency of the Fenton process. The hydroxyl radical generated in the Fenton process is due to the iron catalysed decomposition of H2O2 as shown in the following:
The UV/H2O2 process is a homogeneous advanced oxidation process employing hydrogen peroxide with UV light. Hydrogen peroxide requires activation by an external source such as UV light and the photolysis of hydrogen peroxide generates the effective oxidizing species hydroxyl radical (•OH). The rate of photolysis of H2O2 depends directly on the incident power or intensity. The hydrogen peroxide decomposition quantum yield is 0.5 at UV (254 nm) irradiation. Solar light could also be used as a radiation source but the rate of photolysis may be low compared to UV light. In this process the dosage needs to be optimized, however, since excess H2O2 may scavenge hydroxyl radical.
Heterogeneous catalytic ozonation is a novel type of AOP that combines ozone with the adsorptive and oxidative properties of solid phase catalysts to decompose pollutants at room temperature. Catalytic ozone decomposition at room temperature is advantageous compared to thermal decomposition in terms of energy conservation since it does not require large volumes of air to be heated. It is therefore a promising advanced oxidation technology for water treatment.
Heterogeneous photocatalysis through illumination by UV or visible light on a semiconductor surface generates hydroxyl radicals. The photocatalyst can be used successfully for the effective treatment of pollutants in water and wastewater.
As noted above, ozone reacts with various organic and inorganic compounds in an aqueous solution, either by direct reaction of molecular ozone or through a radical mechanism involving hydroxyl radical induced by the ozone decomposition. Figure
Experimental setup of the ozonation process.
Although hydroxyl radical formation is highly favourable to produce more •OH radicals by ozone self-decomposition at pH 10, a portion of carbonate or bicarbonate ion formation could play a key scavenging role in trapping •OH radicals, appreciably decreasing the degradation rate. Wu et al. found that 2-propanol degradation decreases at pH 10 and suggested bicarbonate formation as the possible reason for the decreasing degradation rate at this pH [
Several investigations were conducted into the effect of temperature on the ozonation process. Changing the temperature generally influences the ozonation process in two ways. Firstly, when the temperature increases, the solubility of ozone may decrease, since Henry’s law coefficient of ozone increases with rising temperatures. Secondly, raising the temperature increases the activation energy which may positively assist the ozonation process. Muruganandham et al. noted that N-methyl pyrrolidone (NMP) mineralization was substantially increased when the ozonation temperature rose from 5 to 50°C [
Another important experimental parameter influencing ozonation process efficiency is influent ozone dosage. Treatment cost increases with a higher applied ozone dose, so it is necessary to optimize this dosage. For semibatch experiments, increasing the ozone dosage will enhance the mass transfer rate of ozone from the gas phase to the liquid phase, which is expected to enhance the degradation rate appreciably. As the ozone concentration in the liquid phase is saturated, however, ozone mass transfer is limited at a very high ozone dosage [
Though the ozonation process is effective for treating some organic compounds, a key problem is the accumulation of refractory compounds which interfere with the mineralization of the organic matter present in water. Some compounds were even found to be refractory to the ozonation process [
It was reported that ozone in the presence of UV light enhances the decomposition rate of pollutants present in wastewater. The hydroxyl radicals generated in the UV/O3 process are shown in (
Recent studies also combined H2O2, and TiO2 with the UV/O3 process [
Degradation of some model pollutants by ozone based AOPs under different experimental conditions.
Reference | AOPs applied | Pollutant(s) | Conclusions |
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O3, UV/H2O2, O3/H2O2, O3/AC | Diethyl phthalate (DEP) in ultrapure water, surface water, and wastewater | The O3/AC process was the most efficient for the removal of DEP in all three types of water. The O3/H2O2 and O3/AC processes are more efficient than ozonation alone. |
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O3, UV photolysis, O3/UV, H2O2/O3, O3/H2O2/UV | 1,4-Dioxane | The O3/H2O2/UV process was most efficient for 1,4-dioxane removal at pH 10, with H2O2 : O3 ratio of 0.5. |
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O3, O3/H2O2, UV/H2O2, UV/O3 UV/H2O2/O3 | Phenol | The UV/H2O2/O3 process at pH 7 with H2O2 = 10 mM was most ecoeffective with 100% of phenol removal within 30 min and 58.0% TOC removal after 1 h. UV/H2O2/O3 was the most effective process for phenol wastewater mineralization. |
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O3, O3/H2O2, UV/H2O2 | Twenty-four micropollutants including endocrine disrupting compounds, pharmaceuticals, and personal care products | The general trend of ozone and hydroxyl radical reactivity with the selected micropollutants was explained. Suitable technology for the removal of these micropollutants was suggested based on the micropollutant reactivity with ozone and hydroxyl radical. |
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O3, UV photolysis, O3/UV, O3/catalyst, UV/catalyst, O3/UV/catalyst, H2O2/UV, H2O2/UV/catalyst | Pyruvic acid | The UV/H2O2 process with or without perovskite catalysts facilitates pyruvic acid removal fastest. The O3/UV/perovskite process was efficient for mineralization. |
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UV, UV/H2O2, UV/O3, UV/H2O2
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Operating conditions such as initial pH, concentration of H2O2, and ferrous salt were optimized for each process. The UV/Fenton and UV/H2O2
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O3, H2O2/O3, UV/ |
Nitroaromatics such as |
Ozonation and/or Fenton’s reagent were found to be efficient for TNT degradation. The O3/H2O2 process at pH > 7 was most efficient for 2-MNT and 2.4-DNT removal. |
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O3/UV, H2O2/UV, O3/H2O2, O3/H2O2/UV | Haloacetic acids (HAAs), dichloroacetic acid (DCAA), and trichloroacetic acid |
The O3/UV process was the most efficient of the six degradation methods for DCAA and TCAA in water. Decomposition by AOPs was easier for DCAA than for TCAA. |
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O3, O3/UV, O3/H2O2, UV/H2O2, O3/UV/H2O2 | O-Nitrotoluene | The optimum H2O2 dosage and solution pH were studied. Adding H2O2 to the ozonation process accelerated the oxidation of O-nitrotoluene by a factor of 8. The O3/UV and UV/H2O2 processes are 20 and 10 times more efficient than the ozonation process, respectively. |
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O3, UV-vis, |
Phenol, |
AOP efficiencies are in the following order: adsorption < TiO2 + UV-vis < UV-vis < O3 + TiO2
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O3, O3/H2O2, O3/activated carbon | Acid Blue 92 (AB92) | Ozone treatment was a very effective method for complete removal of colour but in COD removal it was not efficient. The removal of COD in ozonation, O3/H2O2, and O3/AC processes, 30%, 80% and 100%, respectively. |
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O3 or O3/H2O2, O3/powdered activated carbon (PAC) | Sodium Dodecylbenzenesulfonate(SDBS) | Comparison of the O3/PAC system with the O3 and O3/H2O2 processes showed that the O3/PAC system was more effective in the removal of SDBS. |
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O3, O3/UV, UV/H2O2 | Dye house effluent | The AOP efficiency is dependent on the pH and dosage of H2O2. The UV/H2O2 process is 50 times more efficient than the O3/H2O2 process. |
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Ozonation, sonication, UV photolysis, O3/ultrasound, UV/ultrasound, O3/UV/ultrasound | Phenol | The efficiency of the various AOPs at two different pH was in the following order. At pH 2, US/UV/O3
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US |
Acid Orange 7 | The UV/O3 process was more effective at all times than the US and/or O3 process. The O3/US/UV process was the most efficient for colour and aromatic removal and AO7 dye mineralization. |
The presence of transition metal ions such as Mn2+, Co2+, Ag+, and Fe2+ in the ozonation process has significant catalytic effects in producing hydroxyl radical [
The Fenton process has its root in the finding reported in 1894 that ferrous ion strongly elevated the oxidation of tartaric acid by hydrogen peroxide [
•OH radicals are rapidly generated through (
As iron(II) acts as a catalyst, it has to be regenerated, which seems to occur through the following scheme [
Generally speaking, Fenton’s oxidation process is composed of four stages including pH adjustment, oxidation reaction, neutralization and coagulation, and precipitation. The organic substances are removed at two stages of oxidation and coagulation [
The oxidation rate was influenced by many factors such as pH value, Fe2+ : H2O2 ratio, and the amount of iron salt. Some of these parameters are discussed in detail in the following sections. The Fenton process seems to be the best compromise because it is technologically simple, there is no mass transfer limitation (homogeneous nature), and both iron and hydrogen peroxide are cheap and nontoxic. From the economic point of view, using the Fenton process as a pretreatment can lower the cost and improve biological treatment efficiency [
A batch Fenton reactor essentially consists of a pressurized stirred reactor with metering pumps for the addition of acid, a base, a ferrous sulphate catalyst solution and industrial strength (35–50%) hydrogen peroxide. It is recommended that the reactor vessel be coated with an acid resistant material, because Fenton’s reagent is very aggressive and corrosion can be a serious problem. The pH of the solution must be adjusted to maintain the stability of the catalyst, as at pH 6 iron hydroxide is usually formed. For many chemicals the ideal pH for the Fenton reaction is between 3 and 4, and the optimum catalyst to peroxide ratio is usually 1 : 5 wt/wt. Reactants are added in the following sequence: wastewater followed by dilute sulphuric acid catalyst in acidic solutions, base or acid for the adjustment of pH at a constant value, and lastly hydrogen peroxide (which must be added slowly, maintaining a steady temperature). Since wastewater compositions are highly changeable, there are some design considerations to enable the Fenton reactor to operate within flexible parameters. The discharge from the Fenton reactor is fed into a neutralizing tank to adjust the pH of the stream, followed by a flocculation tank and a solid-liquid separation tank for adjusting the TDS (total dissolved solids) content of the effluent stream. A schematic representation of the Fenton oxidation treatment is shown in Figure
Treatment flow sheet for Fenton oxidation [
As mentioned above, Fenton oxidation was applied to wastewater treatment based on the following observed optimum pH conditions, since this has been shown to affect the degradation of pollutants significantly [
Furthermore, the scavenging effect of hydroxyl radicals by hydrogen ions becomes important at a very low pH, at which the reaction of Fe3+ with hydrogen peroxide is also inhibited. At an operating pH of >3, the decomposition rate decreases because of the decreased free iron species in the solution, probably due to the formation of Fe(II) complexes with the buffer inhibiting the formation of free radicals. At a pH higher than 3, Fe3+ starts precipitating as ferric oxyhydroxides and breaks down the H2O2 into O2 and H2O [
Usually the rate of degradation increases with an increased concentration of ferrous ions [
The concentration of hydrogen peroxide plays a more crucial role in the overall efficacy of the degradation process. Usually it has been observed that the percentage degradation of the pollutant increases with an increased dosage of hydrogen peroxide [
It should be noted that the dose of H2O2 and the concentration of Fe2+ are two relevant and closely related factors affecting the Fenton process. The H2O2 dose has to be fixed according to the initial pollutant concentration. An amount of H2O2 corresponding to the theoretical stoichiometric H2O2 to chemical oxygen demand (COD) ratio is frequently used [
As noted above, as the maximum degradation rates are observed at a pH of approximately 3, the operating pH should be maintained constant around this optimum value. The type of buffer solution used also affects the degradation process [
Not many studies are available depicting the effect of temperature on degradation rates and ambient conditions can safely be used with good efficiency [
Typical findings observed in work related to the use of Fenton.
Reference | Process conditions | Pollutant(s) | Conclusions |
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A temperature controllable magnetic stirrer ensures perfect mixing at a constant rate of 300 rpm during all experiments. The effect of Fe2+ concentration on COD removal varied in the range of 0.5–10 mM (these factors were kept constant: H2O2 = 30 mM; pH = 3; |
Synthetic acid dye baths (SADB) consist of three different acid dyestuffs (C.I. Acid Yellow 242, C.I. Acid Red 360, and C.I. Acid Blue 264) and two dye auxiliaries (a levelling agent and an acid donor) | Optimum experimental conditions for the simulated acid dye bath effluent were established as follows: Fe2+ = 10 mM, H2O2 = 30 mM, and pH = 3 at room temperature ( |
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The Fenton reactor was stirred at room temperature in an open-batch system with a magnetic stirring bar and was treated for 2 h. The Fe+2 : H2O2 ratio was varied in the range of 1 : 5, 1 : 10, 1 : 20, 1 : 30, 1 : 40, and 1 : 50, pH in the range of 2–4, and Fe2+ in the range 0.5 and 1 mM. | RB49 Reactive Blue 49 |
The Fenton process was decolourized more than 90% in all cases. The best mineralization extent, that is, maximal TOC removal, 72.1%, was obtained for degradation of RB49 by Fenton process, Fe2+ : H2O2 = 1 : 20, Fe2+ = 0.5 mM at pH = 3. The molecular structure of the dyes studied plays a significant role in oxidation by Fenton type processes. |
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The oxidation studies were conducted in brown 500 mL glass bottles. The pH of wastewater and bleach was first adjusted to 3 with H2SO4. Degradation of EDTA in distilled water was conducted by Fenton’s reagent with Fe concentrations 0–0.9 mM and a maximum reaction time of 15 min. The temperature reaction and pH were fixed at 60°C and 3, respectively. | Ethylenediamine tetra acetic acid (EDTA), novel complexing agents, namely, BCA5 and BCA6 | Fenton’s process proved highly effective in the degradation of EDTA in spiked integrated wastewater. With an initial molar ratio of 70 : 1 (H2O2 and EDTA) or higher, EDTA degradation was nearly complete within 3 min of reaction time. Lower EDTA degradation levels at pH 4 and low temperature in bleaching effluent are a major drawback in this study. |
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The initial concentrations of Fe(II) used in this study were 8.37, 13.95, 19.53, 25.11, and 33.40 mg/L, the Fe2+ : H2O2 ratios were set at 0.016, 0.028, 0.039, 0.05, and 0.067, and the concentration of H2O2 was kept constant at 500 mg/L. The initial concentrations of H2O2 used in this study were 50, 100, 200, 500, and 700 mg/L, the Fe2+ : H2O2 ratios were set at 0.0199, 0.0279, 0.06975, 0.1395, and 0.279, and the concentration of Fe(II) was fixed at 13.95 mg/L. | Azo dye C.I. Acid Yellow 23 (AY 23) | The decolourization rate is strongly dependent on the initial concentrations of Fe2+ and H2O2. The optimum operational conditions were obtained at pH 3. The results show that as much as 98% of AY 23 can be decolourized by 13.95 mg/L ferrous ions and 500 mg/L H2O2. |
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All tests were conducted in a 200 mL double glass cylindrical jacketed reactor, which allows cycle water to maintain the reaction mixture at a constant temperature. Temperature control was realized through a thermostat and a magnetic stirrer was used to stir reaction solutions. Operating pH was in the range of 2.5–6.0 and decolouration time was 60 min. Hydrogen peroxide in the range of |
Azo dye Orange G (OG) | The results showed a suitable decolourization condition of initial pH 4.0, H2O2 dosage |
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Chemical oxidation of the red dye solutions with Fenton’s reagent was carried out in a closed jacketed batch reactor (1 L capacity). The reactor was provided with constant stirring, accomplished through a magnetic bar and a Falc magnetic stirrer. The temperature of the reaction mixture was kept constant by coupling the reactor to a Huber thermostatic bath. Operating pH and H2O2 concentration were varied in the range of 2–5 and 5.9–8.8 mM, respectively. The effect of the Fe2+ concentration and reaction temperature was investigated in the range of 0.13–1.1 mM and 20–70°C, respectively. | Azo dye (Procion Deep Red H-EXL gran) | Total organic carbon (TOC) reduction occurred after 120 min of reaction; however, the reaction time required to achieve colour removal levels above 95% is around 15 min. Four operating variables must be considered, namely, the pH, the concentration of hydrogen peroxide, the temperature, and the concentration of ferrous ion, between 3-4, 5.9 mM, 20 min, and 0.27 mM, respectively. It was concluded that temperature and ferrous ion concentration are the only-variables that affect TOC removal, and, due to cross interactions, the effect of each variable depends on the value of the other one, thus affecting the process response positively or negatively. |
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Fenton’s reagent experiments were carried out at room temperature ( |
Simazine | At a constant simazine concentration, the percentage of TOC removal increased with increasing H2O2 and Fe(II) concentrations up to 15 mg/L Fe(II) and 50 mg/L peroxide above which mineralization decreased due to the scavenging effects of H2O2 on hydroxyl radicals. Maximum pesticide (100%) and TOC removals (32%) were obtained with H2O2/Fe(II)/simazine ratio of 55 : 15 : 3 (mg/L). Simazine degradation was incomplete, yielding the formation of intermediates which were not completely mineralized to CO2 and H2O. |
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The experiments were performed in an insulated vessel with a capacity of 1 L mounted on a steel frame and stirred at 130 rpm. The pH of initial solutions was set at 3. Gradation efficiencies were compared by varying Fenton’s reagent concentration and ratios. The parallel monitoring of Fenton’s reagent concentrations allowed the evidencing hydrogen peroxide or ferrous ion contents as limiting factors for TNT removal. The |
TNT | Fenton oxidation is an effective method to transform TNT totally in contaminated aqueous solution. This is feasible by the efficient generation of hydroxyl radicals during H2O2 catalytic decomposition with Fe(II) ions. TNT degradation kinetics and efficiency are largely influenced by H2O2 and Fe2+ concentrations. Using [H2O2]0 : [Fe(II)]0 molar ratios equal to or lower than 0.5 leads to the formation of the maximum number of intermediates. The absolute rate constant of the reaction between hydroxyl radicals and TNT is 9.6– |
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The Fenton reactor was a 0.5 L beaker placed in a thermostat water bath with constant temperature and stirred by a magnetic stirrer, with operating pH values of 2.50, 3.00, 3.50, 4.00, and 5.00, initial H2O2 concentration in the range of 0.10 mM to 4.00 mM, initial concentration of Fe2+ from 0.01 mM to 0.10 mM, and initial Amido Black 10B concentration on its degradation in the range of 10–100 mg/L. A series of experiments were conducted by varying the temperature from 15°C to 45°C. | Azo dye Amido Black 10B | The optimal operation parameters for the Fenton oxidation of Amido Black 10B were 0.50 mM |
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Fenton oxidation was performed in a batch reactor under initially anaerobic conditions to determine the effect of [MTBE]0 on the degradation of MTBE with FR: MTBE degradation at different [MTBE]0 in the range of 1, 2, and 5 mg/L when treated with the same amount of FR. This study was performed using solutions containing [MTBE]0 of 11.4 and 22.7 mM, each one in individual experiments at pH values of 3.0, 3.6, 5.0, 6.3, and 7.0. The FR to MTBE molar ratio varied in the range of 0.5 : 1 and 200 : 1. The initial concentration of pollutant was 22.7 |
Methyl tert-butyl ether (MTBE) | FR partially degraded low |
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The experiments were conducted in batch mode. 4L borosilicate reactors were filled with 3.6 L of deionized (DI) water at pH = 3.0 and purged with high-purity nitrogen until the dissolved oxygen (DO) reading was below 0.01 mg/L and the oxygen concentration in the head space was negligible ( |
Methyl tert-butyl ether (MTBE) | The added amount of FR proved to be an important controlling parameter for the overall MTBE degradation mineralization efficiency. An FR to MTBE molar ratio of 20 : 1 was the minimum required to achieve complete MTBE degradation. Kinetic analysis is reported to be pseudo first-order given the good linear correlation found between |
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A series of experiments were conducted at pH 3 for 5, 15, or 60 min of mixing followed by 30 min clarification. The studied H2O2/Fe2+ stoichiometric molar ratios were 1, 2, 3, 4, 5, and 10 with H2O2 dose of 1000 mgL−1, and the H2O2/Fe2+ stoichiometric molar ratios were 0.5, 2, 3, 5, and 10 with H2O2 dose of 500 mgL−1. A further series of experiments were conducted at an initial pH of 3, 4, 5, 6, or 7 with 5 min mixing followed by 30 min clarification. Comparisons between the Fenton process and Fe3+ coagulation were carried out at an initial pH of 3 and 7. | Nuclear laundry water | The experimental data generally indicated decreased removal efficiencies of organic compounds with an increasing H2O2/Fe2+ ratio. Yet taking into account all factors, thermostat cost-effective degradation conditions were at H2O2/Fe2+ stoichiometric molar ratio of 2 with 5 min mixing and an H2O2 dose of 1000 mgL−1. The initial pH of the laundry water can be as high as 7. Fe3+ coagulation experiments were conducted in order to interpret the nature of the Fenton process. Since the removal efficiency of organic compounds in the Fenton process was slightly higher than in coagulation, the treatment of the nuclear laundry water can be called Fenton-based Fe3+ coagulation. |
The photo-Fenton process, as its name suggests, is rather similar to the Fenton one but also employs radiation [
The effectiveness of photo-Fenton processes is attributed to the photolysis of Fe(III) cations in acidic media yielding Fe(II) cations (
In this process, the photolytic decomposition of Fe(OH)2+ (
The first reaction is a reaction of Fe2+ with H2O2, which generates the powerful reactive species •OH radicals and oxidizes Fe2+ to Fe3+. In other words, the hydroxyl radical generation in Fenton processes is due to the iron catalyzed decomposition of H2O2. The first photo-Fenton reaction causes the formation of hydroxyl radicals. The second reaction of the photo-Fenton process is a reaction of Fe3+ with water, which occurs when light is used at a wavelength from 300 nm to 650 nm. This generates •OH radicals and reduces Fe3+ to Fe2+. These two oxidation-reduction reactions occur repeatedly and completely mineralize organic pollutants to CO2 and H2O [
The oxidation power of the photo-Fenton process is attributed to the generation of OH radicals. Without irradiation, a Fenton-like reaction occurred instead of a photo-Fenton reaction. The Fenton-like reaction is a reaction of Fe3+ with H2O2, which causes the reduction of Fe3+ to Fe2+:
Since Reaction (
Reaction pathways of the photo-Fenton process.
Appropriate implementation of the photo-Fenton treatment depends mainly on the operating variables—H2O2/COD molar ratio, H2O2/Fe2+ molar ratio, and irradiation time. The conventional method is to optimize the operating variables by changing one factor at a time; that is, a single factor is varied while all other factors are kept unchanged for a particular set of experiments. Likewise, other variables are individually optimized through single-dimensional searches, which are time consuming and incapable of reaching the actual optimum as interaction among variables is not taken into consideration [
Typical findings observed in work related to the use of photo-Fenton.
Reference | Process conditions | Pollutant(s) | Conclusions |
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Natural pH conditions with phenol concentrations in the range of 180–733 mg/L. The photochemical treatment was mediated with ferrioxalate and peroxide in two photoreactors of different volumes and operation conditions (batch and with closed flow). | Wastewater | Phenol transformation efficiencies of 100% and total COD reduction percentages of 85% were reached within the first hour of phototreatment, with an aromatic free effluent as the final product in both types of reactor. The ferrioxalate type complexes using mass ratios of oxalate/phenol = 1.5, oxalate/Fe3+ = 15, and H2O2/phenol > 5.0 were shown to be very effective in the treatment of these effluents, even at pH conditions close to neutral, the pH region in which Fenton type processes begin to lose efficiency due to the precipitation of iron as a hydroxide. |
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Photo-Fenton process in a CPC solar photoreactor. The effect of solar activated photo-Fenton reagent at pH 5.0 before and after a slow sand filtration (SSF) process in waters containing natural iron species was investigated. | Natural organic matter (NOM) model compounds (dihydroxy-benzene) | The results showed that the total transformation of dihydroxybenzene compounds was obtained with a mineralization of over 80%. The mineralization of organic compounds dissolved in natural water was higher than in Milli-Q water, suggesting that the aqueous organic and inorganic components (metals, humic acids, and photoactive species) positively affect the photocatalytic process. When 1.0 mg/L of Fe3+ was added to the system, photo-Fenton degradation improved. |
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Two laboratory scale photo-Fenton experiments were performed with the solar simulator and SMX dissolved in diluted water (DW) and in seawater (SW) at the same concentration (50 mg/L; DOC = 23.75 mg/L) as in the pilot plant experiments for their comparison with natural solar radiation. The initial DOC of SW was 2.6 mg C/L. The experiments were performed at three different initial concentrations of FeSO4·7H2O (2.6, 5.2, and 10.4 mg/L). Initial H2O2 concentrations ranged from 30 to 210 mg/L. The solar pilot plant reactor consisted of a compound parabolic collector (CPC) with a 3.0 m2 irradiated surface and total volume of 39 L. | Antibiotic sulfamethoxazole (SMX) | The photo-Fenton degradation of SMX was strongly influenced by the seawater matrix when compared to distilled water. Indeed, in seawater it is proposed that degradation occurs mainly through |
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Photo-Fenton oxidation was carried out using a cylindrical Pyrex thermostatic cell with a 300 mL capacity ( |
Homo-bireactive dye (Procion Red H-E7 B) | The results demonstrated that a photo-Fenton reaction can be used successfully as a pretreatment process to biocompatibilize Procion Red H-E7B reactive dye solutions. The best pretreatment results were obtained with 60 min of photo-Fenton irradiation time and 10 mg/L Fe(II) and 125 mg/L H2O2 of initial reagent concentration. Under these conditions, the BOD5/COD index increased from 0.10 to 0.35 units with 39% mineralization and 16.5 mg/L of residual H2O2. The use of photo-Fenton type reactions as a pretreatment allows the SBR system to remove Procion Red H-E7B Reactive Dye from aqueous solution, which improves the low success rate of aerobic biological removal of dye colour. |
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This study explored the application of the solar photoFenton process to the degradation of PNA in water. The operating pH value was varied in the range of 3–6. The effect, of H2O2 and Fe2+ dosage on the degradation of PNA by solar photo-Fenton process were investigated between 2.5–40 and 0.025–0.1 mM, respectively. Also the effect, of temperature and initial pollutant concentration were investigated in the range of 20–50°C and |
P-Nitroaniline (PNA) | The optimum conditions for the degradation of PNA in water were considered to be pH 3.0, 10 mmol/L H2O2, 0.05 mmol/L Fe2+, 0.072–0.217 mmol/L PNA, and temperature 20°C. Under optimum conditions, the degradation efficiencies of PNA were more than 98% within a 30 min reaction time. The degradation characteristic of PNA showed that the conjugated systems of the aromatic ring in the PNA molecules were effectively destroyed. The experimental results indicated that the solar photo-Fenton process has advantages over the classic Fenton process, such as higher oxidation power, a wider working pH range, and a lower ferrous ion usage. |
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During the experiment, H2O2 was added continuously to the reactor at a flow rate of 1 mL/min with a syringe pump. Two 8 W monochromatic UV lamps of 312 nm (with an emission range between 280 and 360 nm) were placed axially in the reactor and kept in place with a quartz sleeve. The UV intensity of one 8 W UV lamp is 60 |
Carbofuran | Under these conditions, the toxicity unit measured by Microtox test with 5 min exposure was decreased from 47 to 6 and the biodegradability evaluated by BOD 5/COD ratio was increased from 0 to 0.76 after a 60 min reaction. The results obtained in this study demonstrate that the photo-Fenton process is a promising pretreatment to biological treatment for carbofuran removal from contaminated water or wastewater. |
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Experiments were carried out in a Pyrex glass cylindrical reactor of 0.10 m diameter and 0.20 m height. The working volume was 1 L and all experiments were conducted in batch mode. The initial solution pH was adjusted to 3 which is the optimal value for the Fenton and photo-Fenton reactions using sulphuric acid. All experiments except those in the dark and at night were carried out between 10 am and 4 pm. The mean solar radiation during the experiments from October to January was in the range of 2.55–3.01 kWh (m2 day)−1. The effects of solar light, initial Fe concentration, and initial H2O2 concentration were investigated. | Acid Orange 7 | With increasing Fe dosage the decolourization rate increased, but the enhancement was not pronounced beyond 10 mg/L. Although the addition of H2O2 increased the decolourization rate up to around 1000 mg/L of H2O2, further additions of H2O2 did not enhance colour removal. At excess dosages of Fenton reagents, colour removal was not improved, due to their scavenging of hydroxyl radicals. It was found that the pseudo first-order decolourization kinetic constant based on the accumulated solar energy is the sole parameter unifying solar photo-Fenton decolourization processes under different weather conditions. |
Like other AOPs, the oxidizing ability of UV/H2O2 may be attributed to the formation of •OH, Initiation (Rate Constant)
Propagation [ Termination [
It is important to note that the effectiveness of UV/H2O2 systems depends on various conditions that affect their ability to degrade chelating agents. The variables include the type and concentration of contaminants or dissolved inorganic substances (such as carbonates and iron cations), organic substances present in surface water, light transmittance in solutions (as indicated by turbidity or colour), pH, temperature, and the optimum oxidant dose [
Tubular reactor configurations are usually employed for direct photolysis and photo-Fenton processes or processes based on H2O2/UV reagent, in order to achieve a good interaction between CPs, other intermediates, and radiation [
Typical findings observed in work related to the use of UV/H2O2.
Reference | Process conditions | Pollutant(s) | Conclusions |
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For photolytic experiments, the samples were irradiated with a UV lamp with an output of 254 nm operating at 50–60 Hz with a current intensity of 0.12 A at ambient temperature. The photolytic decolouration of carmine via UV radiation in the presence of H2O2 was optimized using response surface methodology (RSM) utilizing Design-Expert 7.1. | Carmine |
Under the optimized conditions of 62 |
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UV/TiO2/H2O2, UV/TiO2, and UV/H2O2 were compared as pretreatment processes to detoxification and treatment. The tubes were then irradiated for 40 h (initial concentrations of 50 mg/L) or 56 h (initial concentrations of 100 mg/L) at 300 |
4-Chlorophenol (4CP), |
Chlorophenol photodegradation was well described by a first-order model kinetic ( |
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A 60 W mercury vapour lamp (UV C, 253.7 nm) with a frequency of 50 Hz and a voltage of 240 V was used. The initial concentrations of H2O2 and melanoidin were manipulated while pH, flow rate, irradiated surface area, volume, lamp intensity, and temperature were kept constant. The relative change of each constituent was identified at various initial concentrations of H2O2 (up to 12000 mg/L) and melanoidin (263–5314 mg-Pt Co/L). | Melanoidin | UV/H2O2 was shown to remove the colour associated with melanoidin effectively. The process was less effective in removing the DON and DOC present in the melanoidin solution. At the optimum H2O2 dose (3300 mg/L), with an initial melanoidin concentration of 2000 mg/L, the removal of colour, DOC and DON was 99%, 50%, and 25%, respectively. |
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This study compared the efficacy of UV photodegradation with that of different advanced oxidation processes (O3, UV/H2O2, O3/activated carbon). Photo-irradiations were carried out using a merry-go-round photoreactor (MGRR), DEMA equipped with a 500 WTQ 718 Heraeus medium-pressure mercury lamp (239–334 nm) or a TNN 15/32 Heraeus low-pressure mercury lamp (254 nm). The temperature in the MGRR was kept at |
Naphthalene sulphonic acids | These results demonstrated that the treatment of naphthalene sulphonic acids with UV radiation is not effective in their removal from aqueous solutions. The presence of duroquinone and 4-carboxybenzophenone during the irradiation of naphthalene sulphonic acids increases their elimination rate. O3/activated carbon and UV/H2O2 based systems were found to be more efficient than the irradiation process in the removal of naphthalene sulphonic acids from aqueous solutions. |
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The reactor had a 1 L capacity and was equipped with a mercury medium-pressure steam UV lamp which was 110 mm in length and used 1000 W, 145 V, and 7.5 A. In the UV light/H2O2 flow reactor system, the initial concentration of sulphide was 6.34 mg L−1. The initial concentrations of sulphurous water were 6.34 mg L−1 of HS−, 1000 mg L−1 of |
Sulphurous water | In a batch reactor it was possible to demonstrate that the sulphur compounds of the sulphurous waters could be oxidized to sulphate in a UV light/H2O2 air system with very small concentrations of hydrogen peroxide ( |
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Radiation energy was supplied by two lamps. Two different types of lamp were used: (1) two Philips TUV lamps with an input power of 15 W each and (2) two Heraeus UV-C lamps operated with an input power of 40 W each. Both types of lamp are low pressure mercury vapour lamps with one single significant emission wavelength at 253.7 nm. DCA concentration and radiation absorbing species concentration (H2O2) were 60 ppm, 145 ppm and pH and temperature were kept at 3.4 and 20°C, respectively. | Dichloroacetic acid (DCA) | The fastest degradation rate was obtained with the H2O2/UV40W system, followed by H2O2/UV15W. Although the photocatalytic process was effective in degrading DCA, the reaction rate was much slower when compared with the homogeneous processes. For the H2O2/UV40W reaction, the DCA conversion at |
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Low pressure mercury vapour lamps with a maximum emission primarily at 253.7 nm were used as the light source. The changes in the pH of dye solutions as a function of the irradiation time for different initial pH values are carried out. The effect of the initial H2O2 concentration in a range of 10–100 mM on the rate of RO16 decolourization was investigated. The effect of the initial RO16 concentration in a range from 20 to 80 mg dm−3 on the efficiency of dye degradation was also investigated. The influence of UV light intensity on the decolourization of RO16 azo dye was monitored by varying the light intensity from 730 up to 1950 |
Azo dye Reactive Orange 16 | The UV/H2O2 process could be used efficiently for the decolourization of aqueous solutions of the azo dye Reactive Orange 16. It was found that the rate of decolourization is significantly affected by the initial pH, the initial hydrogen peroxide concentration, the initial dye concentration, and the UV light intensity. The decolourization follows pseudo first-order reaction kinetics. Peroxide concentrations in the range from 20 to 40 mM appear to be optimal. Colour removal was observed to be faster in neutral pH solutions than in acidic and basic ones. The hydroxyl radical scavenging effect of the examined inorganic anions increased in the order phosphate < sulphate < nitrate < chloride. |
Ultimate oxidation of CPs to carbon dioxide and water has rarely been obtained under typical test conditions. As summarized in Table
Recent developments in various homogeneous AOPs have been analysed comprehensively. The principle of individual and combined AOPs and their efficiency on the degradation of various pollutants was discussed. The influence of various experimental parameters such as oxidant dosage, solution pH, flow rates, substrate concentrations, water matrix, and light intensity on the AOPs was explored. This review also listed various AOPs applied for the degradation of contaminants under different experimental conditions. Combined AOPs substantially enhanced the degradation rate by generating more reactive radicals under suitable conditions. The optimum oxidant dosage and solution for efficient removal were reported.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors wish to thank the National Science Council (NSC) in Taiwan for their financial support under the Contract no. NSC-101-2221-035-031-MY3. The Laboratory of Green Chemistry, Mikkeli, Finland, and Water and Environmental Technology (WET) Center, Temple University, are also gratefully acknowledged for their partial financial support of this study.