photoelectro-Fenton Treatment Research Articles

Solar photoelectro-Fenton degradation of the herbicide 4-chloro-2-methylphenoxyacetic acid optimized by response surface methodology

A central composite rotatable design and response surface methodology (RSM) were used to optimize the experimental variables of the solar photoelectro-Fenton (SPEF) treatment of the herbicide 4-chloro-2-methylphenoxyacetic acid (MCPA). The experiments were made with a flow plant containing a Pt/air-diffusion reactor coupled to a solar compound parabolic collector (CPC) under recirculation of 10 L of 186 mg L −1 MCPA solutions in 0.05 M Na 2SO 4 at a liquid flow rate of 180 L h −1 with an average UV irradiation intensity of about 32 W m −2. The optimum variables found for the SPEF process were 5.0 A, 1.0 mM Fe 2+ and pH 3.0 after 120 min of electrolysis. Under these conditions, 75% of mineralization with 71% of current efficiency and 87.7 kWh kg −1 TOC of energy consumption were obtained. MCPA decayed under the attack of generated hydroxyl radicals following a pseudo-first-order kinetics. Hydroxyl radicals also destroyed 4-chloro-2-methylphenol, methylhydroquinone and methyl- p-benzoquinone detected as aromatic by-products. Glycolic, maleic, fumaric, malic, succinic, tartronic, oxalic and formic acids were identified as generated carboxylic acids, which form Fe(III) complexes that are quickly photodecarboxylated by the UV irradiation of sunlight at the CPC photoreactor. A reaction sequence for the SPEF degradation of MCPA was proposed.

Mineralization of Metoprolol by Electro-Fenton and Photoelectro-Fenton Processes

Solutions of about 0.25 mM of the β-blocker metoprolol tartrate (100 mg L(-1) total organic carbon) with 0.5 mM Fe(2+) in the presence and absence of 0.1 mM Cu(2+) of pH 3.0 have been comparatively degraded under electro-Fenton (EF) and photoelectro-Fenton (PEF) conditions. The electrolyses were carried out with two systems: (i) a single cell with a boron-doped diamond (BDD) anode and an air-diffusion cathode (ADE) for H(2)O(2) electrogeneration and (ii) a combined cell with a BDD/ADE pair coupled with a Pt/carbon felt (CF) cell. Overall mineralization was reached in all PEF treatments using both systems due to the efficient production of hydroxyl radical ((•)OH) from Fenton's reaction induced by UVA light and the quick photolysis of Fe(III) carboxylate complexes formed. In EF, the combined cell was much more potent than the single one by the larger (•)OH generation from the continuous Fe(2+) regeneration at the CF cathode, accelerating the oxidation of organics. However, almost total mineralization in EF was feasible using the combined cell in the presence of 0.1 mM Cu(2+), because of the parallel quick oxidation of Cu(II) carboxylate complexes by (•)OH. Metoprolol decay always followed a pseudo-first-order reaction. Aromatic products related to consecutive hydroxylation/oxidation reactions of metoprolol were detected by gas chromatography-mass spectrometry. The evolution of the aromatic 4-(2-methoxyethyl)phenol and generated carboxylic acids was followed by HPLC. The degradation rate and mineralization degree of metoprolol tartrate were limited by the removal of Fe(III) and Cu(II) complexes of ultimate carboxylic acids such as formic, oxalic, and oxamic. NH(4)(+) ion and to a lesser extent NO(3)(-) ion were released in all treatments, being quantified by ionic chromatography.

Mineralization of antibiotic sulfamethoxazole by photoelectro-Fenton treatment using activated carbon fiber cathode and under UVA irradiation

The mineralization of antibiotic sulfamethoxazole (SMX) of concentrations up to 300mgL−1 was examined by photoelectro-Fenton (PEF) using an activated carbon fiber (ACF) cathode with UVA (365nm) irradiation. Comparative mineralization has been studied by different methods: RuO2/Ti anodic oxidation (AO), AO in the presence of electrogenerated H2O2 (AO-H2O2), AO-H2O2 in the presence of UVA (AO-H2O2-UVA), and both the electro-Fenton (EF) and PEF processes. PEF treatment at a low applied current of 0.36A yields a faster and more complete depollution with 80% of the TOC removed after 6h of electrolysis. The higher oxidative ability of the PEF process can be attributed to the additional hydroxyl radicals (OH) produced by the photo-Fenton reaction. The 63% mineralization in the case of EF treatment was due to the formation of short intermediates, such as carboxylic acids, which were difficult to oxidise with OH. In the AO-H2O2-UVA process, about 36% of the TOC was removed after 6h electrolysis, while 28% of the TOC was removed in the AO-H2O2 process. SMX is only slightly mineralized by the AO process, with only 25% of the TOC removed. HPLC–MS analysis allowed for up to six aromatic reaction products to be identified during the SMX degradation in the PEF process, mainly formed from the hydroxylation of the aromatic ring or/and isoxazole ring, accompanied by the substitution of the amine group (on aromatic cycle) or methyl group (on isoxazole ring) by OH. The carboxylic acids generated, including oxalic, maleic, oxamic, formic and acetic acids, were detected by ion-exclusion chromatography. The initial organic nitrogen was mainly converted into NH4+ along with a very small proportion of NO3− ion. Considering all the oxidation intermediates and end products for SMX degradation in the PEF process, a general mineralization mechanism by OH and UVA was proposed.

Electro-Fenton and photoelectro-Fenton degradations of the drug beta-blocker propranolol using a Pt anode: Identification and evolution of oxidation products

The beta-blocker propranolol hydrochloride has been degraded by electrochemical advanced oxidation processes like electro-Fenton (EF) and photoelectro-Fenton (PEF) using a single cell with a Pt anode and an air diffusion cathode (ADE) for H 2O 2 electrogeneration and a combined system containing the above Pt/ADE pair coupled in parallel to a Pt/carbon-felt (CF) cell. Organics are mainly oxidized with hydroxyl radical ( OH) formed from Fenton's reaction between added Fe 2+ and electrogenerated H 2O 2. The PEF treatment in Pt/ADE–Pt/CF system yields almost total mineralization because OH production is enhanced by Fe 2+ regeneration from Fe 3+ reduction at the CF cathode and Fe(III) complexes with generated carboxylic acids are rapidly photodecarboxylated under UVA irradiation. Lower mineralization degree is found for PEF in Pt/ADE cell due to the little influence of UVA light on Fe 2+ regeneration. The homologous EF processes are much less potent as a result of the persistence of Fe(III)–carboxylate complexes. Aromatic intermediates such as 1-naphthol, 1,4-naphthoquinone and phthalic acid and generated carboxylic acids such as pyruvic, glycolic, malonic, maleic, oxamic, oxalic and formic are identified. While chloride ion remains stable, NH 4 + and NO 3 − ions are released to the medium. A reaction sequence for propranolol hydrochloride mineralization is proposed.

Degradation of the beta-blocker propranolol by electrochemical advanced oxidation processes based on Fenton's reaction chemistry using a boron-doped diamond anode

The electro-Fenton (EF) and photoelectro-Fenton (PEF) degradation of solutions of the beta-blocker propranolol hydrochloride with 0.5 mmol dm −3 Fe 2+ at pH 3.0 has been studied using a single cell with a boron-doped diamond (BDD) anode and an air diffusion cathode (ADE) for H 2O 2 electrogeneration and a combined cell containing the above BDD/ADE pair coupled in parallel to a Pt/carbon felt (CF) cell. This naphthalene derivative can be mineralized by both methods with a BDD anode. Almost overall mineralization is attained for the PEF treatments, more rapidly with the combined system due to the generation of higher amounts of hydroxyl radical from Fenton's reaction by the continuous Fe 2+ regeneration at the CF cathode, accelerating the oxidation of organics to Fe(III)–carboxylate complexes that are more quickly photolyzed by UVA light. The homologous EF processes are less potent giving partial mineralization. The effect of current density, pH and Fe 2+ and drug concentrations on the oxidation power of PEF process in combined cell is examined. Propranolol decay follows a pseudo first-order reaction in most cases. Aromatic intermediates such as 1-naphthol and phthalic acid and generated carboxylic acids such as maleic, formic, oxalic and oxamic are detected and quantified by high-performance liquid chromatography. The chloride ions present in the starting solution are slowly oxidized at the BDD anode. In PEF treatments, all initial N of propranolol is completely transformed into inorganic ions, with predominance of NH 4 + over NO 3 − ion.

Mineralization of salicylic acid in acidic aqueous medium by electrochemical advanced oxidation processes using platinum and boron-doped diamond as anode and cathodically generated hydrogen peroxide

Solutions containing 164 mg L −1 salicylic acid of pH 3.0 have been degraded by electrochemical advanced oxidation processes such as anodic oxidation, anodic oxidation with electrogenerated H 2O 2, electro-Fenton, photoelectro-Fenton and solar photoelectro-Fenton at constant current density. Their oxidation power has been comparatively studied in a one-compartment cell with a Pt or boron-doped diamond (BDD) anode and a graphite or O 2-diffusion cathode. In the three latter procedures, 0.5 mM Fe 2+ is added to the solution to form hydroxyl radical ( OH) from Fenton's reaction between Fe 2+ and H 2O 2 generated at the O 2-diffusion cathode. Total mineralization is attained for all methods with BDD and for photoelectro-Fenton and solar photoelectro-Fenton with Pt. The poor decontamination achieved in anodic oxidation and electro-Fenton with Pt is explained by the slow removal of most pollutants by OH formed from water oxidation at the Pt anode in comparison to their quick destruction with OH produced at BDD. OH generated from Fenton's reaction oxidizes rapidly all aromatic pollutants, but it cannot destroy final Fe(III)–oxalate complexes. Solar photoelectro-Fenton treatments always yield quicker degradation rate due to the very fast photodecarboxylation of these complexes by UVA irradiation supplied by solar light. The effect of current density on the degradation rate, efficiency and energy cost of all methods is examined. The salicylic acid decay always follows a pseudo-first-order kinetics. 2,3-Dihydroxybenzoic, 2,5-dihydroxybenzoic, 2,6-dihydroxybenzoic, α-ketoglutaric, glycolic, glyoxylic, maleic, fumaric, malic, tartronic and oxalic acids are detected as oxidation products. A general reaction sequence for salicylic acid mineralization considering all these intermediates is proposed.

Catalytic effect of Fe2+, Cu2+and UVA light on the electrochemical degradation of nitrobenzene using an oxygen-diffusion cathode

The electrochemical treatment of 100 mg L−1 nitrobenzene solutions in 0.05 M Na2SO4 in the pH range 2.0–4.0 at 25 °C has been comparatively studied in the presence of Fe2+, Cu2+ and/or UVA light as catalysts using an undivided cell with a Pt anode and an O2-diffusion cathode able to electrogenerate H2O2. A quite slow degradation is found for the solution with 1 mM Cu2+ due to the low production of oxidizing hydroxyl radical (OH˙) from anodic oxidation of water and the action of the Cu2+/Cu+ system. Electro-Fenton treatment with 1 mM Fe2+ leads to a high concentration of OH˙ in the medium from Fenton's reaction, but less than 70% of the nitrobenzene is mineralized since stable complexes of Fe3+ with products are formed. These complexes are quickly photodecomposed in the photoelectro-Fenton treatment of the same solution under UVA irradiation, leading to overall depollution at low currents. Complete degradation is also feasible using 1 mM Cu2+ and 1 mM Fe2+ at high current because OH˙ can slowly destroy the complexes between Cu2+ and intermediates. The positive synergetic effect of all catalysts allows the quickest nitrobenzene mineralization using 1 mM Cu2+ and 1 mM Fe2+ under UVA irradiation. The nitrobenzene decay always follows a pseudo-first-order reaction. Aromatic products such as o-nitrophenol, m-nitrophenol, p-nitrophenol and 4-nitrocatechol have been followed by reverse-phase chromatography. Nitrate ions are formed from oxidation of nitroaromatic products. Generated carboxylic acids such as maleic, fumaric and oxalic have been detected by ion-exclusion chromatography. The different evolution of complexes of oxalic with Cu2+ and Fe3+ explains the influence of the catalysts on nitrobenzene degradation.

Anodic oxidation, electro-Fenton and photoelectro-Fenton treatments of 2,4,5-trichlorophenoxyacetic acid

The mineralization of acidic aqueous solutions of the herbicide 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) in 0.05 M Na 2SO 4+H 2SO 4 at 35 °C has been studied by different electrochemical methods. Photoelectro-Fenton treatment yields a fast and complete depollution of solutions with 2,4,5-T concentrations up to about saturation within the pH range 2.0–4.0, operating at low current and with 1 mM Fe 2+. This method is carried out in an undivided cell with a Pt anode and an O 2-diffusion cathode under UV irradiation, and its great oxidative ability is due to the fast reaction of organics with the high concentration of oxidizing hydroxyl radicals produced in the medium from Fenton's reaction between added Fe 2+ and H 2O 2 electrogenerated by the cathode. Comparative treatment by an electro-Fenton reaction only leads to 60–65% of mineralization because of the formation of hardly oxidable products with hydroxyl radical. 2,4,5-T is very poorly mineralized by anodic oxidation in the absence and presence of electrogenerated H 2O 2. The herbicide decay always follows a pseudo first-order reaction. 2,4,5-Trichlorophenol, 2,5-dichlorohydroquinone, 4,6-dichlororesorcinol and 2,5-dihydroxy- p-benzoquinone have been identified and quantified as aromatic products by reverse-phase chromatography. Generated carboxylic acids such as glycolic, glyoxylic, formic, malic, maleic, fumaric and oxalic have been detected and followed by ion-exclusion chromatography. All these products are destroyed by electro-Fenton treatment, except oxalic acid which forms stable complexes with Fe 3+. These complexes are rapidly photodecarboxylated by UV light in the photoelectro-Fenton treatment.