Ferrate(VI) oxidation of zinc-cyanide complex

Zinc-cyanide complexes are found in gold mining effluents and in metal finishing rinse water. The effect of Zn(II) on the oxidation of cyanide by ferrate(VI) (Fe(VI)O(4)(2-), Fe(VI)) was thus investigated by studying the kinetics of the reaction of Fe(VI) with cyanide present in a potassium salt of a zinc cyanide complex (K(2)Zn(CN)(4)) and in a mixture of Zn(II) and cyanide solutions as a function of pH (9.0-11.0). The rate-law for the oxidation of Zn(CN)(4)(2-) by Fe(VI) was found to be -d[Fe(VI)]/dt=k[Fe(VI)][Zn(CN)(4)(2-)](0.5). The rate constant, k, decreased with an increase in pH. The effect of temperature (15-45 degrees C) on the oxidation was studied at pH 9.0, which gave an activation energy of 45.7+/-1.5kJmol(-1). The cyanide oxidation rate decreased in the presence of the Zn(II) ions. However, Zn(II) ions had no effect on the cyanide removal efficiency by Fe(VI) and the stoichiometry of Fe(VI) to cyanide was approximately 1:1; similar to the stoichiometry in absence of Zn(II) ions. The destruction of cyanide by Fe(VI) resulted in cyanate. The experiments on removal of cyanide from rinse water using Fe(VI) demonstrated complete conversion of cyanide to cyanate.     

Light-induced disappearance of nitrite in the presence of iron (III)

Understanding of rapid disappearance of nitrite in natural waters and its impact on nitrogen natural cycling has remained limited. We found that NO2- disappeared rapidly in pH 3.2 aqueous Fe(III) solutions both in sunlight and in 356 nm light. Quantum yields of the NO2- loss at 356 nm were 0.049-0.14 for initial levels of 10-80 microns NO2- and 200 microns Fe(III). The NO2- loss (at 356 nm) followed apparent first-order kinetics. The rate constants were 1.3 x 10(-3) (40 microns NO2-) and 4.1 x 10(-4) s-1 (80 microns NO2-) for 100 microns Fe(III), and 2.3 x 10(-3) (40 microns NO2-) and 7.5 x 10(-4) s-1 (80 microns NO2(-1)) for 200 microns Fe(III) (t1/2 = 8.7, 27.9, 5.1, and 15.3 min, respectively). The rate constants were directly proportional to [Fe(III)]0 and inversely proportional to [NO2-]0. Agreement between the rate constants obtained experimentally and those calculated mechanistically supports the hypothesis that NO2- was oxidized to NO2 by .OH radicals from photolysis of FeOH2+ complexes, and at high [NO2-]0 (e.g., 80 microns) relative to [Fe(III)]0, hydrolysis of NO2 or N2O4 to form NO3- and NO2- could be significant. This study showed that light and Fe(III)-induced oxidation of NO2- (rate = approximately 10(-1)-10(-2) microns s-1) was more rapid than its direct photolysis (rate = approximately 10(-4) microns s-1), and the photolysis could be a significant source of .OH radicals only in cases where the Fe(III) level is much lower than the NO2- level ([Fe(III)]/[NO2-] < 1/80). This study suggests that the light and Fe(III)-induced oxidation of NO2- would be one potential important pathway responsible for the rapid transformation of NO2- in acidic surface waters, especially those affected by acid-mine drainage or volcanic activities. This study also may be of interest for modeling certain acidic atmospheric water environments.     

Kinetics of oxidation of chlorobenzenes and phenyl-ureas by Fe(II)/H2O2 and Fe(III)/H2O2. Evidence of reduction and oxidation reactions of intermediates by Fe(II) or Fe(III)

The rates of degradation of 1,2,4-trichlorobenzene (TCB), 2,5-dichloronitrobenzene (DCNB), diuron and isoproturon by Fe(II)/H2O2 and Fe(III)/H2O2 have been investigated in dilute aqueous solution ([Organic compound]0 approximately 1 microM, at 25.0 +/- 0.2 degrees C and pH < or = 3). Using the relative rate method with atrazine as the reference compound, and the Fe(II)/H2O2 (with an excess of Fe(II)) and Fe(III)/H2O2 systems as sources of OH radicals, the rate constants for the reaction of OH* with TCB and DCNB were determined as (6.0 +/- 0.3)10(9) and (1.1 +/- 0.2)10(9) M(-1) s(-1). Relative rates of degradation of diuron and isoproturon by Fe(II)/H2O2 were about two times smaller in the absence of dissolved oxygen than in the presence of oxygen. These data indicate that radical intermediates are reduced back to the parent compound by Fe(II) in the absence of oxygen. Oxidation experiments with Fe(III)/H2O2 showed that the rate of decomposition of atrazine markedly increased in the presence of TCB and this increase has been attributed to a regeneration of Fe(II) by oxidation reactions of intermediates (radical species and dihydroxybenzenes) by Fe(III).     

Comparison of hematite/Fe(II) systems with cement/Fe(II) systems in reductively dechlorinating trichloroethylene

Reactive reductants of cement/Fe(II) systems in dechlorinating chlorinated hydrocarbons are unknown. This study initially evaluated reactivities of potential reactive agents of cement/Fe(II) systems such as hematite (alpha-Fe(2)O(3)), goethite (alpha-FeOOH), lepidocrocite (gamma-FeOOH), akaganeite (beta-FeOOH), ettringite (Ca(6)Al(2)(SO(4))(3)(OH)(12)), Friedel's salt (Ca(4)Al(2)Cl(2)(OH)(12)), and hydrocalumite (Ca(2)Al(OH)(6)(OH).3H(2)O) in reductively dechlorinating trichloroethylene (TCE) in the presence of Fe(II). It was found that a hematite/Fe(II) system shows TCE degradation characteristics similar to those of cement/Fe(II) systems in terms of degradation kinetics, Fe(II) dose dependence, and final products distribution. It was therefore suspected that Fe(III)-containing phases of cement hydrates in cement/Fe(II) systems behaved similarly to the hematite. CaO, which was initially introduced as a pH buffer, was observed to participate in or catalyze the formation of reactive reductants in the hematite/Fe(II) system, because its addition enhanced the reactivities of hematite/Fe(II) systems. From the SEM (scanning electron microscope) and XRD (X-ray diffraction) analyses that were carried out on the solids from hematite/Fe(II) suspensions, it was discovered that a sulfate green rust with a hexagonal-plate structure was probably a reactive reductant for TCE. However, SEM analyses conducted on a cement/Fe(II) system showed that hexagonal-plate crystals, which were presumed to be sulfate green rusts, were much less abundant in the cement/Fe(II) than in the hematite/Fe(II) systems. It was not possible to identify any crystalline minerals in the cement/Fe(II) system by using XRD analysis, probably because of the complexity of the cement hydrates. These observations suggest that major reactive reductants of cement/Fe(II) systems may differ from those of hematite/Fe(II) systems.     

Ferrate(VI): green chemistry oxidant for degradation of cationic surfactant

Iron in its familiar form exists in the +2 and +3 oxidation states, however, higher oxidation state of iron +6, ferrate(VI) (Fe(VI)O(4)(2-)) can be obtained. The high oxidation power of ferrate(VI) can be utilized in developing cleaner ("greener") technology for remediation processes. This paper demonstrates the unique property of ferrate(VI) to degrade almost completely the cationic surfactant, cetylpyridinium chloride (C(5)H(5)N(+)(CH(2))(15)CH(3).H(2)O Cl(-), CPC). The Rate law for the oxidation of CPC by ferrate(VI) at pH 9.2 was found to be: -d[Fe(VI)]/dt = k[Fe(VI)][CPC](2). Ferrate(VI) oxidizes CPC within minutes and molar consumption of ferrate(VI) was nearly equal to the oxidized CPC. The decrease in total organic carbon (TOC) from CPC was more than 95%; suggesting mineralization of CPC to carbon dioxide. Ammonium ion was the other product of the oxidation. This is the first report in which Fe(VI)O(4)(2-) ion opens the pyridine ring and mineralizes the aliphatic chain of the organic molecule giving inorganic ions.     

Degradation of the herbicide 2,4-DP by anodic oxidation, electro-Fenton and photoelectro-Fenton using platinum and boron-doped diamond anodes

This paper reports the degradation of 2,4-DP (2-(2,4-dichlorophenoxy)-propionic acid) solutions of pH 3.0 by environmentally friendly electrochemical methods such as anodic oxidation, electro-Fenton and photoelectro-Fenton with a Pt or boron-doped diamond (BDD) anode. In the two latter techniques an O(2)-diffusion cathode was used and 1.0mM Fe(2+) was added to the solution to give hydroxyl radical (*OH) from Fenton's reaction between Fe(2+) and H(2)O(2) generated at the cathode. All treatments with BDD are viable to decontaminate acidic wastewaters containing 2,4-DP since they give complete mineralization, with loss of chloride ion, at high current due to the great production of oxidant *OH at the BDD surface favoring the destruction of final carboxylic acids. *OH formed from Fenton's reaction destroys more rapidly aromatic products, making the electro-Fenton and photoelectro-Fenton processes much more efficient than anodic oxidation. UVA light in photoelectro-Fenton with BDD has little effect on the degradation rate of pollutants. The comparative procedures with Pt lead to slower decontamination because of the lower oxidizing power of this anode. The effect of current on the degradation rate and efficiency of all methods is studied. The 2,4-DP decay always follows a pseudo-first-order kinetics. Chlorohydroquinone, chloro-p-benzoquinone and maleic, fumaric, malic, lactic, pyruvic, acetic, formic and oxalic acids are detected as products by chromatographic techniques. A general sequence accounting for by the reaction of all these intermediates with the different oxidizing agents is proposed.     

Modeling the reaction kinetics of Fenton's process on the removal of atrazine

The degradation of pesticide, atrazine (ATZ), 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine, by Fenton's reagent (FR) was investigated as a function of reagents' concentrations and ratios in a batch reactor. The degradation of ATZ was effectively achieved by hydroxyl radicals, which were generated in the FR process. The decay rates of ATZ and the oxidation capacities of FR were found to depend on the concentrations of hydrogen peroxide and ferrous ion. The removal kinetics of ATZ are initiated by a rapid decay and then followed by a much slower one. After an extended reaction time (5-10 min), the reactions ceased because the Fe(II) and H(2)O(2) were consumed and would be deactivated in the process. A mathematical model was successfully developed to describe the two-stage reaction kinetics by using two simple but critical parameters: the initial ATZ decay rate and the final oxidation capacity of Fenton's process. In general, higher [Fe(II)] or H(2)O(2) concentrations result in faster initial decay rate and higher oxidation capacity. However, the oxidation capacity is more sensitive to the initial [Fe(II)] due to the presence of side reactions as discussed in the paper.     

Transformation characteristics of refractory pollutants in plugboard wastewater by an optimal electrocoagulation and electro-Fenton process

The treatment of the plugboard wastewater was performed by an optimal electrocoagulation and electro-Fenton. The organic components with suspended fractions accounting for 30% COD were preferably removed via electrocoagulation at initial 5 min. In contrast, the removal efficiency was increased to 76% with the addition of H(2)O(2). The electrogenerated Fe(2+) reacts with H(2)O(2) and leads to the generation of (·)OH, which is responsible for the higher COD removal. However, overdosage H(2)O(2) will consume (·)OH generated in the electro-Fenton process and lead to the low COD removal. The COD removal efficiency decreased with the increased pH. The concentration of Fe(2+) ions was dependent on the solution pH, H(2)O(2) dosage and current density. The changes of organic characteristics in coagulation and oxidation process were differenced and evaluated using gel permeation chromatography, fluorescence excitation-emission scans and Fourier transform infrared spectroscopy. The fraction of the wastewater with aromatic structure and large molecular weight was decomposed into aliphatic structure and small molecular weight fraction in the electro-Fenton process.     

A comparative study of the effects of chloride, sulfate and nitrate ions on the rates of decomposition of H2O2 and organic compounds by Fe(II)/H2O2 and Fe(III)/H2O2

The effects of chloride, nitrate, perchlorate and sulfate ions on the rates of the decomposition of hydrogen peroxide and the oxidation of organic compounds by the Fenton's process have been investigated. Experiments were conducted in a batch reactor, in the dark at pH < or = 3.0 and at 25 degrees C. Data obtained from Fe(II)/H2O2 experiments with [Fe(II)]0/[H2O2]0 > or = 2 mol mol(-1), showed that the rates of reaction between Fe(II) and H2O2 followed the order SO4(2-) > ClO4(-) = NO3- = Cl-. For the Fe(III)/H2O2 process, identical rates were obtained in the presence of nitrate and perchlorate, whereas the presence of sulfate or chloride markedly decreased the rates of decomposition of H2O2 by Fe(III) and the rates of oxidation of atrazine ([atrazine]0 = 0.83 microM), 4-nitrophenol ([4-NP]0 = 1 mM) and acetic acid ([acetic acid]0 = 2 mM). These inhibitory effects have been attributed to a decrease of the rate of generation of hydroxyl radicals resulting from the formation of Fe(III) complexes and the formation of less reactive (SO4(*-)) or much less reactive (Cl2(*-)) inorganic radicals.     

Influence of complex reagents on removal of chromium(VI) by zero-valent iron

The removal of Cr(VI) by zero-valent iron (Fe(0)) and the effect of three complex reagents, ethylenediaminetetraacetic acid (EDTA), NaF and 1,10-phenanthroline, on this reaction were investigated using batch reactors at pH values of 4, 5 and 6. The results indicate that the removal of Cr(VI) by Fe(0) is slow at pH 5.0 and that three complex reagents play different roles in the reaction. EDTA and NaF significantly enhance the reaction rate. The zero-order rate constants at pH 5.0 were 5.44 microM min(-1) in the presence of 4mM EDTA and 0.99 micrM min(-1) in the presence of 8 mM NaF, respectively, whereas that of control was only 0.33 micrM min(-1), even at pH=4.0. This enhancement is attributed to the formation of complex compounds between EDTA/NaF and reaction products, such as Cr(III) and Fe(III), which eliminate the precipitates of Cr(III), Fe(III) hydroxides and Cr(x)Fe(1-)(x)(OH)(3) and thus reduce surface passivation of Fe(0). In contrast, 1,10-phenanthroline, a complex reagent for Fe(II), dramatically decreases Cr(VI) reduction by Fe(0). At pH=4.0, the zero-order rate constant in the presence of 1mM of 1,10-phenanthroline was 0.02 micrM min(-1), decreasing by 99.7% and 93.9%, respectively, compared with the results in the presence and absence of EDTA. The results suggest that a pathway of the reduction of Cr(VI) to Cr(III) by Fe(0) may involve dissolution of Fe(0) to produce Fe(II), followed by reduction of Cr(VI) by Fe(II), rather than the direct reaction between Cr(VI) and Fe(0), in which Fe(0) transfers electrons to Cr(VI).     

Electrochemical degradation of chlorophenoxy and chlorobenzoic herbicides in acidic aqueous medium by the peroxi-coagulation method

The degradation of 4-chlorophenoxyacetic acid (4-CPA), 4-chloro-2-methylphenoxyacetic acid (MCPA), 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) as chlorophenoxy herbicides, as well as of 3,6-dichloro-2-methoxybenzoic acid (dicamba) as chlorobenzoic herbicide, has been studied by peroxi-coagulation. This electrochemical method yields a very effective depollution of all compounds in acidic aqueous medium of pH 3.0 working under pH regulation, since they are oxidized with hydroxyl radicals produced from Fenton's reaction between Fe(2+) and H(2)O(2) generated by the corresponding Fe anode and O(2)-diffusion cathode. Their products can then be removed by mineralization or coagulation with the Fe(OH)(3) precipitate formed. Both degradative paths compete at low currents, but coagulation predominates at high currents. The peroxi-coagulation process of dicamba at I>or=300 mA leads to more than 90% of coagulation, being much more efficient than its comparative electro-Fenton treatment with a Pt anode and 1 mM Fe(2+), where only mineralization takes place. For the chlorophenoxy compounds, electro-Fenton gives a slightly lower depollution than peroxi-coagulation, because more easily oxidable products are produced. Oxidation of chlorinated products during peroxi-coagulation is accompanied by the release of chloride ion to the solution. The efficiency of this method decreases with increasing electrolysis time and current. The decay of all herbicides follows a pseudo-first-order reaction, with a similar constant rate for 4-CPA, MCPA, 2,4-D and 2,4,5-T, and a higher value for dicamba.     

Oxidative transformations of environmental pharmaceuticals by Cl₂, ClO₂, O₃, and Fe(VI): kinetics assessment

Several pharmaceuticals have been detected globally in surface water and drinking water, which indicate their insufficient removal from water and wastewater using conventional treatment methods. This paper reviews the kinetics of oxidative transformations of pharmaceuticals (antibiotics, lipid regulators, antipyretics, anticonvulsants, and beta-blockers) by Cl(2), ClO(2), O(3), and ferrate(VI) (Fe(VI)O(4)(2-),Fe(VI)) under treatment conditions. In the chlorination of sulfonamide antibiotics, HOCl is the major reactive Cl(2) species whereas in the oxidation by Fe(VI), HFeO(4)(-) is the dominant reactive species. Both oxidation processes can oxidize sulfonamides in seconds at a neutral pH (t(1/2)≤ 220 s; 1 mg L(-1) HOCl or K(2)FeO(4)). The reactivity of O(3) with pharmaceuticals is generally higher than that of HOCl (k(app,pH 7) (O(3))=1-10(7)M(-1)s(-1); k(app,pH 7) (HOCl)=10(-2)-10(5)M(-1)s(-1)). Ozone selectively oxidizes pharmaceuticals and reacts mainly with activated aromatic systems and non-protonated amines. Oxidative transformation of most pharmaceuticals by O(3) occurs in seconds (t(1/2)≤ 100 s; 1 mg L(-1) O(3)) while half-lives for oxidations by HOCl differ by at least two orders of magnitude. Ozone appears to be efficient in oxidizing pharmaceuticals in aquatic environments. The limited work on Fe(VI) shows that it can also potentially transform pharmaceuticals in treatment processes.     

Influence of a soil enzyme on iron-cyanide complex speciation and mineral adsorption

Cyanide is commonly found as ferrocyanide [Fe(II)(CN)(6)](-4) and in the more mobile form, ferricyanide [Fe(III)(CN)(6)](-3) in contaminated soils and sediments. Although soil minerals may influence ferrocyanide speciation, and thus mobility, the possible influence of soil enzymes has not been examined. In a series of experiments conducted under a range of soil-like conditions, laccase, a phenoloxidase enzyme derived from the fungi Trametes versicolor, was found to exert a large influence on iron-cyanide speciation and mobility. In the presence of laccase, up to 93% of ferrocyanide (36-362ppm) was oxidized to ferricyanide within 4h. No significant effect of pH (3.6 and 6.2) or initial ferrocyanide concentration on the extent or rate of oxidation was found and ferrocyanide oxidation did not occur in the absence of laccase. Relative to iron-cyanide-mineral systems without laccase, ferrocyanide adsorption to aluminum hydroxide and montmorillonite decreased in the presence of laccase and was similar to or somewhat greater than that of ferricyanide without laccase. Laccase-catalyzed conversion of ferrocyanide to ferricyanide was extensive though up to 33% of the enzyme was mineral-bound. These results demonstrate that soil enzymes can play a major role in ferrocyanide speciation and mobility. Biotic soil components must be considered as highly effective oxidation catalysts that may alter the mobility of metals and metal complexes in soil. Immobilized enzymes should also be considered for use in soil metal remediation efforts.     

Efficient removal of triphenylmethane dyes from aqueous medium by in situ electrogenerated Fenton's reagent at carbon-felt cathode

Fenton's reagent (Fe2+ +H2O2) has been electrogenerated in situ in an undivided electrolytic cell from the effective reduction of Fe3+ and O2 at carbon-felt cathode for the treatment of aqueous solutions of four triphenylmethane dyes (TPMs), namely malachite green (MG), crystal violet (CV), methyl green (MeG) and fast green FCF (FCF), at pH 3.0 and room temperature. MG has been used as a model among them to study the influence of some experimental parameters on the decay kinetics, COD removal and current efficiency. The results in such electro-Fenton system are explained in terms of the many parasitic reactions involving .OH. Higher efficiency values are obtained with rising organic content and decreasing applied current. The first stage of the mineralization process, involving aromatic by-products, leads to fast decoloration as well as quick initial COD removal that fit well to a pseudo-first-order kinetics. At prolonged electrolysis time, the mineralization rate and efficiency decrease due to the formation of hardly oxidizable compounds and the enhancement of wasting reactions. Solutions of all four TPMs are quickly degraded following a pseudo-first-order decay kinetics. The absolute rate constant (kTPM) for their reaction with .OH increases in the order MeG相似文献   

Kinetic assessment of the potassium ferrate(VI) oxidation of antibacterial drug sulfamethoxazole

Sulfamethoxazole (SMX), a worldwide-applied antibacterial drug, was recently found in surface waters and in secondary wastewater effluents, which may result in ecotoxical effects in the environment. Herein, removal of SMX by environmentally-friendly oxidant, potassium ferrate(VI) (K(2)FeO(4)), is sought by studying the kinetics of the reaction between Fe(VI) and SMX as a function of pH (6.93-9.50) and temperature (15-45 degrees C). The rate law for the oxidation of SMX by Fe(VI) is first-order with respect to each reactant. The observed second-order rate constant decreased non-linearly from 1.33+/-0.08 x 10(3) M(-1)s(-1) to 1.33+/-0.10 x 10(0) M(-1)s(-1) with an increase of pH from 7.00 to 9.50. This is related to protonation of Fe(VI) (HFeO(4)(-) <==> H(+) + FeO(4)(2-); pK(a,HFeO(4)) = 7.23) and sulfamethoxazole (SH <==> H(+) + S(-); pK(a,SH)=5.7). The estimated rate constants were k(11)(HFeO(4)(-) + SH) = 3.0 x 10(4) M(-1)s(-1), k(12)(HFeO(4)(-) + S(-)) = 1.7 x 10(2) M(-1)s(-1), and k(13) (FeO(4)(2-) + SH) = 1.2 x 10(0) M(-1)s(-1). The energy of activation at pH 7.0 was found to be 1.86+/-0.04 kJ mol(-1). If excess potassium ferrate(VI) concentration (10 microM) is used than the SMX in water, the half-life of the reaction using a rate constant obtained in our study would be approximately 2 min at pH 7. The reaction rates are pH dependent; thus, so are the half-lives of the reactions. The results suggest that K(2)FeO(4) has the potential to serve as an oxidative treatment chemical for removing SMX in water.     

Decolorization and degradation of H-acid and other dyes using ferrous-hydrogen peroxide system

In this study, advanced oxidation process utilizing Fenton's reaction was investigated for the decolorization and degradation of two commercial dyes viz., Red M5B, Blue MR and H-acid, a dye intermediate used in chemical industries for the synthesis of direct, reactive and azo dyes. Effect of Fe2 +, H2O2, pH, and contact time on the degradation of the dyes was studied. Maximum color and COD removal was obtained for Red MSB, H-acid and Blue MR at 10-25 mg/l of Fe2+ dose and 400-500 mg/l of H2O2 dose at pH 3.0. The initial oxidation reaction was found to fit into first order rate kinetics and the rate of oxidation of H-acid was higher than the other dyes. Release of chloride and sulfate from the Fenton's treated Red M5B dye and sulfate from H-acid and Blue MR indicates that the dye degradation proceeds through cleavage of the substituent group.     

电催化氧化法处理染料废水的影响因素及动力学

以钛涂膜极板为阳极、石墨极板为阴极、Fe2O3/γ-Al2O3为多相催化剂,构建电-多相催化氧化体系,研究了该体系对酸性大红模拟染料废水中COD的去除效果及其影响因素,优化了实验条件,并初步探讨了COD的降解机理。结果表明,在槽电压20 V,pH 4,曝气量0.24 m3/h,极板间距3 cm的条件下,COD的去除率最高,达到64.5%;COD的降解近似符合一级动力学方程:ln(C0/C)=0.0034t+0.719。在电-多相催化氧化体系中,废水中的有机物被直接矿化或降解为小分子有机物。     

Kinetics and mechanism of nitrite oxidation by hypochlorous acid in the aqueous phase

The rate coefficient for the reaction of nitrite with hypochlorite and hypochlorous acid has been studied using spectrophotometric measurements. The reaction rate has been determined in a wide range of H(+) concentration (5< or =-log[H(+)]< or =11). The kinetics were carried out as a function of NO(2)(-), H(+) and total hypochlorite ([HOCl](total)=[HOCl]+[ClO(-)]+[ClNO(2)]) concentrations. The observed overall rate law is described by: -d[HClO](T)dt=[a[NO(2)(-)](2)+b[NO(2)(-)]][H(+)](2)c+d[H(+)]+e[NO(2)(-)][H(+)](2)[HOCl](total)At T=298 K and in Na(2)SO(4) at an ionic strength (I=1.00 M), we obtained using a nonlinear fitting procedure: a=(1.83+/-0.36)x10(7) s(-1), b=(1.14+/-0.23)x10(5) Ms(-1), c=(1.12+/-0.17)x10(-13) M, d=(1.43+/-0.29)x10(-6) M(2) and e=(1.41+/-0.28)x10(3) M where the errors represent 2sigma. According to the overall rate law, a/b=k(1)/k(3), b/e=k(3), c=K(w), d/c=K(a), d=K(a)K(w) and e=K(1)K(a). In Na(2)SO(4) at an ionic strength (I=1.00 M), the values of K(1) and K(a) are (1.1+/-0.1)x10(-4) and 1.28x10(7) M(-1), respectively. A mechanism is proposed for the NO(2)(-) oxidation which involves the reversible initial step: NO(2)(-)+HOCl left harpoon over right harpoon ClNO(2)+OH(-) (K(1)), while ClNO(2) undergoes the two parallel reactions: attack by NO(2)(-) (k(1)) and hydrolysis (k(3)). ClNO(2) and N(2)O(4) are proposed as important intermediates as they control the mechanism. The rate coefficients k(1) and k(3) have been determined at different ionic strengths in NaCl and Na(2)SO(4). The influence of the ionic strength and ionic environment has been studied in this work.     

Degradation of clofibric acid in acidic aqueous medium by electro-Fenton and photoelectro-Fenton

Acidic aqueous solutions of clofibric acid (2-(4-chlorophenoxy)-2-methylpropionic acid), the bioactive metabolite of various lipid-regulating drugs, have been degraded by indirect electrooxidation methods such as electro-Fenton and photoelectro-Fenton with Fe(2+) as catalyst using an undivided electrolytic cell with a Pt anode and an O(2)-diffusion cathode able to electrogenerate H(2)O(2). At pH 3.0 about 80% of mineralization is achieved with the electro-Fenton method due to the efficient production of oxidant hydroxyl radical from Fenton's reaction between Fe(2+) and H(2)O(2), but stable Fe(3+) complexes are formed. The photoelectro-Fenton method favors the photodecomposition of these species under UVA irradiation, reaching more than 96% of decontamination. The mineralization current efficiency increases with rising metabolite concentration up to saturation and with decreasing current density. The photoelectro-Fenton method is then viable for treating acidic wastewaters containing this pollutant. Comparative degradation by anodic oxidation (without Fe(2+)) yields poor decontamination. Chloride ion is released during all degradation processes. The decay kinetics of clofibric acid always follows a pseudo-first-order reaction, with a similar rate constant in electro-Fenton and photoelectro-Fenton that increases with rising current density, but decreases at greater metabolite concentration. 4-Chlorophenol, 4-chlorocatechol, 4-chlororesorcinol, hydroquinone, p-benzoquinone and 1,2,4-benzenetriol, along with carboxylic acids such as 2-hydroxyisobutyric, tartronic, maleic, fumaric, formic and oxalic, are detected as intermediates. The ultimate product is oxalic acid, which forms very stable Fe(3+)-oxalato complexes under electro-Fenton conditions. These complexes are efficiently photodecarboxylated in photoelectro-Fenton under the action of UVA light.     

Reduction of ferrate(VI) and oxidation of cyanate in a Fe(VI)-TiO2-UV-NCO- system

The aqueous photocatalytic degradation of cyanate (NCO(-)), which is a long-lived neurotoxin formed during the remediation of cyanide in industrial waste streams, was studied in the ferrate(VI)-UV-TiO2-NCO(-) system. Kinetics measurements of the photocatalytic reduction of ferrate(VI) were carried out as a function of [NCO(-)], [ferrate(VI)], [O(2)], light intensity (I(o)), and amount of TiO2 in suspensions at pH 9.0. The photocatalytic reduction rate of ferrate(VI) in the studied system can be expressed as -d[Fe(VI)]/dt=kI(o)(0.5) [NCO(-)] [TiO2]. The rate of photocatalytic oxidation of cyanate with ferrate(VI) was greater than the rate in the analogous system without ferrate(VI). The possibility of involvement of reactive ferrate(V) species for this enhancement was determined by studying the reactivity of ferrate(V) with NCO(-) in a homogeneous solution using a premix pulse radiolysis technique. The rate constant for the reaction of ferrate(V) and NCO(-) in alkaline medium was estimated to be (9.60+/-0.07) x 10(2) M(-1) s(-1), which is much slower than the ferrate(VI) self-decomposition reaction (k approximately 10(7) M(-1) s(-1)). An analysis of the kinetic data in the Fe(VI)-UV-TiO2-NCO(-) system suggests that ferrate(V) is not directly participating in the oxidation of cyanate. Possible reactions in the system are presented to explain results of ferrate(VI) reduction and oxidation of cyanate.