Oxidation of diclofenac by permanganate: Kinetics,products and effect of inorganic reductants

The large consumption and discharge of diclofenac (DCF) lead to its frequent detection in surface water and groundwater, posing great threats to humans and ecosystems. This study explored the oxidation kinetics of DCF by permanganate (Mn(VII)), and expounded the underlying reason for the unusual pH-dependency that was unclear in previous studies. The kinetics of DCF analogues (i.e., aromatic secondary amines) by Mn(VII) oxidation were comparatively investigated. Then, a tentative kinetic model involving the formation of an intermediate between Mn(VII) and DCF or its analogues was proposed to fit the pH-rate profile. Since DCF contained two chloro groups, and a carboxyl group which could be ionized by negative electrospray ionization, a precursor ionization scanning approach was used for the first time for detection of N-containing chlorinated oxidation products. New degradation pathways of DCF containing ring opening, carboxylation, carbonylation, electrophilic addition, hydroxylation and dehydrogenation were proposed based on the identified oxidation products. Moreover, it was demonstrated that the introduction of various reducing agents such as Mn(II), Fe(II) and bisulfite significantly improved the oxidation kinetics of DCF by Mn(VII). The positive effects of Mn(II) and Fe(II) were mainly attributed to the accelerated formation of MnO₂ that acted as a catalyst or co-oxidizer contributing to DCF degradation. The presence of bisulfite caused two-stage kinetics, where a sharp drop of DCF concentration followed by a slowdown of DCF removal. In the first stage, potent reactive manganese species (e.g., Mn(III), Mn(V), and Mn(VI)) and sulfate radical were generated during reaction of bisulfite with Mn(VII), whereas bisulfite was depleted fast due to excess Mn(VII) concentrations and the system became the Mn(VII)/MnO₂ system in the second stage. These results provide new insight into reaction mechanism of DCF with Mn(VII) as well as propose a feasible strategy for enhancing the treatment of DCF contaminated water by Mn(VII).     

Effect of Micelles on kinetics and mechanism of the oxidation of Serine by acid Permanganate

The oxidation of serine (HORCO₂H) by acid permanganate was investigated both in the absence and presence of sodium dodecyl sulfate (SDS). It has been observed that the presence of surfactant enhanced the reaction rate. The reaction is first order with respect to [Serine] and [MnO₄^?]. The reaction is retarded by the hydrogen ion in the absence of SDS but catalyzed in the presence of SDS. The overall rate expression for the reduction of Mn(VII) may be written as In the presence of SDS of the rate law is The reaction appears to involve a parallel consecutive reaction mechanism in which Mn(IV) appears as the reaction intermediate. ??′_4f signifies the rate constant for the reaction path leading to the formation of Mn(IV) from Mn(VII) as reaction intermediate, whereas ??′_2f signifies the rate constant for the reaction path leading to the reduction of Mn(VII) to Mn(II) without prior formation of Mn(IV). A mechanism satisfying the various kinetic parameters has been proposed.     

Continuous-flow determination of reserpine by oxidation with periodate ion and catalysis by manganese(II) in solution or by an MnO2(s) reactor

The periodate ion oxidation of reserpine catalyzed by Mn(II) or Mn(IV) ions was used for the continuous-flow determination of the pharmaceutical drug. Spectrophotometric monitoring of the oxidation product, 3,4-didehydroreserpine, at 385 nm served as means of detection. The Mn(II) catalyst was used in solution and the Mn(IV) in immobilized form as crystalline MnO₂(s) was thermally embedded on the walls of Tygon tubing and incorporated in the flow system as a solid catalyst in an open-tubular reactor. The proposed methods were applied to the analyses of single tablets and of Rauwolfia serpentina standard powders.     

Use of amberlite XAD-7HP for the separation of Mn(II) and Mn(VII) in waters

A simple method is proposed for the determination and speciation of Mn(II) and Mn(VII) in waters utilizing a macroporous resin, Amberlite XAD-7HP. The batch method was employed and flame atomic absorption spectrometry was used in all determinations. Amberlite XAD-7HP resin was shown to retain Mn(VII) between pH 4 and 12. If the solution contains only one of the species, either Mn(II) or Mn(VII), the resin behaves selectively depending on the pH of the solution. The elution from the sorbent was realized using K₂C₂O₄ in HNO₃. The efficiency of the method was checked via spike recovery experiments. The proposed method was successfully applied to industrial wastewater samples and quantitative recoveries (≥96.0%) confirmed the accuracy of the method.     

A kinetic study of the oxidation of <Emphasis Type="SmallCaps">l</Emphasis>-methionine by water soluble colloidal MnO<Subscript>2</Subscript>

Aqueous solution of water soluble colloidal MnO₂ was prepared by Perez-Benito method. Kinetics of l-methionine oxidation by colloidal MnO₂ in perchloric acid (0.93 × 10⁻⁴ to 3.72 × 10⁻⁴ mol dm⁻³) has been studied spectrophotometrically. The reaction follows first-order kinetics with respect to [H⁺]. The first-order kinetics with respect to l-methionine at low concentration shifts to zero order at higher concentration. The effects of [Mn(II)] and [F⁻] on the reaction rate were also determined. Manganese (II) has sigmoidal effect on the rate reaction and act as auto catalyst. The exact dependence on [Mn(II)] cannot be explained due to its oxidation by colloidal MnO₂. Methionine sulfoxide was formed as the oxidation product of l-methionine. Ammonia and carbon dioxide have not been identified as the reaction products. The mechanism with the observed kinetics has been proposed and discussed.     

Effect of Mn(II) and Ce(IV) Ions on the Oxidation of Lactic Acid by Chromic Acid

The kinetics and mechanism of lactic acid oxidation in the presence of Mn(II) and Ce(IV) ions by chromic acid were studied spectrophotometrically. The oxidation of lactic acid by Cr(VI) was found to proceed in two measurable steps, both of which gave pyruvic acid as the primary product in the absence of Mn(II). 2Cr(VI)+2CH₃CHOHCOOH → 2CH₃COCOOH+Cr(V)+Cr(III) Cr(V)+CH₃CHOHCOOH → Cr(III)+CH₃COCOOHThe observed kinetics was explained due to the catalytic and inhibitory effects of Mn(II) and Ce(IV) on the lactic acid oxidation by Cr(VI). The reactivity of lactic acid depends upon the experimental conditions. It acts as a two-or three-equivalent reducing agent in the absence or presence of Mn(II). It was examined that Cr(III) products resulting from the direct reduction of Cr(VI) by three-equivalent reducing agents. The oxidation of lactic acid follows the complex order kinetics with respect to [lactic acid]. The activation parameters E_a, ΔH^#, and ΔS^# were calculated and discussed.     

Low-temperature oxidation of carbon monoxide over (Mn1 − x M x )O2 (M = Co, Pd) catalysts

(Mn_{1 ? x} M _x )O₂ (M = Co, Pd) materials synthesized under hydrothermal conditions and dried at 80°C have been characterized by X-ray diffraction, diffuse reflectance spectroscopy, electron microscopy, X-ray photoelectron spectroscopy, and adsorption and have been tested in CO oxidation under CO + O₂ TPR conditions and under isothermal conditions at room temperature in the absence and presence of water vapor. The synthesized materials have the tunnel structure of cryptomelane irrespective of the promoter nature and content. Their specific surface area is 110–120 m²/g. MnO₂ is morphologically uniform, and the introduction of cobalt or palladium into this oxide disrupts its uniformity and causes the formation of more or less crystallized aggregates varying in size. The (Mn,Pd)O₂ composition contains Pd metal, which is in contact with the MnO₂-based oxide phase. The average size of the palladium particles is no larger than 12 nm. The initial activity of the materials in CO oxidation, which was estimated in terms of the 10% CO conversion temperature, increases in the following order: MnO₂ (100°C) < (Mn,Co)O₂ (98°C) < (Mn,Co,Pd)O₂ (23°C) < (Mn,Pd)O₂ (?12°C). The high activity of (Mn,Pd)O₂ is due to its surface containing palladium in two states, namely, oxidized palladium (interaction phase) palladium metal (clusters). The latter are mainly dispersed in the MnO₂ matrix. This catalyst is effective in CO oxidation even at room temperature when there is no water vapor in the reaction mixture, but it is inactive in the presence of water vapor. Water vapor causes partial reduction of Mn⁴⁺ ions and an increase in the proportion of palladium metal clusters.     

Chemical oscillators based on the oxidation of Mn(II) by some halogens

Chemical oscillators based on the oxidation of Mn(II) ions by bromine or chlorine in NaH₂PO₄–NaOH buffer solutions in a CSTR (continuous-flow stirred tank reactor) are described. The oscillations correspond to the two alternating processes. The first process is the oxidation of Mn(II) by HOBr or HOCl to Mn(IV) and the second one is of a micro-heterogeneous nature, consisting of the reduction of Mn _c ⁴⁺ and Mn _c ³⁺ centers on the surface of colloids (MnO₂)_col by halides.     

Kinetics and Mechanism of the Autocatalytic Oxidation of Mn(II) by Bromine

The oxidation of Mn(II) by bromine is an autocatalytic reaction, which seems to be important for a detailed elucidation of chemical oscillators, based on manganese chemistry. With regard to the mechanism proposed previously, an alternative reaction mechanism is proposed, based on a micro-heterogeneous oxidation of Mn(II) ion, adsorbed on a surface of the MnO₂ colloid. This revised version was published online in June 2006 with corrections to the Cover Date.     

Study of the base-catalysed oxidation of the anti-bacterial and anti-protozoal agent metronidazole by permanganate ion in alkaline medium

The mechanism of dismutation of MnO₄ ^2? via the complex [MTZ–MnO₄·OH]^2?, formed during the oxidation of metronidazole (MTZ), has been investigated spectrophotometrically at different temperatures. The stoichiometry of the reaction is 1:1, i.e. 1 mol MTZ reacts with 1 mol Mn(VII).The reaction is first order in permanganate, less than first order in [MTZ] and [alkali]. The effects of added products and the dielectric constant and ionic strength of the reaction medium were investigated. The main products were identified by spot test and FT-IR. A mechanism involving a free radical has been proposed. In the equilibrium step MTZ binds to the MnO₄ ^? species to form a complex (C). Investigation of the reaction at different temperatures enabled determination of the activation data for the slow step of proposed mechanism. The proposed mechanism and the derived rate laws are consistent with the observed kinetics.     

Charge and structural ordering in the brownmillerite phases: La1−xSrxMnO2.5 (0.2<x<0.4)

The topotactic reduction of La_1−xSr_xMnO₃ (0.2<x<0.4) perovskite phases to the corresponding La_1−xSr_xMnO_2.5 brownmillerite phases with NaH is described. Neutron and electron diffraction data show the x=0.25 and 0.2 phases adopt structures with an unusual ordered L-R-L-R alternation of twisted chains of Mn(II) tetrahedra within each anion-deficient layer. This is accompanied by Mn(II)/(III) charge ordering within the remaining MnO₆ octahedral layers. In contrast, the x=0.4 phase adopts a structure in which the twisted chains of tetrahedra are disordered.     

Impact of environmental conditions on the sorption behavior of radionuclide <Superscript>60</Superscript>Co(II) on MnO<Subscript>2</Subscript>

The fate and transport of toxic metal ions and radionuclides in the environment is generally controlled by sorption reactions. The removal of ⁶⁰Co(II) from wastewaters by MnO₂ was studied as a function of various environmental parameters such as shaking time, pH, ionic strength, foreign ions, and humic substances under ambient conditions. The results indicated that the sorption of ⁶⁰Co(II) on MnO₂ was strongly dependent on pH and ionic strength. At low pH, the sorption of ⁶⁰Co(II) was dominated by outer-sphere surface complexation and ion exchange with Na⁺/H⁺ on MnO₂ surfaces, whereas inner-sphere surface complexation was the main sorption mechanism at high pH. The presence of HA/FA enhances ⁶⁰Co(II) sorption at low pH values, whereas reduces ⁶⁰Co(II) sorption at high pH values. The Langmuir and Freundlich models were used to simulate the sorption isotherms of ⁶⁰Co(II) at three different temperatures of 298.15, 318.15 and 338.15 K. The thermodynamic parameters (ΔH ⁰, ΔS ⁰ and ΔG ⁰) calculated from the temperature dependent sorption isotherms indicated that the sorption process of ⁶⁰Co(II) on MnO₂ was endothermic and spontaneous.     

Effect of environmental conditions on 109Cd(II) sorption to MnO2

The sorption of Cd(II) from aqueous solution on MnO₂ was investigated under ambient conditions. Experiments were carried out as a function of contact time, solid content, pH, ionic strength, foreign ions, fulvic acid and temperature. The results indicated that the sorption of Cd(II) was strongly dependent on pH and ionic strength. At low pH, the sorption of Cd(II) was dominated by outer-sphere surface complexation and ion exchange with Na⁺/H⁺ on MnO₂ surfaces, whereas inner-sphere surface complexation was the main sorption mechanism at high pH. The Langmuir, Freundlich and Dubinin–Radushkevich models were used to simulate the sorption isotherms at three different temperatures. The thermodynamic data (ΔG ⁰, ΔS ⁰, ΔH ⁰) calculated from the temperature dependent sorption isotherms suggested that the sorption of Cd(II) on MnO₂ was an spontaneous and endothermic process.     

Continuous Determination of Trace Amounts of Humic Acid in Natural Water by Chemiluminescence Method

A chemiluminescence (CL) method for the determination of humic acid (HA) based on the oxidation of HA with hydrogen peroxide in the presence of formaldehyde in alkaline solution is described. This method is sensitive and selective for the determination of HA in natural water. HA produces strong CL in the oxidation of HA with MnO₄⁻, Br₂, ClO⁻, and Cr₂O₇²⁻, and the H₂O₂. HA-H₂O₂-HCHO system is suitable for the determination of HA because of its high sensitivity and high selectivity. The detection limit was 50 ppb and relative standard deviation for five measurements of 0.5 ppm (w/w) HA was 1.8%. Cations such as Na⁺, K⁺, Mg²⁺, Cu²⁺, and Fe3⁺ and anions such as PO₄³⁻, NO₃⁻, CO₃²⁻, SO₄²⁻, Cl⁻, and Y⁻ (EDTA-Na) did not interfere with the determination of HA. Addition of Mn(II) increased the CL intensity. The concentration of HA in natural water determined with this method is in good agreement with that determined by fluorometric analysis.     

Ozone interaction with manganese acetate solution. Formation of H x MnO2·nH2O layers and microtubes based on them

We present the first study of Mn(II) cations interaction with ozone fed onto the air/aqueous manganese acetate solution boundary. In the reaction, the layer of H _x MnO₂·nH₂O of birnessite structure has been formed at the surface. Subsequent ～1–3 μm thick layers drying at air has led to their rolling up to form microtubes with diameter of 20–50 μm and up to 10 mm long.     

Development of the bifunctional catalyst Mn-Fe-Beta for selective catalytic reduction of nitrogen oxides

The catalytic properties of the Mn-Fe-Beta system with Mn contents in the range 0.1–16 wt.% were studied in the selective catalytic reduction (SCR) of NO _x with ammonia. The catalyst structure was investigated using IR spectra of adsorbed NO, temperature-programmed reduction with hydrogen (H₂-TPR), X-ray diffraction analysis, and ESR. The use of manganese as a promoter substantially increases the activity of iron-containing catalysts in the SCR of NO _x with ammonia. At low contents (<2 wt.%), Mn exists in the cation form and the catalytic activity of the Mn-Fe-Beta system does not increase. At a higher content of Mn, clusters MnO _x begin to form, which are highly active in the oxidation of NO to NO₂ and the low-temperature catalytic activity of the Mn-Fe-Beta system increases. The observed increase in the low-temperature catalytic activity in the process of SCR of NO _x with ammonia is related to a change in the reaction route. The MnO _x clusters favor the oxidation of NO and the iron cations facilitate the reaction of “fast” SCR.     

Sorption behavior of Co(II) on γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> in the presence of humic acid

The fate and transport of toxic metal ions and radionuclides in the environment is generally controlled by sorption reactions. The extent of sorption of divalent metal cations is controlled by a number of factors including cosorbing or complexing. In this work, the effects of pH, humic acid HA/Co(II) addition orders, ionic strength, concentration of HA, and foreign cations on the Co(II) sorption on γ-Al₂O₃ in the presence of HA were investigated. The sorption isotherms of Co(II) on γ-Al₂O₃ in the absence and presence HA were also studied and described by using S-type sorption model. The experimental results showed that the Co(II) sorption is strongly dependent on the pH values, concentration of HA, but independent of HA/Co(II) addition orders, ionic strength, and foreign cations in the presence of HA under our experimental conditions. The results also indicated that HA enhanced the Co(II) sorption at low pH, but reduced the Co(II) sorption at high pH. It was hypothesized that the significantly positive influence of HA at low pH on the Co(II) sorption on γ-Al₂O₃ was attributed to strong surface binding of HA on γ-Al₂O₃ and subsequently the formation of ternary surface complexes such as ≡S-OOC-R-(COO⁻) _x Co^2−x . Chemi-complexation may be the main mechanism of the Co(II) sorption on γ-Al₂O₃ in the presence of HA.     

Amplification effects of magnetic field on hydroxylamine-promoted ZVI/H2O2 near-neutral Fenton like system

This study has demonstrated an interesting amplification effect of magnetic field（MF） on the hydroxylamine（HA）-promoted zero valent iron（ZVI）/H2 O₂ Fenton-like system.Sulfamethoxazole（SMX） could be efficiently degraded at near neutral pH.Conditional parameters affecting the SMX degradation in the ZVI/H₂ O₂/HA/MF system,e.g.,pH and the dosages of ZVI,HA and H₂ O₂,were investigated.Unlike the acid-favorable ZVI/H₂ O₂ and ...     

XAFS characterization of La1−xSrxMnO3±δ catalysts prepared by Pechini’s method

XAFS experiments at the Mn and Sr K-edges were carried out in order to investigate the short-range arrangement of Mn and Sr sites on La_1−xSr_xMnO_3±δ highly doped perovskites (x = 0, 0.2, 0.4 and 0.6). The Mn K-edge EXAFS spectra show a static Jahn–Teller distortion of the MnO₆ for x = 0 and 0.2, which is drastically reduced as x increases. The distortion of perovskite, characterized by the Mn–O–Mn tilt angle, progressively decreases with increasing Sr contents. Sr K-edge results indicated a decrease on the Sr–Mn coordination number upon Sr doping. Based on this and TPD results, a charge compensating mechanism is proposed suggesting a partial Mn oxidation and formation of Mn defect vacancies due to the introduction of Sr.     

Diffusion and reactivity of ZnO-MnOx system

The solid state interaction between ZnO and MnO_x in air was investigated at different temperatures by means of the diffusion couple technique. No diffusion is observed at temperatures below 973 K. Above this temperature, Mn(IV) is already reduced to Mn(III) and the subsequent formation of Mn₂O₃ impels the diffusion of manganese into the ZnO pellet. However, it never enters the wurtzite lattice, so no homogeneous Mn:ZnO solid solution is formed. Simultaneously, Zn greatly diffuses in the manganese pellet, and as a consequence, a new phase layer develops at MnO_x/reaction zone interface. A mixture of cubic and tetragonal spinel-type phases initially comprises this layer. However at higher temperatures, the tetragonal ZnMn₂O₄ spinel is the unique phase present in the interface, and it forms a physical barrier for further diffusion of both zinc and manganese species in the respective pellets of the couple. Differences arising between ZnO, MnO₂ and Mn₂O₃ crystal structures are behind these diffusion behaviors.