Unrecognized role of humic acid as a reductant in accelerating fluoroquinolones oxidation by aqueous permanganate

A great concern has been raised regarding the issue of fluoroquinolones（FQs） in the environment. In this work, the transformation of FQs by commonly used oxidant permanganate(Mn（VII）) in the absence and presence of humic acid（HA）, ubiquitously existing in aquatic environments, was systematically investigated. Here, the catalytic role of in-situ formed MnO₂ on Mn（VII） oxidation of FQs depending on solution pH and co-existing substrates was firstly reported. It was interestingly found t...     

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.     

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.     

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.     

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.     

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.     

Kinetic and mechanistic investigation of the oxidation of the antibacterial agent levofloxacin by permanganate in alkaline medium

The kinetics and mechanism of oxidation of levofloxacin (LF) by manganese(VII) in alkaline medium at constant ionic strength was studied spectrophotometrically. The reaction exhibits 2:1 Mn:LF stoichiometry and is first order in permanganate but fractional order in both LF and alkali. Decrease in the dielectric constant of the medium results in a decrease in the rate of reaction. The effects of added products and ionic strength have also been investigated. The main products identified were hydroxylated LF and Mn(VI). A mechanism involving free radicals is proposed. In a composite equilibrium step, levofloxacin binds to MnO₄ ⁻ to form a complex that subsequently decomposes to the products. Investigations of the reaction at different temperatures allowed the determination of the activation parameters with respect to the slow step of the proposed mechanism.     

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.     

Kinetics and mechanism of the reduction of colloidal MnO2 by glycyl-leucine in the absence and presence of surfactants

The kinetics of the reduction of water-soluble colloidal manganese dioxide by glycyl-leucine (Gly-Leu) has been investigated in the presence of perchloric acid both in aqueous as well as micellar media at 35 °C. The study was carried out as functions of [MnO₂], [Gly-Leu] and [HClO₄]. The first-order-rate is observed with respect to [MnO₂], whereas fractional-order-rates are determined in both [Gly-Leu] and [HClO₄]. Addition of sodium pyrophosphate and sodium fluoride enhanced the rate of the reaction. Further, the use of surfactant micelles is highlighted as, in favourable cases, the micelles help the redox reactions by bringing the reactants into a close proximity due to hydrogen bonding. While the ionic surfactants SDS and CTAB have not shown any effect on the reaction rate, the nonionic surfactant TX-100 has catalytic effect which is explained in terms of the mathematical model proposed by Tuncay et al. (1999). The Arrhenius and Eyring equations are valid for the reaction over the range of temperatures used and different activation parameters (E_a, ΔH^#, ΔS^# and ΔG^#) have been evaluated. Kinetic studies show that the redox reaction between MnO₂ and Gly-Leu proceeds through a mechanism combining one- and two-electron pathways: Mn(IV) → Mn(III) → Mn(II) and Mn(IV) → Mn(II). On the basis of the observed results, a possible mechanism has been proposed and discussed.     

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.     

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.     

The Synthesis and Crystal Structures of New Condensed Phosphates of Manganese

Abstract

This work deals with the synthesis of condensed phosphates of Mn(II), Mn(III), and also their combinations with Na, Cs. The reaction of MnO₂ with molten polyphosphoric acids, partially neutralized with carbonates of Na and Cs is used. In the course of reaction of MnO₂ with molten polyphosphoric acids, Mn(IV) is reduced to Mn(III) or/and Mn(II). The ratio Mn(III)/Mn(II) depends on the conditions of synthesis (temperature, time, presence of oxidants or reductants etc.). By varying these parameters we obtained 7 new manganese phosphates and determined their structures.     

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.     

1.2 Volt manganese oxide symmetric supercapacitor

Capacitance fading of MnO₂ supercapacitor electrode under negative polarization below 0.0 V (versus Ag/AgCl/sat. KCl_(aq)) arises from extensive reduction of Mn(IV) to form inactive Mn(II) species, and this has typically limited the operating voltage window of an aqueous symmetric MnO₂ supercapacitor to be no greater than 0.8 V. As this lower potential limit is close to the onset potential of MnO₂-catalyzed oxygen reduction reaction (ORR), the fading problem can be alleviated by effectively passing the accumulated electrons in the oxide electrode to the dissolved oxygen molecules in electrolyte in order to avoid the formation of the surface Mn(II) species. This has been demonstrated by either increasing the dissolved oxygen content or using the Ti(IV)/Ti(III) redox couple in the electrolyte as a charge-transfer mediator to enhance the electrocatalytic activity of MnO₂ for ORR. Therefore, a MnO₂ symmetric supercapacitor showing remarkable cycling stability over an operating voltage window of 1.2 V has been achieved by using Ti(IV)-containing neutral electrolyte (1 M KCl_(aq)).     

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.     

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.     

Cathodic stripping voltammetry of trace Mn(II) at carbon film electrodes

A sensitive voltammetric method is presented for the determination of tract levels of Mn (II) using carbon film electrodes fabricated from carbon resistors of 2 Ω. Determination of manganese was made by square wave cathodic stripping voltammetry (CSV), with deposition of manganese as manganese dioxide. Chronoamperometric experiments were made to study MnO₂ nucleation and growth. As a result, it was found to be necessary to perform electrode conditioning at a more positive potential to initiate MnO₂ nucleation. Under optimised conditions the detection limit obtained was 4 nM and the relative standard deviation for eight measurements of 0.22 nM was 5.3%. Interferences from various metal ions on the response CSV of Mn(II) were investigated, namely Cd(II), Ni(II), Cu(II), Cr(VI), Pb(II), Zn(II) and Fe(II). Application to environmental samples was demonstrated.     

Structural analysis of colloidal MnO x composites

We report on the detailed structure of MnO _x nanoparticles (MnO _x NP) which are either stabilized by cationic spherical polyelectrolyte brushes or by star-shaped cationic polyelectrolyte chains. In both cases, the polycation is composed of 2-(trimethylammonium)ethyl methacrylate chloride (TMAEMC). The analysis by transmission electron microscopy (TEM), cryogenic transmission electron microscopy (cryoTEM), and powder X-ray diffraction leads to the conclusion that the MnO _x nanoparticles in aqueous dispersed state are composed of only a few or even single lamellae of c-disordered potassium birnessite (birnessite). Using star-shaped pTMAEMC homopolymer for the synthesis of composite particles, we obtain MnO _x NP with an average diameter of about 5 nm. MnO _x NP immobilized on cationic spherical polyelectrolyte brush have a length of about 20 nm and a width of 1.6 nm. Comparison of the extended X-ray absorption fine structure (EXAFS) spectra of the MnO _x composites with reference spectra leads to the conclusion that all materials include c-disordered birnessite-type nanoparticles. A comparison of the energy shift of the Mn K-edge absorption peak of the X-ray absorption near-edge structure spectra of different manganese oxide reference materials with the different MnO _x NP revealed an average oxidation state of about 3.5–3.7 for synthesized compounds. No distinct structural difference is found when comparing the dried samples to samples dispersed in water. A comparison of the EXAFS data of the birnessite nanoparticles with the crystal structure of macroscopic systems showed a compression along the c direction accompanied by a slight elongation within the ab plane of the layered material.     

Rationally designed/constructed MnOx/WO3 anode for photoelectrochemical water oxidation

The activity of WO₃ photoanode could be improved efficiently after loading MnO_x by photodeposition. The maximum photocurrent density of composite photoanode is achieved with a deposition time of 3 min, which is higher than that of pristine WO₃ photoanode around 40%.