Mass transfer and kinetics study on the sulfite forced oxidation with manganese ion catalyst

Wet limestone scrubbing is the most common flue gas desulfurization process (FGD) for control of sulfur dioxide emissions from the combustion of fossil fuels. Forced oxidation, which controls the overall reaction of the sulfur dioxide absorption, is the key path of the process. Manganese which comes from the coal is one of the catalysts during the forced oxidation process. In the present work, the two-film theory was used to analyze the sulfite forced oxidation reaction with an image boundary recognition technique, and the oxidation rate was experimentally studied by contacting pure oxygen with a sodium sulfite solution. There was a critical sulfite concentration 0.328 mol/L without catalyst or at a constant catalyst concentration value. The kinetics study focused on the active energy of the reaction and the reaction constant k; furthermore, we obtained the order with respect to the sulfite and Mn²⁺ concentrations. When the Mn²⁺ catalyst concentration was kept unchanged, the sulfite oxidation reaction rate was controlled by dual film and the reaction kinetics was first order with respect to sulfite while SO₃²⁻ concentration was below 0.328 mol/L; the sulfite oxidation reaction rate was controlled by gas film only and the reaction kinetics was zero order with respect to sulfite while SO₃²⁻ concentration over 0.328 mol/L. When SO₃²⁻ concentration was kept unchanged, the sulfite oxidation reaction rate depended on gas-liquid mass transfer and the reaction kinetics was different in various stages with respect to Mn²⁺ concentrations. This work was presented at the 6 ^th Korea-China Workshop on Clean Energy Technology held at Busan, Korea, July 4–7, 2006.     

Reaction between Hydrosulfide and Iron/cerium (hydr)oxide: Hydrosulfide Oxidation and Iron Dissolution Kinetics

Hydrosulfide oxidation and iron dissolution kinetics were studied at normal pressure, under inert (N₂) atmosphere, in a liquid–solid mechanically-stirred slurry reactor. The kinetic variables undergoing variations were: hydrosulfide initial concentration (0.90–3.30 mmol/L), oxide initial surface area (16–143 m²/L) and pH (8.0–11.0). The hydrosulfide consumption and products (thiosulfate and polysulfide) formation were quantified by means of capillary electrophoresis, while iron dissolution was monitored through atomic absorption spectroscopy. Most of Fe(II) produced at pH = 9.5 remained associated with the oxide surface in the time-scale of the experiments. The hydrosulfide oxidation by the iron/cerium (hydr)oxide was found to be surface-controlled, with rates (R_i) of both sulfide oxidation and Fe(II) dissolution expressed in terms of an empirical rate equation: R_i = k_i[HS⁻]_t=0^−0.5[A]_t=0[H⁺]_t=0^−0.5 , where k_i represents the apparent rate constants for the oxidation of HS⁻ (k_HS) or the dissolution of Fe(II) (k_Fe), [HS⁻]_{t = 0} is the initial hydrosulfide concentration, [A]_{t = 0} is the initial Fe/Ce (hydr)oxide surface area and [H⁺]_{t = 0} is the initial proton concentration. The rate constant, k_HS, for the oxidation of hydrosulfide at pH = 9.5 was (3.4219 ± 0.65) × 10⁻⁴ mol² L⁻¹ m⁻² min⁻¹, with the rate of hydrosulfide oxidation being ca. 10 times faster than the rate of Fe(II) dissolution (assuming a 1:2 stoichiometric ratio between HS⁻ oxidized and Fe(II) produced; k_Fe = (3.9116 ± 0.41) × 10⁻⁵ mol² L⁻¹ m⁻² min⁻¹).     

Oxidation of herbicides by in situ synthesized hydrogen peroxide and fenton’s reagent in an electrochemical flow reactor: study of the degradation of 2,4-dichlorophenoxyacetic acid

This paper reports an investigation of H₂O₂ electrogeneration in a flow electrochemical reactor with RVC cathode, and the optimization of the O₂ reduction rate relative to cell potential. A study of the simultaneous oxidation of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) by the in situ electrogenerated H₂O₂ is also reported. Experiments were performed in 0.3 M K₂SO₄ at pH 10 and 2.5. Maximum hydrogen peroxide generation rate was reached at −1.6 V versus Pt for both acidic and alkaline solutions. Then, 100 mg L⁻¹ of 2,4-D was added to the solution. 2,4-D, its aromatic intermediates such as chlorophenols, chlororesorcinol and chlorinated quinone, as well as TOC were removed at different rates depending on pH, the use of UV radiation and addition of Fe(II). The acidic medium favored the hydroxylation reaction, and first order apparent rate constants for TOC removal ranged from 10⁻⁵ to 10⁻⁴ s⁻¹. In the presence of UV and iron, more than 90% of TOC was removed. This value indicates that some of the intermediates derived from 2,4-D decomposition remained in solution, mainly as more biodegradable light aliphatic compounds.     

Enhanced discharge voltage by CuO and RuO2 modification of ferrate(VI) cathode in alkaline super-iron/TiB2 batteries

The K₂FeO₄/TiB₂ battery has a significant advantage of battery capacity due to their multi-electron discharge reaction both of the cathode K₂FeO₄ (3e⁻) and the anode TiB₂ (6e⁻). However, the more positive reduction potential of TiB₂ anode results in a lower discharge voltage plateau of K₂FeO₄/TiB₂ battery, compared with the K₂FeO₄/Zn battery. The simple modification of Fe(VI) cathode with CuO additive was used to improve the cathode reduction kinetics and decrease the polarization potential in the discharge process. Another electrocatalysis media RuO₂ with excellent electric conductivity is used as additive in K₂FeO₄ cathode to demonstrate which effect is more important for the discharge voltage plateau, electrocatalysis or electron conductivity of additives. The results show that the 5% CuO additive modified K₂FeO₄/TiB₂ battery exhibits an enhanced discharge voltage plateau (1.5 V) and a higher cathode specific capacity (327 mAh/g). The advanced discharge voltage plateau can be due to the electrocatalysis of additives on the electrochemical reduction kinetics of Fe(VI) cathode in the whole discharge process, rather than the good electronic conductivity of additives.     

Antimony Remediation Using Ferrate(VI)

This research investigates a remediation technique for antimony involving the adsorption and co-precipitation of aqueous antimony by in-situ formed ferric hydroxide. Flocculation is initiated by the oxidation of iron(II) with potassium ferrate(VI), K₂FeO₄, along with oxidation of the more toxic Sb(III) to Sb(V). A 3/1 mole ratio of Fe(II)/Fe(VI) and a total Fe/Sb mole ratio of 300/1 was needed to achieve total antimony concentrations below the maximum contaminant levels for drinking water (6 μg/l).     

Preparation, electrochemical properties, and cycle mechanism of Li1? x Fe0.8Ni0.2O2-Li x MnO2 (Mn/(Fe+Ni+Mn)=0.8) material

A new type of Li_1−x Fe_0.8Ni_0.2O₂-Li _x MnO₂ (Mn/(Fe+Ni+Mn)=0.8) material was synthesized at 350 °C in an air atmosphere by a solid-state reaction. The material had an XRD pattern that closely resembled that of the original Li_1−x FeO₂-Li _x MnO₂ ((Fe+Ni+Mn)=0.8) with much reduced impurity peaks. It was composed of many large particles of about 500–600 nm and small particles of about 100–200 nm, which were distributed among the larger particles. The Li/Li_1−x Fe_0.8Ni_0.2O₂-Li _x MnO₂ cell showed a high initial discharge capacity above 192 mAh/g, which was higher than that of the parent Li/Li_1−x FeO₂-Li _x MnO₂ (186 mAh/g). This cell exhibited not only a typical voltage plateau in the 2.8 V region, but also an excellent cycle retention rate (96%) up to 45 cycles. We suggest a unique role of doped nickel ion in the Li/Li_1−x Fe_0.8Ni_0.2O₂-Li _x MnO₂ cell, which results in the increased initial discharge capacity from the redox reaction of Ni²⁺/Ni³⁺ between 2.0 and 1.5 V region.     

The potentiodynamic behaviour of iron in alkaline solutions

The potentiodynamic behaviour of iron in alkaline solutions under carefully controlled perturbation conditions reveals that the overall electrochemical process is more involved than was thought earlier. The electrochemical characteristics of the systems are explained through a series of successive conjugated redox couples principally involving Fe(OH), Fe(OH)₂ and FeOOH as limiting stoichiometric species. The yield of soluble species such as either FeO^2?₂ or HFeO^?₂ increases with the pH. Ageing effects of reactants and products are also distinguished through the potentiodynamic E/I records.     

Electrochemical pretreatment of distillery wastewater using aluminum electrode

Electrochemical (EC) oxidation of distillery wastewater with low (BOD₅/COD) ratio was investigated using aluminum plates as electrodes. The effects of operating parameters such as pH, electrolysis duration, and current density on COD removal were studied. At a current density of 0.03 A cm⁻² and at pH 3, the COD removal was found to be 72.3%. The BOD₅/COD ratio increased from 0.15 to 0.68 for an optimum of 120-min electrolysis duration indicating improvement of biodegradability of wastewater. The maximum anodic efficiency observed was 21.58 kg COD h⁻¹ A⁻¹ m⁻², and the minimum energy consumption observed was 0.084 kWh kg⁻¹ COD. The kinetic study results revealed that reaction rate (k) decreased from 0.011 to 0.0063 min⁻¹ with increase in pH from 3 to 9 while the k value increased from 0.0035 to 0.0102 min⁻¹ with increase in current density from 0.01 to 0.03 A cm⁻². This study showed that the COD reduction is more influenced by the current density. The linear and the nonlinear regression models reveal that the COD reduction is influenced by the applied current density.     

Electrochemical synthesis of 2,2-dinitropropanol

This paper describes electrochemical approaches to the synthesis of 2,2-dinitropropanol (DNPOH) and discusses the potential for pilot-plant scale synthesis. In this work, the anode of the electrochemical cell replaces the chemical oxidants used in the conventional synthesis for the purpose of reducing secondary waste and the consequent disposal cost. The electrosynthesis reactions described in this work use the common starting material, nitroethane (NE). The synthesis of the end-product DNPOH involves two steps: (1) electrochemical oxidative nitration (addition of a geminal NO₂ group); and, (2) condensation with formaldehyde. Electrochemical oxidation of NE was first attempted by direct oxidation on a Pt electrode surface resulting in low yield and significant generation of undesirable by-product. Alternatively, two different mediators were employed resulting in a dramatic improvement of yield for the oxidative nitration step. The two different mediators used, Ag⁺/Ag⁰ and Fe(CN)₆^−3/−4, were derived from the chemical oxidants known to perform the oxidative nitration. The Fe(CN)₆^−3/−4 mediator demonstrated the best promise for scale-up and industrial production due to the lower cost of the mediator and the solubility of the mediator lending it to greater ease-of-use in conventional electrochemical cell designs.     

Ion flotation of dichromate and of complexed cyanide: Surfactants for qualitative analysis

Ion flotation studies have shown that a surface-active agent is useful for qualitative analysis of complex ions in dilute aqueous solution, with the surfactant forming a particulate complex with the complex ion of concern. Experiments with a monovalent, cationic surfactant have established the prevalence of Cr₂O₇ ²⁻ (HCrO₄ ⁻) and not CrO₄ ²⁻; of [Fe(CN)₆]⁴⁻ and [Fe(CN)₅H₂O]³⁻; and of [FeFe(CN)₆]²⁻ and not [FeFe(CN)₆]⁻ or [Fe(CN)₆]³⁻. The results can be contrasted to those with ions that do not form particulate complexes with the surfactant, such as HPO₄ ²⁻ and phenolate; with the latter, no qualitative analytical information can be gained. Ion flotation appears to be a promising technique in general for the determination of ionic species present in aqueous solution; the surfactant must react with the ion of significance to form a particulate complex and the initial surfactant concentration must be controlled carefully.     

Surfactant-Mediated Catalytic Determination of Fe(II) in Herbal and Pharmaceutical Products

In the present work, the catalyzed oxidation of neutral red (NR) by bromate was used to work out a kinetic-based analytical method as an alternative technique for the determination of Fe(II) in real and synthetic samples. A use of a surfactant, N-dodecylpyridinium chloride enhanced the sensitivity of the reaction by becoming involved in the reaction mechanism and providing a more suitable reaction environment. The iron-catalyzed oxidation of NR with potassium bromate was studied kinetically by using a fixed time method. The reaction was followed by measuring the decrease in absorbance at 535 nm. The use of a surfactant in the analytical run showed a five times increase in the sensitivity of the method. It served as a ready reservoir of NR by increasing its solubilization. The salt effect, pH, and reagent concentration were also investigated to achieve a more selective and sensitive analytical procedure. Under optimized conditions (4.2 × 10⁻⁵ mol L⁻¹ NR, 1.4 × 10⁻³ mol L⁻¹ KBrO₃, 1.5 × 10⁻² mol L⁻¹ cationic surfactant, 0.5 mol L⁻¹ LiCl and pH 2.60 at 30 °C), iron(II) was determined in the range 0.1–0.5 μg mL⁻¹ with a detection limit of 0.019 μg mL⁻¹ and a relative standard deviation (n = 6) 1.02% for 0.2 μg mL⁻¹ Fe(II). The influence of foreign ions on the accuracy of the results was investigated. The developed method is extremely sensitive, selective and simple. The method was applied successfully to the determination of iron in the herbal pharmaceutical and synthetic samples. The results showed good agreement with those obtained by atomic absorption spectrophotometry.

The rapid electrochemical preparation of dissolved ferrate(VI): Effects of various operating parameters

The preparation of ferrate(VI) by the anodic dissolution of an iron wire gauze in concentrated NaOH solution is described. An anolyte of 0.35-0.48 M Na₂FeO₄ can be produced during 3-6 h electrolysis in initial 16 M NaOH solution at 35 °C. The experimental results indicate that the Fe(VI) concentration variation rate during electrolysis is close related to the factors such as current density, alkaline concentration, the ratio of effective surface area to anolyte volume, the passivation of iron anode and the decomposition of ferrate(VI), etc., and the relevant empirical equation is given. The alkalinity of anolyte has large effect on the electrogeneration of ferrate(VI), especially during an interval electrolysis.     

The O<Subscript>2</Subscript> + NH<Subscript>3</Subscript> Reaction Over Rh(110): Steady State Kinetics and Oscillatory Behavior

Abstract  

The kinetics of ammonia oxidation with oxygen over a Rh(110) surface were studied in the pressure range 10⁻⁵–10⁻⁴ mbar. Nitrogen was found to be the preferred product at low partial pressures ratios \textp_{\texto 2} :\textp_{\textNH 3} {\text{p}}_{{{\text{o}}_{ 2} }} :{\text{p}}_{{{\text{NH}}_{ 3} }} , while the NO pathway was favored with oxygen rich gas mixtures and at high temperature. The reactive sticking coefficient of O₂ reaches up to 0.05 under steady state conditions. Pronounced hysteresis effects in the reaction rates were found in T-cycling experiments. Sustained oscillations in the reaction rates occurred under isothermal conditions at T = 620 K at a total pressure of 4 × 10⁻⁵ mbar.     

Treatment and reuse of dyeing effluents by potassium ferrate

The use of potassium ferrate, K₂FeO₄, an environmentally-friendly chemical reagent containing iron in the + 6 oxidation state, has been investigated as a new approach for dyeing wastewater purification.The performance of this product, alone or in combination with a cationic organic polymer and/or power ultrasound, was compared to the traditional biological activated sludge process and a tertiary treatment featuring ozonation.Experimental tests showed that, thanks to its unique properties (high redox potential and simultaneous generation of ferric coagulating species), potassium ferrate can be successfully used in dyeing wastewater treatment. In fact, treatment with ferrate at the optimal dose of 70 mg/L as Fe(VI) was found to allow a high removal efficiency of relevant parameters such as turbidity, total suspended solids and chemical oxygen demand (COD).Whilst potassium ferrate alone had a minor effect on colour, the combination of ferrate with the organic polymer allowed a good decolourisation: this suggested the eventual application of this combined process for reuse of dyeing wastewater, resulting in environmental and economic benefits. The possibility of reusing the purified effluent in textile processes that do not require softened water was demonstrated through dyeing tests.     

Advances in electrochemical Fe(VI) synthesis and analysis

Hexavalent iron species (Fe(VI)) have been known for over a century, and have long-time been investigated as the oxidant for water purification, as the catalysts in organic synthesis and more recently as cathodic charge storage materials. Conventional Fe(VI) syntheses include solution phase oxidation (by hyphchlorite) of Fe(III), and the synthesis of less soluble super-irons by dissolution of FeO₄ ²⁻, and precipitation with alternate cations. This paper reviews a new electrochemical Fe(VI) synthesis route including both in situ and ex situ syntheses of Fe(VI) salts. The optimized electrolysis conditions for electrochemical Fe(VI) synthesis are summarized. Direct electrochemical synthesis of Fe(VI) compounds has several advantages of shorter synthesis time, simplicity, reduced costs (no chemical oxidant is required) and providing a possible pathway towards more electro-active and thermal stable Fe(VI) compounds. Fe(VI) analytical methodologies summarized in this paper are a range of electrochemical techniques. Fe(VI) compounds have been explored as energy storage cathode materials in both aqueous and non-aqueous phase in “super-iron” battery configurations. In this paper, electrochemical synthesis of reversible Fe(VI/III) thin film towards a rechargeable super-iron cathode is also summarized.     

Deep desulfurization of diesel oil oxidized by Fe (VI) systems

Fe (VI) compound, such as K₂FeO₄, is a powerful oxidizing agent. Its oxidative potential is higher than KMnO₄, O₃ and Cl₂. Oxidation activity of Fe (VI) compounds can be adjusted by modifying their structure and pH value of media. The reduction of Fe (VI), differing from Cr and Mn, results in a relatively non-toxic by-product Fe (III) compounds, which suggests that Fe (VI) compound is an environmentally friendly oxidant. Oxidation of model sulfur compound and diesel oil by K₂FeO₄ in water-phase, in organic acid and in the presence of phase-transfer catalysts is investigated, respectively. The results show that the activity of oxidation of benzothiophene (BT) and dibenzothiophene (DBT) is low in water-phase, even adding phase-transfer catalyst to the system, because K₂FeO₄ reacts rapidly with water to form brown Fe(OH)₃ to lose ability of oxidation of organic sulfur compounds. The activity of oxidation of the BT and DBT increases markedly in acetic acid. Moreover, the addition of the solid catalyst to the acetic acid medium promotes very remarkably oxidation of organic sulfur compounds. Conversions of the DBT and BT are 98.4% and 70.1%, respectively, under the condition of room temperature, atmospheric pressure, acetic acid/oil (v/v) = 1.0, K₂FeO₄/S (mol/mol) = 1.0 and catalyst/K₂FeO₄ (mol/mol) = 1.0. Under the same condition, diesel oil is oxidized, followed by furfural extraction, the results display sulfur removal rate is 96.7% and sulfur content in diesel oil reduces from 457 ppm to 15.1 ppm.     

Solubility of potassium ferrate in 12 M alkaline solutions between 20°C and 60°C

The solubility of potassium ferrate (K₂FeO₄) was measured in aqueous solutions of NaOH and KOH of total concentration 12 M containing various molar ratios of KOH:NaOH in the range 12:0 to 3:9. Several analytical methods were tested for the determination of ferrate concentration. The final method chosen consisted of potentiometric titration of the ferrate sample with an alkaline solution of As₂O₃. The assumption was made that ferrate dissociates in concentrated KOH solutions predominantly to KFeO₄⁻. The solubility constant, S, defined as the product of the molar concentration of the potassium ion, K⁺, and the ferrate anion, KFeO₄⁻, was found to be 0·044 ± 0·006 mol² dm⁻⁶ for 20°C, 0·093 ± 0·004 mol² dm⁻⁶ for 40°C and 0·15 ± 0·09 mol² dm⁻⁶ for 60°C. From these results the heat of dissolution of K₂FeO₄ was calculated as −14·3 kJ mol⁻¹. At 60°C the enhanced decomposition of the ferrate at the higher temperature led to a greater deviation in solubility values compared with data for either 20°C or 40°C.     

Heterogeneous photocatalytic reduction of Fe(VI) in UV-irradiated titania suspensions: effect of ammonia

Results of the heterogeneous photocatalytic reduction of Fe(VI) in UV-irradiated TiO₂ suspensions in the presence of ammonia are presented. The initial rate of Fe(VI) reduction, R, may be expressed as R = k _Fe(VI)[Fe(VI)]^1.25 where k _Fe(VI) = a[Ammonia]+b), a = 6.0 × 10³ μm ^0.25 s and b = 4.1 × 10⁶ μm ^−1.25s⁻¹. The rate constant, k _Fe(VI), increases with the ammonia concentration. The photocatalytic oxidation of ammonia is enhanced in the presence of Fe(VI). A mechanism involving Fe(V) as a reactive intermediate is presented which explains the faster photocatalytic oxidation of ammonia in the presence of Fe(VI).     

Adsorption behavior of hexavalent chromium on synthesized ethylenediamine modified starch

The starch-based material, ethylenediamine modified cross-linked starch (CAS) was synthesized and employed to remove hexavalent chromium from aqueous solution. Maximum adsorption of total chromium was observed at reaction pH of 4.0 and adsorption equilibrium achieved within 4h. The adsorption process can be described by pseudo-second-order adsorption model and the best-fit isotherm is Freudlich equation. The mechanism is predominately based on electrostatic attraction. The FT-IR spectra indicate that the amino groups of CAS are protonated and the hexavalent chromium ions were effectively adsorbed. Furthermore, studies on chromium release by using inorganic electrolytes confirm the mechanism observed by sorption experiments, which HCrO₄ ⁻ ion plays an important role interacting with CAS. Chromium release increases with increasing negative charge of electrolyte following the sequence PO₄ ³⁻ > SO₄ ²⁻ > B₄O₇ ²⁻ > NO₃ ⁻.     

Uptake and oxidation of malonaldehyde by cultured mammalian cells

Primary cultures of rat skin fibroblasts were used as a model system to investigate the cellular uptake and oxidation of malonaldehyde (MA). The cells were grown in a medium containing 10⁻⁵ M, 10⁻⁴ M or 10⁻³ M concentrations of [1,3-¹⁴C]MA. There was a limited, concentration-dependent uptake of MA by 24 hr (∼4% at all concentrations). The uptake of [1,2-¹⁴C]acetate by 24 hr was ∼24%; 83–89% of the¹⁴C in the MA taken up was oxidized to¹⁴CO₂ by 24 hr and ∼5% was recovered in the major lipids. Despite its low uptake and rapid oxidation to CO₂, pretreatment of the cells with 10⁻³ M MA for 24 hr produced a latent inhibition of [¹⁴C]glucose oxidation. Limited cellular uptake of MA may explain the tolerance of cells grown in culture to relatively high MA concentrations.