Uranium ore processing: Kinetics of the growth and dissolution of a potential host solid from a model surface

As part of an investigation concerning the fate of²²⁶Ra during uranium ore milling and long-term tailings storage, we have investigated the growth of potential host solids-barium sulfate and lead sulfate-on mica, a model surface. The kinetics of dissolution of barium sulfate-and of a barium sulfate solid containing²²⁸Ra as a trace component into 10^–3M sulfuric acid were subsequently investigated using a flow-cell and radiotracer techniques. It was found that ther dissolution of both the barium sulfate host and the²²⁸Ra trace component was congruent. The dissolution reaction appears to be surface controlled.     

A kinetic model for the ultrasound catalyzed hydrolysis of solventless TEOS-water mixtures and the role of the initial additions of ethanol

The acid and ultrasound catalyzed hydrolysis of solventless TEOS-water mixtures are studied, as a function of the initial additions of ethanol to the mixtures, by means of flux calorimetry measurements. A device was specially designed for this purpose. Under acid conditions, our proposed method has been able to resolve hydrolysis from other condensation reactions, by detecting the exothermal hydrolysis reaction heat. The process has been explained by a dissolution and reaction mechanism. Ultrasound forces the dissolution process to start the reaction. The alcohol produced in the reaction helps the dissolution process to further enhance the hydrolysis. Initial amounts of pure ethanol added to the mixtures shorten the start time of the reaction, due to an additional effect of dissolution, and diminish the reaction rate, as a result of the solvent dilution effect. Our dissolution and reaction mechanism modeling describes the main points arising from the experimental data and yields k _H =0.24 M^-1 min^-1 for the second-order hydrolysis rate constant at 39°C.     

Dissolution kinetics of copper(II) oxide in water saturated by sulphur dioxide

The dissolution and the kinetics of dissolution of cooper(II) oxide in water saturated by sulphur dioxide has been studied. In the experiments, the particle size, the flow rate of the gas, the solid to liquid ratio, and the reaction temperature have been chosen as parameters, while the stirring rate was held constant. As a result of present experiments, it was observed that the decrease of the particle size, the solid to liquid ratio, and an increase of the reaction temperature increased the dissolution rate. It was also observed that the flow rate of sulphur dioxide in the range of its flow rate values did not affect the dissolution rate. The reaction kinetics of copper(II) oxide according to the heterogeneous reaction models was examined and it was found that the dissolution rate was controlled by chemical reaction. The calculated activation energy is 66.50 kJmol^?1. © 1994 John Wiley & Sons, Inc.     

The leaching kinetics of brannerite ore in sulfate solutions with iron(III)

A kinetic study of the leaching of powdered brannerite ore by sulfuric acid has been investigated. The effects of stirring speed ranging from 100 to 1,000?min^?1, particle size ranging from 20 to 120???m, concentration of Fe(III) ranging from 0.0025 to 0.20?M, acid concentration ranging from 0.1 to 2.0?M and temperature ranging from 15 to 90?°C on uranium dissolution are reported. The dissolution rate was founded to be significantly influenced by the temperature and concentration of the acid in solution. The experimental data for the dissolution rates of uranium have been analyzed with the shrinking-core model for reaction control. The apparent activation energy for the dissolution of uranium has been evaluated using the Arrhenius expression.     

Electrochemical reactions at a copper electrode in copper-conducting solid electrolytes

Potentiostatic measurements are used to show that, depending on the overvoltage sign, either electrochemical deposition or dissolution of copper occurs at the Cu/Cu₄RbCl₃I₂ interface at overvoltages η > 8–10 mV. At η = 10–100 mV, the reaction rate is limited by the formation and expansion of dissolution centers at the copper surface during anodic polarization and crystallization centers, during cathodic polarization. At η > 120 mV, the reaction rate is limited by charge transfer; the exchange current density is 2.7 mA cm^?2 and the anodic transfer coefficient is ～0.45. Under anodic polarization, formation of electron holes in the electrolyte occurs in parallel with the copper anodic dissolution. Therefore, nonstoichiometry of the electrolyte emerges in the near-electrode layer and divalent copper accumulates there.     

铬(III)离子催化铈(IV)离子氧化四氢糠醇的反应动力学及机理

在酸性介质中用氧化还原滴定法研究了铈(IV)离子在铬(III)离子催化作用下, 于25～40 ℃区间氧化四氢糠醇的反应动力学. 结果表明反应对铈(IV)和四氢糠醇均为一级. 准一级速率常数k_obs随催化剂[Cr(III)]增加而增大, 亦随[H^＋]增加而增大, 而随增加而减小. 在氮气保护下, 反应不能引发丙烯酰胺聚合, 说明在反应中没有自由基产生. 提出了催化剂、底物和氧化剂间生成双核加合物的反应机理. 通过k_obs与的依赖关系, 并结合Ce(IV)在溶液中的平衡, 找到了本反应体系的动力学活性物种是Ce(SO₄)₂. 还计算出一些速率常数及相应的活化参数.     

Studies on the kinetics of UO2 dissolution in carbonate-bicarbonate medium using sodium hypochlorite as oxidant

The dissolution of UO₂ in carbonate-bicarbonate solutions containing sodium hypochlorite as an oxidant has been investigated. The effect of temperature, sodium hypochlorite concentration and stirring speed was examined. In the temperature range of 303 to 318 K, the leaching reaction displayed linear kinetics. Apparent activation energy obtained from the differential approach was found to be 57 kJ mol^?1. This relatively high activation energy value indicates a chemically controlled behavior of UO₂ dissolution. The order of reaction with respect to sodium hypochlorite concentration was found to be unity.     

Corrosion mechanism of pure aluminium in aqueous alkaline solution

The corrosion of pure aluminium in alkaline solution has been explored using an open circuit potential transient, potentiodynamic polarization experiment and a.c. impedance spectroscopy. The steady-state value of the open circuit potential (E _ocp ^ss ) of pure aluminium in alkaline solution was observed to decrease with increasing rotation rate of the specimen, which is ascribed to the enhanced anodic reaction. The extent of anodic polarization for the aluminium dissolution reaction on pure aluminium at E _ocp ^ss was found to be greater than that of cathodic polarization for the water reduction reaction. This indicates that the rate of corrosion of pure aluminium is mainly determined by the anodic reaction in alkaline solution. Based upon the experimental results, a corrosion mechanism for pure aluminium has been proposed in the presence of the native surface oxide film in alkaline solution, involving consecutive oxide film formation and dissolution, and simultaneous water reduction.     

Studies on the kinetics of UO2 dissolution in carbonate-bicarbonate medium using sodium hypochlorite as oxidant

The dissolution of UO₂ in carbonate-bicarbonate solutions containing sodium hypochlorite as an oxidant has been investigated. The effect of temperature, sodium hypochlorite concentration and stirring speed was examined. In the temperature range of 303 to 318 K, the leaching reaction displayed linear kinetics. Apparent activation energy obtained from the differential approach was found to be 57 kJ mol^–1. This relatively high activation energy value indicates a chemically controlled behavior of UO₂ dissolution. The order of reaction with respect to sodium hypochlorite concentration was found to be unity.     

Transpassive dissolution mechanism of ferrous alloys in phosphoric acid/acetic acid mixtures

The transpassive dissolution of Fe-12%Cr and Fe-25%Cr alloys in near-anhydrous phosphoric acid/acetic acid mixtures has been studied by conventional voltammetry and electrochemical impedance spectroscopy. Both steady-state and transient techniques point to two parallel pathways for the process involving oxidative dissolution of Cr as Cr(VI) and isovalent dissolution of Fe most probably mediated by an electrolyte-originating species. A simplified kinetic model of the process including only surface kinetic steps has been found to reproduce successfully both the steady-state and the small-amplitude AC response of the studied materials. The kinetic parameters of the model are determined and their relevance regarding the influence of the alloy and electrolyte composition on the relative importance of the two parallel pathways is discussed. The experimental data and model calculations indicate that the effect of acetic acid on the reaction steps related to dissolution of Fe is more significant.     

Site of protonation in the reaction of gaseous Brønsted acids with halobenzene derivatives

The site of protonation in the reaction of gaseous Brønsted acids with chlorobenzene and fluorobenzene derivatives has been examined using deuterium labelling of chlorobenzene and deuterated reagent gases (D₂ and N₂/D₂). In the protonation of chlorobenzene by [H₃]⁺ ～30% of the [MH? HCl]⁺ fragment ions arise from a chlorine-protonated species while ～70% arise from a ring-protonated species in which complete hydrogen scrambling has occurred. In the reaction of [N₂H]⁺ with chlorobenzene ～75% of the fragment ions arise from the chlorine-protonated form with ～25% arising from the ring-protonated form of [MH]⁺. By contrast fluorobenzene fragments almost entirely from the fluorine-protonated form. Similar results are obtained for dihalobenzenes. The mechanistic implications are discussed.     

Mechanistic aspects of ligand substitution on [(H2O)(tap)2RuORu(tap)2(H2O)]2+ ion {tap = 2‐(m‐tolylazo)pyridine} by some amino acids in aqueous medium at physiological pH

The interaction of the title complex with selected amino acids such as glycine (L¹H), l ‐valine (L²H), and l ‐leucine (L³H) has been studied spectrophotometrically in aqueous medium as a function of [substrate complex], [ligand], and temperature. The reaction has been monitored at 600 nm, where the spectral difference between the reactant and product is maximum. At pH 7.4, the reaction has been found to proceed via two distinct consecutive steps, i.e., it shows a nonlinear dependence on the concentration of ligands: The first process is [ligand] dependent, but the second step is [ligand] independent. The rate constants for the processes are k₁～10^?3 s^?1 and k₂～10^?4 s ^?1. The activation parameters were calculated from Eyring plots. Based on the kinetic and activation parameters, an associative interchange mechanism is proposed for the interaction processes. From the temperature dependence of the outer sphere association equilibrium constant, the thermodynamic parameters were also calculated, which gives a negative Δ G^o value for both the steps at all temperatures studied, supporting the spontaneous formation of an outer sphere association complex. The product of the reaction has been characterized with the help of conductance measurement, IR, NMR, and ESI‐mass spectroscopic analysis. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 612–623, 2012     

Multiple reaction scheme modelling

A simple procedure whereby the energies of activation and pre-exponential factors of the individual reactions, of any kinetic model, comprising a wide variety of multiple reaction sets, is described. The sets belong to one of three classes, such that the single extent of reaction—temperature curves diverge, converge or are equally spaced at one prescribed temperature. The method has been applied to sets of first order reactions, which yield effectiven ^th order reactions with the order ‘n’varying from ～0.6 to ～ 3.0. Variations in Friedman iso-conversional reaction kinetics analytical data are indicative of the designed reaction complexity.     

Anodic passivation of tin in buffered phosphate electrolyte

The anodic behaviour of tin in buffered phosphate electrolyte (pH=3.1) has been studied by a variety of techniques. A number of anodic processes occur depending on potential and the conditions at the electrode/electrolyte interphase. On anodic polarisation the electrode, which is probably filmed with a phosphate layer, initially undergoes dissolution to form probably Sn(H₂PO₄·HPO₄)^? species. Impedance data indicate that this process has a corresponding Tafel slope of ～0.046 V/decade. At more positive potentials three consecutive passivating processes occur.The primary passivating process involves the blocking of the electrode by Sn₃(PO₄)₂ by a dissolution-precipitation mechanism. The formation of SnO by a slow three dimensional nucleation and growth process constitutes the second. It is formed as a result of the attainment of alkaline conditions at the electrode surface. There is also a parallel reaction path involving the formation of soluble Sn(II) species. The tertiary process consists of the oxidation of Sn to Sn(IV) species. Passivation occurs via a dissolution-precipitation mechanism when the electrode is blocked by SnO₂. The relative quantities of SnO and SnO₂ produced is a function of operating conditions.     

N-{4-{N-甲基-N-[2-羟基-3-(2,4-二氧代-1,2,3,4-四氢嘧啶-5-基)氨基]丙基}氨基-3-溴}苯甲酰基-L-谷氨酸二乙酯及其衍生物的合成

报道了N-{4-{N-甲基-N-[2-羟基-3-(2,4-二氧代-1,2,3,4-四氢嘧啶-5-基)氨基]丙基}氨基-3-溴}苯甲酰基-L-谷氨酸二乙酯及其衍生物的简便合成方法. 分别以4-氨基苯甲酸乙酯和4-氨基苯甲酰基-L-谷氨酸二乙酯为起始物, 经甲基化、烯丙基化、溴羟基化、环氧化、开环、脱保护等反应首次合成了6个新型5-取代氨基嘧啶类化合物, 并通过¹H NMR, ¹³C NMR 和MS对其化学结构进行了表征. 初步生物活性结果表明, 苯环侧链的L-谷氨酸酯部分是此类化合物抑制人重组二氢叶酸还原酶的必需结构.     

Kinetic calculations of the Ni anodic dissolution from EIS

Anodic nickel dissolution in acid media has been analyzed by means of electrochemical quartz crystal microbalance and electrochemical impedance spectroscopy techniques. The experimental impedance spectra have been fitted to an equivalent circuit which is related to a mechanism of two consecutive electron transfers followed by a Ni²⁺ mass transfer step. That way, an estimation for values of rate constants and surface concentrations of the Ni(0), Ni(I) and Ni(II) species has been obtained.     

Oxygen and nitrogen in coal by instrumental neutron activation analysis

The feasibility of using fast neutron (14 MeV) activation analysis techniques for the determination of oxygen and nitrogen in coal has been investigated. Conditions that favor instrumental neutron activation analysis (INAA) include the absence of problems associated with sample dissolution and the capability of extremely rapid analyses as compared to older techniques such as the Kjeldahl method for nitrogen. Most previous oxygen determinations have been by difference after major component analyses. In the present study, oxygen was determined in sized coal and its low temperature ash (LTA) with the difference representing the organic oxygen content. Both the oxygen and nitrogen analyses employ a multiscaling technique with the former based on the¹⁶O(n, p)¹⁶N reaction, while the latter utilizes the annihilation radiation produced by the product of the¹⁴N(n, 2n)¹³N reaction. The high-energy gamma-radiation associated with the decay of¹⁶N was essentially free of spectral interferences for coal analysis, although fluorine could cause a primary interference if the F/O ratio exceeds 0.02. In the nitrogen work, experiments were performed to determine correction factors to account for the effects of the¹²C(p, γ)¹³N and¹³C(p, n)¹³N “knock-on” reactions and the³⁹K(n, 2n)³⁸K reaction which produce interfering β⁺ emitting radionuclides. Data are presented for oxygen in Western Kentucky No. 9 and No. 11 coal and coal ash and for nitrogen in eleven different coals.     

The effect of acetohydroxamic acid on stainless steel corrosion in nitric acid

We present the first study of the effect of acetohydroxamic acid (AHA) on the corrosion behaviour of stainless steels. Particularly, studies have been performed using steels and physico-chemical conditions equivalent to those proposed for use in advanced nuclear reprocessing platforms. In these, AHA has been shown to have little effect on either steel passivation or reductive dissolution of both SS304L and SS316L. However, under transpassive dissolution conditions, AHA while in part electrochemically oxidised to acetic acid and nitroxyl/hydroxylamine, also complexes with Fe^3 +, inhibiting secondary passivation and driving transpassive dissolution of both steels.     

A Study on the Dissolution of Ferric Oxide in Aqueous Hydrochloric Acid Solutions

The effect of hydrochloric acid concentration and the effect of temperature on the rate of dissolution of Fe₂O³ have been investigated. Being heterogeneous, the dissolution reaction has been characterized to have an apparent activation energy of 21.5 Kcal/mol for a 7.11 m aqueous hydrochloric acid solution at temperatures between 273 and 373°K. The rate is linearly correlated to the mean activity of hydrochloric acid in the range from 1.3 to 10.8 m at 300°K.     

The kinetics of chlorine evolution and reduction on glassy carbon in molten tetrachloroaluminate at 175°C

The chlorine electrode reaction on glassy carbon in sodium tetrachloroaluminate melt (AlCl₃+NaCl) with near equimolar compositions was investigated at 175°C with voltammetric techniques. The kinetic parameters (Tafel slope and exchange current density) measured as functions of chloride ion activity and partial pressure of chlorine, and the reaction orders with respect to Cl^? and Cl₂ have been collected extensively, being compared with the theoretical kinetic derivatives deduced from the rate equations solved under three different kinds of adsorption isotherms: Langmuir, non-activated Temkin and activated Temkin isotherms. All the evidence collected in this study indicates that the reaction mechanism for both evolution and dissolution of chlorine consists of a fast electron transfer (Cl^?→Cl_ad+e) followed (or preceded) by a slow Heyrovsky-type reaction (Cl^?+Cl_ad→Cl₂+e) on glassy carbon surfaces where the adsorbed intermediate obeys the activated Temkin isotherm. The exchange current density was found as 8.6±0.8 μA cm^?2 at 175°C in the melt of pCl=1.1 under an atmospheric pressure of Cl₂, and its electrode potential (E°_CΓ/Cl2) was determined as 2.182±0.005 V vs. Al.