Comparative dissolution kinetics of biogenic and chemogenic uraninite under oxidizing conditions in the presence of carbonate

The long-term stability of biogenic uraninite with respect to oxidative dissolution is pivotal to the success of in situ bioreduction strategies for the subsurface remediation of uranium legacies. Batch and flow-through dissolution experiments were conducted along with spectroscopic analyses to compare biogenic uraninite nanoparticles obtained from Shewanella oneidensis MR-1 and chemogenic UO_2.00 with respect to their equilibrium solubility, dissolution mechanisms, and dissolution kinetics in water of varied oxygen and carbonate concentrations. Both materials exhibited a similar intrinsic solubility of ∼10⁻⁸ M under reducing conditions. The two materials had comparable dissolution rates under anoxic as well as oxidizing conditions, consistent with structural bulk homology of biogenic and stoichiometric uraninite. Carbonate reversibly promoted uraninite dissolution under both moderately oxidizing and reducing conditions, and the biogenic material yielded higher surface area-normalized dissolution rates than the chemogenic. This difference is in accordance with the higher proportion of U(V) detected on the biogenic uraninite surface by means of X-ray photoelectron spectroscopy. Reasonable sources of a stable U(V)-bearing intermediate phase are discussed. The observed increase of the dissolution rates can be explained by carbonate complexation of U(V) facilitating the detachment of U(V) from the uraninite surface. The fraction of surface-associated U(VI) increased with dissolved oxygen concentration. Simultaneously, X-ray absorption spectra showed conversion of the bulk from UO_2.0 to UO_2+x. In equilibrium with air, combined spectroscopic results support the formation of a near-surface layer of approximate composition UO_2.25 (U₄O₉) coated by an outer layer of U(VI). This result is in accordance with flow-through dissolution experiments that indicate control of the dissolution rate of surface-oxidized uraninite by the solubility of metaschoepite under the tested conditions. Although U(V) has been observed in electrochemical studies on the dissolution of spent nuclear fuel, this is the first investigation that demonstrates the formation of a stable U(V) intermediate phase on the surface of submicron-sized uraninite particles suspended in aqueous solutions.     

Comparative study of dissolution rate-determining mechanisms of limestone and dolomite

The dissolution rate-determining processes of carbonate rocks include: (1) heterogeneous reactions on rock surfaces; (2) mass transport of ions into solution from rock surfaces via diffusion; and (3) the conversion reaction of CO₂ into H⁺ and HCO ₃ ⁻ . Generally, it is the slowest of these three processes that limits the dissolution rate of carbonate rock. However, from experiment and theoretical analysis under similar conditions not only were the initial dissolution rates of dolomite lower by a factor of 3–60 than those of limestone, but also there are different dissolution rate-determining mechanisms between limestone and dolomite. For example, for limestone under the condition of CO₂ partial pressures dissolution rates increased significantly by a factor of about ten after addition of carbonic anhydrase (CA) into solution, which catalysed the conversation reaction of CO₂, whereas CA had little influence on dolomite dissolution. For dolomite, the increase of dissolution rate after addition of CA into solution appeared at Moreover, the enhancement factor of CA on dolomite dissolution rate was much lower (by a factor of about 3). In addition, when dissolution of both limestone and dolomite was determined by hydrodynamics (rotation speed or flow speed), especially under the dissolution of limestone was more sensitive to hydrodynamic change than that of dolomite. These findings are of significance in understanding the differences in karstification and relevant problems of resource and environment in dolomite and limestone areas.     

The mechanism of surface chemical kinetics of dissolution of minerals

This paper deals with the mechanism of dissolution reaction kinetics of minerals in aqueous solution based on the theory of surface chemistry.Surface chemical catalysis would lead to an obvous decrease in active energy of dissolution reaction of minerals.The dissolution rate of minerals is controlled by suface adsorption,surface exchange reaction and desorption,depending on pH of the solution and is directly proportional to δH^n0 ,When controlled by surface adsorption,i.e.,nθ=1,the dissolution rate will decrease with increasing pH;when controlled by surface exchane reaction,i.e.,nθ=0,the dissolution rate is independent of pH;when controlled by desorption,nθis a positive decimal between 0 and 1 in acidic solution and a negative decimal between-1 and 0 in alkaline solution.Dissolution of many minerals is controlled by surface adsorption and/or surface exchange reactions under acid conditions and by desorption under alkaline conditions.     

中低温成矿过程的锰镁碳酸盐矿物化学动力学与矿物成因

文章使用开放流动反应装置研究了中低温热液矿床中常见脉石矿物碳酸盐(白云石、菱锰矿)在25℃～250℃水中化学动力学溶解反应速率。在25℃条件下,碳酸盐矿物的溶解速率r(Ca)>r(Mn)>r(Mg)。碳酸盐矿物在水中200℃时具有最大溶解速率。随着温度的继续升高,Mg、Mn碳酸盐的溶解反应速率会下降,容易形成白云石和锰碳酸盐。实验研究结果有助于理解中低温环境下,金属矿石的主要伴生碳酸盐矿物与水反应的化学动力学限制,试图说明中低温矿石伴随白云石和锰碳酸盐的沉淀机理。     

Siderophore-promoted dissolution of cobalt from hydroxide minerals

Recent research has revealed that siderophores, a class of biogenic ligands with high affinities for Fe(III), can also strongly complex Co(III), an element essential to the normal metabolic function of microbes and animals. This study was conducted to quantify the rates and identify the products and mechanisms of the siderophore-promoted dissolution of Co from synthetic Co-bearing minerals. The dissolution reactions of heterogenite (CoOOH) and four Co-substituted goethites (Co-FeOOH) containing different Co concentrations were investigated in the presence of a trihydroxamate siderophore, desferrioxamine B (DFOB), using batch and flow-through experiments. Results showed that DFOB-promoted dissolution of Co from Co-bearing minerals may occur via pH-dependent ligand-promoted or reductive dissolution mechanisms. For heterogenite, ligand-promoted dissolution was the dominant pathway at neutral to alkaline pH, while production of dissolved Co(II) for pH <6. It was not possible from our data to decouple the separate contributions of homogenous and heterogeneous reduction reactions to the aqueous Co(II) pool. Cobalt substitution in Co-substituted goethite, possibly caused by distortion of goethite structure and increased lattice strain, resulted in enhanced total dissolution rates of both Co and Fe. The DFOB-promoted dissolution rates of Co-bearing minerals, coupled with the high affinity of Co(III) for DFOB, suggest that siderophores may be effective for increasing Co solubility, and thus possibly Co bioavailability. The results also suggest that siderophores may contribute to the mobilization of radioactive ⁶⁰Co from Co-bearing mineral phases through mineral weathering and dissolution processes.     

Dissolution kinetics and the weathering of mafic minerals

Fayalite, hypersthene, basalt, and obsidian were dissolved in buffered solutions (25°C; pH 4.5 and 5.5) under air, N₂ or O₂ atmospheres, in order to follow the kinetics of dissolution. Each dissolved more rapidly at lower pH values, dissolving most rapidly in the initial few days, followed by slower dissolution for periods up to six months. Dissolution was more rapid when air was excluded. In oxygen atmospheres an Fe(OH)₃ precipitate armors mineral surfaces, thus inhibiting further dissolution, and further affects the solution by scavenging dissolved silica and cations. Dissolution reactions include initial exchange between cations and H⁺, incongruent dissolution of silicate structures, oxidation of Fe²⁺ in solution, precipitation of Fe(OH)₃, and scavenging of dissolved silica and cations by Fe(OH)₃. Dissolution kinetics may explain weathering of mafic rocks and minerals at the Earth's surface, the formation of Fe-oxide coatings on mineral grains, weathering of submarine mafic rocks and intrastratal solution of mafic minerals in buried sandstones. Early Precambrian weathering would have been more rapid before the appearance of large amounts of oxygen in the atmosphere, and continental denudation rates may have been higher than at present because of this effect and the predominance of mafic igneous rocks at an early stage of continent formation and growth.     

The effects of organic acids on the dissolution of silicate minerals: A case study of oxalate catalysis of kaolinite dissolution

Most studies agree that the dissolution rate of aluminosilicates in the presence of oxalic and other simple carboxylic acids is faster than the rate with non-organic acid under the same pH. However, the mechanisms by which organic ligands enhance the dissolution of minerals are in debate. The main goal of this paper was to study the mechanism that controls the dissolution rate of kaolinite in the presence of oxalate under far from equilibrium conditions (−29 < ΔG_r < −18 kcal mol⁻¹). Two types of experiments were performed: non-stirred flow-through dissolution experiments and batch type adsorption isotherms. All the experiments were conducted at pH 2.5-3.5 in a thermostatic water-bath held at a constant temperature of 25.0, 50.0 or 70.0 ± 0.1 °C. Kaolinite dissolution rates were obtained based on the release of silicon and aluminum at steady state. The results show good agreement between these two estimates of kaolinite dissolution rate. At constant temperature, there is a general trend of increase in the overall dissolution rate as a function of the total concentration of oxalate in solution. The overall kaolinite dissolution rates in the presence of oxalate was up to 30 times faster than the dissolution rate of kaolinite at the same temperature and pH without oxalate as was observed in our previous study. Therefore, these rate differences are related to differences in oxalate and aluminum concentrations. Within the experimental variability, the oxalate adsorption at 25, 50, and 70 °C showed the same dependence on the sum of the activities of oxalate and bioxalate in solution. The change of oxalate concentration on the kaolinite surface (C_s,ox) as a function of the sum of the activities of the oxalate and bioxalate in solution may be described by the general adsorption isotherm:

     

碳酸盐矿物阴极发光性的控制因素分析

碳酸盐矿物的阴极发光性主要受其Mn²⁺和Fe²⁺含量,以及Mn²⁺/Fe²⁺值的综合控制,但Mn²⁺/Fe²⁺值的控制作用强一些。研究中充填裂缝的方解石胶结物样品主要采自北京西山寒武系和奥陶系的碳酸盐岩地层中。此外,还从北京石花洞中采集了现代溶洞的石笋样品。样品的阴极发光强度可分为不发光、暗、中等和亮,阴极发光颜色也相应地分为不发光、橙红色、橙黄色和亮黄色。通过对野外露头样品进行电子探针研究,测定其中的Mn²⁺和Fe²⁺的含量,并结合样品的阴极发光颜色和强度,得出Mn²⁺含量要在0.01%以上,Fe²⁺含量至少在0.8%以下,Mn²⁺/Fe²⁺在0.05以上,方解石胶结物才能发光。经研究发现,充填裂缝中方解石胶结物的阴极发光性与Mn²⁺和Fe²⁺含量,以及Mn²⁺/Fe²⁺值之间存在如下关系:不含Mn²⁺,Fe²⁺微量,Mn²⁺/Fe²⁺为0时,不发光;0.052+/Fe²⁺<0.2时,发光强度暗,发光颜色为橙红色;0.22+/Fe²⁺<2时,发光强度中等,发光颜色为橙黄色;Mn²⁺/Fe²⁺>2时,发光强度亮,发光颜色为亮黄色。     

The kinetics of chlorite dissolution

A model for the dissolution of chlorite has been developed based on fast ligand assisted proton attack of the alumina tetrahedra within the alumina-silica lattice followed by slower dissolution of the remnant silica lattice. While the rate determining step is within the silica dissolution regime, the rate is a function of the H⁺ and Al³⁺ concentrations and the dominant aqueous Al species. Individual rates may be described by a generic rate equation applicable across the spectrum of Al species:

     

Experimental study of the effect of pH and temperature on the kinetics of montmorillonite dissolution

The effect of pH on the kinetics of smectite (K-montmorillonite) dissolution was investigated at 50 and 70 °C in stirred flow-through reactors over the pH range of 1-13.5. Experiments done at very acidic and very basic pH were far from equilibrium. Near neutral pH experiments were closer to equilibrium. The Al/Si release ratio, while initially being incongruent, ultimately approached the stoichiometric value in most of the experiments. Temperature, extreme pH, and time favor congruency. Rates can be described by:

     

碳酸钙矿物表面润湿性的控制机理

系统论述了碳酸钙矿物浮选中常用的捕收剂和抑制剂及其相关作用机理。这些药剂通过包括静电作用、氢键、半胶束吸附和化学吸附等多种机理,吸附在碳酸钙矿物表面,从而控制矿物表面的润湿性。捕收剂的功能在于增强矿物表面的疏水性,抑制剂的功能在于增强矿物表面的亲水性。选择合适的捕收剂和抑制剂复配方案对高效浮选分离至关重要。     

碳酸盐矿物的阴极发光性与微量元素的关系

碳酸盐矿物的阴极发光特征与其成分有关。笔者用阴极发光与电子探针微区分析法对砂岩中碳酸盐矿物进行测试分析,其结果表明碳酸盐矿物的阴极发光与微量元素含量有如下规律:①碳酸盐矿物在铁含量高于猝灭下限或锰含量低于激活下限时,不具有阴极发光性;②铁的猝灭下限约为0.04 mol,锰的激活下限为小于7×10^-5mol;③铁/锰比值越高,越不利于碳酸盐矿物阴极发光;但是铁/锰比值小于1的碳酸盐一定具有阴极发光;④铁或锰二者之一含量很少时不利于碳酸盐矿物的阴极发光。     

Bank-derived carbonate sediment transport and dissolution in the Hawaiian Archipelago

This investigation used two approaches to examine the flux of bank-derived carbonate particles and determine the potential influence of benthic carbonate particle dissolution on the carbon chemistry of the waters around the Hawaiian Archipelago. First, the particle flux near several representative carbonate banks in the Hawaiian Archipelago was measured and compared with the flux at a distal site (ALOHA) approximately 100 km north of Oahu, Hawaii. The results of four sediment trap deployments on three carbonate banks in the Hawaiian Archipelago demonstrate that the flux of bank-derived carbonate particles are consistently one to two orders of magnitude higher than the fluxes at the distal station. Furthermore, the mineralogy of the carbonate flux near the banks, which includes very soluble bank-derived aragonite and magnesian calcite particles, is distinctly different from that of the distal fluxes. Second, the chemistry of the waters at each bank station along the archipelago was characterized and compared with the chemistry of the distal waters to determine if differences in the particle flux were reflected in the carbon chemistry. Higher alkalinity and carbonate ion concentrations were observed around all of the banks studied. The saturation state of these waters suggests that the dissolution of some magnesian calcite and aragonite phases could explain the higher alkalinity values. Calculations suggest that the dissolution of benthically-derived aragonite and magnesian calcite may be an important component of the North Pacific alkalinity budget and a potential sink for anthropogenic CO₂.     

A database of dissolution and precipitation rates for clay-rocks minerals

Many geoscientific fields use reactive transport codes to set up and interpret experiments as well as to understand natural processes. Reactive transport codes are also useful to give insights in the long term evolution of systems such as radioactive waste repositories or CO₂ storage sites, for which experiments cannot reach the targeted time scale nor the dimension of those systems. The consideration of kinetic reaction rates is often required to reproduce correctly the geochemical and transport processes of interest. However, kinetic data are scattered in the literature, making data and selection a tedious task. Kinetic parameters on a single system are also highly variable depending on data choice, interpretation and chosen kinetic modelling approaches, thus making inter-comparison of modelling studies difficult. The present work aims at proposing a compilation of kinetic parameters to overcome part of above cited problems. The proposed selection was done (i) to ensure consistency of data selection criteria and data treatment and (ii) to ease the use of common kinetic parameters that are independent of the chosen geochemical modelling code. For those two reasons, the kinetic formalism of the transition state theory (TST) was chosen. The selection of minerals is currently limited to those present in clay rich rocks and cements, reflecting the effort made at predicting the evolution of radioactive waste underground storage systems. Still, the proposed compilation should also be useful for other applications such as CO₂ sequestration.     

Magnesium inhibition of calcite dissolution kinetics

We present evidence of inhibition of calcite dissolution by dissolved magnesium through direct observations of the (104) surface using atomic force microscopy (AFM) and vertical scanning interferometry (VSI). Far from equilibrium, the pattern of magnesium inhibition is dependent on solution composition and specific to surface step geometry. In CO₂-free solutions (pH 8.8), dissolved magnesium brings about little inhibition even at concentrations of 0.8 × 10⁻³ molal. At the same pH, magnesium concentrations of less than 0.05 × 10⁻³ molal in carbonate-buffered solutions generate significant inhibition, although no changes in surface and etch pit morphology are observed. As concentrations exceed magnesite saturation, the dissolution rate shows little additional decrease; however, selective pinning of step edges results in unique etch pit profiles, seen in both AFM and VSI datasets. Despite the decreases in step velocity, magnesium addition in carbonated solutions also appears to activate the surface by increasing the nucleation rate of new defects. These relationships suggest that the modest depression of the bulk rate measured by VSI reflects a balance between competing reaction mechanisms that simultaneously depress the rate through selective inhibition of step movement, but also enhance reactivity on terraces by lowering the energy barrier to new etch pit formation.     

Some kinetics of bronzite orthopyroxene dissolution

The kinetics of bronzite orthopyroxene dissolution were investigated in HClKCl solutions having a total concentration of 0.1 M, over the pH range 1–4.5, at temperatures between 42 and 1°C. Dissolution of the pyroxene was incongruent and followed a parabolic rate law. The activation energy of the reaction is 10.5 ± 0.6 kcal mole^?1. The rate dependence on hydrogen ion activity is one-half order. The rate of dissolution is unaffected by substitution of sodium for potassium, or sulfate or nitrate for chloride anion, or by addition of citrate or acetate ions. However, traces of fluoride increase the dissolution rate. The rates observed are one to two orders of magnitude slower than those previously reported by Luceet al. (1972) for dissolution of enstatite.     

On the neutralization of acid rock drainage by carbonate and silicate minerals

The net result of acid-generating and-neutralizing reactions within mining wastes is termed acid rock drainage (ARD). The oxidation of sulfide minerals is the major contributor to acid generation. Dissolution and alteration of various minerals can contribute to the neutralization of acid. Definitions of alkalinity, acidity, and buffer capacity are reviewed, and a detailed discussion of the dissolution and neutralizing capacity of carbonate and silicate minerals related to equilibium conditions, dissolution mechanism, and kinetics is provided. Factors that determine neutralization rate by carbonate and silicate minerals include: pH, P_{CO 2}, equilibrium conditions, temperature, mineral composition and structure, redox conditions, and the presence of foreign ions. Similar factors affect sulfide oxidation. Comparison of rates shows sulfides react fastest, followed by carbonates and silicates. The differences in the reaction mechanisms and kinetics of neutralization have important implications in the prediction, control, and regulation of ARD. Current static and kinetic prediction methods upon which mine permitting, ARD control, and mine closure plans are based do not consider sample mineralogy or the kinetics of the acid-generating and-neutralizing reactions. Erroneous test interpretations and predictions can result. The importance of considering mineralogy for site-specific interpretation is highlighted. Uncertainty in prediction leads to difficulties for the mine operator in developing satisfactory and cost-effective control and remediation measures. Thus, the application of regulations and guidelines for waste management planning need to beflexible.     

碳酸盐岩溶蚀模拟实验技术进展及应用

碳酸盐岩溶蚀作用是指流动的侵蚀性流体与碳酸盐岩之间相互作用的过程及由此产生的结果,从地表到深埋藏地层中均可发生。碳酸盐岩溶蚀模拟实验是指通过模拟地层环境来再现碳酸盐岩溶蚀作用的过程和结果,是研究碳酸盐岩储层规模溶蚀有利条件和分布规律的重要方法。中国石油集团碳酸盐岩储层重点实验室自主研发高温高压溶解动力学模拟装置,最终建成由岩石内部溶蚀、岩石表面溶蚀和高温高压原位可视化检测组成的碳酸盐岩溶蚀模拟实验技术。利用高温高压溶解动力学模拟实验装置,开展了碳酸盐岩埋藏溶蚀温度窗口和孔隙演化样式的实验研究,取得2个方面的认识:①高盐度流体背景模拟实验表明,随着温度增加,碳酸盐岩的溶蚀量具有缓慢下降—缓慢上升—快速下降的特征,由于地层水两种相反离子效应的作用,在80～110℃范围内存在一个有利于碳酸盐岩溶蚀的温度窗口;②通过粒间孔隙型、晶间孔隙型、溶蚀孔洞型、鲕模孔隙型和格架孔隙型5种碳酸盐岩溶蚀模拟的对比实验,认识到连通孔隙是埋藏溶蚀发生的先决条件和有利区域,碳酸盐岩内部组构差异会进一步加剧储集空间在孔、洞和缝组合上的复杂性。     

Dissolution kinetics of experimentally shocked silicate minerals

The effect of lattice strain on mineral dissolution rates was examined by comparing the dissolution rates of shocked and unshocked minerals. Labradorite, oligoclase and hornblende were explosively shocked at mean pressures ranging from 4 to 22 GPa. The labradorite was examined with transmission electron microscopy to estimate the density of dislocations produced by the shock-loading experiment. Subsamples of the labradorite were then thermally annealed to remove some of the dislocations, and to evaluate the importance of such thermal pre-treatment in preparing mineral surfaces for experiments. The dissolution rates of these minerals were measured in batch experiments at pH-values of 2.7 and 4.0.

Shock-loading did not produce extremely high dislocation densities in the labradorite. The density of dislocations in the unshocked labradorite is ≤ 10¹⁰ m⁻². After shocking, the density increases to 10¹²-10¹³ m⁻². The distribution of dislocations is heterogeneous, and the amount of deformation does not increase substantially with shock pressure. These results are highly atypical of shock-modified minerals, where relatively low shock pressures usually induce high ( 10¹⁵ m⁻²) densities of dislocations. Thermal annealing for 1 hr. at 900°C in a dry furnace removes many dislocations from the shocked labradorite.

The difference in observed dissolution rates between shocked and unshocked minerals appears to have a weak correlation with the increase in the density of dislocations on the mineral surface. The unshocked and shocked oligoclase and hornblende samples exhibit limited dissolution enhancement at pH 4.0. Increasing the density of dislocations by several orders of magnitude with shock-loading causes a relatively small increase in dissolution rates for these silicate minerals. These results suggest that the surface dislocations produced by the shock treatment are not the primary sites for dissolution reactions.