Activation of zinc sulphide with CuII,CdII and PbII: I. Activation in weakly acidic media

During the activation of zinc sulphide with the heavy metal ions Cu^II, Cd^II and Pb^II, metal-ion uptake follows a logarithmic dependence on time. A reaction mechanism is proposed which is consistent with experimental observations. Incident radiation has a strong influence on the activation reaction, an effect which is interpreted in terms of semiconductor theory.     

Activation of zinc sulphide with CuII,CdII and PbII: II. Activation in neutral and weakly alkaline media

The results of a study of the activation of zinc sulphide by the heavy metal ions Cu^II, Cd^II and Pb^II are reported here. Metal-ion uptake follows a logarithmic dependence on time, depending on the particular metal ion and pH region. In some cases there is no well-defined rate equation. A mechanism for the activation process is proposed and related to activation in weakly acidic media.     

Diel behavior of rare earth elements in a mountain stream with acidic to neutral pH

Diel (24-h) changes in concentrations of rare earth elements (REE) were investigated in Fisher Creek, a mountain stream in Montana that receives acid mine drainage in its headwaters. Three simultaneous 24-h samplings were conducted at an upstream station (pH = 3.3), an intermediate station (pH = 5.5), and a downstream station (pH = 6.8). The REE were found to behave conservatively at the two upstream stations. At the downstream station, REE partitioned into suspended particles to a degree that varied with the time of day, and concentrations of dissolved REE were 2.9- to 9.4-fold (190% to 830%) higher in the early morning vs. the late afternoon. The decrease in dissolved REE concentrations during the day coincided with a corresponding increase in the concentration of REE in suspended particles, such that diel changes in the total REE concentrations were relatively minor (27% to 55% increase at night). Across the lanthanide series, the heavy REE partitioned into the suspended solid phase to a greater extent than the light REE. Filtered samples from the downstream station showed a decrease in shale-normalized REE concentration across the lanthanide series, with positive anomalies at La and Gd, and a negative Eu anomaly. As the temperature of the creek increased in the afternoon, the slope of the REE profile steepened and the magnitude of the anomalies increased.The above observations are explained by cyclic adsorption of REE onto suspended particles of hydrous ferric and aluminum oxides (HFO, HAO). Conditional partition coefficients for each REE between the suspended solids and the aqueous phase reached a maximum at 1700 hours and a minimum at 0700 hours. This pattern is attributed to diel variations in stream temperature, possibly reinforced by kinetic factors (i.e., slower rates of reaction at night than during the day). Estimates of the enthalpy of adsorption of each REE onto suspended particles based on the field results averaged +82 kJ/mol and are similar in magnitude to estimates in the literature for adsorption of divalent metal cations onto clays and hydrous metal oxides. The results of this study have important implications to the use of REE as hydrogeochemical tracers in streams.     

Structure and oxidation state of hematite surfaces reacted with aqueous Fe(II) at acidic and neutral pH

Structural changes and surface oxidation state were examined following the reaction of hematite (0 0 1), (0 1 2), and (1 1 0) with aqueous Fe(II). X-ray reflectivity measurements indicated that Fe(II) induces changes in the structure of all three surfaces under both acidic (pH 3) and neutral (pH 7) conditions. The structural changes were generally independent of pH although the extent of surface transformation varied slightly between acidic and neutral conditions; no systematic trends with pH were observed. Induced changes on the (1 1 0) and (0 1 2) surfaces include the addition or removal of partial surface layers consistent with either growth or dissolution. In contrast, a <1 nm thick, discontinuous film formed on the (0 0 1) surface that appears to be epitaxial yet is not a perfect extension of the underlying hematite lattice, being either structurally defective, compositionally distinct, or nanoscale in size and highly relaxed. Resonant anomalous X-ray reflectivity measurements determined that the surface concentration of Fe(II) present after reaction at pH 7 was below the detection limit of approximately 0.5-1 μmol/m² on all surfaces. These observations are consistent with Fe(II) oxidative adsorption, whereby adsorbed Fe(II) is oxidized by structural Fe(III) in the hematite lattice, with the extent of this reaction controlled by surface structure at the atomic scale. The observed surface transformations at pH 3 show that Fe(II) oxidatively adsorbs on hematite surfaces at pH values where little net adsorption occurs, based on historical macroscopic Fe(II) adsorption behavior on fine-grained hematite powders. This suggests that Fe(II) plays a catalytic role, in which an electron from an adsorbed Fe(II) migrates to and reduces a lattice Fe(III) cation elsewhere, which subsequently desorbs in a scenario with zero net reduction and zero net adsorption. Given the general pH-independence and substantial mass transfer involved, this electron and atom exchange process appears to be a significant subsystem within macroscopic pH-dependent Fe(II) adsorption.     

Diel behavior of iron and other heavy metals in a mountain stream with acidic to neutral pH: Fisher Creek, Montana, USA

Three simultaneous 24-h samplings at three sites over a downstream pH gradient were conducted to examine diel fluctuations in heavy metal concentrations in Fisher Creek, a small mountain stream draining abandoned mine lands in Montana. Average pH values at the upstream (F1), middle (F2), and downstream (F3) monitoring stations were 3.31, 5.46, and 6.80, respectively. The downstream increase in pH resulted in precipitation of hydrous ferric oxide (HFO) and hydrous aluminum oxide (HAO) on the streambed. At F1 and F2, Fe showed strong diel cycles in dissolved concentration and Fe(II)/Fe(III) ratio; these cycles were attributed to daytime photoreduction of Fe(III) to Fe(II), reoxidation of Fe(II) to Fe(III), and temperature-dependent hydrolysis and precipitation of HFO. At the near-neutral downstream station, no evidence of Fe(III) photoreduction was observed, and suspended particles of HFO dominated the total Fe load. HFO precipitation rates between F2 and F3 were highest in the afternoon, due in part to reoxidation of a midday pulse of Fe²⁺ formed by photoreduction in the upper, acidic portions of the stream. Dissolved concentrations of Fe(II) and Cu decreased tenfold and 2.4-fold, respectively, during the day at F3. These changes were attributed to sorption onto fresh HFO surfaces. Results of surface complexation modeling showed good agreement between observed and predicted Cu concentrations at F3, but only when adsorption enthalpies were added to the thermodynamic database to take into account diel temperature variations. The field and modeling results illustrate that the degree to which trace metals adsorb onto actively forming HFO is strongly temperature dependent. This study is an example of how diel Fe cycles caused by redox and hydrolysis reactions can induce a diel cycle in a trace metal of toxicological importance in downstream waters.     

酸性环境下生物成因施氏矿物稳定性研究

施氏矿物是酸性矿山废水中广泛存在的次生矿物,其形成和转化受环境pH值、温度和共存离子等条件影响。文中研究了酸性环境中生物成因施氏矿物和吸附了三价砷的生物成因施氏矿物,在不同温度和钾离子浓度条件下的稳定性。结果表明,老化温度的增加促进施氏矿物相的转变:4℃条件下,在15周的老化时间里,无砷及含砷施氏矿物均未发生相转变;而在40℃条件下,经过15周的老化,则无砷和含砷施氏矿物均发生了部分相转变。此外,钾离子浓度变化可以导致施氏矿物老化产物不同:生物成因施氏矿物在0.01mM钾离子条件下老化15周后的转化产物主要为针铁矿,在100mM钾离子条件下老化产物为黄钾铁矾和针铁矿。含As(Ⅲ)施氏矿物在0.01mM钾离子条件下老化15周后没有发生相转变,在100mM钾离子条件下发生了部分相转变,产物为黄钾铁矾。生物成因施氏矿物中的As(Ⅲ)使得矿物在环境中更加稳定。     

The development of acidic groundwaters in coal-bearing strata: Part I. Rare earth element fingerprinting

This study examines the rare earth element (REE) pattern of acidic mine discharges in the Durham basin (NE England) as a means of fingerprinting their sources and understanding the water-rock interactions Although whole rock compositions proved to be unhelpful, mimicking acid mine water generation in a series of selective leachates of the coal and coal-bearing strata gave REE patterns similar to those measured in the emergent waters. REE ratios and anomalies were used as indicators of specific interactions in the development of acidic groundwater. The implications of the findings for the classification and evolution of acid mine discharges are discussed.     

Stability and inter-conversion of synthetic dawsonites in aqueous media

Previous works have opened the debate on the importance of dawsonite in CO₂ sequestration by mineral trapping, although there is a lack of systematic studies assessing its long-term stability. With an open eye to this application, we have systematically investigated the stability of synthetic dawsonites (MAlCO₃(OH)₂, M = , Na⁺, and K⁺) in a variety of aqueous media (, Cl⁻, , , and OH⁻) at ambient pressure and 323 K. The three compositions dissolved under strong acid (pH 2) and basic (pH 14) conditions. In water (pH 7), dawsonites were not stable, evolving into boehmite (NH₄-form) and bayerite (Na and K-forms). The most interesting transformations occurred upon contacting the synthesized dawsonites with ammonium, potassium, and sodium carbonate solutions. NH₄-dawsonite was converted into Na-dawsonite and K-dawsonite in Na₂CO₃ and K₂CO₃ (pH 12), respectively. Na-dawsonite transformed into NH₄-dawsonite in (NH₄)₂CO₃ (pH 10). The NH₄-dawsonite and Na-dawsonite can be successively inter-converted in the appropriate carbonate solution. These transformations are thought to follow a dissolution-precipitation mechanism. In the pH range of 10-12, K-dawsonite was the most stable among the three synthesized samples. Besides structural changes, the treatment of dawsonites in carbonate solutions caused remarkable morphology and porosity alterations of the product samples. Our study provides an improved understanding of the chemistry of dawsonite-type compounds. In particular, the narrow pH range at which they are stable poses serious constraints in their potential use for geochemical fixation of CO₂.     

Stability of earth slopes. Part II: three dimensional analysis in closed‐form

A closed‐form stability analysis of earth slopes performed in 3D is proposed. The sliding surface is assumed spherical and treated as a rigid body allowing the internal state of stress to be ignored. The proposed closed‐formed solution (CFS) can be applied to both homogenous and non‐homogenous slopes of either simple or complex geometry and can also deal with any kind of additional loading. Although it is recognized that the critical failure surface is often non‐spherical, the CFS methodology for spheres described herein provides an objective tool for the evaluation of the assumptions made by other limit equilibrium methods including the role of intercolumn forces. Copyright © 2012 John Wiley & Sons, Ltd.     

煤巷弱胶结顶板稳定性分析与变形控制技术

煤层顶板为弱胶结岩体时稳定性差,常在煤炭回采过程中或巷道掘进时发生大变形或失稳等现象,维护难度较大。针对煤巷弱胶结粉砂岩顶板力学特征与变形控制等问题,以广西林场煤矿煤巷弱胶结粉砂岩顶板支护工程为工程背景,采用现场调研、实验室试验、理论分析等方法进行研究。通过现场调研,发现研究巷道有顶板岩层自承能力差、受水环境影响、支护结构失效率高、底板受水影响底鼓量大、受采动影响等特征;顶板岩石点荷载试验表明,其单轴抗压强度仅为1.9～ 2.3 MPa,采用电镜扫描发现,弱胶结粉砂岩以粗粒矿物为骨架;在普氏理论的基础上,推导了巷道顶板和两帮承压极限表达式,提出了提高巷道围岩整体强度和承载能力、确定合理的锚杆支护参数、顶板设计锚索加强支护等弱胶结巷道围岩控制要点。依据试验分析与理论研究成果,提出了以注浆加固为基础,锚杆和锚索联合支护的控制方案,并在林场煤矿进行了工业性试验。现场监测数据表明,设计的支护方案可以有效地控制煤巷弱胶结粉砂岩顶板变形。     

Redox and pH conditions in the water column and in the sediments of an acidic mining lake

Physico-chemical parameters of the acidic mining lake (ML) 111 in the Lusatian region (Germany) were determined in April and August 1997 using a multiparameter probe for the water column and punch-in pH and Eh electrodes for the sediment, respectively. Samples of the sediment profile were taken by a gravity corer. Besides determination of the physico-chemical parameters, grain size and water contents were determined after sediment slicing.Differences in the redox state of the water body, deduced from oxygen saturation between April and August, could not be inferred. However, a shift in pH to lower values was apparent. Data support the conclusion that the enhanced oxygen content was consumed for oxidation of ferrous iron to ferric iron (oxides/hydroxides) coupled with the release of protons.In the sediment, different buffer systems of iron compounds could be detected within separate zones with values below and above pH 3. At higher pH values, the redox conditions are controlled and stabilized by the presence of Fe-oxide-hydroxides and possibly Fe-sulfates.     

Iron oxide incrustations in wells. Part 1: genesis,mineralogy and geochemistry

The formation of incrustations seriously affects the performance of wells, piezometers and drains. Their economic relevance must not be underestimated. Oxides of Fe(III) represent the most common incrustation type. Their formation via oxidation of dissolved Fe(II) is enhanced at zones of elevated flow velocities. The first, “amorphous” precipitates (ferrihydrite) are thermodynamically unstable and re-crystallise with time to form stable phases, mainly goethite (“ageing”). This transformation involves a decrease of surface area and a simultaneous decrease in reactivity and explains why older incrustations are harder to remove by chemical treatment. During formation, Fe oxide incrustations take up large amounts of trace elements from groundwater. Anions such as phosphate and arsenate dominate over cations. Anions are surface-bound and are mostly expelled with the decrease of surface area during ageing. Cations are probably bound in the crystal structure rather than as sorbed surface species. Only those with large ionic radius are mostly not expelled during ageing. Trace element concentrations influence the rate of transformation and the solubility of Fe oxide incrustations.     

Stability of earth slopes. Part I: two‐dimensional analysis in closed‐form

A closed‐form solution (CFS) satisfying both equilibrium of moments and forces for the stability analysis of earth slopes in 2D is proposed. The sliding surface is assumed circular and treated as a rigid body, allowing the internal state of stress to be ignored. The proposed solution can be applied to both homogenous and non‐homogenous slopes of either simple or complex geometry, and can also deal with any kind of additional loading. The method is based on the fact that, all possible forces acting on the slope can be projected onto the failure surface where they are broken into driving and resisting ones. Comparison of the safety factors obtained by the proposed CFS and those obtained by traditional limit equilibrium methods, as applied to several test examples, indicates that the proposed method is more conservative, whereas moreover, it gives a more realistic point of view for the formation of tension crack in slopes. Copyright © 2012 John Wiley & Sons, Ltd.     

pH scales and proton-transfer reactions in saline media such as sea water

The three approaches to defining pH scales for use in sea water: the N.B.S. scale, the pH(SWS) or ‘total’ hydrogen ion concentration scale and the ‘free’ hydrogen ion concentration scale are described, and it is shown how these arise as a direct consequence of alternative experimental procedures for determining practical acidity constants. The advantages of conceptual simplicity and of experimental precision inherent in the use of concentration products to describe proton-transfer reactions in saline media are emphasised. In addition, the problems of theoretical interpretation and of reproducibility which result from the conventional nature of the N.B.S. pH scale are described, and the effect on the corresponding ‘apparent’ constants outlined. Insofar as it is concentration products rather than ‘apparent’ constants that are amenable to prediction using models for activity coefficients, the deliberate use of a ‘free’ hydrogen ion concentration scale should be applicable to many areas of aqueous geochemistry in addition to marine chemistry.     

Fluorite dissolution at acidic pH: In situ AFM and ex situ VSI experiments and Monte Carlo simulations

Dissolution of the fluorite (1 1 1) cleavage surface was investigated by means of in situ atomic force microscopy (AFM) and ex situ vertical scanning interferometry (VSI) experiments at pH range 1-3 in HCl solutions. Surface retreat was quantified at different pH values, yielding dissolution rates that were used to derive an empirical rate law for fluorite dissolution:

     

The combined effect of pH and temperature on smectite dissolution rate under acidic conditions

The main goal of this paper is to propose a new rate law describing the combined effect of pH (1 to 4.5) and temperature (25 to 70 °C) on smectite dissolution rate, under far from equilibrium conditions, as a step towards establishing the full rate law of smectite dissolution under acidic conditions. Dissolution experiments were carried out using non-stirred flow-through reactors fully immersed in a thermostatic water bath held at a constant temperature of 25.0°C, 50.0°C or 70.0°C ± 0.1°C. Smectite 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 smectite dissolution rate. Low Al/Si ratios were obtained in experiments that were conducted at pH ≥4. These low Al/Si ratios are explained by precipitation of gibbsite and/or diaspore.Dissolution rate increases with temperature and decreases with increasing pH. Dissolution rates of experiments in which ΔG_r ≤ −21 kcal mol ⁻¹, are not affected by deviation from equilibrium. Dissolution rates in most experiments are not affected by the addition of up to 0.3 M NaNO₃ to the input solution.A simple model is used to describe the combined effect of pH and temperature on smectite dissolution rate. According to this model, dissolution rate is linearly proportional to the concentration of adsorbed protons on the mineral surface, and proton adsorption is described using a Langmuir adsorption isotherm. All experimental results at pH <4 were fitted to the model using a multiple non-linear regression. The resulting rate law is:

(A1)     

Staurolite Stability in a Part of the System Fe-Al-Si-O-H

The following reactions, believed to be analogous to those whichdefine the maximum extent of staurolite-quartz compatibilityat moderate oxygen fugacity in metamorphic rocks, have beendetermined in terms of hydrous fluid pressure and temperature.The O: H composition ratio of the fluid was controlled withthe quartz-fayalite-magnetite (QFM) buffer assemblage. (I) Fe-staurolitequartz almandine+sillimanite+water. (II) Fe-staurolitequartz Fe-cordierite+sillimanite+water. (III) Fe-chloritoid+sillimanite Fe-staurolite+quartz+water. In addition, two reactions which delineate part of the stabilitylimits of Fe-cordierite have been investigated: (IV) Fe-cordierite almandine+sillimanite+quartz. (V) Fe-cordierite hercynite+sillimanite+quartz. The experimental information has been used to predict boundariesto the PT fields of all quartz and QFM-buffered fluid-bearingassemblages involving Fe-staurolite, Fe-cordierite, Fe-chloritoid,almandine, and sillimanite. Using information from this andother studies, three mineral assemblages are recognized whichare stable at similar temperatures but different fluid pressures.In order of decreasing pressure they are: (a) Above 5 kb: staurolite, quartz, kyanite, fluid; (b) Between 1.5 and 8.5 kb (outer limits; in natural rocks thisfield will have a much narrower pressure range) staurolite,quartz, cordierite, fluid. (c) Below 3.5 kb: Fe-cordierite, andalusite, fluid of oxygenfugacity equivalent to the quartz-fayalite-magnetite assemblage. These phase assemblages may be the equivalents of naturallyoccurring mineral facies, but this must be proven in the field.In addition the absence of cordierite from rocks of appropriatecomposition and temperature of formation betokens total pressuresgreater than 3–5 kb. ¹Present address: Grant Institute of Geology, West Mains Road, Edinburgh 9, Scotland.