Characterization of limestone reacted with acid-mine drainage in a pulsed limestone bed treatment system at the Friendship Hill National Historical Site,Pennsylvania, USA

Armoring of limestone is a common cause of failure in limestone-based acid-mine drainage (AMD) treatment systems. Limestone is the least expensive material available for acid neutralization, but is not typically recommended for highly acidic, Fe-rich waters due to armoring with Fe(III) oxyhydroxide coatings. A new AMD treatment technology that uses CO₂ in a pulsed limestone bed reactor minimizes armor formation and enhances limestone reaction with AMD. Limestone was characterized before and after treatment with constant flow and with the new pulsed limestone bed process using AMD from an inactive coal mine in Pennsylvania (pH=2.9, Fe =150 mg/l, acidity =1000 mg/l CaCO₃). In constant flow experiments, limestone is completely armored with reddish-colored ochre within 48 h of contact in a fluidized bed reactor. Effluent pH initially increased from the inflow pH of 2.9 to over 7, but then decreased to <4 during the 48 h of contact. Limestone grains developed a rind of gypsum encapsulated by a 10- to 30-μm thick, Fe-Al hydroxysulfate coating. Armoring slowed the reaction and prevented the limestone from generating any additional alkalinity in the system. With the pulsed flow limestone bed process, armor formation is largely suppressed and most limestone grains completely dissolve resulting in an effluent pH of >6 during operation. Limestone removed from a pulsed bed pilot plant is a mixture of unarmored, rounded and etched limestone grains and partially armored limestone and refractory mineral grains (dolomite, pyrite). The ∼30% of the residual grains in the pulsed flow reactor that are armored have thicker (50- to 100-μm), more aluminous coatings and lack the gypsum rind that develops in the constant flow experiment. Aluminium-rich zones developed in the interior parts of armor rims in both the constant flow and pulsed limestone bed experiments in response to pH changes at the solid/solution interface.     

An electrochemical study of pyrite oxidation in the presence of carbon coatings in cyanide medium

The anodic and cathodic behaviour of pyrite with clay and different carbon coatings of activated carbon, graphite and carbonaceous matter in cyanide medium was investigated using the potentiodynamic method. The presence of clay coating did not change the polarisation curve appearance for either the anodic oxidation of pyrite or the cathodic reduction of oxygen or the potential of the current plateau, but only decreased the plateau current especially at a higher coating thickness. The presence of the carbon coatings marginally shifted the rest potential for pyrite to a more anodic position and slightly changed the polarisation curve appearance for pyrite oxidation. The current density for pyrite oxidation largely increased in the presence of the carbon coatings, the potential at the plateau shifted to more cathodic positions, and the plateau width became smaller. These effects became more noticeable at a higher coating thickness. The activated carbon, graphite and carbonaceous matter coatings performed similarly in affecting pyrite oxidation at a similar thickness. The carbon coatings significantly increased the limiting current densities for oxygen reduction on pyrite, and the limiting current plateau became steeper at a higher coating thickness. The carbon coatings increased the limiting current density for oxygen reduction to a similar extent at a low coating thickness, but increased to varied extents at a higher coating thickness. The carbon coatings also greatly increased the cathodic current density for gold reduction on pyrite. The enhancement of pyrite oxidation and oxygen or gold reduction on pyrite by the carbon coatings was likely attributed to the electrochemical interaction between pyrite and the carbon materials with electron-rich surfaces and high conductivity. The presence of the carbon coatings significantly increased the oxidation of pyrite in aerated cyanide solutions and the preg-robbing of pyrite especially at a higher coating thickness.     

Mechanism of arsenic release to groundwater,Bangladesh and West Bengal

In some areas of Bangladesh and West Bengal, concentrations of As in groundwater exceed guide concentrations, set internationally and nationally at 10 to 50 μg l⁻¹ and may reach levels in the mg l⁻¹ range. The As derives from reductive dissolution of Fe oxyhydroxide and release of its sorbed As. The Fe oxyhydroxide exists in the aquifer as dispersed phases, such as coatings on sedimentary grains. Recalculated to pure FeOOH, As concentrations in this phase reach 517 ppm. Reduction of the Fe is driven by microbial metabolism of sedimentary organic matter, which is present in concentrations as high as 6% C. Arsenic released by oxidation of pyrite, as water levels are drawn down and air enters the aquifer, contributes negligibly to the problem of As pollution. Identification of the mechanism of As release to groundwater helps to provide a framework to guide the placement of new water wells so that they will have acceptable concentrations of As.     

Diagenetic alteration of a Mesozoic fluvial gold placer deposit,southern New Zealand

Gold paleoplacers become progressively more affected by diagenetic processes with age and burial. Mesozoic paleoplacer deposits in southern New Zealand display intermediate stages of diagenetic transformation compared to little-affected Late Cenozoic paleoplacers and strongly-affected Paleozoic and Precambrian paleoplacers. The Mesozoic (Cretaceous) diagenesis resulted in near-pervasive alteration, cementation and lithification of the paleoplacer. Lithic clasts and matrix have been extensively altered to illite, ferrous iron-bearing smectite-vermiculite, and kaolinite, and the cement consists mainly of clays and calcite. Diagenetic pyrite, marcasite, vivianite, and Mn oxide also contributed to cementation. Alteration occurred under near-surface (<500 m depth) conditions with groundwater that had circumneutral pH, high alkalinity, and elevated dissolved K, Mg and Ca. Detrital albite remained unaffected by alteration. Detrital gold has been variably dissolved and redeposited, with widespread formation of gold overgrowths on the 1–10 μm scales, with 1–3 wt% Ag. Gold mobility was driven by reduced sulphur complexes in the low redox, high pH diagenetic environment. The overgrowth gold locally contributed to cementation of fine clastic grains, and has intergrown with diagenetic clays and Mn oxide. Post-diagenetic oxidation of the paleoplacer deposit has transformed much of the pyrite to ferric oxyhydroxide and deposited some ferric oxyhydroxide coatings on gold. These oxidation processes have had only minor effects on gold mobility and textures. Hence, the low redox conditions of diagenetic gold mobility were distinctly different from those typically associated with oxidation-related supergene gold mobility. Diagenesis can affect economics of paleoplacer mining by hindering rock disaggregation during processing, coating gold particles with secondary minerals, and increasing the clay content of the deposit, all of which can lower the efficiency of gold recovery.     

Experimental study on prevention of acid mine drainage by silica coating of pyrite waste rocks with amorphous silica solution

Acid mine drainage (AMD) is a widespread environmental problem associated with working and abandoned mining operations. It results from the microbial oxidation of pyrite in the presence of water and air, affording an acidic solution that contains toxic metal ions. Pyrite microencapsulation, utilizing silica coating, is a novel approach for controlling AMD that has been shown to be very effective in controlling pyrite oxidation. The roles of the solution pH and silica concentration in the formation mechanism for the AMD-preventing coating were investigated. A silica coating can be formed from silica solution at pH 7, at which the amount of Fe eluted from pyrite into the solution is small. No coating was formed at other pH values, and the amounts of eluted Fe were larger than at pH 7, especially at pH 11. The silica coating forms from 2,500 to 5,000 mg/L silica solutions, but not from 0 or 1,000 mg/L silica solutions. The coating formation rate was slower in the 2,500 mg/L silica solution than in the 5,000 mg/L silica solution. The formation of silica coating on pyrite surfaces depends on three main steps: formation of Fe(OH)₃ on the surface of pyrite, reaction between Fe(OH)₃ and silicate in the solution on the pyrite surface, and growth of the silica layer on the first layer of silica. The best pH condition to enable these steps was around 7, and the silica coating formation rate can be controlled by the concentration of silica.     

Mobilization and speciation of arsenic from hydrothermally altered rock containing calcite and pyrite under anoxic conditions

The effects of water residence time and anoxic conditions on the mobilization and speciation of As in a calcite- and pyrite-bearing altered rock excavated during a road-tunnel project has been evaluated using batch and column laboratory experiments. Higher infiltration rates (i.e., shorter water residence times) enhanced the leaching of As due to the higher pH values of the effluents and more rapid transport of dissolved As through the columns. The concentration of As in the effluent also increased under anoxic conditions regardless of the water residence time. This enhanced leaching of As under anoxic conditions could be attributed to a significant pH increase and decreased Fe oxyhydroxide/oxide precipitation compared to similar experiments done under ambient conditions. Processes that controlled the evolution of pH and the temporal release mechanisms of As under anoxic conditions were identical to those previously observed under ambient conditions: the dissolution of soluble phases, pyrite oxidation, co-precipitation and/or adsorption/desorption reactions. Speciation of As in the column experiments could partly be attributed to the pH-dependent adsorption of As species onto Fe oxyhydroxide/oxide precipitates. Moreover, apparent equilibrium of the total As and As[III] concentrations was delayed under anoxic conditions in both batch and column experiments.     

Surface oxidation of pyrite under ambient atmospheric and aqueous (pH = 2 to 10) conditions: electronic structure and mineralogy from X-ray absorption spectroscopy

The nature of the surface oxidation phase on pyrite, FeS₂, reacted in aqueous electrolytes at pH = 2 to 10 and with air under ambient atmospheric conditions was studied using synchrotron-based oxygen K edge, sulfur L_III edge, and iron L_II,III edge X-ray absorption spectroscopy. We demonstrate that O K edge X-ray absorption spectra provide a sensitive probe of sulfide surface oxidation that is complementary to X-ray photoelectron spectroscopy. Using total electron yield detection, the top 20 to 50 Å of the pyrite surface is characterized. In air, pyrite oxidizes to form predominantly ferric sulfate. In aqueous air-saturated solutions, the surface oxidation products of pyrite vary with pH, with a marked transition occurring around pH 4. Below pH = 4, a ferric (hydroxy)sulfate is the main oxidation product on the pyrite surface. At higher pH, we find iron(III) oxyhydroxide in addition to ferric (hydroxy)sulfate on the surface. Under the most alkaline conditions, the O K edge spectrum closely resembles that of goethite, FeOOH, and the surface is oxidized to the extent that no FeS₂ can be detected in the X-ray absorption spectra. In a 1.667 × 10⁻³ mol/L Fe³⁺ solution with ferric iron present as FeCl₃ in NaCl, the oxidation of pyrite is autocatalyzed, and formation of the surface iron(III) oxyhydroxide phase is promoted at low pH.     

硫化物氧化过程中金的富集实验研究

室温条例上将海绵金用王水溶解形成金氯酸溶液,再用纯净的不含金黄铁矿吸附溶液中的金,粉干后用过氧化氢溶液将其分解,发现溶液中出现粒状、片状及壳皮状金。对溶液中金和铁的含量测定结果表明,随着黄铁矿的分解溶液中铁含量增加,金含量也随之增加,但当溶注保出现小金粒或金片后,溶液中的金含量急剧下降,并保持相对稳定。     

The role of carbonate ions in pyrite oxidation in aqueous systems

The mechanism of pyrite oxidation in carbonate-containing alkaline solutions at 80 °C was investigated with the help of rate experiments, thermodynamic modeling and diffuse reflectance infrared spectroscopy (DRIFTS). Pyrite oxidation rate increased with pH and was enhanced by addition of bicarbonate/carbonate ions. The carbonate effect was found to be limited to moderately alkaline conditions (pH 8-11). Metastable Eh-pH diagrams, at 25 °C, indicate that soluble iron-carbonate complexes (FeHCO₃⁻, FeCO₃⁰, Fe(CO₃)(OH)⁻ and FeCO₃²⁻) may coexist with pyrite in the pH range of 6-12.5. Above pH 11 and 13, the Fe(II) and Fe(III) hydroxocomplexes, respectively, become stable, even in the presence of carbonate/bicarbonate ions. Surface-bound carbonate complexes on iron were also identified with DRIFTS as products of pyrite oxidation in addition to iron oxyhydroxides and soluble sulfate species. The conditions under which thermodynamic and DRIFTS analyses indicate the presence of carbonate compounds also correspond to those in which the fastest rate of pyrite oxidation in carbonate solutions was observed. Following the Singer-Stumm model for pyrite oxidation in acidic solutions, it is assumed that Fe(III) is the preferred pyrite oxidant under alkaline conditions. We propose that carbonate ions facilitate the electron transfer from soluble iron(II)-carbonate to O₂, increase the iron solubility, and provide buffered, favorable alkaline conditions at the reaction front, which in turn favors the overall kinetics of pyrite oxidation. Therefore, the electron transfer from sulfur atoms to O₂ is facilitated by the formation of the cycle of Fe(II)-pyrite/Fe(III)-carbonate redox couple at the pyrite surface.     

Biogeochemical reactive–diffusive transport of heavy metals in Lake Coeur d’Alene sediments

Decades of runoff from precious-metal mining operations in the Lake Coeur d’Alene Basin, Idaho, have left the sediments in this lake heavily enriched with toxic metals, most notably Zn, Pb and Cu, together with As. The bioavailability, fate and transport of these metals in the sediments are governed by complex biogeochemical processes. In particular, indigenous microbes are capable of catalyzing reactions that detoxify their environments, and thus constitute an important driving component in the biogeochemical cycling of these metals. Here, the development of a quantitative model to evaluate the transport and fate of Zn, Pb and Cu in Lake Coeur d’Alene sediments is reported. The current focus is on the investigation and understanding of local-scale processes, rather than the larger-scale dynamics of sedimentation and diagenesis, with particular emphasis on metal transport through reductive dissolution of Fe hydroxides. The model includes 1-D inorganic diffusive transport coupled to a biotic reaction network including consortium biodegradation kinetics with multiple terminal electron acceptors and syntrophic consortium biotransformation dynamics of redox front. The model captures the mobilization of metals initially sorbed onto hydrous ferric oxides, through bacterial reduction of Fe(III) near the top of the sediment column, coupled with the precipitation of metal sulfides at depth due to biogenic sulfide production. Key chemical reactions involve the dissolution of ferrihydrite and precipitation of siderite and Fe sulfide. The relative rates of these reactions play an important role in the evolution of the sediment pore-water chemistry, notably pH, and directly depend on the relative activity of Fe and SO₄ reducers. The model captures fairly well the observed trends of increased alkalinity, sulfide, Fe and heavy metal concentrations below the sediment–water interface, together with decreasing terminal electron acceptor concentrations with depth, including the development of anoxic conditions within about a centimeter below the lake bottom. This effort provides insights on important biogeochemical processes affecting the cycling of metals in Lake Coeur d’Alene and similar metal-impacted lacustrine environments.     

The oxidation of pyrite at pH 7 in the presence of reducing and nonreducing Fe(III)-chelators

The oxidation rate of pyrite at pH 7, 25°C and at constant partial pressure of oxygen (0.21 and 0.177 atm) was measured in the presence of the Fe(III)-chelators NTA, oxalate, leucine, EDTA, citrate, IDA and the Fe(III)-reductant ascorbic acid. With the exception of leucine and EDTA, non-reducing Fe(III)-chelators increased the oxidation rate relative to the reference state of formation of the Fe(OH)₂⁺ complex at pH 7. The rate increase was proportional to the logarithm of the conditional stability constant of the ligands for the complexation of Fe³⁺. No effect on the oxidation rate was observed in the presence of EDTA, which shifted the redox potential of the redox couple Fe²⁺/Fe³⁺ to a value below that in the absence of any ligand at pH 7. Ascorbic acid decreased the pyrite oxidation rate by a factor of 5 at ascorbic acid concentrations between 10⁻⁴ and 10⁻² mol L⁻¹. Comparison of the rate constants for the oxidation of ascorbic acid by surface bound Fe(III) in the absence and presence of pyrite shows that the pyrite surface accelerates this reaction by a factor of 10. The oxidation of both pyrite and ascorbic acid is of fractional order with respect to ascorbic acid (HAsc): r_py=0.55 c(HAsc)^−0.35 r_HAsc=3.6 c(HAsc)^0.59. Both the results from experiments with Fe(III)-chelating ligands and the Fe(III)-reductant, suggest a very efficient interference in the electron cycling between Fe(II) and Fe(III) at the pyrite surface. The interference seems to be mainly related to the reductive side of the iron cycling. It is therefore concluded that the electron transfer between ferric iron and pyritic sulfur limits the pyrite oxidation rate at pH 7.     

Pyrite oxidation: a state-of-the-art assessment of the reaction mechanism

The oxidation of pyrite to release ferrous iron and sulfate ions to solution involves the transfer of seven electrons from each sulfur atom in the mineral to an aqueous oxidant. Because only one or, at most, two electrons can be transferred at a time, the overall oxidation process is quite complex. Furthermore, pyrite is a semiconductor, so the electrons are transferred from sulfur atoms at an anodic site, where oxygen atoms from water molecules attach to the sulfur atoms to form sulfoxy species, through the crystal to cathodic Fe(II) sites, where they are acquired by the oxidant species. The reaction at the cathodic sites is the rate-determining step for the overall process. This paper maps out the most important steps in this overall process.     

Pyrite oxidation by Acidithiobacillus ferrooxidans at various concentrations of dissolved oxygen

Pyrite oxidation rates were examined at various concentrations of dissolved oxygen (DO) in the presence of the sulfur and iron oxidizer Acidithiobacillus ferrooxidans. Five different batch experiments were performed at room temperature for 75 days under various DO levels (273, 129, 64.8, 13.2, and ≤ 0.006 μM), containing pyrite grains (particle size 63–250 μm) and a modified 9K nutrient medium at pH 3. The reactors were inoculated with A. ferrooxidans. In all experiments, pH decreased with time and sulfur and iron were released to the solution, indicating pyrite oxidation at all DO levels. Pyrite oxidation rates (ca. 5 × 10^− 10 mol m^− 2 s^− 1 at 273 μM DO) from all experiments showed positive correlation with DO, Fe(III), and bacterial concentration. These rates were significantly slower than rates presented in other published studies, but this is probably due to the significantly greater Fe(III) concentration at lower pH in these previous studies. The results obtained in this study suggest that ferric iron reduction at the pyrite surface is the primarily mechanism for microbial pyrite oxidation in the presence of DO. The results from our study support the indirect mechanism of sulfide oxidation, where A. ferrooxidans oxidizes ferrous iron in the presence of DO, which then oxidizes pyrite.     

Oxygen isotope evidence for sorption of molecular oxygen to pyrite surface sites and incorporation into sulfate in oxidation experiments

Experiments were conducted to investigate (i) the rate of O-isotope exchange between SO₄ and water molecules at low pH and surface temperatures typical for conditions of acid mine drainage (AMD) and (ii) the O- and S-isotope composition of sulfates produced by pyrite oxidation under closed and open conditions (limited and free access of atmospheric O₂) to identify the O source/s in sulfide oxidation (water or atmospheric molecular O₂) and to better understand the pyrite oxidation pathway. An O-isotope exchange between SO₄ and water was observed over a pH range of 0–2 only at 50 °C, whereas no exchange occurred at lower temperatures over a period of 8 a. The calculated half-time of the exchange rate for 50 °C (pH = 0 and 1) is in good agreement with former experimental data for higher and lower temperatures and excludes the possibility of isotope exchange for typical AMD conditions (T  25 °C, pH  3) for decades.Pyrite oxidation experiments revealed two dependencies of the O-isotope composition of dissolved sulfates: O-isotope values decreased with longer duration of experiments and increasing grain size of pyrite. Both changes are interpreted as evidence for chemisorption of molecular O₂ to pyrite surface sites. The sorption of molecular O₂ is important at initial oxidation stages and more abundant in finer grained pyrite fractions and leads to its incorporation in the produced SO₄. The calculated bulk contribution of atmospheric O₂ in the dissolved SO₄ reached up to 50% during initial oxidation stages (first 5 days, pH 2, fine-grained pyrite fraction) and decreased to less than 20% after about 100 days. Based on the direct incorporation of molecular O₂ in the early-formed sulfates, chemisorption and electron transfer of molecular O₂ on S sites of the pyrite surface are proposed, in addition to chemisorption on Fe sites. After about 10 days, the O of all newly-formed sulfates originates only from water, indicating direct interaction of hydroxyls from water with S at the anodic S pyrite surface site. Then, the role of molecular O₂ is as proposed in previous studies: acting as electron acceptor only at the cathodic Fe pyrite surface site for oxidation of Fe(II) to Fe(III).     

Grain-scale iron isotopic distribution of pyrite from Precambrian shallow marine carbonate revealed by a femtosecond laser ablation multicollector ICP-MS technique: Possible proxy for the redox state of ancient seawater

The redox state of Precambrian shallow seas has been linked with material cycle and evolution of the photosynthesis-based ecosystem. Iron is a redox-sensitive element and exists as a soluble Fe(II) species or insoluble Fe(III) species on Earth’s surface. Previous studies have shown that the iron isotopic ratio of marine sedimentary minerals is useful for understanding the ocean redox state, although the redox state of the Archean shallow sea is poorly known. This is partly because the conventional bulk isotope analytical technique has often been used, wherein the iron isotopic record may be dampened by the presence of isotopically different iron-bearing minerals within the same sample. Here we report a microscale iron isotopic ratio of individual pyrite grains in shallow marine stromatolitic carbonates over geological time using a newly developed, near-infrared femtosecond laser ablation multicollector ICP-MS technique (NIR-fs-LA-MC-ICP-MS).We have determined that the grain-scale iron isotopic distribution of pyrite from coeval samples shows a bimodal (2.7 and 2.3 Ga) or unimodal pattern (2.9, 2.6, and 0.7 Ga). In particular, pyrite from the 2.7 Ga Fortescue Group shows a unique bimodal distribution with highly positive (+1.0‰ defined as Type 1) and negative δ⁵⁶Fe values (−1.8‰ defined as Type 2). Type 1 and 2 pyrites occasionally occur within different siliceous layers in the same rock specimen. Layer-scale iron isotopic heterogeneity indicates that the iron isotopic ratios of the two types of pyrite are not homogenized by diagenesis after deposition. Some cubic pyrites have a core with a positive δ⁵⁶Fe value (1‰) and a rim with a crustal δ⁵⁶Fe value (0‰). The observed isotopic zoning suggests that the positive δ⁵⁶Fe value is a primary signature at the time of stromatolite formation, while secondary pyrite precipitated during diagenesis.The positive δ⁵⁶Fe value of Type 1 and the large iron isotopic difference between Type 1 and 2 (2.8‰.) suggest partial Fe(II) oxidation in the 2.7-Ga shallow sea, i.e., pyritization of ⁵⁶Fe-enriched ferric oxyhydroxide (Type 1) and ⁵⁶Fe depleted Fe²⁺aq in seawater (Type 2). Type 2 pyrite was probably not produced by microbial iron redox cycling during diagenesis because this scenario requires a higher abundance of pyrite with δ⁵⁶Fe of 0‰ than of −1.8‰. Consequently, the degree of Fe(II) oxidation in the 2.7-Ga shallow sea can be estimated by a Fe²⁺aq steady-state model. The model calculation shows that half the Fe²⁺aq influx was oxidized in the seawater. This implies that O₂ produced by photosynthesis would have been completely consumed by oxidation of the Fe²⁺aq influx. Grain-scale iron isotopic distribution of pyrite could be a useful index for reconstructing the redox state of the Archean shallow sea.     

Geochemical response of acid groundwater to neutralization by alkaline recharge

Solute transport and chemical neutralization (pH 3 to 7) within a shallow heterogeneous aquifer producing acid mine drainage (AMD) are examined at an abandoned surface coal mine in West Virginia. The aquifer is undergoing partial neutralization by mixing with alkalinity from a leaking sludge disposal pond, extending in preferential zones controlled by aquifer heterogeneity. Hydraulic heads interpolated from wells indicate leakage from a central alkaline (pH 7.1, 0.72 meq/L alkalinity) sludge pond is a principal source of recharge. Chemically-conservative sodium, added to AMD during treatment and leaked into the aquifer with the sludge, develops a dispersion plume over a restricted portion of the aquifer that correlates with pH, hydraulic head, and dissolved metals distributions. Concentrations of aluminum, iron, sulfate and acidity display higher concentrations downgradient from the pond as sludge alkalinity is consumed along flow paths. Before reaching springs, most dissolved iron is oxidized and hydrolyzed, likely precipitating in the aquifer as a ferric hydroxide or hydroxysulfate phase. The spatial pattern of iron and aluminum concentrations suggests accelerated oxidation caused by gas transport along the outer slopes of the spoil. Dissolved aluminum concentrations increase with total acidity, suggesting that dissolution of silicate minerals results from acidity released by iron hydrolysis. Neutralization reactions and higher pH are favored in more highly permeable portions of the spoil, where ferrihydrite and aluminum hydroxysulfate minerals (such as basaluminite) are supersaturated. In acid-producing zones at pH < 4.5, jurbanite is near equilibrium and an aluminum-sulfate phase with similar properties may limit aluminum concentrations, but become undersaturated in zones of advancing neutralization. At this particular site, ferrous iron produced by pyrite oxidation is almost completely oxidized over short transport distances, allowing hydrolysis of iron and aluminum should sufficient alkalinity be added to these acid waters.     

Hydrogeologic and environmental impact of amjhore pyrite mines,India

Drainage from active and inactive pyrite mines has produced chemical and physical pollution of both ground- and surface water in Amjhore region. In the present case, chemical pollution is caused by exposing pyrite minerals to oxidation or leaching, resulting in undesirable concentrations of dissolved materials. Pyrite mining suddenly exposed large quantities of sulfides to direct contact with oxygen, and oxidation proceeds rapidly, resulting in acidity and release of metal (Fe) and sulfates to the water system, eventually resulting in water pollution in the region. The magnitude and impact of the problem is just being recognized and, as the present and the future projected demand for clean water is of top priority, the present studies were undertaken.Mine drainage includes water flowing from the surface and underground mines and runoff or seepage from the pyrite mines. This article describes the various hydrologic factors that control acid water formation and its transport. The mine drainage is obviously a continuing source of pollution and, therefore, remedial measures mainly consisting of a double-stage limestone-lime treatment technique have been suggested. The present results will be used to develop an alternative and more effective abatement technology to mitigate acid production at the source, namely, the technique of revegetation of the soil cover applied to the waste mine dump material.Water quality change is discussed in detail, with emphasis on acidity formed from exposed pyrite material and on increase in dissolved solids. Preventive and treatment measures are recommended.     

金属矿物的反应动力学与地球化学意义

概述了动力学实验的技术方法和金属矿物的反应动力学研究进展。动力学实验使用的三种基本化学反应装置是间歇反应器（ＢＲ）、活塞流反应器（ＰＦＲ）和混合流反应器（ＭＦＲ）,确定速率定律的数学方法包括积分法、微分法和混合法,以微分法中的初始速率法应用最广。目前主要研究了水溶液中黄铁矿氧化、黄铁矿和黄铜矿形成、晶质铀矿和磁铁矿溶解的速率定律和反应机理,发现：（１）酸性溶液中黄铁矿的氧化速率对Ｆｅ３＋和Ｏ２浓度呈分数依赖并受表面反应的控制;（２）低于３００℃时黄铁矿不能从溶液中直接成核,而需初始地通过ＦｅＳ先驱物的硫化生成,ＦｅＳ与Ｈ２Ｓ反应形成黄铁矿的速率方程为二级;（３）磁黄铁矿或黄铁矿与Ｃｕ２＋反应均可形成黄铜矿,前者经历了一系列准稳的Ｃｕ Ｆｅ硫化物的中间物,后者的速率方程为表观一级并受表面反应的控制;（４）酸性ｐＨ时磁铁矿的非线性溶解行为可采用表面反应扩散输运耦合的收缩核模型（ＳＣＭ）来描述。有关动力学实验成果完善和深化了对矿床中黄铁矿、黄铜矿的形成机理和风化壳中磁铁矿的稳定性等方面的认识。将来的实验研究将向更多的金属矿物和高温高压领域发展。     

Weathering of the New Albany Shale, Kentucky: II. Redistribution of minor and trace elements

During weathering, elements enriched in black shale are dispersed in the environment by aqueous and mechanical transport. Here a unique evaluation of the differential release, transport, and fate of Fe and 15 trace elements during progressive weathering of the Devonian New Albany Shale in Kentucky is presented. Results of chemical analyses along a weathering profile (unweathered through progressively weathered shale to soil) describe the chemically distinct pathways of the trace elements and the rate that elements are transferred into the broader, local environment. Trace elements enriched in the unweathered shale are in massive or framboidal pyrite, minor sphalerite, CuS and NiS phases, organic matter and clay minerals. These phases are subject to varying degrees and rates of alteration along the profile. Cadmium, Co, Mn, Ni, and Zn are removed from weathered shale during sulfide-mineral oxidation and transported primarily in aqueous solution. The aqueous fluxes for these trace elements range from 0.1 g/ha/a (Cd) to 44 g/ha/a (Mn). When hydrologic and climatic conditions are favorable, solutions seep to surface exposures, evaporate, and form Fe-sulfate efflorescent salts rich in these elements. Elements that remain dissolved in the low pH (<4) streams and groundwater draining New Albany Shale watersheds become fixed by reactions that increase pH. Neutralization of the weathering solution in local streams results in elements being adsorbed and precipitated onto sediment surfaces, resulting in trace element anomalies.Other elements are strongly adsorbed or structurally bound to solid phases during weathering. Copper and U initially are concentrated in weathering solutions, but become fixed to modern plant litter in soil formed on New Albany Shale. Molybdenum, Pb, Sb, and Se are released from sulfide minerals and organic matter by oxidation and accumulate in Fe-oxyhydroxide clay coatings that concentrate in surface soil during illuviation. Chromium, Ti, and V are strongly correlated with clay abundance and considered to be in the structure of illitic clay. Illite undergoes minimal alteration during weathering and is concentrated during illuvial processes. Arsenic concentration increases across the weathering profile and is associated with the succession of secondary Fe(III) minerals that form with progressive weathering. Detrital fluxes of particle-bound trace elements range from 0.1 g/ha/a (Sb) to 8 g/ha/a (Mo). Although many of the elements are concentrated in the stream sediments, changes in pH and redox conditions along the sediment transport path could facilitate their release for aqueous transport.     

Role of hydrogen peroxide and hydroxyl radical in pyrite oxidation by molecular oxygen

Hydrogen peroxide and hydroxyl radical are readily formed during the oxidation of pyrite with molecular oxygen over a wide range of pH conditions. However, pretreatment of the pyrite surface influences how much of the intermediates are formed and their fate. Acid-washed pyrite produces significant amounts of hydrogen peroxide and hydroxyl radical when suspended in air-saturated water. However, the hydrogen peroxide concentration shows an exponential decrease with time. Suspensions made with partially oxidized pyrite yield significantly lower amounts of hydrogen peroxide product. The presence of Fe(III)-oxide or Fe(III)-hydroxide patches facilitates the conversion of hydrogen peroxide to oxygen and water. Hence, the degree to which a pyrite surface is covered with patches of Fe(III)-oxide or Fe(III)-hydroxide patches is an important control on the concentration of hydrogen peroxide in solution.Hydrogen peroxide appears to be an important intermediate in the four-electron transfer from pyrite to molecular oxygen. Addition of catalase, an enzyme that decomposes hydrogen peroxide to water and molecular oxygen, to a pyrite suspension reduces the oxidation rate by 40%. By contrast, hydroxyl radical does not appear to play a significant role in the oxidation mechanism. It is estimated on the basis of a molecular oxygen and sulfate mass balance that 5-6% of the molecular oxygen is consumed without forming sulfate.