David W. Blowes

The Geochemistry of Acid Mine Drainage

Mining and mineral processing generates large volumes of waste, including waste rock, mill tailings, and mineral refinery wastes. The oxidation of sulfide minerals in the materials can result in the release of acidic water containing high concentrations of dissolved metals. Recent studies have determined the mechanisms of abiotic sulfide-mineral oxidation. Within mine wastes, the oxidation of sulfide minerals is catalyzed by microorganisms. Molecular tools have been developed and applied to determine the activity and role of these organisms in sulfide-mineral-bearing systems. Novel tools have been developed for assessing the toxicity of mine-waste effluent. Dissolved constituents released by sulfide oxidation may be attenuated through the precipitation of secondary minerals, including metal sulfate, oxyhydroxide, and basic sulfate minerals. Geochemical models have been developed to provide improved predictions of the magnitude and duration of environmental concerns. Novel techniques have been developed to prevent and remediate environmental problems associated with these materials.

The pore-water geochemistry and the mineralogy of the vadose zone of sulfide tailings, Waite Amulet, Quebec, Canada

Low-pH waters, requiring treatment because of high concentrations of dissolved metals, are being discharged from a decommissioned tailings impoundment at the former Waite Amulet ZnCu mine in northeastern Quebec, Canada. A detailed study of the vadose zone of the tailings comparing mineralogical and geochemical analyses of the tailings solids with geochemical analysis of the tailings pore water and pore gas indicates the presence of three zones within the tailings. These zones include an upper sulfide-depleted zone in which the sulfides have been extensively removed and oxidation is largely complete, an intermediate zone, where sulfide oxidation and acid neutralization reactions are occurring, and an unoxidized zone below the depth of oxygen penetration, where sulfides are unoxidized. High aqueous concentrations of dissolved solids are being displaced downward from the sulfide-depleted zone, through the unoxidized zone toward the water table. Sulfide-oxidation and acid-neutralization reactions have generated concentrations of dissolved solids high enough to lead to the precipitation of a series of secondary solid phases. Precipitation-dissolution reactions involving these solid phases control the concentrations of the major ions in the tailings pore water. Concentrations of dissolved metals are controlled by precipitation-dissolution, crystal-structural replacement, and adsorption/coprecipitation reactions. Predictions based on the observed data and numerical simulations suggest that, in the absence of an effective remedial program, sulfide oxidation and H+ production will continue for several centuries.

The formation and potential importance of cemented layers in inactive sulfide mine tailings

Abstract Investigations of inactive sulfide-rich tailings impoundments at the Heath Steele (New Brunswick) and Waite Amulet (Quebec) minesites have revealed two distinct types of cemented layers or “hardpans.” That at Heath Steele is 10–15 cm thick, occurs 20–30 cm below the depth of active oxidation, is continuous throughout the tailings impoundment, and is characterized by cementation of tailings by gypsum and Fe(II) solid phases, principally melanterite. Hardpan at the Waite Amulet site is only 1–5 cm thick, is laterally discontinuous (10–100 cm), occurs at the depth of active oxidation, and is characterized by cementation of tailings by Fe(III) minerals, principally goethite, lepidocrocite, ferrihydrite, and jarosite. At Heath Steele, an accumulation of gas-phase CO2, of up to 60% of the pore gas, occurs below the hardpan. The calculated diffusivity of the hardpan layer is only about 1 100 that of the overlying, uncemented tailings. The pore-water chemistry at Heath Steele has changed little over a 10-year period, suggesting that the cemented layer restricts the movement of dissolved metals through the tailings and also acts as a zone of metal accumulation. Generation of a cemented layer therefore has significant environmental and economic implications. It is likely that, in sulfide-rich tailings impoundments, the addition of carbonate-rich buffering material during the late stages of tailings deposition would enhance the formation of hardpan layers.

Modeling of multicomponent reactive transport in groundwater: 1. Model development and evaluation

MINTRAN is a new model for simulating transport of multiple thermodynamically reacting chemical substances in groundwater systems. It consists of two main modules, a finite element transport module (PLUME2D), and an equilibrium geochemistry module (MINTEQA2). Making use of the local equilibrium assumption, the inherent chemical nonlinearity is confined to the chemical domain. This linearizes the coupling between the physical and chemical processes and leads to a simple and efficient two-step sequential solution algorithm. The advantages of the coupled model include access to the comprehensive geochemical database of MINTEQA2 and the ability to simulate hydrogeological systems with realistic aquifer properties and boundary conditions under complex geochemical conditions. The model is primarily targeted toward groundwater contamination due to acidic mine tailings efiiuents but is potentially also applicable to the full range of geochemical scenarios covered by MINTEQA2. The model is tested with respect to ion exchange chemistry and with respect to precipitation/dissolution chemistry involving multiple sharp fronts. The companion paper presents two-dimensional simulations of heavy metal transport in an acidic mine tailings environment, focusing on environmental implications.

Acid neutralization mechanisms and metal release in mine tailings: a laboratory column experiment

Mining and milling of base metal ore deposits can result in the release of metals to the environment. When sulfide minerals contained in mine tailings are exposed to oxygen and water, they oxidize and dissolve. Two principal antagonistic geochemical processes affect the migration of dissolved metals in tailings impoundments: sulfide oxidation and acid neutralization. This study focuses on acid neutralization reactions occurring in the saturated zone of tailings impoundments. To simulate conditions prevailing in many tailings impoundments, 0.1 mol/L sulfuric acid was passed continuously through columns containing fresh, unoxidized tailings, collected at Kidd Creek metallurgical site. The results of this column experiment represent a detailed temporal observation of pH, Eh, and metal concentrations. The results are consistent with previous field observations, which suggest that a series of mineral dissolution-precipitation reactions control pH and metal mobility. Typically, the series consists of carbonate minerals, Al and Fe(III) hydroxides, and aluminosilicates. In the case of Kidd Creek tailings, the dissolution series consists of ankerite-dolomite, siderite, gibbsite, and aluminosilicates. In the column experiment, three distinct pH plateaus were observed: 5.7, 4.0, and 1.3. The releases of trace elements such as Cd, Co, Cr, Cu, Li, Ni, Pb, V, and Zn were observed to be related to the pH buffering zones. High concentrations of Zn, Ni, and Co were observed at the first pH plateau (pH 5.7), whereas Cd, Cr, Pb, As, V, and Al were released as the pH of the pore water decreased to 4.0 or less.

The potential for metal release by reductive dissolution of weathered mine tailings

Abstract Remediation programs proposed for decommissioned sulphide tailings may include the addition of a cover layer rich in organic-carbon material such as sewage sludge or composted municipal waste. These covers are designed to consume oxygen and prevent the oxidation of underlying sulphide minerals. The aerobic and anaerobic degradation of such organic-carbon-rich waste can release soluble organic compounds to infiltrating precipitation water. In laboratory experiments, and in natural settings, biotic and abiotic interactions between similar dissolved organic compounds and ferric-bearing secondary minerals have been observed to result in the reductive dissolution of ferric (oxy)hydroxides and the release of ferrous iron to pore waters. In weathered tailings, oxidation of sulphide minerals typically results in the formation of abundant ferric-bearing secondary precipitates near the tailings surface. These secondary precipitates may contain high concentrations of potentially toxic metals, either coprecipitated with or adsorbed onto ferric (oxy)hydroxides. Reductive dissolution reactions, resulting from the addition of the organic-carbon covers, may remobilize metals previously attenuated near the tailings surface. To assess the potential for metal release to tailings pore water by reductive dissolution reactions, a laboratory study was conducted on weathered tailings collected from the Nickel Rim mine tailings impoundment near Sudbury, Ontario, Canada. This site was selected for study because it is representative of many tailings sites. Mineralogical study indicates that sulphide minerals originally present in the vadose zone at the time of tailings deposition have been replaced by a series of secondary precipitates. The most abundant secondary minerals are goethite, gypsum and jarosite. Scanning electron microscopy, coupled with elemental analyses by X-ray energy dispersion analysis, and electron microprobe analysis indicate that trace metals including Ni, Cr and Cu are associated with these secondary minerals. To assess the masses of trace metals associated with each of the dominant secondary mineral phases, a series of extraction procedures was used. The masses of metals determined in three fractions (water soluble, reducible and residual) suggest that the greatest accumulation of metals is in the reducible fraction. These measurements indicate that high concentrations of metals are potentially available for release by reductive dissolution of the ferric-bearing secondary minerals. The actual mass of metals that can be released by this mechanism will depend on a number of site-specific characteristics, particularly the intensity of the reducing conditions established near the tailings surface.

Surface chemistry and morphology of poorly crystalline iron sulfides precipitated in media containing sulfate-reducing bacteria

Abstract This study characterizes the surface chemistry and morphology of poorly crystalline iron sulfides precipitated in a chemically defined growth media for sulfate-reducing bacteria. The precipitates were analyzed by X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Based on XRD results, the precipitates exhibit only incipient crystallization with a single broad diffraction peak at about 5 A, characteristic of disordered mackinawite. The iron sulfides generally exhibit a platy habit with particles 100 to 300 nm in diameter; these particles appear often in 1 to 2 μm spherical aggregates. The XPS results indicate that the Fe(2p3/2) spectrum for poorly crystalline iron sulfide can be fitted with Fe(II) and Fe(III) components, both corresponding to Fe–S bond types. The absence of oxide oxygen in the O(1s) spectrum and Fe(III)–O bond types in the Fe(2p3/2) spectrum supports the conclusion that the poorly crystalline iron sulfides are composed of both ferric and ferrous iron coordinated with monosulfide, with lesser amounts of disulfide and polysulfides also present. These results suggest that the precipitates possess a surface composition similar to greigite, with the remaining mineral mass composed of disordered mackinawite.

The hydrogeochemistry of the Nickel Rim mine tailings impoundment, Sudbury, Ontario

From 1953 to 1958, mine tailings were deposited in an elevated impoundment at the Nickel Rim mine, an abandoned Ni–Cu mine near Sudbury, Ontario. The oxidation of sulfide minerals, principally pyrrhotite (8 wt.% of tailings), has generated low-pH waters and released high concentrations of Fe (up to 9.8 g/l), SO4 (up to 24 g/l), dissolved metals (1130 mg/l Al; 698 mg/l Ni) and other dissolved constituents. Groundwater flow through the tailings is mainly horizontal with velocities of 4–8 m/yr. Iron, SO4 and Mn are moving at groundwater velocities and a low-pH front (pH≤4.5) is moving more slowly than the groundwater velocity. A series of pH zones with pH values of 3.0, 4.1, 5.6 and 6.7 is present within the impoundment. The occurrence of these zones is attributed to the successive dissolution of iron (oxy)hydroxides, aluminum hydroxide and aluminosilicates, siderite and calcite. Concentrations of dissolved metals, including Al, Co, Cr, Ni and Zn are controlled by the pore-water pH, except for Cu, which is controlled by the precipitation of covellite. The dominant secondary minerals are goethite, jarosite and gypsum. Accumulations of gypsum and jarosite in the oxidation zone have been great enough to cement the tailings. The cemented layers limit O2 diffusion, resulting in longer oxidation times for the tailings. High concentrations of Fe and SO4 presently in the tailings pore-water will discharge from the impoundment for at least 50 years.

Sulfide mineral oxidation and subsequent reactive transport of oxidation products in mine tailings impoundments: A numerical model

A versatile numerical model that couples oxygen diffusion and sulfide-mineral oxidation (PYROX) has been developed to simulate the oxidation of pyrite in the vadose zone of mine tailings. A shrinking-core oxidation model and a finite element numerical scheme are used to simulate the transport of oxygen and oxidation of pyrite grains. The rate of pyrite oxidation is assumed to be limited by the transport of oxygen to the reaction site. The model determines the spatially variable bulk diffusion coefficient for oxygen on the basis of moisture content, porosity, and temperature, all of which are variable input parameters. The model PYROX has been coupled to an existing reactive transport model (MINTRAN), which uses a finite element scheme for transport of contaminants and MINTEQA2 to solve for the equilibrium geochemistry. The reactions described by MINTRAN are subject to the local equilibrium assumption. The resulting model, MINTOX, is capable of simulating tailings impoundments where the oxidation of pyrite or pyrrhotite is causing acidic drainage and where acid neutralization and attenuation of dissolved metals can be attributed to equilibrium reactions. Because MINTOX uses realistic boundary conditions and hydrogeological properties, the potential benefits of various remediation schemes, such as moisture-retaining covers, can be quantitatively evaluated.

Reactive transport modeling of an in situ reactive barrier for the treatment of hexavalent chromium and trichloroethylene in groundwater

Multicomponent reactive transport modeling was conducted for the permeable reactive barrier at the Coast Guard Support Center near Elizabeth City, North Carolina. The zero-valent iron barrier was installed to treat groundwater contaminated by hexavalent chromium and chlorinated solvents. The simulations were performed using the reactive transport model MIN3P, applied to an existing site-specific conceptual model. Reaction processes controlling the geochemical evolution within and down gradient of the barrier were considered. Within the barrier, the treatment of the contaminants, the reduction of other electron acceptors present in the ambient groundwater, microbially mediated sulfate reduction, the precipitation of secondary minerals, and degassing of hydrogen gas were included. Down gradient of the barrier, water-rock interactions between the highly alkaline and reducing pore water emanating from the barrier and the aquifer material were considered. The model results illustrate removal of Cr(VI) and the chlorinated solvents by the reactive barrier and highlight that reactions other than the remediation reactions most significantly affect the water chemistry in the barrier. In particular, sulfate reduction and iron corrosion by water control the evolution of the pore water while passing through the treatment system. The simulation results indicate that secondary mineral formation has the potential to decrease the porosity in the barrier over the long term and illustrate that the precipitation of minerals is concentrated in the upgradient portion of the barrier. Two-dimensional simulations demonstrate how preferential flow can affect the reduction of electron acceptors, the consumption of the treatment material, and the formation of secondary minerals. In addition, the model results indicate that deprotonation and the adsorption of cations down gradient of the barrier can potentially explain the observed pH buffering.