Ronald V. Nicholson

Pyrite oxidation in carbonate-buffered solution: 2. Rate control by oxide coatings

Abstract The kinetic behaviour of pyrite oxidation in the laboratory was studied over a period of about 10,000 hours in reactors through which a carbonate-buffered solution and air (20% O2) flowed continuously. Three grain size fractions were monitored. The concentration of sulphate and the mass of the effluent solution were measured periodically to calculate oxidation rates. The results indicate that the rates of reaction decreased significantly with time. The rates initially exhibited an inverse dependence on grain size (within 400 h) then became more linear with the square of the inverse grain size at later times (after 8000 h) suggesting a surface-layer control of the reaction with time. Surface analysis by X-ray Photoelectron Spectroscopy revealed the presence of ferric oxide on the pyrite surfaces, and ion boring with auger electron spectroscopy indicated a layer thickness on the order of 0.6 microns on the 215 micron grains. The data are represented by a shrinking core model which includes the effects of the surface rate constant plus the diffusive resistance to oxygen transfer through the accumulating reaction layer as oxidation proceeds. The three grain sizes (representing different specific surface areas) exhibited consistent estimates of the surface rate constant (Ks = 3.07 × 10−6mh−1 ± 46%) and the diffusion coefficient for oxygen through the oxide layer (Ds = 1.08 × 10−12m2h−1 ± 30%). The estimated thickness of the oxide layer at the end of the experiment agreed well with the measured value. Oxide accumulation on the pyrite surfaces under neutral pH conditions results in a significant reduction in oxidation rates over time. This behaviour has important implications for the reduction of the rate of release of oxidation products, including hydrogen ions, to environments where sulphide mineral wastes are exposed to the atmosphere.

Migration of contaminants in groundwater at a landfill: A case study 6. Hydrogeochemistry

Multilevel point-samplers located on a longitudinal cross-section through the plume of contaminated groundwater in the sand aquifer at the abandoned Borden landfill were used to obtain a suite of small-volume samples for analysis of major ions and of minor and trace-level inorganic constituents. Calcium is the dominant cation and sulphate and bicarbonate are the dominant anions in the contaminant plume, with maximum concentrations of 400, 2000 and 1200 mg l−1, respectively. Beneath the landfill the most highly contaminated water in the aquifer has total dissolved solids of ∼4000 mg l−1. Sulphate and iron are the only inorganic constituents that exceed the recommended limits for drinking water. The plume contains above-background concentrations of dissolved zinc and manganese. No heavy metals or other hazardous inorganic trace elements occur at levels above the maximum limits for drinking water. The plume contains levels of total dissolved organic carbon that are above background levels, ranging from 30 mg l−1 beneath the landfill to 5 mg l−1 near the front of the plume. The landfill, which is almost entirely above the water table, contains high partial pressures of carbon dioxide and methane. The high carbon dioxide levels induce substantial dissolution of calcite, which occurs in sand layers in the landfill, and in the aquifer immediately below the landfill. Calcite dissolution is the origin of much of the dissolved inorganic carbon and some of the calcium in the plume. Most of the sulphate and much of the calcium in the plume appears to be derived from gypsum in construction debris in the landfill. Groundwater beneath the landfill is saturated with respect to gypsum and slightly supersaturated with respect to calcite. Some of the dissolved iron in the plume results from the release under reducing conditions of iron from ferric iron coatings on sand grains located in the aquifer beneath the landfill and in the layers in the refuse. Dissolved iron concentrations appear to be limited by the solubility of siderite. Dissolved sulphide is present at only very low concentrations and is probably controlled by ferrous sulphide solubility. Exchange of cations between the aquifer material and the leachate-contaminated groundwater is an apparent cause of calcium release and subsequent precipitation of calcite in the plume. As much of 80% of the K+, 20% of the Mg2+ and 15% of the Na+ in the plume exists on the exchange sites. The effects of dispersive and geochemical attenuation were evaluated from the ratios of chloride to other species along a central flow path from the landfill to the front of the plume. Platinum electrode measurements and electrochemical potentials calculated from equilibrium speciation of analysed constituents varied over a wide range of values and provided poor agreement among methods for the same sample. Only potentials derived from the iron redox couples and Pt-electrode measurements exhibited fair agreement for any one sample. The rather narrow range of Pt-electrode potential values along the central flow line through the plume suggest that the electrochemical potentials are buffered, apparently as a result of Fe(OH)3(s) /ag FeCO3(s) equilibria.

Geochemical characterization of acid mine drainage from a waste rock pile, Mine Doyon, Québec, Canada

Water quality in the unsaturated and saturated zones of a waste rock pile containing sulphides was investigated. The main objectives of the project were (1) the evaluation of geochemical trends including the acid mine drainage (AMD)-buffering mechanism and the role of secondary minerals, and (2) the investigation of the use of stable isotopes for the interpretation of physical and geochemical processes in waste rock. Pore water in unsaturated zone was sampled from suction lysimeters and with piezometers in underlying saturated rocks. The investigation revealed strong temporal (dry period vs. recharge period), and spatial (slope vs. central region of pile) variability in the formation of acid mine drainage. The main secondary minerals observed were gypsum and jarosite. There was a higher concentration of gypsum in solid phase at Site TBT than at Site 6, suggesting that part of the gypsum formed at Site 6 in the early stage of AMD has been already dissolved. Formation of secondary minerals contributed to the formation of AMD by opening of foliation planes in waste rock, thus increasing the access of oxidants like O2 and Fe3+ to previously encapsulated pyrite. The behavior of several dissolved species such as Mg, Al, and Fe2+ can be considered as conservative in the leachate. Stable isotopes, deuterium and 18O, indicated internal evaporation within the pile, and were used to trace recharge pulses from snowmelt. Isotope trends for 34S and 18O(SO4) indicated a lack of sulfate reduction and zones of active oxidation of pyrite, respectively. Results of numerical modeling of pyrite oxidation and gas and water transport were consistent with geochemical and isotopic trends and confirmed zones of high evaporation rate within the rock pile close to the slope. The results indicate that physical and chemical processes within the pile are strongly coupled and cannot be considered separately when oxidation rates are high and influence gas transport as a result of heat generation.

A combined kinetic and diffusion model for pyrite oxidation in tailings : a change in controls with time

Abstract Acidic drainage from the oxidation of mine tailing wastes is an important environmental problem. The purpose of this paper is to develop a model (1) to simulate the rate of oxidation of pyrite over time, (2) to verify the importance of chemical kinetic control and diffusion control on the oxidation rate with time and, (3) to evaluate the sensitivity of the model to critical parameters of the tailings, such as grain size, pyrite content and the effective diffusion coefficient. The source code comprises four main modules including parameter allocation (kinetics, transport), sulphide oxidation (shrinking particle), oxygen transport and pyrite mass balance. The results show that high oxidation rates are observed in the initial time after tailings deposition. During this initial period of high rates, an apparent shift occurs from kinetic to diffusional control over a period of time that depends on the composition and properties of the tailings. Based on the simulation results, it is evident that the overall rate of oxidation after a few years will be controlled dominantly by the diffusion of oxygen rather than by biological or non-biological kinetics in the tailings.

Field determination of sulphide oxidation rates in mine tailings

A field study was conducted to assess the rates of oxygen consumption in the vadose zone of sulphide mine tailings. Oxygen uptake rates were measured directly at the ground surface and were compared to rates of sulphate production in the tailings profile. A nonreactive tracer gas was also used to assess effective diffusion coefficients in the shallow subsurface. The 26 measurement sites were located within a 0.1 km2 area in tailings with a water table depth ranging from 0.6 m to ponding at the surface. Ambient oxygen consumption rates in the 8-year-old tailings were compared to measurements through 0.2-m-thick covers of fine sand and 0.2 m layers of fresh tailings replacing oxidized tailings at ground surface. The oxygen flux across the tailings surface ranged from 0.1 to 250 mol O2 m−2 yr−1 for ambient oxidation conditions. Fresh tailings at the surface produced rates that were as much as twice the ambient rates. The 0.2 m fine sand cover lowered rates by a factor of 100 below ambient rates. Sulphate production rates agreed well with oxygen consumption measurements. Independent diffusion flux measurements with a nonreactive gas exhibited an excellent correlation with oxygen uptake fluxes, indicating the dominance of oxygen diffusion in the control of oxidation rates in the tailings. Spatial trends in oxidation rates were consistent with depth to the water table, and temporal changes by as much as a factor of 2 were attributed to a 0.16 m variation in the depth to the water table over a 2-week period. The results of this study support the use of simple Fickian models to evaluate oxygen diffusion and oxidation rates in sulphide tailings.

An arsenate effect on ferrihydrite dissolution kinetics under acidic oxic conditions

The dissolution behaviour of ferrihydrite and ferrihydrite incorporating arsenate at different levels was investigated under low pH, oxic conditions. The dissolution of pure ferrihydrite was found to be well described by the cube root law for mineral dissolution. The rate of dissolution of ferrihydrite precipitated in the presence of selected levels of arsenate was strongly retarded by the presence of the arsenate. The surface coordinative chemical model of mineral dissolution was applied to interpret and quantify proton dissolution rates of ferrihydrite and the inhibitory effect of arsenate. The inhibitory effect of the arsenate appears to be related to its complexation with the ferrihydrite surface and the consequent involvement of the surface iron centres in competitive equilibria between protons and the surface-complexed arsenate. From the iron:arsenic ratios obtained during dissolution, it has been concluded that arsenic is incorporated into these precipitates by sorption onto existing surfaces of the ferrihydrite during precipitation, rather than by the formation of a solid solution or by co-deposition.

The effect of phosphate on the transformation of ferrihydrite into crystalline products in alkaline media

The presence of phosphate retards the transformation of ferrihydrite into crystalline products. Increasing phosphate from 0 to 1 mole % results in an order of magnitude decrease in the rate of transformation of ferrihydrite at pH 12. Levels of phosphate of ∼1 mol % suppress the formation of goethite (α-FeO(OH)) and result in the formation of a product consisting ofη-Fe2O3. Higher levels of phosphate result in the ferrihydrite remaining amorphous, even after several hundred hours. Phosphate prevents formation of goethite by hindering the dissolution of ferrihydrite rather than by interfering with nucleation and growth of goethite in solution. The transformation rate of pure ferrihydrite is also strongly inhibited in the presence of dissolved phosphate. This is due to surface complexation. The transformation rate was measured at temperatures of 60 °C and 70 °C. The rate of transformation was found to be described by either (i) a solid-state reaction equation for powdered compacts or (ii) a zero-order reaction controlled by desorption. The transformation of the ferrihydrite matrix was accompanied by the loss of the phosphate trace component. X-ray diffraction indicates that no solid solution involving phosphate substitution intoη-Fe2O3 is formed. Transmission electron microphotographs of the original precipitates containing phosphate confirm the presence of the phosphate and demonstrate its involvement in linking together extremely small particles of ferrihydrite.

Origin of Dissolved Groundwater Sulphate in Coastal Plain Sediments of the Rio de la Plata, Eastern Argentina

Groundwater of the coastal plain of the Rio de La Plata, Argentina,contains up to 17 g L-1 SO4 and 37 gL-1 TDS. Some of this SO4 is from paleo-seawater intrusion; however, SO4 : Cl ratios can be>2 : 1, and most of the SO4 must, therefore, have another source. Three possible sources were investigated: gypsum, organic matter,and iron sulphides.Dissolved SO4 showed δ34S valuesfrom -7 to 0‰, typical values for S from iron sulphides or organicmatter, but distinct from that of seawater (+22‰). To test whetherthe SO4 was derived from oxidation of reduced S, four 4-mcores were taken from marine sediments of the coastal plain. Two were takenfrom higher, drier areas where the highest dissolved SO4values were encountered, and two were taken from lower, wetter areas thathad much lower SO4 concentrations. Pore waterSO4, Cl and alkalinity were determined; solids were analyzedfor SO4, sulphide-S and organic-S.Sulphide-S was the dominant form of reduced S, averaging about0.5% S in the lower interval (2.5–4 m) of the cores. Sulphidewas absent in the upper 2.5 m in both topographically higher and lowerareas. Sulphate was present in the entire unit in the higher, drier areas,but almost absent in lower areas. Organic-S was insignificant.Our model for the origin of dissolved SO4 is: fine-grainedpyrite was oxidized during hotter or drier periods. Some resulting dissolvedSO4 was precipitated as gypsum. Iron from the pyriteprecipitated as FeOOH. Lower, wetter areas formed over time where recharginggroundwater dissolved most of the gypsum. In higher areas with low hydraulicgradients and high net evapotranspiration, SO4 remained asgypsum and in the dissolved phase.

Evaluation of bacterial community structure and its influence on sulfide oxidation in a bio-leaching environment.

The mechanism of sulfide oxidation by adhering bacteria (direct oxidation mechanism) and by ferric ion in the aqueous phase was studied by quantitative assessment of bacterial activity on the sulfide surface. To probe for the principal bacterial species on the surface and in the supernatant, a library of DNA genes encoding portions of bacterial 16S rRNA was constructed. The PCR-amplified DNA from the bacterial populations was cloned employing PROMEGAs pGEM-T Easy Vector system. The clone frequency indicated that iron-oxidizing bacteria were dominant in the liquid phase, while Acidithiobacillus ferroixdans, which is both sulfur and iron oxidizer was the most prevalent on the sulfide surface. Samples of crystalline pyrite were exposed to the bacterial consortium to evaluate surface alterations caused by bacteria. Chemical (abiotic) oxidation experiments with ferric ion as the oxidant were carried out in parallel with the biological oxidation tests. Changes in the surface topography were monitored by atomic force microscopy (AFM) while changes in surface chemistry were examined by Raman spectroscopy. Bacterial attachment resulted in a 53% increase in the specific surface area in comparison to a 13% increase caused by chemical (ferric ion) oxidation. The oxidation rate was assessed by evaluating the iron release. After corrections for surface area changes, the specific abiotic (oxidation by Fe3 +) and biotic oxidation rates with adhering bacteria were nearly the same (2.6 × 10− 9 mol O2/s/m2 versus 3.3 × 10− 9 mol O2/s/m2) at pH = 2 and a temperature of 25°C. The equality of rates implies that the availability of ferric ion as the oxidant is rate limiting.

Acid generation modelling : Equity Silver waste rock dumps

The Equity Silver mine near Houston B.C. is proposing to cease operations in 1992. The large open pit operation has produced more than 80 million tonnes of waste rock which for the most part has been placed into surface dumps. The waste contains about 4 percent sulphide minerals and is a strong acid generator. Since 1981 Equity Silver Mines Limited (Equity) have been intercepting and treating seepage during which time there has been a steadily increasing load of acidity.