Rate Of Sulfide Oxidation Research Articles

Large-Scale Oxygen Injection for Odor Control at Primary Clarifiers at the Trinity River Authority of Texas

The Trinity River Authority of Texas owns and operates the 162-MGD Central Regional Wastewater Treatment Plant in Grand Prairie, Texas. The plant is an advanced secondary facility consistently producing nitrified effluent with BOD and TSS concentrations less than 5 mg/L each and an ammonia nitrogen concentration less than 0.5 mg/L. In addition to providing high quality effluent, the Authority is committed to being a good neighbor to the commercial, recreational, and residential areas surrounding the plant. As such, the Authority has committed significant funds and manpower to minimize the emission of odors from the plant. In projects spanning more than 20 years, the plant has eliminated or minimized emissions from major odor sources such as the preliminary treatment areas, influent junction boxes, primary clarifier weirs, and sludge processing areas. The plant eliminated use of its sludge-only landfill except for screenings and implemented beneficial reuse of all its sludge. The next phase of odor control would include the management of odors from aerated grit units and the quiescent areas of the primary clarifiers. Covering the primary clarifiers could involve literally acres of covers and the collection and treatment of well over 150,000 cfm of foul air. As an alternative to the covering of the primary clarifiers and collection and treatment of the air, the Authority, with Alan Plummer Associates, Inc., conducted “full-scale pilot studies” to investigate the feasibility of using pure oxygen for odor control. Pure oxygen has been used for odor control in pressure lines in collection systems and in receiving streams to boost oxygen concentrations during critical low-flow periods. Use of pure oxygen for odor control at a wastewater treatment plant, however, is a new application. Following the success of the pilot study, the Authority conducted a second bench-scale study to identify optimum dosage rates and injection points. This study identified the reaction rate of sulfide oxidation for the wastewater stream and provided a basis of design. The system began operation in June 2007. Following startup, the Authority conducted performance testing to identify the reduction in hydrogen sulfide, reduced sulfide, total odor (dilution to threshold) and other parameters.

Bio-oxidation of pyrite, chalcopyrite and pyrrhotite by Acidithiobacillus ferrooxidans

This paper deals with the bio-oxidation processes by Acidithiobacillus ferrooxidans of pyrite, chalcopyrite and pyrrhotite. Our experimental results show distinctive bio-oxidation characteristics for the three sulfide minerals. In the presence of A. ferrooxidans, the sulfide oxidation rates generally decrease in the order of pyrrhotite, chalcopyrite and pyrite. The pH during bio-oxidation of pyrite tends to decrease as a whole, whereas a rise-fall pattern was recorded for both chalcopyrite and pyrrhotite in their pH variations. No deposition was observed during the bio-oxidation of pyrite, suggesting a possible link to lower pH value in the process. However, large amounts of jarosite and element sulfur were determined in the bio-oxidation processes of chalcopyrite and pyrrhotite. A. ferrooxidans individuals were found directly as attachments to erosion pits on the smooth surface of pyrite. The erosion pits are similar to the bacterium in shape and length, and thus are probably products of dissolution of organic acid secreted by the cells on the mineral surface. More complicatedly, biofilm exists on the surfaces of chalcopyrite and pyrrhotite. This type of structured community of A. ferrooxidans is enclosed in the extracellular polymeric substances (EPS), and covered with the deposition generated in the bio-oxidation processes of chalcopyrite and pyrrhotite. Different bio-oxidation processes of pyrite, chalcopyrite and pyrrhotite may be linked mainly to characteristics of individual minerals and the pH in the reaction solution of the bio-oxidation system.

Effects of different quinoid redox mediators on the removal of sulphide and nitrate via denitrification

The impact of different quinoid redox mediators on the simultaneous conversion of sulphide and nitrate in a denitrifying culture was evaluated. All quinones evaluated, including anthraquinone-2,6-disulphonate (AQDS), 2-hydroxy-1,4-naphthoquinone and 1,2-naphthoquinone-4-sulphonate (NQS) were reduced by sulphide under abiotic conditions. NQS showed the highest reduction rate by sulphide (132 μmol h −1) and promoted the maximum rate of sulphide oxidation (87 μmol h −1) by denitrifying sludge, which represents an increase of 44% compared to the control lacking quinones. The reduced form of AQDS (AH 2QDS) served as an electron donor for the microbial reduction of nitrite and N 2O, which represents the first demonstration of hydroquinones supporting the microbial reduction of denitrifying intermediates. The results taken as a whole suggest that some quinones may significantly increase the rate of removal of S and N under denitrifying conditions.

Removal of malodorous organic sulfides with molecular oxygen and visible light over metal phthalocyanine

Organic sulfides are malodorous compounds in environment. In this work, deodorization of model substrates, methyl phenyl sulfide, 2-mercaptobenzoic acid and benzyl 2-propenyl sulfide, have been studied in an aerated methanolic aqueous solution under visible light irradiation ( λ > 450 nm), using metal phthalocyanine sulfonate (MPcS, M = Al, Pd) as a photocatalyst. The result shows that all the representative sulfides could be efficiently oxidized, with concomitant formation of sulfoxide and sulfone as the main products. Kinetic study using sodium azide and benzoquinone as reactive species scavenger reveals that the sulfide oxidation is mainly initiated by singlet oxygen. It is also observed that the rate of sulfide oxidation increases with increasing the water content in the mixed solvent. Recycle experiments with immobilized PdPcS on organoclay or immobilized AlPcS on anionic resin demonstrates that the sensitizer could be repeatedly used, without significant loss in the photosensitization activity.

Effects of Iron on Chemical Sulfide Oxidation in Wastewater from Sewer Networks

Effects of iron on the kinetics and stoichiometry of aerobic chemical sulfide oxidation in wastewater from two different sites were studied at pH 8 and 20°C. Iron(III) chloride was added to the wastewater in concentrations of up to 20 g Fe m -3 . The rate of aerobic chemical sulfide oxidation increased linearly with the iron(III) additions resulting in equal effects with wastewater from the two sites. Despite the significant effect of the iron(III) additions, the background concentrations of iron cannot explain the significant temporal and spatial variability of aerobic chemical sulfide oxidation kinetics reported in this study and in the literature. In this respect, other metals are probably also important. In addition to the impacts on the oxidation kinetics, the iron(III) additions resulted in a change of the oxidation stoichiometry. With increasing amounts of iron(III) added to the wastewater, less dissolved oxygen was required for the sulfide oxidation.

Autotrophic Removal of Sulphide from Industrial Wastewaters Using Oxygen and Nitrate as Electron Acceptors

The effect of environmental conditions on sulphide removal was studied in laboratory-scale batch experiments. Sulphide removal was carried out by adding two different electron acceptors (oxygen and nitrate) and stock sulphide solution to activated sludge solutions obtained from a wastewater treatment plant treating molasses-based industrial wastewaters. The effects of pH, temperature, and concentration of activated sludge on specific sulphide oxidation rates were studied. It was found that 80% sulphide removal was achieved when oxygen was used as an electron acceptor with activated sludge, whereas almost 100% sulphide removal was obtained when nitrate was used. Sulphide removal without activated sludge was also determined. The specific sulphide oxidation rate increased approximately 88% with increasing activated sludge concentration with nitrate as an electron acceptor and sulphide was completely removed within 6 min. When oxygen was used as an electron acceptor, specific removal increased up to 92%; however, it was observed that an amount of sulphide remained in the medium, even though the activated sludge concentration was increased. Additionally, the stoichiometry of sulphide oxidation both with nitrate and oxygen were calculated assuming different end products based on thermodynamic approach and compared with experimental yield values.

Sustained sulfide oxidation by physical erosion processes in the Mackenzie River basin: Climatic perspectives

The chemical weathering of rocks with sulfuric acid is usually not considered in reconstructions of the past evolution of the carbon cycle, although this reaction delivers cations and alkalinity to the ocean without involvement of atmospheric CO 2 . The contribution of sulfuric acid as a weathering agent is still poorly quantifi ed; the identifi cation of riverine sulfate sources is diffi cult. The use of δ 34 S and δ 18 O of dissolved sulfate allows us to demonstrate that most of the sulfate in surface waters of the Mackenzie River system, Canada, derives from pyrite oxidation (85% ± 5%) and not from sedimentary sulfate. The calculated fl ux of pyrite-derived sulfate is 0.13 ◊ 10 12 mol/yr, corresponding to 20%‐27% of the estimated global budget. This result suggests that the modern global ocean delivery of sulfi de-derived sulfate, and thus chemical weathering with sulfuric acid, may be signifi cantly underestimated. A strong correlation between sulfi de oxidation rates and mechanical erosion rates suggests that the exposure of fresh mineral surfaces is the rate-limiting factor of sulfi de oxidation in the subbasins investigated. The chemical weathering budget of the Mackenzie River shows that more than half of the dissolved inorganic carbon discharged to the ocean is ancient sedimentary carbon from carbonate (62%) and not atmospheric carbon (38%). The subsequent carbonate precipitation in the ocean will thus release more CO 2 in the atmosphere-ocean system than that consumed by continental weathering, typically on glacial-interglacial time scales.

Progressive oxidation of pyrite in five bituminous coal samples: An As XANES and 57Fe Mössbauer spectroscopic study

Naturally occurring pyrite commonly contains minor substituted metals and metalloids (As, Se, Hg, Cu, Ni, etc.) that can be released to the environment as a result of its weathering. Arsenic, often the most abundant minor constituent in pyrite, is a sensitive monitor of progressive pyrite oxidation in coal. To test the effect of pyrite composition and environmental parameters on the rate and extent of pyrite oxidation in coal, splits of five bituminous coal samples having differing amounts of pyrite and extents of As substitution in the pyrite, were exposed to a range of simulated weathering conditions over a period of 17 months. Samples investigated include a Springfield coal from Indiana (whole coal pyritic S = 2.13 wt.%; As in pyrite = detection limit (d.l.) to 0.06 wt.%), two Pittsburgh coal samples from West Virginia (pyritic S = 1.32–1.58 wt.%; As in pyrite = d.l. to 0.34 wt.%), and two samples from the Warrior Basin, Alabama (pyritic S = 0.26–0.27 wt.%; As in pyrite = d.l. to 2.72 wt.%). Samples were collected from active mine faces, and expected differences in the concentration of As in pyrite were confirmed by electron microprobe analysis. Experimental weathering conditions in test chambers were maintained as follows: (1) dry Ar atmosphere; (2) dry O 2 atmosphere; (3) room atmosphere (relative humidity ∼20–60%); and (4) room atmosphere with samples wetted periodically with double-distilled water. Sample splits were removed after one month, nine months, and 17 months to monitor the extent of As and Fe oxidation using As X-ray absorption near-edge structure (XANES) spectroscopy and 57Fe Mössbauer spectroscopy, respectively. Arsenic XANES spectroscopy shows progressive oxidation of pyritic As to arsenate, with wetted samples showing the most rapid oxidation. 57Fe Mössbauer spectroscopy also shows a much greater proportion of Fe 3+ forms (jarosite, Fe 3+ sulfate, FeOOH) for samples stored under wet conditions, but much less difference among samples stored under dry conditions in different atmospheres. The air-wet experiments show evidence of pyrite re-precipitation from soluble ferric sulfates, with As retention in the jarosite phase. Extents of As and Fe oxidation were similar for samples having differing As substitution in pyrite, suggesting that environmental conditions outweigh the composition and amount of pyrite as factors influencing the oxidation rate of Fe sulfides in coal.

ARD generation and corrosion potential of exposed roadside rockmass at Boeun and Mujoo, South Korea

Acid rock drainage (ARD) is a longstanding problem often associated with the resulting corrosion due to the acidity generated from sulfidic oxidation. To evaluate characteristics of ARD and corrosion, samples from the road side rock mass of Boeun and Mujoo were analysed using X-ray diffraction, acid/base accounting and Leaching tests. The results indicated that many samples had a pyritic origin and can be regarded as acid-generating rocks. The Leaching test showed that the average pH of the leachates of samples from both Boeun and Mujoo were moderately acidic, ranging from 3 to 4. Interestingly, as acidity increases from pH 4, the SO4−, Fe, Al and Mg concentrations increase abnormally. Samples from roadside slope of Mujoo showed high corrosive potential. Maximum sulfide oxidation rate of a sample taken from Mujoo was as high as 5,166 mg/kg/week.

Geochemistry of sulfur in an inland acid sulfate soil system

Introduction The Loveday basin was a natural flooded wetland adjacent to the River Murray. However, for three decades the site was used as a disposal basin for saline groundwater, which has led to an increased salinization of the area and accumulation of sulfidic sediments. The purpose of this study is to determine the effect of changing water regimes on S geochemistry. Results and Discussion Surface sediments contain ~ 0.01 to 1% reduced S minerals and up to 10% gypsum, though concentrations are extremely heterogeneous due to the pedal stucture that forms as a result of wetting and drying of the sediments. Total S concentrations generally decrease with depth [1]. Water samples collected from the basin are extremely saline, with Cl ranging from ~ 9000 to 58,000 mg/L. Surface water sulfate is a mix of biological sulfate reduction (BSR) residual (enriched S and O) and sulfide oxidation (depleted S and O). Pit (pore)water sulfate has an oxidized sulfide (depleted S and O) source. Experiments were conducted to determine S-oxidation/ Sflux from sediments collected from ‘wet’ (reduced), intermittantly wet-dry (partially oxidized and slightly acidic), and ‘dry’ parts of the basin to overlying water. Sediments were added to water, dilute HCl (pH ~ 2.4), and 1 mM FeCl3 and allowed to incubate for weeks. The initial stages of the experiments show S flux from the surface sediments is dominated by gypsum dissolution. However, subsequent leaches of surface sediments, and experiments with sediments from deeper in the basin had increasing SO4 concentrations over time, Ca/SO4 ratios < 0.4 (g/g), indicating oxidation of reduced FeS minerals, though solutions did not become acidic and Fe concentration was was below detection. Adding acid did not substantially increase sulfide oxidation rate (< 2 fold) compared to ~ neutral pH conditions. Ferric iron greatly increased Fe release to solution, ~ 2 to 4 orders of magnitude compared to acid conditions, but had minimal effect on S release, < 2 fold increase.

Experimental Oxidative Dissolution of Sphalerite in the Aznalcóllar Sludge and Other Pyritic Matrices

After the collapse on 25 Apr. 1998 of the Aznalcóllar mine tailings dike in southwestern Spain, 45 km2 of the Guadiamar valley were covered by a pyritic sludge containing up to 2% sphalerite (ZnS). Later, the sludge was mechanically removed and calcium carbonate was plowed into the soil to immobilize heavy metals. By June 2001 more than 60% of the sulfides in the residual sludge had oxidized and soil Zn contents reached locally phytotoxic levels. Therefore, the oxidative dissolution of sphalerite in the sludge and other pyritic samples was examined. Flow-through oxidation experiments showed that: (i) about 5 and 17% of the sludge Fe and Zn were in soluble form, respectively, because the sludge sample had been partly oxidized in the field; (ii) the oxidation rates of the residual pyrite and sphalerite were similar; (iii) the overall sulfide oxidation rate was relatively unaffected by the addition of calcite; and (iv) poorly crystalline Fe (hydr)oxides containing Zn in occluded form and Zn (hydroxi)carbonates were formed in the presence of calcite. The rate of oxidation of reference sphalerite greatly increased when it was incorporated in the sludge or in a reference pyrite matrix. This enhancement was due to galvanic interaction because pyrite oxidation was depressed in the presence of sphalerite. Oxidation by Fe3+ ions was less important because the oxidation rates of native sphalerite were not greater at low than at high pH. The fast oxidation rate of sphalerite in the Aznalcóllar sludge indicates a need for quick adoption of remediation measures in similar accidents elsewhere. The use of calcite amendments has little influence on the oxidation rate but does result in the accumulation of Zn in relatively insoluble forms.

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The Meadowbank Gold Project of Cumberland Resources Limited is located 70 km north of Baker Lake in the arid, arctic environment of Nunavut Territory, Canada. With a site annual average temperature of -11C, and 310 mm/year precipitation, the extrapolation of constituent loading rates from laboratory tests is particularly precarious. Three different scales of kinetic leaching tests were performed on waste rock samples: 1-kg and 100-kg laboratory leaching cells and 250-kg field cell tests. Field tests yielded considerably slower rates of buffering capacity depletion and sulfide oxidation than laboratory-derived rates, although the differences were not consistent between the various test scales. Additional

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Source control of acid rock drainage (ARD) requires consideration of both biological and abiotic mechanisms of metal sulfide oxidation. A promising approach is to couple a biocide with phosphate for application to sulfidic waste materials. This approach aims to 1) inhibit or kill with thiocyanate (SCN) the iron-oxidizing acidophilic microorganisms that accelerate sulfide oxidation and 2) precipitate FePO4 and AlPO4, thereby removing ferric ion oxidant and Lewis acidity, and, in the process, armoring the surface of pyrite to retard its abiotic oxidation. Thiocyanate effectively reduces sulfide biooxidation if it is applied efficiently and is not washed out of the system by rainfall or adsorbed to the rock. Several sources of phosphate including phosphate rock, waste material and agricultural products were characterized and tested in combination with SCN for their ability to retard oxidation of sulfidic waste rock. Waste rock sources include samples from base metal and precious metal mines and a coal mine. The acid neutralizing capacities (ANC) of different sources of phosphate were compared and evaluated using an artificial ARD solution which included iron and aluminum sulfates. Thiocyanate alone in laboratory tests sharply reduced ARD production, approaching the abiotic rate of sulfide oxidation. Whether thiocyanate plus phosphate further reduced ARD production depended on the mineral sulfide composition of the waste rock and the parameter measured. The abiotic sulfide oxidation rate of sphalerite-containing waste rock was reduced with phosphate treatment, most likely by removal of residual ferric iron from leach solutions. Phosphate did not further reduce abiotic sulfide oxidation rates with a pyritic waste rock, but did significantly reduce the soluble iron and other metal content of leachates. The combined thiocyanate plus phosphate treatment minimized biooxidation, removed ferric ion oxidant, and restricted formation of Lewis acidity but has not shown evidence of armoring pyrite. The laboratory test work guided the set up and operation of a 3000 t field test. ARD production was reduced over 50% in the first season with combined thiocyanate and phosphate treatment. However, washout of thiocyanate and its adsorption to rock reduced its concentrations in leachates to near zero in the second year of the test, greatly reducing the effectiveness of chemical treatment. Water soluble forms of thiocyanate (NaSCN) are best used to reduce ARD in situations where rainfall infiltration is low. The development of slowor controlled-release thiocyanate products combined with phosphates would be beneficial. Additional key words: ARD prevention, phosphate, thiocyanate ______________________ 1 Paper presented at the 7 th International Conference on Acid Rock Drainage (ICARD), March 26-30, 2006, St. Louis MO. R.I. Barnhisel (ed.) Published by the American Society of Mining and Reclamation (ASMR), 3134 Montavesta Road, Lexington, KY 40502 2 Gregory J. Olson, Vice President, Thomas R. Clark, Senior Scientist, Little Bear Laboratories, Inc., Golden, CO 80403; Terry I. Mudder, Managing Director, TIMES, Ltd., Sheridan, WY 82801; Mark Logsdon, Principal Geochemist, Geochimica Inc., Aptos, CA 95003. 7 th International Conference on Acid Rock Drainage, 2006 pp 1435-1452 DOI: 10.21000/JASMR06021435

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The interaction between geochemical and hydrological processes in waste rock was evaluated in an instrumented waste rock pile as part of ongoing studies into the scale-up of water chemistry predictions. Sulfate and metal concentrations were determined in both pore water and outflow waters collected in sixteen basal lysimeters over a four-year period. The waste rock contained approximately 0.5% sulfur as pyrite/pyrrhotite and was acid generating. The correlation between outflow rate and concentrations was negative during times when macropore flow was active and positive over longer periods. Sulfate concentrations greater than 40,000 mg/L and elevated metal concentrations including cobalt, manganese, nickel, strontium, uranium and zinc were observed in the acidic (pH ~ 3.6) pore water and outflow water. Over the four–year experiment approximately 5% of the initial sulfur was released at the base of the pile. Predicted sulfide oxidation rates determined from laboratory experiments were up to four times higher than the rates inferred from the field experiment (0.1 mg SO4 -2 /kg rock/week to 19 mg SO4 -2 /kg rock/week). A lower-permeability cover placed on the surface of the waste rock pile three years after the experiment started induced a decrease of the dissolved load in the effluent from a pre-cover average of 4.8 mg SO4 -2 /kg rock/week to 1.2 mg SO4 -2 /kg rock/week. Additional

Kinetics and Stoichiometry of Aerobic Sulfide Oxidation in Wastewater from Sewers—Effects of pH and Temperature

Kinetics and stoichiometry of aerobic chemical and biological sulfide oxidation in wastewater from sewer networks were studied. In this respect, the effects of temperature and pH were investigated in the ranges 10 to 20 degrees C and 5 to 9, respectively. The temperature dependency of sulfide oxidation kinetics was described using an Arrhenius relationship. The effect of pH on the rate of chemical sulfide oxidation is related to the dissociation of hydrogen sulfide (H2S) to hydrogen sulfide ion (HS(-)), with HS(-) being more readily oxidized than H2S. Biological sulfide oxidation exhibited the highest rates at ambient wastewater pH, and the reaction was inhibited at both low and high pH values. Chemical sulfide oxidation was found to produce thiosulfate and sulfate, while elemental sulfur was the main product of biological sulfide oxidation. Based on the investigations, general rate equations and stoichiometric constants were determined, enabling the processes to be incorporated to conceptual sewer process models.

Chemistry of acid production in black coal mine washery wastes

Column leaching tests on black coal mine washery wastes were performed, to determine the chemistry of acid generation. Coal mine coarse rejects and tailings were subjected to wet and dry cycle dissolution and subsequently column leached. The rates of iron sulphide oxidation and carbonate mineral dissolution were determined based on the drainage chemistry. The kinetic data from column leach experiments are used to predict the time required to deplete the acid producing and acid consuming minerals in the mine wastes. The acid production in the mine rejects was found to depend upon iron chemistry, carbonate chemistry, diffusion of oxygen, and permeability. The chemistry of the drainage from two different coal mines is compared.

Hydrochemical models of the sulphidic tailings dumps at Matchless (Namibia) and Selebi-Phikwe (Botswana)

The sulphidic tailings dumps at Matchless (Namibia) and Selebi-Phikwe (Botswana) are located in a similar semiarid environment but have a contrasting mineralogical composition. The Matchless tailings are pyrite-rich, whereas the Selebi-Phikwe tailings are dominated by pyrrhotite. Hydrochemical models are established with computer codes for water-balance, sulphide oxidation rate and hydrochemical equilibrium calculations. The data input is based on detailed mineralogical, chemical and kinetic investigations carried out on the core of boreholes drilled in 2000 and 2003. The oxidation of pyrrhotite proceeds at a much faster rate than the oxidation of pyrite. The PYROX code, which is used for kinetic calculations, can take these differences into account by applying different oxide-coating diffusion coefficients (D2) for pyrrhotite and pyrite. Humidity-cell testing is widely used to predict the post-mining composition of drainage water in humid climates. However, the semiarid conditions at Matchless and Selebi-Phikwe only allow a minimal water flux within the dump. Under such conditions, humidity-cell testing is likely to overestimate the seepage-water pH. This is suggested by the hydrochemical equilibrium calculations for the post-mining period at Selebi-Phikwe, which predict a seepage-water pH about one unit lower than the pH at the end of the 26-weeks humidity-cell testing period. The acidity of the seepage water can be reduced by about half a pH unit, if an oxygen barrier below the evaporation zone is installed. A clay layer 0.5 m thick covered by >1.5 m tailings represents the optimal design for a wet barrier. All three computer codes used for water-balance calculations (HELP3, UNSAT-H and HYDRUS-1D), predict >85% average water saturation for such a layer, which diminishes the diffusion of oxygen into the pile and production of SO4−2 and H+. The alternative design for a dry barrier consists of a vegetated silt layer 1 m thick on top of the tailings. This barrier does not significantly influence the diffusion of oxygen although it reduces the net infiltration to ≤11 mm/year.

The role of sulfur in chemical weathering and atmospheric CO 2 fluxes: Evidence from major ions, δ 13C DIC, and δ 34S SO4 in rivers of the Canadian Cordillera

Water samples from the Fraser, Skeena and Nass River basins of the Canadian Cordillera were analyzed for dissolved major element concentrations (HCO 3 −, SO 4 2−, Cl −, Ca 2+, Mg 2+, K +, Na +), δ 13C of dissolved inorganic carbon (δ 13C DIC), and δ 34S of dissolved sulfate (δ 34S SO4) to quantify chemical weathering rates and exchanges of CO 2 between the atmosphere, hydrosphere, and lithosphere. Weathering rates of silicates and carbonates were determined from major element mass balance. Combining the major element mass balance with δ 34S SO4 (−8.9 to 14.1‰ CDT) indicates sulfide oxidation (sulfuric acid production) and subsequent weathering of carbonate and to a lesser degree silicate minerals are important processes in the study area. We determine that on average, 81% of the riverine sulfate can be attributed to sulfide oxidation in the Cordilleran rivers, and that 25% of the total weathering cation flux can be attributed to carbonate and silicate dissolution by sulfuric acid. This result is validated by δ 13C DIC values (−9.8 to −3.7‰ VPDB) which represents a mixture of DIC produced by the following weathering pathways: (i) carbonate dissolution by carbonic acid (−8.25‰) > (ii) silicate dissolution by carbonic acid (−17‰) ≈ (iii) carbonate dissolution by sulfuric acid derived from the oxidation of sulfides (coupled sulfide-carbonate weathering) (+0.5‰). δ 34S SO4 is negatively correlated with δ 13C DIC in the Cordilleran rivers, which further supports the hypothesis that sulfuric acid produced by sulfide oxidation is primarily neutralized by carbonates, and that sulfide-carbonate weathering impacts the δ 13C DIC of rivers. The negative correlation between δ 34S SO4 and δ 13C DIC is not observed in the Ottawa and St. Lawrence River basins. This suggests other factors such as landscape age (governed by tectonic uplift) and bedrock geology are important controls on regional sulfide oxidation rates, and therefore also on the magnitude of sulfide-carbonate weathering—i.e., it is more significant in tectonically active areas. Calculated DIC fluxes due to Ca and Mg silicate weathering by carbonic acid (38.3 × 10 3 mol C · km −2 · yr −1) are similar in magnitude to DIC fluxes due to sulfide-carbonate weathering (18.5 × 10 3 mol C · km −2 · yr −1). While Ca and Mg silicate weathering facilitates a transfer of atmospheric CO 2 to carbonate rocks, sulfide-carbonate weathering can liberate CO 2 from carbonate rocks to the atmosphere when sulfide oxidation exceeds sulfide deposition. This implies that in the Canadian Cordillera, sulfide-carbonate weathering can offset up to 48% of the current CO 2 drawdown by silicate weathering in the region.

Element discharge from pyritic mine tailings at limited oxygen availability in column experiments

Sulfide mineral oxidation in mine tailings deposits poses a long term threat to surrounding ground water and surface waters. Soil or water cover remediation aims at reducing the rate of sulfide mineral oxidation by decreasing the O 2 ingress rate. In this study, the authors addressed the rate of sulfide oxidation and pH buffering in ∼33 months long, well-controlled laboratory studies of water saturated columns of sulfidic mine tailings from the Kristineberg site in Sweden at reduced O 2 availability. The element discharge rates slowly declined towards a quasi-steady state over hundreds of days. Non-reactive tracer tests showed an anomalously large dispersion, indicating strong flow heterogeneity, possibly including preferential flow and/or stagnant water zones. Congruent dissolution of pyrite and sphalerite by injected oxidants (dissolved O 2 and Fe(III)) adequately explained the discharge rate of Fe, S and Zn at quasi-steady state. Arsenic, Pb and Cu were partly retained in the tailings. Base cation discharge rates, and thus pH buffering, were apparently controlled by the rate of acidity production, with actual pH levels, available mineral surface area, and water residence times being of less importance.

Numerical simulations of pyrite oxidation and acid mine drainage in unsaturated waste rock piles

Numerical simulations of layered, sulphide-bearing unsaturated waste rock piles are presented to illustrate the effect of coupled processes on the generation of acid mine drainage (AMD). The conceptual 2D systems were simulated using the HYDRUS model for flow and the POLYMIN model for reactive transport. The simulations generated low-pH AMD which was buffered by sequential mineral dissolution and precipitation. Sulphide oxidation rates throughout the pile varied by about two orders of magnitude (0.004–0.4 kg m − 3 year − 1 ) due to small changes in moisture content and grain size. In the fine-grained layers, the high reactive surface area induced high oxidation rates, even though capillary forces kept the local moisture content relatively high. In waste rock piles with horizontal layers, most of the acidity discharged through vertical preferential flow channels while with inclined fine grained layers, capillary diversion channeled the AMD to the outer slope boundary, keeping the pile interior relatively dry. The simulation approach will be useful for helping evaluate design strategies for controlling AMD from waste rock.