Low molecular weight carboxylic acids in oxidizing porphyry copper tailings

The distribution of low molecular weight carboxylic acids (LMWCA) was investigated in pore water profiles from two porphyry copper tailings impoundments in Chile (Piuquenes at La Andina and Cauquenes at El Teniente mine). The objectives of this study were (1) to determine the distribution of LMWCA, which are interpreted to be the metabolic byproducts of the autotroph microbial community in this low organic carbon system, and (2) to infer the potential role of these acids in cycling of Fe and other elements in the tailings impoundments. The speciation and mobility of iron, and potential for the release of H+ via hydrolysis of the ferric iron, are key factors in the formation of acid mine drainage in sulfidic mine wastes. In the low-pH oxidation zone of the Piuquenes tailings, Fe(III) is the dominant iron species and shows high mobility. LMWCA, which occur mainly between the oxidation front down to 300 cm below the tailings surface at both locations (e.g., max concentrations of 0.12 mmol/L formate, 0.17 mmol/L acetate, and 0.01 mmol/L pyruvate at Piuquenes and 0.14 mmol/L formate, 0.14 mmol/L acetate, and 0.006 mmol/L pyruvate at Cauquenes), are observed at the same location as high Fe concentrations (up to 71.2 mmol/L Fe(II) and 16.1 mmol/L Fe(III), respectively). In this zone, secondary Fe(111) hydroxides are depleted. Our data suggest that LMWCA may influence the mobility of iron in two ways. First, complexation of Fe(III), through formation of bidentate Fe(III)-LMWCA complexes (e.g., pyruvate, oxalate), may enhance the dissolution of Fe(III) (oxy)hydroxides or may prevent precipitation of Fe(III) (oxy)hydroxides. Soluble Fe(III) chelate complexes which may be mobilized downward and convert to Fe(II) by Fe(III) reducing bacteria. Second, monodentate LMWCA (e.g., acetate and formate) can be used by iron-reducing bacteria as electron donors (e.g., Acidophilum spp.), with ferric iron as the electron acceptor. These processes may, in part, explain the low abundances of secondary Fe(III) hydroxide precipitates below the oxidation front and the high concentrations of Fe(II) observed in the pore waters of some low-sulfide systems. The reduction of Fe(III) and the subsequent increase of iron mobility and potential acidity transfer (Fe(II) oxidation can result in the release of H+ in an oxic environment) should be taken in account in mine waste management strategies

Interpretation of Zn Isotope Ratio Measurements in a Complex Geochemical System

Zinc isotope ratios were measured for pore water samples collected from a pilot-scale remediation system designed to asssess the potential benefits of promoting bacterial SO4 reduction and precipitation of metal sulfides. Samples were collected from three test cells at the Greens Creek mine (Alaska, USA), including a control cell and two treatment cells. Cells TC4 and TC7 were amended with organic carbon. The first treatment cell (TC4) contained 5 vol.% organic carbon as peat (2.5 vol. %) and spent brewing grain (2.5 vol. %), and the second treatment cell (TC7) contained 10 vol.% organic carbon as peat (5 Vol. %), spent brewing grain (2.5 vol. %) and municipal biosolids (2.5 vol. %). High concentrations of dissolved Zn (97 to 320 mg L-1 ) and SO4 near the tailings surface indicate Zn release by sphalerite [(Zn,Fe)S] oxidation. Zinc isotope ratios near the tailings surface in all three cells were similar and ranged between +0.25 and +0.35 ‰ (δ66Znavg = +0.3 ±0.05 ‰). At depths equal or below 1 m below surface, Zn concentrations were generally below 2.7 mg L-1 in TC4 and TC7 and below 7.1 mg L-1 in TC2. This decline in Zn concentrations in TC4 and TC7 is attributed bacterial SO4 reduction and concomitant alkalinity production, leading to extensive precipitation of Zn sulfide phases and potentially Zn carbonate phases. Zinc isotope measurements indicate Δ66Zn values of up to -0.35 ‰. Laboratory studies indicate precipitation of Zn sulfide phases results in preferential incorporation of 64Zn, resulting in increasingly positive δ 66Zn values, whereas precipitation of Zn carbonate leads to increasingly negative δ66Zn values. These observations suggest that precipitation of a combination of secondary sulfide and carbonate phases controls Zn mobility and isotope ratios under SO4-reducing conditions within the amended cells

Interpretation of Zn Isotope Ratio Measurements in a Complex Geochemical System

Zinc isotope ratios were measured for pore water samples collected from a pilot-scale remediation system designed to asssess the potential benefits of promoting bacterial SO4 reduction and precipitation of metal sulfides. Samples were collected from three test cells at the Greens Creek mine (Alaska, USA), including a control cell and two treatment cells. Cells TC4 and TC7 were amended with organic carbon. The first treatment cell (TC4) contained 5 vol.% organic carbon as peat (2.5 vol. %) and spent brewing grain (2.5 vol. %), and the second treatment cell (TC7) contained 10 vol.% organic carbon as peat (5 Vol. %), spent brewing grain (2.5 vol. %) and municipal biosolids (2.5 vol. %). High concentrations of dissolved Zn (97 to 320 mg L-1 ) and SO4 near the tailings surface indicate Zn release by sphalerite [(Zn,Fe)S] oxidation. Zinc isotope ratios near the tailings surface in all three cells were similar and ranged between +0.25 and +0.35 ‰ (δ66Znavg = +0.3 ±0.05 ‰). At depths equal or below 1 m below surface, Zn concentrations were generally below 2.7 mg L-1 in TC4 and TC7 and below 7.1 mg L-1 in TC2. This decline in Zn concentrations in TC4 and TC7 is attributed bacterial SO4 reduction and concomitant alkalinity production, leading to extensive precipitation of Zn sulfide phases and potentially Zn carbonate phases. Zinc isotope measurements indicate Δ66Zn values of up to -0.35 ‰. Laboratory studies indicate precipitation of Zn sulfide phases results in preferential incorporation of 64Zn, resulting in increasingly positive δ 66Zn values, whereas precipitation of Zn carbonate leads to increasingly negative δ66Zn values. These observations suggest that precipitation of a combination of secondary sulfide and carbonate phases controls Zn mobility and isotope ratios under SO4-reducing conditions within the amended cells

Modeling controls on the chemical weathering of marine mudrocks from the Middle Jurassic in Southern Germany

Chemical weathering of sedimentary rocks is of great importance in determining seepage water chemistry, carbon, iron, calcium and sulfur turnover, as well as mineral transformation. In this study, we used the numerical code MIN3P to investigate controls on seepage water chemistry during chemical weathering of marine mudrocks. In particular, we focused on the pyrite- and kerogen-bearing formation, Opalinus Clay (with outcrops in the area of the Swabian and Franconian Alb in Southern Germany), a typical fine-grained sedimentary mudrock that had been deposited during the Middle Jurassic in a shallow marine environment. In the geochemical model we considered four reactive minerals, i.e., pyrite, kerogen, calcite and siderite (assuming silicate minerals to be stable), and ran model scenarios over a time period of 10kyrs (since the last ice age). Our numerical results show that chemical weathering of Opalinus Clay is driven by oxygen ingress (which depends on effective gas diffusion, and thus on water saturation). Due to oxidation of pyrite and kerogen seepage water acidifies, which leads to dissolution of carbonate minerals, i.e., calcite and siderite. As a consequence, porosity and groundwater alkalinity increase, and CO2 is released into the atmosphere at early decades. Following the consumption of primary reactive minerals, iron oxides precipitate in the oxic zone. We compared our model results with field data of water saturation, porosity, and water chemistry. The overall reasonable fit between model results and field data demonstrates the applicability of the numerical code MIN3P to quantify chemical weathering of pyrite-bearing sedimentary mudrocks and to predict seepage water chemistry that is impacted by geochemical water-rock interactions

Exposure-time based modeling of nonlinear reactive transport in porous media subject to physical and geochemical heterogeneity

Transport of reactive solutes in groundwater is affected by physical and chemical heterogeneity of the porous medium, leading to complex spatio-temporal patterns of concentrations and reaction rates. For certain cases of bioreactive transport, it could be shown that the concentrations of reactive constituents in multi-dimensional domains are approximately aligned with isochrones, that is, lines of identical travel time, provided that the chemical properties of the matrix are uniform. We extend this concept to combined physical and chemical heterogeneity by additionally considering the time that a water parcel has been exposed to reactive materials, the so-called exposure time. We simulate bioreactive transport in a one-dimensional domain as function of time and exposure time, rather than space. Subsequently, we map the concentrations to multi-dimensional heterogeneous domains by means of the mean exposure time at each location in the multi-dimensional domain. Differences in travel and exposure time at a given location are accounted for as time difference. This approximation simplifies reactive-transport simulations significantly under conditions of steady-state flow when reactions are restricted to specific locations. It is not expected to be exact in realistic applications because the underlying assumption, such as neglecting transverse mixing altogether, may not hold. We quantify the error introduced by the approximation for the hypothetical case of a two-dimensional, binary aquifer made of highly-permeable, non-reactive and low-permeable, reactive materials releasing dissolved organic matter acting as electron donor for aerobic respiration and denitrification. The kinetically controlled reactions are catalyzed by two non-competitive bacteria populations, enabling microbial growth. Even though the initial biomass concentrations were uniform, the interplay between transport, non-

Diavik Waste Rock Project: Scale-up of a reactive transport model for temperature and sulfide-content dependent geochemical evolution of waste rock

The Diavik Waste Rock Project, located in a region of continuous permafrost in northern Canada, includes complementary field and laboratory experiments with the purpose of investigating scale-up techniques for the assessment of the geochemical evolution of mine waste rock at a large scale. As part of the Diavik project, medium-scale field experiments (∼1.5 m high active zone lysimeters) were conducted to assess the long term geochemical evolution and drainage of a low-sulfide waste rock under a relatively simple (i.e. constrained by the container) flow regime while exposed to atmospheric conditions. A conceptual model, including the most significant processes controlling the sulfide-mineral oxidation and weathering of the associated host minerals as observed in a laboratory humidity cell experiment, was developed as part of a previous modelling study. The current study investigated the efficacy of scaling the calibrated humidity cell model to simulate the geochemical evolution of the active zone lysimeter experiments. The humidity cell model was used to simulate the geochemical evolution of low-sulfide waste rock with S content of 0.053 wt.% and 0.035 wt.% (primarily pyrrhotite) in the active zone lysimeter experiments using the reactive transport code MIN3P. Water flow through the lysimeters was simulated using temporally variable infiltration estimated from precipitation measurements made within 200 m of the lysimeters. Flow parameters and physical properties determined during previous studies at Diavik were incorporated into the simulations to reproduce the flow regime. The geochemical evolution of the waste-rock system was simulated by adjustment of the sulfide-mineral content to reflect the values measured at the lysimeters. The temperature dependence of the geochemical system was considered using temperature measurements taken daily, adjacent to the lysimeters, to correct weathering rates according to the Arrhenius equat