A five-year, humidity-cell experiment was used to study the weathering evolution of a low-sulfide, granitic waste rock at 5 and 22 °C. Only the rock with the highest sulfide content (0.16 wt %) released sufficient acid to overcome a limited carbonate acid-neutralization capacity and produce a substantial decline in pH. Leached SO₄ and Ca quickly increased then decreased during the first two years of weathering. Sulfide oxidation continued to release acid and SO₄ after carbonate depletion, resulting in an increase in acid-soluble elements, including Cu and Zn. With the dissolution of Al-bearing minerals, the pH stabilized above 4, and sulfide oxidation continued to decline until the end of the experiment. The variation in activation energy of sulfide oxidation correlates with changes in sulfide availability, where the lowest activation energies occurred during the largest releases of SO₄. A decrease in sulfide availability was attributed to consumption of sulfide and weathered rims on sulfide grains that reduced the oxidation rate. Varying element release rates due to changing carbonate and sulfide availability provide identifiable geochemical conditions that can be viewed as neutralization sequences and may be extrapolated to the field site for examining the evolution of mineral weathering of the waste rock.

The Strathcona Waste Water Treatment System (SWWTS; Sudbury, ON, Canada) has received mill tailings from Ni/Cu ore processing from 1970 to present. Demonstration-scale, multi-layer cover systems were installed on selected tailings deposition cells at the SWWTS. The cover systems are comprised of an upper layer of organic carbon-rich material, composed of a layer biosolids fertilizer along with composted municipal food and yard waste, then a layer of desulfurized, fine-grained tailings. Organic carbon components used in these covers promote microbial communities that consume O₂, thus decreasing sulfide oxidation rates in the underlying tailings. The aim of this study was to investigate the microbiology of the cover systems and the underlying tailings, using a combination of culture-dependent (most probable number) and culture-independent (16S rRNA gene amplicon sequencing) techniques, and assess the impact of the organic component of the cover system four to six years after implementation. Most tailings samples were characterized by circumneutral bulk pH and low concentrations of dissolved metals. The presence of the organic cover resulted in elevated counts of sulfate-reducers (by two orders of magnitude, compared to control samples) immediately below the organic cover, as well as an increased abundance of heterotrophic species (∼10⁸ cells g^-1) at greater depth (∼4 m) in the tailings profile. Mineral-oxidizing microorganisms were also present in the tailings, with neutrophilic sulfur-oxidizers dominating the samples (mean ∼10⁶ cells g^-1). Relative abundances of sulfur- and/or iron-oxidizers determined by sequencing ranged from 0.5 to 18.3% of total reads (mean ∼5.6% in amended tailings) and indicated the presence of local microenvironments with ongoing sulfide oxidation. This work provides a detailed characterization of the microbiology of a tailings impoundment with an organic cover, highlighting the opportunities associated with monitoring microbial processes in such remediation systems.

Chromium, especially in its hexavalent form (Cr(VI)), poses significant health risks due to its carcinogenic properties. Emerging research suggests that biochar, a carbon-rich material derived from biomass pyrolysis, holds promise as an effective and sustainable solution for Cr(VI) remediation. Biochar's unique physicochemical properties, such as its high surface area, porous structure, and functional groups, contribute to its exceptional adsorption capacity for metals. In a series of batch experiments with varying durations, an oak-based biochar was exposed to a 48 mg L⁻¹ Cr(VI) solution. The results demonstrate that the biochar effectively removed 99% of the Cr(VI) after 100 h, with a removal capacity greater than 5 mg g⁻¹. Fourier transform infrared (FTIR) spectroscopy suggests that the removal of Cr(VI) involved aliphatic and aromatic C-H bonds. X-ray photoelectron spectroscopy (XPS) also indicated the role of aliphatic groups in the removal process and suggested the involvement of carbonyl groups. XPS analysis also detected both Cr(III) and Cr(VI) on the surface of the biochar, indicating the occurrence of reduction and sorption processes. The presence of Cr(III) and Cr(VI) was further confirmed at the Canadian Light Source (CLS) synchrotron facility using X-ray absorption near edge structure (XANES) spectroscopy. Cr isotope analysis showed an increase in δ⁵³Cr as the concentration of Cr in solution decreased, indicating Cr(VI) reduction. The isotope data followed a Rayleigh curve with a kinetic fractionation ε⁵³Cr of -1.33‰, highlighting that Cr stable isotopes can effectively be used as an indicator of biochar-Cr reactions.

High-energy-resolution fluorescence-detected (HERFD) X-ray absorption near-edge spectroscopy (XANES) is a spectroscopic method that allows for increased spectral feature resolution, and greater selectivity to decrease complex matrix effects compared with conventional XANES. XANES is an ideal tool for speciation of elements in solid-phase environmental samples. Accurate speciation of As in mine waste materials is important for understanding the mobility and toxicity of As in near-surface environments. In this study, linear combination fitting (LCF) was performed on synthetic spectra generated from mixtures of eight measured reference compounds for both HERFD-XANES and transmission-detected XANES to evaluate the improvement in quantitative speciation with HERFD-XANES spectra. The reference compounds arsenolite (As₂O₃), orpiment (As₂S₃), getchellite (AsSbS₃), arsenopyrite (FeAsS), kaňkite (FeAsO₄·3.5H₂O), scorodite (FeAsO₄·2H₂O), sodium arsenate (Na₃AsO₄), and realgar (As₄S₄) were selected for their importance in mine waste systems. Statistical methods of principal component analysis and target transformation were employed to determine whether HERFD improves identification of the components in a dataset of mixtures of reference compounds. LCF was performed on HERFD- and total fluorescence yield (TFY)-XANES spectra collected from mine waste samples. Arsenopyrite, arsenolite, orpiment, and sodium arsenate were more accurately identified in the synthetic HERFD-XANES spectra compared with the transmission-XANES spectra. In mine waste samples containing arsenopyrite and either scorodite or kaňkite, LCF with HERFD-XANES measurements resulted in fits with smaller R-factors than concurrently collected TFY measurements. The improved accuracy of HERFD-XANES analysis may provide enhanced delineation of As phases controlling biogeochemical reactions in mine wastes, contaminated soils, and remediation systems.

One of the large-scale field waste rock experiments (test piles) conducted as part of the Diavik Waste Rock Project was deconstructed, providing a spatially located set of geochemical, mineralogical, and particle-size distribution samples. Geostatistical analyses were conducted for sulfur and carbon content and saturated hydraulic conductivity, which affect the geochemical evolution of waste rock, to investigate the spatial dependence of these parameters. Analyses included population statistics, experimental semi-variogram estimation, and theoretical semi-variogram fitting. Population statistics were calculated for additional data sets from samples collected during the construction of the test piles. The population statistical analyses indicated that log-normal distribution provided the best fit for all investigated data sets. Experimental semi-variograms were estimated for the spatially located data set (test pile deconstruction) using the classical estimator, and theoretical semi-variograms were fitted. This investigation showed that the spatial distribution of sulfur, carbon, and hydraulic conductivity within the core of the test-pile experiments can be approximated using a log-normal distribution with a mean and standard deviation calculated using the samples collected during construction of the piles, and that little to no spatial relationship was present for these parameters at the scale of sampling. That the saturated hydraulic conductivity of the matrix material can be represented by the same statistical distribution throughout the test pile is significant because water flow, as well as mineral surface area and reactivity are dominantly controlled within the matrix portion of the test pile. Reactive transport simulations are included to demonstrate the influence of the matrix material on effluent geochemistry.