Acid generation and metal(loid) release are growing considerations for oil sands mine closure in northern Alberta, Canada. Oxidative weathering of pyrite-bearing froth treatment tailings (FTT) has potential to promote acid generation and metal(loid) release. However, the acid-neutralization reactions and their influence on pore-water pH and metal(loid) mobility have not yet been reported. Laboratory column experiments examined acid-neutralization reactions and metal(loid) release for variably weathered (i.e., non-weathered, partially weathered, highly weathered) FTT samples collected from a commercial-scale beach deposit. Solvent-washed and non-solvent-washed splits of each sample were included to assess the influence of residual hydrocarbons. Acidic influent (i.e., 0.05 M H2SO4; pH 1.5) was continuously pumped through each column, and effluent samples were collected for geochemical analysis over time. Effluent pH decreased from 7.0 to 5.5 over the first 5 pore volumes for the non-weathered and partially weathered columns, while gradual pH decreases to 4.5 were observed over the following 30 to 70 pore volumes. Subsequent decreases in effluent pH from 4.5 to < 3.0 occurred over the next 2 to 5 pore volumes for these columns. In contrast, effluent pH consistently remained below 2.0 for the highly weathered columns. We attribute these effluent pH trends to the dissolution of Ca- and Mg-bearing carbonates (pH 6.5 to 6), Fe-bearing carbonates (pH 5.6 to 4.5), Al (oxy)hydroxides (pH 4.5 to 4.0), and silicates (pH < 2). Corresponding increases in effluent concentrations of Fe (< 1 to > 500 mg L− 1), Al (< 0.1 to > 10 mg L− 1), Si (< 0.1 to > 10 mg L− 1), and additional metal(loid)s (e.g., Ni, Zn, V, As) were observed with decreasing pH. Cumulative mass releases (e.g., Ca, Mg, Fe) were greatest for the non-weathered samples and solvent-washed splits. These results offer new insights into relationships between acid neutralization reactions and metal(loid) release that can inform FTT management and reclamation.
Perfluorooctane sulfonate (PFOS) is an emerging contaminant frequently detected in subsurface environments, raising significant concern due to its environmental persistence, mobility, and potential human health impacts. This study examines PFOS adsorption onto a range of solid substrates, including pure minerals, mineral assemblages, and natural soils. Specifically, the adsorption behavior of 2-line ferrihydrite, ferrihydrite-coated sand, and soil collected from a PFOS-impacted site in Killingworth, Connecticut was investigated to evaluate their capacity to retain PFOS under varying geochemical conditions. By integrating batch adsorption experiments with surface complexation modeling (SCM) and applying the component additivity approach, this study elucidates the reactive transport mechanisms governing PFOS behavior under a range of geochemical conditions. Our findings demonstrate that PFOS adsorption occurs significantly on both ferrihydrite and quartz surfaces, with the ferrihydrite-coated sand and soil exhibiting retention behavior attributable to contributions from both mineral phases. At lower pH values, sorption is predominantly governed by outer-sphere complexation driven by the surface charge characteristics of ferrihydrite. Specifically, under acidic conditions (pH < 5.5 for ferrihydrite-coated sand and pH < 6.0 for soil), PFOS retention is primarily facilitated through an outer-sphere hydrogen-bonded complex at ferrihydrite’s surface, while a secondary outer-sphere complex involving Na+ co-adsorption contributes to a lesser extent. At elevated pH levels, however, electrostatic interactions become less favorable, and non-electrostatic hydrophobic interactions with quartz surfaces become increasingly dominant, highlighting the transition in sorption mechanisms from charge-driven to hydrophobic partitioning under neutral to alkaline conditions. A comparison with traditional partitioning coefficients (Kd) revealed that their variability closely corresponds with changes in dominant surface complexes across different pH conditions. Given the critical role of solid-phase partitioning in governing PFAS transport in the subsurface, enhanced predictive capabilities are essential for advancing site-specific risk assessments and informing management strategies aimed at protecting both public and private water resources.
Bitumen extraction from mined oil sands ore generates a large volume of fluid fines tailings (FFT) that must be incorporated into either aquatic or terrestrial reclamation landforms. Mine operators are developing various tailings technologies to accelerate FFT dewatering, including the addition of chemical coagulants and flocculants. However, the impacts of these coagulants and flocculants on biogeochemical processes in treated FFT are not fully understood. We conducted anaerobic batch experiments to examine the influence of different doses (i.e., 0, 500, 1000, and 1500 ppm) of sulfate-based coagulants, including aluminum sulfate (alum) [Al2(SO4)3∙nH2O], ferric sulfate (ferric) [Fe2(SO4)3∙nH2O], and calcium sulfate (gypsum) [CaSO4∙2H2O], on biogenic gas production and microbial communities in treated FFT. Our results show that sulfate addition stimulated microbial sulfate reduction, which inhibited methanogenesis in coagulated FFT relative to experimental controls. Sulfate depletion preceded increased methane production in the 500 ppm gypsum experiment, while larger ferric and alum doses produced higher sulfate concentrations and larger pH decreases. 16 S rRNA sequencing revealed that Comamonadaceae, Anaerolineaceae, and Desulfocapsaceae were the major bacterial families, while Methanoregulaceae and Methanosaetaceae dominated the archaeal families in all treatments. Precipitation of iron(II) sulfides limited dissolved hydrogen sulfide concentrations in experiments where Fe availability was not limited. Our results indicate that addition of sulfate-based coagulants can stimulate microbial sulfate reduction and suppress methanogenesis. However, resumption of methane production following sulfate depletion reveals complex interactions among biogeochemical reaction pathways. Overall, this study demonstrates that biogeochemical cycling of carbon, sulfur, and iron are important considerations for the development and implementation of tailings treatment technologies.
The present research work aims to understand the geochemistry of groundwater resources of the Yamuna-Hindon interfluve region of Bagpat district, Western Uttar Pradesh, India. The region is a part of Indo-Gangetic belt, one of the world's most fertile and intensely farmed areas. To investigate the geochemical processes governing groundwater quality, a total of 105 groundwater samples were collected during pre-monsoon season and analyzed for various physico-chemical parameters, namely, pH, electrical conductivity (EC), total dissolved solid (TDS), total hardness (TH), turbidity, major anions (HCO3-, SO42-, F-, Cl-, NO3-), cations (Ca2+, Mg2+, Na+, K+) following the methods outlined in the American Public Health Association (APHA). The dissolved heavy metals (Fe, Mn, Zn, Pb, Cu, Cr, Ni, As, Se, Co, Cd and Al) in groundwater were analyzed by ICP-MS following the instrument manual. The analysis results revealed that the groundwater is pre-dominantly neutral to mildly alkaline in nature. The major cation chemistry majorly followed the occurrence pattern of Na+ > Mg2+ > Ca2+ > K+, while for anions it was HCO3- > Cl- > SO42- > NO3- > F-. The data plotted on Piper triangular diagram indicated that Ca2+-Mg2+-HCO3- and Na+-K+-HCO3--Cl- were major hydrogeochemical facies. Weathering of rock-forming minerals mainly governed the groundwater geochemistry in this region, although part of the cations associated with Cl-, F- and NO3- may originate from anthropogenic sources. TDS, TH, turbidity and F- were identified as the major parameters that violated the prescribed limits for drinking water. Most of the heavy metals were found within the drinking water prescribed limits except for Fe, Mn, Al and Se. Elevated salinity, %Na, and magnesium hazard (MH) at certain sites limit its suitability for agricultural use. The assessment of selected organochlorine and organophosphorus pesticides in five samples indicated presence of lindane, β-endosulfan and DDT isomers in few samples. However, a detailed investigation of possible pesticide contamination in this intensive agriculture area is required before drawing any final conclusions.
This study presents the first kinetic model to predict the solid and pore solution composition of Na-bentonite clay reacting with slaked lime over a period of 720 days. The model successfully accounts for most experimental data using a single kinetic rate constant. The following sequence of reactions was predicted by the model: initial rapid dissolution of portlandite within the first 7 days, leading to a decrease in pH and dissolved calcium, and concurrent formation of calcium silicate hydrates (C-S-H: jennite), calcium aluminate hydrate (C-A-H: C₄AH₁₃), calcium aluminosilicate hydrates (stratlingite) and hydrotalcite. After 7 days, jennite and stratlingite are predicted to transform into tobermorite-II, contributing to strength development up to 28 days. From 28 to 90 days, continued montmorillonite dissolution is predicted, along with minor formation of ettringite, partial tobermorite-II dissolution, and precipitation of secondary phases such as albite and talc. Experimentally, portlandite dissolution was confirmed by TGA and XRD and found to be complete within 7 days, in agreement with model predictions. However, other predicted solid-phase transformations (e.g., tobermorite-II formation and dissolution, ettringite, albite, and talc formation) could not be conclusively verified through experimental techniques. Aqueous phase measurements confirmed that the pH and Ca trends in solution, and that equilibrium was reached by 90 days.
