33 Search Results

Phosphorus limits primary productivity in many (Sub-)Arctic ecosystems and may constrain biological carbon sequestration. Iron (III) oxides strongly bind phosphate in soils but can dissolve under flooded, reducing conditions induced by permafrost thaw and ground collapse. The ability for iron to regulate phosphate storage and solubility in thawing permafrost landscapes remains unclear. Here, iron-rich sediments containing iron oxides and organic-bound iron were incubated with or without added phosphate in soils along a permafrost thaw gradient to evaluate how iron-phosphate associations respond to thaw-induced redox shifts. Iron oxides partially dissolved and released sorbed phosphate when incubated in soils underlain by degraded permafrost. Iron complexed by organic matter remained stable but provided no phosphate binding capacity. Phosphate addition enhanced iron oxide dissolution and phosphorus concentrations in associated microbial biomass. Our study demonstrates that the capacity for iron oxides to immobilize and retain phosphate in permafrost peatlands decreases with permafrost thaw.

Iron (oxyhydr)oxides strongly adsorb phosphate and limit its bioavailability, but interactions between phosphate and various Fe (oxyhydr)oxides are poorly constrained in natural systems. An in-situ incubation experiment was conducted to explore Fe (oxyhydr)oxide transformation and effects on phosphate sorption in soils with contrasting saturation and redox conditions. Synthetic Fe (oxyhydr)oxides (ferrihydrite, goethite and hematite) were coated onto quartz sand and either pre-sorbed with phosphate or left phosphate-free. The oxide-coated sands were mixed with natural organic matter, enclosed in mesh bags, and buried in and around a vernal pond for up to 12 weeks. Redox conditions were stable and oxic in the upland soils surrounding the vernal pond but largely shifted from Fe reducing to Fe oxidizing in the lowland soils within the vernal pond as it dried during the summer. Iron (oxyhydr)oxides lost more Fe (- 41% ± 10%) and P (- 43 ± 11%) when incubated in the redox-dynamic lowlands compared to the uplands (- 18% ± 5% Fe and - 24 ± 8% P). Averaged across both uplands and lowlands, Fe losses from crystalline goethite and hematite (- 38% ± 6%) were unexpectedly higher than losses from short range ordered ferrihydrite (- 12% ± 10%). We attribute losses of Fe and associated P from goethite and hematite to colloid detachment and dispersion but losses from ferrihydrite to reductive dissolution. Iron losses were partially offset by retention of solubilized Fe as organic-bound Fe(III). Iron (oxyhydr)oxides that persisted during the incubation retained or even gained P, indicating low amounts of phosphate sorption from solution. In conclusion, these results demonstrate that hydrologic variability and Fe (oxyhydr)oxide mineralogy impact Fe mobilization pathways that may regulate phosphate bioavailability.

Historical and abandoned coal mine spoil continues to generate acid- and metal(loid)-rich porewaters and represents a geographically large and diffuse non-point source of contamination to local watersheds. A potentially inexpensive approach to treat these materials and soils developing on them is through the application of lime slurries, to neutralize acidity and encourage the (co)precipitation of metal(loids) with Fe(III)-(oxy)hydroxides, and potentially other metal-oxides and/or Ca-bearing phases. Here, the efficacy of this approach was evaluated through parallel field application and laboratory-based flow-through column experiments. The field site is Huff Run sub-watershed 25 located in Tuscarawas County, Ohio and was chosen in part because it was previously classified as one of the most highly AMD-impacted sub-watersheds in the region. Two locations with historical spoil were chosen and suction lysimeters were installed at 25 and 75 cm depth to monitor porewater composition on two sides at the base of each pile. Half of each slope received seven lime slurry treatments from June through October of 2017. A suite of aqueous (ICP-OES, IC, and TOC-L) and solid phase geochemical and mineralogical approaches (quantitative SEM-EDS and synchrotron μ-XRF) were used to determine how composition, texture, morphology, and spatial distribution of mineral coatings differ in pre- and post-lime treated soils, and how that impacts the distribution and transport of trace metal(loid)s. Mine spoil porewater at site 1 was slightly less alkaline (pH ranging from 7.04 to 7.37) than at site 2 (ranging from 7.55 to 7.71), and average electrical conductivity values at site 1 (316–405 μS cm^—1) were slightly lower than at site 2 (358–464 μS cm^—1), although differences between the sites were not significant. Porewater pH and electrical conductivity in all lysimeters decreased over the course of the field season but there was no obvious response to lime treatment at either site or any depth. At site 1, both treatment and depth were significant factors affecting Ca, K, Ni, SO₄^2—, and DOC concentrations while only treatment effects were significant for dissolved Al and Cu (p < 0.05). For all soils, there were no trends in metal concentration observed over time although DOC and SO₄^2— decreased over the field season. Pedon-scale changes in metal porewater concentrations in response to treatment were linked to micrometer-scale changes in mineral surface coatings; specifically, higher concentrations of Ca, Fe, Mn, and Zn were observed in the coatings and no changes were observed in Fe redox speciation, whereas total S decreased likely due to oxidation of S in coal fragments. In contrast to the field experiment, the column experiments exhibited a much greater response in effluent composition with respect to lime treatment. The untreated columns had approximately an order of magnitude more H⁺ leached over the course of the experiment (p < 0.001) and resulted in greater Ca, Al, Cu, and DIC leached and less Mn, Zn, and SO₄^2—. Soils treated with the lime slurry in the column experiments exhibited larger and thicker secondary Fe-coatings, including the addition of Fe-sulfates. Despite clear trends in the laboratory-based column experiment where the lime-to-soil ratio was higher, the effects were either muted or undetected in the field pilot project, suggesting that a higher application rate of lime in the field is needed to achieve a similar effect. This work provides evidence that a less alkaline lime slurry could be a practical and inexpensive method of treating coal mine spoil-impacted soils and represents an important step in linking laboratory-based remediation studies to implemented field-based studies.

Chemical weathering of pyrite via oxidative dissolution is well-known for generating Fe(III)-bearing colloids at acid mine drainage (AMD) sites; however, the potential for physical weathering of pyrite-bearing materials and subsequent release and transport of colloidal pyrite and associated trace metals has not been studied. Here, we monitored the colloidal metal transport in soil developing on abandoned coal mine spoil with a history of AMD generation to systematically study the contribution of colloids to base and trace metal transport and determine the elemental and mineralogical composition of colloids. Additionally, we collected soil pore water using lysimeters with a pore size of approximately 1.3 μm and centrifugation was used to separate the colloids from aqueous fractions. Metal concentrations of Na, Ca, Mg, K, Si, Al, Mn, Fe, Cu, and Zn were analyzed. Our results show only 14% of the total Al and Cu were present in colloidal fractions whereas 23%, 43%, and 54% of Fe, Mn, and Zn were transported in the colloidal phase, respectively. In contrast, all base metals were primarily present in the aqueous concentration with a small fraction (<10%) present in the colloidal phase. The colloidal fractions of the base metals were inferred to be associated with the concentration of clay colloids. The release of colloids exhibited possible sensitivity toward weather conditions such as alternating high temperatures and rainfall during summer (May–July) compared to fall (August–November). The morphology, elemental, and mineral composition of colloids were determined by a scanning electron microscopy equipped with energy dispersive spectroscopy (SEM-EDS) and X-ray diffraction (XRD). Colloids were dominated by phyllosilicates (biotite, muscovite, and kaolinite) with minor quartz and feldspars. Other minerals phases identified in colloidal fractions were hematite, goethite, arsenopyrite, and chalcopyrite. Colloids consisting of Fe, S, and O with structures resembling framboidal pyrite were identified, which is consistent with the non-silicates minerals identified by XRD. Our study suggests that the physical weathering of pyrite in the mine spoil can generate colloidal pyrite, which is mobilized and transported by soil pore water. Our results also indicate that these pyritic colloids are associated with toxic trace metals including Cu, Mn, and Zn. Colloid mobilization is also impacted by changes in temperature and precipitation, where clay mobilization afer rain events is favored. Further, sudden spikes in aqueous and/or colloidal concentrations may be the result of local heterogeneity within soils developing on mine spoil, indicating that further field-based work is necessary to better characterize pore-scale processes that control aqueous and colloid transport at similar sites to have a better understanding of potential contaminant transport from mine spoil systems.

Coal mine spoil is widespread in US coal mining regions, and the potential long-term leaching of toxic metal(loid)s is a significant and underappreciated issue. This study aimed to determine the flux of contaminants from historic mine coal spoil at a field site located in Appalachian Ohio (USA) and link pore water composition and solid-phase composition to the weathering reaction stages within the soils. The overall mineralogical and microbial community composition indicates that despite very different soil formation pathways, soils developing on historic coal mine spoil and an undisturbed soil are currently dominated by similar mineral weathering reactions. Both soils contained pyrite coated with clays and secondary oxide minerals. However, mine spoil soil contained abundant residual coal, with abundant Fe- and Mn- (oxy)hydroxides. These secondary phases likely control and mitigate trace metal (Cu, Ni, and Zn) transport from the soils. While Mn was highly mobile in Mn-enriched soils, Fe and Al mobility may be more controlled by dissolved organic carbon dynamics than mineral abundance. There is also likely an underappreciated risk of Mn transport from coal mine spoil, and that mine spoil soils could become a major source of metals if local biogeochemical conditions change.

Contaminant metals derived from acid mine drainage (AMD) are transported through stream networks in aqueous, colloidal, and/or particulate phases; however, hydrogeochemical processes that regulate export of these phases from headwater catchments are not fully resolved. Here, we investigated metal speciation and transport along redox and pH gradients and as a function of discharge in an AMD-impaired stream. Contaminated groundwater upwelling into the stream mixed first with oxygenated surface water and then with alkaline effluent from a treatment system. Contaminant Fe was effectively removed from the stream as Fe²⁺ oxidation generated Fe(III)-bearing colloids that rapidly aggregated and accumulated in streambed sediments. Iron precipitated first as Fe(III) (oxyhydr)oxides but then as oxyhydroxysulfates as Fe hydrolysis lowered stream pH. Smaller amounts of Fe²⁺ were incorporated into framboidal pyrite in oxygen-poor sediments. Conversely, AMD-derived Mn²⁺ and Al³⁺ were only minimally removed from the stream during seasonal mixing with treated effluent. Although natural attenuation limited Fe export from the watershed, Fe stored in stream sediments has the potential to be remobilized due to oxidative dissolution of Fe-sulfides and/or physical scouring of the streambed. This research demonstrates how colloid formation and mobility respond to geochemical gradients, with implications for how sediment–water interactions influence metal transport through streams.

Acid mine drainage (AMD) from historic and abandoned coal mine spoil represents a potential long-term source of contaminants to surface and groundwater. Determining the risk associated within AMD generation and metal(loid) transport from coal mine spoil is complicated by the heterogeneous natural of spoil heaps and mineralogical and hydro(bio)geochemical factors that may limit or promote metal(loid) transport. The current work aims to determine if primary, reduced phases such as pyrite continue to persist in abandoned and historic coal mine spoil. This objective was accomplished through characterization soils undergoing active weathering while developing on coal mine spoil in Appalachian Ohio to determine the factors that might limit oxidative dissolution. Soils in the Huff Run Watershed (Ohio, USA) were sampled at 0–10 cm, 30–40 cm, 70–80 cm, and 110–120 cm depth. X-ray Diffraction (XRD), Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDS), and synchrotron-based X-ray Microprobe (XMP) analyses were used to determine the speciation and distribution of metal(loid)s and the minerals they are associated with. The XMP analyses included micro-focused XRD (μ-XRD), X-ray Fluorescence (μ-XRF) element and redox state mapping, and X-ray absorption Near Edge Structure (μ-XANES) Spectroscopy. Soil mineralogy was dominated by quartz, muscovite, kaolinite, and feldspar, with minor amounts of chlorite and other phases including pyrite, arsenopyrite, realgar, orpiment, hematite, and goethite. Soils from all depths contained metal(loid)-sulfide particles with secondary mineral surface coatings, often in physically complex and heterogeneous aggregates that were composed of clay minerals and secondary Fe(III)-(oxy)hydroxides. These assemblages were typically 10–20 μm in diameter, with an individual pyrite particle core grain size ranging from 0.5 to 10 μm, and secondary mineral surface coatings ranging in thickness from undetectable to 1 μm. Within these aggregates, S and As were present as: (1) small (<20 μm) phases that were spatially correlated with Fe and other trace metal(loids) (Cu, Se, and Zn) and identified as metal(loid)-sulfide minerals; and (2) As(III), As(VI), and S(VI) associated with secondary Fe(III)-(oxy)hydroxides. Intermediate S oxidation state was also observed to be associated with remnant coal and organic matter. These results indicate that pyrite and other metal(loid) sulfides are present in soils developing on historic coal mine spoil after several decades since waste emplacement. The persistence of the μm-scale pyrite grains is likely the result of the formation of the secondary mineral surface coatings which can limit complete oxidative dissolution. These phases also play a role in re-sequestration of metal(loid)s release from sulfide mineral weathering. Furthermore, this work highlights the importance for considering AMD generation from non-point sources, and the potential for long-term ecosystem impairment.

Abandoned mining operations continue to severely degrade many ecosystems worldwide by releasing acidic water and/or heavy metals into surface and groundwater. Contaminant concentrations in affected streams vary with discharge in patterns that reflect both geochemical reactions and variable mixing of contaminated and non-contaminated waters. However, controls on concentration-discharge (C-Q) patterns remain unclear, particularly for constituents that experience changing solubility across redox and pH gradients. Understanding the C-Q behaviour of contaminants aids in predicting both downstream transport and effects on aquatic life under variable flow. Here, we examined the C-Q behaviours of non-reactive (Na, K, Ca, Mg, Cl^-) and reactive (Fe, Mn, Al, H⁺, SO₄^2-) solutes in a stream contaminated with acid mine drainage in northeastern Ohio, USA. Concentration-discharge patterns at the watershed outlet primarily reflected mixing of contaminated baseflow with intermittent inputs of high pH water draining from a passive limestone treatment system into the stream. The treatment system acted as an ephemeral tributary that mitigated contamination in the stream by diluting solutes, raising pH, and driving metal precipitation, but only when flow was present during wet seasons. Consequently, AMD-derived reactive solutes (H⁺, Fe, Mn, Al) decreased with increasing stream discharge while relatively conservative solutes (e.g., Ca, Mg, K, Na) decreased only slightly or were chemostatic. This study highlights both the unique C-Q patterns of reactive solutes when compared to those of non-reactive solutes and the potential for intermittent streams to control C-Q behaviour in headwater catchments.

Abandoned mine lands continue to serve as non-point sources of acid and metal contamination to water bodies long after mining operations have ended. Although soils formed from abandoned mine spoil can support forest vegetation, as observed throughout the Appalachian coal basin, the effects of vegetation on metal cycling in these regions remain poorly characterized. Iron (Fe) and manganese (Mn) biogeochemistry were examined at a former coal mine where deciduous trees grow on mine spoil deposited nearly a century ago. Forest vegetation growing on mine spoil effectively removed dissolved Mn from pore water; however, mineral weathering at a reaction front below the rooting zone resulted in high quantities of leached Mn. Iron was taken up in relatively low quantities by vegetation but was more readily mobilized by dissolved organic carbon produced in the surface soil. Dissolved Fe was low below the reaction front, suggesting that iron oxyhydroxide precipitation retains Fe within the system. These results indicate that mine spoil continues to produce Mn contamination, but vegetation can accumulate Mn and mitigate its leaching from shallow soils, potentially also decreasing Mn leaching from deeper soils by reducing infiltration. Vegetation had less impact on Fe mobility, which was retained as Fe oxides following oxidative weathering.

Coal mine spoil is a long-lasting legacy of historic coal mining operations that continues to impact water quality across Appalachia. Metal(loid) release from abandoned coal mine spoil, which can be a significant source of acid mine drainage (AMD), is dependent on speciation and distribution within the parent coal shale. This work aimed to determine how the micron- and sub-micron scale mineralogy, morphology, and texture of metal(loid)-bearing phases in a coal-shale control the rate and release of metal(loid)s during subsequent weathering through two primary objectives: (1) determine the microscale speciation and distribution of Fe and other metal(loid)s in a parent coal shale, and (2) determine the amount of metal(loid)s released during simulated weathering of shale physically crushed into silt- to sand-sized particles. This work was accomplished through a combination of electron microscopy and synchrotron-based X-ray microprobe analyses. Furthermore, this suite of techniques also provides insight into the geochemical history of the coal shale that could not be determined by bulk techniques. Trace elements were associated with either Fe-sulfides or with other phases that included metal(loid)-sulfides, aluminosilicates, and organic matter. Three distinct pools of Fe-sulfides were present: (i) larger mm-scale grains, with minimal internal fractures; (ii) μm- to mm-scale aggregates of μm-scale crystals forming secondary coatings on the larger mm-scale grains, and (iii) μm- to mm-scale aggregates forming framboidal grains. Fe-sulfide mineralogy was dominated by pyrite with minor contributions of marcasite and a S(-II)-bearing phase. Larger pyrite grains and their secondary coatings contained homogeneous distributions of Fe and Mn as well as other trace metal(loids) including V, Ti, Cr, As, Se, Cu, and Zn. Additionally, the fine-grained pyrite aggregates were strongly enriched in As and Se and contained discrete Cu- and Zn-bearing particles. Clays and organic matter surrounding the sulfide minerals were identified by micro-XRD and FT-IR spectroscopy. Simulated batch weathering of the physically crushed shale resulted in metal(loid) release that varied as function of size fraction and time. Metal(loid) release increased with decreasing particle size from sand-sized to silt-sized fractions. Although small amounts of Fe were released into solution after two days, the bulk of Fe release occurred after 10 days and continued for six months, after which Fe was gradually removed from solution. The weathering trends for Mn, Cu, Zn, and Ni were similar to Fe. In contrast, As and Se were characterized by rapid release into solution followed by removal prior to 10 days. These results indicate that micrometer-scale metal(loid) distribution within Fe sulfides controlled metal(loid) release into solution during simulated weathering of a coal shale. Specifically, elements concentrated in mineral coatings (As, Se) were rapidly mobilized in a short-lived pulse whereas elements associated with the larger grains (Fe, Mn, Cu, Zn, and Ni) exhibited a delayed but prolonged release into solution. The non-concomitant contaminant release indicates that an understanding of the textural relationships in the source material is required to understand weathering trends. This work highlights the importance of non-point sources of AMD, and addressing these sources is a critical step in improving water quality in the region.