Lime slurry treatment of soils developing on abandoned coal mine spoil: Linking contaminant transport from the micrometer to pedon-scale
Graphical abstract
Introduction
Coal mine spoil, a combination of residual, non-economically viable mined rock or sediment and overburden material, is an obligate byproduct of coal mining. Unregulated coal extraction and waste management occurred for over 150 years in the United States prior to the Surface Mining Control and Reclamation Act (SMCRA) of 1977, resulting in severe environmental consequences including acid mine drainage (AMD) and the leaching of toxic elements (ODNR, 2014). Mine spoil was often left abandoned after cessation of coal production, generating an on-going and critical need to continue mine reclamation and restoration (Griffith et al., 2012). Pyrite (FeS2) and other metal sulfides remaining in these materials can undergo oxidative dissolution generating acid- and metal(loid)-rich (e.g. Al, Mn, Fe, Cu, Ni, and Zn) porewaters (Abraitis et al., 2004; Cravotta et al., 1994; Cravotta, 2008; Nordstrom, 1982; Sánchez España, 2007). These metal(loid)s have been observed to be enriched in soils developing on mine spoil and linked to elevated concentrations in surface soils up to 1 km from their source (Alekseenko et al., 2018), and in the post-closure period of mining operations, leaching of these elements can be a significant long-lived environmental impact (Younger, 2004). These abandoned mine spoil piles therefore represent a geographically large and diffuse non-point source of contamination to local watersheds, although the scope of the problem and potential solutions have been less studied compared to point sources (e.g. abandoned mine ponds and underground tunnels). The extent of abandoned mine spoil can be difficult to quantify, although in Ohio alone, it has been estimated that there are approximately 36 billion tons of known coal mine spoil spread over 600 km2 (Singer et al., 2020b). These materials are widespread in US coal mining regions, and the potential long-term leaching of toxic metal(loid)s to surface and groundwater is a significant and underappreciated issue.
Removal of metal(loid)s by precipitation through the addition of lime and related alkaline materials is well established for a range of contaminated water sources including municipal and industrial wastewater and AMD (Fu and Wang, 2011; Skousen et al., 2019). The addition of lime to AMD aims to neutralize acidity and encourage the (co)precipitation of metal(loids) with Fe(III)-(oxy)hydroxides (to be referred to as Fe-oxides), other metal-oxides of Mn and Al, or Ca-bearing phases such as gypsum or calcite (Johnson and Hallberg, 2005; Khorasanipour et al., 2011; Sánchez España, 2007). Lime slurries applied at the surface can infiltrate into targeted material. Due to the fine particle size of the lime and ability to quickly harden as cement-like compounds, lime slurries have been shown to improve soil stability by filling pore spaces (Consoli et al., 2009; Sephton and Webb, 2017). This results in a decrease in porosity and permeability which can increase resistance to erosion while also decreasing the number of flow paths in the soil for contact with infiltrating rainfall. This approach has been used for waste rock, fly ash, and other AMD-producing materials with aim of increasing pH, decreasing sulfate and metal loadings, decreasing water flow, and increasing the structural stability of these materials (Behera and Mishra, 2012; Jung and Santagata, 2014; Sephton and Webb, 2017; Sephton et al., 2019). These studies commonly report the formation of jarosite and gypsum under acidic conditions and calcite, gibbsite, and ferrihydrite at more circumneutral pH (Davis et al., 1999), and the transformation of trace elements (e.g. Co, Cr, Cu, Ni, Pb, and Zn) into less labile mineral pools during the reactions (Abbott et al., 2001). Amendments such as biochar or fertilizers have also been added to the treatment to improve the response of biota to reclamation (Novak et al., 2018). However, treatment of soils developed on historical mine spoil has been limited despite the expansive distribution of these materials, particularly in the Appalachian region of the US, where a cumulative mining footprint of 5900 km2 has been estimated (Pericak et al., 2018). Lime-based slurries may represent an economically efficient approach to treatment.
Grain-scale and sub-scale geochemical processes likely control the efficacy of lime slurry treatment through changes in the composition, texture, and morphology of secondary minerals that form coatings and/or aggregates with primary grains (Demers et al., 2017). The addition of a lime slurry can result in the rapid precipitation of secondary Fe-bearing phases which can limit metal(loid) release to porewater by coating pyrite (FeS2) grains, thereby limiting further oxidative dissolution, and by metal(loid) uptake by newly formed phase. The potential for these Fe-bearing phases to sequester contaminants or release them back into solution, depends on their behavior during their initial formation, transformation to more stable phases, and their susceptibility to dissolve again and/or be transported as colloids (Chowdhury et al., 2021; Stipp et al., 2002). Rapid precipitation can also promote aggregate formation which can decrease porosity leading to natural passivation as O2 and water are prevented from reaching sulfide mineral surfaces (Elghali et al., 2019; Hakkou et al., 2008). It is therefore likely that mineralogical changes at the micrometer-scale during lime slurry application may impact metal(loid) transport at the pedon-scale.
Undisturbed legacy spoil piles remain across the Huff Run Watershed (the focus of the current study), and previous work has characterized the spoil piles examined in the current work in an effort to understand colloid transport and toxic metal mobility within these non-point sources of contamination to the watershed. Solid phase characterization of these piles found an assemblage of common soil forming minerals including quartz, feldspars, micas, and clays. The persistence of the micrometer-scale pyrite grains in soils developing on mine spoil is likely the result of the formation of the secondary mineral surface coatings which can limit complete oxidative dissolution and play a role in re-sequestration of metal(loid)s release from sulfide mineral weathering (Singer et al., 2020b). Physical weathering of pyrite within the mine spoil has also been shown to generate colloidal pyrite, which is mobilized and transported by soil pore water, and these particles are associated with toxic trace metals including Cu, Mn, and Zn (Chowdhury et al., 2021). Further, the presence of abundant residual coal and Fe- and Mn- (oxy)hydroxides in mine spoil-derived soils control and mitigate trace metal (Cu, Ni, and Zn) transport from the soils, and although Mn is highly mobile in Mn-enriched soils, Fe and Al mobility may be more controlled by dissolved organic carbon dynamics than mineral abundance (Singer et al., 2021).
The overall goal of this work was to determine the efficacy of treating soils developing on abandoned coal mine spoil with lime slurry to neutralize acidity and sequester metal(loid)s through the precipitation of secondary solid phases. Parallel field application and laboratory-based column experiments were conducted to constrain the solid phase and solution variables that control the fate and transport of contaminants in these soils using a suite of mineralogical and geochemical approaches. Quantitative imaging analyses were then conducted 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 of trace metal(loid)s.
Section snippets
Site description
The field site is Huff Run sub-watershed 25 (Fig. 1). The watershed is located in Tuscarawas County, near Mineral City, Ohio and this site was chosen in part because it was previously classified as one of the most highly AMD-impacted watersheds by the Huff Run Watershed Restoration Partnership. The watershed spans a 0.44 km2 area and is underlain by Pennsylvanian sandstones and siltstones, which are interlayered with claystone, limestone, and coal (Lamborn, 1956). The coal seams present were
Porewater composition in untreated and lime-treated mine spoil
Time averaged porewater analyte values are shown in Fig. 2, with the full time series data shown in SI Figs. 5, 6, and 7. 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 (SI Fig. 5A and B) and electrical
Relationship between lime infiltration and porewater chemistry
Soil porewater composition in the column experiments showed a clear response to the addition of the lime slurry, as evidenced by the increase in the pH and Ca concentration of the leachate and decrease in the amount of Mn, Zn, and sulfate that were leached compared to the untreated columns (Fig. 3). This is consistent with previous work which has shown that the addition of lime encourages the sequestration of metal(loid)s via (co)precipitation and/or sorption onto existing and newly formed
Conclusions and environmental implications
This goal of this work was in part to determine the potential for using a lime slurry as a cost-effective treatment of abandoned coal mine spoil, which could ideally be scaled up to abandoned mine sites across coal mining regions. Ultimately, the data from this work provide evidence that a lime slurry added to coal mine spoil can neutralize acidity, increase dissolved Ca, and decrease dissolved Mn and Zn. Despite clear trends in the laboratory-based column experiment where the lime-to-soil
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research was supported by the Ohio Coal Development Office-Ohio Coal Research Consortium (grant #R-16-06 to DMS). We thank Matthew Newville and Tony Lanzirotti and the APS GeoSoilEnviroCARS (GSE-CARS) staff for beamline support. GSE-CARS is supported by the National Science Foundation-Earth Sciences (EAR-1634415) and Department of Energy-GeoSciences (DE-FG02-94ER14466). This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User
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