Colloidal metal transport in soils developing on historic coal mine spoil
Graphical abstract
Introduction
Mine spoils are metal(loid) rich solid waste generated as a result of a mining operation (Dang et al., 2002a). The coal mine spoils often include sulfidic minerals, mostly pyrite. The oxidation of pyrite by water and oxygen results in the formation of acid mine drainage (AMD) with a subsequent release of metal(loid)s associated with pyrite. The acidic nature of AMD can also enhance the chemical dissolution of the surrounding rock body resulting in even higher metal(loid) leaching (Hossner and Hons, 1992). The chemical dissolution of adjacent rock (mostly carbonate rocks) plays a neutralizing effect on AMD (Clark et al., 2018a, Clark et al., 2018b). However, in the absence of sufficient neutralizing rock body, the release of AMD and toxic metal(loid)s into the environment results in serious acidification of surface and groundwater causing harm to aquatic and plant life and also threatens human health and safety. When pyrite (FeS2) reacts with oxygen (O2) in the water, it releases Fe (II), SO4-2, and H+. At high pH (>5) Fe (II) oxidizes to Fe (III) which consumes some H+ and lowers the acidity of the solution. The newly formed Fe (III) then further goes through hydrolysis producing Fe(III) hydroxide (Fe(OH)3) and produces even more H+ than consumed in the previous process, resulting in a decrease in water pH. At lower pH (<3) aqueous Fe3+ can become the main oxidant and generate acid mine drainage (Lindsay et al., 2015; Singer and Stumm, 1970). The microbial activity at low pH can expedite the oxidation of Fe(II) to Fe(III) causing a higher rate of acid generation (Williamson et al., 2006). Under natural condition the dissolution of surrounding rocks and minerals (such as carbonates, sulfates, and oxides) counteracts the decreasing pH; however, if the mine spoils become depleted of carbonate rocks due to persistent pyrite oxidation, mine spoil loses its acid-neutralizing capacity (Lindsay et al., 2015). To neutralize the AMD, additional alkalinity is added by channeling AMD through constructing limestone bed channels and settling ponds. This process can lower the pH (even neutralize) leading to the removal of metal(loids) by precipitation. However, such remediation projects are expensive and often fail to deliver desired results (Kinney, 2013). The limited success of the traditional remediation practices may originate from the lack of understanding of the combined effect of physical and chemical weathering of pyrite. The physical weathering process acting upon pyrite fragments scattered in the mine spoils can potentially generate colloidal pyrite (Dang et al., 2002b; Taylor, 1974). The colloidal pyrite can transport with the pore water away from the site of origin and oxidize elsewhere. They can also act as a vector for other contaminants (Denaix et al., 2001; Lindsay et al., 2015). However, the occurrence, transport, and environmental consequences of colloidal pyrite are still unknown.
Colloids are suspended particles of 1 nm–10 μm which typically include clay minerals, metal oxides, and humic substances (Yin et al., 2000). Colloids may form by chemical precipitation and/or the physical breakdown of rock and mineral fragments (Forstner, 1995). Due to high surface area and adsorption capacity, colloids play a significant role in contaminant transport of sequestered metal(loid)s (Chikanda et al., 2021; McCarthy and McKay, 2004) The mobilization of colloids also depends on several chemical and physical characteristics of the host pore water including ionic strength (IS), pH, flow rate, and wetting and drying (Knappenberger et al., 2014; Lu et al., 2016b; Rousseau et al., 2004). At low IS the electrostatic double layer between the colloidal particle and soil matrix increases causing the release of the colloids whereas an increase in IS causes colloidal retention (Lu et al., 2016b). Changes in pH may not be a significant factor unless pH changes several units and/or the pH change takes place close to the particles ‘point of zero charge’ (pHpzc) (Forstner, 1995). For example, Fe (oxy)hydroxide colloids show positive or neutral surface charge at acidic pH values below approximately pH 7.2. The positively charged Fe (oxy)hydroxide colloids will adsorb negatively charged anion such as phosphate, whereas, at pH > 7.2, the colloids will have a negative surface charge which will lead to adsorption of positively charged cations such as Cu2+ (Forstner, 1995). At lower pH (<4) Fe (oxy)hydroxide will dissolve releasing all the adsorbed metals. According to Lu et al. (2016), alternative drying and wetting of soil can have a significant impact on colloidal release (Lu et al., 2016a; Mohanty et al., 2015b). The higher colloidal release was associated with a longer drying period; the longer the drying period, the more colloids were released in the following wetting period (Lu et al., 2016a). Higher total mass of colloidal release has also been reported from mine spoils due to the excavation, destruction of the soil matrix, loss of aggregability, and enhanced oxidation (Kimball et al., 1995; Rahman et al., 2013; Trostle et al., 2016).
Weathering processes acting upon pyritic minerals present in mine spoils can produce and mobilize colloids, and also potentially liberate toxic metal(loid)s associated with them, as the pyritic mineral colloids oxidize (e.g., Cu, Mn, and Zn) (Dang et al., 2002a). This can cause serious soil, pore water, surface water, and groundwater contamination (Large et al., 2014). The Fe (III) released from the pyrite oxidation can subsequently precipitate as secondary Fe(III)-(oxy)hydroxide coating on the pyrite colloid. The Fe(III)-(oxy)hydroxide coating can potentially inhibit pyrite oxidation by limiting the access of oxidants (O2 and Fe3+) to the core (McCarty et al., 1998; Schaider et al., 2014). Moreover, the Fe(III)-(oxy)hydroxide coating can sequester trace metals from pore water by adsorption.
The impact of colloidal pyrite oxidation can be potentially far-reaching as colloids have been reported to be able to transport faster and further compared to aqueous contaminants from the site of origin, which raises concerns for surface and groundwater contamination around an abandoned mine site (Grolimund et al., 1996). Nonetheless, the mobilization and transport of colloidal pyrite have not been studied yet, to our knowledge. Moreover, most of the studies related to the mobilization and transport of colloids are conducted in laboratory settings leading to a lack of field studies on geochemically complex sites, such as abandoned mine areas with a history of AMD generation and a huge collection of mine spoils (Dang et al., 2002a; Denaix et al., 2001; Mohanty et al., 2015a, 2015b). In the current work, we have collected soil pore water from soils developing on historic coal mine spoil using suction lysimeters, followed by centrifugation to determine metal(loid) distribution of the colloidal and aqueous fractions. The composition and morphology of separated colloids were determined by X-ray diffraction and scanning electron microscopy. The objectives of this study were to: (1) determine the contribution of colloids in the transportation of base and trace metal(loid)s, and (2) determine the mineral composition of soil pore water colloids. We hypothesize that trace metals (Al, Fe, Mn, Cu, and Zn) will be present at higher concentrations in the colloidal fraction whereas the base metals will be present predominantly in the aqueous phase and pyritic minerals will be present in the pore water as colloids in addition to phyllosilicates and metal (Fe, Mn, and Al) oxides.
Section snippets
Site description
The study site HR-25 is a sub-watershed of the Huff Run watershed located near Mineral city in northeastern Ohio covering parts of Carroll County and Tuscarawas County (Fig. 1). The area is underlain by sandstone, siltstone, and shale with interbedded coal seams of the Pennsylvanian system (Habig, 2015; Wise, 2005). Coal mining operations in the Huff Run watershed started in 1850 and continued until 1946. After the underground coal mine at HR-25 was closed, the mine area was abandoned leaving a
pH, electrical conductivity (EC), dissolved oxygen (DO), and precipitation
Field-based soil pore water analyses included dissolved oxygen (DO), electrical conductivity (EC), and pH; seasonally-grouped average values are reported in SI Table 1 and shown in Fig. 2. Pore water analyses were averaged within each season (Summer, 2018; Fall, 2018; Summer, 2019; SI Table 1). The seasonal average values for DO ranged between approximately 10% and 30% saturation (Fig. 2, top). There was no significant trend over time or differences among the six lysimeters. In contrast,
Relationship between pore water pH, EC, and DO and precipitation
The seasonal average pH in lysimeter 1–20 cm decreased from 6.54 in summer 2018 to 5.99 in summer 2019 (Fig. 2). A similar decrease in the seasonal average pH was also observed in lysimeter 1–25 cm, 1–75 cm, and 1–80 cm (Fig. 2). It has been reported by previous studies that the mine spoils of Appalachian coal mines typically contain reactive pyrite (Clark et al., 2018a, Clark et al., 2018b; Diehl et al., 2012). The oxidation of those reactive pyrites in the mine spoil can result in a decrease
Conclusions and implications
For the first time, we demonstrate that colloidal pyrite can be present in the pore water at a mine spoil site. The transport of colloidal pyrite as an AMD source from mine spoil to the surface and/or groundwater may potentially be overlooked as a cause of water quality impairment. We also demonstrate Fe-bearing colloids (oxides or sulfides) can play a more significant role in trace metal transportation compared to clay colloids. We also showed trace metals such as Fe, Mn, Cu, and Zn primarily
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.
Acknowledgement
This research was supported by the graduate research funding from the Geological Society of America (GSA), Kent State University, and Sigma-Gamma-Epsilon Kent State University chapter. We thank the following KSU graduate and undergraduate students for their support in pore water sample collection and analyses: Max Barczok, Daniel Wood, Emily Verhovitz, Nicholas Manning, and Lindsey Yazbek. We also thank Marissa Lautzenheiser (Huff Run Watershed Restoration Partnership) for logistical support at
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