Effect of seepage conditions on chemical attenuation of arsenic by soils across an abandoned mine site
Highlights
► Arsenic leaching from mine soils is kinetically limited. ► Arsenic mobility can be greatly attenuated by downgradient soils. ► The attenuation was found kinetically limited and energetically hysteric.
Introduction
As is a toxic trace element that is widely distributed in soils and aquifers from both geological and anthropogenic sources (Turner, 1981, Smith et al., 1998). A significant amount of As is introduced to soil systems as a result of mining and metals smelting processes (Smith et al., 1998). According to the report of the Ministry of Environment (MOE) in Korea, approximately 900 metal mines are spread throughout the Republic of Korea and only about 150 are still operating or planned to operate (MOE, 2007). Most of the inactive mines have been abandoned for several decades, and those abandoned mine sites are frequently found to be highly contaminated with As at amounts that are well above the regulatory level established by the MOE (i.e., 15 mg of total As kg−1 soil). Further, many abandoned metal mines are located within the Han River watershed, a drinking water reservoir for >10 million citizens living in the Seoul metropolitan area (MOE, 2007). Therefore, unless appropriately managed, the leachate derived from the abandoned mine site could be a potential source of As, causing degradation of the water quality in the watershed.
Seepage is the primary manner by which a portion of As becomes soluble and mobile out of the contaminated solid phases (e.g., mine residues or mine impacted soils). Upon interacting with seepage water, the readily soluble fraction of As in the solid phase is partitioned into the water phase. Once dissolved in the leachate, the aqueous As readily moves through soil pore spaces and eventually enters nearby water bodies. In this sense, many investigations have focused on the characterization and quantification of the easily leachable and/or soluble fractions of heavy metals from mine-impacted soils within various environmental settings (Hu et al., 2008, Yolcubal and Akyol, 2008, Wang and Mulligan, 2009). Much effort has been invested in assessing the mobile As fraction in contaminated soils. Chemical extraction methods have been used to predict the fraction of mobile or readily accessible to ecological components (Wang and Mulligan, 2006, Singh, 2007). For example, a weak neutral salt or slightly acidic solution has been used to quantify the liable (easily leached) fractions of As associated with the exchange sites in the solid phase. However, frequent observation of poor correlations between the total As content in the solid phase and those in the leachates has led to the conjecture that the As retained in the solid phase is not responsible for As leaching. According to the work done by Nam et al. (2010) with several soils collected from different abandoned mine sites, <1% of the total As was water soluble during a 10 stepwise batch leaching experiment and the amount of As leaching was not well correlated with the total As amounts in the soils.
The down-gradient movement of As dissolved in mine leachates can be retarded by soil solid-phase As adsorption. As has been known to strongly adsorb to inorganic constituents in soils (Manning and Goldberg, 1996, Burns et al., 2006, Nam et al., 2010), such as clay minerals and Fe and Al oxides/hydroxides. The results of a batch adsorption study performed with various model adsorbents and natural soils showed that As adsorption was highly correlated with soil pH and iron oxide content (Goh and Lim, 2004, Burns et al., 2006). In addition, X-ray spectroscopy studies demonstrated the formation of strong inner-sphere complexes between adsorbed As and the adsorbent surfaces that were mainly composed of Fe/Al oxides (Wang and Mulligan, 2006, Makris et al., 2007). In a one-dimensional (1-D) column study, Nam et al. (2010) showed that the risk of mine leachate enriched with As(V) was greatly attenuated as travelling through Fe/Al oxide rich downgradient soils.
Most earlier work on As attenuation has been conducted under equilibrium conditions, that is, the potential of As leaching from mine soil and As adsorption onto model/natural adsorbents. However, leaching and adsorption are essentially time-dependent (Brusseau et al., 1989, Zhang and Selim, 2006, Yolcubal and Akyol, 2008), and the attenuation of As from this aspect across abandoned mine sites has not been well studied. Therefore, the objectives of this study were to characterize the fate of As in various leachate seepage settings. Leaching and adsorption experiments were conducted under both batch and 1-D water flow system conditions at different seepage velocities. During column displacement experiments, the flow interruption method was employed to analyze the concentration perturbations of As and a nonreactive solute. Desorption of As from the soil after bath and column leaching studies was also investigated.
Section snippets
Site descriptions
An abandoned DY mine located in the middle of Korea at 36°58′26.2″ N, 128°10′52.7″ E was investigated in this study. This mine had produced tungsten and has been abandoned for several decades (MOE, 2007). In an earlier site survey (Nam et al., 2010), a mixture of soil and residual rock debris was found in front of the mine mouth and scattered along the downslope. It was also revealed that the As in this site was predominantly As(V) and that the content of As(V) in the soils was highest in front
Soil properties
The two mine-impacted soils (DY0 and DY1) and three downgradient soils (DY2, DY4, and DY7) showed distinctly different properties. The mine soils were highly contaminated with As concentrations up to 142,000 mg kg−1, while the downgradient soils had clearly lower As contents, well below the environmental regulatory level established by the MOE (i.e., 15 mg of total As kg−1 soil). The Fe/Al oxide concentrations were an order higher in the mine soils. Site descriptions and soil properties were
Summary and Conclusion
The leaching of As from mine soils and the chemical attenuation of As by downgradient soils were investigated using various 1-D seepage settings. The export of As from mine soil continued to occur and the effect of seepage velocity on export concentration was insignificant. However, chemical attenuation of As by downgradient soils varied with different seepage conditions. From the BTC of As leaching from mine soil, a greater As peak was observed from slower seepage velocity indicating that the
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