Iron reductive dissolution in vadose zone soils: Implication for groundwater pollution in landfill impacted sites
Introduction
In recent years, excessively high concentrations of dissolved iron (Fe) have been observed in the groundwater at many landfill sites, including sites in the state of Florida, USA. However, monitoring data show little to no link between Fe and the chemical tracers of landfill leachates (Geosyntec, 2005), implying that in addition of potential Fe removal from solution by precipitation, Fe sources other than landfilled wastes could be responsible for the groundwater contamination. Based on the high concentrations of dissolved organic compounds commonly encountered in landfill leachates (Mohammadzadeh et al., 2005; He et al., 2006), it is hypothesized that microbial respiration of organic matter present in landfill leachates drives the reduction of different terminal electron acceptors present in soils including iron oxide minerals. It is worth noting that the “iron oxide” terminology used in this paper refers also to iron hydroxides and oxyhydroxides (Alloway, 1995; Lentini et al., 2012). In soils and soil water, Fe occurs predominantly in the +3 and + 2 oxidation states (Viollier et al., 2000; Thompson et al., 2006; Melton et al., 2014; Essington, 2015), with Fe3+ being prevalent in solid phases (Pedersen et al., 2005; Komlos et al., 2007), and Fe2+ occurring as dominant aqueous species (Lovley et al., 1991; Konhauser et al., 2011; Wu et al., 2017).
Iron oxide minerals are commonly found in soils (Fredrickson et al., 1999; Thompson et al., 2006; Diagboya et al., 2015), where they can have a profound impact on both soil and soil water chemistries (Luu and Ramsay, 2003; Larsen et al., 2006). In soils, iron oxide concentrations vary from 0.1 to 50% (Schwertmann, 1991; Cornell and Schwertmann, 2003), and may be present as discrete minerals, coatings on sand and clay particles (Stipp et al., 2002; Xu et al., 2009; Rusch et al., 2010), nodules or fillings in the cracks and veins of minerals (Kabata-Pendias and Pendias, 2000; Hilgers and Urai, 2005), or as major constituents of mottles, concretions and other segregations (Wiriyakitnateekul et al., 2007; Ketrot et al., 2014). Variations exist in the crystallinity, stability, solubility, particle size and specific surface area (SSA) of iron oxide minerals. With regard to crystallinity, Fe-oxides can range from amorphous forms such as ferrihydrite Fe(OH)3 to the highly crystalline goethite (α-FeOOH) and hematite (α-Fe2O3) minerals (Roden, 2004; Colombo et al., 2014). However during the aging process, iron oxides can undergo transformation, in that amorphous iron oxides evolve to the more stable and crystalline forms (Benner et al., 2002; Pedersen et al., 2006). The size of iron oxide particles ranges from 5 to 150 nm (Schwertmann, 1991; Huang et al., 2011), leading to SSA values as high as 250–400 m2/g (Cornell and Schwertmann, 2003; Kappler and Straub, 2005; Huang et al., 2011). Iron oxide surfaces also have the potential to sorb metals such as zinc (Bolan et al., 2014; Schaider et al., 2014; Mukwaturi and Lin, 2015) and oxyanion forming elements such arsenic (Mladenov et al., 2010; Neidhardt et al., 2014; Cozzarelli et al., 2016), and phosphorus (Stipp et al., 2002; Rentz et al., 2009). When used as terminal electron acceptor during microbial respiration under anoxic conditions (Colombo et al., 2014; Meckenstock et al., 2015; Mejia et al., 2016), iron oxides will undergo a reductive dissolution, and compounds previously sorbed onto them get released to solution, with implications for increased mobility, bioavailability, and toxicity (Makris et al., 2005; deLemos et al., 2006; Thompson et al., 2006; Ziegler et al., 2015).
Although landfilling remains the most widely used method for waste disposal in the developed world (Cozzarelli et al., 2000; Omar and Rohani, 2015), the occurrence of groundwater contamination by landfill leachate is problematic and has been the focus of many studies (Lyngkilde and Christensen, 1992; Christensen et al., 2001; Baun and Christensen, 2004; Wiszniowski et al., 2006; Renou et al., 2008; Mukherjee, 2015). Landfill leachate is a liquid solution composed of many components of the landfill wastes that is produced when water infiltrates through the wastes (Renou et al., 2008; Moody and Townsend, 2017). Iron is also found in landfill leachates and groundwater contamination with iron has been observed in leachate plumes from various landfill locations in the world (Christensen et al., 2001; Kjeldsen et al., 2002; Zhang et al., 2013). However, in the monitoring data from the impacted Florida landfills, no clear link appears between iron in the landfill leachates and iron in polluted groundwater (Geosyntec, 2005). The purpose of this study is therefore to investigate the potential effect of organic matter present in landfill leachate to drive the dissolution of iron oxides present in vadose zone soils as a function of degrees of crystallinity of Fe-minerals.
Section snippets
Soil samples collection and handling
Soil samples used in this study were collected from landfill sites with and without Fe groundwater contamination issues. Soil samples were also collected from sites with no landfill activities to obtain a clear gradient in soil's iron mineralogy and crystallinity. All soil samples were collected in the state of Florida, and from various sites including Klondike Landfill site in Escambia County (an unlined landfill site closed in 1982 after 6 years of operation), the New River Regional Landfill
Characteristics of used soils and landfill leachate
As stated earlier, tested soils were grouped in to 3 groups labeled as zone 1, 2 or 3 (Table 1, Table 2). This grouping was based on the assumption of a decrease in the degree of crystallinity of Fe-minerals with decreasing thickness of vadose zone soil (Fig. 1), and possibility of temporal vadose zone soil saturation due water table fluctuations. We therefore anticipated that zone 1 would consist of soil samples with oxidized crystalline iron minerals, while zones 2 and 3 would be dominated by
Acknowledgement
This research was supported by funds from the Hinkley Center for Solid and Waste Management, University of Florida. We thank the staff of the Soil Mineralogy laboratory at the University of Florida for soil physicochemical analyses. Finally, we thank Shiqin Lin, Kevin Njeru, Ian Vicotorio and Joshua Lane for their assistance with sample processing.
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