Residence time and salinity impacts on chromium speciation and mobility in soils of varying iron and manganese oxide contents

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University of Delaware
Chromium (Cr) contamination in soils has inevitably become a threat to human health and the environment over several decades due to its excessive use in industries such as leather tanning, wood preservation, and stainless steel production. Chromium is a redox-reactive metal, predominantly present in two stable forms as Cr(III) and Cr(VI). Chromium(III) is less toxic and less mobile, while Cr(VI) is carcinogenic and can travel a great distance in the environment. The stability and toxicity of Cr are strongly related to its speciation, driven by several soil components (e.g., iron (Fe) oxides, organic matter, manganese (Mn) oxides) as well as environmental factors (e.g., redox conditions, porewater composition). Several studies have indicated that Cr mobility and availability decrease over time through aging. However, the long-term aging effect on Cr speciation and mobility in contaminated soils is still unclear, especially in soils containing high content of Mn oxides, which can persistently oxidize Cr(III) to the readily mobile Cr(VI). This is of particular concern, especially when most soil worldwide is currently threatened by global climate change. Increasing storms and rainfalls may enhance Cr leaching, while inundation and saltwater intrusion of contaminated coastal soils may mobilize Cr through changes in redox conditions and ion competition. A better understanding of how aging and water salinity impact Cr cycling is needed to predict long-term environmental effects and potential remediation strategies under the impacts of global climate change. ☐ The objectives of this work are (1) to determine the residence time effect on Cr speciation and stability in soils and (2) to investigate how Cr in contaminated soils responds to changes in water salinity and redox conditions. Throughout this work, a systematic characterization of soil samples (pre- and post-reactions) was conducted utilizing batch incubations, sequential extractions, and desorption experiments, as well as advanced spectroscopic tools (e.g., bulk and microfocused X-ray absorption techniques) to identify changes in solid-phase Cr speciation and mobility. ☐ Chapters 1 and 2 investigate residence time and water salinity impacts on Cr speciation and mobility in soils. Two soils with varying Fe and Mn contents were reacted with Cr(III) and Cr(VI), as in dissolved CrCl3 and K2CrO4, and allowed to age up to 180 days. Alkaline digestion and sequential extractions were utilized to identify changes in soil Cr(VI) concentrations and Cr partitioning, while bulk and microfocused X-ray absorption techniques were employed to determine shifts in Cr speciation and spatial distribution over time. The results indicated that the speciation of Cr in aged soils was strongly related to the added Cr oxidation state and the presence of Mn oxides, which oxidized Cr(III) to Cr(VI). The increase in more stable Cr phases was observed in all aged soils; however, the aging effect was more pronounced in low Mn soils (LM), in which soil Cr(VI) content decreased over time. The results also suggested the persistence of Cr(VI) in high Mn soils (HM). Reacting Cr-aged soils with artificial seawater resulted in extremely high Cr release (up to 12 mg L-1) in all Cr-aged soils, except Cr(III)-aged LM soils, possibly due to the presence of Cr(VI) outer-sphere complexes. Chromium(VI) remained in these sediments after seawater treatment, suggesting that Cr may be released in case of multiple cycles of exposures. ☐ Chapter 3 investigates Cr speciation and mobility in existing Cr-contaminated coastal urban soils impacted by water salinity and redox conditions. Chromium-contaminated soil samples were collected from a contaminated site in Wilmington, Delaware. Bulk and microfocused X-ray absorption near edge structure (XANES) spectroscopy and sequential extractions were utilized to determine Cr speciation and stability. In addition, desorption experiments and batch incubations were conducted to examine how Cr in these soils responds to changes in water salinity and redox conditions. The results revealed that, although the soils contained very high levels of Cr (up to 4320 mg kg-1), Cr mobility remained very low under non-flooded conditions due to the abundance of chromite (~52%) and Fe-Cr hydroxide coprecipitates (~44%). Chromium(II) was also identified in the soils by bulk and microfocused XANES, potentially derived from the waste materials at the sampling site. Seawater and anoxic conditions resulted in lower Cr release compared to freshwater and aerobic conditions. Three to eight times more Cr was released under aerobic conditions versus anaerobic conditions in the freshwater versus saltwater, respectively. ☐ This work improves an understanding of residence time and water salinity impacts on Cr speciation and mobility in Cr-contaminated soils and, more importantly, raises some concerns regarding a potential long-term health risk from Cr release due to sea level rise and global climate change. Further studies should focus on assessing how salinity and redox fluctuations impact Cr cycling and potential health hazards due to the potential Cr release from these contaminated soils.
Chromium, Cycling, Salinity, Sea level rise, X-ray absorption spectroscopy