Exploring Geochemical Methods to Measure Arsenic-Dissolved Organic Matter Complexes and Coal Combustion Residuals in Surface Water
Type of DegreeMaster's Thesis
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Chapter 1, investigates the geochemical controls governing arsenic (As) mobility, precisely the formation of As-DOM complexes. Dissolved organic matter (DOM) reduces As mobility through equilibrium binding of As to the outer and/or inner sphere of DOM, creating an As-DOM complex (Buschmann et al., 2006; Ren et al., 2017; Tipping, 2002). Conversely, DOM increases As mobility if(1) As binding in the As-DOM complex is reversible, (2) the DOM structure is degraded, (3) there is competitive sorption where DOM will preferentially bind to a mineral surface, inhibiting As binding, and (4) the complex remains soluble in solution (Dowling et al., 2002; Liu and Cai, 2010; Miller et al., 2010; Redman, 2002). To determine As-DOM complex formation, two separate methods were utilized (1) high performance liquid chromatography (HPLC), size exclusion column separation (SEC), inductively coupled mass plasma spectroscopy (ICP-MS), ultra-violet visible spectroscopy (UV-VIS), and fluorescence detection (FLD) and (2) equilibrium dialysis experiments. Additional methods, including spectroscopic methods and functional group quantification, were employed to investigate trends between DOM characteristics and As complexation capacity. Lignite derived DOM was utilized to compare metal binding capacity between the lignite derived DOM and a known DOM standard, Suwannee River Natural Organic Matter (SRNOM). In HPLC-SEC-ICP-MS analysis, a complexed As peak was observed at 17 across all DOM types; a free As peak was observed at 28 minutes across all DOM types and ultra-pure water. Three independent lines of evidence was produced to confirm the detection of free and complexed As: (1) free As elutes later than complexed As which is consistent with SEC theory and (2) HPLC-SEC-FLD analysis and (3) HPLC-SEC-UV analysis produced peaks at a similar retention time, 28 minutes. In both HPLC-SEC-ICP-MS and equilibrium dialysis results, log KD results decreased with increased addition of As which is likely caused by the saturation of As and loss of DOM complexation sites. A larger As-DOM complex, at minute 13, was also identified with lignite derived DOM samples. This study confirmed the use of HPLC-SEC-ICP-MS as a precise tool for determining As-DOM complexes; HPLC-SEC-ICP-MS results were comparable to previous literature and those calculated through equilibrium dialysis. We recommend that future studies focus on the use of HPLC-SEC-ICP-MS to gather more detailed information regarding the molecular characteristics of DOM fraction responsible for complexation. HPLC-SEC-ICP-MS analysis suggests that lignite derived DOM has a larger complexation capacity compared to the DOM standard, SRNOM, implying that complexation may be underestimated in groundwater systems in previous literature. The second study, Chapter 2, investigated coal combustion residual (CCR) contamination at two samples sites, Gadsden, and Gaston impoundments, along the Coosa River in Alabama. CCR is created as a byproduct of coal combustion at electrical powerplants and stored in CCR impoundments across the United States (Wang et al., 2020). CCR contamination produces an enrichment in heavy metals in sediments and surface water after decades of potential contamination in shallow aquifers and flood events (Aguirre, 2019; Harkness et al., 2016; Vengosh et al., 2019; Wang et al., 2019). The research objective of this study was to evaluate the impact of CCR impoundments on the nearby sediment and water quality. Seven samples were collected in proximity to Gadsden impoundment and seven samples collected near the Gaston impoundment. Heavy metal enrichment of surface water and sediment samples were utilized as a diagnostic tool for CCR contamination. The linear relationship between molybdenum (Mo) and antimony (Sb) was also employed as additional evidence. Sediment and surface water produced a similar enrichment pattern as one produced in previous literature, however most elements studied had an enrichment value below 10. A strong linear relationship between Mo and Sb was found in both sediment (R2=0.8758) and surface water samples (R2=0.8215). The evidence is inconclusive to suggest that CCR contamination is contributing to reduced water quality in the Coosa River.