Functional Predictor Variables for the Leaching Potential of Arsenic and Selenium from Coal Fly Ash

Abstract

The release of leachates from intact coal ash impoundments is a concern due to the enrichment and mobilization of toxic elements such as arsenic (As) and selenium (Se). This study aims to explore the intrinsic properties of coal fly ash that correlate with the relative leachability of As and Se. We performed leaching experiments with 52 fly ash samples collected from 15 different U.S. power plants and representing coal feedstocks from the three major domestic regions. We assessed the mobilization potential of As and Se in fly ash based on standardized leaching protocols and performed multivariate and lasso regression analyses to explore correlations of leachable As and Se contents with characteristics such as major element contents, loss on ignition, and pH. The results of regression models indicated that major elements (Fe, Ca, and Al) for a wide range of fly ashes can serve as predictor variables for the leaching potential of As but not for Se. LOI and pH were not important predictive variables in the models. Both regression approaches resulted in relatively strong fits for leachable As (correlation coefficient R 2 = 0.78 for both models) compared to models for leachable Se (R 2 = 0.49). Overall, these results suggest that correlation models combined with on-site elemental analysis with portable analyzers may enable a screening method for leachable As in coal ash.

Nanoscale heterogeneity of arsenic and selenium species in coal fly ash particles: analysis using enhanced spectroscopic imaging and speciation techniques

“…The differences in Se distribution between the two fly ash samples can explain why such wide variation in Se leachability has been observed. 23 These observations that would not have been possible to determine with the resolution of μ-XRF. The spot size for μ-XRF measurements in this study is approximately 5 μm × 5 μm.…”

“… a From Jin et al (2023). 23 b From Taggart et al (2018); 22 sequential extractions corresponded to: F1 deionized water soluble, F2 acid soluble (0.11 M acetic acid), F3 reducible (0.5 M NH 3 OH·HCl pH 2), F4 oxidizable (30% H 2 O 2 followed by 1.0 M ammonium acetate at pH 2). …”

“…Major elements Si, Al, Fe, and Ca contents (expressed in their wt% oxide form in Table 1) were determined by benchtop X-ray fluorescence spectroscopy (XRF). 7,22 These fly ash samples were previously assessed for leachable As and Se contents (Table 2) through multiple extraction methods: 1) overnight leaching in deionized water (modeled after the Leaching Environmental Assessment Framework); 23 2) leaching according to the Toxicity Characteristic Leaching Protocol (TCLP); 23 and 3) modified BCR sequential extraction, 22 as detailed in prior studies. 22,23…”