Mobilization of arsenic from coal fly ash in the presence of dissolved organic matter

In this study, we examined the influence of dissolved organic matter (humic and fulvic acids) on arsenic (As) mobilization from two Class F coal fly ashes under oxic and anoxic conditions. Batch leaching experiments were conducted to examine As leaching behavior and changes in As aqueous and solid phase speciation after exposure to humic and fulvic acids for up to 4 weeks (ash/buffer ratio = 0.01 g/mL). Under oxic conditions, mobilized As concentrations increased compared to the organic-free controls and were positively correlated with percent aliphatic carbon and oxygen/carbon ratio of the humic and fulvic acids, suggesting both non-specific hydrophobic association of the organic matter with the surface of the fly ash particles and competitive desorption between negatively charged organic functional groups and As anions adsorbed to mineral surfaces. Under anoxic conditions, organic matter enhanced the reductive dissolution of As-bearing iron oxides such as hematite, resulting in increased As mobilization. X-ray absorption spectroscopy indicated that the majority of As in the leached and unleached fly ash was As(V) (≥90%), with As(V)-ferrihydrite precipitation occurring under all leaching conditions tested. Our results demonstrated that humic and fulvic acids are capable of mobilizing As from fly ash under both oxic and anoxic conditions, though redox appeared to be a larger driver of As mobilization compared to dissolved organic matter (DOM). DOM is a parameter which should be considered in risk assessment strategies for fly ash that is exposed to water (e.g., subsurface infiltration) or released to aquatic systems.