Monday, November 13, 2006
127-3
Mobility and Distribution of As and Se Species during Transport through Facility Site Soils.
Seunghun Hyun, Korea Univ, Env Science and Ecological Eng, Seoul, 136-713, South Korea and Linda S. Lee, Purdue Univ, 915 W State Street, Dept of Agronomy, West Lafayette, IN 47907.
Management of arsenic and selenium present in fly ash leachate at disposal facilities is a significant environmental concern. Enhancing our ability to predict arsenic and selenium mobility is important in assessing site-specific human and ecological risks. Previously, we assessed the attenuation of arsenic (III and V) and selenium (IV and VI) by soils from 3 ash disposal sites. In the current study, mobility and stability of As and Se in a simulated ash leachate (1 mM CaSO4) was investigated using 1-D columns and soils from three ash disposal sites. The order of elution followed batch isotherm trends with As(V) > Se(IV) > As(III) > Se(VI); however, retardation factors calculated from linearized concentration-specific Freundlich sorption model coefficients were not always in good agreement with observed breakthrough. Later than expected breakthrough of As(III) and early breakthrough of As(V) observed for all cases suggest oxidative transformation of As(III) to As(V). Expected versus observed differences increased with increasing As(V) sorption nonlinearity (N<1). Although it was much lesser extent than observed with As(III), some oxidative transformation of Se(IV) to Se(VI) occurred. However, Se(VI) is already inherently more mobile than Se(IV), thus earlier than expected breakthrough was not observed. A single desorption step with 1 mM CaSO4 (S) of column soil was performed followed by two sequential extractions with 0.1M/0.1M NaH2PO4/Na2HPO4 (P), and finally a HNO3-H2O2 digestion (D) for solute residuals. Relative to the total (S+P+D) for each column section, 69% ± 3% of the total As and 79% ± 7% of the total Se was extractable (S+P). However, a significant fraction appears to be irreversible, suggesting that attenuation by soils downgradient of an ash landfill may be much greater than that predicted from batch isotherms. Additional site-specific factors also need to be considered including groundwater flow and ash-leachate induced changes in soil pH over time.