DISSOLUTION OF AS AND U IN ABANDONED MINE WASTES

Abstract:

The release of U and As from abandoned mine wastes obtained from a site in the Navajo Nation in Northeastern Arizona was assessed by integrating spectroscopy, microscopy, and aqueous chemistry techniques. X-ray fluorescence analyses detected concentrations of 0.6% U, 0.4 % V, and 0.3% Fe. X-ray photoelectron spectroscopy analyses confirmed the presence of 73% Fe(II), 27% Fe(III), and the predominance of U(VI), V(V), and As(0). Transmission electron microscopy analyses detected the presence of amorphous carnotite [K₂(UO₂)₂(VO₄)₂·3H₂O]. The proportional release of U and V into solution was measured for chemical extractions using 10 mM ascorbic acid (~pH 3.5) using inductively coupled plasma optical emission spectroscopy after 96 hours of reaction (sampled periodically). A similar proportional release of As and Fe after 2 hours of reaction was observed for chemical extractions using the same conditions mentioned previously. Lower concentrations of U, V, As, and Fe were released from chemical extractions using 10 mM bicarbonate (~pH 8.3). The integration of spectroscopy, microscopy, and aqueous chemical extractions suggest that carnotite and an As-Fe-bearing mineral phase (likely arsenopyrite) control the dissolution of U and As in these abandoned mine wastes. This has important environmental implications related to contamination of water sources in communities that live in the proximity of abandoned mine wastes.