Uranium(VI) Removal by Zero-Valent Iron Metal
URANIUM(VI) REMOVAL - RESEARCH SUMMARY
the Y-12 site, uranium is the main driver for this groundwater remediation
using the permeable Fe0 reactive barrier because uranium poses
the major potential health and environmental risks within
Uranium retention by Fe0 filings was also evidenced by the analysis of uranium content in Fe0 core samples (Figure 2). These angled cores were collected ~15 months after installation of the Fe0 barrier, and detailed sample preparation and analysis were given elsewhere (Phillips et al., 2000) . Although only trace quantities of uranium are present in the contaminated groundwater, an elevated amount of uranium was detected in the Fe0 core materials, particularly in those samples near the interface (between the soil and Fe0 filings) where groundwater enters the Fe0 barrier. The greatest concentration of uranium in the Fe0 barrier occurred at the shallow, upgradient interface, but uranium concentration decreased dramatically over a short distance. These observations suggest that once uranium enters the Fe0 reactive barrier it is rapidly sequestered in situ as a result of either reductive precipitation of relatively insoluble U(IV) species or surface adsorption of U(VI) species on the Fe0 corrosion products (e.g., iron oxyhydroxides) (Cantrell et al., 1995; Gu et al., 1998) .
Reductive precipitation of U(VI) to U(IV) species by Fe0 is believed to be one of the dominant mechanisms for uranium removal and is thermodynamically favorable, according to the following stoichiometric reactions :
= +0.327 V
e = -0.07
V at pH 8.
However, as the corrosion products of Fe0, such as iron oxyhydroxides, accumulate on Fe0 surfaces, uranium removal through sorption or co-precipitation could not be ruled out (Fiedor et al., 1998; Gu et al., 1998; Hsi and Langmuir, 1985; Morrison et al., 1995) . Unfortunately, because of a relatively low amount of uranium retained by the Fe0 filings and a possible reoxidation of reduced U(IV) species during sample preparation and extraction, no attempts were made to distinguish whether uranium was reductively precipitated or sorbed by iron oxyhydroxides in these Fe0 barrier materials. However, on the basis of previous laboratory studies (Cantrell et al., 1995; Fiedor et al., 1998; Gu et al., 1998) , a conclusion can be made that a majority of uranium could have been retained by the reductive precipitation process because of a strong reducing environment within the Fe0 barrier (with a high solid to solution ratio). Understanding of U(VI) removal mechanisms through either reductive precipitation or sorption/co-precipitation has important environmental implications because the reduced U(IV) species on Fe0 surfaces could be potentially re-oxidized when it is exposed to the air or dissolved O2 in a matter of a few hours or days (Gu et al., 1998) . Similarly, the sorbed U(VI) species could be desorbed and therefore remobilized as groundwater geochemistry changes.