Abstract

Tims Branch, a stream on the U.S. Department of Energy's Savannah River Site located near Aiken, South Carolina, received significant amounts U and Ni as a result of nuclear materials production and refinement. Batch and column experiments were used to evaluate potential remediation scenarios and the migration hazard of U and Ni from flood-plain sediments collected along Tims Branch. Treatment solutions included a low-ionic-strength groundwater surrogate (artificial groundwater), 1 mM ethylenediaminetetraacetic acid (EDTA), 1 mM CaCl2 (pH 3.0), 1 mM Na-phytate (Na12C6H6O24P6), and 1 mM Ca-phytate (CaC6H16O24P6). The two organic phosphate compounds were included in the study because of their potential to serve as in-situ immobilizing agents through the formation of insoluble precipitates in association with the contaminant metals. Repacked columns were leached with a specific treatment solution for 30 pore volumes (PV) at a constant seepage velocity of 8 m day−1 (26 ft day−1). After leaching, solid-phase metal redistribution was evaluated using the toxicity characteristic leaching procedure and sequential extraction methods. For comparison, batch extractions were conducted using a solid-to-solution ratio (1:20) that mimicked the total leaching volume. Despite differences in reaction time, batch and column results were generally consistent. The CaCl2 and EDTA column treatments removed substantial Ni from the labile fractions, i.e., water soluble and exchangeable, as indicted by subsequent extractions for the residual sediments. The effectiveness of the treatments in removing U and Ni was not evident from digestion of the residual column sediments. Ca-phytate significantly reduced U mobility in columns, with evidence for solid-phase redistribution to more recalcitrant fractions. Na-phytate extracted the most U in batch and columns despite inducing colloid dispersion that clogged the column and precluded further leaching after 8–10 PV.

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