PNNL

Abstract

A series of bench-scale batch experiments were conducted to evaluate the potential application of a two-step subsurface remediation approach that can 1) immobilize contaminants in-situ and 2) prevent contaminant remobilization via re-oxidation due to oxygenated recharge. The sequential approach involves application of a reducing agent (e.g., zero valent iron, ZVI; sulfur modified iron, SMI; or calcium polysulfide, CPS), followed by application of sequestering chemical treatments (e.g., calcite or apatite), or a dilute alkaline solution (e.g., NaOH) to promote mineral dissolution and subsequent contaminant co-precipitation. This approach was tested under conditions similar to the perched water zone within the Central Plateau at the Hanford Site located in southeastern Washington State (USA). This zone is located a few meters above the unconfined aquifer, on top of a low-permeability unit, and contains high concentrations of technetium (Tc) (55,000 pCi/L), uranium (U) (150 mg/L), and nitrate (NO3) (200 mg/L). The results demonstrated that combining SMI reduction with either apatite or calcite sequestration was the most effective method for successful sequestration of both U and Tc. The aqueous U removal with the highest dose tested (i.e., 1% SMI by weight) was quantitative. The percent Tc removed was 95.6% ± 2.5% (SMI-apatite) and 98.3% ± 0.0% (SMI-calcite). Examination of the sediment samples at the end of experiments via sequential extractions showed limited re-oxidation capacity by selective extraction of bioavailable mineral phases. This is evidence of how the two-step approach can suppress remobilization by limiting oxidation potential. More specially, for the 1% SMI-Apatite combined treatment mechanism, extractable U was 9.9% ± 1.8% and extractable Tc was 1.9% ± 0.4%, while in the 1% SMI-calcite combination extractable U and Tc were 8.9% ± 5.3% and 0.3% ± 0.1%, respectively. In addition, XANES analysis of the sediment samples treated with SMI-calcite showed the presence of only U(IV), while both U(IV) and U(VI) were present in the SMI-apatite combination [ratio of 0.43 U(IV):0.59 U(VI)].