Effect of organic co-contaminants on technetium and rhenium speciation and solubility under reducing conditions

摘要

The chemical and biogeochemical reduction of pertechnetate (TcO4-) and perrhenate (ReO4-) have been compared alongside complexation of the reduced species by three anthropogenic ligands relevant to nuclear waste (ethylene-diaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), and isosaccharinic acid (ISA)). An HPLC size-exclusion column coupled to ICP-MS was used to separate the species and quantify Tc and Re. During method development, ReO4- showed recalcitrance to direct chemical reduction by Sn(II) under conditions that readily reduced TcO4- and resulted in Tc(IV)-organic complexes. In microcosm experiments of a silty loam soil containing TcO4-, ReO4-, and ISA (3.0 mM), EDTA (0.17 mM), or NTA (2.4 mM), anoxia developed to iron-reducing conditions during the 42 day experimental period. The majority of the TcO4- was reduced to particle-reactive Tc(IV) and removed from solution during nitrate reduction, but there was no chromatographic evidence of Tc(IV)-organic complexes in the porewater. Overall, the excess organic complexants added did not cause a measurable difference in the solubility of Tc( IV) over the control experiments in this organic-rich (12% organic carbon) soil. ReO4- did not undergo reduction, as shown by the constant porewater concentration and the chromatographic data, and thus Re does not function as an analogue for Tc under environmental nitrate- and iron-reducing conditions.