Evidence for the Interaction of Technetium Colloids with Humic Substances by X-ray Absorption Spectroscopy

Abstract

Spectroscopic extended X-ray absorption fine structure (EXAFS) evidence was obtained on the chemical environment of 99Tc(IV) atoms formed upon introduction of TcO4- into four types of laboratory-scale synthetic and natural systems which mimic in situ natural reducing conditions in humic-rich geochemical environments: (a) magnetite/pyrite in synthetic groundwater in the absence of humic substances (HSs), (b) magnetite/pyrite in natural Gorleben groundwater in the presence of HSs, (c) Boom clay sediment mixed with synthetic groundwater, and (d) Gorleben sand mixed with natural Gorleben groundwater. The investigated systems obey to pH 8-9 conditions, and all measured samples show similar EXAFS spectra for Tc, which could be fitted by a hydrated TcO2 x xH2O phase. The results are interpreted as follows: upon introduction of high concentrations (millimolar to micromolar) of TcO4-to chemically reducing environments, small Tc(IV) oxidic polymers are formed, which either may aggregate into larger units (colloids) and finally precipitate or may interact in their polymeric form with (dissolved and immobile) humic substances. This latter type of interaction--Tc(IV) colloid sorption onto HSs--differs significantly from the generally accepted metal--humate complexation and therefore offers new views on the possible reaction pathways of metals and radionuclides in humic-rich environments.

The Geochemistry of Technetium: A Summary of the Behavior of an Artificial Element in the Natural Environment

“…Instead, the authors argue that dissolution of Re(III) results in the formation of Re(IV) polymers or colloids that forestalls the precipitation of ReO 2 . Again, this characteristic is similar to that of technetium, in which dimers [TcO(OH) 2°]2 form at concentrations greater than 10 -5 mol/L (Maes et al 2004). Thus, the data in this study indicate that ReO 2 (cr) has a larger stability field than previously thought and is therefore closer in behavior to technetium than realized.…”

“…Because of the tendency for colloids of technetium to form at concentrations above saturation, it is best to approach solubility measurements from undersaturated conditions. The influence of colloids on the geochemical behavior of technetium was explored through a set of experiments performed by Maes et al (2004). The authors imposed reducing conditions on a groundwater solution spiked with pertechnetate and the resulting solution was equilibrated with natural (Gorleben) sand.…”

Redox interactions of technetium with iron-bearing minerals

“…Normalized Tc X-ray absorption near edge structure (XANES) spectra for the mineral phases are shown in Fig. 3 was no evidence for the pre-edge feature that is characteristic of the forbidden 1s?4d transition seen in TcO 4 À species (Maes et al, 2004); indeed all the spectra were typical of a Tc(IV)-like valence (Fredrickson et al, 2004;Plymale et al, 2011;Morris et al, 2008). Thus, low concentration tracer experiments coupled with higher concentration XANES experiments show that the different Fe(II)-bearing mineral phases have different reactivity to Tc(VII) but that abiotically mediated reductive scavenging of TcO 4 À to Tc(IV) is the dominant retention mechanism for Tc in these systems.…”

EXAFS and DFT: Evidence for the [TcO]2+ core

“…A Tc(III) reference compound Tc(acac) 3 0 , well-known from single crystal structure analysis, was prepared similarly to the method of Abrams et al: 12,13 39 mg of Na 2 S 2 O 4 was added to a mixture of 4.6 mL 2.5 M NaOH, 5.5 mL ethanol, 2.8 mL b-diketone and 200 ml of 6.2 ¥ 10 -2 M NH 4 99 TcO 4 . After 24 h reaction at 65 • C the Tc(acac) 3 0 was extracted from the red solution with 4-methyl-2-pentanone (MIBK) in a glove box (N 2 /H 2 95/5%; < 2 ppm O 2 ).…”