Evidence for the interaction of technetium colloids with humic substances by X-ray absorption spectroscopy

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·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...