Nanoscale mineral/contaminant redox reaction processes: impact on oxyanion contaminant fate in oscillating anoxic environments

Oscillating oxic/anoxic conditions induces processes at mineral-Kuid
interfaces aeecting the speciation and fate of redox active elements. Such
Kuctuations occur in natural environments (at watershed and delta water
table or in lake water chemocline) and in engineered systems (in oil reDnery
drainage water peatbogs or nuclear waste repository). In presence of aqueous
sulDde or ferrous iron ions, Se, Sb, Pu and Re undergo heterogeneous
reductive precipitation. At the nanosecond scale, selenate adsorption onto
the net pyrite surface is shown by ab-initio computations to proceed via the
formation of a chemical bond between the oxyanion oxygen atom and a
surface Fe atom, weakening the other Se-O bonds and reducing Se atom
oxydation state. At hour-to-day scale, mobile Se(VI), Se(IV), Sb(V), Pu(V) and
Re(VII) oxyanions are shown by strictly anoxic wet chemistry experiments,
XAFS and Mossbauer spectroscopy [1] to be reduced, in presence of
nanosized particles (iron sulDdes (mackinawite or pyrite) [2], oxides
(magnetite) [3] [4] [5] or clays [6]) and aqueous iron (II) or S(-II), to solids (Se(0),
Sb O , PuO , Re0 , ReS or Re(0)) or to highly speciDc Sb(III) [7] and Pu(III) [8]
sorption complexes. STEM-HAADF microscopy demonstrates the formation of
Se(0) trigonal gray selenium nanowires after co-adsorption of selenate and
ferrous iron ions on 10 nm nanomagnetite [4], a reaction which results from a
complex, kinetically controlled interplay of intermediate reduced surface
complexes and transport of these reduced species to the tip of the nanowires.
These processes in far-from-equilibrium conditions are of prime importance
in the safety assessment of petroleum industry and geological repositories, as
well as in the bioavailability to crop or human beings.
[1] Charlet et al., 2022, J. Mat. Res. DOI: 10.1557/s43578-022-00823-8 [2] Son et
al., 2022, Geochim. Cosmochim. Acta 338 : 220–228. [3] Goberna-Feron et al.,
2021, Env. Sci. & Tech. 55, 3021−3031. [4] Poulain et al., 2022, Env. Sci. & Tech 56
: 14817-14827 [5] Wang et al., 2022, Environ. Sci. Technol. 2022, 56, 9. [6]
Charlet et al., Geochim. Cosmochim. Acta 71: 5731-49. [7] Kirsch et al., 2008,
Mineral Mag, 72, 185-189. [8] Kirsch et al, 2011, Environ. Sci. Technol. 45 :
7267–7274