Redox reactions of Sb(V) and Sb(III) with mackinawite (FeS) and of aqueous Pu(III) and Pu(V) with various Fe(II)-bearing minerals and Fe(III)-oxides have been investigated under anoxic conditions. X-ray absorption spectroscopy was used to analyze oxidation state and local coordination environment of Sb and Pu associated with the solid phase. After reaction with mackinawite, chukanovite and magnetite, PuO2, Pu(III) or mixtures of the two oxidation states were observed. On magnetite, a tridentate Pu(III) surface complex could be identified from EXAFS combined with Feff-Monte-Carlo simulation. The relative amount of Pu(III) depends on the type of mineral, the solid/solution ratio, the system pe and pH, and, potentially, the particle size and crystallinity of the formed PuO2 solid phase. With mackinawite at pH 6.2 and a surface loading of 67 nmol/m2 and with magnetite up to pH 8.4 and a surface loading of 56 nmol/m2, only Pu(III) was identified associated with the solid phase. With maghemite containing residual Fe(II) at pH6, Pu(III) and Pu(IV) were present in, probably, inner-sphere surface complexes similar to the one formed by Pu(III) on magnetite. Under the given conditions (surface loadings ≤ 77 nmol/m2) formation of PuO2 was not observed. After reaction with hematite and goethite, Pu(IV) was the predominant oxidation state associated with the solid phase. Sorption and reduction of Sb(V) on mackinawite were strongly pH dependent. At acidic pH, sorption was fast and Sb(V) was completely reduced to an Sb(III)-sulfide complex associated with the solid phase. Reduction of Sb(V) was coupled to oxidation of mackinawite and formation of a greigite (Fe3S4) phase could be observed by XRD. At basic pH, Sb(V) was slowly removed from solution and reduction to Sb(III) was complete only at very small Sb/FeS ratios. At higher Sb/FeS, Sb(V) removal occurred partly through reduction to solid phase associated Sb(III)-S3 and partly through co-precipitation with Fe(III). In conclusion, it could be shown that Fe(II) bearing minerals can effectively contribute to the reduction and immobilization of antimony and plutonium, two contaminants of growing environmental importance.
