Interfacial reaction of Sn(II) on mackinawite (FeS)

The interaction of Sn(II) with metastable, highly reactive mackinawite is a complex process due to transient changes of the mackinawite surface in the sorption process. In this work, we show that tin redox state and local structure as investigated by Sn-K X-ray absorption spectroscopy (XAS) change with pH. We observe at pH<7 that divalent Sn forms two short (2.38 Å) Sn-S bonds to the S-terminated surface of mackinawite, and two longer (2.59 Å) Sn-S bonds pointing most likely towards the solution phase, in line with a SnS4 innersphere sorption complex. Precipitation of SnS or formation of a solid solution with mackinawite could be excluded. At pH>9, Sn(II) is completely oxidized to Sn(IV) by an Fe(II)/Fe(III) (hydr)oxide, most likely green rust, forming on the surface of mackinawite. Six O atoms at 2.04 Å and 6 Fe atoms at 3.29 Å indicate a structural incorporation by green rust, with Sn(IV) substituting for Fe in the crystal structure. The transition between Sn(II) and Sn(IV) and between sulfur and oxygen coordination takes place at a pH of 7 to 8 and an Eh of -250 mV, close to the thermodynamically predicted transitions from mackinawite to Fe (hydr)oxide and from sulfide to sulfate. The uptake processes of Sn(II) by mackinawite are largely in line with the uptake processes of divalent cations with soft Lewis-acid character like Cd, Hg and Pb, and lead to a strong retention of Sn with logRd values from 5 to 7 across the investigated pH range of 5 to 11.