Structural incorporation of arsenate into vivianite

Vivianite is a stable ferrous iron phosphate [FeII3(PO4)2·8H2O] that forms in oxygen-poor, Fe2+- and organic-rich environments. Isomorphic substitution of divalent cations (e.g. Mg and Mn) for Fe(II) in the structure are typical in natural vivianites. However, anion substitution is rare; and in particular, arsenate [AsVO43-] substitution has never been reported for natural vivianites and only partial substitution has been reported for synthetic analogues although parasymplesite [FeII3(AsO4)2·8H2O] exist in natural environments. In this study, we synthesized As(V)-substituted vivianites (FeII3[(PO4)1-x(AsO4)x]2·8H2O) with systematically increased degrees of substitution (0.22 ≤ x ≤ 0.95), forming a complete solid solution between end-members vivianite and parasymplesite. The As(V)-substituted vivianites crystallized in a monoclinic C2/m like both end-members, and kept the platelet crystal habit of As-free vivianite even at high degrees of As(V) substitution. The local bonding environment data derived from Fe and As K-edge X-ray absorption spectroscopy and crystal chemical information from infrared spectroscopy analysis confirmed that As(V) was successfully incorporated into the vivianite crystal structure. Furthermore, the scanning transmission electron microscopy coupled with energy dispersive X-ray elemental maps revealed a homogenous distribution of As in the vivianite samples. Our data shows that As(V) can be incorporated into the vivianite structure (up to 95 mol% or 25 wt.%). In natural environments, this could provide a stable sink for arsenic immobilization in both modern and ancient ferruginous settings.