Formation of Ternary Metal-Oxalate Surface Complexes on α-FeOOH Particles

Processes at the aqueous interfaces of metal (hydr)oxide particles greatly influence the mobility, bioavailability, and reactivity of metal ions and ligands. Here we investigated the time-dependent reactions of oxalate or Me(C2O4)33- (Me = Fe(III), Al(III), Ga(III), Co(III)) with goethite in aqueous suspension at pH 4 using attenuated total reflectance infrared (ATR-IR) and extended X-ray absorption fine structure (EXAFS) spectroscopy. The data indicate four coordination modes for oxalate and Fe(C2O4)33- adsorbed at the goethite surface: (1) outer-spherically with a hydration shell similar to aqueous ligand; (2) outer-spherically but hydrogen bonded to a surface site; (3) inner-spherically to surface iron; (4) inner-spherically within a ternary type A surface complex. In the presence of oxalate, the two outer-sphere complexes form rapidly, but with time these species are partially consumed and the ternary inner-sphere complex is formed as a result of a dissolution–readsorption process. We propose that iron in these ternary complexes is more labile than iron that is mostly embedded in the lattice. Thus, ternary complexation may play an important role in iron bioavailabilty in the environment. For goethite reacted with Al(C2O4)33- or Ga(C2O4)33-, these four surface complexes are accompanied by an additional Al(III) or Ga(III) ternary oxalate surface complex.