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(C(2)O(4))(3)(3-) (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(C(2)O(4))(3)(3-) 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(C(2)O(4))(3)(3-) or Ga(C(2)O(4))(3)(3-), these four surface complexes
are accompanied by an additional Al(III) or Ga(III) ternary oxalate
surface complex.