THE MECHANISM OF ANION ADSORPTION ON IRON-OXIDES - EVIDENCE FOR THE BONDING OF ARSENATE TETRAHEDRA ON FREE FE(O,OH)(6) EDGES

Parallel studies conducted recently on the mechanism of adsorption of arsenate and selenate oxyanions on iron oxyhydroxides have resulted in different structural models. On the basis of EXAFS-derived As-Fe distances, Waychunas et al. (1993) have postulated the existence of two kinds of arsenate surface complexes, both of them being bonded to A-type surface functional groups. One surface complex involves the sharing of two adjacent A-type groups (C-2([hk1])[hk0] surface complex, d(As-Fe) = 3.2(5) Angstrom) and the other the sharing of two adjacent A-type group (V-1([hk0])[hk0] surface complex, d(As-Fe) = 3.6(0) Angstrom). While the former complex has also been identified in the case of selenate, single corner 1V linkage between arsenate tetrahedra and Fe(O, OH)(6) octahedra is not observed in the case of selenate. The second Fe-Se distance has been instead attributed to an edge (E) linkage ((1)E([001])([001]) surface complex, d(Se-Fe) = 2.8(0) Angstrom; Manceau and Charlet, 1994). Comparison of the EXAFS results obtained at the As and Se K-edges leads to the conclusion that this difference of structural behavior is not supported by the experimental data. The reason for this discrepancy is shown to result from a mistaken determination by Waychunas et al. (1993) of the As-Fe distance of 3.6(0) Angstrom due to a 2 pi phase shift of the theoretical As-Fe electronic wave constructed for least-square fitting of the experimental data. The sorption of arsenate on free Fe(O, OH)(6) edges is shown to be critical to understanding structural changes of the ferrihydrite structure at increasing As loading and ageing times. Specifically, it provides a molecular-level explanation for the poisoning of particles growth in the direction of octahedral Fe chains and for the slowing down of the transformation of ferrihydrite to hematite.