STRUCTURE AND STABILITY OF CD2+ SURFACE COMPLEXES ON FERRIC OXIDES .1. RESULTS FROM EXAFS SPECTROSCOPY

Extended X-ray absorption fine structure(EXAFS) spectroscopy has been used to study the adsorption and coprecipitation mechanism of Cd(II) on hydrous ferric oxide (HFO) and alpha-FeOOH (goethite) in aqueous solutions. For HFO samples, the pH of adsorption varied from 6.7 to 9.5 and the Cd content from 0.11 to 9.3 wt% of gCd/gFeOOH, corresponding to a surface loading of 1 to 90%. The sorption on alpha-FeOOH was conducted at pH 7.5 and over a concentration range between 0.16 and 1.50 wt% (10-100% surface loading). Fe and Cd K-edge EXAFS spectra of a coprecipitated alpha-(Cd0.005Fe0.995)OOH phase are similar which indicates that both cations have the same structural environment and, hence, that Cd substitutes for Fe within the alpha-FeOOH lattice. Three Fe shells were recognized around Cd atoms: 2.5 +/- 0.7 Fe-I at 3.15 Angstrom, 1.8 +/- 0.4 Fe-II at 3.40 A, and 3.5 +/- 0.7 Fe-III at 3.52 Angstrom. The first two distances are characteristic of edge (E) linkages between Fe and Cd octahedra belonging to the same double chain of the alpha-FeOOH structure. The third distance corresponds to corner (C) linkages between Fe and Cd octahedra belonging to two adjacent double chains. The sorption of Cd on alpha-FeOOH results in two Cd-Fe distances at 3.26 Angstrom (R(1)) and 3.48 Angstrom (R(2)). The ratio of the number of nearest Fe neighbors N-2/N-1 increases with increasing surface loading from 1.3 (11% surface loading) to 3 (100%). This change in the average structure about Cd atoms has been interpreted as the existence of at least two different surface sites. At low coverage, Cd atoms mainly sorb at the termination of alpha-FeOOH chains on (001) planes by sharing edges (R(1)=E) and corners (R(2)=C) with surface Fe octahedra. At medium and high surface loading, Cd atoms sorb along chains on (hk0) planes where they share mainly C with Fe octahedra. As a consequence, crystallographic planes of goethite, and thus surface sites, do not appear to have the same affinity for Cd and it is demonstrated that the number of bonds between the sorbate and the sorbent is at a maximum on high affinity surface sites. As expected from crystal growth considerations, these high affinity sites are situated on small crystallographic planes. On HFO, Cd-Fe distances are similar to those found for alpha-FeOOH (R(1) = 3.32 +/- 0.04 Angstrom) and (R(2) = 3.50 +/- 0.03 Angstrom) but the number of nearest (N-1 = 0.7 +/- 0.1) and next-nearest (N-2 = 0.8 +/- 0.2) Fe cations is independent of solution parameters such as pH and Cd concentration despite a variation of 3 pH units and nearly two [Cd](tot) sorbed. This result indicates that the high affinity to low affinity surface site density ratio is much higher on HFO than on alpha-FeOOH and it is shown that this finding is fully consistent with the structure of ferrihydrite. (C) 1994 Academic Press, Inc.