SURFACE-CHEMISTRY OF FERRIHYDRITE .1. EXAFS STUDIES OF THE GEOMETRY OF COPRECIPITATED AND ADSORBED ARSENATE

EXAFS spectra were collected on both the As and Fe K-edges from sample s of two-line ferrihydrite with adsorbed (ADS) and coprecipitated (CPT ) arsenate prepared over a range of conditions and arsenate surface co verages. Spectra also were collected for arsenate adsorbed on the surf aces of three FeOOH crystalline polymorphs, alpha (goethite), beta (ak aganeite), and gamma (lepidocrocite), and as a free ion in aqueous sol ution. Analyses of the As EXAFS show clear evidence for inner sphere b identate (bridging) arsenate complexes on the ferrihydrite surface and on the surfaces of the crystalline FeOOH polymorphs. The bridging ars enate is attached to adjacent apices of edge-sharing Fe oxyhydroxyl oc tahedra. The arsenic-iron distance at the interface (3.28 +/- 0.01 ang strom) is close to that expected for this geometry on the FeOOH polymo rph surfaces, but is slightly shorter on the ferrihydrite surfaces (3. 25 +/- 0.02 angstrom). Monodentate arsenate linkages (3.60 +/- 0.03 an gstrom) also occur on the ferrihydrite, but are not generally observed on the crystalline FeOOH polymorphs. The proportion of monodentate bo nds appears largest for adsorption samples with the smallest As/Fe mol ar ratio. In all cases the arsenate tetrahedral complex is relatively undistorted with As-O bonds of 1.66 +/- 0.01 angstrom. Precipitation o f arsenate or scorodite-like phases was not observed for any samples, all of which were prepared at a pH value of 8. The Fe EXAFS results co nfirm that the Fe-Fe correlations in the ferrihydrite are progressivel y disrupted in the CPT samples as the As/Fe ratio is increased. Cohere nt crystallite size is probably no more than 10 angstrom in diameter a nd no Fe oxyhydroxyl octahedra corner-sharing linkages (as would be pr esent in FeOOH polymorphs) are observed at the largest As/Fe ratios. C omparison of the number and type of Fe-Fe neighbors with the topologic al constraints imposed by the arsenate saturation limit in the CPT sam ples (about 0.7 As/Fe) indicates ferrihydrite units consisting mainly of Fe oxyhydroxyl octahedra arranged in short dioctahedral chains with minimal interchain linking by octahedra comers. This is consistent wi th an enlarged surface area and a larger proportion of sites for biden tate arsenate bonding in CPT samples as compared to the ADS samples, w hich saturate with arsenate at lower As/Fe ratios. The latter samples have larger crystallite sizes and a definite proportion of ferric octa hedra sharing comers. The ratio of comer-sharing to edge-sharing Fe ox yhydroxyl octahedra in the ADS samples, and CPT samples with small As loadings, is very similar to what would be present in very small parti cles of goethite or akaganeite. The difference in the polymeric struct ure of ADS and CPT samples at higher As/Fe ratios is due to strong ars enate bidentate adsorption that poisons the surface of particles of fe rrihydrite precipitated in the presence of substantial arsenate, limit ing their normal crystallization, and preventing further Fe-O-Fe polym erization. If the arsenate is applied after precipitation much less ad sorption occurs since polymerization has already progressed. In both A DS and CPT samples, Fe-O-Fe polymerization increases with age, though at different rates for each type of sample.