STUDIES OF THE REACTIVITY OF THE FERRIHYDRITE SURFACE BY IRON ISOTOPIC EXCHANGE AND MOSSBAUER-SPECTROSCOPY

Two-line ferrihydrite is an important adsorbent of many toxics in natu ral and anthropogenic systems; however, the specific structural sites responsible for the high adsorption capacity are not well understood. A combination of chemical and spectroscopic techniques have been emplo yed in this study to gain further insight into the structural nature o f sites at the ferrihydrite surface. The kinetics of iron isotopic exc hange demonstrated that there are at least two types of iron sites in ferrihydrite. One population of sites, referred to as labile sites, ap proached iron isotopic equilibrium within 24 hr in Fe-59-NTA solutions , while the second population of sites, referred to as non-labile, exh ibited a much slower rate of isotopic exchange. Adsorbed arsenate redu ced the degree of exchange by labile sites, indicating that the anion blocked or greatly inhibited the rate of exchange of these sites. Moss bauer spectra were collected from a variety of samples including Fe-56 -ferrihydrite samples with Fe-57 in labile sites, samples containing F e-57 throughout the structure, and samples with Fe-57 in non-labile si tes. The spectra showed characteristic broad doublets signifying poor structural order. Refined fits of the spectra indicated that labile si tes have larger quadrupole splitting, hence more local distortion, tha n non-labile sites. In all cases, the spectra demonstrated some degree of asymmetry, indicating a distribution of Fe environments in ferrihy drite. Overall spectral findings, combined with recent EXAFS results ( Waychunas et al., 1993), indicate that labile sites likely are more re active (with respect to iron isotopic exchange) because they have fewe r neighboring Fe octahedra and are therefore bound less strongly to th e ferrihydrite structure. The labile population of sites probably is c omposed of end sites of the dioctahedral chain structure of 2-line fer rihydrite, which is a subset of the entire population of surface sites . Mossbauer spectra of samples containing adsorbed arsenate indicated that the anion may slightly decrease the distortion of labile sites an d stabilized the structure as a whole by bidentate bonding.