Ferrihydrite adsorbed anions

Results from two field investigations illustrate this point. Kent et al. (1995) modeled adsorption of Cr(VI) in the presence of S(VI) by natural aquifer solids using equilibrium constants that were 1.75 Log K units lower than those used to model adsorption by ferrihydrite (Dzombak and Morel, 1990). To model competitive adsorption of Mo(Vl) and P(V) by these same aquifer solids, Stollenwerk (1995) decreased the Dzombak and Morel (1990) equilibrium constants for both anions by 1.2 Log K units. Results from both of these studies indicate that the dominant adsorbents had lower affinities for adsorption of anions than pure ferrihydrite. 

The noncrystalline hydrated iron oxide, ferrihydrite, is indicative of young deposits of Fe oxides in soils, forming by the fast hydrolysis of Fe " salts or rapid oxidation of Fe solubilized by reduction reactions. It is more reddish than goethite, but less reddish than hematite. It is stabilized relative to the more crystalline oxides by adsorbed cations and anions, notably silicate. There is some uncertainty about its structure, but it is composed of tiny (30-70 A) spherical particles that may have short-range hematitelike or goethitelike order. Ferrihydrite is probably a precursor of more crystalline oxides such as hematite. 

Finally, there is the question of sorption of both arsenic species to mineral phases. Arsenate adsorbs strongly to a number of common minerals, such as ferrihydrite, goethite, chlorite, and alumina, which constrains its mobility in aquifers, soils, and sediments. This is a complicated, pH-dependent phenomenon (36). Phosphate has chemical properties similar to arsenate and is a common anion... 

Arsenate adsorption on ferrihydrite consisted of a period of rapid uptake followed by slow adsorption for at least 8 days 43). The rate of the slow adsorption reaction is considered to be limited by diffusion into the ferrihydrite aggregates. Slow adsorption kinetics similar to those for phosphate are expected for arsenate because of the similar chemistry of these two anions. Arsenate adsorption data adhere to the Elovich kinetic model indicating a diffusion limited reaction. Arsenate desorption rates were much slower than arsenate adsorption rates, also consistent with a diffusion limited process. A model was developed that assumes that 63% of adsorbing sites are located at the exteriors of aggregates and reach arsenate equilibrium rapidly, while 37% of adsorbing sites are located in the interiors of aggregates with access being diffusion limited. 

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