Experimental and Theoretical Diffusivities of Cd and Sr in Hydrous Ferric Oxide

Ni and Zn Sorption to Amorphous versus Crystalline Iron Oxides: Macroscopic Studies

“…8). In these experiments, the metal concentration in the bulk aqueous phase was monitored and maintained constant by measuring the activity of the radioisotope in the suspension and filtrate (12)(13)(14)(15)(16). This procedure allows one to investigate the slow sorption process in a convenient time frame (12)(13)(14)(15)(16).…”

“…The longterm or constant boundary condition (CBC) experiments were used to investigate the slow sorption of a metal ion (Ni or Zn) through micropores. In these studies, the metal ion concentration in the bulk phase was maintained constant by continuously monitoring its concentration (12)(13)(14)(15). Throughout all tests, a turbulent hydraulic regime was maintained with Re > 3.0 × 10 5 ; as a result, the ratio of external mass transfer resistance to internal resistance was negligible and a constant boundary condition was sustained (12-15).…”

“…For amorphous oxides like that of Fe as well as Al and Mn, where porosity, micropores, and adsorption of metal ions are significant, intraparticle surface diffusion has been studied experimentally and theoretically (12)(13)(14)(15)(16). Surface diffusivities of metal ions have been found to range between 10 −16 and 10 −9 cm 2 s −1 .…”

“…Estimating diffusivities on the basis of site activation theory (17) as detailed in a recent review (18) may be useful, as the experimentally determined ones often require lengthy periods of time. Surface diffusivity, D s , is a function of the mean distance between sites, λ, and the energy required to jump to the neighboring site, the activation energy (E a ) (13)(14)(15),…”

An Analysis of Zinc Sorption to Amorphous versus Crystalline Iron Oxides Using XAS

“…Manceau and co-workers have shown that even though goethite and hydrous ferric oxide (HFO) have similar structures, the lengths of their octahedral chains and hence their site densities differ (4)(5)(6). A comparison of the macroscopic results shows that amorphous oxides such as ferrihydrite (also known as HFO) have large sorption capacities for metal contaminants as compared to crystalline oxides such as goethite (8)(9)(10)(11)(12)(13). On the other hand, thermodynamic analyses suggest that while transition metals like Zn may be chemically sorbed to goethite (13,14), adsorption to HFO involves physical forces (9,10,13).…”