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

Abstract

Amorphous and crystalline iron oxides are prevalent in subsurface systems and are important surfaces for metal contaminant sorption. While discrete amorphous iron oxide is microporous with particles ranging from 1 to 100 μm resulting in significant contributions from intraparticle diffusion, the degree to which this process is important for crystalline iron oxides such as goethite was studied in this work. The porosity of goethite was found to be only 0.3% with pores ranging from micro to macro; however, measurements are conducted with freeze-dried oxide. The particle size distribution was observed to be bimodal with overall smaller sizes than that observed for the amorphous iron oxide. Long-term sorption studies for Ni and Zn where a constant boundary constant was maintained revealed no change in the amount sorbed as a function of time. On the other hand, sorption due to intraparticle diffusion in amorphous iron oxide accounts for as much as 40% of the total amount sorbed. Interestingly, although ferrihydrite is a precursor to goethite, enthalpies demonstrated that adsorption to goethite involves greater forces than those for the amorphous form. In addition, site densities were observed to be as much as 3 orders of magnitude greater for amorphous iron oxide than for goethite. Overall, this crystalline oxide does not have significant microporosity.