Journal of Colloid and Interface Science 244, 221–229 (2001) doi:10.1006/jcis.2001.7970, available online at http://www.idealibrary.com on Ni and Zn Sorption to Amorphous versus Crystalline Iron Oxides: Macroscopic Studies Paras Trivedi and Lisa Axe 1 Department of Civil and Environmental Engineering, New Jersey Institute of Technology, Newark, New Jersey 07102 Received May 17, 2001; accepted September 14, 2001 Amorphous and crystalline iron oxides are prevalent in subsur- face 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 contribu- tions 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 distribu- tion 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, en- thalpies demonstrated that adsorption to goethite involves greater forces than those for the amorphous form. In addition, site densi- ties 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. C  2001 Elsevier Science Key Words: sorption; intraparticle surface diffusion; zinc; nickel; goethite; amorphous iron oxide. INTRODUCTION In soils and sediments, amorphous and crystalline oxides are relatively abundant as discrete particles and coatings on other mineral surfaces. Because of their chemical nature and structure, iron oxides are efficient sinks for many contaminants including cations such as Ni and Zn (1–5). Therefore, they have been studied extensively to better understand sorption mechanisms and ultimately to model the processes. Some of the more commonly found iron oxides in soils are ferrihydrite, hematite, and goethite (6). The predominance of a particular Fe oxide is a function of pH, soil moisture, temper- ature, redox potential, and adsorbed species. Of these iron ox- ides, ferrihydrite is the only metastable form and the most poorly 1 To whom correspondence should be addressed. Fax: 973.596.5790. E-mail: axe@adm.njit.edu. ordered one often referred to as amorphous iron oxide or hydrous ferric oxide (HFO). Although ferrihydrite is metastable, evi- dence suggests that crystallization is inhibited in the presence of adsorbed species (1, 7–9). From their spectroscopic stud- ies, Manceau and co-workers (10, 11) observed that although goethite and ferrihydrite have similar local structure where three O atoms and three OH groups neighbor the ferric ion, the long-range order in the goethite is due to the long octahedral chains. Because of the much shorter octahedral chains, ferri- hydrite is expected to possess more edge sites while goethite has a higher proportion of sites located along the chain length. Even though these oxides may have similar types of sites, the relative densities of the sites are different. The transformation of ferrihydrite to goethite occurs through dissolution and re- precipitation. On the other hand, transformation to hematite is favored under dehydrated conditions at neutral pH and elevated temperatures (6). For amorphous oxides like that of Fe as well as Al and Mn, where porosity, micropores, and adsorption of metal ions are sig- nificant, intraparticle surface diffusion has been studied experi- mentally and theoretically (12–16). 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 dis- tance between sites, λ, and the energy required to jump to the neighboring site, the activation energy ( E a ) (13–15), D s = λ[ E a /(2m)] 1/2 exp[−E a /( RT )], [1] where m is the molecular mass of the diffusing species and T is temperature. Theoretically, the unknown quantities, λ and E a , can be obtained from experiments and possibly from correlations (14–16). Recently developed relations for predicting site density (14) and activation energy (16) suggest that theoretical surface diffusivities for metal ion sorption to amorphous Al, Fe, and Mn oxides can be predicted. The necessity for including intraparti- cle diffusion in mechanistically modeling sorption to crystalline oxides like goethite is not well understood, and if it should be needed, a theoretically based value may be helpful. While 221 0021-9797/01 $35.00 C  2001 Elsevier Science All rights reserved