EFFECTS OF AGGREGATION-BASED GROWTH ON HEAVY METAL SORPTION AND PRECIPITATION ONTO NANOSCALE IRON OXYHYDROXIDES

Iron oxyhydroxide minerals are present in many natural systems as dispersed and/or aggregated nanoparticles. However, the processes by which iron oxyhydroxides grow from nanosized to bulk particles are not well understood. Furthermore, the effects of particle size and particle growth on heavy metal sequestration, especially during particle aggregation, may have significant implications for contaminant sequestration and mobility.

To study the effects of particle size on metal uptake, a series of iron oxyhydroxide batches ranging from 5-80 nm in average diameter was synthesized using a rapid microwave technique followed by aging in suspension at 90°C. The growth of the nanoparticles under these conditions occurs in two distinct stages: 1) rapid growth from 5-60 nm over the first 4 days followed by 2) slower growth from 60-80 nm over the next 28 days. In conjunction with TEM, XRD, SAXS/WAXS, and EXAFS analysis, these trends suggest that the first stage of growth occurs primarily via oriented aggregation of 5-nm poorly-ordered ferrihydrite-like particles, while the second stage includes ripening-based growth and structural rearrangement into goethite.

Selected iron oxyhydroxide nanoparticle suspensions of 5, 25, and 75 nm in effective diameter were then used in batch uptake experiments featuring As(V), Cu(II), Hg(II), and Zn(II), metal(loid) contaminants frequently associated with acid mine drainage systems. EXAFS spectroscopy shows that while metal speciation on the 25- and 75-nm particles is identical, differences in both second-neighbor interatomic distances and coordination numbers indicate changes in the mode of uptake on the smallest 5-nm particles. This is thought to be due to changing proportions of binding sites (e.g. edges, corners) and particle morphology evolution from oblong (10-nm particles) to more tabular/acicular (25- and 75-nm particles) as particle size increases.

Macroscopic uptake curves of Hg(II) on the same nanoparticle batches demonstrate that the degree of surface coverage varies as a function of particle size independent of surface area effects, also possibly due to different binding site energies. Nanoparticle aging at 90°C in the presence of the metals shows that Hg(II) and Zn(II) form (co-)precipitates during aging while As(V) and Cu(II) do not.