STRUCTURAL INCORPORATION AS A MEANS OF METAL SEQUESTRATION INTO NANOPARTICLE AGGREGATES

Iron oxyhydroxide nanoparticles play an important role in the mobility of metal species through sorption/desorption processes. Additionally, the often rapid aggregation of nanophases in aqueous systems can lead to changes in their structure, surface area, porosity, and reactivity that may modify the mechanisms by which metal ions are retained and therefore the long-term potential of metal sequestration in the solid phase. Interparticle (nano)pore spaces may also present new incorporation mechanisms or preferentially favor specific mechanisms for metal uptake and retention.

Batch adsorption/desorption experiments and spectroscopic analysis were used to investigate the uptake, retention, and speciation of metals onto and within nanoscale iron oxyhydroxides exposed to conditions that induce nanoparticle aggregation. Aqueous Cu(II) or Zn(II) was added to synthetic 5-nm iron oxyhydroxide particle suspensions which were then aggregated through increases in pH, ionic strength, or temperature. A desorption step was then induced by lowering the pH back below each metal’s macroscopic absorption edge.

EXAFS studies of the solid aggregates suggest that the desorption step removes the weakly-held (i.e. outer-sphere, surface-bound) metal fraction but retains more strongly-held metal species that transition from inner-sphere surface complexes to structurally incorporated species within the nanoparticle aggregates. Elevated temperature most dramatically enhances the process, likely due to ripening/healing of aggregated nanoparticles over time. This may represent a primary and common mechanism of metal retention among nanoparticles compared to surface precipitation, occlusion, and solid solution formation, terms often used collectively to describe the “co-precipitation” of metals.

Macroscopic studies of the filtered supernatants reveal metal-specific differences in adsorption/desorption behavior over time, with desorption of Zn(II) followed by re-adsorption and structural incorporation with both temperature and time. Additionally, ion selective electrode in situ measurements indicate that while aggregation pathway does not have a substantial effect on metal ion uptake under the conditions examined, it appears to be the main determinant of Cu(II) retention when desorption is induced.