EFFECTS OF NANOPARTICLE AGGREGATION ON METAL UPTAKE, RETENTION, AND SPECIATION

Iron oxyhydroxide nanoparticles play an important role in the mobility of aqueous metal species through both sorption and desorption processes. However, the natural and often rapid aggregation of such nanophases in aqueous systems can lead to changes in their structure, available 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.

Batch and spectroscopic methods were used to investigate the uptake, release, and speciation of metals onto and within nanoscale iron oxyhydroxides exposed to conditions which induce nanoparticle aggregation and growth. Aqueous Cu(II) or Zn(II) was added to a suspension of synthetic 5-nm iron oxyhydroxide nanoparticles aggregated through increases in pH, ionic strength, or temperature and allowed to adsorb for a period of 24 hours. A desorption step was then induced by lowering the pH back below the macroscopic absorption edge for the specific metal. TEM and BET surface area characterization was also conducted on the aggregates and control (unaggregated) nanoparticles prior to metal exposure.

Analysis of filtered supernatants combined with EXAFS studies of the solid aggregate pastes suggest that the desorption step removes the weakly-held (i.e. surface-bound) metal fraction but retains strongly-held metals that appear to be more structurally incorporated within the nanoparticle aggregates. SAXS analysis of the aggregated nanoparticles also established that variable extents of aggregation based on aggregation method appear to correlate with the macroscopic uptake and desorption experiments.

Results show the relative effects of different aggregation methods on metal uptake and subsequent release, with temperature/time most effective at retaining metals in the solid phase and pH-based aggregation less so, while ionic strength-based aggregation had little effect relative to unaggregated particles. These findings have implications for the removal of hazardous metals from the aqueous phase and the design of remediation strategies targeting contaminated environments such as mine-impacted regions.