Spectroscopic Investigations of Metal Speciation on Nanoscale Iron Oxyhydroxide Aggregates

Iron oxyhydroxide nanomaterials play an important role in the mobility of aqueous metal species through both sorption and desorption processes. The natural aggregation of such nanophases in aqueous environmental systems can lead to changes in their structure, available surface area, and reactivity and may additionally modify the mechanisms by which metal ions are adsorbed and retained. In this study, spectroscopic techniques were used to investigate the speciation of metals onto and within nanoscale iron oxyhydroxides exposed to various geochemical conditions which induce nanoparticle aggregation and growth. Suspensions of ~5 nm diameter iron oxyhydroxide nanoparticles were synthesized and aggregated through variations in pH (7-10), ionic strength (0.001-1 M), and temperature (25,50,75°C)/time (up to 96 hours) in the presence of 0.5 mM Cu(II) or Zn(II). A second set of experiments introduced the metals after aggregation had been induced (pH and ionic strength-based aggregation only). One aliquot of each suspension was then analyzed with atomic absorption spectroscopy to determine the initial uptake, while a second aliquot was subjected to a desorption step in which the pH was lowered to values below the macroscopic absorption edges of the relevant metals prior to AA analysis. X-ray absorption spectroscopy (XAS) analysis of selected samples was then conducted to assess the speciation of the metals associated with the solid phase following both the adsorption and desorption steps and determine the conditions that yielded the most strongly bound, incorporated metal species. Results indicate that the desorption step removes weakly-bound metal species but retains metals that appear to be more structurally incorporated into/onto the nanoparticle aggregates. Samples exposed to metals before aggregation were also found to incorporate metals more thoroughly than those that were aggregated before being exposed to metals. These findings have implications for the long-term removal of hazardous metals from the aqueous phase and the development of possible remediation strategies targeting contaminated environments.