NANOSCALE IRON OXYHYDROXIDE AGGREGATION UNDER RELEVANT ENVIRONMENTAL CONDITIONS AND CORRESPONDING EFFECTS ON METAL ION UPTAKE, RETENTION, AND SPECIATION

Nanoscale iron oxyhydroxides, which are ubiquitous in surface aqueous systems, serve as highly effective sorbents for dissolved metals due to their chemical reactivity, small size and high surface area. Such nanosized particles also rapidly aggregate under natural geochemical conditions, thus affecting their metal sorption and retention capabilities. The broad range of particle aggregation mechanisms and states possible in the environment substantially increases the complexity and variety of interactions between metal ions and iron oxyhydroxide nanoparticle aggregates.

We have undertaken a combination of macroscopic batch uptake experiments and X-ray spectroscopic analyses to explore Zn(II) adsorption on and retention to iron oxyhydroxide nanoparticles, with a range of aggregation states induced by suspension freezing, drying, humic acid (1-250 mg/L), and salinity (0-100% of seawater concentrations). Nanoparticle aggregate state was assessed by dynamic light scattering (DLS) and sedimentation, Zn(II) adsorption/retention behavior through macroscopic batch experiments and inductively coupled plasma-optical emission spectrometry (ICP-OES), and Zn(II) speciation using extended X-ray absorption fine structure (EXAFS) spectroscopy.

Nanoparticle aggregation state was found to result in reduced initial Zn(II) uptake, likely due to lowered surface area, but in increased metal retention, likely due to ion trapping within nanoporous regimes. Quantifiable and statistically significant variations in Zn speciation in the sorbed/retained state indicate that a diversity of Zn(II) sorption complexes exists at the nanoparticle-water interface, with more weakly-bound complexes removed by a pH-based desorption step that leaves behind more strongly-bound species, as primarily evidenced by an increase in iron coordination around the average zinc atom. The presence of common seawater ions sulfate and chloride also serve to enhance Zn(II) uptake and retention through a combination of surface charge reduction and possible ternary surface complex formation. These dynamic changes in surface complexation inform our understanding of environmental sorption/desorption processes to Fe-based nanoparticles and their aggregates.