47-4 Nanoscale iron oxyhydroxide aggregation and effects on metal ion adsorption, desorption, retention, and speciation

Session: Minerals in Motion: Tracking Mineral Reactions Using In Situ and Synchrotron Techniques, A Celebration of the Career of Peter Heaney

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Abstract:

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

We have undertaken a combination of macroscopic batch adsorption/desorption experiments, metal ion-selective electrode real-time measurements, and X-ray spectroscopic analyses to explore Zn(II) and Cu(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 concentration). Nanoparticle aggregate state was assessed by dynamic light scattering (DLS) and sedimentation, metal ion adsorption/retention behavior through macroscopic batch experiments and inductively coupled plasma-optical emission spectrometry (ICP-OES), and metal ion speciation using extended X-ray absorption fine structure (EXAFS) spectroscopy.

Nanoparticle aggregation state was found to result in reduced initial metal ion uptake, likely due to lowered surface area, but in increased metal retention, possibly due to ion trapping within nanoporous regimes. Changes in metal speciation between the sorbed and retained states indicate that a diversity of metal ion sorption complexes exists at the nanoparticle-water interface, with weakly-bound complexes more readily removed by a pH-based desorption step that leaves behind more strongly-bound complexes, as primarily evidenced by an increase in iron coordination around the average sorbed metal atom. With increasing exposure time, metal ion desorption rates decline and the retained fraction increases exponentially, corroborated by speciation evidence suggesting greater proportions of strongly-bound metal ion complexes over time. The presence of common seawater ions sulfate and chloride also serve to enhance metal ion 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 iron oxyhydroxide nanoparticles and their aggregates.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10905

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