Nanoscale iron oxyhydroxides, which are commonly found in surface aqueous systems, act as effective sorbents for dissolved metals due to their chemical reactivity, small size and high surface area which allow them to sequester ions from the aqueous and into the solid phase. However, they also rapidly aggregate under natural geochemical conditions, thus affecting their metal sorption capabilities. The broad range of particle aggregation mechanisms and states possible in the environment, in addition to variations in exposure/reaction time and other environmental factors (such as the increase in salinity associated with the transition from fresh water to seawater), substantially increases the complexity and variety of interactions between metal ions and nanoparticle aggregates.<br/><br/>We have conducted an array of macroscopic batch uptake experiments and spectroscopic analyses using synchrotron X-rays to explore Zn(II) adsorption, desorption, and retention processes associated with aggregated and dispersed iron oxyhydroxide nanoparticles, with a range of aggregation states induced by partial/complete suspension freezing, partial/complete suspension drying, and increasing concentrations of humic acid (1-100 mg/L). Nanoparticle aggregate state was assessed by dynamic light scattering (DLS), Zn(II) adsorption/retention behavior through batch experimentation and inductively coupled plasma-optical emission spectrometry (ICP-OES), and Zn(II) speciation using extended X-ray absorption fine structure (EXAFS) spectroscopy.<br/><br/>Nanoparticle aggregation state was found to result in reduced initial Zn(II) uptake due to lowered surface area but in increased metal retention, likely due to inter-particle nanoporosity. Quantifiable variations in zinc speciation at the solution-solid interface indicate that a range of Zn(II) sorption complexes exists at the nanoparticle-water interface, with the more weakly-bound complexes removed by a pH-based desorption step that leaves behind more strongly-bound species, as evidenced by an increase in iron coordination around the average zinc atom. These dynamic changes in surface complexation inform our understanding of fundamental sorption/desorption processes to nanoparticles and their aggregates as well as the development of potential applied remediation strategies in contaminated systems.

Effects of aggregation through freezing, drying, and organic matter on Zn(II) adsorption/retention to iron oxyhydroxide nanoparticles

2019 Fall National Meeting

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