Iron oxyhydroxide minerals are commonly formed in natural aqueous systems as nanoscale particles, where they can 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, such particles typically aggregate under natural aqueous conditions and are rarely observed as dispersed nanoparticles.  The wide range of particle aggregation mechanisms and states possible in natural systems as a result of variations in ionic strength, pH, temperature, time since formation/aggregation, and other factors substantially increases the complexity and variety of interactions between dissolved metals and particle aggregates. To explore the influences of aggregation on the adsorption, desorption, and retention of dissolved metals we have conducted an array of metal uptake/removal studies using macroscopic batch experiments, kinetic rate measurements using real-time ion selective electrodes, and spectroscopic analysis using synchrotron X-rays.  Synthetic iron oxyhydroxide nanoparticles were prepared using a flash microwave synthesis method and aggregated by placing the particles in conditions of increasing ionic strength (0.001-1.0 M), pH (8-10), and temperature (25�C-75�C) to induce varying states of particle aggregation.  Characterization of the aggregates with dynamic light scattering, BET surface area analysis, and X-ray diffraction was conducted prior to Zn(II) and Cu(II) adsorption/desorption experiments.   Initial Cu(II) uptake rates appeared to correlate positively with the proportion of Cu(II) retained after an induced desorption step (i.e. lowering of pH), suggesting that the strength of binding can be predicted by the rate of metal uptake.  Increasingly saline conditions (mimicking the transition from fresh to brackish to marine waters) result in greater retention of Zn(II), suggesting the formation of more stable and possibly ternary surface complexes.

Influences of aggregation on nanoparticle adsorption and retention of dissolved Zn(II) and Cu(II)

246th National Meeting (2013)

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