Metal adsorption in near surface systems plays a key role in processes including metal transport, mineral precipitation and dissolution, and metal bioavailability. Nanoparticles, which have very high surface areas relative to their total mass, are particularly important sorbents in soils and sediments. In theory, thermodynamic surface complexation models can be used to predict metal adsorption under varying pH and ionic strength and in the presence of competing sorbates and sorbents. In practice, however, relatively few studies have tested stability constants derived for single metal and sorbent systems in the presence of competing ions and substrates. In this study, Cd(II), Cr(VI) and Pb(II) adsorption was measured on nanoparticulate maghemite (γ-Fe<sub>2</sub>O<sub>3</sub>, &lt;50 nm) and maghemite-coated quartz as a function of pH, ionic strength and sorbate/sorbent ratio in open atmosphere. Adsorption was rapid, reaching steady state in ≤1 hr. Pb(II) and Cd(II) adsorption increased with increasing pH; Cr(VI) adsorption decreased with increasing pH. Adsorption of all ions decreased with increasing ionic strength and sorbate/sorbent ratios. Cd(II) and Pb(II) edges on maghemite-coated quartz were similar to those measured on pure maghemite with a similar quantity of total Fe. However, Cr(VI) adsorption was suppressed on the maghemite-coated quartz compared to pure maghemite. Competitive adsorption experiments were conducted using mixtures of Cd(II) and Pb(II) or Cd(II), Pb(II) and Cr(VI) on both solids. Cd(II) adsorption was unchanged in the presence of Pb(II) or Pb(II) and Cr(VI). Cr(VI) adsorption decreased in some experiments with Pb(II) and Cd(II). Pb(II) adsorption typically decreased in the presence of either Cd(II) or Cd(II) and Cr(VI). Potentiometric titrations were used to derive double layer model protonation and deprotonation stability constants for the pure maghemite, and edge data from the single metal experiments were used to constrain Pb(II), Cd(II) and Cr(VI) adsorption stability constants. XANES analysis demonstrated that &gt;85% of adsorbed Cr was present as Cr(VI). Increasing XAS analysis time led to reduction of the Cr(VI) to Cr(III), but Fe remained trivalent. X-ray photoelectron spectroscopy also demonstrated that the oxidation state of near surface Cr(VI), Cd(II), Pb(II) and Fe(III) species remained unchanged by adsorption reactions. Cr likely formed outersphere complexes, while the XPS data suggest that Cd(II) and Pb(II) formed innersphere complexes.

Adsorption of Cr(VI), Cd(II), and Pb(II) on nanomaghemite and maghemite-coated silica

249th National Meeting (2015)

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