Adsorption of Aqueous Cobalt Ethylenediaminetetraacetate by δ‐Al2O3

Abstract

Abstract Enhanced Co migration in the presence of ethylenediaminetetraacetate (EDTA, referred to below as Y ) at U.S. Department of Energy waste sites may be due to the presence of Co(III)‐ Y rather than Co(II)‐ Y chelates in groundwaters. To examine the implication of this for the attenuation of Co in soils and subsoils, the absorption of equimolar Co and Y by δ‐Al 2 O 3 was examined for ranges of pH (5–9.5), adsorbate concentration (0.2–10 µ M ), adsorbent concentration (0.1–10 g/L), and ionic strength (0.1–0.001 M NaClO 4 ) in the presence of dissolved Al (0.2–10 µ M ) in equilibrium with δ‐Al 2 O 3 . The anionic Co(II)‐ Y and Co(III)‐ Y chelates exhibited ligand‐like adsorption behavior as a function of pH; however, under identical conditions, Co(III)‐ Y adsorption was much less than that of Co(II)‐ Y . Dissolved Al competed with Co(II) for Y and, in both Co(II)‐ Y and Y ‐only experiments, Al‐ Y appeared to adsorb, although the affinity of surface hydroxyls ( X OH) is greater for Co(II)‐ Y than for Al‐ Y . Significant variation in Co(II)‐ Y and Al‐ Y adsorption with ionic strength suggests that there is a major electrostatic contribution to adsorption and supports the hypothesis that these chelates form outer‐sphere complexes (ion pairs) with X OH. Evidence for competition between X OH and Y for the Co(II) occurs only at the highest adsorbent‐to‐adsorbate ratio (10 g/L: 0.5 µ M = Co(II) = Y ), where Co(II)‐ and Co(II)‐ Y adsorption occur simultaneously for pH > 8. Outer‐sphere surface complexes between X OH and the free carboxylate groups of the chelated Al and Co are proposed to describe absorption.