Adsorption of Aqueous Cobalt Ethylenediaminetetraacetate by δ-Al2O3

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-Al2O3 was examined for ranges of pH (5–9.5), adsorbate concentration (0.2–10 µM), adsorbent concentration (0.1–10 g/L), and ionic strength (0.1–0.001 M NaClO4) in the presence of dissolved Al (0.2–10 µM) in equilibrium with δ-Al2O3. 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 (XOH) 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 XOH. Evidence for competition between XOH and Y for the Co(II) occurs only at the highest adsorbent-to-adsorbate ratio (10 g/L: 0.5 µM = Co(II) = Y), where Co(II)- and Co(II)-Y adsorption occur simultaneously for pH > 8. Outer-sphere surface complexes between XOH and the free carboxylate groups of the chelated Al and Co are proposed to describe absorption.