ADSORPTION AND DISSOCIATION OF CO-EDTA COMPLEXES IN IRON OXIDE-CONTAINING SUBSURFACE SANDS

The sorption of Co(II)EDTA(2-) (where EDTA is ethylenediaminetetraceti c acid) was investigated on goethite and on eight sand-textured Quater nary and Pliocene fluvial sediments. Dual-label tracer techniques were used to follow the distribution of Co-60(II)-C-14/EDTA added as the p reformed 1:1, Co(II)EDTA(2-) complex. Sorption experiments were perfor med with fixed concentrations of Co(II)EDTA(2-) (10(-5) mol/L) and var iable pH (all materials), and fixed pH (4.4) with variable Co(II)EDTA( 2-) concentrations (two materials), using solids concentrations of 0.5 g/L for goethite and 500 g/L for the sediments and electrolyte concen trations of 0.003 and 0.03 (goethite only) mol/L Ca(ClO4)(2). Aqueous Fe-aq(3+) and Al-aq(3+) were measured at the time of the sorption dete rmination. On goethite, Co(II)EDTA(2-) exhibited anion-like sorption, increasing with decreasing pH. Increasing electrolyte concentration de creased sorption, indicating a weak, ion-pair type surface complex. Be low pH 6, however, the sorption chemistry of Co2+ and EDTA(4-) became complex and disparate as a result of Co(II)EDTA(2-)dissociation. Disso ciation was driven by exchange with Fe-aq(3+). A nonelectrostatic surf ace complexation model that explicitly considered the Fe3+-Co(II)EDTA( 2-) exchange reaction was able to adequately describe the sorption dat a using surface complexes with Co(II)EDTA(2-), FeEDTA(-), and Co2+. Th e subsurface sediments contained variable amounts of grain-coating iro n and aluminum oxides and layer silicates and their substrate mineralo gy was dominated by quartz and plagioclase with some mica. Iron oxides were a dominant grain-coating phase on over half the sorbents, and X- ray diffraction (XRD), chemical extraction, and microscopic techniques documented the presence of poorly crystalline forms as well as goethi te, hematite, and feroxyhite. Aluminum oxides were also present. The s orption behavior of Co(II)EDTA(2-) on the subsurface sediments was muc h weaker than, but analogous in behavior to, goethite on six of the se diments. The mineralogic complexity of the sediments prevented identif ication of the dominant sorbent, but iron and aluminum oxides were imp licated. Complex dissociation occurred with decreasing pH below 6.5 in the sediments. Solution analysis coupled with modeling of aqueous spe ciation showed that the dissociation of Co(II)EDTA(2-) was promoted by Al3+ and Fe3+ that were liberated from the sediments. The dissociatio n, which was not complete, yielded a multicomponent mixture of Co2+ Co (II)EDTA(2-), AIEDTA(-), and FeEDTA(-). Each of these species sorbed t o varying degrees. Complexation of Co2+ by EDTA(4-) gave rise to a net increase in retardation of Co2+ below pH 6.5 and a decrease above. Re tardation coefficients for the intact complex and the extent of dissoc iation both increased as the initial concentration of Co(II)EDTA(2-) d ecreased. The coupled adsorption, dissolution, and dissociation proces s will cause complex distance-variant speciation and retardation behav ior for Co(lI)EDTA(2-) in subsurface environments.