The sorption of Co(II)EDTA 2-- (where EDTA is ethylenediaminetetracetic acid) was investigated on goethite and on eight sand-textured Quaternary and Pliocene fluvial sediments. Dual-label tracer techniques were used to follow the distribution of 60Co(II)- 14C/EDTA added as the preformed 1:1, Co(II)EDTA 2- complex. Sorption experiments were performed with fixed concentrations of Co(II)EDTA 2- (10 -5 mol/L) and variable 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 concentrations of 0.003 and 0.03 (goethite only) mol/L Ca(Cl0 4) 2. Aqueous Fe 3+aq) and Al'(a'q) were measured at the time of the sorption determination. On goethite, Co(II) EDTA 2- exhibited anion-like sorption, increasing with decreasing pH. Increasing electrolyte concentration decreased sorption, indicating a weak, ion-pair type surface complex. Below pH6, however, the sorption chemistry of Co 2+ and EDTA 4- became complex and disparate as a result of Co(II)EDTA 2--dissociation. dissociation was driven by exchange with Fe 3+(aq). A nonelectrostatic surface complexation model that explicitly considered the Fe 3+-Co(II)EDTA 2- exchange reaction was able to adequately describe the sorption data using surface complexes with Co(II)EDTA 2-, FeEDTA -, and Co 2+. The subsurface sediments contained variable amounts of grain-coating iron and aluminum oxides and layer silicates and their substrate mineralogy 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 goethite, hematite, and feroxyhite. Aluminum oxides were also present. The sorption behavior of Co(II) EDTA 2- on the subsurface sediments was much weaker than, but analogous in behavior to, goethite on six of the sediments. The mineralogy complexity of the sediments prevented identification of the dominant sorbent, but iron and aluminum oxides were implicated. Complex dissociation occurred with decreasing pFl below 6.5 in the sediments. Solution analysis coupled with modeling of aqueous speciation showed that the dissociation of Co(II)EDTA 2- was promoted by Al 3+ and Fe 3+ that were liberated from the sediments. The dissociation, which was not complete, yielded a multicomponent mixture of Co 2+, Co(II)EDTA 2-, AAEDTA -, and FeEDTA -. Each of these species sorbed to varying degrees. Complexation of Co 2+ by EDTA 4- gave rise to a net increase in retardation of Co 2+ below pH 6.5 and a decrease above. Retardation coefficients for the intact complex and the extent of dissociation both increased as the initial concentration of Co(II)EDTA 2+- decreased. The coupled adsorption, dissolution, and dissociation process will cause complex distance-variant speciation and retardation behavior for Co(II)EDTA 2- in subsurface environments.