OXIDATION AND ADSORPTION OF CO(II)EDTA(2-) COMPLEXES IN SUBSURFACE MATERIALS WITH IRON AND MANGANESE OXIDE GRAIN COATINGS

Batch interaction experiments were performed under aerobic conditions to characterize the adsorption behavior and valence speciation of CoED TA complexes (equimolar at 10(-5) mol/L) in a series of Pliocene subsu rface sediments containing various amounts of Fe and Mn oxides. The ex periments were performed in 0.003 mol/L Ca(ClO4)(2) with a solids conc entration of 500 g/L at variable pH (4-9) and at the natural pH of the sediments (pH = 8.3). Three of these subaerial sediments (Ringold 1, 2, 3) contained significant quantities of extractable Fe and Mn, while the fourth (Ringold 4) was virtually devoid of sesquioxide precipitat es. Microscopic and mineralogic analyses of the most heavily encrusted material (Ringold 2) showed that the oxides existed as intergrain cem ents and contained crystalline goethite and rancieite/todorokite. Adso rption on a synthetic analog sorbent (0.6 mass % ferrihydrite-coated s and) over a range in pH showed that, while both Co(II)EDTA(2-) and Co( III)EDTA(-) sorb as anions, the divalent Co complex forms stronger sur face complexes with FeOH sites. In the subsurface sediments containing both Fe and Mn oxides, however, the sorption of Co(II)EDTA(2-) and Co (III)EDTA(-) was low and equivalent, suggesting transformation to a co mmon valence form. Ion chromatography documented that Co(III)EDTA(-) w as the equilibrium species and that the oxidation of Co (II)EDTA(2-) w as rapid. Sorption of Co(II)EDTA(2-) in the Ringold 4 sediment was dif ferent: no oxidation was seen and Al-(aq)(3+), promoted dissociation o f the complex. Sorption experiments with Co(III)EDTA(-) and Ni(II)EDTA (2-) on Ringold 2 sediment demonstrated that the natural Fe oxide frac tion was a poor anion sorbent, in contrast to ferrihydrite coated sand . Experimental evidence suggests Co(II)EDTA(2-) remains intact during oxidation and that dissolved Si, and Si coreacted with the Fe oxides, influence MeEDTA sorption. It is concluded that Mn oxides could greatl y accelerate the potential migration of CoEDTA complexes in subsurface systems.