Oxidation and adsorption of Co(II)EDTA 2− complexes in subsurface materials with iron and manganese oxide grain coatings

Abstract

Batch interaction experiments were performed under aerobic conditions to characterize the adsorption behavior and valence speciation of CoEDTA complexes (equimolar at 10 −5 mol/L) in a series of Pliocene subsurface sediments containing various amounts of Fe and Mn oxides. The experiments were performed in 0.003 mol/L Ca(ClO 4) 2 with a solids concentration 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 precipitates. Microscopic and mineralogic analyses of the most heavily encrusted material (Ringold 2) showed that the oxides existed as intergrain cements and contained crystalline goethite and rancieite/todorokite. Adsorption on a synthetic analog sorbent (0.6 mass % ferrihydrite-coated sand) 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 surface 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 common valence form. Ion chromatography documented that Co(III)EDTA − was the equilibrium species and that the oxidation of Co(II)EDTA 2− was rapid. Sorption of Co(II)EDTA 2− in the Ringold 4 sediment was different: no oxidation was seen and Al (aq) 3+ promoted dissociation of the complex. Sorption experiments with Co(III)EDTA − and Ni(II)EDTA 2− on Ringold 2 sediment demonstrated that the natural Fe oxide fraction 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 McEDTA sorption. It is concluded that Mn oxides could greatly accelerate the potential migration of CoEDTA complexes in subsurface systems.