Abstract
The co-disposal of trace metals (e.g., Co), synthetic chelates (e.g., ethylendiaminetetraacetic acid, H4EDTA), and water-miscible organic solvents has occurred at some contamination sites. The present study examines the reactions of Co(II)−EDTA2- with a redox reactive naturally occurring solid, goethite, in aqueous and semi-aqueous (methanol−water, acetone−water) suspensions. UV−vis spectroscopy indicated that goethite catalyzed oxidation of Co(II)−EDTA2- to Co(III)−EDTA- by dissolved O2. The aeorobic reaction was described by a pseudo-first-order rate constant, k, of 0.0078 ± 0.0002 h-1 at p[H] 5.0 and a goethite concentration of 3.09 g L-1. A decreased oxidation rate under N2(g) purge was caused by an accumulation of Fe(II) in the goethite suspensions. The addition of acetone monotonically increased k. In contrast, methanol additions up to 20 wt % decreased k. Further additions of methanol raised k in excess of that observed for fully aqueous suspensions. These reactions have important implications on the fate of the redox-sensitive metal in complex, mixed waste environments.
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