OXIDATION-KINETICS OF CO(II)-EDTA IN AQUEOUS AND SEMI-AQUEOUS GOETHITE SUSPENSIONS

The co-disposal of trace metals (e.g., Co), synthetic chelates (e.g., ethylendiaminetetraacetic acid, H(4)EDTA), and water-miscible organic solvents has occurred at some contamination sites. The present study e xamines the reactions of Co(II)-EDTA(2-) with a redox reactive natural ly occurring solid, goethite, in aqueous and semi-aqueous (methanol-wa ter, acetone-water) suspensions. UV-vis spectroscopy indicated that go ethite catalyzed oxidation of Co((II))-EDTA(2-) to Co(lll)-EDTA(-) by dissolved O-2. The aeorobic reaction was described by a pseudo-first-o rder rate constant, k, of 0.0078 +/- 0.0002 h(-1) at p[H] 5.0 and a go ethite concentration of 3.09 g L(-1). A decreased oxidation rate under N-2(g) purge was caused by an accumulation of Fe(II) in the goethite suspensions. The addition of acetone monotonically increased k. In con trast, methanol additions up to 20 wt % decreased k. Further additions of methanol raised k in excess of that observed for fully aqueous sus pensions. These reactions have important implications on the fate of t he redox-sensitive metal in complex, mixed waste environments.