Determination of oxidation mechanisms of ethylenediaminetetraacetate (EDTA)-metal complexes by alkaline permanganate and structures of in situ formed manganese oxides containing heavy metals

Reactions that occur during the permanganate treatment of Hanford tank wastes that have pH 14 and a very high ionic strength were investigated in this study. Experiments were conducted in conditions representative of these wastes and using several target compounds (EDTA, NTA, Zn2+, Cu2+, Ni2+). The structure of solids formed during the reduction of permanganate by EDTA were examined via SEM, BET surface area and XRD. Structural parameters of the precipitated solids containing incorporated heavy metals were determined using XAFS spectroscopy.; The reduction of alkaline permanganate by EDTA and its metal complexes produced manganate which disproportionated into manganese dioxide and permanganate. This pathway provided a feedback for one-electron permanganate oxidations of organic substrates and formation of solid phases that bind heavy metals present in the system.; The oxidation of EDTA in basic conditions was initiated through N-dealkylation at the ethylene group. Oxalate and CO2 were the major products of EDTA oxidation, and IDA was the major identified intermediate. The unidentified products were hypothesized to be ED3A and EDDA.; EDTA-Me2+ complexes were readily oxidized by alkaline permanganate, with an exception of Ni2+ complexes. Their oxidation was characterized by an incomplete oxidation of the bound EDTA, lower formation of oxalate, and absence of IDA. The resistance of EDTA-Ni2+ complexes may represent a problem for the oxidative treatment of the tank wastes.; The manganese solids formed upon the oxidation of EDTA lacked distinct morphological features. Their average size and surface area were 10 nm in diameter and 11 m2/g, respectively. EXAFS analysis showed that the manganese solids had a phyllomanganate structure similar to that of birnessite (delta-MnO2). EXAFS showed that decomplexed Cu2+ and Zn2+ ions were incorporated in the manganese solid phase via different binding mechanisms. The incorporated Zn2+ was positioned between MnO6 sheets of birnessite and caused the local structure to be altered to that similar to chalcophanite. The Cu2+ was incorporated into the MnO6 layers and exhibited less mobility than Zn2+.