Investigation of Mechanisms of Oxidation of EDTA and NTA by Permanganate at High pH HYUN-SHIK CHANG, GREGORY V. KORSHIN,* AND JOHN F. FERGUSON Department of Civil and Environmental Engineering, Box 352700, University of Washington, Seattle, Washington 98115-2700 Permanganate has been used for oxidation of nuclear wastes containing chelating agents such as ethylenedi- aminetetraacetic and nitrilotriacetic acids (EDTA and NTA) to improve separation of radionuclides and heavy metals from the wastes, but the mechanisms of degradation of these and related organic ligands at high pHs have not been studied. EDTA, NTA, and the model compound ethylenediamine (EN) were found to be readily oxidized by permanganate at pH 12-14. The reduction of permangante was accompanied by formation of unstable manganate and dispersed MnO 2 particles, which constituted the final product of permanganate reduction. The yields and speciation of EDTA, NTA, and EN breakdown products were affected by the pH and permanganate dose. Iminodiacetic acid (IDA), oxalate, formate, and ammonia were the predominant EDTA and NTA oxidation products. Mineralization of EDTA, NTA, and EN to CO 2 was more significant at pH 12. At pH 14 formation of oxalate and deamination to NH 3 were the most important reactions. IDA was released upon the oxidation of both EDTA and NTA, but EDTA oxidation yielded no ethylenediaminediacetic acid (EDDA). The speciation of the reaction products indicated that the ethylene group in EDTA was the preferred attack site in oxidations by alkaline permanganate. Introduction Ethylenediaminetetraacetic and nitrilotriacetic acids (EDTA and NTA, respectively) are found at concentrations as high as 0.03 M in many tanks containing nuclear wastes located at the Hanford Nuclear Reservation (1-4). Formation of complexes of EDTA, NTA, and related ligands with transu- ranium elements (TRU), 90 Sr, other radionuclides, and metals (for instance, chromium, nickel, copper) interferes with re- moval of the target species from the wastes, which is necessary to facilitate waste handling, maximize vitrification of radio- nuclides for long-term disposal, and improve the long-term stability of the vitrification products. These complexes also tend to persist in the environment and exhibit a greater mobility in surface waters and the subsurface zone (5-7). Permanganate treatment of the high pH, high ionic strength wastes has been shown to improve separation of the target species because of the breakdown of EDTA (and other organic ligands present in the wastes) caused by permanganate oxidation and attendant formation of man- ganese solids that bind TRU and some of the other target species (1-4), but intrinsic mechanisms of these processes have not been explored. In acidic or circumneutral solutions, the oxidation of EDTA is accompanied by release of ethyl- enediamine-N,N′,N′-triacetic acid (ED3A) and CO2 as the major products of EDTA degradation, and Mn 2+ as the final product of Mn(VII) reduction (8). Microbiologically mediated degradation of EDTA in circumneutral pHs also proceeds via the formation of ED3A, N,N′-ethylenediaminediacetic acid (EDDA), iminodiacetatic acid (IDA), and low-molecular- weight compounds (9, 10). Experiments with model com- pounds such as amino acids (e.g., 11-14) have shown that the reduction of permanganate by these species at high pHs proceeds via formation of manganate MnO4 2- , hypoman- ganate MnO4 3- (15), and finally MnO2 or mixed Mn(III)/Mn- (IV) solids (16). Identified reaction products formed upon the permanganate oxidation of amino acids and other model compounds include ammonia, oxalate, CO2, and traces of aldehydes (17-20). Similar species and, in addition to them, IDA, glycine, and glycolate were identified in solutions of EDTA oxidized at high pHs by Ag(III) (21), and also at circumneutral pHs in the case of oxidation by hydroxyl radicals generated in the H2O2/UV system (22) or with reactive oxygen species activated by zerovalent iron (23). Despite the extent of studies concerned with the micro- biological and chemical degradation of EDTA, NTA, and other persistent organic ligands at acidic and circumneutral conditions, the nature of processes that govern the oxidative breakdown of such compounds at high pHs has not been ascertained, nor have the identities and yields of breakdown products in these conditions been quantified. The goal of this study was to explore in detail the degradation of EDTA, NTA, and a model compound ethylenediamine (EN) in alkaline media. In addition to the practical utility of such data for modeling of physicochemical processes in nuclear wastes and compartments of the environment affected by them (e.g., the subsurface zone), more detailed and chemi- cally explicit theories of EDTA, NTA, and EN oxidation at high pHs will enhance and complement the current under- standing of the processes that govern the environmental fate of persistent organic ligands. Experimental Section All chemicals were ACS reagent grade. Reagent water was obtained from a Millipore Super-Q water system. All experi- ments were conducted at 20 °C and a constant ionic strength (1.13 M) which was adjusted with sodium perchlorate. Concentrations of sodium hydroxide in solutions with pH values of 12, 13, and 14 were 0.01, 0.1, and 1 M, respectively. EDTA, NTA, IDA, and EN oxidation experiments con- cerned with the quantification of reaction product formation were carried out using 9 mL disposable test tubes to which varying amounts of permanganate were added. Following that, the test tubes were mildly agitated for 40 h to complete reactions between permanganate and organic substrates. Manganese solids formed during the reaction were allowed to precipitate. Changes of pH values during the reaction were less than 0.2 pH units. The supernatant was filtered through a 0.45 μm filter. The retained solids were used in selected cases for morphological and structural analyses. Concentrations of EDTA, NTA, ethylenediaminediacetic acid (EDDA), IDA, acetate, glycolate, formate, oxalate, nitrate, and nitrite were measured using a Dionex DX500 ion chromatograph equipped with a CD20 conductivity detector, GP40 gradient pump, and IonPac AS11 separation column. The loop size was 24 μL. Three degassed eluents, including * Corresponding author e-mail korshin@u.washington.edu; phone: (206) 543-2394; fax: (206) 685-9185. Environ. Sci. Technol. 2006, 40, 5089-5094 10.1021/es0605366 CCC: $33.50  2006 American Chemical Society VOL. 40, NO. 16, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 5089 Published on Web 07/13/2006