The objective of this work is to examine the environmental chemistry of chelating agents and quantify their effects on metal speciation and migration in soils and aquifer sediments. Processes that affect metal speciation include formation of metal complexes, redox reactions with the chelating agent as reductant, and adsorption at the surfaces of metal-hydrous oxides. The synthetic organic chelating agents EDTA (Ethylenediaminetetraacetic acid) and NTA (Nitrilotriacetic acid) enter the environment through different anthropogenic sources and have been found in significant quantities in soil and groundwater (Frimmel et al., 1989; Alder et al., 1990). In this study, EDTA, NTA and the structure-related IDA (Iminodiacetic acid) were chosen to study the role they play in the formation of dissolved Co(III)- and Mn(III)-complexes and their subsequent breakdown. Co(III) and Mn(III) are found adsorbed or incorporated into hydrous oxides in subsurface environments. Co(III)-EDTA is very stable and mobile (Girvin et al., 1993; Jardine et al., 1993; Nowack and Sigg, 1995). In low-level radioactive waste disposal sites the Co(III)-EDTA complex plays an important role in enhanced subsurface 60Co migration mobile (Girvin et al., 1993; Jardine et al., 1993; Means et al., 1978). Concerning Mn(III)-EDTA no data are available which describe interactions of EDTA with Mn(III) on natural surfaces.
a) Source of Co(III) / Mn(III) and analysis of free chelating agents and their complexes: Heterogenite (CoOOH) and manganite (MnOOH) were synthesized to serve as surrogates for Co(III)- and Mn(III)-containing surfaces. Pure hydrous oxides with oxidation states close to +3.0 were achieved and particle sizes of 0.1 µm were found. Two slightly different methods of synthesizing CoOOH produced particles with different morphologies and different reaction behavior. TEM-measurements show that one consists of hexagonal plates, the other of hexagonally ordered needles. MnOOH consists of thin needles.
For the identification and quantification of the organic compounds and their complexes with metal ions, a new method was developed using Capillary Electrophoresis (CE). EDTA, NTA and other aliphatic carboxylic acids can be separated and determined with a detection limit of 10 - 20 µM. Co- and Mn-organic complexes which are substitution inert in the time scale of CE analysis (e.g. Co(III)EDTA, Co(II)EDTA and Mn(II)EDTA) can be easily resolved from free carboxylic acids. Results from more labile complexes (e.g. Co(II)IDA) are more difficult to interpret.
b) Reaction of chelating agents with CoOOH and MnOOH: In experimental reactions of CoOOH with EDTA, NTA and IDA, formation of Co(III)-organic complexes was found as expected. In experiments with EDTA and NTA, results from CE analysis indicate that Co(III) was also reduced to Co(II), whereas the chelating agents, serving as the reductants, were degraded. Co(II)EDTA and Co(II)NTA, respectively, were found as reaction products. In dissolution experiments with EDTA, ED3A (Ethylenediaminetriacetic acid) could be identified as a major degradation product; in experiments with NTA, IDA was produced. More degradation was found in experiments with the chelating agents in excess. These results are surprising, since a solution of Co(III)EDTA (as prepared from the synthesizes potassium salt of the complex) does not show degradation.
In dissolution experiments of MnOOH with EDTA no evidence for soluble Mn(III)EDTA was found. ED3A could be identified as a major degradation product. In experiments with EDTA in excess, formation of Mn(II)EDTA was observed.
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