Sources, sinks, and speciation of chromium(III) (amino)carboxylate complexes in heterogeneous aqueous media.

摘要

In the environment, chromium exists primarily in the trivalent and hexavalent oxidation states. In the trivalent oxidation state, chromium is essential to human metabolism, while in the hexavalent oxidation state, chromium is a known human carcinogen and allergen. It is therefore important that we understand mechanisms responsible for chromium redox cycling.; This study examines two CrIII transformation processes: ligand-assisted dissolution of amorphous chromium (hydr)oxide by aminocarboxylate chelating agents and oxidation of CrIII-aminocarboxylate complexes to CrVI by MnIII,IV (hydr)oxide (MnO2). Dissolution of CrIII (hydr)oxide by multidentate aminocarboxylate chelating agents (e.g. EDTA, NTA and IDA) can facilitate CrIII mobility. Formation of soluble CrIII complexes with organic chelating agents opens the possibility for CrIII oxidation to CrVI by MnIII,IV (hydr)oxides at neutral pH. As part of this work, capillary electrophoresis methods were developed to monitor the products of CrIII-aminocarboxylate transformation reactions.; Capillary Electrophoresis is an excellent analytical tool for examining CrIII speciation because rates of ligand exchange with are sufficiently slow that the metal ion-chelating agent complexes present in the injected sample do not dissociate under the influence of the applied electric field. Subtle differences in protonation level, stoichiometry and stereochemistry of CrIII-aminocarboxylate complexes were discerned.; Dissolution of amorphous chromium (hydr)oxide was examined using seven (amino)carboxylate chelating agents (EDTA, CDTA, TMDTA, NTA, IDA, citrate and tricarballylate) over the pH range of 4 to 10. Long contact times were necessary for chelating agent adsorption onto ACH to reach completion. Rates of ligand-assisted dissolution exhibited by the (amino)carboxylate chelating agents had different pH dependencies. Employing acetate as a pH buffer reduced the extent of adsorption of the smaller chelating agents (IDA, NTA, CIT), however, normalized dissolution rates using acetate were greater for all chelating agents when compared to the non-metal complexing buffer N,N' -diethylpiperazine (DEPP).; Oxidation of CrIII complexes with inorganic ligands (water/hydroxide ion, chloride) and organic chelating agents (IDA, NTA, HEDTA and EDTA) by MnO2 was examined. CrIII complexes with IDA and NTA are readily oxidized by MnO2 to CrVI. However, complete conversion of CrIII to CrVI does not occur in moderately acidic to neutral solutions even in the presence of excess MnO2. MnCl2 addition experiments indicate that the observed reaction inhibition is due to MnII generation during the reaction. Metal ion-chelating agent dissociation occurs after CrIII is oxidized to CrVI and free NTA is oxidized to free IDA by MnO 2. Rates of CrVI formation for u-fac-[Cr III(ida)2]-, CrIII-HEDTA and CrIII-EDTA were much slower than that observed for complexes with water/hydroxide ion, IDA and NTA. Half lives for CrIII oxidation varied by over eight orders of magnitude indicating the importance of CrIII speciation on reactivity.