Oxidation of Cr(III)-citrate/tartrate complexes by δ-MnO2: Production of Cr(VI) and its impact factors

Cr(III)-cit and Cr(III)-tar were synthesized and purified, and then their stability in the presence of δ-MnO2 was further investigated in batch experiments under different conditions to predict the potential oxidation behaviors of Cr(III)–organic acid complexes in the environment. The results indicated that although the rates and extents of Cr(III)-cit and Cr(III)-tar oxidation by δ-MnO2 were much lower than those of aqueous Cr(III), Cr(VI) could be gradually released through the whole reaction. The oxidations of Cr(III)-cit and Cr(III)-tar were affected by the initial concentrations of δ-MnO2, pH and co-existing ions. Lower pH and higher concentrations of δ-MnO2 markedly enhanced the production of Cr(VI). Ammonium ions significantly improved the oxidation of Cr(III)-cit and Cr(III)-tar, but phosphate ions demonstrated an opposite effect due to the formation of more stable CrPO4. The oxidation process of Cr(III)-cit and Cr(III)-tar by δ-MnO2 can be divided into two phases. At the initial phase, a relatively rapid reaction obeyed the first-order model, and then a zero-order one was followed. It was observed that in all the cases the extent of Cr(III)-cit oxidation was lower than that of Cr(III)-tar. Thus, it was concluded that the stability of Cr(III)-cit was higher than that of Cr(III)-tar.