Goethite catalyzed Cr(VI) reduction by tartaric acid via surface adsorption

The surface catalysis of goethite on the Cr(VI) reduction by tartaric acid was examined together with its adsorption characteristics towards the two reactants. The results showed the adsorption of tartaric acid by goethite was favorable at low pH and adsorption isotherm could be properly described by Langmuir model. The adsorption kinetic curves for both reactants obeyed the pseudo second-order rate model (R² >0.99). The FTIR spectrum suggested the formation of bidentate binuclear surface complexes between tartaric acid and goethite. At pH 4.50, the reduction percentage of 0.1 mM Cr(VI) by 1.0 mM tartaric acid alone was about 12% after 72 h, while which was increased to 100% in the presence of goethite within 24 h. Kinetic results revealed the Cr(VI) reduction only occurred between the adsorbed tartaric acid and the aqueous Cr(VI) since the Cr(VI) adsorption was completely inhibited under the examined conditions. Meanwhile, the catalysis of aqueous Fe(III) released from the goethite surfaces was excluded due to its low concentration (<5 μM). With the initial concentration of tartaric acid decreased to 0.1 mM, Cr(VI) reduction could be completed within 4 h, confirmed by the XPS result that only Cr(III) species existed on the goethite surfaces. In this case, electron transfer was suggested to occur directly between the two adsorbed reactants or goethite was believed to serve as an ideal channel to allow electron excited from the adsorbed tartaric acid to transfer to the adsorbed Cr(VI). The findings above were helpful for us to understand the Cr(VI) reduction by organic compounds in soils with rich contents of Fe-oxides.