Catalysis of Manganese(II) on Chromium(VI) Reduction by Citrate

Abstract

The catalysis of manganese(II) (Mn 2+) on chromium(VI) (Cr 6+) reduction by citrate was studied through batch experiments with the concentration of citrate greatly in excess of Cr 6+ at 25°C and in pH ranges of 4.0 to 5.0. Results showed that at pH 4.5 within 22 h direct reduction of Cr 6+ by citrate was not observed, but for the same time when Mn 2+ (50 to 200 μmol L −1) was added, nearly all Cr 6+ was reduced, with the higher initial Mn 2+ concentration having faster Cr 6+ reduction. In the initial stage of the reaction, the Cr 6+ reduction could be described with a pseudo-first-order kinetics equation. In the later stage of the reaction, plots of Inc(Cr 6+) versus t, where c(Cr 6+) is the Cr 6+ concentration in the reaction and t is the reaction time, deviated from the initial linear trend. The deviations suggested that the pseudo-first-order kinetics did not apply to the whole experimental period and that some reaction intermediates could have greatly accelerated Cr 6+ reduction by citrate. The catalysis of the intermediates increased with the reaction time and gradually reached stability. Then, the plot of Inc(Cr 6+) versus t in the presence of Mn 2+ was linear again, with the rate constant increasing by 102 times compared with the absence of Mn 2+. Complexation between Mn 2+ and citrate was likely a prerequisite for the catalysis of Mn 2+ on the reaction. Additional experiments showed that introducing ethylenediaminetetraacetic acid (EDTA) into the reaction system strongly suppressed the catalysis of Mn 2+.