Aqueous Phase Reduction of Mn(VII) by Sodium Dithionite; Kinetics, Thermodynamics, and Stoichiometry
AbstractThe reduction of Mn(VII) by Na2S2O4 in aqueous solution was investigated. Changes in [Mn (VII)] were followed kinetically by UV-Visible Spectrophotometry (λmax=530 nm) at different pH (3-12) and temperatures (298–333 K).Thermodynamic parameters for the reduction were derived from ln kobs/T vs 1/T plots. The stoichiometry was studied with fixed [Mn(VII)] (1.818×10-7 M) with various [Na2S2O4] (7.273×10-5–2.182×10-4 M) at constant pH, temperature and time. The temperature effect showed that the reaction rate increased with increased temperature. The activated entropy obtained was between -104.8887 and -113.6600 JKmol-1 for all reactions showing that the reaction was highly spontaneous suggesting an associative mechanistic pathway. The activated enthalpy obtained varied from -8.680×102 to 4.776×103 Jmol-1.In most cases it was positive showing that the reaction was rather endothermic. The activation energies obtained were 31.5954, 32.6848, 33.7742, 34.3186 and 35.4082 KJmol-1 for respective Kelvin temperature of 298, 308, 318, 323 and 333. This increase in activation energy with increasing temperature showed that the reaction does not necessarily require increased temperature to proceed. The stoichiometry therefore suggested that Mn(VII) reductant ratio was 1:1.05 and did not change under alkaline condition. These values were found to be in agreement with the theoretically obtained results. This implied that the reduction of Mn(VII) does not depend on pH. The findings from the research showed that all the reductants could be used for the reductive detoxification of Mn(VII) in polluted areas even at hyper alkaline pH at a very fast rate.