Aqueous Phase Reduction of Cr (VI) by Sodium Sulphide: A Kinetic, Thermodynamic and Stoichiometric Approach

The ability of Na2S.9H2O to reduce Cr (VI) in aqueous phase was determined spectrophotometrically at different temperatures of (298 ? 323 K) to study the effect of temperature on reduction while, the effect of pH was studied at pH 3, 5, 7, 9 and 12. The effect of concentration was also considered by varying the concentrations of the reductant (i. e.1.67x10-2, 2.5x10-2, 3.33x10-2, 4.17x10-2 and 5x10-2 mM) over a pH of 5 and 9 at 298 K and 5 minutes with concentration of Cr (VI) set at 1.9X10-1 mM. The temperature effect showed that the reaction rate increased slightly with increase in temperature. The kinetic results showed that the reaction was Pseudo first ? order with respect to Cr (VI) with the rate equation of ? (d [Cr (VI))/dt=k^' [Cr (VI)]. The activation entropy obtained from plots of ln (kobs/T) vs.1/T was -197.53 J. K-1. mol-1 agreeing with an associative mechanism of reaction. The activation enthalpy varied from (-7.5x10-6 - 3.3x10-4 kJmol-1) and in most cases it was negative showing that the reaction was rather exothermic. The activation energies obtained were 58.87, 60.84, 62.82, 63.80 and 65.78 kJmol-1 for the respective Kelvin temperatures of 298, 308, 318, 323 and 333. This increase in activation energy with increasing temperature implied that bond breakage and product formation was more favourable at lower temperature. The stoichiometry therefore suggested that the Cr (VI) Na2S.9H2O molar ratio was 1 0.75 Cr (VI), this value was found to be in agreement with the theoretically obtained result. The findings from the research show that Na2S.9H2O could be used for the reductive detoxification of Cr (VI) in polluted areas.