A Kinetic, Thermodynamic and Stoichiometric Study on the Reductive Detoxification of Cr(VI) in Aqueous Phase by Sodium Metabisulphite

Abstract

In this study Na₂S₂O₅, was assessed as an inorganic reductant of Cr(VI) in aqueous phase. The absorbance of Cr(VI) was monitored at 349 nm using a Milton  Roy Spectronic 20D UV/Visible spectrophotometer  at different temperatures (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 reductants (i.e. 4, 6, 8, 10 and 12 mg/L) over a pH of 5, 298 K and 5 minutes with concentration of Cr(VI) set at 10mg/L. The temperature effect showed that the reaction rate increased slightly with increase in temperature. It was found that there was a logarithmic increase in time required to reach a certain percentage reduction of the [Cr(VI)]_initial, as the pH was increased. The kinetic results showed that the reaction was first – order with respect to Cr(VI) with the rate equation of . The activation entropy obtained from plots of ln(k_obs/T) vs. 1/T was -197.53 J.K^-1.mol^-1 agreeing with the formation of a condensed compound. The activation enthalpy varied from (-7.5x10^-6 - 3.3x10^-4 kJmol^-1) and 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 suggests that the Cr(VI) : Na₂S₂O₅ ratio was 1 Cr(VI) : 1.36 Na₂S₂O₅, this value was found to be in agreement with the theoretically obtained result for both acidic and alkaline rigimes.  The findings from the research show that Na₂S₂O₅ could be used for the reductive detoxification of Cr(VI) in polluted areas.