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Research Paper | Chemistry | Nigeria | Volume 3 Issue 6, June 2014 | Popularity: 7 / 10

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Aqueous Phase Reductive Detoxification of Cr (VI) By Sodium Dithionite: Kinetics; Thermodynamics and Stoichiometric Studies

IORUNGWA M.S., WUANA R.A., YIASE S.G., TOR-ANYIIN T.A

Abstract: In this study Na2S2O4; 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 Na2S2O4 (i. e.0.02; 0.04; 0.05; 0.06 and 0.07 mM) over a pH of 5 and 9 at 298 K and 5 minutes with concentration of Cr (VI) set at 0.19 mM. 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 ? (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 the formation of a condensed compound. The activation enthalpy varied from (-5.82x10-5 ? 1.66x10-4 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 therefore suggested that the Cr (VI): Reductant millimolar ratio was 1 Cr (VI): 0.45 Na2S2O4; this value was found to be in agreement with the theoretically obtained result. This implies that the stoichiometry does not depend on pH. The findings from the research show that Na2S2O4 could be used for the reductive detoxification of Cr (VI) in polluted areas.

Keywords: Aqueous phase, Reduction, Kinetics, thermodynamics, stoichiometry

Edition: Volume 3 Issue 6, June 2014

Pages: 2802 - 2807

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