Aqueous Phase Reduction of Mn(VII) by Sulphide Ion: Kinetics, Thermodynamics and Stoichiometric Studies

Abstract

The reduction of Mn(VII) by sulphide ions (S²⁻) in aqueous solution was investigated across pH 2–14, temperatures 303–323 K, and different reaction times. Concentration changes of Mn(VII) were monitored spectrophotometrically at 549 nm. Kinetic analysis confirmed a first-order dependence on Mn(VII), described by the rate law: Rate = –d[MnO₄⁻]/dt = –d[S²⁻]/dt = k[MnO₄⁻][S²⁻]. The reduction progressed with increasing time, temperature, and alkalinity. Higher rate constants (k_obs) were observed in alkaline media, attributed to OH⁻-induced formation of manganese oxyanions and hydroxo complexes that are more reactive toward S²⁻ than MnO₄⁻ itself. Thermodynamic parameters derived from ln(k_obs/T) versus 1/T plots revealed negative activation enthalpy (ΔH° = –7.32 to –43.40 kJ mol⁻¹), confirming the exothermic and spontaneous nature of the process. Positive activation entropy (ΔS° = 1.05–1.15 J K⁻¹ mol⁻¹) indicated a dissociative mechanism with greater molecular randomness, while activation energies (9.91–45.96 kJ mol⁻¹) suggested relatively long transition-state bonds. Stoichiometric analysis showed a consistent Mn(VII):S²⁻ ratio of 0.23:1, independent of pH. Overall, the findings establish that sulphide ions effectively reduce Mn(VII) to more stable and less oxidizing lower oxidation states. The reaction generates intermediate manganese species and sulphate radicals, highlighting the Mn(VII)/S²⁻ system as a promising pathway for pollutant degradation and advanced water treatment applications.