Effect of agitation speed on methylene blue discoloration kinetics via ambient air cold plasma

This research investigates the kinetics of methylene blue (MB) discoloration using ambient air cold plasma, with a focus on the impact of agitation speed (100 and 750 rpm). The study revealed pseudo-first-order kinetics for MB discoloration, pinpointing optimal conditions at 35.00°C and 100 rpm. These parameters minimized half-life times, correlated with observed k_obs values. A decreasing pH trend, more pronounced at 750 rpm, was attributed to increased acidic nitrogen species (HNO₃ and HNO₂) production, adversely affecting dye discoloration. Concurrently, enhanced electrolyte concentration was noted from rising conductivity due to plasma production of reactive species followed by solubilization in the aqueous phase. The calculated thermodynamic activation parameters comprised: E_a = 7.96 kJ mol⁻¹, ΔH^‡ = +5.53 kJ mol⁻¹, ΔS^‡ = −253.23 J K⁻¹ mol⁻¹, and ΔG^‡ = +79.77 kJ mol⁻¹ (100 rpm); and E_a = 12.94 kJ mol⁻¹, ΔH^‡ = +13.78 kJ mol⁻¹, ΔS^‡ = −239.06 J K⁻¹ mol⁻¹, and ΔG^‡ = +80.58 kJ mol⁻¹, (750 rpm). The lowest E_a and ΔG^‡ values at 100 rpm reinforced lower agitation favoring the reaction. The study demonstrated a linear decay of the reaction rate constant with the square root of ionic strength. This result, besides the negative activation entropy and moderate activation enthalpy led to a proposition to the determinant step for the transition state formation, involving an associative step between a solvated electron and the protonated substrate. The optimal dye discoloration rate and energy yield were observed at 35.00°C and 100 rpm, with values of 97.2% and 3.371 × 10⁻² g kW⁻¹ h⁻¹, respectively.