Tuning the thermodynamic and kinetic performance of Fenton process - effects of dissolved anions/gases and temperature.

Inorganic anions such as chloride (Cl^-), nitrate (${\mathrm{NO}}_3^{-}$), sulfate (${\mathrm{SO}}_4^{2-}$), carbonate (${\mathrm{CO}}_3^{2-}$), bicarbonate (${\mathrm{HCO}}_3^{-}$), dihydrogen phosphate ($H_2{\mathrm{PO}}_4^{-}$), fluoride (F^-) are ubiquitous in water matrices, play a significant role in the degradation of organic pollutants by Fenton process. In the present study, the performance of Fenton process in the presence of these anions was studied using phenol as a model compound along with the underlying mechanism and their tolerance limit. The presence of these anions affects the rate constant of the Fenton process and decreases in the following order, ${\mathrm{ClO}}_4^{-}$-${\mathrm{NO}}_3^{-}$-${\mathrm{SO}}_4^{2-}$-Cl^- > ${\mathrm{HCO}}_3^{-}$ > ${\mathrm{CO}}_3^{2-}$ > $H_2{\mathrm{PO}}_4^{-}$ > F^-. Among the anions studied, $H_2{\mathrm{PO}}_4^{-}$ and F^- ions inhibit the oxidation process at a low concentration of 50 mg/L. The chloride ion inhibits the reaction at high concentrations above 1000 mg/L by a factor of 1.1 times for every 500 mg/L. An increase in temperature from 293 to 323 K increases the rate constant of the Fenton process for the phenolic compounds studied (phenol, 2-chlorophenol, 2-nitrophenol and 2-methylphenol) by 1.3-1.5. The energy of activation (E_a), enthalpy of activation (ΔH_a) and entropy of activation (ΔS_a) for the degradation of phenolic compounds were found to be 6.68-10.14 kJ/mol; 4.16-7.56 kJ/mol and -273.36 to -264.30 JK^-1mol^-1.