Phenomenological kinetics of isothermal wet air oxidation: Operating severity factor and recalcitrance index

In the present investigation, the kinetics governing wet air oxidation (WAO) of organic pollutants is modeled by two distinct parallel kinetic terms corresponding to a readily oxidizable and a recalcitrant structural parts of the pollutant molecule involving the concept of thermodynamic severity factor. The developed model was successfully applied to several classes of organic water pollutants including phenolic compounds as well as mono- and di-carboxylic acids. In the offshoot of the model, a recalcitrance index was proposed classifying the pollutant molecules according to their resistance to oxidation. For the carboxylic acids, it was found that the recalcitrance towards WAO was inversely correlated to the molecular size and weight. Acetic acid proved to be the more recalcitrant component. As for the phenolic compounds, the oxidation efficiency was shown to be affected by the nature of the substitutents present on the phenolic ring. The electron donating substitutents such as alkyl groups (CH3 or -C2H5) confer to the molecule an enhanced reactivity towards WAO. On the contrary, electron-accepting substitutents, such as methoxy groups, lower the oxidation and consequently enhance the recalcitrance.