p-nitrophenol degradation by hybrid advanced oxidation process of heterogeneous Fenton assisted hydrodynamic cavitation: Discernment of synergistic interactions and chemical mechanism.

The present study has investigated p-nitrophenol (PNP) degradation by hybrid advanced oxidation process (AOP) of hydrodynamic cavitation with heterogenous FeO nanoparticles. 78.8 ± 1.2% of PNP degradation was obtained at optimum operational conditions: inlet pressure = 8 atm, pH = 3, initial concentration of PNP = 20 mg L, FeO:HO = 1:100. PNP degradation profiles were analyzed using a kinetic model based on the reaction network. The closest match between the simulated and experimental degradation profiles was obtained for the initial concertation of [HO] = 0.6 M, which was far higher than concentration of externally added HO. This was attributed to in-situ generation of HO through transient cavitation. Intense shear and turbulence generated in cavitating flow caused surface leaching of FeO particles that released Fe/Fe ions. The synergy in the hybrid AOP was in-situ Fenton reactions between leached Fe/Fe ions and HO present in the reaction mixture. The mechanism in OH mediated oxidative degradation of PNP was further explored with Density Functional Theory (DFT) simulations. Both OH addition on benzene ring and H-abstraction reactions were simulated to identify the possible pathways for the degradation. On the basis of activation free energy analysis, degradation pathways initiating with both OH addition and H abstraction were determined to be feasible. The ortho-C of benzene ring was the most favourable site for OH addition, while H atom of phenolic hydroxyl group was more susceptible (or more reactive) for H-atom abstraction route.