Naphthalene degradation by Fe2+/Oxone/UV – Applying an unconventional kinetics model and studying the reaction mechanism

This study shows the degradation of naphthalene (Nap) in aqueous solution using Oxone process mediated by Fe2+ with UV-A irradiation (FOU). To elucidate the role of different parameters, Fe2+/Oxone (FO), Fe2+/UV (FU), Oxone/UV (OU) and direct photolysis processes were studied, separately. The degradation efficiency under different dosage of Fe2+, Oxone, initial probe compound concentration and solution pH were evaluated. It is concluded that FOU process has significantly better degradation capacity and efficiency. More than 90% of 0.125 mM Nap was removed in 20 min, under the optimal conditions of FOU ([Fe2+]0 = 0.250 mM, [Oxone]0 = 0.250 mM, wavelength = 350 nm and pH = 2.8). A mathematical model is proposed to describe the two-stage reaction kinetics involving Oxone. To alleviate the problems of radical surge at the initial stage and a radical deficit at later stage, a stepwise addition of oxidants was conducted and achieved a higher removal performance. Besides, the decay pathways of Nap under FOU process were proposed by using LC-ESI/MS analysis. The TOC content was found to be increased initially and decreased after 2 h reaction. It is clarified that the TOC increment was contributed by the partially degraded intermediates rather than the persistent Nap, since the latter was not completely combustible in the TOC analyzer, demonstrating that the FOU process is effective in degrading Nap into more degradable products such naphthoic acids and aldehydes.