Advanced oxidation of the commercial nonionic surfactant octylphenol polyethoxylate Triton™ X-45 by the persulfate/UV-C process: Effect of operating parameters and kinetic evaluation

This study explored the potential use of a sulfate radical (SO^·-₄)-based photochemical oxidation process to treat the commercial nonionic surfactant octylphenol polyethoxylate (OPPE) Triton™ X-45. For this purpose, the effect of initial S₂O^2-₈ (0-5.0 mM) and OPPE (10-100 mg/L) concentrations on OPPE and its organic carbon content (TOC) removal were investigated at an initial reaction pH of 6.5. Results indicated that very fast OPPE degradation (100%) accompanied with high TOC abatement rates (90%) could be achieved for 10 and 20 mg/L aqueous OPPE at elevated S₂O^2-₈ concentrations (≥2.5 mM). S₂O^2-₈/UV-C treatment was still capable of complete OPPE removal up to an initial concentration of 40 mg/L in the presence of 2.5 mM S₂O^2-₈. On the other hand, TOC removal efficiencies dropped down to only 40% under the same reaction conditions. S₂O^2-₈/UV-C oxidation of OPPE was also compared with the relatively well-known and established H₂O₂/UV-C oxidation process. Treatment results showed that the performance of S₂O^2-₈/UV-C was comparable to that of H₂O₂/UV-C oxidation for the degradation and mineralization of OPPE. In order to elucidate the relative reactivity and selectivity of SO^·-₄ and HO·, bimolecular reaction rate coefficients of OPPE with SO^·-₄ and HO· were determined by employing competition kinetics with aqueous phenol (47 μM) selected as the reference compound. The pseudo-first-order abatement rate coefficient obtained for OPPE during S₂O^2-₈/UV-C oxidation (0.044 min^-1) was found to be significantly lower than that calculated for phenol (0.397 min^-1). In the case of H₂O₂/UV-C oxidation however, similar pseudo-first-order abatement rate coefficients were obtained for both OPPE (0.087 min^-1) and phenol (0.140 min^-1). From the kinetic study, second-order reaction rate coefficients for OPPE with SO^·-₄ and HO· were determined as 9.8 × 10⁸ M^-1 s^-1 and 4.1 × 10⁹ M^-1 s^-1, respectively. The kinetic study also revealed that the selectivity of SO^·-₄ was found to be significantly higher than that of HO·.