Photocatalytic TiO2 - assisted decomposition of Triton X-100: inhibition of p-nitrophenol degradation

A decrease in the apparent pseudo first-order rate constant is observed in the photocatalyzed (TiO2) degradation of surfactant Triton X-100 (Triton) when its concentration is increased. The measured rate versus the concentration profile is consistent with a hyperbolic form (rate increases with concentration) as described by the Langmuir-Hinshelwood (L-H) model. The rate is then given by the expression: r=kK[Triton]/(1 + K[Triton]) but the apparent rate constant by k(app)=kK/(1 + K[Triton](o)), where k=0.66 mg L-1 min(-1) and K=0.037 L mg(-1). Therefore, at low [Triton](o), k(app) = kK but at high [Triton](o), k(app) = k/[Triton](o), that is, an inverse function of the reactant concentration. Although, in the latter case the reaction does not follow first-order kinetics, its pseudo first-order deviation is not easily noticeable. Therefore, this decrease in kapp with reactant concentration may limit its use when rate constants are compared to evaluate degradation efficiency or when it is used to show reaction inhibition. However, we have detected p-nitrophenol inhibition induced by Triton using kapp values. Inhibition is observed at [Triton](o) < CMC (critical micelle concentration) and also at [Triton](o) > CMC. These inhibitions are consistent with the LH model given by the expression: r=k'K'[phenol]/(1+K'[phenol](o)+K[Triton](o)), where [phenol] is equal during all kinetic runs. Copyright (C) 2008 John Wiley & Sons, Ltd.