Photocatalytic behavior of TOPO-capped TiO2 nanocrystals for degradation of endocrine disrupting chemicals

In this study, we investigated the photocatalytic kinetics and mechanisms of trioctylphosphine oxide-capped titanium dioxide (TOPO-capped TiO2) for degradation of two endocrine disrupting chemicals (EDCs), phenol and bisphenol A, which contained different hydrophobicity, chemiadsorbability, and degradable capacity. The TOPO-capped TiO2 exhibited high partition coefficients of 2.00 x 10(-4) and 3.42 x 10(-3) l/mg for phenol and bisphenol A, respectively. In addition, the bonded TOPO introduced substantial amounts of trapped holes at the surface. The high affinity toward the target compounds and stabilized charge carriers resulted in high photocatalytic activity of the modified TiO2. The TOPO-capped TiO2 exhibited 12- and 3-fold higher photocatalytic activities than P25 for decomposition of bisphenol A and phenol, respectively. The presence of the hydrophobic modifier inhibited the generation of hydroxyl radicals and led to the photocatalysis undergoing mainly from partition followed by chemisorption and interfacial charge transfer. Partition and interfacial charge transfer were the rate determining steps for the degradation of phenol and bisphenol A, respectively. According to the Langmuir-Hinshelwood model, the adsorption coefficient (K,) and intrinsic rate constant (k(r)) for phenol were 4.17 x 10(-2) l/mg and 5.60 x 10(-2) mg-g/l-min-m(2), respectively, while they were 2.91 x 10(-2) l/m and 2.18 x 10(-1) mg-g/l-min-m(2), respectively for bisphenol A degradation. The product of the k(r) and k(a) for bisphenol A was 2.7 times higher than that for phenol, revealing that the modified TiO2 favored decomposition of the compound which contains high chemiadsorbability and direct photocatalytic tendency in their individual systems. In contrast, the high degradable capability of phenol resulted in its preferential degradation in the competitive system. (C) 2009 Elsevier B.V. All rights reserved.