Horikoshi Satoshi

The biodegradation of 4-nonylphenol polyethoxylate (NPE-n) nonionic surfactant through bacteria results in the formation of toxic 4-nonylphenol (NP) that has been identified as an endocrine disruptor by EPA. NP persists in nature for long periods, and consequently, new wastewater treatment should be established to ensure aquatic environmental protection.<BR>The photodegradation of NPE-n, NP and mixture solutions at the TiO₂/water interface was investigated. The photodecomposition of NP solution and NP/NPE-n mixed solutions (each 0.1mM) in aqueous TiO₂ (100mg) suspensions under UV-irradiation (320nm<λ<387nm) was monitored. After filtering TiO₂ particles, photodegradation kinetics were examined by surface tension, UV-absorbance, CO₂ evolution, TOC, FT-IR, ¹H-NMR and GC-MS. The aggregation of TiO₂ particles (studied by dynamic light scattering) and the formation of carboxylic acid intermediates (with HPLC) were examined. Surface photocatalytic reactions of NPE-n and/or NP structures on the TiO₂ semiconductor were studied by molecular orbital (MO) simulation. Efficient adsorption of the ethoxyl group on the TiO₂ surface was initially noted, followed by cleavage of the aromatic ring and the ethoxyl moiety by direct photooxidation and attack of OH radicals. No NP formation was detected in the photodecomposition of NPE. The photomineralization of the ethoxyl moiety proceeded via formic acid, and other alkyl groups and benzene rings were converted to acetic acids. The photooxidation of hydrophobic NP material proceeded as follows : (i) adsorption of the OH moiety in NP on the TiO₂ surface, (ii) facile ring-opening cleavage of phenol and (iii) CO₂ gas evolution for mineralization of NP. The photooxidation of mixed NPE-n and NP solution could be predicted from TiO₂ photooxidation results with pure NPE-n and/or NP solutions, photocatalytic degradation is thus shown to be a promising route for wastewater treatment of surfactants.