Photodecomposition of surfactants using Ti(IV)-doped calcium hydroxyapatite particles

The decomposition of surfactants under UV irradiation was carried out using Ti(IV)-doped calcium hydroxyapatite (TiHap) particles with various Ti/(Ca+Ti) atomic ratios (X_Ti). The typical cationic, anionic, and nonionic surfactants, namely, cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and polyoxyethylene nonylphenyl ether (NP-15, which has 15 oxyethylene units on average), respectively, were used. CTAB readily decomposed in the presence of TiHap particles with X_Ti values of 0.10 and 0.20. The plot of CTAB concentration versus time for the decomposition was fitted with a first-order reaction rate equation, and rate constants (k) were obtained. k values for the CTAB system were independent of the X_Ti values of TiHap particles, that is, TiHap particles facilitated the decomposition of CTAB under UV irradiation. A considerable amount of the hydrocarbon chains in CTAB molecules decomposed because the percentage of CO₂ produced (PCP) increased to 55–70% after UV irradiation for 240 h. We also obtained k values for the SDS system using the first-order reaction equation, although the fit was slightly inferior to that observed with the CTAB system. This result may be explained by the electrostatic repulsive force between negatively charged SDS and TiHap particles. In addition, the PCPs after UV irradiation for 120 h slowly increased to 20–35%, which are less than those obtained for CTAB. Nevertheless, few NP-15 molecules (15%) decomposed after UV irradiation for 240 h. The decomposition rates for the three surfactants followed the order CTAB > SDS >> NP-15.