Photocatalytic removal of organic phosphate esters by TiO2: Effect of inorganic ions and humic acid.

In this study, the TiO2-mediated photocatalytic removal of organic phosphate esters (OPEs) under UV irradiation was investigated. In addition, the effects of selected inorganic ions and humic acid (HA) were examined. TCEP, TCPP, and TDCPP hardly undergo degradation via photolysis, and TiO2-mediated photocatalysis was confirmed to be effective for removing OPEs present in a water matrix. Inorganic ions (e.g., PO43-, SO42-, Cl-, and NO3-) and HA inhibited the degradation of OPEs. Among these selected ions, PO43- exhibited the strongest inhibition ability, followed by SO42- and then Cl- and NO3-, and the inhibition effect increased with the ion concentration. The inhibition effect was caused by two main reasons: (1) Inorganic ions or HA served as ·OH scavengers and competed with OPEs for ·OH. (2) On account of the adsorption of inorganic ions and HA on TiO2, the available surface-active sites for OPEs decreased, followed by their oxidation with surface-bound ·OH. The ·OH concentrations were measured by electron spin resonance and the probe method, and it was positively correlated to the degradation rate of OPEs. This study emphasized the importance of inorganic species and HA on the photocatalytic degradation of OPEs and clarified the inhibition mechanism for the degradation of OPEs by the inorganic species or HA.