Heterogeneous photocatalytic ozonation of 2,4-D in dilute aqueous solution with TiO2 fiber

Abstract

Photocatalytic ozonation (O3/UV/TiO2) is an emerging oxidation method for recalcitrant organic contaminants in water. However, immobilised TiO2 catalysts suffer from reduced photonic efficiency. Therefore, TiO2 catalysts with excellent mechanical and thermal properties and enhanced photonic efficiencies are sought. This paper aimed to elucidate the mineralisation of low concentration 2,4-D (45.0 μM) by O3/UV/TiO2 using the world's first high-strength TiO2 fibre in laboratory batch experiments. 2,4-D degradation and TOC removal followed pseudo first-order reaction kinetic. The removal rates for 2,4-D and TOC in O3/UV/TiO2 were 1.5 and 2.4-fold larger than the summation of the values for ozonation (O3) and photocatalysis (UV/TiO2), respectively. O3/UV/TiO2 was characterised by few aromatic intermediates with low abundance, fast degradations of aliphatic intermediates and dechlorination as a major step. The significantly enhanced 2,4-D mineralisation in O3/UV/TiO2 was attributed to increased ozone dissolution and decomposition, and reduced electron–hole recombination resulting in large number of hydroxyl radical (•OH) formation from more than one parallel path. The discrepancies in the organic carbon mass budget were attributed to few apparently major unidentified intermediates, while chlorine mass balance was reasonably acceptable. The mineralisation efficiency of O3/UV/TiO2 with the TiO2 fibre can further be enhanced by optimisation of experimental design parameters. The new TiO2 fibre is very promising to overcome the problem of reduced efficiency of TiO2 catalyst in an immobilised state.