Photocatalytic degradation of terephthalic acid on sulfated titania particles and identification of fluorescent intermediates

Highlights

• SO₄²⁻/TiO₂ photocatalysts were efficient for the photocatalytic degradation of TA.

• 5 New intermediates were identified.

• Intermediate A/B ratio provided some clues for the degradation mechanism of TA.

• Formation of fluorescent intermediates was slower than their further degradation.

• Photocatalytic destruction mechanism of TA was proposed.

Abstract

Terephthalic acid (TA) is toxic and known as an endocrine disruptor. In this paper, the photocatalytic degradation of TA using sulfated titanium dioxide SO₄²⁻/TiO₂ photocatalysts was investigated. The photocatalysts were prepared by sol–gel method and characterized by XRD, SEM, BET, ICP/MS and spectroscopic methods. Their activities were compared with bare TiO₂ and Degussa P25. The effects of catalyst sulfur content, the initial TA concentration and pre-treatment conditions (O₂, N₂ or non-pretreated) were studied. O₂ functions were also explored. Since there had been no comprehensive study of fluorescent intermediates reported yet, we investigated the intermediates and discovered 5 new intermediates (4 fluorescent and 1 non-fluorescent) which were identified by gas chromatography–mass spectrometry (GC/MS) and fluorescence spectroscopy. These intermediates are complementary to the previously identified carboxylic acid intermediates and might provide new insights to the mechanism of photocatalytic degradation of TA. Since TA is widely used as a probing molecule for photocatalytically generated ^·OH radicals, the ratios of fluorescent intermediates of TA degradation may provide new clues to the photocatalytic activity and mechanism. Based on the results obtained, the possible destruction pathway of TA is also proposed.

Introduction

Terephthalic acid (TA) is widely used as a raw material to produce polyester fiber, polyethylene terephthalate (PET) bottles, PET films, engineering plastics and medicines, etc. In 2013, polyester fiber and PET bottle resin accounted for 93% global TA demand. Global TA production quantity was 46 million tons in 2013 and was forecasted to reach 66 million tons in 2020. TA is toxic and known as an endocrine disruptor [1], [2]. In addition, it may also interfere with the reproductive system and normal embryonic development of animals and humans. Traditional aerobic biological treatment method is slow and inefficient. Traditional coagulants for removal of TA included FeCl₃, AlCl₃, MgCl₂, CaCl₂, Al₂(SO₄)₃, polyaluminium chloride, FeSO₄ and polyelectrolyte [3], [4]. Recently, several advanced oxidation processes were investigated for the destruction of TA, including UV–TiO₂, UV–H₂O₂, UV–H₂O₂–Fe, O₃, O₃/TiO₂ and UV–O₃–H₂O₂–Fe [5]. The UV–O₃–H₂O₂–Fe method was extended to the degradation of wastewater from TA manufacturing [6]. Besides TiO₂ photocatalysts, ZnO was also studied for TA degradation [7]. Although UV–TiO₂ based method had been investigated, only pure Degussa P25 TiO₂ was evaluated. In this work, we extend the study to sulfated titanium dioxide (SO₄²⁻/TiO₂) photocatalysts with sulfate species on the surface which we reported to have good activities for the degradation of reverse osmosis concentrate (ROC) [8]. We then compare the efficiencies of SO₄²⁻/TiO₂ and Degussa P25 photocatalysts for the destruction of TA.

TA is also widely used as a probing molecule for the important OH radicals generated from photocatalysts [9], [10], [11]. Mason monitored the OH radicals generated from acoustic cavitation with TA by measuring the fluorescent intermediate, namely 2-hydroxyterephthalic acid (2-HTA) [12]. Ishibashi developed a fluorescence method for determining the quantum yield of OH radical production during TiO₂ photocatalysis [10], [11]. They used TANa (TA sodium salt) and coumarin to trap OH radicals to produce fluorescent intermediates, namely 2-HTA and 7-hydroxycoumarin. Since 2008, more researchers adopted the TA or TANa fluorescence probing method to determine OH radical formation and the photocatalytic activity of photocatalysts [9], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26]. Nine intermediates had been reported, including 4 aromatic and 5 aliphatic intermediates. The 4 reported aromatic intermediates were 2-HTA, 4-hydroxybenzoic acid (4-HBA), benzoquinone and benzene [5], [9], [10], [11]. The 5 reported aliphatic intermediates were formic, acetic, oxalic, maleic and fumaric acids [5], [7]. However, there had been no comprehensive study of intermediates to date. In addition, for different photocatalysts the TA degradation pathways might be different and generate different intermediates. Therefore, we investigated the intermediates produced from the photocatalytic degradation of TA by SO₄²⁻/TiO₂ photocatalysts compared with those by bare TiO₂ and P25. 5 new intermediates were discovered and identified through silylation method using GC/MS and fluorescence spectroscopy. These newly discovered intermediates are complementary to the previously discovered ones and might provide new insights to the mechanism of photocatalytic degradation of TA. In addition, the relative ratios of fluorescent intermediates might also shed some light on the photocatalytic activity and different mechanisms of photocatalysis.