Ti₂O₃/TiO₂-Assisted Solar Photocatalytic Degradation of 4-tert-Butylphenol in Water

Abstract

Colored Ti₂O₃ and Ti₂O₃/TiO₂ (mTiO) catalysts were prepared by the thermal treatment method. The effects of treatment temperature on the structure, surface area, morphology and optical properties of the as-prepared samples were investigated by XRD, BET, SEM, TEM, Raman and UV–VIS spectroscopies. Phase transformation from Ti₂O₃ to TiO₂ rutile and TiO₂ anatase to TiO₂ rutile increased with increasing treatment temperatures. The photocatalytic activities of thermally treated Ti₂O₃ and mTiO were evaluated in the photodegradation of 4-tert-butylphenol (4-t-BP) under solar light irradiation. mTiO heated at 650 °C exhibited the highest photocatalytic activity for the degradation and mineralization of 4-t-BP, being approximately 89.8% and 52.4%, respectively, after 150 min of irradiation. The effects of various water constituents, including anions (${\mathrm{CO}}_3^{2-}$, ${\mathrm{NO}}_3$, $\mathrm{Cl}$ and ${\mathrm{HCO}}_3^{-}$) and humic acid (HA), on the photocatalytic activity of mTiO-650 were evaluated. The results showed that the presence of carbonate and nitrate ions inhibited 4-t-BP photodegradation, while chloride and bicarbonate ions enhanced the photodegradation of 4-t-BP. As for HA, its effect on the degradation of 4-t-BP was dependent on the concentration. A low concentration of HA (1 mg/L) promoted the degradation of 4-t-BP from 89.8% to 92.4% by mTiO-650, but higher concentrations of HA (5 mg/L and 10 mg/L) had a negative effect.

1. Introduction

Water pollution by a broad category of organic pollutants is a rising issue of worldwide concern [1]. During the last decade, the consumption of personal care products (PPCPs), pharmaceuticals and endocrine-disrupting compounds (EDCs) has increased owing to economic development and population growth [2,3,4,5]. Their widespread use has increased their appearance in the aqueous environment, including rivers, lakes and reservoirs, at concentrations starting from several nanograms (ng/L) to several micrograms (µg/L) per liter [6,7,8,9,10,11,12]. They can even escape wastewater treatment plants (WWTPs) and drinking water treatment plants (DWTPs), ultimately reaching drinking water sources. These contaminants are termed emerging pollutants (EPs) and can cause severe adverse effects on human health and the aquatic environment [13].

In particular, 4-t-BP is an industrial chemical used as a raw material for the production of synthetic phenol and polycarbonate resins [14,15]. As a representative of EDCs, 4-t-BP has a high estrogenic effect and acute/chronic environmental toxicity [16,17]. Considering its adverse effects on human health and aquatic systems, 4-t-BP, as a highly persistent pollutant, needs to be controlled efficiently.

To date, various methods have been investigated to remove 4-t-BP from water, mainly including advanced oxidation processes (AOPs) and biological processes [16,18,19,20]. Among them, AOPs have attracted great attention for the removal such contaminants by converting them into carbon dioxide and water [21,22]. The high efficiency of the process has mostly been associated with the production of hydroxyl radicals (standard potential, 2.8 V) used as oxidants. AOPs may vary in terms of work conditions, used materials and different paths of hydroxyl radical (${\mathrm{OH}}^{.}$) production [23,24]. Heterogeneous photocatalysis may be considered an economically feasible solution to remove 4-t-BP from water due to the competitive cost of the process and the ambient conditions of temperature and pressure [25,26,27]. Moreover, it is considered an environmentally friendly oxidation process since it allows not only the degradation of the pollutant from the contaminated system but also its total elimination, without generating any undesired by-products, which could be even more toxic compounds than the parent one [28,29].

Although various photoactive materials have been investigated, TiO₂-based photocatalysts remain the most studied ones due to their high photocatalytic oxidation activity, chemical stability and availability [30,31,32]. The P25 form of TiO₂ is one of the most effective photocatalytic materials, which can be attributed to the combination of anatase and rutile phases [33,34,35]. However, the high energy band gap of approximately 3.0–3.2 eV limits the application of TiO₂ under solar light. In this context, numerous strategies have been devoted to extending the absorption wavelength to the visible area for the efficient utilization of sunlight. For example, the introduction of ${\mathrm{Ti}}^{3+}$ into TiO₂ demonstrated the capacity to extend the light response of TiO₂. It has been reported that the formation of ${\mathrm{Ti}}^{3+}$ species is accompanied by the generation of oxygen vacancies (O_v), which can favor the separation of electron–hole pairs and thus improve the visible light activity of TiO₂ [36,37,38]. Moreover, Ti³⁺ and oxygen vacancies can form localized states below the conduction band (CB), which reduces the band gap of TiO₂ (Figure 1), so that it can distinctly expand the absorption to the visible region [39,40,41,42,43]. The reported methods to prepare structurally defective TiO₂ with ${\mathrm{Ti}}^{3+}$ include the partial oxidation of low-valence Ti species (Ti, Ti (II) and Ti (III)), H₂ thermal treatment and the reduction of ${\mathrm{Ti}}^{4+}$ to ${\mathrm{Ti}}^{3+}$ by a chemical reducing agent (NaBH₄), metals (Al, Mg, Li, Zn), etc. [44,45,46,47,48,49]. Although there are numerous preparation methods available, most of them require high consumption of chemicals, as well as multiple steps using specialized equipment. Therefore, it is of significant importance to develop a facile and feasible method to prepare defective TiO₂ with ${\mathrm{Ti}}^{3+}$.

In the present work, Ti₂O₃ alone or in combination with TiO₂ (P25) was thermally treated through a simple one-step method, and their photocatalytic performance towards 4-t-BP degradation under simulated solar light was tested. The as-prepared samples were characterized by means of SEM, TEM, BET, XRD, Raman and UV–VIS spectroscopies to study their morphology, textural properties, crystal structure and optical properties. The effects of the presence of humic acid (HA) and inorganic ions (${\mathrm{CO}}_3^{2-}$, ${\mathrm{NO}}_3^{-}$, ${\mathrm{Cl}}^{-}$ and ${\mathrm{HCO}}_3^{-}$) on 4-t-BP degradation were also investigated.

2. Results and Discussion

2.1. Characterization of Photocatalysts

XRD measurements were conducted to identify the phase structures of the thermally treated Ti₂O₃ and Ti₂O₃/TiO₂ (hereinafter denoted as mTiO) catalysts (Figure 2 and Figure 3). The diffraction peaks at 2θ = 23.823° (012), 33.040° (104), 34.836° (110), 40.219° (113), 48.786° (024), 53.692° (116), 61.42° (214) and 62.64° (300) were attributed to Ti₂O₃ (JCPDS No. 00-043-1033). With the increase in treatment temperature, the intensity of all characteristic peaks corresponding to Ti₂O₃ became weaker in both Ti₂O₃ and mTiO samples. As the temperature further increased to 750 °C, no typical peaks of Ti₂O₃ were observed, indicating the complete transformation of Ti₂O₃ to rutile TiO₂ (JCPDS No. 00-021-1276) [50,51].

In the Ti₂O₃-550 sample (Figure 2), apart from the diffraction peaks of TiO₂ rutile, peaks attributed to the anatase phase of TiO₂ (JCPDS No. 00-021-12-72) also appeared at 2θ = 25.3° (101) and 48.028° (200). These findings reveal that the transformation of Ti₂O₃ into TiO₂ anatase also took place. The results are in good agreement with previously reported ones [52]. These peaks almost completely disappeared at 900 °C, suggesting the transformation of TiO₂ anatase into TiO₂ rutile.

The composition of the catalysts was further investigated by Raman spectroscopy (Figure 4 and Figure 5). The Raman peak at around 143 cm⁻¹ justified the existence of the TiO₂ anatase phase in both types of catalysts. For treated Ti₂O₃ (Figure 4), this peak became more intense with the increase in treatment temperature to 750 °C, confirming the successful transformation of Ti₂O₃ into TiO₂ anatase. In addition, low-intensity peaks corresponding to the TiO₂ anatase phase were observed at 196.85 cm⁻¹, 399.57 cm⁻¹ and 514.54 cm⁻¹ in the spectra of Ti₂O₃-650 and Ti₂O₃-750, while a further increase in temperature to 900 °C led to an increase in the TiO₂ rutile phase. However, no peaks were observed corresponding to Ti₂O₃, which could be attributed to the low intensities of the Raman bands of the Ti₂O₃ structure.

Compared to untreated mTiO, mTiO-900 exhibited a negative shift at 143 cm⁻¹ (Figure 5), indicating the association of Ti₂O₃ with TiO₂, while, for mTiO-550 and mTiO-650, a positive shift in this peak could possibly be attributed to the introduction of Ti³⁺ and oxygen vacancies into the TiO₂ lattice as a result of the thermal treatment [53]. In the photocatalytic process, the presence of such structural defects in TiO₂ can inhibit the recombination of charge carriers and thus improve the photocatalytic activity.

The results from Raman spectroscopy are in general agreement with the ones obtained from XRD analysis, with the exception of the TiO₂ anatase phase, which was detected only with the first technique for the catalysts treated at 900 °C.

The textural properties of all catalysts were evaluated by BET N₂ adsorption/desorption measurements. As presented in Figure 6 and Figure 7, all the catalysts revealed a typical type-III isotherm according to the classification of the international union of pure and applied chemistry (IUPAC). Interestingly, Ti₂O₃ and mTiO catalysts heated at 650 °C exhibited the highest N₂ adsorption capacity and pore volume (V_p). In general, larger values of V_p can be beneficial for the photocatalytic reaction through providing ionic diffusion and charge transfer on the surface of the photocatalyst [54].

Some other characteristics obtained from the BET analysis are displayed in

Table 1. BET surface area and pore volume of as-prepared catalysts.

Photocatalyst	SBET (m2/g)	Vp (cm3/g)
Ti2O3-550	1.629	0.009
Ti2O3-650	1.985	0.017
Ti2O3-750	2.733	0.014
Ti2O3-900	0.974	0.012
mTiO-550	23.012	0.255
mTiO-650	20.894	0.347
mTiO-750	5.593	0.134
mTiO-900	3.443	0.029

, which shows that the treatment temperature had a significant effect on the microstructure of thermally treated Ti₂O₃ and mTiO, particularly on the BET surface area (S_BET) and pore volume (Vp). It could be noticed that the S_BET of treated Ti₂O₃ was relatively low compared to that of mTiO. The S_BET of treated Ti₂O₃ catalysts increased gradually as the treatment temperature increased from 550 °C to 750 °C, which could be likely associated with the formation of a better crystalline framework. However, a further increase in the treatment temperature to 900 °C caused a drastic decrease in S_BET due to the phase transformation of TiO₂ anatase to TiO₂ rutile [55].

The increase in treatment temperature continuously decreased the S_BET of treated mTiO. The lowering of S_BET can be attributed to the increase in particle size as a result of aggregation [56].

The morphology of the prepared catalysts was examined by SEM and TEM. As can be seen from Figure 8, untreated Ti₂O₃ particles exhibited an irregular shape with a smooth continuous morphology. In contrast, heating under different temperatures resulted in the formation of a much rougher surface of Ti₂O₃, which could be associated with the phase transformation from Ti₂O₃ to TiO₂ rutile. Such an increase in surface roughness can increase the surface area of the catalyst and further influence the catalytic activity of the material. These results are consistent with the findings obtained from BET analysis, where the heating of Ti₂O₃ up to 750 °C was accompanied by an increase in S_BET.

SEM images of treated mTiO catalysts clearly revealed that thermal treatment caused a particle size growth in TiO₂, well-distributed on the surface of Ti₂O₃ (Figure 9). The increase in the size of TiO₂ particles may have resulted in the decrease in S_BET.

A UV–VIS absorption study was carried out to assess the light-harvesting ability of the prepared samples (Figure 10 and Figure 11). It can be seen that the temperature variation influenced the light absorption properties of all prepared catalysts. The rise in treatment temperature for Ti₂O₃ catalysts from 550 °C to 650 °C extended the light absorption to the visible region (400–550 nm), while a further increase in the treatment temperature to 900 °C lowered the visible light absorption capacity (Figure 10).

In contrast, all prepared mTiO catalysts demonstrated a good light absorption ability within the wavelength range of 300−400 nm, although to different extents (Figure 11). Specifically, catalysts heated at lower temperatures demonstrated stronger absorption, which is in accordance with the expectations based on the color change of the catalysts. At the same time, mTiO-550 and mTiO-650 were found to be absorbing in the 400–550 nm region. This phenomenon may be attributed to the transformation of Ti₂O₃ to TiO₂ rutile, containing Ti³⁺ (oxygen vacancies) sites [50,56]. It is noteworthy that the light absorption of mTiO-650 in the visible region was substantially enhanced compared with mTiO-550, as a result of the higher concentration of oxygen vacancies in the lattice of TiO₂. Such an enhancement in the light absorption is favorable for improving the photoactivity of the material. On the other hand, the intensities at wavelengths higher than approximately 550 nm gradually weakened for mTiO-550 and mTiO-650.

Moreover, the band gap values of the prepared catalysts were estimated using the Kubelka–Munk equation and the corresponding Tauc plots. As illustrated in Figure 12, the calculated direct band gap energies were found to be 1.76, 1.75, 1.79 and 2.69 eV for Ti₂O₃-550, Ti₂O₃-650, Ti₂O₃-750 and Ti₂O₃-900, respectively. Similar variations in band gap energies were obtained for mTiO catalysts, where mTiO-650 had a lower band gap of 2.01 eV compared to mTiO-550, mTiO-750 and mTiO-900 (Figure 13). These results reveal the possible application of the prepared catalysts in solar-light-driven photocatalytic reactions.

2.2. Photocatalytic Degradation of 4-t-BP in Aqueous Solution

The photocatalytic activity of all the prepared catalysts was evaluated by the photodegradation of 4-t-BP under solar light irradiation, and the results are shown in Figure 14. In the absence of a catalyst, the decomposition of 4-t-BP observed after 150 min of irradiation was only 8.3%.

Treated Ti₂O₃ showed low photocatalytic activity and approximately 13%, 12.7%, 10.5% and 14% of 4-t-BP was decomposed by Ti₂O₃-550, Ti₂O₃-650, Ti₂O₃-750 and Ti₂O₃-900, respectively, after 150 min of irradiation. In contrast, mTiO exhibited much higher photocatalytic degradation efficiency. Among mTiO catalysts, mTiO-650 showed the highest photocatalytic activity, achieving 89.8% of 4-t-BP degradation. These findings are consistent with the results obtained from physico-chemical characterization, where mTiO-650 exhibited better optical properties and a lower band gap and pore volume as compared to mTiO-550, mTiO-750 and mTiO-900, indicating the importance of the treatment temperature on the optical properties and photocatalytic activity of the catalyst [57].

The mineralization efficiency of a photocatalyst is an important indicator for assessing its practical application. Thus, the mineralization of 4-t-BP was evaluated via total organic carbon (TOC) measurements (Figure 15). As in the case of 4-t-BP photodegradation, the treated mTiO catalysts exhibited higher TOC removal than Ti₂O₃ catalysts. In particular, 54.2% of TOC removal was obtained in 150 min using mTiO-650 under solar light irradiation. In the same reaction time, 32.5%, 12.4%, 12.2%, 11.2%, 10%, 7.8% and 5.4% of TOC removal was obtained for mTiO-550, mTiO-900, mTiO-750, Ti₂O₃-750, Ti₂O₃-900, Ti₂O₃-650 and Ti₂O₃-550. The observed photocatalytic efficiency of the catalysts tested for TOC removal was in accordance with the 4-t-BP photodegradation results.

2.3. Effect of HA and Coexisting Ions (${\mathrm{CO}}_3^{2-}$, ${\mathrm{NO}}_3^{-}$, ${\mathrm{Cl}}^{-}$ and ${\mathrm{HCO}}_3^{-}$) on the Degradation of 4-t-BP

The widespread water constituents in wastewater, including natural organic matter (NOM) and inorganic ions (${\mathrm{CO}}_3^{2-}$, ${\mathrm{NO}}_3^{-}$, ${\mathrm{Cl}}^{-}$ and ${\mathrm{HCO}}_3^{-}$), could significantly affect the performance of the reaction system towards the degradation and mineralization of the target pollutants [58,59].

NOM is considered an integral part of natural water bodies and wastewater, and it is mainly composed of humic compounds and proteins [60,61]. In this study, HA was used as a model NOM compound and the effects of different concentrations of HA (1 mg/L, 5 mg/L and 10 mg/L) on the degradation of 4-t-BP were investigated. As shown in Figure 16, the presence of HA in the mTiO-650/solar light system could promote or hinder the degradation of 4-t-BP, depending on its concentration. The presence of a relatively low concentration (1 mg/L) of HA increased the degradation efficiency of 4-t-BP from 89.8% to 92.4%, while higher concentrations (5mg/L and 10 mg/L) of HA decreased the degradation efficiency of 4-t-BP to 84.6% and 70.8%, respectively. The enhanced degradation of 4-t-BP in the presence of HA was also observed for the degradation of Bisphenol A [62] and dimethoate [63] by TiO₂ photocatalytic degradation. The positive effect of HA at low concentrations might be ascribed to the photosensitization of HA, which would produce extra electrons, leading to an improvement in the photocatalytic degradation of organic pollutants [62,64,65]. On the other hand, at higher concentrations, HA adsorbed on the surface of the catalyst could compete with 4-t-BP for active sites, resulting in a reduction in degradation efficiency [66,67].

The presence of ${\mathrm{CO}}_3^{2-}$, ${\mathrm{NO}}_3^{-}$, ${\mathrm{Cl}}^{-}$ and ${\mathrm{HCO}}_3^{-}$ anions in the concentration of 100 mg/L had dual effects on the degradation of 4-t-BP over mTiO-650. ${\mathrm{CO}}_3^{2-}$ and ${\mathrm{NO}}_3^{-}$ ions resulted in a certain degree of negative effect with respect to the degradation of 4-t-BP. As shown in Figure 17, the 4-t-BP degradation decreased from 89.8% to 87.3% and 70.3% in the presence of nitrate and carbonate, respectively. The inhibition effect of ${\mathrm{CO}}_3^{2-}$ and ${\mathrm{NO}}_3^{-}$ was due to: (1) the quenching of oxidizing species, such as hydroxyl radicals (${\mathrm{OH}}^{.}$), and positive holes (${\mathrm{h}}^{+}$) by anions (Equations (1)–(3)); (2) anions could compete with 4-t-BP molecules for the available active sites of the catalyst surface, which further affects the degradation process [68,69,70,71]. Several studies have highlighted that ${\mathrm{NO}}_3^{-}$ ions are usually weakly adsorbed on the surface of the catalyst and, thus, they slightly inhibit photodegradation reactions [69,72].

$${\mathrm{CO}}_3^{2-}+{\mathrm{OH}}^{.}\to {\mathrm{OH}}^{-}+{\mathrm{CO}}_3^{-}$$

(1)

$${\mathrm{NO}}_3^{-}+{\mathrm{h}}_{\mathrm{VB}}^{+}\to {\mathrm{NO}}_3^{.}$$

(2)

$${\mathrm{NO}}_3^{-}+{\mathrm{OH}}^{.}\to {\mathrm{NO}}_3^{.}+{\mathrm{OH}}^{-}$$

(3)

It is noteworthy that both ${\mathrm{Cl}}^{-}$ and ${\mathrm{HCO}}_3^{-}$ accelerated the degradation of 4-t-BP by mTiO-650. The addition of ${\mathrm{Cl}}^{-}$ and ${\mathrm{HCO}}_3^{-}$ to the system resulted in 92.5% and 100% degradation after 150 min of solar light irradiation. ${\mathrm{Cl}}^{-}$ anions reacting with hydroxyl radicals can produce ${\mathrm{ClOH}}^{.-}$ and subsequently transform into ${\mathrm{Cl}}^{.}$ (Equations (4) and (5)) [73]. The generated active chlorine species can selectively attack electron-rich organic compounds [74]. The complete 4-t-BP degradation in the presence of ${\mathrm{HCO}}_3^{-}$ could be most likely attributed to the generated alkaline condition or the formation of more selective radicals (${\mathrm{CO}}_3^{.-}$) by the reaction of ${\mathrm{HCO}}_3^{-}$ with ${\mathrm{OH}}^{.}$ (Equation (6)), which can promote the degradation of 4-t-BP [69,75].

$${\mathrm{Cl}}^{-}+{\mathrm{OH}}^{.}\to {\mathrm{OH}}^{-}+{\mathrm{ClOH}}^{.-}$$

(4)

$${\mathrm{H}}^{+}+{\mathrm{ClOH}}^{.-}\to {\mathrm{Cl}}^{.}+{\mathrm{H}}_2\mathrm{O}$$

(5)

$${\mathrm{HCO}}_3^{-}+{\mathrm{OH}}^{.}\to {\mathrm{CO}}_3^{.-}+{\mathrm{H}}_2\mathrm{O}$$

(6)

3. Materials and Methods

3.1. Materials

The 4-t-BP (99%), titanium (IV) oxide (TiO₂, nanopowder, 21 nm primary particle size, purity ≥ 99.5% trace metals basis, P25), titanium (III) oxide (Ti₂O₃, 100 mesh, 99.9% trace metals basis), Na₂CO₃, NaNO₃, NaCl, NaHCO₃ and HA were purchased from Sigma Aldrich. All chemicals were of analytical grade and used as purchased. All aqueous solutions were prepared with ultrapure water (UPW) using a Milli-Q System (18.2 MΩ. cm).

3.2. Preparation of the Photocatalysts

The preparation process was the same for both Ti₂O₃ alone and mTiO. At first, Ti₂O₃ or a mixture of Ti₂O₃ and TiO₂ was crushed into a fine powder and then heated in a muffle furnace at 550 °C, 650 °C, 750 °C or 900 °C for 3 h in air. For mTiO, the weight ratio between Ti₂O₃ and TiO₂ was 1:1. The final products were denoted as Ti₂O₃-X and mTiO-X (with X being the temperature of the thermal treatment), respectively. The detailed synthesis process is illustrated in Figure 18.

3.3. Characterization of the Photocatalysts

The crystallographic properties and XRD patterns of the prepared catalysts were acquired at 2θ of 20–80° on an X-ray diffraction (XRD, Rigaku Smartlab) system. Raman spectra were recorded with the help of a Raman spectrometer (Horiba, LabRam HR evolution) and excitation energy was λ = 532 nm. Textural properties, including specific surface area (S_BET) and pore volume (V_p), were measured using an automated gas sorption analyzer (Autosorb iQ, Quantachrome, Boynton Beach, FL, USA) by the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) method, respectively. Surface morphology was observed by a Scanning Electron Microscope (SEM, Auriga CrossBeam 540, Carl Zeiss) and Transmission Electron Microscope (TEM, JEOL JEM-1400 Plus). UV–VIS spectroscopy of samples was implemented on a Thermo Scientific Genesys 150 UV–Visible spectrophotometer.

3.4. Photodegradation Tests

The photocatalytic performance of the prepared catalysts was evaluated through the experiments of 4-t-BP degradation under simulated solar light irradiation. First, 100 mg of catalyst was added to 500 mL 4-t-BP (5 mg/L) aqueous solution. Prior to irradiation, the mixture was kept in the dark for 15 min under stirring to reach the adsorption/desorption equilibrium. Then, while stirring, the suspension was exposed to the simulated solar irradiation produced by a 100 W Xenon lamp with an AM1.5G filter (LCS-100 solar simulator). During the experiment, 20 mL of reaction solution was extracted at regular time intervals and filtered by a 0.22 µm Millex syringe filter to remove the photocatalyst for further analysis.

The concentration of 4-t-BP was analyzed by a high-performance liquid chromatography instrument (HPLC, Agilent 1290 Infinity II, Santa Clara, CA, USA) equipped with an SB-C8 column (2.1 mm × 100 mm, 1.8 µm). The mobile phase composition was methanol and UPW (50:50, v/v), which were mixed to compose the mobile phase. The mineralization of 4-t-BP solution was monitored from the decay of TOC content, measured by a TOC analyzer (Multi N/C 3100, Analytic Jena, Jena, Germany).

4. Conclusions

In summary, Ti₂O₃ and mTiO photocatalysts were prepared via a one-step synthesis method and further characterized by different tests. The effect of treatment temperature on the physicochemical properties and photocatalytic performance of the prepared catalysts in the degradation of 4-t-BP under simulated solar light irradiation was investigated. Based on the results obtained, the increase in treatment temperature from 550 °C to 650 °C caused an increase in the pore volume and enhanced light absorbance in the visible region (400–550 nm) for both Ti₂O₃ and mTiO photocatalysts. The improved textural and optical properties related to the anatase to rutile ratio and specific surface area contributed to the enhanced performance of mTiO-650, which exhibited the highest photocatalytic activity at the dosage of 0.2 mg/L, achieving 89.8% degradation and 54.2% mineralization of 4-t-BP after 150 min. The effect of treatment temperature on the catalytic performance of the treated Ti₂O₃ catalysts was almost negligible and resulted in 13%, 12.7%, 10.5% and 14% 4-t-BP degradation by Ti₂O₃-550, Ti₂O₃-650, Ti₂O₃-750 and Ti₂O₃-900, respectively. In addition, the effects of the presence of HA and various inorganic ions, including ${\mathrm{CO}}_3^{2-}$, ${\mathrm{NO}}_3^{-}$, ${\mathrm{Cl}}^{-}$ and ${\mathrm{HCO}}_3^{-}$ on the photodegradation of 4-t-BP by mTiO-650 were also studied. At relatively low concentrations, HA could act as a photosensitizer and therefore promoted the degradation of 4-t-BP, whereas higher concentrations inhibited the degradation. The presence of ${\mathrm{Cl}}^{-}$ and ${\mathrm{HCO}}_3^{-}$ exhibited a positive influence on 4-t-BP degradation, resulting from the favorable formation of additional reactive species, while the presence of ${\mathrm{NO}}_3^{-}$ and ${\mathrm{CO}}_3^{2-}$ slightly inhibited the reaction.