3.1. Effects of experimental factors for Ce-doped TiO2 on 2-CP degradations
Effects of three factors, such as (a) calcination temperature, (b) amount of dopant, and (c) amount of nitric acid added on Ce-doped TiO2 for the degradation of 2-CP under visible light irradiation were shown in Fig. 1. For the Ce-doped TiO2 photocatalysts synthesized by hydrothermal method, cerium amount of 0.35 mol.%, nitric acid amount of 0.15 vol.% and calcined at 600oC, the optimum degradation of 99.1% was obtained (refer to Table 1). Although three factors all showed positive effects with their increasing levels in Fig. 1, it is more significant of the first two factors (A and B) for Ce-doped TiO2 than the third (C). The preliminary investigations were helpful for material scientists to fabricate a more suitable photocatalyst to degrade hazardous materials, such as 2-CP, in this case.
3.2. Effects of synthesized Ce-doped TiO2 and its operational parameters on 2-CP degradations
The Ce-doped TiO2 made by hydrothermal method from previous optimized condition, was further compared to a commercial TiO2 (P25) and undoped TiO2 made by both methods. This is to assess the contribution of cerium doping on the 2-CP degradation efficiency of TiO2 under visible light irradiation. As shown in Fig. 2a, the P25 TiO2 showed the lowest degradation ratio of 2-CP (38.0%) during the continuous monitoring up to 7 hours (420 min). The best fitting of 2-CP degradation by P25 was a polynomial quadratic equation with a R2 of 0.9990 the blue light LED irradiation. The TiO2 photocatalyst made by sol-gel method without doping by cerium had a better of 2-CP degradation at approximately 88.8% during the 7 hours irradiation. Similar trend of 2-CP degradation utilized by the undoped TiO2 photocatalyst synthesized via hydrothermal method found to be more efficient on the degradation of 2-CP (93.1%) with the same time duration. Both undoped TiO2, synthesized by both methods fitted well with exponential decay double equations with five parameters, whereas respective R2 were 0.9930 and 0.9992 for sol-gel and hydrothermal methods, respectively. The most promising degradation (99.1%) at 5 hours was exhibited by utilizing the Ce-doped TiO2 (0.28 mol.%) made via hydrothermal method. An exponential decay double equation with three parameters was found to be interpreted the decay behavior well (R2 of 0.9945). Therefore, the hydrothermal method was confirmed as the most feasible synthetic approach for Ce-doped TiO2 and the same doping amount (0.28 mol.%) was applied in the subsequent studies.
Kosmulski and Tolosa et al. (2011) mentioned the pH-dependent surface charging and pH of point of zero charges (pHpzc) [64, 65] can be a reference in searching of optimal pH either for homogeneous or heterogeneous photocatalysis systems. As the pHpzc were critical in determining the operational pH in wastewater treatment, pHpzc of the pristine (undoped) and Ce-doped TiO2 (0.28 mol.%) was determined and shown in Fig. 2b. The former pHpzc of the undoped TiO2 (3.51) was consistent with previously study [40, 51]. Also, the latter pHpzc of 2.83 for the Ce-doped TiO2 measured in this study was close to TiO2 doped with other dopants, e.g. CuSO4-doped TiO2 (pHpzc=3.84)  and KAl(SO4)2-doped TiO2 (pHpzc=1.90~3.39) .
As photocatalytic degradation efficiency of specific contaminants was affected by actual conditions of the wastewater streams, insights of the effects of operational parameters were examined intensively here. Detailed investigating ranges of three parameters (initial pH, catalyst dosages of Ce-doped TiO2 and initial 2-CP concentrations) that utilizes the previous optimized Ce-doped TiO2 (0.28 mol %, calcined at 600 oC) were listed in Table 2. The residual concentration of 2-CP under visible light irradiation at five initially conditioned pHs were all gradually decreased over time, as displayed in Fig. 3a. The solutions conditioned to neutral (pH 7) and slightly acidic (pH 5.5) performed better than the other three set pH. It can be shown that at initial pH of 5.5 and 7.0, the degradation of 2-CP at the end of 4-hour irradiation is approximately 100%. Highly acidic conditions with the pH of 3.0 and 2.0 have a good removal efficiency also during the 4-hour degradation of 2-CP which is about 83.9% and 85.8%, respectively. It can be seen in Fig. 3a that the degradation profiles with respect to time of pH 3.0 and 2.0 were quite close to each other. Moreover, the solution conditioned to basic (pH 9.0) yielded the lowest degradation efficiency of 2-CP. Aggregation of TiO2 particles occur as the conditioned pH approaches to pHpzc, while it tends to stabilize at both higher and lower pH conditions [64, 65].
For positively charged surface, pH< pHpzc:
For negatively charged surface, pH> pHpzc:
TiO2 + nOH− ↔ TiO2(OH)n−n (3)
From previously determined pHpzc of the Ce-doped TiO2 at 2.83, the surface charge of the Ce-doped TiO2 in the extreme low acidic condition (pH 2.0) was positive. But applying the photocatalyst at pH 3.0 that is very close to its pHpzc, the charge effects may not be significant. As a result, the degradation profiles of pH 3.0 and 2.0 were overlapping, as displayed in Fig. 3a. The best 2-CP degradations in aqueous solutions conditioned to neutral may contributed to the surface charge of Ce-doped TiO2 which became negative and the pKa of 2-CP is 8.56. Similar results of pH-dependent in the degradation of 4-CP were observed by Silva et al.  whereas they found main deactivation mechanism of Ce-doped TiO2 is ceria loss from the catalyst surface during reaction.
The effects various dosages of the Ce-doped TiO2 catalyst that utilized for 2-CP degradations under visible light as shown in Fig. 3b were also investigated. Increasing the photocatalyst dosage from 1 to 3 g·L-1 increased the number of active sites available for surface adsorption and reaction. This results to more OH and·O2- radicals generated to facilitate photocatalytic activity that gives final removal in 4-hour duration of 66.7%, 85.2% and 99.9% shown in Fig. 3b. However, further increasing the Ce-doped TiO2 dosage from 4 to 5 g·L-1, decreases the 2-CP degradation efficiency of 97.0 % and 70.4%, respectively. This phenomenon can be associated with the overcrowding catalysts that could block the light absorption on the catalyst surfaces . The same trends were also found in our previous studies for CuSO4-doped TiO2 catalysts . Effects of initial 2-CP concentration on the degradation performance were also carried out from 10 to 50 mg·L-1 shown in Fig. 3c. Unsurprisingly, higher initial 2-CP concentration yielded lower degradations. This result is obvious since this can be associated with the pore blocking and multi-layer adsorption in the catalysts surface which will limit the release of the OH- and O2- radicals . In summary, optimal degradation of 2-CP was achieved at the dosage of Ce–doped TiO2 catalysts, initial pH and initial 2-CP concentration of 33g, 7.0 and 10·mg·L-1, respectively. Such results would be helpful in practical operation of this system in wastewater treatment facility.
3.3. Characterizations of Ce-doped TiO2 synthesized at various calcination temperatures and doping amounts
As catalysts calcined at various temperatures possess various properties and may affects their photocatalytic degradation efficiencies under visible light irradiation. Characterizations of the Ce-doped TiO2 photocatalyst were conducted by Brunauer–Emmett–Teller (BET), Langmuir, t-plot external and single point methods to measure its surface area, pore volume and pore size. As shown in Table 3, it can be observed no matter which methods analyzed, surface area of the Ce-doped TiO2 photocatalyst generally decreased with the increasing calcination temperatures from 200 to 500 oC. However, there is a different trend in the result observed in 600 oC calcination temperature where the surface area increases. The occurrence is also evident with the pore volume and pore size measurements of the Ce-doped TiO2 photocatalyst. As all surface areas characterized by various methods consistently showed that the Ce-doped TiO2 calcined at 600 oC were higher than that calcined at 200 oC, we can concluded that structures of the Ce-doped TiO2 was not the sole factor affecting the photocatalytic degradation of 2-CP. Instead, the cerium doping amount played a certain role as shown in the previous data in the Section 3.1 and these characterizations obtained here. Tong et al.  have prepared Ce–TiO2 catalysts by controlled hydrolysis of titanium alkoxide based on esterification reaction followed by hydrothermal treatment. They doped various cerium amounts (0.1, 0.2, 0.4, 0.6 and 1.0 wt.%) into TiO2 by the controlled hydrolysis and calcined at 460oC (733 K) can be a reference as well. As shown in Table 3, the Ce-doped TiO2 calcined at 400 oC registered the highest pore volume of 0.2208 cm3/g among the five (5) calcination temperatures. Although there is no clear relationships between calcination temperatures and pore volumes, it was noted that there is significant correlation between small particle size, large surface area and pore volume. Consequently, calcined at either middle temperatures (e.g. 300, 400 oC or in between) can be a feasible option for future large-scale production the Ce-doped TiO2 catalysts.
Morphology of the undoped and the TiO2 doped with low, medium and high amounts of cerium (from 0.07, 0.28 to 0.35 mol.%) can be observed by scanning electron micrographs (SEM) shown in Fig. 4. All SEM images were taken at magnitude of 1,000 times. A more uniform distribution of spherical particles was obtained in the undoped TiO2 as shown in Fig. 4a, while irregular shapes of crystals were obtained in the Ce-doped TiO2 (Fig. 4b, 4c and 4d) regardless of doping amounts added. Even the diverse distributions of the particles of Ce-doped TiO2 revealed by the SEM images. It can also see that the presence of Cerium in the preparation of the catalysts modify the surface morphology of the TiO2 which means that might affect the light absorption of the catalysts. The main effects on performance of 2-CP degradation were affected by their specific surface area.