Visible-Light Degradation of 2-Chlorophenol by TiO2 doped with Neighboring Transition Metal Cerium via Various Approaches and Operation Parameters

TiO 2 -related materials or processes for 2-chlorophenol (2-CP) degradation either under UV or visible light irradiations with key operational conditions was systematically reviewed in the beginning of this study. Cerium (Ce), which was neighboring transition metal elements of titanium (Ti), were individually doped with TiO 2 via various methods. Three synthetic parameters in the two approaches were examined their signi�cance by using experimental designs. It was found that the 2-CP can be 100% removal within 4-h irradiation by visible light in the synthetic condition of 0.35 mol.% Ce, 0.15 vol.% nitric acid and calcined at 600 o C. Moreover, effects of three operation parameters of the as-prepared catalysts were further investigated. The most e�cient condition obtained was 3 g·L − 1 catalysts at initial pH and 2-CP concentration of 7 and 10 mg·L − 1 , respectively. A critical parameter, pHpzc of undoped and Ce-doped TiO 2 , were also determined. In addition, surface area, pore volume and size of both TiO 2 –based catalysts were found affected by the calcination temperatures and consequently degradation e�ciency. The presenting results and mini-review were facilitated the development and applications of TiO 2 in the degradation of 2-CP under lower energy radiations.


Introduction
Chlorophenols (CPs) are listed as priority toxic chemicals [1] and considered to be carcinogenic, mutagenic, low biodegradable and di cult to remove by conventional wastewater treatment processes [2]. They are commonly used as precusor to chemical prducts such as pharmaceuticals, synthetic dyes, biocides, paints, textiles, leather products and wood preservatives [3]. Considering public health risks and ecological impacts, it may address the attentions as chlorophenols release from industrial wastes incineration, residual pesticides and petroleum re ning e uents. Toxicological pro les of CPs and their derivatives were recently evaluated [3,4] as well as their environmental fates and transformations were addressed [5]. Di-chlorophenol (2-CP) is extensively used as precursors of the higher-substituted CPs or being generated as a by-product from plastic, papermaking, insecticidal and petrochemical industries.
Due to its high solubility in water (28 g·L -1 ) at room temperature, concentrations of 2-CP released in natural aquatic environment have been reported to be 103-164 ng·L -1 in Lake Balaton, 31 ng·L -1 in River Danube, Hungary [6], average of 82±20 ng·L -1 in River Pearl, Guangzhou [7] and 6 ng·L -1 in Taihu Lake, China [8]. Five types of phenols have been identi ed in e uents of ve sewage treatment plants [2].
In order to reduce energy cost, directly applying of lower energy demand such as the renewable (sun) or the visible lights as driving sources of photocatlysis were prefered instead of UV [29][30]. The band-gap of commercially available pristine TiO 2 is large and only small fraction of the solar spectrum can be utilized [31]. Consistent efforts to improve visible light activity (VLA) of TiO 2 were diligently worked out [32].
However, there is no detailed comparison of actual 2-CP degradation e ciency under diversed application of irradiations and conditions studied based in literatures. Various ion doping with TiO 2 using different methods of synthesis were the popular approach to enhance its VLA [32,33]. Pristine commercial TiO 2 has been studied for its degradation e ciencies to 2-CP using a variety of parameters from early 1990s [19]. Since then various degradation conditions such as the initial 2-CP concentrations, types of substituted phenols, UV irradiation wavelengths, dissolved oxygen with/out applied external bias voltage, pH and presence of another semiconductor have been investigated [20][21][22]24]. Process integrations with TiO 2 photocatalysis and adsorption have been evaluated for 2-CP removal e ciency [25]. Adsorption of 2-CP by organo-clay combined with Degussa P25 under UV irradiations was also undertaken [26].
Moreover, biological process was also integrated with the commercial TiO 2 under the absence of light and natural sunlight irradiations, whereas the study of complete removal by the sole photocatalysis and by integrated process were achieved after 11 and 3 hours, respectively [11].
Apart from process integration with the commercial TiO 2 (Degussa P25), more researchers take approaches to enhance 2-CP degradation e ciencies and VLA of the most popular semiconductor by direct materials modi cation. Buzby et al., (2006)  . There were studies that modi ed the surface of TiO 2 by doping with Co(III) while others tend to doped it with triple elements [39] [40]. Nanomaterials, such as reduced graphene oxide, carbon nanotube, In 2 O 3 and InVO 4 were used to dope TiO 2 as well as Fe 3 O 4 /SiO 2 /TiO 2 core-shell-shell nanoparticles for 2-CP removal [41][42][43][44][45]. A V 2 O 5 -doped TiO 2 catalyst prepared by impregnation method was used to convert two gaseous 2-CP isomers and obtained completely conversion to CO 2 at 270 o C [46]. Pt-doped TiO 2 prepared by immersed coating on Ti plate and Ag-doped TiO 2 nano bers made by sol-gel and electrospinning was synthesized for the same purpose [47][48]. Ga, Ico-doped TiO 2 was also synthesized for the degradation of 2-CP in aqueous solution [49]. Elsalamony and Mahmoud, (2017) doped TiO 2 with ruthenium and yielded 98% degradation of 2-CP directly under UV light irradiation [50]. Lin et al., (2018) applied CuSO 4 -doped TiO 2 to degrade 2-CP and obtained 100% removal after 6h [51].
Apart from TiO 2 , some nanocomposites also used as photocatalysts to degrade 2-CP. This includes Cunano zeolite was demonstrated excellent adsorption capability in 2-CP reduction in real wastewater (81.8%) with 150 min as well and e cient in the laboratory experiment with an optimized pH of 6 [52]. Thus, in this study, Ce-doped TiO 2 were synthesized and evaluated its performance via photocatalytic degradation of 2-CP. Moreover, three operational parameters were used to evaluate its photocatalytic degradation performance on 2-CP under simulated visible light irradiation. Part of the study is to determine and characterize the synthesized TiO 2 calcined at ve temperatures including the pH pzc of the pristine and Ce-doped TiO 2 . Based on these experiments, utilizations of the doped TiO 2 could not only contribute to e cient photocatalytic degradation of 2-CP from materials modi cation aspects but also facilitated real applications in wastewater treatment processes.

Chemicals and doping methods
In this study, Ce-doped TiO 2 catalysts were made by two methods, a sol-gel and a hydrothermal method. were pulverized and then calcined at various temperatures with a heating rate of 5 o C/min. The second method as reported by Elsalamony and Mahmoud, (2017) was applied to synthesize pristine TiO 2 to compare with Ce-doped made by the rst method [50]. The sample names were given named after individual calcination temperatures with digits after the doping elements thereafter.

Experimental designs, set-up and analytic methods
Design of experiments based on 2 3 full factorial design (FFD) was utilized the same approach with previous study [51]. Two levels (-1 and +1) of three factors for Ce-doped TiO 2 were shown in Table 1. The responses (Y) obtained by individual photocatalytic degradation experiments were calculated based on below equation: where C 0 and C t represent initial and residual concentrations of 2-CP (mg·L -1 ) at the 4 th h. The experimental set-up, sampling procedure, analyzed instruments and method of pH pzc measurements were the same as previously reported [51]. was obtained (refer to Table 1). Although three factors all showed positive effects with their increasing levels in Fig. 1, it is more signi cant of the rst two factors (A and B) for Ce-doped TiO 2 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.

Effects of synthesized Ce-doped TiO2 and its operational parameters on 2-CP degradations
The Ce-doped TiO 2 made by hydrothermal method from previous optimized condition, was further compared to a commercial TiO 2 (P25) and undoped TiO 2 made by both methods. This is to assess the contribution of cerium doping on the 2-CP degradation e ciency of TiO 2 under visible light irradiation. As shown in Fig. 2a can be a reference in searching of optimal pH either for homogeneous or heterogeneous photocatalysis systems. As the pH pzc were critical in determining the operational pH in wastewater treatment, pH pzc of the pristine (undoped) and Ce-doped TiO 2 (0.28 mol.%) was determined and shown in Fig. 2b. The former pH pzc of the undoped TiO 2 (3.51) was consistent with previously study [40,51]. Also, the latter pH pzc of 2.83 for the Ce-doped TiO 2 measured in this study was close to TiO 2 doped with other dopants, e.g. CuSO 4 -doped TiO 2 (pH pzc =3.84) [51] and KAl(SO 4 ) 2 -doped TiO 2 (pH pzc =1.90~3.39) [40].
As photocatalytic degradation e ciency of speci c 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 TiO 2 and initial 2-CP concentrations) that utilizes the previous optimized Ce-doped TiO 2 (0.28 mol %, calcined at 600 o C) were listed in Table 2. The residual concentration of 2-CP under visible light irradiation at ve 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 e ciency 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 pro les 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 e ciency of 2-CP.
Aggregation of TiO 2 particles occur as the conditioned pH approaches to pH pzc , while it tends to stabilize at both higher and lower pH conditions [64, 65].
For positively charged surface, pH< pH pzc : TiO 2 +nH + ↔TiO 2 H n +n (2) For negatively charged surface, pH> pH pzc : From previously determined pH pzc of the Ce-doped TiO 2 at 2.83, the surface charge of the Ce-doped TiO 2 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 pH pzc , the charge effects may not be signi cant. As a result, the degradation pro les of This phenomenon can be associated with the overcrowding catalysts that could block the light absorption on the catalyst surfaces [40]. The same trends were also found in our previous studies for CuSO 4 -doped TiO 2 catalysts [51]. 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 OHand O 2 radicals [40]. In summary, optimal degradation of 2-CP was achieved at the dosage of Ce-doped TiO 2 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.

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 e ciencies under visible light irradiation. Characterizations of the Ce-doped TiO 2 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 TiO 2 photocatalyst generally decreased with the increasing calcination temperatures from 200 to 500 o C. However, there is a different trend in the result observed in 600 o C calcination temperature where the surface area increases. The occurrence is also evident with the pore volume and pore size measurements of the Ce-doped TiO 2 photocatalyst. As all surface areas characterized by various methods consistently showed that the Cedoped TiO 2 calcined at 600 o C were higher than that calcined at 200 o C, we can concluded that structures of the Ce-doped TiO 2 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. [59] have prepared Ce-TiO 2 catalysts by controlled hydrolysis of titanium alkoxide based on esteri cation reaction followed by hydrothermal treatment. They doped various cerium amounts (0.1, 0.2, 0.4, 0.6 and 1.0 wt.%) into TiO 2 by the controlled hydrolysis and calcined at 460 o C (733 K) can be a reference as well. As shown in Table 3, the Ce-doped TiO 2 calcined at 400 o C registered the highest pore volume of 0.2208 cm 3 /g among the ve (5) calcination temperatures.
Although there is no clear relationships between calcination temperatures and pore volumes, it was noted that there is signi cant correlation between small particle size, large surface area and pore volume.
Consequently, calcined at either middle temperatures (e.g. 300, 400 o C or in between) can be a feasible option for future large-scale production the Ce-doped TiO 2 catalysts.

Conclusion
In summary, the degradation e ciency of the synthesized TiO 2 -based photocatalysts is succesful in degrading aqueous 2-chlorophenol via photocatalytic oxidation process.

Declarations
Availability of data and materials All data generated or analyzed during this study are available upon request to the corresponding author.

Competing interests
The authors declare they have no competing interests.