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Aditya Sharma, Minami Yano, Cheng Zhang, Jie Ming, Xiang Sun, [Yunxin Zhu](https://orcid.org/0000-0001-6070-7305), [Guangqi An](https://orcid.org/0000-0003-2726-1369), [Naoki Kawazoe](https://orcid.org/0000-0003-3916-0709), [Guoping Chen](https://orcid.org/0000-0001-6753-3678), Yingnan Yang

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[Efficient and stable immobilization of TiO2-based composite photocatalytic system for separation and purification of organic pollutants in wastewater under solar light](https://mdr.nims.go.jp/datasets/b22febb7-2e8e-4dc5-a77e-fe0e8d0d8ad0)

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1 Efficient and Stable Immobilization of TiO2-Based Composite Photocatalytic System for Separation and Purification of Organic Pollutants in Wastewater under Solar Light  Aditya Sharmaa,c,, Minami Yanoa, Cheng Zhanga, Jie Minga, Xiang Suna, Yunxin Zhua, Guangqi Ana, Naoki Kawazoeb, Guoping Chenb, Yingnan Yang a* aGraduate School of Life and Environmental Science, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571, Japan bResearch Center for Functional Materials, National Institute for Materials Sciences, 1-1-1 Namiki, Tsukuba, Ibaraki 305-0004, Japan cInterdisciplinary Center for Catalytic Chemistry, Namiki, Tsukuba, Ibaraki 305-0004, Japan  *Corresponding Author. Tel/Fax: +81 29 8534650E-mail address: yo.innan.fu@u.tsukuba.ac.jp (Y.Yang)https://www2.cloud.editorialmanager.com/apsusc/viewRCResults.aspx?pdf=1&docID=259110&rev=0&fileID=5572536&msid=ff5e6ac2-2a73-4be4-bb15-d0456c8c407dhttps://www2.cloud.editorialmanager.com/apsusc/viewRCResults.aspx?pdf=1&docID=259110&rev=0&fileID=5572536&msid=ff5e6ac2-2a73-4be4-bb15-d0456c8c407d2 Abstract Photocatalytic removal of organic pollutants emerging from untreated industrial and aquaculture wastewater are still limited by catalytic efficiency and stability in practical application. In this study, we have reported the development of silicone immobilized, hydrothermal and PEG assisted Ag doped TiO2 composite (S-H-PEG/PAgT) system for an effective and sustainable elimination of organic pollutants under solar light illumination. Firstly, the newly developed silicone immobilized TiO2 (S-TiO2) was identified as an ideal strategy. It exhibited higher and stable organic pollutant removal (Rh B) than conventional immobilization methods, including poly-vinyl alcohol TiO2 (PVA/TiO2) and dip-coated TiO2. Furthermore, the developed solar light driven S-H-PEG/PAgT photocatalytic system showed good light absorption, narrow band gap, crystalline phase, surface hydrophobicity, roughness, and excellent catalytic performance than S-TiO2 (control). The S-H-PEG/PAgT was effective in removal of tetracycline (a model emerging organic contaminant ) and exhibited high stability in a continuous (10 cycles) operation, meanwhile achieved good efficiency under different environmental conditions. The mechanism analysis revealed •O2- and h+ as dominant active species involved in the catalytic reaction. Most importantly, a synergistic effect of surface hydrophobicity, roughness and photocatalytic activity contributed to the overall reduction of pollutants. Therefore, the developed novel solar light driven S-H-PEG/PAgT photocatalytic system shows great promise for practical purification of organics pollutants in wastewater. Keywords: Strategy of photocatalytic immobilization process, Efficiency of wastewater treatment, Elimination of antibiotics, Practical feasibility 3 1. Introduction1 Water is the most vital component and valuable resource humans depend on for survival and 2 development. In recent years, due to rapid industrialization and urbanization, water sources have been 3 continuously polluted with emerging organic pollutants and antibiotics such as tetracycline (TC) [1,2]. 4 The discharge of untreated, recalcitrant, toxic and non-degradable organic compounds poses a significant 5 threat to the environment [3,4]. The unjustified use of antibiotics in aquaculture and other industries, as 6 well as untreated wastewater with high concentrations of antibiotics contribute to the development of 7 antibiotic resistant microorganisms that are harmful to human health, aquatic life, and biodiversity. Hence, 8 wastewater treatment is critical for protecting water resources and advancing sustainable development. 9 Therefore, a sustainable and efficient treatment is necessary for complete removal of emerging organic 10 pollutants in wastewater. 11 Photocatalytic technology that utilizes solar as sustainable source of light has become a desirable 12 green solution for treating environmental pollutants. The photocatalysts, including metal oxide 13 semiconductors such as titanium dioxide (TiO2), zinc oxide (ZnO), and tungsten dioxide (WO3) have 14 shown great potential for water treatment under UV irradiation. Among them, TiO2 has gained much 15 attention due to its superior activity, oxidizing ability, economic feasibility, and non-toxicity [5]. However, 16 the recovery and reuse of TiO2 nanoparticles in a slurry system is a fundamental problem, as it is time 17 consuming, poor recoverability and practically difficult [6]. Stabilization of photocatalysts on solid 18 supports can be an ideal option to increase the efficiency and reduce the operational costs in photocatalytic 19 wastewater treatment [7]. 20 In order to improve the feasibility of photocatalysts for practical application, the immobilization 21 strategy should be economical, strongly fix photocatalysts on the surface of the support and have excellent 22 photocatalytic activity. This reduces the loss of photocatalysts and improve the stability for a long term 23 4 use [8]. To achieve the above, several studies have been focused on identifying optimal stabilization 24 technique and improving photocatalytic activity. Traditionally, the photocatalysts are immobilized on the 25 supports by various approaches such as sol-gel technique, chemical vapor deposition (CVD), pulsed laser 26 deposition (PVD), and solvent deposition [9]. The sol-gel method is used to immobilize photocatalysts by 27 dip-coating supports in photocatalyst solution [10]. This method offers good controllability, dependability, 28 and reproducibility in obtaining structured thin films [11]. But with dip-coated TiO2 photocatalysts, it has 29 been noted that formation of amorphous phase frequently resulted in low activity and poor charge 30 separation [12]. To control the crystallinity, temperature and other reaction conditions needs to be 31 monitored to achieve higher activity and stability in pollutant removal. The use polymer such as poly-32 vinyl alcohol (PVA) mixed with TiO2 synthesized by sol-gel method are casted on supports under mild 33 thermal treatment [13,14]. The thermal fixation of PVA/TiO2 showed good activity and stability for 5 34 consecutive cycles. In another aspect, the use of polymeric binder such as polysiloxane is considered to 35 be more convenient as it strongly fixes photocatalysts under ambient conditions and maintain good 36 crystallinity and activity of TiO2. Specifically, poly-siloxane binders seem to be a suitable option given 37 their thermal stability, surface hydrophobicity, optical transparency,  non-toxicity, and UV resistance. The 38 TiO2 photocatalyst uniformly combined with an oxime-based silicone binder in a continuous flow packed-39 bed reactor under UV light increased the photoactivity and durability [15]. Similarly, loading of 40 photocatalysts on the siloxane increased activity and stability in consecutive cycles for organic reduction 41 [16]. While silicone is optically transparent, mixing nanoparticles with the binder lowers the interaction 42 of organics with the photocatalysts. A higher catalyst loading can overcome this problem, but, not feasible 43 from the economical point of view. To date, the studies related to silicone binders are used after 44 homogenization and there is also a lack of comparative studies with conventional immobilization methods. 45 So, fixing the photocatalysts on silicone support could enhance the stability and durability, and their 46 5 activity in the continuous reactor should be compared with the existing immobilization methods. 47 Additionally, the selection of support materials for fixing photocatalysts have an important function for 48 ensuring process stability, catalytic performance, and development of immobilization system [17]. So, the 49 glass beads coated with photocatalysts can be incorporated in multiple photocatalytic system and increase 50 the interaction with the pollutants to achieve higher activity [18,19]. In addition, the solution parameters 51 in an actual environmental also change depending on the pollution source [20]. However, little is known 52 about the process stability and activity of photocatalytic surfaces for the removal of organic contaminants. 53 Besides, pure TiO2 photocatalysts are only activated under UV light, which accounts for less than 54 5% of solar light. The rapid electron-hole recombination, low charge separation and wide band gap limits 55 its scalability. Doping TiO2 with noble metals (Ag, Au and Pt), non-metals (P, C, N and S) and narrow 56 band semiconductors (Ag2O and Ag3PO4) are known to effectively decrease original band gap and 57 enhance photo-induced charge separation [21,22]. In our previous study, the doping of metal (Ag), metal 58 oxides (Ag2O and Ag3PO4) and non-metal (P) with TiO2, synthesized by one-pot sol-gel/hydrothermal 59 method showed superior performance in water splitting and organic dye degradation [23]. The quaternary 60 heterostructure, hydrothermal synthesized Ag species doped TiO2 based photocatalyst (H-PAgT) showed 61 higher charge separation, small crystalline size, low electron-hole pair recombination than pure TiO2. The 62 H-PAgT photocatalyst immobilized on poly-siloxane showed strong adhesion of photocatalysts63 nanoparticles with good disinfection and pollutant removal efficiency [24,25]. In addition, several reports 64 emphasize on the use of morphological tuners polyethylene glycol (PEG) during the synthesis process to 65 further control grain growth and improve photocatalytic activity [26]. Until now, the effect of surface 66 modifiers on H-PAgT immobilized system for removal of antibiotics has not yet been understood. 67 Moreover, the underlying mechanism with different immobilization method is still unclear and requires 68 further clarification. From practical perspective, a comprehensive assessment of catalytic performance and 69 6 stability needs to be evaluated to be adopted commercially. To the best of our knowledge, there is no study 70 on H-PEG/PAgT immobilized system for treatment of wastewater. 71 Therefore, the main objective of this study was to identify the ideal immobilization strategy and 72 develop a silicone immobilized, hydrothermal and PEG assisted Ag species TiO2 based photocatalytic 73 system (S-H-PEG/PAgT) for treatment of organic pollutants under solar light. We observed the reduction 74 of organics with silicone immobilized TiO2 (S-TiO2) and S-H-PEG/PAgT photocatalysts. In the 75 comparative study, the newly developed system showed better physio-chemical characteristics and 76 stability compared to conventional dip-coated-TiO2 and PVA/TiO2 systems. Finally, the practical 77 feasibility of the immobilized system for removal of organics was determined and the mechanism of the 78 reaction was proposed. 79 2. Materials and methods80 2.1 Preparation of hydrothermal synthesized Ag species doped TiO2 based photocatalyst 81 The Ag species doped TiO2 based photocatalysts was synthesized by hydrothermal method 82 according to our previous study [23]. 6 mL of tetra-butyl titanate (TNBT) was added to 46 mL of ethanol 83 was mixed thoroughly for 1 h (Solution A). A known amount of precursor salts AgNO3 and Ag3PO4 was 84 added to 1 mol L-1 of nitric acid (HNO3) and sonicated until the salts were completely dissolved (Solution 85 B). The solution B containing Ag and phosphate precursor mixture was added dropwise to solution A and 86 stirred for 16 h to obtain a homogeneous and clear sol-gel at room temperature. The sol-gel solution was 87 transferred to a hydrothermal reactor and treated at 120ºC for 3 h. After the reaction, the supernatant was 88 washed 3 times with ethanol and distilled water. The supernatant was centrifuged three times at 5000 rpm 89 and dried at 60ºC for 12 h to obtain H/PAgT. For the PEG-modified samples, PEG (1 g: 300 MW) and 90 TNBT (6 mL) was added to 46 mL of ethanol and stirred for 1 hour (Solution A). The remaining steps are 91 the same as those described previously. The sample that was obtained was denoted as H-PEG/PAgT. 92  7 2.2 Immobilization of TiO2, H/PAgT and H-PEG/PAgT photocatalysts on silicone beads 93 The immobilization of photocatalysts on poly-siloxane binder was prepared according to our 94 previous study [24], The glass beads were first washed with ethanol and distilled water and dried at 100ºC 95 for 6 h. A commercially available poly-siloxane binder was coated on glass beads to cover the entire 96 surface. 0.2 g of photocatalyst powders was loaded with the help of lab-designed sieve tube until the 97 silicone supported glass surface was completely coated. The silicone supported beads were blow dried to 98 remove any non-adherent nanoparticles and stored at room temperature for 24 h. After the curing process, 99 the beads were washed with distill water to remove loosely attached nanoparticles on the surface and dried 100 at room temperature. The amount of photocatalysts adhered on the surface on the beads were estimated to 101 be around 0.023 mg. Silicone supported beads for TiO2, PAgT and H-PEG/PAgT were denoted as S-TiO2, 102 S-H/PAgT and S-H-PEG/PAgT (S: silicone, PEG: polyethylene glycol, H: hydrothermal two step method, 103 PAgT: P/Ag/Ag2O/Ag3PO4/TiO2 photocatalyst, and TiO2: commercial P25), respectively. The glass beads 104 coated with only silicone were used as a control.  105 2.3 Synthesis of dip-coated TiO2 and PVA-TiO2 coated beads 106 A sol-gel approach was followed for dip-coating TiO2 on glass beads. Firstly, the glass beads were 107 washed with methanol/water (30/70% w/w) and dried at 60ºC for 3 h. 1 g of P25 (TiO2 Evonik Degussa) 108 was added to methanol/water mixture (30/70% w/w) and sonicated for 15 min. 10 mL of dilute nitric acid 109 (pH 3) was added slowly to above solution and stirred for 1 h. After 1 h of homogenization, glass beads 110 were added to the TiO2 dispersion and dried for 12 h at room temperature. The samples were calcined at 111 500ºC for 2 h. After calcination, the obtained dip coated samples were denoted as dip-coated TiO2.  112 The PVA/TiO2 coating was synthesized by adding 1 g of PVA and 0.5 g of TiO2 to 20 mL of 113 distilled water. The solution was stirred at 70ºC for 3 h with thermal magnetic stirrer. The PVA/TiO2 was 114 8 coated by immersing glass beads in the solution. The coated glass beads were dried at 180ºC for 2 h to 115 obtain PVA/TiO2 beads. 116 2.4 Photocatalytic activity evaluation 117 The catalytic activity of immobilized photocatalysts were evaluated by reduction of rhodamine B 118 (RhB) (model organic pollutant) and tetracycline (TC) (emerging organic pollutant) in a laboratory-119 designed single-column reactor. The immobilized photocatalytic beads (0.14±0.02 g) thus synthesized 120 were packed in glass tubes (10cm, diameter: 1cm), which were connected with each other using plastic 121 connectors. One end of the connector was placed in a beaker containing Rh B (2 mg L-1) /TC solution (10 122 mg L-1) (50 mL). The connector was passed through a pump with a constant flow rate of 150 mL min-1, 123 facilitating the suction of pollutants from one end and through the fixed-bed photocatalytic reactor, 124 forming a cyclic system. The experiments were performed under simulated sunlight (550 W m-2, XC-100, 125 SERIC ltd, USA) and 4 mL samples were taken at constant intervals. The experiments were all carried 126 out at ambient temperature and pH. For the comparative studies, the silicone immobilized TiO2 beads was 127 replaced with dip-coated and PVA/TiO2 coated beads. To effectively compare the catalyst activity, the 128 dosage of photocatalysts was kept constant at 0.16 g and experiment was conducted under the same 129 reaction condition as described above. 130 3. Results and discussion131 3.1 Characterization and identification of the optimal photocatalytic immobilization process 132 The process of photocatalytic immobilization on glass beads by dip-coating, PVA and silicone are 133 described in section 2.3. Commercially available TiO2 (P25) was employed as a model photocatalyst to 134 effectively compare and identify the optimal immobilization process. The immobilized photocatalysts was 135 firstly characterized, and activity of S-TiO2 was compared with dip-coated TiO2 and PVA/TiO2 136 photocatalysts. The SEM measurements were conducted to understand the morphology of the S-TiO2, dip-137 9 coated TiO2, and PVA/TiO2 photocatalysts (Fig. 1a-c). In the dip-coated TiO2, after calcination, two 138 distinct catalysts layers stacked on each other were observed. The irregularity observed on dip-coated TiO2 139 could be to due loss of poorly attached TiO2 nanoparticles after washing (Fig.1a). Previous reports have 140 also indicated the poor fixation of nanoparticles on glass surface by dip coating [27]. While in PVA/TiO2, 141 the crevasses are a result of thermal casting of TiO2 photocatalysts with PVA (Fig.1b), which exhibit 142 rough and porous structure. However, the homogenization of TiO2 with PVA could hinder the interaction 143 of pollutants with the active sites. On the other hand, the S-TiO2 clearly displayed a rough surface with 144 TiO2 photocatalysts firmly attached on the silicone surface (Fig.1c). The silicone coated glass beads 145 (control; No photocatalysts) showed smooth surface, typical for poly-siloxane materials (Fig. S1a-c). The 146 addition of TiO2 photocatalysts on silicone coated beads increased the surface roughness which could 147 improve the light absorption and reaction sites with the pollutants [28]. Therefore, the S-TiO2 could be 148 beneficial for organic removal than dip-coated TiO2 and PVA/TiO2 photocatalysts. 149 To further investigate the surface texture and morphology, atomic force microscopy (AFM) 150 measurement was carried out for the TiO2 photocatalysts immobilized by different methods. As shown in 151 Fig. 1d-i, the S-TiO2 photocatalysts showed the highest surface roughness followed by PVA/TiO2 and dip-152 coated TiO2 photocatalysts. The average surface roughness observed in dip-coated TiO2, PVA/TiO2 and 153 S-TiO2, were 40.4, 69.3 and 74.9 nm, respectively. The surface roughness is consistent with the SEM154 images. The increase in surface roughness could improve the photocatalytic interaction with the pollutants 155 and enhance the catalytic activity [29,30]. In addition, the surface roughness could improve the adsorption 156 of pollutants, which would lead to higher catalytic activity upon irradiation. The higher surface roughness 157 observed in S-TiO2 and PVA/TiO2 may improve overall catalytic adsorption and degradation of organic 158 pollutants. 159 10 The surface wetting property of the immobilized photocatalysts involves the interaction of 160 pollutants with the photocatalysts. The wettability of the surface is also governed by surface texture and 161 roughness, in addition to their chemical property. The angle formed between the solid surface and the 162 tangent drawn at the liquid drop was used to determine the wettability of the photocatalysts. As seen in 163 Fig. 2, S-TiO2 photocatalysts showed hydrophobic property with a contact angle of 121.4º, as compared 164 to dip coated TiO2 (5.3º) and PVA/TiO2 (62.4º) photocatalysts. The pristine TiO2 (0º) and silicone coated 165 beads (92.3º) showed hydrophilic and hydrophobic property, respectively (Figure S1d-e). Usually, in the 166 hydrophobic surface, a solid-liquid-air interface is formed which could be beneficial for interaction of 167 pollutants with photocatalysts, and the oxidative species generated during the reaction process [31,32]. 168 The increase in interaction and reaction with reactive species could drive the photocatalytic process and 169 effectively reduce pollutants in wastewater. So, compared to hydrophilic surface, the hydrophobic surface 170 could improve the photocatalytic activity of TiO2 immobilized catalysts for pollutant removal in 171 wastewater. Therefore, the increased surface roughness, and hydrophobicity of the S-TiO2 immobilized 172 photocatalysts may be beneficial for improving the activity and stability for wastewater treatment.  173 The photocatalytic activity of the immobilized TiO2 photocatalysts was determined by the 174 degradation of model organic pollutant Rh B. The experiments were conducted in a single packed column 175 reactor and the initial dosage of photocatalysts from various immobilization methods were kept constant 176 at 0.16 g/tube. Fig. 3a shows the catalytic performance of immobilized TiO2 photocatalytic reduction of 177 Rh B. As seen in the figure, the S-TiO2 showed higher photocatalytic activity than dip-coated TiO2 and 178 PVA/TiO2 photocatalysts. Rh B removal efficiency for PVA/TiO2, dip-coated TiO2, and S-TiO2 179 photocatalysts were 43.7, 57.4, and 96.5%, respectively. The photocatalytic degradation rate of S-TiO2 180 was 3.0 and 2.5 times faster than that of PVA/TiO2 and dip-coated TiO2 photocatalysts, respectively (Fig. 181 S2). The increase in degradation rate can be attributed to the hydrophobic property of the S-TiO2 182 11 immobilized photocatalyst. The hydrophobicity and surface roughness increased the interaction of the 183 pollutant with the photocatalysts at the solid-liquid interface [33,34]. The generated oxidative species at 184 the solid-liquid interface reduce organic pollutants resulting in higher photocatalytic efficiency. In the case 185 of PVA/TiO2, the homogenization of TiO2 in the PVA matrix could restricts the interaction of 186 photocatalysts with the pollutants and block the active site, leading to low activity. Similarly, the dip-187 coated TiO2 photocatalyst could form amorphous TiO2, which generally exhibits poor activity. These 188 results demonstrated the superior catalytic performance of S-TiO2 immobilized photocatalytic system than 189 PVA/TiO2 and dip-coated TiO2 photocatalysts. 190 In practical application, the stability of the photocatalysts determines its economic feasibility and 191 catalytic performance. A 10-cycle repetitive experiments were conducted to investigate the stability of S-192 TiO2, PVA/TiO2 and dip-coated TiO2 photocatalysts. As seen Fig. 3b, the S-TiO2 photocatalysts showed 193 the highest degradation efficiency than PVA/TiO2 and dip-coated TiO2 photocatalysts after 30 min of 194 irradiation. The Rh B degradation efficiency of S-TiO2 were constant throughout the 10-cycle and reached 195 a degradation efficiency of 96.5% (10th cycle). The degradation efficiency of PVA/TiO2 and dip-coated 196 TiO2 after 10 cycles were 49.7% and 52.3%, respectively. For dip-coated TiO2 and PVA/TiO2, a slight 197 decrease in activity and relatively unstable performance was observed, mainly due to the loss of poorly 198 attached photocatalysts on the glass beads. This was confirmed by SEM images observed before and after 199 10 cycles. In comparison with the SEM of before samples (Fig. 1a-b), a significant loss of photocatalysts 200 could be seen in dip-coated TiO2 and PVA/TiO2 after reaction (Fig. 3c-d). On the other hand, the 201 morphology of S-TiO2 remains unchanged even after 10 cycles, and no photocatalytic loss was observed 202 (Fig. 3e). This suggests that S-TiO2 immobilized strategy strongly fixed the photocatalysts on the support 203 material, which led to improved photocatalytic activity for removal of pollutants. Therefore, the 204 immobilization of TiO2 nanoparticles on silicone was found to be an ideal strategy for improving the 205 12 photocatalytic activity, stability, and reliability than conventional methods for treating organic pollutants 206 in wastewater. 207 3.2 Development of solar light driven S-H-PEG/PAgT photocatalytic system for TC removal 208 In the photocatalytic system, the use of pristine TiO2 (P25) is limited only to UV light, which 209 accounts for less than 5% of the solar light and restricts the potential use of visible light (> 40% solar 210 light). To address this issue, S-H-PEG/PAgT photocatalytic system was developed and used in future 211 experiments using TC as a representative model organic pollutant in wastewater under solar light. The S-212 H-PEG/PAgT photocatalysts was characterized by SEM, AFM, WCA, UV-vis and XRD. The morphology213 of S-H/PAgT (control) and S-H-PEG/PAgT are shown in Fig. S3a-b. As seen in the figure, the S-H/PAgT 214 and S-H-PEG/PAgT showed irregular morphology, as compared to S-TiO2 photocatalysts (Fig. 1c). It has 215 been noted that PEG could effectively control the morphology and crystalline size of TiO2 modified 216 photocatalysts [35]. The PEG addition during hydrothermal synthesis could control the grain growth, and 217 lead to small particle size [36]. The small particle size and irregular structure of S-H-PEG/PAgT could 218 significantly increase surface roughness and improves the pollutant adsorption, and higher catalytic 219 activity. The surface texture of the photocatalysts was determined by the AFM measurement. As illustrated 220 in Fig. S3c-f, the S-H/PAgT and S-H-PEG/PAgT photocatalysts exhibited the highest surface roughness 221 than S-TiO2 (Fig. 1f and i). The higher average peak to valley value (Ra=2349 nm) observed in the S-222 H/PAgT photocatalysts could be due to the large particle size attached on the surface of silicone coated 223 beads. In S-H-PEG/PAgT photocatalysts, the addition of PEG surfactant during the synthesis process 224 controlled the grain growth resulting in smaller NPs fixed on silicone coated beads and lower peak to 225 valley (Ra=1698 nm). The observations are in accordance with the SEM images (Fig. S3a-b). The surface 226 light absorption and scattering characteristics depend on roughness around the wavelength of the incident 227 13 light. For the above reasons, increased surface roughness in S-H/PAgT and S-H-PEG/PAgT could further228 improve light absorption and lead to improved activity in wastewater treatment. 229 Fig. S3g-h shows the surface wetting property of the silicone immobilized TiO2 based composite 230 photocatalysts. The water contact angle of S-H-PEG/PAgT and S-H/PAgT photocatalysts were 134.2º and 231 125.7º, respectively. As mentioned before, the silicone (control) surface is hydrophobic in nature. The S-232 H-PEG/PAgT photocatalysts showed ultra-hydrophobic surface due to the irregular morphology, and233 small particle size. The addition of PEG during the synthesis process could have influenced the surface 234 texture and hydrophobicity. The ultra-hydrophobic surface could improve the adsorption and interaction 235 of pollutants. The improved interaction on the surface also increases adsorption of surface oxygen to 236 generate super oxide anions, leading to higher photocatalytic activity. Hence, the ultra-hydrophobic 237 surface of S-H-PEG/PAgT may improve activity and stability for long-term practical elimination under 238 solar light irradiation. 239 The light absorption ability of TiO2 and TiO2 based composite photocatalysts immobilized on 240 silicone are shown in Fig. 4a. The S-H-PEG/PAgT photocatalysts showed the highest light absorption 241 both in the UV and visible light range compared to S-H/PAgT and S-TiO2 photocatalysts. The strong 242 absorption absorbed in the visible light region (400-800 nm) could be due to the surface plasmon 243 resonance (SPR) effect coming from light responsive Ag salts [37], which is present in both S-H-244 PEG/PAgT and S-H/PAgT photocatalysts. The addition of Ag salts improves the light absorption, and 245 significant red shift into the visible light region, according to the previous reports [22]. Additionally, the 246 better optical property in S-H-PEG/PAgT can be owed to the addition of surfactants (PEG), which reduces 247 grain growth and further improves light absorption. Moreover, the surface texture and roughness in S-H-248 PEG/PAgT could improve light scattering and absorption (Table S1). The calculated band gap of S-H-249 PEG/PAgT, S-H/PAgT and S-TiO2 photocatalysts were 2.98, 3.10 and 3.17 eV. The narrow band gap, and 250 14 improved light absorption ability of S-H-PEG/PAgT could effectively increase the generation of reactive 251 species, and charge separation for reduction of pollutants under solar light irradiation. 252 Fig. 4b shows the XRD patterns of S-TiO2 and TiO2 based composite photocatalysts. As shown in 253 figure, the crystalline phase of S-H-PEG/PAgT and S-H/PAgT showed the presence of only anatase phase 254 of TiO2. While S-TiO2 showed both anatase and rutile phase of TiO2, typical observation for P25 255 photocatalysts [38]. The Anatase phase of TiO2 is considered to be more active under visible light 256 irradiation and could result in higher catalytic performance for wastewater treatment [39]. The slightly 257 better crystallinity observed in S-H-PEG/PAgT photocatalyst are a result of the addition of PEG. The 258 addition of surfactant during the synthesis process control the grain growth and effectively improve its 259 crystallinity [40]. In addition, the S-H-PEG/PAgT and S-H/PAgT showed the presence of Ag salts 260 (Ag2O/Ag3PO4) [24]. The presence of Ag salts in the XRD patterns confirms the increased light absorption 261 resulted from the SPR effect observed in both S-H-PEG/PAgT and S-H/PAgT (Fig. 4a). The superior 262 characteristics of S-H-PEG/PAgT photocatalysts with higher surface roughness, hydrophobicity, narrow 263 band gap, good optical property, and better crystallinity could improve photocatalytic activity in removal 264 of pollutants from wastewater under solar light irradiation. 265 The photocatalytic activity of the silicone immobilized S-H-PEG/PAgT was evaluated by 266 degradation of model emerging organic pollutant tetracycline (TC). As seen in Fig. 4c, higher TC removal 267 was observed with S-H-PEG/PAgT photocatalyst compared to S-H/PAgT, and S-TiO2. The poor removal 268 of TC in silicone (control) suggested that the removal occurred through the photocatalytic generation of 269 oxidative species rather than adsorption. The rate of TC degradation efficiency for S-H-PEG/PAgT, S-270 H/PAgT, and S-TiO2, was 0.0278, 0.0202 and 0.0106 min-1, respectively (Fig. 4d). The irregular 271 morphology and hydrophobicity in S-H-PEG/PAgT photocatalyst could contribute to increased interaction 272 with pollutants, and generation of reactive species, which led to higher photocatalytic activity. In addition, 273 15 the S-H-PEG/PAgT showed higher TOC removal compared of other modified samples (Fig. S4). 274 Furthermore, the S-H-PEG/PAgT photocatalysts showed high activity in comparison with the reported 275 literatures (Table S2). Although, a direct comparison with the existing literature would be difficult 276 considering the difference in various reaction parameters, but the S-H-PEG/PAgT photocatalysts showed 277 better performance under ambient conditions. Thus, the above results clarify the S-H-PEG/PAgT 278 photocatalyst with small particles size, average surface roughness, good light absorption and effective 279 removal of organics from wastewater. 280 3.3 Practical feasibility of S-H-PEG/PAgT photocatalytic system 281 The photocatalytic activity in real environmental is affected by various factors such as flow rate, 282 pH, light intensity, and temperature. Therefore, the efficiency of S-H-PEG/PAgT photocatalysts was 283 investigated under various influencing factors to mimic the environmental conditions. 284 The flow rate in the photocatalytic reaction is one of the important parameters that affects the TC 285 degradation efficiency. The effect of flow rates on TC removal is shown in Fig. 5a. The photocatalytic 286 activity remained significantly higher in flow rates from 50 mL to 250 mL min-1. The degradation 287 efficiency at 50-, 150- and 250-mL min-1 were 85.1, 85.9 and 86.2%, respectively. The pseudo-first order 288 kinetics of the reaction for 50-, 150- and 250-mL min-1 were 2.243 x 10-2, 2.788 x 10-2, and 2.560 x 10-2289 min-1, respectively. The flow rate determines the rate of mass transfer and catalytic interaction with the 290 pollutants [6]. The high activity observed from low-high flow rate suggested effective mass transfer and 291 activity. The high activity achieved under different flow rates illustrated that the S-H-PEG/PAgT 292 photocatalysts could be used in various photocatalytic systems with varying flow rates to obtain effective 293 reduction of pollutants. 294 The photocatalytic removal of organic materials is significantly influenced by the pH of the 295 solution. The influence of initial pH on S-H-PEG/PAgT photocatalytic removal of TC was investigated 296  16 from pH 3 to 9. As shown in Fig. 5b, the degradation efficiency increased from pH 3 to 5, and plateaued 297 at pH 7 before significantly decreasing at pH 9. The TC degradation efficiency observed at pH 3, 5, 7 and 298 9 were 77.4, 96.7, 93.2 and 54.3%, respectively. The optimal pH was observed to be around pH 5 to 7. 299 The pHpzc value of TiO2 photocatalysts are around pH 6.8 [41], and the pKa value of TC pH 5.0 to 6.7. 300 The S-H-PEG/PAgT at pH 5 have a positive charge, which TC is in the zwitterion state. Hence, the 301 interaction between the TC and S-H-PEG/PAgT increases at pH 5 to 7, and led to higher photocatalytic 302 conversion [42]. On the other hand, the degradation is less favorable at acidic (pH 3) and alkaline (pH 9) 303 due to differences the surface property. This shows that the S-H-PEG/PAgT photocatalyst can effectively 304 reduce TC at pH 5 to 7 in environmental conditions.  305 Fig 5c shows dependence of photocatalytic activity on temperature from 10 to 50 °C. The TC 306 removal achieved at 10-20, 20-30, 30-40, and 40-50 °C were 76.3, 85.5, 86.3 and 86.4% respectively. As 307 observed in the figure, the rise in temperature from 10 to 20 °C increased the photocatalytic activity, and 308 remained consistent from 20 to 50 °C. The significant reduction observed at such low temperature (10 °C), 309 infers that the TC reduction is predominantly depended on light activation rather than temperature. These 310 findings are consistent with our previous studies conducted at low to high temperature [43]. The optimal 311 temperature for photocatalytic TC reduction was observed to be in the range of 20 to 50 °C and is efficient 312 under real temperature observed in the environment.  313 The dependence of TC degradation on incident light intensity was investigated in the range of 100 314 to 1000 W m-2. As shown in Fig 5d, a slight increase in photocatalytic activity was observed with increase 315 in light intensity from 100 to 550 W m-2, while TC reduction from 550 to 1000 W m-2 remained unchanged. 316 The TC degradation efficiency observed at 100, 300, 550, 700, and 1000 Wm-2 were 78.4, 79.5, 88.5, 89.1 317 and 89.4% respectively. The marginal difference in catalytic performance at low to high intensity is 318 attributed to the higher catalytic activity of the synthesized photocatalysts. This indicates that the even at 319  17 low light intensity, high charge separation can be achieved for the generation of reactive species resulting 320 in higher TC reduction. The results suggest that the photocatalyst is effective in real environmental 321 conditions from low to high light intensity for reduction of pollutants. The S-H-PEG/PAgT photocatalytic 322 activity demonstrates the ability for effective remediation of organic contaminants under different 323 influencing factors in real environmental (pH, temperature, light intensity) and reactor condition (flow 324 rate) for practical application. 325 The feasibility and stability of S-H-PEG/PAgT photocatalytic system for practical application was 326 investigated by repetitive experiments (10 cycles). As shown in Fig. 6a, the TC degradation efficiency 327 from 1st (94.6%) to 10th  (91.3%) cycle remained significantly higher demonstrating the superior S-H-328 PEG/PAgT catalytic performance in pollutant removal. The high surface roughness and irregular 329 morphology of nanoparticles ensured good contact with the pollutants and improved catalytic removal in 330 repetitive experiments. The XRD patterns showed the crystallinity and crystalline phase remained the 331 same and strong peaks associated with TiO2 (Anatase) was visible after 10 cycles (Fig. 6b). Similarly, 332 SEM images showed no change in morphology and surface texture after the reaction (Fig. 6c-d). The 333 hydrophobic surfaces have high stability and increase reusability of the photocatalyst materials [44]. In 334 general, after prolonged exposure to organic pollutants, the surface is contaminated with unreacted 335 organics which affect the surface wetting property or loss of nanoparticles reduce catalytic activity. In this 336 study, the morphology and surface hydrophobicity were retained after reaction, further illustrating the 337 durability of the photocatalytic materials (Fig. 6e-f). Therefore, the S-H-PEG/PAgT photocatalysts with 338 high stability and activity shows promise for the treatment of organic pollutants in practical application.   339 3.4 Mechanism of S-H-PEG/PAgT photocatalysts for wastewater treatment  340 The purpose of the scavenger studies was to ascertain the contribution of reactive species to the 341 photocatalytic reduction of TC by S-H-PEG/PAgT under solar light. As shown in Figure 7a, the addition 342 18 of 1-4BQ significantly suppressed the S-H-PEG/PAgT photocatalytic degradation of TC, indicating that 343 O2•- as the predominant species involved in the degradation process. Moreover, the activity also reduced 344 after the addition of EDTA, suggesting that h+ was the main species involved in the reaction. On the other 345 hand, the addition of •OH had no effect on TC removal and was not a direct contributor to the 346 photocatalytic reaction. The predominant reactive species observed in this study are in accordance with 347 our previous results with H/PAgT photocatalysts in the slurry system [23]. Therefore, O2•- and h+ were 348 found to be the key species driving S-H-PEG/PAgT photocatalytic pollutant removal in wastewater.  349 Figure 7b-c shows the proposed mechanism for S-H-PEG/PAgT composite photocatalysts for 350 degradation of organic pollutants under solar light irradiation. Based on the aforementioned results and 351 discussion, the S-H-PEG/PAgT showed superior photocatalytic activity and stability than conventional 352 immobilization methods. The improved photocatalytic activity could be attributed to the following: The 353 photocatalyst upon light irradiation, e- and h+ are generated at the CB and VB position. As the flow of e- 354 and h+ in TiO2, Ag3PO4, Ag2O are governed by the fermi levels [45], the h+ flows to the higher fermi level 355 (TiO2 ® Ag2O ® Ag3PO4), while e- move to the lower fermi level (Ag3PO4 ® Ag2O ® Ag) (Fig. 7b). 356 The e- and h+ on reaction with adsorbed water and surface oxygen, O2•- and  h+ are generated which 357 effectively reduce pollutants (Fig. 7a); Subsequently, the increase in surface roughness due to irregular 358 morphology of S-H-PEG/PAgT photocatalysts could enhance the interaction between the pollutants and 359 photocatalysts (Fig. 7c). The addition of PEG effectively controls the grain growth, and particle size of 360 the photocatalysts, which further increase the light absorption and activity under visible light irradiation 361 (Fig. 4a-b). The improved interaction with the photocatalysts could effectively react with generated active 362 species and reduce pollutants; the ultra-hydrophobic property (134.3°) of S-H-PEG/PAgT  photocatalyst 363 was found to improve the photocatalytic stability in long-term treatment of organics. Typically, a solid-364 liquid-air interface is formed in the hydrophobic surface, which may be advantageous for the interaction 365  19 of pollutants with photocatalysts and increased generation of surface oxidative species during the reaction 366 process could affect positively on the reaction process. Thus, an interaction between photocatalytic 367 activity, the irregular morphology, roughness, and ultra-hydrophobicity contributed to the overall activity 368 and stability of S-H-PEG/PAgT photocatalysts. Therefore, the developed photocatalytic immobilized 369 system shows great promise as a green alternative for efficient and durable treatment of organics pollutants 370 in wastewater under solar light irradiation. 371 4. Conclusion  372 In this study, a silicone immobilized TiO2 (S-TiO2) was identified as an ideal strategy of 373 photocatalytic immobilization. In addition, the silicone immobilized, hydrothermal and PEG assisted Ag 374 species doped TiO2 composite photocatalytic system was developed as an efficient and sustainable method 375 for degradation of pollutants under solar light irradiation. The S-H-PEG/PAgT photocatalysts showed 376 good light absorption, narrow band gap, better crystallinity, higher surface roughness, hydrophobicity, 377 and improved catalytic performance than S-TiO2 (control). Moreover, an effective removal of pollutants 378 under different environmental factors and high stability in repetitive treatment demonstrates the feasibility 379 in real application. Scavenger study revealed •O2- and  h+ as the dominant reactive species contributed to 380 the removal of TC. The S-H-PEG/PAgT photocatalyst surface hydrophobicity, surface roughness, and 381 photocatalytic activity work in concert to remove organic contaminants from wastewater. Therefore, the 382 solar light driven S-H-PEG/PAgT photocatalyst shows great potential in practical treatment of emerging 383 organic pollutants in wastewater. 384 Acknowledgements 385 This work was supported by Scientific Research (B) 22H03778 and Grant-in-Aid for Exploratory 386 Research 21k19628 from Japan Society for the Promotion of Science. The authors would like to thank 387 20 National Institute of Materials Science (NIMS) for their technical support with characterization of 388 photocatalyst materials. 389 Author contributions 390 Aditya Sharma: Formal analysis, Investigation, Methodology, Writing-Original Draft and 391 Editing. Minami Yano: Formal analysis, Investigation, Methodology, Validation. Cheng Zhang: 392 Writing-Review and Editing, Investigation. Jie Ming: Investigation, Validation. Xiang Sun: 393 Investigation, Validation. Yunxin Zhu: Writing-Review and Editing. Guangqi An: Writing-Review and 394 Editing. Naoki Kawazoe: Resources, Methodology, Editing. Guoping Chen: Resources, Methodology, 395 Writing-Review and Editing. Yingnan Yang: Conceptualization, Methodology, Validation, Funding 396 acquisition, Resources, Project administration, Supervision, Writing-Review and Editing. 397 References 398 [1] N. Roy, S.A. Alex, N. Chandrasekaran, A. Mukherjee, K. 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Mater. 29 (2017) 1604637. 543 https://doi.org/10.1002/adma.201604637. 544 [45] X. Hu, Q. Zhu, X. Wang, N. Kawazoe, Y. Yang, Nonmetal-metal-semiconductor-promoted545 P/Ag/Ag2O/Ag3PO4/TiO2 photocatalyst with superior photocatalytic activity and stability, J. 546 Mater. Chem. A. 3 (2015) 17858–17865. https://doi.org/10.1039/c5ta05153c. 547 548 Fig. 1. SEM images of (a) Dip-coated TiO2, (b) PVA/TiO2, (c) S-TiO2 (50 x magnification). Inset images (magnification 3000x). AFM images of (d) Dip-coated TiO2, (e) PVA/TiO2, (f) S-TiO2, and corresponding 3 D images (g) Dip-coated TiO2, (h) PVA/TiO2, (i) S-TiO2.  30 µm(A)(B)(C)2(D)(E)(F)30 µm100 nm(G)(H)(I)20 µm100 nm20 µm20 µm100 nm20 µm(a)(b)(c)(d)(e)(f)(g)(h)(i)Ra= 40.43 nmRa= 69.38 nmRa= 74.95 nmFigurehttps://www2.cloud.editorialmanager.com/apsusc/download.aspx?id=5570928&guid=8c2b38ef-12d9-4676-9c2b-ecb6fefe7a04&scheme=1https://www2.cloud.editorialmanager.com/apsusc/download.aspx?id=5570928&guid=8c2b38ef-12d9-4676-9c2b-ecb6fefe7a04&scheme=1Fig. 2. The water contact angle measurements of immobilized photocatalysts. (a) dip-coated TiO2, (b) PVA/TiO2, (c) S-TiO2. (Volume of water: 10 uL)(a) (c)(b)Fig. 3. The comparative study of photocatalytic performance and stability of S-TiO2, PVA/TiO2 and dip-coated TiO2 photocatalyst. (a) photocatalytic reduction of Rh B, (b) recyclability and (c)-(e) scanning electron microscope images of immobilized photocatalysts after 10 repetitive cycles.(Rh B: 2 mg L-1, 50 mL, pH: 7, temperature: ambient, flow rate: 150 mL min-1, light intensity: 550 W m-2) 0 15 300.00.51.0C/C0Time (min)S-TiO2PVA / TiO2Dip coated TiO20 30 60 90 120 150 180 210 240 270 3000.00.51.0C/C0Time (min) S-TiO2 PVA / TiO2 Dip coated TiO2x50 x50x3000x50x400 x300Dip-coated TiO2 PVA/TiO2 S-TiO2(c) (d) (e)(a) (b)Fig. 4.  (a) Diffuse reflectance spectroscopy, (b) x-ray diffraction patterns, (c) TC photocatalytic degradation efficiency, and (d) degradation rate of S-H/PAgT, S-H-PEG/PAgT, and S-TiO2 (control). Temperature: ambient, flow rate: 150 mL min-1, pH: 7, light intensity: 550 Wm-2, TC: 10 ppm, 50 mL. 200 400 600 8000.00.51.01.5Intensity (a.u.)Wavelength (nm) Silicone (S) S-TiO2 S-PAgT S-PEG-PAgT20 40 60 80Ag2O/Ag3PO4*Ag* Rutile** ** ****Silicone (S)S-TiO2S-PAgTS-PEG-PAgTAnataseIntensity (a.u.)2 theta (Degree)*(a) (b)S-TiO2 S-H/PAgT S-H/PEG-PAgTKapp min-10 x 10-25 x 10-210 x 10-215 x 10-220 x 10-225 x 10-230 x 10-20 30 60 900.00.51.0C/C0Time (min) S-TiO2 S-PAgT S-PEG-PAgT(c) (d)Silicone (S)S-TiO2S-H/PAgTS-H-PEG/PAgTS-TiO2S-H/PAgTS-H-PEG/PAgTSilicone (S)S-TiO2S-H/PAgTS-H-PEG/PAgTFig. 5. The S-H-PEG/PAgT photocatalytic degradation of TC under different environmental factors. (a) flow rate, (b) pH, (c) temperature, and (d) light intensity. The reaction conditions of photocatalyst are the same unless mentioned (Temperature: ambient, flow rate: 150 mL min-1, pH: 7, light intensity: 550 Wm-2). 0 30 60 900.00.51.0C/C0Time (min) 100 W/m2 300 W/m2 550 W/m2 700 W/m2 1000 W/m2 10-20oC 20-30oC 30-40oC 40-50oC0 30 60 900.00.51.0C/C0Time (min)0 30 60 900.00.51.0C/C0Time (min) 50 mL/min 150 mL/min 250 mL/min0 30 60 900.00.51.0C/C0Time (min) pH 3 pH 5 pH 7 pH 9(a) (b)(c) (d)Fig. 6. Repetitive experiments for TC degradation using S-H-PEG/PAgT photocatalyst under solar light irradiation. (a) 10 cycle repetitive, (b) XRD patterns, (c-d) SEM (e-f) water contact angle. Temperature: ambient, flow rate: 150 mL min-1, pH: 7, light intensity: 550 Wm-2. 0 90 180 270 360 450 540 630 720 810 9000.00.51.0C/C0Time (min) S-PEG-PAgT10 15 20 25 30 35 40 45 50 55 60 65 70 75 80Intensity (a.u.)Degree (Θ) Before used After 10 cycle used134.2 °134.2 °(a) (b)(c) (d) (e) (f)Before After Before AfterS-H-PEG/PAgT Fig. 7. The proposed mechanism for S-H-PEG/PAgT photocatalytic system for removal of organic pollutants under solar light. (a) The scavenger study, (b) S-H-PEG/PAgT photocatalytic generation of active species and (c) surface characteristics synergistic effect on photocatalysis. Temperature: ambient, flow rate: 150 mL min-1, pH: 7, light intensity: 550 Wm-2, Scavengers concentration: 1M.