TiO 2 /CTS/ATP adsorbent and its application in Adsorption- 1 Ultrafiltration Process for dye wastewater purification

: Industrial dyeing produces highly polluting wastewater and threatens the 16 environment. Effective treatment of dyeing wastewater is a crucial step to prevent toxic 17 chemicals from entering receiving waters. This study aimed to assess a modified 18 attapulgite (ATP)-based adsorbent for dyeing wastewater purification by introducing 19 chitosan (C T S) and titanium dioxide (TiO 2 ) into the adsorbent material named 20 TiO 2 /CTS/ATP. It was found that, after modification, the adsorbent showed a lower 21 hydrophilicity, as demonstrated by an increase in the water contact angle from 9.1° to 22 42°, which could reduce the water adsorption tendency and potentially facilitate 23 contaminants adherence. The modification also led to a significantly increased specific 24 surface area of 79.111 m 2 /g from 3.791 m 2 /g, and exhibited more uniform and smaller 25 particle size (reduced to 2.52 µm from 3.99 µm). When the TiO 2 /CTS/ATP adsorbent 26 was applied to the adsorption of Congo red solution, the adsorption efficiency was 27 observed to reach to 97.6% at the dosage of 0.5 g/L. Furthermore, the combination of 28 adsorption and ultrafiltration was able to achieve 99% Congo red removal. Adsorption 29 pretreatment prior to the ultrafiltration also enabled to reduce membrane fouling, 30 increased the reversible membrane fouling and resulted in a considerably lower 31 transmembrane pressure as compared with the direct ultrafiltration filtration system.

42°, which could reduce the water adsorption tendency and potentially facilitate 23 contaminants adherence. The modification also led to a significantly increased specific 24 surface area of 79.111 m 2 /g from 3.791 m 2 /g, and exhibited more uniform and smaller 25 particle size (reduced to 2.52 µm from 3.99 µm). When the TiO2/CTS/ATP adsorbent 26 was applied to the adsorption of Congo red solution, the adsorption efficiency was 27 observed to reach to 97.6% at the dosage of 0.5 g/L. Furthermore, the combination of 28 adsorption and ultrafiltration was able to achieve 99% Congo red removal. Adsorption 29 pretreatment prior to the ultrafiltration also enabled to reduce membrane fouling, 30 increased the reversible membrane fouling and resulted in a considerably lower 31 transmembrane pressure as compared with the direct ultrafiltration filtration system. 32 7 wastewater after biological treatment was still higher than First class effluent standard 119 of China's "Discharge Standard of Water Pollutants for Dyeing and Finishing of Textile 120   Fig.1, CTS of different masses (0.08 g, 0.16 g, 0.32 g) was dissolved into 129 100 mL CH3COOH (2.0%), and 16 g acidified ATP was then added into the solution. 130 8 The solution was then mixed with 8 g acidified ATP or 8 g CTS/ATP (1.0%) and a 137 buffer solution composed of 0.5 mL HNO3, 10mL ethanol as well as 5 mL distilled 138 water. The mixture was stirred for 30 min and then dried and grinded to get TiO2/ATP 139 or TiO2/CTS/ATP powder. 140 2.3.4 Synthesis of CTS/TiO2/ATP 141 As show in Fig.1 The experiment was carried out by adding 0.05 g TiO2/CTS/ATP into 9 beakers 161 containing 100 ml Congo red solution (100 mg/L) with the adjustment of pH to 3, 4, 5, 162 6, 7, 8, 9, 10 and 11, respectively. The pH of the raw water was adjusted to the initial 163 target pH by the addition of dilute 0.1 M HCl or NaOH as necessary. The experimental 164 conditions were the same as above. (2) 198 Contact angle measuring instrument (OCA15Pro) was used to analysis the 200 wettability of the adsorbents. Distilled water was used as the reagent and the contact 201 angle was measured by the hanging drop method. For particle size analysis, a small 202 amount of adsorbent was initially subjected to ultrasonic dispersion in ultra-pure water 203 for 20 min, and a portion of the upper liquid was then transferred to laser particle size 204 analyzer (Mastersizer2000, Malvern Instruments Ltd., UK) for analysis. Surface Area 205 and Porosity Analyzer (ASAP2460) was used to measure the specific surface area of 206 adsorbent samples. Before testing, the sample was vacuumed at 100 ℃ for about 4 h. 207 N2 was used as adsorption molecules for the tests. It can be seen from Fig. 3a, b and c that modifying ATP achieved considerably 211 enhanced Congo Red removal. Among 0.5%, 1.0% and 2.0% CTS/ATP, the 1.0% had 212 the best Congo red adsorption efficiency (Fig. 3a). The lower Congo red removal at the 213 lower CTS concentration (0.5%) was likely attributed to the lower availability of The particle size distribution of TiO2 before and after modification is shown in Fig.  255 4(c). The average particle size of ATP, TiO2/ATP, CTS/ATP was 3.99 μm, 12.62 μm, 256 14.16 μm respectively, indicating that particle agglomeration occurred in the system 257 after the ATP was treated separately by TiO2 or CTS. In comparison, the TiO2/CTS/ATP 258 adsorbent displayed the lowest average particle size of 2.52 μm. The result suggests 259 that treating ATP with both TiO2 and CTS can not only solve the agglomeration problem, 260 but also distribute adsorbents more uniform in size (Siripatrawan &Kaewklin 2018). 261

Fig. 4(d) illustrates the water contact angle of the laminates of ATP, CTS/ATP and 262
TiO2/CTS/ATP. ATP had the strongest hydrophilicity with a water contact angle of 9.1°, 263 followed by CTS/ATP (11.7°). By contrast, the water contact angle of TiO2/CTS/ATP 264 soared to 42° and thus its hydrophobicity was improved substantially (Cieśliński 265 &Krygier 2014, Ku et al. 2011). Therefore, the TiO2/CTS/ATP modification a hindered 266 was anticipated to hinder the adsorption of water molecules during the treatment 267 process, which facilitated the removal of Congo red. 268

Adsorption efficiency of modified attapulgite for Congo red 269
It can be seen from Fig. 5(a)  The effect of different pH on the absorption of Congo red by TiO2/CTS/ATP is 278 shown in Fig. 5(b). The removal of Congo red by TiO2/CTS/ATP remained above 95% 279 when pH was between 3 and 8, and a higher pH Potential of TiO2/CTS/ATP adsorbent 296

Simulated actual dye wastewater
removal of Congo red from synthetic dyeing wastewater was only 22%. Severe 299 membrane fouling was observed, with TMP rising by 51 kPa within 30 minutes. The 300 resistance Rf, representing irreversible pollution, caused by adsorption or blockage 301 accounted for 70% of the membrane fouling resistance. The filter cake resistance Rc, 302 representing reversible fouling, was only responsible for 30% of the membrane fouling 303 resistance. When the adsorption-ultrafiltration process was combined, the removal of 304 Congo red was as high as 99%. Previous research also reported that a hybrid adsorption 305 membrane process also achieved More than 97 % dye removal in removing dyes from 306 surface pollutants (a-Virgin, b-cake by ultrafiltration, c-cake by ultrafiltration 328 after cleaning, d-cake by adsorption-ultrafiltration；e-cake by adsorption-329 ultrafiltration after cleaning). 330 As shown in Fig.8, the comparison between the adsorption-ultrafiltration process and 331 the controlled ultrafiltration process reveals that the levels of C, Na, which were the 332 main element of Congo red on ultrafiltration membrane, and the element on the 333 ultrafiltration membrane without pretreatment were higher than those measured for the 334 adsorption-UF membrane. This finding indicates that the adsorption-ultrafiltration 335 process alleviated the fouling of Congo red on the ultrafiltration membrane. 336 was significantly higher than its counterpart, as expected due to these four elements 338 being the main components of TiO2/CTS/ATP (Fig. 8 a). When the adsorption-339 ultrafiltration membrane was cleaned, the levels of these four elements decreased 340 significantly, suggesting that these elements contributed to reversible membrane 341 fouling (Fig. 8 b). 342 As shown in Fig. 9(a), when ultrafiltration was used to remove dye molecules, 346 membrane pores were easily blocked by dye molecules due to direct contact. To address 347 the fouling issue, pretreatment with adsorption was considered Fig. 8(b). The adsorbent 348 was produced by modifying ATP with CTS and TiO2. After modification, the water 349 contact angle of ATP increased from 9.1° to 42° (Fig. 4 (d)), indicating a decrease in 350 hydrophilicity, which reduced the adsorption of water molecules and improved Congo 351 red removal. The specific surface area also increased from 3.791 m 2 /g to 79.111 m 2 /g, 20 adsorption, pollutants attached with TiO2/CTS/ATP through electrostatic interaction and 354 hydrogen bonds Fig. 5. During filtration of pre-adsorbed wastewater, a cake layer 355 containing abundant adsorbents was formed on the ultrafiltration membrane. When 356 approaching the membrane, pollutants were intercepted by the cake layer without 357 entering and blocking the membrane pore (Fig. 9 b). As a result, not only enhanced 358 treatment efficiency (99% congo red removal) but also minimized membrane fouling 359 and improved water permeability were achieved. 360 361 Fig. 9 Schematic of mechanism for ultrafiltration in dye wastewater treatment (a) and 362 TiO2/CTS/ATP adsorbent combined with ultrafiltration in dye wastewater treatment 363 (b). 364

365
In current work, TiO2/CTS/ATP adsorbent was developed and applied for 366 adsorption-ultrafiltration process for dye wastewater purification. Our finding was 367 summarized as follows: 368 increased hydrophobicity and specific surface area of the adsorbent, and decreased the 371 average particle size, which was conducive to adsorption of Congo red. 372 (2) When the dosage of TiO2/CTS/ATP was 0.5 g/L, the removal of Congo red was 373 97.6%. Under acidic conditions, the removal of Congo red was almost not impacted. 374 Increasing pH over 10 was found to greatly reduce the treatment efficiency due to the 375 strongest electrostatic repulsion. 376 (3) The removal of Congo red was notably enhanced and reached up to 99% during 377 treatment with by the adsorption-ultrafiltration process. Membrane fouling was 378 considerably mitigated due to adsorption. Adsorption also increased the reversibility of 379 the fouling components.