Bionanocomposite Hydrogel for the Adsorption of Dye and Reusability

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Bionanocomposite Hydrogel for the Adsorption of Dye and Reusability of Generated Waste for the Photo-degradation of Ciprofloxacin: A Demonstration of the Circularity Concept for Water Purification Neeraj Kumar, Hemant Mittal, Saeed M Alhassan, and Suprakas Sinha Ray ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04347 • Publication Date (Web): 22 Oct 2018 Downloaded from http://pubs.acs.org on October 25, 2018

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ACS Sustainable Chemistry & Engineering

Bionanocomposite Hydrogel for the Adsorption of Dye and Reusability of Generated Waste for the Photo-degradation of Ciprofloxacin: A Demonstration of the Circularity Concept for Water Purification Neeraj Kumara*, Hemant Mittalb, Saeed M. Alhassanb, Suprakas Sinha Raya,c* aDST-CSIR

National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria 0001, South Africa bDepartment of Chemical Engineering, Khalifa University of Science and Technology, PO Box 2533, Abu Dhabi, United Arab Emirates cDepartment of Applied Chemistry, University of Johannesburg, Doornfontein 2028, South Africa *Corresponding authors: N. Kumar ([email protected]; [email protected]), and S. S. Ray ([email protected]; [email protected]) Hemant Mittal ([email protected]) Saeed M. Alhassan ([email protected])

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Abstract Adsorption has emerged as a simple and economical approach to water decontamination; however, it creates large amounts of secondary toxic waste following the removal of the effluents from the water. The present investigation introduces an innovative circular approach that tackles the serious problem of environmentally toxic secondary waste. Herein, TiO2 nanorods (NRs) and a functionalized gum ghatti (Gg) biopolymer-based bionanocomposite hydrogel (TGB-hydrogel) was synthesized by freeradical graft polymerization and used to remove brilliant green (BG), which is a toxic dye. The dyeadsorbed TGB-hydrogel waste was then processed at 550 °C for 3 h and re-employed for the photocatalytic degradation of the antibiotic ciprofloxacin (CIP), after which the spent photocatalyst was reinstated for the adsorption of BG dye to complete the cycle. The ability of the TGB-hydrogel to adsorb the dye was studied in detail by varying the adsorbent dosage, initial dye concentration, pH, and temperature. Adsorption kinetics followed a pseudo-second-order kinetics model, whereas, the adsorption isotherm followed the Langmuir isotherm model with a maximum adsorption capacity of 740.97 mg g-1. The thermodynamic studies highlighted that the adsorption process was endothermic in nature. Furthermore, the obtained photocatalyst exhibited high photocatalytic efficiency, with 88.7% CIP degradation over 180 min due to a recombination delay of charge carriers, high light absorption and the high surface area (179.33 m2 g-1). The introduced circular-approach concept is envisaged to be applicable to other processes that need to avoid unwanted secondary waste. Keywords: Hydrogel, Nanocomposite, Circular-approach, Adsorption, Photocatalysis, Water purification, Secondary waste


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INTRODUCTION Water contamination is essentially the result of the alarming increment in world population and rapid industrialization. Water is polluted by a variety of hazardous effluents, such as dyestuffs, toxic heavy metals, inorganic anions, pesticides, cosmetics, and pharmaceuticals, especially antibiotics1-3. Numerous industries involved in metallurgical processes, electroplating, mining, pigments, steel production, textiles, leather, glass, paint, paper, and coal burning, discharge toxic waste pollutants into natural water bodies, which has led to a severe worsening of water quality. Over 7  105 tons of industrial dyes are released as effluents into water bodies every year4-5. In particular, dye-based effluents severely affect aquatic life as well as human health, as they are toxic, non-biodegradable, carcinogenic, and teratogentic6. Among various purification techniques, adsorption has proved to be the best and simplest technique for eliminating pollutants due to its cost-effectiveness, incredible versatility, simplicity in design, accuracy, ease of operation, high efficiency, and adsorbent reusability7-8 over other strategies, such as reverse osmosis9, solvent extraction, ion exchange10-11, coagulation-flocculation12, membrane separation13, ozonation14, chemical reduction, and advanced oxidation processes15. In addition to the many advantages of adsorption processes, one main drawback is the generation of large amounts of environmentally toxic secondary waste. It is imperative that this secondary toxic waste is disposed of in order to improve the practical applicability of the adsorption method. Unprecedented innovative approaches are needed to solve the secondary-pollution problem associated with the adsorption process. Apart from improving the adsorption strategy, the design and fabrication of cost-effective, efficient, and eco-friendly nanoadsorbents is another noteworthy challenge in the water-remediation field. The circular-economy concept has been adopted by many manufacturing industries as it offers solutions to a number of challenges, including resource scarcity, excessive waste generation, and the sustainment of economic advantages16-17. In this concept, the residue or waste material from any process is treated as a resource for a new process unless the material has lost all of its functionality or activity. 3 ACS Paragon Plus Environment


Residues can be processed under different conditions to improve their active sites (indirect route) or used without modification (direct route) in the next process (Scheme 1). We asked ourselves: Is it possible to apply the circularity concept to water-purification processes? Notwithstanding, adsorption is a favorable method for purification of water but its lead to secondary unwanted waste. In order to avoid secondary environmental pollution, the present investigation introduces a new circular approach that involves the following steps. Firstly, TiO2 nanorods (NRs) and a functionalized gum ghatti (Gg) biopolymer-based bionanocomposite hydrogel (TGB-hydrogel) was designed for the removal of the first toxic model pollutant, namely the brilliant green (BG) dye, from aqueous solution. Secondly, the dye-adsorbed TGBhydrogel was processed at high temperatures and reused for the photocatalytic degradation of a second model pollutant, namely the antibiotic ciprofloxacin (CIP). Finally, the spent photocatalyst was reinstated for the adsorption of the BG dye, which completes the cycle (Scheme 1). The use of active pharmaceuticals, especially antibiotics, for the treatment of diseases and infections in humans and animals has increased significantly18. The complete decomposition of most antibiotics is difficult in living organisms, which results in the excretion of residues into water bodies. The daily water consumption in a hospital is typically 260–940 L per bed, and the generated wastewater typically contains antibiotic effluents (viz., up to 101 µg/L ciprofloxacin)19-20. Ciprofloxacin is commonly used for the treatment of sexually transmitted diseases and skin infections21. Even low concentrations of antibiotics (>10 µL as defined by the US Environmental Protection Agency (EPA))22 significantly threaten the safety of entire ecosystems and humans by promoting the proliferation of bacterial drug resistance18. With this background in mind, photocatalysis is considered to provide a better choice for the segregation of antibiotics as it can lead to complete mineralization or less toxic products23-25. Moreover, the selection of a biopolymer, namely Gg, is an excellent choice due to its low-cost, high abundance, and large numbers of surface functional groups. TiO2 NPs were chosen due to their high adsorbing abilities, excellent photocatalytic properties, and cost effectiveness26-27. It is one of the most promising semiconductor photocatalyst due to its high photochemical stability, high catalytic efficiency, and good resistance to photocorrosion28. The structural properties, non-toxicity, stability and easy availability make TiO2 NPs 4 ACS Paragon Plus Environment

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good candidate for adsorption of various water pollutants29. Hence, this study introduces an innovative way to minimize secondary waste in the environment based on the maximum exploitation of materials for a variety of applications. EXPERIMENTAL SECTION Materials Gum ghatti (Gg, >95%), acrylamide (AAM, ≥99.9%), acrylic acid (AA, 99%), potassium persulfate (KPS, ≥99.0%), N,N’-methylenebisacrylamide (99%, MBA), ascorbic acid (ASC, ≥99%), brilliant green (dye content ~90 %), titanium dioxide (mixture of rutile and anatase; TiO2 nanopowder, C=O stretches of carboxylic acids), 1445 cm-1 (-N-H in-plane bending vibrations of amides), 1412 (-CH bending vibrations), 1166 cm-1 (-C-N stretches of amides), 1027 cm-1 (-C-O stretches), and 780 cm-1 (OCN amide deformations) were observed in the spectrum of the TGB-hydrogel. The observed characteristic acid and amide peaks indicate that the grafting and crosslinking of Gg was successful. The presence of broad Ti-O-Ti bands in the 550–800 cm-1 range confirms the successful incorporation of TiO2 NRs in the hydrogel polymer matrix. However, the peaks corresponding to the TiO2 NRs and the TGBhydrogel were shifted to slightly higher wavenumbers in the FTIR spectrum of the dye-adsorbed TGBhydrogel. The broad peaks in the 3480–3170 cm-1 range can be ascribed to –OH, -NH, -NH2 stretching vibrations of amines and associated amines, and hydroxyl groups that are extensively hydrogen bonded. The disappearance of peaks at 1710 cm-1 (>C=O) and 1027 cm-1 (-C-O stretches) indicates that the – COOH groups are involved in the adsorption of the BG dye, which is facilitated by electrostatic interaction between the –COOH groups of the hydrogel and the positively charged nitrogen center of the dye30. The peak at 1652 cm-1 broadened, indicating the presence of water molecules, aromatic -C=C bonds, and the adsorbed =N+ ammonium ion of the BG dye. The emergence of new peaks at 1578 and 1335 cm-1 is attributed to -C=C stretching vibrations of the aromatic ring and the –C-N groups of the 9 ACS Paragon Plus Environment


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adsorbed BG dye, respectively. The peak at 1167 cm-1 (-C-N amide stretch) shifted to 1184 cm-1 and increased in intensity, suggesting the involvement of amide groups in adsorption events. The changes observed for the vibrational peaks in the 1280–1020 cm-1 range (-C-O stretching vibrations of ether and other hydroxyl functionalities), reaffirmed the participation of –OH groups during the adsorption of the BG dye. Moreover, grafting and crosslinking in the TGB-hydrogel were confirmed by XPS, the survey spectrum of which clearly shows distinctive C, O, N, and Ti elemental peaks (Fig. S1a). Deconvolution of the C1s XPS spectrum (Fig. S1b) revealed peaks at 288.41, 286.36, and 284.89 eV that confirm the presence of –COOH/COO¯/-CONH2/-CONHCHNH-, C-N/C-O-C/C-OH, and C(sp3)-C(sp3)/C-H carbons of polymeric chains, respectively31-32. The O 1s spectrum was deconvoluted into two peaks at 532.96 and 531.68 eV that are assigned to –COO-/-COOH and >C=O, respectively32 (Fig. S1c). The high-resolution peaks in the 399.21–405.16 eV range are ascribable to –C-N/–CONH-/-CONH2 groups31-32 (Fig. S1d). As seen in Fig. S1e, the low intensity peak at 458.73 eV, which is assigned to Ti 2p3, confirms the presence of TiO2 on the surface of the TGB-hydrogel33. The

TiO2

NRs,

hydrogel

polymer

matrix,

and

TGB-hydrogel

were

subjected

to

Thermogravimetric analysis (TGA), the results of which are displayed in Fig. 4b. The TiO2 NRs exhibited an initial weight loss in the 25–160 °C range that is attributed to the loss of physisorbed/chemisorbed water. The second weight loss at 160–510 °C is due to the evaporation of residual HMTA present on the surface of the TiO2. No further weight losses were observed, indicating that a pure thermally stable rutile TiO2 phase had formed. On the other hand, the hydrogel polymer matrix exhibited several stages of weight loss in the 106.5–561.6 °C temperature range. The first stage (170.2– 325.2 °C), with a weight loss of 30.4%, is associated with dehydration and initial depolymerization. Thermal decomposition, with a weight loss of 41.1%, occurred in the 325.2–562.5 °C range through further depolymerization and the cracking of the initial polymeric network. The final decomposition, which was observed in the 562.5–892.8 temperature range and accounted for a weight loss of 3.4%, is 10 ACS Paragon Plus Environment

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ascribable to the complete collapse of the polymer matrix. The remaining weight (19.8%) of the obtained sample is possibly due to the formation of graphitic carbon. Moreover, the TGB-hydrogel had a lower initial decomposition temperature (IDT) and a higher final decomposition temperature (FDT) than the polymer matrix (Fig. 4b). The lower IDT value of the TGB-hydrogel is due to the presence of TiO2 NRs and HMTA residues, which releases ammonia and other functionalities, whereas the high FDT value is due to the formation of the stable 3D network of crosslinked polymers embedded with TiO2 NRs. The as-prepared samples were subjected to Brunauer–Emmett–Teller (BET) analysis in order to determine their specific surface areas, pore volumes, and pore diameters. The BET surface area, pore volume, and pore diameter of the TiO2 NRs were found to be 120.65 m2 g-1, 0.6825 cm3/g, and 18.5 nm, respectively, whereas the BET surface area, pore volume, and pore diameter of the TGB-hydrogel were found to be 2.4095 m2 g-1, 0.0085 cm3/g, and 39.95 nm, respectively; these values are higher than those previously reported for the Gg-cl-PAAM hydrogel34. The nature of BET pattern of TGB-hydrogel was similar to other reported nanocomposites35-36. Hence, we expect the as-prepared TGB-hydrogel to exhibit high adsorption capacity for dye removal. Adsorption studies The reaction conditions for the adsorption of the BG dye from aqueous solutions were first optimized. The effects of various parameters such as adsorbent dose and solution pH, on adsorption performance were investigated (Figs. 5a and b). In order to determine the optimum amount of adsorbent, various amounts of TGB-hydrogel (0.1–1.0 g L-1) were suspended in 100 mg L-1 of BG solution. Adsorption efficiency was observed to increase with increasing adsorbent dose and became constant at a specific value (Fig. 5a), which is due to decreases in the active surface area at higher concentrations that limit the adsorption of BG onto the surface. Adsorption sites aggregate and diffusion paths increase at higher concentrations, which also retard adsorption performance; consequently, the optimal concentration of adsorbent was found to be 0.6 g L-1. Furthermore, the effect of solution pH on the adsorption performance of the TGB-hydrogel was investigated using BG dye solutions of different pH, which revealed similar 11 ACS Paragon Plus Environment


trends to those observed in the adsorbent-dose experiments (Fig. 5b). Adsorption efficiency clearly increases with increasing solution pH and plateaued above pH 7.0. The adsorption of the dye on the biopolymer surface basically takes place through ionic interactions that, in turn, are governed by the surface charge of the adsorbent. Hence, the surface charge of the adsorbent was evaluated by determining the point of zero charge (pzc) of the adsorbent. The adsorbent surface is negatively charged when the pH is less than the pzc, which is suitable for the adsorption of cationic molecules. On the other hand, the adsorbent surface is positively charged when pH > pzc, which is suitable for the adsorption of anionic molecules. As shown in Fig. 5c, the pzc of the TGB-hydrogel was found to be 3.75; hence, the surface of the adsorbent carries positive charges under strongly acidic conditions, which reduces its ability to adsorb BG. Around pH 3.8 and below, cationic dye molecules and H+ ions (from the acidic solution) compete healthily for adsorption onto the TGB-hydrogel adsorbent, which lowers the adsorption efficiency as a consequence. Therefore, higher adsorption capacities were observed for the neutral and basic dye solutions. With these results in mind, the remaining adsorption experiments were carried under neutral pH condition. Dye-removal kinetics Efficient adsorbents are characterized by excellent adsorption performance over short periods of time under severe conditions, and rapid interactions between the adsorbent and model dye pollutant. The rate of adsorption is obtained from kinetics studies of dye removal; Fig. 6a displays the relationship between adsorption time and initial dye concentration. Adsorption capacity (qt) was plotted as a function of time at three dye concentrations, namely 50, 100, and 150 mg L-1. The rate of adsorption was observed to increase rapidly, after which it slowed down somewhat as adsorption approached equilibrium. The quicker adsorption equilibrium observed for the lower concentration dye solution due to stronger adsorption interactions. During adsorption, the adsorbate is first transported to the boundary layer of the external surface of the adsorbent and begins to diffuse into the surface and slowly into the internal


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network of porous structures through pore diffusion. This may delay reaching adsorption equilibrium at higher initial dye concentration. To obtain further insight into the adsorption mechanism governing the adsorption of BG onto the TGB-hydrogel, the experimental data were fitted to several available kinetics models, including pseudofirst-order (Fig. 6b), pseudo-second-order (Fig. 6c), intraparticle diffusion (Fig. 6d), liquid-film diffusion (Fig. 6e), and the Elovich model (Fig. 6f). The mathematical expressions for these kinetics models and associated parameters are summarized in Table S2. The kinetics model that fitted best was determined by the coefficient of determination (R2), with the highest value corresponding to the best model. Calculated values of R2 and other kinetic parameters related to the above-mentioned kinetic models are summarized in Table 1, which reveals that the pseudo-second-order kinetics model, with an R2 of 0.999 best fitted the experimental kinetics data. Table 1 also reveals that the pseudo-second-order-calculated values of qe are in close agreement with the experimental values, reaffirming that the adsorption of BG onto the TGBhydrogel is governed by pseudo-second-order kinetics. Similar observations were previously reported for the removal of dye by a chitin hydrogel37 and a gum karaya hydrogel8. The experimental kinetics data were fairly well described by the Elovich model (R2 ~0.95 to 0.98), a result of strong interactions between the molecular BG cation and the adsorbent. Furthermore, the Weber–Morris intraparticle-diffusion kinetics model was applied in order to proper understanding of the external diffusion process. According to this model, the adsorption of a liquid onto the surface of solid substrate involves three stages: (i) initial diffusion to the boundary surface of the adsorbent, (ii) diffusion of the liquid into pores of the adsorbent or intraparticle diffusion, and (iii) adsorption by the interior surfaces of the pores38. As shown in Fig. 6c, the intraparticle-diffusion curve for the adsorption of BG shows three linear regions. The first region of the curve represents surface or film penetration, whereas the second region suggests intraparticle or pore diffusion of the dye molecules, which is the rate-limiting step, while last linear region corresponds to the final equilibrium phase. The qt vs. t0.5 curve must pass through the origin for the intraparticle-diffusion model to solely determine the adsorption mechanism. As seen in Fig. 6c, the fitted lines do not pass



through the origin, indicating that this model does not exclusively describe the adsorption mechanism. The liquid-film diffusion model was also investigated for its applicability to the adsorption mechanism (Fig. 6d); this curve also did not pass through the origin, nor was its R2 value satisfactory, indicating that this model also does not exclusively describe the rate-determining step for cationic BG adsorption onto the TGB-hydrogel. Moreover, it is evident that the present adsorption study follows pseudo-second-order kinetics, along with slight involvements of the Elovich and pseudo-first-order kinetics models. We conclude that this adsorption process is a combination of intraparticle and external mass-diffusion processes. Similar dye-adsorption behavior was exhibited by magnetic nanohydrogels38 and hazelnut powder39. Adsorption-isotherm studies Adsorption-isotherm experiments involving the cationic BG dye and the TGB-hydrogel were performed at different temperatures (30, 40, and 50 °C) at neutral pH. To understand the distribution of BG dye on the surface of the as-prepared TGB-hydrogel at the molecular level and the nature of their interactions, experimental adsorption data were fitted to six isotherms models, namely Langmuir, Freundlich, Halsey, Dubinin–Kaganer–Radushkevich (DKR), Temkin, and Jovanovic. The mathematical expressions for these isotherm models are summarized in Table S3. Isotherms nonlinearly fitted by the abovementioned adsorption models at various temperatures (30, 40, and 50 °C) and shown in Figs. 7a-c. Lower dye removal from solutions of higher initial dye concentration was observed at all three temperatures, while it was higher for solutions of lower initial dye concentration. This is due to the presence of sufficient available adsorption sites at lower initial dye concentrations, whereas these adsorption sites become saturated by occupied dye molecules in solutions of higher initial dye concentration, which results in equilibrium adsorption of the BG dye onto the TGB-hydrogel. Moreover, the adsorption capacity of the TGB-hydrogel increased with temperature, which suggests that the adsorption events are endothermic in nature. By considering the R2 values (R2 ~0.999), the Langmuir adsorption model was observed to fit the experimental adsorption data the best, which assure the


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homogenous monolayer coverage of BG on the TGB-hydrogel. The calculated value of R2 and other parameters relating to the various isotherm models are listed in Table S4. The maximum adsorption capacities (qm) were determined to be 740.97, 773.68, and 817.63 mg g-1 at 30, 40, and 50 °C respectively. The increments in qm observed with increasing temperature are attributed to the effect of induced swelling within the hydrogel, which provides more passages to the binding sites of the adsorbent for the incoming BG dye molecules. Separation factors (RL) in the 0–1 range were obtained, which also validates the Langmuir adsorption model (Table S4). The DKR model provided values of E of about 2.10–2.35 kJ mol-1, which is in the 1–8 kJmol-1 range associated with physical adsorption, suggesting that the BG dye is physically adsorbed onto the surface of the TGB-hydrogel.

Adsorption thermodynamics Thermodynamic data provide statistical details about inherent energetic changes related to the adsorption process. The nature and spontaneity of adsorption process were investigated by determining thermodynamic parameters, including the standard free-energy change (ΔG°), standard entropy change (ΔS°), and standard enthalpy change (ΔH°). The mathematical equations used to calculate these parameters are detailed in Table S5. The slope and intercept of the Van’t Hoff plot (ln 𝐾𝐿vs. 1/T) provide values of ΔH° and ΔS°, respectively (Fig. 7d). The values of ΔG° determined at different temperatures, as well as those of ΔH° and ΔS° are listed in Table 2. The positive values of ΔH° and ΔS° indicate that the adsorption process is endothermic and proceeds with an increase in disorder at the interface between the TGB-hydrogel and the adsorbent. The low calculated value of ΔH° (in the ~10 to 40 kJmol-1 range) indicates that the current adsorption process is physical in nature. The negative value of ΔG° reveals that the adsorption of the BG dye onto the surface of the TBG-hydrogel is a spontaneous process, and increases in ΔG° with increasing temperature implies that the adsorption process is favored at higher temperatures. In addition, the adsorption capacity of the pure polymer matrix or functionalized gum ghatti and used TiO2 NRs was calculated in comparison to TGB-hydrogel. The adsorption experiments were carried 15 ACS Paragon Plus Environment


out using different concentrations of dye solution (100-500 mgL-1) at 30 °C and the obtained data were applied in the Langmuir adsorption isotherm (Fig. S2, ESI). The maximum adsorption capacity of TiO2 NRs was found to be 207.12 mg.g-1, whereas, it was 424.68 mg.g-1 for polymer matrix which was significantly less than TGB-hydrogel. The high adsorption capacity of the TGB-hydrogel in comparison to individual TiO2 NRs and polymer matrix could be attributed to additional functionalities and binding sites on the surface, high surface area, increased porosity and hydrodynamic volume due to the presence of TiO2 NRs. The maximum adsorption capacity (qm) of the TGB-hydrogel towards BG was compared to various reported adsorbents in literature40-49 and listed in Table 3. The adsorption capacity of TGBhydrogel (qm = 740.97 mg.g-1) for removal of BG was significantly higher than various listed adsorbents, suggesting its potential for practical applications. We also analyzed the regeneration and reusability behavior of the TGB-hydrogel, as shown in Fig. S3. TGB-hydrogel reusability was examined in ethanol over five consecutive adsorption-desorption cycles. The data reveal that the adsorption efficiency was constant for the first two cycles, but slightly decreased over the next three cycles; however, the decrease in efficiency was minimal, at ~7.8%. The adsorption capacity of materials depends on numerous aspects: availability of binding sites on the surface, hydrophilic properties, structural and functional nature of adsorbent and adsorbate, diffusion process, mass transport and various type of interactions50. In this work, once the TGB-hydrogel carrying negative functionalities was added into the BG solution, the nanocomposite could favorably adsorb cationic BG dye due to electrostatic interaction. The disappearance of carbonyl and –C-O peaks from FTIR spectrum (Fig. 4a) of dye-adsorbed TGB-hydrogel indicates that the –COO groups are involved in the adsorption of the BG dye, which is facilitated by electrostatic interaction between the – COO groups of the hydrogel and the positively charged nitrogen center of the dye. Additionally, the excellent adsorption capacity of the TGB-hydrogel for BG-dye removal might be collectively attributable to effective electrostatic interactions, van der Waals forces, ion exchange,  interactions, hydrogen


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bonding, and Yoshida hydrogen-bonding interactions between the BG dye molecules and the TGBhydrogel8, 51. Based on the FTIR data, the best possible interaction sites between the BG dye and the TGB-hydrogel adsorbent are depicted in Scheme 3. Photocatalytic degradation of ciprofloxacin (CIP) The waste TGB-hydrogel obtained following adsorption of the BG dye was used as the resource material for the photocatalytic degradation of CIP. Prior to this application, the waste TGB-hydrogel was processed by heat treatment at 550 °C for 3 h to generate active sites; this material was thoroughly characterized by XRD, FTIR, BET, X-ray Photoelectron Spectroscopy (XPS), Photoluminescence (PL), and Raman techniques. For clarity, a sample obtained following heat treatment is referred to as a “carbon TiO2 nanocomposite” (C-TiO2 NC) throughout this article. The crystalline structure of the as-prepared CTiO2 NC was examined by XRD (Fig. 8ai); all high-intensity peaks observed in the XRD pattern are in good agreement with those of crystalline rutile TiO2, whereas the amorphous functionalities of the waste TGB-hydrogel were significantly less obvious in the pattern of C-TiO2 NC, as the broad peak was less intense compared to that of the dye-adsorbed TGB-hydrogel, and its center had shifted to a 2 value of 18.2°, which may be due to the conversion of adsorbed aromatic dye molecules and polymer chains into graphitic carbon during the high-temperature treatment. Fig. 8aii shows the FTIR spectrum of the C-TiO2 NC. The characteristic peaks at 3397, 1636, 1065, and 729 cm-1 correspond to –N-H/O-H stretches, -C=C stretches, -C-N stretching/C-H bending vibrations, and Ti-O-Ti bands or out-of-plane C-H bending modes, respectively, while overtone peaks in 1850–2150 cm-1 range are ascribable to substituted aromatic rings. The FTIR data suggest the presence of TiO2 and graphitic-like carbon arrangements that are partially substituted with N and O. The Raman peaks at 233, 439, and 608 cm-1 correspond to the three active Raman modes of the TiO2 rutile phase52, with respective Eg (both 233 and 439 cm-1) and A1g symmetries (Fig. 8aiii). An additional vibrational mode at 682 cm-1 in the lower-frequency region is attributable to Ti-O-C bending53. The Raman spectrum also exhibited two peaks at 1389 cm-1 (D band) and 1608 cm-1 (G band) that are ascribable to sp3-carbon defects and sp2-carbon vibrations, respectively54. 17 ACS Paragon Plus Environment


The observed D- and G-band peaks are shifted to higher wavenumbers than the literature values, consistent with a high number of defects in the present carbon system55. Moreover, the relatively high intensity of the G band indicates the presence of graphitic of carbon in the C-TiO2 NC. In addition, the XPS results reaffirm the existence of carbon in the form of -C-C/-C=C bonds, and TiO2 NPs in the CTiO2 NC (Fig. S4). The specific BET surface area and pore-size distribution were calculated through N2adsorption/desorption experiments (Fig. 8aiv); the specific surface area, pore volume, and pore diameter were determined to be 179.2352 m² g-1, 1.1162 cm3 g-1, and 55.7746 nm, respectively, which are significantly higher than those of the TGB-hydrogel. The morphology of the C-TiO2 NC was examined by HRTEM; the TEM images displayed in Fig. 8bi-ii reveal the presence of interconnected TiO2 NRs and a layered carbon structure. Large TiO2 NRs were observed, which is ascribable to coalescence during heating. The TiO2 NRs was observed with continuously crystalline rutile phase with a large number of defects (Fig. 8biii). The clearly observed lattice fringe, with a d spacing of 0.328 nm, corresponds to the (101) crystal plane of rutile TiO2. Moreover, the SAED pattern as shown in Fig. 8biv, with bright spots and faded circular rings, confirm the polycrystalline nature of the C-TiO2 NC. The optical properties of the as-prepared C-TiO2 NC were studied by UV-vis spectroscopy, the results of which are shown in Fig. 9a. C-TiO2 NC exhibited a band-edge absorption at ~454 nm and a characteristic rutile-TiO2 peak with a band gap energy of ~2.73 eV. The broad visible-light absorption band to 690 nm is the result of excessive defects in the C-TiO2 NC. According to the literature, TiO2 is an indirect band gap semiconductor and its band gap can be determined by extrapolating the tangent of a Tauc plot of (αhν)1/2 vs. hν to the x-axis (Fig. 9aii); the band gap of the C-TiO2 NC was found to be 2.72 eV by this method. According to its light-absorbing properties, C-TiO2 NC should be an active visiblelight photocatalyst. The apparent valance-band edge of the C-TiO2 NC was calculated by the linear extrapolation of the leading edge of its XPS valence band (VB) spectrum to the x-axis (Fig. 9b). The actual value of the VB was determined to be 2.61 eV versus normal hydrogen electrode after standardization with reference Fermi level, which is smaller than that of standard rutile TiO2 (~2.89 eV 18 ACS Paragon Plus Environment

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vs. normal hydrogen electrode)56. The upward shift in the upper VB region is possibly due to visible absorption by excessive defects and the graphitic C arrangement. The upward shift in VB top is also related to the emergence of its visible absorption. To understand the observed enhancement in charge separation due to the alignment of band positions in a multiphase system, a Mott-Schottky experiment was performed to determine the flat band potential (Efb) of the C-TiO2 NC in 0.5 M KOH electrolyte. Fig. 9c displays the Mott-Schottky plot used to calculate Efb and the charge-carrier density (Nd) of the C-TiO2 NC, as determined by the following equations56-57:

1 𝐶

(

𝑘𝑇

2 𝐸 ― 𝐸𝑓𝑏 ― 𝑒

=

𝑁𝑑 =

𝑁𝑑𝑒0𝜀𝜀0𝐴

2 (𝑒0𝜀𝜀0)

0

( )

𝑑

)

(2),

2

1 𝐶2

[ ]

―1

𝑑𝑣

(3),

where C, ε0, ε, and A represent the space charge capacity, permittivity of a vacuum (8.86  10-12 F m-1), dielectric constant of the used material, and the active surface area, respectively, while e0 refers to the electron charge (1.6  10-19 C), V is the applied potential, T is the absolute temperature (K), and k is the Boltzmann constant (1.38  10-23 J K-1). The Efb for C-TiO2 NC was determined by extrapolating the tangent of the 1/C2 vs. V plot to the x-axis; the slope of this plot provides the carrier-charge density (Nd). The Mott-Schottky plot for C-TiO2 NC exhibits a positive slope that is characteristic of an n-type semiconductor58. The charge density for C-TiO2 NC is found to be 0.15  1018 cm-3. As defect sites always contribute to charge density, the high charge density suggests the presence of a high number of defects sites in C-TiO2 NC. The bottom of the conduction band (~ 0.10 eV) of the C-TiO2 NC was higher than that of standard rutile TiO2 (0.11 eV vs. normal hydrogen electrode), which indicates higher charge density and faster charge transfer59-61. The observed shifts in conduction band edge and valence band are possibly due to the existence of a multiphasic system composed of graphitic C, excessive 19 ACS Paragon Plus Environment


defects, and the TiO2 phase. The band gap energy calculated from the Mott-Schottky plot and the XPS VB was found to be 2.71 eV, which is consistent with the band gap (~2.72 eV) obtained from the Tauc plot. The calculated band energy states for the C-TiO2 NC are shown in Fig. 9d. The photocatalytic properties of the as-prepared C-TiO2 NC toward the degradation of CIP (15 ppm) in water were investigated, the results of which are summarized in Fig. 10. For maximum C-TiO2NC efficiency, the pH was optimized while maintaining other reaction parameters at constant values. As seen in Fig. 10a, the photocatalytic efficiency increased as the solution pH was increased to 6, and decreased at higher pH values. At lower pH, both the C-TiO2 NC and the CIP molecules are protonated, which, in turn, results in electrostatic repulsion and decreased photocatalytic activity62. At neutral pH, the lower adsorption of CIP onto the C-TiO2 NC is due to the electroneutral conditions that lead to slow photodegradation kinetics. The observed decrease in photodegradation rate at high pH is due to electrostatic repulsion between CIP and the C-TiO2 NC, since they both carry similar negative charges under these conditions. Therefore, all experiments were conducted at pH 6 where maximum degradation was observed. Fig. 10b shows the temporal evolution of the UV-vis absorption spectrum of CIP during its photodegradation. The absorbance of CIP was observed to decrease with time, which highlights the photodegradation properties of the photocatalyst. Furthermore, only minimal (~6.1%) CIP photodegradation was observed in the absence of the photocatalyst (blank experiment), confirming that photodegradation was primarily a photocatalytic process (Fig. 10c); a photocatalytic efficiency of 88.7% was observed after 180 min during the C-TiO2-NC-catalyzed degradation of the CIP solution. In comparison to C-TiO2-NC, bare TiO2 NRs have exhibited less photocatalytic efficiency (69.45%) over 180 min (Fig. S5). The enhanced photodegradation performance of nanocomposite was noticed due to high light absorption and photogenerated electrons trapping by graphitic carbon to facilitate long separation of electron and hole pairs. Moreover, the kinetics of the present photocatalyst were characterized by fitting the experimental data to the following first-order kinetics reaction:


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ln

( ) = 𝐾𝑡 𝐶0

(4),

𝐶𝑡

where K represents the apparent rate constant (min-1), and C0, Ct, and T are as previously defined. Fig. 10d reveals that the relationship between ln(C0/Ct) and t is linear, which suggests the experimental data are well fitted by first-order kinetics with a rate constant of 0.0114 m-1. In order to determine the role of active species in the photocatalysis event, sacrificial reagents, namely IPA, EDTA-2Na, and BZQ, were employed as scavengers of hydroxyl radicals, photogenerated holes, and superoxide radical anions, respectively5, 63. The efficiency of the photocatalytic degradation of CIP in the presence of each of these sacrificial agents was found to be much lower than the experiment in the absence of additive (~88.7%), which indicates that all of the above-mentioned reactive species are involved in the photocatalysis process (Fig. 10e). The very low photocatalytic efficiency (~7.6%) observed in the presence of BZQ indicates that superoxide radical anions contribute significantly to the degradation of CIP over the C-TiO2 NC photocatalyst when exposed to UV-vis radiation. In addition, C-TiO2 NC was tested for eleven successive cycles for photodegradation of CIP under similar conditions to investigate the reusability possibility of photocatalyst (Fig. 10f). The photodegradation performance was not significantly decreased after five cycles, which indicates high stability and reusability of the catalyst. In later cycles, the loss in the degradation efficiency of CIP in the reusability experiments was occurred due to a decrease in active sites and mass loss of the catalyst with time. Furthermore, the XRD pattern of the spent C-TiO2 NC revealed no structural or compositional variations after the degradation of CIP (Fig. S6), suggesting good photocatalyst stability. After photocatalysis experiment, the FTIR measurement was performed again (Fig. S7) which did not show any extra peaks of CIP besides the shift in previous peaks of C-TiO2 NC and rearrangements of peaks that related to Ti-O-Ti bands or out-of-plane C-H bending modes were noticed. Low-intensity peaks located at 2927 and 2980 cm-1 might be ascribed to -CH2 stretching of composite carbon or adsorbed degraded products of CIP. This observation suggests that there was no adsorption of CIP onto C-TiO2 NC during photocatalysis rather photodegradation occurred by proposed photocatalyst. 21 ACS Paragon Plus Environment


Photoinduced reactive charge carriers and their migrations in the C-TiO2 NC during the photodegradation of CIP are schematically displayed in Fig. 11 and detail mechanism has been explained using equations. Upon excitation of C-TiO2 NC using light, TiO2 NRs generate the charge carriers (electrons and holes). The isolated charge carriers can shift to the surface of the nanocomposite in order to participate in the reaction of holes with H2O molecules to produce •OH (hydroxyl radicals) and the reaction of dissolved O2 with electrons to yield •O2− (superoxide anion radicals). At the same time, photogenerated e− can transmit to the graphitic carbon of nanocomposite which delays the recombination rate of e−h+ pairs and increase the stability and abundance of photogenerated charge. Moreover, the high amount of reactive oxygen species viz. •OH and •O2− are accountable for photodegradation of CIP to intermediates products and eventually to less harmful products. Therefore, the high photocatalytic efficiency of C-TiO2 NC might be ascribed to several common factors, including enhanced light-absorption capacity via a photosensitizing effect and the delayed recombination of electron-hole pairs due to effective charge transfer via the graphitic carbons network57, 64-67. The waste C-TiO2 NC produced following the photocatalysis experiments was washed thoroughly with water and ethanol, and dried in an air oven to reuse in dye-adsorption experiments, thereby completing the circular approach introduced herein. The removal capacity of the multi-used material toward the BG dye was 386.54 mg g-1, indicating that the C-TiO2 NC has retained sufficient active functional sites for a variety of applications. Conclusion A functionalized Gg-based bionanocomposite hydrogel containing TiO2 nanorods (TGB-hydrogel) was synthesized using free-radical graft polymerization. The synthesized TGB-hydrogel exhibited an excellent adsorption capacity (qm = 740.97 mg g-1) for the removal of BG dye from an aqueous solution. The presence of large numbers of functional groups, such as hydroxyl, carboxylic acid, and amide, as well as TiO2 NRs within the hydrogel are responsible for this exceptionally high adsorption capacity. The


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adsorption of BG follows the monolayer Langmuir isotherm and the pseudo-second-order rate equation, whereas dye diffusion partially follows both intraparticle diffusion as well as liquid-film diffusion mechanisms. The dye-adsorbed waste TGB-hydrogel was calcined at 550 °C for 3 h to generate active sites, and was redeployed as a photocatalyst for the photodegradation of the antibiotic CIP. A MottSchottky experiment, as well as valence-band XPS and optical spectroscopy were used to investigate the band positional alignment in the C-TiO2 NC. The observed shifts in band positions are possibly ascribable to the existence of a multiphase system composed of graphitic C, excessive defects, and the TiO2 phase. The photocatalytic efficiency of the C-TiO2 NC toward the degradation of a CIP solution was 88.7% over 180 min; this high photocatalytic efficiency is attributed to a delay in the recombination of electron-hole pairs due to the effective circulation of photogenerated electrons via graphitic carbons, and the high surface area of the C-TiO2 NC photocatalyst (179.33 m2 g-1). Following the photocatalysis experiments, the produced C-TiO2 NC waste was reemployed for the adsorption of the BG dye (qm = 386.54 mg g-1), which completes the circular water-purification approach introduced herein. In this way, the circular approach is beneficial for the purification of water and related processes and tackles the conundrum associated with secondary environmentally toxic waste, leading to a more sustainable system. Associated Content Supporting Information: The supporting information is accessible free of charge on the ACS websites at Details of characterization techniques used, procedure for adsorption and photocatalytic studies, swelling capacity, synthesis mechanism of hydrogel, list of kinetics models, isotherms models and thermodynamic parameters, calculation for isotherm parameters, comparative adsorption capacity of TiO2 NRs and polymer matrix, XPS of hydrogel and C-TiO2 NC, reusability studies, photocatalytic activity of TiO2 NRs, XRD and FTIR pattern of spent photocatalyst



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45. Mittal, H.; Kumar, V.; Alhassan, S. M.; Ray, S. S., Modification of gum ghatti via grafting with acrylamide and analysis of its flocculation, adsorption, and biodegradation properties. International journal of biological macromolecules 2018, 114, 283-294, DOI 10.1016/j.ijbiomac.2018.03.131. 46. Saif Ur Rehman, M.; Kim, I.; Rashid, N.; Adeel Umer, M.; Sajid, M.; Han, J. I., Adsorption of Brilliant Green dye on biochar prepared from lignocellulosic bioethanol plant waste. CLEAN–Soil, Air, Water 2016, 44 (1), 55-62, DOI 10.1002/clen.201300954. 47. Zhang, M.; Chang, L.; Zhao, Y.; Yu, Z., Fabrication of Zinc Oxide/Polypyrrole Nanocomposites for Brilliant Green Removal from Aqueous Phase. Arabian Journal for Science and Engineering 2018, 111, DOI 10.1007/s13369-018-3258-3. 48. Shah, A. T.; Din, M. I.; Kanwal, F. N.; Mirza, M. L., Direct synthesis of mesoporous molecular sieves of Ni-SBA-16 by internal pH adjustment method and its performance for adsorption of toxic Brilliant Green dye. Arabian Journal of Chemistry 2015, 8 (4), 579-586, DOI 10.1016/j.arabjc.2014.11.046. 49. Azqhandi, M. A.; Ghaedi, M.; Yousefi, F.; Jamshidi, M., Application of random forest, radial basis function neural networks and central composite design for modeling and/or optimization of the ultrasonic assisted adsorption of brilliant green on ZnS-NP-AC. Journal of colloid and interface science 2017, 505, 278-292, DOI 10.1016/j.jcis.2017.05.098. 50. Xu, F.; Cheng, G.; Song, S.; Wei, Y.; Chen, R., Insights into promoted adsorption capability of layered BiOCl nanostructures decorated with TiO2 nanoparticles. ACS Sustainable Chemistry & Engineering 2016, 4 (12), 7013-7022, DOI 10.1021/acssuschemeng.6b01920. 51. Ghorai, S.; Sarkar, A.; Raoufi, M.; Panda, A. B.; Schönherr, H.; Pal, S., Enhanced Removal of Methylene Blue and Methyl Violet Dyes from Aqueous Solution Using a Nanocomposite of Hydrolyzed Polyacrylamide Grafted Xanthan Gum and Incorporated Nanosilica. ACS Applied Materials & Interfaces 2014, 6 (7), 4766-4777, DOI 10.1021/am4055657. 52. Chen, G.; Chen, J.; Song, Z.; Srinivasakannan, C.; Peng, J., A new highly efficient method for the synthesis of rutile TiO2. Journal of Alloys and Compounds 2014, 585, 75-77, DOI 10.1016/j.jallcom.2013.09.056. 53. Kernazhitsky, L.; Shymanovska, V.; Gavrilko, T.; Puchkovska, G.; Naumov, V.; Khalyavka, T.; Kshnyakin, V.; Chernyak, V.; Baran, J., Optical and photocatalytic properties of titanium–manganese mixed oxides. Materials Science and Engineering: B 2010, 175 (1), 48-55, DOI 10.1016/j.mseb.2010.05.034. 54. Denys, N.; Valentinas, S.; Boris, S.; Sanna, A.; Harri, L., Graphene-enhanced Raman imaging of TiO 2 nanoparticles. Nanotechnology 2012, 23 (46), 465703, DOI 10.1088/0957-4484/23/46/465703. 55. How, G. T. S.; Pandikumar, A.; Ming, H. N.; Ngee, L. H., Highly exposed {001} facets of titanium dioxide modified with reduced graphene oxide for dopamine sensing. Scientific Reports 2014, 4, 5044, DOI 10.1038/srep05044. 56. Li, L.; Yan, J.; Wang, T.; Zhao, Z.-J.; Zhang, J.; Gong, J.; Guan, N., Sub-10 nm rutile titanium dioxide nanoparticles for efficient visible-light-driven photocatalytic hydrogen production. Nature Communications 2015, 6, 5881, DOI 10.1038/ncomms6881. 57. Yang, H.; Wang, P.; Wang, D.; Zhu, Y.; Xie, K.; Zhao, X.; Yang, J.; Wang, X., New Understanding on Photocatalytic Mechanism of Nitrogen-Doped Graphene Quantum Dots-Decorated BiVO4 Nanojunction Photocatalysts. ACS Omega 2017, 2 (7), 3766-3773, DOI 10.1021/acsomega.7b00603. 58. Curti, M.; Kirsch, A.; Granone, L. I.; Tarasi, F.; López-Robledo, G.; Bahnemann, D. W.; Murshed, M. M.; Gesing, T. M.; Mendive, C. B., Visible-Light Photocatalysis with Mullite-Type Bi2 (Al1–x Fe x) 4O9: Striking the Balance between Bandgap Narrowing and Conduction Band Lowering. ACS Catalysis 2018, 8 (9), 8844-8855, DOI 10.1021/acscatal.8b01210. 59. Wang, G.; Wang, H.; Ling, Y.; Tang, Y.; Yang, X.; Fitzmorris, R. C.; Wang, C.; Zhang, J. Z.; Li, Y., Hydrogen-Treated TiO2 Nanowire Arrays for Photoelectrochemical Water Splitting. Nano Letters 2011, 11 (7), 3026-3033, DOI 10.1021/nl201766h. 27 ACS Paragon Plus Environment


60. Su, F.; Wang, T.; Lv, R.; Zhang, J.; Zhang, P.; Lu, J.; Gong, J., Dendritic Au/TiO2 nanorod arrays for visible-light driven photoelectrochemical water splitting. Nanoscale 2013, 5 (19), 9001-9009, DOI 10.1039/C3NR02766J. 61. Matsumoto, Y., Energy positions of oxide semiconductors and photocatalysis with iron complex oxides. Journal of solid state chemistry 1996, 126 (2), 227-234, DOI 10.1006/jssc.1996.0333. 62. Zhang, X.; Li, R.; Jia, M.; Wang, S.; Huang, Y.; Chen, C., Degradation of ciprofloxacin in aqueous bismuth oxybromide (BiOBr) suspensions under visible light irradiation: A direct hole oxidation pathway. Chemical Engineering Journal 2015, 274, 290-297, DOI 10.1016/j.cej.2015.03.077. 63. Wang, Y.; Shi, R.; Lin, J.; Zhu, Y., Enhancement of photocurrent and photocatalytic activity of ZnO hybridized with graphite-like C3N4. Energy & Environmental Science 2011, 4 (8), 2922-2929, DOI 10.1039/C0EE00825G. 64. Kumar, N.; Mittal, H.; Reddy, L.; Nair, P.; Ngila, J. C.; Parashar, V., Morphogenesis of ZnO nanostructures: role of acetate (COOH−) and nitrate (NO3−) ligand donors from zinc salt precursors in synthesis and morphology dependent photocatalytic properties. RSC Advances 2015, 5 (48), 3880138809, DOI 10.1039/C5RA04162G. 65. Sultana, S.; Mansingh, S.; Parida, K. M., Rational design of light induced self healed Fe based oxygen vacancy rich CeO2 (CeO2NS–FeOOH/Fe2O3) nanostructure materials for photocatalytic water oxidation and Cr(vi) reduction. Journal of Materials Chemistry A 2018, 6 (24), 11377-11389, DOI 10.1039/C8TA02539H. 66. Ong, W.-J.; Tan, L.-L.; Ng, Y. H.; Yong, S.-T.; Chai, S.-P., Graphitic Carbon Nitride (g-C3N4)Based Photocatalysts for Artificial Photosynthesis and Environmental Remediation: Are We a Step Closer To Achieving Sustainability? Chemical Reviews 2016, 116 (12), 7159-7329, DOI 10.1021/acs.chemrev.6b00075. 67. Hu, Y.; Gao, X.; Yu, L.; Wang, Y.; Ning, J.; Xu, S.; Lou, X. W., Carbon-Coated CdS Petalous Nanostructures with Enhanced Photostability and Photocatalytic Activity. Angewandte Chemie International Edition 2013, 52 (21), 5636-5639, DOI 10.1002/anie.201301709.


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Figures Fig. 1. XRD patterns of the prepared TiO2 NRs, TGB-hydrogel, and dye-adsorbed TGB-hydrogel. Fig. 2. (a, b) TEM images of TiO2 NRs; (c) HRTEM image of TiO2 NRs; (d) SAED pattern of TiO2 NRs; and (e, f) TEM images of the TGB-hydrogel. Fig. 3. SEM images of (a) TiO2 NRs, (b) Gg, (c) the polymer matrix, and (d, e) the TGB-hydrogel; (f) EDS maps and spectrum of the TGB-hydrogel. Fig. 4. (a) FTIR spectra and (b) TGA traces of TiO2 NRs, TGB-hydrogel, and dye-adsorbed TGBhydrogel; BET isotherms for (c) the TiO2 nanorods and (d) the TGB-hydrogel. Fig. 5. Effect of (a) composite-hydrogel dosage (solution pH = 7.0, V = 50 mL) and (b) solution pH on the adsorption efficiency (dose = 0.6 gL-1, V = 50 mL). (c) Determining the point of zero charge (pzc) of the composite hydrogel. Fig. 6. (a) Adsorption kinetics, (b) non-linear pseudo-first-order model fitting, (c) non-linear pseudosecond-order model fitting, (d) linear intraparticle-diffusion model fitting, (e) linear liquid-film diffusion model fitting, and (f) non-linear Elovich-model fitting for the adsorption of BG onto the TGB-hydrogel (dye concentration = 50, 100 and 150 mgL-1, dose = 0.6 gL-1, T = 30 °C, pH = 7).



Fig. 7. Fitting various non-linear isotherm models (Langmuir, Freundlich, Temkin, DKR, Jovanovic, Halsey) to the experimental data at temperatures of (a) 30 °C, (b) 40 °C, and (c) 50 °C (dose = 0.6 gL-1, pH = 7.0, V = 50 mL, and dye concentration =  mgL−1). (d) Van’t Hoff plot for the adsorption of BG onto the TGB-hydrogel. Fig. 8. (ai) XRD, (aii) FTIR, and (aiii) PL spectra of the C-TiO2 NC; (aiv) adsorption/desorption isotherms for the C-TiO2 NC with the inset showing the pore-size distribution; (bi-ii) TEM and (biii) HRTEM images, and (biv) SAED pattern of the C-TiO2 NC. Fig. 9. (a) UV-vis absorbance spectrum of the C-TiO2 NC; (ai) shows a photographic image of fluffy CTiO2 NC in the inset; (aii) depicts the Tauc plot that used to determine the band gap of the C-TiO2 NC. (b) Valence-band XPS spectrum of the C-TiO2 NC (c) Mott–Schottky plot and (d) band-energy states for the C-TiO2 NC. Fig. 10. (a) Optimizing the pH for the photocatalytic degradation of CIP with the C-TiO2 NC; (b) timeevolution of the UV-vis absorption spectrum of CIP during its degradation; (c) photocatalytic activities as a function of time; (d) relationship between ln(C/C0) and time; (e) effects of scavenging agents on photocatalytic activity; (f) reusability experiments over elven successive cycles. Fig. 11. Depicting the photoinduced reactive charge migrations in the C-TiO2 NC during the photodegradation of CIP. Photodegradation mechanism has been discussed using equations.


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Schemes Scheme 1. Depicting the circular-approach concept for water purification. Scheme 2. Illustrating the formation of the TGB-hydrogel. Scheme 3. Depicting the best possible sites for interactions between the BG dye and the TGB-hydrogel adsorbent.

Tables Table 1: Adsorption kinetics data derived from the various models. 31 ACS Paragon Plus Environment


Table 2: Thermodynamic data for the adsorption of the BG dye onto the TGB-hydrogel. Table 3: Comparative adsorption capacities of various adsorbents towards BG dye.

Fig. 1.

Fig. 2.


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Fig. 3.




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Fig. 4.

Fig. 5.



Fig. 6.

Fig. 7.


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Fig. 8.

Fig. 9.



Fig.10


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Fig. 11


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Scheme 1

Scheme 2



Scheme 3


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Table 1: Model

Pseudo-first-order

Pseudo-second-order

Parameter

Dye concentration (mg L-1) 50

100

150

k1 (min-1)

0.241

0.164

0.146

qe,cal (mg g-1)

95.42

131.24

166.14

qe,exp (mg g-1)

99.76

138.05

173.86

Reduced χ2

20.25

47.45

67.28

RSS

263.30

616.66

874.75

R2

0.972

0.969

0.973

K2 (g mg-1 min-1)

3.8×10-3

1.81×10-3

1.26×10-3

qe,cal (mg g-1)

101.60

141.70

180.11

qe,exp (mg g-1)

99.76

138.05

173.86

ho

36.34

39.22

40.87

Reduced χ2

0.173

1.431

2.210

RSS

2.25

18.60

28.73



Elovich

Intraparticle diffusion

Liquid-film diffusion

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R2

0.999

0.999

0.999

B (g mg-1)

0.0875

0.5218

0.0390

kE (mg g-1 min)

894.506

301.433

271.53

Reduced χ2

34.079

64.038

111.107

RSS

443.03

832.50

1444.40

R2

0.954

0.958

0.955

kID (S1) (mg g-1 min-1/2) 14.394

21.65

28.76

C(S1) (mg g-1)

29.94

26.25

25.32

R2 (S1)

0.937

0.959

0.968

kID (S2) (mg g-1 min-1/2) 0.889

1.754

2.348

C(S2) (mg g-1)

89.56

117.81

147.2

R2 (S2)

0.812

0.790

0.774

kFD (min-1)

0.0493

0.0391

0.0436

C1

1.054

0.869

0.752

RSS

2.523

2.079

1.586

R2

0.928

0.926

0.953


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Table 2:

ΔG° (kJmol-1) 30 °C

40 °C

50 °C

-6.607

-7.204

-7.722

ΔH° (kJmol-1)

ΔS° (kJmol-1K-1)

10.306

5.582

Table 3

Adsorbents

qm (mg/g)

T (°C)

Ref

Red clay

125

45

38

Hydrogel loaded with

26.31

35

39

DP/MWCNTs

434.78

30

40

PANI/Ag

49

30

41

kalonite



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Coal/Polyurethane foam

134.95

40

42

Gg-cl-PAAM hydrogel

523.62

25

43

Rice straw biochar

111.11

30

44

ZnO/PPy

140.8

25

45

Ni-SBA-16

322.58

50

46

ZnS-NP-AC

258.7

25

47

TGB-hydrogel

740.97

30

Present study

Graphical Abstract This report highlights a sustainable circular approach to solve the issue of secondary toxic waste generation during the water purification


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This report highlights a sustainable circular approach to solve the issue of secondary toxic waste generation during the water purification. 88x59mm (300 x 300 DPI)

ACS Paragon Plus Environment

Bionanocomposite Hydrogel for the Adsorption of Dye and Reusability

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