Efficient Removal of Toxic Dyes via Simultaneous Adsorption and

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Efficient Removal of Toxic Dyes via Simultaneous Adsorption and Solar Light Driven Photodegradation Using Recyclable Functionalized Amylopectin-TiO2-Au Nanocomposite Amit Kumar Sarkar, Arka Saha, Abhrajyoti Tarafdar, Asit Baran Panda, and Sagar Pal ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01614 • Publication Date (Web): 10 Feb 2016 Downloaded from http://pubs.acs.org on February 17, 2016

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ACS Sustainable Chemistry & Engineering

Efficient Removal of Toxic Dyes via Simultaneous Adsorption and Solar Light Driven Photodegradation Using Recyclable Functionalized Amylopectin-TiO2Au Nanocomposite

Amit Kumar Sarkar†, Arka Saha┴, Abhrojyoti Tarafder‡, Asit Baran Panda┴*, Sagar Pal †*

†

Polymer Chemistry Laboratory, Department of Applied Chemistry, Indian School of Mines,

Dhanbad – 826004, Jharkhand, India ‡

Department of Environmental Science & Engineering, Indian School of Mines, Dhanbad –

826004, Jharkhand, India ┴

Discipline of Inorganic Materials and Catalysis, Central salt and Marine Chemicals Research

Institute (CSIR), G. B. Marg, Bhavnagar-364002, Gujarat, India.

AUTHOR INFORMATION †*

Corresponding author:

Tel: +91-326-2235769; E-mail: [email protected]; [email protected] (S. Pal) ┴*

Corresponding Author: Tel: +91-278-2567760, Ext-704; E-mail: [email protected]

RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required according to the journal that you are submitting your paper to)

1 ACS Paragon Plus Environment


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ABSTRACT Herein, functionalized amylopectin and in-situ generated TiO2 and Au nanoparticles (NPs) based nanocomposite with excellent toxic organic dyes removal capacity, via synergistic effects of adsorption and photocatalytic degradation, along with enhanced antimicrobial activity, is reported. The TiO2 and TiO2-AuNPs incorporated nanocomposites (g-AP-pAA/TiO2, g-APpAA/TiO2-Au) have been synthesized by addition of ethanolic solution of titanium isopropoxide (Ti-iPr) followed by HAuCl4 to the gelling matrix of functionalized amylopectin. Characterization results confirm the formation of uniform distribution of anatase TiO2 (5-8 nm) and Au (7-15 nm) NPs in the polymer matrix. The synthesized g-AP-pAA/TiO2-Au nanocomposite demonstrates excellent potential to remove dyes from concentrated aqueous dye solution via simultaneous adsorption followed by photocatalytic degradation under sunlight through plasmonic photocatalysis. Synthesized g-AP-pAA/TiO2-Au is capable to remove specific one dye from mixture of dyes. Besides, the photo induced bacterial (Escherichia coli) growth inhibition characteristics of composite indicate the development of a novel new generation sustainable and cost effective polymeric nanocomposite. KEYWORDS: Antimicrobial activity; Dye degradation; Nanocomposite; Photocatalytic degradation; TiO2-AuNPs.


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INTRODUCTION Nanocomposites based on organic-inorganic hybrid systems represent new class of advanced materials with novel properties compared to individual counterparts.1-2 Simply with variation of nanofiller concentration, nanocomposites with desired properties can be developed. The immobilization of nanoparticles onto matrix provides an efficient route for the synthesis of nanocomposite. The matrix moiety prevents the agglomeration of NPs and increased the exposed surface area. Among the various matrix materials, polymers are snatching the attention owing to its ability to control the growth and stabilize the in-situ synthesized NPs.3 Efficient removal of concentrated organic dyes through adsorption using different functionalized polymers and polymeric nanocomposites is one of the most attractive and cheapest ways. 4 Although adsorption is considered as a most reliable process, but disposal of desorbed organic dyes has become a serious issue. Thus, it is imperative to design a nanocomposite with both adsorption and degradation characteristics to overcome the disposal problem. Titania (TiO2) is an extensively studied photocatalyst owing to its excellent photocatalytic activity, robust chemical stability, cost effectivity, and abundant nature.5-6 However, its serious drawbacks, like active in UV-light, recombination of photo-generated holes-electron and difficult to separate from aqueous solution, restricts its efficient use for wastewater treatment in large scale.7-8 Further, its photocatalytic efficiency in a high concentrated dye solution is very low, due to low adsorption efficiency and most of the light is absorb by dye.-9-10 Till date, various attempts have been made to address the above mentioned shortcomings of TiO2 as photocatalyst.11-12 Surface Plasmon Resonance (SPR) is occurred, when frequency of incident light resonates with plasmon oscillation frequency and light got absorbed.13 Commonly the noble metals, specifically Au, exhibited SPR property in visible light



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and showed photocatalytic activity under visible light.14-16Sometimes noble metal based nanocomposites showed improved photocatalytic activity through synergism between the support and material.17Ample of reports are there on visible light driven plasmonic photocatalyst based on TiO2-noble metal, specifically TiO2-Au.10,

18, 19

Au in TiO2 not only facilitate the

absorption of visible light through plasmonic effect but also restricts the recombination of photogenerated electron-hole.7, 8, 14 It is reported that TiO2 derived from alkaline conditions showed good adsorption property, but improvement is not so distinctive. 20 Attempts have been made to address the separation problem by incorporating the TiO2 based photocatalyst on polymer matrix, like polythene beads, nafion, polyaniline, epoxy resin, without considering the adsorption efficiency.21-24 Most of the reported composites were synthesized post incorporation of TiO2 NPs in the polymer matrix. 2526

Till date, there are no reports addressing all the above mentioned shortcomings combining in

one work. Therefore, it is imperative to develop a suitable TiO2 based material, which will overcome all the mentioned disadvantages. It has been observed that incorporation of nanoparticles on functionalized natural polymer, the adsorption efficiency increased by several times.27-28 Thus, it is imperious to explore the toxic dye adsorption followed by photocatalytic degradation using TiO2-Au nanoparticle incorporated functionalized natural polymer based nanocomposite. It is expected that such nanocomposite may solve all the shortcomings of TiO2 based photocatalyst. Au-TiO2 nano-structures have been studied extensively for visible light induced efficient plasmonic photo-catalytic degradation of dye, 7, 14-16 as Au suppressed electronhole recombination efficiently via forming a Schottky barrier at the Au-TiO2 interface and capable to transfer electrons from the conduction band of TiO2 to surface adsorbed O2 efficiently. 29-32


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Here in, first time, we report the novel nanocomposite of TiO2/TiO2-Au incorporated copolymer developed from amylopectin and poly (acrylic acid) (g-AP-pAA/TiO2 and g-APpAA/TiO2-Au). A simple low temperature synthetic method is adopted for the synthesis of nanocomposite, through formation of TiO2 and TiO2-Au nanoparticles in-situ, in aqueous gelling matrix of g-AP-pAA, at 50 °C using the ethanoic titanium isopropoxide (Ti-iPr) and HAuCl4 as corresponding precursors. Both the nanocomposites led to higher rate of adsorption of toxic cationic methyl violet (MV)/ anionic acid orange 7 (AO) dyes. Besides, rate of MV removal was accelerated by efficient adsorption followed by sunlight driven degradation. Additionally the nanocomposite displayed excellent regeneration efficiency along with enhanced antimicrobial activity towards E. coli in presence of artificial visible light.

EXPERIMENTAL SECTION. Materials. g-AP-pAA was developed by graft copolymerization of poly (acrylic acid) on amylopectin (AP).33 Titanium isopropoxide (Sigma Aldrich, USA), Gold (III) chloride hydrate (HAuCl4. xH2O) (Spectrochem Pvt. Ltd. Mumbai, India), Terepthalic acid, Sodium hydroxide, hydrochloric acid, acetone (E. Merck, Mumbai, India), Methyl violet (MV), methylene blue (MB), safranine (SF), methyl orange (MO) (Loba Chemie Pvt. Ltd., Mumbai, India) were of analytical grade. Synthesis. Synthesis of copolymer. Amylopectin (AP) was functionalized via grafting of flexible poly (acrylic acid) chains to obtain a copolymer with very high molecular weight and the corresponding grafting was performed following our previously reported method with some modifications.33 After formation of the copolymer g-AP-pAA, it was immersed in 0.1 (N) NaOH



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for 12 h and subsequently it was washed with methanol-water (70:30) followed by acetone to obtain pure anionic copolymer. Then the product was completely dried in a vacuum oven. Synthesis of nanocomposite. g-AP-pAA was dispersed in basic media (pH 9) at 50 °C with constant stirring to obtain homogeneous gelling matrix. For optimization, pH of the suspension was varied. Then, 10 mL ethanoic mixture of Ti-iPr was added to homogeneous gelling matrix of g-AP-pAA drop wise under vigorous stirring for 6 h and the transparent gelling matrix changed to milky white. Afterwards, ethanolic solution of HAuCl4 was added in dark condition at 50°C with stirring for 12 h. The color of reaction mixture was changed from milky white to distinct violet, confirming the in-situ reduction of Au3+ to metallic Au0 (Au NPs). After that the mixture was precipitated in acetone and placed in vacuum oven at 50°C for 24 h. The detailed synthetic procedure has been given in “Supporting Information”. Characterization. The synthesized materials were characterized using XRD, FTIR, FESEM, HR-TEM, AFM, UV-Vis DRS, FT-IR, RAMAN, PL, BET surface area, Zeta Potential and nano particle size analyser. The instrumental details and the techniques are given in “Supporting Information”. Adsorption and photodegradation study. Application of dye removal through adsorption. MV, AO were used for dye removal study through adsorption. Adsorption parameters i.e. pH, temperature, dye concentration, adsorbent dosage and contact time were varied. The detailed experimental procedure has been discussed in “Supporting Information”. Selective removal of cationic MV among the mixtures of cationic and anionic dyes. The selective removal of MV from the mixture of cationic (MV, AR, SF, MB), and anionic dyes (AO) were analysed. The detailed procedure has been explained in “Supporting Information”.


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Dye removal and regeneration study through degradation. After accomplishment of adsorption-desorption equilibrium, the remaining dye (MV) solution with photoadsorbent was placed under sunlight for 240 min (11 AM to 3 PM). The Langmuir-Hinshelwood model (eq. 1) was used to determine the photodegradation rate of used dyes.34 ln (C0/C) = kt

.......................... eq. (1)

Where, k is the pseudo first order rate constant. C0 and C denote the initial and final concentrations and t represents the degradation time. The efficient recyclability of g-APpAA/TiO2-Au was determined under sunlight. The experimental procedure has been described in “Supporting Information”. Detection and analysis of Hydroxyl Radicals (OH•). The generation of hydroxyl radicals (OH•) on the surface of g-AP-pAA/TiO2-Au was detected by photoluminescence (PL) spectra. Terepthalic acid (TA) as probe molecule was taken. The concentration of TA (5×10-4 M) and NaOH (2×10-3 M) were maintained throughout the analysis. The analysis procedure is explained in “Supporting Information”. Treatment of textile effluent. The textile effluent was collected from a textile industry near to Ranchi, India. The removal of effluent was carried out by combining effect of adsorption and degradation under sunlight for 240 min. Evaluation of antimicrobial activity. Escherichia coli (E. coli), a gram negative bacterium was used for investigating the antimicrobial activity of the nanocomposite. We demonstrated the inhibition of normal growth curve of E. coli using synthesized g-AP-pAA/TiO2 and g-AP-pAA/TiO2-Au nanocomposite. Herein, we use external visible light as source (Philips, 60W Equivalent Daylight, 5000K A19 Dimmable LED). The detailed experimental layout has been described in “Supporting Information”. Morphological analyses of the isolated species



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before and after irradiation were examined by FE-SEM analysis. The microscopic slide preparation technique has been discussed in “Supporting Information”.

RESULTS AND DISCUSSIONS Synthesis of g-AP-pAA/TiO2-Au photoadsorbent. The developed strategy is based on the in-situ formation of TiO2 and Au NPs in the polymer matrix through controlled hydrolysis, condensation and crystallization of Ti-iPr and reduction of Au3+ to AuNPs in gelling suspension of freshly prepared g-AP-pAA copolymer. Functionalized amylopectin was observed to be very effective in NPs synthesis owing to its interaction with metal NPs through chelation. Because of chelating effect of functionalized amylopectin, metal ions are dispersed throughout the polymeric matrix. Consequently, the metal ions were reduced to zero valent to form dispersed NPs. During synthesis, the carboxylate groups of polymer chains bind with metal ions, favour the controlled hydrolysis and also stabilized the newly formed nanoparticles and restrict the growth (Scheme S1, Supporting Information). pH 9 was optimized condition (Table S1, Supporting Information). At neutral pH, loss of net surface charge reveals coiled nature of copolymer, which makes the gelling matrix as less efficient with lower hydrodynamic diameter (Figure S1, Supporting Information). While at optimized alkaline media (pH 9), the repulsive interactions among – COO- groups make the copolymer more straightened and expanded. This is obvious from the difference in hydrodynamic diameter in two different media (Figure S1, Supporting Information). The hydrodynamic diameter of g-AP-pAA was 1333 nm at pH 9, which reveals that the copolymer is stretched in nature. Whereas at pH 7, the hydrodynamic diameter was observed as 623 nm, suggesting the collapsed nature of the copolymer and thus acts as less susceptible for proficient matrix. Therefore, higher hydrodynamic diameter at alkaline media facilitates the titanium ion penetration inside the polymer, in-turn homogeneous distribution of


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TiO2 nanoparticles as well as the nanoparticles would bind with the copolymer as depicted in Scheme S1, Supporting Information. At higher pH, because of the presence of higher concentration of counter positive ions, the chelating ability of carboxylate groups towards metal ion was decreased, in-turn the controlled hydrolysis and restriction of growth of nanoparticles were hinder. This resulted the nanoparticles with broad size distribution. During synthesis, the amount of water in reaction mixture and amount of added ethanol have distinct effects on the size distribution of formed TiO2 nanoparticles. Water favours the rate of hydrolysis of Ti-iPr and facilitates the formation and corresponding poly-condensation of hydroxylated titanium. At lower temperature (< 50°C), available energy for crystallization of TiO2 was very less and results amorphous/ lower crystalline TiO2. While at higher temperature, polydispersed nanoparticles were formed due to enhancement of the rate of polycondensation. Scheme S1, Supporting Information represents an overall schematic representation of the formation mechanism of synthesized nanocomposite, comprise of g-AP-pAA, in-situ generated TiO2 and AuNPs. Characterization. XRD analysis. Figure 1a represents the corresponding XRD pattern of synthesized nanocomposites. The XRD pattern of pure g-AP-pAA did not show any distinct peak, indicating its amorphous nature.35 Whereas, in the XRD pattern of g-AP-pAA/TiO2 nanocomposite, distinct diffraction peaks indexed to the respective planes of anatase TiO2 (JCPDS 21-1272), confirm the formation of crystalline TiO2 particle in the polymer matrix. In the XRD pattern of the synthesized g-AP-pAA/TiO2-Au nanocomposite, presence of additional three intense peaks at 44.5°, 64.9° and 77.8° ascribed to (200), (220) and (311) planes of cubic metallic Au, in addition to the peaks correspond to anatase TiO2, indicating the formation of crystalline Au particles with TiO2 particles in the polymer matrix. Further, the intensity of peak at 34.4° is much higher than



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that of (004) of TiO2, suggesting the combined peak of (004) of TiO2 and (111) of Au. In the XRD pattern of both samples, the absence of any other peaks confirmed the phase-pure nature of formed anatase TiO2 and metallic Au. The average crystallite size of formed TiO2 and Au, calculated from the X-ray line broadening of corresponding XRD pattern using the Scherrer equation were found to be 7 nm and 12 nm, respectively. No such change in crystallite size of TiO2 after incorporation of Au was observed. The composite materials synthesized at higher pH demonstrated amorphous nature (Figure S2, Supporting Information).

Figure 1.

(a) XRD pattern of synthesized g-AP-pAA/TiO2 and g-AP-pAA/TiO2-Au

nanocomposites, (b) FESEM image of g-AP-pAA/TiO2-Au, and HR-TEM images of (c)-(d) gAP-pAA/TiO2 (e)-(f) g-AP-pAA/TiO2-Au. 10 ACS Paragon Plus Environment

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FESEM and AFM analyses. FESEM micrographs of the synthesized g-AP-pAA/TiO2 nanocomposite depict the transformation of random fibriller structure of pure g-AP-pAA copolymer to nearly homogeneous spherical/granular like morphology on incorporation of TiO2 in the polymer matrix (Figure S3a and S3b, Supporting Information). The average sizes of spheres are in the range of 250-300 nm. No distinct change in spherical morphology or size of respective spheres was observed after incorporation of Au nanoparticles in the g-AP-pAA/TiO2Au nanocomposite (Figure 1b, Figure S3c, Supporting Information). The topographic image of TiO2-Au based nanocomposite demonstrates small particle shaped regular morphology as similar to that of FESEM images (Figure S3d, Supporting Information). The elemental mapping of gAP-pAA/TiO2-Au confirms the homogeneous distribution of both the Ti as well as Au throughout the polymer matrix (Figure S3e-g, Supporting Information). The existence of Ti and Au peaks in EDAX spectra further confirmed the presence of both Ti and Au in the polymer matrix of g-AP-pAA/TiO2-Au nanocomposite (Figure S4, Supporting Information). TEM and HR-TEM analyses. Figures 1c-f depict the TEM and HR-TEM images of g-APpAA/TiO2 and g-AP-pAA/TiO2-Au nanocomposites. The low resolution image of g-APpAA/TiO2 nanocomposite evidenced the formation of homogeneously distributed very small nanoparticles in the polymer matrix (Figure 1c). Corresponding magnified image of the g-APpAA/TiO2 nanocomposite confirmed the formation of nearly monodispersed TiO2 particles with size range of 5-8 nm (Figure 1d). In the respective HR-TEM image, the distinct lattice fringe with inter-planer distance of 0.35 nm, ascribed to (101) plane of anatase TiO2, confirmed the formation of well crystalline anatase TiO2 in the polymer matrix (inset Figure 1d). In the low resolution TEM image of g-AP-pAA/TiO2-Au, presence of particles with two different contrasts, higher contrast dark particles with bigger size and small particles with low contrast, indicate the



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presence of two types of particles in the composite (Figures 1e and 1f, Fig. S5, Supporting Information). The size of low contrast particles is comparable to that of size of TiO2 particles in g-AP-pAA/TiO2 nanocomposites, and thus it can be ascribed as TiO2 particles. Generally, the AuNPs resulted high contrast in TEM image and thus the bigger particles may be the Au particles. In the HR-TEM image, the inter-planer distance of 0.35 nm of small particles, indexed to (101) plane of anatase TiO2, and 0.23 nm of bigger particles, indexed to (111) planes of Au confirmed the above prediction (Inset Figure 1f). From the TEM analysis, it is evident the size distribution of formed AuNPs are very broad, in the range of 7-15 nm, although no such change in size and size distribution of TiO2 particles were observed. TEM image of the composites synthesized with higher amount of Au, the size distribution was further increased and some cases the aggregated Au particles were also observed (Figure S6, Supporting Information). FTIR analysis. The development of nanocomposite is based on the in-situ growth and fabrication of TiO2 and Au NPs in the polymer matrix, without employing any external reducing agent. Amylopectin was modified by grafting with poly (acrylic acid). The presence of large number of carboxylate and hydroxyl groups in functionalized amylopectin is mainly responsible for synthesis of uniform nanoparticles. From Fig. S7, Supporting Information, it is obvious that the peak at 1732 cm-1 arises from the carbonyl stretching vibration of g-AP-pAA along with peaks at 1648 cm-1 and 1453 cm-1 for carboxylate stretching frequencies. In g-AP-pAA/TiO2-Au, increase in peak intensity as well as the shifting of peak positions of carboxylate groups (1648 cm-1 to 1639 cm-1 and 1453 cm-1 to 1408 cm-1 ) and hydroxyl groups (3445 cm-1 to 3412 cm-1) at lower wave number supports the physical interactions between the copolymer and the metal NPs as proposed in Scheme S1, Supporting Information. Similar type of observation was reported before by Tagad et al. for the synthesis of polysaccharide mediated Au nanoparticles.36


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UV-vis DRS analysis. Figure 2a represents the UV-vis diffuse reflectance spectra of synthesized g-AP-pAA/TiO2 and g-AP-pAA/TiO2-Au. Spectrum of g-AP-pAA/TiO2 showed a sharp absorbance in UV region with a band edge ~324 nm (3.79 eV), which is specifically for TiO2 nanoparticles present in the polymer matrix. The obtained band edge (3.79 eV) is in quite high range, i.e., blue shifted, compared to that of bulk anatase TiO2 (3.2 eV). The blue shifting is for high quantum confinement originating from small size (5-8 nm) of TiO2 nanoparticles formed in the polymer matrix. Whereas, the UV-vis diffuse reflectance spectrum of synthesized g-AP-pAA/TiO2-Au showed similar absorbance characteristic in UV range, but in visible range the absorption characteristic was changed. A broad absorption in the range of 430-650 nm was observed, which can be ascribed to the gold surface plasmon, originating from the collective oscillation of free electrons present on conduction band of gold particles activated through optical excitation.37 Spectra of g-AP-pAA/TiO2-Au confirmed the presence of TiO2 and AuNPs in the composite and band structure of TiO2 remain intact on the incorporation of Au in the gAP-pAA/TiO2 matrix. This further confirmed that g-AP-pAA/TiO2-Au nanocomposite is capable to absorb visible light efficiently. Raman spectral analyses. Raman spectra were performed to elucidate the SPR effect forecasting on surface-enhanced Raman scattering (SERS) from AuNPs based nanocomposite. Raman spectra of g-AP-pAA/TiO2 reveals the characteristics Raman active modes at 147 cm−1 (Eg), 398 cm−1 (B1g), 515 cm−1 (B1g), and 637 cm−1 (Eg), which confirm that anatase TiO2 present in g-AP-pAA/TiO2 (Figure 2b). Interestingly, after incorporation of AuNPs, i.e., in g-APpAA/TiO2-Au, Raman signal were shifted to lower frequency region without any additional signal. The Raman signal shift is mainly because of the SPR field generated by AuNPs present in g-AP-pAA/TiO2-Au. It further confirmed that AuNPs are in close vicinity to TiO2 NPs and could



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enhance the crystalline defect within TiO2 which affect the Raman resonance of TiO2.38-40 Additionally, the above observations indicate that deposition of AuNPs on g-AP-pAA/TiO2 affects the changes in electronic environment, not phase transition.41 Besides, the Raman spectra of pure TiO2 and pure TiO2-Au materials (Figure S8, Supporting Information) suggest that similar phenomenon was observed.

Figure 2. (a) UV-vis diffuse reflectance spectra (b) Raman spectra of synthesized g-APpAA/TiO2 and g-AP-pAA/TiO2-Au. Measurement of Zeta potential and particle size. Zeta potential measurement indicates the stability of the sol or colloidal dispersions. The magnitude of zeta potential reveals the electrostatic repulsions between the similarly charged colloidal suspensions. Suspensions with higher magnitude of zeta potential (either positive or negative) refer to electrically stable, while the colloids with low zeta potential have a tendency to separate. After 12 h of continuous stirring 14 ACS Paragon Plus Environment

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the violet coloured sol was studied for measurement of zeta potential (Figure S9, Supporting Information). Zeta potential was found to be -48.0 mV. The high negative zeta potential reveals that the colloidal dispersions or sols are completely in stable form. Interestingly it was observed that the magnitude of zeta potential of the sol after 12 h stirring is higher than that of 6 h stirring (Figure S9, Supporting Information), indicating the higher stability of synthesized sol. Then the same reaction was continued up to 7 days and the zeta potential was observed to be -36.0 mV (Figure S9, Supporting Information). Although after 7 days, there is decreasing trend in magnitude of zeta potential was obvious, but at the same time the colloidal suspension is in stable form. For investigating the change in particle size with time, we measured the particle size at different time intervals (Figure S10, Supporting Information). After 6 h stirring, the whitish sol was used for DLS measurement and wide distribution curve was found. But, after addition of gold precursor (after 12 h), narrow distribution curve was observed. It is also to be mentioned that after 4 and 7 days, wide distribution curve was perceived. Dye removal via adsorption, and synergistic effect of adsorption and photodegradation. The prime aim of the present work is the development of a novel nanocomposite which is capable to treat the dye contaminated water through simultaneous adsorption followed by photocatalytic decomposition, which is highly dependent on its adsorption efficiency. Dye removal via adsorption. It is established that the extent of dye adsorption is highly dependent on pH of solution, as different adsorption sites were activated.27 Dye adsorption took place through electrostatic interactions and thus the assessment of point to zero charge (pHpzc) of an adsorbent is crucial. The point to zero charge (pHpzc) of the g-AP-pAA/TiO2-Au composite was found to be 4.16. Thus it is apparent that beyond pHpzc (pH>pHpzc), the negatively charged



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surface of the composite favours the adsorption of cationic dyes. While for anionic dyes, adsorption efficacy would increase with pH

Efficient Removal of Toxic Dyes via Simultaneous Adsorption and

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