Synthesis and characterization of guar-alginate hybrid bead templated mercury sorbing titania spheres

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Synthesis and characterization of guar-alginate hybrid bead templated mercury sorbing titania spheres

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Q1

Vandana Singh ∗ , Preeti, Angela Singh, Devendra Singh, Yadveer Singh, Arvind Kumar Pandey

Q3 Department of Chemistry, University of Allahabad, Allahabad-211002, India

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a r t i c l e

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a b s t r a c t

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Article history: Received 4 June 2014 Received in revised form 1 August 2014 Accepted 7 August 2014 Available online xxx

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Keywords: Guar-alginate hybrid bead Titania spheres Mercury

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1. Introduction

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Present communication reports on the synthesis and characterization of Hg(II) sorbing millimeter sized porous titania spheres (TSP). The synthesis utilizes guar gum-alginate hybrid beads as sacrificial template to polymerize titanium(IV) isopropoxide. The hybrid beads are crafted by pouring guar-alginate mixed solution to calcium bath. The mechanical strength of the beads depended on guar to alginate ratio in the mixed solution. The equal weight ratio of the two polysaccharides is appropriate for adequate mechanical strength beads. The unique performance of the templating beads is attributed to the synergistic interaction between guar gum and sodium alginate. FTIR, BET, SEM, TEM, XRD, TGA, and DTG analyses have been used for the characterization of the optimum performance TSP (TSPAG2 ). TSPAG2 is a mesoporous material that has higher surface area and narrower pore size distribution than TSPA . TEM study demonstrated that TSPAG2 spheres are constituted of aggregated TiO2 nanoparticles of ∼10 nm size. TSPAG2 is able to capture >95% Hg(II) from synthetic Hg(II) solution in 10 h at pH 5 as opposed to only 68% removal by pure alginate derived control titania spheres (TSPA ). © 2014 Published by Elsevier B.V.

Sol–gel derived hierarchically hollow inorganic adsorbents possess high specific surface area, excellent transport behavior, crystalline framework, controlled morphology and composition, thin walls, and tunable pore sizes [1,2]. The column clogging problem in adsorption experiments can be alleviated by replacing fine morphology and small particle size adsorbents by hollow inorganic spheres. Sacrificial templation by biomolecular micro/nano spheres [3–5] is known for deriving such materials from metal oxide precursors. The metal oxide network structures frequently cage some of the templating molecules during calcination to form functional spheres [6]. Alginate is brown seaweed polysaccharide having mannuronic (M block) and guluronic acid (G block) building blocks [7]. Guar gum is another abundant and popular commercial polysaccharide that has metal complexing cis-hydroxyl groups at its mannan backbone [8]. While alginate can be easily shaped into beads by Ca2+ complexation, guar gum does not form bead as it lacks uronic acid content. Recently mesoporous titania microspheres [1] have been designed

∗ Corresponding author. Tel.: +91 8127598952. E-mail address: [email protected] (V. Singh).

using alginate bead as sacrificial template. Polysaccharide binary solutions often show new rheological and textural properties due to synergistic interactions at the hydrophilic sites of the components. pH sensitive glutaraldehyde crosslinked alginate–guar gum hybrid hydrogel is reported for the controlled delivery of protein drugs [9], but guar-alginate hybrid beads are not explored yet. Inclusion of guar gum to alginate beads can potentially result into hybrid beads of new functional characteristics. Mercury is associated with its high affinity for biological tissues [10]. It is brought to aquatic systems through atmospheric deposition, erosion, urban discharges, agricultural materials, mining, combustion, and industrial discharges [11]. Many methods for mercury removal have been developed e.g. chemical binding by organically modified mesoporous materials [12], ion adsorption by hydrous manganese oxides [13] manganese dioxide nanowhisker [14], photocatalysis [15], and adsorption at activated carbons [16]. Although TiO2 is used as a photocatalyst in Hg(II) removal [17], its ability to adsorb Hg(II) as such is low [18–20]. Mesoporous films of a ruthenium dye functionalized crystalline TiO2 nanoparticles are known for efficient Hg(II) scavenging [21]. Recently titanate nanoflowers [22] have been synthesized for the effective removal of Pb(II), Cd(II), Zn(II) and Ni(II). In the present study we aim to exploit guar gum-alginate hybrid beads for synthesizing porous adsorbent for Hg(II). This is the first

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report of this kind where synergetic properties of guar gum and alginate have been used for developing metal ion adsorbent.

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2. Experimental

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2.1. Materials and equipments

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Titanium(IV) isopropoxide (97% purity) and 2-propanol (99% purity) were purchased from Aldrich and Merck, India, respectively. Absolute ethanol 99.9% (AR) was purchased from Changshu Yangyuan Chemical China. Sodium alginate (Food grade) and Rhodamine 6G were from Loba Chemie. Guar gum, calcium chloride (fused), mercury(II) chloride (GR), potassium iodide (GR), potassium hydrogen phthalate (GR) were all purchased from Merck, India. Sodium thiosulphate Na2 S2 O3 (AR) and gelatin bacteriological were purchased from central drug house (P) Ltd and Fisher Scientific, respectively. Double distilled water was used in all the procedures. The pH values were adjusted with the help of 5 M HCl (GR, Merck, 35%); or 1 M NaOH (Merck). EUTECH Instruments pH meter (model 510) was used for the pH measurements. Temperature treatment of the hybrid TiO2 spheres was done (in air) in microprocessor controlled electric muffle furnace, Metrex Scientific Instruments (P) Ltd., New Delhi, India. Orbital shaker Incubator, Metrex Scientific Instruments (P) Ltd., New Delhi was used.

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2.2. Characterization

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Brunauer–Emmett–Teller (BET) surface area and porosity of the TiO2 spheres were determined by N2 adsorption–desorption isotherms using a Quantachrome Instruments Autosorb 1 C surface area analyzer at 77 K. All the samples were degassed (at 393 K for 3 h) before the measurements. Scanning electron microscopy (SEM) was employed to observe microscopic morphology of the synthesized beads on FEI ESEM QUANTA 200 instrument with an accelerating voltage of 25 kV. The samples were gold coated to avoid charging. Transmission Electron Microscopy (TEM) was conducted using a FEI Tecnai U-Twin 20 TEM instrument. The TiO2 spheres were dispersed in ethanol and the dispersed sample was sonicated before placing on the grids for the TEM measurements. UV/Vis Spectrophotometer UV 100, Cyber lab, USA was used to determine mercury concentrations in the solution. TGA was done using SETARAM TGA analyzer (model SETSYS Evolution 2400). The samples were heated at the rate of 10 ◦ C/min in Pt basket 170 ␮l crucible using Argon as carrier gas. X-Ray diffraction (Cu/Ka-source) was carried out on PXRD-Bruker-D8 Advance diffractometer in 2theta angle range 5–80◦ with 0.5 step/minute scanning rate. FTIR was done on Perkin Elmer spectrophotometer. Viscosity measurements were performed using LVDVE Brookfield viscometer using small sample adapter (spindle no. S-18).

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2.3. Viscosity measurements

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The guar gum or/and sodium alginate (2% w/v) of were prepared by overnight soaking of a calculated weight of polysaccharide/s in a known volume of distilled deionized water. After homogenizing, the solutions were made up to known volumes so that the concentrations were 2% (w/v). The viscosity of the solutions were measured at 0.3 rpm at room temperature.

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2.4. Synthesis of polysaccharide beads

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Alginate (BA ) or alginate-guar hybrid gel beads (BAG to BAG9 ) (Table 1) were synthesized by dropping 25 ml of 2% w/v polysaccharide solution (alginate or/and guar gum) through 1.2 mm diameter

nozzle into aqueous Ca2+ baths that contained 100 ml of CaCl2 solution of a known concentration (0.13–0.41 M). The beads were left (for 2 h) inside their parent Ca2+ bath for curing and then separated. The beads were washed well with distilled water to remove the adhered Ca2+ ions. The average size of the prepared beads was ∼1 mm. The swollen beads were dehydrated by equilibration through solvent exchange from water (75 ml) to ethanol and finally in 2-propanol (for 3 h in each solvent). 2.5. Synthesis of titania hollow spheres (TSP) The polysaccharide beads were soaked in a mixture of titanium(IV) isopropoxide and 2-propanol (7:3 v/v) for 16 h at room temperature. The impregnated titanium isopropoxide precursor was hydrolyzed and polycondensed within the beads on equilibration with 1:1 (v/v) water/2-propanol solution. The composite beads (BAT and BAGT to BAGT9 ) were dried in an oven at 35–40◦ for 8 h C. Titania spheres (TSPA and TSPAG to TSPAG9 ) were obtained by calcining the dried beads (alginate beads and hybrid beads, respectively) at known temperature (ranging from 300 ◦ C to 650 ◦ C) for 2 h at each temperature under air flow. 2.6. Measurement of pHZPC For pH drift experiments, 500 ml of CaCl2 solution (0.005 M) was boiled to remove the dissolved CO2 . After cooling to room temperature, small aliquots (20 ml) from this solution were adjusted to different pH (ranging from pH 2 to pH 10) with the help of 2.5 M HCl or 5 M NaOH. Each of these pH adjusted CaCl2 aliquots were separately equilibrated with TSPAG2 (50 mg) for 48 h in capped vials. The final pH values of these solutions were measured and plotted against respective initial pH values. The pH at which the curve crosses the pHinitial = pHfinal line is taken as pHZPC [23]. 2.7. Mercury removal A stock solution of 1000 mg/l Hg(II) was prepared by dissolving a known amount of HgCl2 in deionized double-distilled water. Batch adsorption experiments were carried out using TSP (50 mg) as adsorbent with 20 ml of 100 mg/l Hg(II) solution on a temperaturecontrolled incubator shaker set at 150 rpm and maintained at 30 ◦ C for a desired time and then filtered through a Whatman 0.45 mm filter paper. After suitable dilution, the remaining Hg(II) was estimated spectrophotometrically (at -575 nm) using Rhodamine 6G dye in the presence of iodine buffer [23]. Since the adsorption equilibrium for TSPAG2 (the optimum sample) was achieved in 10 h (results not shown), 10 h contact time was chosen for all sets of experiments. To understand the adsorption behavior of TSPAG2 , the adsorption was monitored in the pH range of pH 2 to pH 7. The adsorbent showed 94–95% adsorption in the pH range of pH 3–6, being optimum at pH 5 (95%). The adsorption marginally decreased as the solution pH was increased to pH 6.5 (92%). On further increasing the pH to pH 7, 88% Hg(II) was adsorbed. In light of above results, pH 5 was chosen for all the experiments in the present study.

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3. Results and discussion

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3.1. Material synthesis

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Alginate is a copolymer of homopolymeric blocks of (1→4)linked ˇ-D-mannuronate (M) and ˛-L-guluronate (G) residues, respectively. It exists in special zig-zag conformation with “egg box” cavities formed by G blocks. The cavities can interact with calcium ions to form spherical calcium alginate beads [24]. While

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Table 1 Optimization of TSP synthesis by varying various process parameters while keeping the volume of H2 O fixed (25 ml); Hg(II) adsorption was done in batch adsorption Q6 experiment using 50 g adsorbent, 20 ml of 100 mg/l Hg(II) solution, at 30 ◦ C, Contact time 10 h, rpm 150.

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Alginate (g)

Guar gum (g)

CaCl2 (M)

Polysaccharide bead

TIP: iPrOH (3:1) (v/v) (ml)

Hybrid Titania beads

Calcination temp (◦ C)

Porous TiO2 sphere

% Hg(II) Ads (±1.0)

0.5 0.4 0.3 0.25 0.2 0.25 0.25 0.1 – 0.25 0.25 0.25 0.25 0.25 0.25 0.25

– 0.1 0.2 0.25 0.3 0.25 0.25 0.4 0.5 0.25 0.25 0.25 0.25 0.25 0.25 0.25

0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.20 0.13 0.27 0.34 0.41 0.20 0.20 0.20

BA BAG BAG1 BAG2 BAG3 BAG4 BAG5 – – BAG6 BAG7 BAG8 BAG9 BAG2 BAG2 BAG2

3.0 3.0 3.0 3.0 3.0 4.0 5.0 – – 3.0 3.0 3.0 3.0 3.0 3.0 3.0

BAT BAGT BAGT1 BAGT2 BAGT3 BAGT4 BAGT5 – – BAGT6 BAGT7 BAGT8 BAGT9 BAGT2 BAGT2 BAGT2

450 450 450 450 450 450 450 – – 450 450 450 450 300 550 650

TSPA TSPAG TSPAG1 TSPAG2 TSPAG3 TSPAG4 TSPAG5 – – TSPAG6 TSPAG7 TSPAG8 TSPAG9 TSPAG10 TSPAG11 TSPAG12

68.6 70.0 80.3 95.1 93.4 89.3 92.7 – – 92.7 76.2 77.6 81.7 92.7 92.5 1.0

guar gum has a ˇ-(1→4) link mannan backbone to which ␣glactopyranosyl groups are glycosidically attached at C6 of every other mannose unit. The viscosities of guar gum and sodium alginate solutions (2% w/v) were 8930 cP and 340 cP, respectively, while the viscosity of the mixed polysaccharide solution was found to be 4200 cP. This clearly evidenced the synergistic interaction between the two polysaccharides. It is reasonable to assume that the secondary interaction between the two polysaccharides is responsible for change in the texture of the hybrid beads from that of pure alginate beads. Guar gum-alginate hybrid beads were obtained by adding guar gum-alginate binary solution to CaCl2 bath. The calcium ions interacted with uronic acid residues at alginate G block to form the hybrid beads, while hydroxyls at the polysaccharides

O

OH

O

O OH

O

O

O OH O

C

HO O

HO

OH

O

O O

O OH

C

Ca2+

O

O 2+

O O

O O

OH

O

O

C HO O

HO

O OH

C

OH O

O OH

O

Alginate back bone O OH O

C

O

O O

O OH

O

O

Ca O

O

OH

O

C O

OH

O HO

OH

O

O C

O C

OH

Ca

O

C

O O

O

Alginate back bone

O

O C

(alginate and guar gum), due to their electron rich nature further hold the calcium ions (Scheme 1). The cis hydroxyls at guar gum mannan backbone offered extra functionality to the hybrid beads. The texture and the mechanical strength of the hybrid beads depended on the weight ratio of guar gum to alginate in the mixed polysaccharide solution and on the CaCl2 concentration in the bath. The pure guar gum did not form any bead while poor mechanical strength beads were obtained when the guar gum to alginate weight ratio was 3:2 (Table 1). No beads are formed in absence of alginate or if the weight ratio of guar to alginate was 4:1. Equal weight ratio of alginate to guar gum was most suited for the formation of the hybrid beads of adequate mechanical strength that suited the templating. The mechanical strength of the hybrid beads

OH

2+

O

HO HO

HO

O O HO

OHO OH

Guar gum back bone O OH O

O

OH OHO

Ca2+

O

O

O HO

Scheme 1. Schematic diagram for the formation of BAG2 .

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decreased with the decrease of the alginate content in the beads which may be due to less availability of uronic acid groups for complexing the calcium ions. Titanium(IV) isopropoxide when polymerized in presence of polysaccharide beads formed titania coated hybrid beads via water and alcohol condensation around the beads. Hybrid TiO2 beads (BAT and BAGT to BAGT9 ) were obtained by hydrolyzing titanium(IV) isopropoxide in presence of alginate or alginate-guar gum hybrid beads of adequate mechanical strength (Table 1). The titania coated alginate bead (BAT ) were identically synthesized as control beads to understand the advantage of guar gum inclusion in the beads. All the synthesized titania hybrid beads (BAGT to BAGT9 and BAT ) were calcined at 450 ◦ C in air to obtain porous titania spheres (TSPA or TSPAG to TSPAG9 ) which were screened for Hg(II) adsorption from aqueous solution as described in section 2.7. Since TSPAG2 (Table 1) showed best performance among the synthesized TSP, the corresponding hybrid titania beads, BAGT2 were subjected to calcination at different temperatures to obtain TSPAG10 to TSPAG12 . It was observed that calcination at 450 ◦ C was most appropriate for obtaining optimum performance TSP, the TSPAG2 . It can be assumed that during titania hybrid bead formation, titania surface hydroxyl groups associate with hydroxyls at the polysaccharides through hydrogen bonding. The conditions for obtaining the optimum performance spheres are: equal weight amount of guar gum (0.25 g) and alginate (0.25 g); 25 ml H2 O; 100 ml of 0.20 M CalCl2 ; 3 ml of 3:1 (v/v) TIP:iPrOH; calcination temperature 450 ◦ C.

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3.2. Characterization

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FTIR spectrum of BAGT2 showed two strong absorption peaks at 3422 cm−1 and 1619 cm−1 (O–H stretching and bending, respectively) due to surface-adsorbed water at TiO2 [17,25] and polysaccharide hydroxyls (Fig. 3). Presence of alginate in TSPAG2 and TSPAG2 -Hg is evidenced by the observation of a strong absorption peak at 1428 cm−1 for (s ), COO, while (as ) COO (∼1590) is seen merged with O–H bending [26]. Other relevant peaks were observed at 2923 cm−1 (C–H stretching at polysaccharides) and below 1000 cm−1 (from TiO2 ) [27]. The peaks between 500 and 900 cm−1 can be ascribed to absorption bands of Ti–O and O–Ti–O flexion vibration [28]. A broad peak ranging from 500 to 800 cm−1 is characteristic absorbance of O–Ti–O network in TiO2 [29]. Presence of terminal and bridging isopropoxide ligands in BAGT2 is indicated [30] by the absorptions at 1085 cm−1 and 1031 cm−1 , respectively. These absorptions are very weak in the calcined sample (TSPAG2 ). TSPA showed the elongation vibration of C–O groups, merged with a peak at 1085 cm−1 . An absorption band at 800–890 cm−1 (s ), Ti–O–Ti stretching [31] is visible in all the spectra. In the calcined BAGT2 (TSPAG2 ), the peaks are observed at 3384 cm−1 (O–H stretching), 2923 cm−1 (C–H stretching), 1615 cm−1 (COO− symmetrical stretching), 1407 cm−1 (COO− ), asymmetrical stretching), 1075 cm−1 (C–O stretching), 1021 cm−1 (terminal and bridging isopropoxide ligands), 869 cm−1 (Ti–O–Ti stretching), and 586 cm−1 (Ti–O stretching). The existence of the OH bending and stretching modes and C–H stretching in the calcined samples indicated that the polysaccharide did not fully escape during the calcination and moisture is adsorbed at TSPAG2 . The Ti–O stretching mode (at 624 cm−1 ) in the hybrid shifted toward a lower frequency (586 cm−1 ) on calcination that indicated change in the crystalline lattice of TiO2 [32]. Moreover, intensities of terminal and bridging isopropoxide ligand peaks are seen highly reduced in the calcined sample as these ligands are lost during condensation polymerization of Titanium(IV) isopropoxide (Scheme 1). TSPA show similar IR spectrum, the peaks being at 3434 cm−1 (s) (O–H stretching), 2917 cm−1 , 2969 cm−1 (C–H stretching), 1428 cm−1

Fig. 1. FTIR spectra (a) BAGT2 ; (a ) TSPA ; (b) TSPAG2 ; (c) TSPAG2 -Hg.

(COO− symmetrical stretching), 1500 cm−1 (COO− asymmetrical stretching are seen merged with O–H bending). Titanium related peaks are seen at 874 cm−1 (Ti–O–Ti stretching) and 617 cm−1 (Ti–O stretching). In mercury loaded TSP (TSPAG2 -Hg), O–H stretching peak is seen shifted to 3419 cm,−1 which indicated the involvement of titania surface hydroxyls in Hg(II) binding. Other significant shift in mercury loaded sample is for Ti–O–Ti (peak at 869 cm−1 shifted to 874 cm−1 ) and Ti–O stretching (shifted from 586 cm−1 to 617 cm−1 ), indicating role of TiO2 lattice in Hg+2 uptake (Fig. 1). Q4 Nitrogen adsorption–desorption isotherms and pore size distribution plots for TSPAG2 and TSPA are shown in Fig. 2. Both samples showed a type-IV isotherm [27] which is representative of mesoporous solids. Using the Brunauer–Emmett–Teller (BET) method, the specific surface area of the TSPAG2 spheres was determined to be 11.75 m2 /g. The hybrid spheres showed a bimodal pore distribution ranging from 1.2 to 9 nm, while pure alginate templated spheres (TSPA ) acquired wider pore distribution that ranged between 2.5 nm and 15 nm (the peak centers around 11 nm) and a relatively lower specific surface area (7.79 m2 /g). The pore volume of the TSPAG2 (0.0209 cc/g) was found almost double to that of TSPA (0.0135 cc/g). The incorporation of guar gum in the beads increased both BET surface area, and pore volume, while narrowed the pore size distribution. This difference can be attributed to the different constitution of the TSPA and TSPAG2 spheres. SEM study (Fig. 3) revealed that the TSPA (Fig. 3(a)) have quite different morphology and much less porosity as compared to TSPAG2 . TSPAG2 showed complex rough porous external surface that might have resulted from the loss of the templating polysaccharides during the calcination. Different textures of TSPAG2 and TSPA may be due to the more complex network formed by calcium with

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Fig. 2. (a) BET surface area, pore width and pore volume of TSPAG2 ; (b) BET surface area and pore width and volume of TSPA .

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the mixed alginate-guar gum system (Scheme 1). Fig. 3(c) illustrates the structure of a single fractured TSPAG2 bead and the porous internal structure can be observed in Fig. 3(b). The images indicated that the size of the voids between the ridges is ∼50 ␮m. The beads retained their spherical morphology even after the weight

loss during calcination. On mercury deposition the surface becomes less fibrillar and more granular in nature as seen in Fig. 3(d). The BAGT2 beads have been converted into hollow spheres by calcination at 450 ◦ C (TSPAG2 ). TEM images, Fig. 4 (a) showed that TSPAG2 has a wormhole-like mesoporous structure and mesopores

Fig. 3. (a) SEM picture of TSPA ; (b) TSPAG2 ; (c) fractured TSPAG2 ; (d) TSPAG2 -Hg.

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Fig. 4. (a) TEM picture of TSPAG2 (SAED seen in the inset); (b) Particle size histogram & (c) TSPAG2 ; (d) TSPAG2 -Hg.

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(intercrystalline pores) are well dispersed in the entire sphere, while Fig. 4(c) make obvious that the mesoporous TiO2 are formed by the aggregation of TiO2 nanoparticles of ∼10 nm size TEM histogram (Fig. 4b) revealed that the average size of those nanoparticles is ∼10 (most of the particles are between 10 and 12 nm size), which is consistent with the average crystallite size estimated from the XRD pattern (10 nm). Due to embedment of nanocrystals, the as-prepared spheres show a clearly rough and fuzzy surface, which is consistent with the SEM results. The corresponding SAED pattern (seen as inset in Fig. 4(a)) has also established the crystalline nature of porous TSPAG2 , which accords well with the XRD results [33,34]. X-Ray diffraction (XRD) was employed to further investigate the crystalline structure of the hybrid bead (BAGT2 ) and titania spheres, (TSPA and TSPAG2 ). The as-synthesized TiO2 microspheres (TSPA ) and TSPAG2 exhibited good crystallinity (Fig. 5). The Debye–Scherrer formula [35] has been used for the estimation of the average size of the crystallite (d) from the XRD patterns which have been summarized in Table 2. TSPAG2 displayed well-defined diffraction peaks Fig. 5(a) shows that the BAGT2 is amorphous but acquires crystalline structure on calcination. The crystalline peaks corresponded to anatase phase (JCPDF-84-1285) [36] while a brookite reflection peak is also observed at 2 29.5◦ . TSPA also showed the same crystalline phase similar to TSPAG2 . In mercury loaded samples, the effect of mercury incorporation to the titania lattice can be observed. There is change in XRD diffraction in terms of both peak positions and intensities (Fig. 5(d)). The peak intensity of mesoporous TSPAG2 -Hg is much larger than that of TSPAG2 and the full width at half-maximum (fwhm) of TSPAG2 -Hg is

smaller than that TSPAG2 , indicating that Hg doping improved the order of the mesophase [37]. In mercury loaded sample (TSPAG2 Hg), besides peaks simlar to TSPAG2 , some new peaks also appear that can be assigned to the adsorbed mercury species. Thermogravimetric analysis of BAGT2 (titania coated BAG2 ) (Fig. 6(a)) essentially revealed three distinct zones of weight loss. The initial weight loss (∼15%) occurred due to the moisture traces and the solvent present in the sample from 25 ◦ C to 175 ◦ C, the corressponding DTG peak is seen at 84.9 ◦ C. The second step represents the degradation of the guar gum backbone and organics, that Table 2 2 and particle sizes (d) in nm for TSPA , TSPAG2 and TSPAG2 -Hg. TSPA 2 25.2 29.2 37.8 47.9 54.01 55.1 – – – – – – – –

TSPAG2 (d) in nm 15 28 20 9 84 26 – – – – – – – –

2 25.4 29.5 38.1 48.1 54.2 55 – – – – – – – –

TSPAG2 -Hg (d) in nm 9 21 11 7 1 10 – – – – – – – –

2

(d) in nm

25.3 29.3 37.8 48.4 54.1 54.8 23.03 31.35 35.96 39.38 43.14 47.42 57.4 23.03

12 50 14 51 13 11 32 41 35 35 38 19 34 32

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Fig. 5. XRD of BAGT2 (a); TSPA (b); TSPAG2 (c); TSPAG2 -Hg (d).

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started at 230 ◦ C and lasted until 335 ◦ C, amounting ∼36% weight loss [38]. The corresponding DTG peak is seen at 273 ◦ C. Next weight loss (∼22%) is observed between 423 ◦ C and 535 ◦ C due to the loss of sodium alginate along with the exothermal crystallization of the amorphous gel (c.f. XRD) and further condensation of TiO2 oligomer and the unreacted TiO2 precursor, the corresponding DTG peak is seen at 513 ◦ C. This transition temperature is significantly higher than that of TSPA where the peak is seen at 495 ◦ C. It appears that the presence of guar gum has retarded the crystallization of the pore wall [39]. In TSPA first weight loss (∼8%) is seen due to the moisture traces and the solvent present in the sample which is lesser as compared to TSPAG2 , the corressponding DTG peak is seen at 97 ◦ C, while second DTG peak (at 425 ◦ C) indicated the loss of residual alginate, while the peak corressponding to phase change of TiO2 is visible at 495 ◦ C. TGA of TSPAG2 , (Fig. 6(b)) showed no weight loss in the region of 230–330 ◦ C, which indicated that guar gum content has fully escaped during the calcination but complete loss of alginate can not be assumed. Weight loss (∼32%) between 450 and 675 ◦ C can be attibuted to the leftover alginate and due to exothermal crystallization of the amorphous gel and further condensation of TiO2

oligomer and the unreacted TiO2 precursor. The corresponding DTG peak is seen at 533 ◦ C. Another weight of ∼17% is seen between 615 and 800 ◦ C. This weight loss can be ascribed to condensation of defective –OH groupings which may not be very close [39]. TGA of TSPAG2 -Hg show same pattern of weight loss as TSPAG2 except the phase change took place at sightly lower temperature due to presence of mercury species, DTG peak is seen at 411 ◦ C and weight loss corressponding to third peak is higher (∼65%) due to loss of mercury species in comparion to weight loss in TSPAG2 (∼32%). 3.3. Mercury removal Hg(II) show complex behavior in aqueous solutions. Hg2+ is the dominant species in the solution at pH < 3.0 whereas Hg2+ , HgOH+ and Hg(OH)2 exist in the pH range of 3–5, and mostly as Hg(OH)2 at pH 5.0 [40,41] pH drift experiments indicated that pHZPC of TSPAG2 is pH 7.4 (Fig. 7). It means that the surface charge of the adsorbent will be positive at pH < 3. Since the adsorbent will have least affinity with the similarly charged Hg2+ species (the major species at pH <3), the adsorption is low below pH 3. In the pH range of pH 3–6,

Fig. 6. (a) TGA and (b) DTG of the hybrids beads/spheres.

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investigation. The titania spheres derived from hybrid beads showed narrower pore size distribution, higher surface area, pore volume, and thermal stability than the identically synthesized spheres from pure alginate beads, indicating broader aspects of their utility as adsorbent.

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concentration of Hg(OH)2 increases in the solution and it is the principal adsorbate in the pH range. It can be assumed that the hydrogen binding interactions are involved between the Hg(OH)2 and Hg(OH)+ and surface O–H groups at TiO2 spheres (which become available after the loss of the polysaccharides during calcination) from the hybrid titania beads. The adsorption varied from 94 to 95% in the pH range of 3–6 and then it marginally declined (∼88% at pH 7) (Fig. 8). The adsorbents synthesized in the present study have been evaluated at pH 5. It was observed that on the addition of the adsorbent to Hg(II) solution of pH 5, the pH of the mercury solution immediately increased to pH 5.9 and after equilibration time (10 h) it reached to pH 7.5. This pH change is due to the possible formation of calcium oxide during the calcination of the hybrid beads (BAGT2 ), since beads have been calcined in air. It can be assumed that the calcium component of the beads have been oxidized to calcium oxide that hydrolyzed to Ca(OH)2 during equilibration time and thus the observed change in the pH is expected. The presence of calcium has been confirmed by the formation of white precipitate on addition of (NH)2 C2 O4 solution to the contact mixture. Mercury removal by the adsorbent can be further improved by optimization of the batch adsorption experiment conditions that we aim to undertake in a separate study.

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The authors are grateful to the Council of Scientific & Industrial Q5 research, New Delhi, India for the financial support to carry out this research work. Author acknowledge IISER, Pune for IR, XRD facilities, Indian Institute of Sciences (IISC) Banglore for SEM, Indian institute of Technology (IIT) Kanpur for TEM, Defence Materials and Stores Research and Development establishment (DMSRDE) Kanpur for TGA and Indian Cultivation of Science (ICS), Kolkata for BET facilities.

Presence of guar gum altered the characteristics and texture of the alginate beads. The guar-alginate hybrid beads were used as sacrificial template to derive mesoporous titania spheres that have been evaluated as efficient Hg(II) sorbent in a preliminary

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