Synthesis and characterization of ZnO:CeO2:nanocellulose:PANI bionanocomposite. A bimodal agent for arsenic adsorption and antibacterial action

Carbohydrate Polymers 148 (2016) 397–405

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Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol

Synthesis and characterization of ZnO:CeO2 :nanocellulose:PANI bionanocomposite. A bimodal agent for arsenic adsorption and antibacterial action B.K. Nath a , C. Chaliha a , E. Kalita a,∗ , M.C. Kalita b a b

Department of Molecular Biology and Biotechnology, Tezpur University, Tezpur, Assam 784028, India Department of Biotechnology, Gauhati University, Guwahati, Assam 781014, India

a r t i c l e

i n f o

Article history: Received 13 October 2015 Received in revised form 28 March 2016 Accepted 29 March 2016 Available online 1 April 2016 Keywords: Nanocellulose Polyaniline Arsenic Adsorption Antibacterial

a b s t r a c t In the present study we report the generation of a bimodal, ZnO:CeO2 :nanocellulose:polyaniline bionanocomposite having an appreciable remediation efficiency for dissolved Arsenic along with a noticeable antibacterial activity. The microstructural analysis of the synthesized bionanocomposite was carried out by TEM, XRD and FTIR studies, which confirmed the incorporation of the nanoscaled ZnO and CeO2 in the polymeric nanocellulose:polyaniline matrix. The bionanocomposite exhibited a remediation efficiency above ∼95% against As under different adsorbent concentrations and pH conditions. The biosorption mechanism of As on the nanobiosorbent was found to conform to the Freundlich and DubininRadushkevich isotherms. Antibacterial assays for the bionanocomposite showed a high antibacterial activity with MIC50 values of 10.6 ␮g ml−1 against the Gram-positive Bacillus subtilis and 10.3 ␮g ml−1 against the Gram-negative Escherichia coli. Thus, the bionanocomposite shall be of high interest as a novel and sustainable matrix for the design of coats/devices that effectuate arsenic adsorption and microbial control, to generate contaminant free potable water. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Arsenic is a ubiquitously distributed metalloid, found in both organic and inorganic forms in the environment. The presence of arsenic in natural water sources is generally associated with the geological weathering of lithospheric deposits rich in Arsenic, leading to the leaching of Arsenic into vicinal water sources (Baig, Sheng, Hu, Xu, & Xu, 2015). Besides these, anthropogenic activities such as mining-wastes, use of arsenical pesticides, paints, dyes etc. contribute significantly towards the release of Arsenic into the aquatic ecosystems (Hokkanen, Repo, Lou, & Sillanpää, 2015). Arsenic is known to be acutely toxic, mutagenic and carcinogenic, and its presence in potable water sources leads to skin lesions, developmental defects, cardiovascular anomalies and chronic neurotoxicity (Hokkanen et al., 2015). The conventional approaches employed for the removal of Arsenic from aqueous systems involve chemical precipitation processes, reverse osmosis, microfiltration, distillation etc. which are deemed to be energy intensive with high recurring costs. The existence of several natural and synthesized

∗ Corresponding author. E-mail addresses: [email protected], [email protected] (E. Kalita). http://dx.doi.org/10.1016/j.carbpol.2016.03.091 0144-8617/© 2016 Elsevier Ltd. All rights reserved.

adsorbents like activated carbon, chitosan, zeolite, peat, silica, clay etc. (Song, Kong, & Jang, 2011) that provide a simple and efficient mechanism for adsorption are emerging as sustainable alternatives for the remediation of contaminants like arsenic. In this context, biosorption has garnered a significant attention in the recent years, for the remediation of heavy metal contaminated water systems. The use of adsorbents derived from lignocellulose rich agro-waste based biopolymers is a viable solution which exploits the three dimensional microstructure of the biological polymers for the chelation and subsequent removal of heavy metal species from polluted wastewaters (Sadeek, Negm, Hefni, & Wahab, 2015). Moreover, the wide availability of these plant based lignocellulosic sources, their renewability, biodegradability and biocompatibility provides enough impetus for their acceptance as potential substitutes to conventional adsorbents (Singh, Sinha, & Srivastava, 2015). However, these polymers in their native states, are often compromised on mechanical strength and performance owing to the properties associated with their hierarchical structure (Moon, Martini, Nairn, Simonsen, & Younglblood, 2011). The reduction of these macroscopic structures to nanoscale dimensions results in the alleviation of these drawbacks while contributing to a significant increase in the surface to volume ratio, strength to weight ratio, elastic modulus, and sorption efficiency

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(Boufi, Kaddami, & Dufresne, 2014). Furthermore, the presence of surface hydroxyl groups in cellulose has been seen to present the scope for the incorporation of diversified inorganic materials and other biopolymers and for the chelation of diversified metallic species (Eichhorn et al., 2010). Celluloses isolated from a wide array of lignocellulosic sources have been extensively used in the construction of specialized microporous membranes for water purification applications (C¸ifci and Kaya, 2010, Sánchez, Bastrzyk, Rivas, Bryjak, & Kabay, 2013). Nanoscaled metal oxides have also been applied to a diversified range of water treatment processes for the rapid removal of heavy metals. In addition to their low production and regeneration costs, the high surface area of nanoscaled metal oxides contributes significantly towards their use as sorption-ready systems for the remediation of aqueous contaminants (Haldorai, Kharismadewi, Tuma, & Shim, 2014). Furthermore, potable water is often contaminated with microflora which may cause diarrhoea, anemia, kidney failure, urinary tract infections etc. in humans. There have been reports where the native Gold (Au), Silver (Ag) and Titanium (Ti) based nanoscaled metal oxides were used to confer antibacterial activity under such conditions (Li et al., 2008). The high synthesis costs for these systems have shifted the focus to search for economical alternatives like Zinc oxide (ZnO), Cerium oxide (CeO2 ), Copper oxide (CuO) etc. having similar properties. Of these, the antibacterial activity of the native ZnO (Salem et al., 2015) and CeO2 nanoparticles (Leung et al., 2015) is dependable. The use of petrochemical based matrices for the incorporation of natural fibers have been extensively studied quite recently and various polymeric matrices composed of polyacrylamide, polyvinyl chloride, polyvinyl alcohol, polyaniline and polypyrrole have been explored for the generation of fiber-reinforced polymer composites for use in water treatment applications (C¸ifci and Kaya, 2010, Castillo-Ortega et al., 2011). Moreover, studies on the use of nanostructured polyaniline (PANI) as support matrices for metallic nanoparticles have also reported of late which are known to contribute towards achieving a superior performance over native metallic nanoparticles for use as nanoelectrocatalysts, sensors, adsorbents in separation and purification systems (Bhaumik, Noubactep, Gupta, McCrindle, & Maity, 2015). Additionally, the fabrication of these native petrochemical based matrices is generally achieved using facile approaches, thus presenting a significant scope for the use of these systems as blends and composites, exhibiting dual properties of remediation and antibacterial action. In the present study, we report the synthesis and characterization of a novel ZnO:CeO2 :PANI:nanocellulose bionanocomposite using polyaniline as the supporting matrix for the efficient removal of Arsenic from water, also demonstrating antibacterial activity. The microstructural properties of the synthesized nanosystems were investigated using Transmission electron microscopy (TEM), X-ray diffraction spectroscopy (XRD) and Fourier transform infrared spectroscopy (FTIR). The adsorption efficiency of the bionanocomposite for dissolved Arsenic was estimated using Atomic emission spectroscopy, while the antibacterial efficacy of the synthesized nanosystems were quantitatively assessed against both Gram-positive bacteria (Bacillus subtilis ATCC 11774) and Gramnegative bacteria (Escherichia coli NCTC 10538).

2. Materials and methods 2.1. Isolation of nanocellulose (NCs) from rice husk The isolation of NCs from rice husk was carried out based on isolation procedures reported by us earlier (Kalita et al., 2015). The rice husk samples were freshly collected, in sterile containers from

local rice mills of Tezpur, Assam, India. Analytical grade reagents viz. Sulphuric acid (H2 SO4 ), Hydrochloric acid (HCl), Sodium hypochlorite (NaOCl), Sodium hydroxide (NaOH) and Acetic acid (CH3 COOH) were purchased from Merck India Pvt., Ltd. The rice husk samples were extensively washed with distilled water for the removal of impurities present on the surface. A finite volume of the rice husk was subsequently weighed into a stainless steel grinding container (50 ml) and wet milled in a planetary ball mill (EGOMA, India) at a frequency of 8.33 Hz. The finely milled rice husk samples were then soaked in a 2% NaOH solution for 14 h and were subsequently autoclaved in a SS-316 Mechomine Teflon lined-autoclave (India) under 20 Psi at 210 ± 5 ◦ C using 5% H2 SO4 for a period of 8 h. The resultant mixture was allowed to cool down to room temperature and was successively bleached using a solution containing stoichiometric amounts of NaOH, CH3 COOH and NaOCl. The bleached sample was acidified using 10% HCl solution and the slurry was sonicated for 2 h at room temperature in a Labsonic M Ultrasonic Homogenizer from Sartorius (Germany). An operating voltage of 240 kV and a frequency of 24 kHz was employed for the homogenization which finally resulted in the yield of nanocellulose (NC). The samples were finally washed with copious amounts of distilled water to neutralize the pH before being vacuum dried. 2.2. Synthesis of ZnO and CeO2 nanoparticles The ZnO nanoparticles were synthesized using a modified solgel approach (Gayen, Sarkar, Hussain, Bhar, & Pal, 2011). The reagents were purchased from Merck India Pvt., Ltd. and used without purification. To an aqueous solution of Zinc acetate dihydrate (Zn(C2 H3 O2 )2 ·2H2 O), a stoichiometric amount of Sodium Dodecyl Sulphate (SDS) was added to obtain a molar ratio of Zn: SDS = 1:1. Aqueous solution of 0.2 M Sodium Hydroxide (NaOH) was added drop-wise to the above to bring about immediate precipitation of ZnO nanoparticles. The precipitate was washed copiously with deionized water to eliminate traces of the precursors and was vacuum dried. The CeO2 nanoparticles were synthesized by modifying a previously reported approach (Kaneko et al., 2007). Cerium nitrate hexahydrate (Ce(NO3 )3 ·6H2 O) and urea (CO(NH2 )2 ) purchased from Sigma Aldrich, were weighed in the molar ratio of 1:5 and were separately dissolved in equal volumes of distilled water. The two solutions were then mixed together and the resultant mixture was heated in an oil bath at 115 ◦ C for a period of 1.5 h. The white precipitate formed was filtered, washed with deionized water and vacuum dried in an oven at 90 ◦ C for 6 h. The dried precipitate was then calcined at a temperature of 500 ◦ C for 1 h, in a muffle furnace to obtain CeO2 nanoparticles having a light yellowish colouration. 2.3. Synthesis of ZnO:CeO2 :nanocellulose:PANI bionanocomposites The ZnO:CeO2 :nanocellulose:PANI bionanocomposite (ZnO:CeO2 :NC:PANI) was synthesized using a modified polymerization process (Hu, Chen, Yang, Liu, & Wang, 2011). The NCs isolated from rice husk was thoroughly dispersed in deionized water at 25 ◦ C. To a definite volume of the resultant dispersion, ZnO and CeO2 nanoparticles (1000 ␮g ml−1 ) were added and the resultant mixture was sonicated at 4 ◦ C using a Sartorius Labsonic M Ultrasonic Homogenizer (Germany) to obtain a stable suspension. The suspension was then added to a solution containing Aniline monomer (3.2 mM) in Chloroform. Subsequently Ammonium peroxydisulfate (0.8 mM) and Hydrochloric acid (1 M) were added to initiate the polymerization. The reaction was allowed to proceed overnight. The resultant precipitate was filtered and washed with copious amounts of distilled water, absolute alcohol and acetone until the filtrate was clear. The as-prepared bionanocomposites

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were vacuum dried in an oven at 60 ◦ C to ensure the complete removal of moisture. An outline of the synthesis is depicted in the data in brief section of the article (Fig. 1). 2.4. Characterization of ZnO, CeO2 nanoparticles and ZnO:CeO2 :nanocellulose:PANI bionanocomposites The synthesized ZnO nanoparticles, CeO2 nanoparticles and the ZnO:CeO2 :nanocellulose:PANI bionanocomposite were analyzed for their structural attributes using a JEOL JEM-2100 transmission electron microscope (TEM). Dilute suspensions of each of the samples was prepared and a droplet of each was deposited on a carbon-coated copper microgrid (400 mesh) and dried under vacuum prior to examination. To assign the constitutive elemental composition, the bionanocomposite was analyzed using Energy Dispersive X-ray Spectroscopy (EDX) on a JSM6390, JEOL Scanning electron Microscope, Singapore. Additionally, the ZnO:CeO2 :nanocellulose:PANI bionanocomposite was subjected to Atomic Emission Spectroscopy using an Agilent 4100 Microwave Plasma-Atomic Emission Spectrophotometer (USA). The percentage incorporation of the nanoscaled ZnO and CeO2 onto the nanocellulose and the percentage of incorporation of the nanocellulose:ZnO:CeO2 onto the polyaniline matrix was thereby calculated. Furthermore, powder X-ray diffraction studies for the samples was carried out on a RIGAKU Miniflex Benchtop X-Ray Diffractometer (Japan) using Ni filtered Cu K␣ radiation (␭= 1.5406 Å). An operating voltage and current of 40 kV and 15 mA was employed to generate the diffractograms of the samples. A Perkin Elmer Spectrum 100 Optica FT-IR Spectrometer (USA) was used for the Fourier transform infrared (FTIR) spectroscopic study of the samples. The nanoparticles and the bionanocomposite was ground with Potassium Bromide (KBr) and pelletized with the help of a Qwik Handi-Press Kit. The pellets were used for the FTIR analysis in the range of 400–4000 cm−1 . 2.5. Adsorption of Arsenic on ZnO:CeO2 :nanocellulose:PANI bionanocomposite Batch adsorption experiments were performed to assess the effect of adsorbate concentration (As), adsorbent concentration (ZnO:CeO2 :nanocellulose:PANI bionanocomposite), solvent pH and contact time with the adsorbent. The amount of As adsorbed on the bionanocomposite, was measured by Microwave Plasma-Atomic Emission Spectroscopy (MP-AES). To determine the sorption capacity of the adsorbent, the following equation was used (Raul et al., 2014): Qe =

V (Ci − Cf ) W

where, Qe is the adsorption capacity of the adsorbent (mg g−1 ); v is the volume of the adsorbate solution (l); Ci is the initial adsorbate concentration in solution (mg l−1 ), Cf is the final adsorbate concentration in solution (mg l−1 ) and w is the weight of the adsorbent (g). This data obtained was used to model the adsorption for the pseudo-first order and the pseudo-second order kinetic equations. The results from the other batch adsorption studies were fitted against the linear forms of Langmuir, Freundlich, Temkin and Dubinin-Radushkevich adsorption isotherms to determine the nature of the adsorptive uptake of dissolved As by the bionanocomposite. 2.6. Test for antibacterial properties and determination of MIC The antibacterial properties of the nanoparticles and the bionanocomposite was comparatively assessed using MTT (3(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) cell

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viability assay, against both Gram-positive B. subtilis ATCC 11774 and Gram-negative E. coli NCTC 10538. The bacterial strains were grown aerobically in Luria-Bertani broth (Sigma-Aldrich, USA) at 35 ◦ C for 18 h. The cultures were diluted to obtain a final concentration in the range ∼106 cells ml−1 for the assay. The suspensions were aliquoted into the wells of a microtiter plate and equal working concentrations of the ZnO nanoparticles, CeO2 nanoparticles and the ZnO:CeO2 :NC:PANI bionanocomposites were added. Following an overnight incubation at 37 ◦ C, MTT solution (10 ␮l of 5 mg ml−1 , Sigma-Aldrich, USA) was added to each well and incubated for an additional period of 4 h at 37 ◦ C. The absorbance at 570 nm was determined in each well with a Multiskan FC 96well plate reader from Thermo Scientific (USA). The background absorbance was recorded at 690 nm and the viability of the bacterial cells in suspension was determined. The minimum inhibitory concentration (MIC50 ) for the ZnO nanoparticles, CeO2 nanoparticles and the ZnO:CeO2 :nanocellulose:PANI bionanocomposite was calculated from the results. Additionally, agar-cup diffusion assay against the aforementioned bacterial strains was also carried out. The test organisms were incubated overnight at 37 ◦ C in Luria-Bertani broth (SigmaAldrich, USA) and 0.6 ml of the incubated cultures was utilized to prepare Agar test plates (Luria Agar, Sigma-Aldrich, USA) of each test organism. The requisite number of wells were bored into the agar with the help of a sterile cork borer and different concentrations of test samples, the standard antibiotic (Ampicillin: 10 mg ml−1 ) and a test blank was aliquoted into the wells. The plates were subsequently incubated at 37 ◦ C overnight and checked for the presence of prominent zone of inhibition.

3. Results and discussions 3.1. TEM analysis and elemental composition Fig. 1a,b shows the TEM micrographs of the native ZnO and CeO2 nanoparticles, prior to their use for the generation of the cellulose based antibacterial bionanocomposite. The micrograph of the as synthesized ZnO nanoparticles (Fig. 1a) shows their wurtzite-like morphology while that of the CeO2 (Fig. 1b) nanoparticles reveal their cuboidal surface topology. The average size of the native ZnO and CeO2 nanoparticles, as estimated from the micrographs was found to be in the range of ∼85–124 nm for ZnO nanoparticles and ∼1.6 nm for CeO2 nanoparticles respectively. Interestingly, the size of the ZnO nanoparticles appear to be significantly reduced to an average size of ∼10.5 nm, in the as prepared ZnO:CeO2 :nanocellulose:PANI bionanocomposite (Fig. 1c). It has been reported previously that the native ZnO nanosystems undergo dissolution, releasing Zn+2 ions, when suspended in acidic aqueous solutions to attain an equilibrium around 10 h post-incubation (Bian, Mundunkotuwa, Rupasinghe, & Grassian, 2011). The size reduction for ZnO particles observed in the asprepared bionanocomposite may be attributed to the dissolution of the ZnO under the highly acidic conditions (pH ∼ 1.6) during the in situ polymerization. However the sizes of the CeO2 nanoparticles (∼1.05 nm) do not seem to be effected by acidic pH owing to their high resistance to chlorination, as reported earlier (Amrute et al., 2012). Thus, the in situ polymerization of PANI, along with the nanocellulose, ZnO and CeO2 , helped in the size reduction of ZnO as well as the incorporation of the nanoparticles onto the PANI nanocellulose matrix. The elemental composition of the synthesized bionanocomposite was measured by EDX analysis (Fig. 1d). The dominance of Carbon and Oxygen signatures and the characteristic presence of signals pertaining to zinc (Jin, Gao, Zhou, & Wang, 2014;Zhang, Liu, Bao, Tu, & Dai, 2013) and cerium in the EDX spectrum thus

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Fig. 1. TEM micrographs of ZnO nanoparticles (a), CeO2 nanoparticles (b), and the ZnO:CeO2 :nanocellulose:PANI bionanocomposite (c); SEM-EDS elemental analysis of ZnO:CeO2 :nanocellulose:PANI bionanocomposite (d).

Fig. 2. Comparative X-ray diffraction (a) and FTIR spectra (b) of ZnO:CeO2 :nanocellulose:PANI bionanocomposite, Polyaniline, and the isolated Nanocellulose (NC) respectively.

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indicates the successful polymerization of the nanoscaled ZnO and CeO2 with the carbon rich nanocellulose. To further estimate the constitutive percentage of the components present, the ZnO:CeO2 :nanocellulose:PANI bionanocomposite was analyzed using AES and the percentage of incorporation of the ZnO and CeO2 onto nanocellulose was calculated (Table 1, data in brief). As evident, ∼95.75% of the ZnO/CeO2 nanoparticles used were successfully incorporated onto the nanocellulose used. Further, a total of ∼98.33% of the ZnO/CeO2 /nanocellulose nanoparticles were successfully incorporated onto the porous polyaniline matrix during the in situ polymerization (Table 1, data in brief). 3.1.1. XRD analysis The synthesized nanosystems were subjected to powder X-Ray Diffraction analysis and their structural attributes were subsequently analyzed. The characteristic peaks for zinc oxide (Fig. 2a, data in brief) were observed at 2 = 31.7◦ , 34.3◦ , 36.1◦ , 47.4◦ , 56.5◦ , 62.7◦ , 66.3◦ , 67.8◦ , 69◦ , corresponding to the (100), (002), (101), (102), (110), (103) and (201) diffraction planes. These planes corroborate to the hexagonal wurtzite structure of ZnO (JCPDS No: 01-079-0207). The crystallinity peaks for cerium oxide (Fig. 2a, data in brief) were observed at 2␪ = 28.6◦ , 33.1◦ 47.6◦ , 56.3◦ corresponding to the (111), (200), (220), and (311) diffraction planes and corroborate to the cubic fluorite structure of CeO2 crystal (JCPDS No: 34-0394). The X-ray diffractogram of the ZnO:CeO2 :nanocellulose:PANI bionanocomposite (Fig. 2a) shows crystallinity peaks at 2␪ = 15.1◦ , 22.4◦ , 34.1◦ that correspond to the 101, 002 and 004 cellulose diffraction planes and is highly similar to the diffractogram of the isolated NCs (Kalita et al., 2015). Although no prominent peaks are observed for ZnO and CeO2 in the bionanocomposite the shift in the cellulose signatures towards the lower 2␪ angles (22.3◦ and 15.1◦ ), indicating the increase in strain, can be attributed to the incorporation of the metal oxides on the otherwise dominant cellulose. 3.1.2. FTIR analysis The synthesized nanosystems were subjected to Fourier transform infrared (FTIR) spectroscopy to identify the functional groups of the active components based on their characteristic signatures in the infra-red region. Prominent peaks for the native ZnO (Fig. 2b, data in brief) were observed at 3374 cm−1 , 2919 cm−1 which correspond to the OH stretching mode of hydroxyl groups (Xiong, Pal, Serrano, Ucer, & Williams, 2006) and at 2848 cm−1 corresponding to C H bond stretching frequencies (Gayen et al., 2011). Moreover, the peak for Zn-O absorption at 430 cm−1 (Fig. 2b, data in brief) also indicates the formation of the ZnO nanoparticles (Segala, Dutra, Franco, Pereira, & Trindade, 2010). The native CeO2 nanoparticles, shows prominent peaks at 3434 cm−1 and 450 cm−1 (Fig. 2b, data in brief) for the OH stretching associated with physical adsorption of water and the Ce-O stretching band, respectively, confirming the formation of the CeO2 nanoparticles. Additionally, the minor signatures at 1622 cm −1 corresponds to OH stretching of adsorbed water (Goharshadi, Samiee, & Nancarrow, 2011). The ZnO:CeO2 :NC:PANI bionanocomposite showed dominant peaks at 3369 cm−1 and 1110 cm−1 (Fig. 2b). These infrared signatures correspond to the OH vibration of the O3:H O5 intramolecular hydrogen bond and C O stretching of cellulose, respectively (Li & Renneckar 2011). Additional peaks in the range of 2921–2922 cm−1 , 1650–1652 cm−1 , and 608–610 cm−1 were also observed which correspond to the aliphatic saturated C H stretching vibration, the bending mode of adsorbed water and the ␤-glycosidic linkages of the glucose ring in nanocelluloses, respectively. This peak profile is highly similar to the FTIR spectra of the isolated NCs, suggesting the dominant presence of nanocellulose in the as-prepared bionanocomposite. However, the peaks at 1570 cm−1 and 1495 cm−1 observed in the

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FTIR spectra for the pristine polyaniline, corresponding to the C C stretching of the quinone ring and benzene ring, are found to be superimposed by the 1650–1390 cm−1 signatures of nanocellulose in the bionanocomposite. Further, the PANI peaks at 1130 cm−1 and 818 cm−1 , corresponding to NH+ stretching of aniline (Izumi, Constantino, & Temperini, 2005) and C H bending vibration for polyaniline (Liu, Sui, & Bhattacharya, 2014) are present in both pristine PANI as well as in the bionanocomposite. However, no prominent peaks for ZnO and CeO2 could be observed in the bionanocomposite. Thus, the dominance of cellulosic signatures and the presence of infrared peaks for polyaniline in the FTIR spectra of the ZnO:CeO2 :nanocellulose:PANI bionanocomposite indicates the encapsulation of the metallic nano oxides by the nanocellulose: polyaniline matrix during the process of in situ polymerization. 3.2. Adsorption studies The ZnO:CeO2 :nanocellulose:PANI bionanocomposite was evaluated for potential use as a remediation-ready nanobiosorbent for the removal of dissolved Arsenic from aqueous systems. To investigate the various factors that may have an effect on the adsorption capacity of the bionanocomposite, the adsorbate (As) concentration, contact time for adsorption, adsorbent (bionanocomposite) concentration and the solution pH were investigated through batch adsorption studies at room temperature (Fig. 3). 3.2.1. Effect of the contact time with the adsorbent on the adsorption capacity of the ZnO:CeO2 :nanocellulose:PANI bionanocomposite The effect of the contact time on the adsorption capacity of the adsorbent was studied by incubating it in As solutions at different time intervals (0 h–24 h), while keeping other parameters viz. adsorbate (As) concentration, adsorbent (bionanocomposite) concentration and solution pH constant (Fig. 3a). The maximum admissible presence of dissolved As in potable water has been fixed at 10 ppb by the WHO and UESPA. As such, the concentration of the As present in working solutions used for incubation at different time points was fixed at 10 ppb to ensure its removal to acceptable limits. The removal of As by the bionanocomposite was found to be very rapid, with the mean adsorption capacity of ∼53% in most cases, immediately after the incubation. This efficiency increased progressively with the increase in the contact time attaining a ∼80% removal at 4 h, while saturating (>95%) around the 12 h time interval. The adsorption of As from the 12–24 h interval showed trivial changes in the adsorption which indicates that the adsorption had attained an equilibrium state at this stage. 3.2.2. Effect of adsorbate (As) concentration on the adsorption capacity of the ZnO:CeO2 :nanocellulose:PANI bionanocomposite To study the effect of the adsorbate (As) concentration w.r.t the adsorption capacity of the bionanocomposite, different concentrations of Arsenic (5 ppb–40 ppb) were used, while keeping the other parameters, viz. adsorbent (bionanocomposite) concentration and solution pH, unchanged for a contact time of 24 h (Fig. 3b). The ZnO:CeO2 :PANI bionanocomposite showed a ∼90–99% removal efficiency for the low concentrations of As (5 ppb). With the increase in the adsorbate concentration up to 40–ppb, there was a gradual decline in the adsorption efficiency which was limited up to 70–80%. The presence of a large number of hydroxyl groups in cellulose and the polymeric chain configuration of cellulose has been reported to function as an excellent adsorbent for As with high selectivity by the formation of covalent bonds as well as by efficient complexation with As ions (Mohan & Pittman, 2007). Additionally, the presence of the constitutive primary amines and tertiary imines of the PANI binding matrix is also known to bind covalently with metal ions like As thereby promoting their uptake from aqueous

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Fig. 3. Effect of contact time with the adsorbent (a), adsorbate concentration (b), adsorbent concentration (c), and solution pH (d) on the adsorption of Arsenic on the ZnO:CeO2 :nanocellulose:PANI bionanocomposite.

environments (Jia, Shan, Jiang, Wang, & Li, 2012). Thus the decline in the adsorption as observed, may be attributed to saturation of the available binding sites for As on the ZnO:CeO2 :nanocellulose: PANI bionanocomposite, with the progressive increase in As concentrations.

nanocomposite was not affected by the change in the H+ and OH− ions of the solution, unlike many adsorptive processes where pH plays an important role in the adsorption of metal ions by the adsorbent (Zhu et al., 2015). 3.3. Adsorption kinetics and isotherm

3.2.3. Effect of adsorbent concentration on the adsorption capacity of the ZnO:CeO2 :nanocellulose:PANI bionanocomposite To optimize the minimum dosage of the bionanocomposite required to bring down the As to acceptable limits (>10 ppb), batch adsorption studies using different concentrations of the bionanocomposite (0.01–0.1 g l−1 ) were carried out while keeping other parameters viz. adsorbate (As) concentration and solution pH constant, for a contact time of 24 h (Fig. 3c). At low initial concentrations (0.01–0.02 g l−1 ), the ZnO:CeO2 :nanocellulose:PANI bionanocomposite demonstrated a sorption efficiency in the range ∼79–90%. The efficiency of As adsorption progressively increased up to ∼98% with the increase in the adsorbent concentration in the range of 0.03–0.1 g l−1 (Raul et al., 2014). 3.2.4. Effect of solution pH on the adsorption capacity of the ZnO:CeO2 :nanocellulose:PANI bionanocomposite To study the effect of the pH of the solution on the adsorption efficiency of the bionanocomposite, As solutions at different pH (2–12) were used, keeping other parameters viz. adsorbate (As) concentration, adsorbent (bionanocomposite) concentration and contact time constant (Fig. 3d). The ZnO:CeO2 :nanocellulose:PANI bionanocomposite demonstrated an average adsorption efficiency of ∼97.5% across the pH range, with the maximum adsorption efficiency (99.5%) at the pH 8. The sorption efficiency of the bionanocomposite was found to remain fairly consistent throughout the pH range. This indicates that adsorption of As on the bio-

3.3.1. Study of adsorption kinetics To investigate the controlling mechanism of the adsorption process, the pseudo-first order and pseudo-second order equations were applied to model the kinetics of As adsorption on the ZnO:CeO2 :nanocellulose:PANI bionanocomposite (Kumar & Kirthika, 2009). The results show that the pseudo second order linear model gave better data correlation than that of the of the pseudo-first order linear model with correlation coefficients of 0.95 and 0.98 respectively (Fig. 4a,b). Additionally, the better fit obtained for the pseudo second order linear model, signified by the lower values for ␹2 (2.00E-08) and RMSE (0.00001) also validate the above findings. Thus, it may be inferred that the adsorption of As on the bionanocomposite occurs primarily due to the formation of covalent bonds between the dissolved As and the bionanocomposite since chemisorption is assumed to be the rate-limiting step for adsorption processes that follow pseudo second-order kinetics (Kumar & Kirthika, 2009). 3.3.2. Adsorption isotherms To establish the relationship between the amount of Arsenic adsorbed and the equilibrium concentration of the bionanocomposite in the reaction mixture, Langmuir, Freundlich, Temkin and Dubinin-Radushkevich adsorption isotherms were applied to the experimental data obtained from the batch adsorption studies (Fig. 5). The results show well fit straight lines for the linear

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Fig. 4. Pseudo first order (a) and pseudo second order (b) adsorption kinetics curves for the adsorption of Arsenic on the ZnO:CeO2 :nanocellulose:PANI bionanocomposite.

Fig. 5. Langmuir adsorption isotherm plot (a), Freundlich adsorption isotherm plot (b), Temkin adsorption isotherm plot (c), Dubinin- Radushkevich adsorption (d) for the adsorption of Arsenic by the ZnO:CeO2 :nanocellulose:PANI bionanocomposite.

plots of the Freundlich and Dubinin-Radushkevich adsorption isotherms with correlation coefficients of 0.99 and 0.98 respectively (Fig. 5b,d). Since the linear plots of the Langmuir and Temkin adsorption isotherms showed comparatively lower values for correlation of the data (0.94, 0.88), it may be inferred that the adsorption isotherms were successfully described by the Freundlich and the Dubinin-Radushkevich equations. Additionally, it may be inferred that the sorption of As onto the ZnO:CeO2 :nanocellulose:PANI bionanocomposite follows a multilayer mechanism, occurring over the heterogeneous surface of the adsorbent, since Freundlich adsorption isotherm is generally associated with the multilayer adsorption for a heterogeneous surface (Dada, Olalekan, Olatunya, & Dada, 2012). Also, the adsorption intensity was determined from the slope of the Freundlich adsorption isotherm and the value was estimated to be 1, thus indicating

a favorable linear correlation between the concentration of the adsorbent and the extent of adsorption (Dada et al., 2012). Similar results were obtained from the batch adsorption studies for the effect of adsorbent concentration on the sorption capacity of the bionanocomposite, thus further validating the results. Furthermore, the mean free energy of adsorption was determined from the Dubinin-Radushkevich adsorption isotherm and was found to in the range of ∼8.781 kJ mol−1 . Thus, it may be inferred that chemisorption plays a dominant role in the sorption of As by the bionanocomposite since a mean free energy value in the range of ∼8–16 kJ mol−1 is indicative of chemisorptive process (Ibrahim & Sani, 2014). The results further corroborated with the data obtained from the pseudo second order kinetic studies which assumes chemisorption to be the rate limiting step.

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Fig. 6. Comparison of the inhibitory concentrations of the ZnO:CeO2 :nanocellulose:PANI bionanocomposite, ZnO nanoparticles, CeO2 nanoparticles and native PANI against Gram positive Bacillus subtilis (a) and Gram negative Escherichia coli (b) using MTT based cell viability assay. Comparative zones of inhibition of Agar cup diffusion assay for varying concentrations of the ZnO:CeO2 :nanocellulose:PANI bionanocomposite, ZnO nanoparticles, CeO2 nanoparticles and native PANI against Gram-positive Bacillus subtilis (c) and Gram-negative Escherichia coli (d).

The components of the bionanocomposite are expected to harbor the bimodal potential of behaving both as an Arsenic biosorbent as well as an antibacterial matrix. Fig. 6a,b shows the comparison of the antibacterial properties of the ZnO:CeO2 :nanocellulose:PANI bionanocomposite w.r.t the native ZnO nanoparticles, CeO2 nanoparticles and PANI. These assays were performed against both Gram positive B. subtilis ATCC 11774 and Gram negative E. coli NCTC 10538, using the MTT based cell viability assay. The results show that the ZnO:CeO2 :NC:PANI bionanocomposite has a higher antibacterial activity in comparison to the native ZnO and CeO2 nanoparticles while the native PANI did not show any perceptible antibacterial activity. The MIC50 values for the ZnO:CeO2 :NC:PANI bionanocomposite, ZnO nanoparticles and CeO2 nanoparticles were estimated to be 10.6 ␮g ml−1 , 23.7 ␮g ml−1 , 22.8 ␮g ml−1 against B. subtilis, and 10.3 ␮g ml−1 , 13.47 ␮g ml−1 , 14.65 ␮g ml−1 against E. coli respectively. To further validate the above findings, the agar cup diffusion assay was carried for the ZnO:CeO2 :nanocellulose:PANI bionanocomposite along with the native ZnO nanoparticles, CeO2 nanoparticles and PANI. The comparative results (Fig. 6c,d) consistently showed the highest zones of inhibition for the wells containing the ZnO:CeO2 :nanocellulose:PANI bionanocomposite, in the concentration range ∼10–40 ␮g ml−1 , against both B. subtilis and E. coli. The zone of inhibition for the native ZnO and CeO2 nanoparticles were smaller, with the antibacterial efficacy of the ZnO being slightly better than the CeO2 nanoparticles. The wells containing the native PANI did not show any detectable zone of inhibition. It was interesting to note that the bionanocomposite and the native nanosystems were able to show a significant antibacterial action within the concentration range 10–40 ␮g ml−1 , which is significantly lower than the concentration for Ampicillin (10 mg ml−1 ), used as the pogsitive control in the assay.

4. Conclusion Nanocellulose isolated from the lignocellulose rich, natural Arsenic adsorbent rice husk, was effective as a matrix for the further integration of the remediation efficient and antibacterial ZnO and CeO2 nanoparticles, using Polyaniline as the binding agent. The batch adsorption studies reveal the rapid removal of As by the bionanocomposite that appeared to be independent of the pH variations, while indicating a linear correlation between the adsorption efficiency and the adsorbent concentration. The kinetic studies of the bionanocomposite showed conformity to the pseudo-second order kinetics and is believed to be mediated by covalent chemisorption. Furthermore, the biosorption followed a non-uniform and multi-layered mechanism, based on chemisorption as elucidated by the Freundlich and Dubinin-Radushkevich adsorption isotherms. The antibacterial activity of the nanoscaled ZnO and CeO2 incorporated into the bionanocomposite was found to be effective against both Gram-positive and Gram-negative bacteria with MIC50 values of 10.6 ␮g ml−1 and 10.3 ␮g ml−1 respectively. These properties demonstrate the bimodal potential of the ZnO:CeO2 :nanocellulose:PANI bionanocomposite both as an Arsenic adsorbent as well as an antimicrobial agent which are likely to find potential application as coating agents for efficient and economical water purification and remediation strategies. Acknowledgement The authors wish to acknowledge DBT, Govt. of India, for the Research Grant (Grant No. BT/258/NE/TBP/2011), UGC for the Research Grant (TU/Fin/MBBT/116/05/11-12/64), DBT Strengthening Grant, Tezpur University, for the TU Startup Research Grant (TU/Fin/P/12-13/05) and SAIF-NEHU. They also thank Mr. Ratan

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