New glucosamine Schiff base grafted poly(acrylic acid) as efficient Cu2+ ions adsorbent and antimicrobial agent

Accepted Manuscript Title: New glucosamine Schiff base grafted poly(acrylic acid) as efficient Cu2+ ions adsorbent and antimicrobial agent Authors: Yashwant Shandil, Umesh K. Dautoo, Ghanshyam S. Chauhan PII: DOI: Reference:

S2213-3437(18)30542-6 https://doi.org/10.1016/j.jece.2018.09.011 JECE 2632

To appear in: Received date: Revised date: Accepted date:

13-7-2018 3-9-2018 7-9-2018

Please cite this article as: Shandil Y, Dautoo UK, Chauhan GS, New glucosamine Schiff base grafted poly(acrylic acid) as efficient Cu2+ ions adsorbent and antimicrobial agent, Journal of Environmental Chemical Engineering (2018), https://doi.org/10.1016/j.jece.2018.09.011 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

New glucosamine Schiff base grafted poly(acrylic acid) as efficient Cu2+ ions adsorbent and antimicrobial agent Yashwant Shandil, Umesh K. Dautoo and Ghanshyam S. Chauhan*

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Department of Chemistry, Himachal Pradesh University, Shimla, India – 171 005 e‒mail: [email protected]; [email protected]

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Graphical Abstract

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HIGHLIGHTS

New Schiff base-based polymer synthesized from glucosamine and poly(acrylic acid) The synthesized polymer exhibit bi-functional characteristics Polymer exhibits excellent Cu2+ ions adsorption apart from good antibacterial activity Polymer shows excellent reusability and selectivity in the presence of competing ions

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   

Developing multifunctional materials to deal with the complex problem of water pollution has

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drawn significant concerns in recent time. In this study we report the synthesis of a new bi-

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functional polymer derived from N,N-dimethylaminobenzaldehyde and glucosamine-based Schiff

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base grafted poly(acrylic acid). The polymer besides affecting active adsorption of Cu2+ ions in

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the aqueous solution was also found to possess significant antibacterial properties. A high qm value

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of 208.33 mg g‒1for Cu2+ ions adsorption by the polymer was observed in the present work under

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the optimal conditions of contact time, temperature, pH of the medium and initial Cu2+ ions concentration. Presence of competing Mg2+ and Ca2+ ions was not found to have any appreciable

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effect on the adsorption capacity of the polymer towards Cu2+ ions and shows that the polymer could be an ideal and selective Cu2+ ions adsorbent under hard water conditions. The polymer also

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exhibited significant antibacterial activity against Gram (+), S. aureus and Gram (‒) P. vulgaris. The key findings reported in the present work reveal the bi-functional attributes of polymer and

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emphasize the role of Schiff base functionality in designing such materials.

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Keywords: Bi‒functional polymer, glucosamine, Schiff base, adsorption capacity, antimicrobial properties

1. Introduction

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Adsorbents derived from polymeric materials have attracted the attention of researchers’ worldwide and these also reflect contemporary advances in the adsorption chemistry [1]. Such adsorbents synthesized through the simple polymer analogous reactions possess active functional groups as selective ligands [2-4]. Therefore, tailor-made structural and functional design impart

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characteristic adsorption properties to the polymers [5,6]. In view of the preceding arguments, the

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synthesis of a new polymer as efficient Cu2+ ions adsorbent is reported in the present work. Cu2+

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ions present in the drinking water can be a huge health threat and have been reported to cause

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many life-threatening diseases in human beings such as the impairment of thyroid gland, Wilson disease, bronchitis, hypertension, kidney damage, cirrhosis and many stomach and intestinal

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cancers [7-9]. These ions are also associated with the malfunctioning of many critical enzymes

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through binding with nitrogen or sulfur atoms present in the proteins and nucleic acids [10,11].

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The extent of toxicity of Cu2+ ions in drinking water can be evidenced from the very low permissible limit, less than 2mg/L, set by WHO [12]. Industries like steel, mining, copper

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electrorefining, etc., release enormous amount of copper into fresh water streams everyday [13]. Copper may also add to the drinking water by the corrosion of interior pipes or fittings used in

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water distribution systems and copper concentration in such incidences has been reported to be lethal to the newborns [14]. Moreover, copper is also used as a corrosion resistant coating material in nuclear reactors and the traces of these ions have been reported in water present in the vicinity of the nuclear reactors [15,16]. Hence, it is imperative to develop effective materials and techniques for the efficient removal of Cu2+ ions from wastewater before consumption. 3

A number of adsorbents such as activated carbon [17,18], zeolites [19], those based on biowaste [20,21] and polymer-based materials [4,22] have been reported for the removal of toxic heavy metal ions from the wastewater. Besides this, the polymeric materials consisting of active

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functional groups like amine [23], amide [5], azomethine [24,25], thiourea [26,27], amidoxime [28-30], pyridyl [31], etc., also exhibit excellent metal ions binding affinities and have been extensively used for adsorption applications. Therefore, controlling of the functional attributes of polymers significantly affect their attractive interactions with the targeted heavy metal ions and can be handy in designing new selective polymeric adsorbents. Water contamination by the

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harmful microorganisms poses another serious problem that requires serious attention and

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immediate redressal. Hence, synthesizing materials with multifunctional properties reflect

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contemporary inventions in the field of water treatment.

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In view of the above arguments a new Schiff base-based polymer was synthesized from N,N-dimethylaminobenzaldehyde, glucosamine (a monomer of chitosan) and poly(acrylic acid)

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[p-AAc] in the present work. The synthesized polymer exhibited bi-functional characteristics as

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apart from being efficient Cu2+ ions adsorbent it also possessed significant antimicrobial

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properties. Ease of the synthesis of Schiff base structures makes them an important precursor to develop target oriented materials. Schiff bases are generally considered less stable and usually

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perform low on reusability [32]. Hence, the as-synthesized Schiff base was grafted to a preformed polymer, poly(acrylic acid) [p-AAc], which significantly enhanced its structural stability and the

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resulting polymer, SB-g-p-AAc, exhibited excellent reusability and water holding capacity. Pendent tertiary amine functionalities in the form of N,N-dimethylbenzaldehyde attract cationic Cu2+ ions with ease and favors initial interactions of the polymer with these ions. High local concentration of azomethine groups of Schiff base structure on the polymer imprison Cu2+ ions

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firmly and the attractive interactions are reinforced by the adjacent lying –OH functional groups of glucosamine moieties, Scheme 1. Therefore, tailor-made structural and functional design of the polymer, SB-g-p-AAc,

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favors active adsorption of targeted Cu2+ ions. The use of chitosan derived Schiff bases for active Cu2+ ions adsorption has also been reported elsewhere [33,34]. In their work Monier et al. have shown that the Schiff base derivatives of chitosan exhibited preferential adsorption of Cu2+ ions in the presence of Co2+, Ni2+, and Zn2+ ions [35,36]. Therefore, it can be stated that the Schiff basebased polymeric materials can affect preferential adsorption of Cu2+ ions in the presence of some

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other bivalent ions present in the solution. However, more structure-activity based functional

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modifications are indicative of inducing more selectivity and specificity in polymer functions. The

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Schiff bases have also been reported in literature to possess excellent antimicrobial properties [37-

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39]. Hence, the antimicrobial activity of SB-g-p-AAc was also evaluated. In view of the above stated, the synthesis of a new Schiff base-based bi-functional polymer as efficient Cu2+ ions

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2.1 Materials

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2. Experimental

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adsorbent and antimicrobial agent is reported in the present work.

Glucosamine hydrochloride, N,N-dimethylaminobenzaldehyde (SD Fine Chemicals, Ltd.,

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Mumbai, India), acrylic acid (AAc), hydrochloric acid and lipase [Hi Media Lab Pvt. Ltd., Mumbai, India) were of reagent grade and used as received. Sodium hydroxide (Rankem,

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RFCL Ltd., Faridabad, India), sodium borohydride (Merck India Ltd., Mumbai, India) and ammonium per-sulfate (APS) [Sigma-Aldrich Inc., St. Lucia US], were used as received. Potato dextrose broth, inoculating loop (Hi Media Lab Pvt. Ltd., India) and the Mueller Hinton agar medium was used as received. Copper sulfate, copper regent [1-(2-pyridyazo)-2-naphthol] (Orlab

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Instruments Pvt. Ltd., Hyderabad, India) were of analytical grade and used as received. pH meter (Eutech 20) was used to adjust the pH of the solutions and the concentration of Cu 2+ ions was determined with a Photolab 6600 ultraviolet−visible (UV−VIS) programmable spectrophotometer.

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The test microorganisms, Gram (+) bacterium, Staphylococcus aureus (S. aureus), Gram (−) bacterium, Proteus vulgaris (P. vulgaris) and a fungus Candida albicans (C. albicans) were obtained from the Department of Biotechnology, Himachal Pradesh University Shimla, India. Ciprofloxacin, Ciprodac-500 (Zydus Cadila Healthcare Ltd., Ahmadabad, India) and fluconazole, Fluka-150 (Cipla Ltd., Haridwar, India) were used as antibacterial and antifungal references,

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respectively.

base

was

synthesized

from

glucosamine

hydrochloride

and

N,N-

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Schiff

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2.2 Synthesis of Schiff base

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dimethylaminobenzaldehyde following the procedure reported by Applet et al [39]. To 1.0 g of glucosamine hydrochloride (4.6 mM) was added 1N NaOH (4.6 mL) in a reaction vessel and stirred

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at room temperature for 10 min. To this was added 0.7 g of N,N-dimethylaminobenzaldehyde with

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continuous stirring. Precipitates of glucosamine Schiff base (Glu-SB) appears immediately. After

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15-20 min of stirring the reaction mixture was maintained below 4 ºC temperature for about 5h. The final product was purified by repeated washings with water/methanol (1:1) mixture, Scheme

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2.

2.3.Grafting of Glu-SB to p-AAc

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p-AAc was prepared by thermal polymerization of the monomer, acrylic acid (AAc). Initiator

ammonium persulphate (APS) [0.1 wt% of the monomer content] was added to 10 mL of AAc and allowed to polymerize for 5h at 70 ºC temperature. The Schiff base (Glu-SB) was grafted onto the p-AAc through enzymatic route using lipase as the catalyst. A new ester-linkage between free –

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COOH groups of p-AAc and primary –OH groups of glucosamine at C6 was induced as a result of grafting. The polymer, p-AAc, was mixed with Glu-SB in a weight ratio of 1:2 in hexane. Lipase 1% of the reaction content was added to the reaction mixture which was maintained at 45 ºC for

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12h. The final product, SB-g-p-AAc, was repeatedly washed with de-ionized water to remove lipase and dried below 50 ºC in hot air oven, Scheme 3. 2.4.Characterization of the polymer SB-g-p-AAc

The as-synthesized polymer, SB-g-p-AAc, was characterized with FTIR (Fourier transformation infrared) spectroscopy, NMR (Nuclear magnetic resonance) spectroscopy, XRD

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(X-ray diffraction) and FESEM (Field emission scanning electron microscope) imaging and EDX

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(Energy-dispersive X-rays) analysis to get evidence of the structure and formation at different

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stages of the synthetic protocol. FTIR spectrum was recorded with Nicolet 5700 in transmittance

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mode in KBr. 13C NMR spectrum was recorded with BRUKER AV 400 MHz spectrometer. X‒ ray diffraction spectrum was recorded with a PANalytic XRD with XPERT‒PRO diffractrometer

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system at 1.54060 Å (Cu‒Kα radiation). The diffraction angle 2θ was varied from 10 to 55 degrees.

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2.5. Swelling studies

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FESEM was recorded with NOVA NANO SEM-450.

The ability of a polymer to adsorb water is an essential characteristic and ensures its better

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interaction with the water dissolved pollutant species. Besides this, active adsorptive sites become more exposed in a swollen polymer which enhances its adsorption efficiency [5]. In view of the

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above arguments; the swelling behavior of SB-g-p-AAc was studied as a function of time, temperature and pH. 10 mL of the double distilled water was added to the accurately weighed (0.01 g) polymeric sample. pH of water was adjusted with 0.1 N HCl and 0.1 N NaOH solutions and %swelling (Ps) was calculated using the formula [40]:

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𝑃𝑠 =

𝑊𝑠 − 𝑊0 × 100 𝑊0

(1)

Where Wo and Ws stands for weights of the dry and the swollen polymers, respectively. 2.6. Determination of Cu2+ ions

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The stock solution (500 ppm) of Cu2+ ions was prepared by dissolving 1.964 g of copper sulfate [CuSO4.5H2O] in 1L of de–ionized water. The resultant solution was used to prepare Cu2+ ions solutions of different concentrations by dilution process. ORLAB copper reagent kit (ORREGT-Cu) was used for spectrophotometric determination of Cu2+ ions in its aqueous solution.

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The reagent has a working range between 0.1‒6.0 mg L-1 of Cu2+ ions present in the solution. The

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spectrophotometric determination at λmax 545 nm.

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Cu2+ ions form a violet colored complex with this reagent which can be used for their

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The adsorption capacity (q) of SB-g-p-AAc was studied as a function of contact time in batch experiments. 0.01 g of the polymer sample was added to a reaction vessel containing 10 mL of

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Cu2+ ions solution of 100 ppm concentration. After specific time intervals (5 to 120 min) the

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residual Cu2+ ion concentration of the solution was determined spectrophotometrically by

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measuring absorbance at λmax 545 nm. The adsorption capacity was calculated as [41]:

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𝑞=

(𝐶0 − 𝐶𝑡 ) ×𝑉 𝑤

(2)

Where q is the adsorption capacity (mg g‒1); C0 and Ct, respectively, are the initial and residual

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Cu2+ ions concentrations (mg L‒1) after time t, V is the volume of the solution in liters and w is weight of the adsorbent polymer in grams. Effect of temperature on adsorption capacity was also studied and the temperature was varied as 20, 30, 40, 45, 50 and 60 ºC. The role of pH is crucial in adsorption study of the metal ions. Hence, the Cu2+ ions adsorption was determined at 2.5, 4.0,

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5.5, 6.0 and 7.0 pH values. To study the effect of Cu2+ ions concentration on adsorption capacity, q value was determined at different Cu2+ ions concentrations ranging from 25 to 350 ppm. The mechanism of adsorption of Cu2+ ions by the polymer was studied using kinetic models (pseudo-

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first and pseudo-second order), adsorption isotherm models (Langmuir and Freundlich) and various thermodynamic parameters were calculated from the van’t Hoff plot. The different equations used for studying these models are given in the supplementary data. Reusability and effect of competing ions (Mg2+, Ca2+) on adsorption capacity of the polymer was also studied. 2.7. Evaluation of antimicrobial activity

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As already discussed, the Schiff bases possess antimicrobial characteristics hence, the

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antimicrobial activity of SB-g-p-AAc was also evaluated. To determine the effect of structure on

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antimicrobial activity a new reduced Schiff base derivative, Glu-RSB, was also synthesized and

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grafted to p-AAc following the similar reaction protocol as in the case of Glu-SB (supplementary data). The antimicrobial activity of SB-g-p-AAc and RSB-g-p-AAc was determined by ‘agar well‒

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diffusion method’ in parallel experiments [42]. Potato dextrose broth (4.1% w/v) and nutrient broth

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(2% w/v) were respectively used as the media for growing fungal and bacterial suspension cultures.

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The Mueller Hinton agar medium (38 g/L; pH 7.0) was poured in the petri plates (20 mL/plate) and was used for developing surface colony growth. The plates were swabbed (sterile cotton

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swabs) with 8h broth culture of respective bacteria and fungus in laminar flow on solidified media. Wells (8 mm diameter) were made using sterile cork borer in each of these plates. Samples, 50 mg

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(for fungus) and 30 mg (for bacteria) of the respective polymers and reference drugs were placed in the wells. The plates were incubated at 37 ºC for 18‒24h in the case of bacteria and for 48h in case of fungus. Antimicrobial activity was assessed by measuring the diameter of the growth‒ inhibition zone in millimeter (mm) and the actual zone of inhibition was reported after subtracting

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the diameter of the well. Pathogenic microorganisms used in the study were a gram (+) bacterium, S. aureus, gram (‒) bacterium, P. vulgaris and a fungus, C. albicans.

3. Results and Discussion

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3.1. Characterization studies

A new Schiff base-based polymeric adsorbent, SB-g-p-AAc, was synthesized from glucosamine hydrochloride, N,N-dimethylaminobenzaldehyde and AAc. The polymer was characterized by FTIR, NMR, XRD and FESEM techniques to get evidence of the structure and formation. The FTIR spectra of glucosamine Schiff base (Glu-SB), and its polymeric derivative

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SB-g-p-AAc are presented in Fig. 1.

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Glu-SB show characteristic band due to >C=N stretching of Schiff base structure at 1640 cm ‒1

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[43]. The broad absorption bands around 3420 cm-1 in its spectrum represent the O-H stretching of

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the hydroxyl groups of glucosamine moieties. While the other bands around 2920 cm-1 are due to

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C-H stretching vibrations. On the other hand, a new band at 1740 cm‒1 was observed in the

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spectrum of SB-g-p-AAc which represents the >C=O stretching of the ester-linkage formed between free carboxylic groups of p-AAc and hydroxyl group of Glu-SB at C6. Also a band at

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1631 cm‒1 indicates >C=N stretching of azomethine functionality of Schiff-base. Similarly, a

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broader band near 3410 cm-1 represents the O-H stretching of free hydroxyl groups of glucosamine moieties.

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C NMR spectra of Glu-SB and SB-g-p-AAc are presented in Fig. 2(a) and (b),

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respectively. The spectrum of Glu-SB has a signal near 158 ppm due to the carbon of azomethine group. Other signals between 110-150 ppm represent the ring carbon atoms of N,Ndimethylaminobenzaldehyde and those between 20-80 ppm represent the carbons of glucosamine structure. The 13C NMR spectrum of SB-g-p-AAc shows the presence of a new signal around 190 10

ppm δ-value which is due to >C=O carbon of ester linkage induced as a result of grafting reaction. The new signals appearing between 10-15 ppm represent the vinylic carbons of p-AAc, Fig. 2(b).

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The other signals in the 13C NMR spectrum of SB-g-p-AAc are similar as discussed for Glu-SB.

The XRD spectra of Glu-SB and SB-g-p-AAc are presented in Fig. 3. The sharp peaks of high intensity were observed in the spectrum of Glu-SB in the region 15° - 35° 2θ which indicate comparatively crystalline nature. No such peaks were observed in the XRD spectrum of SB-g-pAAc indicating the amorphous nature induced as a result of grafting reaction.

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Adsorption is a surface phenomenon and hence the study of the surface structure of the

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polymer was made using SEM imaging technique and the images are presented in Fig. 4. The SEM

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analysis of SB-g-p-AAc reveals rough, porous and foam like surface morphology. Such surface

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structure is advantageous for adsorption of Cu2+ ions onto the polymer surface.

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3.2. Swelling studies

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Swelling behavior of SB-g-p-AAc was studied as function of time, temperature and pH. The polymer exhibited hydrophilicity and excellent swelling in water due to the presence of polar

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hydroxyl (–OH) groups of the glucosamine moieties in its structure. The degree of swelling

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depends upon the polymer/water interactions and the presence of polar groups in the polymer structure is supposed to increase the swelling [44]. The swelling was rapid as after 60 min the

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maximum weight increase was observed, Fig. 1.1S (Supplementary data). Hence, 60 min was taken as the optimum time for further studies. Ps was found to increase with increase in temperature from 25 to 45 ºC while equilibrium was observed after 50 ºC. Increase in temperature results in improved interactions of the polymer functional groups with water molecules [Fig. 1.2S (supplementary data)]. Effect of the pH of the medium (2.5, 4.0, 5.5, 6.0, 7.0 and 9.0) on Ps was 11

also studied [Fig. 1.3S (supplementary data)]. The polymers exhibited higher Ps at lower pH i.e. in acidic medium. At lower pH, the protonation of the amine groups develop repulsive interactions between the adjacent polymer chains and they remain fully expanded in the solution. As a result

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more water molecules can reach the active absorptive sites of the polymer and consequently higher Ps is observed at lower pH [45]. A similar trend in the swelling behavior of SB-g-p-AAc was observed in the present case. From the perusal of the reported results it can be stated that the synthesized polymer behave as an attractive stimuli responsive hydrogel with higher swelling observed at the lower pH. These results portray well for the adsorption properties of the

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synthesized polymer as targeted Cu2+ ions are adsorbed generally under weak acidic pH.

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3.3. Cu2+ ions adsorption studies

Effect of different parameters such as contact time, temperature, pH and initial Cu2+ ions

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of Cu2+ ions from its aqueous solution, Fig. 5.

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concentration on q value was studied to evaluate the best conditions for the maximum adsorption

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Effect of contact time (5–120 min) on adsorption capacity was studied at temperature 20 ºC,

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pH 6.0 and at Cu2+ ions concentration of 100 ppm. Adsorption capacity (q) increased with time

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initially and attained equilibrium after 60 min, Fig. 5(a). Hence, 60 min was taken as the optimum time for further studies. Temperature has positive effect on adsorption and the q value was found

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to increase with increase in temperature, Fig. 5(b).The increased flexibility of the polymeric chains and high thermal mobility of Cu2+ ions at higher temperature enhance their effective interactions

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[46]. Moreover, the polymer structure opens up and more adsorption sites become exposed for interaction with the targeted ions at higher temperature. Hence, the polymer exhibit higher q value at elevated temperature.

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The pH of the medium has significant effect on the adsorption capacity of the polymer which increased with pH and reached the maximum near pH 5.5 in the present study, Fig. 5(c). Effect of pH on adsorption capacity was studied between 2.5 to 7.0. Under the acidic medium, protonation

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of active groups such as –NH2, –OH or >C=N– generate cationic sites on the polymer surface and increase the electrostatic repulsions between the adsorbent and cationic Cu2+ ions that consequently decrease the q value [32]. The de‒protonation of the active groups with increase in pH favors adsorption and hence q reached maximum near the pH 5.5. A further decrease in the q values beyond pH 6.0 can be ascribed to the precipitation of Cu2+ ions as Cu(OH)2 [47]. Therefore, the

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adsorption of Cu2+ ions by SB-g-p-AAc was found to be pH-dependent.

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Effect of initial concentration of Cu2+ ions on adsorption capacity of SB-g-p-AAc is

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presented in Fig. 5(d). The increase in q values with increase in initial Cu2+ ions concentration

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from 25 to 200 ppm was observed while it became nearly constant beyond that. This shows that the active sites of the polymer matrices become saturated at this concentration and equilibrium

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was reached beyond that. The EDX spectra of the polymer supports active adsorption of Cu2+ ions

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by SB-g-p-AAc as distinct peak of Cu is observed in Cu-loaded-SB-g-p-AAc Fig. 2S

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(supplementary data). The functional groups like amine, azomethine and hydroxyl present in the polymer structure possess donor N and O atoms which can easily coordinate with cationic metal

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ion species [48]. This shows that the functional design of SB-g-p-AAc favors active adsorption of Cu2+ ions from the aqueous solution. The role of Schiff base structures in active metal ions

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adsorption from the aqueous solution has also been documented by Salisu et al [49]. 3.4. Reusability studies The reusability of the polymer, SB-g-p-AAc, was studied by leaching out the adsorbed Cu2+ ions with 0.1N EDTA solution (10 mL) for 10 min at 25 ºC. Excess of the EDTA solution

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was removed by centrifuging and decanting-off. The regenerated polymer was washed with double distilled water and dried below 50 ºC before using for the next feed cycle. The polymer exhibited excellent reusability and a negligible decrease in its q values was observed up to eighth repeat

enhanced stability and adsorption efficiency of the polymer.

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cycle, Fig. 6(a). Thus, it can be stated that the grafting of Schiff base to p-AAc significantly

3.5. Effect of competing Mg2+ and Ca2+ ions on adsorption capacity

The adsorption capacity of SB-g-p-AAc was also studied in a solution consisting of 50 ppm concentration of Mg2+ and Ca2+ ions besides Cu2+ ions. The polymer exhibited a minor decrease

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of 4.2% in q value in the presence of competing Mg2+ and Ca2+ ions while a decrease of 1.4% was

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observed for the Cu2+ ions solution prepared in the tap water, Fig. 6(b). This shows that the polymer

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is an ideal Cu2+ ions adsorbent under hard water conditions as Mg2+ and Ca2+ ions are the common

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constituents of hard water. Hence, it can be stated that the azomethine linkage of Schiff base structure imparts more selectivity to the polymer for active Cu2+ ions adsorption in the presence of

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competing Mg2+ and Ca2+ ions. Besides this, the high concentration of adjacent lying amino and

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hydroxyl groups present on benzaldehyde and glucosamine moieties of the polymer ensure

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efficient entrapment of cationic Cu2+ ions from the aqueous solution [50]. Shokrollahi et al in their work studied various ‘Schiff base‒Cu ion complexes’ and reported that the stability of such

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complexes is affected by a number of factors such as hard-soft interactions, ligand topology, ionic radii, charge on metal ion species, etc. [51]. The role of geometrical factors in forming the stable

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Schiff base-metal ion complexes is also documented by Reglinski et al [52]. Similarly, Schiff base modified nano-silica particles for selective copper ion removal is also reported by Moftakhar et al [53].

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Therefore, it can be concluded from the forgone discussion that the tailored-made structural and functional design of the polymer makes it more selective adsorbent for Cu2+ ions. 3.6. Mechanism of adsorption

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The adsorption behavior of the polymer was studied by applying pseudo‒first and pseudo‒ second order kinetic and Langmuir and Freundlich adsorption isotherm models and various thermodynamic parameters were also calculated from van’t Hoff plot.

The non-linear plots of pseudo‒first and pseudo‒second order kinetic models are presented in Fig. 7. The values of various parameters calculated from these plots are given in Table 1. The

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goodness of the fit of the experimental q values to those obtained from pseudo‒first and pseudo‒

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second order kinetic models were determined from the R2 (determination coefficient) values of the

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respective models and error analysis was also performed by estimating normalized deviation (ND)

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and normalized standard deviation (NSD) according to the method reported by Simha et al [54].

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This observation is further supported by the non-linear plots showing correlation of the

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experimental q values to those obtained from pseudo‒first and pseudo‒second order kinetic models. It can be seen that the experimental q values correlate well with the q values obtained

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from the pseudo–second order kinetic model which suggests that the adsorption of the Cu2+ ions

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by the polymer can be best described by the pseudo–second order model.

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The thermodynamic parameters ΔH°, ΔS° and ∆Gº were calculated from the intercept and

slope of van’t Hoff plot [Fig. 3S (supplementary data)] and their values are presented in Table 2.

From Table 2, it can be concluded that ∆Gº exhibit negative values and indicate the spontaneity of the adsorption process at all temperatures. The positive value of ∆Hº confirms that

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the adsorption is favored at high temperature and indicates the endothermic nature of adsorption process. The positive value of ∆Sº suggests an increased randomness at the solid–solution interface during the adsorption process.

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The adsorption isotherm models, Langmuir and Freundlich, were applied to interpret the adsorption behavior of the polymer and their comparison with experimental values is shown in Fig. 8.

The values of various parameters calculated by applying Langmuir and Freundlich isotherm

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models are presented in Table 3.

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As can be seen from Table 3 the value of R2 (determination coefficient) is high and quite

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close to unity in case of Langmuir model than that obtained from the Freundlich isotherm model.

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The results are reported with error analysis estimation in the form of ND and NSD. The goodness

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of the fit of the Langmuir model is also supported by better comparison of the q values obtained

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from this model to the experimental q values, Fig. 8. Also, the values of RL factor as calculated from the Langmuir isotherm model exists between

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0 and 1 for all the concentrations of Cu2+ ions studied and indicate the favorability of this model for the adsorption of Cu2+ ions by the polymer in the present case, Fig. 4S (supplementary data).

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A comparison of the maximum adsorption capacity values of similar materials is also shown in

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Table 4.

3.7.Antimicrobial activity studies As already stated the Schiff bases possess inherent antimicrobial properties. Hence, the antimicrobial activity of the as-synthesized polymer, SB-g-p-AAc was also evaluated in the present work. To further investigate the effect of Schiff base structure on antimicrobial action of 16

the polymer, a new reduced-Schiff base derivative, RSB-g-p-AAc was also synthesized and the antimicrobial action of SB-g-p-AAc and RSB-g-p-AAc was evaluated in parallel experimental sets. Two bacteria, S. aureus [Gram (+)], P. vulgaris [Gram (−)] and a fungus, C. albicans, were

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used as the test microorganisms. The microorganisms used were of pathogenic nature and have been reported to cause many life threatening diseases to the living beings including humans [5759]. The microbes were allowed to grow on agar media in petriplates and zone of inhibition was measured after incubation period of 24h for bacteria and 48h for fungus, under optimum conditions of growth, Table 5.

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The antibacterial action of SB-g-p-AAc and RSB-g-p-AAc was comparable and they formed

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prominent zones of 7 ±1 and 5 ±1 mm, respectively, against the Gram (+) bacterium, S. aureus. A

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smaller zone of 2 ±1 mm was formed by SB-g-p-AAc against the Gram (‒) bacterium, P. vulgaris

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while no prominent zone was observed for the polymer, RSB-g-p-AAc, against this bacteria. Both, SB-g-p-AAc and RSB-g-p-AAc were not able to affect fungal inhibition and no prominent zones

D

were shown by them against the fungus, C. albicans, Fig. 5S (supplementary data). This shows the

TE

better antibacterial character of SB-g-p-AAc as it could affect the inhibition of Gram (‒) bacterium,

EP

P. vulgaris while no such activity was observed for RSB-g-p-AAc. Also, SB-g-p-AAc exhibited larger zone of inhibition than RSB-g-p-AAc against the Gram (+) bacterium, S. aureus. These

CC

findings emphasize the role of Schiff base structure in better antimicrobial action and reveal the bi-functional character of SB-g-p-AAc. Nartop et al also reported the synthesis of bi-functional

A

poly(styrene) attached Schiff bases which exhibited the uptake of Mn(II) and Ni(II) ions besides showing antibacterial properties [60]. The synthesize of such bi-functional polymers possessing dual properties of adsorption of heavy metal ions and antimicrobial action has also been reported in a few other literature reports [5,61].

17

4. Conclusions A new Schiff base was synthesized from glucosamine hydrochloride and N,Ndimethylaminobenzaldehyde followed by its grafting to p-AAc which generated a new Schiff base-

SC RI PT

based bi-functional polymer, SB-g-p-AAc. Latter was used as an adsorbent for the adsorption of Cu2+ ions in the aqueous solution. The polymer was found to be efficient Cu2+ ions adsorbent and exhibited excellent qmax value of 208.33 mg g-1. It was also found to be selective Cu2+ ions adsorbent in the presence of competing Mg2+ and Ca2+ ions present in the solution and asserts its suitability under hard water conditions. The polymer exhibited excellent water uptake capacity and

U

shows high Ps value due to the presence of hydrophilic –OH groups of glucosamine moieties

N

present in its structure. Also the polymer, SB-g-p-AAc, was found to possess better antibacterial

A

activity than RSB-g-p-AAc which indicate the significance of Schiff base structure in efficient

M

antimicrobial action. Therefore, the synthesis of a new bi-functional polymer possessing dual properties of adsorption of Cu2+ ions and possessing significant antibacterial character is reported

D

in the present work. The findings highlight the multifunctional attributes of the Schiff base

TE

structure and could pave way for developing more efficient and multifunctional materials from

EP

these potential precursors in the future.

CC

Conflicts of interest

A

The authors declared that there are no competing interests.

Acknowledgement The authors thank Professor Duni Chand, Department of Biotechnology, Himachal Pradesh University, Shimla, India, for providing microorganisms and laboratory facilities for the

18

antimicrobial studies. YS, expresses his gratitude to the University Grants Commission for BSR fellowship and the facilities provided under the UGC(SAP)-DST FIST programme.

SC RI PT

References

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26

A

N

U

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Figure captions

A

CC

EP

TE

D

M

Fig. 1. FTIR spectra of Glu-SB and SB-g-p-AAc

Fig. 2. 13C NMR spectra of Glu-SB, (a), and SB-g-p-AAc, (b) 27

SC RI PT U

N

Fig. 3. XRD spectra of Glu-SB and SB-g-p-AAc

EP

TE

D

M

A

(a)

A

CC

Fig. 4. FESEM images of SB-g-p-AAc at 35.0k, (a), and 50.0k, (b)

28

(b)

(a)

70

90

(b)

60

85

40

30

20

80

75

70

10 0

20

40

60

80

100

120

65 20

time (min)

SC RI PT

-1

q (mg g )

-1

q (mg g )

50

30

40

50

60



Temperature ( C)

90

(d)

180

(c)

140 120 100

80

A

-1

70

N

-1

q (mg g )

80

q (mg g )

U

160

60

50

2

3

4

5

6

7

8

M

60

9

pH

40 20

0

50

100

150

200

250

300

350

2+

TE

D

Cu ions (ppm)

EP

Fig. 5. Adsorption capacity, q, (mg g‒1) of SB-g-p-AAc at 100 ppm Cu2+ ions concentration as a function of: time at temperature 20 ºC and pH 6.0, (a); temperature at time 60 min and pH 6.0,

concentration at time 60 min, temperature 45 ºC and pH 5.5, (d)

A

CC

(b); pH at time 60 min and temperature 45 ºC, (c); and q as a function of initial Cu2+ ions

29

(a)

(b)

SB-g-p-AAc

A: Distilled water B: Tap water 2+ 2+ C: Competing Ca and Mg ions

100

89.8% 160

Cu ions removal (%)

q (mg g -1)

120 80

60

40

20

7

0

8

SC RI PT

1 2 3 4 Nu mb 5 er 6 of cyc les

85.6%

2+

40 0 0

88.4%

80

B

A

C

Fig. 6. Plots of reusability study of SB-g-p-AAc, (a), and the comparison of % Cu2+ ions removal

U

by the polymer in 50 ppm Cu2+ ions solution prepared in double distilled water, tap water and in

N

a competing ions solution consisting 50 ppm each of Ca2+, Mg2+ and Cu2+ ions under the

M

A

optimized conditions, (b)

70 60

Experimental pseudo-first order pseudo-second order

D

50

TE

qt

40 30 20

0 0

20

40

60

80

100

120

t (min)

A

CC

EP

10

Fig. 7. Non-linear plots showing correlation of various kinetic models with the experimental data

30

Experimental Langmuir Freundlich

240

200

120

80

40

0 0

20

40

60

80

100

120

140

U

Ce

SC RI PT

qe

160

160

N

Fig. 8. Non-linear plots of Langmuir isotherm and Freundlich isotherm showing correlation

A

CC

EP

TE

D

M

A

of these models with the experimental data

31

D

M

A

N

U

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Scheme captions

EP

TE

Scheme 1. Cu2+ ions adsorption on the polymer, SB-g-p-AAc

NH 2 .HCl

A

OH O

NaOH

CC

OH O

OH O

O C H N

<4 oC/5h

CH NH 2

N H 3C

CH 3 N

Glucosamine hydrochloride

N ,N -dimethyl benzaldehyde

Scheme 2: Synthesis of Glu-SB

32

H 3C

CH 3

Glu-SB

CH-CH2 O=C O O



CH

H2 C

H C

n

initiator COOH

N

Glu-SB

CH

Lipase (45 ºC/12h)

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H2 C

COOH

N

H 3C

p -AAc

AAc

CH3

SB-g-p -AAc

A

CC

EP

TE

D

M

A

N

U

Scheme 3. Polymerization of AAc and subsequent esterification with Glu-SB using lipase

33

n

Table Table 1: Various kinetic parameters for the adsorption of Cu2+ ions by SB-g-p-AAc Pseudo-first order qe

4.56

63.13

R2

ND/NSD

K2(× 10-3)

0.970

14.47/22.85

R2

qe

ND/NSD

SC RI PT

k1(× 10-2)

Pseudo-second order

0.55

84.75

0.984

6.61/6.73

Table 2: Different thermodynamic parameters for adsorption of Cu2+ ions by SB-g-p-AAc -∆Gº (KJ mol-1)

∆Hº (KJ mol-1)

∆Sº (J mol-1 K-1)

U

Temperature (K) 303

313

318

323

333

1.79

2.43

4.20

5.58

5.61

5.10

29.06

105.72

M

A

N

293

D

Table 3: Different parameters calculated from the Langmuir and Freundlich isotherm models for

TE

the adsorption of Cu2+ ions by SB-g-p-AAc Langmuir

0.058

CC

208.33

R2

ND/NSD

0.994

12.87/13.45

EP

qm (mg g-1) b (L mg-1)

Freundlich Kf (mg g-1)

n

R2

ND/NSD

20.37

2.05

0.882

28.58/28.62

A

Table 4: A comparison of maximum adsorption capacity values of reported polymeric adsorbent and other similar materials

S. No.

Adsorbent

1. 2.

Chitosan derived Schiff bases Formaldehyde cross-linked modified chitosan-thioglyceraldehyde Schiff base 34

qmax (mg g-1) Antibacterial properties 48.30 Not reported 76.00

Not reported

Source [33] [35]

4. 5. 6.

Dialdehyde starch aminopyrazine Schiff base Chitosan-g-poly(acrylic acid) Poly(N-isopropylacrylamide-co-acrylic acid) hydrogels Glucosamine Schiff base-g-poly(acrylic acid)

24.00

Not reported

[49]

333.33

Not reported

[55]

67.25

Not reported

[56]

208.33

Effective

Present work

SC RI PT

3.

Table 5: Zone of inhibition of SB-g-p-AAc and RSB-g-p-AAc, ciprofloxacin and fluconazole against different microorganisms

S. aureus

7 ±1

2.

P. vulgaris

2 ±1

3.

C. albicans

‒

Ciprofloxacin

Fluconazole

N

1.

RSB-g-p-AAc 5 ±1

44 ±2

‒

‒

38 ±3

‒

‒

‒

35 ±2

A

SB-g-p-AAc

M

Sr. No. Microorganism

U

Zone of Inhibition* (mm)

A

CC

EP

TE

D

*diameter in mm (excluding the diameter of the well).

35