Development of high alginate comprised hydrogels for removal of Pb(II) ions

Journal Pre-proof Development of high alginate comprised hydrogels for removal of Pb(II) ions

Kokkarachedu Varaprasad, Dariela Nùñez, Walther Tippabattini Jayaramdu, Emmanuel Rotimi Sadiku

Ide,

PII:

S0167-7322(19)35081-0

DOI:

https://doi.org/10.1016/j.molliq.2019.112087

Reference:

MOLLIQ 112087

To appear in:

Journal of Molecular Liquids

Received date:

9 September 2019

Revised date:

29 October 2019

Accepted date:

7 November 2019

Please cite this article as: K. Varaprasad, D. Nùñez, W. Ide, et al., Development of high alginate comprised hydrogels for removal of Pb(II) ions, Journal of Molecular Liquids(2019), https://doi.org/10.1016/j.molliq.2019.112087

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© 2019 Published by Elsevier.

Journal Pre-proof 1 Development of high alginate comprised hydrogels for removal of Pb(II) ions Kokkarachedu Varaprasada*, Dariela Nùñeza, Walther Idea, Tippabattini Jayaramdub, Emmanuel Rotimi Sadikuc a

Centro de Investigación de Polímeros Avanzados, Av.Collao 1202, Edificio Laboratorio CIPA, Concepción, Bío-Bío, Chile b

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Laboratory of Material Sciences, Instituto de Química de Recursos Naturales, Universidad de Talca, P.O. Box 747, Talca, Chile c Institute for Nanoengineering Research, Department of Polymer Technology, Tshwane University of Technology, CSIR campus, Building 14 D, Private Bag X025, Lynwood, 0040 Pretoria, South Africa *Corresponding author emails: [email protected]; [email protected]

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Abstract

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Natural polymers have been used for the development of innovative hydrogels in order to

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improve their properties, principally, their swelling capacity. Herein, an alginate-

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acrylamide hydrogel was developed by employing a high proportion of alginate via a freeradical polymerization method. Water soluble alginate was extracted from Laminaria

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digitata and used for the development of alginate hydrogels in order to obtain superior

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hydrogels. Water adsorption capacity (swelling) and diffusion kinetics performance of the

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hydrogels were investigated. The hydrogel containing a high amount of alginate had the highest swelling capacity (109.3 g/g) and diffusion coefficient (3.5116 cm2s-1), which has high water absorption capacity. This hydrogel exhibited super case II diffusion and other hydrogels showed anomalous diffusion water transport phenomena. Additionally, the hydrogel with high alginate content, showed high adsorption of Pb(II) ions. Owing to their swelling and adsorption capacity of Pb(II) ions, this hydrogel could serve as a strong absorbent material in water purification and agricultural applications. Keywords: Alginate; Hydrogels; Swelling capacity; Laminaria digitata; Heavy metal removal

Journal Pre-proof 2 Introduction For several decades, natural polymers have been employed to prepare biomaterials for specific applications, owing to their biocompatibility, biodegradability and non-toxicity nature [1–4]. Owing to these properties, research groups were extracted and developed biomaterials [5–10]. Macroalgaes (seaweeds) are one of the natural sources (critical biomass), which are available in many coastal areas worldwide [11]. The cell wall of

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macroalgae contains hydrophilic colloidal polysaccharides, mainly alginic acid (alginate

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polymer), which is insoluble in water. In order to obtain a water-soluble polymer, this could

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be extracted from the brown macroalgae by alkaline extraction method [11]. Brown

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macroalgae are an excellent source of water-soluble sodium alginate polymer with

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outstanding physicochemical characteristics.

Alginic acid is a natural anionic biopolymer, which contains β-D-mannuronic acid (M-

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block) and α-L-guluronic acid (G-block) units [12]. These monomer units are combined by

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(1→4)-glycosidic linkages, playing a vital role in the development of biomaterials [13,14].

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According to the literature, these characteristics provide biodegradable, hydrophilic and pores nature to the biomaterials [15]. In addition, the molecule binds, easily with divalent ions and other materials via electrostatic, ionic interaction, in a covalent-like bonding, redox-reactions and coordination bonding [16]. However, alginate biopolymers are employed toward the development of polymeric biomaterials, especially in the development of hydrophilic (hydrogels) composites. They have been combined with synthetic and natural components [17]. Alginate-based materials have been shown to possess a three-dimensional network with swelling capacity in different mediums, being also able to adsorb different metal ions [17]. Alginate (hydroxyl, carboxyl functional

Journal Pre-proof 3 groups) can improve the hydrogels porous network, swelling and sorption properties via ionic crosslinking with other materials [18]. According to the literature, increasing the natural hydrophilic polymer content in the hydrogel, its physical and chemical properties and mainly, the sorption characteristics are venerable [15]. Whereas synthetic polymerbased absorbent hydrogels have serious concerns regarding environmental degradation and their economic problems, include: water retention, high evapotranspiration and moisture

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leaching [19]. In addition, most of hydrogels have been prepared with acrylamide, which

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provides high stability, hydrophilic nature and high swelling capacity to the hydrogels [20].

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However, a high amount of acrylamide can induce some level of toxicity to the

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biomaterials [21,22]. Thus, the combination of natural polymer with acrylamide hydrogels can reduce toxicity, cost of the hydrogels and can produce novel semi interpenetrating

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network hydrogels with controlled release property and result in environmentally-friendly

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substitute to synthetic polymers [19]. Instead, the control of the natural polymer content in the hydrogel composition is a challenging issue. In addition, to increase the porous

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structure, swelling and adsorption property of the semi interpenetrating hydrogels are a

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challenging issue. Generally, semi-interpenetrating hydrogels are formed via physical bond formation within the polymer that is synthesized in the presence of another polymer [23,24]. These type of semi-interpenetrating hydrogels have been employed for advanced technology applications. In this investigation, we aim to reduce acrylamide content in the hydrogels and develop high alginate-comprised hydrogel with a significant porous structure, high swelling and good adsorption property. Herein, alginate-acrylamide hydrogels were prepared via free-radical polymerization method. The required water-soluble alginate was extracted from Laminaria digitata brown

Journal Pre-proof 4 seaweed (LDBS), which was collected from the sea coast of Lenga, Concepción, Bío-Bío Region, Chile. The alginate collected was analysed by FTIR spectra. Furthermore, it was used to prepare SAX1-AMX2 (X1 = 0.0, 0.25, 0.50, 0.75,

X2

= 1.00, 0.75, 0.50, 0.25)

hydrogels. Alginate hydrogels swelling capacity was analysed by using a gravimetric method. In addition, swelling and diffusion kinetics data were achieved from the swelling capacity of the hydrogels. Finally, the SA0.75-AM0.25 hydrogel was used for the removal of

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lead, Pb(II) ions from polluted water.

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Experimental section

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Materials

Acrylamide (AM), Ammonium Persulfate (APS) and N,N`-Methylenebis(acrylamide)

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Method

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(MBA) and isopropanol were purchased from Sigma-Aldrich, Chile.

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of alginate.

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Alginate extraction from Laminaria digitata brown seaweed (LDBS) and Purification

Brown seaweeds were collected from the sea coast of Lenga, Concepción, Bío-Bío Region, Chile. The assembled weeds were rinsed with purified water, air-dried at ambient condition and powdered into small pieces (1 mm) by using a laboratory mill. Initially, 30 g of LDBS powder was placed in the oven at 60 oC for 5 min. Then, 2% formaldehyde solution was added to eliminate pigments and kept for 24 h, thereafter the mixture was washed with purified water and added to a 0.5 M H2SO4 solution for 24h. The product obtained was washed again with distilled water in order to eliminate excess acid, following which, the

Journal Pre-proof 5 solution was filtered and a 4 % sodium carbonate solution was added up to pH 11. The resulting product was dissolved in the sodium carbonate solution at room temperature. In this solution, sulphuric acid was added in order to precipitate alginic acid. The precipitated material obtained was washed with 80 % of isopropanol and centrifuged at 4 oC 10, 000 rpm for 5 min. The product obtained was air-dried at ambient condition [25]. The final

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material was characterized by FTIR.

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Synthesis of alginate hydrogels

Scheme 1: Synthesis of high alginate comprised hydrogels

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Alginate hydrogels were synthesized through a free-radical polymerization method by using alginate and acrylamide (Scheme 1). In this process, 0.75 g of extracted alginate and 0.25 g of acrylamide were suspended in a 10 mL of purified water, following a condition of continuous stirring. Then, N,N`-methylenebis(acrylamide) (0.648 mM) cross-linker and ammonium

persulfate (2.191 mM) initiator were sequentially added to the earlier

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suspension, at 21 ± 2 oC. Furthermore, it was placed in the oven at 45 oC for 30 mins in

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order to produce the hydrogel. The hydrogel developed was soaked with purified water and

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dipped in purified water in order to eliminate the unreacted elements towards the end of

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480 mints. To this end, the hydrogel (SA0.75-AM0.25) obtained was removed and dried at

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ambient temperature. Similarly, SA0.50-AM0.50, SA0.25-AM0.75 and SA0.00-AM1.00 were subsequently developed by using the equivalent method. The feed compositions of the

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alginate hydrogels are defined in Table 1.

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Table 1. Raw materials composition in the alginate hydrogel and the related swelling data Alginate comprised hydrogels code SA0.00-AM1.00

SA (g)

APS (mM)

Swelling ratio, 𝑺𝒈

0.648

2.191

8.89

6

0.648

2.191

26.68

0.50

6

0.648

2.191

86.91

0.25

6

0.648

2.191

109.3

AM (mM)

H2 O (mL)

MBA (mM)

0.00

1.00

6

SA0.25-AM0.75

0.25

0.75

SA0.50-AM0.50

0.50

SA0.75-AM0.25

0.75

Swelling kinetics of alginate hydrogels

𝒈

Journal Pre-proof 7 Swelling exponen t (n)

Diffusion coefficient (D) cm2 s-1

Initial swelling rate (ri) (g water per g hydrogel) per min

SA0.00-AM1.00

0.7078

0.2646

2.1060

Theoretical equilibrium swelling (TSeq) (g water per g hydrogel) 0.4748

SA0.25-AM0.75

0.7101

0.7912

0.9197

1.0872

0.0035

SA0.50-AM0.50

0.9252

2.4239

0.1487

6.7208

0.0002

SA0.75-AM0.25

1.1266

3.5116

0.1916

5.2186

0.0002

The swelling rate constant (ks) ((g hydrogel per g water) per min)

0.0296

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Alginate comprised hydrogels code

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Characterizations

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The functional groups of the alginate and hydrogels were recorded by using the Attenuated

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Total Reflection-Fourier Transforms Infrared (ATR-FTIR, Perkin Elmer, UATR two). The morphologies (pores network) of the alginate hydrogels were characterized by using an

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Olympus BX43 microscope. Thermal properties of the alginate hydrogels were resolved on a TGA (thermogravimetric analyser), by using the TGA Q50 TA instrument-water LLC,

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Newcastle, DE, USA, at a heating rate of 10 oC/min and passing N2 gas at a flow rate of

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100 mL/min in the temperature range of between 25-600 oC. Swelling behaviour of alginate hydrogels Swelling behaviour of synthesized alginate hydrogels were characterized via the gravimetric method. Briefly, the dry weighted hydrogels (W0) were dipped in 25 mL of purified water at 21±2 oC. At particular time intervals, the swollen hydrogel was removed and an excess of surfaces water was wiped with tissue paper. Subsequently, the hydrogels were immediately weighed (W) and the swelling capacity (Sg/g) was calculated, thus:

Journal Pre-proof 8 𝑆𝑔/𝑔 =

𝑊−𝑊0

(1)

𝑊0

Swelling kinetics in distilled water Water adsorption kinetics of the alginate hydrogels were examined according to earlier studies [3,26]. In order to investigate the water uptake mechanism of the hydrophilic polymeric materials, various water adsorption kinetic models were employed to examine

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the empirical data. Amongst these models, a second order kinetic study, is simple and it

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gives a considerable amount of information; the equation of the second order is written as

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follows:

(2)

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𝑑𝑆/𝑑𝑡 = 𝑘𝑠 (𝑆𝑒𝑞 − 𝑆)2

In equation 2, ks indicates the swelling rate constant, Seq indicates the equilibrium swelling

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and S indicates the swelling at an unspecified time. The upper equation over the limits S=0,

𝑡 𝑆

= 𝐴 + 𝐵𝑡

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at time t=0 and S=Seq at equilibrium time t=t, provides the resulting equation. (3)

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In equation 3, B(1/Seq) and A(1/ks, S2eq), indicate the equilibrium swelling and the initial

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swelling rate of the alginate hydrogels, respectively. In order to follow the upper swelling kinetic pattern for the alginate hydrogels, the graph of t/S versus T was plotted. From the graphs, the theoretical equilibrium (Seq), initial rate swelling (ri), the swelling rate constant (ks) were determined from the slopes and the intercepts of the straight lines obtained from the plots of t/S versus T graph. The swelling kinetics of alginate hydrogels were examined via a gravimetric method (explained in swelling behaviour of alginate hydrogels section). This data was used to study the hydrogels water diffusion transport characteristics. In addition, only 60% of the swelling results obtained were employed for the swelling curves.

Journal Pre-proof 9 𝑆𝑤𝑒𝑙𝑙𝑖𝑛𝑔 𝑟𝑎𝑡𝑖𝑜 (𝑆) = (𝑊𝑆 − 𝑊𝑑 )/𝑊𝑑 = 𝑘𝑡 𝑛

(4)

In equation 4, S indicates the swelling ratio at time t, Ws refers to the weight of the swollen alginate hydrogel at time t and Wd is the original weight of the alginate hydrogel at time t (t=0). By extension, k is a constant of the sample and n is the swelling exponent, respectively. Herein, `n’ principally reveals the water transport mechanism of the hydrogels. For example, if the `n’ value corresponds to 0.5, the hydrogel displays the

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Fickian water transport characteristic, which is a controlled diffusion. If `n’ value is within

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0.5 1.0, it symbolises an anomalous diffusion or Fickian diffusion water transports. If

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the hydrogel has a value `n’=1, it connotates a Case II diffusion and when n>1, it exhibits a super Case II diffusion. In order to determine the value of `n’ by employing equation 4 up

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to ~60% swelling ratio data and ℓnS as a function of ℓnt diagrams were plotted to obtain

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straight lines. The hydrogel n data were determined from the slope of the straight lines of

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ℓnS vs. ℓnt graphs.

The diffusion coefficient (D) value of the hydrogels was obtained from the graphs of the

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swelling ratio (Sg/g) and T1/2. From the slopes of the graphs, the D value was obtained. This

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graph was drawn by applying a short time approximate process. The D values of the hydrogels developed were calculated by using the subsequent equalisation [3,26]: 𝑆 = 4⌈𝐷/𝜋𝑟 2 ⌉1/2 𝑡1/2

(5)

In equation 5, D is the diffusion coefficient, r is the radius of the sample, S is the swelling ratio and t is the time. All the kinetic values data are shown exhibited in Table II. Pb(II) solution preparation and measurement Aqueous lead solutions were prepared by employing lead, Pb(II) nitrate standard solution (Merck Co., Germany). All measurements were performed by using an atomic absorption

Journal 10 Pre-proof spectrometer (AAS) (PinAAcle 900F, Perkin Elmer), equipped with an electrode discharge lamp for lead analysis, operated at a wavelength of 283.3 nm, by using an air-acetylene flame. The adsorbed amount of Pb(II) on the hydrogels, q (mg/g), was estimated by utilizing the subsequent equation. (𝐶0 −𝐶) 𝑉

(6)

𝑋

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q=

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where C0 (mmol/L) and C (mmol/L) represent the initial and residual concentrations of

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Pb(II), respectively, V (L) is the volume of the solution and X (g) is the mass of the

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adsorbent.

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pH studies

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5 mL solutions of 500 ppm of lead, Pb(II) were prepared at pH 3, 4 and 5. They were in contact with 20 mg each of alginate hydrogel (SA0.00-AM1.00, SA0.25-AM0.75, SA0.50-AM0.50,

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SA0.75-AM0.25) at 25°C and at 140 rpm. Pb(II) residual concentration was measured after 24 h of contact time and this was done by using atomic absorption spectroscopy. Higher pHs were not evaluated to avoid metal precipitation. The experiment was carried out in duplicates. Kinetic studies 25 mL of a solution of 500 ppm, prepared at pH 5 was contacted with 100 mg of the selected hydrogel (SA0.75-AM0.25). 1 mL of the solution was withdrawn from the

Journal 11 Pre-proof experiment at the following time intervals: 10, 20, 40, 60, 180, 240, 360 and 1440 min and then analyzed by atomic absorption spectroscopy. The experiment was performed in duplicates. Results were fitted by using the pseudo-first-order and pseudo-second-order kinetic models to determine the adsorption mechanisms of Pb(II) on the hydrogels. The pseudo-first-order equation (Largergen) is written as: ln(𝑞𝑒 − 𝑞𝑡 ) = ln 𝑞𝑒 − 𝑘1 𝑡

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(7),

=𝑘

1 2 2 𝑞𝑒

1

+𝑞 𝑡 𝑒

(8)

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𝑡 𝑞𝑡

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while the pseudo-second-order equation (Ho) is written thus:

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where 𝑞𝑒 (mg●g-1) is the removal capacity of Pb(II) at equilibrium and 𝑞𝑡 (mg●g-1) is the

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removal capacity of Pb(II) at time, t (min). k1 (min-1) is the kinetic constant of the pseudofirst-order kinetic model, which is calculated by plotting ln(qe-qt) as a function of t. k2

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(g●mg-1●min-1) is the constant for the pseudo second-order kinetic model and its value is

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calculated by plotting t/qt versus t.

Results and discussions

Biopolymer alginate was developed from Laminaria digitata brown seaweed. The sodium alginate synthesized was investigated by FTIR analysis (Fig 1A). The resulting alginate absorption peaks were observed at: 3312.97 (O-H), 2923.82 (C-H), 1590.09 (-COO-), 1407.54 (COO-), 1291.50 (C-C-H), 1088.30 (C-O), 1024.50 (C-C), 950.47 (C-O) and 883.96 (C-H), 816 (Na-O) cm-1 [27–29]. According to literature, the strong 1590.09 and 1407.54 cm-1 bands confirm that the polymer obtained is water-soluble because the

Journal 12 Pre-proof seaweed carbonyl functional group shifted as carboxylate anion in the alginate [30]. The 1291.50 (C-C-H), 1088.30 (C-O), 1024.50 (C-C) cm-1 bands are related to the pyranose rings of alginate [31]. The 950.47 (C-O) 883.96 (C-H) and 816 cm-1 indicate the presence of uronic acid and mannuronic acid in the alginate [27]. In addition, the results of the FTIR measurements are in good agreement with previous reports [27,32]. However, FTIR spectroscopy has been employed to calculate the mannuronic acid/guluronic acid (M/G)

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ratio in the alginate [31]. Herein, M/G ratio value was calculated from the band intensity

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(1024.50/1088.30) of the alginate FTIR spectra. According to previous reports, M/G (ratio

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of alginate) < 1, which indicates the fact that alginate has a large amount of guluronic

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acid, that can provide a rigid structure to the biomaterials [31]. If M/G > 1, alginate has a

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low amount of guluronic acid and that can provide soft structure to the biomaterials [31]. Furthermore, the extracted alginate was used to prepare the alginate hydrogels. Pure

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acrylamide hydrogel (Fig 1B) showed the main characteristic peaks at: 3348 (N-H), 3191 (NH stretching), 2925 (CH stretching), 2852 (CH stretching), 1657 (C=O stretching),

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1605 (NH bending) and 1500-1300 cm-1 (several CH bendings) were also observed [32].

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However, these characteristic peaks slightly shifted in the SA-AM hydrogels (Fig 1C). Mainly, it is observed that when the alginate content in the hydrogels was increased, some of the characteristic peaks of the hydrogels become broad, while others became sharp. Similarly, the transmittance of the hydrogels were changed slightly, due to the alginate colour. In addition, SA peaks were observed at 936 and 820 cm-1.

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Fig 1: FTIR spectra of: A) alginate, B) pure (SA0-AM1) hydrogel, C) alginate hydrogels, D) DSC curves and E) TGA curve of hydrogels

Fig 1D. is a DSC curve of SA0.00-AM1.00 and SA0.75-AM0.25 hydrogels under nitrogen atmosphere at the heating rate of 10 oC/min. The DSC curve of SA0.00-AM1.00 hydrogels exhibited an endothermic peak at 65 oC which could be due to the elimination of entrapped water molecules in the hydrogel network. The thermogram shows two exothermic peaks at 165 oC and 225 oC can be related to the melting temperature (Tm) and thermal

Journal 14 Pre-proof decomposition. On the other hand, the SA0.75-AM0.25 hydrogel also showed similar exothermic peaks with temperature differences at 153 oC and 252 oC, respectively. By comparison of these two DSC curves, the SA decreased the melting temperature and increased the decomposition temperature of the SA0.75-AM0.25 hydrogel. The DSC study revealed that the hydrogel network was composed with alginate and acrylamide. Moreover, the SA0.75-AM0.25 hydrogel DSC thermogram also showed an endothermic peak at around

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63 oC due to elimination of water.

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The thermogravimetric analysis (TGA) profile of pure and alginate hydrogels are shown in

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Fig 1E. Pure acrylamide hydrogels showed the onset of thermal degradation at 219 oC due

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to the liberation of ammonia and other elements of acrylamide in the hydrogels. On the

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other hand, alginate hydrogels showed that the onset of weight loss occurred in the range of between 160-200 oC, due to degradation of hydrophilic functional (carboxyl groups) in the

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hydrogels and other degradation followed by the release CO2 and this is mainly due to the

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volatile components of hydrogels and fragmentation of alginate [33]. However, the final degradation of SA0.00-AM1.00, SA0.25-AM0.75, SA0.50-AM0.50, and SA0.75-AM0.25 hydrogels,

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were observed at: 550 oC with 0, 79.03, 73 and 69.04 %, respectively. The results explain the fact that increasing the alginate content in the hydrogels, the weight loss of hydrogels decreases due to the strong interaction between the acrylamide and alginate [34]. Consequently, it was observed that alginate can improve the thermal properties of the hydrogels and that alginate can improve the hydrophilic nature of the hydrogels. Optical microscopic images of pure and alginate hydrogels are shown in Fig 2. Fig 2A the pure hydrogels (SA0.00-AM1.00) showed the smooth surface structure. Whereas, the SA0.75AM0.25 hydrogel was showed a highly porous structure (Fig 2C). However, the porous

Journal 15 Pre-proof structure was depending on the concentration of the SA used in the synthesis of the hydrogels. For example, the SA0.50-AM0.50 hydrogel was showed smooth and small porous structure as shown in Fig 2B. This behaviour may be observed due to the equal concertation of the SA and AM. In general, the anionic alginate can easily create a poreforming capacity [35–37]. These porous structures can increase the adsorption ability of the

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hydrogels developed and its applicability in several fields [38–41].

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Fig 2: Optical microscopic images of: A) SA0.00-AM1.00, B) SA0.50-AM0.50 and C) SA0.75AM0.25 hydrogels

Hydrogels swelling capacity plays an important role in medical, biomedical and agricultural

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applications [17]. This is in addition to using them for toxic metals removal from polluted

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water [42,43]. Fig 3A shows the swelling kinetics of the hydrogels developed, i.e. (SA0.00AM1.00, SA0.25-AM0.75, SA0.50-AM0.50, and SA0.75-AM0.25) as plots of Sg/g versus time (minutes). Fig 3A explains the fact that by increasing the alginate content in the hydrogel composition, the swelling capacity of alginate hydrogels was increased. This phenomenon occurs due to the hydrophilic functional groups (COO-, Na+) of alginate, which can increase the electrostatic repulsive force in the hydrogel network, thereby, improving the swelling capacity of the alginate hydrogels. A similar phenomenon was observed in earlier studies [23,44]. In this study, in order to increase the swelling capacity of the hydrogels, the

Journal 16 Pre-proof alginate content was increased (0, 25, 50, 75) in the hydrogel composition (SA0.00-AM1.00, SA0.25-AM0.75,

SA0.50-AM0.50

and

SA0.75-AM0.25),

or

decreasing

the

acrylamide

concentration, thus: (100, 75, 50, 25). When compared with our earlier studies, in this investigation, we achieved higher swelling capacity (109.3 g/g) of alginate hydrogels,

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prepared by the free-radical polymerization method [23,44].

Fig 3: Influence of alginate and acrylamide contents on the swelling capacity of hydrogels: A) swelling capacity and time, B) time/swelling capacity (T/S) and time (T), C) ℓnS vs. ℓnT and D) Sg/g vs. T1/2 graphs of SA-AM hydrogels

Journal 17 Pre-proof Furthermore, swelling (initial rate of swelling (ri), the swelling rate constant (ks) and the theoretical equilibrium swelling (TSeq)) and diffusion kinetics (swelling exponent (n), diffusion coefficient (D)) values of the hydrogels were calculated from the swelling capacity values. Primarily, ri, ks and TSeq values were obtained from Fig 3B. The results explain the fact that the swelling parameters are changed with the hydrophilic polymer (alginate) content in the hydrogel composition [45]. The n values (Table 1) of the

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hydrogels, were obtained from the Fig 3C. The results indicate the fact that SA0.00-AM1.00,

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SA0.25-AM0.75, SA0.50-AM0.50 hydrogels have n values between 0.7-1.0. Therefore, they

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showed a non-Fickian (anomalous) diffusion water transport phenomenon, which indicates

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that the relative rate of diffusion and SA-AM relaxation, control the fluid transport [46]. On the other hand, SA0.75-AM0.25 has 1.126 (n value), which indicates the fact that the hydrogel

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has a super Case II diffusion (relaxation-controlled) water transport, which means that the

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diffusion of the water is faster than the relaxation of the hydrogel network [46,47]. According to literature, hydrogels swelling transport can be controlled by using an anionic

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alginate polymer, which can increase the n value of such hydrogels [48]. Similarly, the

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values of D of the pure and alginate hydrogels, varied between 0.26469-3.51161 cm2s-1 (Fig 3D). From the results, it is observed that alginate-based hydrogels have higher D values than pure hydrogels, which means that they absorbed a significant amount of water. Among them, SA0.75-AM0.25 hydrogels (3.51161 cm2s-1) have higher water absorption capacity than other hydrogels. Similar results were reported in previous studies [47]. Furthermore, Pb(II) adsorption capacity onto alginate hydrogels was studied by using different pH solutions. The highest adsorption capacity was obtained by using the SA0.75AM0.25 hydrogel, with values of: 101.8, 109.2, 103.6 mg●g-1, for pH 3, 4 and 5,

Journal 18 Pre-proof respectively and no significant difference was observed on the adsorption capacity at different pH by using this hydrogel (Fig 4A). However, the enhancement and detraction of adsorption capacity depend mainly on the: osmotic pressure inside the hydrogel, hydrophilic groups of the hydrogel, electrostatics repulsion (-COOH) functional groups of alginate (which can ionize and give –COO– ions and leads the swelling capacity) hydrogels [49–51]. Carboxyl groups present in the alginate molecule also act as active sites for Pb(II)

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adsorption [52]. Therefore, an increment in alginate composition, leads to a rise in metal

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ions adsorption. A decrease in the adsorption capacity was observed when the alginate

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content of the hydrogels also decreased; SA0.50-AM0.50 hydrogels had adsorption capacities of: 42.0, 51.0 and 68.0 mg●g-1 at pH 3, 4 and 5, respectively. SA0.25-AM0.75 hydrogel had

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adsorption capacities of: 25.8, 31.8 and 26.8 mg●g-1 at pH 3, 4 and 5, respectively.

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Similarly, SA0.00-AM1.00 hydrogel had: 1.4, 4.0 and 3.9 mg●g-1 of adsorption capacities at

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pH 3, 4 and 5, respectively. Overall, it can be inferred that SA0.75-AM0.25 has a higher adsorption capacity than other hydrogels. Therefore, the SA0.75-AM0.25 hydrogels were

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selected for the removal of Pb ions from polluted water.

Journal 19 Pre-proof Fig 4: A) Adsorption capacity of Pb (II) on SA0.00-AM0.00, SA0.25-AM0.75, SA0.50-AM0.50, and SA0.75-AM0.25 hydrogels using solutions prepared at 500 ppm, 25 °C at different pHs 3, 4 and 5. Experiments were carried out in duplicates. B) Adsorption kinetics of Pb(II) on SA0.75-AM0.25 hydrogel at 25 °C by using a solution with an initial concentration of 500 ppm at pH5, adsorption capacity at time, t were obtained at: 10, 20, 40, 60, 180, 240, 360 and 1440 min. The dashed line corresponds to the fitting to the pseudo second order adsorption kinetic model. Error bars correspond to the standard deviation of duplicates.

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Table 2: Pseudo-first and pseudo-second order kinetic parameters obtained for Pb(II) adsorption on SA0.75-AM0.25 hydrogel, by using a solution of 500 ppm at pH 5 and 25 °C Pseudo-first-order kinetic model -1

-1

Pseudo-second order kinetic model 2

qe (mg/g-1)

k1 (min )

R

87

0.0026

0.9444

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qe (mg●g )

0,0001

0.9965

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110

k2 (g●mg-1 R2 min-1)

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The adsorption of Pb(II) occurred rapidly and at 60 min, 50 % of Pb(II) was absorbed on

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the SA0.75-AM0.25 hydrogel and at 180 min, 75 % of Pb(II) was adsorbed, reaching maximum at 1440 min with an adsorption capacity of 103 mg●g-1 (Fig 4B) and at an

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efficiency of 96%. The experimental data were fitted by using pseudo-first and pseudo-

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second-order kinetic models; the best correlation coefficient was obtained by using the pseudo-second order kinetic model (0.9965). In addition, the calculated qe was the closest to the experimental data when using this model, i.e. 110 when compared with 103 mg●g-1 (Table 2). This reveals that the rate-limiting step of the adsorption process is related to a chemisorption process rather than physical sorption, involving the exchange of electrons between the sorbent and sorbate [53,54]. This phenomenon has been observed in other alginate-based composite materials [55]. Overall, it can be inferred that alginate increases the hydrogel swelling and hence, the adsorption properties of the resulting composites.

Journal 20 Pre-proof The removal of Pb(II) ions adsorption capacity was compared to with previous studies and presented in Table 3. From these studies the alginate hydrogels are promising materiasls fo the removal of Pb(II) ions. The maximum capacity of lead removal will be determined in future studies. Table 3. Comparison of alginate-based materials for Pb(II) ions adsorption capacity with other studies in the literature.

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Pb(II) ions adsorption (mg/g)

Refs

60.27

Calcium alginate- silica nano powders

83.33

Sodium alginate-Nano chitosanmicrocrystalline cellulose beads

114.47

[58]

Chitosan coated calcium alginate

106.9

[59]

Calcium Alginate-graphene oxide Composite Aerogel

368.2

[60]

140.84

[61]

325

[62]

263.2

[63]

244.6

[64]

110

In this study

Alginate-halloysite beads

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Biochar-alginate capsule

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Alginate-MCM-41

Alginate- Montmorillonite

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Alginate-chitosan beads

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Alginate-based adsorbents

Laminaria digitata brown seaweedbased alginate hydrogels

[56] [57]

Conclusion In this investigation, high alginate contained porous hydrogel was successfully synthesized via the free-radical free-radical polymerization method. The required water soluble alginate biopolymer was extracted from Laminaria digitata Brown seaweed (LDBS) by alkaline extraction method. Extracted water soluble alginate functional groups

Journal 21 Pre-proof and M/G ratio were calculated by FTIR analysis. The M/G ratio of the extracted alginate value is M/G > 1, which can provide the soft nature to the hydrogels. Optical microscope image results explain that by increasing the alginate content in the hydrogels, the porous nature of the hydrogels increased. The swelling studies showed that the hydrogel swelling is transport-controlled by water soluble anionic alginate polymer. The resulting hydrogel, containing 75% of alginate and 25% of acrylamide, showed the highest swelling capacity

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and exhibited a super Case II diffusion (relaxation-controlled) water transport

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phenomenon. In addition, it showed a better adsorption capacity of Pb(II) from the media.

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These results suggest that the alginate hydrogels are promising materials for the removal

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of the studied heavy metal, i.e. Pb(II) in water purification applications. We believe that high water uptake capacity with an adsorption capacity of toxic heavy metal can be

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achieved by alteration of hydrogel composition in the future.

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Acknowledgements

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The Kokkarachedu Varaprasad wishes to acknowledge Centro de Investigaciòn de

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Polìmeros Avanzados (CIPA). Conicyt Regional Gore Biò-biò, R17A10003, Fondecyt 11160073 and Programa de Cooperación Internacional/REDES180165. References [1]

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Journal 28 Pre-proof Ms. Ref. No.: MOLLIQ_2019_4873 Title: Development of high alginate comprised hydrogels for removal of Pb(II) ions

Disclosure statement No potential conflict of interest was reported by the authors

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Looking forward to hearing from you.

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Thanking you Sincerely yours

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Dr. Kokkarachedu Varaprasad

Journal 29 Pre-proof

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Graphical abstract

Journal 30 Pre-proof

Research highlights Water soluble alginate extraction from Laminaria digitata



High alginate contained hydrogel was generated via a free-radical polymerization



SA0.75-AM0.25 hydrogel has the highest swelling capacity (109.3 g/g)



SA0.75-AM0.25 hydrogel has a diffusion coefficient (3.51161 cm2s-1)

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SA0.75-AM0.25 showed noble adsorption ability of the Pb(II) ions.

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