acid-activated montmorillonite composite hydrogel for the adsorption and removal of Pb+2 and Ni+2 ions accommodated in aqueous solutions

Accepted Manuscript Title: Chitosan-based hydrogel and chitosan/acid-activated montmorillonite composite hydrogel for the adsorption and removal of Pb+2 and Ni+2 ions accommodated in aqueous solutions Authors: Ricardo M. Vieira, Pˆamela B. Vilela, Valter A. Becegato, Alexandre T. Paulino PII: DOI: Reference:

S2213-3437(18)30194-5 https://doi.org/10.1016/j.jece.2018.04.018 JECE 2314

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

10-2-2018 31-3-2018 7-4-2018

Please cite this article as: Ricardo M.Vieira, Pˆamela B.Vilela, Valter A.Becegato, Alexandre T.Paulino, Chitosan-based hydrogel and chitosan/acid-activated montmorillonite composite hydrogel for the adsorption and removal of Pb+2 and Ni+2 ions accommodated in aqueous solutions, Journal of Environmental Chemical Engineering https://doi.org/10.1016/j.jece.2018.04.018 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.

Chitosan-based hydrogel and chitosan/acid-activated montmorillonite composite hydrogel for the adsorption and removal of Pb+2 and Ni+2 ions accommodated in aqueous solutions

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Santa Catarina State University, Graduate Program in Environmental Sciences, Av. Luiz de

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Camões, 2090, Conta Dinheiro, CEP: 88520-000, Lages – SC, Brazil 2

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Ricardo M. Vieira1, Pâmela B. Vilela1, Valter A. Becegato1, Alexandre T. Paulino1,2*

Santa Catarina State University, Department of Food and Chemical Engineering, BR 282, Km

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574, CEP: 89870-000, Pinhalzinho – SC, Brazil

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*Corresponding author: Phone: +55 (49) 2049-9589; Fax: +55 (49) 2049-9593

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E-mail: [email protected]

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Graphical abstract

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Highlights Chitosan-based hydrogel and chitosan/montmorillonite composite hydrogels were synthesized

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Adsorption studies of Pb2+ and Ni2+ ions in the hydrogels were efficiently performed

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An efficient removal of Pb2+ and Ni2+ ions from aqueous solutions was achieved

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Chitosan/acid-activated montmorillonite composite hydrogel was applied without precipitating the metals as hydroxides The hydrogels are efficient adsorbents for the purification of aqueous solutions

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Abstract

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A chitosan-based hydrogel and chitosan/acid-activated montmorillonite composite hydrogel

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were synthesized by copolymerization of radical chitosan, acrylic acid and N,N’-

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methylenebisacrylamide and tested with regard to the adsorption and removal of Pb2+ and Ni2+

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ions accommodated in aqueous solutions. Fourier-transform infrared spectra confirmed the hydrogel synthesis. The swelling degrees of the chitosan-based hydrogel were 40.86, 20.97 and

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4.16 g water per g dried hydrogel at pH 8.0, 6.5 and 4.5, respectively, after 1440 min of contact, whereas the swelling degrees for the chitosan/acid-activated montmorillonite composite hydrogel

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were 80.01, 25.97 and 14.76 g water per g dried composite hydrogel at pH 8.0, 6.5 and 4.5,

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respectively, after 5760 min of contact. The water absorption mechanism was strongly influenced by the pH of the aqueous solution, varying between Fickian and non-Fickian transports. The Pb2+ and Ni2+ adsorption capacities of the chitosan-based hydrogel ranged from

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41.06 to 30.20 and 42.38 to 36.45 mg metal per g dried hydrogel in the pH range of 5.5 to 3.5, respectively. The adsorption capacities of the chitosan/acid-activated montmorillonite composite hydrogel ranged from 35.22 to 26.11 and 37.16 to 42.04 mg metal per g dried composite hydrogel in the pH range of 5.5 to 3.5, respectively. The Pb2+ and Ni2+ adsorption mechanism was evaluated using linear and non-linear Langmuir, Freundlich, Redlich-Peterson and Sips

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isotherms, with the best fit determined for the non-linear Redlich-Peterson isotherm with both hydrogels. The best kinetic fit to both hydrogels was observed using the non-linear pseudosecond order kinetic model and confirmed by Chi-square test statistics.

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Keywords: Heavy metal; Hydrogel; Montmorillonite; Adsorption; Aqueous Solution

1. Introduction

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The contamination of soil, water and air by heavy metals is a major concern of environmental regulation agencies around the world due to the highly toxic effects on the environment, animals and humans [1]. Lead bioaccumulation, for example, causes damage to the

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central nervous system, kidneys, liver, reproductive system, simple cellular processes and brain

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of animals and humans and its bioaccumulation in soil, water and air increases environmental

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pollution indices [2]. Nickel is carcinogenic to humans and toxic to the environment due to its

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capacity for bioaccumulation and persistence in living tissues [3]. As many heavy metals have high economic value in certain industrial processes, it is necessary to study methods for their

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effective removal from water, wastewater and air in order to aggregate value to industrial feedstock [4].

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Several methods can be employed for the removal of heavy metals from aqueous solutions,

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including chemical precipitation [5-7], physiochemical adsorption [8-9], filtration membranes [10], ionic exchange [11], photocatalysis [12], electrochemical processes [13] and so forth [14]. However, some of these methods are economically and industrially infeasible due to the high

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costs involved. In contrast, natural polysaccharide-based hydrogels are eco-friendly and capable of removing large amounts of metals from aqueous solutions at low cost and with a high performance due to their considerable swelling capacity and physiochemical properties [15]. Adsorbents commonly used in physiochemical adsorption processes include mineral, organic and

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biological residues, industrial byproducts, agricultural residues, biomass, polymer materials, hydrogels and composites [15-17]. Natural polysaccharide-based hydrogels and composite hydrogels are commonly formed by hydrophilic or hydrophobic three-dimensional polymer networks that can absorb large amounts of water, dyes, proteins, agricultural nutrients and heavy metals [14, 18-20]. Composite

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hydrogels, such as those containing clays, are efficient adsorbents due to the bioavailability of active functional groups [21-22], which increases their adsorption capacity [23]. Mineral clays

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can alter the amount of metals adsorbed to composite hydrogels due to ionic exchange capacities, surface areas and chemical/mechanical stability [16].

The aim of this work was to synthesize a chitosan-based hydrogel and chitosan/acid-

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activated montmorillonite composite hydrogel for the adsorption and removal of Pb2+ and Ni2+

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ions accommodated in aqueous solutions. The water absorption capacity and mechanism were

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studied through swelling assays at different pH values. Pb2+ and Ni2+ ions adsorption capacities

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were studied with different contact times and pH values as well as initial concentrations of

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metallic solutions and hydrogel masses. Metal adsorption mechanisms were evaluated using linear and non-linear Langmuir, Freundlich, Redlich-Peterson and Sips isotherm models, and

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non-linear pseudo-first and pseudo-second order kinetic models.

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2. Materials and Methods 2.1. Reagents

Chitosan (CS - deacetylation degree: 92.0 wt-%; 1.0 x 106 Dalton molar mass), ammonium

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persulfate (APS), N,N’-methylenebisacrylamide (MBA) and montmorillonite K10 (MMT surface area: 250.0 m2 g-1) were purchased from Aldrich. Acetic acid (Ac), acrylic acid (AAc), sulfuric acid (SAc), lead nitrate (Pb(NO3)2) and nickel nitrate (Ni(NO3)2) were purchased from Merck. Sodium hydroxide (NaOH) was purchased from Dinamica. All reagents were used as acquired and aqueous solutions were prepared in ultrapure water (Megapurity MEGA-UP). 4

2.2. Acidic activation of montmorillonite Acid-activated montmorillonite was prepared by dissolving 20.0 g of pure montmorillonite K10 in a glass flask for refluxing containing 200.0 mL of a 0.25 mol L-1 sulfuric acid solution. This refluxing system was kept under an acidic reaction at 100.0 ± 1.0 °C for 3 h. The resulting montmorillonite was centrifuged and washed several times with ultrapure water to remove free

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SO42- ions and dried in an air circulation oven (Ethik technology 400/5TS) at 110.0 ± 1.0 °C until achieving a constant mass. The acid-activated montmorillonite sample was calcined in a muffle

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furnace at 500.0 ± 1.0 °C for 10 h prior to being applied in the synthesis of the composite hydrogel.

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2.3. Hydrogel synthesis

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The chitosan-based hydrogel was synthesized by placing 30.0 mL of 1.0 wt-% chitosan

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solution in a three-neck closed glass flask and maintaining a nitrogen flux for 30 min to generate

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an inert atmosphere. Next, 0.5215 mmol of ammonium persulfate were added, keeping the gas nitrogen flux for an additional time of 10 min under constant magnetic stirring at 70.0 ± 1.0 °C

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to form radical chitosan. A deaerated solution containing 15.0 mL of ultrapure water, 3.5 mL of acrylic acid and 0.150 g of N,N’-methylenebisacrylamide was added under constant magnetic

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stirring and kept at 70.0 ± 1.0 °C for 4 h for the completion of the copolymerization and the

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formation of the chitosan-based hydrogel. A deaerated solution containing 15.0 mL of ultrapure water, 3.4 mL of acrylic acid, 0.150 g of N,N’-methylenebisacrylamide and 50.0 mg of either pure

or

acid-activated

montmorillonite

was

used

for

the

synthesis

of

the

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chitosan/montmorillonite composite hydrogels. The hydrogels were transferred to 200.0-mL glass beakers containing specific volumes of 2.0 mol L-1 sodium hydroxide solution and left for 15 min to neutralize the hydrophilic three-dimensional hydrogel networks. The hydrogels were then washed several times with ultrapure water and dried in an air circulation oven at 60.0 ± 1.0

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°C for 72 h prior to use in the swelling assays and application for the adsorption and removal of Pb2+ and Ni2+ ions accommodated in aqueous solutions. 2.4. Swelling assays and water absorption mechanism Approximately 100.0 mg of dried hydrogel were immersed in 100.0 mL of either ultrapure

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water or buffer solutions at pH 4.0, 6.5 and 8.0 for different contact times. The swelling degree (SD) was calculated from Equation 1: ms − md md

(1)

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SD =

in which ms and md are the swollen and dried hydrogel masses, respectively.

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The water diffusion mechanism was studied using the modified Fick Equation for

(2)

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wt = kt n weq

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hydrophilic three-dimensional hydrogel networks as follows:

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in which wt and weq are the water masses in the hydrogel network at a specific time and

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equilibrium, respectively, k is the water diffusion constant, t is the swelling time and n is the parameter describing the water diffusion mechanism.

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2.5. Fourier-transform infrared spectroscopy

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Fourier-transform infrared spectroscopy was conducted with the Bomem Easy MB-100 Nichelson spectrometer, using 1.0 wt-% KBr pellets and 20 runs per minute at resolution of 4.0 cm-1.

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2.6. Pb2+ and Ni2+ adsorption assays For the Pb2+ and Ni2+ adsorption assays, 1000 mg dm-3 metal stock solutions at pH 3.5, 4.5

and 5.5 were prepared. Known masses of hydrogels were immersed in open glass beakers containing 100.0 or 50.0 mL of either Pb2+ or Ni2+ ion solution at specific initial concentrations. The Pb2+ and Ni2+ ions concentrations during adsorption and removal assays from aqueous 6

solutions were determined by flame atomic absorption spectrometry (FAAS - Analytik Jena AG, Jena, Germany, contrAA 700, air-acetylene flame, wavelengths for Pb2+ and Ni2+ of 362.5 and 261.4 nm, respectively). The Pb2+ and Ni2+ adsorption capacities (qe) in the hydrogel networks were calculated using Equation 3: Ci − Ce .V m

(3)

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qe =

in which Ci and Ce are the initial and equilibrium concentrations of the metal in the aqueous

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solution, respectively, m is the dried hydrogel mass and V is the metallic solution volume. 2.7. Adsorption isotherms

The Langmuir isotherm model describes adsorption to a monolayer for a defined number of

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active sites on the adsorbent surface with absence of physiochemical interactions between

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adsorbed neighbor ions/molecules [24]. Each active site contains one adsorbed ion/molecule.

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The non-linear form for this isotherm is represented by Equation 4: q max K L Ce 1 + K L Ce

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qe =

(4)

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in which qmax is the maximum adsorption capacity, KL is the Langmuir adsorption rate and Ce is

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the equilibrium ion/molecule concentration. The Freundlich isotherm model describes multilayer adsorption phenomena with the

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occurrence of physiochemical interactions between adsorbed neighbor ions/molecules at different active sites. In this case, a single active site can support more than one chemical species

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[25]. The non-linear Freundlich isotherm model is represented by Equation 5: qe =

1 n K F Ce

(5)

in which KF is the Freundlich constant related to adsorption capacity and 1/nF is the Freundlich constant related to the surface heterogeneity of the specific adsorbate.

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Favorable Freundlich adsorption processes generate nF values ranging from 1 to 10, with a higher nF indicative of stronger physiochemical interactions between the adsorbent and adsorbate. The adsorption energy is linear and similar at all adsorption sites when nF is 1. For nF lower than 1, significant molecular attractions are found between the adsorbent and solvent,

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which decreases the adsorption capacity [25]. The Redlich-Peterson isotherm model describes three parameters that are derivatives of the

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Langmuir and Freundlich isotherm models. This isotherm is commonly applied when adsorption equilibriums are studied in large concentration intervals in homogeneous and heterogeneous systems [26]. In this case, monolayer and multi-site adsorptions occur concomitantly [27]. The

K RP Ce

(6)



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1 + RP Ce

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qe =

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non-linear Redlich-Peterson isotherm model is represented by Equation 6:

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in which KRP, αRP and β are Redlich-Peterson constants determined by a trial-and-error optimization and solver add-in using an ExcelTM (Microsoft) spreadsheet.

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The Redlich-Peterson isotherm model has specific properties of the Langmuir and

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Freundlich isotherm models. This model tends toward the Langmuir isotherm for low adsorbate

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concentrations with β  1. Otherwise, β  zero for high adsorbate concentrations [28]. The Sips isotherm model describes a combination of the Langmuir and Freundlich isotherm

models. This model tends toward the Freundlich isotherm for low adsorbate concentrations and

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Langmuir isotherm for high adsorbate concentrations [28]. The Sips isotherm model is represented by Equation 7: 1

𝑞𝑒 = 𝑞𝑚𝑎𝑥

(𝐾𝑠 𝐶𝑒 )𝑛𝑠 1 + (𝐾𝑠 𝐶𝑒

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1 )𝑛𝑠

(7)

in which Ks is the Sips isotherm constant related to energy of adsorption and ns is a parameter describing the homogeneity/heterogeneity of an adsorption process. For 0 < ns < 1, the adsorption phenomenon is heterogeneous with possible multi-site adsorption. For ns = 1, the adsorption phenomenon is homogeneous and the Sips isotherm model tends toward the Langmuir model. Finally, for n > 1, it takes place multi-layer formation of

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adsorbate on the adsorbent surface [29].

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2.8. Adsorption kinetics

Pb2+ and Ni2+ adsorption kinetics in the hydrogels at different pH values were studied using pseudo-first [30] and pseudo-second order kinetic models [31]. The non-linear pseudo-first order

(8)

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q t = q e (1 − e−kt )

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kinetic model describes one-site adsorption phenomena [32] and it is represented by Equation 8:

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in which k is the adsorption rate, t is the adsorption time and qt is the adsorption capacity at

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adsorption time t.

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In the pseudo-second order kinetic model, the adsorption rate-limiting phenomenon is chemisorption due to physiochemical interactions between the adsorbate and adsorbent [33]. The

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non-linear pseudo-second order kinetic model is represented by Equations 9: qt =

kq2e t 1 + kq e t

(9)

2.9. Error analysis

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An error analysis based on Chi-square test statistic (2) was applied to compare

experimental data and predictions from the non-linear pseudo-first and pseudo-second kinetic models. This error function is represented by Equation 10: (𝑞𝑒𝑥𝑝𝑒𝑟𝑖𝑚𝑒𝑛𝑡𝑎𝑙 − 𝑞𝑒,𝑡ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 ) 𝜒2 = ∑ 𝑞𝑒,𝑡ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 9

2

(10)

in which qexperimental is the experimental adsorption capacity and qe,theoretical is the theoretical adsorption capacity predicted from the models. The correlation coefficient (R2) based on this error analysis is represented by Equation 11: ∑(𝑞𝑒,𝑡ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 − ̅̅̅) 𝑞𝑒

2

2

∑(𝑞𝑒,𝑡ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 − ̅̅̅) 𝑞𝑒 + ∑(𝑞𝑒,𝑡ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 − 𝑞𝑒𝑥𝑝𝑒𝑟𝑖𝑚𝑒𝑛𝑡𝑎𝑙 )

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in which ̅̅̅ 𝑞𝑒 is the average of 𝑞𝑒𝑥𝑝𝑒𝑟𝑖𝑚𝑒𝑛𝑡𝑎𝑙 . 3.

2

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𝑅 =

2

Results and Discussion

3.1. Fourier-transform infrared spectroscopy

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Figure 1 displays the Fourier-transform infrared spectra for the pure and acid-activated

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montmorillonite (a), chitosan-based hydrogel, chitosan/pure montmorillonite composite hydrogel

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and chitosan/acid-activated montmorillonite composite hydrogel (b). The absorption bands at

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3600 and 3450 cm-1 are related to the axial deformation and stretching vibration of hydroxyl

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groups from pure or acid-activated montmorillonite molecules (OHclay) and absorbed water (OHwater), respectively. The intensities of these bands in clay molecular structures decrease with

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the absorption of water due to formation of interlamellar hydrogen bonds (OH---H) [34-35]. The band at 1640 cm-1 is also related to absorbed water in the montmorillonite chemical structure.

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The absorption band at 1050 cm-1 corresponds to the stretching of Si-O groups. Absorption bands at approximately 840 cm-1 are associated with OH-MgAl-OH groups and the bands at approximately 800 cm-1 are associated with OH-Fe2Fe3-OH groups. The absorption band at 550

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cm-1 corresponds to Si-O-Al groups containing Si bonded to tetrahedral sites and Al bonded to octahedral sites of clays. The bands at 840 and 800 cm-1 decreased after the acidic activation of montmorillonite due to the addition of protons and the increase in interlamellar hydrogen bonds in the molecular structure [36-37].

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As displayed in Figure 1b, the absorption bands at 3600 and 3450 cm-1 disappeared and a new band appeared at 3400 cm-1 after recording the spectra of the chitosan-based hydrogel and composite hydrogel due to stretching vibrations of hydroxyl groups from chitosan and acrylic acid molecules. Absorption bands ranging from 2920 to 2850 cm-1 correspond to vibrations of aliphatic compounds containing C-H groups [38]. At 1750 cm-1, stretching vibrations of carbonyl

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groups (C=O) from polyacrylic acid occurred. The absorption band at 1577 cm-1 is related to the stretching deformation of N-H groups from chitosan and N,N’-methylenebisacrylamide

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molecules and the absorption band at 1463 cm-1 was attributed to the stretching of C-O-C groups. The increase in the absorption bands at 1050 cm-1 and the appearance of new absorption bands at 840 cm-1 confirm the montmorillonite incorporation into the chitosan/pure montmorillonite and

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chitosan/acid-activated montmorillonite composite hydrogels. This occurrence was also

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confirmed by the disappearing of the carbonyl band in the chitosan/pure montmorillonite

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montmorillonite ionic chemical groups.

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composite hydrogel due to formation of interlamellar hydrogen bonds between the hydrogel and

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3.2. Hydrogel swelling degree and water absorption mechanism Figure 2 displays the swelling degrees (a), water diffusion rates (b) and linear regression

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curves (c) for the chitosan-based hydrogel at different pH values. Figure 3 displays the swelling

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degrees (a), water diffusion rates (b) and linear regression curves (c) for the chitosan/acidactivated montmorillonite composite hydrogel at different pH values. The swelling degrees at pH 8.0, 6.5 and 4.5 were 40.86, 20.97 and 4.16 g water per g dried chitosan-based hydrogel,

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respectively, after 1440 min of contact. The swelling degrees for the chitosan/acid-activated montmorillonite composite hydrogel were 80.01, 25.97 and 14.76 g water per g dried composite hydrogel, respectively, after 5760 min of contact. The swelling degrees and equilibrium time increased in the composite hydrogel due to the presence of montmorillonite in the polymer network [22]. Significant electrostatic repulsions are generated between deprotonated carboxyl 11

groups when water molecules from alkaline aqueous solutions enter the hydrophilic threedimensional hydrogel network. This also increases the swelling degree [15] and hydrogel size [20]. In contrast, a reduction in pH increases the proton ion concentration in the solution and the amounts of protonated carboxyl groups, decreasing the electrostatic repulsion forces between carboxyl groups and swelling degree. Hydrophilic carboxyl and amide/amine groups contained

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in hydrogel networks also favor the absorption of water due to formation of hydrogen bonds [39].

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Table 1 displays the water diffusion coefficients (n), water diffusion constants (k) and the determination coefficients (R2) for the swelling degree of the chitosan-based hydrogel and chitosan/acid-activated montmorillonite composite hydrogel. The water absorption mechanism in

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the chitosan-based hydrogel at pH 4.5 was Fickian, since the n value was lower than 4.5. This

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occurs due to protonation of the carboxylic groups in the hydrogel network, which decreases the

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electrostatic repulsion forces between anionic groups. In this case, water molecules diffuse

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throughout the hydrogel preferably by diffusion processes, with no significant influence from

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macromolecular relaxation [40]. The water diffusion mechanism in the chitosan/acid-activated montmorillonite composite hydrogel at pH 4.5 was non-Fickian, since the n value was 0.5731.

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One may conclude that the addition of montmorillonite to the hydrogel network increases the swelling degree and changes the water absorption mechanism. All other n values ranged from

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0.45 to 0.89, indicating non-Fickian water transports, in which water enters the hydrogel network through combination of diffusion processes and macromolecular relaxation. The water diffusion

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constant (k) was much higher for the swelling of the chitosan-based hydrogel at pH 4.5, confirming that the water diffusion mechanism was exclusively governed by diffusion processes, since this parameter is related to the diffusion capacity of molecules throughout a solution or porous material [41]. 3.3. Effect of contact time and pH on Pb2+ and Ni2+ adsorption capacity 12

Figure 4 displays the Pb2+ and Ni2+ adsorption capacities of the chitosan-based hydrogel (a and b) and chitosan/acid-activated montmorillonite composite hydrogel (c and d) as a function of time at different pH values. The experimental conditions were 100.0 mg L-1 initial metal concentration, 100.0 mg hydrogel mass and room temperature. The Pb2+ and Ni2+ adsorption capacities of the chitosan-based hydrogel ranged from 41.06 to 30.20 and 42.38 to 36.45 mg

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metal per g dried hydrogel, respectively, in the pH range of 5.5 to 3.5. The adsorption capacities of the chitosan/acid-activated montmorillonite composite hydrogel ranged from 35.22 to 26.11

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and 37.16 to 42.04 mg metal per g dried composite hydrogel, respectively, in the pH range of 5.5 to 3.5. The adsorption equilibrium of Pb2+ ions in both hydrogels was approximately 2880 min, whereas the adsorption equilibrium of Ni2+ ions was approximately 7200 min. As Pb2+ and Ni2+

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ions precipitate as hydroxides at pH higher than 6.0 and 8.0, respectively, all adsorption studies

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were carried out at pH lower than 6.0 [42]. The chitosan/pure montmorillonite composite

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hydrogel was not used for adsorption studies, since Pb2+ and Ni2+ ions precipitated as hydroxides

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even in acid pH solutions. This occurred due to the alkaline properties of the pure

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montmorillonite incorporated into the hydrogel network, which increased the solution pH after water absorption. As the carboxyl and amine groups of chitosan-based hydrogels are protonated

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at acid pH, lower adsorption capacities were determined in most of the adsorption studies when decreasing the pH. In acidic conditions, the active sites of hydrogels do not have sufficient

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amounts of free electrons for donation during the adsorption of metals [43]. In contrast, active sites and electrons are available for adsorption in alkaline pH solutions, leading to an increase in

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adsorption capacity. The Pb2+ and Ni2+ adsorption capacities of the composite hydrogels were lower than those of the chitosan-based hydrogels due to the increase in hydrogen bonds and crosslinking points after incorporating montmorillonite [23]. Adding clays to a hydrophilic threedimensional hydrogel network can also decrease electrostatic repulsion forces due to physiochemical interactions among anionic groups of the hydrogels and Al3+, Ca2+ and Mg2+

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ions from clay molecules. The Ni2+ adsorption capacities of the composite hydrogel increased slightly with the decrease in pH, as the electron affinity of this metal (-1.161 eV) for active sites of the hydrogel network is higher than the electron affinity of H+ (-0.757 eV), Ca2+ (+0.104 eV) and Mg2+ (+0.197 eV) ions by the same active sites. Lastly, slightly higher equilibrium adsorption capacities were determined for Ni2+ ions in comparison to Pb2+ ions, since the ionic

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radius of Ni2+ ion (0.69 Å) is lower than the ionic radius of Pb2+ ion (1.19 Å), which facilitates adsorption and diffusion processes throughout the polymer matrix. It is also related with the

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lower electron affinity of Pb2+ ion (-0.363 eV) in comparison to Ni2+ ion. Finally, the Pb2+ adsorption capacities were more affected in acidic media than the Ni2+ adsorption capacities, as

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the electron affinity of Pb2+ ions is lower than the electron affinity of H+ ions.

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3.4. Effect of initial metal concentration and hydrogel mass on Pb2+ and Ni2+ adsorption

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Figure 5 displays the Pb2+ and Ni2+ adsorption capacities of the chitosan-based hydrogel (a)

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and chitosan/acid-activated montmorillonite composite hydrogel (b) as a function of the initial metal concentration. The Pb2+ and Ni2+ adsorption capacities ranged from 40.46 to 177.86 and

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30.29 to 100.71 mg metal per g dried chitosan-based hydrogel, respectively, with the increase in the initial metal concentration from 100.0 to 400.0 mg dm-3. The Pb2+ and Ni2+ adsorption

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capacities of the chitosan/acid-activated montmorillonite composite hydrogel ranged from 35.11

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to 107.86 and 26.30 to 71.12 mg metal per g dried composite hydrogel, respectively, with the increase in the initial metal concentration from 100.0 to 400.0 mg dm-3. The adsorption capacities of the composite hydrogels tended toward equilibrium for initial concentrations higher

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than 400.0 mg dm-3 due to the faster saturation of active sites in the hydrogel network. However, the same was not found for chitosan-based hydrogels due to the absence of montmorillonite and larger amounts of available active sites. Figure 6 displays the Pb2+ and Ni2+ adsorption capacities of the chitosan-based hydrogel (a) and chitosan/acid-activated montmorillonite composite hydrogel (b) as a function of hydrogel 14

mass. The Pb2+ and Ni2+ adsorption capacities decreased from 277.94 to 32.51 and 87.66 to 20.06 mg metal per g dried chitosan-based hydrogel, respectively, with the increase in hydrogel mass from 50.0 to 400 mg. These adsorption capacities ranged from 248.52 to 33.16 and 98.62 to 14.15 mg metal per g dried composite hydrogel, respectively, with the increase in hydrogel mass from 50.0 to 400.0 mg. The variation in the adsorption capacity with the initial metal

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concentration and hydrogel mass may be explained by one-site/multi-site adsorption phenomena and adsorption kinetics [14]. A greater hydrogel mass leads to a lower metal adsorption capacity,

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since a polysaccharide-based hydrogel absorbs water first and heavy metals second [9, 15]. Thus, the metal concentration in the remaining aqueous solution increases with the fast absorption of water, which causes a decrease in adsorption capacity. Other physiochemical properties that

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affect the metal adsorption capacity of hydrogels include the metal ionic radius and

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electron/chemical affinity [9].

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3.5. Adsorption isotherms

Figure 7 displays the results of the non-linear Langmuir, Freundlich, Redlich-Peterson and

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Sips isotherms for the adsorption of Pb2+ and Ni2+ ions to the chitosan-based hydrogel (a and b) and chitosan/acid-activated montmorillonite composite hydrogel (b and c). The coefficients of

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determination obtained for the non-linear Langmuir, Freundlich, Redlich-Peterson and Sips

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isotherms were much higher than those obtained for the respective linear isotherms (data not shown), since the linearization of an isotherm affects normality assumptions, changing the curves to either better or worse [9]. In this case, linear curves were worse than non-linear curves

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and a significant difference was found between maximum adsorption capacities. The higher maximum Ni2+ ion adsorption capacity in comparison to the Pb2+ ion (Table 2) to the hydrogels can be related with the ionic radius and affinity of metal species besides is related with the porous structure of hydrophilic three-dimensional hydrogel networks. Overall, the best fits for the adsorption of Pb2+ and Ni2+ ions to both hydrogels were found when applying the non-linear 15

Redlich-Peterson isotherm, indicating that adsorption takes place by the formation of a monolayer concomitantly to multi-site-interaction phenomena, typical for adsorption into hydrogels [9]. 3.6. Adsorption kinetics

IP T

Tables 3 and 4, and Figures 8 and 9 display results of the non-linear pseudo-first and pseudo-second order kinetic models for the adsorption of Pb2+ and Ni2+ ions to the chitosanbased hydrogel (CBH) and chitosan/acid-activated montmorillonite composite hydrogel experimental)

Pb2+ and Ni2+

SC R

(CAAMCH). Most experimental (qe,

and theoretical (qe,

theoretical)

adsorption capacities of the hydrogels were similar at pH 3.5, 4.5 and 5.5 when employing the

U

non-linear pseudo-first order kinetic model. In contrast, theoretical (qe, experimental)

adsorption

adsorption capacities when

N

capacities were relatively higher than experimental (qe,

theoretical)

A

employing the non-linear pseudo-second order kinetic model. Moreover, higher adsorption

M

capacities were found for Ni2+ when compared to Pb2+, confirming that the physiochemical properties of metals, such as ionic radius and electron affinity, affect the adsorption capacity [9],

ED

as also already previously described in this work. The best kinetic fit for the adsorption of Pb2+ and Ni2+ ions to both hydrogels was found with the non-linear pseudo-second order kinetic

PT

model due to higher determination coefficients and lower Chi-square test statistics. In this case,

CC E

it was concluded that the limiting-phase of the adsorption process involves chemical bonds through electron sharing between the adsorbent and adsorbate [32].

A

Conclusion

Chitosan-based hydrogel and chitosan/acid-activated montmorillonite composite hydrogel

can be efficiently employed in adsorption studies without precipitating Pb2+ and Ni2+ ions in an aqueous solution. The water diffusion mechanism ranged from Fickian transport in the chitosanbased hydrogel at pH 4.5 to non-Fickian transport in the chitosan-based hydrogel and chitosan/acid-activated montmorillonite composite hydrogel at different pH values. The Pb2+ and 16

Ni2+ adsorption capacities of both hydrogels decreased with the decrease in the pH of the aqueous solutions. However, a slight increase was found in the adsorption of Ni2+ ions to the chitosan/acid-activated montmorillonite composite hydrogel due to differences in the ionic radii and electron affinities. The adsorption mechanism was evaluated using linear and non-linear Langmuir, Freundlich, Redlich-Peterson and Sips isotherms, with the best fit determined for the

IP T

non-linear Redlich-Peterson isotherm for both hydrogels, indicating the formation of a monolayer concomitantly to multi-site interaction phenomena, typical for adsorption into

SC R

hydrogels. The best kinetic fit for the adsorption of Pb2+ and Ni2+ ions to both hydrogels was observed by using the non-linear pseudo-second order kinetic model, indicating that the limitingphase of the adsorption process involves chemical bonds through electron sharing between the

N

U

adsorbent and adsorbate.

A

Acknowledgments

M

ATP and RMV thank the Brazilian fostering agency FAPESC for the master scholarship and research support. ATP and PBV thank the Brazilian fostering agency CAPES for the master

References

PT

financial support.

ED

scholarship and ATP thanks the Brazilian fostering agency CNPq (Grant No 312356/2015-3) for

CC E

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reduced-charge Na/Li-montmorillonites, Clays Clay Miner. 31 (1983) 9 – 16.

[38] N.E. Valderruten, J.D. Valverde, F. Zuluaga, E. Ruiz-Durántez, Synthesis and

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characterization of chitosan hydrogels cross-linked with dicarboxylic acids, React. Funct. Polym.

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[39] L.H. Sperling, Introduction to physical polymers science, third ed., John Wiley &Sons, Inc.

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[40] A.T. Paulino, A.G.B. Pereira, A.R. Fajardo, K. Erickson, M.J. Kipper, E.C. Muniz, L.A. Belfiore, E.B. Tambourgi, Natural polymer-based magnetic hydrogels: Potential vectors for

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remote-controlled drug release, Carbohydr. Polym. 90 (2012) 1216 – 1225.

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[41] P.L. Ritger, N.A. Peppas, A simple equation for description of solute release II. Fickian and anomalous release from swellable devices, J. Controlled Release. 5 (1987) 37 – 42.

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[42] A.T. Paulino, M.R. Guilherme, A.V. Reis, E.B. Tambourgi, J. Nozaki, E.C. Muniz, Capacity

of

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22

[43] R. Chang, Físico-Química para as ciências químicas e biológicas, third ed., McGraw-Hill, São Paulo, 2010. Figure Captions Figure 1 – Fourier-transform infrared spectra for the pure and acid-activated montmorillonite (a),

IP T

chitosan-based hydrogel and chitosan/acid-activated montmorillonite composite hydrogel (b). Figure 2 – Swelling degrees (a), water diffusion rates (b) and linear regression curves (c) for the

SC R

chitosan-based hydrogel at different pH values.

Figure 3 – Swelling degrees (a), water diffusion rates (b) and linear regression curves (c) for the

A

N

U

chitosan/acid-activated montmorillonite composite hydrogel at different pH values.

M

Figure 4 – Pb2+ and Ni2+ adsorption capacities of the chitosan-based hydrogel (a and b) and chitosan/acid-activated montmorillonite composite hydrogel (c and d) as a function of time at

ED

different pH values. Experimental conditions: 100.0 mg dm-3 initial metal concentration, 100.0

PT

mg hydrogel mass and room temperature. Figure 5 – Pb2+ and Ni2+ adsorption capacities of the chitosan-based hydrogel (a) and

CC E

chitosan/acid-activated montmorillonite composite hydrogel (b) as a function of the initial metal concentration. Experimental conditions: 2880 min contact time, 100.0 mg hydrogel mass, pH 5.5

A

and room temperature. Figure 6 – Pb2+ and Ni2+ adsorption capacities of the chitosan-based hydrogel (a) and chitosan/acid-activated montmorillonite composite hydrogel (b) as a function of hydrogel mass. Experimental conditions: 7200 min contact time, 300 mg dm-3 initial metal concentration, pH 5.5 and room temperature.

23

Figure 7 – Non-linear Langmuir, Freundlich, Redlich-Peterson and Sips isotherms for the adsorption of Pb2+ and Ni2+ ions to the chitosan-based hydrogel (a and b) and chitosan/acidactivated montmorillonite composite hydrogel (c and d). Figure 8 – Non-linear pseudo-first order kinetic model for the adsorption of Pb2+ and Ni2+ ions to

IP T

the chitosan-based hydrogel (a and b) and chitosan/acid-activated montmorillonite composite hydrogel (c and d) at different pH values.

SC R

Figure 9 – Non-linear pseudo-second order kinetic model for the adsorption of Pb2+ and Ni2+ ions to the chitosan-based hydrogel (a and b) and chitosan/acid-activated montmorillonite

A

CC E

PT

ED

M

A

N

U

composite hydrogel (c and d) at different pH values.

24

OH

Si-O-Al

C-H

C=O C-O-C N-H

Si-O

3000

2500

2000

1500

Wavenumber (cm-1)

1000

4000

500

3500

3000

2500

2000

1500

Wavenumber (cm-1)

A

CC E

PT

ED

M

A

N

U

SC R

Figure 1

25

1050

840

1750 1577 1463

Chitosan/acid-activated montmorillonite composite hydrogel

Si-O

1000

500

IP T

3500

2925

3400

Fe2Fe3OH

MgAlOH

b) Chitosan-based hydrogel Chitosan/pure montmorillonite composite hydrogel

Transmitance (%)

550

OHwater OHwater OHclay

4000

840 800 1050

3600

Pure montmorillonite

3450

Transmitance (%)

1640

a) Acid-activated montmorillonite

(a)

pH 4.5 pH 6.5 pH 8.0

40

(b) 1.0 0.8

70 % water absorbed

30

wt/weq

20

0.4 0.2

10

0

0.6

pH 4.5 pH 6.5 pH 8.0

0.0 0

500

1000

1500

2000

2500

3000 0

min

500

1500

min 0.50 0.25 0.00

(c) pH 4.5 pH 6.5 pH 8.0

70 % water absorbed

-0.50 -0.75 -1.00 -1.25

-1.75 0.5

1.0

1.5

2.0

2.5

N

log (t)

U

-1.50

-2.00 0.0

2000

2500

SC R

-0.25

log (wt/weq)

1000

A

CC E

PT

ED

M

A

Figure 2

26

3.0

3.5

3000

IP T

-1 Swelling degree(g g )

50

(a)

pH 4.5 pH 6.5 pH 8.0

80

(b)

1.0

0.8

70% water absorbed 0.6

60

wt/weq

-1 Swelling degree (g g )

100

40

0.2

20

0

0.4

0.0 0

1000

2000

3000

4000

5000

6000

7000

pH 4.5 pH 6.5 pH 8.0 0

1000

2000

3000

5000

6000

7000

0.0

IP T

0.4

(c)

pH 4.5 pH 6.5 pH 8.0

70 % water absorbed

SC R

-0.4

log (wt/weq)

4000

min

min

-0.8 -1.2 -1.6

-2.4 0.0

0.5

1.0

1.5

2.0

2.5

3.0

N

log (t)

U

-2.0

A

CC E

PT

ED

M

A

Figure 3

27

3.5

4.0

(a)

per g hydrogel)

40

pH 3.5 pH 4.5 pH 5.5

10

1000

1500

2000

2500

30

20 pH 3.5 pH 4.5 pH 5.5

10

0

3000

0

1000

2000

3000

10

pH 3.5 pH 4.5 pH 5.5

0

500

1000

1500

5000

6000

2000

2500

3000

30

20

10

0

0

1000

N

min

40

SC R

20

(d)

50

U

30

2+ qe(mg Ni per g composite hydrogel)

40

0

4000

7000

min

(c)

50

qe(mg Pb

2+

per g composite hydrogel)

min

A

CC E

PT

ED

M

A

Figure 4

28

IP T

500

40

qe(mg Ni

2+

20

qe(mg Pb

2+

30

0 0

(b)

50

per g hydrogel)

50

2000

3000

4000

min

5000

pH 3.5 pH 4.5 pH 5.5 6000

7000

100

120 90 60 2+

Ni 2+ Pb

30 100

150

200

250

300

350

400

450

-3 Initial concentration (mg dm )

80 60 40 2+

Ni 2+ Pb

20 100

150

200

250

300

350

A

CC E

PT

ED

M

A

N

U

SC R

Figure 5

29

400

-3 Initial concentration (mg dm )

IP T

150

50

(b)

120

qe(mg metal per g composite hydrogel)

qe (mg metal per g hydrogel)

(a) 180

2+

Ni 2+ Pb

250 200 150 100 50 0 0.0

0.1

0.2

0.3

0.4

Hydrogel mass (g)

(b)

300

2+

Ni 2+ Pb

250 200 150 100 50 0 0.0

0.1

0.2

0.3

A

CC E

PT

ED

M

A

N

U

SC R

Figure 6

30

0.4

Hydrogel mass (g)

IP T

qe (mg metal per g hydrogel)

qe (mg metal per g composite hydrogel)

(a)

300

(a)

(b)

180

40

0

0 20

40

60

80

100

-3 Ce (mg dm )

120

140

80

(d)

60

80

100

20

120

-3 Ce (mg dm )

U

40

A

CC E

PT

ED

M

A

Figure 7

31

120

Experimental Langmuir Freundlich Redlich-Peterson Sips

40

N

20

40

2+

50

100

SC R

Experimental Langmuir Freundlich Redlich-Peterson Sips

qe (mg Ni

2+

60

60

75

qe (mg Pb

40

80

100

25

20

-3 Ce (mg dm )

(c)

125

Experimental Langmuir Freundlich Redlich-Peterson Sips

qe (mg Ni

qe (mg Pb per g composite hydrogel)

2+

Experimental Langmuir Freundlich Redlich-Peterson Sips

60

80

per g composite hydrogel)

2+

120

IP T

per g hydrogel)

per g hydrogel)

120

80

120

160

-3 Ce (mg dm )

200

240

per g hydrogel)

30

20

10

0

500

1000

1500

2000

2500

2+

qt (mg Pb

pH 3.5 pH 3.5 pH 3.5 non-linear fit at pH 3.5 non-linear fit at pH 4.5 non-linear fit at pH 5.5

40

30

20

pH 3.5 pH 3.5 pH 3.5 non-linear fit at pH 3.5 non-linear fit at pH 4.5 non-linear fit at pH 5.5

qt (mg Ni

per g hydrogel) 2+

40

0

(b)

50

10

0

3000

0

1000

2000

3000

500

1000

1500 min

2000

2500

3000

10

0

0

A

CC E

PT

ED

M

Figure 8

32

5000

6000

7000

SC R

20

U

0

30

1000

A

0

2+

pH 3.5 pH 4.5 pH 5.5 non-linear fit at pH 3.5 non-linear fit at pH 4.5 non-linear fit at pH 5.5

10

qt (mg Pb

2+

20

40

2000

N

per g composite hydrogel)

30

(d)

50

qt (mg Ni

per g composite hydrogel)

(c)

40

4000

min

min

IP T

(a)

50

3000

pH 3.5 pH 4.5 pH 5.5 non-linear fit at pH 3.5 non-linear fit at pH 4.5 non-linear fit at pH 5.5

4000

min

5000

6000

7000

(a)

per g hydrogel)

40

10

0

500

1000

1500

2000

2500

30

20

pH 3.5 pH 4.5 pH 5.5 non-linear fit at pH 3.5 non-linear fit at pH 4.5 non-linear fit at pH 5.5

qt (mg Ni

pH 3.5 pH 4.5 pH 5.5 non-linear fit at pH 3.5 non-linear fit at pH 4.5 non-linear fit at pH 5.5

40

10

0

3000

0

1000

2000

3000

0

500

1000

1500

2000

2500

3000

min

20

10

0

0

A

CC E

PT

ED

M

Figure 9

33

5000

6000

7000

SC R

30

1000

A

0

2+

pH 3.5 pH 4.5 pH 5.5 non-linear fit at pH 3.5 non-linear fit at pH 4.5 non-linear fit at pH 5.5

10

qt (mg Pb

2+

20

40

pH 3.5 pH 4.5 pH 5.5 non-linear fit at pH 3.5 non-linear fit at pH 4.5 non-linear fit at pH 5.5

U

30

(d)

50

N

40

qt (mg Ni per g composite hydrogel)

per g composite hydrogel)

(c)

50

4000

min

min

IP T

20

2+

30

0

(b)

50

qt (mg Pb

2+

per g hydrogel)

50

2000

3000

4000

min

5000

6000

7000

Table 1 – Water diffusion coefficients (n), water diffusion constants (k) and determination coefficients (R2) for the swelling degree of the chitosan-based hydrogel (CBH) and chitosan/acid-activated montmorillonite composite hydrogel (CAAMCH).

n

R2

n

k(min-1)

pH 8.0 R2

n

k(min-1)

R2

0.3900 0.0794 0.9194 0.6088 0.0207

0.9779 0.6110 0.0223 0.9592

CAAMCH 0.5731 0.0175 0.9648 0.5754 0.0132

0.9817 0.7915 0.0049 0.9726

A

CC E

PT

ED

M

A

N

U

SC R

CBH

k(min-1)

pH 6.5

IP T

pH 4.5

34

Table 2 – Results of the non-linear Langmuir, Freundlich, Redlich-Peterson and Sips isotherms for the adsorption of Pb2+ and Ni2+ ions to the chitosan-based hydrogel (CBH) and chitosan/acid-activated montmorillonite composite hydrogel (CAAMCH). Non-linear Langmuir isotherm

CBH

R2

αL (L g-1)

KL (L mg-1)

qmax (mg g-1)

Ni2+

0.9652

0.606

4.53 x 10-3

133.8

Pb2+

0.9369

4.784

3.43 x 10-2

139.6

Ni2+

0.9366

6.026

7.64 x 10-2

Pb2+

0.9842

1.679

4.45 x 10-3

IP T

CAAMCH

Metal

78.87 76.80

SC R

Adsorbent

Non-linear Freundlich isotherm bF

KF (mg g-1)

nF

Ni2+

0.9836

0.579

2.917

1.725

Pb2+

0.9833

0.385

18.29

2.597

Ni2+

0.9933

0.578

4.283

1.730

Pb2+

0.9914

1.363

0.206

0.733

U

CBH

R2

N

CAAMCH

Metal

A

Adsorbent

Non-linear Redlich-Peterson isotherm R2

αRP (L mg-1)g

KRP (L g-1)



Ni2+

0.9997

1.324

5.974

0.494

Pb2+

0.9993

-1.030

0.042

-0.012

Ni2+

0.9993

1040.2

451.2

0.422

Pb2+

0.9991

2588.6

1723.3

-0.117

ED

CAAMCH

Metal

PT

CBH

CC E

Adsorbent

CAAMCH

A

CBH

M

Adsorbent

Non-linear Sips isotherm

Metal

R2

ns

Ks (L mg-1)

qmax (mg g-1)

Ni2+

0.9908

1.454

4.15 x 10-3

127.9

Pb2+

0.9632

0.570

3.34 x 10-2

120.3

Ni2+

0.9732

1.638

1.49 x 10-2

98.70

Pb2+

0.9587

0.882

2.87 x 10-3

94.25

35

Table 3 – Kinetic parameters, determination coefficients (R2) and Chi-square test statistics (2) for the non-linear pseudo-first order kinetic models during adsorption of Pb2+ and Ni2+ ions to the chitosan-based hydrogel (CBH) and chitosan/acid-activated montmorillonite composite hydrogel (CAAMCH).

K2 Adsorbent

Metal

qe-experimental

-1

-1

-1

(g mg min )

-1

(mg g )

(mg g ) 44.89

38.16

4.5

2.314 X 10-4

0.9947

38.59

5.5

2.671 X 10-4

0.9955

38.74

3.5

2.682 X 10-3

0.9992

4.5

2.239 X 10-3

0.9944

5.5

9.655 X 10-4

0.9904

3.5

4.549 x 10-4

SC R

0.9893

50.16

0.7941

45.03

0.6513

26.57

26.32

0.0857

N

3.407 x 10-4

31.18

0.6673

33.94

38.75

0.9638

0.9999

34.78

37.61

0.1220

5.417 x 10-4

0.9882

33.59

32.41

1.6794

3.356 x 10-4

0.9882

41.59

44.74

1.8537

PT

3.5

2

1.7385

2.431 x 10-3

0.9980

29.86

30.97

0.2289

4.5

3.313 x 10-3

0.9963

34.73

32.33

0.5646

5.5

2.633 x 10-3

0.9874

38.02

37.02

1.8256

Ni2+

Ni2+

4.5 5.5 3.5

28.82

A

M

A

CC E

Pb2+

ED

Pb2+

U

CAAMCH

CBH

qe-theoretical

R2

pH

IP T

Non-linear pseudo-first order kinetic model

36

Table 4 – Kinetic parameters, determination coefficients (R2) and Chi-square test statistics (2) for the non-linear pseudo-second order kinetic model during adsorption of Pb2+ and Ni2+ ions to the chitosan-based hydrogel (CBH) and chitosan/acid-activated montmorillonite composite hydrogel (CAAMCH). Non-linear pseudo-second order kinetic model qe-experimental

-1

-1

-1

2

-1

(mg g )

(mg g )

4.468 x 10-6

0.9905

38.16

62.27

4.5

2.231 x 10-6

0.9922

38.59

74.90

0.1467

5.5

3.164 x 10-6

0.9951

38.74

64.84

0.6074

3.5

9.976 x 10-5

0.9999

30.80

0.0013

Pb2+ 4.5

5.346 X 10-5

0.9946

28.82

38.79

0.6436

5.5

1.610 X 10-5

0.9932

33.94

51.90

0.6886

3.5

9.060 X 10-6

0.9999

34.78

48.34

0.0170

M

ED

1.5540

4.5

1.541 X 10-5

0.9946

33.59

39.42

0.7961

5.5

5.230 X 10-6

0.9939

41.59

58.55

0.9851

PT

Ni2+

26.57

N

CAAMCH

U

Ni2+

SC R

3.5

A

(g mg min )

3.5

7.194 X 10-5

0.9994

29.86

36.79

0.0746

Pb2+ 4.5

9.388 X 10-5

0.9987

34.73

41.21

0.1949

5.5

7.472 X 10-5

0.9965

38.02

43.37

0.5229

A

CC E

CBH

qe-theoretical

R2

pH

IP T

K2 Adsorbent

37