Physically-crosslinked polyvinyl alcohol composite hydrogels containing clays, carbonaceous materials and magnetic nanoparticles as fillers

Journal Pre-proof PHYSICALLY-CROSSLINKED POLYVINYL ALCOHOL COMPOSITE HYDROGELS CONTAINING CLAYS, CARBONACEOUS MATERIALS AND MAGNETIC NANOPARTICLES AS FILLERS Laura M. Sanchez (Conceptualization) (Methodology) (Validation) (Investigation) (Writing - original draft) (Writing - review and editing) (Supervision) (Project administration), Peter S. Shuttleworth (Conceptualization) (Methodology) (Validation) (Investigation) (Writing - review and editing) (Resources) (Funding acquisition), Carolina Waiman (Conceptualization) (Methodology) (Validation) (Investigation) (Writing - review and editing), Graciela Zanini (Conceptualization) (Methodology) (Validation) (Investigation) (Writing - review and editing), Vera A. Alvarez (Conceptualization) (Writing - review and editing) (Resources) (Funding acquisition), Romina P. Ollier (Conceptualization) (Methodology) (Validation) (Investigation) (Writing - original draft) (Writing - review and editing) (Supervision) (Project administration)

PII:

S2213-3437(20)30143-3

DOI:

https://doi.org/10.1016/j.jece.2020.103795

Reference:

JECE 103795

To appear in:

Journal of Environmental Chemical Engineering

Received Date:

29 October 2019

Revised Date:

28 January 2020

Accepted Date:

16 February 2020

Please cite this article as: Sanchez LM, Shuttleworth PS, Waiman C, Zanini G, Alvarez VA, Ollier RP, PHYSICALLY-CROSSLINKED POLYVINYL ALCOHOL COMPOSITE HYDROGELS CONTAINING CLAYS, CARBONACEOUS MATERIALS AND MAGNETIC NANOPARTICLES AS FILLERS, Journal of Environmental Chemical Engineering (2020),

doi: https://doi.org/10.1016/j.jece.2020.103795

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PHYSICALLY-CROSSLINKED POLYVINYL ALCOHOL COMPOSITE HYDROGELS CONTAINING CLAYS, CARBONACEOUS MATERIALS AND MAGNETIC NANOPARTICLES AS FILLERS

Laura M. Sanchez1,*, Peter S. Shuttleworth2, Carolina Waiman3, Graciela Zanini3, Vera A.

1Materiales

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Alvarez1 and Romina P. Ollier1

Compuestos Termoplásticos (CoMP), Instituto de Investigaciones en Ciencia y

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Tecnología de Materiales (INTEMA), CONICET - Universidad Nacional de Mar del Plata (UNMdP). Av. Colón 10890, Mar del Plata, 7600, Argentina.

de Física de Polímeros, Elastómeros y Aplicaciones Energéticas, Instituto

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2Departamento

3Instituto

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de Ciencia y Tecnología de Polímeros, CSIC. c/ Juan de la Cierva, 3, 28006, Madrid, España. de Química del Sur (INQUISUR), CONICET - Departamento de Química, Universidad

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Nacional del Sur (UNS), Av. Alem 1253, Bahía Blanca, 8000, Argentina

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*Corresponding author: Dr. Laura Mabel Sanchez. Phone Nº: (+54223) 6260627; e-mail: [email protected].

Abstract

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Polyvinyl alcohol (PVA)/ Bentonite, acid-modified Bentonite, magnetic nanoparticles

and mesoporous carbonaceous composite hydrogels were synthesized via an eco-friendly freeze-thawing technique, which is a simple and non-toxic crosslinking methodology. After thorough characterisation using X-Ray Diffraction (XRD), Scanning Electron microscopy (SEM), Dynamic Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA) and swelling assessment.

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Furthermore, the prepared materials were tested in order to explore their cadmium adsorption potential, a heavy metal usually found as a pollutant in water bodies. The data obtained showed that PVA has no affinity for cadmium ions but the incorporation of the different fillers makes the materials develop important adsorbent characteristics. Under the selected working conditions, and for a contact time of 1440 min, the maximum amount of cadmium ions adsorbed was 50.9 mg/g, 33.9 mg/g, 42.6 mg/g and 30.5 mg/g for composites containing bentonite; acid-modified bentonite; magnetic nanoparticles and carbonaceous

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materials, respectively. For all cases the higher the filler amounts the higher the adsorption

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efficiency.

INTRODUCTION

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Keywords: Composite hydrogels; polyvinyl alcohol; magnetic nanoparticles; bentonite; carbonaceous materials; cadmium adsorption.

Toxic heavy metals such as cadmium, zinc, copper, nickel, mercury, lead and

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chromium are of particular concern in the field of water remediation[1]. According to the World Health Organization (WHO), cadmium (Cd) a metal with an oxidation state of +2 and chemically similar to zinc, is naturally found with zinc and lead in sulphide ores[2]. Cd is

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released into the environment from anthropogenic sources such as the combustion of fossil fuels, metal production, application of phosphate fertilizers, electroplating, and the manufacturing of batteries, pigments, and alloys[3,4]. Consequently, this heavy metal has resulted in serious contamination of both soil and water. Hence, Cd can affect human health

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directly through drinking water or indirectly by bioaccumulation in living organisms entering the human food chain[5,6]. According to the International Agency for Research on Cancer, Cd has also been classified as a human carcinogen impacting lungs, kidneys, liver and reproductive organs[7–9]. As a consequence, the WHO has established a maximum concentration level of 0.003 milligrams per litre (mg/L) for Cd in drinking water[2]. As for most heavy metals, Cd tends to accumulate and for this reason, there is considerable interest in the development of techniques to be able to remove it effectively 2

from contaminated water. Many treatment processes, such as chemical precipitation[10], ion-exchange[11],

adsorption[12],

biosorption[6],

electrodialysis[13]

and

reverse

osmosis[14], are currently used. Among these methods, adsorption is a promising and widely applied method due to its cost-effectiveness[4]. Hydrogels typically are hydrophilic crosslinked three-dimensional (3D) polymer networks, with the ability to absorb large quantities of water or biological fluids, resulting in their drastic swelling, whilst maintaining their 3D structure and without dissolving[15,16].

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Therefore, cross-linking provides the polymeric hydrogels insolubility in water, as well as mechanical strength and physical integrity. The versatile and outstanding properties of

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these polymer-based systems have attracted the attention of researchers for numerous miscellaneous applications including drug delivery devices[17], tissue engineering[18],

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separation processes, sensors[19], water remediation technologies[20], agriculture[21] and many other fields. In addition, research on these materials in a wide range of scientific areas

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is continuously growing.

With increasing technological demand for newer and better materials, incorporation of different fillers has also been found to result in tailored or improved hydrogel properties

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(mechanical, thermal, optical, electrical etc.) and responsiveness (such as, magnetic response)[22]. These fillers vary in shape and dimension from micro- to nanometre, and type such as, carbon-based materials, polymeric nanoparticles, inorganic/ceramic nanoparticles and metal/metal-oxide nanoparticles[23,24]. Chemical compatibility between the polymer and the filler plays a key role in the enhancement of the structure of

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composite hydrogel, and in some cases, pre-surface functionalization of the filler is required. In addition, multiple interactions between the filler and hydrogel, including hydrogen bonding, van der Waals and electrostatic interactions may take place. As a result, a large amount of research has been conducted in the field of composite hydrogel understanding[25], with nanotechnology offering many opportunities to develop optimized composite materials with synergistic properties. Controlling the design of the 3D hydrogel

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structure at the nanometre scale provides a powerful strategy to incorporate versatility and customized functionality into the resulting nanocomposite system[26]. Polyvinyl alcohol (PVA) based hydrogels have also attracted a lot of interest in recent years, due to the large diversity of hydrogels that can be made, depending largely on the preparation/processing route adopted[27–29]. These materials have been widely studied for their use in drug delivery[30], tissue engineering[31], dressing for antibacterial applications[32,33], separation or recovery of heavy metals,[34] immobilization of

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microorganisms[35], etc. Particularly, non-toxic and low-cost crosslinked PVA hydrogels

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have attracted attention in the area of wastewater treatment[36]. This is likely due to its ability to selectively retain contaminants from water through adsorption. Other adsorbents, such as, clays[37,38], iron oxide nanomaterials[39] and carbonaceous materials[40,41] have

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also been well documented as adsorbents for water or wastewater treatment, though their recovery after treatment can be complicated. This limitation can be corrected by confining

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the particles within a hydrogel that also has its own sorption capacity, such as PVA-based hydrogels, and in some cases, filler inclusion can increase the composite specific surface

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area[42] as well as improve the chemical interaction with the contaminant[43]. The fillers or reinforcing materials of PVA hydrogels can be nanoparticles[44], inorganic solids as

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natural clays[45], synthetic clays as layered double hydroxide (LDH)[46], among many other possibilities.

On the other hand, the usefulness of clays[37,38], iron oxide nanomaterials[39] and carbonaceous materials[40,41] by themselves for water or wastewater treatment by pollutant adsorption is well documented. However, their recovery from wastewater could

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be complicated, whereby this limitation can be corrected by confining the particles in a hydrogel that also has its own sorption capacity, such as PVA-based hydrogels. Regarding magnetic PVA hydrogel composites, they have been shown to be useful for the adsorption of heavy metals, such as, Cu(II)[47], Cd(II)[48,49], Cr(VI)[50] and Cs(I)[51] etc., whilst PVA hydrogel - graphene oxide composites are able to absorb Cu(II), Ni(II)[52]and Pb(II)[36] and those containing clay for the removal of Cu(II), Ni(II) and Co(II)[53].

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In this work, we will focus particularly on the development and characterization of composite PVA-based hydrogels with fillers of different physical and chemical nature: clays, iron oxides and carbonaceous materials. In addition, we will evaluate the potential of these materials as adsorbents for Cd removal from water.

2- PREPARATION OF POLYVINYL ALCOHOL COMPOSITE HYDROGELS

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Four different composite hydrogels were prepared through physical crosslinking using polyvinyl alcohol as the matrix and different contents of bentonite; acid-modified

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bentonite, magnetic nanoparticles and a sustainable mesoporous carbonaceous material.

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2.1 Materials.

PVA was supplied by Sigma-Aldrich (Molecular weight 89,000-98,000 g/mol,

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hydrolysis degree of 98-99%). The clay employed in this work was bentonite (Bent) supplied by Minarmco S.A. from Neuquén, Argentina. The raw clay consisted predominantly of

found to be 0.939 mEq/g.

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montmorillonite[54]. The basal spacing was 1.3 nm and its cation exchange capacity was

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Acid-treated bentonite (Bent-H) was prepared according to our previously reported methodology [54]. Bent (5 g), after being dried at 80 ºC during 24 h was dispersed in 200 mL of distilled water. To this, 10 mL of concentrated H2SO4 was added and constant magnetic stirring was maintained for 6 h. The resulting aqueous Bent-H suspension was separated by centrifugation at 10,000 rpm for 10 min. The clay was then washed with 5

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aliquots of distilled water and again centrifuged. Finally, the wet Bent-H was frozen, lyophilized (72 h at -50 ºC, 100 mTorr) and sieved through a 150 µm mesh. For the preparation of the iron oxide magnetic nanoparticles (MNP), two different

iron salts were used as starting materials: FeSO4·7H2O and FeCl3·6H2O (both from Cicarelli Laboratory, Argentina). Polyacrylic acid (PAA, Mw 1800 g/mol) was purchased from Polysciences. The co-precipitation was conducted according to a previous reported

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technique [55]. In summary, 4.75 g of Fe3+ and 2.45 g of Fe2+ were dissolved in 80 mL of milli-Q water at 60 ºC, under N2 atmosphere. Then, 0.49 g of PAA was added to the system and vigorous stirring was maintained for 30 min. After, 10 mL of NH4OH was quickly added to the mixture, resulting in a colour change from orange to black, yielding the desired magnetic nanoparticles. Starch-derived mesoporous carbonaceous materials (C) were prepared according to previous reports[56–58]. Briefly, 3 stages were followed: gelation of starch, its

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retrogradation and then exchange of the water present with a lower surface tension solvent (EtOH etc.). The mesoporous material was then dried under vacuum in an oven at 50ºC to

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of an organic acid and heated under vacuum to 800 ºC.

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generate the mesoporous starch. Finally, the material was doped with a catalytic amount

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2.2 Preparation of the hydrogels.

PVA hydrogels were prepared as follows: For the blank PVA hydrogels, PVA (10 wt. %)

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was dissolved in water at 80 ºC under constant stirring, and then cast into a series of antiadherent containers. The samples were then frozen (-18 ºC, 1 h) and afterwards thawed at room temperature (25 ºC, 1h). This freezing-thawing (F-T) cycle was repeated 3 times to

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obtain the final physically cross-linked hydrogels [59]. For the preparation of the composite hydrogels, an analogous protocol was followed, but different amounts of the selected filler (1, 3 and 5 wt.% regarding the PVA mass) were added to the initial aqueous solution of PVA[48,60,61]. Although the MNPs and the

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carbonaceous material were directly dispersed onto the already dissolved PVA, the clays required previous swelling treatments. Bent and Bent-H were left overnight in order to swell sufficiently, and then the solid PVA was added to each system. Again, each composite hydrogel was cross-linked via 3 F-T cycles, with the overall employed procedure shown in Figure 1.

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Figure 1. Schematic view of the preparation route of the composite PVA hydrogels.

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2.3- Characterization of the polyvinyl alcohol composite hydrogels X-ray diffraction (XRD) of the dried hydrogels was performed using an X-Pert pro diffractometer, operating at 40 kV and 40 mA, with a monochromatic CuKα radiation (λ = 1.54 Å), at a scanning speed of 1º/min. The basal spacing distances (d001) were calculated

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from the 2θ values using the Bragg equation. The starting materials Bent, Bent-H and MNPs were also analysed by XRD in the same diffractometer. The morphology of the different hydrogels was analysed by Scanning Electron

Microscopy (SEM) in a JEOL JSM-6460 LV instrument. The previously swollen, frozen and lyophilized samples were cryo-fractured with liquid N2 and then sputtered with gold. The swelling degree (SD%, Equation 1) capacities of the prepared hydrogels were determined by placing 0.5 g of the dried hydrogel in distilled water at 25 ºC under stirring 7

(330 rpm) for defined periods of time (t). Afterwards, the hydrogel were carefully filtered, and then weighed. In each case, samples were run in triplicate, with the swelling degree calculated using Equation 1: 𝑆𝐷 (%) =

𝑊𝑡 −𝑊𝑖 𝑊𝑖

× 100

(Equation 1)

where wi is the weight of the samples before immersion and wt is the weight of the sample at t period of time. Additionally, gel fraction determinations (GF%, Equation 2) were

sample. 𝑊𝑓 −𝑊𝑓𝑖𝑙𝑙 𝑊𝑖 −𝑊𝑓𝑖𝑙𝑙

× 100

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𝐺𝐹 (%) =

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carried out under the same experimental conditions. Three specimens were tested for each

(Equation 2)

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where Wi and Wf are the weights of the dried hydrogels before and after immersion, respectively, whereas Wfill is the weight of the filler contained in the studied sample (that

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can be obtained in different ways, for example, in the case of having inorganic fillers, TGA results are commonly used).

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Thermogravimetric analysis (TGA) was carried out on 7–15 mg sample using a TA Instruments, Q500 from room temperature to 800 ºC at a heating rate of 10 ºC/min under

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a nitrogen atmosphere, in order to avoid thermo-oxidative oxidation reactions. Additionally, Differential Scanning Calorimetric (DSC) of the dried hydrogels (5–10 mg) were carried out on a TA-Q2000 DSC equipment, under N2 atmosphere. Each sample was heated from room temperature to 40 ºC, maintained isothermally for 10 minutes, cooled to -20 ºC, and then heated from -20 ºC to 255 ºC at 5 ºC/min, followed by cooling.

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The crystallinity degree (Xcr %, Equation 3) was calculated from the obtained DSC

thermograms (Figure 11), taking into account that ΔH° is the heat (138.6 J/g) required to melt a 100% crystalline PVA sample [62,63].

𝑋𝑐𝑟 (%) = 𝑊

𝛥𝐻

𝑚

𝛥𝐻 0

× 100

(Equation 3)

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where ΔH is determined by integrating the area under the melting peak, ΔH0 is the heat required to melt a 100% crystalline sample, and Wm is the weight fraction of PVA (obtained from TGA curves).

2.4- Cd removal efficiency tests With the aim of assessing the properties of the synthesised materials as Cd

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adsorbents, batch adsorption experiments were performed as a function of time. All experiments were conducted in polypropylene tubes by submerging 15 mg of the dried

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hydrogel films in a Cd aqueous solution (initial concentration 8 ppm, final volume 15 mL) at pH=5, and then shaking them for 0, 60, 240, 420, 720 and 1440 min at 25 ºC respectively.

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Afterwards, the films were removed from the polypropylene tubes and the Cd water solution tested to quantify the remaining Cd present within the solution using an atomic

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absorption spectrometer (Perkin Elmer Analyst 200). The amount of Cd adsorbed, q (mg/g) was calculated by employing the following equation: (𝐶0 −𝐶𝑒𝑞 )

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𝑞=

𝑚

×𝑣

(Equation 4)

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where C0 (mg/L) is the initial Cd concentration, Ceq (mg/L) is the Cd concentration of the water after 0, 60, 240, 420, 720 and 1440 min respectively, v (L) the volume of Cd

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solution and m (g) is the mass of the adsorbent employed.

3- RESULTS AND DISCUSSION The XRD measurements were made in order to determine the dispersion of the clay

and iron oxide –containing particles within the hydrogel matrix, with the results shown in Figure 2.

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PVA 1 wt.% PVA/Bent 3 wt.% PVA/Bent 5 wt.% PVA/Bent Bent

1000

Counts (a.u.)

800

600

a)

400

0 2

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2 (°)

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Counts (a.u.)

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PVA 1 wt. % PVA/Bent-H 3 wt. % PVA/Bent-H 5 wt. % PVA/Bent-H Bent-H

3

4

5

6

7

8

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10

2 (°)

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15

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25

30

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45

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55

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c)

Counts (a.u.)

PVA 1 wt. % PVA/MNPs 3 wt. % PVA/MNPs 5 wt. % PVA/MNPs MNPs

Figure 2. XRD patterns of the composite hydrogels: a) PVA/Bent; b) PVA/Bent-H and c)

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PVA/MNPs.

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From Figure 2a, for 1 and 3 wt.% Bent, it can be seen that a good dispersion of the filler inside the PVA matrix is observed as no characteristic clay diffraction peak appears at

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2θ = 2-10°. This peak absence is related to the possibility of having exfoliated or mixed exfoliated-intercalated silicate layers of the clay dispersed within the PVA matrix [64]. A

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minor increase in the interlamellar space of the clay is also observed in the case of the nanocomposite with 5 wt.% Bent from the presence of a small peak around 4°, which is associated with the polymer inclusion within the clay galleries [65]. From Figure 2b, it can be seen that the 001 diffraction peak of Bent-H inside the composite hydrogels shifted towards lower angle values compared to that of the neat clay,

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with this effect being slightly lower for the 5 wt.% Bent-H nanocomposite. This indicates that the basal spacing of the clay has increased due to the intercalation of the PVA polymer chains [66]. In addition, Bent-H can interact with the PVA chains via hydrogen bonding due to the presence of hydroxyl and silanol groups on the surface layer of the clay.

The main characteristic diffraction peaks of semicrystalline PVA are located at 19.8° and 22.9° (2θ), that correspond to the (1 0 1) and (2 0 0) reflection planes respectively, and 11

are present in both the PVA and the ferrogel diffractograms as shown in Figure 2c [67,68]. In the case of the ferrogel, a very intense peak centred at 35.5 ° (2θ) was observed, which can be attributed to the (3 1 1) reflection plane, the most intense diffraction peak for magnetite (19-0629 JCPDS) and confirms the presence of this iron oxide in the PVA matrix. In addition, other minor diffraction peaks corresponding to a mixture of iron oxides coated with PAA are present in the ferrogels samples (2θ=20 °, 30.2 °, 32.8 °, 43.4 °, 53.7 °, 57.4 °

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and 62.9 °) [55,69].

The obtained SEM micrographs of the neat PVA, and the composite PVA hydrogels are

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shown in Figures 3-7. It is clear that both the structure and morphological characteristics of the hydrogels are strongly affected by the addition of the fillers. The PVA/Bent-H and

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PVA/MNPs (Figures 5 and 6) materials pore walls appear to be thicker and more welldefined in comparison to the neat PVA (Figure 3) and to the composite materials PVA/Bent

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and PVA/C (Figures 4 and 7). It can therefore be assumed that the addition of certain types of filler can stabilize the hydrogel pore structure better than other filler types. This can be

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explained in terms of the well-known three-phase model widely accepted to describe the composition of F–T PVA hydrogels[70,71]. In this case, the stronger interaction capability of the MNPs and Bent-H via hydrogen bonding to PVA matrix compared to the other employed

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fillers would explain their morphology differences. The authors have previously presented this hypothesis in terms of an easy to follow schematic[60].

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×10,000

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×1000

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Figure 3. SEM micrographs of the neat PVA hydrogel.

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×10,000

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b)

Figure 4. SEM micrographs of the PVA/Bent nanocomposite hydrogels: a) 1 wt. %; b) 3 wt. %; c) 5 wt. %.

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×10,000

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Figure 5. SEM micrographs of the PVA/Bent-H nanocomposite hydrogels: a) 1 wt. %; b) 3 wt. %; c) 5 wt. %.

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Figure6. SEM micrographs of the PVA/MNPs nanocomposite hydrogels: a) 1 wt. %; b) 3 wt. %; c) 5 wt. %.

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Figure 7. SEM micrographs of the PVA/C nanocomposite hydrogels: a) 1 wt. % and b) 3 wt.

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%.

An interesting characteristic of the PVA-based hydrogels is their swelling properties. The swelling degree as a function of time for each set of composite materials considered is shown in Figure 8. The SD of all the samples except for those containing MNPs rapidly increases during the first 60 minutes of immersion in the distilled water and then plateaus

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from 120 minutes onwards. In addition, the SD of the PVA/bent, PVA/Bent-H, and PVA/C composite hydrogels all decrease with filler concentration. For the sample containing the MNPs the SD plateaus after 120 minutes except for the 5 wt.% that continues to increase with time. The MNPs samples also show the opposite tendency in regards to SD and filler concentration, were SD decreases rapidly initially with lower filler addition, and then increases again with increasing amounts of filler. This phenomenon may be associated with the formation of MNPs agglomerates, which would result in an increase in filler content but 17

not an increased interaction with the hydrogel network, and hence, effecting the SD to a lower degree. The relation between gel fraction and the filler content for each composite hydrogel is shown in Figure 9. As can be seen, in some of the studied cases, the gel fraction remains almost constant or it presents a slight increase when filler content increases. This could be due to the fact that the incorporated clay interacts with the polymer and acts as crosslinking knots, thus leading to an hydrogel network with a more entangled structure [72,73]. On the

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other hand, the gel fraction notably decreases when MNPs content increases. Again as in the case in Figure 8, this is likely due to agglomerate formation and the increase in the filler

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content not resulting in an increase in these ‘crosslinking knots’.

b)

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a) 250

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250

150

100

PVA 1 wt.% PVA/Bent 3 wt.% PVA/Bent 5 wt.% PVA/Bent

50

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0

0

60

120

180

240

360

420

150

100

PVA 1 wt.% PVA/Bent-H 3 wt.% PVA/Bent-H 5 wt.% PVA/Bent-H

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0 0

60

120

180

240

300

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420

Time (min)

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Time (min)

300

SD (%)

200

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SD (%)

200

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d)

250

250

200

200

150

150

SD (%)

100

PVA 1 wt.% PVA/MNPs 3 wt.% PVA/MNPs 5 wt.% PVA/MNPs

50

0 0

60

120

180

240

300

360

100

50

PVA 1 wt.% PVA/C 3 wt.% PVA/C

0

420

0

Time (min)

60

120

180

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SD (%)

c)

240

300

360

420

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Time (min)

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Figure 8. Swelling degree(%) as a function of time for a) PVA/bent; b) PVA/Bent-H; c)

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PVA/MNPs and d) PVA/C composite hydrogels.

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90

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Gel Fraction (%)

100

PVA/Bent PVA/Bent-H PVA/MNPs PVA/C

80

1

3

5

Filler content (%)

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Figure 9. Gel Fraction dependence on filler content for the composite hydrogels.

The swelling capabilities of the hydrogels are strongly dependent on their network characteristics, such as crystallinity degree, pore size and gel fraction[73]. For the presented materials, with the exception of just one sample (PVA/Bent 1 wt.%), the results demonstrate 19

that composite materials have reduced water uptake capabilities compared to the standard PVA hydrogel. As seen in Figure 8, the swelling capacity of the samples decreases with increase in filler content, regardless of the filler’s nature. Hui Wong et al. observed that the higher the gel fraction the lower the swelling (see Swelling dependence on gel fraction hydrogel characteristics in Figure 10)[74]. PVA/Bent offered the highest swelling values that

260

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were related to the higher water affinity of Bent compared to the other fillers.

PVA/Bent PVA/Bent-H PVA/MNPs PVA/C

240

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200 180

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Swelling (%)

220

160 140

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80

85

90

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100

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Gel Fraction (%)

Figure 10. Swelling degree dependence on gel fraction of the composite hydrogels.

Characteristic thermal properties of the hydrogels, including the temperature for the

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maximum degradation rate (Tdeg) obtained from TGA and melting temperature (Tm) as well as crystallinity degree (Xcr) obtained from DSC measurements, are summarized in Table 1. The 5 wt.% PVA/MNPs composite material was not thermally characterized due to MNPs agglomeration.

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a)

Heat flow (mW)

0

-2

-4

PVA 1 wt.% bent 3 wt.% bent 5 wt.% bent

-6 0

50

100

150

200

250

PVA 1 wt.% bent-H 3 wt.% bent-H 5 wt.% bent-H 50

100

150

200

250

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0

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Heat flow (a.u)

b)

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Temperature (°C)

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c)

Heat flow (a.u.)

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Temperature (°C)

1 wt.% MNPs 3 wt.% MNPs PVA

0

50

100

150

200

250

Temperature (°C)

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Heat flow (a.u)

d)

1 wt.% C 3 wt.% C PVA 0

50

100

150

200

250

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Temperature (°C)

Figure 11. DSC thermograms of the composite hydrogels (Endo down): a) PVA/Bent; b)

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PVA/Bent-H; c) PVA/MNPs and d) PVA/C.

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Table 1. Thermal parameters of the prepared hydrogels. Tdeg

Tm

Xcr

(ºC)

(ºC)

(%)

282

227

42.7

1 wt.%

278

226

46.0

3 wt.%

274

222

35.3

5 wt.%

279

225

31.1

1 wt.%

277

228

49.6

3 wt.%

359

227

46.3

5 wt.%

355

226

46.5

1 wt.%

312

213

29.2

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PVA

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Material

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PVA/Bent

PVA/Bent-H

PVA/MNPs

22

3 wt.%

303

212

30.5

1 wt.%

295

228

50.9

3 wt.%

294

229

56.8

PVA/C

The incorporation of filler could increase, decrease or not alter the crystallinity of a

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semi-crystalline polymer. For the sample containing bentonite, a small amount of the filler favours heterogeneous nucleation of the PVA chains, whilst further increase seems to

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induce a steric hindrance on the PVA matrix, resulting in a decrease in Xcr. This tendency appears for most of the samples except the samples containing porous carbon, where the

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crystallinity continued to increase with filler content and opposite for the samples containing MNPs where a large decrease in crystallinity was observed. The incorporation of

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magnetic nanoparticles and Bent also significantly modified the melting temperature of the PVA gel. Interactions between the gel and filler can cause the disruption of the lamellar

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arrangement of the PVA crystallites, resulting in a diminution of average crystallite size and consequently a lowering of the PVA melting point[75]. Though, in general an increase in the

PVA.

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enthalpies of crystallization and melting was observed after incorporation of filler in the

It has also been reported previously that PVA hydrogels could exhibit a double glass transition (Tg), as in the case for the samples tested[60]. Generally, it was found that the Tg values of the samples slightly increased with filler incorporation, which can be attributed to

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the fact that the PVA chains have reduced mobility due to their confinement between the clay layers and also due to the strong interactions between both materials[61]. Regarding the degradation of PVA nanocomposites, the first step in the process

involves an elimination reaction [65], which is accelerated when polymer chains have enhanced mobility [76]. In some of the analysed materials, this mobility has been reduced by the addition of fillers, leading to an increase in the thermal stability and consequently degradation temperature Tdeg. The modification of the Tdeg is strongly dependent on the 23

filler interaction with the PVA matrix, with Tdeg increasing when the interactions become stronger or more significant. In the case of Bent-H and MNPs, these fillers can form hydrogen bonds to both water and PVA [77], which results in an significant reduction in PVA chain mobility, and hence, the Tdeg increases for the hydrogels containing these fillers compared to those obtained for PVA/Bent and PVA/C.

Finally, Figure 12 shows the results obtained in terms of Cd adsorption (q) as a

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function of time. The studies were performed at pH=5 to ensure the presence of the divalent ion Cd2+ ([78,79]). It can be seen that the PVA hydrogel sample shows the lowest

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Cd adsorbed efficiency. However, the addition of fillers (natural and modified clay, magnetic nanoparticles or the mesoporous carbon) into the polymeric matrix significantly improved

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its adsorptive capacity, and in general, this improved further with higher filler concentrations. This behaviour could indicate that the main function of the PVA is to be the

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supporting material.

60

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a)

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PVA 1 wt. % PVA/Bent 3 wt. % PVA/Bent 5 wt. % PVA/Bent

60

q (mg/g)

q (mg/g) 20

20

0

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0

200

400

600

800

Tiempo (min)

1000

1200

PVA 1 wt. % PVA/Bent-H 3 wt. % PVA/Bent-H 5 wt. % PVA/Bent-H

40

40

0

b)

1400

0

200

400

600

800

1000

1200

1400

Time (min)

24

c)

d) 60

60

PVA 1 wt. % PVA/C 3 wt. % PVA/C

PVA 1 wt. % PVA/MNPs 3 wt. % PVA/MNPs 40

q (mg/g)

q (mg/g)

40

20

20

0

0

0

200

400

600

800

1000

1200

200

1400

400

600

800

1000

1200

1400

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0

Time (min)

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Time (min)

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Figure 12. Cd adsorption (q) as a function of time for a) PVA/Bent; b) PVA/Bent-H; c)

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PVA/MNPs and d) PVA/C composite hydrogels.

From more detailed analysis of Figure 12 it can be seen that the sample after 1440

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minutes containing 5wt.% Bent shows the highest Cd adsorption (50.9 ± 1.6 mg g-1), followed by 3wt.% (42.6 ± 8.4 mg g-1) and 1wt% PVA/MNPs (36.3 ± 4.6 mg g-1), 3wt.%

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PVA/Bent-H (33.9 ± 2.7 mg g-1) and then 3wt.% PVA/C (30.5 ± 9.5 mg g-1). In all cases the adsorption capacity of Cd2+ for these materials are better than that reported by Burham et. al [80]. For Na–Bentonite (8.2 mg g-1) or 26.2 mg Cd/g on a similar sodium modified bentonite and 7.28 mg Cd g-1 on Na-bentonite for Alvarez and Sanchez respectively [81]. It is also interesting to note, that despite the fact that the 5wt.% Bent shows the highest Cd

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adsorption, the 1wt.% Bent sample shows the lowest compared to the other samples with 1wt.% filler added. In this case the sample containing 1wt.% MNPs has the best Cd adsorption, with a value of 36.3 ± 4.6 mg g-1, followed by the PVA/C (27.4 ± 7.0 mg g-1) > PVA/Bent-H (15.9 ± 1.2 mg g-1) > PVA/Bent (14.2 ± 1.0 mg g-1). In both the PVA/MNPs and PVA/C case were the adsorption values for both the 1 and 3wt.% samples are similar it is likely that this is due to the particles agglomerating to some degree reducing their potential

25

and accessibility of the contaminate into their porous structure. In the case of PVA/C, the sample is hydrophilic and wetting could be an issue.

In summary, pollutant adsorption using a polymeric hydrogel is a complex process that depends on its surface area, pores size, distribution and structure, crosslinking degree, diffusion capacity and on its specific chemical composition since the pollutant can interact with the adsorbent through electrostatic attraction and/or complexes formation, for

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example [82–84]. Since the addition of filler to the hydrogel may change more than one of the above-mentioned parameters, establishing the key pollutant adsorption factors is not

ro

trivial. Additionally, in this case generalizing the ion adsorption mechanism when each of the employed fillers have very differential characteristics from leads to further

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4-

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complications.

CONCLUSIONS AND FUTURE REMARKS

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PVA composite hydrogels containing natural and modified bentonite, magnetic nanoparticles and mesoporous carbonaceous particles derived from starch with filler loadings of 1, 3 and 5wt.% have been prepared, characterised and tested for cadmium

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adsorption. It was found that each filler imparts to the hydrogel a defined morphology (pore size, swelling degree, gel fraction and crystallinity) and thermal properties. In general, with filler addition the swelling degree decreased, while the thermal stability of the materials increased SEM of the samples demonstrated that the pore walls of the PVA/Bent-H and MNPs were thicker and pore structure more regular. These fillers can interact with the PVA

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via hydrogen bonding and stabilise the hydrogel structure better via the widely accepted three-phase model when describing F-T prepared PVA hydrogels. It was also found that all composite hydrogels displayed enhanced Cd(II) adsorption capacities compared to the neat PVA hydrogel and a clear correlation between the filler loading and the adsorption capability was found, with PVA/Bent 5wt.% and 3wt.% displaying the highest values of 50.9 ± 1.6 mg g-1 and 42.6 ± 8.4 mg g-1 respectively. Though, at the low filler concentration of 1

26

wt.% the PVA/MNPs (36.3 ± 4.6 mg g-1) and C (27.4 ± 7.0 mg g-1) easily outperformed the other tested materials. It was found that all composite hydrogels displayed an improved behaviour regarding to the neat PVA hydrogel for Cd adsorption, even when filler concentrations as low as 1 wt.% are tested, which turns the validity of the work. In addition, a clear correlation between the filler loading and the adsorption capability of the materials was found. Considering the facile preparation procedure, desirable structural properties and

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improved adsorption capacity, it can be assumed that the so obtained results are promising and thus, the developed materials could be tested for the remediation of other problematic

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heavy metals, such as, chromium, lead, etc. and another important pollutant like arsenic.

CRediT author statement

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Laura M. Sanchez: Conceptualization, Methodology, Validation, Investigation, Writing Original Draft, Writing - Review & Editing, Supervision, Project administration.

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Peter S. Shuttleworth: Conceptualization, Methodology, Validation, Investigation, Writing Review & Editing, Resources, Funding acquisition.

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Carolina Waiman: Conceptualization, Methodology, Validation, Investigation, Writing Review & Editing. Graciela Zanini: Conceptualization, Methodology, Validation, Investigation, Writing Review & Editing.

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Vera A. Alvarez: Conceptualization, Writing - Review & Editing, Resources, Funding acquisition. Romina P. Ollier: Conceptualization, Methodology, Validation, Investigation, Writing Original Draft, Writing - Review & Editing, Supervision, Project administration.

Declaration of interests 27

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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ACKNOWLEDGEMENTS

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The authors acknowledge the financial support of CONICET, Universidad Nacional de Mar del Plata, ANPCyT and Fundación Bunge & Born. PSS also acknowledges financial

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support from the Spanish Ministry Economy and Competitivity (MINECO) for a Ramón y Cajal fellowship (RYC-2014-16759) and a Proyecto de Investigación en el Programa Estatal

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lP

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de Fomento de la Investigación Científica y Técnica de Excelencia (MAT2017-88382-P).

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