Chitosan-grafted nonwoven geotextile for heavy metals sorption in sediments
Introduction
Massive amounts of sediments are dredged in order to maintain the depth of the navigational waterways, harbors and estuaries worldwide. Land disposal of these dredged materials may affect the surrounding environment due to the presence of harmful components such as organic compounds and heavy metals [1]. In some harbors, a lot of sediments and sludge can be dredged, but nobody can use them further because of their high concentration in heavy metals such as Arsenic, Cadmium, Chromium, Copper, Mercury, Nickel, Lead, and Zinc [2]. All these metals can be found in the sediments from seas [3] or from rivers [4]. To recover sediments and sludge, heavy metals must be removed.
Many synthetic and natural molecules can trap these heavy metals. For example, poly(dimethylaminoethyl methacrylate – cross linked pregelled starch graft copolymers), or poly(DMAEM-CPS), can be used to trap Copper, Cadmium, Mercury or Lead by putting it into the contaminated solution followed by a filtration step [5]. Poly(2-hydroxyethyl methacrylate-n-vinyl imidazole), or poly(HEMA-VIM), can also be used to trap Copper, Lead, Zinc and Cadmium when it is in cryogel form [6]. These polymers are synthetic, but natural polymers such as cellulose and alginate, under beads form, can trap heavy metals like Copper, Lead and Zinc when beads are stirred up into the polluted solution [7]. The most well-known natural polymers are lignin, alginate, cellulose and chitin, generally put directly into the polluted solution [8], [9]. They can be activated or modified to improve their ability to remove heavy metals. For example, chitosan (Fig. 1) which is the deacetylation product of chitin and one of the most interesting polysaccharide in medical sciences because of its antibacterial capacity [10], [11], can remove Cadmium, Cobalt, Copper, Chromium, Iron, Lead, Nickel or Zinc [12], [13], [14]. Its ability to chelate heavy metals is due to the amino and hydroxyl groups [15], [16]. Indeed, chitosan can be up to three times more efficient than chitin to adsorb some heavy metals, which proves the importance of the amino groups [17].
The aim of this study is to functionalize textiles with chitosan in order to decontaminate fluvial or marine wastewater. Geotextiles can be used in this field because they consist in permeable structures that possess filtration and draining capacities. Polypropylene (PP) nonwoven is often used as geotextile because of PP chemical stability. The chosen biopolymer, i.e. chitosan, must be grafted onto the PP nonwoven, without altering its ability to trap heavy metals. The capacity of this new chitosan-grafted PP nonwoven to trap heavy metals is determined thanks to an artificial solution prepared with a model metal: copper.
However, PP is a hydrophobic inert material and it is necessary to make active functional groups on its surface before immobilizing any molecule. For this purpose, irradiation techniques, especially cold plasma treatments, are used to create radicals upon the surface of polypropylene that improve surface wettability. Though, plasma treatment effectiveness decreases more or less rapidly, and furthermore does not generate a sufficient density of functional groups on the fibers surface to allow the direct modification with molecules such as chitosan. Therefore, the solution is the graft polymerization of a functional monomer such as acrylic acid (AA) [18], [19], [20] or N-vinyl-2-pyrrolidone [21] onto the plasma activated surface, a strategy that leads to a coating rich in functional groups available for a further chemical modification. In most cases, this grafting is carried out by immersing the activated samples in a bath of monomer with different time and temperature conditions. This approach presents several drawbacks: slow reaction kinetics, risks of homopolymerization etc. Thus, another grafting pathway has been developed in our research group to avoid these disadvantages [22].
In this study, acrylic acid has been chosen to be grafted onto PP nonwoven [22]. The carboxylic functions brought on the textile were then used to graft chitosan by formation of an amide group. Then, the copper sorption ability of the textile was analyzed using an artificial polluted solution. Kinetics of copper adsorption at different temperatures was carried out to evaluate the influence of temperature on copper chelation. Then, the effect of pH on copper sorption was studied, as well as the NaCl concentration, which allows evaluating the effect of the ionic strength.
Section snippets
Preparation of the sample
Squares (5 * 5 cm2) were cut from polypropylene nonwoven INTN50 (50 g/m2, provided by PGI nonwovens, France). The surface, observed using a 3D digital microscope (Keyence). PP samples were first washed by Soxhlet extractor for 3 h with ethanol and for 3 h with distilled water and then dried under vacuum before use (at room temperature (RT) for 16 h).
Preparation of the contaminated solution
Artificial contaminated water was prepared by diluting 40 mL of CuSO4 at 0.1 M (purchased from Sigma–Aldrich) in distilled water in a 1L-volumetric flask. A
Characterization of the grafting of acrylic acid and chitosan
Textiles were grafted according to the protocol described in the experimental part. Then, some analyses were made to check if the chitosan was grafted on the textile. SEM was carried out on fibers after each step of grafting. Differences between samples can be clearly seen (Fig. 3). If the acrylic acid-grafted-polypropylene sample (PP-g-AA) (Fig. 3B) is compared to the initial PP fiber (Fig. 3A), a homogeneous coating is observed all around the fiber. After weighting the samples, this coating
Conclusion
Chitosan was chemically grafted on acrylic-acid-grafted-polypropylene nonwoven geotextiles in order to trap copper of artificially polluted solutions. Different surface analyses show that chitosan was successfully grafted on PP-g-AA fibers. Sorption of copper succeeded, giving a maximum quantity of 30 mg/g PP at pH = 4.10 and at 20 °C. Langmuir isotherm was proven to be the best model to describe the process of copper sorption by PP-g-AA-chitosan. ΔG0 = −20 kJ/mol at 20 °C, so the adsorption phenomenon
Acknowledgements
The authors gratefully acknowledge the support of FEDER (Fonds Européen de Développement Régional), Nord-Pas-de-Calais region, and FUI (Fonds Unique Interministériel) for funding this work. We also would like to deeply acknowledge the support of the Up-Tex Competitiveness Cluster and all DEPOLTEX partners for helpful collaboration and discussion.
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