Synthesis and characterization of magnetic alginate beads for the removal of organic pollutants from effluent by magnetic separation
Synthèse et caractérisation de billes d?alginate magnétiques pour l'élimination de polluants organiques dans les effluents par séparation magnétique.
Résumé
The goal of this work is to prepare and study a magnetic composite material capable of adsorbing organic molecules. This work takes part in the development of new pollution-reduction methods through magnetic separation.
The composite material is synthesised by encapsulating iron oxide nanoparticles and activated carbon particles within alginate beads. Alginate is a natural polysaccharide produced by brown algae that can form a gel in presence of divalent cations such as calcium ions. Iron oxide nanoparticles are used in the form of a stable colloidal suspension called a ferrofluid. The nanoparticles are synthesised by coprecipitating Fe(II) and Fe(III) ions with ammonia. After an oxidation step leading to the formation of maghémite (Γ-Fe2O3), the nanoparticles are functionalised by citrate ions in order to stabilise them in a neutral solution. The inclusion of activated carbon particles in the polymer matrix allows for the efficient removal of organic molecules by adsorption.
Several formulations are used for the preparation of the beads. Different amounts of iron oxide nanoparticles or activated carbon particles are encapsulated in the alginate beads. Variation of the formulation also includes the chemical modification of the polymer matrix: alginate is either reticulated by calcium ions or with a bifunctional organic molecule, epichlorohydrin. Finally, the beads are used directly after their preparation in a humid form or are dried in an oven.
Chemical characterisation of the beads shows that encapsulation of the precursors is total, with no loss of materials with time. Determination of the amounts of calcium and sodium cations within the beads allows for the quantification of negative charges borne by alginate and the citrate-functionalised iron oxide nanoparticles. Physical characterisation includes the study of the porosity of the beads and the determination of their magnetic properties. The beads are found to be superparamagnetic, with characteristic parameters close to those of the ferrofluid precursor: the encapsulation has no impact on the magnetic behaviour of the iron oxide nanoparticles.
Adsorption properties of the beads are evaluated through the adsorption of two organic dyes, negatively-charged methyl orange and positively-charged methylene blue. Adsorption at equilibrium is studied by building adsorption isotherms: these curves give access to the adsorption capacity of the beads, which is compared to the capacity of the precursors, and gives hints of the adsorption mechanisms involved. Both dyes are found to be adsorbed on the encapsulated activated carbon through Van der Walls interactions, whereas methylene blue can also be adsorbed on alginate and on the iron oxide nanoparticles through ion-exchange interactions with sodium and calcium ions. Those two mechanisms may be modelled by independent Langmuir equations. Influence of the formulation of the beads on the adsorption capacity and the adsorption mechanisms is studied. In particular, the amount of activated carbon encapsulated within the beads directly influences the ratio of the adsorption capacities of methylene blue and methyl orange: low amount of encapsulated activated carbons allows the beads to adsorb more methylene blue than methyl orange which gives the beads some selectivity properties. Finally, the influence of external factors such as pH is studied; it is found to impact the ion-exchange reaction, but not adsorption on activated carbon.
Adsorption kinetics is studied for each dye, and shows that equilibrium is reached in the scale of several hours. The variation of the amount of dye adsorbed with time may be modelled with the pseudo-second order equation as well as the intraparticular diffusion model. This highlights the need to control precisely the porosity of the composite material the speed up the adsorption reaction.
Finally, comparison with existing adsorption materials shows that our composite beads have an interesting potential for use in adsorption, with good adsorption capacities, ability to modulate selectivity and additional magnetic properties that may be useful in innovative industrial processes.
The composite material is synthesised by encapsulating iron oxide nanoparticles and activated carbon particles within alginate beads. Alginate is a natural polysaccharide produced by brown algae that can form a gel in presence of divalent cations such as calcium ions. Iron oxide nanoparticles are used in the form of a stable colloidal suspension called a ferrofluid. The nanoparticles are synthesised by coprecipitating Fe(II) and Fe(III) ions with ammonia. After an oxidation step leading to the formation of maghémite (Γ-Fe2O3), the nanoparticles are functionalised by citrate ions in order to stabilise them in a neutral solution. The inclusion of activated carbon particles in the polymer matrix allows for the efficient removal of organic molecules by adsorption.
Several formulations are used for the preparation of the beads. Different amounts of iron oxide nanoparticles or activated carbon particles are encapsulated in the alginate beads. Variation of the formulation also includes the chemical modification of the polymer matrix: alginate is either reticulated by calcium ions or with a bifunctional organic molecule, epichlorohydrin. Finally, the beads are used directly after their preparation in a humid form or are dried in an oven.
Chemical characterisation of the beads shows that encapsulation of the precursors is total, with no loss of materials with time. Determination of the amounts of calcium and sodium cations within the beads allows for the quantification of negative charges borne by alginate and the citrate-functionalised iron oxide nanoparticles. Physical characterisation includes the study of the porosity of the beads and the determination of their magnetic properties. The beads are found to be superparamagnetic, with characteristic parameters close to those of the ferrofluid precursor: the encapsulation has no impact on the magnetic behaviour of the iron oxide nanoparticles.
Adsorption properties of the beads are evaluated through the adsorption of two organic dyes, negatively-charged methyl orange and positively-charged methylene blue. Adsorption at equilibrium is studied by building adsorption isotherms: these curves give access to the adsorption capacity of the beads, which is compared to the capacity of the precursors, and gives hints of the adsorption mechanisms involved. Both dyes are found to be adsorbed on the encapsulated activated carbon through Van der Walls interactions, whereas methylene blue can also be adsorbed on alginate and on the iron oxide nanoparticles through ion-exchange interactions with sodium and calcium ions. Those two mechanisms may be modelled by independent Langmuir equations. Influence of the formulation of the beads on the adsorption capacity and the adsorption mechanisms is studied. In particular, the amount of activated carbon encapsulated within the beads directly influences the ratio of the adsorption capacities of methylene blue and methyl orange: low amount of encapsulated activated carbons allows the beads to adsorb more methylene blue than methyl orange which gives the beads some selectivity properties. Finally, the influence of external factors such as pH is studied; it is found to impact the ion-exchange reaction, but not adsorption on activated carbon.
Adsorption kinetics is studied for each dye, and shows that equilibrium is reached in the scale of several hours. The variation of the amount of dye adsorbed with time may be modelled with the pseudo-second order equation as well as the intraparticular diffusion model. This highlights the need to control precisely the porosity of the composite material the speed up the adsorption reaction.
Finally, comparison with existing adsorption materials shows that our composite beads have an interesting potential for use in adsorption, with good adsorption capacities, ability to modulate selectivity and additional magnetic properties that may be useful in innovative industrial processes.
Ce travail de recherche s'intègre dans un projet de développement d'un procédé de traitement des effluents par séparation magnétique s'inscrivant dans une démarche d'écoconception. Pour ce faire, nous avons préparé des billes d'alginate encapsulant des nanoparticules magnétiques et du charbon actif. Les matériaux composant ces billes possèdent des propriétés adsorbantes permettant l'extraction de polluants ; leurs propriétés magnétiques permettent de les séparer magnétiquement de l'effluent à traiter. La formulation et la caractérisation des billes magnétiques est le point essentiel de ce travail. Différentes formulations ont été testées en modifiant les quantités de matériaux précurseurs ainsi que le mode de réticulation de l'alginate. Les capacités d'adsorption des billes vis-à-vis de deux colorants utilisés comme modèles de polluant organique, le bleu de méthylène et le méthyl orange, ont été étudiées. La modélisation des isothermes d'adsorption a permis également de comprendre les mécanismes mis en jeu. Les cinétiques d'adsorption ont également été étudiées et modélisées.
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