Preparation of polyaminated Fe3O4@chitosan core-shell magnetic nanoparticles for efficient adsorption of phosphate in aqueous solutions
Graphical abstract
This scheme shows the adsorption mechanism of polyethylenimine-grafted Fe3O4@chitosan core-shell magnetic particle (Fe3O4/CS/PEI) toward phosphate and the recovery of the used particles under an applied magnetic field. The largest adsorption of phosphate was achieved at an equilibrium pH of 3.0–4.0, where more than 92% of phosphate exists in the H2PO4− form. This is because the free amino and hydroxyl groups on the Fe3O4/CS/PEI surface are protonated to NH3+ (major) and OH2+ (minor) groups under such low pH conditions. In this regard, the Fe3O4/CS/PEI surface has a strong affinity to H2PO4− anions. Moreover, the prepared Fe3O4/CS/PEI nanoparticles could be rapidly separated from the solution environment, which again demonstrated their promising application potential.
Introduction
Phosphorus (P) is ubiquitously present in nature as phosphates and has been extensively used in various electronic and traditional industries, such as agricultural (approximately 82%), medical, and food processing [1]. Phosphate primarily damages the ecosystem because of the abuse of P-containing chemical fertilizers in agricultural cultivation and the inadequate discharge of process or wastewater. In a water environment, phosphate can be detected in rivers, reservoirs, lakes, soil, and groundwater [2], [3]. Excessive phosphates in water bodies can cause eutrophication, which is the major reason for the oxygen-depleting red tides that decimate fish and other aquatic organisms. For humans, on the other hand, P plays an essential role in cellular signaling, bone metabolism, and possible energy metabolism. Approximately 0.1% of the total phosphorus body presents in the circulating P level (i.e., the fraction measurable in clinical practice) [4]. Even within the normal range, an increase in serum P level for patients may occur quite early in the progression of chronic kidney disease. Hence, hyperphosphatemia (P level >55 mg L−1) – the late indicator of phosphorus retention – can increase some potential risks regarding cardiovascular morbidity and mortality in patients undergoing hemodialysis [4]. However, a standard 4-h thrice-weekly prescription of maintenance hemodialysis only partially remove phosphates [5]. Moreover, the currently available P binders – aluminum hydroxide, calcium carbonate, lanthanum carbonate, and calcium acetate – corresponds to neither high costs nor hypercalcemia, which prevents their utilization for a large number of hemodialysis patients [5]. Thus, an effective reduction and control of P level in serum plays an important role in avoiding secondary hyperparathyroidism and soft tissue calcification, at least in the thrice-weekly prescription of hemodialysis, although serum P level will be continuously released for Ca-P balance in the body.
The removal of phosphate from contaminated water has been recently received wide attention from researchers because of the bloom of eutrophication phenomenon around the world. As reported by the USA Environmental Protection Agency [6], the eutrophication in surface waters (i.e., rivers and lakes) might occur when the phosphate concentration exceeds 0.03 mg L−1. This value is remarkably lower than the maximum allowable concentration of phosphate in drinking water (5 mg L−1 of P2O5, equivalent to 2.2 mg L−1 of P) established by the European Communities Regulation (S.I. No. 81/1988). Numerous alternative techniques have been developed to efficiently remove phosphate from water bodies including chemical precipitation [7], electrodialysis [8], electrocoagulation [9], biological treatment [10], microbial remediation [11], membrane separation [12], anion exchange [13], and adsorption [14]. Among them, adsorption has been recognized as a promising and low-cost technique for effective removal of phosphate from water because it can minimize secondary pollution (i.e., sludge) and remove phosphate at low concentrations [15], [16], [17]. Although carbonaceous porous materials (i.e., biochar and activated carbon) have been widely applied for environmental remediation, especially water treatment, their applications in the elimination of phosphate from water might be limited to some extent. This is because they exhibited a relatively low adsorption capacity to phosphates, such as 4.20–10.8 mg g−1 with biochars [16] and 2.77–10.7 mg g−1 with activated carbons [14]. The low adsorption capacity resulted from the negligible role of pore filling in the adsorption mechanism of phosphate onto porous materials [14], [16], [18]. To improve the adsorption capacity of phosphate, some scholars have developed the functionalized or modified adsorbents that exhibited the abundant adsorption sites on their external surface. Such adsorbents include the Fe3O4/ZrO2/chitosan nanocomposites [19], chitosan/Al2O3/Fe3O4 nanofibers and beads [20], cross-linked chitosan beads [21], tetra-amine copper-grafted chitosan bead [22], polyacrylamide-modified magnetite [23], and tetraethylenepentamine-functionalized magnetic Fe3O4 polymers [24], and amine-functionalized epichlorohydrin-grafted cellulose [25].
Chitosan (CS), a typical natural and green biopolymer, is often extracted from the shell wastes of arthropods and crustaceans or produced from the deacetylation of the abundant chitin, an amino-polysaccharide polymer. The hydroxyl (OH) and amino (NH2) groups in CS make it not only effective for the adsorption of dyes and transition metals, but also accessible for chemical modifications — including carboxyalkyl substitution, aldehyde crosslinking, and ligand (e.g., EDTA) crosslinking [26], [27]. More importantly, chemical modifications of CS can inhibit it from spontaneous dissolution in certain acidic solutions (often, pH < 4.0) and extend its application for some biochemical and biomedical fields because CS is biocompatible, biodegradable, and nontoxic to the environment [28], [29]. In principle, the cross-linking of CS with a double functional reagent (i.e., glutaraldehyde) will make CS resistant to bases, acids, and other chemicals; however, this step often results in inevitably reducing free amino groups in CS (acting as potential adsorbing sites). Therefore, the crosslinked CS materials can serve as promising adsorbents to effectively adsorb some negatively charged species such as dyes (methyl orange) [30], organic acids (clofibric acid) [31], and metallic anions [32] in aquatic environments if they are further grafted or modified by suitable amines. For example, we have prepared glutaraldehyde-crosslinked polyethylenimine (PEI)-modified CS beads to remove trace Cu(II)-EDTA chelate anions from aqueous solutions [33]. In this regard, such types of modified CS beads are expected to have a high affinity to phosphates in aqueous solutions because the branched PEI does contain abundant secondary and tertiary amino groups.
The aim of this study was to prepare the branched PEI-grafted Fe3O4@CS core-shell (called as Fe3O4/CS/PEI) magnetic nanoparticles (MNPs) as the promising nano-scaled adsorbents for efficient removal of phosphate from water. The magnetic characteristics of the prepared nanoparticles enable us to easily recover and reuse them and/or to solve the disposal problems of the conventional adsorbents. To our best knowledge, few studies on phosphate adsorption using such MNPs were reported because the nanocomposites functionalized by amine-based polymers (e.g., PEI) are mainly applied for the removal of heavy metals from water due to the mechanism of complexation (coordination) [34], [35]. The physicochemical, structural, and magnetic features of the as-synthesized MNPs were first determined. The equilibrium amounts of phosphate adsorbed on the prepared MNPs were comprehensively measured, and the adsorption isotherms obtained at various equilibrium pH values were modeled. The effects of the concentrations of co-existing anions (i.e., chloride, nitrate, sulfate, and carbonate) on phosphate adsorption were investigated, which will reveal the adsorption affinity of Fe3O4/CS/PEI MNPs toward phosphate. The desorption efficiency and the reusability of the as-prepared adsorbents were finally evaluated.
Section snippets
Chemicals
Analytical reagent-grade inorganic chemicals – FeCl2·4H2O and FeCl3·6H2O, sodium tripolyphosphate (TPP), KH2PO4, CS, and branched PEI (molecular weight 800 g mol−1) – were purchased from Sigma Aldrich. The molecular weight of CS is determined as 2.4 × 105 by using high-performance size exclusion chromatography, and the degree of deacetylation is measured in the range of 75–84 mol%. The ratio of primary, secondary, and tertiary amines in the branched PEI compound is estimated to be 1:2:1,
Textural property
On the basis of the IUPAC definition, the nitrogen physical sorption isotherms of three as-prepared particles (Fig. 1a) were classified as the IV type, with the wide knee (H3-type hysteresis loop) being visible in the isotherms at a relative pressure (P/Po) higher than 0.30. The result indicated the Fe3O4, Fe3O4/CS, and Fe3O4/CS/PEI samples were highly mesoporous materials. An analogous finding was previously reported elsewhere [38], [39]. In essence, an adsorbent with its mesoporous nature is
Conclusions
This work has successfully developed polyethylenimine (PEI)-grafted Fe3O4@chitosan core-shell magnetic nanoparticles (Fe3O4/CS/PEI). Sodium tripolyphosphate was adopted for ionic cross-linking between Fe3O4 and chitosan (CS) and epichlorohydrin was used for activating and grafting between PEI and chitosan. The FTIR and XPS results verified that CS and PEI were attached to the surface of Fe3O4; meanwhile, the XRD patterns indicated that the modification did not change the spinel structure of Fe3O
Acknowledgments
Financial support for this work through a grant from the Chang Gung Medical Foundation, Taiwan (No. CMRPD2H0201) is gratefully appreciated. The authors also thank the Microscope Core Laboratory, Chang Gung Memorial Hospital, Linkou, Taiwan for the TEM measurements of the prepared samples.
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