polymers Article
Preparation of Functionalized Magnetic Fe3O4@Au@polydopamine Nanocomposites and Their Application for Copper(II) Removal Yanxia Li *
ID
, Lu Huang, Wenxuan He, Yiting Chen and Benyong Lou
Department of Chemical Engineering and Materials, Ocean College, Minjiang University, Fuzhou 350108, China;
[email protected] (L.H.);
[email protected] (W.H.);
[email protected] (Y.C.);
[email protected] (B.L.) * Correspondence:
[email protected]; Tel.: +86-136-650-34805
Received: 7 April 2018; Accepted: 21 May 2018; Published: 23 May 2018
Abstract: Polydopamine (PDA) displays many striking properties of naturally occurring melanin in optics, electricity, and biocompatibility. Another valuable feature of polydopamine lies in its chemical structure that incorporates many functional groups such as amine, catechol and imine. In this study, a nanocomposite of magnetic Fe3 O4 @Au@polydopamine nanopaticles (Fe3 O4 @Au@ PDA MNPs) was synthesized. Carboxyl functionalized Fe3 O4 @Au nanoparticles (NPs) were successfully embedded in a layer of PDA through dopamine oxypolymerization in alkaline solution. Through the investigation of adsorption behavior to Cu(II), combined with high sensitive electrochemical detection, the as-prepared magnetic nanocomposites (MNPs) have been successfully applied in the separation and analysis of Cu(II). The experimental parameters of temperature, Cu(II) concentration and pH were optimized. Results showed that the as-prepared MNPs can reach saturation adsorption after adsorbing 2 h in neutral environment. Furthermore, the as-prepared MNPs can be easily regenerated by temperature control and exhibits a good selectivity compared to other metal ions. The prepared Fe3 O4 @Au@PDA MNPs are expected to act as a kind of adsorbent for Cu(II) deep removal from contaminated waters. Keywords: magnetic nanocomposites; copper(II); polydopamine; Fe3 O4 @Au nanoparticles
1. Introduction It is important that freshwater be free from toxic chemicals for industry, agriculture and human health [1]. The contamination of toxic heavy metals in aqueous systems brings serious threats to the environment, even though a trace intake of different metals is key for human health [2]. These metals are in the form of ions that interact with proteins, nucleic acids and other biological ligands to form metal proteins, metal enzymes and other biological complexes and also play an important biochemical and physiological role in the life process. These metals might lead to morbidity if the content of metals in the human body is too high or too low [3]. The development of technologies for water purification is critical to meet the global challenges of insufficient water supply and inadequate sanitation. Among all treatments for heavy metals, adsorption is globally recognized as a very attractive technique because of its simplicity, reversibility and economic feasibility [3,4]. Therefore, development of novel materials as adsorbents for removing heavy metals from wastewater has been widely addressed [5,6]. Copper ions play an important role in many areas, such as chemical, biological and environmental fields. However, excessive intake of copper ions produces severe toxicological effects, such as nausea, diarrhea, vomiting, stomach cramps, or even death. Along with the extensive use of copper in industry, copper contamination is an important environmental problem and has attracted more and more attention [7–9]. The World Health Organization recommends the maximum limit of Cu ions in drinking
Polymers 2018, 10, 570; doi:10.3390/polym10060570
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water as 1.5 mg/L, whereas the US EPA defines it as 1.3 mg/L [10,11]. Therefore, effective removal of concentrated copper from industrial and agricultural wastewater is of fundamental importance. There are many reports on Cu(II) removal from wastewater with natural, modified, or composite adsorbents [12–16]. Datta et al. used rioctylamine supported sontmorillonite for adsorptive separation of Cu(II) from an aqueous solution [12]. Dichiara et al. describes the aqueous-phase adsorption of Cu(II) on free-standing hybrid papers comprised of both graphene and single-wall CNTs [13]. These composite materials show great promise for separation and enrichment, environmental administration and wastewater treatments. The intrinsic properties of materials and the physicochemical parameters such as the pH of aqueous solution, initial metal ion concentration, and time can affect the adsorption of copper ions. Environmentally friendly materials with higher adsorption capacity, higher selectivity, and that are more cost-efficient are in urgent need. Recently, various inorganic nanomaterials as carriers, such as iron oxide, carbon nanotubes, metal oxide, etc., have been applied for removal of metal ions through surface modification [4]. Among these nanostructured materials, core-shell magnetic materials have been widely favored by domestic and overseas researchers owing to their outstanding performance [17]. Firstly, these magnetic materials can be easily removed from the system by an external magnetic field. Secondly, they are cheap and easy to synthesize. Finally, they can be functionalized with various chemical species on the particle surface to potentiate the specific affinities to metal ions [15,18,19]. In modern materials science, surface coatings and modifications allow control of the surface properties to confer new functionalities for them. Polydopamine displays many striking properties of naturally occurring melanin in optics, electricity, and magnetics, and biocompatibility. Another valuable feature of polydopamine lies in its chemical structure that incorporates many functional groups such as catechol, amine, and imine. These functional groups can serve as both the starting points for covalent modification with desired molecules and the anchors for the loading of transition metal ions [20]. In fact, previous literature reported that magnetic graphene@polydopamine composites exert excellent adsorption efficiency to Cu(II) and were applied to enrich and identify low concentration standard peptides [21]. This phenomenon indicates that PDA can effectively combine with copper ions. In addition, PDA exhibits potential to immobilize metal ions (e.g., Ti4+ , Fe3+ , Cu2+ ) [22,23]. In the present study, novel magnetic Fe3 O4 @Au@PDA nanocomposites were synthesized. The properties of the composites were investigated in detail. Through the investigation of adsorption behavior to Cu(II), combined with high sensitive electrochemical detection, the prepared Fe3 O4 @Au@PDA MNPs have been successfully applied in the separation and analysis of copper ions. Results demonstrate the great potential of the composites for versatile water purification and treatment. 2. Experimental Section 2.1. Materials CuCl2 , AgNO3 , Mn(NO3 )2 , MgCl2 , NiCl2 ·6H2 O, CdCl2 , Pb(NO3 )2 , FeCl3 ·6H2 O, FeCl2 ·4H2 O, 25–28% NH3 ·H2 O, HCl, NaOH, KCl, sodium citrate, gold chloride tetrahydrate (HAuCl4 ·4H2 O), dopamine hydrochloride (DA·HCl, 98%), mercaptopropionic acid (MPA) of analytical grade were received from Fuzhou Xinyuhua Experimental Instrument Co., Ltd. (Fuzhou, China), and the solvent is deionized water. 2.2. Preparation of Fe3 O4 @Au-COOH NPs The sodium citrate dispersed Fe3 O4 NPs were prepared according to the published literature [24]. Sodium citrate (0.069 g) was then added to 40 mL of dispersed Fe3 O4 NPs under vigorous stirring. A total of 32 mg gold chloride tetrahydrate was added rapidly and continued under reflux for 30 min before being allowed to cool to normal atmospheric temperature. The resulting colloidal solution was dissolved with 1 mol/L HCl to remove unwrapped Fe3 O4 NPs, then isolated in a magnetic field to remove independent Au NPs and washed several times with water until the solution is neutral.
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The resulting Fe3 O4 @Au colloidal solution (Fe3 O4 @Au) in 40 mL deionized water was added to 0.5 mL of MPA stirred for 30 min, and washed several times with deionized water, then dried at 60 ◦ C overnight, producing MPA modified Fe3 O4 @Au NPs (Fe3 O4 @Au-COOH NPs). 2.3. Preparation of Fe3 O4 @Au@PDA MNPs First, 20 mg of dopamine was dissolved in the suspension of Fe3 O4 @Au-COOH NPs (30 mg) and dispersed by ultrasound in a 5 mL buffer solution (10 mmol/L Tris, pH = 8.5). The mixture was shaken at room temperature. After the reaction, the Fe3 O4 @Au@PDA MNPs were collected by magnetic separation and washed with water several times to remove unreacted reagents. Finally, the products were dried at 60 ◦ C for 24 h for further use. 2.4. Adsorption Experiments To investigate the binding capacity, 3 mg of Fe3 O4 @Au@PDA MNPs was incubated with 1.0 mL Cu(II) solution at different concentrations for an optimized time. After separation, the final Cu(II) concentration of the supernatant was determined by cyclic voltammetry and calculated by peak current. The amount of Cu(II) adsorbed by the Fe3 O4 @Au@PDA MNPs was calculated from the following formula [25,26]: (Ci − C f ) × V Q= (1) m where: Q—mass of Cu(II) adsorbed by unit mass of dry particles, mg/g Ci —Cu(II) concentrations of the initial solutions, mg/L Cf —Cu(II) concentrations of the final solutions, mg/L V—total volume of the adsorption mixture, L m—is the mass of the used particles, g To investigate the effect of adsorption, the removal efficiency of ions is another evaluation parameter to study the adsorption properties of Fe3 O4 @Au@PDA MNPs. Removal efficiency of ions was calculated using the following formula [27,28]: η=
Ci − C f × 100 Ci
(2)
where: η—Removal efficiency, % Ci —ion concentration before treatment, mg/L Cf —ion concentration after treatment, mg/L 2.5. Selective Removal of Cu(II) from Water Ag+ , Mn2+ , Mg2+ , Fe3+ , Ni2+ , Cd2+ and Pb2+ were selected as interfering ions. The experimental procedure is in accordance with the above adsorption experiment at 5.0 mmol/L of initial concentration. The copper ion is replaced by other ions. During the experiment, the contents of these metal ions including Cu2+ which were diluted 50-fold were measured by flame atomic absorption spectrometry at WFX-120 (Beijing Rayleigh Analytical Instrument Co., Ltd., Beijing, China). 2.6. Electrochemical Characterization The electrochemical analysis was performed with an electrochemical workstation (CHI 660D, Shanghai, China). A conventional three-electrode system was used, comprising a bare glassy carbon electrode(GCE) as working electrode, an Ag/AgCl electrode as reference electrode, a platinum wire
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2.6. Electrochemical Characterization The electrochemical analysis was performed with an electrochemical workstation (CHI 660D, Polymers 2018, 10, 570 4 of 15 Shanghai, China). A conventional three-electrode system was used, comprising a bare glassy carbon electrode(GCE) as working electrode, an Ag/AgCl electrode as reference electrode, a platinum wire as as auxiliary auxiliary electrode. electrode. The The GCE GCE was was polished polished with with 0.05 0.05 mm mm alumina alumina slurry slurry followed followed by by sonicating sonicating and rinsing with water, then drying at room temperature. Cyclic voltammetry(CV) was carried out and rinsing with water, then drying at room temperature. Cyclic voltammetry(CV) was carried out in in 1-mL Cu(II) solutions (containing 100 μL supernatant, 1 mol/L KCl, pH = 2). 1-mL Cu(II) solutions (containing 100 µL supernatant, 1 mol/L KCl, pH = 2). 3. 3. Results Results and and Discussion Discussion 3.1. Synthetic Synthetic Strategy Strategy of of Magnetic Magnetic Nanocomposites Nanocomposites 3.1. In virtue virtue of of unique unique properties properties such such as as extraordinary extraordinary biocompatibility, biocompatibility, excellent excellent dispersibility dispersibility in in In aqueous phase, phase, etc., etc., surface surface modification modification on on nanoparticles nanoparticles by by PDA PDA has has been been proved proved to to be be an an effective effective aqueous method [29,30]. active groups, especially catechol groups, can interact with metals method [29,30]. PDA PDAwith withabundant abundant active groups, especially catechol groups, can interact with ions through electrostatic, hydrogen bonding interactions or bidentate chelating. Therefore, PDA metals ions through electrostatic, hydrogen bonding interactions or bidentate chelating. Therefore, 4+ 4+,, demonstrates potential applications in immobilization and separation of metal ions (e.g., PDA demonstrates potential applications in immobilization and separation of metal ions Ti (e.g.,, Fe Ti3+ 2+ 2+ 2+ 2+, Pb 2+, Cd Cu3+, Cu , Pb , Cd )2+[21–23,31]. with Fe ) [21–23,31].InInthis thismanuscript, manuscript,we wefound foundaa novel novel magnetic magnetic nanocomposite with PDA that can effectively remove Cu(II). Herein, we prepared PDA-coated carboxyl functionalized PDA that can effectively remove Cu(II). carboxyl functionalized Fe33O O44@Au @AuNPs NPsfor forremoval removalofofCu(II), Cu(II),The Thesynthesis synthesisstrategy strategyisisshown shownin inFigure Figure1. 1.Firstly, Firstly, aa chemical chemical Fe 3+ 3+ coprecipitation of ofFe Fe2+2+and andFeFe under sodium citrate media adopted for the preparation of coprecipitation under sodium citrate media waswas adopted for the preparation of the the The sodium dispersed NPs favoured the formation of a hydrophilic Fe 3OFe 4 NPs. The sodium citratecitrate dispersed Fe3O4Fe NPs the formation of a hydrophilic core3 O4 NPs. 3 O4favoured core-shell @Auwhich NPs which was prepared in reduction situ reduction of chloroauric Secondly, shell Fe3O4Fe @Au was prepared via invia situ of chloroauric acid.acid. Secondly, the 3 O4NPs the purpose of Au addition to Fe O NPs was to protect Fe O in a harsh environment and prevent purpose of Au addition to Fe3O43 NPs was to protect Fe3O34 in 4 4 a harsh environment and prevent oxidation of of Fe(II). Fe(II). The The uncoated uncoated gold gold nanoparticles nanoparticlescan can be be removed removed by by magnetic magnetic field field separation. separation. oxidation Furthermore, Au Au coating coating on on Fe Fe33O O44NPs NPsisisbeneficial beneficialfor forsurface surfacefunctionalization functionalizationby byAu-S Au-Sbonding. bonding. Furthermore, Through MPA functionalization on the surface of Fe O @Au NPs, carboxyl groups can be easily Through functionalization 3 44@Au NPs, carboxyl groups can be easily introduced to interactions with the the amino group of dopamine. Moreover, the surface introduced to form formelectrostatic electrostatic interactions with amino group of dopamine. Moreover, the of Fe3 Oof @Au is easily wrapped with PDA because PDA is easy to deposit on the surface. surface Fe 3 O 4 @Au is easily wrapped with PDA because PDA is easy to deposit on the metal surface. 4 Finally, the thecarboxyl carboxylfunctionalized functionalizedFeFe NPs were easily wrapped by a layer of after PDAbeing after Finally, 3O34O @Au NPs were easily wrapped by a layer of PDA 4 @Au being dispersed in dopamine solution the alkaline environment (10 mmol/L pHwhich = 8.5) dispersed in dopamine solution under under the alkaline environment (10 mmol/L Tris, pHTris, = 8.5) which initiated polymerization of dopamine. layer effectively adsorbCu(II) Cu(II)with with good good initiated polymerization of dopamine. The The PDAPDA layer cancan effectively adsorb selectivity and and reproducibility. reproducibility. selectivity
Figure Figure 1. 1. Synthesis Synthesisroute route of of magnetic magnetic nanocomposites. nanocomposites.
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3.2. Characterization of MNPs The morphologies, structures, components and other physicochemical properties of the Fe3 O4 @Au@PDA MNPs were characterized by various techniques. The morphologies and structures of the MNPs were characterized by field emission transmission electron microscope (TEM, Tecnai F30 G2 300 KV, Hillsboro, OR, USA). The TEM images of the MNPs are shown in Figure 2A,B to investigate morphological structures. As we can see from the figures, it is obvious that all of particles are nano-sized and roughly spherical in shape. Figure 2C shows that the diameter range of the Fe3 O4 NPs is about 1.5~6.4 nm and the average diameter is 3.2 nm. After being wrapped by PDA, Figure 2B,D shows an average diameter about 29.1 nm of Fe3 O4 @Au@PDA MNPs with the range of 21.2~39.7 nm. From Figure 2B, the layer of PDA was visible, and no free Fe3 O4 or Fe3 O4 @Au-COOH NPs were observed which indicated that magnetite NPs were successfully wrapped by PDA. Fourier transform infrared spectroscopy (FT-IR, Nicolet, Madison, WI, USA) was employed to characterize the preparation procedure of Fe3 O4 @Au@PDA MNPs. As shown in Figure 3A, FT-IR spectra of Fe3 O4 NPs (a), Fe3 O4 @Au NPs (b), Fe3 O4 @Au-COOH NPs (c), Fe3 O4 @Au@PDA MNPs (d), were compared. The peaks at 455 and 670 cm−1 in curves a–b were related to the Fe–O group, and the peak around 3400 cm−1 was assigned to the –OH vibrations on the surface of Fe3 O4 NPs. Characteristic absorption peaks of Fe3 O4 NPs decreased significantly in curve c and d, which means the surface of Fe3 O4 @Au NPs was modified. The absorption band at 1698 cm−1 corresponds to the carbonyl group of functionalized carboxyl groups (curve c and d). The absorption bands of 1430 to 1398 cm−1 related to –CH2 bending vibration. After being coated with PDA, many new infrared absorption peaks are generated in curve d. The broad and weak absorption bands near 3030 cm−1 originated from the benzene ring in PDA. The absorption peak at 1241 cm−1 contain a C-N stretching vibration. An Ar−H bending vibration (807 and 644 cm−1 ) was assigned to 1,2,4-substitued aromatic compounds. These results confirmed that Fe3 O4 @Au NPs had been successfully encapsulated by PDA via in situ oxidative polymerization. Thermo-gravimetric analysis (TGA, STA 449C Netzsch, Bavaria, Germany) was used to determine the relative composition of the Fe3 O4 @Au@PDA MNPs. TGA was performed using dry powder samples with a heating rate of 10 ◦ C/min up to 600 ◦ C under a nitrogen atmosphere. As we can see from Figure 3B, the weight loss of Fe3 O4 (curve a) and Fe3 O4 @Au NPs (curve b) from 100 to 600 ◦ C was about 26% and 14%, respectively, which may be due to the loss of water and citrate ions on nanomaterial surface. Au coated on the Fe3 O4 NPs leads to less weight loss. However, after being coated with PDA, Fe3 O4 @Au@PDA MNPs (curve c) show higher weight loss than Fe3 O4 NPs and Fe3 O4 @Au NPs, which were about 36% mass percent. The result indicates that the content of the PDA coating was about 22% which further supported that PDA successfully wrapped the Fe3 O4 @Au NPs. The structure of magnetic nanomaterials was also characterized by X-ray diffraction (XRD, MiniFlex 600, Tokyo, Japan) as shown in Figure 4. No obvious diffraction peak in Figure 3a indicated the crystal was not produced which may be due to the low temperature of the nanomaterial treatment. After being wrapped by Au, a series of obvious diffraction peaks appeared. The peaks at 38.20, 44.31, 64.63, 77.64 and 81.85 in Figure 3b–d are ascribed to (111), (200), (220), (311) and (222) reflections of the Au face-centered cubic crystallographic structure (JCPDS card No. 65-2870). All the patterns illustrate that Au has been successfully loaded onto the Fe3 O4 NPs. No change in the peak positions of Fe3 O4 , Fe3 O4 @Au NPs and Fe3 O4 @Au@PDA MNPs indicated that the surface modification has no effect on the Au crystal form. Magnetization was detected with a LDJ9600 vibrating sample magnetometer (VSM, Troy, MI, USA) at ambient temperature. The vibrating sample magnetization (VSM) curves of Fe3 O4 NPs, Fe3 O4 @Au NPs, Fe3 O4 @Au-COOH NPs, Fe3 O4 @Au@PDA MNPs are shown in Figure 4B. The superpara-magnetism nature of these materials can be proved by the hysteresis loops. All these materials have obvious magnetism. After wrapping with Au, the magnetism of Fe3 O4 @Au NPs decreased slightly. After cross-linking with COOH and PDA step by step, the saturation magnetization
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of Fe3 O4 @Au-COOH NPs, Fe3 O4 @Au@PDA MNPs decreases accordingly. However, it is still strong enough for magnetic isolation. Polymers 2017, 9, x FOR PEER REVIEW 6 of 14
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3O NPs(A);Fe Fe O @Au@PDAMNPs MNPs(B); andparticle particlesize sizedistribution distribution Figure2.2.2.TEM TEMimages imagesof Fe Figure TEM images ofofFe Fe O NPs(A); Fe (B);and and particle size distribution 33O 444NPs(A); 33 O3O 4@Au@PDA Figure 44@Au@PDA 3 O 4 NPs (C) with total 33 particles; Fe 3 O 4 MNPs total 50 particlesby by diagram of Fe diagram of Fe O NPs (C) with total 33 particles; Fe O @Au@PDA (D) with total 50 particles by MNPs (D) with total 50 particles diagram of Fe3O 3 4 4NPs (C) with total 33 particles; Fe3O 3 4@Au@PDA 4 Nano Measurer 1.2.5. The y-axis % represents the proportion of nanoparticles in a certain size range. NanoMeasurer Measurer1.2.5. 1.2.5.The They-axis y-axis%%represents representsthe theproportion proportionof ofnanoparticles nanoparticlesin inaacertain certainsize sizerange. range. Nano
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Figure 3. FTIRspectra spectra (A)ofofFeFe 3O4 NPs(a), (a), Fe 3O4@AuNPs NPs(b), (b),FeFe 3O 4@Au-COOHNPs NPs(c), (c), Figure 3O 4@Au-COOH Figure 3.3. FTIR FTIR spectra(A) (A) of Fe33OO4 4 NPs NPs (a), Fe Fe33OO4@Au 4 @Au NPs (b), Fe3 O4 @Au-COOH NPs (c), Fe 3 O 4 @Au@PDA MNPs (d), TGA curves (B) of Fe 3 O 4 NPs (a), Fe 3 O 4 @Au NPs (b), Fe 3 O 4 @Au@PDA Fe 4@Au@PDA MNPs (d), TGA curves (B) of Fe3O4 NPs (a), Fe3O4@Au NPs (b), Fe3O4@Au@PDA Fe3O 3 O4 @Au@PDA MNPs (d), TGA curves (B) of Fe3 O4 NPs (a), Fe3 O4 @Au NPs (b), Fe3 O4 @Au@PDA MNPs(c). (c). MNPs MNPs (c).
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(A) (B) Figure 4. XRD spectra (A) and magnetization hysteresis loops (B) of the magnetic nanomaterials. Figure XRD spectra (A)(b), and hysteresis of the magnetic (a),4.Fe 3O4@Au NPs Femagnetization 3O4@Au-COOH NPs (c),loops Fe3O(B) 4@Au@PDA MNPsnanomaterials. (d). Fe3O4 NPs Figure 4. XRD spectra (A) and magnetization hysteresis loops (B) of the magnetic nanomaterials. Fe3O4 NPs (a), Fe3O4@Au NPs (b), Fe3O4@Au-COOH NPs (c), Fe3O4@Au@PDA MNPs (d). Fe3 O4 NPs (a), Fe3 O4 @Au NPs (b), Fe3 O4 @Au-COOH NPs (c), Fe3 O4 @Au@PDA MNPs (d).
3.3. Electrochemical Detection of Cu(II)
3.3. Electrochemical Detection of Cu(II) 3.3. Electrochemical Detection oftoCu(II) There are many methods detect copper ions, such as fluorescent probes, atomic adsorption There are many methods to detect copper ions, such as fluorescent probes, atomic adsorption spectrophotometry, electrochemistry, etc.copper Among these due probes, to highatomic sensitivity and easy There are many methods to detect ions, suchmethods, as fluorescent adsorption spectrophotometry, electrochemistry, etc. Among these methods, due to high sensitivity and easy spectrophotometry, electrochemistry, etc. Among these methods, due previous to high sensitivity andconditions easy operation, electrochemical analysis is the best analytic method. In our study [32], operation, electrochemical analysis is the best analytic method. In our previous study [32], conditions operation, electrochemical analysis is the best analytic method. In our previous study [32], conditions of electrochemical detection to Cu(II) were optimized. peakcurrent currentintensity intensity Cu(II) of electrochemical detection to Cu(II) were optimized.We Wefound foundthat that the the peak of of Cu(II) of electrochemical detection to Cu(II) wereinoptimized. We found to that the peak current intensity of is largely affected by the electrolytes and pH solution. According the literature [32], a Cu(II) solution is largely affected by the electrolytes and pH in solution. According to the literature [32], a Cu(II) solution Cu(II) is0.01 largely affected by the1.0 electrolytes and pH scanned in solution. According to the literature(CV) [32], ausing Cu(II)bare containing HCl HCl and KCl was containingmol/L 0.01 mol/L and mol/L 1.0 mol/L KCl was scannedby bycyclic cyclic voltammograms voltammograms (CV) using bare solution containing 0.01 mol/L HCl and 1.0 mol/L KCl was scanned by cyclic voltammograms (CV) glassyglassy carbon electrode withwith highhigh signal response. the CV CVofof5.0 5.0mmol/L mmol/L Cu(II) carbon electrode signal response.Figure Figure 55 shows shows the Cu(II) using bare glassy carbon electrode with high signal response. Figure 5 shows the CV of 5.0 mmol/L solution containing mol/L mol/L KCl.Two Twopairs pairs of irreversible at 0.25 solution containing 0.01 0.01 mol/L HClHCl andand 1.01.0 mol/L KCl. irreversibleredox redoxpeaks peaks at 0.25 Cu(II) solution containing 0.01 mol/L HCl and 1.0 mol/L KCl. Two pairs of irreversible redox peaks at and −0.15 canclearly be clearly from Figure Consideringthe the convenience convenience ofofdetection, thethe peak and 0.25 −0.15 V canVbe seenseen from Figure 5. 5. Considering detection, peak and −0.15 V can be clearly seen from Figure 5. Considering the convenience of detection, the peak potential at −0.15 V was selected for quantitative analysis. potential at −0.15 V was selected forfor quantitative potential at −0.15 V was selected quantitativeanalysis. analysis. 25
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Potential /V and 1.0 mol/L KCl, scan rate: 100 mV·s−1. Figure 5. CV of 5.0 mmol/L Cu(II) in 0.1 mol/L HCl
Figure CV of5.0 5.0mmol/L mmol/L Cu(II) in in 0.1 0.1 mol/L HCl scan rate: 100100 mVmV·s ·s−1 . −1. 5. 5. CV Cu(II) mol/L HCland and1.0 1.0mol/L mol/LKCl, KCl, scan rate: 3.4.Figure Interaction ofofMagnetic Nanomaterials with Cu(II)
Nanomaterials may interact with molecules due to their nano-size and large surface-to-mass ratio. The adsorption properties of nanomaterials are highly affected not only by the weak Nanomaterials may interact with molecules due to and large surface-to-mass intermolecular interaction, such as molecules hydrophobic interactions, electrostatic interactions, hydrogen Nanomaterials may interact with due totheir theirnano-size nano-size and large surface-to-mass ratio. The adsorption properties of nanomaterials are highly affected not only by the weak van der waals, but alsoofbynanomaterials the intrinsic characteristics charge,not size,only shape, ratio. bonding, The adsorption properties are highly(e.g., affected byelectronic the weak intermolecular interaction, suchsurface as hydrophobic interactions, electrostatic interactions, hydrogen states, crystallinity, coatings, modifications with active groups, surface wrapping in the intermolecular interaction, such as hydrophobic interactions, electrostatic interactions, hydrogen bonding, van der waals, but also by theand intrinsic characteristics (e.g., charge, size, shape, biological medium, hydrophobicity, hydrophilicity). Therefore, we investigated theelectronic adsorption 3.4. Interaction of Magnetic Nanomaterialswith with Cu(II) Cu(II) 3.4. Interaction of Magnetic Nanomaterials
bonding, van der waals, but also by the intrinsic characteristics (e.g., charge, size, shape, electronic properties of Cu(II) on several kinds of magnetic nanomaterials. As shown in Figure 6, all the states, crystallinity, coatings, surface modifications with active groups, surface wrapping in the nanomaterials have a certain adsorption mass to Cu(II), but the Fe3O4@Au@PDA MNPs exhibit a biological medium, hydrophobicity, and hydrophilicity). Therefore, we investigated the adsorption properties of Cu(II) on several kinds of magnetic nanomaterials. As shown in Figure 6, all the nanomaterials have a certain adsorption mass to Cu(II), but the Fe3O4@Au@PDA MNPs exhibit a
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states, crystallinity, coatings, surface modifications with active groups, surface wrapping in the biological medium, hydrophobicity, and hydrophilicity). Therefore, we investigated the adsorption properties on REVIEW several kinds of magnetic nanomaterials. As shown in Figure 6, all Polymers 2017, of 9, xCu(II) FOR PEER 8 ofthe 14 nanomaterials have a certain adsorption mass to Cu(II), but the Fe3 O4 @Au@PDA MNPs exhibit distinct adsorption a distinct adsorptioncapacity capacitycompared comparedtotoother other magnetic magnetic nanomaterials, nanomaterials, which which indicates indicates that Fe33O O44@Au@PDA @Au@PDAMNPs MNPshave haveaaspecific specificinteraction interactionwith withCu(II). Cu(II). 40 35
Q mg/g
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Fe3O4@Au-COOH Fe3O4@Au@PDA
i= Figure magnetic nanomaterials. V =V1.0 m =m 3.0=mg, Figure 6. 6. Adsorption Adsorptionmass massofofCu(II) Cu(II)onondifferent different magnetic nanomaterials. = mL, 1.0 mL, 3.0 C mg, 10.0 mmol/L, time 2 h, temperature RT. Ci = 10.0 mmol/L, time 2 h, temperature RT.
3.5. Effects Effects of of DA DA Polymerization Polymerization Time Time on Cu(II) Removal Dopamine aqueous solution, it can be oxidized by Dopamine is is aa kind kindofofbiological biologicalneurotransmitter. neurotransmitter.In In aqueous solution, it can be oxidized dissolved oxygen andand undergoes an oxidation crosslinking reaction, forming a composite layer of by dissolved oxygen undergoes an oxidation crosslinking reaction, forming a composite layer PDA thatthat strongly attaches to atosubstrate. The The PDAPDA layerlayer contains abundant catechol groups. The of PDA strongly attaches a substrate. contains abundant catechol groups. adsorption massmass change withwith DA polymerization timetime was was calculated (Figure 7A) 7A) to investigate the The adsorption change DA polymerization calculated (Figure to investigate adsorption effect of Cu(II). As As shown in in Figure 7A, ananincrease the adsorption effect of Cu(II). shown Figure 7A, increaseininDA DApolymerization polymerization time time brings brings about a significant increase in adsorption capacity which reaches a threshold corresponding to best the about a significant increase in adsorption capacity which reaches a threshold corresponding to the best adsorption condition when the DA polymerization h. Therefore, h was selected as adsorption condition when the DA polymerization timetime is 12ish.12Therefore, 12 h12 was selected as the the optimized polymerization time. optimized polymerization time. Effects of of Temperature Temperature on Cu(II) Adsorption 3.6. Effects Metal ion ion absorption absorption is is often often influenced influenced by by temperature. temperature. To To investigate investigate the the influence influence of Metal temperature, the adsorption behavior was examined in an aqueous aqueous medium medium at at different different temperatures. temperatures. Figure 5B 5Bpresents presents effect of temperature the adsorption of Fe@PDA @PDA Figure thethe effect of temperature on theon adsorption capacitycapacity of Fe3O4@Au MNPs. As 3 O4 @Au MNPs. As the temperature increased 15adsorption to 100 ◦ C,capacity the adsorption capacity first decreased the temperature increased from 15 to 100from °C, the first decreased dramatically, then ◦ C, and lost the adsorption performance completely by dramatically, then declined overthe 25 adsorption declined slowly over 25 °C,slowly and lost performance completely by 80 °C. Results ◦ 80 C. Results indicated that the high adsorption of Fe on Cu(II) at indicated that the high adsorption capacity of Fe3O4capacity @Au@PDA MNPs on Cu(II) atMNPs a low temperature 3 O4 @Au@PDA ◦C a low is duenature to the of exothermic naturereaction of the adsorption reaction [33]. Therefore, 15the is due temperature to the exothermic the adsorption [33]. Therefore, 15 °C was selected as was selected as thetemperature. optimal adsorption temperature. At the same adsorption of Cu(II) optimal adsorption At the same time, the adsorption of time, Cu(II)the decreased dramatically decreased dramatically from Fe O @Au@PDA MNPs, indicating that it is a physical interaction at from Fe3O4@Au@PDA MNPs, indicating that it is a physical interaction at low temperature. 3 4 low temperature. 3.7. Effects of pH on Cu(II) Adsorption 3.7. Effects of pH on Cu(II) Adsorption Solution pH is another important factor affecting the adsorption characteristics of the adsorbents Solution pH is charges another being important factor affecting by thethe adsorption adsorbents due to the surface largely influenced solutioncharacteristics environment.ofTothe evaluate the due to the surface charges being largely influenced by the solution environment. To evaluate effect effect of pH values on Cu(II) adsorption to Fe3O4@Au@PDA MNPs, we conducted the a set of of pH values on Cu(II) adsorption to Fe O @Au@PDA MNPs, we conducted a set of experiments in 3 4 experiments in different pH solutions containing the same initial concentrations of the 1.0 mmol/L Cu(II) solution. Considering the stability of Cu(II) ion in acidic condition, pH values were adjusted from 4.0 to 7.0 (Figure 7C). From the result, it can be found that with increasing pH values, the surface charges of Fe3O4@Au@PDA MNPs became more negative, and the adsorption capacities of Cu(II) dramatically increased in the range of pH values from 4.0 to 7.0. When pH exceeded 7.0, with pH increasing, metal oxide is gradually formed, and produces precipitate. In this condition, the removal
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different pH solutions containing the same initial concentrations of the 1.0 mmol/L Cu(II) solution. Considering the stability of Cu(II) ion in acidic condition, pH values were adjusted from 4.0 to 7.0 (Figure 7C). From the result, it can be found that with increasing pH values, the surface charges of Fe3 O4 @Au@PDA MNPs became more negative, and the adsorption capacities of Cu(II) dramatically increased in9,the range ofREVIEW pH values from 4.0 to 7.0. When pH exceeded 7.0, with pH increasing, metal Polymers 2017, x FOR PEER 9 of 14 oxide is gradually formed, and produces precipitate. In this condition, the removal mechanism of metal ions will become and it will beand difficult thebetween adsorption mechanism of metal ionscomplicated, will become complicated, it willtobedistinguish difficult to between distinguish the and precipitation of metal ions. Therefore, 7.0 was selected asselected the optimized solution pH. Generally, adsorption and precipitation of metal ions. Therefore, 7.0 was as the optimized solution pH. materials for removing require anrequire environment of pH > 7;ofthe to needs be adjusted Generally, materials for copper removing copper an environment pHpH > 7;needs the pH to be repeatedly, and a waste alkali. pH of 7.0 can simplify the Cu(II) adsorption process and be conducive adjusted repeatedly, and a waste alkali. pH of 7.0 can simplify the Cu(II) adsorption process and be to industrial conducive to application. industrial application. 9
10
8
8
8
5 4 3
Q mg/g
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6
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Q m g/g
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4 2
2 1
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polymerization time /h
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T / oC
(B)
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6
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pH
(C)
Figure 7. Adsorption mass of Cu(II) changed with DA polymerization time (A); temperature (B); pH Figure 7. Adsorption mass of Cu(II) changed with DA polymerization time (A); temperature (B); (C). V = 1.0 mL, m = 3.0 mg, Ci = 1.0 mmol/L, time 2 h. pH (C). V = 1.0 mL, m = 3.0 mg, Ci = 1.0 mmol/L, time 2 h.
3.8. Adsorption Kinetic Studies 3.8. Adsorption Kinetic Studies Adsorption kinetics describe the solute uptake rate which in turn controls the residence time of Adsorption kinetics describe the solute uptake rate which in turn controls the residence time of adsorbate uptake at the solid-solution interface. Therefore, the kinetics can provide valuable insights adsorbate uptake at the solid-solution interface. Therefore, the kinetics can provide valuable insights into the mechanism and reaction pathway of adsorption process [34,35]. To gain further insight into into the mechanism and reaction pathway of adsorption process [34,35]. To gain further insight into the adsorption mechanism of MNPs, adsorption kinetics were investigated. The adsorption tests are the adsorption mechanism of MNPs, adsorption kinetics were investigated. The adsorption tests are carried out in a Cu(II) solution with 1.0 mmol/L at pH 7.0. The effect of contact time on the adsorption carried out in a Cu(II) solution with 1.0 mmol/L at pH 7.0. The effect of contact time on the adsorption of the Fe3O4@Au@ PDA MNPs for Cu(II) ions is shown in Figure 8A. As can be seen, the adsorption of the Fe3 O4 @Au@ PDA MNPs for Cu(II) ions is shown in Figure 8A. As can be seen, the adsorption occurs rapidly in the first 30 min, and then the adsorption rate slows down. Finally, the adsorption occurs rapidly in the first 30 min, and then the adsorption rate slows down. Finally, the adsorption capacity achieves a state of equilibrium after 2 h. It is found that the adsorption capacity of the capacity achieves a state of equilibrium after 2 h. It is found that the adsorption capacity of the Fe3O4@Au@ PDA MNPs reaches 7.90 mg/g. Fe3 O4 @Au@ PDA MNPs reaches 7.90 mg/g. The pseudo-first order and the pseudo-second order kinetic models are used to simulate the The pseudo-first order and the pseudo-second order kinetic models are used to simulate the adsorption kinetics of the Fe3O4@Au@ PDA MNPs for Cu(II) ions. These two rate equations are shown adsorption kinetics of the Fe3 O4 @Au@ PDA MNPs for Cu(II) ions. These two rate equations are shown below. The pseudo-first order kinetic model suggested by Lagergren for the adsorption of below. The pseudo-first order kinetic model suggested by Lagergren for the adsorption of solid/liquid solid/liquid systems can be expressed as [36]: systems can be expressed as [36]: (3) qt =qtq=e q[1e [1 −−e(e−(−kk11t)] (3) Ho be expressed expressed as: as: Ho and and McKay’s McKay’s pseudo-second pseudo-second order order kinetic kinetic model model can can be tt tt 11 = + qq qqe e kk2 q2eq2e 2 tt
(4) (4)
where k11 is the Lagergren rate rate constant constant of of adsorption adsorption (min (min−−11),), kk22 is is the the pseudo-second-order pseudo-second-order rate rate − 1 − 1 −1 −1 constant adsorption (g (g·mg adsorbed (mg (mg·g constant of adsorption ·mg ·min arethe the amounts amounts of of Cu(II) adsorbed ·g−−11)) at at ·min ). ).qe qand e andqtqtare equilibrium equilibrium and and at at time time t,t, respectively. The values of k11, k22 and and the the correlation correlation coefficient coefficient (R) (R) can can be be determined determined experimentally by plotting qtt versus versus tt and and t/q t/qtt versus versus t,t, respectively. respectively. A plot (Figure 8B) of qt versus t according to the pseudo-first-order kinetic model gives a fitting curve in the initial 120 min. The correspondence with the pseudo-first-order kinetic model substantiates that Cu(II) adsorption onto the Fe3O4@Au@ PDA MNPs is a diffusion-based process. However, the pseudo-second-order kinetic model (Figure 8C) is suitable for the whole adsorption process, which indicates that the adsorption of Cu(II) onto the Fe3O4@Au@ PDA MNPs is controlled by chemical adsorption. The kinetic parameters of the Fe3O4@Au@ PDA MNPs calculated from
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A plot (Figure 8B) of qt versus t according to the pseudo-first-order kinetic model gives a fitting curve in the initial 120 min. The correspondence with the pseudo-first-order kinetic model substantiates that Cu(II) adsorption onto the Fe3 O4 @Au@ PDA MNPs is a diffusion-based process. However, the pseudo-second-order kinetic model (Figure 8C) is suitable for the whole adsorption process, which indicates that the adsorption of Cu(II) onto the Fe3 O4 @Au@ PDA MNPs is controlled by chemical adsorption. The kinetic parameters of the Fe3 O4 @Au@ PDA MNPs calculated from Equations (3) and (4) are listed in Table 1, which shows that the value of R (R > 0.99) was high, suggesting that both models Polymers 2017, 9, x FOR PEER REVIEWare well fitted to the experimental results. Therefore, the adsorption 10 of 14 process can essentially be divided into two steps. The first step is mass transfer through a water film to thethe adsorbent surface (film diffusion) in the initial 80 80 min; thethe second oneone is occupation at aatsite on the to adsorbent surface (film diffusion) in the initial min; second is occupation a site on the surface through chemical adsorption 80 min surface through chemical adsorption overover 80 min [37].[37]. 9
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Figure 8. Effect of time (A), pseudo-first-order (B) and pseudo-second-order (C) kinetic models for Figure 8. Effect of time (A), pseudo-first-order (B) and pseudo-second-order (C) kinetic models for Cu(II) adsorption. V = 1.0 mL, m = 3.0 mg, Ci = 1.0 mmol/L, temperature 15 °C,◦ pH 7.0, time 2 h. Cu(II) adsorption. V = 1.0 mL, m = 3.0 mg, Ci = 1.0 mmol/L, temperature 15 C, pH 7.0, time 2 h. Table 1 Adsorption kinetic parameters for Cu(II) adsorption on MNPs. Table 1. Adsorption kinetic parameters for Cu(II) adsorption on MNPs.
Qe, exp Pseudo-first-order Pseudo-second-order Pseudo-first-order Pseudo-second-order
Qe , exp
7.90
7.90
Qe, cal
k
R
k0.044
Qe , 8.01 cal
R 0.99942
0.0074 0.044 0.0074
8.76 8.01 8.76
0.99839 0.99942 0.99839
3.9. Adsorption Isotherms 3.9. Adsorption Isotherms Adsorption isotherms describe how the process of adsorption proceeds on the adsorbent surface [38]. The adsorption isothermdescribe experiments theprocess prepared were carried out on at different initial Adsorption isotherms howfor the of MNPs adsorption proceeds the adsorbent concentrations ofadsorption Cu(II), ranging fromexperiments 0 to 20 mmol/L. Asprepared shown in Figure 9A,carried it was out observed that surface [38]. The isotherm for the MNPs were at different the adsorption amount of Cu(II) ions on MNPs rapidly increased with the increase of Cu(II) initial concentrations of Cu(II), ranging from 0 to 20 mmol/L. As shown in Figure 9A, it was observed concentration from 0amount to 10 mmol/L, and reached equilibrium 10 mmol/L. Inincrease this case,ofaCu(II) high that the adsorption of Cu(II) ions on MNPs rapidly over increased with the saturated adsorption of 37.86 and mg/greached was obtained when the Cu(II) concentration 10 mmol/L. concentration from 0capacity to 10 mmol/L, equilibrium over 10 mmol/L. In thisiscase, a high As shown adsorption in Figure 9B, the removal ofobtained Cu(II) reached 100% when the Cu(II) concentration saturated capacity of 37.86efficiency mg/g was when the Cu(II) concentration is 10 mmol/L. was belowin 0.05 mmol/L, which meets the standard forreached purified100% drinking and concentration then rapidly As shown Figure 9B, the removal efficiency of Cu(II) whenwater, the Cu(II) decreased increasewhich in themeets Cu(II)the concentration. thedrinking concentration 1.0 mmol/L, the was belowwith 0.05the mmol/L, standard forWhen purified water,isand then rapidly removal rate canthestill reach in 36%, good removal efficiency for Cu(II) in mmol/L, a water decreased with increase the which Cu(II) shows concentration. When the concentration is 1.0 environment. An increase the removal rate shows for high concentrations of Cu(II)for canCu(II) be achieved by the removal rate can still in reach 36%, which good removal efficiency in a water increasing the amount of adsorbent and with the repeated use of adsorbents. environment. An increase in the removal rate for high concentrations of Cu(II) can be achieved by increasing the amount of adsorbent and with the repeated use of adsorbents. 100
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As shown in Figure 9B, the removal efficiency of Cu(II) reached 100% when the Cu(II) concentration was below 0.05 mmol/L, which meets the standard for purified drinking water, and then rapidly decreased with the increase in the Cu(II) concentration. When the concentration is 1.0 mmol/L, the removal rate can still reach 36%, which shows good removal efficiency for Cu(II) in a water environment. An increase in the removal rate for high concentrations of Cu(II) can be achieved Polymers 2018, 10, 570 11 of by 15 increasing the amount of adsorbent and with the repeated use of adsorbents. 100
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(A)
(B)
Figure 9. Adsorption isotherms (A) and removal rate (B) of Cu(II) on the MNPs. Adsorption conditions: V = 1.0 mL, m = 3.0 mg, Ci = 0–20 mmol/L, time 2 h, temperature 15 °C, pH 7.0. V = 1.0 mL, m = 3.0 mg, Ci = 0–20 mmol/L, time 2 h, temperature 15 ◦ C, pH 7.0.
Polymers 2017,9.9,Adsorption x FOR PEER isotherms REVIEW (A) and removal rate (B) of Cu(II) on the MNPs. Adsorption conditions: 11 of 14 Figure
The theoretical adsorption capacity of MNPs can be described by Langmuir and Freundlich equations. The Langmuir modelcapacity is a model assumes of a finite number of The theoretical adsorption of that MNPs can bemonolayer described coverage by Langmuir and Freundlich identical sites on the surface that noassumes further adsorption placeof [39]. The Freundlich equations. Thepresent Langmuir model is asuch model that monolayertakes coverage a finite number of model describes non-ideal and reversible adsorption, not limited to monolayer formation. It can be identical sites present on the surface such that no further adsorption takes place [39]. The Freundlich applieddescribes to multilayer adsorption, with non-uniform distribution adsorptionformation. heat and It affinities model non-ideal and reversible adsorption, not limited toofmonolayer can be over a heterogeneous surface [40]. The Langmuir and Freundlich equations are expressed as follows: applied to multilayer adsorption, with non-uniform distribution of adsorption heat and affinities over Langmuir equations: a heterogeneous surface [40]. The Langmuir and Freundlich equations are expressed as follows: ce
Langmuir equations:
ce
1
(5) (5)
ce q e ce q m q1m kL = + qe qm qm k L
Freundlich equation: Freundlich equation:
Ince InkF+ Ince (6) (6) Inqe Inq = eInk F nn where ccee (mg/mL) (mg/mL) is where is the the equilibrium equilibrium concentration of Cu(II) ions, qee (mg/g) (mg/g)isisthe theadsorption adsorptioncapacity, capacity, qm m (mg/g) is the theoretical saturation adsorption capacity, k L is the Langmuir constant, the (mg/g) is the theoretical saturation adsorption the Langmuir constant, kkFF is the L energy constant constant and and nn is is the the Freundlich Freundlich constant. constant. binding energy The linear fitting curves of the Langmuir and Freundlich models are shown in Figure 10A,B, respectively. As we we can can see, see,according accordingtotothe thevalues valuesofofcorrelation correlationcoefficients coefficients(rLangmuir (rLangmuir = 0.95678, respectively. Freundlich = = 0.90058), rFreundlich 0.90058),the theLangmuir Langmuirmodel modelgave gaveaabetter betterfit, fit,indicating indicatingthat that the the adsorption adsorption of Cu(II) ions MNPs is is homogeneous homogeneous adsorption adsorption on on the the surface surface of of MNPs. MNPs. on MNPs 50
35
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Inqe
Ce(mg/L)/qe(mg/g)
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(A)
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1400
-1
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InCe
(B)
Figure 10. The linear fitting curves of the Langmuir (A) and Freundlich (B) models. Figure 10. The linear fitting curves of the Langmuir (A) and Freundlich (B) models.
3.10. Selective Adsorption of Cu(II) Ions Selectivity is another index to evaluate the performance of an adsorbent. In general, the metalion sorbents have a good adsorption capacity for a certain kind of ions, such as heavy metal ions [41], (Cu2+, Ag+, and Hg2+) [42], (Cr5+ and Cu2+) [43], but the selective adsorption of copper ion has not discussed in depth. Chouyyok et al. reported that a kind of nanoporous sorbent functionalized with chelating diamines had excellent selectivity for Cu2+ over other metal ions (e.g., Ca2+, Fe2+, Ni2+, and
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3.10. Selective Adsorption of Cu(II) Ions
40
40
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35
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30
Q mg/g
Q mg/g
Selectivity is another index to evaluate the performance of an adsorbent. In general, the metal-ion sorbents have a good adsorption capacity for a certain kind of ions, such as heavy metal ions [41], (Cu2+ , Ag+ , and Hg2+ ) [42], (Cr5+ and Cu2+ ) [43], but the selective adsorption of copper ion has not discussed in depth. Chouyyok et al. reported that a kind of nanoporous sorbent functionalized12with Polymers 2017, 9, x FOR PEER REVIEW of 14 chelating diamines had excellent selectivity for Cu2+ over other metal ions (e.g., Ca2+ , Fe2+ , Ni2+ , and Zn2+ ) [44]. In orderthe to reuse investigate the selective of MNPs on different performed to evaluate possibility of MNPsadsorption for Cu(II) capacity adsorption. As shown in Figuremetal 11B, + , Mn2+ , Mg2+ , Fe3+ , Cd2+ , Ni2+ , ions, some metal ions were selected as interfering ions, including Ag the regenerative MNPs still possessed a high adsorption capability, which declined slightly with 2+ and Cu2+ . As shown in Figure 11A, it was observed that the adsorption amount of Cu(II) ions Pb increasing cycle times. The adsorption capacity decreased to 5.59 mg/g (about 70% of the initial value) on MNPs is significantly higher than formg/g other(about metal58% ions.ofIn particular, the MNPs have almost no after five regeneration periods and 4.60 the initial value) after five regeneration + , Mn2+ , Ni2+ and a weak adsorption to Fe3+ , Cd2+ , Pb2+ . This result confirms that adsorption to Ag periods. Result confirmed the good reusability and stability of the adsorbent. Regeneration studies the MNPs showed to [45] theseand interfering metal ions. giveproposed better results than Khangood Rao’sselectivity three cycles Wu’s five cycles [46].
25 20
25 20
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Metal ions
(A)
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Cu2+
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Recycle times (B)
Figure 11. Adsorption of MNPs toward metal ions (A) and effect of regenerative times on the Figure 11. Adsorption of MNPs toward metal ions (A) and effect of regenerative times on the adsorption capacity (B). Adsorption conditions: V = 1.0 mL, m = 3.0 mg, Ci = 10.0 mmol/L, time 2 h, adsorption capacity (B). Adsorption conditions: V = 1.0 mL, m = 3.0 mg, Ci = 10.0 mmol/L, time temperature 15 °C, pH 7.0. 2 h, temperature 15 ◦ C, pH 7.0.
4. Conclusions 3.11. Regeneration Studies In this study, novel magnetic Fe3O4@Au@PDA nanocomposites were synthesized which can To keep the processing cost down and for potential practical application, it is preferable to examine effectively adsorb Cu(II). Through high sensitive electrochemical monitoring, the adsorption the possibility of desorbing Cu(II) ions from MNPs for its reuse. A desorption experiment was carried performance of the MNPs was found to be greatly dependent on temperature, solution pH and initial out by controlling the temperature above 60 ◦ C. Seven adsorption-desorption consecutive cycles were Cu(II) concentration. The excellent adsorption behaviors were dominated by rich catechol groups of performed to evaluate the reuse possibility of MNPs for Cu(II) adsorption. As shown in Figure 11B, polydopamine. In addition, MNPs can be easily desorbed and repeatedly used by controlling the the regenerative MNPs still possessed a high adsorption capability, which declined slightly with temperature above 60 °C. Furthermore, the as-prepared MNPs shows a good selectivity for removal increasing cycle times. The adsorption capacity decreased to 5.59 mg/g (about 70% of the initial value) Cu(II). Results indicate that the MNPs are efficient and environmentally friendly adsorbents for the after five regeneration periods and 4.60 mg/g (about 58% of the initial value) after five regeneration selective removal of Cu(II) in aqueous solutions. periods. Result confirmed the good reusability and stability of the adsorbent. Regeneration studies give better results than Khan Rao’sdesigned three cycles [45] and Wu’s [46]. Author Contributions: Y.L. and W.H. the experiments. Y.L.five and cycles Y.C. performed the experiments. L.H. revised the English spelling. B.L. provided an experiment platform. Y.L. analyzed the data and wrote the paper.
4. Conclusions
Acknowledgments: This project was financially supported by NSFC (21405075); Fujian province natural science In this study, novel magnetic Feprovincial nanocomposites were synthesized which 3 O4 @Au@PDA foundation (2016J05040, 2017J01418); Fujian youth natural fund key project (JZ160468); Outstanding can effectively adsorb Cu(II). Through high sensitive electrochemical monitoring, the adsorption young scientific research personnel training plan of Fujian Province colleges and universities (2015); and New performance of the MNPs wasplan found to be greatly onuniversities temperature, solution pH and initial century excellent talents support of Fujian provincedependent colleges and (2017).
Cu(II) concentration. The excellent adsorption behaviors were dominated by rich catechol groups Conflicts of Interest: The authors declare no conflict of interest. of polydopamine. In addition, MNPs can be easily desorbed and repeatedly used by controlling the ◦ temperature References above 60 C. Furthermore, the as-prepared MNPs shows a good selectivity for removal 1. 2. 3.
Machell, J.; Prior, K.; Allan, R.; Andresen, J.M. The water energy food nexus-challenges and emerging solutions. Environ. Sci. Water Res. Technol. 2015, 1, 15–16. Callender, E. 11.3—Heavy metals in the environment—Historical trends. Treatise Geochem. 2014, 1, 59–89. Mahmud, H.N.E.; Huq, A.K.O.; Yahya, R.B. Cheminform abstract: The removal of heavy metal ions from wastewater/aqueous solution using polypyrrole-based adsorbents: A review. RSC Adv. 2016, 6, 14778–
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Cu(II). Results indicate that the MNPs are efficient and environmentally friendly adsorbents for the selective removal of Cu(II) in aqueous solutions. Author Contributions: Y.L. and W.H. designed the experiments. Y.L. and Y.C. performed the experiments. L.H. revised the English spelling. B.L. provided an experiment platform. Y.L. analyzed the data and wrote the paper. Acknowledgments: This project was financially supported by NSFC (21405075); Fujian province natural science foundation (2016J05040, 2017J01418); Fujian provincial youth natural fund key project (JZ160468); Outstanding young scientific research personnel training plan of Fujian Province colleges and universities (2015); and New century excellent talents support plan of Fujian province colleges and universities (2017). Conflicts of Interest: The authors declare no conflict of interest.
References 1. 2. 3.
4. 5. 6.
7. 8. 9. 10. 11.
12. 13.
14. 15. 16.
17. 18.
Machell, J.; Prior, K.; Allan, R.; Andresen, J.M. The water energy food nexus-challenges and emerging solutions. Environ. Sci. Water Res. Technol. 2015, 1, 15–16. [CrossRef] Callender, E. 11.3—Heavy metals in the environment—Historical trends. Treatise Geochem. 2014, 1, 59–89. Mahmud, H.N.E.; Huq, A.K.O.; Yahya, R.B. Cheminform abstract: The removal of heavy metal ions from wastewater/aqueous solution using polypyrrole-based adsorbents: A review. RSC Adv. 2016, 6, 14778–14791. [CrossRef] Ray, P.Z.; Shipley, H.J. Cheminform abstract: Inorganic nano-adsorbents for the removal of heavy metals and arsenic: A review. RSC Adv. 2015, 5, 29885–29907. [CrossRef] Pourbeyram, S. Effective removal of heavy metals from aqueous solutions by graphene oxide-zirconium phosphate (GO-Zr-P) nanocomposite. Ind. Eng. Chem. Res. 2016, 55, 5608–5617. [CrossRef] Chavan, A.A.; Li, H.; Scarpellini, A.; Marras, S.; Manna, L.; Athanassiou, A.; Fragouli, D. Elastomeric nanocomposite foams for the removal of heavy metal ions from water. ACS Appl. Mater. Interfaces 2015, 7, 14778–14784. [CrossRef] [PubMed] Liu, X.; Zhang, N.; Bing, T.; Shangguan, D. Carbon dots based dual-emission silica nanoparticles as a ratiometric nanosensor for Cu2+ . Anal. Chem. 2014, 86, 2289–2296. [CrossRef] [PubMed] Rong, M.; Mu, L.; Zhang, H.; She, G.; Zhou, B.; Xu, H.; Wang, P.; Shi, W. Silicon nanowire-based fluorescent nanosensor for complexed Cu2+ and its bioapplications. Nano Lett. 2014, 14, 3124–3129. Inglese, A.; Lindroos, J.; Savin, H. Accelerated light-induced degradation for detecting copper contamination in p-type silicon. Appl. Phys. Lett. 2015, 107, 41–46. [CrossRef] World Health Organization. Guidelines for Drinking-Water Quality: Recommendations; WHO: Geneva, Switzerland, 2004. Griffiths, C.; Klemick, H.; Massey, M.; Moore, C.; Newbold, S.; Simpson, D.; Walsh, P.; Wheeler, W. U.S. environmental protection agency valuation of surface water quality improvements. Rev. Environ. Econ. Policy 2012, 6, 130–146. [CrossRef] Datta, D.; Uslu, H.; Kumar, S. Adsorptive separation of Cu2+ from an aqueous solution using trioctylamine supported montmorillonite. J. Chem. Eng. Data 2015, 60, 3193–3200. [CrossRef] Dichiara, A.B.; Webber, M.R.; Gorman, W.R.; Rogers, R.E. Removal of copper ions from aqueous solutions via adsorption on carbon nanocomposites. ACS Appl. Mater. Interfaces 2015, 7, 15674–15680. [CrossRef] [PubMed] Klein, T.Y.; Wehling, J.; Treccani, L.; Rezwan, K. Effective bacterial inactivation and removal of copper by porous ceramics with high surface area. Environ. Sci. Technol. 2013, 47, 1065–1072. [CrossRef] [PubMed] Xu, Y.; Zhou, Y.; Ma, W.; Wang, S. A highly sensitive and efficient Fe3 O4 @SiO2 nanoparticles chemosensor for Cu2+ removal. Integr. Ferroelectr. 2013, 147, 110–114. [CrossRef] Zhang, T.; Wang, M.; Yang, W.; Yang, Z.; Wang, Y.; Gu, Z. Synergistic removal of copper(II) and tetracycline from water using an environmentally friendly chitosan-based flocculant. Ind. Eng. Chem. Res. 2014, 53, 14913–14920. [CrossRef] Levin, C.S.; Hofmann, C.; Ali, T.A.; Kelly, A.T.; Morosan, E.; Nordlander, P.; Whitmire, K.H.; Halas, N.J. Magnetic-plasmonic core-shell nanoparticles. ACS Nano 2009, 6, 1379–1388. [CrossRef] [PubMed] Padervand, M.; Gholami, M.R. Removal of toxic heavy metal ions from waste water by functionalized magnetic core-zeolitic shell nanocomposites as adsorbents. Environ. Sci. Pollut. Res. 2013, 20, 3900–3909. [CrossRef] [PubMed]
Polymers 2018, 10, 570
19.
20. 21.
22. 23.
24.
25.
26. 27. 28.
29. 30.
31.
32.
33.
34. 35. 36.
37. 38.
14 of 15
Zhao, M.; Deng, C.; Zhang, X. Synthesis of polydopamine-coated magnetic graphene for Cu2+ immobilization and application to the enrichment of low-concentration peptides for mass spectrometry analysis. ACS Appl. Mater. Interfaces 2013, 5, 13104–13112. [CrossRef] [PubMed] Liu, Y.; Ai, K.; Lu, L. Polydopamine and its derivative materials: Synthesis and promising applications in energy, environmental, and biomedical fields. Chem. Rev. 2014, 9, 5057–5115. [CrossRef] [PubMed] Ou, J.; Wang, J.; Zhang, D.; Zhang, P.; Liu, S.; Yan, P.; Liu, B.; Yang, S. Fabrication and biocompatibility investigation of TiO2 films on the polymer substrates obtained via a novel and versatile route. Colloids Surf. B 2010, 76, 123–127. [CrossRef] [PubMed] Zhang, M.; Zhang, X.; He, X.; Chen, L.; Zhang, Y. A self-assembled polydopamine film on the surface of magnetic nanoparticles for specific capture of protein. Nanoscale 2012, 4, 3141–3147. [CrossRef] [PubMed] Farnad, N.; Farhadi, K.; Voelcker, N.H. Polydopamine nanoparticles as a new and highly selective biosorbent for the removal of copper(II) ions from aqueous solutions. Water Air Soil Pollut. 2012, 223, 3535–3544. [CrossRef] Yu, Q.; Shi, M.; Cheng, Y.; Wang, M.; Chen, H.Z. Fe3 O4 @Au/polyaniline multifunctional nanocomposites: Their preparation and optical, electrical and magnetic properties. Nanotechnology 2008, 19, 265702–265707. [CrossRef] [PubMed] Kan, X.; Zhao, Q.; Shao, D.; Geng, Z.; Wang, Z.; Zhu, J.J. Preparation and recognition properties of bovine hemoglobin magnetic molecularly imprinted polymers. J. Phys. Chem. B 2010, 114, 3999–4004. [CrossRef] [PubMed] Bai, Y.; Rong, F.; Wang, H.; Zhou, Y.; Xie, X.; Teng, J. Removal of copper from aqueous solutions by adsorption on elemental selenium nanoparticles. J. Chem. Eng. Data 2011, 56, 2563–2568. [CrossRef] Zhou, X.Y.; Huang, F. Study on absorption of Cu2+ and Cd2+ in wastewater by modified bentonite. Adv. Mater. Res. 2013, 779–780, 1238–1241. [CrossRef] Wang, X.; Wang, C. Chitosan-poly(vinyl alcohol)/attapulgite nanocomposites for copper(II) ions removal: PH dependence and adsorption mechanisms. Colloid Surf. A Physicochem. Eng. Asp. 2016, 500, 186–194. [CrossRef] Lynge, M.E.; van der Westen, R.; Postma, A.; Städler, B. Polydopamine—A nature-inspired polymer coating for biomedical science. Nanoscale 2011, 3, 4916–4928. [CrossRef] [PubMed] Li, M.; Liu, Q.; Jia, Z.; Xu, X.; Shi, Y.; Cheng, Y.; Zheng, Y. Polydopamine-induced nanocomposite ag/cap coatings on the surface of titania nanotubes for antibacterial and osteointegration functions. J. Mater. Chem. B 2015, 3, 8796–8805. [CrossRef] Lin, J.H.; Yu, C.J.; Yang, Y.C.; Tseng, W.L. Formation of fluorescent polydopamine dots from hydroxyl radical-induced degradation of polydopamine nanoparticles. Phys. Chem. Chem. Phys. 2015, 17, 15124–15130. [CrossRef] [PubMed] Li, Y.; Hong, M.; Lin, Y.; Bin, Q.; Lin, Z.; Cai, Z.; Chen, G. Highly sensitive electrochemical immunoassay for H1N1 influenza virus based on copper-mediated amplification. Chem. Commun. 2012, 48, 6562–6564. [CrossRef] [PubMed] Liu, X.J.; Li, H.Q.; Lin, X.Y.; Liu, H.Y.; Gao, G.H. Synthesis of siloxane-modified melamine-formaldehyde microsphere and its heavy metal ions adsorption by coordination effects. Colloid Surf. A Physicochem. Eng. Asp. 2015, 482, 491–499. [CrossRef] Ho, Y.S.; Mckay, G. Pseudo-second order model for sorption processes. Process Biochem. 1999, 34, 451–465. [CrossRef] Zhang, H.; Wang, X.; Liang, H.; Tan, T.; Wu, W. Adsorption behavior of Th(IV) onto illite: Effect of contact time, ph value, ionic strength, humic acid and temperature. Appl. Clay Sci. 2016, 127–128, 35–43. Li, Y.; Bin, Q.; Lin, Z.; Chen, Y.; Yang, H.; Cai, Z.; Chen, G. Synthesis and characterization of vinyl-functionalized magnetic nanofibers for protein imprinting. Chem. Commun. 2015, 51, 202–205. [CrossRef] [PubMed] Hui, L.I.; Xiao, D.L.; Hua, H.E.; Lin, R.; Zuo, P.L. Adsorption behavior and adsorption mechanism of Cu(II) ions on amino-functionalized magnetic nanoparticles. Trans. Nonferrous Met. Soc. China 2013, 23, 2657–2665. Hamad, H.; Ezzeddine, Z.; Lakis, F.; Rammal, H.; Srour, M.; Hijazi, A. An insight into the removal of Cu (II) and Pb(II) by aminopropyl-modified mesoporous carbon cmk-3: Adsorption capacity and mechanism. Mater. Chem. Phys. 2016, 178, 57–64. [CrossRef]
Polymers 2018, 10, 570
39. 40. 41. 42.
43. 44.
45.
46.
15 of 15
Langmuir, I. The constitution and fundamental properties of solids and liquids. Part I. Solids. J. Am. Chem. Soc. 1916, 38, 2221–2295. [CrossRef] Freundlich, H. Über die adsorption in lösungen. Z. Phys. Chem. 1907, 57, 385–470. [CrossRef] Dong, C.; Zhang, F.; Pang, Z.; Yang, G. Efficient and selective adsorption of multi-metal ions using sulfonated cellulose as adsorbent. Carbohydr. Polym. 2016, 151, 230–236. [CrossRef] [PubMed] Ma, L.; Wang, Q.; Islam, S.M.; Liu, Y.; Ma, S.; Kanatzidis, M.G. Highly selective and efficient removal of heavy metals by layered double hydroxide intercalated with the MoS4 2− ion. J. Am. Chem. Soc. 2016, 138, 2858–2866. [CrossRef] [PubMed] Chen, Y.; Song, Y.F. Highly selective and efficient removal of Cr(VI) and Cu(II) by the chromotropic acid-intercalated Zn-Al layered double hydroxides. Ind. Eng. Chem. Res. 2013, 52, 4436–4442. [CrossRef] Chouyyok, W.; Shin, Y.; Davidson, J.; Samuels, W.D.; Lafemina, N.H.; Rutledge, R.D.; Fryxell, G.E. Selective removal of copper (II) from natural waters by nanoporous sorbents functionalized with chelating diamines. Environ. Sci. Technol. 2010, 44, 6390–6395. [CrossRef] [PubMed] Rao, R.A.K.; Khatoon, A. Adsorption characteristics of chemically modified caryota urens seeds for the removal of Cu(II) from aqueous solution: Isotherms and kinetic studies. Groundw. Sustain. Dev. 2016, 2–3, 42–52. Wu, Z.C.; Wang, Z.Z.; Liu, J.; Yin, J.H.; Kuang, S.P. Removal of Cu(II) ions from aqueous water by L-arginine modifying magnetic chitosan. Colloid Surf. A Physicochem. Eng. Asp. 2016, 499, 141–149. [CrossRef] © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
