Adsorption of Cadmium Ions from an Aqueous Solution on a Highly Stable Dopamine-Modified Magnetic Nano-Adsorbent
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Magnetic nanomaterials were functionalized with dopamine hydrochloride as the functional reagent to afford a core–shell-type Fe3O4 modified with polydopamine (Fe3O4@PDA) composite, which was used for the adsorption of cadmium ions from an aqueous solution. In addition, the effects of environmental factors on the adsorption capacity were investigated. Furthermore, the adsorption kinetics, isotherm, and thermodynamics of the adsorbents were discussed. Results revealed that the adsorption of cadmium by Fe3O4@PDA reaches equilibrium within 120 min, and kinetic fitting data are consistent with the pseudo-second-order kinetics (R2 > 0.999). The adsorption isotherm of Cd2+ on Fe3O4@PDA was in agreement with the Freundlich model, with the maximum adsorption capacity of 21.58 mg/g. The thermodynamic parameters revealed that adsorption is inherently endothermic and spontaneous. Results obtained from the adsorption–desorption cycles revealed that Fe3O4@PDA exhibits ultra-high adsorption stability and reusability. Furthermore, the adsorbents were easily separated from water under an enhanced external magnetic field after adsorption due to the introduction of an iron-based core. Hence, this study demonstrates a promising magnetic nano-adsorbent for the effective removal of cadmium from cadmium-containing wastewater.
KeywordsMagnetic nanomaterials Dopamine modification Adsorption Cadmium
Fe3O4 modified with polydopamine
Fourier transform infrared spectroscopy
Transmission electron microscopy
Vibrating sample magnetometer
X-ray photoelectron spectroscopy
Pollution by Cd(II) metals has become one of the serious environmental problems. A majority of the cadmium pollution originates from the smelting of non-ferrous metals, sintering of ores, discharge of wastewater from the electroplating industry, and preparation of phosphate fertilizers from phosphate rock [1, 2, 3, 4]. With a long half-life, cadmium is slowly metabolized in the human body; hence, it can be easily accumulated in organs such as the kidneys of the human body . The long-term exposure of humans or animals to low cadmium concentrations can lead to health issues, including kidney dysfunction and reproductive organ and bone damage, as well as malformation of the development of offspring . Hence, Cd(II) is designated as a carcinogen by the World Health Organization, the International Agency for Research on Cancer, and the National Toxicology Program (USA) [7, 8, 9]. Therefore, an effective removal method of cadmium ions for the purpose of reducing environmental pollution and damage to humans and animals is an interesting topic for environmental governance.
Currently, cadmium can be removed by chemical precipitation, ion exchange, adsorption, solvent extraction, and membrane separation [10, 11, 12, 13, 14]. In particular, adsorption has been widely employed due to its simple operation, high efficiency, and cost-effectiveness . In the past two decades, various adsorbent materials have been developed and utilized, including natural soil materials, inorganic minerals, activated carbon, zeolites, silica gel, chitosan, and polymer materials [16, 17, 18, 19, 20, 21, 22]. Compared to these adsorbents, magnetic nano-adsorbents can be developed and used for treating industrial wastewater due to their high specific surface area, good biocompatibility, cost-effectiveness, and rapid separation and recovery under an external magnetic field . However, the superparamagnetism and high surface energy of magnetic Fe3O4 lead to facile agglomeration or corrosion and poor stability [24, 25]. Hence, Fe3O4 needs to be functionalized for the improvement of its dispersibility, stability, and contaminant removal rate. Currently, the main surface modifiers of magnetic nanoparticles include organic small molecules, high molecular-weight polymers, inorganic materials, and metal–organic frameworks [26, 27, 28, 29].
In the past decade, dopamine (DA) has been reported to form a stable polydopamine (PDA) film with controlled thickness by self-polymerization under weakly alkaline conditions. Studies have revealed that PDA is a highly adhesive biopolymer with functional groups such as catechol, amine, and imine, which can adhere to the surface of organic or inorganic materials via the formation of covalent and non-covalent interactions (e.g., chelation, hydrogen bonds, van der Waals forces, and π–π stacking) [30, 31]. In addition, these interactions between PDA and the carrier exist between PDA and water pollutants, thereby providing a method for removing water-containing contaminants. Farnad et al.  have synthesized PDA nanoparticles with an average diameter of 75 nm that can efficiently adsorb Cu2+ from wastewater. The maximum adsorption capacity of 34.4 mg/g is obtained after the reaction is conducted for 270 min at pH 5. Zhang et al.  have obtained PDA-modified magnetic nanoparticles (Fe3O4/PDA) and subsequently used them for the removal of methylene blue, lemon yellow, Cu2+, Ag+, and Hg2+ from sewage. At an optimum pH, the maximum adsorption capacities of Fe3O4/PDA for these contaminants are 204.1, 100.0, 112.9, 259.1, and 467.3 mg/g, respectively. This study demonstrated the immense potential of Fe3O4/PDA for the removal of multiple pollutants. Huang et al.  have prepared a PDA-coated clay (D-clay/Fe3+) with a three-dimensional network structure by using Fe3+ for coordination with PDA. As-prepared material exhibits good elastic response and self-repairing ability. Rhodamine 6G (Rh6G) can be removed from water through the π–π stacking interactions of the aromatic moiety between PDA and Rh6G. Gao et al.  have prepared a PDA-modified graphene hydrogel (PDA-GH) by a one-step approach. This material exhibits high adsorption capacities for Pb2+, Cd2+, rhodamine B, and p-nitrophenol. This material is also easily regenerated using low-cost desorbents such as acids and alcohols. Hence, according to previous studies, PDA exhibits a good adsorption capacity for various pollutants (i.e., heavy metals and organic pollutants), and it demonstrates broad application prospects for wastewater treatment.
Sodium acetate anhydrous, ethylene glycol, and nitric acid (65–68 wt%) were purchased from Guangdong Chemical Reagent Engineering-Technological Research and Development Center (Guangdong, China). Trisodium citrate dihydrate and hydrochloric acid (36–38 wt%) were purchased from Tianjin Fengchuan Chemical Reagent Technologies Co., Ltd. (Tianjin, China). Sodium hydroxide, ferric chloride hexahydrate, and cadmium chloride hydrate (99%) were obtained from Guangfu Technology Development Co., Ltd. (Tianjin, China). Tris (hydroxymethyl) aminomethane and 3-hydroxytyramine hydrochloride were purchased from Aikeda Chemical Reagent Co., Ltd. (Chengdu, China). All chemicals were of analytical grade or better, and they were used without further purification. Ultrapure water was used throughout the experimental process.
Synthesis of Fe3O4 Microspheres
Fe3O4 magnetic nanoparticles were prepared by a solvothermal reaction. Briefly, FeCl3·6H2O (8.1 g), Na3C6H5O7·2H2O (6 g), and CH3COONa (21.6 g) were first dissolved in ethylene glycol (240 mL) with magnetic stirring. Second, after vigorous magnetic stirring for 30 min, the homogeneous orange-red solution was divided into three parts (80 mL/part) and transferred into three Teflon-lined stainless-steel autoclaves (100 mL) and sealed for heating at 200 °C. After reaction for 8 h, the autoclave was cooled to room temperature. The obtained Fe3O4 particles were collected using an external magnet and washed several times with ethanol and H2O. Finally, the products were stored in an appropriate amount of ethanol for further use.
Synthesis of Fe3O4@PDA Particles with Core–Shell Nanostructures
PDA-coated Fe3O4 nanoparticles were obtained by the polymerization of DA in an alkaline buffer at 25 °C. Briefly, the synthesized Fe3O4 particles were added into a 1000-mL three-necked flask containing 200 mL of Tris-HCl buffer (10 mM, pH 8.5) and subjected to sonication for 5 min. Then, 2 g of dopamine hydrochloride was weighed and dispersed in a 500-mL beaker containing 400 mL of Tris-HCl buffer (10 mM, pH 8.5) and subjected to sonication for 1 min. Then, it was added into the three-necked flask and mechanically stirred for 24 h. The synthesized Fe3O4@PDA particles were separated and collected using a magnet and washed several times with deionized water and ethanol, followed by drying under vacuum at 50 °C for 4 h.
X-ray diffraction (XRD) was employed to identify the crystalline structure and phase composition of the synthesized samples in the 2θ range from 10 to 90° using Co Kα radiation. The morphology and dimensions of the samples were observed by transmission electron microscopy (TEM). Fourier transform infrared (FTIR) spectra of the samples were recorded in the wavenumber range of 400–4000 cm−1. The chemical states of the samples were examined by X-ray photoelectron spectroscopy (XPS). The specific surface area was measured by N2 adsorption using the Brunauer–Emmett–Teller (BET) method. The magnetization curve was measured at room temperature under a varying magnetic field from − 20,000 to 20,000 Oe using an MPMS-3 vibrating sample magnetometer (VSM).
Batch Adsorption Studies
Adsorption properties of the two adsorbents (i.e., Fe3O4 and Fe3O4@PDA, respectively) for the cadmium ions from an aqueous solution under various operating conditions were investigated. Adsorption experiments were carried out in an Erlenmeyer flask with 20.0 mg of the adsorbent and 50 mL of a 20 mg/L Cd2+ solution (pH 7). The Erlenmeyer flask was placed in a constant-temperature shaker and subjected to shaking at 250 rpm for 120 min at 25 °C. After attaining equilibrium, the magnetic nano-adsorbent was separated from the Cd2+ solution by using an external magnetic field. Then, the supernatant was removed, and the concentration of cadmium ions in the initial and adsorbed solutions was estimated by atomic absorption spectrometry.
To investigate the effect of the reaction time, the reaction time was set between 15 and 2160 min. The effect of the initial cadmium solution concentration on the adsorption capacity of the adsorbent was investigated by the variation in the initial Cd2+ concentration between 3 and 30 mg/L. In addition, the effect of the adsorbent dose was investigated by the variation in the adsorbent dose (10–50 mg). The effect of pH on adsorption was investigated by the addition of 0.1 M NaOH or 0.1 M HCl to adjust the solution pH in the range of 4.0–9.0. Experiments were carried out at 20–45 °C to examine the effect of temperature on the adsorption performance.
Reusability and Stability Studies
The reusability and stability of the adsorbent were investigated by performing 10 adsorption–desorption cycles using the Fe3O4@PDA adsorbent. The adsorption capacity of the adsorbent for Cd2+ in each cycle was analyzed. Experimental conditions for the adsorption reaction were as follows: 20 mg Fe3O4@PDA, 50 mL of a 20 mg/L CdCl2 solution (pH 7), and reaction at 250 rpm for 120 min at 25 °C. Desorption was carried out using 50 mL of 0.5 M HCl as the desorbent, and the reaction was carried out at 250 rpm for 60 min at 25 °C. The adsorbent was separated by using a magnet after desorption and washed with deionized water and ethanol until neutral pH was achieved. After drying the adsorbents at 60 °C for 30 min, the next adsorption–desorption experiment was carried out.
Results and Discussion
Characterization of the Magnetic Adsorbents
Furthermore, TEM images were recorded to observe the morphology of the adsorbent and to confirm the polymerization of DA on the Fe3O4 core. Figure 1b shows the TEM image of bare Fe3O4 particles. As can be clearly observed in the TEM image, as-synthesized Fe3O4 exhibited a cluster of nanospheres comprising several secondary Fe3O4 nanoparticles. With the addition of sodium citrate as the stabilizer during the synthesis, the as-obtained Fe3O4 particles exhibited a uniform size (average particle diameter of 250–300 nm) and good dispersibility without agglomeration. The Fe3O4@PDA composites exhibited a core–shell structure (Fig. 1c). As the core, dark Fe3O4 particles were uniformly coated by light-colored PDA, with a 40–50-nm-thick PDA layer. Furthermore, the Fe3O4@PDA nanoparticles tended to agglomerate, likely related to the magnetic attraction  and hydrogen bonding between the Fe3O4@PDA particles. In addition, owing to the high surface energy of the nanoparticles, the system automatically changed in the direction of the decreased surface area, leading to the nanoparticle agglomeration . Figure 1d shows the Fe3O4@PDA particles after the adsorption of cadmium ions. The PDA coating was darker in color, and the core structure was unchanged. Moreover, the dispersibility of Fe3O4@PDA-Cd2+ was better than that of Fe3O4@PDA particles due to the decrease in the surface energy of Fe3O4@PDA after the adsorption of Cd2+. By the comparison of Fig. 1d and e, after 10 adsorption–desorption cycles, the PDA coating remained intact, but the Fe3O4 core density decreased because the desorbent (0.5 mol/L HCl) corroded a part of the secondary Fe3O4 nanoparticles. Although the core was corroded, the Fe3O4@PDA structure was retained after 10 adsorption–desorption cycles, indicating that PDA exhibits a good protective effect on the exposed Fe3O4 particles.
The BET specific surface areas of Fe3O4 and Fe3O4@PDA were estimated to be 61.84 m2/g and 14.23 m2/g, respectively. The decrease in the specific surface area of Fe3O4@PDA corresponded to the increase in the particle size of the magnetic nanoparticles after coating with DA. In addition, the agglomeration of Fe3O4@PDA led to the decrease in the specific surface area .
Hence, the O 1s, N 1s, and Cd 3d XPS spectra confirmed the results obtained from FTIR spectroscopy; Cd2+ is adsorbed by the action of the amino and hydroxyl groups on the Fe3O4@PDA surface; and the hydroxyl group plays a major role.
Adsorption of Cd2+ in Batch Systems
The adsorption capacity of the adsorbents for heavy metal ions is mainly affected by factors such as reaction time, heavy metal-ion concentration, adsorbent dose, pH, and reaction temperature. Hence, batch adsorption experiments are carried out on the adsorbent to examine the effect of the above factors on the reaction, as well as the kinetics, isotherm, and thermodynamic properties of the adsorbent.
Effect of Contact Time and Kinetics Study
Kinetics adsorption parameters of Cd2+ by Fe3O4@PDA
qe exp (mg/g)
qe cal (mg/g)
qe cal (mg/g)
Effect of Concentration and Adsorption Isotherms
In addition, the equilibrium parameter RL [1/(1 + bCo)], highest initial solute concentration in the concentration gradient C0 (mg/L)), and the mean free energy of adsorption, Ed [(2Kd)−1/2, kJ/mol] were estimated. RL was utilized to determine whether the adsorption process is favorable. Ed can be used to determine the type of adsorption. Ed values of 1–8 kJ/mol were indicative of physical adsorption (such as van der Waals forces), while those of 8–16 kJ/mol were indicative of ion exchange. An Ed value of between 20 and 40 kJ/mol revealed that the adsorption reaction is chemisorption .
The Langmuir, Freundlich, and D-R isotherm parameters for the adsorption of Cd2+ by Fe3O4@PDA
1.319 × 10–3
6.02 × 10–3
Comparison of the maximum adsorption capacities of Fe3O4@PDA with some adsorbents cited in the literature
C0 (Cd) (mg/L)
Nano-hydroxyapatite chitosan composites
Fly ash/chitosan (A-FA/Ch) composite
Surface-modified Eucalyptus seeds by sulfuric acid (SMES-S)
Surface-modified Eucalyptus seeds by hydrochloric acid (SMES-H)
Nano zero-valent iron particles
Biopolymeric sorbent: sporopollenin
Poly (sodium acrylate)–graphene oxide
Zero-valent iron-coated biochars (magnetic ones)
Multiwalled carbon nanotubes (MWCNTs)
1.260 ± 0.02
22.39 ± 0.36
21.67 ± 0.40
Sawdust of Pinus sylvestris
Mercaptoacetic-acid-modified orange peel (MOP)
Effect of Temperature and Thermodynamic Parameters
Thermodynamic parameters for the adsorption of Cd2+ by Fe3O4@PDA
Effect of Adsorbent Dose
Effect of pH
The solution pH is also one of the most important factors affecting adsorption. The adsorption of Cd2+ is mainly affected by the surface charges on the adsorbents, and the surface charges of the adsorbents are affected by the solution pH. Considering the degree of tolerance for adsorbents to acid and base, the chemical states of cadmium ions in an aqueous solution [i.e., Cd2+, Cd (OH)+, Cd (OH)2, and Cd (OH)3−] , and the actual conditions of environmental water samples, pH values of between 4 and 9 were selected to investigate the adsorption of Cd2+ by the adsorbents.
Figure 7b shows the experimental results. With the increase in the solution pH, the adsorption amount of the adsorbents on Cd2+ significantly increased. At low solution pH, the concentration and activity of H+ in the solution were extremely high, which can compete with Cd2+ for adsorption and occupy the active site on the adsorbent surface, leading to the low adsorption capacity of Cd2+ by the adsorbent . At low solution pH, the adsorption of cadmium must overcome the repulsive force between the positively charged Fe3O4@PDA surface and Cd2+ through chemical interactions with sufficient energy rather than through electrostatic attractions . At solution pH values of between 6.0 and 8.0, the main chemical states of cadmium were Cd2+ (minor) and Cd (OH)+ (major) . As the affinity of Cd (OH)+ was better than that of Cd2+, it can be adsorbed on the adsorbent surface not only by electrostatic adsorption and ion exchange but also by hydrogen bonds. With the further increase in the solution pH, the protonation sites on the adsorbent surface decreased, and the negative charge increased, facilitating the adsorption of Cd2+ and Cd (OH)+ on the deprotonation active sites of the adsorbent by electrostatic adsorption . Hence, the increase in pH is beneficial to the adsorption of heavy metal cadmium. The precipitation of cadmium at pH 8.8 was calculated from the precipitation constant of Cd (OH)2(s) (Ksp = 7.2 × 10−15) and the initial Cd2+ concentration (20 mg/L). Thus, at a solution pH from 8.0 to 9.0, the adsorption capacities of both adsorbents sharply increase (Fig. 7b). Moreover, the high adsorption amount at pH 9.0 resulted from the formation of a Cd (OH)2 precipitate rather than the adsorption of cadmium on the adsorbent. Nevertheless, at a solution pH between 4.0 and 8.8, the adsorption capacity of Fe3O4@PDA was greater than that of bare Fe3O4, indicating that the Fe3O4@PDA adsorbent can be used in the wide pH range of 4.0–8.8 for treating Cd2+-containing wastewater.
To verify the occurrence of ion exchange, the pH of the residual solutions at pH 6, 7, and 8 was measured after the completion of the experiment. The result revealed that the solution pH slightly decreases after adsorption, confirming the presence of the proposed mechanism for (17) and (18). This result indicated that the reactions of (17) and (18) considerably contribute to the overall adsorption process.
Reusability and Stability Studies
The reusability and stability of adsorbents are crucial for the industrial treatment of heavy metal wastewater. The inhibition of the heavy metal adsorption on Fe3O4@PDA at low pH indicated that acid treatment is a viable method for regenerating heavy metal-loaded adsorbents. Hence, in this experiment, 0.5 M HCl is used as the desorbent, and 10 adsorption–desorption cycles are carried out using the Fe3O4@PDA adsorbent. With the increase in the number of experiments, the adsorption capacity gradually decreased (Fig. 7c), possibly related to the incomplete desorption of cadmium ions adsorbed on the adsorbent surface. After the completion of the 10th cycle, the adsorption capacity of the adsorbents was reduced from 6.25 to 4.25 mg/g, and the adsorption rate only decreased by 3.6% compared with that of the initial cycle, indicating that Fe3O4@PDA exhibits good reusability and provides a basis for the practical applications of Fe3O4@PDA.
TEM images of the adsorbent were recorded after 10 adsorption–desorption cycles (Fig. 1e). After 10 desorption cycles of Fe3O4@PDA in an acidic environment of 0.5 M HCl, the PDA layer was preserved, but the Fe3O4 core was corroded. Although the Fe3O4@PDA core structure was damaged, the core–shell structure was complete, and the adsorption efficiency did not change considerably, indicating that PDA exhibits a good protective effect on the Fe3O4 core and that Fe3O4@PDA can stably exist in an acidic environment. The results showed that Fe3O4@PDA exhibits excellent stability and adsorption properties, providing the basis for the practical applications of Fe3O4@PDA because if the adsorbent exhibits good reusability and stability, it will effectively decrease the cost of industrial applications.
In conclusion, a highly stable, hydrophilic functionalized magnetic nano-adsorbent (Fe3O4@PDA) was synthesized by a simple, safe, and environmentally friendly method in this study. Results revealed that the Cd2+ adsorption is dependent on the contact time, initial Cd2+ concentration, temperature, adsorbent dose, and solution pH. The adsorption performance of Cd2+ on Fe3O4@PDA was better than that of bare Fe3O4, which was related to the presence of active sites such as phenolic hydroxyl groups (electron negative groups) and amino groups on the Fe3O4@PDA surface. In kinetics studies, adsorption equilibrium was achieved at 120 min, and the adsorption capacity of Cd2+ onto Fe3O4@PDA reached up to 9.176 mg/g. The adsorption of Cd2+ followed the pseudo-second-order kinetics model. The adsorption of Cd2+ onto Fe3O4@PDA was consistent with the Freundlich isotherm, and the maximum adsorption capacity obtained by the Langmuir model was 21.58 mg/g. Thermodynamic analyses indicated that the reaction is spontaneous and endothermic. Meanwhile, the possible adsorption mechanism was also proposed on the basis of the kinetics, D-R isotherm model, and thermodynamic results, i.e., Cd2+ was adsorbed on the Fe3O4@PDA surface-active site by electrostatic adsorption, ion exchange, and chelation. Furthermore, 10 adsorption–desorption cycles were carried out using the Fe3O4@PDA nano-adsorbent for water samples containing cadmium. The adsorption rate of the adsorbent was only decreased by 3.6%, indicating that Fe3O4@PDA exhibits good adsorption stability and reusability, thereby reducing costs. The dissolution of Fe3O4@PDA in 0.5 M HCl indicated that the adsorbent can be treated harmlessly, avoiding secondary environmental pollution. The results revealed that Fe3O4@PDA exhibit immense potential for the treatment of cadmium-containing wastewater.
The authors acknowledge the Advanced Analysis and Measurement Center of Yunnan University.
This study was carried out in collaboration of all the authors. TL designed the study, conducted experiments and analysis, and prepared the manuscript. FJ and XJY revised the manuscript. SJL, ZXR, and LLW played a part in the experimental process. LHT and SXW provided advice for the experimental design and participated in the revision of the manuscript. All authors read and approved the final manuscript.
This study was supported by the Natural Science Foundation of Yunnan Province (project no. 2018FB014), Scientific Research Foundation of Yunnan Provincial Department of Education (project no. 2018JS005), Natural Science Foundation of Yunnan University (project no. 2017YDQN01), and Major Project of Kunming Science and Technology Bureau (project no. 2017-1-S-12305).
The authors declare that they have no competing interests.
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