Fabrication and Characterization of Magnetic Hydroxyapatite

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Fabrication and characterization of M-HAP/Agar composite beads with high adsorption capacity for heavy metal removal Qi Zhang, Shunmin Dan, and Kaifeng Du Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01635 • Publication Date (Web): 10 Jul 2017 Downloaded from http://pubs.acs.org on July 11, 2017

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Industrial & Engineering Chemistry Research

Fabrication and characterization of M-HAP/Agar composite beads with high adsorption capacity for heavy metal removal

Qi Zhang, Shunmin Dan, Kaifeng Du*

Department of Pharmaceutical & Biological Engineering, School of Chemical Engineering, Sichuan University, Chengdu 610065, P. R. China

Corresponding author: Dr. Kaifeng Du Department of Pharmaceutical & Biological Engineering, School of Chemical Engineering, Sichuan University, Chengdu 610065, China Tel.: +86-28-85405221; Fax: +86-28-85405221 Email: [email protected]

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Abstract Magnetic hydroxyapatite entrapped agarose composite beads (M-HAP/Agar composite beads) have been successfully synthesized by emulsification of magnetic HAP nanoparticles with agarose suspension. In the process, the magnetic HAP nanoparticles served as main resource for high adsorption performance, which were constructed by surface modification of Fe3O4 with N-(phosphonomethyl) iminodiacetic acid (PM-IDA) and followed by coating with HAP. This strategy integrates the distinct advantages of large-size beads and magnetic response for easy recovery as well as nanoparticles for high adsorption capacity. The resulting M-HAP/Agar composite beads display large specific surface area (90 m2 g-1) and uniform spherical shape (150 µm). These magnetic hydroxyapatite nanoparticles in beads can provide more adsorption sites due to their suitable porous structure. As results, the adsorbent exhibits excellent performance in adsorption of Pb2+, Co2+, Cu2+, showing the maximum binding capacity as high as 842.6 mg g-1, 105.1 mg g-1 and 71.6 mg g-1, respectively. All these results suggest that these magnetic nanoparticles entrapped in beads have a positive effect on improving the adsorption capacity. Moreover, the beads possess superparamagnetism and large size, allowing them to be easily recovered from solution. Therefore, this work provides a promising approach for the design and synthesis of multifunctional hydroxyapatite composite beads for high efficient removal of heavy metal in the field of wastewater treatment.

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Introduction The emerging heavy metal ion pollution in the environment demands high effective water purification technologies since heavy metal ions are potentially hazardous to human health and ecosystem even in minute quantities. Traditional technologies being applied to remove heavy metal ions involve precipitation, solvent extraction, membrane separation, adsorption, and so on. Among them, the adsorption is generally preferred to address heavy metal ions pollution due to its simple operation, potential recovery and reuse of metals. In this context, a vast variety of adsorbents including activated carbons, clay material, zeolites, and biosorbents have been developed to remove heavy metal ions from various industrial effluents in past years.1-4 Recently, synthetic hydroxyapatite, a bioactive and biocompatible ceramic, has emerged as one of most promising adsorbents for heavy mental ions and protein adsorption. The popularity lies in that the hydroxyapatite possesses phosphate sites and exhibits negative charge, which allows high affinity to varied metal ions (Cu, Cd, Co, Pb, Zn and Ni).5-10 Hence, extensive researches have devoted to develop varied hydroxyapatite-type adsorbent for heavy metal ions adsorption in recent years. One of most popular HAP-type adsorbent is the magnetic HAP nanoparticles. Together with large specific surface of HAP nanoparticles, entrapping magnetic core into HAP particles endows the adsorbent with magnetic separation convenience, which enables easy separation through an external magnetic field. For example, Dong et al. reported a kind of magnetic nano-HAP with high sorption capacity of Pb2+ up to 598.8 mg g−1.

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Besides,

Feng and Cui groups investigated the adsorption of Cd2+, Zn2+ and Cu2+ onto magnetic HAP using batch experiments.12-13 Recently, Cui and coworkers reported a graphene oxide composite magnetic HAP, which showed excellent Pb2+ adsorption performance.14 Wang’s group developed 3



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an oxidized multi-walled carbon nanotubes immobilized magnetic hydroxyapatite, which displayed a high capacity about 698.4 mg g-1 for Pb2+.15 However, with the repeated use and increasing requirement of treating large-volume effluents, the magnetic response tends to be weakened and then compromises the recovery efficiency of adsorbent by an external magnetic field. Therefore, it is necessary to explore a novel method to construct high-performance HAP adsorbent with large adsorption capacity and easy recovery for large-volume effluent. To resolve this problem, a design strategy of fabricating the magnetic hydroxyapatite agarose composite beads (M-HAP/Agar composite beads) was proposed, which would enjoy the environmental-friendly property and be suitable for high-performance separation in the industrial field. In the design strategy, a surface modification technique was applied to construct the magnetic HAP nanoparticles, which guaranteed the HAP particles with suitable porous structure and high specific surface for large adsorption capacity.16-18 The agarose material as a host support can enclose the magnetic HAP nanoparticles into its networks, leading to the generation of large-size magnetic HAP agarose composite adsorbent.19-22 The large size of composite beads together with the magnetic cores conduce to easy recovery of adsorbent from effluent by the filtration or the external magnetic field. In this regard, it is expected that the proposed M-HAP/Agar composite beads are suitable application in both laboratories and industries. The present work aims at fabricating high-performance magnetic HAP agarose composite beads and exploiting their advantageous properties of both HAP nanoparticles and large agarose spherical support in the composite material. For this purpose, bead morphology, HAP structure and surface property of the prepared M-HAP/Agar composite beads were evaluated by varied methods of SEM, XRD, N2 adsorption and FTIR. The influence of pH and initial metal ions 4


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concentration on materials’ Zeta potential change, adsorption isotherm and kinetics were studied systematically to explore the possibility of the novel adsorbent for large scale industrial use for the removal of heavy metal ions from industrial effluents.

2. Experiment Section 2.1. Chemicals. N-(phosphonomethyl) iminodiacetic acid (PM-IDA) was obtained from TCI Delelopment (Shanghai, China). Agarose powder was purchased from Biowest (Shanghai, China). Polyoxyethylene sorbitanmonooleate (Tween 80) and Sorbitan trioleate (Span 85) were purchased from Dengfeng Chemical Co. (Tianjin, China). Other reagents were received from Kelong Chemical (Chengdu, Sichuan, China). All chemical agents listed above were of analytical grade. 2.2. Preparation of M-HAP Nanoparticles. The magnetic Fe3O4 nanoparticles were prepared by the solvothermal reaction.23 Magnetic HAP were prepared as follows: firstly, 0.10 g Fe3O4 nanoparticles and 0.10 g PM-IDA were dispersed in alkaline medium (pH 8) with ultrasonication for 30 min.24-25 The resulting dark particles were then homogeneously mixed with 100 mL 0.01M Ca(OH)2 solution (ethanol/water: 1:1,v/v). After that, 100 mL 0.006M Na2HPO4 was added into the solution with a molar Ca/P ratio of 1.67, and the mixture was stirred for 6 h at 60 ℃. In this process, the solution pH was maintained at 11 by dilute ammonia water. Finally, the product, named magnetic hydroxyapatite nanoparticles (M-HAP), were collected and dried in vacuum oven at 35 ℃ for 12 h. 2.3 Preparation of M-HAP/Agar Composite Beads. The M-HAP/Agar composite beads were prepared by the emulsification method. Typically, 6 g M-HAP nanoparticles and 2 g agarose 5



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were well dispersed in 48 mL pure water, and then heated at 95 ℃ to completely dissolve the agarose. After that, the resulting hot M-HAP/Agarose suspension was quickly transferred into a 250 mL three-neck flack containing 100 mL cyclohexane in the presence of 3 g Span 85, 1 g Tween 80 and was stirred at 60 ℃ for 30 min. Finally, the formed emulsification was rapidly cooled below 10 ℃, washed by ethanol and distilled water and screened with 100 mesh standard metal screen. 2.4 Characterization. Optical microscopy (OM) observation was conducted on Nikon ECLPSE600 microscope. The microscopic morphology of M-HAP/Agar composite beads was investigated by Scanning electron microscopic SEM (Philips XL 30, Netherlands). Pore size distributions and pore volume were analyzed by the adsorption branch of N2 isotherm adsorption curve using surface area and porosity analyzer (Quantachrome, Autosorb-1C-VP, USA), and the information of chemical bonds of samples was evaluated by Fourier-transform infrared spectroscopy (FTIR, Bruker Optics Tensor 27, Germany). X-ray diffraction (XRD) patterns of the samples were analyzed on X-ray diffraction (XRD, Rigaku D/max-2550pc, Japan) with a scan range between 20º and 70º. The zeta potential was measured by the Zetasizer (Malvern Zetasizer Nano ZS90, UK). 2.5 Adsorption Experiments. The adsorption experiments of Cu2+ 、Co2+ and Pb2+ by M-HAP/Agar composite beads were performed by batch experiments at different pH (3.0-7.0). Adsorption equilibrium isotherms were carried out with the initial concentrations in range of 10-200 mg L-1 for Cu2+ and Co2+, 50-1200 mg L-1 for Pb2+, sorption kinetics was obtained in different time intervals ranging from 0 to 10 h. In the typical procedure, the experiments were carried on the conical flasks containing 50 mg adsorbent and 50.00 mL metal ions solution at 6


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room temperature with shaking for defined time intervals. The initial pH values were adjusted using 0.1 M HNO3 and NaOH solutions. All the metal ion loaded composite beads in this work were regenerated with 50 mL 0.0035 M ethylenediaminetetraacetic acid (EDTA) solution and 100 mL pure water at room temperature, successively.12 After being separated, the residual heavy metal concentrations in solutions were determined by a UV-Vis spectrophotometer (TU-1901, Beijing Purkinje General Instrument Co., Ltd, China). The adsorption capacity of heavy metal ions to M-HAP/Agar composite beads was calculated by the following equation:

qe =

(C o − C e )V

(1)

M

where qe is equilibrium adsorption capacity of metal ions (mg g-1), Co and Ce (mg L-1) are initial and equilibrium concentration of heavy metal ions in solution, respectively. V (L) is volume of adsorbed solution, and M (g) is weight of dried M-HAP/Agar composite beads.

3. Results and discussions 3.1. Characterization of M-HAP/Agar composite beads. The synthesis process of M-HAP/Agar composite beads was divided into two main steps. Figure 1 illustrates the synthesis strategy of the composite beads. Firstly, PM-IDA was directly coated on the surface of Fe3O4 nanoparticles to generate the carboxyl groups. These nanoparticles were highly negatively charged when the carboxyl groups were anchored. Once the calcium ions were added into the reaction system, the anchored carboxylate groups on magnetic cores chelated with Ca2+ and further electrostatically attracted PO43-. With this fabrication process, the magnetic HAP nanoparticles were constructed. Finally, those obtained M-HAP nanoparticles were mixed with 7



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agarose solution and then emulsified into M-HAP/Agar composite spherical adsorbent. One appealing feature is that the proposed composite beads possess both large size and magnetic properties, which allows the easy recovery of the adsorbent. To elucidate the structure formation, the M-HAP/Agar composite beads were observed carefully by optical microscopy (OM) and scanning electron microscopy (SEM). From OM image (Figure 2A), the composite beads all exhibited perfectly uniform spherical shape with an average diameter of about 150 µm. The SEM images (Figure 2B) further confirmed the observation. As shown in SEM images, the surface of composite beads was not very smooth compared with the pure agarose beads (Figure 2C). It indicated that many M-HAP nanoparticles have been successfully encapsulated in the agarose network. These M-HAP /Agar composite beads could be dispersed in water by vigorous shaking, while be quickly assembled by an external magnetic field, as proved by the observation of Figure S1. The excellent magnetic responsivity and large size of the adsorbent would endow the easy recovery from the effluents during the adsorption process.

Figure 1. Schematic illustration of M-HAP/Agarose composite beads fabricated by modification with PM-IDA.

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Figure 2.

OM (A) and SEM (B) of M-HAP/Agar composite beads; SEM (C) of pure agarose beads.

The crystallographic structure of Fe3O4 and M-HAP were determined using X-ray (XRD). As shown in Figure 3, the characteristic diffraction peaks of face-centered cubic structure magnetite (Fe3O4) were appeared in both samples (JCPDS 19-0629), which agreed well with the result of Yang et al.17 Meanwhile, coating HAP shell via deposition process did not affect the phase structure of Fe3O4, due to the existing of typical diffraction peaks of Fe3O4 on M-HAP samples. What’s more, the XRD pattern of the recovered one showed new peaks, which corresponded to the hydroxyapatite (HAP, Ca10(PO4)6(OH)2) structure (Figure 3b) according to JCPDS card No. 19-0432. Therefore, it concluded that the Fe3O4 nanoparticles were successfully coated by HAP through a series of surface modification reactions.

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Figure 3. XRD patterns of Fe3O4 particles and M-HAP nanoparticles along with hydroxyapatite reference. In order to further verify the coating of HAP on Fe3O4 nanoparticles, the obtained Fe3O4 and M-HAP nanoparticles were characterized by FTIR measurement, respectively, as seen in Figure S2. The Fe3O4 spectrum showed a strong and broad peak at 3140 cm−1, and it was assigned to the O−H bending vibration peak caused by hydrogen bonds. A weak signal at 1619 cm-1 was the characteristic peak originated from the stretching vibration of OH group. The peak at 576 cm−1 was ascribed to the stretching vibration of Fe−O bonding. For M-HAP nanoparticles, the absorption peaks at 1113 cm-1、563 and 1035 cm-1 corresponded to the molecular vibration of PO43- groups, confirming the successful deposition of HAP on the surface of magnetic nanoparticles.16 Based on the XRD and FTIR results, the composite agarose beads entrapped with magnetic hydroxyapatite nanoparticles have been successfully synthesized through a series of chemical reactions. Nitrogen sorption measurements were conducted to characterize the pore parameters of the samples. The nitrogen adsorption-desorption isotherm and pore size distribution of M-HAP 10


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nanoparticles were shown in Figure 4. It was found that the M-HAP nanoparticles exhibited a typical IV isotherm with a H1 type hysteresis loop according to the IUPAC classification. The pore-size distribution plot confirmed that the M-HAP nanoparticles have well-developed pores with a narrow size distribution centering at 14.5 nm (inset in Figure 4). It was estimated that the relatively large pores within HAP particles were derived from the aggregated particles.26 The BET surface and pore volume of M-HAP were determined to be about 90.00 m2 g-1 and 0.398 cm3 g-1, respectively, suggesting highly porous structure in the HAP sample. The relatively high BET surface area together with nanoscale size would endow the adsorbent with high adsorption capacity for heavy metal ions.

Figure 4. N2 sorption isotherms and pore size distribution (inset) of M-HAP nanoparticles.

3.2. High adsorption capacity for heavy metal ions. The as-prepared composite beads were employed to adsorb metal ions from aqueous solution at pH 7.0 in this experiment. To ensure the adsorption equilibrium, the composite beads were incubated in the metal ions solution for 8 h. It is well known that the surface chemistry of adsorbent has great effects on the ions adsorption 11



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behavior. Zeta potential has been widely used to characterize the surface charge of particles. Figure 5 presented the zeta potential on composite beads after the adsorption equilibrium with different initial heavy metal ions solution. It was observed that the zeta potential of composite beads was negative before adsorption. After heavy metal ions being adsorbed, the value of zeta potential on the composite beads further reduced with an increase of initial metal ions concentration, and finally reached a plateau after the saturation adsorption. More interestingly, it was worth noting that with more cations being adsorbed, the zetal potential of composite beads became more negative. Such a different phenomenon can be ascribed to the anchored cations on HAP, which shows strong affinity to OH-1 in the solution. As a result, more cations adsorbed more OH-1 and then contributed larger negative potential on the adsorbent surface. Moreover, as shown in Figure 5, the composite beads had very high adsorption capacities for both Pb2+ and Co2+, up to about 806.7 mg g-1 and 90.6 mg g-1, respectively. Compared with the previous reports, the adsorption capacities of the M-HAP/Agar composite adsorbent were relatively higher than other similar hydroxyapatite ones.11, 14, 15, 27 The high adsorption capacity for heavy metal ions on this adsorbent might be ascribed to the following reason. During the fabrication, the PM-IDA served as a bridge and facilitated the deposit of hydroxyapatite on the magnetic cores surface. In addition, the M-HAP was synthesized under relatively low temperature. It has been reported that lowering the reaction temperature generated the nanolayer-like hydroxyapatite with lower crystallinity and smaller crystallite size.7 With that, the prepared M-HAP/Agar composite beads enjoyed the advantages of both large specific surface and suitable pore size and then achieved high adsorption capacity for Pb2+ and Co2+. However, the M-HAP/Agar composite beads did not display obvious improvement for Cu2+ adsorption. The significant difference may derive from 12


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the unique interaction between HAP and Cu2+, and we are currently working on finding the reason.

Figure 5. The dependence of zeta potential ( ) and metal ion adsorption capacities ( ) on adsorbent upon initial metal ion concentration in solution.

3.3. Effect of pH The pH value is an important parameter for the adsorption capacity of an adsorbent because it might affect not only the protonation degree of functional groups on the adsorbent but also the ion forms in the solution.11, 28, 29 A series of batch equilibrium experiments 13



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were carried out to confirm the effect of pH on the adsorption of heavy metal ions at the initial concentrations of 30 mg L-1, 35 mg L-1 and 400 mg L-1 for Cu2+, Co2+ and Pb2+, respectively. The effect of solution pH in this work was adjusted in the range of 3.0-7.0 using dilute HNO3 or NaOH solution, and the results were indicated in Figure 6. The metal ions adsorption by M-HAP/Agar composite beads mainly relied on the ion-exchange of M-HAP through substituting Ca2+ ions from the apatite lattice by a diffusion process according to Eq. (1): Ca10(PO4)6(OH)2 + xMe2+

(Ca10-x)Mex(PO4)6(OH)2 + xCa2+

（1）

With an increase of the initial pH, all metal adsorption capacities increased dramatically. It might be assigned to the reducing competition between positively H3O+ and metal ions on the same adsorption sites on M-HAP microspheres, as reported previously.13 While being in the range of lower pH values, a large number of H3O+ emerged and then resulted in a fierce competition in the ion-exchange sites on M-HAP adsorbent. Because of repulsive force, the metal ions would be hindered from approaching to the adsorbent sites. Generally, the adsorption capacity of M-HAP/Agar composite beads shows the strong dependency upon pH variation of metal ions solution. Figure 6 presented the relationship between the relative amounts of metal ionic species and the initial pH in solution. As shown in Figure 6A, up to pH 5, the copper in solution was present mainly as the form of Cu2+ ions. While the positively charged Cu(OH)+, Cu2(OH)22+ and Cu3(OH)42+ hydrolytic products appeared at pH 6 and then became the dominant formation at pH 7. Besides, the neutral Cu(OH)2 began to produce at pH 8 (Figure 6A). For the species of lead, the same phenomenon can be seen in Figure 6C. The hydrolytic species of lead appeared at pH 5, and the amount of Pb2+ reduced remarkably at pH 7 because of the appearance of hydrolytic species Pb(OH)+, Pb4(OH)44+, 14


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Pb3(OH)42+ and Pb6(OH)84+. What’s more, Pb(OH)2 only occurred after pH 8.2. More interestingly, the adsorption capacity of lead and copper at pH 7 showed no decrease, which suggested the existence of lead and cooper adsorption process by the sorption of positively charged hydrolytic species. On the contrary, the effect of pH on cobalt was slightly different from the copper and lead adsorption. The hydrolytic products of cobalt appeared up to pH 7, and the cobalt ions started to precipitate at pH 8. Thus, there was no significant decrease for Co2+ adsorption at pH 7 (Figure. 6B). As a result, the competition between H3O+ and Co2+ worked crucially at pH 7. In view of the role of initial solution pH on the adsorption efficiency, all these results indicated that the adsorption ability of the adsorbent was strong in nearly neutral conditions and poor in acidic conditions.

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Figure 6. Distribution of heavy metal ionic species as a function of initial pH and effect of initial pH on immobilized capacity of M-HAP/Agar composite beads (insert).

3.4. Equilibrium studies on heavy metal adsorption. Adsorption isotherm is of very importance in providing the sufficient information for diagnosing the adsorption behavior and optimizing the separation operation. For this purpose, two different isotherm models, Langmuir (Eq. 2) and Freundlich adsorption isotherms (Eq. 3) were applied to analyze the experimental adsorption data and then gave the adsorption parameters.3 16


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qe =

q m K LC e 1 + K LC e

q e = K FC e

(2)

1/n

(3) where KL is the Langmuir adsorption constant (L mg−1) related to the affinity of binding site, Ce represents the solute equilibrium concentration (mg L−1), qe is the adsorption capacity of metal ions, qm is the equilibrium capacity (mg g-1), KF is the characteristic constant related to the adsorption capacity (mg1-1/n L1/n g-1), n is a constant related to the adsorption intensity. To judge whether the adsorption process is favorable or not for Langmuir isotherm model, the dimensionless constant separation factor (RL) has also been calculated using Eq.4.30

RL =

1 1 + K LC O

(4)

where C0 represents the initial concentration of adsorbate. The RL value shows whether the type of the isotherm is favorable (0< RL1), linear (RL=1), or irreversible (RL=0). The adsorption isotherms and the quantitative parameters were summarized in Figure 7 and Table 1. It revealed that all the values of ‘n’ fitted by Eq. 2 were greater than 2, indicating that these metal ions were facile to be adsorbed onto M-HAP/Agar composite beads.31 Meanwhile, the values of RL were determined to be less than 1. The results indicated that this adsorption behavior matched better with Langmuir model and then proved the monolayer coverage of the metal ions on the adsorbent. Fitting the experimental adsorption data in Langmuir model gave the maximum adsorption capacities of 71.6 mg g-1, 842.6 mg g-1, and 105.1 mg g-1 for Cu2+, Pb2+, and Co2+, respectively. The maximum adsorption capacities for varied metal ions are relatively high and endow the M-HAP/Agar composite beads with great potential for the heavy metal ions adsorption. 17



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Figure 7.

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Adsorption equilibrium isotherms of Co2+(A), Cu2+(B), and Pb2+(C) onto M-HAP/Agar composite beads.

Table 1. Adsorption equilibrium isotherm parameters of Co2+, Cu2+, and Pb2+ removal onto M-HAP/Agar composite beads. Isotherm equation

Langmuir

Freundlich

Parameter

Co2+

Cu2+

Pb2+

qm (mg g-1) KL (L mg-1) R2 RL KF (mg1-1/n L1/n g-1) n R2

105.1 0.104 0.992 0.046-0.490 20.068 2.815 0.877

71.6 0.456 0.986 0.011-0.180 27.088 3.876 0.661

842.6 0.052 0.995 0.016-0.280 137.024 3.167 0.863

Dynamic adsorption capacity and adsorption time are two important parameters that evaluate the adsorbent in the fields of its adsorption efficiency and help to design the adsorption process. Figure 8 shows the effect of time change on the adsorption of varied heavy metal ions on the M-HAP/Agar composite beads. It revealed that the adsorption capacity of all three metal ions increased sharply within the first period of about 2 h and then slowly reached equilibrium after 8 h. In this way, 8 h of adsorption time was selected as the optimum operation time to guarantee the satisfied adsorption. Given the mesoporous structure of M-HAP/Agar composite beads (Figure 4), the adsorption behavior is mainly dominated by intraparticle diffusion. The initial 18


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faster adsorption can be ascribed to the larger free surface available for metal ions. Once the available free surface is covered by adsorbates, other metal ions have to penetrate through the matrix and be adsorbed inside the inner pores, which leads to the slower adsorption kinetics at later adsorption stage. In order to elucidate clearly the adsorption mechanism, two adsorption models in terms of pseudo-first order (Eq. 5) and pseudo-second order (Eq. 6) were applied to fit the experimental data, which were expressed by the following equations:32

q t = q e(1 − e −k t ) 1

(5) 2

k 2q e t qt = 1 + k 2q et

(6)

where qe and qt represent the amount of metal ions absorbed (mg g-1) at equilibrium and time t (h), respectively. k1 (h-1) and k2 (g mg-1 h-1) are the rate constants of adsorption. The fitting results of adsorption kinetics were summarized in Table 2. It revealed that the values of rate constant for all metal ions decreased regularly with an increase of the initial metal ions concentration. Differently, the correlation coefficient (R2) for pseudo-second order was higher than that of pseudo-first order, which confirmed that the pseudo-second order described better for the metal ions adsorption on the M-HAP/Agar composite beads. It was seen from Figure 8 that the fitting curves by pseudo-first order model matched better with the experimental adsorption data, which further confirmed the conclusion stated above. All the results suggested that the adsorption of metal ions on HAP might be dominated by chemical sorption process.17, 33

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Figure 8.

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Adsorption kinetics of Cu2+(A), Pb2+(B), and Co2+(C) onto M-HAP/Agar composite beads.

Table 2. Kinetic parameters of Cu2+, Pb2+, and Co2+ removal onto M-HAP/Agar composite beads. Cu2+

isotherm equation

100 mg L-1

400 mg L-1

1000 mg L-1

40 mg L-1

150 mg L-1

28.3

66.4

387.7

748.2

34.6

89.0

qe,cal (mg g )

25.8

62.8

358.7

714.7

32.9

83.6

k1 (h-1)

1.183

0.644

1.244

1.145

1.619

1.110

0.986

0.974

0.856

0.959

0.985

0.962

28.3

66.4

387.7

748.2

34.6

89.0

qe,cal (mg g )

28.6

69.3

396.6

754.4

35.8

93.8

k2 (g mg-1 h-1)

0.061

0.011

0.005

0.003

0.071

0.019

0.999

0.987

0.984

0.991

0.999

0.992

-1

2

R

qe,exp (mg g-1) Pseudo-secondorder model

Co2+

30 mg L-1

qe,exp (mg g-1) Pseudo-first-ord er model

Pb2+

-1

2

R

4. Conclusions In this study, the magnetic hydroxyapatite (M-HAP) nanoparticles have been successfully fabricated by surface modification method, in which the PM-IDA as intermediate contributed the unique structure formation of HAP-based adsorbent for high capacity. To meet the criterion of industry application, the M-HAP nanoparticles were mixed with agarose solution and then was emulsified into M-HAP/Agar composite beads with large size. As expected, the entrapped HAP nanoparticles in the beads provided an efficient adsorption of heavy metal ions. And, the large 20


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size and the magnetic response of M-HAP/Agar composite beads allowed being simply separated by means of an external magnetic field or filtering. The as-prepared adsorbents were employed in the adsorption of Pb2+, Co2+, Cu2+, showing an ultra-high adsorption capacity and efficiency. All of these splendid properties proved that the prepared composite beads could be used as novel, highly efficient and environmentally friendly sorbents for the removal of heavy metal ions from waste water.

Acknowledgements The work was funded by Natural Science Foundation of China (Grant 21676170, Grant 21476144),.

Supporting Information Magnetic separation-redispersion images showing the excellent magnetic responsivity of composite beads; FTIR results confirming the successful deposition of HAP on the Fe3O4.

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Fabrication and Characterization of Magnetic Hydroxyapatite

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