Efficient heavy metal ion removal by triazinyl-β-cyclodextrin functionalized iron nanoparticles

Abstract

In this contribution, novel nano-adsorbent, triazinyl-β-cyclodextrin modified magnetic nanoparticles (T-β-CD–MNPs) were fabricated for the removal of heavy metal ions from aqueous solutions by reacting mono-chlorotriazinyl-β-cyclodextrin (T-β-CD) with magnetite Fe₃O₄ nanoparticles (MNPs) via the nucleophilic substitution of the chloride leaving group of T-β-CD by the attacking hydroxyl group of the MNPs. T-β-CD–MNPs exhibit excellent removal ability for heavy metal ions (Pb²⁺, Cu²⁺, Zn²⁺ and Co²⁺). The grafted T-β-CD on the MNP surface contributes to an improvement of the adsorption capacity due to the strong abilities of the multiple hydroxyl and azinyl nitrogen groups in T-β-CD to adsorb metal ions. They also exhibited a relatively good saturation magnetization (67 emu g⁻¹), which allowed their highly-efficient magnetic separation from wastewater. Equilibrium data for metal ion adsorption fit well with the Langmuir isotherm model with maximum adsorption capacities of 105.38, 58.44, 51.30 and 33.33 mg g⁻¹ for Pb²⁺, Cu²⁺, Zn²⁺ and Co²⁺, which is larger than MNPs that had been modified by other natural compounds. The excellent adsorption capacity of T-β-CD–MNPs, together with other advantages such as reusability, easy separation, environmentally friendly composition, and their simple preparation method, make them suitable adsorbents for the removal of heavy metal ions from environmental and industrial waste.

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Introduction

Nowadays, considerable attention is paid to water treatment. Clean water, especially free of highly toxic heavy metal ions, is vital for people’s health.^1,2 Industrial wastewater from different industries such as chemical manufacturing, battery manufacturing, metallurgical, leather tanning, and mining industries, contain various heavy metal ions. Wastewater containing heavy metal ions is discharged into natural water directly and will harm human health through the food chain.^3,4 So, toxic metal ion removal from wastewater is becoming an important subject.

Heavy metal ions can be removed using several techniques, including chemical precipitation, reverse osmosis, electrochemical treatment techniques, ion exchange, membrane filtration, coagulation, extraction, irradiation, and adsorption.^5–9 Among the mentioned methods, the adsorption technique is the most extensively approved one owing to its simplicity, high efficiency and low cost.^10–14 Recently, nanomaterial sorbents exhibited higher adsorption properties than bulk materials due to their higher surface areas and many more surface active sites.^15,16

Nanosized metal oxides, as one of the inorganic materials, have been used broadly for heavy metal ion removal in wastewater treatment in recent decades. These nanoparticles have high surface areas and specific affinities. They possess minimal environmental impact and low solubility with no secondary pollution, and have been adopted as sorbents to remove heavy metals.^17–21 Among the different kinds of nanoparticles, magnetic nano-adsorbents have attracted significant attention due to their specific features, especially the absence of internal diffusion resistance, which provides better kinetics for metal ion adsorption from aqueous solutions. Moreover, as they combine the benefit of magnetic separation techniques and nano-sized materials, they can be easily recovered or manipulated with an external magnetic field.^22–27

Furthermore, incorporation of magnetic particles with other functionalized materials can be an effective way for pollutant removal and also can potentially be a promising technique to facilitate the adsorbent separation and recovery. The functionalization and modification of magnetic nanoparticles with various biocompatible and biodegradable materials in many different ways has been reported.^22,28–30 Among various natural compounds for the functionalization of magnetic nanoparticles, β-cyclodextrin (β-CD) is an important family of cyclic oligosaccharides containing a cavity that is limited by hydroxyl groups of different chemical character. This apolar cavity has primary hydroxyl groups lying on the outside and secondary hydroxyl groups on the inside. In addition, this structure significantly raises the capability to form inclusion complexes with organic molecules, through host–guest interactions.^31–34

The aim of this work is the preparation and characterization of magnetic nanoparticles (MNPs) functionalized with a reactive β-cyclodextrin derivative, monochlorotriazinyl-β-cyclodextrin (MCT-β-CD), and studying their applicability for heavy metal ion adsorption. These β-CD grafted MNPs (T-β-CD–MNPs) were used as separable, recyclable and highly selective nano-adsorbents for the effective removal of metal ions, i.e., Pb²⁺, Cu²⁺, Zn²⁺ and Co²⁺. The obtained results revealed that the adsorption capacity for the metal ions was larger than in many other reports on the adsorption of metal ions by magnetic materials that were modified by other natural compounds.

Experimental

Materials

MCT-β-CD was purchased from Ebo-chem. Fe₃O₄ nanoparticles with an average particle size of about 25–30 nm were purchased from Neutrino Co. Cobalt chloride (CoCl₂·6H₂O), copper chloride (CuCl₂·2H₂O), zinc acetate (Zn(OAc)₂·2H₂O), lead thiocyanate (Pb(SCN)₂), Na₂CO₃ and NaCl were purchased from Merck.

Instrumentation

FT-IR spectra of the materials were recorded on a Jasco-680 (Japan) spectrometer at 4 cm⁻¹ resolution using pressed KBr pellets, and scanned in the wavenumber range 400–4000 cm⁻¹. Elemental analysis was performed using a CHNS-932, Leco. Thermogravimetric analysis (TGA) was performed with a STA503 win TA (Bahr-Thermoanalyse GmbH, Hüllhorst, Germany) at the heating rate of 10 °C min⁻¹ from 25 °C to 800 °C under a nitrogen atmosphere. X-ray diffraction (XRD) was used to characterize the crystalline structure of nanoparticles. XRD patterns were collected using a Bruker, D8 Advanced diffractometer with a copper target at the wavelength, λ, of CuK_α = 1.54 Å and a tube voltage of 40 kV and tube current of 35 mA, in the range of 10–100° at the speed of 0.05° min⁻¹. Nanoparticle morphology was observed using field emission scanning electron microscopy (FE-SEM, HITACHI S-4160, Japan). A Perkin-Elmer 2380-Waltham flame atomic absorption spectrophotometer (FAAS) was used for determination of the metal ion concentrations. Magnetic properties of the prepared nanoparticles were determined using a vibrating sample magnetometer (VSM) Kavir Kashan (Iran).

Functionalization of Fe₃O₄ nanoparticles

Fe₃O₄ nanoparticles (0.1 g), MCT-β-CD (0.025 g), H₂O (4 mL) and Na₂CO₃ (0.015 g) were refluxed for 90 min. During this time, two portions of NaCl (each time 0.015 g) were added gradually and then Na₂CO₃ (0.015 g) was added. The reaction was refluxed for another 2 h. The product was filtered and washed several times with water, and eventually dried under vacuum at 70 °C for 24 h.

Adsorption measurements

Pb²⁺, Cu²⁺, Zn²⁺ and Co²⁺ ion adsorption experiments were carried out using a batch equilibrium technique in aqueous solutions at 25 °C using T-β-CD–MNPs. Analyzing the adsorption behavior of the MNPs involved adding T-β-CD functionalized Fe₃O₄ nanoparticles (5 mg) to 50 mL of a solution of each metal ion at different concentrations at room temperature and stirring for 120 min. Thereafter, T-β-CD–MNPs with adsorbed heavy metal ions were separated from the mixture with a permanent hand-held magnet. The residual heavy metals in the solution were determined with a Perkin-Elmer 2380-Waltham flame atomic absorption spectrophotometer (FAAS).

The removal efficiency (% R_e) and the amount of heavy metal ions adsorbed q_m (mg g⁻¹) were calculated according to eqn (1) and (2), respectively:

where C₀ and C_t are the concentrations of the metal ions in the aqueous phase before and after the adsorption period, respectively (mg L⁻¹); V is the volume of the aqueous phase (mL), and m is the amount of dry T-β-CD–Fe₃O₄ nanoparticles used (g).

Desorption and reusability

A desorption study was conducted by stirring the metal ion loaded MNPs (0.05 g) in a nitric acid solution (10 mL, 0.01 M) at room temperature for 3 h to desorb the metal ions. The solid phase T-β-CD–MNPs were collected using a magnet and neutralized with dilute NaOH, washed with deionized water, and again subjected to the adsorption process to determine the reusability of the MNPs. The reusability was checked by following the above adsorption–desorption process for six cycles for Pb²⁺ ions.

Results and discussion

Functionalization of MNPs with MCT-β-CD

Among the different kinds of iron oxide nanoparticles, Fe₃O₄ demonstrates more favorable properties owing to the existence of non-equivalent iron cations in two valence states, Fe²⁺ and Fe³⁺, in the crystal structure, which causes a magnetic structure. Fe₃O₄ nanoparticles show better magnetic separation behavior compared to γ-Fe₂O₃ and amorphous Fe₂O₃ nanoparticles.^35–37 MCT-β-CD contains a reactive site which was used to graft β-cyclodextrin onto the surface of the MNPs through forming covalent bonds with the –OH groups on the Fe₃O₄ surface. As shown in Scheme 1, each –OH group of Fe₃O₄ surface can displace the reactive triazine Cl⁻ leaving group of a MCT-β-CD molecule.

Scheme 1 MNP functionalization with MCT-β-CD and their use as a facile tool for heavy metal ion removal with the help of an external magnetic field.

Characterization of T-β-CD–Fe₃O₄

T-β-CD grafted on the MNPs was confirmed using FT-IR spectroscopy. Fig. 1 shows the FT-IR spectra of MCT-β-CD, and pure and CD coated Fe₃O₄ nanoparticles in the 4000–400 cm⁻¹ wavenumber range. In the FT-IR spectrum of pure MNPs (Fig. 1a), the bands at around 1627 and 3434 cm⁻¹ can be assigned to the H–O–H stretching modes and bending vibrations of the free or adsorbed water, respectively. In addition, the characteristic absorption band of the MNPs is 588 cm⁻¹, which is due to the Fe–O bonds in the tetrahedral sites. In the MCT-β-CD FT-IR spectrum (Fig. 1b), the significant bands are assigned to –OH groups (3420 cm⁻¹), C–H aliphatic groups (2923 cm⁻¹) and C=N and C–N triazine ring (1611 and 1469 cm⁻¹). After functionalization of the Fe₃O₄ nanoparticles by T-β-CD, appearance of some characteristic peaks of sp³ C–H groups, C=N and C–N triazine ring (1626 and 1453 cm⁻¹) and C–O–C (1027 cm⁻¹) confirms that T-β-CD grafts onto the MNP surface. Moreover, the characteristic absorption band of the Fe–O bonds in the tetrahedral sites is shifted to 579 cm⁻¹ after surface modification with T-β-CD.

Fig. 1 FT-IR spectra of (a) pure Fe₃O₄, (b) MCT-β-CD and (c) T-β-CD–MNPs.

Elemental analysis provides further evidence of successful T-β-CD grafting onto the MNP surface. Elemental analysis of carbon, hydrogen and nitrogen before and after functionalization of the MNPs with T-β-CD are presented in Table 1. The obtained data prove T-β-CD has grafted onto the Fe₃O₄ nanoparticle surface. In addition, the grafted amount of T-β-CD onto the Fe₃O₄ nanoparticle surface was calculated using eqn (3),^38–40 using the T-β-CD–MNP nitrogen content:

where W_t is the weight percent of the measured element, X is the theoretical weight percent of the element in the molecule and Y is the theoretical M_w of the molecule. T-β-CD has carbon, hydrogen and nitrogen content of 42.01, 5.41 and 3.27 wt%, respectively, with a M_w of 1285.32 g mol⁻¹. Based on this equation, the degree of T-β-CD bound onto the MNP surface is 12.4 mmol g⁻¹.

Table 1 Elemental analysis of Fe₃O₄ nanoparticles before and after functionalization with T-β-CD

Sample	C%	N%	H%
Pure Fe3O4	0.370	0.000	0.652
T-β-CD–MNP	1.445	0.451	0.406

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The VSM technique was used to measure the magnetization value of the magnetic particles under an applied magnetic field. Magnetic measurements of MNPs and the T-β-CD–MNPs were investigated at room temperature in an applied magnetic field sweeping from −8 to 8 kOe. The hysteresis loops of MNPs and the T-β-CD–MNPs are displayed in Fig. 2. According to the figure, both of them show ferromagnetic behavior. The magnetic saturation values of these nanoparticles are 80 and 67 emu g⁻¹, respectively. The obtained magnetization for T-β-CD–MNPs is lower than that of uncoated Fe₃O₄ at the same field. This is mainly assigned to the existence of the T-β-CD layer (a nonmagnetic material) on the surface of the nanoparticles. Observing such remarkable magnetic properties signifies that the prepared T-β-CD–MNPs have strong magnetic responsivity and can be separated easily from solution with the help of an external magnetic force.

Fig. 2 Hysteresis loops of (a) MNPs and (b) T-β-CD–MNPs.

TGA analysis was used to estimate the amount of T-β-CD grafted onto the Fe₃O₄ MNPs by contrasting the uncoated and T-β-CD coated MNP thermograms. As shown in Fig. 3a, the TGA curve of the uncoated MNPs shows a total weight loss of 2% over the full temperature range that could be due to the loss of the adsorbed water as well as the loss of surface OH groups and could also be attributed to the oxidation of Fe₃O₄ to Fe₂O₃. The TGA curve of T-β-CD–MNPs (Fig. 3b) demonstrates two steps to the weight loss; the first step is due to the loss of absorbed water in the 25–110 °C range and hydroxyl groups up to 230 °C and the second step is assigned to the loss and thermal decomposition of T-β-CD in the range of 250–750 °C. Compared with that of uncoated MNPs, the weight loss of T-β-CD–MNPs is 16 wt%. Therefore, TGA analysis also proves the successful grafting of T-β-CD onto the MNP surface.

Fig. 3 TGA thermograms of (a) MNPs and (b) T-β-CD–MNPs.

In addition, according to eqn (4),^41,42 the number of T-β-CD molecules grafted on the MNP surface could be calculated, where N is the number of T-β-CD molecules immobilized on each MNP, R is the mean radius of the T-β-CD–MNPs (50 nm based on the FE-SEM results), ρ is the nanoparticle density (5.18 g cm⁻³), N_A is Avogadro’s number, W_T-CD is the weight loss of the T-β-CD–MNPs and M_T-CD is the molar mass of the T-β-CD immobilized on the T-β-CD–MNPs. The number of T-β-CD molecules grafted on each T-β-CD–MNP calculated was about 1024.

The XRD patterns of the MNPs and T-β-CD–MNPs indicate some characteristic peaks, as shown in Fig. 4. These peaks are associated to their corresponding indices (JCPDS no. 85-1436 for comparison), which reveal that the modification of MNP surface with T-β-CD does not change the Fe₃O₄ phase. In addition, a low intensity broad peak centered at 2θ = 20° is observed in the XRD pattern of T-β-CD–MNPs, which could be due to the presence of cyclodextrin molecules on the MNP surface. The crystal size of T-β-CD–MNPs, determined from the XRD pattern using Scherrer’s equation (eqn (5)), is found to be 57 nm, which is almost consistent with the observed result from the FE-SEM images.

Fig. 4 XRD patterns of (a) MNPs and (b) T-β-CD–MNPs.

Fig. 5 displays FE-SEM images of T-β-CD–MNPs at different magnifications. The FE-SEM images show that the Fe₃O₄ particles differed in size to some extent and that the particles had a mean diameter of about 50 nm (spheroid).

Fig. 5 FE-SEM images of T-β-CD–MNPs.

Adsorption of heavy metal ions

Effect of pH. The pH value plays a very significant role in the adsorption process and especially on the adsorption efficiency or affinity in the use of nano-adsorbents as supports in the metal ion adsorption process. The effect of the initial solution pH on Pb²⁺, Cu²⁺, Zn²⁺ and Co²⁺ adsorption onto T-β-CD–MNPs was investigated at pH 2–6, 25 °C, and an initial M²⁺ ion concentration of 300 mg L⁻¹. Fig. 6 displays the effects of pH on the adsorption of Pb²⁺. As observed, the removal efficiency of Pb²⁺ ions from water samples by MNPs increases with increasing pH from 2 to 6 and the maximum adsorption efficiency was obtained at pH 6 at room temperature. Decreasing Pb²⁺ removal at pH < 6 may be due to hydronium ion competition toward complexation with CD. In an acidic solution, the protonation of CD occurs and there is a weak tendency for the reaction between Pb²⁺ and CD, which leads to the decrease in the extraction yield. At pH > 6, precipitation of lead hydroxide, Pb(OH)₂, causes the removal efficiency to decrease.

Fig. 6 Effect of pH on Pb²⁺ removal efficiency of the T-β-CD–MNPs.

Removal of heavy metal ions

The scheme for heavy metal ion removal from aqueous solution using T-β-CD–MNPs as the adsorbent is shown in Scheme 1. T-β-CD–MNP adsorption capacities for the metal ions were measured individually at pH 6 with 5 mg of functionalized Fe₃O₄ and various metal concentrations (20–450 mg L⁻¹), and using the data of the heavy metals adsorbed at equilibrium (q_e, mg g⁻¹) and the equilibrium metal concentration (C_e, mg L⁻¹) with the Langmuir adsorption model (eqn (6)):where q_e is the amount of adsorbate adsorbed per mass of adsorbent at equilibrium (mg g⁻¹), C_e is the equilibrium concentration of adsorbate in aqueous solution (mg L⁻¹), q_m is the maximum capacity of the adsorbent (mg g⁻¹), and K_L is the Langmuir equilibrium constant. The data fit well to the model with correlation coefficients (R²) in the range of 0.97–0.99, and maximum adsorption capacities of 105.38, 58.44, 51.30 and 33.33 mg g⁻¹ for Pb²⁺, Cu²⁺, Zn²⁺ and Co²⁺, respectively (Fig. 7, S1† and Table 2).

Fig. 7 Langmuir isotherm fitting for the T-β-CD–MNPs for Pb²⁺, Cu²⁺, Zn²⁺, and Co²⁺ ion removal processes.

Table 2 Langmuir constant values for heavy metal ion removal

Metal ion	qm (mg g^−1)	KL (L mg^−1)	R^2
Pb^2+	105.38	0.0828	0.9801
Cu^2+	58.44	0.2200	0.9940
Zn^2+	51.30	0.0994	0.9997
Co^2+	33.33	1.0610	0.9772

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Moreover, to examine competitive adsorption for mixtures of Pb²⁺, Cu²⁺, Zn²⁺, and Co²⁺ ions, T-β-CD–MNPs (0.050 g) were added to a solution (50 mL) containing equal initial concentrations (30 mg L⁻¹) of each metal ion. The pH was maintained at 6, and the equilibrium time was 2 h. Finally, the adsorbents were separated by powerful magnets and the removal efficiencies (% R_e) were calculated using eqn (1). The obtained removal efficiencies for metal ions decreased in the order Pb²⁺ (94%) > Cu²⁺ (49.6%) > Zn²⁺ (44%) > Co²⁺ (33%).

These results reveal that modification of MNP surface by T-β-CD shows good adsorption capabilities because of the strong abilities of the multiple hydroxyl and azinyl nitrogen groups in T-β-CD to adsorb metal ions. The significant difference among the values of q_m can be attributed to the different complexation capacity of the oxygen and nitrogen groups on the MNP surface with the metal ions. The extent to which a metal ion will bind to a ligand depends strongly on the chemistry of the metal ion and its preference to form covalent or ionic bonding. Metal ions act as Lewis acids by accepting electron pairs from ligands. Pb²⁺ is known as soft acid which can strongly interact with CD oxygen and nitrogen groups to form a complex. Cu²⁺, Zn²⁺, and Co²⁺ are on the borderline between soft and hard.

Desorption and reusability

From a practical point of view, regeneration of adsorbents is an important feature to evaluate their repeatability in use; therefore, the reusability of T-β-CD–MNPs was checked with an adsorption–desorption process using a HNO₃ solution (0.01 M) for six cycles for Pb²⁺ ions and the removal efficiency in each cycle was analyzed. In acidic conditions, the bonding between the active sites of MNP and metal ion is not sufficiently strong to be held. Under these conditions, H⁺ ions protonate the adsorbent surface, leading to the desorption of positively charged metal ions. Pb²⁺ removal efficiency is slightly reduced in the later cycles, however the removal efficiency is still above 92% in the final cycle. The T-β-CD–MNPs adsorbent kept its adsorption capability after repeated adsorption–regeneration cycles with small changes, signifying that there are almost no irreversible sites on the T-β-CD–MNP surface. Recyclability studies reveal that these nanoadsorbents can be repeatedly used for the removal of heavy metal ions in wastewater treatment.

The maximum adsorption capacity of T-β-CD–MNP was compared with previously reported MNPs that had been modified with other natural materials, as shown in Table 3. According to the reported data, the T-β-CD–MNPs possess significant improvements over the existing adsorbents for heavy metal ion removal. In addition, one of the noteworthy advantages of this study is that the CD molecules were anchored on the Fe₃O₄ nanoparticles in a one-step method, which is a simple and inexpensive technique. Prepared T-β-CD–MNPs show higher adsorption capacities and they could be regenerated and recycled using simple magnetic separation technology which is clearly a great advantage.

Table 3 Comparison of the maximum adsorption capacities of T-β-CD–MNPs with those of some other adsorbents reported in literature

Adsorbents	Target metals	Maximum adsorbed amount, qm (mg g^−1)	References
Iron oxide nanoparticles	Cu^2+, Pb^2+, Cd^2+ and Ni^2+	19.30, 29.0, 18.59 and 11.34	23, 43 and 44
CM-β-CD modified MNPs	Cu^2+	47.20	30
CDpoly-MNPs	Pb^2+, Cd^2+ and Ni^2+	64.50, 27.70 and 13.20	45
Chitosan/magnetite nanocomposite	Pb^2+ and Ni^2+	63.33 and 52.55	46
Alginate beads magnetic nanoparticles	Pb^2+	99.5	47
Magnetic Fe3O4 baker’s yeast biomass	Pb^2+ and Cd^2+	89.2 and 41.0	8
T-β-CD–MNPs	Pb^2+, Cu^2+, Zn^2+, and Co^2+	105.38, 58.44, 51.30 and 33.33	This study

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Conclusions

A novel magnetic nano-adsorbent containing Fe₃O₄ nanoparticles functionalized with MCT-β-CD was fabricated and exhibited remarkable enhancement of heavy metal removal efficiency from aqueous solutions. T-β-CD provides numerous surface hydroxyl and azinyl nitrogen groups on the MNPs, which provide a strong affinity for metal ions. The T-β-CD–MNP synthesis procedure is simple and cost-effective and the resulting T-β-CD based Fe₃O₄ nanoparticles are very cheap and environmentally friendly as their main components, Fe₃O₄ and T-β-CD, are abundant and have no unfavorable effects on the environment. Adsorption of heavy metal ions onto T-β-CD–MNPs reaches an equilibrium within 120 min at pH 6.0, and fits well to a Langmuir adsorption model with maximum adsorption capacities of 105.38, 58.44, 51.30 and 33.33 mg g⁻¹ for Pb²⁺, Cu²⁺, Zn²⁺ and Co²⁺, respectively. The as-prepared T-β-CD–MNPs can be widely used in the removal of heavy metals from various waters. Moreover, since β-CD has inclusion capabilities with a wide variety of organic molecules, these nano-adsorbents can also be used for the removal of organic pollutants from wastewater.

Acknowledgements

We gratefully acknowledge the partial financial support from the Research Affairs Division, Isfahan University of Technology (IUT), Isfahan. Further partial financial support from the Iran Nanotechnology Initiative Council (INIC), National Elite Foundation (NEF) and Center of Excellency in Sensors and Green Chemistry (IUT) is also gratefully acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra15134a

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