Synthesis of an activated carbon based urea formaldehyde resin and its adsorption and recognition performance towards Fe(III)

Abstract

Rare earths are very important strategic resources. However, impurities, such as Fe(III), have great adverse impacts on the properties of rare earth materials. Therefore, the efficient and easy removal of non-rare earth impurities from rare earths is extremely important. A novel activated carbon, AC_UF700, was synthesized using a homemade urea formaldehyde resin carbonized at 700 °C. The AC_UF700 was characterized using surface area analyzer, FT-IR, elemental analysis, and SEM methods. The adsorption and recognition properties of AC_UF700 towards Fe(III) ions were studied. The BET special surface area of AC_UF700 was 702.3 cm³ g⁻¹, and the average pore diameter was 2.044 nm. AC_UF700 possesses strong adsorption affinity and excellent recognition selectivity for Fe(III). The adsorption capacity could reach 12.8 mg g⁻¹, and the relative selectivity coefficient relative to La(III) was 28.0. Besides, AC_UF700 was easily regenerated using a diluted hydrochloric acid solution as the eluent, and AC_UF700 possesses good reusability.

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1. Introduction

Rare earth elements (REEs) are composed of scandium, yttrium, and the lanthanide metallic elements. As some of the most important strategic and critical mineral resources, REEs have a significant role in luminescence, electronics, magnetism, catalysis, metallurgy, and the ceramic industry.^1,2 For high technology materials, non-rare earth impurities, such as Fe(III), have great adverse impacts on the properties of rare earth materials.³ The removal of non-rare earth impurities gets more and more attention. Therefore, the efficient and easy removal of non-rare earth impurities from high purity rare earths is extremely important. Researchers have carried out some related studies in the last century.^4,5 Solvent extraction methods and extraction-elution resin (solvent impregnated resin) methods are mainly used. However, some shortcomings, such as a low separating efficiency, are concomitant. Therefore, researching a new effective separation material is important for economic and social benefits.

Porous carbons have been widely used as adsorbents,^6,7 catalyst supports,⁸ and electrode materials⁹ due to their developed pore structure, high-specific surface area and easy surface modifications. The pore structure and surface chemical property of a porous carbon determine its application properties.^10–12 Direct preparation using nitrogen-containing compounds as precursors is an effective route to introduce nitrogen into a carbon matrix.^13,14

Urea formaldehyde resins (UF resins) have been widely used in adhesives, finishes, medium density fiberboard (MDF), and molded objects. Taking into account their high nitrogen content, UF resins can be used as raw materials for the fabrication of nitrogen-containing activated carbons.^15–18

In this study, activated carbon was synthesized using UF resins as raw materials. The pore structure and surface chemical groups of the resultant porous carbon were characterized, and its adsorption and recognition properties for Fe(III) were investigated.

2. Experimental

2.1 Preparation and characterization of AC_UF700

To prepare the UF resin, 18 g of NaHCO₃, 35 ml of a formaldehyde aqueous solution (37%) and 15 g of urea were mixed in a beaker. The mixture was heated to 95 °C for 5 h. The resultant solid resins were placed in a vacuum oven for 24 h at 80 °C to ensure complete dryness. The powdered resin was carbonized in a charcoal furnace at 700 °C for 2 h with a heating rate of 2 °C min⁻¹ and N₂ flow of 50 ml min⁻¹. Finally, the resultant samples were washed with distilled water until the pH was neutral and dried at 80 °C for 24 h.

The nitrogen adsorption–desorption isotherm was measured with a surface area analyzer (Beijing JWGB BF-JW132F) with nitrogen absorption at 77 K using the Brunauer–Emmett–Teller (BET) method. Before the measurements, the sample was degassed at 180 °C under vacuum for more than 3 h. The surface area was calculated using the BET method from the adsorption branch in the relative pressure range (P/P₀) of 0.005 to 0.1. The SEM images were obtained on a S-4800 Field Emission-Scanning Electron Microscope (Hitachi, Japan) operated at 1 kV. Fourier transform infrared (FTIR) spectra of the samples were measured on a Nicolet FT-IR 4800s (Shimadzu, Japan) spectrometer using the conventional KBr pellet method. The element content was measured with a Vario EL elemental analyzer (Elementar, Germany).

2.2 Batch adsorption of the activated carbon towards Fe(III)

2.2.1 Kinetic adsorption curve and adsorption isotherm. About 0.05 g of AC_UF700 was introduced into a conical flask directly. 25 ml of an Fe(III) aqueous solution with a concentration (C₀) of 100 mg L⁻¹ and pH of 2 was then added into the conical flask. This conical flask was placed in a shaker at a presettled temperature. At different times, the concentrations (C_t) of the Fe(III) solution were determined by inductively coupled plasma emission spectrometry. The adsorption capacity (Q) was calculated according to eqn (1).where V is the volume of the solution (L), and m is the weight of the absorbent carbon material (g).

Several similar conical flasks were prepared and about 0.05 g of AC_UF700 was introduced into each flask directly. 25 ml of Fe(III) aqueous solutions with different concentrations (C₀) and a same pH of 2 were then added into each conical flask. The conical flasks were placed in a shaker at a presettled temperature. After the adsorption reached equilibrium, the equilibrium concentrations (C_e) of the Fe(III) solutions in the different flasks were determined using an inductively coupled plasma emission spectrometer. The equilibrium adsorption capacities (Q_e) were calculated according to eqn (1).

2.2.2 Selectivity experiments. A binary mixed solution of Fe(III)/La(III) was prepared, and the concentrations of Fe(III) and La(III) were 10 mg L⁻¹ and 100 mg L⁻¹, respectively. The batch adsorption experiment was performed for the mixed solution. After adsorption reached equilibrium, the concentrations of Fe(III) and La(III) in the remaining solutions were determined. The distribution coefficients (K_d) of Fe(III) and La(III) were calculated using eqn (2).

The selectivity coefficient (k) of AC_UF700 for Fe(III) with respect to the competitor species La(III) can be obtained using eqn (3).

2.3 Dynamic adsorption experiment

2.0 g of AC_UF700 was filled in a glass column with 10 ml of the bed volume. The mixture solution of Fe(III) and La(III) with the initial concentrations of 10 mg L⁻¹ and 100 mg L⁻¹, respectively, and a pH of 2, was allowed to flow gradually through the column at a rate of 5 BV h⁻¹. 1 BV of the effluent was collected and the concentration was determined. Then the dynamic adsorption curve was plotted.

2.4 Repeated use experiment

Reusability is an important factor for a good adsorption material. Desorption of the adsorbed Fe(III) on AC_UF700 was also studied through a batch experiment. 5 ml of a hydrochloric acid solution with a concentration of 2 mol L⁻¹ was used as the eluent for 0.2 g of adsorbent, and the desorption time was 30 min. In order to test the reusability of AC_UF700, the Fe(III) adsorption–desorption procedure was repeated nine times.

3. Results and discussion

3.1 Characterization of AC_UF700

The Fourier transformed infrared (FTIR) spectra of UF and AC_UF700 are shown in Fig. 1. As for the UF resin synthesized here, the band assignment is in good agreement with literature reports.¹⁹ The band between 3200 and 3500 cm⁻¹ was attributed to NH stretching vibrations, and the band between 1500 and 1600 cm⁻¹ was attributed to C=C stretching vibrations. Three bands at 1080, 1039 and 875 cm⁻¹ were assigned to the asymmetric stretching vibration of the ether, C–O stretch of C–OH, and cyclic ether linkages. AC_UF700 exhibits broader and overlapping bands due to the strong IR absorption of carbon and the complex rearranged structure. It can be seen in the spectrum of AC_UF700 that AC_UF700 still retains the amine functional group after carbonization.

Fig. 1 FTIR spectra of UF and AC_UF700.

The morphologies of AC_UF700 were characterized using scanning electron microscopy at different magnifications and are shown in Fig. 2.

Fig. 2 SEM images of AC_UF700 with different magnifications. (a) x2000; (b) x5000; (c) x10 000; (d) x20 000.

The developed pore structure was easily observed. The relative data are listed in Table 1.

Table 1 Chemical composition and structure parameters of AC_UF

Sample	Elemental analysis (wt%)			Structure parameters				Yield (%)
	C	N	H	SBET (m^2 g^−1)	d (nm)	Vmicro (cm^3 g^−1)	Vtotal (cm^3 g^−1)
ACUF400	55.46	24.54	4.26	3	20.49	0.001	0.015	7.73
ACUF700	53.42	15.11	2.68	702	2.04	0.267	0.359	4.22
ACUF800	67.80	9.98	2.41	1920	2.77	0.229	1.328	1.21

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The surface properties of the samples were measured using the N₂ adsorption–desorption isotherms (Fig. 3). The chemical composition and pore structure parameters of AC_UF700 are listed in Table 1.

Fig. 3 N₂ adsorption–desorption isotherms.

AC_UF700 exhibited type I isotherms, and the pore size distribution is narrow.

3.2 Adsorption properties of AC_UF700 towards metal ions

3.2.1 Kinetic adsorption curve. The kinetic adsorption curve is shown in Fig. 4.

Fig. 4 Kinetic adsorption curves of AC_UF700 for Fe(III) and La(III). Temperature: 25 °C; pH = 2.

The adsorption rates of AC_UF700 toward the Fe(III) and La(III) ions were vary fast, the adsorption reached equilibrium within 80 min and 100 min, respectively. It can be seen that the saturated adsorption capacity of AC_UF700 for Fe(III) and La(III) is 12.8 mg g⁻¹ and 10.3 mg g⁻¹, respectively. It implies that AC_UF700 possesses a very strong adsorption ability for the Fe(III) and La(III) ions.

3.2.2 Adsorption isotherms. The Langmuir equation is as follows:where C_e (mg L⁻¹) is the equilibrium concentration, Q_e (mg g⁻¹) is the equilibrium adsorption capacity, Q_m (mg g⁻¹) is the saturated adsorption capacity, and k is the combined constant.

The data of Fe(III) and La(III) in Fig. 5 were regressed linearly according to eqn (4) and Table 2 was obtained. The data in Table 2 satisfactorily fit the Langmuir equation, and this clearly indicates that the Fe(III) and La(III) are adsorbed on AC_UF700 as monomolecular layers.

Fig. 5 Adsorption isotherms of AC_UF700 for Fe(III) and La(III). Temperature: 25 °C; pH = 2; adsorption time: 12 h.

Table 2 Fitting parameters of the Langmuir isotherm

Ion	Qm	k	R^2	Linear equation
Fe^3+	29.66	0.02395	0.9974	Y = 1.4077 + 0.03371X
La^3+	22.71	0.01136	0.9985	Y = 3.8745 + 0.04403X

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3.2.3 Adsorption selectivity. The competitive adsorptions of AC_UF700 for Fe(III) from Fe(III)/La(III) mixtures were researched in batch systems. Table 3 summarizes the data of the distribution coefficients K_d and the selectivity coefficient k.

Table 3 Distribution coefficient and selectivity coefficient data of AC_UF700

Adsorbent	Kd (L g^−1)		k
	Fe(III)	La(III)
ACUF700	2.80	0.10	28.0

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It can be seen that AC_UF700 has high selectivity for Fe(III). This suggests that the adsorption recognition of AC_UF700 for Fe(III) is very strong.

3.3 Column adsorption characteristics of AC_UF700 towards Fe(III)

3.3.1 Dynamic adsorption curve. Fig. 6 shows the dynamic adsorption curve of AC_UF700 for a mixture solution of Fe(III) and La(III) with initial concentrations of 10 mg L⁻¹ and 100 mg L⁻¹, respectively.

Fig. 6 Dynamic adsorption curve of AC_UF700 towards the mixture of Fe(III) and La(III). Temperature: 25 °C; pH = 2.

The leaking bed volume for Fe(III) is 120 BV and for La(III) is 18 BV. In addition, the leaking concentration of Fe(III) is lower than 5 mg L⁻¹ before 100 BV. This indicated that Fe(III) can be better recognized by AC_UF700, and that AC_UF700 can selectively remove Fe(III) from the La(III) solution.

3.4 Desorption and reusability

When hydrochloric acid was used as the eluent, the interaction between Fe(III) and AC_UF700 is disrupted and subsequently Fe(III) ions are released into the desorption medium. In order to show the reusability of AC_UF700, the adsorption–desorption cycle was repeated 9 times by using the same carbon material. The adsorption–desorption cycle of Fe(III) on AC_UF700 is shown in Fig. 7. The result clearly shows that AC_UF700 could be used repeatedly without significantly losing its binding ability.

Fig. 7 Adsorption–desorption cycle of AC_UF700.

4. Conclusions

In this paper, the high-performance activated carbon AC_UF700 was synthesized successfully using a homemade urea formaldehyde resin carbonized at 700 °C. The BET special surface area reached 702.3 m² g⁻¹. The average pore size was 2.044 nm. AC_UF700 possesses strong adsorption affinity, specific recognition ability, and excellent selectivity for Fe(III). The adsorption capacity could reach 12.8 mg g⁻¹, and the relative selectivity coefficient relative to La(III) is 28.0. Besides, AC_UF700 was easily regenerated using diluted hydrochloric acid solution as the eluent, and AC_UF700 possesses good reusability.

Acknowledgements

The authors gratefully acknowledge the financial support for this work by the International Science & Technology Cooperation Program of China (No. 2011DFA51980), Shanxi Science & Technology Cooperation Program of China (No. 2015081043), and Science Foundation of Shanxi Province (No. 20130321022-02, 2013021012-3, 20140313002-4).

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