Selective removal of transition metal ions from aqueous solution by metal–organic frameworks

Abstract

Water stable Zr-based metal–organic frameworks (MOFs) with different functional groups were used for selective removal of Cu²⁺ over Ni²⁺ from aqueous solution. Due to the unique chelation effect of two carboxyl groups on the adjacent organic ligand as well as the Jahn–Teller effect, UiO-66(Zr)–2COOH exhibits the highest selectivity (up to about 27) for Cu²⁺/Ni²⁺ in aqueous solution among the reported adsorbents as far as we know. In addition, the removal process is fast with less than 60 min for equilibration, and the stability and regenerability are good. These results may be helpful not only for the efficient treatment of wastewater containing heavy transition metal ions, but also for metal enrichment and recycling.

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1. Introduction

With the rapid development of industry, the pollutants in water are receiving more and more concern. Among them, heavy transition metal ions contamination has become a world-wide environmental problem. These ions can enter into the human body through inhalation, ingestion and skin adsorption to induce various diseases.¹ Therefore, wastewater containing heavy metal ions should be treated before being released to the main stream of water, which is a great contribution to public health and environmental sustainable development. Up to now, several methods have been applied for the capture of heavy metal ions from water, including chemical precipitation,² electrochemical treatment,³ filtration,⁴ electro-deposition,^5,6 and adsorption,^7–11 where adsorption process is considered as one of the most attractive methods with low operation cost and efficient capture ability. However, although various kinds of adsorbents have exhibited good performance, the study on the selective adsorption and separation ability for these metal ions is very rare,^12,13 which is of great importance for the recycle of these industrial resources.

As a new class of porous materials constituted by metal ions or clusters with organic ligands, metal–organic frameworks (MOFs) have been explored to be excellent adsorbents for adsorption and separation, in both gas^14–16 and liquid^17–22 phases. Great efforts were made focusing on further improving the performance of MOFs.²³ The introduction of metal ions may be an efficient way to increase the number of active sites of materials,²⁴ fully utilizing their intrinsic features including special pore structure and high specific surface area. In this respect, it is instructive to investigate the selective capture of heavy metal ions using MOFs. On the one hand, wastewater can be treated using this kind of adsorbents. On the other hand, the selectively entrapped heavy metal ions in MOFs may be enriched and then recycled through providing additional active sites on the frameworks. Therefore, as a demonstration, we employed several stable MOFs to selectively capture Cu²⁺ and Ni²⁺ from aqueous solutions in this study. So far, the related work on the selective removal of heavy metal ions is very limited in MOFs,²⁵ where the selectivity is too low to meet the practical requirement. Moreover, the information obtained in this work may also be useful for removal of Cu²⁺ from the nickel electrolysis anolyte, which is still a challenging and intriguing task in electrolytic nickel refining process.²⁶

2. Experimental section

2.1 Reagents and solutions

All reagents of analytical grade (ZrCl₄, terephthalic acid (BDC), benzene-1,2,4,5-tetracarboxylic acid (BTEC), 2-bromoterephthalic acid (Br-BDC), 1,2,4-benzenetricarboxylic acid (1,2,4-BTC), 1,3-propanesultone (1,3-PS), N,N′-dimethylformamide (DMF), acetone, trichloromethane) were purchased from J&K Chemical (Beijing, China), Sinopharm Chemical Reagent Beijing Co., Ltd or Aladdin (Shanghai), and trichloromethane was further purified using distillation technology. The stock solutions of Cu²⁺ and Ni²⁺ were prepared using their nitrate salts, Cu(NO₃)₂·3H₂O and Ni(NO₃)₂·6H₂O. All solutions were prepared using double distilled water.

2.2 MOFs preparation

Considering that the adsorption is performed in aqueous solution, the adsorbents should be water stable. Therefore, a series of MOFs with different functional groups were selected on the basis of UiO-66(Zr), which have exhibited excellent stability.²⁷ UiO-66(Zr)–X (X = H, Br, NH₂, NH–(CH₂)₂–SO₃H, 2COOH) were synthesized according to the previously reported procedures.^28–30 UiO-66(Zr)–COOH was synthesized and activated according to literatures with modifications.³¹ The functionalized terephthalic acids (1.8 g, 8.6 mmol) along with ZrO(NO₃)₂·2H₂O (1.0 g, 4.3 mmol), benzoic acid (15.8 g, 129.4 mmol) in DMF (30 mL) were placed in a Teflon lined autoclave and heated at 423 K for 24 h. After the solution was cooled down to room temperature, the white precipitate was collected by filtration, washed with DMF and acetone with the Soxhlet extraction, and dried under vacuum at 353 K.

2.3 Instruments

Power X-ray diffractions of the MOFs were recorded on a D8 Advance X diffractometer with Cu Kα radiation (λ = 1.5406 Å) from 3° to 50° (0.02° per step). The BET data were measured on specific surface and pore size analysis instrument using nitrogen adsorption by 3H-2000PS2 static volume method at 77 K. The concentrations of metal ions were detected by inductive coupled plasma emission spectrometer (ICP). The air constant temperature oscillator was used to react at 303 K.

2.4 Adsorption experiments

Batch adsorption experiments were conducted using 10 mg of absorbent with 10 mL solution in a 20 mL glass bottle. The solution contained heavy metal ions mixture (the ratio of mass is 1 : 1) and was shaken in a speed of 130 rpm at 303 K. For the study of the effect of interfering ions, the concentration of interfering ion varied from 50 to 1000 mg L⁻¹. The kinetic parameters of the adsorption were collected with the contact time of 5–600 min. Adsorption isotherms were obtained with Cu²⁺ or Ni²⁺ concentration up to about 60 mg L⁻¹. To confirm the effect of pH on the stability and adsorption, solutions with different pH ranging from 2.0 to 6.0 were prepared using 0.1 mol L⁻¹ NaOH or HCl. All the experiments were repeated for at least 3 times to determine the accuracy and veracity of the experimental data. For competitive experiments, selectivity α_i,j can be calculated using eqn (1),where q_i and q_j are the amounts (mg g⁻¹) of metal ions i and j adsorbed per gram of MOF, and c_j and c_i are the equilibrium concentrations (mg L⁻¹) of metal ions j and i in aqueous solution. The accuracy of selectivity calculated using eqn (1) decreases in cases where the relative concentration differences between solutions with and without adsorbent approach the experimental error.³²

2.5 Recycling experiments

After the typical adsorption test, in order to regenerate the metal-binding property, UiO-66(Zr)–2COOH with adsorbed metal ions was dispersed into 10 mL of 0.25 M hydrochloric acid solution for at least three times. Then, the solids were collected using a centrifuge and washed repeatedly with excessive amount of deionized water (pH = 7) to neutralize the acidic condition.³³ After drying in an oven at 75 °C, the solids were re-added into the fresh solution with metal ions to investigate the recovered separation capacity.

3. Results and discussion

Fig. 1 shows the XRD patterns of as-synthesized MOFs. It is obvious that they are in good agreement with those reported in literature.³⁴ The N₂ adsorption data at 77 K indicate that the BET of these MOFs decrease with the increase of functional group size, as listed in Table 1. At the first step, the adsorption of pure component was investigated and the results are shown in Fig. 2, as a function of the initial metal ion concentration (C₀) in the solutions. It can be seen that the functional group plays an important role on the uptake of Cu²⁺. The groups of –2COOH, –NH–(CH₂)₂–SO₃H, and –NH₂ can improve the adsorption capacity compared to the pristine UiO-66(Zr), among which –2COOH exhibits the best performance. It should be noted that UiO-66(Zr)–COOH hardly adsorbs Cu²⁺. This may be attributed to the fact that it is hard for single carboxyl group to coordinate to Cu²⁺. Recent work indicated that the uncoordinated free carboxyl group chelated Cu²⁺ with the help of carboxyl group on the ligand,²⁵ which contributes greatly to the high adsorption capacity of transition metal ions.³⁵ In UiO-66(Zr)–COOH, there are very few free carboxyl groups besides the one on ligand. In contrast, carboxyl groups in the adjacent ligand are close to each other in UiO-66(Zr)–2COOH, showing similar chelation effect with Cu²⁺ as uncoordinated free carboxyl group. Thus, the adsorption behavior of Cu²⁺ in UiO-66(Zr)–COOH is totally different from that in UiO-66(Zr)–2COOH. For Ni²⁺, most of these MOFs have poor adsorption capacities, except for UiO-66(Zr)–2COOH, while it is still very low compared to Cu²⁺. This may be ascribed to the difference in coordination ability for these two transition metal ions. According to the Jahn–Teller effect theory, Cu²⁺ undergoes a significant Jahn–Teller distortion in contrast to Ni²⁺, leading to the stronger coordination ability.^36,37 As a result, UiO-66(Zr)–2COOH can entrap more Cu²⁺ than Ni²⁺ using carboxyl groups.

Fig. 1 PXRD patterns for UiO-66(Zr)–X (X = H, Br, NH₂, COOH, NH₂–(CH₂)₂–SO₃H, and 2COOH): (a) as-synthesized, (b) after adsorption.

Table 1 BET surface areas and pore volumes of six functional Zr-MOFs

MOFs	BET surface area^a (m^2 g^−1)	Pore volume^b (cm^3 g^−1)
a Calculated from N2 adsorption isotherms at 77 K in range of P/P0 = 0.005–0.05.b Calculated at N2 adsorption amounts at P/P0 = 0.95.
UiO-66(Zr)	1204	0.6
UiO-66(Zr)–NH2	1079	0.4
UiO-66(Zr)–Br	966	0.5
UiO-66(Zr)–COOH	653	0.4
UiO-66(Zr)–NH–(CH2)2–SO3H	690	0.3
UiO-66(Zr)–2COOH	488	0.2

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Fig. 2 Adsorption uptakes of Cu²⁺ (a) and Ni²⁺ (b) in different MOFs as a function of the initial metal ion concentration.

On the basis of data for single components mentioned above, competitive adsorption experiments in Cu²⁺/Ni²⁺ system were performed in UiO-66(Zr)–2COOH to further investigate the selectivity for these metal ions. As illustrated in Fig. 3, the selectivity increases with the increase of the initial ion concentration, which can reach up to 26.96 on the condition that the initial concentration is about 50 mg L⁻¹. To the best of our knowledge, such separation performance for the Cu²⁺/Ni²⁺ system in aqueous solution is much better than all the adsorbents reported so far, as shown in Table 2.

Fig. 3 Adsorption uptakes (a) and selectivity (b) of the Cu²⁺/Ni²⁺ system in UiO-66(Zr)–2COOH as a function of the initial metal ion concentration.

Table 2 Comparison of adsorption and selectivity in UiO-66(Zr)–2COOH with those in other reported adsorbents in aqueous solution

Material	Adsorption capacity (mg g^−1)		Cu^2+/Ni^2+ selectivity	Ref.
	Cu^2+	Ni^2+
a The max adsorption capacity ratio of single metal ion adsorption.
CHB	15.38	8.68	2.63	38
RDS-50	83.33	71.43	1.17^a	39
Clay	28.00	21.30	1.31^a	40
Peat moss	7.52	5.94	1.41	41
Modified chitosan	66.70	15.30	4.36^a	42
Clinoptilolite	5.24	1.12	12.22	43
(MAAm-co-AA)/MMT	27.10	13.60	2.10	44
MMT	3.11	2.80	1.11^a	45
Nature zeolite	9.81	5.99	1.79	46
Nature zeolite (with Ca^2+)	8.84	4.75	2.05	46
[Zn(trz)(H2betc)0.5]·DMF	32.01	3.12	11.02	25
UiO-66(Zr)–2COOH	11.04	0.52	26.96	This work

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To further analyze the separation performance, the adsorption kinetics for the Cu²⁺/Ni²⁺ system is also studied. Fig. 4 shows the effect of contact time. It is obvious that adsorption for these two ions can reach equilibrium in less than 60 min. In addition, the experimental data were fitted using pseudo-first-order model and pseudo-second-order model,

Pseudo-first-order model: ln(Q_e − Q_t) = ln Q_e − k₁t (2)

where Q_e and Q_t (mg g⁻¹) are the adsorption capacity for equilibrium and at time t (min) respectively, k₁ and k₂ are rate constants of the pseudo-first-order model and pseudo-second-order model respectively. The kinetic parameters are given in Table 3. It can be seen that the experimental data fitted well with pseudo-second-order model, indicating the rate-limiting step may be the chemisorption process.⁴⁷ This further confirms that these transition metal ions are captured through chelation effect discussed above.

Fig. 4 Effect of contact time on the adsorption (a) and selectivity (b) of Cu²⁺/Ni²⁺.

Table 3 Kinetic parameters of the adsorption of Cu²⁺ and Ni²⁺ in UiO-66(Zr)–2COOH

	Pseudo-first-order-model			Pseudo-second-order-model
	k1 (min^−1)	Qe,cal (mg g^−1)	R^2	k2 (g mg^−1 min^−1)	Qe,cal (mg g^−1)	R^2
Cu^2+	0.0022	2.4081	0.2349	0.0139	11.3507	0.9993
Ni^2+	0.0062	64.3030	0.9135	0.8974	0.4813	0.9996

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Previous studies have demonstrated that the adsorption process of metal ions would be influenced by the pH value of the initial solution (pH₀).⁴⁸ Therefore, the measurements were further performed at different initial pH values. From Fig. 5a, the adsorption capacity for Cu²⁺ increases with the pH₀ changing from 3.1 to 6.0, while for Ni²⁺ the change is not evident. As a result, the selectivity increases and remains nearly constant with the increasing pH₀ from 4.0 to 6.0 (Fig. 5b). In order to further explore the potential adsorption mechanism of Cu²⁺ in UiO-66(Zr)–2COOH, the pH value of the solution before and after (pH_f) adsorption was detected. As shown in Fig. 5c, pH_f is lower than pH₀ and keeps nearly constant, indicating that the amount of H⁺ produced in solution increases with the increase of the pH₀. Due to the chelation between carboxyl groups and heavy metal ions, adsorption should be occurred with the ion exchange mechanism, as illustrated in eqn (4). The –COOH group transforms into –COO⁻ through ionization process in solution at the first step. The electrostatic attraction might also accelerate the migration of Cu²⁺ and promote the adsorption process. However, according to the Le Châtelier's principle, the lower solution pH can restrain the above response and the adsorption of Cu²⁺.⁴⁹

MOF − 2COOH + Cu²⁺ = MOF − 2(COO)Cu + 2H⁺ (4)

Fig. 5 Effect of pH₀ on the adsorption (a) and selectivity (b) of Cu²⁺/Ni²⁺ in UiO-66(Zr)–2COOH; the change of pH before and after the adsorption (c).

In aqueous medium, there are several kinds of other metal ions, such as Zn²⁺, Mg²⁺, Ca²⁺. Bearing these in mind, we further investigated the influence of the interfering ions in this study. The concentrations of interfering metal ions were equal to the initial concentrations of target metal ions.⁵⁰ As can be seen from Table 4, these three metal ions (Zn²⁺, Mg²⁺, Ca²⁺) only induce a slight decrease of adsorption of Cu²⁺ and Ni²⁺, and do not significantly interfere the selectivity of Cu²⁺/Ni²⁺ under the experimental conditions. It may be due to the specific Jahn–Teller effect for Cu²⁺. In contrast, for the other reported adsorbents, for example nature zeolite, it is found that Ca²⁺ in the solution leads to an obvious decrease in the uptake of Cu²⁺ and Ni²⁺ and an increase in the selectivity of Cu²⁺/Ni²⁺.⁴⁶ To further study such effect, we increased the concentration of interfering ions by taking Ca²⁺ as an example. The results in Table 4 revealed that the effect is still not evident.

Table 4 Effects of the interfering ions on the adsorption and selectivity of Cu²⁺/Ni²⁺

Metal ions	Concentration (mg L^−1)	Adsorption capacity (mg g^−1)		Selectivity
		Cu^2+	Ni^2+
—	—	11.04	0.52	26.96
Zn^2+	50	10.61	0.49	27.21
Mg^2+	50	10.96	0.51	27.24
Ca^2+	50	10.79	0.50	27.24
Ca^2+	1000	10.67	0.48	27.95

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From the practical point of view, an excellent adsorbent should have good regenerability and reusability besides high adsorption capacity and selectivity. Thus, the regeneration of UiO-66(Zr)–2COOH was further investigated. Through the simple regeneration processes using hydrochloric acid solution and deionized water, the Cu²⁺ and Ni²⁺ were washed out and UiO-66(Zr)–2COOH recovered the metal ion binding performance. As can be seen from Fig. 6, the selectivity changes a little after five cycles and the variation is within 2.0%. In addition, the PXRD patterns of the samples after adsorption and after desorption are displayed in Fig. 7, indicating its good stability. The above results show that UiO-66(Zr)–2COOH can be used as a recyclable adsorbent for the separation of heavy metal ions.

Fig. 6 Selectivity of Cu²⁺/Ni²⁺ in adsorption–desorption cycles in UiO-66(Zr)–2COOH.

Fig. 7 PXRD patterns of as-synthesized UiO(Zr)-66–2COOH, and the samples after adsorption and after desorption (the 5^th cycle).

4. Conclusions

In this work, two typical transition heavy metal ions, Cu²⁺ and Ni²⁺, were adsorbed using water stable Zr-MOFs with different functional groups. UiO-66(Zr)–2COOH exhibited high selective capture performance of Cu²⁺ from aqueous solution, due to the chelation effect of two carboxyl groups on the adjacent organic ligand as well as the significant Jahn–Teller distortion. The selectivity in aqueous solution is highest among the adsorbents so far. In addition, the stability and regenerability are good. The results may provide not only an efficient approach for the treatment of wastewater with heavy metal ions contamination, but also a method for metal enrichment and recycling in MOFs.

Acknowledgements

Financial support by the National Key Basic Research Program of China (“973”) (2013CB733503) and the Natural Science Foundation of China (no. 21136001 and 21276008) are greatly appreciated. D. L. also thanks the support of Beijing Higher Education Young Elite Teacher Project (no. YETP0486).

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