Solvothermal synthesis of the defect pyrochlore KNbWO₆·H₂O and its application in Pb²⁺ removal

Abstract

The octahedral defect pyrochlore KNbWO₆·H₂O is prepared by a two-step solvothermal process. It shows an excellent performance in the removal of toxic Pb²⁺ by ion exchange. The removal process fits well with the Langmuir isotherm, and the maximum removal capacity is 86.95 mg g⁻¹ at pH 5.0. High selectivity to Pb²⁺ is observed in an aqueous solution also containing Mn²⁺, Cd²⁺ and Co²⁺. The kinetics of the ion exchange process can be expressed by the pseudo-second-order rate kinetic model.

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1. Introduction

The effective removal of heavy metal ions from aqueous systems is important for the protection of the environment and public health.^1,2 Heavy metal ions, such as Pb²⁺, Cu²⁺, Ni²⁺, Cd²⁺, Zn²⁺, Hg²⁺, As⁵⁺ and Cr⁶⁺, are common contaminants of industrial wastewater.^3–7 Pb²⁺ is highly toxic and tends to accumulate in living tissues, causing various diseases and disorders.⁸ Chemical precipitation, membrane filtration, flocculation, ion exchange, reverse osmosis, precipitation, evaporation and adsorption are common methods for the removal of heavy metals from aqueous solutions.^9–13 The ion exchange technique has been well developed, due to its high efficiency and low cost.^14,15 Highly selective inorganic materials also play an increasing role when high concentrations of competing ions are present.¹⁶ Many natural mineral compounds, such as clays (e.g. bentonite, kaolinite and illite), vermiculite and zeolites (e.g. analcite, chabazite, sodalite and clinoptilolite), have received considerable interest for heavy metal removal.^17–20

Pyrochlore is a kind of oxide mineral with an ideal formula of A₂B₂O₇. Its structure is built of two corner-sharing BO₆ octahedra and an interpenetrating A₂O′ chain, forming a three-dimensional network.^21–23 A wide variety of mixed metal oxides adopt this structure, depending on the choice of the metal cations A and B. For defect pyrochlores the A and O′ atom of the chain are not essential in the stabilization of the ideal structure, and so vacancies may occur, leading to the defect. Defect pyrochlores can possess interesting properties, such as ion exchange, electrical conductivity and magnetic properties.^24,25

Here we report a two-step solvothermal process to prepare the defect pyrochlore KNbWO₆·H₂O. The as-obtained KNbWO₆·H₂O exhibits a high capacity for the removal of toxic Pb²⁺ in aqueous solution. To the best of our knowledge, this is the first report on the ion exchanging property of a defect pyrochlore with a high selectivity.

2. Experimental

2.1 Reagents

Nb₂O₅ (A.R.), H₂WO₄ (A.R.), C₈H₁₈O (A.R.), Pb(NO₃)₂ (A.R.), HCl (A.R. wt% 36.0–38.0), HNO₃ (A.R. wt% 65.0–68.0), KOH (A.R.), MnCl₂·4H₂O (A.R.), Co(NO₃)₂·6H₂O (A.R.) and CdCl₂·2.5H₂O (A.R.) were all purchased from Sinopharm Chemical Reagent Co. (Shanghai, China) and used without further purification.

2.2 Synthetic procedures

The soluble niobium source is prepared according to ref. 26, in which 1.0 g Nb₂O₅ and 5.0 g KOH are mixed in 60 ml distilled water. The mixture is then transferred into a 100 ml autoclave and reacted at 180 °C for 2 days. After cooling down naturally, a clear colorless solution of K₈Nb₆O₁₉ is obtained after separating out the insoluble residue by centrifugation. For the synthesis of KNbWO₆·H₂O, 2.4 ml of the above K₈Nb₆O₁₉ solution (0.0186 mol l⁻¹) is taken and dissolved in 20 ml of n-octanol, and then H₂WO₄ is added with different Nb⁵⁺ : W⁶⁺ molar ratios from 1 : 1 to 1 : 3. HCl is used to adjust the pH value to 7.2. The solvothermal treatment is undertaken in a 40 ml autoclave at 220 °C for 2 days. After cooling down to room temperature naturally, the products are centrifuged, washed with distilled water and dried under vacuum at 60 °C for 10 h.

2.3 Characterization

Powder X-ray diffraction (XRD) analysis is carried out on a PANalytical BV Empyrean X-ray diffractometer with Cu Kα radiation over the range 10–80°. Scanning electron microscopy (SEM) is performed with a HITACHI SU8020 electron microscope. Ion concentration is determined with an Inductively Coupled Plasma Spectrometer (ICP, OPTIMA 3300DV, Perkin Elmer).

2.4 Ion exchange experiments

A typical procedure for the ion exchange experiment is as follows: 0.02 g KNbWO₆·H₂O is added to 50 ml of Pb(NO₃)₂ solution (50 mg l⁻¹). The experiment is carried out at a pH from 2.0 to 5.0, under magnetic stirring at room temperature. After 30 h, the solids are separated from the solution by centrifugation. EDX measurements are taken with an acceleration voltage of 20 kV, to confirm the existence of Pd in the centrifuged KNbWO₆·H₂O after the ion exchange experiment (Fig. S1†).

3. Results and discussion

In order to investigate the influence of reaction time on the product, experiments were carried out with Nb⁵⁺ : W⁶⁺ in a molar ratio of 1 : 2.5, at 220 °C from 1 to 5 days. Fig. 1a shows the XRD pattern of the product made when the reaction time is 1 day. The marked peaks match well with those of KNbWO₆·H₂O (JCPDS no. 25-0668), but the reflection at 22° (as indicated by the arrow) is not indexable. As the reaction time is prolonged to 2 days, as shown in Fig. 1b, the XRD pattern of the resulting powder matches well with that of KNbWO₆·H₂O; it has a calculated lattice parameter of a = 10.499 Å, which is very close to the standard card value of 10.507 Å (JCPDS no. 25-0668). The product keeps its defect pyrochlore structure as the reaction time increases to 5 days, as shown in Fig. 1c.

Fig. 1 XRD patterns of the products prepared at 220 °C for (a) 1 day with Nb⁵⁺ : W⁶⁺ in a molar ratio of 1 : 2.5, (b) 2 days with Nb⁵⁺ : W⁶⁺ in a molar ratio of 1 : 2.5, (c) 5 days with Nb⁵⁺ : W⁶⁺ in a molar ratio of 1 : 2.5, (d) 2 days with Nb⁵⁺ : W⁶⁺ in a molar ratio of 1 : 1, (e) 2 days with Nb⁵⁺ : W⁶⁺ in a molar ratio of 1 : 2 and (f) 2 days with Nb⁵⁺ : W⁶⁺ in a molar ratio of 1 : 3.

The influence of the reactant molar ratios of Nb⁵⁺ to W⁶⁺ on the products was also investigated. When the Nb⁵⁺ : W⁶⁺ molar ratios are 1 : 1 and 1 : 2, several weak reflections on the XRD pattern (Fig. 1d and e) cannot be indexed. When the Nb⁵⁺ : W⁶⁺ molar ratio is 1 : 2.5, the XRD pattern (Fig. 1b) matches well with that of KNbWO₆·H₂O (JCPDS no. 25-0668). When the Nb⁵⁺ : W⁶⁺ molar ratio is 1 : 3, there are also several unidentified weak peaks (Fig. 1f). Therefore, the optimized molar ratio of Nb⁵⁺ to W⁶⁺ for the formation of KNbWO₆·H₂O is 1 : 2.5.

Fig. 2 shows the SEM images of the products obtained with a Nb⁵⁺ : W⁶⁺ molar ratio of 1 : 2.5 from 1 to 5 days. All of the products exhibit octahedral morphology. There are some irregular adsorbents on the surface of the octahedron when a reaction time of 1 day is used (Fig. 2a), which should match the unidentified peaks in the XRD pattern. With a reaction time of 2 days, as shown in Fig. 2b and c, octahedrons with particle sizes varying from 0.1 to 2 μm are observed. There is no significant increase in particle size when the reaction time increases to 5 days (Fig. 2d).

Fig. 2 SEM images of the KNbWO₆·H₂O with different reaction times: (a) 1 day, (b) 2 days, (c) high-magnification SEM image of b and (d) 5 days.

3.1 Ion-exchange properties

The pH value of an aqueous solution usually has an influence on the ion exchange efficiency. To optimize the pH value for maximum removal efficiency, batch experiments are carried out from pH 2.0 to 5.0, with an initial Pb²⁺ concentration of 50 mg l⁻¹ at room temperature. The pH value is adjusted by adding HNO₃ solution (0.5 mol l⁻¹). The ion exchange efficiency of Pb²⁺ is estimated by eqn (1) as follows:where C₀ (mg l⁻¹) is the initial concentration of the Pb²⁺ solution, and C_t (mg l⁻¹) is the concentration at a time of t. As shown in Fig. 3, after an ion exchange time of 30 h, the ion exchange efficiency increases from 41.0% to 65.4% as the pH value varies from 2.0 to 5.0. This behavior demonstrates that a high pH value is favorable for Pb²⁺ exchange. Similar behavior is also reported in the literature.²⁷ Consequently, a pH value of 5.0 is the optimized condition for the ion exchange experiment.

Fig. 3 Effect of pH on the adsorption efficiency (the initial concentration is 50 mg l⁻¹, the ion exchange time is 30 h).

The experimental equilibrium data for the exchange of Pb²⁺ are measured at a pH value of 5.0, with different initial concentrations ranging from 20 to 100 mg l⁻¹. The equilibrium adsorption capacity q_e (mg g⁻¹) is determined using eqn (2):

where C_e (mg l⁻¹) is the equilibrium concentration, v (l) is the volume of the solution and m (g) is the adsorbent mass. The Langmuir adsorption model is employed to analyse the ion exchange behavior, which is expressed as eqn (3):²⁸where q_m (mg g⁻¹) is the maximum adsorption capacity, b (l mg⁻¹) is the constant of Langmuir adsorption, and q_e (mg g⁻¹) is the equilibrium adsorption capacity. Besides the Langmuir model, the Freundlich isotherm model is also applied to analyse the experimental data.^28,29 This is presented as eqn (4):

ln q_c = 1/n ln C_e + ln K_F (4)

The values of K_F and n are Freundlich constants relating to the adsorption capacity and the adsorption intensity, respectively. Fig. 4a shows that the experimental data fit well with the Langmuir adsorption model isotherm and that the correlation coefficient (R²) is 0.9997, from which the maximum adsorption capacity is calculated to be 86.95 mg g⁻¹. The adsorbents do not fit the Freundlich equation well, with a correlation coefficient of 0.6271 (Fig. 4b).

Fig. 4 Application of (a) the Langmuir and (b) the Freundlich isotherm models to the experimental data. Inset of (a) is the variation in equilibrium concentration with a changing equilibrium adsorption capacity.

The kinetics of ion exchange is one of the most important characteristics in this reaction. The pseudo-second order model is proposed to describe the exchange process of Pb²⁺, which can be expressed as eqn (5):²⁹

t/q_t = 1/(k₂qe²) + 1/q_e (5)

where q_t and q_e (mg g⁻¹) are the exchange amounts at time t and at the equilibrium, respectively, and k₂ is the pseudo-second-order rate constant for the exchange process. As shown in Fig. 5, the exchange kinetics data can be fitted well by the pseudo-second-order rate kinetic model, with a high correlation coefficient of 0.9947.

Fig. 5 Removal kinetics for adsorption of Pb²⁺ with an initial concentration of 30 mg l⁻¹ (pH = 5.0).

3.2 Effect of coexisting ions

It is well known that selectivity plays a very important role in ion exchange. KNbWO₆·H₂O was added into a solution of Pb²⁺ coexisting with Mn²⁺, Cd²⁺ and Co²⁺ ions. The concentrations of the coexisting ions were the same as that of Pb²⁺ (30 mg l⁻¹) at pH 5.0. The exchange efficiency of Pb²⁺ is 80.8% without the competitive ions. With the coexisting ions, the exchange efficiency of Pb²⁺ is 78.6%, whereas the ion exchange efficiencies for Mn²⁺, Cd²⁺ and Co²⁺ are 7.2%, 0.6% and 21.2%, respectively. This implies a higher selectivity of KNbWO₆·H₂O to Pb²⁺ than to other cations in an aqueous solution.

4. Conclusions

We have prepared a defect pyrochlore, KNbWO₆·H₂O, via a two-step solvothermal process. The pyrochlore KNbWO₆·H₂O exhibits an excellent performance in the removal of toxic Pb²⁺ by ion exchange. The maximum removal capacity is 86.95 mg g⁻¹ at pH 5.0. Selective removal of Pb²⁺ is achieved in solutions also containing Mn²⁺, Cd²⁺ and Co²⁺.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (no. 21071058, 21371066).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra47190j

‡ Permanent address: Department of Chemistry, Kim Il Sung University, Pyongyang, Democratic People's Republic of Korea.

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