Rational structural design of ZnOHF nanotube-assembled microsphere adsorbents for high-efficient Pb2+ removal

Yingying Guoa, Nan Liua, Tongming Sunab, Huihui Cuiab, Jin Wangab, Miao Wang*ab, Minmin Wang*ab and Yanfeng Tang*ab
aCollege of Chemistry and Chemical Engineering, Nantong University, Nantong 226019, P. R. China. E-mail: hi_wangmiao@163.com; mmwang0528@ntu.edu.cn; tangyf@ntu.edu.cn
bNantong Key Lab of Intelligent and New Energy Materials, Nantong 226019, P. R. China

Received 3rd September 2020 , Accepted 17th September 2020

First published on 17th September 2020


Adsorbents with high effective adsorption capabilities have attracted extensive interest and are promising for the removal of heavy metal ions from wastewater. Herein, we prepared well-dispersed ZnOHF microspheres with efficient adsorptive removal toward Pb2+ in an aqueous environment via an amino acid assisted hydrothermal method. The obtained microspheres were self-assembled by numerous nanotubes with an outer diameter of ∼10 nm and a wall thickness of about 3.5 nm. By taking advantage of the large specific surface areas of nanotube-assembled microspheres (271.06 m2 g−1), the maximum adsorption capacity for Pb2+ was up to 285.7 mg g−1 in solution. Further analysis revealed the key role of the geometry structure of fine nanotube-assembled microspheres in boosting their adsorption ability. Furthermore, we proposed a potential growth mechanism of the as-prepared microspheres. The thermodynamic study of Pb2+ adsorbed on nanotube-assembled ZnOHF microspheres suggests that it is an endothermic and spontaneous process. This finding may open a new avenue to design efficient sorbents with high surface areas and effective heavy metal or organic dye removal ability in solution.


1. Introduction

Nowadays, efficient removal of heavy metal ions from industrial wastewater and drinking water is of great importance in solving environmental crises and healthcare related issues.1–3 Many technologies, such as adsorption, photocatalysis, biological treatment, ion exchange and membrane separation, have been explored to remove or mimic heavy metal ions from wastewater.4–10 Among these approaches, adsorption is widely used owing to its simplicity, high efficiency and economic feasibility. However, previously reported adsorbents always suffer from low efficiency and adsorption capacity, which limit their application. Therefore, designing and preparing adsorbents with high-efficient adsorption capacity is highly desirable for practical applications.11–13

In general, fine nanostructures with high specific surface area, well-defined active site, delimited void space, and tunable mass transfer rate properties are demonstrated to facilitate adsorption.14 Meanwhile, semiconductor nanocrystals can be easily and quickly exchanged with some heavy metal ions which are also widely used in sewage treatment processes.15,16 Although vast advancements have been made in designing and applying semiconductor nanocrystals or micro/nanostructures with high specific surface area as effective adsorbents, there are still limited reports on materials with one stone two birds properties.

Herein, we rationally designed and synthesized ZnOHF semiconductor nanocrystals with a fine nanotube-assembled microsphere geometry structure via a simple DL-alanine (DL-Ala) assisted method. The obtained ZnOHF nanotube-assembled microspheres with a high specific surface area of 271.06 m2 g−1 showed efficient adsorption performance. The Pb2+ removal efficiency was over 65% in 20 min at 100 mg L−1. The maximum adsorption capacity was 285.7 mg g−1, which was calculated using the Langmuir adsorption isotherm. By taking advantage of the unique capillary action and electrostatic adsorption of the as-prepared 3D ZnOHF nanotube-assembled microspheres, an extremely high adsorption efficiency (up to 100% within 90 min at 100 mg L−1) can be achieved.

2. Experimental section

2.1 Synthesis of ZnOHF nanotube-assembled microspheres

All chemicals were of analytical grade and used without further purification. Typically, 3.0 mmol zinc acetate and 1.0 mmol DL-Ala were dissolved in 25 mL distilled water with stirring for 20 min, and then 1.0 mmol (NH4)2SiF6 was added. After stirring vigorously for another 20 min, the homogeneous solution was transferred into a 30 mL Teflon-lined stainless-steel autoclave and reacted at 120 °C for 12 h, and the autoclave was gradually cooled to room temperature. The final white precipitate was obtained by centrifugation and washed with distilled water and ethanol. The final product was dried at 70 °C for 3 h. Comparably, experiments were carried out at different reaction times (0.5 h, 1 h, 3 h, 6 h, and 9 h).

2.2 Adsorption experiments

The as-obtained hierarchical ZnOHF microspheres were evaluated as the adsorbents for the removal of Pb2+ ions. In a typical procedure, 20.0 mg of ZnOHF powders were dispersed into 40.00 mL of a Pb(NO3)2 solution (concentration: 100.0 mg L−1) under magnetic stirring at room temperature. After adsorption, the resultant slurry was centrifuged at 5000 rpm for 5 min, and the supernatant and precipitated powder were collected individually so as to monitor the composition of the solution and adsorbents. In addition, the adsorption isotherms were obtained by conducting adsorption experiments with a series of initial concentrations of Pb2+ solution from 50.0 to 250.0 mg L−1 under magnetic stirring for 120 min.

2.3 Characterization

The crystalline phases of the products were analyzed by XRD on a Bruker D8-Advance powder X-ray diffractometer (Cu Kα radiation λ = 0.15418 nm). The morphologies and microstructures of the samples were studied by scanning electron microscopy (SEM, Hitachi S-4800) employing an operating voltage of 15 kV and transmission electron microscopy (TEM, JEOL-2100F) at 200 kV accelerating voltage. Nitrogen adsorption–desorption isotherms were collected using a Micromeritics ASAP 2020C apparatus and the specific surface area was calculated using the Brunauer–Emmett–Teller (BET) method. The XPS spectra were collected on an ESCALAB MK II X-ray photoelectron spectrometer, using a non-monochromatized Mg K X-ray source as the excitation source. A Malvern Zetasizer Nano ZS90 was used to measure the zeta potential of nanotube-assembled hierarchical ZnOHF. A pH meter (Mettler Toledo Co., Ltd., Shanghai, China) was used to determine the pH values of organic dye aqueous solutions. The actual residual concentration of Pb2+ ions within the supernatant after adsorption and centrifugation was determined by atomic absorption spectroscopy (Analytik Jena Nova A350/ZEEnit650p).

3. Results and discussion

The ZnOHF nanotube-assembled microspheres were prepared by a simple amino acid-assisted method. The detailed synthetic procedures are shown in the experimental section. The fine nanotube-assembled microsphere morphology of ZnOHF was determined by SEM and TEM. As shown in Fig. 1a–c, the obtained nanotube-assembled microspheres have a mean size distribution of 1 μm and the resultant microspheres are composed of numerous uniform nanotubes. The fine nanotube structure of the unit ZnOHF nanocrystal was observable from the rim of a single microsphere by TEM analysis. As shown in Fig. 1d, the nanotubes with an outer diameter of ∼10 nm and a wall thickness of about 3.5 nm are tightly assembled into a 3D micro-sized sphere. The fine tube geometry structure facilitates the adsorption of heavy metal ions for its capillarity. X-ray diffraction analysis demonstrated the crystal structure of the ZnOHF nanotube-assembled microspheres. As shown in Fig. 1e, all of the diffraction peaks can be indexed to the orthorhombic phase of ZnOHF (JCPDS card No. 74-1816) and there are no impurity peaks appeared, which demonstrates the high crystallinity and high purity properties of the 3D ZnOHF nanotube-assembled microspheres prepared via an amino acid-assisted method. Aiming to evaluate ZnOHF nanotube-assembled microspheres as adsorbent candidates, N2 adsorption–desorption tests were performed. The as-prepared 3D nanotube-assembled microspheres show a large BET surface area of 271.06 m2 g−1 (Fig. 1f).
image file: d0ce01279c-f1.tif
Fig. 1 (a–c) SEM images of the as-prepared 3D ZnOHF nanotube-assembled microspheres at various magnifications; (d) TEM image of ZnOHF; (e) the XRD pattern and (f) nitrogen adsorption–desorption isotherms of the as-prepared ZnOHF nanotube-assembled microspheres.

Experimentally, we found that the reaction time and the concentration of DL-Ala are vital to obtaining high adsorption efficiency of the adsorbents. Time-dependent experiments were carried out. The XRD results indicate that the reaction time has no great effect on the phase of the products (Fig. S1). Moreover, the diffraction peaks of the samples obtained after 12 h are much sharper than those of the samples obtained after 1 h and 3 h, indicating that crystallinity is improved by prolonging the reaction time. The SEM images of the corresponding samples are shown in Fig. 2. Irregular 3D microspheres composed of ZnOHF nanoparticles are obtained in the initial 0.5 h (Fig. 2a). The nanoparticles on the 3D microsphere surface further grow along the (110) plane, and flake-like stripes and fringes appear on the surface of the microspheres for a further 6 h (Fig. 2b–d). Further prolonging the reaction time to 9 h, 3D nanoflake-assembled microspheres are formed fully (Fig. 2e) and the edges of the nanoflakes are curled. On further increasing the reaction time to 12 h, 3D tube-like microspheres are totally generated (Fig. 2f). Therefore, a mechanism influenced by DL-Ala and reaction time is proposed, as shown in Fig. 2g. In our system, Zn2+ will coordinate with carboxyl and amino groups to form the complex and the resulting complex molecules will aggregate into a layered arrangement by hydrogen-bond.17 As a structure-directing agent, DL-Ala molecules selectively adsorb onto certain crystallographic planes of the initial ZnOHF nanoparticles to form 2D lamellar structures during the nuclei growth process, consequently, nanoflake-assembled microspheres are prepared. With the perspective of geometric structural evolution, 3D hollow nanotubes can be considered as curling and sealing of nanoflakes. Driven by the surface energy of nanostructures, there is an altering tendency from 2D nanoflakes to 3D nanotubes under higher temperature and longer reaction time. Actually, this model has already been demonstrated and proven to be feasible to fabricate hollow structures.18,19


image file: d0ce01279c-f2.tif
Fig. 2 SEM images of ZnOHF obtained at different reaction times: (a) 0.5 h; (b) 1 h; (c) 3 h; (d) 6 h; (e) 9 h; (f) 12 h. (g) Illustration of the growth process of ZnOHF nanotube-assembled microspheres.

To further demonstrate the role of DL-Ala in preparing ZnOHF nanotube-assembled microspheres, the experiments at different amounts of DL-Ala were performed keeping all other conditions identical. As shown in Fig. 3a, nanoflake-assembled microspheres are obtained without the addition of DL-Ala. As shown by XRD analysis (Fig. 3c), there is no apparent loss of crystallinity for ZnOHF with the assistance of DL-Ala. The as-prepared 3D nanotube-assembled microspheres show a higher BET surface area of 271.06 m2 g−1 (Fig. 1f) than nanoflake-assembled microspheres of 129.26 m2 g−1 (Fig. 3d). When 2 mmol DL-Ala is used, the yield of the as-obtained samples is much lower than that of 1 mmol. As shown in Fig. 3b, obviously, the morphologies of tube-assembled microspheres are maintained. All results demonstrate the vital role of DL-Ala in the morphology control process.


image file: d0ce01279c-f3.tif
Fig. 3 The SEM images of ZnOHF obtained from (a) 0 mmol and (b) 2 mmol DL-Ala within 12 h. (c) The XRD pattern and (d) nitrogen adsorption–desorption isotherms of the as-prepared ZnOHF nanotube-assembled and nanoflake-assembled microspheres.

To verify the contribution of the fine geometry of 3D nanotube-assembled microspheres to the adsorption performance of metal ions in water solution, the removal performance of the samples for Pb2+ from water was investigated. 20 mg of 3D ZnOHF nanotube-assembled microspheres was added into 40 mL Pb2+ solution (100 mg L−1) with stirring at pH 4.5 and the temperature was 25 °C. As shown in Fig. 4a, for ZnOHF nanotube-assembled microspheres, 65% of Pb2+ is removed after only 20 min and the Pb2+ removal efficiency is about 100% in 90 min, which is twice than that for nanoflake-assembled microspheres prepared without DL-Ala. To further understand the adsorption efficiency of our prepared ZnOHF nanotube-assembled microspheres, the adsorption isotherms of Pb2+ at different initial concentrations were obtained. As shown in Fig. 4b, the adsorption isotherms are obtained by conducting the adsorption experiments with a series of initial concentrations of Pb2+ solution from 50.0 to 250.0 mg L−1 under magnetic stirring for 120 min. Obviously, the adsorption amount is increased greatly with the increase of the initial concentration of Pb2+ until it reaches the saturation value. The adsorption activity for ZnOHF nanotube-assembled microspheres fits well with the pseudo-second-order kinetic model and Langmuir adsorption isotherm at various initial Pb2+ concentrations, with correlation coefficients of 0.991 and 0.997, respectively (Fig. 4c and d). According to the complete monolayer coverage theory, the maximum adsorption capacity (qm) for Pb2+ calculated by the Langmuir isotherm model is up to 285.7 mg g−1, which is higher than those of other previously reported adsorbents (Table 1).


image file: d0ce01279c-f4.tif
Fig. 4 (a) Adsorption kinetics and (b) adsorption isotherm of Pb2+ for 120 min; (c) the pseudo-second-order kinetic model at 100 mg L−1; (d) Langmuir model with concentrations of Pb2+ (50–250 mg L−1); (e) the XRD patterns of the samples after adsorption of Pb2+; (f) XPS spectra of ZnOHF and high-resolution of Pb 4f (inset).
Table 1 Comparison of Pb2+ adsorption efficiency on ZnOHF nanotube-assembled microspheres with other reported sorbents
Materials SBET (m2 g−1) Adsorption capacity (mg g−1)
Porous Ca4B10O19·7H2O microspheres21 32.79 252.4
α-FeOOH hollow spheres22 96.9 80.0
Layer coated graphene oxide23 83 53.6
BiOBr microspheres24 59.3 6.5
Snowflake-shaped ZnO@SiO2@Fe3O4/C micro/nanostructures25 79.2 94.3
Magnetic porous MnFe2O4 nanowires26 37.8 131.0
ZnOHF nanotube-assembled microspheres 271.06 285.7


In order to investigate the reaction mechanism of Pb2+ by the as-prepared ZnOHF nanotube-assembled microspheres, the XRD pattern after the adsorption process was tested. As shown in Fig. 4c, both species of ZnOHF (JCPDS No. 74-1816) and 2PbCO3·Pb(OH)2 (JCPDS No.13-0131) exist in the final products, which demonstrate that the adsorption occurs on the adsorbents. The result of 2PbCO3·Pb(OH)2 on the adsorbents was formed by the capture of Pb2+ by OH located on the surface of ZnOHF, and then the obtained basic insoluble Pb(OH)2 further reacted with CO2 dissolved in the solution to form 2PbCO3·Pb(OH)2.20,21 The elemental composition and electronic state of the adsorbent after adsorption was investigated by X-ray photoelectron spectroscopy (XPS). As shown in Fig. 4d, the survey spectra reveal the presence of the Zn, O, F, and Pb elements in the adsorbents. The high-resolution XPS spectrum was further implemented to understand the chemical identity and detailed chemical and electronic states of Pb 4f and Zn 2p. As shown in Fig. 4d (inset), the binding energy of the observed photoelectron peaks of Pb 4f7/2 and 4f5/2 are 137.9 eV and 143.1 eV, respectively, revealing the existence of the Pb2+ state. Therefore, in our system Pb2+ was adsorbed by the ZnOHF nanotube-assembled microspheres and formed adsorptive xPb·ZnOHF. The unique capillarity of the as-prepared fine nanotube structure facilitates the adsorption process and accelerates the diffusion of Pb2+ toward the adsorbents.

The influence of the pH value of the contaminated fluid on the adsorption activity was investigated as shown in Fig. 5a. It could be seen that the best Pb2+ removal performance is obtained at a pH value of 4.56. We should note that the original pH value of the Pb(NO3)2 solution (100 mg L−1) is 4.56. Therefore, we did not increase the alkalinity of the solution further. To explain the dependency between the adsorption amount and pH value, it is necessary to analyze the relationship between the adsorption amount and zeta potential. Fig. S2 presents the zeta potentials of ZnOHF nanotube-assembled microspheres at different pH solutions. It is obvious that with increasing pH value from 3 to 10, the zeta potentials of ZnOHF are increased. The zeta potential of the ZnOHF nanotube-assembled microspheres is measured to be −13.8 mV in water. There may exist a large number of negatively charged OH groups on the surface of ZnOHF microspheres. With the increase of solution pH, driven by electrostatic interactions, the amount of cationic dyes adsorbed on ZnOHF is increased. Therefore, the electrostatic attraction occurring between negative ZnOHF and positive Pb2+ may have great contribution to the high adsorption performance.


image file: d0ce01279c-f5.tif
Fig. 5 Pb2+ removal performance dependent on (a) pH and (b) temperature with concentrations of Pb2+ (100 mg L−1).

To understand the effect of temperature on adsorption performance, we studied the adsorption process at 273 K and 313 K, respectively. Obviously, higher temperature is beneficial for the Pb2+ removal (Fig. 5b). Table 2 lists the detailed thermodynamic parameters of Pb2+ adsorption by ZnOHF microspheres at three temperatures. The ΔG0, ΔH0 (16.92 kJ mol−1) and ΔS0 (122.58 J mol−1 K−1) values suggest that the adsorption of Pb2+ onto ZnOHF is an endothermic and spontaneous process.

Table 2 The thermodynamic parameters of Pb2+ adsorbed on nanotube-assembled ZnOHF microspheres
T (K) ΔG0 (kJ mol−1) ΔH0 (kJ mol−1) ΔS0 (J mol−1 K−1)
273 −3.20 16.92 81.99
298 −10.78
313 −15.82


4. Conclusions

Herein, we reported an effective adsorbent based on 3D ZnOHF nanotube-assembled microspheres by a simple DL-Ala assisted hydrothermal method and used it to remove Pb2+ efficiently. As experimentally demonstrated, the geometry structure of fine nanotube-assembled microspheres contributes greatly to Pb2+ adsorption. Moreover, the unique electrostatic adsorption can accelerate the diffusion of contaminants towards adsorbents. Up to 100% removal is achieved after 90 min standing, and the maximum adsorption capacity for Pb2+ is up to 285.7 mg g−1 in water solution, which is much higher than those of many previously reported adsorbents. Such adsorbents are envisioned to be very promising for practical, versatile and clean water regeneration applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 21776140), the Natural Science Foundation of Jiangsu Province (BK20190918) and the Natural Science Research Projects of Universities in Jiangsu Province (19KJB430030).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ce01279c

This journal is © The Royal Society of Chemistry 2020