A three-dimensional hierarchically structured flowerlike zeolite was synthesized using naturally occurring nanohalloysite (HNT) by hydrothermal methods. Halloysite a hydrated aluminum silicate with nanohollow morphology, microporosity, and environmentally friendly properties was chosen to be the sole precursor. The morphology and structure of the composite that was prepared was characterized using XRD, FT-IR, BET, TG, SEM, HRTEM, and NMR. SEM and HRTEM images indicated that the synthesized zeolite has a flowerlike hierarchical structure, with well-defined edges and uniform pore channels. FT-IR and NMR spectra indicated that different species of silicon and aluminum were present in the synthesized zeolite. The zeolite was applied in fluoride (F-) removal from aqueous solutions. Single-factor studies, including the initial concentration of F-, initial adsorbent concentration, and the effect of pH value on the adsorption properties, were investigated to evaluate the removal behavior of F- by the zeolite. The zeolite exhibited strong adsorption properties for fluoride ions (F-), with an adsorption capacity up to 161 mg g-1. The pseudo-second-order kinetics and Freundlich models were the best fit to the kinetics and isotherm experimental data, respectively.
Zeolites are aluminum silicate crystals with a framework structure of three-dimensional tetrahedral units that have abundant micropores with molecular dimensions. Furthermore, this porous crystalline solid has high surface area for unique reactions and adsorption [
Various morphologies of the nanomaterials [
Halloysite, a multiple-layered aluminosilicate clay with a natural nanohollow shape, consists of one alumina octahedron sheet and one silica tetrahedron sheet alternating in a 1 : 1 stoichiometric ratio. The structure and composition of halloysite are similar to kaolinite, except that the unit layers of halloysite are separated by a monolayer of water molecules [
Groundwater is an essential resource for ecosystems and human beings. However, some anions in the groundwater are undesirable and often responsible for serious environmental and health problems [
Industrial and domestic wastewater treatment systems usually employ biological (activated sludges), physical (adsorption, filtration, flocculation, etc.), and chemical (coagulation and electrolysis) processes, which are inefficient in terms of fluoride removal [
The goals of this work are (i) to find an optimum method for the synthesis of hierarchical flowerlike zeolite using halloysite and (ii) to investigate its adsorption potential for F-. The prepared zeolite was characterized using various methods, including XRD, FT-IR, and NMR. And the prepared zeolite was used as an adsorbent for fluoride removal from aqueous solution. The factors impacting removal efficiency, including pH, initial F- concentration, equilibrium time, and mass of adsorbent, were evaluated. Kinetics and isotherm experiments were conducted.
Natural halloysite, used as silicon and aluminum sources in this work, was purchased from Clay Mineral in Jiangsu Province, China. All other chemicals were of analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd.
The halloysite was dispersed with 0.2 wt% sodium hexametaphosphate (used as a dispersant) and stirred at 60°C for 12 h. After resting for 24 h at room temperature, the impurities (gibbsite, quartz, and feldspar) were removed and the remainder was dried. A certain amount of pure halloysite was added into 2 wt% NaOH solution (60 mL). The mixed solution was magnetically stirred for 10 min until the reaction gel was homogenous. Then, the mixture was placed into a Teflon reactor (80 mL) and crystallized at 120°C for 24 h. The solid powder was filtered and washed with distilled water and then dried in a temperature-controlled oven at 70°C for 12 h. Then, the products were obtained and labeled as zeolite NaA.
The crystalline characteristics of the samples were investigated using wide-angle X-ray diffraction (XRD). The diffraction patterns were obtained using a D/max 2550 X-ray diffractometer (Rigaku, Japan) with Cu K
Fourier Transformed Infrared Spectroscopy (FT-IR) was recorded on an Equinox 55 spectrometer in the range 400− 4000 cm-1. Nuclear magnetic resonance (NMR) was performed using a Bruker Avance III at the National Center for Magnetic Resonance in Wuhan. Surface areas and pore distributions were measured by nitrogen adsorption and desorption at 473 K using an ASAP 2020 surface area and pore analyzer (Micromeritics, USA). Thermal gravimetric (TG) data were measured using a STA 449 F3 Jupiter (NETZSCH, Germany) by heating a certain weight of sample in a silica crucible from room temperature to 800°C at a heating rate of 10°C min-1.
The prepared zeolite NaA was used as adsorbent for fluoride removal. A certain amount of adsorbent was added into a fluoride solution, which was placed into an Erlenmeyer flask and shaken in a thermo shaker incubator. The initial and remaining fluoride ion concentrations were determined by an ion meter (PXSJ-226, INESA Scientific Instrument Co., Ltd).
A series of single-factor studies were performed by maintaining the volume of the solution at 50 mL, the temperature at 35°C, and the reaction time at 24 h. The influence of pH on the F- adsorption was investigated by varying the pH from 2.0 to 9.0 using 10 mg of adsorbent and a 20 mg L-1 F- solution. The pH values were adjusted using 0.1 M HCl or 0.1 M NaOH solutions. The effect of adsorbent mass was studied by varying the concentrations of prepared zeolite NaA from 0.1 to 0.5 g L-1. The influence of the initial fluoride concentration on F- removal was studied by varying the initial concentrations from 1.05 to 51.7 mg L–1. The adsorption kinetics experiment was carried out using initial F- concentrations of 20 mg L–1 (pH 2.0) and by shaking for durations ranging from 5 to 180 min.
The amount of adsorption
1 g of zeolite NaA loaded with fluoride was shaken with 0.5 mol L-1 NaOH solution for 2 h. The adsorbent was separated by filtration, and the residue on filter paper was washed with deionized water and dried at 70°C for 10 h.
The X-ray diffraction patterns of unreacted halloysite powders indicate a d001 peak at 12.1°
X-ray diffraction pattern and TEM image of raw B and pure A halloysite.
Previous studies for fluoride removal from water.
Adsorbent | Type of water | Adsorption capacity (mg g-1) | Contact time | References |
---|---|---|---|---|
Al(III)-Zr(III) binary oxide | Aqueous solutions | 114.5 | 4 h | [ |
Regenerated aluminum oxide-coated media | Aqueous solutions | 34.24 | 5 days | [ |
3D hierarchical amorphous aluminum oxide microspheres | Aqueous solutions | 126.9 | 600 min | [ |
Synthetic Fe-Mg-La trimetal nanocomposite | Aqueous solutions | 47.2 | — | [ |
TiO2-ZrO2 | Groundwater/synthetic water system | 13.1 | 1.5 h | [ |
Zeolite NaA | Aqueous solutions | 161 | 30 min | In this study |
The XRD patterns of the products obtained from hydrothermal reaction are shown in Figure
XRD pattern of prepared zeolite NaA.
Halloysite crystals are hollow and open-ended nanotubes. The morphological parameters of the halloysite sample, measured from the TEM image (Figure
The SEM images of zeolite NaA (a, c), TEM image and lattice fringes (b, d), and the elemental composition (e).
Figure
FT-IR spectra of pure HNTs (a), zeolite NaA (b), and zeolite NaA after reaction (c).
The 29Si solid-state MAS NMR spectra of the halloysite (Figure
29Si solid-state magic-angle spinning (MAS) nuclear magnetic resonance (29SiMAS NMR) spectra of the halloysite and zeolite NaA (a). 27Al solid-state magic-angle spinning (MAS) nuclear magnetic resonance (27Al MAS NMR) spectra of the halloysite and zeolite NaA (b).
Chemical shift (ppm)
Chemical shift (ppm)
Nitrogen adsorption and desorption analyses were conducted to investigate the surface area and pore volume of natural and alkali-treated halloysite. According to IUPAC [
N2 adsorption-desorption isotherms and pore size distribution of the halloysite and zeolite NaA (estimated from the BJH method) of halloysite (a) and zeolite NaA (b).
Thermal gravimetry (TG) curves of pure halloysite and alkali-treated products are shown in Figure
TG curves of halloysite (a) and zeolite NaA (b).
The impact of solution pH is an important parameter for understanding the interaction between target molecules and adsorbents. The pH variation can promote changes in the surface charges of adsorbents and influence the protonation of functional groups present on the surfaces of materials. The experimental result in Figure
Effects of pH (a), initial concentration (b), adsorbent mass (c), and time (d).
Zero charge of zeolite NaA in different pH solutions.
Effect of regeneration on equilibrium adsorption capacity.
Kinetics studies can provide useful information regarding the speed and mechanism of an adsorption process. The experimental kinetics data were fitted with nonlinear forms of pseudo-first-order and pseudo-second-order models as described by Equation (
Table
Kinetics models and related parameters.
Pseudo-first-order | Pseudo-second-order | |||||
---|---|---|---|---|---|---|
106 | 9.85 | 0.018 | 0.862 | 111 | 6.23 × 10-3 | 0.999 |
Both Langmuir and Freundlich models were used for the evaluation of the experimental results. The linear form of the Langmuir equation is given as
The linear form of the Freundlich equation can be written as
The linear models of Langmuir and Freundlich were fitted to the experimental data (Fig.
Related parameters determined from isotherm models.
Isotherm | Model parameters | ||
---|---|---|---|
Langmuir | |||
— | — | 0.953 | |
Freundlich | |||
2.558 | 0.720 | 0.989 |
As mentioned in the literature [
The diffusion mechanism of fluoride onto zeolite NaA was investigated by applying the intraparticle diffusion model proposed by Weber and Morris [
If the adsorption process follows the intraparticle diffusion model, then
Multilinearity is apparent in the plots obtained from the kinetics study (Figure
Related parameters determined from intraparticle diffusion models.
First stage | Second stage | Third stage | |
---|---|---|---|
30.21 | 3.314 | 0.4509 | |
0 | 85.60 | 100.37 | |
1 | 0.9214 | 0.8010 |
Furthermore, the results of our studies on the adsorption mechanism indicate that the adsorption consists of both physisorption and chemisorption. At pH 2, the charge of zeolite NaA is positive (zeta potential: 19.05 mV), and the adsorption capacity of zeolite NaA is the highest. When the adsorption process is operated at lower pH range, several reactions are expressed in Equations (
The adsorbent was subjected to fluoride adsorption experiment to determine the fluoride removal efficiency after regeneration (Figure
The hierarchy flowerlike zeolite was synthesized successfully from natural halloysite with NaOH by hydrothermal methods. We employed several characterization methods for the raw and as-prepared materials, including XRD, FT-IR, HRTEM, and NMR for investigating the morphology and the structure in detail. It could be seen that silicon atoms dissociate from halloysite, so halloysite changes its internal structure and morphology gradually in the course of the reaction under moderate conditions (2 wt% NaOH, 120°C). The adsorption performance showed that the hierarchical flowerlike zeolite is an efficient material for fluoride removal: the adsorption isotherms showed that the sorption capacity of fluoride is 106 mg/g at the equilibrium fluoride concentration of 20 mg/L, and the maximum adsorption capacity is 161 mg/g in batch adsorption study. The equilibrium data were best described by the Freundlich isotherm model, and the adsorption kinetics were best described by pseudo-second-order kinetics. The adsorption mechanism was studied using the intraparticle diffusion model and FT-IR characterization. Combining the effect of pH and the adsorption isotherm, the results suggested that the mechanism is based on electrostatic attraction and ion exchange. And it is possible to regenerate zeolite NaA with NaOH solution treatment. It is concluded that the prepared zeolite from halloysite can be used as a low-cost and relatively effective adsorbent for the removal of fluoride from polluted water.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare no conflict of interest.
The research work was financially supported by National Natural Science Foundation of China (Nos. 41472217 and 41521001).
Fig. S1: growing process, 12 h (A), 16 h (B), and 24 h (C). Fig. S2: pseudo-first-order kinetic plots (a) and pseudo-second-order kinetic plots (b) for the adsorption. Fig. S3: Langmuir model (a) and Freundlich model (b). Fig. S4: plot of the intraparticle diffusion model for adsorption of fluorine adsorbed onto zeolite NaA. Fig. S5: this is a flowchart for synthesizing zeolite from raw nanohalloysite by using hydrothermal methods.