Summary
This article presents a novel and convenient route to synthesize Fe2O3/faujasite (FAU)-type zeolite composite material from red soil. The detailed synthesis parameters have been finely tuned. The obtained composite material can be used for efficient heavy metal-contaminated water remediation, indicating its potential applications in environmental engineering.
Abstract
Heavy metal-polluted water is of great concern to human health and the eco-environment. In situ water remediation techniques enabled by highly efficient adsorption materials are of great importance in these circumstances. Among all the materials used in water remediation, iron-based nanomaterials and porous materials are of great interest, benefiting from their rich redox reactivity and adsorption function. Here, we developed a facile protocol to directly convert the widely spread red soil in south China to fabricate the Fe2O3/faujasite (FAU)-type zeolite composite material.
The detailed synthesis procedure and synthesis parameters, such as reaction temperature, reaction time, and the Si/Al ratio in the raw materials, have been carefully tuned. The as-synthesized composite materials show good adsorption capacity for typical heavy metal(loid) ions. With 0.001 g/mL Fe2O3/FAU-type zeolite composite material added to different heavy metal(loid)-polluted aqueous solutions (single type of heavy metal(loid) concentration: 1,000 mg/L [ppm]), the adsorption capacity was shown to be 172, 45, 170, 40, 429, 693, 94, and 133 mg/g for Cu (II), Cr (III), Cr (VI), As (III), Cd (II), Pb (II), Zn (II), and Ni (II) removal, respectively, which can be further expanded for heavy metal-polluted water and soil remediation.
Introduction
Heavy metal(loid)s from anthropogenic and natural activities are ubiquitous in the air, water, and soil environment1. They are of high mobility and toxicity, posing a potential health risk to human beings by direct contact or via food chain transportation2. Water is vital for the life of human beings since it is the feedstock of every family. Restoring water health is crucial. Therefore, it is of great importance to decrease the mobility and bioavailability of toxic heavy metal(loid)s in water. To maintain good health in water, water remediation materials, such as biochar, iron-based materials, and zeolite, play an essential role in immobilizing or removing heavy metal(loid)s from aqueous environments3,4,5.
Zeolites are highly crystalline materials with unique pores and channels in their crystal structures. They are composed of TO4 tetrahedra (T is the central atom, usually Si, Al, or P) connected by shared O atoms. The negative surface charge and exchangeable ions in the pores make it a popular adsorbent for ion capture, which has been extensively used in heavy metal-polluted water and soil remediation. Benefiting from their structures, the remediation mechanisms involved in contaminant removal by zeolites mainly include chemical bonding6, surface electrostatic interaction7, and ion exchange8.
Faujasite (FAU)-type zeolite has relatively large pores, with a maximum pore diameter of 11.24 Å. It shows high efficiency and broad applications for contaminant removal9,10. In recent years, extensive research has devoted to developing green and low-cost routines for zeolite synthesis, such as using industrial solid wastes11 as raw material to provide silicon and aluminium sources, or adopting directing agent-free recipes12. The reported alternative industrial solid wastes that can be silicon and aluminum sources include coal gangue13, fly ash11, waste molecular sieves14, mining and metallurgical wastes15, engineering-abandoned soil8, and agricultural soil6, etc.
Herein, red soil, an abundant and easily obtained silicon and aluminum-rich material, was adopted as the raw material, and a facile green chemistry approach was developed for Fe2O3/FAU-type zeolite composite material synthesis (Figure 1). The detailed synthesis parameters have been finely tuned. The as-synthesized material shows high immobilization capacity for heavy metal-contaminated water remediation. The present study should be instructive for related researchers who are interested in this area to use soil as a raw material for eco-material synthesis.
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Protocol
1. Raw material collection and treatment
- Red soil collection
- Collect the red soil. Remove the 30 cm top layer of the soil containing plants and residual organic matter.
NOTE: In this experiment, the red soil was collected at the campus of Southern University of Science and Technology (SUSTech), Shenzhen, Guangdong, China (113°59' E, 22°36' N).
- Collect the red soil. Remove the 30 cm top layer of the soil containing plants and residual organic matter.
- Red soil treatment
- Air-dry the collected red soil at room temperature and filter it through a 30-mesh sieve. Remove most of the large stones and leaves. Measure the heavy metal(loid) concentration (Table 1) in the red soil with inductively coupled plasma mass spectrometry (ICP-MS)16 to make sure that there is no unwanted pollution introduced.
NOTE: A sieve with small holes is recommended since few large non-silicon or aluminum-containing objects will be in the raw material. Here, a 30-mesh sieve is sufficient to treat the raw material in this experiment.
- Air-dry the collected red soil at room temperature and filter it through a 30-mesh sieve. Remove most of the large stones and leaves. Measure the heavy metal(loid) concentration (Table 1) in the red soil with inductively coupled plasma mass spectrometry (ICP-MS)16 to make sure that there is no unwanted pollution introduced.
2. Fe2O3/FAU-type zeolite synthesis
- Preparation of alkali mixture powder
- Weigh 5 g of pretreated red soil, 1 g of SiO2, and 7.63 g of NaOH, and add them to a natural agate mortar. Grind them for 2-3 min into a fine powder. Ensure the relative humidity in the laboratory is 65%-72%.
NOTE: Be careful of the grinding time since NaOH is very hygroscopic. It can easily absorb water from the air atmosphere. A medium-moist alkali powder is crucial for the next step of the experiment. The grinding time is related to the humidity in the laboratory.
- Weigh 5 g of pretreated red soil, 1 g of SiO2, and 7.63 g of NaOH, and add them to a natural agate mortar. Grind them for 2-3 min into a fine powder. Ensure the relative humidity in the laboratory is 65%-72%.
- Alkali fusion/activation
- Transfer the alkali mixture into a 100 mL Teflon reactor liner without the stainless steel outer covering. Heat it in a 200 °C oven for 1 h.
NOTE: The purpose of this step is to make use of the strong base NaOH to activate the Si-O bond and Al-O bond17 so that the Al, Si, and O atoms reassemble to form the desired aluminosilicate zeolite.
- Transfer the alkali mixture into a 100 mL Teflon reactor liner without the stainless steel outer covering. Heat it in a 200 °C oven for 1 h.
- Preparation of zeolite precursor
- Add 60 mL of deionized water into the Teflon reactor liner containing the activated alkali mixture. Add a stir bar of the appropriate size and stir the mixture at 600 rpm on the magnetic stirrer for 3 h at 25 °C. Wait for a homogeneous gel to be formed as the zeolite precursor18.
- Crystallization
- Transfer the homogeneous gel into a 100 mL stainless steel autoclave and heat the gel in a 100 °C oven for 12 h. Wait until the oven cools to room temperature following the default cooling program to open the oven's door and take the autoclave out.
NOTE: The autoclave generates high pressure under high temperatures to boost the crystallization process. Always wait for it to reach room temperature to prevent a high pressure-generated explosion.
- Transfer the homogeneous gel into a 100 mL stainless steel autoclave and heat the gel in a 100 °C oven for 12 h. Wait until the oven cools to room temperature following the default cooling program to open the oven's door and take the autoclave out.
- Wash the obtained zeolite with deionized water several times until the solution pH is close to 7. Use a centrifuge to separate the solid and liquid, and collect the solid at the bottom of the 50 mL centrifuge tube. Finally, dry the obtained product for 8 h in an 80 °C oven and grind it into fine powder for subsequent characterization.
- Characterization
- Acquire the X-ray fluorescence- (XRF) spectrometer result for the red soil (Figure 2). It is used to accurately measure the soil's inorganic element concentration19.
- Acquire the crystal information file (CIF) of Fe2O3 from the Inorganic Crystal Structure Database (ICSD). Acquire the CIF file of FAU-type zeolite from the Database of Zeolite Structures.
NOTE: Mercury and Materials Studio (MS) can both be used as crystal structure visualization tools. In this work, Mercury was used for the visualization of the Fe2O3 structure, and MS was used for the FAU-type zeolite (Figure 3). - Acquire a powder X-ray diffraction (PXRD) pattern to confirm the phase of the as-synthesized Fe2O3/FAU-type zeolite composite material (Figure 4)20. Compare it with the simulated PXRD pattern of Fe2O3 and FAU-type zeolite using JADE 6.5 software.
NOTE: The Mercury software developed by the Cambridge Crystallographic Data Centre (CCDC) can calculate the PXRD pattern based on the CIF file of the standard materials obtained from the ICSD-the world's largest database for completely identified inorganic crystal structures. - Acquire a scanning electron microscopy (SEM) image (Figure 5) to confirm the morphology20.
- Acquire transmission electron microscope (TEM) energy-dispersive X-ray spectroscopy (EDS) mapping (Figure 6) to determine the chemical composition6.
NOTE: Compared to SEM-EDS mapping, TEM-EDS mapping can detect low amounts of elemental composition.
3. Batch adsorption experiment
- Prepare 50 mL of 1,000 ppm Cu (II), Cr (III), Cr (VI), As (III), Cd (II), Pb (II), Zn (II), and Ni (II) aqueous solutions. Note the pH of each solution.
- Add 50 mg of zeolite to each heavy metal(loid) solution. Finely adjust the pH of the mixture solution with 0.1 M HCl or 0.1 M NaOH. Stir the mixture at 600 rpm for 48 h at 25 °C.
NOTE: Each heavy metal(loid) ion has a stable pH range without the metal hydroxide precipitation. Adjust the pH of the final mixed solution to a pH range so that the decrease in heavy metal(loid) concentration can be attributed to the performance of the zeolite. - Adjust the pH of the final mixed solutions of Cu (II), Cr (III), Cr (VI), As (III), Cd (II), Pb (II), Zn (II), and Ni (II) to 4.2, 3.9, 6.4, 7.8, 5.8, 5.2, 5.7, and 6.4, respectively.
- Filter the mixed solutions through 0.22 µm membranes. Dilute them 1,000x by adding 2% HNO3 solution.Measure the residual heavy metal(loid) concentrations (Figure 6) with inductively coupled plasma mass spectrometry (ICP-MS)16, with a testing range of 0.001 ppm to 1 ppm. See Table 2 for the ICP-MS operating parameters.
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Representative Results
Figure 1 illustrates the overall synthesis route of zeolite based on the "soil for soil remediation" strategy6. With a simple organic-free route, red soil can be converted to Fe2O3/FAU-type zeolite composite material without adding any Fe or Al source. The as-synthesized zeolite composite material exhibits excellent removal capacity for heavy metal-polluted water remediation and can be used for soil remediation.
Figure 2 presents the result of XRF analysis for red soil. The main composition of red soil is SiO2, Al2O3, and Fe2O3.
Figure 3 shows the crystal structure of the FAU-type zeolite framework and Fe2O3. FAU-type zeolite belongs to the cubic crystal system, the space group is Fd-3m, and the unit cell parameter is a = 24.3450 Å. The framework of FAU zeolite is composed of three-dimensional, 12-membered rings. The crystal structure-related information was obtained from the International Zeolite Association (IZA)21, which provides an exhaustive database of all zeolite structures.
Figure 4 presents the experimental PXRD pattern of the as-synthesized Fe2O3/FAU-type zeolite composite material and simulated patterns of FAU-type zeolite and Fe2O3. The great match of this sample with the simulated standard materials shows the success of the synthesis. The SEM image is shown in Figure 5. The Fe2O3/FAU-type zeolite composite material shows needle-like morphology with high purity.
The result of energy-dispersive X-ray spectroscopy (EDS) mapping is shown in Figure 6. The typical zeolite composition elements-Si, Al, Na, and O-are evenly distributed on the material, and Fe is distributed discretely in the composite material. This also confirms the successful synthesis of Fe2O3/FAU-type zeolite composite material.
Figure 7 demonstrates the adsorption capacity of Fe2O3/FAU-type zeolite composite material for eight typical heavy metal(loid) solutions. In particular, it shows a fascinatingly high capacity for Pb (II) and Cd (II) ion adsorption. The pH of the metal ion solution was carefully adjusted, so no precipitation was observed in the solutions.
Figure 1: Preparation method of the Fe2O3/FAU-type zeolite composite material and its potential application. Fe2O3/FAU-type zeolite composite material was synthesized by the typical alkali-activation hydrothermal method. Please click here to view a larger version of this figure.
Figure 2: XRF anaylsis of the red soil. Abbrevation: XRF = X-ray flurorescence. Please click here to view a larger version of this figure.
Figure 3: Crystal structure of the FAU-type zeolite framework and Fe2O3 crystal structure. (A) The spatial structure and, especially, the pore architecture of the FAU-type zeolite framework; (B) Fe2O3 crystal structure along the c-axis. Please click here to view a larger version of this figure.
Figure 4: XRD pattern of the Fe2O3/FAU-type zeolite composite material. Abbreviation: XRD = X-ray diffraction. Please click here to view a larger version of this figure.
Figure 5: SEM image of the Fe2O3/FAU-type zeolite composite material. The surface morphology was characterized by SEM. Scale bar = 2 µm. Abbreviation: SEM = scanning electron microscopy. Please click here to view a larger version of this figure.
Figure 6: TEM-EDS mapping image of the Fe2O3/FAU-type zeolite composite material. The element distribution is characterized by TEM-EDS mapping. Scale bar = 1 µm. Abbreviation: TEM-EDS = transmission electron microscopy energy-dispersive X-ray spectroscopy. Please click here to view a larger version of this figure.
Figure 7: Adsorption capacities of the as-synthesized Fe2O3/FAU-type zeolite composite material for eight typical heavy metal(loid) solutions. The adsorption capacity of this material was examined in different heavy metal(loid) water solutions. Some similar studies5,9 have tested the applicability of this type of material in soil environments. Please click here to view a larger version of this figure.
| Bioavailable heavy metal(loid)s concentrations in the red soil | |
| Heavy metal(loid)s | Concentration (mg/L) |
| Pb | 19.30 |
| Cu | 1.56 |
| Cd | 0.16 |
| Zn | 11.73 |
Table 1: Heavy metal(loid) concentration in the red soil.
| ICP-MS operating parameters | |
| Parameter | Value |
| Forward power | 1500 W |
| Plasma gas flow | 14.0 L min-1 |
| Carrier gas flow | 0.78 L min-1 |
| Dilution gas flow | 1.06 L min-1 |
| Total carrier gas flow | 1.84 L min-1 |
| He gas flow | 4.8 mL min-1 |
| QP bias | -98 V |
| Oct bias | -100 V |
| Cell entrance | -130 V |
| Cell exit | -150 V |
| Deflect | -80 V |
| Plate bias | -150 V |
| Nebulizer type | Micro mist |
| Sample uptake rate | 1.0 mL min-1 |
| m/z isotopes monitored in Cu speciation | 63 Cu, 65 Cu |
| m/z isotopes of internal standards | 115 In, 175 Lu |
| Total acquisition time | 8 s per sample |
Table 2: ICP-MS operating parameters. Abbreviation: ICP-MS = inductively coupled plasma mass spectrometry.
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Discussion
Zeolite is typically an aluminosilicate material. In theory, materials that are rich in silicate and aluminate can be chosen as raw materials for zeolite synthesis. The Si/Al ratio of the raw material must be similar to that of the selected type of zeolite to minimize the usage of additional silicon/aluminum sources6,8,16. The Si/Al ratio of FAU-type zeolite is 1.2, and the Si/Al ratio of red soil is 1.3. Therefore, red soil is a perfect Si and Al source for FAU-type zeolite synthesis. However, in this method, not all SiO2 in the red soil was successfully transferred to zeolite. And in our protocol, extra SiO2 is needed for the zeolite synthesis. Moreover, as the red soil contained 7.65 wt% Fe2O3, there was no need to add extra Fe source in the composite material preparation.
NaOH, SiO2, and red soil must be well mixed before the alkali-activation step. The existence of large granules in the mixture may negatively impact the activation efficiency. Stirring time is a somewhat loosely controlled parameter in the synthesis route. In theory, longer stirring time provides better mixing but is more energy-consuming.
The crystallization time and temperature were carefully tuned in the experiment. A small deviation of these two synthesis parameters may cause the synthesis of different types of zeolites19. The as-synthesized Fe2O3/FAU-type zeolite composite material was tested for applicability in adsorbing metal ions in this study. It can be extended for ammonium or organic matter removal10,22.
PXRD, SEM, and TEM-EDS mapping are commonly used techniques for material characterization. PXRD is often used for phase identification23. The position and intensity of the diffraction peaks indicate rich structure information of the detected sample, such as the interplanar spacing and crystallinity. The SEM image is mainly used to show the morphology24. Meanwhile, the size and uniformity can also be confirmed. TEM-EDS mapping25 was used to confirm the elemental composition. Analyzing the mapping reveals a clear distribution of elements. ICP-MS is an extremely sensitive technique for detecting trace concentrations of heavy metal(loid)s8. The key to data accuracy is a well-constructed standard curve. For quantitative analysis, selecting a suitable internal standard can effectively compensate for general matrix effects and correct the drift of the analytical signal, thereby improving the accuracy of the analytical results.
This paper describes the development of a facile protocol to directly convert the widely-spread red soil in South China to fabricate the Fe2O3/FAU-type zeolite composite material. By this method, the abundant soil resource was successfully transformed into the high-value zeolite composite material under conditions of a relatively low temperature and short reaction time for heavy metal(loid) removal. However, the traditional hydrothermal method used may not be efficient and environmentally friendly enough compared to other zeolite synthetic approaches, such as the solvent-free26 or the microwave-assisted approach27. In the future, it can be further expanded for heavy metal-polluted water and soil remediation to finally achieve the "soil for soil remediation" strategy6.
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Disclosures
The authors have no conflicts of interest to disclose.
Acknowledgments
This work was financially supported by the Natural Science Funds for Distinguished Young Scholar of Guangdong Province, China, No. 2020B151502094; National Natural Science Foundation of China, No. 21777045 and 22106064; Foundation of Shenzhen Science, Technology and Innovation Commission, China, JCYJ20200109141625078; 2019 youth innovation project of Guangdong universities and colleges, China, No. 2019KQNCX133 and a special fund for the science and technology innovation strategy of Guangdong Province (PDJH2021C0033). This work was sponsored by the Shenzhen Key Laboratory of Interfacial Science and Engineering of Materials (No. ZDSYS20200421111401738), Guangdong Provincial Key Laboratory of Soil and Groundwater Pollution Control (2017B030301012), and State Environmental Protection Key Laboratory of Integrated Surface Water-Groundwater Pollution Control. In particular, we acknowledge the technical support from the SUSTech Core Research Facilities.
Materials
| Name | Company | Catalog Number | Comments |
| Chemicals | |||
| Cadmium nitrate tetrahydrate | Shanghai Aladdin Bio-Chem Technology Co., LTD | C102676 | AR, 99%. Make 1,000 ppm stock solution for the test of adsorption performance of zeolite. |
| Chromium(III) nitrate nonahydrate | Shanghai Aladdin Bio-Chem Technology Co., LTD | C116446 | AR, 99%. Make 1,000 ppm stock solution for the test of adsorption performance of zeolite. |
| Copper sulfate pentahydrate | Shanghai Aladdin Bio-Chem Technology Co., LTD | C112396 | AR, 99%. Make 1,000 ppm stock solution for the test of adsorption performance of zeolite. |
| Lead nitrate | Shanghai Aladdin Bio-Chem Technology Co., LTD | L112118 | AR, 99%. Make 1,000 ppm stock solution for the test of adsorption performance of zeolite. |
| Nickel nitrate hexahydrate | Shanghai Aladdin Bio-Chem Technology Co., LTD | N108891 | AR, 98%. Make 1,000 ppm stock solution for the test of adsorption performance of zeolite. |
| Nitric acid | Shanghai Aladdin Bio-Chem Technology Co., LTD | N116238 | AR, 69.2%. Used as solvent in ICP-MS test. |
| Potassium dichromate | Shanghai Aladdin Bio-Chem Technology Co., LTD | P112163 | AR, 99.8%. Make 1,000 ppm stock solution for the test of adsorption performance of zeolite. |
| Silicon dioxide | Shanghai Aladdin Bio-Chem Technology Co., LTD | S116482 | AR, 99%. For synthesis of zeolite. |
| Sodium (meta)arsenite | Sigma-aldrich | S7400-100G | AR, 90%. Make 1,000 ppm stock solution for the test of adsorption performance of zeolite. |
| Sodium hydroxide | Shanghai Aladdin Bio-Chem Technology Co., LTD | S111502 | Pellets. For the synthesis of zeolite. |
| Zinc nitrate hexahydrate | Shanghai Aladdin Bio-Chem Technology Co., LTD | Z111703 | AR, 99%. Make 1,000 ppm stock solution for the test of adsorption performance of zeolite. |
| Equipment | |||
| Air-dry oven | Shanghai Yiheng Technology Instrument Co.,LTD. | DHG-9075A | Used for hydrothermal crystallization and drying of sample |
| Analytical balance | Sartorius Scientific Instruments Co.LTD | BSA224S-CW | Used for weighing samples |
| Centrifuge tubes | Nantong Supin Experimental Equipment Co., LTD | ||
| High speed centrifuge | Hunan Xiang Yi Laboratory Instrument Development Co.,LTD | H1850 | Used for separation of solid and liquid samples |
| Multipoint magnetic stirrer | IKA Equipment Co.,LTD. | RT15 | Used for stirring samples |
| Oscillator | Changzhou Guohua Electric Appliances Co.,LTD. | SHA-B | For uniform mixing of samples |
| Syringe-driven filter | Tianjin Jinteng Experimental Equipment Co.,LTD. | 0.22 μm. For filtration. | |
| Softwares | |||
| JADE 6.5 | Materials Data& (MDI) | ||
| Mercury | Cambridge Crystallographic Data Centre (CCDC) | ||
| Materials Studio | Accelrys Software Inc. | ||
| Websites | |||
| Database of Zeolite Structures: http://www.iza-structure.org/databases/ | |||
| ICSD: https://icsd.products.fiz-karlsruhe.de/en |
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