Resource Recycling of Red Soil to Synthesize Fe2O3/FAU-type Zeolite Composite Material for Heavy Metal Removal

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 environments^3,4,5.

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 removal^9,10. 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.

Protocol

1. Raw material collection and treatment

1. Red soil collection
   1. 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).
2. Red soil treatment
   1. 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.

2. Fe₂O₃/FAU-type zeolite synthesis

1. Preparation of alkali mixture powder
   1. 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.
2. Alkali fusion/activation
   1. 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.
3. Preparation of zeolite precursor
   1. 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.
4. Crystallization
   1. 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.
5. 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.
6. Characterization
   1. 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.
   2. 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).
   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.
   4. Acquire a scanning electron microscopy (SEM) image (Figure 5) to confirm the morphology²⁰.
   5. 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

1. 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.
2. 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.
3. 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.
4. 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.

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.

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 sources^6,8,16. 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 ...

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