Emerging Nanomaterials for Recovery of Toxic and Radioactive Metal Ions from Environmental Media

Emerging Nanomaterials for Recovery of Toxic and Radioactive Metal Ions from Environmental Media covers nanomaterials used in the environmental remediation of sites contaminated by toxic or radioactive heavy metals. The book comprehensively covers the use of MOF-based nanomaterials, COF-based nanomaterials, MXene-based nanomaterials, nZVI-based nanomaterials and carbon-based nanomaterials in remediation techniques and details the main interaction mechanisms between toxic/radioactive metal ions and the described novel nanomaterials through kinetic analysis, thermodynamic analysis, spectroscopic techniques and theoretical calculations. It provides a thorough reference on the use of the described novel nanomaterials for academics, researchers and advanced postgraduates in the environmental sciences and environmental chemistry.

• Provides a comprehensive and systematic reference on various novel nanomaterials that are available for use in the treatment of heavy metal ions and radioactive wastes
• Presents the latest knowledge on the interaction of toxic and radioactive metal ions with novel nanomaterials, including how to choose different materials for specific uses
• Covers the principles and functionalization of nanomaterials in environmental remediation, enabling an understanding of methodologies and best choice in nanomaterials

• Language: English
• Publisher: Elsevier Science
• Release date: Nov 26, 2021
• ISBN: 9780323854856

Book preview

Emerging Nanomaterials for Recovery of Toxic and Radioactive Metal Ions from Environmental Media

Editor

Xiangke Wang

Hebei Key Lab of Power Plant Flue Gas Multi-Pollutants Control, Department of Environmental Science and Engineering, North China Electric Power University, Baoding, P.R. China

Table of Contents

Cover image

Title page

Copyright

Contributors

Chapter 1. Novel nanomaterials for environmental remediation of toxic metal ions and radionuclides

1. Introduction

2. Chemical behaviors of toxic metal ions and radionuclides in solution

3. Remediation technologies for radioactive and toxic metal ions

4. Novel nanomaterials for the elimination of heavy/radioactive ions

5. Conclusions

Chapter 2. Water-stable metal–organic framework–based nanomaterials for removal of heavy metal ions and radionuclides

1. Introduction

2. Synthesis methods of water-stable metal–organic framework–based nanomaterials

3. Applications of metal–organic framework–based nanomaterials for wastewater treatment

4. Heavy metal ion uptake in metal–organic framework–based nanomaterials

5. Radionuclide uptake in metal–organic framework–based nanomaterials

6. Practical application possibilities

7. Conclusions and perspectives

Chapter 3. Applications of covalent organic framework–based nanomaterials as superior adsorbents in wastewater treatment

1. Introduction

2. Synthesis methods of covalent organic frameworks

3. Properties of covalent organic frameworks

4. Removal of heavy metal ions by covalent organic framework–based nanomaterials

5. Removal of radionuclides by covalent organic framework-based nanomaterials

6. Conclusions and perspectives

Chapter 4. Two-dimensional transition metal carbide/nitride (MXene)-based nanomaterials for removal of toxic/radioactive metal ions from wastewater

1. Introduction

2. General synthesis and processing of MXene

3. Properties of MXene

4. Application of MXene in removal of environmental contaminants

5. Concluding remarks and perspectives

Chapter 5. Adsorptive and reductive removal of toxic and radioactive metal ions by nanoscale zero-valent iron–based nanomaterials from wastewater

1. Introduction

2. Synthesis and properties of nanoscale zero-valent iron–based nanomaterials

3. Applications of nanoscale zero-valent iron–based nanomaterials for environmental remediation

4. Interaction mechanism between heavy metals and nanoscale zero-valent iron–based nanomaterials

5. Fate and toxicity of nanoscale zero-valent iron–based nanomaterials

6. Conclusions and expected future research

Chapter 6. Water treatment and environmental remediation applications of carbon-based nanomaterials

1. Introduction

2. Properties of carbon-based nanomaterials

3. Removal of heavy metal ions by carbon-based nanomaterials

4. Removal of radionuclides by carbon-based nanomaterials

5. Dispersion/aggregation behaviors of carbon-based nanomaterials in aquatic environments

6. Challenges and perspectives

Chapter 7. Theoretical calculation of toxic/radioactive metal ion capture by novel nanomaterials

1. Fundamental theory of quantum mechanics

2. Density functional theory

3. Exchange-correlation functional

4. Molecular dynamics simulation

5. Removal mechanisms of toxic/radioactive metal ions

Index

Copyright

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Contributors

Yuejie Ai,     MOE Key Laboratory of Resources and Environmental Systems Optimization, College of Environmental Science and Engineering, North China Electric Power University, Beijing, P.R. China

Zhifang Chai

Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, P.R. China

Engineering Laboratory of Advanced Energy Materials, Ningbo Institute of Industrial Technology, Chinese Academy of Sciences, Ningbo, Zhejiang, P.R. China

Lanhua Chen,     State Key Laboratory of Radiation Medicine and Protection, School for Radiological and Interdisciplinary Sciences (RAD-X) and Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou, P.R. China

Lixi Chen,     State Key Laboratory of Radiation Medicine and Protection, School for Radiological and Interdisciplinary Sciences (RAD-X) and Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou, P.R. China

Long Chen,     State Key Laboratory of Radiation Medicine and Protection, School for Radiological and Interdisciplinary Sciences (RAD-X) and Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou, P.R. China

Ke Du,     Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, P.R. China

Ruoxuan Guo,     MOE Key Laboratory of Resources and Environmental Systems Optimization, College of Environmental Science and Engineering, North China Electric Power University, Beijing, P.R. China

Yanan Han,     MOE Key Laboratory of Resources and Environmental Systems Optimization, College of Environmental Science and Engineering, North China Electric Power University, Beijing, P.R. China

Jiahui Hong,     MOE Key Laboratory of Resources and Environmental Systems Optimization, College of Environmental Science and Engineering, North China Electric Power University, Beijing, P.R. China

Yingzhong Huo,     MOE Key Laboratory of Resources and Environmental Systems Optimization, College of Environmental Science and Engineering, North China Electric Power University, Beijing, P.R. China

Zheng Jiang,     MOE Key Laboratory of Resources and Environmental Systems Optimization, College of Environmental Science and Engineering, North China Electric Power University, Beijing, P.R. China

Xuewei Liu,     MOE Key Laboratory of Resources and Environmental Systems Optimization, College of Environmental Science and Engineering, North China Electric Power University, Beijing, P.R. China

Yang Liu,     MOE Key Laboratory of Resources and Environmental Systems Optimization, College of Environmental Science and Engineering, North China Electric Power University, Beijing, P.R. China

Yue Liu,     MOE Key Laboratory of Resources and Environmental Systems Optimization, College of Environmental Science and Engineering, North China Electric Power University, Beijing, P.R. China

Feng Luo,     School of Chemistry, Biology and Materials Science, East China University of Technology, Nanchang, P.R. China

Ran Ma,     MOE Key Laboratory of Resources and Environmental Systems Optimization, College of Environmental Science and Engineering, North China Electric Power University, Beijing, P.R. China

Hongwei Pang,     MOE Key Laboratory of Resources and Environmental Systems Optimization, College of Environmental Science and Engineering, North China Electric Power University, Beijing, P.R. China

Nannan Shen,     State Key Laboratory of Radiation Medicine and Protection, School for Radiological and Interdisciplinary Sciences (RAD-X) and Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou, P.R. China

Weiqun Shi,     Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, P.R. China

Shuang Song,     MOE Key Laboratory of Resources and Environmental Systems Optimization, College of Environmental Science and Engineering, North China Electric Power University, Beijing, P.R. China

Xiaoli Tan,     MOE Key Laboratory of Resources and Environmental Systems Optimization, College of Environmental Science and Engineering, North China Electric Power University, Beijing, P.R. China

Hao Tang,     MOE Key Laboratory of Resources and Environmental Systems Optimization, College of Environmental Science and Engineering, North China Electric Power University, Beijing, P.R. China

Jiaqi Wang,     Hebei Key Lab of Power Plant Flue Gas Multi-Pollutants Control, Department of Environmental Science and Engineering, North China Electric Power University, Baoding, P.R. China

Lin Wang,     Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, P.R. China

Shuao Wang,     State Key Laboratory of Radiation Medicine and Protection, School for Radiological and Interdisciplinary Sciences (RAD-X) and Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou, P.R. China

Siyi Wang,     Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, P.R. China

Xiangke Wang,     MOE Key Laboratory of Resources and Environmental Systems Optimization, College of Environmental Science and Engineering, North China Electric Power University, Beijing, P.R. China

Xiangxue Wang,     Hebei Key Lab of Power Plant Flue Gas Multi-Pollutants Control, Department of Environmental Science and Engineering, North China Electric Power University, Baoding, P.R. China

Xin Wang,     MOE Key Laboratory of Resources and Environmental Systems Optimization, College of Environmental Science and Engineering, North China Electric Power University, Beijing, P.R. China

Tao Wen,     MOE Key Laboratory of Resources and Environmental Systems Optimization, College of Environmental Science and Engineering, North China Electric Power University, Beijing, P.R. China

Bo Wu,     MOE Key Laboratory of Resources and Environmental Systems Optimization, College of Environmental Science and Engineering, North China Electric Power University, Beijing, P.R. China

Mengjia Yin,     School of Chemistry, Biology and Materials Science, East China University of Technology, Nanchang, P.R. China

Shujun Yu,     MOE Key Laboratory of Resources and Environmental Systems Optimization, College of Environmental Science and Engineering, North China Electric Power University, Beijing, P.R. China

Anrui Zhang,     MOE Key Laboratory of Resources and Environmental Systems Optimization, College of Environmental Science and Engineering, North China Electric Power University, Beijing, P.R. China

Pengcheng Zhang,     Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, P.R. China

Ruihong Zhang,     MOE Key Laboratory of Resources and Environmental Systems Optimization, College of Environmental Science and Engineering, North China Electric Power University, Beijing, P.R. China

Shu Zhang,     Hebei Key Lab of Power Plant Flue Gas Multi-Pollutants Control, Department of Environmental Science and Engineering, North China Electric Power University, Baoding, P.R. China

Chaofeng Zhao,     MOE Key Laboratory of Resources and Environmental Systems Optimization, College of Environmental Science and Engineering, North China Electric Power University, Beijing, P.R. China

Chapter 1: Novel nanomaterials for environmental remediation of toxic metal ions and radionuclides

Shujun Yu ¹ , Yue Liu ¹ , Hongwei Pang ¹ , Hao Tang ¹ , Jiaqi Wang ² , Shu Zhang ² , and Xiangxue Wang ²       ¹ MOE Key Laboratory of Resources and Environmental Systems Optimization, College of Environmental Science and Engineering, North China Electric Power University, Beijing, P.R. China      ² Hebei Key Lab of Power Plant Flue Gas Multi-Pollutants Control, Department of Environmental Science and Engineering, North China Electric Power University, Baoding, P.R. China

Abstract

The global environment has been compromised in various ways, leading to problems such as heavy metal ions and radionuclide contamination. Novel nanomaterials are promising for the treatment of polluted water owing to their special physicochemical properties. This chapter focuses on novel nanomaterials and remediation technologies for the treatment of heavy and radioactive ions. First, the chemical behavior of different metal ions and radionuclides (e.g., Cr, As, Pb, Hg, U, Tc, and Sr) in solution are discussed. Then, advantages and disadvantages of various remediation technologies including adsorption, membrane filtration, ion exchange, chemical precipitation, electrochemical treatment, and bioremediation are discussed. Subsequently, the properties, preparation methods, and applications of novel nanomaterials such as metal-organic framework, covalent organic framework, MXene, nanoscale zero-valent iron, and carbon-based nanomaterials are systematic discussed. Finally, expectations are briefly discussed and the work is summarized.

Keywords

Carbon material; COF; Environmental remediation; Heavy metal ions; MOF; MXene; Nanomaterial; nZVI; Radionuclides; Remediation technologies

1. Introduction

1.1. Toxic metal ions and radionuclides

Over the past few decades, toxic heavy metal ions and radionuclides have been released into the hydrosphere owing to the rapid development of industry and the increasing demand for global energy. Metal contaminants with poor ability to degrade usually exist in their original state, posing a huge threat to biological and environmental safety [1,2].

Representative trace heavy metal ions include lead (Pb), zinc (Zn), nickel (Ni), manganese (Mn), mercury (Hg), chromium (Cr), copper (Cu), cadmium (Cd), and arsenic (As). Natural and anthropogenic activities are the main sources of dissociated or fixed heavy metal ions in water bodies [3,4]. Natural sources include volcanic eruption, crustal movement, rock weathering, soil erosion, and dry/wet settlement [5]. Heavy metal ions are injected into the environment through natural activities with the background value of metal elements. Sources of anthropogenic activities include ore extraction, oil field extraction, electroplating metallurgy, chemical waste discharge, and the use of pesticides or fertilizers [6,7]. Fig. 1.1 briefly describes the major sources and migration/transformation paths of heavy metal ions in the environment. With the characteristics of wide sources, long residence time, and easy accumulation and enrichment, heavy metal ions are difficult to be remediated and degraded in an aqueous environment [9]. The accumulation and enrichment effects of heavy metal ions appear in aquatic animals, plants, and microorganisms. When the concentration and toxicity level of pollutants are greater than the range that aquatic organisms can bear, the adverse effects or serious toxic effects on the life activities of organisms may cause genetic mutations or variations, resulting in changes in biodiversity and the survival rate of biological species [10]. Heavy metals also pose a serious threat to human health, because they can be passed on to advanced organisms such as humans through the food chain. Heavy metals are not easily discharged after they enter the human body; toxicants reaching the physiologic load of the human body through a cumulative effect cause physiologic structural changes and acute, chronic, or long-term harm [5,11]. In the presence of a high concentration of heavy metal ions, functional loss of the brain and nervous system, damage to internal organs, physical weakness, memory loss, skin allergies, hypertension, and other symptoms are easily triggered [12]. Table 1.1 showed the maximum concentration and toxicity of heavy metal ions in drinking water, as defined by the World Health Organization (WHO) and US Environmental Protection Agency (USEPA). Based on this, preventive and remedial measures to reduce the pollution of heavy metal ions in water bodies have received worldwide attention.

Figure 1.1 Flowchart showing how soil and freshwater and groundwater systems redistribute heavy metal of an anthropogenic origin [8].

Because of the tense situation of energy shortages, developing clean nuclear power become an urgent trend across the world. The application of nuclear technology in all fields, which exploits and processes radioactive minerals, produced a large number of radionuclides [15]. When the dose of radioactive contaminants in the human body reaches a certain level, it may cause headaches, depression, and a reduction in white blood cells and platelets, resulting in leukemia or cancer in the most severe cases [16]. For people who are not exposed to radiation in daily work, the normal natural radiation dose per year is 1000–2400   μSv. A radiation dose of less than 100   μSv has no effect on humans [16]. For workers in contact with radiation, the maximum annual radiation dose is 50   mSv [17], whereas one-time exposure to 4000   mSv can cause sudden death [18]. Migration behaviors and adverse effects on biological health as a result of various radioactive pollutants such as uranium (U), europium (Eu), technetium (Tc), cesium (Cs), curium (Cm), strontium (Sr), americium (Am), and plutonium (Pu), have aroused wide concern in many countries. Uranium is the most common pollutant. The WHO has classified uranium as a carcinogen and has set the maximum contamination threshold for uranium in drinking water at 30   μg   L −¹. The proper discharge and treatment of radionuclide wastewater is an urgent task facing humanity.

Table 1.1

NA, not available.

1.2. Common treatment technologies for contaminants

The pollution of heavy metal ions and radionuclides in water has become a serious public health problem in the world. Because the types and concentrations of metal contaminants that are discharged from different sources are different, single traditional water treatment processes lead to low efficiency and substandard effluent quality. Therefore, it is urgent to preconcentrate or recover valuable metal ions from wastewater by appropriate water treatment technologies so that treated water can meet discharge standards or be recycled.

To meet the effluent standards of environmental protection regulations, various water treatment technologies such as adsorption, membrane filtration, ion exchange, chemical precipitation, electrochemical treatment, and bioremediation have been proposed [19]. However, different water treatment processes have advantages and disadvantages. Adsorption method is recognized as a promising technology for treating low-concentration metal-contaminated wastewater, but the low efficiency and actual availability of adsorbents are problems that need to be solved [20]. Compared with other water purification processes, membrane filtration technology requires a smaller space and has high removal efficiency for metal ions, but the initial investment and maintenance are high, the operation is complicated, and long-term use will cause membrane pollution and reduce the efficiency of pollutant removal [21]. Ion exchange can be used to treat high-concentration metal pollution, and it has fast kinetics. However, regeneration of the resin bed requires a large number of acid-base reagents, which not only increases the cost but also inevitably causes secondary pollution [22]. Chemical precipitation is simple and inexpensive, and it is suitable for large-scale polluted wastewater treatments. However, the generation of a quantity of sludge in the process increases the difficulty of subsequent treatment, and the quality of the effluent is unsatisfactory [23]. Electrochemical treatment is considered to be an ideal technology to control the concentration of metal ions quickly in water, and no additional chemical reagents are needed in the process. However, the large amount of electrical energy consumption and short electrode life in the electrochemical process severely restrict its use in actual wastewater [24]. Bioremediation, with the advantages of low cost, environmental friendliness, and no secondary pollution, is the priority choice for restoring damaged ecosystems and achieving sustainable environmental development. In fact, different types of metal pollution and required remediation sites must be specifically investigated and analyzed in actual remediation, which limits the applicability of bioremediation technology [25]. Table 1.2 lists advantages and disadvantages of various water treatment processes in the treatment of wastewater contaminated by heavy metal ions and radionuclides, so that readers may compare and choose among them.

In the design of actual wastewater treatment, factors such as the type and concentration of metal pollutants, the capital investment for initial construction and later maintenance, the water quality requirements of the effluent, and the water environment all should be considered. In many cases, it is difficult for a single water treatment process to achieve the desired results. Therefore, the combination of multiple technologies can remove heavy metal ions and radionuclides in water more quickly and efficiently. As shown in Fig. 1.2, water treatment units and processes can be combined to form primary, secondary, and tertiary water treatment technologies [27]. In general, water purified by primary and secondary treatment can meet normal domestic and industrial needs. After tertiary treatment, almost all pollutants can be removed from water, which will be converted into high-quality safe water that can be used for many specific purposes (e.g., recycled water for power plants).

Table 1.2

1.3. Novel nanomaterials for water purification

A nanometer is a unit of measurement of length, one-billionth of a meter (10 −⁹   m). The term nanometer was first proposed by Zsigmondy to describe the size of nanoscale particles [28]. In 1982, German scientists including Gerd Binnig and Heinrich Rohrer [29] invented an important tool for studying nanoscience at IBM Zurich Research Laboratory: scanning tunneling microscopy (STM). The birth of STM has greatly promoted the development of modern nanoscience. Subsequently, research on nanoscience and nanotechnology grew explosively. Many studies have shown that nanoscaled materials exhibit different physical and chemical properties relative to their equivalents of normal size [30]. When the particle size reaches the nanometer level, the material will show a strong small size effect, quantum effect, surface effect, macroquantum tunneling effect, and so on, so that nanomaterials exhibit light, electricity, heat, magnetism, and absorption, reflection, catalysis, and biological activity different from macroscopic substances. With continuous advancements in nanoscience, nanotechnology has been successfully applied in many fields, including medicine, pharmacy, chemistry, monitoring, manufacturing, optics, and national defense [31]. As a platform technology, nanotechnology is often integrated with other technologies to achieve more practical applications. Nanotechnology has gradually been applied to wastewater treatment. As shown in Fig. 1.3, developed countries demand to adopt more advanced nanotechnology to deal with more complex pollutant systems than developing countries [32]. Hence, developing cheaper, higher-quality water treatment units is pushing the boundaries of current treatment paradigms. Although most nanotechnology-based water treatment processes have good performance, their high costs limit their application. Scientific researchers around the world focus on continuously improving industrial production efficiency, reducing energy consumption, and limiting environmental pollution through nanotechnology, and attempt to provide solutions to help countries achieve sustainable development.

Figure 1.2 Classification of chemical treatment and water recycling technologies [27].

Figure 1.3 Conceptual improvements to water treatment through nanotechnology. Arrows represent specific strategies or drivers that can enhance performance and/or decrease costs through the use of nanotechnology [32]. ENM, engineered nanomaterials: EHS, environmental health and safety: POU, point-of-use devices in water treatment process.

In recent years, nanomaterials as a water purifier to treat pollution in a water environment have attracted widespread attention. Nanomaterials generally refer to substances with a structural unit size of 1–100   nm, which have unique nanostructures and excellent physical and chemical properties. Owing to high specific surface areas and abundant surface atoms, nanomaterials have excellent adsorption capacities and chemical affinities compared with traditional adsorbent materials [33]. High specific surface areas provide more opportunities for interfacial reactions, which is crucial for improving the adsorptive performance and removal speed of soluble pollutants by nanomaterials. In addition, insufficiently coordinated atoms on the surface of the nanomaterial generate a large number of unsaturated bonds, which create more active sites for bonding pollutants. Moreover, for different pollutants, active sites on the surface of nanomaterials can be functionalized with different chemical groups to achieve the targeted removal of contaminants. Extensively used nano-adsorbents can be roughly divided into four common categories according to the structure and composition of the materials, including carbon family nanomaterials, inorganic nanomaterials, organic polymers, and porous framework materials [34]. Among them, common carbon family nanomaterials include carbon nanotubes (CNTs), graphene oxide (GO) materials, and biochar materials. Carbon family nanomaterials are the most popular nano-adsorbents and have high adsorption capacity, fast removal speed, and good regeneration ability in water purification. However, because the technology is not yet mature, biological toxicity and separation from water after use still pose problems for carbon family materials [35]. Inorganic nanomaterials include nanosized natural materials (clays, zeolites, birnessite, etc.), metal nanoparticles (nanoscale zero-valent iron [nZVI], silver nanoparticles, etc.), and nanoscale metal oxides (such as Al2O3, TiO2, ZnO, and layered double hydroxide). Inorganic nanomaterials have the advantage of low cost, wide sources, and simple preparation methods, which are suitable for large-scale production and use. However, because of deficient functional groups on the surface of inorganic nanomaterials, there is an urgent need to functionalize the materials [36]. Commonly used organic polymers mainly refer to cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs). CNCs and CNFs can be widely obtained from agricultural waste with high economic efficiency. However, in practical application, organic polymers may face problems such as low kinetics and instability under harsh conditions [37]. Porous framework materials are emerging nanomaterials with permanent porosity, an adjustable pore structure, a huge specific surface area, and abundant functional groups, include metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and porous organic polymers (POPs). Although many reports on porous framework materials have shown excellent removal ability for pollutants, their high cost, potential threats, and rebellious stabilities limit their applications in real water bodies. Adopting appropriate modified technologies to remediate the disadvantages of porous framework materials is of great significance for their regeneration and reuse [38].

1.4. Aims of this chapter

In this chapter, information on the chemical behavior of typical toxic metal ions and radionuclides such as chromium (Cr), arsenic (As), lead (Pb), mercury (Hg), uranium (U), technetium (Tc), and strontium (Sr) is summarized in detail. Subsequently, remediation technologies including adsorption, membrane filtration, ion exchange, chemical precipitation, electrochemical treatment, and bioremediation for radioactive and toxic metal ions will be introduced. Finally, we will focus on several emerging nanomaterials (MOFs, COFs, MXene, nZVI, and carbon-based nanomaterials), as well as their physical and chemical properties, development history, synthetic technologies, and applications in water treatment. We hope that this chapter will help researchers to gain a deeper understanding of these emerging nanomaterials and bring inspiration to the promising research field of materials and environmental remediation.

2. Chemical behaviors of toxic metal ions and radionuclides in solution

2.1. Chromium

Anthropogenic sources of chromium include chrome plating of metal parts, industrial pigments, and the production of tanned leather and rubber and ceramic raw materials [39]. The oxidation states of chromium include Cr(II), Cr(III), and Cr(VI), whereas Cr(II) is not stable in the environment; hence, Cr(III) and Cr(VI) are dominant species in natural aqueous solutions. The hydrolysis process of Cr(III) is relatively complicated, which may produce cations, neutral, and anions in water (such as CrOH²+, Cr[OH]2 +, Cr3[OH]4 ⁵+, Cr[OH]3, and Cr[O])4 − ). However, the hydrolysis products of Cr(VI) are only neutral and anions, including CrO4 ²− , HCrO4 − , and Cr2O7 ²− [40]. Fig. 1.4 provides the chemical morphology of Cr in a water environment under different redox conditions and pHs. Under the reduction condition, Cr(III) is the dominant species in the aqueous phase; it mainly exists in the forms of Cr³+, CrOH²+, Cr(OH)2 +, Cr(OH)3 ⁰, and Cr(OH)4 − . In natural water, Cr(III) mainly exists in the form of insoluble Cr(OH)3 precipitation. Because Cr(III) is easily adsorbed in the stationary phase or deposited in the form of precipitation in natural water, it poses little threat to the environment and biological safety. When the total Cr concentration is not high, Cr(VI) will widely exist in water with highly stable chemical forms (HCrO4 − and CrO4 ²− ). The form of Cr(VI) has high fluidity, solubility, and toxicity, causing a degree of ecological damage greater than that of Cr(III) [41]. Cr(III) and Cr(VI) can easily convert into each other in aqueous solution, so water quality standards are usually specified based on the total content of chromium [42]. For instance, the WHO stipulates that the concentration of total chromium in drinking water for residents should be limited in 50   μg   L −¹. Based on the high toxicity stability of Cr(VI) in water, the USEPA declared hexavalent chromium to be one of 129 chemicals that damage humans [43].

Figure 1.4 Eh-pH diagram for chromium [40].

2.2. Arsenic

Arsenic is not only the main component of rocks and minerals, it also widely exists in groundwater owing to the elastic action of the water–rock interface [44]. Arsenic is a huge potential threat to human health and is listed as a priority pollutant by the USEPA. According to relative reports of the water environment, As(V) has good thermodynamic stability and usually exists stably in aqueous solutions and sediments. Based on the Eh-pH diagram for As species in the As–H2O system at 25°C (Fig. 1.5), in the conventional pH range of natural waters, As(V) mainly exists in the anion form of H2AsO4 − (pKa1   =   2.30) and HAsO4 ²− (pKa2   =   6.99), whereas As(III) manifests as H3AsO3 (pKa1   =   9.17) [45,46]. In neutral and weakly acidic water environments, adsorbents with a positive surface charge can attract negatively charged H2AsO4 − and HAsO4 ²− through electrostatic attraction, increasing the amount of As(V) removed. However, the adsorption of electrically neutral H3AsO3 from aqueous solution is little affected by the pH value, especially in an alkaline liquid. The natural attenuation of arsenic mainly relies on precipitation and adsorption, which is greatly affected by microbial transformation and redox cycling. The effective way to reduce the amount of arsenic in a water environment is to convert moving arsenic into fixed arsenic and store it underground to prevent it from entering groundwater. Reisinger et al. [46] pointed out that arsenic pollution in water can be reduced by adsorbing arsenic in a fixed layer of water or by converting arsenic into a main/trace component of solids for precipitation (Fig. 1.6). Generally speaking, redox conditions in the process always have a vital role. Therefore, a more detailed understanding of the elemental morphology of arsenic at different pHs and Eh is crucial for controlling the degree of arsenic adsorption and precipitation.

Figure 1.5 Eh-pH diagram for aqueous As species in the system As–H2O at 25°C and As   =   0.1   mol   L −¹, calculated using HSC Chemistry (version 4.0, Outotec Research Oy, Pori, Finland) [45].

2.3. Lead

In nature, lead usually exists in the oxidized form of Pb(II) and Pb(IV). According to Ochs' research [47], Pb usually exists in the chemical form of Pb²+ and PbOH+ in the conventional pH range, whereas Pb(IV) would be exposed to the environment in the state of plattnerite (PbO2) only under very oxidizing conditions. As a heavy metal that cannot be degraded by biology, Pb can be transferred and transformed only by dispersion, enrichment, and mutual transformation among various forms. Lead ions and their compounds can undergo mechanical, physicochemical, and biological migration and transformation in a water background. In mechanical migration, lead in a dissolved or particulate form is encapsulated by minerals/organic colloids or adsorbed in suspended matters in water, and then migrates with the water flow. In physicochemical migration and transformation, lead undergoes precipitation, adsorption, and redox to achieve diffusion or deposition in water through different factors, such as Eh, pH, and dissolved oxygen. Biological migration of Pb relates to the food chain and its enrichment [48]. Through biological metabolism in nature, lead can be transferred in the food chain from one level to another, and the content of lead will also be gradually accumulated during transfer. Understanding the rule of lead migration and transformation is helpful for scientists to prevent and manage lead pollution in water bodies more scientifically [49]. With improvements in science and technology, people have gradually mastered the treatment technologies of lead. For actively treatment, lead-free production technology should be advocated and promoted; a reduction in lead pollution from the beginning is also worth thought and research.

Figure 1.6 Possible processes in biogeochemical cycling of arsenic [46]. Mobile arsenic species are shown in red (light gray in print), and immobile in blue (dark gray in print). Mobilization pathways can be affected by microbial transformations and redox conditions.

2.4. Mercury

Mercury is a natural element widely found in air, water, and soil in the form of elementary substance, inorganic matter, and organic matter. The form and solubility of Hg in natural water mainly depend on the redox condition. According to investigations, the three valence forms of Hg (Hg[0], Hg[I], and Hg[II]) may generate different forms of inorganic mercury and organic mercury species (CH3Hg+, [CH3]2Hg, CH3HgCl, and CH3HgOH). Generally, OH − and Cl − have a significant influence on the chemical morphology of Hg in water systems. Without considering the complexation of organic matter to Hg, Allard and Arsenie [50] provided Eh/pH diagrams for Hg at different Cl − concentrations based on thermodynamic data from the National Bureau of Standards. The result showed that Hg(OH)2(aq) is the dominant species in natural water at a low Cl − concentration. However, because Hg's effective Eh is generally lower than the theoretical Eh set by the partial pressure of oxygen, Hg also exists in air as gas. In chlorine-rich water bodies, HgCl2(aq) and HgOHCl(aq) become mainstream species under weakly acidic conditions, which greatly improves the overall stability of Hg(II) and increases the difficulty of water purification.

Mercury is the only liquid metal at room temperature, and its elementary substance is generally nontoxic. However, after entering the blood, metallic mercury will be rapidly oxidized to monovalent or divalent mercury ions to produce toxic effects, which can bind to sulfhydryl or disulfide groups in cell membranes or enzyme proteins and then affect cell functions. Inorganic mercury compounds are generally soluble in water and mainly damage the kidneys and liver after they are adsorbed by human, but they do not stay or accumulate in the body for a long time. The toxicity of organic mercury depends on the internal bond stability of the compound. Common organic forms include methyl mercury and ethyl mercury [51]. Most toxic methylmercury is formed by the complexation of organic compounds and inorganic mercury dissolved by microorganisms in a natural environment. Fat-soluble methylmercury maintains a high concentration in a water environment. After it enters the biological chain through aquatic organisms, it can easily be digested and absorbed by humans. Prolonged exposure to high levels of various forms of mercury can cause permanent damage to the human brain, kidneys, and developing fetus [52]. Fig. 1.7 showed the interaction mechanism of Hg in natural water bodies and some specific species in temperate lakes [53], which is helpful for further study of the biogeochemical cycle of Hg in the marine system.

2.5. Uranium

As one of the most common radionuclides, uranium is a representative actinide element with a half-life of millions or even billions of years. In addition to spent fuel produced by nuclear power plants, the source of uranium comes from the mining and processing of granite and uranium ore, nuclear weapons manufacture, industry, and medical industries [34]. Because uranium is extremely stable in a natural environment and easily soluble in aqueous solutions, it easily migrates in water bodies and extends into the food chain, posing a health threat to humans and organisms. The threat of uranium comes not only from its own radiation properties, but also from its chemical properties. Epidemiologic and toxicologic studies have shown that uranium in drinking water is closely related to chronic kidney disease [54]. Based on radiologic limits, the WHO and USEPA have set a limit of 30   μg   L −¹ for uranium content in drinking water.

Figure 1.7 Generalized view of mercury biogeochemistry in the aquatic environment [53].

The migration and transformation of uranium in water will largely affect its enrichment, mobility, and hazards. Under normal circumstances, uranium exists in a water environment in U(IV) and U(VI) valence states. Under reducing conditions, U(IV) will produce insoluble UO2. In contrast, oxidizing U(VI) has better solubility, mainly in the form of uranyl (UO2 ²+) in aqueous solution [55]. Under different pH and redox conditions, U(VI) will form different kinds of complexes with water components, mainly hydroxides or carbonates. Understanding the exact species form of U(VI) in water is necessary for treating uranium pollution. Under high pH and Eh conditions, U(VI) is present in the form of anionic UO2(CO3)2 ²− and UO2(CO3)3 ⁴− (Fig. 1.8) [34]. A large number of studies have shown that uranium carbonate complex in water will significantly inhibit the removal of adsorbents [56].

2.6. Technetium

Technetium is a typical radionuclide with a half-life of 2.13   ×   10⁵   years, with high toxicity, fluidity, and complex environmental behavior [57]. Isotopes of Tc such as ⁹⁷Tc and ⁹⁸Tc are unstable in nature, except for long-lived ⁹⁹Tc. ⁹⁹Tc-contaminated wastewater poses a major safety hazard to public health and has aroused widespread concern around the world [58]. Therefore, understanding the pollution sources as well as the migration and transformation modes of the radionuclide ⁹⁹Tc is important to protect human and ecological health.

Figure 1.8 Eh-pH diagram for uranium species as a function of pH in the U–CO2–H2O system: total uranium (UTot)   =   10–6   M, PCO2   =   10–2   atm, and T   =   25°C; Dashed lines represent the limits of the stability range of water [34].

Technetium has been proven to exist in the oxidation states of Tc(0), Tc(II), Tc(IV), Tc(V), Tc(VI), and Tc(VII). Once Tc is released into the environment, it will exist as Tc(VII) under oxidized or hypoxic conditions, presenting as pertechnetate (TcO4 − ) with stable physical and chemical properties and good solubility [59,60]. Studies have shown that the dissociation constant (pKa) of TcO4 − is −8, which indicates that when the pH of the water is 4.0–8.0, the surface of these oxygen-containing anions is negative. When the mineral pH is lower than the pH zero potential charge (pHzpc), there will be electrostatic attraction between the oxygen-containing anions of Tc and the surface of the positively charged mineral [61]. As the pH increases, the adsorption of Tc by minerals will gradually decrease, which is attributed to the increase of negative charges on the mineral surface. TcO4 − can move almost at the same speed as water flow, so it is widely distributed in the water environment, which increases the difficulty of technetium pollution treatment. However, under reducing conditions, the electron donor in the aqueous solution can convert Tc(VII) to Tc(IV), generating insoluble TcO2·nH2O solids. In short, the retention rate of Tc(VII) toward minerals is low. On the contrary, Tc(IV) can easily be adsorbed by minerals because of the precipitation reaction [62,63]. Understanding the chemistry behavior of Tc provides information about the existence and transportation of Tc under different environmental conditions and also meaningful suggestions for removing Tc pollution from water.

2.7. Strontium

Strontium is an alkaline earth metal with white luster and active chemical properties. It can easily be oxidized into stable and colorless Sr(II). Natural stable isotopes of strontium include ⁸⁴Sr, ⁸⁶Sr, ⁸⁷Sr, ⁸⁸Sr, and 31 unstable isotopes [64]. Among them, the most common ⁹⁰Sr has the longest life, with a half-life of 28.9   years [65].

In a natural environment, Sr is a trace element in the lithosphere composition of the crustal, but it is the most abundant trace element (375   ppm) in the upper part of the lithosphere. Rocks are the main source of strontium in groundwater, and it is widely distributed in nature. Sr²+ is almost the dominant species in the entire pH range, whereas SrOH+ exists only in a strongly alkaline environment [66]. The water chemistry type of the strontium-rich water environment is a heavy carbon water environment. Water containing Ca²+ and HCO3 − is the most widely distributed on earth, and the content of Sr is generally low [67–69]. This is because of the influence of Ca(II) and K(I) on the migration of Sr(II) in a water background. Sr can be released from Ca- and K-rich rocks along with Ca and K under the influence of groundwater and surrounding media. In addition, owing to changes in cation adsorption, Sr is desorbed into water from the surface of highly dispersed particles such as clay materials. The distribution of Sr in water depends on the extent of Sr(II) replaced by Ca(II) in Ca-containing minerals, and the extent of Sr(II) captured by K(I) in potassium feldspar. When Ca(II) in the local groundwater reaches the saturation concentration and precipitates, Sr(II) will replace part of the Ca(II) and produce coprecipitates. Hence, the Sr content of carbonate-rich aqueous solution is generally low. In addition, the pH value of aqueous solutions affect the migration ability of Sr(II). Studies have shown that when the pH is between 7 and 8.5, the concentration of Sr in water is usually higher. In addition, the distribution of strontium-rich rocks, the degree of weathering and fragmentation of rock minerals, water erosion, and temperature affect the migration and transformation of Sr(II).

3. Remediation technologies for radioactive and toxic metal ions

3.1. Adsorption

Adsorption is defined as an economical, efficient, and highly selective method for treating heavy metal- and radionuclide-containing wastewater. The design and operation of the adsorption process are flexible, and heavy metals and radionuclides in the water can be well-recovered [70]. Adsorption is a typical solid–liquid mass transfer process in which heavy metals and radioactive contaminants are transferred to active sites on the surface of solid adsorbents through physical or chemical reactions. Physical adsorption results from weak van der Waals force between metal ions and adsorbents, whereas chemical adsorption is due to the generation of covalent bonds or other chemical bonding [71]. The key technology of adsorption is the design and selection of the adsorbent materials.

Various adsorbents with excellent specific surface area, ordered pore structure, and abundant surface functional groups have been developed for the enrichment and removal of heavy metal ions and radionuclides from sewage. Commonly used traditional adsorbent materials include clay minerals, metal oxides, activated carbon, resins, cellulose, and chitosan [34]. Environmentally friendly clay minerals (such as kaolinite and montmorillonite) and metal oxides (such as aluminum oxide, iron oxide, titanium oxide, and manganese oxide) widely exist in the hydrosphere; they are excellent adsorbents to control the migration and transportation of metal ions [72,73]. Carbon-based material is the most popular and widely used adsorbent material in wastewater treatment [74]. Organic polymers such as cellulose and chitosan have a wide range of sources. They are nontoxic and inexpensive, promising adsorbents [75]. Porous framework materials such as COFs, POPs, and MOFs have gradually attracted the attention of researchers. Porous framework materials have a permanent pore structure attributed to their covalent bonds or organic linkers, which endow them with many outstanding characteristics, such as ultra-high specific surface area, designable pore size and framework structure, and functional crystal. These excellent physical and chemical properties give porous framework materials broad application prospects for removing heavy metal ions and radionuclides [76–78].

Although the development and improvement of various adsorbent materials have attracted the attention of scientific researchers, most of these materials cannot be mass-produced and used in practice. Therefore, the preparation and promotion of more efficient and practical adsorbents require more effort. In addition, adsorption of heavy metal ions and radionuclides is reversible, and the regeneration of adsorbents can be achieved through proper desorption technology, which reduces the cost of adsorption [77]. There are many technologies for regenerating adsorbents, including thermal, pressure swing, and electrochemical, which indicates that adsorption is an environmentally acceptable water treatment strategy.

3.2. Membrane filtration

Membrane filtration is a common wastewater treatment technology with high efficiency, simple operation, and a small equipment area. According to the pore size range and separation method, membrane filtration can be divided into microfiltration, ultrafiltration, nanofiltration, reverse osmosis, and electrodialysis [79]. Among them, microfiltration is used to remove pollutants with a particle size of 100–1000   nm. The main function is to pretreat wastewater to remove large particles and suspended solids. The remaining several membrane technologies have been successfully promoted to separate heavy metal ions and radionuclides from sewage. The complex structure of the membrane determines the water permeability and sieving capability. Also, the size of the metal ions, pollutant concentration, the initial pH, and the pressure applied during the process affect the effectiveness of membrane filtration [80].

Ultrafiltration is a technology that removes metal pollution in wastewater under low transmembrane pressure. It can separate particles with a particle size of 10–100   nm. Because the pore size of the ultrafiltration membrane is larger than the metal ions, metal ions cannot be intercepted by ultrafiltration. Therefore, chemical reagents or polymers are usually added to form large-structure complexes with the metal ions before elimination. Micellar enhanced ultrafiltration (MEUF) [81] and polymer enhanced ultrafiltration are proposed to capture metal ions [82]. When using MEUF to treat wastewater, excess surfactant is added to the wastewater until its concentration exceeds the concentration of the critical micelle. Surfactants with opposite charges will form complexes with metal ions that are larger in size than the ultrafiltration membrane and will be retained. Sodium dodecyl sulfate, as a traditional negatively charged surfactant, is frequently used in MEUF to remove metal ions [83].

Nanofiltration, a membrane separation technique between ultrafiltration and reverse osmosis, has great potential in the treatment of heavy metal solutions [84]. For nanofiltration, the particle size of the separated solute is smaller than that of ultrafiltration, and the energy consumption of the process is lower than that of reverse osmosis. Membranes used in nanofiltration are usually composed of synthetic polymers and the membrane surface is positively or negatively charged. The pore size and surface characteristics of nanofiltration membranes determine its unique properties. For example, nanofiltration membranes have different Donann exclusion for metal ions with different charges and different valences [85]. The mechanism of separating metal ions by nanofiltration membrane is particle repulsion and charge repulsion, which can effectively remove divalent and multivalent metal ions [86]. Moreover, owing to electrification of the nanofiltration membrane, it maintains a high retention capacity for metal ions at a lower pressure and solute concentration. As a green water treatment technology, nanofiltration can replace traditional sewage treatment methods that are expensive and cumbersome, and it has bright application prospects.

Reverse osmosis has been widely applied to remove heavy metal ions and radionuclides from sewage [87]. The pore size of the reverse osmosis membrane ranges from 0.1 to 1.0   nm. It can remove most dissolved substances in water. The mechanism of reverse osmosis to remove metal ions from water is similar to that of nanofiltration; particle size repulsion and charge repulsion are the dominant process. However, reverse osmosis requires a lot of energy input, which limits its application on a large scale [88]. In addition, the small pore size of reverse osmosis membrane is easily