Application of Nanomaterials and Nanotechnology in the Reutilization of Metal Ion From Wastewater

5 APPLICATION OF NANOMATERIALS AND NANOTECHNOLOGY IN THE REUTILIZATION OF METAL ION FROM WASTEWATER Fang Deng*, Xu-Biao Luo*, Lin Ding†, Sheng-Lian Luo* *Key Laboratory of Jiangxi Province for Persistent Pollutants Control and Resources Recycle, Nanchang Hangkong University, Nanchang, P. R. China † School of Environmental Science and Engineering, Huazhong University of Science and Technology, Wuhan, P. R. China

CHAPTER OUTLINE 5.1 Introduction 150 5.1.1 The Demand for Recovery of Silver 150 5.1.2 The Demand for Recovery of Gold, Platinum, and Palladium 151 5.1.3 The Demand for Recovery of Li, Co, and Ni From Lithium Ion Batteries 151 5.2 Adsorption 152 5.2.1 The Types of Adsorption 152 5.2.2 The Application of Selective Adsorption in Recovery of Metals 152 5.2.3 Classification of Selective Adsorption for Heavy Metals 153 5.3 Ion-Imprinting Technology 155 5.3.1 The Introduction of Ion-Imprinting 155 5.3.2 The Materials for Preparation of Ion-Imprinted Polymers 157 5.3.3 Major Polymerization Techniques for the Preparation of Ion Imprinted Polymers 160 5.3.4 The Application of Ion-Imprinting Technology in the Resource Utilization 165 5.4 Photocatalytic Reduction 170 5.4.1 Heterogeneous Photocatalysis for the Reduction of Metal Ion to Metal Element 170 5.4.2 Homogeneous Photocatalysis for the Reduction of Metal Ion to Metal Element 171 Nanomaterials for the Removal of Pollutants and Resource Reutilization. https://doi.org/10.1016/B978-0-12-814837-2.00005-6 Copyright # 2019 Elsevier Inc. All rights reserved.

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5.5 The Combination of Ion-Imprinting Technology With Photocatalytic Reduction 175 5.6 Conclusions 176 References 177

5.1

Introduction

The treatment of heavy metal pollution in water is an important branch of water pollution control. Continuous developments in industry and agriculture and unreasonable discharge of wastewater lead to more and more serious pollution of water with heavy metals. The wastewater from metal smelting and processing, leather manufacturing, electroplating, electronic products processing, photographic processes, printing, dyeing, and pesticide manufacturing usually contains toxic heavy metal ions. Heavy metal pollution is concealed and can be accumulated. Two of the distinctive features of heavy metal pollution that separate it from other types of pollution are the enrichment effect and the fact that heavy metal ions are difficult to be biodegraded. Even a low concentration of heavy metal ions can be enriched in humans and animals through the food chain and cause harmful effects. However, heavy metals have high economic value and strategic value as an important nonrenewable resource. Therefore discovering how to effectively control heavy metal pollution in water body, protect human health and the ecological environment, and reuse the metal in wastewater could ease the pressure on our resources and the environment.

5.1.1

The Demand for Recovery of Silver

Silver (Ag) is extensively applied in many fields, such as communication, aerospace, medical equipment, the chemical industry, photographic materials, electroplating, and the electronic industry because of its excellent malleability, thermal conductivity, and superior electrical. However, silver resources are extraordinarily scant since its applications have greatly increased in number in various fields, and silver is often used in combination with many other metals, such as copper, lead, and antimony deposits. Due to its usefulness in a wide range of applications in industry, silver ion has been detected in various industrial wastewater from electronics, electroplating, galvanic industries, and hazardous waste disposal sites. Therefore from the perspective of sustainable development and comprehensive utilization of

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resources, silver recovery from wastewater is of great importance for social development. However, in most cases, the concentration of silver ions in wastewater is very low and needs to be enriched and separated before recovery. Generally, conventional techniques for the extraction of silver from wastewater include precipitation, electrodeposition, ion exchange, and solvent extraction. The main shortcomings of these methods are high cost, low selectivity, the fact that they are time-consuming, and the massive labor forces involved. Other disadvantages include low recovery efficiency, toxic waste generation, and secondary pollution. Therefore it is essential to develop some effective methods to enrich and recover Ag from silver-containing wastewater.

5.1.2

The Demand for Recovery of Gold, Platinum, and Palladium

In the recent years, precious metals such as gold, platinum, and palladium have been widely used not only in traditional jewelry manufacture, but also in many advanced applications, such as decoration, watchmaking, dentistry, catalysts, electric and electronic devices, and medical and precision instruments. The great increase in industrial need for gold, palladium, and platinum has resulted in a steady growth in the demand for refining these metals using a cost-effective technique. However, gold, platinum, and palladium are scarce, and crude gold, platinum, and palladium metals are usually associated with other metal deposits. Thus there is an urgent need to develop effective methods for the recovery of gold, platinum, and palladium from various wastes.

5.1.3

The Demand for Recovery of Li, Co, and Ni From Lithium Ion Batteries

Secondary or rechargeable lithium-ion batteries (LIBs) are the best choice in the consumer electronics market, including portable electronic devices such as cell phones, laptops, and camcorders due to their outstanding characteristics, such as long cycle life, high energy density, low self-discharge, and safe handling. With the considerable market demand for these electronics products, the number of secondary or rechargeable batteries being produced has tremendously increased resulting in a large quantity of spent LIBs being discarded worldwide. In spent LIBs, the content of cobalt and lithium is 5–15 wt% and 2–7 wt%,

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respectively, which is higher than those in natural ores or even concentrated natural ores. Thus spent LIBs are considered as an attractive secondary resource for cobalt, nickel, and lithium. Spent LIBs can cause environmental pollution, thus reuse and recycling of the valuable metals in LIBs not only can result in economic benefits, but also reduces pollution.

5.2 5.2.1

Adsorption The Types of Adsorption

Physisorption is caused by the intermolecular force that exists between adsorbates and adsorbents. The adsorption is known as van der Waals adsorption and the force is called van der Waals force. Since van der Waals force exists between any two molecules, physical adsorption can occur on any solid surface. Since physisorption is caused by intermolecular forces, the binding force is weak with less adsorption heat, and the rate of adsorption and desorption is fast. The adsorbed substance is also easier to desorb, so the physical adsorption is reversible to a certain extent. For example, adsorbed gas on activated carbon is easily extricated without any changes in nature. Chemisorption involves the transfer, exchange, or sharing of electrons between adsorbates and adsorbents (atoms or molecules), and the adsorption of adsorbates on adsorbents is due to the formation of chemical bonds between them. Adsorption selectivity refers to the preferential adsorption ability of adsorbents for some substances because of their particular composition and the structure of the adsorbents. For example, activated carbon is composed of covalently linked carbon atoms, and can preferentially adsorb high-molecular-weight organic molecules due to its large aperture.

5.2.2

The Application of Selective Adsorption in Recovery of Metals

Commonly used methods to control heavy metal pollution in water include chemical precipitation, ion exchange, and membrane separation and adsorption. Among these adsorption is a widely used, effective, and low-cost method. This method involves a wide range of raw materials, low energy consumption, and no secondary pollution, it also offers high flexibility, and requires little equipment. Various natural materials, such as rice stem, clay, algae, microorganisms, and synthetic materials (such

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as activated carbon, polymeric microspheres, and nanocomposites), have been successfully used for the removal of heavy metals through adsorption. In addition, the regeneration methods for adsorbents are various and the cost is low, and in most cases the treated wastewater is suitable for reuse and the adsorbed toxic heavy metal or valuable metal can be further processed or utilized by desorption and concentration. The adsorption of heavy metals on adsorbents can be divided into two methods: selective adsorption and nonselective adsorption. At present, most of the studies focus on the nonselective adsorption using adsorption materials, such as acrylic modified deacetyled konjac glucomannan, chemically treated bentonite, magnetic hydroxyapatite nanoparticles, and ethylenediamine-modified chitosan microspheres, can effectively adsorb metal ions, such as Cu, Zn, Cd, and Pb. However, due to the nonselective adsorption of heavy metal pollutants, they cannot remove specific heavy metal ions from wastewater. However, real wastewater often contains more than two kinds of heavy metal, thus it is of great importance to treat wastewater containing heavy metals using selective adsorption from the perspective of environmental protection and resource recycling.

5.2.3

Classification of Selective Adsorption for Heavy Metals

The selective adsorption of heavy metals in solution requires the preparation of specific adsorbents according to the nature of the separated materials or the creation of a specific adsorption environment. On the basis of a number of pieces of literature about the selective adsorption of heavy metals, selective adsorption for heavy metals can be classified into four types. The first type: selective adsorption is based on the specific functional groups of adsorbate and adsorbate. When the adsorbents were modified with amine (dNH2), carboxyl (COOH), and thiol (SH) functional groups, it can give priority to some heavy metals. Generally, this adsorption is achieved by complexation or chelation between metal ions and functional groups in a certain solution condition. The second type: selective adsorption of heavy metals by adsorbents based on ion-imprinting technique. The ionimprinting polymers have a binding site similar to that of enzymes or receptors and they exhibit specific selectivity and recognition capability for imprinted metal ions. The third type: selective removal of heavy metals based on the ion-exchange principle.

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Ion-exchange resin is widely used in wastewater treatment, and selective removal of heavy metal ions can be carried out by displacement and adsorption. Different heavy metal ions were selectively removed by different types of resins. For example, the cation exchange resin containing the dSO3H group has the characteristics of high selectivity, large exchange capacity, and good reversibility of adsorption and desorption processes for lead ion. Imac TMR resin, Srafion resin, ALM-525 resin, and NMRR Nisso exchange resin can selectively remove mercury ion from wastewater. Imac TMR resin is a macroporous structure styrene copolymer resin with thiol groups and acidic functional groups, and it has a strong affinity for mercury ion because of the existence of SH functional groups. Cadmium in wastewater was selectively removed by cationic resins such as Dowex50 W-X4 and Purolite S-950. Amberlite IRA-420 strong acid cation exchange resin, strong basic anion exchange resin, and Amberlite IR-67 RF weakly basic anion exchange resin are commonly used in the selective removal of chromium. Amberlite IRC-718 chelating resin and Dowex 508 strong acid anion exchange resin are commonly used for selective removal of copper from wastewater. Awual et al. [1] prepared 3-(3-(methoxycarbonyl)benzylidene) hydrazinyl) benzoic acid ligand (MBHB) containing specific functional groups, and immobilized it onto highly ordered mesoporous silica monoliths successfully for rapid Au(III) recognition and recovery from municipal waste. During recognition and adsorption operations, this novel adsorbent boosted the color formation by stable complexation as [Au(III)-MBHB]n+ complexes. The selectivity and sensitivity of this adsorbent for ultra-trace Au(III) ions are dependent on solution pH. The optimal pH for Au(III) ions recognition and adsorption was 2.0, and the maximum adsorption capacity was about 177.94 mg/g. The processes of recognition and recovery were also decided by contact-time and initial concentrations of Au(III) ions. The color change in the recognition system is related to the Au(III) concentration. The MBHB-modified mesoporous silica monoliths can preferentially adsorb Au(III) ion, and thus show high adsorption selectivity even in the presence of co-existing competing cations, such as Ag(I), Ca(II), Mg(II), Ni(II), Zn(II), Co(II), Pt(II), Al(III), Cr(III), Fe(III), and Ru(III), and the interfering anions, such as chloride, nitrate, bicarbonate, sulfate, carbonate, citrate, phosphate, and perchlorate. The adsorbed Au(III) was extracted and recovered as pure Au(III) with two-step elution. After extraction the MBHB-modified mesoporous silica monoliths was simultaneously regenerated, and showed excellent reusability and exhibited the same capture capacity as fresh adsorbent after washing

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with water. The high adsorption capacity and selectivity, excellent efficiency, and recyclability of this mesoporous adsorbent indicate great potential for a simple method of recovery of Au(III) at low-cost. rez et al. [2] prepared a new type of chiral threeLeyva-Pe dimensional bioMOF, which has functional channels with thioalkyl chains derived from the natural amino acid L-methionine (1). The network of 1 shows hexagonal channels with a virtual diameter of about 0.3 nm and highly flexible ethylenethiomethyl arms. This novel chiral three-dimensional bioMOF shows strong affinity for Au(III) and Au(I) in aqueous solution, even in the presence of Pd(II), Cu(II), Ni(II), Zn(II), and Al(III) ions, which are regularly present in electronic wastes, indicating its high adsorption selectivity for Au(III) and Au(I). The three-dimension network of 1 is stable during the adsorption process. Parajuli et al. [3] prepared two kinds of adsorption gels based on plant lignin extracted from waste wood powder, containing primary amine (PA-lignin) and ethylenediamine functional groups (EN-lignin). Both of PA-lignin and EN-lignin adsorption gels are effective for the adsorption of Pd(II), Au(III), and Pt(IV) in weak and strong hydrochloric acid media. On the contrary, PA-lignin and EN-lignin gels almost do not have adsorption capacity for Ni(II), Cu(II), Zn(II), and Fe(III). The adsorption of Pd(II), Au(III), and Pt(IV) on these two gels obeys the Langmuir adsorption model, and the gels show the highest adsorption capacity for Au(III) with a reductive adsorption mechanism.

5.3 5.3.1

Ion-Imprinting Technology The Introduction of Ion-Imprinting

The ion-imprinting method is an important technique for the preparation of adsorbents to separate ions selectively. Ionimprinted polymers (IIPs) are a new class of adsorbents with high selectivity and affinity for the target metal ions. IIPs are similar to molecularly imprinted polymers (MIPs), and show all the merits of MIPs, but they distinguish metal ions after imprinting. IIPs can be synthesized using proper monomers, a crosslinker, and ion templates (various anions, cations, and ionic organic ligand). The monomers and ion templates can form a ligand-metal complex via electrostatic interaction or coordination, then copolymerize in the presence of initiators, and finally the obtained polymers are washed with proper eluents to remove template ions so that three-dimensional recognition cavities are created inside the polymer network. The detailed process for IIP preparation

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Fig. 5.1 Schematic representation of ion-imprinted polymer synthesis.

includes three steps: (1) the complexation of metal ions with polymerizable ligands via electrostatic interaction or coordination, (2) polymerization of this complex by thermal initiation or photo-initiation in the presence of crosslinking agents, and (3) removal of the template ions after polymerization to form three-dimensional imprinted cavities (Fig. 5.1). There are three approaches to molecular imprinting: (1) The common method is a covalent or preorganized approach. Prior to polymerization, the template-monomer complexes are maintained in solution by reversible covalent bonds, and the recognition of the templates depends on the formation and cleavage of these covalent bonds. In general, IIPs are synthesized by free-radical polymerization, so vinylcontaining monomers are typical of this goal. The polymerizable ligands usually have two functions: their chelating ability and their vinyl function. (2) A simpler way for synthesis of IIPs is noncovalent imprinting or a self-assembly method using nonpolymerizable ligand, where the prearrangement between the templates and monomers is obtained by noncovalent interactions and subsequent recognition lies in these interactions. In such a case, the ligand is embedded in the polymer matrix by the capture process, and the interaction between polymer framework and the complexed ion is based on the coordination bond from some electron-donating heteroatoms (such as oxygen, nitrogen or sulfur) to the outer unfilled orbitals of the metal ions. (3) The semicovalent approach is a recently developed method, which is termed the sacrificial spacer methodology. This approach combines the advantages of the above two approaches. In this case, strong covalent bonds play a role in the imprinting process, and noncovalent interactions occur in the recognition step after the templates are cleaved from the polymer. Since the recognition site is created by the self-assembly of ligands around the template ions and

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subsequent crosslinking, this arrangement makes it possible to effectively match the size, shape, charge, coordination number, and binding sites of the ions. Moreover, the crosslinking and leaching steps preserve the complex geometry to rebind a favorable environment for template ions. IIPs show high selectivity to target ions due to the specific memory effect. Two major factors are responsible for this high selectivity: the selective affinity of the ligand for the imprinted metal ion and the geometric and chemical matching between the template metal ions and the generated cavities.

5.3.2

The Materials for Preparation of Ion-Imprinted Polymers

The design of IIPs refers to the choices of the chemical reagents for the polymerization process: template ions, monomers (including crosslinkers), initators, and solvent (porogen). Their relative proportions can not only affect the morphology of IIPs, but also have significant influence on the adsorption capacities and selectivity of the prepared IIPs. The details of some general characteristics about the IIP design will be outline in the following paragraphs.

5.3.2.1

Template Ions

Template ions are generally our target of study. Many different types of IIPs have been synthesized successfully. The main metal ions (K(I), Cs(I), and Sr(II)), transition metal ions (Cu(II), Cd(II), Ni(II), Cd(II), Fe(III), Co(II), Zn(II), and Ga(III)), lanthanide metals (La(III), Er(III), Nd(III), Sm(III), Dy(III)), actinide metals (U(VI), Th(IV)), heavy metals (Pb(II), Ag(I)), and anions (AsO4 3
,AsO3 3
, PO4 3
, CN
) have been used as template ions. The coordination number of different metal ions in different environments is different, so the different template ions are influential factors for the selection of functional monomers and preparation methods.

5.3.2.2

Ligands or Monomers

In the synthesis of IIPs, selection of a functional monomer is crucial, and the choice of functional monomers is mainly determined by the template ions. Functional monomers should be rationally chosen according to the nature of the template ions, coordination number, coordination force, and the interaction between the functional monomer and the template ions.

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Functional monomers usually contain dNd, dOd, dSd, and other groups to guarantee the interaction with target ions via ion chelation or electrostatic force in the recognition process, which enables the copolymerizing with crosslinking agent to form an imprinted polymer. In addition to the nonspecific 4-vinylpyridine (4-VP), acrylamide, 1-vinylimidazoleand acrylic acid, the general trend is to employ ligands with one or more chelating groups. Some of the functionalized ligands used for chemical immobilization and capture procedure are summarized in the following list: (1) Nonvinylated ligands used for the capture method without separating the complex between the ligands and the template ions: 8-aminoquinoline, formamidoxime, 5,7-dichloroquinoline-8-ol, diaza-dibenzo-13-crown-4-ether; (2) Nonvinylated ligands used for the capture method with prior formation of the complex between the ligands and the template ions: N,N 0 -ethylenebis(pyridoxylideneiminato), dithizone, thiosemicarbazide, acetaldehydethiosemicarbazone and 8-hydroxyquinoline; (3) Functionalized ligands used in chemical immobilization method without separating the complex between the functionalized ligands and the template ions: benzo-15-crown-5-acrylamide, 1-hydroxy-2-(prop-20 -enyl)-9, 10-anthraquinone, 1-hydroxy-4-(prop-20 -enyloxy)-9, 10-anthraquinone, 4-[(E)methacrylate 2-(40 -methyl-2,20 -bipyridin-4-yl)vinyl]phenyl (BSOMe), 5-vinyl-8-hydroxyquinoline and 4-vinylphenylazo2-naphthol; (4) Functionalized ligands used in the chemical immobilization method with prior formation of the complex between the functionalized ligands and the template ions: N-methacryloyl-(L)-histidine, N-methacryloyl-(L)-cysteine, N-methacryloyl-(L)-glutamic acid, N-methacryloyl-(L)-cysteine methyl ester, N-methacryloyl-2-mercaptoethylamine, Salen, SalenOMe, 6-(4-vinylphenylcarbamoyl)pyridine-2-carboxylic acid (HL1), pyridine-2,6-dicarboxylic acid bis(4-vinylphenyl)amide (L2), (4-ethenylphenyl)-4-formate-6-phenyl-2, 20 -bipyridine, 5-vinylthenoyltrifluoroacetone and [N-(4-vinylbenzyl)imino] diacetic acid.

5.3.2.3 Crosslinker The crosslinker also plays an important part in the IIP preparation. In the synthesis of IIPs, crosslinking agents are used to fix the space of the functional monomers and the template ions after the functional monomers and the template ions are complexed,

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giving the IIP a certain rigidity and specific spatial configuration. The crosslinking agent largely determines the imprinting effect, the shape, and mechanical stability of the IIPs. In order to obtain an IIP with good rigidity and imprinted sites that are stable enough, the proper crosslinking degree of the IIPs should be 70%–90%. A low degree of crosslinking cannot meet the rigidity requirement. The structure of polymers is loose, the imprinted cavities will collapse easily, and a large number of nonspecific cavities will be produced if there is a low degree of crosslinking, leading to poor imprinting effect and repeatability. Too high a crosslinking degree is also not beneficial to the performance of IIP, as this makes the structure of imprinted polymer very dense, hydrophilic properties decrease, and the imprinted cavities are too deep and are not easily accessible. Ethylene dimethacrylate (EDMA) and ethylene glycol dimethacrylate (EGDMA) are the most commonly used crosslinkers in IIP preparation. The second most popular bivinylated monomer is divinylbenzene (DVB). 3-(acryloyloxy)-2-hydroxypropyl methacrylate was chosen by some reseachers because of its lower polarity and enhanced water compatibility. N,N 0 -methylenebis(acrylamide) (MBA), trimethylolpropane trimethacrylate (TRIM) and pentaerythritol triacrylate (PETRA) are also used as crosslinkers but to a lesser extent. In order to improve the performance of IIPs a two-component mixture of different crosslinking agents is also usually used. Moreover, the different combinations of functional monomers and crosslinkers, as well as the different ratios of functional monomer to crosslinker have significant effects on the adsorption capacity, selectivity, rigidity, and morphology of IIPs, thus the combinations of functional monomers and crosslinkers, and the different ratios of functional monomer for specific template ions should be explored and optimized.

5.3.2.4

Choice of Solvent

Solvent is also referred to as porogen. In the process of ionimprinting, solvent is used to fully dissolve the functional monomers, crosslinking agents, template ions, and other components in the system. The choice of solvent is closely related to the polymerization methods, and has the dissolution capabilities of template ions, as well as the morphology and internal cavity structure of IIPs. The solvent should be able to dissolve various reagents in the reaction, and also promote the formation of pores and the coordination between template ions and functional monomers. Therefore it is very important to find a suitable solvent to optimize the amount of solvent in the synthesis process. If the

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amount of solvent used in the reaction is too large, too many solvent molecules inside the imprinted polymers will make for a relatively loose structure with reduced mechanical properties, which will influence the adsorption and regeneration performance. If the solvent amount is too small, the equivalent ratio of crosslinking agent would rise sharply, which will lead to a high degree of crosslinking of IIPs resulting in dense imprinted cavities, which will cause trouble in the post processing. Some commonly used solvent molecules are water, methanol, acetonitrile, toluene, chloroform, N,N-dimethylformamide (DMF), etc. Multiple kinds of solvents have been used for the preparation of IIPs. They can be divided into three categories: nonpolar solvents, polar aprotic solvents, and alcohols. Toluene and chloroform are nonpolar solvents. Tetrahydrofuran (THF), acetonitrile, 1,2-dichloroethane, N,Ndimethylformamide (DMF), and dimethyl sulfoxide (DMSO) fall into polar aprotic solvent group. Cyclohexanol,2-methoxyethanol, ethanol, methanol, and isopropanol belong to the alcohol group. Moreover, mixtures of a nonpolar solvent and a more polar one are usually adopted in the preparation of IIPs.

5.3.3

Major Polymerization Techniques for the Preparation of Ion Imprinted Polymers

Generally, the methods for synthesis of IIPs can be divided into five categories: bulk, precipitation, suspension, emulsion, and dispersion polymerization. Apart from some individual examples of IIPs prepared by polycondensation, organic IIPs are largely synthesized via free-radical polymerization. The morphology of IIPs can vary with the polymerization process. Monolithic IIPs materials can be prepared by bulk polymerization, whereas heterogeneous (suspension or emulsion) or homogeneous (dispersion or precipitation) polymerization can produce well-defined IIP particles.

5.3.3.1 Bulk Polymerization Bulk polymerization is the simplest and most convenient approach to synthesize MIPs. The general procedure of bulk polymerization for preparation of IIPs is as follows: template ions and functional monomers are dissolved in the solvent to make their full contact for a period of time, and then a certain amount of crosslinking agent and initiator are added. The above mixture is purged with nitrogen gas to displace oxygen, and then polymerized in the presence of an initiator under photoinitiation or

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thermal initiation to obtain block an IIP solid. The prepared block IIP solid is grounded and sieved to a suitable size, and finally is eluted with proper solvent to remove template ions. The resultant particles have a wide range of molecular weights and sizes. Bulk polymerization has the advantages of simple operation, obvious adsorption effect, and good selectivity for target ions. However, there are some disadvantages, which are as follows: (1) the obtained polymers require to be comminuted, milled and sieved, and the process is cumbersome and time-consuming, and yields <50%; (2) the particle size distribution of MIPs is heterogeneous and the shape is irregular, which are disadvantageous to many chromatographic and separation applications; (3) there is destruction of some binding sites during grinding, leading to a great loss of adsorption capacity; and (4) the adsorption sites in the polymers are too far from the surface, so that binding capacity for the target ions is poor.

5.3.3.2

Precipitation Polymerization

Precipitation polymerization is one of the most straightforward and effective methods for synthesis of a spherical molecularly imprinted polymer. Precipitation polymerization and bulk polymerization is very similar, but a large amount of solvent is required in the precipitation polymerization, whereas a small amount of solvent is introduced in bulk polymerization, thus the possibility of contact between the functional monomers and the templates is reduced. Moreover, the reaction time of precipitation polymerization is usually longer than bulk polymerization. The relatively large solvent volume leads to a decreased viscosity in the reaction mixture and enhances heat transfer, which makes precipitation polymerization easier to scale-up to industrial levels. In the precipitation polymerization, the choice of solvent is crucial, and the requirement is that target ions, functional monomer, crosslinking agent, and initiator can be completely soluble, whereas the formed polymer is insoluble and can precipitate out of solution easily. The main process of precipitation polymerization involves the dissolution of a template ion and functional monomer in a certain proportion in the solvent during prepolymerization, followed by adding a certain amount of crosslinking agent and initiator. Thereafter polymerization under photoinitiation or thermal initiation occurs and the polymer precipitates are obtained. Finally, the polymer precipitate is eluted with proper solvent to remove template ions. Some advantages of precipitation polymerization compared to conventional bulk

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and emulsion polymerization method are as follows: (1) there is no need for any surfactants or stabilizers in this synthesis, which could reduce environmental pollution in the imprinting process; (2) the obtained polymers are spherical particles; (3) the formation of highly crosslinked polymer networks around imprinted metal ions can lead to the easy removal of imprinted metal ions by IIPs; (4) the prepared particles are in a uniform range; (5) the size and the shape of the prepared particles are closely related to the ratios of monomers to solvents and the stirring of the polymerization mixture—optimization steps of these parameters are usually required to obtain the desired size and shape; and (6) production by direct precipitation polymerization of imprinted polymer microspheres is rapid and produces an almost quantitative yield—however, the disadvantages of this method include large solvent volume, long reaction time, and difficulties in removing the solvent from the product.

5.3.3.3 Suspension Polymerization Suspension polymerization is one of the most popular methods for systhesis of IIPs. Suspension polymerization is a heterogeneous polymerization process, and the reaction mixture is usually composed of a liquid matrix and monomer droplets. The monomer and initiator are not soluble in the liquid phase, and monomer droplets can be generated within the liquid matrix and suspended as the viscosity increases under continuous mechanical agitation. The polymerization occurs in the droplets of the dispersed phase, which are used as small-sized reactors, resulting in polymer beads. The general suspension imprinting procedure is as follows: (1) a functional monomer, crosslinker, and initiator are dissolved in an organic solvent as an organic phase; (2) the dispersant is dissolved in water as an water phase; (3) water phase and organic phase are mixed by stirring to form a suspension; and (4) finally, polymerization reaction occurs in the single and very tiny polymerizable droplets, yielding large uniform particles. The particle size of imprinted polymer microspheres can be controlled by adjusting stirring speed and the volume rate of the organic and aqueous phases. In suspension polymerization, heat transfer within the droplets is relatively fast due to the large surface area and volume ratio, thus the hindrance of heat transfer can be overcomed. Because the monomer droplets are independent under stirring, a more uniform suspension is formed and a relatively narrow size distribution of beads can be easily obtained.

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5.3.3.4

Emulsion Polymerization

In the process of emulsion polymerization for preparation of IIPs, the selection of emulsifiers is critical, which affects the particle size, hydrophilicity, and rigidity of IIPs greatly. The main process of emulsion polymerization for IIPs preparation involves the dissolution of template ions and functional monomer in organic solvents and the prepolymerization between them. After the crosslinking agent and initiator are introduced water and emulsifier are added and the solution is stirred for emulsification, thereafter polymerization occurs under photoinitiation or thermal initiation in the presence of the initiator. The polymer microspheres are eluted and dried to obtain IIP microspheres. Oil-in-water emulsion is the most common type of emulsion, and the monomer is dispersed in an inert liquid in the presence of an emulsifying agent to form small micelles (about 0.1–1 μm in diameter). Some monomers are located within the micelles, while the others are suspended in water. The water-soluble initiator is dissolved in the solvent phase. The latex particles are spontaneously formed in the first few minutes of the process, and then the polymerization process proceeds within these latex particles. Each particle is surrounded by a surfactant, which prevents coagulation of the particles. During the process of emulsion polymerization, the viscosity of the reaction mixture remains unchanged even when the solid content is up to 60%. The resultant IIPs are highly uniform in particle size. Nevertheless, there are disadvantages to emulsion polymerization which include the difficulty of removing emulsifiers, the low rigidity of the polymers, and low reusability, leading to the infrequent implementation of emulsion polymerization.

5.3.3.5

Dispersion Polymerization

In dispersion polymerization, the monomers, initiator, and template ions are mixed with each other in solvent and stabilizer. Proper stabilizers are essential in dispersion polymerization. The dispersion particles produced are not stable enough and may be formed during condensation without any stabilizer of situation. The stabilizers can effectively prevent the aggregation of polymer granules. The moment when the growing polymer network precipitates out of solution depends on its solubility in the polymerization solvent. Although the dispersion polymerization is relatively time-consuming, the IIP particles prepared by dispersion polymerization have good sphericity, large particle size, narrow particle size distribution, high purity, and high molecular weight.

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5.3.3.6 Reversible Addition-Fragmentation Chain-Transfer Polymerization Reversible addition-fragmentation chain-transfer (RAFT) polymerization has become a popular method for the synthesis of IIPs. RAFT polymerization shows the general characteristics of reactive polymerization, for example, the molecular weight of the polymer is proportional to the concentration ratio of the monomer to addition fragmentation chain transfer agent, the molecular weight of the polymer increased linearly with monomer conversion efficiency, and there is a relatively narrow molecular weight distribution. Its characteristics are as follows: (1) there is a wide range of suitable monomers; (2) there is a mild operating condition; (3) polymerization can be achieved by various methods such as bulk polymerization, suspension polymerization, emulsion polymerization; and (4) the polymer structures are well-controlled. At present RAFT polymerization has been mainly adopted to control the structures and improve the properties of the IIPs. Reactive functional groups and hydrophilic polymer brushes can be attached to MIP polymers. Yan et al. [4] synthesized a novel graphene oxide-based hydrophilic strontium ion imprinted polymer (RAFT-IIP) for the removal of Sr(II) from an aqueous solution using the combination of RAFT polymerization and surface imprinting technique. The RAFT-IIP showed good reusability, excellent adsorption capacity, and selectivity for Sr(II) ions.

5.3.3.7 Surface Imprinting IIPs materials synthesized by conventional polymerization methods (bulk polymerization, precipitation polymerization, and suspension polymerization) exhibit relatively high selectivity for target ions. However, they usually show low rebinding capacities due to the fact that the recognition sites are embedded within highly rigid polymer networks and there is limited accessibility. The poor site accessibility leads to slow mass transfer and binding kinetic. Moreover, some template ions may release outside the IIP because the template is not completely deleted. To solve these problems, surface imprinting has been introduced by creating a binding cavity on or near the surface of IIPs. The silica gel, nanoporous alumina membranes, chitosan, and Fe3O4 nanoparticles are always adopted as the supports of surface imprinting. A thin film of IIPs is then grafted onto or from the surfaces of carrier. Compared with conventional methods for the preparation of IIPs, surface imprinting shows some advantages: more accessable recognition sites, good binding capacity and elution rate, complete

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removal of templates, high adsorption selectivity for target ions, low mass-transfer resistance, and high adsorption rate.

5.3.4

The Application of Ion-Imprinting Technology in the Resource Utilization

5.3.4.1

The Resource Utilization for Au

Marjanovic et al. [5] prepared N-(2-(1-imidazolyl)ethyl)chitosan (IECS), then synthesized a novel ethylenediamineN-(2-(1-imidazolyl)ethyl)chitosan (IECS-GLA) IIP using chitosan as a scaffold material and gold(III) as an ion template, which was used for the uptake or recovery of the Au(III) ions from aqueous solutions. The contact time, pH, initial concentration of Au(III) ions, and temperature have some effect on the adsorption performance of this novel chelating ion imprinted resin. The adsorption process followed a pseudo-second-order kinetic model, while the adsorption data obeyed the Langmuir and Temkin model. According to the Langmuir equation, the maximum adsorption capacities are 810.67 and 649.35 mg/g at pH 3 and 6, respectively. The adsorption capacity of the novel chelating ion imprinted resin is much higher than the reported values. The thermodynamic parameters (4 G < 0 and 4 S > 0) indicate the spontaneous and endothermic adsorption processes. Furthermore, the IECS-GLA imprinted polymer shows high adsorption selectivity for Au(III) in the presence of Ni(II), Pb(II), Mn(II), Cu(II), and Fe(III). This high adsorption selectivity for Au(III) is due to the specific recognition cavities in IECS-GLA imprinted polymer for Au(III) ions. IECS-GLA imprinted polymer exhibited good reusability, there was no obvious decline in the adsorption efficiency of IECS-GLA for Au(III) ions after up to five repeated cycles, and desorption efficiency can be maintained up to 95%. IECS-GLA IIPs can treat simulated mining solutions and acid mining drainage with nearly 95% adsorbtion efficiency for Au(III) ions and complete removal of Pd(II) and Pt(IV), but it cannot adsorb Cu(II) and Fe(III) ions. The results indicate that IIP-IECS-GLA has a promising prospect in the uptake and recovery of precious metals in acidic mining drainage and stimulated mining fluids.

5.3.4.2

The Resource Utilization for Ag

Ahamed [6] synthesized Ag(I)-IIPs using a precipitation polymerization approach with Ag(I) ions as the template, 4-vinyl pyridine (4-VP) and 1-vinyl imidazole (1-VID) as functional monomers, and N,N-ethylene bisacrylamide (EBAm) as the crosslinker. When the IIP was used for selective adsorption of Ag(I) ions

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from water its adsorption capacity was influenced by the amount of Ag(I) imprinted polymer, agitation time, pH, and initial concentration of Ag(I) ions. The adsorption kinetics followed the pseudosecond-order kinetic model, and the adsorption data fitted well with the Langmuir isotherm. The IIPs had a higher absorption capacity than the control polymers. In the presence of competing ions, such as Cd(II), Cu(II), and Pb(II), the adsorption capacity of IIPs for Ag(I) was several tens times that for Cd(II), Cu(II), and Pb(II), while the selectivity coefficient of NIPs for Ag(I) to the competing ions is relatively low, indicating IIPs exhibit much higher selectivity for Ag(I) ions due to the well matched geometry between the imprinted cavities and the Ag(I) ions. The cost of Ag(I)-IIPs prepared using 4-VP, 1-VID as functional monomers and EBAm as the cross linker is high, and it is difficult to recycle IIP nanomaterials, thus it is necessary to seek inexpensive and available materials to prepare Ag(I) ion-imprinted polymers. Chitosan (CS) is a major component of crustacean shells and fungal biomass. It is also regarded as one of the most affluent biopolymers in nature, and the amine groups make CS different from other biopolymers. It has been widely used for the adsorption of various metal ions from wastewater. The combination of ionimprinting technology and chitosan can enhance the adsorption selectivity. Huo et al. prepared Ag(I)-imprinted biosorbent by surface molecular imprinting technology with Ag(I) as the template ions, chitosan and mycelium as the matrix, and epichlorohydrin as a crosslinking agent. The Ag(I)-imprinted biosorbent was applied for treatment of the Ag(I)-contaminated wastewater and it demonstrated higher adsorption selectivity and affinity for Ag(I) ions. The imprinted Ag(I) concentration, agitation time, and initial concentration of Ag(I) have an obvious effect on the adsorption capacity of the Ag(I)-imprinted biosorbent, while temperature does not influence its adsorption capacity. The optimal imprinted Ag(I) concentration for preparing Ag(I)-imprinted biosorbent was 2 mg/g (biomass). When the initial concentration of Ag(I) was 1200 mg/L, the maximum adsorption capacity was 199.2 mg/g and the biosorbent dosage was 3 g/L. Compared with pure chitosan, surface Ag(I)-imprinted biosorbent has the advantages of requiring a lower dosage of chitosan, costing less, and having broad application prospects in the treatment and recovery of Ag(I)-containing wastewater. However, the mechanism of interaction between biosorbent and Ag(I) before or after desorption is not clear and needs further investigation [7]. Moreover, the CS-based Ag(I)-imprinted biosorbent has the drawbacks of poor

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chemical resistance and mechanical strength, which significantly reduces the reusability of chitosan beads. On the other hand, poly(vinyl alcohol) (PVA) is a well-known membrane-forming material with excellent chemical resistance, high hydrophilicity, and good mechanical properties. Glutaraldehyde (GA) is a common crosslinker, which is usually used for the crosslinking of polypeptide, protein, PVA, and CS. CS/PVA combination in the presence of GA can provide high mechanical strength, pH sensitivity, swelling and shrinkage, and biodegradability. Shawky [8] synthesized Ag(I) ion-imprinted membranes by crosslinking CS and PVA alone and blended. The competitive ability of the imprinted membrane for Ag(I) is higher than nonimprinted membranes, and the removal efficiency of a ion-imprinted membrane is significantly affected by pH and temperature. The imprinted membranes are good enough for selective Ag(I) removal in complex matrices containing interfering ions such as Cu(II) and Ni(II). For the ion-imprinted CS membrane, the relative selectivity coefficient of Ag(I)/Cu(II) and Ag(I)/Ni(II) are 9 and 10.7, respectively. The relative selectivity coefficients of Ag(I)/Cu(II) and Ag(I)/Ni(II) for ion-imprinted CS/PVA membrane is 11.1 and 15. Moreover, ion-imprinted membranes have good reusability and stability, showing >85% of the original removal capacity even after using five cycles. Therefore Ag(I)-imprinted CS/PVA membranes are expected to be useful in the preconcentration of Ag(I) ions. Although Ag(I)-imprinted polymers have broad prospects for the recovery of Ag(I), the tedious separation process and recovery limit its practical application. If ion-imprinting technology and membrane separation can be combined, the ion-imprinted membrane will not only exhibit high selectivity for the guest ion, but also can be easily recovered, realizing the goals of easy separation and reusability. Wang et al. [9] prepared the Ag(I) imprinted membrane (Ag(I)-IIM) using Ag(I) as the template and a blend of chitosan and polyvinyl alcohol as film-forming materials. During the process of imprinting, metal complexing plays an important part and the coordination atoms may be N atom of –NH2 in CTS and O atom of –OH in PVA. The adsorption capacity of Ag(I)-IIM for Ag(I) is much higher than that of the corresponding nonimprinted membrane (NIIM). In the presence of interfering ions, such as Pb(II), Cu(II), and K(I), the adsorption capacity of Ag(I)-IIM for Ag(I) is much higher than interfering ions, while the adsorption capacity of NIIM for all ions is poor and shows no adsorption selectivity. The high selectivity of Ag(I)-IIM and poor selectivity of NIIM can be attributed to the matching of the cavities imprinted of Ag(I)-IIM with Ag(I) in shape, size, and the spatial

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arrangement of the recognition sites, while NIIM does not contain the cavities that match with Ag(I). Ag(I)-IIM has specific recognition for Ag(I), which can be used for selective enrichment and separation of Ag(I), and Ag(I)-IIM can effectively recover Ag(I) from hydrometallurgy industrial spent liquors. Although Ag(I)-imprinted polymers offer a promising prospect in the recovery of Ag(I), the tedious separation process limits their practical application. Recently magnetic separation technology has received increasing attention. Magnetic separation technology is another way to solve the problem of difficult separation and recovery of Ag(I)-imprinted polymers. When magnetically susceptible materials are encapsulated in IMIPs, magnetic MIPs exhibit magnetically susceptible characteristic and selectivity for the guest ions. Fan [10] first prepared magnetic thioureachitosan (TCM), then synthesized Ag(I)-imprinted magnetic thiourea-chitosan (Ag-TCM) using Ag(I) as imprinted ions. The maximum adsorption capacity of Ag-TCM for Ag(I) was 4.93 mmol/g, and adsorption equilibrium could be achieved within 50 min. The kinetic data matched well with a pseudosecond order equation, and the adsorption isotherms followed the Langmuir model. The capacity ratio of Ag-TCM showed high adsorption selectivity for Ag(I), but TCM could easily adsorb other ions as well as Ag(I), which indicates that the specific recognition cavities for Ag(I) were created in Ag-TCM, and the specific recognition cavities were related to the size, shape, and coordination geometry of the imprinting Ag(I) ion. Moreover, the structure of Ag-TCM is stable, and it can be used repeatedly. There is no obvious decrease in the adsorption capacity of Ag-TCM after being used five times, and it can be easily recovered under an external magnetic field, indicating that Ag-TCM could act as a good adsorbent for the separation and enrichment of Ag(I) from aqueous solution.

5.3.4.3 The Resource Utilization for Lithium There is a growing demand for the recovery of lithium (Li) from spent lithium-ion batteries. Currently the materials and technology for the recovery of Li(I) are lacking. We have synthesized a magnetic lithium IIP (Fe3O4@SiO2-IIP) using a surface imprinting technique using 2-(allyloxy)methyl-12-crown-4 as functional monomer. The optimum pH for adsorption is about 6. Fe3O4@SiO2-IIP shows rapid binding kinetics for Li(I) with complete equilibrium being reached within 10 min, and the saturation adsorption capacity is 0.586 mmol/g. The adsorption isotherms follows the Langmuir model, and the adsorption kinetics of Li

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ions on Fe3O4@SiO2-IIP obeys an external mass transfer model with rapid external mass transfer coefficients as high as kf ¼ 5.56  10
4 m/s. The Fe3O4@SiO2@IIP has excellent adsorption selectivity for Li(I) with the selectivity separation factor of Li(I) relative to Na(I), K(I), Cu(II), and Zn(II) is 50.88, 42.38, 22.5, and 22.2, respectively. Fe3O4@SiO2@IIP is stable during the adsorption process, and its adsorption capacity is >92% after five cycles. A fixed-bed column adsorption experiment indicated that the effective treatment volume is 140 bed volumes (BV) for the first run and 110 BV is treated for the second run under optimal conditions. Fe3O4@SiO2@IIP is also used in real wastewater, ensuring the wastewater meets Li emission standards. Moreover, the application of Fe3O4@SiO2@IIP for the recovery of Li(I) from wastewater is good economic value for enterprises, indicating Fe3O4@SiO2-IIP has promising prospects in recycling Li(I) from industrial waste [11]. The removal of heavy metal pollutants from water by the adsorption method, especially the selective removal of some or some heavy metal ions, is of great significance to alleviate the current levels of heavy metal pollution and to allow reuse of water resources. At present the selectivity of adsorbents is mainly focused on ion-exchange resins. In addition, the development of IIPs is also an important way to develop high performance adsorbents. However, in the adsorption of heavy metal ions, the adsorbent usually needs to be strengthened. When doing this kind of research, we should consider the following aspects: (1) The nature of heavy metal ions, especially the chemical properties (such as electronegativity), and the differences between the properties of various metal ions to be removed are the starting points for designing adsorbents. (2) The adsorbent usually needs to be modified with proper functional groups, which can be single or multiple containing amino, imine, carboxyl, hydroxyl, and sulfhydryl groups, and the modification of functional groups is beneficial to the high adsorption capacity and high selectivity. (3) The regeneration performance of adsorbents and the reusability of adsorbates are important aspects when evaluating the method of selective adsorption of heavy metals. (4) The research on the mechanism is still not complete, and main models remain the traditional adsorption models. Thus analysis of the various advanced methods and tools, the mathematical models, and the changes of interaction between adsorbent and adsorbate in the adsorption process, are essential for understanding the nature and development of selective adsorbents.

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5.4 5.4.1

Photocatalytic Reduction Heterogeneous Photocatalysis for the Reduction of Metal Ion to Metal Element

5.4.1.1 Principle of Heterogeneous Photocatalysis for the Reduction of Metal Ion The reduction by semiconductor photocatalysis technology is a relatively new technique in the removal or recovery of dissolved metal ions in wastewater [12, 13]. The process couples low-energy ultraviolet light with semiconductor particles as catalysts and is based on the principle of reduction by the photogenerated electrons. When the semiconductor particles are irradiated with ultraviolet or visible light having an energy greater than the bandgap energy of the photocatalyst, electron and hole pairs are generated inside the catalyst particles. Reduction by photogenerated electrons occurs simultaneously with oxidation by the photogenerated holes during heterogeneous photocatalysis. Photocatalytic oxidation is when an electron scavenger such as oxygen is used to suppress electron/hole recombination and photogenerated holes undergo anodic reactions. In contrast, photocatalytic reduction occurs when hole scavengers are adopted, and cathodic reactions occur with photogenerated electrons. A potential and very attractive practical application of lightdriven photocatalytic reduction is the deposition of toxic metals that are harmful to the environment and the recovery of precious metals from industrial wastewater. Those metals are deposited onto the surface of semiconductor catalyst powders and can subsequently be extracted from the slurry by mechanical and/or chemical means.

5.4.1.2 Application of Heterogeneous Photocatalysis in Noble Metal Recovery Chen et al. [12] researched the physisorption and photocatalytic reduction of eight metal ions including Ag(I), Pb(II), Hg(II), Ni(II), Cu(II), Cr(VI), Fe(III), and Fe(II) by TiO2 suspensions of Degussa P25 and Hombikat UV100. UV100 TiO2 showed no activity in the photocatalytic reduction of these metal ions, while P25 TiO2 can efficiently reduce Fe(III), Hg(II), Ag(I), Fe(III), and Cr(VI). After reduction, mercury and silver were deposited on TiO2, while iron and chromium still remained in solution in the form of Fe(II) and Cr(III) ions, respectively. Both photocatalysts strongly adsorb Fe(III) ions, which can prevent the photocatalytic reduction of

Chapter 5 NANOMATERIALS AND NANOTECHNOLOGY IN REUTILIZATION

mercury ion. Photocatalytic reduction occurs on the catalyst surface rather than in bulk solution. The presence of Fe(II) increases the reduction efficiency of Hg(II) ions. Dissolved oxygen can significantly inhibit the photocatalytic reduction of metal ions, and the reduction potential is low. An organic reducing agent could promote photocatalytic reduction. EDTA is the most efficient promoter among these four organic species. The promoting effect may be due to the direct transfer of electrons to the valence band of the semiconductor photocatalyst, which is dependent on the concentration of EDTA and the optimal EDTA concentration for the reduction of mercury is 0.2 mmol/L. Herrmann [14] explored the effect of the initial concentration and temperature of Ag(I) on the photoassisted reduction of silver ions to metallic Ag deposits on powder TiO2. According to a Langmuir-Hinshelwood mechanism, the initial deposition rate of Ag varied with the initial concentration of Ag(I) and was not dependent on the temperature around 300 K, but was proportional to the radiant flux (300 nm < A < 400 nm) of photons absorbed by TiO2. Under the same irradiation conditions, Ag(I) ions completely covered the surface at room temperature with an apparent initial quantum yield of 0.16. The diameter of the initially formed silver particles is between 3 and 8 nm, and larger crystallites (up to 400 nm) can be formed at longer illumination time. It may be possible to use this method for the recovery of silver from dilute aqueous solutions. Moreover, silver can be deposited onto other semiconductors, such as CdS, WO3, and Fe2O3, under visible-light illumination.

5.4.2

Homogeneous Photocatalysis for the Reduction of Metal Ion to Metal Element

5.4.2.1

Homogeneous Catalytic Reduction

The homogeneous catalytic reduction process has attracted great attention. It usually refers to gas-liquid phase oxidation reaction, which is commonly called liquid-phase reduction reaction. It has the following characteristics: (1) even distribution of the active center on the solid surface due to the reactants and catalysts in the same phase; (2) high activity and good selectivity; (3) a mild reaction condition, steady reaction, and simple reaction equipment; (4) small volume and high production capacity; and (5) relatively low reaction temperature and lower energy consumption.

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5.4.2.2 Polyoxometalate Polyoxometalates (POM), including heteropolyacid and its salts, can be formed by the dehydration and condensation of oxyacid salt under certain pH conditions. POM obtained by the condensation and dehydration of the same oxyacides is called isopolyacid, while POM obtained by the condensation and dehydration of two or more different oxyacid salts is called heteropolyacid. With the development of advanced instruments, there is a deeper understanding of the properties of heteropolyacids. The chemistry of heteropolyacid has become an important field. There are many kinds of heteropolyacids and their salts. They can be divided into six structures according to the ratio of their center atoms to the hetero atoms: Keggin structure, Dawson structure, Anderson structure, Silverton structure, Waugh structure, and Lindqvist structure. The structure of heteropolyacid also can be divided into primary, secondary, and tertiary structures. The primary structures are mainly in the form of anions. The secondary structures are the crystal structures of POMs and salts obtained by the combination of multianions and anticharged ions. The tertiary structures of heteropolyacid are composed of three parts: multianions, anticharged ions, and crystalline water. In these three structures, the primary structure is the most stable, and the secondary and tertiary structures are very easy to change. Understanding and mastering these structural characteristics play an important guiding role in the study of the catalytic properties of heteropolyacids. Heteropolyacid, together with its salts, is a multielectron body, which can obtain six electrons continuously, thus it shows a strong oxidizing property. Heteropolyacid can easily oxidize many materials by means of electrocatalysis and photocatalysis, and can transform itself into a reduction state, a process that is reversible so that it is easily regenerated. Heteropolyacid has become an excellent catalyst because of its excellent properties, which cannot be found in traditional catalysts: (1) it has a defined structure, such as Keggin structure and Dawson structure; (2) it is usually soluble in polar solvent; (3) it can be used as an acid, in oxidation, or as a bi-functional catalyst; (4) it has a unique reaction field; (5) its heteropoly anions are flexible; and (6) it is a kind of environmentally friendly catalyst, which can reduce environmental pollution and equipment corrosion, and can be easily prepared and regenerated. The transition metal-substituted Keggin-type heteropolyacid salt has a general formula of [ZXM11O40H2]n
(X is a heteroatom, Z is a transition metal ion instead of W and Mo, M is Mo and W),

Chapter 5 NANOMATERIALS AND NANOTECHNOLOGY IN REUTILIZATION

abbreviated as “ZXM11,” is one of the largest and most widely used classes of vacant heteropolyacid anions. The formula can be written as [XM11O39Z(H2O)] (X ¼ P, Si, Co, Fe; M ¼ W, Mo; Z ¼ Mn, Fe, Co, Ni), Keggin-type vacant heteropolyacid salts can be complexed with most transition metal ions to form the skeleton of heteropolyacid anion, the heteropolyacid complexes presented reduction characteristics, but their basic structure is not changed.

5.4.2.3

POM-Based Homogeneous Photocatalysis

Photocatalysis is an effective method for the recovery of metals from wastewater. Photoelectron reduction of metal ions and the formation of elementary metal particles make it possible to recover the metal from wastewater. Many heterogeneous photocatalysts such as TiO2 and WO3 can recover gold under light irradiation, however, due to the deposition and accumulation (poisoning) of the recovered metal particles at the active sites of the catalyst, the rate of recovery is reduced when the reaction proceeds. POM is one of the most important metal oxides and has a wide range of applications in catalysis, optics, magnetics, electronics, and medicine. In particular, POM is highly a reactive acid catalyst for various reactions and is widely used in industries. The good photocatalytic activity of POM in the green catalytic process has also received widespread attention. POM-based photocatalysts are used to destroy organic contaminants in solution, decompose water to hydrogen, recover precious metals from water, etc. Water-soluble W- or Mo-based POMs have been used as photocatalysts instead of heterogeneous photocatalyst particles for the photoreduction of gold ions. This method allows the spontaneous separation of deposited metal particles from a water-soluble photocatalyst and avoids catalyst poisoning due to metal deposition. Another interesting feature is that POM is considered as a nanoscale component for creating a functional nanostructured composite or inorganic/organic hybrid materials. Anionic POMs readily hybridize to cationic surfactants and polycations via simple electrostatic interactions.

5.4.2.4

Application of Homogeneous Photocatalysis in Noble Metal Recovery

The recovery of Ag(I) ions from aqueous solutions can be achieved by means of a homogeneous photocatalytic process that involves the addition of organic substrates of POM ðPOM ¼ PW 12 O40 3
, SiW 12 O40 4
or P2 Mo18 O62 6
Þ, such as propan-2-ol, and illumination with near-visible and UV-light. This method is effective for a wide range of Ag(I) concentrations ranging from about

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three to 1300 ppm. Prolonged exposure resulted in complete removal of Ag(I) to undetected traces (<0.2 ppm). The removal of silver follows the thermodynamic method and depends on the redox potential difference of POM and Ag(I), after photochemical reduction in the thermal reaction [POM(e
) + Ag+ ! POM + Ag]. Atmospheric oxygen has no effect on the rate of silver recovery. In contrast, thiosulfate complexes with Ag(I), reduce the redox potential and hinder the reduction and precipitation of silver. POM anions are different from the TiO2 particulates, are not contaminated by the precipitated silver, and can retain their ability to remove large amounts of pure metal [15]. Kida et al. [16] prepared an amphiphilic POM/surfactant mixed photocatalyst based on SiW 12 O40 4
and dissolved this inorganic/organic hybrid photocatalyst in an organic solution to reduce gold ions. This inorganic/organic hybrid photocatalyst can reduce gold ions to large microsized sheet-like gold particles in aqueous solutions at the liquid-liquid interface under nearvisible UV light. These large microsized sheet-like gold particles can be easily collected, and the catalysts can be easily separated from the reaction system. Their group also prepared an amphiphilic photocatalyst based on a polyoxometallate (W 10 O32 4
)/surfactant (dimethyldioctadecylammonium) hybrid, then dissolved it in chloroform. The inorganic/organic hybrid successfully photoreduced gold ions in a two-phase system that was exposed to UV light (λ > 320 nm). The reduction of gold ions to gold particles can be carried out at an environmentally friendly pH of 3–6 and recovery depends on the concentration of catalyst and sacrificial agents. The amphiphilic catalyst has morphological guidance in the formation of the gold particles due to its preferential adsorption of gold, allowing the gold nuclei to form at the interface and grow into tiny, large particles that can be easily collected by filtration. Hybridization of POMs with surfactants allows repeated use of the catalyst. Moreover, the addition of thiol compounds can significantly increase gold recovery as they covalently adsorb onto the gold and assist in the aggregation of the particles. Kida [17] also investigated the photochemical recovery of noble metals from water using inorganic-organic hybrid photocatalysts based on polyoxometalates such as PMo12 O40 3
, SiW 12 O40 4
, and γ
SiW 10 O36 8
, coupled with the cationic surfactant dimethyldioctadecylammonium (DODA). The γ-SiW10O36/ DODA, SiW12O40/DODA, and PMo12O40/DODA dissolved in chloroform were successful in the photoreduction of gold ions from aqueous systems in a two-phase (chloroform/water) system under UV irradiation (λ < 475 nm) (Fig. 5.2). The γ-SiW10O36/DODA photocatalysts exhibited the best photocatalytic reduction activity

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175

Fig. 5.2 Metal recovery scheme using an amphiphilic POM/surfactant photocatalyst under UV irradiation at the interface between water and chloroform.

for efficient recovery of gold from solution. Due to the increase of reaction interface area, the efficiency of the photoreduction of the gold is remarkably improved by stirring the reaction system. Therefore it is possible to selectively recover gold by controlling the pHand oxygen concentration in the reaction system. Zhang et al. [18] fabricated a crystalline porphyrinic POM hybrid {H8[C40H26N8]3}{[SiW12O40]2}•(H2O)5(CH3CN)2, and this POM hybrid can reduce AuIII ions (AuCl4
) to triangular Au0 nanosheets. The color of the solution changes from the paleyellow at the start (AuIII) to pink (Au0) under light illumination. This POM hybrid can reduce Ag+ ions and the spheric Ag nanoparticles were aggregated. The different morphology of Ag nanoparticles (isolated Au nanosheets and aggregated Ag nanoparticles) can be attributed to the different electrostatic repulsion or attraction interactions of this POM hybrid with the starting materials AuCl4
/Ag+ ions.

5.5

The Combination of Ion-Imprinting Technology With Photocatalytic Reduction

Although Ag(I)-imprinted polymers can enrich Ag(I) even at a very low Ag(I) concentration, practical application and resource utilization remains far away. The combination of ion-imprinting technology with photocatalytic reduction can realize resource utilization of Ag(I). When magnetically susceptible materials can be encapsulated in a photocatalytic IIP, a magnetic photocatalytic IIP will not only have magnetically susceptible characteristics, but also have adsorption selectivity for the guest ions and

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photocatalytic activity. In our work we aim to prepare novel magnetic photocatalytic Ag(I)-imprinted polymers that can reduce the adsorbed Ag(I) ions using TiO2 photocatalysis in place of acid elution. In view of the strong interaction between Ag(I) and -SH, we fabricated a new type of SH-functionalized photocatalytic Ag(I) IIP (Fe3O4@SiO2@TiO2-IIP) with magnetic performance by using Fe3O4@SiO2@TiO2 as support, 3-mercaptopropyl trimethoxysilane (MPTS) as a functional monomer and Ag(I) as a template, which was used for selective removal and recycling of Ag(I) ions from real wastewater. The Fe3O4@SiO2@TiO2-IIP exhibited high adsorption capacity of 35.475 mg/g for Ag(I) under the optimum pH of 6 within 80 min and showed excellent adsorption selectivity for Ag(I) ions. The selectivity separation factor for Ag(I) with respect to Li(I), Co(II), Cu(II), and Ni(II) is 10.63, 27.83, 13.28, and 68.11, respectively. Using methanol as the sacrificial agent (methanol/water 15:40), the adsorbed Ag(I) ions on Fe3O4@SiO2@TiO2-IIP can be reduced to metallic Ag(0), and then separated from Fe3O4@SiO2@TiO2-IIP by the means of ultrasound. The reduction of Ag(I) followed the pseudo-first-order kinetic model with a reduction rate of 0.00566 min
1. The adsorption capacity of the Ag-IIP retained 68.51% after one instance of photocatalysis and ultrasound, which was close to three times that of acid elution. Fe3O4@SiO2@TiO2-IIP was also applied in the treatment of real wastewater, and 1.3 mg of silver was recovered from 100 mL of 50 mg/L AgNO3 solution with 0.1 g of Fe3O4@SiO2@TiO2-IIP. Fe3O4@SiO2@TiO2-IIP is promising in terms of selective adsorption capacity for Ag, excellent magnetic separation ability, and evironmentally friendly regeneration, which indicates great potential in practical wastewater treatment [19].

5.6

Conclusions

Nanomaterials and nanotechnology have attracted a great deal of attention as alternative methods for the recovery of precious metal ions and other useful metal ions from wastewater. Ionimprinting technology exhibits high selective adsorption capacities for recovery of metal ions, and photocatalytic reduction based on nanosized semiconductors shows efficiency in the reduction of metal ions to metallic elements, indicating nanomaterials and nanotechnology have a prospective future in the reutilization of metal ions extracted from wastewater. However, nanomaterials and nanotechnology still present many challenges when used for the reutilization of metal ions in terms of the mechanisms

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of selective adsorption and photocatalytic reduction, and the development of highly efficient and cheap nanomaterials and nanotechnology to enhance the recovery efficiency. Therefore there is an urgent need to develop the novel nanomaterials and nanotechnology required to reduce the cost and improve the recovery efficiency. Therefore more investigations are needed to understand the underlying mechanisms involved.

References [1] M.R. Awual, M.A. Khaleque, M. Ferdows, A.M.S. Chowdhury, T. Yaita, Rapid recognition and recovery of gold(III) with functional ligand immobilized novel mesoporous adsorbent, Microchem. J. 110 (2013) 591–598. rez, J. Gascon, A. Leyva[2] M. Mon, J. Ferrando-Soria, T. Grancha, F.R. Fortea-Pe rez, D. Armentano, E. Pardo, Selective gold recovery and catalysis in a highly Pe flexible methionine-decorated metal-organic framework, J. Am. Chem. Soc. 138 (2016) 7864–7867. [3] D. Parajuli, H. Kawakita, K. Inoue, M. Funaoka, Recovery of gold(III), palladium(II), and platinum(IV) by aminated lignin derivatives, Ind. Eng. Chem. Res. 45 (2006) 6405–6412. [4] Y. Liu, X.G. Meng, M. Luo, M.J. Meng, L. Ni, J. Qiu, Z.Y. Hu, F.F. Liu, G.X. Zhong, Z.C. Liu, Y.S. Yan, Synthesis of hydrophilic surface ion-imprinted polymer based on graphene oxide for removal of strontium from aqueous solution, J. Mater. Chem. A 3 (2015) 1287–1297. [5] M.E.H. Ahamed, X.Y. Mbianda, A.F. Mulaba-Bafubiandi, L. Marjanovic, Selective extraction of gold(III) from metal chloride mixtures using ethylenediamine- N-(2-(1-imidazolyl)ethyl)chitosan ion-imprinted polymer, Hydrometallurgy 140 (2013) 1–13. [6] M.E.H. Ahamed, X.Y. Mbianda, A.F. Mulaba-Bafubiandi, L. Marjanovic, Ion imprinted polymers for the selective extraction of silver(I) ions in aqueous media: kinetic modeling and isotherm studies, React. Funct. Polym. 73 (2013) 474–483. [7] H.Y. Huo, H.J. Su, T.W. Tan, Adsorption of Ag+ by a surface molecularimprinted biosorbent, Chem. Eng. J. 150 (2009) 139–144. [8] H.A. Shawky, Synthesis of ion-imprinting chitosan/PVA crosslinked membrane for selective removal of Ag(I), J. Appl. Polym. Sci. 114 (2009) 2608–2615. [9] X.W. Wang, L. Zhang, C.L. Ma, R.Y. Song, H.B. Hou, D.L. Li, Enrichment and separation of silver from waste solutions by metal ion imprinted membrane, Hydrometallurgy 100 (2009) 82–86. [10] L.L. Fan, C.N. Luo, Z. Lv, F.G. Lu, H.M. Qiu, Removal of Ag+ from water environment using a novel magnetic thiourea-chitosan imprinted Ag+, J. Hazard. Mater. 194 (2011) 193–201. [11] X.B. Luo, B. Guo, J.M. Luo, F. Deng, S.Y. Zhang, S.L. Luo, J. Crittenden, Recovery of lithium from wastewater using development of Li ion-imprinted polymers, ACS Sustain. Chem. Eng. 3 (2015) 460–467. [12] D.W. Chen, A.K. Ray, Removal of toxic metal ions from wastewater by semiconductor photocatalysis, Chem. Eng. Sci. 56 (2001) 1561–1570. [13] L.X. Chen, T. Rajh, O. Micic, Z. Wang, D.M. Tiede, M. Thurnauer, Photocatalytic reduction of heavy metal ions on derivatized titanium dioxide nano-particle surface studied by XAFS, Nucl. Instrum. Meth. B 133 (1997) 8–14.

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[14] J.M. Herrmann, J. Disdier, P. Pichat, Photocatalytic deposition of silver on powder titania: consequences for the recovery of silver, J. Catal. 113 (1988) 72–81. [15] A. Troupis, A. Hiskia, E. Papaconstantinou, Photocatalytic reduction— recovery of silver using polyoxometalates, Appl. Catal. Environ. 42 (2003) 305–315. [16] T. Kida, R. Oshima, K. Nonaka, K. Shimanoe, M. Nagano, Photoinduced recovery of gold using an inorganic/organic hybrid photocatalyst, J. Phys. Chem. C 113 (2009) 19986–19993. [17] T. Kida, H. Matsufuji, M. Yuasa, K. Shimanoe, Efficient photorecovery of noble metals from solution using a γ SiW10O36/surfactant hybrid photocatalyst, Langmuir 29 (2013) 2128–2135. [18] J.Y. Zhang, C.H. Gong, X.H. Zeng, J.H. Shu, P.X. Xiao, J.L. Xie, Fabrication of crystalline porphyrinic polyoxometalate cluster with high thermal stability and exploration of its photocatalytic activity, Polyhedron 121 (2017) 95–100. [19] X.C. Yin, J. Long, Y. Xi, X.B. Luo, Recovery of silver from wastewater using a new magnetic photocatalytic ion-imprinted polymer, ACS Sustain. Chem. Eng. 5 (2017) 2090–2097.