Micro–Nanocomposites in Environmental Management€¦ · has become an urgent global environmental issue.[1–14] This problem has become more serious with the development of industry

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Micro–Nanocomposites in Environmental Management

Dongyun Chen, Haiguang Zhu, Shun Yang, Najun Li, Qingfeng Xu, Hua Li, Jinghui He, and Jianmei Lu*

Prof. D. Chen, Dr. H. Zhu, Dr. S. Yang, Prof. N. Li, Prof. Q. Xu, Prof. H. Li, Prof. J. He, Prof. J. LuCollege of ChemistryChemical Engineering and Materials ScienceCollaborative Innovation Center of Suzhou Nano Science and TechnologySoochow UniversitySuzhou 215123, ChinaE-mail: [email protected]

DOI: 10.1002/adma.201601486

and photocatalytic activity. For instance, adsorbent materials fabricated by the modification of low-cost commercial mate-rials, including inorganic nanomaterials, polyurethane sponge,[15–17] melamine foam,[18–21] cellulose foam,[22–25] and some carbon materials[12,26–30] (e.g., graphene, carbon nanotubes) have been utilized to remove oil, organic solvents, and dyes for the purification of the water environment due to their large surface area, chemical stability, and effective adsorption. Some materials with photocatalytic activity fur-ther extend the application of micro–nano materials in the field of environmental remediation, for example some traditional semiconductor materials (TiO2, ZnO, etc)

can photodegrade organic dyes to purify wastewater.[31] How-ever, the function of these micro–nano materials is singular, and the recovery of the materials is difficult. These drawbacks strongly hinder their application for the wastewater treatment. Hence, development of novel multifunctional micro–nano materials for wastewater remediation has become an impor-tant global concern and has attracted significant interest. Many researchers have prepared novel multifunctional composites for the treatment of environment problems via combining micro–nano materials with different functions, or immobi-lizing micro–nano materials on substrate materials.

The micro–nano composites exhibit a better performance than a single material, such as superwetting property, higher adsorption capacity, and photodegradation activity, which makes these micro–nano composites more applicable to deal with environment accidents. Here, we give an overview of the application of micro–nano materials, as well as micro–nano material composites in environmental remediation, such as oil–water separation, and adsorption and degradation of organics and heavy-metal ions in wastewater. Furthermore, the research trends and future prospects are briefly discussed.

2. Adsorbent Materials

2.1. Foam Absorbent Materials

Efficient technologies are urgently needed to deal with various types of environmental accidents. In recent years, adsorption, as a chemical–physical method, has been widely studied to treat wastewater, especially in the field of wastewater polluted by heavy-metal ions and organics. In the past decades, acti-vated carbon has been proven to be a traditional and efficient adsorbent for the treatment of wastewater. Although activated

Water pollution, a worldwide issue for the human society, has raised global concerns on environmental sustainability, calling for high-performance mate-rials for effective treatments. Since the traditional techniques have inherent limitations in treatment speed and efficiency, nanotechnology is subsequently used as an environmental technology to remove pollutants through a rapid adsorption and degradation process. Therefore, here, various adsorbent and photodegradation composite materials leading to effective water remediation are summarized and predicted. Notably, recent advances in simultaneous adsorption and photodegradation micro–nanocomposites are outlined. Such materials can not only completely adsorb and remove contaminants, but the micro–nanocomposites can also be directly reused without further treatment. Finally, the future development of this unique system is discussed.

1. Introduction

In recent years, water pollution caused by oils spills, heavy-metal wastewater discharge, and chemical-reagent leakage has become an urgent global environmental issue.[1–14] This problem has become more serious with the development of industry. For example, the occurrence of the Minamata disease event in Japan in early 1956 caused by the discharge of indus-trial wastewater (mercury-containing wastewater) damaged the environment and public health seriously. In the Deepwater Horizon Oil accident in 2010, about 4.9 million barrels of crude oil were spilled in the Gulf of Mexico, resulting in sig-nificant damage to important habitats. Various methods such as oil–water separation technology, dispersants, and sorbents have been widely adopted. However, the restrictions in both time and space, and secondary pollution to water have inhib-ited their application in the field of environmental remediation. Thus, effective strategies to deal with these wastewaters are still urgently needed.

In the last decades, micro–nano materials have been inves-tigated widely and exhibited great potential in the field of envi-ronmental remediation due to their excellent chemical and physical properties, such as large surface area, superwetting,

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vents, oil and dyes efficiently, the high price of activated carbon and its lack of selectivity, as well as secondary pollution, limit its application in the treatment of wastewater, especially in environmental accidents. Therefore, it is urgent that novel and cost-effective materials are developed for wastewater treatment, instead of traditional absorbents. In the past few years, with the development of science and technology, three-dimensional (3D) foams with polymer skeletons have been widely researched and applied as adsorbents due to their porous and flexible struc-ture, high surface area, and functional groups for modification of their surface and so on. The high adsorption rate and selec-tivity, as well as the quick adsorption rates of these 3D foams also make them ideal substrates for the preparation of multi-functional adsorbents. On the other hand, 3D foams can be prepared on a large scale in a short time, owing to the develop-ment of mature production technologies to deal with sudden environmental accidents. The following is a short summary and overview of different commercialized and surface-modified 3D foams and their application in the treatment of pollutions.

2.1.1. Polyurethane-Based Absorbent Materials

Polyurethane (PU) sponge is a commercially available 3D mate-rial with porous structure and large specific surface area, as well as abundant functional polar groups, such as carbonyl, car-boxyl, and amine groups. Therefore, it can efficiently achieve adsorption of organic solvents and metal ions by van der Waals forces or hydrogen bonding from wastewater. In addition, the high flexibility, low density, chemical stability, and low cost make it an ideal substrate to construct various adsorbent mate-rials for removal of oils and organic dyes from wastewater.[32–34] Up to now, PU or modified PU foams have been extensively researched and applied in sewage treatment. For example, Shahat et al. prepared iodo–PU foam by replacing the amino functional groups of PU with iodine atoms. The iodo–PU foam could effectively remove organic dyes, including aniline blue and crystal violet, from wastewater. In addition, the foam could be regenerated many times without obvious decrease in the absorption capacity.[35] In order to enhance the adsorption selec-tivity and efficiency for removal of organic dye from wastewater, Ke et al. synthesized an attapulgite-modified PU foam by the foaming technique.[36] Attapulgite, as a natural adsorbent with high surface area and exchangeable cations, as well as reac-tive hydroxyl on its surface, can effectively absorb cations dyes from wastewater. Hence, the as-prepared PU foam exhibited high adsorption efficiency for malachite green from wastewater, and the highest adsorption efficiency was 99.51% at optimal conditions.

To recover spilled oils or organic solvents, an absorbent mate-rial with special wettability, especially for superhydrophobic and superoleophilic surfaces, is a crucial factor. Generally speaking, to achieve superhydrophobicity, a material surface with a low surface energy is a prerequisite, which is further enhanced by the rough structure. Flowing this strategy, Pan et al. fabricated a robust superhydrophobic PU sponge as an oil-absorption material by simple one-step solution immersion in methyl-trichlorosilane/hexane.[34] Scanning electron microscopy (SEM)

characterization showed that the material had a hierarchical and rough structure with pores sizes ranging from hundreds of nanometers to hundreds of micrometers. The chemical compo-sition and the surface roughness are two critical factors for the preparation of absorbents with special wettability. Hence, after coating hydrophobic polysiloxane on the rough PU surface, the PU sponge exhibited superhydrophobicity and superoleophi-licity. The water contact angle (WCA) was measured to be 157°, while the oil contact angle (OCA) was ca. 0°. Oils floating on water were completely absorbed by the PU sponge within a few seconds. The adsorption capacity was 15–25 times its own weight for various types of oils. Furthermore, the adsorbed oils could be recovered and collected by simple mechanical squeezing owing to the flexible compressibility of the PU sponge.

2.1.2. Melamine-Foam-Based Absorbent Materials

Melamine foam (MF) is a melamine–formaldehyde polymer. It has promising environmental applications because of its excel-lent properties, such as low density, porosity, high thermal sta-bility, corrosion resistance, and flame retardancy.[21,37] Zhang et al. synthesized poly(melamine–formaldehyde) (pMF) with high densities of amine and triazine groups.[38] These groups can act as chelating ligands to bind metal ions (Figure 1a), and pMF showed excellent performance for the removal of heavy-metal ions from water. As shown in Figure 1b, pMF removed >98% of lead ions for initial lead concentrations of 100–5000 ppm to an equilibrium concentration of <25 ppm with a high absorption capacity (up to 363 μg g−1). Dickerson et al. fabricated a superhydrophobic MF by the silanization reaction between alkylsilane compounds and the secondary amine groups in the sponge skeletons (Figure 2a).[39] The MF exhibited a high absorption capacity (82–163 times its own weight) for various types of oils and organic solvents, as well as outstanding recyclability (more than 1000 cycles) (Figure 2b). Similarly, Lu et al. fabricated fluorinated MF with a low-surface-energy material (1H,1H,2H,2H-perfluorodecanethiol).[19] The fluorinated MF sponge exhibited superhydrophobicity, chem-ical stability, and high absorption capacity for various types of oils and organic solvents (Figure 2c,d), making it a promising absorbent for treatment of oily wastewater. Wang et al. pre-pared a multifunctional foam by pyrolysis of commercial MF

Jianmei Lu received her Ph.D. degree in polymer chemistry from Zhejiang University in China and was then appointed as a full professor at the College of Chemistry, Chemical Engineering and Materials Science, Soochow University in 2000. She has research interests in materials for adsorption of oils, non-volatile electronic memory

devices, organic sensors, and smart materials.

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and subsequent treatment with chlorotrimethylsilane to obtain a hydrophobic surface.[14] Owing to its porous structure, hydro-phobicity, compressibility, and flame resistance, the foam exhib-ited high performance for the removal of oil from wastewater.

2.1.3. Cellulose-Based Absorbent Materials

Cellulose is the most abundant organic polymer on Earth, and it has been widely used since its discovery in 1838. It can be

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Figure 1. a) Illustration of chelation between lead ions and the amine and triazine groups in pMF. b) Correlation between pMF adsorption capacity and equilibrium concentration of lead ions. Reproduced with permission.[38] Copyright 2013, The Royal Society of Chemistry.

Figure 2. a) Illustration of preparation of silanized MF. b) Recyclability of silanized MF for oil and organic solvents. a,b) Reproduced with permission.[39] Copyright 2014, ACS. c) Adsorption capacities of fluorinated MF for various types of oils and organic solvents. d) Absorbent mass and remnant mass of cyclohexane over 100 cycles of absorption/squeezing tests. The inset shows the water contact angle (158.3°) for fluorinated MF after 100 cycle tests. c,d) Reproduced with permission.[19] Copyright 2014, Wiley-VCH.


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some species of bacteria. In recent years, cellulose nanofibrils (CNFs) extracted from cellulose pulp have become a promising can-didate for environmental applications because of their high surface areas, high aspect ratios, and biodegradability.[22,40–44] For example, Chin et al. prepared CNFs from kenaf core.[44] The CNFs exhibited rapid removal of meth-ylene blue (MB) with a maximum adsorp-tion capacity of 122.2 mg g−1 at pH 9. More importantly, the absorbed MB could be easily desorbed at low pH, and >70% of the dye des-orbed after six adsorption–desorption cycles. Similarly, Zhao et al. fabricated maleic-anhydride-modified cellulose with high removal capacity for basic dyes, such as MB and methyl violet, and high removal efficiencies (up to 97.5%) under optimal conditions.[23] The porous structure makes it an ideal material for removal of oils or organic solvents from water. Hsieh et al. reported an amphiphilic CNF aerogel pre-pared using rice-straw cellulose as the raw material.[45] The CNF aerogel had ultralow density (1.7–8.1 mg cm−3) and an ultraporous structure. After modification with hydrophobic triethoxyl(octyl)silane, the aerogel showed excellent hydropho-bicity and high adsorption capacity for oils and organic solvents (135–356 times its own weight). Yu et al. produced CNF aerogels on a large scale from bacterial cellulose.[46] The aerogels were ultralight, flexible, and fire-resistant, and they were capable of absorbing various types of oils and organic solvents with high capacity and excellent recyclability.

2.1.4. Carbon-Based Absorbent Materials

Owing to their unique and tunable properties, carbon nano-materials, including fullerenes and carbon nanotubes (CNTs), have attracted considerable attention because of their poten-tial applications to address environmental challenges.[47–52] For example, the theoretical surface areas of fullerene and single-walled CNTs (SWCNTs) are both about 3000 m2 g−1,[48,53] which makes them excellent candidates for treatment of polluted water.

The difference between fullerenes and CNTs is mainly the distinct geometry of the monomer, which leads to different adsorption sites. The monomer structure of fullerenes is a closed graphite ball and the adsorption sites are on the external surface (Figure 3A). CNTs are rolled-up graphite sheets forming a coaxial tube, and different numbers of rolled-up graphite sheets form SWCNTs (Figure 3B) and multiwalled CNTs (MWCNTs) (Figure 3C). For both SWCNTs and MWCNTs, available adsorption sites not only exist on their external sur-face but also in the inner cavity. Furthermore, the interwall spaces of MWCNTs are also available.[53]

Compared with conventional carbonaceous sorbents (e.g., active carbon), carbon nanomaterials can overcome many intrinsic limitations, such as the low density of the surface active sites, activation energy of sorptive bonds, slow kinetics and non-equilibrium sorption in heterogeneous systems, and mass transfer rate to the sorbent surface. When used to remove

organics from aqueous solution, carbon nanomaterials show rapid equilibrium rates, high adsorption capacity, and effective-ness over a wide pH range owing to their high surface area, controlled pore size distribution, and surface active sites.[54–56]

Interactions between organic contaminants and carbon nanomaterials can be classified into five types: the hydrophobic effect, π–π bonds, hydrogen bonds, and covalent and electro-static interactions. Compared with conventional nanomaterials and activated carbon, π-electron polarizability or π–π electron–donor acceptor (EDA) interactions determine the high equi-librium rates of carbon nanomaterials.[57,58] The work of Yang et al.,[59] comparing C-60, nC-60 nanoparticles, SWCNTs, and MWCNTs with various dimensions, verified this conclusion.

Graphene is one of the allotropes of elemental carbon, and has received considerable attention since it was discovered in 2004. It has a wide range of environmental, energy, and other applications because of its unique and intriguing properties, such as high thermal conductivity, unique electronic conduc-tivity, good biocompatibility, and excellent mechanical flex-ibility.[60–65] In recent years, graphene foam (GF), which can be prepared by chemical vapor deposition, and chemical or thermal reduction of graphene oxide (GO), has been extensively studied to address environmental issues, especially in wastewater treat-ment and water purification owing to its porous structure, large surface area, and hydrophobicity.[28,66,67] Kim et al. reported a 3D reduced-GO (rGO) foam prepared by chemical reduction of GO for removal of organic dyes (MB and rhodamine B) from aqueous solution.[68] Because of strong anion–cation interac-tions and π–π stacking, organic dyes showed excellent chemical adsorption to the rGO foam with a rapid adsorption rate and high adsorption capacity. To investigate its practicability, the toxicity of the aqueous solution of the organic dye after puri-fication by rGO foam was investigated using bacterial cells. The results showed that bacterial cells can grow in the puri-fied aqueous solution, indicating successful removal of organic dyes from water by the rGO foam. Cheng et al. prepared gra-phene sponges (GSs) by hydrothermal treatment of aqueous GO with the assistance of thiourea.[69] The GS had a tunable pore structure, a large surface area, and excellent mechanical properties, allowing it to remove various types of water pollut-ants with high adsorption capacity, such as organic dyes, oils, and organic solvents. Huang et al. reported a graphene-based

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Figure 3. Schematic structures of C60 fullerene (A), SWCNTs (B), and MWCNTs (C) showing the inner cavities, interwall spaces, and external surfaces. Only the external surface of C60 fullerene is accessible for adsorption. Reproduced with permission.[53] Copyright 1999, Amer-ican Chemical Society.


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hydrogel prepared by self-assembly of two-dimensional GO sheets using agarose as the stabilizer and crosslinking agent.[27] Owing to the hydrogen bonding and π–π stacking interactions between graphene sheets and agarose, GS exhibited excellent removal efficiency for malachite green and high adsorption capacity (242 mg g−1). Similarly, Duan et al. prepared a gra-phene hydrogel (GH) using dopamine as the reductant and surface-functionalization agent.[9] Owing to abundant oxygen functional groups (O− and COO−) on both the surfaces of polydopamine and GO sheets, the GH showed high adsorption capacity for heavy metals by electrostatic adsorption at pH from 3 to 6. By simply decreasing the solution pH value, absorbed heavy metals could be completely desorbed from GH because of electrostatic repulsion between the heavy-metal and proto-nated oxygen groups on the GH surface.

Because most oxygen functional groups can be removed during self-assembly of GO sheets to rGO foam by hydro-thermal treatment, the rGO foam exhibits good hydropho-bicity. Because of its porous structure and hydrophobicity, rGO foam shows high performance for effective removal of oils and organic solvents from water. For example, Ruoff et al. reported shape-moldable spongy graphene (SG) prepared by thermal treatment of GO solution at 180 °C for 24 h.[70] The SG exhibited good hydrophobicity with a WCA of 114° and a large specific surface area (432 m2 g−1). Because of these properties, it shows rapid adsorption and high adsorption capacity (20–86 times its own weight) for various common oils and organic solvents. More importantly, because of its excellent thermal stability, the SG can be regenerated and reused by heat treatment to release the absorbate. Chen et al. prepared rGO foam using a leavening strategy.[71] A freestanding GO layered film was firstly fabri-cated by the flow-directed assembly method (Figure 4a), and then heat treatment with the assistance of hydrazine gave the rGO foam. The rGO foam had an open porous structure with pore sizes ranging from sub-micrometer to several micrometers (Figure 4c). The rGO foam possessed good hydrophobicity and capillary action, which are crucial for selective absorption of oils and organic solvents from water. As shown in Figure 4d, the foam rapidly and completely removed oils floating on water

while repelling water. Owing its excellent chemical stability in harsh conditions, such as in organic solvents, the rGO foam can be reused and regenerated by simply immersing the foam in hexane to release the absorbed oils or organic solvents. Figure 4e shows that the adsorption capacity of rGO remained unchanged after ten adsorption–desorption cycle tests.

These foams have a large surface area and 3D porous structure, and they have been used as the substrate to prepare composite materials for the treatment of wastewater. Compared with surface-modified foams, these composite materials exhibit good functionality and performance in sewage treatment.

2.1.5. Boron Nitride and Its Composite Materials

With the recent surge in research in graphene, another 2D nanostructured material, boron nitride (BN), so-called “white graphene”, has attracted a great deal of attention due to the structure, which is similar to graphene; thus, it exhibits poten-tial application for removal of organic pollutants and oils from wastewater because of its polarity, as well as its large surface area.[72–74] Meanwhile, it is the lightest group-III–V compound, with high chemical and thermal stability, which make it a prom-ising absorbent with high capacity and excellent reusability. The forms of BN materials mainly include nanosheets, nano-tubes, nanoribbons, and nanomeshes. These BN materials and their composites have been researched and exploited in water purification and treatment. For example, Chen et al. prepared porous BN nanosheets for water cleaning via a simple thermal-treatment method.[74] Compared with other absorbents, such as activated carbon and BN particles, the as-prepared porous BN nanosheets exhibited higher adsorption capacity ranging from 2000 to 3300 wt%. In addition, it can effectively absorb organic dyes from water, and the maximum adsorption capacity reaches 556 mg g−1 due to its large surface area. Meanwhile, owing to its high resistance to oxidation and thermal stability, the as-prepared BN nanosheets showed excellent reusability through regeneration upon burning in air. To achieve the separation of oil from polluted water, superhydrophobic BN-nanotube-coated

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Figure 4. a) Schematic diagram showing the preparation of rGO foam using a leavening process. b) Photograph of rGO foam. c) SEM image of rGO foam showing the porous structure. d) Photographs showing rGO foam absorbing a layer of motor oil floating on water. e) Recyclability of rGO foam for absorption of oil (the triangles indicate the weight gain of the foam for the absorption of oil and the squares indicate the restored weight of the foam after washing with hexane). Reproduced with permission.[71] Copyright 2012, Wiley-VCH.


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liquid–solid growth process.[75] Since the BN nanotubes on the surface of a stainless-steel mesh showed nanometer-scale roughness, the as-developed mesh exhibited superhydropho-bicity with a WCA over 150°, which means the mesh is featured with excellent performance in the separation of oils and organic solvents from water. Similarly, by integrating the unique prop-erties of BN and the special wettability of poly(vinylidene fluo-ride) (PVDF), a novel porous-BN-nanosheets/PVDF composite material was prepared, which exhibited excellent performance in clean-up of oils from oily wastewater.[76]

2.2. Inorganic Nanomaterials

Owing to the rapid growth of nanotechnology in the past few years, inorganic nanomaterials have been widely used for envi-ronmental remediation. These nanomaterials have large spe-cific surface areas and high reactivities, and they are excellent adsorbents and catalysts for treatment of wastewater. The high surface-area/mass ratio of inorganic nanomaterials can greatly improve the adsorption capacities of sorbent materials. Some inorganic nanomaterials with enhanced redox and photocata-lytic properties (e.g., silver nanoparticles and photocatalytic TiO2) can also be used to degrade contaminants in wastewater.

2.2.1. Silica Nanoparticles

To be an excellent absorbent, a nanomaterial should have not only a large surface area, but also an ordered and open-pore structure, with accessible adsorption sites and enhanced mechanical and chemical stability under adsorption and regen-eration conditions, e.g., treatment with acidic or basic solutions should not cause any structural damage. Mesoporous silicas have proven to be excellent absorbents for wastewater recovery.

Mesoporous silicas were first discovered in the early 1990s,[77,78] partly in response to the need to extend the appli-cations of zeolites. With the development of nanotechnology, various types of mesoporous silica nanoparticles (Figure 5)[79] have been fabricated and used for catalysis, separation, ion exchange, molecular sieving, and adsorption. Mesoporous silica nanoparticles have received considerable attention in the field of wastewater treatment because of their large surface area and well-defined pore size and pore shape. Mesoporous silica nano-particles can be defined based on their pore structure and size. Silica nanoparticles with pore sizes between 2 and 5 nm are 2D hexagonal p6m, 3D cubic Ia3d, and lamellar p2, which are defined as MCM-41, MCM-48, and MCM-50 materials, respec-tively (Figure 5b,c).[80] Silicas with pore sizes between 6 and 20 nm are 2D-hexagonal-p6mm-structured SBA-15.[81] Other 3D cubic silica materials with large pores have also been reported, such as IBN-2 (Fm3m).[82]

In the last decade, much attention has been focused on using MCM-41 as a sorbent trap for binding volatile organic compounds in indoor air[83–85] by physical adsorption or chemisorption after chemical modification. MCM-41 has also been successfully used to remove toxic heavy metals from wastewater. Mesoporous silicas have been proposed as adsorbents to remove dyes from aqueous

solutions.[86–88] After modification with organic functional groups (e.g., amino, diamino, triamino, malonamide, carboxyl, dithi-ocarbamate, humic acid, and imidazole groups), they can achieve specific purposes, and they have attracted much attention for the removal of dye molecules from water.

Compared with MCM-41, SBA-15 has larger adjustable pores (5–30 nm) that allow targeted adsorbates easier access to the inner surface of the silica nanoparticles. This results in rapid kinetics and non-equilibrium sorption, and SBA-15 shows higher adsorption capacity than MCM-41 silica. Moreover, the pore walls are very thick (around 4 nm), resulting in enhanced mechanical stability.[89]

Although mesoporous silicas are promising adsorbents com-pared with traditional absorbents (e.g., active carbon) because of their large surface area and unique pore structure, mesoporous silica lacks surface functional groups, which greatly limits its adsorption applications, especially when high adsorption selec-tivity and capacity are required. Chemical modification of the surface of mesoporous silica is a promising method to over-come this limitation. The surface of mesoporous silica has been modified with functional small organic molecules, such as dodecylamine,[90] 2-mercaptopyridine,[91] tetrakis(4-carboxy-phenyl) porphyrin,[92] and 3-aminopropyltrimethoxy silane,[89] to remove dyes and heavy metal ions (e.g., Cu2+, Cd2+, Pb2+ and Hg2+) from aqueous solution.

2.2.2. Magnetic Iron Oxide Nanomaterials

The recyclability of treatment agents should be considered when selecting the best material for wastewater treatment, especially for nanomaterials. Magnetism aids in water purification by

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Figure 5. Different particle and pore morphologies of MSN: a) rods with 3 nm wide helicoidal pores, b) spheres with 4 nm wide hexagonal pores, c) spheres with 3 nm wide cubic pores, and d) hexagonal plates with 10 nm wide hexagonal pores. Reproduced with permission.[79] Copyright 2010, The Royal Society of Chemistry.


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influencing the physical properties of contaminants in water. Combining adsorption with magnetic separation has been extensively used in water treatment and environmental cleanup.

In the last few decades, Fe3O4 nanomaterials have attracted considerable attention in industrial-scale wastewater treatment owing to their high adsorption capacity, easy separation, and enhanced stability.[93,94] The ability of iron oxide nanomaterials to remove contaminants from wastewater has been investigated not only in the the laboratory, but also at the field scale.[95] Both laboratory and field-scale tests have shown that Fe3O4 magnetic nanomaterials possess higher capability to treat large volumes of wastewater than carbon nanotubes, activated carbon, and zero-valent iron,[96] and magnetic separation using a strong external magnetic field can easily separate the sorbent and sorbate. In previous reports, Fe3O4 nanomaterials have shown good adsorption capability and fast magnetic separation for the treatment of wastewater polluted with both heavy-metal ions and organic contaminants.[97,98]

Adsorption of contaminants occurs by surface exchange reactions until the surface functional sites are fully occupied, and then contaminants can diffuse into the adsorbent for fur-

ther interactions with functional groups. Thus, surface modi-fication of Fe3O4 nanomaterials with functional groups to enhance the adsorption capability has attracted much attention. The surface of Fe3O4 nanomaterials has been modified with small organic groups to enhance the adsorption capability by surface site binding, magnetic selective adsorption, electrostatic inter actions, and modified ligand combination. Furthermore, attachment of some inorganic materials to the surface of Fe3O4 nano particles not only stabilizes the nanoparticles and eventu-ally prevents their oxidation, but it also enhances the adsorption capacity because of the larger surface area. For example, carbon-coated Fe3O4 nanoparticles (Fe3O4/C) have been fabricated to absorb polycyclic aromatic hydrocarbons (PAHs) from water.[99] The results showed that the adsorption capacity is much higher than for pure Fe3O4 nanoparticles, and the removal efficiencies of PhA, FluA, Pyr, BaA, and BbF are also high. A silica shell has also been coated on the surface of Fe3O4 nanoparticles for the removal of microcystins from aqueous solution. The micro-spheres could be reused with a removal efficiency of >90% even after they were used eight times, owing to the large surface area and magnetic sensitivity (Figure 6).

In summary, nanosorbents have been widely investigated for contaminant removal from wastewater because of their high adsorption capacity and other favorable properties (Figure 7). However, widespread application of these nanomaterials is still limited owing to the difficulty in separating the contaminants from the nanomaterials, which increases the cost and hinders reuse of the nanosorbents. Thus, it is important to develop nanomaterials that can be directly reused without separation from contaminants. Furthermore, the adsorption process cannot completely remove contaminants, especially in low-concentra-tion systems, and an effective process should be developed to deal with trace contaminants to minimize their accumulation, particularly prior to indirect or direct reuse of reclaimed water.

3. Simultaneous Adsorption and Degradation

3.1. Photocatalytic Nanomaterials

Over the past few decades, the heterogeneous photocatalytic oxidation (HPO) process using semiconductor photocatalysts

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Figure 6. Effect of the amount of Fe3O4 and Fe3O4/C sorbents on the extraction efficiencies of PAHs. Reproduced with permission.[99] Copy-right 2010, Elsevier.

Figure 7. A) Magnetic hysteresis loops of Fe3O4 particles (a), Fe3O4@nSiO2 (b), and Fe3O4@nSiO2@mSiO2 (c) microspheres. B) Separation re-dispersion process of Fe3O4@nSiO2@mSiO2 microspheres. Reproduced with permission.[100] Copyright 2008, American Chemical Society.


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and light has attracted much attention because of its low-cost, environmental friendliness, and sustainable treatment tech-nology to treat wastewater. In this process, contaminants in aqueous solution are degraded to more biologically degradable and less toxic substances without secondary pollution.[101,102] This process can occur at ambient operating temperature and pressure, and it has low operating costs.

The hydroxyl radicals generated under ambient conditions are converted to organic compounds during the HPO process, and generation of this oxidizing agent is driven by UV–vis-ible light based on semiconductor photocatalysis. Under an energetic light source, photons with energies higher than the bandgap energy of the semiconductor (ΔE) can lead to excita-tion of valence-band (VB) electrons, promoting reactions. When the energies of the absorbed photons are lower than ΔE, energy dissipates in the form of heat.

A positive hole (hv+) in the VB and an electron (e−) in the conduction band (CB) form when the surface of the photocata-lyst absorbs sufficient energy (Figure 8). The produced hv+ can directly react with organics or with water to generate hydroxyl radicals, and the electron in the CB reduces oxygen adsorbed on the catalyst.

Photoactivation of semiconductor photocatalysts by UV/vis-ible light and the oxidative/reductive reaction of organics can be expressed by the following steps:

Photoactivation:

photocatalysis light e h+ → +− + (1)

e O O2 2+ →− −• (2)

Oxidative reaction:

h organics CO2+ →+ (3)

h H O OH H2+ → ++ • + (4)

Reductive reaction:

OH organics CO2+ →• (5)

Over the past few decades, many semiconductor catalysts have been developed for application in wastewater treatment, such as metal oxides (e.g., TiO2, ZnO, and Fe2O3) and metal sulfides (e.g., CdS and SnS2).

Among the inorganic photocatalysts, titanium dioxide (TiO2) has attracted much attention for photocatalytic water treat-ment owing to its chemical and thermal stability, and excellent mechanical properties. TiO2 powders have been used as white pigments from ancient times,[103] and Fujishima and Honda first discovered photocatalytic splitting of water on a TiO2 electrode under ultraviolet (UV) light.[104,105] Since the 1970s, application of TiO2 photocatalysts for clean-up of pollution in wastewater and photodegradation of many organic compounds (e.g., phenol, chlorophenol, and oxalic acid) has been exten-sively investigated over the TiO2 surface.[106,107] As well as its stability, TiO2 is the most active photocatalyst in the photon energy range 300 nm < λ < 390 nm (Figure 9).[108]

However, recombination of the electron and hole in the absence of proper electron acceptors still limits the indus-trial application of TiO2 because it causes a waste of energy. Although some electron acceptors (e.g., H2O2, KBrO3, and (NH4)2S2O8)[109–111] have been investigated to overcome this limitation, mineralization photoefficiencies are still low. Mod-ification and doping of TiO2 has proven to be an appropriate strategy to enhance the photoefficiency.

In the past few decades, noble metals have been used to modify the surface of TiO2, which has been shown to be effec-tive for enhancing the photoefficiency. The excited electrons can be transferred from the CB to the deposited metal with a lower Fermi level to avoid electron/hole recombination, leading to efficient charge separation and high photocatalytic reaction rates.[112]

Another strategy to enhance the photoefficiency is ion doping, because ions doped in the TiO2 lattice can extend the photoresponse of TiO2 into the visible spectrum. During the last few years, transition-metal ions (e.g., Cr, Mn, Fe, Co, Ni, and Cu) and rare-earth-metal ions (e.g., La, Ce, Er, Pr, Gd, Nd, and Sm) have also been investigated to enhance the photocata-lytic activity of TiO2. Anions (e.g., N, F, C, and S) have also been used to improve the photocatalytic activity of TiO2 under visible light.[108]

ZnO is extensively used as a photocatalyst for wastewater treatment owing to its high photocatalytic activity, nontoxic nature, low-cost, and excellent chemical and mechanical sta-bility. Although TiO2 is considered to be the most active photo-catalyst, ZnO shows better photodegradation of some organic contaminants in certain situations.[113,114] Furthermore, ZnO can absorb a larger fraction of the solar spectrum than TiO2, so ZnO is the most suitable photocatalyst for photodegrada-tion in the presence of sunlight.[115] Many studies have focused on applying ZnO as a catalyst in the photocatalytic degrada-tion of organic pollutants, such as kraft black liquor,[116] 2-phe-nylphenol fungicide,[117] phenol and chlorophenols,[118] and lignin.[119] Dyes such as the azo dyes acid red 14 and acid yellow 23, rhodamine 6G, and methyl orange (MO) can also be photo-degraded.[120,121] In addition, photodegradation of some antibi-otics has also been investigated, such as sulfamethoxazole and lincomycin. Elmolla and Chaudhuri reported UV/ZnO photo-catalytic degradation of amoxicillin, ampicillin, and cloxacillin

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Figure 8. Schematic diagram illustrating the generation of a positive hole and an electron.


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in aqueous solution, and found that the pH has a great effect on the degradation (Figure 10).[122]

The bandgap of Fe2O3 is about 2.2 eV. Therefore, Fe2O3 is an interesting n-type semiconducting material and a suitable candidate for photodegradation under visible light. Because of the excellent photocatalytic performance of iron oxide nano-materials, they have attracted much attention in the field of organic-pollutants degradation. Many Fe(III) species have been proposed to treat water polluted by organics, such as α-Fe2O3, γ-Fe2O3, α-FeOOH, β-FeOOH, and γ-FeOOH.[123] Photodegra-dation of Congo red (C32H24N6O6S2) by iron oxide nanoparti-cles has been investigated, and the results showed that the max-imum removal efficiency is 96%.[124] In addition, the catalytic decomposition capacity is not affected by the introduction of light, while the rate of photodegradation is accelerated.

The drawback of iron oxide is the fast recombination of elec-tron–hole charges on the surface of nanoparticles. To overcome this problem, a noble metal can be deposited on the metal oxide support. For example, gold/iron oxide aerogels have been used as photocatalysts to degrade disperse blue 79 azo dye in water under UV light.[125] The gold nanoparticles are considered to be the sites for electron accumulation under UV-light irradiation, and can facilitate the transfer of surface electrons. Better sepa-ration between electrons and holes will allow higher efficiency photoreactions.

Some metal sulfides and nitrides have also been developed as photocatalysts for wastewater treatment, such as CdS, SnS2, and C3N4. The bandgap of CdS is 2.42 eV, and its valence elec-tron can be easily transferred to the CB by light with a wave-length of ≤495 nm.[126,127] In recent years, CdS semiconductor nanoparticles have attracted intense interest as photocata-lysts to treat dye-containing wastewater owing to their unique photochemical and photophysical properties.[128,129] Compared with CdS, SnS2 is more stable against oxidation and photocorro-sion, and Li reported that SnS2 exhibits a much higher activity than CdS during visible-light-induced photocatalytic degrada-tion of organic dyes containing the NN double bond.[130]

However, most of these photocatalysts are either UV-light responsive or are made of expensive or scarce metal species, which limits their long-term and large-scale applications. Gra-phitic carbon nitride (g-C3N4) overcomes the problems of

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Figure 9. a) Electrons capture by a metal in contact with a semiconductor surface. b) UV–vis spectra of iron-doped titania catalysts. Reproduced with permission.[108] Copyright 2009, Elsevier.

Figure 10. Effect of pH on degradation of amoxicillin (A), ampicillin (B), and cloxacillin (C). Reproduced with permission.[122] Copyright 2009, Elsevier.


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iew metals, and has been used as a metal-free polymeric photo-

catalyst for water splitting and organic synthesis.[131] Recently, application of g-C3N4 has been extended to reduction of CO2. Photodegradation of heavy-metal ions and organic pollutants has shown that g-C3N4 is a promising semiconductor for envi-ronmental applications.

As discussed above, inorganic nanoparticles have been widely used in the field of environmental remediation either as absorbents or photocatalysts. However, pure inorganic nano-particles cannot meet specific requirements in some environ-mental cases, such as high concentrations of contaminants, selective adsorption, continuous adsorption, and photodegrada-tion. Modification of pure inorganic nanoparticles with some functional organic polymers to form core–shell nanocompos-ites is a potential way to overcome these limitations.

Compared with inorganic nanoparticles with mesoporous structures, the adsorption capacity of some solid particles is much lower and limits their applications. Modification of the surface of solid nanoparticles with organic groups or poly-mers to form a core–shell structure can enhance the adsorp-tion capacity. For example, Fe3O4 nanoparticles acting as a core have been modified with some functional polymers or groups to form magnetic nanocomposites, which not only improves the adsorption performance but also allows for magnetic sepa-ration.[132] Moreover, modification can also prevent particle–particle aggregation of the core parts and improves the disper-sion stability of the core–shell nanostructures in a suspension medium, leading to better adsorption properties.[133,134]

The most important advantage of inorganic nanomate-rials in the field of environmental remediation is their high adsorption capacity owing to their large specific surface area. However, these pure nanomaterials are not able to achieve selective adsorption of some contaminants in practical applica-tions. This problem not only reduces the adsorption efficiency, but also affects the recovery of adsorbates and the reuse of absorbents. Over the past few years, to overcome these limita-tions, researchers have investigated modification of the surface of nanomaterials with functional groups. The most commonly used functional groups are protonic acids/bases that can bind to contaminants by electrostatic or van der Waals inter-actions. For example, the amino group has been wildly used to functionalize nanoadsorbents, and the functionalized nano-adsorbents show excellent ability to remove heavy-metal ions from wastewater owing to easy protonation under acidic con-ditions.[135] In addition, particles with attached free carboxylate functional groups are also excellent candidates for positively charged pollutant complexation, including complexation of dye molecules.[136]

Another strategy for selective adsorption is functionaliza-tion of inorganic nanomaterials with molecular imprinted

polymers (MIPs). The molecular imprinting technique is a method to prepare highly selective polymer receptors for given molecules.[137–140] MIPs have been used as materials for mole-cular recognition in many scientific and technical fields, such as solid-phase extraction, chromatograph separation, membrane separation, sensors, drug release, and catalysts.[141–143] Modifi-cation of inorganic nanomaterials by surface-imprinting tech-niques not only gives the nanomaterials selective adsorption properties, but also overcomes some disadvantages, such as the relatively low number of recognition sites per unit volume of the polymer and template molecules embedded too deeply in the matrices.

Based on the active groups of nanomaterials or polymeriza-tion processes, MIP–inorganic nanomaterial nanocomposites have been prepared and used for environmental remediation. Silica and carbon nanomaterials have been modified with MIP for the removal of heavy-metal ions and organic molecules from aqueous solution.[144,145] In addition, the hydrophilic sur-face of the silica shell of hollow mesoporous silica nanoparti-cles can also improve dispersion of MIP in water when MIP is introduced into the hollow core.[146] It should be noted that GO has a 2D structure that is more suitable as a platform than 3D platforms, and provides complete removal of templates and a very high rebinding capacity (Figure 11).[147] Other inorganic nanomaterials with unique properties have also been used to prepare nanocomposites. As well as the advantages mentioned above, these nanomaterials make nanocomposites more prac-tical. For example, magnetic Fe3O4 nanoparticles and MIP can be magnetically separated after adsorption.[148]

As mentioned above, some pure inorganic nanomaterials (e.g., TiO2, ZnO, and SnS2) can act as photocatalysts and are able to photodegrade contaminants to nontoxic species. There-fore, for wastewater treatment, these nanomaterials can be reused after photodegradation without further treatment. Many advanced techniques have been developed to control the struc-ture of these inorganic nanomaterials and increase the surface area, leading to higher adsorption capacity and photodegra-dation efficiency. However, they may not meet the demands of practical application, especially for wastewaters with a high concentration of contaminants, owing to low binding efficiency, and non-selective adsorption and degradation. On the other hand, it is well known that adsorption is a dynamic process and adsorbates on the surface permeate into the absorbents until adsorption equilibrium is reached, where release and adsorp-tion of adsorbates are in dynamic equilibrium. Therefore, it is difficult to completely remove contaminants only by adsorp-tion, especially when the contaminant concentration is high.[149]

Combining the advantages of photocatalysts and adsorbent materials would overcome these problems. The most promising method is to functionalize the surface of inorganic nanomaterials

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Figure 11. Preparation of GO–MIP hybrids. Reproduced with permission.[147] Copyright 2010, Elsevier.


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with polymers to form core–shell nanocomposites, which leads to a higher local concentration of contaminants on the photo-catalyst surface and promotes the photodegradation process (Figure 12). Adsorption and photodegradation of contaminants would be combined into continuous process: i) the surface of the absorptive polymer would efficiently bind contaminants, and then contaminants absorbed on the surface would per-meate into the polymer and be close to the photocatalytic core. ii) Under the irradiation of UV–visible light, the encapsulated photocatalysts would be excited, and photodegradation of the contaminants would occur. iii) The lower concentration inner of the polymer would lead to non-equilibrium conditions and continuous adsorption would occur. The continuous adsorp-tion–photodegradation process could not only completely remove contaminants within a relatively short time, but the photocatalyst–polymer nanocomposite could also be directly reused without further treatment. This process involves two main steps when these core–shell nanocomposites are used in practical applications: contaminants first efficiently adsorb to the attached polymer and the concentration of contaminants decreases to a relatively low level, and then the continuous adsorption–photodegradation process occurs until complete degradation.

In the past few years, photocatalysts have been combined with various polymers for environmental applications. For example, TiO2 nanoparticles have been encapsulated by MIP for selective adsorption and biodegradation of organic mole-cules,[150,151] and EDTA–TiO2 core–shell microspheres have been fabricated to improve photocatalytic performance.[152] Although these functional polymers showed attractive selec-tivity and adsorption capacity, the preparation method is relatively long and complicated, which is not quite suitable for scale-up. Thus, carbon materials and some commercial foams with low density, high surface area, excellent flexibility and stretchability, good chemical and thermal stability, and environ-mental friendliness make them particularly suitable for waste-water treatment.

3.2. Carbon-Based Composite Materials

Photocatalysts composited with carbon of various forms, such as graphite, carbon nanotubes, and graphene, have been widely investigated to obtain improved adsorption capacity and photodegradation performance in the visible region. As known to all, activated carbon can adsorb organic pollutants effectively, but the regeneration cost is relatively high. Thus, activated carbon was initially loaded with TiO2 for photodeg-radation studies.[153] The modification of TiO2 on activated carbon showed high adsorption capacity and highly improved photocatalytic efficiency since the activated carbon can help to enrich the local pollutant concentration of the catalyst and subsequently promote the pollutant transfer process. Moreover, the synergistic effect between adsorption and photocatalytic decomposition is very important in the carbon-based TiO2 composite.[154,155] Matos et al. confirmed the synergistic effect of activated carbon (surface area: 775 m2 g−1)-based TiO2.[156] The enhancement of the phenol-degradation process could be attributed to the strong adsorption capability of phenol and its intermediate species on the activated carbon followed by the subsequent pollutant transfer to the photocatalytically active TiO2.

On the basis of the synergistic effect, fullerene is very promising among all the types of carbon supports because the charge-transfer property could be increased between the fullerene and photocatalyst (e.g., TiO2) by acidification of sur-face groups. Moreover, the energy sensitization of fullerene also improves the quantum efficiency. Oh et al. prepared a fullerene–TiO2 composite and investigated its adsorption and photodegradation performance.[157] The results showed the excellent activity of fullerene–TiO2 composites for the effi-cient removal of organic dye, which could be attributed to the synergistic effect between the adsorbent fullerene and the photocatalyst TiO2.

When compared with activated carbon, carbon nanotubes show a larger specific surface area and thus can further enhance the improved adsorption capacity and photodegradation perfor-mance as explained above. In addition, carbon nanotubes can be tailored to enhance specificity toward pollutants through the surface groups (e.g., alcohol, keto, and acid moieties) modifica-tion.[158] In this context, carbon nanotubes are excellent candi-dates, allowing deeper insight into the semiconductor junction of carbon materials with photocatalysts.

Because of its unique properties, such as photonic and elec-tronic properties, graphene is also used as an additive compo-nent to prepare graphene composite materials with required or particular performance. With respect to this, some graphene composites have excellent photocatalytic performance when incorporated with metal nanoparticles, such as TiO2, AgX (X = Cl, Br), Cu2O, and SnO2.[7,159–163] These composites have extensive applications in water remediation, including photo-degradation of organic pollutants and reduction of toxic metal ions. Niu et al. fabricated a 3D AgX/graphene composite mate-rial by coating AgX nanoparticles on the surface of 3D gra-phene by a simple method.[12] The photocatalytic performance of the composite was investigated by degradation of MO and reduction of Cr(VI). The results showed that the composite exhibited higher photocatalytic performance for removal of

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Figure 12. Schematic diagram of the continuous adsorption–photodegra-dation process within photocatalyst–polymer core–shell nanocomposites.


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iew pollutants than pristine AgX nanoparticles under visible-

light illumination. More importantly, the 3D structure of the composite means that it can be easily recycled from aqueous solution, which is important for practical applications. Dong et al. synthesized a Cu2O–rGO composite with tunable Cu2O crystal facets for photocatalytic degradation of organic con-taminates.[7] The photodegradation performance of the com-posite with different Cu2O crystal facets was investigated by degradation of MB under visible light. The result showed that the o-Cu2O/rGO composite exhibited excellent performance, which was attributed to its enhanced visible-light absorp-tion and the rapid photogenerated electron transfer. As well as these metal nanoparticle composites, other nonmetal (e.g., g-C3N4 and CNT)[67,164,165] composite materials exhibit similar photocatalytic degradation. For instance, Jiang et al. fabricated a g-C3N4/GO composite by hydrothermal coassembly of g-C3N4 and GO.[67] GO was used as a frame to support the g-C3N4 and a carrier to promote electron transfer, while the g-C3N4 acts as the photocatalyst for the photodegradation of MO in waste-water. Under the synergetic effect of the g-C3N4 and the GO, the removal ratio of MO over the composite was up to 92% within 4 h under irradiation by sunlight, which is better than that of pure g-C3N4 (12%).

Considering GFs have high surface, chemical, and thermal stability, as well as hydrophobic and oleophilic properties, several GF-based composites have been fabricated as adsorbents for the removal of oils, organic solvents, and water-soluble organic dyes from wastewater with exceptional performance. Chen et al. syn-thesized a superhydrophobic graphene/CNT composite by two-step chemical vapor deposition.[28] The composite showed high absorption capacities (80–130 times its own weight) for various types of oils and organic solvents. Qiu et al. fabricated a similar graphene/CNT composite but with excellent compressibility by the microwave-irradiation-mediated method.[166] The composite not only exhibited high absorption capacities for oils, but also showed excellent recyclability in oil-cleanup applications.

3.3. PU Composite Materials

Compared with PU-based materials, PU composite mate-rials exhibit better performance by combining to one or more material with a special functionality for targeted adsorption or removal of pollutions from sewage. For example, Lee et al. pre-pared PU/chitosan composite foams by reacting PU prepolymer with various amounts of chitosan.[167] Chitosan, or its deriva-tives, possess high performance in sewage treatment as their surface contains a large number of amine and hydroxyl groups, which features it with high adsorption for organic dyes and metal ions via electrostatic attraction or chelating groups. As a result, the synergistic effect between those two materials ren-dered as-prepared composite foams with high performance for adsorption of acid violet 48. The adsorption capacity increased with increasing concentration of chitosan, and reached a max-imum (30 mg g−1) with 20 wt% chitosan. In addition, as a result of chemisorption of sulfonic ions of acid violet 48 to protonated amine groups of chitosan, the foams exhibited high adsorp-tion capacity in low-pH aqueous solution. To remove Pb2+ ions from wastewater, Lee et al. developed composite foams using

immobilized hydroxyapatite, an inorganic compound with a high capacity for removal of heavy-metal ions, on the PU struc-ture.[16] The removal capacity of Pb2+ ions was investigated. The results showed that the composite foams exhibited excellent per-formance for removal of Pb2+ ions at pH 5, and the maximum adsorption capacity of the composite foams was 150 mg g−1 with 50 wt% hydroxyapatite. Imae et al. synthesized PU–SiO2/TiO2 composite materials for photocatalytic degradation of acid black 1 by immobilizing the SiO2/TiO2 composite in the PU matrix.[10] Because of the large amount of sub-micrometer-sized TiO2 attached to the surface of the PU matrix, the composite materials exhibited good photocatalytic degradation of acid black 1 under UV-light irradiation. In addition, the recyclability of the composite material was investigated, and the results indi-cated that it maintained its initial photocatalytic activity after three photocatalytic degradation tests. Liu et al. fabricated PU composite foams by coating with a film of copper and subse-quent hydrophobic treatment in an ethanol solution of AgNO3 and n-dodecanoic acid.[32] Because a large amount of copper–C11H23COOAg nanoparticles agglomerated on the surface of the PU structure to form a rough and hierarchical structure, the foams exhibited superhydrophobicity and superoleophilicity. The WCA was higher than 170° and the time taken to adsorb a 6 μL oil droplet was only 1.76 s. These properties allowed it to rapidly and selectively absorb oils and organic solvents from wastewater. Furthermore, the robust and compressible proper-ties resulted in excellent recyclability for oil–water separation by simple mechanical squeezing. Wang et al. fabricated magnetic superhydrophobic PU composite foams by coating with Fe3O4 and subsequent dip coating in fluoropoly mer aqueous solu-tion to obtain a hydrophobic surface.[168] The surface-modified composite layer made the PU foams magnetically responsive, superhydrophobic, superoleophilic, and chemically stable. The composite foam could be magnetically driven to adsorb floating oils or organic solvents from polluted water with high selec-tivity. Moreover, it exhibited excellent oil–water separation per-formance when used as a membrane.

3.4. MF Composite Materials

Similar to PU foams, MF has an open porous structure, which makes it another ideal matrix for the preparation of composite materials for environmental-pollution treatment. Generally, to maintain its original porosity, 1D or 2D materials are commonly adopted to be composited with the MF. Yu et al. fabricated PS@Ag/AgCl composite foams as a photoreactor for continuous photocatalytic degradation of wastewater pollution by immobili-zation of photocatalysts on the backbone of MF (Figure 13a).[169] SEM images showed that the backbone was completely coated with Ag/AgCl nanowires (Figure 13b). Figure 13c shows a schematic illustration of foams used as a photoreactor for the photodegradation of MO under illumination by sunlight. Com-pared with 2D laminar Ag/AgCl films, the contact area between the Ag/AgCl nanowires and the contaminated water increases when wastewater flows through the photoreactor, which results in a photodegradation efficiency 4 times higher than that of 2D Ag/AgCl nanowire films. More importantly, the interconnected porous structures of the photoreactor allow contaminated water

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to rapidly flow through the photoreactor and the throughput can be up to 9600 L h−1 m−2, which is much higher than that of other microfluidic photoreactors.

Kuo et al. used MF as a frame to fabricate graphene com-posite foams using the simple dip-coating method to anchor graphene sheets to the MF skeletons.[170] The foams inherit not only the porous microstructure and chemical stability of the matrix, but also the superhydrophobicity and superoleophilicity of graphene. Therefore, when the composite foam was used as an absorbent for sewage treatment, it showed high performance for the adsorption of various types of oils and organic solvents with high adsorption capacity, good selectivity, and excellent recyclability. Similarly, our group prepared rGO-modified foam by thermal reduction of GO on the skeleton of MF.[33] The preparation method is simple and cost-effective, and the perfor-mance for clean-up of oils from water and oil–water separation is high, which make it an ideal absorbent for sewage treatment.

3.5. Cellulose Composite Materials

Cellulose is a green and inexhaustible material with good prop-erties, such as biodegradability, low toxicity, eco-friendliness,

and chemical stability. It is a commonly used matrix to prepare adsorbents for water purification, including the removal of heavy metals and organic dyes, and clean-up of oils and organic solvents from water. For example, Yao et al. prepared a cellulose-composited adsorbent by combining cellulose with acrylamide and acrylic acid for removal of nickel ions from wastewater.[22] Because there was a large amount of functional groups (e.g., hydroxyl, amino, and carboxyl groups) exposed on the surface of the composite, it exhibited high performance for the removal of nickel ions. Tam et al. synthesized cellulose-composited alginate hydrogel beads for removal of organic dyes from sewage.[44] Owing to the ionic charge of the alginate hydrogel beads, the composite exhibited a high adsorption capacity for MB. In addition, the inherent properties of the large surface area, porous structure, and mechanical stability of the cellulose foam further enhanced the adsorption efficiency and recycla-bility. The maximum adsorption capacity of the composite was 256.4 mg g−1. More importantly, it exhibited more than 97% MB removal efficiency after five successive adsorption–desorption tests. Chen et al. fabricated a hydropropyl cellulose hydrogel composited with GO for MB removal.[25] Owing to the abundant functional groups and large surface area of GO, the hydrogel exhibited high adsorption capacity for MB. Sharma et al. used

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Figure 13. a) Schematic diagram of fabrication processes of the PS@Ag/AgCl composite using MF as the frame. b) SEM images of Ag and Ag/AgCl nanowires coated on MF. c) Schematic diagram of the composite used as a photoreactor for photodegradation of wastewater. Reproduced with permis-sion.[169] Copyright 2015, The Royal Society of Chemistry.


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iew cellulose acetate as a frame to prepare a tin(IV) phosphate com-

posite for photodegradation of MB in wastewater under irradia-tion by sunlight.[8] Owing to the high ion-exchange capacity and photocatalytic activity of the tin(IV) phosphate nanocomposite, the composite simultaneously absorbed and photodegraded MB under sunlight irradiation.

4. Conclusions and Outlook

Here, we have given a brief summary of recent developments in the fabrication and application of various micro–nanocom-posites in environmental management. Through years of considerable efforts, great achievements have been made via environmental nanotechnology to remove pollutants through rapid adsorption and degradation processes. Particular atten-tion has been devoted to simultaneous adsorption and photodegradation micro–nanocomposites because the pollutant could be totally removed via this technology. However, many technical challenges and obstacles still need to be addressed.

Based on the aforementioned challenges, it is suggested that future research work be carried out in the following aspects. First and foremost, the tiny nanomaterial itself is also a kind of potential hazard for the environment. To resolve this issue, the micro–nanocomposites should be fabricated into fibers, membranes, or foams. In the meantime, through the fabrica-tion process, the practical applicability of composites will be solved. It is worth noting that the porous nanostructure needs to be kept with a high surface area for rapid adsorption and degradation of pollutants. Secondly, although the performance of traditional environmental remediation material is not high, the price is very cheap and easy to scale-up. Thus, it should not be technically difficult or too costly for large-scale produc-tion of the obtained environmentally friendly micro–nano-composites, which is very important for environmental reme-diation. Thirdly, regarding high concentrations of pollutant, the recovery process plays a crucial role in sustainable develop-ment. Recently, a great breakthrough in smart adsorbent mate-rials was introduced. For instance, using stimuli-responsive composite materials is a promising candidate for oils, organic solvents, and recovery of heavy metals in future work. In addi-tion, metal–organic frameworks (MOFs), a new class of highly crystalline porous adsorbents formed via strong metal-ligand bonds between organic linkers and metal cations, have been developed and explored in adsorptive removal/separation and purification, owing to its promising adsorption capacity and extremely high surface area (ranging from 1000 to 5400 m2 g−1). Significant progress has been made in this area, but much more research needs to be done, which will provide smart and convenient recovery technology. In the area of environmental remediation a lot of challenges and unknowns remain waiting for us to explore and discover.

AcknowledgementsThis work is supported by the Collaborative Innovation Center of Suzhou Nano Science and Technology. The authors gratefully acknowledge the financial support provided by the National Natural Science Foundation

of China (21336005, 21301125), Natural Science Foundation of the Education Committee of Jiangsu Province (15KJB150026), Environmental Protection Research Foundation of Suzhou, and Suzhou Nano-project (ZXG2013001, ZXG201420).

Received: March 16, 2016Revised: July 13, 2016

Published online: October 26, 2016

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