Environmental remediation of heavy metal ions by novel-nanomaterials: A review

Environmental Pollution 246 (2019) 608e620

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Environmental remediation of heavy metal ions by novel-nanomaterials: A review* Yihan Wu a, Hongwei Pang a, Yue Liu a, Xiangxue Wang a, b, Shujun Yu a, Dong Fu b, Jianrong Chen c, Xiangke Wang a, * a

MOE Key Laboratory of Resources and Environmental Systems Optimization, College of Environmental Science and Engineering, North China Electric Power University, Beijing, 102206, PR China Department of Environmental Science and Engineering, North China Electric Power University, Baoding 071003, PR China c College of Geography and Environmental Science, Zhejiang Normal University, Jinhua, 321004, China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 September 2018 Received in revised form 23 December 2018 Accepted 23 December 2018 Available online 26 December 2018

Recently, novel-nanomaterials with excellent sorption capacities, mild stability, and environmentalfriendly performance, have enabled massive developments in capturing heavy metal ions. This review firstly introduces the preparation and modification of novel-nanomaterials (e.g., MOFs, nZVI, MXenes, and g-C3N4). Then, the heavy metal ions’ sorption properties and the impact of environmental conditions have been discussed. Subsequently, the sorption mechanisms are verified through batch experiments, spectral analysis, surface complexation models, and theoretical calculations. Finally, the applications prospects of novel-nanomaterials in removing heavy metal ion polluted water have also been discussed, which provide perspective for future in-depth research and practical applications. © 2018 Elsevier Ltd. All rights reserved.

Keywords: Nanomaterials Heavy metal ions Sorption Interaction mechanism

1. Introduction The development of metallurgical, mining, nuclear energy, and chemical manufacturing has released large amounts of toxic heavy metal ions into the natural environment, posing serious threats to the surface and subsurface environments (Li et al., 2018a; Vilardi et al., 2018a; Yin et al., 2018; Yu et al., 2017a, 2018a). Due to the continuous accumulation, high toxicity, and strong permeability, heavy metal ions are easily accumulated in living organisms, which cause long-term damage to humans and other species (Wang et al., 2015a, 2018a; Zhang et al., 2018a; Zou et al., 2016a). Thereby, it is necessary to eliminate these heavy metal ions from contaminated water. To date, the techniques to remove heavy metal ions mainly include sorption (Minju et al., 2015; Song et al., 2019; Wang et al., 2015b, c; Yu et al., 2015), chemical precipitation (Neeraj et al.,  et al., 2016; Yu et al., 2017b), membrane filtration (Miku
sova ~ a et al., 2013), ion exchange (Radchenko et al., 2014; Montan

*

This paper has been recommended for acceptance by Eddy Y. Zeng. * Corresponding author. E-mail address: [email protected] (X. Wang).

https://doi.org/10.1016/j.envpol.2018.12.076 0269-7491/© 2018 Elsevier Ltd. All rights reserved.

2015; Zhang et al., 2014a), photocatalytic degradation (Kumar et al., 2018a,b), coagulation (Yu et al., 2017c; Zou et al., 2016b, c), oxidation/reduction (Sheng et al., 2016a, b), and solvent extraction (Huang et al., 2016). Among these methods, sorption has become one of the most common and effective techniques due to low cost, simple operation, strong practicality, environment-friendly, and the simple regeneration of the adsorbents (Hu et al., 2015; Wang et al., 2016, 2018b; Zhao et al., 2018). Novel-nanomaterials with superior performance are constantly applied to improve their applications in environmental pollution cleanup. Metal-organic frameworks (MOFs) with abundant functional groups and tailorable structure, have represented a new field of capturing various heavy metal ions (Li et al., 2018a). Nanoscale zero-valent irons (nZVI) are environment-friendly materials with excellent mobility, high reactivity, low toxicity, controlled particle size, and abundant surface active sites, and thereby being used as effective adsorbents to eliminate heavy metal ions (Zou et al., 2016d). MXenes own large specific surface areas, excellent mechanical properties. As the most stable allotrope of C3N4 under environmental conditions (Masih et al., 2017), the abundant functional groups (e.g., eNH2/eNHe/¼NA-) in g-C3N4 provide basic active sites for eliminating pollutants (Hu et al., 2015; Yu et al., 2018b). Although novel-nanomaterials have been increasingly

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studied and reported in removing heavy metal ions, the systematic summaries and reviews of heavy metal ions’ removal onto novelnanomaterials are relatively lacking. Therefore, this review mainly introduces the related research and progress of several novel-nanomaterials (e.g., MOFs, nZVI, gC3N4, MXenes) for eliminating heavy metal ions, and impact of environmental conditions on sorption behavior. At the same time, the interaction mechanisms are discussed in detail. Finally, the opportunities and challenges are presented to motivate more researchers to enter this promising field. 2. Novel-nanomaterials 2.1. Metal-organic frameworks (MOFs) MOFs materials are formed by the coordination of metal ion precursors (e.g., Fe(III), Cu(II), Ca(II), Al(III), Mg(II), Zn(II), Cd(II), Co(II), Zr(IV), Ln(III), and Ti(III)) and organic ligand (e.g., p-phthalic acid, benzoic acid, pyridine, imidazole, piperazine, amines, carboxylates, sulfonates, and phosphates, etc.) (Li et al., 2018a, b; Wu et al., 2018) (Fig. S1A(a)). Owing to the high porosity, large surface areas, well-defined structures, the ease of process-ability and structural diversity, MOFs are much more attractive in sorption compared with other materials (Ricco et al., 2015; Yang and Yin, 2017). Up to now, MOFs have been successfully obtained through various methods shown in Table S1. Among them, solvothermal synthesis is the most typical method. To obtain MOFs with better performance, five categories (Fig. S1A(b)) have been widely used (Li et al., 2018a): (I) MOFs designed with long organic linkers. Howarth et al. (2015) carefully designed a series of Zr-MOFs with long organic linkers and found that the obtained MOFs with very high surface areas, porosity, dispersity, and stability, but the organic linkers are often expensive; (II) MOFs created from defects. Specifically, the defects created from adding modulators to the precursor mixture will enlarge the active sites and make MOFs more hydrophilic. As shown in Fig. S1B, the UiO-66 was synthesized through a monocarboxylic acid modifier under the most common conditions and showed excellent As(V) sorption capacity of 303 mg/g at pH 2.0 (Wang et al., 2015a); (III) MOFs derived from organic linkers functionalization. Zhang et al. (2015) found that two carboxyl groups modified MOF (UiO-66(Zr) e2COOH) can selectively and significantly remove Cu(II) due to chelation; (IV) MOFs originated from metal nodes functionalization. Liang et al. (2016) synthesized sulfur-functionalized MOFs using Co(NCS)2 4 for Hg(II) removal, and demonstrated the chemically soft S atom was good for adsorbing heavy metal ions; (V) MOFs composites. MOFs hybridize with functional materials (e.g., Fe3O4, GO, and g-C3N4, etc.) can enhance the practical applications such as easily recycle and prevent column clogging. Yang et al. (2016) used Fe3O4 to grow the MOF to obtain the Fe3O4@MIL-100 (also called MMCs) for removing Cr(VI) (Fig. S1C), and the sorption results indicated that Fe3Oþ and Cr(VI) could form electrostatic interactions at pH 2.0, which led to a higher sorption capacity compared to Fe3O4 and MIL-100. 2.2. Nanoscale zero-valent iron (nZVI) Due to low cost, large specific surface area, rich surface binding sites, fast reaction rate, and without secondary pollution, nZVI has been used as highly efficient and environment-friendly adsorbent (Li et al., 2015a; Pang et al., 2018; Sheng et al., 2016c). Kanel et al. (2006) found that the reaction rate constant of nZVI removing As(V) was ~1000 times that of ordinary ZVI. Therefore, nZVI is a kind of nanomaterial which can efficiently remove As(V) from aqueous solutions. Conventional nZVI preparation methods are

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shown in Table S2, and among them, the liquid phase reducing method has been widely used. Fig. S2A shows the reaction process of bare nZVI generated by reducing the dissolved iron with sodium borohydride (NaBH4) solutions (Bae et al., 2016). Experimentally, the produced nZVI was highly reactive and the NaBH4 content significantly increased the reactivity of nZVI in an oxygen environment. However, the expensive reagents and the production of large amounts of hydrogen hamper their large-scale industrial application (Zou et al., 2016a). Although their effectiveness in removing pollutants, nZVI still has certain limitations in practical application such as easy aggregation, instability, oxidation and secondary pollution (Hu et al., 2016; Li et al., 2016; Sheng et al., 2016a). In order to overcome the above problems, functional groups or stabilizers are added to the surface of nZVI. And the common modification methods (Fig. S2B) can be summarized into surface modified nZVI, porous materials loaded nZVI, and inorganic clay mineral supported nZVI. Liu et al. (2015) applied nZVI@Mg(OH)2 composite to improve the efficiency of Pb(II) elimination, which presented an excellent sorption capacity of 1986 mg/g and sorption rate was about 94% within 15 min. Besides, the high specific surface area and pore structure characteristics of Mg(OH)2 can alleviate nZVI particles aggregation and oxidation. As shown in Fig. S2C, due to magnetic properties and the van der Waals’ force, the unsupported particles tend to aggregate together. But, the stabilizer of Mg(OH)2 makes the nZVI particles evenly distributed. Li et al. (2015b) applied bentonite pillar modified nZVI (nZVI/Al-bent) to adsorb Se(VI) from aqueous solutions. The synergetic removal efficiency (95.7%) of Se(VI) onto nZVI/Al-bent was much higher than the sum (72.0%) of Al-bent sorption (9.9%) and nZVI reduction (62.1%), which was mainly attributed to the sorption and reduction of both Al-bent and nZVI co-contribution. In addition, Fu et al. (2015) found the sepiolite with special surface properties, chemical stability and low cost, so the sepiolite-supported nZVI (S-nZVI) was used to remove Cr(Ⅵ) and Pb(Ⅱ). And the results showed that sepiolite not only improved the reunion performance of nZVI and can efficiently remove multiple heavy metal ions. 2.3. MXenes Owing to the weak binding force between the A layer and the MAX layer in the MAX phase, the MXenes are commonly prepared through selectively etching of the A element layers using appropriate etching agents (e.g., HF, LiF þ HCl, NH4HF2, etc.) from the MAX phases at room temperature. The formula of MXenes can be expressed as (MX)nM, where M represents the transition metal elements, X is C or N element, and n ¼ 1, 2, or 3 (Peng et al., 2014). Fig. 1A indicates the three different formulae of MXenes: M2X, M3X2, and M4X3. Ascribe to the high electrical conductivity, large specific surface area, stability, and mechanical properties, MXenes show promising application in sorption (Ng et al., 2017; Zhang et al., 2018b). The specific surface areas of materials directly affect its sorption capacity, so the quality of MXenes is a key factor in environmental application. In 2011, Naguib et al. (2011) firstly used a simple chemical etching to obtain high-quality MXenes, where the A layers were selectively etched from corresponding MAX matrix phases by sonication at room temperature, using aqueous HF (Fig. 1B). Recently, the derivatives of MAX phases (d-MAX) called laminated phase have also been applied to synthesize the MXenes. For example, Zhou et al. (2016 & 2017) successively etched the AleC and Si-alloyed AleC sublayers of nano-laminated Zr3Al3C5 and Hf3[Al(Si)]4C6, and synthesized the Zr3C2 and Hf3C2 MXenes, respectively. Fig. 1E shows the fabrication process of Hf3C2 MXene from Hf3[Al(Si)]4C6. During the etching process, through weakening

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Fig. 1. (A) Schematic diagram of MXenes synthesized from MAX phases (Naguib et al., 2014); (B) Schematic of the exfoliation process for Ti3AlC2 (Naguib et al., 2012); (C) Schematic diagram of Hf3C2Tz MXene synthesized from nanolaminated phase Hf3[Al(Si)]4C6. (a) Structure of Hf3Al4C6, (b) structure of pure Hf3Al4C6 monomer, (c) and (d) structure of two different siliconized Hf3Al4C6 monomers, and (e) profile of Hf3C2 MXene (Zhou et al., 2017).

the interfacial adhesion between the HfeC and Al(Si)eC sublayers, the Si-introduction can obviously promote the etching of layered ternary carbide Hf3Al4C6, and the obtained MXenes own highly active and easy to react with pollutants (Gu et al., 2018a; Peng et al., 2014). However, due to the highly toxic and corrosive of HF, the above preparation methods own risks. Recently, NH4HF2 has been used as etching fluid for etching MAX phase to reduce the risk of the experiments (Halim et al., 2014). Additionally, researchers have applied ball-milling for shortening the preparation time and improving the sorption performance of MXenes (Naguib et al., 2012, 2013). Despite the excellent performance of MXenes, the high cost, low yield, high toxicity, and cumbersome preparation methods have restricted the development and application of MXenes, so it is critical to improve the preparation technology. 2.4. Graphitic carbon nitride (g-C3N4) As a valuable extension of carbon in material application, carbon nitride materials have attracted a revival activity recently. Teter and Hemley (1996) found that the C3N4 compounds mainly contained cubic phase c-C3N4, quasi-cubic phase q-C3N4, a-C3N4, b-C3N4, and graphite phase g-C3N4. Among them, the g-C3N4 has been regarded as the most stable allotrope of C3N4 with controllable structure, environment-friendly products, excellent thermal and chemical stability, and diverse physicochemical properties, and thus it has been widely applied in sorption (Wang et al., 2010a, b; Zou et al., 2017). In recent years, researchers have made great efforts in mass preparation and modification of g-C3N4. And the main preparation methods of g-C3N4 include thermal polymerization, pyrolytic organic compound, deposition and solvothermal methods (Wang et al., 2010a; Zou et al., 2017). Among them, thermal polymerization is the most common preparation method. The g-C3N4 prepared by this method is generally a nano-sheet structure, and has a large specific surface area, so the sorption performance is superior. To further improve the structure and performance, three various strategies have been developed to modify g-C3N4: (I) Nano-casting to increase the specific surface area of g-C3N4. Generally, g-C3N4 directly synthesized from organic precursors tends to own smaller specific surface areas (<10 m2/g). Zhang et al. (2014b) synthesized porous g-C3N4 with controlled and highly ordered porosity through

simple polymerization of urea and dicyandiamide by nano-casting technology. And the g-C3N4 with specific surface area 11 times that of the original (5.4 m2/g) through adjusting the proportion of urea/ dicyandiamide and calcining temperature, which increased the degradation efficiency of pollutants. (II) Template production to improve the uniformity of porosity and aperture. Feng et al. (2014) obtained sulfur-doped g-C3N4 micron rod with abundant active sites and highly ordered porous structure (Fig. S3A); (III) Composites with functional materials to increase the specific surface area and oxygen-containing functional groups. Fig. S3B shows the onestep hydrothermal synthesis strategy of cotton-like gC3N4@NieMgeAl-LDH with sufficient metal-oxygen functional groups and free metal functional groups from g-C3N4 and NieMgeAl-LDH (Zou et al., 2017). To summarize, we briefly introduce the preparation and modification methods of novel-nanomaterials, and find that novelnanomaterials own excellent structure and physicochemical properties. In addition, the limitations of novel-nanomaterials and corresponding solutions have also been summarized in Table S3, which provide theoretical supports for the efficient removal of heavy metal ions in wastewater. 3. Heavy metal ions sorption The modern industrial mining, agricultural production, and human activities generate large amounts of toxic heavy metal ions, most of them will eventually harm human health. Therefore, efficient removal of them is significant for human beings and ecological environment. In this review, we deeply discuss the behavior of removing several common highly toxic heavy metal ions (e.g., Cr, As, Pb, U, Hg, Cd, and Ni, etc.) onto novel-nanomaterials, and the sorption properties of different materials are listed in Table S4. 3.1. Chromium (Cr) sorption Chromium ions and their corresponding compounds are released into the environment as carcinogens by oxyanions (CrO2 4 , 3þ Cr2O2 or HCrO 7 4 ) and cations (Cr ). To prevent chromium diffusing into the environment, a series of novel-nanomaterials have been applied to safely dispose of chromium pollution. Oliver's group reported the superior sorption capacity for trapping

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chromate (CrO2 4 ) (~60 mg/g) by cationic MOF (SLUG-21) within 48 h (Fei et al., 2011). Besides, the first anion-exchangeable cationic metal-organic solid material (SLUG-35) also showed great potential of capturing chromate in high capacity (68.5 mg/g) even with nontoxic anions (nitrate and sulfate) in excess of 50-fold concentration (Fei et al., 2013). Therefore, MOFs is a kind of novelnanomaterials that can efficiently remove Cr(VI) from groundwater. Due to the abundant functional groups and reaction sites, modified nZVI has attracted wide attention (Gueye et al., 2016; Vilardi et al., 2017, 2018b). Specifically, graphene supported nZVI (G-nZVI, 162 mg/g) (Jabeen et al., 2011), multi-walled carbon nanotube modified nZVI (MWCNT-nZVI, 200 mg/g) (Lv et al., 2011), and cellulose-supported nZVI (562.8 mg/g) (Sharma et al., 2015) showed highly efficient removal of Cr(VI). In addition, the activated hydroxyls on the surface enable MXenes to own the characteristics of fast sorption rate, large sorption capacity, high sensitivity and reversible sorption. For the first time, 2D Ti3C2Tx was demonstrated to be a novel promising material for removing Cr(VI) (~250 mg/g) and the residual concentration was less than 5 ppb (<0.05 ppm) (Ying et al., 2015). Interestingly, these reductive 2D Ti3C2Tx materials usually achieve the purpose of removing pollutants through transforming them into low oxidation states, which greatly expand the potential application of 2D Ti3C2Tx materials in actual pollution treatment. 3.2. Arsenic (As) sorption Arsenic is one of the most poisonous environmental pollutants, and often exists in the forms of As(V) and As(III) (Cheng et al., 2016; Li et al., 2018a; Zou et al., 2016d). The first MOF used as an adsorbent for As(V) removal was Fe-BTC polymer with relatively high stability and excellent sorption characteristic, which was more than 6 times higher than iron oxide nanoparticles and 36 times higher than commercial iron oxide powders (Li et al., 2018a). Subsequently, the MIL families such as MIL-53(Fe) (21.3 mg/g) (Vu et al., 2014), MIL-53(Al) (105.6 mg/g) (Li et al., 2014) and MIL-100(Fe) (100.0 mg/g) (Cai et al., 2016) also showed excellent sorption performance for As(V). Compared with As(V), As(III) is more toxic and difficult to remove, and thereby mesoporous CoFe2O4@MIL-100(Fe) hybrid magnetic nanoparticles (MNPs) with large nanoscale size and mesoporous was also applied to adsorb 0.1 mg/L As(III), and the maximum sorption capacity reached 143.6 mg/g within 2 min (Yang and Yin, 2017), which further demonstrate the huge potential of MOFs for removing heavy metal ions pollution. Additionally, Choi's group synthesized the nZVI for the removal of As(III) and As(V) (Kanel et al., 2006, 2005). Although the assynthesized nZVI reached the sorption equilibrium within minutes, the low sorption capacities for As(III) (3.5 mg/g) (Kanel et al., 2005) and As(V) (1.0 mg/g) (Kanel et al., 2006) restricted its wide application. Thus, it is of great significance to improve the sorption ability of As(III) and As(V) through modifying nZVI. Zhu et al. (2009) investigated the removal capacity of As(III) and As(V) onto activated carbon loaded nZVI (named as nZVI/AC), and found that the adsorbent was effective for the removal of As(III) (~18.2 mg/g) and As(V) (~12.0 mg/g) with relatively fast kinetic compared with other As(III) and As(V) adsorbents. However, such sorption capacity is still not ideal, more researches should be devoted to improving the sorption performance of nZVI through modification in the future. 3.3. Lead (Pb) sorption Lead is non-degradable, toxic, widespread and accumulative in living organisms, so the removal of lead contaminants is of great significance. Luo et al. (2015a) firstly revealed the ED-MIL-101 with an appropriate amount of ethylene-diamine (ED) to selectively

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remove Pb(II) and Cu(II). The Pb(II) sorption capacity of ED-MIL-101 (81.09 mg/g) was more than five times higher than MIL-101 (15.78 mg/g). Li et al. (2015c) found that nZVI@Mg(OH)2 nanocomposite can remove 94% of Pb(II) within 15 min, and the maximum sorption capacity was 1986 mg/g. The synergistic removal processes of Pb(II) involved three paths: (1) Mg(OH)2 adsorb Pb(II), usually accompanied with ion exchange reaction; (2) nZVI reduced Pb2þ to Pb0; (3) Pb(OH)2 precipitation was formed. The multirole of Mg(OH)2 or nZVI and the synergistic effect of nZVI@Mg(OH)2 led to the exceptional sorption performance for Pb(II). MXenes surfaces are usually attached with functional groups such as eO, eF and eOH, which significantly affect its sorption capacity. Peng et al. (2014) reported that 2D alk-MXene (Ti3C2(OH/ ONa)xF2-x) material exhibited preferential Pb(II) sorption behavior when competitive cations (Ca(II)/Mg(II)) co-existed at high levels. The alk-MXene presented an ultrahigh removal capacity within 120 s at 4500 kg water per alk-MXene, and the contents of Pb(II) in treated wastewater were far below than the water standard (10 mg/ L), which made MXenes promising in effectively purify drinking water. Subsequently, Wang's group (Hu et al., 2015) successively synthesized g-C3N4 and g-C3N4/b-CD to eliminate Pb(II), due to large specific surface areas, abundant ¼ NH and eNH groups, gC3N4 owned a maximum sorption capacity of 65.6 mg/g for Pb(II), higher than activated carbon (21.8 mg/g), iron oxide (36 mg/g), and sodium bentonite (47.8 mg/g). And the higher sorption capacity of g-C3N4/b-CD (100.5 mg/g) was attributed to its more oxygencontaining functional groups than g-C3N4. Therefore, g-C3N4 is very promising for pretreatment of pollutants in future pollution management. 3.4. Other heavy metal sorption In addition to the common heavy metal ions mentioned above, heavy metal ions such as Hg(II), U(VI), Cd(II), and Ni(II) are nonbiodegradable and highly toxic though at low concentration, which are easy to accumulate in environment and organisms. Therefore, effective removal of these toxic heavy metal ions is also essential for human and environmental protection. As a highly toxic heavy metal ion, Hg(II) is a widespread environmental pollutant from coal and automobile exhaust, and mainly found in surface water (Luo et al., 2015b). Recently, amide- and hydroxyl-functionalized MOF [Zn(hip)(L)(DMF)(H2O)] showed a higher capacity (up to 278 mg/g) for Hg(II), and the removal efficiency of 66.5% can be achieved under 2 ppb Hg(II) concentration (Luo et al., 2015c). Owing to a coupled Fe-M(II) redox reaction, Azoll-NaOH-nZVI showed potential for capture and removal of Hg(II) (~459.3 mg/g, within 20min), and the reusability analysis revealed that the AzollaeOHenZVI still owned high sorption efficiency after seventh cycles (Arshadi et al., 2017). Liu et al. (2014) further synthesized pumice modified nZVI (P-NZVI) with higher capacity for Hg(II) sorption (332.4 mg/g), which was ascribed to the reduction reaction and physical sorption. These results indicate that P-NZVI was effective for metal ions remediation. Due to the high toxic, a large number of novel-nanomaterials have been reported for U(VI) sorption to date. For example, the first example of MOFs (MOF-2 and MOF-3) applied as novel adsorbents to extract U(VI) from water and simulated seawater, and the UiO-68-type MOFs maintained efficient sorption at pH 2.5, which was attributed to the possible coordination mode of the phosphoryl urea moiety with U(VI) (Carboni et al., 2013). In addition, the sorption, separation, and enrichment of U(VI) onto nZVI based materials are also the hot research topic. Xu et al. (2014) studied the separation and enrichment of U(VI) on montmorillonite-supported nZVI (M-nZVI) composite from

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aqueous solutions. Compared with the primitive nZVI, the removal rate on M-nZVI reached 97.8%, which indicated that the sorption performance was improved significantly. To further explore the sorption capacity of novel-nanomaterials for other heavy metal ions (Cd(II), Ni(II)), cubic mesoporous graphitic carbon nitride (c-mpg-C3N4) has recently been synthesized by Lee et al. (2010) for absorbing metal ions (e.g., Co(II), Mg(II), Ni(II)). And the surface functionalities (e.g., eNH2, eNHe, ¼N-) endow c-mpg-C3N4 with excellent sorption capacity for metal ions through chelation or redox reaction. In addition, the FJI-H9 MOFs showed a high sorption performance (225 mg/g) due to the unusual synergy between active and confined cavity. And Cd(II) sorption on FJI-H9 was not affected by other background cations (Hg(II), Cd(II), Hg(II), Ca(II), Mg(II), Co(II), Ni(II), Mn(II), Zn(II), Fe(II), and Pb(II)) (Xue et al., 2016). Therefore, it is further verified that the novel-nanomaterials are effective in removing heavy metal ions pollution in environment. 4. Effect of environmental conditions Generally, the reaction mechanisms between novelnanomaterials and heavy metal ions can be demonstrated by batch sorption experiments in surrounding environments such as contact time, temperature, solution pH, and dosage of adsorbent etc. Environment changes can alter the properties of materials and affect the migration efficiency or bioavailability of heavy metal ions. Therefore, evaluating the environmental effect of pollutants onto novel-nanomaterials is crucial in practical applications.

uniformly covered by single molecular layer. Besides, the maximum sorption amount of heavy metal ions can be obtained through Langmuir isotherm simulation. Distinguishingly, Shahzad et al. (2017) investigated Cu(II) sorption on Ti3C2Tx through fitting three isotherm models and demonstrated that the sorption was most suitable for Freundlich isotherm, which assumed that the heterogeneous surface contained terminal/functional groups to offer stable binding sites for removing Cu(II). The maximum sorption capacity derived from Langmuir model reached 78.45 mg/g (Fig. 2B), illustrating the promising potential for removing toxic metals from wastewater. So as to further confirm the reaction process between the novel-nanomaterials and heavy metal ions, gibbs free energy (DG0), enthalpy change (DH0) and entropy change (DS0) are obtained by fitting the temperature-dependent sorption isotherms. The sorption process of Ba(II) on the bare-nZVI surface fitted D-R isotherm, implying that the removal process is chemical and physical sorption, and the former is favored (Celebi et al., 2007). Xiong et al. (2014) demonstrated that Cr(VI) sorption on g-Fe2O3/ C@HKUST-1 followed Langmuir sorption model, and the sorption process was endothermic, suggesting that higher temperature was favorable for sorption. Besides, the negative DG0 value and positive DS0 value indicated that the sorption process was spontaneous, irreversible and favored sorption stability. To better understand the thermodynamic behaviors of heavy metal ions binging onto different nano-sorbents, the sorption isotherms and Langmuir sorption capacities are sorted out and compared in Table S5.

4.1. Effect of contact time

4.3. Effect of solution pH

To examine the reaction rate and practical application potential for heavy metal ions’ sorption on novel-nanomaterials, pseudofirst-order, pseudo-second-order, Zeldowitsch, Elovich, and Lagergren kinetic models are applied to explore the effect of contact time on metal ion sorption (Kumar et al., 2018a,b; Nithya et al., 2017). Commonly, the heavy metal ion sorption rate onto novelnanomaterials ranges from fast to slow with time, and finally reaches the sorption equilibrium. The time to reach sorption equilibrium is usually uncertain for different adsorbent materials, and mostly accords with the pseudo-second-order kinetic model, demonstrating the chemical adsorption is the dominant mechanism. Specifically, Lv et al. (2012)Lv et al. (2012) found that Cr(VI) removal on nZVI-Fe3O4 fitted well with pseudo-second-order model, and the Cr(VI) sorption could run up to 96.4% within 2 h. Similarly, the time-dependent sorption of Cu(II), Cd(II), Co(II), and Ni(II) onto ZIF-8 in Fig. 2A all showed an initial rapid sorption process followed by a slower elimination process and reached sorption equilibrium within 20 min. Besides, the better fitting pseudo-second-order model demonstrated that the sorption behaviors were chemisorption (Zhao et al., 2015). The rapid sorption equilibrium and superior sorption capacity made ZIF-8 a promising material for selective sorption of Cu(II).

Solution pH not only affects the surface charge of as-prepared materials, but also changes the distribution of heavy metal ions, thus leading to the electrostatic interaction between novelnanomaterials and pollutants. Interestingly, when pH < pHPZC (point of zero charge), the adsorbents exhibit positive charge, while they own negative charge at pH > pHPZC. Generally speaking, under the extreme pH conditions, the removal rate of heavy metal ions onto novel-nanomaterials tends to be decreased sharply (Bhowmick et al., 2014; Jabeen et al., 2011) which is mainly ascribed to the strong electrostatic repulsion produced by the adsorbate and adsorbent. While under the neutral pH conditions, strong surface coordination, electrostatic attraction, and co-precipitation result in the high removal rate. According to Zou et al. (2016e) the Pb(II) removal capacities of g-C3N4 and g-C3N4/b-CD were associated with solution pH at pH < 7.0, and unassociated with solution pH at pH > 7.0. Besides, the sorption rate increased slowly in the range of pH 2.0e4.0 and increased rapidly in the range of pH 4.0e7.0, and finally, the removal capacity maintained at a high level. Commonly, at pH < 7.0, the presence of Pb2þ and Pb(OH)þ led to surface complexation and electrostatic attraction between Pb(II) and hydroxyl groups or heptazine rings. Furthermore, the presence of Pb3(OH)2þ 4 , Pb(OH)2(aq), and Pb(OH)3 at pH > 7.0, suggesting the Pb(II) removal mechanism was surface complexation (Zou et al., 2016e). Besides, the pH values can also alter the reaction rate of novel-nanomaterials with heavy metal ions. Fig. 2C indicates that the elimination rates of Hg(II) onto pumice supported nZVI (P-nZVI) increase as pH from 3.11 to 8.13, which may due to the electrostatic attraction between Hg(II) and the negatively charged P-nZVI. Fig. 2D shows that the Cr(VI) removal rate decreases as pH increases, which may owe to the electrostatic attraction between HCrO 4 (1.0 < pH < 6.0) and positively charged P-nZVI, and electrostatic repulsion between CrO2 4 (pH > 6.0) and negatively charged P-nZVI (Liu et al., 2014).

4.2. Effect of temperature To our knowledge, the temperature can alter the energy of reaction system, removal rate and sorption quantity, and the sorption isotherms are curve that describe the influence of temperature about sorption behaviors. The common sorption isotherms include Langmuir model, Freundlich model and Dubinin Redushcke-vich model etc. (Devi et al., 2016). Notably, the heavy metal ion sorption onto novel-nanomaterials are mainly in line with the Langmuir model, showing that the removal behaviors on nanomaterials are

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Fig. 2. (A) Effect of contact time on the adsorption of single metal ion onto ZIF-8 nanocrystals (Zhao et al., 2015); (B) Sorption isotherm of Cu(II) onto Ti3C2Tx nanosheets (Shahzad et al., 2017); Effect of pH on removing 60 mg/L Hg(II) (C) and 60 mg/L Cr(VI) (D) by P-nZVI (Liu et al., 2014); Effect of MXenes dosage on sorption capacity (E) and removal efficiency (F) of Ba(II) (Fard et al., 2017).

4.4. Effect of adsorbent dosage Dosage of adsorbent is also a key factor that affects the removal capacities, and many publications are using an optimum dosage of adsorbent to improve the removal efficiency of heavy metal ions. To determine the correlation of Cr(VI) sorption with adsorbent dosage, Fu et al. (2015) applied various dosages (0.05e3.20 g) of S-nZVI to adsorb Cr(VI), and the results indicated that as the adsorbent dosage increased from 0.05 to 3.20 g, the removal efficiency of Cr(VI) increased significantly from 45.1% to 99.2%. In another study, due to the more available active sites for surface complexation or ion exchange as solid content increases, the g-C3N4 shows that the removal percentages increases from ~8% to ~35% for Pb(II) as the content of g-C3N4 increases from 0.02 to 0.9 g/L (Hu et al., 2015). Fig. 2E shows the effect of MXenes dosage for removing Ba(II) at different pH values. With the increases of MXenes content, the removal efficiency is improved with the increase of active sites. However, the reaction sites on MXenes surface are occupied with the increases of the MXenes dose, which lead to the sorption capacity decreases (Fard et al., 2017) (Fig. 2F). To sum up, the effect of environmental conditions on the sorption of novel-nanomaterials for heavy metal ions has been fully

studied, and we find the removal capacity has been improved with the increase of time and temperature. Under neutral pH condition, the novel-nanomaterials own higher removal efficiency for heavy metal ions due to surface coordination, electrostatic sorption, and precipitates. However, the water environment conditions in practical applications are usually very complex, which result in the infiltration and migration of heavy metal ions in aqueous environment are complex and non-linear. Therefore, the sorption performances and potential reaction mechanisms of pollutants onto novel-nanomaterials under different environmental conditions should be fully investigated to provide references for further studies on the sorption behavior of water interface.

4.5. Interaction mechanism between novel-nanomaterials and heavy metal ions Mechanistic studies of heavy metal ions’ sorption by novelnanomaterials are critical for evaluating the treatment efficiency of heavy metal contaminated wastewater, which are beneficial for choosing the optimal removal conditions. Up to now, various studies have focused on exploring the possible interaction mechanism of heavy metal ions on novel-nanomaterials, and the

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common reaction mechanisms include adsorption, redox, aggregation, ion exchange, and precipitation (Li et al., 2018a; Yu et al., 2018c; Zou et al., 2016d). Particularly, adsorption, reduction and oxidation process play the most critical role. 4.6. Adsorption mechanism Adsorption technique has become one of the most common and effective methods for removing heavy metal contamination ascribe to sufficient oxygen-containing functional groups (Yu et al., 2016a, 2016b). To our knowledge, the abundant reaction sites and functional groups on novel-nanomaterials surfaces have led them as typical adsorbents for efficiently removing heavy metal ions, and the possible mechanism could attribute to adsorption reaction. To determine the main adsorption mechanism, advanced spectroscopy analysis, surface complexation models and theoretical calculations have been applied to investigate the adsorption behavior. The adsorption mechanisms commonly include: ion exchange, electrostatic adsorption, hydrogen bonding, specific surface bonding, etc. Apparently, spectral analysis technology including Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and X-ray absorption fine structure (XAFS) can detect the chemical state and structure of surface complexation. Recently studied the interaction mechanism of CoFe2O4@MIL100(Fe) for simultaneously removing As(III) and As(V), and found that the zeta potential of CoFe2O4@MIL-100(Fe) decreased rapidly after eliminating As(V), which was direct evidence for the formation of specific adsorption and inner-sphere surface complexes on material surface. Besides, FTIR results (Fig. 3A) confirm that the

eOH groups of CoFe2O4@MIL-100(Fe) are replaced by the deprotonated as in the hydroxyl exchange process. Owning to the hydroxyl substitution with the formation of FeeOeAs bonds and hydrogen bonding, the As(III) sorption is higher than As(V), which is a monolayer reaction process. Han et al. (2016) applied adsorbent mixed from acid-washed ZVI and zero-valent aluminum (ZVAl) to remove heavy metal ions (e.g., Cr(VI), Cu(II), Cd(II), Ni(II) and Zn(II)) from groundwater. XPS analysis (Fig. 3B) demonstrated the appearance of Zn, Cr, and Cd, while the peaks of Cu and Ni could not be found, which may be attributed to the heavy metal precipitates and hydroxide precipitates. To further determine the sorption mechanisms of heavy metal ions onto novel-nanomaterials, XAFS analysis has attracted extensive research interests (Yu et al., 2017d, 2018b). Chen et al. (2018) applied XANES and EXAFS techniques (Fig. 3C) to explore the interaction mechanisms between Cr(VI) and nZVI. The XANES and EXAFS spectra analysis demonstrated that Cr(VI) removal on nZVI was achieved through adsorption and reduction reaction. For Cr(VI)-reacted MCM-41, the peak for CreO at 1.68 Å was better accord with Cr(VI)eO bond distance of 1.66 Å. For Cr(VI)-reacted nZVI sample, Cr was coordinated with ~4.0 O at RCr-O ~1.97 Å and ~1.9 Cr/Fe at RCr-Cr/Fe ~ 3.01 Å, indicating that adsorption and reduction played an equally important role in Cr(VI) immobilization. Additionally, the spectrum of Cr(VI)-reacted nZVI/MCM sample illustrated that MCM-41 could act as a scavenger to reduce the accumulation of the insoluble products on nZVI surface. Yan et al. (2012) deduced that the reduction process of As(V) consisted of two stages of transformation. The first stage corresponded to AseO bonds fractured on particle surface, and the thin oxide layer around the nanoparticles could further reduction and diffusion of As(III)

Fig. 3. (A) FTIR spectra of (a) CoFe2O4 and (b) CoFe2O4@MIL-100(Fe) (Yang, & Yin, 2017); (B) XPS spectra for original acid-washed 80 g/40 g ZVI/ZVAl mixture, and acid-washed 80 g/ 40 g ZVI/ZVAl mixture used in PRBs for 20 d (Han et al., 2016); (C) Cr K-edge XANES spectra (a), Cr K-edge EXAFS spectra (b) of Cr-reacted samples (Chen et al., 2018).

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and As(V), leading to the formation of intermetallic phase between Fe(0) core and As(III) or As(V) in the second stage. The surface complexation models (SCMs) can simulate the surface characteristics, adsorption mechanism and morphological characteristics of the solid-water interface. Thereby, four SCMs, i.e., constant capacitance model (CCM), diffusion layer model (DLM), basic stern model (BSM), and triple layer model (TLM) are widely used to simulate the adsorption mechanism of novelnanomaterials (Gu et al., 2018b) (Fig. 4A). Among them, the DLM model is widely used to explore the interaction between nanomaterials and heavy metal ions due to simple operation and high accuracy. Boparai et al. (2013) explored the interaction mechanisms between Cd(II) and nZVI by using DLM, CCM, and TLM, and found that nZVI were occupied with abundant hydroxyl groups, which can efficiently eliminate Cd(II) from aqueous solutions. Three models revealed that Cd(II) formed inner-sphere complexes with the formation of ≡SOCdþ, ≡SOCdOH, and ≡(SO)2Cd species. The main species at pH 6.0e10.0 were formed due to surface complexation of Cd(II) with nZVI surface, and the surface reactions are expressed below: Cd2þþ≡SOH#≡SOCdþþHþ

(1)

615

Cd2þþ2(≡SOH)#≡(SO)2Cdþ2Hþ

(2)

Cd2þþ≡SOH þ H2O#≡SOCdOHþ2Hþ

(3)

Recently, DFT has been used to investigate the microstructure of novel-nanomaterials interaction with heavy metal ions. Guo et al. (2015) illustrated that Pb2þ could occupy the center of a hydroxyl potential trap for replacing Hþ of Ti3C2(OH)2 to obtain Ti3C2(O2H22mPbm) structure, and the hydroxyl potential trap included eight hydroxyl groups and two oxygen atoms (the hydroxyls lose H). Lazar and Otyepka (2012) used DFT theory to calculate the adsorption and chemical reactions of single water molecules with Fe(100) and Fe(111), and the adsorbed water molecules could be transferred to H þ OH (HeFeeOH) at the activation potentials of 15.7 and 21.1 kcaL/mol, respectively, which promoted the adsorption process (Fig. 4B). Similarly, the adsorption mechanisms between heavy metal ions and g-C3N4 are confirmed by Lee et al. (2010) and Hu et al. (2015). 4.7. Oxidation and reduction mechanism 4.7.1. Reduction mechanism Generally, two main mechanisms for reducing heavy metal ions on novel-nanomaterials are as follows: (1) novel-nanomaterials

Fig. 4. (A) Schematic diagram of a solid-liquid interface for the surface complex models (Gu et al., 2018b); (B) Dissociation of water molecule on the Fe(100) surface calculated PW91 density functional (black curve, dots) and HSE06 hybrid functional (red curve, dashed lines). The arrows show the heights of energy barriers along the reaction path (Lazar, & Otyepka, 2012); (C) Reaction mechanisms and process of Cr(VI) onto MP@ZIF-8 (Zhu et al., 2017); (D) Schematic illustration of the As(III) removal process on MnO2@ZIF-8 NWs. The blue part represents ZIF-8 particles, and the red particles stand for As(III) (Jian, et al., 2016). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

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directly reduce heavy metal ions; (2) heavy metal ions adsorb onto novel-nanomaterials, and then cause valence changes on the surface of nanomaterials to gradually reduce adsorbed heavy metal ions from high valent to low valent. Zhu et al. (2017) reported that MP@ZIF-8 showed possible reduction process of Cr(VI) to Cr(III) with the reduction of nitrogen atom groups on ZIF-8 and PDA (Fig. 4C), and the main reduction reaction included the following steps: (I) the nitrogen atom groups of ZIF-8 partially reduced Cr(VI) to Cr(III), and (II) Cr(VI) correspondingly retarded diffusion to PDA, and converted Cr(VI) to less toxic Cr(III) by reducing amine groups. Luo et al. (2014) investigated the nZVI immobilized by alginate microcapsules for the uptake of Pb(II), and Pb(II) was reduced to Pb(0) was also found, which may be attributed to the reduction reaction:

The montmorillonite-supported nZVI (Mt-nZVI) has been proved to own the tendency of oxidizing As(III) to As(V), which demonstrates the oxidation mechanism between novelnanomaterials and heavy metal ions (Bhowmick et al., 2014). In the water environment, nZVI can spontaneously undergo a series of reactions to produce oxidizing intermediate, and thereby oxidizing the hard-removal As(III) to easy-removal As(V) so as to efficiently remove As(III). On the one hand, Fe(0) spontaneously reacts with water and oxygen to form Fe2þ and Fe3þ, which further forms iron oxides, hydroxides, and oxy-hydroxides depending on reaction conditions. On the other hand, Fe2þ formed by the oxidation of Fe(0) can also react with water and O2 to form oxidized intermediates (e.g., H2O2, HO, O 2 ) in solution, which are expressed as follows:

2Fe0þ3 Pb(C2H3O2)2þ4H2O/3 Pb0þ2FeOOHþ6CH3COOH

2Fe(0)þO2þ2H2O/2Fe2þþ4OH

(10)

(4)

Furthermore, based on the XPS analysis, Li et al. (2015c) explored the removal mechanisms of Cr(VI) on nZVI/rGOs, and found that Fe(0) could be oxidized to Fe(II) and Fe(III), and can reduce Cr(VI) to Cr(III), finally to form a surface complex on nZVI/ rGOs surface. The possible reactions were as follows:

Fe2þþO2þ2H2O/2Fe3þþ4HO

(11)

Fe2þþO2/Fe3þþO 2

(12)

þ 3þ  Fe2þþ O 2 þ2H /Fe þHO þ HO

(13)

0 þ 2þ 3þ 2HCrO 4 þ3Fe þ14H /3Fe þ2Cr þ8H2O

(5)

þ 3þ Fe2þþ O 2 þ2H /2Fe þH2O2

(14)

2þ þ 3þ 3þ HCrO 4 þ3Fe þ7H /3Fe þCr þ2H2O

(6)

(1-x)Fe3þþxCr3þþ3H2O/(CrxFe1-x)(OH)3Yþ3Hþ

(7)

(1-x)Fe3þþxCr3þþ2H2O/CrxFe1-xOOHYþ3Hþ

(8)

In practical environmental applications, the interaction mechanisms between novel-nanomaterials and heavy metal ions are often multi-factorial. To fully evaluate the migration and transformation behavior of heavy metal ions in nature, multiple analysis techniques should be applied to the systematical research.

For a deep understanding of the Cr(VI) removal mechanism onto novel-nanomaterials, Ying et al. (2015) characterized the bonding states of C 1s, Ti 2p, O 1s, and Cr 2p of 2-D Ti3C2Tx nanosheets through XPS technology, and found that after reaction with Cr(VI), the Cr 2p peak could be attributed to Cr(III) rather than Cr(VI). These results illustrated that Cr(VI) was converted to Cr(III) and finally led to strong interaction with TieO, which endowed Ti3C2Tx excellent Cr(VI) removal capacity (~250 mg/g). With the further research of the possible reduction behavior onto novel-nanomaterials, EXAFS analysis has also been used for the determination of molecular level microstructures. Li et al. (2015b) found that the positively charged nZVI/Al-bent could reductively transform and remove Se(VI) into a more insoluble Se(II) from wastewater. The EXAFS analysis indicated that SeeO bond was formed from Se(II) rather than Se(VI) due to the reduction reaction.

4.7.2. Oxidation mechanism In some exceptional environmental systems, besides the experimental studies of the reduction process for removing heavy metal ions from aqueous solutions, the oxidation process is also crucial react mechanism. Obviously, As(III) is more toxic and more difficult to eliminate than As(V). In order to fully understand the removal mechanism of As(III), firstly synthesized MnO2@ZIF-8 NWs to achieve concurrent oxidation and adsorptive removal of As(III). To further understand the oxidation mechanism, XPS was applied to determine the elemental species of MnO2@ZIF-8 NWs before or after As(III) sorption, which indicated that the oxidation of As(III) to As(V) occurred on MnO2 core (Fig. 4D). Similar research of removing As(III) has also been conducted by Zhang et al. (2007). The reaction could briefly represent as follows: 2MnO2þH3AsO3þH2O ¼ 2MnOOH þ H3AsO4

(9)

5. Possibilities for commercial applications Due to excellent surface properties, novel-nanomaterials can efficiently remove heavy metal ions from wastewater. However, there are still significant differences between industrial production and experimental environment. The cost, toxicity, and large scale applications of novel-nanomaterials will affect their commercial applications, which should be further discussed. To our knowledge, MOFs materials are synthesized through the coordination of expensive metal precursors and organic ligand, which leads to the high cost of MOFs in wastewater treatment. Therefore, we should choose a kind of cheap organic solvent as the organic ligand and select cheap metal salts as metal precursors. nZVI is mainly synthesized from cheap ferric salt and the synthesis process is simple, which make nZVI a kind of very economical adsorbent. The harsh synthesis conditions of MXenes lead to high preparation costs. Therefore, using ball mill instead of etching method can reduce cost, and deeper investigations on the preparation technology of MXenes are needed. The raw material of gC3N4 is cheap and the synthesis method is very simple, making gC3N4 a very affordable adsorbent. While in environmental pollution cleanup, novelnanomaterials inevitably enter the ecological environment, and the features of small size, good fluidity, high reactivity, and strong penetration may lead to potential toxic risks in environment. MOFs materials are formed by the coordination of metal precursors and organic ligand, and studies have illustrated that the leached metal ions directly affect the toxicity of MOFs (Ruyra et al., 2015; Sajid, 2016). Baati et al. (2013) evaluated the toxicity of three iron(III) carboxylate MOFs (MIL-88A, MIL-88Be4CH3 and MIL100) with high dosage, found that all experimental parameters agreed with a low toxicity. Therefore, non-toxic metal salts such iron salts should be selected as metal precursors to avoid the toxicity. nZVI is mainly synthesized from ferric salt and the iron is

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usually non-toxic. However, when it reaches to nanoscale, the change of iron's properties will lead to toxicity to organisms. Li et al. (2009) tested the antioxidant enzyme activity and lipid peroxidation, confirming that nZVI interfered with the antioxidant system of embryos and adult fish. Chen et al. (2011) found that natural organic matter can greatly reduce the toxicity of nZVI. The above studies mainly focus on the toxicity of bare nZVI, but these common nano-iron particles are easy to agglomerate and oxidize, own poor dispersion stability, thus limiting further applications. In fact, the modified nZVI is more commonly used in environmental remediation. Phenrat et al. (2009) investigated the neurotoxic effects of different types of nZVI and found that fresh nZVI owned the greatest toxicity while surface modified nZVI had the least toxicity. MXenes are commonly prepared through selectively etching of the A element layers using appropriate etching agents (e.g., HF, LiF þ HCl, NH4HF2, etc.) from the MAX phases at room temperature. It is well known that, HF is highly corrosive, which not only increase the risk of the preparation, the residual HF are also toxic to the environment (Dutta et al., 2017). Therefore, it is of great significance to develop a fluorine-free MXene synthetic method for commercial application. The gC3N4 is mainly synthesized from non-toxic urea and without toxicity, so it is usually widely used as an environmental friendly material. Due to excellent properties, novel-nanomaterials have been widely used for removing pollutants in column, pilot and full-scale. Rapti et al. (2016, 2017) found that the hybridization materials of amino MOFs and alginate (MOR-1-HA and MOR-2-HA) can be used as stationary phase, mixed with sand, in an anion-exchange column, and showed good sorption performance for Cr(VI) with reproducibility. The high sorption capacity further illustrated their potential application in treating wastewater. As an effective adsorbent, nZVI has already been used in field application for metal ion removal. Puls et al. (1999) reported that the continuous monitoring of the suspended zero-valent ferric permeation reaction grid effectively restored the local organochlorine and chromate pollution after two years of continuous work. In Europe, nZVI has been used to remove chlorinated hydrocarbons, and the pilot test results showed that the concentration of chlorinated hydrocarbons decreased by 40e80%. Full-scale remediation showed that the concentration of chlorinated hydrocarbons had dropped to 20% of the initial value (Mueller et al., 2012). According news reports, the electroplating wastewater was treated by nZVI in supercharger plant of China. And the pH (6e8), Cr(VI) (0.09 mg/L) and Zn(II) (1.53 mg/L) concnetrations of the treated wastewater all met the national discharge standards, which demonstrated the practical application of nZVI in removing heavy metal ions. At present, although the practical application of g-C3N4 has not been reported, researchers have conducted simulated water experiments. Thomas and Sandhyarani (2015) developed the g-C3N4/TiO2 with the surface area of 147 m2/g and found it can remove 81% of Cr(VI) within 2 min in simulated water, which provided further support for the actual wastewater treatment. Due to the harsh synthesis process and conditions, MXenes lack relevant reports on actual wastewater treatment, which requires further research input from researchers in the future. In summary, novel-nanomaterials can basically achieve the requirements of low cost and low toxicity through optimizing experimental conditions. And the applications of nanomaterials for removing heavy metal ions from wastewater in column, pilot and full-scale have been basically realized. In near future, with the development of technology, the synthesis of nanomaterials will be become easy in large scale at low price. Based on these results, commercial scale applications of novel-nanomaterials will become possible as long as extensively study.

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6. Conclusions and future perspectives To our knowledge, novel-nanomaterials with large specific surface area, abundant surface active sites, high removal rate, and good utilization, have shown excellent sorption capacity for heavy metal ions. Batch experiment results illustrated that the optimal pH values of the sorption process were 4.0e7.0, which accord with the aquatic environment range (pH 5.0e9.0). Therefore, novelnanomaterials have great potential for removing pollutants and the in-situ restoration of the environment. Although the novel-nanomaterials own many advantages, they still face various challenges in removing heavy metal contaminated wastewater, such as: (I) The nanosize of novel-nanomaterials made it easy to aggregate and then cause blocking and fouling problems in practical applications, which will reduce the sorption capacity and make secondary pollution; (II) The surface properties of some novel-nanomaterials are unstable and easy to oxidize; (III) Most nanomaterials are made of chemical methods, which may lead to toxic effects; (IV) Large-scale industrial production on wastewater treatment is limited to biological incompatibility and high cost; (V) Low selectivity of metal ions from complicated conditions, etc. According to the research status of removing heavy metal ions through novel-nanomaterials at home and abroad, a long way still to go to further overcome its drawbacks and achieve the goals of commercial scale applications: (1) Green methods to synthesize novel-nanomaterials should be extensively explored. To our knowledge, the agricultural wastes (e.g., bagasse pith, maize cob, coconut shell, rice husk, peanut shell etc.), mycelial microorganism (Dictyophora indusiata and Bacillus), mineral slag (coal and fly ash) and clay materials have been applied for synthesizing nanosorbents due to non-toxic and economical. However, it has not been widely used, and the goals of large-scale utilization should be achieved in future. (2) To improve the stability, sorption efficiency, and selectivity sorption capacity for heavy metal ions under complicated conditions by optimizing the synthesis scheme, surface properties and subsequent geometric arrangement of nanoparticles. (3) It is very important to recycle the novel-nanomaterials through desorption technology to achieve cost-effectiveness. At present, the researches on desorption of nano-adsorbents are lack and it is urgent to find an efficient desorption method that can recycle the active form of nano-adsorbents. As a novel field, the nanomaterials own broad applications prospects in wastewater treatment. Therefore, as long as we stick to research and overcome the above difficulties, novel-nanomaterials will be widely applied in commercial production in the future. Acknowledgments This work was supported by National Key Research and Development Program of China (2017YFA0207002), the National Natural Science Foundation of China (21836001; 21577032, 21607042), the Fundamental Research Funds for the Central Universities (2018ZD11, 2018MS114). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.envpol.2018.12.076. References Arshadi, M., Abdolmaleki, M., Mousavinia, F., Foroughifard, S., Karimzadeh, A., 2017. Nano modification of NZVI with an aquatic plant azolla filiculoides to remove Pb(II) and Hg(II) from water: aging time and mechanism study. J. Colloid Interface Sci. 486, 296e308. Baati, T., Njim, L., Neffati, F., Kerkeni, A., Bouttemi, M., Gref, R., Najjar, M.,

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