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Chemical Engineering Journal

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Review

Boron nitride-based materials for the removal of pollutants from aqueoussolutions: A review

Shujun Yua, Xiangxue Wanga,b,⁎, Hongwei Panga, Rui Zhanga, Wencheng Songa, Dong Fub,Tasawar Hayatc, Xiangke Wanga,c,⁎

a College of Environmental Science and Engineering, North China Electric Power University, Beijing 102206, PR Chinab Department of Environmental Science and Engineering, North China Electric Power University, Baoding 071003, PR Chinac NAAM Research Group, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia

G R A P H I C A L A B S T R A C T

A R T I C L E I N F O

Keywords:Boron nitrideOrganic pollutantsInorganic pollutantsInteraction mechanismReview

A B S T R A C T

Water pollution, a worldwide issue for the human society, has raised global concerns on environmental sus-tainability, calling for high-performance materials in effective pollution treatments. Boron nitride (BN) with astructure similar to graphene possesses many extraordinary properties such as high surface areas, high oxidi-zation resistance at high temperature, and high chemical stability. This review presents the outstanding removalpercentage and environmental restoration of BN-based nanomaterials for the elimination of various pollutantsfrom the last ten years. Notably, recent advances in the removal of organic/inorganic pollutants and interactionmechanism are outlined. BN-based materials can not only preferably remove contaminants, but also can bedirectly regenerated by burning in air. The BN-based materials have satisfactory sorption capacities for inorganicpollutants (e.g. heavy metal ions) and organic pollutants (e.g. dyes and pharmaceutical molecules). The inter-action mechanisms between pollutants and BN-based materials are mainly surface complexation, π-π stacking,and electrostatic interactions. This paper is beneficial to further comprehend the interactions of pollutants withBN-based materials, which is also helpful for the improvement of BN-based materials and potential areas forfuture applications in environment remediation.

http://dx.doi.org/10.1016/j.cej.2017.09.163Received 17 August 2017; Received in revised form 23 September 2017; Accepted 25 September 2017

⁎ Corresponding authors at: College of Environmental Science and Engineering, North China Electric Power University, Beijing 102206, PR China.E-mail addresses: wang730304@fg.com (X. Wang), xkwang@ncepu.edu.cn (X. Wang).

Chemical Engineering Journal 333 (2018) 343–360

Available online 28 September 20171385-8947/ © 2017 Elsevier B.V. All rights reserved.

MARK

1. Introduction

Water resource is the fundamental basis for vital activities of livingbeings on the earth. Unfortunately, the precious resource has beengreatly polluted due to the development of population, agriculturalactivities, industrialization, and other geological and environmentalchanges [1,2]. A great deal of organic, inorganic, and biological pol-lutants were entered into aquatic environment [3–6]. Some of themwith high mobility would move into soils and are accumulated inplants, ultimately becoming a part of animals and human beings [7]. Itis generally known that most of them are highly toxic or carcinogenic[8–10]. For instance, cobalt poisoning can cause thyroid, gastro-intestinal, and liver problems via its multiple-toxicity and progressiveaccumulation [11]. Zinc is a necessary micronutrient element to thegrowth of human beings, however, high concentrations (> 3.0 mg/L)impair mental fever as well as poor growth [12]. The negative effects onecological environment caused by specific organic pollutants such aspesticides, herbicides, detergents, oils, and phenolic compounds havebeen considered as tough questions [13–15]. The world may be sufferedfrom acute water shortages in 2020 when the world’s population isexpected to top 7.9 billion more than its resources can support [1].Consequently, the removal of these contaminants from wastewater isindispensable for supplying disease-free health to our society.

During the past few decades, many approaches including coagula-tion [16–18], precipitation [19], oxidation/reduction [20,21], filtration[22], solvent extraction [23], electrolysis [24], sedimentation [25], ionexchange [26], sorption [27–30], and membrane [31] have been pro-posed and used for water treatment. Of all the known methods, sorptionhas clear superiority over other methods for eliminating pollutants atrelatively low concentration with respect to high efficiency, economyand environmental friendliness [32–34]. Besides, sorption can also beused for the preconcentrating of soluble and insoluble organic, in-organic, and biological contaminants [35–37]. To the best of ourknowledge, many current studies on clay minerals [38–40], activatedcarbon [41–43], carbon nanotubes [44–46], graphene [47–49], layereddouble hydroxides [50–52], metal oxides [53–55], and carbon nitride[56,57] as adsorbents to separate pollutants from aqueous solutions.Boron nitride (BN), an environmentally benign material, has been de-monstrated that porous BN nanomaterials can treat a wide range ofpollutants (such as dyes, organic solvents, heavy metal ions, oils, etc.)from water.

BN is a chemical compound constructed from equal numbers ofboron (B) and nitrogen (N) atoms. Balmain [58,59] first demonstratedthe synthesis of BN in 1842 by using the reaction between moltenH3BO3 and potassium cyanide (KCN). Since then, an enormous amount

of investigations has been carried out on the preparation of various BNnanostructures including nanotubes [60], nanosheets [61], nano-particles [62], nanofibers [63], nanoflowers [64], etc. These nanos-tructures exhibited excellent physical and chemical properties, such ashigh specific surface area (SSA), electrical insulation, wide energy bandgap, high thermal stability and conductivity, ultraviolet photo-luminescence, and superb resistance to oxidation as well as chemicalinertness. The advantages make BN a promising material for applica-tions in various areas (e.g. sensor, hydrogen storage, catalysis andcontaminant removal in very harsh environments) [65–68]. In the lastdecade, more and more reviews were published about BN materials,indicating the rising enthusiasm for this unique and significant material[68–76]. For instance, Golberg’s group [68–70,72,75,76] had done asignificant amount of valuable work to develop BN materials and re-viewed the synthesis, morphology, structure, physicochemical proper-ties and applications of BN nanomaterials. Samantaray and Singh [74]reviewed the synthesis and properties of cubic BN thin films. To the bestof our knowledge, the summary about the sorption properties, inter-action mechanism and application of BN-based nanomaterials in pol-lutant removal is still scarce.

Considering the importance of wastewater treatment and emergingutilities of BN nanomaterials, this paper presents a comprehensive in-troduction of review BN materials on five key sections: (I) structuralcharacterizations and properties, (II) utilization of BN for the elimina-tion of organic and inorganic pollutants, (III) interaction mechanisms,(IV) effect of environmental conditions, (V) regeneration of BN mate-rials and (VI) bio-compatibility of BN materials. Finally, the typicalchallenges and perspectives of this exciting new field are highlightedand discussed.

2. Structural characterization and properties of BN-basedmaterials

To understand the interaction mechanism and potential applicationof the BN materials, it is very important to investigate the physical andchemical properties of BN. BN is isostructural to carbon and generallyexists in four crystalline forms including graphite-like hexagonal BN (h-BN), diamond-like cubic BN (c-BN), rhombohedral BN (r-BN), andwurtzite BN (w-BN). The h-BN and r-BN are a dense phase with sp2

hybridized B–N bonds, while c-BN and w-BN are low-density phase withsp3 hybridized bonds. Among them, h-BN has attracted special interestsbecause it is analogue of graphite, so-called “white” graphite [77].Additionally, two-dimensional (2D) crystals of h-BN can be wrapped upinto 0D BN fullerenes and rolled into 1D BN nanotubes (BNNTs)(Fig. 1). Compared with graphene, BN has some unique properties

Fig. 1. Structural models of 2D, 1D and 0D BN nanostructures. The edge of a nanosheet or nanoribbon can be either zigzag (B- or N-edged) or armchair (BN pair-edged) [75].

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including high SSA, numerous structural defects, chemical inertnessand high resistance to oxidation, which render BN material a promisingcandidate for applications in pollutant sorption [78–80]. For single-layered BN, both sides of the BN surface supply sorption sites. However,the large-scale production of BN sheets with a considerable proportionof single or few layer materials is still a challenge due to its oxidation-resistant (stable up to 900 °C in air) and intercalation-resistant [77].Therefore, multilayer BN sheets have great possibilities to enter aquaticenvironments because of the difficult and complicated process of iso-lating single-layer BN. Generally, the sorption of pollutants on BN is acritical physiochemical process at the BN-water interface owning to SSAand surface-active properties of the adsorbents.

Zhao et al. [32] reported that the sorption of heavy metal ions andorganic molecules on graphene-based nanomaterials was directly pro-portional to the SSA of materials. Other investigators also documentedthat SSA was the primary factor determining the magnitude of sorptiononto adsorbents [13,81]. Similar to the fabrication of other materials,BN could be received by soft-/hard-template and non-templatemethods. The main characteristics of each method were summarized inTable 1. Han et al. [82] first prepared the porous BN using activatedcarbon as a hard template method in 2004 and the SSA of the preparedBN was measured to be 167.8 m2/g. Since then, a series of orderedmaterials including SBA-15 silica, CMK-3 carbon, zeolite and grapheneaerogels were selected as the hard templates [83–86]. For example,Rushton and Mokaya [87] selected SBA-15 as a hard template andammonia borane as the BN precursor to obtain BN with high SSA of327 m2/g. Soft-templates were also designed for manufacturing BN.Miele et al. [88,89] fabricated a highly porous BN material with SSA of820 m2/g through utilizing the polycondensation property of tris(monomethylamino)borazine. Malenfant et al. [90] prepared the or-dered porous BN nanostructures with a high SSA value of 950 m2/g bytaking advantage of polynorbornene-decaborane, and self-assembly intetrahydrofuran (THF). In the last several years, remarkable achieve-ments were achieved by taking the non-template strategy [91,92]. In2013, Golberg’s group [93] first produced the BN porous microbeltmaterial with the SSA of 1488 m2/g after a direct reaction betweenboron acid-melamine precursor and ammonia. And interestingly en-ough, the use of other precursor systems (e.g. B2O3-guanidine hydro-chloride and boric acid-dicyandiamide) also reached up to an incon-ceivable SSA values [91,94]. Weng et al. [91] prepared the BN

microsponge (BNMSs) by a facile, one-step, template-free reaction ofboric acid (BA) and dicyanamide (Dcy). The authors demonstrated theBNMSs showed the largest SSA and total pore volume (up to 1900 m2/gand 1.070 cm3/g, respectively) at the Dcy/BA = 3:17 and 800 °C, andthe results of BET analysis were shown in Fig. 2A. Recently, a greatvariety of BN materials were directly synthesized using non-templatetechnique rather than using templates [79]. Dai et al. [95] used a globalstructural search and first-principle calculations simulated two types ofporous BN networks built upon zigzag BN nanoribbons (Fig. 2B). Thetheoretic calculation found that the SSA of porous BN structures canreach up to 4800 m2/g, suggesting the potential to further texturalproperties improvements and application to pollutant treatment.

Because of its hydrophobic surface and bulk π systems on its surface,the BN could interact with the aromatic rings of organic compoundsthrough hydrophobic interaction and π-π interactions. Xue et al. [96]observed higher removal of methylene blue (MB) on h-BN fibers, whichwas a result of the strong π-π stacking interaction. In addition, thestrong π-π interactions between 2,3,7,8-tetrachlorodibenzo-p-dioxinand BNNTs were explored theoretically by Wang et al. [97] Never-theless, the aggregation and hydrophobicity of BN in the aquatic en-vironments significantly reduced its removal capacity. Coupling the BNwith the various functional groups could change the surface propertiesof materials and resulted in higher sorption capacity. It is well knownthat amino and oxygen groups can bind with pollutants via electrostaticinteractions, surface complexation and Lewis acid-base interactions[32]. Hitherto, abundant functional groups (such as amino (–NH2),amine (–NHR), hydroxyl (–OH), ether (–OR) as well as arcyl (–COR))have been experimentally attached to BN skeletons by chemical mod-ification [98–108]. As shown in Fig. 3, Weng et al. [68] summarized thefrequently-used chemical functionalization strategies of h-BN bulk-/nanomaterials. In 2007, Zettl’s group [109] first demonstrated that–NH2 groups could be introduced into the BNNTs’ surfaces via ammoniaplasma. Lei et al. [98] reported ultralight NH2-BN aerogels with adensity of 1.4 mg/cm3 using urea via ball milling method exhibitedexcellent solubility in aqueous solutions, and the microstructure char-acteristics of the BN aerogel were shown in Fig. 4. The –OH group canalso be covalently introduced into the electrophilic B sites of the BN,which is the most efficient oxygen group for contaminants’ removal[110]. For example, Li et al. [80] prepared the OH-BN nanomaterial viausing the mixed solution of H2SO4 and HNO3 as activating reagent

Table 1Summary of growth methods of BN-based materials.

Methods Materials T (°C) Template B source N source BET surfacearea (m2/g)

Total porevolume (cm3/g)

Average poreradius (nm)

Ref.

Hard-template porous BN 1580 activated carbon B2O3 N2 167.8 0.27 3.216 [82]BN aerogel 1600 graphene aerogel B2O3 N2 431 – – [83]BN 1200 zeolite borazine NH3 570 0.78 – [84]porous BN 1200 silica monolith borazine NH3 299 0.41 4.1 [85]BN 1000 SBA-15 silica BCl3 NH4Cl 140 0.16 – [86]

CMK-3 carbon 540 0.35 4.9mesoporous BN 1150 SBA-15 BH3·NH3 BH3·NH3 327 0.50 – [87]

Soft-template mesoporous BN 1000 cetyltrimethylammoniumbromide

BCl3 NH4Cl 820 0.74 6.0 [89]

mesoporous BN 1400 cetyltrimethylammoniumbromide

BCl3 NH4Cl 600 0.60 7.0

mesoporous BN 1600 cetyltrimethylammoniumbromide

BCl3 NH4Cl 510 0.59 6.7

mesoporous BN 1700 cetyltrimethylammoniumbromide

BCl3 NH4Cl 30 0.15 –

porous BN 1000 PNB-b-PDB30 decaborane N2 950 – 2.0 [90]

non-template BNMSs 800 – boric acid dicyanamide 1900 1.07 – [91]h-BNNs 900 – boric acid urea 1900 – 2.0 [92]BN 1000 – boric acid melamine 1488 0.86 1.1 [93]BN 1100 – boric acid melamine 1144 0.88 1.1BN 1100 – boron trioxide guanidine

hydrochloride1427 1.09 50 [94]

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assisted by ultrasonication, which exhibited enhanced MB (up to392.2 mg/g) removal performance. Specially, the sorption rate andcapacity of the material for MB were about 10 and 2 times higher thanthose of the commercial activated carbon, respectively [111]. Fur-thermore, theoretical studies indicated that the band gaps between the

highest occupied molecular orbital (HOMO) and the lowest unoccupiedmolecular orbital (LUMO) for the –OH terminated BN was narrow thanthat of h-BN [112,113]. Therefore, both experimental and theoreticalinvestigations have demonstrated the feasibility of BN bandgaps functionalization by edge-modification. In addition, BN-based

Fig. 2. (A) Nitrogen adsorption-desorption isotherms (a) and (b) BET specific surface areas (SSAs) and total pore volumes of BNMSs obtained with different Dcy/BA ratios at a fixedsynthesis temperature of 1000 °C. (c) Nitrogen adsorption-desorption isotherms and (d) BET SSAs and total pore volumes of BNMSs obtained at different synthesis temperatures whilekeeping the ratio of Dcy/BA = 3:1 [91]. (B) Predicted highly stable porous BN structures: (a) dz2-BN, (b) dz4-BN, (c) lz1-BN, (d) lz1z2-BN, and (e) lz2-BN. The green and silver spheresdenote B and N atoms, respectively [95].

Fig. 3. Summary of chemical functionalization strategies of h-BN bulk-/nanomaterials. A charge is denoted when the compensating functional group is unknown [68].

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composites have been successfully synthesized with TiO2 [114,115],Fe3O4 [116], ZIF-8 [117], ZnO [118], etc. to expand its applicationareas.

According to literature survey, we can conclude that the sorptionsites of BN-based nanomaterials are various and complex, and theycontain high SSA, bulk π systems, defects, and functional groups. Thesorption availability of these sites was regulated by different adsorbateswith various sizes and polarities surface properties, and experimentalconditions, such as pH, ionic strength, temperature, concentration ofadsorbents and adsorbates. The interactions between BN-based nano-materials and pollutants under different conditions were described inmore detail below.

3. Water treatment by BN-based materials

The global occurrence in water resources containing toxic organicand inorganic pollutants has raised concerns about potential effects onaquatic ecosystems and public health. For elimination of those con-taminants, a variety of carbon-based materials including carbon nano-tubes [119–121], graphene [122–124] and graphene oxides [125–127]have been widely investigated in previous years and they exhibitedhigh removal capacity. In addition, there are many reviews on carbon-based materials for the removal of pollutants [81,128–130]. Despite thestructural similarity between BN and graphene, application of the BNmaterials on the removal of contaminants has been far less well ex-plored. Although being very challenging, there have been a great dealof work done for BN’s application in wastewater treatment. Differentkinds of BN-based materials have exhibited efficient sorption capacityfor organic and inorganic pollutants, and the main sorption propertieswere summarized in Table 2.

3.1. Removal of inorganic pollutants

The contamination of aqueous environment by toxic heavy metal

ions (such as As, Pb, Cu, Cd, Ni, Zn, Co and Cr) pose enormous damageto the environment, especially in developing countries with less strin-gent regulations [131]. Hence, the elimination of heavy metal ions fromwater solutions is crucial to our health. Chen et al. [132] loaded Fe3O4

nanoparticles onto BNNTs and the sorption capacity for As(V) wasapproximately 32.2 mg/g at pH 6.9 (Fig. 5a). The sorption data fittedwell to pseudo-second-order kinetics with D-R isotherms. The combi-nation of the high removal efficiency of BN and the magnetic propertiesof Fe3O4 can provide a promising candidate for the preconcentration ofheavy metal ions in wastewater management. Li et al. [78] found themaximum sorption of Cu(II) (373 mg/g) on a porous BN material,which was significantly higher than other adsorbents including CNT-graphene nanocomposites (250 mg/g) [131], GO (22 mg/g) [133],carbonaceous nanofibers (204 mg/g) [134], TiO2 (32 mg/g) [8], andmesoporous silica (182 mg/g) [135]. The excellent sorption capacity ofBN was resulted from the high SSA (1687 m2/g), structural defects andpore volume (0.45 cm3/g). The authors declared this material as anideal one for the wastewater cleanup. Furthermore, the same group[136–138] used activated BN and fluorinated activated BN (F-ABN) forthe removal of various heavy metal ions from aqueous solutions.Sorption experiments of Cr(III) on F-ABN with varying contact time andtemperature were shown in Fig. 5b, and 82.6% of Cr(III) was removedwithin 3 h at 30 °C [137]. To further estimate the filtration sorptioncharacteristics of BN-based materials, Fig. 5c expressed the break-through curves for Pb(II) solution (5 mg/L) through activated BN andmicrofibrillated cellulose (ABN/MFC) composite and activated carboncolumn at a constant flow rate of 2.0 mL/min, respectively [138]. Theresults indicated that the sorption ability of the BN-based materialssignificantly exceeded that of activated carbon by an order of magni-tude at least. The sorption capacity of Cr(III) in the activated BN wasextremely high up to 352 mg/g, which was significantly higher than thereported removal of Cr(III) using activated carbon (40 mg/g) [139],chitosan/attapulgite composites (27 mg/g) [140], vineyard pruningwaste (12 mg/g) [141], polyvinyltetrazole-grafted resin (175 mg/g)

Fig. 4. Characterization of BN aerogel. (a) Photo of an aerogel with a low density (1.4 mg/cm3) placed on the spike of a plant. (b) Photo of an aerogel adhering to a beaker wall. (c) Photoof an aerogel of a higher density (20 mg/cm3) pressing the spike of a plant. (d,e) SEM images of aerogels with densities of 1.4 and 20 mg/cm3, respectively. Scale bars, 2 mm (d,e) [98].

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[142], 4-aminoantipyrine immobilized bentonite (39 mg/g) [143],cross-linked acidic tetrapolymer (49 mg/g) [144]. Even more im-portant, the final equilibrium concentration of Cr(III) was only0.052 mg/L (52 ppb), below the WHO drinking water standard. Again,Liu et al. [145] reported nanosheet-structured BN spheres (NSBNSs) asan effective adsorbent for Cu(II), Pb(II), and Cd(II) removal from water.The sorption capacities of Cu(II), Pb(II), and Cd(II), calculated byLangmuir sorption isotherms, were 678.7, 536.7, and 107.0 mg/g, re-spectively. According to the authors, the excellent sorption efficiencyand capability of the NSBNSs mainly arises from the polarity of the B–Nbonds as well as the functional groups (–NH2 and –OH) of the NSBNSs.Specifically, about 100 mg of the NSBNSs was obtained in the smallcrucible (5 × 2.5 cm2) in one single run, indicating the possibility oflarge-scale production and practical application. Currently, Xue et al.[79] used BN-based porous monoliths (BNPMs) for the preconcentra-tion of Cd(II) from wastewater and the sorption capacity reached up to561 mg/g. The BNPMs had unsaturated atoms and -OH/-NH2 groups,which were responsible for ion exchange. For comparison, the max-imum uptake capacities of porous BN, activated BN and activatedcarbon for different heavy metal ions were shown in Fig. 5d. It wasclearly seen that the sorption capacities of activated BN were sig-nificantly higher than those of the porous BN and activated carbon,because of the “lop-sided’’ densities characteristic of ionic B–N bonding,and the polyelectron nitride can transfer more electron density to theheavy metal ions [136]. The aforementioned analyses suggested that

the BN-based materials were suitable for fast and efficient heavy meatalions cleanup.

Like heavy metal ions, anions including sulfate, phosphate, nitrate,chloride, fluoride, and oxalate could cause some side influences anddiseases once their concentrations exceed the standard. For instance,elevated nitrate concentrations in drinking water are a concern be-cause, once ingested by babies, it could interfere with the blood oxy-genation levels and result in methemoglobinemia (blue baby syndrome)[146]. On the other hand, phosphate is one of necessary macronutrientfor living organisms; however, it has become important water pollutionowning to excessive discharge from various anthropogenic activities.High concentration of phosphate (above 0.02 mg/L) could cause eu-trophication of water bodies, which in turn, causes overgrowth of algaeor algal blooms and reduction of dissolved oxygen [147]. While, itshould be noted that to the best of our knowledge, there have been noexperimental data associated with the removal of anions by BN-basedmaterials. Future advances in this field may broaden the application ofBN and bring revolutionary breakthroughs in wastewater treatment.

3.2. Removal of organic pollutants

Organic dyes discharged from paper, textile, paints, leather, tan-nery, and plastics industries are regarded as the dominating con-taminants in water sources. Dye pollution has become a very grievousissue at present, resulting in disaster for aquatic life [148]. The

Table 2The sorption of pollutants on BN-based materials.

BN-based materials Adsorbates m/V (g/L)

C0 (mg/L)

pH T (h) SSA (m2/g)

qmax (mg/g)

Type of Sorption Ref.

Fe3O4/BNNT As(V) 1.0 1 6.9 12 218.6 32 chemical surface complexation, physicalelectrostatic attraction

[132]

Porous BN Cu(II) 4.0 1864 – 48 1687 373 – [78]Activated BN Cr(III) 0.25 52 5.5 6 2078 352 chemisorption [136]

Pb(II) 6.0 225Ce(III) 6.0 282Co(II) 6.0 215Ni(II) 6.0 235

Fluorinated activated BN Cr(III) 0.25 52 5.5 5 1250 387 electrostatic attraction, ion exchange [137]BN spheres Cu(II) 0.4 200 5.5 12 196.5 537 – [145]

Pb(II) 679Cd(II) 107

BNPMs Cd(II) – – 7.0 – 1406 561 electrostatic, complex, hydrogen-bondinginteractions

[79]

porous BN MO 0.4 40 – 2 1687 298 electrostatic attraction, complex interactions [78]activated BN tetracycline – – 6.0 – 2078 305 – [136]BN spheres malachite green 0.4 110 – 3 196.5 324 – [145]

methylene blue 233BN nanocarpets methylene blue 0.4 10 – 0.2 117.2 272 π-π stacking interaction [149]porous BN congo red 0.4 130 – 3 1427 782 adsorption and pore filling [94]

methylene blue 313basic yellow 1 556

BN nanofibers methyl blue 0.5 20 – 2 114.5 328 rapid diffusion, strong electrostatic interaction [150]3D BN basic yellow 1 0.4 90 – 1 1156 424 electrostatic attraction, π-π stacking interaction [151]

congo red 110 3 718h-BN methyl blue 0.1 10 – 0.3 627 631 π-π stacking interaction, stacking interaction [96]Activated BN methyl blue 0.24 25 8.0 1 1104 392 electrostatic interactions [80]BN fibers 2433D C-BN methyl blue 1 50 – 1.5 – 313 – [153]h-BN whiskers rhodamine-B 0.06 3 7.0 1.5 964.4 210 π-π stacking interaction, electrostatic attraction [154]

methyl blue 0.02 60 0.08 13973BNHSs basic yellow 1 0.4 40 – 3 215 192 electrostatic interaction, complex interaction [155]

methylene blue 117g-BN gatifloxacin 1 80 – 0.25 – 89 π-π interaction, electrostatic interaction [156]BNNSs Lysozyme 0.4 60 – 3 273 312 electrostatic attraction, van der Waals forces,

Lewis acid-base interaction[160]

BNNSs tetracycline 0.2 50 – 5 1427 284 π-π interaction, Lewis acid-base interaction [157]chlortetracyclinehydrochloride

2 1170

ciprofloxacin 10 206 π-π interactionnorfloxacin 174

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following section highlights the results of various investigations on theremoval of organic dyes from wastewater using BN-based materials.Zhang et al. [149] synthesized BN nanocarpets by a new hot-pressbenzene-thermal method and applied for the removal of MB. The ki-netic studies indicated that 99% of MB was removed after slightlyshaking for 10 s, which was caused by the large specific surface areaand high density of structural defects of BN nanocarpets. Lei et al. [94]investigated the removal of anionic (congo red, CR) and cationic (MBand basic yellow 1, BY) dyes using a porous BN nanosheets. This BNnanomaterial exhibited much higher sorption capacity for CR (782 mg/g) than MB (313 mg/g) and BY (556 mg/g). Lian et al. [150] tested thesorption of MB in aqueous solutions using BN nanofibers. The in-vestigation revealed that BN not only had effective sorption ability forMB but also had superior removal capacities as compared to conven-tional adsorbents (e.g. activated carbon, graphite, TiO2, silica, chitin,chitosan and bentonite). Another study by Liu et al. [151] reported thesynthesis of a three-dimensional (3 D) BN architecture and tested theremoval performances of BY and CR in aqueous solutions. The 3 D BNshowed excellent sorption capacities for anionic and cationic dyes dueto its high SSA, hierarchical porous structure, and large pore volume.MB was successfully removed from aqueous solutions using BN and BNfibers [80]. As shown in Fig. 6a, activated BN was capable to eliminateMB than the BN fibers. The result was ascribed to the favorable texturalproperties of activated BN, i.e. higher surface defects and numeroushydroxyl and amino groups. In addition, the sorption capacity of MBonto the activated BN was much higher than that of neutral red (NR)and methyl orange (MO), as shown in Fig. 6b. The results were due tothe activated BN with an overall negatively charged surface at all pHvalues, which could easily adsorb positively charged dye throughelectrostatic interaction. Singla et al. [152] studied the sorption ofbrilliant green (BG) and MO on BN nanoparticles and BN nanosheets,and investigated the interaction mechanism by theoretical calculation.

More recently, the removal of MB was studied by Jia et al. [153] using3 D c-BN with the broken hollow spherical shell, and the most extremeadvantages were that the sorption rate of the MB became faster whenthe material was used by the second time. Li et al. [154] tested theremoval of rhodamine-B (RhB) and MB over h-BN whiskers, and foundthat only 13% RhB was removed by commercial BN powder from thesolution within 90 min at room temperature (Fig. 6c). By comparison,almost 99% RhB was removed using the same amount of BN whiskers(Fig. 6c), which was also evidenced from the visually express in Fig. 6d.The sorption capacity of MB was calculated to be 13973 mg/g, whichwas much higher than other today’s materials. The high sorption ca-pacity makes it a potentially attractive adsorbent in environmental andwater source protection. Generally, multiple contaminants, such as dyesand heavy metal ions, are often simultaneously present in naturalaqueous solution. However, reports on selective removal of dyes frommixed solutions of dyes and heavy metal ions by BN-based materialsfrom wastewaters are still scarce [96,149,155]. Lian et al. [155] testedthe sorption of basic yellow 1 (BY1) from a mixture of BY1 (40 mg/L)and CuSO4 (5 g/L) using BN hollow spheres (BNHSs). Interestingly, theconcentration of Cu(II) did not change obviously before and aftersorption by BNHSs, indicating that high separation selectivity of BNHSsfor dyes. Similar interesting phenomena was also observed by Xue et al.[96] that the h-BN fibers can also selectively adsorb MB from mixedsolution of MB and CuSO4. Spectra of the MB-CuSO4 mixed solutionbefore and after treatment with the BN nanocarpets were shown inFig. 7a [149]. An interesting phenomenon was observed in the BNnanocarpets: the absorption band at 800 nm remained almost un-changable after the mixed solution was treated with the BN nano-carpets, while the other band at 664 nm disappeared. In other words,the BN nanocarpets selectively adsorbed MB from a mixed solution ofMB and CuSO4, which was attributed to the structural similarity be-tween MB and BN. The possible mechanism of the competitive sorption

Fig. 5. (a) The effect of pH on As(V) sorption while using the sample BNNT and the equilibrium pH (initial As = 1 mg/L; dosage = 1 g/L; contacting time = 24 h; T = 25 °C) [132]; (b)Removal amount of Cr(III) on F-ABN as a function of time and temperature (pH: 5.5) [137]; (c) Breakthrough curves for Pb(II) solution through ABN/MFC-70 and the AC column. Feedconcentration 5 mg/L, flow rate 2.0 mL/min, pH 6.0 [138]; (d) Comparison of sorption capacities of the activated BN, porous BN and activated carbon for Co(II), Ni(II), Ce(III), and Pb(II),respectively [136].

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between MB and Cu2+ on the (0 0 2) plane of BN was shown in Fig. 7b.These results indicated that the BN-based materials preferentially ad-sorbed organic pollutants from mixed solutions of dyes and heavy metalions, therefore they can be applied to recover and separate some va-luable organic compounds from wastewater.

There are still some other organic pollutants those were removedefficiently by BN-based materials. For example, psychotherapeutics inaquatic ecosystems can influence key behavior traits of organisms af-fecting their growth, reproduction, and survival. Chao et al. [156] re-ported the sorption of fluoroquinolone antibiotic gatifloxacin (GTF) ongraphene-like layered hexagonal BN (g-BN), with more than 90% of

GTF adsorbed on g-BN. Sorption isotherms of GTF at 288, 303, and318 K (Fig. 8a) showed that the removal percentage increased in highertemperature, indicating the sorption was exothermic. The GTF sorptionwas discovered to be decreased with increase of ionic strength, sug-gesting the main removal mechanism was electrostatic interaction. Astudy from Liu et al. [157] demonstrated that BN nanosheets (BNNSs)were efficient for the preconcentration of tetracycline (TC), chlorte-tracycline hydrochloride (CTC), ciprofloxacin (CIP), and norfloxacin(NOR). The sorption capacities for CTC (1170 mg/g) and TC (284 mg/g) were higher than for CIP (206 mg/g) and NOR (174 mg/g) owning tothe greater number of aromatic units in CTC and TC than CIP and NOR.

Fig. 6. (a) Comparison of sorption rates of MB on the activated BN and deactivated BN, respectively; (b) Comparison of sorption rates of MB, NR, and MO on the chemically activated BN(pH value: 8, adsorbent dosage: 60 mg, sorption temperature: 30 °C, dye concentration: 25 mg/L, solution volume: 250 mL), respectively [80]. (c) Comparison of sorption rates of RhB onBN whiskers and commercial BN powder; (d) UV–vis sorption spectra and photos (inset) of the aqueous RhB solution (3 mg/L, 100 mL) in the presence of BN whiskers at different timeintervals [154].

Fig. 7. (a) Absorption spectra of the MB-CuSO4 mixed solution before and after treatment with the BN nanocarpets; (b) A schematic diagram of the competitive sorption between MB andCu(II) [149].

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Besides that, the porous BNNSs showed excellent recyclability aftermultiple antibiotic sorption and regeneration steps. Proteins are one ofprimary components of natural organic matter (NOM) which can causebiofouling during membrane filtration of drinking water and waste-water. Recently, Zhi et al. [158] reported that proteins can be im-mobilized on BNNTs through π-π interactions and electrostatic inter-actions. Lin et al. [159] also indicated that BNNSs in water exhibitedstrong affinity toward proteins by Lewis acid-base interactions. Fur-thermore, lysozyme (LYS), a model protein, was removed from water byBNNSs, and the balance time was 120 min and the reaction date fittedwell with pseudo-second-order, and the maximum removal capacitywas 312 mg/g at room temperature [160]. The authors indicated thatBNNSs was an ideal candidate as adsorbent for water purification due toits outstanding sorption capacities for proteins and excellent recyclingstability. Poly- and perfluoroalkyl substances (PFASs) are almostnaturally un-degradable and have become the focus of global concerndue to their worldwide occurrence, toxicity, high persistency, andbioaccumulation. Feng et al. [161] examined the removal of per-fluorooctanesulfonate (PFOS) and perfluorodecanoate (PFDA) (two re-presentative PFASs) under different experimental conditions by h-BNfor the first time. The removal process was fitted well with pseudo-second-order equation and Freundlich equation. The sorption capacitiesof PFDA and PFOS were 0.72 mg/m2 and 0.45 mg/m2, respectively atpH 6.0. Very recently, h-BN was employed to remove dibenzothiophene(DBT) by Xiong and co-authors [162], and the influence of environ-mental conditions such as the number of BN layers, DBT initial con-centration, and temperature on DBT removal was systematically in-vestigated. The results showed that the sorption process was best fittedwith the pseudo-second-order kinetic model. The main interactionmechanism was Lewis acid-base interaction, which was further provedby density functional theory (DFT) calculation. Furthermore, the samegroup prepared the BN mesoporous nanowires with doped oxygenatoms and investigated its sorption desulfurization performance forDBT and 4,6-dimethyldibenzothiophene (4,6-DMDBT) [163]. Com-pared with the commercial BN and graphene-like BN adsorbents(Fig. 8b), the BN mesoporous nanowires exhibited higher sorption de-sulfurization performance due to the large number of low coordinatedatoms along the nanowire surface and mesoporous, and the doped Oatoms further strengthen the interaction with DBT. The above-mentioned analyses indicated that the BN-based nanomaterials havehigh efficiency to remove pollutants in wastewater.

4. Removal mechanism

Numerous studies indicated that BN-based materials were effectiveadsorbents for wastewater treatment due to their high SSA, large porevolume, and high chemical stability. The strong interaction between BN

and pollutants could greatly change the mobility, bioavailability, andenvironmental risk of heavy metal ions and organic chemicals.Nevertheless, the reaction mechanism between pollutants and BN-basedmaterials is still in debate. Mechanistic investigation of contaminants’sorption by BN-based materials is domination in the explanation of thesorption process, which is vital important for the selection of the re-action conditions and desorption/regeneration. The interaction me-chanisms have been performed with either the assistance of the DFTtheoretical calculation, spectroscopy analysis or comprehensive ex-perimental observations. The potential reaction mechanism of pollu-tants with BN-based materials was summarized in Table 2.

For the interaction of heavy metal ions with BN-based materials, theinteraction was mainly dominated by electrostatic interaction andsurface complexation. Compared to the carbon-based materials (such asactivated carbon, carbon nanotube and graphene) with covalent C–Cbonds, BN-based materials with polar B–N bonds were more favorablefor the chemisorption of heavy metal ions, because the BN showed the“lop-sided” densities properties of ionic B–N bonding, and the poly-electron nitride could transfer more electron to the heavy metal ions[136,145]. Xue and co-worker [79] measured that BNPMs were nega-tively charged at overall pH, which came from a small number of -OH/-NH2 on the BNPMs surfaces and plentiful non-saturated atomic bondsinside the BN structures. The BNPMs showed high sorption capacity tocation ions due to ion-exchange interactions including electrostatic,complexation, or hydrogen-bonding interactions. In another study,electrostatic interaction between Cr(III) cation and the negativelycharged nitrogen atoms of F-ABN played an important role in Cr(III)sorption, which was also demonstrated by theoretical calculations[137]. Chen et al. [132] indicated that As(V) was tightly adsorbed ontofunctionalized Fe3O4/BNNT nanocomposites through surface com-plexation (Eqs. (1) and (2)):

+ ↔ + +− +BNNSs-FeOH(s) H AsO BNNSs-FeHAsO H H O3 4(aq) 4 (aq) 2 (1)

+ + +− +BNNSs-FeOH(s) H AsO BNNSs-FeAsO 2H H O3 4(aq) 4

2(aq) 2 (2)

Besides the experimental studies of the efficient sorption of heavymetal ions from aqueous solutions, the theoretical calculation of theinteraction mechanism between heavy metal ions and BN-based mate-rials is also crucial [164–167]. The most important advantage of DFTtheoretical calculation is that it can simulate the interaction processesand to probe the species and microstructures at molecular levels undervery complicated conditions. Wu and Zeng [164] systematically studiedthe interaction between the BNNTs and all 3d transition metals (Sc, Ti,V, Cr, Mn, Fe, Co, Ni, Cu, and Zn) and two group-VIIIA metals (Pd andPt) using spin-polarized DFT, and configurations corresponding to thelocal minima of the ten 3d transition metals adsorbed on the BNNTswere displayed in Fig. 9. The results showed that the binding energies

Fig. 8. (a) Sorption isotherms for GTF onto g-BN fitting by Langmuir model (msorbent = 10 mg; C0 = 20–80 mg/L; 15 min) [156]; (b) Sorption capacities of different BN samples.Experimental conditions: 500 ppm initial sulfur concentration, V (oil) = 20 mL, m(adsorbent) = 0.05 g, T = 298 K, and atmospheric pressure [163].

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Fig. 9. Local minima configurations of the 3d transition-metal atom [(a)-(j)] adsorbed on the outer surface of the(8,0) BNNTs [164].

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of Sc, Ti, Ni, Pd, and Pt atoms were relatively higher than 1.0 eV, whilethose of V, Fe, and Co atoms were modest, ranging from 0.62 to0.92 eV. Mn atom formed a weak bond with the BNNT, while Zn atomcannot be chemically adsorbed on the BNNT. Zhao and Ding [166]firstly reported the interaction of a Ni atom on three intrinsic defects(single vacancies, Stone-Wales, and antisite defects) in an (8,0) BNNTsby DFT calculations, and they found that the reactivity of the defectiveBNNTs toward Ni atom was higher than that of the perfect BNNTs to-ward Ni, demonstrating that the defects in BNNTs were benefit to Nisorption. These results can provide not only fundamental understandingbut also further insight into environment research fields.

Generally, the strong π-π stacking interaction was the dominantmechanism for the interaction of organic pollutants with BN-basedmaterials. Considering the structures of the sp2-hybridized B and Natoms, the high chemical stability/inertness, and excellent oxidationresistance of BN-based materials, it is expected that the BN can strongly

interact with the delocalized π bonds of organic pollutants. Xue and co-work [96] reported that efficient removal of MB by h-BN was attributedto the strong π-π stacking interaction in the form of face-to-facestacking. The sorption ability of MB enhanced with the pH increasingand reached the maximum point at pH 8.0, indicating the strong elec-trostatic interaction between MB and active BN [80]. Lian et al. [150]firstly synthesized novel BN ultrathin fibrous networks via a one-stepsolvothermal process and used for the removal of MB. The materialexhibited high sorption capacity (327.8 mg/g) and ultrafast removalrate (1 min) due to the strong electrostatic interaction and rapid dif-fusion of MB from the solution to the external surfaces of BN nanonets.Another study also indicated that the electrostatic attraction and π-πstacking interaction between the 3 D BN surface and the dyes in solu-tion were responsible for the CR and BY removal [151]. The solution pHcan influence the surface charges and active sites of the adsorbents andthe structures of the dye molecule, so it had great effect on the sorption

Fig. 9. (continued)

Fig. 10. Optimized geometries of dye adsorbed BN nanosheet complexes (a) brilliant green and (b) methyl orange adsorbed BN nanosheet [152].

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process. Singla et al. [152] reported that the decrease of sorption ca-pacity with pH decreasing for cationic dye (BG) while opposite wasobserved in case of anionic dye (MO). The nature of interaction be-tween the dye molecule and the BN nanomaterial surface was deducedfrom the optimized geometries of the concerned molecules, as shown inFig. 10. It was clearly seen that H atom for BG (3.18 Å) and O atom forMO (3.13 Å) was close to the BN surface. Furthermore, the sorptionenergies for BG and MO were 104.58 and 90.97 kcal/mol, respectively,indicated the thermodynamic feasibility of the sorption process. Fenget al. [161] found that Ca2+ had a significant promotion effect on theremoval of PFDA and PFOS, suggested that electrostatic interaction wasresponsible for the sorption by h-BN. The structures, bonding nature,and binding energies of rhodamine 6 G (R6 G) with BNNSs were sys-tematically investigated using DFT calculations, near-edge X-ray ad-sorption fine structure (NEXAFS) spectroscopy, and Raman spectro-scopy [168]. The C K-edge NEXAFS spectra of monolayer R6 Gmolecules (9 Å thick) adsorbed on the BNNSs at different X-ray incidentangles (θ) were shown in Fig. 11. The π∗ resonances of the R6 G mo-lecules were highly angularly dependent, indicated that the orientation

of the R6 G on BN was not random. The DFT calculations and Ramanspectroscopy analysis further demonstrated that the high removalpercentage of R6 G was due to strong π-π interaction. Lei et al. [160]reported that BNNSs exhibited strong affinity toward LYS due to theelectrostatic attraction, van der Waals forces, and Lewis acid-base in-teractions. DFT calculations were performed to study the sorption be-haviors of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on the pristineand Ni-doped (8, 0) BNNT surfaces, and the results showed that com-pared with the pristine BNNTs, the NiB- and NiN-doped BNNTs ex-hibited strong interaction toward TCDD molecule with higher sorptionenergy and smaller interaction distances [97]. These results might besignificant for exploiting new materials in the detection and removal ofhighly toxic dioxin pollutants.

Based on the abovementioned analyses, the interaction mechanismsof pollutants with BN-based materials were shown in Fig. 12. Thesorption of heavy metal ions on BN-based materials was mainly domi-nated by strong surface complexation and electrostatic interaction. Forthe sorption of organic pollutants, the interaction was mainly governedby π-π stacking interactions. Organic molecules with benzene-con-taining structures have strong π-π stacking, which increases with theincrease of benzene number. In fact, various mechanisms may si-multaneously control pollutants’ sorption on BN-based materials. Thedominating interaction could be ascribed to one or two types of inter-actions because of the reaction medium. Therefore, the true removalmechanism could be understood via to real environmental conditions.

5. Effect of environmental conditions

Generally, ambient environments such as solution pH, ionic strength,reaction time, amount of adsorbent, coexisting ions, and temperature havegreat influence on the removal of pollutants. Environment changes not onlyexert impact on the surface properties of adsorbents, but also on the mo-bility, bioavailability, and environmental risk of contaminants. Therefore,understanding the different interactions between BN-based materials and

Fig. 11. a) Fittings to the π* resonance of the angularly dependent NEXAFS spectra of the 9 Å R6 G deposited on the BN nanosheet, with the geometry of the incidence shown in the inset;b) the C atoms corresponding to the six fitted π* transition subpeaks in (a); c) cosine-squared dependence of the intensities of the fitted C1, C4, C5, and C6 1 s-π* transitions [168].

Fig. 12. Schematic of the interaction mechanism of pollutants with BN-based materials.

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pollutants as a function of various conditions will provide important in-formation on assessing BN environmental risks and in exploring theirpractical applications.

Solution pH is one of the key factors that controls the eliminationbehavior of organic weak electrolytes and heavy metal ions on BN-based materials because it controls the surface charge of BN-basedmaterials and the species distribution of the electrolyte, and furtherdominates the electrostatic interactions between the materials and thepollutants. At pH value lower than the pHPZC (point zero charge), theadsorbent surfaces were positively charged, whereas they were negativeat pH>pHPZC. Besides, the solution pH also controls the species dis-tribution of the heavy metal ions and the dissociation or ionization ofthe organic compounds through its pKa. Hence, the solution pH had aprofound effect on the removal process through controlling the elec-trostatic interactions between adsorbents and pollutants. The removalpercentage of heavy metal ions on BN-based materials quickly in-creased with increasing pH value, and the maximum sorption occurredat neutral pH as confirmed in Refs. 132 and 137. The lower sorptioncapacity under low pH conditions was resulted from the competitivesorption between heavy metal ions and the protons for the limited ac-tive sites on the BN-based materials surface. The strong sorption atneutral pH was attributed to the strong surface complexation interac-tions and electrostatic interactions. The obvious pH influence on or-ganic chemical sorption relied on how the enhance in attractive forces(e.g., π-π interactions) neutralizes the enhancement of repulsive forces(e.g., charge repulsion) and/or the decrease of certain attractive in-teractions (e.g., hydrophobic interaction). Significant data may be de-duced by comparing pH values, the pKa of the organic compounds, andthe pHpzc of BN-based materials [80,152,154,156]. Singla et al. [152]found that the maximum removal efficiency of BG and MO on BN na-noparticles was obtained at pH 4.0 and 7.0, respectively. In general,low sorption was obtained at pH > pKa and pHpzc due to both ad-sorbent and adsorbate were negatively charged and electrostatic re-pulsion was the dominant interaction mechanism. The organic chemi-cals exhibited high removal efficiency under conditions atpHpzc > pH > pKa owning to their electrostatic attraction with BN-based materials.

In order to investigate the sorption rate-controlling steps (e.g. masstransport or chemical reaction), kinetic models were applied to simu-late the reaction data. On the one hand, the kinetic described the soluteuptake rate which in turn determined the residence time of pollutantsuptake at the adsorbents. On the other hand, one can know the scale ofa sorption apparatus according to kinetic information. In general, theremoval of pollutants on BN-based materials increased rapidly at thefirst contact time and then slowed down upon reaching the balancestate (Fig. 6). Different kinds of kinetic models have been applied to fitthe reaction process, such as pseudo-first-order, pseudo-second-order,Zeldowitsch, Elovich, and Lagergren kinetic models [8–13]. Chen et al.[132] showed that the As(V) sorption was fast at the first 60 min, thenslowed down between 60 and 240 min and gradually reached theequilibrium after 720 min. The authors described that the sorption datawas well approximated to pseudo-second-order kinetic model. Anotherstudy reported by Li and co-worker [137] found that the Cr(III) removalwas in good agreement with the pseudo-second-order model (Fig. 5b),suggesting that the sorption process was surface-reaction controlled.The sorption of four pharmaceuticals (e.g. TC, CTC, CIP, and NOR) onBNNSs fitted the pseudo-second order model better than others [157].The ultrafast removal of BG and MO on the surface of BN-based na-nomaterials indicated that the BN-based nanomaterials would be ex-tremely useful for efficient and significant elimination of harmfulcompounds [152]. Chao et al. [156] showed that approximately 95%GTF was removed by g-BN in 15 min, while commercial BN exhibitedonly less than 10% GTF sorption. This consequence predicted that theBN-based nanomaterial might have remarkable sorption potency for theelimination of pollutants from wastewater.

In general, temperature can modify the removal capacity of

pollutants and the sorption energy of the interaction system. Thesorption isotherm is one of the efficient approaches to understand theinteraction mechanism, to evaluate how the adsorbate attached to theadsorbent and to detect the application of the materials. The frequently-used sorption isotherms of pollutants on BN-based materials wereLangmuir, Freundlich, Temkin, and Dubinin-Radushkevich (D-R)models [128]. At the same time, the important thermodynamic para-meters, such as the Gibbs free energy (ΔG), entropy (ΔS) and enthalpy(ΔH), deduced from temperature-dependent reaction data, were crucialto investigate the reaction mechanisms. Sorption isotherm showed theinteraction behavior of adsorbates with adsorbents, which was neces-sary in optimizing the application of materials. The As(V) sorption onFe3O4/BNNT nanocomposites followed D-R isotherm, implied that thesorption mechanism was a combination of chemical and physicalsorption with a preference for the former [132]. Li et al. [137] reportedthat the removal percentage of Cr(III) on F-ABN increased when tem-perature increased from 10 to 30 °C (Fig. 5b) and the negative values ofΔG confirmed that the sorption was a spontaneous process. The sorptionisotherms of MB on activated BN and BN fibers proved that the elim-ination occurred on the surface of the material that was made up ofspecific homogeneous sites, each of which can adsorb one adsorbatemolecule, i.e. monolayer sorption [80]. Oepen et al. [169] measuredthe ranges of sorption energies from different forces: van der waalsforces of 4–10 kJ/mol, hydrophobic bond forces about 5 kJ/mol, hy-drogen bond forces of 2–40 kJ/mol, coordination exchange about40 kJ/mol, dipole bond forces of 2–29 kJ/mol, chemical bondforces> 60 kJ/mol. In the sorption of RhB on BN whiskers, ΔG was inthe range of 12.11∼9.95 kJ/mol, indicated that the interaction me-chanism might be hydrogen bond forces [154]. In addition, the nega-tive value of ΔH suggested that the sorption process was exothermicand the negative value of ΔS implied a decreased randomness at thesolid/solution interfaces. Generally, the temperature had a positiveinfluence on the sorption capacity of pollutants on BN-based materials,which was beneficial for insitu remediation in environmental pollutioncleanup.

The aforementioned researches were mainly focused on the in-vestigation of common environmental conditions (e.g. pH, contact timeand temperature). However, the content of water environment is verycomplex and its pervasion and transference is complex and non-line-arity process by the effect of multiple factors. A variety of metal ions(e.g. coexisting oxyanions and cations) and natural organic materials(NOM) in aqueous environments may affect the removal mechanism ofpollutants onto BN-based materials. Thereby, a lot of work should becarried out on the sorption properties of contaminants on BN-basedmaterials at different environmental conditions as well as the under-lying reaction mechanisms, providing basis for further investigation onthe physicochemical behavior of pollutants at solid/water interfaces.

6. Regeneration of BN materials

As is well known, the feasibility of applying the adsorbent systemsin large-scale operations is determined by the cost of their disposal orregeneration. At present a large number of investigations indicate thatthe BN-based materials could be readily cleaned for reuse by burning orheating in air due to their strong resistance to oxidation. A few im-portant studies dealing this issue were discussed herein.

Lei et al. [94] investigated the regeneration of BN nanosheets aftersorption of oil and CR. The nanosheets were recycled completely by asimple thermal treatment route at 600 °C for 2 h in air. The XRD(Fig. 13A) analysis indicated that the crystallite size and the porosity ofcalcined samples were unchanged after regeneration. In addition, thesame group [151] evaluated fifteen sorption/desorption cycles for BYimmobilized on 3D BN architecture by using easily heating at 500 °C inair for 2 h. The regenerated adsorbents could retain almost the sameremoval efficiency after the second regenerations, and still remained upto 88% even after 15 cycles, indicated that BN-based material had good

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reusability. Li et al. [80,136] reported the regeneration of activated BNby desorption of tetracycline and MB. Only∼14% sorption efficiency oftetracycline was loss after six cycles by heating in air at 400 °C for 2 h.Furthermore, the same group [137] investigated the desorption of Cr(III) from F-ABN by acid elution using 0.1 M HNO3 solution, and foundthat the removal efficiency of approximately 55.6% was retained evenafter ten sorption-regeneration cycles. Xue et al. [96] got desorbed MBfrom h-BN adsorbents by heating at 600 °C for 2 h in air. Remarkably(Fig. 13B), the h-BN fibers were highly stable and the removal per-centage could maintain over 88% after 5 cycles of sorption-desorption.Xue et al. [79] regenerated BNPMs after the removal of oil and RhB,and the BNPMs showed excellent sorption capacity after 10 regenera-tion cycles, and levels of eluted oil and RhB were consistently high.Chao and co-worker [156] measured the B content less than 0.02 mg/Lin water, implied that the proportion of g-BN leached into the aqueoussolution was negligible. These consequences showed that BN-basedmaterials could be regenerated and applied to pollutant control, whichmade these materials as economic tools. This property of BN-basedmaterials may be regarded as an extra advantage for their popularity inwastewater treatment.

7. Bio-compatibility of BN materials

With the widespread use of BN and BN-based materials, BN may beable to release into the environment at significant levels. Therefore,biocompatibility and toxicity should be considered before its employ-ment in any practical applications. Many investigations have performedto study the biological effect of BN under the conditions of differentpurities and geometry as well as different dispersion reagents and cell

types [170–174]. Based on the toxicological results, one can concludethat the BN-based materials were nontoxic or had very low toxicity. Forthe first time, Ciofani et al. [175,176] investigated the cytocompat-ibility of polyethyleneimine (PEI)-coated BNNTs with human neuro-blastoma cell line (SH-SY5Y), they found that the BNNTs materialsexhibited very good cell viability in the culture medium up to a dose of5.0 μg/mL. Subsequently, they also identified the outstanding cyto-compatibility of ploy-L-lysine modified BNNTs with both glioblastomaand primary fibroblast cells [177–179]. Furthermore, Horváth et al.[180] examined the in vitro cytotoxicity of BNNTs to four different celllines (murine embryonic fibroblast cells, murine alveolar macrophagecells, human lung adenocarcinoma epithelial cells, and human em-bryonic kidney (HEK) cells). They found that the BNNT induced mod-ifications of the metabolic activity as well as of the cell morphology.The toxicity of the BNNT was strongly dependent on the cell type, andwas more pronounced in cells with high endocytic (phagocytic) abilitysuch as macrophages. On the basis of these differences, further worksneed to be done to reveal the biotoxicity of BN-based materials underdifferent conditions.

Actually, the biocompatibility and toxicity of nanomaterials havebecome hot topics in the last few years. However, owning to naturalcomplexity of nanomaterials, generally, the significant differences inconsequence were obtained during different tests on the same kind of ananomaterial. For instance, CNT with different functional groups, sur-face topology and impurities exhibited different biocompatibilities[181]. In addition, the real environment is more complex in actualusage of BN-based materials. Consequently, we firmly believe that moreinvestigations should be carried on BN-based materials with differentproperties to ensure their biocompatibility and practical applications.

Fig. 13. (A) Structural evolutions of porous BN nanosheets. XRD patterns of porous BN nanosheets during (a, left) used engine oil removal: before sorption (a), after sorption (b), afterregeneration at 600 °C in air (c), and (b, right) congo red removal: before sorption (a), after sorption (b), after regeneration at 400 °C in air (c) [94]. (B) The C/C0 ratio of MB solutionusing recyclable h-BN fibers within 5 times (a). FT-IR spectra of the h-BN fibers after MB sorption and desorption (b) [96].

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8. Conclusion and outlook

This review outlines the potential of BN-based materials as adsor-bents and the plentiful advances in the removal and preconcentration ofvarious inorganic and organic pollutants. Because of the high surfacearea, excellent chemical inertness and distinguished oxidation re-sistance, the BN-based materials showed much higher sorption capacityand removal performance to heavy metal ions and organic pollutantsfrom aqueous solutions than its structural analogue graphene-basedmaterials. It is necessary and significant to study the removal behaviorand sorption mechanism of pollutants on BN-based materials fromwater system, and it is favorable for us to understand the migration andtransformation rules of contaminants in wastewater. The removal pro-cesses of heavy metal ions on BN-based materials were mainly attrib-uted to electrostatic interaction and surface complexation. The inter-action mechanism between BN-based materials and organic compoundswas π-π interaction and Lewis acid-base interactions. In addition, thesaturated BN-based materials could be easily cleaned for regenerationby burning or heating in air and maintained high sorption capacity afterseveral cycles. This easy recyclability further confirmed the potential ofBN-based materials for water purification and treatment.

Despite the bright future of BN-based materials in water purifica-tion, there are some issues should be taken into consideration. Thepreparation methods of functionalized BN and BN-based nanomaterialsare still rather difficulty, particularly compared to graphene analogues.This situation is primarily explained by high chemical inertness of BN-based materials that prevents them from direct modifications. Hitherto,a variety of post-synthetic methods were applied for preparation ofmodified BN and BN-based nanomaterials, however, the yield and ef-ficiency of these methods are not quite satisfied. Alternatively, the re-cently proposed ‘‘self-bubbling precursor solidification and high-tem-perature pyrolysis’’ route may supply a new strategy toward moreefficient preparation of BN and BN-based nanomaterials [79]. Manymore ways need to be introduced to deal with this challenge.

In addition to develop new methods appropriate for the preparationand modification of BN and BN-based nanomaterials, the application ofanalytical techniques that can precisely confirm the interaction me-chanisms between adsorbent and adsorbate is also critical. For example,the X-ray absorption fine structure (XAFS) spectroscopy, which containsX-ray absorption near edge structure (XANES) and extended X-ray ab-sorption fine structure (EXAFS) spectroscopy, is beneficial to obtain themicrostructures and species at molecular level. The XANES spectra cansupply the evidence for the distinction of oxidation-reduction state ofheavy metal ions at solid adsorbents directly, whereas the EXAFSspectra can supply the knowledge of specific bonding type, coordina-tion number and the corresponding microstructures at molecular level.By analysis of the fluorescence lifetime, the time resolved laser fluor-escence spectroscopy (TRLFS) can directly supply the information ofthe number of water molecules in the first coordination sphere, which isvital to identify the formation of outer-sphere or inner-sphere surfacecomplexes. Surface complexation models (SCMs) are the valuable in-struments to receive chemical species of contaminants on solid surfaces.All of these approaches should become valuable tools for uncoveringinteractions between BN-based materials and pollutants.

Considering the above-mentioned challenges and restrictions of BNand BN-based nanomaterials, functionalization of the materials, andpotential applications, investigators should devote more time to theexploitation of more scalable and effective BN functionalization stra-tegies. This would provide the guidance for the design of functionalizedBN nanomaterials with exact property to meet the ever growing de-mands for water treatment and to solve water contamination globally.The outstanding properties of BN and BN-based nanomaterials makethem promising candidates in environmental pollution managementwhen they can be synthesized in large scale at low price with the de-velopment of technology in near future.

Acknowledgements

This work was supported by the National Key Research andDevelopment Program of China (2017YFA0207002), the NationalNatural Science Foundation of China (21577032), and the FundamentalResearch Funds for the Central Universities (JB2015001). X. Wangacknowledged the CAS Interdisciplinary Innovation Team of ChineseAcademy of Sciences.

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