Adsorption of dyes by nanomaterials: Recent developments and adsorption mechanisms

Adsorption of dyes by nanomaterials: Recent developments and adsorption mechanisms

Separation and Purification Technology 150 (2015) 229–242 Contents lists available at ScienceDirect Separation and Purification Technology journal hom...

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Separation and Purification Technology 150 (2015) 229–242

Contents lists available at ScienceDirect

Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur

Review

Adsorption of dyes by nanomaterials: Recent developments and adsorption mechanisms Kok Bing Tan a, Mohammadtaghi Vakili b, Bahman Amini Horri a, Phaik Eong Poh a, Ahmad Zuhairi Abdullah c, Babak Salamatinia a,⇑ a b c

Chemical Engineering Discipline, School of Engineering, Monash University Malaysia, Jalan Lagoon Selatan, Bandar Sunway, 47500 Subang Jaya, Selangor Darul Ehsan, Malaysia School of Industrial Technology, Universiti Sains Malaysia, 11700 Pulau Pinang, Malaysia School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Pulau Pinang, Malaysia

a r t i c l e

i n f o

Article history: Received 23 January 2015 Received in revised form 20 June 2015 Accepted 5 July 2015 Available online 6 July 2015 Keywords: Nanomaterials Dye removal Adsorption mechanism Adsorption capacity Environment

a b s t r a c t Application of nanomaterials in dye wastewater treatment has received wide attention in recent years. This review highlights recent developments in the use of nanomaterials for the adsorption of dyes from wastewater. Specific adsorption mechanisms, improvements, particularly for increasing adsorption capacities, and toxicity are discussed for each nanomaterial. The accumulated data indicate that nanomaterials can be effectively used for treating dye wastewater. Nanochitosan, in particular, has a huge potential for commercial application due to its sustainability with respect to excellent adsorption performance, non-toxicity and low cost. Although the applications using nanomaterials have been developing rapidly, the technology is still far from achieving the ultimate goal of commercialization. Other considerations, such as regeneration methods and treatment of actual commercial textile dye wastewater, have not been sufficiently researched. Ó 2015 Elsevier B.V. All rights reserved.

Contents 1. 2.

3.

4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1. Categorization of dyes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carbonaceous nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Carbon nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1. Effect of charge, structure, and surface area on CNT adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2. CNT nanocomposites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Nanodiamond . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Metallic nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Nano titanium dioxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1. Adsorption phase of nano titanium dioxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2. Surfactant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.3. Doping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Nano zero-valent iron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Nano zinc oxide. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Nano magnesium oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Magnetic nanomaterials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Importance of separation and the use of magnetic properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Nanomaterials with magnetic properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⇑ Corresponding author. E-mail address: [email protected] (B. Salamatinia). http://dx.doi.org/10.1016/j.seppur.2015.07.009 1383-5866/Ó 2015 Elsevier B.V. All rights reserved.

230 231 231 231 231 232 233 233 233 233 235 235 235 236 236 237 237 237

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6. 7.

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Bionanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Nanochitosan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.1. Effect of the chitosan to sodium tripolyphosphate ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.2. Cross-linked nanochitosan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Toxicity of nanomaterials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions and future perspectives. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Dyes have been extensively used for thousands of years for textile, paint, pigment and many other applications [1]. Today, dyes play a critical role in textile, paint and pigment manufacturing industries, and at least 100,000 different dye types are commercially available currently [2]. To meet industrial demand, it is estimated that 1.6 million tons of dyes are produced annually, and 10– 15% of this volume is discarded as wastewater [3]. As a result, dyes are major water pollutants. Excessive exposure to dye causes skin irritation, respiratory problems, and, for some dyes, increase cancer risk in humans [4]. In addition, the presence of dyes in wastewater also contributes to high chemical oxidation demand and causes foul odor [5]. Thus, it is of utmost importance to remove dyes from wastewater effectively to ensure safe discharge of treated liquid effluent into watercourses. Typically, dye wastewater is treated using coagulation–floccula tion [6], aerobic or anaerobic treatment [7], electrochemical treatment [8], membrane filtration [9] and adsorption methods [10]. Adsorption is the most popular of these methods, due to the effectiveness and the simplicity of the process. Dye manufacturing factories commonly use commercial activated carbon for dye removal, due to its high porosity and large surface area (500–2000 m2/g) [11]. However, commercial activated carbon is relatively expensive because of high production cost [12]. Additionally, regeneration of activated carbon requires high-pressure stream, which contributes to the operation cost of this treatment system [13]. This high cost has motivated the search for alternative adsorbents that are both economical and efficient for dye removal. Low-cost adsorbents derived from solid and agricultural wastes have received widespread attention from researchers in the last decade [14]. Most of these wastes have been shown to effectively remove dyes as well as heavy metals. For example, an adsorbent derived from palm oil waste removes copper [15] and zinc [16], as well as reactive dyes [17]. These potentially low-cost adsorbents

237 238 238 238 238 239 239 239

have been intensively reviewed by Gupta [2]. However, most of the low-cost adsorbents are microparticles [18,19], and the small contact surface area requires considerable time to achieve maximum removal of pollutants. As most industries require a fast removal rate to sustain increasing pollutant capacities, developing these adsorbents for industrial applications is not feasible. Therefore, the need to develop sustainable adsorbents that are economical and offer both high removal rates and high adsorption capacities is urgent. Nanomaterials, also referred to as nanoparticles, are particles that fall within the size range of 1–100 nm. Generally, well known nanomaterials are valued for their strength, highly active sites, low mass [20]. In addition to wastewater treatment, current research focuses on the development of nanomaterials for optical data storage [21], sensors [22] and durable and light construction materials. Although both nanomaterials and activated carbon have considerably high surface areas, some nanomaterials have two main advantages over activated carbon as adsorbents: they can be easily synthesized at a lower cost and smaller amounts are required for effective removal of pollutants [23]. Thus, it is expected that nanomaterials will become more economical than activated carbon for adsorption applications. The adsorption capacities of different nanomaterials for various heavy metals have been reviewed intensively by Hua et al. [24]. As reviewed by these researchers, the heavy metals Cu(II), Ni(II) and Cr(VI) are easily adsorbed by ZnO, c O2O3 and CeO, which exhibited adsorption capacities of 1600 mg/g, 171.1 mg/g and 121.15 mg/g, respectively [24]. These high adsorption capacities raise the expectation that these materials can be used effectively in wastewater treatment. In related research, the removal of different dyes using various nanomaterials has also shown promising results, recently which are discussed in this manuscript. The aim of this present work is to discuss the application of different nano-materials for the adsorption of dyes. Although the different parameters related to adsorption, isotherm and kinetics are

Fig. 1. Categorization of dyes according to ionic charge [3].

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K.B. Tan et al. / Separation and Purification Technology 150 (2015) 229–242 Table 1 Specific properties, applications and toxicities of various dyes [3]. Dye

Examples

Properties

Application

Toxicity

Acidic

Acid red 183, acid orange 10, acid orange 12, acid orange 8, acid red 73, acid red 18, sunset yellow, acid green 27, methyl orange, amido black 10B, indigo carmine

Water soluble, anionic

Nylon, wool, silk, paper, leather, ink-jet printing

Carcinogenic (benign and malignant tumors)

Cationic

Methylene blue, janus green, basic green 5, basic violet 10, rhodamine 6G

Water-soluble, releasing colored cations in solution. Some dyes show biological activity

Paper, polyacrylonitrile, modified nylons, modified polyesters, used in medicine as antiseptics

Carcinogenic (benign and malignant tumors)

Disperse

Disperse orange 3, disperse red, disperse red 1, disperse yellow 1

Water-insoluble, non-ionic; for hydrophobic aqueous dispersion

Polyester, nylon, cellulose, cellulose acetate, acrylic fibers

Allergenic (skin), carcinogenic

Direct

Congo red, direct red 23, direct orange 39, direct blue 86

Water soluble, anionic, improves wash fastness by chelating with metal salts

Cotton, regenerated cellulose, paper, leather

Bladder cancer

Reactive

Reactive black 5, reactive green 19, reactive blue 4, reactive red 195, reactive red 198, reactive blue 19, reactive red 120

Extremely high wash fastness due to covalent bond formation with fiber, brighter dyeing than direct dyes

Cotton, wool, nylon, ink-jet printing of textiles

Dermatitis, allergic conjunctivitis, rhinitis, occupational asthma

Vat

Vat blue 4, vat green 11, vat orange 15, vat orange 28, vat yellow 20

Use soluble leuco salts after reduction in an alkaline bath (NaOH)

Cellulosic fibers



typically reported, this review paper emphasizes the adsorption mechanism of each individual nanomaterial instead to facilitate fundamental understanding of how nanomaterials function as adsorbents for dye removal. Recent developments and improvements of each nanomaterial to further increase their adsorption efficiency are also discussed. Ultimately, our hope is that this review will provide ideas and inspiration to spark rapid development of nanomaterials for the adsorption dyes, eventually leading to the commercialization of nanomaterial-based adsorbents as alternatives to activated carbon. In the following, nanomaterial adsorbents are categorized as carbonaceous, metal oxide, bio or magnetic nanomaterials for simplicity and clearer review. 1.1. Categorization of dyes Dyes are typically classified according to structural or function groups and color [25], as well as by ionic charge upon dissolution in aqueous solution [25]. Because the ionic categorization of dyes strongly influences the efficiency of dye adsorption, this categorization is used in this review. As shown in Fig. 1, dyes are categorized into ionic and non-ionic dyes. Non-ionic dyes are further categorized into vat dyes and disperse dyes, and ionic dyes into cationic (basic), and anionic dyes (reactive, direct and acidic) [3]. The specific properties, application and toxicity for each dye are listed in Table 1. Most of the dyes mentioned are carcinogenic, highlighting the need for effective treatment of dye wastewater in dye manufacturing factories. 2. Carbonaceous nanomaterials Carbonaceous nanomaterials consist of graphite as the core component. These nanomaterials contain strong covalently bonded

pure carbon molecules and generally exhibit strong mechanical and thermal properties. Carbonaceous nanomaterials are typically synthesized from organic hydrocarbon raw materials, such as methane and acetylene. Carbon nanotubes (CNTs) and nanodiamonds have been studied recently. 2.1. Carbon nanotubes Synthesis of CNTs was first reported by Iljima in 1991, even so CNTs were synthesized prior to that date [26]. Subsequently, CNTs have been intensively studied to elucidate their interesting mechanical, electronic and optical properties [27]. CNTs have been widely applied by various industries for wastewater treatment as adsorbents. CNTs are well known for their favorable physicochemical stability, high selectivity and structural diversity [28]. Extensive research of CNTs for wastewater treatment only began in the last decade. CNTs are promising adsorbents for the treatment of major polluting heavy metals, such as Cd(II) [29], Zn(II) [29] and Pb(II) [30], as they exhibit high removal efficiencies. Research using CNTs for treatment of dyes began in 2004: Fugetsu et al. [31] investigated the adsorption of acridine orange, ethidium bromide, eosin bluish, and orange G as model dyes using CNTs. 2.1.1. Effect of charge, structure, and surface area on CNT adsorption As CNTs are carbonaceous nanomaterials, the adsorption mechanism between the chemical functional group of dyes and the CNT adsorbent is expected to be influenced by hydrophobic effects, p–p bonds, hydrogen bonds, covalent and electrostatic interactions [32]. However, careful experiments by Liu and group [33] showed that the main driving force of interaction between CNTs and dyes involves the molecular structure, which is heavily influenced by the p–p bonds, as well as the charge load of the CNTs and dyes

Table 2 Structural properties of dyes which are removed by CNTs. Dyes

Type

Structure

Initial concentration, ppm

Dosage, g/L

Surface area, m2/g

Adsorption capacity, mg/g

Reference

Methylene blue Acid red 183 Congo red Reactive green 19 Reactive Yellow 44 Methyl Orange

Cationic Anionic Anionic Anionic Anionic Anionic

Planar Non-planar Planar Planar Planar Non-Planar

10 10 250 250 250 2

0.2 0.2 0.4 0.4 0.4 0.3

217 217 92 91.2 91.2 160

59.7 49.2 148.0 152.0 148 27.6

[36] [36] [41] [41] [41] [34]

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Table 3 CNT nanocomposite functions and dyes. Composites

Function

Dyes

Initial concentration, ppm

Dosage. g/L

Adsorption capacity, mg/g

Reference

Encapsulated nanoFe3O4

Separation of adsorbent from adsorbate

Methylene blue Janus green

20

0.75

48.1

[46]

20

0.75

250

Graphene Oxide

Separation of adsorbent from adsorbate

10

0.4

81.97

[56]

RVC

Prevention of leakage of CNTs into dye wastewater Minimization of CNT aggregation

Methylene blue Methylene blue Methylene blue Congo red Direct red 23

10



43.5

[55]

37.4

1

61.92

[49]

500 20

– 0.2

450.4 50

[59] [52]

Nano-Fe3O4 with guar gum Chitosan Fe3–C

CNTs as chitosan support Separation of adsorbent from adsorbate

[34]. Generally, CNTs are positively charged between 10 and 30 mV in a wide pH range, based on zeta potential measurement [35]. Therefore, it is expected that CNTs are more favorable for the adsorption of anionic azo dyes than that of cationic dyes. However, as shown in Table 2, studies conducted by Wang and colleagues [36] revealed a higher adsorption capacity for cationic dye methylene blue (59.7 mg/g) than anionic acid red 183 (45.2 mg/g) at initial concentration of 10 ppm and adsorbent dosage of 0.2 g/L at 298 K. Further investigation showed that the planar structure of methylene blue provided face-to-face conformation which is favorable for p–p bonds interactions between the chromophore of methylene blue and CNTs. It should be noted that the chromophore is the key functional group of the dye that determines the color and the type of the dye [37]. Thus, the structure of the chromophore indirectly affects dye adsorption. The non-planar structure of acid red 183 has weak interaction with CNTs as compared to Methylene Blue due to spatial restriction [38]. Similarly, the adsorption capacity of anionic methyl orange dye (27.6 mg/g) [39] was found to be lower than cationic dye of methylene blue studied by Wang et al. [36] due to the non-planar structure of methyl orange [40]. These indicate that the conformational structure of azo dyes plays a more important role than the charge load of both azo dyes and CNTs. Table 2 presents a comparison of the adsorption capacities of methylene blue [36], congo red, reactive yellow 44 and reaction green 19 [41]. As these dyes are anionic dyes with planar structures [42,43], extra driving force was contributed by both the negative charge and the planar structure of these dyes. The extra driving force due to electrostatic attraction enhanced the CNT adsorption capability of congo red and reactive green 19 significantly. This resulted in significantly higher adsorption capacity of congo red (148 mg/g) reactive green 19 (152 mg/g), reactive yellow 148 (150 mg/g) compared to methylene blue (59.7 mg/g), despite higher surface area of CNTs used for methylene blue adsorption. From these comparisons, it can be concluded that the structure and surface charge of dyes played major roles in the adsorption capabilities of CNTs for different dyes: CNTs removed anionic dyes with planar structure most efficiently. 2.1.2. CNT nanocomposites Most of the CNTs used for adsorption are pure, i.e., they are not composited or grafted with other materials. It is almost unnecessary to add any nanocomposite to CNTs, as pure CNTs absorb well. However, the separation procedures for CNTs are rather complicated [44], due to stable dispersion of CNTs in water. The limited recovery of CNTs from treated dye wastewater results in uneconomical material loss and toxicity to the environment [45]. Therefore, some researchers have attempted to produce magnetic nanocomposite with CNTs, to allow effective separation. Magnetic separation was studied intensively by Madrakian et al.

[46], using nano-Fe3O4 composited with CNTs through encapsulation. As shown in Table 3, the adsorption capacity of CNT-nano-Fe3O4 nanocomposite with methylene blue and janus green were found to be 48.1 mg/g and 250 mg/g, respectively. Although the CNT-nano-Fe3O4 composite could be easily separated due to the magnetic properties of nano-Fe3O4, a comparison of adsorption performance should be conducted between CNTs and CNT nanocomposite using the same dyes. Materials composited on CNTs might occupy some of the adsorption sites, which could greatly affect the adsorption capacity. A comparison of results using pure CNT adsorption capacities with methylene blue by Wang and group [36] (Table 2) and its CNT nanocomposite by Madrakian et al. [46] (Table 3) is provided. It was found that the adsorption capacity with methylene blue was higher for pure CNTs (59.7 mg/g) than for the CNT-nano-Fe3O4 nanocomposite (48.1 mg/g). In general, pure CNTs are hydrophobic and aggregate due to van der Waals force, limiting contact between the dye molecule and the adsorbent surface [47]. However, with the incorporation nano-Fe3O4 into CNTs, further agglomeration of nano-Fe3O4 reduces the contact between dye molecule and the adsorbent surface [48], resulting in a reduction in adsorption capacity. To solve this problem, nano-Fe3O4 composited CNTs can be composited with hydrophilic guar gum to reduce the hydrophobicity [49] for the adsorption of methylene blue. In comparison to the work of Madrakian et al., the adsorption capacity was improved to 61.92 mg/g [49], despite the higher initial dye concentration of 37.4 ppm. By grafting with guar gum, the hydrophobicity of CNTs decreased, thus reducing agglomeration and providing better contact between the dye molecule and the adsorbent surface [49]. Based on Tables 2 and 3, it can be concluded that the adsorption capacity of methylene blue improved from 59.7 mg/g to 61.92 mg/g with the use of guar-gum-CNT-nano-Fe3O4 nanocomposite. The variation in adsorption capacity was not significant, as the composited nano-Fe3O4 still exhibited agglomeration, which could only be reduced by using surfactants or an ionic liquid, such as humic acid [48]. The adsorption of nano-Fe3O4 treated with surfactant on dyes will be further discussed in Section 4.2. Iron carbide (Fe3C) is another magnetic component that can be composited with CNTs and which functions as a magnetic separator in a manner similar to nano-Fe3O4. In comparison to nano-Fe3O4, which has a magnetic value of 80 emu/g [50], Fe3C has a higher magnetic value of 140 emu/g [51]. Thus, Fe3C is a more efficient magnetic separator than nano-Fe3O4. The adsorption capacities of CNT-Fe3C or CNT-nano-Fe3O4 nanocomposite were compared and, as shown in Table 3, direct red 23 was adsorbed at 20 ppm and 0.2 g/L dosage using CNT-Fe3C nanocomposite, resulting in adsorption capacity of 50 mg/g [52]. This adsorption capacity is significant lower than the adsorption capacity of janus green on CNT-nano-Fe3O4, which is 250 mg/g [46]. As an anionic dye with planar structure [53], it is expected that direct red 23

K.B. Tan et al. / Separation and Purification Technology 150 (2015) 229–242

would perform better than janus green: the adsorption of direct red 23 is reinforced by both electrostatic attraction and p–p stacking on the CNT surface, whereas the adsorption of janus green is only reinforced by p–p stacking on the CNT surface. However, as the CNT-Fe3C nanocomposite had a lower surface area (38.7 m2/g) than that of CNT-nano-Fe3O4 (144.68 m2/g), the adsorption performance of CNT-Fe3C nanocomposite is limited. This limitation contributes to the lower adsorption capacity of direct red 23 on CNT-Fe3C nanocomposite. Furthermore, Fe3C is synthesized from iron oxides, such as Fe3O4, at high pressure, making it a more expensive to produce than nano-Fe3O4 [54]. Thus, in terms of adsorption performance and economic feasibility, nano-Fe3O4 was a better choice than Fe3C as a magnetic component for compositing with CNTs. The separation of magnetic CNTs from treated dye using magnets results in additional costs for the adsorption process [55]. Therefore, some research has been focused on designing aggregated nanomaterials using CNTs composited with other materials. Ai and Jiang [56] provide a method to composite CNTs with graphene oxide. In this method, three dimensional random stacking between graphene oxide and the highly flexible CNTs produce a self-assembled CNT-graphene-oxide composite of cylindrical shape. This huge cylindrical structure eliminated the complicated separation process necessary for pure CNTs. It was found that, at initial concentration of 10 ppm, the adsorption capacity for methylene blue was 81.97 mg/g [56], which is higher than the adsorption capacity of methylene blue using pure CNTs (59.7 mg/g), as shown in Table 2. However, Vijwani et al. [55] reported that this method produced CNT-graphene-oxide that is still loosely attached, and thus lacks structural integrity of a covalent bonded solid. The method resulted in leakage of CNTs into the treated dye wastewater. Therefore, these researchers proposed the formation of hierarchical hybrid CNTs as a robust and reusable adsorbent. By pre-coating the silica buffer layer followed by chemical vapor deposition (CVD), an array of CNTs carpets was attached strongly onto the surfaces of porous carbon foam. By compositing CNTs into reticulated vitreous carbon foam (RVC) at CVD run time of 40 min, the resulting material was used to adsorb methylene blue. It was found that the maximum adsorption capacity was 43.13 mg/g [55] at an initial dye concentration of 16 ppm, which is lower than for both pure CNTs (59.7 mg/g) and CNT-graphene oxide (81.97 mg/g). Despite the decrease in adsorption capacity, the strong attraction between CNTs and RVC prevented the leakage of CNTs into the dye wastewater, which solved most of the leakage problems occurring with pure CNTs and CNT-graphene oxide [55]. However, further development is necessary to improve the adsorption capacity. In addition to separation purposes, CNTs have been composited with other materials because of its mechanical strength. CNTs have a Young modulus as high as 1 TPa and a strength of 68 GPa [57], making them an excellent choice as a nanofiller for improving the mechanical stability of other adsorbents. For example, the addition of CNTs to chitosan increased the force at complete breakdown in compression tests from 1.87 N to 7.62 N [58]. The adsorption capability of CNT-chitosan nanocomposite was studied by Chatterjee et al. [59] for the adsorption of congo red at an initial concentration of 500 ppm. The adsorption capacity was found to be 450.4 mg/g, which was significantly greater than the adsorption capacity of congo red (148 mg/g) reported for other research, in which pure CNTs were used to treat congo red dye [41], as shown in Table 2. Chitosan is an excellent adsorbent for anionic dyes, such as congo red due to the strong electrostatic attraction between the positively charged chitosan [60] and the anionic dye. Using only pure chitosan, the adsorption capacity for congo red was found to be 178.32 mg/g at the same conditions [58]. Therefore, the

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CNTs act as a nanofiller for chitosan, as well as an adsorbent providing additional sites for the removal of congo red. 2.2. Nanodiamond Like CNTs, nanodiamond is a carbonaceous nanomaterial that is able to adsorb dyes. Traditionally, synthetic diamond is formed through the transformation of graphite under high pressure and temperature [61]. However, this method is expensive and impractical for commercialization. As such, nanodiamond is mainly derived from the detonation of 2,4,6-trinitrotoluene. Soot forming at detonation is at sufficiently high temperature and pressure to form nanodiamond [62]. Using this method, production of nanodiamond is relatively cheaper than CNTs [63]. Raw nanodiamond from detonation requires purification to remove graphite, which is usually accomplished using oxidizing acid. As a result, many functional groups such as carboxylic acid, lactones, ketones and hydroxyl groups, are present on the surface of the nanodiamond [64]. In addition to the high specific area, these functional groups are the main driving force behind the adsorption capabilities and high adsorption efficiencies of nanodiamond for peptides [65], drugs [66], enzymes [67] and antibodies [68,69]. The adsorption of acid orange 7 using nanodiamond was studied by Wang et al. [63]. Using a high concentration of acid orange 7 (2627.4 ppm) and adsorbent dosage of 1 g/L, this dye was completely adsorbed by nanodiamond, due to hydrogen bonding between the sulfonate group and the functional group on the nanodiamond. This bonding occurred despite negative zeta potential values (2.5 mV to 30 mV) in all ranges of pH for nanodiamond [63], which otherwise would repel the incoming dye molecule due to electrostatic forces. Overall, the study of the adsorption of dye on nanodiamond is relatively new and, to our knowledge, only one group of researchers has reported on it to date. Further investigation is required to close this gap in knowledge, especially regarding the mechanism of adsorption. In particular, it is still not known whether the surface charge of nanodiamond has a strong effect on the adsorption capacity of dyes. Thus, it is suggested that the adsorption of basic dyes, such as methylene blue, be investigated using nanodiamond. 3. Metallic nanomaterials Metallic nanomaterials have a metallic element as one of the components. To date, nano titanium dioxide (nano-TiO2) [70], nano zinc oxide (nano-ZnO) [71] and nano magnesium oxide (nano-MgO) [72] and one pure metal nanomaterial, nano-zero-valent iron (nZVI) [73], have been studied for dye adsorption. These metals are not from the same group in the periodic table, and their physical and chemical properties as well as their adsorption mechanisms are significantly different. Some adsorbents, such as nano-TiO2, rely on physical and structural morphology, resulting in a predominantly physical mechanism for dye adsorption. In contrast, adsorbents, such as nZVI, adsorb dyes through an oxidation–reduction mechanism, resulting in predominantly chemical adsorption. The mechanism of these nanomaterials in adsorption will be discussed in the next few sections. 3.1. Nano titanium dioxide 3.1.1. Adsorption phase of nano titanium dioxide Nano-TiO2 is widely used as a photocatalyst to degrade dye under UV radiation. Upon exposure to UV light, hydroxyl radical forms on the surface of nano-TiO2, which are capable of oxidizing toxic compounds, such as dye molecules [74], into non-harmful

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Table 4 Dyes relying heavily on nano-TiO2 photocatalysis phase for dye removal. Dyes

Time, min

Acid orange 10 Acid orange 12 Acid orange 8 Orange G Amido black 10B Indigo carmine Methyl orange

Removal, %

Reference

Adsorption

Photocatalytic

Adsorption

Photocatalytic

60 60 60 30 30 30 0

130 160 210 120 90 90 120

18 18 18 0 0 0 0

100 100 100 100 100 100 100

[146] [146] [146] [76] [76] [76] [78]

Table 5 Dyes completely adsorbed by mesoporous nano-TiO2 without photocatalysis. Synthesis method

Structure

Dye

Time, min

Initial concentration, ppm

Dosage, g/L

Adsorption capacity, mg/g

Reference

Titanium butoxide Hydrothermal Hydrothermal Hydrothermal

Nanoparticle Nanotube Nanotube Nanotube

Reactive red 195 Basic green 5 Basic violet 10 Methylene blue

10 240 240 60

50 2000 2000 100

2 1 1 0.5

86.96 377.9 294 133.33

[80] [83] [83] [84]

molecules [75]. Therefore, most studies related to nano-TiO2 have focused more on the photocatalysis degradation of dyes. However, of the adsorbent properties of nano-TiO2 should also be taken into consideration, as efficient nano-TiO2 adsorbent might facilitate better contact between dye molecules and surfaces of reactive species on nano-TiO2 in the pre-photocatalysis phase [76]. Thus, large amounts of dye might be adsorbed on nano-TiO2 surface, increasing the reaction probability between photo-excited holes and adsorbed dyes. This in turn, would increase the overall photocatalytic efficiency [77]. Furthermore, with an effective nano-TiO2 adsorbent, it is possible that dyes could be adsorbed completely, eliminating the dependence on the photocatalytic phase for the removal of dyes. Thus in this section, the ability of nano-TiO2 as an adsorbent to remove dyes will be reviewed. Most studies have shown that, despite the fact that synthesized nano-TiO2 was able to completely degrade dye by photocatalysis, the adsorption capability was very low. The dyes that require photocatalysis for complete degradation of dyes are listed in Table 4. From Table 4, it is clear that, as a result of weak adsorption capability, these dyes were heavily dependent on the photocatalytic phase for complete removal of dyes, and, in addition, required more time in the photocatalytic phase. For example, as dyes did not adsorb on the surface of nano-TiO2 in the first 30 min of the adsorption period, and orange G [76], amido black 10B [76], indigo carmine [76] and methyl orange [78] required 120 min, 90 min and 90 min, respectively, for complete degradation of dyes in the photocatalytic phase. A long photocatalytic time is deemed not sufficiently economical, as more energy is required for the UV lamp to supply UV light for the photocatalytic process [79]. To reduce the dependency on photocatalysis, the structure of nano-TiO2 could be modified to increase the number of adsorption sites. By using titanium butoxide, Belessi et al. [80] managed to synthesize a highly mesoporous pure nano-TiO2, which was able

to adsorb reactive red 198 completely within 10 min, achieving an adsorption capacity of 86.96 mg/g [80]. As a result, the photocatalytic reaction was not required to degrade the dye. A hydrothermal method is commonly used to synthesize highly mesoporous nano-TiO2, which results in the production of titanate nanotubes (TNTs), first described by Kasuga and group [81]. Compared to normal nano-TiO2, pure TNTs generally have a relatively higher pore volume, ranging from 0.67 to 0.89 cm3/g [82]. The surface area is also very high, ranging from 157.9 to 379 m2/g [83,84]. As a result, as with the mesoporous nano-TiO2 synthesized by Belessi et al. [80], many dyes can be removed completely solely based on adsorption of dye on the TNTs. The dyes that were successfully adsorbed without photocatalytic reaction are listed in Table 5. The dyes listed in Table 5 were adsorbed completely with relatively high adsorption capacity. Basic green 5 and basic violet 10, with an initial concentration of 2400 ppm, were removed completely with adsorption capacities of 377.9 mg/g and 294 mg/g, respectively [83]. In contrast, Xiong and group [84] synthesized TNTs for methylene blue adsorption that exhibited an adsorption capacity of 133.33 mg/g. The adsorption of methylene blue on pure CNTs, CNT nanocomposites and TNTs can be compared within Tables 2, 3 and 5. It was found that the adsorption capacity of methylene blue on TNTs was far superior to that of CNTs. On one hand, the only force driving the adsorption of planar structured methylene blue are the p–p bonds of CNTs. On the other hand, the highly mesoporous TNTs are highly negatively charge due to the presence of many OH– ions on the surface of the TNTs, originating from the NaOH used during hydrothermal process [85]. The electrostatic attraction between the cationic methylene blue and mesoporous TNTs is further reinforced by a cationic exchange mechanism facilitated by the huge amount of Na+ attached to the surface [85], leading to a higher adsorption capacity for methylene blue on TNTs than that on CNTs.

Table 6 Anionic dyes adsorbed by TNTs requiring a photocatalytic phase. Dyes

Acid orange 7 Congo red Reactive blue 4 Methyl orange

Time, min

Removal, %

Reference

Adsorption

Photocatalytic

Adsorption

Photocatalytic

50 30 30 30

180 60 60 60

40 75 60 20

100 100 50 12.5

[86] [147] [147] [147]

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However, the presence of many OH– ions on the surface the TNTs limits the adsorption of anionic dyes due to electrostatic repulsion. As a result, a photocatalytic phase is still required for degradation of dyes. Table 6 list the anionic dyes adsorbed by TNTs that require photocatalysis. A comparison of the data in Tables 4 and 6 suggests that the mesoporous structure of TNTs enables a portion of the dye to be absorbed by TNTs, resulting in a generally shorter period of photocatalytic dye degradation required. However, the removal of acid orange 7 requires longer photocatalytic degradation than any dyes listed in Table 4. This longer time is due to the prolonged hydrothermal process of 48 h required to produce TNTs with the low surface area of 50 m2/g [86]. 3.1.2. Surfactant Due to the electrostatic repulsion between anionic dyes and TNTs, the efficiency of anionic dye adsorption on TNTs is limited. Thus, mesoporous nano-TiO2 as synthesized by Belessi and group [80], instead of TNTs, should be used for the adsorption of anionic dyes. As shown in Table 5, reactive red 195, an anionic dye, could be adsorbed completely without relying on the photocatalytic phase, and, thus, it might possible to use nano-TiO2 for other anionic dyes as well. Alternatively, surfactants can be used with TNTs to increase the adsorption capacity. Lee and group [70] used hexadecyltrimethylammonium (HDTMA) as a surfactant for adsorption using TNTs [70]. In this work, 0.0001 mol/dm3 HDTMA was added to TNTs. Despite the decrease in surface area of the TNTs from 243.3 m2/g to 45.3 m2/g, the adsorption capacities of anionic dyes acid red 1 and acid blue 9 (both at initial concentrations of 2000 ppm dye and 1 g/L of adsorbent) increased from 19.8 mg/g and 0 mg/g to 396 mg/g and 84.5 mg/g, respectively. The significant increase in adsorption capacities was due to the interaction of TNTs with the hydrophobic tails of the HDTMA ions, which substitutes the Na+ cation on the surface of TNTs, causing the TNTs surfaces to be positively charged [70]. As a result, anionic dyes could be adsorbed due to electrostatic attraction between the adsorbate and the adsorbent. However, as HDTMA-modified TNTs exhibit dense packing of HDTMA on the nanotubes, adsorption selectivity due to adsorbate size might become significant, resulting in higher adsorption capacities for dyes of lower molecule weight, such as acid red 1. With the significant increases in the adsorption capacities, HDTMA-modified TNTs are promising for the potential expansion of TNTs applications for the removal of anionic dyes. These results suggest that the removal of other common anionic dyes, such as methyl orange, acid orange 10 and reactive black 5, using HDTMA-modified TNTs could also be very promising. As such, the removal of these dyes should also be investigated in the future. 3.1.3. Doping Several studies have shown that pure nano-TiO2 is able to degrade dyes completely through photocatalysis using UV irradiation. These dyes include azure A, azure B, sudan III, sudan IV [87], indigo carmine, eosin Y, amido black 10B, alizarin cyanine green, orange G, malachite green, pyronin Y, and rhodamine 6G [76]. In fact, photocatalytic degradation of dyes only occurred when pure nano-TiO2 was exposed to UV radiation. However, the use of UV radiation provides several drawbacks. High exposure to UV radiation is known to cause acute and chronic eye and skin damage, which can eventually lead to skin cancer [88]. In addition, the complete degradation of some dies requires many hours of irradiation, and, due to the high energy requirements of the UV lamps [79], is not economical at commercial scale. However, nano-TiO2 doping might modify the band gap to increase the efficiency of dye degradation using UV radiation.

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Furthermore, the absorption spectrum could be extended to visible light, allowing the photocatalytic process to be conducted under visible light, which is otherwise not possible with pure nano-TiO2 [89]. Theoretically, sunlight, which consists of 95% visible light and 5% UV radiation, could be a cost-free source for photocatalysis. Reduction in the UV requirement for the photocatalytic process results in a more economical and safer process. Elements such as tungsten [90], nitrogen [91] and sulfur [90] have been reported to satisfactorily degrade dyes under visible light [92]. Although the photocatalytic efficiency has been studied intensively, not much information is available for the adsorption efficiency using doped nano-TiO2. It has been suggested that dopants attached to the nano-TiO2 surface might reduce the number of adsorption sites, affecting both the adsorption efficiency and capacity. Thus, the adsorption performance of doped nano-TiO2 is discussed further here. Adsorption efficiency studies reported the use of nitrogen-doped nano-TiO2 under visible light for photocatalysis. At pH 6, the maximum percentage removal of reactive yellow 125 [93] (initial concentration 100 mg/L, dosage 1 g/L), methylene blue (initial concentration 6 ppm) and reactive red 198 (initial concentration 20 ppm) [94] during the adsorption phase were 30%, 6% and 43%, respectively. The low efficiency removal of methylene blue was due to weak electrostatic attraction between the adsorbent, which has 10 mV zeta potential at pH 6, and the positively charged dye molecule. By increasing the pH to 9, the zeta potential of the adsorbent decreases to 30 mV [94], enhancing the removal to 96%. Despite the good adsorption performance, the photocatalysis efficiency using visible light is still generally lower than UV light: as the band gap increases, the efficiency of photocatalysis using UV light and visible light increases as well. In general, visible light could not completely degrade the dyes, and further investigation is needed to determine the parameters allowing doped nano-TiO2 to completely degrade dyes. 3.2. Nano zero-valent iron Zero valent iron and its nanoscale counterpart, nano zero-valent iron (nZVI) have been shown to be a strong reducing agents useful for removing several pollutants [95]. The synthesis of nZVI, which is simple and straightforward, involves the reaction of any ferric ion with NaBH4, as first described by Wang and Zhang [95]. The chemical reaction is described in (Eq. (1)).

2Fe2þ þ BH4 þ 3H2 O ! 2Fe0 þ H2 BO3 þ 4Hþ þ 2H2

ð1Þ

nZVI is also a relatively cheap adsorbent. Therefore, much research has been conducted to remove pollutants such as nitrates [96], chlorinated compounds [97], heavy metals (such as arsenic from groundwater [98]), lead ions [99], and copper ions [100].

Fig. 2. Schematic diagram for the three step chemisorptions mechanism for removal of anionic dyes using nZVI.

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Table 7 Dyes successfully removed by nZVI and nZVI composite. Composite

Dye

Initial concentration, ppm

Dosage

Equilibrium time, min

Removal, %

Reference

N/A N/A N/A Cationic exchange resin Cationic exchange resin Cationic exchange resin

Reactive black 5 Reactive red 198 Light green Acid orange 7 Acid orange 8 Sunset yellow

100 100 100 2500 2500 2500

0.5 0.5 10 0.05 0.05 0.05

120 120 360 4 4 5

100 100 90 97 97 97

[73] [73] [102] [104] [104] [104]

nZVI has been used effectively for dye removal of anionic dyes, such as reactive black 5 and reactive red 198 [73]. The chemisorption mechanism involves three simple steps as illustrated in Fig. 2. First, dyes are reduced by nZVI into dyes with 4 charges, while the iron is oxidized into Fe2+. Next, the chromophore group and conjugated systems of these dyes are destroyed, generating hydrogen atoms, in a reaction between Fe0 and water. Finally, dyes adsorb on the intermediate products of Fe0 (Fe2+, Fe3+) and the passive iron oxides layers [101]. For example, reactive black 5 and reactive red 198 [73], at initial concentrations of 100 ppm, were completely removed by nZVI within 360 min (Table 7). In addition, 90% removal was achieved for adsorption of light green dye [102]. Despite efficient removal and low cost, the time required to achieve equilibrium using nZVI is usually long and, thus, uneconomical and inefficient especially for treating high-capacity wastewater rapidly. This long equilibration time is due to the tendency of nZVI to aggregate, leading to limited mobility of the nanoparticles in the aqueous solution and reduction of the surface area of the reaction site. Thus, adsorption efficiency is limited and longer contact time is essential to achieve maximum removal of dyes [103]. This issue has been addressed by applying a solid matrix to decrease the tendency of nZVI to aggregate: Zhao and group [104] used cation exchange resin composited with nZVI for the removal of acid orange 7, acid orange 8 and sunset yellow. Table 7 presents comparisons between the adsorption performance of pure nZVI and nZVI-complexed cationic exchange resin. Unlike other nanomaterials, no data are available for nZVI, and, thus, the adsorption performance has been evaluated using percentage removal and equilibrium time. Data in Table 7 indicate that the removal rates (91–100%) for all dyes were very high. Although different dyes were investigated, a common trend was observed: all dyes were removed within 120–360 min using pure nZVI. However, significant improvement in equilibrium time was observed with all dyes by using nZVI-complexed cationic exchange resin, which generally required only about 4–5 min. The cationic exchange resin provides electrostatic repulsion or steric stabilization between nZVI particles by providing organic surfactants [105]. Thus, agglomeration was minimized, resulting in a rapid adsorbent mass diffusion rate to the adsorption sites on nZVI. Therefore, nZVI should be grafted with other supports to further develop rapid treatment of high-concentration industrial dye wastewater [106]. 3.3. Nano zinc oxide Nano zinc oxide (nano-ZnO) is typically used as a photocatalytic nanomaterial in the degradation of dyes. However, nano-ZnO has

also been used as a nanocomposite together with chitosan for the removal of acid black 26 and direct blue 78 [71]. In most conventional chitosan adsorption studies, the time required to achieve equilibrium is usually at least 7 h, regardless of the type of dye [107]. However, by immobilizing nano-ZnO on the surface of chitosan, equilibrium was achieved in only 3 min [71]. In addition to strong electrostatic attraction between the positive charged NH3-group from chitosan and the negatively charged anionic dye, the physical properties of nano-ZnO also contributed to the rapid adsorption of these dyes. These properties include high surface charge of nano-ZnO (+40 mV at pH 2) [108] and high surface area of nanoscale ZnO. Nano-ZnO was used to adsorb acid black 26 and direct blue 78 at initial dye concentrations of 25 ppm, 0.5 g/L adsorbent and pH 2. The adsorption capacities of nano-ZnO on acid black 26 and direct blue 78 were 52.63 and 34.48 mg/g, respectively [71]. The adsorption capacities, however, decreased significantly at pH higher than 2, due to increasing numbers of negative charged ions and the resulting increase in electrostatic attraction. As such, it was concluded that the main adsorption mechanism for nano-ZnO on these dyes is due to electrostatic attraction. 3.4. Nano magnesium oxide For a decade, nano magnesium oxide (nano-MgO) has been used a destructive adsorbent for many toxic chemical agents, an antibacterial nanomaterial [109] and adsorbent for treatment of dye wastewater since 2009 [110]. Importantly, the cost of production is low, as it is easily synthesized from abundant natural alkaline minerals [110,111]. Pure nano-MgO adsorbents have been synthesized as nanoparticles and nanoplates by Moussavi and Mahmoudi [112] and Hu et al. [113], respectively. As shown in Table 8, Hu et al. [113] used MgO nanoplates to remove congo red, while Moussavi and Mahmoudi [112] used MgO nanoparticles to remove reactive blue 19 and reactive red 198. These two studies have shown that these dyes could be adsorbed rapidly, achieving equilibrium within 5–10 min, with very high adsorption capacities: 131.3 mg/g, 166.7 mg/g and 123.5 mg/g for congo red, reactive blue 19 and reactive red 198, respectively. MgO nanoparticles provide a high surface area (150–200 m2/g), and the relatively high adsorption capacities and rapid removal of these dyes are due to its strong basic surface sites. Accordingly, the zero point charge occurs at pH 12.4, making nano-MgO very suitable for adsorbing anionic dyes by electrostatic attraction [114]. Despite the rapid adsorption rate of nano-MgO, the adsorption capacity observed was not as high as that of pure CNTs. For

Table 8 Dyes successfully removed by nano-MgO. Surfactant

Dye

Surface area, m2/g

Initial concentration, ppm

Dosage, g/L

Time, min

Adsorption capacity, mg/g

Reference

– – – Sodium surfactant (AOT) Sodium surfactant (AOT) Sodium surfactant (AOT)

Congo red Reactive blue 19 Reactive red 198 Congo red Methyl orange Sudan III

198 153.7 153.7 191.36 191.36 191.36

100 100 100 25 25 25

0.75 2 2 0.75 0.75 0.75

10 5 5 2.5 2.5 2.5

131.3 166.7 123.5 588 370 180

[113] [112] [112] [72] [72] [72]

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example, the adsorption capacity of CNTs for congo red using pure (148 mg/g, Table 2) is higher than that of nano-MgO (131.3 mg/g, Table 8), despite the fact that nano-MgO has a higher surface area (198 m2/g) than pure CNTs (91.96 m2/g). It was found that, planar CNTs provide extra driving force due to p–p bonds, as well as greater electrostatic attraction than nano-MgO, which provides only electrostatic attraction. To improve the performance of nano-MgO, Li and group [72] used a sodium surfactant, sodium bis(2-ethylhexyl)sulfosuccinate (AOT), as a template to synthesize a highly mesoporous MgO nanoplates. By testing various AOT concentrations, Li and group [72] found that 0.064 M AOT provide the best surface area (191.36 m2/g), pore volume (0.24 cm3/g) and pore size (7.52 nm) [72]. Despite the slight decrease in surface area reported by Hu and colleagues [113], the synthesized nano-MgO removed congo red, methyl orange and sudan III dyes rapidly within only 2.5 min with adsorption capacities of 588 mg/g, 370 mg/g and 180 mg/g, respectively. The AOT synthesized nano-MgO was highly mesoporous, providing extra adsorption sites, in addition to electrostatic attraction and significantly improved the adsorption capacity of congo red compared to pure nano-MgO (131.3 mg/g Table 8), pure CNTs (148 mg/g, Table 2) and CNT-chitosan nanocomposite (450.4 mg/g Table 3). Considering its high adsorption capacity, low cost, and rapid adsorption rate, nano-MgO has a huge potential to be commercialized as a superior adsorbent for rapidly treating high-capacity wastewater, as compared to more common nanomaterials such as CNTs. Other than electrostatic attraction, little is known about the adsorption mechanism, due to the relatively recent attention received by this material. In addition, nano-MgO has been show to effectively remove only anionic dyes and further research is necessary to characterize treatment of cationic dyes, such as methylene blue. 4. Magnetic nanomaterials Because of the importance of magnetism for applications, nanomaterials with magnetic properties are categorized separately from other metal nanomaterials. The importance of magnetic nanomaterials for adsorption and separation is discussed in the following sections. 4.1. Importance of separation and the use of magnetic properties When using adsorption columns for dye wastewater treatment, some of the adsorbent flows through with the treated wastewater if it is not properly supported. This is a major problem, especially for nanomaterials, as the purged adsorbents constitute contamination and defeat the purpose of treating toxic dye wastewater [115]. Furthermore, to ensure that adsorption efficiencies are maintained, adsorbents must be replenished after use, increasing the operational and maintenance costs. An external magnetic field can be used to prevent magnetic nanomaterial adsorbents from contaminating treated wastewater and has proven to be one of the most economical ways to separate adsorbent from treated wastewater. Further research on magnetic nanomaterials for dye adsorption is required to ensure that such

adsorbents have desirable characteristics with respect to treatment and separation [116]. 4.2. Nanomaterials with magnetic properties The application of magnetic nanomaterials to the adsorption of dyes is relatively new, and, to date, only ilmenite FeTiO3 and Fe3O4 nanoparticles have been investigated. Ilmenite FeTiO3 nanoparticles adsorb cationic dyes, such as methylene blue, at initial dye concentration as low as 5 ppm [23], with an adsorption capacity of 71.9 mg/g, as shown in Table 9. This adsorbent exhibits magnetization values of 0.5 emu/g. Fe3O4 nanoparticles (nano-Fe3O4) are stronger than ilmenite nano-FeTiO3, with magnetic strength values as high as 80 emu/g [50], suggesting it would be more effective in terms of separation. Recent studies have focused on modifying nano-Fe3O4 to provide better surface specificity. Furthermore, nano-Fe3O4 has the tendency to agglomerate, which could greatly limit the adsorption capacity. Agglomeration was minimized by using surfactants and ionic liquids to modify the surface of nano-Fe3O4 [117]. Polyacrylic acid [50] and humic acid [118] successfully remove dyes, such as rhodamine 6G (8 ppm initial concentration) and methylene blue (64 ppm initial concentration). Table 9 provides a comparison of ilmenite nanoparticles and nano-Fe3O4 for the adsorption of methylene blue. Nano-Fe3O4 demonstrated better magnetic and adsorption capabilities than ilmenite FeTiO3 nanoparticles. The adsorption capacity for methylene blue was higher for nano-Fe3O4 (93.08 mg/g) than ilmenite FeTiO3 nanoparticles (71.9 mg/g). The positive charge of nano-Fe3O4 was increased in the presence of an ionic liquid, 1-hexyl-3-methylimidazolium bromide ([C6MIM][Br]), and the resulting adsorbent rapidly and effectively adsorbed anionic reactive red 120 (initial concentration 20 ppm) in only 2 min with an adsorption capacity of 166.67 mg/g [117]. This performance was due to the electrostatic attraction between nano-Fe3O4 and the negatively charged anionic dyes. Table 9 provides information regarding dyes which were effectively removed using magnetic nanomaterials. 5. Bionanomaterials Due to their abundance, biomaterials — organic materials derived from living substances — have attracted a lot of research attention as potential non-toxic, low-cost and environmentally friendly adsorbents. The adsorption of biomaterials have been reviewed intensively by Crini [119]. Typically, polysaccharides have been used for dye adsorption. Consisting of several complex functional groups, polysaccharides allow a large range of modification, such as cross-linking and polymerization. Modifying functional groups by grafting could improve the performance of biomaterials as dye adsorbents. However, the adsorption performance is generally limited by the low surface area, resulting in low adsorption capacities. Therefore, nanochitosan has recently been synthesized and studied for the adsorption of dyes, as it exhibits an improved surface area. Other bionanomaterials, such as nanocellulose, are expected to be studied in the future.

Table 9 Dyes removed by magnetic nanomaterials. Nanomaterials

Modifier

Dye

Initial concentration, ppm

Dosage, g/L

Adsorption capacity, mg/g

Reference

Ilmenite FeTiO3 Nano-Fe3O4 Nano-Fe3O4 Nano-Fe3O4

– Humic-acid coated Ionic liquid Polyacrylic acid

Methylene blue Methylene blue Reactive red 120 Rhodamine 6G

2 64 20 4.8

– 4 60 0.5

71.9 93.08 166.67 55.8

[23] [118] [117] [50]

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5.1. Nanochitosan Chitosan has a structure that is similar to cellulose, consisting of repeating b-(1–4)-linked D-glucosamine and N-acetyl-D-glucosamine units [120]. It is produced by deacetylating chitin, the main structural component in crustacean shells, with sodium hydroxide [121]. As a biodegradable, low-cost and abundant biomaterial, it has been used successfully in a wide range of applications, including medicine [122], agriculture, manufacture and wastewater treatment. Chitosan has reportedly removed several types of dyes successfully. The dye adsorption to chitosan is further elaborated here, as it has been reviewed intensively by Vakili and group [123]. Briefly, chitosan effectively adsorbs reactive dyes and acid dyes, especially at low pH. In the presence of high concentration H+, the amino group of chitosan is protonated. Under these ionic conditions, anionic reactive or acid dye molecules are dissociated in aqueous medium, and sulfonate groups (acid dyes) and reactive dyes are converted into anionic dye ions. The anionic dye ions attach by electrostatic attraction to the protonated amino group of chitosan. The adsorption mechanism is shown in Eqs. (2)–(4) [124]. Hþ

—NH2Chitosan ƒƒƒƒƒ ƒƒƒƒƒ ƒ! ƒ —NHþ3

ð2Þ

Protonation

H2 O

 Dye—SO3 Na ƒƒƒƒƒ ƒƒƒƒƒ ƒ! ƒ dye—SO3 þ Naþ

ð3Þ

—NH3þ Chitosan þ dye—SO3 ƒƒƒƒƒƒƒƒƒƒƒƒƒ! ƒƒƒƒƒƒƒƒƒƒƒƒƒ —NHþ3  O3 S—dye

ð4Þ

Dissociation

Electrostatic attraction

As commercial chitosan has very low surface area (<20 m2/g) [124], it is expected that nanoscale chitosan, having a larger surface area, would exhibit drastically increased adsorption efficiency and capacities due to an increase in adsorption sites [125]. This expectation has fueled much research on the dye-adsorption performance of nanochitosan. The most common method to synthesize nanochitosan was first described by Calvo and group [126] and involves the addition of sodium tripolyphosphate (STPP) into chitosan powder. Removal by nanochitosan of anionic dyes, such as acid green 27 at pH 5 [125] and acid orange 10, acid orange 12, acid red 73, and acid red 73 [127] at an adsorbent dosage of 0.75 g/L at pH 4, has been tested. The adsorption capacities of nanochitosan and microsized chitosan [128] are presented in Table 10. As expected, the adsorption capacities of nanochitosan are higher than those of microsized chitosan. Among these dyes, the highest adsorption capacity was observed for acid orange 12 for both micro- and nanochitosan. This dye has the smallest molecular weight, allowing the dye molecule to penetrated easily and deeply into the internal pore structure of the nanochitosan [127]. The adsorption capacity both micro- and nanochitosan are similar: acid orange 12> acid red 73> acid orange 10> acid red 18. 5.1.1. Effect of the chitosan to sodium tripolyphosphate ratio In a further work, it was found that the ratio of chitosan to STPP is important for synthesizing nanochitosan that is more homogenous and exhibits a narrower size distribution [129]. This is

Table 10 Adsorption capacity of micro- and nanochitosan. Dye

Acid Acid Acid Acid

Adsorption capacity, mg/g

orange 10 orange 12 red 73 red 18

Nanochitosan [127]

Microsized chitosan [128]

1060.74 1516.93 1185.32 828.14

922.9 973.33 728.2 693.2

important as homogenous adsorbent ensures that the adsorption performance does not vary much. If the concentration of chitosan is too high compared to that of STPP, a clear solution results, indicating that the nanochitosan is not formed due to lack of STPP. Otherwise, aggregation occurs, indicating non-homogenous adsorbent is formed [129]. The optimized ratio of chitosan to STPP was found to be 3:1, with a narrow size distribution of mean diameter 86 nm. At this ratio, an opalescent suspension is observed, due to electrostatic interaction between chitosan and STPP [130]. It is also at this ratio that nanodispersion occurred, and the highest removal (80%) of reactive red 120 was observed (initial concentration 80 ppm and adsorbent dosage of 12 g/L). 5.1.2. Cross-linked nanochitosan Both micro- and nanochitosan exhibit less protonation of the amino group at pH 3 than at higher pHs, resulting in chitosan dissolution in acidic medium and gelation [131]. This tendency could reduce efficiency, especially for treatment of acid dyes in wastewater at very low pH. One common solution is to cross-link chitosan with other chemicals such as glutaraldehyde [132] and epochlorohydrin [133] to prevent gelation at low pH. However, cross-linking might compromise the adsorption capacity by decreasing the concentration of amino and hydroxyl groups. This problem was resolved in a report using templated cross-linked nanochitosan, in which maximum adsorption capacities achieved at pH 3 were 5572 mg/g and 5392 mg/g for remazol black 5 and remazol brilliant orange 3R, respectively. These results indicate that the cross-linked nanochitosan was still intact even at very low pH and was able to adsorb these dyes efficiently with significantly high adsorption capacities [131]. In the few articles describing dye adsorption using nanochitosan, only anionic dyes have been reported. However, one important consideration for commercialization is the ability of nanochitosan to adsorb basic dyes as well. Due to the amino group, chitosan has high positive surface charge that would repel approaching basic dye molecules, which are also positive charged. As expected, chitosan does not adsorb basic dyes well, as compared to anionic dyes [123]. However, adsorption of basic dyes is reportedly improved using grafted chitosan, due to significantly increased adsorption capacity [124]. Nanochitosan grafting is expected to be an important area of study with respect to adsorption of basic dyes. Dyes which have been removed using nanochitosan, as well as the conditions used for adsorption are listed in Table 11. 6. Toxicity of nanomaterials Compared to their bulk counterparts, nanomaterials exhibit a high surface area to volume ratio and very high reactivity upon exposure, which increases dye absorbance performance as well as toxicity [134]. As such, toxicity of nanomaterials has become a major concern despite their potential application in several areas, particularly wastewater treatment. Metal nanomaterials, particularly metal oxide nanomaterials and magnetic nanomaterials are well known for their nanotoxicity. Although bulk TiO2 is non-toxic, nano-TiO2 has been reported to cause unusual sedimentation, hemagglutination and hemolysis of erythrocytes in humans [135]. Nano-TiO2 has also been shown to affect physical changes in organisms in aquatic ecosystem as even at low concentrations [136]. Adverse health effects of other nanomaterials, such as nano-Fe3O4, nano-ZnO and nano-MgO have been reported for human lung alveolar epithelial cells [137] and neurons [138] as well as animal cells [139]. Although no nZVI toxicity has been reported for humans, toxicity has been reported for aquatic environments [140]. Comparatively, the most toxic nanomaterial

239

K.B. Tan et al. / Separation and Purification Technology 150 (2015) 229–242 Table 11 Adsorption of dyes using nanochitosan. Adsorbent

Dyes

pH

Nanochitosan

Acid orange 10 Acid orange 12 Acid red 18 Acid red 73 Reactive red 120 Acid green 27 Remazol black 5 Remazol brilliant orange 3R

4 4 4 4 4.5 5 6 3

Initial concentration, mg/L

Dosage concentration, mg/L

Adsorption capacity, mg/g

80

0.75 0.75 0.75 0.75 12

6000 6000

9 9

696.67 1516.93 828.14 1185.32 910 2103 5572 5392

is nano-ZnO, followed by nano-TiO2 and the least toxic is nano-MgO [141]. Nano-Fe3O4, which has been found to have low toxicity in humans, has been used to cure lung cancer. Thus, nano-MgO, nano-Fe3O4 and nZVI pose very low health risk to humans, as to compare with nano-TiO2 and nano-ZnO. The use of carbonaceous nanomaterials are also of concern due to their toxicity. CNTs are toxic to human dermal fibroblast cells [142], and nanodiamond shows low level of toxicity in human kidney cells [143]. Comparatively, CNTs are more toxic than nano-TiO2, making CNTs and nano-TiO2 relatively more toxic adsorbents compared to other nanomaterials [144]. The least toxic nanomaterial appears to be nanochitosan, which is virtually non-toxic and has been used extensively in drug delivery systems [145].

7. Conclusions and future perspectives This review has addressed the developments in the use of different nanomaterials for dye removal from wastewater, discussing relevant data available for adsorption mechanism. Clearly, the adsorption capabilities for dyes of the reviewed nanomaterials is at least comparable with activated carbon, and many are superior. With their high adsorption capacities, nanomaterials show great potential as commercial adsorbents. Some adsorbents, such as nano-MgO, have rapid adsorption rates, making them potentially desirable adsorbents for high-capacity dye wastewater treatment. One question that remains to be answered is whether the use of developed nanomaterials is sustainable. Nano-TiO2 and CNTs have been most intensively used for the adsorption of dyes. However, these two adsorbents are among the most toxic as well as the most expensive nanomaterials, as synthesis of both requires high temperature and pressure. Most dyes that can be treated with nano-TiO2 require photocatalysis for complete degradation of dyes, imposing additional utility cost for the necessary UV lamps. Although cheaper and less toxic adsorbents, such as nZVI and nano-MgO, are highly effective in removing certain dyes, the applications using them are currently quite limited. In particular, adsorption of other dyes, especially cationic dyes, should be investigated using these nanomaterials in the future. Although research has shown that nanomaterials effectively remove specific dyes, the major challenge is yet to be faced: Can these nanoadsorbents be used with comparable effectiveness in treating a real industrial textile dye wastewater? In a lab-scale study, the dye wastewater is normally prepared synthetically and only consists of a single dye. However, real-world dye wastewater consists of mixture several dyes, as well as other pollutants, such as ionic compounds. The presence of these dyes and ionic compound presents a challenge as they compete for the same adsorption sites on the adsorbent surfaces. These issues have been resolved long ago for activated carbon, allowing its commercialization. To date, there are extremely few studies dedicated to the treatment of real-world dye wastewater. One close approximation is the work of Wang et al. [36], which reported the removal of dye

Time, min

Reference [127]

4000 4000

[129] [125] [131]

with CNTs from a mixture resembling actual dye wastewater containing methylene blue and acid red 183 [36]. Until nanomaterial research can demonstrate effective treatment of actual textile dye wastewater with a performance that is comparable to activated carbon, the ultimate goal of commercialization of nanomaterials as alternative adsorbents has yet to be achieved. Clearly, more research efforts should be focused on the treatment of real-world textile dye wastewater using nanomaterials. Another important consideration for future research is adsorbent-regeneration methods. Regeneration ensures the reuse of adsorbents, ultimately reducing the cost of raw materials and resulting in more economical processes. The high cost of the regenerating activated carbon has inspired much research on potential alternative adsorbents, including nanomaterials. Very few reports describing regeneration methods of these nanomaterials are available, and clearly more research is warranted. Such research should focus on developing regeneration methods that are not only more cost-effective than activated carbon regeneration, but also safer. Overall, nanochitosan has the largest potential as a commercial adsorbent, due to its non-toxicity and significantly lower price. Most importantly, it has a much higher adsorption capacity than many adsorbents, ranging between 800 and 5000 mg/g. However, as for other adsorbents, its use in treating actual dye wastewater, as well as cheap and effective regeneration methods have not yet been intensively researched. Furthermore, the challenge of nanochitosan separation from treated dye wastewater has not been sufficiently addressed. In conclusion, the development of nanomaterials has received intensive attention recently, but it has a very long way to go before achieving the ultimate goal of commercialization. Acknowledgement The authors would like to gratefully acknowledge the Ministry of Education Malaysia (MOE) for providing research funding under the FRGS scheme grant number FRGS/2/2013 TK05/MUSM/03/1. References [1] J. Liu, D. Guo, Y. Zhou, Z. Wu, W. Li, F. Zhao, X. Zheng, Identification of ancient textiles from Yingpan, Xinjiang, by multiple analytical techniques, J. Archaeol. Sci. 38 (2011) 1763–1770. [2] V.K. Gupta, Suhas, Application of low-cost adsorbents for dye removal – a review, J. Environ. Manage. 90 (2009) 2313–2342. [3] Hunger, Industrial Dyes – Chemistry, Properties, Application, Wiley, 2003. [4] H.S. Rai, M.S. Bhattacharyya, J. Singh, T.K. Bansal, P. Vats, U.C. Banerjee, Removal of dyes from the effluent of textile and dyestuff manufacturing industry: a review of emerging techniques with reference to biological treatment, Crit. Rev. Environ. Sci. Technol. 35 (2005) 219–238. [5] V. Midha, A. Dey, Biological treatment of tannery wastewater for sulfide removal journal of international, Chem. Sci. 6 (2009) 472–486. [6] J. García-Montaño, X. Domènech, J.A. García-Hortal, F. Torrades, J. Peral, The testing of several biological and chemical coupled treatments for Cibacron Red FN-R azo dye removal, J. Hazard. Mater. 154 (2008) 484–490. [7] E. Hosseini Koupaie, M.R. Alavi Moghaddam, S.H. Hashemi, Post-treatment of anaerobically degraded azo dye Acid Red 18 using aerobic moving bed biofilm process: enhanced removal of aromatic amines, J. Hazard. Mater. 195 (2011) 147–154.

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