14 Treatment of Reactive Dyes from Water and ...

14 Treatment of Reactive Dyes from Water and Wastewater through Chitosan and Its Derivatives Mohammadtaghi Vakili1, Mohd Rafatullah,1,* Zahra Gholami,2 and Hossein Farraji3 1

School of Industrial Technology, Universiti Sains Malaysia, Penang, Malaysia 2 Centralized Analytical Laboratory, Universiti Teknologi PETRONAS, Perak, Malaysia 3 School of Civil Engineering, Universiti Sains Malaysia, Penang, Malaysia

Abstract Adsorption is a technique used to remove dyes from polluted aqueous solutions. In recent years, the use of low-cost adsorbents, such as chitosan, in wastewater treatment has increased. Chitosan is a safe, environmentally friendly and costeffective substance produced by alkaline de-acetylation(DD) of chitin, which can be used as an adsorbent. Chitosan and its derivatives have attracted considerable attention as an appropriate adsorbent for dye removal due to their specific characteristics such as non-toxicity, cationicity, biodegradability and high absorption capability. Many chitosan derivatives have been obtained by chemical and physical modifications of raw chitosan, including cross-linking and grafting new functional groups on the chitosan backbone to absorb dyes. This review summarizes the applications of chitosan and its grafted and cross-linked derivatives in removing dyes from wastewater. It also highlights notable examples in the use of chitosan and its derivatives for dye removal from aqueous solutions. Keywords: Adsorption, chitosan, reactive dye, wastewater treatment

*Corresponding author: [email protected] Ajay Kumar Mishra (ed.) Smart Materials for Waste Water Applications, (347–378) © 2016 Scrivener Publishing LLC

347

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Smart Materials for Waste Water Applications

14.1 Introduction Water is a vital substance for survival of life and health on earth. About 55% of human body, 65% of animal’s tissue and 60% of plants are composed of water. Although more than 70% of Earth’s surface is covered by water, but a lot of people suffer from a shortage of potable water, because most of the existing water on Earth is saline water in oceans, which cannot be used for drinking, cooking, farming and industrial activities. The amount of available fresh water on the Earth is very limited, and only 1% of the total existing water on Earth is freshwater and usable, which unfortunately is not readily available [1]. In recent decades, increases in the world’s population, rapid technological development, unplanned urbanization, industrialization, agricultural activities and expanded use of chemicals have resulted in threatening the environment on Earth via emission of wastes and pollutants [2]. The generated wastes by human activities contributed to the contamination of water, which is increasing day by day. This phenomenon leads to increasing the pollution of limited fresh water resources and thus threatens human health and other living organisms [3]. The sources of freshwater could be polluted by pollution transferred from an identifiable, confined and discrete source (point source) like well, channel, tunnel pipe or container or tank from which pollutants are discharged. In addition, the pollution conveyed from diffuse sources caused by human activities or naturally, which occurred over a wide area and is not attributed to a point source (nonpoint source) like agricultural or urban runoff [4]. Various industries discharge their generated waste and untreated effluents in water flows, which is recognized as one of the environmental concerns and major sources of water pollution [5]. Dyeing industries such as paper, rubber, plastic, food, leather or textile are the main sources of industrial wastewater. The wastewater generated by these industries, with main characteristics of high salinity, high chemical oxygen demand (COD) concentrations, high temperature, high fluctuation in pH (2–12) and strong coloration is one of the most important environmental concerns [6]. Some of the used dyes in these industries directly or during the dyeing process are released to effluents. In the dyeing process, due to the low level of dye–fibre fixation, about 10–15% of the used dyes are lost in wastewater. Eventually, generated highly coloured wastewater gets its way to the environment, which is problematic because of the high visibility, resistance and toxic impact of the dyes exist in the wastewater [7, 8]. The presence of dyes in the water, even at low concentration, reduces the penetration of oxygen and light, which resulted in endanger the

Treatment of Reactive Dyes from Water and Wastewater 349 environment by affect on biological cycles and photosynthetic activities. In addition, they lead to toxic effects on human health such as jaundice, skin irritation, allergies, heart defects and mutations [9]. According to the synthetic origin and complex aromatic structures of dyes, they are resistant to biological degradation. Therefore, dyes can remain stable under different conditions and biological processes are not able to eliminate the dyes easily and completely [10]. Therefore, a lot of efforts have been dedicated to the development of different type of treatment processes in order to degrade the harmful compounds by converting them to small chemical products.

14.2

Dyes

The dye is defined as a coloured chemical organic compound used for imparting and provides colour to other substances [11]. The dyes can bind to other materials surface by mechanical retention, by physical adsorption, by forming covalent bond or complexes with salts or metals [12]. Dyes can be classified according to their chemical structure. There are two major components in the dyes molecule including chromophores (quinoid, carbonyl (–C=O), azo (–N=N–), nitro (–NO2) and methine (–CH=) groups) and auxochromes (hydroxyl (–OH), amine (–NH3), sulfonate (–SO3H) and carboxyl (–COOH) groups) [13]. The chromophores are responsible to impart colour to the dye and auxochromes are used as a supplement of chromophores to deepen the colour and enhance the dye attachment towards the fibres. It should be noted that the sulfonate groups confer very high aqueous solubility to the dyes [14]. The ability to absorb light in the visible region is a feature of all dyes [15]. Due to the structural diversity of dyes, they can be classified in several ways such as chemical structure, application class and their solubility. However, the classification based on their usage is the most common method and they can be divided into anionic (acid, reactive and direct dyes), cationic (basic dyes) and non-ionic (dispersed dyes) [11]. Anionic dyes normally have similar characteristics, e.g. negative charge, ionic substituent, high water solubility and contain sulfonate (SO3Na) group but possess dissimilar structure characteristics such as xanthenes, anthraquinone, azonic and triphenylmethane, which increase dyes resistance to degradation [16]. Anionic dyes are extensively used in dyeing of polyamide and protein materials (acid dyes) and cellulosic substances (reactive and direct dyes) [17]. The process of dyeing is frequently carried out in acidic conditions due to the interaction between protonated amino

350


groups in fibres and negatively charged sulfonate groups of anionic dyes structure in acidic solution [18]. On the other hand, positively charged dyes are known as cationic or basic dyes [16]. Transformation of amino (–NH2) to ammonium (–NH3) groups is the responsible for their basic and positive properties. This class of dyes is widely used for acrylic fibres dyeing due to negative charge of these fibres, which interact with the positively charged dye molecules [18]. The presence of these functional groups in cationic dyes makes them more water soluble and provides more visibility, brilliance and intensity of colours [19]. The term of non-ionic is applied for the other group of dyes (disperse dye) with small, planar and not ionized molecules (free from ionizing groups). Due to the hydrophobic properties of disperse dyes (limited water solubility and the presence of polar groups such as –NO2 and –CN), they are more suitable for dyeing the hydrophobic fibres such as nylon, polyamide, polyester and polyurethane [20, 21].

14.3 Reactive Dyes Reactive dye was discovered in 1954 and in 1956 was entered into the commercial market [22]. Afterwards, due to the suitable dyeing properties, this class of dyes has become one of the most popular and extensively applied dyes for dyeing the cellulosic substrate such as polyamides, wool and cotton [23]. Reactive dyes are possess interesting dyeing features such as wide shade range, bright colours, ease of application and high colour fastness [24]. On the other hand, they suffer from some drawbacks such as high cost of dye, long time for batch processing, a high salt content of the wastewater, low adsorption ability, non-biodegradability, high water solubility and low degree of fixation on the surfaces, which resulted in generating highly coloured wastewater [25]. These kinds of dyes are called reactive dyes due to the presence of reactive groups on dye molecules and capability of chemically interaction (covalent bonds) with functional groups of fibre [26]. Reactive dyes are characterized by azo-based chromophores with aromatic structure combined with various types of reactive groups such as chlorotriazine, vinyl sulfone, difluorochloropyrimidine and trichloropyrimidine [27]. The other difference of reactive dyes with other dyes is in the dyeing process. The anionic properties of both reactive dyes and cellulose reduce the interaction between them, so the dyeing process is conducted in high concentrated alkaline conditions (pH 9–12), salt concentration (40– 100 g/L) and at high temperatures (30–70 °C) [13]. However, in the presence of water, some of dye molecules do not attach to surface of fibre due

Treatment of Reactive Dyes from Water and Wastewater 351 to the hydrolysis of their reactive group with the hydroxyl group of water. Consequently, a high amount of applied reactive dyes are wasted and discharged in the effluent [28]. The presence of reactive dyes in environmental can threat the ecosystem by their toxicity effects and sunlight transmission reduction through aquatic environment. Therefore, this leads to complications and environmental problems if the effluent passes to the environment without suitable treatment for dye elimination.

14.4 Dye Treatment Methods In recent years, the rapid expansion of the industry has led to an increase in industrial effluents, which is considered as one of the environmental and water pollution sources. Textile effluents constitute a major part of industrial wastewater. Release of dyes to the environment by untreated wastewater poses serious threat to the freshwater sources, aquatic life and human beings [29]. Hence, given the importance of water for human life, an effective method for treatment of dyes wastewater is necessary to control water pollution in many countries. There are various methods have been applied for the removal of dyes from contaminated waters and industrial effluents, which are generally classified as physical, chemical and biological [30]. Biological wastewater treatment is the most common method for removing dyes from wastewater [31, 32]. In this method, bacteria are used to prepare the required energy for microbial activities through various wastewater components [33]. This method is affected by factors such as dye concentration, temperature and initial pH of the wastewater. Compared with other methods, biological wastewater treatment is more environmental friendly, cost effective and appropriate for the removal of different dyes. However, this method also has drawbacks, including large area requirement, long decolourization time, and lack of flexibility in operation and design [9, 34]. Oxidation methods, electrokinetic coagulation, electroflotation, irradiation or electrochemical processes are some chemical treatment methods. This method is effective in eliminating dyes from wastewaters by using chemical reagents, such as aluminium, calcium, chlorine, lime or ferric ions [35]. This method is useful for treating industrial wastewaters. Disadvantages of this method include large volume of sludge generated as waste, pH dependence, excessive chemical use and expensive reagents [36]. Physical methods involve separation process, including sedimentation, membrane and adsorption, do not require any chemical reagent, bacteria or microorganisms to improve the quality of wastewater [2, 37]. Nevertheless,

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these methods shows some limitations such as low efficiency, high operating or investment costs, needing special equipment, use of chemicals and high sludge production. All these limitations lead to inadequacy of these methods for dye wastewater treatment in small-scale industries [38]. Adsorption is a simple and effective process for elimination of dyes from wastewater. This method is preferred by researchers over other methods and widely used in wastewater treatment since the discharged effluent has high quality as well as the use of low-cost and effective adsorbents instead of applying commercial adsorbents.

14.5 Adsorption Adsorption is a simple and effective process of dye removal from wastewater. The first application of the adsorption is not clear. However, the capability of some special materials to eliminate the colour of solution, ability of bone char for removal of colour from sugar solution and use of wood charcoal for hospitals air purification is known in 15st, 18st and 19st centuries, respectively. In addition, the first use of adsorption process in large scale was in the early 1920 in Germany and United states [39]. Over the past few decades, application of absorption has gained more importance in the industry and has been further developed for environmental protection. Adsorption is a separation process, in which the amount of chemical components (adsorbate) is increased at the surface of a solid (adsorbent) [40]. This adsorption process involves both physical and chemical actions that involve a van der Waals force or are action between an adsorbate and an adsorbent [41]. Adsorption can function in solid or liquid matrices and certainly can be used to remove pollutants from polluted aqueous solutions. Adsorption is preferred over other methods because it is rapid, conveniently designed and operated, impenetrable to toxic contaminants, discharging high-quality effluent and does not produce hazardous by-products [42].

14.6 Adsorbents for Dye Removal The degree of adsorption is affecting by many parameters such as adsorbent, adsorbate and aqueous phase properties. The type and nature of the adsorbent are the main parameters affecting the adsorption efficiency. Generally, adsorbents with sufficient pore volume and size, large surface area, mechanical stability, ease of regeneration, cost effectiveness and easy

Treatment of Reactive Dyes from Water and Wastewater 353 accessibility, high selectivity and high adsorption capacity are acceptable and appropriate adsorbents for elimination of dyes [43, 44]. Activated carbon, activated alumina, silica gel and zeolite are the commonly used commercial adsorbents for dye removal. Activated alumina is synthesized by the thermal treatment of hydrous alumina granules. Specifically, thermal treatment removes hydroxyl groups, thereby leaving a porous solid structure of activated alumina with a large surface area (200–300 m2/g). The adequate surface area of activated alumina makes it an appropriate adsorbent to remove pollutants from aqueous solutions. Previous studies have evaluated the capacity of activated alumina to remove dyes [45–47]. Zeolites are hydrated aluminosilicate minerals with a porous structure. They are naturally formed through changes in glass-rich volcanic rocks (tuff ) by sea or playa lake water. Zeolites can also be synthesized. They are an appropriate adsorbent for removing pollutants from wastewaters because of their effective properties, including high ion exchange, and their applications in molecular sieving, catalysis and sorption [48, 49]. Some zeolites can be used for the removal of dyes [50, 51] and other pollutants, such as heavy metals [52, 53]. Silica gel, invented in the 1920s, is a concentration of Si(OH)4 in siloxane chains. It can have regular, intermediate, or low density with a surface area of 750, 300–350 and 100–200 m2/g, respectively. Silica gel is a suitable adsorbent because of its valuable physicochemical properties, such as stability under acidic conditions, rapid adsorption and porous structure with high surface area. Although it is also nontoxic, nonflammable and chemically ineffective, the use of silica gel is limited by its high cost [54, 55]. Gaikwad and Misal [56] and Samiey and Toosi [57] used silica gel for dye adsorption. Activated carbon, with an outstanding capacity to absorb various chemicals, is one of the oldest and important adsorbents utilized for wastewater treatment worldwide [19]. Carbon is activated through dehydration and carbonization in the presence of heat and in the absence of oxygen. Produced activated carbon has an amorphous structure with tiny pores and a large surface area of 300–4000 m2/g. Although activated carbon is an effective adsorbent for eliminating different dyes, it is still limited by its high cost and requirement of regeneration after adsorption, which leads to decreased adsorption capability and increased cost [58]. Although activated carbon is an effective and most widely used adsorbent for eliminating of different type of dyes from wastewater due to its specific adsorption properties, but application of this adsorbent is restricted due to its pricey nature and requirement of regeneration after adsorption, which lead to decrease adsorption capability and increase the cost [58] as well as the need to the disinfection, precipitation, filtration and adjust the

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pH [59]. Moreover, due to the bacterial growth increases, the carbon needs to be reactivated at high temperatures to burn off the bacterial growth, which leads loss and oxidation of material as well as increase the cost [60, 61]. Hence, due to these drawbacks during the recent years, many efforts have been carried out by researchers in order to production of more cost-effective alternative adsorbents with effective adsorption capacity that can be used in dye wastewater treatment. Recently, researchers have been focussed on utilization of adsorbents composed of natural polymers that are not harmful to the environment and also can be obtained in abundance especially polysaccharides such as chitosan to serve as alternative adsorbents [62].

14.7 Chitosan Chitosan is de-acetylated form of chitin (the second most abundant polymer in the world). Chitin, as a naturally abundant and low-cost polymer, can be used in adsorption because of the presence of acetamide and hydroxyl groups in its structure [63]. Nevertheless, it is very resistant to biodegradation, which has become a major environmental concern [62]. Converting this polymer to chitosan is a suitable way to overcome this problem. In the 19th century, Rouget boiled chitin in potassium hydroxide and produced an acid-soluble product called chitosan [64]. Chitosan is chemically expressed as a heterogeneous, linear, cationic and polysaccharide biopolymer with high molecular weight. Compared with chitin, chitosan is chemically more versatile because of its attractive inherent properties, including biodegradability, biocompatibility, film-forming ability, bio-adhesivity, polyfunctionality, hydrophilicity and adsorption capacity [65]. These favourable properties make chitosan suitable for use in different industries, including agriculture [66], food [67], cosmetics [65], biomedical [68], pharmaceutical [69] and wastewater treatment [70]. To date, researchers are interested in selecting and using natural, effective alternative materials as adsorbents in wastewater treatment [9]. Low-cost, environmentally friendly and nontoxic materials with high surface area and high adsorption capacity are preferred [71]. Chitosan has been gained a lot of attention by researchers for use as an appropriate adsorbent for dye removal due to its specific adsorption properties such as the presence of different adsorption sites on chitosan chain, versatility, biodegradability, cationicity, high adsorption capability and selectivity, macromolecular structure, abundance and low price [72]. Thus, there has been a growing interest in use of chitosan as adsorbent in adsorption process for removal of dyes.

Treatment of Reactive Dyes from Water and Wastewater 355 However, some weaknesses of chitosan such as low surface area, low mechanical strength and solubility in acid limit the adsorption performance of this material. Therefore, favourable adsorption characteristics of chitosan could be modified through physical modification (conversion of raw chitosan flakes into beads, film and membrane) or chemical modification (cross-linking, impregnation and fictionalization) [73]. Many researchers have investigated the adsorption performance of different forms of chitosan for elimination of reactive dyes (Table 14.1).

14.7.1 Unmodified Chitosan The obtained chitosan from chitin, a solid material with high crystallinity called chitosan flakes, has been used by a few researchers as an adsorbent for reactive dye removal from aqueous solutions (Table 14.1). Juang et al. [74] analysed the sorption performance of reactive dyes on chitosan flakes as an adsorbent. Results showed that the concentrations of adsorbate and adsorbent affect the adsorption capacity and also increase in particle size of adsorbent decreases adsorption capacity. The highest adsorption capacities of chitosan flakes for Reactive Red 222 (RR222), Reactive Yellow 145 (RY145) and Reactive Blue 222 were 380, 179 and 87 g/kg, respectively. Sakkayawong et al. [75] investigated the ability of chitosan to remove Reactive Red 141 (RR141) from textile wastewater under acidic and caustic conditions. Results indicated that the process is affected by pH of dye solution. Under acidic conditions, electrostatic interaction occurs between the effective functional groups (amino groups) and the dye. Adsorption under caustic conditions is also affected by the covalent bonding of the dye and the hydroxyl groups of chitosan. In addition, the adsorption mechanism under acidic conditions is chemical, whereas that under caustic conditions is both physical and chemical. However, the maximum adsorption capacities of chitosan increased with increasing temperature. The maximum adsorption capacities of chitosan in the study were 68, 110 and 156 mg/g under a system pH of 11 at 20 °C, 40 °C and 60 °C, respectively. The adsorption of Reactive Yellow 2 (RY2) and Reactive Black 5 (RB5) by chitosan flakes was studied by Uzun [76]. He found that for maximum adsorption, the dye adsorption by chitosan from aqueous solutions must be studied at high temperature. Annadurai et al. [77] investigated adsorption and desorption of a reactive dye (Remazol Black 13) using chitosan flakes from aqueous solutions in batch system. Adsorption experiments performed under different conditions including contact time, initial dye concentration (100–300 mg/L), particle size (0.177, 0.384 and 1.651 mm), pH (6.7–9.0), and temperature

Reactive black 5

Reactive red 141

Reactive black 5

Remazol yellow Gelb 3RS

Reactive black 5

Reactive black 5

Flake

Powder

Flake

Flake

–

–

– –

–

–

–

–

–

–

–

–

– –

–

–

–

–

–

–

–

–

–

–

–

–

–

Modification reagent

–

–

–

Reactive black 8

Powder

–

Reactive black 5

Flake

–

Reactive red 11

– –

Reactive black 5

Reactive yellow 84

Flake

Reactive yellow 145

Powder

– –

Reactive red 222

Flake

– –

Reactive Blue 222

Reactive red 222

Flake

– –

Reactive yellow 145

Flake

Cross-linker

Reactive red 222

Dye

Chitosan

19.91

62.92

353

417

477

156

110

68

1100

387

650

450

500

200

188

339

494

87

179

380

Adsorption capacity (mg/g)

Table 14.1 Treatment of reactive dyes from aqueous solutions by chitosan and its derivatives.

33

33

25

25

25

60

40

20

–

–

–

–

–

–

30

30

30

30

30

30

Temperature (°C)

9

5

2

2

2.3

11

11

11

3

5

5

5

5

7

–

6

6

6

pH

[113]

[79]

[111]

[78]

[75]

[112]

[85]

[93]

[84]

[83]

[74]

Ref.

356 Smart Materials for Waste Water Applications

– –

Reactive red 222

Reactive red 222

Reactive yellow 145

Bead

Bead

– –

Reactive black 5

Reactive black 8

Bead

Reactive blue 15

ECH

ECH ECH

Reactive yellow 2

Bead

ECH

ECH

Reactive yellow 86

Reactive red 222

Bead ECH

Reactive red 189

Bead

– –

ECH

Reactive yellow

Bead

Reactive blue 2

Reactive black 5

Bead

–

Reactive red 2

Reactive black 5

– –

Reactive red

Reactive yellow

Bead

Bead

–

Reactive yellow 84

Reactive red 11

Bead

–

Reactive red 189

Bead

–

Reactive red 189

Bead

–

–

Flake

–

Reactive red 3

Remazol black 13

Flake

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

–

722

2436

1911

2422

2498

2252

1936

334

4.83

201.90

430

648

487

480

480

690

950

1189

885

1653

1106

96.0

151.5

30

30

30

30

30

30

30

25

25

30

25

25

–

–

–

–

30

30

30

30

30

60

20

5

[99]

[101]

[98]

[97]

[111]

[88]

[106]

[105]

[85]

[102]

[97]

[84]

[83]

[81]

(Continued)

4

4

3

3

3

3

3

2

7

4

2

2

3

3

3

3

6

6

–

–

–

6.7

Treatment of Reactive Dyes from Water and Wastewater 357

GLA

Reactive black 5

Reactive yellow

Powder

Poly(acrylic acid) Poly(acrylamide)

–

Poly(acrylamide)

–

Poly(acrylic acid)

–

Reactive blue –

–

Reactive yellow

PEI

Acrylamide

PMMA

–

–

–

–

–

–

–

Modification reagent

–

–

Reactive red

Reactive yellow

–

Reactive blue

Bead

–

Reactive yellow

– –

Reactive blue H5G

Reactive red

–

Bead

–

Remazol Brilliant violet

Remazol Brilliant violet

Bead

GLA

Reactive red GLA

GLA

Reactive Orange 16

GLA

Reactive yellow

Reactive blue

Flake

Bead

GLA

Reactive black 5

Powder

ECH

Reactive black 5

Bead

Cross-linker

Dye

Chitosan

Table 14.1 Cont.

1211

527

1058

456

1329

1392

1412

1125

1160

1185

204

384

357

1060

7.5

10

9

198

109

2043

Adsorption capacity (mg/g)

25

25

25

25

25

25

25

25

25

25

–

–

–

25

50

25

50

–

–

35

Temperature(°C)

2

2

2

2

2

2

2

2

2

2

5

5

7

4

2

2

2

4

4

3

pH

[111]

[105]

[107]

[95]

[94]

[93]

[100]

Ref.

358 Smart Materials for Waste Water Applications

Treatment of Reactive Dyes from Water and Wastewater 359 (30–60 °C). The maximum adsorption capacity was 91.47–130.0 mg/g. The amino group nature of the chitosan provided reasonable dye removal capability. Desorption studies elucidated the mechanism and recovery of the adsorbate and adsorbent. The number of negatively charged sites increased with increasing system pH. A negatively charged surface site on the adsorbent favours the adsorption of dye electrostatic repulsion. Szygula et al. [78] also investigated adsorption and desorption potentiality of an anionic dye on chitosan. They employed RB5 and 87% de-acetylated chitosan flakes as adsorbate and adsorbent, respectively, in a batch technique. Initial dye concentration, pH and contact time were the parameters studied in the experiment. Results showed that the maximum sorption capacity was 477 mg/g at optimum pH 1–3 and room temperature. Desorption studies explained that this adsorbent can be regenerated easily by 0.01 M NaOH solution and that adsorbed dye can be desorbed. Adsorption of RB5 onto chitosan flakes in a fixed-bed column system was as well analysed by Barron-Zambrano et al. [79]. Results showed that initial dye concentration, superficial flow velocity, bed height and particle size significantly affect the adsorption process. Analysis of the breakthrough curves indicated that adsorption is affected by mass transfer limitations, probably because of intraparticle diffusion. Regeneration experiment using 0.01 mol/L NaOH represented that the chitosan to be regenerated and the dye to be recovered. Several cycles of adsorption elution showed that chitosan is able to be regenerated and retain good adsorption efficiency. Li and Ding [80] conducted batch tests using chitosan flakes to remove Reactive Black M-2R (RBM) from wastewater. In this experiment, the effects of different temperatures (25–50 °C), chitosan dosage and degree of de-acetylation (DD) (55%, 66% and 88%) on RBM removal were studied. The sorption data showed that adsorption capacity decreases with increasing temperature. Chitosan with 66% DD exhibited the highest sorption capacity (146 mg/g) within 1 h by using 0.01 g adsorbent dosage, 298 K, and 19 mg/L initial concentration. Ignat et al. [81] also studied the adsorption behaviour of chitosan for elimination of reactive dyes, Reactive Red 3 (RR3) and Direct Brown 95 (DB95) in a batch system. In this study, effect of chitosan structure, contact time, initial dye concentration, pH, addition of sodium chloride and temperature was assessed. Results showed that increase in temperature, pH and concentration of sodium chloride decreased the adsorption of both dyes on chitosan. Accordingly, the highest adsorption values for RR-3 and DB-95 were 151.52 and 41.84 mg/g at 20 °C and 50 °C, respectively. In this study, the chitosan flakes were ground and sieved to 0.10–0.15 mm and used for adsorption experiments in a batch system.

360


14.7.2 Physically Modified Chitosan Despite dye adsorption capability of chitosan, the applications of chitosan in form of flakes are limited by some drawback such as low surface area, hydrophilicity and adsorption capacity non-porosity, high crystallinity and resistance to mass transfer. These drawbacks of chitosan flakes make them unsuitable adsorbents. To overcome these problems, chitosan flakes are usually subjected to physical modification by converting them into gel beads to increase surface area, porosity and expand chitosan polymer chains, decrease crystallinity, improve access to internal sorption sites and easily separation of beads from the solution after adsorption. All these modifications improve the adsorption capability of chitosan [73]. Since during the bead preparation, conversion of chitosan flakes to dissolvedstate forms leads to breaking of hydrogen bonds between hydroxyl groups and between amino groups. Therefore, accessibility of adsorption sites for interacting with molecules of dyes and consequently adsorption capacity of chitosan beads could be increased [82]. Other comparative studies found that the adsorption capacity of chitosan flakes is lower than that of chitosan beads. Numerous comparative studies have been carried out by researchers explaining the adsorption performance of chitosan in form of flakes and beads for removal of reactive dyes from aqueous solutions and it is reported that chitosan beads have higher adsorption capacity than that of chitosan flakes. Adsorption performances of chitosan flakes and beads for RR222 dye removal at 30 °C were analysed by Wu et al. [83]. Results illustrated that the adsorption capacity of chitosan beads (1036 mg/g) is higher than that of chitosan flakes (494 mg/g). This could be due to the higher due to the higher surface area of chitosan beads (4–6 m2/g) than that of chitosan flakes (30–40 m2/g). In other study, Wu et al. [84] also reported that chitosan beads have higher adsorption capability to eliminate reactive dyes [RR222, RY145 and Reactive Black 222 (RB222)] compared with that of chitosan flakes. Experimental data proved that compared with chitosan flakes, chitosan beads increases the immobilization rate by 14 times, increases the absorption rate by 10% (RB222, RY145) to 40% (RR222), and increases the absorption capacity by approximately 5 times. Filipkowska [85] evaluated the adsorption capacity of reactive dyes [Reactive Yellow 84 (RY84), Reactive Red 11 (RR11), RB5 and Reactive Black 8 (RB8)] on chitin, chitosan flakes and chitosan beads. Results showed that pH is crucial in the adsorption and the highest dye removal efficiency of chitin occurred at pH 3, whereas that of chitosan flakes and beads occurred at pH 5. Chitosan beads showed the highest adsorption efficiency. Maximum

Treatment of Reactive Dyes from Water and Wastewater 361 adsorption capacities of chitin, chitosan flake and chitosan beads were 350 mg/g for removal of RY84 and 450 mg/g and 690 mg/g for removal of RB5, respectively. The adsorption capacity of chitosan beads for the removal of Reactive Red 195 (RR195) from wastewater in the presence of other substances was studied by Wen et al. [86]. It is found that the presence of Ca+, Mg2+ and Fe2+ decreases adsorption capacity, whereas the presence of Na+ does not affect RR195 removal by chitosan. This result can be attributed to the chelation between these cations and chitosan chains. Chelation decreases the electrostatic interaction between RR195 and chitosan. Moreover, increasing chitosan dose has a dramatic positive effect on dye removal. Phung et al. [87] identified the adsorption ability of chitosan beads to remove RB5 from aqueous solution in a batch system under different reaction conditions. Experimental data revealed that the maximum adsorption capacity was 8.14 mg/g (more than 99% removal) obtained at optimum conditions (pH 4, 200 rpm agitation rate, 1.0 g sorbent dosage, 300 min contact time and initial dye concentration of 25 mg/L). Ong and Seou [88] also assessed adsorption of RB5 using chitosan beads. In present research, the maximum RB5 removal percentage was about 96.22% with an initial RB5 concentration of 60 mg/L for a duration of 182.5 min at pH 7 and agitation rate of 200 rpm.

14.7.3

Chemically Modified Chitosan

14.7.3.1 Cross-Linking The pH value of the dye solution usually has a great effect on the adsorption behaviour of the adsorbent. It changed the surface charge of the adsorbent and influenced the chitosan structure at specific functional groups (OH and –NH2) [89]. It is found by researchers that at lower pH, the maximum reactive dye removals by chitosan were found to be higher than that of at higher pH. This could be due to the electrostatic interactions between anionic groups in reactive dyes and cationic groups in chitosan. Sulfonate groups (–SO3H) of reactive dyes could be converted in water to active negative sulfonate groups (–SO3–). In addition, protonation of amine groups in the chitosan at low pH solution leads to increase in the adsorption of reactive dyes molecules on the chitosan [90]. However, increasing the acidity of dye solution leads to a decrease in the reactive dyes removal. This could be due to the instability and dissolution tendency of chitosan under acidic environment occurred by the protonation of its amine groups at low pH values. This is considered as one of the weaknesses of chitosan that

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could limit the successful use of it as an adsorbent in an acid environment for reactive dyes removal [73]. Therefore, cross-linking is an effective way to overcome this problem. Cross-linkers links chitosan chains resulted in improve the mechanical strength and acid stability of chitosan [91]. There are some cross-linking agents have been used for cross-linking the chitosan within adsorption of reactive dyes such as epichlorohydrin (ECH), glutaraldehyde (GLA), tripolyphosphate (TPP) and dimethylol dihydroxyethyleneurea (DMDHEU) [73]. Fahmy et al. [92] analysed adsorption of a reactive dye (Brilliant Red M5BR-2) on chitosan cross-linked with DMDHEU. Experimental data showed that cross-linked chitosan presented high affinity for the reactive dye removal and the dye removal percentage decreased significantly by increasing the pH and decreasing the time of process. The highest dye removal percentage (full removal) was occurred after 30 min of adsorption process at low pH value of 4. 14.7.3.1.1 GLA Cross-Linking Guibal et al. [93] evaluated the capability of chitosan powder cross-linked with GLA for RB5 removal from wastewater. Sorption was dependent on the acidity of the dye solution. Protonation of amino groups on chitosan in the solution with low pH leads to electrostatic interaction between these protonated groups and anionic groups presented on reactive dye resulted in the sorption of RB5 on chitosan. Cestari et al. [94] assessed adsorption behaviour of GLA cross-linked chitosan beads for elimination of Reactive Red (RR), Reactive Blue (RB) and Reactive Yellow (RY) dyes from aqueous solutions at pH 2. The results revealed that the adsorption mechanisms and adsorption quantities were affected strongly by contact times, the temperatures and the chemical structures of the dyes. Increased the temperature from 25 °C to 50 °C led to decrease adsorption of the RB, however, sorption of the RY decreased by increasing the temperature. In the case of RR removal, decreased from 25 °C to 35 °C and increased from 45 °C to 50 °C. Rosa et al. [95] analysed the removal of Reactive Orange 16 (RO16) dye from textile effluents on GLA chitosan flakes. The adsorption experiments were conducted at different pH values and initial dye concentrations. Adsorption was independent of solution pH. The adsorption rate was dependent on dye concentration at the surface of the adsorbent for each time period and on the amount of dye adsorbed. The maximum adsorption capacity was 1060 mg/g, corresponding to 75% occupation of the adsorption sites. The results demonstrated that the adsorbent material can remove dyes from textile effluents independent of the pH of the aqueous medium.

Treatment of Reactive Dyes from Water and Wastewater 363 14.7.3.1.2 TPP Cross-Linking Momenzadeh et al. [96] investigated the potentiality of removing an azo reactive dye, Reactive Red 120 (RR120), from aqueous solution by using chitosan and chitosan nanoparticles. In this study, sodium TPP was used as ionic cross-linker, and results showed that the chitosan nanoparticles have much higher adsorption capacity and faster adsorption kinetics than dissolved chitosan. The adsorption capacity of the nanoparticles and dissolved chitosan were 910 and 51 mg/g, respectively, at a pH of 4–5. 14.7.3.1.3 ECH Cross-Linking Removal of Reactive Red 189 (RR189) using ECH cross-linked chitosan beads as adsorbent studied by Chiou and Li [97]. Adsorption experiment was conducted in a batch system under different reaction conditions such as pH (1, 3, 6 and 9), temperature (30 °C, 40 °C and 50 °C), initial dye concentration (4320, 5760 and 7286 g/m3), particle sizes (2.3–2.5, 2.5–2.7 and 3.5–3.8 mm) and cross-linking ratio (ECH/chitosan weight: 0.2, 0.5, 0.7 and 1.0). Results showed that effect of both the pH of dye solution and the initial dye concentration on adsorption capacity was significantly higher than that of the cross-linking ratio and the temperature. A decrease in the solution pH and also an increase in initial concentration of dye led to increase the adsorption capacity of adsorbent. The maximum adsorption capacities of small, medium and large particle sizes were 1936, 1686 and 1642 g/kg, respectively, at 0.2 cross-linking ratio, 30 °C and pH 3. Chiou et al. [98] applied ECH cross-linked chitosan beads for the adsorption of RR222, from aqueous solution using a batch technique. The initial dye concentration and the solution pH significantly affect the adsorption of RR222. An increase in initial dye concentration results in the increase of adsorption capacity, which also increases with decreasing pH. The maximum adsorption capacity was 2252 g/kg at pH 3 and 30 °C. Chiou and Chuang [99] analysed the adsorption behaviour of an acid dye [Metanil Yellow (MY)] and a reactive dye, Reactive Blue 15 (RB15), on ECH crosslinked chitosan beads in a batch system. The adsorption capacities for MY and RB15 were 1334 and 722 mg/g, respectively, at pH 4 and 30 °C. Adsorption of RB5 on chitosan beads cross-linked by ECH from aqueous solution was reported by Kim et al. [100]. Results indicated that the RB5 removal strongly affected by changing pH and temperature. An increase in temperature and a decrease in pH of dye solution led to increase the adsorption capacity of adsorbent. The maximum adsorption capacity of RB5 onto the ECH cross-linked chitosan beads was 2.06 mol/kg at pH 3 and 35 °C. The average pore size and surface area of the cross-linked chitosan beads were 70.9 Å and 315 m2/g, respectively. Chiou et al. [101]

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synthesized an adsorbent with high reactive dye adsorption capacity using ECH cross-linked chitosan beads. This adsorbent was used to remove four reactive dyes, namely, Reactive Blue 2 (RB2), Reactive Red 2 (RR2), RY2 and Reactive Yellow 86 (RY86) from acid solutions in a batch system. The adsorption capacity increased largely with the increase in dye initial concentration as well as decreasing solution pH and adsorbent dosage. The maximum adsorption capacities values at pH 3 and 30 °C for RY86, RR2, RY2 and RB2 were 1911, 2422, 2436 and 2498 kg/g, respectively. In acidic solutions the major adsorption site of chitosan (–NH2), easily protonated and converted to (–NH3+). Therefore, the strong electrostatic interaction between the –NH3+ of chitosan and dye anions can explain the high adsorption capacity of anionic dyes onto chemically cross-linked chitosan beads. In a comparative study, removal of RR189 dye from aqueous solution using cross-linked chitosan beads with different cross-linking reagents (ECH, TPP and EDGE) was investigated by Chiou and Li [102]. Results revealed that a decrease in pH of dye solution and also increasing concentration of dye increased the adsorption capacity of adsorbents. Using ECH for cross-linking the chitosan beads led to improve their adsorption performance, while TPP chitosan beads were more rigid. The maximum adsorption capacity of ECH–TPP cross-linked chitosan beads was 1840 g/ kg at pH 3 and 30 °C, while non-cross-linked chitosan beads exhibited lower adsorption capacity (950 g/kg) at pH 6. In addition, desorption experiment showed that prepared cross-linked chitosan beads are able to be reused to adsorb the dye and to reach the same capacity as that before desorption. ECH as a proper cross-linking agent is an organic molecule contains a highly reactive three-membered oxirane ring and oxygen and chlorine heteroatoms. It is reported by many researchers that cross-linked chitosan by ECH compared with other cross linkers can be used as a suitable adsorbent with high acid stability and adsorption capacity. This could be attributed to the fact that since ECH mostly binds with –OH group, the availability of major adsorption sites (amine groups) is not compromised. These sites are not eliminated during the cross-linking process of chitosan, while other cross-linkers interact with the amine groups [73].

14.7.3.2 Functionalization In order to improve the chemical resistance and mechanical strength of chitosan against acids, alkali and chemicals, chitosan must be cross-linked to avoid dissolution and to allow its use in acidic media [91]. Cross-linkers contain minimum two reactive functional groups which links chitosan chains with covalent bond. Since cross-linkers bind with functional

Treatment of Reactive Dyes from Water and Wastewater 365 groups presented in chitosan chain, the availability of the free adsorption sites especially amine groups and consequently adsorption capacity may decrease [82]. Therefore, further chemical modification such as functionalization is required to overcome the limitation of cross-linking and improve the adsorption capacity of adsorbents. Owing to presence of different reactive functional groups such as acetamido, amino and hydroxyl groups on chitosan backbone, hence new molecules are able to be coupled to chitosan chains. Therefore, the basic properties of chitosan do not change after functionalization and the physicochemical characteristics of the chitosan are maintained, it could bringing new or improved properties. Introducing new functional groups can impart new functionality to chitosan by increasing the density of sorption sites or by increasing the selectivity for a target sorbate [82]. After modification by functionalization, chitosan achieves much improved water solubility, which increases the chelating or complexation properties or enhances adsorption properties. Moreover, functionalization may modify the chitosan properties such as mucoadhesivity, biocompatibility and biodegradability [103]. Table 14.1 presents various chitosan derivatives synthesized by functionalization used for reactive dye elimination from wastewater. 14.7.3.2.1 Chitosan Functionalized Amino Group Elwakeel [104] analysed the elimination of an anion dye, RB5, from aqueous solutions on magnetic resin derived from chemically modified chitosan using batch and column methods. In this study, chitosan was cross-linked using GLA and then chemically modified through the reaction with tetraethylenepentamine followed by glycidyltrimethylammonium chloride to produce chitosan/amino adsorbent (R1) and chitosan bearing both amine and quaternary ammonium chloride moieties (R2), respectively. Results showed that the nature of interaction between the anions and the adsorbent is dependent upon the acidity and temperature of the medium. Both adsorbents showed high affinity for the adsorption of RB5 and the maximum adsorption capacities of R1 (0.70 mmol/g) and R2 (0.90 mmol/g) occurred at pH 3 and 45 °C. The higher adsorption capacity of R2 than that of R1 could be due to the presence of both trimethylammonium and glycidyltrimethylammonium chloride in R2 adsorbent. This led to improve adsorption performance of R2 in all pH ranges (acidic/neutral/basic), while R1 is just a suitable adsorbent in acidic mediums. Kyzas et al. [105] studied the adsorption and desorption performance of modified chitosan beads for treatment of industrial wastewater containing reactive dyes [Remazol Red 3BS (RR), Remazol Blue RN (RB), and Remazol Yellow Gelb 3RS (RYG3RS)]. In this study, chitosan beads first

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cross-linked with both ECH and GLA and then modified using acrylamide and poly ethylene imine (PEI). Results revealed that cross-linking led to develop the capability of adsorbent for reuse for at least 10 cycles without significant capacity loss (5%). In addition, modification of cross-linked chitosan beads led to increase the sorption sites (amido and imino groups) on adsorbents resulted in enhancing the adsorption capacity. Dye uptake efficiency of absorbents presented highest level at acidic conditions (pH 2). This could be due to the protonation of amine groups of chitosan and increase the cationicity of adsorbents at acidic conditions. The adsorption capacity followed the order chitosan–PEI>chitosan–acrylamide>chitosan. This was attributed to the higher basicity (electro positivity) of imino groups (chitosan–PEI) versus amide groups (chitosan–acrylamide). Thus, more positively charged groups had the chance to occur on PEI derivative. Chatterjee et al. [106] investigated the adsorption capacity of modified chitosan beads to remove RB5 from aqueous solutions. Chitosan obtained from crab shells was cross-linked using ECH, modified by PEI and sodium dodecyl sulphate (SDS), and then used in the adsorption process. The maximum adsorption capacity values of PEI–chitosan (709.27 mg/g) and PEI–SDS–chitosan (413.23 mg/g) were higher than that of chitosan (201.90 mg/g) and SDS–chitosan (168.07 mg/g), indicating that the adsorption performance of chitosan and cross-linked SDS–chitosan can be highly enhanced by PEI grafting. 14.7.3.2.2 Chitosan Functionalized Carboxyl Group Singha et al. [107] assessed adsorption performance of modified chitosan using poly (methyl methacrylate) (PMMA) for elimination of reactive dyes from textile industry effluent. Experimental results revealed that that modification led to improve adsorption performance of chitosan. Moreover, modified chitosan was insoluble in water or acidic solution (pH range of 4–10). The adsorption rate was dependent on dye concentration, temperature and pH of dye solution. The maximum adsorption capacities of chitosan for RY86 (126 mg/g), Reactive Violet 5R (122 mg/g) and Reactive Blue 81 (RB81) (123 mg/g) dyes were obtained at pH 4. After modification, adsorption of yellow, violet and blue dyes on modified chitosan increased to 263, 384 and 204 mg/g, respectively, at pH 4. In addition, increase in temperature and dye concentration decreased the adsorption capacity of adsorbents. Modification of chitosan using some poly (alkyl methacrylate), such as poly(hexyl methacrylate) (PHMA), poly(ethyl methacrylate) (PEMA), poly(butyl methacrylate) (PBMA) and PMMA was evaluated by Konaganti et al. [108]. Prepared modified chitosan were applied for adsorption of a

Treatment of Reactive Dyes from Water and Wastewater 367 reactive dye [Remazol Brill Blue R (RBBR)]. It was found that the adsorption capacity followed the order chitosan–PMMA > chitosan–PEMA > chitosan–PBMA > chitosan–PHMA > chitosan, with the adsorption equilibrium capacity of chitosan–PMMA 4.5 times that of chitosan. Modification of chitosan by PMMA for elimination of Reactive Blue 19 (RB19) from aqueous solutions also was investigated by Jiang et al. [109]. Results indicated that the RB19 adsorption on the chitosan–PMMA adsorbent is pH dependent and the maximum adsorption capacity of modified chitosan (1498 mg/g) obtained at pH 3 and 30 °C. In strong acidic solution (pH < 3), beside the conversion of the amine groups (–NH2) of chitosan to (–NH3+) and dissolution of the chitosan in acidic solutions, the active negative sulfonate groups –SO3– in reactive dye in acidic solution (pH (chitosan–Aam) beads, 97% > (chitosan–Aa) powder, 95% > (chitosan–Aa) beads, 90% > (chitosan) powder, 90% > (chitosan) beads, 82%. Chitosan powders should be used only in a batch-mode stirred reactor because of the high swelling degrees, whereas the beads could be used in either batch mode or in fixed-bed configuration.

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14.8 Conclusions and Future Perspectives In today’s world, the wastewater generated by the dyeing industries with main characteristic of strong coloration, high COD concentration and high fluctuation in pH (2–12), is one of the most important environmental concerns. The presence of dyes in wastewater even in small amount can be very harmful to the aquatic environment due to high visibility, toxicity and non-biodegradability under different conditions. Among the wide range of dyes, reactive dyes are one of the most used dyes in dyeing processes due to their high stability and simple dyeing procedures. Nevertheless, removal of this kind of dyes from wastewater by using conventional physiochemical methods is difficult due their high solubility. Therefore, it is necessary to select an appropriate treatment method to improve the quality of discharged wastewater into the environment. Among all treatment strategies, adsorption is proven to be an effective method for dye removal. It is found that adsorption, using efficient adsorbents, is an effective method for elimination of reactive dyes from wastewater. Adsorption efficiency is affected by the nature and type of adsorbent. Generally, an ideal sorbent for the removal of dyes from water is an adsorbent which is characterized by its low cost, availability, environmentally friendly and high adsorption capability. Therefore, adsorption of dyes on chitosan and its derivatives has been considered by several researchers due to their specific adsorption characteristics (high absorption capability, environmentally friendly, biodegradability and low cost). Chitosan and its derivatives are efficient adsorbents with extraordinary absorption capability for dye elimination. Present study aimed to review and compare the reactive dyes adsorption on chitosan and its derivatives. The comparison between the two adsorbents is not possible because of the different experimental conditions, scarcity of information and different chemical agents used to create chitosan derivatives. We tried to highlight some points that may be useful for future research. We found that chitosan possess ideal properties for reactive dye removal. However, the applications of chitosan in form of flakes or powder are limited by their low surface area, low hydrophilicity, high crystallinity, non-porosity, low adsorption capacity and resistance to mass transfer. To overcome these problems and improve the favourable adsorption characteristics, chitosan could be modified through physical or chemical modification. Conversion of chitosan into gel beads (physical modification) leads to decreased crystallinity, expanded chitosan polymer chains, improved access to internal sorption sites, increased porosity and increased surface

Treatment of Reactive Dyes from Water and Wastewater 369 area. Chemical modification of chitosan by cross-linking and introducing various functional groups is an effective approach to improve its adsorption properties and achieve the success in use of chitosan as an effective adsorbent. All these modifications lead to enhanced adsorption capacity. In addition, it is found that few studies have been reported regeneration of chitosan and its derivatives after reactive dyes adsorption. Therefore, regeneration studies also need to be performed in detail to determine the reusability and improve the economic feasibility of adsorbents.

Acknowledgement The authors acknowledge the research grant provided by the Universiti Sains Malaysia under the Short Term Grant Scheme (Project No. 304/ PTEKIND/6312118).

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