Keywords

12.1 Introduction

Approximately 7,00,000 tons of dyes are manufactured worldwide annually; wherein about 60% is azo dye out of which 10–15% of azo dyes is passed in the water system (Langhals 2004). Azo dyes are classified as acidic, basic, disperse, reactive, direct, and solvents/food dyes. An azo dye which is derived from benzidine, C12H12N2 is the main segment of carcinogenicity. Textile industries use a huge amount of freshwater for dyeing and processing, which discharge large amount of chemical pollutants like organic surfactants, dyes, dispersants, alkalis, acids, solvents, and various salts into the water bodies. The presence of dyes in water creates acute aesthetic and aquatic problems by disturbing the photosynthetic reaction due to light penetration. So, it is essential to focus on wastewater treatment technologies for its reuse and recycle. From the view of an environment, severe problems such as toxicity to water bodies and foremost to carcinogenic and mutagenic effects on humans are created by the textile-based industries (Dalvand et al. 2011; Du et al. 2017). The removal/treatment of dyes from polluted wastewater is comparatively more significant than the other soluble polluted compounds, which also add the portion of biochemical oxygen demand (Holkar et al. 2016). Many methods exist to eradicate harmful dyes from wastewater such as coagulation, an electrochemical process, membrane separation process, chemical oxidation, adsorption, reverse osmosis, and biodegradation. But most of these processes are not so acceptable due to additional chemical demand, producing large amount of secondary by products with their disposal problem and monetary disadvantages. A massive amount of such dyes in wastewater is nonbiodegradable because of their complex and stable molecular structures.

The drawback with the conventional biological methods (Shankar et al. 1999) is the treatment period. Treatment period typically varies from 1 day to 6 days. In addition, traditional processes (physical and chemical) are found to be comparatively expensive and may also lead to other secondary pollution due to the requirement of additional chemicals (Oller et al. 2011). Conventional biological procedures in association with physical/chemical processes to obtain improved decolorization of effluents may generally be costly and limited applicable. The adsorption processes using suitable adsorbents (activated carbons, charcoals, clays, diatomaceous earth, microbial biomass, compost, unmodified and modified lignocellulose, etc.) for the wastewater treatment have disadvantage of regeneration, which also increases the cost of the process. At times, it demands a very long time, and it may be ineffective for low concentrations of pollutants. Oxidation can be used to decolorize the dye effluent using various oxidizing agents such as hydrogen peroxide and ozone activated with Fe(II) salts. However, the pollutant removal mechanisms involve flocculation, and contaminants are shifted from the wastewater to the sludge, which further demands disposal in landfill (Boczkaj and Fernandes 2017; De Lima et al. 2017). Hence, the abovementioned techniques may not provide sustained solution with ecological point of view. Very recently, electrochemical treatment methods have achieved a good position as effective and efficient treatment technologies for dye effluents. However, the application of electrochemical techniques warrants a higher investment in equipment and energy requirement (Vaghela et al. 2005; Butler et al. 2011; Gautam et al. 2019).

In the recent years, researchers have used ionic liquids for various applications such as biocatalytic transformation, chemical synthesis, electrochemistry, analytical chemistry, and separation processes, because of the green characteristics of ionic liquids (Wasserscheid and Keim 2000; van Rantwijk et al. 2003; Pandey 2006; Hernández-Fernández et al. 2015). Main advantages of ionic liquids are the better possibilities of reusing and relatively easy recovery of ionic liquids. Therefore, application of ionic liquids effectively reduces the amount of waste generated during treatment process. The photodegradation of dye molecules using ionic liquids (ionic liquids), also regarded as green solvents, has advantageous over other processes with the view of environment protection. However, ionic liquids are still quite expensive. Hence, regeneration and recycling of ionic liquids are important to make a technology economically viable (Fernández et al. 2010). The scheme to reduce the amount of pollutants from industrial effluents using ionic liquids and potential sources of environmental release is presented in Fig. 12.1.

Fig. 12.1
figure 1

The scheme to reduce the amount of pollutants from industrial effluents using ionic liquids (IL)

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12.2 Ionic Liquids

An ionic liquid is a molten salt comprising of positive charged ions, cations and negative charged ions, anions having melting point below 100 °C. Combination of bulky and asymmetrical cations such as imidazolium, pyrrolidinium, and pyridinium and evenly shaped symmetrical anions such as halides, triflate, and nitrate lowers the lattice energy which is accounted for low melting points. Ionic liquids are also known as liquid electrolytes, room temperature molten salts, low temperature molten salts, ambient temperature molten salts, ionic melts, ionic fluids, fused salts, liquid salts, ionic glasses, or neoteric solvents (Wilkes 2002). Room temperature ionic liquids are salts which are in liquid state at or below room temperature. The structures of common cations and anions of ionic liquids are presented in Fig. 12.2. Ionic liquids are gaining worldwide acknowledgement from researchers as well as people in the industries for a large number of applications (Heintz and Wertz 2006).

Fig. 12.2
figure 2

Structure of common cations and anions of ionic liquids. (Sun and Armstrong 2010)

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Depending on the cation and anion, the characteristics of ionic liquids, e.g., viscosity, solubility, and cloud point, may differ. The main benefit of ionic liquids is the provision of the replacement of anion that enables developing compounds with characteristics necessary for specific application. These tuned salts are referred to as task-specific ionic liquids/functionalized ionic liquids. Moreover, non-volatility, non-flammability, and better thermal stability make ionic liquids very attractive for industrial applications. Ionic liquids also have excellent features for removal of organic pollutants and metal ions because of its low volatility, chemical stability, and chelating abilities (Domanska and Rekawek 2009; Egorov et al. 2010; Regel-Rosocka and Wisniewski 2011). Ionic liquids consist of loosely coordinating bulky ions with the polarities comparable to alcohols. In addition to negligible vapor pressure, ionic liquids are nonexplosive and nonflammable. Ionic liquids have application in fields such as electrochemical, photo-degradation of organic compounds, catalysis and/or reaction media, drug delivery, biotransformation, solar cells, and various areas of separations (Patel and Lee 2012). The tailored ionic liquids used in reaction media and separation processes are reported to show better activity, selectivity, yields, stability, and environmental safety than conventional solvents (Dietz 2006; Han and Row 2010; Bollin and Viamajala 2012). An overview of physical properties such as density and viscosity of selected ionic liquids used in separations are presented in Table 12.1 (Han and Row 2010). The design ability of the functionalized ionic liquids/task-specific ionic liquids provides better opportunities to change the structures in cation and anion, and the length of the side chain can also be altered to the organic cation to achieve tailored/ functionalized ionic liquid of special properties.

Table 12.1 Physical properties of ionic liquids at 25 °C
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12.3 Functionalization of Ionic Liquids

The chemo-physical properties of ionic liquids depend on the cation-anion combination and the length of alkyl chain. The functionality of ionic liquids also has a strong effect on the properties. Thus, the synthesis of functionalized ionic liquids is generally performed by the integration of functional groups into the cation of ionic liquid. Till date, mostly, the imidazolium cations are functionalized, and a few functionalized anions are known. Imidazolium cations can be modified using functionalized groups such as alkyl halides, hydroxyl groups, carboxylic groups, thiol groups, alkene groups, diene groups, and so on (Ye and Shreeve 2004; Fei et al. 2005). Using the quaternization method, most of the functional groups directly added imidazolium moiety and give the desired functionalized ionic liquids. Certain functionalities require other synthetic routes to be connected to the imidazole backbone (Liddle and Arnold 2005). The imidazolium salts with SiOMe3 groups can also be synthesized with greater yield using direct quaternization. The hydrophilicity of these imidazolium salts may be controlled on the surfaces of Si/SiO2 by anion exchange (Gao et al. 2004).

Less attention was given on the synthesis of pyridinium-based functionalized ionic liquids rather than imidazolium systems. Pyridinium-based ionic liquids are successfully functionalized as the nitrile salt, 1,2,4-substitute-triazolium salts, and phosphazene-based ionic liquids. Other low melting salts have also been formed using quaternization of pyrazine and pyrimidine with alkyl and polyfluoroalkyl halides. Functionalized ammonium ionic liquids, also known as ammonium salts, have been synthesized with ether groups. The ammonium salts show enhancement in cathodic and anodic stability and provide larger electrochemical window (Zhao 2007).

12.4 Applications of IL in Wastewater Treatment

Ionic liquids have excellent features for extraction of metal ions/organic pollutants from aqueous solutions because of its low volatility, chemical stability, and chelating abilities. In this section, few recent studies are discussed particularly extraction of heavy metals/others from wastewater using functionalized and modified ionic liquids as presented in Table 12.2. Major findings of the study appeared in the literature are presented in this table.

Table 12.2 Few selected studies on the wastewater treatment using ionic liquids
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12.5 Degradation of Dyes Using Functionalized Ionic Liquids

Ionic liquids have found the recent applications in fields such as electrochemical, photo-degradation of organic compounds, catalysis and/or reaction media, drug delivery, biotransformation, and various areas of separations (Patel and Lee 2012). In this section of the chapter, recent and selected studies have been presented in Table 12.3 for the degradation of various dyes such as methyl orange, methylene blue, rhodamine B, indigo blue, and Sudan III using the functionalized ionic liquids. Applicable ionic liquids are also assisted by incorporating different nanoparticles such as Ag and TiO2. It is clear from Table 12.3 that ionic liquids can be effectively used for the effective removal of the dyes. The following section describes a case study on the photodegradation of organic dyes, namely, methylene blue and congo red, using a polymer-supported ionic liquid Fe-porphyrin complex.

Table 12.3 Recent studies on degradation of dyes using functionalized ionic liquids
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12.6 Case Study: Photodegradation of Organic Dyes, Methylene Blue, and Congo Red Using a Polymer-Supported Ionic Liquid Fe-Porphyrin Complex

In this case study, a polymer-supported ionic liquid Fe-porphyrin complex was synthesized using the procedure described below. Ionic liquid complex was tested for the photodegradation of the organic dyes, namely, methylene blue and congo red. Results were analyzed using several characterization techniques.

12.6.1 Synthesis of a Polymer-Supported Ionic Liquid Fe-Porphyrin Complex

  • Synthesis of Poly 1-(4-(sec-butyl) benzyl)-3-(3-formyl-4-hydroxybenzyl)-1H-benzo [d] imidazol-3-ium chloride, 1-A

For the synthesis of a polymer-supported ionic liquid Fe-porphyrin complex, firstly, equal moles of poly 1-(4-s-butyl) benzyl)-1H-benzo[d]imidazole, 0.02 mol/5.284 g and 5-(chloromethyl)-2-hydroxybenz-aldehyde, 0.02 mol/3.411 g were taken and mixed in the round bottle flask of 100 mL with acetonitrile of 25 mL. The complete reaction mixture was agitated 24 h using a magnetic stirrer at ambient temperature. After completion of reaction, a new compound was formed and washed using a solvent, diethyl ether to acquire white powder form of 1D (Balinge et al. 2017, 2018). The yield and melting point of the resultant compound were determined as 68.99% and 180–198 °C, respectively.

1H NMR (400 MHz, DMSO-d6): δ 0.52 (s, 1H), 0.74 (br. 1H), 1.08 (br. 1H), 1.29 (s, 1H), 1.92 (s, 1H), 5.65 (s, 4H), 7.36 (br. 6H), 7.73 (br. 1H), 10.13 (d, 2H, J = 8.97 Hz), 15.79 (br. 1H). 13C NMR (100 MHz, DMSO-d6): 190.77, 161.74, 159.18, 136.94, 135.46, 131.29, 129.18, 127.15, 122.94, 118.64, 114.49, 49.76. FT-IR – (KBr, v/cm−1): 3377.36, 3018.60, 2926.01, 1654.92, 1612.49, 1558.48, 1487.12, 1444.68, 1371.39, 1282.66, 1249.87, 1188.15, 1151.50, 1016.49,929.69, 842.89, 744.52, 698.23, 632.65, 540.07, 497.63, 424.34.

  • Synthesis of 3-(2-carboxyethyl)-1-(3-formyl-4-hydroxybenzyl)-1H-benzo[d]imidazol-3-ium bromide, 1-B

The synthesis of 3-(2-carboxyethyl)-1-(3-formyl-4-hydroxybenzyl)-1H-benzo[d]imidazol-3-ium bromide was carried out according to the procedure given in (Muskawar et al. 2013; Khiratkar et al. 2016) with slight modification. 5-(1-H-benzo[d]imidazole-1-yl) methyl)-2-hydroxybenzaldehyde-(BMSA), 7.56 g/30 mmol was stirred for 1 h in toluene solution of 50 ml at 60 °C. Then, 3-bromopropionic acid of 6.08 g/40 mmol in toluene was added drop wise for 30 min and stirred for next 14 h at 85 °C. After this, the whitish solid product was obtained, after the decantation of toluene and washing with ethyl acetate, 3 × 20 mL and diethyl ether, 3 × 25 mL with yield of 73.33%, and melting point of 205 °C.

1H NMR (400 MHz, DMSO-d6): δ 2.63 (s, 2H), 4.33 (s, 2H), 5.35 (s, 2H), 6.67 (s, 1H),7.15 (s, 1H), 7.29 (s, 1H), 7.50 (d, 1H, J = 19.52 Hz), 7.75 (s, 1H), 9.24 (s, 1H), 9.58 (s, 1H), 9.88 (s, 1H), 10.55 (s, 1H), 12.32 (1H, br), 15.58 (1H, br), 13C NMR (100 MHz, DMSO-d6): 190.97, 172.31, 161.49, 161.32, 142.40, 142.57, 136.53, 129.55, 127.12, 126.01, 125.24, 122.90, 118.48, 116.31, 114.47, 114.34, 113.51, 65.36, 49.56, 33.21, 15.63. FT-IR – (KBr, v/cm−1): 3360, 3120, 1720, 1560, 1440, 1249, 1184, 1128, 989, 823, 744, 626, 495.

  • Synthesis of 1,1′1″-(((20-(5-((3-(4-(sec-butyl)benzyl)-1H-benzo[d]imidazole-3-ium-1-yl)methyl)-2-hydroxyphenyl)porohyrin-5,10,15-triyl)tris(4-hydroxybenzene-3,1-diyl))tris (methylene))tris(3-(2-carboxyethyl)-1H-benzo[d]imidazole-3-ium) tribromide chloride, (PSILPP) ligand 1-C

1-(4-(sec-butyl)benzyl)-3-(3-formyl-4-hydroxybenzyl)-1H-benzo[d]imidazol-3-ium chloride, 1-A of 1.01 g/2.5 mmol and 3-(2-carboxyethyl)-1-(3-formyl-4-hydroxybenzyl)-1H-benzo[d]imidazol-3-ium bromide, and 1-B of 3.039 g/7.5 mmol were mixed thoroughly in propionic acid, 20 mL at 60 °C for 30 min. To the mixture, pyrrole of 0.67 g/10 mmol in propionic acid of 15 mL was fed gradually for 30 min with controlled stirring for next 60 min at 130–140 °C. The purple solid product was then obtained. This was separated by removal of 2/3 propionic acid using rotary evaporator and finally washing with methanol followed by diethyl ether, 3 × 25 mL. The NH protons inside the ring of porphyrins acquired acidic character and therefore can get deprotonated to provide porphyrin to ions (Rabbani et al. 2016; Min et al. 2019). %Yield = 71.99% and melting point = above 250 °C were found.

1H NMR (400 MHz, DMSO-d6): δ −1.006 (s, 2H), 0.05(t, 5H, J = 7.62 Hz), 0.24(s, 2H), 1.21(q, 3H, J = 7.88 Hz), 1.54(s, 3H), 1.99(q, 2H, J = 6.1 Hz), 3.74(q, 3H, J = 7.21 Hz), 4.72(brs, 5H), 6.17(s, 7H), 6.57(m, 9H), 6.69(q, 4H, J = 3.17 Hz), 6.84(q, 7H, J = 3.00 Hz), 6.98(d, 4H, J = 8.58 Hz), 7.07(q, 4H, J = 3.19 Hz), 7.10(s, 5H), 8.49(s, 6H), 8.86(s, 2H), 12.21(s, 2H). FT-IR – (KBr, v/cm−1): 3026, 2929, 1712, 1610, 1560, 1502, 1438, 1330, 1255, 1182, 991, 827, 744, 601, 464.

  • Synthesis of 1,1′1″-(((19-(5-((3-(4-(sec-butyl)benzyl)-1H-benzo[d]imidazole-3-ium-1-yl)methyl)-2-hydroxyphenyl)-12,13-dihydro-112H,32H-11λ2–1(12,3)-1λ2–5,8-(azeno)pyrrolo[1,2-b][1]zinc[2,11]diazacycloundecina-3(5,2)-pyrrolacycloheptaphan-5-ene-14,2,4-triyl)tris(4-hydroxybenzene-3,1-diyl))tris(methylene))tris(3-(2-carboxyethyl)-1H-benzo[d]imidazole-3-ium) tribromide chloride, polymer supported ionic liquid iron porphyrin complex 1-D

Polymer-supported ionic liquid iron porphyrin complex was successfully prepared and characterized by the different methods. The complex ligand 1-C of 0.903 g/0.5 mmol and FeCl3 of 0.097 g/0.6 mmol were charged to methanol and stirred initially for 30 min at atmospheric temperature. Later, above mixture was subjected to reflux at a temperature of 75 °C for 5 h in the inert atmosphere of nitrogen. The synthesized product was then isolated by periodic washing using methanol, and it was dried in the oven for 2 h at a temperature of 80 °C. Finally, a purple color powder of polymer-supported ionic liquid iron porphyrin complex was obtained as given in Fig. 12.3 with yield of 79.93% and melting point of above 250 °C.

Fig. 12.3
figure 3

Synthesis of polymer-supported ionic liquid Fe-porphyrin complex

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FT-IR – (KBr, v/cm−1): 3248, 1714, 1608, 1562, 1506, 1438, 1259, 1189, 1132, 993, 829, 746, 592, 505, 422.

12.6.2 Photodegradation of Methylene Blue and Congo Red

The synthesized polymer-supported ionic liquid iron porphyrin catalyst was used to photodegrade the two azo dyes, methylene blue and congo red under the simulated light and normal air at specific values of pH. The photodegradation results are shown in Figs. 12.4 and 12.5. The process parameters used in photodegradation were optimized to maximize the degradation. In the process of optimization, photodegradation of dyes in visible light was investigated by changing one variable, and remaining variables were kept constant. After getting one optimum parameter, next parameter study was carried out keeping fixed other variables to find optimum photodegradation. First, the photodegradation of methylene blue using this catalyst with different quantities was optimized under simulated light at temperature of 25 °C. The results are presented in Table 12.4 as serial numbers 1–5 and Fig. 12.4, 1B.

Fig. 12.4
figure 4

Methylene blue, MB dye (A) R-square value, (B) catalyst loading, (C) time variation of 10 mg catalyst and atmospheric air, and (D) time variation of atmospheric air. (Carried out at pH of ~5 in the presence of 5 W LED light)

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Fig. 12.5
figure 5

Congo red, CR dye (A) R-square value, (B) catalyst loading, (C) time variation of 12.5 mg catalyst and atmospheric air, and (D) time variation of atmospheric air. (Carried out at pH of ~5 in the presence of 5 W LED light)

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Table 12.4 Total optimum conditions of methylene blue dye degradation with or without polymer-supported ionic liquid Fe-porphyrin complex catalyst (10 ppm and 25 mL)
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It was found from Table 12.4 as serial number 12 and Fig. 12.4, 1D that only 22.42% of methylene blue was degraded in 60 min without using catalyst, under atmospheric air irradiation. On the other hand, 46.78% degradation efficiency of methylene blue was obtained with 5 mg catalyst as shown in Table 12.4, serial number 1 and Fig. 12.4, 1B. The improvement in the degradation efficiencies was observed as 65.31% and 77.76% with higher catalyst loadings of 7.5 mg and 10 mg, respectively, as given in Table 12.4, serial numbers 2–3 and Fig. 12.4, 1B. Comparatively less improvement was observed in photodegradation of methylene blue with the catalyst loading above 10 mg as shown in Table 12.4, serial numbers 4–5, and Fig. 12.4, 1B. This implies that the photo-induced electrons and holes are produced at lesser rate with high catalyst loadings and leads to a limiting factor for degradation. Catalyst loading of 10 mg is found to be a better choice for the photodegradation of methylene blue.

The photodegradation of congo red using polymer-supported ionic liquid iron porphyrin with different amounts of catalyst at atmospheric temperature of 25 °C under simulated sunlight was studied as given in Table 12.5 from serial numbers 1–5 and Fig. 12.5, 2B. Without catalyst, 21% of congo red was only able to degrade under atmospheric air irradiation in 60 min, whereas 41.7% removal was accomplished with 5 mg of catalyst as presented in Table 12.5 and Fig. 12.5, 2B. The increased removal was noticed at higher loadings of the catalyst, where −8.41% and 51.76% degradation were observed with catalyst loadings of 7.5 mg and 10 mg, respectively, as represented in Table 12.5, serial numbers 2–3, and Fig. 12.5, 2B. The negative value of degradation efficiencies may be attributed to the formation of few chromophore groups during the photodegradation. Similar findings were obtained by  Kharazi et al. (2018); Khajone et al. (2019).

Table 12.5 Optimization of parameters for degradation of congo red dye with or without polymer-supported ionic liquid Fe-porphyrin complex catalyst (25 mL of 10 ppm)
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The rate of improvement was enhanced with 12.5 mg photocatalyst but found lesser thereafter, in serial numbers 4–5 of Table 12.5 and Fig. 12.5, 2B, which means that the photodegradation rate can be controlled by the production rate of photo-induced electrons and holes at these higher catalyst dosages. Next, the influence of duration was also influenced on photodegradation of congo red using the optimized catalyst loading of 12.5 mg in 25 mL at 10 ppm concentration. The progress in the degradation efficiencies was detected from 41% to 98.82% with respect to duration from 15 min to 75 min as presented in Table 12.5, s. no. 6–10. However, there was no appreciable enhancement with more duration, Table 12.5 at s. no. 11. It was found that there was no significant degradation in absence of photocatalyst as given in Table 12.5, s. no. 12–17.

12.6.3 Possible Mechanism

The possible mechanism of the dye degradation by polymer-supported ionic liquid iron porphyrin catalyst is given in Fig. 12.6. When exposed to replicated sunlight, electrons could reduce O2 to O2−· and the OH− and OH· radicals could be formed with the photo-generated holes. More radicals are generated due to the higher separation efficacy of electron–hole pairs in the developed photocatalyst. These generated radicals such as · OH· and O2− and 1O2 were found to be responsible for the photodegradation of methylene blue and congo red. Equations (12.1, 12.2, 12.3, 12.4, 12.5, 12.6, 12.7, and 12.8) illustrate the creation of radicals.

Fig. 12.6
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Plausible mechanism for dye degradation by polymer-supported ionic liquid Fe-porphyrin complex (PSILFePP)

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$$ catalyst+ h\nu \to {e^{-}}_{CB}+h{\nu^{+}}_{VB} $$
(12.1)
$$ {h^{+}}_{VB}+{H}_2O\to {H}^{+}+{}^{-}OH $$
(12.2)
$$ h\nu +{}^{-}OH\to {OH}^{\bullet } $$
(12.3)
$$ {e^{-}}_{CB}+{O}_2\to {O_2}^{-} $$
(12.4)
$$ {O_2}^{-}+{H}^{+}\to {{}^{\bullet }O}_2H $$
(12.5)
$$ {{}^{\bullet }O}_2H+{e^{-}}_{CB}+{H}^{+}\to {H}_2{O}_2 $$
(12.6)
$$ {H}_2{O}_2+{e}^{-}\to {OH}^{\bullet }+{}^{-}OH $$
(12.7)
$$ Dye+{OH}^{\bullet}\to degraded\ product $$
(12.8)

12.7 Conclusions

This study presents the recent applications of functionalized ionic liquids for the removal of metal ions and different dyes. An up-to-date review of the recent articles is presented in the first part. Later, a case study focusing on the application of a polymer-supported ionic liquid Fe-porphyrin complex for the photodegradation of organic dyes, namely methylene blue and congo red, is presented. It was observed that a polymer-supported ionic liquid Fe-porphyrin complex could be effectively used for the removal of organic dyes. This may pave a way for the efficient removal of the organic pollutants from the wastewater.