The Coffee Residues and the Esparto Fibers as a Lignocellulosic Material for Removal of Dyes from Wastewater by Adsorption

Abstract

Biosorption onto lignocellulosic products such as coffee residues and esparto fibers in natural and modified forms have been identified as a potential alternative to the existing biosorbents applied for dye removal from wastewater. The efficiency of each material has been discussed with respect to the operating conditions and the chemical modifications. The investigated thermodynamics and kinetics studies were exposed also in terms of equilibrium isotherms and fitted kinetic models. Moreover, the crucial role of the chemical structures of the cellulosic fibers as an affecting factor on the mechanism of the adsorption process was evaluated and compared. The different treatment methods showed an improvement in terms of removal and maximum adsorption capacity. In fact, in some cases the removal capacity can be increased to 99% and the maximum adsorption capacity can reach 67 mg/g. On the other hand, the different investigations showed that the study data fitted to the known model such as Langmuir isotherm and pseudo-second-order kinetic.

1. Introduction

Water pollution due to the rejected industrial hazard has become global issue of concern. Many industries such as textile industry use dyes to color their products and thus produce wastewater containing organics with a strong color. Dyes are used in different industries such printing, textile and cosmetic. Therefore, an important quantity of these dyes is rejected in the environment, thus causing a real ecotoxicity for human and animal health [1, 2]. Dyes are reported toxic, non-degradable, and harmful to the environment and human and animal health [3, 4]. Generally, based on their solubility and chemical characteristics, the dyes are categorized into acid, basic, direct, mordant, vat, reactive, disperse, azo and sulfur dyes [5, 6]. Among the most common methods for the removal of dyes from wastewaters include adsorption, chemical precipitation, coagulation-flocculation, ion exchange, membrane technology, and electrochemical methods [7]. Adsorption is one of the most commonly used techniques for the removal of dyes from wastewater [4]. While other methods limit their applications based on the high cost of materials used, researchers have turned their attention to develop easily available, low-cost, renewable, green, and efficient adsorbents such as agricultural wastes [8, 9, 10, 11]. The coffee residue and the esparto fiber were the typical lignocellulosic wastes and by-products, respectively. The coffee residue can be obtained during the treatment of raw coffee powder with hot water or steam for the instant coffee preparation. Every year an important quantity of coffee can be collected and different wastes can be rejected during the preparation of coffee powders from coffee beams which can cause environment pollution [12, 13, 14, 15, 16, 17, 18, 19]. So, collection of these wastes was investigated by different researchers for their use as adsorbent materials in the wastewater treatment. On the other hand, Esparto grass (Stipa Tenacissima L) is a tussock grass widely distributed in semi-arid and arid regions in North Africa and southern Spain. The leaves are cylindrical, tough and very tenacious reaching up to 1.5 m in height. Traditionally, esparto has been employed for crafts. Humankind has used natural lignocellulosic materials for an amount of applications in daily life [20]. These wastes contain some organic compounds, such as cellulose, hemicellulose, lignin, and waxes. Recently research has been carried out on the application of coffee waste and esparto grass as an adsorbent in water treatment using the adsorption technology to provide references for future researches on the recycling and utilization of these materials in water treatment. This paper reviewed and analyzed the effect of coffee waste and esparto fiber on removal dye from wastewater. In addition, the pretreatment method of coffee waste and esparto fiber was discussed to solve the problem of limited adsorption efficiency. Moreover, this current chapter compares the adsorptive capacity of various forms of coffee wastes and esparto fiber. The most favorable conditions for the decontamination process for each hazardous dye were also discussed. Finally, the existing knowledge on coffee waste and esparto fiber as sorbent was analyzed to provide new directions for further applications of lignocellulosic materials in water treatment.

2. Characterization of the coffee waste and esparto fiber

The interests in utilization of agricultural wastes have been significantly increased and many attempts have been made regarding the use of lignocellulosic materials (either natural substances or agro-industrial wastes and by-products) as economic and eco-friendly options. Agricultural wastes such as lignin, cellulose and hemicelluloses are characterized by high molecular weights. The major chemical constituents of this lignocellulosic biomass include cellulose; hemicellulose, lignin, and their percentage dry weight composition are approximately 39, 31, and 18 wt%, respectively [1]. Cellulose is a linear polymer of β-d-glucopyranose sugar units. The average chain has a degree of polymerization about 9000–10,000 units. Hemicelluloses are amorphous polysaccharide polymers with a low degree of polymerization compared to cellulose. Lignin is a heterogeneous, complex and large molecular structure with cross-linked three-dimensional phenyl-propane polymer of phenolic monomers. Lignins have a highly branched structure, amorphous and made by cross-linking phenolic precursors. The chemical composition of the two lignocellulosic materials is presented in Table 1.

2.1. Composition and functional groups

The type of functional groups and chemical components in lignocellulosic wastes and by-products are similar but in different amounts [21]. They play an important role in dyes sorption. Lignocellulosic based cellulose hemicellulose and lignin contain different functional groups such as hydroxyl, ether, and carbonyl [22, 23]. The cellulose in the esparto fiber form long chains (or elemental fibrils) linked together by hydrogen bonds and van der Waals interactions Cellulose is usually presented as a major crystalline fraction associated to a minor amorphous. Lafi et al. [24] found that the composition of esparto fiber was 45.3% cellulose, 23.7% hemicelluloses, 23.9% lignin, and 2.1% ash. According to Boehm method [25], the functional groups at the surface of esparto were carboxylic 0.58 mmol/g and basic 0.4 mmol/g, followed by phenolic 0.96 mmol/g and lactonic 0.03 mmol/g. However, the composition of coffee residues was 45% water, 13% protein, 14% lipids, 26% carbohydrates, and 2% ash [23]. The functional groups at the surface of coffee residue were carboxylic 0.225 mmol/g and basic 0.1 mmol/g, followed by phenolic 0.27 mmol/g and lactonic 0.015 mmol/g. These results indicate that the differentiation of the surface chemistry of coffee residues and esparto fibers is directly related to the surface treatment before the adsorption process (Table 2).

		Coffee residue	Esparto fiber
Elemental analysis (wt%)	C	65.8	44.3
	H	28.4	6.5
	N	—	0.6
	O	—	46.5
	Others	5.7	2.1
Cellulose		8.6 (glucose)	45.3
Hemicellulose		36.7	23.7
Lignin		—	23.9
Protein		13	—
Lipids		14	—
Carbohydrates		26	—
Water		45	—
Ash		2	2.1

Table 1.

Chemical composition of lignocellulosic materials.

Adsorbant	pHPZC	SBET (m^2/g)	Boehm titration
			Carboxylic	Lactonic	Phenolic
Esparto fiber	6.3	20.7	0.58	0.03	0.96
Coffee residue	5.3	2.9	0.225	0.015	0.27

Table 2.

BET and pH_PZC parameters of various coffee adsorbents.

2.2. FTIR and SEM

FTIR technique has been used to identify functional groups before and after modification of lignocellulosic materials (Table 3). These functional groups are responsible for the removal of dyes from water. For example, after crystal violet (CV) biosorption by Esparto fiber [24], the infrared bands 3334, 1721, 1653, 1431, 1381, and 1161 had shifted to 3344, 1737, 1666, 1437, 1369, and 1166 cm⁻¹, respectively, indicating a chemical interaction was occurred between CV molecules and the carboxylate and the hydroxylate anions. This result suggests that carboxyl and hydroxyl functions are predominant contributors in dye uptake. Lafi et al. [26] concluded that after the toluidine blue (TB) adsorption by coffee residues, the bands shifted from 3446, 1728, 1658, and 1029 cm⁻¹ to 3444, 1737, 1629, and 1031 cm⁻¹, respectively, due to TB adsorption. These indicates that the corresponding functions (▬O▬H, C〓O, COO, C▬O and C▬O, respectively) are involved on the mechanism of dye adsorption. Based on another study, Lafi et al. [27] studied the comparison of FTIR of coffee residue (CR) and CR modified with cationic surfactants such as cetylpyridinium chloride (CPC) and cetyltrimethylammonium bromide (CTAB). The result shows that with CPC-CR the intensity of ▬CH₃ (2929 and 1368 cm⁻¹) increased while the peak of ▬CH₂ (2853 cm⁻¹) was slightly resolved due to the increase in the aliphatic carbon content in CPC-CR. The others conclude that CPC molecules exist on the surface. The FTIR shows that the band from ▬CH₂ of CTAB (at 2853 and 1465 cm⁻¹) becomes stronger while the band from ▬NH₂ and ▬OH (at 3414 cm⁻¹) is broadened. In this investigation, two new peaks at 1607 cm⁻¹ and 1520 cm⁻¹ (assigned to aromatic skeletal vibrations and ▬N〓N▬ stretching vibrations, respectively) appear after adsorption of MR into MCRs. This result is probably related to the new quaternary ammonium group introduced in the adsorbent material after modification. The peaks at 1039 and 1195 cm⁻¹ attributed to S〓O stretching observed after adsorption of MR into MCR_s indicate that the ▬SO₃⁻ groups of MR are involved in the adsorbent material [28]. The FTIR of the modification of extracted cellulose from Stipa tenacissima with cetyltrimethyl ammonium bromide (CTAB) showed that the quaternary ammonium group of CTAB was introduced at the surface of modified extracted cellulose (MEC). In fact, the vibration peak from ▬CH₂ of CTAB (at 2908 and 1427 cm⁻¹) became stronger while the band at 3313 cm⁻¹ (from ▬NH₂ and ▬OH) is broadened. The new quaternary ammonium group is responsible for the electrostatic interactions between the MEC and the methyl orange (MO) during the adsorption process [29]. Due to their various treatment methods (washing etc.), the adsorbent based coffee residues and esparto fibers present different morphologies before adsorption process. Figure 1(left) showed the structure of coffee residue indicating that a porous and homogenous structure with deep pores exists. The surface was not smooth, but scraggy with a variety of cavities involves a small surface area [28]. Regarding the esparto fiber, the others found cellular and irregular textures, related to the existence of heterogeneous layer surfaces with large number of pores characterized by various sizes at the surface [29].

Adsorbant	Wavenumber (cm^−1)	Assignment (functional groups)	Ref.
Coffee residue	3446	Bonded O▬H
	2924	C▬H
	2852	CH2
	1728	C〓O stretching vibration
	1658	COO
	1535	N▬H	[26]
	1454	C〓O
	1382	COO
	1165	C▬O▬C stretching vibration
	1029	C〓O stretching vibration
Esparto fiber	3700–3000	free O▬H, O▬H stretch, and inter-chains H-bonds
	2918	Asymmetrical stretch vibration ▬CH3
	2844	Symmetrical stretch vibration\▬CH2
	1721	C〓O stretching vibration
	1658	COO	[24]
	1514	N▬H amine/amide groups
	1246	Symmetric stretching COO
	1165	C▬O▬C vibration
	1033	Stretching vibration of C▬O▬H

Table 3.

Interactions of functional groups of coffee waste and esparto fiber and their shifts in FTIR spectra.

Figure 1.

SEM of Coffee waste (left) and Esparto fiber (right).

3. Treatment methods of coffee waste and the esparto fiber

After the material is collected, simple pretreatment is required, including washing and drying. The purpose of washing is to remove impurities that can affect the adsorption process [26, 27]. Coffee waste and esparto fiber have the following advantages: (i) porous or various cavity structures, (ii) cellulose, hemicellulose, and lignin structures with abundant functional groups (e.g., ▬C〓O, ▬COO⁻, ▬COOH and ▬OH). These functional groups can be involved in adsorption process after simple pretreatment. Lafi et al. [26] used coffee waste after washing and drying to remove CV and TB and obtained adsorption amounts of 125 and 142.5 mg/g, respectively. On the other hand, esparto grass fibers were used to remove CV and TB [24]. The investigation showed that the adsorption capacity was more important with coffee waste (CW), and this result was attributed to the higher lignocellulosic amount in CW. It can be seen that unmodified material have also the potential to adsorb dyes. The treatment methods of coffee waste and the esparto fiber include physical, acid alkali, and others treatment. The physical treatment changes the particle size and surface area of the adsorbent to increase the removal rate [30]. Delil et al. [31] used ultrasonic technology to treat SCG. The particle size was reduced and the surface area was increased from 3.58 to 1.13 m²/g. In addition, the Zeta potential of this treated SCG became more negative, which enhanced the adsorption of Cd (II). Nabais et al. [32] concluded that esparto fibers have an interesting potential for the production of activated carbons using carbon dioxide as activation agent. The acid treatment leads to structural changes in coffee waste. These changes are accompanied by an increase in the number of acid groups such as the carboxyl group or an introduction of other functional groups to improve its adsorption capacity [33, 34, 35]. Ahsan et al. [35] modified spent coffee grounds (SCG) using sulfuric acid as a sulfonating agent. The introduction of sulfonic acid as polar functional groups made the adsorbent surface electronegative with this modification, the adsorption capacity of methylene blue (MB), tetracycline (TC), and chromium (VI) reached 812, 462, and 302 mg/g, respectively. In another study, Raffas et al. [36] prepared activated carbons by the pyrolysis of coffee grounds impregnated by phosphoric acid at 450°C with different impregnation ratios: 30, 60, 120, and 180 wt.%. The lower impregnation ratios (<120 wt.%) led to the best microporous and acidic activated carbons whereas higher impregnation ratios (>120 wt.%) yielded to mesoporous carbons with specific surface areas as high as 925 m²/g, pore volume as large as 0.7 cm³/g, and neutral surface. The adsorption uptake of Nylosan Red (N-2RBL: anionic dye of diameter ≈ 2 nm) onto this carbon is 1.75-fold higher than that of a commercial activated carbon (S_BET ≈ 1400 m²/g). Sodium hydroxide (NaOH) and potassium hydroxide (KOH) are the commonly used reagents for alkali modification that can increase the surface area, pore volume, and some functional groups of coffee waste [37]. An alkali treatment using NaOH was used to modify coffee husk (NaOH-CFCB) [38]. This investigation showed functional groups at the surface of adsorbent and an increase of surface area and pore volume. In another study, Lafi et al. [39] used potassium acetate to activate coffee waste to prepare activated carbon coffee waste (ACCW). The potassium acetate modification created functional groups on the surface of the adsorbent, and also increased the surface area and pore volume, which showed an imported adsorption performance for CR. Several researchers applied cationic surfactant, magnetite nanoparticles, or other chemicals (such as cetyltrimethyl ammonium bromide (CTAB), cetylpyridinium chloride (CPC), dodecyltrimethyl ammonium bromide (DTAB), and N, N-dimethyldodecylamine N-oxide (DDAO) [28, 27] to modify coffee wastes for removing dyes. Cationic surfactant can enhance the hydrophilicity of the surface of coffee wastes, and alter the surface charge characteristics. Khataee et al. [40] synthesized a new adsorbent based Fe₃O₄-loaded waste coffee (Fe₃O₄-CHC) by precipitation method. Using this adsorbent, the decolorization rate of acid red 17 (AR 17) decreased from 100% to 74% with the increase of the initial dye concentration. Methylene blue (MB), methyl orange (MO) and rhodamine B (Rh B) were removed from water using CG4 adsorbent abtained from catalyzed waste coffee grounds by FeCl₃ [41]. The removal rates of MB, MO, and Rh B by CG4 were 93.8%, 92.9%, and 94.1%, respectively, after ten cycles. The adsorbent prepared by using a magnetic treatment, the adsorbent can be also prepared. However, the continuous adsorption–desorption cycle reduces the adsorption efficiency under acid condition by ion leaching [42]. Moreover, the use of large quantities of magnetic adsorbents can also lead to serious environmental problems [43].

The acid treatment introduces some functional groups and increases the porosity and the surface area. These modifications improve the adsorption capacity. However, the strong acid reagents used are expensive and corrosive, which limit their industrial application. The alkali treatment can transform coffee waste materials to adsorbents with high adsorption capacity. However, the appropriate ratio must be used when the base solution is used.

4. Adsorption of dyes

Most of dyes are not degradable and can cause carcinogenicity, mutagenesis in humans, and can affect the photosynthesis of aquatic organisms [44, 45]. Therefore, it is necessary to treat toxic dyes in wastewater. The following investigations describe the use of coffee waste and esparto fiber to adsorb dyes form wastewater.

Coffee husk was activated using H₃PO₄ to remove MB [46]. Under these acidic conditions, the negative charges on the surface of the adsorbent attract cationic MB contrary to alkaline conditions that increase the repulsive forces. Therefore, the MB removal rate and the maximum adsorption capacity were about 96.9% and 6.82 mg/g, respectively. Tran et al. [47] also used coffee husk to remove MB. It was found that the activated carbon at 108°C followed by KOH treatment had a high adsorption capacity for MB. The adsorption reached equilibrium rapidly (<30 min) and the removal rate was about 99.28% at pH 7.8 at an initial dye concentration of 200 mg/L. The experimental data showed that the maximum adsorption capacity of AH on MB was 418.78 mg/g. This value is very close to the theoretical data (418.78 mg/g) obtained by the Langmuir model. In term of adsorption kinetic, the experimental date fitted to the pseudo-second-order kinetics to describe the adsorption process. Coffee ground powder (CGP) was used as adsorbents to remove Rh B and Rh 6G [48]. In this study, the chemical structure of dye played a crucial role on the adsorption process. In fact, the adsorption capacity of Rh 6G (17.37 μmol/g) was higher than that of Rh B (5.26 μmol/g). This result was related to the dissociated carboxyl group (▬COOH) of Rh B that play a repulsion role between adsorbent and dye, and to the dissociated ester group (▬COOCH₃) of Rh 6G that was responsible to the hydrophobic interactions between the adsorbent and the dye. The adsorbent based on CGW/PPy composite was prepared using the pyrrole polymerization method. The coffee grounds were the raw material and the potassium persulfate was the oxidant [49]. The CGW/PPy composite was used to remove Rh B. The result showed that when pH (6) > pHPZC (3.2), the electrostatic interaction between CGW/PPy and Rh B increased the adsorption capacity; however under a constant pH of 9, the adsorption capacity increased by 1.7 times and the maximum adsorption capacity was about 50.59 mg/g. In term of adsorption equilibrium, the experimental data fitted to the Langmuir and Redlich-Peterson models. Regarding the adsorption kinetic, the experimental data fitted to the pseudo-second-order model. Coffee grounds (CG) were investigated as adsorbent to remove MG [50]. The FTIR analysis showed different functional groups and the SEM morphology revealed cavities with different sizes which provided channels for the adsorption of MG. The study showed that the removal rate of MG increased with the increase in contact time and adsorbent dosage; however, the increase of the initial MG concentration from 50 to 250 mg/L implies a decrease of the removal capacity from 99.63% to 98.69%. In term of adsorption isotherm and adsorption kinetic, the experimental data fitted to the Langmuir and to the pseudo-second-order models, respectively. Concentrated sulfuric acid-activated coffee husk (ACH) as an adsorbent to remove MG was investigated by Murthy et al. [51]. Using an adsorbent dosage of 0.5 g/L, the removal rate of MG reached 90% and the maximum adsorption capacity was about 263 mg/g. Regarding the influence of the pH; when the pH > pH_PZC (5.4), the negative charge on the ACH surface attracts MG, which increases the removal rate and reaches the maximum at pH 6.8. Finally, in this it was found the experimental data fitted to the Sips isotherm model and the pseudo-second order kinetic model. SCG as adsorbent was used to remove CR dye [52]. The results showed that removal rate increased with the increase of adsorbent dosage and contact time until reaching equilibrium. Regarding the adsorption process, it was found that the removal rate decreased with the increase of the initial dye concentration. The optimization of the experimental data was performed using RSM, and the results showed that the removal rate was about 89.17% when the adsorbent dosage was 1.87 g, the initial concentration was 48.18 mg/L, and the contact time was 57.96 min. Potassium acetate was used to modify coffee waste [39]. The obtained activated carbon (ACCW) was used as adsorbent to remove CR. After 120 min the adsorption equilibrium was reached. Furthermore, the authors found that the acid condition lead to the protonation of the oxygen-containing functional groups (▬OH and ▬COOH). Under this condition, the positive charged adsorbent surface attract the R-SO₃⁻ functional groups of the dissociated CR, and the maximum adsorption capacity of CR was about 90.90 mg/g. The authors found that the experimental data fitted to the Langmuir isotherm model and the pseudo-second-order kinetic model. Coffee husks (WCH) were used as adsorbent to remove CR [53]. The result showed that the adsorption efficiency was about 96% at 25°C when the dye concentration was about 12.24 mg/L and the pH was about 4. In term of adsorption isotherm and kinetic, the authors found that the experimental data fitted to the pseudo-second-order model and both the Langmuir model and the Freundlich model. Untreated SCG was used to adsorb CV [54]. The authors indicated that when the SCG concentration was about 2 g/L, the removal rate was about 90%. When the pH was above 5.3, the surface of the adsorbent acquired a negative charge (pH_PZC = 5.3) to attract the cationic CV dye by electrostatic interaction. The FTIR analyze showed the appearance of functional groups such as hydroxyl and carbonyl that could be responsible for the adsorption of CV. The experimental data fitted to the Langmuir isotherm model and the pseudo-second-order model. Regarding the thermodynamic parameters, the data showed the adsorption process was exothermic and spontaneous. Pagalan et al. [55] used KOH activation of SCG to adsorb aniline yellow dye (AYD). The SEM indicated that the KOH-modified SCG formed mesoporous and microporous structures, which were favorable for AYD removal by activated carbon. The factors affecting the adsorption were optimized using response surface methodology (RSM) methodology. The AYD removal rate reached 88.72% and the adsorption capacity was about 2.58 mg/g, when the initial AYD concentration was 35 mg/L. The obtained experimental data fitted to the Freundlich and the pseudo-first-order kinetic models. KOH-activated SCG as adsorbent was investigated to remove orange G [56]. Using this adsorbent, the adsorption capacity reached 100 mg/g at 45°C.The experimental data fitted to the Langmuir and Redlich-Peterson models in term of adsorption equilibrium. In terms of adsorption kinetic the experimental data fitted to the pseudo-second-order model and the diffusion with intraparticles was not the unique rate-limiting step. CW was modified by cetyltrimethyl ammonium bromide (CTAB) and cetyl pyridine chloride (CPC) cationic surfactant [27]. Using these adsorbents, the maximum adsorption capacities of MO by CTAB–CW and CPC–CW were 58.82 and 62.5 mg/g, respectively. In term of adsorption kinetic, the experimental data fitted to the pseudo-second-order kinetic model. In another study, Lafi et al. [28] used cationic surfactant, Dodecyltrimethyl ammonium bromide (DTAB) and a zwitterionic surfactant, N, N-dimethyldodecylamine N-oxide (DDAO) to modify coffee residue (CW) to increase affinity for MO anionic dyes. The maximum adsorption capacities of MR using DTAB-CR and DDAO-CR were 76.22 and 66.22 mg/g, respectively. The pseudo-second-order kinetic model fitted to the experimental data. Lafi et al. [24] studied the performance of Esparto grass fibers (EGF) as adsorbent to remove TB and CV from aqueous solutions. Under optimum conditions (25°C, pH 7.0, contact time of 150 min, and adsorbent dose of 2 g/L), the adsorption capacity was about 40.00 mg/g for TB and 43.47 mg/g for CV, and the two equilibrium data were fitted to the Langmuir isotherm. Adsorption of MO dye onto modified extracted cellulose using cationic surfactants was investigated by Lafi et al. [29]. The investigation showed that the maximum adsorption capacity (16.95 mg/g) was obtained under pH 3.7 with 4 g/L of adsorbent at 25°C. Experimental data showed better agreement with the pseudo-second-order kinetic model and Langmuir adsorption isotherm model. Table 4 presents the optimal adsorption conditions and maximum adsorption capacities of different studies using coffee wastes and esparto fibers to remove dyes from wastewater [57, 58, 59, 60, 61, 62, 63, 64, 65].

Adsorbent	Adsorbate	Process variables of adsorption					Adsorption isotherm	Adsorption kinetic	qmax	Ref.
		m (g/L)	C (mg/L)	Contact time	pH	T (°C)
Coffee ground powder	RhB	1	7.18	3 h	2	19	L	PSO2;PSO1	5.26	[48]
Coffee ground powder	Rh6G	1	6.65	3 h	2	19	L	PSO2;PSO1	17.37	[48]
Coffee wastes	CV	5	80–400	20 min	6	20	L	PSO2	125	[26]
Coffee wastes	TB	5	80–400	20 min	6	20	L	PSO2	142.5	[26]
Coffee waste CTAB	MO	2	20–120	4 h	3.5	25	L	PSO2	58.82	[27]
Coffee wastes-CPC	MO	2	20–120	4 h	3.5	25	L	PSO2	62.5	[27]
Coffee residues AC at 400°C	MR	0.05	5–50	60 min	10	25	L-F	PSO2	—	[57]
Coffee residues AC at 500°C	MR	0.05	5–50	60 min	10	25	L-F	PSO2	—	[57]
Coffee residues AC at 600°C	MR	0.05	5–50	60 min	10	25	L-F	PSO2	—	[57]
Magnetic coffee silverskin	MB	0.005	25–1000	6 h	8	20	L	PSO2	556	[58]
Magnetic green coffee	MB	0.5	50	30–300 min	5.5	20	—	PSO2	66.2	[59]
Magnetic coffee silverskin	MB	0.5	50	30–300 min	5.5	20	—	PSO2	99	[59]
Magnetic spent coffee grounds	MB	0.5	50	30–300 min	5.5	20	—	PSO2	78.1	[59]
Coffee residues	MB	20	10–100	18 min	10	30	L	—	4.68	[60]
Coffee residues	MB	0.5	50	24 h	7.5	25	L	PSO2	112	[61]
Pyrolized coffee residues	MB	0.5	50	24 h	7.5	25	L	PSO2	132	[61]
Coffee residues	MB	7	5–60	180 min	—	27	F	PSO2	6.69	[62]
Coffee husks	BY 3G-P	2	15–550	—	2	20	L	PSO2	24.04	[63]
Coffee wastes	RR 3BS	0–15	0–1000	24 h	2–12	25	L–F;L	PSO2; PSO1	179–241	[64]
	RBRN	0–15	0–1000	24 h	2–12	25	—	—	—	[64]
	RR 3BS	0–15	0–1000	24 h	2–12	25	—	—	—	[64]
Coffee husks	FG	1	10–100	100 min	2	—	L–F	PSO1	96.6	[65]
Coffee husks	MB	1.6	20	50 min	5	30	L	PSO2	6.82	[46]
Coffee husk	MB	1.2	—	—	—	—	F	PSO2	78	[47]
Coffee husk waste	MB	—	500	720 min	7.8	—	L	PSO2	418.78	[47]
Coffee grounds waste	RhB	—	200	—	9	20	L, R–P	—	50.59	[49]
Spent coffee grounds	MG	0.001	50–250	90 min	—	30	L	PSO2	—	[50]
Coffee husks	MG	0.2	25–150	120 min	6.8	50	S	PSO2	264.81	[51]
Spent coffee grounds	CR	2	50	60 min	5	37	L	PSO2	—	[52]
Coffee waste	CR	4	—	120 min	3	25	L	PSO2	90.90	[39]
Coffee husks	CV	—	12.24	—	4	25	L–F	PSO2	—	[53]
Spent coffee grounds	CV	2	100	40 min	6	—	L	PSO2	63.3	[54]
Spent coffee grounds	AYD	0.6	35	150 min	—	25	L	PSO2	2.58	[55]
Coffee grounds	OG	1	20–200	250 min	6	25	L	PSO2	72	[56]
Coffee residue -DDAO	MR	2	—	240 min	3.5	25	L	PSO2	76.22	[22]
Coffee residue -DTAB	MR	2	—	240 min	3.5	25	L	PSO2	66.66	[22]
Esparto grass fiber	CV	2	20–100	150 min	7	25	L	PSO2	43.47	[24]
Esparto grass fiber	TB	2	20–100	150 min	7	25	L	PSO2	40	[25]
Modified extracted cellulose	MR	4	20–100	4 h	3.7	25	L	PSO2	16.95	[29]

Table 4

Optimal adsorption conditions, maximum adsorption capacities and removal rates for dyes removal using coffee wastes and esparto fibers.

5. Adsorption isotherm and kinetic

5.1. Adsorption isotherms

The determination of the adsorption isotherms allows an understanding of the interaction between dyes and the adsorbents. Different adsorption isotherm models (Table 5) were used to describe the adsorption process such as Langmuir, Freundlich, Temkin, Redlich-Peterson, and Dubinin-Radushkevich [4, 5, 6, 8, 66, 67, 68, 69, 70]. Langmuir and Freundlich are the most widely used as adsorption isotherm models. These models are only applicable where sufficient time is provided to allow equilibrium between the dye in solution and the dye adsorbed on the adsorbent. During the adsorption process, the dye are expected to be in contact with the adsorption sites and thus retained on the adsorbent surface [5]. Due to the different sources and processing conditions of coffee waste and the different nature of various dyes, the adsorption process could be different. Murthy et al. [71] describes the adsorption process of MB removal using coffee husks by the Freundlich model, which suggests that the adsorption is heterogeneous and multi-layered. However, the Langmuir model describes the adsorption process of MB removal using coffee husks waste, which can describe that the adsorption process follows monolayer adsorption with a uniform distribution of active sites at the surface of adsorbent [47]. In the process of adsorbing CV, both the Langmuir and Freundlich isotherm models fitted to the experimental data of CV removal using coffee husks as adsorbent. However, other investigations describe that the Langmuir model fitted more to the experimental data regarding the CV removal [53, 54].

Isotherm	Non-linear form	Ref.
Langmuir	$q_e=\frac{q_m{\mathit{bC}}_e}{\left(1+b{C}_e\right)}$	[67]
Freundlich	$q_e=K_F{C_e}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$n$}\right.}$	[68]
Temkin	$q_e=\mathit{\text{Bln}}\left(A{C}_e\right)$	[69]
D–R	$q_e=q_m\mathit{\text{exp}}\left(-\beta {\epsilon }^2\right)$	[70]

Table 5.

Lists of adsorption isotherms mostly discussed in the present study.

5.2. Adsorption kinetic

The kinetic model gives information about the rate of the adsorption process, and can also explain the mechanism involved in the removal of dye. Several kinetic models have been applied to the adsorption process, such as pseudo-first-order kinetic, pseudo-second-order kinetic, intraparticle diffusion model, and Elovich model [72, 73, 74, 75, 76]. In the case of coffee waste, pseudo-second kinetic describe the adsorption process for the removal of dyes. The adsorption by coffee waste is mainly physiosorption, which is the main limiting factor affecting the adsorption rate of the whole adsorption process. In the adsorption processes using modified coffee waste to remove MO, pseudo-second-order kinetic model describes the process, indicating that physiosorption and hydrophobic interactions are involved [27]. In the case of OG, the quasi-second-order kinetic model describes well the adsorption process. On another note, the results of the diffusion model showed that the adsorption of OG by modified coffee grounds was also controlled by intraparticle diffusion [56]. In the case of adsorption of Fast green dye using coffee husks, the process can be described by a pseudo-first-order kinetic model [65]. Table 6 presents a list of equations used to determine the kinetic behavior of various dyes from wastewater by using coffee wastes and esparto fibers.

Kinetics model	Equation form	Ref
Pseudo-first-order	$\mathit{log}\left(q_e-q_t\right)=\mathit{log}\left(q_e\right)-\left(\frac{k_1}{2.303}\right)t$	[73]
Pseudo-second-order	$\frac{t}{q_t}=\frac{1}{k_2{q}_e^2}+\frac{t}{q_e}$	[74]
Elovich	$q_t=\frac{1}{\beta }\mathit{ln}\left(\mathit{\alpha \beta }\right)+\frac{1}{\beta }\mathit{ln}\left(t\right)$	[75]
Intra-particle diffusion	$q_t=k_{\mathit{id}}{t}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}+C$	[76]

Table 6

Lists of kinetic equations.

6. Mechanism of adsorption

The main constituents of lignocellulosic materials contain various functional groups that play an essential role in dyes adsorption. The mechanism of adsorption relies on the adsorbent characteristics (e.g., surface area, porosity, and surface functional groups) [23]. Molecular properties of dyes (e.g., molecular size, aliphatic vs. aromatic and hydrophobicity), solution properties (pH, ionic strength, and temperature), and the interactions between functional groups of adsorbent and dyes are often involved in the adsorption mechanism [77]. The surface charges of adsorbent and adsorbate id related to the pH, and it is one of the important factors that control the adsorption process. The adsorption regulation could be achieved by measuring the point of zero charges (pH_PZC) of sorbents using the zeta potential [78]. At pH < pH_PZC, protonation of functional groups leads to a positively charged adsorbent that can successfully remove the anionic sorbates. Kyzas et al. [79] reported that the determination of pH_PZC played an important role to explain the possible pH-mechanism regarding the adsorption of reactive dye (RB) onto UCR. The pH_pzc was about 3.2–3.4 and negatively charged surface of UCR was occurred when the pH values above of 3; however, positively charged occurred at lower pH values. Therefore, at pH < 3.5, the adsorption process involves electrostatic interaction between UCR⁺ and SO₃⁻ of dyes, and at pH > 3.5. The interactions decrease because the dye is still negatively charged, illustrating the reduction of dye (RB) removal. In the case of basic dye (BB), the molecule presented constant positive charge; the UCR was protonated in acidic pH values. Therefore, the adsorption process is very limited. However, increasing the pH of the solution (pH > 3.5) the surface of UCR is charged negatively. In this case, the deprotonation of the surface of UCR is taken place, and transformed to the negatively charged form which provides electrostatic interactions favorable for adsorbing cationic species. In alkaline conditions, the increase of pH solution would convert more groups to ▬O⁻ and ▬COO⁻, providing electrostatic interactions for cationic species removal. Based on these findings, the more significant adsorption mechanisms of dyes onto coffee residue is through electrostatic interaction. If the pH > pK_a, the sorbate surface charge is negative; however, at pH < pK_a, the sorbate exists in a positively charged surface. Thus, the adsorption capacity of the adsorbent can be altered by changing the pH solution. Reffas et al. [36] investigated the adsorption of methylene blue and Nylosan Red N-2RBL onto activated carbons prepared by the pyrolysis of coffee grounds. In the case of MB, the mechanism can be explained on the basis of an electrostatic interaction between the ionic dye molecule and the charged carbon substrate. At pH 6, the CGAC30, CGAC60 and CGAC120 activated carbons are negatively charged (pH > pH_PZC ≈ 3.7) while the CGAC180 and CAC are positively charged (pH < pH_PZC). Therefore, electrostatic repulsion between the MB cation (at pH 6) and CGAC180 (or CAC) is not in favor of adsorption.

In another study Murthy et al. [51] reported that the adsorption of MG by ACH was carried out by van der Waals force and electrostatic interaction. It was also controlled by membrane and intra-particle diffusion. The adsorption was accelerated with the increase of temperature and concentration. Furthermore, Shen and Gondal [48] examined the adsorption mechanism of Rh B and Rh 6G using CGP as adsorbents. The results involve electrostatic interaction and molecular interaction and the adsorption capacity decreased to 0.9 μmol/g for Rh B and 5.5 μmol/g for Rh 6G after 5 cycles. Based on FTIR analysis, Lafi et al. [43] found that the mechanism of adsorption could be explained by the hydrogen bonding and the electrostatic interaction between CR and oxygen-containing functional groups on the activated carbon surface. Cheruiyot et al. [53] provide that the electron sharing or exchange between WCH and CV indicates that the adsorption process is controlled by chemisorption. In another study [27], through the FTIR characterization and mechanism analysis, it was indicated that the process of adsorption of MO may involve hydrophobic–hydrophobic interaction and electrostatic interaction. Based on the FTIR characterization and the proposed adsorption mechanism, Lafi et al. [28] concluded that zwitterionic surfactant (DDAO) is the most efficient for the adsorption of MR onto coffee residues (CR). The mechanism involves different types of interaction such as hydrogen bonds, electrostatic and hydrophobic interactions. Lafi et al. [35] also investigated the adsorption capacity of esparto grass fibers (EGF) for TB and CV removal. Infrared spectroscopy demonstrated that several functional groups were involved in CV and TB binding on EGF such as ester, hydroxyl and amino groups. Binding seemed to be more related to chemisorption with hydrogen atoms of non-ionized carboxyl groups. Sorption behavior of modified extracted cellulose (MEC) from Stipa Tenacissima L by cetyltrimethyl ammonium bromide was investigated by Lafi et al. [39]. In this case of MO adsorption on the MEC, the mechanism could be related to hydrophobic interaction and electrostatic interaction.

Therefore, isotherms and kinetics models, along with thermodynamic parameters and activation energy, can clarify the physisorption or chemisorption characteristics of an adsorption process.

Different methods of dyes adsorption on coffee waste and esparto fiber adsorbents or and the adsorption mechanisms are schematically presented in Figure 2.

Figure 2.

Schematic illustration for the adsorption mechanism of dyes onto coffee waste and esparto fiber based materials.

7. Conclusions

This chapter was devoted to the use of lignocellulosic material based coffee residues and esparto fibers in the adsorption of dyes from wastewater. This Lignocellulosic material was investigated in natural and modified forms, and showed a potential use as adsorbents in the detoxification of water using the adsorption process. These adsorbents were characterized using different analytical methods such as FTIR, SEM, and BET. In term of kinetic and thermodynamic, this chapter demonstrated that the different experimental data fitted to the known models. We believe that Lignocellulosic material could be used in industrial water purification by removing undesirable chemicals, biological contaminants, and gases from water.

Conflict of interest

The authors declare no conflict of interest.

Abbreviations

AC	activated carbon
AH	activated the hydrothermal carbon
ACH	acid-activated coffee husk
AR17	acid red 17
AYD	aniline yellow dye
ACCW	activated carbon coffee waste
BC	biochar
BET	Bruner-Emmett-Teller
BPA	bisphenol A
BY	Brilliant yellow 3G-P
CH	coffee husk
CR	congo red
CV	crystal violet
CW	coffee waste
CR	coffee residue
CGW	coffee ground waste
CGP	coffee ground powder
CTAB	cetyl trimethyl ammonium bromide
CPC	cetyl pyridine chloride
CGAC	coffee grounds activated carbon
CAC	commercial activated carbon
DTAB	dodecyltrimethyl ammonium bromide
DDAO	N, N-Dimethyldodecylamine N-oxide
ECW	exhausted coffee waste
EGF	esparto grass fiber
FG	Fast green
FTIR	Fourier transform infrared spectroscopy
Fe3O4-CHC	Fe3O4-loaded coffee waste hydrochar
HC	hydrochar
HAC	coffee husk
KOH	potassium hydroxide
ICO	International Coffee Organization
MB	methylene blue
MO	methyl orange
MR	methyl red
MG	malachite green
MEC	modified extracted cellulose
MCW	Fe3O4 loaded magnetic coffee waste
MCRs	modified coffee residue
NaOH	sodium hydroxide
N-2RBL	Nylosan Red
NaOH-CFCB	NaOH-modified coffee husk
NaOH-SCG	NaOH-modified waste coffee grounds
OG	orange G
pHPZC	pH the point of zero charge
PBD	Plackett-Burman design PPy polypyrrole
PSO1	pseudo-first-order
PSO2	pseudo-second-order
Rh B	rhodamine B
Rh 6G	rhodamine 6G
RH	rice husk
RSM	response surface methodology
RB	Remazol Brilliant Blue RN
RR	Remazol Red 3BS
RB	reactive dye
BB	basic dye
SAC	spent coffee
SCG	spent coffee ground
SEM	scanning electron microscope
SCGB	spent coffee grounds derived biochar
SMCW	surface modified coffee waste
SCBC	spent coffee grounds biochar
TC	tetracycline
TB	toluidine blue
UCR	untreated coffee residue
UACW	ultrasonic assisted coffee waste
WCH	waste coffee husk