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

*Ridha Lafi, Hajer Chemingui, Imed Montasser and Amor Hafiane*

## **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.

**Keywords:** dye, biosorption, lignocellulosic products, adsorption isotherms, kinetics

## **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 [8–11]. 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 [12–19]. 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**.

*The Coffee Residues and the Esparto Fibers as a Lignocellulosic Material for Removal... DOI: http://dx.doi.org/10.5772/intechopen.111420*


#### **Table 1.**

*Chemical composition of lignocellulosic materials.*

#### **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**).

#### **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<sup>1</sup> , respectively, indicating a chemical interaction was occurred between CV molecules and the carboxylate


**Table 2.**

*BET and pHPZC parameters of various coffee adsorbents.*


#### **Table 3.**

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

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<sup>1</sup> to 3444, 1737, 1629, and 1031 cm<sup>1</sup> , respectively, due to TB adsorption. These indicates that the corresponding functions (dOdH, C]O, COO, CdO and CdO, 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 dCH3 (2929 and 1368 cm<sup>1</sup> ) increased while the peak of dCH2 (2853 cm<sup>1</sup> ) 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

#### *The Coffee Residues and the Esparto Fibers as a Lignocellulosic Material for Removal... DOI: http://dx.doi.org/10.5772/intechopen.111420*

FTIR shows that the band from dCH2 of CTAB (at 2853 and 1465 cm<sup>1</sup> ) becomes stronger while the band from dNH2 and dOH (at 3414 cm<sup>1</sup> ) is broadened. In this investigation, two new peaks at 1607 cm<sup>1</sup> and 1520 cm<sup>1</sup> (assigned to aromatic skeletal vibrations and dN]Nd 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<sup>1</sup> attributed to S]O stretching observed after adsorption of MR into MCRs indicate that the dSO3 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 dCH2 of CTAB (at 2908 and 1427 cm<sup>1</sup> ) became stronger while the band at 3313 cm<sup>1</sup> (from dNH2 and dOH) 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].

#### **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., dC]O, dCOO, dCOOH and dOH). 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 m<sup>2</sup> /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 [33–35]. Ahsan et al. [35]

#### **Figure 1.**

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

#### *The Coffee Residues and the Esparto Fibers as a Lignocellulosic Material for Removal... DOI: http://dx.doi.org/10.5772/intechopen.111420*

higher impregnation ratios (>120 wt.%) yielded to mesoporous carbons with specific surface areas as high as 925 m<sup>2</sup> /g, pore volume as large as 0.7 cm<sup>3</sup> /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 (SBET ≈ 1400 m<sup>2</sup> /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 (dCOOH) of Rh B that play a repulsion role between adsorbent and dye, and to the dissociated ester group (dCOOCH3) 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-secondorder 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 > pHPZC (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 (dOH and dCOOH). 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

#### *The Coffee Residues and the Esparto Fibers as a Lignocellulosic Material for Removal... DOI: http://dx.doi.org/10.5772/intechopen.111420*

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 (pHPZC = 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 [57–65].



*The Coffee Residues and the Esparto Fibers as a Lignocellulosic Material for Removal... DOI: http://dx.doi.org/10.5772/intechopen.111420*

#### **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 [4–6, 8, 66–70]. 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


**Table 5.**

*Lists of adsorption isotherms mostly discussed in the present study.*

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].

#### **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 [72–76]. 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


**Table 6** *Lists of kinetic equations.* 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.
