* Corresponding author: E-mail: [email protected] (A.M. Ayuba)
2550-4800/© 2021 The Authors. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Journal homepage:http://revues.imist.ma/?journal=jasi ISSN: 2550-4800
https://doi.org/10.48442/IMIST.PRSM/jasi-v9i1-3.24438
Adsorption of Congo red dye from aqueous solution using raw cowpea (Vigna Unguiculata) husk
Abdullahi Muhammad Ayuba
*, Bridget Idoko
Department of Pure and Industrial Chemistry, Bayero University, Kano, Nigeria.
Received 09 January 2021; Revised 15 July 2021; Accepted 17 July 2021.
Abstract: An agricultural residue, cowpea (Vigna unguiculata) husk (CPH), was used for the removal of Congo red dye from aqueous solution using a batch adsorption method. The adsorbent was characterized using FTIR and SEM techniques. The optimization of adsorption variables including pH, contact time, adsorbent dosage and initial dye concentration at 25 °C were also carried out. The results indicated the dependence of the adsorption system on the studied variables and equilibrium was attained in 60 min. The adsorption kinetics was fitted to four models (pseudo first order, pseudo second order, Elovich and intra-particle diffusion) to validate the kinetics, which resulted in pseudo second order as the best model for the description of Congo red dye uptake. Equilibrium isotherm modeling was also carried out using the Langmuir, Freundlich, and Temkin models. The Langmuir isotherm relatively giving the best fitting to the experimental results. The maximum loading capacity (qm) of the adsorbent for Congo red obtained from the Langmuir isotherm model is 161.29 mg/g. This result indicates that the CPH residue could serve as a good adsorbent for the removal of Congo red dye from aqueous system.
Keywords: Adsorption; Congo red; Isotherm; Kinetics; Vigna unguiculata.
1. Introduction
Many industries including food, textile, paper, rubber, plastics, cosmetics, pharmaceutical, etc.
generate colored effluents containing various dyes and pigments and discharge them into natural water bodies.
A small amount of dyes is sufficient to color the large water bodies, which affects aesthetic merit and reduces the light penetration and photosynthesis in aquatic bodies. Many colored dyes are toxic, carcinogenic and mutagenic. Therefore, there is an increasing demand of economical and efficient technologies for the removal of dyes from wastewater in the environment [1]. Thus, several physical, chemical and microbial methods including membrane separation, ion exchange, biological degradation; coagulation-flocculation, electrochemical techniques, chemical oxidation and adsorption have been tested for the treatment of effluents containing dyes, including Congo red (CR) dye [2-4]. Among these processes, adsorption methods have gained a considerable attention due to simplicity, high sorption capacity, eco-friendliness, non-toxicity and efficiency for the removal of a wide range of sorbents. Dyes are typically classified into cationic, anionic and non-ionic [5].
Among various kinds of anionic dyes, CR is a sodium salt of benzidinediazobis-1-naphthylamine-4-sulfonic acid, that is benzidine-based anionic azo dye. It was selected in this study because of its complex chemical structure, high solubility in aqueous solution and its pluck, once it is discharged into natural environment. CR is metabolized to benzidine, a known human carcinogen and exposure
to this dye can cause some allergic responses [6]. Congo red is mainly found in the textile, paper, printing, and leather industrial effluents and about 15 % of dye enters in wastewaters during dyeing process [7].
Many researchers have studied the removal of this carcinogenic dye through adsorption process using various adsorbents of plant origin including: coconut residual fiber [8], Phoenix dactylifera date stones and Ziziphus lotus jujube shells [9], Solanum tuberosum and Pisum sativum peels [10], common Beach (Fagus sylvatica L.) [11], Ulva lactuca biomass [12], Mango leaves [13], Litchi peel biochar [14], Pomelo peel [15], Brewer’s grains [16], Magnolia-leaf [17], Pineapple peel [18], Banana peel [19], Walnut shell [20], Pine bark [21], Vetiveria zizanioides [22], and Cabbage waste [23].
Vigna unguiculata (Cowpea) is a versatile African crop; it feeds people and their livestock. It is a high protein food and very popular in West Africa. As a nitrogen fixing legume, Cowpea improves soil fertility and consequently helps to increase the yields of cereal crops when grown in rotation. It is referred to as the
“hungry season crop” given that it is the first crop to be harvested before the cereal crops are ready. It is a crop that offers farmers great flexibility. They can choose to apply more input and pick more beans or- if cash and inputs are scarce, they can pick fewer beans and allow the plants to produce more foliage [24-25]
The aim of this research is to investigate the adsorptive, kinetic, thermodynamic and equilibrium isotherm characteristics of Congo red removal from aqueous solution onto raw cowpea husk.
10 2. Materials and methods
2.1. Material
All reagents used for this study were of Analar grade and were used without further purification. Double distilled water was used for all the solution preparations (both stock and working). Congo red was obtained from BDH Chemical, England. Fig.1 shows the chemical structure of the used dye. Cowpea husk was collected, treated and utilized as the adsorbent.
Fig.1. Molecular structure of Congo Red dye 2.2. Adsorbent collection and preparation
The cowpea husk (CPH) was obtained from a local market in Kano, Nigeria. The shells obtained after removing the seeds from the pods were washed and air dried. CPH was first washed with water to remove dirt from its surface and subsequently dried at 105 °C for 24 h in an oven to remove the moisture content. The dried CPH was ground and sieved to the desired particle size of 1-2 mm, stored in an airtight container.
2.3. Preparation of dye solution
The stock solution of Congo red dye was prepared by dissolving accurately weighed 1 g of the dye into a 1 L to produce 1000 mg/L using distilled water. The experimental solutions of desired concentration (50-500 mg/L) were prepared accordingly by diluting the stock solution with distilled water. The concentration of the un-adsorbed CR dye was measured at λmax = 498.3 nm using UV–Visible spectrophotometer (Model Hitachi 2800).
2.4. Characterization of the adsorbent
The surface morphological properties of the adsorbent sample were investigated using Scanning Electron Microscope (Phenom World Eindhoven).
Scanned micrographs of adsorbents before and after adsorption were taken at an accelerating voltage of 15 kV and x500 magnification. Fourier transform infrared (FTIR) analyses of the adsorbent before and after adsorption were carried out using Cary 630 Spectrophotometer Agilent Technology. The analysis was done by scanning the sample through a wave number range of 650 – 4000 cm-1; 32 scans at 8 cm-1 resolution.
2.5. Batch adsorption experiments
Batch experiments were carried out to determine the optimum conditions for the equilibrium adsorption of CR onto raw cowpea husk. The variables studied
included the adsorbent dosage, contact time, initial concentration and pH. For each of these variables studied, all other variables were fixed at a constant value for the optimization experiments. The results obtained after the optimization experiments were used to conduct the batch adsorption experiments. Each of these systems was separately run in a 250 cm3 conical flask differently.
The conical flasks were covered during the equilibration period and placed on a temperature-controlled tightly Innova 4000 incubator shaker for the earlier reported period.
After reaching adsorption equilibrium, the content was filtered through Whatman No 1 filter paper. The filtrate was analyzed using Perkin-Elemer UV-vis spectrophotometer at maximum absorbance wavelength of 498.3 nm [26]. The extent of adsorption was calculated using:
q = ( )× V (1)
where qe is the adsorption capacity (mg/g), Co and Ce are the initial and final equilibrium concentration of Congo red in solution (mg/L), V is the volume of solution (L), and m is the mass of the adsorbent (g).
3. Results and discussion 3.1. FTIR and SEM analysis
FTIR spectra from 600 cm-1 to 4000 cm-1 presented in Fig.2 were carried out to identity the functional groups present in the adsorbent. The adsorption capacity of adsorbent depends upon the porosity as well as the chemical reactivity of the functional groups at the adsorbent surface [11, 27]. In the FTIR spectrum of CPH before adsorption of CR, there is a broad adsorption band at 3279 cm-1 which is assigned to O–H of hydroxyls, and also prominent peaks at 2918 cm-1 assigned to C – H bonds of methyl and methylene groups [15, 28].
The hydroxyl group however is an important adsorption site [29]. After adsorption, there was a shift and broadening of absorption peaks. The shift of the OH peak from 3279 cm-1 to 3331 cm-1 indicates the involvement of the hydroxyl groups in the adsorption.
However, shift in bands and changes in wavelength between the before and after adsorption of samples indicate that there was an interaction between the CR dye and the adsorbent surface through adsorption as presented in Table 1.
The SEM of CPH before adsorption shows a moderately smooth surface which is characterized with defects (pores, cracks and cavities) with presence of some adhering particles on the adsorbent surface [11, 15, 30]. To reduce this, proper and more vigorous washing of the husk was conducted before use in the adsorption process. The micrograph of CPH after adsorption of CR showed leveling of the defects by depositions of adsorbed dyes in smooth regular formations on the surfaces.
11 Table 1
Different functional group recognized before and after adsorption of CR onto CPH
Functional
Group Wavelength
range (cm-1) Before
adsorption After adsorption C-H stretch 2950- 2800 2918 2918
C≡C Alkynes 2100-2260 2117 2117
Carboxylic group 1690- 1760 1722 1722 C=C Aromatics 1500- 1700 1547 - C-O stretch 1080- 1300 1100 1100
3.2. Batch Adsorption
3.2.1. Effect of adsorbent dosage
Fig.4a shows the effect of the quantity of CPH used on the adsorption of Congo red which was varied from 0.1 to 0.6 g, while the dye concentration was fixed at 50 mg/L. A dosage study is an important experiment in adsorption studies because it determines the capacity of an adsorbent for a given initial concentration of dye in solution. The net quantity of adsorbate removed
increased with increasing mass of CPH which is attributed to an increase in the specific surface area and the availability of more active binding sites. The net equilibrium amount adsorbed however is an expression of the efficiency of an adsorbent which may not show increase in the amount adsorbed per unit mass as the adsorbate dose increases [31].
3.2.2. Effect of contact time
The effect of contact time on the adsorption of the CR by CPH was studied by keeping the dye concentration, adsorbent dosage, pH and temperature constant as shown in Fig.4b. In this study, the adsorption process of CR using CPH was studied for various time intervals (10, 20, 30, 60, 90 and 120 min). It can be observed that, initially, increase in time enhances the rate of adsorption and its equilibrium was attained at a dye adsorption capacity removal of 18.962 mg/g within 60 min.
Thereafter, there were no appreciable changes in adsorption. Therefore, 60 min was the sufficient time for the maximum adsorption of CR dye to occur [32].
Fig.2. FTIR spectra of CPH before and after adsorption of CR
(a) (b) Fig.3. SEM Micrographs of CPH (a) before and (b) after adsorption of CR
12
Fig.4. Effect of adsorbent dosage (a) and contact time (b) on the adsorption of CR onto the CPH 3.2.3. Effect of initial dye concentration
Fig.5a shows the effect of initial dye concentration on the adsorption of CR onto CPH. The initial concentration of the adsorbate plays an important role, as a given mass of adsorbent can adsorb only a certain amount of a solute. The concentrations tested for the optimization ranged from 50 mg/L to 500 mg/L. The adsorption capacity was found to increase from 35.904 to 153.789 mg/g when the dye concentration was increased from 50 mg/L to 500 mg/L. The increase of dye concentration enhances the interaction between the dye and CPH providing necessary driving force to overcome the resistance to mass transfer of dye [33].
3.2.4. Effect of solution pH
Fig.5b shows the effect of pH on the adsorption of CR onto CPH. pH is an important parameter in adsorption study because it controls the degree of ionization and speciation of the adsorbate. The effect of pH on the adsorption of CR dye onto CPH was studied in the range of 2-9. The result showed that the amount of dye adsorbed decreased with increase in the pH of dye solution. The maximum adsorption occurred at pH 2 leading to adsorption capacity of 24.51 mg/g. For comparison, the maximum uptake capacity of CR by surfactant-modified zeolites was achieved at pH = 3 (for a range of pH tested between 3 and 11) [34].
Fig.5. Effect of initial dye concentration (a) and pH of solution (b) on the adsorption of CR onto CPH.
3.3. Adsorption kinetics
To further understand the mechanism of adsorption of CR onto cowpea husk, kinetic investigation of the adsorption processes was conducted. Pseudo-first order, pseudo second order, Elovich and Weber and Morris intra-particle diffusion models were used to model the generated experimental data.
3.3.1. Pseudo-first order kinetic model
This kinetic model can be applied to liquid/solid system adsorption and it assumed the adsorption rate of the solutes to be proportional to the number of unoccupied sited on the adsorbent by solutes [11, 15, 35-
37]. The pseudo-first order rate expression popularly known as Lagergren equation is generally expressed by [38]:
ln(𝑞𝑒 − 𝑞𝑡) = ln𝑞𝑒 − k1𝑡 (2) where qe and qt are the amount of dye adsorbed at equilibrium time and at any time t, respectively (mg/g);
k1 is the pseudo-first order adsorption rate constant (min-
1); and t is the contact time (min). The Pseudo-first order rate constants, k1 and qe were determined from the slope and intercept of the plot of ln (qe -qt) versus t as presented in Fig.6a. The results evaluated were listed in Table 2. The values of the correlation coefficient R2
13 showed that pseudo-first order kinetic model does not describe the adsorption process. Similar results were obtained by other authors [39].
3.3.2. Pseudo-second order kinetic model
Adsorption may be second order and the rate limiting step may be chemical adsorption involving valence forces through sharing or the exchange of electrons between the adsorbate and adsorbent. In addition, this type of adsorption follows the Langmuir equation. The rate of adsorption is dependent upon the amount of adsorbate on the surface of adsorbent and that adsorbed at equilibrium [11, 15, 35-36, 40]. The pseudo-second order equation is expressed as:
= 𝑘 (𝑞 𝑞 )2 (3)
where k2 is the rate constant of pseudo second-order adsorption (g/mg.min). For the boundary conditions t = 0 to t = t and qt = 0 to qt = qt, the integrated form of Eq.3 becomes:
( ) = + kt (4)
the equation can be rearranged to obtain Eq.5, which has a linear Form:
= + (𝑡) (5) If the initial adsorption rate, h (mg/g.min) is;
ℎ = 𝑘 𝑞 (6)
then Eq.6 becomes:
( ) = + (t) (7) The plot of (t/qt) versus t gave a linear relationship from which qe and k2 were determined from the slope and intercept of the plot (Fig.6b), respectively. The calculated qe values agree with the experimental values.
The correlation coefficient value R2 was found to be 0.998. These results suggest that the pseudo-second
order fits the adsorption process quiet well. Similar results were reported by other authors [41].
3.3.3. Elovich kinetic model
This is a kinetic equation of chemisorption which describes the rate of adsorption of adsorbate on adsorbent that decreases exponentially with an increase in the amount of the adsorbate adsorbed. It is applied to determine the kinetics of chemisorption onto heterogeneous surfaces [35, 40]. This model gives useful information on the extent of both surface activity and activation energy for adsorption process. The Elovich kinetic model is described by the following relation [42]:
qt = 1/β ln (αβ) + (1/β) ln t (8) The parameters (α) and (β) was calculated from the slope and intercept of the linear plot of qt versus ln(t) (Fig.6c).
3.3.4. Intraparticle diffusion equation
This model, describes mass transfer in an amorphous and homogenous sphere, which is initially free from solute and the concentration of the solute at the surface remains constant [11, 15, 35-36, 40]. The slowest step in an adsorption process is usually taken as the rate determining step. This step is often attributed to pore and intra particle diffusion. Since pseudo first and pseudo second order models cannot provide information on effect of intra particle diffusion in adsorption, intra particle diffusion model can be used. Possibility of involvement of intra particle diffusion model as the sole mechanism was investigated according to Weber-Morris equation [43]:
qe= C + kintt1/2 (9)
where the constant kint (mg/g min0.5) is the intra particle diffusion rate and C is the boundary layer thickness. If the rate-limiting step is only due to the intra particle diffusion, then qt versus t1/2 will be linear and the plot passes through the origin (Fig.6d).
Table 2
Kinetic models for the adsorption of CR onto CPH
Kinetic models Parameters Values
Pseudo-first order qeExp (mg/g) 19.447
qeCal (mg/g) 4.603
k1 (min-1) 0.0253
R2 0.971
Pseudo-second order qeExp (mg/g) 19.447
qeCal (mg/g) 19.531
k2 (g/mg.min) 0.021
R2 0.999
Elovich B 0.918
A 0.860
R2 0.541
Intra particle diffusion kint (mg/g min0.5) 0.329
C 15.847
R2 0.503
14
Fig.6. Pseudo-first order (a), pseudo-second order (b), Elovich (c) and Intra particle diffusion (d) Linear plots for the kinetics of CR adsorption onto CPH
3.4. Adsorption Isotherms
Adsorption isotherms provide helpful information to identify the uptake mechanism and characteristics of the adsorbent surface for the design of sorption systems [44- 45]. Three adsorption isotherm models; Langmuir, Freundlich and Temkin were applied to fit the experimental data for the removal of CR dye by CPH.
3.4.1. Langmuir model
This model assumes monolayer chemical adsorption which can only occur at a finite number of definite localized sites, that are identical and equivalent, with no lateral interaction and steric hindrance between the adsorbed molecules, even on adjacent sites. It refers to homogenous adsorption, with each molecule possessing constant enthalpies and sorption activation energies with no transmigration of the adsorbate in the plane of the surface [11, 15, 35-36, 46-48]. This model is widely used to describe the uptake of pollutants from aqueous media. The Langmuir isotherm model is given by:
q = (10)
where Ce is the equilibrium concentration of CR (mg/L), qe is the amount of dye adsorbed per gram of the adsorbent at equilibrium (mg/g), qm is the maximum monolayer coverage capacity (mg/g) and KL is the Langmuir isotherm constant (L/mg). The values of qm and KL were computed from the slope and intercept of the Langmuir plot of 1/qe versus 1/Ce.
3.4.2. Freundlich model
This empirical model describes the non-ideal and reversible adsorption, and it’s not restricted to the formation of monolayer. It can be applied to multi-layer adsorption, with non-uniform distribution of adsorption heat and affinities over the heterogeneous surface. The amount adsorbed is the summation of adsorption on all sites with the stronger binding sites occupied first, until adsorption energy is exponentially decreased upon the completion of the adsorption process [11, 15, 35, 46-48].
The Freundlich isotherm model is generally used for multilayer sorption on heterogeneous surface of the adsorbent. This uptake model can be illustrated by the following equation:
𝑞 = 𝐾 𝐶 1/n (11)
where qe (mg/g) is the adsorbed amount at equilibrium, Ce (mg/L) is the concentration of the adsorbate in the solution at equilibrium, Kf is the Freundlich constant, n is the adsorption intensity. Its linear form is given by the following equation:
Log q = log 𝐾 + log C (12) 3.4.3. Temkin model
This model is an example of an empirical isotherm. It contains a factor that explicitly takes into account of adsorption of adsorbent-adsorbate interactions by ignoring the extremely low and large value of concentration. The model assumes that the heat of
15 adsorption as a function of temperature of all molecules in the layer would decrease linearly rather than logarithmic with coverage [11, 15, 35, 46-48]. Temkin's model is based on the hypothesis that the heat of adsorption due to interactions with the adsorbate decreases linearly with the recovery rate. Temkin equation is given by the following expression:
q = B lnA + B lnC (13)
where AT is the Temkin isotherm equilibrium binding constant (L/g), B is the Temkin isotherm constant related to heat of adsorption (J/mol), R is the universal gas constant (8.314 J/mol/K) and T is the temperature.
Comparing the values of the correlation coefficient, R2, for the three tested isotherms as shown in Table 3.
From the table, it can be concluded that the adsorption data fitted well to the Langmuir adsorption isotherm.
Table 3
Isotherm constants for the adsorption of CR onto Raw Cowpea Husk
Langmuir Freundlich Temkin
qm (mg/g) KL (L/mg) R2 1/n n Kf R2 A B R2
161.29 0.013 0.958 1.569 0.637 0.676 0.937 0.114 40.43 0.854 3.5. Comparison with other studies
Table 4 illustrates the comparison of adsorption capacities of CR by various adsorbents. As observed, the maximum monolayer uptake capacity (qm, mg/g) of CPH was found to be above the other types of sorbents here reported. Thus, this adsorbent can be considered as an effective option for the removal of CR from aqueous media
Table 4
Comparison of the maximum sorption capacity of CPH with other adsorbents for adsorption of Congo red.
Adsorbent qm(mg/g) Ref.
Eichhornia 1.58 [49]
Activated carbon (Laboratory grade) 1.88 [50]
Bagasse fly Ash 11.89 [50]
Cashew nutshell 5.18 [51]
Montmorillonite 12.70 [52]
Cattail root 38.79 [53]
Neem leaf powder 41.20 [54]
Ca-bentonite 107.41 [55]
Cowpea husk powder 161.29 This study 3.6. Thermodynamics
A study of temperature dependence for the adsorption process gives information on whether the reaction is spontaneous or not. With the aid of thermodynamic parameters such as change in Gibbs free energy (∆G), enthalpy (∆H), and entropy (∆S), the thermodynamic of adsorption reaction towards spontaneity can be evaluated. These parameters were calculated using the following equations [56]:
∆G = −RT ln Kc (14)
Ln Kc= -∆G/RT = −(∆H/RT) + (∆S/R) (15) where R is the universal gas constant (8.314 J/mol.K), T is the absolute temperature (K) and Kc is the distribution coefficient equals to qe/Ce. The values of ∆H and ∆S were calculated from the slope and intercept of the plot of ln Kc versus 1/T and are listed in Table 5. The negative values of ∆G indicates that the process is feasible and the adsorption is thermodynamically spontaneous.
Table 5
Thermodynamic parameters of CR Adsorption onto CPH T (K) ΔG (kJ/mol) ΔH (kJ/mol) ΔS (J/mol.K)
303 -2188.13 288.09 -0.0076
313 -2948.13 323 -4627.52 333 -5917.25 4. Conclusion
In this research, cowpea husk was applied as a cheap adsorbent for the removal of Congo red (CR) from aqueous media. Influential parameters on the removal, such as the contact time, pH, adsorbent dosage, and initial adsorbate concentrations were evaluated and optimized. The equilibrium adsorption (18.962 mg/g) was obtained for a contact time of 60 min and the maximum sorption occurred at pH of 2. The uptake of CR onto the CPH surface was accurately described by the pseudo-second-order kinetic and Langmuir isotherm models. The maximum loading capacity (qm) of the adsorbent for Congo red obtained from the Langmuir isotherm model was 161.29 mg/g. This natural adsorbent has advantages including low-cost, eco-friendly, high- sorption capacity and non-toxicity. Therefore, it can be considered as an effective sorbent for treating wastewater containing CR dye.
Acknowledgement
The contribution of the staff of the research laboratory, Bayero University, Kano is highly acknowledged by the authors.
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