# Adsorptive Removal of Methylene Blue Dye Using Biowaste Materials: Barley Bran and Enset Midrib Leaf

Authors: Dereje Mekuria; Abebe Diro; Fekadu Melak; Tsegaye Girma Asere
Journal: Journal of Chemistry
 (2022)

---

## Abstract

In this study, several biowaste materials are screened for adsorptive removal of methylene blue (MB) from synthetic water. Among the tested adsorbents, barley (Hordeum vulgare) bran (BB) and enset (Ensete ventricosum midrib leaf, EVML) were selected for further evaluation of MB (a model cationic dye) adsorption. Batch MB adsorption performance of BB and EVML adsorbents was significantly high in a wide pH range (4-9). The well fitting of experimental data with pseudosecond-order kinetic model suggests a monolayer adsorption of MB. The MB adsorption onto both adsorbents was fit well with the Langmuir isotherm model with maximum MB adsorption capacities of 63.2 mg/g (BB) and 35.5 mg/g (EVML). The biowaste materials exhibit considerable adsorption capacity for cationic dye (MB), perform well under acidic and basic conditions, and are reusable. Therefore, the use of these materials as adsorbents may have an environmental benefit in terms of the conversion of wastes into valuable materials. Further studies are suggested to investigate the performance of these adsorbents in a continuous mode using real wastewater.


---

## Body

## 1. Introduction

Discharging industrial effluents directly into water bodies without proper treatment can create water pollution [1]. For instance, textile manufacturing uses various types of dyes, chemicals, and large volumes of water in fabric production. Direct release of this dye-contaminated water into the environment can affect human health, soil, and aquatic life [2]. Dye effluents are among those contaminants that have caused significant water pollution worldwide [3]. Dyes are substances obtained either naturally from extracts of flowers, fruits, certain insects, etc. or from synthetic sources that give colors [4]. Natural dyes have inherent weaknesses such as offering a narrow range of colors, low fastness to fabrics, fade on washing, and exposure to light [5]. On the other hand, synthetic dyes provide several colorfast dyes in a wide color range, which led to their mass industrial production [4]. The wide production of various synthetic dyes leads to the discharge of a considerable volume of dye-contaminated wastewater into the environment, which becomes one of the causes of water contamination [6].Generally, synthetic dyes have low biodegradability due to the presence of complex aromatic molecular structures [2]. They are water-soluble and produce very bright colors in the water, particularly those dyes used in textile industries. Textile manufacturing consumes greater than 1000 tons of dyes per annum, where about 10-15% of them are directly discharged into the water bodies through the sewerage system [7]. These effluents contain a mixture of different dyes, auxiliaries, additives, and additional chemicals that may disturb the aquatic system. Small amounts of dyes in water (as low as 1.0 ppm) could bring substantial color, making it unsuitable for human usage [8]. Dyes can also reduce the light transmission and thus diminish the photosynthetic processes of water bodies and imbalance the aquatic system [9, 10]. The problems become serious due to them are stable in different situations such as heat, light, microbes, and even oxidizing agents [11]. Hence, the removal of dyes from water/wastewater by using different technologies is still an ongoing research topic.Many treatment methods have been used for removing dyes from waterwater. The typical treatment methods comprise ion exchange, membrane system (reverse osmosis, electroflotation, nanofiltration), adsorption, coagulation/flocculation, advanced oxidation, ozonation, electrokinetic coagulation, microbial degradation, electrochemical destruction, and precipitation [9, 12, 13]. Adsorption is the commonly utilized treatment technology owing to its flexibility, user friendly, economical, nice performance, recyclability, and being ecofriendly [14]. The common adsorbent, a commercial activated carbon, is costly to use in real applications in different regions of the globe, particularly in developing nations. This has motivated researchers for exploring low-cost, efficient, high carbon content, and renewable adsorbents, which has led to increasing attention in the usage of locally accessible resources, for example, natural materials and agricultural debris [15]. Therefore, this study was intended to evaluate the methylene blue (MB, model dye) adsorption efficiency of selected biowastes accessible in Ethiopia: the petiole of enset (Ensete ventricosum) and barley bran (Hordeum vulgare L. (Poaceae).


## 2. Materials and Methods

### 2.1. Chemicals and Apparatuses

A stock solution of 1000 mg/L MB was prepared, wrap with aluminum foil, and put in a cabinet to protect it from light interaction. Standard and working solutions were prepared by diluting the stock solution. The pH of working solutions was adjusted using 0.1 M HCl (37%, Riedel-deHaën, Germany) and 0.1 M NaOH (90%, BDH, England) solutions. Distilled water was used to prepare and dilute the stock solution. Freshly prepared different concentrations of MB were immediately utilized for the desired experiments. Horizontal GFL shaker (memmert, D-30938, Germany) for the batch adsorption, digital pH meter ORION star A211 for pH measurement, respectively.


### 2.2. Preparation of Biowaste Materials

Biowaste materials such as avocado (Persea americana) seed (AS), enset (Ensete ventricosum) midrib leaf (EVML), barley (Hordeum vulgare L. (Poaceae) bran (BB), coffee (Coffea arabica L) residue (CR), “Tella” residue (TR), eucalyptus tree ash (ETA), acacia tree charcoal (ATC), pea (Pisum sativum) seed shell (PSS), and Erythrina abyssinica bark (EAB) locally called “Korch” were obtained from various locations of Jimma city, Ethiopia. “Tella” residue (TR) and the pea (Pisum sativum) seed shell (PSS) were treated as in literature [16]. Both AS and CR were obtained from local cafeterias, whereas BB, TR, ETA, ATC, and PSS were collected from local household wastes. “Korch” is a wild plant that is also cultivated in different regions of Ethiopia and used to make fences around houses and farmlands in rural areas. Enset belongs to the banana family Musaceae, which is widely domesticated in Ethiopia. It is the primary source of food for many people living in the South and Southwest Ethiopia. The root and stem parts of enset are used as food, whereas the leaves are used to wrap cheese, butter, etc. However, the midrib of the leaf is usually a local waste material. “Tella” is one of the traditionally fermented beverages that is regularly prepared and drunk in Ethiopia. Barley, maize, wheat, hops, and other spices are among the commonly used raw materials in the preparation of “Tella.” Preparation of “Tella” requires 5–7 days, wherein the final stage, the potable “Tell” is filtered to remove any suspended solid while the settled residue is used to feed cows or discard [16].All the collected adsorbents were cleaned and rinsed with distilled water before chopped (in the case of AS, EAB, and EVML) and then dried in air. Mortar and pestle were used to ground the adsorbents, then sieved to themeshsize≤300μm, and kept in closed vessels until further use. In the case of EVML, 50 g of the adsorbent (size of ≤300 μm) was added to 250 mL of 0.1 M HCl containing flask and shaken for 6 h to remove colored components. Then, the adsorbent was thoroughly washed with distilled water to get rid of the excess acid and allowed to dry in an oven at 80°C.


### 2.3. Characterization of Adsorbents

#### 2.3.1. Physicochemical Characteristics

The pH and point of zero charges (pHpzc) of BB and EVML adsorbents were evaluated following the procedures used in [17, 18]. The surface area of the biomaterials was estimated following Sear’s method [19]. Moisture and ash contents of the adsorbents were calculated following the common procedures ASTM D2867-91 method [20] and ASTM D2866-94 [21], respectively.


#### 2.3.2. FTIR Spectral Analysis

The BB and EVML adsorbents (before and after adsorption of MB) were characterized using Fourier transform infrared (FTIR) to locate functional groups that might participate in the MB adsorption. KBr pellet of each adsorbent of an appropriate thickness for infrared analysis was prepared. FTIR spectrophotometer (Perkin Elmer Spectrum 2.0) in a wavenumber range of 400 to 4000 cm-1 was used.


### 2.4. Batch Adsorption Study

Preliminary screening investigations were done to select biowastes (adsorbents) with high MB adsorption capacity at fixed equilibrium experimental conditions. For this purpose, nine (9) different biowaste materials, namely, avocado (Persea americana) seed (AS), Ensete ventricosum midrib leaf (EVML), barley (Hordeum vulgare L. (Poaceae) bran (BB), coffee (Coffea arabica L) residue (CR), “Tella” residue (TR), eucalyptus tree ash (ETA), acacia tree charcoal (ATC), pea (Pisum sativum) seed shell (PSS), and Erythrina abyssinica bark (EAB) were tested for MB adsorption from contaminated water. The adsorbents were tested under similar settings: 10 mg/L of MB solution, 2.5 g/L of adsorbent dose, and pH 5.7. The batch adsorptions were conducted at room temperature agitated on a horizontal thermostat water bath shaker (GrantGLS400, England) for 24 h at a speed of 200 rpm. Then, it was centrifuged and filtered, and the concentration (mg/L) of the residual MB in the filtrate was measured at λmax (665 nm) after each calibration curve using a double beam UV-VIS spectrophotometer (Analytic Jena Specord200 Plus, Germany). The MB removal effectiveness (%) and the MB adsorption capacity (mg/g) were evaluated using Eqs. (1) and (2), respectively.
(1)MBremoval%=C0−CeC0×100(2)qe=C0−CeVmwhereC0 and Ce are the initial and equilibrium concentration of MB solution (mg/L), respectively, m is the mass (g) adsorbent and V is the volume of solution (L).Subsequently, the adsorption behaviors of BB and EVML toward anionic dyes (methyl orange (MO), Erichrome black T (EBT)), and cationic dyes (malachite green (MG), brilliant green (BG) were studied in addition to MB. This study was also performed for each dye using 10 mg/L of initial concentration without pH adjustment, an adsorbent dose of 2.5 g/L, and a contact time of 24 h. All experiments were done in duplicate, and the obtained results were reported asmean±standarddeviation.


### 2.5. Data Analysis

#### 2.5.1. Adsorption Kinetics

To examine the effect of contact time, batch adsorption studies were performed employing 10 mg/L MB in polyethylene containers at the dose of 2.5 g/L BB and EVML and shaking in a water bath at 200 rpm from 5 min to 24 h and 1 min to 2 h, respectively. Samples were collected at predetermined distinct contact times. Then, it was centrifuged and filtered, and the residual-MB concentration of the filtrate was measured atλmax (665 nm). Pseudofirst-order (PFO) and pseudosecond-order (PSO) adsorption kinetic models were employed to evaluate the kinetics of MB adsorption onto BB and EVML adsorbents. The nonlinear expressions of the PFO and PSO models are given in Eqs. (3) and (4), respectively.
(3)qt=qe1−exp−K1t(4)qt=K2qe2t1+K2qetwhereqt and qe are the MB adsorption capacity (mg/g) at any time and at equilibrium, respectively, and k1 and k2 are the PFO (L/min) and PSO (g/mg min) rate constants, respectively.To predict the potential rate-limiting steps in the adsorption progressions, intraparticle diffusion was evaluated using the Weber Morris model as presented in Eq. (5) [22]. Pore or internal diffusion was stipulated by employing the relationship between specific sorption (qt) and time (t0.5) in the model.
(5)qt=kpt0.5+cwhereqt is the amount of MB adsorbed on BB or EVML (mg/g) at time t, kp (mg/(g.min0.5) is the intraparticle diffusion rate constant; if the plot of qt against t1/2 is a linear and pass through the origin, it suggests that intraparticle diffusion is the rate-limiting step of MB adsorption.


#### 2.5.2. Adsorption Isotherms

Adsorption isotherms were determined at24±1°C by employing BB and EVML adsorbents. Constant adsorbent dosage (2.5 g/L) was added to a series of increasing MB concentrations from 10 to 165 mg/L and 10 to 105 mg/L for BB and EVML adsorbents, respectively. The mixtures were agitated until the equilibrium time, and the remaining MB in the filtrate was measured after centrifuging it. The equilibrium experimental data of MB adsorption onto BB and EVML were studied using Langmuir and Freundlich models. The Langmuir isotherm is a monolayer adsorption phenomenon, whereas the Freundlich isotherm model assumes multilayer adsorption. The nonlinear forms of the Langmuir and Freundlich isotherm models are as given in Eqs. (6) and (7), respectively [17].
(6)qe=QmaxbCe1+bCe(7)qe=KFCe1/nwhereCe (mg/L) is the equilibrium concentration of MB, Qmax (mg/g) is the maximum adsorption capacity based on the Langmuir equation, b (L/mg) is the Langmuir constant, KF (mg1-1/n L1/n/g) is the sorption coefficient based on the Freundlich equation, and 1/n is the sorption intensity based on the Freundlich equation.


#### 2.5.3. Statistical Test

In addition to the correlation coefficient (R2), the nonlinear chi-square (χ2) statistic test was employed to distinguish the best model fit to the obtained experimental data. A small χ2 value shows the similarity of data between the modeled and the experimental data, whereas a greater χ2 means variation within the modeled and experimental data. The χ2 is computed using Eq. (8) [17].
(8)χ2=∑qe−qe,cal2qe,calwhereqe,cal is the equilibrium adsorption capacity computed from the model, and qe is the experimental equilibrium adsorption capacity.


### 2.6. Adsorption Cycle

Reusability of BB and EVML adsorbents was tested by keeping the optimum adsorption operating conditions like contact time, adsorbent dose, pH of MB solution, and initial concentration of MB (10 mg/L) dye. The adsorption cycles were determined without desorbing the previously 10 mg/L MB-loaded adsorbents. The experiments were done by carefully decanting the supernatant residual MB solution and then allowing the MB-loaded adsorbents to dry to open air before the 2nd cycle MB adsorption test and so on.


## 2.1. Chemicals and Apparatuses

A stock solution of 1000 mg/L MB was prepared, wrap with aluminum foil, and put in a cabinet to protect it from light interaction. Standard and working solutions were prepared by diluting the stock solution. The pH of working solutions was adjusted using 0.1 M HCl (37%, Riedel-deHaën, Germany) and 0.1 M NaOH (90%, BDH, England) solutions. Distilled water was used to prepare and dilute the stock solution. Freshly prepared different concentrations of MB were immediately utilized for the desired experiments. Horizontal GFL shaker (memmert, D-30938, Germany) for the batch adsorption, digital pH meter ORION star A211 for pH measurement, respectively.


## 2.2. Preparation of Biowaste Materials

Biowaste materials such as avocado (Persea americana) seed (AS), enset (Ensete ventricosum) midrib leaf (EVML), barley (Hordeum vulgare L. (Poaceae) bran (BB), coffee (Coffea arabica L) residue (CR), “Tella” residue (TR), eucalyptus tree ash (ETA), acacia tree charcoal (ATC), pea (Pisum sativum) seed shell (PSS), and Erythrina abyssinica bark (EAB) locally called “Korch” were obtained from various locations of Jimma city, Ethiopia. “Tella” residue (TR) and the pea (Pisum sativum) seed shell (PSS) were treated as in literature [16]. Both AS and CR were obtained from local cafeterias, whereas BB, TR, ETA, ATC, and PSS were collected from local household wastes. “Korch” is a wild plant that is also cultivated in different regions of Ethiopia and used to make fences around houses and farmlands in rural areas. Enset belongs to the banana family Musaceae, which is widely domesticated in Ethiopia. It is the primary source of food for many people living in the South and Southwest Ethiopia. The root and stem parts of enset are used as food, whereas the leaves are used to wrap cheese, butter, etc. However, the midrib of the leaf is usually a local waste material. “Tella” is one of the traditionally fermented beverages that is regularly prepared and drunk in Ethiopia. Barley, maize, wheat, hops, and other spices are among the commonly used raw materials in the preparation of “Tella.” Preparation of “Tella” requires 5–7 days, wherein the final stage, the potable “Tell” is filtered to remove any suspended solid while the settled residue is used to feed cows or discard [16].All the collected adsorbents were cleaned and rinsed with distilled water before chopped (in the case of AS, EAB, and EVML) and then dried in air. Mortar and pestle were used to ground the adsorbents, then sieved to themeshsize≤300μm, and kept in closed vessels until further use. In the case of EVML, 50 g of the adsorbent (size of ≤300 μm) was added to 250 mL of 0.1 M HCl containing flask and shaken for 6 h to remove colored components. Then, the adsorbent was thoroughly washed with distilled water to get rid of the excess acid and allowed to dry in an oven at 80°C.


## 2.3. Characterization of Adsorbents

### 2.3.1. Physicochemical Characteristics

The pH and point of zero charges (pHpzc) of BB and EVML adsorbents were evaluated following the procedures used in [17, 18]. The surface area of the biomaterials was estimated following Sear’s method [19]. Moisture and ash contents of the adsorbents were calculated following the common procedures ASTM D2867-91 method [20] and ASTM D2866-94 [21], respectively.


### 2.3.2. FTIR Spectral Analysis

The BB and EVML adsorbents (before and after adsorption of MB) were characterized using Fourier transform infrared (FTIR) to locate functional groups that might participate in the MB adsorption. KBr pellet of each adsorbent of an appropriate thickness for infrared analysis was prepared. FTIR spectrophotometer (Perkin Elmer Spectrum 2.0) in a wavenumber range of 400 to 4000 cm-1 was used.


## 2.3.1. Physicochemical Characteristics

The pH and point of zero charges (pHpzc) of BB and EVML adsorbents were evaluated following the procedures used in [17, 18]. The surface area of the biomaterials was estimated following Sear’s method [19]. Moisture and ash contents of the adsorbents were calculated following the common procedures ASTM D2867-91 method [20] and ASTM D2866-94 [21], respectively.


## 2.3.2. FTIR Spectral Analysis

The BB and EVML adsorbents (before and after adsorption of MB) were characterized using Fourier transform infrared (FTIR) to locate functional groups that might participate in the MB adsorption. KBr pellet of each adsorbent of an appropriate thickness for infrared analysis was prepared. FTIR spectrophotometer (Perkin Elmer Spectrum 2.0) in a wavenumber range of 400 to 4000 cm-1 was used.


## 2.4. Batch Adsorption Study

Preliminary screening investigations were done to select biowastes (adsorbents) with high MB adsorption capacity at fixed equilibrium experimental conditions. For this purpose, nine (9) different biowaste materials, namely, avocado (Persea americana) seed (AS), Ensete ventricosum midrib leaf (EVML), barley (Hordeum vulgare L. (Poaceae) bran (BB), coffee (Coffea arabica L) residue (CR), “Tella” residue (TR), eucalyptus tree ash (ETA), acacia tree charcoal (ATC), pea (Pisum sativum) seed shell (PSS), and Erythrina abyssinica bark (EAB) were tested for MB adsorption from contaminated water. The adsorbents were tested under similar settings: 10 mg/L of MB solution, 2.5 g/L of adsorbent dose, and pH 5.7. The batch adsorptions were conducted at room temperature agitated on a horizontal thermostat water bath shaker (GrantGLS400, England) for 24 h at a speed of 200 rpm. Then, it was centrifuged and filtered, and the concentration (mg/L) of the residual MB in the filtrate was measured at λmax (665 nm) after each calibration curve using a double beam UV-VIS spectrophotometer (Analytic Jena Specord200 Plus, Germany). The MB removal effectiveness (%) and the MB adsorption capacity (mg/g) were evaluated using Eqs. (1) and (2), respectively.
(1)MBremoval%=C0−CeC0×100(2)qe=C0−CeVmwhereC0 and Ce are the initial and equilibrium concentration of MB solution (mg/L), respectively, m is the mass (g) adsorbent and V is the volume of solution (L).Subsequently, the adsorption behaviors of BB and EVML toward anionic dyes (methyl orange (MO), Erichrome black T (EBT)), and cationic dyes (malachite green (MG), brilliant green (BG) were studied in addition to MB. This study was also performed for each dye using 10 mg/L of initial concentration without pH adjustment, an adsorbent dose of 2.5 g/L, and a contact time of 24 h. All experiments were done in duplicate, and the obtained results were reported asmean±standarddeviation.


## 2.5. Data Analysis

### 2.5.1. Adsorption Kinetics

To examine the effect of contact time, batch adsorption studies were performed employing 10 mg/L MB in polyethylene containers at the dose of 2.5 g/L BB and EVML and shaking in a water bath at 200 rpm from 5 min to 24 h and 1 min to 2 h, respectively. Samples were collected at predetermined distinct contact times. Then, it was centrifuged and filtered, and the residual-MB concentration of the filtrate was measured atλmax (665 nm). Pseudofirst-order (PFO) and pseudosecond-order (PSO) adsorption kinetic models were employed to evaluate the kinetics of MB adsorption onto BB and EVML adsorbents. The nonlinear expressions of the PFO and PSO models are given in Eqs. (3) and (4), respectively.
(3)qt=qe1−exp−K1t(4)qt=K2qe2t1+K2qetwhereqt and qe are the MB adsorption capacity (mg/g) at any time and at equilibrium, respectively, and k1 and k2 are the PFO (L/min) and PSO (g/mg min) rate constants, respectively.To predict the potential rate-limiting steps in the adsorption progressions, intraparticle diffusion was evaluated using the Weber Morris model as presented in Eq. (5) [22]. Pore or internal diffusion was stipulated by employing the relationship between specific sorption (qt) and time (t0.5) in the model.
(5)qt=kpt0.5+cwhereqt is the amount of MB adsorbed on BB or EVML (mg/g) at time t, kp (mg/(g.min0.5) is the intraparticle diffusion rate constant; if the plot of qt against t1/2 is a linear and pass through the origin, it suggests that intraparticle diffusion is the rate-limiting step of MB adsorption.


### 2.5.2. Adsorption Isotherms

Adsorption isotherms were determined at24±1°C by employing BB and EVML adsorbents. Constant adsorbent dosage (2.5 g/L) was added to a series of increasing MB concentrations from 10 to 165 mg/L and 10 to 105 mg/L for BB and EVML adsorbents, respectively. The mixtures were agitated until the equilibrium time, and the remaining MB in the filtrate was measured after centrifuging it. The equilibrium experimental data of MB adsorption onto BB and EVML were studied using Langmuir and Freundlich models. The Langmuir isotherm is a monolayer adsorption phenomenon, whereas the Freundlich isotherm model assumes multilayer adsorption. The nonlinear forms of the Langmuir and Freundlich isotherm models are as given in Eqs. (6) and (7), respectively [17].
(6)qe=QmaxbCe1+bCe(7)qe=KFCe1/nwhereCe (mg/L) is the equilibrium concentration of MB, Qmax (mg/g) is the maximum adsorption capacity based on the Langmuir equation, b (L/mg) is the Langmuir constant, KF (mg1-1/n L1/n/g) is the sorption coefficient based on the Freundlich equation, and 1/n is the sorption intensity based on the Freundlich equation.


### 2.5.3. Statistical Test

In addition to the correlation coefficient (R2), the nonlinear chi-square (χ2) statistic test was employed to distinguish the best model fit to the obtained experimental data. A small χ2 value shows the similarity of data between the modeled and the experimental data, whereas a greater χ2 means variation within the modeled and experimental data. The χ2 is computed using Eq. (8) [17].
(8)χ2=∑qe−qe,cal2qe,calwhereqe,cal is the equilibrium adsorption capacity computed from the model, and qe is the experimental equilibrium adsorption capacity.


## 2.5.1. Adsorption Kinetics

To examine the effect of contact time, batch adsorption studies were performed employing 10 mg/L MB in polyethylene containers at the dose of 2.5 g/L BB and EVML and shaking in a water bath at 200 rpm from 5 min to 24 h and 1 min to 2 h, respectively. Samples were collected at predetermined distinct contact times. Then, it was centrifuged and filtered, and the residual-MB concentration of the filtrate was measured atλmax (665 nm). Pseudofirst-order (PFO) and pseudosecond-order (PSO) adsorption kinetic models were employed to evaluate the kinetics of MB adsorption onto BB and EVML adsorbents. The nonlinear expressions of the PFO and PSO models are given in Eqs. (3) and (4), respectively.
(3)qt=qe1−exp−K1t(4)qt=K2qe2t1+K2qetwhereqt and qe are the MB adsorption capacity (mg/g) at any time and at equilibrium, respectively, and k1 and k2 are the PFO (L/min) and PSO (g/mg min) rate constants, respectively.To predict the potential rate-limiting steps in the adsorption progressions, intraparticle diffusion was evaluated using the Weber Morris model as presented in Eq. (5) [22]. Pore or internal diffusion was stipulated by employing the relationship between specific sorption (qt) and time (t0.5) in the model.
(5)qt=kpt0.5+cwhereqt is the amount of MB adsorbed on BB or EVML (mg/g) at time t, kp (mg/(g.min0.5) is the intraparticle diffusion rate constant; if the plot of qt against t1/2 is a linear and pass through the origin, it suggests that intraparticle diffusion is the rate-limiting step of MB adsorption.


## 2.5.2. Adsorption Isotherms

Adsorption isotherms were determined at24±1°C by employing BB and EVML adsorbents. Constant adsorbent dosage (2.5 g/L) was added to a series of increasing MB concentrations from 10 to 165 mg/L and 10 to 105 mg/L for BB and EVML adsorbents, respectively. The mixtures were agitated until the equilibrium time, and the remaining MB in the filtrate was measured after centrifuging it. The equilibrium experimental data of MB adsorption onto BB and EVML were studied using Langmuir and Freundlich models. The Langmuir isotherm is a monolayer adsorption phenomenon, whereas the Freundlich isotherm model assumes multilayer adsorption. The nonlinear forms of the Langmuir and Freundlich isotherm models are as given in Eqs. (6) and (7), respectively [17].
(6)qe=QmaxbCe1+bCe(7)qe=KFCe1/nwhereCe (mg/L) is the equilibrium concentration of MB, Qmax (mg/g) is the maximum adsorption capacity based on the Langmuir equation, b (L/mg) is the Langmuir constant, KF (mg1-1/n L1/n/g) is the sorption coefficient based on the Freundlich equation, and 1/n is the sorption intensity based on the Freundlich equation.


## 2.5.3. Statistical Test

In addition to the correlation coefficient (R2), the nonlinear chi-square (χ2) statistic test was employed to distinguish the best model fit to the obtained experimental data. A small χ2 value shows the similarity of data between the modeled and the experimental data, whereas a greater χ2 means variation within the modeled and experimental data. The χ2 is computed using Eq. (8) [17].
(8)χ2=∑qe−qe,cal2qe,calwhereqe,cal is the equilibrium adsorption capacity computed from the model, and qe is the experimental equilibrium adsorption capacity.


## 2.6. Adsorption Cycle

Reusability of BB and EVML adsorbents was tested by keeping the optimum adsorption operating conditions like contact time, adsorbent dose, pH of MB solution, and initial concentration of MB (10 mg/L) dye. The adsorption cycles were determined without desorbing the previously 10 mg/L MB-loaded adsorbents. The experiments were done by carefully decanting the supernatant residual MB solution and then allowing the MB-loaded adsorbents to dry to open air before the 2nd cycle MB adsorption test and so on.


## 3. Results and Discussion

### 3.1. Characterization of Adsorbents

Determining the physicochemical properties and/or nature of adsorbents is basic to understanding the adsorption efficiency or capacity of the adsorbents. Hereby, the major physicochemical properties of BB and EVML are illustrated in Table1. Adsorbent features, including low moisture content and high value of dry matter, are important for the adsorption processes.Table 1
Physiochemical properties of BB and EVML adsorbents.
ParameterBBEVMLMoisture content %3.04.0Dry matter %97.096.0Ash %10.28.8Surface area (m2/g)26.220.34pH in water6.474.53pHPZC5.93.5The FTIR of BB and EVML was analyzed, and the obtained spectral data is shown in Figure1. Several bands with different intensity were observed in the spectra region of 4000 cm-1 to 500 cm-1. Figure 1(a) displays the FTIR results of BB before and after MB adsorption, whereas Figure 1(b) gives the spectra of EVML before and after MB adsorption. The observed change in intensity of absorption bands and/or a shift in frequency after MB adsorption may indicate the interaction of MB with the functional groups available on the surfaces the BB and EVML [23].Figure 1
FTIR spectra of (a) BB and (b) EVML before and after adsorption.
(a)(b)The band near 3428 cm-1 in the FTIR of BB shows the existence of –OH groups in BB (Figure 1(a)). The bands in the region 2922 cm-1 and 2852 cm-1 could be due to the asymmetrical and symmetrical stretching vibrations of the CH2 group with a saturated carbon [24]. The peak around 1640 cm−1 may either be ascribed to C=C stretching of alkene or N–H bending of amine [23]. The weak bands around 1383 cm−1 and 1418 cm−1 could be due to -CH2 bending. The band at 1040 cm−1 was assigned to be the C–O and COO– stretching, indicating the presence of carboxylic groups in BB Figure 1(a) [25]. From the FTIR spectrum of EVML (Figure 1(b)), the band at 3410 cm-1 is because of the O-H (hydroxyl) group stretching vibration, seemingly N-H medium peak overlapping near to 3480 cm-1. The band near 2920 cm-1 indicates the notable C-H stretching in the lignin functionality as explained in the literature [26]. The band around 1620 cm−1 is ascribed to the C=O (carbonyl) group stretching. The weak band near 1316 may be due to C-N bending in the amino group. The adsorption band observed around 1055 cm−1 matches the C–O stretching [27].


### 3.2. Screening of Biowaste Materials

The MB removal efficiencies of nine locally available biowaste materials were under similar conditions, and the obtained results are demonstrated in Figure2. The MB adsorption efficiency of BB, EAB, CR, and EVML was ≥95%. The PSS and TR showed similar MB removal efficiency (~90%) under the specified conditions. However, if any of these conditions (initial MB concentration, pH, etc.) change, the two adsorbents may not have similar MB removal potential.Figure 2
Adsorbents screening test for MB adsorption: initial concentration of MB 10 mg/L, adsorbent dose 2.5 g/L, pH 5.7, agitation time 24 h, shaking speed 200 rpm, and temperature25±1°C).Among the considered adsorbents, BB and EVML were selected because of their high MB removal efficiency. To the best of our knowledge, there was no previous report on these biowastes as adsorbents. The selected adsorbents (BB and EVML) were also evaluated for their anionic dyes methyl orange (MO) and Erichrome Black T (EBT) and cationic dyes malachite green (MG) and brilliant green (BG) removal efficiency at two different concentrations (10 mg/L and 20 mg/L) in their natural (nonadjusted) pH 5.62 and 5.68, 5.59 and 5.53, 4.82 and 4.55, and 4.99 and 4.26, respectively. Figure3 shows the obtained results.Figure 3
Adsorption performance of BB (a) and EVML (b) for different cationic and anionic dyes (conditions: initial concentration of the dyes 10 mg/L, adsorbent dose 2.5 g/L, agitation time 24 h, shaking speed 200 rpm, and temperature25±1°C).
(a)(b)Both adsorbents effectively removed the cationic dyes MB, BG, and MG compared to the anionic dyes MO and EBT. It may be due to the surface of the adsorbents being negatively charged under the above-specified pH since the pH of zero charge of adsorbents, BB, and EVML was found to be 5.9 and 3.5, respectively. The negatively charged surface of the adsorbents imparts electrostatic interactions to bind cationic dyes (MB, BG, and MG) while repelling the anionic dyes MO and EBT). The negative charge on the anionic dyes is due to the functional group SO3-. Note that the SO3H group having a pKa of -2.1 is expected to be dissociated to SO3- even at pH 2 [28]. Therefore, the adsorption properties of BB and EVML were further studied using MB as a model cationic dye.


### 3.3. Effect of Agitation Time

The influence of agitation time on the removal of MB was evaluated for the agitation time of 5 min to 24 h and 1 min to 2 h for BB and EVML adsorbents, respectively. The obtained results were presented in Figure4(a) and 4(b). As the contact time increases, the MB adsorption onto both adsorbents increases. Rapid adsorption at the beginning was observed until 20 min and 2 h for EVML and BB, respectively. Afterward, only a gradual increment was observed for both adsorbents because of attaining adsorption equilibrium. That is, the MB adsorption and desorption proceeded nearly at equal rates. However, to ensure maximum MB adsorption and avoid uncertainties in the equilibrium, 4 h was considered for BB, whereas 1 h for EVML for the subsequent adsorption analysis and other adsorption parameter optimizations.Figure 4
Effect of agitation time on MB removal (a) BB and (b) EVML (conditions: initial concentration (10 mg/L), adsorbent dose (2.5 g/L), shaking speed 200 rpm, and temperature25±1°C).
(a)(b)


### 3.4. Kinetics

Kinetics governing the adsorption procedure is among the critical factors that must be considered in designing an adsorption system. The time-dependent MB adsorption onto BB and EVML was estimated by fitting the laboratory obtained results into the nonlinear form of pseudofirst-order (PFO) and pseudosecond-order (PSO) models (Eqs. (3) and (4)). The obtained information from the fitting models is shown in Figures 5(a) and 5(b). The better-fitted model was identified by comparing their correlation coefficient (R2) and the nonlinear chi-square (χ2) values (Table 2). Table 2 indicates that the kinetics of obtained experimental data for both adsorbents can be described by using the PSO model rather than the PFO. Besides, the experimental qe values matched with the calculated qe values found in theory, proposing chemisorption as the main MB removal mechanism [27]. The initial MB removal rates (V0, mg/g min) were computed using the formula V0=k2qe2, and the MB adsorption rate of BB (512 mg/g min) was about 42 times faster than that of EVML (12 mg/g min). This can be due to the variation in the interaction of MB to the active functional group present in BB and EVML. The slope of the curves for both adsorbents is very sharp at the initial stage indicates fast adsorption process with high capacity, which is vital for real uses of the adsorbents to treat dye-contaminated water.Figure 5
The kinetic models represented by PFO and PSO for (a) BB and (b) EVML.
(a)(b)Table 2
PFO and PSO model parameters for MB adsorption onto BB and EVML.
ParameterBBEVMLPFOPSOPFOPSOCo (mg/L)10101010qe,exp (mg/g)3.7443.7443.6073.607qe,cal (mg/g)3.6863.7203.5733.616k1 (min-1)33.057—0.635—k2 (g/(mg.min))—36.995—0.926V0 (mg/(g.min))—511.95—12.11R20.569040.837290.566760.92953χ26.02×10−32.29×10−32.02×10−33.29×10−4The Weber and Morris model was used to identify the rate-determining step in the MB adsorption process [29] (Eq. (5)). The adsorption process of a certain adsorbate usually proceeds through four steps, and these are bulk diffusion, film diffusion, intraparticle or pore diffusion, and adsorption of adsorbate on the adsorbent surface [30]. However, film and/or intraparticle diffusion are usually determine the overall rate of the adsorption process [22, 27]. Figures 6(a) and 6(b) demonstrate the intraparticle diffusion phenomena for the adsorption of MB onto BB and EVML. It is clearly shown that the curves are linear and do not pass through the origin, demonstrating that intraparticle diffusion was not the only rate-determining step. That is, MB adsorption occurs through the engagement of more than one step of the aforementioned four adsorption steps. The obtained results match with previous research findings on adsorption of MB onto various biomasses such as biowaste Papaya stem [27] and chemically modified Bamboo [31].Figure 6
Intraparticle diffusion model plot for the (a) BB and (b) EVML.
(a)(b)


### 3.5. Effect of pH

Adsorption processes is significantly affected by pH, since it determine whether an adsorbate is in anionic, neutral, or cationic form and also the surface charge of adsorbents [7]. Increasing the initial pH of MB solution from 2 to 9 for the case of both adsorbents, MB removal efficiency increases from 46% to 97% for BB and from 60% to 97% for EVML (Figures 7(a) and 7(b)). The only exception is pH 3, where 87% and 91% of MB were removed by BB and EVML, respectively. The smaller MB adsorption below pH 3 may be due to the amino and carboxylic acid functional groups being largely protonated, thus resulting in greater electrostatic repulsion for the cationic MB dye [31]. The high MB adsorption onto the adsorbents was observed when the initial pH increases from 4 to 9. The observations were in accordance with earlier findings on MB removal using chemically modified bamboo [31], spent tea leaves [6], and chemically treated agricultural waste [7]. This is interesting in real applications because it avoids the possibility of pH adjustment using an acid and/or a base that may cause secondary pollution. Therefore, the natural pH of MB solution (pH=5.07) was considered as an ideal pH and used it in further optimization procedures. The high MB removal by both adsorbents in a wide pH range, above and below their pHpzc, suggests the involvement of several interactions like complexation, electrostatic force, and the dipole of the adsorbent surface-polar (cation of the dye) interaction [6].Figure 7
Effect of solution pH on the adsorption of MB onto (a) BB and (b) EVML (conditions: initial concentration 10 mg/L MB, adsorbent dose 2.5 g/L, shaking speed 200 rpm, and temperature25±1°C).
(a)(b)


### 3.6. Change in Initial MB Concentration

The impact of initial concentration in the adsorption process was assessed by varying the MB concentrations in the range of 10 mg/L to 165 mg/L for BB and 10 mg/L to 105 mg/L for EVML. The obtained results are illustrated in Figures8(a) and 8(b). As the initial concentration of MB changed from10 mg/L to 165 mg/L, the adsorption capacity of BB improved from 3.68 to 49.11 mg/g, whereas the adsorption capacity of EVML raised from 3.69 to 31.48 mg/g along with an increase in MB concentration from 10 mg/L to 105 mg/L. On the other hand, the percent of MB removal decreased for both adsorbents as the initial concentrations of MB increased. At a lower initial concentration of MB, a few of the adsorbent molecules occupy the adsorbent’s surface; therefore, the percent of adsorbate removal is high. However, an increase in the initial MB concentration resulted in coverage of the adsorbent surface by MB molecules, causing in a decline in MB adsorption. This is a common phenomenon in the adsorption process and revealed in results of previous studies on removal of MB by acid-treated banana peel [32], chemically activated Palm fibers [24], and Parthenium hysterophorus (an agricultural waste) [7].Figure 8
Initial MB concentration variation (a) BB and (b) EVML(conditions: adsorbent dose 2.5 g/L, shaking speed 200 rpm, and temperature25±1°C).
(a)(b)


### 3.7. Isotherm

The adoption capacity of an adsorbent usually predicted from isotherm study. Thus, nonlinear Langmuir and Freundlich isothermal equations (Eqs. (6) and (7)) were employed to the experimental MB adsorption data analysis at equilibrium conditions. The plots are shown in Figures 9(a) and 9(b). Table 3 provides the obtained isotherm parameters. The isotherm model that better describes the MB adsorption process was identified by comparing the obtained correlation coefficients (R2) and chi-square (χ2) values. The experimental data of MB adsorption onto both adsorbents suited to the Langmuir isotherm, indicating a monolayer adsorption of MB on the surface of the adsorbents [33]. The maximum MB adsorption capacity was 63.2 mg/g and 35.5 mg/g for BB and EVML, respectively. The separation factor (RL), calculated by the formula RL=1/1+bC0, is used to anticipate the feature of the adsorption procedure [17]. The adsorption process is irreversible if RL=0, favorable if 0<RL<1, linear if RL=1, and unfavorable if RL>1. In this case, the calculated RL values are between 0 and 1 (Table 3), demonstrating the favorable nature of the adsorption process.Figure 9
Plot of isotherm models: (a) BB and (b) EVML.
(a)(b)Table 3
Important parameters derived from Langmuir and Freundlich model fittings.
IsothermParametersBBEVMLLangmuirqmax (mg/g)63.2335.52b (L/mg)0.19780.2438RL0.00123−0.02020.00232-0.0252R20.989730.98029χ23.432271.79086FreundlichKF ((mg1-1/nL1/n)/g)11.9989.784N2.0182.677R20.93110.9579χ223.04443.82589MB is among the toxic cationic basic dyes that are widely used in different industries that requires effective treatment. The effectiveness of BB and EVML adsorbents was compared to some natural and modified adsorbents (Table4). In general, the maximum adsorption capacity of MB onto the BB and EVML adsorbents was comparable to that of several other natural adsorbents. However, compared with some activated adsorbents such as palm stem and H2SO4 activated banana peel, the adsorption capacity of BB and EVML adsorbents was lower. Besides the relatively good adsorption capacities of the biowaste materials, they have also advantages from the perspective of waste reuse to be economically and environmentally safe. The possibility to reuse the adsorbents is an additional asset for these adsorbents.Table 4
The maximum adsorption capacity of some adsorbents for MB adsorption.
AdsorbentspHQmax (mg/g)Equilibrium time (h)Dose (g/L)ReferenceWheat shell5-916.6110[34]Rice husk—40.60.674[35]Activated carbon from palm stem6110.80.31.25[24]Date palm leaf powder6.543.12.6710[36]Neem leaf powder5-88.8——[37]Rice husk5-721.9246[38]Rice husk ash5-712.8246[38]H2SO4 activated banana peel10250.0240.8[32]Sugarcane bagasse—9.4242.0[39]Tea waste8113.1244.0[40]BB5.0763.242.5Present studyEVML5.6635.512.5Present study


### 3.8. Adsorption Cycles

The reuse of adsorbents is very vital in producing cost-effective and safe adsorbents. 0.1 M NaOH and 0.1 M HCl solutions were employed in the desorption study. However, the obtained desorption efficiency was not satisfactory. Therefore, the reuse of the adsorbents was assessed without desorption through successive adsorption cycles. The reusability of BB and EVML is presented in Figure10. The MB removal efficiency of both BB and EVML was not significantly reduced until the 5th cycle. This indicated that the biowaste materials (BB and EVML) were efficient for MB adsorption from contaminated water. The reusability of biowaste materials and their natural accessibility at a very low price makes this method further attractive.Figure 10
Adsorption cycle of (a) BB and (b) EVML for MB removal.
(a)(b)


### 3.9. Feasibility Insights

#### 3.9.1. Economic Feasibility

The cost of water treatment is the most important criterion for an adsorbent or treatment method employed. In adsorption technologies, adsorbent cost-effectiveness is the key constraint that must be considered during adsorbents selection. The cost of an adsorbent can be affected by several factors including its local availability, type of processes required, treatment conditions, recyclability or reusability options, and a lifetime of the adsorbent [41, 42]. The adsorption capacity has also an impact on the cost of the adsorbent material. The techniques to enhance the applicability of the adsorbent material that has a relation to the cost including regeneration, reusability; and high capacity of the adsorbents should be taken into account in cost-benefit analysis. Table 5 presents cost estimates for some reported adsorbents for MB removal. Most often, the cost estimation is rarely reported in literature briefing simply “low-cost” that might be obtained by considering the cost of the raw materials. However, all considerations of local availability, processing, transportation costs, recyclability, and a lifetime of adsorbent material are found vital [43]. The costs of BB and EVML are quite good in comparison with other adsorbents (Table 5). For instance, it was found to have only processing costs as both adsorbents are sourced from byproducts/wastes. Comparison of the costs of the adsorbents is not straightforward due to variations in adsorption capacity, regional processing fees, experimental level, and large-scale conditions. Furthermore, material costs, energy costs, capital costs, and other miscellaneous detail costs are required to be evaluated for large-scale applications. Hence, locally available and low-cost materials as is the case in this study predicted from lab-scale experiments have paramount importance as an alternative adsorbent. In general, the reusability of both adsorbents, BB and EVML, can work for more than five cycles in the adsorption process, local availability, and abundance. Enset and barley are widely cultivated in Ethiopia [44] indicating the promising availability of BB and EVML and their feasibility for the targeted application. Both BB and EVML adsorbents could have a potential to substitute commercially available activated carbon at a lower cost. Industrial-scale treatments of dye wastewater using bio-adsorbents were considered promising at large due to availability and cost benefits [43].Table 5
Comparative presentation of costs of adsorbents.
AdsorbentsCost USD/kgReferenceActivated carbon from nutshells2.15[45]Virgin carbon0.86-2.95[46]Reactivated carbon0.43-0.70[47]Alumina0.03[48]Sludge-based adsorbents0.1-0.2[41]Clay minerals0.04-0.12[49]Zeolite0.14-0.8[48, 50]Diatomite0.011-1.10[48]BB<0.012 (processing cost only)Current studyEVML<0.02 (processing cost only)Current study


#### 3.9.2. Technical Feasibility and Environmental Safety

Most often, carbonization, physical activation, and chemical activation are commonly employed in adsorbent preparation processes that incur costs to a certain extent. The leaching from the adsorbents could be a limiting factor in water treatment applications; however, the adsorbents, BB, and EVML are part of edible crops that are expected to be less likely to leach toxic chemicals. This is because enset and barley are among the staple foods of Ethiopians, and the level of contaminants including heavy metal contents is well studied [44]. Processing and putting into batches and column set up of these adsorbents are easier and adaptable due to the traditional use of these materials in water storage clay pot preparations.


#### 3.9.3. Accessibility

Accessibility of the adsorbent material in adsorption technology could contribute to cost savings. Hence, understanding the local availability and abundance of materials BB and EVML is vital. Enset is one of the staple foods of Ethiopia consumed by more than 20 million people due to its high starch content [51], from where EVML is produced. The dry matter content of lamina and midribs of leaves of enset is estimated to be 15–17% [52]. It is a multipurpose tree that has been found abundantly, especially in South Ethiopia. Enset grows in East and Southern Africa, and the genus extends across Asia to China; however, it is domesticated only in the Ethiopian [53]. And Ethiopia is the second-largest barley producer in Africa [54]. Barley bran is a byproduct of barley processes, and it is brittle and shatters of barley.


## 3.1. Characterization of Adsorbents

Determining the physicochemical properties and/or nature of adsorbents is basic to understanding the adsorption efficiency or capacity of the adsorbents. Hereby, the major physicochemical properties of BB and EVML are illustrated in Table1. Adsorbent features, including low moisture content and high value of dry matter, are important for the adsorption processes.Table 1
Physiochemical properties of BB and EVML adsorbents.
ParameterBBEVMLMoisture content %3.04.0Dry matter %97.096.0Ash %10.28.8Surface area (m2/g)26.220.34pH in water6.474.53pHPZC5.93.5The FTIR of BB and EVML was analyzed, and the obtained spectral data is shown in Figure1. Several bands with different intensity were observed in the spectra region of 4000 cm-1 to 500 cm-1. Figure 1(a) displays the FTIR results of BB before and after MB adsorption, whereas Figure 1(b) gives the spectra of EVML before and after MB adsorption. The observed change in intensity of absorption bands and/or a shift in frequency after MB adsorption may indicate the interaction of MB with the functional groups available on the surfaces the BB and EVML [23].Figure 1
FTIR spectra of (a) BB and (b) EVML before and after adsorption.
(a)(b)The band near 3428 cm-1 in the FTIR of BB shows the existence of –OH groups in BB (Figure 1(a)). The bands in the region 2922 cm-1 and 2852 cm-1 could be due to the asymmetrical and symmetrical stretching vibrations of the CH2 group with a saturated carbon [24]. The peak around 1640 cm−1 may either be ascribed to C=C stretching of alkene or N–H bending of amine [23]. The weak bands around 1383 cm−1 and 1418 cm−1 could be due to -CH2 bending. The band at 1040 cm−1 was assigned to be the C–O and COO– stretching, indicating the presence of carboxylic groups in BB Figure 1(a) [25]. From the FTIR spectrum of EVML (Figure 1(b)), the band at 3410 cm-1 is because of the O-H (hydroxyl) group stretching vibration, seemingly N-H medium peak overlapping near to 3480 cm-1. The band near 2920 cm-1 indicates the notable C-H stretching in the lignin functionality as explained in the literature [26]. The band around 1620 cm−1 is ascribed to the C=O (carbonyl) group stretching. The weak band near 1316 may be due to C-N bending in the amino group. The adsorption band observed around 1055 cm−1 matches the C–O stretching [27].


## 3.2. Screening of Biowaste Materials

The MB removal efficiencies of nine locally available biowaste materials were under similar conditions, and the obtained results are demonstrated in Figure2. The MB adsorption efficiency of BB, EAB, CR, and EVML was ≥95%. The PSS and TR showed similar MB removal efficiency (~90%) under the specified conditions. However, if any of these conditions (initial MB concentration, pH, etc.) change, the two adsorbents may not have similar MB removal potential.Figure 2
Adsorbents screening test for MB adsorption: initial concentration of MB 10 mg/L, adsorbent dose 2.5 g/L, pH 5.7, agitation time 24 h, shaking speed 200 rpm, and temperature25±1°C).Among the considered adsorbents, BB and EVML were selected because of their high MB removal efficiency. To the best of our knowledge, there was no previous report on these biowastes as adsorbents. The selected adsorbents (BB and EVML) were also evaluated for their anionic dyes methyl orange (MO) and Erichrome Black T (EBT) and cationic dyes malachite green (MG) and brilliant green (BG) removal efficiency at two different concentrations (10 mg/L and 20 mg/L) in their natural (nonadjusted) pH 5.62 and 5.68, 5.59 and 5.53, 4.82 and 4.55, and 4.99 and 4.26, respectively. Figure3 shows the obtained results.Figure 3
Adsorption performance of BB (a) and EVML (b) for different cationic and anionic dyes (conditions: initial concentration of the dyes 10 mg/L, adsorbent dose 2.5 g/L, agitation time 24 h, shaking speed 200 rpm, and temperature25±1°C).
(a)(b)Both adsorbents effectively removed the cationic dyes MB, BG, and MG compared to the anionic dyes MO and EBT. It may be due to the surface of the adsorbents being negatively charged under the above-specified pH since the pH of zero charge of adsorbents, BB, and EVML was found to be 5.9 and 3.5, respectively. The negatively charged surface of the adsorbents imparts electrostatic interactions to bind cationic dyes (MB, BG, and MG) while repelling the anionic dyes MO and EBT). The negative charge on the anionic dyes is due to the functional group SO3-. Note that the SO3H group having a pKa of -2.1 is expected to be dissociated to SO3- even at pH 2 [28]. Therefore, the adsorption properties of BB and EVML were further studied using MB as a model cationic dye.


## 3.3. Effect of Agitation Time

The influence of agitation time on the removal of MB was evaluated for the agitation time of 5 min to 24 h and 1 min to 2 h for BB and EVML adsorbents, respectively. The obtained results were presented in Figure4(a) and 4(b). As the contact time increases, the MB adsorption onto both adsorbents increases. Rapid adsorption at the beginning was observed until 20 min and 2 h for EVML and BB, respectively. Afterward, only a gradual increment was observed for both adsorbents because of attaining adsorption equilibrium. That is, the MB adsorption and desorption proceeded nearly at equal rates. However, to ensure maximum MB adsorption and avoid uncertainties in the equilibrium, 4 h was considered for BB, whereas 1 h for EVML for the subsequent adsorption analysis and other adsorption parameter optimizations.Figure 4
Effect of agitation time on MB removal (a) BB and (b) EVML (conditions: initial concentration (10 mg/L), adsorbent dose (2.5 g/L), shaking speed 200 rpm, and temperature25±1°C).
(a)(b)


## 3.4. Kinetics

Kinetics governing the adsorption procedure is among the critical factors that must be considered in designing an adsorption system. The time-dependent MB adsorption onto BB and EVML was estimated by fitting the laboratory obtained results into the nonlinear form of pseudofirst-order (PFO) and pseudosecond-order (PSO) models (Eqs. (3) and (4)). The obtained information from the fitting models is shown in Figures 5(a) and 5(b). The better-fitted model was identified by comparing their correlation coefficient (R2) and the nonlinear chi-square (χ2) values (Table 2). Table 2 indicates that the kinetics of obtained experimental data for both adsorbents can be described by using the PSO model rather than the PFO. Besides, the experimental qe values matched with the calculated qe values found in theory, proposing chemisorption as the main MB removal mechanism [27]. The initial MB removal rates (V0, mg/g min) were computed using the formula V0=k2qe2, and the MB adsorption rate of BB (512 mg/g min) was about 42 times faster than that of EVML (12 mg/g min). This can be due to the variation in the interaction of MB to the active functional group present in BB and EVML. The slope of the curves for both adsorbents is very sharp at the initial stage indicates fast adsorption process with high capacity, which is vital for real uses of the adsorbents to treat dye-contaminated water.Figure 5
The kinetic models represented by PFO and PSO for (a) BB and (b) EVML.
(a)(b)Table 2
PFO and PSO model parameters for MB adsorption onto BB and EVML.
ParameterBBEVMLPFOPSOPFOPSOCo (mg/L)10101010qe,exp (mg/g)3.7443.7443.6073.607qe,cal (mg/g)3.6863.7203.5733.616k1 (min-1)33.057—0.635—k2 (g/(mg.min))—36.995—0.926V0 (mg/(g.min))—511.95—12.11R20.569040.837290.566760.92953χ26.02×10−32.29×10−32.02×10−33.29×10−4The Weber and Morris model was used to identify the rate-determining step in the MB adsorption process [29] (Eq. (5)). The adsorption process of a certain adsorbate usually proceeds through four steps, and these are bulk diffusion, film diffusion, intraparticle or pore diffusion, and adsorption of adsorbate on the adsorbent surface [30]. However, film and/or intraparticle diffusion are usually determine the overall rate of the adsorption process [22, 27]. Figures 6(a) and 6(b) demonstrate the intraparticle diffusion phenomena for the adsorption of MB onto BB and EVML. It is clearly shown that the curves are linear and do not pass through the origin, demonstrating that intraparticle diffusion was not the only rate-determining step. That is, MB adsorption occurs through the engagement of more than one step of the aforementioned four adsorption steps. The obtained results match with previous research findings on adsorption of MB onto various biomasses such as biowaste Papaya stem [27] and chemically modified Bamboo [31].Figure 6
Intraparticle diffusion model plot for the (a) BB and (b) EVML.
(a)(b)


## 3.5. Effect of pH

Adsorption processes is significantly affected by pH, since it determine whether an adsorbate is in anionic, neutral, or cationic form and also the surface charge of adsorbents [7]. Increasing the initial pH of MB solution from 2 to 9 for the case of both adsorbents, MB removal efficiency increases from 46% to 97% for BB and from 60% to 97% for EVML (Figures 7(a) and 7(b)). The only exception is pH 3, where 87% and 91% of MB were removed by BB and EVML, respectively. The smaller MB adsorption below pH 3 may be due to the amino and carboxylic acid functional groups being largely protonated, thus resulting in greater electrostatic repulsion for the cationic MB dye [31]. The high MB adsorption onto the adsorbents was observed when the initial pH increases from 4 to 9. The observations were in accordance with earlier findings on MB removal using chemically modified bamboo [31], spent tea leaves [6], and chemically treated agricultural waste [7]. This is interesting in real applications because it avoids the possibility of pH adjustment using an acid and/or a base that may cause secondary pollution. Therefore, the natural pH of MB solution (pH=5.07) was considered as an ideal pH and used it in further optimization procedures. The high MB removal by both adsorbents in a wide pH range, above and below their pHpzc, suggests the involvement of several interactions like complexation, electrostatic force, and the dipole of the adsorbent surface-polar (cation of the dye) interaction [6].Figure 7
Effect of solution pH on the adsorption of MB onto (a) BB and (b) EVML (conditions: initial concentration 10 mg/L MB, adsorbent dose 2.5 g/L, shaking speed 200 rpm, and temperature25±1°C).
(a)(b)


## 3.6. Change in Initial MB Concentration

The impact of initial concentration in the adsorption process was assessed by varying the MB concentrations in the range of 10 mg/L to 165 mg/L for BB and 10 mg/L to 105 mg/L for EVML. The obtained results are illustrated in Figures8(a) and 8(b). As the initial concentration of MB changed from10 mg/L to 165 mg/L, the adsorption capacity of BB improved from 3.68 to 49.11 mg/g, whereas the adsorption capacity of EVML raised from 3.69 to 31.48 mg/g along with an increase in MB concentration from 10 mg/L to 105 mg/L. On the other hand, the percent of MB removal decreased for both adsorbents as the initial concentrations of MB increased. At a lower initial concentration of MB, a few of the adsorbent molecules occupy the adsorbent’s surface; therefore, the percent of adsorbate removal is high. However, an increase in the initial MB concentration resulted in coverage of the adsorbent surface by MB molecules, causing in a decline in MB adsorption. This is a common phenomenon in the adsorption process and revealed in results of previous studies on removal of MB by acid-treated banana peel [32], chemically activated Palm fibers [24], and Parthenium hysterophorus (an agricultural waste) [7].Figure 8
Initial MB concentration variation (a) BB and (b) EVML(conditions: adsorbent dose 2.5 g/L, shaking speed 200 rpm, and temperature25±1°C).
(a)(b)


## 3.7. Isotherm

The adoption capacity of an adsorbent usually predicted from isotherm study. Thus, nonlinear Langmuir and Freundlich isothermal equations (Eqs. (6) and (7)) were employed to the experimental MB adsorption data analysis at equilibrium conditions. The plots are shown in Figures 9(a) and 9(b). Table 3 provides the obtained isotherm parameters. The isotherm model that better describes the MB adsorption process was identified by comparing the obtained correlation coefficients (R2) and chi-square (χ2) values. The experimental data of MB adsorption onto both adsorbents suited to the Langmuir isotherm, indicating a monolayer adsorption of MB on the surface of the adsorbents [33]. The maximum MB adsorption capacity was 63.2 mg/g and 35.5 mg/g for BB and EVML, respectively. The separation factor (RL), calculated by the formula RL=1/1+bC0, is used to anticipate the feature of the adsorption procedure [17]. The adsorption process is irreversible if RL=0, favorable if 0<RL<1, linear if RL=1, and unfavorable if RL>1. In this case, the calculated RL values are between 0 and 1 (Table 3), demonstrating the favorable nature of the adsorption process.Figure 9
Plot of isotherm models: (a) BB and (b) EVML.
(a)(b)Table 3
Important parameters derived from Langmuir and Freundlich model fittings.
IsothermParametersBBEVMLLangmuirqmax (mg/g)63.2335.52b (L/mg)0.19780.2438RL0.00123−0.02020.00232-0.0252R20.989730.98029χ23.432271.79086FreundlichKF ((mg1-1/nL1/n)/g)11.9989.784N2.0182.677R20.93110.9579χ223.04443.82589MB is among the toxic cationic basic dyes that are widely used in different industries that requires effective treatment. The effectiveness of BB and EVML adsorbents was compared to some natural and modified adsorbents (Table4). In general, the maximum adsorption capacity of MB onto the BB and EVML adsorbents was comparable to that of several other natural adsorbents. However, compared with some activated adsorbents such as palm stem and H2SO4 activated banana peel, the adsorption capacity of BB and EVML adsorbents was lower. Besides the relatively good adsorption capacities of the biowaste materials, they have also advantages from the perspective of waste reuse to be economically and environmentally safe. The possibility to reuse the adsorbents is an additional asset for these adsorbents.Table 4
The maximum adsorption capacity of some adsorbents for MB adsorption.
AdsorbentspHQmax (mg/g)Equilibrium time (h)Dose (g/L)ReferenceWheat shell5-916.6110[34]Rice husk—40.60.674[35]Activated carbon from palm stem6110.80.31.25[24]Date palm leaf powder6.543.12.6710[36]Neem leaf powder5-88.8——[37]Rice husk5-721.9246[38]Rice husk ash5-712.8246[38]H2SO4 activated banana peel10250.0240.8[32]Sugarcane bagasse—9.4242.0[39]Tea waste8113.1244.0[40]BB5.0763.242.5Present studyEVML5.6635.512.5Present study


## 3.8. Adsorption Cycles

The reuse of adsorbents is very vital in producing cost-effective and safe adsorbents. 0.1 M NaOH and 0.1 M HCl solutions were employed in the desorption study. However, the obtained desorption efficiency was not satisfactory. Therefore, the reuse of the adsorbents was assessed without desorption through successive adsorption cycles. The reusability of BB and EVML is presented in Figure10. The MB removal efficiency of both BB and EVML was not significantly reduced until the 5th cycle. This indicated that the biowaste materials (BB and EVML) were efficient for MB adsorption from contaminated water. The reusability of biowaste materials and their natural accessibility at a very low price makes this method further attractive.Figure 10
Adsorption cycle of (a) BB and (b) EVML for MB removal.
(a)(b)


## 3.9. Feasibility Insights

### 3.9.1. Economic Feasibility

The cost of water treatment is the most important criterion for an adsorbent or treatment method employed. In adsorption technologies, adsorbent cost-effectiveness is the key constraint that must be considered during adsorbents selection. The cost of an adsorbent can be affected by several factors including its local availability, type of processes required, treatment conditions, recyclability or reusability options, and a lifetime of the adsorbent [41, 42]. The adsorption capacity has also an impact on the cost of the adsorbent material. The techniques to enhance the applicability of the adsorbent material that has a relation to the cost including regeneration, reusability; and high capacity of the adsorbents should be taken into account in cost-benefit analysis. Table 5 presents cost estimates for some reported adsorbents for MB removal. Most often, the cost estimation is rarely reported in literature briefing simply “low-cost” that might be obtained by considering the cost of the raw materials. However, all considerations of local availability, processing, transportation costs, recyclability, and a lifetime of adsorbent material are found vital [43]. The costs of BB and EVML are quite good in comparison with other adsorbents (Table 5). For instance, it was found to have only processing costs as both adsorbents are sourced from byproducts/wastes. Comparison of the costs of the adsorbents is not straightforward due to variations in adsorption capacity, regional processing fees, experimental level, and large-scale conditions. Furthermore, material costs, energy costs, capital costs, and other miscellaneous detail costs are required to be evaluated for large-scale applications. Hence, locally available and low-cost materials as is the case in this study predicted from lab-scale experiments have paramount importance as an alternative adsorbent. In general, the reusability of both adsorbents, BB and EVML, can work for more than five cycles in the adsorption process, local availability, and abundance. Enset and barley are widely cultivated in Ethiopia [44] indicating the promising availability of BB and EVML and their feasibility for the targeted application. Both BB and EVML adsorbents could have a potential to substitute commercially available activated carbon at a lower cost. Industrial-scale treatments of dye wastewater using bio-adsorbents were considered promising at large due to availability and cost benefits [43].Table 5
Comparative presentation of costs of adsorbents.
AdsorbentsCost USD/kgReferenceActivated carbon from nutshells2.15[45]Virgin carbon0.86-2.95[46]Reactivated carbon0.43-0.70[47]Alumina0.03[48]Sludge-based adsorbents0.1-0.2[41]Clay minerals0.04-0.12[49]Zeolite0.14-0.8[48, 50]Diatomite0.011-1.10[48]BB<0.012 (processing cost only)Current studyEVML<0.02 (processing cost only)Current study


### 3.9.2. Technical Feasibility and Environmental Safety

Most often, carbonization, physical activation, and chemical activation are commonly employed in adsorbent preparation processes that incur costs to a certain extent. The leaching from the adsorbents could be a limiting factor in water treatment applications; however, the adsorbents, BB, and EVML are part of edible crops that are expected to be less likely to leach toxic chemicals. This is because enset and barley are among the staple foods of Ethiopians, and the level of contaminants including heavy metal contents is well studied [44]. Processing and putting into batches and column set up of these adsorbents are easier and adaptable due to the traditional use of these materials in water storage clay pot preparations.


### 3.9.3. Accessibility

Accessibility of the adsorbent material in adsorption technology could contribute to cost savings. Hence, understanding the local availability and abundance of materials BB and EVML is vital. Enset is one of the staple foods of Ethiopia consumed by more than 20 million people due to its high starch content [51], from where EVML is produced. The dry matter content of lamina and midribs of leaves of enset is estimated to be 15–17% [52]. It is a multipurpose tree that has been found abundantly, especially in South Ethiopia. Enset grows in East and Southern Africa, and the genus extends across Asia to China; however, it is domesticated only in the Ethiopian [53]. And Ethiopia is the second-largest barley producer in Africa [54]. Barley bran is a byproduct of barley processes, and it is brittle and shatters of barley.


## 3.9.1. Economic Feasibility

The cost of water treatment is the most important criterion for an adsorbent or treatment method employed. In adsorption technologies, adsorbent cost-effectiveness is the key constraint that must be considered during adsorbents selection. The cost of an adsorbent can be affected by several factors including its local availability, type of processes required, treatment conditions, recyclability or reusability options, and a lifetime of the adsorbent [41, 42]. The adsorption capacity has also an impact on the cost of the adsorbent material. The techniques to enhance the applicability of the adsorbent material that has a relation to the cost including regeneration, reusability; and high capacity of the adsorbents should be taken into account in cost-benefit analysis. Table 5 presents cost estimates for some reported adsorbents for MB removal. Most often, the cost estimation is rarely reported in literature briefing simply “low-cost” that might be obtained by considering the cost of the raw materials. However, all considerations of local availability, processing, transportation costs, recyclability, and a lifetime of adsorbent material are found vital [43]. The costs of BB and EVML are quite good in comparison with other adsorbents (Table 5). For instance, it was found to have only processing costs as both adsorbents are sourced from byproducts/wastes. Comparison of the costs of the adsorbents is not straightforward due to variations in adsorption capacity, regional processing fees, experimental level, and large-scale conditions. Furthermore, material costs, energy costs, capital costs, and other miscellaneous detail costs are required to be evaluated for large-scale applications. Hence, locally available and low-cost materials as is the case in this study predicted from lab-scale experiments have paramount importance as an alternative adsorbent. In general, the reusability of both adsorbents, BB and EVML, can work for more than five cycles in the adsorption process, local availability, and abundance. Enset and barley are widely cultivated in Ethiopia [44] indicating the promising availability of BB and EVML and their feasibility for the targeted application. Both BB and EVML adsorbents could have a potential to substitute commercially available activated carbon at a lower cost. Industrial-scale treatments of dye wastewater using bio-adsorbents were considered promising at large due to availability and cost benefits [43].Table 5
Comparative presentation of costs of adsorbents.
AdsorbentsCost USD/kgReferenceActivated carbon from nutshells2.15[45]Virgin carbon0.86-2.95[46]Reactivated carbon0.43-0.70[47]Alumina0.03[48]Sludge-based adsorbents0.1-0.2[41]Clay minerals0.04-0.12[49]Zeolite0.14-0.8[48, 50]Diatomite0.011-1.10[48]BB<0.012 (processing cost only)Current studyEVML<0.02 (processing cost only)Current study


## 3.9.2. Technical Feasibility and Environmental Safety

Most often, carbonization, physical activation, and chemical activation are commonly employed in adsorbent preparation processes that incur costs to a certain extent. The leaching from the adsorbents could be a limiting factor in water treatment applications; however, the adsorbents, BB, and EVML are part of edible crops that are expected to be less likely to leach toxic chemicals. This is because enset and barley are among the staple foods of Ethiopians, and the level of contaminants including heavy metal contents is well studied [44]. Processing and putting into batches and column set up of these adsorbents are easier and adaptable due to the traditional use of these materials in water storage clay pot preparations.


## 3.9.3. Accessibility

Accessibility of the adsorbent material in adsorption technology could contribute to cost savings. Hence, understanding the local availability and abundance of materials BB and EVML is vital. Enset is one of the staple foods of Ethiopia consumed by more than 20 million people due to its high starch content [51], from where EVML is produced. The dry matter content of lamina and midribs of leaves of enset is estimated to be 15–17% [52]. It is a multipurpose tree that has been found abundantly, especially in South Ethiopia. Enset grows in East and Southern Africa, and the genus extends across Asia to China; however, it is domesticated only in the Ethiopian [53]. And Ethiopia is the second-largest barley producer in Africa [54]. Barley bran is a byproduct of barley processes, and it is brittle and shatters of barley.


## 4. Conclusion

In this paper, several biowaste materials were screened for MB removal from an aqueous solution, which is in line with the concept of waste reuse. Among them, BB and EVML were selected based on their performances. The obtained results show that the MB dye was adsorbed between pH 4 and 9. The maximum predicted adsorption capacities were found to be 63.2 mg/g for BB and 35.5 mg/g for EVML as obtained from the well-fitting Langmuir isotherm model. Results of the kinetic studies suggested that the adsorption phenomena can be explained by using PSO kinetics, suggesting chemisorption as the main adsorption mechanism. The biowaste materials (BB and EVML) can be reused in multiple adsorption cycles (without desorption), being MB removal efficiencies above 80% until the 5th cycle. Therefore, it can be concluded that the use of these biowaste materials has promising feasibility. Further studies should focus on the performance and scalability of using adsorbents on real wastewaters in a column study.



---

*Source: 4849758-2022-04-28.xml*