# Utilization of a Novel Low-Cost Gibto (Lupinus Albus) Seed Peel Waste for the Removal of Malachite Green Dye: Equilibrium, Kinetic, and Thermodynamic Studies

Authors: Adugna Nigatu Alene; Gietu Yirga Abate; Adere Tarekegne Habte; Desiew Mekuanint Getahun
Journal: Journal of Chemistry
 (2021)

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## Abstract

The aim of this study was to investigate the adsorption characteristics of malachite green (MG) dye onto the raw (RLAPW) and activated (ALAPW) surface ofLupinus albus seed peel waste prepared via physicochemical activation under alkaline condition as a dye adsorbent. Proximate analysis, surface area (Sears’ method), point of zero charge (pHzpc), and FTIR analysis were used to characterize the adsorbents. The effects of operational parameters such as pH (4) for ALAPW and pH (6) for RLAPW, adsorbent dose (0.2 g), initial dye concentration (30 mg/L), contact time (60 min), and temperature (298 K) were optimized. The experimental data well fitted with the Freundlich adsorption isotherm with the adsorption capacity of 7.3 mg/g for activated Lupinus albus seed peel waste (ALAPW) and Sips isotherm for raw Lupinus albus seed peel waste (RLAPW) with the adsorption capacity of 6.6 mg/g. The kinetics data well fitted to pseudo-second-order kinetic model for both adsorbents. Thermodynamic study revealed that the bioadsorption process using bioadsorbents was spontaneous and exothermic in nature. Desorption experiment was conducted and showed desorption efficiency at an acidic pH of 2. The results showed that the prepared adsorbents exhibited good adsorption capacity and can be used as an alternative adsorbent for the adsorptive removal of malachite green dyes.


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## Body

## 1. Background

The aquatic environment is significantly affected by the presence of various toxic chemicals such as dyes and pigments, metals, organics, and pharmaceuticals and is a major environmental concern due to industrialization and urbanization [1]. Colored effluents (mainly synthetic dyes) discharged from textiles, cosmetics, leather, pulp mills, printing, dye synthesis, food processing, hair dying, mineral processing, and plastic industries have become a major global problem [2, 3]. Dyes are classified as cationic (basic dyes), anionic (acid, direct and reactive dyes), and nonionic (disperse dyes) dyes on the basis of their dissociation in an aqueous solution [1]. Most of these synthetic dyes are toxic/hazardous and have genotoxic, mutagenic, and carcinogenic effects on the aquatic life and human health [4, 5]. Malachite green (MG) is a basic (cationic) dye containing triphenyl methane water soluble group, widely used for the dyeing of wool and silk, leather, paper, acrylic industries, and distilleries [6]. Malachite green (MG) is also used as an antimicrobial, antifungal, antiprotozoal, antiparasitic, and antiseptic agents in the aquacultural areas [7–9]. However, MG dye is environmentally persistent and highly toxic to aquatic and mammalian cells and also acts as carcinogenic, mutagenic, a liver tumor enhancing agent, and teratogenic effects on human health and biota [9–11]. Therefore, the removal of such synthetic dyes from effluents discharged from various industries is a great concern for the environmental viewpoint [12].Appropriate methods of dye removal from various effluents such as physical (precipitation, membrane filtration, electrochemical destruction, ion exchange, irradiation, ozonation, and adsorption), chemical (coagulation and flocculation), and biological (decolourisation-fermentation) effluents are used in the removal of colored effluents (dyes) from waste water [13]. Amongst those possible methods, adsorption has been superior to others in terms of economic feasibility, simplicity of design, ease of operation, high efficiency, and insensitivity to toxic pollutants [14, 15].According to different scholar findings, waste peels from agricultural areas are potential bioadsorbents for the removal of dyes due to their availability in nature, renewability, nontoxicity, easy conversion, and preparation with simple methods [16]. Some of the bioadsorbents, such as citrus limetta peel waste [17], yellow passion fruit peel [18], pomelo peel [19], garlic peel [15], lime peel [20], citrus peel [21], orange peel [22], banana peel [2], jackfruit peel [23], pomegranate peel [12], potato peel [23], rambutan peel [13], water chest nut peel [24], and green pea peel [25], are waste peels used to treat toxic pollutants. Agricultural wastes (adsorbents) aided the uptake of colored effluents (dyes) via various binding mechanisms (hydrogen bonding, electrostatic attraction, and π-π interaction) due to the presence of various surface functional groups such as hydroxyl, carboxyl, carbonyl, amine, amide, alcohol, and phenol [16].Gibto (Amharic name),Lupinus albus plant, is a member of the genus Lupinus in the family of Fabaceae mostly grown in northern Europe, Russia, Mediterranean countries, North America, Australia, and Africa (Kenya, South Africa, Tanzania, Zimbabwe, Mauritius, and Ethiopia) [26]. In Ethiopia, Gibto is a traditional crop mostly produced and consumed by small holder/scale farmers and mainly used for soil fertility maintenance values (green manure), food source, making local alcoholic drink “katikala” or “Gibto areke,” and pharmaceutical issues. Before consumption, the Lupinus albus seeds are first roasted and soaked with running water for 3–5 days until the bitter taste (alkaloid part) is removed to make edible seeds. The edible seeds were consumed by mostly low-income classes, users of local alcoholic drinks, and as a traditional medicine by removing skin/peel part of the seed. The peels are directly disposed as a waste, and it is absolutely noneconomical. To our knowledge, there is no study that deals with waste peels of Lupinus albus being used for adsorbents for the removal of malachite green dye (cationic dye).In this study, Gibto (Lupinus albus) seed peel waste (LAPW) was prepared by physicochemical activation and evaluated as a novel bioadsorbent for the removal of malachite green (MG) dye from aqueous solution. The adsorbents were characterized by proximate analysis, surface area (Sears’ method), point of zero charge (pHzpc), and FTIR studies. The influence of experimental parameters such as pH, adsorbent dose, contact time, temperature, and initial dye concentration was evaluated. The adsorption process (thermodynamics, kinetics, and isotherm models) and desorption study were performed.


## 2. Materials and Methods

### 2.1. Collection and Preparation of Materials

All the chemicals, malachite green dye (99%, Sigma Aldrich), NaCl (99.9%, SD Fine Chemicals Ltd., India), HCl (36–38%, Ranchem Industry and Trading, India), and NaOH (99.8%, Alpha Chemicals, India) are analytical grade and used without further purification. Distilled water was used for the entire experimental procedures.Lupinus albus seed peel waste adsorbents were prepared via physicochemical activation process (ALAPW) and without any activation (RLAPW) using the method reported in [26–30] with some modifications, and its process is presented in Figure 1. Gibto (Lupinus albus) seed peel wastes were collected in “Mesheta Bet” from Debre Markos town, Amhara regional state, Ethiopia, and washed with distilled water till the dirt species and soluble impurities were completely removed. The cleaned sample was allowed to dry at room temperature and oven-dried at 378 K until constant weight reached. The dried sample was crushed into small pieces, powdered, and sieved to ≤1 mm mesh size. The powder was physically activated in an oven at 573 K for 4 hours to develop porosity and enhance adsorption efficiency. Thereafter, the carbonized (physically activated) sample was soaked with 1N NaOH with an impregnated ratio of 1 : 5 w/v for 24 hours for chemical activation. The activated sample was filtered, washed with distilled water repeatedly until the pH of the solution reached to neutral, dried at 378 K for 24 hours, and kept in a desiccator for further analysis.Figure 1
Lupinus albus seed peel waste (a), powdered Lupinus albus seed peel waste, RLAPW (b), and physicochemically activated Lupinus albus seed peel waste, ALAPW (c).


### 2.2. Description and Preparation of Malachite Green (MG) Dye

Analytical grade malachite green (MG) dye (Scheme 1) was obtained from the chemistry laboratory, Woldia University, Ethiopia. Stock solution of the MG dye (500 mg/L) was prepared and further diluted to the experimental solutions of different concentrations ranging from 10–50 mg/L. The maximum wavelength (618 nm) was obtained after scanning of the dye using UV/VIS spectrometer (Lamda 35 Perkin Elmer), and a standard curve was developed through the measurement of the MG dye solution absorbance and the maximum wavelength (618 nm).Scheme 1
Overview of malachite green (MG) dye.


### 2.3. Characterization of Adsorbents

The proximate analysis (moisture content (MC), ash content (AC), volatile matter (VM), and fixed carbon (FC)) that determines the characteristics ofLupinus albus seed peel waste adsorbents (RLAPW and ALAPW) was performed according to [27, 31–34]. Specific surface area (SA) of the adsorbents was analyzed by Sears’ method [35]. In brief, 1.5 g of adsorbents and 30 g NaCl were added in a 250 ml conical flask and dissolved by 100 ml of distilled water. Then, the pH of solutions was adjusted to 4 using 0.1 M HCl, and the solutions were titrated by 0.1 M NaOH until pH of the solution reaches to 9. The volume of NaOH required to change pH value from 4 to 9 was recorded. The specific surface area of the adsorbents (RLAPW and ALAPW) was examined using the following formula:(1)specific surface aream2g=32⋅V−25,where V = volume of NaOH (0.1 M) required to raise the pH from 4–9.The surface charge analysis (point of zero charge (pHzpc)) was evaluated according to [24, 36, 37]. In brief, 0.2 g of adsorbents and 50 mL of 0.1 M NaCl solution were placed into different 250 mL Erlenmeyer flasks. The initial pH of the mixtures were adjusted between 2 and 12 by the addition of 1M HCl or 1M NaOH solution and then left to equilibrate for 24 h. The final pH of the solutions was measured and plotted the graph as pH final versus pH initial gave the pHzpc of the adsorbents. The surface functional groups of the adsorbents before and after adsorption were interpreted by Fourier transform infrared (FTIR) spectroscopy (Jasco-FT/IR-6600A), and the spectra in terms of percent transmittance were recorded in the range of 4000–400 cm−1.


### 2.4. Batch Adsorption Experiments

All adsorption experiments were carried out in batch mode by optimizing different variable parameters: pH (2–12), adsorbent dosage (0.0–0.4 g), initial dye concentration (10–50 mg/L), contact time (10–70 min), and temperature (298 K–323 K). The initial pH of all the solutions was adjusted with 1M NaOH or 1M HCl. The solid-liquid separation was performed by centrifugation (4000 rpm for 10 min) followed by filtration. At the end of each experiment, a small amount of the solutions (supernatants) was withdrawn at predetermined time, and the absorbance was determined by using UV-vis spectrophotometer at a maximum wavelength of 618 nm. The malachite green (MG) dye removal by the adsorbents (%) and the adsorption capacity at equilibrium,qe (mg/g), are shown in equations (2) and (3), respectively [38–40].(2)removal efficiency%=CO−CeCO×100,(3)qemgg=CO−CeVm,where Co = initial concentration of MG dye (mg/L); Ce = liquid-phase concentrations of the MG dye (mg/L) at equilibrium; V = volume of the MG dye (L); and m = adsorbent mass (g).


### 2.5. Desorption Experiments

Desorption experiments are applicable to elucidate the nature of adsorption process and examine the possibility to recover adsorbate and to regenerate/recycle the adsorbent [41]. Desorption of MG dye was investigated on the adsorbents with preadsorbed dye at optimum conditions (pH 4 (ALAPW) and pH 6 (RLAPW), adsorbent dose 0.2 g, MG dye concentration 30 mgL−1/50 mL, contact time 60 min, and temperature 298 K), and the mixture was shaken with a magnetic stirrer on digital hot plate at 200 rpm. The preadsorbed MG dye and adsorbent was isolated from the mixture by centrifugation at 4000 rpm for 5 min and then placed into 25 mL Me-OH (99%), NaOH (0.1 M), and HCl (0.1 M), at pH 2. The suspensions were shaken on a rotary shaker at 150 rpm for 24 hrs, and the supernatant solutions were analyzed using the UV-vis spectrophotometer. The amount of desorbed dye can be calculated by(4)desorption%=C∗Vqm∗100,where C is MG dye concentration in the desorption solution (mg/L), V is the volume of the desorption solution (L), q is the amount of MG dye adsorbed on the adsorbents before desorption experiment (mg/g), and m is the amount of the adsorbent used in the desorption experiment (g).


## 2.1. Collection and Preparation of Materials

All the chemicals, malachite green dye (99%, Sigma Aldrich), NaCl (99.9%, SD Fine Chemicals Ltd., India), HCl (36–38%, Ranchem Industry and Trading, India), and NaOH (99.8%, Alpha Chemicals, India) are analytical grade and used without further purification. Distilled water was used for the entire experimental procedures.Lupinus albus seed peel waste adsorbents were prepared via physicochemical activation process (ALAPW) and without any activation (RLAPW) using the method reported in [26–30] with some modifications, and its process is presented in Figure 1. Gibto (Lupinus albus) seed peel wastes were collected in “Mesheta Bet” from Debre Markos town, Amhara regional state, Ethiopia, and washed with distilled water till the dirt species and soluble impurities were completely removed. The cleaned sample was allowed to dry at room temperature and oven-dried at 378 K until constant weight reached. The dried sample was crushed into small pieces, powdered, and sieved to ≤1 mm mesh size. The powder was physically activated in an oven at 573 K for 4 hours to develop porosity and enhance adsorption efficiency. Thereafter, the carbonized (physically activated) sample was soaked with 1N NaOH with an impregnated ratio of 1 : 5 w/v for 24 hours for chemical activation. The activated sample was filtered, washed with distilled water repeatedly until the pH of the solution reached to neutral, dried at 378 K for 24 hours, and kept in a desiccator for further analysis.Figure 1
Lupinus albus seed peel waste (a), powdered Lupinus albus seed peel waste, RLAPW (b), and physicochemically activated Lupinus albus seed peel waste, ALAPW (c).


## 2.2. Description and Preparation of Malachite Green (MG) Dye

Analytical grade malachite green (MG) dye (Scheme 1) was obtained from the chemistry laboratory, Woldia University, Ethiopia. Stock solution of the MG dye (500 mg/L) was prepared and further diluted to the experimental solutions of different concentrations ranging from 10–50 mg/L. The maximum wavelength (618 nm) was obtained after scanning of the dye using UV/VIS spectrometer (Lamda 35 Perkin Elmer), and a standard curve was developed through the measurement of the MG dye solution absorbance and the maximum wavelength (618 nm).Scheme 1
Overview of malachite green (MG) dye.


## 2.3. Characterization of Adsorbents

The proximate analysis (moisture content (MC), ash content (AC), volatile matter (VM), and fixed carbon (FC)) that determines the characteristics ofLupinus albus seed peel waste adsorbents (RLAPW and ALAPW) was performed according to [27, 31–34]. Specific surface area (SA) of the adsorbents was analyzed by Sears’ method [35]. In brief, 1.5 g of adsorbents and 30 g NaCl were added in a 250 ml conical flask and dissolved by 100 ml of distilled water. Then, the pH of solutions was adjusted to 4 using 0.1 M HCl, and the solutions were titrated by 0.1 M NaOH until pH of the solution reaches to 9. The volume of NaOH required to change pH value from 4 to 9 was recorded. The specific surface area of the adsorbents (RLAPW and ALAPW) was examined using the following formula:(1)specific surface aream2g=32⋅V−25,where V = volume of NaOH (0.1 M) required to raise the pH from 4–9.The surface charge analysis (point of zero charge (pHzpc)) was evaluated according to [24, 36, 37]. In brief, 0.2 g of adsorbents and 50 mL of 0.1 M NaCl solution were placed into different 250 mL Erlenmeyer flasks. The initial pH of the mixtures were adjusted between 2 and 12 by the addition of 1M HCl or 1M NaOH solution and then left to equilibrate for 24 h. The final pH of the solutions was measured and plotted the graph as pH final versus pH initial gave the pHzpc of the adsorbents. The surface functional groups of the adsorbents before and after adsorption were interpreted by Fourier transform infrared (FTIR) spectroscopy (Jasco-FT/IR-6600A), and the spectra in terms of percent transmittance were recorded in the range of 4000–400 cm−1.


## 2.4. Batch Adsorption Experiments

All adsorption experiments were carried out in batch mode by optimizing different variable parameters: pH (2–12), adsorbent dosage (0.0–0.4 g), initial dye concentration (10–50 mg/L), contact time (10–70 min), and temperature (298 K–323 K). The initial pH of all the solutions was adjusted with 1M NaOH or 1M HCl. The solid-liquid separation was performed by centrifugation (4000 rpm for 10 min) followed by filtration. At the end of each experiment, a small amount of the solutions (supernatants) was withdrawn at predetermined time, and the absorbance was determined by using UV-vis spectrophotometer at a maximum wavelength of 618 nm. The malachite green (MG) dye removal by the adsorbents (%) and the adsorption capacity at equilibrium,qe (mg/g), are shown in equations (2) and (3), respectively [38–40].(2)removal efficiency%=CO−CeCO×100,(3)qemgg=CO−CeVm,where Co = initial concentration of MG dye (mg/L); Ce = liquid-phase concentrations of the MG dye (mg/L) at equilibrium; V = volume of the MG dye (L); and m = adsorbent mass (g).


## 2.5. Desorption Experiments

Desorption experiments are applicable to elucidate the nature of adsorption process and examine the possibility to recover adsorbate and to regenerate/recycle the adsorbent [41]. Desorption of MG dye was investigated on the adsorbents with preadsorbed dye at optimum conditions (pH 4 (ALAPW) and pH 6 (RLAPW), adsorbent dose 0.2 g, MG dye concentration 30 mgL−1/50 mL, contact time 60 min, and temperature 298 K), and the mixture was shaken with a magnetic stirrer on digital hot plate at 200 rpm. The preadsorbed MG dye and adsorbent was isolated from the mixture by centrifugation at 4000 rpm for 5 min and then placed into 25 mL Me-OH (99%), NaOH (0.1 M), and HCl (0.1 M), at pH 2. The suspensions were shaken on a rotary shaker at 150 rpm for 24 hrs, and the supernatant solutions were analyzed using the UV-vis spectrophotometer. The amount of desorbed dye can be calculated by(4)desorption%=C∗Vqm∗100,where C is MG dye concentration in the desorption solution (mg/L), V is the volume of the desorption solution (L), q is the amount of MG dye adsorbed on the adsorbents before desorption experiment (mg/g), and m is the amount of the adsorbent used in the desorption experiment (g).


## 3. Result and Discussion

### 3.1. Characterization ofLupinus albus Seed Peel Waste Adsorbents (RLAPW and ALAPW)

Physicochemical characteristics of the adsorbents such as proximate analysis, point of zero charge, and specific surface area are presented in Table1. The moisture content (MC), ash content (AC), and volatile matter (VM) of ALAPW were found to be lower than RLAPW, whereas the fixed carbon (FC) content is higher than the RLAPW. This indicates that most of the moistures and heat-sensitive molecules present in the sample were removed up on physiochemical activation. The volatile matter of ALAPW was found to be low due to the organic components present in adsorbents which become less stable and the release of volatile matter as gas and liquid products which evaporates off leaving the material during the physicochemical activation process [27]. The ash content of ALAPW was also low, due to the removal of significant amount of mineral components, certain oxides, carbonates, and sulfides in the adsorbent during the physicochemical activation process [31]. The fixed carbon content of the ALAPW was high, which indicates the adsorbent material has good quality which enhances the surface area as well as the adsorption performance [33]. The specific surface area of the prepared adsorbent was 1703 ± 0.56 m2/g and 1170 ± 0.40 m2/g for ALAPW and RLAPW, respectively. The specific surface area of ALAPW was so high, indicating the adsorbent has good efficiency to adsorb a dye. Similar result was observed for methylene blue dye removal using activated carbon [42].Table 1
Proximate analyses of the adsorbents.
SampleCharacteristicsMoisture content (%)Ash content (%)Volatile matter (%)Fixed carbon (%)Surface area (m2/g)pHzpcRLAPW5.24 ± 0.118.23 ± 0.0512.12 ± 0.2774.41 ± 0.131170 ± 0.404.3 ± 0.04ALAPW0.22 ± 0.023.25 ± 0.011.16 ± 0.0095.4 ± 0.281703 ± 0.563.2 ± 0.02Point of zero charge (pHzpc) is explained as the situation in which the density of electric charge on the surface of the adsorbent becomes zero. The graph of pHzpc was plotted as “pH final versus pH initial,” and the pHzpc values of the adsorbents was obtained at the intersection point of the curves of “pH final versus pH initial.” The pHzpc plot of the adsorbents (ALAPW and RLAPW) is presented in Figure2. The pHzpc was found to be 3.2 and 4.3 for ALAPW and RLAPW biosorbents, respectively. The low pHzpc for ALAPW may be due to the effects of physiochemical activation process. When pH < pHzpc, the adsorbents surface will become positively charged, and when the solution pH > pHzpc, the adsorbents surface will become negatively charged [43]. Below pHzpc, the surface of the adsorbents (ALAPW and RLAPW) becomes positively charged, and they compete with a cationic MG dye for vacant adsorption sites causing a decrease in dye uptake due to electrostatic repulsion. Above pHzpc, the adsorbent surface is negatively charged and favors uptake of cationic MG dye due to increased electrostatic force of attraction [44].Figure 2
The pHzpc plot of the adsorbents (ALAPW and RLAPW).


### 3.2. FTIR Analysis

Fourier transform infrared (FT-IR) spectral analysis was conducted to determine the functional groups that exist on the surface of the materials. The FTIR spectra of RLAPW, ALAPW, RLAPW-loaded MG, and ALAPW loaded MG dye were recorded in the range of 4000–400 cm−1 and shown in Figure 3. In both spectra (before and after adsorption), the broad and intense adsorption peaks in the range of 3450–3350 cm−1 were obtained due to the hydroxyl (–O-H) or amine (–N-H) functional groups [2, 3, 45], the absorption peaks at 2950–2850 cm−1 can be reflected to the –C-H group of alkane, stretching vibrations at 2400–2050 cm−1 assigned to C≡C and C≡N−H bonds [20], the peaks at 1620–1610 cm−1 were attributed to stretching vibrations of carbonyl/carboxyl (C=O) groups [1], and intense peaks at 1460–1440 cm−1 are supposed to the presence of (C-H) vibration of aliphatic and aromatic groups and absorption bands in the range 1420–1000 cm−1 can be assigned to the C-O and C-N stretching vibration of carboxylic acids (-COOH) and/or alcohols and amine groups [1–3].Figure 3
FT-IR spectra of LAPW adsorbents before and after adsorption of MG dye.
(a)(b)The surface/characteristic peaks of RLAPW and ALAPW were found to be different (Figure3) as some of the functional groups disappeared and the intensity of the peaks altered due to the physicochemical activation process, which shows that these functional groups were chemically protonated/deprotonated and thermally unstable [20]. The spectra of the RLAPW, ALAPW, RLAPW-loaded MG, and ALAPW-loaded MG dye showed similar characteristics of adsorption regions except for slight differences/changes. The FTIR spectra before and after adsorption indicate that the peaks are slightly shifted from their positions and the intensity gets changed. These results indicated the binding of some functional groups (hydroxyl, carbonyl, amine, and carboxyl) in the adsorption of malachite green (MG) dye on the adsorbents surface through weak electrostatic interaction or Van der Waals forces [1, 45]. The FTIR spectral analysis revealed that both of the adsorbents (RLAPW and ALAPW) contain several functional groups such as hydroxyl, carbonyl, carboxyl, and amine groups, and these groups act as potential active sites for interaction with the malachite green (MG) dye. Those functional groups (potential active sites) were found in the adsorbents having high affinity towards pollutants (organic and inorganic), and the MG dye removal was carried via hydrogen bonding, electrostatic, and π-π interactions [46].


### 3.3. Batch Adsorption Study

#### 3.3.1. Effect of Solution pH

The effect of solution pH plays an important controlling parameter in the adsorption processes [1, 47]. The effect of pH on the adsorption of MG dye by the ALAPW and RLAPW at pH between 2 and 12 is shown in Figure 4. The MG dye removal efficiency (Figure 4) was increased at the pH range of 2–4 for ALAPW and 2–6 for RLAPW at a given concentration. The adsorbent surface was positively charged at pH under 3.2 (ALAPW) and 4.3 (RLAPW) and showed negative charge over the pHzpc. The MG is a positively charged (cationic) dye and provides positive ions in the solutions. Thus, below pHzpc, the amounts of adsorption were lesser owing to electrostatic repulsion between the MG dye ions and the positively charged surface of the adsorbents.Figure 4
pH effect on the adsorption of MG dye by ALAPW and RLAPW at adsorbent dose 0.2 g. MG dye concentration 10 mg/L, contact time 60 min, agitation speed 200 rpm, and room temperature (298 K).The FT-IR analysis (Figure3) showed that the ALAPW surface contained excess acidic functional groups due to physicochemical activation [30]. The FT-IR analysis also showed that the RLAPW surface contained acidic functional groups (C=O at 1617 cm−1, O-H at 3407 cm−1, and C-O at 1028 cm−1). The results revealed that the adsorption process may be carried out via a dominant acidic active surface of the adsorbents and the cationic MG dye. The high removal efficiency at lower pH (4) for ALAPW and pH (6) for RLAPW is probably due to electrostatic attraction, hydrogen bonding, or π-π interaction between adsorbent and adsorbate (dye) [2]. In addition, the mechanism also responsible for the adsorption of MG may be more related to textural properties due to the presence of high surface area [14]. As a result, maximum MG dye removal efficiency was carried at acidic regions (pH <7). Similar behaviors were observed for malachite green dye adsorption on activated sintering process red mud [10, 48] and methylene blue cationic dye on jackfruit peel [49].


#### 3.3.2. Effect of Adsorbent Dose

In the adsorption process, adsorbent dose is a very important parameter due to the dosage effect on the adsorbent and adsorbate [25]. The effect of adsorbent dose on the removal of MG dye by ALAPW and RLAPW is presented in Figure 5. When the adsorbents dose increased from 0.05 to 0.2 g, an increase in the MG dye removal from 76.3 to 90.14% for ALAPW and 55.63 to 78.7% for RLAPW was observed. Such increase of MG dye removal with adsorbent dose is due to the presence of high surface area and availability of several adsorption sites in the adsorbents [16]. Beyond the optimum adsorbent dose (0.2 g), the MG dye removal was not significantly changed or affected due to conglomeration/aggregation of adsorbent particles which limits the active surfaces for adsorption [50].Figure 5
Effect of adsorbent dose on MG dye removal by ALAPW and RLAPW at pH 4 (ALAPW) and pH 6 (RLAPW), initial MG dye concentration 10 mg/L, contact time 60 min, and temperature (298 K) with agitation speed 200 rpm.


#### 3.3.3. Effect of Initial Dye Concentration

The effect of initial MG dye concentration on the adsorption of MG onto ALAPW and RLAPW was carried out in the concentration range of 10–50 mg/L as shown in Figure6. Percent removal efficiency increased with an increase in MG dye concentration from 10 to 30 mg/L for both adsorbents. The percent removal of MG dye decreases beyond 30 mg/L of dye concentrations. When initial dye concentration increases (beyond 30 mg/L), the active sites presented in the adsorbents required for bioadsorption of the dye molecules may be occupied and further adsorption is hindered or prevented by repulsion force of dye or steric hindrance on the adsorbent phase and on the bulk phase [50, 51].Figure 6
Effect of initial MG dye concentration on MG dye removal by ALAPW and RLAPW at pH 4 (ALAPW) and pH 6 (RLAPW), adsorbent dose 0.2 g, contact time 60 min, and temperature (298 K) with agitation speed 200 rpm.


#### 3.3.4. Effect of Contact Time

The effect of contact time on the adsorption processes was examined in the range of 10–70 min at optimum values of adsorbent dose 0.2 g, pH 4 (ALAPW) and pH 6 (RLAPW), initial MG dye concentration 30 and 40 mg/L, and temperature 298 K with an agitation speed of 200 rpm. As shown in Figure7, MG dye removal efficiency increases with an increase in contact time up to 60 min and then nearly constant. The results revealed that the adsorption of MG dye onto ALAPW and RLAPW was increased up to 60 min due to the availability of free/vacant surface sites of functional groups [52], and nearly constant beyond 60 min due to the saturation of the available free adsorbing sites or the remaining vacant surface sites are hard to be adsorb due to repulsive forces between the dye molecules on the adsorbents and the bulk phase [50]. Hence, 60 min was the equilibrium time obtained for MG dye adsorption in this study.Figure 7
Contact time effect on MG dye removal by ALAPW and RLAPW at adsorbent dose 0.2 g, pH 4 (ALAPW) and pH 6 (RLAPW), initial MG dye concentration 30 and 40 mg/L, and temperature (298 K) with agitation speed 200 rpm.


#### 3.3.5. Effect of Temperature

The adsorption process is temperature-dependent [53]. The temperature effect was explained using 298 K, 303 K, 313 K, and 323 K at MG dye concentration 30 mg/L, adsorbent dose 0.2 g, pH 4 (ALAPW) and 6 (RLAPW), and contact time 60 min with agitation speed 200 rpm. Figure 8 shows the removal of MG dye by ALAPW and RLAPW was decreased with increasing the temperature from 298 to 323 K. The results clearly suggested that the adsorption of malachite green (MG) dye on to the adsorbents (ALAPW and RLAPW) was favorable at low temperature and exothermic in nature and such effects were explained [44].Figure 8
Temperature effect on MG dye removal by ALAPW and RLAPW at adsorbent dose 0.2 g, pH 4 (ALAPW) and pH 6 (RLAPW), initial MG dye concentration 30 mg/L, and contact time 60 min with agitation speed 200 rpm.


### 3.4. Thermodynamic Study

Thermodynamic study shows the favorability and feasibility of the adsorption process [52]. The values of thermodynamic parameters evaluated for MG adsorption onto the adsorbents (RLAPW and RLAPW) such as change in free energy (ΔG°, kJ mol−1), change in enthalpy (ΔH°, kJ mol−1), and change in entropy (ΔS°, kJ·mol−1·K−1) were determined by the change of equilibrium temperature using the following equations [54]:(5)ΔG°=−RTLnKc=−2.303RTLogKc,(6)KC=qeCe,(7)ΔG°=ΔH°−TΔS°,(8)LnKc=−ΔH°RT+ΔS°R,where R (R = 8.314 J/molK) is the universal gas constant, T is the absolute temperature (K), qe (mg/g) is the amount of MG dye adsorbed on the RLAPW and ALAPW of the solution at equilibrium, Ce (mg/L) is the equilibrium concentration of the dye in the solution, and Kc (qe/Ce) is the thermodynamic equilibrium constant. The values of ΔH° and ΔS° were determined from the slope (−ΔH°/R) and intercept (ΔS°/R) of the plot of Ln (Kc) versus 1/T (Figure 9). As shown in Table 2, the negative ΔG° values indicated spontaneous feasible adsorption process in nature; negative ΔH° values suggested the exothermic nature of the adsorption; and the negative ΔS° values suggest the decrease in adsorbate concentration.Figure 9
Plots of lnKc vs. 1/T for the estimation of thermodynamic parameters.
(a)(b)Table 2
The values of thermodynamic parameters.
Type of adsorbentTemperature (K)Thermodynamic parameterslnKcΔG° (kJ/mol)ΔH° (kJ/molK)ΔS° (J/mol K)ErrorsALAPW2988.083−1.395−4.697−11.0830.140713037.815−1.3390.140713136.694−1.2280.140713236.000−1.1180.14071RLAPW2981.830−5.200−8.879−12.3460.03023031.67−5.1390.025653131.59−5.0160.02323231.54−4.8920.0216


### 3.5. Kinetic Study

Adsorption kinetics for the adsorbents of ALAPW and RLAPW were conducted at pH 4 (ALAPW) and pH 6 (RLAPW), adsorbent dose 0.2 g, initial MG dye concentration 30 mg/L and 40 mg/L, contact time 10–70 minute, and temperature 298 K. The adsorption kinetic parameters are useful for defining the adsorption rate and give important information on the mechanism of the sorption process [16, 55], and the adsorption dynamics was studied by the kinetics in terms of the order of the rate constant [56]. In order to investigate the mechanism of dye adsorption onto RLAPW and ALAPW, adsorption kinetics was studied using the pseudo-first-order (equation (9)), pseudo-second-order (equation (10)), and intraparticle diffusion model (equation (11)), respectively [46, 52].(9)logqe−qt=logqe−k1t,(10)tqt=1k2qe2+tqe,(11)qt=kdifft1/2+C,where qt is amount adsorbed at time t (mg/g), qe is amount adsorbed at equilibrium time (mg/g), k1 is the pseudo-first-order rate constant (min−1), k2 is the pseudo-second-order rate constant (g/mg·min), kdiff is the intraparticle diffusion rate constant (mg/g·min1/2), and C is the intercept. The kinetic parameters (Table 3) for each model were calculated by plotting graph log (qe − qt) vs. t for pseudo-first-order, t/qt vs. t for pseudo-second-order (Figure 10), and qt vs. t1/2 for intraparticle diffusion models (Figure 11).Table 3
Kinetic parameters for MG adsorption onto ALAPW and RLAPW adsorbents.
KineticsParametersALAPWRLAPWConcentration (mg/L)30403040qe exp.(mg/g)7.279.386.558.30Pseudo-first orderR20.980540.813560.993340.97849k1 (min−1)−0.0145−0.0259−0.00906−0.0178qe cal.(mg/g)0.41420.8451.9753.296Pseudo-second orderR20.999940.999010.99910.99762qe cal. (mg/g)7.3968.4756.1638.948k2 (g/mg·min)0.000330.034630.051930.02365Intraparticle diffusion modelR20.988720.939440.955840.97933kdiff (mg/g·min1/2)0.057910.514190.299790.38931C (mg/g)6.795764.815713.913445.11084Figure 10
Pseudo-first-order (a) for ALAPW and (b) for RLAPW and pseudo-second-order (c) for ALAPW and (d) for RLAPW for the adsorption of MG dye at different contact time intervals.Figure 11
Intraparticle diffusion model (a) for ALAPW and (b) for RLAPW at different contact times (10–70 min).It was observed that, for the pseudo-first-order and intraparticle diffusion model of MG dye adsorption,R2 was relatively low and the calculated qe (qe calc.) value was dramatically lower than the experimental qe (qe exp.) value. However, the pseudo-second-order kinetic model provided a near perfect match between the calculated (qe calc.) and experimental (qe exp.) values, and its R2 value is close to unity for both adsorbents. As a result, the adsorption of MG dye on both adsorbents fitted well into the pseudo-second-order kinetic model.


### 3.6. Adsorption Isotherms

Equilibrium adsorption isotherms were studied with MG dye concentrations (10–50 mg/L) with a fixed adsorbent mass (0.2 g), pH 4 (ALAPW) and 6 (RLAPW), contact time 60 min, and temperature 298 K. In this study, the adsorption of MG dye onto ALAPW and RLAPW was analyzed by various isotherm models such as Langmuir, Freundlich, Redlich–Peterson, and Sips isotherms. The Langmuir model assumes the formation of a monolayer of adsorbate on the outer surface of the adsorbent (uniform energies of adsorption) and no further adsorption thereafter (applicable to homogeneous sorption or all sorption sites are identical) and expressed by equation (12) [46].(12)Ceqe=1bqm+Ceqm,where Ce (mg/L) is the equilibrium solute concentration of dye in solution, qe (mg/g) is the adsorbed value of dye at equilibrium concentration, qm (mg/g) is the maximum monolayer adsorption capacity, and b is the Langmuir adsorption constant (L/mg). Slope (1/qm) and intercept (1/bqm) of the straight line plot of Ce/qe versus Ce is shown in Figure 12. The values of Langmuir isotherm constants qm and b are presented in Table 4.Figure 12
Langmuir isotherm (a) for ALAPW and (b) for RLAPW and Freundlich isotherm (c) for ALAPW and (d) RLAPW.Table 4
Isotherm parameters for the adsorption of MG dye onto ALAPW and RLAPW.
Isotherm modelsParametersALAPWRLAPWLangmuirR20.127550.455qm (mg/g)74.629.0b (L/mg)0.0620.042FreundlichR20.754560.87413Kf (mg/g)4.811.25n1.211.14SipsQmax.(mg/g)36.1419.43Ks (L/mg)1.40.05n11R20.590140.9022Redlich–Petersonβ0.217960.13014A (mg/L)38.051.68R20.283810.26368The type of the Langmuir isotherm could be predicted based on whether the adsorption was favorable or unfavorable in terms of equilibrium parameter or dimensionless constant separation factorRL [45], which is presented as follows:(13)RL=11+bCo,where RL is the Langmuir constant and Co is the initial concentration of adsorbate. The values of RL indicates whether the isotherm is unfavorable (RL > 1), linear (RL = 1), favorable (RL < 1), or irreversible (RL = 0). As shown in Table 5, the values of RL (between 0 and 1) indicate that the isotherm was favorable.Table 5
RL (dimensionless constant separation factor) values at different initial concentration.
AdsorbentRL valuesConcentration (mg/L)1020304050ALAPW0.2 g0.620.450.350.290.24RLAPW0.2 g0.700.540.440.370.32Freundlich isotherm is used to describe the adsorption surface becoming heterogeneous (nonuniform) during the adsorption process [45] and expressed in(14)logqe=logkf+logCen,where Ce is the equilibrium concentration of dye in solution (mg/L), qe is the adsorbed value of dye at equilibrium concentration (mg/g), and Kf and n are Freundlich constants characteristics of the system, indicating the adsorption capacity and the adsorption intensity. The Freundlich coefficients, Kf (related to adsorption capacity) and n (related to adsorption intensity), obtained from the slope (1/n) and the intercept (log Kf) of the linearized plots of log qe versus log Ce (Figure 12) are shown in Table 4. The value n > 1 suggests that adsorbate is favorably adsorbed on the adsorbent. The higher the n value is, the stronger the adsorption intensity is [13].The Sips isotherm model is a combination of both Langmuir and Freundlich isotherm, and the model is valid for localized adsorption without adsorbate-adsorbate interactions [57, 58]. This model is also used for predicting the heterogeneous adsorption systems and circumventing the limitation of the rising adsorbate concentration associated with the Freundlich isotherm model. At the low adsorbate concentrations (at low Ce), the Sips isotherm model effectively reduces to the Freundlich isotherm, and at high adsorbate concentrations (at high Ce), this model predicts a monolayer sorption capacity characteristic of the Langmuir isotherm. The Sips isotherm model is expressed in(15)1qe=1QmaxKs1Ce1/n+1Qmax,where Ks (L/mg) is the Sips equilibrium constant and Qmax (mg/g) is maximum adsorption capacity values obtained from the slope and the intercept of the plot as shown in Figure 13. The Sips isotherm equation is characterized by the dimensionless heterogeneity factor, n, which can also be employed to describe the system’s heterogeneity when n is between 0 and 1. When n = 1, the Sips equation reduces to the Langmuir equation, and it implies a homogeneous adsorption process.Figure 13
Sips isotherm (a) for ALAPW and (b) for RLAPW, Redlich–Peterson isotherm (c) for ALAPW and (d) RLAPW.Redlich–Peterson isotherm is a hybrid isotherm featuring both Langmuir and Freundlich isotherms, which incorporate three parameters into an empirical equation which may be used to represent adsorption equilibria over a wide concentration range and can be applied either in homogeneous or heterogeneous systems due to its versatility [57]. The linear form of the Redlich–Peterson isotherm is represented by(16)lnCeqe=βlnCe−lnA,where ln A is the Redlich–Peterson isotherm constant obtained from the intercept of ln (Ce/qe) versus ln Ce graphs and β is the exponent between 0 and 1. The experimental data analyzed by Sips and Redlich–Peterson isotherms are shown in Figure 13.Isotherm parameters and correlation factor (R2) for both models are presented in Table 4. The results revealed that the Freundlich isotherm correlation factor for ALAPW relatively close to unity confirms Freundlich isotherm model which better describes the interaction between adsorbent and adsorbate in the aqueous system. For RLAPW, the Sips isotherm correlation factor close to unity better describes the interaction between adsorbent and adsorbate in the aqueous system.Dimensionless constant separation factor (RL) value indicates the adsorption nature to be either favorable if 0 < RL < 1, unfavorable if RL > 1, linear if RL = 1, and irreversible if RL = 0 [59]. For this study, all the RL valves were obtained between 0 and 1 which confirming that the adsorption of MG dye over the ALAPW and RLAPW was favorable [59]. In the present study, the calculated RL values for the adsorption of MG on ALAPW and RLAPW adsorbents are presented in Table 5 at initial concentrations of 10, 20, 30, 40, and 50 mg/L. These RL values confirmed that adsorbents (ALAPW and RLAPW) are favorable for adsorbing MG dye from aqueous solution under the conditions applied in this study.


### 3.7. Desorption Study

High desorption efficiency and good reusability after adsorption are desirable for an adsorbents. Desorption studies are helpful to explain the nature of adsorption and recycling of the spent adsorbent and the dye [51, 60–63]. The regeneration of ALAPW and RLAPW was conducted by immersing the adsorbents into different solvents Me-OH (99%), NaOH (0.1 M), and HCl (0.1 M), and the results are presented in Figure 14. As shown in Figure 14, the regeneration of adsorbents by 0.1 M NaOH was more efficient (compared to the other solvents). These phenomena are consistent with the results observed for the effect of pH. Since the maximum removal efficiency of MG is attained at weakly acidic conditions, it is expectable that desorption is favored at high pH values. At pH 2, a significantly high electrostatic repulsion exists between the positively charged surfaces of both adsorbents and the cationic MG dye so that regeneration is carried out as a result of charge competition. As described in the figure, ALAPW has high regeneration ability as compared to RLAPW. This is because the high fixed carbon of the activated carbon gives a better strength and porosity nature. Therefore, ALAPW shows excellent adsorption performance and regeneration, and its use can be extended to environmental applications for wastewater treatment.Figure 14
Desorption efficiency (%) using different solvent solutions.


### 3.8. Proposed Adsorption Mechanism

The bioadsorption of MG dye from aqueous solutions by RLAPW and ALAPW is strongly dependent on the various polar functional groups on the surface of the adsorbents such as hydroxyl (cellulose, pectin, hemicellulose, adsorbed water, and lignin), phenols, amines, and aromatics, which are supported by FTIR spectral results described in Figure3. The surface functional groups of the adsorbents may be charged, neutral upon protonation/deprotonation, and multiple bonded upon delocalization. The possible adsorption mechanism of MG dye on the adsorbents surface is summarized in Figure 15. The probable adsorption mechanisms between the adsorbents surface functional groups and MG dye can be related to the various interactions such as electrostatic attractions, hydrogen bonding interaction, and π-π interactions [38]. Similar observation was reported for the adsorption on MG on chemically modified rice husk [64]. The comparison of previously reported adsorption capacities of various wastes for MG dye removal is presented in Table 6.Figure 15
Possible adsorption mechanisms between LAPW and MG dye.Table 6
Previously reported adsorption capacities of various wastes for MG dye removal.
AdsorbentsPHCapacity (mg/g)ReferencesWood apple shell7.534.56[53]Neem sawdust7.24.35[65]Tamarind fruit shell51.95[66]Cellulose powder7.22.42[67]Avena sativa hull883[68]Arunda donax root carbon58.7[69]Rice husk (HNO3-treated)818.1[70]Saw dust (NaOH-treated)2.958.479[70]Prosopis cineraria sawdust665.8[70]Pine needles752.91[70]Walnut shellAmbient90.8[71]Modified rice straw425.6[72]Maize husk leaf681.5[73]Waste pea shellsNatural6.2[74]ALAPW47.3This studyRLAPW66.6This study


## 3.1. Characterization ofLupinus albus Seed Peel Waste Adsorbents (RLAPW and ALAPW)

Physicochemical characteristics of the adsorbents such as proximate analysis, point of zero charge, and specific surface area are presented in Table1. The moisture content (MC), ash content (AC), and volatile matter (VM) of ALAPW were found to be lower than RLAPW, whereas the fixed carbon (FC) content is higher than the RLAPW. This indicates that most of the moistures and heat-sensitive molecules present in the sample were removed up on physiochemical activation. The volatile matter of ALAPW was found to be low due to the organic components present in adsorbents which become less stable and the release of volatile matter as gas and liquid products which evaporates off leaving the material during the physicochemical activation process [27]. The ash content of ALAPW was also low, due to the removal of significant amount of mineral components, certain oxides, carbonates, and sulfides in the adsorbent during the physicochemical activation process [31]. The fixed carbon content of the ALAPW was high, which indicates the adsorbent material has good quality which enhances the surface area as well as the adsorption performance [33]. The specific surface area of the prepared adsorbent was 1703 ± 0.56 m2/g and 1170 ± 0.40 m2/g for ALAPW and RLAPW, respectively. The specific surface area of ALAPW was so high, indicating the adsorbent has good efficiency to adsorb a dye. Similar result was observed for methylene blue dye removal using activated carbon [42].Table 1
Proximate analyses of the adsorbents.
SampleCharacteristicsMoisture content (%)Ash content (%)Volatile matter (%)Fixed carbon (%)Surface area (m2/g)pHzpcRLAPW5.24 ± 0.118.23 ± 0.0512.12 ± 0.2774.41 ± 0.131170 ± 0.404.3 ± 0.04ALAPW0.22 ± 0.023.25 ± 0.011.16 ± 0.0095.4 ± 0.281703 ± 0.563.2 ± 0.02Point of zero charge (pHzpc) is explained as the situation in which the density of electric charge on the surface of the adsorbent becomes zero. The graph of pHzpc was plotted as “pH final versus pH initial,” and the pHzpc values of the adsorbents was obtained at the intersection point of the curves of “pH final versus pH initial.” The pHzpc plot of the adsorbents (ALAPW and RLAPW) is presented in Figure2. The pHzpc was found to be 3.2 and 4.3 for ALAPW and RLAPW biosorbents, respectively. The low pHzpc for ALAPW may be due to the effects of physiochemical activation process. When pH < pHzpc, the adsorbents surface will become positively charged, and when the solution pH > pHzpc, the adsorbents surface will become negatively charged [43]. Below pHzpc, the surface of the adsorbents (ALAPW and RLAPW) becomes positively charged, and they compete with a cationic MG dye for vacant adsorption sites causing a decrease in dye uptake due to electrostatic repulsion. Above pHzpc, the adsorbent surface is negatively charged and favors uptake of cationic MG dye due to increased electrostatic force of attraction [44].Figure 2
The pHzpc plot of the adsorbents (ALAPW and RLAPW).


## 3.2. FTIR Analysis

Fourier transform infrared (FT-IR) spectral analysis was conducted to determine the functional groups that exist on the surface of the materials. The FTIR spectra of RLAPW, ALAPW, RLAPW-loaded MG, and ALAPW loaded MG dye were recorded in the range of 4000–400 cm−1 and shown in Figure 3. In both spectra (before and after adsorption), the broad and intense adsorption peaks in the range of 3450–3350 cm−1 were obtained due to the hydroxyl (–O-H) or amine (–N-H) functional groups [2, 3, 45], the absorption peaks at 2950–2850 cm−1 can be reflected to the –C-H group of alkane, stretching vibrations at 2400–2050 cm−1 assigned to C≡C and C≡N−H bonds [20], the peaks at 1620–1610 cm−1 were attributed to stretching vibrations of carbonyl/carboxyl (C=O) groups [1], and intense peaks at 1460–1440 cm−1 are supposed to the presence of (C-H) vibration of aliphatic and aromatic groups and absorption bands in the range 1420–1000 cm−1 can be assigned to the C-O and C-N stretching vibration of carboxylic acids (-COOH) and/or alcohols and amine groups [1–3].Figure 3
FT-IR spectra of LAPW adsorbents before and after adsorption of MG dye.
(a)(b)The surface/characteristic peaks of RLAPW and ALAPW were found to be different (Figure3) as some of the functional groups disappeared and the intensity of the peaks altered due to the physicochemical activation process, which shows that these functional groups were chemically protonated/deprotonated and thermally unstable [20]. The spectra of the RLAPW, ALAPW, RLAPW-loaded MG, and ALAPW-loaded MG dye showed similar characteristics of adsorption regions except for slight differences/changes. The FTIR spectra before and after adsorption indicate that the peaks are slightly shifted from their positions and the intensity gets changed. These results indicated the binding of some functional groups (hydroxyl, carbonyl, amine, and carboxyl) in the adsorption of malachite green (MG) dye on the adsorbents surface through weak electrostatic interaction or Van der Waals forces [1, 45]. The FTIR spectral analysis revealed that both of the adsorbents (RLAPW and ALAPW) contain several functional groups such as hydroxyl, carbonyl, carboxyl, and amine groups, and these groups act as potential active sites for interaction with the malachite green (MG) dye. Those functional groups (potential active sites) were found in the adsorbents having high affinity towards pollutants (organic and inorganic), and the MG dye removal was carried via hydrogen bonding, electrostatic, and π-π interactions [46].


## 3.3. Batch Adsorption Study

### 3.3.1. Effect of Solution pH

The effect of solution pH plays an important controlling parameter in the adsorption processes [1, 47]. The effect of pH on the adsorption of MG dye by the ALAPW and RLAPW at pH between 2 and 12 is shown in Figure 4. The MG dye removal efficiency (Figure 4) was increased at the pH range of 2–4 for ALAPW and 2–6 for RLAPW at a given concentration. The adsorbent surface was positively charged at pH under 3.2 (ALAPW) and 4.3 (RLAPW) and showed negative charge over the pHzpc. The MG is a positively charged (cationic) dye and provides positive ions in the solutions. Thus, below pHzpc, the amounts of adsorption were lesser owing to electrostatic repulsion between the MG dye ions and the positively charged surface of the adsorbents.Figure 4
pH effect on the adsorption of MG dye by ALAPW and RLAPW at adsorbent dose 0.2 g. MG dye concentration 10 mg/L, contact time 60 min, agitation speed 200 rpm, and room temperature (298 K).The FT-IR analysis (Figure3) showed that the ALAPW surface contained excess acidic functional groups due to physicochemical activation [30]. The FT-IR analysis also showed that the RLAPW surface contained acidic functional groups (C=O at 1617 cm−1, O-H at 3407 cm−1, and C-O at 1028 cm−1). The results revealed that the adsorption process may be carried out via a dominant acidic active surface of the adsorbents and the cationic MG dye. The high removal efficiency at lower pH (4) for ALAPW and pH (6) for RLAPW is probably due to electrostatic attraction, hydrogen bonding, or π-π interaction between adsorbent and adsorbate (dye) [2]. In addition, the mechanism also responsible for the adsorption of MG may be more related to textural properties due to the presence of high surface area [14]. As a result, maximum MG dye removal efficiency was carried at acidic regions (pH <7). Similar behaviors were observed for malachite green dye adsorption on activated sintering process red mud [10, 48] and methylene blue cationic dye on jackfruit peel [49].


### 3.3.2. Effect of Adsorbent Dose

In the adsorption process, adsorbent dose is a very important parameter due to the dosage effect on the adsorbent and adsorbate [25]. The effect of adsorbent dose on the removal of MG dye by ALAPW and RLAPW is presented in Figure 5. When the adsorbents dose increased from 0.05 to 0.2 g, an increase in the MG dye removal from 76.3 to 90.14% for ALAPW and 55.63 to 78.7% for RLAPW was observed. Such increase of MG dye removal with adsorbent dose is due to the presence of high surface area and availability of several adsorption sites in the adsorbents [16]. Beyond the optimum adsorbent dose (0.2 g), the MG dye removal was not significantly changed or affected due to conglomeration/aggregation of adsorbent particles which limits the active surfaces for adsorption [50].Figure 5
Effect of adsorbent dose on MG dye removal by ALAPW and RLAPW at pH 4 (ALAPW) and pH 6 (RLAPW), initial MG dye concentration 10 mg/L, contact time 60 min, and temperature (298 K) with agitation speed 200 rpm.


### 3.3.3. Effect of Initial Dye Concentration

The effect of initial MG dye concentration on the adsorption of MG onto ALAPW and RLAPW was carried out in the concentration range of 10–50 mg/L as shown in Figure6. Percent removal efficiency increased with an increase in MG dye concentration from 10 to 30 mg/L for both adsorbents. The percent removal of MG dye decreases beyond 30 mg/L of dye concentrations. When initial dye concentration increases (beyond 30 mg/L), the active sites presented in the adsorbents required for bioadsorption of the dye molecules may be occupied and further adsorption is hindered or prevented by repulsion force of dye or steric hindrance on the adsorbent phase and on the bulk phase [50, 51].Figure 6
Effect of initial MG dye concentration on MG dye removal by ALAPW and RLAPW at pH 4 (ALAPW) and pH 6 (RLAPW), adsorbent dose 0.2 g, contact time 60 min, and temperature (298 K) with agitation speed 200 rpm.


### 3.3.4. Effect of Contact Time

The effect of contact time on the adsorption processes was examined in the range of 10–70 min at optimum values of adsorbent dose 0.2 g, pH 4 (ALAPW) and pH 6 (RLAPW), initial MG dye concentration 30 and 40 mg/L, and temperature 298 K with an agitation speed of 200 rpm. As shown in Figure7, MG dye removal efficiency increases with an increase in contact time up to 60 min and then nearly constant. The results revealed that the adsorption of MG dye onto ALAPW and RLAPW was increased up to 60 min due to the availability of free/vacant surface sites of functional groups [52], and nearly constant beyond 60 min due to the saturation of the available free adsorbing sites or the remaining vacant surface sites are hard to be adsorb due to repulsive forces between the dye molecules on the adsorbents and the bulk phase [50]. Hence, 60 min was the equilibrium time obtained for MG dye adsorption in this study.Figure 7
Contact time effect on MG dye removal by ALAPW and RLAPW at adsorbent dose 0.2 g, pH 4 (ALAPW) and pH 6 (RLAPW), initial MG dye concentration 30 and 40 mg/L, and temperature (298 K) with agitation speed 200 rpm.


### 3.3.5. Effect of Temperature

The adsorption process is temperature-dependent [53]. The temperature effect was explained using 298 K, 303 K, 313 K, and 323 K at MG dye concentration 30 mg/L, adsorbent dose 0.2 g, pH 4 (ALAPW) and 6 (RLAPW), and contact time 60 min with agitation speed 200 rpm. Figure 8 shows the removal of MG dye by ALAPW and RLAPW was decreased with increasing the temperature from 298 to 323 K. The results clearly suggested that the adsorption of malachite green (MG) dye on to the adsorbents (ALAPW and RLAPW) was favorable at low temperature and exothermic in nature and such effects were explained [44].Figure 8
Temperature effect on MG dye removal by ALAPW and RLAPW at adsorbent dose 0.2 g, pH 4 (ALAPW) and pH 6 (RLAPW), initial MG dye concentration 30 mg/L, and contact time 60 min with agitation speed 200 rpm.


## 3.3.1. Effect of Solution pH

The effect of solution pH plays an important controlling parameter in the adsorption processes [1, 47]. The effect of pH on the adsorption of MG dye by the ALAPW and RLAPW at pH between 2 and 12 is shown in Figure 4. The MG dye removal efficiency (Figure 4) was increased at the pH range of 2–4 for ALAPW and 2–6 for RLAPW at a given concentration. The adsorbent surface was positively charged at pH under 3.2 (ALAPW) and 4.3 (RLAPW) and showed negative charge over the pHzpc. The MG is a positively charged (cationic) dye and provides positive ions in the solutions. Thus, below pHzpc, the amounts of adsorption were lesser owing to electrostatic repulsion between the MG dye ions and the positively charged surface of the adsorbents.Figure 4
pH effect on the adsorption of MG dye by ALAPW and RLAPW at adsorbent dose 0.2 g. MG dye concentration 10 mg/L, contact time 60 min, agitation speed 200 rpm, and room temperature (298 K).The FT-IR analysis (Figure3) showed that the ALAPW surface contained excess acidic functional groups due to physicochemical activation [30]. The FT-IR analysis also showed that the RLAPW surface contained acidic functional groups (C=O at 1617 cm−1, O-H at 3407 cm−1, and C-O at 1028 cm−1). The results revealed that the adsorption process may be carried out via a dominant acidic active surface of the adsorbents and the cationic MG dye. The high removal efficiency at lower pH (4) for ALAPW and pH (6) for RLAPW is probably due to electrostatic attraction, hydrogen bonding, or π-π interaction between adsorbent and adsorbate (dye) [2]. In addition, the mechanism also responsible for the adsorption of MG may be more related to textural properties due to the presence of high surface area [14]. As a result, maximum MG dye removal efficiency was carried at acidic regions (pH <7). Similar behaviors were observed for malachite green dye adsorption on activated sintering process red mud [10, 48] and methylene blue cationic dye on jackfruit peel [49].


## 3.3.2. Effect of Adsorbent Dose

In the adsorption process, adsorbent dose is a very important parameter due to the dosage effect on the adsorbent and adsorbate [25]. The effect of adsorbent dose on the removal of MG dye by ALAPW and RLAPW is presented in Figure 5. When the adsorbents dose increased from 0.05 to 0.2 g, an increase in the MG dye removal from 76.3 to 90.14% for ALAPW and 55.63 to 78.7% for RLAPW was observed. Such increase of MG dye removal with adsorbent dose is due to the presence of high surface area and availability of several adsorption sites in the adsorbents [16]. Beyond the optimum adsorbent dose (0.2 g), the MG dye removal was not significantly changed or affected due to conglomeration/aggregation of adsorbent particles which limits the active surfaces for adsorption [50].Figure 5
Effect of adsorbent dose on MG dye removal by ALAPW and RLAPW at pH 4 (ALAPW) and pH 6 (RLAPW), initial MG dye concentration 10 mg/L, contact time 60 min, and temperature (298 K) with agitation speed 200 rpm.


## 3.3.3. Effect of Initial Dye Concentration

The effect of initial MG dye concentration on the adsorption of MG onto ALAPW and RLAPW was carried out in the concentration range of 10–50 mg/L as shown in Figure6. Percent removal efficiency increased with an increase in MG dye concentration from 10 to 30 mg/L for both adsorbents. The percent removal of MG dye decreases beyond 30 mg/L of dye concentrations. When initial dye concentration increases (beyond 30 mg/L), the active sites presented in the adsorbents required for bioadsorption of the dye molecules may be occupied and further adsorption is hindered or prevented by repulsion force of dye or steric hindrance on the adsorbent phase and on the bulk phase [50, 51].Figure 6
Effect of initial MG dye concentration on MG dye removal by ALAPW and RLAPW at pH 4 (ALAPW) and pH 6 (RLAPW), adsorbent dose 0.2 g, contact time 60 min, and temperature (298 K) with agitation speed 200 rpm.


## 3.3.4. Effect of Contact Time

The effect of contact time on the adsorption processes was examined in the range of 10–70 min at optimum values of adsorbent dose 0.2 g, pH 4 (ALAPW) and pH 6 (RLAPW), initial MG dye concentration 30 and 40 mg/L, and temperature 298 K with an agitation speed of 200 rpm. As shown in Figure7, MG dye removal efficiency increases with an increase in contact time up to 60 min and then nearly constant. The results revealed that the adsorption of MG dye onto ALAPW and RLAPW was increased up to 60 min due to the availability of free/vacant surface sites of functional groups [52], and nearly constant beyond 60 min due to the saturation of the available free adsorbing sites or the remaining vacant surface sites are hard to be adsorb due to repulsive forces between the dye molecules on the adsorbents and the bulk phase [50]. Hence, 60 min was the equilibrium time obtained for MG dye adsorption in this study.Figure 7
Contact time effect on MG dye removal by ALAPW and RLAPW at adsorbent dose 0.2 g, pH 4 (ALAPW) and pH 6 (RLAPW), initial MG dye concentration 30 and 40 mg/L, and temperature (298 K) with agitation speed 200 rpm.


## 3.3.5. Effect of Temperature

The adsorption process is temperature-dependent [53]. The temperature effect was explained using 298 K, 303 K, 313 K, and 323 K at MG dye concentration 30 mg/L, adsorbent dose 0.2 g, pH 4 (ALAPW) and 6 (RLAPW), and contact time 60 min with agitation speed 200 rpm. Figure 8 shows the removal of MG dye by ALAPW and RLAPW was decreased with increasing the temperature from 298 to 323 K. The results clearly suggested that the adsorption of malachite green (MG) dye on to the adsorbents (ALAPW and RLAPW) was favorable at low temperature and exothermic in nature and such effects were explained [44].Figure 8
Temperature effect on MG dye removal by ALAPW and RLAPW at adsorbent dose 0.2 g, pH 4 (ALAPW) and pH 6 (RLAPW), initial MG dye concentration 30 mg/L, and contact time 60 min with agitation speed 200 rpm.


## 3.4. Thermodynamic Study

Thermodynamic study shows the favorability and feasibility of the adsorption process [52]. The values of thermodynamic parameters evaluated for MG adsorption onto the adsorbents (RLAPW and RLAPW) such as change in free energy (ΔG°, kJ mol−1), change in enthalpy (ΔH°, kJ mol−1), and change in entropy (ΔS°, kJ·mol−1·K−1) were determined by the change of equilibrium temperature using the following equations [54]:(5)ΔG°=−RTLnKc=−2.303RTLogKc,(6)KC=qeCe,(7)ΔG°=ΔH°−TΔS°,(8)LnKc=−ΔH°RT+ΔS°R,where R (R = 8.314 J/molK) is the universal gas constant, T is the absolute temperature (K), qe (mg/g) is the amount of MG dye adsorbed on the RLAPW and ALAPW of the solution at equilibrium, Ce (mg/L) is the equilibrium concentration of the dye in the solution, and Kc (qe/Ce) is the thermodynamic equilibrium constant. The values of ΔH° and ΔS° were determined from the slope (−ΔH°/R) and intercept (ΔS°/R) of the plot of Ln (Kc) versus 1/T (Figure 9). As shown in Table 2, the negative ΔG° values indicated spontaneous feasible adsorption process in nature; negative ΔH° values suggested the exothermic nature of the adsorption; and the negative ΔS° values suggest the decrease in adsorbate concentration.Figure 9
Plots of lnKc vs. 1/T for the estimation of thermodynamic parameters.
(a)(b)Table 2
The values of thermodynamic parameters.
Type of adsorbentTemperature (K)Thermodynamic parameterslnKcΔG° (kJ/mol)ΔH° (kJ/molK)ΔS° (J/mol K)ErrorsALAPW2988.083−1.395−4.697−11.0830.140713037.815−1.3390.140713136.694−1.2280.140713236.000−1.1180.14071RLAPW2981.830−5.200−8.879−12.3460.03023031.67−5.1390.025653131.59−5.0160.02323231.54−4.8920.0216


## 3.5. Kinetic Study

Adsorption kinetics for the adsorbents of ALAPW and RLAPW were conducted at pH 4 (ALAPW) and pH 6 (RLAPW), adsorbent dose 0.2 g, initial MG dye concentration 30 mg/L and 40 mg/L, contact time 10–70 minute, and temperature 298 K. The adsorption kinetic parameters are useful for defining the adsorption rate and give important information on the mechanism of the sorption process [16, 55], and the adsorption dynamics was studied by the kinetics in terms of the order of the rate constant [56]. In order to investigate the mechanism of dye adsorption onto RLAPW and ALAPW, adsorption kinetics was studied using the pseudo-first-order (equation (9)), pseudo-second-order (equation (10)), and intraparticle diffusion model (equation (11)), respectively [46, 52].(9)logqe−qt=logqe−k1t,(10)tqt=1k2qe2+tqe,(11)qt=kdifft1/2+C,where qt is amount adsorbed at time t (mg/g), qe is amount adsorbed at equilibrium time (mg/g), k1 is the pseudo-first-order rate constant (min−1), k2 is the pseudo-second-order rate constant (g/mg·min), kdiff is the intraparticle diffusion rate constant (mg/g·min1/2), and C is the intercept. The kinetic parameters (Table 3) for each model were calculated by plotting graph log (qe − qt) vs. t for pseudo-first-order, t/qt vs. t for pseudo-second-order (Figure 10), and qt vs. t1/2 for intraparticle diffusion models (Figure 11).Table 3
Kinetic parameters for MG adsorption onto ALAPW and RLAPW adsorbents.
KineticsParametersALAPWRLAPWConcentration (mg/L)30403040qe exp.(mg/g)7.279.386.558.30Pseudo-first orderR20.980540.813560.993340.97849k1 (min−1)−0.0145−0.0259−0.00906−0.0178qe cal.(mg/g)0.41420.8451.9753.296Pseudo-second orderR20.999940.999010.99910.99762qe cal. (mg/g)7.3968.4756.1638.948k2 (g/mg·min)0.000330.034630.051930.02365Intraparticle diffusion modelR20.988720.939440.955840.97933kdiff (mg/g·min1/2)0.057910.514190.299790.38931C (mg/g)6.795764.815713.913445.11084Figure 10
Pseudo-first-order (a) for ALAPW and (b) for RLAPW and pseudo-second-order (c) for ALAPW and (d) for RLAPW for the adsorption of MG dye at different contact time intervals.Figure 11
Intraparticle diffusion model (a) for ALAPW and (b) for RLAPW at different contact times (10–70 min).It was observed that, for the pseudo-first-order and intraparticle diffusion model of MG dye adsorption,R2 was relatively low and the calculated qe (qe calc.) value was dramatically lower than the experimental qe (qe exp.) value. However, the pseudo-second-order kinetic model provided a near perfect match between the calculated (qe calc.) and experimental (qe exp.) values, and its R2 value is close to unity for both adsorbents. As a result, the adsorption of MG dye on both adsorbents fitted well into the pseudo-second-order kinetic model.


## 3.6. Adsorption Isotherms

Equilibrium adsorption isotherms were studied with MG dye concentrations (10–50 mg/L) with a fixed adsorbent mass (0.2 g), pH 4 (ALAPW) and 6 (RLAPW), contact time 60 min, and temperature 298 K. In this study, the adsorption of MG dye onto ALAPW and RLAPW was analyzed by various isotherm models such as Langmuir, Freundlich, Redlich–Peterson, and Sips isotherms. The Langmuir model assumes the formation of a monolayer of adsorbate on the outer surface of the adsorbent (uniform energies of adsorption) and no further adsorption thereafter (applicable to homogeneous sorption or all sorption sites are identical) and expressed by equation (12) [46].(12)Ceqe=1bqm+Ceqm,where Ce (mg/L) is the equilibrium solute concentration of dye in solution, qe (mg/g) is the adsorbed value of dye at equilibrium concentration, qm (mg/g) is the maximum monolayer adsorption capacity, and b is the Langmuir adsorption constant (L/mg). Slope (1/qm) and intercept (1/bqm) of the straight line plot of Ce/qe versus Ce is shown in Figure 12. The values of Langmuir isotherm constants qm and b are presented in Table 4.Figure 12
Langmuir isotherm (a) for ALAPW and (b) for RLAPW and Freundlich isotherm (c) for ALAPW and (d) RLAPW.Table 4
Isotherm parameters for the adsorption of MG dye onto ALAPW and RLAPW.
Isotherm modelsParametersALAPWRLAPWLangmuirR20.127550.455qm (mg/g)74.629.0b (L/mg)0.0620.042FreundlichR20.754560.87413Kf (mg/g)4.811.25n1.211.14SipsQmax.(mg/g)36.1419.43Ks (L/mg)1.40.05n11R20.590140.9022Redlich–Petersonβ0.217960.13014A (mg/L)38.051.68R20.283810.26368The type of the Langmuir isotherm could be predicted based on whether the adsorption was favorable or unfavorable in terms of equilibrium parameter or dimensionless constant separation factorRL [45], which is presented as follows:(13)RL=11+bCo,where RL is the Langmuir constant and Co is the initial concentration of adsorbate. The values of RL indicates whether the isotherm is unfavorable (RL > 1), linear (RL = 1), favorable (RL < 1), or irreversible (RL = 0). As shown in Table 5, the values of RL (between 0 and 1) indicate that the isotherm was favorable.Table 5
RL (dimensionless constant separation factor) values at different initial concentration.
AdsorbentRL valuesConcentration (mg/L)1020304050ALAPW0.2 g0.620.450.350.290.24RLAPW0.2 g0.700.540.440.370.32Freundlich isotherm is used to describe the adsorption surface becoming heterogeneous (nonuniform) during the adsorption process [45] and expressed in(14)logqe=logkf+logCen,where Ce is the equilibrium concentration of dye in solution (mg/L), qe is the adsorbed value of dye at equilibrium concentration (mg/g), and Kf and n are Freundlich constants characteristics of the system, indicating the adsorption capacity and the adsorption intensity. The Freundlich coefficients, Kf (related to adsorption capacity) and n (related to adsorption intensity), obtained from the slope (1/n) and the intercept (log Kf) of the linearized plots of log qe versus log Ce (Figure 12) are shown in Table 4. The value n > 1 suggests that adsorbate is favorably adsorbed on the adsorbent. The higher the n value is, the stronger the adsorption intensity is [13].The Sips isotherm model is a combination of both Langmuir and Freundlich isotherm, and the model is valid for localized adsorption without adsorbate-adsorbate interactions [57, 58]. This model is also used for predicting the heterogeneous adsorption systems and circumventing the limitation of the rising adsorbate concentration associated with the Freundlich isotherm model. At the low adsorbate concentrations (at low Ce), the Sips isotherm model effectively reduces to the Freundlich isotherm, and at high adsorbate concentrations (at high Ce), this model predicts a monolayer sorption capacity characteristic of the Langmuir isotherm. The Sips isotherm model is expressed in(15)1qe=1QmaxKs1Ce1/n+1Qmax,where Ks (L/mg) is the Sips equilibrium constant and Qmax (mg/g) is maximum adsorption capacity values obtained from the slope and the intercept of the plot as shown in Figure 13. The Sips isotherm equation is characterized by the dimensionless heterogeneity factor, n, which can also be employed to describe the system’s heterogeneity when n is between 0 and 1. When n = 1, the Sips equation reduces to the Langmuir equation, and it implies a homogeneous adsorption process.Figure 13
Sips isotherm (a) for ALAPW and (b) for RLAPW, Redlich–Peterson isotherm (c) for ALAPW and (d) RLAPW.Redlich–Peterson isotherm is a hybrid isotherm featuring both Langmuir and Freundlich isotherms, which incorporate three parameters into an empirical equation which may be used to represent adsorption equilibria over a wide concentration range and can be applied either in homogeneous or heterogeneous systems due to its versatility [57]. The linear form of the Redlich–Peterson isotherm is represented by(16)lnCeqe=βlnCe−lnA,where ln A is the Redlich–Peterson isotherm constant obtained from the intercept of ln (Ce/qe) versus ln Ce graphs and β is the exponent between 0 and 1. The experimental data analyzed by Sips and Redlich–Peterson isotherms are shown in Figure 13.Isotherm parameters and correlation factor (R2) for both models are presented in Table 4. The results revealed that the Freundlich isotherm correlation factor for ALAPW relatively close to unity confirms Freundlich isotherm model which better describes the interaction between adsorbent and adsorbate in the aqueous system. For RLAPW, the Sips isotherm correlation factor close to unity better describes the interaction between adsorbent and adsorbate in the aqueous system.Dimensionless constant separation factor (RL) value indicates the adsorption nature to be either favorable if 0 < RL < 1, unfavorable if RL > 1, linear if RL = 1, and irreversible if RL = 0 [59]. For this study, all the RL valves were obtained between 0 and 1 which confirming that the adsorption of MG dye over the ALAPW and RLAPW was favorable [59]. In the present study, the calculated RL values for the adsorption of MG on ALAPW and RLAPW adsorbents are presented in Table 5 at initial concentrations of 10, 20, 30, 40, and 50 mg/L. These RL values confirmed that adsorbents (ALAPW and RLAPW) are favorable for adsorbing MG dye from aqueous solution under the conditions applied in this study.


## 3.7. Desorption Study

High desorption efficiency and good reusability after adsorption are desirable for an adsorbents. Desorption studies are helpful to explain the nature of adsorption and recycling of the spent adsorbent and the dye [51, 60–63]. The regeneration of ALAPW and RLAPW was conducted by immersing the adsorbents into different solvents Me-OH (99%), NaOH (0.1 M), and HCl (0.1 M), and the results are presented in Figure 14. As shown in Figure 14, the regeneration of adsorbents by 0.1 M NaOH was more efficient (compared to the other solvents). These phenomena are consistent with the results observed for the effect of pH. Since the maximum removal efficiency of MG is attained at weakly acidic conditions, it is expectable that desorption is favored at high pH values. At pH 2, a significantly high electrostatic repulsion exists between the positively charged surfaces of both adsorbents and the cationic MG dye so that regeneration is carried out as a result of charge competition. As described in the figure, ALAPW has high regeneration ability as compared to RLAPW. This is because the high fixed carbon of the activated carbon gives a better strength and porosity nature. Therefore, ALAPW shows excellent adsorption performance and regeneration, and its use can be extended to environmental applications for wastewater treatment.Figure 14
Desorption efficiency (%) using different solvent solutions.


## 3.8. Proposed Adsorption Mechanism

The bioadsorption of MG dye from aqueous solutions by RLAPW and ALAPW is strongly dependent on the various polar functional groups on the surface of the adsorbents such as hydroxyl (cellulose, pectin, hemicellulose, adsorbed water, and lignin), phenols, amines, and aromatics, which are supported by FTIR spectral results described in Figure3. The surface functional groups of the adsorbents may be charged, neutral upon protonation/deprotonation, and multiple bonded upon delocalization. The possible adsorption mechanism of MG dye on the adsorbents surface is summarized in Figure 15. The probable adsorption mechanisms between the adsorbents surface functional groups and MG dye can be related to the various interactions such as electrostatic attractions, hydrogen bonding interaction, and π-π interactions [38]. Similar observation was reported for the adsorption on MG on chemically modified rice husk [64]. The comparison of previously reported adsorption capacities of various wastes for MG dye removal is presented in Table 6.Figure 15
Possible adsorption mechanisms between LAPW and MG dye.Table 6
Previously reported adsorption capacities of various wastes for MG dye removal.
AdsorbentsPHCapacity (mg/g)ReferencesWood apple shell7.534.56[53]Neem sawdust7.24.35[65]Tamarind fruit shell51.95[66]Cellulose powder7.22.42[67]Avena sativa hull883[68]Arunda donax root carbon58.7[69]Rice husk (HNO3-treated)818.1[70]Saw dust (NaOH-treated)2.958.479[70]Prosopis cineraria sawdust665.8[70]Pine needles752.91[70]Walnut shellAmbient90.8[71]Modified rice straw425.6[72]Maize husk leaf681.5[73]Waste pea shellsNatural6.2[74]ALAPW47.3This studyRLAPW66.6This study


## 4. Conclusion

This study proposed to use a naturally available, noncost, and environmental friendlyLupinus albus seed peel waste as a novel adsorbent to remove malachite green dye for the first time. The effects of experimental parameters such as pH, adsorbent dose, initial dye concentration, contact time, and temperature on the percentage of MG dye removal were investigated. The best fit adsorption isotherm models for RLAPW and ALAPW were Freundlich and Sips with the adsorption capacity of 6.6 mg/g and 7.3 mg/g, respectively. The best fit kinetic model for both adsorbents was pseudo-second-order. The thermodynamics parameters indicated that the adsorption was spontaneous and exothermic in nature. Desorption studies were conducted, and the results showed that the adsorbents have regeneration ability at acidic pH 2. This study provides a good indication that the prepared adsorbents are an efficient adsorbent for the removal of dyes from aqueous solution.



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