Effective removal of methyl green from aqueous environment using activated residual Dodonaea Viscosa: equilibrium, isotherm, and mechanism studies

ABSTRACT

In this study, activated residual Dodonaea Viscosa (ARDV) was synthesized by the simple chemical activation process as a cost-effective and sustainable adsorbent for the efficient elimination of methyl green molecule (MG) from water. The prepared ARDV was characterized by TGA, SEM, EDX, FTIR, and BET surface area analysis. The adsorption properties and mechanisms toward MG were examined using the batch method. The optimal conditions for adsorption of Pb(II) were pH= 6.6, [ARDV] =0.1 g, time=180 min, and temperature= 65°C. The adsorption isotherms and kinetics study was conducted and the resulting data fitted well with Langmuir and PSO model. The maximum sorption capacity of 99 mg/g was obtained from Langmuir isotherm for MG at 60 °C. According to thermodynamic parameters, the adsorption of MG on ARDV was endothermic and spontaneous. Hence, it could be concluded that ARDV is an inexpensive and effective bio-adsorbent for the removal of MG from polluted water.         

KEYWORDS:

• Adsorption
• dodonaea viscosa
• endothermic
• isotherm
• methyl green (MG)

1. Introduction

Dye wastewater is generated by different industries such as textile, plastics, printing, electroplating, paper dye synthesis, leather and cosmetics [1]. ~10% to 20% of these dyes are existing in the effluent of these industries [2]. Disposal of these dyes into the environment without any treatment is hazardous to the ecosystem and health due to some of the dyes being toxic, carcinogenic and degradation resistant [3–7]. Besides, the presence of a very low concentration of methyl green, in water, is undesirable. Methyl green (MG) [C₂₇H₃₅BrClN₃ZnCl₂] is a cationic dye that is used in medicine and biology such as staining DNA and cell nuclei, as well as used as a photochromophore to sensitize gelatinous films [8,9]. Owing to the negative effects of organic dyes on the ecosystem and human health, it is vital to eliminate these organic dyes before releasing them into the water environment.

Various dyes removal methods, such as photodecolorization [10], membrane [11], photocatalytic degradation [12], UV photolysis [13] and adsorption [14], were developed to limit their effect on the environment. Among them, adsorption is believed to be the most promising method owing to its effectiveness, low cost, easy operation, and efficient technique for the removal of nonbiodegradable pollutants [15,16]. Many adsorbents have been studied for the elimination of methyl green from the water systems, namely, activated carbon [17], reduced graphene oxide [18], MCM-41 [17] and graphene sheets [19]. However, most of them have drawbacks such as high cost and low adsorption efficiency. The efficiency of the adsorption process mainly depends on the abundance and cost in nature, easy regenerability and stability [20].

Recently, biosorbents made from plants are getting much more interest as effective adsorbents for the elimination of molecule dyes from water systems owing to their advantages of low cost, availability, eco-friendliness and accessibility [21]. Therefore, many researchers have examined the elimination of pollutants from aqueous solutions by bio adsorbent materials like, parviflora plant leaves [22], Platanus orientalis leaves [23], algal biomass [24], pine leaves [25], phoenix tree leaf powder [26], Solanum incanum [22], activated Aerva javanica and dead leaves of Ficus racemosa [27].

Dodonaea viscosa is used traditionally as folk medicine in many countries to treat many diseases like rheumatism, malaria, cold, fever, indigestion, headaches, toothaches, diarrhea, constipation and dysmenorrhea [28]. This is maybe a plant container much important like tannins, flavonoids, alkaloids, saponins, triterpenoids, steroids, volatile oil and phytosterols [29]. Therefore, Dodonaea viscosa has numerous highly reactive chemical functions which use them as adsorption sites for adsorption pollutants.

Usually, after extraction of medicine materials from raw plants, the residues materials are thrown out. To the best of our knowledge, this is the first time to use residual waste of a medicinal plant after their extraction and use it for dye adsorption after activation by hydrogen peroxide and sodium hydroxide. The extraction of the active medicine materials had been used as therapeutic compounds for anticancer and antimicrobial activities [30]. Therefore, this work has thus focused on the development a new biosorbent from residual waste of a medicinal plant (Dodonaea viscosa). The residual waste of a medicinal plant was activated by H₂O₂ and NaOH reagents to enhance the adsorption properties of RDV toward MG adsorption from aqueous solutions. The activated residual Dodonaea viscosa (ARDV) was characterized by SEM, EDX, BET, TGA and FTIR. Besides, the influence of adsorbent dose, solution pH, temperature, contact time and initial MG concentration on the methyl green adsorption using activated residual Dodonaea viscosa (ARDV) was achieved. The thermodynamics, kinetics and isotherm adsorption were investigated to establish the mechanism and nature of the adsorption process of MG on synthesized bio adsorbent (ARDV).

2. Experiments

2.1. Chemicals

Hydrogen peroxide (H₂O₂, 30%), methyl green (MG) and ethanol (C₂H₅OH, 99.8%) were gotten from Sigma Aldrich. Nitric acid (HNO₃, 68.0–70.0%), hydrochloric acid (HCl, 36%) and dodium hydroxide (NaOH, ≥ 97%) were procured from BDH, England.

2.2. Characterization techniques

The surface morphology and elemental analysis of the DV, RDV, ARDV and MG/ARDV powders were evaluated by a scanning electron microscope (SEM) and energy-dispersive X-ray (JEOL 200KV, Tokyo, Japan), respectively. Fourier transform infra-red spectra of the Dodonaea viscosa (DV), Residual Dodonaea viscosa (RDV), Activated Residual Dodonaea viscosa (ARDV) and MG-loaded ARDV adsorbents were recorded by Nicolet iS50, Thermo Scientific (Madison, WI, USA). Surface area, pore volumes and pore radius of DV, RDV and ARDV were determined using BET method and BJH (Barrett-Joyner-Halenda) method. The thermal stability of DV and ARDV adsorbents were carried out by thermogravimetric analyzer (Mettler Toledo GA/SDTA851) in nitrogen atmosphere under the heating rate of 10 ͦC min⁻¹ varying from room temperature to 800 ͦC.

2.3. Chemical treatment of Dodonaea viscosa

Dodonaea viscosa (DV) was collected from Albaha, Saudi Arabia. The Dodonaea viscosa was washed several times with distilled water and then dried and ground into powder using an electrical grinder. Stock samples were prepared by extracting 80 g of Dodonaea viscosa powder using hexane (nonpolar solvent) and ethanol (polar solvent) at room temperature. All DV extracts were prepared in the Soxhlet apparatus and refluxing for one day according to the boiling point of each solvent. After extraction, the solvent was evaporated using a rotary evaporator [30]. The extracted fraction in each solvent was calculated using the following equation 1(1) $\mathrm{W}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{o}\mathrm{f}\mathrm{e}\mathrm{x}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{c}\mathrm{t}\left(\mathrm{\%}\right)=\hspace{0.17em}\frac{\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{o}\mathrm{f}\mathrm{e}\mathrm{x}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{c}\mathrm{t}}{\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{o}\mathrm{f}\mathrm{r}\mathrm{a}\mathrm{w}\mathrm{m}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{i}\mathrm{a}\mathrm{l}}\hspace{0.17em}\times 100\hspace{0.17em}$(1) :

(1) $\mathrm{W}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{o}\mathrm{f}\mathrm{e}\mathrm{x}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{c}\mathrm{t}\left(\mathrm{\%}\right)=\hspace{0.17em}\frac{\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{o}\mathrm{f}\mathrm{e}\mathrm{x}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{c}\mathrm{t}}{\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{o}\mathrm{f}\mathrm{r}\mathrm{a}\mathrm{w}\mathrm{m}\mathrm{a}\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{i}\mathrm{a}\mathrm{l}}\hspace{0.17em}\times 100\hspace{0.17em}$(1)

The use of ethanol in the extraction from Dodonaea viscosa ensured a better extraction yield around 25.45%. The residual of the Dodonaea viscosa (RDV) (not extracted) portion underwent activation treatment and was used for the adsorption experiments. To improve the adsorption properties of RDV powder and eliminate the impurities from its surface, the dried RDV was treated with H₂O₂ and 0.1 M NaOH [31]. Briefly, 20 g of RDV powder was immersed in 150 mL of H₂O₂ under magnetic stirring for 24 h, then, the RDV powder was washed several times with deionized water. Following that, 20 g of activated residual Dodonaea viscosa (ARDV) powder was immersed in 150 mL of 0.1 M NaOH under stirring for 24 hrs. Finally, the ARDV was washed several times with deionized water and then dried in an oven at 60°C for 24 hrs. (Scheme 1).

Scheme 1. Schematic representation of adsorbent preparation.

2.4. Adsorption studies

A standard curve of absorbance-methyl green concentration was plotted based on the measured absorbance of MG at five concentration levels (2–10 mg/L) by UV/Visible spectrometer at λ_max 632 nm. This standard curve was plotted to obtain the concentration of MG at any time throughout the period, and the MG dye exhibited satisfactory linearity with correlation coefficients (R² = 0.999). The adsorption process was carried out using batch method. The influence of various parameters on adsorption of MG dye by using ARDV adsorbents, namely, contact time (5–240 min), pH (3.4–11), adsorbent mass (0.01–0.2 g), initial MG concentration (45–200 mg/L and temperature (25–65°C), was investigated. All adsorption experiments were carried out three times to ensure accuracy and repeatability, and to minimize errors. The general procedure was as follows: 0.1 g of ARDV adsorbent was added to 100 mL of MG dye solutions of 15 to 200 mg/L concentration. Then the contents were adjusted using 0.1 M HCl or NaOH to the wanted pH. After that the contents were shaken at 100 rpm using a mechanical shaker to 180 min. Then the solution was filtered and the residual MG concentration was measured. The adsorbent capacity (q_e mg/g) and removal efficiency (R%) were calculated according to Eq. 2(2) ${\mathrm{q}}_{\mathrm{e}}\hspace{0.17em}=\left({\mathrm{C}}_{\mathrm{o}}-{\mathrm{C}}_{\mathrm{e}}\right)\frac{\mathrm{V}}{\mathrm{m}}\hspace{0.17em}$(2) and 3(3) ${\mathrm{R}}_{\mathrm{e}}\hspace{0.17em}\left(\mathrm{\%}\right)\hspace{0.17em}=\frac{{\mathrm{C}}_{\mathrm{o}}-{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{C}}_{\mathrm{o}}}\hspace{0.17em}\times 100\hspace{0.17em}$(3) , respectively.

(2) ${\mathrm{q}}_{\mathrm{e}}\hspace{0.17em}=\left({\mathrm{C}}_{\mathrm{o}}-{\mathrm{C}}_{\mathrm{e}}\right)\frac{\mathrm{V}}{\mathrm{m}}\hspace{0.17em}$(2)

(3) ${\mathrm{R}}_{\mathrm{e}}\hspace{0.17em}\left(\mathrm{\%}\right)\hspace{0.17em}=\frac{{\mathrm{C}}_{\mathrm{o}}-{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{C}}_{\mathrm{o}}}\hspace{0.17em}\times 100\hspace{0.17em}$(3)

where C_o (mg/L) and C_e (mg/L) refer to the initial concentrations of MG and the equilibrium concentrations of MG, respectively; V(L) is the volume of the MG solution, and m (g) is the mass of ARDV.

For the influence of the adsorbent dose: different ARDV adsorbent quantities (0.01–0.2 g) were added into a series of flasks containing 100 mL of MG dye solution of 15 mg/L concentration at pH = 6.6, separately. The contents of the conical flasks were shaken at 100 rpm under 25 ͦC for 1440 min. After the adsorption process, the residual concentration of the MG dye ions was determined. For the influence of contact time, the adsorption experiments were performed by placing 0.1 g of ARDV adsorbent in a series of conical flasks containing 100 mL (15 mg/L) MG solution at pH 6.6. The time gradient was set to 5, 10, 15, 30, 45, 60, 120, 180 and 240 min, then the samples were taken at time intervals for the analysis of residual MG concentration. The influence of initial MG concentration and temperature on the MG adsorption was carried out by placing 0.1 g of ARDV in a series of conical flasks containing 100 mL of MG dye at definite concentrations (45, 60, 75, 100, 150, 200 mg/L) at pH 6.6. The contents of the conical flasks were shaken at 100 rpm at 25 ͦC for 180 min. After the adsorption process, the residual MG concentration of the MG dye ions was determined. These steps were repeated at 45 ͦC and 65 ͦC, and residual MG concentration was determined.

3. Results and discussion

Figure 1a shows the FTIR of RDV, ARDV and MG-saturated ARDV. For RDV adsorbent, the band at 3302 cm⁻¹ represents the – OH group in tannins, flavonoids, alkaloids, saponins, triterpenoids, steroids, volatile oil, and phytosterols, indicating the existence of -OH and -COOH groups over ARDV surface. This band slightly augmented and shifted to 3330 cm⁻¹ after being activated via H₂O₂ and NaOH reagents. The bands at 1926 and 2852 cm⁻¹ were assigned to the asymmetric and symmetric stretching vibration of–C-H in methylene. The weak and strong characteristic absorption bands at 1726 cm⁻¹ represent the C = O group [32]. Additionally, the characteristic band at 1618–1516 cm⁻¹ was attributed to aromatic C = C bond stretching which is due to the phenolic compounds [33]. The band of C = C was reduced in intensity and shifted to 1612 cm⁻¹ after being activated with chemical reagents. The absorption bands at 1442, (1367–1311), and 1238 cm⁻¹ were assigned to asymmetric stretch vibrations of COO⁻ [34], the bending vibration of – CH₃ and – CH₂, and the stretch vibrations of C–OH [33]. The strong absorption band at 1154 cm⁻¹ and a weak band at 1024 cm⁻¹ were corresponding to C-O-C groups in tannins, saponins and flavonoids [35]. The bands around 800–500 cm⁻¹ assigned to CH = CH₂ stretching vibrations in aromatic structures [36]. After MG adsorption on ARDV adsorbent, it was observed that some bands were reduced or increased in their intensities and shifted. In detail, the strong adsorption band at 1600 cm⁻¹ was decreased owing to π–π interactions between the MG molecules with – C = C–in ARDV. The band of–OH group with intermolecular hydrogen bond was shifted from 3330 to 3335 cm⁻¹. Besides, the band of carboxylic acid had a slight increase in intensity due to the electrostatic attraction between a cationic MG molecule and the negative charge of COO⁻ onto the ARDV surface. The intensity band of C-O-C was significantly increased in intensity after MG adsorption onto ARDV adsorbent. These bands provide enough evidence to support the MG adsorption onto the ARDV surface by π-π interactions, hydrogen bonding and electrostatic interaction.

Figure 1. (a) FTIR spectra (a), and TGA analysis (b).

Figure 1b represents the thermal gravimetric analysis of RDV and ARDV adsorbents. It was observed that the total weight loss for RDV and ARDV adsorbents were 80% and 85%, respectively, in the range of 30–800°C, indicating that the RDV was activated using the chemical reagent sodium hydroxide and hydrogen peroxide. For more details, a two-stage thermal decomposition can be observed for both RDV and ARDV adsorbents. In the first stage, a 9% weight loss was observed for both RDV and ARDV adsorbents in the range of 30–230°C and 30–285°C, respectively, due to the moisture evaporation and solvent. In the second stage, a weight loss of 61% in the range of 230 to 475°C and 285 to 400°C was observed in both RDV and ARDV adsorbents, which might be due to the decomposition of organic compounds like flavonoids, alkaloids, saponins, triterpenoids, steroids, volatile oil and phytosterols. These organic materials underwent continuous degradation until 800°C with weight loss of 10% and 15% for RDV and ARDV adsorbents, respectively. However, the rate of loss of volatiles for ARDV adsorbent was much higher as compared to the DV. The total weight losses of DV and were 80% and 85%, respectively. Thus, the oxygen-content functional in the ARDV adsorbent was about 5% and that is due to the activation of DV adsorbent with sodium hydroxide and hydrogen peroxide.

Figure 2 depicts the morphological characteristics of RDV, ARDV and MG-saturated ARDV using SEM technology. It was observed that the RDV surface exhibits a rough surface with pores (Figure 2a). While the surface of RDV becomes highly porosity due to the activation of RDV with H₂O₂ and NaOH reagents (Figure 2b). After MG adsorption over ARDV, the surface looks smooth and dark which can be taken as a sign of adsorption of MG dye in the pores of ARDV (Figure 2c).

Figure 2. SEM images of (a) RDV, (b) ARDV, and (c) MG loaded ARDV.

The elemental analysis of the RDV, ARDV and MG saturated ARDV were checked by EDX as shown in Figure 3. In the case of RDV, two peaks with content (75.79%) and (24.21%) for carbon and oxygen were observed (Figure 3a). After activation of RDV with hydrogen peroxide, the oxygen content was increased to 27%, while the carbon content was reduced to 72.16% (Figure 3b).

Figure 3. EDX analysis of (a) RDV, and (b) ARDV adsorbents.

The BET specific surface area, pore volume and average pore radius were observed to be (0.425 m²/g, 0.021 cc/g and 3.10 nm), (5.167 m₂/g, 0.018 cc/g and 2.148 nm), (12.789 m²/g, 0.031 cc/g and 1.996 nm) for DV, RDV and ARDV respectively. The surface area of the ARDV adsorbent is 96.7% greater than DV, and 59.6% higher than RDV, indicating the ARDV adsorbent has a greater adsorption capacity toward MG dye due to the adsorption capacity increased with the surface area.

3.1. Adsorption studies

3.1.1. Adsorbent selectivity

The prepared adsorbents namely, Dodonaea viscosa (DV), activated Dodonaea viscosa (ADV) residual Dodonaea viscosa (RDV) and activated residual Dodonaea viscosa (ARDV) powders, were tested for adsorption of MG dye. Tests were performed at C_o = 15 mg/L; adsorbent; 0.1 g; pH = neutral; T = 25°C; agitated speed = 100 rpm; contact time = 1440 min) as indicated in Figure 4a. The maximum elimination efficiency of MG on the DV, ADV, RDV and ARDV powders were 88.57%, 95.55%, 91.74% and 96.35%, respectively. Therefore, the elimination of MG by ARDV is considerably favored owing to its good performance. This increase in the elimination efficiency of MG dye onto ARDV adsorbent is owing to the usage of NaOH and H₂O₂ as activation agents [37].

Figure 4. (a) Adsorbent selectivity toward MG [C_o = 15 mg/L; adsorbent mass; 0.1 g; pH = neutral; T = 25°C; agitated speed = 100 rpm; contact time = 1440 min), (b) effect of pH on adsorption of MG by ARDV [C_o = 15 mg/L; adsorbent mass = 0.1 g; T = 25°C, agitated speed = 100 rpm; contact time = 1440 min], (c) Zeta potential at different pH, (c) effect of adsorbent dose [at C_o = 15 mg/L; adsorbent; pH = 6.6; T = 25°C; agitated speed = 100 rpm; contact time = 180 min)], (e) contact time [adsorbent mass = 0.1 g; pH = 6.6; T = 25°C, agitated speed = 100 rpm)], and (f) initial MG concentration on adsorption of MG by ARDV adsorbent [adsorbent mass = 0.1 g; agitated speed = 100 rpm; contact time = 180 min; pH = 6.6].

3.1.2. Effect of pH

The influence of the solution pH on the adsorption process of MG using ARDV powder was studied in the range of 3.11 to 11 as indicated in Figure 4b. Tests were performed at C_o = 15 mg/L; adsorbent mass = 0.1 g; T = 25°C, agitated speed = 100 rpm; contact time = 1440 min. The results showed that the efficiency of MG removal increased from 44.16% to 95.71% with the increase in pH from 3.4 to 6.6 then, the removal efficiency decreased to 91.78% with increased pH from 6.6 to 8.2, owing to precipitation of the methyl green. Besides, when the pH is greater than 8.2, the efficiency of MG removal was augmented to 97.73, due to the cationic MG molecule being converted into the colorless carbinol base (CB) [38]. Therefore, the pH of the solution had a great influence on the ARDV surface and MG dye which can be confirmed by correlating the zeta potential with the pH (Figure 4c). The pH_pzc of ARDV was found to be 3.9; thus at a pH< pH_pzc (3.9), the ARDV surface has a positive surface charge. Consequently, the electrostatic repulsion increase between a cationic MG molecule and the positive charge of the surface ARDV, resulting in a reduction in the removal efficiency of MG dye and the adsorption capacity of ARDV. With the rise in pH value from 3.4 to 6.6, the adsorbent surface of ARDV became negatively charged owing to the deprotonation of the ARDV surface. Consequently, the electrostatic attraction increases between a cationic MG molecule and the negative charge of the surface ARDV, increasing the removal efficiency of MG dye. Thus, the maximum removal efficiency of MG dye and adsorption capacity of ARDV toward MG dye was 95.71% and 19.14 mg/g, respectively, at pH = 6.6. Similar results have been reported by Alalwan et al. (2021), who investigated MG removal by eggshell waste [39]. In addition, the obtained outcomes were consistent with those articles by researchers who achieved the MG adsorption using various adsorbents [40,41].

3.1.3. Effect of dose

The effect of the adsorbent amount, ranging from 0.01 to 0.2 g, on the MG adsorption onto ARDV adsorbent was studied as shown in Figure 4d. Tests were performed at C_o = 15 mg/L; adsorbent; pH = 6.6; T = 25°C; agitated speed = 100 rpm; contact time = 180 min). It is clear that the elimination efficiency of MG dye improved from 79.52% to 95.55% as the amount of the ARDV in the solution increased from 0.01 to 0.1 g and reached a maximum value of 95.6% at 0.1 g. After 0.1 g, the elimination efficiency remains constant. This increase in removal efficiency is basically owing to the existence of more adsorption sites on the ARDV surface with increasing adsorbent dose. On the other hand, the adsorption capacity of ARDV toward MG dye was reduced from 119.29 to 95.55 mg/g as the adsorbent amount increased to 0.1 g. This is owing to the aggregation of adsorbent at a high adsorbent mass which results in a reduction in the total surface area of the ARDV adsorbent. Thus, 0.1 g was selected as the operating adsorbent mass for subsequent studies.

3.1.4. Effect of contact time

The impact of equilibrium time on the efficiency of MG removal by ARDV was analyzed in the time range of 5–240 min at different initial MG concentrations as indicated in Figure 4e. Tests were performed at adsorbent mass = 0.1 g; pH = 6.6; T = 25°C, agitated speed = 100 rpm). The results exhibited that the efficiency of MG removal of MG from its aqueous solution was increased from (56.82% to 93.01%) and (62.06% to 91.74%) with the increased contact time from 5 to 180 min at 15 and 30 mg/L, respectively. The adsorption of MG onto ARDV was rapid at first contact time owing to the availability of vacant sites on the ARDV surfaces. ~62.06% of MG dye was adsorbed in the first 5 min at 30 mg/L. In addition, the results exhibited that the quantity of MG removal rises fast with the initial MG concentration increases may be owing to the increase in the driving force of MG dye and availability of maximum active sites. Thus, the equilibrium time was set at 180 min for the subsequent experiments. Similar findings were reported by Satlaoui et al. [41], who achieved the MG adsorption using activated bentonite from aqueous solutions.

3.1.5. Effect of initial concentration and temperatures

The impact of MG concentrations on the adsorption process was studied over a range of initial MG concentrations from 45 to 200 mg/L at various temperatures (25,45, 65°C) as shown in Figure 4f. Tests were performed at adsorbent mass = 0.1 g; agitated speed = 100 rpm; contact time = 180 min. The results exhibited that the amount of MG adsorption increases from 41.45 to 81.70 mg/g while increasing the concentration of MG from 45 to 200 mg/L at 25°C. This may be due to increasing the driving force that overcomes the mass transfer resistance of MG molecules from the solution to the ARDV surface [42]. The influence of temperature on the MG adsorption on ARDV was also achieved at various concentrations and temperatures. Figure 4f also shows that the amount of MG adsorption increases from 79.21 to 82.71 mg/g at 100 mg/L and from 81.70 to 92.54 mg/g, at 200 mg/L, while increasing the temperature from 25 to 65°C, respectively, indicated that the MG adsorption is endothermic. This is because, with an increase in temperature, the rate of diffusion of the MG ions to the external boundary layer and pores of ARDV was increased [43]. Similar findings were reported by Satlaoui et al. [44], who achieved the MG adsorption using raw and purified sejnane clay type at various temperatures.

3.2. Adsorption modeling

3.2.1. Adsorption isotherm

To determine the maximum adsorption amount of ARDV toward MG dye, three nonlinear isotherm models, namely, Freundlich Eq. 4(4) $q_e=K_F\hspace{0.17em}{C}_e^{1/n}$(4) [45], Langmuir Eq. 5(5) $q_e=\frac{Q_m{K}_L{C}_e}{1+K_L{C}_e}$(5) [46], and Dubinin- R (Eq. 6(6) $q_e=\hspace{0.17em}{q}_o\hspace{0.17em}{e}^{-K_{D-R}\hspace{0.17em}{\epsilon }^2}$(6) , 7(7) $\epsilon =RTln\left(1+\frac{1}{C_e}\hspace{0.17em}\right)\hspace{0.17em}$(7) , and 8(8) $E=\frac{1}{\sqrt{2K_{D-R}}}\hspace{0.17em}$(8) ) [47], were used. Tests were performed at various MG concentrations (45–200 mg/L) and different temperatures (25, 45, 65°C); adsorbent mass = 0.1 g; agitated speed = 100 rpm; contact time = 180 min.

(4) $q_e=K_F\hspace{0.17em}{C}_e^{1/n}$(4)

(5) $q_e=\frac{Q_m{K}_L{C}_e}{1+K_L{C}_e}$(5)

(6) $q_e=\hspace{0.17em}{q}_o\hspace{0.17em}{e}^{-K_{D-R}\hspace{0.17em}{\epsilon }^2}$(6)

(7) $\epsilon =RTln\left(1+\frac{1}{C_e}\hspace{0.17em}\right)\hspace{0.17em}$(7)

(8) $E=\frac{1}{\sqrt{2K_{D-R}}}\hspace{0.17em}$(8)

where q_m (mg/g) is the maximum adsorption capacity of the ARDV; C_e (mg/L) is the content of MG at equilibrium; K_L (L/mg) refers to the heat of adsorption (Langmuir constant). K_F (mg/g) and n refers to the Freundlich isotherm constant (bonding energy) and the adsorption intensity, respectively. K_D-R (mol²/kJ⁻²) is the D-R constant, ε refers to the Polanyi potential; and E represents the mean free energy (kJ mol⁻¹). The results of the isotherm models employed are listed in Table 1. Figure 5a-c exhibits the model graphs describing the isotherm data for the MG adsorption onto ARDV.

Figure 5. Nonlinear isotherms at different temperature, 25 ͦC (a), 45 ͦC (b), 65 ͦC (c) [conditions: [MG]: 45 − 200 mg/L, adsorbent dose: 0.1 g, time: 180 min, pH:6.6 and agitation: 100 rpm], and (b) Nonlinear kinetic models at 15 mg/L (d) and 30 mg/L for adsorption of MG using ARDV adsorbent (e) [conditions: [MG]: 15 and 30 mg/L, pH:6.6, adsorbent dose: 0.1 g, time: 180 min, T: 25°C and agitation: 100 rpm).

Table 1. Isotherm data for MG adsorption by ARDV adsorbent [conditions: [MG]: 45 − 200 mg/L, adsorbent dose: 0.1 g, time: 180 min, pH:6.6 and agitation: 100 rpm].

Model	MG dye
	298 K	318 K	338 K
Langmuir
qm, mg/g	81.43	91.72	99.03
KL (L/mg)	0.2836	0.2586	0.2058
R^2	0.99225	0.99563	0.99257
Freundlich
Kf, (mg/g) (L/mg)^1/n	38.23	41.69	40.15
n	6.02	5.89	5.18
R^2	0.98486	0.94375	0.93463
Dubinin-R
qs, mg/g	73.96	82.67	87.76
KD-R(mol^2 KJ^−2)	4.361	4.951	6.454
E (kJ mol^−1)	0.339	0.318	0.278
R^2	0.95134	0.96663	0.96457

Based on the high value of R², the order of isotherm models are as follows: Langmuir (R² = 0.99257)> Dubinin-R (R² = 0.96457)> Freundlich (R² = 0.93463) models at 65 ͦC, indicating that the Langmuir model exhibited the best fit to the adsorption process which suggests a monolayer adsorption of MG occurred onto the ARDV surface. The maximum monolayer coverage was 99.03 mg/g at 65°C. Similar trends have been reported by Al-Tameemi et al. (2022) and by Naushad et al. (2019) for the removal of textile dye onto Saccharomyces cerevisiae (Baker’s yeast) [48] and para-aminobenzoic acid functionalized activated carbon, respectively [49]. Table 3 shows the results of monolayer maximum adsorption of MG dye on various adsorbents, such as Saccharomyces cerevisiae (20.04 mg/g) [48], almond shell (1.1 mg/g) [50], mobil composition of matter No. 41 (18.45 mg/g) [9], bamboo(20.41 mg/g) [51], activated carbon (67.93 mg/g) [52], activated carbon (71.43 mg/g) [53], mesoporous SBA-S4 (39.4 mg/g) [54]. It was observed that the ARDV is a good adsorbent to eliminate MG dye compared with the other adsorbents, which was because of the high oxygen-containing functionalities on the ARDV surface. According to Dubinin-R model, the mean free energy of adsorption was in the range of 0.228–0.339 kJ mol⁻¹ below 8 kJ mol⁻¹ (Table 1), indicating that the adsorption of MG by ARDV occurred by physical adsorption.

Table 2. Kinetic data for MG adsorption by ARDV adsorbent [conditions: [MG]: 15 and 30 mg/L, pH:6.6, adsorbent dose: 0.1 g, time: 180 min, T: 25°C and agitation: 100 rpm).

Co (mg/L)	qe,exp (mg/g)	Pseudo-first-order			Pseudo-second-order
		qe1, cal. (mg/g)	K1 (1/min)	R^2	qe2, cal. (mg/g)	K2 (g/mg-min)	R^2
15	14.31	13.27	0.1557	0.95687	14.11	0.0182	0.99301
30	27.69	25.55	0.1892	0.92674	27.11	0.0117	0.97868

Table 3. Comparison of ARDV with other reported adsorbents.

Adsorbent	Kinetic/Isotherm models	Endo/Exothermic process	qm (mg/g)	Reference
Mobil Composition of Matter No. 41	PSO/ Temkin	Endothermic	18.45	[9]
Saccharomyces cerevisiae.	PSO/ Langmuir	Endothermic	20.04	[48]
Almond shell	PSO/ Langmuir	Endothermic	1.1	[50]
Bamboo	-/ Temkin	Room temperature	20.41	[51]
Activated carbon	PSO/ Langmuir	Endothermic	67.93	[52]
Activated carbon	PSO/ Langmuir	Exothermic	71.43	[53]
Mesoporous SBA-S4	PSO/ Langmuir	Room temperature	39.4	[54]
ARDV	PSO/ Langmuir	Endothermic	99.03	This study

3.2.2. Kinetics adsorption

To predict the mechanism of the adsorption of MG dye onto ARDV, nonlinear kinetics models namely, pseudo-first-order Eq. 9(9) $q_t=q_e\left(1-e^{-K_{1}t}\right)$(9) [55] and pseudo-second-order Eq. 10(10) $q_t=\frac{q_2^2{k}_{2}t}{1+\hspace{0.17em}{q}_e{K}_{2\hspace{0.17em}}t}$(10) [56], were applied. Tests were performed at C_o = 15 and 30 mg/L; adsorbent mass = 0.1 g; contact time = 180 min; agitated speed = 100 rpm; T = 25°C.

(9) $q_t=q_e\left(1-e^{-K_{1}t}\right)$(9)

(10) $q_t=\frac{q_2^2{k}_{2}t}{1+\hspace{0.17em}{q}_e{K}_{2\hspace{0.17em}}t}$(10)

where q_e and q_t (mg/g) represent the adsorption capacity of ARDV toward MG at equilibrium and time t (min), respectively. k₁ and k₂ represent the rate constants of the PFO and PSO, respectively. Figure 5d and e exhibits the model graphs describing the kinetic data for the MG adsorption onto ARDV and Table 2 shows the kinetic data of the adsorption process. The results revealed that the best kinetic plot using the proposed equation was of the PSO kinetic model due to the PSO having a good regression coefficient (R² > 0.99) compared with PFO (R² > 0.92), respectively. Besides, the adsorbed quantity calculated (q_e,cal = 14.11 mg/g) at 15 mg/L and (q_e,cal = 27.11 mg/g) at 30 mg/l by the PSO model is closer to the experimental value (q_e,exp = 14.31 mg/g) at 15 mg/L and (q_e,exp = 27.69 mg/g) at 30 mg/L, indicating the MG dye adsorption might be chemisorption. The existence of oxygen-containing functionalities on the ARDV surface would accelerate the kinetic process of MG molecule adsorption, resulting in a stronger adsorption ability for the MG dye. Similar findings were reported in the literature for elimination of MG by Saccharomyces cerevisiae [48], and activated carbon [52].

3.2.3. Adsorption thermodynamics

The degree of the spontaneity of the adsorption process could be determined by Gibbs free energy (ΔG°) (Eq. 11)(11) ${\mathrm{\Delta }\mathrm{G}}^{\circ }=-\mathrm{R}\mathrm{T}\mathrm{l}\mathrm{n}\mathrm{K}\mathrm{c}$(11) and the entropy change (∆S°) and enthalpy change (∆H°) can be calculated by using the van’t Hoff equation Eq. 12(12) $\mathrm{l}\mathrm{n}{\mathrm{K}}_{\mathrm{c}}=-\frac{\mathrm{\Delta }{H}^0}{RT}+\frac{\mathrm{\Delta }{S}^0}{R}$(12) [57]. Tests were performed at different MG concentrations (45–200 mg/L) and temperatures (25, 45, 65°C); adsorbent mass = 0.1 g; contact time = 180 min; agitated speed = 100 rpm.

(11) ${\mathrm{\Delta }\mathrm{G}}^{\circ }=-\mathrm{R}\mathrm{T}\mathrm{l}\mathrm{n}\mathrm{K}\mathrm{c}$(11)

(12) $\mathrm{l}\mathrm{n}{\mathrm{K}}_{\mathrm{c}}=-\frac{\mathrm{\Delta }{H}^0}{RT}+\frac{\mathrm{\Delta }{S}^0}{R}$(12)

where K_c refers to the thermodynamic equilibrium constant [58]. The values of (ΔH°) and (ΔS°) are acquired by the plot of lnK_c versus 1 /T (Figure 6). Table 4 shows the thermodynamic parameters for the adsorption process.

Figure 6. Van’t Hoff plot for the adsorption of MG by ARDV adsorbent.

Table 4. Thermodynamic parameters for MG adsorption by ARDV adsorbent [conditions: [MG]: 45 − 200 mg/L, adsorbent dose: 0.1 g, time: 180 min, pH:6.6 and agitation: 100 rpm].

Co (mg/L)	ΔH° (kJ/mol)	ΔS° (J/mol.K)	ΔG° (kJ/mol)
			298 K	318 K	338 K
75	33.81	126.22	−3.75	−5.37	−6.66
100	28.29	102.34	−2.10	−3.54	−4.40
150	13.43	45.937	−0.22	−0.83	−1.21

The negative values of ΔG° indicate the feasibility of adsorption process and its spontaneous nature. These values reduced with rise in temperatures, indicating favorable adsorption at a higher temperature. The positive values of ΔH° indicate that the adsorption process was endothermic, which was supported by the improvement of the removal efficiency of MG with an rise in temperature. The positive values of ΔS° indicate the good affinity of ARDV adsorbent toward MG and the increasing randomness during the adsorption process. Besides, the ΔH° values were in the range between 13.43 and 33.81 kJ mol⁻¹ which indicated that the adsorption process is in the physical adsorption range due to the ΔH° <40 kJ mol⁻¹.

3.3. Adsorption mechanism

To understand the mechanism of the adsorption process, it is necessary to know the structure of the adsorbent and adsorbate. MG molecule dye is a basic triphenylmethane-type dicationic dye with amine groups in its structure. On the other hand, the ARDV adsorbent is a plant material consisting of tannins, flavonoids, alkaloids, saponins, triterpenoids, steroids, volatile oil and phytosterols. These important materials have the majority of functional groups, such as COOH, -OH, C = O groups and highly aromatic rings. Based on the structure of the MG dye, ARDV and experimental results of kinetic, isotherm, thermodynamic, FTIR and EDX analysis, we can determine the adsorption mechanism of MG onto ARDV adsorbent. Based on FTIR results, the strong adsorption band of at – C = C–group was decreased in intensity and shifted from 1681 to 1600 cm⁻¹ owing to π–π interactions between MG molecules with – C = C–onto the surface of ARDV. While the band of – OH groups was increased in intensity and a slight shift from 3330 to 3335 owing to hydrogen bond formation between the – N(CH₃)₂ group of MG molecules and the – OH group in the RDV surface. The band of C = O stretch vibration of carboxylic acid was a slight increase in intensity due to electrostatic attraction between a cationic ⁺N(CH³)₃⁺ group of MG molecules with the negative charge of COOH group onto the ARDV surface. According to the results of thermodynamics, the adsorption mechanism is a physical process due to a low ΔH° values. Therefore, these results provide enough evidence to support the MG adsorption onto the ARDV surface by different mechanisms such as hydrogen bonding, electrostatic interaction and π-π interactions (Figure 7).

Figure 7. Mechanism adsorption of MG onto ARDV adsorbent.

4. Conclusion

In conclusion, activated residual Dodonaea Viscosa (ARDV) is a promising adsorption material for methyl green molecule (MG) removal from aqueous solutions. The batch method with the influence of parameters such as pH, temperature, contact time, adsorbent dose and initial MG concentration on the adsorption of MG dye were optimized. The results exhibited that the high adsorption of MG dye was found at pH = 6.6, [ARDV] = 0.1 g, time = 180 min and temperature = 65°C. Adsorption equilibrium was satisfactorily represented by the PSO and the Langmuir models, with adsorption capacity of 99 mg/g. The interaction between ARDV and MG dye occurred through physical adsorption through π–π interactions, hydrogen bonding and electrostatic attractions. The thermodynamic data confirms that the MG adsorption was endothermic and spontaneous, and their values support the physical adsorption mechanism.

Acknowledgments

We gratefully acknowledge the financial support by Albaha University (Project No. 1441/3) and are grateful to the Scientific Research Deanship.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Correction Statement

This article has been corrected with minor changes. These changes do not impact the academic content of the article.