Efficient Removal of Methyl Red Dye by Using Bark of Hopbush

isotherm appeared to be the best ﬁt for describing the adsorption of methyl red onto D. viscosa plant bark.


Introduction
All living things need water to survive. Animals and birds, as well as humans, depend on clean water for survival. The water bodies become contaminated over time. When waste is dumped into water directly or when noxious chemicals and microorganisms enter water bodies, water pollution occurs. Water-related illnesses claim the lives of more than 5 million people annually, which is approximately 10 times as many as are killed in wars. Two-thirds of the world's population is predicted to reside in nations with moderate to severe water shortages by 2025 [1]. Due to water pollution brought on by various human activities, particularly industrial pollutants, just 0.01 percent of the 3% of pure water on Earth may be utilized for human consumption [2]. The most significant cause of water pollution (from several chemical kinds, the others of which have been excluded from this study), from the perspective of wastewater treatment, is dyes.
Dyes are colored organic compounds, mainly used in the textile, dyeing, tannery, paper, pulp, and paint industries. Their effluents include various dyes, similar to those from dye-manufacturing facilities, which cause numerous ecological issues [3]. Azo dyes are the most prevalent of these dyes [4]. If these dyes are ingested through water, they can result in neurochemical issues, allergies, infections of the eyes or skin, or irritations [4,5]. For the removal of dyes, a variety of treatment techniques are used, including ion exchange, biological treatment, photocatalytic degradation, coagulation, etc. [6]. However, each of them has its own limitations. Ion exchange, for instance, is non-selective, sensitive to pH, and unable to handle highly-concentrated wastewater, while biological treatment takes a very long time to complete the decolorization and fermentation processes, needs a lot of space, and is less adaptable [7]. Unlike coagulation-which uses coagulants that are typically non-biodegradable and monomers that have neurotoxic and cancerous consequences-photocatalytic degradation requires an effective photocatalyst and controlled circumstances, making the procedure very expensive. Adsorption is one of the most efficient, cost-effective, and well-known processes for treating wastewater; as a result, it is frequently used to remove dyes from wastewater [6].
Organic substances with the functional group RN=NR , in which R and R are typically aryl, are known as azo dyes. They are a family of commercially-significant azo compounds, or substances with the bond C-N=N-C. Methyl red (MR) is a mono-azo dye with a (-N=N-) linkage, which is used in the production of textiles and other commercial goods [8]. If ingested or inhaled, it might irritate the pharynx or digestive tract and induce skin and ocular sensitivities [9,10]. Additionally, MR undergoes bioconversion into 2-aminobenzoic acid and N-N-dimethyl-p-phenylene diamine in environments where oxygen is present, because it is mutagenic [11,12]. Many scientists, including Rajoriya et al. [13], Xiang et al. [14], Dawadi et al. [15], Santhi et al. [16] and Mahmoud et al. [17], have tried to discover a means by which to remove MR dye from water before releasing it into recipient water bodies. Numerous bio-materials have been described as adsorbents in the literature, including plant leaves [18][19][20], chitosan and chitin [20,21], crab shells [22], nutshells [22,23], fruit pods [24,25], egg shells [26,27], fruit shells [28], agricultural wastes [29][30][31], fish scales [32][33][34], and various others [35][36][37][38][39]. Plant barks are one of the most often employed adsorbents in investigations involving the removal of contaminants from aqueous environments. Numerous investigations have been conducted in this area [40]. Eucalyptus [41,42], flamboyant pod [43], African border [44], pine [45], Sycamore [46], Azollapinnata [47], and many more plant species are among those researched. Plant barks can be used as adsorbents for water treatment because they are inexpensive and have a number of other benefits as well. Biomaterial-based adsorbents are less labor-intensive, as well as being environmentally benign, and renewable [42,48]. Plant bark has the ability to absorb wastewater contaminants on nonliving cells by extracellular and intracellular accumulation through mild forces of contact, similar to the majority of other plant-based biomaterials. They are suitable as an adsorbent for the desired adsorption process, as a result [49].
Bark from the D. viscosa plant was used as an adsorbent in this study to remove MR dye. Common names for this evergreen plant with a quick growth rate include Hopbush and Sanatha. It is both wild and ornamental, and wind and bees are among its pollinators. It thrives in environments with temperatures between 18 and 38 degrees Celsius, although it can even withstand conditions between 7 and 45 degrees Celsius [50]. The D. viscosa plant, which is often found in mountainous locations, was collected in the wild, in the Swabi district of Pakistan's Khyber Pakhtun Khwa (KPK). To maximize the rate of MR adsorption, the adsorption kinetics was studied, and the impact of pH, concentration, contact time, and ionic strength was examined. The adsorption isotherm was also determined in order to understand the adsorption process on our chosen adsorbent. The adsorbent demonstrated outstanding dye-removal efficiency. Additionally, it demonstrated strong MR dye adsorption capacity in both tap water and river water. By using the bark of the D. viscosa tree as an effective adsorbent, which can also be utilized directly for the treatment Water 2022, 14, 2831 3 of 12 of wastewater released from various industries containing MR, this work intends to protect aquatic bodies from pollution caused by dyes from industrial effluents.

Materials and Methods
Methyl Red dye, which was used as an adsorbate in this study, is commonly used as an indicator (see Supplementary Materials: Text E1 and Figures S1 and S2). The adsorbent used was the bark of the D. viscosa plant. All the solutions were prepared in distilled water. To maintain the pH of the MR dye solution, 0.5 M hydrochloric acid (Merck) and 0.5 M sodium hydroxide (KOSDAQ listed company, Korea) solutions were used. For comparative study, distilled water, tap water, and river water were used. The sample water was collected from a canal in Hamlet district, Swabi, Pakistan. Sodium chloride salt was also used to check the ionic effect of salt on the rate of adsorption.
The bark from the plant was taken and washed with distilled water five times and then completely dried in the sunlight. After that, the bark was ground into powdery form ( Figure 1). The leaves and flowers were also washed with distilled water and then dried and ground, following the same process as used for the bark of the D. viscosa plant. The D. viscosa used in this research was wild, as shown in Figure 1, wherein (a) shows the complete appearance of the plant, (b) shows the bark of the plant, and (c) shows the powdered bark used during experiments. The 500 ppm solution of MR was prepared by dissolving MR in water after shaking at 298 rpm for 40 min. To find the wavelength maximum (λ max ), the absorbance of the stock solution of adsorbate was noted in the wavelength range of 380-465 nm. According to Figure S3 (see Supplementary Materials), the wavelength; 433 nm shows maximum absorption. Therefore, further experiments were carried out at 433 nm to record a time-course graph, showing decrease in absorbance as a function of time. The batch experiment was performed by adding 100 mg of adsorbent to the 10 mL solution of MR dye under continuous stirring (298 rpm), at room temperature (25 • C), and the adsorbent was removed by centrifugation after adsorption. A series of experiments was performed to investigate the effect of the different parameters: pH; initial dye concentration; contact time; and ionic strength; on the adsorption of MR. The value of the amount adsorbed or adsorption capacity; Qe was identified by Equation (1) [51,52].
Ci is the initial concentration, Ce is the final or equilibrium concentration, V is the total volume of solution taken, and W is the amount of adsorbent used.
The percent removal of the adsorbate was determined by Equation (2) [52].
Water 2022, 14, x FOR PEER REVIEW 4 of 13

Characterizations
Various instruments were used during the study. A digital orbital shaker was used to ensure full mixing of biosorbent and dyes at 298 rpm at room temperature (a product of PCSIR, Pakistan), while centrifugation was carried out at 1000 rpm for 50 min (Centrifuge 80-2, China). The absorbance was measured by the UV-visible spectrophotometer (Model UV 3000, Hamburg, Germany).

Characterizations
Various instruments were used during the study. A digital orbital shaker was used to ensure full mixing of biosorbent and dyes at 298 rpm at room temperature (a product of PCSIR, Islamabad, Pakistan), while centrifugation was carried out at 1000 rpm for 50 min (Centrifuge 80-2, Changzhou, China). The absorbance was measured by the UV-visible spectrophotometer (Model UV 3000, Hamburg, Germany).

Effect of pH on Adsorption
The solution of 500 ppm concentration was prepared and divided into ten beakers. The total volume of each solution was 10 mL. Each solution was maintained at a different pH, at a value between 1 and 10, i.e., beaker 1: pH = 1 and beaker 10: pH = 10, by using 0.5 M HCl (for acidic pH, i.e., from pH 1-6) and 0.5 M NaOH (for basic pH). A total of 100 mg adsorbent (bark of D. viscosa plant) was added to each solution, to check the effect of pH on adsorption. The adsorption capacity was determined and plotted against the pH profile. Figure 2 shows the relationship of pH to the adsorption capacity of the adsorbent (Qe). The adsorption capacity decreased with increasing pH and with lowering the strength of protons. At pH = 1, where the adsorption capacity was high (i.e., 34 mg/g), the greatest adsorption was observed. The adsorption capacity declined markedly from pH = 2 (23 mg/g) to pH = 3 (e.g., 8 mg/g), then decreased noticeably less, until pH = 9 (e.g., from 8 mg/g to 2.5 mg/g), before remaining constant at pH = 10. The gradual decrease in the adsorption capacity of bark may be due to electrostatic attraction in the negatively charged dye and the positively charged surface of the adsorbent at pH 1-6. When the pH of the solution increases, it results in an increased number of hydroxyl groups; hence the number of positively charged sites decreases and results in less electrostatic attraction between charges of adsorbate and surface area of adsorbent [12]. Further research was conducted at pH = 1, where the adsorbent's adsorption capacity was at its highest, in order to follow the maximum adsorption rate in the shortest amount of time possible.

Characterizations
Various instruments were used during the study. A digital orbital shaker was used to ensure full mixing of biosorbent and dyes at 298 rpm at room temperature (a product of PCSIR, Pakistan), while centrifugation was carried out at 1000 rpm for 50 min (Centrifuge 80-2, China). The absorbance was measured by the UV-visible spectrophotometer (Model UV 3000, Hamburg, Germany).

Effect of pH on Adsorption
The solution of 500 ppm concentration was prepared and divided into ten beakers. The total volume of each solution was 10 mL. Each solution was maintained at a different pH, at a value between 1 and 10, i.e., beaker 1: pH = 1 and beaker 10: pH = 10, by using 0.5 M HCl (for acidic pH, i.e., from pH 1-6) and 0.5 M NaOH (for basic pH). A total of 100 mg adsorbent (bark of D. viscosa plant) was added to each solution, to check the effect of pH on adsorption. The adsorption capacity was determined and plotted against the pH profile. Figure 2 shows the relationship of pH to the adsorption capacity of the adsorbent (Qe). The adsorption capacity decreased with increasing pH and with lowering the strength of protons. At pH = 1, where the adsorption capacity was high (i.e., 34 mg/g), the greatest adsorption was observed. The adsorption capacity declined markedly from pH = 2 (23 mg/g) to pH = 3 (e.g., 8 mg/g), then decreased noticeably less, until pH = 9 (e.g., from 8 mg/g to 2.5 mg/g), before remaining constant at pH = 10. The gradual decrease in the adsorption capacity of bark may be due to electrostatic attraction in the negatively charged dye and the positively charged surface of the adsorbent at pH 1-6. When the pH of the solution increases, it results in an increased number of hydroxyl groups; hence the number of positively charged sites decreases and results in less electrostatic attraction between charges of adsorbate and surface area of adsorbent [12]. Further research was conducted at pH = 1, where the adsorbent's adsorption capacity was at its highest, in order to follow the maximum adsorption rate in the shortest amount of time possible.

Effect of Concentration of MR Dye on Adsorption
Using the standard dilution formula, solutions of MR at various concentrations (ranging from 20 ppm to 560 ppm) were prepared from a stock solution (600 ppm). HCl solution (0.5 M) kept the liquids' pH at 1. The D. viscosa bark powder (100 mg) was added to each solution. The solutions were shaken for 60 min at 298 rpm and the suspended adsorbent was separated from the solution by centrifugation. Figure 3 illustrates the relationship between the increasing MR concentrations and the adsorption capacity (Qe) of the adsorbent we chose. Figure 3 demonstrates that as the initial concentration of the adsorbate increases, the adsorption capacity also rises. This phenomenon may be a result of effective collisions between the adsorbate and the adsorbent, which enhanced the adsorption capacity. The obtained results indicate that when employing 100 mg of powdered D. viscosa bark powder, the highest adsorption occurs in a 500 ppm solution. adsorbent was separated from the solution by centrifugation. Figure 3 illustrates the relationship between the increasing MR concentrations and the adsorption capacity (Qe) of the adsorbent we chose. Figure 3 demonstrates that as the initial concentration of the adsorbate increases, the adsorption capacity also rises. This phenomenon may be a result of effective collisions between the adsorbate and the adsorbent, which enhanced the adsorption capacity. The obtained results indicate that when employing 100 mg of powdered D. viscosa bark powder, the highest adsorption occurs in a 500 ppm solution.

Effect of Contact Time on Adsorption
The experiments were conducted under optimized experimental conditions, in order to ascertain how contact duration affects the adsorption process. Eight solutions containing 500 ppm of MR dye,at pH-1, were prepared for this purpose, and 100 mg of bark powder was added to each solution. Each solution's adsorbent-adsorbate contact time ranged from 20 to 105 min. The association between contact time and amount adsorbed is shown in Figure 4. The adsorption capacity of the adsorbent was found to increase with increasing contact time, possibly as a result of the effective collisions and time for the adsorption process to be completed. The comparative analysis of Figures 2-4 helps to identify that the average adsorption capacity of the adsorbent is 35 mg/g.

Effect of Contact Time on Adsorption
The experiments were conducted under optimized experimental conditions, in order to ascertain how contact duration affects the adsorption process. Eight solutions containing 500 ppm of MR dye, at pH-1, were prepared for this purpose, and 100 mg of bark powder was added to each solution. Each solution's adsorbent-adsorbate contact time ranged from 20 to 105 min. The association between contact time and amount adsorbed is shown in Figure 4. The adsorption capacity of the adsorbent was found to increase with increasing contact time, possibly as a result of the effective collisions and time for the adsorption process to be completed. The comparative analysis of Figures 2-4 helps to identify that the average adsorption capacity of the adsorbent is 35 mg/g.

Effect of Ionic Strength on Adsorption
To determine the charge on the adsorbent, the impact of ionic strength on the adsorption capacity was observed. To explore the impact of ionic strength, the concentrations of adsorbent, adsorbate, and protons (pH) were kept constant at 100 mg, 500 ppm, and pH = 1, respectively. The concentration of NaCl was changed from 0.1 to 0.9 M. With increasing salt concentration, a reduction in adsorption capacity was seen. Figure 5 depicts the relationship between Qe and molar concentration of NaCl. The findings reveal that the adsorption capacity reduced from 39.9 mg/g to 38.3 mg/g, possibly as a result of the positive charge on the adsorbent's surface because the MR dye has a negative charge. According to the primary salt effect's kinetic phenomenon, a decrease in the rate of the reaction or process occurs when opposite charge carriers contact. This is evidence that the adsorbent's positively charged surface interacts with the negatively charged MR dye through an electrostatic interaction, favoring the high adsorption capacity in an acidic solution.

Effect of Ionic Strength on Adsorption
To determine the charge on the adsorbent, the impact of ionic strength on the adsorption capacity was observed. To explore the impact of ionic strength, the concentrations of adsorbent, adsorbate, and protons (pH) were kept constant at 100 mg, 500 ppm, and pH = 1, respectively. The concentration of NaCl was changed from 0.1 to 0.9 M. With increasing salt concentration, a reduction in adsorption capacity was seen. Figure 5 depicts the relationship between Qe and molar concentration of NaCl. The findings reveal that the adsorption capacity reduced from 39.9 mg/g to 38.3 mg/g, possibly as a result of the positive charge on the adsorbent's surface because the MR dye has a negative charge. According to the primary salt effect's kinetic phenomenon, a decrease in the rate of the reaction or process occurs when opposite charge carriers contact. This is evidence that the adsorbent's positively increasing salt concentration, a reduction in adsorption capacity was seen. Figure 5 depicts the relationship between Qe and molar concentration of NaCl. The findings reveal that the adsorption capacity reduced from 39.9 mg/g to 38.3 mg/g, possibly as a result of the positive charge on the adsorbent's surface because the MR dye has a negative charge. According to the primary salt effect's kinetic phenomenon, a decrease in the rate of the reaction or process occurs when opposite charge carriers contact. This is evidence that the adsorbent's positively charged surface interacts with the negatively charged MR dye through an electrostatic interaction, favoring the high adsorption capacity in an acidic solution.

Effect of Water obtained from Different Resources on the Adsoprption of MR Dye
While the solutions were being prepared in various aqueous media (i.e., tap water, distilled water, river water, and filtered river water), the MR dye concentration was maintained at 500 ppm, the pH of the solutions was kept at 1, and the contact time for adsorption on a shaker was maintained at 75 min. The percentage of dye removal is shown in all media, on a graph, to assess how different types of water affected equilibrium concentration after adsorption ( Figure 6). Filtered river water had the highest level

Effect of Water Obtained from Different Resources on the Adsoprption of MR Dye
While the solutions were being prepared in various aqueous media (i.e., tap water, distilled water, river water, and filtered river water), the MR dye concentration was maintained at 500 ppm, the pH of the solutions was kept at 1, and the contact time for adsorption on a shaker was maintained at 75 min. The percentage of dye removal is shown in all media, on a graph, to assess how different types of water affected equilibrium concentration after adsorption ( Figure 6). Filtered river water had the highest level of dye removal at 85.45%, which could be attributed to the best performance of the adsorbent in real water samples rather distilled water. of dye removal at 85.45%, which could be attributed to the best performance of the adsorbent in real water samples rather distilled water.

Comparison between Leaves, Flowers, and Bark Powder of D. viscosa Plant
Three MR dye solutions at 500 ppm concentration, and at pH-1, were made in preparation for the adsorption process, using three various adsorbents. The D. viscosa plant's dry and powdered bark, leaves, and flowers served as the adsorbents in this experiment. The purpose of this experiment was to evaluate the ability of other D. viscosa plant components to adsorb substances. Observation and calculations show that the bark of the D. viscosa plant removes 73% of the MR dye, while the leaves remove 60.8% of the dye and the flowers remove 53.38% of the dye (Figure 7). It was determined that the MR dye had a 36.64 mg/g adsorption capability on the bark of the D. viscosa plant. Comparatively, the adsorption capacities for leaves and flowers were determined to be 30.49 mg/g and 26.70 mg/g, respectively.

Comparison between Leaves, Flowers, and Bark Powder of D. viscosa Plant
Three MR dye solutions at 500 ppm concentration, and at pH-1, were made in preparation for the adsorption process, using three various adsorbents. The D. viscosa plant's dry and powdered bark, leaves, and flowers served as the adsorbents in this experiment. The purpose of this experiment was to evaluate the ability of other D. viscosa plant components to adsorb substances. Observation and calculations show that the bark of the D. viscosa plant removes 73% of the MR dye, while the leaves remove 60.8% of the dye and the flowers remove 53.38% of the dye (Figure 7). It was determined that the MR dye had a 36.64 mg/g adsorption capability on the bark of the D. viscosa plant. Comparatively, the adsorption capacities for leaves and flowers were determined to be 30.49 mg/g and 26.70 mg/g, respectively.
periment. The purpose of this experiment was to evaluate the ability of other D. viscosa plant components to adsorb substances. Observation and calculations show that the bark of the D. viscosa plant removes 73% of the MR dye, while the leaves remove 60.8% of the dye and the flowers remove 53.38% of the dye (Figure 7). It was determined that the MR dye had a 36.64 mg/g adsorption capability on the bark of the D. viscosa plant. Comparatively, the adsorption capacities for leaves and flowers were determined to be 30.49 mg/g and 26.70 mg/g, respectively.

Comparison of Adsorbents: Bark Powder of D. viscosa Plant, Animal Charcoal and Silica Gel
To remove MR dye, we compared our chosen adsorbent to other commercially available adsorbents, and we calculated and compared each one's adsorption capacity. The D. viscosa plant bark showed a large adsorption capacity of 36.64 mg/g, whereas animal charcoal showed 32.36 mg/g in the form of pellets and 21.59 mg/g in the form of powdered charcoal, and silica gel demonstrated 24.60 mg/g in powder form. This was

Comparison of Adsorbents: Bark Powder of D. viscosa Plant, Animal Charcoal and Silica Gel
To remove MR dye, we compared our chosen adsorbent to other commercially available adsorbents, and we calculated and compared each one's adsorption capacity. The D. viscosa plant bark showed a large adsorption capacity of 36.64 mg/g, whereas animal charcoal showed 32.36 mg/g in the form of pellets and 21.59 mg/g in the form of powdered charcoal, and silica gel demonstrated 24.60 mg/g in powder form. This was determined from a comparative analysis of the adsorption capacities of the bark of the D. viscosa plant, animal charcoal, and silica gel.

X-ray Diffraction Analysis
Before and after adsorption, the bark samples were subjected to X-ray diffraction (XRD) examination ( Figure 8). Peaks at 2θ = 15 show the presence of amorphous hemicelluloses, whereas the peak at 2θ = 22 indicates the presence of cellulose in the sample [53][54][55]. In comparison to the unloaded adsorbent, the MR dye-loaded adsorbent's XRD pattern shows somewhat different peak locations. According to the XRD analyses, the adsorbent's crystallinity has changed as a result of the adsorption process.

X-ray Diffraction Analysis
Before and after adsorption, the bark samples were subjected to X-ray diffraction (XRD) examination ( Figure 8). Peaks at 2θ = 15 show the presence of amorphous hemicelluloses, whereas the peak at 2θ = 22 indicates the presence of cellulose in the sample [53][54][55]. In comparison to the unloaded adsorbent, the MR dye-loaded adsorbent's XRD pattern shows somewhat different peak locations. According to the XRD analyses, the adsorbent's crystallinity has changed as a result of the adsorption process.

Order of Adsorpton Kinetics
To determine the adsorption kinetics, pseudo-first-order (PFO) and pseudo-second-order (PSO) kinetic models were applied to the data (Figure 9). For the adsorption of MR dye on the bark powder of Hopbush, the linear fit shows that the adsorption process follows a PSO kinetics rather than a PFO (Figure 9a,b). When compared to the pseudo-first-order reaction straight line graph, the value of R 2 for the pseudo-second-order reaction straight line graph was found to be relatively near to 1. In light of the findings, a PSO kinetics can be drawn. The pseudo-second-order kinetic model states that the adsorption rate depends on both Qt (adsorption capacity at any time point "t"), and Qe (adsorption capacity at equilibrium) [56]. The following equation

Order of Adsorpton Kinetics
To determine the adsorption kinetics, pseudo-first-order (PFO) and pseudo-secondorder (PSO) kinetic models were applied to the data (Figure 9). For the adsorption of MR dye on the bark powder of Hopbush, the linear fit shows that the adsorption process follows a PSO kinetics rather than a PFO (Figure 9a,b). When compared to the pseudo-firstorder reaction straight line graph, the value of R 2 for the pseudo-second-order reaction straight line graph was found to be relatively near to 1. In light of the findings, a PSO kinetics can be drawn. The pseudo-second-order kinetic model states that the adsorption rate depends on both Qt (adsorption capacity at any time point "t"), and Qe (adsorption capacity at equilibrium) [56]. The following equation illustrates the linear version of the pseudo-second-order adsorption kinetics: where, k 2 denotes the pseudo-second-order rate constant, with dimension g mg −1 min −1 .
A linear plot of t/Qt versus t yields Qe and k 2 (Figure 9b), from the slope and intercept of the plot, respectively.

Adsorption Isotherms
The experimental data were examined using Langmuir and Freundlich's adsorption isotherms. Table 2 displays the parameters derived from these isotherms. All adsorbent sites must be equal and the adsorbent surface must be homogenous in order for the Langmuir adsorption isotherm to exist [57]. As a result, the surface of the adsorbent develops a monolayer of adsorbate. The well-known Langmuir equation applied is as follows: Ce/Qe= Ce/Qm + 1/QmKL (4) Ce is the dye concentration at equilibrium, expressed as ppm, while Qe provides the adsorption capacity at equilibrium, which is defined as the quantity of dye adsorbed per gram of adsorbent (mg g −1 ). Qm stands for the maximum amount of dye adsorbed per gram of adsorbent (mg g −1 ). Similarly, KL stands for the Langmuir adsorption constant. The value 0.694 was the calculated coefficient of determination (R 2 ) for Langmuir adsorption isotherm, thus indicating that the adsorption behaviour of the examined system does not follow the presumptions of the Langmuir method.
The experimental data were additionally fitted with Freundlich asorption isotherm to take into consideration surface heterogeneity, surface roughness, and the presence of different kinds of adsorption sites. The Freundlich adsorption isotherm has the following linear form: log Qe = log KF + 1/n log Ce (5) The Freundlich constants KF and n are related to the capacity and intensity of adsorption, respectively. The linear fit (R 2 ) value of 0.983 shows that the adsorption

Adsorption Isotherms
The experimental data were examined using Langmuir and Freundlich's adsorption isotherms. Table 2 displays the parameters derived from these isotherms. All adsorbent sites must be equal and the adsorbent surface must be homogenous in order for the Langmuir adsorption isotherm to exist [57]. As a result, the surface of the adsorbent develops a monolayer of adsorbate. The well-known Langmuir equation applied is as follows: Ce/Qe= Ce/Qm + 1/QmK L (4) Ce is the dye concentration at equilibrium, expressed as ppm, while Qe provides the adsorption capacity at equilibrium, which is defined as the quantity of dye adsorbed per gram of adsorbent (mg g −1 ). Qm stands for the maximum amount of dye adsorbed per gram of adsorbent (mg g −1 ). Similarly, K L stands for the Langmuir adsorption constant. The value 0.694 was the calculated coefficient of determination (R 2 ) for Langmuir adsorption isotherm, thus indicating that the adsorption behaviour of the examined system does not follow the presumptions of the Langmuir method.
The experimental data were additionally fitted with Freundlich asorption isotherm to take into consideration surface heterogeneity, surface roughness, and the presence of different kinds of adsorption sites. The Freundlich adsorption isotherm has the following linear form: log Qe = log K F + 1/n log Ce (5) The Freundlich constants K F and n are related to the capacity and intensity of adsorption, respectively. The linear fit (R 2 ) value of 0.983 shows that the adsorption process obeys the Freundlich adsorption isotherm. In general, favourable Freundlich adsorption is indicated by n (adsorption intensity) being less than unity. Table 2 displays the values of these constants. According to Table 2, the MR dye adsorption on the bark powder of the D. viscosa plant is a physisorption process. It also shows that not all adsorption sites are homogenous, suggesting that multilayer adsorption may take place on the surface of the adsorbent.  Table 3 shows a comparison of D. viscosa plant bark powder's ability to adsorb MR dye with that of other inexpensive adsorbents from earlier studies [58][59][60]. It was discovered through this comparison that utilizing 100 mg of adsorbent per 10 mL of solution MR dye (500 ppm and at pH-1) during a contact time of 75 min offers outstanding results. Additionally, it is notable that this work's adsorbent dose is lower than that of earlier adsorbents and may remove a higher concentration of dye.

Conclusions
This study demonstrates the superior adsorption capacity of D. viscosa (Hopbush) plant's bark powder for MR dye removal. Without any chemical treatment, the adsorbent is cost-effectively prepared by washing, drying in sunlight and grinding. The findings showed an overall adsorption capacity of 36.64 mg/g at pH-1, and as a result, this adsorbent may be better able to treat acidic waste water, with a high concentration of dye (500 ppm), within 75 min. The bark of the D. viscosa plant has a greater capacity for adsorption than its leaves and flowers. When compared to other adsorbents, however, the adsorption capacity of the leaves and flowers, which is 30.49 mg/g and 26.70 mg/g, respectively, is still rather good. The outcomes also demonstrate that Hopbush bark powder, with a percentage MR dye removal value of 73.15%, has a greater adsorption capacity, when compared to charcoal powder, silica gel powder, and charcoal in the form of pellets, which have values of: 64.72%, 49.06%, and 43.18%, respectively. The high availability, high percentage of dye removal, ease of cultivation, and lack of need for chemical treatment make the bark of the D. viscosa plant an efficient, affordable adsorbent. Additionally, the pseudo-second-order kinetic model and the Freundlich adsorption isotherm both accurately depict the MR dye's adsorption process on the bark of the D. viscosa plant. Thus, water contamination can be reduced using this natural, inexpensive adsorbent.
Supplementary Materials: The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/w14182831/s1, Figure S1: Colour of methyl red at different pH; Figure S2: Protonation of methyl red in acidic solutions; Figure S3: Plot of absorbance as a function of wavelength to determine wavelength maximum (λ max ); Explanation E1.