Utilizing Novel Lignocellulosic Material from Hart’s-Tongue Fern (Asplenium scolopendrium) Leaves for Crystal Violet Adsorption: Characterization, Application, and Optimization

In this work, a new lignocellulosic adsorbent was obtained and tested for crystal violet dye removal from water. The material was obtained from hart’s-tongue fern (Asplenium scolopendrium) leaves after minimal processing, without chemical or thermal treatment. The surface of the material was characterized using a variety of techniques, including FTIR, SEM, and color analysis. The effect of various factors on the adsorption capacity was then investigated and discussed. The kinetic and equilibrium studies showed that the general-order kinetic model and the Sips isotherm are the most suitable to describe the adsorption process. The equilibrium time was reached after 20 min and the maximum calculated value of the adsorption capacity was 224.2 (mg g−1). The determined values for the thermodynamic parameters indicated physical adsorption as the main mechanism involved in the process. The Taguchi method was used to optimize the adsorption conditions and identify the most influential controllable factor, which was pH. ANOVA (general linear model) was used to calculate the percentage contribution of each controllable factor to dye removal efficiency. Analysis of all the results shows that hart’s-tongue fern (Asplenium scolopendrium) leaves are a very inexpensive, readily available, and effective adsorbent for removing crystal violet dye from aqueous solutions.


Introduction
In contemporary times, the utilization of chemical dyes has witnessed a notable surge across a spectrum of industries including textiles, paper, plastics, printing, tanning, pigments, food, and pharmaceuticals.This proliferation, while catering to diverse industrial needs, has concurrently raised substantial concerns regarding its ecological ramifications presenting serious risks to the environment due to the inevitable leakage of effluents containing dangerous substances for soil and natural waters [1][2][3][4].
In addition to their well-documented harmful effects on health, the presence of dyes in aquatic environments leads to water discoloration and reduces oxygen levels, causing significant harm to aquatic life.This is especially concerning as most dyes used in industries are synthetic and contain aromatic rings, making their degradation in the environment a slow process.Moreover, the degradation of these synthetic dyes can produce secondary toxic substances that further damage the environment.Consequently, the effective removal of these hazardous dyes from industrial wastewater is an urgent issue, leading to the protection of aquatic ecosystems and also carries economic benefits [1][2][3][4][5][6][7].
Crystal violet, a versatile dye used in a wide range of industries, including textiles, printing, and pharmaceuticals, is also used in human and veterinary medicine [8,9].However, crystal violet is a toxic and non-biodegradable dye, and its presence in residual Polymers 2023, 15, 3923 3 of 17 powder.SEM analysis was carried out with a Quanta FEG 250 microscope (FEI, Eindhoven, The Netherlands) at 800× magnification and the color analysis with a Cary-Varian 300 Bio UV-VIS colorimeter (Varian Inc., Mulgrave, Australia) under D65 (natural light) illumination and with 10 observer angles.The point of zero charge (pH PZC ) was calculated according to the solid addition method [27] using a WTW Ino-lab pH-meter (model 7310, Xylem Analytics Germany, Weilheim, Germany).The granulometric analysis of the powder adsorbent was carried out according to standard ASTM D6913-04(2009)e1 [28].
All adsorption experiments were carried out in batch mode using three independent replicates, using a volume of dye solution of 50 mL.The values of the main parameters that affect the process varied as follows: pH = 2-12, stirring time = 1-60 min, initial dye concentration = 25-700 (mg L −1 ), adsorbent dose = 1-6 (g L −1 ), temperature = 285-319 K, and ionic strength = 0-0.25 (mol L −1 ).Dilute solutions of HCl 0.1 (mol dm −3 ) and NaOH 0.1 (mol dm −3 ) were used for pH adjustment and the ionic strength was modified by adding solid NaCl.The crystal violet concentration was measured with a Specord 200 PLUS UV-VIS spectrophotometer (Analytik Jena, Jena, Germany) at a wavelength of 590 nm.
In all the experimental determinations carried out, a control sample was also used in which the adsorbent material was not introduced.The absorbance and concentration of the control sample remained practically constant throughout the determinations, which proves that there are no degradation processes of the dye in the solution.
Equilibrium and kinetics of adsorption were analyzed by modeling the experimental data using several adsorption isotherms and kinetic models.In the Supplementary Materials, Table S1, these isotherms and models along with their corresponding non-linear equations are presented [29,30].In order to establish which isotherm and kinetic model is the most appropriate to characterize the dye adsorption process on adsorbent material obtained from hart's-tongue fern, the values for determination coefficient (R 2 ), sum of square error (SSE), chi-square (χ 2 ), and average relative error (ARE) were calculated.The calculation equations of these parameters are detailed in the Supplementary Materials, Table S2 [30].
The main adsorption mechanism was identified based on the thermodynamic parameters, calculated with the equations described in Supplementary Materials, Table S3 [29].
The Taguchi method, a powerful experimental design technique, was used to optimize the critical factors influencing the adsorption process, maximizing the removal efficiency of the dye.For this purpose, the L27 orthogonal array with six factors at three levels was used and the signal-to-noise ratio (S/N) was analyzed to assess the experimental results.
ANOVA (general linear model) analysis was used to evaluate the Taguchi method results and to calculate the percentage contribution of each controllable factor on the crystal violet removal efficiency.The required mathematical calculations were conducted with the Minitab 19 Software (version 19.1.1,Minitab LLC, State College, PA, USA).
In the desorption study, the dye-loaded adsorbent was stirred continuously for two hours with three different desorbing agents: distilled water, HCl (0.1 M), and NaOH (0.1 M) and then the amount of the dye released into the solution was measured.The equation used to calculate the desorption efficiency is shown in the Supplementary Materials, Table S4.
Polymers 2023, 15, 3923 5 of 17 values higher than pHPZC, the surface is negatively charged, favoring the adsorption of cationic dyes, while at lower values, the effect is the opposite [19,47].The value determined for the adsorbent obtained from hart's-tongue fern leaves was 7.4 (Figure 2), being comparable to the values determined in other studies for similar adsorbents such as 6.98 for weeping willow (Salix babylonica) leaves [47], 7.1 for chicory (Cichorium intybus) leaves [48], 7.5 for gulmohar (Delonix regia) leaves [49], and 7.7 boxwood (Buxus sempervirens) leaves [50].Figure S1, in the Supplementary Materials, shows the granulometric distribution of the hart's-tongue fern leaves powder.As the dry leaves are brittle and easy to grind, the granulometric distribution of the powder is very narrow after grinding with an average particle size of 0.056 mm.
Scanning electron microscopy (SEM) images of the adsorbent surface before and after dye adsorption are shown in Figure 3. Initially, the surface is heterogeneous, with many irregularities and different pores, which may provide a large number of adsorption sites (Figure 3A).After adsorption, the surface is more compact and uniform because the ad- Figure S1, in the Supplementary Materials, shows the granulometric distribution of the hart's-tongue fern leaves powder.As the dry leaves are brittle and easy to grind, the granulometric distribution of the powder is very narrow after grinding with an average particle size of 0.056 mm.Scanning electron microscopy (SEM) images of the adsorbent surface before and after dye adsorption are shown in Figure 3. Initially, the surface is heterogeneous, with many irregularities and different pores, which may provide a large number of adsorption sites (Figure 3A).After adsorption, the surface is more compact and uniform because the adsorbed dye molecules fill and cover the pores and irregularities (Figure 3B). Figure S1, in the Supplementary Materials, shows the granulometric distribution of the hart's-tongue fern leaves powder.As the dry leaves are brittle and easy to grind, the granulometric distribution of the powder is very narrow after grinding with an average particle size of 0.056 mm.
Scanning electron microscopy (SEM) images of the adsorbent surface before and after dye adsorption are shown in Figure 3. Initially, the surface is heterogeneous, with many irregularities and different pores, which may provide a large number of adsorption sites (Figure 3A).After adsorption, the surface is more compact and uniform because the adsorbed dye molecules fill and cover the pores and irregularities (Figure 3B).

The Influence of pH, Ionic Strength, and Adsorbent Dose on Adsorption Capacity
The influence of pH and ionic strength on adsorption capacity at different adsorbent doses is illustrated in Figure 5.As expected, at pH values higher than pHPZC, the adsorption capacity had the highest values, highlighting the positive effect of the electrostatic attraction between the adsorbate surface and the cationic dye [51,52].Increasing the ad-

The Influence of pH, Ionic Strength, and Adsorbent Dose on Adsorption Capacity
The influence of pH and ionic strength on adsorption capacity at different adsorbent doses is illustrated in Figure 5.As expected, at pH values higher than pH PZC , the adsorption capacity had the highest values, highlighting the positive effect of the electrostatic attraction between the adsorbate surface and the cationic dye [51,52].Increasing the adsorbent dose increases the number of available adsorption sites, but a significant portion of these sites remained unsaturated.This fact, along with the agglomeration of the adsorbent particles that can appear when the dose increases, lead to a decrease of the adsorption capacity [46,53,54].Similar effects of pH and adsorbent dose were reported in other studies where the same type of adsorbent materials were used to retain crystal violet dye [51,[55][56][57].The increase in ionic strength results in a negligible decrease in adsorption capacity, indicating the adsorbent material's affinity for the cationic dye.

Equilibrum Isotherms
The adsorption equilibrium was examined by modeling the experimental data using the isotherm models described in Table 1, which lists the isotherms' constants and the corresponding error parameter.The information in the table indicates that the Sips isotherm best describes the process, this isotherm having the highest value for R 2 and the lowest values for SSE, χ 2 and ARE.
Figure 6 depicts the fitted Sips isotherm curves at various temperatures.According to the figure, the adsorption capacity increases with temperature.The solutions' viscosity decreases as the temperature rises, therefore the mobility of the dye molecules increases.This has a beneficial impact on the adsorption capacity with the process being endothermic [58][59][60].While increasing the ionic strength of the solution may introduce competition between crystal violet cations and sodium ions for the occupation of adsorption sites on the adsorbate surface, this does not significantly affect the retention of the dye.This suggests that the adsorption process is not solely driven by electrostatic attraction.This is further supported by the observation that the adsorption capacity does not increase significantly at pH values above the point of zero charge (pH PZC ).
These results indicate that the proposed adsorbent material is suitable for practical applications, such as the removal of cationic dyes from wastewaters containing other salts together with the considered dye.

Equilibrum Isotherms
The adsorption equilibrium was examined by modeling the experimental data using the isotherm models described in Table 1, which lists the isotherms' constants and the corresponding error parameter.The information in the table indicates that the Sips isotherm best describes the process, this isotherm having the highest value for R 2 and the lowest values for SSE, χ 2 and ARE.
Figure 6 depicts the fitted Sips isotherm curves at various temperatures.According to the figure, the adsorption capacity increases with temperature.The solutions' viscosity decreases as the temperature rises, therefore the mobility of the dye molecules increases.This has a beneficial impact on the adsorption capacity with the process being endothermic [58][59][60].
the isotherm models described in Table 1, which lists the isotherms' constants and the corresponding error parameter.The information in the table indicates that the Sips isotherm best describes the process, this isotherm having the highest value for R 2 and the lowest values for SSE, χ 2 and ARE.
Figure 6 depicts the fitted Sips isotherm curves at various temperatures.According to the figure, the adsorption capacity increases with temperature.The solutions' viscosity decreases as the temperature rises, therefore the mobility of the dye molecules increases.This has a beneficial impact on the adsorption capacity with the process being endothermic [58][59][60].The maximum absorption capacities of various similar adsorbents obtained from plant leaves and used for crystal violet adsorption are compared in Table 2.The data indicate that the adsorbent obtained from hart's-tongue fern (Asplenium scolopendrium) leaves has an adsorption capacity higher than other adsorbents, demonstrating the usefulness of the new adsorbent proposed in this work.

Kinetic Study
The effect of contact time on the ability of the adsorbent material to retain the dye at different initial dye concentrations is illustrated in Figure 7.The adsorption capacity increases quickly in the first minutes, due to a large number of adsorption sites accessible for the dye retention, and then more slowly as it approaches equilibrium.This is reached after 20 min when it is assumed that the adsorbent surface is almost completely coated by dye molecules [55,60].The obtained equilibrium time is lower than those reported in the scientific literature for other adsorbents based on plant leaves (Table 3).Five kinetic models were tested to model the experimental results.Table 4 presents these models together with their constants and the corresponding error parameter.The highest value for R 2 and the lowest values for SSE, χ 2 , and ARE indicate that the generalorder model is the proper model to describe the crystal violet adsorption.Figure 7 shows the fitted curves of this model at various initial dye concentrations.In the scientific literature, it is mentioned that the general-order kinetic model characterizes the adsorption of the crystal violet dye on similar adsorbent materials such as motherwort biomass [67] and sour cherry leaf [68].Increasing the initial concentration of the dye has a positive effect on the adsorption capacity due to the increase in the concentration gradient and the number of collisions between the dye molecules and the adsorbent material particles [8,46,51,53,55].
Five kinetic models were tested to model the experimental results.Table 4 presents these models together with their constants and the corresponding error parameter.The highest value for R 2 and the lowest values for SSE, χ 2 , and ARE indicate that the generalorder model is the proper model to describe the crystal violet adsorption.Figure 7 shows the fitted curves of this model at various initial dye concentrations.In the scientific literature, it is mentioned that the general-order kinetic model characterizes the adsorption of the crystal violet dye on similar adsorbent materials such as motherwort biomass [67] and sour cherry leaf [68].

Thermodynamic Study
The thermodynamic parameters listed in Table 5 were calculated from the slope and intercept of the plot of ln K L versus 1/T, which is shown in Figure S2 of the Supplementary Materials, based on experimental data collected at three different temperatures: 282, 293, and 307 K.The standard Gibbs free energy change (∆G 0 ) is negative and varies with increasing temperature, indicating that the adsorption process is spontaneous and favorable.The standard enthalpy change (∆H 0 ) and standard entropy change (∆S 0 ) are both positive, indicating that the adsorption process is endothermic and increases randomness at the solid-liquid interface [55,58].The value of ∆H 0 lower than 20 (kJ mol −1 ) indicates that the main mechanism is physical adsorption, with van der Waals interaction implied in the process [69,70].The standard Gibbs free energy change (∆G 0 ) of the adsorption process is between −80 and −20 (kJ mol −1 ), but closer to −20 (kJ mol −1 ).This suggests that there is a small chemical effect that may enhance the adsorption [43,71].

Optimization Using the Taguchi Method
The Taguchi method was used to determine the optimal adsorption conditions, which were based on an L27 orthogonal array experimental design.The six controllable factors that formed the basis of this array, together with their levels, are shown in Table 6.Table 6.Controllable factors and their levels used to realize the L27 orthogonal array experimental design.The Taguchi method is a powerful way to design experiments.It focuses on finding the signal-to-noise ratio (S/N), which is a measure of how accurate and reliable the results are.The S/N ratio is calculated by comparing the response to the noise, which is any factor that can affect the accuracy of the results.

Factor
The Taguchi method has two main advantages: it minimizes the number of experiments needed and it provides a visual representation of the best conditions.
In this study, the Taguchi method was used to improve the efficiency of dye removal.The "larger is the better" option for the S/N ratio was used [72][73][74].Table 7 details the L27 orthogonal array experimental design, the results of the experiments, and the S/N ratio for each experiment.
Table 8 shows the signal-to-noise (S/N) ratios for each factor at each level and their significant ranks.These ratios indicate how much each factor affects the effectiveness of dye removal.The higher the S/N ratio, the greater the impact of the factor.Based on the S/N ratios and significant ranks, pH has the most impact on dye removal efficiency, while temperature has the least.The Taguchi approach leads to the following optimal adsorption conditions: pH of 12, contact time of 60 min, adsorbent dose of 6 (g L −1 ), initial dye concentration of 200 (mg L −1 ), temperature of 319 K, and ionic strength of 0.0 (mol L −1 ).Table 8 also shows the ANOVA analysis results and the contribution percent of each controllable factor on crystal violet removal efficiency.Their value indicates the same hierarchy of influence of the controllable factors as the Taguchi technique.By correlating the experimental dye removal efficiency values to those predicted by optimization, the validity of the Taguchi experimental design was confirmed (Figure 8).The value of determination coefficient R 2 obtained for linear regression demonstrates a high degree of accuracy of the Taguchi approach.

Desorption Study
The desorption of crystal violet dye from the absorbent material was inefficient, regardless of the desorbing agent used, suggesting that it is not practical to reuse it.The desorption efficiencies were 7.83%, 29.38%, and 17.35% for distilled water, HCl, and NaOH, respectively.However, the low cost and abundance of hart's-tongue fern leaves offset this drawback.Additionally, the absorbent material can be incinerated to generate energy, which is a simple and efficient way to reuse it.

Conclusions
Within this study, a novel lignocellulosic adsorbent was proposed to remove crystal violet dye from water.The source material for this adsorbent was derived from the leaves of the hart's-tongue fern (Asplenium scolopendrium), having undergone a procedure of minimal processing that deliberately avoided both chemical and thermal treatments.
FTIR analysis identified different functional groups specific to the main constituents of the adsorbent (cellulose, hemicellulose, and lignin) that can interact with the crystal violet dye.SEM and color analysis, before and after adsorption, revealed changes in the morphology and color of the adsorbent, confirming the retention of the dye on its surface.The augmentation of specific parameters, such as pH, contact time, initial dye concentration, and temperature, exerts a favorable impact on the enhancement of the adsorption capacity value.The increase of the ionic strength resulted in a nearly negligible reduction in the adsorption capacity, underscoring the inherent affinity of the adsorbent towards crystal violet dye.Sips isotherm was the most suitable to characterize the process compared to other isotherms tested: Langmuir, Freundlich, Temkin, Sips, and Redlich-Peterson.The adsorbent exhibits an adsorption capacity (224.2 mg g −1 ) surpassing that of com-

Desorption Study
The desorption of crystal violet dye from the absorbent material was inefficient, regardless of the desorbing agent used, suggesting that it is not practical to reuse it.The desorption efficiencies were 7.83%, 29.38%, and 17.35% for distilled water, HCl, and NaOH, respectively.However, the low cost and abundance of hart's-tongue fern leaves offset this drawback.Additionally, the absorbent material can be incinerated to generate energy, which is a simple and efficient way to reuse it.

Conclusions
Within this study, a novel lignocellulosic adsorbent was proposed to remove crystal violet dye from water.The source material for this adsorbent was derived from the leaves of the hart's-tongue fern (Asplenium scolopendrium), having undergone a procedure of minimal processing that deliberately avoided both chemical and thermal treatments.
FTIR analysis identified different functional groups specific to the main constituents of the adsorbent (cellulose, hemicellulose, and lignin) that can interact with the crystal violet dye.SEM and color analysis, before and after adsorption, revealed changes in the morphology and color of the adsorbent, confirming the retention of the dye on its surface.The augmentation of specific parameters, such as pH, contact time, initial dye concentration, and temperature, exerts a favorable impact on the enhancement of the adsorption capacity value.The increase of the ionic strength resulted in a nearly negligible reduction in the adsorption capacity, underscoring the inherent affinity of the adsorbent towards crystal violet dye.Sips isotherm was the most suitable to characterize the process compared to other isotherms tested: Langmuir, Freundlich, Temkin, Sips, and Redlich-Peterson.The adsorbent exhibits an adsorption capacity (224.2 mg g −1 ) surpassing that of comparable adsorbents, suggesting the efficacy and practical value of the new proposed adsorbent.Equilibrium is reached after 20 min and the general-order model is the most proper model to describe the crystal violet adsorption.The thermodynamic parameters indicate a spontaneous and favorable process, the main mechanism being physical adsorption, with van der Waals interaction involved, along with a small chemical effect that may enhance adsorption.The Taguchi method and ANOVA analysis were used to determine the best conditions for adsorption (using an L27 orthogonal array experimental design) and the relative importance of each controllable factor on the removal efficiency of crystal violet, respectively.pH had the greatest impact on dye removal efficiency (75.84%), while temperature had the least (0.22%).The Taguchi method had good accuracy, with a good match between the experimental dye removal efficiency values and those predicted by the optimization.
The comprehensive assessment of the acquired data suggests that the hart's-tongue fern (Asplenium scolopendrium) leaves serve as a cost-effective, readily accessible, and efficient adsorbent for eliminating crystal violet dye from aqueous solutions.

Supplementary Materials:
The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/polym15193923/s1,Table S1: The non-linear equations of the adsorption isotherms and kinetic models used to assess the adsorption process; Table S2: The calculation equations for error parameters R 2 , χ 2 , SSE, and ARE; Table S3: The equations of specific thermodynamic parameters; Table S4: The equation used to calculate the desorption efficiency; Figure S1

Figure 2 .
Figure 2. Determination of point of zero charge (pHPZC) for the studied adsorbent using the solid addition method.

Figure 2 .
Figure 2. Determination of point of zero charge (pH PZC ) for the studied adsorbent using the solid addition method.

Figure 2 .
Figure 2. Determination of point of zero charge (pHPZC) for the studied adsorbent using the solid addition method.

Figure 4
Figure 4 shows the CIELab* color analysis results for the studied adsorbent before and after dye adsorption.The analysis reveals changes in the L*, a*, and b* parameters after the adsorption of crystal violet dye.Point (1) represents the color of the cationic dye, and points (2) and (3) represent the color of the adsorbent before and after adsorption,

Figure 4
Figure 4 shows the CIELab* color analysis results for the studied adsorbent before and after dye adsorption.The analysis reveals changes in the L*, a*, and b* parameters after the adsorption of crystal violet dye.Point (1) represents the color of the cationic dye, and points (2) and (3) represent the color of the adsorbent before and after adsorption, respectively.The point representing the adsorbent color moves towards the color quadrant of the crystal violet dye after adsorption, indicating that the adsorbent color is modified by the dye adsorption.Polymers 2023, 15, x FOR PEER REVIEW 7 of 18

Polymers 2023 , 18 Figure 8 .
Figure 8.The relationship between the dye removal efficiency values measured in the experiment and those predicted by the Taguchi method.

Figure 8 .
Figure 8.The relationship between the dye removal efficiency values measured in the experiment and those predicted by the Taguchi method.
: The particle size distribution of hart's-tongue fern (Asplenium scolopendrium) leaves powder; Figure S2: Plot of ln K L vs. 1/T for the dye adsorption onto hart's-tongue fern (Asplenium scolopendrium) leaves powder.Author Contributions: Conceptualization, G.M., M.D. and S.B.; methodology, G.M. and M.D.; software, G.M. and C.V.; validation, G.M. and M.D.; formal analysis, G.M., C.V. and S.B.; investigation, G.M., S.P., M.D. and S.B.; resources, G.M. and M.D.; data curation, G.M. and S.P.; writing-original draft preparation, G.M., C.V. and S.B.; writing-review and editing, G.M., M.D. and S.B.; visualization, G.M.; supervision, G.M. All authors have read and agreed to the published version of the manuscript.Funding: This work was supported by a grant of the Romanian Ministry of Research, Innovation and Digitalization, project number PFE 26/30.12.2021,PERFORM-CDI@UPT100-The increasing of the performance of the Polytechnic University of Timisoara by strengthening the research, development and technological transfer capacity in the field of "Energy, Environment and Climate Change" at the beginning of the second century of its existence, within Program 1-Development of the national system of Research and Development, Subprogram 1.2-Institutional PerformanceInstitutional Development Projects-Excellence Funding Projects in RDI, PNCDI III.Institutional Review Board Statement: Not applicable.

Table 1 .
The constants and the corresponding error parameters for the tested adsorption isotherms.

Table 2 .
The maximum absorption capacities of various similar adsorbents obtained from plant leaves and used for crystal violet adsorption.

Table 3 .
The equilibrium times reported in the scientific literature for various adsorbents based on plant leaves and used for crystal violet adsorption.

Table 3 .
The equilibrium times reported in the scientific literature for various adsorbents based on plant leaves and used for crystal violet adsorption.

Table 4 .
The constants and the corresponding error parameters for the tested kinetic models.

Table 5 .
The thermodynamic parameters for the dye adsorption on the adsorbent obtained from hart's-tongue fern (Asplenium scolopendrium) leaves.

Table 7 .
The L27 orthogonal array experimental design, the experimental value obtained for dye removal efficiency, and corresponding S/N ratios after each run.

Table 8 .
Response table for signal-to-noise S/N ratios (larger is better) and the ANOVA results.