Removal of Azo Dyes from Water Using Natural Luffa cylindrica as a Non-Conventional Adsorbent

Reducing high concentrations of pollutants such as heavy metals, pesticides, drugs, and dyes from water is an emerging necessity. We evaluated the use of Luffa cylindrica (Lc) as a natural non-conventional adsorbent to remove azo dye mixture (ADM) from water. The capacity of Lc at three different doses (2.5, 5.0, and 10.0 g/L) was evaluated using three concentrations of azo dyes (0.125, 0.250, and 0.500 g/L). The removal percent (R%), maximum adsorption capacity (Qm), isotherm and kinetics adsorption models, and pH influence were evaluated, and Fourier-transform infrared spectroscopy and scanning electron microscopy were performed. The maximum R% was 70.8% for 10.0 g L−1 Lc and 0.125 g L−1 ADM. The Qm of Lc was 161.29 mg g−1. Adsorption by Lc obeys a Langmuir isotherm and occurs through the pseudo-second-order kinetic model. Statistical analysis showed that the adsorbent dose, the azo dye concentration, and contact time significantly influenced R% and the adsorption capacity. These findings indicate that Lc could be used as a natural non-conventional adsorbent to reduce ADM in water, and it has a potential application in the pretreatment of wastewaters.


Introduction
Processing and manufacturing industries are sources of generated wastewater, which includes a mixture of pollutants, including heavy metals, pesticides, azo dyes, drugs, and oils.Wastewater can be classified depending on the type of compounds it contains.For instance, according to the World Bank, textile dyeing generates between 17% and 20% of industrial wastewater [1].Mainly, textile wastewater contains dyes and salts and largely aqueous soluble inorganic-colored complexes.Some authors have reported that wastewater from the leather industry may contain up to 200 mg L −1 of dyes [2].
Most color-based industries commonly employ, at the end of the process, a wastewater treatment to reduce the pollutant concentrations before leaving water in open sources.
Molecules 2024, 29,1954 2 of 16 However, the complexity and heterogeneity of the pollutants in wastewater need highly efficient technologies.Hence, water treatment plants are closely monitored and regulated by standard protocols to meet the required standards for water quality [3].Higher pollutant concentration disposed in water is correlated with a huge risk and hazardous impacts on the environment and human health [2,4].Consequently, water treatment is a leading factor in the accomplishment of sustainable water provision [5].
The presence of dyes in water is highly visible.Moreover, even at concentrations of <1.0 mg L −1 [6], there can be significant changes in the physicochemical properties of water.For example, dyes reduce the transmission and penetration of sunlight into the water and thus affect the biochemical oxygen demand (BOD), chemical oxygen demand (COD), and pH [7,8].Synthetic textile dyes are widely used due to their easy application and ability to give clothing brighter and more durable colors [9].
Azo dyes are the largest and the most important group of synthetic dyes [10].They are highly toxic and stable under different environmental conditions (light, heat, etc.) and resist degradation processes (aerobic digestion and oxidizing agents) [11].
Several technologies are being studied as part of wastewater treatment, including the combination of pre-treatments and primary treatments to reduce the organic load and improve the efficiency of secondary processes [12].The most acceptable approaches should be eco-friendly and non-polluting.Moreover, the removal techniques that are used should consider the types of pollutants and their concentration [13].
Adsorption is a widely accepted technique because it is easy to apply, efficient for pollutant removal, does not require supplements, and can be low cost when a natural absorbent is applied [13][14][15].The adsorbent, a highly porous material, has a large surface area and many free functional groups that provide adequate and selective adsorption [16][17][18].To ensure sustainability in wastewater treatment, for nearly two decades, natural sources have been studied as non-conventional adsorbent materials to remove or reduce pollutants in water, including dyes; they have demonstrated good adsorption capacity [19,20].For example, Acid Blue 25 was removed using waste tea residue (50.8 mg g −1 ) [21].Crystal violet and methylene blue were removed using sugarcane bagasse (107.5 and 112.9 mg g −1 , respectively) [22].Malachite green was removed by employing grape stalks (214.1 mg g −1 ) [23].Methyl red was removed using eggshell powder (1.26 mg g −1 ) [24].Cibacron blue was removed by utilizing pistachio shells (1.63 mg g −1 untreated and 4.53 mg g −1 chemically treated) [25].Crystal violet was removed using activated carbon from pomegranate peels (23.26 mg g −1 ) [26].Tartrazine was removed with activated carbon powder from cola nut shells (19.26 mg g −1 ) [27].While these reports show that non-conventional adsorbents represent emerging and eco-friendly alternatives for water decontamination, most of them focused on the removal of only one dye.The purpose of the present study was to establish the adsorption capacity of the non-conventional natural and green material Luffa cylindrica (Lc) to remove an azo dye mixture.

Azo Dye Mixture Removal
Figure 1 shows the removal of ADM, considering 2.5, 5.0, and 10.0 g L −1 Lc doses.

The Effect of pH
The effect of pH is shown in Figure 2. pH 2.0 led to the lowest ADM removal (47.3%); there was 60-70% ADM removal at pH 4.0-8.0.The highest removal (79.3%) occurred at pH 3.0.

The Effect of pH
The effect of pH is shown in Figure 2. pH 2.0 led to the lowest ADM removal (47.3%); there was 60%-70% ADM removal at pH 4.0-8.0.The highest removal (79.3%) occurred at pH 3.0.

Adsorption Capacity
The adsorption capacity for 2.5, 5.0 and 10.0 g L −1 Lc dose is showing in Figure 3a The time effect respect to adsotprion capacity for 2.5 g L −1 Lc is showing in Figure 3b.

Adsorption Capacity
The adsorption capacity for 2.5, 5.0 and 10.0 g L −1 Lc dose is showing in Figure 3a.The time effect respect to adsotprion capacity for 2.5 g L −1 Lc is showing in Figure 3b.

Adsorption Capacity
The adsorption capacity for 2.5, 5.0 and 10.0 g L −1 Lc dose is showing in Figure 3a.The time effect respect to adsotprion capacity for 2.5 g L −1 Lc is showing in Figure 3b.The relation between concentration (Ce) at the equilibrim and adsorption capacity (Qe), starting from 0.5 g L −1 ADM, are showing in Figure 4.The relation between concentration (C e ) at the equilibrim and adsorption capacity (Q e ), starting from 0.5 g L −1 ADM, are showing in Figure 4.

Adsorption Isotherms
Figure 5 shows how the adsorption isotherms fit the data.According to the correlation coefficient (R 2 ) values, the Langmuir model (Figure 5a) provided the best fit.Thus, ADM adsorbs to the Lc surface by forming a homogeneous monolayer.

Adsorption Isotherms
Figure 5 shows how the adsorption isotherms fit the data.According to the correlation coefficient (R 2 ) values, the Langmuir model (Figure 5a) provided the best fit.Thus, ADM adsorbs to the Lc surface by forming a homogeneous monolayer.

Adsorption Isotherms
Figure 5 shows how the adsorption isotherms fit the data.According to the correlation coefficient (R 2 ) values, the Langmuir model (Figure 5a) provided the best fit.Thus, ADM adsorbs to the Lc surface by forming a homogeneous monolayer.

Adsorption Kinetics
The kinetic parameters from the models evaluated using 2.5 g L −1 Lc are presented in Table 1, and Figure 6 shows the best fit for the data.Table 2 compares the results from the present study with other adsorption studies using Luffa spp. to remove dyes in water [28][29][30][31][32][33].

Kinetic Model Parameters Value R 2
Pseudo-first order k 1 (min

Study
R

FTIR Spectroscopy
FTIR spectra (Figure 7) were acquired to analyze Lc with and without ADM and ADM only.

Morphology of Lc
The analysis of FESEM micrographs of the Lc surface before and after ADM adsorption is shown in Figure 8.

FTIR Spectroscopy
FTIR spectra (Figure 7) were acquired to analyze Lc with and without ADM ADM only.

Morphology of Lc
The analysis of FESEM micrographs of the Lc surface before and after ADM ad tion is shown in Figure 8.

Morphology of Lc
The analysis of FESEM micrographs of the Lc surface before and after ADM adsorption is shown in Figure 8.

Discussion
After 24 h as contact time, the highest Lc dose (10.0 g L −1 ) had removed 70.8% of ADM azo dye (line blue, Figure 1c).Based on previous studies, other lignocellulosic adsorbents could remove approximately 90% of dyes; however, most of these studies focused on only one dye or used chemically modified adsorbents [34][35][36].
pH is a crucial parameter in the adsorption process [37,38] because it impacts the binding sites on the adsorbent and alters the electronic environment of dissolved molecules.In acidic media, the surface is positively charged, and H + ions compete with dye cations, meaning that less adsorbent interacts with the adsorbate [39,40].On the other hand, in basic media, the lignin and cellulose chains of Lc fibers become negatively charged, and thus, the azo dyes cannot interact well with the adsorbent.Analysis of variance showed that pH significantly affects ADM removal (p < 0.05).
The adsorption capacity is enhanced at the lowest Lc dose and the highest ADM concentration (Figure 3a), and it increases with respect to time (Figure 3b).The highest Q e was obtained when using 2.5 g L −1 Lc.
The adsorption of ADM along the entire surface of Lc is mediated by the formation of multiple, different, and strong physicochemical interactions like van der Waals forces [41].Similar observations have been reported when using natural adsorbents from agricultural waste such as Citrus limetta peel [42], wheat straw [43], shaddock husk [44], and cottonseed husk [45], to mention a few examples, to remove dyes in solution.In each of the aforementioned studies, the Langmuir isotherm provided the best fit for the data.
According to the R 2 values (Table 1 and Figure 6), the pseudo-second-order model shows the best fit for the data.Hence, the removal process involves chemisorption [28].In most of them, the Langmuir isotherm and the pseudo-second-order model best described the dye adsorption processes, even with other pollutants or adsorbents [14,[46][47][48].
Unmodified Lc (green line, Figure 7) shows a broad absorption band at 3328 cm −1 characteristic of O-H.A broad shorter peak at 2894 cm −1 is attributable to C-H stretching vibrations.The bands at 1370, 1317, and 1243 cm −1 represent vibrations for -CH-and -CH 2 -groups [49].The peaks at 1620 and 1424 cm −1 show conjugated double bonds.The strong peak at 1027 cm −1 is attributed to C-O-C and to C-O vibrations, bonds present in polysaccharides [50].All these signals describe a lignocellulosic material [51].The FTIR spectrum of ADM (blue line, Figure 7) shows the vibrations of the π conjugation system of C=C bending (1606 cm −1 ) and of N=N stretching (1482 cm −1 ), characteristics of amines.The sulfonate group vibrations are observed at 1396 cm −1 (the S=O double bond) and 1052 cm −1 (the S-O bond) [52].A broad signal at 3354 cm −1 is attributed to N-H bonds for amines.The FTIR spectra of ADM adsorbed to Lc (reddish-brown line, Figure 7) show a broad signal at 3332 cm −1 assigned to N-H and O-H vibrations due to dye-adsorbent interactions.The azo group (-N=N) vibrates at 1580 cm −1 , as confirmed by stretching vibrations at 1243 and 1158 cm −1 .The signals at 1405, 1316, and 1031 cm −1 indicate the stretching vibrations for the sulfonate group present in the dye [53].
FTIR signals corresponding to hydroxyl, carbonyl, and alkyl functional groups, and aromatic vibrations are generally present in cellulose, hemicellulose, and lignin [54,55].All of those vibrations are present in the Lc-ADM FTIR spectrum (reddish-brown line, Figure 7).Moreover, the peaks at 3328, 2894, and 1732-1482 cm −1 indicate that ADM involves stronger interactions with the hydroxyl and carbonyl groups of Lc and methylene hydrogens of the aromatic ring of ADM dye on the Lc surface.
Figure 8c (enclosed zone in yellow) shows the fibrous structure of Lc with many small holes (0.5-4 µm in diameter).The smooth amorphous waxy/gummy layer indicates the presence of lignin [56].Porosity increases the surface area of adsorbent material [17].Figure 8d (enclosed zone in yellow) shows the changes in the surface of Lc after ADM adsorption.It is possible to observe that the holes are occupied by the ADM molecules.The large, brilliant zones could correspond to salts [42,57].
The data of composition Lc obtained by EDXA (Table 3) are consistent with the composition of cellulosic and hemi-cellulosic materials [15].After adsorption (Figure 8d), Lc exhibited a composition of 53.4% carbon, 15.2% nitrogen, and 24.7% oxygen as the chief elements, along with trace amounts of other elements.
Thus, azo dye mixture likely adsorbs to Lc via hydrogen, electrostatic, chemical, and van der Waals interactions [54,58].In the literature, researchers have reported various adsorption mechanisms depending on the chemical composition of the adsorbent, the nature of the pollutant, and the environmental media [11,18,59].The adsorption process in solution involves water molecules forming hydrogen bonds between dyes and free functional groups on the Lc lignin-cellulose surface [60][61][62].In agreement with the results of the isotherms and the FTIR spectra of the adsorbent after ADM adsorption, it is necessary to consider that the molecules and free ions play an important role in the adsorption process.Figure 9 shows a general mechanism for the Lc-ADM interaction to understand how adsorption occurs.holes (0.5-4 µm in diameter).The smooth amorphous waxy/gummy layer indicates the presence of lignin [56].Porosity increases the surface area of adsorbent material [17].Figure 8d (enclosed zone in yellow) shows the changes in the surface of Lc after ADM adsorption.It is possible to observe that the holes are occupied by the ADM molecules.The large, brilliant zones could correspond to salts [42,57].
The data of composition Lc obtained by EDXA (Table 3) are consistent with the composition of cellulosic and hemi-cellulosic materials [15].After adsorption (Figure 8d), Lc exhibited a composition of 53.4% carbon, 15.2% nitrogen, and 24.7% oxygen as the chief elements, along with trace amounts of other elements.
Thus, azo dye mixture likely adsorbs to Lc via hydrogen, electrostatic, chemical, and van der Waals interactions [54,58].In the literature, researchers have reported various adsorption mechanisms depending on the chemical composition of the adsorbent, the nature of the pollutant, and the environmental media [11,18,59].The adsorption process in solution involves water molecules forming hydrogen bonds between dyes and free functional groups on the Lc lignin-cellulose surface [60][61][62].In agreement with the results of the isotherms and the FTIR spectra of the adsorbent after ADM adsorption, it is necessary to consider that the molecules and free ions play an important role in the adsorption process.Figure 9 shows a general mechanism for the Lc-ADM interaction to understand how adsorption occurs.

Luffa Cylindrica (Lc)
Lc (Figure 10) is a fibrous, highly porous natural material composed of lignocellulose [13].It has a large surface area with a large number of hydroxyl groups that can bind organic and inorganic molecules [63,64].Dried Lc was acquired from Buenavista de Cuellar, Guerrero, Mexico (18°27′32″ N 99°24′20″ W).It was cleaned, the peel and seeds were removed, and it was cut into small 1 cm 3 fragments, which were exhaustively washed using distilled water and dried using an oven (RIOSSA made in Mexico) at 40 °C for 24 h.These Lc fragments were used in the subsequent experiments as untreated natural fiber.

Luffa Cylindrica (Lc)
Lc (Figure 10) is a fibrous, highly porous natural material composed of lignocellulose [13].It has a large surface area with a large number of hydroxyl groups that can bind organic and inorganic molecules [63,64].Dried Lc was acquired from Buenavista de Cuellar, Guerrero, Mexico (18 • 27 ′ 32 ′′ N 99 • 24 ′ 20 ′′ W).It was cleaned, the peel and seeds were removed, and it was cut into small 1 cm 3 fragments, which were exhaustively washed using distilled water and dried using an oven (RIOSSA made in Mexico) at 40 • C for 24 h.These Lc fragments were used in the subsequent experiments as untreated natural fiber.

Azo Dye Mixture
A commercial mixture of azo dyes commonly used to renew the blue color of jeans at home was acquired from a local supermarket; the commercial mixture is commonly named 'Indigo Blue'.Its formulation contains nine azo dyes: blue 71, 86, 102, 151, 200, and 201; black 22; red 23; and yellow 50 (Figure 11) as the major ingredients.They can be classified as mono-, di-, tri-, and tetra-azo dyes, one of which is a phthalocyanine class.This commercial azo dye mixture was renamed as 'Azo dye mixture' (ADM).A stock solution of ADM (1.0 g L −1 ) was prepared and used for all experiments.The maximum absorbance of ADM was determined using an ultraviolet-visible (UV-vis) spectrophotometer; it was detected at 550 nm.

Azo Dye Mixture
A commercial mixture of azo dyes commonly used to renew the blue color of jeans at home was acquired from a local supermarket; the commercial mixture is commonly named 'Indigo Blue'.Its formulation contains nine azo dyes: blue 71, 86, 102, 151, 200, and 201; black 22; red 23; and yellow 50 (Figure 11) as the major ingredients.They can be classified as mono-, di-, tri-, and tetra-azo dyes, one of which is a phthalocyanine class.This commercial azo dye mixture was renamed as 'Azo dye mixture' (ADM).A stock solution of ADM (1.0 g L −1 ) was prepared and used for all experiments.The maximum absorbance of ADM was determined using an ultraviolet-visible (UV-vis) spectrophotometer; it was detected at 550 nm.
A commercial mixture of azo dyes commonly used to renew the blue color of jeans at home was acquired from a local supermarket; the commercial mixture is commonly named 'Indigo Blue'.Its formulation contains nine azo dyes: blue 71, 86, 102, 151, 200, and 201; black 22; red 23; and yellow 50 (Figure 11) as the major ingredients.They can be classified as mono-, di-, tri-, and tetra-azo dyes, one of which is a phthalocyanine class.This commercial azo dye mixture was renamed as 'Azo dye mixture' (ADM).A stock solution of ADM (1.0 g L −1 ) was prepared and used for all experiments.The maximum absorbance of ADM was determined using an ultraviolet-visible (UV-vis) spectrophotometer; it was detected at 550 nm.

Experimental Design
In prior studies of adsorbents, researchers have generally used azo dye concentrations <0.100 g L −1 [28][29][30]65].However, in real-world scenarios, dye concentrations are commonly >0.100 g L −1 .For this study, it was necessary to establish a factorial experimental design for batch adsorption kinetic assays considering several azo dye and Lc doses (Table 4).

Adsorption Kinetics
The experiments were carried out in conical borosilicate glass flasks with a 50 mL working volume in static conditions at 28 ± 2 • C and pH 7.0; the contact time was 24 h.The residual ADM concentration was measured at 550 nm.All experiments were performed in quadruplicates, and the results were processed using Origin Pro-8 version 2018 and Minitab version 20 software.
The removal percent (R%) of ADM was calculated with Equation (1): where C 0 is initial concentration, C f is final concentration, and R% is the removal percent.
The adsorption kinetics was evaluated to understand ADM removal.The amount of ADM adsorbed by Lc (mg g −1 ) was calculated with Equation (2): where Q is the adsorption capacity (mg g −1 ), C 0 and C f are the initial and final concentrations (mg L −1 ), respectively, V is the volume (L) used, and m is the Lc mass (g) employed [28].
Several kinetic models were analyzed with the degree of adsorption controlled as a function of time.Equation (3) was used for the pseudo-first-order model: where Q t is the ADM amount adsorbed by Lc at time t (mg g −1 ), and k 1 is the adsorption rate constant (min −1 ).The pseudo-second-order kinetic assumes that chemisorption is the rate-limiting step; it is represented in Equation ( 4) [34,42]: where t is the time (min), and k 2 is the rate constant of pseudo-second order (g min mg −1 ).Weber and Morris [66] proposed a theory based on interparticle diffusion and mentioned that adsorption occurs through a diffusion mechanism, as represented by Equation (5): where k id is a rate constant of intraparticle diffusion (mg min 1/2 g −1 ), and C is a dimensionless constant.
The Elovich model provides an explanation for adsorption of an adsorbate on the surface of an adsorbent.This model assumes that adsorbents have heterogeneous active sites with different active energies [67].The model is represented in Equation ( 6): where α (mg g −1 min −1 ) is initial adsorption rate, β (g mg −1 ) is desorption constant, and t (min) is time [67,68].

Adsorption Isotherms
Adsorption equilibrium was analyzed to determine the adsorption capacity of Lc for ADM in aqueous solution.The amount of ADM adsorbed at equilibrium is expressed by Equation (7): where Q e and C e are the adsorption capacity and adsorbate concentration at equilibrium, respectively.The type of adsorbent-adsorbate interaction, as well as the experimental conditions, is vital to determine which isotherm best fits the data.The equilibrium data were analyzed by considering the linearized Langmuir, Freundlich, and Temkin isotherms.The Langmuir isotherm has been commonly used to discuss various adsorbate-adsorbent combinations (liquid and gas phase adsorption) [29,30,65,69] and is represented by Equation ( 8): where Q e (mg g −1 ) is the adsorption capacity of the material at equilibrium; Q m (mg g −1 ) is the maximum adsorption capacity to form a complete monolayer on the surface at the adsorbate concentration at equilibrium C e (mg L −1 ); and K L (L mg −1 ) is the Langmuir constant related to the affinity between the adsorbate and adsorbent [26].
The essential characteristics of the Langmuir isotherm model are known as the separation factor RL, which can be expressed by Equation ( 9): The Freundlich isotherm is represented by Equation ( 10): logQ e = 1 n logC e + logK F (10) where K F is the Freundlich constant [(mg g −1 ) (L mg −1 ) 1/n ], and n is a dimensionless constant that is related to the absorption intensity of the adsorbent-adsorbate relationship, sometimes called the adsorption intensity or surface heterogeneity [26,70].The Temkin isotherm is presented in Equation ( 11): Q e = B * LnC e + B * LnK T (11) where B (J mol −1 ) is the Temkin constant, obtained from RT/b, where R is the universal gas constant (8.314J mol −1 ), T is the temperature in Kelvin (298 • K), b is the variation in the absorption energy, and K T is the Temkin equilibrium constant (L mg −1 ) [70].

Influence of pH on ADM Removal
The pH effect of ADM removal was evaluated.A solution of 0.500 g L −1 ADM was prepared using 0.01 M KNO 3 as a buffer [28,30].Then, the pH was adjusted between 2 and 12 by adding 0.1 M NaOH or HCl.Lc (2.5 g L −1 ) was added to each flask and incubated for 24 h.Samples were taken at 0 and 24 h and analyzed to calculate the percent ADM removal.

Fourier-Transform Infrared (FTIR) Spectroscopy
FTIR spectra were obtained with a Spectrum One FTIR spectrophotometer (Perkin Elmer, Wellesley, MA, USA).The spectra included 128 scans from 500 to 4000 cm −1 at a resolution of 4 cm −1 .

Field Emission Scanning Electron Microscopy (FESEM)
The internal three-dimensional (3D) morphology and the surface of Lc before and after dye adsorption were obtained with FESEM coupled to energy-dispersive X-ray spectroscopy using a Hitachi SU5000 microscope manufactured in Tokio, Japan.The fiber samples were sprayed with gold coating, and the analysis of samples was acquired by topographic contrast and registered at 100 and 500 magnification.

Conclusions
Lc could remove 70.8% of ADM dissolved in water in static conditions at 25 ± 2 • C, pH 7.0, and a contact time of 24 h.The adsorption capacity of Lc was 161.29 mg g −1 ; adsorption occurs according to the Langmuir isotherm, and the kinetics obeys the pseudosecond-order model.The statistical analysis showed that the adsorbent dose, the azo dye mixture concentration, pH, and contact time significantly influence azo dye mixture removal and the adsorption capacity.Lc is an eco-friendly and sustainable adsorbent.It acts as a non-conventional material to reduce azo dyes dissolved in water.It could be applied to coupled treatments to ensure pollutant concentrations are sufficiently reduced and that wastewater treatment performs efficiently.

Figure 3 .
Figure 3. (a) Effect of the adsorbent dose and (b) effect of contact time on Lc adsorptive capacity (Q).

Figure 3 .
Figure 3. (a) Effect of the adsorbent dose and (b) effect of contact time on Lc adsorptive capacity (Q).

Figure 4 .
Figure 4. Concentration (C e ) versus adsorptive capacity at the equilibrium (Q e ).

Figure 7 .
Figure 7. FTIR spectra of non-conventional and untreated Lc (pink line), ADM (reddish-brow and ADM adsorbed to Lc (blue-green line).

Figure 9 .
Figure 9. Mechanism of adsorption proposal to Lc-ADM in aqueous solution.

Figure 9 .
Figure 9. Mechanism of adsorption proposal to Lc-ADM in aqueous solution.

Figure 10 .
Figure 10.Luffa cylindrica; longitudinal and cross-sectional views of its structure.

Figure 10 .
Figure 10.Luffa cylindrica; longitudinal and cross-sectional views of its structure.

Table 2 .
Adsorption studies with Luffa spp. as adsorbent for dye removal.

Table 2 .
Adsorption studies with Luffa spp. as adsorbent for dye removal.

Table 3 .
Principal elements identified on the biomass surface by FESEM/EDXA.
N.D.No detected