Lignin Extracted from Green Coconut Waste Impregnated with Sodium Octanoate for Removal of Cu²⁺ in Aqueous Solution

Abstract

Investigating viable processes for the use of lignocellulosic biomass in clean fuels and high-value-added chemical products is essential for sustainable development. Large amounts of lignin are available every year as by-products of the paper and biorefinery industries, causing a series of problems, particularly environmental ones. Its structure and composition make lignin compatible with the concept of sustainability, since it can be used to produce new chemical products with high added value. As such, this study aims to extract lignin from green coconut fiber (LIG), with the subsequent impregnation of a sodium-octanoate-based surfactant (LIG-SUR), and determine its applicability as an adsorbent for removing copper ions from synthetic waste. To this end, the green coconut fiber lignocellulosic biomass was initially subjected to alkaline pre-treatment with 2% (w/v) sodium hydroxide in an autoclave. Next, the surface of the lignin was modified by impregnating it with sodium octanoate, synthesized from the reaction of octanoic acid and NaOH. The physical and chemical traits of the lignin were studied before and after surfactant impregnation, as well as after copper ion adsorption. The lignin was analyzed by X-ray fluorescence (XRF), Fourier transform infrared (FTIR) and scanning electron microscopy (SEM). The adsorption tests were carried out using lignin pre-treated with surfactant in a batch system, where the effects of pH and adsorbent concentration were investigated. XRF and SEM analyses confirmed surfactant impregnation, with Na₂O partially replaced by CuO after Cu²⁺ adsorption. FTIR analysis revealed shifts in O–H, C–H, C=O, and C=C bands, indicating electrostatic interactions with lignin. Adsorption kinetics followed the pseudo-second-order model, suggesting chemisorption, with equilibrium reached in approximately 10 and 60 min for LIG-SUR and LIG, respectively. The Langmuir model best described the isotherm data, indicating monolayer adsorption. LIG-SUR removed 91.57% of Cu²⁺ and reached a maximum capacity of 30.7 mg·g⁻¹ at 25 °C and a pH of 6. The results of this research showed that pre-treatment with NaOH, followed by impregnation with surfactant, significantly increased the adsorption capacity of copper ions in solution. This technique is a viable and sustainable alternative to the traditional adsorbents used to treat liquid waste. In addition, by using green coconut fiber lignin more efficiently, the research contributes to adding value to this material and strengthening practices in line with the circular economy and environmental preservation.

1. Introduction

The presence of heavy metals in water has emerged as a major environmental concern, impacting both ecosystems and human health [1,2]. Consequently, significant research efforts in recent decades have been directed toward the development of green and sustainable technologies aimed at producing energy and chemicals from renewable sources [3,4,5].

In this context, the use of lignocellulosic biomass (LB) for adsorption presents a promising approach, offering both environmental and economic benefits [6]. This is primarily because LB is a widely available, renewable, and cost-effective natural resource [7,8,9,10]. Its chemical composition (including lignin, cellulose, and hemicellulose) and structural features (such as functional groups) enable it to effectively adsorb dyes and metals from aqueous solutions [11,12].

Among the various types of LB, green coconut fiber (GCF) stands out as an effective adsorbent. Brazil, along with countries like India, Indonesia, the Philippines, and Sri Lanka, ranks among the top coconut producers globally, with an estimated production of 63.7 million tons in 2021 [13]. After extracting the water and meat from coconuts, the green coconut shells, which account for a significant portion of the total coconut mass (80–85%), are often discarded in landfills [6,14,15,16] or burned as a low-grade fuel for cooking in many developing nations [17]. This practice can lead to environmental issues such as greenhouse gas emissions and slow decomposition [14,15].

To address these environmental challenges, numerous studies have explored the use of coconut shells for biofuel production [14,18,19,20,21,22], as well as for the removal of dyes [23], heavy metals [24,25,26,27], and emulsified oils [16,28].

Among the various heavy metals commonly found in industrial effluents, copper (Cu²⁺) stands out due to its widespread use in manufacturing industries, including the manufacturing of electrical conductors, pharmaceuticals, paint, and fertilizers [29,30,31,32,33]. Although copper is an essential micronutrient for biological systems, it becomes highly toxic in high concentrations, affecting aquatic life and posing serious risks to human health, including the accumulation of oxidizing species within the structure of cells [34], Prion disease [35], Alzheimer’s disease [36], lung cancer, central nervous system irritation, jaundice and, in more severe cases, death [37,38].

In light of these issues, the maximum concentration of Cu²⁺ ions present in drinking water should not be greater than 1.3 mg·L⁻¹ or 2 mg·L⁻¹ according to the US Environmental Protection Agency (EPA) (EPA, 2020) and the World Health Organization (WHO) [39], respectively. In Brazil, the National Environmental Council (CONAMA) sets the allowable concentration of dissolved copper ions in wastewater at 1.0 mg·L⁻¹ [40].

In this regard, Nascimento et al. [26] investigated Cu²⁺ adsorption using GCF, both with and without chemical treatment with amines. Their findings revealed that amine treatment significantly increased Cu²⁺ adsorption capacity, as it provided more available electrons to attract metal ions. Similarly, Nascimento et al. [25] studied the chemical modification of GCF by impregnating it with a sodium octanoate surfactant through microemulsion, which enhanced the adsorbent’s ability to remove Cu²⁺ from wastewater. Surfactant impregnation increased Cu²⁺ removal capacity by a factor of three, due to the strong interaction between the carboxylate anions in the surfactant and the positively charged Cu²⁺ ions in the aqueous medium.

The use of GCF as an adsorbent is advantageous because it has a higher lignin content compared to other types of biomass, which demonstrates its potential as a raw material for producing lignin-based materials [17]. Biorefineries could play a pivotal role in the integrated conversion of biomass, ensuring the full utilization of lignocellulosic materials [41].

Although lignin is generally insoluble and has limited dispersion in its native form, it contains surface groups that enable chemical modifications, which can improve its polarity and enhance its application in wastewater treatment [42]. The chemical and physical modification of lignin using surfactants aim to increase its adsorption capacity and overall efficiency by increasing surface charge density, promoting electrostatic interactions and ionic exchange, and generating new functional groups [43,44]. Additionally, surfactants can alter the pore volume, surface area, and diameter of adsorbents [45].

Padilha et al. [23] studied lignin extracted from GCF in the development of nanostructured adsorbents with Fe₃O₄ for dye removal. Their results demonstrated that lignin/Fe₃O₄ nanoparticles exhibited rapid equilibrium times and high adsorption capacities for various dyes, including methylene blue (203.66 mg/g), Cibacron blue (112.36 mg/g), and Remazol red (96.46 mg/g). Kumar and Kumar [24] evaluated the Cr(VI) adsorption potential of lignin from GCF, reporting a maximum adsorption capacity of 30.94 mg/g and a 92.8% Cr(VI) removal efficiency after 80 min of exposure.

These studies highlight the importance of chemically modifying lignin to improve its efficiency in adsorbing heavy metals from aqueous solutions. However, there is a gap in the literature regarding the use of lignin extracted from GCF with surfactant impregnation for metal ion removal. This gap underscores the need for further adsorption studies to enhance the applicability of these materials and contribute to cleaner production.

Thus, this study aims to evaluate the potential of lignin extracted from GCF, chemically modified by impregnation with a sodium octanoate surfactant (LIG-SUR), as an efficient and sustainable adsorbent for Cu²⁺ removal from aqueous solutions. The study will assess the performance of this modification by evaluating the effect of solution pH and adsorbent dosage on Cu²⁺ removal efficiency. Additionally, the adsorption process will be analyzed using kinetic models (pseudo-first-order, pseudo-second-order, and intraparticle diffusion) and isothermal models (Langmuir and Freundlich) to better understand the underlying adsorption mechanisms. Finally, the physicochemical properties of the adsorbents, including their surface morphology, functional groups, and chemical composition, will be examined to correlate material characteristics with adsorption performance.

2. Materials and Methods

2.1. GCF Collection and Processing

Dried coconut samples were collected from urban areas in Natal, Northeast Brazil. The epicarp and mesocarp were first cut into smaller pieces (approximately 5 cm), washed twice under running water, and then dried in an air-circulation oven (Model TE-394/1-MP, manufactured by TECNAL, Brazil) at 50 °C for 72 h. After drying, the material was ground using a knife mill (Model R-TE-680, manufactured by TECNAL, Brazil). The resulting biomass was then sieved through a 20-mesh sieve (48 µm) and stored in plastic bags at room temperature (~25 °C). This process followed the procedure described in a previous study [28].

2.2. Alkaline Pre-Treatment of GCF

To extract lignin, GCF was subjected to alkaline treatment with sodium hydroxide (NaOH) as described in [28]. For this, 200 mL of a 2% (w/v) sodium hydroxide solution and 20 g of GCF were combined in a 500 mL Erlenmeyer flask. The mixture was maintained at 121 °C in an autoclave for 30 min. The resulting lignin-rich liquid fraction (black liquor) was then recovered through fabric filtration, acidified to a pH of 2.0, and centrifuged at 1500 rpm using an SL-700 model centrifuge (manufactured by SOLAB, Brazil). During centrifugation, the precipitated lignin was washed with acidified water (pH 2.0) in eight cycles, followed by drying in an oven at 80 °C for 24 h. The final product from this process was referred to as GCF lignin (LIG).

2.3. Synthesis of Sodium Octanoate

Sodium octanoate was synthesized following procedures from previous studies [25,46,47]. The reaction diagram for obtaining sodium octanoate from lab-grade octanoic acid (C₈H₁₆O₂) and sodium hydroxide (NaOH) is shown in Figure 1.

In the synthesis, a 1:1 molar ratio of NaOH and octanoic acid was added to a round-bottom flask, which was connected to a reflux condenser. Approximately 40 mL of ethanol was added to ensure the uniform mixing of the reactants. The reaction mixture was boiled for 2 h. Afterward, it was transferred to a beaker and stirred until the ethanol and water completely evaporated. The precipitated sodium octanoate was then dried in an oven at 100 °C for 24 h and ground manually in an agate mortar and pestle. The choice of sodium octanoate was based on a previous study. Nascimento et al. [26] impregnated sodium octanoate directly into the coconut shell. Their results were promising and encouraged the realization of the present work.

2.4. Impregnation of Sodium Octanoate in Lignin Surface

The impregnation of sodium octanoate into the lignin surface followed methods described in previous works [25,48]. A microemulsion was prepared using a solution of 25% organic phase (kerosene), 25% aqueous phase (distilled water), 40% co-surfactant (butyl alcohol), and 10% active phase (sodium octanoate). Lignin (10 g) was then immersed in 20 mL of the microemulsion, stirred, and dried in an oven at 65 °C for 48 h. The volume of the microemulsion used was the minimum necessary to ensure the complete wetting of the lignin. The resulting material was named LIG-SUR.

2.5. Study of Heavy Metal Adsorption

2.5.1. Batch Adsorption

LIG-SUR was tested as an adsorbent for the removal of Cu²⁺ ions. Adsorption experiments were carried out in batch mode (duplicates), using an incubator (Nova Ética) with controlled temperature and agitation. Typically, 0.2 g of adsorbent was mixed with 50 mL of a Cu²⁺ solution, stirred at 150 rpm, and maintained at 30 °C. After adsorption, the samples were filtered under vacuum, and the copper concentration was measured using an atomic absorption spectrometer (Model AA-6300, manufactures by SHIMADZU, Japan). The effects of pH (2 to 7) and adsorbent mass (10, 20, and 30 mg) on adsorption were investigated.

Additionally, the influence of bioadsorbent concentration and pH on the biosorption process was evaluated. For the bioadsorbent concentration, levels of 1.0, 2.0, 4.0, 6.0, and 8.0 g/L were tested. The pH was adjusted from 2.0 to 7.0 using NaOH and HCl solutions, and only the initial pH was corrected, with no further adjustments made during the process. pH was monitored with a DM-32 model pH meter (manufactured by DIGMED, Brazil). The experimental conditions were kept constant, with an initial metal ion concentration of 50 g/L, stirring at 150 rpm, a temperature of 30 °C, and particle size between 0.10 and 0.07 mm, with the pH set at 6.0.

2.5.2. Adsorption Kinetics

A kinetic study was conducted to determine the time required for Cu²⁺ ions to reach adsorption equilibrium in LIG-SUR. For this, 0.20 g of adsorbent and 50 mL of Cu²⁺ solution (50 mg/L) were added to a 125 mL Erlenmeyer flask. The flask was stirred at 150 rpm and maintained at 30 °C. Aliquots were collected at various time intervals (0.5, 1, 2, 3, 4, 5, 10, 20, 30, 60, 90, 120, 180, 240, and 300 min), filtered, and analyzed to determine the metal concentration. The data were then fitted to pseudo-first-order, pseudo-second-order, and intraparticle diffusion models.

2.5.3. Adsorption Equilibrium Study

Equilibrium studies were performed using Cu²⁺ concentrations of 50, 100, 150, and 200 mg/L. Copper solutions were added to 20 mg of LIG-SUR, and the system was stirred under constant temperature until equilibrium was reached. The studies were carried out at temperatures of 30, 40, 50, and 60 °C. Adsorption isotherms were analyzed by applying the Langmuir and Freundlich models.

2.6. Bioadsorbent Characterization

The physicochemical properties of the lignins (LIG and LIG-SUR) and adsorbent after Cu²⁺ adsorption (LIG-SUR-Cu²⁺) were evaluated using X-ray fluorescence (XRF) and an EDX-720 spectrometer (Shimadzu) with a detection range from Na (11) to U (92), with a sensitivity greater than 0.001. Scanning electron microscopy (SEM) was performed using a CARL ZEISS AURIGA device, coupled with energy-dispersive X-ray spectroscopy (EDS). The functional groups present in the adsorbents were identified through Fourier transform infrared spectroscopy (FTIR) using a Bruker Tensor II spectrometer, with a wavenumber range of 4000–400 cm⁻¹ and a resolution of 4 cm⁻¹.

3. Results and Discussion

3.1. Bioadsorbent Characteristics

3.1.1. Chemical Analysis by XRF

Table 1. Chemical composition of lignins (% by mass).

Lignins	CuO	K2O	Na2O	SO3	NiO	Fe2O3	SiO2	CaO	Cr2O3	ZrO2	P2O5
LIG	0.40	11.44	-	76.04	0.41	4.02	3.52	1.69	0.46	0.08	1.94
LIG-SUR	0.24	11.01	22.30	61.25	0.27	2.23	2.70		-
LIG-SUR-Cu2+	68.14	3.32	-	15.10	0.21	1.48	7.17	4.36	0.22

shows the chemical composition obtained by X-ray fluorescence (XRF) for the lignins before and after sodium octanoate impregnation and after adsorption to remove Cu²⁺ ions. The presence of sodium oxide (Na₂O) in LIG-SUR reveals that the surfactant was introduced onto the adsorbent surface. After adsorption, Na₂O was replaced by copper oxide (CuO), indicating an ion exchange mechanism, as observed in previous studies [25,49,50]. The presence of sulfur trioxide (SO₃) in both LIG and LIG-SUR demonstrates the formation of sulfonic groups [51]. From the perspective of chemical species, this shows that alkaline pre-treatment and surfactant impregnation were effective in Cu²⁺ removal.

3.1.2. Adsorbent Surface Analysis by SEM

The adsorbent surfaces before and after surfactant impregnation and after adsorption can be seen in Figure 2. LIG has a smooth surface, with clusters of smaller particles (Figure 2a). By contrast, LIG-SUR and LIG-SUR-Cu²⁺ exhibit rougher surfaces with more pores and cracks, as shown in Figure 2b and Figure 2c, respectively.

Using the scanning methodology on the images shown in Figure 2, the EDS chemical composition of the lignins was obtained, as shown in

Table 2. EDS chemical composition of lignins (% by mass).

Lignins	O	C	Na	Cu	Ca	Fe	Cr	Ni
LIG	37.0%	61.9%	1.0%	-	-	-	-	-
LIG-SUR	35.8%	61.2%	3.0%	-	-	-	-	-
LIG-SUR-Cu2+	81.9%	-	5.5%	12.1%	0.2%	0.2%	0.1%	0.1%

. EDS analysis confirms that the main composition of the adsorbents is carbon (C) and oxygen (O). This is consistent with a previous study by Nascimento et al. [52], which assessed the potential of using macadamia nuts modified with a sodium-dodecyl-sulfate-based surfactant for methylene blue removal. However, the sodium (Na) percentage increased when the lignin was pre-treated, indicating that the active phase of the microemulsion (surfactant) was introduced onto the LIG surface. Additionally, copper (Cu) was identified on the LIG-SUR-Cu²⁺ surface, showing that Cu²⁺ was adsorbed onto the LIG-SUR surface. These results confirm those obtained by XRF chemical analysis.

3.1.3. Molecular Structure Analysis by FTIR

The FTIR spectra of the adsorbents (LIG and LIG-SUR) can be seen in Figure 3. In both adsorbents, a broad band around 3392 and 3411 cm⁻¹ can be attributed to the stretching vibration of the hydroxyl group (O–H) from the alcohols and phenols in the lignin [25,51,53]. The bands around 2917–2919 and 2848–2850 cm⁻¹ correspond to the symmetric and asymmetric stretching of the C–H bonds in the methyl and methylene groups of lignin and cellulose [25,51,54], and those at 1608 and 1618 cm⁻¹ correspond to the C=O vibration of the carboxyl group and the C=C stretching of the aromatic group [55]. The band at 1124 cm⁻¹ is attributed to the asymmetric stretching of the C–O–C bond [54].

There is also a slight shift and a small decline in the intensity of the O–H stretching, C–H stretching vibration, and C=O and C=C stretching bands. This indicates that there was an electrostatic interaction between the surfactant molecules and the lignin constituents extracted from the GCF, as was also observed in a previous study assessing GCF with surfactant impregnation [25].

3.2. Adsorption Parameter Effects

3.2.1. Bioadsorbent Mass

Figure 4 shows the effects of using different adsorbent mass on the percentage Cu²⁺ removal and adsorption capacity of LIG-SUR. When a concentration of up to 4.0 g/L of LIG-SUR was used, Cu²⁺ removal increased linearly due to the availability of active sites [56,57,58].

The maximum metal ion removal efficiency was 92.63%. On the other hand, from 6.0 g/L of LIG-SUR, the percentage Cu²⁺ removal remained almost unchanged, indicating that the adsorbate molecules available in solution were completely adsorbed. These results are consistent with a previous study [52] that assessed the adsorption capacity of surfactant-modified macadamia nut shells in removing methylene blue.

This behavior is more evident when analyzing adsorption capacity, given that increasing the amount of LIG-SUR results in a linear decrease in the adsorption capacity for Cu²⁺. This is due to the lower adsorbate/active sites ratio and, consequently, the smaller amount of Cu²⁺ available for complete distribution among the available active sites, as well as a possible interaction between binding sites [59]. Thus, these results suggest that 4.0 g of LIG-SUR is enough to achieve adsorption equilibrium when the initial Cu²⁺ concentration is 50 mg·L⁻¹.

3.2.2. pH of Solution

One of the key factors influencing adsorption is pH [60]. As such, the effect of pH on the adsorption capacity of LIG-SUR for Cu²⁺ was investigated by varying the solution pH from 2 to 7, as shown in Figure 5. This pH range was selected because copper or copper oxide precipitation occur at a pH of 6 or higher, as previously observed by [53].

As shown in Figure 5, adsorption was favored by increasing the pH of the solution due to the attraction between the negative charges on the adsorbent surface and the copper ions. Modifying the lignin surface with the anionic surfactant increases its negative charge due to carboxylate anion formation (RCOO⁻), which explains this behavior. These observations are consistent with previous studies [25,61].

It is important to underscore that the adsorption capacity increased substantially with a rise in pH from 2 to 3, which can be explained by the direct competition between metal and H⁺ ions for active sites on the adsorbent surface. At a low pH, H⁺ ions occupy the adsorption sites and limit access to these sites due to repulsive forces, thereby reducing adsorption capacity [43]. Similarly, at higher pH values, adsorption capacity also declines due to the formation of metal ion complexes with hydroxide [62].

Thus, the highest efficiency in removing copper ions using LIG-SUR was obtained at a pH of 7. However, a pH of 6 was used in subsequent experiments to avoid the precipitation of copper ions, as discussed above. This observation was also consistent with a previous study [25].

3.2.3. Kinetics

The results of the kinetic study of Cu²⁺ ion adsorption for GCF lignin with (LIG-SUR) and without (LIG) surfactant impregnation is shown in Figure 6. For both bioadsorbents, Cu²⁺ concentration declines over time, with equilibrium reached rapidly at approximately 10 and 60 min for LIG-SUR and LIG, respectively. This suggests that adsorption occurs predominantly on the adsorbent surfaces [63]. A number of factors may explain the rapid adsorption of these adsorbents: (i) the initial availability of a large number of vacant active sites, including sulfonic and hydroxyl groups for Cu²⁺ adsorption [24,51]; (ii) the high initial copper concentration and the ion exchange mechanism [64,65]; (iii) the strong electrostatic attraction between the carboxylate anion present in the surfactant head (RCOO⁻) and the positive charge of the Cu²⁺ ions in the aqueous solution [25].

Surfactant impregnation improved copper adsorption performance, with LIG-SUR able to remove 91.57% of the initial metal content in the liquid medium. In comparison, LIG (without surfactant impregnation) achieved a removal percentage of 41.2%. These results demonstrate the efficiency of surfactant impregnation in GCF lignin in copper removal from solutions.

Table 3. Kinetic models for copper adsorption on bioadsorbents at 30 °C.

Kinetic Parameters	LIG	LIG-SUR
Pseudo-first-order
$K_1$ (min−1)	0.083	0.059
$q_{e\left(\mathrm{e}\mathrm{x}\mathrm{p}\right)}$ (mg·g−1)	5.152	11.889
$q_{e\left(\mathrm{c}\mathrm{a}\mathrm{l}\mathrm{c}\right)}$ (mg·g−1)	2.349	1.469
R2	0.949	0.673
Pseudo-second-order
$K_2$ (mg·g−1·min)	0.059	0.134
$q_{e\left(\mathrm{e}\mathrm{x}\mathrm{p}\right)}$ (mg·g−1)	5.152	11.889
$q_{e\left(\mathrm{c}\mathrm{a}\mathrm{l}\mathrm{c}\right)}$ (mg·g−1)	5.640	12.077
R2	0.999	0.999
Intraparticle diffusion
$K_d$ (mg·g−1·min−1/2)	0.195	0.223
$c$ (mg·g−1)	3.006	9.369
R2	0.591	0.189

shows the kinetic parameters for the pseudo-first-order, pseudo-second-order, and intraparticle diffusion models obtained after fitting the experimental data. The experimental data for both adsorbents fit the pseudo-second-order kinetic model best. For this model, the $q_{e\left(\mathrm{c}\mathrm{a}\mathrm{l}\mathrm{c}\right)}$ and $q_{e\left(\mathrm{e}\mathrm{x}\mathrm{p}\right)}$ values are similar, and the correlation coefficient (R²) is 0.999 for both studies. This indicates that the adsorption process for both LIG and LIG-SUR can be described as chemisorption, involving electron sharing between the adsorbent and the metal ions [66,67,68]. In addition, the kinetic constant values of the pseudo-second-order model (K₂) confirm that the chemical modification of the adsorbent surface favors Cu²⁺ ion adsorption, given that a higher K₂ value correspond to a faster adsorption rate.

Figure 7 shows the curves fitted to the intraparticle diffusion (ID) kinetic model for LIG and LIG-SUR. It is important to note that the ID curves for both bioadsorbents do not intersect at the origin, indicating that the C constant is not zero, suggesting an external diffusion control. The results also reveal that there was an increase in the C constant after surfactant impregnation, indicating greater resistance to mass transfer during the diffusion transport of ions through the boundary layer to the adsorbent surface. There was also an increase in the diffusion constant (K_d), corresponding to the rapid adsorption by LIG-SUR. These observations are also consistent with a previous study [25]. Thus, it can be inferred that the ID model does not represent the adsorption control step.

3.2.4. Adsorption Isotherms

To describe the adsorption equilibrium of Cu²⁺ ions by LIG-SUR, adsorption isotherms were analyzed, since this study provides essential adsorption mechanism data and helps design an efficient adsorption system [69]. The experimental data were fitted to the Langmuir and Freundlich isotherm models, and the results of the parameters obtained are presented in

Table 4. Estimated parameters of Langmuir and Freundlich isotherms of LIG-SUR.

Temperature	Langmuir Isotherm			Freundlich Isotherm
	${\mathit{q}}_{\mathit{m}\mathit{a}\mathit{x}}$ (mg·g−1)	${\mathit{K}}_{\mathit{L}}$ (L·mg−1)	R2	$\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$\mathit{n}$}\right.$	${\mathit{K}}_{\mathit{F}}$ (L·mg−1)	R2
298.15	30.675	0.0159	0.9897	0.362	4.365	0.6579
303.15	25.445	0.0811	0.9749	0.400	3.495	0.5767
313.15	26.248	0.0504	0.9324	0.433	2.961	0.5269
323.15	30.581	0.0202	0.7531	0.539	1.773	0.5284

.

The results in Table 4 show that the Langmuir model provided a better fit for the experimental data, with a higher correlation coefficient (R²) for all the temperatures studied when compared to the Freundlich isotherm model. As the temperature increased, so did the Langmuir constant (K₁), indicating greater affinity between the adsorbent and adsorbate, thereby favoring Cu²⁺ ion adsorption by LIG-SUR. The low R² values of the Freundlich isotherm model suggest that this model also does not fit the experimental data. It can also be seen that the best results were obtained at 25 °C, with a maximum copper ion adsorption capacity of 30.675 mg·g⁻¹. Thus, the Langmuir isotherm model best represents Cu²⁺ ion adsorption by LIG-SUR. This indicates monolayer adsorption with identical and energetically equivalent sites, where each adsorbate molecule occupies only one site.

Nascimento et al. [25] report that Cu²⁺ ion adsorption using a modified adsorbent via anionic surfactant impregnation is enhanced by the insertion of carboxylate anions (RCOO⁻), given that this insertion increases the number of available sites for Cu²⁺ adsorption, which may explain the good performance of LIG-SUR.

4. Conclusions

This study evaluated the adsorption of copper ions (Cu²⁺) using lignin extracted from green coconut fiber (LIG), impregnated with a sodium-octanoate-based surfactant (LIG-SUR). The main conclusions are as follows:

• Chemical analysis by XRF and morphological analysis by SEM found that the impregnation of the surfactant, evidenced by the presence of Na₂O, was partially replaced by CuO after adsorption.
• Microstructural analysis by FTIR indicated a slight shift and reduction in the intensity of the vibration bands of various chemical bonds in the lignin, such as O-H, C-H, C=O, and C=C, suggesting electrostatic interaction between the surfactant and the constituents of the lignin extracted from the green coconut fiber (GCF).
• The adsorption kinetics followed the pseudo-second-order model, indicating chemisorption. In addition, equilibrium was reached in around 10 and 60 min for LIG-SUR and LIG, respectively.
• The Langmuir model best described the isotherm data, suggesting monolayer adsorption with equivalent sites.

Surfactant impregnation significantly increased the removal efficiency, removing 91.57% of Cu²⁺ (LIG-SUR) compared to 41.2% (LIG). In terms of maximum adsorption capacity, the LIG-SUR bioadsorbent adsorbed around 30.7 mg-g⁻¹ of Cu²⁺. These conclusions demonstrate that GCF impregnation with a sodium-octanoate-based surfactant is a viable solution, given that it significantly improves copper ion adsorption capacity. Moreover, treating solid waste to improve its adsorption properties is a sustainable approach because it uses materials produced in significant volumes that are normally improperly discarded in nature.