Multiscale Evaluation of Raw Coconut Fiber as Biosorbent for Marine Oil Spill Remediation: From Laboratory to Field Applications

Abstract

This study provides the first comprehensive multiscale evaluation of raw coconut fibers as biosorbents for crude oil removal, encompassing laboratory adsorption tests, mesoscale hydrodynamic simulations, and field trials in marine environments. Fibers were characterized by SEM, FTIR, XRD, XPS, and chemical composition analysis (NREL method), confirming their lignocellulosic nature, high lignin content, and functional groups favorable for hydrocarbon adsorption. At the microscale, a 2⁵⁻¹ fractional factorial design evaluated the influence of dosage, concentration, contact time, temperature, and pH, followed by kinetic and equilibrium model fitting and regeneration tests. Dosage, concentration, and contact time were the most significant factors, while low sensitivity to salinity highlighted the material’s robustness under marine conditions. Adsorption followed pseudo-second-order kinetics, with an equilibrium adsorption capacity of 4.18 ± 0.19 g/g, and it was best described by the Langmuir isotherm, indicating chemisorption and monolayer formation. Mechanical regeneration by centrifugation allowed for reuse for up to five cycles without chemical reagents, aligning with circular economy principles. In mesoscale and field applications, fibers maintained structural integrity, buoyancy, and adsorption efficiency. These results provide strong technical support for the practical use of raw coconut fibers in oil spill response, offering a renewable, accessible, and cost-effective solution for scalable applications in coastal and marine environments.

1. Introduction

Environmental contamination caused by oil spills remains a major global concern due to their potential to cause extensive damage to both terrestrial and aquatic ecosystems [1,2]. Several remediation strategies have been developed to mitigate oil spills, each with specific advantages and limitations. Mechanical containment and recovery using booms and skimmers are effective for large-scale spills, although their efficiency decreases under rough sea conditions and they involve high operational costs [3]. Chemical dispersants promote the breakdown of oil into smaller droplets and enhance biodegradation, but they may also introduce ecotoxicological risks due to their own toxicity [3,4]. In situ burning is a rapid method that reduces the oil volume; however, it generates secondary air pollution and can only be applied under favorable weather conditions [4]. Bioremediation, through the stimulation of native microbial communities or the addition of specialized microorganisms, is considered environmentally friendly, but its efficiency is limited by the time required for effective degradation and by environmental constraints such as nutrient availability [5,6,7,8]. Sorbent-based approaches also play a central role in oil spill remediation. Synthetic sorbents such as polypropylene mats and foams are widely used due to their high efficiency, but they are costly, non-biodegradable, and generate secondary disposal problems [9]. In contrast, natural materials such as peat moss, straw, sawdust, and lignocellulosic residues have been investigated as low-cost, biodegradable, and abundant alternatives, although they typically present lower adsorption capacity and durability when compared to synthetic materials [10]. These comparisons reinforce the importance of developing sustainable, efficient, and scalable biosorbents for marine oil spill remediation.

Among the various strategies developed to mitigate the adverse effects of such events, oil adsorption using porous materials has emerged as a promising, efficient, and cost-effective approach [11]. In recent years, natural fibers have gained increasing attention as adsorbent materials for oil removal in marine environments [12,13,14]. Derived from renewable sources, these fibers offer several advantages, including low cost, wide availability, biodegradability, and high adsorption capacity [15,16]. Furthermore, they represent a sustainable alternative to synthetic adsorbents, contributing to the reduction in environmental impacts associated with oil pollution. Among the most promising biosorbents is coconut fiber (Cocos nucifera L.), a lignocellulosic residue abundant in tropical regions such as Brazil [17,18].

Most studies have focused on the physicochemical modification of these fibers through treatments with acids, bases, organic solvents, or thermal processes aimed at enhancing surface properties and improving adsorption efficiency [19,20]. While these modifications may improve performance, they require additional preparation steps, chemical reagents, and laboratory infrastructure, which can limit their applicability in real-world oil spill scenarios that demand immediate response. Moreover, even when treated, these biodegradable fibers tend to have a limited shelf life, which compromises their effectiveness after long storage periods [21]. For these reasons, the use of coconut fibers in their raw form stands out as a practical and sustainable alternative, particularly in regions where this agricultural residue is widely available [22]. Using untreated fibers eliminates the need for processing, reduces operational costs, and enables immediate deployment in emergency situations. Additionally, the potential for regeneration and reuse through physical methods enhances material circularity and reduces environmental impacts related to disposal [23,24]. Despite previous studies evaluating raw coconut fibers, there remains a need for research that assesses their performance under realistic operational conditions, especially in dynamic and complex marine environments.

The performance of natural biosorbents is determined by a set of interdependent factors that involve not only the physicochemical properties of both the adsorbent and the adsorbate, but also the environmental conditions under which the interaction occurs, such as temperature, salinity, pH, and the hydrodynamic behavior of the system [25]. These environmental factors play a crucial role, as they directly influence the interaction mechanisms between organic compounds and the biosorbent surface, affecting diffusion, interface stability, and overall adsorption efficiency [24,25,26]. In the case of coconut fibers, which simultaneously exhibit oleophilic and hydrophilic properties, sensitivity to environmental variables becomes even more relevant in aquatic systems, particularly in complex environments such as marine settings [16,26,27,28,29].

Despite the well-known influence of environmental variables on biosorbent performance, most studies involving coconut fibers remain limited to micro-scale experiments conducted under static and controlled laboratory conditions, which do not reflect the complexity of real environments. For example, De Sousa et al. [27] evaluated diesel oil adsorption by raw fibers in static systems with reduced volumes. Hoang et al. [30] developed a porous composite using pre-treated coconut fibers, tested under simple immersion for 30 min, without accounting for operational factors such as sorbent dosage or medium dynamics, where medium dynamics refers to the physical behavior of the aquatic system, including water movement (waves, currents, and turbulence) and other hydrodynamic conditions that directly influence oil–sorbent interactions. While these studies contribute to understanding the sorptive capacity of the fibers, investigations advancing toward more realistic conditions, such as meso-scale tests with hydrodynamic simulation or field trials that assess the physical response of the materials in natural environments, remain scarce.

To help close this gap in the literature, the present study conducted meso-scale adsorption tests with simulated agitation and buoyancy trials in a real marine environment, in order to evaluate the physical behavior of the material under realistic application conditions. In addition, micro-scale tests were carried out to evaluate the performance of raw coconut fiber in crude oil adsorption under saline conditions, to characterize its physicochemical and structural properties relevant to the adsorption process, to investigate the influence of operational factors through a fractional factorial design, and to perform kinetic and equilibrium studies as well as reuse tests by centrifugation. Initially, the fiber was subjected to physicochemical and structural characterization techniques to assess properties relevant to the adsorption process. At the micro-scale, a fractional factorial design was applied to evaluate the influence of key operational factors on adsorption capacity, followed by kinetic studies, equilibrium isotherms, and regeneration by centrifugation. As a distinguishing feature, the study was expanded to include a meso-scale adsorption test with simulated agitation, as well as a buoyancy test conducted in a real marine environment to assess the physical behavior of the material under realistic application conditions. The findings indicate that raw coconut fibers, despite their natural hydrophilic nature, exhibit structural features and oleophilic behavior that allow effective oil removal in expanded-scale scenarios. These attributes highlight their viability as an environmentally friendly, widely available, and economically advantageous material for mitigating oil spills in coastal and marine settings.

2. Materials and Methods

2.1. Preparation and Characterization of Coconut Fibers

Coconut fibers were collected from agricultural residues in the tropical coastal region of Salvador, Bahia, Brazil. The material was processed in the laboratory, initially undergoing manual selection to remove impurities commonly found mixed with the residues, such as pieces of husk, dust, or deteriorated fragments, which are not part of the fiber structure. This step ensured greater uniformity of the samples while maintaining the natural state of the material. The fibers were then ground using a Willye TE-680 mill and sieved to obtain lengths between 2 and 3 mm. This procedure ensured a uniform surface area distribution for the adsorption tests. The processed fibers were subsequently subjected to physicochemical and structural characterization techniques, including scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), chemical composition analysis by the NREL method (NREL/TP-510-42618) [31], X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), all of which are essential for understanding their performance in the adsorption process.

2.1.1. Scanning Electron Microscopy (SEM)

Scanning electron microscopy (SEM) was employed to examine the surface morphology of the raw coconut fibers. Prior to imaging, the samples were coated with a thin layer of gold using a Deten Vacuum sputter coater to improve electrical conductivity. SEM images were acquired at magnifications between 250× and 700×, operating at an accelerating voltage of 10 kV, using a JSM-6610LV microscope (JEOL Ltd., Tokyo, Japan).

2.1.2. Fourier-Transform Infrared Spectroscopy (ATR-FTIR)

The surface functional groups of the coconut fiber were analyzed by Fourier-transform infrared (FTIR) spectroscopy in attenuated total reflectance (ATR) mode. Spectra were recorded on a PerkinElmer Spectrum BX II spectrometer (PerkinElmer Inc., Shelton, CT, USA). equipped with a Universal ATR Sampling Accessory (PerkinElmer Inc., Shelton, CT, USA). Measurements were collected over 4000–600 cm⁻¹ at 4 cm⁻¹ resolution, with 20 co-added scans per spectrum. The fiber was pressed directly onto the ATR crystal to ensure intimate contact, and spectra were acquired at room temperature.

2.1.3. Chemical Composition Analysis by the NREL Method

The chemical characterization of coconut fiber was carried out according to the protocol of the National Renewable Energy Laboratory (NREL/TP-510-42618, [31]). Initially, the biomass was subjected to a pretreatment for extractives removal with water and ethanol. The extractive-free material was then used for the determination of the structural components (cellulose, hemicelluloses, and lignin) by a two-step acid hydrolysis procedure.

In the first step, the sample was treated with concentrated sulfuric acid (72% w/w) at 30 °C for 1 h to promote partial depolymerization of the lignocellulosic matrix. In the second step, the suspension was diluted with deionized water to a final concentration of 4% (w/w) H₂SO₄ and autoclaved for 1 h to ensure complete hydrolysis of polysaccharides into their monomeric sugars. After cooling to room temperature, the mixture was vacuum-filtered through pre-dried and weighed filter paper. The solid residue retained on the filter was used to determine the acid-insoluble lignin fraction by gravimetric measurement. The acid-soluble lignin fraction present in the filtrate was quantified by ultraviolet–visible spectrophotometry (Shimadzu UV-1800, Shimadzu Corp., Kyoto, Japan) at 280 nm. The filtrate was further analyzed by high-performance liquid chromatography (HPLC) equipped with a refractive index detector and an Aminex HPX-87H column (Bio-Rad Laboratories, Hercules, CA, USA), using 5 mM H₂SO₄ as the mobile phase at 0.6 mL·min⁻¹ and 60 °C. Under these conditions, the released monomeric sugars (glucose, xylose, arabinose, mannose, and galactose) were quantified. The glucose content was used to estimate the cellulose fraction, while the sum of xylose, arabinose, mannose, and galactose was used to calculate the hemicelluloses fraction. This procedure allowed a complete quantification of the main structural components of the coconut fiber: cellulose, hemicelluloses, and lignin. The corresponding HPLC chromatogram, as well as the concentrations of the quantified sugars used for these calculations, are provided in the Supplementary Material (Figure S1 and Table S1, respectively).

2.1.4. X-Ray Diffraction (XRD)

The crystalline structure of the fibers was analyzed using a Shimadzu X-ray (Shimadzu XRD-6000, Shimadzu Corp., Kyoto, Japan) diffractometer (model XRD-6000), with a scanning range from 2θ = 10° to 80°, at a rate of 2° per minute, in a fixed-sample configuration, and using slits DS 1, S 1, and RS 0.15. The crystallinity index (CrI) was calculated according to Equation (1).

$$CrI={\displaystyle \frac{I_{\left(002\right)}-I_{\left(am\right)}}{I_{\left(002\right)}}}\times 100$$

(1)

where $I_{\left(002\right)}$ corresponds to the intensity of the crystalline peak (2θ ≈ 22.5°), specifically the (200) reflection of cellulose I, and $I_{\left(am\right)}$ represents the minimum intensity of the signal associated with the amorphous region, typically observed around 2θ ≈ 18° [32,33,34]. The peaks at approximately 2θ = 14.8° and 16.5° correspond to the (1–10) and (110) reflections of cellulose I, respectively [33].

2.1.5. X-Ray Photoelectron Spectroscopy (XPS)

The surface chemical composition of raw coconut fiber was analyzed by X-ray Photoelectron Spectroscopy (XPS) using a monochromatic Al Kα radiation source (1486.6 eV). Survey spectra were acquired with a pass energy of 200 eV, an energy step size of 0.40 eV, and an analysis area of approximately 400 µm, with a total of 8 accumulated scans. Calibration was performed using the C 1s peak at 285.0 eV as a reference [35,36,37]. Atomic concentrations were calculated from peak areas corrected by the sensitivity factors of each element. Elements detected included C, O, N, Ca, S, and Si, whose atomic distribution offers insights into the surface composition and adsorption potential of the material.

2.2. Physicochemical Characterization of the Petroleum Sample

The physicochemical characterization of the crude oil employed in the experiments was performed. The oil was sourced from the Campos Basin (Brazil), and parameters such as pour point (ASTM D97 [38]) and density (ASTM D4052 [39]) were analyzed. The API gravity was calculated from the measured density according to the ASTM D1250 [40] standard, as these two parameters are inversely related and provide a measure of the oil’s relative density.

2.3. Fractional Factorial Experimental Design

A two-level coded fractional factorial design (−1: low level, +1: high level) was used to investigate the influence of five experimental factors on oil adsorption by raw coconut fibers. The evaluated factors included salinity (1.5% and 3.5%), contact time (5 min and 90 min), adsorbent dosage (0.5 g and 1.0 g), pH (5 and 9), and oil concentration in water (25 and 80 mL·L⁻¹). The response variable was the oil adsorption capacity (g·g⁻¹) of raw coconut fibers. The fractional factorial design employed in this study followed a 2⁵⁻¹ configuration, representing half of a full factorial design, in order to improve experimental efficiency. A total of 16 experimental trials were performed, ensuring a resolution V design, meaning that main effects and two-factor interactions were not aliased with each other [41].

All tests were conducted in duplicate, resulting in 32 randomized experimental runs. Statistical analysis was conducted using Minitab™ version 14.1.0 (Minitab Inc., State College, PA, USA) [42]. The analysis aimed to determine the most significant factors influencing oil adsorption onto coconut fibers. Additionally, data normality was assessed using a normal probability plot of standardized residuals.

2.4. Adsorption Experiments

Adsorption experiments were conducted on a reciprocating shaking table operating at approximately 126 cycles per minute. All tests were performed under laboratory bench conditions (micro-scale), meaning small-volume assays carried out in controlled glassware (100 mL oil/water mixtures) with defined adsorbent dosage (0.5 g). These experiments were designed to provide a preliminary evaluation of the adsorption behavior of coconut fibers prior to future applications in meso-scale units (mesocosms with larger volumes and more complex hydrodynamics) and, ultimately, in field-scale scenarios. Coconut fibers were confined within mini-containment barriers made of polypropylene mesh (2 × 5 cm) to minimize mass loss during gravimetric adsorption tests. Each adsorbent sample (0.5 g of fiber) was exposed to a mixture of saline water (3.5% salinity, pH ≈ 7) and crude oil from the Campos Basin (Brazil) (80 mL·L⁻¹) for 5 min at room temperature, following the ASTM F726-99 [43] standard procedure for testing sorbents used in oil spill cleanup.

After the adsorption step, the samples were subjected to freeze-drying to remove residual moisture [16]. The dried samples were then weighed, and the oil adsorption capacity (S) of the coconut fibers was calculated using Equation (2).

$$q={\displaystyle \frac{m_f-m_o}{m_o}}$$

(2)

where $q$ represents the adsorption capacity, expressed in grams of oil per gram of sorbent (g·g⁻¹), $m_o$ is the initial mass of the sorbent assembly (fiber + polypropylene mesh) prior to adsorption (g), and $m_f$ is the final mass of the same assembly (fiber + polypropylene mesh + retained oil) (g) after adsorption and freeze-drying.

For the kinetic study, the experiments were conducted under the same setup, maintaining constant the parameters that provided the best performance according to the fractional factorial design: oil concentration, adsorbent dosage, salinity, and pH. Contact time was varied at intervals of 5, 20, 40, 60, 90, and 120 min to assess the rate of adsorption. The experimental data were fitted to kinetic models in their linearized forms to evaluate the adsorption mechanism. The pseudo-first-order model is given by Equation (3), and the pseudo-second-order model by Equation (4).

$$\ln \left(q_e-q_t\right)=\ln \left(q_e\right)-k_1\times t$$

(3)

$${\displaystyle \frac{t}{q_t}}={\displaystyle \frac{1}{k_2\times q_e^2}}+{\displaystyle \frac{t}{q_e}}$$

(4)

where $q_e$ (g·g⁻¹) is the amount adsorbed at equilibrium, $q_t$ (g·g⁻¹) is the amount adsorbed at time $t$, $k_1$ (min⁻¹) is the pseudo-first-order rate constant, and $k_2$ (g·g⁻¹·min⁻¹) is the pseudo-second-order rate constant. The statistical significance of the model fits was verified using the Tukey test at a 5% probability level.

To determine the adsorption capacity under equilibrium conditions, adsorption isotherms were constructed. The tests were performed by varying the volume of crude oil (0 to 10 mL, in 2 mL increments), while keeping the total volume of the oil/water mixture constant at 100 mL. Other parameters such as adsorbent dosage (0.5 g), temperature (ambient), salinity (3.5%) and pH (≈7) were maintained at constant levels. The experimental data were fitted to three widely used equilibrium models: Langmuir, Freundlich, and Sips, in their nonlinear forms (Equations (5–7)).

$$q={\displaystyle \frac{q_{max}\times K_L\times C_e}{1+K_L\times C_e}}$$

(5)

where $q$ is the amount adsorbed at equilibrium (g·g⁻¹), $q_{max}$ is the maximum adsorption capacity (g·g⁻¹), $C_e$ is the equilibrium concentration (g·L⁻¹), and $K_L$ is the Langmuir constant (L·mg⁻¹).

$$q=K_F\times C_e^{1/n}$$

(6)

where $K_F$ and $1/n$ are Freundlich constants related to adsorption capacity and surface heterogeneity, respectively.

$$q={\displaystyle \frac{q_{ms}\times b\times C^n}{1+b\times C^n}}$$

(7)

where $q_{ms}$ is the maximum adsorption capacity for the Sips model (g·g⁻¹), $b$ is the Sips constant, $C$ is the equilibrium concentration (mg·L⁻¹), and $n$ is the model exponent.

These kinetic and isothermal models were employed to describe the adsorption behavior of raw coconut fibers under controlled micro-scale conditions and to provide baseline data for comparison with future pilot- and field-scale applications.

2.5. Reuse Test of Bioadsorbent Mini-Barriers in Subsequent Cycles

The recovery and reuse of the adsorbent material were evaluated over five consecutive adsorption–desorption cycles. The desorption process was carried out by centrifugation, a physical technique that can be advantageous for practical applications since it does not require the use of chemical reagents to remove the adsorbed oil. This procedure is feasible due to the simultaneous occurrence of physical and chemical adsorption on the biosorbents, allowing efficient oil removal without the need for additional substances [24]. After each desorption cycle, gravimetric analysis was performed to quantify the desorbed oil. The biosorbent material, already separated from the oil, was reused in a new adsorption cycle, and the adsorption tests were repeated while maintaining the experimental parameters defined in the previous trials. The adsorption capacity of the material was monitored throughout the cycles to evaluate the maintenance of its efficiency. The variation in adsorption capacity between cycles was analyzed to verify the stability and feasibility of the adsorbent material for reuse in oil spill remediation.

2.6. Performance of Raw Coconut Fiber Barriers in Meso-Scale and Field Marine Conditions

To evaluate the buoyancy, structural integrity, and oil adsorption performance of barriers made from raw coconut fiber, two sets of experiments were conducted: (i) a meso-scale test using a hydrodynamic simulation tank (mesocosm) and (ii) a field test in a natural marine environment.

In the meso-scale simulation, mesocosms were filled with 140 L of artificial saline water (salinity 35‰, room temperature) and equipped with wave maker pumps. The system, illustrated in Figure S2 (Supplementary Information), generated water recirculation and wave dynamics, simulating nearshore hydrodynamic conditions. A barrier (15.5 cm × 20 cm), produced from raw coconut fiber, was fabricated specifically for this experiment and placed freely on the water surface. Crude oil from the Campos Basin (Brazil) was added at a fixed point on the surface (80 mL), allowing natural dispersion driven by the hydrodynamic conditions established by the pumping system. The contact time between the sorbent and the oil was standardized at 5 min, based on previous findings indicating the rapid adsorption capacity of natural fibers under short exposure durations. After the test, the barrier was carefully removed and subjected to freeze-drying, as performed in the micro-scale tests, to eliminate residual water mass. Oil adsorption capacity was determined gravimetrically according to Equation (2), using the difference in dry mass before and after exposure. The procedure was performed in triplicate to ensure reproducibility and enable statistical validation.

For the field experiment, a large-scale sorbent barrier (300 cm long, 20 cm in diameter) was constructed entirely from raw coconut fibers. The test was conducted in a tropical coastal region (Tinharé, Cairu, Bahia, Brazil), known for its high salinity and rich marine biodiversity. The barrier was placed on the sea surface and remained floating for 20 min. The local water presented a salinity of approximately 35‰ and a temperature of 26 °C. After field exposure, the barrier was stored for 12 months under controlled conditions (ambient temperature, protected from light and moisture) and later inspected visually and physically to assess its structural integrity and potential degradation.

2.7. Statistical Analyses

All adsorption experiments were conducted in triplicate, and statistical analysis was performed using multifactor analysis of variance (ANOVA) at a significance level of p = 0.05. Data points in each graph represent the mean values obtained from the replicates, with error bars indicating the corresponding standard deviations. To confirm significant differences in adsorption between the fibers, the Tukey–Kramer test for multiple comparisons was applied, considering a probability level of 5%.

3. Results and Discussion

3.1. Physicochemical and Structural Characterization of Raw Coconut Fibers

The morphological characterization of raw coconut fibers is presented in Figure 1a–c. Figure 1a shows the fiber extremity at 700× magnification, highlighting pronounced surface irregularities and a rough texture. Figure 1b,c display longitudinal views of the fiber at 250× and 700× magnifications, respectively, where a structure with well-defined fissures and cavities can be observed. The presence of superficial fissures and cavities, together with irregular roughness, plays a crucial role in oil adsorption. These structural features increase the specific surface area, promoting greater contact between the fiber and oil molecules. This roughness is intrinsically related to the chemical composition of coconut fibers, since their lignocellulosic structure is composed of cellulose microfibrils bound by hemicelluloses and lignin, which naturally generate heterogeneous surfaces with irregularities, fissures, and cavities [12,14]. SEM analysis confirms that raw coconut fibers exhibit highly favorable morphological characteristics, making them effective candidates for bioadsorbents in oil spill remediation.

Figure 1. Scanning Electron Microscopy (SEM) images of raw coconut fibers: (a) fiber extremity at 700× magnification; (b) longitudinal view at 250× magnification; (c) longitudinal view at 700× magnification.

The surface chemistry of the raw coconut fiber was investigated by ATR-FTIR (Figure 2). The dominant broad band at ~3342 cm⁻¹ is assigned to ν(O–H) stretching from hydrogen-bonded hydroxyls in cellulose, hemicelluloses, and lignin [44,45]. The band near ~2941 cm⁻¹ corresponds to aliphatic ν(C–H) of methyl/methylene groups in the polysaccharide backbone [44,45]. In the fingerprint region, a distinct peak at ~1732 cm⁻¹ is assigned to ν(C=O) of acetyl/uronic ester groups in hemicelluloses and non-conjugated carbonyls from lignin [45]. Lignin is indicated by bands at ~1604 and ~1510 cm⁻¹, attributed to aromatic skeletal vibrations [46]. The polysaccharide framework is supported by bands at ~1420 cm⁻¹ and ~1343 cm⁻¹ (δ(C–H)) [44,47], as well as a strong band at ~1241 cm⁻¹ assigned to C–O stretching in lignin/hemicellulose [46]. The most intense absorption occurs at ~1036 cm⁻¹, dominated by ν(C–O–C)/ν(C–O) of the pyranose ring in cellulose [44]. Altogether, these features confirm the lignocellulosic composition of raw coconut fiber, with a predominant polysaccharide framework and a lignin fraction [44,45,46,47].

Figure 2. Fourier Transform Infrared (FTIR) spectrum of coconut fibers.

The chemical composition of coconut fiber, expressed as a percentage of dry matter, is summarized in Table 1. These results are consistent with values reported in the literature for coconut fiber, which is characterized by its high lignin content (approximately 40–55%), moderate cellulose (30–40%), hemicelluloses (10–30%), and ash content typically ranging from 2 to 5%. The levels of ash and extractives often show considerable variation among studies, which can be attributed to the washing and pretreatment steps applied to the fiber. It is important to note that the relative proportions of these components may vary depending on the fiber’s origin, age, and processing conditions [21,22].

Table 1. Chemical composition of coconut fiber.

Component	Fraction (%)
Cellulose	28.29 ± 2.46
Hemicelluloses	17.79 ± 1.23
Lignin	46.08 ± 1.64
Ash	3.10 ± 0.17
Extractives	9.38 ± 0.18

Lignin, in particular, is a key component for contaminant adsorption due to its hydrophobic nature, rigid structure, high specific surface area, porosity, and eco-compatibility [16,48]. As a complex, branched, amorphous, and tridimensional macromolecule composed of phenolic units derived from phenylpropane, lignin is rich in aromatic rings and ether and carbon–carbon linkages, which favor interactions with non-polar organic compounds such as petroleum hydrocarbons [49]. The high lignin content (46.08%) observed in this study supports the potential use of coconut fiber as an efficient bioadsorbent, since its aromatic backbone and hydrophobicity facilitate interactions with crude oil and other hydrocarbon compounds [16,48].

Although cellulose (28.29%) and hemicelluloses (17.79%) do not exhibit the same hydrophobicity and affinity for hydrocarbons as lignin, they contribute significantly to the structural properties of coconut fiber. Cellulose, with its crystalline structure, provides mechanical strength and rigidity, while hemicelluloses, being more amorphous and flexible, contributes to water absorption and structural resilience. These properties together make coconut fiber both durable and adaptable for environmental applications [50].

The ash content (3.10%) indicates the presence of mineral elements, which may influence adsorption behavior. Although present in small quantities, these minerals could participate in ion exchange or adsorption enhancement, depending on the environmental conditions [21,22]. Further investigation is needed to clarify their specific role in adsorption mechanisms involving petroleum compounds.

X-ray diffraction (XRD) analysis was performed to investigate the crystalline structure of raw coconut fiber, and the profile is provided Figure S3. The pattern exhibits an intensity distribution characteristic of a semi-crystalline material. A prominent diffraction peak was observed near 2θ = 22.5°, corresponding to the (200) reflection of cellulose I, confirming the semi-crystalline nature of the fiber matrix [33,51]. Additionally, peaks at approximately 2θ = 14.8° and 16.5° are observed, corresponding to the (1–10) and (110) reflections of cellulose I, respectively [33]. CrI of coconut fiber was calculated as 63.59% using the Segal method. While widely used for its simplicity, it is important to note the limitations of the Segal method, as discussed by [34]. The intensity at the minimum (I(am)) in the Segal method is not solely representative of the amorphous content but also includes contributions from overlapping crystalline reflections [34]. Despite this, the CrI provides a useful comparative measure of crystallinity. This result indicates that the material contains a substantial proportion of amorphous regions, which are advantageous for adsorption processes due to their higher surface area and molecular flexibility. In contrast, crystalline regions are more ordered and less interactive, making them less effective for adsorption applications [52]. When compared to other cellulosic materials such as energycane bagasse (CrI = 32.4%) [53], curaua fibers (CrI = 58.1%) [54], tucum fibers from the Amazon (CrI = 55.7%) [55], and sisal fiber (CrI = 70.68%) [56], coconut fiber demonstrates a favorable balance between crystallinity and surface area. This combination supports its potential as a low-cost, high-efficiency bioadsorbent for environmental remediation.

The XPS analysis of raw coconut fiber is presented in Figure S4. The analysis confirmed the predominance of carbon (C 1s, 285.0 eV; 72.8% atomic) and oxygen (O 1s, 533.1 eV; 25.2% atomic) on the material’s surface, which is consistent with its lignocellulosic composition based on cellulose, hemicelluloses, and lignin. A strong peak at 285.0 eV corresponds to C–C and C–H bonds, which are typical of aliphatic and aromatic chains present in these biopolymers. The O 1s signal indicates the presence of oxygen-containing groups such as hydroxyls, ethers, and carbonyls [36,37]. High-resolution spectra and elemental quantification are presented in Figure S5, including minor peaks for nitrogen (N 1s, 400.1 eV; 1.0%), calcium (Ca 2p, 347.9 eV; 0.3%), sulfur (S 2p, 168.4 eV; 0.2%), and silicon (Si 2p, 102.5 eV; 0.6%). Although present in low concentrations, these elements are commonly found in plant fibers as natural constituents or residues from cultivation and harvesting processes. The presence of calcium and silicon, for example, may be associated with the plant cell wall structure or with the deposition of soil-derived mineral [35,57]. The surface chemical profile revealed by XPS indicates an abundance of polar functional groups, imparting a hydrophilic character to the fiber surface. This characteristic may limit the adsorption of nonpolar compounds, such as petroleum, by reducing hydrophobic interactions. On the other hand, oxygen-containing functional groups serve as reactive sites for chemical modifications that can enhance hydrophobicity and, consequently, improve the fiber’s affinity for hydrocarbons.

The results highlight the complexity of coconut fiber as a lignocellulosic material, in which hydrophilic and oleophilic characteristics coexist. The high lignin content (46.08%) favors hydrophobicity, whereas XPS analysis (O/C = 25.2%/72.8%) indicates surface hydrophilicity [58]. This duality shows that surface and bulk properties act differently in adsorption, an essential aspect for evaluating the fiber’s performance as a bioadsorbent in oil spill response strategies.

3.2. Physicochemical Characterization of Petroleum

The main physicochemical properties of the crude oil from the Campos Basin are summarized in Table 2. The oil presented an API gravity of 21.7°, classifying it as a heavy oil. API gravity (American Petroleum Institute gravity) is a measure of petroleum density relative to water, where values lower than 22.3° indicate heavy oils [59]. Under the conditions of the adsorption tests (approximately 23 °C), the oil remained fluid, indicating low viscosity. This property enhances the ability of coconut fibers to retain oil on their rough surfaces and within their cavities [17], supporting the use of raw coconut fibers for petroleum adsorption. Detailed characterization of both the crude oil and the coconut fibers was essential to understand their interactions and to define appropriate experimental conditions. The structural and functional properties of the fibers demonstrated their potential as bioadsorbents, providing a foundation for the development of an efficient, sustainable, and low-cost adsorption process within oil spill response strategies.

Table 2. Physicochemical properties of the crude oil from Campos Basin.

Property	Value
Density (g·mL−1, 20 °C)	0.9225
API gravity (°)	21.7 (Heavy oil)
Pour point (°C)	–39

3.3. Fractional Factorial Design for Oil Adsorption

The results of the fractional factorial design revealed that Time, Adsorbent Dose, Adsorbate Concentration, and the Time × Adsorbent Dose interaction were statistically significant (p ≤ 0.05), whereas Salinity and pH showed no significant effect. The Pareto chart (Figure 3) confirms that adsorbent dose had the most pronounced effect on oil adsorption, followed by oil concentration and contact time. This clear presentation highlights that the adsorption capacity of raw coconut fibers is strongly dependent on operational conditions, with dose, concentration, and contact time being the most relevant factors.

Figure 3. Pareto chart of standardized effects (Response: Oil adsorption, α = 0.05). The red vertical line represents the critical t-value (α = 0.05), indicating the threshold above which effects are statistically significant.

The main effects plot (Figure S6) highlights the influence of adsorbent dose, showing that a lower dose resulted in greater adsorption. This is attributed to the increased internal void volume in less compact barriers, which favors oil diffusion. Although a higher dosage increases the number of available active sites, dense fiber packing can restrict capillary action and oil accessibility [60,61]. Adsorption capacity is intrinsically related to the sorbent packing factor (ϕ), defined as the ratio between the total fiber volume and the total sorbent volume. As described in Equation (8), an increase in the packing factor corresponds to a reduction in the Adsorption capacity of the fibers:

$$Adsorption capacity=V_S\left({\displaystyle \frac{1-\varphi }{k_0\rho }}\right)$$

(8)

where $V_S$ represents the specific volume of air within the sorbent, $\rho$ denotes the fiber density, $\varphi$ corresponds to the sorbent packing factor, and $k_0$ is a constant.

Oil concentration also exhibited a positive effect on adsorption, which is consistent with the literature [62]. However, excessive concentrations may saturate the sorbent surface, suggesting the need for regeneration strategies during extreme spill events. Contact time showed an inverse relationship, in which shorter durations resulted in higher adsorption. This behavior is favorable in emergency scenarios where rapid oil capture is crucial. Adsorption tends to be most effective during the initial minutes due to the abundant availability of active sites [16,63].

Among the investigated factors, the study also revealed less intuitive insights regarding salinity and pH. Although not statistically significant, salinity consistently exerted a negative effect on adsorption. This finding is in line with reports indicating that elevated salinity reduces oil mobility and capillary penetration [64]. In addition, the presence of dissolved ions can promote the formation of water clusters strongly bound to the functional groups on the sorbent surface, thereby blocking contaminant access to micropores [65]. Therefore, even under conditions that could theoretically favor adsorption through the reduction in organic compound solubility (salting-out), factors such as physical obstruction and altered interaction dynamics may prevail, explaining the negative effect observed [66]. This apparent resistance to salinity variations, albeit with slightly reduced performance, demonstrates the robustness of the material under adverse environmental conditions. Similarly, pH showed a slight positive effect. In alkaline environments, natural fibers tend to acquire a more negative surface charge, which facilitates oil droplet deposition [67]. The literature reports that the point of zero charge (pHpzc) of activated carbon derived from coconut fiber is approximately 7.1 [68]. At pH values below this point, the surface tends to be protonated, favoring the adsorption of negatively charged compounds, whereas at pH above this value, the surface becomes deprotonated and more negatively charged, which is consistent with the improved adsorption observed under alkaline conditions. While the material investigated here was not activated, the observed trend of improved adsorption under alkaline conditions is consistent with the expected behavior of lignocellulosic adsorbents in relation to their surface charge characteristics. Although subtle, these findings highlight the importance of considering physicochemical mechanisms beyond statistical thresholds. The interaction plots (Figure S7) revealed that significant interactions occurred mainly between adsorbent dose and contact time. Other combinations showed consistent trends, particularly the combination of higher pH and lower salinity, which favored adsorption.

Analysis of variance (ANOVA) confirmed that the factors adsorbent dose, adsorbate concentration, and contact time, as well as the time × dose interaction, were statistically significant (p ≤ 0.05), whereas salinity and pH were not significant. The model presented a coefficient of determination of R² = 91.5% and an adjusted R² = 83.5%, indicating high explanatory capacity. The plot of observed versus predicted values (Figure 4) further supports the quality of the fit, showing that the points are distributed close to the line of equality without relevant systematic deviations. These results demonstrate the adequacy of the linear model in representing the adsorption process and confirm its predictive utility under conditions similar to those evaluated experimentally.

Figure 4. Plot of observed versus predicted values for the linear regression model. The scatter plot compares observed and predicted values. The blue dots represent experimental data points, while the red line indicates the line of perfect agreement (y = x), showing the correlation between the predicted and observed results.

Therefore, the factor levels adopted in the subsequent experiments, based on the significant effects observed in the experimental design stage, were oil concentration of 80 mL·L⁻¹, contact time of 5 min, and adsorbent dose of 0.5 g. These parameters provide a valuable basis for the development of practical, scalable, and cost-effective sorbent barriers using raw coconut fibers.

3.4. Kinetic Study and Adsorption Isotherms

The kinetic and adsorption results are shown for coconut fiber in contact with crude oil from the Campos Basin, Brazil. Figure 5 illustrates the evolution of the adsorbed oil quantity (qt) as a function of time (t), ranging from 0 to 120 min. It can be observed that as the contact time increases, the amount of oil adsorbed also increases, reaching a plateau after approximately 120 min with an adsorption of 4.18 ± 0.19 g·g⁻¹, indicating that the system had reached equilibrium, where no further significant adsorption occurred.

Figure 5. Kinetic evolution of oil adsorbed quantity (qt) as a function of time (t) for adsorption using coconut fiber and oil from the Campos Basin, Brazil.

Despite this behavior, statistical analysis of the data using the Tukey test revealed no significant difference in the results over time. In just 5 min, the adsorption was 3.01 ± 0.15 g·g⁻¹, suggesting that in a short period, the adsorption process approaches equilibrium. This is particularly advantageous in situations that require a rapid response, such as oil spill containment. These findings are consistent with the literature, as previously mentioned, where time is the third most significant main effect, showing better adsorption results in the low configuration. This conclusion is particularly relevant because, during an oil spill, minimizing the response time of the applied technology is crucial. By doing so, the risks of further oil spread and secondary contamination are reduced. This behavior is in agreement with several studies in the literature, which report that adsorption tends to increase significantly during the initial minutes of contact. This phenomenon occurs because, initially, the active sites are abundantly available and are quickly occupied. Over time, the interaction proceeds more gradually [16,63].

The experimental data were fitted to the pseudo-first-order and pseudo-second-order kinetic models, both in their linear forms, to describe the oil adsorption behavior using coconut fiber. For the pseudo-first-order model, the linear fit (Figure S8) yielded an R² of 0.741, which is considered low, indicating that this model did not adequately represent the experimental data. The kinetic constant (k₁) was determined as 0.010 min⁻¹, and the calculated equilibrium adsorption capacity (qₑ) was 1.719 g·g⁻¹, as shown in Table 3. These results demonstrate that the pseudo-first-order model does not satisfactorily describe the adsorption process under the studied conditions.

Table 3. Fitted parameters of pseudo-first-order and pseudo-second-order kinetic models.

Kinect Model	R2	k (Rate Constant)	qₑ (g·g−1)
Pseudo-first-order	0.741	0.010 min−1	1.719
Pseudo-second-order	0.990	0.043 g·g−1·min−1	4.182

On the other hand, the pseudo-second-order model, which assumes that the adsorption rate is proportional to the square of the difference between the maximum adsorption capacity and the amount adsorbed at time t, provided a more robust fit. The corresponding plot (Figure S8) showed an R² of 0.990, indicating excellent agreement with the experimental data. The adsorption rate constant (k₂) was determined to be 0.043 g·g⁻¹·min⁻¹, and the calculated equilibrium adsorption capacity (qₑ) was 4.182 g·g⁻¹ (Table 2), a value more consistent with those observed experimentally. These results indicate that the pseudo-second-order model more accurately describes the oil adsorption process onto coconut fiber, reflecting a predominant chemisorption mechanism characterized by stronger interactions. However, the simultaneous occurrence of weaker interactions associated with physisorption cannot be excluded. This finding reinforces the effectiveness of coconut fiber as an adsorbent and highlights the relevance of the pseudo-second-order model in predicting performance for oil spill remediation applications [16,28,69].

Adsorption equilibrium is another essential parameter for evaluating the efficiency of an adsorbent, as it defines the maximum adsorption capacity under specific process conditions. The adsorption equilibrium curve for oil removal using coconut fiber is presented in Figure 6. From the graph analysis, it is observed that adsorption increases with the increase in oil concentration in the solution, reaching a saturation point around 0.05 g·mL⁻¹. Above this concentration, a slight reduction in adsorption capacity occurs, indicating the maximum occupancy of the active sites on the coconut fiber and possible competitive effects between the adsorbed molecules. This phenomenon can be explained by the progressive saturation of the active sites and the aggregation of oil molecules at high concentrations, reducing adsorption efficiency [70].

Figure 6. Oil adsorption equilibrium curve on raw coconut fiber.

The curve suggests a typical adsorption system behavior, where adsorption capacity increases with the initial adsorbate concentration until reaching a plateau, characteristic of Langmuir or Freundlich isotherms. The Langmuir model assumes the formation of a monolayer on the active sites of the adsorbent and the absence of interactions between the adsorbed molecules [70]. In contrast, the Freundlich model, applied to heterogeneous systems, considers multiple layers and different adsorption energies at the active sites [71]. The slight reduction in adsorption capacity at high concentrations may be associated with the saturation of the active sites on the coconut fiber, as observed in studies on oil adsorption in lignocellulosic materials [72] and the formation of micelles or oil aggregates in the aqueous medium, hindering interaction with the active sites [73].

In addition to the Langmuir and Freundlich models, the data were fitted to the Sips model, which combines characteristics of both. This model is useful for describing systems with both homogeneous and heterogeneous behavior, providing a better fit over a wide range of adsorbate concentrations [74]. The Sips model is frequently used to describe adsorption processes in lignocellulosic materials and is suitable for representing oil adsorption on natural fibers [75]. The model fitting plots for Langmuir, Freundlich, and Sips isotherms are available in the Supplementary Information (Figure S9). The adjusted parameters for each equilibrium isotherm and the correlation coefficients are presented in Table 4. The high correlation of the models with the experimental data reinforces the reliability of the isotherms used to describe oil adsorption by coconut fiber.

Table 4. Fitted parameters of Langmuir, Freundlich, and Sips isotherm models for oil adsorption by coconut fiber.

Isotherm Model	R2	qₘₐₓ or qₘₛ (g·g−1)	K or b	n
Langmuir	0.919	5.535	Kᴸ = 78.992 (L·mg−1)	–
Freundlich	0.886	–	Kᶠ = 10.54 (L1/n.g−1/n)⋅(g.g)−1	3.409
Sips	0.908	5.286	b = 12,008.02 (L·g−1)n	105.47

Among the evaluated models, the Langmuir model provided the best fit to the experimental data, with a determination coefficient (R²) of 0.919. The Sips model yielded an R² of 0.908, while the Freundlich model showed a lower R² of 0.886. These results indicate that adsorption predominantly occurs through the formation of a monolayer on the active sites of the coconut fiber, suggesting a homogeneous distribution of adsorption sites consistent with Langmuir-type behavior. The suitability of the Langmuir model for oil adsorption onto natural fibers has been previously reported in the literature, as lignocellulosic materials often contain specific functional groups that interact in a structured and saturable manner with oily compounds [72,76]. The small difference between the Langmuir and Sips model fits suggests some degree of heterogeneity in the adsorption sites, justifying the competitive performance of the Sips model.

In contrast, the lower fit of the Freundlich model (R² = 0.886) suggests that the system does not exhibit strongly heterogeneous characteristics, unlike other natural adsorbents that present high variability in the distribution of active sites and adsorption energies [72,73]. The Freundlich model is more appropriate for highly heterogeneous surfaces and multilayer adsorption processes, which do not appear to apply as the predominant mechanism for the coconut fibers investigated in this study, although the occurrence of multilayer interactions cannot be excluded. The slight difference in R² values between the Langmuir and Sips models may be attributed to secondary interactions among the adsorbed oil molecules, such as Van der Waals forces or hydrophobic effects, which can influence the molecular distribution on the adsorbent surface [77].

3.5. Reusability of Coconut Fiber Mini-Barriers in Consecutive Adsorption–Desorption Cycles

The results of the reuse experiments with coconut fiber over six consecutive adsorption and desorption cycles are presented in Figure 7. A progressive decline in adsorption capacity was observed, possibly associated with residual oil accumulation within the fiber matrix and with alterations in the surface properties of the material [78]. Between the first and second cycle, coconut fiber exhibited a sharp decrease of 52.70% in adsorption capacity. In the third cycle, the reduction was more moderate, at 18.86%, followed by stabilization in the subsequent cycles. Although a significant loss of performance was recorded in the initial cycles, the maintenance of relatively constant values from the third cycle onwards suggests the potential for reusing coconut fiber in successive adsorption processes.

Figure 7. Adsorption performance of coconut-based biosorbent fibers over six reuse cycles.

The quantification of the recovered oil mass in each cycle is presented in Figure 8. The increasing trend in desorbed oil mass across the cycles suggests that, even after multiple reuse stages, a portion of the oil can still be mechanically recovered, reinforcing the potential for adsorbent material reutilization. In the first cycle, a smaller amount of oil was recovered compared to the subsequent cycles. This may be attributed to the initial occupation of active sites and the occurrence of chemisorption, in which strong interactions between the oil and the fiber lead to partial retention of the oil, hindering its mechanical desorption. In contrast, during the subsequent adsorption tests, the amount of desorbed oil remained statistically constant according to Tukey’s test and was higher than in the first cycle. This suggests that after the initial chemisorption phase fewer active sites were available for strong interactions, enabling more consistent mechanical recovery in the following cycles [79]. However, with the exception of the fifth cycle, where a transient increase in adsorption was observed, the amount of oil removed did not remain constant.

Figure 8. Desorbed oil mass (g) from coconut fiber after each adsorption–desorption cycle. Different lowercase letters indicate statistically significant differences between cycles according to Tukey’s test (p < 0.05).

When comparing the adsorption and desorption results, an opposite trend was observed between the adsorption capacity of the fiber and the amount of oil recovered, indicating that adsorption decreases as mechanical desorption becomes progressively more effective over successive cycles. This behavior suggests that as the fiber becomes partially saturated, a fraction of the adsorbed oil becomes more susceptible to mechanical recovery, as evidenced by the progressive increase in desorbed oil mass. However, the continuous reduction in adsorption capacity indicates that part of the oil remains irreversibly retained within the fiber structure. The results support the hypothesis of monolayer adsorption, suggesting that the interaction between oil and coconut fiber occurs predominantly through van der Waals forces or limited surface bonding, which restricts adsorption beyond a certain threshold. Additionally, the presence of strongly retained fractions indicates the occurrence of chemisorption, in which oil molecules interact more intensely with the fiber’s functional groups, hindering removal by mechanical means [69]. Thus, although mechanical desorption is advantageous due to its operational simplicity and reduced residue generation under real-scale conditions such as oil spill scenarios, the complete removal of oil may require the use of chemical reagents, as reported in other studies [25,80]. However, a strictly mechanical desorption approach was chosen in order to reduce costs and facilitate operation under real-scale conditions, such as oil spill scenarios. Mechanical oil removal decreases the need for chemical reagents, which simplifies the process and contributes to reducing waste generation, favoring its application on a large scale [17,81].

3.6. Meso- and Field-Scale Evaluation of Coconut Fiber Sorbent Barriers

In the mesoscale tests, coconut fiber sorbents exhibited average adsorption capacities of 2.36 g·g⁻¹ within only 5 min (Figure 9). At this scale, a 40.35% reduction in adsorption was observed compared with the values obtained at the microscale. This performance decrease can be attributed to the intensification of hydrodynamic forces present in the mesoscale system. These forces directly affect the residence time of oil at the fiber–water interface due to the wave simulation in the mesocosm. This movement increases turbulence, favoring the interaction of oil droplets with suspended particulate matter, dispersing part of the oil into the water column, and consequently reducing the effective contact time of the biosorbent applied on the surface with the oil [7,59,82].

Figure 9. Comparison of the average petroleum adsorption capacity of coconut fibers at microscale and mesoscale. The images highlight (a) the mesocosm system used for wave simulation and (b) the tests conducted in Erlenmeyer flasks at the microscale.

Although the microscale tests employed reciprocating agitation to simulate marine hydrodynamics, the mesoscale setup represents more realistic conditions, with wave simulation and turbulence, hydrodynamic features inherent to the oceanic environment [7,59,82,83]. Moreover, this simulation, under conditions closer to reality, enabled a wider spreading of oil, introducing a variable that is seldom considered in adsorption tests, which are generally conducted in experimental units with reduced surface areas, such as Erlenmeyer flasks or beakers [83]. This difference can be observed in Figure 9a,b. The mesoscale system (Figure 9a) allows for wide dispersion of oil across the water surface, more closely reflecting the action of waves and the spreading dynamics observed in coastal environments. In contrast, the Erlenmeyer flasks (Figure 9b) highlight the limitations of the microscale, where spatial restriction prevents natural oil spreading. This limitation may overestimate sorbent efficiency by promoting forced and unrepresentative contact between the biosorbent and petroleum.

The results obtained in mesoscale tests highlight critical gaps for the translation of lignocellulosic biofibers into effective technologies for oil spill response in marine environments. It is essential to perform tests that more realistically consider the effects of weathered oil, such as spreading, which reduces the interface between oil and sorbent [83,84]; emulsification, which promotes the formation of water-in-oil emulsions and decreases oil hydrophobicity due to the creation of heterogeneous compounds [84,85]; aggregate formation and dispersion, which remove oil from the sea surface by increasing density and dispersing it as droplets in the water column or onto the seabed [7,82]; and geochemical and rheological changes in petroleum, since oil tends to become more viscous, with reduced surface area and fluidity, directly affecting the adsorption efficiency of biosorbents [59,83].

Although rarely explored in the literature, Abdelwahab et al. [86] performed a mesoscale adsorption experiment simulating an oil spill in a basin connected to the sea, where treated luffa cylindrica (LCLA) and palm fibers (PFLA) were packed in polyester pads and exposed to natural wave movement for 30 min. Adsorption capacities decreased by 38.89% (LCLA) and 42.35% (PFLA) compared to laboratory tests, highlighting the influence of hydrodynamics. Similarly, in the present study, mesoscale tests also showed reduced adsorption efficiency, evidencing the impact of spreading, emulsification, and rheological changes in crude oil on fiber performance. These results reinforce the importance of validating biosorbents across multiple scales and hydrodynamic scenarios to ensure their effectiveness in real oil spill applications.

In addition to the mesoscale results, field trials further demonstrated the operational feasibility of the raw coconut fiber barrier under real marine conditions. The barrier exhibited excellent buoyancy when placed in the marine environment, as shown in Figure S10, remaining fully afloat for 20 min without sinking or showing signs of saturation. This duration is four times longer than the minimum of 5 min required for effective oil adsorption, a value confirmed in this study based on the experimental design. The barrier’s stability throughout the entire immersion period, even under the influence of natural currents, demonstrated its physical resistance and ability to maintain adsorption function under real environmental conditions.

Moreover, after 12 months of storage in a closed room, with the material kept properly packaged and protected, the material showed no signs of degradation, compaction, or loss of structural integrity, reinforcing its potential for reuse in future oil spill events, with the capacity to complete multiple adsorption cycles without compromising performance. This is one of the few studies that provides experimental evidence supporting the feasibility of natural biosorbents under real operational conditions, contributing significantly to the advancement of sustainable and scalable solutions for emergency oil spill response.

Several studies have reported the use of lignocellulosic biosorbents for oil removal, with adsorption capacities varying according to the material, pretreatment strategy, and experimental conditions. Table 5 presents a comparative summary of adsorption capacities reported in the literature for different biosorbents, together with the results obtained in the present study. This comparison highlights the diversity of approaches, ranging from raw fibers to chemically modified materials and biochars, as well as the influence of regeneration methods on practical applicability.

Table 5. Summary of recent studies on oil adsorption using lignocellulosic biosorbents.

Biosorbent	Treatment/Modification	Adsorbate	Adsorption Capacity (g·g−1)	Experimental Conditions	Regeneration	Efficiency Loss	Desorption Method	Ref.
Banana Cellulose	Succinic anhydride in ionic liquid	Crude oil	32.12	Dose: 0.05 g; Time: 200 min; Initial oil conc.: 25 g	Not reported	—	—	[87]
Jute Fiber	None (raw)	Crude oil	7.12	Dose: 1 g; Time: 15 min (ASTM F-726-12)	Not reported	—	—	[88]
Bamboo Fiber	Delignification + CVD	Paraffin oil	18.80	Time: 10 min; Oil conc.: 50 mL	Yes	96% after 5 cycles	Mechanical squeezing	[89]
Coconut Fiber	Hydrothermal treatment (170 °C)	Diesel oil	3.20	Dose: 0.5 g; Time: 120 min; Temp.: 180 °C (ASTM F726-17)	Not reported	—	—	[90]
Coconut Fiber	None (raw)	Petroleum (crude oil)	4.18 ± 0.19	Dose: 0.5 g; Time: 5 min; Conc.: 80 mL·L−1; pH: 7	Yes, 5 cycles	52.70%	Centrifugation	Present study

Compared with values reported in the literature (Table 5), the oil adsorption capacity of raw coconut fiber obtained in this study (4.18 ± 0.19 g·g⁻¹ at equilibrium) is lower than that of chemically modified biosorbents, such as banana cellulose treated with succinic anhydride (32.12 g·g⁻¹ [87]) or bamboo fibers subjected to delignification and CVD (18.80 g·g⁻¹ [89]), but it is similar to other untreated or mildly treated materials, such as jute fiber (7.12 g·g⁻¹ [88]) and hydrothermally treated coconut fiber (3.20 g·g⁻¹ [90]). Importantly, unlike most studies that did not include regeneration tests, our work showed that raw coconut fiber maintained structural integrity and reusable capacity after centrifugation, despite an initial 52.70% reduction, highlighting advantages of low cost, wide availability, and operational simplicity for rapid emergency response. This practical relevance is reinforced by moving beyond laboratory batch assays to include meso-scale simulations and real marine field tests, offering performance evidence under hydrodynamic complexity and natural variability. Nevertheless, the absence of standardized protocols for scaling and environmental testing still hampers cross-study comparability and technology transfer; recent efforts such as the BSEE field-oriented protocol for Type I sorbents [91] and the emphasis by Vasić et al. on the need for meso-, pilot-, and field-scale validation [92] underscore the importance of broader adoption of harmonized testing frameworks.

4. Conclusions

Raw coconut fibers demonstrated to be effective biosorbents for crude oil removal, with consistent performance across multiple experimental scales. At the microscale, the fractional factorial design identified adsorbent dosage, contact time, and oil concentration as the most significant factors influencing adsorption. Kinetic modeling revealed that the adsorption process followed a pseudo-second-order model, indicating chemisorption as the predominant mechanism, while equilibrium data fitted best to the Langmuir isotherm, confirming monolayer adsorption on the fiber surface. The maximum equilibrium capacity reached 4.18 ± 0.19 g·g⁻¹, comparable to values reported for other untreated or mildly treated lignocellulosic materials.

Regeneration by centrifugation enabled reuse for up to five cycles, with an initial capacity loss of 52.70% but subsequent stabilization, confirming the feasibility of mechanical recovery without chemical reagents and aligning with circular economy principles. At larger scales, coconut fiber barriers-maintained buoyancy, structural integrity, and adsorption efficiency under simulated hydrodynamic conditions in mesocosms, with average capacities of 2.36 g·g⁻¹, and in real marine field trials, where they remained stable and fully functional. These findings highlight the robustness of the material under environmentally complex and dynamic scenarios, rarely addressed in previous studies.

Altogether, the results demonstrate that, beyond adsorption efficiency, practical feasibility, low cost, wide availability, and environmental compatibility position raw coconut fibers as a sustainable and scalable alternative for oil spill remediation. This study contributes significantly to bridging the gap between bench-scale research and real-world applications, reinforcing the importance of integrating multiscale approaches and standardizing methodologies for biosorbent evaluation in marine environments.