Synthesis and application of adsorbent pads for removal of oil pollutants from water

Abstract

Oil is a crucial foundation for energy, significantly influencing both industrial production and daily human activities. However, oil pollution inflicts considerable harm on the environment. To protect the environment, it is essential to implement effective cleaning strategies to remove oil from aquatic systems. Among the various remediation techniques, the adsorption process has emerged as one of the most efficient approaches for removing oil contaminants from water sources. This research used animal hair, modified palm fibers, and polyurethane particles as adsorbent pads to remediate water contaminated with oil. The characterization of the synthesized adsorbent and the assessment of the impact of modifications at each stage were conducted utilizing various analytical techniques, including Fourier transform infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), thermogravimetric analysis (TGA), X-ray diffraction (XRD), and Brunauer-Emmett-Teller (BET) surface area analysis. The key factors influencing absorption, such as pH, temperature, contact time, and oil concentration, have been assessed. The analysis revealed that the optimal conditions for absorbing 50 g of oil are a temperature of 40 °C, a pH level of 7, and a contact duration of 10 min. The results of isotherm studies indicated that the adsorbent has the best fit with the Langmuir model. In this research, the maximum absorption capacity was 3333 mg/g based on Langmuir adsorption model. Furthermore, the absorbent follows the pseudo-second-order kinetic model with rate constant of 0.06 g/(mg.min) and presents the chemisorption process. The developed adsorbent demonstrated significant potential as an effective and economical solution for oil contamination remediation.

1. Introduction

Oil and oil products play a major part in the global economy because they are the main drivers of various industries around the world. Oil pollution is an unavoidable risk associated with oil production and transportation If they occur, it can cause significant environmental damage [1]. Furthermore, the harmful materials in these contaminants and their breakdown products can cause aquatic toxicity [2]. By using these polluted waters, toxic substances can be absorbed into the human body, which will ultimately cause serious damage to human health [3]. Oil spills can be called as one of the main problems that occur due to human errors and negligence. All these events cause serious damage to the health of humans, animals, and the environment [[4], [5], [6], [7]]. Oil spills in the sea are usually much more damaging than on land, due to the special properties of oil such as low density, hydrophobicity, coagulation, and flocculation properties, non-biodegradation, and interaction with microorganisms, there is a possibility of forming a large oil slick on the surface of the water and marine oil snow (MOS), which can endanger marine organisms and the marine environment [8]. Oil slicks progressively lead to the development of sludge that includes a variety of harmful substances, including sulfides, phenols, alkanes, aromatics, asphaltenes, resins, and polycyclic aromatic hydrocarbons, all of which are known to possess carcinogenic and mutagenic properties. These events can severely impact marine organisms by inflicting significant harm on the marine environment, resulting in enduring secondary consequences for aquatic ecosystems. Additionally, the presence of volatile organic hydrocarbons, which are found in oil sludge, poses risks to the central nervous system, potentially leading to symptoms such as headaches, dizziness, and memory impairment [9]. Nitrophenols can also be generated through photochemical reactions involving aromatic compounds and volatile organic hydrocarbons. These compounds contribute to the creation of organic aerosols in the atmosphere. Additionally, this combination poses significant health risks, including disruptions to the body's circulatory system and an increased risk of cancer. The swift removal of hydrocarbon contaminants and aromatic compounds from water surfaces is crucial in mitigating the formation and spread of nitrophenols [10,11]. Cleaning water systems and seas from such pollution is one of the basic challenges that requires a lot of attention [5]. Therefore, one should always find suitable methods to quickly remove oil stains after each spill [8,12].

There are various methods for cleaning hydrocarbon contaminants in water, such as direct burning, dispersers, mechanical skimmers, canvases, polymerized materials, and adsorbents [5,[13], [14], [15], [16]]. Most of these methods are time-consuming, expensive, and may require a lot of personnel and equipment, and most of the time may lead to environmental damage [4]. For example, we can refer to uncontrolled in-situ burning that can lead to severe air pollution [17], so it is essential to find more efficient, cost-effective, cleaner, and environmentally friendly methods for cleaning and removing oily stains is always necessary [18]. When oil comes into contact with water, it forms an oil-in-water emulsion or floating film due to the lower density of oil than water, which must be removed before discharge into the environment [19]. Also, there are various adsorbent materials applied as effective oil spill separators in the literature such as carbon materials [2], zeolites [20], clay [21], keratin [22], nanoparticles [23] and so on. For example, Mubarak et al. prepared a polyvinyl chloride membrane based on copper oxide magnetic nanocomposite, which is durable and anti-fouling. This membrane showed an oil rejection of 98 %. Other magnetic membranes prepared from polyvinyl chloride showed an oil rejection of about 90 % [24].

Among the available methods, adsorbents, which are among the physical methods, have attracted a lot of attention in recent years because of their simplicity of operation and lower cost. Commercial adsorbents are widely used in cleaning oily stains. In recent years, the use of natural adsorbents due to their biocompatibility, availability, low cost, biodegradability, reusability, and high adsorption capacity as high-efficiency adsorbents used for containment and removal of oil stains have been noticed [25]. Using biomasses as natural absorbents will lead to sustainable consumption and as a result sustainable economy and environment [26]. Among the biomasses that have been used in various studies to make natural adsorbents are kapok, rice husk, coconut husk, barley husk, cotton plant, sugarcane bagasse, sawdust, wool fibers, walnut shell, feathers, and garlic skin pointed out [1]. Adsorbents of natural origin, if effectively used, can be more efficient than synthetic ones [27]. For example, Paulauskiene et al. prepared a natural absorbent from barley straw and wood chips, which is processed with methoxy trimethyl silanes and increases the hydrophobicity of the absorbent. To separate the adsorbents from the water, they were placed on a plastic mesh [16]. In another study, Alhassani and colleagues prepared a composite film for the removal of oil pollutants using diesel particulate filters, cellulose acetate, and zinc oxide nanoparticles, the maximum absorption capacity of which is about 65 mg/g [28]. Similarly, Haryanto and colleagues used corncob powder to purify cooking oil and the research showed that the maximum absorption capacity is 12.37 mg/g [29]. Polyurethane foams, and polyurethane (PU) as a foam will provide a suitable enough space and surface area for absorption. Because of their superior characteristics such as low density, open cell, high porosity, and industrial making, polyurethane foams (PUF) show significant oil adsorption capability [30]. Hair is a substance that is considered useless in most societies, and for this reason, it is found in almost all urban wastes in all cities of the world. Hair with a diameter of 50–100 mm has attracted the attention of researchers as a bio-adsorbent with high water-repellency properties and a very porous layer. The property of low density of endoticol in combination with its hydrophilic state makes hair strands have a high floating property on water. Also, this substance, as a natural polymer, will not leave new negative side effects on the water when it is used as an adsorbent on the water surface [31]. Another type of natural adsorbent is palm fiber, which is used as an adsorbent for oil stains [32]. According to the report of the Ministry of Agriculture Jihad, large areas of southern Iran have suitable weather conditions for planting palm trees. These fibers are often burned by farmers or thrown away as waste [33]. Meanwhile, palm fiber is biodegradable, so it is an environmentally friendly and safe material [32]. Therefore, the purpose of this study is to produce a low-cost, accessible, simple, and biocompatible pad. It is made of natural and biodegradable materials and contains hair, activated palm fiber, and polyurethane particles. Different weights of engine oil were used as model pollutants in 400 mL of water. Then determining the adsorption of engine oil stains from aqueous solutions in different concentrations by the made pads is considered. In this regard, the variables of pH, temperature, contact time, adsorbent mass, and oil concentration are evaluated. The effect of temperature, concentration and time was evaluated and discussed through thermodynamic studies, adsorption isotherms and kinetic models.

2. Materials and methods

Deionized water was prepared by Elixir Hirad water distiller model 2112. All materials were prepared by Merck. Sulfuric acid (2 % by volume, 97 %) and Sodium hydroxide (2 % by weight, 37 %) to process palm fibers were used [34].

2.1. Activation of palm fibers

First, the fibers are sourced from the palm groves of Fars province, Zarin Dasht city, then the red palm fibers are pieced into small parts (in length of 4–5 cm) to gain an adsorbent with the appropriate size for the composition. The parts are cleaned with water to eliminate any sticky material and dried in the oven until constant weight. The fibers undergo separate treatment with sodium hydroxide and a 2 % sulfuric acid solution to enhance their active surface area, a process that is carried out overnight at ambient temperature. Following this treatment, the fibers are subsequently dried in an oven set to 80 °C [34,35]. Next, the obtained materials were kept in a dry place until use.

2.2. Hair and polyurethane particles

To improve the performance of the adsorbent, animal hair particles of 4–5 cm were used. Small polyurethane granular particles with sizes of 4–5 mm and animal hair were added to the activated date fibers. In this study, at first, equal-weight amounts of animal hair, activated palm fibers, and polyurethane particles were mixed and poured into a bag-like cover with the following specifications (square shape with dimensions of 13 × 13 cm), and as the plate was sewn with a thickness of two to 3 cm.

2.3. Adsorption experiments

First, the adsorbent pad was weighed before adsorbing the oil. Then a certain amount of engine oil was decanted into a pan comprising 400 mL of water with different concentrations of oil (5, 10, 15, 30, 50, g) and pH (3, 5, 7, 9). Then, the adsorbent pad was slowly placed at the interface of oil and water for a certain contact of time. After the specified time, the pad was placed in an oven at 80 °C for 2 h until it was completely dry and the weight of it was recorded. In order to investigate the effect of temperature, it was investigated in the range of 10–45 °C. The adsorption efficiency and capacity were examined by the following equations (Eq. (1), (2))),

$$\text{Adsorption}\text{efficiency}=\frac{Ci-Ce}{\text{Ci}}\ast 100$$ (1)

$$\text{Adsorption}\text{capacity}=\frac{\left(Ci-Ce\right)v}{\mathrm{m}}$$ (2)

for reusability of the pads, by mechanically squeezing adsorbent pads after each adsorption step, they can be used again (4).

The obtained data in this experiment were investigated employing Langmuir, Freundlich, and Temkin isotherm models. These models can be described as equations of (3–5):

Langmuir adsorption isotherm:

$$\frac{{\mathrm{C}}_{\mathrm{e}}}{{\mathrm{q}}_{\mathrm{e}}}=\frac{1}{\mathrm{Q}°\mathrm{b}}+\frac{{\mathrm{C}}_{\mathrm{e}}}{\mathrm{Q}°}$$ (3)

where q_e (mg/g) is defined as the quantity of oil adsorbed (mg) per adsorbent (g). C_e (mg/L) is the equilibrium concentration of oil in the equilibrium state. The b, Langmuir parameter, which is correlated to adsorption correlation energy. Q^o can be achieved from a plot of Ce/q_e versus C_e. The equilibrium parameter (R_L), without dimension, is the main characteristic of the constant Langmuir isotherm, which is expressed by equation (4):

$${\mathrm{R}}_{\mathrm{L}}=\frac{1}{1+{\text{bC}}_0}$$ (4)

C_o is the initial concentration of the oil. The amount of R_L specifies the type of isotherm. If 1>R_L > 0 adsorption is favorable, R_L > 1 shows unfavorable adsorption. Linear and irreversible adsorption are applied if R_L = 1 and R_L = 0, respectively.

Linear form of Freundlich adsorption isotherm:

$$\log {\mathrm{q}}_{\mathrm{e}}=\log {\mathrm{K}}_{\mathrm{f}}+\frac{1}{\mathrm{n}}\log {\mathrm{C}}_{\mathrm{e}}$$ (5)

where n and K_f are the adsorption constants. K_f and 1/n are the adsorption capacity in a unit concentration and intensity of adsorption, respectively. In this equation, if 1 < n < 10, it is optimal and favorable adsorption, and if n < 1 indicates poor adsorption.

Temkin model can be presented as equation (6):

$$q_e=\frac{\text{RT}}{b_T}\ln \left(K_T\right)+\frac{\text{RT}}{b_T}\ln \left(C_e\right)$$ (6)

For this equation, equilibrium binding energy and the heat of adsorption are designed by the K_T (L/g) and b_T (J/mol), respectively.

To examine the adsorption kinetics, Pseudo-first-order, Pseudo-second-order, and Elovich kinetic equations were applied, which are as follows:

Pseudo-first order kinetic equation (Eq. (7)):

$$\log \left({\mathrm{q}}_{\text{eq}}-\mathrm{q}\right)=\log {\mathrm{q}}_{\text{eq}}-\frac{{\mathrm{K}}_1\mathrm{t}}{2.303}$$ (7)

q (mg/g) and q_e are the quantity of adsorbed oil at time t and equilibrium, respectively. The kinetic constant is defined as K₁ (1/min). Assuming that the changes are linear, k₁ is obtained from the slope of the plot of log (q_e -q) vs t.

Pseudo-second-order kinetic equation (Eq. (8)):

$$\frac{\mathrm{t}}{\mathrm{q}}=\frac{1}{{\mathrm{K}}_2{\mathrm{q}}_{\text{eq}}^2}+\frac{1}{{\mathrm{q}}_{\text{eq}}}\mathrm{t}$$ (8)

If this model is applicable, the plot of t/q_eq vs t from the above equation should show a linear relationship. The kinetic constant of this equation is defined as K₂ (g/(mg. min)). Q_eq and k₂ are determined from the plot [36].

The Elovich model is expressed as equation (9):

$$q_t=\frac{1}{\beta }\ln \left(\alpha \beta \right)+\frac{1}{\beta }\ln t$$ (9)

where α and β are the primary rate of adsorption (mg/(g.min)) and the desorption constant (g/mg), respectively.

Thermodynamic studies were performed to assay the effect of temperature on the adsorbent capacity using enthalpy (ΔH), entropy (ΔS), and Gibbs free energy (ΔG) calculations and Vanʼt Hoff equation (Eq. (10), (11), (12))). K and R are the thermodynamic constant for temperatures from 10 to 40 °C and the gas constant, respectively.

$$K=\frac{q_e}{C_e}$$ (10)

$$\ln K_0=\frac{{\Delta S}^0}{R}-\frac{{\Delta H}^0}{RT}$$ (11)

$${\Delta G}^0={-RTLnK}_0$$ (12)

2.4. Chemical modification

There is a growing desire to modify natural adsorbents to advance their efficiency for oil removal. Acidic and alkaline conditions are among the used methods. H₂SO₄ and NaOH were used to modify the natural adsorbent in order to better the mechanical and surface properties. It causes removing all compounds covering the external surface of the adsorbent such as lignin, pectin, and wax. In this research, palm fibers were processed using H₂SO₄ and NaOH. As can be seen in Fig. S1, it was observed that palm fibers processed with H₂SO₄ have a higher adsorption capacity than NaOH.

2.5. Instruments

XRD was used to analyze the produced adsorbent. XRD analysis was performed using an XRD device, model Dutch PW1730, made in the Netherlands, with the wavelength of the copper X-ray lamp-1.54056 Å, current 30 mA, voltage 40 KV, and at angles of 10–80°. FT-IR was used for quantitative and qualitative identification of adsorbent organic materials. FTIR device was the Nicolet Avatar model, made in the United States, and it was analyzed in the range of 400–4000 cm⁻¹. FE-SEM was applied to examine the surface morphology of palm leaf particles treated and untreated with sulfuric acid. SEM device was applied by model Tescan MIRA II-MIRA III-VEGA 3 made in the Czech Republic. TGA measurement was done using a TGA device, model Q600, made in the United States at a temperature of 700 °C and a speed of 30 °C/min. The BET surface of adsorbents was measured using a BEL SORP mini II surface analyzer made in Japan. in which N₂ gas was used as an adsorbent.

3. Results and discussion

3.1. Characterization of pad synthesized

FT-IR spectrum of unmodified and modified palm fibers is shown in Fig. 1 a) and b), respectively. The functional groups of palm fibers and the corresponding infrared adsorption frequency have been determined. The depressions, due to the attached OH groups, are observed in the range of 3400–3990 cm⁻¹. Also, the increase in the intensity of the OH peak confirms the activation of the surface and the increase of hydroxyl functional groups on the surface as a result of modification with sodium hydroxide. The observed band around 886-884 cm⁻¹ in both graphs can be attributed to the functional group of alkenes C=C. The increase in the peak intensity of the double bands after modification with acid is due to the absorption of alcohols. Wavenumbers of 1239–1240 cm⁻¹ in both fiber samples are related to the functional group of amines (C-N). The depression 1373 cm⁻¹ in Fig. 1 b) indicates the sulfonate functional group (S=O). It is related to modification with sulfuric acid, which increases effective functional groups. The wavenumber of 1368 cm⁻¹ in Fig. 1 a) and the band observed in the range of 2851–2976 cm⁻¹ in both graphs indicate the functional group of alkanes. Along with the increase in the peak intensity of alkenes, the decrease in the intensity of the peak related to alkanes confirms the increase in the number of alkene functional groups. The wavenumbers of 1513–1509 cm⁻¹ in both graphs designate the functional group nitro (N-O). The observed band in the range of 1642–1640 cm⁻¹ indicates the imine functional group. The large depression of 1728 cm⁻¹ in the graph of Fig. 1 a) is the strong stretching state of the C=O bond related to the unsaturated ester. Since unsaturated esters are reactive, its intensity has been reduced as a result of modification. Wavenumbers of 1723 and 1956 cm⁻¹ in the graph of modified palm fibers and wavenumber of 1957 cm⁻¹ in the graph of unmodified palm fibers are related to the weak C-H bond of the aromatic group. The 2001 cm⁻¹ wavenumber in the modified palm fibers is the stretch mode of the C=C=N bond of ketene imine.

Fig. 1.

FTIR of Unmodified palm fibers a) and of Processed palm fibers b).

The morphology of the obtained materials was checked by the comparison between the SEM images of raw fibers and fibers processed with sulfuric acid is shown in Fig. 2. Raw palm fibers have a homogeneous and porous texture. It is clear that after the modification, the surface of the fibers became smoother, and deeper cracks can be seen on the surface of the modified fibers, the pores are enlarged and provide favorable conditions for the absorption of target pollutants.

Fig. 2.

FESEM image of a, b) unprocessed and c, d) processed palm fiber with H₂SO₄.

The TGA curve is shown in Fig. 3. The curve was categorized by two different temperature phases: the first stage shows that the weight loss of 10 % from the temperature range of 50–250 °C was due to the loss of water remaining in the sample (palm fiber). In the second stage of temperature 300–450 °C, about 38 % weight loss was due to the splitting and destruction of fiber structure.

Fig. 3.

TGA analysis of processed palm fibers.

As can be seen in Fig. 4, the XRD spectrum of the sample has broad peaks with weak intensities. Most of the sample is amorphous (amorphism and irregularity of particles) and only one degree of data peak. This characteristic shows that the sample does not have a high crystal quality and the average size of the crystals is small. The peak at the angle of 22.7° has the highest intensity. The distance between the plates from which the reflection led to the formation of this peak is 3.91 Å.

Fig. 4.

XRD analysis of processed palm fibers for 2Ɵ of 10–80°.

The specific surface area of the palm fibers (modified and unmodified palm fibers) was measured using BET analysis and shown in Fig. 5. The absorbent demonstrates a type V isotherm, indicating that the interactions between the adsorbate and the adsorbent are relatively weak, comparable to the interactions occurring among the adsorbates themselves. Furthermore, it is observed that the absorption capacity rises as the pressure increases [37]. Based on the obtained results, the specific area of unmodified palm fibers and the volume of adsorbed gas were 0.639 m²/g and 0.14 cm³/g, respectively. While the specific area and volume of adsorbed gas of processed palm fibers were 0.424 m²/g and 0.097 cm³/g, respectively. This means that the specific surface area of unprocessed palm fibers is higher than the specific surface area of processed palm fibers and there are more adsorption holes in the surface of the unmodified adsorbent. Nevertheless, the pore diameter and pore volume in unprocessed palm fibers are equal to 399.4 nm and 0.063 cm³/g, respectively, and in processed palm fibers, the pore diameter and pore volume are 904.8 nm and 0.095 cm³/g, respectively. The hysteresis loop has increased after modification, which indicates the need to reduce the pressure for desorption. This phenomenon suggests an increase in active absorption sites and an increase in pore diameter. These results show that the pores of palm fiber after chemical modification have become larger and the diameter of the pores has increased. Based on the IUPAC classification, holes can be separated into three types, micropores (d < 2 nm), mesopores (2 < d < 50 nm), and macropores (d > 50 nm). The pore size distribution assayed based on the BJH method displayed that the pore diameter in the mesopore range with the average pore diameter is equal to 7.99 nm.

Fig. 5.

The N₂ adsorption-desorption and pore size distribution of unprocessed (a) and processed palm fiber (b).

To reuse the adsorbent pad under consistent conditions—namely temperature, duration, oil quantity, and pH—the experiment was carried out in five cycles. In each cycle, the adsorption efficiency was evaluated by applying compression to the adsorbent pad (Fig. 6). The findings indicated a satisfactory recovery rate of 76 % during the second cycle; however, this rate began to decline starting from the third cycle.

Fig. 6.

Reusability of the adsorbent pad for removing engine oil.

3.2. Effect of contact time

Experiments were conducted at different contact times in order to know the effect of time on oil removal. Fig. 7 shows this effect on oil removal efficiency. As seen from Fig. 7a the quantity of oil uptake was very high at the beginning and then has been reached out to almost constant trend with increasing contact time. The oil uptake efficiency is due to the high active site at the beginning of the adsorption process. After the initial time, an equilibrium is formed between the adsorbed and the adsorbent [38]. Therefore, a contact time of 10 min was chosen as an optimum for further study. Nwabueze et al. (2015) investigated the properties of acetylated middle wood (roasted corn) as an adsorbent for cleaning oil stains in simulated conditions. The kinetic data are best matched to the pseudo-second-order kinetic, which all indicate the successful use of this waste material as an adsorbent for oil spill cleanup. In this study, the maximum amount of adsorption was observed in 10 min, which is consistent with this study [18].

Fig. 7.

Effect of time (a), oil concentration (b), temperature (c), and pH (d) on removal efficiency.

3.3. Effect of oil concentration

Fig. 7b shows the removal efficiency as a function of oil concentration. Fig. 7b shows that the adsorption efficiency decreased with increasing oil concentration, which indicates that all the active sites of the adsorbent are saturated, there are no empty pores to adsorb more oil, and the adsorbent and the adsorbed material are in balance. Abdelwahab et al. (2017) during a study to investigate the properties of adsorbents made of palm tree fibers and modified adsorbents of these fibers in acidic and alkaline environments to remove and purify diesel fuel, crude oil, and oil. The results showed that the adsorption capacity of fibers increases with the increase in oil layer concentration, which is completely consistent with the results of the present study [32].

3.4. Effect of oil concentration

Fig. 7b shows the removal efficiency as a function of oil concentration. Fig. 7b shows that the adsorption efficiency decreased with increasing oil concentration, which indicates that all the active sites of the adsorbent are saturated, there are no empty pores to adsorb more oil, and the adsorbent and the adsorbed material are in balance. Abdelwahab et al. (2017) during a study to investigate the properties of adsorbents made of palm tree fibers and modified adsorbents of these fibers in acidic and alkaline environments to remove and purify diesel fuel, crude oil, and oil. The results showed that the adsorption capacity of fibers increases with the increase in oil layer concentration, which is completely consistent with the results of the present study [32].

3.5. Effect of pH

The pH level of a solution significantly influences of the active sites present on its surface, making it a crucial factor in the adsorption process. In this research, the experiments were carried out in diverse pH values in the range (3-5-7-9) as shown in Fig. 7d. As the pH increased from 3 to 9, the adsorption capacity increased. At lower pH levels, due to competition between H⁺ ions, these are adsorbed on the sites and reduce the adsorbent's capacity for further adsorption. The optimal pH in this experiment is equal to 7. Toamah et al. (2021) investigated the effect of the adsorbent made from papyrus plants during a study. Activated carbon made from papyrus plants was used as adsorbent to clean crude oil from water. According to the results of this study, the maximum uptake capacity is 96 % at pH of 7, which is consistent with the present study [39].

3.6. Optimal ratio of components of adsorbent pads

Fig. S2 shows the effect of different proportions of adsorbent pads. Amounts of each pad are as follows for goat hair, polyurethane, fiber and lace, respectively. Pad (I) (4-3-5-3) - Pad (II) (2-3-7-3) - Pad (III) (6-3-3-3) - Pad (IV) (3-6-3-3). Each of these pads was tested at the same temperature and time. Each pad was weighed before and after oil adsorption. Based on this Fig., Pad (II), which was made of more palm fibers, adsorbed more oil than other pads, and as a result, it had a higher adsorption efficiency and was selected as the optimal pad. The reason for this is that palm fibers have more active sites (porosity) and specific surfaces than hair and polyurethane particles. Also, by keeping the parameters (temperature, pH, contact time, and oil amount) constant, we obtained the adsorption efficiency of each component of the adsorbent (hair, palm fibers, polyurethane particles). The adsorption efficacy of the palm fibers, polyurethane foams, and hairs was 50 %, 15 %, and 13 %, respectively.

3.7. Adsorption isotherm

Adsorption isotherm is significant in defining the interaction between adsorbate equilibrium concentration and adsorbent. In this study, the equilibrium adsorption of engine oil was investigated by adsorbent pads, and the results can be seen in Fig. 8 a), b), and c). Also, the degree of their correspondence was analyzed in terms of correlation coefficient. In order to check the validity of the models, R² values were applied to compare isotherm models [40]. A linear form of each model can be used for the constant of all isotherms.

Fig. 8.

Langmuir (a), Freundlich (b), and Temkin isotherm models (c) for oil removal by pad adsorbent.

According to Table (1), the examination of the adsorption isotherms in the oil adsorption process using adsorbent pads showed that this process follows the Langmuir isotherm and this shows that mutual interactions occur on the surface of the adsorbent and adsorption takes place in a single layer and the attractive site are assumed to be homogeneous. A value of n higher than 1 indicates good adsorption potential and a strong bond between the adsorbent and the adsorbed substance. In this study, the parameter n equal to 3.06 was obtained, which indicates the good potential of the adsorbent for adsorbing oil. Also, the maximum adsorption capacity in optimal conditions and at the initial concentration of 12,500 mg/L, temperature of 40 °C, pH of 7, and time of 10 min was obtained as 3333 mg/g, which confirmed the good efficiency of this adsorbent for oil adsorption. Mahmoud et al. (2022) reported the application of Lawsonia leaves for oil spill cleanup of seawater. In their study, several isotherm models were investigated and the best model was Langmuir which is consistent with this research [41]. The R_L parameter was calculated to be 0.998, which designates that the adsorption process is favorable. Moreover, the Temkin model and the results obtained from heat of sorption indicate physisorption.

Table 1.

Adsorption isotherm parameters for oil adsorption using pads.

Langmuir				Freundlich			Temkin
RL	qmax mg/g	KL L/mg	R2	n	KF (mg/g)(mg/L)1/n	R2	bT(j/mol)	KT(L/g)	R2
0.998	3333.3	0.00012	0.9831	3.06	74.57	0.7752	4.806	0.0026	0.8953

3.8. Adsorption kinetics

Based on Fig. 9 a), b), and c), the study of the kinetics of oil adsorption by adsorbent pads provided a detailed study of the efficiency and mechanism of the adsorption process. Three pseudo-first-order, pseudo-second-order, and Elovich kinetic models were investigated, and it is clear from the correlation coefficient of 0.9946 results that the absorbent pad follows second-order kinetics. The pseudo-second-order model is known for its ability to understand complex adsorption mechanisms, which include multiple interactions between oil molecules and adsorbent active surfaces. Transfer or exchange of electrons between the adsorbent and the adsorbate is considered to limit the process rate. The pseudo-second-order kinetic model indicates chemical adsorption. The correlation coefficient calculated under this model showed a high correlation with the experimental data, indicating the high accuracy of the model in describing oil adsorption by adsorbent pads. Based on this model, the calculated maximum absorption capacity is 232.55 mg/g and the absorption rate constant is −0.06 g/(mg. min). Abel et al. (2020) reported the application of coconut coir activated carbon for the adsorption of crude oil from water. In their work, the kinetic data were appropriately matched into various kinetic models with the pseudo-second-order model indicating the best fit having a correlation coefficient that is consistent with the result of this work [42].

Fig. 9.

Pseudo-first-order (a), Pseudo-second-order (b), and Elovich model (c).

3.9. Thermodynamics study

This study successfully determined and analyzed the thermodynamic parameters associated with the adsorption process. As indicated in Table 2 and illustrated in Fig. S3, the enthalpy change (ΔH) was found to be negative at 9867.89 J/mol, signifying that the adsorption process is exothermic. This implies that the attachment of adsorbed molecules to the surface of the adsorbent results in the release of energy. The exothermic nature of the adsorption process is generally advantageous, as it suggests that the process can occur spontaneously without requiring additional energy input [43]. This characteristic not only enhances the energy efficiency of the process but also offers economic benefits. A positive entropy changes of 20.51 J/(mol·K) indicates a shift towards greater disorder during the adsorption process. This phenomenon can be attributed to multiple underlying mechanisms depending on the characteristics of the system involved. For instance, it may reflect the movement of adsorbent molecules from a highly ordered state in the bulk phase to a less organized state upon adsorption, or it could suggest modifications in the interactions among adsorbent particles that result in increased entropy. The Gibbs free energy change (ΔG) is consistently negative across all temperature ranges, which inherently enhances the adsorption process at these temperatures. Furthermore, it is important to highlight that as the temperature rises, the values of ΔG become increasingly negative, suggesting that the adsorption process becomes more spontaneous at elevated temperatures. In a relevant study, Abel et al. (2020) explored the use of coconut coir-activated carbon for the adsorption of crude oil from water, evaluating the thermodynamic parameters to determine the characteristics of the adsorption process [42].

Table 2.

Thermodynamics of pads adsorption.

T (K)	ΔG° (J/mol)	ΔH° (J/mol)	ΔS° (J/mol K)
283	−15672.2	−9867.89	20.51
293	−15877.3
303	−16082.4
313	−16287.5

3.10. Comparison with other works

In this research, an adsorbent made of goat hair, palm fibers, and polyurethane particles was used to remove hydrocarbon contaminants from aqueous solutions. The purpose of this study is to first produce a cheap, accessible, and compatible adsorbent and then to determine the amount of adsorption of oil stains from aqueous solutions by adsorbent pads. In order to compare some other studied works with the presented work, maximum adsorption capacity, contact time, pH, cost, temperature, reusability, and advantages were considered. As can be seen in Table 3, the advantage of this study compared to previous studies was that a comparable maximum adsorption capacity in the contact time of 10 min. Absorption rate is one of the important parameters in oil absorbents in order to prevent oil evaporation. Unlike other absorbents, the amount of absorption increases with increasing temperature, which can be a suitable candidate for use in tropical areas. Also, isotherm investigations show monolayer adsorption on a homogeneous surface. Therefore, the presented adsorbent could be a capable and quick adsorbent for removing oil from an aqueous solution.

Table 3.

Comparison of some other works with the presented work for oil adsorption.

Adsorbent	qm (mg/g)	Time (min)	Temp. (°C)	advantages	performance	cost	Ref.
Palm fibers	3.571 × 104	60	45	Biodegradable, ecofriendly, safe	monolayer coverage	cost-effective	[32]
Human Hair	7917 ± 72	5	18	biosorbent, high reusability	Chemisorption, better in African origin	low-cost	[45]
Coconut husk/human- hair modified	6530	60	18	Improved hydrophobicity, environmentally friend,	Chemisorption, Heterogenic	Low-cost	[46]
Chicken feather	4097	10	18	environmentally sustainable, eco-friendliness, availability,	Chemisorption	Low-cost	[47]
Treated modified Solvay effluent	2800	–	25	eco-friendly, sustainable, Regenerated, high surface area, Low density, water solubility	physical and chemical adsorption	cost-effectiveness	[48]
Straw	3.62 × 103	1440–20160	25	Recyclable, operational stability, Hydrophobic	Oil recovery	Moderate	[16]
Lauric acid-modified oil palm leaves	1176 ± 12.92	20	30	Biodegradable, Large Hydrophobic, surface area	internal transport mechanism, film-diffusion, Heterogenic	low-cost	[44]
Adsorbent pads	3333	10	40	biodegradable, environmentally friendly, safe material	Chemisorption, homogeneous surface, mono-layer coverage	low-cost	This work

3.11. Mechanism of adsorption

The mechanism of removing oil from water can be explained through absorption, adsorption, or both of them. In the case of absorption, oil has the potential to infiltrate the porous regions of the absorbent materials. The movement of oil into the voids of the absorbent is facilitated by capillary forces, which can be enhanced by the application of pressure and the influence of gravitational forces. Adsorption is believed to take place when sorbents come into contact with oil. The oil tends to accumulate on the surface of the adsorbent primarily due to lipophilic interactions or aggregation phenomena. In the process of adsorption, oil is retained or gathered by surface forces, remaining on the exterior of the adsorbent rather than infiltrating its structure [24]. The dynamics of these forces are governed by the attractive interactions between the external surfaces of the adsorbents and the oil, which arise from a combination of physical and chemical phenomena. These phenomena include van der Waals forces, polarity, hydrogen bonding, spatial interactions, hydrophobic characteristics, etc. The activation of palm fiber surfaces facilitates interactions with oil through the presence of hydroxyl groups and other activated surface characteristics. Furthermore, studies conducted in this field have demonstrated that the presence of various functional groups, such as hydroxyl and carboxylic groups, significantly influences the adsorption properties by establishing bonds between the adsorbent and these functional groups. The adsorption of oil onto the surface of an adsorbent pad is significantly influenced by two primary factors. Firstly, the activation of the adsorbent surface enhances its hydrophobic properties, thereby facilitating oil retention. Secondly, modifications that increase the pore diameter of the adsorbent further contribute to its efficacy in oil adsorption [44]. From the studies done in kinetic models, the chemical adsorption of oil in the absorbent pad confirmed that the reduction of pad recycling after the second use completely confirmed that most absorbent surfaces and pores cannot be recycled and reused due to chemical absorption. Considering the high absorption rate in the first two recycling, it seems that the adsorbent pad is suitable and efficient for industrial use.

4. Conclusion

In this research, an adsorbent pad made of goat hair, palm fibers, and polyurethane granule particles was used to remove engine oil as a Pollutant model from water. Optimum adsorption conditions in this method included pH of 7, contact time of 10 min, oil concentration of 50 g, and optimal temperature of 40 °C. The maximum experimental absorption capacity in optimal conditions was 1160 mg/g. Isothermal studies were performed with Freundlich, Langmuir, and Temkin linear models. The results show that the Langmuir model adsorption isotherm was better to fit. Langmuir correlation coefficient and the maximum calculational adsorption capacity were 0.983 and 3333 mg/g, respectively, which indicates that this model is suitable for adsorbent pads, and also, shows monolayer adsorption mechanisms. Based on the data obtained from the research, adsorption kinetics was also investigated and it shows that it follows the pseudo-second-order kinetic model and has a high regression coefficient (R² = 0.994). In this kinetic model, chemical adsorption occurs through the sharing or transfer of electrons between the adsorbent and the adsorbed. The data related to the recycling of absorbent pads show that the efficiency of absorption from the third recycling has decreased significantly, which confirms the chemical nature of the absorption. Considering the high efficiency of adsorbent pads made of hair, polyurethane particles, and processed palm fibers in removing engine oil from aqueous solutions, the prepared adsorbent can be used as a suitable and biocompatible adsorbent to remove hydrocarbon contaminants from the laboratory scale. Since the only chemical reaction required to prepare this effective absorbent is the treatment of palm fiber with acid and base, it will have a high ability to be used on an industrial scale.

For future studies, the change in the components or the ratio between the contents of the adsorbent to increase the adsorption rate and the ability to recycle the product can be investigated.

CRediT authorship contribution statement

Esmat Askari: Writing – original draft, Investigation, Formal analysis. Vali Alipour: Writing – review & editing, Funding acquisition, Conceptualization. Omid Rahmanian: Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Project administration, Methodology, Conceptualization.

Additional information

No additional information related to this article is available.

Data availability statement

Data will be made available on request.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary data

The following is the supplementary data to this article: