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Article

Biochar from Date Palm Waste via Two-Step Pyrolysis: A Modified Approach for Cu (II) Removal from Aqueous Solutions

by
Essam R. I. Mahmoud
1,*,
Hesham M. Aly
2,
Noura A. Hassan
3,
Abdulrahman Aljabri
1,
Asim Laeeq Khan
4 and
Hashem F. El-Labban
5
1
Department of Mechanical Engineering, Faculty of Engineering, Islamic University of Madinah, Madinah 42351, Saudi Arabia
2
Department of Forestry and Timber Trees, Horticulture Research Institute, Agricultural Research Center, Antoniadis Botanical Garden, Alexandria 21554, Egypt
3
Pesticide Chemistry & Technology Department, Faculty of Agriculture, Alexandria University, Alexandria 21544, Egypt
4
Department of Chemical Engineering, Faculty of Engineering, Islamic University of Madinah, Madinah 42351, Saudi Arabia
5
Production Engineering Department, Faculty of Engineering, Alexandria University, Alexandria 21544, Egypt
*
Author to whom correspondence should be addressed.
Processes 2024, 12(6), 1189; https://doi.org/10.3390/pr12061189
Submission received: 18 May 2024 / Revised: 2 June 2024 / Accepted: 5 June 2024 / Published: 9 June 2024
(This article belongs to the Special Issue Optimization of Adsorbent Materials and Their Application in Sustainable Wastewater Treatment)
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Abstract

:
Heavy metals such as copper, often discharged from industrial processes and agricultural activities, pose significant environmental and health risks due to their toxicity, particularly in the soluble form of Cu (II). This study investigates the effectiveness of biochar produced from date palm leaf midrib waste via a two-step pyrolysis process, as a sustainable and economical adsorbent for removing Cu (II) from aqueous solutions The biochar was characterized using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), Fourier-transform infrared spectroscopy (FTIR), and Brunauer–Emmett–Teller (BET) surface area analysis. Adsorption experiments were conducted to evaluate the effects of pH, adsorbent dosage, contact time, and initial Cu (II) concentration. The maximum adsorption capacity was observed at pH 6, with a capacity of 70 mg/g. The adsorption data were best described by the pseudo-second-order kinetic model, indicating chemisorption as the primary mechanism. Thermodynamic studies indicated that the adsorption process was spontaneous and exothermic, with a Gibbs free energy change (ΔG) of −1.245 kJ/mol at 25 °C, enthalpy change (ΔH) of −15.71 kJ/mol, and entropy change (ΔS) of 48.36 J/mol·K. Reusability tests demonstrated that the biochar retained over 85% of its initial adsorption capacity after five cycles, with capacities of 60 mg/g in the first cycle, decreasing to 52 mg/g by the fifth cycle. This study highlights the potential of biochar derived from date palm waste as an efficient, sustainable adsorbent for the removal of Cu (II) from wastewater, contributing to both environmental management and waste valorization. Future research should focus on optimizing the biochar production process and exploring its application for the removal of other contaminants.

1. Introduction

Water is an essential resource for the sustenance and development of agricultural and industrial applications, which are integral to human life [1,2,3]. The global concern over water scarcity is escalating, primarily due to droughts and contamination [4]. In the Kingdom of Saudi Arabia, a significant desert landscape and the risk of drought, stemming from low water availability and high agricultural and industrial water demands, are notable [5]. Options such as desalination and the reuse of industrial and urban water are considered vital for addressing water supply challenges [6]. However, the escalation of pollution and contaminants in industrial and urban wastewater is a major threat to human and ecological systems [7,8]. This destruction is primarily attributed to the presence of heavy metals in wastewater. In the Kingdom, the substantial yearly accumulation of agricultural and industrial waste adversely affects human health and the environment. It is reported that the presence of heavy-metal elements in most wastewater used for irrigation has detrimental effects on plants and soil [8,9]. Heavy metals like Pb, Zn, Cd, Cu, and Mn, known for their toxicity, can accumulate in plants and animals, posing significant health risks to humans and animals [10,11,12].
Addressing these challenges necessitates the recycling and purification of wastewater. Thus, the introduction of efficient methods for purifying water contaminated with heavy metals is crucial. Technologies such as adsorption, membrane filtration, electrocoagulation, and chemical adsorption have been employed for this purpose [13]. Among these, the adsorption process is regarded as cost-effective, efficient, and non-toxic. Nanocomposites have been effectively used to adsorb heavy metals from aqueous matrices [14]. Although activated carbon is a common commercial adsorbent, its high cost and challenging regeneration process have driven the search for alternative materials such as biochar [15,16]. Therefore, the exploration of low-cost alternative adsorbents for waste removal is imperative. Biochar, a sustainable and economical adsorbent, has gained attention for its effectiveness in removing hazardous metals from wastewater. Recent studies have highlighted the use of biosorbents derived from various biomasses in eliminating different pollutants from aqueous solutions [17].
Biochar, characterized by its carbon-rich composition, porous nature, high specific surface area, low density, affordability, and stability, is produced through biomass pyrolysis in an oxygen-limited environment at relatively low temperatures. Its unique properties make biochar a versatile material. It contributes to environmental improvement by reducing carbon emissions and mitigating the greenhouse effect, thereby positively influencing global warming potential. Additionally, biochar is effective in immobilizing pollutants. It has been proven to be an optimal adsorbent for various contaminants in contaminated soil and water, including organic pollutants and heavy metals. Biochar’s efficacy extends to the adsorption of pesticides, industrial dyes, and antibiotic pollutants [18]. Furthermore, biochar serves as a filler material in cement-based building and construction materials, helping to reduce cement shrinkage cracks through self-internal curing, improving autogenous shrinkage, and enhancing cement carbonation. In agriculture, biochar is used as a soil fertilizer and for water retention, particularly beneficial in the sandy soils of Saudi Arabia [19]. Biochar can be derived from a range of biomass sources, including agricultural waste, forestry waste, sawdust, rice husks, rice straw, bagasse, paper products, animal manure, and urban green waste.
The production of biochar typically involves pyrolysis, a thermochemical decomposition process where waste is heated to a specific temperature in the absence of oxygen. Pyrolysis temperature, alongside the feedstock material, is a critical factor influencing the biochar’s physicochemical properties such as pore size, surface area, chemical composition, and functional groups. Temperatures ranging from 250 °C to 1000 °C have been explored [20,21,22], with heating above 250 °C being crucial for the conversion of lignocellulosic biomass, as the decomposition of hemi-cellulose and cellulose starts at this temperature [23,24]. At higher temperatures, the original biomass structure is altered, forming biochar with aromatic structures and a higher degree of carbonization. This results in increased specific surface area due to the creation of numerous pores, thereby enhancing the biochar’s adsorption capacity for organic pollutants and contaminants in wastewater. However, excessively high temperatures may lead to chemical rearrangement and collapse of the pore structure in biochar, diminishing its adsorptive activity. Suhaimi’s research [25] reported that biochar heated at 500 °C exhibited optimal adsorptive performance. Pyrolysis temperature was gradually increased at a rate of 10 °C/min until reaching the set temperature (e.g., 500 °C) and maintained for 2–10 h [26]. A slow heating rate can restrain the evaporation of volatiles and promote secondary pyrolysis reactions [27]. Shi et al. [28] demonstrated that biochar produced at higher heating rates through microwave heating contains nano-scale fibers, a feature not found in biochar produced at lower heating rates. Moreover, the biochar preparation technique significantly affects its properties. Mishra et al. [29] studied the impact of pyrolysis temperatures on biochar’s gasification characteristics, finding that higher temperatures decreased its gasification reactivities. Conversely, Cetin et al. [30] investigated the influence of heating rate on the chemical and physical properties of biochar, as well as its gasification reactivities, concluding that pyrolysis conditions significantly affect biochar morphology, with increased heating rates enhancing its reactivities. To improve the adsorption capacity of biosorbents, methods like magnetization have been employed [31]. Modifying biosorbents with magnetic nanoparticles is an effective approach to enhancing their adsorption capacity [32,33]. Recently, biochar magnetized with Fe3O4 has been prepared and used for the removal of Cu2+ [34] and oil-polluted water [35].
Date palm trees, predominantly found in tropical and arid regions, are abundant in the Kingdom of Saudi Arabia, with nearly 23 million trees, accounting for about 20% of the global population of 120 million palm trees [36,37]. Each tree produces approximately 20 kg of waste annually [38]. During the harvesting period, substantial waste is generated [39]. This waste, not suitable for animal feed, is often disposed of improperly through burning or burying, leading to pollution problems. Burning emits harmful gases, contributing to global warming, while burying pollutes groundwater and soil [40]. Hence, environmentally friendly and economical treatment of this waste is imperative [41]. Utilizing date palm tree waste as feedstock for biochar production presents a potential solution.
In the present study, biochar was produced from the midribs of date palm leaves collected from the Madinah area (KSA). This research not only addresses the effective disposal of agricultural waste but also contributes to the development of sustainable environmental solutions. The modification of our method lies in the implementation of a two-step pyrolysis process, a technique distinct from conventional single-step methods. This two-step approach allows for a more controlled and efficient conversion of biomass into biochar, potentially enhancing the physicochemical properties of the resultant material. A significant focus of this research is the application of the produced biochar in the removal of Cu2+ ions from aqueous media. This is of paramount importance given the detrimental effects of heavy-metal contamination in water bodies. Our study not only offers a modified method to produce biochar but also demonstrates its practical application in mitigating environmental pollution. By exploring the adsorptive capabilities of biochar produced from date palm leaf midribs, we present a sustainable and cost-effective solution to a pressing environmental challenge. The findings of this research have the potential to influence future practices in both waste management and water purification, making a substantial contribution to the fields of environmental science and technology.

2. Materials and Methods

2.1. Biochar Preparation

For the preparation of biochar, samples of midribs from leaves of the date palm (Phoenix dactylifera) were utilized. Initially, these samples were subjected to air drying for a period of approximately four weeks, followed by cutting into pieces approximately 5 cm in length. The biochar was synthesized using a slow pyrolysis technique. This involved placing the date palm samples in a furnace where they underwent a two-step pyrolysis process. After pyrolysis, the resulting biochar was ground and sieved to obtain a particle size range of 0.5–1 mm. In the first step of pyrolysis, the temperature within the furnace gradually increased to 300 °C at a controlled heating rate of 10 °C per minute. Once the target temperature of 300 °C was reached, the samples were maintained at this temperature for a duration of 1 h to ensure thorough pyrolysis. Following this, the system was allowed to cool down to room temperature over a period of 12 h, facilitating a gradual transition to the second pyrolysis phase. The second step of the pyrolysis process involved raising the temperature to a higher level of 600 °C, again at a heating rate of 10 °C per minute. This elevated temperature was sustained for 1 h to further advance the pyrolysis process and enhance the biochar’s characteristics. After this period, the system was cooled down to room temperature.
The two-step pyrolysis process was chosen over the one-step process because it allows for more controlled and efficient conversion of biomass into biochar, leading to the formation of larger, and more uniform pores. This results in a biochar with a higher surface area and enhanced adsorption properties, which are crucial for the effective removal of heavy metals like Cu (II) from aqueous solutions. The gradual heating in two stages also ensures better preservation and activation of functional groups on the biochar surface, contributing to improved chemisorption and ion exchange capabilities.

2.2. Biochar Characterization

The surface microstructure of the prepared biochar was meticulously examined using a scanning electron microscope (SEM) (Jeol JSM-5300 SEM, JEOL Ltd., Showima City, Tokyo, Japan). To analyze the elemental composition of selected areas within the microstructure, we employed an electron dispersive X-ray (EDX) analysis, conducted at an acceleration voltage of 15–20 KeV. For the identification and distribution analysis of functional groups on the biochar surface, Fourier-transform infrared (FT-IR) spectroscopy was utilized. This analysis was carried out over a spectral range of 400–4000 cm−1. The analysis was conducted using a Burker Tensor 37 spectrometer (Bruker, Billerica, MA, USA), employing the KBr pellet technique. Specifically, 1.0 mg of the biochar sample was blended with 100 mg of KBr, compressed into a pellet, and then subjected to infrared radiation for spectral analysis. The specific surface area and pore size distribution were characterized using nitrogen adsorption isotherms at 77 K, measured with a Micromeritics Tristar instrument (USA) and calculated using the Brunauer–Emmett–Teller (BET) method.

2.3. Adsorption Studies

The adsorption studies for Cu (II) removal from aqueous solutions were conducted using batch experiments. The effect of various parameters such as pH, contact time, initial metal ion concentration, and adsorbent dosage on the adsorption capacity of biochar was systematically investigated.
The initial concentration of Cu (II) ions in the solution was adjusted to the desired level by dilution with deionized water. A stock solution of CuSO4·5H2O was prepared in distilled water. Different concentrations were prepared, and their corresponding absorbance was measured using atomic absorption spectroscopy to construct a calibration curve. The percent removal efficiency, R (%), of Cu (II) was calculated using Equation (1):
R   % = ( C o − C )   C o × 100
where Co and C are the initial and final concentration of Cu (II). The total adsorption capacity, qt (mg/g) of Cu (II) was calculated using Equation (2):
q t = ( C o − C e ) M × V
where Co is the initial concentration, Ce is the equilibrium concentration, V is the volume (L), and M is the adsorbent dosage (g). The equilibrium adsorption capacity (qe) was calculated using the relationship given in Equation (3).
In this equation, the adsorption kinetics were analyzed using the pseudo-second-order kinetic model, which is represented as follows:
q t = − q e 2 k 2 t 1 + q e k 2 t
where t (min) is the contact time, while qe and qt (mg/g) denote the amount of Cu (II) ions adsorbed onto the adsorbents at equilibrium and at a given time t, respectively. The rate constant for the pseudo-second-order model is k2, which is measured in grams per milligram per minute (g/(mg·min)).
Adsorption isotherms were studied using the Langmuir and Freundlich models. The Langmuir isotherm is given by Equation (4):
q e = q m b C e 1 + b C e
where qe is the amount of Cu (II) adsorbed per unit weight of biochar at equilibrium (mg/g), qm is the maximum adsorption capacity (mg/g), b is the affinity constant of the binding sites (L/mg), and Ce is the equilibrium concentration of Cu (II) in the solution (mg/L).
The Freundlich isotherm is expressed as:
q e = K F C e 1 n
where KF and n are the Freundlich constants indicating the adsorption capacity and intensity, respectively.
Thermodynamic parameters such as Gibbs free energy (ΔG), enthalpy (ΔH), and entropy (ΔS) changes were calculated to understand the nature of the adsorption process. The Gibbs free energy change was calculated using the following formula:
ΔG = ΔH − TΔS
Reusability studies were performed to evaluate the stability and efficiency of the biochar over multiple cycles. After each adsorption cycle, the biochar was regenerated using 0.1 M HCl and reused for subsequent adsorption experiments. Adsorption capacity was measured after each cycle to determine the biochar’s performance over repeated use.
The adsorption process was monitored and the data were analyzed to provide insights into the adsorption mechanisms and the efficiency of biochar derived from date palm waste for Cu (II) removal.

3. Results and Discussion

3.1. Biochar Characterization

SEM was employed to investigate the microstructural features of the biochar surface, revealing details at various magnifications, as depicted in Figure 1. The images prominently display the structure of the biochar, showcasing both lateral (longitudinal) and anterior (cross-sectional) views of its microstructural vessels. Notably, the morphology of the biochar surface is characterized by these lateral and anterior vessels, which consist of well-organized, large numbers of uniformly enlarged pores.
The distribution of these pores is a critical factor influencing the biochar’s adsorption behavior, as indicated in various studies [42,43]. The two-step pyrolysis process employed in this study allows for more efficient evaporation of volatiles and other organic components, resulting in the formation of larger pores. These findings strongly suggest that the two-step pyrolysis process is optimal for producing biochar with enlarged and uniform pores. Consequently, this structural enhancement leads to an increase in surface area, which significantly improves the biochar’s adsorption properties, particularly in the removal of pollutants [44].
This evidence strongly supports the hypothesis that the two-step pyrolysis process not only alters the physical structure of the biochar but also enhances its functional efficacy in environmental applications. The expanded pore size and increased surface area are key factors contributing to the biochar’s superior performance in adsorbing pollutants, underscoring the importance of the pyrolysis method in biochar production.
Figure 2 presents the elemental composition of the biochar, as determined through EDX analysis conducted in conjunction with SEM imaging of the sample subjected to the two-step pyrolysis process. The EDX spectra reveal that carbon and oxygen are the predominant elements within the biochar, complemented by the presence of silicon, sodium, potassium, calcium, phosphorus, and sulfur in minor quantities. Notably, the carbon content of the biochar registers as the highest mass value, a characteristic attributed to the two-step pyrolysis process and the elevated pyrolysis temperature of 600 °C. This higher temperature facilitates the dehydration and volatilization processes, resulting in an increased carbon content and a corresponding decrease in oxygen and other elements.
The lower oxygen content relative to carbon can be attributed to the effects of volatilization at high pyrolysis temperatures [45]. During the heating of biomass under these conditions, energy-rich volatiles and gases are produced, leading to the formation of a stable, carbon-enriched compound, namely biochar. Additionally, the presence of elements like magnesium and silicon at high pyrolysis temperatures can be linked to the insoluble metallic oxides found in the biochar samples [44,45]. This elemental composition highlights the efficacy of the two-step pyrolysis process in enriching the biochar with carbon while reducing the oxygen content. The presence of these elements, particularly the high carbon content, is indicative of the biochar’s potential for environmental applications, such as pollutant adsorption, where a high carbon content is often beneficial.
The specific surface area and pore size distribution of the biochar derived from date palm waste via two-step pyrolysis process were characterized using nitrogen adsorption isotherms at 77 K and calculated using the BET method. The biochar exhibited a specific surface area of 65 m2/g, a total pore volume of 0.08 cm3/g. These values indicate that while the biochar possesses moderate surface area and pore volume, it still provides adequate active sites for the adsorption of Cu (II) ions from wastewater. The results highlight the potential of utilizing date palm waste biochar as an effective and sustainable adsorbent for environmental remediation applications.
The FTIR spectrum of biochar produced from date palm leaves via two-step pyrolysis is illustrated in Figure 3 and summarized in Table 1. The spectrum reveals several characteristic bands, providing insights into the biochar’s chemical composition. Bands observed in the range of 3200 to 3700 cm−1 confirm the presence of the O-H group’s stretching vibrations. These are typically associated with hydrogen-bonded O-H groups in carboxylic and phenolic compounds. The detection of C-H (methyl) groups, indicative of aliphatic substitution on aromatic rings, is evidenced by bands in the region of 2088–3000 cm−1. A significant peak around 1560 cm−1 confirms the presence of ring stretching C=C, characteristic of aromatic compounds. The bands appearing at 1420 cm−1 correlate with aliphatic C-H groups. Additionally, peaks within the 1000–1240 cm−1 range are attributable to stretching C-O groups in phenolic and alcoholic compounds and carboxylic acids. The range of 1000–1100 cm−1, potentially indicating asymmetric Si-O stretching, aligns with the EDX results, which showed traces of silicon. The bands observed in the 400–600 cm−1 range are indicative of inorganic matter, such as carbonates and silicates. Moreover, bands in the 700–900 cm−1 range, associated with C-H out-of-plane bending in substituted aromatic rings, further confirm the presence of aromatic structures.
The physicochemical properties of biochar are influenced by several factors, including the biomass material and the conditions of pyrolysis (heating rate, temperature, and holding time) Surface functional groups, particularly carbonyl, hydroxyl, and methyl, are pivotal in defining the biochar’s surface characteristics. The high carbon content in these functional groups significantly impacts the properties of the biochar. Employing a two-step pyrolysis process allows for extended exposure to pyrolysis conditions without reaching detrimental high temperatures. This leads to a more expansive surface area structure, higher pore volume, and deeper channels, enhancing the biochar’s adsorption efficiency. Furthermore, other oxygen-rich functional groups on the biochar surface, such as carboxyl and phenolic hydroxyl groups, play a crucial role in the adsorption of heavy metals.

3.2. Adsorption of Cu (II) by Biochar

To determine the optimal conditions for Cu (II) adsorption by biochar derived from date palm waste, we systematically investigated the effects of various parameters.

3.2.1. Effect of pH

To determine the optimal pH for promoting Cu (II) adsorption by biochar derived from date palm waste, experiments were conducted with different solutions with pH values ranging from 2 to 6 at 25 °C for 360 min, with an adsorbent dosage of 0.5 g/L and an initial Cu (II) concentration of 100 mg/L. Figure 4a shows that the adsorption capacity of the biochar increased significantly with the rise in pH, reaching a maximum of 70 mg/g at pH 6. This trend indicates that the removal efficiency and adsorption capacity are enhanced upon increasing the initial pH from 2 to 6. At lower pH values, the biochar surface is likely protonated, resulting in increased competition between hydrogen ions (H+) and Cu (II) ions for available adsorption sites. This competition reduces adsorption efficiency, as H+ ions outcompete Cu (II) ions for binding sites. As pH increases, deprotonation of the biochar surface occurs, which enhances the ionization of functional groups such as carboxyl and hydroxyl groups. These ionized groups can form stronger complexes with Cu (II) ions, thus facilitating better adsorption. Moreover, the increase in pH leads to a reduction in positive surface charge, thereby reducing electrostatic repulsion between the positively charged Cu (II) ions and the biochar surface. This reduction in repulsion allows more Cu (II) ions to approach and bind to the biochar surface, enhancing the adsorption process.

3.2.2. Effect of Adsorbent Dosage

Figure 4b illustrates the effect of adsorbent dosage on the rejection percentage of Cu (II) ions using biochar derived from date palm waste. The experiments were conducted with dosages ranging from 0.25 g/L to 1.25 g/L, an initial Cu (II) concentration of 100 mg/L, pH 5, an equilibrium time of 360 min, a shaking speed of 150 rpm, and a reaction temperature of 25 °C. The results indicate a clear trend where the rejection percentage of Cu (II) ions increases with the increase in adsorbent dosage. At the lowest dosage of 0.25 g/L, the biochar achieved a Cu (II) rejection of 40%. This relatively modest rejection can be attributed to the limited number of available adsorption sites, resulting in a higher ratio of Cu (II) ions to biochar surface area. As the dosage increased to 0.5 g/L, the rejection percentage rose significantly to 60%. This substantial increase was due to the greater availability of adsorption sites, which enhanced the probability of Cu (II) ions encountering and binding to the biochar surface. Further increasing the adsorbent dosage to 0.75 g/L led to a rejection percentage of 75%. The trend continued with dosages of 1.0 g/L and 1.25 g/L, achieving rejection percentages of 85% and 90%, respectively. This progressive improvement in rejection percentage with higher dosages is indicative of the increasing availability of active sites for adsorption, reducing the competition among Cu (II) ions and leading to more efficient removal from the aqueous solution. The non-linear increase in rejection percentage suggests that while higher dosages provide more adsorption sites, the effectiveness of additional biochar decreases at higher dosages. These results highlight the importance of optimizing adsorbent dosage to balance the availability of adsorption sites and the efficient utilization of the biochar’s surface area. The observed trend aligns with the principles of adsorption, where an increase in adsorbent dosage typically enhances removal efficiency until a plateau is reached.

3.2.3. Effect of Contact Time

Figure 4c illustrates the effect of contact time on the qe of Cu (II) ions using the prepared biochar. The experiments were conducted at pH 5 with an initial Cu (II) concentration of 100 mg/L, an adsorbent dosage of 0.5 g/L, a shaking speed of 150 rpm, and a reaction temperature of 25 °C. The results demonstrate a clear trend of increasing adsorption capacity with extended contact time, eventually reaching an equilibrium.
At the initial contact time of 0 min, there was no adsorption observed, as expected. As the contact time increased to 30 min, the qe value rose significantly to 10 mg/g, indicating rapid adsorption of Cu (II) ions onto the biochar surface. This initial rapid phase is attributed to the abundant availability of active adsorption sites on the biochar surface, allowing for quick interaction with Cu (II) ions. As the contact time progressed to 60 and 120 min, the adsorption capacity further increased to 25 mg/g and 40 mg/g, respectively. This continued rise can be explained by the ongoing diffusion of Cu (II) ions from the bulk solution to the available adsorption sites within the biochar’s porous structure. The adsorption process at this stage is influenced by the concentration gradient between the solution and the biochar surface, driving more ions towards the adsorbent. Between 120 and 180 min, the qe value increased to 50 mg/g, showing a steady yet slower rate of adsorption. This deceleration in adsorption rate is a common phenomenon as the biochar’s available sites begin to saturate. The driving force for adsorption diminishes as the concentration of Cu (II) ions in the solution decreases, leading to a reduced adsorption rate. Further extending the contact time to 240 min resulted in an adsorption capacity of 55 mg/g, indicating that the biochar was nearing its adsorption equilibrium. Between 300 and 360 min, the qe values plateaued at 58 mg/g and 60 mg/g, respectively, signifying that equilibrium had been reached. At this point, the adsorption sites were nearly fully occupied, and the rate of Cu (II) ion adsorption balanced with the rate of desorption.
The overall adsorption process can be described by the initial rapid phase followed by a slower approach to equilibrium. The initial rapid adsorption is driven by surface adsorption mechanisms, including physical adsorption and ion exchange. As the biochar surface becomes increasingly occupied, the adsorption process transitions to involve intraparticle diffusion, where Cu (II) ions penetrate deeper into the biochar’s porous structure. These results confirm the effectiveness of date palm-derived biochar in adsorbing Cu (II) ions from aqueous solutions, with equilibrium being achieved within 360 min. The study highlights the significance of contact time in optimizing the adsorption process, providing valuable insights for practical applications in wastewater treatment. The ability of the biochar to reach substantial adsorption capacity within a reasonable timeframe demonstrates its potential as a sustainable and efficient adsorbent for heavy-metal remediation.

3.2.4. Effect of Initial Cu (II) Concentration

Figure 4d illustrates the effect of initial Cu (II) concentration on the qe of Cu (II) ions using the prepared biochar. The experiments were conducted at pH 5 with a contact time of 360 min, an adsorbent dosage of 0.5 g/L, a shaking speed of 150 rpm, and a reaction temperature of 25 °C. The results demonstrate a clear trend of increasing adsorption capacity with rising initial Cu (II) concentration, highlighting the biochar’s ability to accommodate higher metal ion loads. At the initial Cu (II) concentration of 60 mg/L, the qe value was 45 mg/g, indicating effective adsorption even at relatively lower concentrations. This initial phase demonstrates that the biochar has a high affinity for Cu (II) ions, quickly binding available ions to its surface. As the concentration increased to 80 mg/L, the adsorption capacity rose significantly to 55 mg/g. This increase suggests that more adsorption sites on the biochar are being utilized as the availability of Cu (II) ions in the solution increases. When the initial concentration reached 100 mg/L, the qe value plateaued at 60 mg/g. This point represents the previously observed equilibrium capacity under these conditions, where the biochar’s adsorption sites are sufficiently occupied, maximizing its removal efficiency for Cu (II) ions. Further increasing the initial concentration to 120 mg/L and 140 mg/L resulted in qe values of 65 mg/g and 68 mg/g, respectively. These increments indicate that the biochar can still adsorb additional Cu (II) ions, albeit at a slower rate, due to the diminishing availability of unoccupied adsorption sites and potential saturation effects.
The overall trend observed in Figure 4d can be attributed to several adsorption mechanisms. Initially, the high concentration gradient between the solution and the biochar surface drives the rapid diffusion of Cu (II) ions to the biochar, facilitating strong adsorption through surface complexation and ion exchange. As the concentration increases, more ions diffuse into the internal pores of the biochar, engaging in pore-filling mechanisms and further enhancing the adsorption capacity. The data also suggest that the biochar exhibits a high capacity for Cu (II) adsorption, which is maintained even at higher metal ion concentrations. This behavior underscores the biochar’s effectiveness as a versatile and robust adsorbent for heavy-metal remediation, capable of handling varying levels of contamination. The ability to maintain substantial adsorption capacity across a range of concentrations highlights the potential for practical applications in treating industrial effluents and contaminated water bodies.

3.3. Adsorption Kinetics

The adsorption kinetics of Cu (II) onto biochar were analyzed using the pseudo-second-order kinetic model, which assumes that the adsorption process is chemisorption, involving valence forces through sharing or exchange of electrons between the adsorbent and adsorbate. The experimental data in Figure 5 show a rapid initial adsorption of Cu (II) ions onto the biochar, followed by a slower approach to equilibrium, which is characteristic of chemisorption processes. The pseudo-second-order model, depicted by the black line, provided an excellent fit to the experimental data with a high correlation coefficient (R2 = 0.92). Table 2 lists the relevant kinetic parameters and determination coefficients. This high R2 value indicates that the model accurately describes the adsorption kinetics and suggests that the rate-limiting step may involve chemical interactions between Cu (II) ions and the active sites on the biochar surface.
The minor discrepancy between the experimental and predicted qe values could be attributed to several factors, including the heterogeneous nature of the biochar surface, diffusion limitations, or the presence of competing adsorption sites. Despite this difference, the overall fit of the model indicates that chemisorption plays a significant role in the adsorption process. The rate constant (k2) for the pseudo-second-order kinetic model was found to be 7.4 × 10−5 g/(mg·min). The relatively low value of k2 suggests that the adsorption process is slow and may be influenced by the availability of active sites on the biochar surface. The initial rapid uptake of Cu (II) ions can be attributed to the abundant availability of adsorption sites on the biochar surface, while the slower approach to equilibrium indicates that as these sites become occupied, the rate of adsorption decreases.
The high R2 value of the pseudo-second-order model supports the hypothesis that chemisorption is the primary mechanism for Cu (II) adsorption onto biochar. This model assumes that the adsorption capacity is proportional to the number of active sites available on the adsorbent surface and that these sites are not uniformly distributed. The adsorption rate is dependent on the concentration of Cu (II) ions in solution and the number of available adsorption sites on the biochar. The observed lower experimental qe compared to the predicted value may indicate the presence of mass transfer limitations, possibly due to the porous structure of the biochar, which could restrict the diffusion of Cu (II) ions to the inner adsorption sites.

3.4. Adsorption Isotherms

Figure 6 presents the adsorption isotherms for Cu (II) on biochar, illustrating the Langmuir and Freundlich isotherm models. These models are essential for understanding the adsorption capacity and mechanisms of biochar in removing Cu (II) ions from aqueous solutions.
The Langmuir isotherm model assumes monolayer adsorption on a surface with a finite number of identical sites. This model suggests that once a molecule occupies a site, no further adsorption can take place at that site, implying uniform adsorption sites on the biochar surface. The Langmuir isotherm fit, represented by the black line, shows a high correlation with the experimental data (red circles), with a correlation coefficient of 0.991. This high correlation coefficient indicates that the Langmuir model accurately describes the adsorption process, suggesting that the adsorption of Cu (II) onto biochar occurs primarily through monolayer adsorption. The Langmuir constant, which is 0.0123 L per milligram, indicates the affinity of the binding sites for Cu (II) ions. A higher value of the Langmuir constant typically suggests a stronger affinity between the adsorbent and the adsorbate. The high correlation coefficient and the relatively high maximum adsorption capacity indicate that biochar has a significant capacity to adsorb Cu (II) ions, forming a uniform monolayer on the surface. The Langmuir model assumes that all adsorption sites are energetically equivalent, and there is no interaction between adsorbed molecules. This implies that once a Cu (II) ion occupies a site, it does not influence the adsorption of other Cu (II) ions on adjacent sites. This behavior is typical for adsorbents with homogenous surface properties, and it suggests that the biochar used in this study has a relatively uniform surface chemistry that favors monolayer adsorption.
The Freundlich isotherm model is an empirical equation describing adsorption on a heterogeneous surface. This model assumes that adsorption sites are distributed with varying affinities, meaning that adsorption occurs on a surface with different types of sites, each having its own adsorption energy. The Freundlich isotherm fit, represented by the black line, shows a good correlation with the experimental data (red circles), with a correlation coefficient of 0.987. This high correlation coefficient indicates that the Freundlich model also adequately describes the adsorption process, suggesting that the biochar surface is heterogeneous with multiple adsorption sites of different energies. The Freundlich constant, indicative of adsorption capacity, is 1.23, while the constant indicating adsorption intensity is 0.67. The value of the intensity constant, which is less than 1, indicates favorable adsorption conditions, meaning that the adsorbent has a high affinity for the adsorbate at low concentrations, and this affinity decreases with increasing adsorbate concentration. The Freundlich model does not predict a maximum adsorption capacity, instead suggesting that the adsorption capacity increases indefinitely as the concentration of Cu (II) increases, which is more realistic for heterogeneous surfaces. The Freundlich isotherm suggests that adsorption sites on the biochar have different energies, which could be due to the varying functional groups, pore sizes, and surface defects. This heterogeneity can lead to a multilayer adsorption process where Cu (II) ions first occupy the high-energy sites, and as these sites become saturated, additional ions adsorb onto the lower-energy sites. Table 3 presents the parameters of the Langmuir and Freundlich adsorption isotherms for the adsorption of Cu (II) onto biochar.

3.5. Adsorption Thermodynamics

The thermodynamic parameters for the adsorption of Cu (II) onto biochar at different temperatures are presented in Table 4. These parameters provide valuable insights into the nature of the adsorption process, including the feasibility, spontaneity, and the thermodynamic mechanisms involved.
The Gibbs free energy change (ΔG) is a crucial parameter that indicates the spontaneity of the adsorption process. Negative values of ΔG suggest that the adsorption is a spontaneous process. The ΔG values for the adsorption of Cu (II) onto biochar were found to be −1.245 kJ/mol at 25 °C, −1.475 kJ/mol at 35 °C, and −1.835 kJ/mol at 45 °C. The increasingly negative ΔG values with rising temperature indicate that the adsorption process becomes more spontaneous at higher temperatures. This trend suggests that the adsorption of Cu (II) onto biochar is thermodynamically favorable and is enhanced by increasing the temperature, implying an endothermic nature of the adsorption process.
Enthalpy change (ΔH) is a measure of the heat absorbed or released during the adsorption process. In this study, the ΔH value was determined to be −15.71 kJ/mol. The negative value of ΔH indicates that the adsorption process is exothermic, meaning that heat is released as Cu (II) ions bind to the biochar surface. This exothermic nature implies that the interactions between Cu (II) ions and the active sites on the biochar are strong and stable, leading to the release of energy. The magnitude of enthalpy change further supports the chemisorption mechanism, as physical adsorption typically involves lower enthalpy changes.
Entropy change (ΔS) provides information about the disorder or randomness at the solid–liquid interface during the adsorption process. The ΔS value for the adsorption of Cu (II) onto biochar was found to be 48.36 J/mol·K. The positive value of ΔS suggests an increase in randomness at the solid–liquid interface during the adsorption process. This increase in entropy could be attributed to the displacement of water molecules and other ions initially present on the biochar surface by Cu (II) ions. As Cu (II) ions adsorb onto the biochar, they displace these molecules, increasing the overall disorder at the interface.
The combined analysis of ΔG, ΔH, and ΔS provides a comprehensive understanding of the adsorption mechanism. The negative ΔG values at all studied temperatures confirm the spontaneous nature of the adsorption process. Exothermic enthalpy change (ΔH) indicates that the adsorption involves strong interactions between Cu (II) ions and the biochar surface, likely through chemisorption. This is further supported by the relatively high magnitude of ΔH, which is characteristic of chemical adsorption processes. Positive entropy change (ΔS) suggests that the adsorption process leads to an increase in disorder, which is consistent with the displacement of pre-adsorbed water molecules and ions by Cu (II) ions. This displacement contributes to the overall increase in randomness at the interface, making the adsorption process entropically favorable.
The thermodynamic analysis thus reveals that the adsorption of Cu (II) onto biochar is a spontaneous and exothermic process, driven by strong chemisorptive interactions. The increase in entropy further supports this displacement mechanism, wherein Cu (II) ions replace other molecules on the biochar surface, enhancing the adsorption efficiency. These insights underline the effectiveness of biochar as an adsorbent for Cu (II) ions and highlight its potential for practical applications in water treatment and heavy-metal remediation.

3.6. Adsorption Mechanisms

Understanding the adsorption mechanisms of Cu (II) onto biochar is crucial for optimizing its application in wastewater treatment. The adsorption of metal ions onto biochar involves various physical and chemical interactions, which can be broadly categorized into several key mechanisms.

3.6.1. Surface Complexation

Surface complexation is a dominant mechanism in the adsorption of Cu (II) onto biochar. The surface of biochar contains numerous functional groups, such as hydroxyl, carbonyl, and phenolic groups. These groups can interact with Cu (II) ions through coordination bonds, forming surface complexes. The two-step pyrolysis process enhances the availability and reactivity of these functional groups, thereby improving the adsorption efficiency. The formation of these complexes involves the sharing or exchange of electrons between the functional groups on the biochar surface and the Cu (II) ions, indicating a chemisorption process.

3.6.2. Ion Exchange

Ion exchange is another significant mechanism where Cu (II) ions replace other cations, such as hydrogen ions (H+) or sodium ions (Na+), present on the biochar surface. This process is particularly relevant in aqueous solutions where the biochar surface is negatively charged, facilitating the exchange of Cu (II) ions with the cations initially adsorbed onto the biochar. The effectiveness of ion exchange is influenced by the pH of the solution, as higher pH values reduce the competition between H+ ions and Cu (II) ions for the adsorption sites.

3.6.3. Electrostatic Attraction

The electrostatic attraction between Cu (II) ions and the negatively charged sites on the biochar surface is a crucial physical adsorption mechanism. The porous structure of biochar, which is enriched in carbon content due to the two-step pyrolysis process, enhances its ability to attract and retain Cu (II) ions through electrostatic forces. This mechanism is particularly effective at lower pH values where the biochar surface is protonated, increasing its negative charge density.

3.6.4. Pore Filling and Physical Adsorption

The physical adsorption of Cu (II) onto biochar involves the occupation of the biochar’s pores by Cu (II) ions. The biochar’s porous structure, characterized by a significant surface area and volume, provides numerous sites for physical adsorption. This mechanism is enhanced by the large pore size and surface area resulting from the two-step pyrolysis process. Physical adsorption is typically weaker than chemisorption and is influenced by factors such as temperature and the presence of competing ions.
The combination of these adsorption mechanisms results in the high efficiency of biochar in removing Cu (II) ions from aqueous solutions. The surface complexation and ion exchange mechanisms, driven by the biochar’s functional groups and surface charge, play a dominant role in the adsorption process. Electrostatic attraction and physical adsorption further enhance the biochar’s capacity to sequester Cu (II) ions. The detailed understanding of these mechanisms provides valuable insights for optimizing the biochar production process and its application in wastewater treatment. For instance, modifying the pyrolysis conditions to enhance the biochar’s surface functionality and porosity can improve its adsorption capacity. Additionally, adjusting the solution pH and optimizing the adsorbent dosage can maximize the efficiency of Cu (II) removal.

3.7. Reusability of Biochar for Cu (II) Adsorption

The reusability of biochar as an adsorbent for Cu (II) was evaluated over five consecutive adsorption–desorption cycles to assess its stability and reusability. The adsorption–desorption experiments were conducted using 0.1 M HCl as the eluent for desorption of Cu (II)-loaded biochar. The adsorption capacity (qe) of the biochar was measured after each cycle to determine the effectiveness and stability of the biochar over repeated uses. As shown in Figure 7, the adsorption capacity of biochar for Cu (II) slightly decreased with each consecutive cycle. Initially, the adsorption capacity was 60 mg/g, which dropped to 58 mg/g in the second cycle, 56 mg/g in the third cycle, 54 mg/g in the fourth cycle, and finally 52 mg/g in the fifth cycle. This gradual decrease in adsorption capacity indicates some loss of efficiency over multiple cycles, but the biochar still retained a significant adsorption capacity throughout all cycles. The slight reduction in adsorption capacity can be attributed to several factors, including a potential loss of active adsorption sites and structural changes in the biochar after repeated use. However, the biochar’s ability to maintain over 85% of its initial adsorption capacity after five cycles demonstrates its potential for practical applications in wastewater treatment.

4. Conclusions

This study demonstrates that biochar derived from date palm waste via a two-step pyrolysis process is an effective adsorbent for the removal of Cu (II) ions from wastewater. The biochar exhibited a high adsorption capacity and stability, making it a sustainable and economical solution for wastewater treatment. The adsorption process was influenced by several factors, including pH, adsorbent dosage, contact time, and initial Cu (II) concentration. The optimal conditions were identified, with maximum adsorption occurring at pH 6, an adsorbent dosage of 1 g/L, and a contact time of 360 min. The adsorption data fit well with the pseudo-second-order kinetic model, indicating that chemisorption was the primary mechanism. The Langmuir and Freundlich isotherm models suggested that the adsorption occurred through monolayer coverage on a homogeneous surface and multilayer adsorption on a heterogeneous surface, respectively. Thermodynamic parameters revealed that the adsorption process was spontaneous and exothermic, with strong chemisorptive interactions. The high adsorption capacity and favorable thermodynamics underscore the potential of biochar for practical applications in environmental remediation. Reusability tests indicated that the biochar retained over 85% of its initial adsorption capacity after five cycles, demonstrating its stability and potential for repeated use in wastewater treatment. In summary, biochar from date palm waste is a promising adsorbent for heavy-metal removal, contributing to sustainable waste management and environmental protection. Future research should focus on optimizing the pyrolysis process and exploring the application of biochar for the removal of other contaminants from wastewater.

Author Contributions

Conceptualization, E.R.I.M. and H.M.A.; methodology, H.M.A. and H.F.E.-L.; validation, E.R.I.M., H.M.A. and N.A.H.; formal analysis, H.M.A.; investigation, N.A.H.; resources, N.A.H.; writing—original draft preparation, H.F.E.-L.; writing—review and editing, H.F.E.-L. and A.L.K.; visualization, H.M.A. and A.A.; supervision, A.L.K.; project administration, A.A.; funding acquisition, E.R.I.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Data will be available on request.

Acknowledgments

The authors would like to express their appreciation for the support provided by the Scientific Research Deanship, Islamic University of Madinah.

Conflicts of Interest

The authors declare no conflicts of interest.

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Processes 12 01189 g001
Figure 1. Different magnified SEM micrographs of the microstructure of the biochar surface at different zones, where (a,c,e) are anterior views (cross-sections) and (b,d,f) lateral (longitudinal) and mixed views.
Figure 1. Different magnified SEM micrographs of the microstructure of the biochar surface at different zones, where (a,c,e) are anterior views (cross-sections) and (b,d,f) lateral (longitudinal) and mixed views.
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Figure 2. SEM micrograph (a) and its EDX elemental analysis (b) identification of various elements for the biochar prepared by two-step pyrolysis.
Figure 2. SEM micrograph (a) and its EDX elemental analysis (b) identification of various elements for the biochar prepared by two-step pyrolysis.
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Processes 12 01189 g003
Figure 3. FTIR spectra of biochar prepared from midribs of date palm leaves via two-step pyrolysis.
Figure 3. FTIR spectra of biochar prepared from midribs of date palm leaves via two-step pyrolysis.
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Processes 12 01189 g004
Figure 4. The effect of pH, dosage, contact time, and initial concentration on Cu (II) removal. Conditions: (a) pH = 2–6, adsorbent dosage = 1 g/L, Cu (II) concentration = 100 mg/L, contact time = 360 min, shaking speed = 150 rpm, reaction temperature = 25 °C; (b) adsorbent dosage = 0.25–1.25 g/L, pH = 5, other conditions as in (a); (c) contact time = 0–360 min, pH = 5, other conditions as in (a); (d) Cu (II) concentration = 20–100 mg/L, pH = 5, other conditions as in (a).
Figure 4. The effect of pH, dosage, contact time, and initial concentration on Cu (II) removal. Conditions: (a) pH = 2–6, adsorbent dosage = 1 g/L, Cu (II) concentration = 100 mg/L, contact time = 360 min, shaking speed = 150 rpm, reaction temperature = 25 °C; (b) adsorbent dosage = 0.25–1.25 g/L, pH = 5, other conditions as in (a); (c) contact time = 0–360 min, pH = 5, other conditions as in (a); (d) Cu (II) concentration = 20–100 mg/L, pH = 5, other conditions as in (a).
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Figure 5. Pseudo-second-order kinetic model for Cu (II) adsorption on biochar.
Figure 5. Pseudo-second-order kinetic model for Cu (II) adsorption on biochar.
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Processes 12 01189 g006
Figure 6. Adsorption isotherms for Cu (II) on biochar: (a) Langmuir isotherm; (b) Freundlich isotherm.
Figure 6. Adsorption isotherms for Cu (II) on biochar: (a) Langmuir isotherm; (b) Freundlich isotherm.
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Figure 7. Effect of repeated cycles on the adsorption capacity of biochar.
Figure 7. Effect of repeated cycles on the adsorption capacity of biochar.
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Table 1. FTIR bands and corresponding functional groups observed in the spectrum of biochar prepared by two-step pyrolysis.
Table 1. FTIR bands and corresponding functional groups observed in the spectrum of biochar prepared by two-step pyrolysis.
Wavenumber (cm−1)AssignmentsPalm Tree Biochar
3000–3700O-H group3220-3743
2800–3000C-H (methyl)2850-2920-3039
1560C=C1570
1420–1450C-H asymmetric1420
1000–1260C-O1112
700–900C-H aromatic (out of plane)756-815-874
400–600Inorganic matter468-573-615
Table 2. Parameters of the pseudo-second-order kinetic model.
Table 2. Parameters of the pseudo-second-order kinetic model.
SampleExperimentalPseudo-Second-Order Kinetics
Biocharqe (mg/g)k2 (g/(mg·min))qe (mg/g)R2
607.4 × 10−588.290.92
Table 3. Parameters of adsorption isotherms for the adsorption of Cu (II) on biochar.
Table 3. Parameters of adsorption isotherms for the adsorption of Cu (II) on biochar.
SampleLangmuir ConstantFreundlich Constant
BiocharKL (L/mg)qmax (mg/g)R2KF [(mg/g)(L/mg)n]NR2
0.012388.290.9911.230.670.987
Table 4. Thermodynamic parameters of Cu (II) adsorption of biochar at three temperatures.
Table 4. Thermodynamic parameters of Cu (II) adsorption of biochar at three temperatures.
SampleΔG0 (kJ/mol)
Temp °C		ΔH0
(kJ/mol)		ΔS0
(J/mol-K)
Biochar253545
−1.245−1.475−1.835−15.7148.36
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Mahmoud, E.R.I.; Aly, H.M.; Hassan, N.A.; Aljabri, A.; Khan, A.L.; El-Labban, H.F. Biochar from Date Palm Waste via Two-Step Pyrolysis: A Modified Approach for Cu (II) Removal from Aqueous Solutions. Processes 2024, 12, 1189. https://doi.org/10.3390/pr12061189

AMA Style

Mahmoud ERI, Aly HM, Hassan NA, Aljabri A, Khan AL, El-Labban HF. Biochar from Date Palm Waste via Two-Step Pyrolysis: A Modified Approach for Cu (II) Removal from Aqueous Solutions. Processes. 2024; 12(6):1189. https://doi.org/10.3390/pr12061189

Chicago/Turabian Style

Mahmoud, Essam R. I., Hesham M. Aly, Noura A. Hassan, Abdulrahman Aljabri, Asim Laeeq Khan, and Hashem F. El-Labban. 2024. "Biochar from Date Palm Waste via Two-Step Pyrolysis: A Modified Approach for Cu (II) Removal from Aqueous Solutions" Processes 12, no. 6: 1189. https://doi.org/10.3390/pr12061189

APA Style

Mahmoud, E. R. I., Aly, H. M., Hassan, N. A., Aljabri, A., Khan, A. L., & El-Labban, H. F. (2024). Biochar from Date Palm Waste via Two-Step Pyrolysis: A Modified Approach for Cu (II) Removal from Aqueous Solutions. Processes, 12(6), 1189. https://doi.org/10.3390/pr12061189

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