1. Introduction
Over the past two decades, the adsorption of phosphate using iron-based oxides has been extensively studied due to increasing environmental concerns associated with eutrophication and water quality deterioration [1]. Phosphorus (P), an essential but potentially harmful nutrient, is commonly introduced into aquatic environments in the form of orthophosphate (PO43−), primarily originating from both point sources (e.g., municipal and industrial effluents) and non-point sources (e.g., agricultural runoff and soil erosion) [2]. These phosphate species often bind to soil particles and are transported via surface runoff into rivers, lakes, and reservoirs, where they accumulate in sediments and persist in bioavailable forms, leading to long-term ecological impacts [3].
The sustained release of phosphorus from sediments, combined with increasing anthropogenic inputs, has raised significant concerns about long-term nutrient pollution in aquatic environments. According to the Korean Ministry of Environment’s effluent discharge standards, total phosphorus concentrations in treated wastewater must be maintained below 0.2–0.5 mg/L, depending on the classification of the treatment facility and the sensitivity of the receiving water body [4]. Therefore, effective phosphorus removal in wastewater treatment processes is crucial to ensure regulatory compliance and to minimize ecological damage to downstream ecosystems [5]. Among various phosphorus remediation technologies, adsorption has emerged as a highly effective approach for the removal of phosphorus and other inorganic contaminants due to its operational simplicity, reversibility, and minimal sludge generation [6].
A wide range of adsorbents, including activated carbon, biochar, metal-organic frameworks (MOFs), and various metal oxides, have been explored for phosphate removal [7, 8]. However, these materials often suffer from limitations such as low adsorption selectivity, high production or regeneration costs, or insufficient performance at low phosphate concentrations (<1 mg/L) [9, 10]. Recent attention has turned toward the development of iron oxide-based adsorbents, particularly hematite (α-Fe2O3), due to its strong affinity for oxyanions like phosphate via surface complexation and ligand exchange mechanisms [11, 12].
Expanded graphite (EG), a novel carbonaceous material with high thermal stability and unique layered morphology, has traditionally been used in flame retardants and thermal management applications. Its extremely high surface area-to-mass ratio, electrical conductivity, and porous architecture render it a promising support material for adsorbent design, although its potential in water treatment remains underexplored [13, 14].
In this context, hematite-coated expanded graphite (α-Fe2O3/EG) was developed as a composite adsorbent to synergize the phosphate-binding capacity of hematite with the high surface area and structural stability of EG. The nanocomposite structure facilitates enhanced adsorption kinetics and capacity through both chemical (inner-sphere complexation) and physical (surface diffusion) mechanisms [15, 16]. Moreover, the combination addresses key limitations of traditional adsorbents by improving removal efficiency, especially at trace concentrations, while reducing the need for chemical coagulants and repeated regeneration cycles. The primary objective of this research is to investigate the adsorption performance of α-Fe2O3/EG for phosphate removal at different temperatures, with the goal of understanding both the equilibrium characteristics and kinetic behavior of the process. To achieve this, adsorption isotherm models (e.g., Langmuir and Freundlich) are applied to determine the maximum adsorption capacity, while kinetic models (pseudo-first-order and pseudo-second-order) are employed to analyze the rate-controlling mechanisms. Furthermore, thermodynamic parameters—including Gibbs free energy (ΔG), enthalpy (ΔH), entropy (ΔS), and activation energy (Ea)—are calculated to assess the spontaneity, energetic feasibility, and temperature dependence of the adsorption process. Through this comprehensive evaluation, the study aims to provide insight into the potential application of α-Fe2O3/EG as an effective and thermally stable adsorbent for phosphate removal in water treatment systems.
2. Experimental
2.1. Preparation of Expanded Graphite
A stock quantity of expanded graphite (EG) was prepared through chemical intercalation followed by rapid thermal expansion. Initially, natural graphite flakes were immersed in concentrated sulfuric acid (H2SO4), which facilitated the intercalation of sulfate ions between the graphite layers. This reversible anionic insertion caused an increase in interlayer spacing by disrupting the van der Waals forces holding the graphite planes together, thereby expanding the crystal lattice structure. Following the intercalation step, the acid-treated graphite flakes were subjected to rapid thermal shock by placing them in a preheated muffle furnace (Digital Furnace, LAB HOUSE) at 800 °C for 1 minute. The sudden temperature increase induced the volatilization of intercalated species, resulting in the exfoliation and expansion of graphite, yielding the desired EG material [17]. To prevent thermal degradation or burning of the graphite, the flakes were immediately removed from the furnace after thermal expansion. The natural graphite flakes used in this study were commercially obtained from Hyundai Coma Inc. (Republic of Korea).
2.2. Preparation of Adsorbent α-Fe2O3/EG
FeOOH, a nanostructured precursor of hematite (α-Fe2O3), was synthesized via a hydrothermal method. Specifically, 68.6 mL of deionized water, 15.6 g of ferric chloride (FeCl3), 1.4 mL of ethanol, and 0.5 g of expanded graphite (EG) were added into a conical flask and stirred using a magnetic stirrer at 150 rpm for 30 minutes. This procedure yielded a 0.06 M aqueous FeCl3 solution with a uniform dispersion of EG. The homogeneous mixture was subsequently transferred into a 60 mL Teflon-lined stainless-steel autoclave, sealed, and subjected to hydrothermal treatment at 120 °C for 12 hours. Upon completion, the resulting black precipitate was cooled to below 30 °C, rinsed three times with deionized water to remove residual ions and unreacted species, and then dried using a freeze dryer for 4 hours. The obtained yellow FeOOH solid was collected and further subjected to calcination to convert it into hematite. The dried FeOOH/EG composite was heated at 520 °C for 8 hours with a ramping rate of 10 °C/min under ambient atmosphere. This thermal treatment facilitated the crystallization of α-Fe2O3 and its uniform deposition onto the EG surface. The final Fe2O3/EG composite was rinsed thoroughly with deionized water and dried completely using a vacuum freeze dryer (Model OPR-FDβ-5503, OPERON, Republic of Korea) for subsequent characterization and adsorption experiments [18].
2.3. Phosphorous Isotherm Experimental Procedure
To investigate the effect of temperature on phosphorus adsorption by the hematite-coated expanded graphite (α-Fe2O3/EG) adsorbent, adsorption isotherms were obtained by varying the initial phosphorus concentration. Specifically, aliquots ranging from 0.1 to 20.0 mL of a phosphorus stock solution were added to 100 mL glass bottles containing 10.0 mg of the α-Fe2O3/EG suspension. The baseline phosphorus stock solution, prepared at an initial concentration of 100 mg/L, was formulated by dissolving 30.221 mg of NaH2PO4 and 23.758 mg of Na2HPO4 in deionized water, adjusted to fit the volume constraints of the conical flasks. Each conical flasks was tightly sealed with silicon-coated caps to prevent leakage and minimize oxidation throughout the adsorption process. Adsorption experiments were conducted in a temperature-controlled shaking incubator at three distinct temperatures (298 K, 308 K, and 318 K). After 24 hours of agitation, samples were collected, and the residual phosphorus concentrations were quantified using appropriate analytical techniques. The amount of phosphorus adsorbed was calculated based on the difference between the initial and final concentrations. Additionally, the pH of the solutions was measured before and after the adsorption to evaluate the buffering capacity of phosphate in the system. Minimal pH variation indicated that phosphate ions functioned effectively as a buffer during the adsorption process. All experiments were performed in duplicate to ensure reproducibility. Prior to analysis, samples were filtered through 0.45 μm pore size membrane filters to remove suspended solids, and the filtrates were subjected to subsequent phosphorus concentration measurements.
2.4. Phosphorous Kinetic Experimental Procedure
To determine the Gibbs free energy (ΔG) and activation energy (Ea) associated with phosphorus adsorption, a kinetic adsorption experiment was conducted to monitor the removal rate of phosphorus in real time. A 500 mL reaction vessel containing 400 mL of deionized water with phosphorus stock solution diluted to 400 mg/L (prepared by dissolving 120.882 mg of NaH2PO4 and 95.032 mg of Na2HPO4) was used as a control. Two additional reaction vessel with identical solutions were prepared, each supplemented with 100 mg of α-Fe2O3/EG adsorbent.
All reaction vessels were promptly transferred to a temperature-controlled shaking incubator to initiate the adsorption process, minimizing any pre-adsorption occurring outside the shaker by rapid sample handling. Aliquots of 6 mL were withdrawn at predetermined time intervals (5, 10, 15, 30, 45, 60, 90, 120, 180, 240, and 300 minutes) using an electronic timer to ensure precise sampling. Upon completion of the kinetic experiment, the collected samples were filtered and analyzed to determine the residual phosphorus concentration. The control reaction vessels, which contained no adsorbent, exhibited negligible phosphorus loss, confirming that adsorption to the glass surface was insignificant.
2.5. Analytical Methods
Phosphorus adsorption was analyzed using SEM, XRD, TGA, and IC. The surface topography of the adsorbent samples was analyzed using a Scanning Electron Microscope equipped with (LEO SUPRA 55, Carl Zeiss). The collected samples were placed onto a carbon tape supported by gold metallic disks. The composition of α-Fe2O3/EG before and after phosphorus adsorption was determined by selecting surfaces that contained black solid areas. Then, samples were characterized by X-ray powder diffraction (XRD) on a (D8 Advance, Bruker). Then, thermal characterization of the synthesized adsorbents was performed using TGA. TGA determines temperature and weight change of the decomposition reaction and allow quantitative composition analysis (TGA Q5000 IR, TA Instruments). The samples were conducted at a temperature range of 298K to 1298K at a heating rate of 1K/min. The specific adsorption rate from the kinetic experiment was measured by ion chromatography (ICS-1100, Thermo Scientific). All analytical experiments, including SEM, XRD, and adsorption measurements, were performed using instruments at the Central Research Facility of Gyeongsang National University.
3. Results and Discussion
3.1. Adsorption Isotherms
Adsorption of phosphorus ions by hematite/EG was analyzed using the Langmuir and Freundlich isotherms. The Langmuir adsorption isotherm model [19] is founded on the assumption that the maximum adsorption corresponds to the formation of a saturated monolayer of solute molecules on the adsorbent surface. The linearized Langmuir equation is expressed as: where qe symbolizes qmax (mg.g−1), qm is the maximum amount of phosphorus per unit mass of sorbent (from model), K is the adsorption capacity (L.mg−1), and Ce (mg.L−1) is the equilibrium concentration (experimental value). Through a linearized plot of (Ce/qe) versus Ce shown in Fig. 1(a) demonstrates that the adsorption behavior of phosphorus on α-Fe2O3/EG conforms well to the Langmuir model. Table 1 presents correlation coefficients (R2) of approximately 0.95, indicating a high degree of fit and a favorable adsorption process under the experimental conditions. The Freundlich isotherm is an empirical equation that is expressed as the following [20]: where KF and n are Freundlich constants. KF (mg/g (L/mg)1/n) relates to the adsorption capacity of sorbent while n determines the adsorption process as favorable or not. Dividing this constant by 1 gives the magnitude of the exponent which tells the adsorption favorability. Isotherms with n > 1 is likely to be a favorable condition [21]. KF and n values are calculated using a plot of logqe versus logCe (Fig. 1(b), Table 1).
Comparative analysis reveals that the Langmuir isotherm model provides the best fit for phosphorus adsorption in this system, as further elucidated by the graphical data and statistical parameters discussed subsequently. This suggests monolayer adsorption onto a homogenous adsorbent surface predominates under the experimental conditions.
To further evaluate the favorability of phosphate adsorption, the dimensionless Langmuir separation factor (RL) was calculated RL values were illustrated for each temperature and are now included in Table 1. The dimensionless separation factor RL defined as where C0 is the initial phosphorus concentration (mg/L), characterizes the nature of the adsorption process. Specifically, RL = 1 indicates linear adsorption RL = 0 corresponds to irreversible adsorption, and 0 < RL < 1 suggests favorable adsorption [22]. The results show that RL values at 298 K, 308 K, and 318 K were 0.014, 0.037, and 0.014, respectively—all within the range of 0 < RL < 1—indicating favorable adsorption behavior across all tested conditions. These values further confirm that the phosphate adsorption onto αFe2O3/EG proceeds efficiently and is thermodynamically favorable.
3.2. Adsorption Kinetics
In order to evaluate the adsorption process, pseudo-first order and pseudo-second order kinetic models were applied to the obtained experimental data. The pseudo first-order equation, also termed as the Lagergren’s equation [23] can be applied to correlate kinetic adsorption data in systems near equilibrium stage [24]. The equation is stated as:
where k1 is the pseudo first-order rate constant (g/mg h) and qe, qt is calculated as the adsorption capacity at equilibrium and at a specific time t. Using the slope and intercept of plot log(qe−qt) versus t (Fig. 2, Table 2), parameters k1 and qe were calculated.
Pseudo-second order rate expression is dealt to analyze chemisorption kinetics [25, 26]. The equation can be expressed as:
where k2 is the pseudo second order rate constant (g/mg h) and k2qe2 is the initial rate of adsorption. Adsorption parameters qe and k2 were determined by plotting t/qt versus t (Fig. 2, Table 2). Comparing the two models, it was observed that the pseudo second-order model tended to be a better interpretation of kinetic adsorption because pseudo first-order model did not fit well with the experimental data. Presented in Table 2, the calculated qe values are very off to the experimental qe values [27]. In the pseudo first-order model, the plot appears to have a non-linear shape leading to lower correlation coefficients (R2) compared to pseudo second-order model (Fig. 2, Table 2). This suggests poor phosphorus adsorption using the pseudo first-order fit.
3.3. Adsorption Gibb’s Free Energy
Gibb’s free energy [28] plays an important role in chemical reactions. The change in the Gibbs free energy of the system that occurs during a reaction equals to the change in the enthalpy of the system minus the change in the product of the temperature times the entropy of the system which is expressed as: ΔGo = ΔHo − TΔSo.
This reaction measures the balance between the two driving forces that determine whether a reaction is spontaneous or not. Favorable, or spontaneous reactions have ΔG < 0 and unfavorable, non-spontaneous reactions have ΔG > 0. Reactions are classified as either exothermic (ΔH < 0) or endothermic (ΔH > 0) on whether they release or absorb heat. The results of this theory will be further discussed in the thermodynamic studies section.
3.4. Adsorption Activation Energy
The magnitude of activation energy is a useful tool to distinguish between physisorption and chemisorption. The primary method to tell between a particular adsorption depends on the binding energy of the adsorbate to the substrate. Physisorption, caused by van der waals forces has weak interaction energy ranging from 5 to 40 kJ mol−1 [29]. On the other hand, chemisorption expresses a much stronger force compared to physisorption and can occur in higher temperatures. Chemisorption requires higher activation energy, typically ranging from 40 to 800 kJ/mol [29]. Phosphorus adsorption on hematite led to an activation energy of 79.66 kJ/mol which follows the method of chemisorption. Fig. 3 shows a linear plot of ln k2 versus 1/T, where the slope corresponds to −Ea/R, with Ea representing the activation energy (kJ/mol) and R the universal gas constant (8.314 J/mol·K).
3.5. Thermodynamic Studies
The increasing temperature resulted in a decreasing rate of phosphorus adsorption meaning that the process is an exothermic reaction. At three different temperatures, K0 was examined by plotting ln (Cs/Ce) versus Cs (Fig. 4). After observing the graph and referencing Table 3, K0 decreased with increasing temperature. This indicates that this adsorption model released heat during the reaction. Also, Gibb’s free energy (ΔG) turned out to be negative which leads to a spontaneous adsorption. In order to calculate ΔH and ΔS, the slope and intercept of lnK0 versus 1/T (K−1) were plotted (Fig. 4). The standard enthalpy change came out to be negative (−89.4584 kJ mol−1), suggesting that the adsorption of phosphorus by hematite is exothermic (Table 3).
3.6. SEM, XRD, TGA Analysis
To confirm the occurrence of phosphorus adsorption on the surface of the adsorbent, scanning electron microscopy (SEM) analysis was performed before and after the adsorption process. As illustrated in Fig. 5, a clear morphological distinction can be observed between the two states. Prior to adsorption, the hematite-coated expanded graphite (α-Fe2O2/EG) exhibits a highly porous and rough surface morphology, characterized by numerous voids and interlayer spaces [25, 30]. These pores provide a high surface area conducive to the adsorption of phosphorus species. After the adsorption process, a noticeable reduction in surface roughness and partial pore coverage was observed, suggesting the successful attachment of phosphorus onto the adsorbent surface. The pore size distribution is estimated to fall within the mesoporous range (2–50 nm), which supports effective diffusion and surface interaction of phosphate ions.
For X-ray powder diffraction (XRD) analysis, the D8 Advance by Bruker instrument was used to carry out the results. The XRD pattern for hematite/EG is shown in Fig. S1c. All the diffracted intensity peaks match very well with standard patterns of pure hexagonal hematite. As previously described, thermogravimetric analysis (TGA) was performed to determine the thermal stability and compositional properties of the synthesized α-Fe2O3/EG nanocomposite [31]. The analysis was conducted under an air atmosphere using a heating rate of 1 K/min, with the temperature ramped from 298 K to 1298 K. For comparison, TGA curves were also obtained for pure expanded graphite (EG) and pure hematite (α-Fe2O3). The TGA curve of the α-Fe2O3/EG sample showed a significant mass loss occurring between approximately 673 K and 973 K, which corresponds to the complete combustion of the carbonaceous EG component. The derivative thermogravimetric (DTG) curve revealed a sharp peak near 973 K, indicating the rapid oxidation and vaporization of graphite under the oxidative atmosphere. In contrast, pure EG exhibited a similar mass loss profile, whereas pure α-Fe2O3 remained thermally stable throughout the entire temperature range with negligible weight change. The residual weight fraction observed after 973 K in the composite sample represents the thermally stable iron oxide content. As summarized in Table 4, the remaining mass corresponds to the amount of hematite successfully deposited on the EG surface. These results confirm the effective loading of α-Fe2O3 onto EG and the composite’s thermal decomposition behavior, validating the structural integrity of the metal oxide component under high-temperature conditions (Table S1 and Fig. S2).
3.7. Comparison of Phosphate Adsorption Capacity with other Adsorbents
To evaluate the performance of the synthesized αFe2O3/EG composite, a comparative analysis was conducted against various iron oxide- and EG-based adsorbents previously reported in the literature. Table 4 summarizes representative materials, their synthesis methods, operating temperatures, and maximum phosphate adsorption capacities.
As shown in Table 5, the phosphate adsorption capacities of iron-based adsorbents vary considerably depending on the synthesis method, support material, and physicochemical properties. Conventional αFe2O3 nanoparticles prepared via co-precipitation typically exhibit limited adsorption capacities (~5.6 mg/g), whereas EG-supported composites demonstrate improved performance due to the high surface area and porosity of the carbonaceous support. In particular, CuFe2O4/EG and magnetite/EG composites synthesized via hydrothermal routes report capacities ranging from 8 to 12 mg/g. In comparison, the αFe2O3/EG composite developed in this study achieved a significantly higher maximum adsorption capacity of 33.3 mg/g at 298 K. This enhancement is attributed to the optimized synthesis procedure, in which FeOOH was hydrothermally deposited onto expanded graphite and subsequently converted to crystalline αFe2O3 via calcination. The resulting nanocomposite features highly dispersed iron oxide nanoparticles and a porous graphite matrix, which together provide abundant active sites and facilitate strong chemisorptive interactions with phosphate ions. The superior performance of αFe2O3/EG underscores its potential as a promising and scalable adsorbent for phosphate removal in water and wastewater treatment applications.
4. Conclusions
This study confirms that hematite-coated expanded graphite (α-Fe2O3/EG) is a highly efficient and environmentally friendly adsorbent for the removal of phosphorus from contaminated water. The material’s high surface area, nano-scale layered structure, and porous morphology enabled rapid and effective adsorption. The Langmuir isotherm model exhibited the best fit, with a maximum adsorption capacity of 33.3 mg/g at 298 K, indicating monolayer chemisorption. Thermodynamic analysis showed that the process is exothermic and spontaneous, as evidenced by negative ΔG and ΔH values, suggesting strong chemical interactions between phosphate and the hematite surface. Material characterizations using SEM, XRD, and TGA supported the successful synthesis and thermal stability of the α-Fe2O3/EG composite. SEM images confirmed morphological changes post-adsorption, XRD patterns matched standard hematite structures, and TGA analysis quantified residual hematite content and carbon loss during heating. While a slight decrease in adsorption capacity was observed with increasing temperature, the material’s robustness was demonstrated, maintaining effective adsorption under varying environmental conditions. Given its strong performance, α-Fe2O3/EG presents a cost-effective and scalable option for phosphate removal in municipal, agricultural, and industrial wastewater treatment. Future research should explore its reusability, performance in complex multi-contaminant systems, and large-scale application through pilot-scale testing or integration into fixed-bed columns. Additionally, the development of regeneration protocols and long-term performance assessments will be essential to validate its practical viability in real-world water treatment infrastructures [35].
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