Adsorption of Phosphates from Wastewater Using MgAlFe-Layered Double Hydroxides

Abstract

Phosphates pollution, primarily from agricultural runoff and wastewater discharge, is a major contributor to water eutrophication, adversely affecting aquatic ecosystems. This study reports the synthesis, characterization, and phosphates adsorption performance of a MgAlFe-layered double hydroxide (MgAlFe-LDH) with a 2:1:1 cationic ratio. The material was prepared via co-precipitation and characterized using digital microscopy, XRD, BET, XPS, and FTIR. Adsorption experiments were conducted at pH 3 and 9 to investigate equilibrium, kinetics, and reusability. The MgAlFe-LDH exhibited a high maximum adsorption capacity (q_max ≈ 215 mg/g) largely independent of pH, with adsorption well described by the Langmuir model. Kinetic studies revealed a pseudo-first-order mechanism, indicating that adsorption is dominated by surface diffusion and electrostatic interactions. Phosphate removal occurs through a dual mechanism involving rapid electrostatic attraction at protonated surface sites and slower ion exchange in the LDH interlayers. The material retained over 75% of its adsorption capacity after five consecutive adsorption–desorption cycles, highlighting its potential for sustainable phosphate recovery. Overall, the MgAlFe-LDH represents a promising, reusable adsorbent for phosphorus removal from wastewater, supporting circular economy strategies.

1. Introduction

Nutrient pollution, primarily from nitrogen and phosphorus, is a major driver of water quality degradation and refers to contamination caused by excessive nutrient inputs. It is a leading cause of eutrophication in surface waters, where surplus nutrients stimulate biomass growth and support harmful cyanobacterial blooms [1,2,3]. The resulting biomass reduces water transparency and depletes dissolved oxygen, ultimately causing the loss of aquatic fauna and, in extreme cases, the transition to swamp-like conditions. Although eutrophication is naturally a very slow process unfolding over centuries, human activities have greatly accelerated it, rapidly altering aquatic ecosystems. Modern agriculture, rapid urbanization, and industrialization have disrupted the nutrient balance, especially for nitrogen and phosphorus, by markedly increasing their concentrations from both diffuse and point sources in surface and groundwater [4,5,6,7,8]. This influx, particularly evident in lakes, has led to a global decline in water quality.

The need to develop new technologies for phosphorus removal and recovery from wastewater arises from growing demand for this critical resource, the tightening of discharge limits to curb eutrophication, and stricter restrictions on sewage-sludge disposal. Two main strategies are pursued to mitigate eutrophication: direct immobilization of bioavailable phosphorus using inactivating agents [9], and treatment of wastewater to reduce phosphorus levels [10,11,12,13]. Enhancing the performance of wastewater treatment plants can be achieved by implementing high-efficiency processes that lower phosphate concentrations in effluents; available options include coagulation [10], adsorption [11], electrochemical precipitation [12], and biological phosphorus removal [13]. In line with circular economy principles, priority is given to methods that enable recovery and reuse of phosphorus compounds [14], such as coupling biological phosphorus removal with chemical processes (e.g., precipitation) [15] or employing adsorption [16]. Because biological processes mainly transfer phosphorus from wastewater to sludge, a subsequent recovery step is required. In practice, recovering phosphorus is more challenging than simply removing it: chemical precipitation and coagulation typically require ≥100 mg·L⁻¹ phosphorus in the feed, which is above the concentrations commonly found in wastewater [17] and in sewage sludge [18].

Compared to other approaches, adsorption offers straightforward operation, high efficiency, and the potential to reuse both the adsorbent and the adsorbate. A variety of adsorbents have been evaluated, with emphasis on natural or repurposed materials such as alginate [19,20], red clay [21], and biochar [11,22,23].

Phosphate speciation is strongly pH-dependent: below pH 2.15 H₃PO₄ predominates; between pH 2.15 and 7.20 H₂PO₄⁻ is dominant; and from pH 7.20 to 12.33 HPO₄²⁻ prevails [24]. The pH-controlled solubility of iron, aluminum, and magnesium oxides also influences adsorption. At pH < 6, the acidic environment may partially dissolve LDH layers, releasing Fe³⁺ and Mg²⁺ that can interact with phosphate in solution, potentially enhancing uptake. In this system, the pH effect is linked to LDH structural stability and the solubility of its constituent cations rather than to the presence of separate iron- or magnesium-oxide phases [25,26].

Layered double hydroxides (LDHs) have been widely studied and shown to be effective for removing contaminants from wastewater [27,28,29,30,31]. LDH anionic clays comprise natural and synthetic layered compounds of general formula [M²⁺_1−xM³⁺_x(OH)₂]^x+(Aⁿ⁻)^x/ⁿ·mH₂O, where M²⁺ (e.g., Mg, Ni, Zn, Cu) and M³⁺ (e.g., Al, Cr, Fe, V, Ga) form brucite-like layers, and interlayer anions (e.g., NO₃⁻, CO₃²⁻, Cl⁻) can be exchanged with anionic contaminants in water. Their large specific surface area, high porosity, substantial anion-exchange capacity, and good stability make them well suited for the harsh conditions typical of wastewater treatment [32,33].

To evaluate circular economy applicability, adsorption–desorption testing is essential. Despite extensive LDH research, relatively few studies report pollutant desorption and adsorbent reuse [29,34,35].

Against this backdrop, most phosphate-removal studies have focused on binary LDH systems or varied cation ratios, with limited attention to MgAlFe-LDH formulations. In particular, the 2:1:1 molar ratio of Mg²⁺:Al³⁺:Fe³⁺ remains underexplored for phosphate adsorption, though it may balance surface reactivity, structural stability, and ion-exchange capacity. Moreover, few works jointly address high adsorption capacity and reusability under realistic aqueous conditions. Therefore, this study synthesizes and characterizes a MgAlFe-LDH with a 2:1:1 cation ratio and evaluates its phosphate-adsorption performance, including regeneration over multiple cycles. The material was characterized by digital microscopy, XRD, XPS, FTIR, and nitrogen-adsorption BET analysis. The effects of initial phosphate concentration and contact time were investigated; kinetics were modeled using two approaches; and reusability was assessed over five adsorption–desorption cycles. To our knowledge, a similar LDH has not previously been tested for phosphates removal.

2. Materials and Methods

2.1. Materials

The reagents used in this study were prepared following the Romanian standard ISO protocol (Romanian Institute for Standardization, SR 11411-2:1998) [36]. These included 63% nitric acid (ρ = 1.41 g·cm⁻³), vanadomolybdate color reagent, and KH₂PO₄ solutions with concentrations ranging from 10 to 100 mg·L⁻¹. Diluted solutions of sodium hydroxide and sulfuric acid were used for pH adjustment.

For the synthesis of the adsorbent, Mg(NO₃)₂·6H₂O, Al(NO₃)₃·9H₂O, Fe(NO₃)₃·6H₂O, Na₂CO₃, and NaOH were employed, all purchased from Sigma-Aldrich (Merck KGaA, Darmstadt, Germany) and used without further purification. Additional sulfuric acid, NaOH, and 63% HNO₃ were supplied by Merck (Merck KGaA, Darmstadt, Germany). All reagents were of analytical grade. Distilled water was used throughout the preparation of the adsorbent, solutions, and synthetic wastewater.

2.2. Synthesis and Characterization of the MgAlFe-LDH Material

2.2.1. Synthesis of MgAlFe-LDH Material

MgAlFe-LDH materials were synthesized via the chemical co-precipitation method at a constant pH of 8.45 (pH meter: JK-PH009, JKI, Shanghai, China). Stoichiometric amounts of Mg(NO₃)₂·6H₂O, Al(NO₃)₃·9H₂O, and Fe(NO₃)₃·6H₂O were dissolved in distilled water to achieve a total cation concentration of 1 M, with a Mg²⁺:Al³⁺:Fe³⁺ molar ratio of 2:1:1. Although this ratio deviates from the typical divalent/trivalent cation ratio (commonly 2:1 to 4:1) for well-crystallized LDH structures, it was intentionally selected to investigate the effect of unconventional composition on adsorption properties. The potential formation of secondary phases is discussed based on XRD results.

The mixed cation solution was slowly added to a 2 × 10⁻³ M Na₂CO₃ solution under vigorous stirring (200 rpm), while maintaining the pH at 8.45 by the dropwise addition of 0.1 M NaOH. The suspension was stirred for an additional 30 min at room temperature (22 ± 1 °C), and the resulting precipitate was allowed to mature for 12 h. The solid was then filtered, washed repeatedly with distilled water, and air-dried at room temperature. Finally, the dried material was calcined at 650 °C for 8 h and subsequently cooled to room temperature over 8 h before use in phosphate adsorption experiments [17].

A schematic representation of the LDH synthesis procedure is shown in Figure 1.

2.2.2. Characterization of the MgAlFe-LDH Material

The surface morphology of the samples (pristine LDH and phosphate-loaded LDH) was examined using a USB Digital Microscope with portable stand, full HD, 400× magnification, Andoer, Shenzhen, China (by Oxford Instruments, Hillsboro, AZ, USA).

Crystallographic structure was analyzed by X-ray diffraction (XRD) using a Rigaku MiniFlex II diffractometer with Cu-Kα radiation (λ = 1.5406 Å), calibrated with an Al₂O₃ standard (SRM 676 A) (Rigaku Corporation, Tokyo, Japan). Samples were scanned over a 2θ range of 10–70° with a step size of 0.02° and a counting time of 100 s/step.

Specific surface area was determined via nitrogen adsorption–desorption at 77 K using a Micromeritics TriStar II Plus analyzer (Micromeritics Instrument Corporation, Norcross, GA, USA). Samples were outgassed at 40 °C for 17 h prior to measurement, and the surface area was calculated using the Brunauer–Emmett–Teller (BET) method within a relative pressure (P/P₀) range of 0.08–0.25.

Surface elemental composition and chemical states were analyzed by X-ray photoelectron spectroscopy (XPS) using an AXIS Supra+ instrument (Kratos Analytical Ltd., Manchester, UK) with a monochromatic Al Kα source (hν = 1486.6 eV). Spectra were acquired under ultra-high vacuum and calibrated using the C 1s peak at 284.8 eV, with a binding energy range of 0–1400 eV.

Fourier-transform infrared (FTIR) spectra were recorded in transmittance mode using a Perkin Elmer spectrophotometer (PerkinElmer, Inc., London, UK) equipped with a Golden Gate unit, over the range 4000–500 cm⁻¹ at a resolution of 4 cm⁻¹.

The point of zero charge (pHpzc) of MgAlFe-LDH was determined using the pH drift method. In brief, 0.05 g of the adsorbent was added to 50 mL of 0.01 M NaCl solution, and the initial pH of the suspension was adjusted to values between 2 and 12 using HCl or NaOH. The suspensions were stirred for 24 h at room temperature, after which the final pH was measured. The pHpzc was identified as the pH at which the change in pH (ΔpH = pH_f − pH_i) was zero.

2.3. Preparation of the Simulated Wastewater Solution

A simulated wastewater solution containing phosphate ions was prepared by dissolving 1.9175 g of KH₂PO₄ in 1000 mL of distilled water to create a stock solution. Working solutions were then prepared by diluting the stock solution up to concentrations from 10 to 100 mg/L. Deionized water, sulfuric acid and sodium hydroxide diluted solutions were used to adjust the pH of the working solutions to 3 and 9, as measured with a JK-PH009 pH meter (Shanghai Ltd., Shanghai, China). The resulting solutions were used immediately in experiments.

2.4. Equilibrium Adsorption Isotherms

The adsorption performance of MgAlFe-LDH clay toward phosphorus was investigated through batch equilibrium experiments. Isotherm studies were carried out at two pH values (3 and 9) using an adsorbent dosage of 50 mg in 50 mL solution with a contact time of 24 h. The samples were kept in an incubator at 25 °C under continuous shaking (150 rpm). Equilibrium data were evaluated by applying the Langmuir and Freundlich models. For the equilibrium assessment, the adsorbent was exposed to KH₂PO₄ solutions with initial concentrations ranging from 10 to 100 mg/L under identical experimental conditions. Following equilibration, suspensions were filtered through 0.45 μm membrane filters, and the residual phosphate concentrations in the filtrates were quantified using an UV-VIS spectrophotometer (UV-1900 Shimadzu, Shimadzu Europa GmbH, Duisburg, Germany) at a wavelength of 470 nm, following the SR 11411-2:1998 standard method [36]. Each experiment was performed in triplicate. The equilibrium adsorption capacity (q_e, mg/g) was calculated using the following equation:

$${\mathrm{q}}_{\mathrm{e}}={\displaystyle \frac{\left({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{e}}\right)\mathrm{V}}{\mathrm{m}}}$$

(1)

where C₀ is the initial concentration (mg/L), C_e the equilibrium concentration (mg/L), V (L) is the volume of the solution and m (g) is the adsorbent dose.

2.4.1. Langmuir Model

The model assumes that the adsorbent surface possesses active sites capable of binding adsorbate ions via physisorption or chemisorption. It is based on the premise that adsorption occurs through physical or chemical bonds between the surface and the adsorbate, without interactions among the adsorbed molecules. The model describes monolayer adsorption, where each site accommodates only one molecule, and adsorbate species do not interact with each other.

The Langmuir isotherm is mathematically represented by the following equation:

$${\mathrm{q}}_{\mathrm{e}}={\displaystyle \frac{{{\mathrm{q}}_{\mathrm{m}\mathrm{a}\mathrm{x}}\cdot \mathrm{K}}_{\mathrm{L}}\cdot {\mathrm{C}}_{\mathrm{e}}}{1+{\mathrm{K}}_{\mathrm{L}}\cdot {\mathrm{C}}_{\mathrm{e}}}}$$

(2)

where q_e is the amount adsorbed at equilibrium, mg/g, q_max is the maximum adsorption capacity, mg/g, C_e is the concentration at equilibrium, mg/L, and K_L is Langmuir constant, L/mg [37].

2.4.2. Freundlich Model

The model is based on the hypothesis that chemical equilibrium is established through a dynamic exchange between molecules adsorbed on the surface and those remaining in solution. It assumes a heterogeneous adsorbent surface, allowing adsorption to occur in multiple layers, with adsorption sites exhibiting different affinities for the adsorbate species. The Freundlich isotherm is mathematically expressed as follows:

$${\mathrm{q}}_{\mathrm{e}}={\mathrm{K}}_{\mathrm{F}}\cdot {\mathrm{C}}_{\mathrm{e}}^{1/\mathrm{n}}$$

(3)

where q_e is the amount adsorbed at equilibrium, mg/g, C_e is the concentration at equilibrium, mg/L, K_F is Freundlich constant (mg/g)⋅(L/mg)^1/n, and n is the adsorption intensity [37].

2.5. Adsorption Kinetics

Kinetic studies were carried out at two pH values (3 and 9) using an adsorbent dos-age of 50 mg in 50 mL solution with a contact time ranging from 10 to 180 min. The samples were kept in an incubator at 25 °C under continuous shaking (150 rpm). Kinetic data were evaluated by applying the PFO and PSO kinetic adsorption models. For the kinetic assessment, the adsorbent was exposed to KH₂PO₄ solutions with an initial concentration of 100 mg/L. At specific time intervals ranging from 10 to 180 min, 50 mL of solution was filtered using vacuum filtration through a 0.22 μm pore size filter paper (Merck SRL, Bucharest, Romania, affiliated with Merck KGaA, Darmstadt, Germany). The filtered solutions were analyzed using a UV-VIS spectrophotometer (UV-1900 Shimadzu, Shimadzu Europa GmbH, Duisburg, Germany) to determine the phosphate concentration at a wavelength of 470 nm, following the SR 11411-2:1998 standard method [38]. Each experiment was performed in triplicate.

The amount of phosphates adsorbed at time t (q_t, mg/g) was calculated using the following equation:

$${\mathrm{q}}_{\mathrm{t}}={\displaystyle \frac{\left({\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{t}}\right)\mathrm{V}}{\mathrm{m}}}$$

(4)

where C₀ is the initial concentration (mg/L), C_t the residual concentration at different contact times (mg/L), V (L) is the volume of the solution and m (g) is the adsorbent dose.

The experimental adsorption data were evaluated using two kinetic models, namely the pseudo-first-order (PFO) and pseudo-second-order (PSO) models. PFO adsorption kinetic model assumes that the rate of adsorption is proportional to the number of unoccupied adsorption sites. It is governed by the following equation [39,40]:

$${\displaystyle \frac{\mathrm{d}{\mathrm{q}}_{\mathrm{t}}}{\mathrm{d}\mathrm{t}}}={\mathrm{k}}_1\left({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}}\right)$$

(5)

where k₁ (min⁻¹) is the rate constant, q_e (mg/g) is the equilibrium adsorption capacity, q_t (mg/g) is the amount of phosphates adsorbed at time t (min).

PSO adsorption kinetic model assumes that the adsorption follows chemisorption (involving valence forces through sharing or exchange of electrons). It is described by following equation [39,40]:

$${\displaystyle \frac{\mathrm{d}{\mathrm{q}}_{\mathrm{t}}}{\mathrm{d}\mathrm{t}}}={\mathrm{k}}_2{\left({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}}\right)}^2$$

(6)

where k₂ (g.mg⁻¹min⁻¹) is the rate constant of the pseudo-second-order kinetic model.

2.6. Adsorbent Reusability

The reusability of MgAlFe-LDH was evaluated through five consecutive adsorption–desorption cycles. In each cycle, 50 mg of the adsorbent was contacted with 50 mL of phosphate solution (50 mg/L) at 25 °C and 150 rpm for 24 h. Following adsorption, phosphate desorption was carried out using 1 mol/L NaOH solution. The material was then thoroughly washed with deionized water, dried at 105 °C, ground, and reused in the subsequent cycle [41].

3. Results and Discussion

3.1. Characterization of MgAlFe-LDH Materials

3.1.1. Digital Microscope Analysis

The surface morphology of the samples (pristine LDH and phosphate-loaded LDH) was examined at 400× magnification (Figure 2). The pristine LDH displayed irregular, heterogeneous structures with uneven cavities and rough surfaces. In contrast, the phosphate-loaded LDH exhibited stacked fine particles with brighter surfaces, suggesting the adsorption of phosphate ions from the solution and confirming the material’s potential as an adsorbent.

The textural properties of LDH are strongly influenced by synthesis conditions. Factors such as solution pH, the type of cations, and their initial concentrations play a key role in determining the morphological characteristics of LDH-type clays.

3.1.2. X-Ray Diffraction (XRD) Analysis

XRD analysis was performed to investigate the crystallographic characteristics of the MgAlFe-LDH material before and after phosphate adsorption (Figure 3).

The analysis aimed to identify the crystalline structure based on raw data (2θ vs. intensity) and the peaks obtained from the diffraction pattern. Key extracted parameters included the peak positions (2θ), interplanar spacings (d), intensity, full width at half maximum (FWHM), and crystallite size (

Table 1. Extracted XRD peak parameters for the MgAlFe-LDH sample, ordered by interplanar distance (d).

2θ (deg)	d (Å)	Height (cps)	FWHM (°)	Size(Å)
35.3369	2.53793	50.29	1.8911	46.05
36.8636	2.43624	177.19	0.5715	153.02
42.8816	2.10724	2482.47	0.5535	161.05
62.2742	1.48965	1221.87	0.6042	160.43

).

XRD analysis of the MgAlFe-LDH material revealed sharp and well-defined peaks, characteristic of well-crystallized LDH structures. The main reflections were observed at 2θ = 35.34° (d = 2.538 Å, height = 50.29 cps, FWHM = 1.89°, crystallite size ≈ 46 Å), 36.86° (d = 2.436 Å, height = 177.19 cps, FWHM = 0.57°, ≈153 Å), 42.88° (d = 2.107 Å, height = 2482.47 cps, FWHM = 0.55°, >161 Å), and 62.27° (d = 1.490 Å, height = 1221.87 cps, FWHM = 0.60°, ≈160 Å). These results indicate an ordered arrangement of Mg²⁺, Al³⁺, and Fe³⁺ cations in brucite-like layers with intercalated small anions (NO₃⁻ or CO₃²⁻), confirming the high crystallinity and structural quality of the synthesized LDH material [24,25,42].

3.1.3. Specific Surface Area

The synthesized MgAlFe-LDH exhibited a BET specific surface area of 4.9 m²/g prior to phosphate adsorption. Although relatively modest, this value is typical for LDH-type materials and can be attributed to both the selected cationic ratio and the synthesis conditions, which strongly influence textural properties. As shown in

Table 2. BET surface areas of different LDH materials.

Material	SBET (m2/g)	Vt (cm3/g)	Reference
LDHs/Alginate	2.76	0.001293	[43]
FeMg-LDH	0.62	-	[32]
CaAl0.25Ga0.75-LDH	4	-	[42]
MgAlFe-LDH	4.9	0.0018	Present work

, the measured surface area is in good agreement with values reported in the literature for comparable MgAlFe-LDH materials.

3.1.4. XPS Analysis

To elucidate the interaction mechanisms between the adsorbent and phosphate, XPS analysis was performed on MgAlFe-LDH, with full-scale spectra of Mg, Al, Fe, O, and C shown in Figure 4. Before phosphate adsorption, the XPS spectra confirmed the presence of Mg, Al, Fe, O, and C, consistent with the expected surface composition of the synthesized LDH [39]. After phosphate exposure, distinct P signals appeared, demonstrating effective phosphate uptake. The high-resolution P2p spectrum (Figure 4b) exhibited a peak at 133.28 eV, slightly shifted from the characteristic KH₂PO₄ peak (134 eV), indicating ligand exchange between surface groups and phosphate, leading to the formation of new phosphate-containing complexes. Moreover, the P2p peak at 133.28 eV closely corresponds to that of AlPO₄·2H₂O (133.40 eV), suggesting that precipitation involving Al and phosphate may occur. This is supported by the very low solubility product (Ksp = 3.7 × 10⁻²³ to 1.3 × 10⁻²²) of AlPO₄, which reflects its high affinity for phosphate and propensity to form stable precipitates in solution.

The XPS spectra revealed a phosphorus signal within the binding energy range of 130–135 eV, confirming its retention on the MgAlFe-LDH surface. The P2p peak at ~133 eV is characteristic of inorganic phosphates, suggesting their incorporation through multiple mechanisms, including electrostatic interactions with positively charged LDH layers, ion exchange with interlayer anions, or complexation with surface metal cations (Mg, Al, Fe) [37]. A strong O1s signal (~531 eV) further supports the presence of hydroxyl groups, intercalated water, and oxygen species from phosphates, consistent with phosphate binding to the LDH matrix. The characteristic Mg, Al, and Fe signals confirmed the integrity of the layered structure after adsorption. Quantitative assessment of elemental composition was obtained by integrating the peak areas of Mg, Al, Fe, P, and O, with results summarized in

Table 3. Quantitative analysis of XPS data.

Element	Peak BE	Area (P) CPS·eV	Area (N) KE^0.6	Atomic %	Relative Concentration (%)
Mg1s	1302.96	229,336.8	898.53	22.04	28.35
O1s	531.1	362,606.8	2015.62	49.45	66.83
Ca2p	351.4	74,768.17	216.59	5.31	9.24
Fe2p	711.06	78,045.53	87.74	2.15	9.65
C1s	284.52	31,397.89	445.58	10.93	3.88
Al2p	74.17	12,601.85	302.32	7.42	1.55
Na1s	74.17	14,109.69	44.45	1.09	1.75
P2p	132.94	5881.45	65.21	1.6	0.75

.

3.1.5. FTIR Analysis

The FTIR spectrum of the synthesized MgAlFe-LDH shows distinct absorption bands characteristic of layered double hydroxides and functional groups relevant to phosphate adsorption. A broad band around 3440 cm⁻¹ corresponds to O–H stretching vibrations from interlayer water and hydroxyl groups in the layers (Figure 5a) [44], while the bending mode of H–O–H near 1630 cm⁻¹ further indicates adsorbed water [26,45]. After phosphate adsorption (Figure 5b), a new peak emerges at ~1045 cm⁻¹, assigned to the asymmetric stretching of phosphate groups (PO₄³⁻) [46,47], confirming their incorporation into the LDH structure via anion exchange or surface complexation. Additional weaker bands below 800 cm⁻¹, attributed to M–O and O–M–O vibrations (M = Mg, Al, Fe), verify the integrity of the metal hydroxide framework [48]. Overall, the FTIR data demonstrate that MgAlFe-LDH retains its structural stability while effectively adsorbing phosphate.

3.1.6. The Point of Zero Charge (pHpzc) of the MgAlFe-LDH

LDH materials remove anionic species via electrostatic attraction and/or anion exchange, which are influenced by surface charge and pH. The MgAlFe-LDH exhibited a pHpzc of ~6.0 (Figure 6), indicating a positively charged surface at pH < 6, favoring adsorption of negatively charged phosphate ions.

3.2. Adsorption Isotherms

The adsorption of phosphate onto MgAlFe-LDH was evaluated by fitting the experimental equilibrium data to the Langmuir and Freundlich isotherm models at two different pH values, namely 3 and 9 (Figure 7). The fitting parameters are summarized in

Table 4. Adsorption isotherm parameters for MgAlFe-LDH material.

pH	Langmuir				Freundlich
	KL (L/mg)	qmax (mg/g)	R2	% RMSE	KF, (mg/g)⋅(L/mg)1/n	n	R2	% RMSE
3	0.0535	215.179	0.9750	2.849	13.3574	1.3585	0.9518	9.653
9	0.0333	213.829	0.9719	3.563	8.8425	1.3128	0.9505	12.523

.

According to the Langmuir model, the maximum adsorption capacities (q_max) were 215.18 mg/g at pH 3 and 213.83 mg/g at pH 9. These values are very similar, indicating that the overall adsorption capacity of MgAlFe-LDH is high and largely independent of solution pH. However, the Langmuir affinity constant (K_L) decreased from 0.0535 L/mg at pH 3 to 0.0333 L/mg at pH 9, suggesting stronger interaction between phosphate anions and the LDH surface under acidic conditions. This can be attributed to the higher degree of surface protonation at low pH, which enhances the electrostatic attraction toward negatively charged phosphate species.

The Freundlich model also provided a good fit to the experimental data, with K_F decreasing from 13.36 (mg/g)(L/mg)^1/n at pH 3 to 8.84 at pH 9. The Freundlich exponent n was slightly above unity (1.36 at pH 3 and 1.31 at pH 9), confirming that phosphate adsorption is favorable under both conditions.

Goodness-of-fit statistics indicated that both models describe the equilibrium data adequately (R² > 0.95), although the Langmuir model exhibited slightly higher determination coefficients (R² ≈ 0.97) compared to Freundlich (R² ≈ 0.95). On the other hand, the percentage values of RMSE (Root Mean Square Error) clearly indicate that the Langmuir model fits the experimental results better. This result implies that phosphate adsorption onto MgAlFe-LDH proceeds predominantly through monolayer coverage of homogeneous adsorption sites, in agreement with the assumptions of the Langmuir model.

Table 5. Comparison between the maximum adsorption capacities for phosphates of different types of LDH (nonlinear regression).

Material	Maximum Adsorption Capacity (mg/g)	Reference
La–MgAl–LDH/BC	249.3	[51]
La/Fe-chitosan	87.23	[49]
Alginate/kaolin	22.51	[20]
AC/MgAl-3 LDH	337.2	[52]
CO3–LDHs	184.0	[50]
MgAlFe–LDH	215.18	Present work

summarizes the maximum adsorption capacities of different phosphate adsorbents reported in the literature. The MgAlFe-LDH developed in the present work shows a capacity of 215.18 mg/g, which is significantly higher than that of natural or biopolymer-based materials, such as alginate/kaolin (22.51 mg/g [20]). It also exceeds the performance of several composite LDH-based adsorbents, for example, La/Fe-chitosan (87.23 mg/g [49]). Furthermore, its adsorption capacity is comparable to carbonate-intercalated LDHs (184.0 mg/g [50]) and to La–MgAl–LDH/BC (249.3 mg/g [51]). Although the AC/MgAl-3 LDH composite shows the highest reported capacity (337.2 mg/g [52]), the MgAlFe-LDH synthesized in this study still demonstrates a competitive performance, highlighting its potential as an efficient and promising material for phosphate removal.

3.3. Kinetics

The kinetic data obtained at pH 3 and pH 9 were analyzed using both the pseudo-first-order (PFO) and pseudo-second-order (PSO) models through nonlinear regression. The results (

Table 6. Kinetic parameters for phosphate adsorption on MgAlFe-LDH (nonlinear regression).

	Pseudo-First-Order (PFO)				Pseudo-Second-Order (PSO)
pH	k1 (min−1)	qe (mg/g)	R2	% RMSE	k2 (g/(min · mg))	qe (mg/g)	R2	% RMSE
3	0.010290	101.944	0.9882	2.315	1.2283 × 10−4	110.351	0.9563	7.654
9	0.006765	102.410	0.9914	2.695	1.2685 × 10−4	96.302	0.9417	8.015

and Figure 8) clearly indicate that the PFO model provides a better description of the adsorption process at both pH values, as reflected by the higher determination coefficients (R² > 0.98). At pH 3, the adsorption rate constant (k₁ = 0.0103 min⁻¹) was higher than at pH 9 (k₁ = 0.0068 min⁻¹), suggesting a faster adsorption process under acidic conditions. The equilibrium adsorption capacity (q_e) estimated by the PFO model remained relatively stable (~102 mg/g) across the investigated pH values, further supporting the good fit of this model.

In contrast, the PSO model yielded slightly lower determination coefficients (R² = 0.9563 at pH 3 and R² = 0.9417 at pH 9), indicating a weaker agreement with the experimental data. Moreover, the predicted equilibrium capacities varied more strongly with pH, with higher q_e at pH 3 (110.35 mg/g) and a noticeable decrease at pH 9 (96.30 mg/g). This suggests that the PSO model does not fully capture the adsorption mechanism in this case. On the other hand, the percentage values of RMSE clearly indicate that the PFO model fits the experimental results better. Overall, the results demonstrate that the adsorption kinetics follow the pseudo-first-order model, implying that the process is mainly governed by physisorption and diffusion-controlled mechanisms rather than chemisorption. The influence of pH is evident: under acidic conditions, surface protonation enhances electrostatic attraction between the adsorbent and adsorbate, leading to faster and slightly higher adsorption, whereas at alkaline pH the deprotonation of the surface reduces adsorption efficiency.

3.4. Adsorption Mechanism

The adsorption mechanism of phosphate on MgAlFe-LDH can be linked to the surface charge characteristics of the material. With a pHpzc of ~6.0 (Figure 6), the surface becomes protonated under acidic conditions (pH < 6), which enhances electrostatic attraction toward anionic phosphate species. This electrostatic interaction, together with the material’s layered structure and exchangeable interlayer anions, provides a favorable environment for phosphate uptake.

The kinetic and equilibrium results provide complementary insights into the mechanism of phosphate adsorption onto MgAlFe-LDH. The kinetic data demonstrated that the pseudo-first-order model best described the adsorption process, indicating that surface diffusion and electrostatic interactions dominate the initial stages of uptake. This interpretation is consistent with the affinity trends observed in the Langmuir isotherms, where the adsorption affinity constant (K_L) was higher at acidic pH, reflecting stronger electrostatic attraction between protonated LDH surface sites and negatively charged phosphate species. At the same time, the high and pH-independent maximum adsorption capacities (q_max > 210 mg/g) suggest that electrostatic attraction alone cannot fully explain phosphate retention. The layered structure of MgAlFe-LDH is well known to facilitate anion exchange, and phosphate, with its high charge density, can readily replace interlayer anions such as carbonate (CO₃²⁻), nitrate (NO₃⁻), and hydroxide (OH⁻). This ion-exchange mechanism accounts for the high equilibrium capacities predicted by the Langmuir model and explains why phosphate uptake remains substantial even at alkaline pH, despite reduced surface protonation and increased competition with hydroxide ions.

Therefore, phosphate adsorption onto MgAlFe-LDH can be described by a dual mechanism: rapid, diffusion-controlled electrostatic adsorption onto protonated surface sites, followed by slower but more stable ion-exchange processes in the interlayer region. The presence of Mg²⁺, Al³⁺, and Fe³⁺ centers in the LDH layers further enhances adsorption by creating sites capable of both electrostatic stabilization and specific interactions with phosphate. This combined mechanism reconciles the kinetic data, which emphasize fast surface processes, with the isotherm data, which highlight high adsorption capacity and the contribution of ion exchange to long-term phosphate retention. A sketch of the adsorption mechanism is shown in Figure 9.

Although partial dissolution of Mg²⁺ from the LDH structure under acidic conditions cannot be ruled out, the formation of magnesium phosphate precipitates was not confirmed in this study. Verification would require solid-phase evidence, such as the detection of new crystalline phases by SEM–EDX elemental mapping. Thus, while interactions between dissolved Mg²⁺ and phosphate anions remain possible, they cannot be considered a demonstrated mechanism in the current system.

Evidence of ion exchange was observed in comparative FTIR spectra before and after phosphate adsorption, where carbonate and nitrate bands were partially replaced by phosphate-specific bands (~1045 cm⁻¹), indicating displacement of interlayer anions by phosphate. This supports the notion that anion exchange acts as a secondary mechanism contributing to the overall removal process, in agreement with previous studies.

Collectively, these observations suggest a dual-mechanism adsorption process involving electrostatic attraction and ion exchange, consistent with earlier research on LDH materials for phosphate removal [47].

3.5. Reusability of MgAlFe-LDH

The evaluation of adsorbent performance and cost-effectiveness largely depends on cyclic regeneration, which was investigated for MgAlFe-LDH in this study. As shown in Figure 10, the material exhibited a high phosphate removal efficiency of ~90% in the first cycle and maintained over 75% even after five consecutive cycles. The gradual decline in adsorption capacity may result from the permanent occupation of active sites over successive cycles. Phosphate desorption efficiency remained consistently above 70% throughout all five cycles, demonstrating the material’s potential for phosphate recovery upon repeated use. These findings are consistent with previous reports on other LDH materials, which show a removal efficiency of ~70% after three adsorption–desorption cycles [34].

4. Conclusions

This study demonstrates that a synthesized MgAlFe-LDH material with a 2:1:1 cationic ratio possesses a high phosphate adsorption capacity of approximately 215 mg/g, which is largely independent of solution pH, indicating its robust performance under diverse environmental conditions. Phosphate removal occurs through a dual mechanism—fast electrostatic attraction between protonated surface sites and negatively charged phosphate ions—followed by slower but stable anion exchange within the interlayer, where phosphate replaces interlayer anions such as carbonate and nitrate. Kinetic analysis revealed that the adsorption process follows a pseudo-first-order model, suggesting that surface diffusion and physical adsorption dominate, while chemisorption plays a minor role. Structural and chemical characterizations using XRD, FTIR, and XPS confirmed the integrity of the LDH framework after adsorption and provided clear evidence of phosphate incorporation through complexation and interlayer exchange. The material demonstrated excellent reusability, maintaining over 75% of its adsorption capacity after five consecutive adsorption–desorption cycles, with consistent phosphate desorption efficiency above 70%, highlighting its potential for sustainable phosphate recovery. These findings establish MgAlFe-LDH as an effective, stable, and reusable adsorbent for wastewater treatment, offering a promising approach to mitigate eutrophication and support circular economy strategies in nutrient management.