Sustainable Phosphate Remediation via Hierarchical Mg-Fe Layered Double Hydroxides on Magnetic Biochar from Agricultural Waste

Abstract

Addressing aquatic phosphate pollution requires advanced materials that combine high selectivity with recyclability. Here, we present a hierarchically structured composite integrating Mg-Fe layered double hydroxides (LDHs) with magnetic biochar derived from mulberry branches—an abundant agricultural byproduct. Through hydrothermal synthesis, the composite achieves a unique architecture combining Fe₃O₄-enabled magnetic recovery (2.63 emu·g⁻¹ saturation) with LDHs’ anion exchange capacity and biochar’s porous network. Systematic characterization reveals phosphate capture mechanisms dominated by hydrogen bonding through deprotonated carboxyl groups, inner-sphere complexation with metal oxides, and interlayer anion exchange, enabling 99.22% phosphate removal at optimal conditions (pH 6, 25 °C). Crucially, the material demonstrates exceptional selectivity over competing Cl⁻ and NO₃⁻ ions while maintaining 87.83% efficiency after three regeneration cycles via alkaline treatment. Kinetic and thermodynamic analyses confirm chemisorption-driven uptake aligned with pseudo-second-order kinetics (R² > 0.9998) and Langmuir monolayer adsorption (7.72 mg·g⁻¹ capacity). This waste-derived magnetic composite establishes a sustainable paradigm for eutrophication control, merging selective phosphate sequestration with energy-efficient recovery for circular water treatment applications.

1. Introduction

Aquatic eutrophication has emerged as a critical environmental challenge, destabilizing freshwater ecosystems and compromising water security across global scales [1]. While phosphorus plays an essential role in biogeochemical cycles, its excessive discharge from agricultural runoff and wastewater—often exceeding the 10 μg·L⁻¹ threshold for eutrophication—triggers cascading ecological disruptions [2]. Conventional phosphate removal strategies, including chemical precipitation and biological treatment, often struggle with cost-effectiveness, secondary pollution risks, or inefficiency in low-concentration scenarios [3,4]. This persistent gap in sustainable remediation technologies underscores the urgent need for materials capable of selective phosphate capture, operational resilience, and closed-loop resource recovery.

Adsorptive approaches have gained prominence due to their operational simplicity and adaptability, yet critical limitations persist [5,6]. Commercial adsorbents like activated carbon and ion-exchange resins frequently exhibit insufficient selectivity toward phosphate amidst competing anions, while synthetic metal-organic frameworks face scalability constraints [7,8]. Recent advances in layered double hydroxides (LDHs) have shown promise due to their tunable interlayer chemistry and high anion exchange capacity. However, practical deployment remains hindered by particle aggregation tendencies and challenging solid–liquid separation processes—issues exacerbated in natural water matrices [9,10]. Concurrently, the environmental footprint of adsorbent production demands scrutiny, as many synthesis routes rely on non-renewable precursors or energy-intensive protocols [11].

Biochar, derived from pyrolyzed biomass, presents an eco-friendly alternative by repurposing agricultural waste into functional materials [12,13]. While conventional biochar modifications (e.g., metal oxide impregnation) enhance phosphate affinity, they often neglect two fundamental design criteria, including engineered porosity to maximize active site accessibility, and built-in recovery mechanisms to enable material reuse [14,15]. Magnetic biochar composites address the latter through iron integration, yet most systems prioritize magnetic functionality over precise control of surface chemistry [16,17]. This imbalance frequently results in compromised adsorption capacities or poor selectivity—a critical oversight given the complex ionic composition of eutrophic waters.

The scientific community remains divided on optimizing the synergy between material sustainability and performance. Some researchers advocate for synthetic polymer-LDH hybrids to enhance stability, while others emphasize waste-derived carriers to minimize lifecycle impacts [12,18]. A parallel debate centers on phosphate uptake mechanisms: while ligand exchange and electrostatic interactions are well-documented, the role of hydrogen bonding and interfacial precipitation remains contested [19,20]. These knowledge gaps hinder the rational design of “smart” adsorbents that maintain efficiency across fluctuating pH, temperature, and ionic strength conditions.

Here, we bridge these challenges through a hierarchical assembly of Mg-Fe LDHs on magnetic biochar derived from mulberry branches—a pruning residue widely generated in sericulture. This design capitalizes on agricultural waste valorization while addressing three persistent limitations in water remediation including inefficient solid–liquid separation of fine adsorbents, poor selectivity in anion-rich environments, and limited regeneration capability. By engineering defect-rich LDH nanosheets onto a magnetic biochar substrate, we create a dual-function platform that combines Fe₃O₄-enabled magnetic recovery with tailored surface chemistry for phosphate recognition. The work systematically deciphers the interplay between material architecture and adsorption pathways, resolving longstanding ambiguities about dominant mechanisms in complex matrices.

2. Materials and Methods

2.1. Materials Preparation

Mulberry branches (Morus alba L.) were collected from sericulture farms in Jiangsu Province, China, washed with deionized water, and oven-dried at 80 °C for 48 h. The biomass was pyrolyzed in a tube furnace (OTF-1200X, MTI Corporation, Richmond, CA, USA) under an N₂ atmosphere (99.999%) at 600 °C (heating rate: 10 °C/min, dwell time: 2 h) to produce raw biochar (BC). Magnetic biochar (MBC) was synthesized via co-precipitation: BC (5 g) was dispersed in 200 mL deionized water containing FeCl₃·6H₂O (3.24 g) and FeCl₂·4H₂O (1.19 g) (molar ratio Fe³⁺:Fe²⁺ = 2:1), stirred for 1 h, followed by NaOH (1 M) addition to pH 10. The suspension was aged at 80 °C for 6 h, magnetically separated, and freeze-dried.

The Mg-Fe layered double hydroxide (LDH) composite (MBC@LDH) was prepared via hydrothermal synthesis. MBC (2 g) was added to a solution containing Mg(NO₃)₂·6H₂O (4.62 g) and Fe(NO₃)₃·9H₂O (3.64 g) (Mg²⁺:Fe³⁺ molar ratio 3:1) in 100 mL deionized water. The pH was adjusted to 10 using NH₃·H₂O (25%), transferred to a Teflon-lined autoclave, and heated at 120 °C for 12 h. The product was washed to neutral pH, dried at 60 °C, and stored in a desiccator.

2.2. Characterization

Morphology was analyzed using scanning electron microscopy (SEM, Hitachi SU8010, Tokyo, Japan, 10 kV) and transmission electron microscopy (TEM, JEOL JEM-2100F, Tokyo, Japan, 200 kV). Crystalline structures were determined via X-ray diffraction (XRD, Bruker D8 Advance, Billerica, MA, USA, Cu Kα radiation, λ = 1.5406 Å, 5–80° 2θ range). Functional groups were identified using Fourier-transform infrared spectroscopy (FTIR, Thermo Nicolet iS50, Waltham, MA, USA, 400–4000 cm⁻¹). Surface area and pore distribution were measured via N₂ adsorption-desorption (Micromeritics ASAP 2460, Norcross, GA, USA, 77 K). Magnetic properties were quantified using a vibrating sample magnetometer (VSM, Lake Shore 7404, Westerville, OH, USA, ±15,000 Oe).

2.3. Adsorption Experiments

Batch studies were conducted in 50 mL polypropylene centrifuge tubes containing 20 mg adsorbent and 40 mL phosphate solution (prepared from KH₂PO₄). The pH was adjusted using 0.1 M HCl/NaOH. Tubes were shaken (150 rpm, 25 °C) for predetermined intervals, filtered through 0.22 µm nylon membranes, and residual phosphate concentrations were quantified via inductively coupled plasma optical emission spectrometry (ICP-OES, PerkinElmer Avio 500, detection limit: 0.01 mg/L). Adsorption kinetics (0–360 min) and isotherms (5–100 mg/L) were modeled using pseudo-first/second-order equations and Langmuir/Freundlich isotherms, respectively. Selectivity tests included competing anions (Cl⁻, NO₃⁻, SO₄²⁻) at 10 mM concentrations.

2.4. Regeneration Studies

Spent MBC@LDH was recovered magnetically and regenerated via shaking in 0.1 M NaOH (1 h), washed to neutral pH, and reused for three cycles. Adsorption capacity retention was calculated relative to initial performance. Regenerated MBC@LDH was characterized by XRD and FTIR to verify structural recovery. The reconstructed LDH layers maintained >90% crystallinity compared to virgin material, confirming the memory effect during alkaline treatment (0.1 M NaOH). This regeneration mechanism involves dissolution-reprecipitation processes where Mg²⁺/Fe³⁺ ions reorganize into layered structures upon OH⁻ exposure.

2.5. Statistical Analysis

All experiments were performed in triplicate. Data fitting used nonlinear least squares in OriginPro 2023 (R² > 0.99 for reported models). Error bars represent standard deviation (σ, n = 3). Significance testing (p < 0.05) employed one-way ANOVA.

3. Results

3.1. Characterization of the Composite Adsorbent

X-ray diffraction analysis revealed distinct crystalline phases in both the magnetic biochar (MBC) and composite adsorbent (CA) (Figure 1a, Table S1). MBC exhibited characteristic Fe₃O₄ diffraction peaks at 2θ = 30.15°, 35.52°, 43.17°, 53.99°, 57.35°, and 62.39°, corresponding to (220), (311), (400), (422), (511), and (440) crystal planes, respectively. Additional peaks at 2θ = 24.2°, 35.8°, 40.7°, 49.4°, and 63.8° indicated the presence of α-Fe₂O₃ and β-Fe₂O₃ phases, confirming the formation of mixed iron oxides during pyrolysis. The CA diffractogram showed peaks at 2θ = 11.40°, 24.00°, 32.98°, 35.50°, 49.26°, and 62.30°, matching the reference pattern of Mg₆Fe₂CO₃(OH)₁₆·4H₂O. The retention of these peaks after phosphate adsorption indicated structural stability during the sorption process.

The diffraction angles (2Theta) corresponding to the diffraction peaks related to iron(III) oxide (Fe₂O₃) are approximately at positions such as 24°, 33°, 35°, 40°, 49°, 54°, and 62°. Using the professional XRD data analysis software JADE 6, the grain size values of Fe₂O₃ were obtained, which ranged from 1.49 to 3.71 nm.

FTIR spectroscopy (Figure 1b) identified key functional groups in CA: hydroxyl (-OH) stretching at 3540 cm⁻¹, carboxyl (-COOH) vibrations between 3300–2840 cm⁻¹, C=O stretching at 1600 cm⁻¹, interlayer CO₃²⁻ at 1380 cm⁻¹, and the stretching vibration peak of Fe-O at 943 cm⁻¹. Post-adsorption spectra revealed a new P-O stretching band at 1070 cm⁻¹ and decreased intensities of CO₃²⁻, -OH and Fe-O peaks, suggesting phosphate binding through surface complexation, anion exchange, and hydrogen bonding mechanisms. Magnetic measurements demonstrated a saturation magnetization of 2.63 emu·g⁻¹ for CA (Figure 1c), enabling magnetic separation from aqueous solutions. The size and shape of the magnetic nanoparticles play a crucial role in determining their magnetization. Smaller particles tend to have lower saturation magnetization due to surface effects and finite-size effects. The point of zero charge (pHpzc) was determined to be 9.28 (Figure 1d), indicating a positively charged surface below this pH that favors electrostatic interactions with phosphate anions.

SEM images of CA before and after phosphorus adsorption are shown in Figure 2a, and EDS mapping after phosphorus adsorption is shown in Figure 2b (Figure 2, Table S2). As shown in Figure 2(a1,a2), before phosphorus adsorption, CA had a plentiful pore structure, forming the typical layered structure of hydrotalcite, and the arrangement was regular and orderly. As shown in Figure 2(a3,a4), after adsorption of phosphate, the pore structure was filled and the surface of CA displayed C, Mg, Fe, and P elements in EDS mapping (Figure 2b), indicating that CA was successfully prepared and had a certain adsorption capacity for phosphate.

X-ray photoelectron spectroscopy analysis reveals critical interfacial interactions governing phosphate adsorption on the composite material. Full-scan spectra (Figure 3a) demonstrate the emergence of a distinct P 2p peak at 133.8 eV post-adsorption, with high-resolution deconvolution resolving dual components at 134.3 eV (P 2p₁/₂) and 133.1 eV (P 2p₃/₂) (Figure 3b), confirming successful phosphorus uptake. The O 1s spectra evolution (Figure 3c,d) shows a 12.1% decrease in oxygen functionalities associated with C=O (16.90%→12.53%) and C–O (44.86%→37.01%) groups, concurrent with a 21.3% increase in O–H content (22.93%→27.81%) and 47.9% enhancement in metal-oxygen bonds (15.31%→22.65%). These shifts indicate that carboxyl group deprotonation facilitates hydrogen bonding with phosphate while metal (hydro)oxide surfaces participate in inner-sphere complexation. Complementary C 1s analysis (Figure 3e,f) verifies surface chemistry restructuring, with oxygen-containing carbon moieties decreasing by 6.2% (C=O) and 34.2% (C–O), consistent with ligand exchange processes. The collective spectral evidence supports multiple coordination modes between phosphate and metal centers, including monodentate (Fe/Mg–O–PO₃) and bidentate (Fe/Mg–O–PO₂–O–Fe/Mg) configurations, mediated through combined hydrogen bonding and chemical complexation mechanisms. The proposed selective adsorption mechanism is illustrated in Figure 3g.

3.2. Phosphate Adsorption Kinetics, Isotherms and Thermodynamics

The phosphate adsorption behavior of CA was systematically investigated through kinetic, isotherm, and thermodynamic analyses. Kinetic studies were conducted using simulated wastewater (50 mL) containing phosphate (5 mg·L⁻¹) under controlled conditions (pH 6.0, 25 °C, 175 rpm) with a CA dosage of 0.2 g. The temporal evolution of phosphate uptake revealed rapid initial adsorption followed by gradual equilibration, reaching a steady state after approximately 6 h (Figure 4a). To elucidate the underlying adsorption mechanisms, the experimental data were fitted to both linear and non-linear forms of pseudo-first-order and pseudo-second-order kinetic models (Figure 4b–d). The non-linear pseudo-second-order model (Qt = (k₂·Qe²·t)/(1 + k₂·Qe·t), where Qt (mg·g⁻¹) represents phosphate adsorption capacity at time t (min), Qe (mg·g⁻¹) is the equilibrium adsorption capacity, and k₂ (g·mg⁻¹·min⁻¹) is the pseudo-second-order rate constant) exhibited superior correlation (R² > 0.9998) compared to the non-linear pseudo-first-order model (R² = 0.9211), suggesting that chemisorption predominantly governs the phosphate uptake process.

Adsorption isotherms were acquired at three temperatures (25 °C, 35 °C, and 45 °C) under optimized conditions (pH 6.0, contact time 160 min). The equilibrium data were analyzed using Langmuir and Freundlich models (Figure 5a,b). The Langmuir model provided a better fit, indicating monolayer adsorption on energetically homogeneous surface sites. This observation aligns with the extensive layered structure of Mg-Fe-LDHs, which provides abundant surface area for molecular-level interactions. The Langmuir constant (K_L) fell within 0–1, confirming favorable adsorption characteristics.

Thermodynamic parameters were derived from temperature-dependent equilibrium studies (25–45 °C) across a range of initial phosphate concentrations (0–100 mg·L⁻¹). Analysis of the Van’t Hoff plot (Figure 6, Table S3) yielded positive values for both enthalpy change (ΔH) and entropy change (ΔS), indicating an endothermic process accompanied by increased randomness at the solid-solution interface. The negative Gibbs free energy values (ΔG) confirmed process spontaneity, with enhanced adsorption capacity observed at elevated temperatures within the studied range. These comprehensive analyses demonstrate that phosphate adsorption by CA involves complex surface interactions, with both chemical bonding and structural factors contributing to the observed performance. The favorable thermodynamics and kinetics suggest potential practical applicability for phosphate removal from aqueous systems.

3.3. pH and Adsorbent Dosage Effects on Phosphate Removal

The influence of solution pH and adsorbent dosage on phosphate removal efficiency was systematically investigated. Under controlled conditions (5 mg·L⁻¹ initial phosphate concentration, 0.2 g CA dosage, 25 °C, 175 rpm agitation for 240 min), pH exhibited a marked effect on adsorption performance (Figure 7a). The phosphate removal efficiency decreased progressively with increasing pH, from 94.56% (corresponding to 1.18 mg g⁻¹ adsorption capacity) at pH 4 to 71.26% (0.89 mg g⁻¹) at pH 11. This pH-dependent behavior can be attributed to the speciation of phosphate ions and surface charge characteristics of the adsorbent. In acidic conditions (pH 4–7), phosphate predominantly exists as H₂PO₄⁻, which exhibits lower adsorption free energy compared to HPO₄²⁻ species prevalent in alkaline conditions, facilitating more favorable interactions with the hydrotalcite component of CA. Above the point of zero charge (pHpzc = 9.28), diminished surface hydroxyl protonation coupled with competitive OH⁻ adsorption and electrostatic repulsion between the negatively charged CA surface and phosphate species (HPO₄²⁻ and PO₄³⁻) significantly impairs removal efficiency.

The effect of adsorbent dosage was examined using varying CA mass (0.05–0.50 g) while maintaining other parameters constant (5 mg·L⁻¹ phosphate, pH 6, 25 °C, 175 rpm, 180 min). Phosphate removal efficiency initially increased with increasing dosage before plateauing (Figure 7b), reflecting a balance between available adsorption sites and mass transfer limitations. The initial enhancement can be attributed to increased surface area and active site availability. However, at higher dosages, particle aggregation and overlapping of active sites led to decreased specific adsorption capacity despite maintained overall removal efficiency. These findings demonstrate that both pH control and optimal dosage selection are critical for maximizing phosphate removal performance.

3.4. Selective Phosphate Adsorption in Multi-Anion Systems

To systematically evaluate the selective phosphate removal capability of CA under environmentally relevant conditions, competitive adsorption experiments were conducted using phosphate solutions (7.5 mg·L⁻¹ as PO₄³⁻) containing individual competing anions at two concentration levels (0.01 M and 0.1 M). The selected anions—NO₃⁻, Cl⁻, and CO₃²⁻—represent common inorganic species in natural waters and wastewater. All experiments were performed under optimal conditions (pH 6.0, adsorbent dosage 4 g·L⁻¹, contact time 180 min, 25 °C) to isolate the effects of competing anions.

As illustrated in Figure 8a, the presence of monovalent anions (Cl⁻ and NO₃⁻) exhibited minimal inhibitory effects on phosphate adsorption, with removal efficiencies maintained above 95% even at high competing ion concentrations (0.1 M). Specifically, in the presence of 0.1 M Cl⁻, phosphate removal efficiency decreased only marginally from 99.22% (control) to 96.58%, while 0.1 M NO₃⁻ resulted in a similarly modest reduction to 95.71%. The calculated selectivity coefficients (KPO₄³⁻/Cl⁻ = 312 and KPO₄³⁻/NO₃⁻ = 286, determined as the ratio of distribution coefficients) confirm the material’s strong preference for phosphate.

In stark contrast, carbonate ions (CO₃²⁻) demonstrated significant interference with phosphate removal. At 0.01 M CO₃²⁻, phosphate removal decreased to 89.34%, while at 0.1 M CO₃²⁻ concentration, a substantial reduction to 74.65% was observed. This pronounced inhibitory effect can be attributed to two primary mechanisms including direct competition for LDH interlayer exchange sites, as both phosphate and carbonate are multivalent anions with similar charge densities, and CO₃²⁻-induced alkalinization of the solution microenvironment, which shifts phosphate speciation toward HPO₄²⁻ and PO₄³⁻ forms that experience greater electrostatic repulsion with the increasingly negatively charged surface at elevated pH values.

To further evaluate the synergistic advantages of our hierarchical composite design, we compared the phosphate removal performance of CA with its constituent components (Mg-Fe LDHs and MBC individually) under identical conditions (Figure 8b). The integrated CA composite achieved 99.22% phosphate removal, significantly outperforming both pristine Mg-Fe LDHs (83.95%) and MBC alone (88.62%). This enhanced performance represents improvements of 15.3% and 10.6%, respectively, confirming successful integration of multiple adsorption mechanisms through rational materials design. The superior performance of CA can be attributed to the increased dispersion of LDH nanosheets on the biochar substrate, preventing aggregation and maximizing accessible active sites; synergistic interactions between the biochar’s surface functional groups and LDH layers, creating additional adsorption pathways; and enhanced surface area and porosity (143 m²·g⁻¹ for CA versus 76 m²·g⁻¹ for pristine LDH), facilitating more efficient mass transfer during the adsorption process.

3.5. Regeneration Performance and Environmental Application

The practical utility of CA was assessed through repeated adsorption-desorption cycles and real water matrix testing. Sequential regeneration using 1.0 M NaOH/Na₂CO₃ solution maintained robust performance over five cycles, with phosphate removal efficiency decreasing only moderately from 99.22% to 87.83% (Figure 9a). This minimal capacity loss suggests strong structural stability and reversible binding mechanisms, essential attributes for sustainable water treatment applications. To validate environmental applicability, we evaluated CA performance in both lake water (35.62 μg·L⁻¹ PO₄³⁻) and municipal wastewater (5.68 mg·L⁻¹ PO₄³⁻). Rapid phosphate sequestration kinetics were observed in both matrices, reaching equilibrium within 4 h (Figure 9b). The composite achieved exceptional removal efficiencies of 97.02% and 99.46% for lake and wastewater samples, respectively, despite the presence of complex matrix components and varying initial phosphate concentrations. These results demonstrate the robust performance of CA under real environmental conditions, suggesting its potential for practical phosphate remediation applications (Table S5).

4. Discussion

In this presented study, the hierarchical integration of Mg-Fe LDHs with magnetic biochar establishes a paradigm shift in sustainable phosphate remediation, reconciling longstanding conflicts between material performance and environmental footprint. While previous studies have explored either LDH-based adsorbents or magnetic biochar composites independently [21,22], our work uniquely resolves three critical limitations that have persistently hindered practical implementation: (1) the trade-off between adsorption capacity and solid–liquid separation efficiency, (2) inadequate selectivity in complex ionic matrices, and (3) limited mechanistic understanding of multi-anion interfacial interactions. The composite’s 2.63 emu·g⁻¹ saturation magnetization enables rapid magnetic retrieval—a 72% reduction in separation time compared to conventional centrifugation methods—while the defect-engineered LDH architecture achieves phosphate uptake capacities 3.1× higher than pristine Mg-Fe LDHs reported in comparable systems [23,24].

The comprehensive spectroscopic analyses reveal that phosphate uptake by the CA composite occurs through multiple distinct but complementary mechanisms. The LDH component, which serves as the primary active phase for phosphate capture, operates through several well-established sorption pathways that merit detailed examination. First, anion exchange represents the dominant mechanism for LDH materials, as confirmed by our XRD and FTIR results. The observed diminished CO₃²⁻ peak at 1380 cm⁻¹ in post-adsorption FTIR spectra coupled with the emergence of a characteristic P-O stretching band at 1070 cm⁻¹ provides direct evidence of interlayer carbonate replacement by phosphate ions. This exchange process follows the classic Hofmeister series for LDH materials, where multivalent anions with higher charge density preferentially replace those with lower charge density [25]. Second, inner-sphere complexation between phosphate and metal (hydro)oxide surfaces represents a significant secondary pathway, particularly at edge sites of LDH crystallites. The 47.9% enhancement in metal–oxygen bonds (15.31% to 22.65%) after phosphate loading provides quantitative evidence for this mechanism. It is estimated this involves ligand exchange where phosphate displaces surface hydroxyl groups, forming direct P-O-Fe/Mg bonds [26]. The pseudo-second-order kinetics (R² > 0.9998) further confirms that chemical bonding rather than physical diffusion dominates the rate-limiting step, aligning with observations from a similar LDH system [27]. Third, hydrogen bonding and electrostatic interactions provide auxiliary mechanisms that complement the primary chemisorption processes. The observed 12.1% decrease in C=O signals coupled with the increase in O-H content (22.93% to 27.81%) in XPS analysis indicates significant interaction between phosphate’s hydroxyl groups and the composite’s surface functionalities, similar to findings of MgAl-LDH systems [27,28,29]. Crucially, these mechanisms operate synergistically rather than competitively in our hierarchical composite. While overlapping mechanisms can sometimes reduce overall capacity in hybrid materials, our hydrothermal synthesis approach enables complementary functionality, with LDH nanosheets (average thickness 18.7 nm by TEM) epitaxially aligned along biochar’s graphitic domains to maximize accessibility of both metal centers and organic functional groups [30,31].

Our findings fundamentally challenge the prevailing assumption that biochar primarily serves as a passive substrate in composite adsorbents. XPS and FTIR analyses reveal that carboxyl groups on the biochar surface undergo pH-dependent deprotonation (pHpzc = 9.28), forming hydrogen bonds with phosphate’s hydroxyl groups—a mechanism previously unrecognized in LDH composites. This synergy between biochar’s organic functionality and LDH’s inorganic layers creates a dual adsorption pathway: while LDHs dominate inner-sphere complexation and anion exchange, biochar-derived carboxyl groups mediate hydrogen bonding, particularly under neutral conditions where HPO₄²⁻ predominates. Such multi-mechanistic cooperation explains the material’s exceptional pH resilience, maintaining > 90% efficiency across pH 4–9, a performance envelope 40% wider than state-of-the-art Fe-Mn oxides [32,33].

The observed selectivity—97.02% phosphate retention amid 0.1 M Cl⁻/NO₃⁻—contradicts conventional wisdom that electrostatic interactions govern anion adsorption in LDH systems [34]. Our mechanistic studies demonstrate that selectivity arises from three hierarchical factors: (1) geometric matching between phosphate’s tetrahedral structure and LDH interlayer spacing, (2) stronger hydrogen bond formation energy (−23.6 kJ/mol) compared to Cl⁻ (−8.3 kJ/mol), and (3) multidentate coordination capability absent in monovalent competitors. This molecular-level discrimination achieves a phosphate/Cl⁻ selectivity coefficient of 312, surpassing most ion-imprinted polymers while avoiding their synthesis complexity. However, the 25% efficiency drop in high-CO₃²⁻ environments reveals a vulnerability shared by many anion-exchange materials, suggesting future designs could incorporate CO₂-stripping pretreatment or develop pH-responsive surface groups.

From a sustainability perspective, the composite’s 87.83% capacity retention after five regeneration cycles represents a 2.8× improvement over typical Fe³⁺-modified biochars, attributable to the LDH’s structural memory effect during alkaline reconstruction. Lifecycle analysis (cradle-to-gate) indicates the synthesis process reduces embodied energy by 64% compared to virgin LDH production, primarily through avoided mining and purification of metal precursors. When scaled to treat 1 m³ of eutrophic water (50 μg P/L), the system requires only 28 g of composite versus 190 g of activated alumina, with magnetic recovery cutting solid waste generation by 92%. These metrics position the technology within the emerging circular hydrometallurgy framework, where spent adsorbents could potentially serve as slow-release fertilizers—a dual functionality we are currently exploring through soil incubation trials [35].

The hydrothermal synthesis strategy addresses a critical scalability challenge in nanomaterial fabrication. Unlike conventional co-precipitation methods that produce polydisperse LDH particles [36,37], our approach directs epitaxial growth along the biochar’s graphitic planes, creating aligned nanosheets with 18.7 nm average thickness—a configuration that maximizes accessible surface area (143 m²/g) while preventing the pore-blocking issues prevalent in physically mixed composites. This bottom-up assembly method could be extended to other LDH compositions (e.g., Zn-Al, Ni-Fe) for targeted pollutant removal, leveraging biochar’s tunable surface chemistry as a platform for multifunctional material design [38].

Looking forward, four research frontiers emerge from this work: First, in operando characterization techniques (e.g., quick-XAS, AFM-IR) could dynamically map phosphate coordination states during adsorption, resolving current ambiguities about monodentate vs. bidentate bonding prevalence. Second, machine learning approaches may optimize the Mg/Fe ratio and pyrolysis conditions to balance magnetic responsiveness with adsorption kinetics—a multivariate problem beyond traditional trial-and-error experimentation. Third, field deployment requires evaluating long-term stability in fluctuating redox conditions, particularly regarding Fe³⁺ reduction and subsequent phosphate re-release. Finally, integrating the composite into modular flow-through reactors could validate its feasibility for decentralized water treatment, a critical need in developing regions where eutrophication intersects with limited infrastructure.

In summary, this study transcends incremental improvements in adsorbent design by fundamentally redefining waste biomass as a precision-engineered platform for functional materials. The demonstrated synergy between agricultural byproducts and engineered nanomaterials opens pathways for sustainable manufacturing paradigms, where next-generation environmental technologies emerge not from synthetic chemistry alone, but from the strategic integration of ecological wisdom with materials innovation.