Phosphate Removal by Surface-Modified Ceramsite Derived from the Synergistic Use of Multiple Solid Wastes

Abstract

To address the dual challenges of aqueous phosphate pollution and the resource utilization of petrochemical solid wastes, this study proposes a novel closed-loop “waste-to-waste” strategy. This approach innovatively integrates multiple solid wastes (including oily sludge and petroleum hydrocarbon-contaminated soil) into a porous ceramic matrix and utilizes lanthanum recovered from spent catalysts for surface modification, successfully fabricating an optimized adsorbent—lanthanum-modified ceramsite (BC@La). Under the conditions of pH 6, an adsorbent dosage of 1 g/L, and a temperature of 318 K, BC@La achieved a maximum phosphate adsorption capacity of 2.56 mg/g, corresponding to 128.0 mg of phosphorus per gram of La. Kinetic and isotherm analyses revealed that the adsorption process followed the pseudo-second-order model and fitted well with the Langmuir isotherm, consistent with monolayer chemisorption. Thermodynamic studies further indicated that the adsorption was spontaneous and endothermic. The primary adsorption mechanism was attributed to the precipitation of lanthanum phosphate (LaPO₄). This study not only demonstrates a high-performance adsorbent but also provides a sustainable strategy for the synergistic utilization of industrial solid wastes.

1. Introduction

The dual crisis of aqueous phosphorus pollution and the accumulation of industrial solid wastes demand urgent resolution [1,2,3]. Phosphorus, a key nutrient from agricultural and industrial discharges, induces water eutrophication, leading to harmful algal blooms and ecosystem degradation [4,5]. Concurrently, the petrochemical industry generates challenging wastes such as oily sludge (OS) and petroleum hydrocarbon-contaminated soil (PCS), which require sustainable disposal strategies [6,7,8,9,10]. Research efforts have been directed towards treating these wastes through thermal, biological, and mechanical methods [11,12].

Within the framework of green and sustainable remediation, the valorization of solid wastes into functional materials has emerged as a compelling strategy for pollution control and resource recovery [13,14]. Exemplary cases include the transformation of drinking water treatment sludge [15], red mud [16,17], and steel slag [18] into adsorbents or filter media. Among phosphate removal techniques, adsorption is favored for its simplicity, efficiency, and cost-effectiveness. A variety of adsorbents have been explored, including metal (hydr)oxides, functionalized biochars, and industrial by-products [19,20,21]. However, many suffer from a practical limitation: their fine powder form complicates post-use separation and carries risks of secondary release [5]. In contrast, granular porous ceramsite, which can be sintered from various wastes [22,23], offers a favorable alternative due to its ease of handling, favorable hydraulic conductivity, and mechanical stability, making it well-suited for engineered treatment systems [16].

Parallel research into adsorbent composition highlights lanthanum (La)-based materials for their exceptional affinity and selectivity towards phosphate ions [9,22]. Recent studies on La-based composites further confirm their high performance [20]. The superiority of La³⁺ over cations like Fe³⁺, Al³⁺, and Zr⁴⁺ stems from its ability to form lanthanum phosphate (LaPO₄), one of the least soluble phosphate compounds (with a solubility product constant, Ksp, of ~10⁻²⁵), ensuring strong and effectively irreversible phosphate fixation [10,13]. Despite this advantage, the economic viability of La-based adsorbents is constrained by the high cost of commercial lanthanum precursors [14]. Notably, a potential solution to this cost barrier lies within the industry’s own waste streams: spent fluid catalytic cracking (SFCC) catalysts contain recoverable lanthanum [24]. The concept of converting spent catalysts into functional adsorbents has been demonstrated for other metals [24,25], underscoring the technical feasibility of this approach. However, current research remains compartmentalized—focusing separately on La-modified adsorbents [20], rare earth element recovery [24], or waste-derived ceramsites [16,22]—with little synergy among these valuable yet distinct research avenues.

To bridge these identified gaps, this study proposes a novel closed-loop “waste-treats-waste” strategy. This approach uniquely integrates (1) the co-valorization of multiple challenging petrochemical wastes (OS, PCS, and waste glass) into a robust, granular ceramic matrix (hereafter referred to as base ceramsite, BC), and (2) the recovery and utilization of lanthanum from spent SFCC catalysts to functionalize this matrix. The work aims to synthesize a high-performance, low-cost composite adsorbent (designated as BC@La), simultaneously addressing phosphate pollution and solid waste management in an integrated and sustainable manner.

2. Materials and Methods

2.1. Materials

The main raw materials employed in this study comprised oily sludge (OS) and petroleum hydrocarbon-contaminated soil (PCS) (petrochemical plant in Wuhan, China). Waste glass powder (GP) was introduced as a fluxing agent to improve ceramsite performance and lower sintering energy consumption. Furthermore, lanthanum (La) recovered from spent fluid catalytic cracking (SFCC) catalyst was utilized for ceramsite surface modification. All waste feedstocks (OS, PCS, and SFCC) originated from a petrochemical plant in Wuhan, Hubei Province, China. OS was stored under refrigerated conditions, while SFCC and PCS were processed by crushing, drying at 105 °C for 24 h, and ball milling for 30 min. The milled powders were then passed through a 200-mesh sieve and stored in sealed bags.

Table 1. Chemical compositions of raw materials (wt.%).

Chemical Composition (wt.%)	PCS	OS	GP	SFCC
SiO2	56.99	24.31	63.66	37.61
Al2O3	23.20	21.81	6.29	54.73
K2O	4.49	0.69	0.45	0.16
Fe2O3	3.47	15.01	1.37	0.47
CaO	2.26	15.75	12.80	0.16
Na2O	1.84	1.26	10.73	0.16
P2O5	0.13	2.04	0.02	0.56
MgO	0.71	1.88	3.69	0.94
La2O3	/	/	/	2.18
CeO2	/	/	/	1.14

Note: “/” indicates that the element was not detected or its content is below the detection limit of the XRF instrument.

summarizes the chemical compositions of OS, PCS, SFCC, and GP. All chemicals used were analytical grade, sourced from Sinopharm Chemical Reagent Co., Ltd. (Wuhan Xinshi Chemical Technology Co., Ltd., Wuhan, China). Primary reagents included potassium dihydrogen phosphate (KH₂PO₄, AR), ammonium molybdate ((NH₄) ₂MoO₄, AR), and ascorbic acid (C₆H₈O₆, AR). pH adjustments were performed using hydrochloric acid (HCl, 36–38 wt.%) and sodium hydroxide (NaOH, ≥96%). Deionized water was used throughout solution preparation.

Figure 1a presents the phase compositions of the OS and PCS samples as determined by X-ray diffraction (XRD, Malvern PANalytical Empyrean, Almelo, The Netherlands). The diffraction patterns reveal that the primary crystalline phases in OS are quartz (SiO₂) and calcite (CaCO₃), while PCS consists predominantly of quartz and albite (NaAlSi₃O₈). Figure 1b shows the XRD pattern of the SFCC material used for lanthanum extraction. The identified diffraction peaks correspond to crystalline silica (SiO₂) and corundum (α-Al₂O₃) as the major phases. In contrast, the GP sample exhibits a characteristic broad hump in the XRD profile, with no discernible sharp diffraction peaks, confirming its predominantly amorphous, glassy structure.

2.2. Preparation and Modification of Ceramsite

(1)

Preparation of Ceramsite

As illustrated in Figure 2, OS, PCS, and GP were initially mixed uniformly according to predetermined mass ratios. An appropriate amount of deionized water was added to form a plastic slurry, which was subsequently hand-rolled into spherical green pellets with a diameter of approximately 1 cm. The green pellets were dried at 105 °C for 2 h and then subjected to high-temperature calcination in a muffle furnace. After cooling to room temperature, the resultant porous ceramic matrix was obtained and designated as base ceramsite (BC).

(2)

Extraction of Lanthanum

Using spent FCC catalyst as the lanthanum source, lanthanum was recovered via a “calcination–acid leaching–precipitation–conversion” process. The spent catalyst was first calcined at 600 °C to remove carbon, followed by ball milling for particle size reduction. Selective leaching of lanthanum was then performed under acidic conditions (pH ≈ 1) using hydrochloric acid. After filtration, sodium sulfate was added to the leachate to precipitate La³⁺ as NaLa (SO₄)₂·xH₂O, achieving initial enrichment and purification. This precipitate subsequently underwent steps such as alkali conversion and acid dissolution, ultimately yielding a purified LaCl₃ solution suitable for subsequent modification purposes [26].

(3)

Modification of Ceramsite

The aforementioned LaCl₃ solution was diluted to a La concentration of 2 wt.% and employed to impregnate the BC for 4 h under continuous shaking at 120 rpm. After impregnation, the mixture was statically aged. Subsequently, a NaOH solution was added dropwise to adjust the pH to 10, followed by continuous stirring for 2 h to facilitate the complete precipitation and loading of lanthanum species onto the surface and within the pores of the ceramsite. The resulting material was then washed thoroughly with deionized water, dried at 105 °C, and finally calcined at 550 °C for 4 h to obtain the lanthanum-modified ceramsite, designated as BC@La, which was used for subsequent adsorption experiments [26].

2.3. Characterization

The physicochemical properties of the synthesized ceramsite, such as water absorption, apparent density, bulk density, porosity, breakage and wear rate, mud-carrying capacity, and hydrochloric acid solubility, were evaluated in compliance with the Chinese standard “Artificial Ceramsite Filter Media for Water Treatment” (CJ/T 299-2008) [27]. Quantitative analysis of phosphate concentration post-adsorption was performed via UV-Vis spectrophotometry (UV-8000, Shanghai Mepuda Instruments Co., Ltd., Shanghai, China) at 700 nm. Bulk elemental composition was determined by X-ray fluorescence spectrometry (XRF, Zetium, Malvern Panalytical, Almelo, The Netherlands), while crystalline phases were identified through X-ray diffraction (XRD, Malvern PANalytical Empyrean, Almelo, The Netherlands). Surface functional groups were characterized using Fourier transform infrared spectroscopy (FT-IR, Beijing Beifen-Ruili Analytical Instrument (Group) Co., Ltd., Beijing, China). Additionally, X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher Scientific Inc., Waltham, MA, USA) was employed to analyze the surface chemical states and elemental composition before and after adsorption. Microstructural features and elemental distribution were examined by scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy (SEM-EDS, Regulus 8100, Hitachi High-Tech Corporation, Minato-ku, Tokyo, Japan).

2.4. Batch Adsorption Experiments

A phosphorus standard stock solution with a concentration of 100 mg/L was prepared by dissolving KH₂PO₄ in deionized water, and subsequent working solutions at varying concentrations were derived via serial dilution. Batch adsorption tests were conducted in triplicate using 250 mL Erlenmeyer flasks, each containing 50 mg/L of phosphate solution and a precisely weighed amount of the optimal ceramsite. The initial pH of the solutions was regulated by adding 0.01 M HCl or NaOH. The mixtures were agitated at 120 rpm in a temperature-controlled shaker for 24 h. At specified intervals, aliquots of the supernatant were extracted, filtered through a 0.45 μm membrane, and analyzed for residual phosphate concentration via the ammonium molybdate spectrophotometric method. The precision of this analytical method was validated through repeated calibration standards, yielding a relative standard deviation (RSD) of less than 2%. Therefore, the reported removal efficiencies and adsorption capacities are presented as the mean ± standard deviation of triplicate measurements, with associated uncertainty primarily attributed to this analytical precision and minor operational variations. The effects of adsorbent dosage (0.2–10.0 g/L), initial phosphate concentration (5–50 mg/L), temperature (298–318 K), and initial pH (3–11) were systematically investigated. This concentration range was selected to span from levels slightly above those typical of highly eutrophic waters (e.g., ~0.2 mg/L) to significantly higher concentrations relevant to concentrated wastewater discharges or acute pollution events. This approach enables a comprehensive evaluation of the adsorbent’s capacity and performance under both environmentally relevant and stressed conditions. Adsorption isotherms, kinetics, and thermodynamics were subsequently analyzed based on the experimental data.

The uncertainty in the reported phosphate concentration was propagated to calculate the uncertainty in the removal efficiency ($\eta$) using the standard formula for error propagation. The removal efficiency is defined as $\eta =\left(1−{\displaystyle \frac{C_e}{C_0}}\right)\times 100\%$, where $C_0$ and $C_e$ are the initial and equilibrium concentrations, respectively. Given that the standard deviation of the concentration measurement (${\sigma }_C$) was derived from the triplicate analyses (with an RSD < 2%), the combined standard uncertainty ($u_{\eta }$) for the removal efficiency was calculated as:

$$u_{\eta }=\left({\displaystyle \frac{C_e}{C_0}}\right)\times \sqrt{{\left({\displaystyle \frac{{\sigma }_{C_0}}{C_0}}\right)}^2+{\left({\displaystyle \frac{{\sigma }_{C_e}}{C_e}}\right)}^2}$$

(1)

This $u_{\eta }$ represents the standard uncertainty associated with each efficiency value reported in

Table A1. Raw material mix proportion of ceramsite.

Run	OS (%)	GP (%)	PCS (%)	Removal (%)
1	39.8	20.7	39.5	80.9 ± 0.2
2	39.8	20.7	39.5	80.6 ± 0.2
3	40	30	30	78.6 ± 0.2
4	30	28.7	41.4	79 ± 0.4
5	43.8	23	33.1	79.8 ± 0.2
6	50	18.4	31.7	77.9 ± 0.2
7	46.2	10	43.8	76.7 ± 0.1
8	33.6	22.2	44.2	79.9 ± 0.2
9	33.7	16.3	50	77.7 ± 0.2
10	39.8	20.7	39.5	80.7 ± 0.3
11	30	28.7	41.4	78.9 ± 0.2
12	33.7	16.3	50	77.9 ± 0.1
13	40	10	50	76.8 ± 0.2
14	49.3	13.2	37.4	76.5 ± 0.3
15	36.5	27.1	36.4	79.3 ± 0.2
16	50	18.4	31.7	77.9 ± 0.2

Note: Values are presented as mean ± standard deviation (n = 3).

 (see Appendix A for details) and throughout the study.

3. Results and Discussion

3.1. Optimization of Ceramsite Composition

3.1.1. Model Construction and Significance Analysis

To more intuitively analyze the interactive effects of OS, GP, and PCS incorporation levels on the adsorption performance of the ceramsite, a single-factor test was first conducted to determine the approximate ranges for each component and the sintering conditions (OS: 30–50%, PCS: 30–50%, GP: 10–30%; Temperature: 1040 °C). The optimization of the mixture proportions was implemented using the Optimal (Custom) design model within the Mixture design techniques module of Design-Expert 13.0 software. Sixteen simulated formulation combinations were generated randomly, as shown in Table A1. With the ceramsite adsorption capacity as the evaluation index, a model equation was fitted relating the three raw material proportions to the adsorption capacity. The optimal ceramsite formulation (BC) was ultimately determined, as illustrated in Figure A1 (see Appendix B for details).

As presented in

Table 2. ANOVA and model fitting statistics of phosphate removal efficiency.

Source	F Value	p Value	Remark
Model	400.28	<0.0001	significant
Linear Mixture	632.56	<0.0001
AB	103.03	<0.0001
AC	337.93	<0.0001
BC	110.71	<0.0001
ABC	31.16	0.0014
AB (A − B)	51.82	0.0004
AC (A − C)	26.83	0.0021
BC (B − C)	39.12	0.0008
Lack of Fit	0.1668	0.6999	not significant
$R^2$			$0.9983$
$R_{Adj}^2$			$0.9958$
RMSE			0.0009
Reduced Chi-square			8.6320 × 10−7

, the established model for phosphate removal efficiency exhibits exceptionally high statistical significance (F-value = 400.28, p < 0.0001) and an excellent goodness of fit (R² = 0.9983). The non-significant lack of fit (p = 0.6999) confirms the model’s reliability. A crucial finding is that all linear terms, as well as binary and ternary interaction terms (AB, AC, BC, ABC), are highly significant (p < 0.01). This demonstrates that the effect of the proportions of the three solid waste materials—OS, PCS, and GP—on phosphate removal is not merely additive but involves strong nonlinear synergistic interactions. Variations in the ratio of any two components, and even specific combinations of all three, cooperatively regulate the ultimate adsorption structure and performance of the ceramsite through deep physicochemical coupling mechanisms. This quantitative relationship is precisely described by the following mixture regression model (Equation (2)):

$$\begin{array}{c}Y=0.61A+0.78B+0.61C+0.40AB+0.64AC+0.38BC-0.46ABC+0.36AB\left(A-B\right)\hfill \\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em} -0.21AC\left(A-C\right)-0.25BC\left(B-C\right)\hfill \end{array}$$

(2)

where Y is the phosphate removal efficiency; A, B, and C are the pseudo-component coded values for OS, GP, and PCS, respectively. This model provides a precise quantitative basis for the formulation design and process optimization of adsorbent materials prepared from such multi-component solid wastes.

3.1.2. Analysis of Response Surface Interactions

Figure 3 visually reveals the complex interactive effects of the three raw material proportions—OS, PCS, and GP—on the phosphate removal efficiency through response surface and contour plots. The typical elliptical contour lines presented in the figure confirm the existence of significant synergistic effects among the factors, and the surface exhibits a clear region of maximum response. Specifically, the phosphate removal efficiency shows a nonlinear trend of initially rapid increase followed by a slow decrease with variations in the OS-to-PCS ratio.

This graphical characteristic aligns with the high statistical significance of the model (F-value = 400.28, p < 0.0001). The underlying mechanism lies in the synergistic interaction among the organic matter and metal oxides in OS, the clay minerals in PCS, and the glass phase in GP during the sintering process, which collectively governs the final structure of the ceramsite. OS and PCS primarily contribute to constructing the porous skeleton and providing adsorption sites, while GP acts as a flux, promoting matrix consolidation and pore channel formation. A precise balance among the three is essential. An excess of any single component—such as excessive OS leading to over-gassing or excessive GP causing surface over-vitrification—disrupts the synergy, resulting in a decline in removal efficiency. This is manifested in the figure as the attenuation of the response surface from the peak region toward the periphery.

Ultimately, the global optimal formulation obtained through model solving (OS:PCS:GP = 39.8:39.5:20.7) precisely coincides with the peak region of the response surface in the figure. This formulation quantitatively achieves the optimal synergy among the three components: it ensures a sufficiently porous skeleton and active surface while also providing structural stability through an appropriate amount of the glass phase. This not only validates the predictive capability of the response surface analysis but also practically demonstrates that the precise regulation of the proportions of multiple solid wastes can lead to the preparation of phosphate adsorbent materials with optimal performance.

3.1.3. Model Validation

In mixture design experimental methodology, residual analysis and prediction verification are core steps for assessing model reliability. As shown in Figure A2 (see Appendix B for details), in the normal probability plot of residuals for the phosphate removal efficiency model, all observed points are closely distributed around the reference line. This indicates that the residuals follow a normal distribution, with no systematic deviation or significant outliers present. This confirms that the regression analysis meets the fundamental assumptions of the least squares method, the established mathematical model can unbiasedly reflect the true experimental system, and the parameter estimates are robust.

The validity and reliability of the model were further assessed by comparing its predicted values with the experimental measurements. As shown in Figure A3 (see Appendix B for details), a high level of consistency is observed between the actual and predicted phosphate removal efficiencies, with the data points uniformly distributed along the diagonal line and exhibiting minimal deviation. This close agreement confirms that the optimal formulation derived from the mixture design is both experimentally reproducible and statistically robust.

3.1.4. Experimental Optimization and Verification Characterization

The numerical optimization module of Design-Expert software (version 13.0.5) was employed to determine the raw material formulation that maximizes phosphate removal efficiency within the experimental range. As summarized in

Table A2. Optimal solution under comprehensive conditions generated by software and actual value.

Factor	OS (%)	PCS (%)	GP (%)	Removal (%)
Predicted	39.8	39.5	20.7	79.4
Actual				80.3
Relative deviation (%)	-	-	-	1.13

 (see Appendix A for details), the model-predicted optimal conditions are: OS content 39.8%, PCS content 39.5%, and GP content 20.7% (by mass). The predicted phosphate removal efficiency under this formulation is 79.4%. For practical experimentation, the formulation was slightly adjusted and fixed as OS:PCS:GP = 39.8:39.5:20.7. Three independent validation experiments conducted under this optimized formulation yielded an average measured phosphate removal efficiency of 80.3%. The relative deviation between the experimental and predicted values is only 1.13%, which is well below the conventional 5% threshold. This strongly confirms that the established mixture model possesses high predictive accuracy and reliability for practical application.

In summary, the optimal preparation protocol for the synthetic ceramsite is established as follows: OS, PCS, and GP are homogenized and pelletized at a mass ratio of 39.8:39.5:20.7. The mixture is heated to 110 °C at a ramp rate of 10 °C/min, held for 1 h for drying, then further heated to 550 °C at 10 °C/min and maintained for 20 min to facilitate organic matter removal. Subsequently, the temperature is increased to 1040 °C at 5 °C/min and sintered for 30 min to achieve complete vitrification and structural consolidation. After cooling to room temperature, the obtained porous ceramic matrix is designated as BC. Lanthanum surface modification is subsequently conducted to yield the final composite adsorbent, denoted as BC@La.

3.2. Characterization of Ceramsite

A preliminary assessment of the phosphate adsorption efficacy was conducted for BC and BC@La. Under fixed experimental conditions, the fundamental properties of the different ceramsite products are listed in

Table 3. Physical characteristics of BC and BC@La.

Samples	BC	BC@La	Standard
Specific surface area (cm2/g)	4.556	5.261	≥0.5
Total pore volume (cm3/g)	0.051	0.047	≤2
Porosities (%)	24.443	23.593	≥0.5
Solubility in hydrochloric (%)	1.753	1.563	≤2
Apparent density (cm3/g)	1.667	1.743	/
Bulk density (cm3/g)	0.912	0.963	/
Cylinder compressing strength (MPa)	12.452	13.593	/
Water absorption (%)	23.347	24.007	/

Note: “/: No specified limit in the standard (CJ/T 299-2008)”.

. All listed properties—hydrochloric acid solubility, specific surface area, porosity, and the sum of breakage and wear rates—meet the limits specified by the Chinese standard “Artificial Ceramsite Filter Media for Water Treatment” (CJ/T 299-2008) [27]. Specifically, following lanthanum modification, the specific surface area of the ceramsite increased from 4.556 cm²/g to 5.261 cm²/g, the apparent density rose from 1.667 g/cm³ to 1.743 g/cm³, the cylindrical compressive strength improved from 12.452 MPa to 13.593 MPa, and the water absorption increased from 23.347% to 24.007%. Notably, the increase in specific surface area was accompanied by a slight decrease in total pore volume. This is primarily attributed to the deposition of loaded lanthanum species (e.g., La(OH)₃) on the internal pore surfaces of the base ceramsite, a process that introduces new surface area while partially occupying the pore space. The concurrent improvement in these key performance indicators demonstrates that lanthanum modification not only imparts chemical adsorption activity to the ceramsite but also enhances its physical structure and mechanical stability. As BC@La exhibited significantly superior structural and mechanical properties compared to BC, all subsequent adsorption characterization experiments were carried out using the higher-performing BC@La for in-depth investigation.

3.3. Leaching Safety Performance of Ceramsite

The environmental safety of the ceramsite was assessed through heavy metal leaching toxicity testing, a critical evaluation of its potential to release contaminants. The test measures the leachability of heavy metals from the ceramsite, which could otherwise pose potential hazards to ecosystems and human health.

Table 4. Heavy metal leaching concentrations.

Heavy Metals	Ceramsite (mg/L)	Limits (mg/L)
Zn	0.5264	≤100
Cu	0.0623	≤100
Ni	0.0338	≤5
Cd	0.0221	≤1
Cr	0.0040	≤15
Sb	0.0048	≤5
La	0.0037	/

Note: “/: No limit specified”.

presents the leaching concentrations of the targeted heavy metals (Zn, Cu, Ni, Cd, Cr, Sb) and the characteristic modifying element, La, for the ceramsite synthesized under optimal conditions. All measured values are substantially below the regulatory limits defined by the “Standard for Identification of Hazardous Wastes—Identification of Leaching Toxicity” (GB 5085.3-2007) [28]. Notably, the leaching concentration of the key functional element, La, is remarkably low (0.0037 mg/L). This minimal release suggests that La is primarily stabilized within the ceramsite matrix, likely incorporated into stable crystalline phases (e.g., LaPO₄) or solid solutions, thereby rendering it non-leachable. Consequently, the prepared ceramsite exhibits low leaching toxicity and a low environmental risk profile, confirming that its application does not pose a risk of secondary pollution from heavy metals or rare earth elements.

3.4. Adsorption Characteristics

3.4.1. Comparison of Adsorption Performance Between BC and BC@La

To delineate the contribution of the lanthanum modification, the adsorption capacities of the unmodified base ceramsite (BC) and the modified BC@La are directly compared across concentrations. The optimally formulated BC (OS:PCS:GP = 39.8:39.5:20.7) reached an adsorption capacity of ≈1.6 mg/g at a low phosphate concentration of 2 mg/L, with a removal efficiency of 81% (Table A1 (see Appendix A for details)). This value likely represents the saturation capacity ($Q_{max}^{BC}$) of the ceramic support itself, as its limited, non-specific sites are easily saturated.

In stark contrast, the BC@La ceramsite exhibited a dynamic adsorption profile: its capacity increased from 0.88 mg/g at 2 mg/L to 2.78 mg/g at 50 mg/L (under standard conditions: dosage 1 g/L, 24 h equilibrium) (Section 3.4.2 and Figure 4b), demonstrating abundant and high-affinity active sites. Most decisively, the lanthanum-specific capacity of BC@La reached 128.0 mg P per gram of La—two orders of magnitude higher than the total adsorption capacity of the BC support (≈1.6 mg/g). This quantitative comparison provides unambiguous evidence that the porous BC matrix primarily serves as a mechanical host, while the surface-immobilized lanthanum species are responsible for the high-capacity, high-affinity phosphate adsorption.

3.4.2. Phosphorus Adsorption Characteristics of BC@La Ceramsite

During adsorption, pH serves as a critical parameter governing the surface charge and speciation of functional groups on the adsorbent. Figure 4a presents the phosphate removal efficiency of BC@La from simulated wastewater across different initial pH levels. The results indicate that removal efficiency first increases and then decreases as pH varies from 3 to 11. This trend is primarily attributed to the increasing negative charge on phosphate species as pH rises from acidic to neutral. This enhances their electrostatic attraction to the positively charged adsorbent surface, thereby promoting adsorption. With increasing alkalinity, electrostatic repulsion and competitive adsorption by OH⁻ ions become predominant. Although phosphate ions exist predominantly as PO₄³⁻ with higher negative charge under alkaline conditions, their access to and binding with adsorption sites are hindered, leading to a pronounced reduction in removal efficiency. The plausible chemical interactions between La and P are described by Equations (3)–(5) [29].

$$\equiv La–OH+H_{2}P{O}_4^{-}\leftrightarrow \equiv La–H_{2}P{O}_4+O{H}^{-}$$

(3)

$$\equiv La–OH+HP{O}_4^{2-}\leftrightarrow \equiv La–{HPO}_4^{-}+{OH}^{-}$$

(4)

$$2(\equiv La–OH)+{PO}_4^{3-}\leftrightarrow {(\equiv La)}_2–{HPO}_4^{2-}+2{OH}^{-}$$

(5)

As shown in Figure 4b, 1 g of ceramsite was added to 1 L of phosphate-containing wastewater with varying initial concentrations. With increasing phosphate concentration, the adsorption uptake (q) of the ceramsite increased progressively, while the removal efficiency exhibited a gradual decline (Figure 4b). At a concentration of 50 mg/L, BC@La achieved a removal efficiency of 88.34, corresponding to a q of 2.78 mg/g under these specific conditions. Figure 4c illustrates the influence of adsorbent dosage and different initial phosphate concentrations on removal efficiency. It is crucial to distinguish the nature of the values reported here from the standardized q_e. The q values plotted in Figure 4b,c represent the observed phosphate uptake under the specific, non-standardized conditions of each individual experiment (varying dosage or concentration). These values are presented to illustrate trends and are distinct from the equilibrium adsorption capacity (q_e) obtained under the controlled, standardized conditions of the adsorption isotherm experiments (fixed dosage of 1 g/L, 24 h equilibrium), as detailed in Section 3.4.3 and Figure 5.

With increasing adsorbent dosage, the observed uptake decreased for all initial phosphate concentrations tested (5, 10, 20, and 50 mg/L). For the wastewater containing 50 mg/L phosphate, the observed uptake declined gradually when the ceramsite dosage increased from 0.3 to 1 g/L. However, when the dosage was further increased from 1 to 2 g/L, the observed uptake decreased markedly from 2.87 mg/g (obtained at the low, non-standard dosage of 0.3 g/L) to 1.32 mg/g. This value (2.87 mg/g) should not be compared directly with the standardized equilibrium adsorption capacity (q_e) of 2.78 mg/g obtained at the optimal dosage of 1 g/L, as they originate from fundamentally different experimental protocols. Similar trends were observed for the other concentration groups. The optimal ceramsite (BC@La) developed in this study demonstrated excellent adsorption performance, highlighting its significant application potential. Therefore, considering both adsorption effectiveness and economic feasibility, a dosage of 1 g/L was selected as the optimal condition for the static adsorption experiments with the ceramsite.

3.4.3. Adsorption Isotherm

To investigate the temperature effect (298, 308, and 318 K) on phosphate adsorption, experiments were conducted under standardized conditions: an initial phosphate concentration range of 0.5–50 mg/L, a constant adsorbent dosage of 1 g/L, and an equilibrium period of 24 h. The equilibrium adsorption capacity (q_e), determined under this fixed-dosage protocol, serves as a robust metric for comparative analysis and modeling. Subsequently, the obtained equilibrium data were analyzed by fitting to both the Langmuir and Freundlich isotherm models (Equations (A1) and (A2) (see Appendix C for details)).

The fitted curves (Figure 5a,b) exhibit a characteristic shape: adsorption capacity increased rapidly at low equilibrium concentrations and then approached a plateau, indicating the saturation of available adsorption sites [30]. To quantitatively compare the models and gain physical insights, key parameters are summarized in

Table 5. The parameters of the Langmuir and Freundlich models.

T/K	Langmuir			Freundlich
	qm/mg·g−1	KL/L·mg−1	R2	KF/L·mg−1	1/n	R2
298	2.20	2.402	0.9805	1.3386	0.33	0.9592
308	2.32	2.593	0.9743	1.4589	0.30	0.9389
318	2.56	2.697	0.9873	1.6854	0.28	0.9589

. The higher correlation coefficients (R²) for the Langmuir model across all temperatures suggest that it provides a better description of the adsorption data than the Freundlich model [31]. The maximum monolayer adsorption capacity ($q_m$) derived from the Langmuir model increased from 2.20 mg/g at 298 K to 2.56 mg/g at 318 K. Concurrently, the Langmuir constant ($K_L$), which reflects adsorption affinity, also increased with temperature. This consistent trend confirms the endothermic nature of the phosphate adsorption process on BC@La [32].

It is important to critically discuss the implications of the Langmuir fit. A good fit primarily indicates that the adsorption system conforms to the model’s core assumptions: monolayer coverage and a surface with relatively uniform site energy [33]. It does not, by itself, exclude the contribution of other simultaneous processes. In our system, the predominance of La³⁺ active sites—recovered and uniformly dispersed from waste catalyst—provides a reasonable basis for the “uniform sites” approximation. More importantly, the strong, specific adsorption behavior implied by the Langmuir model is directly corroborated by the formation of LaPO₄ precipitates observed via XRD and the corresponding chemical state changes detected by XPS (Section 3.5). This multi-technique convergence supports that the adsorption is dominated by site-specific chemisorption, consistent with the physical picture of the Langmuir model.

In summary, the isotherm analysis, supported by complementary characterization, indicates that phosphate adsorption onto BC@La is best described as an endothermic, monolayer process driven by strong chemisorption onto La³⁺ sites, ultimately leading to surface precipitation.

Figure 5a,b show that at lower initial phosphorus concentrations, the adsorption capacity of the ceramsite increases rapidly. As the initial concentration further increases, the curve flattens, and the adsorption capacity approaches saturation. This indicates that at low initial concentrations, the adsorption sites on the ceramsite are not fully occupied, whereas at higher concentrations, the sites become saturated, leading to the plateau in the curve.

Table 5 presents the fitted parameters of the Langmuir and Freundlich models, which describe the adsorption thermodynamics of phosphate onto BC@La. The standard equilibrium adsorption capacity (q_e) increased progressively with temperature, reaching a maximum value of 2.56 mg P per gram of adsorbent at 318 K (

Table A3. Comparison of the maximum adsorption capacity of different types of ceramsite for phosphorus.

Adsorbent	La Loading (wt.%)	Adsorption Capacity (mg P/g)	La Efficiency Index [(mg P/g La)/(wt.% La)]	Refs.
BC@La	2.0	2.6	64.0	This study
Fe3O4/La(OH)3	27.8	83.5	10.8	[26]
La-MB	23.1	48.4	9.1	[51]
La2O2CO3@biochar	45.0	81.7	4.0	[52]
LOMP	68.8	165.5	2.4	[53]

 (see Appendix A for details) provides a comparative overview with other adsorbents on this basis). This moderate gravimetric capacity results from the deliberately low lanthanum loading (2 wt.%), sourced from spent catalyst, reflecting a design philosophy focused on the efficient and sustainable utilization of a critical rare-earth resource. To accurately assess the effectiveness of the active phase, the adsorption performance was also evaluated in terms of lanthanum utilization efficiency. In this regard, BC@La exhibits an exceptional lanthanum-specific capacity of 128.0 mg P per gram of La, a metric that surpasses many reported La-based adsorbents when normalized to the mass of the active component. Beyond its intrinsic adsorption metrics, BC@La offers practical engineering advantages: its granular morphology facilitates solid–liquid separation compared to powdered analogues, and its synthesis via solid waste valorization represents a cost-effective and environmentally synergistic materials strategy. Finally, the superior fitting of the Langmuir model (R²) over the Freundlich model indicates that the adsorption process is predominantly monolayer chemisorption [34].

3.4.4. Adsorption Kinetics

Table 6. The parameters of the pseudo-first-order and pseudo-second-order models.

Pseudo-First-Order			Pseudo-Second-Order
qe/mg·g−1	k1/min−1	R2	qe/mg·g−1	K2/g·(mg·min)−1	h (mg·g−1·min−1)	R2
2.3940	0.0079	0.8745	2.6392	0.0045	0.0313	0.9615

summarizes the fitting parameters of the pseudo-first-order (PFO) and pseudo-second-order (PSO) kinetic models. The PSO model demonstrates superior performance, exhibiting a higher calculated equilibrium adsorption capacity (q_e = 2.64 mg/g) and a more satisfactory correlation coefficient (R² = 0.9615) compared to the PFO model. Consequently, the adsorption kinetics of phosphate onto the adsorbent are better described by the PSO model, suggesting a process predominantly governed by chemisorption [35,36]. This finding aligns with recent kinetic studies on pharmaceutical adsorption using activated carbon [37]. A distinct merit of the PSO model is its provision of the initial adsorption rate (h), a parameter with explicit physical significance. As defined by h = k₂q_e², the calculated value was 0.0313 mg·g⁻¹·min⁻¹ (Table 6). This rate quantitatively characterizes the adsorption velocity at the very beginning of the process (t → 0). The obtained h value corroborates the rapid initial uptake phase evident in the kinetic profile (Figure 5c), underscoring the high accessibility and efficacy of the active sites on the BC@La surface for phosphate sequestration.

3.4.5. Adsorption Thermodynamics

The temperature-dependent adsorption isotherms (Figure 5d) were employed to determine the thermodynamic parameters for phosphate adsorption onto the ceramsite, utilizing Equations (A5)–(A7) (see Appendix C for details). These equations incorporate the distribution coefficient (K_D), the initial and equilibrium phosphate concentrations (c₀ and c_e), the universal gas constant R (8.314 J·mol⁻¹·K⁻¹), and the absolute temperature T. The derived parameters are summarized in

Table 7. The parameters of thermodynamics.

ΔH° (KJ·mol−1)	ΔS° (J/(mol·K)−1	R2	ΔG° (KJ·mol−1)
			298 K	308 K	318 K
8.4481	36.3366	0.9999	−2.3687	−2.7368	−3.0917

. The calculated Gibbs free energy change (ΔG°) was negative at 298, 308, and 318 K (−2.3687, −2.7368, and −3.0917 kJ·mol⁻¹, respectively), confirming the spontaneity of the adsorption process. Furthermore, the increasingly negative values of ΔG° with rising temperature indicate enhanced thermodynamic favorability at higher temperatures. The adsorption was found to be endothermic, as evidenced by a positive enthalpy change (ΔH° = 8.4481 kJ·mol⁻¹) [38]. Additionally, the positive entropy change (ΔS° = 36.3366 J·mol⁻¹·K⁻¹) suggests an increase in the overall disorder of the system following adsorption.

3.5. Characterization of Ceramsite Before and After Phosphate Adsorption

3.5.1. XRD Characterization Analysis

Figure 6 shows the XRD patterns of the ceramsite before and after phosphate adsorption. Before adsorption, the main crystalline mineral phases of the ceramsite matrix were quartz and albite. The presence of a distinct broad diffraction hump in the low-angle region of the XRD pattern, combined with the synthesis process, indicates that the ceramsite surface is coated with a layer of poorly crystalline, amorphous lanthanum-based compounds (primarily amorphous lanthanum hydroxide and its complexes) formed by the hydrolysis of lanthanum ions. After adsorption, clear characteristic diffraction peaks corresponding to lanthanum phosphate (LaPO₄, JCPDS 32-0493) [39] appear at multiple positions (e.g., 2θ = 25.9°, 29.7°, 31.2°). This provides direct crystallographic evidence for the formation of LaPO₄ precipitates on the surface, strongly supporting the precipitation mechanism. The intensity and sharpness of these new peaks confirm the crystallinity of the formed LaPO₄. Conversely, the positions and intensities of the primary quartz and albite peaks remained largely unchanged, indicating that the core aluminosilicate matrix structure was not disrupted during the adsorption process.

$${La}^{3+}+{PO}_4^{3-}\to {LaPO}_4\downarrow$$

(6)

3.5.2. FTIR Characterization Analysis

Figure 7 presents the FTIR spectra of the ceramsite at three stages: before sintering, before phosphate adsorption, and after adsorption, scanned over the range of 4000–400 cm⁻¹. Key molecular vibrational information derived from these spectra elucidates the phosphate adsorption mechanism. A new absorption band emerges at 468 cm⁻¹ post-adsorption, which is assigned to the bending vibration of the P–O bond [40]. This provides direct spectroscopic evidence for the successful fixation of phosphate ions onto the surface. The aluminosilicate framework of the ceramsite maintains its structural integrity, as indicated by the stability of the characteristic peaks at 1026 cm⁻¹ (symmetric Si–O–Si stretching) and 762 cm⁻¹ (Al–O vibration) [41,42]. Prior to adsorption, the sintering process effectively purifies the surface, evidenced by the significant attenuation of the C=O peak at 1474 cm⁻¹, which denotes the elimination of organic impurities [43]. Such purification exposes more inorganic active sites. Additionally, the broad absorption in the 3629–3693 cm⁻¹ region, associated with O–H stretching vibrations, points to the presence of surface hydroxyl groups and water molecules [44]. These hydrophilic groups are instrumental in facilitating the initial approach and subsequent reaction of phosphate ions. In summary, the FTIR results collectively demonstrate that phosphate adsorption proceeds through chemical interaction on a surface that is structurally stable, purified from organics, and enriched with hydroxyl groups.

3.5.3. SEM Characterization Analysis

The surface microstructural images of the ceramsite before (a, c) and after (b, d) phosphate adsorption are presented in Figure 8. Before adsorption, the interior of the ceramsite exhibits a rough and porous texture, which affords a high specific surface area and abundant pathways for mass transport. Following lanthanum surface modification, the deposited nanoparticles established a hierarchical porous architecture, further enhancing the surface roughness and the population of accessible reactive sites. Notably, a marked reduction in surface porosity is observed post-adsorption. This distinct pore-filling phenomenon provides direct morphological evidence supporting an in situ precipitation mechanism. The reaction between surface-immobilized lanthanum species and phosphate ions yields lanthanum phosphate (LaPO₄) precipitates, which deposit within and subsequently occlude the pore spaces [45]. Consequently, SEM analysis offers visual corroboration of the chemical pathway deduced from spectroscopic data: the observed pore infilling represents the physical manifestation of LaPO₄ precipitation, as verified by XRD, while FTIR spectroscopy confirmed the concomitant formation of P–O bonds. Collectively, these findings confirm a chemisorption process governed by surface chemical reaction and pore-blocking precipitation.

3.5.4. XPS Characterization Analysis

To elucidate the phosphate removal mechanism at the electronic-structure level, X-ray photoelectron spectroscopy (XPS) was employed to probe the chemical states of the BC@La ceramsite before and after adsorption. The survey spectrum (Figure 9a) confirms the emergence of phosphorus post-adsorption. Concurrently, the attenuation of the La signal and a positive chemical shift of ~0.8 eV observed in the La 3d region signify a decrease in electron density around La centers. This provides direct evidence for electron transfer from La³⁺ to phosphate oxygen, characteristic of La–O–P bond formation in LaPO₄ [46]. The C 1s spectrum (Figure 9b) shows a reduction in the C=O peak area by about 18%, supporting the ligand competition mechanism where phosphate ions displace carbonyl-containing adsorbates from metal sites [47]. The Ca 2p spectrum (Figure 9c) shows a subtle shift of 0.3 eV to lower binding energy. This suggests a change in the chemical environment of residual Ca²⁺ ions, likely due to weak secondary coordination (Ca–O–P) induced by the primary LaPO₄ precipitation, rather than forming a distinct crystalline phase [48]. The P 2p spectrum (Figure 9d) evolves into a well-defined doublet. Peak deconvolution shows the main component at a binding energy of 133.4 ± 0.2 eV, which is exclusively assigned to phosphorus in the orthophosphate (PO₄³⁻) state, as in LaPO₄ [46,49]. The quantitative area ratio of the P 2p doublet (2p₃/₂:2p₁/₂) approximates 2:1, further confirming the validity of the peak assignment. Deconvolution of the O 1s spectrum (Figure 9e) offers semi-quantitative insights into the surface transformation. Post-adsorption, the proportion of the component attributed to metal–OH (e.g., La–OH) decreases significantly, while a new component attributable to P–O bonds in La–O–P emerges. This ligand exchange, quantified by the notable decrease in the (–OH)/(O²⁻) ratio, directly evidences the replacement of surface hydroxyls by phosphate ligands [50].

In summary, the deepened XPS analysis elucidates a multi-level fixation mechanism: (1) The core mechanism is La³⁺-driven chemisorption via La–O–P bond formation, quantitatively evidenced by La 3d chemical shift and the characteristic P 2p doublet. (2) This process triggers a synergistic ligand-exchange reaction, quantified by the transformation of O 1s and C 1s spectra. (3) A secondary interfacial interaction involving matrix-derived Ca²⁺ is also indicated. These results provide electronic-structure-level verification that the adsorption is predominantly an irreversible chemisorption process dominated by surface complexation and precipitation, which is fully consistent with the monomolecular layer adsorption behavior described by the best-fit Langmuir isotherm model [30,51].

3.6. Mechanism of Phosphorus Adsorption by Ceramsite

The core innovation of this study lies in implementing a “waste-treats-waste” closed-loop strategy. More importantly, the comprehensive characterization data (XRD, FTIR, XPS) collectively reveal that the BC@La ceramsite exhibits a multi-level adsorption mechanism centered on La³⁺-dominated surface precipitation and complexation, accompanied by matrix synergy. As illustrated in Figure 10, the fixation of phosphorus on the BC@La ceramsite primarily involves the following synergistic stages:

Stage 1: Surface Coordination and Rapid Precipitation: Phosphate ions (PO₄³⁻) from the solution diffuse to the ceramsite surface and preferentially undergo specific coordination with the highly dispersed active sites of amorphous lanthanum hydroxide. The XPS evidence of La 3d peak shift and O 1s component change confirms this ligand exchange and bond formation. Due to the extremely high affinity between La³⁺ and PO₄³⁻, this coordination rapidly induces local supersaturation, leading to the in situ formation of thermodynamically stable lanthanum phosphate (LaPO₄) on the surface. The appearance of sharp LaPO₄ XRD peaks provides definitive proof of this precipitation pathway. This stage aligns with the characteristics of Langmuir monolayer chemisorption.

Stage 2: Matrix Induction and Synergistic Fixation: The rapid precipitation in the first stage significantly reduces the phosphate concentration at the solid–liquid interface, disrupting the micro-interfacial equilibrium of the ceramsite’s aluminosilicate matrix (which contains calcium-bearing mineral phases). This induces the dissolution of trace cations such as Ca²⁺, as suggested by the XPS Ca 2p peak shift. The dissolved Ca²⁺ then combines with phosphate, forming Ca–O–P coordination structures around the LaPO₄ crystals or on the matrix surface, achieving secondary phosphorus fixation. This process does not occur independently but is a synergistic effect triggered and enhanced by the initial La³⁺ precipitation.

Stage 3: Electrostatic Interaction and Surface State Reconstruction: At near-neutral pH, the positively charged La–OH₂⁺/La–OH⁺ sites on the ceramsite surface provide favorable electrostatic attraction for negatively charged phosphate species (H₂PO₄⁻, HPO₄²⁻), facilitating their approach to the surface-active sites and explaining the high removal efficiency at pH 6–7 observed in Figure 4a. The aforementioned precipitation and coordination processes are accompanied by the dynamic reconstruction of the chemical environment at the outermost surface layer. The decrease in La–OH signal and increase in hydrophilicity (O 1s) post-adsorption, as seen in XPS and FTIR, reflect this surface chemical state change, which further facilitates subsequent phosphate contact.

In summary, phosphorus removal by BC@La is not a simple homogeneous precipitation process. Instead, it is a multi-step, multi-phase interfacial reaction sequence that begins at La³⁺ active sites, triggers changes in the interfacial microenvironment, and drives synergistic interactions among multiple components. The combined XRD, FTIR, and XPS analyses move the mechanistic understanding from qualitative description to a data-supported model involving specific surface complexation, crystallization precipitation, electrostatic attraction, and matrix-assisted fixation.

3.7. Practical Application Potential: Stability and Resource Recovery Perspective

For practical implementation, the environmental stability and end-of-life strategy of an adsorbent are as critical as its removal capacity. The BC@La ceramsite demonstrates strong potential in these aspects, grounded in the fundamental chemistry of the adsorption product.

• Chemical Stability and Leaching Resistance. The core removal mechanism, confirmed by XRD and XPS, is the precipitation of lanthanum phosphate (LaPO₄). This phase is thermodynamically stable due to its exceptionally low solubility product (K_sp ~ 10⁻²⁵), which inherently secures the captured phosphate and immobilizes La³⁺ ions. Empirical leaching tests (Table 4) corroborate this stability, showing negligible La release (<0.1 mg L⁻¹) across a broad pH range (4–9). This performance underscores a key advantage over many Fe- or Al-based adsorbents, particularly in mildly acidic conditions where the stability of LaPO₄ offers superior leaching resistance.
• From Reusability Challenge to Direct Resource Recovery. The high-affinity, irreversible chemisorption that grants BC@La its excellent performance also limits conventional regenerability via desorption. This presents an opportunity to shift the application paradigm. Instead of aiming for multiple adsorption cycles, the spent BC@La—now a phosphorus-enriched composite—can be directly utilized as a value-added soil amendment or a slow-release phosphate fertilizer. This approach transforms the “spent adsorbent” into a secondary resource, effectively closing the phosphorus loop and avoiding the energy and chemical costs associated with regeneration. The stabilized LaPO₄ within the robust ceramic matrix ensures minimal trace metal impact while gradually releasing plant-available phosphate.

In summary, BC@La exhibits excellent environmental stability derived from the formation of insoluble LaPO₄, effectively preventing secondary pollution. Its most sustainable application pathway may involve a single, high-efficiency use for phosphate removal, followed by the direct resource recovery of the laden material in agriculture, aligning with circular economy principles for nutrient management.

4. Conclusions

Through a two-stage process involving calcination and surface modification, a novel porous adsorbent ceramsite—lanthanum surface-modified ceramsite (BC@La)—was developed. The results demonstrate:

• Under the conditions of an adsorbent dosage of 1 g/L, pH = 6, and a temperature of 318 K, BC@La demonstrated excellent phosphate removal performance (128.06 mg P/g La). The experimental data were best fitted by the Langmuir isotherm model (R² = 0.987) and the pseudo-second-order kinetic model (R² = 0.956), indicating a monolayer chemisorption process occurring on the BC@La surface.
• BC@La effectively removed phosphate even under alkaline conditions. When applied to real wastewater, no pH adjustment is required to promote phosphate removal.
• Leaching toxicity tests confirmed that nearly all toxic and harmful elements within the BC@La ceramsite were immobilized during the sintering process.
• Due to its granular form, BC@La exhibits excellent liquid–solid separation performance, showing clear practical advantages over powdered adsorbents in phosphorus removal applications.
• The adsorption process is predominantly irreversible, leading to the formation of stable LaPO₄ and Ca–O–P phases that ensure long-term phosphate fixation. While this grants the material excellent stability, it also presents a challenge for conventional desorption-based regeneration. Therefore, exploring effective regeneration strategies or alternative resource recovery pathways (e.g., direct utilization of spent adsorbent as a phosphorus source) constitutes a key focus for the future development and practical application of this material.