The Performance and Mechanism of Solvothermal Synthesis of a Ca-Fe-La Composite for Enhanced Removal of Phosphate from Aqueous Solutions

Abstract

Since it is a limiting nutrient element in rivers and lakes, the effective removal of phosphorus is key to alleviating eutrophication. In this study, the one-pot solvothermal method was adopted to prepare an environmentally friendly Ca-Fe-La composite. This is an amorphous material with a large specific surface area of 278.41 m² g⁻¹. The effects of coexisting anions and pH on the phosphate removal performance were explored. Phosphate adsorption mechanisms were revealed by various characterization techniques. The phosphate adsorption obeyed the fractal-like pseudo-second-order (PSO) kinetic model, implying that the overall adsorption system was highly heterogeneous. In this work, the maximum adsorption capacity predicted by the Langmuir model was 93.0 mg g⁻¹ (as PO₄³⁻-P). The phosphate-loaded Ca-Fe-La composite could be used as a slow-release fertilizer, achieving waste management and resource utilization. The presence of SO₄²⁻, CO₃²⁻ and HCO₃⁻ anions inhibited the phosphate adsorption significantly. It was unfavorable for phosphate removal at a high pH value. Inner-sphere complexation and electrostatic attraction were mainly responsible for phosphate adsorption onto the Ca-Fe-La composite.

1. Introduction

Phosphorus is an essential nutrient element for living organisms and plays a decisive role in modern agriculture and industry [1]. However, excessive phosphorus discharged into rivers, lakes and reservoirs inevitably induces eutrophication in the water, leading to algal bloom, the deterioration of water quality, a decrease in dissolved oxygen and the destruction of the aquatic community structure [2]. In the USA, the phosphorus level is below 0.02 mg L⁻¹ in surface water [3]. In Australia and New Zealand, water quality guidelines require that the concentration of phosphorus be less than 0.025 mg L⁻¹ in water environments [4]. Reducing the emission of phosphorus may be the most efficient way of controlling phosphorus pollution due to the weak self-purification of eutrophic waters. Hence, it is urgent to find an effective method for phosphate removal.

Among various phosphate removal techniques, adsorption has received increasing attention due to its advantages of a high adsorption capacity, fast kinetics and simple operation [5,6]. Many metal cations such as Ca²⁺, Fe³⁺, Al³⁺, Mg²⁺ and La³⁺ have been employed to prepare various types of adsorbents for the advanced removal of phosphate because of their high selectivity and affinity for phosphate [7]. The adsorption materials currently used for phosphate removal include metal (hydr)oxides [8], metal-cation-modified biochar [9], layered double hydroxides [10] and metal–organic frameworks [11]. Recently, the solvothermal method has frequently been employed to synthesize metal oxides with good crystallinity, high purity and low agglomeration. The NaLa(CO₃)₂/Fe₃O₄ composite was synthesized by Hao et al. (2019), and its maximum adsorption capacity was 77.85 mg g⁻¹ [12]. Wu and Lo (2020) synthesized CeO₂ particles for enhanced phosphate adsorption, and their maximum adsorption capacity was 94.9 mg g⁻¹ [13]. Feng et al. (2024) prepared Ca²⁺-doped LaMnO₃ perovskites (Ca_xLa_1−xMnO₃) for the efficient removal of low concentrations of phosphate, and their maximum adsorption capacity was 63.01 mg g⁻¹ [14].

On the other hand, some transition metal cations, such as Mn²⁺, Zn²⁺ and Cu²⁺, have not been widely applied because of their potential toxicity. It is widely accepted that calcium and iron are inexpensive and nontoxic, and their contents in the Earth’s crust are also abundant [15]. However, the Ca-Fe composite synthesized by the solvothermal method was not ideal for phosphate removal and presented a low adsorption capacity (17.8 mg g⁻¹ in this study). Recently, rare earth elements such as lanthanum exhibited a strong affinity for phosphate [16]. The doping of La³⁺ contributed to enhancing its phosphate adsorption capacity significantly. Zhu et al. (2024) synthesized La/Al-BTC by the solvothermal method, and its phosphate removal performance increased by about 5.9 times relative to Al-BTC [17].

In this study, La³⁺ was incorporated into the Ca-Fe composite by the one-pot solvothermal method to enhance its phosphate removal performance. The Ca-Fe-La composite was a highly efficient and environmentally friendly adsorbent for phosphate removal. The effects of coexisting anions and pH on the phosphate adsorption were investigated in batch experiments. Phosphate adsorption processes such as kinetic and isotherm studies were carried out. Some characterization methods were adopted to analyze the physicochemical properties of the Ca-Fe-La composite and reveal its phosphate adsorption mechanisms.

2. Materials and Methods

2.1. Adsorbent Preparation

All chemicals used in this study are of analytical reagent grade. The Ca-Fe-La composite was obtained by the one-pot solvothermal method according to our previous work [18]. As shown in Figure 1, 1.2928 g of Fe(NO₃)₃·9H₂O, 0.9446 g of Ca(NO₃)₂·4H₂O and 2.0784 g of La(NO₃)₃·6H₂O were dissolved in a beaker containing 70 mL of anhydrous ethanol. After the addition of 0.1 g of urea, the mixed solution was continuously stirred for 20 min and was then transferred into a 100 mL Teflon-lined stainless steel autoclave. After sealing, the autoclave was placed into a thermostatic oven and was naturally heated from room temperature to the specified temperature. After centrifugation, washing and dying, the obtained precipitate was referred to as the Ca-Fe-La composite. The effects of La³⁺ content (0–6 mmol), solvothermal temperature (100–180 °C) and solvothermal time (1–6 h) on its phosphate removal performance were explored.

2.2. Batch Experiments

First, 25 mg of the Ca-Fe-La composite was added to a series of conical flasks containing 100 mL of the phosphate solution (30 mg L⁻¹) and then the batch experiments were conducted in a thermostatic oscillator (150 rpm and 30 °C). The effects of the initial pH of the solution (2–11) and coexisting anions (Cl⁻, SO₄²⁻, CO₃²⁻, NO₃⁻ and HCO₃⁻; 1, 2 and 3 mmol L⁻¹) on the phosphate adsorption were investigated. All water samples to be measured were filtered by a 0.45 μm membrane. The concentration of phosphorus was measured by a UV/vis spectrophotometer. To avoid confusion, the amount of phosphorus uptake ($q_t$ and $q_{\mathrm{e}}$) and the concentration of phosphorus ($C_0$, $C_t$ and $C_{\mathrm{e}}$) reported in this study were measured as PO₄³⁻-P. The amounts of phosphorus uptake at time t and at equilibrium are expressed as

$$q_t={\displaystyle \frac{\left(C_0-C_t\right)V}{m}}$$

(1)

$$q_{\mathrm{e}}={\displaystyle \frac{\left(C_0-C_{\mathrm{e}}\right)V}{m}}$$

(2)

where $q_t$ (mg g⁻¹) and $q_{\mathrm{e}}$ (mg g⁻¹) are the amounts of phosphorus uptake at time t and at equilibrium, respectively; $C_0$ (mg L⁻¹) is the concentration of phosphorus at the initial time; $C_t$ (mg L⁻¹) is the concentration of phosphorus at time t; $C_0$ (mg L⁻¹) is the concentration of phosphorus at equilibrium; V (mL) is the volume of the solution; m (g) is the adsorbent mass.

2.3. Kinetic and Isotherm Studies

The kinetic experiments for the Ca-Fe and Ca-Fe-La composites were carried out at the phosphorus concentration of 30 mg L⁻¹. The residual phosphorus concentration was measured at a time range of 2 to 1440 min. The pseudo-second-order (PSO) and fractal-like pseudo-second-order (PSO) models are used to analyze the kinetic data in the batch system, which are expressed as [19]

$$q_t=q_{\mathrm{e}}\cdot \left(1-{\displaystyle \frac{1}{1+q_{\mathrm{e}}{k}_{2}t}}\right)$$

(3)

$$q_t=q_{\mathrm{e}}\cdot \left[1-{\displaystyle \frac{1}{1+\frac{q_{\mathrm{e}}{k}_{2,0}}{1-h}\cdot t^{\left(1-h\right)}}}\right]$$

(4)

where $k_2$ (g mg⁻¹ min⁻¹) and $k_{2,0}$ (g mg⁻¹ min^−(1−h)) are the PSO and fractal-like PSO rate constants, respectively; h is the fractal-like exponent; t (min) is the reaction time.

The equilibrium experiments for the Ca-Fe and Ca-Fe-La composites were carried out at different phosphorus concentrations (10–50 mg L⁻¹). The Langmuir and Freundlich models are adopted for modeling of the equilibrium data, which are given as [20]

$$q_{\mathrm{e}}={\displaystyle \frac{q_{\max }{K}_{\mathrm{L}}{C}_{\mathrm{e}}}{1+K_{\mathrm{L}}{C}_{\mathrm{e}}}}$$

(5)

$$q_{\mathrm{e}}=K_{\mathrm{F}}{C}_{\mathrm{e}}^{1/n}$$

(6)

where $q_{\max }$ (mg g⁻¹) is the maximum adsorption capacity; $K_{\mathrm{L}}$ (L mg⁻¹) is the equilibrium constant; $K_{\mathrm{F}}$ (L^1/n g⁻¹ mg^−(1+1/n)) and n are the unknown parameters.

2.4. Error Analysis

In this study, the model parameters were determined by the iteration algorithm of Levenberg–Marquardt in OriginPro 2021 software. The adjusted coefficient of determination (Adj. R²) and the root of mean squared error (RMSE) are used to evaluate the fitting quality of various kinetic and isotherm models, which are expressed as

$$\mathrm{Adj}{. R}^2=1-{\displaystyle \frac{{\displaystyle \sum _{i=1}^n}{\left(y_i-{ý}_i\right)}^2}{{\displaystyle \sum _{i=1}^n}{\left(y_i-\stackrel{-}{y}\right)}^2}}\left({\displaystyle \frac{n-1}{n-p}}\right)$$

(7)

$$\mathrm{RMSE}=\sqrt{{\displaystyle \frac{1}{n}}{\displaystyle \sum }_{i=1}^n{\left(y_i-{ý}_i\right)}^2}$$

(8)

where $y_i$ are the observed values (y = $q_t$ or $q_{\mathrm{e}}$), ${ý}_i$ are the predicted values, $\stackrel{-}{y}$ is the average value of all observed values, n is the number of data points, p is the number of model parameters.

2.5. Adsorbent Characterization

The surface morphologies of Ca-Fe and Ca-Fe-La composites were observed by a scanning electron microscope (SEM, JSM-7800F Prime, JEOL, Akishima-shi, Japan). Their crystalline structures were measured by an X-ray diffractometer (XRD, SmartLab 9, Rigaku, Akishima-shi, Japan). Their surface area and porosity were determined by an automatic surface area and porosity analyzer (BET, Quadrasorb-evo^TM, Quantachrome, Boca Raton, FL, USA). Before and after phosphate adsorption, the point of zero charge of the Ca-Fe-La composite was measured by a zeta potential analyzer (Zetasizer Nano S90, Malvern Panalytical Ltd., Malvern, UK). Its surface functional groups were determined by Fourier transform infrared spectroscopy (FTIR, Nicolet 670, Thermo Fisher Scientific, Waltham, MA, USA). The chemical states of La 3d, O 1s and Fe 2p were identified by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher Scientific, Waltham, MA, USA).

3. Results and Discussion

3.1. Optimization of the Ca-Fe-La Composite

As shown in Figure 2a, the amount of phosphate uptake for the Ca-Fe composite is only 17.8 mg g⁻¹. After the doping of La³⁺, the phosphate removal performance is greatly improved. The amount of phosphate uptake is 88.9 mg g⁻¹ using 4.8 mmol of La(NO₃)₃·6H₂O during the solvothermal synthesis. This was because La³⁺ showed a very high affinity for phosphate. However, the amount of phosphate uptake does not increase significantly with further increasing of the La³⁺ content. By contrast, the amount of phosphate uptake first increases and then decreases with increasing of the solvothermal temperature. The amount of phosphate uptake is 88.1 mg g⁻¹ at 160 °C. Longer solvothermal time also contributes to the phosphate removal. The amount of phosphate uptake is 87.2 mg g⁻¹ at 4 h. In summary, the optimal conditions were 4.8 mmol of La(NO₃)₃·6H₂O, solvothermal temperature of 160 °C and 4 h of solvothermal time.

3.2. Physical Properties of Ca-Fe and Ca-Fe-La Composites

It can be clearly seen from Figure 3a that the Ca-Fe composite is mainly composed of a number of approximately spherical particles with different degrees of agglomeration. Poor porosity occurs on its surface. By contrast, the surface of the Ca-Fe-La composite is covered by some fine particles, resulting in the exposure of a larger available surface area (see Figure 3b). After the phosphate adsorption, the surface of the Ca-Fe-La composite becomes loose and porous (see Figure 3c). The N₂ adsorption–desorption isotherms and the pore size distribution of the Ca-Fe and Ca-Fe-La composites are illustrated in Figure 3d,e, respectively. According to the classification of the isotherms [21], the two curves are type IV isotherms with H4 hysteresis loops, indicating that the Ca-Fe and Ca-Fe-La composites were the mesoporous materials, accounting for 87% and 72.5%, respectively (see

Table 1. BET results of the Ca-Fe and Ca-Fe-La composites.

Absorbent	BET Surface Area (m2 g−1)	Average Pore Diameter (nm)	Pore Volume (cm3 g−1)	Micropores	Mesopores	Macropores
Ca-Fe composite	73.31	3.72	0.067	8.9%	87%	3.8%
Ca-Fe-La composite	278.41	3.51	0.197	21.5%	72.5%	6.0%

). After the doping of La³⁺, the volume of micropores increases to 21.5%. Thus, the filling of micropores also played an important role in N₂ adsorption at a low relative pressure. The pore size distributions of the Ca-Fe and Ca-Fe-La composites are very narrow, which are mainly concentrated in the range of 2–5 nm. The average pore diameters are 3.72 and 3.51 nm, respectively. After the doping of La³⁺, the BET specific surface area of the Ca-Fe composite increases from 73.31 to 278.41 m² g⁻¹. The larger surface area contributed to providing more available active sites for the phosphate adsorption [18].

3.3. Adsorption Processes

3.3.1. Kinetic Studies

The kinetic curves of the phosphate adsorption onto the Ca-Fe and Ca-Fe-La composites are illustrated in Figure 4a. The amount of phosphate uptake undergoes a rapid rise in the first 1.5 h and then a slow increase until the equilibrium is finally established. In the early stage, this trend indicated that there were sufficient active sites for the fast capture of phosphate ions [22]. Subsequently, a decrease in both the concentration gradient at the solid/solution interface and the number of active sites led to a slower rate of increase. Finally, the net adsorption capacity was zero at the point of the adsorption equilibrium. The fitted curves obtained from the fractal-like PSO model agree well with all data points, which is superior to the PSO model. From a mathematical point of view, the extra adjustable parameter h could enhance the fitting ability of the fractal-like PSO model. The PSO model assumed a time-independent rate constant, implying that the reaction process occurred in a dilute system where the reactants were uniformly distributed in space [23]. This assumption was unsatisfactory in practice, especially for a diffusion-limited process in a heterogeneous system. In general, the heterogeneity was mainly caused by the presence of pores with different sizes and shapes and different types of active sites on the solid surface. The value of h of more than zero indicated that the surfaces of the Ca-Fe and Ca-Fe-La composites were highly heterogeneous. As shown in

Table 2. Parameters and error statistics obtained by PSO, fractal-like PSO, Langmuir and Freundlich models.

Models	Parameters	Ca-Fe-La	Ca-Fe	Models	Parameters	Ca-Fe-La	Ca-Fe
PSO	$q_{\mathrm{e}}$ (mg g−1)	94.5	16.3	Fractal-like PSO	$q_{\mathrm{e}}$ (mg g−1)	104.3	19.2
	$k_2$ (g mg−1 min−1))	1.28 × 10−3	4.77 × 10−3		$k_{2,0}$ (g mg−1 min−(1−h))	1.50 × 10−3	5.73 × 10−3
	Adj. R2	0.9689	0.9539		h	0.423	0.486
	RMSE	4.78	1.05		Adj. R2	0.9996	0.9961
					RMSE	0.575	0.303
Langmuir	$q_{\max }$ (mg g−1)	93.0	15.2	Freundlich	$K_{\mathrm{F}}$ (L1/n g−1 mg−(1+1/n))	68.7	7.26
	$K_{\mathrm{L}}$ (L mg−1)	5.69	0.72		n	8.87	4.26
	Adj. R2	0.9882	0.9897		Adj. R2	0.9320	0.9388
	RMSE	3.40	0.523		RMSE	8.14	1.27

, the fractal-like PSO model has a higher value of Adj. R² and a lower value of RMSE than the PSO model. Hence, the phosphate adsorption obeyed the fractal-like PSO model based on both the fitted curves and the error statistics.

3.3.2. Isotherm Studies

As shown in Figure 4b. The amount of phosphate uptake increases with increasing of the equilibrium concentration. This was because the active sites could be sufficiently utilized at a high concentration. From a mathematical perspective, a good fit could be obtained when the distribution of the data points agreed well with the function curves represented by the isotherm models [24]. The Freundlich model was obtained by the appropriate transformation of a power function (y = x^α) [25]. Due to its divergent property, the Freundlich model did not describe the isotherm data at a high concentration. Moreover, it was also criticized for a lack of a sound thermodynamic basis because the Freundlich model did not follow Henry’s law at a low concentration. Therefore, the Freundlich model provided a good fit for the isotherm data only in a moderate concentration range. The Langmuir model not only met Henry’s law at a low concentration but also reached a plateau at a high concentration. As a result, the Langmuir model was superior to the Freundlich model for modeling of the complete isotherm data. As shown in Table 2, the Langmuir model also has a higher value of Adj. R² and a lower value of RMSE. The predicted maximum adsorption capacities for the Ca-Fe and Ca-Fe-La composites were 15.2 and 93.0 mg g⁻¹, respectively. As shown in

Table 3. Comparison of maximum adsorption capacities from different adsorbents for phosphate removal.

Adsorbents	Dosage (g L−1)	Concentration (mg L−1)	Temperature (°C)	pH	Reaction Time (h)	qmax (mg g−1)	Refs.
OMC-MgO	1.0	0–400	25	5	24	107	[3]
LC-80	0.02	0.2–3.0	23.0 ± 0.5	−	5	82.7	[4]
ML-10	0.33	25–200	25	7	24	67.6	[10]
MLC	0.1	0.5–50	25	7.0 ± 0.2	24	77.85	[12]
CeO2 (2 mL H2O)	0.1	1–20	25	4.8 ± 0.2	24	80.5	[13]
Ca0.4La0.6MnO3	0.4	5–100	25	5	24	63.01	[14]
Ca-Fe-La composite	0.5	20–120	25	−	24	93.0	This study

, the maximum adsorption capacities of different adsorbents vary considerably in a range of 63.01–107 mg g⁻¹. On the whole, the Ca-Fe-La composite had a moderate adsorption ability among these adsorbents, which was mainly attributed to the incorporation of La³⁺. In this study, the Ca-Fe-La composite exhibited a poor reusability due to the strong affinity of Ca²⁺, Fe³⁺ and La³⁺ for phosphate. It was worth noting that the Ca-Fe-La composite did not contain any toxic elements. Thus, the phosphate-loaded Ca-Fe-La composite could be used as a slow-release fertilizer and did not result in secondary pollution, thereby achieving waste management and resource utilization. To sum up, the environmentally friendly Ca-Fe-La composite had a good prospect for advanced removal of phosphate from aqueous solutions.

3.3.3. Effect of pH

Obviously, the phosphate adsorption depends highly on the initial pH of the solution (see Figure 5a). The amount of phosphate uptake is only 12.0 mg g⁻¹ at pH = 2 since a part of the Ca-Fe-La composite was dissolved in the solution. The amount of phosphate uptake gradually decreases when the initial pH of the solution increases from 3 to 11. The pH of the solution affected not only the chemical forms of the solute but also the charge state of the solid surface [26]. As shown in Figure 5b, H₂PO₄⁻ dominates in a pH range of 3–7, which has a strong binding ability for the surface active sites [27]. The Ca-Fe-La composite before the phosphate adsorption has an isoelectric point at a pH of 8.0 (see Figure 5c). In this case, the protonated surface for the Ca-Fe-La composite caused it to carry more positive charges [28], which contributed to enhancing the electrostatic attraction between H₂PO₄⁻ and the adsorbent. After the phosphate adsorption, numerous negatively charged phosphate anions adhered to the adsorbent and thus the isoelectric point decreased to 5.8. When the pH value of the solution was more than 7.20, H₂PO₄⁻ was converted to HPO₄²⁻. The higher free energy of adsorption for HPO₄²⁻ resulted in its weaker ligand-exchange ability with surface hydroxyl groups [29,30]. Meanwhile, at a higher pH, the electrostatic repulsion caused by the negatively charged surface inhibited the phosphate adsorption [31] and the competition between OH⁻ and HPO₄²⁻ for the active sites further interfered with the phosphate adsorption.

3.3.4. Effect of Coexisting Anions

Real wastewater always contains a variety of inorganic anions that interfere with phosphate adsorption because of their competition for the active sites on the adsorbent surface [32]. Therefore, it was essential to evaluate the selectivity of the Ca-Fe-La composite for phosphate anions in a binary system. As shown in Figure 5d, the amount of phosphate uptake is 92 mg g⁻¹ (see dashed line) in the absence of other anions. In all cases, the coexisting inorganic anions have a negative effect on the phosphate adsorption and the interference effects become more significant with an increase in their concentrations. The phosphate adsorption is slightly affected in the presence of Cl⁻ and NO₃⁻. This was due to the weak outer-sphere complex between Cl⁻/NO₃⁻ and the Ca-Fe-La composite [33]. By contrast, SO₄²⁻ inhibits the phosphate adsorption strongly. This was because (i) SO₄²⁻ (r = 2.30 Å) and PO₄³⁻ (r = 2.38 Å) had the similar ionic radius and tetrahedral structure [10], resulting in the occupation of the active sites by SO₄²⁻ as an effective ligand; (ii) lone pair electrons provided by SO₄²⁻ could occupy the unfilled d orbitals of metal ions in the Ca-Fe-La composite [34]. CO₃²⁻ and HCO₃⁻ also interfere with the phosphate removal significantly. On the one hand, the hydrolysis of CO₃²⁻ and HCO₃⁻ occurred in the solution and thus produced more OH⁻ ions, which were not favorable for the phosphate removal [35]. On the other hand, La³⁺ had a higher affinity for CO₃²⁻ and HCO₃⁻ because the solubility product constant of La₂(CO₃)₃ (K_sp = 3.98 × 10⁻³⁴) was much lower than that of LaPO₄ (K_sp = 3.72 × 10⁻²³) [36].

3.4. Adsorption Mechanisms

As shown in Figure 6a, there are no obvious diffraction peaks, indicating that the Ca-Fe and Ca-Fe-La composites are amorphous structures. After the phosphate adsorption, the new diffraction peaks appear at 2θ = 20°, 25°, 29°, 30°, 42° and 48°, confirming the presence of LaPO₄ precipitates. Figure 6b shows the FTIR spectra of the Ca-Fe-La composite before and after the phosphate adsorption. A broad peak near 3390 cm⁻¹ belongs to the stretching vibration of hydroxyl groups (–OH). The characteristic peaks at 670 and 615 cm⁻¹ are assigned to the bending vibration of La–O and the stretching vibration of Fe–O, respectively [12,37]. The peak at 848 cm⁻¹ is assigned to the La–OH vibration [38]. The peaks at 1418 and 1565 cm⁻¹ are associated with the asymmetric stretching vibration of CO₃²⁻ [39]. The presence of CO₃²⁻ in the Ca-Fe-La composite might be caused by the decomposition of urea during the solvothermal reaction. After the phosphate adsorption, the peak of –OH stretching vibration is shifted to 3393 cm⁻¹, indicating the interaction between phosphate and hydroxyl groups through the ligand exchange [40]. The peak at 1636 cm⁻¹ is attributed to the bending vibration of –OH [41]. The intensity of CO₃²⁻ stretching vibration weakens significantly after the phosphate adsorption, implying that the ion exchange between CO₃²⁻ and phosphate occurred [39]. Additionally, the absorption peaks of La–OH, La–O and Fe–O almost disappear due to the interaction between the Ca-Fe-La composite and phosphate. The new peaks at 1056, 616 and 540 cm⁻¹ are associated with the P–O bending vibration in PO₄³⁻ and the O–P–O stretching vibration in H₂PO₄⁻ or HPO₄²⁻ [42].

As shown in Figure 7a, a new peak at 133.3 eV appears after the phosphate adsorption, further confirming the presence of phosphorus on the Ca-Fe-La composite. It is observed from Figure 7b that the XPS spectra of La have two sets of peaks: La 3d_5/2 (835.0 and 838.7 eV) and La 3d_3/2 (851.9 and 855.4 eV). After the phosphate adsorption, the binding energies of two peaks for La 3d_5/2 were changed to 835.2 and 838.9 eV, while those of two peaks for La 3d_3/2 were changed to 852.2 and 855.7 eV. All peaks of La 3d were chemically shifted toward higher binding energies, implying that the valence band of La 3d may undergo electron transfer during the adsorption process and thus form the La–O–P inner-sphere complexation [43]. As shown in Figure 7c, two peaks of iron at 710.1 and 723.8 eV belong to the Fe 2p_3/2 and Fe 2p_1/2 orbitals, respectively, indicating that the iron element in the La-Ca-Fe composite mainly existed in the form of Fe³⁺ [44]. After the phosphate adsorption, the peaks of Fe 2p_3/2 and Fe 2p_1/2 were also shifted toward higher binding energies, implying that Fe³⁺ might be involved in the phosphate adsorption through the formation of Fe–O–P bonds [45]. As shown in Figure 7d, three peaks at 529.5, 531.5 and 533.0 eV correspond to the lattice oxygen (M–O), the hydroxyl oxygen (M–OH) and the surface-adsorbed H₂O, respectively [35,46]. After the phosphate adsorption, the percentages of lattice oxygen decrease from 14.7% to 2.2%, which was attributed to the combination of phosphate with La³⁺/Fe³⁺.

4. Conclusions

The Ca-Fe-La composite was successfully prepared by the one-pot solvothermal method. It was an amorphous material with the specific surface area of 278.41 m² g⁻¹. During the adsorption processes, the amount of phosphate uptake was easily affected by the pH of the solution. The coexisting inorganic anions had a negative effect on the phosphate adsorption, especially SO₄²⁻, CO₃²⁻ and HCO₃⁻. In this study, the phosphate adsorption obeyed the fractal-like PSO kinetic model and the Langmuir isotherm model. The fractal-like PSO rate constant was 1.50 × 10⁻³ g mg⁻¹ min^−(1−h). The doping of La³⁺ contributed to promoting the adsorption capacity of the Ca-Fe composite, reaching 93.0 mg g⁻¹. The phosphate-loaded Ca-Fe-La composite could be used as a slow-release fertilizer, achieving waste management and resource utilization. The phosphate removal mechanisms were mainly inner-sphere complexation and electrostatic attraction.