Removal of Phosphate from Wastewater with a Recyclable La-Based Particulate Adsorbent in a Small-Scale Reactor

Highlights

• Tailored La-based particles (La-CZ) sorb phosphate in wastewater.
• The adsorption capacity of La-CZ particles was 18.2 mg g⁻¹ (PO₄³⁻).
• The adsorbed phosphate mainly existed in a stable state.
• La-CZ particles were recovered easily in the small-scale reactor designed in this study.
• Effluent concentrations <0.1 mg/L PO₄³⁻ were achieved in the reactor when treating river water.

Abstract

It is crucial to develop an effective and easily recoverable phosphate absorbent for the control of eutrophication problems in polluted rivers. In this study, a stable particulate adsorbent with a diameter of 5 mm synthesized from lanthanum, activated carbon, and zeolite (La-CZ) was developed, characterized, and tested for the removal of phosphate from wastewater in a small-scale reactor, which was designed to easily recycle La-CZ with a basket. Batch studies showed that La-CZ could reach adsorption equilibrium within 2 h and the maximum phosphate sorption capacity was 18.2 mg g⁻¹. The experimental data showed good compliance with the Langmuir isotherm model and pseudo-second-order kinetic model, implying that chemisorption dominates the phosphate uptake process. La-CZ exhibited a stable adsorption capacity over a wide pH range (3–7), while the adsorption capacity decreased slightly under alkaline conditions. Although Nitrates (NO₃⁻) and Carbonate (CO₃²⁻) had some effects, normal coexisting ions such as Chloride (Cl⁻), Sulfate (SO₄²⁻), and Fluorine (F⁻) had no significant effects on the phosphate adsorption capacity of La-CZ. The main form of phosphate removed from the reaction system was HCl-P (77.68%), as determined through phosphorus fractionation. In particular, this study designed a replaceable filler-type reactor integrating a reflux and aeration system, 98.8% of phosphorus could be removed from actual wastewater, and La-CZ could be reclaimed easily. This work provides an excellent reference for particulate adsorbents that can efficiently remove phosphate in practical applications in the future.

1. Introduction

Currently, excessive release of phosphate to river bodies leads to outbreaks of algae and the aggravation of water quality [1,2], which has resulted in difficulties in improving the water quality of 80% of rivers and lakes in China [3]; accordingly, phosphate pollution has been regarded as a pressing environmental problem [4]. Although urban sewage treatment plants have adopted various interception measures to treat phosphorus-containing wastewater to keep the concentration of phosphorus from tailwater in as low a range as possible, with the release of endogenous phosphorus from sediments, the eutrophication of river water bodies has not improved [5].In view of this situation, some strategies, such as physical, chemical, and other biological methods, were investigated to remove phosphate from slightly polluted wastewater in recent years [6,7]. In contrast to biological methods commonly used for denitrification, it is difficult to remove phosphorus by using microorganisms [8], and the use of chemical methods will significantly increase the amount of sewage sludge. Therefore, the adsorption method is the most widely used method in physical wastewater treatment and can remove phosphate from wastewater by the formation of precipitation of phosphorus [9].

The adsorption method has attracted wide attention due to its low cost, high efficiency, low energy consumption, and easy recycling, and it is considered to be one of the most practical technologies. The relevant absorbents include bauxite, limestone, zeolites (minerals) [10], laterite, marl (soils) [11], red mud, fly ash (industrial byproducts) [12], and active carbon (manufactured products) [13]. Activated carbon and zeolite, due to their rich pore structure and huge specific surface area, are widely used as adsorbents in metallurgical, chemical and environmental protection industries, which have a certain adsorption capacity for phosphate. Activated carbon, as hydrophobic and non-polar material, has a good removal effect on color, odor, and organic matter in wastewater, while it has a poor effect on polar substances such as ammonia nitrogen. Zeolite, as a hydrophilic and polar material, has a good selective adsorption effect on ammonia nitrogen. Therefore, in recent years, authors have continuously conducted research on the combination of activated carbon and zeolite for wastewater treatment, or explored the combination of both into a new material for wastewater treatments, studies have not only considered the improvement in the overall adsorption capacity after combining adsorbents [14] but also investigated the use of multiple rare metal oxides to effectively remove contaminants in water bodies, profiting from the high adsorption capacities and certain special physical and chemical traits of such materials [15]. Lanthanum (La) is an abundant and cheap rare element with a special electronic layer structure [16], which is an electropositive element with a large atomic radius. These special structures make lanthanum highly chemically active and well biocompatible. La has strong Lewis acidity that exhibits strong ligand adsorption to phosphate, thus forming lanthanum-phosphate complexes [17,18], and its compounds, such as LaCl₃, La (OH)₃, and La₂O₃, exhibit good adsorption properties for phosphate. Zou et al. [19] used two different methods to load La₂O₃ onto ceramic particles, and the removal efficiencies of the adsorbents reached 98.1% and 99.8% at pH = 4, respectively. The energy spectrum results showed that the La content of the modified adsorbent was 42% (mass %), some of which were present in the endopores while others were loaded on the surface of the ceramic particles. Elsergany et al. [20] prepared lanthanum-modified chitosan (La-Ct) as a phosphorus adsorbent and compared it with aluminum-modified bentonite (Al-Bt) for the removal of phosphorus from wastewater. The result showed that both adsorbents could effectively remove phosphorus, and La-Ct was more effective than the Al-Bt adsorbent, in which adsorption capacity reached 17.9 mg/g. Therefore, developing a La-based absorbent for removing phosphate might be an effective strategy with prospects. Therefore, a composite prepared from the above materials could be a new type of adsorbent for removing phosphate.

Studies on the abovementioned adsorbent materials have mainly focused on improving the adsorption capacity by material modification or on investigating the effect of different reaction conditions on adsorption [21,22] but have seldom considered the turbidity pollution of water bodies caused by the actual application of powdered materials, the difficulty of recycling or other engineering issues. Because most adsorbent particles are very small, the water may become cloudy, affecting aquatic organisms, and particle grinding will increase the engineering costs. With regard to engineering applications, we should comprehensively control the phosphorus effect, ecological risk, grinding energy consumption, and other factors and consider the use of large particle sizes and easy-to-recover adsorbent materials. Avoiding increases in the turbidity and suspended solids (SS) content of the treated water is also a necessary feature of adsorbents. Therefore, developing highly efficient, stable, easily recovered adsorbents to remove phosphate from wastewater is significant for practical engineering applications.

In this study, La was loaded on activated carbon and zeolite (CZ) by a simple precipitation method, and prepared as a granular material; In particular, this study designed a replaceable filler-type reactor integrating a reflux and aeration system, which was designed to easily recycle La-CZ with a basket. In addition, this study explored the adsorption kinetics and thermodynamic characteristics of La-CZ for phosphorus in an aqueous solution, investigated the binding form and mineral composition before and after phosphorus adsorption, considered the effect of pH and coexisting anions on the adsorption effect and stability, and revealed the effect of La-CZ. The adsorption mechanism provides a theoretical basis for subsequent engineering applications. This study is also preparatory work for the application of particle absorbents in the treatment of phosphorus-polluted wastewater.

2. Materials and Methods

2.1. Preparation of Adsorbents

La-CZ was prepared by a modified stirring method [23]. After pre-experiments (Figures S1 and S2), we determined the proportion of coal-based activated and calcium zeolite and the La concentration. Active carbon (30 g) and zeolite (30 g) were added to a triangular flask containing 0.6 mg L⁻¹ LaCl₃ (200 mL). The mixed solution was continuously stirred for 12 h and dried at 80 °C to obtain the mixed powder. Then, the mixed powder was calcined at 100 °C for 2 h, washed with deionized water three times, and dried at 60 °C to obtain La-CZ in the powder state. The powder was shaped into a granular material with a diameter of approximately 5 mm by using a pill-making machine and then fixed with epoxy resin to obtain the final La-CZ in the particle state.

2.2. Characterization of the Adsorbents

The surface morphologies were observed with scanning electron microscopy (SEM; Supra 55; Sapphire, Germany; accelerating voltage: 0.02~30 kV). The structure of La-CZ was analyzed by X-ray diffraction (XRD) with a 1θ scan rate of 8 min⁻¹ (rated power, 12 kW, diffraction patterns: 2θ = 10–100°). The physicochemical properties and microstructure of the La-CZ were measured by energy dispersive spectroscopy (EDS) (Kuroki et al., 2014).

2.3. Adsorption Batch Studies

Adsorption batch studies were conducted to test the properties of La-CZ for the removal of phosphate. Experiments on the influence of dosage on reaction time, adsorbent dosage (0.1, 0.2, 1.0, 2.0, 5.0 g L⁻¹), solution pH (3, 5, 6, 7, 8, 10), and coexisting anions with the same concentration as phosphate (CO₃²⁻, NO₃⁻, Cl⁻, SO₄²⁻, F⁻) on the removal of phosphate were examined with 20 mg L⁻¹ phosphate solution. In the adsorption kinetics experiment, the phosphate concentration was 20 mg L⁻¹. The initial concentrations of phosphate in the adsorption isotherm experiment ranged from 1 to 20 mg L⁻¹.

The molybdenum blue method was utilized to investigate the phosphate concentration with Spectrophotometer-UV-2550 at 700 nm. In the experimental results, the concentration of phosphorus was calculated as phosphate (PO₄³⁻). All of the experiments were practiced in triplicate.

The equilibrium adsorption capacity and removal rate were calculated as Formulas (1) and (2):

$$r=\frac{C_0-C_t}{C_0}\times 100\%$$

(1)

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

(2)

r is the phosphorus removal rate; Ct is the adsorption equilibrium phosphorus concentration (mg L⁻¹); C₀ is the initial phosphorus concentration (mg L⁻¹); q_e is the phosphorus adsorption capacity (mg g⁻¹); m is the adsorbent dosage (g); V is the water sample volume (L).

2.4. Fractionation of the P-Saturated Absorbent

The La-CZ phosphorus sorption retention mechanism could be performed by the fractionation of the P-saturated absorbent. First, 1.0 g La-CZ was added to 100 mL of a 10 mg L⁻¹ PO₄³⁻ solution, and a shaker-controlled temperature (at 25 ℃) was used to thoroughly mix the components at 150 rpm. After reaching adsorption equilibrium, the solution was centrifuged at 4000 rpm for 5 min. The precipitate obtained by centrifugation was successively extracted with 1 mol L⁻¹ NH₄Cl, 0.11 mol L⁻¹ Na₂S₂O₄/NaHCO₃, 0.1 mol L⁻¹ NaOH and 0.5 mol L⁻¹ HCl to obtain loosely bound phosphorus (NH₄Cl-P), redox-sensitive phosphorus (BD-P), aluminum-bound phosphorus and some organic phosphorus (NaOH-P) and calcium-bound phosphorus (HCl-P). All of the experiments were carried out in triplicate.

3. Results and Discussion

3.1. Absorbent Characterization

The morphology and structure of absorbents are significant for the adsorption of phosphate. Therefore, stereoscopy, polarizing microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy dispersive spectroscopy (EDS) were employed to verify the properties of La-CZ. A stereoscope was used to observe the overall shape of the particulate material (Figure 1a). The diameter of the material was approximately 5 mm, and the surface was rough. It seemed that some structures (Figure 1b) had formed on the surface of the adsorbent. As shown in Figure 1a,b, a layer of the particulate matter covered the surface of the material, and uneven folds were formed on the surface. From the porosimetry data (

Table 1. The BET surface area, total pore volume, and average pore diameter of La-CZ and CZ.

Sample	BET Surface Area (m2·g−1)	Total Pore Volume (cm2·g−1)	Average Pore Diameter (nm)
CZ	15.53	0.05	1.41
La-CZ	32.34	0.20	16.68

), compared with the CZ, La-CZ has improved the BET Surface area, total pore volume, and average pore diameter. The surface performance of the adsorbent material under these conditions was good, the surface material distribution was relatively uniform, and there was no clumping, a phenomenon that was indicated by white spots on the surface of the materials in Figure 1a,b.

To confirm that the supporting material was indeed loaded with the key element La during synthesis, we further tested the actual composition of La-CZ with XRD and EDS. The crystalline structures of La-CZ were characterized by XRD (Figure 2). The observed diffraction peaks of La correspond to JCPDS Card 36–0815 (La), and La was proven to be well loaded on CZ as the diffraction peaks of La appeared in La-CZ. The point „spectrum 1” was selected to perform EDS energy spectrum analysis on the element composition of the La-CZ surface [24]. The weight percentages and atomic percentages of La could confirm that La was successfully loaded on La-CZ, SEM mapping also showed that La is uniformly distributed on the surface of the material (Figure 3). The weight percentages and atomic percentages of the elements also confirmed that La was successfully loaded on La-CZ.

3.2. The Adsorption Capacity of La-CZ

The adsorption capacity of La-CZ was verified via phosphate adsorption. The dose of La-CZ was an important factor for phosphate removal. As shown in Figure 4a, different dosages all reached adsorption equilibrium within 2 h, and the amount added did not seem to affect the time when the material reached adsorption equilibrium. The removal efficiency of phosphate increased with increasing dosage: values of 51.80%, 70.50%, 73.38%, 79.14%, and 84.89% were achieved when adding 0.1 g L⁻¹, 0.2 g L⁻¹, 0.5 g L⁻¹, 1.0 g L⁻¹, and 2.0 g L⁻¹ La-CZ, respectively. The removal efficiency of phosphate did not double when the amount of La-CZ added increased from 1.0 to 2.0 g L⁻¹ (Figure 4b). When an absorbent is put into production and application, the utilization efficiency and the economic budget are important factors that need to be considered [25]. Therefore, although the maximum adsorption capacity was slightly higher at a dosage of 2.0 g L⁻¹, 1.0 g L⁻¹ absorbent served as the optimal dosage for future technical applications. The maximum adsorption capacity of La-CZ toward phosphate reached 18.2 mg g⁻¹. De* is defined as the dosage of material per liter of wastewater that degrades 20 mg L⁻¹ phosphorus to the sewage discharge standard (0.5 mg L⁻¹) (

Table 2. Comparison of phosphorus removal capacity of particulate adsorbents and some La-based adsorbents.

Adsorbent	Initial Phosphorus Concentration/(mg·L−1)	Dosage/(g·L−1)	Particle Diameter/(mm)	qe/(mg·g−1)	Equilibration Time/(h)	De*/(mg)	References
ATP Clay particles	20	8	3	2.8	14	6.96	[26]
fine particles of waste concrete	20	20	1	4.96	5	3.93	[27]
Activated carbon fiber-LaOH	30	2.5	powdered form	16.4	6	1.18	[28]
La-doped activated carbon fiber	20	5	2	9.41	4	2.07	[29]
La-modified bentonite	100	2.5	0.84	10.6	20	1.84	[30]
NT-25La	0.5	0.75	powdered form	14.4	2	1.35	[31]
La-CZ	20	1	5	18.1	2	1.07	This study

D_e^*_: The dosage of materials that degrade 20 mg L⁻¹ phosphorous to the sewage discharge standard (0.5 mg L⁻¹) of per liter wastewater.

). The De* of La-CZ was higher than that of most other particle adsorbents and other La-based adsorbents. As an adsorbent material with large particles, La-CZ not only could have a superior adsorption effect but will not be affected by the reduction in surface area. Moreover, the aggregation of large particles will avoid the loss of phosphorus binding sites, so the phosphorus adsorption efficiency of La-CZ may not be evidently reduced.

3.3. Matrix Effects on Phosphate Removal Capacity

3.3.1. Effect of pH

The optimum conditions for phosphate adsorption were determined under different initial pH values from 3 to 10 (Figure 5). La-CZ showed high stability in near-neutral-pH solutions; the adsorption of phosphate was approximately 18.0 mg g⁻¹ and was significantly inhibited under neutral or acidic conditions (initial pH = 5~7). However, under low-pH conditions (pH = 3), the adsorption capacity decreased slightly to 16.7 mg g⁻¹. In addition, an increase in pH led to a decrease in phosphate sorption due to a change in the phosphate species and a reduction in the number of surface sites, so when the pH increased to 8, the adsorption capacity decreased to 16.2 mg g⁻¹; when the pH was higher, the adsorption capacity decreased to 7.1 mg g⁻¹.

Under acidic conditions, phosphate in water mainly exists in the form of H₃PO₄, making it difficult for phosphate to be adsorbed by La-CZ [32]. As the pH increases, the hydroxyl ions in water gradually increase as well [25] and will compete with phosphate for adsorption, which might be the reason that pH conditions that were too low or too high inhibited the adsorption of phosphate by La-CZ. In addition, some authors have found that when the environmental pH is lower than the isoelectric point of an adsorbent, the surface of the adsorbent proves to be positively charged [33], and vice versa. The isoelectric point of most lanthanum-based adsorbents is approximately 6. Acid-base potentiometric titration was used to determine the isoelectric point of La-CZ, and the results showed that La-CZ has an isoelectric point of approximately 6.8. Therefore, when the pH in the solution was lower than 5, most of the phosphate ions were present in the form of H₃PO₄, and the adsorption of La-CZ decreased slightly; when the pH was higher than 7, there was a certain amount of electrostatic repulsion between the negative charges on the surface of the adsorbent and phosphate ions. Moreover, the lanthanum ions underwent hydrolysis to form -OH [34], which led to a decrease in the number of phosphorus adsorption sites, which resulted in inhibition of the adsorption capacity of La-CZ for phosphate ions. Previous studies found that phosphorus removal agents have the highest adsorption efficiency for phosphorus in a solution with a pH of 5–7, and when the pH of the solution increases to 9, the adsorption efficiency of the phosphorus removal agent decreases. Hamdi et al. [35] also found that when the pH increased, the adsorption capacity of phosphorus removal agents for phosphorus was lower, which is similar to the results of this experiment. The results of this experiment showed that a slightly acidic water environment promotes the adsorption of phosphorus by La-CZ. La-CZ is suitable for slightly acidic or neutral water bodies but should be used with caution in alkaline water bodies.

3.3.2. Effect of Coexisting Anions

There are many kinds of anions in actual water bodies, and these anions may affect the adsorption effect of La-CZ. Competitive ion experiments were performed to assess the selectivity of La-MC for phosphate adsorption. Figure 6 shows the fluoride and phosphate removal adsorption capacities of La-CZ in the presence of some common anions (CO₃²⁻, NO₃⁻, Cl⁻, SO₄²⁻, F⁻). The results showed that there was insignificant interference from competitive anions (Cl⁻, SO₄²⁻, F⁻) at the same concentration of PO₄³⁻. The adsorption capacity of phosphate remained at approximately 18.2 mg g⁻¹. Some studies have shown that the presence of fluoride may have some effect on phosphoric acid adsorption in the case of physical adsorption, but La-CZ maintained a high adsorption capacity for phosphoric acid in the presence of fluoride ions. A minor effect of F⁻ on the removal of phosphate could demonstrate that only a handful of phosphate is removed by physical adsorption [36] and could also infer that the process of phosphate removal was chemical precipitation [37] (Jiang et al., 2019). HCO₃⁻ is the hydrolyzed form of CO₃²⁻; and because of the competition from active sorption sites on the surface of the adsorbent [38], HCO₃⁻ can form an inner-sphere compound with phosphate. In addition, after the hydrolysis of CO₃²⁻, the solution is partially alkaline [39]. The pH experiment verified that the adsorption ability of La-CZ was reduced with high pH, so the coexistence of CO₃²⁻ and NO₃⁻ will reduce adsorption. In the treatment of industrial, electroplating, and other forms of wastewater containing a large number of nitrates, the treatment efficiency might be affected, in which case the dosage may have to be increased or combined denitrification methods may need to be implemented to achieve superior selectivity toward phosphate.

3.4. Phosphate Adsorption Mechanism

3.4.1. Sorption Kinetics

To better understand the phosphate sorption process in La-CZ, two typical adsorption kinetic models were utilized to fit the experimental data in this study: pseudo-first-order kinetics (Equation (3)) and pseudo-second-order kinetics (Equation (4)) (Wu et al., 2006).

$$\ln \left(q_e-q_t\right)=\ln q_e-k_{1}t$$

(3)

$$\frac{t}{q_t}=\frac{1}{k_2{q}_e^2}+\frac{t}{q_e}$$

(4)

In Equations (3) and (4), t is the adsorption time, h; q_t is the time, t; q_t is the adsorption capacity of the adsorbent for phosphorus, mg g⁻¹; q_e is the phosphorus adsorption capacity of the adsorbent at adsorption equilibrium, mg g⁻¹; and k₁ and k₂ are pseudo-first-order and pseudo-second-order constants, respectively.

The results of the kinetic models with R² values are shown in Figure 7a and

Table 3. Kinetic parameters of phosphate adsorption by LA-CZ.

Pseudo-First-Order Model			Pseudo-Second-Order Model
qe (mg·g−1)	K1 (min−1)	R2	qe (mg·g−1)	K2 (min−1)	R2
17.9202 ± 0.3084	0.0179 ± 0.0012	0.9942	20.9195 ± 0.2465	0.0011 ± 0.0001	0.9989

. The pseudo-second-order model (R² value of 0.998) provided a better fit than the pseudo-first-order model (R² value of 0.993) to the experimental data. Previous studies [40,41] have suggested that when the experimental data fit the pseudo-first-order and pseudo-second-order models (with R² values usually higher than 0.990), the sorption system proved to be chemical sorption by share or interchange of electrons between the adsorbent and adsorbate.

3.4.2. Sorption Isotherm

Adsorption thermodynamic model fitting: the Langmuir (Equation (5)) and Freundlich (Equation (6)) equations were utilized to fit the experimental data.

$$Q_e=\frac{k_L{Q}_m{C}_e}{1+k_L{C}_e}$$

(5)

$$Q_e=K_F+C_e^n$$

(6)

In Equations (5) and (6), Q_e is the adsorption amount of phosphorus by the adsorbent at adsorption equilibrium, mg g⁻¹; Q_m is the theoretical saturated adsorption amount of phosphorus by the adsorbent, mg g⁻¹; C_e is the concentration of phosphorus in solution at adsorption equilibrium, mg L⁻¹; and K_L, K_F, and n are parameters corresponding to the Langmuir and Freundlich equations.

As shown in Figure 7b and

Table 4. LA-CZ adsorption isotherm parameters of phosphate.

Langmuir			Freundlich
Qmax (mg·g−1)	KL (L·mg−1)	R2	Ff (L·mg−1)	b	R2
14.6417 ± 0.732	−0.1196 ± 0.0247	0.9669	20.7702 ± 0.3196	3.4686 ± 0.2201	0.9975

, the isotherm data fitted both in the Langmuir and Freundlich models, and the Langmuir isotherm model (R² = 0.996) showed a higher correlation coefficient than the Freundlich isotherm model (R² = 958), proving the homogeneous monolayer adsorption on the surface of La-CZ [35,42]. The maximum adsorption capacity of La-CZ toward phosphate was 18.2 mg g⁻¹. The sorption isotherm results combined with previous studies showed that the La-based material adsorbed phosphate via precipitation, electrostatic interactions, ligand exchange, and inner-sphere complexation [43].

3.5. Fractionation of the P-Saturated Absorbent

Absorbents intended for engineering applications must be subjected to an environmental safety assessment.

Table 5. Phosphorus fractionation after adsorbed by La-CZ.

Initial Concentration (mg L−1)	NH4Cl-P		BD-P		NaOH-P		HCl-P
	Concentration (μg g−1)	Percentage (%)	Concentration (μg g−1)	Percentage (%)	Concentration (μg g−1)	Percentage (%)	Concentration (mg g−1)	Percentage (%)
10.0	9.80 ± 0.21	6.24	0.46 ± 0.01	0.29	24.52 ± 0.06	15.61	122.29 ± 0.27	77.86
20.0	4.19 ± 0.37	2.22	1.00 ± 0.01	0.53	19.06 ± 0.03	10.09	166.37 ± 0.31	88.09

showed the content of each form of phosphorus adsorbed by La-CZ and the percentage of the total extractable form. At an initial phosphate concentration of 10 mg L⁻¹, the phosphorus adsorbed by the phosphorus removal agent was mainly in the form of HCl-P, accounting for 77.86% of the total extractable forms. The phosphorus content of NH₄Cl-P was 15.61%, and the contents of the NaOH-P and BD-P forms were relatively low, accounting for 6.24% and 0.29%, respectively. NH₄Cl-P is loosely sorbed phosphorus, including dissolved phosphorus in interstitial water and loosely adsorbed phosphorus [44]. This part of phosphorus is an easily released form, which may adhere to the surface of La-CZ in the form of ion exchange and is susceptible to wind and waves. BD-P is redox-sensitive phosphorus [45], mainly including phosphorus combined with ferrite (hydroxide) compounds and manganese compounds. When the external redox conditions change or the iron-manganese compound is oxidized or reduced, this part of the phosphorus may also be fixed or released [46]. NaOH-P mainly includes aluminum phosphorus and some complex organic phosphorus that mineralizes easily [47].

In this experiment, a small amount of the phosphorus adsorbed by La-CZ existed in the form of BD-P and NH₄Cl-P because activated carbon and zeolite can bind a small amount of phosphorus. The lanthanum ions in La-CZ captured and fixed phosphorus [48,49], forming anhydrous monazite (LaPO₄) and water-containing rhabdophane (LaPO₄·nH₂O, n ≤ 3). The HCl-P bound by La-CZ mainly existed in the form of lanthanum phosphorus minerals. HCl-P is relatively stable and is hard to release from sediments. According to the discussion above, most of the phosphorus adsorbed by La-CZ presented a stable form that was not affected by changes in hydrodynamics and redox conditions and is more practical than other forms.

3.6. Application of the Phosphate-Removal Reactor

Past studies on adsorbents have typically focused on laboratory-scale research, so La-CZ was designed and prepared to be put into practical application. After considering the diameter of La-CZ, referring to the general process of sewage treatment and integrated sewage treatment devices, and considering factors such as material recovery and replacement during sewage treatment, we designed a replaceable filler-type integrated reflux aeration reactor. As shown in Figure 8, the reactor was composed of a sand filter unit, reaction unit 1, and reaction unit 2. The two reaction units were equipped with a basket of suitable size to facilitate the replacement of La-CZ with aeration and reflux devices inside. The reaction unit parameters, such as the unit number, inlet water flow rate, aeration flow rate and return flow rate, could be adjusted according to the phosphate concentration of the influent water.

In previous experiments, we determined the best water quality parameters for La-CZ. In this experiment, we measured the best reactor parameters reactor by adjusting the aeration volume and the reflux flow rate. And then used the optimal parameters to determine the effects of backflow and aeration factors. As shown in Figure 9a, the aeration backflow group is designated group A, the nonaerated reflow group is designated group B, the aeration non-reflow group is designated group C, and the non-aeration and non-reflow group is designated group D. The time to reach the maximum removal rate followed the order A < B < C < D, and the maximum phosphate removal efficiency followed the order D < C < B < A; there was no significant difference between group B and group C. In the experiment assessing the effect of aeration rate (Figure 9b), the aeration rate was set to 5 m³/m² h, 10 m³/m² h, and 15 m³/m² h. As the aeration rate increased, the contact between La-CZ and the water body also increased gradually, so the time to reach the equilibrium was shorter, and the maximum phosphate removal efficiency increased. When the reflux rate increased (Figure 9c), the same result was also obtained. Therefore, in the actual experiment, the best removal efficiency could be attained in the shortest time when the aeration rate was adjusted to 15 m³/m² h and the reflux rate was 400 dm³/s.

The reactor was also applied in the phosphorus removal of actual river water (low phosphorus pollution, approximately 2 mg L⁻¹). After treatment with La-CZ in the reactor, the phosphate removal efficiency was nearly 100%, and the effluent phosphorus concentration was maintained at a low level that could not be detected by the molybdenum-blue ascorbic acid method. Compared with the experimental results obtained using synthetic wastewater (Figure 10), the time to equilibrium and removal efficiency were essentially not affected, while the replacement and recycling process of La-CZ was quite simple. Moreover, the turbidity of the effluent decreased from 256 to 28 FTU, which indicated the one-step removal of both phosphate and suspended solids. The efficacy of La-CZ was not interfered with by complex water factors, which indicates a high practical application value.

4. Conclusions

A stable La-based particle adsorbent named La-CZ was synthesized through a modified stirring method and used to remove phosphate from actual river water. La-CZ exhibited excellent adsorption ability over a wide pH range (3–7). Phosphate adsorption was minimally influenced by normal coexisting ions. The experimental data showed good compliance with the Langmuir isotherm model and pseudo-second-order kinetic model, suggesting that the precipitation formed by the interaction of La³⁺ with phosphate was the removal mechanism, and implying that chemisorption dominates the phosphate uptake process. As La-CZ is a large-particle material, the accumulation of materials during the adsorption process was avoided, was a reduction in phosphorus binding sites; thus, the material maintained a high adsorption efficiency for phosphorus, with an adsorption capacity of 18.3 mg g⁻¹. In addition, our study developed a replaceable filler-type integrated reflux aeration reactor and applied La-CZ in the treatment of actual river water. A nearly 100% phosphorus removal efficiency could be obtained by this reactor, and La-CZ could be recycled easily; moreover, the turbidity of the effluent was reduced from 256 to 28 FTU. The study showed that La-CZ has excellent development prospects for efficiently removing phosphate in practical applications.