Next Article in Journal
Molecular Dynamics Studies on the Inhibition of Cholinesterases by Secondary Metabolites
Previous Article in Journal
Unveiling the Photocatalytic Behavior of PNTP on Au-Ag Alloy Nanoshells Through SERS
Previous Article in Special Issue
Enhanced Peroxydisulfate Activation via Fe-Doped BiOBr for Visible-Light Photocatalytic Degradation of Paracetamol
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Catalysts
Volume 15
Issue 8
10.3390/catal15080706
catalysts-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Synergistic Sequestration and Hydroxyapatite-Based Recovery of Phosphorus by the Coupling Process of CaCl2/Modified Oyster Shell and Circulating Fluidized Bed Reactor

by
Xuejun Long
1,†,
Nanshan Yang
1,†,
Huiqi Wang
1,
Jun Fang
1,
Rui Wang
1,
Zhenxing Zhong
1,2,3,4,*,
Peng Yu
5,
Xuelian Xu
1,*,
Hao Huang
6,
Jun Wan
1,3,
Xiejuan Lu
6 and
Xiaohui Wu
6
1
School of Resources and Environment, Wuhan Textile University, Wuhan 430200, China
2
Hubei Key Laboratory of Biomass Fibers and Eco-Dyeing & Finishing, Wuhan Textile University, Wuhan 430200, China
3
Engineering Research Center of Ministry of Education for Clean Production of Textile Dyeing and Printing, Wuhan Textile University, Wuhan 430200, China
4
State Key Laboratory of New Textile Materials and Advanced Processing, Wuhan Textile University, Wuhan 430200, China
5
School of Urban Construction, Wuchang Shouyi University, Wuhan 430064, China
6
School of Environmental Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
*
Authors to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Catalysts 2025, 15(8), 706; https://doi.org/10.3390/catal15080706
Submission received: 12 May 2025 / Revised: 10 July 2025 / Accepted: 18 July 2025 / Published: 24 July 2025
(This article belongs to the Special Issue Nanocatalysts for the Degradation of Refractory Pollutants, 2nd Edition)
Browse Figures
Versions Notes

Abstract

A novel modified oyster shell (MOS-800) was developed to enhance phosphorus sequestration and recovery from wastewater. Approximately 33.3% of phosphate was eliminated by the MOS-800, which also exhibited excellent pH regulation capabilities. In semicontinuous tests, a synergistic phosphorus separation was achieved through the coupling process of CaCl2/MOS-800 and a circulating fluidized bed (CFB), resulting in an 86.5% phosphate separation. In continuous flow experiments, phosphorus elimination reached 98.2%. Material characterization revealed that hydroxyapatite (HAP) was the primary component of the crystallized products. Additionally, MOS-800 released 506.5–572.2 mg/g Ca2+ and 98.1 mg/g OH−. A four-stage heterogeneous crystallization mechanism was proposed for the coupling process. In the first stage, Ca2+ quickly reacted with phosphate to form Ca-P ion clusters, etc. In the second stage, these clusters packed randomly to form spherical amorphous calcium phosphate (ACP). In the third stage, the ACP spheres were transformed and rearranged into sheet-like HAP crystallites, Finally, in the fourth stage, the HAP crystallites aggregated on the surface of crystal seeds, also with the addition of crystal seeds and undissolved MOS-800, potentially catalyzing the heterogeneous crystallization. These findings suggest that the CaCl2/MOS-800/CFB system is a promising technique for phosphate recovery from wastewater.

1. Introduction

With the rapid development of modern manufacturing industries, sectors such as the phosphorus chemical industry, pharmaceutical production, livestock and poultry breeding, textile printing, and dyeing industries are generating significant amounts of phosphorus-containing wastewater. The wastewater poses a serious threat to the ecological environment and is raising increasing concern [1,2,3,4]. Excessive phosphorus entering urban surface water bodies can lead to eutrophication, resulting in harmful algal blooms, fish death, and disruption to the balance of aquatic ecosystems [5,6]. Additionally, phosphorus is a non-renewable strategic resource, and the irrational discharge of high-concentration phosphorus-containing wastewaters raises concerns about the sustainability of this valuable resource [7]. As a result, developing novel, eco-friendly technologies for recycling and recovering phosphorus from high-content phosphorus wastewater has become a prominent aera of research.
Conventional phosphorus sequestration methods include biological methods, chemical adsorption, chemical precipitation, etc. [8]. Biological methods are often preferred for wastewater with low phosphorus concentrations due to their effectiveness, cost-efficiency, and sustainability [9]. In contrast, adsorption techniques offer notable advantages such as ease of operation and high P-capturing efficiency [10], but face challenges related to the high cost of adsorbents and difficulties in recycling phosphorus after adsorption [11]. Chemical precipitation, typically involving Al-based or Fe-based reagents, is commonly used to treat wastewater containing high phosphorus concentrations [12]. While these methods are recognized for their rapid reaction rates and high elimination efficiencies, they require large quantities of chemicals, and the disposal of precipitate residues presents significant challenges. These limitations hinder their widespread application in phosphate-rich wastewater treatment.
In recent years, innovative phosphorus recovery methods have emerged, such as the extraction of phosphorus from wastewater in the form of struvite (MgNH4PO4·6H2O, MAP) [13], vivianite (Fe3(PO4)2·8H2O) [14,15], and hydroxyapatite (Ca5(PO4)3OH, HAP) [16,17]. For example, Qian [18] demonstrated the efficient recovery of both phosphorus and potassium from synthetic urine as MgNH4PO4 and MgKPO4 using fluidized bed crystallization (FBC). Riewklang [19] successfully reclaimed phosphorus from anaerobically digested wastewater from cassava starch processing using FBC, achieving a recovery rate of over 85%. Moreover, the HAP-based P-recovery technologies have gained increasing attention due to their low cost and potential for reusing HAP as a fertilizer or soil conditioner. This approach not only achieves high elimination efficiency, but also offer a sustainable solution for resource recovery, with potential socioeconomic benefits for developing economies [20,21]. However, there is still limited research on the characteristics and mechanisms of HAP crystallization, and further investigation is needed.
Additionally, large amounts of calcium-containing solid wastes, such as slag, fly ash, cow bones, and eggshells, are generated from industrial and agricultural activities [22,23,24,25]. From a “circular economy” perspective, reusing these calcium-rich solid wastes for phosphorus capture and recycling could yield multiple environmental benefits by reducing both the cost of solid waste disposal and the need for chemicals. Oyster shell (OS), a by-product of marine aquaculture, is primarily composed of calcium carbonate (90–95%) [26]. OS is an ideal calcium-based material that has recently been applied in wastewater treatment. However, the low reactivity of untreated oyster shells limits their direct application. Previous research showed that thermal treatment under various conditions (400–700 °C) not only removes impurities and improves physicochemical properties, but also shifts the isoelectric point (pHIEP) of oyster shells to a more alkaline range, thereby activating the material [27]. Nevertheless, most studies have focused on the simple utilization of modified OS, and its potential for phosphorus elimination and reclamation in combination with others technologies remains underexplored.
Therefore, the objectives of this study were as follows: (1) to optimize the modification conditions of oyster shell (OS) and evaluate the phosphorus separation performance of modified oyster shell (MOS); (2) to investigate the synergistic mechanisms of P sequestration in the coupling process of CaCl2/MOS and a circulating fluidized bed reactor (CFB), and to assess the influences of coexisting ions on P removal; (3) to analyze the main physicochemical properties of the MOS, the profiles of Ca2+ release, and explore the potential mechanisms of HAP crystallization in the CaCl2-MOS/CFB process.

2. Results and Discussion

2.1. Characterization of Oyster Shell (OS) and Modified Oyster Shell (MOS)

As shown in Figure 1a–d, the original oyster shell (OS) exhibited a regular lamellar structure with dimensions of approximately 30 × 62 μm. In contrast, the modified oyster shell (MOS-800) exhibited granule formations ranging from 2 to 20 μm, with a noticeable agglomeration [28]. This change in structure can be attributed to a crushing and high-temperature modification process, which resulted in smaller particle diameters and the formation of agglomeration [29].
SEM-Mapping analysis of both pristine OS and MOS-800 revealed that Ca was the dominant element of both materials (Figure 1e,f). In addition to Ca, trace amounts of Mg and Fe were also present in both the OS and MOS-800. As for OS, the Ca content was 97.2%, while the Mg and Fe contents were 1.8% and 1.2%, respectively (Figure 2a,b). In MOS-800, the Ca content slightly increased to 98.3%, while the contents of Mg and Fe decreased to 0.9% and 0.5%, respectively. These results suggest that modification led to a slight increase in Ca content, while Mg and Fe contents were reduced due to the decomposition of organic matter and the scavenge of impurities during high-temperature treatment. In addition, in Figure 2a,b, the characteristic peaks at 0.27, 0.5, and 2.1 keV in EDS corresponded to the elements C, O, and P, respectively. Furthermore, after modification, MOS-800 exhibited significantly higher BET surface area, micropore surface area, total pore volume, and average pore diameter compared to OS (Table S1). This improvement in the material’s properties can be attributed to changes in its composition and microstructure following the high-temperature modification.
As illustrated in Figure 2c,d, the characteristic peaks at 23°, 29°, and 47° for the OS corresponded to CaCO3, indicating that CaCO3 was the main component in OS. In contrast, the peaks for MOS-800 were primarily associated to CaO [30], confirming the successful conversion of CaCO3 to CaO during the modification process.
The FT-IR spectrum of OS exhibited characteristic peaks for CO32− stretching bands (Figure 3), including 710 cm−1 (v4CO32−), 876 cm−1 (v2CO32−), and 1430 cm−1 (v3CO32−) [28,31], while vibrational bands at 1799 and 2514 cm−1 corresponded to combined modes of CO32− [32]. In contrast, the FT-IR spectrum of MOS-800 showed the disappearance of CO32− peaks, while new peaks emerged. The bands between 1430 and 1480 cm−1 were attributed to the decomposition of CO2 during calcination [33], while the peak at 3640 cm−1 corresponded to the O-H stretching mode. Additionally, the spectral band between 3330 and 3542 cm−1 represented the O-H stretching and H-O-H bending vibration of H2O [34].
Figure 4a presents the wide scan XPS spectra of OS and MOS-800, revealing prominent peaks of O 1s, Ca 2p, and C 1s. As for the OS, the characteristic peak at 346.7 eV corresponded to CaCO3 (Figure 4b) [35]. However, for MOS-800, the CaCO3 at 346.9 eV decreased to 29.2%. Two new peaks appeared at 346.3 and 347.4 eV, corresponding to CaO and Ca(OH)2 at proportions of 19.5% and 51.3%, respectively [21,36]. These results indicated that CaCO3 was converted to CaO during the modification, and Ca(OH)2 was likely formed due to the reaction of CaO with moisture in the air.
Regarding the OS, the O 1s peak at 531.1 eV probably corresponded to CaCO3 with a proportion of 86.2% (Figure 4c), which is consistent with the CaCO3 composition observed in the Ca 2p spectrum. For MOS-800, the CaCO3 proportion decreased to 22.7% at 531.2 eV, while new peaks emerged at 530.6 and 531.7 eV, corresponding to CaO and Ca(OH)2 with proportions of 20.9% and 47.4%, respectively [37,38]. These results further confirmed the conversion of CaCO3 to CaO and Ca(OH)2 following modification of the OS. Notably, a weak P 2p peak was observed (Figure 4d), which may indicate trace amounts of phosphorus naturally present in the OS.

2.2. Separation Profiles of Phosphorus by the OS-Based Materials

2.2.1. Effect of Modification Temperature

The phosphorus elimination rates for OS, MOS-500, MOS-600, MOS-700, and MOS-800 after 120 min of reaction were 5.7%, 7.7%, 11.0%, 9.8%, and 33.3%, respectively (Figure 5a). MOS-800 exhibited the best performance, capturing 30.5% of phosphorus in 5 min and stabilizing after 30 min. This demonstrates that MOS-800 can rapidly and efficiently eliminate phosphate from wastewater.
Figure 5b shows that the pH value stabilized at 9.1 after OS addition, implying minimal release of alkaline substances like OH−. However, after adding MOS-500, MOS-600, and MOS-700, the pH increased to around 9.7. The addition of MOS-800 caused a rapid pH increase from 9.0 to approximately 10.9, suggesting that MOS-800 released more alkaline substances, such as OH−, contributing to the phosphate precipitation. Previous studies showed that calcite (CaCO3) transforms into CaO at 700–800 °C [39]. Jang and Kang [22] demonstrated that bovine bone calcined at 400 °C achieving 80% phosphorus sequestration in 4 h, while increasing the temperature to 600 °C reduced the reaction time to 1 h and improved pH regulation. Taking sequestration efficiency and pH regulation ability into consideration, MOS-800 was chosen for further studies.

2.2.2. Effect of Particle Size

Figure 5c illustrates that phosphorus sequestration rates for MOS-800 with particle sizes of 100–200, 50–100, 30–50, and 12–20 mesh were 33.3%, 30.4%, 27.2%, and 32.0%, respectively, after 120 min of reaction. The 100–200 mesh MOS-800 exhibited the highest phosphorus sequestration efficiency. This improvement is attributed to the smaller particle size, which increased the specific surface area and enhanced the reaction rate and phosphorus sequestration efficiency. In addition, all MOS-800 particle sizes caused a rapid increase in pH from 9.0 to 10.9, followed by pH stabilization, implying strong pH adjustment ability (Figure 5d).
Considering both elimination efficiency and pH adjustment, 100–200 mesh MOS-800 granules were selected for further study.

2.2.3. Effect of MOS-800 Dosage

As shown in Figure 5e, increasing the MOS-800 dosage from 0.05 g/L to 0.10–0.15 g/L and a further increase to 0.20 g/L, apparently improved phosphate capture from 20.1% to 33.3%, 41.8%, and 51.7%, respectively. In the initial reaction stage (0–15 min), the phosphorus capture rates reached 20.0%, 31.7%, 38.8%, and 48.4% at MOS-800 dosages of 0.05, 0.10, 0.15, and 0.20 g/L, respectively. This suggests that phosphorus capture predominantly occurred during the early reaction phase, significantly reducing the required reaction time. Moreover, a good linear correlation between dosage and phosphorus elimination was observed (R2 = 0.9915, Figure S1). These findings highlight the significant impact of dosage on phosphorus sequestration efficiency.
Moreover, after 120 min of reaction, the final pH increased markedly from 9.0 to 10.4–10.7, 10.8–11.1, 11.0–11.2, and 11.2–11.3 at MOS-800 dosages of 0.05, 0.10, 0.15, and 0.20 g/L, respectively (Figure 5f). The increase in pH was directly proportional to the MOS-800 dosage, with pH stabilization observed as the reaction approached equilibrium.
Taking both dosage and sequestration efficiency into consideration, an MOS-800 dosage of 0.1 g/L was regarded as the optimum for further studies.

2.3. Phosphorus Separation Profiles in Semicontinuous-Flow Testes of Coupling Processes

As displayed in Figure 6a, the phosphorus sequestration rates were 33.3%, 46.0%, and 86.5% in the semicontinuous MOS-800/CFB, CaCl2/CFB, and CaCl2/MOS-800/CFB processes, respectively. The synergistic effect between CaCl2 and MOS-800 significantly enhanced phosphorus capture. This synergy can be attributed to three factors: (1) the release of a relatively large number of calcium ions from MOS-800 materials, enhancing phosphate recycling; (2) the addition of crystal seeds reducing the activation energy barrier for crystallization, thus catalyzing Ca-P crystal formation on the seed surfaces; and (3) undissolved micron-sized MOS-800 particles (75–150 μm) acting as crystal seeds, facilitating heterogeneous crystallization between Ca2+ and PO43−. Previous research showed that heterogeneous crystallization promoted by seed addition significantly enhances crystal growth [15].
As seen in Figure 6b, after 10 h of reaction, the final pH in the MOS-800/CFB, CaCl2/CFB, and CaCl2/MOS-800/CFB processes changed from 9.0 to 10.8, 6.5, and 7.4, respectively. The rapid pH drop in the presence of CaCl2 can be attributed to the continuous release of H+ from H2PO4− and HPO42− during the precipitation process. In contrast, MOS-800 caused a rapid increase in pH due to its ability to release substantial amounts of OH−. As a result, the CaCl2/MOS-800/CFB process maintained a neutral pH range of 7.3–7.6 throughout the reaction, which is advantageous for phosphate precipitation and water quality management in practical applications.

2.4. Effects of Coexisting Ions on Phosphate Sequestration in the Semicontinuous CaCl2/MOS-800/CFB Process

2.4.1. Effects of Coexisting Anions

The phosphorus capture rates in the CaCl2/MOS-800/CFB process, in the presence of 0, 1, and 10 mM NO3−, were 86.5%, 85.6%, and 86.0%, respectively (Figure 7a), indicating that NO3− had a negligible effect on the phosphorus elimination efficiency. Similarly, the phosphate sequestration efficiencies in the presence of 0, 1, and 10 mM SO42− were 86.5%, 84.8%, and 80.0%, respectively, suggesting a slight inhibitory effect of SO42− on the phosphate sequestration. This may be due to the higher SO42− concentrations promoting the formation of CaSO4 precursors with some Ca2+ ions in the solution [40], thereby, reducing available Ca2+ for the phosphorus capture.
When the coexisting HCO3− concentrations were 1 and 10 mM, the phosphorus sequestration rates decreased apparently from 86.5% to 85.4% and 68.5%, respectively (Figure 7b). This suggests that higher HCO3− concentrations notably inhibit the formation of Ca-P precipitates. This inhibitory effect may stem from HCO3− converting to CO32− under alkaline conditions, which preferentially reacts with Ca2+ to form CaCO3, thereby reducing the availability of Ca2+ for phosphorus capture [41]. This finding is consistent with the XRD results (Figure 8a) where hydroxyapatite and CaCO3 were identified as the reaction products.

2.4.2. Effects of Coexisting Cations

In the presence of 0, 1, and 10 mM Mg2+, the phosphorus separation rates for the CaCl2/MOS-800/CFB system were 84.6%, 82.5%, and 87.4%, respectively (Figure 9a), indicating that the presence of Mg2+ had minimal impact on the phosphorus elimination efficiency. Similarly, the addition of 0, 1, and 10 mM NH4+-N resulted in phosphorus separation rates of 84.6%, 83.5%, and 80.9%, respectively (Figure 9a), suggesting that NH4+-N did not obviously affect the phosphorus sequestration efficiency, but did influence the reaction pH values (Figure 9b).
Furthermore, the phosphorus elimination rates increased to 84.6%, 86.4%, and 98.6% upon adding 0, 1, and 10 mM Cu2+ to the CaCl2/MOS-800/CFB system (Figure 9a), respectively, indicating that higher Cu2+ concentrations appreciably enhanced the phosphorus sequestration. Particularly, at 10 mM Cu2+ and a pH of 9.0, the phosphorus capture rate reached 91.7% without MOS-800 or CaCl2, likely due to the high reactivity of phosphate with Cu2+ to form precipitates under alkaline conditions [42].
Figure 8b shows the XRD results of various reaction products after four cations were added to the coupling process. While co-existing NH4+-N exhibited negligible influence on the XRD profiles, the simultaneous presence of 10 mM of Mg2+ and Cu2+ resulted in predominantly amorphous products, identified as amorphous calcium phosphate (ACP)). These findings align with previous studies: Lei [43] found that co-precipitation of Mg2+ with calcium phosphate significantly inhibits the transformation of ACP to HAP. In addition, Mg2+ can stabilize ACP, thereby preventing its conversion to HAP, consequently, limiting the accumulation of HAP crystals [44].

2.5. Phosphorus Recovery Profiles in Continuous Flow CFB Processes

To assess phosphate recovery from high-concentration phosphorus-containing wastewater, the CaCl2/CFB process was continuously operated for 24 h; the matching results are shown in Figure 10a. Phosphate sequestration gradually stabilized after 1 h, with an average phosphorus recovery rate (Xr) of 96.1% and an average crystallization rate (Xc) of 95.6%. The addition of crystal seed particles facilitated the crystallization process and reduced the energy barrier for crystallization. Additionally, the microcrystalline rate (Xm) of 0.5% indicated that flocculated microcrystals were formed in small proportions within the reactor.
To explore the role of the novel MOS-800 material, the CaCl2/MOS-800/CFB process was continuously operated for 24 h. From Figure 10b, under the same Ca/P molar ratio, the average phosphorus recovery (Xr) of this system was 98.2%, higher than that of the CaCl2/CFB process (96.1%), indicating that MOS-800 enhanced phosphorus sequestration. The higher sequestration efficiency may be due to the catalytic effect of Ca-based MOS materials, which increase the rate of heterogeneous nucleation during phosphorus recycling. The rough surfaces of the crystal seeds may further catalyze heterogeneous crystallization of Ca-P complexes.
The average Xc values for the CaCl2/CFB and CaCl2/MOS-800/CFB systems were 95.6% and 94.8%, respectively, indicating that both the systems exhibited good phosphorus recovery [45,46]. Furthermore, the average Xm values for the two systems were 0.5% and 3.5%, respectively, showing a moderate increase in flocculated microcrystals after adding MOS-800.
The pH variations in the systems are shown in Figure 10c, with average pH values of 8.0 and 9.3 for the CaCl2/CFB and CaCl2/MOS-800/CFB systems, respectively. The increase in pH for the CaCl2/MOS-800/CFB system was likely due to the continuous release of alkaline substances such as OH− from the MOS-800.

2.6. Characterization of Reclaimed Products

2.6.1. SEM

As shown in Figure 11a,b, the surface of the crystal seeds was relatively smooth. However, after crystallization, crystallization particles (30–50 nm) adhered to the surface, roughing it (Figure 11c–f), suggesting successful loading of HAP crystallites onto the quartz sand surfaces [47].

2.6.2. XRD, Raman, and FT-IR

Figure 12a,b show the XRD patterns of two reclaimed products (RP-CaCl2/CFB and RP-CaCl2/MOS-800/CFB), with peaks at 26°, 31°, 33°, and 47° corresponding to the Ca5(PO4)3OH crystal structure, confirming the presence of HAP crystallites on the crystal seed surfaces.
Raman spectra (Figure 12c) show peaks at 465 and 963 cm−1, associated with Si-O and P-O stretching vibrations, respectively [48,49], while the additional peaks at 425, 590, 963, and 1043 cm−1 corresponded to the vibrations of v2PO43−, v4PO43−, v1PO43−, and v3PO43− groups in HAP, respectively [50,51].
FTIR spectra (Figure 12d) showed characteristic peaks at 3580 and 647 cm−1 corresponding to -OH stretching vibration, while the peaks at 1630 and 3440 cm−1 were ascribed to H2O molecules [52]. Peaks at 1030 and 1095 cm−1 were due to PO43− asymmetric stretching [53], and those at 574 cm−1 corresponded to the phosphate bending vibration [54]. In addition, the characteristic peak at 877 cm−1 corresponded to the bending vibrational mode of v2CO32− [55], and the presence of peaks at 1420 and 1460 cm−1 indicated the presence of CaCO3 [56].

2.6.3. XPS

Figure 13a shows the wide scan XPS spectra of crystal seeds and two reclaimed products (RP-CaCl2/CFB and RP-CaCl2/MOS-800/CFB). The presence of Ca 2p and P 2p, demonstrated in both reclaimed products, suggests the formation of calcium phosphorus complexes. In the O 1s spectrum (Figure 13b), characteristic peaks at 531.2 eV in RP-CaCl2/CFB were attributed to P-O bonds [57], and peaks at 532.5 eV corresponded to SiO2 [58]. Peaks at 530.1 and 533.4 eV indicated the presence of CaCO3 and H2O molecules, respectively [59]. In RP-CaCl2/MOS-800/CFB, peaks at 532.6, 531.1, and 529.9 eV corresponded to SiO2, P-O, and CaCO3, respectively, with relative intensities of 18.5%, 23.4%, and 21.2%.
Figure 13c shows new Ca 2p peaks at 348.0 and 348.1 eV in both reclaimed products attributed to HAP [60]. The content of CaSiO3 decreased from 61.3% in quartz sand to 37.5% and 29.2% in the respective products [61]. Peaks at 346.4 and 346.3 eV were attributed to CaCO3 [19].
In Figure 13d, characteristic peaks at 132.9 and 133.9 eV corresponded to PO43− and HPO42− [30,62], with proportions of 73.5% and 26.5%, respectively. As for RP-CaCl2/MOS-800/CFB, the proportions changed to 68.0% and 32.0%, respectively, indicating that HAP was the predominant phosphate form in the crystallized products.

2.7. Leaching Properties of MOS-800 and Supersaturation Index

2.7.1. Leaching Characterization of MOS-800

When MOS-800 was dissolved in 1 mol/L HCl, Ca2+ was rapidly released, reaching a concentration of 56.8 mg/L at 5 min and stabilizing at 57.8 mg/L. The Mg2+ concentration remained consistently below 0.3 mg/L, and total Fe was undetectable (Figure 14a). In a dilute hydrochloric acid solution with a pH of 2.5, the Ca2+ concentration initially reached 43.6 mg/L at 5 min, ultimately stabilizing at 55.4 mg/L, with the pH rising from 2.5 to 3.7 over this period (Figure 14b).
As shown in Figure 14c,d, when MOS-800 was dissolved in neutral (pH = 7.0) and weakly alkaline (pH = 9.0) solutions, the initial Ca2+ concentrations were 36.1 and 36.7 mg/L, respectively, both lower than those observed under acidic conditions. As the reaction progressed, the Ca2+ concentration stabilized at 36.4 and 50.3 mg/L, while the pH values increased from the pH 7.0 and 9.0 to 11.8 and 11.5, respectively. These results indicated that MOS-800 effectively releases significant amounts of Ca2+ and alkalinity under neutral and weakly alkaline conditions, which enhances the phosphate sequestration efficiency [28].
Interestingly, after 60 min, both the Ca2+ concentrations and pH values decreased simultaneously. This was likely due to the reaction between OH− and CO32− with Ca2+ in the strongly alkaline solution, leading to the formation of Ca(OH)2 and CaCO3 precipitates, which in turn reduced both Ca2+ levels and pH values [63]. As shown in Figure S2b, the Ca2+ fraction decreased dramatically when the pH exceeded 11, aligning with the results of the leaching tests.
Figure 14e shows that the highest amount of Ca2+ released (572.2 mg/g) occurred when MOS-800 was immersed in 1 mol/L HCl. The release of Ca2+ also reached 544 and 506.5 mg/g when the pH was 2.5 and 9, respectively. These results suggest that MOS-800 has a strong capacity to release Ca2+ under both weakly acidic and weakly alkaline conditions.
Additionally, Figure 14f illustrates the total alkalinity (as CaCO3) and OH− concentrations released by MOS-800 in deionized water. After 30 min, the total alkalinity reached 125.1 mg/g, with 111.6 mg/g OH− released. After 120 min, both the total alkalinity and OH− concentrations declined to 109.1 and 98.1 mg/g, respectively. These findings indicate that MOS-800 effectively releases substantial amounts of alkaline substances, such as OH− and CO32−, which increases the system’s pH and facilitates phosphorus elimination.

2.7.2. Supersaturation Index of Possible Ca-P Complexes

The supersaturation index (SI) for various Ca-P complexes, shown in Figure 15a, reveals that the SI for HAP was considerably higher than those for other Ca-P complexes within the pH range of 8–10, suggesting that HAP is the dominant crystalline product [64]. These results are consistent with the characterization findings from XRD, Raman, and XPS.
As shown in Figure 15b, when Ca2+ reacted with phosphate for just 1 min, the resulting precipitate formed was primarily amorphous. However, after 15 min, the precipitate was converted to poorly crystalline hydroxyapatite, which is consistent with previous observations [65]. Interestingly, HAP crystals and ACP coexisted during the early stage of crystallization [48]. Furthermore, in the case of RP-CaCl2/MOS-800/CFB, the proportion of ACP decreased, while the crystalline HAP content increased (Figure 15c,d), confirming the shift toward more crystalline HAP, as reported in earlier studies on hydroxyapatite crystallization [66].

2.8. Heterogeneous Crystallization Mechanisms in the CaCl2/MOS-800/CFB Process

When the CaCl2/MOS-800/CFB coupling process was applied to eliminate and reclaim phosphate from high-phosphorus wastewater, the crystalline degree of crystallized products was significantly enhanced. This can be attributed to the substantial release of Ca2+, OH−, and other ions from MOS-800, which facilitated the formation of HAP crystals.
Combining the experimental findings with the SI index, the heterogeneous crystallization processes of phosphate in the CaCl2/MOS-800/CFB system can be divided into four phases (Figure 16).
In the first stage (S1), the pH sharply decreased from about 9.0 to 7.7 upon mixing the Ca-based materials and the phosphate solutions (Figure 16a). This rapid pH decrease is likely due to the consumption of free PO43− ions and the release of H+, driven by the formation of Ca-P ion clusters (Ca9(PO4)6), amorphous calcium phosphate (ACP), and Ca-P complexes [44]. Notably, the various Ca-P precipitates predominantly existed as flocs (Figure 16b and Figure S2c).
In the second stage (S2), the pH stabilized at a relatively constant level, which was mainly due to the randomly packed Ca-P ion clusters into sphere-like ACP particles, leading to the stabilization of ACP (Figure 16c). Similar observations were noted in previous studies [67].
In the third stage 3 (S3), the pH gradually declined from 7.7 to about 7.4 as the sphere-like ACP particles transformed into sheet-like crystallite HAP (Figure 16d). It is believed that the Ca2+ and PO43− in ACP rearranged, consuming additional Ca2+ and OH− ions in the solution, extending the order of ACP to longer distances, and forming HAP embryos [66]. It is important to note that MOS-800 likely played a role in facilitating this transformation by releasing substantial amounts of Ca2+ and OH− ions [68].
In the fourth stage (S4), the pH stabilized again, and crystal growth of the HAP embryos dominated. The crystallinity of the HAP crystals gradually increased (Figure 16e), indicating that HAP maturation occurred progressively. This is consistent with findings by Ding et al., who observed the transformation of the ACP intermediate phase into HAP crystals [44]. Furthermore, the continuous collisions and interactions between HAP embryos and crystal seed surfaces likely accelerated the attachment of these embryos to the seed surfaces via electrostatic attraction and surface sorption, especially under the circulating flow conditions [25].
As a result, the HAP crystallites aggregated on the crystal seed surfaces, ultimately encapsulating the micrometric and millimetric seed materials with HAP crystallites [69,70,71]. It should be noted that the addition of crystal seeds (quartz sand with diameters of 0.2–0.6 mm) and undissolved micro-sized MOS-800 particles likely catalyzed the heterogeneous crystallization process by acting as crystal seeds in the CaCl2/MOS-800/CFB system. The underlying mechanisms of HAP crystallization are illustrated in Figure 16f.

3. Materials and Methods

3.1. Reagents and Materials

Potassium dihydrogen phosphate (KH2PO4, ≥99.5%), calcium chloride (CaCl2, ≥99.0%), and all the other reagents used were of analytical grade, and were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Pristine oyster shell (OS) was purchased from Wuhan Xingkeda Biotechnology Instrument Co. Ltd. All aqueous and standard solutions were prepared using deionized water (Exceed-BD-16, Aike, Chengdu, China).

3.2. Preparation of Modified Oyster Shell (MOS)

Modified oyster shell (MOS) was prepared via a mechanical crushing and thermal modification method. First, original OS were washed, air-dried and crushed. The resulting granules were sieved using a series of stainless-steel sieves with different sizes: 12–20 mesh (0.83–1.4 mm), 30–50 mesh (0.3–0.6 mm), 50–100 mesh (0.15–0.3 mm), and 100–200 mesh (0.075–0.15 mm). Finally, the sieved granules were then calcined in a muffle furnace (SX2-12-12A, Jianing, Shanghai, China) at temperatures of 500 °C, 600 °C, 700 °C, and 800 °C for 2 h. The materials obtained at these temperatures were labeled as MOS-500, MOS-600, MOS-700, and MOS-800, respectively.

3.3. Circulating Fluidized Bed System

Figure 17 illustrates the laboratory-scale circulating fluidized bed (CFB) system, which consists of a feed tank, an inlet pump, a crystallization reactor, a recycle pump, a reflux basin, an outlet pump, and a settling tank.
The crystallization reactor was constructed from glass with a height of 55 cm and an outside diameter of 5 cm. The effective volume of the reactor was about 600 mL, and it was divided into three layers, each filled with different media. The bottom layer with a height of 10 cm was filled with glass beads (Φ: 2–4 mm) for support. The middle layer, 4 cm high, was filled with light yellow quartz sand (Φ: 0.2–0.6 mm), which formed the fluidized layer. The toper layer, containing the crystals, was positioned above the fluidized bed. During the experiments, the CFB reactors were operated at room temperature (20 °C), the circulating flow rate of the mixture was set to about 5.5–6 L/min, while the influent flow rate was maintained at 10 mL/min, yielding a hydraulic retention time (HRT) of around 50 min.

3.4. Experimental Setup and Design

The preparation technique of MOS was optimized by exploring the effects of various modification temperatures and particle sizes. In addition, the impacts of varying MOS dosages (0.05–0.20 g/L) were also investigated. All batch experiments involved adding MOS and/or CaCl2 into 500 mL of phosphate solution (100 mg P/L, initial pH = 9.0) at 25 °C with magnetic stirring. At different time intervals, 3 mL samples were taken out for phosphate content analysis after filtration through a 0.45 μm membrane. pH variations during the reactions were also recorded using a pH meter (PB-10, Sartorius, Germany).
To evaluate the phosphorus sequestration profiles of CaCl2 alone, MOS alone, and the CaCl2/MOS combination in the CFB reactors, the CaCl2/CFB, MOS/CFB, CaCl2/MOS/CFB processes were implemented through semicontinuous flow experiments. The effects of coexisting ions on phosphorus capture in the semicontinuous CaCl2/MOS-800/CFB system were assessed by adding varying concentrations (1 mM and 10 mM) of NO3−, SO42−, HCO3−, Mg2+, NH4+-N, and Cu2+ to the simulated phosphorus-containing wastewater (100 mg P/L), with the pH adjusted to 9.0.
Moreover, phosphorus recycling properties of the CaCl2/CFB and CaCl2/MOS-800/CFB processes were evaluated via continuous flow experiments. Two samples were collected simultaneously: one was filtered through a 0.45 μm membrane, and the other was unfiltered and acidified using a 1 mol/L HCl solution. pH changes were recorded, the phosphorus recovery rate (Xr), crystallization rate (Xc), and microcrystalline rate (Xm) were calculated based on the formulas in Text S1.
To reveal the mechanisms of phosphate recovery, the reclaimed products obtained from the CaCl2/CFB and CaCl2/MOS-800/CFB processes were dried at 30 °C for 24 h in a vacuum drying oven. These products were labeled as RP-CaCl2/CFB and RP-CaCl2/MOS-800/CFB, respectively. Subsequently, leaching experiments and characterization analyses was performed to elucidate the potential mechanisms behind phosphate elimination in the CaCl2/MOS-800/CFB process.

3.5. Analysis and Characterization

Phosphate concentrations were analyzed using the molybdenum blue spectrophotometric method with a UV/VIS spectrometer (UV-1600PC, Mapada, Shanghai, China). The released concentrations of Ca2+, Mg2+, and total Fe from MOS were determined using an inductively coupled plasma emission spectrometer (ICP-OES, 2060T, Skyray Instruments, Suzhou, China). The released alkalinity from MOS-800 was measured by an acid–base titration method. The surface morphologies of OS, MOS-800, and reclaimed products (RP-CaCl2/CFB and RP-CaCl2/MOS-800/CFB) were examined by a scanning electron microscope (SEM, MIRA LMS, TESCAN, Brno, Czech Republic). Elemental distribution in OS and MOS-800 was studied using X-ray energy dispersive spectroscopy (EDS, Xplore30, Oxford Instruments, Oxford, UK). Additionally, the crystal structures of OS, MOS-800, and reclaimed products were analyzed by X-ray diffraction (XRD, Smartlab, Rigaku, Tokyo, Japan). A Fourier-transform infrared spectrometer (FT-IR, IRTracer-100, SHIMADZU, Kyoto, Japan) was used to assess the spectroscopic characteristics of OS, MOS-800 and reclaimed products. Raman spectroscopy (Raman, LabRAM HR Evolution, Horiba Scientific, Kyoto, Japan) and X-ray photoelectron spectroscopy (XPS, ESCALAB Xi+, Thermo Fisher Scientific, Waltham, USA) were employed to measure the Raman and XPS spectra of the reclaimed products. Finally, the supersaturation index (SI) of the various Ca-P complexes was calculated using Visual MINTEQ 3.1 software [72].

4. Conclusions

In this study, a novel MOS-800 material was developed for efficient phosphorus sequestration and recovery from wastewater with high levels of phosphorus. Batch experiments demonstrated that MOS-800 alone achieved a phosphate removal efficiency of 33.3%, while also exhibiting excellent pH regulation capabilities. The CaCl2/MOS-800/CFB coupling process significantly enhanced phosphate removal, reaching 86.5% in the semicontinuous mode. Moreover, the phosphate sequestration in the CaCl2/MOS-800/CFB process was only minimally affected by common coexisting ions such as NO3−, SO42−, Mg2+, and NH4+-N, but it decreased notably in the presence of 10 mM HCO3− and increased in the presence of 10 mM Cu2+. Continuous flow experiments revealed that the phosphate recovery and crystallization rates of the CaCl2/MOS-800/CFB process reached 98.2% and 94.8%, respectively. Material characterization and the supersaturation index (SI) confirmed that hydroxyapatite (HAP) was the primary crystallized product. Additionally, MOS-800 released significant amounts of Ca2+ (506.5–572.2 mg/g), with total alkalinity of 109.1 mg/g as CaCO3, and OH− (98.1 mg/g), which greatly facilitated phosphorus sequestration. The heterogeneous crystallization process in the CaCl2/MOS-800/CFB system was divided into four stages (S1–S4). In S1, Ca2+ rapidly reacted with phosphate to form Ca-P complexes, mainly including Ca-P ion clusters (Ca9(PO4)6) and amorphous calcium phosphate (ACP). In S2, the Ca-P ion clusters randomly packed to form sphere-like ACP particles. In S3, sphere-like ACP particles transformed into sheet-like HAP crystallites, and MOS-800 played a key role by releasing large amounts of Ca2+ and OH− ions. In S4, HAP embryos grew into well-formed crystals, which aggregated on the surface of the crystal seeds through continuous collisions. The presence of crystal seeds and undissolved MOS-800 particles further catalyzed this process by acting as additional seed material, reducing the energy barrier for the formation of HAP granules. Overall, the novel Ca-rich MOS-800 proves to be an eco-friendly and highly efficient material for enhancing phosphate recovery. Further investigations elucidated the underlying mechanisms of hydroxyapatite (HAP) crystallization in the CaCl2/MOS-800/CFB coupling process.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15080706/s1, Figure S1: The correlation between phosphorus removal efficiencies and MOS-800 doses; Figure S2. Phosphate species (a), Ca2+ species (b), and Ca-P species (c) under different pH conditions. (Experimental conditions for (a,b) Ca/P = 2, 100 mg P/L); Table S1: Primary physiochemical properties of the OS and MOS-800; Text S1: The three evaluation parameters of the crystallization process.

Author Contributions

X.L. (Xuejun Long): Writing—review and editing, Supervision; N.Y.: Data curation, Investigation, Writing—original draft; H.W.: Data curation, Investigation, Writing—original draft; J.F.: Data curation, Investigation; R.W.: Data curation, Investigation; Z.Z.: Conceptualization, Methodology, Software, Writing—review and editing; P.Y.: Software; X.X.: Resources; H.H.: Software, Supervision; J.W.: Supervision; X.L. (Xiejuan Lu): Conceptualization, Supervision; X.W.: Resources. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Guided project of the Education Department of Hubei Province (grant number B2022083), the Opening Project of Hubei Key Laboratory of Biomass Fibers and Eco-Dyeing & Finishing (Project Number: STRZ202316), and the Opening Project of Hubei Integrative Technology and Innovation Center for Advanced Fibrous Materials (XC202412).

Data Availability Statement

The data presented in this study are available upon request from the corresponding author. The data are not publicly available due to privacy restrictions.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

References

  1. Xing, C.; Shi, J.; Cui, F.; Shen, J.; Li, H. Fe2+/H2O2-Strengite method with the enhanced settlement for phosphorus removal and recovery from pharmaceutical effluents. Chemosphere 2021, 277, 130343. [Google Scholar] [CrossRef]
  2. Song, Y.; Wang, L.; Qiang, X.; Gu, W.; Ma, Z.; Wang, G. An overview of biological mechanisms and strategies for treating wastewater from printing and dyeing processes. J. Water Process Eng. 2023, 55, 104242. [Google Scholar] [CrossRef]
  3. Liu, B.; Zhou, H.; Li, L.; Ai, J.; He, H.; Yu, J.; Li, P.; Zhang, W. Environmental impact and optimization suggestions of pig manure and wastewater treatment systems from a life cycle perspective. Sci. Total Environ. 2023, 905, 167262. [Google Scholar] [CrossRef]
  4. Jupp, A.R.; Beijer, S.; Narain, G.C.; Schipper, W.; Slootweg, J.C. Phosphorus recovery and recycling—Closing the loop. Chem. Soc. Rev. 2021, 50, 87–101. [Google Scholar] [CrossRef]
  5. Cakmak, E.K.; Hartl, M.; Kisser, J.; Cetecioglu, Z. Phosphorus mining from eutrophic marine environment towards a blue economy: The role of bio-based applications. Water Res. 2022, 219, 118505. [Google Scholar] [CrossRef] [PubMed]
  6. Wurtsbaugh, W.A.; Paerl, H.W.; Dodds, W.K. Nutrients, eutrophication and harmful algal blooms along the freshwater to marine continuum. WIREs Water 2019, 6, e1373. [Google Scholar] [CrossRef]
  7. Barbieri, P.; MacDonald, G.K.; Bernard De Raymond, A.; Nesme, T. Food system resilience to phosphorus shortages on a telecoupled planet. Nat. Sustain. 2021, 5, 114–122. [Google Scholar] [CrossRef]
  8. Jia, Y.; Sun, S.; Wang, S.; Yan, X.; Qian, J.; Pan, B. Phosphorus in water: A review on the speciation analysis and species specific removal strategies. Crit. Rev. Environ. Sci. Technol. 2023, 53, 435–456. [Google Scholar] [CrossRef]
  9. Izadi, P.; Izadi, P.; Eldyasti, A. Design, operation and technology configurations for enhanced biological phosphorus removal (EBPR) process: A review. Rev. Environ. Sci. Bio/Technol. 2020, 19, 561–593. [Google Scholar] [CrossRef]
  10. Du, M.; Zhang, Y.; Wang, Z.; Lv, M.; Tang, A.; Yu, Y.; Qu, X.; Chen, Z.; Wen, Q.; Li, A. Insight into the synthesis and adsorption mechanism of adsorbents for efficient phosphate removal: Exploration from synthesis to modification. Chem. Eng. J. 2022, 442, 136147. [Google Scholar] [CrossRef]
  11. Deng, L.; Dhar, B.R. Phosphorus recovery from wastewater via calcium phosphate precipitation: A critical review of methods, progress, and insights. Chemosphere 2023, 330, 138685. [Google Scholar] [CrossRef]
  12. Cordell, D.; Rosemarin, A.; Schröder, J.J.; Smit, A.L. Towards global phosphorus security: A systems framework for phosphorus recovery and reuse options. Chemosphere 2011, 84, 747–758. [Google Scholar] [CrossRef]
  13. Riewklang, K.; Kaan Dereli, R.; Nakason, K.; Jin, G.; Panyapinyopol, B. Assessing phosphorus recovery from anaerobic digestion effluent of tapioca starch processing in a pilot—Scale fluidized—Bed homogeneous crystallizer: Effects of operation modes. Chem. Eng. J. 2024, 488, 150825. [Google Scholar] [CrossRef]
  14. Wilfert, P.; Dugulan, A.I.; Goubitz, K.; Korving, L.; Witkamp, G.J.; Van Loosdrecht, M.C.M. Vivianite as the main phosphate mineral in digested sewage sludge and its role for phosphate recovery. Water Res. 2018, 144, 312–321. [Google Scholar] [CrossRef] [PubMed]
  15. Yang, X.; Zhang, C.; Zhang, X.; Deng, S.; Cheng, X.; Waite, T.D. Phosphate Recovery from Aqueous Solutions via Vivianite Crystallization: Interference of FeII Oxidation at Different DO Concentrations and pHs. Environ. Sci. Technol. 2023, 57, 2105–2117. [Google Scholar] [CrossRef] [PubMed]
  16. Cichy, B.; Kużdżał, E.; Krztoń, H. Phosphorus recovery from acidic wastewater by hydroxyapatite precipitation. J. Environ. Manag. 2019, 232, 421–427. [Google Scholar]
  17. Eggers, E.; Dirkzwager, A.H.; Van Der Honing, H. Full-Scale Experiences with Phosphate Crystallization in a Crystalactor®. Water Sci. Technol. 1991, 23, 819–824. [Google Scholar] [CrossRef]
  18. Guan, Q.; Zeng, G.; Song, J.; Li, Y.; Yang, L.; Wang, Z.; Liu, C. Highly efficient phosphorus and potassium recovery from urine via crystallization process in a fluidized bed reactor system. J. Environ. Chem. Eng. 2021, 9, 105623. [Google Scholar] [CrossRef]
  19. Riewklang, K.; Polprasert, C.; Nakason, K.; Polprasert, S.; Kwonpongsagoon, S.; Mahasandana, S.; Panyapinyopol, B. Enhancing chemical phosphorus precipitation from tapioca starch anaerobic digestion effluent in a modified pilot-scale fluidized bed reactor. Environ. Res. 2023, 231, 116277. [Google Scholar] [CrossRef]
  20. Cruz-Hernández, M.; Velázquez-Herrera, F.D.; Fetter, G. Synthetic hydroxyapatites as high-performance fertilizers for lettuce plant growth. Rhizosphere 2023, 26, 100718. [Google Scholar] [CrossRef]
  21. Lee, J.-I.; Kang, J.-K.; Oh, J.-S.; Yoo, S.-C.; Lee, C.-G.; Jho, E.H.; Park, S.-J. New insight to the use of oyster shell for removing phosphorus from aqueous solutions and fertilizing rice growth. J. Clean. Prod. 2021, 328, 129536. [Google Scholar] [CrossRef]
  22. Jang, H.; Kang, S.-H. Phosphorus removal using cow bone in hydroxyapatite crystallization. Water Res. 2002, 36, 1324–1330. [Google Scholar] [PubMed]
  23. Kim, E.; Yim, S.; Jung, H.; Lee, E. Hydroxyapatite crystallization from a highly concentrated phosphate solution using powdered converter slag as a seed material. J. Hazard. Mater. 2006, 136, 690–697. [Google Scholar] [CrossRef]
  24. Santos, A.F.; Lopes, D.V.; Alvarenga, P.; Gando-Ferreira, L.M.; Quina, M.J. Phosphorus removal from urban wastewater through adsorption using biogenic calcium carbonate. J. Environ. Manag. 2024, 351, 119875. [Google Scholar] [CrossRef]
  25. Su, P.; Huo, Q.; Zhang, J.; Zhao, G.; Quan, B.; Zhang, C. Recovery of phosphorus from public toilet press filtrate using Ca-rich fly ash through the formation of hydroxyapatite (HAP). Resour. Conserv. Recycl. Adv. 2023, 17, 200138. [Google Scholar] [CrossRef]
  26. Ramakrishna, C.; Thenepalli, T.; Han, C.; Ahn, J.-W. Synthesis of aragonite-precipitated calcium carbonate from oyster shell waste via a carbonation process and its applications. Korean J. Chem. Eng. 2017, 34, 225–230. [Google Scholar] [CrossRef]
  27. Martins, M.C.; Santos, E.B.H.; Marques, C.R. First study on oyster-shell-based phosphorous removal in saltwater—A proxy to effluent bioremediation of marine aquaculture. Sci. Total Environ. 2017, 574, 605–615. [Google Scholar]
  28. Inthapanya, X.; Wu, S.; Han, Z.; Zeng, G.; Wu, M.; Yang, C. Adsorptive removal of anionic dye using calcined oyster shells: Isotherms, kinetics, and thermodynamics. Environ. Sci. Pollut. Res. 2019, 26, 5944–5954. [Google Scholar] [CrossRef]
  29. Currie, J.A.; Harrison, N.R.; Wang, L.; Jones, M.I.; Brooks, M.S. A preliminary study of processing seafood shells for eutrophication control. Asia-Pac. J. Chem. Eng. 2007, 2, 460–467. [Google Scholar] [CrossRef]
  30. Mao, Y.; Zhang, X.; Bai, J.; Li, X.; Wang, C.; Cai, J.; Li, Y.; Liang, W.; Chang, S.; Jiang, Y. Utilization of oyster shell nano-hydroxyapatite modified red-brick waste as an environmentally friendly composite filler for Cd(II) and Cr(VI) adsorption: Preparation, property and mechanism. J. Water Process Eng. 2024, 59, 104955. [Google Scholar] [CrossRef]
  31. Legodi, M.A.; De Waal, D.; Potgieter, J.H.; Potgieter, S.S. Rapid determination of CaCO3 in mixtures utilising FT—IR spectroscopy. Miner. Eng. 2001, 14, 1107–1111. [Google Scholar] [CrossRef]
  32. Park, K.; Sadeghi, K.; Thanakkasaranee, S.; Park, Y.I.; Park, J.; Nam, K.H.; Han, H.; Seo, J. Effects of calcination temperature on morphological and crystallographic properties of oyster shell as biocidal agent. Int. J. Appl. Ceram. Technol. 2021, 18, 302–311. [Google Scholar] [CrossRef]
  33. Khiri, M.Z.A.; Matori, K.A.; Zainuddin, N.; Abdullah, C.A.C.; Alassan, Z.N.; Baharuddin, N.F.; Zaid, M.H.M. The usability of ark clam shell (Anadara granosa) as calcium precursor to produce hydroxyapatite nanoparticle via wet chemical precipitate method in various sintering temperature. SpringerPlus 2016, 5, 1206. [Google Scholar] [CrossRef] [PubMed]
  34. Santos, V.H.J.M.D.; Pontin, D.; Ponzi, G.G.D.; Stepanha, A.S.D.G.E.; Martel, R.B.; Schütz, M.K.; Einloft, S.M.O.; Dalla Vecchia, F. Application of Fourier Transform infrared spectroscopy (FTIR) coupled with multivariate regression for calcium carbonate (CaCO3) quantification in cement. Constr. Build. Mater. 2021, 313, 125413. [Google Scholar] [CrossRef]
  35. Chang, L.; Feng, Y.; Wang, B.; Huang, X.; Yang, D.-P.; Lu, Y. Dual functional oyster shell-derived Ag/ZnO/CaCO3 nanocomposites with enhanced catalytic and antibacterial activities for water purification. RSC Adv. 2019, 9, 41336–41344. [Google Scholar] [CrossRef]
  36. Inoue, Y.; Yasumori, I. Catalysis by Alkaline Earth Metal Oxides. III. X-Ray Photoelectron Spectroscopic Study of Catalytically Active MgO, CaO, and BaO Surfaces. Bull. Chem. Soc. Jpn. 1981, 54, 1505–1510. [Google Scholar] [CrossRef]
  37. Sugama, T.; Kukacka, L.E.; Carciello, N.; Hocker, N.J. Study of interactions at water-soluble polymer/Ca(OH)2 or gibbsite interfaces by XPS. Cem. Concr. Res. 1989, 19, 857–867. [Google Scholar] [CrossRef]
  38. Supamathanon, N.; Boonserm, K.; Lisnund, S.; Chanlek, N.; Rungtaweevoranit, B.; Khemthong, P.; Wittayakun, J.; Osakoo, N. Development of CaO supported on modified geopolymer catalyst for transesterification of soybean oil to biodiesel. Mater. Today Commun. 2021, 29, 102822. [Google Scholar] [CrossRef]
  39. Tran, T.-T.; Tran, N.-N.T.; Sugiyama, S.; Liu, J.-C. Enhanced phosphate removal by thermally pretreated waste oyster shells. J. Mater. Cycles Waste Manag. 2021, 23, 177–185. [Google Scholar] [CrossRef]
  40. Liu, Y.; Sheng, X.; Dong, Y.; Ma, Y. Removal of high-concentration phosphate by calcite: Effect of sulfate and pH. Desalination 2012, 289, 66–71. [Google Scholar] [CrossRef]
  41. Cao, X.; Harris, W. Carbonate and Magnesium Interactive Effect on Calcium Phosphate Precipitation. Environ. Sci. Technol. 2008, 42, 436–442. [Google Scholar] [CrossRef]
  42. Dai, J.; Yang, H.; Yan, H.; Shangguan, Y.; Zheng, Q.; Cheng, R. Phosphate adsorption from aqueous solutions by disused adsorbents: Chitosan hydrogel beads after the removal of copper(II). Chem. Eng. J. 2011, 166, 970–977. [Google Scholar] [CrossRef]
  43. Lei, Y.; Zhan, Z.; Saakes, M.; Van Der Weijden, R.D.; Buisman, C.J.N. Electrochemical Recovery of Phosphorus from Acidic Cheese Wastewater: Feasibility, Quality of Products, and Comparison with Chemical Precipitation. ACS EST Water 2021, 1, 1002–1013. [Google Scholar] [CrossRef] [PubMed]
  44. Ding, H.; Pan, H.; Xu, X.; Tang, R. Toward a Detailed Understanding of Magnesium Ions on Hydroxyapatite Crystallization Inhibition. Cryst. Growth Des. 2014, 14, 763–769. [Google Scholar] [CrossRef]
  45. Ha, T.-H.; Mahasti, N.N.N.; Lin, C.-S.; Lu, M.-C.; Huang, Y.-H. Enhanced struvite (MgNH4PO4·6HXO) granulation and separation from synthetic wastewater using fluidized-bed crystallization (FBC) technology. J. Water Process Eng. 2023, 53, 103855. [Google Scholar] [CrossRef]
  46. Le, V.-G.; Vu, C.-T.; Shih, Y.-J.; Bui, X.-T.; Liao, C.-H.; Huang, Y.-H. Phosphorus and potassium recovery from human urine using a fluidized bed homogeneous crystallization (FBHC) process. Chem. Eng. J. 2020, 384, 123282. [Google Scholar] [CrossRef]
  47. Zhang, B.; Tian, S.; Wu, D. Phosphorus harvesting from fresh human urine: A strategy of precisely recovering high-purity calcium phosphate and insights into the precipitation conversion mechanism. Water Res. 2022, 227, 119325. [Google Scholar] [CrossRef]
  48. Montes-Hernandez, G.; Renard, F. Nucleation of Brushite and Hydroxyapatite from Amorphous Calcium Phosphate Phases Revealed by Dynamic In Situ Raman Spectroscopy. J. Phys. Chem. C 2020, 124, 15302–15311. [Google Scholar] [CrossRef]
  49. Ostroumov, M.; Faulques, E.; Lounejeva, E. Raman spectroscopy of natural silica in Chicxulub impactite, Mexico. Comptes Rendus. Géoscience 2002, 334, 21–26. [Google Scholar] [CrossRef]
  50. Kazanci, M.; Fratzl, P.; Klaushofer, K.; Paschalis, E.P. Complementary Information on In Vitro Conversion of Amorphous (Precursor) Calcium Phosphate to Hydroxyapatite from Raman Microspectroscopy and Wide-Angle X-Ray Scattering. Calcif. Tissue Int. 2006, 79, 354–359. [Google Scholar] [CrossRef]
  51. McMillan, P.; Wolf, G.; Lambert, P. A Raman spectroscopic study of shocked single crystalline quartz. Phys. Chem. Miner. 1992, 19, 71–79. [Google Scholar] [CrossRef]
  52. Mir, M.; Leite, F.L.; Herrmann Junior, P.S.D.P.; Pissetti, F.L.; Rossi, A.M.; Moreira, E.L.; Mascarenhas, Y.P. XRD, AFM, IR and TGA study of nanostructured hydroxyapatite. Mater. Res. 2012, 15, 622–627. [Google Scholar] [CrossRef]
  53. Zhao, W.; Xu, Y.; Gu, L.; Zhu, M.; Yang, P.; Gu, C.; Liu, Z.; Feng, X.; Tan, W.; Huang, Q.; et al. Elucidating Phosphate and Cadmium Cosorption Mechanisms on Mineral Surfaces with Direct Spectroscopic and Modeling Evidence. Environ. Sci. Technol. 2024, 58, 20211–20223. [Google Scholar] [CrossRef]
  54. Choi, D.; Marra, K.G.; Kumta, P.N. Chemical synthesis of hydroxyapatite/poly(ε-caprolactone) composites. Mater. Res. Bull. 2004, 39, 417–432. [Google Scholar] [CrossRef]
  55. Gibson, I.R.; Bonfield, W. Novel synthesis and characterization of an AB-type carbonate-substituted hydroxyapatite. J. Biomed. Mater. Res. 2002, 59, 697–708. [Google Scholar] [CrossRef] [PubMed]
  56. Pal, A.; Paul, S.; Choudhury, A.R.; Balla, V.K.; Das, M.; Sinha, A. Synthesis of hydroxyapatite from Lates calcarifer fish bone for biomedical applications. Mater. Lett. 2017, 203, 89–92. [Google Scholar] [CrossRef]
  57. Cao, J.; Zhao, W.; Wang, S.; Xu, R.; Hao, L.; Sun, W. Effects of calcium on phosphorus recovery from wastewater by vivianite crystallization: Interaction and mechanism analysis. J. Environ. Chem. Eng. 2023, 11, 110506. [Google Scholar] [CrossRef]
  58. Navío, J.A.; Macías, M.; Colón, G.; Sánchez-Soto, P.J.; Augugliaro, V.; Palmisano, L. Combined use of XPS, IR and EDAX techniques for the characterization of ZrO2-SiO2 powders prepared by a sol-gel process. Appl. Surf. Sci. 1994, 81, 325–329. [Google Scholar] [CrossRef]
  59. Kačiulis, S.; Mattogno, G.; Pandolfi, L.; Cavalli, M.; Gnappi, G.; Montenero, A. XPS study of apatite-based coatings prepared by sol–gel technique. Appl. Surf. Sci. 1999, 151, 1–5. [Google Scholar] [CrossRef]
  60. Mucalo, M.R.; Toriyama, M.; Yokogawa, Y.; Suzuki, T.; Kawamoto, Y.; Nagata, F.; Nishizawa, K. Growth of calcium phosphate on ion-exchange resins pre-saturated with calcium or hydrogenphosphate ions: An SEM/EDX and XPS study. J. Mater. Sci. Mater. Med. 1995, 6, 409–419. [Google Scholar] [CrossRef]
  61. Chaudhary, B.; Kshetri, Y.K.; Dhakal, D.R.; Lee, S.W.; Kim, T.-H. Synthesis and characterization of red-emitting Yb/Ho-CaSiO3 upconversion phosphors. Prog. Nat. Sci. Mater. Int. 2022, 32, 594–601. [Google Scholar] [CrossRef]
  62. Landis, W.J.; Martin, J.R. X-ray photoelectron spectroscopy applied to gold-decorated mineral standards of biological interest. J. Vac. Sci. Technol. Vac. Surf. Film. 1984, 2, 1108–1111. [Google Scholar] [CrossRef]
  63. Luo, J.; Peng, J.; Zhong, Z.; Long, X.; Yang, J.; Li, R.; Wan, J. A novel calcium peroxide/attapulgite-Fe(II) process for high concentration phosphate removal and recovery: Efficiency and mechanism. J. Environ. Manag. 2023, 343, 118166. [Google Scholar] [CrossRef] [PubMed]
  64. Dai, H.; Lu, X.; Peng, Y.; Yang, Z.; Zhsssu, H. Effects of supersaturation control strategies on hydroxyapatite (HAP) crystallization for phosphorus recovery from wastewater. Environ. Sci. Pollut. Res. 2017, 24, 5791–5799. [Google Scholar] [CrossRef] [PubMed]
  65. Dorozhkin, S.V. Amorphous calcium (ortho)phosphates. Acta Biomater. 2010, 6, 4457–4475. [Google Scholar] [CrossRef]
  66. Yun, J.; Holmes, B.; Fok, A.; Wang, Y. A Kinetic Model for Hydroxyapatite Precipitation in Mineralizing Solutions. Cryst. Growth Des. 2018, 18, 2717–2725. [Google Scholar] [CrossRef]
  67. Liu, X.; Yang, S.; Liu, S.; Yang, Y. Performance and mechanism of phosphorus removal by slag ceramsite filler. Process Saf. Environ. Prot. 2021, 148, 858–866. [Google Scholar] [CrossRef]
  68. Gholami, M.; O’Sullivan, A.D.; Mackey, H.R. Calcinated sea urchin shell waste for rapid phosphate removal from greywater for application to nature-based systems. Process Saf. Environ. Prot. 2025, 194, 955–966. [Google Scholar] [CrossRef]
  69. Yang, J.; Long, X.; Feng, X.; Wan, J. Simultaneous sorption of orthophosphate and phosphonate from RO concentrate by kaolin/lanthanum carbonate composites: Experimental investigation and multi-objective artificial neural network modeling. J. Environ. Chem. Eng. 2023, 11, 109776. [Google Scholar] [CrossRef]
  70. Guo, S.; Liu, Y.; Li, J. Nanocatalysts for the Degradation of Refractory Pollutants. Catalysts 2024, 14, 444. [Google Scholar] [CrossRef]
  71. Guo, S.; Chen, M.; Wei, Y.; You, L.; Cai, C.; Wei, Q.; Zhou, K. Designing hierarchically porous zero-valent iron via 3D printing to degrade organic pollutants by activating peroxymonosulfate using high-valent iron-oxo species. Chem. Eng. J. 2023, 476, 146523. [Google Scholar] [CrossRef]
  72. Gustafsson, J.P. Visual MINTEQ 3.0 User Guide; KTH, Department of Land and Water Recources: Stockholm, Sweden, 2011. [Google Scholar]
Catalysts 15 00706 g001
Figure 1. SEM-Mapping images of pristine OS (a,b,e) and MOS-800 (c,d,f).
Figure 1. SEM-Mapping images of pristine OS (a,b,e) and MOS-800 (c,d,f).
Catalysts 15 00706 g001
Catalysts 15 00706 g002
Figure 2. EDS and XRD patterns of pristine OS (a,c) and MOS-800 (b,d).
Figure 2. EDS and XRD patterns of pristine OS (a,c) and MOS-800 (b,d).
Catalysts 15 00706 g002
Catalysts 15 00706 g003
Figure 3. FT-IR patterns of pristine OS and MOS-800.
Figure 3. FT-IR patterns of pristine OS and MOS-800.
Catalysts 15 00706 g003
Catalysts 15 00706 g004
Figure 4. XPS spectra of pristine OS and MOS-800: wide spectrum (a), Ca 2p (b), O 1s (c), and P 2p (d).
Figure 4. XPS spectra of pristine OS and MOS-800: wide spectrum (a), Ca 2p (b), O 1s (c), and P 2p (d).
Catalysts 15 00706 g004
Catalysts 15 00706 g005
Figure 5. Effects of modified temperature (a,b), particle size (c,d), and dosage (e,f) on phosphate sequestration by OS-based materials; (Experimental conditions for (a,b): 25 °C, 100 mg P/L, 0.1 g/L, initial pH = 9.0, 100–200 mesh, 500–800 °C; (c,d): 25 °C, 100 mg P/L, 0.1 g/L, initial pH = 9.0, 800 °C, 12–200 mesh; (e,f): 25 °C, 100 mg P/L, initial pH = 9.0, 800 °C 100–200 mesh, 0.05–0.20 g/L).
Figure 5. Effects of modified temperature (a,b), particle size (c,d), and dosage (e,f) on phosphate sequestration by OS-based materials; (Experimental conditions for (a,b): 25 °C, 100 mg P/L, 0.1 g/L, initial pH = 9.0, 100–200 mesh, 500–800 °C; (c,d): 25 °C, 100 mg P/L, 0.1 g/L, initial pH = 9.0, 800 °C, 12–200 mesh; (e,f): 25 °C, 100 mg P/L, initial pH = 9.0, 800 °C 100–200 mesh, 0.05–0.20 g/L).
Catalysts 15 00706 g005
Catalysts 15 00706 g006
Figure 6. Comparison of phosphate separation (a) and variations of pH values (b) in various processes. (Reaction conditions: 100 mg P/L, initial pH = 9.0, Ca/P = 1 (molar ratio), 0.1 g/L of MOS-800 in the various processes above).
Figure 6. Comparison of phosphate separation (a) and variations of pH values (b) in various processes. (Reaction conditions: 100 mg P/L, initial pH = 9.0, Ca/P = 1 (molar ratio), 0.1 g/L of MOS-800 in the various processes above).
Catalysts 15 00706 g006
Catalysts 15 00706 g007
Figure 7. Effects of different anions on phosphorus separation (a) and pH (b) in the semicontinuous CaCl2/MOS-800/CFB process; (Experimental conditions: Ca/P = 1 (molar ratio), 0.1 g MOS-800/L, 100 mg P/L, pH = 9.0).
Figure 7. Effects of different anions on phosphorus separation (a) and pH (b) in the semicontinuous CaCl2/MOS-800/CFB process; (Experimental conditions: Ca/P = 1 (molar ratio), 0.1 g MOS-800/L, 100 mg P/L, pH = 9.0).
Catalysts 15 00706 g007
Catalysts 15 00706 g008
Figure 8. XRD patterns of different products when 10 mM of anions (a) and cations (b) coexisted in the semicontinuous CaCl2/MOS-800/CFB process.
Figure 8. XRD patterns of different products when 10 mM of anions (a) and cations (b) coexisted in the semicontinuous CaCl2/MOS-800/CFB process.
Catalysts 15 00706 g008
Catalysts 15 00706 g009
Figure 9. Effects of different cations on phosphorus separation (a) and pH (b) in the semicontinuous CaCl2/MOS-800/CFB process; (Experimental conditions: Ca/P = 1 (molar ratio), 0.1 g MOS-800/L, 100 mg P/L, pH = 9.0 ± 0.1).
Figure 9. Effects of different cations on phosphorus separation (a) and pH (b) in the semicontinuous CaCl2/MOS-800/CFB process; (Experimental conditions: Ca/P = 1 (molar ratio), 0.1 g MOS-800/L, 100 mg P/L, pH = 9.0 ± 0.1).
Catalysts 15 00706 g009
Catalysts 15 00706 g010
Figure 10. Profiles of phosphate recovery in the continuous flow CaCl2/CFB (a) and CaCl2/MOS-800/CFB processes (b), and variations of pH values (c) in above two processes. (Experimental conditions for (a) Ca/P = 2, 100 mg P/L, pH = 9.0, 200 mg/L NH4+-N; (b) (Ca(MOS-800) + Ca(CaCl2))/P = 2, 100 mg P/L, pH = 9.0, 200 mg/L NH4+-N).
Figure 10. Profiles of phosphate recovery in the continuous flow CaCl2/CFB (a) and CaCl2/MOS-800/CFB processes (b), and variations of pH values (c) in above two processes. (Experimental conditions for (a) Ca/P = 2, 100 mg P/L, pH = 9.0, 200 mg/L NH4+-N; (b) (Ca(MOS-800) + Ca(CaCl2))/P = 2, 100 mg P/L, pH = 9.0, 200 mg/L NH4+-N).
Catalysts 15 00706 g010
Catalysts 15 00706 g011
Figure 11. SEM micrographs of crystal seeds (a,b), RP-CaCl2/CFB (c,d), and RP-CaCl2/MOS-800/CFB processes (e,f) (RP-CaCl2/CFB—reclaimed products from the CaCl2/CFB process, RP-CaCl2/MOS-800/CFB—reclaimed products from the CaCl2/MOS-800/CFB process).
Figure 11. SEM micrographs of crystal seeds (a,b), RP-CaCl2/CFB (c,d), and RP-CaCl2/MOS-800/CFB processes (e,f) (RP-CaCl2/CFB—reclaimed products from the CaCl2/CFB process, RP-CaCl2/MOS-800/CFB—reclaimed products from the CaCl2/MOS-800/CFB process).
Catalysts 15 00706 g011
Catalysts 15 00706 g012
Figure 12. XRD (a,b), Raman (c), and FT-IR patterns (d) of reclaimed products in the various processes.
Figure 12. XRD (a,b), Raman (c), and FT-IR patterns (d) of reclaimed products in the various processes.
Catalysts 15 00706 g012
Catalysts 15 00706 g013
Figure 13. XPS spectra of crystal seeds and reclaimed products: wide scan XPS spectra (a), O 1s (b), Ca 2p (c), and P 2p (d).
Figure 13. XPS spectra of crystal seeds and reclaimed products: wide scan XPS spectra (a), O 1s (b), Ca 2p (c), and P 2p (d).
Catalysts 15 00706 g013
Catalysts 15 00706 g014
Figure 14. Concentrations of released cations and pH of MOS-800 (0.1 g/L) at 1 mol/L HCl (a), pH = 2.5 (b), pH = 7 (c), and pH = 9 (d), comparison of Ca2+ released under different conditions (e), and the contents of total alkalinity, OH−, CO32− (f).
Figure 14. Concentrations of released cations and pH of MOS-800 (0.1 g/L) at 1 mol/L HCl (a), pH = 2.5 (b), pH = 7 (c), and pH = 9 (d), comparison of Ca2+ released under different conditions (e), and the contents of total alkalinity, OH−, CO32− (f).
Catalysts 15 00706 g014
Catalysts 15 00706 g015
Figure 15. Supersaturation index of possible Ca-P complexes (a), XRD patterns of recovered products with different reaction times (b), HAP flocs (c), and HAP crystalline particles (d) in the CaCl2/MOS-800/CFB process. (Experimental conditions for (a) Ca/P = 2, 100 mg P/L; (b–d) (Ca(MOS-800) + Ca(CaCl2))/P =2, 100 mg P/L, pH = 9.0, 200 mg/L NH4+-N).
Figure 15. Supersaturation index of possible Ca-P complexes (a), XRD patterns of recovered products with different reaction times (b), HAP flocs (c), and HAP crystalline particles (d) in the CaCl2/MOS-800/CFB process. (Experimental conditions for (a) Ca/P = 2, 100 mg P/L; (b–d) (Ca(MOS-800) + Ca(CaCl2))/P =2, 100 mg P/L, pH = 9.0, 200 mg/L NH4+-N).
Catalysts 15 00706 g015
Catalysts 15 00706 g016
Figure 16. Variations of pH values in the CFB reactor (a), the various Ca-P precipitates predominantly existed as flocs in the S1 stage (b), sphere-like ACP particles (c) and sheet-like HAP crystallites (d) in the CFB reactor (e), the reclaimed products RP-CaCl2/MOS-800/CFB in the system, and the underlying mechanisms of HAP crystallization in the CaCl2/MOS-800/CFB process (f).
Figure 16. Variations of pH values in the CFB reactor (a), the various Ca-P precipitates predominantly existed as flocs in the S1 stage (b), sphere-like ACP particles (c) and sheet-like HAP crystallites (d) in the CFB reactor (e), the reclaimed products RP-CaCl2/MOS-800/CFB in the system, and the underlying mechanisms of HAP crystallization in the CaCl2/MOS-800/CFB process (f).
Catalysts 15 00706 g016
Catalysts 15 00706 g017
Figure 17. Schematic diagram of circulating fluidized bed (CFB) system.
Figure 17. Schematic diagram of circulating fluidized bed (CFB) system.
Catalysts 15 00706 g017
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Long, X.; Yang, N.; Wang, H.; Fang, J.; Wang, R.; Zhong, Z.; Yu, P.; Xu, X.; Huang, H.; Wan, J.; et al. Synergistic Sequestration and Hydroxyapatite-Based Recovery of Phosphorus by the Coupling Process of CaCl2/Modified Oyster Shell and Circulating Fluidized Bed Reactor. Catalysts 2025, 15, 706. https://doi.org/10.3390/catal15080706

AMA Style

Long X, Yang N, Wang H, Fang J, Wang R, Zhong Z, Yu P, Xu X, Huang H, Wan J, et al. Synergistic Sequestration and Hydroxyapatite-Based Recovery of Phosphorus by the Coupling Process of CaCl2/Modified Oyster Shell and Circulating Fluidized Bed Reactor. Catalysts. 2025; 15(8):706. https://doi.org/10.3390/catal15080706

Chicago/Turabian Style

Long, Xuejun, Nanshan Yang, Huiqi Wang, Jun Fang, Rui Wang, Zhenxing Zhong, Peng Yu, Xuelian Xu, Hao Huang, Jun Wan, and et al. 2025. "Synergistic Sequestration and Hydroxyapatite-Based Recovery of Phosphorus by the Coupling Process of CaCl2/Modified Oyster Shell and Circulating Fluidized Bed Reactor" Catalysts 15, no. 8: 706. https://doi.org/10.3390/catal15080706

APA Style

Long, X., Yang, N., Wang, H., Fang, J., Wang, R., Zhong, Z., Yu, P., Xu, X., Huang, H., Wan, J., Lu, X., & Wu, X. (2025). Synergistic Sequestration and Hydroxyapatite-Based Recovery of Phosphorus by the Coupling Process of CaCl2/Modified Oyster Shell and Circulating Fluidized Bed Reactor. Catalysts, 15(8), 706. https://doi.org/10.3390/catal15080706

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop