Resource recovery of water treatment plant sludge and river sediment as phosphorus removal material: feasibility and mechanisms

Excessive phosphorus levels in water bodies are recognized as the primary contributor to eutrophication (Jiao et al. 2021), and reducing phosphorus concentrations is crucial for mitigating and restoring eutrophication in aquatic ecosystems. Currently, the main methods for reducing phosphorus in water bodies include chemical precipitation, biological removal (Yang et al. 2022), and adsorption (Zhang et al. 2022). The adsorption method has gained significant attention due to its simplicity, stability, low cost, low energy consumption, and low environmental impact (Xu et al. 2022). Adsorbent materials can be categorized into three groups based on their sources: natural materials, solid waste (Sun et al. 2022), and synthetic materials (Jiao et al. 2021). Among these, solid waste stands out for its advantages such as cost-effectiveness, environmental friendliness, renewability, and excellent adsorption performance, leading to increasing interest and research in this area (Li et al. 2021).

Water treatment plant sludge (WTPS) is a type of solid waste produced during the water treatment process in water treatment plants (Wang et al. 2022b), which exhibits a favorable adsorption capacity for phosphorus, as evidenced by studies indicating phosphorus adsorption amounts ranging from 2 to 43 mg/g and phosphorus removal rates exceeding 96% (Ren et al. 2020; Lin et al. 2021). However, the scarcity of WTPS in practical applications poses a challenge. Taking Lake Taihu as an example, the ‘2022 China Ecological and Environmental Status Bulletin’ identifies total phosphorus (TP) as the primary pollution index for the lake. With a capping strength of WTPS estimated at 2 kg/m², approximately 4.89 million tons of WTPS would be required for the treatment of Lake Taihu. In contrast, the average annual production of WTPS in China amounts to only 2.35 million tons (Ren et al. 2020).

River sediment (RS) is a byproduct generated in large quantities during river dredging projects. It contains iron and aluminum oxides, which can potentially serve as phosphorus adsorbents (Wang et al. 2022a). Pérez et al. (2014) found that the adsorption characteristics of the sediment on phosphorus conformed to the Langmuir model through experiments, and estimated that the maximum phosphorus adsorption of the sediment was 0.96 and 1.13 mg/g at 25 and 35 °C, respectively, indicating that the RS has a certain adsorption effect on phosphorus, but the effectiveness is limited.

The phosphorus adsorbent derived from WTPS demonstrates high phosphorus removal efficiency; however, the raw material is scarce. On the other hand, RS is abundant but shows lower phosphorus removal capabilities. By combining WTPS and RS to prepare phosphorus removal materials, it is possible to create materials with abundant raw materials and effective phosphorus removal capabilities for practical water treatment applications. Additionally, this approach enables the conversion of WTPS and RS from waste materials to valuable resources and achieves waste reuse.

However, there is a risk of releasing organic matter and ammonia nitrogen when solid wastes such as WTPS and RS are used directly for phosphorus removal (Fan et al. 2022). Thermal modification can be used for removal of water, organic matter, and other impurities from materials by calcining them at high temperatures, resulting in an increase in the specific surface area of the material and a reduction in the release of pollutants from the materials (Liu et al. 2019).

The adsorption mechanism of phosphorus adsorption materials can be divided into physical adsorption and chemical adsorption. Physical adsorption occurs when phosphorus is attracted to the surface of adsorption materials through electrostatic or gravitational forces, but it can be easily desorbed (Zhang et al. 2011). Chemical adsorption relies on chemical bonding forces, making it more selective toward pollutants and less prone to desorption. Typically, both physical and chemical adsorption occur simultaneously during the process of phosphorus removal (Vohla et al. 2011).

This study centers on the preparation of a novel phosphorus removal material, named C-WTPS/RS phosphorus removal material, through a process of mixing and calcining of WTPS and RS. The aim is to address the shortage of raw materials for phosphorus removal in practical projects, as well as the resource utilization of WTPS and RS. The research investigated the phosphorus removal efficiency of C-WTPS/RS in real water scenarios, the effectiveness of C-WTPS/RS in reducing phosphorus release from sediment, the cyclic phosphorus removal capabilities of C-WTPS/RS, the phosphorus release from C-WTPS/RS under varying dissolved oxygen (DO) and pH conditions.

Additionally, the transformation of phosphorus forms in C-WTPS/RS before and after adsorption was analyzed, followed by an investigation into the phosphorus removal mechanism of C-WTPS/RS. The development of this innovative phosphorus removal material offers a resourceful solution for treating WTPS and RS. Additionally, it provides mass-producible phosphorus removal materials for actual water body phosphorus removal projects, aiding in the prevention of eutrophication.

The WTPS was collected from a water treatment plant in Xiamen, while the RS was sourced from a reservoir in the same city. After the removal of impurities, both samples were air-dried for a duration of 30 days. Subsequently, they were combined in a 1:1 mass ratio and calcined at 600 °C for 1 h in a muffle furnace under aerobic conditions. Following this process, the samples were cooled, crushed, and filtered to produce particles ranging from 1 to 2 mm, which served as the final experimental material.

The water samples were taken from Xinglinwan Reservoir, Xiamen, with a TP concentration of 0.72 mg/L and concentration of 0.56 mg/L. The dosage of C-WTPS/RS was 2.50 g/L. Around 200 mL of water was poured into a conical flask and sealed with tin foil. The adsorption was carried out at 25 °C and 150 r/min, and the and TP concentrations were determined from 0 to 48 h. The and TP concentrations were determined by the ammonium molybdate spectrophotometric method. Three sets of parallel samples were set up for the experiment.

The effect of C-WTPS/RS in reducing phosphorus release from the sediment was investigated by constructing a sediment pollutant release simulation system, and the schematic diagram of the in-situ capping experiment device is shown in Supplementary Figure S1.

As shown in Supplementary Figure S1, the in-situ capping device was a transparent wide-mouth glass bottle with a diameter of 20 cm and a volume of 10 L. Prior to the experiment, the sediment was continuously stirred to ensure a thorough and uniform mixture. The in-situ capping experiment was divided into two groups: the uncovered system (control group) and the C-WTPS/RS capping system, with the latter having a strength of 2 kg/m². The experiment device contained approximately 2.58 ± 0.01 kg of sediment (with a thickness of approximately 5 cm), and the height of the overlying water was maintained at 25 cm. The experiment was conducted for a duration of 50 days, with three parallel samples set up for each system. Regular measurements of the TP concentration in the overlying water were taken throughout the experiment.

The dosage of C-WTPS/RS was 2.5 g/L, with an initial phosphorus concentration of 2 mg/L. The experiment was conducted at 25 °C and 150 r/min, and the concentration in the solution was measured after 24 h of adsorption. Subsequently, the C-WTPS/RS was extracted from the reaction vessel, rinsed thrice with deionized water, and dried. The above experiment was repeated five times and the amount of phosphorus adsorbed in each cycle was calculated. The experiments were conducted in three parallel groups.

C-WTPS/RS was adsorbed for 24 h according to the procedure outlined in Section 2.2.3. Following this, C-WTPS/RS underwent three washes with deionized water and was subsequently dried. A conical flask was then filled with 200 mL of deionized water at varying pH levels (adjusted using 0.1 mol/L NaOH and 0.1 mol/L HCl) and 1 g of C-WTPS/RS. The phosphorus concentration in the solution was measured after 24 h at 25 °C and 150 r/min to determine the phosphorus release rate of C-WTPS/RS. Each experimental group had three parallel sets.

C-WTPS/RS was adsorbed for 24 h according to the procedure outlined in Section 2.2.3. Subsequently, the material was washed three times with deionized water and dried. After that, a transparent screw cap glass bottle was filled with 1 L of deionized water and 5 g of C-WTPS/RS. High purity N₂ was then introduced to adjust the DO concentration in the water to less than 1 mg/L for the anaerobic group, 1–3 mg/L for the anoxic group, and 5–7 mg/L for the aerobic group. The experiments lasted for 30 days, during which the phosphorus concentration in the water was continuously monitored in the overlying water. DO concentration was determined using a portable dissolved oxygen meter. Each experimental group had three parallel groups for comparison.

Five grams of C-WTPS/RS were accurately weighed and added to 200 mL of KH₂PO₄ solution with a concentration of 100 mg/L. The conical flask was then sealed with tin foil and subjected to adsorption at a temperature of 25 °C and a rotational speed of 150 r/min for 24 h. After the adsorption process, the C-WTPS/RS was carefully removed from the solution and washed three times using deionized water. The study measured the TP, inorganic phosphorus (IP), organic phosphorus (OP), non-apatite inorganic phosphorus (NAIP), and apatite phosphorus (AP) content of the C-WTPS/RS using The Standards, Measurements and Testing (SMT) method (Gonz Lez Medeiros et al. 2005), both before and after adsorption.

The elemental analysis of the materials was conducted using ARLAdvant'X Intellipower 3600 (Thermo Fisher Scientific, USA). The zeta potential was analyzed using NanoBrook Omni (Brookhaven, USA). Scanning electron microscopy (SEM) was conducted with XFlash 6,130 (Bruker, USA). The N₂ adsorption–desorption measurement was performed using a gas adsorption analyzer (Micromeritics, ASAP 2020 M, USA). Several physical properties, including the specific surface area, the pore volume, and the pore size distribution, were calculated with Brunauer–Emmett–Teller (BET) equation and Barrett–Joyner–Halenda (BJH) model, respectively. XPS was conducted with K-Alpha⁺ (Thermo Fisher Scientific, USA). FT-IR was conducted with Thermo Nicolet iS 5 FT-IR (Thermo Fisher Scientific, USA).

Significant difference analysis was performed using Origin 8.5 software.

As shown in Figure 1, both TP and concentrations in water decreased with the increase in adsorption time. After 48 h of adsorption, the concentration of TP decreased from 0.72 to 0.16 mg/L, with a removal rate of 78.10%, and the concentration of decreased from 0.56 mg/L to below the detection limit, with a removal rate of 99.99%, and the adsorbed overlying water met the standards of surface class Ⅵ water (TP < 0.20 mg/L). The results demonstrate the effective removal capability of C-WTPS/RS for both TP and in actual water bodies, indicating its practical value for application in water treatment processes.

As shown in Figure 2, the TP concentration in the overlying water of the control system exhibited an increasing trend followed by a leveling off, reaching 0.72 mg/L on the 50th day. The TP release intensity in the control system ranged from 2.16 to 15.86 mg/(m² d), with an average release intensity of 4.19 mg/(m² d).

The TP concentration in the overlying water during the experiment of the C-WTPS/RS capping system was consistently complied with the standard of surface class Ⅵ water (TP < 0.20 mg/L). Analysis of variance showed that there was a significant difference between the C-WTPS/RS capping system and the control system (p < 0.05). Among the existing sediment capping materials, Han et al. used hydrous ferric oxide as a sediment capping material and achieved 80.9% removal of TP in the overlying water of the experimental setup after 91 days of incubation. Kong et al. used lanthanum-modified bentonite for sediment capping and reduced TP and IP in the overlying water by 60 and 45%, respectively, after 90 days of incubation. When C-WTPS/RS was used as an in-situ sediment capping material, the average reduction rate of TP in the overlying water was 89.51%, demonstrating a superior phosphorus removal effect compared with the materials used in the studies above. Therefore, C-WTPS/RS has been shown to be an effective active cover material for controlling phosphorus release from sediment and reducing TP concentrations in the overlying water.

The cyclic removal effect of C-WTPS/RS is shown in Supplementary Figure S2.

The adsorption capacity of C-WTPS/RS for gradually decreased with each additional cyclic removal experiment, as illustrated in Supplementary Figure S2. Following two cycles of removal, the removal rate decreased from 82.80 to 65.90%, and after the 5th cycle, it further decreased to 45.23%. This decline in removal efficiency can be attributed to the saturation of adsorption sites on the surface of C-WTPS/RS as the number of experiments increased. Furthermore, throughout multiple cycles of removal experiments, there was no observed disintegration or detachment of C-WTPS/RS, indicating its exceptional mechanical properties.

release from C-WTPS/RS at different pHs is shown in Supplementary Figure S3(a). release from C-WTPS/RS at different DO concentrations is shown in Supplementary Figure S3(b).

In accordance with the findings presented in Supplementary Figure S3(a), it was observed that release rates remained below 2.00% within a pH range of 3–10. Consequently, it was deduced that C-WTPS/RS posed a low risk of release with a pH value ranging from 3 to 10. Conversely, the C-WTPS/RS release rate began to rise once the pH surpassed 10, with a notable spike to 30.02% at a pH of 11. This suggested that adsorption was more prone to release under these specific pH conditions. Therefore, C-WTPS/RS increases the risk of release when the pH value exceeds 10. The pH value of the actual water bodies falls within the range of 6–9. Within this pH range, the release rate of C-WTPS/RS remained below 0.5%, suggesting a low risk of release from C-WTPS/RS in the natural aquatic environment. This finding highlights the practical value of C-WTPS/RS in practical engineering applications.

As shown in Supplementary Figure S3(b), the concentration showed a gradual increase during the initial phase of the release experiment, followed by stabilization in the later stages for all three experiment groups. This suggests that C-WTPS/RS did not continuously release into the water under varying DO concentrations. By the 30th day, the release rates were 0.54% for the anaerobic group, 0.38% for the anoxic group, and 0.24% for the aerobic group. The highest release rate observed in the anaerobic group was only 0.04 mg/L in the overlying water, indicating that DO concentration had a minimal impact on release from C-WTPS/RS.

As illustrated in Figure 3, the initial TP content in C-WTPS/RS was 1.96 mg/g, which subsequently increased to 5.08 mg/g post-adsorption. This observation signified the effective removal of phosphorus from the solution by C-WTPS/RS. Furthermore, there was an increase in IP, OP, NAIP, and adsorbed phosphorus (AP) by 3.07, 0.007, 3.01, and 0.060 mg/g, respectively. It is noteworthy that IP was further categorized into NAIP and AP, with IP constituting 98.5% of TP and NAIP representing 96.60% of TP after adsorption. This suggests that the NAIP content within IP predominantly increased following phosphorus adsorption by C-WTPS/RS.

C-WTPS/RS elemental mass fractions are shown in Table 1.

As shown in Table 1, the main components of C-WTPS/RS were Al₂O₃ (42.49%), SiO₂ (44.64%), and Fe₂O₃ (8.51%). Al₂O₃ and Fe₂O₃ can provide a large number of iron and aluminum adsorption sites for the material. Combined with the changes of different forms of phosphorus content in C-WTPS/RS after the adsorption of phosphorus, it can be seen that the adsorption of phosphorus by C-WTPS/RS was mainly an increase in the content of NAIP, which is generally referred to as phosphorus bound to Fe, Al, and Mn (Li et al. 2015). It is inferred that phosphorus removal by chemical precipitation of Fe and Al compounds in C-WTPS/RS was one of the phosphorus removal mechanisms in C-WTPS/RS.

The pH at the point of zero charge (pH_pzc) for C-WTPS/RS was determined to be 4.65, as depicted in Supplementary Figure S4. At pH levels below 4.65, the surface of C-WTPS/RS displayed a positive charge, enabling the adsorption of negatively charged species through electrostatic attraction (Jiang et al. 2023). Conversely, when the pH surpassed 4.65, the surface of C-WTPS/RS acquired a negative charge. Therefore, electrostatic adsorption was one of the mechanisms of phosphorus removal by C-WTPS/RS.

As shown in Figure 4, the surface of C-WTPS/RS exhibited numerous protrusions, cracks, a complex texture, and rough surface features on C-WTPS/RS. The presence of pore structures on the surface led to an increase in the material's specific surface area and enhanced the number of surface active adsorption sites, thereby facilitating the adsorption of phosphorus (Jiang et al. 2021).

The N₂ adsorption/desorption hysteresis loop and pore size distribution of C-WTPS/RS are shown in Supplementary Figure S5, and the specific surface area, pore volume, and pore size of C-WTPS/RS are shown in Table 2.

As shown in Supplementary Figure S5, the N₂ adsorption/desorption hysteresis loops of C-WTPS/RS conformed to the typical H3 type hysteresis loop. C-WTPS/RS produces monolayer adsorption in a low-pressure environment, and the gas adsorption quantity rises with the increasing pressure. Then, the adsorption reaches equilibrium when the single adsorption pore is filled, and the material shows the phenomenon of capillary coalescence and capillary evaporation at different pressures, which produces the H3 type hysteresis loop (Li et al. 2019).

From Table 2, it can be seen that the specific surface area of C-WTPS/RS was 35.81 m²/g, the pore volume was 0.11 cm³/g, and the pore size distribution was below 20 nm, with an average pore size of 15.44 nm. Based on this, it can be concluded that C-WTPS/RS was a mesoporous material (Jiang et al. 2021).

XPS scanning spectrograms were all calibrated using C1s (284.6 eV). As shown in Figure 5, elements Fe, Al, and O were present on C-WTPS/RS. After adsorption of phosphorus, C-WTPS/RS showed a distinct P 2p peak, which indicated that the material adsorbed phosphorus.

The P 2p peak of C-WTPS/RS after adsorption of phosphorus appeared at 133.83 eV, and the binding energy was higher than that of the adsorbed by electrostatic attraction of NaH₂PO₄·2H₂O (132.9 eV) (Mallet et al. 2013), and meanwhile was lower than that of the P 2p peak of (134.0 eV), which indicated that the force between C-WTPS/RS and was greater than that of the electrostatic interaction. This suggests that C-WTPS/RS was not only physisorbed but also chemisorbed on phosphorus (Koh et al. 2020).

As shown in Figure 6 and Supplementary Table S1, the O1s before and after the adsorption of C-WTPS/RS can be divided into three overlapping peaks; the absorption peaks at 532.77 and 532.35 cm⁻¹ corresponded to the oxygen (H₂O) in water. The absorption peaks at 531.76 and 531.73 cm⁻¹ at 531.76 and 531.73 cm⁻¹ corresponded to hydroxyl oxygen (–OH), and the absorption peaks at 530.58 and 530.93 cm⁻¹ corresponded to oxygen of oxides (O²⁻) (Fan et al. 2020; Yang et al. 2020; Lu & Jin 2022). After adsorption, there was no significant change in the percentage of H₂O peak area. However, the percentage of –OH decreased from 28.27 to 14.45%, and the percentage of O²⁻ increased from 22.29 to 38.03%. This suggests that the –OH in the material reacted with to form O²⁻ bonds, indicating that one of the mechanisms of phosphorus removal by C-WTPS/RS was ligand exchange (Zhao et al. 2023).

As shown in Figure 7, the peak of telescopic vibration between M–OH and water molecules was at 1,640 cm⁻¹ (Gu et al. 2021), the peak of telescopic vibration of intermolecular hydrogen bonding O–H was at 3,100–3,700 cm⁻¹ (Fu et al. 2022), and the peak of Al–OH–Al was near 1,080 cm⁻¹, Fe–OH–Fe or Si–O stretching vibration peaks (Gu et al. 2021), and the vibration peak of Si–O bond was near 470 cm⁻¹ (Lian et al. 2021). The large area of the absorption peaks at 3,100–3,700 cm⁻¹ of the material before and after adsorption indicated the presence of a large number of Al–OH and Fe–OH bonds on the surface of C-WTPS/RS. The bending vibrational peaks of O–P–O bonds at 561 cm⁻¹ after the adsorption of phosphorus on the C-WTPS/RS can prove that phosphorus was adsorbed by C-WTPS/RS (Lian et al. 2021).

After adsorption of phosphorus removal materials, a distinct stretching vibration peak appears near 1,080 cm⁻¹ (Gu et al. 2021; Fu et al. 2022). However, it is not found in the infrared spectrum of C-WTPS/RS after adsorption. The reason is that the stretching vibration peaks of Al–OH–Al, Fe–OH–Fe, and Si–O in C-WTPS/RS covered the stretching vibration peaks of P–O in (Zhu et al. 2024).

C-WTPS/RS exhibited abundant specific surface area and large pore volume, which can adsorb phosphorus by physical adsorption. Additionally, when pH was below 4.65, the surface of the material was positively charged, which can adsorb phosphorus by electrostatic adsorption. A large amount of iron and aluminum oxides existed on the surface of C-WTPS/RS, and after the adsorption of phosphorus, the phosphorus increased in C-WTPS/RS was dominated by IP in NAIP, which indicated that C-WTPS/RS could remove phosphorus by chemical precipitation. Moreover, the percentage of oxygen present in the form of –OH decreased, the percentage of oxygen present in the form of O²⁻ increased, the M–O–P peaks were enhanced, and the O–H and M–O peaks were weakened, indicating that C-WTPS/RS can adsorb phosphorus by ligand exchange.

• (1) It was feasible to prepare phosphorus removal materials by mixing WTPS and RS.

• (2) The phosphorus increased by C-WTPS/RS adsorption was dominated by NAIP.

• (3) The main phosphorus removal mechanisms of C-WTPS/RS were physical adsorption, electrostatic adsorption, chemical precipitation, and ligand exchange.

• (4) This study primarily focuses on small-scale laboratory experiments that investigate the feasibility of C-WTPS/RS preparation and the associated adsorption mechanisms. In future research, it is recommended that C-WTPS/RS be utilized in seine experiments conducted in real water bodies to assess its in-situ phosphorus removal effectiveness.

This work was supported by the National Natural Science Foundation of China (Grant No. 51878300) and the National Natural Science Foundation of Xiamen City (Grant No. 3502Z202373041).

All authors contributed to the study conception and design. Investigation and data curation were performed by J.Z., X.L., and W.C. Writing – original draft, visualization, and formal analysis were completed by J.L. Validation and writing – review and editing were completed by L.Z., H.J., and F.L. Conceptualization and supervision were completed by Z.Z. All authors read and approved the final manuscript.

The authors confirm that the manuscript has been read and approved by all authors. The authors declare that this manuscript has not been published and is not under consideration for publication elsewhere.

The study protocol was approved by the appropriate Committee for the Protection of Human Participants [Huaqiao University].

All authors participated and approved the final manuscript to be published.

Data cannot be made publicly available; readers should contact the corresponding author for details.

The authors declare there is no conflict.