Phosphate Removal from Polluted Water via Lanthanum-Modified Sludge Biochar

Citation: Jiang, Y.; Sun, X.; Zhang, H.; Li, Q.; Mo, J.; Xing, M.; Dong, B.; Zhu, H. Phosphate Removal from Polluted Water via Lanthanum-Modified Sludge Biochar Sustainability 2024 , 16 , 5667. https://doi.org/10.3390/ su 16135667 Academic Editors: Giuseppe Bonura, George Z. Kyzas, Catia Cannilla, John Vakros and Evroula Hapeshi Received: 23 May 2024 Revised: 24 June 2024 Accepted: 26 June 2024 Published: 3 July 2024 Copyright: © 2024 by the authors Licensee MDPI, Basel, Switzerland This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/) sustainability Article Phosphate Removal from Polluted Water via Lanthanum-Modified Sludge Biochar Yufan Jiang 1,2 , Xiaojie Sun 1,2, * , Hongxia Zhang 1,2, *, Qian Li 1,2 , Jingjing Mo 1,2 , Meiyan Xing 3 , Bin Dong 1,3 and Hongxiang Zhu 4,5 1 Guangxi Key Laboratory of Environmental Pollution Control Theory and Technology, Guilin University of Technology, Guilin 541004, China; 2120220510@glut.edu.cn (Y.J.); lq 1643136880@outlook.com (Q.L.); 1020210416@glut.edu.cn (J.M.); dongbin@tongji.edu.cn (B.D.) 2 Guangxi Collaborative Innovation Center for Water Pollution Control and Water Safety in Karst Area, Guilin University of Technology, Guilin 541004, China 3 School of Environmental Science and Engineering, Tongji University, Shanghai 200092, China; xingmeiyan@tongji.edu.cn 4 College of Light Industry and Food Engineering, Guangxi University, Nanning 530004, China; zhx@gxu.edu.cn 5 Modern Industry College of Ecology and Environmental Protection, Guilin University of Technology, Guilin 541004, China * Correspondence: sunxiaojie@glut.edu.cn (X.S.); zhx 75@glut.edu.cn (H.Z.) Abstract: Biochar has attracted attention for its capability to remove phosphorus (P) from wastewater However, the poor dispersion and limited adsorption capacity of unmodified biochar prevent its wide usage in water remediation. Herein, sludge biochar was modified using lanthanum nitrate to improve the removal of P from aqueous solutions. Scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) were used to elucidate the modification and adsorption mechanisms of biochar. Furthermore, the adsorption performance of the modified biochar was determined through adsorption kinetics and isotherm model fitting. The results showed that the modification process successfully introduced lanthanum-containing functional groups into biochar and considerably improved the complexation performance and ion-exchange capacity. The maximum experimental adsorption capacity for phosphate was 140.237 mg/g at pH 3.0. The adsorption processes of the modified biochar accorded with the Freundlich adsorption isotherm model, which indicates the successful adsorption of phosphate onto the modified biochar via multilayer adsorption. The adsorption mechanism was dominated by chemisorption, which mainly involved inner-sphere complexation, precipitation, and electrostatic attraction. Meanwhile, the adsorption and desorption experiments demonstrated the satisfying recycling performance of the modified biochar and the 72.3% adsorption capacity retention after the sixth desorption cycle. The dynamic adsorption study revealed that the modified biochar had long sustainable treatment durations of 7.58 and 9.08 h at adsorbent dosages of 1 and 2 g, respectively, which proves the feasibility of using biochar as a cost-effective and efficient adsorbent for phosphate-polluted water Keywords: adsorption; phosphate; modified biochar; lanthanum; sludge 1. Introduction Phosphorus (P) is an essential nutrient in aquatic ecosystems [ 1 – 3 ]; however, excess P levels can cause severe eutrophication, which threatens ecosystems and human health [ 4 , 5 ]. Therefore, to regulate the discharge standards for sewage, the discharge requirements for P emissions have become increasingly stringent [ 6 , 7 ]. In particular, the current maximum permissible P level for discharged effluents of wastewater treatment plants (WWTPs) is 0.5 mg/L [ 8 ]. Hence, P levels must be reduced and controlled [ 8 – 10 ]. Sustainability 2024 , 16 , 5667. https://doi.org/10.3390/su 16135667 https://www.mdpi.com/journal/sustainability

Sustainability 2024 , 16 , 5667 2 of 15 P may be removed from water through various techniques, including membrane separation, ion exchange, chemical precipitation, and electrocoagulation [ 11 , 12 ]. However, the unstable separation effect and high energy consumption limit the practical application of these methods [ 13 , 14 ]. In comparison, phosphate removal via adsorption technology is considerably promising. This process has the advantages of easy application, high removal efficiency, and cost effectiveness [ 15 , 16 ]. However, certain commonly used adsorbents, such as minerals and carbon compounds, which generally have low renewability or pose biodegradation difficulties, can also cause secondary pollution [ 17 , 18 ]. In addition, the majority of traditional adsorbents exhibit poor adsorption or high operation expenses Therefore, an effective, affordable, and ecologically safe adsorbent must be created for the removal of phosphates from wastewater The rapid development of the urban economy has resulted in the generation of enormous quantities of municipal sewage sludge waste [ 19 ]. Sewage sludge waste contains high concentrations of nitrogen, P, organic micropollutants, heavy metals, and pathogens, which pose a major threat to human health and the environment if left untreated [ 20 ]. As a result, the treatment and reuse of sewage sludge have increasingly become the focus of attention in recent years [ 19 , 21 ]. To meet increasing population demands, a cost-effective and environmentally friendly disposal method must be development. Common methods used in sewage sludge treatments in developing countries, such as incineration and landfilling, cause secondary environmental pollution [ 19 , 22 ]. By contrast, thermal treatment, which is an economical and environmentally friendly method for sewage sludge recycling, enables the conversion of sewage sludge waste into functional biochar and valuable biofuels [ 23 ]. Biochar is a type of carbon-rich, porous adsorption material widely used in wastewater treatment and pollution remediation because of its exceptional structural qualities, abundant supplies, environmental friendliness, recyclability, and affordability [ 24 – 26 ]. Therefore, the thermal treatment of sewage sludge waste to yield biochar is important, as its aromatic hydrocarbon surface and oxygen-containing groups can be used to attain effective phosphate adsorption from wastewater and reduce the leaching of heavy metals, organic pollutants, and pathogens in sewage sludge products into soil and water for utilizing sewage sludge waste resources. Nevertheless, the poor dispersion and limited adsorption capacity of unmodified biochar prevent its wide application in water remediation [ 27 , 28 ]. Therefore, further modifications of sewage sludge biochar (SBC) are essential for its improved performance Given the high affinity of phosphate for metal oxides and its reaction with them to produce stable metal phosphates, metal-modified adsorbents are often used for phosphate removal [ 29 ]. Lanthanum (La) is an environmentally friendly and benign rare earth element [ 30 , 31 ], which has an abundant content and is inexpensive compared with other rare earth materials [ 32 ]. Meanwhile, chemical bonds between La and phosphate are stronger than those between phosphate and other metallic elements, such as aluminum and iron [ 33 ]. La-containing materials have garnered considerable attention for P removal because of their superior stability in aqueous solutions and outstanding adsorption selectivity. Fang et al (2018) used a new magnetically recoverable magnetite/lanthanum hydroxide [M-La(OH) 3 ] for phosphate retention and recovery from lakes, demonstrating an adsorption capacity of up to 52.7 mg-P/g at pH 7.0 in water. La exhibits excellent selectivity, forms strong ionic bonds between La (III) and phosphate, and achieves good stability in aqueous solutions [ 34 ]. Min et al. (2019) synthesized a novel La-doped UiO-66, in which 0.2 La-UiO-66 exhibited a high phosphate adsorption capacity, with the maximum adsorption capacity reaching 348.43 mg · g − 1 . La modification increased the number of exposed adsorption sites and improved the phosphate adsorption of adsorbents [ 35 ]. Moreover, the addition of La may cause the biochar surface to become substantially less negatively charged; thus, the repulsion between the phosphate and biochar surface is weakened, increasing the likelihood of collisions [ 36 ]. Therefore, La-modified biochar can exhibit improved phosphate removal capacity from water. Therefore, in this study, SBC was synthesized through low-temperature oxygen-limited pyrolysis and was further

Sustainability 2024 , 16 , 5667 3 of 15 modified with lanthanum nitrate to form La-modified sludge biochar (LaSBC). The effects of the starting dose, solution pH, contact duration, and phosphate initial concentration on the adsorption process were investigated. The adsorption mechanism was discussed, together with the investigation of adsorption isotherms, kinetics, and thermodynamics. Furthermore, the dynamic adsorption capacity of adsorbents for actual wastewater in a fixed-bed column was evaluated. Thus, an innovative and cost-effective LaSBC using lanthanum nitrate as a modifier, proposed in this study, will provide a foundation for the modification of inexpensive and highly effective adsorbents 2. Materials and Method 2.1. Preparation of Materials SBC was produced via a low-temperature limited-oxygen pyrolysis. The dewatering sludge samples were collected from a sewage treatment plant in Guangxi, China, and then dried at 105 ◦ C for 24 h. The dried sludge was ground and passed through a 120-mesh sieve. The sieved sludge powder was pyrolyzed in a tubular quartz furnace (SK 2-6-12, Zhongyang Machinery Factory, Zhengzhou, China) at a rate of 10 ◦ C/min to 600 ◦ C, held at the target temperature for 3 h, surrounded by nitrogen gas [ 37 , 38 ], and then cooled to room temperature. The material acquired was labeled SBC The LaSBC was produced as follows: the SBC and analytical-grade La nitrate hexahydrate (La(NO 3 ) 3 6 H 2 O) were dissolved in 40 mL ultrapure water at a 1:2 mass ratio. A magnetic stirrer was used to constantly agitate the mixture for 3 h at room temperature When the solid had fully settled, the supernatant was drained off. After drying at 50 ◦ C for 12 h and washing with ultrapure water, the LaSBC was finally produced 2.2. Material Characterization The surface morphology of various samples was determined via scanning electron microscopy (SEM; JSM-7900 F, JEOL Ltd., Akishima, Japan). The elemental distribution of the adsorbent was analyzed using a field emission scanning electron microscope–energydispersive spectrometry (EDS; HitachiS-4800, Hitachi High Technology, Akishima, Japan) The surface functional groups of various samples before and after phosphate adsorption were investigated using Fourier transform infrared spectroscopy (FTIR; Nicolet Nexus 470, GMI-Technology Solutions, Phoneix, AZ, USA) in the 4000–500 cm − 1 range. X-ray photoelectron spectroscopy (XPS; ESCALAB 250 xi, Thermo Fisher Scientific, Waltham, MA, USA) was employed to analyze changes in functional groups and surface elemental contents. X-ray diffraction (XRD; X’Pert 3 Powder, DKSH Group, Zurich, Switzerland) was utilized to determine the surface crystal structure of the samples. Further, the point of zero charge (pH pzc ) of various samples was ascertained via a zeta potential analyzer 2.3. Batch Adsorption Stock solutions containing 1000 mg · L − 1 P(V) were prepared by dissolving potassium dihydrogen phosphate (KH 2 PO 4 ) in ultrapure water. The stock solutions were then diluted to prepare the phosphate solutions required for the subsequent studies. The pH in this experiment was adjusted by adding 0.1 mol · L − 1 HCl or NaOH. The entire adsorption process was performed in the oscillator of a thermostatic incubator Adsorption kinetics and isotherm models were used to investigate the overall interaction between phosphate and the adsorbents. The effects of the environmental factors (pH and adsorbent dosage) on the adsorption of phosphate on LaSBC were explored in this study. All experiments were repeated thrice. The operation steps of bath adsorption, calculation, and the fitting of graphs are described in the Supplementary Materials 2.4. Adsorption and Desorption Experiments For adsorption experiments, a 250 mL beaker was filled with a 140 mL 100 mg · L − 1 phosphate solution, and 0.7 g LaSBC was added to it, and the resulting mixture was shaken

Sustainability 2024 , 16 , 5667 4 of 15 at 25 ◦ C for 24 h. After filtration, the concentration of P in the supernatant liquid was determined and q e was calculated For desorption experiments, filtered LaSBC was washed with ultrapure water for the removal of residual phosphate solutions, and adsorbents were subsequently dried in a drying oven at 50 ◦ C for 12 h. The obtained product is the material, adsorbed once Subsequently, 140 mL 0.2 mol · L − 1 NaCl solution was mixed with 0.7 g of the material adsorbed once in 250 mL beakers. In addition, desorption was performed via constant shaking at 25 ◦ C and 200 r · min − 1 for 24 h. The filtered adsorbent, which is the material recycled once, was washed with ultrapure water until the pH became constant and dried The LaSBC was recycled six times to assess the recyclability of the adsorbents 2.5. Fixed-Bed Column Experiments To determine the dynamic adsorption capacity of the adsorbents, we used a plexiglass column with an inner diameter of 10 mm and a height of 100 mm as a fixed-bed column reactor. Continuous fixed-bed column experiments were performed using an 8 cm deep fixed-bed column with various adsorbent amounts (1 and 2 g). The P removal effect of the modified biochar bed was studied at a certain flow rate (5 mL · min − 1 ) and the phosphate concentration (3.08 mg P/L) used in WWTPs. Dynamic adsorption involved the use of a peristaltic pump to pump the phosphate solution upward through the column and the collection of 5 mL samples at a given time interval for its analysis 3. Results and Discussion 3.1. Material Characterization The SEM characterizations of the SBC and LaSBC are displayed in Figure 1 a–c. In addition, the EDS image of the LaSBC is presented in Figure 1 d. Figure 1 a shows a comparatively uniform block structure with a smooth surface and no holes, which resulted from the preservation of a substantial amount of ash and volatiles at a low preparation temperature [ 39 ]. After the modification with La, this smooth block structure was converted into irregular thick lumps (Figure 1 b). The surface of the LaSBC contained abundant layered pores with numerous rough protuberances and grooves, demonstrating the effective grafting of La onto the LaSBC surface. This La grafting could also be directly proven by the presence of La in the EDS image (Figure 1 d). Fine pore structures were well developed on the LaSBC surface after the loading of La 3+ . The increase in fine pore structures provided more active sites for phosphate adsorption. After phosphate adsorption, the LaSBC surface was clustered, and the pores were completely occupied by phosphate (Figure 1 c). The FTIR spectra primarily represent changes in functional groups [ 40 ]. Figure 2 displays the FTIR spectra of LaSBC before and after adsorption. The peaks at 3430 and 1399 cm − 1 can be attributed to the stretching vibrations of –OH [ 29 , 41 ], and those at 1633 and 1034 cm − 1 can be associated with the C=C stretching vibration of the aromatic rings and the C–O stretching vibration, respectively [ 42 – 45 ]. The LaSBC spectra show an increase in the intensity and a shift in their –OH peaks compared with the SBC spectrum, suggesting the existence of several –OH groups that effectively adsorb phosphate [ 46 ]. Two distinct bands are observed at ~766 and ~711 cm − 1 when comparing the FTIR spectra of SBC and LaSBC, which are characteristics of La–OH bond vibrations [ 47 ]. Moreover, the absorption peaks at 1461 and 1337 cm − 1 are related to the vibration mode of the NO 3 − anion in the feedstock La(NO 3 ) 3 . After phosphate adsorption, the bands at 621 and 538 cm − 1 are attributed to O–P–O bending vibrations [ 48 , 49 ]. Furthermore, a new peak at 593 cm − 1 in the LaSBC-P spectrum originates from the La-O coordination between the active La site and oxygen anion in the phosphate molecule [ 47 , 50 ]. The observed alterations can be attributed to the phosphate-active La site reaction, which signifies the effective loading of La groups onto the SBC surface after La modification.

Sustainability 2024 , 16 , 5667 5 of 15 Sustainability 2024 , 16 , x FOR PEER REVIEW 4 of 16 For adsorption experiments, a 250 mL beaker was fi lled with a 140 mL 100 mg·L − 1 phosphate solution, and 0.7 g LaSBC was added to it, and the resulting mixture was shaken at 25 °C for 24 h. After fi ltration, the concentration of P in the supernatant liquid was determined and q e was calculated For desorption experiments, fi ltered LaSBC was washed with ultrapure water for the removal of residual phosphate solutions, and adsorbents were subsequently dried in a drying oven at 50 °C for 12 h. The obtained product is the material, adsorbed once. Subsequently, 140 mL 0.2 mol·L − 1 NaCl solution was mixed with 0.7 g of the material adsorbed once in 250 mL beakers. In addition, desorption was performed via constant shaking at 25 °C and 200 r·min − 1 for 24 h. The fi ltered adsorbent, which is the material recycled once, was washed with ultrapure water until the pH became constant and dried. The LaSBC was recycled six times to assess the recyclability of the adsorbents. 2.5. Fixed-Bed Column Experiments To determine the dynamic adsorption capacity of the adsorbents, we used a plexiglass column with an inner diameter of 10 mm and a height of 100 mm as a fi xed-bed column reactor. Continuous fi xed-bed column experiments were performed using an 8 cm deep fi xed-bed column with various adsorbent amounts (1 and 2 g). The P removal e ff ect of the modi fi ed biochar bed was studied at a certain fl ow rate (5 mL·min − 1 ) and the phosphate concentration (3.08 mg P/L) used in WWTPs. Dynamic adsorption involved the use of a peristaltic pump to pump the phosphate solution upward through the column and the collection of 5 mL samples at a given time interval for its analysis. 3. Results and Discussion 3.1. Material Characterization The SEM characterizations of the SBC and LaSBC are displayed in Figure 1 a–c. In addition, the EDS image of the LaSBC is presented in Figure 1 d. Figure 1 a shows a comparatively uniform block structure with a smooth surface and no holes, which resulted from the preservation of a substantial amount of ash and volatiles at a low preparation temperature [39]. After the modi fi cation with La, this smooth block structure was converted into irregular thick lumps (Figure 1 b). The surface of the LaSBC contained abundant layered pores with numerous rough protuberances and grooves, demonstrating the e ff ective grafting of La onto the LaSBC surface. This La grafting could also be directly proven by the presence of La in the EDS image (Figure 1 d). Fine pore structures were well developed on the LaSBC surface after the loading of La 3+ . The increase in fi ne pore structures provided more active sites for phosphate adsorption. After phosphate adsorption, the LaSBC surface was clustered, and the pores were completely occupied by phosphate (Figure 1 c). Sustainability 2024 , 16 , x FOR PEER REVIEW 5 of 16 Figure 1. SEM images of SBC ( a ), LaSBC ( b ), and LaSBC-P ( c ), and EDS image of LaSBC ( d ). The FTIR spectra primarily represent changes in functional groups [40]. Figure 2 displays the FTIR spectra of LaSBC before and after adsorption. The peaks at 3430 and 1399 cm − 1 can be a tt ributed to the stretching vibrations of ‒ OH [29,41], and those at 1633 and 1034 cm − 1 can be associated with the C=C stretching vibration of the aromatic rings and the C ‒ O stretching vibration, respectively [42–45]. The LaSBC spectra show an increase in the intensity and a shift in their ‒ OH peaks compared with the SBC spectrum, suggesting the existence of several ‒ OH groups that e ff ectively adsorb phosphate [46]. Two distinct bands are observed at ~766 and ~711 cm − 1 when comparing the FTIR spectra of SBC and LaSBC, which are characteristics of La–OH bond vibrations [47]. Moreover, the absorption peaks at 1461 and 1337 cm − 1 are related to the vibration mode of the NO 3- anion in the feedstock La(NO 3 ) 3 . After phosphate adsorption, the bands at 621 and 538 cm − 1 are attributed to O ‒ P ‒ O bending vibrations [48,49]. Furthermore, a new peak at 593 cm − 1 in the LaSBC-P spectrum originates from the La ‒ O coordination between the active La site and oxygen anion in the phosphate molecule [47,50]. The observed alterations can be attributed to the phosphate-active La site reaction, which signi fi es the e ff ective loading of La groups onto the SBC surface after La modi fi cation. Figure 2. FTIR spectra of SBC, LaSBC, and LaSBC-P. Additional information regarding the degree of crystallinity and material structure of the biochar materials was revealed by the XRD spectra. The XRD spectra of LaSBC and LaSBC-P (Figure 3) showed di ff raction peaks at 13.238°, 18.901°, 20.917°, 31.240°, and 36.594°, which can be con fi dently matched with the characteristic peaks of La(OH) 3 [51]. Remarkably, the di ff raction peaks corresponding to crystalline carbon structures, such as Figure 1. SEM images of SBC ( a ), LaSBC ( b ), and LaSBC-P ( c ), and EDS image of LaSBC ( d ) Sustainability 2024 , 16 , x FOR PEER REVIEW 5 of 16 Figure 1. SEM images of SBC ( a ), LaSBC ( b ), and LaSBC-P ( c ), and EDS image of LaSBC ( d ). The FTIR spectra primarily represent changes in functional groups [40]. Figure 2 displays the FTIR spectra of LaSBC before and after adsorption. The peaks at 3430 and 1399 cm − 1 can be a tt ributed to the stretching vibrations of ‒ OH [29,41], and those at 1633 and 1034 cm − 1 can be associated with the C=C stretching vibration of the aromatic rings and the C ‒ O stretching vibration, respectively [42–45]. The LaSBC spectra show an increase in the intensity and a shift in their ‒ OH peaks compared with the SBC spectrum, suggesting the existence of several ‒ OH groups that e ff ectively adsorb phosphate [46]. Two distinct bands are observed at ~766 and ~711 cm − 1 when comparing the FTIR spectra of SBC and LaSBC, which are characteristics of La–OH bond vibrations [47]. Moreover, the absorption peaks at 1461 and 1337 cm − 1 are related to the vibration mode of the NO 3- anion in the feedstock La(NO 3 ) 3 . After phosphate adsorption, the bands at 621 and 538 cm − 1 are attributed to O ‒ P ‒ O bending vibrations [48,49]. Furthermore, a new peak at 593 cm − 1 in the LaSBC-P spectrum originates from the La ‒ O coordination between the active La site and oxygen anion in the phosphate molecule [47,50]. The observed alterations can be attributed to the phosphate-active La site reaction, which signi fi es the e ff ective loading of La groups onto the SBC surface after La modi fi cation. Figure 2. FTIR spectra of SBC, LaSBC, and LaSBC-P. Additional information regarding the degree of crystallinity and material structure of the biochar materials was revealed by the XRD spectra. The XRD spectra of LaSBC and LaSBC-P (Figure 3) showed di ff raction peaks at 13.238°, 18.901°, 20.917°, 31.240°, and 36.594°, which can be con fi dently matched with the characteristic peaks of La(OH) 3 [51]. Remarkably, the di ff raction peaks corresponding to crystalline carbon structures, such as Figure 2. FTIR spectra of SBC, LaSBC, and LaSBC-P Additional information regarding the degree of crystallinity and material structure of the biochar materials was revealed by the XRD spectra. The XRD spectra of LaSBC and LaSBC-P (Figure 3 ) showed diffraction peaks at 13.238 ◦ , 18.901 ◦ , 20.917 ◦ , 31.240 ◦ , and 36.594 ◦ , which can be confidently matched with the characteristic peaks of La(OH) 3 [ 51 ]. Remarkably, the diffraction peaks corresponding to crystalline carbon structures, such as cellulose and lignin, were detected at a 2 θ 26.662 ◦ [ 39 , 52 ]. This LaSBC peak was comparatively weaker than that of SBC, most likely because the heavier La 3+ cations increased the absorption of X-ray radiation. Such a mechanism implies that lanthanide atoms are more effective at the diffusion of X-ray energy compared with sodium cations in biochar After the exchange of Na + with La 3+ , the electron density distribution varied dramatically This condition resulted in the variation in beam interference, which reflects the relative intensities of individual reflections [ 53 ].

Sustainability 2024 , 16 , 5667 6 of 15 Sustainability 2024 , 16 , x FOR PEER REVIEW 6 of 16 cellulose and lignin, were detected at a 2 θ 26.662° [39,52]. This LaSBC peak was comparatively weaker than that of SBC, most likely because the heavier La 3+ cations increased the absorption of X-ray radiation. Such a mechanism implies that lanthanide atoms are more e ff ective at the di ff usion of X-ray energy compared with sodium cations in biochar. After the exchange of Na + with La 3+ , the electron density distribution varied dramatically. This condition resulted in the variation in beam interference, which re fl ects the relative intensities of individual re fl ections [53]. Figure 3. XRD spectra of SBC, LaSBC, and LaSBC-P. XPS examination was performed to investigate changes in elements and chemical states on the LaSBC surface during adsorption (Figure 4). The peak of P 2 p, which was clearly detected in LaSBC-P at a binding energy of 133.61 eV, con fi rmed the e ff ective adsorption of phosphate on LaSBC. Figure 3. XRD spectra of SBC, LaSBC, and LaSBC-P XPS examination was performed to investigate changes in elements and chemical states on the LaSBC surface during adsorption (Figure 4 ). The peak of P 2 p, which was clearly detected in LaSBC-P at a binding energy of 133.61 eV, confirmed the effective adsorption of phosphate on LaSBC Sustainability 2024 , 16 , x FOR PEER REVIEW 6 of 16 cellulose and lignin, were detected at a 2 θ 26.662° [39,52]. This LaSBC peak was comparatively weaker than that of SBC, most likely because the heavier La 3+ cations increased the absorption of X-ray radiation. Such a mechanism implies that lanthanide atoms are more e ff ective at the di ff usion of X-ray energy compared with sodium cations in biochar. After the exchange of Na + with La 3+ , the electron density distribution varied dramatically. This condition resulted in the variation in beam interference, which re fl ects the relative intensities of individual re fl ections [53]. Figure 3. XRD spectra of SBC, LaSBC, and LaSBC-P. XPS examination was performed to investigate changes in elements and chemical states on the LaSBC surface during adsorption (Figure 4). The peak of P 2 p, which was clearly detected in LaSBC-P at a binding energy of 133.61 eV, con fi rmed the e ff ective adsorption of phosphate on LaSBC. Sustainability 2024 , 16 , x FOR PEER REVIEW 7 of 16 Figure 4. XPS spectra of LaSBC before and after adsorption. Full-spectrum diagram, ( a ) XPS spectra of La 3 d ( b , c ) before and after adsorption, and XPS spectra of P 2 p ( d ) after adsorption. 3.2. E ff ect of Environmental Parameters 3.2.1. E ff ect of pH The solution pH in fl uences the surface charge properties of the adsorbent, thereby changing the interaction between the adsorbent and adsorbate and enabling the determination of the main types of phosphate (including H 3 PO 4 , H 2 PO 4 − , HPO 42 − , and PO 43 − ) [36,54]. Figure 5 a illustrates the e ff ect of pH on the phosphate absorption on SBC and LaSBC. The adsorption capacity ( q e ) and removal rate of LaSBC in the pH 3–11 range were considerably improved compared with those of the SBC. In an acidic environment, the capability of the LaSBC to adsorb phosphates increased with an increase in the pH from 3 to 5. Neutral H 3 PO 4 was the predominant phosphate species in low-pH solutions (pH < 3). Furthermore, it exhibited poor binding to the adsorption sites of LaSBC, due to the absence of ion exchange and electrostatic forces during the removal process. Meanwhile, the main phosphate species transformed into H 2 PO 4 − and HPO 42 − at a pH between 5 and 7, and protonation left adsorbent surfaces positively charged. As a result, phosphate species were adsorbed onto the biochar via ion exchange and electrostatic forces. The maximum removal e ffi ciency of LaSBC reached 99.06% at pH 5. The ion exchange and electrostatic interactions were less pronounced in an alkaline environment because of the substantial competition between hydroxide and phosphate and the deprotonation of the adsorbent surface. The electrostatic a tt raction between anions and adsorbents weakened with an increase in the OH − concentration [53]. Further, the precipitation of La(III) hydroxide impeded LaSBC adsorption at pH 11. Figure 4. XPS spectra of LaSBC before and after adsorption. Full-spectrum diagram, ( a ) XPS spectra of La 3 d ( b , c ) before and after adsorption, and XPS spectra of P 2 p ( d ) after adsorption.

Sustainability 2024 , 16 , 5667 7 of 15 3.2. Effect of Environmental Parameters 3.2.1. Effect of pH The solution pH influences the surface charge properties of the adsorbent, thereby changing the interaction between the adsorbent and adsorbate and enabling the determination of the main types of phosphate (including H 3 PO 4 , H 2 PO 4 − , HPO 4 2 − , and PO 4 3 − ) [ 36 , 54 ]. Figure 5 a illustrates the effect of pH on the phosphate absorption on SBC and LaSBC. The adsorption capacity ( q e ) and removal rate of LaSBC in the pH 3–11 range were considerably improved compared with those of the SBC. In an acidic environment, the capability of the LaSBC to adsorb phosphates increased with an increase in the pH from 3 to 5. Neutral H 3 PO 4 was the predominant phosphate species in low-pH solutions (pH < 3). Furthermore, it exhibited poor binding to the adsorption sites of LaSBC, due to the absence of ion exchange and electrostatic forces during the removal process. Meanwhile, the main phosphate species transformed into H 2 PO 4 − and HPO 4 2 − at a pH between 5 and 7, and protonation left adsorbent surfaces positively charged. As a result, phosphate species were adsorbed onto the biochar via ion exchange and electrostatic forces. The maximum removal efficiency of LaSBC reached 99.06% at pH 5. The ion exchange and electrostatic interactions were less pronounced in an alkaline environment because of the substantial competition between hydroxide and phosphate and the deprotonation of the adsorbent surface. The electrostatic attraction between anions and adsorbents weakened with an increase in the OH − concentration [ 53 ]. Further, the precipitation of La(III) hydroxide impeded LaSBC adsorption at pH 11 Sustainability 2024 , 16 , x FOR PEER REVIEW 7 of 16 Figure 4. XPS spectra of LaSBC before and after adsorption. Full-spectrum diagram, ( a ) XPS spectra of La 3 d ( b , c ) before and after adsorption, and XPS spectra of P 2 p ( d ) after adsorption. 3.2. E ff ect of Environmental Parameters 3.2.1. E ff ect of pH The solution pH in fl uences the surface charge properties of the adsorbent, thereby changing the interaction between the adsorbent and adsorbate and enabling the determination of the main types of phosphate (including H 3 PO 4 , H 2 PO 4 − , HPO 42 − , and PO 43 − ) [36,54]. Figure 5 a illustrates the e ff ect of pH on the phosphate absorption on SBC and LaSBC. The adsorption capacity ( q e ) and removal rate of LaSBC in the pH 3–11 range were considerably improved compared with those of the SBC. In an acidic environment, the capability of the LaSBC to adsorb phosphates increased with an increase in the pH from 3 to 5. Neutral H 3 PO 4 was the predominant phosphate species in low-pH solutions (pH < 3). Furthermore, it exhibited poor binding to the adsorption sites of LaSBC, due to the absence of ion exchange and electrostatic forces during the removal process. Meanwhile, the main phosphate species transformed into H 2 PO 4 − and HPO 42 − at a pH between 5 and 7, and protonation left adsorbent surfaces positively charged. As a result, phosphate species were adsorbed onto the biochar via ion exchange and electrostatic forces. The maximum removal e ffi ciency of LaSBC reached 99.06% at pH 5. The ion exchange and electrostatic interactions were less pronounced in an alkaline environment because of the substantial competition between hydroxide and phosphate and the deprotonation of the adsorbent surface. The electrostatic a tt raction between anions and adsorbents weakened with an increase in the OH − concentration [53]. Further, the precipitation of La(III) hydroxide impeded LaSBC adsorption at pH 11. Figure 5. Effect of pH on the phosphate adsorption capacities and removal rate of SBC and LaSBC ( a ): zeta potential of SBC and LaSBC before phosphate sorption at various pH levels ( b ) As a result of zeta potential changes, the surface charge of the LaSBC was altered dramatically with pH (Figure 5 b). Over a broad pH range, the electronegativity of the LaSBC was lower than that of the SBC, indicating that the addition of La likely decreased the negative charge on the SBC surface via the formation of metal oxides on the surface [ 11 , 55 ]. Given the decrease in the negative charge, phosphate adsorption improved due to a reduction in the electrostatic repulsion between the biochar and anions [ 11 ]. However, in an extremely alkaline environment (pH 11), the electronegativity of the SBC and the LaSBC was incredibly high, which revealed that excessively alkaline surroundings negatively influence phosphate adsorption [ 11 ]. 3.2.2. Effect of Adsorbent Dosages The influence of LaSBC dosage on the adsorption process is presented in Figure 6 . With an increase in adsorbent dosage from 0.004 to 0.06 g, the adsorption capacity ( q e ) of the LaSBC dropped from 30.77 mg/g to 7.44 mg/g under the same starting phosphate concentration of 30 mg/L. However, the percentage removal of phosphate improved

Sustainability 2024 , 16 , 5667 8 of 15 from 68.37% to 99.17%. This result may be due to the quantity of active sites on the adsorbent and that were accessible for the adsorption process. The active sites accessible for phosphate adsorption increased with the increase in adsorbent dosage, which allowed for the adsorption of more phosphate and led to a better removal efficiency. As the amount of adsorbate in the solution was fixed, a decrease in the saturation of these active sites with phosphate occurred concurrently with this rise in active sites, which resulted in the observed decrease in q e values [ 56 ]. Sustainability 2024 , 16 , x FOR PEER REVIEW 8 of 16 Figure 5. E ff ect of pH on the phosphate adsorption capacities and removal rate of SBC and LaSBC ( a ): zeta potential of SBC and LaSBC before phosphate sorption at various pH levels ( b ). As a result of zeta potential changes, the surface charge of the LaSBC was altered dramatically with pH (Figure 5 b). Over a broad pH range, the electronegativity of the LaSBC was lower than that of the SBC, indicating that the addition of La likely decreased the negative charge on the SBC surface via the formation of metal oxides on the surface [11,55]. Given the decrease in the negative charge, phosphate adsorption improved due to a reduction in the electrostatic repulsion between the biochar and anions [11]. However, in an extremely alkaline environment (pH 11), the electronegativity of the SBC and the LaSBC was incredibly high, which revealed that excessively alkaline surroundings negatively in fl uence phosphate adsorption [11]. 3.2.2. E ff ect of Adsorbent Dosages The in fl uence of LaSBC dosage on the adsorption process is presented in Figure 6. With an increase in adsorbent dosage from 0.004 to 0.06 g, the adsorption capacity ( q e ) of the LaSBC dropped from 30.77 mg/g to 7.44 mg/g under the same starting phosphate concentration of 30 mg/L. However, the percentage removal of phosphate improved from 68.37% to 99.17%. This result may be due to the quantity of active sites on the adsorbent and that were accessible for the adsorption process. The active sites accessible for phosphate adsorption increased with the increase in adsorbent dosage, which allowed for the adsorption of more phosphate and led to a be tt er removal e ffi ciency. As the amount of adsorbate in the solution was fi xed, a decrease in the saturation of these active sites with phosphate occurred concurrently with this rise in active sites, which resulted in the observed decrease in q e values [56]. Figure 6. E ff ect of dosage on phosphate adsorption capacities and removal rate of LaSBC. 3.3. Adsorption Kinetics To be tt er understand the adsorption process, we used several adsorption models for investigating the adsorption behavior of adsorbents (Figure 7). The reversible physical adsorption process is commonly described using a pseudofi rst-order model, while the chemisorption process between an adsorbent and an adsorbate is explained via a pseudosecond-order model [57]. Figure 6. Effect of dosage on phosphate adsorption capacities and removal rate of LaSBC 3.3. Adsorption Kinetics To better understand the adsorption process, we used several adsorption models for investigating the adsorption behavior of adsorbents (Figure 7 ). The reversible physical adsorption process is commonly described using a pseudo-first-order model, while the chemisorption process between an adsorbent and an adsorbate is explained via a pseudosecond-order model [ 57 ]. Sustainability 2024 , 16 , x FOR PEER REVIEW 9 of 16 Figure 7. Kinetic process for phosphate adsorption on LaSBC. Table S 1 displays the fi tt ing results obtained using the pseudofi rst-order and pseudo-second-order kinetic models. The coe ffi cient of determination (R 2 = 0.978, 0.979, and 0.961, respectively) of the pseudo-second-order kinetic model at 15 °C, 25 °C, and 45 °C was higher than that of the pseudofi rst-order kinetic model (R 2 = 0.951, 0.946, and 0.958, respectively), indicating that chemisorption was predominant in phosphate adsorption on LaSBC. Additionally, the adsorption rate was controlled by the number of unoccupied sites on the LaSBC. The primary cause for this phenomenon was that a signi fi cant number of La-based surface active sites was created following La doping, which paved the way for the chemical adsorption process [52]. 3.4. Adsorption Isotherms Phosphate adsorption from aqueous solutions on the biochar surface is described mathematically using the Langmuir or Freundlich isotherm, which facilitates the analysis of the adsorbent–adsorbate interaction and the structural properties of the adsorption layer [58]. The fi tt ing curve of unary systems are shown in Figure 8, and the model parameters are listed in Table 1. The Langmuir model assumes the single-layer adsorption of the adsorbate on adsorbents, while the Freundlich model implies multilayer adsorption. Table 1 shows the multilayer adsorption of phosphate on the LaSBC surface, with the fi tt ed R 2 of the Freundlich model for phosphate adsorption at 15 °C, 25 °C, 35 °C, 45 °C, and 65 °C being greater than those of the Langmuir model. As a result, the adsorption of phosphate onto LaSBC was heterogeneous, including interactions between adsorbed molecules, and an uneven surface energy [59,60]. The a ffi nity of the adsorbate for the adsorbent is determined using the heterogeneity factor (1/ n ): the closer the factor is to zero, the stronger the a ffi nity [58]. According to Table 1, the adsorbent had a su ffi cient adsorption capacity for phosphate, as the values of 1/ n were less than one. As can be seen from Figure 8, the adsorption capacity of LaSBC rose with an increase in the phosphate initial concentration. This condition might have resulted from the high phosphate concentration, associated with a high driving-force concentration gradient [61]. In addition, more phosphate ions mean more opportunities for adsorbent capture. Furthermore, when the adsorption temperature increased from 15 °C to 65 °C, the adsorption capacity of the adsorbent for phosphate gradually increased, indicating that an elevated temperature is conducive for phosphate removal. This is because an increase in temperature accelerates the di ff usion rate of phosphate to the active sites on the LaSBC surface [40]. As shown in Table 1, according to the Langmuir model results, the maximum adsorption capacities of LaSBC toward phosphate were 84.249, 99.910, 114.647, 130.314, and Figure 7. Kinetic process for phosphate adsorption on LaSBC Table S 1 displays the fitting results obtained using the pseudo-first-order and pseudosecond-order kinetic models. The coefficient of determination (R 2 = 0.978, 0.979, and 0.961, respectively) of the pseudo-second-order kinetic model at 15 ◦ C, 25 ◦ C, and 45 ◦ C was higher than that of the pseudo-first-order kinetic model (R 2 = 0.951, 0.946, and 0.958, respectively), indicating that chemisorption was predominant in phosphate adsorption on LaSBC. Additionally, the adsorption rate was controlled by the number of unoccupied sites

Sustainability 2024 , 16 , 5667 9 of 15 on the LaSBC. The primary cause for this phenomenon was that a significant number of La-based surface active sites was created following La doping, which paved the way for the chemical adsorption process [ 52 ]. 3.4. Adsorption Isotherms Phosphate adsorption from aqueous solutions on the biochar surface is described mathematically using the Langmuir or Freundlich isotherm, which facilitates the analysis of the adsorbent–adsorbate interaction and the structural properties of the adsorption layer [ 58 ]. The fitting curve of unary systems are shown in Figure 8 , and the model parameters are listed in Table 1 . The Langmuir model assumes the single-layer adsorption of the adsorbate on adsorbents, while the Freundlich model implies multilayer adsorption Table 1 shows the multilayer adsorption of phosphate on the LaSBC surface, with the fitted R 2 of the Freundlich model for phosphate adsorption at 15 ◦ C, 25 ◦ C, 35 ◦ C, 45 ◦ C, and 65 ◦ C being greater than those of the Langmuir model. As a result, the adsorption of phosphate onto LaSBC was heterogeneous, including interactions between adsorbed molecules, and an uneven surface energy [ 59 , 60 ]. The affinity of the adsorbate for the adsorbent is determined using the heterogeneity factor (1/ n ): the closer the factor is to zero, the stronger the affinity [ 58 ]. According to Table 1 , the adsorbent had a sufficient adsorption capacity for phosphate, as the values of 1/ n were less than one Sustainability 2024 , 16 , x FOR PEER REVIEW 10 of 16 140.237 mg/g at the adsorption temperatures of 15 °C, 25 °C, 35 °C, 45 °C, and 65 °C, respectively. This result indicates that phosphate adsorption by LaSBC is an endothermic process [62]. Figure 8. Isotherm process for the phosphate adsorption of LaSBC. Table 1. Isotherm parameters for the phosphate adsorption of LaSBC. Samples Langmuir Freundlich Q m (mg/g) K L (L/g) R 2 K F (L/g) 1/ n R 2 LaSBC 15 °C 84.249 0.346 0.848 22.670 0.280 0.941 LaSBC 25 °C 99.910 0.213 0.871 24.876 0.288 0.940 LaSBC 35 °C 114.647 0.206 0.922 29.111 0.284 0.943 LaSBC 45 °C 130.314 0.103 0.905 28.458 0.304 0.952 LaSBC 65 °C 140.237 0.077 0.924 28.347 0.310 0.960 The maximum capacity of LaSBC at 65 °C of 140.237 mg/g was determined using the Langmuir model. The greater ion mobility in the solution and increased likelihood of adsorption sites colliding with phosphate ions accounted for the enhanced adsorption capacity at higher temperatures [36]. The P adsorption capacity of LaSBC is be tt er than the majority of other previously published metal-modi fi ed sludge adsorbents. Speci fi c metalmodi fi ed adsorbents and their values are listed in Table 2. Table 2. Comparison of P adsorption capacity of various metal-modi fi ed sludge biochars. Biochar Q m (mg/g) References La-modified sludge biochar (LaSBC) 140.237 This study Alkaline sludge biochar 42.51 [63] Calcium-containing paper-sludge-based biochar 68.49 [64] Iron-modified waste-biochar-sludgebased biochar 111.0 [25] Egg shell-modified sludge biochar 154.18 [65] Oyster shell-modified sludge biochar 129.03 [65] Dolomite-modified sludge biochar 29.18 [66] 3.5. Adsorption Thermodynamics Figure 8. Isotherm process for the phosphate adsorption of LaSBC Table 1. Isotherm parameters for the phosphate adsorption of LaSBC Samples Langmuir Freundlich Q m (mg/g) K L (L/g) R 2 K F (L/g) 1/ n R 2 LaSBC 15 ◦ C 84.249 0.346 0.848 22.670 0.280 0.941 LaSBC 25 ◦ C 99.910 0.213 0.871 24.876 0.288 0.940 LaSBC 35 ◦ C 114.647 0.206 0.922 29.111 0.284 0.943 LaSBC 45 ◦ C 130.314 0.103 0.905 28.458 0.304 0.952 LaSBC 65 ◦ C 140.237 0.077 0.924 28.347 0.310 0.960 As can be seen from Figure 8 , the adsorption capacity of LaSBC rose with an increase in the phosphate initial concentration. This condition might have resulted from the high phosphate concentration, associated with a high driving-force concentration gradient [ 61 ]. In addition, more phosphate ions mean more opportunities for adsorbent capture. Furthermore, when the adsorption temperature increased from 15 ◦ C to 65 ◦ C, the adsorption capacity of the adsorbent for phosphate gradually increased, indicating that an elevated temperature is conducive for phosphate removal. This is because an increase in temperature

Sustainability 2024 , 16 , 5667 10 of 15 accelerates the diffusion rate of phosphate to the active sites on the LaSBC surface [ 40 ]. As shown in Table 1 , according to the Langmuir model results, the maximum adsorption capacities of LaSBC toward phosphate were 84.249, 99.910, 114.647, 130.314, and 140.237 mg/g at the adsorption temperatures of 15 ◦ C, 25 ◦ C, 35 ◦ C, 45 ◦ C, and 65 ◦ C, respectively. This result indicates that phosphate adsorption by LaSBC is an endothermic process [ 62 ]. The maximum capacity of LaSBC at 65 ◦ C of 140.237 mg/g was determined using the Langmuir model. The greater ion mobility in the solution and increased likelihood of adsorption sites colliding with phosphate ions accounted for the enhanced adsorption capacity at higher temperatures [ 36 ]. The P adsorption capacity of LaSBC is better than the majority of other previously published metal-modified sludge adsorbents. Specific metal-modified adsorbents and their values are listed in Table 2 . Table 2. Comparison of P adsorption capacity of various metal-modified sludge biochars Biochar Q m (mg/g) References La-modified sludge biochar (LaSBC) 140.237 This study Alkaline sludge biochar 42.51 [ 63 ] Calcium-containing paper-sludge-based biochar 68.49 [ 64 ] Iron-modified waste-biochar-sludge-based biochar 111.0 [ 25 ] Egg shell-modified sludge biochar 154.18 [ 65 ] Oyster shell-modified sludge biochar 129.03 [ 65 ] Dolomite-modified sludge biochar 29.18 [ 66 ] 3.5. Adsorption Thermodynamics The adsorption process is spontaneous and entails modifications of thermodynamic functions. To establish the spontaneity and nature of the adsorption process, we calculated the thermodynamic parameters Gibbs free energy, enthalpy change, and entropy change using the Van ’t Hoff equation, and the obtained thermodynamic parameters are given in Table S 2. As presented in Table S 2, the reaction enthalpy ( ∆ H ) was 954.963 kJ/mol, which suggests the rate of the endothermic phosphate adsorption process on the surface of the LaSBC. This parameter’s positive and large value (>50 kJ mol − 1 ) implies that a chemical reaction underlies the adsorption process [ 56 ]. The free enthalpy of negative adsorption ( ∆ G ) values means that phosphate adsorption onto the LaSBC occurred spontaneously. The positive values of ∆ S indicated the increased unpredictability at the solid–liquid interface during adsorption. Thus, the adsorption of P ions on La-modified biochar is driven by entropy [ 67 ]. 3.6. Adsorption and Desorption Studies The stability and recyclability of adsorbents in the adsorption–desorption process are vital to the cost effectiveness of the adsorption process [ 6 , 56 ]. To assess the potential of the adsorbent for reuse in the subsequent phosphate adsorption study, 0.2 mol · L − 1 NaCl was employed in desorption investigations. The adsorption capacity following desorption is shown in Figure S 1, demonstrating a relatively stable adsorption capacity in six cycles. Specifically, 88.0% of the initial q e was found to have been retained after the first desorption regeneration. The adsorption capacity after six cycles was 72.3% of the initial q e , and it decreased by 15.7% in total. This trend could be due to the formation of La-P compounds and multilayer structures, saturating adsorption sites and preventing some sites from further adsorption [ 56 ]. The adsorption capacities of LaSBC remained above 70% after repeated adsorption and desorption, demonstrating that LaSBC is economical and practical for phosphate adsorption 3.7. Dynamic Adsorption Studies To evaluate the practical feasibility of LaSBC, continuous dynamic adsorption experiments were carried out on actual wastewater (P concentration of 3.08 mg/L) using a fixed-bed column. At the adsorbent dosages of 1 and 2 g, the breakthrough curves for

Sustainability 2024 , 16 , 5667 11 of 15 LaSBC are displayed in Figure S 2. The effective treatment times of the adsorbent were 7.58 and 9.08 h, respectively, when the phosphate content in the effluent surpassed the current maximum permitted P concentration (0.5 mg · L − 1 ). The difference in time could be attributed to an increase in the amount of LaSBC, increasing the surface area of the adsorption layer and binding sites for phosphate adsorption [ 40 ]. The results showed that LaSBC had a long sustainable treatment time, which proves the feasibility of its practical application in polluted water 3.8. Adsorption Mechanisms Figure 9 illustrates the possible mechanisms underlying phosphate adsorption by LaSBC from an aqueous solution. The appearance of LaPO 4 peaks in the XRD of LaSBC-P suggests that LaPO 4 precipitation was one of the key processes for phosphate removal by LaSBC. La exhibits a strong affinity for phosphate because of the low Ksp value of LaPO 4 (3.7 × 10 − 23 ) [ 29 , 68 ]. La produces complex compounds with PO 4 3- ions, even at low concentrations in solutions [ 69 ]. Thus, precipitation serves as a crucial adsorption process for phosphate removal [ 29 ]. Sustainability 2024 , 16 , x FOR PEER REVIEW 12 of 16 where pK 1 = 2.13, pK 2 = 7.20, and pK 3 = 12.33 Adsorbents containing metal are positively charged at low pH levels because metal hydroxides are readily protonated, and at high pH levels, they are negatively charged, as metal hydroxides are easily deprotonated under this condition [11,40,70]. As displayed in Figure 4 b, pH pzc was 7.65 for LaSBC. The LaSBC was positively charged and protonated (La-OH + H + ↔ La-OH 2+ ) at a pH of 2.0–7.65. This condition allowed the LaSBC to adsorb phosphate species H 2 PO 4 − or HPO 42 − through electrostatic a tt raction. In addition, in the pH range of 7.65–12.0, LaSBC was negatively charged and deprotonated. The adsorption of phosphate tended toward a lower pH, and the surface anion exchange process was followed by OH - release [71]. The solution pH rose as anion exchange advanced [11]. With an increase in pH, the e ff ect of electrostatic a tt raction diminished rapidly. However, hydroxyl groups present in the solution would undergo anion exchange with phosphate ions and form chemical bonds through inner-sphere complexation to achieve phosphate removal. Thus, anion exchange and inner-sphere complexation are inseparable from one another. FTIR and XPS measurements demonstrated that inner-sphere complexation was a key process in the phosphate removal process [16,36]. The FTIR spectra of LaSBC-P are displayed in Figure 2. The peak observed at approximately 621 and 538 cm − 1 corresponded to the bending vibration of O-P-O, which proved that phosphate can be adsorbed e ff ectively on adsorbents [69,72]. The peak noticed at 1399 cm − 1 generated by -OH vibration was a tt enuated, evidently. This result might be caused by the exchange of -OH with phosphate to enable inner-sphere complexation [73]. The XPS spectra of LaSBC are displayed in Figure 4. The characteristic peaks of La 3 d 5/2 for LaSBC were observed at 835.68 and 839.23 eV, and those of La 3 d 3/2 were observed at 852.28 and 856.08 eV. After phosphate adsorption, the peaks of La 3 d 5/2 were observed at 835.78 and 839.33 eV, whereas those of La 3 d 3/2 were observed at 852.48 and 856.17 eV. The binding energies of La 3 d 5/2 and 3 d 3/2 shifted to higher values, which indicated a possible electron transfer in the valence band of La 3 d and the occurrence of La-O-P innersphere complexation [16,35,74]. These results demonstrate that precipitation, electrostatic a tt raction, and inner-sphere complexation occurred during phosphate adsorption. These adsorption mechanisms are the same as several adsorption mechanisms reported in previous studies [75,76]. Figure 9. Possible mechanisms for phosphate adsorption on LaSBC. 4. Conclusions After the modi fi cation of lanthanum nitrate, the capability of LaSBC to remove phosphate from water considerably improved. The maximum experimental adsorption capacity for phosphate was 140.237 mg/g at pH 3.0. In addition, the adsorption mechanisms were predominantly precipitation, electrostatic a tt raction, and inner-sphere complexation. LaSBC possessed excellent recycling ability, and 72.3% of the initial q e was found to Figure 9. Possible mechanisms for phosphate adsorption on LaSBC The mechanism was further explained through the analysis of the types of phosphate at various pH levels and through determining the zero charge of LaSBC. The types of phosphate were varied at different pH levels, as shown in the following equation (Equation (1)) [ 51 ]: H 3 PO 4 ↔ K 1 H 2 PO 4 − ↔ K 2 HPO 4 2 − ↔ K 3 PO 4 3 − (1) where pK 1 = 2.13, pK 2 = 7.20, and pK 3 = 12.33 Adsorbents containing metal are positively charged at low pH levels because metal hydroxides are readily protonated, and at high pH levels, they are negatively charged, as metal hydroxides are easily deprotonated under this condition [ 11 , 40 , 70 ]. As displayed in Figure 4 b, pH pzc was 7.65 for LaSBC. The LaSBC was positively charged and protonated (La-OH + H + ↔ La-OH 2+ ) at a pH of 2.0–7.65. This condition allowed the LaSBC to adsorb phosphate species H 2 PO 4 − or HPO 4 2 − through electrostatic attraction. In addition, in the pH range of 7.65–12.0, LaSBC was negatively charged and deprotonated. The adsorption of phosphate tended toward a lower pH, and the surface anion exchange process was followed by OH − release [ 71 ]. The solution pH rose as anion exchange advanced [ 11 ]. With an increase in pH, the effect of electrostatic attraction diminished rapidly However, hydroxyl groups present in the solution would undergo anion exchange with phosphate ions and form chemical bonds through inner-sphere complexation to achieve phosphate removal. Thus, anion exchange and inner-sphere complexation are inseparable from one another. FTIR and XPS measurements demonstrated that innersphere complexation was a key process in the phosphate removal process [ 16 , 36 ]. The FTIR spectra of LaSBC-P are displayed in Figure 2 . The peak observed at approximately 621 and

Sustainability 2024 , 16 , 5667 12 of 15 538 cm − 1 corresponded to the bending vibration of O-P-O, which proved that phosphate can be adsorbed effectively on adsorbents [ 69 , 72 ]. The peak noticed at 1399 cm − 1 generated by –OH vibration was attenuated, evidently. This result might be caused by the exchange of –OH with phosphate to enable inner-sphere complexation [ 73 ]. The XPS spectra of LaSBC are displayed in Figure 4 . The characteristic peaks of La 3 d 5/2 for LaSBC were observed at 835.68 and 839.23 eV, and those of La 3 d 3/2 were observed at 852.28 and 856.08 eV. After phosphate adsorption, the peaks of La 3 d 5/2 were observed at 835.78 and 839.33 eV, whereas those of La 3 d 3/2 were observed at 852.48 and 856.17 eV. The binding energies of La 3 d 5/2 and 3 d 3/2 shifted to higher values, which indicated a possible electron transfer in the valence band of La 3 d and the occurrence of La-O-P inner-sphere complexation [ 16 , 35 , 74 ]. These results demonstrate that precipitation, electrostatic attraction, and inner-sphere complexation occurred during phosphate adsorption. These adsorption mechanisms are the same as several adsorption mechanisms reported in previous studies [ 75 , 76 ]. 4. Conclusions After the modification of lanthanum nitrate, the capability of LaSBC to remove phosphate from water considerably improved. The maximum experimental adsorption capacity for phosphate was 140.237 mg/g at pH 3.0. In addition, the adsorption mechanisms were predominantly precipitation, electrostatic attraction, and inner-sphere complexation. LaSBC possessed excellent recycling ability, and 72.3% of the initial q e was found to have been retained after the sixth desorption cycle. The dynamic adsorption study revealed that the modified biochar has a long sustainable treatment period. Moreover, our findings prove the feasibility of biochar as an economical and efficient adsorbent for phosphate-polluted water. Future studies are expected to improve the effect of pH on LaSBC adsorption capacity Supplementary Materials: The following supporting information can be downloaded at https: //www.mdpi.com/article/10.3390/su 16135667/s 1 , Table S 1: Kinetic parameters for the phosphate adsorption of the LaSBC; Table S 2: Thermodynamic parameters for the phosphate adsorption of the LaSBC; Figure S 1: Adsorption capacity after desorption of LaSBC; Figure S 2: Effects of dosages on breakthrough point of LaSBC. Reference [ 77 ] is cited in Supplementary Materials Author Contributions: All authors contributed to the study conception and design. Conceptualization, methodology, and software were performed by Y.J. Writing—review and editing were performed by X.S. and H.Z. (Hongxia Zhang). Investigation, resources, and methodology were performed by Q.L. Supervision was performed by J.M., M.X., B.D. and H.Z. (Hongxiang Zhu). The first draft of the manuscript was written by Y.J. and all authors commented on previous versions of the manuscript All authors have read and agreed to the published version of the manuscript Funding: This research was funded by Natural Science Foundation of Guangxi (No. 2018 GXNS- FGA 281001) Data Availability Statement: The datasets used or analyzed during the current study are available from the corresponding author upon reasonable request Acknowledgments: The authors thank Shiyanjia Lab ( www.shiyanjia.com ) for their help with XPS and the technical guidance of the Guangxi Engineering Research Center of Comprehensive Treatment for Agricultural Non-Point Source Pollution 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 Sun, J.; Yang, Q.; Wang, D.; Wang, S.; Chen, F.; Zhong, Y.; Yi, K.; Yao, F.; Jiang, C.; Li, S.; et al. Nickel toxicity to the performance and microbial community of enhanced biological phosphorus removal system Chem. Eng. J 2017 , 313 , 415–423. [ CrossRef ] 2 Wang, D.; Yang, G.; Li, X.; Zheng, W.; Wu, Y.; Yang, Q.; Zeng, G. Inducing mechanism of biological phosphorus removal driven by the aerobic/extended-idle regime Biotechnol. Bioeng 2012 , 109 , 2798–2807. [ CrossRef ] [ PubMed ] 3 Xu, Q.; Liu, X.; Wang, D.; Wu, Y.; Wang, Q.; Liu, Y.; Li, X.; An, H.; Zhao, J.; Chen, F.; et al. Free ammonia-based pretreatment enhances phosphorus release and recovery from waste activated sludge Chemosphere 2018 , 213 , 276–284. [ CrossRef ]

Sustainability 2024 , 16 , 5667 13 of 15 4 Mayer, B.K.; Baker, L.A.; Boyer, T.H.; Drechsel, P.; Gifford, M.; Hanjra, M.A.; Parameswaran, P.; Stoltzfus, J.; Westerhoff, P.; Rittmann, B.E. Total Value of Phosphorus Recovery Environ. Sci. Technol 2016 , 50 , 6606–6620. [ CrossRef ] [ PubMed ] 5 Shang, Y.; Xu, X.; Qi, S.; Zhao, Y.; Ren, Z.; Gao, B. Preferable uptake of phosphate by hydrous zirconium oxide nanoparticles embedded in quaternary-ammonium Chinese reed J. Colloid Interface Sci 2017 , 496 , 118–129. [ CrossRef ] [ PubMed ] 6 Jung, K.-W.; Lee, S.; Lee, Y.J. Synthesis of novel magnesium ferrite (MgFe 2 O 4 )/biochar magnetic composites and its adsorption behavior for phosphate in aqueous solutions Bioresour. Technol 2017 , 245 , 751–759. [ CrossRef ] [ PubMed ] 7 Xie, T.; Mo, C.; Li, X.; Zhang, J.; An, H.; Yang, Q.; Wang, D.; Zhao, J.; Zhong, Y.; Zeng, G. Effects of different ratios of glucose to acetate on phosphorus removal and microbial community of enhanced biological phosphorus removal (EBPR) system Environ Sci. Pollut. Res 2017 , 24 , 4494–4505. [ CrossRef ] [ PubMed ] 8 Wu, Y.; Li, X.; Yang, Q.; Wang, D.; Xu, Q.; Yao, F.; Chen, F.; Tao, Z.; Huang, X. Hydrated lanthanum oxide-modified diatomite as highly efficient adsorbent for low-concentration phosphate removal from secondary effluents J. Environ. Manag 2019 , 231 , 370–379. [ CrossRef ] [ PubMed ] 9 Huang, W.; Yu, X.; Tang, J.; Zhu, Y.; Zhang, Y.; Li, D. Enhanced adsorption of phosphate by flower-like mesoporous silica spheres loaded with lanthanum Microporous Mesoporous Mater 2015 , 217 , 225–232. [ CrossRef ] 10 Zhang, M.; Song, G.; Gelardi, D.L.; Huang, L.; Khan, E.; Mašek, O.; Parikh, S.J.; Ok, Y.S. Evaluating biochar and its modifications for the removal of ammonium, nitrate, and phosphate in water Water Res 2020 , 186 , 116303. [ CrossRef ] 11 Xu, Q.; Chen, Z.; Wu, Z.; Xu, F.; Yang, D.; He, Q.; Li, G.; Chen, Y. Novel lanthanum doped biochars derived from lignocellulosic wastes for efficient phosphate removal and regeneration Bioresour. Technol 2019 , 289 , 121600. [ CrossRef ] [ PubMed ] 12 Yang, S.; Jin, P.; Wang, X.; Zhang, Q.; Chen, X. Phosphate recovery through adsorption assisted precipitation using novel precipitation material developed from building waste: Behavior and mechanism Chem. Eng. J 2016 , 292 , 246–254. [ CrossRef ] 13 Shang, Y.; Kan, Y.; Xu, X. Stability and regeneration of metal catalytic sites with different sizes in Fenton-like system Chin. Chem Lett 2023 , 34 , 108278. [ CrossRef ] 14 Shrestha, R.; Ban, S.; Devkota, S.; Sharma, S.; Joshi, R.; Tiwari, A.P.; Kim, H.Y.; Joshi, M.K. Technological trends in heavy metals removal from industrial wastewater: A review J. Environ. Chem. Eng 2021 , 9 , 105688. [ CrossRef ] 15 Li, R.; Wang, J.J.; Zhou, B.; Awasthi, M.K.; Ali, A.; Zhang, Z.; Lahori, A.H.; Mahar, A. Recovery of phosphate from aqueous solution by magnesium oxide decorated magnetic biochar and its potential as phosphate-based fertilizer substitute Bioresour Technol 2016 , 215 , 209–214. [ CrossRef ] [ PubMed ] 16 Qiu, H.; Liang, C.; Yu, J.; Zhang, Q.; Song, M.; Chen, F. Preferable phosphate sequestration by nano-La(III) (hydr)oxides modified wheat straw with excellent properties in regeneration Chem. Eng. J 2017 , 315 , 345–354. [ CrossRef ] 17 Biswas, B.K.; Inoue, K.; Ghimire, K.N.; Harada, H.; Ohto, K.; Kawakita, H. Removal and recovery of phosphorus from water by means of adsorption onto orange waste gel loaded with zirconium Bioresour. Technol 2008 , 99 , 8685–8690. [ CrossRef ] [ PubMed ] 18 Lu, S.; Bai, S.; Shan, H. Mechanisms of phosphate removal from aqueous solution by blast furnace slag and steel furnace slag J. Zhejiang Univ.-Sci. A 2008 , 9 , 125–132. [ CrossRef ] 19 Chen, Y.; Wang, R.; Duan, X.; Wang, S.; Ren, N.; Ho, S.-H. Production, properties, and catalytic applications of sludge derived biochar for environmental remediation Water Res 2020 , 187 , 116390. [ CrossRef ] [ PubMed ] 20 Wu, Q.; Bao, X.; Guo, W.; Wang, B.; Li, Y.; Luo, H.; Wang, H.; Ren, N. Medium chain carboxylic acids production from waste biomass: Current advances and perspectives Biotechnol. Adv 2019 , 37 , 599–615. [ CrossRef ] 21 Chen, Y.; Ho, S.-H.; Nagarajan, D.; Ren, N.; Chang, J.-S. Waste biorefineries—Integrating anaerobic digestion and microalgae cultivation for bioenergy production Curr. Opin. Biotechnol 2018 , 50 , 101–110. [ CrossRef ] [ PubMed ] 22 Chen, Y.; Li, S.; Ho, S.-H.; Wang, C.; Lin, Y.-C.; Nagarajan, D.; Chang, J.-S.; Ren, N. Integration of sludge digestion and microalgae cultivation for enhancing bioenergy and biorefinery Renew. Sustain. Energy Rev 2018 , 96 , 76–90. [ CrossRef ] 23 Chen, Y.; Yang, Z.; Ren, N.; Ho, S.-H. Optimizing the production of short and medium chain fatty acids (SCFAs and MCFAs) from waste activated sludge using different alkyl polyglucose surfactants, through bacterial metabolic analysis J. Hazard. Mater 2020 , 384 , 121384. [ CrossRef ] [ PubMed ] 24 Huang, Y.; Xia, S.; Lyu, J.; Tang, J. Highly efficient removal of aqueous Hg 2+ and CH 3 Hg + by selective modification of biochar with 3-mercaptopropyltrimethoxysilane Chem. Eng. J 2019 , 360 , 1646–1655. [ CrossRef ] 25 Yang, Q.; Wang, X.; Luo, W.; Sun, J.; Xu, Q.; Chen, F.; Zhao, J.; Wang, S.; Yao, F.; Wang, D.; et al. Effectiveness and mechanisms of phosphate adsorption on iron-modified biochars derived from waste activated sludge Bioresour. Technol 2018 , 247 , 537–544 [ CrossRef ] [ PubMed ] 26 Zama, E.F.; Li, G.; Tang, Y.-T.; Reid, B.J.; Ngwabie, N.M.; Sun, G.-X. The removal of arsenic from solution through biochar-enhanced precipitation of calcium-arsenic derivatives Environ. Pollut 2022 , 292 , 118241. [ CrossRef ] [ PubMed ] 27 Yin, Q.; Liu, M.; Ren, H. Biochar produced from the co-pyrolysis of sewage sludge and walnut shell for ammonium and phosphate adsorption from water J. Environ. Manag 2019 , 249 , 109410. [ CrossRef ] [ PubMed ] 28 Yin, Q.; Liu, M.; Li, Y.; Li, H.; Wen, Z. Computational study of phosphate adsorption on Mg/Ca modified biochar structure in aqueous solution Chemosphere 2021 , 269 , 129374. [ CrossRef ] [ PubMed ] 29 Liao, T.; Li, T.; Su, X.; Yu, X.; Song, H.; Zhu, Y.; Zhang, Y. La(OH) 3 -modified magnetic pineapple biochar as novel adsorbents for efficient phosphate removal Bioresour. Technol 2018 , 263 , 207–213. [ CrossRef ]

Sustainability 2024 , 16 , 5667 14 of 15 30 He, J.; Wang, W.; Sun, F.; Shi, W.; Qi, D.; Wang, K.; Shi, R.; Cui, F.; Wang, C.; Chen, X. Highly Efficient Phosphate Scavenger Based on Well-Dispersed La(OH) 3 Nanorods in Polyacrylonitrile Nanofibers for Nutrient-Starvation Antibacteria ACS Nano 2015 , 9 , 9292–9302. [ CrossRef ] 31 Liu, B.; Nan, J.; Zu, X.; Zhang, X.; Huang, W.; Wang, W. La-based-adsorbents for efficient biological phosphorus treatment of wastewater: Synergistically strengthen of chemical and biological removal Chemosphere 2020 , 255 , 127010. [ CrossRef ] [ PubMed ] 32 Yu, J.; Xiang, C.; Zhang, G.; Wang, H.; Ji, Q.; Qu, J. Activation of Lattice Oxygen in LaFe (Oxy)hydroxides for Efficient Phosphorus Removal Environ. Sci. Technol 2019 , 53 , 9073–9080. [ CrossRef ] [ PubMed ] 33 Li, R.; Wang, J.J.; Gaston, L.A.; Zhou, B.; Li, M.; Xiao, R.; Wang, Q.; Zhang, Z.; Huang, H.; Liang, W.; et al. An overview of carbothermal synthesis of metal–biochar composites for the removal of oxyanion contaminants from aqueous solution Carbon 2018 , 129 , 674–687. [ CrossRef ] 34 Fang, L.; Liu, R.; Li, J.; Xu, C.; Huang, L.-Z.; Wang, D. Magnetite/Lanthanum hydroxide for phosphate sequestration and recovery from lake and the attenuation effects of sediment particles Water Res 2018 , 130 , 243–254. [ CrossRef ] [ PubMed ] 35 Min, X.; Wu, X.; Shao, P.; Ren, Z.; Ding, L.; Luo, X. Ultra-high capacity of lanthanum-doped UiO-66 for phosphate capture: Unusual doping of lanthanum by the reduction of coordination number Chem. Eng. J 2019 , 358 , 321–330. [ CrossRef ] 36 Wang, Z.; Shen, D.; Shen, F.; Li, T. Phosphate adsorption on lanthanum loaded biochar Chemosphere 2016 , 150 , 1–7. [ CrossRef ] [ PubMed ] 37 Jin, J.; Li, Y.; Zhang, J.; Wu, S.; Cao, Y.; Liang, P.; Zhang, J.; Wong, M.H.; Wang, M.; Shan, S.; et al. Influence of pyrolysis temperature on properties and environmental safety of heavy metals in biochars derived from municipal sewage sludge J Hazard. Mater 2016 , 320 , 417–426. [ CrossRef ] [ PubMed ] 38 Li, J.; Li, B.; Huang, H.; Zhao, N.; Zhang, M.; Cao, L. Investigation into lanthanum-coated biochar obtained from urban dewatered sewage sludge for enhanced phosphate adsorption Sci. Total Environ 2020 , 714 , 136839. [ CrossRef ] [ PubMed ] 39 Guo, J.; Chen, B. Insights on the Molecular Mechanism for the Recalcitrance of Biochars: Interactive Effects of Carbon and Silicon Components Environ. Sci. Technol 2014 , 48 , 9103–9112. [ CrossRef ] 40 Jia, Z.; Zeng, W.; Xu, H.; Li, S.; Peng, Y. Adsorption removal and reuse of phosphate from wastewater using a novel adsorbent of lanthanum-modified platanus biochar Process Saf. Environ. Prot 2020 , 140 , 221–232. [ CrossRef ] 41 Li, X.; Liu, W.; Zhang, J.; Wang, Z.; Guo, Z.; Ali, J.; Wang, L.; Yu, Z.; Zhang, X.; Sun, Y. Effective removal of microplastics by filamentous algae and its magnetic biochar: Performance and mechanism Chemosphere 2024 , 358 , 142152. [ CrossRef ] 42 Fan, J.; Li, Y.; Yu, H.; Li, Y.; Yuan, Q.; Xiao, H.; Li, F.; Pan, B. Using sewage sludge with high ash content for biochar production and Cu(II) sorption Sci. Total Environ 2020 , 713 , 136663. [ CrossRef ] [ PubMed ] 43 Pasumarthi, R.; Sawargaonkar, G.; Kale, S.; Kumar, N.V.; Choudhari, P.L.; Singh, R.; Davala, M.S.; Rani, C.S.; Mutnuri, S.; Jat, M.L. Innovative bio-pyrolytic method for efficient biochar production from maize and pigeonpea stalks and their characterization J Clean. Prod 2024 , 448 , 141573. [ CrossRef ] 44 Qian, G.; Xu, L.; Li, N.; Wang, K.; Qu, Y.; Xu, Y. Enhanced arsenic migration in tailings soil with the addition of humic acid, fulvic acid and thiol-modified humic acid Chemosphere 2022 , 286 , 131784. [ CrossRef ] 45 Yu, H.; Zhang, D.; Gu, L.; Wen, H.; Zhu, N. Coupling sludge-based biochar and electrolysis for conditioning and dewatering of sewage sludge: Effect of char properties Environ. Res 2022 , 214 , 113974. [ CrossRef ] 46 Chen, L.; Li, Y.; Sun, Y.; Chen, Y.; Qian, J. La(OH) 3 loaded magnetic mesoporous nanospheres with highly efficient phosphate removal properties and superior pH stability Chem. Eng. J 2019 , 360 , 342–348. [ CrossRef ] 47 Xiao, Y.-F.; Zhang, Y.; Wang, D.-C.; Su, Y.-M.; Wu, J.; Liu, J.-Q.; Yang, L.-L.; Jin, Z. Hydrothermal synthesis of lanthanum oxide nanoparticles modified pumice: High lanthanum oxide loading ratio and efficiency phosphate removal J. Environ. Chem. Eng 2024 , 12 , 111587. [ CrossRef ] 48 Chen, K.; Guo, S.; Zeng, Y.; Huang, W.; Peng, J.; Zhang, L.; Yin, S. Facile preparation and characterization of lanthanum oxide powders by the calcination of lanthanum carbonate hydrate in microwave field Ceram. Int 2020 , 46 , 165–170. [ CrossRef ] 49 Li, L.; Jiang, W.; Pan, H.; Xu, X.; Tang, Y.; Ming, J.; Xu, Z.; Tang, R. Improved Luminescence of Lanthanide(III)-Doped Nanophosphors by Linear Aggregation J. Phys. Chem. C 2007 , 111 , 4111–4115. [ CrossRef ] 50 Shan, S.; Chen, Z.; Yuen Koh, K.; Cui, F.; Paul Chen, J. Development and application of lanthanum peroxide loaded sepiolite nanocomposites for simultaneous removal of phosphate and inhibition of cyanobacteria growth J. Colloid Interface Sci 2022 , 624 , 691–703. [ CrossRef ] 51 Tang, Q.; Shi, C.; Shi, W.; Huang, X.; Ye, Y.; Jiang, W.; Kang, J.; Liu, D.; Ren, Y.; Li, D. Preferable phosphate removal by nano-La(III) hydroxides modified mesoporous rice husk biochars: Role of the host pore structure and point of zero charge Sci. Total Environ 2019 , 662 , 511–520. [ CrossRef ] 52 Li, S.; Luo, C.; Yan, F.; Yang, Y.; Guo, B.; Wang, L.; Xu, S.; Wu, F.; Ji, P. Remediation of Pb(II) and Cd(II) in polluted waters with calcium thioglycolate–modified straw biochar Environ. Pollut 2023 , 338 , 122638. [ CrossRef ] 53 Goscianska, J.; Ptaszkowska-Koniarz, M.; Frankowski, M.; Franus, M.; Panek, R.; Franus, W. Removal of phosphate from water by lanthanum-modified zeolites obtained from fly ash J. Colloid Interface Sci 2018 , 513 , 72–81. [ CrossRef ] 54 Zhang, J.; Shen, Z.; Shan, W.; Chen, Z.; Mei, Z.; Lei, Y.; Wang, W. Adsorption behavior of phosphate on Lanthanum(III) doped mesoporous silicates material J. Environ. Sci 2010 , 22 , 507–511. [ CrossRef ] [ PubMed ]

Sustainability 2024 , 16 , 5667 15 of 15 55 Wang, Z.; Guo, H.; Shen, F.; Yang, G.; Zhang, Y.; Zeng, Y.; Wang, L.; Xiao, H.; Deng, S. Biochar produced from oak sawdust by Lanthanum (La)-involved pyrolysis for adsorption of ammonium (NH 4+ ), nitrate (NO 3 − ), and phosphate (PO 4 3 − ) Chemosphere 2015 , 119 , 646–653. [ CrossRef ] [ PubMed ] 56 Aryee, A.A.; Mpatani, F.M.; Zhang, X.; Kani, A.N.; Dovi, E.; Han, R.; Li, Z.; Qu, L. Iron (III) and iminodiacetic acid functionalized magnetic peanut husk for the removal of phosphate from solution: Characterization, kinetic and equilibrium studies J. Clean Prod 2020 , 268 , 122191. [ CrossRef ] 57 Yin, Q.; Wang, R.; Zhao, Z. Application of Mg–Al-modified biochar for simultaneous removal of ammonium, nitrate, and phosphate from eutrophic water J. Clean. Prod 2018 , 176 , 230–240. [ CrossRef ] 58 Jang, H.M.; Yoo, S.; Choi, Y.-K.; Park, S.; Kan, E. Adsorption isotherm, kinetic modeling and mechanism of tetracycline on Pinus taeda-derived activated biochar Bioresour. Technol 2018 , 259 , 24–31. [ CrossRef ] 59 Cui, X.; Dai, X.; Khan, K.Y.; Li, T.; Yang, X.; He, Z. Removal of phosphate from aqueous solution using magnesiumalginate/chitosan modified biochar microspheres derived from Thalia dealbata Bioresour. Technol 2016 , 218 , 1123–1132 [ CrossRef ] 60 Yang, H.I.; Lou, K.; Rajapaksha, A.U.; Ok, Y.S.; Anyia, A.O.; Chang, S.X. Adsorption of ammonium in aqueous solutions by pine sawdust and wheat straw biochars Environ. Sci. Pollut. Res 2018 , 25 , 25638–25647. [ CrossRef ] 61 Qu, J.; Akindolie, M.S.; Feng, Y.; Jiang, Z.; Zhang, G.; Jiang, Q.; Deng, F.; Cao, B.; Zhang, Y. One-pot hydrothermal synthesis of NaLa(CO 3 ) 2 decorated magnetic biochar for efficient phosphate removal from water: Kinetics, isotherms, thermodynamics, mechanisms and reusability exploration Chem. Eng. J 2020 , 394 , 124915. [ CrossRef ] 62 Meng, J.; Feng, X.; Dai, Z.; Liu, X.; Wu, J.; Xu, J. Adsorption characteristics of Cu(II) from aqueous solution onto biochar derived from swine manure Environ. Sci. Pollut. Res 2014 , 21 , 7035–7046. [ CrossRef ] [ PubMed ] 63 Liu, Z.; Liu, H.; Zhang, Y.; Lichtfouse, E. Efficient phosphate recycling by adsorption on alkaline sludge biochar Environ. Chem Lett 2023 , 21 , 21–30. [ CrossRef ] 64 Wang, Z.; Miao, R.; Ning, P.; He, L.; Guan, Q. From wastes to functions: A paper mill sludge-based calcium-containing porous biochar adsorbent for phosphorus removal J. Colloid Interface Sci 2021 , 593 , 434–446. [ CrossRef ] 65 Li, J.; Cao, L.; Li, B.; Huang, H.; Yu, W.; Sun, C.; Long, K.; Young, B. Utilization of activated sludge and shell wastes for the preparation of Ca-loaded biochar for phosphate removal and recovery J. Clean. Prod 2023 , 382 , 135395. [ CrossRef ] 66 Li, J.; Li, B.; Huang, H.; Lv, X.; Zhao, N.; Guo, G.; Zhang, D. Removal of phosphate from aqueous solution by dolomite-modified biochar derived from urban dewatered sewage sludge Sci. Total Environ 2019 , 687 , 460–469. [ CrossRef ] 67 Cegłowski, M.; Schroeder, G. Preparation of porous resin with Schiff base chelating groups for removal of heavy metal ions from aqueous solutions Chem. Eng. J 2015 , 263 , 402–411. [ CrossRef ] 68 Yang, J.; Zhou, L.; Zhao, L.; Zhang, H.; Yin, J.; Wei, G.; Qian, K.; Wang, Y.; Yu, C. A designed nanoporous material for phosphate removal with high efficiency J. Mater. Chem 2011 , 21 , 2489. [ CrossRef ] 69 Wang, Z.; Wang, H.; Li, Q.; Xu, M.; Guo, Y.; Li, J.; Wu, T. pH effect on Re(VII) and Se(IV) diffusion in compacted GMZ bentonite Appl. Geochem 2016 , 73 , 1–7. [ CrossRef ] 70 Xiong, W.; Tong, J.; Yang, Z.; Zeng, G.; Zhou, Y.; Wang, D.; Song, P.; Xu, R.; Zhang, C.; Cheng, M. Adsorption of phosphate from aqueous solution using iron-zirconium modified activated carbon nanofiber: Performance and mechanism J. Colloid Interface Sci 2017 , 493 , 17–23. [ CrossRef ] 71 Hiemstra, T.; Van Riemsdijk, W.H. Surface Structural Ion Adsorption Modeling of Competitive Binding of Oxyanions by Metal (Hydr)oxides J. Colloid Interface Sci 1999 , 210 , 182–193. [ CrossRef ] 72 Zhang, L.; Zhou, Q.; Liu, J.; Chang, N.; Wan, L.; Chen, J. Phosphate adsorption on lanthanum hydroxide-doped activated carbon fiber Chem. Eng. J 2012 , 185–186 , 160–167. [ CrossRef ] 73 Chen, Y.; Xu, R.; Li, Y.; Liu, Y.; Wu, Y.; Chen, Y.; Zhang, J.; Chen, S.; Yin, H.; Zeng, Z.; et al. La(OH) 3 -modified magnetic CoFe 2 O 4 nanocomposites: A novel adsorbent with highly efficient activity and reusability for phosphate removal Colloids Surf. A Physicochem. Eng. Asp 2020 , 599 , 124870. [ CrossRef ] 74 Wu, B.; Fang, L.; Fortner, J.D.; Guan, X.; Lo, I.M.C. Highly efficient and selective phosphate removal from wastewater by magnetically recoverable La(OH) 3 /Fe 3 O 4 nanocomposites Water Res 2017 , 126 , 179–188. [ CrossRef ] 75 Mahmoud, A.S.; Mostafa, M.K.; Nasr, M. Regression model, artificial intelligence, and cost estimation for phosphate adsorption using encapsulated nanoscale zero-valent iron Sep. Sci. Technol 2019 , 54 , 13–26. [ CrossRef ] 76 Zhang, Y.; Pan, B. Modeling batch and column phosphate removal by hydrated ferric oxide-based nanocomposite using response surface methodology and artificial neural network Chem. Eng. J 2014 , 249 , 111–120. [ CrossRef ] 77 Qi, G.; Pan, Z.; Zhang, X.; Chang, S.; Wang, H.; Wang, M.; Xiang, W.; Gao, B. Microwave biochar produced with activated carbon catalyst: Characterization and adsorption of heavy metals Environ. Res 2023 , 216 , 114732. [ CrossRef ] 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). 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