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Article

Capacity and Mechanisms of Phosphate Adsorption on Lanthanum-Modified Dewatered Sludge-Based Biochar

by
Jingjing Mo
1,2,
Qian Li
1,2,
Xiaojie Sun
1,2,*,
Hongxia Zhang
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
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
4
College of Light Industry and Food Engineering, Guangxi University, Nanning 530004, China
5
Modern Industry College of Ecology and Environmental Protection, Guilin University of Technology, Guilin 541004, China
*
Authors to whom correspondence should be addressed.
Water 2024, 16(3), 418; https://doi.org/10.3390/w16030418
Submission received: 4 January 2024 / Revised: 18 January 2024 / Accepted: 22 January 2024 / Published: 27 January 2024
(This article belongs to the Section Wastewater Treatment and Reuse)
Browse Figures
Versions Notes

Abstract

:
Using sewage sludge to produce biochar-based adsorbents to remove phosphate (P) from water can be a sustainable and cost-effective method of waste management. However, the adsorption efficiency of sewage sludge biochar is not high. In this study, lanthanum-modified sludge-based biochar (La-SBBC) was synthesized by combining lanthanum nitrate with dewatered sludge. La-SBBC exhibited the highest removal efficiency of 99.06% for an initial P concentration of 15 mg/L at pH 3.0 with a dosage of 1.3 g/L. The maximum adsorption capacity of La-SBBC for P was 152.77 mg/g at 35 °C. The adsorption process followed the pseudo-second-order kinetic model (R2 ≥ 0.973) and the Freundlich isothermal adsorption model (R2 ≥ 0.928). Multilayer chemisorption was identified as the controlling process. The primary mechanisms of P adsorption by La-SBBC involved electrostatic interactions, precipitation, and inner sphere complexation. Thermodynamic analysis revealed that the adsorption process of La-SBBC was a spontaneous endothermic reaction. The fixed-bed experiment demonstrated that La-SBBC had significant practical utility. La-SBBC maintained 76.6% of the original P removal efficiency after six cycles. Therefore, La-SBBC can be used as a promising adsorbent for P in practical applications.

1. Introduction

The presence of phosphate (P) is crucial for the proper functioning of aquatic ecosystems [1,2,3]. However, excessive discharge of P can induce severe eutrophication, threatening both ecosystems and human health [4,5]. Therefore, stricter regulations regarding P emissions have been explicitly required to control water pollution [6,7]. The current maximum permissible P concentration in effluents discharged from municipal wastewater treatment plants (WWTPs) in China is set at 0.5 mg/L. Hence, reducing and controlling the content of P is crucial [8].
Diverse methodologies have been employed to eliminate P from effluent, such as biological removal, membrane technology, ion exchange, chemical precipitation, and adsorption [9,10]. Among these approaches, adsorption was considered a promising method due to its high selectivity, cost-effectiveness, and high efficiency in removing low concentrations of P from wastewater [11,12].
With the rapid development of urbanization and industrialization, a large amount of municipal sludge has been produced in recent decades [13]. The sewage sludge contains a significant quantity of nitrogen, P, heavy metals, and organic matter, which can pose a substantial environmental threat if not carefully treated [14]. Sludge treatment methods, such as incineration and landfills, are widely used in developing countries, leading to the generation of undesirable secondary pollution [13,15]. Therefore, there is an urgent need to develop a green and sustainable technology for sludge disposal [16]. The recycling and utilization of sewage sludge have emerged as prominent research topics in recent years. Using sewage sludge to prepare biochar adsorbents has emerged as an economical and environmentally friendly approach. However, the capacity of sludge biochar to remove P is very limited. It was found that the maximum P adsorption capacity of sludge-derived biochar is only 5.93 mg/g at the optimal pyrolysis temperature of 700 °C [17]. Therefore, addressing the issue of improving the adsorption performance of sludge biochar has become necessary.
Surface modification of sludge is a crucial method to enhance adsorption performance [18,19,20]. Metal-modified biochar exhibited impressive P adsorption capacity [21]. It has been reported that biochar with an Al content of 20% displayed an optimal P adsorption capacity of 57.49 mg/g [22]. Ca-rich sludge-derived biochar demonstrated a high P removal ability, with a maximum sorption capacity of 153.85 mg/g and minimal impact on solution pH [23]. The absorption capacity of biochar produced from orange peels and activated by a Ca/Zn composite was found to be 52.96 mg/g, representing an eightfold increase compared to the untreated biochar [24]. The sludge biochar modified with FeCl3 exhibited a maximal P adsorption capacity of 111.0 mg/g [25].
Lanthanum (La) is a rare earth material that is environmentally friendly and has gained attention for its ability to remove P from wastewater. This is because it has excellent selectivity, forms a strong ionic bond between La (III) and PO43−, and has good stability in aqueous solutions [26,27,28,29]. A recent study reported a La-based magnetic adsorbent exhibiting a remarkable P adsorption capacity of 44.8 mg/g. When treating real domestic wastewater with a P concentration of 1.7 mg/L using 0.2 g/L of La-based magnetic adsorbent, the removal efficiency reached 98.8% [30]. Therefore, La-modified sludge may enhance the performance of P adsorption. Studies have demonstrated that the maximum P adsorption of La-modified sludge biochar prepared by the impregnation–coprecipitation method at 600 °C is 93.91 mg/g [31]. The maximum P adsorption capacity of La-modified water treatment sludge hydrochar, prepared through one-step hydrothermal loading, reached 72.69 mg/g at 30 °C, which is significantly higher than that of the raw sludge [32]. However, in the existing research, most researchers first produce biochar from sludge and then modify it with La.
In this study, La was first used to modify the sludge and then used to prepare biochar. Thus, La-modified sludge-based biochar (La-SBBC) was obtained. The effects of solution pH and La-SBBC dosage on the adsorption capacity of P were investigated. The adsorption isotherms, kinetics, and thermodynamics were also studied, and the adsorption mechanism was revealed through XRD, XPS, and SEM analyses. The regeneration of La-SBBC was inspected, and the dynamic adsorption experiment of La-SBBC in a fixed-bed column with wastewater was evaluated.

2. Materials and Methods

2.1. Preparation of La-SBBC

The dewatered sludge (DS) was collected from the wastewater treatment plants in Guangxi province, China. It was washed, air-dried at 105 °C until all moisture was completely removed, then ground and sieved to achieve a size of less than 0.125 mm.
The coprecipitation method was used to prepare La-SBBC. The dried sludge particles and lanthanum nitrate (La(NO3)3·6H2O) were immersed in 40 mL of deionized water at mass ratios of 5:1, 5:3, 5:5, 5:7, 5:10, and 5:15, respectively. The mixture was subsequently agitated uniformly for 3 h at room temperature. Subsequently, the filtered sediment was oven-dried at 50 °C and then subjected to pyrolysis in a quartz tube electric furnace (SK2-6-12, Yixing Zhongyang Machinery Factory, Yixing, China). The pyrolysis process involved heating at a rate of 10 °C/min until reaching 600 °C, which was maintained for 3 h [33].

2.2. Characterization of La-SBBC

The surface area of the materials was determined using a fully automated specific surface area and microporous pore size analyzer (ASAP2020, Micromeritics Instrument Corporation, Norcross, GA, USA). A scanning electron microscope (SEM, JSM-7900F, JEOL Ltd., Akishima, Japan) was utilized for surface morphology analysis of the samples. The surface crystal structure of the La-SBBC, both before and after the batch adsorption reaction, was investigated using an X-ray diffractometer (XRD, X’Pert3 Powder, DKSH Group, Zurich, Switzerland). Functional groups and chemical bonds of La-SBBC, before and after P adsorption, were observed using Fourier transform infrared spectroscopy (FTIR, Nicolet Nexus 470, GMI-Technology Solutions, Phoneix, AZ, USA) within the range of 4000–500 cm−1. The elemental chemical state of the P-adsorbed La-SBBC was analyzed using X-ray photoelectron spectroscopy (XPS, ESCALAB 250xi, Thermo Fisher Scientific, Waltham, MA, USA) with an Al Kα monochromatic X-ray source (150 W). The point of zero charge (pHpzc) for La-SBBC was determined utilizing a zeta potential analyzer. In addition, a computer-controlled thermogravimetric analyzer (TG, NETZSCH-Proteus-80, NETZSCH Group, Selb, Germany) was used to assess the thermal stability of the materials within a temperature range from room temperature to 1073.15 K under an air/nitrogen atmosphere.

2.3. Batch Adsorption Tests

Preparation ratio: The 1000 mg/L stock solution was formulated through the dissolution of KH2PO4 in ultra-pure water. Experiments were carried out in batches to investigate the adsorption efficiency of La-SBBC for P. The entire adsorption process was conducted within the incubator’s oscillator. The preparation ratio of La-SBBC was determined through an adsorption experiment. La-SBBC of different preparation proportions were weighed to 0.02 g, respectively, and mixed with 15 mL of a 10 mg/L P solution. The mixtures were shaken in a thermostatic oscillator at 200 rpm and 25 °C for 24 h and then filtered using a water filtration membrane with a pore size of 0.22 μm. The P concentration in the filtrate was determined using molybdate blue spectrophotometry and UV-Vis spectrophotometry at a wavelength of 700 nm. The optimal ratios of La-SBBC were used for subsequent experiments.
Dose effect: La-SBBC (0.004, 0.008, 0.01, 0.015, 0.02, 0.025, 0.03, 0.04, 0.05, and 0.06 g) was accurately weighed and placed into polyethylene centrifuge tubes, then mixed with 15 mL of a 30 mg/L P solution. This solution was subjected to continuous agitation at a constant temperature of 200 rpm for a duration of 24 h in order to achieve equilibrium.
pH effect: The impact of the solution’s pH was investigated by creating a 15 mL P solution with a concentration of 15 mg/L. The acidity levels were modified to 3, 5, 7, 9, and 11 using HCl and NaOH. After adding 0.02 g of La-SBBC, the solution was shaken at 25 °C for 24 h. After achieving adsorption equilibrium, the residual levels of P and the ultimate pH were documented. The P adsorption capacity (qe, mg/g) was calculated using Equation (1):
qe = (C0 − Ce)V/W
where C0 and Ce (mg P/L) represent the initial and final concentrations of P in the solution, respectively; V (L) represents the volume of the solution; and W (g) denotes the mass of the La-SBBC.
Adsorption kinetics: Experiments were performed in a shaking incubator at 200 rpm using 0.02 g of La-SBBC and a P solution (30 mg/L) with a volume of 15 mL. The corresponding sample tube was set for different times, and the supernatant of the corresponding sample tubes was taken at 5, 10, 15, 20, 30 min, and every 30 min thereafter, until the total time was 720 min, and the P concentration analysis was performed. The experimental data were fitted using both the pseudo-first-order (Equation (2)) and pseudo-second-order models (Equation (3)).
ln(qe − qt) = lnqe − k1t
t/qt = 1/(k2qe2) + t/qe
where qt (mg P/g) represents the quantity of adsorbed P at a specific time; k1 (min−1) and k2 (g mg−1 min−1) represent the corresponding model’s rate constants.
Adsorption isotherm: The P stock solution was subjected to dilution at different specific concentrations (5, 10, 15, 20, 30, 50, 70, 80, 100, 150, 250, 300, and 400 mg/L) for investigation. Each of these solutions (15 mL) was placed separately into polyethylene centrifuge tubes. The pH of each solution was adjusted to 3.0 by adding HCl and NaOH before the addition of La-SBBC, which was fixed at a dosage of 0.02 g. Each solution underwent agitation at a speed of 200 revolutions per minute for a duration of 24 h until it reached a state of equilibrium. This reaction was performed at various temperatures (15, 25, 35, 45, and 65 °C). The isotherm results were fitted to the Langmuir (Equation (4)) and Freundlich models (Equation (5)).
qe = qmbCe/(1 + bCe)
qe = KFCe1/n
where qm (mg P/g) denotes the maximum capacity for adsorption; Ce (mg/L) represents the concentration of P in the solution at equilibrium; b and KF represent the Langmuir and Freundlich constants, respectively; and n denotes the linearity constant in the Freundlich model.
Adsorption thermodynamics: Experiments were conducted at temperatures of 15, 25, 35, 45, and 65 °C to investigate the effect of temperature on the uptake of P by La-SBBC. A total of 0.02 g of La-SBBC was added to 15 mL of P solutions with various initial concentrations (5, 10, 15, 20, 30, 50, 70, 80, 100, 150, 250, 300, and 400 mg/L). These solutions were then incubated for 24 h in an incubator to reach equilibrium. The Van’t Hoff equation was utilized to compute thermodynamic parameters, such as alterations in free energy (ΔG), enthalpy (ΔH), and entropy (ΔS) [6].
KD = qe/Ce
∆G = −RTlnKD
lnKD = ∆S/R − ∆H/RT
where KD represents the equilibrium constant; R represents the gas constant with a value of 8.314 J/mol K; and T denotes the absolute temperature measured in Kelvin (K).
Cycle regeneration test: To explore the recoverability and stability of La-SBBC, a 0.2 mol/L NaCl solution was utilized as an eluent to perform regeneration experiments on La-SBBC.

2.4. Fixed-Bed Column Experiments

Experiments were carried out using real wastewater (with a P concentration of 3.08 mg/L) to determine the continuous adsorption capacity of La-SBBC. The practical application of La-SBBC was evaluated by analyzing breakthrough curves in bed volume concentration models. Continuous fixed-bed column experiments were performed in a glass column measuring 10 mm in diameter and 100 mm in height, with varying amounts of La-SBBC packed (1 g and 2 g). The wastewater with a P concentration of 3.08 mg/L was introduced into the fixed-bed column in a downward direction at a flow rate of 5 mL/min, facilitated by a peristaltic pump. The breakthrough point was set at 0.5 mg P/L, adhering to China’s effluent discharge standard.

3. Results and Discussion

3.1. Preparation Ratio of La-SBBC

Figure 1 illustrates the P adsorption capacity of La-SBBC, which was synthesized using varying ratios of DS and (La(NO3)3·6H2O). It can be observed that the removal efficiency of P increased proportionally with higher concentrations of La in La-SBBC. When the ratio of DS to (La(NO3)3·6H2O) reached 5:10, the P removal rate of La-SBBC attained its peak. This outcome was due to the saturation of La loading capacity in La-SBBC when the mass ratio was 5:10. Hence, the optimal mass ratio of DS to (La(NO3)3·6H2O) was determined to be 5:10.

3.2. Characterization of La-SBBC

The BET analysis results for the surface area and porosity are shown in Table 1. The specific surface area of La-DS is increased by 2.36 times compared with DS, but the average pore diameter is reduced by half because the specific surface area of La-DS is increased after loading La, and La ions are also present in the pores.
SEM was employed to analyze the changes in surface morphology and composition of DS before and after La modification and of La-SBBC before and after P adsorption (Figure 2). In comparison with DS, numerous layered porous structures, along with rough protrusions and grooves, were detected on the surface of La-SBBC. The successful doping of La into the DS was indicated by the emergence of a well-developed pore structure after modification, which provided more active sites for P adsorption due to an increase in the fine pore structure. The P fraction in La-SBBC-P rose significantly. Floccule formation was observed on the surface of La-SBBC-P particles, which may be related to metal-P structures that appeared on the La-SBBC-P. The SEM results were corroborated using FTIR (Figure 3) and XRD analyses (Figure 4).
The identification of the functional groups in La-SBBC was conducted using FTIR spectroscopy (Figure 3) [34]. A peak at 3400 cm−1 corresponds to -OH and -NH stretching vibrations [35]. The stretching vibrations of the carboxyl group are detected at 1640 cm−1, indicating the presence of C=O vibrations [16]. The peak at 1035 cm−1 is induced by the bending vibrations of the C-O bond in the alcohol group [25]. The stretching vibration peaks of -CH2 and aromatic hydrocarbon C=C in the fat structure are observed at 2928 cm−1 and 1546 cm−1 [16,21,35,36]. The main difference between La-SBBC and DS is observed at 1486 cm−1 and 788 cm−1; the peak at 1486 cm−1 corresponds to the residual nitrate NO₃⁻ remaining after the calcination of (La(NO3)3·6H2O), while the peak observed at 788 cm−1 corresponds to the oscillation of La-O linkages. The presence of a peak at 1457 cm−1 in La-SBBC also suggests that La3+ has been incorporated into the structure of DS. Furthermore, as evidenced by the FTIR spectrum (Figure 3), the disappearance of the peak at 1486 cm−1 can be observed in La-SBBC-P, while a noticeable emergence of a peak at 616 cm−1 is evident, indicating the presence of O-P-O bending vibration. This particular peak is not detected in the spectra analysis of La-SBBC [37,38]. This indicates the presence of P in the La-SBBC-P compound.
Figure 4 presents the XRD patterns of DS, La-SBBC, and La-SBBC-P. The DS exhibits two main diffraction peaks, which are centered at 2θ values of 20.84° and 26.06°. After loading La into the DS, the diffraction peak of La-SBBC at 2θ values of 26.52° was enhanced, and a new diffraction peak associated with La (NO3)3 emerged, along with a newly observed diffraction peak of La-O at 2θ values of 32.35°. After adsorption of P, the diffraction peak at 2θ values of 26.52° in La-SBBC-P was significantly weakened, a diffraction peak associated with LaPO4 was detected, and a new characteristic peak of LaP5O14 was generated at 2θ values of 32.19°.
Derivative thermogravimetric analysis (DTG) by thermogravimetry (TG). The thermogravimetric and derivative thermogravimetric (TG-DTG) curves are shown in Figure 5. The pyrolysis of DS occurred in one stage, which was characterized by an evaporation–dehydration reaction. The pyrolysis process of La-SBBC primarily consisted of two stages. The first stage (30~150 °C) involved the evaporation–dehydration reaction of La and DS. In the second stage (150~700 °C), La and DS combined to form biochar. The difference in the two stages of decomposition between DS and La-SBBC was likely due to a combination of biomass decomposition and the synthesis of La-O during the pyrolysis process.
The XPS spectrum is shown in Figure 6. The scanning spectrum of La-SBBC shows the existence of the elements La, O, C, and N. The observed binding energy of the peak at 836.31 eV confirmed the successful doping of La3+ into DS. After adsorption, peaks of P 2p (133.55) were observed at 135.20 and 132.30 eV, corresponding to HPO42− and H2PO4−, respectively, indicating the presence of P 2p on La-SBBC and successful adsorption of P onto La-SBBC [39].

3.3. Phosphate Adsorption

3.3.1. Effect of pH on Phosphate Adsorption

As the pH is widely recognized as a key factor governing the adsorption of P on biochar, it has an impact on both the chemical composition of P in the solution and the electrical charge of the biochar [36,40]. The results presented in Figure 7a indicate a significant influence of pH on P adsorption, with a noticeable decrease in adsorption observed when the pH exceeded 5. The maximum removal rate of La-SBBC reached 99.06% at pH 3.0. Studies by Kaljunen et al. [41] and Krishnan et al. [40] have reported similar findings, indicating that the optimal pH level for achieving the highest possible adsorption of P from aqueous solutions is found to be 3.0.
Following the La modification of DS, the surface charge of La-SBBC changed significantly (Figure 7b). The relative electronegativity of La-SBBC was observed to be lower compared to DS in a range, suggesting that the presence of La resulted in a decrease in the negative charge on the surface of DS. This reduction can possibly be attributed to the formation of La-O species on the surface of La-SBBC [42]. The decrease in the negative charge weakens the electrostatic repulsion between La3+ and PO43−, thereby facilitating the adsorption of P. The electronegativity of La-SBBC became stronger in a highly alkaline environment (Ph > 10), indicating the negative impact of such an environment on P adsorption [9].

3.3.2. Effect of La-SBBC Dosage on Phosphate Adsorption

The effect of La-SBBC dosage on the P adsorption process is illustrated in Figure 8. After adding 0.008 g of La-SBBC to the solution initially containing a concentration of 30 mg/L, the highest adsorption capacity achieved for P was 37.44 mg/g. However, as the La-SBBC dosage increased from 0.008 g to 0.06 g, the qe decreased from 37.44 mg/g to 7.47 mg/g, while the removal rate rose from 66.56% to 99.53%. Among these data points, when the La-SBBC dosage was 0.015 g, the P removal rate reached 99.17%. From that point onwards, further increases in La-SBBC dosage had a minimal impact on the rate of P removal. This trend may be attributed to the dosage, which has increased the number of adsorption sites for PO43− molecules and thereby enhanced the removal rate. However, since the initial concentration of P ions remained constant, the amount of adsorption did not increase proportionally. This led to unoccupied adsorption sites due to insufficient exposure to P and, consequently, a decrease in adsorption capacity [43]. From an economic perspective, the optimal dosage of La-SBBC is 0.015 g.

3.4. Adsorption Kinetics

Figure 9 demonstrates the impact of temperature (15, 25, and 45 °C) and contact time on the adsorption quantity. The curves all follow the same pattern. The relevant kinetic parameters are presented in Table 2. The rapid adsorption of P at the start was primarily owed to the even distribution of La-SBBC and its higher zero potential, which provided sufficient adsorption sites for P. This ease of access enabled the P to be transferred from the solution to the La-SBBC via electrostatic attraction [44].
The correlation coefficients (R2) of the kinetic models indicate that the pseudo-second-order kinetic models (R2 = 0.973, 0.994, and 0.997) were more fitting to depict the P adsorption process than the pseudo-first-order kinetic models (R2 = 0.949, 0.958, and 0.977). Surface chemisorption processes played a role in the P adsorption of La-SBBC [45]. This effect was primarily attributed to the doping of La, which altered the physicochemical structure of the DS and resulted in the generation of numerous active surface sites, making the chemisorption process a crucial step [25,35,46]. The calculated qe was generally consistent with the experimental data (22.32 mg/L).

3.5. Adsorption Isotherms

The utilization of the adsorption isotherm facilitates the examination of both the interplay between adsorbent and adsorbate as well as the structural characteristics exhibited by the adsorbent layer [47]. The P adsorption isotherms of La-SBBC were studied at varying initial concentrations (5–400 mg P/L) at 15, 25, 35, 45, and 65 °C, respectively.
Figure 10 illustrates the P adsorption isotherm process of La-SBBC, and the corresponding results are presented in Table 3. The adsorption efficiency of La-SBBC demonstrates an upward trend with the rise in the initial concentration, as depicted in Figure 10. This was likely due to the high surface concentration of P, which provided the driving force for adsorption [37,48]. Additionally, the temperature was found to have a positive impact on the adsorption capacity of P by La-SBBC, suggesting that higher temperatures favor the removal of P.
The Langmuir model primarily examines the adsorption of a single layer, such as through precipitation and hydrogen bonding interactions. On the other hand, the Freundlich model encompasses adsorption behaviors involving multiple layers, including van der waals force and electrostatic attraction [35,49,50]. The P adsorption process of La-SBBC was governed by multilayer adsorption actions because the Freundlich model demonstrated a greater level of precision in fitting than the Langmuir model (as shown in Table 3), suggesting a non-uniform nature of the adsorption process [36,51]. This occurrence can be sufficiently elucidated by the alterations in the structure of biochar. After doping with La, the number of La-SBBC surface active sites increased significantly, which led to the precipitation of La-P through a multilayer adsorption process [52]. It is generally accepted that the smaller the value of the Freundlich constant 1/n, the better the adsorption performance. Specifically, when 1/n falls in the range of 0.1 to 0.5, adsorption is easier, whereas when 1/n exceeds 2, adsorption is more difficult [44,53]. The 1/n values in this experiment ranged from 0.2 to 0.35, indicating that the La-SBBC exhibited an effective adsorption capacity for P [47]. The maximum capacity of La-SBBC was calculated to be 136.999 mg/g, which is higher than that of the majority of previously reported La-based sludge adsorbents. The adsorption capacity of La-SBBC for P removal outperforms that of other metal-modified sludge adsorbents reported in the literature (Table 4).

3.6. Adsorption Thermodynamics

The calculated thermodynamic parameters are provided in Table 5. The positive ∆H and ∆S values suggest that the adsorption phenomena were endothermic, which was corroborated by the increase in the P adsorption onto La-SBBC with a rise in temperature. The positive ∆S indicates increased spontaneity during P adsorption onto La-SBBC at the liquid-solid interface. The negative ∆G values confirmed the spontaneous and thermodynamically favorable nature of the adsorption process [58]. Generally, physisorption has an activation energy between 5 and 40 kJ/mol, whereas chemisorption typically involves an energy range of 40–800 kJ/mol [59]. The activation energy of this study was found to be 1059.37 kJ/mol, indicating that La-SBBC mainly underwent chemisorption during P adsorption. This conclusion was further substantiated by the emergence of additional chemical bond formation, as indicated by the FTIR spectra, and phase changes, as indicated by XRD data. In studies of P adsorption on sludge-based adsorbents, the same phenomena were observed [60].

3.7. Adsorption–Desorption Cycles

Stabilizing and regenerating an adsorbent is essential to improving the economics of the adsorption process. The regeneration of P-saturated La-SBBC was investigated through six cycles of adsorption and desorption using a 0.2 mol/L NaCl solution as the regenerant. As shown in Figure 11, the sorbed P could be effectively removed, and the renewed La-SBBC exhibited excellent performance in P removal. Specifically, the adsorption capacity retained 88.7% of its initial value after the first regeneration. The first cycle exhibited a marked decline in adsorption capacity, whereas subsequent cycles demonstrated relative stability. After six cycles, the adsorption capacity remained at 76.6% of the original qe, indicating a strong interaction between PO43− and La-SBBC. This behavior might be attributed to the formation of La-P compounds and multilayer structures. Some active sites on La-SBBC failed to regenerate easily, indicating the existence of both invertible and non-invertible adsorption sites on La-SBBC [43]. The re-usability of La-SBBC for removing P over multiple usage cycles was acceptable.

3.8. Fixed-Bed Column Dynamic Adsorption

To assess the practical feasibility of La-SBBC, adsorption experiments were conducted using successively dynamic actual wastewater with a P concentration of 3.08 mg/L. The breakthrough curves of La-SBBC at different adsorbent dosages (1 g and 2 g) are shown in Figure 12. An augmentation in the quantity of La-SBBC was noted to lead to a rise in the surface area of the adsorption layer, thereby augmenting the count of binding sites that are accessible for P adsorption. This increment also increased the contact time between La-SBBC and the effluent, thereby enhancing the efficiency of P adsorption [34]. Furthermore, once the effluent P concentration reached 0.5 mg/L, the breakthrough and depletion points were recorded at 9.58 h and 30.08 h for 1 g of La-SBBC, respectively, while for 2 g of La-SBBC, they were recorded at 51.75 h and 69.75 h. The results show that La-SBBC has a long-lasting adsorption performance for P, which can be extended to practical applications.

3.9. Phosphate Adsorption Mechanism on La-SBBC

Figure 13 illustrates the mechanism of P adsorption from an aqueous solution by La-SBBC. Precipitation plays an essential part in the adsorption of P by most adsorbents comprising metal ions or metal oxides [21]. LaPO4 has a low Ksp value (3.7 × 10−23), suggesting a strong affinity between La3+ and PO43−, predisposing them to precipitation in solution [38,61]. The XRD characteristic peaks observed in La-SBBC-P confirm that the precipitation of LaPO4 was a crucial mechanism for P removal by La-SBBC. Further evidence of P adsorption on La-SBBC-P was provided by the SEM results, which distinctly showed the presence of the P element. At high pH, metal ions are easily deprotonated, and the adsorbent is usually negatively charged [34], whereas at low pH, the adsorbent containing metal ions is easily protonated, resulting in a positively charged adsorbent [23,62]. The zeta potentials of La-SBBC, as depicted in Figure 7b, indicate that the point of zero charge (pHpzc) for La-SBBC is 10.2. Thus, the protonation reaction of La-SBBC (La-OH + H+ = La-OH2+) gave it a positive charge and promoted the adsorption of H2PO4− or HPO42− by electrostatic attraction within the pH range of 2.0 to 10.2. This led to an increased amount of adsorbed P. However, the electrostatic attraction’s impact diminished gradually as the pH rose.
Further, complexation, as corroborated by FTIR spectra and XPS analyses, played a pivotal role in P removal [12]. The FTIR spectra of La-SBBC-P, depicted in Figure 3, exhibit peaks at about 620 cm−1 and 546 cm−1 corresponding to the O-P-O bending vibration, which signifies successful adsorption of P by La-SBBC-P [37,38]. The peak at 1384 cm−1, attributed to -OH vibration, was significantly attenuated as a result of the creation of inner-sphere complexes involving P.
The peaks for La 3d5/2 in the XPS spectra (Figure 6) were positioned at 835.23 and 838.93 eV prior to P adsorption, while those for La 3d3/2 were located at 852.13 and 855.78 eV. After P adsorption, the La 3d5/2 and 3d3/2 peaks shifted to higher energies at 835.68, 839.48 eV, and 852.33, 855.93 eV, respectively. The shift of the La 3d binding energy towards higher values indicates an electron transfer within the La 3d valence band, culminating in the formation of a La-O-P inner-sphere complex. This led to increased affinities and interactions between P and La [39]. These findings align with prior research [12,21,63].

4. Conclusions

In this study, DS was modified with (La(NO3)3·6H2O), followed by pyrolysis, resulting in the production of La-SBBC with a significant adsorption capacity for P. The adsorption process of La-SBBC was found to be consistent with both the Freundlich isotherm and the pseudo-second-order model, suggesting a multilayer chemisorption process as the rate-determining step. The maximum adsorption capacity of La-SBBC was found to be 152.77 mg/g. Thermodynamic investigations have shown that the adsorption process has both endothermic characteristics and spontaneous tendencies. Additionally, the adsorption mechanism involved precipitation, electrostatic interaction, and inner-sphere complexation. Furthermore, the adsorption efficiency of La-SBBC remained at 76.6% of its initial capacity after six regeneration cycles. Consequently, these findings highlight the potential of La-SBBC as a practical, efficient, and recyclable adsorbent for removing P from actual wastewater.

Author Contributions

Conceptualization, software, data curation, formal analysis, and writing—original draft, J.M.; methodology, Q.L.; writing—review and editing, H.Z. (Hongxiang Zhu) and M.X.; supervision, X.S., B.D. and H.Z. (Hongxia Zhang); funding acquisition, X.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Natural Science Foundation of Guangxi (No. 2018GXNSFGA281001) and the Science and Technology Major Project of Guangxi (GuikeAA18118013). other.

Data Availability Statement

Data available on request due to restrictions eg privacy or ethical The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

Correction Statement

This article has been republished with a minor correction to resolve spelling and grammatical errors. This change does not affect the scientific content of the article.

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Water 16 00418 g001
Figure 1. Removal rate of P by adsorbents in different proportions.
Figure 1. Removal rate of P by adsorbents in different proportions.
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Figure 2. The SEM images: (a) DS; (b) La-SBBC; (c) La-SBBC-P.
Figure 2. The SEM images: (a) DS; (b) La-SBBC; (c) La-SBBC-P.
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Figure 3. FTIR analyses of DS (blue line), La-SBBC (black line), and La-SBBC-P (red line).
Figure 3. FTIR analyses of DS (blue line), La-SBBC (black line), and La-SBBC-P (red line).
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Figure 4. XRD analyses of DS (red line), La-SBBC (black line), and La-SBBC-P (green line).
Figure 4. XRD analyses of DS (red line), La-SBBC (black line), and La-SBBC-P (green line).
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Figure 5. TG-DTG curves of DS and La-SBBC.
Figure 5. TG-DTG curves of DS and La-SBBC.
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Figure 6. XPS spectra: (a) survey scan of DS (blue line), La-SBBC (black line) and La-SBBC-P (red line); (b) La 3d of La-SBBC; (c) P 2p of La-SBBC-P; (d) La 3d of La-SBBC-P.
Figure 6. XPS spectra: (a) survey scan of DS (blue line), La-SBBC (black line) and La-SBBC-P (red line); (b) La 3d of La-SBBC; (c) P 2p of La-SBBC-P; (d) La 3d of La-SBBC-P.
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Figure 7. pH effect: (a) effect of pH on P adsorption capacities and removal rate of La-SBBC; (b) zeta potential of DS and La-SBBC at different pH values.
Figure 7. pH effect: (a) effect of pH on P adsorption capacities and removal rate of La-SBBC; (b) zeta potential of DS and La-SBBC at different pH values.
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Figure 8. Effect of La-SBBC dosage on P adsorption capacities and removal rate.
Figure 8. Effect of La-SBBC dosage on P adsorption capacities and removal rate.
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Figure 9. Kinetic process of P adsorption by the La-SBBC.
Figure 9. Kinetic process of P adsorption by the La-SBBC.
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Figure 10. Isotherm process of P adsorption of the La-SBBC.
Figure 10. Isotherm process of P adsorption of the La-SBBC.
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Figure 11. Adsorption capacity after desorption of La-SBBC-P.
Figure 11. Adsorption capacity after desorption of La-SBBC-P.
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Figure 12. Effects of dosages on the breakthrough point of La-SBBC.
Figure 12. Effects of dosages on the breakthrough point of La-SBBC.
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Figure 13. Mechanism of P adsorption of La-SBBC.
Figure 13. Mechanism of P adsorption of La-SBBC.
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Table 1. Surface area and porosity results.
Table 1. Surface area and porosity results.
MaterialSpecific Surface AreaAperturePore Volume
DS7.9220.960.6
La-SBBC18.410.030.6
Table 2. Kinetic parameters for the P adsorption of the La-SBBC.
Table 2. Kinetic parameters for the P adsorption of the La-SBBC.
SamplesPseudo-First-OrderPseudo-Second-Order
Qe (mg/g)k1 (h−1)R2Qe (mg/g)K2 (h−1)R2
La-SBBC 15 °C21.6452.3350.94922.3150.1510.973
La-SBBC 25 °C21.5156.2860.95822.2560.4880.994
La-SBBC 45 °C21.98510.1610.97722.4670.9380.997
Table 3. Isothermal parameters for the adsorption of P on La-SBBC.
Table 3. Isothermal parameters for the adsorption of P on La-SBBC.
SamplesLangmuirFreundlich
Qm (mg/g)KL (L/g)R2KF (L/g)1/nR2
La-SBBC 15 °C95.8290.2470.81519.5690.3400.928
La-SBBC 25 °C108.9540.1650.86122.9580.3250.956
La-SBBC 35 °C136.6340.0670.85525.4580.3250.950
La-SBBC 45 °C123.1770.2020.87429.8160.3000.953
La-SBBC 65 °C136.9990.1420.88431.0760.3050.948
Table 4. Comparison of P adsorption capacity of different metal-modified sludge biochars.
Table 4. Comparison of P adsorption capacity of different metal-modified sludge biochars.
AdsorbentsQm (mg/g)References
La-modified sludge-based biochar (La-SBBC)136.999This study
La-coated sewage sludge biochar93.91[31]
La-modified water treatment sludge hydrochar72.69[32]
oyster shell-modified activated sludge biochar129.03[33]
sludge biochar modified with FeCl3111.0[25]
Aluminum-impregnated sludge biochar11.9[54]
Ferric sludge biochar36.67[55]
Aluminum sludge biochar36.303[56]
Mg-modified sludge biochar97.45[57]
Table 5. Thermodynamic parameters for adsorption P of the La-SBBC.
Table 5. Thermodynamic parameters for adsorption P of the La-SBBC.
SamplesΔG (kJ/mol)ΔS (J/mol·K)ΔH (kJ/mol)
La-SBBC 15 °C−105.4104.2731059.370
La-SBBC 25 °C−334.641
La-SBBC 35 °C−222.890
La-SBBC 45 °C−283.026
La-SBBC 65 °C−382.348
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Mo, J.; Li, Q.; Sun, X.; Zhang, H.; Xing, M.; Dong, B.; Zhu, H. Capacity and Mechanisms of Phosphate Adsorption on Lanthanum-Modified Dewatered Sludge-Based Biochar. Water 2024, 16, 418. https://doi.org/10.3390/w16030418

AMA Style

Mo J, Li Q, Sun X, Zhang H, Xing M, Dong B, Zhu H. Capacity and Mechanisms of Phosphate Adsorption on Lanthanum-Modified Dewatered Sludge-Based Biochar. Water. 2024; 16(3):418. https://doi.org/10.3390/w16030418

Chicago/Turabian Style

Mo, Jingjing, Qian Li, Xiaojie Sun, Hongxia Zhang, Meiyan Xing, Bin Dong, and Hongxiang Zhu. 2024. "Capacity and Mechanisms of Phosphate Adsorption on Lanthanum-Modified Dewatered Sludge-Based Biochar" Water 16, no. 3: 418. https://doi.org/10.3390/w16030418

APA Style

Mo, J., Li, Q., Sun, X., Zhang, H., Xing, M., Dong, B., & Zhu, H. (2024). Capacity and Mechanisms of Phosphate Adsorption on Lanthanum-Modified Dewatered Sludge-Based Biochar. Water, 16(3), 418. https://doi.org/10.3390/w16030418

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