Dual-Functioned Magnesium-Enriched Biochar Hydrogels for Phosphate Recovery and Slow-Release Nutrient Delivery

Abstract

Excessive phosphate from agriculture and industry has led to widespread eutrophication, posing a serious environmental threat. To address this issue, metal-modified biochars have emerged as promising adsorbents due to their high affinity for phosphate ions. This study investigates the application of two magnesium-modified biochar hydrogels denoted as magnesium–bamboo biochar hydrogel (Mg-BBH) and magnesium–pulp biochar hydrogel (Mg-PBH) for phosphate recovery from aqueous solutions, with an additional aim as slow-release fertilizers. The adsorbents were synthesized by impregnating Mg-modified biochars into sodium-alginate-based hydrogel. The influence of initial phosphate concentration, contact time, and temperature were investigated to determine optimal adsorption conditions. Both adsorbents exhibited excellent adsorption performance, with maximum capacities of 309.96 mg PO₄/g (Mg-BBH) and 234.69 mg PO₄/g (Mg-PBH). Moreover, the adsorption performance of the adsorbents was greatly influenced by the magnesium content. The adsorption process followed the Temkin isotherm and pseudo-second-order kinetics, suggesting that the adsorption energy decreases proportionally with surface coverage and the phosphate uptake was governed by chemisorption. Thermodynamic study confirmed the process was spontaneous and endothermic at 40 °C. A slow-release study further demonstrated a great release of phosphate in soil over time. These findings highlight the dual functionality of Mg-BBH and Mg-PBH as effective materials for both phosphate recovery and controlled nutrient delivery, contributing to sustainable phosphate management.

1. Introduction

A planetary boundaries diagram (Figure 1) serves as a method for assessing the sustainability of the global environment. It is based on the concept that human activities, when exceeding the Earth’s tolerable limits, will cause irreversible damage to nature [1]. Among the ten identified boundaries, biogeochemical flows of phosphorus (P) and nitrogen (N) have surpassed the safe limits, posing serious risks to global sustainability. As can be seen from Figure 1, the phosphorus situation is far more serious than the climate change situation, where it is at the highest risk of “tipping”, along with the “geochemical cycling of nitrogen”. Excess phosphorus generally originates from fertilizers and livestock manure, which are not fully absorbed by crops and eventually become wasted in runoff of agricultural land. This nutrient can accelerate eutrophication of water systems, leading to algal bloom and degradation of aquatic systems [2]. To mitigate these risks, both phosphorus and nitrogen application to agricultural land have been strictly regulated in Europe and the United States [3]. However, Japan has yet to adopt strict regulations to control excess phosphorus from polluting local water bodies.

To mitigate this problem, various approaches have been utilized to control phosphate contamination in water such as membrane separation [4], flocculation/ precipitation [5], ion exchange resin [6], and adsorption [7]. Among these, extensive works on adsorption technologies have been explored, particularly using biomass-derived materials for adsorbing phosphate in the environment due to their green approach, low cost, and abundant material availability. For example, bamboo powder and eggshell composites have been thermally treated to form calcium, which can bind to phosphorus, allowing for its recovery from water and subsequent usage as phosphorus fertilizers [8]. Another study has explored the use of bamboo activated carbon modified with zirconia chloride octahydrate (ZrOCl₂·8H₂O) and cetyltrimethylammonium bromide (CTAB) for removal of phosphate from wastewater [9].

Biochar serves as an effective adsorbent material for many applications due to its high surface area and oxygen-rich functional groups [10]. However, its negative surface charge often limits its affinity for anionic contaminants such as phosphate. Phosphate ions (PO₄) require a substance with high affinity such as metal-oxide to yield a better adsorption capacity. To overcome this limitation, many researchers have chemically modified the biochar with metals such as iron (Fe), calcium (Ca), and magnesium (Mg) to enhance its binding capacity. For instance, Fe modification of bamboo biochar with FeCl₃ has been reported to increase its potential in removing phosphate, with a maximum phosphate adsorption capacity of 11.57 mg/g [11]. Similarly, metal-modified biochar (ZFCO-BC) loaded with Fe and Ca under 900 °C has shown a promising adsorption capacity and improved binding of phosphate [12]. Magnesium is also particularly effective in forming stable complexes with phosphate, thereby being able to enhance the adsorption capacity of biochar materials. Metal-modified biochar such as magnesium (Mg)-/Fe-doped biochar has shown promising results for phosphate uptake, with an adsorption capacity of 64.65 mg/g [13].

While many studies have been conducted on the adsorption and recovery of phosphorus by adsorbents, limited research is available on its direct application in soil such as in the form of slow-release fertilizer. Therefore, we propose a dual-function approach to recover phosphate from water for reuse in agriculture to effectively address both issues related to wastewater treatment and agricultural nutrient recycling. A previous study has investigated the adsorption of phosphate using spent coffee ground modified with calcium hydroxide and its direct application as phosphate fertilizer [14]. The effectiveness of this fertilizer was validated through pot tests and the plant germination index, confirming its potential for agricultural application. Even though coffee residue represents a promising biomass source in the study, concerns regarding the inhibitory effects from residual caffeine limit its widespread use in soil [15].

Given this limitation, this current study intends to investigate the recovery and slow release of phosphate by using magnesium-modified biochar hydrogels from two biomass sources: pulp manufacturing residues (Mg-PBH) and bamboo powder (Mg-BBH). In this context, bamboo and pulp processing residues represent underutilized lignocellulosic wastes with significant potential for environmental remediation and agricultural reuse. Bamboo, in particular, has become increasingly abundant in Japan and East Asia due to the expansion of abandoned bamboo forests. Simultaneously, pulp residues are abundant wastes and are widely available from the paper industry. The byproducts of the paper industry are commonly rich in cellulose and residual chemicals like calcium and magnesium from the pulping process [16,17], which might be beneficial for interaction with phosphate. Converting these wastes into functional biochar adsorbents can provide a sustainable solution for waste management, biomass valorization, and nutrient recycling.

In this study, Mg-BBH was synthesized by impregnating the bamboo with magnesium chloride and carbonization followed by hydrogel formation using sodium alginate. Meanwhile, Mg-PBH was derived directly from pulp manufacturing residue, which contains residual magnesium from the pulping process. The pulp residue was simply carbonized and then immobilized into a sodium alginate hydrogel matrix without additional magnesium supplementation. The resulting biochars were immobilized within the alginate structure to provide structural integrity, enhance adsorption efficiency, and facilitate recovery of these adsorbents in aqueous conditions. This hydrogelation not only enhances PO₄ adsorption efficiency but also facilitates recovery and ease of handling as a slow-release phosphate nutrient. In brief, this work aims to develop and evaluate Mg-modified biochar hydrogels for phosphate adsorption from aqueous solutions and their subsequent controlled release in the soil environment for effective nutrient cycling. Overall, this research could contribute to the reduction in excessive limits of biogeochemical flows of phosphorus while promoting a recycling approach of turning waste into resource using locally abundant biomass materials.

2. Materials and Methods

2.1. Chemicals and Reagents

The raw material of bamboo (Phyllostachys bambusoides) was obtained from Nanatsuka-cho, Shobara, Hiroshima. The dried bamboo was cut into small pieces of 1 to 2 cm and crushed into fine powder to pass through 80-mesh screens, in accordance with JIS Z 8801 [18] and ASTM E11 standards [19], with an aperture width of 180 µm. Meanwhile, the pulp residue, containing magnesium at about 1.364 mg/g, was supplemented from the pulping industry known as Europiren B.V (Schiedam, The Netherlands). Reagents such as such as sodium hydroxide (NaOH, 99.99%) and hydrochloric acid (HCl, 35–37%), calcium chloride (CaCl₂, 95.0%), and magnesium chloride hexahydrate (MgCl₂·6H₂O, 99.0%) were obtained from Kanto Chemical Co. (Tokyo, Japan). Sodium alginate (90.8–106.0%) was purchased from Matsuba Pharmaceutical Co., Ltd. (Nara, Japan). All the chemicals and reagents used in this study were of chemical grade.

2.2. Preparation of Mg-Modified Biochar

The preparation of magnesium-modified bamboo biochar involved several steps and was prepared according to [20] with slight modification. First, the bamboo powder (50 g) was impregnated with 0.5 M MgCl₂ solution (1 L) overnight under constant stirring without any pH adjustment. Then, the resultant Mg-modified bamboo was carbonized at 600 °C for 2 h under limited-oxygen conditions by using a muffle furnace (FO 100, Yamato Scientific Co., Tokyo, Japan) to yield Mg-BB. Meanwhile, the pulp residue, which contained residual Mg, was also carbonized under similar conditions to obtain Mg-PB. After the heating treatment, the biochar materials were crushed into fine particles by using a mortar for subsequent hydrogel preparation.

2.3. Preparation of Mg-Modified Biochar Hydrogel

Firstly, a 2% sodium alginate (SA) solution (w/v) was prepared by dissolving 4 g of sodium alginate in 200 mL of distilled water. The solution was magnetically stirred overnight to ensure proper dissolution. Then, the prepared biochar powder (Mg-BB) beforehand was added into the SA solution in the amount of 1.5% (w/v) and stirred homogenously for 2 h. Subsequently, the mixture was injected into a 3% (w/v) calcium chloride solution using a 10 mL syringe. The formed beads (Mg-BBH) were left to stand in the CaCl₂ solution for 90 min before washing with distilled water and drying in an oven. Similar procedures were repeated for preparation of Mg-PBH.

2.4. Characterization of Mg-Modified Biochar and Mg-Modified Biochar Hydrogel

The amount of Mg contained in the Mg-BB and Mg-PB was assessed using a soil analyzer (Air Water Biodesign Co., Ltd., EW-THA1J, Saitama, Japan). The morphological structures of the Mg-modified biochar and Mg-modified biochar hydrogel were analyzed by scanning electron microscopy (SEM, miniscope TM3000, Hitachi-hitech, Tokyo, Japan). Additionally, the surface functional groups present in both the Mg-BBH and Mg-PBH, before and after phosphate adsorption were confirmed by Fourier transform infrared spectroscopy (FTIR, Thermo Scientific Nicolet iS10, Waltham, MA, USA).

2.5. Adsorption Experiments (Aqueous Solution Condition)

The adsorption experiment was performed based on a batch removal experiment. Initially, a 0.1 M phosphate (PO₄) stock solution (pH 7.4) was diluted to the desired concentrations. A total of 100 mg of the Mg-BBH and Mg-PBH was shaken in 50 mL of diluted PO₄ solution at varying concentrations to investigate the effect of the initial PO₄ concentration in the adsorption process analysis. After shaking in a temperature-regulated rotator (30 °C) at 100 rpm for 1 h, the residual PO₄ concentration was determined by using the molybdenum blue colorimetric method and ion chromatography (Metrohm, Tokyo, Japan). The effect of contact time and temperature were also investigated by using an initial PO₄ concentration of 700 mg/L at different temperatures (20 °C, 30 °C, 40 °C) to determine the sufficient time and optimal temperature for the adsorption process. Other variables such as pH: 7.4–7.6, rotation speed: 100 rpm, adsorbent dosage: 0.1 g, and solution volume: 50 mL were maintained at optimal conditions. All experiments were conducted in duplicate, and the experimental standard deviations are reported in the graph. The adsorption capacity (q_e) and the removal rate of phosphate were calculated using Equations (1) and (2) below, respectively.

$$\mathrm{Adsorption}\mathrm{capacity},q_e(\mathrm{mg}/\mathrm{g})={\displaystyle \frac{C_i-C_e}{W}}\times V$$

(1)

$$\mathrm{Removal}\mathrm{rate}(\%)={\displaystyle \frac{C_i-C_e}{C_i}}\times 100$$

(2)

where $C_i$ and $C_e$ are the initial and final concentrations of the solution (mg/L), respectively. q_e is the adsorption capacity of the adsorbents in mg/g, W is the weight of Mg-BBH or Mg-PBH (g), and $V$ is the volume of the solution (L).

2.6. Release of Phosphate into Soil

The amount of phosphate released from the PO₄-loaded biochar hydrogel into soil was investigated through a soil incubation experiment. Initially, the Mg-modified biochar hydrogels (Mg-BBH and Mg-PBH) were loaded with phosphate through the adsorption process using an initial PO₄ concentration of 2000 mg/L under optimum conditions to ensure the adsorbents reached saturation. The incubation experiment was carried out in a plastic container (12 oz) for uniform water distribution and interaction with soil, and the type of soil used in this study was Andosol. Briefly, 0.2 g of PO₄-loaded biochar hydrogel was mixed into soil (20 g) and watered daily with 15 mL of distilled water. The samples were incubated under ambient temperature and airflow. The analysis of available phosphorus in the soil was performed at days 0, 2, 4, and 6. The available phosphate inside the soil was extracted using a 0.5 M NaHCO₃ solution (adjusted to pH 8.5) for 30 min at a ratio of 1:2 (w/v) following the widely adopted Olsen extraction procedure [21]. The phosphate content was measured by using the colorimetric ammonium molybdate blue method.

2.7. Statistical Analysis

The data analyses were performed using MINITAB 21. The mean values were subjected to analysis of variance (ANOVA) using one-way and two-way analyses of variance to test the statistical significance (p-value < 0.05) of the phosphate recovery between both adsorbents (Mg-BBH and Mg-PBH).

3. Results and Discussion

3.1. Characterization of the Adsorbent Materials

The results for the Mg composition contained in the Mg-modified biochar samples are presented in

Table 1. Chemical components contained in Mg-BB and Mg-PB (mean values ± standard error).

Elements	Mg-Modified Bamboo Biochar (mg/100 g)	Mg-Containing Pulp Biochar (mg/100 g)
Nitrogen	0.4 ± 0.06	0.1 ± 0.08
Phosphoric acid	378.9 ± 8.3	<0.1
Potassium	168.2 ± 6.6	90.1 ± 7.2
Calcium	20.4 ± 2.9	152.7 ± 4.9
Magnesium	>140.0	82.8 ± 8.9

. Based on the results, Mg-BB exhibited a significantly higher Mg content, exceeding 140 mg/100 g, compared to the Mg-containing pulp residue biochar. This confirms the successful incorporation of Mg into the bamboo during the modification process. Additionally, the pulp residue biochar revealed a higher content of calcium (152.7 mg/100 g) than the Mg-modified bamboo biochar, which was likely due to the residual chemicals used from the pulping process. Calcium hydroxide or calcium carbonate are typically used during the pulping process for controlling pH or precipitating lignin; therefore, some calcium may have remained in the pulp residue. Overall, these findings confirm that the Mg-modified bamboo biochar possesses a greater magnesium content, which is expected to enhance its adsorption performance due to the increased availability of metal (Mg) active sites.

The morphological structures of the materials were characterized by scanning electron microscopy (SEM) and are illustrated in Figure 2. The bamboo and pulp biochar samples (Mg-BB and Mg-PB) prior to immobilization into sodium alginate exhibited highly porous structures with well-defined micropores and macropores (Figure 2a,b). The carbonization treatment of the bamboo and pulp at 600 °C resulted in large pore formation due to the decomposition and collapse of the original lignocellulose structure. After immobilization, the surface view of both immobilized biochar hydrogels (Mg-BBH and Mg-PBH) exhibited uneven surfaces with coarse textures and a few formations of pores. However, these pores became significantly visible and well distributed in the interior structure of both biochar hydrogels (Figure 2e,f). These porous structures can promote their potential functions to facilitate the diffusion of molecules into their void cavitied and interact with active sites on the adsorbents’ structure.

FTIR spectroscopy was employed to characterize the specific functional groups present in the Mg-BBH and Mg-PBH prior to and after interaction with phosphate ions. Based on Figure 3, the FTIR spectrum of both adsorbents showed similar absorption properties, particularly at 3218 cm⁻¹ (stretching of OH group), 1588 cm⁻¹ (asymmetric stretching of COO⁻ anions), 1416 cm⁻¹ (symmetric vibration of C=O), and 1293 cm⁻¹ (C–O–C stretching) [22]. In addition, the high-intensity peaks found at 1588 cm⁻¹ in both adsorbents may also show the presence of aromatic C=C for a benzene ring [23,24]. Meanwhile, adsorption peaks found between 1023–1026 cm⁻¹ were assigned to the stretching vibration of the C–O–C group of the pyranose ring of the sodium alginate [25,26]. The formation of a peak at around 871–877 cm⁻¹ in both adsorbents was assigned to the Mg-O vibration band as a result of the treatment with Mg metal salt [22,27]. These FTIR results confirmed the presence of various oxygen-containing functional groups in Mg-BBH and Mg-PBH, providing available functional groups for electrostatic interactions with H₂PO₄⁻, contributing significantly toward enhancement for PO₄ adsorption. After adsorption with phosphate, additional changes can be seen in the O-H region (3240 cm⁻¹), where the bands become weakened and narrower after phosphate adsorption, suggesting that ion exchange occurred between PO₄³⁻/PO₄⁻ ions and surface −OH groups from Mg–OH [28]. Additionally, there was a slight decrease in the intensity of the peak at the C–O–C and Mg-O regions, which may indicate the formation of complexes between Mg, C–O, and phosphate. The high availability of oxygen functional groups in the adsorbents provides active sites for various interactions with H₂PO₄⁻ and HPO₄²⁻, thereby contributing significantly toward enhancement in terms of phosphate adsorption [29].

3.2. Adsorption of PO₄ (Influence of Initial PO₄ Concentration)

Figure 4 illustrates the phosphate adsorption capacity of both adsorbents (Mg-BBH and Mg-PBH) after being subjected to different concentrations of PO₄. In this experiment, the adsorption capacity of PO₄ by these two adsorbents progressively increased with the increase in the initial PO₄ concentration from 196 mg/L to 980 mg/L. This could be explained by the saturated amount of adsorbate in the surrounding, which enhances the deposition and precipitation of phosphate ions on the surface of the metal-modified biochar hydrogels, thus demonstrating an increasing adsorption performance. Moreover, notable differences can be observed in the adsorption performance between these two adsorbents, where Mg-PBH achieved a highest adsorption capacity of 295.75 mg PO₄/g, whereas Mg-BBH recorded a significantly (p < 0.05) higher capacity of 465.93 mg PO₄/g. This corresponds to an overall difference of 57.5% in terms of PO₄ adsorption due to the higher content of Mg present in the Mg-BBH sample, as evidenced in Section 3.1, leading to a better ion exchange ability and more functional groups for interaction with PO₄ [13]. Similarly, ref. [30] has reported a finding that a higher content of Mg in corncob biochar increased the phosphate removal by up to 76.14% compared to 2.64% in pristine biochar. The adsorption capacity observed in this study suggests that these Mg-BBH and Mg-PBH samples exhibit superior capacities in terms of removing phosphate from the environment.

3.3. Adsorption of PO₄ (Influence of Contact Time and Temperature)

Contact time is a crucial factor in determining the efficiency of the adsorption process. To investigate its effect on the phosphate adsorption performance of both adsorbents, experiments were conducted under optimal conditions by using a fixed initial phosphate concentration of 700 mg/L with a 0.1 g adsorbent dosage at a pH range of 7.4–7.6. The adsorption process was also simultaneously performed at three different temperatures (20 °C, 30 °C, 40 °C) to examine the effect of temperature. As illustrated in Figure 5 and Figure 6, the adsorption of phosphate by both Mg-BBH and Mg-PBH was likely to achieve equilibrium within 20 min.

The adsorption performance of Mg-BBH at 20 °C increased abruptly from 5.0 to 90.0 mg PO₄/g within the first 30–60 s, then gradually decreased and fluctuated before reaching a maximum adsorption of 150 mg/g. The fluctuating adsorption trend observed for Mg-BBH at 20 °C (Figure 5) may be attributed to the weaker binding interactions, which are more pronounced at lower temperatures, leading to reversible binding [31]. Interestingly, increasing the adsorption temperature to 30 °C led to more stable and consistent adsorption by Mg-BBH, with a significant increase in phosphate uptake observed in the first minute. Following this initial phase, the adsorption capacity of phosphate showed a slight fluctuation but continued to increase steadily, eventually reaching equilibrium adsorption (300.13 mg PO₄/g) at around 20 min. Generally, the faster adsorption kinetics observed in the first minutes of adsorption are primarily contributed by the abundant active sites that were accessible during the initial phase of the adsorption process [32]. Subsequently, a further increase to 40 °C resulted in a minor increase in the adsorption trend for Mg-BBH, with an adsorption capacity of 309.96 mg PO₄/g.

In contrast to Mg-BBH, Mg-PBH exhibited more consistent adsorption data at 20 °C, gradually increasing until reaching an equilibrium capacity of 146.56 mg PO₄/g at 20 min (Figure 6). At approximately 30 °C, Mg-PBH demonstrated a significantly improved performance, with a maximum capacity of 227.19 mg PO₄/g. Similar to Mg-BBH, increasing the temperature to 40 °C contributed to a slight increase in the adsorption capacity of Mg-PBH, achieving 234.69 mg PO₄/g. These findings are consistent with a previous study [33], which reported that increasing the adsorption temperature to a certain optimum range of 35 °C enhances the mobility and binding of phosphate onto the active sites of the adsorbent. Conclusively, the adsorption capacities of both adsorbents at 30 °C and 40 °C were significantly (p < 0.05) greater than those at 20 °C.

Notably, the comparatively lower adsorption capacity observed in Mg-PBH is likely due to its lower content of magnesium, resulting in fewer active sites available for interaction with phosphate. Despite this, the adsorption performances observed in this study surpass the maximum adsorption capacity of 109.35 mg/g reported by [34] using bamboo biochar over a 2 h contact time. Overall, the higher Mg content and optimum temperature contributed to improved surface characteristics of adsorbent and superior adsorption performance.

3.4. Adsorption Isotherm Studies

The distribution of phosphate molecules in the liquid and solid phases at equilibrium can be described using adsorption isotherm analysis. This analysis provides insight into the adsorption mechanism and surface characteristics of the adsorbents and can be assessed by isotherm models such as Langmuir, Freundlich, and Temkin, which are represented by Equations (3)–(5). The linear fitting results for Langmuir, Freundlich, and Temkin are shown in Figure 7, along with the corresponding isotherm parameters being summarized in

Table 2. Parameters for adsorption isotherm studies.

Isotherm Models	Parameters	Mg-BBH	Mg-PBH
Langmuir	$q_m$	563.1725	193.703
	$K_L$	0.0103	0.0018
	$R_L$	0.4921	0.8470
	R2	0.2918	0.2014
Freundlich	$K_f$	1.9935	0.026
	1/$n$	1.4438	1.6094
	R2	0.8528	0.7344
Temkin	$b_T$	341.52	218.1893
	$K_T$	0.0812	0.0106
	R2	0.9131	0.937

.

$$\mathrm{Langmuir},\hspace{1em}\hspace{1em}{\displaystyle \frac{C_e}{q_e}}={\displaystyle \frac{1}{q_m.K}}+{\displaystyle \frac{C_e}{q_m}}$$

(3)

$$\mathrm{Freundlich},\hspace{1em}\hspace{1em}ln q_e=ln K_f+{\displaystyle \frac{1}{n}} ln C_e$$

(4)

$$\mathrm{Temkin},\hspace{1em}\hspace{1em}{q}_e={\displaystyle \frac{RT}{b_T}}\mathit{ln}{K}_T+{\displaystyle \frac{RT}{b_T}}\mathit{ln}{C}_e$$

(5)

where q_e (mg/g) is the amount of Cd (II) adsorbed at equilibrium, C_e represents the concentration of phosphate at the adsorption equilibrium (mg/L), q_m is the maximum adsorption capacity (mg/g), K_L is the Langmuir constant (L/mg), K_F ((mg/g)(L/mg)^1/n) and 1/n are Freundlich constants referring to adsorption capacity and intensity, respectively, R represents the universal gas constant (8.314 J·mol⁻¹·K⁻¹), T is the absolute temperature (K), K_T is the Temkin maximum binding constant in L/mg, and $b_T$ is the Temkin constant of heat of adsorption (KJ mol⁻¹).

Among the tested isotherm models, the Temkin model exhibited the best fit for both Mg-BBH and Mg-PBH, as evidenced by the highest correlation coefficient values (R² = 0.9131 for Mg-BBH, and R² = 0.937 for Mg-PBH). This model, which considers the adsorbent–adsorbate interactions, implies that moderate interaction between physical adsorption and chemisorption may have occurred between phosphate molecules and the adsorbents, and the heat of adsorption decreases linearly with increasing coverage on the surface. The Temkin isotherm model is also appropriate for adsorption systems in which chemisorption occurs through electrostatic interactions [35]. The large values of the constant ($b_T$) presented in Table 2 signifies a strong interaction between phosphate and the adsorbents.

3.5. Kinetic Studies

Kinetic study analyses were applied to understand the adsorption mechanism of phosphate onto Mg-BBH and Mg-PBH. In this study, three kinetic models, such as pseudo-first-order (Equation (6)) and pseudo-second-order (Equation (7)) models, were implemented to evaluate the adsorption behaviour and potential rate-limiting steps of phosphate binding onto both adsorbents. The fitting results of the kinetic models for both adsorbents at different temperatures are illustrated in Figure 8, and the corresponding kinetic parameters are summarized in

Table 3. Parameters for kinetic models of phosphate adsorption by Mg-BBH and Mg-PBH.

Kinetic Models	Parameter	20 °C (293 K)		30 °C (303 K)		40 °C (313 K)
		Mg-BBH	Mg-PBH	Mg-BBH	Mg-PBH	Mg-BBH	Mg-PBH
Pseudo-first-order	$q_e$	185.6987	39.3709	67.2336	193.4196	84.8002	215.8872
	$K_1$	0.2295	0.1672	0.0623	0.2345	0.0877	0.2819
	R2	0.8454	0.6241	0.237	0.8266	0.4582	0.8415
Pseudo-second- order	$q_e$	173.9835	152.9544	302.0623	226.2074	313.1222	236.8047
	$K_2$	0.0009	0.0101	0.0302	0.0024	0.0091	0.0025
	R2	0.64	0.9981	0.9998	0.9315	0.9994	0.9604

.

$$\mathrm{Pseudo}\text{-}\mathrm{first}\text{-}\mathrm{order},\mathit{log}\left(q_e-q_t\right)=\mathit{log}{q}_e-\left(k_{1}t\right)$$

(6)

$$\mathrm{Pseudo}\text{-}\mathrm{second}\text{-}\mathrm{order},{\displaystyle \frac{t}{q_t}}={\displaystyle \frac{1}{k_2{q}_e^2}}+{\displaystyle \frac{t}{q_e}}$$

(7)

where q_e refers to the adsorption capacity at equilibrium time (mg/g), k₁ is the rate constant for the pseudo-first-order model (min⁻¹), k₂ is the rate constant for the pseudo-second-order model (g mg⁻¹ min⁻¹), and t is the time.

Based on the findings, good linear fittings were observed in the pseudo-second-order model plots for both Mg-BBH and Mg-PBH at the temperatures studied (30 °C and 40 °C), with correlation coefficient (R²) values approaching 1, indicating that the pseudo-second-order model provided an excellent fit for the phosphate adsorption kinetics. This suggests that the adsorption of phosphate at these temperatures was primarily controlled by chemisorption, involving electron exchange or surface complexation with the metal ions present on the adsorbent surface [36]. However, at a temperature of 20 °C, the Mg-BBH experimental data exhibited a higher R² value for pseudo-first-order kinetics, indicating that this model provides a better fit compared to the other two kinetic models. This proposes that the adsorption process by Mg-BBH at lower temperature was influenced by physical interactions, where the phosphate was weakly bound to the adsorbents’ surface through van der Waals forces, diffusion, or surface affinity, rather that chemisorption, as the rate-controlling mechanism [31]. Overall, these metal-containing adsorbents appears to enhance the overall adsorption performance by promoting metal–adsorbate complexation and interactions as well as facilitating physical adsorption [29].

3.6. Thermodynamic Studies

Thermodynamic studies of phosphate adsorption onto Mg-BBH and Mg-PBH were carried out across a temperature range of 20 °C to 40 °C. The Van’t Hoff equations, represented in Equations (8) and (9), were employed to determine the thermodynamic parameters.

$$\mathit{ln}\left(K_d\right)=-{\displaystyle \frac{\Delta H°}{R}}·{\displaystyle \frac{1}{T}}+{\displaystyle \frac{\Delta S°}{R}}$$

(8)

$$\Delta G°=\Delta H°-T\Delta S°$$

(9)

where $K_d$ is the thermodynamic equilibrium constant, R is the universal gas constant (8.314 J/mol K), T is the absolute temperature (K), $\Delta G°$ is the standard Gibbs free energy (kJ mol⁻¹), $\Delta H°$ is the standard enthalpy change (kJ/mol), and $\Delta S°$ is the standard entropy change (J/mol K).

Thermodynamic variables and parameters, including change in enthalpy (ΔH°) and entropy (ΔS°), were derived from the Van’t Hoff plot, as presented in Figure 9 and Figure 10 for both Mg-BBH and Mg-PBH, respectively. As shown in

Table 4. Thermodynamic parameters of phosphate adsorption by Mg-BBH and Mg-PBH.

Adsorbents	Temperature (K)	${\mathit{K}}_{\mathit{d}}$	ΔG° (kJ mol−1)	ΔH° (kJ/mol)	ΔS° (J/mol K)
	293	0.3750	2.3893	89.7663	300.5511
Mg-BBH	303	3.000	−2.7676
	313	3.8750	−3.5249
	293	0.3602	2.4873	39.9330	128.8919
Mg-PBH	303	0.9249	0.1966
	313	1.0176	−0.0454

, the calculated Gibbs free energy (ΔG°) for Mg-BBH was negative as the temperature increased to 30 °C and 40 °C, while Mg-PBH exhibited negative value at 40 °C, indicating that the adsorption processes are thermodynamically spontaneous at these temperatures, demonstrating that PO₄ adsorption was favourable as the temperature increased [34]. These results correspond with the positive values of ΔS°, suggesting a spontaneous disorder reaction at the solid–liquid interface during the adsorption. Moreover, the positive enthalpy change (ΔH°) confirms that the adsorption is endothermic, implying that the uptake of phosphate is enhanced at elevated temperatures. This result aligned with [37], who reported a similar result for endothermic adsorption of phosphate by Mg/Al–palm waste frond biochar nanocomposite.

From a mechanistic perspective, the endothermic nature of adsorption and the positive value of entropy change indicate that more energy is dispersed over a large number of microstates, and the higher temperature increases the interaction and mobility of phosphate, enhancing its diffusion onto the adsorbents’ surfaces and pores [38]. The adsorption process is likely governed by a combination of ligand exchange, physical adsorption, and surface precipitation. Stronger interactions, such as ligand exchange or surface complexation between phosphate and surface hydroxyl or water molecules on Mg, generally formed stable Mg-PO₄ complexes. Additionally, Mg ions (Mg²⁺) on the adsorbents’ surface may also interact with phosphate anions via surface precipitation [28].

3.7. Release of Phosphate in Soil

The available phosphate content in soil was analyzed on day 0, 2, 4, and 6 to evaluate the phosphate release behavior from the hydrogels over time. Based on the results in Figure 11, the concentration of available phosphate in the soil showed a significant increase over time across treatment with both P-loaded biochar hydrogels. Specifically, after incubation for 2 days, the mixture of soil with PO₄-loaded hydrogels (Mg-BBH and Mg-PBH) showed a high increase in available PO₄ content compared to day 0. This confirms the desorption of phosphate nutrients from the hydrogel toward the surrounding soil. This phosphate value in soil continued to increase in the subsequent interval days upon incubation with the hydrogel enriched with phosphate. By the sixth day of incubation, the phosphate concentration in soil incubated with Mg-BBH increased from 35 mg/L (day 0) to 86 mg/L (day 6), representing a 145.7% rise in phosphate release relative to the starting point. Meanwhile, for Mg-PBH, the value rose to 50 mg/L compared to 16 mg/L on day 0, indicating a 212.5% increase in phosphate release compared to the initial value. This result is consistent with a published study which reported 40% phosphorus bioavailability from corn-stover biochar after 5 days of incubation [39]. Other research by the authors of [40] also utilized Mg-enriched biochar as a slow-release fertilizer and resulted in a 16% release of phosphate after 48 h. These results demonstrate the potential application of Mg-BBH and Mg-PBH as a slow-release fertilizer in agricultural practice.

3.8. Estimation of Cost Analysis of Adsorbents

Estimation of cost analysis was conducted to assess the economic efficiency and feasibility of preparing magnesium-modified biochar hydrogels using bamboo (Mg-BBH) and pulp residue (Mg-PBH). As shown in

Table 5. Estimation of costing of adsorbents (Mg-BBH and Mg-PBH) per 100 g.

Items	Unit Cost (JPY)	Mg-BBH		Mg-PBH
		Amount Used	Cost (JPY)	Amount	Cost (JPY)
Bamboo	0	100 g	0	–	–
Pulp residue	0	–	–	100 g	0
MgCl2$·$6H2O	1900/500 g	102 g	387.6	–	–
CaCl2	3300/500 g	45 g	297.0	45 g	297.0
Sodium alginate	2900/300 g	50 g	483.3	50 g	483.3
Heating cost	18.84 JPY/kWh	6.0 kWh (1 kW, 6 h)	113.04	6.0 kWh (1 kW, 6 h)	113.04
Drying cost	18.84 JPY/kWh	12 kWh (1 kW, 12 h)	226.08	12 kWh (1 kW, 12 h)	226.08
Total cost (JPY)	–	–	1507.02	–	1119.42

below, the total preparation cost of Mg-BBH and Mg-PBH were estimated to be JPY 1507.02 and JPY 1119.42, respectively. The higher cost observed in Mg-BBH was due to the additional magnesium modification process for the bamboo, whereas Mg-PBH utilized pulp residue, which inherently contained residual magnesium from the pulping process. Both adsorbents shared similar cost for CaCl₂, sodium alginate, and energy consumption. From an economic efficiency perspective, this analysis demonstrates that both materials were produced at a relatively low cost and with reasonable expenses, serving as cost-efficient adsorbents with simple preparation processes.

4. Conclusions

This study investigated the recovery of phosphate from aqueous solutions using two types of magnesium-modified biochar hydrogels (Mg-BBH and Mg-PBH). The effect of the initial concentration, contact time, and temperature were evaluated, as well as adsorption isotherms, kinetic modelling, and thermodynamic studies. Both Mg-BBH and Mg-PBH demonstrated excellent adsorption performance, with highest capacities of 309.96 mg PO₄/g and 234.69 mg PO₄/g, respectively. The superior performance of Mg-BBH was attributed to its higher magnesium content, which enhanced the number of active binding sites. Optimal phosphate recovery occurred at 40 °C, with adsorption capacity increasing as temperature increased. The adsorption process followed Temkin isotherms, suggesting multiple binding interactions of phosphate on the adsorbents’ surface functionalized with randomly distributed binding sites. The adsorption mechanisms at 40 °C were best describes by a pseudo-second-order (PSO) kinetic model, indicating chemisorption, primarily ion exchange, and surface complexation as the dominant mechanisms. Thermodynamic analysis revealed that the adsorption of phosphorus was thermodynamically favourable and occurred spontaneously at 40 °C. Additionally, the slow-release study of phosphate showed a great ability for P release, with a gradual increase in phosphate concentration in soil over 6 days. This study contributes to sustainable waste valorization and nutrient recovery, as well as slow-release fertilizer, demonstrating their potential an ideal approach for phosphate cycling management.