Basic Research on the Adsorption Capacity and Enhancement of Bamboo Charcoal for the Prevention of Nitrate Groundwater Pollution

Abstract

Pollution of soil and groundwater by chemical fertilizers is an alarming environmental problem. Both bamboo powder and charcoal are known to adsorb nitrates. This study aimed to recommend an effective method by applying a mixture of chemical fertilizers and bamboo charcoal to soil to prevent NO₃ leaching through adsorption. Magnesium treatment and hydrogelation were investigated to increase the amount of NO₃ adsorption and improve handling properties, and subsequently, their behavior in soil was examined. The maximum adsorption of nitrate in bamboo charcoal powder (BC) with a particle size of 15 µm or less was 4.44 mg/g. When the BC was treated with magnesium chloride (Mg-BC), the maximum adsorption capacity was 99.09 mg/g. The Langmuir adsorption model fits well for both BC and Mg-BC. When Mg-BC was hydrogelized (Gel-Mg-BC), the Freundlich equation provided a better fit, with the maximum adsorption estimated at 25–30 mg/g. When the soil was mixed with Mg-BC hydrogel and treated with a nitric acid solution, the nitrate concentration in the leachate decreased by approximately 15–60% (depending on the feed concentration) compared to that in the leachate from the soil alone.

1. Introduction

Nitrate (NO₃) refers to nitrogen in its oxidized form, primarily as nitrate ions. It usually exists in the form of nitrate salts, in which a metal cation is bound to the nitrate anion (NO₃⁻). This form of nitrogen is typically the final product of the oxidation of nitrogen compounds. It is readily soluble in the water present in soil and can be leached from soils treated with excess chemical fertilizers and livestock manure, contributing to water pollution [1]. Nitrogen, phosphorus, and potassium are the three primary nutrients essential for plant growth. Among these, nitrogen is absorbed by plants as nitrate-nitrogen (NO₃⁻-N) [2]. Similar to how animals store excess nutrients as fat in their bodies in preparation for starvation during periods of abundance, plants also accumulate excess nitrate when nutrient uptake is limited. Consequently, vegetables cultivated with high levels of fertilizer, whether chemical or organic, may contain high levels of nitrate [3,4,5,6]. Nitrate can be converted to nitrite ions through a reduction reaction. These nitrite ions may react with aliphatic amines to form nitrosamines. Nitrite ions can also oxidize hemoglobin to produce methemoglobin, which lacks oxygen-carrying capacity and can result in methemoglobinemia (blue baby syndrome). Some nitroso compounds are also known to cause health problems such as cancer, liver damage, and reproductive dysfunction [7,8,9,10,11,12,13].

Currently, nitrate removal techniques include biological denitrification, reverse osmosis, and ion exchange. The biological denitrification process consists of two stages: nitrification and denitrification. During nitrification, nitrifying bacteria oxidize ammonia nitrogen to nitrite or nitrate under aerobic conditions. In this process, the bacteria obtain energy by oxidizing ammonia and nitrite while converting carbon dioxide into carbohydrates. In the denitrification process, denitrifying bacteria reduce nitrate to nitrogen gas under anaerobic conditions. This reduction occurs because the bacteria utilize the oxygen in the NO₃ compound to oxidize organic matter in the absence of molecular oxygen (O₂) [14]. The reverse osmosis method separates ions and low-molecular-weight organic matter using a membrane. By applying pressure greater than the osmotic pressure to the concentrated solution side, only water molecules pass from the concentrated to the diluted solution side, thereby achieving separation [15]. Meanwhile, ion exchange is a technique in which an ion from a substance is released into a solution, and simultaneously replaced by another ion from the solution, which is absorbed by the solid substance [16].

However, the main concern of these techniques, such as high energy consumption, complicated processing, and maintenance, has made them less favorable for sustainable application. Moreover, these techniques commonly address the post-effect of nitrate contamination only, but the key question is how to prevent the contamination from occurring. Therefore, in this study, we proposed a preventive approach to control the problem at its source by applying a mixture of bamboo charcoal treated with magnesium chloride to help bind with the excess nitrate. Charcoal is a carbon-rich material that is produced through pyrolysis (limited oxygen) treatment of biomass. It has been widely used as a soil amendment due to its highly porous structure and excellent ion exchange properties [17]. Their porous structure and presence of various oxygen-containing functional groups contribute to their high cation exchange capacity. These properties enable the charcoal to effectively bind and exchange ions on its surface. Chemical modification with metal salts such as magnesium chloride (MgCl₂) has been shown to improve its exchange capacity by introducing additional functional groups and increasing ion exchange sites [18]. In many studies, raw biomass has been treated with magnesium chloride and then calcined to form magnesium oxide. During hydration, the hydroxyl groups undergo hydration and are exchanged for nitrate ions [16,19,20,21,22].

In this study, magnesium chloride was directly loaded onto bamboo charcoal to improve its adsorption capacity and minimize changes to its physical properties caused by carbonization. The aim of this study was to investigate the effect of magnesium chloride-loaded bamboo charcoal and its hydrogelation with sodium alginate for the recovery of nitrate ions, as well as to deposit excess nitrate ions in agricultural soils. To the best of the authors’ knowledge, limited information is available on the nitrate adsorption efficiency by alginate hydrogel containing Mg-loaded bamboo charcoal. Therefore, three materials were evaluated for their sorption properties: untreated bamboo charcoal (BC), magnesium chloride-treated bamboo charcoal (Mg-BC), and hydrogel magnesium chloride-treated bamboo charcoal (Gel-Mg-BC).

2. Materials and Methods

2.1. Bio-Char (BC), Magnesium Treated Char (Mg-BC), and Hydrogel of Magnesium Treated Bamboo Charcoal (Gel-Mg-BC)

“Bamboo Charcoal Powder MM”, manufactured by LATEST Co., Ltd. (Wakayama, Japan), was purchased and used in the experiment. This product is made by processing moso bamboo into cosmetic-grade ingredients, which are typically supplied to cosmetic manufacturers.

Table 1. Physical properties of bamboo charcoal powder (data published by manufacturer).

Physical Properties	Value
Particle size (D50)	Approximately 10 μm
Apparent density	350–550 g/L
Moisture	$\le$12%
Ash content	$\le$4%

lists the physical properties provided by the manufacturer, while the composition details are presented in

Table 2. Chemical properties of bamboo powder.

Components	P2O5	MgO	K2O	CaO	TN
Compositions (mg/100 g)	1.2	700.4	773.2	227.5	0.8

. The components listed in Table 2 were analyzed in the laboratory using a soil analyzer that is capable of simultaneously measuring six types of soil constituents (Air Water Biodesign Co., Ltd., EW-THA1J, Saitama, Japan). The measurable components included nitric and ammoniacal nitrogen, phosphate, potassium, calcium, and magnesium. Since nitrogen exists in soil in both nitric and ammoniacal forms, each component was measured separately. As this is an absorbance method, the reported values represent the concentrations of the components extracted using the 0.5 M hydrochloric acid extraction method.

Measurements were conducted using Soil Analyzer (Air Water Biodesign Co., Ltd., EW-THA1J, Saitama, Japan).

2.2. Adsorption Experiment and Sample Preparation

2.2.1. Evaluation of BC

(1)

pH dependence of nitrate adsorption

This experiment aimed to investigate the effect of pH on the equilibrium adsorption of nitrate by the biomass adsorbent and to determine the optimal pH conditions for maximum adsorption performance. In this study, 0.1 g of biomass adsorbent was weighed and placed into a centrifuge tube. A 1000 ppm nitric acid standard (Kanto Chemical Co., Ltd., Tokyo, Japan) (nitrate concentration: 225 mg/L) was diluted to 100 ppm (nitrate: 22.5 mg/L). The pH of each solution was adjusted to a range of 2 to 13 by adding a few drops of either 1 M sulfuric acid or 1 M NaOH. Each prepared solution (50 mL) was placed in a centrifuge tube and stirred at room temperature for 10 h using a rotary stirrer (ROTATOR RT-50/TAITEC, Saitama, Japan). The solutions were then filtered using filter paper (∅150 mm/AS ONE CORPORATION), followed by a membrane filter (∅0.45 μm/ADVANTEC). Ion chromatography analysis was conducted to determine the nitrate concentration using an 861 Advanced Compact IC system (Metrohm Japan, Tokyo, Japan) equipped with a Metrosep A Supp 5 column (150 mm/4.0 mm).

(2)

Effect of mixing time on nitrate adsorption

One gram of biomass adsorbent was weighed and placed in a 500 mL beaker. A few drops of 1 M sulfuric acid were added to the solution to adjust the pH to 2. The beaker containing the adjusted solution and the adsorbent was stirred for 24 h at a controlled temperature of 25 °C using a magnetic stirrer. At predetermined time intervals (1, 2, 3, 4, 5, 6, 7, 10, 11, 15, 18, and 24 h), the supernatant was collected for ion chromatographic analysis using the 861 Advanced Compact IC.

(3)

Effect of temperature on nitrate adsorption

A total of 0.1 g of biomass adsorbent was weighed and placed in a 50 mL beaker. The solution was adjusted to 100 ppm (nitrate: 22.5 mg/L) using a 1000 ppm nitric acid standard solution [Kanto Chemical Co., Ltd.]. A few drops of 1 M sulfuric acid were added to adjust the pH to 2. Similar to the procedure (2), the beaker containing the adsorbent and the adjusted solution was stirred for 24 h at the target temperature (15~60 °C) using a heated magnetic stirrer at 500 rpm. After stirring, each solution was filtered using filter paper [∅ 150 mm/AS ONE CORPORATION], followed by a membrane filter [∅ 0.45 μm/ADVANTEC], and ion chromatographic analysis was performed using the 861 Advanced Compact IC.

(4)

Isothermal adsorption studies

Isothermal adsorption experiments were conducted to evaluate the adsorption interaction between nitrate ions and the adsorbents at equilibrium. A fixed amount of adsorbent (0.1 g) was added to 50 mL of nitrate solutions with varying initial concentrations. The pH of each solution was adjusted to the previously optimized value (pH 2) and shaken at a controlled temperature of 25 °C. The suspensions were filtered, and the nitrate concentration in the filtrates was determined using ion chromatography (861 Advanced Compact IC, Metrohm).

2.2.2. Preparation of Mg-BC and Adsorption Tests

The Mg-BC treatment procedure was performed according to the method from [23] with slight modification by mixing a total of 101.65 g of magnesium chloride hexahydrate with 1 L of distilled water in a 1 L beaker. Next, 50 g of bamboo charcoal was placed in a 1 L beaker, and 900 mL of the prepared magnesium chloride aqueous solution was added. The mixture was stirred at 60 °C using a heated stirrer for 24 h. After stirring, the Mg concentration in the filtrate was measured using ion chromatography. Magnesium was analyzed by replacing the analytical column in the ion chromatography with one designed for cation analysis. After Mg modification, the sample was washed with tap water and used for adsorption experiments.

2.2.3. Preparation of Hydrogel of Mg-BC and Adsorption Test

A total of 200 mL of 3% Mg-BC was added to 200 mL of a 4% sodium alginate solution and stirred at 500 rpm using a magnetic stirrer to prevent precipitation. A microtube pump (Tokyo Rika MP-100; Eyela Tokyo Rikakikai. Co., Ltd., Tokyo, Japan) was used to inject the mixture into a 3% calcium chloride solution at a flow rate of 1 mL/min to form the beads. The resulting beads were washed with tap water and used in subsequent experiments. Solutions of 1000 mg/L nitrate were prepared and used for the adsorption tests.

2.3. Characterization of the Material

The Boehm titration was applied to quantitatively determine the amounts of various acidic and basic surface functional groups present on this bamboo charcoal sample. Additionally, the functional groups of Mg-loaded bamboo charcoal prior to and after adsorption of nitrate were assessed by using FT-IR analysis (FTIR, Thermo Scientific Nicolet iS10, Waltham, MA, USA). The physical characteristics of Mg-BC, including specific surface area, pore volume, and average pore diameter, were determined using the Brunauer–Emmett–Teller (BET) method. The particle diameter of Gel Mg-BC was measured by using an electronic digital caliper.

2.4. Soil Column Experiment

Andosol-type soil, with an average pH of 5.65 $\pm$ 0.07 was used in this study. A total of 10 g of soil and bamboo charcoal hydrogel mixture was placed in a 35 mm diameter glass column connected to a rapid soil cation exchange capacity measuring device (Fujihara Industry Co., Osaka, Japan). A perforated metal plate was placed at the bottom of the filtration column, followed by a layer of glass fiber filter paper on top. A mixture of soil and bamboo charcoal hydrogel was then placed on top and pressed evenly using a soil compression rod. Adsorption experiments were carried out by varying the amount of hydrogel from 0 to 0.8 g. A 10 mL of nitrate ion solution of the specified concentration (22 mg/L) was gently poured into the column [24]. After 24 h, the pinchcock was opened to collect the filtrate and measure the available nitrate content (Figure 1). Then, further adsorption experiments in soil were carried out by varying the initial nitrate concentration at a fixed hydrogel content of 0.4 g.

Volume of soil and nitrate solution soil are 10 g and 10 mL, respectively.

2.5. Evaluation Parameter

The amount of nitrate adsorption (q) was calculated using the following equation;

$$\mathrm{q}=-{\displaystyle \frac{V}{W}}\left(C_e-C_0\right)$$

(1)

q: adsorption capacity (mg/g), $C_e$: equilibrium concentration (mg/L), $V$: volume of the liquid (L), $C_0$: initial concentration (mg/L), $W$: adsorbent (g).

The adsorption rate (d) of nitrate was calculated using the following equation;

$$\mathrm{d}(\mathrm{\%})={\displaystyle \frac{(C_0-Ce)}{C_0}}\times 100$$

(2)

d: adsorption rate (%) $C_e$: equilibrium concentration (mg/L) $C_e$: equilibrium concentration (mg/L).

The Langmuir (3) and Freundlich (4) formulas were calculated using the following equations;

$$\mathrm{q}={\displaystyle \frac{Q^{\mathrm{\infty }}{K}_{L}Ce}{1+K_{L}Ce}}$$

(3)

$$log\mathrm{q}=\log K_f+nlog{C}_e$$

(4)

q: adsorption capacity (mg/g), $K_L$: Langmuir constant (-), $K_f$: Freundlich constant (-), n: constant (-), $C_e$: equilibrium concentration (mg/L), Q^∞: maximum adsorption capacity (mg/g).

Acidic functional group (A) (meq/g);

$$\mathrm{A}={\displaystyle \frac{\left(b-a\right)\times 0.1 \left[\mathrm{N}\right]}{0.4 \left[\mathrm{g}\right]}}\times {\displaystyle \frac{40 \left[\mathrm{m}\mathrm{L}\right]}{volume supernatant \left[\mathrm{m}\mathrm{L}\right]}}$$

(5)

a: titration volume (mL), b: blank (mL).

Basic functional group B (meq/g);

$$\mathrm{B}={\displaystyle \frac{\left(b-a\right)\times 0.1\left[\mathrm{N}\right]}{0.4\left[\mathrm{g}\right]}}\times {\displaystyle \frac{40\left[\mathrm{m}\mathrm{L}\right]}{volumesupernatant\left[\mathrm{m}\mathrm{L}\right]}}$$

(6)

a: titration volume (mL), b: blank (mL).

3. Results and Discussion

3.1. BC (Biochar)

3.1.1. Effect of pH, Mixing Time, and Temperature

Based on the results of ion chromatography analysis, the adsorption rate of nitrate by the biomass adsorbent was calculated using Equation (2), and the results are presented in Figure 2. The adsorption rate was as high as 52% at pH 2, which was identified as the optimum pH. In the acidic region, pH adjustment with sulfuric acid promotes anion-exchange reactions with basic functional groups, thereby enhancing adsorption. In neutral and alkaline reagents, the adsorption rate was lower, likely due to the inhibition of these reactions, including basic functional groups by hydroxyl groups during pH adjustment with sodium hydroxide [24]. Figure 3 shows the time required to reach adsorption equilibrium. The adsorption rate rapidly increases to 14% within the first hour, after which the adsorption rate increases gradually. A saturated adsorption rate of 47% was achieved after 10 h. This result suggests that nitrate can be sufficiently adsorbed within a minimum contact time of at least 10 h. Furthermore, it is expected that excess nitrate ions leached from the chemical fertilizers would be gradually adsorbed and later released as the charcoal decomposes, thus acting as a storage and controlling agent. This function is attributed to the porous structure of this bamboo charcoal enables the interaction with nitrate ions.

The effect of solution temperature on the adsorption performance of nitrate was also investigated, and the results are presented in Figure 4. The adsorption rates improved from 52% at 15 °C (room temperature) to 59% at 60 °C, indicating that the adsorption performances were enhanced by an increase in surrounding temperature. However, this amount alone was not sufficient to promote significant adsorption. In a previous study by the authors, equilibrium adsorption was achieved within 15 min when the biochar was treated with magnesium chloride [25], highlighting the critical role of chemical modification in enhancing adsorption.

3.1.2. Isothermal Adsorption Curve

Adsorption experiments were conducted by varying the initial concentration of nitrate, as shown in Figure 5. Based on the equilibrium plateau, the maximum nitrate adsorption capacity was estimated to be 4.34 mg/g. Previous studies using rice bran charcoal and bamboo powder charcoal reported adsorption capacities of 2.77 mg/g and 1.25 mg/g, respectively [26,27], which are comparable to the present findings. Figure 5 shows a comparison of the experimental data with the theoretical isothermal adsorption curve calculated from the constants in the Langmuir equation (Figure 6). The difference between the maximum adsorption capacity in the steady state and the value calculated from the constants is 0.1 mg/g, indicating good agreement with the adsorption data. The high correlation coefficient of the linear regression further suggests that the adsorption mechanism of the biomass adsorbent follows monolayer adsorption behavior. A similar finding was also reported by [28], where nitrate adsorption onto corncob biochar adhered to the Langmuir model, with a maximum adsorption capacity reaching 14.46 mg/g.

3.2. Characterization of Material

To investigate the bonding with surface functional groups, the Boehm method was used, which revealed the presence of basic and acidic functional groups at concentrations of 1.45 meq/g and 0.2 meq/g, respectively (Figure 7). The FT-IR spectroscopy results for the functional groups on the surface of the bamboo charcoal adsorbent are shown in Figure 8. The red line at the top of the figure shows the biomass adsorbent spectrum before adsorption, while the green line at the bottom represents the spectrum after adsorption. In the IR analysis prior to adsorption, notable bands and peaks observed at 1776, 1438, and 811 cm⁻¹ were attributed to C=O stretching, C=C groups, and Mg-O bonds, respectively [19]. However, these peaks become reduced after adsorption, indicating that adsorption occurred in this region. In addition, a significant reduction in magnesium concentration was observed after 24 h of treatment of magnesium chloride hexahydrate with bamboo charcoal, indicating that substantial chemical modification of the adsorbent had occurred. The residual Mg concentration in the solution was measured using ion chromatography, and the corresponding adsorption capacity was calculated to be 105 mg/g. The physical characteristics of Mg-BC revealed a specific surface area of 421.04 m²/g, with a total pore volume of 0.1944 cm³/g, and an average pore diameter of 1.8468 nm, measured by the BET method. These characteristics indicate a highly porous structure and relatively large surface area, which is favorable for nitrate adsorption. The average particle diameter of the Gel-Mg-BC was determined to be 1.74 mm.

3.3. Mg-BC and Hydrogel of Mg-BC (Gel-Mg-BC)

Based on the optimum conditions identified in Section 3.1, adsorption experiments were carried out using Mg-loaded bamboo charcoal (Mg-BC) for nitrate at an initial concentration of 360 mg/L. The maximum adsorption capacity increased 22-fold to 99.0 mg/L after treatment with magnesium chloride, compared to the untreated bamboo charcoal. This result highlights the effectiveness of magnesium modification in enhancing the nitrate adsorption performance. Similar improvement trends have been reported in previous studies. For example, surface modification of water-hyacinth biochar with magnesium ions has been shown to increase the nitrate adsorption capacity to 19.1 mg/g [19]. Moreover, modification with other metal salts such as lanthanum has shown similar effect, able to enhance adsorption through hydration-related valence increase, raised the capacity to 100.0 mg/g [29]—nearly ten times higher than that of untreated oak sawdust biochar, demonstrating the substantial effectiveness of chemical modification in enhancing biochar performance.

Isothermal adsorption curves were constructed to determine the maximum adsorption capacity, and the data were analyzed using Langmuir and Freundlich models (Figure 9 and Figure 10). The results indicated that the Langmuir model provided the best fit as evidenced by a higher R² value, and yielded the highest adsorption capacity compared to the Freundlich model. This suggests that nitrate adsorption occurred via monolayer adsorption on the homogeneous surface of adsorption sites. This Mg modification increases the number of hydration-based hydroxyl groups on the biochar surface, thereby facilitating the exchange of hydroxyl ions with nitrate ions [30]. Previous findings also reported that magnesium oxide modification increased the adsorption capacity to 95 mg/g in peanut shell biochar [20].

Additionally, besides the modification of magnesium, the great adsorption capacity observed in this study can be partly attributed to the use of finer cosmetic particles of bamboo charcoal, which provide a larger surface area for adsorption, as evidenced by the BET results. However, the use of fine bamboo charcoal powder is difficult to handle, especially for practical application in agricultural soil. It was thus immobilized within a hydrogel matrix for better handling. Although previous studies have utilized polymer membranes with slow-acting fertilizers, challenges and concerns regarding chemical leakage from materials into the environment remain [31]. Therefore, the use of biodegradable materials in this study presents a desirable alternative to tackle those environmental concerns.

Figure 11 and Figure 12 illustrate the adsorption capacity of Gel-Mg-BC by varying the initial nitrate concentration and the application of isothermal studies using the Freundlich model, respectively. Unlike Mg-BC, determining a constant maximum adsorption capacity for Gel-Mg-BC was more challenging. However, the estimated maximum value was approximately 30 mg/g, which is about one-third of the capacity observed for Mg-BC. Comparable results have been reported in previous studies, where a chitosan-polyvinyl alcohol (PVA) gel exhibited nitrate adsorption capacity of 35.03 mg/g at pH 3 [32]. Additionally, adsorption of nitrate by using chitosan hydrogel beads has demonstrated an adsorption capacity of 92.1 mg/g after 10 h contact time [33]. These comparisons suggest that the adsorption performance of Gel-Mg-BC remains within a reasonable and acceptable range.

Table 3. Summary of isothermal results obtained in this study.

Adsorbent	Model	Q∞ (cal)	K	R2	n	Q∞ (Graph)
BC	Langmuir	4.44	3.82	0.995	n.d.	n.d.
Mg-BC	Langmuir	99.09	0.023	0.932	n.d.	n.d.
	Freundlich	n.d.	1.18	0.681	0.668	86–90
Gel-Mg-BC	Freundlich	n.d.	6.25	0.8981	1.0365	25–35

Note: n.d.: no data.

summarizes the isothermal results obtained in this study. The Freundlich model has been used to describe the adsorption of nitrate by Gel-Mg-BC, indicating a multilayer adsorption on the heterogeneous surface of this hydrogel.

Figure 13 shows the results of varying amounts of hydrogel at a constant concentration of nitrate in the soil column permeation experiment. The nitrate concentration decreased sequentially when the hydrogel dosage increased from control (0.0 g) to 0.4 g, but no significant changes were observed thereafter. Therefore, the optimum amount of 0.4 g of hydrogel was employed for further experiments. Figure 14 shows the results of a soil column permeation experiment, in which 0.4 g of hydrogel was mixed with 10 g of Andosol soil at varying initial nitrate concentrations. After allowing the mixture to stand for 24 h, the liquid (leachate) from the column was collected and analyzed to measure the residual concentration of nitrate. The results in Figure 14 are compared across three conditions: liquid only (nitrate solution), soil only (control), and the hydrogel–soil mixture. A decrease in nitrate concentration was observed in both the soil-only and hydrogel-soil mixture, with the following order of nitrate concentration: soil-only (control) > hydrogel-soil mixture. In conditions 1 and 2 (higher initial nitrate concentration), the nitrate content in the leachates from the hydrogel-soil mixture was reduced significantly by 50–70% compared to the soil-only (control). However, in condition 3 with a lower initial concentration, the reduction of nitrate content was approximately 15%, showing only a slight improvement against the control. This may be attributed to the lower nitrate load and less saturated conditions in the surrounding environment soil, which limit the ion-exchange interactions in the soil system [34].

Table 4. Comparison of adsorption capacities of this study with reported references.

Reference	Adsorbent Materials	Adsorption Capacity [mg/g]
[16]	Mg-Bio char	9.13
[19]	Water hyacinth/Mg-modified biochar	19.1
[20]	MgO-biochar nanocomposites	95
[26]	Rice bran treated with iron (III) chloride	2.77
[27]	Bamboo powder charcoal	1.25
[28]	Biochar derived from agricultural residuals	14.46
[29]	Lanthanum-oak sawdust biochar	8.94 (Char) → 100.0 (La-Char)
[30]	Mg-loaded cassava straw biochar	24.04
[31]	HCl impregnation of corn straw-biochar	1.958
[32]	Chitosan/PVA	35.03
[33]	Chitosan hydrogel	92.1
[34]	Chitosan/zeolite/Fe3+/Mg2+ bead	62.23
[35]	Bio Char Fe Coated	43.66
This study	Mg-Bamboo charcoal	99.0
This study	Gel-Mg-Bamboo charcoal	32.6

presents a comparison between the findings of this study and those reported in previous studies involving Mg-treatment or other metal salt modifications for improving adsorption capacities.

4. Conclusions

The treatment with magnesium chloride has significantly enhanced the nitrate adsorption capacity of untreated bamboo charcoal from 4.44 mg/g to 99.0 mg/g. Although hydrogelation (Gel-Mg-BC) reduced the capacity to 32.6 mg/g, it still represented an increase of approximately 7.34 times compared to bamboo charcoal and improved the material’s handling properties for practical application. Isothermal adsorption analysis showed that the Langmuir model describes the data of BC and Mg-BC samples well, while the Freundlich model was more suitable for explaining the isothermal data of Gel-Mg-BC. Soil column experiments further confirmed the hydrogel’s ability to reduce nitrate leaching, supporting its potential to address the problem related to leaching and pollution risk caused by nitrate. Overall, Mg-loaded bamboo charcoal hydrogel presents an effective, improved usability and environmentally friendly solution for mitigating nitrate pollution in soil and water.