Revealing the Role of Coal Gangue-Biochar Composite for Removing SO₄²⁻ from Water: Adsorption Mechanisms and Application Effects

Abstract

Sulfate ions are commonly found in water, but excessive concentrations of these ions can have detrimental effects on the aquatic environment. In this study, reed straw was chosen as the raw material for producing biochar, which was then pyrolyzed with coal gangue to create environmentally friendly adsorption materials. This innovative approach aims to combat the issue of elevated SO₄²⁻ concentrations in water while efficiently utilizing solid waste. The results showed that the adsorption process best fit the when the was pH 2 and the dosage was 8 g·L⁻¹, and the adsorption capacity reached the maximum of 49.56 mg·g⁻¹. Via extensive characterization, kinetic studies, and isotherm experiments on the modified biochar, we determined that the mechanism of SO₄²⁻ adsorption primarily involves electrostatic adsorption, physical adsorption, and chemical adsorption. The toxicology experiments showed that the activity of antioxidant enzymes and the amount of malondialdehyde decreased, and the content of chlorophyll and soluble protein increased. These findings suggest that modified biochar can mitigate the harmful effects of SO₄²⁻ on reeds to a certain extent, ultimately promoting the growth of these plants.

1. Introduction

Sulfate (SO₄²⁻) is a common anion found in natural water systems. Elevated concentrations of SO₄²⁻ can potentially pose hazards to organisms and the ecological environment [1,2,3]. Prolonged exposure to high levels of SO₄²⁻ in drinking water can lead to symptoms such as diarrhea, indigestion, and other symptoms, posing a threat to human health [4,5]. In addition, high levels of SO₄²⁻ can acidify water when used to irrigate farmland, causing soil hardening and harming crop growth [6,7]. Reports indicate that SO₄²⁻ concentrations exceeding 400 mg·L⁻¹ in wastewater far exceed China’s surface water quality standards, which set a limit of 250 mg·L⁻¹ for SO₄²⁻ [8,9]. Therefore, it is essential to employ an efficient and cost-effective method for removing SO₄²⁻ from water to mitigate potential risks to organisms and the environment.

Various methods are available for removing SO₄²⁻ from water, including ion exchange, flocculation, chemical precipitation, and adsorption [10,11,12,13]. Among these, adsorption has garnered significant attention due to its advantages of affordability, wide availability, high efficiency, and minimal secondary contamination. A number of different adsorbents have been studied, including activated carbon [14], zeolite [15], bentonite [16], and metal oxides [17]. However, the high cost and low efficiency of these adsorbents limit their large-scale application [18]. Consequently, it is imperative to develop a cost-effective sulfate adsorption material for practical use.

Recently, biochar has garnered great attention as an appealing alternative adsorbent due to its porous structure and cost-effectiveness [19,20]. Numerous prior studies have focused on the utilization of biochar for removing pollutants such as phosphorus, heavy metals, and organic contaminants from water sources [21,22,23]. Biochar is typically prepared from agricultural and forestry wastes, such as wood, crop straw, and fruit husks, and organic wastes produced in industrial applications and urban life [24,25]. Reed, a readily available low-cost biomass waste, is well suited for producing porous biochar. However, its original form lacks sufficient adsorption capacity for SO₄²⁻, necessitating modification. Previous studies have shown that solid wastes containing elements such as calcium, magnesium, and iron are effective for biochar modification [26,27]. These solid wastes can not only enhance the stability, carbon fixation capabilities, and pollutant adsorption efficiency of biochar [28,29], but also the nutrients and metal ions it contains may have a certain positive effect on crops’ effect. Industrial waste, like coal gangue, poses a threat to land resources and environmental pollution when stacked randomly [30,31]. Co-pyrolyzing coal gangue, a metal-rich solid waste, with biomass alters the functional groups and surface charge of biochar, affecting its pollutant adsorption capacity [32,33,34,35]. Notably, there are limited reports on the removal of SO₄²⁻ from water using this modified approach. Therefore, the novel biochar composites were prepared via the co-pyrolysis method using reed straw and gangue as raw materials, which is beneficial to provide a new type of efficient and environment-friendly adsorbent for the removal of sulfate.

Reed, recognized as an ornamental aquatic herb, extensively thrives in wetland areas such as rivers and lakes, possessing ecological and economic significance [36,37]. Reed growth is, however, susceptible to environmental conditions. Elevated sulfate concentrations can lead to slow growth, reduced height, and leaf yellowing in reeds, diminishing their value [38,39]. Therefore, selecting reed as a test plant and conducting pot experiments to investigate the impact of biochar on reed growth holds paramount importance.

We have chosen reed straw and coal gangue as raw materials to prepare the modified biochar and investigate its effectiveness in removing SO₄²⁻. This choice is primarily based on the following reasons: (1) Untreated straw, resulting from the aging of reed, can lead to environmental pollution and resource wastage. By using it as a raw material for biochar production, the SO₄²⁻ can be efficiently absorbed. (2) Coal gangue, classified as industrial waste, can pose a threat to land resources and the environment if not managed properly [30,31]. It contains elements such as Ca, Fe, Mg, Si, Mn, K, and others, which can enhance the stability and sulfate adsorption capacity of the adsorbent. (3) Reeds are ornamental and practical and are susceptible to SO₄²⁻ concentrations. Therefore, we designed a co-pyrolysis experiment of reed straw with coal gangue and a pot experiment to remove the pollutants.

In this study, the modified biochar was obtained via the co-pyrolysis of reed straw and coal gangue, and the adsorption performance of SO₄²⁻ was investigated. The aims of this study were (1) to determine how modification methods enhance the biochar’s SO₄²⁻ adsorption capacity; (2) to analyze the mechanism of SO₄²⁻ adsorption via kinetic and thermodynamic analysis; and (3) to examine the effect of modified biochar on the growth of reed by conducting SO₄²⁻ toxicity experiments using reed as a test plant. This work is expected to provide valuable insights into the removal of SO₄²⁻ in the water and the resource utilization of solid waste.

2. Materials and Methods

2.1. Materials and Chemicals

Reed seedlings were purchased from Hefei Nursery Flower Wholesale Market. Reed straw was collected from Qingyuan Campus of Anhui University. The coal gangue used was obtained from Linhuan Mining Area, Huaibei City, China. All reagents used in the experiments were of high purity. Deionized water was used to prepare all the chemical solutions and to rinse and clean the samples. The solutions of NaOH (98%), HCl (95%–99%), and Na₂SO₄ (99.5%) were purchased from Merck (Darmstadt, Germany) and used as supplied.

2.2. Preparation of Coal Gangue-Modified Biochar

First, the reed straw was washed three times with deionized water and dried at 80 °C. Then, it was crushed and sieved using a 100 mesh sieve to obtain reed straw powder. The reed straw powder was pyrolyzed in a muffle furnace at 600 °C for 2 h in an oxygen-free environment to obtain reed straw biochar (BC). For the preparation of coal gangue-modified biochar (MBC), the coal gangue was crushed to less than 2 cm with a stone crusher and then ball-milled with water for 3 h. After drying at 80 °C, the agglomerated gangue powder was fully ground into powder. Then, the reed straw powder and coal gangue powder with a mass ratio of 5:2 were mixed evenly and calcined at 600 °C for 2 h in a muffle furnace under an oxygen-free environment. After cooling, it was soaked with 1 mol·L⁻¹ of HCl solution for 24 h, then washed to neutral and dried to obtain the modified biochar.

2.3. Characterization of Adsorbent

The surface morphology of the samples was observed via a scanning electron microscope (SEM) [40]. At the same time, an energy-dispersive X-ray spectrometry (EDS) was used to analyze the composition and proportion of the original biochar (BC) and modified biochar (MBC) [41]. A Fourier transform infrared spectrometer (FTIR) was used to characterize the functional groups on the surface of the samples in the spectral range of 500–4000 cm⁻¹ [42]. The specific surface area and pore size distribution of the adsorbent were determined via a specific surface area and aperture texter (BET) [43].

2.4. Batch Adsorption Experiment

The adsorption experiments were carried out in thermostatic oscillators conditioned at 25 °C and 200 r·min⁻¹, using Erlenmeyer flasks containing 0.4 g of BC and MBC and 100 mL 500 mg·L⁻¹ of SO₄²⁻ solution. The effects of the adsorbent dosage (0.05–0.5 g), adsorption time (0.05–7 h), initial SO₄²⁻ concentration (60–600 mg·L⁻¹), and the pH (2–12) on SO₄²⁻ adsorption were investigated, respectively. The pH was adjusted with 0.1 mol·L⁻¹ of NaOH and 0.1 mol·L⁻¹ of HCl. After SO₄² adsorption, the supernatant was filtered with a 0.45 µm filter membrane.

The quasi-first-order kinetic model (Equation (1)) and the quasi-second-order kinetic model (Equation (2)) were taken to fit the data of kinetic studies, as shown below.

$$\ln (q_{\mathrm{e}}-q_t)=\ln q_{\mathrm{e}}-k_{1}t$$

(1)

where q_e is the fitted adsorption capacity after adsorption, mg·g⁻¹; t refers to the adsorption time, min; and k₁ is the quasi-first-order reaction rate constant.

$$\frac{t}{q_t}=\frac{1}{k_2{q}_{\mathrm{e}}^2}+\frac{1}{q_{\mathrm{e}}}t$$

(2)

where q_e (mg·g⁻¹) is the fitted adsorption capacity after adsorption and at time t (min), respectively; t refers to the adsorption time, min; and k₂ is the quasi-second-order reaction rate constant.

The Langmuir model (Equation (3)) and the Freundlich model (Equation (4)) were used for analysis, as shown below.

$$q_{\mathrm{e}}=\frac{q_{\mathrm{m}}{k}_{\mathrm{L}}{c}_{\mathrm{e}}}{1+k_{\mathrm{L}}{c}_{\mathrm{e}}}$$

(3)

where q_e is the adsorption capacity of biochar when the adsorption is saturated, mg·g⁻¹; c_e is the equilibrium mass concentration, mg·L⁻¹; k_L is the Langmuir equilibrium adsorption constant, and q_m is the saturated adsorption capacity of the single-molecular-layer, mg·g⁻¹.

$$q_{\mathrm{e}}=k_{\mathrm{F}}{c}_{\mathrm{e}}^{\frac{1}{n}}$$

(4)

where q_e is the adsorption capacity of biochar when the adsorption is saturated, mg·g⁻¹; c_e is the equilibrium mass concentration, mg·L⁻¹; and k_F and 1/n are the Freundlich empirical constants.

2.5. Pot Experiment

2.5.1. Experimental Design

Pot experiments were conducted to study the effects of modified biochar on the physiological characteristics of reeds under SO₄²⁻ stress. The initial step involved subjecting reeds to SO₄²⁻ toxicity experiments. Purchased reed seedlings were washed with clean water to remove the adhering soil and placed in several plastic buckets for seedling retarding treatment for 30 days. After the end of the treatment, the plants with the same growth vigor were selected for toxicological tests. The different concentrations of SO₄²⁻ were set to cultivate the reed (S1 = 0 mg·L⁻¹, S2 = 200 mg·L⁻¹, S3 = 400 mg·L⁻¹, S4 = 600 mg·L⁻¹, S5 = 800 mg·L⁻¹, and S6 = 1000 mg·L⁻¹) and three groups in parallel for each treatment. Ten reed seedlings were placed in each bucket and pebbles were used as the base to fix the root system of the reed. The test phase lasted for 60 days, during which the SO₄²⁻ solution was added at 2-day intervals to maintain the total amount of the solution; urea was added at 5-day intervals to provide nutrients for reed growth; and the physiological indexes of plants were measured at 10-days interval. After 60 days of planting, the leaves from different plants in each treatment group were randomly selected for evaluating the physiological indicators. A control group was established for comparison, and modified biochar was added to the culture solution while keeping the other experimental conditions constant.

2.5.2. Measurement of Physiological Indicators

The measured physiological parameters mainly include total chlorophyll, superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), malondialdehyde (MDA), and soluble protein (SP). Chlorophyll content was determined via spectrophotometry [44], SOD was determined via the photochemical reduction method of nitrogen blue tetrazole [45], POD was conducted via the guaiacol method [46], CAT was performed via the ultraviolet absorption method [47], MDA was determined via the thio-barbituric acid method [48], and SP content was determined via the Coomassie Brilliant Blue G-250 staining method [49].

2.6. Statistical Analysis

Excel 2016 was used to collate the data, origin 2019 was used to draw charts, and one-way analysis of variance (ANOVA) in SPSS 20.0 was used to analyze the pot experiment data, and significant differences between the different treatments were detected at a significant level of 0.05.

3. Results and Discussion

3.1. Characterization of Adsorbents

The BC and MBC were characterized via SEM to observe the change in surface morphology of the biochar before and after modification. As can be observed in Figure 1, BC presented a dense structure with a smooth surface. After co-pyrolysis with the coal gangue, the surface of MBC was rough and formed coal gangue flocculent particles, which was consistent with those obtained [26]. The structure parameters of the adsorbent were obtained via BET. The specific surface area of BC and MBC were 273.99 m²·g⁻¹ and 658.91 m²·g⁻¹, respectively, and the specific surface area of the modified biochar increased by a factor of 2.4. This is due to the fact that the coal gangue, as one kind of metal-rich solid waste, changes the surface and pore structure of biochar after the co-pyrolysis with biomass. The amount of SO₄²⁻ adsorption was related to the specific surface area. The larger the specific surface area, the stronger the surface ability of adsorption to promote SO₄²⁻ adsorption [50]. As a result, MBC with a high specific surface area was suitable as the materials for wastewater treatment [51].

EDS analysis displayed the main components of biochar. It is observed that the major components of BC and MBC were C and O (Figure 2), and the content of Si and Al was increased after the modification of coal gangue, which was the reason for the addition of coal gangue.

The FTIR diagrams acquired before and after the adsorption on BC and MBC showed that the vibration bands of the two adsorbents were predominantly concentrated around 3400 cm⁻¹, 1631 cm⁻¹, 1589 cm⁻¹, 1350 cm⁻¹, 1087 cm⁻¹, and 762 cm⁻¹ (Figure 3), which were caused by the -OH stretching vibration [52], -COOH stretching [53], C=C stretching [52,54], C-H deformation [54], Si-O antisymmetric stretching [55], and Al-O symmetric stretching [56], respectively. The decreased peak intensities observed in MBC at the vibrational bands associated with -OH, -COOH, and C=C indicated that the modification process resulted in dehydrogenation and oxidation reactions, leading to a reduction in the structural water content and an enhancement of the pore structure. The increased strength of Si-O and Al-O could be attributed to the addition of coal gangue. In addition, a decrease in the intensity of the C=O, C=C, Si-O, and Al-O bands in MBC implied their involvement in the adsorption of SO₄²⁻.

3.2. Adsorption Experiments

The pH level of the solution may affect the surface charge of the adsorbent, thereby altering its adsorption capacity. To explore the effect of pH on the adsorption effect, the solution pH was adjusted to a range of 2 to 12 to obtain the adsorption effect of the adsorbent on the SO₄²⁻ in water at different pH (Figure 4). Both BC and MBC exhibited a decreasing trend in adsorption capacity as the pH increased. When the pH increased from 2 to 12, the adsorption capacity of MBC for SO₄²⁻ dropped from 49.56 mg·g⁻¹ to 20.14 mg·g⁻¹, and that of BC for SO₄²⁻ dropped from 20.99 mg·g⁻¹ to 13.17 mg·g⁻¹. Notably, the decline rate at pH = 4.0–6.0 was more pronounced than in the other intervals. It can be explained by the fact that the lower pH levels are conducive to the protonation of the adsorbent surface, resulting in an increased presence of positive charges. This enhancement in positive charges strengthens the electrostatic interaction between the adsorbent surface and SO₄²⁻, thus facilitating SO₄²⁻ adsorption [57]. Conversely, the OH⁻ can compete with SO₄²⁻ for the active site when the pH of the solution is too high, consequently diminishing the adsorption capacity [58].

The adsorption effect of the MBC dosage on SO₄²⁻ is shown in Figure 4b. The removal rate of SO₄²⁻ gradually increased from 31% to 79.68% with the rising adsorbent dosage, primarily due to the concurrent increase in active adsorption sites [59]. Equilibrium in the removal rate was achieved at the dosages exceeding 8 g·L⁻¹. This equilibrium results from the saturation of the contact-specific surface area between SO₄²⁻ and the adsorbent, rendering any additional active sites unattainable [60]. In contrast, the adsorption capacity exhibited a decreasing trend, attributed to the incomplete occupation of adsorbable substances on the biochar surface as the dosage increased [61]. The optimal adsorbent dosage was determined to be 8 g·L⁻¹, considering both economic and effective factors, resulting in a higher removal rate (79.15%) and adsorption capacity (49.56 mg·g⁻¹) for SO₄²⁻.

Table 1. SO₄²⁻ adsorption capacity of different types of biochar.

Adsorbent	Concentration Range (mg/L)	pH	Adsorption Capacity (mg/g)	Refs.
CA-MB	10–250 mg/L	2–10	10.16 mg·g−1	[62]
ZrBC	5–300 mg/L	2–12	35.21 mg·g−1	[2]
Fe-(Ba-BC)	1500 mg/L	2–11	226.80 mg·g−1	[42]
Carbon residue	50–1000 mg/L	2–8	19.50 mg·g−1	[63]
MBC	60–600 mg·L-1	2–12	49.56 mg·g−1	This study

presents the comparison of the effect of different types of biochar on the adsorption of SO₄²⁻ in water. The results reveal that the MBC proposed in this study exhibits superior SO₄²⁻ adsorption capacity compared to the other biochar types, highlighting MBC has excellent SO₄²⁻ removal ability.

3.3. Adsorption Kinetics

The change in MBC on the SO₄²⁻ adsorption capacity as a function of reaction time is shown in Figure 5. The adsorption process of MBC SO₄²⁻ can be divided into three stages: rapid adsorption, slow adsorption, and adsorption equilibrium stages (

Table 2. Adsorption kinetic parameters of MBC.

Sample	The Quasi-First-Order Kinetic Parameters			The Quasi-Second-Order Kinetics Parameters
	qe/(mg·g−1)	k1	R2	qe/(mg·g−1)	k2	R2
MBC	49.153	2.157	0.957	55.522	0.056	0.977

). The adsorption capacity increased to 49.56 mg·g⁻¹ at 2 h. This observation suggests the presence of numerous adsorption sites on the adsorbent surface that can quickly bind with SO₄²⁻. However, these adsorption sites gradually become saturated over time, eventually reaching equilibrium. The quasi-first-order kinetic model and the quasi-second-order kinetic model were used to fit the adsorption results to further explore the mechanism of MBC adsorption on SO₄²⁻. Both models exhibited good fits to the experimental data, with R² being greater than 0.9, (0.957 and 0.977, respectively), and the R² of the quasi-second-order kinetic model was closer to 1 (Table 2). Therefore, the quasi-second-order kinetic model may better match the adsorption process, suggesting that SO₄² adsorption via MBC is dominated by chemisorption [64]. The results described above illustrate that the adsorption process is dominated by ion exchange and electrostatic interaction between biochar and SO₄²⁻.

3.4. Adsorption Isotherms

Langmuir and Freundlich isotherm adsorption models were used to fit the adsorption data. In Figure 6, a sharp increase in the adsorption capacity is observed when the initial concentration of SO₄²⁻ is lower. However, as the initial concentration gradually increases, the increasing trend of the adsorption capacity tends to be flat and eventually plateaus. Since the adsorption site provided via the adsorbent is fixed, it led to a reduced SO₄²⁻ occupation capacity with the increasing concentration and the eventual saturation of the adsorption capacity.

Table 3. Adsorption isotherm parameters of MBC.

Sample	Langmuir Parameters			Freundlich Parameters
	qe/(mg·g−1)	KL/(L·mg−1)	R2	qe/(mg·g−1)(mg·L−1)−1/n	1/n	R2
MBC	74.564	0.013	0.968	12.687	0.264	0.858

illustrates the results of isothermal model fitting. The R² of the Langmuir model (R² = 0.968) surpasses that of the Freundlich model, indicating that the SO₄²⁻ adsorption process aligns better with the Langmuir model. The maximum SO₄²⁻ adsorption of MBC obtained from the isotherm study was 74.56 mg·g⁻¹. This fitting outcome implies that the sulfate adsorption on MBC is dominated by monolayer adsorption [65].

3.5. Adsorption Mechanism

As expected from the results on the effect of pH on the adsorption performance, the adsorption capacity shows a large variation with a function of pH, emphasizing that the adsorption of MBC on SO₄²⁻ is dominated by electrostatic adsorption. Large specific surface areas and developed pore structures have been observed for MBC, suggesting that MBC exhibits excellent adsorption capabilities. Infrared characterization reveals significant changes in functional group intensity before and after MBC adsorption, indicating the involvement of functional groups in the reaction and the presence of chemical adsorption during the adsorption process. The fitting results for the adsorption kinetic and isotherm models further confirm the appropriateness of the quasi-second-order kinetic model and the Langmuir model in describing the adsorption process, and the adsorption process is dominated by electrostatic adsorption and chemical adsorption. In summary, Figure 7 presents the mechanism of SO₄²⁻ adsorption via MBC, which encompasses physical adsorption, electrostatic adsorption, and chemical adsorption.

3.6. Pot Experiment

3.6.1. Effect of Modified Biochar on Chlorophyll Content of Reed under SO₄²⁻ Stress

Figure 8a shows the chlorophyll content at different SO₄²⁻ concentrations measured on the 60th day of the experiment. The chlorophyll content exhibited a significant decrease with the increasing SO₄²⁻ concentration (p < 0.05). High salt stress can disrupt the ultrastructure and membrane system of chloroplasts and reduce the activity of chlorophyll synthase, leading to a significant decline in chlorophyll content. The chlorophyll content proved to decrease by 55.43% due to SO₄²⁻ treatment alone with increasing the SO₄²⁻ concentration until 1000 mg·g⁻¹, while the application of MBC caused the chlorophyll content to decrease by 38.02%, indicating that MBC could increase the salt tolerance of reed. The addition of MBC increased the chlorophyll content by 4.35%, 8.43%, 18.75%, 27.20%, 21.14%, and 45.12%, compared to the plant at SO₄²⁻ stress alone. The intensity of plant photosynthesis is determined via the chlorophyll content, such as the higher chlorophyll content can improve the photosynthetic rate, and thus, increased the growth and development of plants. Therefore, the application of MBC exhibits a positive effect on the growth of reed.

3.6.2. Effect of Modified Biochar on Antioxidant Enzyme Activity of Reed under SO₄²⁻ Stress

SOD is an important reactive oxygen species (ROS) scavenger, converting superoxide free radicals into H₂O₂ to reduce oxidative damage and improve plant stress tolerance [66]. The SOD activity of CK and MBC in reed showed a trend of rising first and then falling with the increasing SO₄²⁻ concentration (Figure 8b). The damage induced by plant stress is mitigated by increased SOD activity, but the plant cell membrane system can be irreversibly damaged by a certain level of stress, resulting in a decline in SOD activity in the reed. The POD activity of the reed under SO₄²⁻ stress was significantly increased (p < 0.05) compared with the reed without SO₄²⁻, indicating that salt stress promoted the occurrence of an enzymatic oxidative stress reaction in the reed. Furthermore, the SOD activity of MBC was lower than that of the CK group and decreased by 37.84%, 38.62%, 37.03%, 44.39%, 26.73%, and 33.01% respectively, implying that MBC could alleviate the stress on reed.

H₂O₂ generated via SOD can exert a toxic effect on plants, while the lower toxicity can be achieved by decomposing the H₂O₂ into H₂O and O₂ via POD and CAT [67]. The POD activity in the two groups of experiments showed a trend of increasing first and then decreasing with the increasing SO₄²⁻ concentration, and the highest POD activity was found at the SO₄²⁻ concentration of 600 mg·g⁻¹. Upon the addition of MBC, the POD activity of reed decreased by 25.61%, 30.87%, 34.90%, 21.05%, 22.04%, and 24.39%, to some extent (Figure 8c). The effect of MBC on the CAT activity of reed under SO₄²⁻ stress is depicted in Figure 8d. The effect of MBC addition on CAT activity was not significant (p < 0.05) at very high or low SO₄²⁻ concentrations. Specifically, the CAT activity of MBC increased by 7.26% at the SO₄²⁻ concentration of 600 mg·g⁻¹, while that decreased by 10.28% at the SO₄²⁻ concentration of 400 mg·g⁻¹.

3.6.3. Effect of Modified Biochar on MDA Content of Reed under SO₄²⁻ Stress

MDA is the final product of lipid peroxidation, and the content of MDA is in direct proportion to the degree of cell membrane oxidation when plants suffer stress [68]. The effect of MBC on the MDA content of reed under SO₄²⁻ stress is shown in Figure 8e. Both the CK and the MBC groups showed significant increases in MDA content with the rising SO₄²⁻ concentration, suggesting that higher concentrations impose greater stress on reed. The MDA content of reed with MBC was lower than that without MBC and decreased by 29.55%, 9.40%, 28.38%, 19.31%, 13.75%, and 13.64%, respectively, under different SO₄²⁻ concentrations. Therefore, the addition of MBC can be mitigated by adding MBC to reduce SO₄²⁻ stress on plants, which is in agreement with the previous studies [69,70].

3.6.4. Effect of Modified Biochar on SP Content of Reed under SO₄²⁻ Stress

Salt stress induces osmotic potential imbalances in plants, hindering their ability to absorb essential water and mineral elements from the environment, ultimately leading to plant death. Soluble proteins can dissolve in water at a small molecular level, stabilizing the internal environment and preserving normal cell morphology and function. Figure 8f shows the effect of MBC on the SP content of reed under SO₄²⁻ stress. The SP content exhibited an initial increase followed by a decrease with the increasing SO₄²⁻ concentrations. Compared with the CK group without modified biochar, SP content in the MBC group increased by 3.88%, 2.15%, 7.09%, 11.70%, 6.24%, and 7.14%, respectively, at different SO₄²⁻ concentrations. The elevated SP content contributes to maintaining cellular homeostasis and resilience against the adverse effects of SO₄²⁻ stress.

4. Conclusions

In the present study, a cost-effective adsorbent, MBC, was synthesized via a co-pyrolysis method for the purpose of removing sulfates from aqueous solutions. An optimal adsorption performance was achieved at a pH of 2 with a dosage of 8 g·L⁻¹. The adsorption process adhered to the quasi-second-order kinetic model and the Langmuir model, with a maximum adsorption capacity of 49.56 mg·g⁻¹. The adsorption mechanism mainly encompassed electrostatic adsorption, physical adsorption, and chemical adsorption. The pot experiment demonstrated that MBC effectively mitigates the stress induced by SO₄²⁻ on reeds, as evidenced by an increase in chlorophyll and soluble protein content, coupled with a reduction in the activity of SOD, POD, CAT, and MDA. Therefore, MBC exhibits substantial potential in the remediation of SO₄²⁻ pollution in wastewater.