Removal of Methylene Blue from Simulated Wastewater Based upon Hydrothermal Carbon Activated by Phosphoric Acid

Abstract

Cationic dyes pose potential health risks to humans due to their higher toxicity levels. Most current research focuses on the utilization of biomass waste in the preparation of multifunctional materials to mitigate the adverse impact of cationic dye wastewater on the environment. However, conventional methods of biochar preparation require elevated pyrolysis temperatures and greater energy consumption. Accordingly, this study aims to investigate the effectiveness of the removal of methylene blue (MB) from simulated wastewater using a one-step phosphoric acid activation hydrothermal carbonization technique. SEM, BET, XRD, FTIR, and XPS analyses were conducted to investigate the surface morphology and chemical composition of pine sawdust (PS) biomass as a raw material, pine sawdust with hydrothermal carbon (HTC-PS), and pine sawdust with phosphoric acid-activated hydrothermal carbon (PHTC-PS). The results demonstrate that PHTC-PS exhibits a maximum adsorption capacity of 268.4 mg/g for MB at 298 K. The experimental data demonstrate its consistency through both the Langmuir isotherm model and pseudo-second-order kinetic model, suggesting that its adsorption mechanism predominantly involves monolayer formation through chemical interactions. Additionally, thermodynamic parameters reveal that the MB adsorption of PHTC-PS is a spontaneous endothermic reaction. Thus, this study demonstrates that the one-step phosphoric acid activation hydrothermal carbonization method can achieve satisfactory adsorption efficiency with the advantages of lower energy consumption, simplicity to the operation, and mild preparation conditions.

1. Introduction

The acceleration of industrialization has exacerbated water pollution, particularly through dye contamination [1], with over 10,000 synthetic dyes produced globally and annual dye output exceeding 700,000 tons [2,3]. Notably, 10–15% of dye wastewater is discharged into ecosystems untreated, posing significant environmental risks [4]. Dyes can be classified according to the charge they carry when dissolved in water-based media into cationic, anionic, and non-ionic dyes [5]. Among them, cationic dyes such as methylene blue (MB), malachite green, and crystal violet have been widely used in the industries of textiles, printing and dyeing, and papermaking. As a result, concerns about their environmental residues have become particularly prominent [6,7].

MB, with a chemical formula of C₁₆H₁₈N₃ClS, is a widely utilized cationic dye belonging to the family of phenothiazine dyes. It possesses a stable structure, high water solubility, intense chroma, and various toxicities. In various industries, such as textiles, printing, and leather production, MB serves as a colorant. Previous studies have demonstrated that wastewater containing MB can lead to the death of aquatic plants and significantly decrease oxygen levels in water bodies. Prolonged exposure to MB may also adversely impact human health by causing symptoms like dyspnea, nausea, vomiting, blindness, migraines, and nerve damage [8]. Furthermore, there is an increased risk of cancer or genetic mutations in humans due to MB exposure [9]. Consequently, the effective removal of MB is imperative.

The treatment of dye-contaminated wastewater currently utilizes multiple technologies including electrochemical processes, ozonation, photocatalytic decomposition, membrane filtration, biological treatment, and adsorptive removal. Among these approaches, the adsorption method is considered highly effective in removing MB from contaminated wastewater due to its economic viability, superior contaminant removal performance, and straightforward implementation [10,11]. The efficacy of these adsorption techniques predominantly depends on their selection of adsorbent materials, with carbon-based materials (e.g., activated carbon, carbon nanotubes, graphene, carbon nanofibers, biochar, and biomass-derived carbon composites) having garnered significant attention due to their high surface area, tunable surface chemistry, and versatile structural modifications [12,13,14].

Common biomass materials include forestry waste, agricultural waste, shell and fruit core materials, animal manure, and cellulose-based materials [15]. Among them, forestry waste occupies an important position in the utilization of biomass resources because of its wide range of sources and outstanding renewability. Pine wood, for example, is widely distributed in cold northern areas, and wood processing produces a large amount of wood chips. Compared with other seasonal agricultural wastes, PS has the significant advantage of having a stable supply throughout the year. Chemical analysis shows that pine wood chips are rich in carbon (40–50%), lignin (24–32%), cellulose, and hemicellulose (with a combined content of 64–77%). This high proportion of structural carbohydrates makes it an excellent biomass raw material. It is especially suitable for utilization through thermochemical conversion or biodegradation processes [16,17]. The accumulation of traditional biomass waste is a substantial undertaking and requires appropriate treatment methods such as landfilling, incineration, or composting to prevent environmental pollution and resource wastage. Therefore, the transformation of waste into biomass raw materials for rational resource utilization is a relatively cost-effective and reasonable approach [18]. In general, the utilization of waste materials in adsorption processes offers both environmental and economic benefits. Wood waste is widely employed due to its non-toxic nature, negligible cost, high renewability, and abundant availability [19]. These wastes primarily consist of lignin, cellulose, and hemicellulose structures that can effectively adsorb aqueous MB dye through their functional groups and pore structures [20].

However, the efficiency of adsorbing MB dye using solely wood waste biomass is relatively low. Therefore, various thermochemical technologies are employed to convert biomass into biochar, including pyrolysis, gasification, hydrothermal processes, and other techniques [21]. Pyrolysis technology involves converting biomass into a highly aromatic and carbon-rich porous carbon material [22,23], typically at temperatures ranging from 300 to 1000 °C and over a residence time exceeding 0.5 h in an inert atmosphere or with limited oxygen availability. Gasification technology refers to the reaction of biomass at temperatures above 700 °C primarily for syngas and bio-oil production while simultaneously recovering energy from the syngas [24]. The hydrothermal processes mentioned encompass hydrothermal carbonization (HTC), liquefaction (HTL), and gasification (HTG) [25]. The key differentiating factor among these technologies is their operating temperature range [26]. HTC is generally performed within a temperature range of 180–250 °C under sub-critical conditions and in a completely closed, oxygen-free environment where reactions such as hydrolysis, dehydration, decarboxylation, aromatization, and recondensation may occur [27]. Furthermore, this technology offers advantages such as lower energy consumption, ease of operation, mild preparation conditions, and minimal pollution emissions, along with higher carbonization efficiency. These benefits not only contribute to cost reduction but also promote environmentally friendly practices [28].

The physical and chemical properties of hydrothermal carbon can be enhanced through activation, which is carried out using either physical or chemical methods. Physical activation involves using mild oxidizing gases such as CO₂ or H₂O at high temperatures to activate the product [29]. However, due to the higher activation temperature, longer heat treatment time, and lower carbon yield associated with physical activation, chemical activation methods are commonly adopted to enhance the physical and chemical properties of hydrothermal carbon [30]. Chemical activation can be achieved in two ways: (1) one-step synthesis, where chemical reagents are directly added for activation during the HTC process [31], and (2) two-step synthesis, where, after completing HTC, chemical reagents are used to wash the hydrothermal carbon. Following washing, the hydrothermal carbon can either be used directly after rinsing with distilled water or heated in an inert atmosphere for further activation [32]. However, this method has disadvantages such as a complex preparation process and higher energy consumption. For instance, Alhawtali et al. [33] utilized date palm (Phoenix dactylifera) as a raw material and employed a two-step synthesis method that involved hydrothermal carbonization followed by high-temperature calcination activation with phosphoric acid. The hydrothermal reaction was carried out at 230 °C for 3–8 h, and the product was dried at 110 °C for 12 h. After impregnation with a certain amount of phosphoric acid, it was placed in a tubular furnace and calcined at 450–650 °C (protected by N₂) for 1.5 h and finally dried at 80 °C for 24 h to obtain the activated material, which had a maximum adsorption capacity of 409.8 mg/g for MB. Zhou et al. [34] used bagasse as their raw material and adopted a two-step synthesis method of phosphoric acid-assisted HTC followed by NaOH activation. Briefly, the bagasse powder was mixed with the phosphoric acid solution at a ratio of 1:19 (mass ratio) and reacted in a closed system at 513 K for 10 h. The initial hydrothermal carbon was obtained after drying for 12 h. Subsequently, the initial hydrothermal carbon was placed in the NaOH solution, stirred at room temperature for 2 h, and then dried at 373 K for 12 h to prepare activated hydrothermal carbon. Under these conditions, its maximum adsorption capacity for MB was 357.14 mg/g.

In response to the challenges of high energy consumption, complex preparation processes, and excessive use of chemical activators encountered in the traditional methods used for treating MB dye wastewater, this study proposes a “low-temperature, low-concentration, one-step” hydrothermal co-activation technique utilizing waste PS. The material was synthesized via in situ catalysis using low-concentration phosphoric acid (≤15 wt%) in a subcritical water environment (180–220 °C), with the aim of achieving the following objectives: (1) analyzing the surface morphology and chemical composition of the materials before and after carbonization and activation using SEM, BET, XRD, FTIR, and XPS techniques; (2) investigating single influencing factors such as the optimal phosphoric acid concentration, adsorbent dosage, solution pH, and the co-existing ions in the solution; and (3) briefly elucidating the adsorption mechanism of MB removal through fitting kinetic, isotherm, and thermodynamic models.

2. Materials and Methods

2.1. Raw Materials and Chemicals

PS was obtained from waste branches of pine trees at the campus of Shenyang University of Technology. Firstly, the pine wood branches were cut, washed, dried, crushed with a multifunctional grinder, and ground through a 200-mesh sieve. The resulting precursor samples were then placed in a blast drying oven at 120 °C for 4 h before further use. MB was derived from Tianjin Damao Chemical Reagent Factory (Tianjin, China). Phosphoric acid (H₃PO₄) and anhydrous ethanol were purchased from Tianjin Fuyu Fine Chemical Co., Ltd. (Tianjin, China). The chemicals and reagents employed are of analytical purity. All experimental solutions were prepared using Millipore-sourced (Bedford, MA, USA) ultrapure water demonstrating an 18.2 MΩ/cm resistivity.

2.2. Preparation of Hydrothermal Carbon

We prepared the hydrothermal carbon by taking 3 g of pretreated PS powder, adding ultrapure water according to the ratio of solid–liquid of 1:5, putting it on the magnetic stirrer, and stirring for 1 h to make sure it was fully mixed. Moreover, the prepared sample was put into a 100 mL para-polystyrene (PPL) inner liner, tightened in the reactor, and then placed in the oven at 185 °C for 6 h (a low-temperature hydrothermal carbonization process). After cooling, the obtained filter cake was repeatedly and alternately washed with anhydrous ethanol and ultrapure water until the filtrate was colorless. After the filter cake was fully washed and dried at 60 °C for 12 h, a loose hydrothermal carbon material was obtained, i.e., HTC-PS. The process of synthesizing activated carbon via a one-step hydrothermal method with phosphoric acid activation was similar to the traditional hydrothermal synthesis of activated carbon. Specifically, ultrapure water was substituted with low-concentration phosphoric acid (≤15 wt%), and the polystyrene (PPL) lining was replaced with an acid- and alkali-resistant polytetrafluoroethylene (PTFE) lining. The resultant material was called PHTC-PS.

2.3. Characterization Methods

The materials’ microstructural features and surface morphology were characterized via field emission scanning electron microscopy (Thermo Fisher Scientific, Apreo 2S, Waltham, MA, USA). The materials’ specific surface area and pore architecture parameters were determined via static volumetric gas adsorption measurements performed on a BSD PM analyzer (Beijing Beishide Instrument Co., Ltd., Beijing, China). The crystal phase structure of the prepared materials was characterized and analyzed using an X-ray diffractometer (Panalytical, X-Pert Pro MPD, Almelo, The Netherlands) under the following test conditions: a scanning speed of 10°/min and scanning angle range of 10° to 90°. The functional groups in the prepared materials were examined using a Fourier transform infrared spectrometer (IRPrestige-21, Shimadzu Corporation, Tokyo, Japan). XPS characterization was conducted using an X-ray photoelectron spectrometer (Shimadzu AXIS SUPRA+, Kyoto, Japan) with spectral deconvolution performed via XPS Peak 4.1 software.

2.4. Adsorption Experiments

In order to investigate the mechanism behind the adsorption of MB onto PHTC-PS, a series of experiments were conducted to investigate its influencing factors, adsorption kinetics, adsorption isotherm, and adsorption thermodynamics. The single-factor method was employed in all experiments. The adsorption experiments were performed in 250 mL conical bottles at a constant temperature, with the shaker oscillating at 150 rpm until equilibrium was reached after 48 h. MB solution concentrations were quantified via UV-Vis spectroscopic detection at 665 nm using a UV-6000PC spectrophotometer (Shanghai Metash Instruments Co., Ltd., Shanghai, China) [35]. Subsequently, the equilibrium adsorption capacity (q_e; mg/g) and removal rate (R; %) for MB dye were calculated using the following formulas:

$$q_{\mathrm{e}}={\displaystyle \frac{\left(C_0-C_{\mathrm{e}}\right)V}{m}}$$

(1)

$$R={\displaystyle \frac{\left(C_0-C_{\mathrm{e}}\right)100\%}{C_0}}$$

(2)

where C₀ (mg/L) and C_e (mg/L) represent the initial concentration of the MB solution and the concentration of the MB solution after reaching adsorption equilibrium, respectively. V (mL) refers to the volume of the MB solution, while m (g) indicates the amount of adsorbent present.

2.4.1. Influencing Factors

Phosphoric acid solutions with different concentrations of 0.2 mol/L, 0.5 mol/L, and 1.0 mol/L (≤15 wt%) were prepared and added to the hydrothermal reactor to explore the optimal concentration of phosphoric acid-activated material to apply for the adsorption of MB. We weighed 1 mg, 5 mg, 10 mg, 15 mg, 20 mg, 30 mg, 50 mg, 70 mg, and 100 mg of PHTC-PS into separate conical bottles containing an MB solution and shook them until adsorption equilibrium was reached. Then, we determined the residual concentration of MB to confirm the optimal dosage of PHTC-PS for MB adsorption. The pH of the MB solution was regulated to different levels (3, 5, 7, 9, 11) through titration with 0.1 M HCl and NaOH. The effect of the initial pH on the adsorption efficiency of the MB solution was investigated. The coexisting ions of Na⁺, K⁺, Ca²⁺, and Mg²⁺ were 10 mmol/L, and a blank control group was set up. The initial concentration of the solution was 100 mL, containing 20 mg/L of MB solution. After the supernatant was filtered with a 0.45 μm organic filter membrane, the concentration of each sample was determined by the UV-VIS spectrophotometer [36].

2.4.2. Adsorption Kinetics

The study of adsorption kinetics is highly significant for exploring the adsorption mechanism and describing the rate-controlling steps in this reaction. The change in concentration of MB in the wastewater through its adsorption by the prepared material was measured within a 48 h period. The experimental data were fitted using pseudo-first-order and pseudo-second-order dynamics models. The formulas of the dynamics models can be seen in Equations (3) and (4).

$$q_t=q_e\left[1-\exp \left(-k_{1}t\right)\right]$$

(3)

where q_t (mg/g) represents the adsorption capacity at time t, t (min) denotes the reaction time, q_e (mg/g) signifies the equilibrium adsorption capacity, and k₁ (min⁻¹) indicates the reaction rate constant of the pseudo-first-order kinetic equation.

$$q_t={\displaystyle \frac{q_e^2{k}_{2}t}{1+q_e{k}_{2}t}}$$

(4)

where k₂ [g·(mg·min)⁻¹] represents the reaction rate constant in the pseudo-second-order kinetic model.

2.4.3. Adsorption Isotherms

The adsorption isotherm can be used to define the interaction between an adsorbent and adsorbate. PHTC-PS was added to MB solutions of different concentrations (10 mg/L, 20 mg/L, 30 mg/L, 40 mg/L, 50 mg/L, 100 mg/L, and 150 mg/L) at temperatures of 298 K, 308 K, and 318 K to explore its adsorption isotherm. The experimental data were fitted using the Langmuir and Freundlich isotherm models, and their corresponding formulas are provided in Equations (5) and (6).

$$q_e={\displaystyle \frac{q_m{K}_{\mathrm{L}}{C}_e}{1+K_{\mathrm{L}}{C}_e}}$$

(5)

where q_e (mg/g) represents the equilibrium adsorption capacity, q_m (mg/g) denotes the theoretical saturated adsorption capacity, k_L (L/mg) signifies the constant of the Langmuir isotherm model, and C_e (mg/L) indicates the concentration of MB after adsorption equilibrium was reached.

$$q_e=K_{\mathrm{F}}{C}_e^{1/n}$$

(6)

where K_F (L/g) is the Freundlich adsorption constant and n is the adsorption strength of the adsorbent.

2.4.4. Adsorption Thermodynamics

Thermodynamic parameters play a crucial role in controlling adsorption behavior. The Gibbs free energy (ΔG°), the enthalpy change in adsorption (ΔH°), and the entropy change in adsorption (ΔS°) are calculated as follows:

$$K_{\mathrm{D}}={\displaystyle \frac{q_e}{C_e}}$$

(7)

where K_D represents the distribution coefficient, q_e (mg/g) denotes the equilibrium adsorption capacity, and C_e (mg/L) refers to the concentration of MB after adsorption equilibrium was reached.

$$\mathsf{\Delta }G°=-RT\ln K_{\mathrm{D}}$$

(8)

where R represents the ideal gas constant (8.314 J/mol), T denotes the thermodynamic temperature (K), and ∆G° indicates the standard Gibbs free energy change (kJ/mol).

$$\ln K_{\mathrm{D}}={\displaystyle \frac{\mathsf{\Delta }S°}{R}}-{\displaystyle \frac{\mathsf{\Delta }H°}{RT}}$$

(9)

where ΔS° represents the standard entropy change (J/(mol·K)) and ΔH° denotes the standard enthalpy change (kJ/mol). The parameters were calculated by performing the linear regression of lnK_D against 1/T.

3. Results and Discussion

3.1. Characterizations of Materials

The pore structures exhibited by the three materials in Figure 1 primarily consist of macropores and mesoporous regions. Figure 1a,d illustrate that the surface of PS appears relatively smooth, displaying a simple fragmented stacking structure with noticeable holes and voids [37]. Figure 1b,e reveal that the surface of the HTC-PS is comparatively rough, resulting in some debris and microsphere structures, as well as relatively dense pore and void formations. These findings are consistent with those reported by Nguyen et al. [38]. Figure 1c,f demonstrate that PHTC-PS materials possess a more distinct skeleton structure, fragmented stacked arrangements, and smaller pores. This leads to an increased surface area, providing more adsorption sites beneficial for MB adsorption.

According to Figure 2a–c, the specific surface area and pore structure of PS, HTC-PS, and PHTC-PS have observable effects on their adsorption isotherms. Based on the IUPAC classification, all materials exhibit type IV isotherms, which indicates mesoporous capillary condensation. The hysteresis isotherms for all materials are classified as type H3, characterized by the absence of a distinct saturation adsorption plateau. This suggests the presence of slit pores and an irregular pore structure. The pore distribution diagram confirms that these three materials predominantly possess mesopores [39]. As shown in

Table 1. Texture characteristics of PS, HTC-PS, and PHTC-PS.

Samples	BET Surface Area (m2/g)	Total Pore Volume (mL/g)	Average Pore Size (nm)
PS	1.041	0.0057	21.90
HTC-PS	2.440	0.0138	22.62
PHTC-PS	11.56	0.0549	19.00

, the specific surface areas for PS, HTC-PS, and PHTC-PS are 1.041 m²/g, 2.440 m²/g, and 11.56 m²/g, respectively; thus demonstrating that the phosphoric acid-activated hydrothermal carbonization method effectively enhances the specific surface area of the material.

X-ray diffraction (XRD) was used to detect the crystal structures of PS, HTC-PS, and PHTC-PS, as shown in Figure 3. The spectra of PS and HTC-PS exhibit broad peaks at 2 θ values of 15.7, 22.7, and 34.5, which correspond to the characteristic (110), (200), and (004) crystal faces of cellulose structures, and the type of peak exhibited by HTC-PS was sharper than that of PS, which may be because PS is a primitive biomass material. The cellulose structure may be surrounded by a large number of hemicellulose, lignin, and other impurities (ash, volatiles), which weakens the strength of the cellulose’s characteristic peak. Moreover, the HTC-PS material, after hydrothermal carbonization, went through hydrolysis, condensation, and other reactions to remove impurities, so its cellulose characteristic peak is clearer [40,41]. In contrast, for PHTC-PS, which contains hydrothermal carbon with phosphorus (P), the three peaks representing the cellulose structure disappear and are replaced by a wide peak, indicating the presence of amorphous carbon microcrystals. This change can be attributed to H₃PO₄ acting as a catalyst during the hydrothermal carbonization process, accelerating biomass raw material decomposition and promoting biomass carbonization [42].

Fourier transform infrared spectroscopy (FTIR) was employed to analyze the surface chemical properties of PS, HTC-PS, and PHTC-PS. As seen in Figure 4, the peak near 3344 cm⁻¹ may be caused by the O-H stretching vibration of the hydroxyl group, caused by adsorbed water molecules. The two adsorption peaks located at 2923 cm⁻¹ and 2849 cm⁻¹ are related to the stretching vibrations of -CH₂ and -CH₃, respectively [43], and, among these, the peaks appearing only in the PHTC-PS material may be related to the addition of H₃PO₄ to promote the conversion of hydrocarbons [44]. The clear characteristic peak at 1703 cm⁻¹ is attributed to the C=O stretching vibration of the carboxyl group, and the vibration bands of 1600 cm⁻¹ and 1518 cm⁻¹ correspond to the C=C stretching vibrations in different ring structures [45]. The cellulose-related peaks of PS and HTC-PS appear between 1000 cm⁻¹ and 1250 cm⁻¹ (consistent with the XRD results), but disappear in the PHTC-PS materials, indicating that H3PO4 accelerates the decomposition of biomass. The peak at 1219 cm⁻¹ in the PHTC-PS material may be attributed to the P=O bond, C-O-P bond, or P=OOH bond [46]. Finally, two new peaks appeared at 858 cm⁻¹ and 793 cm⁻¹ in the PHTC-PS, possibly due to the formation of new phosphate groups as a result of the addition of H₃PO₄ [42].

The compositions and chemical states of the three materials were studied using X-ray photoelectron spectroscopy (XPS). In the C 1s of Figure 5a, three peaks were detected for all three materials, which are, from low to high binding energies, C-C, C-O, and C=O, respectively. Compared to PS, both HTC-PS and PHTC-PS exhibited varying degrees of displacement towards higher binding energy positions, indicating the direction of electron transfer in the valence band during the adsorption process [47]. A higher binding energy implies a stronger attraction between the nucleus and electrons, resulting in a decrease in electron cloud density [48,49]. Two peaks, C=O and C-O, were detected in the O 1s (Figure 5b), and the C-O peak was stronger than the C=O peak in the three materials [45], indicating that their main structure may be polyfuran [50,51].

3.2. Effects of Different Influencing Factors

The optimal concentration of phosphoric acid for activation can be observed in Figure 6a to be 0.5 mol/L, but the overall difference of this concentration is not significant. This may be due to the incomplete activation caused by phosphoric acid at lower concentrations, while higher concentrations may damage the material’s structure and trigger other reactions. Therefore, the PHTC-PS material used in this study was activated with 0.5 mol/L of phosphoric acid. The removal rate increased from 59.95% to 97.25% as the dosage of PHTC-PS elevated from 10 mg to 20 mg, as shown in Figure 6b. However, the adsorption capacity of the material reduced from 110.8 mg/g to 96.80 mg/g. This is due to the rise in dosage, which results in a considerable increase in both adsorption sites and unoccupied adsorption sites. Consequently, the removal rate of MB increased rapidly, while the adsorption capacity of the material decreased. The pH value of the solution is vital in the adsorption process, as variations in pH can influence the charge of the adsorbent (PS, HTC-PS, and PHTC-PS) and the adsorbed pollutant (MB). As shown in Figure 6c, the adsorption capacity is lower under acidic conditions (pH < 7). At pH = 7, the adsorption capacities of PS, HTC-PS, and PHTC-PS for MB were 25.50 mg/g, 50.72 mg/g, and 125.8 mg/g, respectively. When the pH exceeds 7, there is a rapid increase in the capacity for MB to be adsorbed onto all three materials. This may be attributed to MB being a cationic dye. Under acidic conditions, with high H⁺ concentrations in the solution, hydroxyl groups on the material surface are surrounded by H⁺, hindering the adsorption of MB. After increasing the pH to an alkaline condition, the concentration of H⁺ in the solution decreases, while the concentration of OH⁻ increases and the degree of protonation decreases. This strong electrostatic force significantly enhances the adsorption effect of the material. The results indicate that MB removal is facilitated under alkaline conditions, with electrostatic attraction being a key mechanism for its adsorption [52]. However, coexisting ions such as K⁺, Na⁺, Mg²⁺, and Ca²⁺ exist in dye pollution [36]. Therefore, coexisting ions with a concentration of 10 mmol/L were adopted to investigate their impact on the adsorption of MB by PHTC-PS. As Figure 6d shows, these other ions have no significant influence on its adsorption, except for Na⁺, which has a slightly inhibitory effect.

3.3. Adsorption Kinetics

The adsorption of MB on PHTC-PS reached equilibrium within 48 h, as shown in Figure 7. The adsorption process consists of two stages: the first stage is a rapid adsorption phase, which accounts for over 90% of MB adsorbed within 24 h. Subsequently, the adsorption process gradually transitions into a slower stage and approaches equilibrium. As

Table 2. Kinetic model parameters.

MB Concentration	Pseudo-First Order			Pseudo-Seond-Order
	k1	qe (mg/g)	R2	k2	qe (mg/g)	R2
10 mg/L	0.0221	86.38	0.860	3.48 × 10−4	91.52	0.950
20 mg/L	0.0012	116.3	0.806	1.53 × 10−4	123.8	0.914
30 mg/L	0.0350	173.0	0.872	3.34 × 10−4	179.6	0.942

show, both the pseudo-first-order kinetic model and the pseudo-second-order kinetic model exhibit a high degree of fit, with an R² value above 0.9 for the pseudo-second-order kinetic model, which is larger than that of the pseudo-first-order kinetic model. Therefore, it can be concluded that the adsorption of MB by PHTC-PS is mainly controlled by chemical processes involving electron sharing or transfer between the adsorbent and absorbate, together with physical adsorption control such as electrostatic interactions [53,54,55].

3.4. Adsorption Isotherms

Two common Langmuir and Freundlich isothermal adsorption equations were used to linearly fit the experimental data of the adsorption of MB by PHTC-PS at 298, 308, and 318 K. The obtained isotherm results are shown in Figure 8, and the fitting parameter values are shown in

Table 3. Isotherm model parameters.

Temperature (Kelvin)	Langmuir Model			Freundlich Model
	qm (mg/g)	KL (min−1)	R2	KF (mg/g)	1/n	R2
298	268.4	0.293	0.978	92.50	0.178	0.956
308	339.7	0.267	0.965	106.54	0.193	0.947
318	376.6	0.311	0.987	123.32	0.183	0.964

. As shown in Figure 8 and Table 3, the R² values of the Langmuir isothermal adsorption model were 0.978, 0.965, and 0.987, respectively, which were slightly higher than those of the Freundlich isothermal adsorption model at the different temperatures. This indicates that the adsorption process of MB on PHTC-PS mainly involves monolayer adsorption. Additionally, in the Langmuir isothermal adsorption model, it was found that K_L (the equilibrium constant) falls within a range of 0 < K_L <1, suggesting that PHTC-PS exhibits favorable MB adsorption.

3.5. Adsorption Thermodynamics

Adsorption thermodynamics explores the principles behind sorption processes, emphasizing their reaction direction and adsorption limits. Key thermodynamic parameters—Gibbs free energy (ΔG°), enthalpy change (ΔH°), and entropy change (ΔS°)—determine three main aspects of these processes: (1) reaction spontaneity, (2) thermal energy exchange (endothermic/exothermic), and (3) the level of disorder in the system.

Table 4. Adsorption thermodynamic parameters.

Temperature (Kelvin)	KD (L/g)	ΔG° (kJ·mol−1)	ΔH° (kJ·mol−1)	ΔS° (J·mol−1·K−1)
298	3.900	−3.372	14.79	61.04
308	4.878	−4.058
318	5.674	−4.590

shows that ΔG° is negative, indicating that the adsorption of MB by PHTC-PS is spontaneous. The absolute value of free energy increases with temperature, suggesting that higher temperatures promote the adsorption process and increase spontaneity. A positive ΔH° indicates an endothermic adsorption reaction. The positive value of ΔS° suggests an increase in randomness at the interface between the absorbent and absorbed substances during the adsorption of MB by PHTC-PS [56].

3.6. Adsorption Efficiency of PHTC-PS for Other Dyes

Although this study mainly focused on the efficient removal of MB, further experiments were also performed targeting other dyes, which revealed that under the same experimental conditions, the adsorption capacity of PHTC-PS for other cationic dyes such as crystal violet and malachite green was significantly higher than its absorption capacity for two anionic dyes, namely alizarin yellow and methyl orange (Figure 9), indicating that the significant adsorption selectivity of PHTC-PS is based upon the charge characteristics of the dyes.

4. Conclusions

In the present study, PHTC-PS was successfully synthesized via a one-step hydrothermal method with phosphoric acid activation for the removal of MB from simulated wastewater and exhibited a good adsorption performance. Our characterization results indicated that, compared to PS and HTC-PS, PHTC-PS exhibited a coarser fragmented stacking structure in the SEM images, providing more adsorption sites. BET tests showed it had a larger surface area, while the XRD analysis demonstrated more complete carbonization, with the formation of amorphous carbon microcrystals. Additionally, FTIR spectroscopy and XPS analyses were employed to investigate changes in the surface chemical properties of these materials and the introduction of functional groups. Subsequently, the adsorption experiment of MB on PHTC-PS revealed that the adsorption process is pseudo-second-order kinetic adsorption and is primarily a chemical adsorption process. At room temperature, its Langmuir adsorption capacity is 268.4 mg/g, surpassing that of general adsorbents. The ΔG° at three different temperatures was negative, indicating a spontaneous and endothermic reaction. As a result, this study determined that PHTC-PS materials could be energy-saving, environmentally friendly, and efficient at adsorbing MB dye from wastewater.

This study emphasizes the “waste treatment by waste” closed-loop concept of treating dye wastewater with biomass waste. The utilization of PS could avoid secondary pollution from landfill or incineration. One-step activation technology using a low-temperature (185 °C) hydrothermal treatment and low-concentration phosphoric acid (0.5 mol/L) was developed, which could reduce energy consumption compared with the traditional high-temperature pyrolysis processes. While maintaining a certain adsorption performance, it not only avoids the environmental risk of strongly corrosive reagents but reduces energy consumption and subsequent treatment costs. Moreover, future systematic work is needed to investigate the structure–activity relationship between the dye’s molecular characteristics and adsorption performance, the material’s competitive adsorption behavior in multi-dye systems, and regeneration strategies for adsorbents loaded with cationic dyes.