Highly performant biochar prepared via co-activation designed for the removal of antibiotics

3.1.1. The influence of activator dosage on adsorption capacity

The mass ratio among RS, K₂FeO₄, and KHCO₃ will affect the adsorption capacity of the adsorbent. Thus, the mass ratio among RS, K₂FeO₄, and KHCO₃ had been screened, and the results have been shown in Figure 2. From Figure 2, one can understand that the adsorption capacity increased with the mass ratio between RS and K₂FeO₄. It could be due to K₂FeO₄ being a strong oxidant that has a synergistic effect with BC during adsorption. K₂FeO₄ can oxidize some pollutants into forms that are more easily adsorbed by the BC. For example, for some organic pollutants, K₂FeO₄ can oxidize and decompose them. K₂FeO₄ can cause changes in their molecular structure, increase their polarity, or generate more active sites, and make them more easily adsorbed by the BC [24].

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Figure 2.

The mass ratio of RS to K₂FeO₄ is 2:1, 1:1, 1:2. The heating rate is 5°C min^-1, the activation time is 120 min, and the temperature is 800°C (dosage: 0.015 g, initial concentration: 0.50 g L^-1).

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The adsorption capacity is the largest (1124.74 mg/g) when the mass ratio between RS and K₂FeO₄ is 1:2 and the mass ratio between RS-Fe and KHCO₃ is 1:2. The adsorption capacity is also higher when the mass ratio between RS and K₂FeO₄ is 1:1 and the mass ratios between RS-Fe and KHCO₃ are 1:5 and 1:6. This may be because KHCO₃ can generate abundant mesopores in BC by etching micropores. However, excessive KHCO₃ cannot continuously promote the formation of mesoporous structures [25]. To save costs and reduce waste, a mass ratio of RS:K₂FeO₄ = 1:2 and RS-Fe:KHCO₃ = 1:1 and 1:2 were chosen for the following experiments.

3.1.2. The effect of holding time on adsorption capacity

To investigate the effect of holding time on the adsorption capacity of the samples, holding times of 60, 90, 120, 150, and 180 min were selected, and the results have been shown in Figure 3. In Figure 3, as the holding time increases, the adsorption capacity first increases and then decreases. The adsorption capacity is the highest when the holding time is 120 min for RS-Fe:KHCO₃ = 1:1 (1147.45 mg g^-1). This may be due to the short insulation time, low carbonization degree of straw BC, and residual large amounts of volatile components; However, if the insulation time is too long, it may lead to a decrease in the yield of straw BC and an increase in energy consumption. At the same time, excessive insulation time can cause a serious loss of organic groups on the surface of BC, which is not conducive to the interaction between BC and ions [26]. Although the adsorption capacity for a holding time of 180 min is slightly higher than that of 120 min for the mass ratio of RS-Fe to KHCO3 of 1:2, to save time and energy, a holding time of 120 min was chosen for the subsequent experiments.

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Figure 3.

The mass ratios between RS-Fe and KHCO₃ are 1:1, 1:2, and 1:3. The mass ratio between RS and K₂FeO₄ is 1:2, with a heating rate of 10°C min^-1 and a temperature of 800°C (dosage: 0.015 g, initial concentration: 0.50 g L^-1).

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3.1.3. The effect of activation temperature on adsorption capacity

The activation temperature plays a critical role in determining the adsorption performance of the material, which led to the investigation of various thermal conditions in this study. The experimental results, presented in Figure 4, reveal distinct adsorption behaviors depending on the RS-Fe to KHCO₃ mass ratio. At a 1:1 ratio, the adsorption efficiency exhibits an initial increase, followed by a decline as the temperature rises. In contrast, a 1:2 ratio demonstrates a continuous improvement in adsorption with increasing temperature. This behavior can be explained by thermal-induced modifications in the surface morphology of BC, which result in the exposure of additional active sites for molecular interactions, thereby enhancing its adsorption potential. Additionally, elevated temperatures may induce transformations or increased activity in the surface chemical functional groups of BC. Given the negligible difference in adsorption performance between 800 and 900°C, and to optimize energy consumption, 800°C was selected for the further experimental investigations.

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Figure 4.

The mass ratio between RS-Fe and KHCO₃ is 1:1 and 1:2. The mass ratio between RS and K₂FeO₄ is 1:2. The heating rate is 5°C min^-1, and the activation time is 120 min (dosage: 0.015 g, initial concentration: 0.50 g L^-1).

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3.1.4. The effect of heating rate on adsorption capacity

To study the effect of the heating rate on the adsorption capacity of the material, the heating rates of 5°C /min and 10°C /min were selected. The results have been shown in Figure 5. It was determined that the effect of heating rate on adsorption capacity was minimal. The adsorption capacity was the highest when the RS-Fe:KHCO₃ = 1:1 at 10°C min^-1. This may be due to the decrease in crystal stacking height and surface area with increasing heating rate, which limits the interaction between volatile matter and carbon within the particles, and the destruction of pore structure by volatile matter released violently at higher heating rates, leading to an increase in pore structure [27]. As shown in Table S1, a temperature of 800°C and a heating rate of 10°C min^-1 were selected for subsequent experiments based on the adsorption effect and S_BET results. Consequently, a heating rate of 10°C min^-1 was selected for further experimental procedures.

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Figure 5.

Heating rate of activation 5°C min^-1 and 10°C min^-1. The mass ratio between RS and K₂FeO₄ is 1:2, the activation time is 120 min, and the temperature is 800°C (dosage: 0.015 g, initial concentration: 0.50 g L^-1).

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3.1.5. Comparison of adsorption performance among different samples

The adsorption performance of the samples was systematically evaluated throughout the preparation process, with results presented in Figure 6. Initial measurements revealed that RS exhibited limited TH adsorption (8.14 mg g^-1) in the absence of KHCO₃ and magnetic components. Modification with KHCO₃ significantly enhanced the performance, yielding RS-K with a capacity of 25.76 mg g^-1, while the incorporation of magnetic particles further improved adsorption capacity to 29.77 mg g^-1 (RS-Fe). The optimized composite material, 1RS-2Fe-1K, demonstrated exceptional adsorption characteristics, achieving 34.24 mg g^-1 for TH with an 85.93% removal efficiency. It is the highest among the tested BC variants. This enhanced the performance derived from the synergistic integration of KHCO₃, K₂FeO₄, and BC components, which collectively increase active site availability and functional group diversity. Thereby, it promotes the adsorption processes. Comparative data in Table S2 confirm that 1RS-2Fe-1K outperforms previously reported adsorbents, demonstrating its potential as an effective material for TH removal applications.

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Figure 6.

Removal rates of different BC samples (dosage: 0.050 g, initial concentration: 0.020 g L^-1).

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3.2.1. Scanning electron microscopy (SEM) analysis

The SEM was employed to analysis the morphological of the obtained samples, and the results have been present in Figure 7. The RS possessed an amorphous appearance with a sheet structure and smooth surface (Figure 7a). The introduction of KHCO₃ to RS can make the sheet structure breakage and form new pores on the surface (Figure 7b) [28]. When the K₂FeO₄ was successful loaded to RS, the surface of RS-Fe becomes rougher and appears flower-like structure, which may be more favorable for the adsorption process (Figure 7c) [23]. When the K₂FeO₄ and KHCO₃ were added in sequence to RS, the surface of 1RS-2Fe-1K became rough and uneven (Figure 7d).

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Figure 7.

(a-d) SEM images and (e-h) EDX images of different samples: (a) RS; (b) RS-K; (c) RS-Fe; (d) 1RS-2Fe-1K; (e) RS; (f) RS-K; (g) RS-Fe; and (h) 1RS-2Fe-1K.

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The elemental analysis was employed for the obtained adsorbent using Energy dispersive X-ray (EDX) spectroscopy. As shown in Figure 7 and Table 1, compared with the RS, RS-K undergoes high-temperature pyrolysis during the preparation process and contains a large amount of carbon. Under specific conditions, it can continue to adsorb organic carbon, thereby increasing the content of the C element. Nitrogen-containing compounds may also decompose and evaporate at high temperatures, leading to a decrease in nitrogen content. Oxygen depletion occurs due to thermal degradation, leading to a reduction in the BC’s oxygen-containing functional groups. The decrease in K element may be due to the washing of some soluble potassium compounds during the process of cleaning RS-K, or due to the reaction of K at high temperatures, which produces a gas, resulting in a decrease in K element content in BC (Figure 7f). During the high-temperature pyrolysis process of RS-Fe and 1RS-2Fe-1K, some elements may evaporate in gaseous form. For example, the C element may evaporate in the form of gases such as carbon dioxide and carbon monoxide, leading to a decrease in the C content in BC. Nitrogen-containing compounds may also decompose and evaporate at high temperatures, leading to a decrease in nitrogen content. Oxygen is released from BC in the form of O₂, reducing the content of the O element. The decrease in K element may be due to the washing of some soluble potassium compounds during the preparation of BC, or this may be due to the reaction of K at high temperatures, which produces a gas, resulting in a decrease in K element content in BC. The significant increase in Fe element indicates that Fe has been successfully loaded onto BC, and the magnetic properties of BC have increased (See Eqs. (1)-(3) and Figure 7g).

The reaction Eqs. (1)-(11) are as follows:

3.2.3. X-ray diffraction (XRD) analysis

The crystalline evolution of the material was systematically investigated through the XRD analysis at different preparation stages, as depicted in Figure 8. Distinct diffraction features at 22° and 36° confirm lignocellulose as the primary component in RS [29]. Both RS and RS-K exhibit a characteristic broad reflection at 40.6°, corresponding to graphitic carbon formation [30]. Activation-induced structural modifications are evident in RS-K, manifested by broader and less intense diffraction features. This indicates that the cellulose is decrystallized and subsequently converted to amorphous carbon. The XRD profiles of magnetic composites (RS-Fe and 1RS-2Fe-1K) demonstrate characteristic reflections at 35.7° (Fe₃O₄/γ-Fe₂O₃) [31], 64.9° (Fe₂O₃), and 82.3° [Fe (110), (300)] [32], which confirms the successful incorporation of iron species into the carbon matrix through the activation process.

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Figure 8.

XRD of different BC samples: (a) RS; (b) RS-K; (c) RS-Fe; (d) RS-Fe-K.

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3.2.4. FT-IR analysis

The surface chemical characteristics of RS and its derivatives (RS-K, RS-Fe, and 1RS-2Fe-1K) were investigated through FT-IR spectroscopy. Spectral analysis (Figure 9) revealed the distinct vibrational features: a broad band at 3414 cm^-1 corresponding to O-H stretching, while C-H stretching modes were identified at 2368 cm^-1 [33]. The region between 1560-1642 cm^-1 displayed characteristic absorption associated with C=O/C=C stretching vibrations, with an additional peak at 1060 cm^-1 indicative of aromatic C-O bonds [34]. Modified samples (RS-K, RS-Fe, and 1RS-2Fe-1K) exhibited additional spectral features, including C-H stretching vibrations near 2914 cm^-1 [35] and characteristic Fe-O stretching at 560 cm^-1 [23,36,37]. The presence of C≡C stretching vibrations around 2077 cm^-1 was observed in RS-K and 1RS-2Fe-1K. Furthermore, a weak but distinct absorption at 1217 cm^-1 confirmed Fe-O bond formation, which demonstrates successful iron incorporation. The quantitative analysis reveals significant changes in the intensities of various functional groups after activation (Table S3, Supporting Information). The increases in peak intensities for O-H, C-H, C=O, and Fe-O groups indicate enhanced surface functionalization and the successful introduction of additional active sites. This is consistent with our hypothesis that the co-activation process significantly modifies the surface chemistry of the BC composite, enhancing its adsorption capacity.

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Figure 9.

FT-IR of different BC samples: (a) RS; (b) RS-K; (c) RS-Fe; (d) RS-Fe-K.

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3.2.5. X-ray photoelectron spectroscopy (XPS) analysis

The surface composition of different BCs was studied using XPS analysis. The XPS spectra show that the surfaces of the samples are mainly composed of C, N, and O elements, which aligned well with the elemental analysis results (Table 1). The XPS spectrum shows that compared with the RS (Figure S1), the disappearance of the C-O-C group (on the C1s spectrum) and the N-H and N-C groups (on the N1s spectrum) of RS-K (Figure S2) may be due to the generation of gas [38]. However, for RS-Fe (Figure S3) and 1RS-2Fe-1K (Figure 10), the disappearance of the N-C group (on the N1s spectrum) may be due to two reasons: (1) In the preparation process of BC, pyrolysis is usually required at a certain temperature. If the temperature is high, nitrogen-containing compounds may decompose and evaporate in the form of gas. For example, nitrogen-containing organic small molecules such as amines and amides may decompose into gases such as nitrogen and ammonia at high temperatures and escape, resulting in a decrease or even disappearance of the N element content in BC. The disappearance of C-NH₂ characteristic peaks indicates that amine groups may participate in bonding interactions with TH, potentially through coordination or complexation reactions. According to the results of elemental analysis (Table S4, Supporting Information), the content of the N element decreases when K₂FeO₄ or KHCO₃ is added separately. With the increase of pyrolysis temperature and the prolongation of pyrolysis time, this volatilization effect will become more pronounced. (2) Under special conditions, such as high temperature, the N element may react with carbon element in BC to form carbon-nitrogen compounds. These carbon-nitrogen compounds may have high stability and exist in a form that is not easily detectable in BC, or undergo further reactions during subsequent processing, leading to the loss of N elements. The addition of C=O on the O1s spectrum may be due to the increase of oxygen-containing functional groups on BC [39]. The increase of Fe indicates that Fe has been successfully loaded on the surface of BC. The Fe in MB mainly exists in the mixed valence states of Fe³⁺ (Fe₂O₃/Fe₃O₄) and Fe²⁺ (FeO/Fe₃O₄), and its proportion is controlled by the pyrolysis temperature (for example, the proportion of Fe²⁺ can reach 35% at 600°C). Fe³⁺ coordinates with pollutants through ≡Fe-OH groups, while Fe²⁺ catalyzes the degradation of organic compounds through a Fenton-like reaction (Fe²⁺ + H₂O₂ → Fe³⁺ +·OH). The Fe²⁺-Fe³⁺ electron transition in Fe₃O₄↔ Fe³⁺ not only enhances the conductivity of the material and promotes electron transfer but also stabilizes the carbon matrix by forming Fe-O-C bonds.

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Figure 10.

XPS of 1RS-2Fe-1K: (a) total survey scans of XPS spectra; (b) C1s; (c) Fe2p; (d) N1s; (e) O1s.

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3.3.1. Effects of different sample dosages

The sample dosage has a great influence on the adsorption effect. Thus, different dosages were investigated, and the results have been shown in Figure 11(a). In Figure 11(a), with the increase of sample dosage, the removal rate shows a gradual increasing trend. This may be because increasing the dosage of 1RS-2Fe-1K can increase the number of functional groups involved in adsorption and the number of effective active sites, thereby improving the contact opportunity between 1RS-2Fe-1K and TH per unit volume of solution. However, as the dosage of 1RS-2Fe-1K gradually increased to 60 mg, the removal rate slightly decreased from 74% to 71%. This was because excessive samples lead to a decrease in the adsorption competitiveness of the unit adsorbent. However, when the dosage continues to increase above 80 mg, the increase in removal rate may be due to the high number of active sites, which provide more adsorption sites for antibiotic molecules [40]. Because there was not much difference in the removal rate between the dosage of 0.050 g (73.51%) and 0.10 g (75.83%), the former dose was chosen for subsequent experiments.

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Figure 11.

The impact of different conditions on removal efficiency: (a) different dosage; (b) different initial concentration; (c) different pH values; (d) coexisting ions.

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3.3.2. Effects of different initial concentrations

To evaluate the influence of initial concentration on removal efficiency [41,42], experiments were conducted across the varying concentrations, with results presented in Figure 11(b). The removal efficiency exhibited a nonlinear relationship with initial concentration. It initially increases before reaching a maximum (98.33% at 0.020 g/L) and subsequently decreases. This behavior can be attributed to the finite amount of available adsorption sites on the sample surface. At lower concentrations, the abundant adsorption sites prevent surface saturation, enabling efficient contaminant removal. As concentration increases, enhanced mass transfer drives more adsorption reactions, which improves removal efficiency until reaching the optimal conditions. However, beyond the optimal concentration, the limited availability of active sites becomes insufficient for complete contaminant adsorption, results in reduced removal efficiency. Based on these findings, subsequent experiments were performed using a 0.020 g/L TH solution concentration.

3.3.3. Effects of different pH values

The influence of solution pH value on the removal efficiency was systematically investigated, and the results have been presented in Figure 11(c). The removal efficiency demonstrated a pH-dependent behavior, which initially increases before reaching an optimum (98.95% at pH 5) and subsequently decreases. This phenomenon can be explained by the fact that the amphiphilic nature of TH and its pH-dependent speciation: TH⁺ predominates below pH 3.3, TH⁰ between pH = 3.3-7.7 [43], TH⁻ from pH = 7.7-9.7, and TH²⁻ above pH = 9.7 [29,44]. The adsorption behavior is further influenced by the surface charge characteristics of 1RS-2Fe-1K, which exhibits a pH_pzc of 5.93 (Figure S4). Under acidic conditions (pH < pH_pzc), protonation of the adsorbent surface creates favorable electrostatic interactions, enhancing TH adsorption. Conversely, when the pH values exceed the pH_pzc value, the surface deprotonation generates electrostatic repulsion, reducing adsorption capacity. As shown in the Figure S5, the Zeta potential decreased from +16.93 mV to -0.75 mV when the pH value increased from 4 to 6, indicating that the surface charge of the particles changed from positive to negative with the increase of pH. The point where the isoelectric point (IEP) potential approaches 0 is located between pH 5 and 6. Consistent with the zero charge point results mentioned above. Low pH (acidic), positively charged particle surface, possibly due to protonation (such as amino -NH₃⁺). High pH (neutral), charge is neutralized or reversed, possibly due to deprotonation (such as carboxyl –COO^- formation). Based on these findings, subsequent experiments were conducted at pH = 5 to optimize the removal efficiency.

3.3.4. Effects of coexisting ions

The effect of co-existing ions in solution on the removal rate was studied, and the results have been present in Figure 11(d). The selected co-existing ions are commonly found in natural and polluted water environments (such as Ca²⁺/Mg²⁺ representing hard water, Cl^-/SO₄^2- simulating wastewater salinity) [34].

From Figure 11(d), the K⁺ and CO₃^2- have inhibitory effects on the adsorption of TH for the adsorbent. It may be because the metal ions can bind to TH molecules and compete for the active sites. Some sites were occupied by the metal ions, thereby reducing the degradation rate of the TH solution. The NO₃^- and SO₄^2- promote the adsorption of TH for the adsorbent, possibly due to the electrostatic interactions. Moreover, Na⁺ and Ca²⁺ have no significant effect on the adsorption of TH for adsorbent [24].