Preparation of S-doped biochar with sodium thiosulfate as activator and sulfur source and its highly efficient adsorption for Hg²⁺

2.1 Chemicals and reagents

The reagents used in the experiment are analytical pure reagents, which have not been further purified. The mercury reserve solution with a concentration of 1 g/L was prepared by dissolving HgCl₂ (purchased from Jiangyan Huanqiu Chemical Reagent Company, Jiangsu Province) in pure water. Na₂S₂O₃·5H₂O, HCl and NaOH are provided by Tianjin Comel Chemical Reagent Co. Ultrapure water used in experiment was prepared by ULUPURE water purification system (Chengdu, Sichuan, China).

2.2 Synthetic approach of the SBC

The cherry kernel powder of 80 mesh placed in a corundum crucible was heated to 500 °C in a tubular furnace under nitrogen atmosphere at a heating rate of 10 °C/min, and then maintained for 2 h. The biochar (BC) was obtained and used in next experiments.

For activation and sulfur doping, 1.0 g BC was mixed with a certain amount of Na₂S₂O₃·5H₂O (0.5 g, 1.0 g, 2.0 g, 4.0 g, 5.0 g) and fully ground, and then transferred to a corundum crucible. The mixture was heated to 500 °C, 600 °C, 700 °C, 800 °C and 900 °C at a heating rate of 5 °C/min in nitrogen atmosphere, and then kept for two hours. The obtained material was washed with 1 mol/L HCl and deionized water until the pH value was 7, and then most of the water was removed by vacuum filtration. Finally, the materials were dried at 90 °C for 24 h, and marked as sulfur-doped biochar (S_XBC-Y, where X represents the weight ratio of BC to Na₂S₂O₃·5H₂O and Y represents the activation temperature). The ingredients list of S_XBC-Y is shown in Table 1. The yield was calculated by the ratio of final weight of product to the origin mass of BC.

2.3 Characterization of adsorbents

The specific surface area and pore size were analyzed by ASAP2460 instrument (Micromeritics Instruments Corporation, USA), and the samples were degassed in vacuum at 120 °C for 6 h before analysis. The specific surface area is calculated by Brunauer Emmett Teller (BET) equation and the pore size distribution is calculated by Barret Joyner Halenda (BJH) model. X-ray photoelectron spectroscopy (XPS) analysis adopts Thermo ESCALAB 250XI instrument (Thermo Fisher Scientific, USA), using monochromatic Al Ka source, and the binding energy is calibrated at C1s 284.8 eV. Scanning electron microscope (SEM, Hitachi, Japan) was used to observe the surface morphology of BC and S₄BC-800. X-ray diffraction (XRD) analysis is carried out on an Rigaku D/max-2500 instrument (Nippon Corporation, Japan), and the radiation source is Cu Ka ray, and the scanning range is 10–80° with the scanning speed of 10°/min. Fourier transform infrared spectroscopy (FT-IR) analysis adopts American PerkinElmer Spectrum FT-IR instrument, using potassium bromide tabletting method, and deducts potassium bromide background and air background. Raman spectrometer (Renishaw Invia, UK) was used to measure the Raman spectra of materials excited by 532 nm laser. Elemental analysis (C, H, S) was performed on an Elementar Vario EL III analyzer (Elementar Analysensysteme Comp., Hanau, Germany).

2.4 Batch adsorption experiments

3.1 Characterization of S_XBC-Y

The ratio of activator to raw BC may affect the morphology and pore structure of SBC materials, and SEM images of S_XBC-800 with different proportion of Na₂S₂O₃·5H₂O are shown in Fig. S1(a–d). It can be seen that the morphology of BC is relatively smooth, and seldom pores appear on the surface. With the weight ratio of Na₂S₂O₃·5H₂O increasing, the roughness and pores of the materials enhance obvious, indicating that as a chemical activator, more Na₂S₂O₃·5H₂O will create more pores by reaction with BC. Especially, when the weight ratio of Na₂S₂O₃·5H₂O to BC is 4:1 (Fig. S1c), well-developed macropores are generated with randomly distributed micropores in stacked and folded irregular layers. Fig. S1(d) shows that excessive Na₂S₂O₃·5H₂O (5:1) will result in the collapse of carbon skeleton structure and destroy of micropores, which may lead to the decrease of its adsorption performance. Thus, the weight ratio 4:1 of Na₂S₂O₃·5H₂O to BC is selected for next experiments.

Pyrolysis temperature is another key factor to affect the property of the SBC materials. Fig. 1(a–f) shows SEM images of BC and S₄BC-Y under different reaction temperature. Compared with BC, S₄BC-500 and S₄BC-600 exhibit some pores on the surface, but the pores are shallow and not interconnected each other, indicating the weak activation ability under low temperature. When the reaction temperature is elevated over 700 °C, a great deal of developed and inter-connected macropores with stacking and folding can be observed on S₄BC-700, S₄BC-800, and S₄BC-900, and even plenty of micropores and mesoporous pores occur on the side wall of macropores (Fig. S2), which will lead to high specific surface area and provide abundant active sites for adsorption. This is mainly because the pyrolysis and reaction with BC of Na₂S₂O₃ at high temperature produce hierarchical pore structure.

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Fig. 1

SEM images of (a) BC, (b) S₄BC-500, (c) S₄BC-600, (d) S₄BC-700, (e) S₄BC-800 and (f) S₄BC-900

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The specific surface area and pore size are further investigated, and the N₂ adsorption–desorption diagrams and pore size distributions are shown in Fig. 2 and Fig. S3. The N₂ adsorption–desorption diagram of BC conforms to type II of non-porous or macroporous adsorption structure (Qu et al., 2021), while that of S₄BC-800 belongs to type I, and at low relative pressure (P/P₀), it has a clear platform and a large N₂ adsorption capacity, and then a narrow hysteresis loop appears in the range of P/P₀ of 0.4–1.0, indicating that there are abundant micropores and a small number of mesopores and macropores (Li et al., 2016), which is consistent with the SEM results. Generally, activated carbon with definite pore size distribution can be produced by activation with chemical activator. For further proof, the pore size distribution of S₄BC is shown in inset of Fig. 2(b). It can be seen that all sulfur-doped activated carbons are bimodal, and the pore sizes mainly composed of micropores (<0.8 nm) and certain mesopores (2–3 nm), which is consistent with the adsorption isotherm of N₂. Table 1 shows the detailed information of specific surface area, element content, product yield and porosity. Macropores are very important if biochar is used as an adsorbent to remove pollutants in water or as an electrode in energy storage system. They act as carriers for the solution to pass through pores until it reaches micropores and mesopores (dos Reis et al., 2021, 2024). The prepared S₄BC has a certain amount of macropores and abundant micropores, which can further improve the adsorption of mercury ions. The yield of S₄BC decreased with the increase of preparation temperature, and it decreased from 92.61 % at 500 °C to 24.41 % at 900 °C. This is mainly because the reaction between carbon in BC and Na₂S₂O₃ will produce carbon sulfide and carbon oxide at high temperature, and the water and organic matter in BC will volatilize, resulting in carbon weight loss. The surface area of BC is very low (only 1.76 m²/g), indicating the lack of pore structure. When the BC is activated by Na₂S₂O₃ under different temperature, the surface area enhanced significantly, indicating the pivotal role of Na₂S₂O₃ for pore formation. With the reaction temperature increasing from 500 to 900 °C, the surface area of S₄BC boosts rapidly from 82.8 to 1303.1 m²/g, which may result from the strong reactivity between BC and Na₂S₂O₃ under high temperature, and the promotion of surface area will be conducive to improve the adsorption property of material. As for elemental composition, with the activation temperature increasing from 500 °C to 800 °C, the sulfur content is relatively steady (about 20 wt%), while it decreases sharply to 11.05 % when temperature up to 900 °C, which results from the pyrolysis of S-containing groups under too high temperature(Guo et al., 2018). Compared with previous studies on S-doped carbon (Guo et al., 2017; Park et al., 2014; Seredych and Bandosz, 2011; Yang et al., 2021; Zeng et al., 2018), S₄BC shows a high amount S-doping, which will contribute to create more adsorption sites. The results demonstrate dual functions of Na₂S₂O₃ on activation pore-forming and S-doping during S₄BC material preparation.

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Fig. 2

N₂ adsorption–desorption isotherms of (a) BC and (b) S₄BC-800, and (inset of b) pore size distributions of S₄BC-800

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According to the above description, it can be inferred that there are two main mechanisms for the activation of Na₂S₂O₃·5H₂O: Firstly, Na₂S₂O₃·5H₂O loses its crystal water and becomes anhydrous sodium thiosulfate; Then, Na₂S₂O₃ will consume a certain amount of BC to generate hydrogen sulfide, sulfur dioxide, water, sodium sulfate, and oxygen. Part of the gas rushes out of BC, causing BC to break or form larger holes for preliminary pore-making, and in this process, S atoms are bonded to the carbon skeleton through covalent bonds, which is partly carried out under the catalysis of alkali metal Na. In the second part, Na⁺ is reduced to free metal Na, and it enters the carbon grid. Through the insertion of metal Na, the carbon grid expands, and the inserted substances in the carbon matrix are quickly removed, thus forming a large number of pores. After the residue is removed by acid washing, numerous micropores are formed in the carbon framework to complete the process of activation and pore-making.

The XRD patterns of BC and S₄BC are shown in Fig. 3(a). It can be seen that all samples have two broad diffraction peaks near 24° and 44°, which correspond to the characteristics of (0 0 2) and (1 0 0) planes of graphite (Ma et al., 2013), indicating that graphitization occurred in the process of carbonization and activation of these materials. The relatively low graphitization degree of S₄BC indicates that the addition of sulfur may destroy the graphitization structure of BC to some extent, thus increasing the number of defects, which may generate more active sites and promote the adsorption performance. Compared with BC and S₄BC-500 samples, the diffraction peaks of impurities in S₄BC materials prepared at other temperatures are not obvious, indicating that high activation temperature is helpful to remove impurities. The sum Raman spectra of BC and S₄BC also show that sulfur doping can deform the carbon skeleton and produce more defects, which can enhance the adsorption performance of S₄BC for mercury ions (Text S1 and Fig. S4).

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Fig. 3

(a) XRD and (b) FT-IR spectra patterns of BC and S₄BC

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The FT-IR spectra of BC and S₄BC also confirmed the successful doping of sulfur. As can be seen from Fig. 3(b), the FT-IR spectra of all materials show a peak at 3800 cm⁻¹, which is attributed to the joint action of O—H bond and O—H stretching vibration (Li et al., 2018a). In comparison, the tensile vibration from O⚌S⚌O (1155 cm⁻¹) is observed only at SBC, which was caused by sulfur oxide (Guo et al., 2018), and the peak is apparent when the pyrolysis temperature is below 800 °C, while it shrinks sharply under temperature of 900 °C, indicating the decomposition of the S-containing groups at high temperature, which is in accord with the decrease of sulfur content from elemental analysis (Table 1). The S₄BC-500 shows the C-S tensile vibration peak located at 785 cm⁻¹ from thiophene sulfur (Uygun et al., 2009), while the peak disappears when the pyrolysis temperature is over 600 °C due to the destroy of the groups. C—O and C—C stretching vibration on saturated carbon at 2668 cm⁻¹, O—C—O stretching vibration at 1577 cm⁻¹ and C⚌O stretching vibration peaks at 2102 cm⁻¹ are also observed, which indicate that the prepared SBC adsorbent contained abundant oxygen-containing functional groups on the surface of the material. Sulfur and oxygen functional groups will be conducive to coordinate with Hg²⁺.

Generally speaking, the adsorption properties of carbonaceous materials are closely related to their chemical composition and surface properties, especially the functional groups containing nitrogen and sulfur (Wang et al., 2015; Yang et al., 2012), so the elemental composition and chemical valence states of S₄BC and BC were characterized by XPS. As shown in Fig. 4(a), the XPS survey spectrum of BC exhibits C and O elements, and no S peak is detected, indicating that the raw material of cherry stone contains no S element. While all S₄BC materials show peaks of C, O and S element. The C peak appears because BC and S₄BC are mainly composed of C, and the S peak in S₄BC demonstrates that S element has been successfully doped into carbon skeleton. In order to highlight the surface C, O, and S configurations in the sample, the high-resolution XPS spectra of C 1s, O 1s and S 2p (S₄BC-800) are deconvolved and divided into several peaks. As shown in Fig. 4(b), the high-resolution spectrum of C 1s has four peaks at 284.6, 285.6, 287.3, and 290.02 eV, which result from graphite and aliphatic carbon (C—C/C⚌C, 72.16 %), alcohol or ether group (C—O, 14.93 %), carbonyl group (—C—O—, 6.13 %), and carboxyl or ester group (O—C⚌O, 6.78 %), respectively (Li et al., 2018b). In Fig. 4(c) of O 1s, peaks were observed at 530.68 eV, 531.78 eV, and 532.68 eV, corresponding to various oxygen-containing functional groups of C-O (45.41 %), O—C⚌O (50.55 %) and C⚌O (4.04 %), respectively (Barczak et al., 2015). As shown in Fig. 4(d), the spectrum of S 2p can be deconvolved into three peaks. The peak at 164 eV just splits into two characteristic peaks, namely, 163.98 eV (2p_3/2, 50.59 %) and 165.09 eV (2p_1/2, 21.86 %). Another fitting peak with higher binding energy (168.58 eV) can be attributed to sulfur oxide (—C—SO_x—C, x = 2,3,4, 27.55 %), which indicates that sulfur atoms are successfully bound to the carbon framework(Chen et al., 2018). Moreover, these electron-withdrawing groups can adjust the charge distribution of adjacent carbon atoms and improve the adsorption capacity of carbon materials for Hg²⁺.

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Fig. 4

(a) XPS survey spectra, (b) High-resolution spectra of C 1 s, (c) O 1 s, (d) S 2p of S₄BC-800.

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3.2 Mercury adsorption performance

3.2.1

3.2.1 Effect of synthesis conditions of materials on Hg²⁺ adsorption

SBC adsorbents synthesized with different amount of Na₂S₂O₃ was applied for Hg²⁺ adsorption. The concentration of Hg²⁺ was set at 100 mg/L and the pH value varied from 3 to 7. As shown in Fig. 5(a), as the ratio of BC to Na₂S₂O₃ gradually increased from 1:0.5 to 1:4, the adsorption capacity of the prepared materials for Hg²⁺ also gradually increased in the range of pH 3–7. The reason may be that the doped sulfur atoms own the characteristics of large atomic radius, low electronegativity, lone pair electrons for coordination bonds, which can introduce more active sites on the surface of BC. Moreover, due to the high stability constant between S atom and soft acid metal, it greatly contributes to the adsorption of Hg²⁺ by SBC (Lee et al., 2017). Therefore, when the ratio of Na₂S₂O₃ is low, less S atoms are doped into caron frame, resulting in lower adsorption capacity. However, when the ratio further increases from 1:4 to 1:5, the adsorption capacity declines unexpectedly. The reason is that too much Na₂S₂O₃ will corrode most carbon element to produce carbon disulfide and carbon oxide, which leads to the reduction of sulfur content and product yield, thus reducing the adsorption property for Hg²⁺. Therefore, 1: 4 was chosen as the optimal ratio for the subsequent material preparation.

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Fig. 5

Hg²⁺ adsorption on (a) S_XBC-800 and (b) S₄BC-Y under different solution pH

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Activation temperature is another key factor to affect material property, so S₄BC adsorbent prepared under different pyrolysis temperature for Hg²⁺ intake is investigated. As shown in Fig. 5(b), the adsorption properties of all materials are reduced at low pH, which should be attributed to the competitive adsorption of hydrogen ions (Li et al., 2017). With the increase of synthesis temperature from 500 °C to 800 °C, the adsorption capacities of S₄BC exhibit obvious rising trend, and the S₄BC-800 owns superior adsorption efficiency to other materials, but it reduces sharply when pyrolysis temperature further increases to 900 °C. The Hg²⁺ adsorption performance on these materials can be well cleared up by above mentioned S content and BET specific surface area (Table 1). The S content of materials under temperature of 500–800 °C exhibit no apparent difference besides of a little fluctuation, but their specific surface area boosts rapidly, so the exposed active sites corresponding to S functional groups will enhance, thus resulting in the increase of adsorption capacity. Although the S₄BC-900 material attained the highest specific surface area of 1303.1 m²/g, the S content decreases sharply from around 20 % to 11.05 %, which may result from decomposition of functional groups containing S element under excessive high temperature. Thus, the reduction of active sites leads to the decrease of adsorption capacity. Based on the above analysis, S₄BC-800 adsorbent is selected for the subsequent experiments. Previous studies have proved that when the pH value exceeds 7, mercury (II) will precipitate in the solution (Inbaraj and Sulochana, 2006), which cannot be clear what causes the disappearance of mercury (II), so we chose 6.5 as the best pH value in the follow-up experiments.

3.2.2

3.2.2 Adsorption isotherms

Adsorption isotherm describes the adsorption behavior of adsorbents for target pollutants at different temperatures and initial concentrations, and adsorption capacity is one of the most important indexes to evaluate adsorbents (dos Reis et al., 2023). In order to study the adsorption isotherm, the initial concentration of Hg²⁺ varied from 20 to 300 mg/L, the temperature was set at 293 K, 303 K and 313 K and the pH value was 6.5. As is shown in Fig. 6(a), the adsorption capacity enhances with the increase of Hg²⁺ concentration, and finally attains equilibrium. The increase of initial metal ions concentration will accelerate the chelation balance between adsorbents and target ions to form coordination compound, thus improving the adsorption capacity (Song et al., 2017). With temperature rising from 293 K to 313 K, the adsorption capacity of S₄BC-800 for Hg²⁺ increase obviously, indicating the endothermic character of the adsorption.

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Fig. 6

(a) Adsorption isotherm of Hg²⁺ by S₄BC-800 (b) Fitting curve of adsorption isotherm of pollutants by S₄BC-800.

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Langmuir and Freundlich adsorption isotherm models were used to fit the equilibrium adsorption data. Langmuir model is usually used for monolayer adsorption, in which a large number of adsorption sites have the same affinity with the adsorbate, while Freundlich model is mainly used to describe heterogeneous adsorption process (Wu et al., 2022). Fig. 6(b) showed the Langmuir linear fitting of Hg²⁺ adsorbed on S₄BC-800 adsorbent, showing a good linear relationship. Table 2 summarizes the relevant parameters and constant values calculated by the above isothermal adsorption equation. It can be seen that R² of Langmuir model is closer to 1 than that of Freundlich model, indicating that Langmuir model is more suitable for fitting adsorption data. It shows that the adsorption of mercury by S₄BC-800 is monolayer adsorption. The maximum adsorption capacity of Hg²⁺ calculated by Langmuir model is 389, 448, and 724 mg/g at 293, 303, and 313 K, respectively, which is higher than that of various adsorbents in Table S3. The results demonstrate that the S₄BC-800 adsorbent possesses great application potential for the elimination of aqueous Hg²⁺ pollutants.

Table 2 Adsorption isotherms parameters of the Langmuir and Freundlich model for the adsorption of Hg²⁺ on S₄BC-800.

Temperature (K)	Langmuir model			Freundlich model
	θ (mg/g)	b (g/mL)	R2	KF (mL1/nµg1−1/n)	n	R2
293	389	0.048	0.992	6.64	3.00	0.968
303	448	0.595	0.998	8.20	2.84	0.850
313	724	0.052	0.998	6.52	2.12	0.882

3.2.3

3.2.3 Desorption and regeneration

The regenerative performance is an important factor during practical application of adsorbent (Jiang et al., 2020), and it is very important to achieve efficient reuse after Hg²⁺ adsorption on S₄BC-800. Therefore, recycling adsorption experiments were carried out to observe the reusability of S₄BC-800. In order to reuse the adsorbent, it is necessary to effectively desorb the adsorbed Hg²⁺ without destroying the adsorbent. Thus, after each adsorption test, the adsorbent was eluted with 5 mL of 0.2 mol/L HCl containing 3 % thiourea (w/t%), then washed with ultra-pure water and dried in vacuum at 50 °C for reuse. As shown in Fig. 7, the adsorption capacity of the S₄BC-800 was 280 mg/g at the first use, and then decreased to 270 and 260 mg/g at the second and third reuse, respectively, and finally remained stable at the following reuse. When the adsorbent is reused for the first and second time, the adsorption capacity of Hg²⁺ exhibits a slight decrease, which may result from the loss of a small number of sulfur-containing groups that are not firmly fixed on the adsorbent. The remaining sulfur-containing groups are firmly combined with the material and are not easily destroyed by eluent, so the adsorption capacity remains unchanged in the subsequent reuse process. After recycling for 6 times, the adsorption capacity still maintained over 250 mg/g, which showed that the S₄BC-800 adsorbent owned good reusability and great practical application potential.

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Fig. 7

Effect of S₄BC-800 reuse on Hg²⁺ adsorption capacity.

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3.3 Adsorption mechanism and environmental significance

In order to understand the adsorption mechanism, the XPS analysis of S₄BC-800 adsorbed Hg²⁺ was carried out. It can be seen from Fig. 8(a) that two Hg 4f peaks after adsorption are clearly displayed in the survey spectrum, indicating that a large amount of Hg²⁺ are adsorbed on S₄BC-800. Specifically, as shown in Fig. 8(b), two peaks at 104.95 eV and 101.07 eV belong to Hg 4f_7/2 and Hg 4f_5/2, which proves that Hg exists in its metallic form. Therefore, the result shows that Hg²⁺ is reduced to Hg(0) at a certain moment in the adsorption process (Arshadi et al., 2018). In Fig. 8(c), however, the binding energy peaks of O 1s atoms move from 530.68, 531.78, and 532.68 eV to 531.74, 532.58, and 532.83 eV, which shows that oxygen-containing groups play a role in Hg²⁺ removal by electrostatic interaction between oppositely charged ions. That is, positively charged functional groups can interact with Hg²⁺ through cation exchange reaction (Bao et al., 2017). In Fig. 8(d), after adsorption of Hg²⁺, the binding energy of S 2p shifts from 164.00 eV to 164.56 eV, and the alteration is due to the sharing of lone pair electrons S atoms with Hg²⁺ (Zhang et al., 2013). According to the HSAB, soft base can combine with soft acid to form a stable complex. Hg²⁺ has relatively large ionic size, low electronegativity and high polarizability, which are all characteristics of soft acids (Hakami et al., 2012), while as a soft base, sulfur atoms with lone electron pairs belong to soft Lewis base sites (Wang et al., 2019b). Therefore, the basic sulfur groups have strong coordination with acidic Hg²⁺, and it is easy to form strong Hg-S bonds through covalent bonds (Delacote et al., 2009; Ram and Chauhan, 2018; Wajima and Sugawara, 2011), which leads to the decrease of the contents of S 2p_3/2 and S 2p_1/2 from 50.59 % and 21.86 % to 46.92 % and 17.75 % respectively. However, the —C—SO_X—C in S₄BC-800 increased from 27.55 % to 35.33 %, which indicated that the sulfur oxides produced during adsorption adhered to the surface of S₄BC-800. Based on the above results, we can conclude that the adsorption of Hg²⁺ on S₄BC-800 material is a chemical adsorption process, which is driven by coordination, ion exchange, and electrostatic interaction between the oxygen and sulfur-containing groups of S₄BC-800 material and Hg²⁺.

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Fig. 8

(a) XPS survey spectra of used S₄BC-800.High-resolution (b) Hg4f(c) O 1s and (d) S 2p spectra of used S₄BC-800.

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Although the S₄BC-800 material exhibits excellent adsorption performance, its feasibility for low level Hg²⁺, which usually presents in actual polluted water, and dynamic operation should be further examined. Thus, a microfiltration membrane device base on S₄BC-800 (Fig. 9a) has been designed for removal of low concentration of Hg²⁺ (10 μg/L), and the results are shown in Fig. 9(b). When Hg²⁺ solution passes through PVDF membrane without S₄BC-800, the removal rate of Hg²⁺ is almost negligible, indicating that PVDF membrane hardly adsorbs Hg²⁺. When Hg²⁺ solution passes through the PVDF membrane supported by S₄BC-800, most of Hg²⁺ is adsorbed on S₄BC-800, showing excellent adsorption performance for low concentration Hg²⁺. It shows that even after 1 L water sample passes through the membrane, the residual Hg²⁺ concentration of the filter membrane loaded with 0.4 g S₄BC-800 is still zero. With the increase of water sample loading, the residual Hg²⁺ in the filter membrane loaded with 0.05 g and 0.1 g adsorbents increase slightly by 3.3 and 1.8 μg/L, respectively, which were far below the maximum allowable concentration of total mercury (50 μg/L) in the comprehensive wastewater discharge standard of China (Liu et al., 2021). Although the water sample stays in the active layer of the membrane for a short time, it can still obtain high removal efficiency of Hg²⁺, which is attributed to the excellent adsorption capacity and rapid adsorption rate of S₄BC-800. The feasibility of the designed water treatment device based on S₄BC-800 adsorbent provides a beneficial exploration for the purification of Hg²⁺ polluted water (Fig. 10).

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Fig. 9

Adsorption mechanism schematic diagram of S₄BC-800 for Hg^2+.

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Fig. 10

(a) Membrane filtration system, (b) Hg²⁺ removal performance of S₄BC-800 /PVDF membrane

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