Preparation of BC and SBC

The woody peat used in this experiment was acquired from Fafard, Canada. This type of peat is composed of brown, fibrous, porous moss. The woody peat was dried, crushed, and sifted through a 0.9 mm sieve. Afterwards, it was washed three times with neutral deionized water to remove impurities. The peat was then extracted, filtered, and dried in a 60°C blast drying oven for eight hours. After cooling to room temperature, it was stored in a self-sealing bag for future use.

The powder was placed in a rectangular porcelain boat with nitrogen and then transferred to a muffle furnace, where it was heated at a rate of 10 °C/min until it reached 600°C. The powder was held at this temperature for 2 hours before being allowed to cool to room temperature. It was subsequently removed and stored in a self-sealing bag for further analysis as BC.

The sodium hydroxide solution (0.4 mol/L) and carbon disulfide (CS₂) were combined in a 2:3 ratio and stirred on a magnetic stirrer at 25°C for 4 hours. The resulting modified solution was obtained via ultrasonication for 1 hour. Next, 10 g of BC was added to 100 ml of the modified solution at a solid-liquid ratio of 1:10, stirred on a magnetic stirrer at 40°C for 24 hours, removed, and dried in a blast drying oven at 60°C for 12 hours to obtain SBC.

Characterization

The soil was measured in deionized water at a ratio of 1:2.5 (W/V) using a pH meter (PHS-3E, Shanghai Jingke Lei Magnetic Instrument Factory). The element analyzer (Vario EL, Germany Elementa) was used to analyze the contents of C, N, H, and S in BC. The ash content was determined by heating the sample at 600°C for 2 hours. The specific surface area was measured using a specific surface and pore size analyzer (Quadrasorb EVO, Anton Paar Instruments, USA). The specific surface area and porosity of biochar prior to and after modification were calculated using the BET model at 200°C, utilizing a N₂ adsorption-desorption curve of 77 K with an automatic gas adsorption instrument. Surface functional groups, elemental composition, and binding state of biochar before and after adsorption were analyzed by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS, ESCALAB 250 Xi). The crystalline minerals on the surface of the biochar were analyzed using the D8 Advance X-ray diffractometer from Bruker AXS in Germany. For improved clarity and concision, technical term abbreviations were explained upon their initial use. Additionally, all language variants adhere to American spelling, grammar, and style conventions. The surface morphology was observed via a scanning electron microscope (GeminiSEM 300, Carl Zeiss, Germany). The content of available cadmium in the soil was determined by using inductively coupled plasma mass spectrometry (ICP-MS) with diethylene triamine pentaacetic acid (DTPA) as the solvent.

Adsorption Kinetics and Adsorption Isotherms

One hundred milliliters of Cd(II) solution with concentrations ranging from 0 to 100 mg/L were placed into conical flasks with a capacity of 250 ml. Each treatment was repeated three times. A 0.01 mol/L NaNO₃ solution was used as the electrolyte, and the pH was adjusted to 6. The flasks were thoroughly mixed and then placed on an oscillator at 25°C and 180 r/min for 24 hours. The solutions underwent filtration through a 0.45 μm membrane, and the concentrations that remained were measured through ICP-MS analysis.

In the formula: C₀ – The initial concentration of Cd(II), mg/L; C_e – The concentration of Cd(II) at adsorption equilibrium, mg/L; V – Volume of solution, L; m – Biochar quality, g; E – The removal rate of Cd(II), %.

In the formula: Q_e – Adsorption capacity of Cd(II) at adsorption equilibrium, mg/g; Q_t – Adsorption amount of time, mg/g; k₁, k₂ – Rate of adsorption;

In the formula: Q_m – The maximum adsorption capacity, mg/g; b – Langmuir constant; K_f – Freundlich constant; n – Adsorption strength constant.

Soil Culture Experiment

The air-dried and sieved soil samples were supplemented with an aqueous solution of CdCl₂·2.5H₂O at a concentration of 10 mg/kg. The samples were evenly stirred and cultured, with regular stirring occurring every 7 days for 28 days. Soil water content was maintained at 75%. After the culture period, the total amount of Cd in the soil was 7.5 mg/kg. To investigate the impact of different amounts of SBC on the remediation of Cd-contaminated farmland, we added BC and SBC to 100 g of soil contaminated by Cd in plastic flowerpots at the ratios of 0.5%, 1.0%, 2.0%, and 5%. Each treatment was repeated three times. The passivation material was evenly mixed with the soil and cultured at room temperature (25°C) for 30 days while maintaining a soil water holding capacity of 70% during the culture period. After a 30-day period of culturing, the soil was allowed to dry naturally, then ground and transferred to a self-sealing bag for testing.

To further study the remediation effect of BC and SBC on Cd-contaminated soil, the best addition amount of peat biochar before and after modification on the remediation effect of Cd-contaminated farmland was selected according to the soil culture test. Three treatment trials were set up: CK, 1.0% BC, and 1.0% SBC, and each treatment was replicated three times. 1.0% BC and 1.0% SBC were added to plastic pots containing 1.5 kg of Cd-contaminated passivated soil with phosphorus fertilizer, potash, and urea at 1.25 mg/kg, 0.158 mg/kg, and 0.324 mg/kg, respectively, mixed evenly with the soil, and incubated for 30d. In each pot, 100 full ryegrass seeds were selected, soaked in a 2% H₂O₂ solution for 2 h, and then removed. The ryegrass seeds were washed three times with distilled water, placed on a culture dish with a filter membrane, and germinated in the dark at room temperature. After 3 days, the young buds were transplanted into the pot soil for growth. During the period, the soil moisture content was maintained at 70%, and the ryegrass was harvested after 50 days.

TGA Analysis

As shown in Fig. 1, the results of the TG/DTG curve indicate that the pyrolysis process of BC can be categorized into three evident pyrolysis stages. The initial stage temperature range is from 30 to 230°C, during which the dehydration and drying stages are predominantly due to cell tissue and surface tension in the woody peat raw material. Gradually increasing temperature causes free water in the internal free state to be rapidly desorbed [21]. During the second stage, woody peat raw materials experience the highest mass loss at a temperature range of 230-600°C. This is primarily due to the pyrolysis of various elements, such as carbon, cellulose, lignin, fatty acid, humic acid, and minerals found in organic matter [22]. After reaching a temperature range of 600°C in the third stage, the weight loss rate of BC stabilizes and remains unchanged with further temperature increases. This indicates that BC completes pyrolysis nearly entirely at 600°C. The combustion reaction of organic residues produces biochar abundant in carbon and with favorable pore structure, which enhances Cd adsorption. Research has indicated that the quantity of functional groups on the surface of biochar decreases with rising pyrolysis temperatures [23]. Therefore, the pyrolysis temperature of BC was 600°C in this experiment.

Fig. 1.

TG / DTG curve of woody peat raw materials.

SEM and EDS Analysis

As depicted in Fig. 2a, the surface of BC is smooth, and there is no coarse white crystal particle precipitation. In Fig. 2c, the surface of SBC is rough, the stripe distribution is obvious, and there are white crystals. This may be because woody peat is loaded with sulfur and attached to the surface of BC. From the EDS diagrams of Figs. 2a and c, the sulfur content is increased from 0% (Wt) to 1.06% (Wt). It is further shown that after sulfur modification of BC, the sulfur element is successfully loaded on the surface of the material. Compared with Fig. 2a and c, the surface particles and white crystals of Fig. 2b and Fig. 2d increased and became rougher, which may be due to the co-precipitation, adsorption, chelation, and complexation of Cd with functional groups on the surface of biochar after adsorption to form new compounds attached to the surface of biochar, such as Cd (OH)₂, CdS, and CdCO₃ [24]. It can be seen from the EDS diagram that the absorption characteristic peaks of Cd in Fig. 2a and c were weak before adsorption, and the EDS characteristic peaks of Fig. 2b and d became stronger after adsorption, and the Cd content increased to 2.15% and 6.58%, respectively.

From Table 1, the content of C, N, and O in SBC decreased, while the content of H and S increased. The content of the S element increased from 0.09% to 2.09%, which was 22 times more than BC. The EDS diagram also showed that the sulfur content on the surface of SBC increased. Studies have shown that the sulfur content on the surface of biochar increases to 0.7% after sulfur modification [25]. The atomic molar ratio of H/C in BC was 0.063. After sulfur modification, the atomic molar ratio of H/C was 0.07, and the relative aromaticity increased after modification. The atomic molar ratio of H/C is an indicator of aromaticity and polarity [26]. The molar ratio of BC and SBC is less than 0.5, indicating that the adsorption characteristics of biochar have strong carbonization and high aromaticity [27]. The specific surface area and pore volume of BC and SBC were 2.86 m²/g, 0.0015 cm³/g, and 4.74 m²/g, 0.0044 cm³/g, respectively. The specific surface area and pore volume of SBC increased by 65.6% and 193.3%, respectively.

FTIR Analysis

Studies [28] have shown that the type and strength of functional groups on the surface of biochar have a great influence on the performance of chemical adsorption of heavy metals. The more functional groups on the surface of biochar, the stronger the ion exchange capacity of heavy metals and the better the ability to adsorb Cd(II).

The surface functional groups of BC mainly include: 3687-3000 cm^-1 (N-H and O-H stretching vibration absorption peaks [29]), 2915 cm^-1 (-CH₂ aliphatic stretching vibration peak [30]), 2848 cm^-1 (-CH causes symmetrical stretching vibration absorption), 1560 cm^-1 (C=C double bond caused stretching vibration [31]), 1415 cm^-1 (COO^- caused stretching vibration absorption [32]).

The surface functional groups of SBC mainly include: 3687-3000 cm^-1 (N-H and O-H stretching vibration absorption peak [33]), 2915 cm^-1 (-CH₂ aliphatic stretching vibration peak [34]), 2854 cm^-1 (-CH caused symmetric stretching vibration absorption [35]), 1567 cm^-1 (C=C double bond caused stretching vibration [36]), 1410 cm^-1 (COO-caused stretching vibration absorption), 1240 cm^-1 (C=S double bond absorption vibration peak [37]). 1020 cm^-1 (C=S double bond absorption vibration peak [38]), 880 cm^-1 (C-O caused stretching vibration peak [39]).

Fig. 2.

SEM-EDS before and after modification and adsorption of woody peat biochar at 600°C. (a) Woody peat biochar BC; (b) BC-Cd, after adsorption of Cd by woody peat biochar; (c) SBC, sulfur-modified woody peat biochar; (d) SBC-Cd, sulfur-modified biochar after adsorption of Cd.

As depicted in Fig. 3, after the modification of woody peat by sulfur, the range of 3687-3000 cm^-1 becomes wider, the amplitude becomes stronger, and the functional groups of C=S double bond and C-O double bond are increased. The number of functional groups on the surface of SBC and the adsorption strength increased significantly, and the peak absorption peak shifted to the left on the original basis. After sulfur modification, the C-O- induced stretching vibration peak appeared at 880 cm^-1 of BC, which helped to increase the adsorption sites of biochar for heavy metals [40]. In addition, the absorption peak of the Fe-O functional group appeared at 460 cm^-1, which increased the ion exchange capacity of Cd(II), thus improving the adsorption performance of Cd(II) [41].

After the adsorption of Cd(II) by BC, some peaks changed significantly. The C=C double bond group at 1560 cm^-1 shifted to 1585 cm^-1, and the symmetrical absorption peak group was more obvious. After the adsorption of Cd(II) by sulfur-modified biochar, the original peaks at 1567, 1020 and 880 cm^-1 were moved to 1583, 1030, and 865 cm^-1, respectively. This may be due to the complexation and Π-Cd²⁺ interaction between aromatic C=C, C=S groups, C-O bonds, and Cd(II) in sulfur-modified biochar to form CdS precipitates [42]. In addition, after the adsorption of Cd(II), the peak shifts at 1410 and 880 cm^-1 were weakened, which may be due to the adsorption of Cd(II) by COO- and C-O bonds [43].

XRD Analysis

According to the standard card (ICSD 98-061-9760), BC has an amorphous diffusion diffraction peak at 27.89° (Fig. 5), and the amorphous graphite C crystal plane appears according to the standard card comparison. Compared with the standard card (CaCO₃: PDF # 83-0577), the diffraction peak crystal plane of CdCO₃ appeared at 50.5° in BC-Cd and SBC-Cd, indicating that CdCO₃ crystals were precipitated on the surface of the material after adsorption of Cd by BC and SBC. According to the standard card (ICSD 98-004-2550), an S diffraction peak appeared in SBC, indicating that S was loaded on the surface of BC after S modification. Compared with previous studies [51], CdCO₃, CdSO₄, and CdS diffraction peaks appeared after SBC adsorbed Cd, indicating that SBC adsorbed Cd by precipitation complexation.

Fig. 5.

XRD patterns of BC and SBC before and after adsorption of Cd.

Fig. 6.

Adsorption kinetics for Cd (II) adsorption by BC and SBC.

Adsorption Kinetics

In Fig. 6, when the initial concentration was set to 40 mg/L, the adsorption capacity of BC and SBC increased rapidly in the early stage of adsorption and then increased slowly, reaching adsorption equilibrium after saturation at 90 min and 100 min, respectively. The adsorption capacity of BC and SBC increased rapidly in the early stage of adsorption, which was mainly related to the mass transfer drive of Cd²⁺ caused by the adsorption sites on the surface and the initial concentration difference between water and biochar [52]. The ability of SBC to adsorb Cd is about 6 times that of BC (Table 2). Since the second-order adsorption kinetic model R² = 0.973 - 0.987 and the first-order adsorption kinetic model R² = 0.969 - 0.971, the second-order kinetic model is used to fit the adsorption process of Cd. The second-order kinetic model fitted the adsorption process of Cd better, indicating that the adsorption of Cd by BC and SBC was chemical adsorption [53]. The K² value of SBC is greater than that of BC, indicating that the adsorption rate of Cd by SBC is faster than that of BC, which also indicates that the specific surface area and the number of adsorption sites on the surface of BC are increased after sulfur modification[54].

Adsorption Isotherm Analysis

To study the adsorption effect of BC and SBC on Cd at different concentrations, Langmuir and Freundlich models were used to fit the adsorption of Cd. The fitting parameters are shown in Table 3. R² of the two adsorption models are 0.994 - 0.998 and 0.980 - 0.991, respectively, indicating that the Langmuir model is more suitable for the fitting of the Cd isothermal adsorption effect, which indicates that the adsorption process of Cd by the material belongs to monolayer adsorption [55]. The adsorption sites, oxygen-containing functional groups, and elemental sulfur on the surface of SBC materials increased significantly, which significantly improved the adsorption capacity of Cd. When the solution concentration was less than 100 mg/L, the Qe of SBC (99.866 mg/g) was significantly higher than that of BC (41.982 mg/g), indicating that the adsorption capacity of SBC was higher than that of BC.

Effect of the Available Cd Concentration and pH in Soils.

After 30 days of incubation with different amounts of BC and SBC and passivation treatment, the soil pH values were affected to varying degrees (Fig. 8). BC and SBC were put into the soil at 0.5%, 1%, 2%, and 5%, respectively, and the pH showed a significant first rising trend. Compared with CK treatment, after BC treatment, the pH increased by 1.3%, 10.2%, 5.8%, and 6.9%, respectively. After SBC treatment, compared with CK treatment, it increased by 5.3%, 12.5%, 7.9%, and 9.8%, respectively. Among them, after adding 1% SBC, the soil pH increased by 12.5%, and the soil pH was 7.52. This shows that the change in pH is not only related to the addition amount, but also to the material itself. BC and SBC are alkaline materials that are applied to the soil and neutralize the H⁺ in the soil colloid to reduce the amount of H⁺ in the soil solution and increase the soil pH. The soil pH gradually increased with the increase in dosage, but when the dosage increased to a certain extent, the increase in soil pH gradually weakened, which may be related to the buffering degree of the soil. In addition, the higher content of sulfur in SBC may lead to soil acidification and salinization. This result is basically consistent with previous studies [56].

After 30 days of BC and SBC passivation culture, the available Cd content in the soil was affected to varying degrees (Fig. 9). After BC and SBC were applied to the soil at addition amounts of 0.5%, 1%, 2%, and 5% for 30 days, the content of available Cd in the soil showed a trend of decreasing first and then increasing. Compared with CK treatment, the available Cd content decreased by 14.8%, 20.7%, 15.5%, and 20.4%, respectively, after BC treatment. After SBC treatment, the available Cd content in the soil decreased by 19.6%, 25.7%, 21.9%, and 19.7%, respectively. Among them, after 1% SBC treatment, the available Cd content in soil decreased the most, only 4.01 mg/kg, and the difference was significant compared with CK treatment (P < 0.05). In addition, in the same proportion of BC and SBC addition amount treatments, the available Cd content in the soil after SBC treatment was significantly lower than that after BC treatment. This shows that after sulfur modification of BC, the surface functional groups and adsorption mechanism of biochar were changed. When SBC was applied to the soil, the organic sulfur loaded on the surface of biochar chelated, complexed, and coordinated with Cd in the soil, thus forming CdS precipitation and reducing the bioavailability and fluidity of Cd [57]. In addition, studies have shown that metal sulfides reduce the mobility of their heavy metals in soil and are less soluble and less susceptible to oxidation than metal oxides [58]. Previous studies have shown that sulfur reacts with Cd to form CdS precipitates under flooding conditions. Under aerobic conditions, the oxygen in the soil oxidizes the sulfur element into sulfate. When the oxygen in the soil is reduced by other conditions, the sulfate in the soil is reduced to organic sulfur under anaerobic conditions, which reacts with Cd in the soil to form CdS precipitation [59]. In addition, sulfate-active bacteria in soil microorganisms produce hydrogen sulfide through their own physiological mechanisms, change soil Eh, reduce sulfate in soil to sulfide, and increase the adsorption performance of Cd in soil [60].

Fig. 7.

Adsorption isotherms of Cd (II) by BC and SBC.

Fig. 8.

Effect of different treatments on the pH in soils.

The above results showed that the addition of BC and SBC could not only increase soil pH, but also reduce the available Cd content and biological mobility in soil. Among them, SBC had a better effect on soil pH improvement and the largest decrease in soil available Cd content.