Chemical speciation distribution, desorption characteristics, and quantitative adsorption mechanisms of cadmium/lead ions adsorbed on biochars

2.1 Biochar preparation

The procedure of pristine and demineralized biochars preparation could be seen in our previous study (Liu and Fan, 2018), which was shown in the Supplementary Information (SI).

2.2 Biochar characterization

The biochars was fully analyzed and the specific information was shown in SI, including scanning electron microscope and energy dispersive spectroscopy (SEM–EDS), N₂ adsorption–desorption isotherms (BET), X-ray diffraction (XRD), Raman, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and element analysis.

2.3 Adsorption experiments

The adsorption experiments were also showed in the SI.

2.4 Tessier sequential extraction and risk assessment code

To assess the adsorption stability of Cd²⁺/Pb²⁺ on biochar and the potential release risk of Cd²⁺/Pb²⁺ from biochar, Tessier sequential extraction procedure (Tessier et al., 1979) and the risk assessment code (RAC) method (Liu et al., 2016) was performed. The detailed experimental processes and calculation procedure was shown in the SI.

2.5 Desorption study

The detailed desorption study was shown in the SI.

2.6 Exploration of adsorption mechanisms

Many characterizations and measurements were performed to elucidate the adsorption mechanisms. In particular, Cd/Pb-loaded biochars were characterized using SEM–EDS, XRD, XPS, FTIR, and Raman analysis. In addition, the pH values of supernatants after Cd²⁺/Pb²⁺ adsorption onto biochars and acid-washed biochars were also measured. The concentration of PO₄^3– in the supernatant after Cd²⁺/Pb²⁺ adsorption was determined using ion chromatography (ICS-3000, Dionex Inc., USA). The concentrations of CO₃^2– and HCO₃^– in the supernatant were obtained through the titration method, whereas the concentrations of Ca²⁺, Mg²⁺, Na⁺, and K⁺ in the supernatant were measured using ICP–OES.

2.7 Quantification of different adsorption mechanisms

Based on the previous literatures (Wang et al., 2015b; Cui et al., 2016), the main adsorption mechanisms of Cd²⁺/Pb²⁺ onto biochars are ion exchange, precipitation with minerals, complexation with organic functional groups (OFGs), and Cd/Pb–π interaction. Quantification of different adsorption mechanisms was performed and shown in Supplementary Information (SI).

2.8 Practical application of biochars

The detailed practical application of biochars was presented in the SI.

3.1 Characterization of biochars

The SEM-EDS of biochars are displayed in Fig. S1. As the increase in pyrolysis temperature, the surface of the biochar became more irregular, coarse, and porous. More particles could be observed on the surface of the biochars. Moreover, more minerals could be condensed in high-temperature biochars.

As shown in Fig. S2a, the biochars exhibit typical I/IV-type isothermal curves, indicating the presence of micropores structure (Ye et al., 2019). Moreover, the small hysteresis loop of the desorption branch extending from P/P₀ at 0.5 to 0.9 demonstrates the existence of abundant mesopores structure (Cheng et al., 2020). Thus, the pore structure of biochars showed multilayer and hierarchical. According to Fig. S2b, adsorption pores of biochars are mainly distributed in micropores and mesopores with the pore size ranging from 2 to 50 nm. The pore size distribution of biochar at three temperatures was different, and RSBC700 presented a high mesoporous volume, followed by RSBC500 and RSBC300.

The textural parameters of biochars are listed in Table 1. With the pyrolysis temperature increasing from 300 to 700 °C, the surface area of biochars increased from 70 to 155 m²·g⁻¹, and the pore volume of biochars increased from 0.045659 to 0.100370 cm³·g⁻¹. The micropore surface area and micropore volume also increased with the increase in pyrolysis temperature, suggesting that they were predominant in the biochars. The average pore diameter of RSBC700 was less than that of RSBC300 and RSBC500, which was beneficial for the diffusion of metal ions (Yuan et al., 2024).

The contents of elements, ash, and elemental ratio are presented in Table 1. The contents of C, S, and ash in the biochars increased, whereas the contents of H, O, and N decreased with increasing pyrolysis temperature. These changes were attributed to several reactions such as decarboxylation, dehydrogenation, decarbonylation, and deoxygenation that occurred during the pyrolysis process (Chen et al., 2020; Gupta et al., 2022). The H/C, O/C, and (N + O)/C ratios are generally used to evaluate the aromaticity, polarity, and hydrophilicity of biochars (Wang et al., 2016). As the pyrolysis temperature increased, the atomic ratios of H/C, O/C, and (N + O)/C decreased, suggesting that higher aromaticity, lower polarity, and hydrophilicity were observed in RSBC700. The pH value of the biochars increased with increasing pyrolysis temperature, which was ascribed to the condensation of mineral components in biochars (Yuan et al., 2011).

Fig. 1a shows the main minerals in biochars, namely, SiO₂, CaCO₃, and KCl. Two broad and weak peaks at 2θ = 24° ((0 0 2)) and 42° (1 0 0) were observed in the XRD pattern of the biochars (Hu et al., 2020; Tang et al., 2020). The appearance of the (0 0 2) peak implies the formation of amorphous carbon in the biochars. The intensity of peak (1 0 0) became more obvious in RSBC700, suggesting that more graphitic carbon structure was formed in biochar prepared at higher temperatures.

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Fig. 1

XRD (a), Raman (b), FTIR (c), and XPS survey (d) of biochars.

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Two prominent peaks, the D band (≈1360 cm⁻¹) and G band (≈1580 cm⁻¹) in biochars, were found in the Raman spectra (Fig. 1b). The D-band intensity originated from sp³ hybridization, and the G band was induced by crystalline graphitic/sp² carbon atoms (Ma et al., 2020). The area ratio of the D band to the G band (A_D/A_G) of RSBC300 was larger than that of RSBC500 and RSBC700, suggesting the existence of structural defects and disorder in RSBC300. As the pyrolysis temperature increased from 500 to 700 °C, the A_D/A_G ratio increased from 3.71 to 5.17, indicating the increase in the proportion of condensed aromatic ring structures with defects (Mei et al., 2021; Xu et al., 2022; Huang et al., 2023).

The surface functional groups of the biochars are presented in Fig. 1c; the pyrolysis temperature also influenced the surface functional groups in biochars. In the FTIR spectra, the position of the C⚌C (1630 cm⁻¹), −CH₂ (1401 cm⁻¹), Si—O—Si (1111 and 557 cm⁻¹) significantly changed with the increasing pyrolysis temperature (Fan et al., 2018). The change of surface functional groups may influence the binding ability toward Cd²⁺ and Pb²⁺ ions.

The valence states of the surface elements and composition of the biochars were measured using XPS (Fig. 1d). The peaks of C 1s, O 1s, N 1s, Si 2p, Ca 2p, and Mg 2p were observed on the surface of the biochars. The relative content of various elements also changed with the increase in pyrolysis temperature. Obviously, minerals also could be found on the surface of the biochars.

According to the XPS peak fitting results (Fig. S2), the high-resolution spectra of the C 1s peak of RSBC300 show that the C 1s peaks located at 284.73, 285.66, and 288.35 eV are ascribed to C–C/C⚌C/C–H, C–O, and C⚌O, respectively (Yang et al., 2016). The O 1s spectrum of RSBC300 could be decomposed into two peaks at 533.08 and 531.42 eV, which are ascribed to the C⚌O and C–O, respectively (Yao et al., 2022), respectively. In addition, pyrolysis temperature affected the types and relative percentage of functional groups (C⚌O and C—O).

3.2 Adsorption capacity of Cd²⁺ and Pb²⁺ by biochars and acid washed biochars

The adsorption capacity of Cd²⁺ and Pb²⁺ by biochars and acid-washed biochars are presented in Fig. 2a and 2b. Specifically, RSBC700 exhibited the highest adsorption capacity for Cd²⁺ and were 62.45 mg∙g⁻¹, whereas RSBC500 showed the greatest adsorption capacity for Pb²⁺ and reached 167.49 mg∙g⁻¹. However, the adsorption capacity of Cd²⁺ and Pb²⁺ by biochars extremely decreased after acid demineralization treatment, implying that soluble minerals played a crucial role in the adsorption process of Cd²⁺ and Pb²⁺, which contributed 50 %–80 % for Cd²⁺ adsorption, 61 %–75 % for Pb²⁺ adsorption. In addition, RSBC300-A showed the highest adsorption capacity toward Cd²⁺ and Pb²⁺ among the acid-washed biochars, which could be ascribed to the presence of functional groups in low-temperature biochars.

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Fig. 2

Adsorption capacity of Cd²⁺ (a) and Pb²⁺ (b) by biochars and acid washed biochars, and the chemical speciation distribution of Cd²⁺ (c) and Pb²⁺ (d) on biochars.

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Other studies have also confirmed that minerals could play an crucial role and take part in the adsorption process through ion exchange and precipitation mechanisms, especially for Cd²⁺, Pb²⁺, Cu²⁺, etc (Xu et al., 2017; Gholizadeh and Hu, 2021; Wu et al., 2021). Meanwhile, surface functional groups and the aromatic graphite structure of biochars could bind with heavy-metal ions via complexation and cation–π interaction. (Berslin et al., 2022).

3.3 Speciation distribution and RAC of Cd²⁺ and Pb²⁺ on biochars

Fig. 2c and 2d shows the chemical distribution of Cd and Pb adsorbed on the Tessier fractions of biochars. The majority of adsorbed Cd²⁺ on rice straw biochars was in the F1 and F2 fractions. Cd²⁺ was bound to the carbonate fraction (45.52 %–61.77 %) and the exchangeable fraction (33.79 %–44.70 %). For Pb²⁺, the majority of adsorbed Pb²⁺ on RSBC300 was in the F1 (39.9 %) and F3 (29.96 %) fractions, followed by the F2 fraction (18.30 %). However, most of the adsorbed Pb²⁺ on RSBC500 and RSBC700 was distributed in the F2 fraction (82.00 %−85.56 %). The speciation of heavy metals adsorbed on biochars also could reflect the adsorption mechanisms. In this work, the majority of adsorbed Cd²⁺ and Pb²⁺ on rice straw biochars were in the F1 and F2 fractions, suggesting that occurrence of precipitation and ion exchange (Frišták et al., 2014; Ke et al., 2023).

Among these five fractions, the exchangeable fraction is the most threatening fraction, and the sum of the exchangeable and carbonate fractions can be regarded as the bioavailable fraction (Sun et al., 2022). In this work, Cd²⁺ and Pb²⁺ adsorbed on rice straw biochars were not stable and easily released to the environment, which poses a potential environmental threat. Compared with Pb²⁺, Cd²⁺ displayed a high environmental risk due to its high F1 fraction.

Furthermore, the values of RAC for the Cd and Pb in metal-loaded biochars are displayed in Table 2. Cd and Pb in all metal-loaded biochars showed a high risk with RAC values greater than 50 %. In particular, Cd²⁺ and Pb²⁺ loaded on RSBC700 presented a higher risk. Furthermore, the environmental risk of Cd was greater than that of Pb. Therefore, the biochars after Cd²⁺ and Pb²⁺ adsorption should be disposed of suitably to avoid secondary pollution or environmental risk.

3.4 Desorption characteristics of Cd²⁺ and Pb²⁺ from biochars after adsorption

The desorption amount of Cd²⁺ from Cd-laden biochars with different desorption agents is shown in Fig. 3a-3d. With the increase in desorption time, more Cd²⁺ ions were released into the solution. The desorption capacity was larger at the initial stage, and gradually tended to be stable at the later stage. The desorption deficiency of different desorption agents showed the order of HCl > EDTA-2Na > H₂O > NaOH. When H₂O acted as the desorption reagent, RSBC500 presents a high desorption capacity. In contrast, when HCl, NaOH, and EDTA-2Na were used as the desorption reagents, RSBC700 showed a high desorption capacity due to its high adsorption capacity toward Cd²⁺.

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Fig. 3

Desorption kinetics of Cd²⁺ from Cd-laden biochars (a-d), and Pb²⁺ from Pb-laden biochars (e-h).

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For Pb²⁺, the desorption capacity from Pb-laden biochars with different desorption agents is shown in Fig. 3e-3 h. As desorption time increased, the desorption amount was larger at the initial stage, and gradually decreased at a later stage. The order of different desorption agents toward Pb desorption from biochar was EDTA-2Na > HCl > NaOH > H₂O. When H₂O was acting as the desorption reagent, RSBC700 showed a high desorption capacity, followed by RSBC500 and RSBC300. RSBC500, RSBC700, and RSBC300 exhibited a high desorption amount when HCl was used as the desorption agent. The desorption capacity from RSBC500 and RSBC700 was greater than that from RSBC300. When NaOH acted as the desorption agent, the desorption amount first increased but then decreased with increasing desorption time. The desorption capacity of Pb from biochars became low when the desorption time was 12 h because most Pb had been desorbed from the metal-laden biochars or the desorbed Pb²⁺ occurred secondary adsorption on the surface of biochars. When EDTA-2Na was applied as the desorption agent, the desorption amount of Pb from RSBC300 and RSBC700 first increased, then tended to become stable with the increase in desorption time. The desorption amount of Pb from RSBC500 using EDTA-2Na was stable.

The desorption extent of Cd/Pb from the Cd/Pb-laden biochars is listed in Table 3. Herein, HCl and EDTA-2Na exhibited the highest desorption extent, indicating that most of the Cd/Pb can be desorbed from biochars. HCl can destroy the mineral composition and the structure of the biochar, resulting in the release of Cd/Pb into the solution (Zhang et al., 2019). EDTA-2Na has a strong chelating ability and realizes desorption by chelating with Cd/Pb on the surface of the biochar (Pal and Maiti, 2019).

For RSBC300 and RSBC700, the desorption extent of Pb²⁺ by EDTA-2Na was larger than that of HCl. For RSBC500, the desorption extent of Pb²⁺ by HCl was slightly greater than that of EDTA-2Na. The results of the desorption experiment stayed the same with the analysis of the Tessier sequential extraction procedure. Exchangeable and carbonate fractions of Cd/Pb on biochars could be extracted when HCl and EDTA-2Na were used as desorption reagents.

In this study, the desorption kinetic experiment lasted 720 min due to the stable leaching of metal ions. However, future study was required to be performed in a long-term time to validate the results of desorption kinetics.

3.5 Adsorption mechanisms of Cd²⁺ and Pb²⁺

• Precipitation with minerals

Based on the SEM–EDS of biochars after Cd²⁺ and Pb²⁺ adsorption (Fig. S4 and Fig. S5), many particles and crystal minerals were found on the surface of the biochars. The Cd and Pb could be directly found on the surface of the biochars, which confirmed that precipitation with minerals occurred during the adsorption process.

The typical anion concentrations in the supernatant after Cd²⁺ and Pb²⁺ adsorption are showed in Table S1. Compared with the control group, the concentrations of CO₃^2–, HCO₃^–, and PO₄^3– decreased after Cd²⁺ adsorption, indicating that potential precipitation occurred. Furthermore, the decrease in concentration of HCO₃^– and PO₄^3– after Pb adsorption directly confirmed the presence of precipitation.

As shown in Fig. 4a, CdCO₃ in RSBC300, and CdCO₃ and Cd(OH)₂ in RSBC500 and RSBC700 were formed after Cd adsorption. Therefore, precipitation with minerals was key for the high removal of Cd²⁺ by rice straw biochars (Yang et al., 2021). As shown in Fig. 5a, much Pb-containing precipitation was observed in biochars after Pb²⁺ adsorption. To be specific, PbC₂O₄, Pb₃(PO₄)₂, Pb₂(SO₄)O, and 3PbCO₃⋅2Pb(OH)₂⋅H₂O were detected on the surface of RSBC300. For RSBC500 and RSBC700, Pb₅(PO₄)₃Cl, Pb₃(CO₃)₂(OH)₂, PbCO₃, Pb₄O(PO₄)₂, Pb₃C₂O₇, Pb₄(SO₄)(CO₃)₂(OH)₂, and Pb₅(PO₄)₃OH were found on the surface of the biochars. Thus, precipitation with minerals was responsible for the Pb²⁺ adsorption (Chang et al., 2020).

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Fig. 4

XRD (a), FTIR (b), XPS (c), and pH change on biochars after Cd²⁺ adsorption.

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Fig. 5

XRD (a), FTIR (b), XPS (c), and pH change on biochars after Pb²⁺ adsorption.

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Overall, the above characterization and analysis confirmed that the occurrence of precipitation during the adsorption process was caused by the anions in the biochars.

• Ion exchange

Comparing the results of Fig. S1 with Fig. S4 and Fig. S5, typical minerals cation ions on the surface of biochar disappeared after Cd²⁺ and Pb²⁺ adsorption, suggesting the occurrence of ion exchange.

Comparing the XPS analysis of biochar in Fig. 1d with Fig. 4c and Fig. 5c, the relative content of Ca 2p and Mg 2p of the biochars decreased after Cd and Pb adsorption, which suggested that ion exchange took place during the Cd²⁺ and Pb²⁺ adsorption process.

The release of minerals cation ions by all biochars was measured after Cd²⁺ and Pb²⁺ adsorption and listed in Table S1. More Ca²⁺, Mg²⁺, and K⁺ were released to the solution after Cd²⁺ and Pb²⁺ adsorption on biochars, implying the occurrence of an ion exchange mechanism.

In short, the above results also confirmed ion exchange happened during the adsorption process.

• Complexation interaction

FTIR and XPS of biochars after heavy metal ions adsorption were characterized to elucidate the complexation interaction.

Compared with the FTIR of biochars in Fig. 1c, some peaks shifted after Cd adsorption on biochars (Fig. 4b). For RSBC300, the peaks at 1630, 1111, and 557 cm⁻¹ shifted to 1618, 1104, and 563 cm⁻¹, respectively. For RSBC500, 1420 and 554 cm⁻¹ shifted to 1407 and 580 cm⁻¹, respectively. For RSBC700, 3449, 1625, 1098, and 464 cm⁻¹ shifted to 3442, 1630, 110, and 467 cm⁻¹, respectively. Compared with the FTIR of biochars in Fig. 1c, some peaks also shifted after Pb adsorption on biochar (Fig. 5b). For RSBC300, the peaks at 3443, 1630, and 557 cm⁻¹ shifted to 3439, 1639, and 553 cm⁻¹, respectively. For RSBC500, 3440, 1617, 1420, and 554 cm⁻¹ shifted to 3443, 1629, 1401, and 550 cm⁻¹, respectively. For RSBC700, 3449, 1625, 1098, and 464 cm⁻¹ shifted to 3443, 1633, 1117, and 450 cm⁻¹, respectively. These changes suggested that OFGs in biochar toke prat in the binding of Cd²⁺/Pb²⁺ through complexation.

Compared with the XPS of biochars in Fig. S3, the binding energy and distribution of carbon- and oxygen-related groups changed after Cd²⁺ adsorption on biochars (Fig. S6). In addition, the OFGs also changed after Cd²⁺ adsorption. Moreover, the Cd 3d_3/2 and Cd 3d_5/2 peaks were observed on the surface of biochars after Cd²⁺ adsorption (Liu et al., 2022a; Tan et al., 2022). Compared with the XPS of biochars in Fig. S3, the binding energy and distribution of carbon- and oxygen-containing groups changed after Pb²⁺ adsorption on biochars (Fig. S7). Furthermore, the Pb 4f_5/2 and Pb 4f_7/2 peaks were found on the surface of biochars after Pb²⁺ adsorption (Wu et al., 2021).

As shown in Fig. 4d and Fig. 5d, the pH of RSBC-A after Cd²⁺ and Pb²⁺ adsorption was less than the control groups without Cd²⁺ and Pb²⁺ adsorption, illustrating the complexation interaction between Cd²⁺/Pb²⁺ and OFGs. The complexation reactions can be seen in the following reactions. $$-\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H}+{\mathrm{C}\mathrm{d}}^{2+}/\mathrm{P}\mathrm{b}2++{\mathrm{H}}_2\mathrm{O}\to -\mathrm{C}\mathrm{O}\mathrm{O}{\mathrm{C}\mathrm{d}}^{+}/\mathrm{P}\mathrm{b}++{\mathrm{H}}_3{\mathrm{O}}^{+}$$ $$-\mathrm{O}\mathrm{H}+{\mathrm{C}\mathrm{d}}^{2+}/\mathrm{P}\mathrm{b}2++{\mathrm{H}}_2\mathrm{O}\to -\mathrm{O}{\mathrm{C}\mathrm{d}}^{+}/\mathrm{P}\mathrm{b}++{\mathrm{H}}_3{\mathrm{O}}^{+}$$

• Cd-π interaction

Based on the FTIR and XPS, the aromatic carbon structure (1630 cm⁻¹ in FTIR) and graphene-like structure (C–C/C—H/C⚌C in XPS) in biochars changed after Cd²⁺ and Pb²⁺ adsorption, suggesting that Cd/Pb–π interaction took place during the adsorption process. In addition, the Cd/Pb–π interaction could be observed directly in the XPS analysis of Cd 3p and Pb 4f.

Raman spectroscopy analysis also could be used to revela the adsorption mechanism (Fan and Zhang, 2021). According to the Raman spectra (Fig. S8) of biochar after Cd²⁺ and Pb²⁺ adsorption, the decrease in A_D/A_G values indicated the presence of Cd/Pb–π interaction when compared with the results in Fig. 1b.

3.6 Quantitative contributions of different sorption mechanisms

The pyrolysis temperature significantly influenced the heavy metal adsorption mechanisms on biochars. The Q_exc, Q_pre, Q_com, and Q_π values of biochars were 0.02–2.36, 21.63–51.44, 0.64–4.60, and 10.35–12.93 mg∙g⁻¹, respectively (Fig. 6a.). As the pyrolysis temperature increases, the Q_exc values of biochars decreased due to the minerals becoming stable at high temperatures, and Q_com values of biochars decreased due to the lower abundance of functional groups. The Q_pre values of biochars also greatly increased with the increasing pyrolysis temperature. Precipitation with minerals played a key role in the adsorption of Cd²⁺, especially for the high-temperature biochar. As shown in Fig. 6b, the relative contribution percentage of Q_exc, Q_pre, Q_com, and Q_π values of biochars were 0.03 %–5.75 %, 52.77 %–82.37 %, 1.03 %–11.22 %, and 16.57 %–30.26 %, respectively. The contributions of Q_exc, Q_com, and Q_π decreased with increasing temperature, whereas the contribution of Q_pre was gradually predominant at high pyrolysis temperatures.

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Fig. 6

Adsorption capacity and quantitative contributions of different sorption mechanisms, (a) and (b) for Cd²⁺, (c) and (d) for Pb^2+.

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For Pb²⁺ (Fig. 6c), the Q_exc, Q_pre, Q_com, and Q_π values of biochars were 30.42–75.93, 25.14–93.17, 0.88–26.22, and 17.00–34.14 mg∙g⁻¹, respectively. As the increasing of pyrolysis temperature, the Q_pre and Q_π values of biochars increased, whereas the Q_com values of biochars decreased. The Q_exc values of biochars first increased and then decreased with the increasing pyrolysis temperature. Thus, minerals in biochar could participate in the interaction of precipitation and exchange, which became the dominant mechanism during the adsorption process. As shown in Fig. 6d, the relative contribution percentages of Q_exc, Q_pre, Q_com, and Q_π values of biochars were 19.18 %–48.81 %, 18.82 %–58.71 %, 0.55 %–19.63 %, and 12.73 %–21.52 %, respectively. As the pyrolysis temperature increased, the relative contributions of Q_pre and Q_π increased, whereas the relative contributions of Q_exc and Q_com decreased. The increased contributions of Q_pre and Q_π were attributed to the soluble minerals in biochar and graphitic structure, but the increased contributions of Q_exc and Q_com were ascribed to the passivation of minerals in biochar, and the disappearance of OFGs.

Therefore, quantitative contributions of different sorption mechanisms could be used to explain the speciation distribution and environmental risk of Cd²⁺ and Pb²⁺ adsorbed on biochars, and the desorption characteristics of Cd²⁺ and Pb²⁺ from biochars after adsorption.

3.7 Practical application of biochars

The regeneration performance of biochars for metal ions is shown in Fig. 7a and Fig. 7b. The adsorption performance of biochars toward metal ions decreased significantly due to the loss of minerals since the precipitation with minerals and ion exchange were the dominant mechanisms for metal ions adsorption, especially Cd²⁺. Thus, the recycle potential of biochars was limited due to the decrease of binding sites.

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Fig. 7

Practical application of biochars, (a) and (b) for recycle, (c) and (d) for real waterbodies.

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The real application of biochars toward metal ions removal in various waterbodies is presented in Fig. 7c and Fig. 7d. As seen in the results, biochars exhibited a good adsorption performance toward Cd²⁺ and Pb²⁺ in domestic wastewater and tap water. Furthermore, biochars displayed a high removal rate toward Pb²⁺ in lake water and river water. Further study should be conducted to modify the biochars to improve their adsorption performance.