The Solidification and Stabilization of Pb in Soil using Apatite-modified Biochar

doi:10.21203/rs.3.rs-4668711/v1

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The Solidification and Stabilization of Pb in Soil using Apatite-modified Biochar

https://doi.org/10.21203/rs.3.rs-4668711/v1

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In this study, biochar (BC) was prepared by pyrolysis using sesame straw as the precursor, and modified with hydroxyapatite and chloroapatite to prepare hydroxyapatite-modified biochar (HBC) and chloroapatite-modified biochar (BC-Cl), respectively. The changes in functional groups before and after biochar modification were investigated using various characterization methods, and the passivation effects of BC, HBC, and BC-Cl on lead (Pb) in soil were studied. The three types of biochar were introduced into the contaminated soil at ratios of 3% and 5% to passivate the soil. After remediation, the effective Pb content in the soil decreased by 45.45%, 76.70%, and 82.38%, respectively, compared with the control (CK) group. Moreover, the effective Pb content decreased with increased of biochar dosage. When the soil was cultured for 90 d, the reducible Pb content of BC, HBC, and BC-Cl decreased by 22.03%, 22.97%, and 26.36%, respectively, while the residual state content increased by 76.22%, 88.31%, and 103.53%, respectively, compared with CK. BC, HBC, and BC-Cl effectively passivated Pb in soil, with the 5%BC-Cl soil sample exhibiting the most pronounced passivation effect. This study’s findings offer a new method for efficient utilization of sesame straw and provide a reference for developing apatite-based soil remediation materials.

chlorapatite

hydroxyapatite

biochar

lead pollution

solidification

stabilization

Soil is the cornerstone of human survival and development, along with industrial and agricultural and other anthropogenic activities at the same time, a large number of heavy metal pollutants through different ways into the soil environment, the soil every moment suffers the risk of environmental damage. According to the 2014 Soil Survey Bulletin, the exceedance rate of soil environment reached 16.1%, among which the exceedance rate of inorganic substances is more serious, accounting for 82.8% of the total exceedance rate; the current pollution status of heavy metals cadmium (Cd), lead (Pb) and zinc (Zn) in soil is more serious in the inorganic substances, and the exceedance rate reaches 7%, 1.5% and 0.9%.The accumulation rate of Cd, Pb and Zn in the soil is high The source of pollution is wide and complex, and the toxic effect on human body is large. Industry, pesticides and unregulated discharge in life can cause soil heavy metal pollution (Jiang and Wang et al., 2024; Liu and Zheng et al., 2024).

Pb is a heavy metal polluting element that is widely distributed in the environment. Over the past few decades, worldwide emissions of lead have reached 780,000 tons, of which 34,600 tons are discharged into the natural environment each year in China (Song and Li et al., 2024). Most of the lead migrates to water bodies and soil, of which more than 70% eventually pools in the soil (Carusso and Rodriguez et al., 2024; Xia and Gao et al., 2025). Currently, the main sources of lead pollution in China are: primary lead smelting, emissions from burning leaded gasoline, electroplating wastewater and lead in pigments (Liu and Zhao et al., 2024). Lead is highly toxic and the pollution lasts for a long time, and lead also has strong accumulative properties and can be enriched through the food chain. Excessive lead content in the human body will harm the human nervous system, digestive system, cardiovascular system and kidneys, leading to irreversible and serious consequences such as neurological disorders, endocrine disorders, and affecting children's intelligence, which is a great threat to the health of human beings (Liu and Ai et al., 2024; Yin and Zhao et al., 2024).

Biochar is an excellent soil amendment, playing a crucial role in remediating heavy metal-contaminated soil. Its application improves both the physical and chemical properties of the soil (Meng and Wu et al., 2024), such as hardness, structure, and bulk density, consequently altering the water-holding capacity and permeability of the soil and increasing tillage performance. Nutrient elements, such as N, K, P, and Ca, present in biochar are released into the soil, absorbed and utilized by crops, and promote crop growth. Additionally, the unique physical and chemical properties of biochar (porous (Tong and He et al., 2020; Yao and Ye et al., 2023). high pH (Kang and Chun et al., 2024), multi-functional groups (He and Li et al., 2024), highly aromatic structure (Li and Li et al., 2023), etc.) not only provide a good adsorption function for heavy metal ions in soil (Yang and Luo et al., 2022), but can also fix heavy metals through ion exchange (Yuan and Hong et al., 2020; Wu and Wang et al., 2021; Bhardwaj and Nag et al., 2022; Gope and Das et al., 2022), precipitation (Liu and Chen et al., 2021; Fan and Wang et al., 2023; Sun and Yang et al., 2023), complexation (Mahdi and Yu et al., 2019; Jia and Wu et al., 2021), etc., making it potentially valuable in the remediation of heavy metal-contaminated soil. However, biochar possesses inherent shortcomings, such as the loss of functional groups, uneven pore structure, and limited adsorption capacity for some heavy metals. Therefore, the modification of biochar presents an avenue to enhance its effectiveness in soil heavy metal remediation (Wang and Li et al., 2019; Qiu and Tao et al., 2021).

Most studies have shown that phosphorus-containing substances are more effective in immobilizing Pb in soil and are prone to form lead phosphate precipitates. Soluble phosphate compounds and granular phosphate minerals are widely used in passivation studies of heavy metals. In principle, phosphorus-containing materials immobilize heavy metals by direct metal adsorption/displacement, phosphate-induced metal adsorption or surface complexation, and metal chemical precipitation (Li and Yu et al., 2024; Yu and Wang et al., 2024). Phosphates can form complexes or precipitates such as Pb₅(PO₄)₃Cl, Cd₃(PO₄)₂OH, Cu₅(PO₄)₃Cl, etc. with many common heavy metals (e.g., Pb, Cd, Cu, etc.) (Gao and Kang et al., 2024; Peng and Zhang et al., 2024). When phosphorus-based materials are applied alone (hydroxyapatite, calcium magnesium phosphate, calcium superphosphate, phosphate rock powder, etc.), the higher application amount may cause the accumulation of phosphorus and cause some environmental risks, such as water eutrophication caused by phosphorus leaching, nutrient imbalance, lack of medium and trace elements necessary for crops, and soil acidification. The ability of adsorption, precipitation and complexation of heavy metals by composite materials formed by biochar and other organic substances is generally greater than that of single organic or inorganic substances (Chen and Mao et al., 2023; Li and Qiu et al., 2023). Ahmed et al (Ahmed and Xu et al., 2022). combined rice straw biochar (BC) with nano-hydroxyapatite (HAP). Batch experiments showed that the saturated adsorption capacity of Pb(II) in BC@nHAP reached 335.88 mg/g, which was regarded as a new adsorbent material with the potential of large-scale removal of heavy metals from wastewater. Labrag et al (Labrag and Abbadi et al., 2023). prepared porous and magnetic Fe₃O₄-hydroxyapatite (wFeHAp) nanocomposites. The results showed that the combination of Fe₃O₄ and apatite changed the surface properties of wFeHAp nanocomposites and produced effective antibacterial activity against Staphylococcus aureus, Bacillus subtilis, Escherichia coli and Klebsiella pneumoniae strains.

In this study, sesame straw was used as a raw material to prepare biochar, while hydroxyapatite and chloroapatite were used to modify the biochar. An indoor soil passivation test was conducted to examine the characteristics of the modified biochar and its effect on the availability and speciation of heavy metals in the soil. These findings provide a theoretical reference for the passivation and remediation of Pb-contaminated soils using apatite biochar materials.

Experimental materials

Sesame straw from a farm in Henan Province, China, was selected as the raw material, washed, dried, crushed, and sieved through 20 mesh and then stored as the original biomass material. The main chemical reagents are anhydrous calcium chloride (CaCl₂), diammonium hydrogen phosphate ((NH₄)₂HPO₄), ethanol (C₂H₅OH), and concentrated ammonia (NH₃\(\bullet\)H₂O); the laboratory water is deionized water.

Preparation and characterization of biochar

The biomass was filled and compacted in a crucible (it could not be completely isolated from oxygen, so it was oxygen-limited), placed in a muffle furnace, pyrolyzed at 550°C for 2 h, reduced to room temperature and then removed, washed with deionized water for 2 ~ 3 times to remove the impurities, dried to a constant weight, and ground through a 20-mesh sieve, and the samples obtained were labeled as BC.

5 g BC was weighed in a beaker, and 0.334 mol L^− 1 CaCl₂ solution was added to it. The magnetic stirrer was used to stir violently for 30 min, and then 0.2 mol L^− 1 (NH₄)₂HPO₄ solution was added to it. The solution was adjusted with ammonia to keep its pH = 10(the two would co-precipitate to form hydroxyapatite), centrifuged and dried to constant weight, grinded through a 20-mesh sieve, and the obtained sample was labeled as HBC.

5 g BC was weighed in a beaker, and 0.334 mol L^− 1 CaCl₂ solution was added to it. The mixture was stirred violently with a magnetic stirrer for 30 min, and then 0.2 mol L^− 1 (NH₄)₂HPO₄ solution was added to the mixture (the two will co-precipitate to form chloroapatite), centrifuged and dried to constant weight, grinded through a 20-mesh sieve, and the obtained sample was labeled BC-Cl.

The contents of total C, H, O, N and S in biochar were determined by elemental analyzer. The infrared spectra of biochar before and after modification were tested by Fourier transform infrared spectrometer (Nicolet IS10, thermoelectric company, United States). The crystal structure of biochar materials was analyzed by X-ray diffraction spectrometer (Smartlab, Rigaku, Japan).

Methods of soil sample collection and characterization

The heavy metal-contaminated soil was collected from a city in Henan Province, China, at a depth of 0–20 cm. After the collected soil was air-dried, the debris in it was removed, and it was crushed through a 10-mesh sieve.

Determination of pH value of soil samples: 10 ± 0.02 g of soil was weighed in a 10 mL centrifuge tube with an electronic balance, 25 mL of deionized water was added and placed in a thermostatic oscillator for 2 min. After standing for 30 min, all samples were measured with a pH meter within 1 h.

Determination of available phosphorus content in soil samples: The determination of available phosphorus in soil was determined according to the standard of “determination of ammonium nitrogen, available phosphorus and available potassium in neutral and calcareous soil combined with extraction-colorimetric method” (NYT 1848–2010). After the soil was cured for a certain period of time, 2.5 ± 0.01 g air-dried samples were accurately weighed in a 100 ml conical flask, about 0.5 g phosphorus-free activated carbon and 50 ml combined extractant were added successively, sealed with a preservative film, and placed in a constant temperature oscillator at 20°C, 220 r/min. The oscillation was carried out for 10 min, and the filtrate was collected after filtration for the determination of available phosphorus content.

Determination of the content of heavy metal Pb in soil samples: According to the “13 trace elements in soil and sediment sequential extraction procedure” standard measurement. In this paper, the heavy metal forms are divided into: weak acid extraction state F₁, reducible state F₂, oxidizable state F₃ and residual state F₄ according to the BCR four-step continuous step-by-step extraction method. The specific steps are shown in Table 1:

Table 1

BCR four-step continuous extraction procedure
Morphological Name	Extraction Method
F₁	0.5 g of soil to be tested was weighed in a centrifuge tube, and 0.11 mol/L acetic acid solution was added. After oscillation and centrifugation, the supernatant was taken for analysis.
F₂	The 0.5 mol/L NH₂OH·HCl solution was added to the remaining solid after the first step of extraction, fully mixed, shaken and centrifuged, and the supernatant was taken for analysis.
F₃	5 mL of H₂O₂ was added to the remaining solids after the second step of extraction and digested at room temperature for 1 h, and then heated in a water bath for 1 h until the liquid in the centrifuge tube was less than 1.5 ml; after cooling, 5 mL of H₂O₂ was added in three times, fully mixed, and heated in a water bath for 1 h until the liquid in the centrifuge tube was less than 0.5 mL. After cooling, 25 ml of 1 mol/L ammonium acetate solution was added. After shaking and centrifugation, the supernatant was taken for analysis.
F₄	The residual solids of 0.1 g step 3 after drying were weighed and determined after digestion with hydrochloric acid-nitric acid-hydrofluoric acid-perchloric acid.

Determination of heavy metal content in soil samples: According to the standard method of “Determination of 22 metal elements in solid waste inductively coupled plasma emission spectrometry” (HJ 781 − 206). A certain amount of concentrated hydrochloric acid, concentrated nitric acid, perchloric acid and hydrofluoric acid solutions were successively taken to digest soil samples, and the Pb concentration in the digested samples was determined by inductively coupled plasma emission spectrometer.

Determination of available Pb in soil samples: According to the standard method of “Determination of available Pb, Cd, Cu and Zn in soil by TCLP extraction-atomic absorption spectrometry”. TCLP extraction method developed two different pH extractants according to the difference of soil pH and buffer amount: When the soil pH was 5, extractant B was selected (5.7 mL glacial acetic acid was diluted to 1 L with deionized water to ensure that the pH value of the solution was 2.64 ± 0.05). Due to the pH > 5 of the tested soil, extractant B was selected for extraction.

After preliminary determination of the physical and chemical properties of the soil, the pH was 8.42, and the total Pb content in the soil was 241.77 mg/kg. The Pb content of the tested soil exceeded the screening value (170 mg/kg) set by the Soil Environmental Quality Agricultural Land Soil Pollution Risk Control Standard (GB 15618 − 2018), indicating a more severe level of pollution.

Experimental and analytical methods for soil passivation

To examine the passivation and remediation effects of original biochar and apatite-modified biochar on soil, 200 g of spare soil samples were weighed and distributed into 21 polyethylene plastic pots. Subsequently, different quantities of sesame straw biochar and modified biochar were weighed and added to the soil samples. Maintaining a mass ratio of 3% and 5% of biochar to soil, respectively, it was established two treatment levels, with three parallel samples for each treatment. After mixing the biochar with the soil samples, a specific volume of deionized water was added, and the mixture was continuously stirred with a glass rod for 10 min to mix evenly. There were four groups of soil in this experiment: blank treatment (CK, no biochar was applied), BC treatment (3%, 5% original sesame straw biochar was applied), HBC treatment (3%, 5% hydroxyapatite biochar was applied), and BC-Cl treatment (3%, 5% chloroapatite-modified biochar was applied).

The experimental soil samples from each group were placed under laboratory environmental conditions for 90 d, during which an uncertain volume of deionized water was added regularly to maintain a soil moisture content of 40%. When the incubation time was 30 d, 60 d, and 90 d, appropriate quantities of soil samples were dried and ground, respectively, through 20-mesh and 100-mesh nylon sieves, collected, and stored for use. Table 2 presents the experimental design.

Table 2

Summary of experimental treatment
Number	Biological carbon dosage/g	Proportion/%
CK	0	0
3%BC	6	3
5%BC	10	5
3%HBC	6	3
5%HBC	10	5
3%BC-Cl	6	3
5%BC-Cl	10	5

Analysis of biochar characterization results

Physicochemical properties of biochar

The differences in the specific surface area and internal pore structure of the adsorbent affect its adsorption performance. A larger specific surface area and improved porosity of biochar provide more active sites for the adsorption reaction, enhancing the adsorption performance of heavy metals (Park and Kim et al., 2021; Zhang and Tran et al., 2023). Table 3 presents the physicochemical properties of the three biochar samples. Changes in the biochar pH values revealed a significant decrease in the pH of modified biochar. The number ratio of different atoms can be calculated using the proportion of C, H, O, and N. H/C reflects the degree of aromatization of the material, while O/C reflects the number of oxygen-containing functional groups (Ghysels and Ronsse et al., 2019; Venkatesh and Gopinath et al., 2022). The H/C and O/C values of BC-Cl were higher than those of BC, with HBC having a higher O/C than BC. This suggests that the addition of chloroapatite and hydroxyapatite effectively increases the number of hydrogen-containing and oxygen-containing functional groups on the surface of biochar, and the H/C value was less than 0.6, indicating that the material has good stability in the environment (Stylianou and Christou et al., 2020).

Table 3

pH value and element composition of biochar samples
Sample	pH	C	H	O	N	S	H/C	O/C
Sample	pH	(%)	(%)	(%)	(%)	(%)	H/C	O/C
BC	10.88	73.40	5.215	18.914	1.99	0.481	0.071	0.258
HBC	8.18	72.873	5.719	20.020	1.10	0.288	0.078	0.275
BC-Cl	6.18	74.265	5.152	18.304	1.87	0.409	0.069	0.246
FTIR Analysis

FTIR was used to analyze the changes in functional groups before and after biochar modification. Figure 1 shows the FTIR spectra of BC, HBC, and BC-Cl. All three biochar samples showed stretching vibrations of alcohol hydroxyl or phenolic hydroxyl -OH (3440 cm^− 1), aliphatic C-H (2926 cm^− 1), and C = O in the lactone group or C = C skeleton (1579 cm^− 1) in aromatic compounds (Shi and Han et al., 2019; Iqbal and Batool et al., 2023). The peak at 3440 cm^− 1 was relatively wide, indicating that the three biochars contained a large amount of -OH. The hydrophilicity of the carbon material was good, which is conducive to the adsorption of pollutants. Additionally, in the infrared spectra of HBC and BC-Cl, the characteristic peak of P-O-P appeared at 1035 cm^− 1 (Wang and Sun et al., 2022), and the peak intensity at 566 cm^− 1(P = O) was significantly enhanced, which is related to the bending vibration of PO₄³⁻ (Yang and Chen et al., 2022). These findings indicate that chloroapatite was successfully loaded onto the biochar, increasing the number of phosphate groups containing oxygen functional groups on the biochar surface.

X-ray Diffraction(XRD) Analysis

The XRD patterns of BC, HBC, and BC-Cl are shown in Fig. 2. The figure reveals that the peak intensity of the BC-Cl diffraction peak is strong and the peak width is narrow, and a new peak and a narrow peak of Ca₅(PO₄)₃Cl appear in the spectrum, indicating the preparation of a pure BC-Cl sample with fine crystallinity (Sha and Li et al., 2023). The appearance of the characteristic peak indicates that the BC-Cl surface was successfully loaded with chloroapatite, which can provide more effective adsorption sites for biochar and improve the adsorption of heavy metal cations. FTIR spectra also confirmed this result. Furthermore, the XRD pattern of BC showed that the sesame straw biochar contained calcium carbonate, likely stemming from the composition of sesame straw. After hydroxyapatite modification, the characteristic peak of Ca₅(PO₄)₃OH appeared, indicating that hydroxyapatite was successfully loaded onto the HBC surface.

Passivation remediation effect of biochar on Pb contaminated soil

Effect on soil pH

Figure 3 depicts the effect of different biochar materials on soil pH at the end of the soil culture experiment. The figure shows that the pH value of the control group (CK) soil was 8.36 at the end of the culture, which was weakly alkaline, while the pH of most soil groups decreased to a certain extent after the addition of original biochar and modified biochar. After 90 days of curing, soil treated with 3%HBC, 5%HBC, 3%BC-Cl, and 5%BC-Cl had pH values of 7.81, 7.65, 7.78, and 8.05, respectively, which were 0.55, 0.71, 0.58, and 0.31 units lower than CK, respectively.

The overall experiment revealed that after the modified biochar was added to the soil, although hydrogen phosphate ions could increase soil pH by resolving OH⁻ from the soil colloid via ion exchange, the addition of phosphate increased the concentration of calcium ions and ammonium ions in the soil, and their mobility in the soil was greater than that of hydrogen phosphate ions. Therefore, a large amount of hydrogenions can be preferentially resolved on the soil colloid by ion exchange, inhibiting the release of hydroxide ions and reducing the soil pH (Teng and Zhao et al., 2021). Furthermore, with the extension of incubation time, the oxygen-containing functional groups on the biochar surface, such as -COOH, -OH, and PO₄³⁻, fully reacted with the metal cation Pb²⁺ in the soil. This process promotes the formation of heavy metal hydroxides and phosphate precipitation in the soil, enhances the binding capacity of biochar to heavy metals, and reduces the migration and transformation of Pb in the soil. These reactions reduce the soil pH to a certain extent (Teng and Zhao et al., 2023).

Effect on available phosphorus content in contaminated soil

Phosphorus-based materials not only reduce the activity of heavy metals but also promote the growth of crops (Vuong and Stephen et al., 2023). In recent years, they have been widely used in soil remediation and the removal of heavy metals from wastewater. Figure 4 shows the effects of BC, HBC, and BC-Cl on available phosphorus content in the soil. From the figure, when the soil was cultured for 90 d, the soil available phosphorus content in the 3% and 5% BC, HBC, and BC-Cl groups increased to 18.95, 22.07, 22.74, 26.16, 23.95, and 30.47 mg/kg, respectively, with an increase of 26.12%, 46.85%, 51.31%, 74.12%, 59.38%, and 102.78%, respectively. It is evident that compared with BC, HBC and BC-Cl significantly increased the available phosphorus content in the soil. After the soil culture experiment, the available phosphorus content in each group positively correlated with the amount of biochar applied. The 5%BC-Cl treatment exhibited the highest available phosphorus content (30.47 mg/kg), which was 2.03 times greater than that of the CK treatment(Hong and Li et al., 2022).

Effect on available Pb in contaminated soil

Figure 5 illustrates the effect of biochar materials on the available soil state when treating Pb-contaminated soil under different dosage conditions. From the figure, upon completion of the culture, the TCLP extractable content of Pb in CK treatment was 21.53 mg/kg, and the TCLP extractable content of Pb in the soil of 3%BC and 5%BC groups was 45.45% and 68.18% lower than that of CK, respectively. The TCLP extractable content of Pb in the soil of 3%HBC and 5%HBC groups was 76.70% and 86.64% lower than that of CK, respectively. The TCLP extractable content of Pb in the soil of 3%BC-Cl and 5%BC-Cl groups was 82.38% and 93.75% lower than that of CK, respectively.

Figure 5 shows that the prolongation of soil curing time significantly reduced the TCLP extractable content of Pb in the three groups of soils treated with biochar materials, thereby reducing the ecological environment risk of Pb. After chlorapatite-modified biochar to the soil, the TCLP extractable content of Pb decreased the most, and the overall passivation repair effect was BC-Cl > HBC > BC. In terms of biochar dosage, the treatment effect of each group was 5%>3%, and the TCLP extractable content of Pb in the 5%HBC and 5%BC-Cl treatment groups was 2.87 mg/kg and 1.35 mg/kg, respectively. Sesame straw, rich in holocellulose and minerals, has a greater remediation effect when added to contaminated soil(Liu and Chen et al., 2022). Moreover, the loading of apatite materials improves the binding ability of HBC and BC-Cl to heavy metal cations in soil without inducing any toxic effects on plants. In summary, the application of BC, HBC, and BC-Cl reduces the leaching toxicity of heavy metals in soil, thereby mitigating pollution and harm to the surrounding environment and significantly aiding the fixation of Pb.

Effect on the speciation of Pb in contaminated soil

The effects of different modified materials and biochar dosages on the chemical forms of Pb in soil are shown in Fig. 6. Pb in the soil exists in various chemical forms, including weak acid extractable, reducible, oxidizable, and residual states, according to the BCR continuous extraction method(Chen and Zhang et al., 2022; Chen and Zhang et al., 2023). Among these, the migration and transformation of the weak acid extractable state are the strongest, making it easily absorbable by plants. Furthermore, the bioavailability of reducible and oxidizable forms is high, enabling indirect absorption by plants under certain conditions. However, the residual state is more stable in the soil, and it is the least prone to migration and transformation(Liu and Yu et al., 2022).

From Fig. 6, it is evident that after adding the different types of biochar, the chemical forms of Pb showed different trends with the change in culture time. In the control soil, Pb mainly existed in the reducible state (accounting for 67.08% of the total Pb content). Furthermore, with the extension of biochar curing time, the proportion of residual state in the three experimental soil groups increased continuously, suggesting that the addition of biochar significantly affects the migration and transformation of Pb chemical forms.

In the later stages of the experiment (60–90 d), the content of weak acid extractable and reducible states of Pb in the biochar group soil decreased compared with CK. The content of the oxidizable state did not change significantly, while the content of the residual state increased significantly. When the soil was cultured for 90 d, the reducible Pb content of CK soil was 162.18 mg/kg. After adding biochar, the reducible Pb content decreased by 19.58–22.03%, while the residual Pb content increased by 71.42–76.22%. Compared with CK, the reducible Pb content of the HBC group decreased by 20.63–22.97%, while the residual Pb content increased by 76.11–88.31%. Compared with CK, the reducible Pb content of the BC-Cl group decreased by 22.63–26.36% and the residual Pb content increased by 81.43–103.53%. The soil in the three biochar groups showed a change in the proportion of Pb chemical forms with increasing biochar dosage. Notably, the 5% biochar dosage group exhibited a superior soil fixation effect, although the difference between each dosage was not significant. Among the modified biochar treatments, the treatment effect of the 5%HBC and 5%BC-Cl groups was better, which significantly reduced the proportion of the Pb reducible state and increased the proportion of the residual state.

Overall, the aforementioned experimental results indicate that biochar application in the soil promotes the transformation of Pb from weak acid extractable and reducible states to a more stable residual state and reduces the migration and transformation of Pb in soil, as well as its bioavailability.

The above overall experimental results show that the application of biochar in soil can promote the transformation of heavy metal Pb from weak acid extractable and reducible to more stable residual state, and reduce the migration and transformation of heavy metal Pb in soil and the bioavailability.

In this study, Pb-contaminated soil was solidified and stabilized by adding biochar with different modification treatments. The effects of biochar on the basic physical and chemical properties of soil and the distribution of heavy metal Pb in soil were investigated. The results of 90 d soil conservation experiment are as follows:

(1) The addition of biochar can improve the physical and chemical properties of soil. After adding BC, HBC and BC-Cl, on the whole, the change of soil pH after the addition of three kinds of biochar is not large, which is still a weak alkaline soil, and the content of available phosphorus in soil is improved.

(2) After 90 d of cultivation, compared with CK, the reducible content of Pb in BC, HBC and BC-Cl groups decreased by 19.58%~22.03%, 20.63%~22.97% and 22.63%~26.36%, respectively, the residual content increased by 71.42%~76.22%, 76.11%~88.31%, 81.43%~103.53%, respectively. BC-Cl is more conducive to the transformation of Pb in soil from the reducible state and weak acid extractable state with high bioavailability and leaching toxicity to the residual state with low activity.

In summary, BC-Cl promoted the transformation of Pb in soil to a stable state, and effectively reduced the environmental risk of Pb by changing its occurrence form, indicating that apatite, which is easy to combine with heavy metal cations, plays a positive role in biochar remediation of soil, and can be used as a new adsorbent in Pb-contaminated soil.

Conflicts of interest

The authors have no conflicts of interest to declare that are relevant to the content of this article.

Funding

The authors declare that no funds, grants, other support were received during the preparation of this manuscript.

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The Solidification and Stabilization of Pb in Soil using Apatite-modified Biochar

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Introduction

Materials and Methods

Experimental materials

Preparation and characterization of biochar

Methods of soil sample collection and characterization

Experimental and analytical methods for soil passivation

Result and Discussion

Analysis of biochar characterization results

Passivation remediation effect of biochar on Pb contaminated soil

Conclusions

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