Fate and health risk assessment of heavy metals in Brassica chinensis L. (pak-choi) and soil amended by sludge-based biochar

Biochar is widely used in agriculture to efficiently solve the problem of sludge. In this study, sludge-based biochar (referred to as BC1, BC2, and BC3) was prepared by mixing sludge with FeCl3, Na2SiO3, and Ca (H2PO4)2, respectively. Then, it was mixed with fresh soil to plant Brassica chinensis L. The analysis of the effects of the three biochar types showed that all of them were beneficial to the growth of Brassica chinensis L. We added the biochar to the soil and found that the concentration of heavy metals did not exceed the recommended threshold. Additionally, the aboveground part of Brassica chinensis L. met the standard requirement for food safety (GB 2761–2017). Notably, BC3 stood out with the best effect on the growth of Brassica chinensis L. and resulted in the improvement of the physical and chemical properties of soil such as ammonium nitrogen, available phosphorus, and available potassium (BC3 was followed by BC2 and BC1). BC3 could efficiently inhibit the migration of heavy metals, thereby reducing the overall heavy metal pollution level and ameliorating the soil nutrients. BC3 could increase the organic carbon by 258.92%, available phosphorus by 234.45%, and available potassium by 37.12% compared with the CK group. The THQ and TTHQ estimates of Brassica chinensis L. were lower than one, indicating that the health risk of heavy metal intake was not prominent. Additionally, the application of the proposed biochar could reduce the form of F1 (acid extracted state) and increase the form of F4 (residue state) in soil. Overall, we conclude that the application of the proposed biochar can promote the root absorption of heavy metals and inhibit the migration of heavy metals.


Introduction
With the rapid development of industrialization and urbanization, sewage treatment in China is also continuously increasing. Correspondingly, sludge production is also increasing as an inevitable by-product in the process of sewage treatment (Qu et al. 2019). According to recent forecasts, China's annual sludge production will exceed 60 Mt by 2020 (Yang et al. 2015). Sludge contains multiple heavy metals, organic pollutants, microplastics, antibiotics, and pathogens and causes severe secondary pollution if sludge production is not efficiently controlled. However, sludge is also a potential resource. More specifically, high value sludge utilization has gained considerable attention among researchers in recent years. For instance, sludge-based biochar, prepared by pyrolysis, can be used as a soil conditioner (Ahmad et al. 2022;Rathnayake et al. 2021;Su et al. 2019), adsorbent , and building material (Gupta and Kua 2019).
Biochar is rich in functional groups (Ma et al. 2014), has a large specific surface area (Li et al. 2018), is rich in nutrients, and is widely used in soil improvement (Laird et al. 2010) and soil remediation (Jing et al. 2020). Numerous studies have focused on the preparation of biochar via sludge pyrolysis for soil improvement worldwide. Biochar can also improve the soil cation exchange capacity and increase soil organic carbon and the contents of available potassium, phosphorus, and other nutrients. It is essential for improving soil quality to promote crop growth and development (Abd Elwahed et al. 2019;/ Published online: 18 August 2022 Environmental Science andPollution Research (2023) 30:5621-5633 Naeem et al. 2017;Wang et al. 2018). Sludge-based biochar has been most frequently used for the remediation of heavy metal-contaminated soil (Muller-Stover et al. 2021;Tomczyk et al. 2020;Wang et al. 2020). Research has been conducted to evaluate the residual effects of sludge-based biochar pyrolyzed at different temperatures on the accumulation, availability, and bioaccumulation of heavy metals in Maize in tropical soil. The biochar only increased the availability of heavy metals that are also essential elements for plants (Cu, Ni, and Zn) (Chagas et al. 2021). Xing et al. (2021) reviewed the differences between lignocellulose and sludge biochar in soil application, and found that the former was beneficial for carbon sequestration, biodegradation, and stabilization of heavy metals, while the latter could release minerals to stabilize heavy metals and improve soil nutrients. However, the risk brought by the application of sludge-based biochar to soil cannot be ignored. Wang et al. (2021) repaired soil via the co-pyrolysis of sludge and straw, and achieved better results than those achieved when applying sludge-based biochar, with a low effective state of metal. However, the nutrient supply was similar. Velli et al. (2021) studied the effect of sludge-based biochar on tomato growth and soil physical and chemical properties, and found that sludge-based biochar increased the contents of organic carbon, ammonium nitrogen, and available phosphorus to a greater extent. Additionally, the content of heavy metals in tomato tissue did not exceed the allowable standard, but the yield was not significantly increased. Sludge-based biochar has a certain risk in agricultural resource utilization as it contains high levels of heavy metals. However, little research has been conducted on this aspect, and the type of sludge biochar is an important factor.
To this end, the safe utilization of sludge-based biochar and its effect on the migration and transportation of heavy metals were examined in this study via pot experiments with Brassica chinensis L. We analyzed the effects of three kinds of biochar on the physical and chemical properties of soil and the growth of Brassica chinensis L. to evaluate the environmental and ecological risks in the biochar-plant system. The research objectives were to (a) elucidate the effects of different sludge-based biochars on the nutrient and heavy metal contents in soil, (b) examine the effects of different sludge-based biochars on the yield and heavy metal concentration of crops, and (c) assess the heavy metal pollution and human health risks of using sludge-based biochar in agriculture.

Material source and pretreatment
The experimental sludge was acquired from a sewage treatment plant in Huizhou City, Guangdong Province, China, with an industrial and domestic sewage treatment capacity of approximately 3:7. The sludge was dried in an oven at 105 °C until constant weight, ground, and sieved (2 mm), and stored in a self-sealing bag. The experimental soil was obtained from a park in Pudong New Area, Shanghai. The fast-growing plant Brassica chinensis L. was selected as the experimental crop. The physical and chemical properties of the mixed soil are shown in Table 1. Application of sludgebased biochar will not exceed the values of Chinese National Standards (GB 15618-2018).

Preparation of sludge-based biochar
Three reagents were selected for co-pyrolysis with sludge to prepare the biochar. The pyrolysis parameters are shown in Table 2.

Experimental design
The amount of FeCl 3 (BC1), Na 2 SiO 3 (BC2), and Ca (H 2 PO 4 ) 2 (BC3) added was 5% of the mass of potted soil. Three groups of parallel experiments were established, and a control experiment without additives was set up. The sludge-based biochar was mixed with the soil and loaded into pots. The mass in each pot was 1 kg. After watering off the seed water and stabilizing for 1 week, it was sown with 100 seeds (growth days = 60 days). No fertilizer was applied during the experiment.

Collection of soil and plant samples
After measuring its fresh weight at harvest, we washed the aboveground part and underground part of Brassica chinensis L. with distilled water. Then, we killed it at 105 °C and dried it at 65 °C. We then collected it after crushing, collected the soil at sowing and harvest, and dried it naturally before analysis. Table 3 Data analysis

Test items, methods, and instruments
Microsoft Excel 2007 was used to sort out the data. SPSS 22.0 was used for analyzing data, including Duncan's new multiple range method and the independent samples t-tests, and Origin 9.0 was used to plot figures.

Heavy metal pollution assessment of crops and soil
The single-factor and Nemero comprehensive pollution index methods (Derakhshan-Babaei et al. 2022;Wu et al. 2022) were used to evaluate the pollution status of Cu, Zn, Pb, and Cd in crops and soil. The calculation formula for the single-factor pollution index method is given as Eq. (1): where P is the single pollution index of heavy metal I in crops or soil, C is the measured value of heavy metal I in (1) P i = C i S i crops or soil (mg/kg), and S is the limit value of heavy metal I in crops or soil (mg/kg). P < 1 indicates that it has not been polluted by heavy metals, while P > 1 indicates that it has been polluted. Note that the greater P is, the more severe the pollution. The Nemero comprehensive pollution index was calculated using Eq. (2): where P Z is the comprehensive pollution index of heavy metals in crops or soil, P imax is the maximum pollution index of heavy metals in crops or soil, and P iave is the average value of all heavy metal single pollution indexes in crops or soil. 0 < P Z ≤ 0.7 means that the pollution level is safe, indicating that the crop or soil is clean; 0.7 < P Z ≤ 1 means that the pollution level is at the warning line, indicating that the crop or soil is still clean; 1 < P Z ≤ 2 means that the pollution level is light, indicating that crops or soil is exposed to pollution; 2 < P Z ≤ 3 means that the pollution level is medium, indicating that the crop or soil is moderately polluted; and P Z > 3 means that the pollution level is severe, marking the level at which the crops or soil are seriously polluted.

Brassica chinensis L. health and safety assessment
Heavy metals in soil are persistent, undergo enrichment, and are irreversible, which are important factors for food safety. According to the health risk concentration set by the U.S. Environmental Protection Agency (USEPA, 2000) in 2000, the target hazard factor (THQ) method can be used to evaluate the health risk of heavy metals in residents through vegetable intake. The evaluation formula is shown in Eq. (3) below: (2) where THQ is the target hazard coefficient of a single heavy metal element, E f is the exposure frequency (365 d), E d is the exposure period (25 years), E ir is the daily average crop intake (0.14 kg/d), C is the mass fraction of heavy metals in crops (mg/kg), R fd is the reference measurement of the daily average intake of heavy metals (mg/kg d), W ab is the adult weight (70 kg), T a is the life expectancy (70), and TTHQ is the composite hazard coefficient of various heavy metals. At THQ < 1, the health risk of heavy metal intake is not prominent, while at THQ > 1, there is a health risk of heavy metal intake. That is, the greater the THQ, the greater the health risk.

Effect of sludge-based biochar on soil physical and chemical properties
Impact on soil organic carbon Figure 1 shows the measurement results of soil organic carbon. Before planting, the soil organic carbon of each group was greatly improved compared with CK. Moreover, the promotion effects of the three biochar types on the organic carbon in soil were similar (247.05-258.92%). At the harvest of Brassica chinensis L., the soil organic carbon content of each component increased by 21.08%, 16.19%, 26.60%, and 26.57%. Although the increase in BC1 and BC2 was less than that in the CK group, the content of soil organic carbon was notably higher than that in the CK group. Notably, BC3 exhibited a substantial increase both before and after planting, thereby exerting the best effect on soil organic carbon. We surmise the possible drivers of the best performance. First, biochar itself is rich in organic carbon. Second, the addition of biochar can promote the adsorption of organic matter in soil (Wiesmeier et al. 2019). Overall, these results resonate with the findings of Zhang et al. (2018).

Effect on soil ammonium nitrogen
The soil ammonium nitrogen measurement results are shown in Fig. 2. The three treatment methods have no prominent effect on the improvement of soil ammonium nitrogen (almost the same as CK) before planting. Although sludge-based biochar can release nitrogen to the soil, it cannot be converted into ammonium nitrogen to be absorbed by plants. Taghizadeh-Toosi et al. (2011) previously conducted an isotope labeling experiment on N adsorption by biochar. We found that the labeled N is stable and non-volatile in air. Therefore, it can be used by plants when applied to the soil. The utilization rate of ammonia adsorbed by plant leaves and roots is 20-40%, which indicates that biochar reduces the loss of N and improves the utilization rate of N by adsorbing ammonia. Doydora et al. (2011) confirmed that biochar has a strong adsorption effect on NH 3 /NH 4+ , which can reduce the loss of gaseous ammonia nitrogen in soil. The ammonium nitrogen content of the soil after harvest increased by 84.67%, 67.17%, 23.54%, and 12.60%, respectively, compared with the conditions before the planting. The main driver was the fixation of ammonium nitrogen by biochar, which reduced the loss. This finding is consistent with the results of previous studies (Song et al. 2014 nutrients provided by biochar itself, the increase of soil organic matter and microbial nitrogen is conducive to the adsorption and fixation of more NH 4+ (Pan et al. 2022).

Effect on soil available phosphorus
We determined the content of available phosphorus in soil (Fig. 3). Compared with the CK group, before planting Brassica chinensis L., the addition of three kinds of biochar would increase the available phosphorus in soil by 17.63%, 38.01%, and 247.30%, respectively. During the harvest of Brassica chinensis L., the available phosphorus in soil increased greatly compared with that before the planting (by 219.19%, 369.41%, and 231.45%, respectively). Of them, treatment BC3 led to the largest increase in available phosphorus, which finally reached 56.48 mg/kg. This may be due to the conversion of inorganic phosphorus into organic phosphorus. Cheng et al. (2015) elucidated the contribution of inorganic phosphorus components to available phosphorus and reported that Ca 2 -P contributed the most to the available phosphorus, confirming that Ca 2 -P was the most effective phosphorus source for crops. Therefore, BC3 most efficiently increased the available phosphorus in soil, consistent with the results of Ge et al. (2022).

Effect on soil available potassium
We determined the soil available potassium as well (Fig. 4).
In particular, before the planting, the soil available potassium of each group has increased by ~ 5-11%, compared with CK. As seen, various biochar types increased the available potassium in the soil roughly with the same efficiency. Moreover, during the harvest of Brassica chinensis L., the soil available potassium in each component increased by 26.45%, 32.08%, 39.04%, and 37.12%, respectively. The increase ratio is similar to that of organic carbon. Although the increase ratio of BC1 and BC2 is less than that of the CK group, the content of soil available potassium is notably higher than that of the CK group. BC3 led to the greatest increase before and after the planting. As seen, BC3 exhibited the best effect on the soil available potassium. Notably, Lei and Zhang (2013) have already suggested that the addition of biochar can increase the content of available potassium in soil.

Impact of sludge-based biochar on the yield of Brassica chinensis L.
The yield of Brassica chinensis L. under different biochar conditions is shown in Fig. 5. CK is the control group without biochar addition. We found that the yield and chlorophyll of Brassica chinensis L. grown in the presence of biochar were higher than those of the control group. According to the determination of soil physical and chemical properties, the application of biochar can more efficiently improve the soil nutrients. In turn, it can promote the growth of Brassica chinensis L. Additionally, the application of biochar can also modify the soil structure and may even increase the types of microorganisms and enzymes (Zhang et al. 2019a Fig. 4 Soil available potassium content. Bars with different letters indicate significant differences at the 5% level was 5%, the yield and chlorophyll of BC3 were highest. This finding is consistent with previous research results on soil physical and chemical properties. The co-pyrolysis of calcium dihydrogen phosphate and sludge greatly increased soil nutrients, thereby promoting the increase of yield and chlorophyll in Brassica chinensis L.

Effect of sludge-based biochar on heavy metals in Brassica chinensis L.
Figure 6(a) demonstrates that, with the increase in biochar, the Cu content in the underground part for the BC1 and BC2 treatments increased significantly (172% and 232%, respectively) compared with CK. At the same time, there was no significant change in the Cu content for the BC3 treatment, and no significant change in the Cu content in the aboveground part of Brassica chinensis L., both being less than 12 mg/kg. Note that GB36783-2018 does not specify the Cu content in the usable parts of vegetables. However, given the results of the safety limit of heavy metal content in Chinese cabbage studied by Zhou et al. (2020), the experiment yielded a value (33 mg/kg) below the safety limit.
The Zn content in the aboveground and underground parts of Brassica chinensis L. is shown in Fig. 6(b). Compared with the CK, the Zn content in the underground part of Brassica chinensis L. greatly increased after applying the biochar. Among the treatments, the content increased the most for BC1 (42.18%), followed by BC2 (33.96%) and BC3 (13.65%). The Zn content in the aboveground part of Brassica chinensis L. was lower than that evaluated in the CK group (57.76 mg/kg). The Zn content in the aboveground part of the plant for the BC3 was lowest (− 40.84%), followed by BC1 (− 24.51%) and BC2 (− 21.64%). Except in the CK group, the Zn content in the aboveground part of Brassica chinensis L. was below the safe value (50 mg/kg) in all cases.
The Ni content of the aboveground part and underground part of Brassica chinensis L. is shown in Fig. 6(c). There was no significant change in the Ni content of the aboveground part and underground part after treatment with the three biochar types. This finding suggests that the addition of biochar did not affect the enrichment and migration of Ni by Brassica chinensis L. Moreover, the heavy metal content of the aboveground part of Brassica chinensis L. was also below the safe threshold (9.4 mg/kg). Figure 6(d) shows that the Pb content in the aboveground and underground parts of Brassica chinensis L. increased by 4.08% and 25%, respectively. Additionally, the Pb content in the aboveground and underground parts for the BC1 treatment was lower than that of the CK group. The Pb content in the underground part of the plant for the BC2 and BC3 treatments decreased by approximately 5.61% and 3.53%. Moreover, the Pb content in the aboveground part of Brassica chinensis L. decreased by approximately 25.22% and 36.53%. The values for the three components are below the safe value (0.3 mg/kg).
As shown in Fig. 6(e), the Cr content in the aboveground and underground parts of Brassica chinensis L. only slightly decreased by approximately 4.24%, 3.58%, and 32.03% for the BC1, BC2, and BC3 treatments, respectively. Moreover, the Cr content in the aboveground part of Brassica chinensis L. was less than that in the CK group. For BC1, BC2, and BC3, the values decreased by approximately 25.42%, 30.82%, and 13.83%. The values were all below the safety value (0.5 mg/kg).
As shown in Fig. 6(f), the Cd content in the aboveground and underground parts of Brassica chinensis L. slightly decreased by approximately 6%, and the Cd content of the aboveground part of Brassica chinensis L. was also lower than that evaluated in the CK group. The Cd contents for the BC1, BC2, and BC3 cases decreased by approximately 36.82%, 61.45%, and 66%, respectively. The values were all below the safe value (0.2 mg/kg).
The addition of biochar affected the content of heavy metals (except Ni) in the aboveground and underground parts of Brassica chinensis L. In line with the results of Gomez-Munoz et al. (2016), the addition of biochar inhibited the aboveground absorption of heavy metals by plants (Gomez-Munoz et al. 2016). Organic acids produced by plant roots can activate heavy metals in soil, which are more easily absorbed by roots . At the same time, organic acids can also chelate with heavy metals in soil, fix heavy metals in roots, and inhibit heavy metal transport (Chu et al. 2021). Compared with BC3, the Cu content in the underground part for cases BC1 and BC2 greatly increased. The possible driver is the ability of the BC1 and BC2 treatments to promote the fixation of Cu in plant roots.

Transport and enrichment of heavy metals in Brassica chinensis L.
The transfer coefficient (TF) is the ratio of heavy metal content in a plant part to that in the root, which can indicate the ability of plants to transport heavy metals from the root to the shoot. The enrichment coefficient EC is the ratio of the content of heavy metals in a plant to the content of corresponding heavy metals in soil. One can use this index to evaluate the ability of plants to enrich heavy metals.
The formulae for calculating the transfer coefficient are shown below in Eq. (4): As shown in Fig. 7, the TFs for six heavy metals in Brassica chinensis L. treated by the three treatment methods were less than one. Except for Cu, the TFs of Brassica chinensis L. treated by BC3 were lowest, indicating that BC3 can more efficiently inhibit the transport of heavy metals, while the TF force for Cu is higher. The possible driver is that the organic acids produced by the roots of Brassica chinensis L. treated by BC1 and BC2 activate more Cu, causing excessive Cu accumulation in the roots. However, the Cu content in the aboveground part was nearly the same, yielding a large TF for BC3. Additionally, P has a good synergistic effect on Cu absorption. For BC3, the inhibition of biochar on Cu and the promotion of P on Cu absorption cancel each other out, resulting in a similar transport of Cu by BC3 to the CK group. Cu is a necessary trace element for plant growth. We assumed that the Cu content should not exceed the standard, thereby allowing the promotion of plant growth due to the transportation of Cu. The enrichment effect (EC) of Brassica chinensis L. on six types of heavy metals was also weak, and the EC value was significantly reduced after adding biochar. This finding confirms the findings of Zheng et al. (2020), who had prepared biochar using attapulgite and sludge pyrolysis. Moreover, compared with the EC value of the CK group, the enrichment factors of Brassica chinensis L. for heavy metals decreased in the order of Zn > Cu > Ni > Cd > Pb > Cr.

Risk assessment of heavy metal pollution in Brassica chinensis L.
From single pollution index perspective, the pollution index (except Cu) of Brassica chinensis L. after various treatments decreased compared with the CK group (see Table 4). However, the Pb pollution index after the BC1 treatment increased, seemingly due to the activation of heavy metals by adding BC1 biochar and organic acids produced by plant roots. Although the roots can form an iron film and fix Pb to a certain extent, some Pb would be transported to the plant. The pollution index of Cu increased without causing Cu pollution. The low concentration of Cu will not expose the plants to toxicity, while promoting plant growth instead (Xiong et al. 2006). From the perspective of comprehensive pollution index, the pollution level of each treatment component was lower than that of the CK group. Additionally, the Brassica chinensis L. treated by BC2 and BC3 was at the safe level, indicating that the addition of biochar can inhibit the pollution of Brassica chinensis L. by heavy metals. Notably, the comprehensive pollution index of BC3 was the lowest, which could be because PO 4 3− and HPO 4 2− exhibit good adsorption on heavy metals and prevent the toxic effect of heavy metals on plants. Table 5 shows that the THQ values of various heavy metals are less than 1, indicating that the risk of heavy metals produced by eating Brassica chinensis L. is not significant. The TTHQ value was also less than 1, which decreased by 5.19%, 8.64%, and 12.80%, respectively, compared with the CK group. The application of biochar can inhibit the migration of heavy metals to the aboveground parts of plants and reduce the health risk of human intake of heavy metals.

Effects on heavy metals in soil
The addition of sludge-based biochar increased the soil heavy metals, but did not exceed the agricultural standard (GB 15618-2018). The soil heavy metals before and after planting are shown in Fig. 8. The reduction ratio of heavy metal content in the soil after harvesting Brassica chinensis L. with biochar is greater than that in the CK group. This finding can prove that adding biochar is beneficial to plants for absorbing heavy metals in the soil.
Notably, the contents of various heavy metals in the soil after harvesting Brassica chinensis L. with BC3 treatment exhibited the most significant decrease. Additionally, BC1 and BC2 absorb heavy metals (except Pb) for Brassica chinensis L. The promotion effect of BC1 is almost the same because the biomass and content of the aboveground and underground parts of plants after BC1 treatment are higher than BC2, resulting in a decrease of the soil Pb content after planting. The significant decrease of the Cd content after planting was driven by the addition of biochar, which increased the cation exchange capacity of soil and enhanced the adsorption capacity of Cd 2+ (Mahabadi et al. 2007). The reduction effect of the three treatment methods on soil decreased in the following order: BC3 > BC2 > BC1. When there are few absorbable states, the exchangeable state is converted to the absorbable state. Otherwise, it will be converted to the "difficult to absorb" state ( Fig. 9). The left side of each treatment method is the form of heavy metals in the soil before planting, while the right side is the form of heavy metals in the soil after planting. We found that, after planting Brassica chinensis L., the F1 in soil decreases, F4 increases, and the sum of F2 and F3 (exchange state) also decreases, which is in line with the findings of previous studies.

Biochar-plant system mechanism
As shown in Fig. 10, biochar is rich in organic matter and releases organic matter when mixed with soil. The release of organic matter can promote the fixation of ammonium nitrogen by plant roots. At the same time, the addition of biochar has a weak effect on the improvement of soil ammonium nitrogen, but biochar can fix ammonium nitrogen and reduce flow loss, thereby ameliorating soil fertility. While releasing available phosphorus, biochar can also convert inorganic phosphorus in soil into available phosphorus. The main mechanism is the conversion of inorganic phosphorus into soluble phosphorus through organic or inorganic acid produced by plant respiration or organic matter decomposition, which can be absorbed by plants. The application of biochar can fix more carbon and improve the soil C/N ratio. In short, the effect of biochar on soil fertility is mainly manifested in two forms. First, there is the adsorption of biochar as biochar adsorbs N, P, K, and other nutrients to avoid loss with water. Second, there is a spontaneous release of biochar as it contains more nutrients. Applying biochar to soil can alter soil fertility. The addition of biochar can stabilize heavy metals and increase the form of F4. Moreover, the application of biochar will promote the root to produce organic acids and fix more heavy metals. In high-phosphorus soil, the absorption of copper by plants will increase.

Conclusion
We found that the sludge-based biochar (BC1, BC2, and BC3 prepared from FeCl 3 , Na 2 SiO 3 , and Ca (H 2 PO 4 ) 2 , respectively) promoted the yield and soil physical and chemical properties of Brassica chinensis L. The promotion degree of three biochar decreased in the following order: BC3 > BC2 > BC1. Adding biochar will not cause the content of heavy metals in soil to exceed the standard (GB 15618-2018), and the usable part of plants after adding biochar also meets the standard (GB 2761-2017). Biochar can reduce the heavy metal pollution and human health risks and hinder the transport and enrichment of heavy metals in the edible part of Brassica chinensis L. while increasing the enrichment of heavy metals in plant roots. Notably, it is conducive to the utilization of biochar in soil remediation. Biochar can also promote the absorption of heavy metals in the form of F1 by plant roots, while the forms of F2 and F3 will decrease, and the form of F4 will increase. P in soil can promote the absorption of Cu by plants. PO 4 3− and HPO 4 2− contained in BC3 were conducive to the adsorption and coprecipitation of heavy metals in soil. Phosphorus in calcium dihydrogen phosphate exists in the form of Ca 2 -P, which can greatly improve the available phosphorus in soil. To conclude, we identified that BC3 was a better soil conditioner, paving the way toward its application in agriculture.
Funding Results incorporated in this paper received funding from the National Key R & D Project of China (grant number 2019YFC1906100), the National Natural Science Foundation of China (grant numbers 51578397, 51808496), and the foundation of Shanghai Polytechnic University (grant number EGD22DS13).
Data availability All data generated or analyzed during this study are included in this article.

Declarations
Ethics approval and consent to participate Not applicable.

Competing interests
The authors declare no competing interests.