Remediation effect of modified biochar on Cd-contaminated soil in post-mining land on the Loess Plateau, China

Biochar (BC) has been widely used in remediating soil Cd contamination, and the efficiency of remediation can be improved by modification. However, Cd immobilization mechanism of modified biochar and its influence factor in coal mine soil receive little attention. In this study, characterizations of four modified biochars (H 2 O 2 -BC, HNO 3 -BC, KMnO 4 -BC and CaCl 2 -BC) and their remediation effects on soil Cd contamination at different addition ratio (1%, 2% and 3%) were investigated. Modified biochar addition significantly affected soil pH and CEC. The acidic functional group and carboxyl group of CaCl 2 -BC and HNO 3 -BC were higher ( p <0.05) than those of BC and other modified biochars. There were significant differences in soil Cd fractions under different modified biochar treatments. The exchangeable fraction with high bioavailability and Fe-Mn oxides bound fraction with potential ecotoxicity of soil Cd decreased under HNO 3 -BC and CaCl 2 -BC treatments at a 2% addition ratio, respectively. The diethylene triamine pentaacetic acid -extractable Cd (DTPA-Cd) in soil, ready to be absorbed by plants, decreased under modified biochar treatments, and the largest reduction occurred in HNO 3 -BC addition at 2% ratio. RDA analysis indicated that soil pH and carboxyl group significantly ( p < 0.05) affected Cd fractions in coal mining soil. Our study suggests that HNO 3 -BC increased main functional groups, leading to high adsorption and complexation capacity, which promoted the removal of soil Cd with high mobility and activity. Therefore, HNO 3 -BC could serve as an effective measure for Cd pollution remediation in degraded ecosystems, especially in the alkaline soil in northern China. 1 Introduction Soil contamination with heavy metal is a severe concern for human health due to its non-biodegradable, persistent and accumulative nature (Tang et al., 2019), which may damage ecosystems and their functions. Cadmium (Cd) is considered as a pervasive, mobile and hazardous element in soils, which is easily absorbed by crops, then endanger food security and human health (Wang et al., 2022). In China, more than 13 million hectares of cultivated lands were contaminated by Cd (Yu et al., 2006). Cd in soil exists different fractions such as exchangeable, carbonate, Fe-Mn oxides and organic fractions, among which exchangeable and carbonate fractions are considered bioavailable and easily absorbed by plants, while Fe-Mn oxides and organic fractions are hardly utilized by plants and exhibit low health risk (Wu et al., 2019; Liu et al., 2025). Therefore, it is necessary to remediate Cd-contaminated soil for increasing biodiversity, restoring soil fertility and ecological functions. Coal mining caused a series of ecological and environmental problems such as heavy metal contamination, vegetation deterioration and water loss and soil erosion. Soil near coal gangue is easily contaminated by heavy metals, and Cd receives particular attention due to its toxicity and biomagnification (Zheng et al., 2024). Heavy metals (e.g., Cd, Pb) pollution altered the composition of bacterial communities in a coal gangue site, and Actinomarinales, Pedomicrobium and Alphaproteobacteria may be resistant to Cd (Kou et al., 2023). Cd showed high activity and ecological risk in coal gangue dump area, with progressive accumulation in plants over time (Wang et al., 2025). However, studies on Cd contamination in coal mining sites are insufficient, especially the toxicity and transformation of different Cd fractions have not been explored. Therefore, it is urgent to develop effective Cd removal techniques in coal mining sites to alleviate ecological toxicity and minimize accumulation risks. Biochar (BC) is a porous and carbon-rich solid material, produced by pyrolyzing organic waste under limited oxygen conditions (Gao et al., 2022). Biochar can remediate soil Cd contamination due to its high specific surface area and active surface functional groups (e.g. hydroxyl, carboxyl and carbonyl), which facilitate immobilization of Cd 2+ in soil through complexation, electrostatic attraction, cation exchange and precipitation (El-Naggar et al., 2022; Wang et al., 2021). Until now, biochar application plays an important role in decreasing Cd bioavailability in croplands, wetlands and mining areas (Bian et al., 2014; Meng et al., 2023; Liu et al., 2020), and studies mainly focus on the Cd immobilization, speciation distribution in soil and accumulation in plants. Nevertheless, application of large amounts of biochar may lead to soil compaction and structure degradation due to the alkalinity of biochar (Wei et al., 2021). To enhance the remediation efficiency of biochar in Cd-contaminated soils, studies on modified biochar with higher surface area, more porosity and functional groups are critical for improving the adsorption and immobilization of soil Cd. Modification methods of biochar include physical, chemical and microbial modification. Chemical modification of biochar, including acid/alkali, oxidizer and metal salt modification, has been widely applied in environmental remediation, particularly for Cd pollution removal (Panwar and Ashish, 2022). Acid/alkali modification can increase the number of functional groups, improve pore structure and provide more cation exchange or attachment sites on biochar surface, therefore enhanced adsorption efficiency of pollutants (Lan et al., 2024). Oxidizer modification enhances redox potential of biochar by increasing carbonyl groups, which play vital roles in catalytic oxidation (Zhou et al., 2021). Metal salts modification could increase the specific surface area and adsorption sites of biochar, thereby improving its adsorption capacity for heavy metals (Zhang et al., 2022). Overall, biochar modification with acids, oxidizers or metal salts enhances its pollutant removal efficiency by increasing the surface area, functional groups and active sites. Modified biochar serves as a promising material for Cd immobilization in soil, facilitating the transformation of Cd fractions while reducing Cd mobility and bioavailability (Maharlouei et al., 2021; Da et al., 2023; Wei et al., 2024). In recent years, modified biochar has received some attention for Cd remediation in agricultural and wetland ecosystems, with research focusing on modification methods, immobilization mechanism and key influencing factors. Previous studies indicated that modified biochar can passivate Cd in yellow brown soil in southern Shaanxi province, while improving soil physical and chemical properties (Tang et al., 2022). Modified biochar application decreased acid extractable Cd and reducible Cd, and increased residual Cd in soil by improving soil physicochemical properties and functional microorganisms (Chen et al., 2025; Liang et al., 2018). Modified biochar decreased the concentrations of diethylene triamine pentaacetic acid (DTPA) extractable Cd and exchangeable Cd in paddy soils (Wu et al., 2019). Moreover, different modification methods affect the adsorption and fixation capacity of biochar for soil Cd. Hence, the remediation of modified biochar for soil Cd contamination is influenced by multiple factors, particularly specific modification method, soil type and contamination level. However, few studies focus on the application of modified biochar in coal mine soil (Gong et al., 2022), and its remediation potential for Cd contamination in such environments has yet been sufficiently investigated. In this study, modified biochars (H 2 O 2 -BC, HNO 3 -BC, KMnO 4 -BC and CaCl 2 -BC) were prepared and characterized by different analytical methods to assess their physical and chemical properties. Meanwhile, the remediation effects of modified biochars on soil Cd contamination in coal mine sites were studied at different addition ratio. We hypothesized that (1) modified biochar would improve pore structure, increase the number and type of functional groups, thereby enhancing adsorption efficiency of Cd; (2) modified biochar addition significantly change soil Cd fractions and effectively remediate soil Cd contamination. The results of this study are of great significance to promote the ecological restoration in coal mine areas, and expand a new way of “utilization of waste resources”. 2 Materials and methods 2.1 Preparation of soil and modified biochar Soils for the experiment were obtained from top layer (0-20 cm) of a reclaimed area in Xishan coal mine (112°27′E, 37°49′N), located in Taiyuan City, Shanxi Province, China. Soil type is classified as Ochrept (USDA Soil Taxonomy) originating from loess sediments. Coal mining produces coal gangue and tailings containing large amounts of heavy metals such as Cd, Pb and As, leading to Cd concentration exceeding regulatory thresholds in surrounding soils. Furthermore, Cd contamination exhibits a belt-like distribution, with the most severe contamination occurring in areas near mines and coal gangue piles. Based on similar parent material, slope direction and manual management, three sampling areas were set up around the coal mining sites. Three plots (10 m×10 m) were randomly set up in each sampling area, and three quadrats (1 m × 1 m) were established in each plot. Soil samples were collected using soil auge after removing surface vegetation. The collected soil samples were mixed, air-dried, removed plant roots and gravels, then ground and sieved through a 2 mm mesh. The basic properties of original soil were shown in Table 1. Biochar (BC) was produced from corn straw by pyrolysis in a muffle furnace at 550℃ for 30 min, followed by drying at 80℃ for 24 h and sieving through a 100-mesh screen (Wu et al., 2019). Modified biochars were prepared by treating biochar with solutions (2M H 2 O 2, 2M HNO 3, 2M CaCl 2 and 0.1 M KMnO 4 ) at the ratio of 1:6 (m/v), followed by oscillation at 25℃ for 24 h. Modified biochars were obtained by ultrasonic cleaning, filtering, washing with distilled water and drying at 80℃ for 24 h. Furthermore, KMnO 4 -modified biochar was pyrolyzed in a muffle furnace under anaerobic conditions at 400℃ for 3 h. 2.2 The characterization of biochar and determination of soil properties The surface morphology of biochar was examined using a scanning electron microscope (SEM). The content of acid functional groups on biochar surface was determined by titration (He et al., 2022). Fourier transform infra spectroscopy (FITR) was used to identify functional groups of biochar with the KBr method (1: 100, w/w). Biochar and KBr were mixed, ground and tableted under infrared lamp irradiation, followed by FTIR analysis (4000-400 cm −1 scan range, 0.2 cm −1 spectral resolution) (Cao et al., 2019). The pH of soil and biochar were measured by mixing solid and deionized water at a 1:20 ratio (w:v) with a pH meter (PHS-3E, Shanghai, China). The cation exchange capacity (CEC) of soil and biochar were determined using 0.1 M BaCl 2 at the ratio of solid to solution 1:50 (Trakal et al., 2016). The ash content of biochar was analyzed using an automatic measuring analyzer according to Zhang et al. (2020a). 2.3 Soil incubation experiment The modified biochars were added into soil samples (0.5 kg) according to 1%, 2% and 3% of soil quality. The mixture of biochar and soil were loaded into plastic pot and cultivated in an incubator with constant temperature (25℃). The treatments consisted of CK (no biochar addition), BC (1%, 2%, 3% addition), KMnO 4 -BC (1%, 2%, 3%), HNO 3 -BC (1%, 2%, 3%), H 2 O 2 -BC (1%, 2%, 3%), CaCl 2 -BC (1%, 2%, 3%). Three replicates were prepared for each treatment. Soil moisture was maintained at 60% of field water holding capacity throughout the 75-day incubation period. 2.4 Measurement of soil Cd fractions The different fractions of soil Cd were measured using a Tessier sequential extraction (Tessier et al., 1979). Soil samples (0.25 g) were weighed into 50 mL polyethylene centrifuge tubes. The extraction processes involved four stages as follows: Exchangeable fraction, 20 mL 0.11 mol/L CH 3 COOH at 25℃ for 16 h; Fe-Mn oxides bound fraction, 20 mL 0.5 mol/L NH 2 OH·HCl (pH=1.5) at 25℃ for 16 h; Organic fraction, 5 mL 30 % H 2 O 2 at 25℃ for 30 min and 85℃ for 30 min, followed by 5 mL 30% H 2 O 2 until solution nearly dry and 25 mL 1 mol/L CH 3 COONH 4 (pH=2) at 25℃ for 16 h; Residual fraction, calculate the content of residual state using the difference subtraction method. Following each extraction stage, solutions were centrifuged for 20 min at 5000 rpm/min and the supernatant was measured. Cd contents in the supernatants were measured by flame atomic absorption spectrophotometer (FAAS) (Wu et al., 2019). The soil samples were digested with a mixture of HCl, HNO 3 and HClO 4, and soil total Cd was measured using FAAS. The available Cd in soil was extracted using the solution of DTPA (diethylene triamine pentaacetic acid), and determined by FAAS (Liu et al., 2024). 2.5 Statistical analyses SPSS 23.0 software was used for statistical analysis. The significant differences among modified biochar treatments were tested by one-way analysis of variance, and pairwise comparisons were performed using the least significant difference (LSD). The data shown in the tables and figures were analyzed using Excel 2013 and Origin 2021. Redundancy analysis (RDA) was used to assess the contribution of soil and biochar characteristics to soil Cd fractions. In addition, the Pearson correlation coefficient method was used for the correlation analysis between measured variables. Statistically significant was considered at p < 0.05 level. 3 Results 3.1 Characteristic for modified biochar The characteristics of biochar and modified biochar are presented in Table 2. The pH of modified biochars (H 2 O 2 -BC, HNO 3 -BC and CaCl 2 -BC) were lower ( p <0.05) than biochar (BC), and HNO 3 -BC showed the greatest decrease in pH from 9.24 to 4.36 (Table 2). A significant ( p <0.05) increase in pH was observed for KMnO 4 -BC relative to BC. The CEC of HNO 3 -BC and CaCl 2 -BC decreased ( p <0.05) with respect to BC. Meanwhile, the ash content of HNO 3 -BC has an obviously ( p <0.05) decrease compared with BC (Table 2). 3.2 SEM images, acidic groups and FTIR spectra of modified biochar The surface morphology of biochar and modified biochars were characterized by SEM (Fig.1). Compared to BC, H 2 O 2 -BC improved pore structure and specific surface area, due to H₂O₂-induced micropore formation. HNO 3 -BC exhibited obvious skeleton structure and smooth surface relative to BC, which could be attributed to HNO 3 oxidation reducing mineral content and damaging pore structure. Compared with other biochars, CaCl 2 -BC had some transparent particulates on its surface (Fig.1), which were attributed to the formation of Ca 2+ and its oxides. KMnO 4 -BC exhibited a distinct layered surface structure, which increased its specific surface area. Overall, KMnO 4 -BC, H 2 O 2 -BC and CaCl 2 -BC had more pore structure, which enhanced their specific surface area and improve their capacity for soil Cd remediation. The acidic functional groups contents of biochar and modified biochar were given in Table 3. Phenolic hydroxyl was the most abundant functional group in BC, accounting for 67.9% of the total acidic functional groups. The significant ( p <0.05) increase in acidic functional group and carboxyl group of HNO 3 -BC were observed in comparison with those of BC. The CaCl 2 -BC exhibited significantly ( p <0.05) higher acidic functional group and carboxyl group but lower phenolic hydroxyl group ( p < 0.05) compared to BC (Table 3). The acidic functional groups in KMnO 4 -BC were significantly ( p <0.05) lower than those in BC, with reductions in carboxyl and phenolic hydroxyl groups and an increase in internal lipid group. The carboxyl group increased ( p <0.05) and phenolic hydroxyl group decreased ( p <0.05) in H 2 O 2 -BC with respect to BC. Overall, acidic functional groups of modified biochar significantly increased compared to BC, except KMnO 4 -BC. The FTIR spectrum of biochar and modified biochar are plotted in Fig.2. The shifts in stretching vibration peaks and their positions in modified biochar indicated changes in functional groups of biochar after modification. HNO 3 -BC showed stretching vibration of –NO 2 at 1335.87 cm -1 (Fig.2), resulting from HNO 3 decomposition. H₂O₂-BC exhibited two new peaks at 3428.76 cm⁻¹ and 1423.47 cm⁻¹, corresponding to –OH stretching vibrations from H₂O molecules generated during H₂O₂ decomposition. The stretching vibration peak of CaCl 2 -BC was enhanced at 468.43 cm -1, likely due to enhanced C-C stretching and the formation of Ca oxide on biochar surface. There are significant differences in the number and stretching vibration of absorption peak in KMnO 4 -BC and H 2 O 2 -BC than those in BC. 3.3 pH, CEC and Cd fractions in soils after adding modified biochar 3.3.1 Soil pH and CEC Soil pH significantly ( p <0.05) decreased after adding H 2 O 2 -BC, HNO 3 -BC and CaCl 2 -BC relative to BC addition (Table 4). KMnO 4 -BC increased ( p <0.05) soil pH at 1% addition ratio compared to BC. Soil CEC decreased ( p <0.05) after HNO 3 -BC addition and increased ( p <0.05) after CaCl 2 -BC addition compared with BC addition. H 2 O 2 -BC decreased ( p <0.05) soil CEC at 2% addition ratio (Table 4). 3.3.2 Cd fractions in soils Cd was mainly bound to exchangeable and residual fractions in soils, and the sum of two fractions accounted to more than 60% of total Cd. The application of BC and modified biochar obviously changed soil Cd fractions, decreasing exchangeable Cd fraction by 6.37% to 37.79% (Fig. 3). Compared to CK (no biochar addition) and BC, the greatest decrease in soil exchangeable Cd occurred with the addition of HNO 3 -BC at 2% ratio. Compared to CK and BC, the Fe-Mn oxides bound fractions of Cd in soil decreased by 15.16%-59.02% and 14.70%-30.37% respectively, with the greatest decrease observed in CaCl 2 -BC at 2% addition ratio. In addition, organic fraction of Cd was detected, accounting for 0.76%-4.86% of soil total Cd, and showed a positive correlation with modified biochar addition ratio. Compared to BC, modified biochars increased the organic fraction of soil Cd by 46.51%-84.88%, with the largest increase observed for HNO 3 -BC and CaCl 2 -BC addition at 2% ratio. In general, additions of BC and modified biochar decreased the exchangeable fraction and Fe-Mn oxides bound fraction of soil Cd, among which HNO 3 -BC significantly reduced exchangeable Cd fraction and CaCl 2 -BC decreased Fe-Mn oxides bound fraction of Cd at 2% addition ratio. 3.3.3 DTPA-Cd in soils Compared to CK, the application of BC and modified biochars decreased soil DTPA-Cd by 12.04%-47.33% (Fig.4). Compared to BC, modified biochars addition decreased soil DTPA-Cd by 0.88%-33.56%, with the significant decrease observed for HNO 3 -BC addition at 2% ratio. During incubation period, soil DTPA-Cd decreased with increasing application ratio of BC, H 2 O 2 -BC and HNO 3 -BC. Overall, soil DTPA-Cd decreased under the treatments with BC and modified biochars relative to CK. There was a significant decrease in soil DTPA-Cd under HNO 3 -BC addition at 2% ratio than that BC addtion. 3.4 Relationships between the properties of soil and modified biochar and soil Cd fractions RDA analysis indicated that soil properties and biochar characteristics explained 44.7% of the variations in soil Cd fractions, and soil pH, carboxyl and lactone groups on biochar surface were significant factors for soil Cd fractions (Fig. 5, Table 5). Soil pH was positively ( p < 0.05) related to exchangeable Cd fraction and Fe-Mn oxides bound fractions of Cd, and negatively related to organic fraction of Cd in soils. Carboxyl group was positive ( p < 0.01) factor on soil DTPA-Cd, and had a significant ( p < 0.01) inhibitory effect on Fe-Mn oxides bound fraction of Cd. Lactone group of biochar positively ( p < 0.05) affected Fe-Mn oxides bound fraction of Cd, and negatively ( p < 0.05) affected DTPA-Cd in soils (Fig. 5, Table 5). According to pearson correlation analysis, soil CEC was negatively ( p < 0.01) related to Fe-Mn oxides bound fraction of Cd in soil; the pH and ash of biochar were negatively ( p < 0.05) related to organic fraction of Cd in soil; lactone group was negatively ( p < 0.01) related to soil DTPA-Cd (Fig. 6). 4 Discussion 4.1 Effects of modification on biochar characteristics Previous studies have indicated that modification can increase the number of acidic functional groups on biochar surface, enhancing its sorption capacity and remediation efficacy for soil Cd by complexation (Li et al., 2014; Xu et al., 2018). In this study, biochar modification with HNO 3 and CaCl 2 significantly increased acidic functional group and carboxyl group, which were in agreement with previous studies (Li et al., 2014; Zhuo et al., 2022). Nitrogen-containing functional groups were attained on biochar surface by HNO 3 modification, facilitating the complexation of Cd cations (Shafeeyan et al., 2010). Biochar modification with CaCl 2 enhanced Cd removal efficiency, which was associated with the complexation of functional groups and strong ion exchange capacity of Ca 2+ on biochar surface (Gao et al., 2023). The stretching vibration of nitro (–NO 2 ) was detected at 1339 cm −1 in HNO 3 /H 2 SO 4 -modified biochar (He et al., 2021), which was consistent with our study. The peaks at 1315 cm -1 and 1700 cm -1 corresponded to carboxylic acid functionality of H 2 O 2 -modified biochar (Huff and Lee, 2016), thereby a new peak of H 2 O 2 -BC at 1423.47 cm -1 may be carboxyl group in this study. The lower pH observed in HNO 3 -BC and CaCl 2 -BC can be explained by the acidic nature of acidic functional group and carboxyl group on biochar surface. Modification increased the surface area and pore volume of biochar, which significantly improved its Cd sorption capacity in soils (Zhong et al., 2023). Biochar modification with CaCl 2 showed larger specific surface area and excellent adsorption capacity, supporting our first hypothesis. These improvements are attributed to Ca/ CaO deposition and cation exchange on biochar surface (Zhang et al., 2023). The higher surface area, micropore creation and layered structures of KMnO 4 -BC could be explained by the presences of manganese oxide particles and oxidation of biochar (Zhang et al., 2012; Song et al., 2014). The larger surface area and microporous volume of modified biochars provided more adsorption sites for heavy metals, playing important roles in enhancing the remediation efficiency of soil Cd contamination. 4.2 Regulation of modified biochar on soil Cd fractions The removal of soil Cd by biochar primarily involve two mechanisms: (1) physical adsorption within porous structure of biochar; (2) chemical reactions (e.g., ion exchange or redox) immobilize Cd cations as precipitates or reduce Cd toxicity by altering its oxidation state (Yang et al., 2019). Cd fractions include the extractable Cd fraction with high mobility and activity, Fe-Mn oxides bound fraction and organic fraction of Cd with potential ecological risks. Functional groups such as hydroxyl, amino and carbonyl functional groups on biochar surface play important roles in immobilizing soil Cd fractions (Yu et al., 2021). Previous studies have indicated that modified biochar promotes the fraction changes in soil Cd, significantly reducing the exchangeable and acid-soluble fractions while increasing Fe-Mn oxides bound fraction of Cd (Wei et al., 2024). In this study, application of modified biochars changed the speciation distribution of soil Cd, decreasing the exchangeable fraction while increasing residual fraction, driven by enhanced acidic functional group and carboxyl group on modified biochars surface, which was consistent with our second hypothesis. The surface functional groups of modified biochar could enhance the specific adsorption of soil Cd, with cation exchange and electrostatic attraction being the main mechanism for Cd immobilization (He et al., 2021; Bian et al., 2015). Moreover, HNO 3 -modified biochar facilitated the conversion of water soluble and exchangeable Cd fractions into metal oxide- and organic bound fractions, which were attributed to increasing oxygen functional groups on modified biochar surface and promoting Cd complexation (Boostani et al., 2024). The removal of Cd by biochar mainly occurred through Ca 2+ cation exchange and physical attachment of Cd compound precipitation (Liu et al., 2023). In this study, the exchangeable fraction and Fe-Mn oxides bound fraction of Cd decreased 23.33% and 30.37% under HNO 3 -BC and CaCl 2 -BC treatments at 2% addition ratio, which can be explained by cation exchange between Ca 2+ and Cd 2+, the larger specific surface area of CaCl 2 -BC and the increased carboxyl functional groups on HNO 3 -BC surface (Zhang et al., 2020b; He et al., 2021). In addition, the increase in soil organic matter (SOM) contributed to more Cd converting to a highly stable state (Zhang et al., 2017). 4.3 Remediation of modified biochar on soil DTPA-Cd Soil DTPA-Cd is used to assess Cd bioavailability and potential toxicity, which reflects the transformation ability of Cd from soil to plant (Meng et al., 2018). Acid modification significantly increased carboxyl functional groups on biochar surface compared to oxidant modification, and surface functional groups of biochar could enhance the specific adsorption for DTPA-Cd in soil (He et al., 2021). HNO 3 -BC enriched Cd sorption affinity and capacity due to more active sites generated by acidic functional groups on biochar surface (Boostani et al., 2024). The dissociation of carboxyl groups generated organic anions that consumed exogenous protons, thereby increasing soil pH and reducing soil available DTPA-Cd (He et al., 2023). In this study, the content of soil DTPA-Cd had obviously decrease under HNO 3 -BC addition at 2% ratio, which could be explained by the increase in acidic functional group and carboxyl group of HNO 3 -BC. The Cd removal capacity of biochar is primarily determined by inorganic mineral composition and organic functional groups on biochar surface (Sun et al., 2022). Previous studies showed that cation exchange between Ca 2+ and Cd 2+, and physical attachment of Cd compound play important roles in Cd removal (Liu et al., 2023; Ben Salem et al., 2025). Functional groups (e.g. –OH, –COOH) on biochar surface enhance Cd immobilization in soils through adsorption and complexation mechanisms, thereby reducing Cd bioavailability (Peng et al., 2017). In this study, soil DTPA-Cd was positively and significantly related to carboxyl group on biochar surface according to RDA analysis. The carboxyl group content on CaCl 2 -BC surface was significantly higher than that on BC. Thus, the enhanced removal of soil DTPA-Cd by CaCl 2 -BC can be attributed to Ca 2+ cation exchange and the increase in carboxyl groups. 5 Conclusions Compared to BC, HNO 3 -BC and CaCl 2 -BC showed significant increase in acidic functional group and carboxyl group. The changes in the stretching vibration peaks and their positions in biochar were attributed to alterations in their functional group composition. The exchangeable Cd exhibited high solubility and bioavailability, leading to significant ecological toxicity; the Fe-Mn oxides bound fraction and organic fraction of Cd could be converted under certain conditions, potentially producing ecological toxicity. The exchangeable fraction and Fe-Mn oxides bound fraction of Cd decreased under HNO 3 -BC and CaCl 2 -BC addition at 2% ratio, respectively. The addition of modified biochars reduced soil DTPA-Cd content, and the largest reduction was observed at a 2% addition ratio of HNO 3 -BC. Soil pH and carboxyl group significantly affected soil Cd fractions in post-mining land. Thus, the application of HNO 3 -BC could serve as an effective strategy for Cd pollution remediation in coal mine sites. Our results imply that selection of appropriate remediation measures are crucial for heavy metal-contaminated soils in degraded ecosystems, especially in northern China’s alkaline regions.

This work was jointly supported by the Youth Science Research Program of Shanxi Province (grant No. 202303021222280). Conflict of interest The authors declare that they have no competing interests in this paper. Data availability statement The data are available from the corresponding author upon request.

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Table 1 Basic physical and chemical properties of collected soils (results are means ± SE, n=3). | Items | Soil | | pH | 8.17±0.02 | | CEC (cmol·kg -1 ) | 8.02±0.38 | | Electrical conductivity (ds·m -1 ) | 0.034±0.005 | | Total Cd (mg·kg -1 ) | 14.84±0.36 | | Exchangeable Cd (mg·kg -1 ) | 5.91±0.39 | | Fe-Mn oxides bound fraction of Cd (mg·kg -1 ) | 1.53±0.07 | | Organic fraction of Cd (mg·kg -1 ) | – – | | Residual fraction of Cd (mg·kg -1 ) | 7.40±0.22 | | DTPA-Cd (mg·kg -1 ) | 2.44±0.10 | Table 2 The pH, CEC and ash content of biochar and modified biochars (results are means±SE, n=3). | Items | Biochar | H 2 O 2 -BC | HNO 3 -BC | KMnO 4 -BC | CaCl 2 -BC | | pH | 9.24±0.16b | 8.82±0.12c | 4.36±0.16e | 10.13±0.21a | 7.50±0.08d | | CEC (cmol/kg) | 24.00±0.60ab | 22.88±1.20b | 17.44±0.70c | 25.76±1.25a | 19.04±1.30c | | Ash content (%) | 35.50±2.40a | 34.55±0.35a | 26.60±1.80b | 37.20±4.90a | 33.85±0.65a | Notes: H 2 O 2 -BC, HNO 3 -BC, KMnO 4 -BC and CaCl 2 -BC represent the biochar modification with H 2 O 2, HNO 3, KMnO 4 and CaCl 2, respectively. Different lowercase letters in the same row indicated significant ( p <0.05) differences in properties of biochar and modified biochars. Table 3 The contents of acidic functional groups on the surface of biochar and modified biochars (results are means ± SE, n=3). | Biochar (mmol/g) | acidic functional group | carboxyl group | phenolic hydroxyl group | internal lipid group | | Biochar | 1.12±0.13c | 0.08±0.02d | 0.76±0.22a | 0.28±0.06bc | | HNO 3 -BC | 1.80±0.17a | 0.62±0.13b | 0.78±0.07a | 0.40±0.05b | | KMnO 4 -BC | 0.69±0.07d | 0.02±0.00d | — | 0.67±0.12a | | H 2 O 2 -BC | 1.20±0.16bc | 0.36±0.08c | 0.44±0.11b | 0.40±0.13b | | CaCl 2 -BC | 1.38±0.15b | 1.14±0.05a | 0.02±0.01c | 0.22±0.05c | Note: H 2 O 2 -BC, HNO 3 -BC, KMnO 4 -BC and CaCl 2 -BC represent the biochar modification with H 2 O 2, HNO 3, KMnO 4 and CaCl 2, respectively. Different lowercase letters in the same column indicated significant ( p <0.05) differences in functional groups of biochar and modified biochars. Table 4 The pH and CEC in soil with modified biochar treatments at different addition ratios (results are means±SE, n=3). | Items | Biochar | H 2 O 2 -BC | HNO 3 -BC | KMnO 4 -BC | CaCl 2 -BC | | | Soil pH | 1% | 8.11±0.05aA | 7.94±0.14bA | 7.88±0.02bA | 8.22±0.04cA | 7.93±0.03bA | | 2% | 8.20±0.02aB | 7.98±0.03bA | 7.84±0.03cA | 8.23±0.03aA | 7.71±0.01dB | | | 3% | 8.22±0.02aB | 7.99±0.03bA | 7.73±0.02cB | 8.24±0.04aA | 7.65±0.03dB | | | Soil CEC | 1% | 8.36±0.20aA | 8.30±0.10aA | 8.00±0.10bA | 8.50±0.10aA | 10.00±0.20cA | | 2% | 8.82±0.11aB | 8.20±0.40bA | 8.50±0.17cB | 8.70±0.17acA | 10.20±0.20dAB | | | 3% | 8.89±0.10aB | 8.87±0.04aB | 8.70±0.10abB | 9.00±0.26acB | 10.43±0.15dB | Notes: Different lowercase letters in the same row indicated significant ( p <0.05) differences among different modified biochar treatments. Different uppercase letters in the same column indicated significant ( p <0.05) differences among modified biochars addition ratios. Table 5 The conditional term effects of the properties of soil and modified biochars on soil Cd fractions. | Name | Explains % | pseudo-F | p | | Carboxyl group | 22.3 | 14.6 | 0.002 | | Soil pH | 13.2 | 6.5 | 0.010 | | Lactone group | 5.3 | 3.6 | 0.024 | | Soil CEC | 3.3 | 2.4 | 0.098 | | pH of biochar | 2.0 | 1.4 | 0.194 | | CEC of biochar | 0.7 | 0.5 | 0.598 | Fig. 1 SEM maps of biochar and modified biochars (5000×). (a), (b), (c), (d) and (e) represent the BC, H 2 O 2 -BC, HNO 3 -BC, CaCl 2 -BC and KMnO 4 -BC. Fig. 2 FITR spectra for biochar and modified biochars. BC, biochar; H 2 O 2 -BC, HNO 3 -BC, CaCl 2 -BC and KMnO 4 -BC represent modified biochars by H 2 O 2, HNO 3, CaCl 2 and KMnO 4, respectively. Fig. 3 Different Cd fractions in soils after biochar and modified biochar additions at 1%, 2% and 3% ratios. Notes : E-Cd, R-Cd, O-Cd and S-Cd represent exchangeable Cd, Fe-Mn oxides bound fraction of Cd, organic fraction of Cd and residual fractions of Cd in soil. CK, no biochar addition; BC, biochar; H 2 O 2 -BC, HNO 3 -BC, CaCl 2 -BC and KMnO 4 -BC represent modified biochars by H 2 O 2, HNO 3, CaCl 2 and KMnO 4, respectively. Fig. 4 The contents of DTPA-Cd in soils after biochar and modified biochar additions at 1%, 2% and 3% ratios. Notes : CK, no biochar addition; BC, biochar; H 2 O 2 -BC, HNO 3 -BC, CaCl 2 -BC and KMnO 4 -BC represent modified biochars by H 2 O 2, HNO 3, CaCl 2 and KMnO 4, respectively. Different lowercase letters indicated significant ( p <0.05) differences between biochar and modified biochars. Bars show standard error (n = 3). Fig. 5 Redundancy analysis (RDA) between soil Cd fractions and the properties of soil and modified biochars. Notes : S-pH, S-CEC represent soil pH and CEC; B-pH and B-CEC represent the pH, and CEC of biochar; CG and LG represent carboxyl and lactone groups on biochar surface. E-Cd, R-Cd, O-Cd and DTPA-Cd represent exchangeable Cd, Fe-Mn oxides bound fraction of Cd, organic fraction of Cd and diethylene triamine pentaacetic acid extractable Cd in soils, respectively. Fig.6 Pearson correlation coefficients among pH, CEC of soil and biochar, the contents of functional groups of biochar and soil Cd fractions. Notes : S-pH, S-CEC represent soil pH and CEC; B-pH, B-CEC and Ash represent the pH, CEC and ash content of biochar; AFG, CG, PHG and LG represent acidic functional group, carboxyl, phenolic hydroxyl and lactone groups on biochar surface. E-Cd, R-Cd, O-Cd and DTPA-Cd represent exchangeable Cd, Fe-Mn oxides bound fraction of Cd, organic fraction of Cd and diethylene triamine pentaacetic acid extractable Cd in soils, respectively. Information & Authors Information Version history Copyright This work is licensed under a Non Exclusive No Reuse License.

Authors Metrics & Citations Metrics Article Usage 113views 81downloads Citations Download citation Lu Wang, Xiuqin Jia, Jinyu Yan, et al. Remediation effect of modified biochar on Cd-contaminated soil in post-mining land on the Loess Plateau, China. Authorea. 22 September 2025. DOI: https://doi.org/10.22541/au.175856067.71445067/v1 DOI: https://doi.org/10.22541/au.175856067.71445067/v1 If you have the appropriate software installed, you can download article citation data to the citation manager of your choice. Simply select your manager software from the list below and click Download. For more information or tips please see 'Downloading to a citation manager' in the Help menu.