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. 2026 Sep 15;11(38):56755–56766. doi: 10.1021/acsomega.6c00172

Insights into Remediation of Cd Contamination in Vegetable Soils by Red Mud-Modified Biochar

Tao Tian †,‡, Jiali Liu †, Songsong Jia †, Jiming Jia †, Qixiao Wu †, Chuangye Yao †,‡,*
PMCID: PMC13629065  PMID: 42825068

Abstract

Recently, heavymetal contamination in soil has been widely concerned. Herein, three types of biochar-based materials, including biochar (BC), Fe-modified biochar (Fe-BC), and red mud-modified biochar (RM-BC), were prepared and characterized, and the effects of BC, Fe-BC, and RM-BC on the bioavailability of Cd in vegetable soils were studied by pot experiments. Results showed that the RM-BC treatment exhibited better immobilization effect on soil Cd than BC and Fe-BC, and reduced the bioavailability of soil Cd and effectively inhibited the migration of Cd from soil to the edible parts of onion plants. After RM-BC treatment, the acid-soluble state of soil Cd decreased by 23%, while the residual state of soil Cd increased by 21%, promoting the form transformation of soil Cd from the acid-soluble state to the residual state. Furthermore, the RM-BC treatment raised the soil pH, soil enzyme (β-glucosidase, alkaline phosphatase, and urease) activities, soil organic carbon, and the biomass of onion plants. The immobilization mechanism of soil Cd following RM-BC treatment was explored, involving in surface adsorption, ion exchange, and complexation reactions. Importantly, the Cd concentration of onion stem is about 0.13 mg/kg following RM-BC treatment, which did not exceed China’s food safety standard of 0.20 mg/kg. Therefore, the RM-BC treatment shows potential for the remediation of Cd-contaminated soils.

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1. Introduction

Remediation of heavy-metal-contaminated soils is of great concern worldwide. Heavy metals in mine tailings, generated during the mining process, were transported and diffused into the soil through atmospheric sedimentation, leaching, and infiltration. The Hunan province in China has abundant mineral resources and enjoys the reputation of being the “hometown of non-ferrous metals”. Previous research showed that the concentration of heavy metals in soil near mining areas was relatively high in the Hunan province, among which Cd contamination is one of the serious pollutants in soil. Soil Cd could continue to accumulate through the food chain, posing a serious threat to human health. Therefore, it is crucial to remediate Cd-contaminated soils.

Until now, various methods have been employed for remediating Cd-contaminated soils, including physical, biological, and chemical methods. Physical methods usually involve high-cost engineering, such as surface capping with soil. Biological methods, such as bioremediation and phytoremediation, were limited to a small number of functional microorganisms and plants, respectively, and it was not universal for the remediation of Cd-contaminated soils. Chemical remediation typically adopts substrate addition to reduce the bioavailability of soil Cd. For the substrate addition application, organic amendments have a significant effect on the immobilization of soil Cd. Recently, biochar, as a novel organic amendment material, has been widely applied in the remediation of contaminated soils. Biochar can be obtained using the biomass pyrolysis method, and the raw material (biomass, e.g., straw) for biochar preparation belongs to large-scale solid waste in agricultural production. Biochar typically features a porous structure and large surface area, making it effective in improving soil properties and remediating Cd-contaminated soils. , Wang et al. found that biochar could reduce the available Cd content in soil. Lu et al. also reported that biochar has an impact on the adsorption of soil Cd.

To improve the effectiveness of stabilization and immobilization of soil Cd, recent studies have confirmed that material modification of biochar is necessary for enhancing the remediation of soil Cd, which can regulate the bioavailability of soil Cd. , Especially, metal-modified biochar composites, such as iron–carbon composites, were found as one of the most effective materials for reducing the bioavailability of soil Cd. , Iron–carbon composites, synthesized by combining biochar with Fe-containing materials, have gained extensive attention for their enhanced ability to immobilize soil Cd. , However, with the increasing demand for such composites, the current large-scale preparation of iron–carbon composites still suffers from the shortcomings of high cost and poor stability. Thus, it is crucial to develop a cheap and highly efficient source of Fe to prepare Fe-based materials. Given these issues, red mud is one of the most feasible candidate materials. Generally, red mud is a large-scale industrial solid waste generated during the production process of alumina, which mainly contains the oxides of aluminium, iron, sodium, and magnesium. Among these oxides, the content of Fe-based oxide can be 20%–50%, relying on the composition of the initial red mud. , Currently, red mud, more than 4.6 billion tons in the world, is stored in open-air disposal areas, with an additional 200 million tons generated annually. Besides, current techniques for red mud disposal, such as metal extraction and preparation of roadbed materials, were limited to realize widespread and long-term applications due to low recovery rates and high impermeability costs. Because of red mud being an Fe-rich waste, utilizing Fe-rich red mud as a source of Fe for biochar modification is an efficient alternative to solve the problem of large-scale accumulation of red mud. In addition, red mud has the characteristics of a large specific surface area and high porosity, which is desirable for immobilization of soil Cd due to high adsorption capacity and complexation with soil Cd. To update, most studies have focused on biochar-based materials to remediate Cd-contaminated soils, and there are few reports on the synergistic effect of biochar and red mud on the remediation of Cd-contaminated soils. Moreover, the remediation mechanism of biochar-based materials for Cd-contaminated soils is uncertain. Thus, a better understanding on the remediation mechanism of Cd-contaminated soils following biochar-based material application is essential.

In this study, it is hypothesized that the synergic remediation of biochar and red mud can immobilize Cd availability in soil and promote plant growth. To verify this hypothesis, the synergic remediation effect of biochar and red mud on the bioavailability of soil Cd was evaluated by pot experiments, where onion plants were grown in Cd-contaminated soils. The specific objectives of the study were (i) to explore the effects of biochar-based materials as amendments on the bioavailability of soil Cd and the Cd concentration in different parts of onion plants and (ii) to understand the remediation mechanism of the red mud-biochar combination on the Cd-contaminated soil.

2. Materials and Methods

2.1. Soil Collection and Analysis

The tested soil samples were collected from vegetable fields near a lead–zinc mining area in Chenzhou, Hunan province, China (26°10′ N, 113°62′ E). This area is a typical subtropical monsoon humid climate zone with an average annual precipitation about 1524.2 mm. After natural drying, the soil samples were measured, and their characteristics were as follows: pH value of 5.45, cation exchange capacity (CEC) of 10.5 cmol/kg, organic matter of 2.0%, and total Cd concentration of 10.3 mg/kg. The Cd concentration of soil samples exceeded the standard value specified by the Soil Environmental Quality Standard (China Standard GB15618-2018), belonging to severe pollution.

2.2. Preparation of Biochar-Based Materials

This study explored the impacts of biochar-based materials on the bioavailability of soil Cd, and the biochar-based materials were prepared as follows:

  • (i)

    Biochar preparation. The raw materials (biomass) for the preparation of the biochar were collected from rice straw near Bailutang Town, Suxian District, Chenzhou, China. Before preparing biochar, the rice straw was washed and dried at 80 °C and then crushed using a grinder. The resultant rice straw powders were placed in a muffle furnace for 5 h of pyrolysis to obtain biochar.

  • (ii)

    Iron-modified biochar preparation. The prepared rice straw powders were added to a 1 mol/L ferric chloride solution, along with a mass ratio of ferric chloride to rice straw maintained at 1:1. After the magnetic stirring for 4 h, the obtained samples were dried at 70 °C and calcined at 500 °C for 5 h in a muffle furnace to synthesize Fe-modified biochar.

  • (iii)

    Red mud-modified biochar preparation. Red mud and rice straw powders were mixed and ball-milled with a mass ratio of 1:1, then calcining the mixture at 500 °C for 5 h in a muffle furnace. Red mud was provided by an alumina factory in the Shandong Province, and no Cd content was examined in red mud. The main compositions of red mud included Fe2O3 (41.6%), Al2O3 (20.0%), SiO2 (26.8%), Na2O (6.5%), MgO (1.7%), and other oxides. The values of pH and specific surface area were about 11.53 and 20.96 m2/g, respectively.

2.3. Experimental Design

In this research, pot experiments were conducted at the School of Chemistry and Environmental Science, Xiangnan University. The experiment adopted plastic pots with an inner diameter of 100 mm and a height of 60 mm, with each pot containing 1 kg of vegetable soils. Four different treatments were set as follows: (i) CK treatment: control group, pure Cd-contaminated soils without addition of biochar-based materials. (ii) BC treatment: addition of 4% biochar to the Cd-contaminated soils. (iii) Fe-BC treatment: addition of 4% Fe-modified biochar to Cd-contaminated soils. (iv) RM-BC treatment: addition of 4% redmud-modified biochar to Cd-contaminated soils. Among them, an appropriate amendment proportion of 4% addition was used for the remediation of Cd-contaminated soils, in terms of the preliminary experiment (Supporting Information, Figure S1). Each treatment was repeated 3 times, for a total of 12 samples with random arrangement. This study selected onion plants as the test vegetable (Four Seasons Onion, provided by Chenzhou Jimian Seed Industry in the Hunan Province, China). Each treatment of mixing biochar-based materials and Cd-contaminated soils was cultivated in plastic pots for about 1 month for stabilization prior to planting the onions, starting from September 10, 2024. Treatment details are provided in Table . Onion plants were sown on October 10, 2024, with 5 seeds evenly distributed per pot. All treatments were conducted under the same experimental conditions, such as the water management, the size and weight of plant seeds, and the number of seeds sown. The management measures were consistent with the local agricultural practices, and onion plants were harvested on November 10, 2024. After harvesting the onion plants for 1 month, the physicochemical properties and Cd fractions of contaminated soils were examined to explore the stability of biochar-based materials.

1. Treatments of Different Types of Modified Biochar.

Treatment Unmodified biochar Fe-modified biochar Red mud-modified biochar Sample size
CK – – – n = 3
BC + – – n = 3
Fe-BC – + – n = 3
RM-BC – – + n = 3
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2.4. Sample Analysis

The crystal structure of biochar-based materials was characterized by X-ray diffractometry (XRD, Rigaku D-MAX 2200) with Cu Kα radiation. The morphology characteristics of biochar-based materials were observed by means of scanning electron microscopy (SEM, ZEISS Sigma 300), where energy-dispersive spectroscopy (EDS) and surface-related mapping were applied to identify the element analysis of samples. The organic functional groups associated with biochar-based materials were characterized using Fourier transform infrared spectroscopy (FTIR, Thermo Fisher Scientific Nicolet iS20) within a range of 400–4000 cm–1. The Brunauer–Emmett–Teller (BET) specific surface area of biochar-based materials was determined using nitrogen (N2) adsorption–desorption measurements in a surface area and pore size analyzer (Micromeritics ASAP 2460). Zeta potential was determined using a Malvern Panalytical ZS90 system, with triplicate measurements ensuring result accuracy. Measurements with approximately 100% peak area coverage were selected for analysis. X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific ESCALAB 250xi) was used to examine the composition and valence state of sample elements. After onions were harvested, the fresh soil samples were collected for enzyme activity analysis through UV-Vis spectrophotometer measurements. Soil samples were analyzed for β-glucosidase by means of determining the release of p-nitrophenol (PNP) using the hydrolysis of p-nitrophenyl-β-glucopyranoside as a substrate, along with an absorbance measured at 400 nm, and the results were expressed as milligram of PNP per gram of soil per hour. Alkaline phosphatase activity was determined via phenyl disodium phosphate colorimetric method, and its activity was quantified through the amount of phenol released, which was expressed as milligrams of phenol per gram of soil per hour. Urease activity was examined by indophenol blue colorimetry at 578 nm, and the NH3–N (mg) generated by urease hydrolysis per gram of soil per hour was adopted to express its activity. Then, the soil samples were naturally dried. Soil pH was measured through a pH meter (PHS-3C, REX, China) with a solid-to-liquid ratio of 1:2.5, and the cation exchange capacity (CEC) of soil was determined by means of the ammonium acetate method after washing with alcohol. Determination of soil organic matter was conducted by the potassium dichromate oxidation-weighted method. Total Cd amount in soil was determined using a boiling digestion process with aquaregia and perchloric acid as reagents. After extraction, the supernatant was filtered via quantitative filter paper. Different fractions of soil Cd were examined through the European Community Bureau of References (BCR) sequential extraction method. The Cd concentration in onion plants was digested and extracted using the dry ashing method. The analysis for measuring Cd concentrations within digested solutions was conducted through an ICP-OES analyzer (ICP-OES 8300, PerkinElmer, USA). The sample analysis processes were in accordance with quality control based on the national standard substance (GBW­(E)-070009) of soil, with a Cd recovery rate of 97.2%–102.3%. Simultaneously, the blank experiment was set for comparative study. The onion plants were washed with distilled water and dried at 60 °C to a constant weight, and the weights of onion plant subsamples were recorded after their separation into root and stem (stem-leaf) parts.

2.5. Statistical Analysis

The experimental data were analyzed using Office 2010 and SPSS 15.0. The means of three replicates complied with one-way ANOVA analysis to compare the significant differences. All values in the results were displayed as means ± standard deviations (SDs). Duncan’s multiple comparisons were performed at a significance level of P < 0.05. Differing lowercase letters (abcd) were used to indicate the significant differences between different treatments. Pearson correlation analysis was used to obtain the positive or negative relationship between soil pH and available Cd concentration in soil. In addition, the graphs were made using Origin 8.5 software.

3. Results and Discussion

3.1. Characterization of Biochar-Based Materials

To probe the crystal structure of the synthesized biochar-based materials, XRD characterization was carried out, and its patterns can be seen in Figure a. The pure BC sample exhibited an amorphous structure. For the Fe-BC sample, two characteristic peaks correspond to the crystal planes of (220) and (400) of the spinel Fe3O4 structure in the database (JCPDS No. 19-0629), which suggested the coexistence of Fe2+ and Fe3+ ions in iron-modified biochar. However, there are no obvious peaks from red mud in the RM-BC sample. Usually, red mud materials are composed of many oxides rather than a single crystalline structure, such as Fe2O3, Al2O3, SiO2, Na2O, MgO, and other metal oxides. This weakens the contents of each single-phase structure and the diffraction peak intensity of red mud in XRD patterns. Also, biochar performed an obvious amorphous diffraction peak, resulting in high background in XRD patterns, which maybe cover the weak crystalline peaks from red mud materials. In addition, the phase transformation from crystalline structure to amorphous phase may be occurred during pyrolysis process. The composition of red mud in the RM-BC sample will be characterized through subsequent analysis (e.g., EDS). Figure b depicts the FTIR analysis of biochar-based materials. The obvious absorption peak at the high-frequency region (3434 cm–1) was observed, which is ascribed to the −OH stretching vibration. , Followed by that, the absorption peaks located at 2922 cm–1 and 2845 cm–1 were attributed to the C–H stretching vibration. An absorption band at 1610 cm–1 was assigned to CC stretching vibration. The peaks at 1091 cm–1 and 1002 cm–1 corresponded to the C–O stretching vibration of biochar and red mud, respectively. ,, In the low-frequency region, an absorption peak located at 785 cm–1 was assigned to the CC stretching vibration. The absorption peaks at 551 cm–1 and 473 cm–1 were attributed to the Si–O–Si and O–Si–O stretching vibrations. , Wang et al. reported that the biochar prepared from rice straw is rich in Si minerals, which is in accordance with our FTIR results. The intensity of absorption bands through FTIR analysis can reflect the soil-related organic functional group characteristics (i.e., amounts and quantities). In this study, the RM-BC sample increased the intensity of absorption bands relative to other samples, which suggested that the RM-BC sample obtained enough amounts of organic functional groups. After the RM-BC amendment was applied in Cd-contaminated soils, these reactive groups will improve organic matter contents and form stable complexes with Cd­(II) through complexation reactions, further lowering and immobilizing the soil Cd activity. To compare the differences of specific surface area of the synthesized biochar-based materials, the N2 adsorption-desorption isotherms were analyzed, as shown in Figure c. The inset of Figure c lists the BET surface areas with different biochar-based materials. It can be seen that the sequence of the BET surface area is RM-BC > Fe-BC > CK. The BET surface area of pure BC sample is the smallest, only 30.6 m2/g, while the Fe-BC sample increased the BET surface area to 931.4 m2/g. The maximum of the BET surface area was found in the RM-BC sample, approximately 1242.5 m2/g. Similar results were reported by Guo et al. and Li et al. , The significantly enhanced BET surface area in the Fe-BC and RM-BC samples is due to that the fibrous tissue in the raw material (biomass) was destroyed into many small parts. The ferric chloride of Fe-BC and the red mud of RM-BC destroyed cellulose and other macromolecular organic matters during the pyrolysis process, generating a large number of micropores and mesopores to enhance surface area. The modified biochar samples acquired a higher specific surface area than pure biochar, especially in the RM-BC sample, which can provide more adsorption and reaction active sites on the surface of modified biochar materials. Comparison of the BET surface areas in different biochar-based materials suggested that the RM-BC treatment exhibited superiority in surface adsorption ability for immobilization of soil Cd. Furthermore, the morphology characteristics of biochar-based materials were observed by SEM measurement, as displayed in Figure d–f. The surface architecture of the BC sample showed significant porous configurations. Unlike the BC sample, the modified biochar samples exhibited an obvious alteration in surface morphology characteristics relative to unmodified biochar. A rough and dense surface texture with a small aperture was observed in Fe-BC and RM-BC samples, where iron oxide and red mud particles were evenly distributed across the biochar surface or pore structure, indicating that iron oxide and red mud particles were successfully loaded on the biochar matrix. Furthermore, the element compositions of biochar-based materials were characterized by EDS analysis, as depicted in Figure g–i. For the BC sample, biochar contains elemental C, O, and Si, in accordance with the FTIR results. Previous research reported that the rice straw-synthesized biochar is rich in elemental Si. Elemental Au originates from sputtering some ultrafine Au nanoparticles onto the sample surface for enhancing conductivity and improving image resolution during the EDS measurement process. As observed, the Fe element was identified in the Fe-BC sample. For the RM-BC sample, similar to BC characterization, these elements with C, O, Si, and Au were found. Additionally, the other elements with Na, Fe, Mg, and Al originate from the red mud of the RM-BC sample.

1.

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Biochar-based material characterizations. (a) XRD patterns of biochar-based materials. (b) FTIR spectra of biochar-based materials. (c) N2 adsorption-desorption isotherms of the synthesized biochar-based materials. (d–f) SEM images of biochar-based materials. (g–i) Corresponding EDS analysis of biochar-based materials.

3.2. Remediation of Cd-Contaminated Soils

In general, soil pH plays an important role in the remediation of Cd contamination following biochar application. Figure shows the effect of biochar-based materials on soil pH. It can be seen that there was no significant difference in soil pH between CK and BC treatments (P > 0.05). However, following Fe-BC and RM-BC treatments, there was a significant difference in soil pH (P < 0.05), compared to the CK treatment. In addition, it was found that Fe-BC and RM-BC treatments can increase soil pH relative to CK and BC treatments, irrespective of the treatment time being 1, 2, or 3 months. The Cd-contaminated soils in the CK treatment exhibit acidic characteristics. After Fe-BC and RM-BC treatments, the biochar is alkaline with high nutrient content, and the red mud has strong alkalinity, which induced the increase of soil pH through a neutralization reaction. Raising the soil pH is conducive to regulating the bioavailability of soil Cd. Hong et al. found that the increase of pH in soil following an organic fertilizer–apatite–biochar mixture treatment could reduce Cd bioavailability, where the pH was negatively correlated with bioavailable Cd in contaminated soils in their research. After harvesting onion plants for 1 month, the soil pH also maintains a higher level, suggesting that the modified biochar materials applied in Cd-contaminated soils exhibit stability in pH regulation.

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Effect of biochar-based materials on soil pH following different treatment periods. (a) Biochar-based materials were applied to the soil for 1 month; (b) Biochar-based materials were applied to the soil for 2 months, along with onion plants for 1 month. (c) Biochar-based materials were applied to the soil for 3 months, along with the harvest of onion plants for 1 month. Data are the means of three measurements from three replicates and differing lowercase letters indicated significant differences between different treatments at P < 0.05.

As shown in Figure , the forms of soil Cd mainly include residual Cd (R-Cd), organic-bound Cd (OM-Cd), Fe/Mn-bound Cd (Fe/Mn–Cd), and acid-soluble Cd (AC-Cd). Generally, the active state of soil Cd, such as acid-soluble Cd, is easy to migrate and transport from soil to plants, further posing threats to humans and animals. The stable state of soil Cd (e.g., residual Cd) is difficult to migrate and transport from soil to plants, which inhibits the environmental risks of soil Cd. During the onion plants for 1 month, for the CK treatment, soil Cd was mainly in the form of the residual state (40%) and acid-soluble state (38%), while the proportions of the Fe/Mn-bound state and organic-bound state were relatively low. With the application of biochar, the proportion of acid-soluble Cd decreased to 34%, and the residual state of soil Cd increased to 44% in the BC-treated soils. Compared with CK treatment, Fe-BC and RM-BC treatments decreased the acid-soluble Cd by 17% and 23%, while the proportion of residual Cd increased by 16% and 21%, respectively. Similar results were also observed in other treatment times of modified biochar materials applied in Cd-contaminated soils for 1 month and 3 months. It can be concluded that the modified biochar promotes the form transformation of soil Cd, which reduces the proportion of acid-soluble Cd and raises the residual Cd content, further effectively reducing the bioavailability of Cd in contaminated soils. Compared with other treatments, the RM-BC treatment has the most significant reduction in bioavailability of soil Cd, which exhibited superiority in the remediation of Cd-contaminated soils. In the Cd-contaminated soils treated for 3 months, the concentration of acid-soluble Cd under the treatment of Fe-BC and RM-BC treatments suffered by 48.65% and 67.57% reduction when compared with the control groups. Moreover, for the RM-BC treatment, the proportions of the lowest acid-soluble Cd (12%) and the highest residual Cd (66%) were obtained, which exhibited a stable effect of RM-BC role in immobilization of Cd in contaminated soils. Table S1 shows the immobilization results of soil Cd treated by biochar-based materials following different modification methods. It would be obtained that the RM-BC treatment in this research exhibited superiority in the reduction of bioavailable Cd concentration.

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Effect of biochar-based materials on the form transformation of soil Cd following different treatment periods. (a) Biochar-based materials were applied to the soil for 1 month; (b) Biochar-based materials were applied to the soil for 2 months, along with onion plants for 1 month. (c) Biochar-based materials were applied to the soil for 3 months, along with the harvest of onion plants for 1 month. Data are the means of three measurements from three replicates and differing lowercase letters indicated significant differences between different treatments at P < 0.05.

It can be observed from Figure that Cd concentration decreased irrespective of root or stem parts of onion plants with the applications of biochar or modified biochar materials, compared to the CK treatment. Compared with CK treatment, after applying biochar and modified biochar materials, there were significant differences (P < 0.05) for Cd concentrations of root- and stem-parts of onion plants. The BC, Fe-BC, and RM-BC treatments decreased the Cd concentration by 3.37, 5.16, and 7.00 mg/kg in the root part of onion plants compared to those of CK treatment (9.35 mg/kg), respectively. Meanwhile, the BC, Fe-BC, and RM-BC treatments decreased the Cd concentration by 0.05, 0.09, and 0.16 mg/kg in the stem part of onion plants, respectively, compared to those of the CK treatment (0.29 mg/kg). Importantly, for the RM-BC treatment, the Cd concentration of onion stem is about 0.13 mg/kg, which did not exceed China’s food safety standard of 0.20 mg/kg (GB2762-2012). These results suggested that RM-BC treatment effectively decreased the Cd concentration irrespective of root or stem parts of onion plants, which can impede the migration and accumulation of Cd from soil to onion plants. As shown in Table S2, based on the comparison of Cd concentration in the stem parts, the compared results of RM-BC treatment with other common remediation materials such as bentonite, urea, and fertilizer were listed. Among the different remediation materials, the RM-BC treatment in this work exhibited a significant reduction of Cd accumulation in the stem part of plants.

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Effect of biochar-based materials on Cd concentrations in different parts of onion plants. Data are the means of three measurements from three replicates and differing lowercase letters indicated significant differences between different treatments at P < 0.05.

As shown in Figure , the amounts of Cd accumulation in different parts of onion plants were investigated. It can be observed that the biochar and modified biochar materials decreased the accumulation amounts of Cd in the root and stem parts of onion plants. The amounts of Cd accumulation following BC treatment in the root and stem parts of onion plants decreased by 22.9% and 13.3% than those of CK treatment, respectively, and there were significant differences (P < 0.05) for the Cd accumulation amounts. There was no significant difference in Cd accumulation amounts of root and stem parts of onion plants between BC and Fe-BC treatments (P > 0.05), while the amounts of Cd accumulation in the root and stem parts of onion plants following Fe-BC treatment were slightly higher than that of BC treatment. Following RM-BC treatment, the amounts of Cd accumulation in the root part of onion plants decreased by 46.4% in comparison to CK treatment, exhibiting an obvious inhibition effect for Cd accumulation. Therefore, the RM-BC treatment can effectively impede the migration and accumulation of soil Cd from the roots through the food chain route to the stems, animals, and humans.

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Effect of biochar-based materials on Cd accumulation amounts in different parts of onion plants. Data are the means of three measurements from three replicates and differing lowercase letters indicated significant differences between different treatments at P < 0.05.

Figure shows the influence of biochar-based materials on biomass in different parts of onion plants. After adding biochar and modified biochar into Cd-contaminated soils, there were significant differences in the biomass of root and stem parts of onion plants in comparison with CK treatment (P < 0.05). The biomass of onion roots with CK treatment was 0.15 g/pot, while the biomass following BC, Fe-BC, and RM-BC treatments increased by 1.20, 1.73, and 2.13 times, respectively. The biomass in the stem part of onion plants following CK treatment was 12.73 g/pot, while the biomass following BC, Fe-BC, and RM-BC treatments increased by 1.05, 1.28, and 1.78 times, respectively, suggesting that the addition of biochar-based materials is beneficial to the growth of onion plants.

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Effect of biochar-based materials on biomass in different parts of onion plants. Data are the means of three measurements from three replicates and differing lowercase letters indicated significant differences between different treatments at P < 0.05.

3.3. Soil Enzyme Activity and Organic Carbon

The enzyme activity and organic carbon of soil altered significantly (P < 0.05) when biochar and modified biochar were applied, as shown in Figure . It can be found from Figure a that there is high β-glucosidase activity in RM-BC treatment, followed by Fe-BC, BC, and CK treatments. Compared with CK treatment, the β-glucosidase activity after BC treatment increased by 70.8%, while it increased by 75.1% and 76.3% following Fe-BC and RM-BC treatments, respectively. The influence of biochar-based materials on alkaline phosphatase and urease activities was similar to β-glucosidase activity. In Figure b, compared with CK treatment, the alkaline phosphatase activity increased by 65.2%, 76.0%, and 78.2% following BC, Fe-BC, and RM-BC treatments, respectively. Moreover, in Figure c, the urease activity increased by 76.7%, 83.9%, and 85.0% following BC, Fe-BC, and RM-BC treatments relative to CK, respectively. These results of the biochar-based amendment effect on soil enzyme activity were similar to a recent research work reported by Nabi et al. Generally, the biochar application was beneficial for the growth of microorganisms, which can raise nutrient content and improve soil quality. Considering that, the variations of soil organic carbon following biochar-based material applications were investigated, as shown in Figure d. Compared with CK treatment, the soil organic carbon increased by 40.4%, 50.2%, and 64.4% following BC, Fe-BC, and RM-BC treatments, respectively. Therefore, the RM-BC treatment significantly increased the β-glucosidase, phosphatase, urease activities, and organic carbon content in Cd-contaminated soils, which may be ascribed to the stronger immobilization ability of RM-BC amendment to soil Cd. Song et al. adopted iron-based biochar to enhance enzyme activity in soil contaminated by multiple heavy metals, and reported that the reason for promoting soil enzyme activity was due to the enhanced immobilization ability of modified biochar to heavy metals.

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Effect of biochar-based materials on soil enzyme activity and soil organic carbon. (a) β-glucosidase activity, (b) alkaline phosphatase activity, (c) urease activity, and (d) variations of soil organic carbon. Data are the means of three measurements from three replicates and differing lowercase letters indicated significant differences between different treatments at P < 0.05.

Although red mud is a strongly alkaline material, and the high content of red mud can obtain a negative effect on the plant growth of agricultural soils. In this study, the red mud content is relatively low (<4%), and red mud can be used to enhance soil quality. Red mud is rich in minerals such as iron oxide, which is beneficial for water and nutrient absorption in agricultural soils, further increasing the organic carbon content in soil (Figure ) and soil fertility. Therefore, a small amount of red mud applied in agricultural soils can raise plant biomass (Figure ).

3.4. Correlation Analysis

The above results demonstrated that the RM-BC treatment raised soil pH, decreased the acid-soluble state of soil Cd, and enhanced the residual state of soil Cd. To explore the relationship between soil pH and forms of soil Cd, correlation analysis was studied, as shown in Figure . For the different forms of soil Cd, different correlations between soil pH and Cd form were observed. Soil pH showed a strong positive correlation with the residual state of soil Cd, along with a correlation coefficient of 0.90. There was a strong negative correlation between soil pH and the acid-soluble state of soil Cd, associated with a correlation coefficient of −0.87. The correlations between soil pH and the Fe/Mn-bound state (organic-bound state) of soil Cd were insignificant. These results indicated that the application of RM-BC treatment to contaminated soils can raise soil pH, and variation in soil pH has an important impact on the forms of soil Cd. For the RM-BC treatment, the addition of biochar to soil can increase the concentration of salinity ions in soil, and the salinity ions easily combine with Al3+ and H+ in acidic soils through cation exchange reaction and synergistic effect of the abundant alkaline functional groups on the surface of biochar. ,, Moreover, red mud has strong alkalinity and large cation exchange capacity, which can release cations to realize neutralization reaction with acidic soils. Therefore, the synergistic effect of red mud and biochar leads to an increase in soil pH. This suggested that the RM-BC treatment plays an important role in raising soil pH and immobilizing Cd activity. In this research, the RM-BC treatment increased soil pH and enhanced the adsorption ability of Cd2+ due to that the alkaline red mud has a porous structure with plenty of cations (see Figures and ), promoting the formation of carbonate or hydroxide precipitation, which decreased the acid-soluble state of soil Cd, increased the residual state of soil Cd, and suppressed the bioavailability of soil Cd. Meanwhile, for the RM-BC treatment, the increase of soil pH induced the form transformation of soil Cd from the acid-soluble state to the residual state (Figure ), further weakening the bioavailability of soil Cd.

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Correlation analysis between soil pH and Cd concentrations with different forms.

The acid-soluble state of soil Cd is considered as the main threatening form in contaminated soils due to its strong mobility and easy absorption to plants, which was used to evaluate the bioavailability of soil Cd. To unveil the relationship between the acid-soluble state of soil Cd and Cd concentrations in different parts of onion plants, correlation analysis was conducted, as shown in Figure . Following RM-BC treatment, there was a highly significant positive correlation between the acid-soluble state of soil Cd and Cd concentrations of root/stem parts of onion plants, associated with correlation coefficients of R root = 0.882 for roots and R stem = 0.725 for stems. This suggested that RM-BC can decrease the concentration of acid-soluble Cd in soil to lower the Cd concentrations in different parts of onion plants, further demonstrating that the RM-BC treatment can inhibit the migration and accumulation of soil Cd into plants. The above results can be explained as follows: biochar has large specific surface areas with organic functional groups such as C–H, C–O, and −OH, based on FTIR analysis (Figure ), and red mud contains a large amount of highly adsorption substances such as Fe/Al oxides from EDS element analysis, which can effectively adsorb cadmium ions to form complexation, ,, further decreasing the acid-soluble state of soil Cd and boosting the transformation from the acid-soluble state to the residual state. Therefore, the bioavailability of soil Cd can be suppressed.

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Correlation analysis between acid-soluble Cd concentration and Cd concentrations in different parts (root and stem) of onion plants.

3.5. A Potential Mechanism of RM-BC Role in Immobilization of Soil Cd

To investigate the influence of biochar-based materials on Cd distribution in contaminated soils, Figure shows the morphology images of Cd-contaminated soils following different treatments and the corresponding elemental mapping characterization through EDS analysis. The Cd mapping of contaminated soils treated by different biochar-based materials showed that elemental Cd was homogeneously dispersed onto the surface of biochar and modified biochar, exhibiting a surface adsorption effect.

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SEM images of Cd-contaminated soils treated by (a1) BC, (a2) Fe-BC, and (a3) RM-BC. The corresponding elemental mapping analysis of (b1–b3) C and (c1–c3) Cd elements.

To further explore the influence of RM-BC treatment on the chemical binding state of soil Cd, an XPS analysis was conducted. The XPS survey spectra indicated the Cd 3d binding energy peak (Supporting Information, Figure S2). Usually, the XPS survey spectra are adopted to quickly cover a wide energy range, with short dwell time and rough signal, and naturally lower signal-to-noise ratio than narrow high-resolution XPS spectra. Furthermore, the high-resolution XPS spectra were analyzed, as shown in Figure . Before RM-BC treatment, it can be seen from Figure a that three binding energy peaks at 405.6, 406.7, and 407.6 eV could be assigned to CdO, Cd­(OH)2, and CdCO3, respectively. Quantitative analysis was carried out by fitting peak area, and the proportion of active CdO to all Cd-based compounds was about 25%. After RM-BC treatment in Figure b, the XPS spectra of the Cd 3d region also exhibited three characteristic peaks, corresponding to CdO, Cd­(OH)2, and CdCO3. Similarly, the proportion of active CdO to all Cd-based compounds was quantified at approximately 17%. This suggested that the RM-BC treatment promoted active CdO in contaminated soils transforming into stable Cd-based compounds such as Cd­(OH)2 and CdCO3, which may be due to the fact that the ion exchange reaction occurs between red mud and soil Cd. Incorporating red mud of RM-BC treatment can promote the release of cations (i.e., Na+, Mg2+, Fe2+, and Al3+). These cations into soil can alter cation-exchange energy and facilitate the immobilization of soil Cd via the cation exchange effect, further promoting the transformation of Cd from active-state CdO to stable-state Cd­(OH)2 and CdCO3. This result further confirmed the conclusion that the RM-BC treatment promoted the form transformation of soil Cd from the acid-soluble state to the residual state.

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High-resolution XPS spectra of Cd 3d region in contaminated soils (a) before RM-BC application and (b) after RM-BC application.

Based on the anterior results and discussion, the remediation mechanism in Cd-contaminated soils following RM-BC treatment was proposed, as shown in Figure . Usually, the prepared biochar materials possess adequate negative surface charges, confirmed by zeta-potential analysis (Supporting Information, Figure S3). This negative surface charge exhibited the high stability of the biochar materials. The presence of these negatively charged surfaces can generate favorable conditions for biochar-based materials to efficiently adsorbed positively charged metal cations (i.e., Cd2+) from contaminated soils. Compared with the other treatments such as BC and Fe-BC, the RM-BC treatment has the advantages of abundant organic functional groups, large specific surface area, and ample surface active sites, as well as multiple cations originated from red mud (Figure ), which generated favorable conditions to immobilize soil Cd. In this study, the RM-BC treatment improved the specific surface area of biochar, which significantly enhanced its surface adsorption ability. Moreover, the RM-BC treatment obtained abundant organic functional groups (e.g., C–O, C–H, and −OH) from the FTIR analysis, the enhanced surface adsorption promoted that the RM-BC treatment has stronger complexing effect to immobilize active cadmium ions. On the other hand, the abundant surface active sites due to the enhancement of surface adsorption ability in RM-BC treatment can facilitate the formation of ion exchange reaction between free Cd2+ in soil and cations (e.g., Na+, Mg2+, Fe2+, and Al3+) in red mud, , which results in the generation of CdO converting Cd­(OH)2 and CdCO3, confirmed by XPS analysis. The ion exchange reaction induces the transformation of active cadmium ions into a stable state, which suppresses the bioavailability of cadmium ions, further impeding the migration and accumulation of soil Cd to onion plants. Therefore, the enhanced surface adsorption effect is dominant for the immobilization mechanism of soil Cd following RM-BC treatment, accompanied by ion exchange and complexation reactions. In addition, the RM-BC treatment through the synergistic effect between red mud and biochar effectively suppressed the migration and accumulation of soil Cd to onion plants.

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Schematic diagram showing the remediation mechanism in Cd-contaminated soils following red mud-modified biochar application.

4. Conclusions

In summary, the biochar-based materials were applied to Cd-contaminated soils in pot experiments. The effects of biochar-based materials on the form transformation of soil Cd, and migration and accumulation of Cd in different parts of onion plants were investigated. Results showed that biochar-based materials increased soil pH, decreased the acid-soluble state of soil Cd, and raised the residual state of soil Cd. It is noteworthy that the RM-BC treatment exhibited superiority in the remediation of Cd-contaminated soils as compared to other treatments. The RM-BC treatment decreased the acid-soluble state of soil Cd, and reduced the accumulation amounts of Cd in the root and stem parts of onion plants. Particularly, a significant reduction of Cd accumulation concentration in onion stem was found, about 0.13 mg/kg, which is lower than the China’s food safety standard of 0.20 mg/kg. Correlation analysis suggested that the increase in soil pH is beneficial for weakening the bioavailability of soil Cd. Moreover, the RM-BC treatment can effectively inhibit the migration and accumulation of Cd from contaminated soil to the edible parts of onion plants. The proposed mechanisms of RM-BC treatment on the immobilization of soil Cd involve in surface adsorption, complexation reaction, and ion exchange reaction. These findings suggest that the RM-BC treatment is a facile approach for the remediation of Cd-contaminated soils. However, the current research was conducted in pot experiments under controlled conditions, which do not capture the full complexity and variability of practical field-scale environments, such as different soil types, microbial communities, and soil water conditions. These environmental factors may impact the efficiency of RM-BC treatment in field-scale application. Further studies conducting field trails to investigate more factors would be valuable for elucidating the remediation mechanism following RM-BC application in practical field environments.

Supplementary Material

ao6c00172_si_001.pdf (221.7KB, pdf)

Acknowledgments

This research was supported by the Open Fund of Key Laboratory of Data Science and Intelligence Education (Hainan Normal University), Ministry of Education (DSIE202304), the Hunan Provincial Natural Science Foundation of China (2022JJ40414 and 2024JJ7512), the Scientific Research Fund of Hunan Provincial Education Department (25A0603), the Research Foundation of the Department of Natural Resources of Hunan Province, China (20230140ST), and the Applied Characteristic Discipline of Electronic Science and Technology of Xiangnan University (XNXY20221210).

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.6c00172.

  • Soil exchangeable Cd content; XPS survey spectra of Cd-contaminated soils; zeta potential of biochar; research novelty; comparison of modified biochar materials on soil Cd immobilization; performance comparison of red mud-modified biochar with other materials (PDF)

The authors declare no competing financial interest.

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