Iranian Journal of Soil and Water Research

Iranian Journal of Soil and Water Research

Evaluation of The Efficacy of Biochar- Mineral Composites on Immobilizing Heavy Metals (Lead, Zinc, and Cadmium) in A Contaminated Soil

Document Type : Research Paper

Authors
mohamad malehmir chegini
Ahmad Golchin
mohammad babaakbari sari
Department of Soil Science and Engineering, Faculty of Agriculture, University of Zanjan, Zanjan, Iran
10.22059/ijswr.2026.413464.670127
Abstract
Heavy metals are among the most persistent soil contaminants, posing serious threats to environmental health, soil quality, and food security. The application of biochar– mineral composites has attracted considerable attention as a novel strategy for the immobilization of heavy metals in contaminated soils because of the synergistic interactions between their constituent components. In this study, the effects of two types of biochar (wheat straw biochar and common bean (Phaseolus vulgaris L.) straw and pod biochar), three mineral materials (diatomite, triple superphosphate, and zeolite), and six biochar– mineral composites produced through a synergistic approach on the immobilization of lead (Pb), zinc (Zn), and cadmium (Cd) in a heavy metal-contaminated soil were evaluated under controlled conditions. The biochars were produced at 400 °C with a residence time of 3 h, mixed with the mineral materials at a 1:1 ratio (50:50, w/w), and applied to the contaminated soil at rates of 0, 1.25, 2.5, and 5% (w/w). The results showed that all treatments significantly (P< 0.05) reduced the bioavailable concentrations of Pb, Zn, and Cd; however, their effectiveness depended on the type of biochar– mineral composite. The biochar– mineral composites were more effective in immobilizing heavy metals than the individual sorbents applied separately. The lowest DTPA- extractable Pb, Zn, and Cd concentrations were obtained with the common bean biochar– triple superphosphate composite applied at 5% (w/w), with values of 0.913, 27.83, and 3.7 mg kg-1 soil, respectively, corresponding to reductions of 99.70, 86.10, and 86.78%, respectively, compared with the control. This treatment also enhanced maize growth and reduced heavy metal concentrations in the shoots. Overall, biochar– mineral composites, owing to their synergistic effects involving surface adsorption, ion exchange, precipitation, and stable complex formation, represent a promising approach for the immobilization of heavy metals and the reduction of their transfer to plants in contaminated soils.
Keywords
Adsorption
Ion exchange
Precipitation
Surface functional groups
Synergistic
Subjects

Introduction

Heavy metal contamination of soils poses a serious threat to environmental quality and human health because of the toxicity, persistence, and bioaccumulative nature of these elements. Conventional soil remediation methods are generally expensive, may cause secondary pollution, and often exhibit limited long-term effectiveness. In recent years, biochar, a carbon-rich material produced through biomass pyrolysis, has received considerable attention as an effective soil amendment for the immobilization of heavy metals owing to its high specific surface area, porous structure, abundance of oxygen-containing functional groups, and high cation exchange capacity. However, the efficiency of biochar alone may be limited under certain conditions. Recent studies have demonstrated that combining biochar with mineral amendments to produce biochar– mineral composites through a synergistic approach can substantially improve heavy metal stabilization via several mechanisms, including ion exchange, surface adsorption, electrostatic interactions, surface complexation, and precipitation. Therefore, this study was conducted to evaluate and compare the efficiency of different biochar– mineral composites in immobilizing heavy metals in contaminated soils.

Objective

This study was based on the hypothesis that biochar– mineral composites could serve as a practical remediation strategy with high application potential for heavy metal-contaminated soils. It was further hypothesized that these composites would be more effective in immobilizing heavy metals than the individual application of either biochar or mineral amendments. Therefore, the objective of this study was to investigate and compare the effects of biochars, mineral amendments, and their corresponding composites on the immobilization of lead (Pb), cadmium (Cd), and zinc (Zn) in contaminated soil.

Materials and Methods

The effectiveness of synergistically produced biochar– mineral composites in immobilizing Pb, Cd, and Zn was evaluated using a contaminated soil collected from the vicinity of the Zanjan Lead and Zinc Industrial Complex, Iran. A factorial pot experiment was conducted using a completely randomized design with three replications.

Eleven adsorbent treatments were evaluated, including two biochars produced from wheat straw and common bean straw and pod residues through pyrolysis at 400°C with a residence time of 3 h, three mineral amendments consisting of zeolite, diatomite, and triple superphosphate (TSP), and six biochar– mineral composites prepared by mixing biochar and mineral powders at a 50:50 (w/w) ratio. Each amendment was incorporated into the contaminated soil at four application rates (0, 1.25, 2.5, and 5%, w/w). Each pot contained 3 kg of contaminated soil. After thoroughly mixing the amendments with the soil, the samples were incubated for two months under alternating wet and dry conditions (field capacity and 5% gravimetric water content, respectively) to allow equilibrium between the soil and the applied amendments. At the end of the incubation period, soil samples were collected, air-dried, and analyzed for DTPA-extractable heavy metals using 0.005 M DTPA (pH= 7.3). The concentrations of Pb, Cd, and Zn were determined by atomic absorption spectrophotometry (AAS). The immobilization efficiency was calculated as the percentage reduction in DTPA-extractable metal concentrations relative to the untreated control. Subsequently, three seeds of maize (Zea mays L.) cultivar BC 678 were sown in each pot. Plants were harvested after eight weeks of growth. Plant samples were transferred to the laboratory, washed, oven-dried, and separated into shoots and roots. Shoot and root dry weights were recorded. The dried plant tissues were ground and digested using a wet digestion procedure with concentrated nitric acid (HNO₃) and hydrogen peroxide (H₂O₂). Heavy metal concentrations in the digested samples were then determined using atomic absorption spectrophotometry.

Results and Discussion

The elemental composition and molar ratios of the biochars produced from wheat straw and common bean straw and pod residues showed that they contained 48– 62% carbon and approximately 31– 33% oxygen, while hydrogen and nitrogen contents were relatively low. These characteristics indicate that the biochars were formed during the pyrolysis process through dehydration, deoxygenation, decarboxylation, depolymerization, and aromatization reactions. The molar ratios of H/C< 0.7 and O/C< 0.6 indicate a high degree of carbonization, low polarity, and excellent stability of the produced biochars, suggesting their long-term environmental persistence. Furthermore, the aromaticity index (0.76– 0.94) confirms the predominance of condensed aromatic carbon structures and the high resistance of these biochars to biological degradation. Analysis of variance (ANOVA) revealed that adsorbent type, application rate, and their interaction had significant effects (P < 0.05) on the concentrations of DTPA- extractable Pb, Cd, and Zn in the contaminated soil. Increasing the application rate of the amendments up to 5% (w/w) significantly reduced the concentrations of bioavailable heavy metals. The highest immobilization efficiency was obtained with the common bean residue biochar–triple superphosphate composite applied at the 5% rate. Under this treatment, the concentrations of DTPA-extractable Pb, Cd, and Zn decreased to 0.913, 27.83, and 3.70 mg kg-1 soil, representing reductions of 99.70, 86.10, and 86.78%, respectively, compared with the untreated control. Overall, the effectiveness of the adsorbents in stabilizing heavy metals followed the order: Biochar– triple superphosphate composite > Biochar– zeolite composite > Biochar– diatomite composite. The stabilization efficiency also depended on both the heavy metal species and the type of amendment. In the biochar–triple superphosphate and common bean biochar– zeolite composites, the immobilization efficiency followed the order Pb > Zn ≈ Cd, whereas in the biochar– diatomite and wheat straw biochar–zeolite composites, the order was Zn > Pb ≈ Cd. The superior performance of the triple superphosphate-based composites was mainly attributed to the formation of insoluble metal-phosphate precipitates together with electrostatic interactions between negatively charged functional groups and metal cations. The results further demonstrated that the synergistic interaction between biochar and mineral amendments was substantially more effective than the individual application of either component. In addition, biochar produced from common bean straw and pod residues exhibited greater immobilization efficiency than wheat straw biochar. This superior performance was attributed to its higher cation exchange capacity, greater abundance of oxygen-containing functional groups, and more favorable surface characteristics for heavy metal adsorption. Overall, the immobilization of cationic heavy metals by biochar– mineral composites was governed by several complementary mechanisms, including ion exchange, physical adsorption, surface adsorption, electrostatic interactions, surface complexation, and phosphate precipitation.The highest maize shoot dry weight (3.16 g. pot-1) was obtained under the common bean residue biochar–triple superphosphate composite applied at the 5% rate, whereas the greatest root dry weight (0.50 g. pot-1) was recorded in the wheat straw biochar– diatomite composite at the 1.25% application rate. These results indicate that the reduction in heavy metal bioavailability alleviated metal toxicity and consequently promoted maize growth.

Regarding heavy metal accumulation in maize shoots, the lowest Pb and Zn concentrations (35 and 138 mg. kg-1, respectively) were observed in the wheat straw biochar– triple superphosphate composite applied at the 1.25% rate, whereas the lowest Cd concentration (14 mg. kg-1) was recorded in the triple superphosphate treatment applied at the 5% rate. Overall, the application of biochar– mineral composites, particularly the common bean residue biochar– triple superphosphate composite, showed considerable potential for reducing heavy metal uptake by plants. This superior performance can be attributed to the synergistic effects of surface adsorption, ion exchange, phosphate precipitation, electrostatic interactions, and the formation of stable surface complexes, all of which effectively reduced heavy metal mobility and their transfer from soil to maize plants.

Conclusion

The findings of this study demonstrated that biochar– mineral composites, particularly those containing triple superphosphate, are highly effective, economical, and environmentally friendly amendments for the in situ immobilization of heavy metals in contaminated soils. Combining the high specific surface area and adsorption capacity of biochar with the phosphate precipitation capability of triple superphosphate substantially enhanced heavy metal stabilization compared with the individual application of either biochar or mineral amendments. Among all evaluated treatments, the common bean residue biochar– triple superphosphate composite exhibited the greatest effectiveness in reducing the bioavailability of Pb, Cd, and Zn and was therefore identified as the most efficient amendment for the remediation of heavy metal-contaminated soils. Overall, the results highlight the considerable potential of biochar– mineral composites as sustainable soil amendments for restoring contaminated soils and minimizing the transfer of heavy metals into the food chain.

Funding

This research has not received any specific financial support from funding organizations in the government, commercial, or non-profit sectors. This study was funded by the University of Zanjan, Faculty of Agriculture, Department of Soil Science and Engineering.

Authorship contribution

Conceptualization, M. M. C, A. G and M. B. S.; methodology , M. M. C, A. G and M. B. S.; software, , M. M. C, A. G and M. B. S.; validation, , M. M. C, A. G and M. B. S.; formal analysis, , M. M. C, A. G and M. B. S.; investigation, M. M. C, A. G and M. B. S.; resources, M. M. C, A. G and M. B. S.; data curation, M. M. C, A. G and M. B. S.; writing—original draft preparation, M. M. C.; writing—review and editing, M. M. C, A. G and M. B. S.; visualization, M. M. C, A. G and M. B. S.; supervision, M. M. C, A. G and M. B. S.; project administration, M. M. C, A. G and M. B. S.; funding acquisition, M. M. C, A. G and M. B. S. All authors have read and agreed to the published version of the manuscript.

Declaration of Generative AI and AI-assisted technologies in the writing process

During the preparation of this work, the authors Mohammad Malemir Chegini, Ahmad Golchin, Mohammad Baba Akbari Sari did not use any artificial intelligence-based tools to prepare this article.

Data availability statement

Data available on request from the authors.

Acknowledgements

We would like to thank the Honorable Vice Chancellor of Research, Faculty of Agriculture, University of Zanjan, and the Zeytoun Rudbar Research Station (Gilan Agricultural Research Center) for their financial support, moral support, and cooperation in carrying out this research.

Ethical considerations

The authors avoided data fabrication, falsification, and plagiarism, and any form of misconduct.

This study was approved by the Ethics Committee of Zanjan University (ethical code: 1941172).

Conflict of interest

The authors declare no conflict of interest.

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Iranian Journal of Soil and Water Research
Volume 57, Issue 7
September 2026
Pages 1637-1681