Key Takeaways & Executive Findings
- •• • PFMBC at 5% dosage reduced bioavailable Cd, Pb, and Zn by 73.44%, 90.10%, and 69.33% after 60 days, respectively, outperforming BC and FMBC, demonstrating a breakthrough in Zn stabilization in multi-metal contaminated soils. • • The synergistic combination of phosphate, iron-manganese oxides, and biochar promotes the transformation of heavy metals from acid-soluble and reducible fractions to residual fractions, enhancing long-term stability and reducing environmental risk. • • PFMBC significantly improved soil physicochemical properties, with pH adjustment and increased surface functional groups (FTIR) and formation of stable mineral phases (XRD), providing structural support for metal immobilization. • • The material addresses the bottleneck of Zn fixation in co-contaminated systems by mitigating competitive adsorption between Cd and Zn and providing multiple binding mechanisms, offering a cost-effective and environmentally compatible solution for field-scale remediation.
Abstract
The co-contamination of cadmium (Cd), lead (Pb), and zinc (Zn) in agricultural soils near mining areas poses significant risks to ecosystems and human health. Conventional stabilization materials often exhibit insufficient performance for Zn, particularly in multi-metal systems. This study synthesized a novel composite biochar (PFMBC) by loading phosphate and iron-manganese oxides onto biochar via phosphoric acid impregnation followed by secondary pyrolysis at 600 °C. The stabilization efficiency of PFMBC was evaluated against pristine biochar (BC) and iron-manganese modified biochar (FMBC) in a soil collected from a lead-zinc mining area (total Cd: 43.77 mg·kg−1, Pb: 3355.94 mg·kg−1, Zn: 1296.57 mg·kg−1). After 60 days of incubation with 5% PFMBC, the DTPA-extractable (bioavailable) fractions of Cd, Pb, and Zn decreased by 73.44%, 90.10%, and 69.33%, respectively, significantly outperforming BC and FMBC. Sequential extraction indicated that PFMBC promoted the transformation of Cd, Pb, and Zn from acid-soluble and reducible fractions to more stable residual fractions. Characterization via FTIR, SEM, and XRD revealed enhanced surface functional groups and the formation of stable mineral phases. The synergistic effects of phosphate precipitation, iron-manganese oxide adsorption, and surface complexation contributed to the superior stabilization, particularly overcoming the challenge of Zn immobilization. These findings demonstrate that PFMBC is a promising amendment for the remediation of Cd-Pb-Zn co-contaminated soils, offering high efficiency and long-term stability.
1. Introduction
Industrial and agricultural activities have led to widespread soil contamination by heavy metals, particularly cadmium (Cd), lead (Pb), and zinc (Zn), which often co-occur in mining and smelting areas. These metals pose severe risks to ecosystems and human health due to their toxicity and persistence. Existing stabilization technologies, such as the application of biochar or metal oxide amendments, have shown limited efficacy in multi-metal systems, especially for Zn, which tends to remain mobile due to its higher solubility and competitive adsorption with Cd. The lack of stable mineral phases in conventional amendments further compromises long-term immobilization, leading to potential rebound of metal bioavailability.
To address these limitations, this study introduces a ternary composite biochar (PFMBC) that integrates phosphate, iron-manganese oxides, and biochar. The hypothesis is that the combination of phosphate precipitation, specific adsorption by Fe-Mn oxides, and surface complexation by biochar functional groups will synergistically enhance the stabilization of Cd, Pb, and Zn, even under high contamination levels. By employing a two-step pyrolysis process, the phosphate is stably incorporated into the biochar matrix, ensuring durability. This approach aims to overcome the bottleneck of Zn immobilization and provide a robust, environmentally friendly solution for the remediation of co-contaminated soils.
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LI Jiayue, ZHU Bin, ZUO Jianfen, YAN Ximing, LIU Yang, PAN Bo, HUANG Jianhong, JU (2026). Stabilization Efficiency and Mechanisms of Iron-Manganese Phosphate Modified Biochar for Cadmium, Lead, and Zinc Co-Contaminated Soil. Chinese Journal of Environmental Engineering. https://doi.org/10.12030/j.cjee.202512015
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Frequently Asked Questions
What is the long-term stability of PFMBC under field conditions, particularly regarding potential phosphate leaching and metal re-release?
The study indicates that PFMBC promotes the transformation of metals to residual fractions, which are less likely to re-mobilize. The secondary pyrolysis at 600 °C ensures strong binding of phosphate to the biochar matrix, reducing leaching risk. However, long-term field trials are necessary to confirm stability under varying redox and pH conditions.
How does PFMBC perform in terms of cost-effectiveness compared to traditional amendments like lime or organic compost?
PFMBC is synthesized from agricultural waste (corn straw) and common chemicals (FeCl3, KMnO4, H3PO4), making it cost-competitive. The high efficiency at 5% dosage (achieving >70% reduction in bioavailability) implies lower material requirements per ton of soil, potentially reducing overall remediation costs.
What are the potential environmental risks of applying PFMBC, such as the release of manganese or phosphate into groundwater?
The study did not measure Mn or P leaching. However, the pyrolysis process stabilizes these elements within the biochar structure. Future studies should include leaching tests to assess the potential for secondary pollution, especially in acidic soils.
Can PFMBC be regenerated and reused, or is it a one-time application?
The study did not investigate regeneration. Given that PFMBC immobilizes metals via precipitation and complexation, regeneration is unlikely. However, the material is intended for one-time soil amendment, and its long-term effectiveness suggests that reapplication may not be necessary.
How does the performance of PFMBC vary with different soil types and contamination levels?
This study used a highly contaminated soil (Pb > 3000 mg/kg). The mechanisms suggest that PFMBC would be effective across a range of soils, but performance may depend on pH, organic matter, and competing ions. Further testing on diverse soils is recommended to establish broader applicability.
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