Turning plastic-contaminated farm waste into safer biochar for soil remediation
image: Converting plastic-contaminated agricultural residues into fit-for-purpose biochar soil amendment: an initial study
Credit: Qiuyu Yu, Xuhui Zhang, Tao Gao, Xueliu Gong, Jiarong Wu, Shuai Tian, Biao Ma, Lujiang Xu, Stephen Joseph, Jufeng Zheng, Rongjun Bian & Lianqing Li
Agricultural plastics help farmers grow crops more efficiently, but they also create a difficult waste problem when they become tangled with plant residues after harvest. A study published in Biochar suggests that one promising solution may be to convert plastic-contaminated crop waste into biochar, a carbon-rich material that can improve soil quality and help reduce heavy metal risks in contaminated farmland.
The study, titled “Converting plastic-contaminated agricultural residues into fit-for-purpose biochar soil amendment: an initial study,” was conducted by Qiuyu Yu, Xuhui Zhang, Tao Gao, Xueliu Gong, Jiarong Wu, Shuai Tian, Biao Ma, Lujiang Xu, Stephen Joseph, Jufeng Zheng, Rongjun Bian, and Lianqing Li. The researchers focused on honeydew melon vines contaminated with plastic hanging ropes, a common agricultural waste in greenhouse production systems.
Plastic ropes are widely used to support climbing crops such as melon and watermelon. After harvest, however, these ropes often become entangled with vines, making them difficult to separate and recycle. If improperly burned, buried, or abandoned in fields, they may contribute to soil pollution and microplastic contamination. The research team explored whether co-pyrolysis, a heating process carried out under limited oxygen, could convert this mixed waste into useful biochar.
The researchers produced biochar at 300, 500, and 700 degrees Celsius. Biochar made at 300 degrees showed incomplete carbonization of the plastic ropes, so the main comparisons focused on biochars produced at 500 degrees Celsius and 700 degrees Celsius. The 500-degree biochar, named BC500, showed especially promising performance. It had a higher yield than the 700-degree biochar and contained higher levels of available phosphorus and potassium, two nutrients important for plant growth.
“Our goal was to test whether a waste stream that is usually difficult to handle could be transformed into a useful and safer soil amendment,” said corresponding author Rongjun Bian. “The results suggest that selecting the right pyrolysis temperature is critical. At 500 degrees Celsius, the biochar showed strong potential for both heavy metal immobilization and soil fertility improvement.”
In laboratory adsorption tests, BC500 showed the highest capacity for removing lead and cadmium from water, with adsorption capacities of 127 mg g⁻¹ for lead and 36 mg g⁻¹ for cadmium. These results indicate that the material has strong potential to bind heavy metals.
The team then tested the biochar in a pot experiment using Chinese cabbage grown in soil contaminated with lead and cadmium. Adding BC500 and BC700 increased soil pH, improved nutrient availability, and enhanced microbial biomass. The effects on plant growth were substantial: cabbage biomass increased by 119% with BC500 and 86% with BC700 compared with the untreated control. At the same time, lead and cadmium concentrations in the edible cabbage tissues decreased by more than 80% and 29%, respectively.
The study also examined potential safety concerns related to polycyclic aromatic hydrocarbons, or PAHs, which can form during pyrolysis. PAH concentrations in the biochars increased with higher pyrolysis temperature, but remained within recommended limits. PAH levels in cabbage leaves also increased after biochar application, yet benzo(a)pyrene and total PAHs remained below relevant safety thresholds.
“These findings do not mean that all plastic-contaminated agricultural residues can be directly converted into soil amendments without careful evaluation,” Bian said. “But they provide an important first step showing that, under controlled conditions, co-pyrolysis may offer a practical pathway for recycling difficult agricultural waste while reducing heavy metal uptake by crops.”
The authors conclude that pyrolyzing plastic-contaminated honeydew melon vines at 500 degrees Celsius may be a feasible strategy for producing fit-for-purpose biochar. Further work is needed to evaluate different plastic types, larger-scale systems, long-term field effects, and emissions from pyrolysis gas to ensure full environmental safety.
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Journal Reference: Yu, Q., Zhang, X., Gao, T. et al. Converting plastic-contaminated agricultural residues into fit-for-purpose biochar soil amendment: an initial study. Biochar 6, 98 (2024).
https://doi.org/10.1007/s42773-024-00382-7
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About Biochar
Biochar (e-ISSN: 2524-7867) is the first journal dedicated exclusively to biochar research, spanning agronomy, environmental science, and materials science. It publishes original studies on biochar production, processing, and applications—such as bioenergy, environmental remediation, soil enhancement, climate mitigation, water treatment, and sustainability analysis. The journal serves as an innovative and professional platform for global researchers to share advances in this rapidly expanding field.