Feedstock choice decides how well biochar locks zinc and cadmium in polluted soil

Biochar made from wheat straw turned out to be the most effective at immobilizing zinc in a heavily contaminated soil, while peanut shell biochar did the best job of locking up cadmium, according to a pot experiment conducted by researchers at Shandong Agricultural University, the Research Center for Eco-Environmental Sciences and the Agro-Environmental Protection Institute in China. The study, published in Frontiers in Agronomy, compared four biochars produced from apple wood, peanut shell, corn straw and wheat straw in a soil polluted with both metals, and found that the raw material a biochar is made from strongly shapes which metal it stabilizes and how much of the metal ends up in the vegetable growing in the treated soil.

The test soil was collected from the top 20 centimeters of a field roughly five kilometers from a copper-zinc smelter in Huanshan Township, Fuyang District, Zhejiang Province. Total zinc in the soil was 500 milligrams per kilogram and total cadmium was 1 milligram per kilogram, both above the third-level contamination thresholds of the Chinese national soil standard GB 15618-2018. Available zinc stood at 191 milligrams per kilogram and available cadmium at 0.36 milligrams per kilogram. The soil had a pH of 6.89 and an organic matter content of 1.49 percent. According to the authors, this moderate-to-severe zinc-cadmium combination, originating from smelting-related atmospheric deposition, is a typical co-contamination scenario across many industrial-impacted farmlands in China.

The four feedstocks were cleaned, dried at 60 degrees Celsius, ground and pyrolyzed at 400 degrees Celsius for two hours. Characterization showed clear differences among the resulting biochars. Corn straw and wheat straw biochars had ash contents of about 20 percent, nearly three times the roughly 7 percent measured for apple wood and peanut shell biochars. All four biochars were alkaline, with pH values ranging from 7.35 for apple wood biochar to 8.74 for corn straw biochar; apple wood and peanut shell biochars did not differ significantly from each other. Scanning electron microscopy revealed that pyrolysis transformed the smooth surfaces of the raw materials into etched, porous networks with voids, cracks and roughened pore walls, structures the authors associate with greater adsorption capacity.

As controls, the uncarbonized raw materials were also applied at 5 percent, allowing the researchers to separate the effect of pyrolysis itself from the effect of simply adding organic matter. An unamended control received neither. Each treatment was replicated three times, and seedlings of Brassica chinensis L., a leafy vegetable that accounts for 19 percent of China’s leafy vegetable production and is known to accumulate cadmium in its edible parts, were grown for 60 days before harvest.

The metal fractionation results were striking. At the 5 percent application rate, the four biochars reduced water-extractable zinc by 20 to 50 percent and water-extractable cadmium by 15 to 35 percent, while residual, chemically stable fractions of zinc rose by 30 to 90 percent and residual cadmium by 110 to 330 percent. Wheat straw biochar was the strongest zinc immobilizer, cutting weak acid-extractable zinc by up to 47.8 percent and raising residual zinc by up to 91.8 percent at the highest dose. Peanut shell biochar was the standout for cadmium, significantly increasing residual cadmium at every application rate tested, by up to 121 percent. Two-way analysis of variance showed that biochar type, application rate and their interaction all had highly significant effects on soil extractable and residual zinc and cadmium contents, with P values below 0.0001.

Those soil changes translated into less metal in the plant. Across the dose range, apple wood, peanut shell, corn straw and wheat straw biochars reduced shoot zinc concentrations by 6.7 to 27.7 percent, 2.0 to 34.8 percent, 8.8 to 27.8 percent and 8.0 to 18.8 percent respectively, and lowered shoot cadmium by 28.8, 28.8, 41.9 and 30.7 percent. At the 5 percent rate, plant zinc fell from an initial 13.25 milligrams per kilogram to between 9.58 and 10.75 milligrams per kilogram, and cadmium dropped from 0.13 to between 0.07 and 0.09 milligrams per kilogram. Bioconcentration factors for both metals declined steadily with increasing biochar dose. Notably, plants grown in soils amended with the uncarbonized raw materials accumulated 10 to 31 percent more zinc and 29 to 59 percent more cadmium than plants in the corresponding 5 percent biochar treatments, indicating that carbonization itself, not just the added biomass, drives the immobilization.

The team also estimated dietary target hazard quotients, a US Environmental Protection Agency measure relating estimated daily intake of a pollutant to its reference dose, for adults and children consuming the vegetable. All biochar treatments kept both adults’ and children’s THQ values for zinc and cadmium below the risk threshold of 1. However, in the untreated control, the children’s cadmium THQ approached that threshold, signaling that eating leafy vegetables from unremediated soil could pose a notable potential health hazard for this vulnerable group. Children showed higher THQ values than adults under identical treatments, reflecting their lower body weight relative to consumption. Apple wood biochar was most effective at reducing zinc-related dietary risk, while corn straw biochar performed best against cadmium risk.

Soil enzyme activity responded positively as well. Urease activity rose in a dose-dependent manner, increasing by 1.4 to 31.8 percent depending on biochar type and rate, and catalase activity at the 5 percent rate increased by 16.3 to 19.9 percent. Sucrase activity followed a different pattern, rising and then falling with dose and peaking at the 1 percent application rate, where increases ranged from 27.0 percent for apple wood biochar to 79.4 percent for peanut shell biochar. The authors attribute the sucrase decline at 5 percent to biochar-induced alkalinity moving soil pH beyond the enzyme’s optimal range and to enzyme adsorption onto biochar surfaces. Correlation analysis showed that weak acid-extractable zinc and cadmium concentrations explained 53 to 77 percent of the variance in plant metal accumulation, and that these labile fractions, together with residual fractions, explained 65 to 77 percent of the variation in sucrase activity, identifying the weak acid-extractable pool as the key link between soil chemistry, plant uptake and biochemical function.

The authors link the differing performances to feedstock chemistry. Straw-derived biochars, rich in alkaline ash minerals such as carbonates, silicates and phosphates, can raise soil pH and shift zinc and cadmium toward insoluble precipitates, while their rough pore walls add physical adsorption sites. Apple wood biochar, with high lignin and low ash, likely fixes metals through microporous adsorption and surface complexation involving phenolic and carboxyl groups, and peanut shell biochar’s medium lignin and ash content is thought to provide mesopores and silanol coordination sites suited to cadmium. The researchers caution, however, that they did not measure surface area, functional groups or elemental composition directly, so these mechanism explanations remain hypotheses rather than demonstrated causes. They also note that a 5 percent application rate is not economically or operationally realistic for routine farming, and that the findings come from a single pot experiment with one soil and one plant species. Field trials across different soils and cropping systems will be needed before the feedstock-specific recommendations can be applied at scale. As a practical takeaway, the study suggests that no single feedstock biochar simultaneously optimizes both metals, and that blended mixed-feedstock biochars may deserve testing for combined zinc-cadmium remediation.

Subject of Research: A pot experiment comparing four feedstock-derived biochars for immobilizing zinc and cadmium in co-contaminated soil and reducing metal uptake in Brassica chinensis.

Article Title: Effects of biochars derived from different raw materials on Zn and Cd bioavailability and soil enzyme activities in Zn-Cd co-contaminated soil

Article References: Xu, Y., Wang, C., Xu, C., Bao, Q., & Sun, T. (2026). Effects of biochars derived from different raw materials on Zn and Cd bioavailability and soil enzyme activities in Zn-Cd co-contaminated soil. Frontiers in Agronomy, 8, Article 1899914. https://doi.org/10.3389/fagro.2026.1899914

Image Credits: AI Generated

DOI: 10.3389/fagro.2026.1899914

Keywords: biochar, zinc, cadmium, soil remediation, heavy metals, Brassica chinensis, soil enzymes, pyrolysis, health risk assessment, co-contaminated soil, feedstock, pot experiment