Zinc-Enhanced Rice Husk Biochar Locks Cadmium in Contaminated Calcareous Soil

A new open-access study in Scientific Reports reports that biochar composites modified with zinc outperformed ordinary, pristine biochars at stabilizing cadmium in a contaminated calcareous soil, and that a zinc-rice husk biochar produced at 600 degrees Celsius was the single most effective amendment tested. The work, led by Hamid Reza Boostani and published on 26 October 2026, systematically compared two agricultural residues and three pyrolysis temperatures to address a question the authors identify as unexplored: which combination of feedstock and temperature produces a Zn-biochar composite best tailored to cadmium immobilization in calcareous soils.

Rice husk and wheat straw were each pyrolyzed at 300, 450, and 600 degrees Celsius, both as pristine biochar and as Zn-biochar composites, and the resulting materials were applied to the contaminated calcareous soil. The researchers then combined a sequential extraction procedure, which partitions cadmium into fractions of decreasing availability, with a release kinetics experiment that tracks how cadmium desorbs into soil solution. Spectroscopic analyses by FTIR and XRD and microscopic imaging by SEM were employed to identify the Zn-biochar composite most effective at reducing soil cadmium availability.

The central result was consistent across every pyrolysis temperature: Zn-biochar composites reduced the labile cadmium fractions, namely the soluble and exchangeable, carbonate-bound, and Fe-Mn oxide-bound pools, while increasing the recalcitrant residual fraction more than pristine biochars did. This pattern held for composites derived from both rice husk and wheat straw, meaning the zinc modification, not the choice of residue alone, drove the advantage over unmodified biochar.

Among all treatments, the Zn-rice husk biochar composite produced at 600 degrees Celsius, labeled R60z, stood out. It raised the cadmium content of the residual fraction by 28.9 percent relative to the untreated control, a statistically significant increase, and it produced the lowest mobility factor of any amendment, at 25.08 percent. A lower mobility factor corresponds to a smaller share of cadmium in the most available pools, the fractions most prone to moving through soil and into solution.

The kinetics data reinforced that picture. R60z delivered the greatest significant reductions compared to the control in the parameters of the simple Elovich model, decreasing the parameter ae by 17.8 percent and 1/beta by 29.4 percent. The authors state that these findings identify R60z as the most effective amendment for suppressing both the initial magnitude of cadmium release and the rate at which cadmium desorbs into the soil solution, the two quantities the Elovich parameters describe.

Why did the high-temperature Zn-rice husk composite win? The authors attribute its superior performance to the potentially greater accessibility of R60z surface sites, which may have enhanced the retention of cadmium in less mobile soil fractions. The abstract frames this mechanism in hedged language, presenting the surface site accessibility as likely rather than proven, while the extraction and kinetic results all point in the same direction.

The temperature contrast within the study is notable. Because composites were produced at 300, 450, and 600 degrees Celsius from the same residues, the results indicate that high-temperature pyrolysis, when combined with zinc modification of rice husk, yielded the material that shifted the most cadmium into the residual pool in this experiment.

The authors conclude that high-temperature Zn-biochar composites derived from rice husk represent a promising amendment for mitigating cadmium mobility in contaminated calcareous soils. The study offers a clear, evidence-based recipe for this soil: when the goal is locking cadmium in place, a zinc-modified rice husk biochar pyrolyzed at 600 degrees Celsius was the most effective material tested.

Subject of Research: Comparing the pristine biochar and Zn-biochar composite for stabilizing Cd in a contaminated calcareous soil

Article Title: Comparing the pristine biochar and Zn-biochar composite for stabilizing Cd in a contaminated calcareous soil

Article References: Boostani, H. R. (2026). Comparing the pristine biochar and Zn-biochar composite for stabilizing Cd in a contaminated calcareous soil. Scientific Reports. https://doi.org/10.1038/s41598-026-77316-9

Image Credits: AI Generated

DOI: 10.1038/s41598-026-77316-9

Keywords: Comparing, pristine, biochar, Zn-biochar, composite, stabilizing, contaminated, calcareous, soil, scientific research