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

The optimal pyrolysis temperature and feedstock type for producing a Zn-biochar composite tailored to Cd immobilization in calcareous soils remains unexplored. This study aimed to evaluate the effects of Zn-biochar composites derived from two agricultural residues (rice husk and wheat straw) pyrolyzed at three temperatures (300, 450, and 600 ° C) on Cd immobilization in a contaminated calcareous soil. Sequential extraction procedure, release kinetics experiment, and spectroscopic (FTIR, XRD) and microscopic (SEM) analyses were employed to identify the Zn-biochar composite most effective at reducing soil Cd availability. Zn-biochar composites outperformed pristine biochars in reducing labile fractions (soluble and exchangeable, carbonate, FeMn-oxides) while increasing the recalcitrant residual fraction across all pyrolysis temperatures. Among all treatments, Zn-rice husk biochar composite produced at 600 ° C (R60z) was the most effective. It caused the highest statistically significant increase in the Cd content of residual fraction relative to the control (28.9%) and showed the lowest mobility factor (25.08%). R60z also exhibited the greatest significant reductions compared to the control in the parameters of the simple Elovich kinetic model, corresponding to decreases of 17.8% in a e and 29.4% in 1/β. These findings identify R60z as the most effective amendment for suppressing both the initial magnitude of Cd release and the rate of Cd desorption into the soil solution. This superior performance was likely associated with the potentially greater accessibility of surface sites of the R60z, which may have enhanced the retention of Cd in less mobile soil fractions. Finally, it seems that high-temperature Zn-biochar composites derived from rice husk represent a promising amendment for mitigating Cd mobility in contaminated calcareous soils.