Can the aging process necessarily weaken the effect of biochar on cadmium-contaminated soil remediation: considering biochar at different pyrolysis temperatures and aging treatment

Biochar has widely used to immobilize soil heavy metals in recent years, while the properties of biochar varied with environmental conditions. The influence of biochar aging on fixation and speciation transformation of Cd in soil remains unclear. This study explores how biochar aging affects the fixation and speciation transformation of Cd in soil. Rice straw biochar (RBC) prepared at different pyrolysis temperatures (300 degrees C, 500 degrees C, and 700 degrees C) was aged under three treatments (drying and watering cycle (DW), H2O2 oxidation (HO), and citric acid acidification (CA)) to investigate the effects of the aging process on the adsorption and passivation capacity for Cd. Results showed that the aging treatment increased Cd adsorption on RBC300 by 73.69% to 216.15%, while adsorption on RBC500 and RBC700 decreased by 11.52% to 74.56% and 7.40% to 75.89%, respectively. The addition of both fresh and aged RBC raised pH, DOC, and TOC in Cd-contaminated soil, aiding in Cd fixation. Either fresh or aged RBC addition enhance the stability of Cd in soil. Compared to CK treatment, residual Cd content rose by 28.63% to 43.71%, while both acid-extractable and reducible Cd contents decreased by 9.144% to 10.95%. Furthermore, the available Cd content in the soil saw a reduction of 10.45% to 30.77%, and high-temperature pyrolytic RBC exhibited a stronger capacity for Cd passivation in the soil. Both fresh and aged RBC indirectly reduced Cd bioavailability by affecting soil pH, DOC, and TOC, and the nature aging process (DW) did not weaken the effect of biochar on Cd-contaminated soil remediation. Thus, biochar has a long-term potential for mitigating Cd pollution in farmland.