Impact of citric acid on cadmium immobilization in soil amended with biochar

Cadmium (Cd) contamination in agricultural soils poses severe threats to human health and ecosystem integrity. While biochar is recognized for its ability to immobilize heavy metals, the impact of plant-derived low molecular weight organic acids (LMWOAs), such as citric acid (CA), on this process remains unclear. This study shows that CA and biochar synergistically immobilize Cd in alkaline soil from extensively polluted southeastern Hubei, China. Soils were amended with maize stover biochar at rates of 0, 0.5 %, 1 %, and 2 % (W/W), with or without CA addition (1 mmol/L), and incubated for 80 days. Results show that biochar significantly reduced bioavailable Cd, with a 2 % application decreasing extractable Cd by 26.84 %. Unexpectedly, CA alone increased Cd mobility by 9.38 %; however, when combined with high-dose biochar, CA and biochar synergistically enhanced Cd immobilization, reducing bioavailable Cd by 27.40 %. Advanced analytical techniques, including BCR sequential extraction, X-ray diffraction, Fourier-transform infrared spectroscopy, and scanning electron microscopy with energy-dispersive X-ray spectroscopy, confirmed that the interaction between biochar and CA altered Cd speciation, promoting its stabilization. This study is the first to show that CA and biochar synergistically immobilize Cd in alkaline soils, disproving the idea that LMWOAs hinder biochar's metal remediation. Our findings show that biochar can prevent or even harness LMWOAs' activating effects on Cd, indicating its great potential for sustainable Cd remediation in alkaline soils. This study improves our understanding of biochar-soil interactions in organic acids and gives new scientific bases for enhancing heavy metal-contaminated soil remediation procedures.