Remediation of cadmium-contaminated soil via the coupling of marigold–water spinach intercropping and passivation

High-accumulation plants intercropped with economic crops, coupled with passivators, represent a novel approach for remediating cadmium (Cd)-contaminated soil. This study aimed to explore the effects of intercropping and passivation coupling on plant growth, Cd accumulation, and Cd content in soil. We prepared hydroxyapatite-modified biochar with loading concentrations of 2.5%, 5%, and 10% as a passivator and interplanted marigold (Tagetes erecta L.) with water spinach (Ipomoea aquatica Forsk.) to remediate Cd-contaminated soil. Eight treatments were established, and the experiments were performed in triplicate. Samples were collected on three different time points following the planting of water spinach. Only the aboveground parts were collected at the first two time points, whereas the entire plant was collected together with the marigold at the third time point. The coupling of intercropping and passivation reduced soil available and total Cd by 45.07–48.78% and 13.12–17.95%, respectively, facilitating Cd transformation from labile fractions to stable oxidized and residual forms. The proportion of residual Cd was markedly higher than that in single treatments. Additionally, the combined method substantially increased Cd accumulation in water spinach roots. Compared with the sole intercropping and sole passivation treatments, the coupled intercropping and passivation treatment significantly increased Cd accumulation in marigold and water spinach by 4.80–25.53% and 106.44–134.90%, respectively. These findings highlight the potential of combining intercropping with passivation for the remediation of Cd-contaminated soil, particularly demonstrating that the marigold and water spinach intercropping system is an effective strategy for alleviating soil Cd pollution.