Phosphorus-Rich Calcium-Based Bentonite Coupled with Smooth Vetch ( <i>Vicia villosa</i> var.) for Enhanced Improvement of Cd-Contaminated Farmland: Interaction and Synergistic Passivation Mechanism

Phosphorus-rich calcium-based bentonite (BM-PCB) was synthesized via a ball milling process and synergistically integrated with smooth vein (SV) to establish a coupled remediation system for Cd-contaminated soil. The combined application of BM-PCB and SV resulted in a 61.28% reduction in bioavailable Cd content, exhibiting 2.81- and 5.13-fold enhancements compared with those of BM-PCB (21.81%) and SV (11.94%) alone, respectively, confirming the existence of a strong synergistic passivation effect. BM-PCB enhanced the Cd enrichment capacity of SV by 2.83 times by promoting root development, subsequently increasing humic acid (HA) production by 33.33% by catalyzing the condensation reactions between phenolic compounds and amino acids following SV incorporation. The interaction between HA and BM-PCB enhanced Cd adsorption by improving the interlayer spacing, pore structure, functional group content, and charge characteristics of BM-PCB. Additionally, the beneficial microorganisms recruited by the coupling of BM-PCB and SV reduced Cd bioavailability as well as facilitated the decomposition and transformation of SV residues, thereby improving soil nutrients. This approach consequently increased the maize yield by 61.31% while concurrently reducing the Cd content in grains to comply with food safety standards. Life cycle assessment confirmed that the proposed BM-PCB+SV system represented a cost-effective and low-environmental-risk “three-in-one” strategy, simultaneously achieving Cd immobilization, soil fertility enhancement, and food security assurance.