Waste Algae-Derived Biochar Composites for Synergistic Soil Cd–As Immobilization: Feasibility, Dose–Response Thresholds, and Mechanism

The antagonistic geochemical behaviors of cadmium (Cd) and arsenic (As) in co-contaminated soils complicate their simultaneous remediation. This study aimed to develop a synergistic immobilization strategy by converting Spirulina residue into a magnetic biochar-layered double hydroxide composite (FSRBL). The composite was applied to both acidic red and calcareous black soils, and its effects on Cd and As, immobilization efficiency, and ecotoxicity were evaluated. The results showed that FSRBL effectively transforms Cd and As from mobile fractions to stable residual forms. At a 2.5% application rate, FSRBL achieved remarkable immobilization efficiencies of 39.2% for Cd and 57.5% for As, representing effectiveness 3.55 and 5.97 times higher than that of unmodified biochar, respectively. A dose–response relationship between the application amount of FSRBL and the immobilization efficiency of As and Cd was observed and further quantified using a logistic model. The results indicate that while increased FSRBL application enhances immobilization efficiency, the marginal benefit of each additional unit diminishes as the application rate increases, demonstrating a significant diminishing marginal effect. According to the ecotoxicity assessment experiment, the soil leachate from FSRBL-amended soil remarkably decreased the ecological toxicity to rice (Oryza sativa L.). Mechanistic investigations employing SEM/TEM-EDS, XRD, and XPS revealed that the synergistic immobilization could be ascribed to the multi-component cooperation within FSRBL, which resolved the conflicting pH/Eh requirements for the immobilization of Cd and As: (1) the LDH phase efficiently immobilized As oxyanions through anion exchange and isomorphic substitution; (2) the magnetic Fe phase concurrently immobilized Cd2+ and As oxyanions via redox transformation and coprecipitation, resulting in the formation of precipitates such as Fe/Ca/Cd–As(V). This work demonstrates a feasible approach to upcycle biomass waste into a value-added material for sustainable remediation of Cd–As co-contaminated soil.