Valorization of Spirulina residue for magnetic layered double hydroxide functionalized biochar: Synergistic cadmium and arsenic adsorption, mechanism insights, and soil phytotoxicity mitigation.

The complex contamination of cadmium (Cd) and arsenic (As) in agricultural systems poses a serious threat to food security and human health. Developing materials capable of simultaneously immobilizing these two chemically distinct pollutants remains a major challenge in the field of environmental remediation. Inspired by the "waste treats waste" concept, this study innovatively utilizes industrial waste, Spirulina residue, as biomass material. Through magnetic modification and functionalization with layered double hydroxide (LDH), Fe-modified Spirulina residue biochar loaded with CaMgAl-LDH (FSRBL) was successfully manufactured for synergistic remediation of Cd-As co-contamination. The FSRBL achieved maximum adsorption capacities of 155 and 87.5 mg/g for Cd(II) and As(III), which were 1.95 and 8.1 times greater than those of unmodified biochar, respectively. Furthermore, FSRBL demonstrated excellent magnetic separation properties and cycle stability. Advanced characterizations and spectroscopic analyses revealed multi-mechanistic synergies: Magnetic component synergistically adsorbed Cd/As through mineralization precipitation (e.g., CdCO3, FeAsO4, and Ca(AsO3OH)·H2O), coordination complexation (e.g., Fe-O-Cd/As and Fe-As-Cd), and redox processes; while LDH component enhanced the selectivity via interlayer anion exchange, isomorphic substitution, and multinuclear complexation (Ca/Mg/Al-O-Cd/As). Soil pot experiments further verified the application potential of FSRBL in actual environments. FSRBL reduced the contents of available Cd and As in the soil by 55.6 % and 27.9 %, respectively. At the same time, the accumulation in the aboveground parts decreased by 69.2 % and 39.9 % (within safe levels), thereby effectively alleviated the oxidative stress and toxic effects of plants. This study develops a low-cost material from industrial biowaste to remediate Cd-As contamination, thus enabling its high-value valorization.