Purpose <p>To address the dual challenges of remediating cadmium (Cd)-arsenic (As) co-contaminated soil and utilizing mining waste resources, this study developed a novel remediation material from natural low-grade molybdenum tailings (MT).</p> Materials and methods <p>MT was subjected to high-temperature alkaline fusion to obtain amorphous silicate (ASi), onto which nanoscale zero-valent iron (nZVI) was subsequently loaded to form nZVI/ASi. Batch adsorption experiments were carried out evaluate its Cd(II) and As(III) adsorption capacities, while soil incubation tests assessed its effectiveness in reducing the bioavailability of Cd and As.</p> Results and discussion <p>Alkaline fusion disrupted the crystalline structures of quartz, mica, feldspar, and calcite in MT, increasing its surface area 37.83-fold and enhancing Cd(II) adsorption, whereas nZVI incorporation improved As(III) removal. nZVI/ASi exhibited maximum adsorption capacities of 139.10 mg·g^− 1 for Cd(II) and 113.57 mg·g^− 1 for As(III). Cd(II) immobilization occurred via coordination with Si-O/Si-OH, complexation with iron oxides/hydroxides, and ion exchange, while As(III) adsorption involved complexation, oxidation, and electrostatic interactions. Co-adsorption experiments revealed synergistic effects, where As(III) enhanced the adsorption rate of nZVI/ASi for Cd(II). Cd(II) notably boosted the ability of nZVI/ASi to adsorb As(III), attributed to electrostatic interactions and the creation of A-type ternary surface complexes. In soil, 2% nZVI/ASi reduced Cd and As bioavailability by 14.84% and 15.56%, respectively, accompanied by elevated pH.</p> Conclusion <p>The nZVI/ASi composite synthesized from low-grade MT can effectively immobilize both Cd and As simultaneously, offering a sustainable and resource-efficient strategy for remediating co-contaminated soils.</p> Graphical Abstract <p></p>