Effective immobilization of Cd(II) in soil by biotic zero-valent iron and coexisting sulfate

In this work, zero-valent iron (ZVI) combined with anaerobic bacteria was used in the remediation of Cd(II)-polluted soil under the mediation of sulfate (SO₄^2-). Owing to hydrogen-autotrophic sulfate reduction, serious corrosion occurred on sulfate-mediated biotic ZVI in terms of solid phase characterization as massive corrosive products (e.g., goethite, magnetite and green rust) were formed, which were crucial in the immobilization of Cd(II). Thus, this integrated system achieved a 4.9-fold increase in aqueous Cd(II) removal and converted more than 53% of easily available Cd(II)-fractions (acid-extractable and reducible) to stable forms (oxidizable and residual) based on the sequential extraction results as compared to the sulfate-mediated ZVI system. Increasing SO₄^2- concentration and ZVI dosage both demonstrated positive correlation to Cd(II) immobilization, which further reflected that hydrogenotrophic desulfuration acted an essential role in improving Cd(II) immobilization. It indicated that hydrogenotrophic desulfuration could accelerate iron corrosion and promote reactive mineral formation through biomineralization, as well as generate cadmium sulfide precipitates (CdS) to achieve excellent immobilization performance for Cd(II). Besides, this reaction was favorable under neutral pH condition. Our results highlighted the promoted effect of hydrogen-autotrophic desulfuration on ZVI corrosion to immobilize Cd(II) and offered a practicable technique in Cd(II)-polluted soil remediation.