Column study of Cd(II) removal and longevity by nitrate-mediated zero-valent iron with mixed anaerobic microorganisms

In this study, hydrogen-autotrophic microorganisms and zero-valent iron (Fe⁰) were filled into columns to investigate hydrogenotrophic denitrification effect on cadmium (Cd(II)) removal and column life-span with sand, microorganisms, Fe⁰ and bio-Fe⁰ columns as controls. In terms of the experiment results, the nitrate-mediated bio-Fe⁰ column showed a slow Cd(II) migration rate of 0.04 cm/PV, while the values in the bio-Fe⁰ and Fe⁰ columns were 0.06 cm/PV and 0.14 cm/PV respectively, indicating much higher Cd(II) removal efficiency and longer service life of the nitrate-mediated bio-Fe⁰ column. The XRD and SEM-EDX results implied that this improvement was attributed to hydrogenotrophic denitrification that caused more serious iron corrosion and larger amount of secondary mineral generation (e.g., green rust, lepidocrocite and goethite). These active minerals provided more reaction sites for Cd(II) adsorption and further immobilization. In addition, the decrease of Cd(II) migration front and the increase of removal capacity along the bio-Fe⁰ column mediated by nitrate presented an uneven distribution in reactive zone. The latter half part was identified to be a more active region for Cd(II) immobilization. The above results indicate that the introduction of nitrate and microorganisms will improve the performance of iron-based permeable reactive barriers for the remediation of Cd(II)-containing groundwater.