Production of Fe-biochar from paper-mill sludge and its application to Se(VI) and Se(IV) removal.

[Display omitted] • Fe-containing waste (paper mill sludge, PS) was pyrolyzed in the N 2 atmosphere. • Fe in PS changed into Fe0 during pyrolysis to form Fe0-carbon composite (PS/BC-750). • PS/BC-750 exhibited high adsorption capacity for Se(VI) and Se(IV). • Se(VI) and Se(IV) were chemisorbed onto different binding sites on PS/BC-750. This study aimed to fabricate zero-valent iron (Fe0)-loaded biochar from an industrial waste and further demonstrate its applicability as an environmental medium to remove Se(VI) and Se(IV). The biochar (PS/BC-750) was produced via the pyrolysis of paper-mill sludge under N 2 atmosphere. The characterization revealed that PS/BC-750 has a porous carbon structure embedded with reduced Fe phases (Fe0 and FeO). The adsorptive removal of Se(VI) and Se(IV) by PS/BC-750 was highly dependent on pH condition and progressively decreased with the pH increase. The adsorption kinetics of Se(VI) and Se(IV) reached equilibrium within 1440 and 60 min, respectively, following a pseudo-second-order kinetics. Se(VI) and Se(IV) adsorption isotherms fit to the Freundlich and Langmuir models, respectively, highlighting the importance of chemisorption in the adsorption processes. X-ray photoelectron spectroscopy and competing anions experiments revealed that Se(VI) was adsorbed mainly on the Fe phases while Se(IV) was adsorbed on both Fe phases and carbon surface by specific binding mechanisms, with those adsorbed on Fe phases being subsequently reduced to Se0. The results demonstrated that the pyrolytic process can be of a viable platform for converting Fe-containing waste materials into environmental media with a wide application potential. [ABSTRACT FROM AUTHOR]