Tetracycline removal from aqueous solution by magnetic biochar modified with different iron valences: A comparative study

Magnetic activation is widely used to prepare highly efficient adsorbents for easy solid–liquid separation. However, knowledge on the effect of various iron valences (nZVI, Fe2SO4, FeCl3·6H2O, and K2FeO4) on the properties of magnetic biochars (Fe-BCs) and their performance is still inadequate. This study aimed to compare the properties and tetracycline (TC) removal performances of four pre-magnetic biochars derived from spent mushroom substrate (SMS) of Lentinus edodes (named as Fe0-BC, Fe2+-BC, Fe3+-BC, and Fe6+-BC). The results revealed that doping with various Fe valences formed Fe-BCs with different elemental contents, morphologies, and structures. The type of Fe valence used for doping biochars plays an important role in TC adsorption. TC adsorption capacity (qm) followed the sequence: Fe6+-BC (37.95 mg·g−1) > Fe2+-BC (31.89 mg·g−1) > Fe0-BC (25.36 mg·g−1) > Fe3+-BC (19.63 mg·g−1) > BC (11.06 mg·g−1). Fe6+-BC exhibited excellent TC adsorption capacity with the best separation performance, which was attributed to its high iron-loading and large surface area from strong corrosiveness during pyrolysis. Mechanistic analysis revealed that pore filling, π–π interactions, complexation, and hydrogen bonding mainly contribute to TC adsorption by Fe-BCs. Furthermore, Fe2+-BC showed acceptable TC adsorption capacity; thus, Fe2SO4 could be considered a promising alternative reagent to K2FeO4 in Fe-BC preparation because it is cost-effective. These results may help guide the selection of an optimal Fe source doped into BC for TC adsorption by wastewater.