Biomass facilitated phase transformation of natural hematite at high temperatures and sorption of Cd2+ and Cu2

Phase changes of natural hematite are often practiced to improve heavy metal removal and magnetism for easy recycling. In this work, pinewood biomass (PB) and natural hematite (H) admixtures were pyrolyzed at 300, 450 and 600 °C under N2 environment to prepare HBC nanocomposites (HBC300, HBC450 and HBC600). The X-ray diffraction (XRD) confirmed the reductive transformation of hematite (crystallite size ≈ 47 nm) into magnetite (25 nm) and further to wustite (25 nm) and zerovalent iron (48 nm). The Langmuir isotherms showed that the maximum sorption capacities of HBC300, HBC450, and HBC600 were 173, 138, and 130 mmol kg−1 for Cd2+, and 359, 172, and 197 mmol kg−1 for Cu2+, respectively. The higher pH up to 5 increased sorption of both Cd2+ and Cu2+, whereas the higher ionic strength (0.05–0.4 M) decreased Cd2+ sorption. Sorption of Cd2+ and Cu2+ by HBC300 was accompanied by one order of magnitude greater cation release than HBC450 and HBC600. In a binary system, Cd2+ sorption was depressed by over four times in presence of Cu2+. Overall, ion exchange was more pronounced for HBC300, and Cu2+ was more favorably retained by specific sorption than Cd2+. The greater magnetism of HBC nanoparticles favors separation from aqueous solutions. [Display omitted] •Pinewood facilitated transformation of natural hematite as α-Fe2O3 → Fe3O4 → FeO → Feo.•Crystallite size of Fe oxide dropped from 47 to 25 nm between 300 and 600 °C.•Cd2+ and Cu2+ sorption by biochar composites was over five times higher than pristine precursors.•Biochar composites prepared at 300 °C over 450 and 600 °C was more favorable for Cd2+ and Cu2+ removal.•Cu2+ was more preferentially retained by specific retention mechanisms.