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.