Toward zero-waste steel slag: simultaneous production of iron oxide nanocomposites for dye adsorption and recyclable gypsum

This study presents a transformative zero-waste paradigm for up-cycling steel slag (SS) which is an abundant industrial waste, by strategically deconstructing it into two valuable products. The established process involves the targeted sulfuric acid leaching of SS to separate its major components: iron ions and calcium. The iron-rich leachate serves as a precursor for the plant-mediated synthesis of α-Fe 2 O 3 /SiO 2 nanocomposites using Gingko biloba leaf extract, while the solid residue is transformed into gypsum (CaSO 4 ·2H 2 O), a huge applicable material in construction and agriculture. Optimization of methylene blue (MB) adsorption using the Taguchi method achieved an experimental equilibrium capacity of 37.03 mg/g under the investigated conditions. The adsorption behavior was best described by the pseudo-second-order kinetic model and the Freundlich isotherm model, indicating dominant chemisorption on a heterogeneous surface. The Freundlich constants ( K F  = 75.5 (mg/g)(L/mg) 1/n and n  = 2.04) confirmed strong adsorption affinity and favorable adsorption characteristics. A Langmuir monolayer capacity of 102.04 mg/g was estimated from model fitting. Density functional theory (DFT) and molecular mechanics simulations provided atomistic-level confirmation of a strong, exothermic interaction between methylene blue (MB) and the Fe 2 O 3 surface, albeit moderated in an aqueous environment. Furthermore, the nanocomposite showed remarkable stability, retaining over 70% of its removal efficiency after five consecutive adsorption-desorption cycles. This study establishes a comprehensive “waste-to-wealth” pipeline beyond simple waste modification and demonstrates a viable circular economy model simultaneously designed to control the solid waste management and water pollution.