Sustainable synthesis of engineered carbon from date palm leaves and steel industry residues for methylene blue removal

Agricultural residues from date palms and alkaline by-products from steelmaking are abundant yet underutilised wastes. This study couples both streams to produce functional adsorbents via a fully aqueous, low-energy activation: date-palm biochar is treated directly with minimally processed leachates from five steel residues (baghouse dust, ladle-furnace slag, cyclone dust, electric-arc-furnace slag, and carbide-lime slurry). Temperature-controlled adsorption tests (35–61 °C) assessed thermodynamic behaviour and its influence on kinetics and capacity at environmentally relevant, dilute methylene blue (MB) levels (≈5–21 mg L⁻¹). Kinetics follow pseudo-second-order (R² > 0.98), confirming chemisorption-dominated uptake. Under identical conditions, slurry- and EAF-activated carbons achieved the highest removals (~64% and ~58%), surpassing untreated biochar (~55%). Equilibrium data (400 min, 25 °C) fit Langmuir > Freundlich models, while pH ≈ 9 optimised uptake and finer fractions (38–150 µm) enhanced rates. SEM/FTIR/XRD revealed mesoporous textures and surface functionalities (–OH/–COO⁻ and metal–oxygen motifs) linked to leachate chemistry. A harmonised workflow connects ionic composition (K⁺/Na⁺/Mg²⁺-rich vs. Ca²⁺-dominant leachates) to activation strength and performance. This proof-of-concept demonstrates dual-waste valorisation yielding reproducible dye removal without added acids/bases or high-temperature calcination, offering a foundation for scalable, circular water treatment integrating regeneration and life-cycle assessment.