<p>To address the persistent challenge of sulfate (SO₄²⁻) removal from acid mine drainage (AMD), a novel calcium-functionalized composite biochar (CaCl₂-BC) was synthesized via the co-pyrolysis of sunflower heads, coal fly ash, and CaCl₂. The composite prepared under optimal conditions (600 °C, 1:1 mass ratio) exhibited an exceptional SO₄²⁻ removal capacity of 188.73 mg/g. The removal kinetics followed the pseudo-second-order model, while the equilibrium data fitted the Langmuir isotherm well. Crucially, microscopic characterizations and Density Functional Theory (DFT) calculations explicitly decoded a synergistic "adsorption-precipitation" mechanism. The fly-ash-derived Si–O groups acted as high-affinity anchors (− 174.3 kJ/mol) to initially capture and enrich SO₄²⁻, effectively overcoming electrostatic repulsion. Subsequently, the surface-loaded CaCl₂·2H₂O phase served as a reactive engine, driving the in-situ crystallization of stable gypsum (CaSO₄·2H₂O). Comparative system matrix evaluations demonstrated that this synergy far exceeded the simple sum of individual components. Furthermore, CaCl₂-BC sustained a formidable capacity of 147.56 mg/g in complex real AMD, while leaching tests confirmed its environmental safety. This study establishes a rigorous theoretical framework for mineral-carbon synergy and provides a highly practical material for industrial AMD remediation.</p>