A novel (NH₄)₂SO₄-NaOH co-activation process converts coal gangue into an efficient silicon fertilizer with optimized...

Coal gangue (CG), a silicon-rich solid waste, contains silicon in a crystalline form unavailable to plants. This study developed an energy-efficient (NH₄)₂SO₄-NaOH co-activation process to convert CG into an efficient silicon fertilizer. Thermodynamic analysis confirmed the feasibility of (NH₄)₂SO₄ as an activator above 400°C. Optimal activation was achieved at a (NH₄)₂SO₄/CG mass ratio of 0.2, 450°C for 30 min (AS), yielding citric acid-leachable Si and Al concentrations of 429.15 and 729.44 mg/L, respectively. The addition of 5% NaOH further enhanced Si leaching to 446.50 mg/L by promoting siloxane network depolymerization. Characterization (XRD, XPS, FTIR) confirmed the transformation of silicon to more bioavailable forms. The co-activated sample (AS-CS) exhibited a superior biphasic release pattern with high initial release and sustained cumulative release (494.53 mg/L). While (NH₄)₂SO₄ activation increased leachable ammonia nitrogen and sulfate, NaOH addition effectively reduced the NH₃-N (sum of the nitrogen content in both free ammonia and ammonium ions in aqueous solution). levels via volatilization. Soil incubation revealed that AS-CS moderated the shifts in microbial community structure induced by (NH₄)₂SO₄ alone. This work provides a sustainable strategy for CG valorization into silicon fertilizer, balancing activation performance with environmental safety, supporting circular economy goals.