Red mud supported CuxO composites for rapid and selective nitrate reduction: Enhancing in-situ Cu0 generation and hydrogen-mediated pathways

The large-scale accumulation of red mud, a waste by-product from aluminum production, presents critical environmental challenges and opportunities for resource valorization. In this study, a Cu x O/RM composite was synthesized via wetness impregnation and applied for rapid and selective NO 3 − -N reduction. Complete NO 3 − -N removal was achieved within 9 min under mild conditions (25 °C, initial pH 7.0, 30 mg(N)/L), with NO 2 − -N as the primary intermediate. With addition of NH 2 SO 3 H, NO 2 − -N was further reduced to N 2 gas within 3 min. Notably, the Cu x O/RM system maintained high NO 3 − -N removal efficiency even at lower temperatures (5–15 °C) and across a wide pH range (3−11), demonstrating excellent adaptability to real-world wastewater conditions. Stability and reusability tests showed that the composite retained over 95 % removal efficiency after five reuse cycles. Mechanistic investigations revealed that in-situ generated Cu 0 and H 2 -mediated pathways synergistically enhanced NO 3 − -N reduction performance. This study not only provides an effective and sustainable strategy for NO 3 − -N remediation but also offers a promising route for the large-scale valorization of red mud.