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_xO/RM composite was synthesized via wetness impregnation and applied for rapid and selective NO₃⁻-N reduction. Complete NO₃⁻-N removal was achieved within 9 min under mild conditions (25 °C, initial pH 7.0, 30 mg(N)/L), with NO₂⁻-N as the primary intermediate. With addition of NH₂SO₃H, NO₂⁻-N was further reduced to N₂ gas within 3 min. Notably, the Cu_xO/RM system maintained high NO₃⁻-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 Cu⁰ and H₂-mediated pathways synergistically enhanced NO₃⁻-N reduction performance. This study not only provides an effective and sustainable strategy for NO₃⁻-N remediation but also offers a promising route for the large-scale valorization of red mud.