Coal Fly Ash-Supported Mn-Doped CuOS Nanocomposite for Efficient NaBH4-Assisted Catalytic Reduction of Organic Dyes and Hexavalent Chromium Under Dark Conditions

The discharge of toxic organic dyes and heavy metals into aquatic environments poses serious risk to ecosystems and human health, necessitating efficient, sustainable, and low-cost remediation technologies. Herein, a coal fly ash (CFA)-supported Mn-doped copper oxysulfide (MnCuOS@CFA) nanocomposite was synthesized via a facile hydrothermal method and evaluated for NaBH4-assisted catalytic reduction of methylene blue (MB), rhodamine B (RhB), and hexavalent chromium [Cr(VI)] under dark conditions. XRD, XPS, SEM, HR-TEM, EDS, and N₂ adsorption–desorption analyses confirmed successful Mn incorporation into the CuOS lattice and the formation of a mesoporous MnCuOS@CFA nanocomposite. XPS analysis revealed Mn2+/Mn4+, Cu⁺, S2-, lattice oxygen species, and preserved Si4⁺/Al3⁺ aluminosilicate frameworks, while binding-energy shifts indicated interfacial electronic interactions and charge redistribution between MnCuOS and CFA. The porous CFA support effectively anchored MnCuOS nanocrystallites, suppressing aggregation, improving dispersion and pore accessibility, and providing abundant active sites, resulting in an increased surface area (10.73 m2 g-1) and improved catalytic performance. Thus, MnCuOS@CFA achieved reduction efficiencies of 97.03%, 96.43%, and 95.92% for MB, RhB, and Cr(VI) within 9, 12, and 12 min, respectively. The enhanced activity was attributed to improved pollutant adsorption, accelerated BH4⁻ activation, efficient electron transfer, hydrogen-assisted reduction, and abundant active sites. Moreover, the MnCuOS@CFA maintained excellent stability, retaining 92–97% removal efficiency after five cycles. This study demonstrates a sustainable strategy for converting CFA waste into a durable, high-performance nanocomposite catalyst for environmental remediation.