Iron-Regulated (FeCoNiCuZn)O Spinel High-Entropy Oxides: A Magnetically Recyclable Dark Catalyst for Efficient Dye Wastewater Treatment

High-entropy oxides (HEOs) have shown extensive applications in catalysis. However, most catalytic reactions rely on external energy inputs such as peroxides, ozone, electricity, light, or high temperatures. This study investigates the dark catalytic performance of spinel HEO catalysts for the degradation of Rhodamine B (RhB) dye under ambient temperature and in the absence of peroxides, ozone, or photochemical energy. Spinel-structured magnetic HEO catalysts (HEO-Fe-x) were synthesized via a citric acid gel combustion method by incorporating equimolar amounts of Zn, Ni, Cu, and Co elements while varying the Fe/(ZnNiCuCo) molar ratio (x = 0.25, 0.5, 1.0, and 1.5). The resulting catalysts consist of a spinel-phase HEO and CoCu alloy phase, forming nano-heterojunction structures. Under strongly acidic conditions, HEO-Fe-1 exhibited superior dark catalytic activity, degrading more than 90% of RhB within 20 minutes. This enhanced performance is attributed to the redox reactions between surface electrons on the catalyst and O₂ in the solution, generating reactive radicals. Moreover, the active sites at the CoCu alloy and heterojunction interfaces under acidic conditions further facilitated redox reactions, producing significant amounts of ·OH, ¹O₂, and ·O₂⁻ radicals, thereby accelerating RhB degradation. At a pH of 2.5, HEO-Fe-1 retained excellent degradation efficiency after five consecutive cycles. These results demonstrate that HEO-Fe-1 exhibits stable and efficient dark catalytic activity for RhB degradation in strongly acidic environments, highlighting its excellent magnetic properties, reusability, and acid resistance. This catalyst offers promising potential for the treatment of organic pollutants in acidic media.