Citric acid modified red mud for valorization as a sustainable catalyst in bisulfite-activated congo red degradation

Bisulfite (BS)-based advanced oxidation processes (AOPs) are attractive for pollutant degradation, but often depend on costly transition metals with leaching risks. Herein, we report a citric acid-modified red mud catalyst (RMAC) for efficient Congo Red (CR) removal. Citric acid acted Simultaneously as an acid activator and carbon template, enlarging the surface area from 31.10 to 116.40 m² g^-1 (3.74-fold increase). Under optimal conditions (5 mM BS, pH = 5, 80 mg L^-1 CR), RMAC3-800 achieved 98.8% CR removal with a pseudo-first-order rate constant of 0.1399 min^-1 and retained > 80% efficiency after three reuse cycles. Radical scavenging and EPR analyses confirmed SO₄^•- (53.7%) and •OH (46.3%) as the dominant species, whereas XPS identified Fe⁰ as the principal active site. GC-MS detected six intermediates, supporting the proposed oxidative cleavage and mineralization pathways of the degradation process. A preliminary bench-scale cost analysis estimated an operating cost of ~ 13.94 RMB m^-3 (≈ 1.95 USD m^-3), underscoring its economic feasibility. This study demonstrates a cost-effective, recyclable, and sustainable catalytic system for wastewater treatment and red mud valorization.