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

Abstract

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.