Colour removal of tartrazine by citric acid assisted Fe(II)/SPS oxidation: Effect of water matrix and kinetics study

Abstract

This study investigates the degradation of Tartrazine, a widely used synthetic azo dye, via an advanced oxidation process (AOP) employing a citric acid (CA)-chelated Fe(II)/sodium persulfate (SPS) system. Optimization of key operational parameters, SPS concentration, Fe(II) concentration, and CA concentration was conducted using face-centered central composite design (FCCD) within the response surface methodology (RSM) framework. The incorporation of CA as a biodegradable chelating agent enhanced Fe(II) stability, reduced Fe(III) accumulation, and minimized sulfate radical ) scavenging, resulting in improved reactivity as it forms a steric shield that inhibits hydrolytic attack, preserving Fe(II) solubility under near-neutral conditions. The predictive model exhibited high accuracy (R2 = 0.92), with optimal conditions identified as 3.602 mM SPS, 0.590 mM Fe(II), and 0.123 mM CA. Under these conditions, significant Tartrazine removal was achieved across a broad pH range (3–9), with maximum colour removal (84.3 ± 1.4%) at pH 3.0, whereas 73.1 ± 1.9% was achieved in a real water matrix. The presence of inorganic anions, particularly chloride, significantly inhibited the process, reducing efficiency to 5.6 ± 1.0% at 1.0 g/L. Kinetic analysis revealed a two-stage removal pattern: a rapid initial stage (57.6 ± 1.7% at 5 min increasing to 91.4 ± 1.9% at 60 min) followed by a slower degradation stage reaching 96.8% at 480 min. A two-stage behavior was effectively described by the Behnajady-Modirshahla-Ghanbery (BMG) kinetic model, after traditional approaches failed to capture it accurately. These results demonstrate that the CA/Fe(II)/SPS system is a cost-effective, environmentally sustainable, and scalable approach for azo dye-contaminated wastewater treatment.