Spectrophotometric determination of thallium in flue gas: Insights into the I2/I- enhanced stable color development mechanism of ABTS·+ radicals.

Currently, thallium (Tl) in flue gas is primarily monitored by liquid phase absorption followed by Inductively Coupled Plasma (ICP) analysis, a process characterized by operational complexity, long turnaround times, and high costs. Here, a rapid and accurate UV-Vis spectrophotometric method was developed for the determination of Tl in flue gas. A 5%HNO3/10%H2O2 solution was employed to capture gaseous Tl and ensure its complete oxidative conversion to trivalent Tl(III). The chromogenic reaction between Tl(III) and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) was systematically investigated. At approximately pH 3.0, Tl(III) oxidized colorless ABTS to stable green ABTS•+ radicals via I2/I- catalysis, enabling spectrophotometric quantification at 415 nm. Under optimized conditions, the Tl(III) concentration exhibited a strong linear relationship with absorbance in the range of 0.05-5 mg·L-1 (R2 = 0.9998), with a limit of detection (LOD) of 0.035 mg·m-3 for Tl in flue gas. The method showed high selectivity, with no significant interference from common metal ions or anions. The accuracy of this method was validated through comparison with ICP, showing highly consistent results. This study provides a cost-effective and efficient approach for the routine monitoring of Tl emissions in industrial flue gas.