Iodometric spectrophotometric determination of peroxydisulfate in hydroxylamine-involved AOPs: 15 min or 15 s for oxidative coloration?

In this study, iodometric spectrophotometry, the most-used method for detecting peroxydisulfate (PDS), was modified by increasing the concentration of potassium iodide (KI) for realizing the immediate PDS determination and avoiding the interference of hydroxylamine. Kinetic studies showed that the reaction between PDS and I⁻ to generate the yellow-colored I₃⁻ followed the kinetic equation as [Formula presented]. Detection time of the iodometric spectrophotometry was shortened from 15 min to 15 s when KI concentration was increased from 0.6 M to 4.8 M. Different with the previous iodometric spectrophotometry, the modified method using 4.8 M KI as the indicator was well tolerable to the interference of hydroxylamine at acidic pH conditions. The calibration curve of the modified method showed a well linear relationship (R² = 0.999) between the absorbance of I₃⁻ at 352 nm and PDS concentration in the range of 0–80 μM. The modified method was highly sensitive with the absorptivity of 2.5 × 10⁴ M⁻¹ cm⁻¹ and the limit of detection of 0.11 μM. Moreover, the modified method was successfully applied for monitoring the change of PDS concentration during the degradation of diclofenac with four different PDS-based AOPs, the calculated reaction stoichiometric efficiency ([Formula presented]) followed the order as heat/PDS system > hydroxylamine/Fe²⁺/PDS system > hydroxylamine/Cu²⁺/PDS system > Fe²⁺/PDS system.