Synergy of sulfate and hydroxyl radicals in UV/S 2O 82−/H 2O 2 oxidation of iodinated X-ray contrast medium iopromide

Abstract

The degradation of iodinated X-ray contrast medium, iopromide, by UV irradiation with the assistance of combined oxidants of S 2O 8 2− and H 2O 2 has been investigated. The effects of various parameters, including different wavelengths of UV irradiation, UV intensities, initial solution pH levels, dosages of oxidants, dosing sequence, and the presence of non-target organic matters, have been evaluated. The iopromide decay follows pseudo-first-order kinetics. The UV at 254 nm exhibits higher decay rate of iopromide than the others (300 and 350 nm) due to stronger photon energy of 254 nm and the relatively higher absorptivity of S 2O 8 2−, H 2O 2 and iopromide nearby this wavelength. Optimum pH level was determined to be around 4.34. The mechanism is complicated because of the involvement of the scavenging, recombination, self-decomposition and/or stability of the involved oxidant (S 2O 8 2− and H 2O 2) and radicals (SO 4 − and HO ) as discussed in the paper. Iopromide decay rate is linearly proportional to the [S 2O 8 2−], however a nonlinear-optimal rate was observed by varying [H 2O 2] due to the formation of weaker radicals via overdosing of H 2O 2. Additionally, the sequential addition of S 2O 8 2− to UV/H 2O 2 or H 2O 2 to UV/S 2O 8 2− was found no better than the UV/S 2O 8 2−/H 2O 2. This is because the quenching of already formed SO 4 − by newly added H 2O 2 (or vice versa the quenching of HO by S 2O 8 2−). The simultaneous addition of S 2O 8 2− and H 2O 2, however, will slightly delay the generation rates of SO 4 − and OH due to the competition of photons between S 2O 8 2− and H 2O 2, which can reduce both the radical-scavenging reactions and the peak radical concentration in the solution, thus maximizing the utilization of precious radicals. The presence of non-target organics, especially the humic acid, in the UV/S 2O 8 2−/H 2O 2 process will quench the radicals, a pre-treatment is recommended to lessen this problem in real application.