Synergy of sulfate and hydroxyl radicals in UV/S₂O₈²-/H₂O₂oxidation of iodinated X-ray contrast medium iopromide

The degradation of iodinated X-ray contrast medium, iopromide, by UV irradiation with the assistance of combined oxidants of S2O82- and H2O2has 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 S2O82-, H2O2and 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 (S2O82- and H2O2) and radicals (SO4•- and HO•) as discussed in the paper. Iopromide decay rate is linearly proportional to the [S2O82-], however a nonlinear-optimal rate was observed by varying [H2O2] due to the formation of weaker radicals via overdosing of H2O2. Additionally, the sequential addition of S2O82- to UV/H2O2or H2O2to UV/S2O82- was found no better than the UV/S2O82-/H2O2. This is because the quenching of already formed SO4•- by newly added H2O2(or vice versa the quenching of HO• by S2O82-). The simultaneous addition of S2O82- and H2O2, however, will slightly delay the generation rates of SO4•- and OH• due to the competition of photons between S2O82- and H2O2, 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/S2O82-/H2O2process will quench the radicals, a pre-treatment is recommended to lessen this problem in real application.