A solution of 6 mM of potassium ferrioxalate in 50 mM sulfuric acid (Sigma Aldrich) solution, using demineralized water as solvent, was prepared. The solution was loaded in the batch reactor or pumped through the microcapillary/photo rotor-stator spinning disk reactor (pRS-SDR), under the irradiation of UV-light source (Batch reactor: Kessil PR160L 370 nm, input power 43 W; microcapillary reactor: LuxaLight UV-A 365 nm LED strip, input power 52 W; pRS-SDR: UV-A curing 365 nm floodlight, input power 175 W). The reactor was placed at a known position relative to the light source. The batch test tube (Pyrex, inner diameter 13.2 mm, outer diameter 15.2 mm, filled with 4 mL reaction mixture) was stirred, using a magnetic stirrer and stirrer bar. This tube was removed after the set irradiation time, whereafter a sample was taken. A new unirradiated tube was used to obtain the subsequent data point to ensure that the volume inside the reactor remained constant. For the continuous reactor types, samples were taken at the outlet after waiting several residence times to ensure steady-state conditions. The microcapillary reactor (PFA, 1.6 mm inner diameter, 3.2 mm outer diameter, 7.5 mL) was operated in continuous mode rather than using stop-flow, due to increased mixing and prevention of unwanted dilution. For the pRS-SDR the total volume was used to express the residence time (59 mL excluding connections, ~28 mL irradiated volume). All experiments and the preparation of the solutions and samples were conducted in a darkroom.