The kinetics and efficiency of UV assisted advanced oxidation of various types of commercial organic dyes in water

• Dyes with urea group form adducts with H2O2.
• In the absence of Fe(II), side carbonyl groups in dyes react with H2O2.
• Reaction between carbonyl group and H2O2 is competitive to H2O2 photolysis.
• Organic dyes primarily degrade via OH attack.
• Reaction between Fe2+ and H2O2 is a main source of OH radicals in UV/Fenton.

Five model solutions formulated from the commercial dyes: C. I. Reactive Black 1 (RB1), C. I. Direct Red 23 (DR23), C. I. Acid Blue 25 (AB25), C. I. Basic Red 1 (BR1) and C. I. Mordant Orange 1 (MO1) were treated by UV-C assisted homogeneous AOPs: UV/Fe(II), UV/H2O2 and UV/Fe(II)/H2O2 (hereafter: UV/Fenton). Dye degradation in acidic medium (pH 3) was evaluated by the decrease in Total Organic Carbon (TOC) content and colour, measured by the decrease in chromophore absorption bands. In comparison to photolysis (UV alone), UV/Fe(II) and UV/H2O2, the highest extents were observed when UV/Fenton process was applied.Optimal experimental conditions were taken from our previous work: 0.5 mM FeSO4·7H2O and 2.5 mM of hydrogen peroxide. Under these experimental conditions, more than 90% of TOC and 100% of colour removal were obtained for AB25 and MO1 dye after 60 min of UV/Fenton. The lowest extents were observed for DR32; only 34% of TOC and 48% of colour removal.Mineralization and decolourization kinetics were modelled according to the comprehensive kinetic model introduced in our previous study [1], taking into account crucial reactions of Fenton catalytic cycle, UV-C induced photolysis and reaction between dye/organic molecule and OH radicals. Direct photolysis was modelled according to semiempirical model based on Lambert–Beer's law (LLM). The results were discussed in respect to each dye.