The practical applicability of decolorization techniques for synthetic textile wastewater (STW) mimicking real conditions in terms of organic matter and carbonate alkalinity content remains unclear. To address this issue, this study investigated the decolorization of an anthraquinone dye, Acid Blue 25 (AB-25), in STW using advanced oxidation processes (AOPs). The oxidation kinetics of AB-25 in STW and ultra-pure water (UPW) under UV-H2O2 and UV-TiO2 were comparatively studied. In UPW, the first-order rate constant with UV-TiO2 (0.5 g/L; k: 2.79 × 10−2 1/min) was more than twice that with UV-H2O2 (100 mg/L; k: 1.14 × 10−2 1/min), corresponding to 85% and 72% decolorization, respectively. Interestingly, in STW, the decolorization was slightly higher with UV-H2O2 than with UV-TiO2; at neutral pH, the first-order rate constants were 2.1 × 10−3 1/min and 1.8 × 10−3 1/min, accounting for 29% and 21% decolorization, respectively. This behavior can be attributed to the influence of background constituents, especially alkaline components (carbonate, bicarbonate). The combined influence of alkalinity and pH accounted for ~ 45%. For practical application, these results suggest that pH should be kept under 4.5 to ensure feasible treatment of STW via AOPs. Furthermore, the oxidation patterns reveal that decolorization of AB-25 results in the formation of sulfates due to desulfonation. Graphical abstract: [Figure not available: see fulltext.].