A good query! Some thoughts, I would like to suggest:

Here are some scavengers you could use:

OH Scavengers: Isopropanol (IPA) or ethanol precursor can be used.

O2•- Scavengers: Superoxide dismutase (SOD) or 4-Hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPOL) can be used.

Singlet Oxygen Scavengers: Sodium azide (NaN3) or histidine can be used.

For all of the above, typical concentrations of the precursor might be around 0.1 M.

More exotically....

Electron Scavengers: Oxygen (O2) is a common electron scavenger. In an oxygen-saturated solution, the concentration of dissolved oxygen is approximately 0.0013 M.

Remember to add the scavenger to the dye solution before irradiation begins. After adding the scavenger, you should mix the solution thoroughly to ensure the scavenger is evenly distributed.

Please note that the concentrations suggested above are just starting points. The optimal concentration for your specific system may be different and should be determined experimentally.

This ties in nicely with my own research concerning AOPs and fouling control in desalination modules. A review should be coming out soon, which overlaps with this area.

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Thank you very much, Dr Robin Chrystie, for answering my query.

I decompose methylene blue in a photodegradation process under titania nanotubes (or just titania), which generates reactive oxygen species in aqueous solutions after its irradiation. You can also take it into account.