The Fe(III)-photoinduced degradation of 4,4′-diaminostilbene-2,2′-disulfonic acid (DSD), used as a model compound of fluorescent whitening agents, was investigated in aqueous solution. The mixing
of DSD and iron(III) solutions immediately led to DSD degradation into two oxidation products 5-amino-2-
formyl-benzene sulfonic acid (1) and DSD epoxide (2) together with the formation of Fe(II). The behaviour
of the latter solution was investigated under monochromatic excitation at 365 nm. The irradiation of the
solution resulted in a photoredox process that yielded Fe(II) and HO• radicals; [Fe(H2O)5(OH)]2+ being the most photoactive species. The disappearances of the oxidation products were shown to only involve the
attack of HO• radicals; the quantum yields of disappearances are wavelength dependent as the formation
of the hydroxyl radicals is. For prolonged irradiation, the total mineralisation of the oxidation products
was achieved with the formation of sulphate and ammonium ions. The half-lives of products 1 and 2 when
submitted to such a process under solar irradiation were estimated to be respectively 12 and 11 min in the
mixture with 100% of monomeric species [Fe(H2O)5(OH)]2+. Under these conditions, the total mineralisation is reached within 2 days of exposure.
