Photocatalytic degradation of 1,5-naphthalenedisulfonate on colloidal titanium dioxide.

Photocatalytic degradation of 1,5-naphthalenedisulfonate (NDS) was investigated by monitoring the absorption and emission spectral changes, chemical oxygen demand, total organic carbon (TOC) content as well as pH and sulfate concentration. Intermediates formed during the irradiation were also detected by liquid chromatographic-mass spectrometric analysis. The results obtained by the applied analytical techniques clearly indicate that the initial step of degradation is oxygenation (hydroxylation) of the starting surfactant resulting in the formation of an 8-hydroxy derivative, although desulfonation and some mineralization, that is, decrease of TOC indicating carbon dioxide generation, also take place at this stage. Further oxygenation and desulfonation lead to the destruction of the diaromatic naphthalene system, then to ring fission, producing diols, aldehydes, and carboxylic acids on the side-chains. A tentative scheme involving possible pathways of degradation is proposed, taking the intermediates detected by mass spectrometry into consideration. On the basis of the results of quantum chemical calculations, the most possible points of attack by HO radical were identified, supplementing the MS results, and elucidating the initial oxidation step in the degradation of NDS and the benzenesulfonate (BS) intermediate. Thus, in the case of NDS para position is favored for hydroxylation, while for BS, formation of the ortho-hydroxy derivative is preferred.

Photocatalytic degradation of benzenesulfonate on colloidal titanium dioxide.

Titanium dioxide-mediated photocatalyzed degradation of benzenesulfonate (BS) was investigated by monitoring chemical oxygen demand (COD), total organic carbon (TOC) content, sulfate concentration, pH as well as the absorption and emission spectral changes in both argon-saturated and aerated systems. Liquid chromatography-mass spectrometry analysis was utilized for the detection of intermediates formed during the irradiation in the UVA range (λ(max) = 350 nm). The results obtained by these analytical techniques indicate that the initial step of degradation is hydroxylation of the starting surfactant, resulting in the production of hydroxy- and dihydroxybenzenesulfonates. These reactions were accompanied by desulfonation, which increases [H(+)] in both argon-saturated and aerated systems. In accordance with our previous theoretical calculations, the formation of ortho- and meta-hydroxylated derivatives is favored in the first step. The main product of the further oxygenation of these derivatives was 2,5-dihydroxy-benzesulfonate. No decay of the hydroxy species occurred during the 8-h irradiation in the absence of dissolved oxygen. In the aerated system much more efficient desulfonation and hydroxylation, moreover, a significant decrease of TOC took place at the initial stage. Further hydroxylation led to cleavage of the aromatic system, due to the formation of polyhydroxy derivatives, followed by ring fission, resulting in the production of aldehydes and carboxylic acids. Total mineralization was achieved by the end of the 8-h photocatalysis. It has been proved that in this photocatalytic procedure the presence of dissolved oxygen is necessary for the cleavage of the aromatic ring because hydroxyl radicals photochemically formed in the deaerated system too alone are not able to break the C-C bonds.