Photocatalytic degradation of seawater organic matter using a submerged membrane reactor

A TiO2 based advanced oxidation process is expected to degrade organic matter in seawater and thereby fouling would be mitigated in the membrane reactor. The effects of seawater sources on photocatalytic microfiltration treatment were investigated with respect to the removal of particulate and organic matter, as well as the control of membrane permeability. The TiO2 mediated photocatalysis of seawater organic matter (SOM) was compared with that of natural organic matter (NOM) present in fresh water. The photocatalytic degradation of SOM was marginal, but it was enhanced significantly when sea salts were removed by electrodialysis. It revealed that the SOM photocatalysis was governed not by the molecular structure of SOM itself, but by the presence of ionic species in the seawater. To identify the ions responsible for the considerably reduced SOM photocatalysis, some major ionic species selected were dosed to desalted seawater. It was thus found that chloride ions were serving as a scavenger of photoinduced strong oxidizing radicals (e.g., hydroxyl or superoxide radicals), although they may contribute to the partial oxidization and chlorination of SOM molecules. The alteration of SOM molecular structures before and after the photocatalysis was confirmed by means of fluorescence spectroscopy. The X-ray fluorescence of TiO2 surfaces showed that no significant adhesion of chloride ions onto the TiO2 surface occurred during SOM photocatalysis.