The photodegradation kinetics of aqueous sodium oxalate solution using TiO2 catalyst Original Research Article

The kinetics of the photodegradation of sodium oxalate have been investigated in a semi-batch annular photoreactor using commercial titania as the catalyst. Sodium oxalate is an important toxic pollutant in Bayer liquor during alumina processing and its removal and possible rejuvenation of the caustic solution is vital to plant economics. Experiments carried out under UV irradiation (250–310 nm wavelength) over 3 h runs showed that initial solution pH, light intensity, oxygen partial pressure, catalyst loading and oxalate concentration all have strong effects on the decomposition rate. The catalyst exhibited a relatively low activation energy of about 45.8 kJ mol−1. The degradation rate decreased exponentially with pH, but both O2 and catalyst loading went through a maximum at about 45% O2/N2 and 1.5 gcat l−1, respectively. The rate–oxalate concentration behaviour indicates that the catalyst has a high adsorption constant for oxalate species. This was attributed to the increased density of positively charged sites at pH values below the isoelectric point (pH=6). The essential features of the proposed mechanism involve aqueous phase dissociation of sodium oxalate followed by adsorption of the C2O2−4 ions onto positively charged sites. However, adsorption of O2 to electron-rich sites yields the superoxide ions which are protonated to give highly active peroxy radicals. The data suggested a bimolecular surface reaction between adsorbed oxalate and peroxy species as the rate-controlling step in the homogeneous–heterogeneous mechanism. The subsequent production of HCO−3 ions gave increased pH with time-on-stream.