PHOTOCATALYTIC CAUSTICIZATION OF SODIUM OXALATE USING COMMERCIAL TIO2PARTICLES

Oxidative photocatalytic degradation has proven to be an effective met hod for the removal of toxic sodium oxalate, a major impurity in the B ayer liquor obtained during alumina processing. The proposed route als o simultaneously converts the oxalate to causticizable solution. The r eaction was carried out in an annular multiphase photoreactor with com mercial TiO2 powder (99% anatase) as the catalyst. There was a strong influence of both catalyst content and initial pH of the reaction mixt ure. Conversion generally decreased with increased pH. Complete minera lization of the oxalate to caustic solution was achieved within a very short period of similar to 2 h at 303 K, 0.5 gl(-1) catalyst loading and an initial solution pH of similar to 2. We believe that the reacti on proceeds via a homogeneous-heterogeneous mechanism involving intera ction between adsorbed oxalate ions (generated from liquid phase disso ciation of sodium oxalate) on the titania surface (holes) and hydroxyl species to give adsorbed hydrogen carbonate ions which later desorb t o form the caustic NaHCO3 solution. (C) 1997 Elsevier Science B.V.