Advanced Oxidation of Typical Dicarboxylic Acids in Highly Alkaline, High-Ionic-Strength Solutions: Effects and Possible Decomposition Mechanisms

The presence of organics in Bayer liquors has a notable adverse effect on alumina production. Among these organics, dicarboxylic acids are a class of low molecular weight compounds that have yet to be studied for their decomposition in Bayer liquors using advanced oxidation techniques. This study aims to investigate the oxidation behavior of four dicarboxylic acids, including malonic acid (C3), succinic acid (C4), glutaric acid (C5), and adipic acid (C6), in Bayer liquors. The influences of different variables, such as sodium persulfate (SPS) dosage, initial organic concentration, caustic soda concentration, reaction temperature, and reaction time, on the advanced oxidation process of dicarboxylic acids were also studied. The results of the experiment indicate that an increase in SPS dosage, initial organic concentration, reaction temperature, and reaction time promoted the removal of total organic carbon (TOC). However, the concentration of caustic soda had an inhibitory effect on TOC removal, and it also affected the type of decomposition products of dicarboxylic acids. The EPR results revealed that the reactive groups responsible for organic decomposition were \({\text{SO}}_{4}^{ - \cdot }\) ${\text{SO}}_4^{-·}$ and \({\text{HO}}^{ \cdot }\) ${\text{HO}}^{·}$ . Dicarboxylic acids were found to produce several decomposition products, including carbon dioxide (in the form of carbonate), formic acid, and oxalic acid. Additionally, malonic and fumaric acids were observed as the main oxidation intermediates for dicarboxylic acids with C-atom numbers > 3. A reaction mechanism for succinic acid was also proposed. Advanced oxidation techniques have shown promise as a means of removing organics from Bayer liquors.