Degradation of spent caustic by Fenton and persulfate oxidation with zero‐valent iron

BACKGROUND This study investigated the oxidation of spent caustic, strong alkaline liquid waste from refinery and petrochemical plants, by hydrogen oxide and persulfate activated with zero‐valent iron (Fe(0)) as an alternative treatment process. Using batch experiments, the mineralization of spent caustic was evaluated by monitoring the total organic carbon (TOC). RESULTS TOC removal from the spent caustic was not successfully achieved by treatment with hydrogen peroxide or persulfate only; spent caustic was mineralized when Fe(0) was added. With 1 g of Fe(0), 25 and 35% of the initial TOC was reduced in the presence of hydrogen peroxide and persulfate, respectively. The optimal Fe(0)‐to‐hydrogen peroxide/persulfate molar ratio was 1:5. Control experiments with Fe2+ under identical conditions indicated that Fe(0) was more effective for activating hydrogen peroxide and persulfate, probably due to the continuous release of Fe2+ in the presence of the oxidants. Increased temperatures enhanced the oxidation of the spent caustic by Fe(0)‐activated persulfate, resulting in greater than 95% TOC removal in 5 h at 80 °C at the optimal ratio. A five‐fold increase in the amounts of Fe(0) and hydrogen peroxide or persulfate while retaining the optimal ratio resulted in the complete removal of TOC in 12 and 2 h, respectively. CONCLUSION Results suggest that Fe(0)‐assisted Fenton and persulfate oxidation may be a promising option for treating spent caustic.