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 degrees 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. (C) 2012 Society of Chemical Industry