Characterization of Homogeneous-Nucleation Fludized-bed Fenton

Ordinary fluidized-bed Fenton process eliminates bulky and high-moisture-content Fe(OH)₃ sludge, generated after neutralization of the Fenton process, by introducing foreign solid particles to stimulate heterogeneous nucleation. Obtained iron oxide-coated pellets which are rigid and contain very low moisture content can be more easily to manage and dispose. Nonetheless, the foreign material at the pellet core still increases the overall disposal cost. Homogeneous-nucleation fluidized-bed Fenton, on the other hand, employs in situ freshly-formed Fe(OH)₃ precipitates as the cores for nucleation and crystallization. As a result, no external supplement of solid particles is needed. This study varied the superficial upflow velocity in the fluidized-bed and H₂O₂ concentration from 1.6 to 38.3 m/d (equivalent to the HRT of 24 to 1 h) and 2.5 to 5.0 mM, respectively. Ferrous and pH were maintained constantly at 1 mM and 4.0. It was found that most Fe²⁺ was oxidized to Fe³⁺ and sequentially precipitated out via homogeneous nucleation after the initiation of Fenton reaction. Iron oxide particles obtained at the initial stage were very small; hence, the superficial upflow velocity had to be maintained at 1.6 m/d to prevent significant solid wash out. The upflow velocity was increased sequentially to 38.3 m/d once the fluidized solids became larger. Total iron removal efficiency of the fluidized-bed reactor was at 65% at the upflow velocity of 1.6 m/d and increased to 70% and 80% when the upflow velocity rose to 2.4 and 3.2 m/d, respectively, in the presence of 10 mM H₂O₂. However, with 5 mM H₂O₂, the performance was better around 78% to 80% for the upflow velocities from 1.6 to 3.2 m/d. This is because oxygen gas bubbles from H₂O₂ decomposition was higher at high H₂O₂ and, as a result, intensifying the carry over of iron oxide particles. Regardless of H₂O₂ concentration, total iron removal reached a plateau of approximately 85% at upflow velocity of 4.8 m/d and higher. Therefore, to successfully stimulate the homogeneous nucleation in the fluidized-bed Fenton reactor, superficial upflow velocity is considered a very important factor. Formic acid, the final soluble intermediate from the reaction between hydroxyl radicals and organic pollutants before being mineralized to carbon dioxide, at 1 mM did not cause any significant deterioration in total iron removal as initially expected. This might be because most of formic acid being added was oxidized by hydroxyl radicals through Fenton reaction leaving very low residuals to form complexes with Fe³⁺. In conclusion, homogeneous-nucleation of iron oxide in a fluidized-bed Fenton reactor is feasible. Total iron and organic removals can be simultaneously achieved under optimum conditions.