Interaction patterns in fluidized-bed Fenton process for the degradation of recalcitrant pollutants: theoretical and experimental insights

The interaction of carriers (SiO2) and Fenton’s reagent during the degradation of recalcitrant pollutant by fluidized-bed Fenton process was investigated. Reactive Black 5 (RB5) was selected as a model pollutant since dyes are recalcitrant to conventional treatment technologies. Quantum chemical simulation and experimental approach were employed to predict the potential interactions among the chemical species involved in the process. Quantum chemical parameters such as highest occupied molecular orbital (HOMO) energy, lowest unoccupied molecular orbital (LUMO) energy, HOMO–LUMO energy gaps were computed and analyzed. Response surface methodology was utilized to investigate the effect of operational parameters and optimize the process performance. The analysis of quantum chemical parameters shows that with a comparatively higher EHOMO of − 8.72 eV, SiO2 can interact with Fe3+ (EHOMO = − 18.74 eV), which explains the crystallization of iron oxide on the carriers. The higher EHOMO–LUMO energy gap value of SiO2 (7.92 eV) indicates its potential interaction with Fe2+, Fe3+ and RB5 due to their lower EHOMO–LUMO energy gaps. RB5 exhibited the lowest hardness value (1.23 eV), indicating that it can be degraded by the hydroxyl radical (2.11 eV). For the experimental part, the process can remove up to 80% and 99.94% of the initial COD and color, respectively, under optimum conditions. The most significant parameters affecting the process performance are [Fe2+] and [Dye]. This study has provided insights into the potential interactions occurring in fluidized-bed Fenton process. The results will be useful toward optimization and scale-up of fluidized-bed Fenton process.