Heterogeneous Fenton's oxidation using Fe/ZSM-5 as catalyst in a continuous stirred tank reactor

This work is the first known report dealing with the heterogeneous Fenton-like process in a continuousstirred tank reactor. A Fe/ZSM-5 zeolite was used as catalyst for degradation of an azo dye (Orange II, OII)-containing solution.A parametric study was carried out to evaluate the effect of the main operating conditions in the basketreactor performance, namely temperature (in the range 10-70 C), pH (1.5-4.0), feed hydrogen peroxideconcentration - [H2O2]feed (1.75-20.0 mM), contact time - W/Q (10-200 mg min/mL), residence time -tresid (30-180 min) and the size of the catalyst particles (0.25 5 mm), for a OII feed concentration of 0.1 mM. Under the best operating conditions found(pH = 3.0, T = 70 C, [H2O2]feed = 6 mM, W/Q = 200 mg min/mL, dp2 and tresid = 90 min), it was achieved91% of discoloration and 36% of mineralization, at steady-state. Moreover, it was found a removal of29% in terms of the chemical oxygen demand (COD), being worth noting the improvement in the effluentbiodegradability (k0 - oxygen uptake rate - increased from 9.3 to 23.2 mgO2/(gVSS h)) and the fact that thefinal effluent is non-toxic (0.0% of Vibrio fischeri inhibition).The stability of the catalyst performance was checked during five consecutive runs. The crucial factorfor the catalyst long-term use is the leaching of iron, which in all runs reached very low levels (e.g. only0.173 mg/L of iron for the run in the optimized conditions, corresponding to only 0.13% of leaching). Thecatalyst was characterized by different techniques before and after the reactions (namely SEM/EDS andN2 adsorption); textural and chemical transformations during its use can be considered negligible (exceptfor very acidic conditions of pH = 1.5) favoring the catalytic stability of the Fe-zeolite