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

•Dye-containing wastewater is degraded by heterogeneous Fenton’s oxidation.•The Fe-ZSM5 zeolite is a stable and active catalyst.•A continuous catalytic stirred tank reactor configuration is implemented.•Final effluent is biodegradable and non-toxic (0.0% of Vibrio fischeri inhibition).

This work is the first known report dealing with the heterogeneous Fenton-like process in a continuous stirred 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 basket reactor performance, namely temperature (in the range 10–70 °C), pH (1.5–4.0), feed hydrogen peroxide concentration – [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 < dp1 < 0.60, 0.60 < dp2 < 0.80 mm and as pellets, dp3 > 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 achieved 91% of discoloration and 36% of mineralization, at steady-state. Moreover, it was found a removal of 29% in terms of the chemical oxygen demand (COD), being worth noting the improvement in the effluent biodegradability (k′ – oxygen uptake rate – increased from 9.3 to 23.2 mgO2/(gVSS h)) and the fact that the final effluent is non-toxic (0.0% of Vibrio fischeri inhibition).The stability of the catalyst performance was checked during five consecutive runs. The crucial factor for the catalyst long-term use is the leaching of iron, which in all runs reached very low levels (e.g. only 0.173 mg/L of iron for the run in the optimized conditions, corresponding to only 0.13% of leaching). The catalyst was characterized by different techniques before and after the reactions (namely SEM/EDS and N2 adsorption); textural and chemical transformations during its use can be considered negligible (except for very acidic conditions of pH = 1.5) favoring the catalytic stability of the Fe–zeolite.

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