Naturally-occurring iron minerals as inexpensive catalysts for CWPO

- Magnetite, hematite and ilmenite are active catalysts in CWPO.
- Increasing the temperature improves both H2O2 decomposition and mineralization.
- CWPO using iron minerals is more efficient than conventional homogeneous Fenton.
- Magnetite and hematite catalysts showed a good reusability upon CWPO of phenol.
- Magnetite and ilmenite catalysts can be easily recovered from water by magnetic separation.

This work explores the potential application of naturally-occurring minerals as inexpensive catalysts in heterogeneous Fenton, namely catalytic wet peroxide oxidation (CWPO). The availability, low cost and environmentally friendly character of those materials make them interesting candidates for such application. The performance of magnetite, hematite and ilmenite as CWPO catalysts has been tested under different working conditions, which include temperature (25-90 °C), H2O2 dose (250-1000 mg L−1) and catalyst concentration (1-4 g L−1). The operating temperature plays a key role on the rate of H2O2 decomposition so that with magnetite H2O2 conversion after 4 h increased from 8 to 99% by increasing the temperature from 25 to 90 °C. Based on the reaction mechanism proposed, a kinetic model was developed which successfully described the experimental results on H2O2 decomposition. The catalytic performance of the minerals tested at temperatures above the ambient was demonstrated using phenol (100 mg L−1) as target pollutant. Unprecedented efficiencies of H2O2 consumption, higher than 80% were achieved, allowing high oxidation and mineralization, i.e. complete phenol conversion and almost 80% TOC reduction at 75 °C with a catalyst loading of 2 g L−1 and the theoretical stoichiometric amount of H2O2 for complete mineralization of phenol (500 mg L−1). Magnetite is particularly attractive, since it showed the highest activity and can be easily separated from the liquid phase given its magnetic properties. All the minerals tested suffered low iron leaching and magnetite and hematite showed a good reusability upon three consecutive runs. However, in this case long-term durability is not a crucial issue, given the availability and low cost of these minerals.

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