Challenges of dye removal treatments based on IONzymes: Beyond heterogeneous Fenton

Abstract

Dyes are recalcitrant organic compounds that generate environmental damage. Heterogeneous Fenton-like catalysts, catalytic systems using reusable solid-state catalysts as an alternative for Fe2+ in solution, have gained widespread attention in dye removal. This article reviews and discusses the catalytic activity and stability enhancement in Fenton-like processes. The reported strategies to improve performance were coating and functionalization of magnetic nanoparticles, substitution of magnetite with transition metals, as well as the use of organic supports with a focus on biomass-derivatives. The precursors and the synthetic routes of iron oxide nanoparticles can be adjusted to tune the properties of resulting solids. Hence, the common techniques applied in the synthesis and characterization of nanosized iron oxides were briefly summarized. Besides, the application and efficiency of IONzymes (Iron Oxides Nanozymes) in azo and anthraquinone dye removal were also analyzed, highlighting the synergistic effect between adsorption and Fenton-like processes. The knowledge to apply IONzymes in the remediation of colored effluents is still in continuous development due to several unsolved problems. The main challenges to overcome are the acidic pH as an optimal reaction condition, the dissolution of metal ions in the reaction medium, the deactivation of catalysts and the generation of contaminating by-products. Finally, an obstacle identified in the process output was the disposal of exhausted catalysts and adsorbents. These could be applied in further remediation of other contaminants. This will imply a huge innovation effort to minimize the resulting waste and to simultaneously reduce the consumption of raw material in another process.