Mesoporous Ca–Mn–O as an efficient scavenger toward organic pollutants and heavy metals: ion exchange provoking ultrafast Fenton-like reaction based on the synergy of alkaline earth/transition metals

Abstract

Intricate Fenton-like catalysts composed of mixed transition metal oxides (TMOs) frequently outperform simple TMOs in scavenging organic contaminants in wastewater. Nonetheless, their activity and/or reusability may sharply decline when abundant heavy metal ions in wastewater adsorb on the catalytic surface and passivate it. This study shows that mesoporous Ca–Mn mixed oxides prepared from a facile combination of Mn and an alkaline earth metal (i.e., Ca) (denoted as CaMn_x) can readily overcome this problem. CaMn_x without any additive (e.g., H₂O₂) was able to degrade organic contaminants (e.g., indigo carmine as a model contaminant) fast within 10 seconds, outperforming many other reported heterogeneous Fenton catalysts. Besides organics, the coexisting heavy metal ions (e.g., Cu²⁺ and Pb²⁺ as indicators) were also rapidly extracted. The outstanding performance was attributed to the synergy of Ca and Mn. CaO in CaMn_x efficiently extracted metal ions via an ion exchange process, and MnO_x generated abundant H₂O₂ and degraded the organics through an uncommon Fenton-like reaction. Therein, Cu species extracted by CaO catalyzed the decomposition of H₂O₂ into HO˙ radicals, speeding up the degradation reaction of the organics by 81 times. Benefiting from the gradual exposure of fresh Mn-sites protected by unreacted CaO, CaMn_x without regeneration treatment maintained high efficiency in scavenging organics and heavy metal ions in cycling tests. CaMn_x also showed great promise in treating complex water samples containing various heavy metal ions (e.g., Pb²⁺, Cu²⁺, Cd²⁺, Ag⁺, Zn²⁺ and Cr³⁺) and organic pollutants. The findings of this work enrich the rational design of related functional nanomaterials.