CoFe2O4 prepared from cobalt-doped coal-gangue-leached iron as an effective peroxymonosulfate activator to degrade rhodamine B: preparation, application and degradation mechanism.

The typical organic dye rhodamine B (RhB), which poses a serious threat to freshwater safety, could be effectively eliminated via an advanced oxidation technology based on peroxymonosulfate (PMS). Developing efficient catalysts is the key to this process. Herein, CoFe2O4 (CFO) catalysts were synthesized via co-precipitation, hydrothermal, and sol-gel methods using an activated acid leaching/extracting/back-extracting process to recover iron from coal gangue as the Fe source and chemically pure cobalt chloride hexahydrate as the cobalt source. The prepared catalysts were applied as a high-efficiency activator of PMS for RhB degradation. As a result, the CFO catalyst prepared by the co-precipitation method could essentially achieve a degradation efficiency of 99.38% for RhB within 20 min; the optimal reaction parameters were a reaction temperature of 20 °C, catalyst concentration of 0.2 g L-1, dye concentration of 30 mg L-1, initial pH of 7, and PMS concentration of 0.7 g L-1. A series of characterizations demonstrated that the catalyst had a large specific surface area, a rich pore structure, and excellent magnetic properties. Combined with radical quenching experiments, EPR and XPS, a possible PMS mechanism was proposed. 1O2 was the main active substance for the catalytic system, and SO4 - and ˙OH were also produced during the PMS activation process. The work not only provides a valuable reference for the high-value utilization of high-iron coal gangue but also lays the foundation for the wet leaching of iron from coal gangue to prepare efficient PMS activators for treating organic dye wastewater.