Undiscovered Multiple Roles of Multivalent Cations in the Pollutant Removal from Actual Water by Persulfate Activated by Carbon Materials

Enormous works have focused on the influence of natural organic matter (NOM) and anions (e.g., Cl^– and HCO₃^–) on the pollutant removal efficiency by persulfate activation technologies. However, the impact of multivalent cations in actual water, e.g., Ca²⁺, on the pollutant removal has been overlooked in past research. In this work, we surprisingly found that pollutant degradation by the carbon materials activating persulfate in some actual water (e.g., groundwater) was higher than that in the buffer solution, and the bridging effect of multivalent cations in the actual water was the main reason. Through the bridging effect of multivalent cations, persulfate was bound on the surface of carbon materials to accelerate its decomposition. However, the promotion mechanism of multivalent cations on the radical and nonradical pathways of persulfate activation was different. Furthermore, the NOM aggregation was also promoted by multivalent cations, thereby inhibiting its quenching effect on the pollutant degradation. Besides that, these multivalent cations could lead to the coagulation of carbon materials, which was harmful for its catalytic performance. This work highlights the multiple roles of multivalent cations in the actual water in the persulfate activated by the carbon material process, and it was conducive to the practical application of Fenton-like technologies in actual water treatment.