Hydrodynamic cavitation-enhanced heterogeneous activation of persulfate for tetracycline degradation: Synergistic effects, degradation mechanism and pathways

Mass transfer and oxidant utilization are perhaps two of the most critical issues in sulfate radical (SO4•−) based advanced oxidation technologies (AOTs) and their scaled-up implementation. In this study, we propose using hydrodynamic cavitation (HC), considered a green, effective method, to promote both mass transfer and oxidant utilization in zero-valent iron (Fe0) activated persulfate (PS) system. Whilst the BET surface area of Fe0 was increased by 8 times after HC treatment, concentration of Fe2+ derived from Fe0 oxidation is greatly increased for effective PS activation. The reappearance of Fe0 and Fe2+ after cavitation ensured a good reusability of the catalyst. Likewise, the impact of pH revealed that TC adsorption on catalyst at acidic pH favored its degradation compared with that at higher pH. With respect to oxidant utilization, it is observed that PS even at a high dosage (2.8 mM) was completed converted within 30 min in the HC-Fe0/PS system. According to SEM, TEM, and BET analysis, we conclude that the microjets induced by cavitation bubbles or direct abrasion by HC agitation have contributed to the removal of hydroxide/oxide layers on the Fe0 surface, thus reactivating its catalytic activity. Given these reasons, we observed up to 97.80% removal of Tetracycline (TC), the model pollutant, with a synergistic coefficient as high as 2.62. After confirming SO4•− as the most dominant reactive species, five degradation pathways of TC were proposed given the intermediate evidence from LC-MS/MS analysis and density functional theory (DFT) calculations. Results from this study could provide new insights into the role of HC on PS activation and shed light on the potential implementation of the SO4•−-based AOTs for scaled-up wastewater treatments.