Micro-nano bubbles enhanced degradation of emerging contaminants by ferrous-oxalate complexes: synergistic interaction between oxidation and coagulation

Abstract

The activation of oxygen by ferrous (Fe²⁺) to generate ·OH for contaminants degradation was inhibited due to the low utilization of oxygen, thus limiting its application in the practical environment. In this study, with the superior oxygenation capacity of micro-nano bubbles (MNBs) and the stronger O₂ activation capacity of Fe²⁺-oxalate complexes, the MNBs/Fe²⁺/oxalate (Ox) system was constructed with 4,4′-sulfonyldiphenol (BPS) as the main target emerging contaminants (ECs), and to investigate the enhancement contribution and reinforcement mechanism of the involvement of MNBs to the removal efficiency of ECs in the Fe²⁺/Ox system. It was shown that the MNBs/Fe²⁺/Ox system could effectively degrade four structurally diverse ECs. In this case, with BPS as the main target contaminant, adding MNBs could increase the BPS removal efficiency by about 35%. In the MNBs/Fe²⁺/Ox system, the degradation rate of BPS depended on the concentration of Fe^II(Ox)₂²⁻, while the extent of degradation was mainly governed by Fe^II(Ox)₂²⁻ and Fe^II(Ox)⁰. EPR and probe experiments showed that the reactive oxygen species (ROS) produced by the system and the iron hydroxide complexes produced by Fe³⁺ hydrolysis contributed to the degradation of BPS by oxidation and coagulation, respectively. In particular, ·OH and O₂^•− were the main reactive oxygen species produced by this system. Moreover, the involvement of MNBs significantly increased the formation of ROS and iron hydroxide complexes in the Fe²⁺/Ox system. The oxygenation process of MNBs used in this study enhanced the contaminants degradation performance of the Fe²⁺/Ox system and has broadened the application scope of MNBs.

Graphical abstract

Keywords

Micro-nano bubbles / Ferrous / Molecular oxygen activation / Enhanced oxidation / Synergistic interaction

Highlight

	● The superior oxygenation capacity of MNB greatly improved pollutants degradation.
	● Degradation kinetics of BPS depended on the speciation of Fe2+-oxalate complexes.
	● Fe2+-oxalate activated O2 to yield ·OH by one and two electron transfer pathways.
	● MNBs boosted ROS formation while promoting iron hydroxide complexes generation.

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