Trace amino-containing indoline-mediated Fe(VI) activation toward effective abatement of 6-methylquinoline: A new application scenario of Fe (VI) without the involvement of external activators

Recently, ferrate (Fe(VI)), an emerging and environmentally friendly oxidant, has been applied to degrade organic micropollutants in various scenarios due to its high reactivity and elimination of byproduct formation. This study developed a novel Fe(VI)-based process by using a new-found pollutant in shale gas wastewater (SGW), i.e., indoline, to activate Fe(VI), termed as Fe(VI)/indoline process, to enhance the degradation of 6methylquinoline (6-MQ). The Fe(VI)/indoline process can remove 97.6 % of 6-MQ in 20 min at pH 9, while the degradation rate constant significantly increased 6.45 times compared with the Fe(VI) process. Scavenging and probing tests and electron paramagnetic resonance analysis were conducted to exclude the generation of radicals and the formation of reactive meta-stable complexes upon activation of Fe(VI) by indoline, and to identify the accelerated production of Fe(IV)/Fe(V) from Fe(VI) conversion at the existence of indoline responsible for the enhanced 6-MQ degradation. The interaction between Fe(VI) and indoline underwent an oxygen atom transfer (OAT) process primarily to generate Fe(IV) and a single electron transfer (SET) process secondarily to produce Fe(V), collectively contributing to the degradation of 6-MQ. The Fe(VI)/indoline process performed a high anti-interference ability toward anions existing in SGW, suggesting its potential in practical application for SGW treatment. Of particular significance is that the contaminant-induced Fe(VI) activation endows Fe(VI) with a new application scenario in SGW for self-decontamination.