Novel activated system of ferrate oxidation on organic substances degradation: Fe(VI) regeneration or Fe(VI) reduction

Abstract

Ferrate(VI) oxidation has caught more attention because of its high-efficiency degradation capacity on organic substances, while previous studies overlooked the effect of NaClO and NaClO2 on Fe(VI) oxidizability. This study discovered that NaClO could react with intermediate iron species, i.e., Fe(V) and Fe(IV), to regenerate Fe(VI), which reduced the unnecessary consumption of Fe(VI) by its self decomposition and thus improved the oxidizability of ferrate(VI) on organic substances. Then, the degradation content of sulfamethoxazole (SMX) and carbamazepine (CBZ) increased from 60.77 % and 68.42 % of 50 μM of Fe(VI) group to 100 % and 73.21 % of Fe(VI)/NaClO (50/5 μM) system within 30 min, respectively. However, NaClO2 contributed to reducing Fe(VI) to form more intermediate iron species in the reaction system and then elevated the degradation rate of SMX and CBZ at the initial stage. NaClO2 also further reduced intermediate iron species, leading to the decrease of active species. Besides, OH was involved into organics degradation in Fe(VI)/NaClO2 system, due to the rapid decomposition of ferrate(VI). By frontier orbital theory analysis, Fe(VI) had the lowest ELUMO (-5.50 eV) so it was more liable to be reduced by NaClO2 than intermediate iron species. Alpha orbital of Fe(V) possessed the highest EHOMO (−7.36 eV), which was more prone to be attacked by NaClO. Two coupling systems still achieved a synergistic effect on SMX degradation in authentic water. Meanwhile, the acute toxicity and disinfection by-products (DBPs) was further decreased in coupling systems. Therefore, Fe(VI)/NaClO and Fe(VI)/NaClO2 system may be a viable method for abating micropollutants with their unique advantages in water treatment.