Superior photo–Fenton activity towards chlortetracycline degradation over novel g–C3N4 nanosheets/schwertmannite nanocomposites with accelerated Fe(III)/Fe(II) cycling

The low Fe(III)/Fe(II) conversion rate seriously hinders the progress of heterogeneous Fenton reactions. In this work, g–C3N4 nanosheets/schwertmannite (CNNS/SCH) nanocomposites were fabricated by means of an in situ growth strategy to enhance the interfacial synergistic effect of photo–Fenton systems for chlortetracycline (CTC) degradation. More specifically, the photogenerated electrons in g–C3N4 can quickly transfer to the SCH via Fe–N bonds formed at interfaces of g–C3N4 and SCH for viable Fe(III) reduction under visible irradiation, and thus to promote the production of active radicals. Compared with an individual CNNS or SCH, CNNS/SCH nanocomposites exhibit superior photo–Fenton catalytic activity. In optimized conditions, 98.7% degradation efficiency of 50 mg L–1 CTC and up to 96.6% of TOC removal have been achieved in 120 min at an optimized pH. CNNS/SCH nanocomposites also display excellent long–term stability without obvious deactivation in five consecutive runs of photo–Fenton reactions. Quenching experiments and EPR analyses identify that hydroxyl radicals (OH) and singlet oxygen (1O2) are the main active species contributing to the degradation of CTC. Mechanism of the photo–Fenton–like processes with CNNS/SCH nanocomposite catalysts has been tentatively explored based on various photoelectrochemical measurements. It is concluded that combinational use of CNNS and SCH could significantly promote the separation of photogenerated electron–hole pairs, the interfacial charge transfer and the Fe(III) reduction at surfaces. This study might offer a new insight for the construction of highly efficient photo–Fenton catalysts to achieve effective treatment of priority pollutants in wastewater.