A novel quaternary ammonium structure of carbon dots modified TiO2 for fast reduction of Cr(VI) over a wide pH range under sunlight

Cr(VI) pollution in water caused by industrialization poses a serious threat to human health and ecosystems, necessitating a rapid reduction of Cr(VI) treatment method. Therefore, We have established a photocatalytic method for the rapid reduction of Cr(VI) by modifying TiO2 with carbon dots (CPC-CDs) that possess a quaternary ammonium structure in the first time. The CPC-CDs widens the band gap of TiO2 and increases the electrostatic interactions between CPC-CDs/TiO2 and Cr(VI), thus improving the reduction efficiency. CPC-CDs/TiO2 (0.1 g, 2 % CPC-CDs) exhibited excellent reduction performance and fully reduced Cr(VI) (50 mg/L) under neutral conditions within 150 min using a xenon lamp to simulate sunlight. Moreover, CPC-CDs/TiO2 achieved more than 95 % reduction efficiency over a wide pH range (pH = 3, 5, 7, and 9) and showed excellent reduction of Cr(VI) at various concentrations (10, 20, 30, 40 and 50 mg/L). CPC-CDs/TiO2 maintained high Cr(VI) reduction efficiency (93.57 %) after five cycles. Tests under real sunlight revealed that CPC-CDs/TiO2 thoroughly reduced Cr(VI) (10 mg/L) within 30 min, and exhibited excellent structural stability under conditions with interfering ions. Furthermore, CPC-CDs/TiO2 exhibited prominent antibacterial activity against Escherichia coli. In addition, free-radical trapping experiments verified that the superoxide radicals (•O2-) was the main active species. This study aims to overcome the limitations of photocatalytic reduction under conditions of high concentration Cr(VI), neutral and alkaline environments, and natural sunlight, thereby providing technical support for the rapid reduction of Cr(VI).