Photolysis and photocatalysis of tetracycline by sonochemically heterojunctioned BiVO4/reduced graphene oxide under visible-light irradiation

The widespread use of antibiotics in pharmaceutical therapies and agricultural practice has led to severe environmental pollution. In this study, the simultaneous photolysis and photocatalysis behaviors of tetracycline (TC), one of the most frequently prescribed groups of antibiotics, were investigated using BiVO4 (BVO) supported on reduced graphene oxide (rG0). The resulting BVO/rGO nanocomposite (NC) showed prominent adsorption performance and photocatalytic ability under wide initial pH conditions (from acidic to alkaline: pH 2.5, 6.7, 9.2 and 10.5). This study analyzed the kinetics and proposed a mechanism for the photolytic and photocatalytic degradation of TC under visible light irradiation with BVO and BVO/rGO. The photolysis and photocatalytic degradation efficiency of TC was largely influenced by the solution pH and increased with increasing initial pH. The TC was stable without significant photolysis at pH 2.5, while TC photolysis increased up to 17% at pH 9.2. With further increase in the solution pH from 9.2 to 10.5, the light absorption of TC at 356 nm showed a red shift to 372 nm and new absorption peaks at around 533 nm were formed due to the formation of new colored intermediates. The photocatalytic degradation activities of TC by BVO/rGO under visible light irradiation reached 55, 67, 92 and 99% at initial pH 2.5, 6.7, 9.2 and 10.5, respectively. However, when using BVO only, the photocatalytic degradation of TC was 42, 61, 73 and 85% at pH 2.5, 6.7, 9.2 and 10.5, respectively. The great improvement of photocatalytic activity of BVO/rGO is attributed to the reduced particle size, increased adsorption ability of rGO, extended photo responding range of BVO, and efficient separation of photogenerated charge carriers, which are derived from the ultimate coverage of the BVO by the rGO.