Benzotriazole (BT) and benzothiazole (BTH) derivatives as emerging marine pollutants in the Pearl River Estuary: Spatiotemporal distribution, environmental fates, and ecological risks

The estuarine environment is a complex and dynamic ecosystem in the transition zone where rivers meet the sea. However, information on the distribution, fate, and risks of novel organic ultraviolet filters and stabilizers (OUVs) in estuaries remains limited. In this study, the occurrence of benzotriazole (BT) and benzothiazole (BTH) derivatives in seawater, suspended particulate matter (SPM) and sediments in the Pearl River Estuary (PRE) was analysed. The total concentrations of both types of derivatives were much lower than those of their parent compounds in all the samples. 2-(2-Hydroxy-3-tert-butyl-5-methyl-phenyl)-5-chloro-benzotriazole (UV-326) and 5-methyl-1H-benzotriazole (5-TTri) were the main contributors to the total occurrence of BT derivatives in seawater, whereas UV-326 and 5-chloro-2-(3,5-di-tert-butyl-2-hydroxyphenyl) benzotriazole (UV-327) emerged as significant derivatives in sediments. 2-mercapto-benzothiazole (2-SH-BTH) and 2-thiocyanomethylthio-benzothiazole (2-SCNMeS-BTH) were the most important BTH derivatives in seawater and sediments, respectively. The mass balance modelling results indicated that riverine and coastal discharges serve as the primary input pathways for BT derivatives (1.50 × 10⁴ and 8.29 × 10³ kg/yr in summer and winter, respectively) and BTH derivatives (1.32 × 10⁴ and 9.46 × 10³ kg/yr in summer and winter, respectively) into the PRE. Moreover, outputs to the outer ocean play a decisive role in determining their main fates, with contributions ranging from 69.3 % to 84.3 %. The sedimentation and degradation of BT and BTH derivatives were not the primary sinks in seawater, suggesting that the estuary cannot serve as a significant barrier for these land-based pollutants. The risk quotient (RQ) results indicated that 2-SCNMeS-BTH has a moderate ecological risk (RQ of 0.105) in sediments, whereas all other BTs and BTHs have a relatively low risk (RQ < 0.1). This investigation offers pivotal insights into the environmental behaviours and ecological impacts of emerging marine pollutants within estuarine ecosystems.