Insights into the Role of Photobleaching on the Photophysical and Photochemical Properties of Dissolved Black Carbon

The photochemical behavior of dissolved black carbon (DBC) has attracted increasing attention since DBC plays an important role in the migration and transformation of environmental contaminants in natural waters. However, little is known about the influence of photobleaching on the physicochemical properties of DBC and DBC-involved photodegradation of micropollutants. Herein, photobleaching of DBC was characterized systematically by spectroscopy, chromatography, and probe methods. Size exclusion chromatography and reactive intermediate (RI) determination results suggested that low molecular weight (MW) moieties (<1k Da) in DBC and high MW moieties (>10k Da) in NOM played a pivotal role in the formation of RIs. Following 120 h of photobleaching, the formation of RIs in DBC all decreased remarkably, while the RI yields for NOM were increased. In addition, the decay rate constants for triplet-labile and HO•-labile pharmaceutical and personal care products in a DBC solution after 48 h of bleaching were about an order of magnitude lower than those values in a DBC solution without bleaching, probably due to the attenuation of the intermolecular absorption. This study will not only provide useful insights into the photochemical properties of DBC but also reveal the pivotal role of photobleaching in the phototransformation of organic contaminants in natural waters.