Role of Photodegradation in the Fate of Fluorescent Whitening Agents (FWAs) in Lacustrine Environments

Abstract

To understand the behavior of fluorescent whitening agents (FWAs) in a lake environment, we measured the quantities of two FWAs, DSBP, and DAS1, in water samples collected monthly from six depths of the water column, in sediment trap sample, and a sediment core sample from Lake Biwa, the largest lake in Japan, and in sewage, effluent, and river water in the lake's catchment. We conducted a sunlight exposure experiment and developed a method to estimate the degree of photodegradation by using DSBP/DAS1 ratio in environmental samples. The observed seasonal pattern of the vertical distributions of the FWAs in the water column can be explained by stratification of the water, photodegradation in the euphotic zone, the subsurface loading of river water, and their seasonal changes. The DSBP/DAS1 ratio was much lower in the lake water (0.12-0.52) than in sewage (6.4 ± 1.1), indicating intensive photodegradation in rivers and the lake. A mass balance calculation and DSBP/DAS1 ratio demonstrated that ∼95% of DSBP and ∼55% of DAS1 supplied in sewage were photodegraded in inflowing rivers and the lake, and that sedimentation to the lake bottom is insignificant for DSBP and ∼35% for DAS1. More intensive photodegradation of FWAs, especially more photodegradable DSBP, in Lake Biwa than in Greifensee, a lake in Switzerland, was suggested, attributable to the longer residence time of water in and the larger size of Lake Biwa. These results demonstrate that photodegradation is important to the fate of FWAs in lacustrine environments, and that FWAs and the DSBP/DAS1 ratio are useful markers for understanding the role of direct photodegradation in the behavior of water-soluble chemicals in aquatic environments.