Environ. Sci. Technol. 2010, 44, 7796–7801
Role of Photodegradation in the Fate of Fluorescent Whitening Agents (FWAs) in Lacustrine Environments NOBUHISA YAMAJI,† KAZUHIDE HAYAKAWA,‡ AND H I D E S H I G E T A K A D A * ,† Laboratory of Organic Geochemistry, Faculty of Agriculture, Tokyo University of Agriculture and Technology, Fuchu, Tokyo 183-8509, Japan, and Lake Biwa Environmental Research Institute, Otsu, Shiga 520-0022, Japan
Received February 11, 2010. Revised manuscript received August 17, 2010. Accepted August 23, 2010.
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.
Introduction Lakes are of enormous importance for human development and for the preservation of sound ecosystems and biodiversity. They are Earth’s major sources of fresh water, they sustain aquatic biodiversity, and they provide livelihoods and socio-economic benefits to many millions. Human activities, however, are increasingly reducing the ecological * Corresponding author phone: +81-42-367-5825; fax: +81-42360-8264; e-mail:
[email protected]. † Tokyo University of Agriculture and Technology. ‡ Lake Biwa Environmental Research Institute. 7796
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integrity of lakes. In particular, lakes suffer various anthropogenic threats from chemical contaminants which may degrade the ecosystem. Causing special concern is the ubiquitous occurrence of pharmaceuticals and personal care products (PPCPs) in sewage and effluent, as has been demonstrated during past decade (1-3). In modern societies, most sewage receives activated-sludge treatment, whereby contaminants are removed by microbial degradation and sorption to sludge. Therefore, PPCPs found in sewage effluent are basically water-soluble (i.e., not removed by sorption) and resistant to microbial degradation. Several studies demonstrated the photodegradation of PPCPs in surface waters (e.g., refs 2, 4, 5). Thus, photodegradation could be one of the important processes for the removal of PPCPs in receiving waters. Factors affecting the photodegradation of PPCPs (e.g., light conditions, dissolved organic matter, inorganic ions) have been studied (6, 7). An understanding of the capacity and characteristics of photodegradation (e.g., accumulated light irradiation) in aquatic systems is therefore important to assessing the fate of PPCPs. During recent decades, a number of molecular markers have been discovered and used in various studies (8). Two fluorescent whitening agents (FWAs), viz., 4 4′-bis(2-sulfostyryl) biphenyl (DSBP) and 4 4′-bis[(4-anilino-6-morpholino1 3 5,-triazine-2-yl)amino]stilbene-2 2′-disulfonate (DAS1), have been used as molecular markers of sewage (9). Their structures are shown in Supporting Information (SI) Figure S1. FWAs are minor components (∼0.15%) of laundry detergents, and are discharged into domestic wastewater. FWAs are hydrophilic contaminants, and approximately 90% of FWAs in secondary wastewater effluent were found in the dissolved phase (10). FWAs are resistant to biodegradation (11). Because of their widespread use and persistent nature, they have been used as molecular markers for assessing the discharge and transport of domestic wastewater to rivers, lakes, and oceans. Furthermore, FWA molecules are used as probes to assess photochemical reactions in the surface water of rivers and lakes, owing to their sensitivity to photodegradation (9, 12, 13). Since DSBP is more labile in photochemical reactions than DAS1 (12), we proposed that the ratio of DSBP to DAS1 (DSBP/DAS1) could be used as an indicator of the degree of photodegradation and of the residence time of a water mass (14). Our studies of FWAs in coastal environments (15, 16) demonstrated the utility of DSBP/DAS1 as a geochemical indicator of photochemical processes in aquatic environments. However, these previous uses of the DSBP/DAS1 ratio were qualitative. Here we developed a method to quantitatively estimate the degree of photodegradation by using DSBP/DAS1 ratios based on our photodegradation experiment and those in the literature. We applied this quantitative approach to a mass balance study in a lacustrine system to understand the role of photodegradation in the behaviors of FWAs. We surveyed FWA behavior in Lake Biwa, Japan’s largest lake. The only previous study of FWA behavior in a lacustrine environment was conducted in Greifensee, a small lake in Switzerland (13). The contribution of photodegradation to the mass balance of FWAs in Greifensee was determined by a combination of intensive monitoring of FWAs and model calculation (13). Lakes have different geological and geochemical characteristics and receive different magnitudes of anthropogenic impacts, which affect the behaviors of FWAs. Thus, a comparison of FWA behaviors between lakes can aid a general understanding of the behaviors of PPCPs in lakes. We intensively monitored FWAs in lake water, inflowing river water, and sewage effluent in the catchment of Lake Biwa. 10.1021/es100465v
2010 American Chemical Society
Published on Web 09/01/2010
We placed special emphasis on understanding the contribution of photodegradation to FWA removal. Through the comparison of our results with those from Greifensee, we address the role of photodegradation in the behavior of PPCPs in lacustrine environments.
Materials and Methods Study Site. Sampling locations in Lake Biwa and its catchment are shown in SI Figure S2. Lake Biwa holds 27.5 km3 of water, has surface and catchment areas of 670 and 3174 km2, respectively, and has average and maximum depths of 41 and 104 m (17). The human population of the catchment is 1.3 million. Although there are more than a hundred inflowing rivers, the only outflowing river is the Seta River. Mean residence time of water in Lake Biwa is 5.5 years. It is a warm monomictic lake with turnover from February to March. The lake’s geophysical parameters are shown in SI Table S1. Nutrient levels are mesotrophic, and concentrations of dissolved organic carbon are 80-130 µM C (18). Three types of municipal sewage treatment are present in the study area. Regional sewage treatment plants (STPs) treat wastewater from tens of thousands of families. Local STPs treat wastewater from hundreds of families. Both use physical treatment followed by activated sludge treatment. On-site septic tanks treat wastewater from single families. Hydraulic retention times are ∼10, 6, and 30 h, respectively. Together they serve 73.6% of the population (19). Untreated sewage (gray water) from the balance of the population (26.4%) is discharged to streams and rivers in the catchment. Regional STPs discharge secondary effluent directly to the lake. Local STPs and on-site treatment systems discharge to rivers that flow into the lake. Sampling. To trace the pathways of FWAs from the sources to the lake, we sampled sewage influent, sewage effluent, river water, lake water (surface and water column), settling particles, and a sediment core sample in the lake. We obtained 24 h composite samples of influent and effluent from five regional STPs. A set of grab water samples of influent and effluent were collected from one of the local STPs with a 6 h delay between influent and effluent to catch the same water mass. Two sets of grab water samples of influent and effluent were collected from one of the on-site treatment systems. We collected 24 h time-series water samples at 3 h intervals from the Yasu River (station Y) in February 2002. Grab water samples were also collected from five main inflowing rivers and the outflowing Seta River in August and December 2001 and in February 2002. Lake water samples were collected from six depths (2.5, 7.5, 15, 30, 40, 70 m) at Station N (35°23.45′N, 136°7.73′E) monthly from July 2001 to July 2002 in a Niskin water sampler. Vertical thermal profiles were monitored simultaneously. All water samples were transferred to amber glass bottles immediately after sampling and filtered through prebaked glass fiber filters (Whatman GF/F, UK) in a darkroom. The filtrates were stored at 5 °C without preservatives, and the filters were preserved at -30 °C. Sinking particles were collected in two cylindrical sediment traps (30 cm diameter, 50 cm length, described elsewhere in detail (20)), and launched at a depth of 70 m (19 m above the bottom) at station N from 1 to 5 July (96 h) 2002. The traps were covered with a 1 mm mesh net at the mouth to exclude large organisms. A sediment core sample was collected at station N in 2000 by using a gravity corer with a plastic pipe of 5 cm i.d. Upon recovery onboard, the core was sliced into 1 cm segments, transferred to a plastic container, and stored at -30 °C. Both filter and sediment samples were freezedried before analysis. Analytical Methods. FWAs were analyzed according to previous methods (14). The FWAs in the filtrates were extracted through an octadecyl silica (ODS) cartridge and the FWAs were eluted with MeOH. The eluent was concentrated and analyzed for DSBP and DAS1 by high-performance
liquid chromatograph (HPLC) with fluorescence detector. DSBP and DAS1 concentrations are reported as sum of cisand trans-isomers, respectively, though both isomers were determined following postcolumn irradiation. The FWAs in particulate phases and sediments were extracted with tetrabutylammonium/MeOH by ultrasonication. The extracts were rotoevaporated and redissolved in distilled water and the FWAs were analyzed by the same procedure as the filtrate samples. Details are described in SI Section SI-1. Analytical precision was examined through replicate (3-4×) analyses of 100 mL river water and river sediment. Relative standard deviations for DSBP and DAS1 were
