Presentation Abstract

Occurrence, Environmental Fate, and Exposure Assessment of Selective Serotonin Reuptake Inhibitors (SSRIs) in Aquatic Environments
Kevin L. Armbrust and Jeong Wook-Kwon
State Chemical Laboratory of Mississippi, Mississippi State, MS

Pharmaceuticals can enter aquatic environments after their prescribed use and lead to negative effects on aquatic organisms. Ideally, information on the environmental fate and effects of these chemicals would be useful prior to their registration so that exposure assessments, and ultimately risk assessments, could be conducted proactively. The Subdivision N battery of tests required by the U.S. Environmental Protection Agency for pesticide registration provide a convenient model to use for pharmaceutical products, as environmental degradation processes are not discriminatory to a chemical’s product-use pattern. Selective serotonin reuptake inhibitors (SSRIs) are among the most heavily prescribed drugs. They are biologically active, low concentrations have been shown to affect aquatic organisms, and evidence indicates that they can be present in effluents from wastewater treatment plants. The physical and chemical properties and rates of degradation of five SSRIs (citalopram, fluoxetine, fluvoxamine, paroxetine, and sertraline) were measured in investigations modeled after those used for pesticide registration. Additionally, their occurrence in raw wastewater, treated effluent, and downstream receiving waters also was measured. The salts of SSRIs had high water solubilities ranging from 3,022-15,460 mg/L and relatively low octanol-water partition coefficients (log K_ow) ranging from 1.12-1.39. Two sediments and three soils with organic matter contents ranging from 0.16 to 1.77 percent and pH ranging between 5.6 and 7.8 were used to measure adsorption coefficients. Values of K_f, K_d, and K_oc ranged from 39 to 18,342, from 60 to 42,579, and from 2,256 to 1,053,380, respectively. No significant hydrolytic degradation was detected for any drug in any aqueous solution.

Photolysis was a potential route of degradation for several SSRIs. Paroxetine and fluvoxamine rapidly photodegraded, with half-lives of 0.5-0.7 days and 3.6-6.0 days, respectively. Fluoxetine and citalopram were stable to photolysis at all pH values tested. Most SSRIs except fluvoxamine did not show enhanced photodegradation in synthetic humic water and natural water, possibly because of light attenuation by natural water materials. Several degradation products for each SSRI were detected and identified by liquid chromatography-electrospray ionization-mass spectrometry (LC/ESI/MS). Hydroxyl radical rate constants, measured by competition kinetic methods, ranged from 1.41 x 10¹² to 1.99 x 10¹³ M^-1 h^-1. All SSRIs treated to irradiated water/sediment systems dissipated rapidly, in part as a result of photolysis but mostly because of adsorption to sediment. Nearly constant SSRI residues over time indicated that SSRIs are resistant to microbial metabolism in sediments. No degradation was observed for any drug over a 28-day exposure period in ready biodegradability investigations using activated sludge from a wastewater plant. Methods that employ solid-phase extraction and LC/MS/MS, using ESI in positive ion mode for the determination of five SSRIs and their metabolites in surface water samples, have been developed. The limits of quantitation ranged from 0.4-2.4 ng/L. Samples of influent, effluent, upstream, and downstream water were collected monthly and analyzed. Fluoxetine and sertraline were detected in all samples, ranging in concentration from 0.006-0.076 μg/L and 0.007-0.061 μg/L, respectively. Citalopram also was detected at a concentration of 0.006-0.064 μg/L in selected samples. Laboratory data indicate that the SSRIs as a general class resist most forms of degradation in environmental systems and would partition to sediment where residues of these compounds would persist.

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